WO2000052791A9 - Rare-earth doped phosphate-glass lasers - Google Patents

Abstract

Apparatus and method for integrating rare-earth doped lasers and optics on glass substrates. An optical (e.g., laser) component formed from a glass substrate doped with an optically active lanthanides species with a plurality of waveguides defined by channels within the substrate. The laser component may constitute a monolithic array of individual waveguides in which the waveguides of the array form laser resonator cavities with differing resonance characteristics. The channels defining the waveguides are created by exposing a surface of the substrate to an ion-exchange solvent through a mask layer having a plurality of line apertures corresponding to the channels which are to be formed. Another aspect is directed toward pumping the laser. A laser component formed from a glass substrate doped with a laser species and having one or more substrate waveguides defined therein, and superstrate waveguide cavity, or cladding, positioned adjacent the substrate waveguide for supplying the latter with pump light. Another aspect provides a closed crucible processing of optical waveguides on a glass substrate. Waveguides are created by exposing a surface of the substrate to an ion-exchange solvent (e.g., a molten potassium or sodium salt). A tightly sealed multi-part crucible made, for example, of aluminum, is sealed with a graphite gasket tightly clamped between flanges on opposing portions of the crucible, in order that gas does not leak in or out of the crucible during cooling or heating of the system. In one embodiment, a potassium-doped waveguide is buried by a thin sodium-doped layer.

Description

RARE-EARTH DOPED PHOSPHATE-GLASS LASERS

This application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application Seπal Number 60/117,477 filed January 27, 1999 and to Provisional Application Seπal Number 60/162,458 filed October 29, 1999, both of which are incorporated by reference.

Statement as to Rights to Inventions Made Under Federally Sponsored Research and Development

Certain aspects of these inventions were developed with support from NIST (National Institute for Standards and Technology). The U.S. Government may have πghts in certain of these inventions.

Field of the Tnvention

This invention relates to the field of optics and lasers, and more specifically to a method and apparatus for pumping optical waveguide lasers formed on a glass substrate.

Background of the Invention The telecommunications industry commonly uses optical fibers to transmit large amounts of data in a short time. One common light source for optical-fiber communications systems is a laser formed using erbium-doped glass. One such system uses erbium-doped glass fibers to form a laser that emits at a wavelength of about 1.536 micrometer and is pumped by an infrared source operating at a wavelength of about 0.98 micrometer. One method usable for forming waveguides in a substrate is descπbed in U.S. Patent 5,080,503 issued Jan. 14, 1992 to Najafi et al., which is hereby incorporated by reference. A phosphate glass useful in lasers is descπbed m U.S. Patent 5,334,559 issued Aug. 2, 1994 to Joseph S. Hayden, which is also hereby incorporated by reference. An integrated optic laser is described in U.S. Patent 5,491,708 issued Feb. 13, 1996 to Malone et al , which is also hereby incorporated by reference. There is a need in the art for an integrated optical system, including one or more high- powered lasers along with routing and other components, that can be inexpensively mass-produced The system should be highly reproducible, accurate, and stable.

Summary of the Invention

The present invention is embodied by a laser component that includes a glass substrate doped with one or more optically active lanthamde species and having a plurality of waveguides defined by channels withm the substrate (As used herein, a "channel withm the substrate" is meant to broadly include any channel formed on or in the substrate, whether or not covered by another structure or layer of substrate.) Each substrate waveguide (or "channel") is defined withm the substrate as a region of increased index of refraction relative to the substrate. The glass substrate is doped with one or more optically active lanthamde species which can be optically pumped (typically a rare-earth element such as Er, Yb, Nd, or Pr or a combination of such elements such as Er and Yb) to form a laser medium which is capable of lasmg at a plurality of frequencies. Mirrors or distπbuted Bragg reflection gratings may be located along the length of a waveguide for providing feedback to create a laser-resonator cavity One or more of the mirrors or reflection gratings is made partially reflective for providing laser output.

The laser component may constitute a monolithic array of individual waveguides m which the waveguides of the array form laser resonator cavities with differing resonance characteπstics (e.g., resonating at differing wavelengths). The component may thus be used as part of a laser system outputtmg laser light at a plurality of selected wavelengths. In certain embodiments of the invention, the resonance characteπstics of a waveguide cavity are vaπed by adjustmg the width of the channel formed in the substrate which thereby changes the effective refractive mdex of the waveguide. The effective refractive index can also be changed by modifying the diffusion conditions under which the waveguides are formed as descπbed below. A diffraction Bragg reflector (DBR) grating formed into or close to the waveguide is used, in some embodiments, to tune the wavelength of light supported in the waveguide cavity. Changing the effective refractive index thus changes the effective wavelength of light m the waveguide cavity which determines the wavelengths of the longitudinal modes supported by the cavity In another embodiment, the resonance characteπstics of the waveguide cavities are individually selected by varying the pitch of the DBR reflection gratings used to define the cavities which, along with the effective refractive index for the propagated optical mode, determines the wavelengths of light reflected by the gratings. In still other embodiments, the location of the gratings on the waveguide is vaπed m order to select a laser-resonator cavity length that supports the desired wavelength of light

In some embodiments, the laser element is constructed from a glass substrate which is a phosphate alkali glass doped with a rare-earth element such as Er or Yb/Er. The channels defining the waveguides are created by exposing a surface of the substrate to an ion-exchange solvent through a mask layer having a plurality of line apertures corresponding to the channels which are to be formed. The ion exchange may be earned out through an aluminum mask layer m an aluminum or borosihcate glass crucible using molten potassium nitrate as a solvent. Lessened etching of the substrate by the ion-exchange melt has been found to occur m some embodiments that use a tightly sealed aluminum crucible having a graphite gasket between opposing flanges that are tightly bolted together, and having two reservoirs, one for holding the salt melt away from the glass wafers duπng heating (and later cooling) and another reservior for holding the salt melt m contact with the glass wafers during ion-exchange processing In other embodiments, a borosihcate crucible is used and if the potassium nitrate is pre -baked at a temperature of at least 120 degrees C for 24-48 hours in an inert argon atmosphere In other embodiments, the crucible is placed inside a fully enclosed chamber duπng the ion-exchange process, with the chamber filled with an inert atmosphere. Carrying out the ion-exchange process in an enclosed chamber has been found to lessen surface etchmg due to oxidation reactions. The exchange of K for Na in the substrate produces a channel m the substrate of higher refractive index than the rest of the substrate, thus defining a waveguide. In another particular embodiment, a sodium nitrate electrode is used to carry out electπcal field-assisted diffusion of Na ions mto the substrate after the K-diffused waveguides are formed. This has the effect of dπvmg the waveguides deeper into the substrate and giving them a more circular cross section. The buπed waveguides thus avoid the effects of corrosive processes that result m surface etchmg.

In one embodiment, a surface-relief grating forming a distπbuted Bragg reflection grating is fabricated on the surface of the waveguide by coating the surface with photoresist, defining the grating pattern in the photoresist holographically or through a phase mask, developing the photoresist pattern, and etching the grating pattern into the waveguide with a reactive ion system such as an argon ion mill. In certain embodiments, a more durable etch mask allowing more precise etching and higher bias voltages is obtained by depositing chromium on the developed photoresist pattern using an evaporation method which causes the chromium to deposit on the tops of the grating lines.

Brief Description of the Drawings

Fig. 1-A1 shows an lsometπc view of a distπbuted Bragg reflector waveguide laser array realized using a single pitch grating and diffused waveguides with varying effective mdex. Fig. 1-A2 shows the single frequency output power at 1536.3 nm as a function of launched 977 nm pump power for a laser described in Example A. Fig. 1-A3 is a Fabry Perot (FP) interferometer scan of the output of a laser descπbed in Example A showing single frequency operation Fig. 1-A4 shows the self heterodyne beat spectrum of a laser m Example A with a 75 MHz frequency shift.

Fig. 1 -A5 shows laser wavelength as a function of waveguide diffusion aperture width for Example

A, with data taken using an automatic spectrum analyzer with a resolution of 0.1 nm. Fig. 1 -B 1 shows the index depth profile at the center of a waveguide described in Example B. Fig. 1 -B2 shows the output power characteπstics of a laser descπbed in Example B for two different pump wavelengths .

Fig. 1-B3 shows the output spectrum of a laser descπbed m Example B.

Fig. 1-B4 shows the output power of the laser descπbed in Example B as a function of time for two pump power levels. Fig. 1-Cl shows a plot of laser signal power vs. launched pump power for two different output couplers in Example C. Fig. 1-C2 is plot of slope efficiency vs. output coupler reflectance for each host glass in Example

C. Fig. 2 A shows in an exploded view the components of diode pumped laser in accordance with the invention. Fig. 2B shows the constructed view of a diode pumped laser m accordance with the invention. Fig. 3 shows an isometric view of an optical chip 200 having a laser 202 compπsmg waveguide

220, DBR mirror 230 and optional input mirror 240. Fig. 4 shows a top view of a laser 900 using direct (butt) coupling that includes optical chip

200. Fig. 5 shows a top view of a laser 1000 using lensed coupling that includes an optical chip 200.

Fig. 6 shows a top view of a laser 1100 using a fiber coupling that includes an optical chip 200.

Description of Preferred Embodiments

In the following detailed descπption of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.

The present invention provides a process for forming waveguides onto (or mto) the surface of a glass substrate. In one embodiment, photolithographic techniques define waveguides by changmg the index of refraction of waveguide channels formed into the surface of the substrate. In one such embodiment, a glass wafer, approximately 10 cm by 10 cm by 1 mm is cut from a slab of IOG-1 laser glass available from Schott Glass Technologies, Inc., of Duryea, PA, USA The surfaces of interest, including a "top" major surface (where "top" refers to an orientation in the Figures of this discussion, and not necessarily to an orientation used in the process or operation of the devices) are polished to optical smoothness.

The present invention is embodied by a laser component that includes a glass substrate doped with one or more optically active lanthamde species, or a laser species that is not a lanthamde, and having a plurality of waveguides defined by channels withm the substrate. (As used herein, a "channel withm the substrate" is meant to broadly include any channel formed on or in the substrate, whether or not covered by another structure or layer of substrate. As used herein, when an embodiment reciting optically active lanthamde species is described, other embodiments may use a laser species that is not a lanthamde.) Each substrate waveguide (or "channel") is defined withm the substrate as a region of increased index of refraction relative to the substrate. The glass substrate is doped with one or more optically active lanthamde species which can be optically pumped (typically a rare-earth element such as Er, Yb, Nd, or Pr or a combination of such elements such as Er and Yb) to form a laser medium which is capable of lasmg at a plurality of frequencies. Mirrors or distributed Bragg reflection gratings may be located along the length of a waveguide for providing feedback to create a laser-resonator cavity. One or more of the mirrors or reflection gratings is made partially reflective for providmg laser output. The laser component may constitute a monolithic array of individual waveguides in which the waveguides of the array form laser resonator cavities with differing resonance characteπstics (e.g., resonating at differing wavelengths). The component may thus be used as part of a laser system outputtmg laser light at a plurality of selected wavelengths. In certain embodiments of the invention, the resonance characteristics of a waveguide cavity are vaπed by adjusting the width of the channel formed in the substrate which thereby changes the effective refractive index of the waveguide. The effective refractive index can also be changed by modifymg the diffusion conditions under which the waveguides are formed as described below. Changmg the effective refractive index thus changes the effective DBR spacmgs length of the waveguide cavity which in some embodiments determines the wavelengths of the longitudinal modes supported by the cavity. In another embodiment, the resonance characteπstics of the waveguide cavities are individually selected by varying the pitch of the reflection gratings used to define the cavities which, along with the effective refractive index of the waveguide under the DBR for the propagated optical mode, determines the wavelengths of light reflected by the gratings. In still other embodiments, the location of the reflectors on the waveguide is vaπed in order to select a laser-resonator cavity length that supports the desired wavelength of light.

In some embodiments, the laser element is constructed from a glass substrate which is a phosphate alkali glass doped with a rare-earth element such as Er or Yb/Er. In certain vaπous embodiments, Schott glass type IOG1 or IOG10 (available from Schott Glass Technology, Inc. of Duryea, PA) is used. In the case of Yb Er doped glass, maximal lasing efficiency has been found to occur when the Yb/Er ratio is between approximately 3: 1 and 8: 1 in some embodiments. The channels defining the waveguides are created by exposing a surface of the substrate to an ion- exchange solvent through a mask layer having a plurality of line apertures corresponding to the channels which are to be formed. The ion exchange may be earned out through an aluminum mask layer in an aluminum or borosihcate glass crucible using molten potassium nitrate as a solvent. Lessened etching of the substrate by the ion-exchange melt has been found to occur m some embodiments if a borosihcate crucible is used and if the potassium nitrate is pre-baked at a temperature of at least 120 degrees C for 24-48 hours in an inert argon atmosphere. In other embodiments, the crucible is placed inside a fully enclosed chamber duπng the ion-exchange process, with the chamber filled with an inert atmosphere. Carrying out the ion-exchange process an enclosed chamber has been found to lessen surface etching due to oxidation reactions. The exchange of K for Na m the substrate produces a channel in the substrate of higher refractive index than the rest of the substrate, thus defining a waveguide. In another particular embodiment, a sodium nitrate ion-exchange melt having a pair of electrodes is used to carry out electπcal field-assisted diffusion of Na ions into the substrate after the K-diffused waveguides are formed. This has the effect of dnvmg the waveguides deeper mto the substrate and giving them a circular cross section. The buπed waveguides thus avoid the effects of corrosive processes that result m surface etchmg. In one embodiment, a surface-relief grating forming a distributed Bragg reflection grating is fabricated on the surface of the waveguide by coating the surface with photoresist, defining the grating pattern in the photoresist holographically or through a phase mask, developing the photoresist pattern, and etching the grating pattern into the waveguide with a reactive ion system such as an argon ion mill. In some embodiments, an Sι0₂ layer, approximately 1 to 2 nm

(nanometers), is deposited on the glass before the photoresist coating to improve adhesion to the glass. In certain embodiments, a more durable etch mask allowing more precise etchmg and higher bias voltages is obtained by depositing chromium on the developed photoresist pattern using an evaporation method which causes the chromium to deposit on the tops of the grating lmes. In some embodiments, a colhmated laser beam passed through a suitably defined hologram which then transforms colhmated laser beam into one or more interference patterns, such as one corresponding to the grating 230 of Figure 1A1. In yet another embodiment, a suitably defined hologram simultaneously provides one or more other grating patterns as defined herein. Fabπcation and operation of the above devices, parts and/or systems may be better understood with reference to the following non-hmitmg examples, each of which descπbe numerous individual embodiments of the present invention. The improved devices output laser power up to and exceeding 170 milliwatts, and provide slope efficiencies of up to and exceeding 26% in vaπous embodiments, far exceeding the output power and slope efficiency of previous devices that lase m the 1.5 micrometer range when pumped in the 0.98 micrometer range. Further, the devices of the present invention provide reproducible and stable output wavelengths, and can be mass produced inexpensively, as compared to previous devices.

Example A

An array of monolithic, single-frequency distπbuted-Bragg-reflector (DBR) waveguide lasers has been successfully demonstrated operating near 1536 nm wavelengths. In this embodiment, lasers 202 were fabricated by forming waveguides 201 in Yb/Er co-doped phosphate glass by ion exchange. The slope efficiency for each laser 202 as a function of launched pump power is 26% and the thresholds occur at 50 mW of launched pump power. An output power of 80 mW was achieved with 350 mW of coupled pump power. Each laser 202 exhibits stable operation on a single longitudinal mode and all have lmewidths less than 500 kHz. A comb 211 of waveguides with varying effective indices allows the selection of wavelength using a single-peπod grating 230 (i.e., a multiple-groove grating having the same groove spacing for all waveguides).

This descπption is the first reported monolithic, single-frequency distnbuted-Bragg- reflector (DBR), waveguide lasers m Yb³⁺ /Er³⁺ -co-doped phosphate glass. Over the past several years, the growth m the demand for telecommunications bandwidth generated by new telecommunication services, cable television (CATV), and data communication has dπven the rapid development of wavelength-division multiplexing (WDM) and dense-wavelength-division multiplexing (DWDM) where information is simultaneously transmitted on several different wavelengths in the same optical fiber channel. The development of WDM systems has placed demands on laser sources that are difficult to meet using conventional technologies such as semiconductor distπbuted feedback (DFB) lasers. Issues of concern for such systems include wavelength selectivity, wavelength stability over laser lifetime, tunability, process yield, power limitations, and costs

Integrated, single-frequency, solid-state lasers using the Er³⁺ ion offer a very promising and competitive alternative to DFB lasers for use in future WDM communications systems and for optically-distπbuted CATV (cable television). Several demonstrations of waveguide-laser and fiber- laser technology have been discussed in the literature. One pπmary advantage of solid-state waveguide lasers is that they offer the possibility for arrays of lasers operatmg on many wavelengths on a smgle glass chip Rare-earth-doped waveguide lasers can also provide kilohertz lmewidths with high radiance, low noise, and easy coupling to optical fibers.

In some embodiments, smgle-transverse-mode waveguides at 1535 nm wavelength were fabπcated in a commercially available phosphate alkali glass that was co-doped with 0.99 x 10²⁰ Er³⁺ ions/cm³ and 3.97 x 10²⁰ Yb³⁺ ions/cm³ Phosphate glass is a very good host material for ytterbium and erbium ions since the sensitization efficiency is nearly unity and large doping concentrations are possible before the onset of concentration quenching The guides were formed by ion exchange of K⁺ for Na" using line apertures 3 to 8 μm wide etched m a 200 nm thick aluminum mask layer The exchange time was 4 hours in an aluminum crucible containing molten KN0₃ at 375 °C. Inspection of the samples using refractive near-field scanning after the ion exchange revealed that the regions of the glass surface corresponding to the location of the mask openings had become recessed by approximately 1 μm during the exchange process The mechanism behind the etchmg of the glass dunng the exchange is currently under investigation, and it is thought that it is caused by residual water in the hygroscopic nitrate melt The surface quality of the glass m the recessed regions, as observed using a lOOOx Nomarski contrast microscope, appears identical to the ongmal surface of the glass and apparently does not cause significant scattering losses. In one embodiment, the length of the waveguides prior to the grating fabπcation step was 2.2 cm The waveguide end faces were polished perpendicular to the channels. Measurements of the waveguide mode-field dimensions showed that a single transverse mode was supported in each of the waveguides For the guide formed with the 6.5 μm mask aperture, the 1/e full- widths of the mode-field intensity were 16 μm wide by 11 μm deep at 1.54 μm wavelength. It supported multiple transverse modes at the 977 nm pump wavelength However, when the device was lasmg, the pump energy was confined pπmaπly withm the lowest-order transverse mode, which had 1/e dimensions of 13 μm wide by 9.5 μm deep. All measurements of the intensity profile are withm an estimated expenmental error of ± 10%.

In this embodiment, a DBR surface relief grating 230 was fabπcated holographically in a 0.5 μm thick layer of Shipley 1805 photoresist using a 90° corner that split a colhmated laser beam into two beams. The corner was mounted on a rotation stage so that the angle of the two beams could be vaπed. One surface of the corner was a mirror, and the other surface was a vacuum chuck for holding the sample. Light from a 457.8 nm Ar-ion laser was spatially filtered by focusing through a 15 μm p hole using a 20x objective lens The beam was colhmated using a 76 mm diameter lens with a 350 mm focal length. The exposure angle was set to wnte a grating with a pitch of Λ = 507.8 nm. For a waveguide with estimated effective index of 1.515 ± 0.003, this pitch should provide laser operation at λ = 1538.6 nm ± 3 nm. The exposure time for the photoresist was 18 s with 3.85 mW incident in each arm on the 0 44 cm² exposed region (0.8 cm long x 0.55 cm wide). The grating was developed m undiluted Shipley CD-30 developer. During the development, the diffraction of light from a 632 8 nm HeNe laser was monitored When the first-order diffracted power reached a peak, the grating was removed, rinsed, and dried.

Before the DBR grating was formed by transferring the photoresist pattern into the glass by Ar-ion sputtering, 40 nm of Cr was deposited on the surface with the specimen inclined 60° to the electron-beam evaporation source. Mounting the specimen in this way causes Cr to accumulate only on the tops of the grating lines and not m the grooves, thus providing a durable etch mask that will perform better in the Ar-ion sputtering step The grating was etched in the glass for 20 minutes using a reactive ion etching system with a 6.67 Pa (50 mTorr) Ar-ion plasma. The low-pressure plasma created a large self-bias voltage of 1700 V when running at 365 W of coupled power with frequency 13.5 MHz. The electrode spacing was 3.2 cm After etching, the sample 200 was cleaned ultrasonically in photoresist stripper at 85 °C. In one such embodiment, a 5 minute πnce in TCE (tπchloroethylene), is followed by a 5 minute πnce m acetone, and then followed by a 5 minute πnce in TCE methyl alcohol

Figure 1-A1 is an illustration of the completed DBR laser array. The waveguide laser cavities 202 were formed by placing a thin, highly reflecting (R=99.9% at 1540 nm, R=15% at 977 nm) dielectric mirror 240 on the pump input facet. The mirror 240 was held in place by a spring clip, and index-matching fluid was used between the mirror 240 and the waveguide facet. The DBR grating 230 was used as the laser output coupler. The laser 202 was tested by coupling light from a Tι:Al₂0₃ laser tuned to a wavelength of 977 nm using a 4x objective lens with a numeπcal aperture of 0.1. The launching efficiency was estimated to be between 65 and 71 percent. To determine the launching efficiency we measured the Fresnel reflectance of the mput mirror, the loss of the launching objective, and the excess coupling loss. Figure 1 -A2 shows the laser output power as a function of launched pump power and the spectrum of the laser. The waveguide diffusion aperture for this waveguide was 8 μm. The slope efficiency as a function of launched pump power is calculated to be 26 percent when we take the coupling factor to be 71 percent.

The reflectance of the grating was estimated using the simplified laser formula denved from the theory of Rigrod:

where P, is the output power at the grating end and P₂ is the output power at the end opposite the grating. R_[ is the grating reflectance and R₂ is the reflectance of the attached mirror. Two mirrors were used with reflectances of 80 and 90 percent for R₂. For both cases the grating reflectance, R,, was calculated to be 65 percent. To investigate the longitudinal mode structure of the laser, the laser output was coupled into an optical fiber scanning Fabry-Perot interferometer with a free spectral range of 124 GHz. Figure 1 -A3 shows that the laser operated on a single longitudinal mode when the coupled pump power did not exceed 300 mW. The laser was robustly single frequency with TE polarization, and no mode hopping was observed The mset in Figure 1-A3 shows that a second longitudinal mode appeared when the coupled pump power exceeded 300 mW. In this pump regime, the laser was unstable and exhibited mode hopping, single-frequency operation, and dual-frequency operation. By measuring the frequency spacing between the longitudinal modes, the physical length of the laser cavity was determined to be 1.4 cm.

The lmewidth of the laser was measured using a conventional self-heterodyne configuration with a 75 MHz frequency shift. The path length difference between the two arms was 10 km corresponding to a lmewidth resolution limit of 30 kHz for a Gaussian line shape. Optical isolators were used m both arms to prevent optical lmewidth narrowing due to feedback; however, the output end of the laser was not beveled.

Figure 1-A4 shows the self-heterodyne spectrum. The laser lmewidth obtained from this measurement was 500 kHz.

Finally, the laser wavelengths of other waveguides on the chip were measured using an automatic spectrum analyzer with a resolution of 0.1 nm Seven of the eleven waveguides on the chip exhibited laser oscillation. The waveguides formed through the smaller apertures did not achieve threshold because the smaller mode volumes caused a reduction of the gam such that the 45 percent transmittance loss of grating could not be overcome.

Figure 1-A5 shows the change in wavelength trend as the waveguides were scanned. The wavelength increases as the diffusion aperture width mcreases, which is consistent with increasing effective index as the aperture width increases.

This example has demonstrated an array of high power, robustly single-frequency, integrated, DBR waveguide lasers operating near 1536 nm wavelength. The slope efficiencies of the lasers are 26 percent based on launched pump power, and the threshold is less than 50 mW when pumped at a wavelength of 977 nm. The lmewidths of the lasers were measured to be 500 kHz, and the outputs were linearly polanzed m a TE mode. The temperature stability of the lasers and the relative intensity noise (RIN) are currently being investigated. It is expected that with diode laser pumping, the RIN will be similar to other results presented for smgle-fluency fiber lasers and will fall below - 150 dB/Hz above 10 MHz. It is anticipated that the output power and efficiency will increase and the threshold power will decrease when the grating reflectance is increased. This is possible with only minor adjustments to the grating fabπcation process. Further improvements will also be realized by directly coating the high reflector onto the waveguide end facets. It has been shown that lasers with several output wavelengths can be integrated on a single glass substrate. This example shows that stable, multi-wavelength, WDM sources with wavelengths falling on the International Telecommunications Union (ITU) grid can be realized by wπtmg several gratings of varying period in Yb/Er-co-doped glass waveguides formed by ion exchange.

Example B

Compact solid-state lasers based on the 1.5 μm Er^3τ transition hold promise as sources for optical fiber communication systems Yb³⁺ is commonly used as a sensitizer in Er³⁺ lasers because it has a much larger absorption cross section near 980 nm than Er³⁺, and it efficiently transfers its excited state energy to the upper level of the Er³⁺ laser. The Er³⁺/Yb³⁺ glass waveguide laser, in particular, has several advantages over lasers in Er³⁺ -doped or Er³⁺ Yb³⁺ -co-doped glass fiber and bulk crystalline or glass hosts Ion-exchanged waveguides can be fabncated in glasses with large ytterbium concentrations (approximately 5-15%) which allows the devices to be substantially shorter than fiber lasers This results in lower polanzation and output power noise, caused by thermal and mechanical stress-induced birefringence, and a smaller device volume. Short (approximately 1 -2 cm) laser cavities are also of interest because of the potential for realizing high- pulse-repetition rate (GHz), passively mode-locked lasers. Unlike bulk devices, waveguide lasers can be designed to operate in a single transverse mode independent of the operating power or pump laser transverse mode profile, and do not require the alignment of bulk mirrors. In addition, the mode field sizes m waveguides can be designed to closely match those of optical fiber for efficient coupling with fiberoptic systems. One disadvantage of Er³⁺/Yb³⁺ glass waveguide lasers, up to this point, has been the relatively low output powers (up to a few milliwatts) available from these devices Increased output power will greatly expand the utility of these devices. A cw Er³⁺/Yb³⁺ -co-doped phosphate glass waveguide laser which has produced 168 mW of output power at around 1540 nm for 611 mW of launched pump power at 979 nm has been descπbed.

Waveguides were fabricated in a commercially available phosphate glass. The glass was co-doped with 1.15 wt % Er₂0₃ (0.99 x 10²⁰ ions/cm³) and 4.73 wt % Yb₂0₃ (3.97 x 10²⁰ ions/cm³). Waveguides were formed by K⁺-Na⁺ exchange through a 200 nm thick Al mask layer with channel apertures ranging from 3 to 8 μm in width. The exchange occurred in a KN0₃ melt at 375 °C for 4 hours in an Al crucible. The laser results reported here are for a 6.5 μm wide mask aperture. Inspection of the samples after the ion exchange process revealed that the regions of the glass surface corresponding to the location of the mask openings had become recessed by one to two microns duπng the exchange process. The widths of the etched channels were close to the widths of the mask apertures and uniform in width and depth.

In one embodiment, the refractive index as a function of position withm the exchanged sample was analyzed using a refractive near-field scanning method. Figure 1-B1 shows the index depth profile at the center of the waveguide formed with the 6.5 μm mask aperture for a wavelength of 633 nm. This method allows the relative position and absolute index values to be determined with an accuracy of 0.7 μm and 0.001 , respectively.

In one embodiment, the transverse modes of the waveguides were characteπzed by coupling light at the wavelength of interest mto one end of the waveguide and imaging the light emerging from the other end onto a calibrated infrared camera. The uncertainty of the mode dimensions determined using this method are approximately 10%. The device supported a single transverse mode at 1.54 μm having dimensions of 14.5 μm wide by 7.5 μm deep (measured at the 1/e points). The waveguide supported multiple transverse modes at 980 nm. However, when the device was lasmg, the pump energy was confined primarily withm the lowest order transverse mode which had dimensions of 6.4 μm wide by 3.6 μm deep

In one embodiment, the device was pumped with a Tι³⁺:sapphιre laser. The waveguide laser cavities were formed by placing thin dielectric mirrors on the polished waveguide end faces. The mirrors were held m place by small spring clips, and index matching oil was used between the mirror and waveguide end face to reduce losses. The pump laser was launched through one of the mirrors with a 4X microscope objective. The laser output and unabsorbed pump were colhmated with a 16x microscope objective and separated using filters. The laser cavity was 20 mm in length. The mirror through which the pump was launched had reflectivities of >99.9% and 15% at 1536 and 980 nm, respectively. The output coupler had a reflectivity of 80% at 1536 nm and transmitted 85% of the incident pump power. Neither the waveguide length nor the cavity output couplmg has been optimized. The launching efficiency was estimated to be <71%, including losses due to the transmission of the input mirror and launching objective. The laser output power characteπstics for two different pump wavelengths are illustrated in Figure 1-B2. When pumped at 979 nm, the launched pump power threshold was 51 mW. A maximum output power of 168 mW was obtained for 611 mW of launched 979 nm pump power. A lower threshold could be obtained by tuning the pump laser off of the Yb^3" absorption peak. For a pump wavelength of 960 nm, the threshold was 23 mW. The slope efficiency for both pump wavelengths was approximately 28%.

Using the broad-band cavity described above, the Er³7Yb³⁺ laser usually operated at several wavelengths simultaneously. A typical laser spectrum showing simultaneous operation at

1536.0, 1540.7, and 1544.8 nm is depicted Figure 1-B3. The wavelength(s) of operation could be shifted by passing some of the colhmated 1.5 μm laser output through a pπsm and reflecting it back through the prism and mto the waveguide using a dielectric mirror. This formed a weakly coupled, external cavity. By rotating the pπsm, it was possible to produce wavelengths rangmg from 1536 to 1595 nm.

A common feature of many three-level, rare-earth lasers is sustained relaxation oscillations which can be caused by small fluctuations in the pump laser power. Fluctuations in output power at frequencies ranging from approximately 0.5 to 1.5 MHz were observed in this laser. The amplitude of the fluctuations decreased with pump power. Figure 1-B4 shows the output power as a function of time for pump power levels just above threshold and 9.4 times threshold At the low pump power, the output power fluctuations of approximately 30% (peak to peak) of the average power were observed. At the high pump power, the fluctuations decreased to approximately 5% (peak to peak) of the average power. The Tι^3τ:sapphιre pump laser exhibited output power fluctuations of approximately 2-3%. Using a diode laser as the pump source should result in much quieter operation of the Er³⁺ laser.

Output powers exceeding 160 mW at 1.5 μm are now available from glass waveguide lasers fabncated using a simple thermal ion exchange process. Improvements in the waveguide fabπcation process to optimize the waveguide geometry (such as incorporating a field-assisted ion exchange and fabricating buried waveguides), as well as adjustments in the cavity length and coupling, should improve the performance of these devices.

Example C

Waveguide lasers and amplifiers in glasses codoped with Er³⁺ and Yb³⁺ are promising candidates for compact multifunctional devices operating near 1.5 μm. The large gam bandwidth resulting from the inhomogeneously broadened glass host makes these devices ideal for narrow-line sources useful m wavelength division multiplexing applications In addition, due to the short cavity lengths, these waveguide lasers offer high repetition rate (GHz) mode-locked lasers usmg semiconductor saturable absorbers. Such lasers are ideal as sources for sohton communications systems. Other applications requiring an eye-safe wavelength, such as remote sensmg and range finding, could benefit from compact, high power continuous-wave (cw) or Q-switched waveguide laser sources based on these matenals. Additionally, optical amplifiers offering gam m the range of 1530 to 1550 nm are realized for some embodiments of the present invention.

It is known that the Er³⁺ concentration must be kept relatively low (approximately 1 wt %) in these devices m order to reduce the deleteπous effects of cooperative upconversion. However, the concentration of sensitizing Yb³⁺ is not limited due to any ion-ion interaction, and is expected to have a significant effect on device performance. Vanous authors have investigated this problem theoretically. This example reports experimental results for waveguide lasers fabncated by K⁺ -Na⁺ ion exchange m silicate glasses with Yb³⁺:Er³⁺ ratios of 3 : 1 , 5 : 1 , and 8:1. In addition, we show how it is possible to increase the signal mode volume and optimize the pump-signal overlap through judicious choice of laser host mateπal and ion exchange processing parameters. The result is an Er³⁺/Yb³⁺ waveguide laser producing as much as 19.6 mW at 1.54 μm with 398 mW of launched pump power at 974.5 nm.

The devices were fabricated m a commercially available laser glass. The glass is a phosphorus-free, mixed-alkali, zmc-sihcate glass. Nominally, all three glasses were doped with 1 wt% (one percent by weight) Er₂0₃ (0.85 x 10²⁰ cm^"3) and the glasses designated NIST10A, NIST10C, and NIST10E contain Er³⁺ :Yb³⁺ ratios of 3:1 (2.47 x 10²⁰ Yb3⁺ ions cm^"3), 5:1 (4.16 x 10²⁰ cm^"3), and 8:1 (6.83 x 10²⁰ cm^"3), respectively. The results reported were obtained by ion exchange through 3 μm apertures in 150 nm thick Al mask layers. The ion exchange was earned out in a melt of 100% KN0₃ for 14 hours at 400 °C

The optical modes of the waveguides at the signal wavelength were evaluated by coupling a 1.5 μm LED into a waveguide while the output was imaged onto an infrared camera. The waveguides supported a single transverse mode of dimensions 20.5 ± 2.1 μm wide by 1 1.5 ± 1.2 μm deep (measured at the 1/e points) at the signal wavelength. Since the host glass is a mixed alkali glass which contains potassium, the introduction of additional potassium by the ion-exchange process leads to a very small mdex change. As a result, the optical mode is not tightly confined. Although the waveguides supported multiple transverse modes at the pump wavelength, an examination of the pump mode while the device was lasmg showed that only the lowest-order mode was excited. The result is excellent overlap between the pump and signal modes. The pump mode measured 15.2 ± 1.5 μm wide by 7.0 ± 0.7 μm deep

Waveguide losses were estimated at 860 nm, away from the broad Yb³⁺ absorption peak. Cutback measurements performed on samples prepared identically to the laser sample indicated an average waveguide loss of 0.25 ± 0.11 dB/cm for NIST10A, 0.32 ± 0.08 dB/cm for NIST10C, and 0.66 ± 0.12 dB/cm for NIST10E The coupling efficiency for the pump light was determined by measuring the pump throughput at 860 nm and correcting for losses from the input and output optics, as well as waveguide loss usmg the above-reported loss figures. Coupling efficiencies typically fell between 50% and 70%). The coupling efficiency was assumed to be identical at 860 nm and 975 nm. For laser characterization, dielectnc mirrors were attached to the polished end facets of the waveguide laser sample with index matchmg fluid and held m place by a small clip. The input mirror had a reflectance at 1536 nm of 99.9% and a transmittance at the pump wavelength of >90% Vanous output couplers with reflectances ranging from 60 to 98% were used. All output couplers were also transmissive at the pump wavelength. The waveguide devices were pumped by a Tι:sapphιre laser operating at 974.5 nm, which is the peak of the Yb³⁺ absorption spectrum m this glass host. Pump light was coupled mto the waveguide with a 4X (0.10 NA) microscope objective, and the output signal light was collected by a 20X objective. For signal power measurements, the output from the waveguide was focused onto an InGaAs power meter.

The laser performance was investigated as a function of device length as well as output coupler reflectance. Figure 1-Cl shows a plot of laser signal power vs. launched pump power for two different output couplers, for a 1.68 cm long device fabncated m the glass with 5 Yb³⁺ per Er^ ion. The slope efficiencies and laser thresholds were determined by fitting a line to the laser data. The lowest threshold was achieved when using a 98% reflector as output coupler. This device lased with a launched pump power threshold of approximately 59 mW. The slope efficiency of this device was 2.0% with respect to launched pump power. The highest slope efficiency was realized with a

70% reflector used as an output coupler In this case, a slope efficiency of 6.5% was achieved with a launched pump power threshold of 86 mW. For a launched pump power of 398 mW, this laser produced 19.6 mW of output power

A plot of slope efficiency vs. output coupler reflectance for each host glass appears in Figure 1-C2. Data for device lengths in each glass which were expenmentally determined to give the highest slope efficiency are plotted. Highest slope efficiency performance in each host is also compared in Table 1.

Table 1: Performance data for highest slope efficiency devices fabricated in IOG10 silicate glass with different Yb³⁺:Er³⁺ dopant ratios.

30

The expeπmental results indicate that the optimal dopant ratio is close to 5 Yb³⁺ for each Er³". Increasing the dopant ratio from 3.1 to 5:1 leads to an improvement in slope efficiency. Further increasing the dopant ratio to 8:1 does not improve the efficiency of the device, but does lead to a substantial penalty in pump power requirements. Recent efforts have been directed toward expanding the above results using a πgorous scalar model. In addition, alteration of the potassium content of the host glass is being investigated as a way to adjust the modal volume and decrease pump threshold requirements.

Some Unique Aspects of the Example Embodiments

The present invention provides the first demonstration of a monolithic single-frequency waveguide laser m a spectroscopically superior phosphate glass, and provides predictable emission wavelength if effective index of the waveguide is known. Embodiments of the invention as descπbed above provide a high-power laser up to 80 mW (previous state-of-the-art in phosphate glass was 5 mW by D. Barbier, et al. at Conference on Optical amplifiers and applications, 1995, and for an Er-doped DBR laser was approximately 2 mW by Technical University of Denmark Microelectronic Centret usmg sputter-deposited silica doped with Er), a very high-slope efficiency laser of 26% (previous state-of-the-art in phosphate glass was 16% by D. Barbier, et al. at Conference on Optical amplifiers and applications, 1995, and about 11 percent by Yeniay, et. al., Lehigh University & Lucent), and a uniquely high-coupling efficiency of pump light from optical fiber as enabled by the low index change caused by the potassium-sodium ion-exchange process used in forming the waveguides.

One embodiment provides making the photoresist grating by evaporation of a chromium coating with the sample inclined 60 degrees to the normal of the evaporation source beam. This results in a hard metal coating on the tops of the photoresist bars that constitute the grating, thus allowing a more selective sputter etch of the grating and the use of higher DC bias voltages. One embodiment provides a DBR grating exposure m standard photoresist using an optical phase mask. Another embodiment provides a uniquely high diffraction efficiency of photoresist grating by actively monitoring during photoresist developing to optimize diffraction efficiency and to ensure proper resist development. Also, etching of the grating by Ar-ion sputtering in a standard reactive-ion-etching system results in no CFC (chloπnated fluorocarbon) emissions as with reactive-ion-etching of silica glass. Using only argon gas at low pressure also provides a nearly sinusoidal grating without excess loss due to improved lsotropic plasma etching. Isotropic etching in an argon ion plasma (the inert Ar ions come in at a range of angles rather than a single angle) also leaves a smooth surface (approximately sinusoidal) resulting in lower grating scattenng losses. In some embodiments, the effective index of refraction of one or more waveguides on a chip are measured before applying the grating, and a grating pitch (lme spacmg) is chosen to achieve the desired wavelength.

Further Embodiments and Improvements As descπbed above, some embodiments of the invention utilize a potassium-sodium ion- exchange process. In some situations, this process can result in the glass surface being etched significantly when exchange is done in an aluminum crucible. Accordingly, a further improvement provides a borosihcate (a.k.a. Pyrex) crucible for performing the ion exchange as opposed to an aluminum crucible. This has been found to greatly reduce the amount of surface attack and etchmg of the phosphate glass caused by the KN0₃ ion-exchange melt. As noted above, water content is also a factor in producing surface etchmg. Therefore, another embodiment provides an improved control of water content of melt by baking the KN0₃ at a temperature of at least approximately 120 °C for a penod of 24-48 hours in an inert argon atmosphere.

Another factor involved in surface etching is due to oxidation reactions occurring during the ion-exchange process. Accordingly, in another embodiment, the crucible containing the molten ion-exchange solvent together with the substrate is placed inside a fully enclosed and sealed chamber (containing an inert atmosphere such as argon) duπng the ion-exchange process.

In another particular embodiment of the invention, the waveguides are buπed within the substrate. This not only results in reduced scattering losses in the waveguide, but also largely avoids the corrosive effects that are another factor m surface etching. In this embodiment, the surface of the glass substrate is covered by a deposited titanium mask with narrow (about 2-5 microns in width) photohthographically applied apertures used to locally control the ion-exchange process. The waveguides are first formed just beneath the substrate surface by immersing the substrate, at an elevated temperature, in a molten salt bath of potassium nitrate. Diffusion of K ions from the solvent into the substrate through the mask apertures m exchange for Na ions results in a channel being formed with a higher refractive index believed due to the higher electronic polaπzability of the K ions relative to Na as well as a possible stress effect. The surface waveguides thus formed are then buπed below the substrate surface by a second, electπc-field-assisted ion-exchange usmg molten sodium nitrate as an ion source. An electrode immersed in a sodium-nitrate bath is used to carry out the electπc-field-assisted diffusion of Na ions into the substrate which exchanges for K ions diffused in the previous step. This has the effect of causing further diffusion of K ions of the waveguides deeper into the substrate, covenng the waveguides with a lower refractive index matenal due to the Na ions diffused mto the substrate surface. This also gives the waveguides a roughly oval or circular cross-section which minimizes surface scattering losses. The use of K diffusion (as opposed to diffusion with a more mobile element such as Ag) to form the waveguides results in a stable structure that is able to be migrated deeper mto the waveguide under the influence of the applied electnc field.

Another embodiment provides a PVD (physical vapor depostion process) that includes electron beam or sputter deposition or CVD (chemical-vapor deposition) deposited Sι0₂ buffer layer coating over the surface of optical chip 200 to reduce surface scattenng losses from the surfaces of waveguides 220. It is also difficult to get standard photoresist to properly adhere to phosphate glass It has been found that providing a 1-2 nm of sputter-deposited Sι0₂ on the surface of the phosphate glass greatly improves adhesion of photoresist duπng processing

One embodiment provides an ion-exchange process that results in a slightly buπed waveguide with the highest index in the waveguide occurring slightly below the surface. This reduces scattering loss. Another embodiment provides a mode field shape and size that is optimized for laser operation using a field-assisted, ion-exchange process where the electnc field is varied as a function of time.

In an Yb/Er doped waveguide laser, there are performance tradeoffs related to the Yb- doping concentration, the Er-doping concentration, and the Yb/Er-doping ratio. It has been generally thought that more Yb doping will result in better laser performance. An alternate approach to optimizing the amount of Er and the Yb/Er ratio involves evaluating the relative performance tradeoffs between Yb-Er cross-relaxation efficiency and the total number of inverted Er ions in the laser cavity. An example of the doping selection process is as follows:

As more Yb is incorporated into a glass with a constant Er concentration, the average distance between Er ions and Yb ions decreases, thus causing a rise in the cross-relaxation efficiency of Yb-Er energy transfer The disadvantage of this is that more of the pump power is absorbed in a shorter distance. The result is that less Er ions will be inverted in a certain length, thus there will be less gam available as well as additional reabsorption loss in the 3-level Er ion laser system. Laser performance will suffer. The penalty will be higher thresholds and lower slope efficiencies. To obtain the best possible performance, both the Yb-Er cross relaxation and the total number of inverted ions as a function of pump power must be optimized simultaneously. For one particular device, a 4:1 ratio of Yb/Er was chosen with an Er concentration of 1x10²⁰ ions/cm³. The total doping concentration is constrained since the glass is doped by substitution of Yb and Er for Na, and the total doping concentration cannot exceed 10 wt%.

By adding feedback elements to the waveguides in the form of reflection gratings, a vanety of laser structures can be obtained using a process m accordance with the invention. In one embodiment, many gratings can be formed by exposing the photoresist fabricated on each single glass chip of a wafer using a silica plate that has multiple pitch phase masks printed on it. Accurate lmewidth control and control of differential lmewidth can be provided using phase masks with more than one period on a single substrate. Another embodiment provides a laser wavelength that is selected by pπnting a single pitch grating over an array of optical waveguides with each waveguide m the array having a different refractive index. Still another embodiment provides a laser wavelength that is selected by fabncating a single pitch grating on an array of identical optical waveguides where each waveguide crosses the grating at varying angles.

Another embodiment of the invention relates to a multi-compositional glass substrate formed with regions of diffeπng concentrations of the same or different dopants, which regions may be used to form complex mtegrated-optic structures. The glass substrate may be doped by a selected amount in certain regions with one or more optically active lanthamde species or a laser species

(which may be a rare-earth element such as Er, a combination of Er and Yb, or Pr) and undoped in others in such a manner that allows the independent control of gain, loss, and absorption m vaπous regions of the glass. In regions that are undoped, light of many wavelengths can propagate with virtually no loss, while in doped regions various wavelengths can be absorbed causing gam to occur at other wavelengths for creating laser amplifiers or resonators.

The optical devices integrated into the substrate may constitute both active and passive devices. Active devices such as laser amplifiers and resonators use the excited laser medium to amplify or generate light while passive devices such as filters, splitters, and waveguides use the substrate as a transmission medium. Passage of a light signal at a lasing wavelength through a doped region of the substrate results m absorption of the light by the laser species, however, unless the population inversion is maintained by pump light. In order to avoid loss of signal, the doped region must be pumped which adds to the energy requirements of the system. Whether the region is pumped or not, however, transmitting the light signal through a doped region also adds noise to the signal by amplified stimulated emission The present invention overcomes these problems by providing an undoped region through which light signals can be transmitted and in which passive devices can be located. The undoped region can also provide a pathway by which pump light from an external source can be directed to laser amplifiers or resonators in the substrate without intervening absorption of the light by a laser or sensitizer species.

Another embodiment of the present invention is a waveguide laser that includes a waveguide defined withm a glass substrate and a diode pump laser with an extended waveguide cavity for supplying pump light to the waveguide laser. Figure 2A shows in an exploded view of the components of such a laser which includes a doped substrate S with a substrate waveguide W defined therein, and a diode pump laser DPL 2401 with its output facet abutted to an extended waveguide laser-resonator cavity EWC 2403. The substrate waveguide W 202 is again defined withm the substrate as a region of increased index of refraction relative to the rest of the substrate. In one embodiment, the glass substrate is doped with one or more optically active lanthanides species that can be optically pumped, such as Er or a combination of Er and Yb, to form a laser medium. In the case of Yb/Er co-doped glass, pump light at 980 nm supplied to the laser medium results m laser output centered near 1536 nm. The aspect of the invention to be descnbed here, however, can be used with other combinations of dopants require different pump light wavelengths and outputtmg laser light centered at another wavelength.

In accordance with the invention, the output facet of the diode pump laser DPL 2401 m Figure 2A has an anti-reflection coating applied thereto and the extended cavity EWC 2403 abutted to the output facet 2402 so that the laser-resonator cavity of the diode pump laser includes the cavity EWC 2403. The diode pump laser in this example produces 980 nm light, and the cavity EWC 2403 is terminated with a reflective element Rl that reflects at a wavelength centered near 980 nm. As shown in Figure 12F2, in the operational device, the extended cavity EWC 2403 is positioned adjacent the waveguide W 202 to form a superstrate thereon. The cavity EWC 2403 includes a cladding layer CL interposed between the cavity and the waveguide W 202 that prevents pump light from uniformly leakmg out of the cavity EWC 2403 and into the waveguide W 202. Instead, apertures AP are located in the layer CL that selectively leak pump light from the cavity EWC 2403 to the waveguide W 202. The apertures are placed so as not to interfere with the gratmg G 230 of the substrate wave guide 202. As pump light 2401 resonates within the extended diode laser cavity EWC 2403, a portion of the light is transmitted to the substrate waveguide W 202 along a portion of its length to cause lasmg action therein. The diode pump laser 2401 cavity EWC 2403 in some embodiments has a lower index of refraction than the substrate waveguide and constitutes part of the cladding thereof. Pump light 2401 is thus transmitted to the substrate waveguide W 202, but laser light from the laser species withm the substrate waveguide 202 is contained by the total internal reflection of the propagating mode due to the relatively higher refractive index of the substrate waveguide with respect to the substrate and diode pump laser cavity EWC 2403 In this embodiment, the substrate waveguide 202 and diode laser cavity 2403 are separated by a layer of interposed cladding having apertures AP defined therein for transmitting the pump light into the substrate waveguide 202 at selected locations In another embodiment, the substrate waveguide 202 and diode laser cavity are separated by a gap at apeture AP with transmission of pump light mto the substrate waveguide 202 occurnng via evanescent coupling. In still other embodiments, the cladding layer CL may be fabricated as a resonant ARROW structure to produce the same effect. In one embodiment, end facet EF2 is highly reflective at pump wavelength 980 nm; end facet EF3 is highly reflective at output wavelength 1536 nm nm, end facet EF1 is anti-reflective at pump wavelength 980 nm; end facet EF4 is anti-reflective at output wavelength 1536 nm.

In the embodiment shown m Figures 2 A and 25, reflective element EF3, (which may be mirrors or distributed Bragg reflection gratings) and gratmg G 230 are located along the substrate waveguide 202 for providing feedback in order to form a laser-resonator cavity withm the waveguide W 202, with the grating G 230 made partially reflective for providmg laser output. Other embodiments may omit the feedback elements EF3 and G to form a laser amplifier. Other possible modifications of this aspect of the invention include the use of a bulk optic component to couple light from the diode pump laser gam section to the extended cavity EWC, and the incorporation of grating stabilization of the diode pump laser via a distπbuted- feedback gratmg formed in the extended cavity EWC. Other embodiments include fabricating an amplifier section m place of the laser cavity 202. The embodiment of the invention described with reference to Figures 2A and 2B thus presents an improvement over prior methods of cladding pumping of waveguide lasers and amplifiers. By usmg the diode pump laser cavity 2403 as a secondary cladding for the substrate waveguide W 202, as opposed to simply coupling the output of the diode pump laser 2401 to the secondary cladding, cladding pumping of very short (i.e., 1 cm or less) substrate waveguide lasers is thereby made possible. The configuration also permits mass production of a number of separate devices fabricated from a single superstrate formed on a single substrate having an array of waveguides defined therein. Figure 3 shows an isometric view of an optical chip 200 having a laser 202 compnsmg waveguide 220, DBR mirror 230 and optional input mirror 240 according to one embodiment of the present invention. Figure 6 shows more detail of a single laser 202 having an external launch mirror 240, which is transmissive at the pump light wavelength but highly reflective at the lasing wavelength of laser 202, and is used to launch the pump light mto the laser cavity. Other embodiments include redundant waveguides all operating at a single wavelength, other waveguides 220 each having a DBR 230 tuned to a unique wavelength, or combinations thereof, all integrated on a single optical chip 200. In vanous embodiments, optical chip 200 of Figure 6 is made with one of the configurations of Figures 2A-2H described above. In some embodiments, a plurality of operable lasers 202 are provided on each chip 200. Figure 4 shows a top view of a laser 900 using direct (butt) coupling of pump laser diode

310 to optical chip 200 according to one embodiment of the present invention.

Figure 5 shows a top view of a laser 1000 using lensed coupling of pump laser diode 310 to optical chip 200 according to one embodiment of the present invention.

Figure 6 shows a top view of a laser 1100 usmg a fiber coupling of pump laser diode 310 to optical chip 200 according to one embodiment of the present invention.

It is understood that the above description is intended to be illustrative, and not restπctive. Many other embodiments will be apparent to those of skill m the art upon reviewing the above descπption. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

Claims

What is claimed is:

1. A laser compnsmg: a glass substrate doped with a laser species; a waveguide defined withm the substrate; and a diode pump laser with an extended waveguide laser resonator cavity, the extended diode laser cavity being positioned adjacent the substrate waveguide so that pump light from the diode laser is absorbed along a length thereof.

2. The laser of claim 1 wherein the substrate is doped with Yb and Er.

3. The laser of claim 1 wherein the substrate is doped with Er.

4. The laser of claim 1 wherein the substrate waveguide forms a laser resonator cavity withm the substrate.

5. The laser of claim 4 further comprising a reflection grating formed on the substrate surface along the substrate waveguide for providing feedback to the laser resonator cavity.

6. The laser of claim 5 further comprising: a cladding deposited on the reflection gratmg of the substrate waveguide, the cladding being composed of an electro-optic polymer with a vaπable index of refraction; and electrodes for applying an electncal potential across the grating cladding to vary the mdex of refraction m accordance therewith and thereby vary the wavelength of light reflected by the grating

7. The laser of claim 5 further comprising electrodes and a resistive element for heatmg and thermally expanding the reflection grating of the substrate waveguide to alter the wavelength reflected by the grating.

8 The laser of claim 5 further comprising electrodes for applying an electrical potential to a piezo- electnc coating applied to the reflection grating to thereby vary the wavelength of light reflected by the gratmg

9. The laser of claim 5 wherein the reflection grating is composed of an electro-optic polymer and further comprising electrodes for applying an electrical potential across the grat g to vary the index of refraction in accordance therewith and thereby vary the wavelength of light reflected by the grating.

10. The laser of claim 5 further compnsmg. one or more additional reflection gratings formed on the substrate waveguide, each grating havmg a claddmg composed of an electro-optic polymer with a vanable index of refraction deposited thereon; and electrodes for selectively applying an electπcal potential across each grating cladding to vary the index of refraction in accordance therewith and render the gratmg transparent or reflective at a wavelength corresponding to a longitudinal mode of the substrate waveguide laser cavity.

11. The laser of claim 4 further comprising a mirror coupled to a location along the substrate waveguide for providmg feedback to the laser resonator cavity.

12. The laser of claim 1 wherem the extended waveguide cavity of the pump diode laser is a dielectric waveguide abutted at one end to an antireflection-coated gam section of the diode laser and at another end to a highly reflective mirror.

13. The laser of claim 1 wherein the extended waveguide cavity of the pump diode laser is a dielectπc waveguide abutted at one end to an antireflection coated gam section of the diode laser and at another end to a reflection gratmg.

14. The laser of claim 1 wherein the extended waveguide cavity of the pump diode laser has a lower index of refraction than the substrate waveguide and forms part of a claddmg thereof.

15. The laser of claim 1 wherein the extended waveguide cavity is abutted to the surface of the substrate waveguide and separated therefrom by a layer of claddmg with apertures for transmitting pump light mto the substrate waveguide.

16. The laser of claim 1 wherein the separation between the extended waveguide cavity and the substrate waveguide is such that pump light is transmitted by evanescent coupling.

17. A method for operating a waveguide laser compnsmg transmitting pump light from an extended waveguide laser cavity of a diode laser into a substrate waveguide laser cavity along a length of the substrate cavity, wherein the extended diode laser cavity forms part of a lower refractive index claddmg of the substrate waveguide laser cavity

18. The method of claim 17 wherem the substrate waveguide laser cavity is composed of glass doped with Er and Yb, with the diode laser and extended cavity thereof bemg tuned to provide pump light at a wavelength appropnate to cause lasing in the substrate waveguide cavity.

19. The method of claim 17 wherem pump light from the extended laser cavity of the diode laser is transmitted mto the substrate waveguide laser cavity via evanescent coupling.

20. The method of claim 17 wherem pump light from the extended laser cavity of the diode laser is transmitted mto the substrate waveguide laser cavity through apertures in a layer of claddmg mateπal interposed therebetween.

21. The method of claim 17 wherem the substrate waveguide cavity has a reflection gratmg at one end for providing optical feedback to the cavity and further wherem an electro-optic polymer having a vanable index of refraction is deposited on the gratmg to form a cladding therefor, the method further comprising tuning the laser by applying an electrical potential to the gratmg cladding to select a wavelength reflected by the grating that corresponds to a longitudinal mode of the substrate waveguide cavity.

22. The method of claim 17 wherem the substrate waveguide cavity has a plurality of spaced apart reflection gratings at one end for providing optical feedback to the cavity and further wherem an electro-optic polymer havmg a vanable index of refraction is formed on the gratings to constitute claddings therefor, the method further compnsmg tuning the laser by selectively applying an electπcal potential to the grating claddings to render one grating reflective at a wavelength that corresponds to a longitudinal mode of the substrate waveguide cavity.

23. A laser component compnsmg a glass substrate doped with a laser species and having one or more waveguides defined by channels withm the substrate, the one or more waveguides forming one or more laser-resonator cavities with distinct resonance charactenstics to provide lasing action at a selected wavelength when pumped wherein the substrate is an alkali phosphate glass doped with Er and Yb and wherein the channels are formed at a surface of the substrate as regions of increased refractive index, the laser component further comprising: one or more feedback elements for providing optical feedback to the waveguides to form the one or more laser-resonator cavities, wherein injection of pump light at one or more suitable wavelengths into the laser-resonator cavity causes output of laser light at a wavelength in accordance with a longitudinal cavity mode of the cavity, wherein the laser-resonator cavities have a plurality of widths on the substrate surface to thereby define a plurality of effective indices of refraction for the cavities, the wavelength of a longitudinal cavity mode being dependent thereon and wherem the laser-resonator cavities have a plurality of widths on the substrate surface adjacent a diffraction Bragg reflector (DBR) to thereby define a plurality of different wavelengths defined at spaced-apart wavelength intervals to match standard wavelength spacmgs.

24. The laser component of claim 23 wherem the laser-resonator cavities are fabncated m a plurality of groups, wherem the cavities m each group have a plurality of widths on the substrate surface adjacent a DBR to thereby define a plurality of different wavelengths defined at spaced-apart wavelength intervals, such that one cavity per group matches a standard wavelength associated with that group.

25. The laser component of claim 23 wherem the feedback element compπses a reflection gratmg formed on the substrate surface along the length of the waveguide and wherem a reflection gratmg of a smgle pitch is formed on the surface of the substrate at diffeπng angles to a plurality of waveguides to form laser-resonator cavities of diffeπng lasing wavelengths.

26. An optical substrate for fabncation of optical devices therein compnsmg a block of glass having distinct regions doped with varying concentrations of one or more laser species, where the substrate is constructed by fusing together a plurality glass blocks having diffeπng concentrations of the same or different dopants wherem at least one region is undoped, the substrate further compnsmg a optic amplifier and a waveguide for conveying pump light to the amplifier fabncated therein, wherem at least a portion of the waveguide is located in an undoped region of the substrate

27. An optical substrate for fabπcation of optical devices therein compnsmg a block of glass having distinct regions doped with varying concentrations of one or more laser species, where the substrate is constructed by fusing together a plurality glass blocks having diffenng concentrations of the same or different dopants wherem at least one region is undoped, the substrate further compnsmg a laser resonator and a first waveguide for conveying pump light to the resonator fabncated therein, wherein at least a portion of the first waveguide is formed in an undoped region of the substrate.

28. An optical substrate for fabrication of optical devices therein compnsmg a block of glass having distinct regions doped with varying concentrations of one or more laser species, wherem the substrate is constructed by fusing together a plurality glass blocks havmg differing concentrations of the same or different dopants, further comprising a laser resonator and a laser amplifier fabncated therein, wherein the resonator and amplifier are formed in regions of the substrate with different dopant concentrations.

29. An optical substrate for fabncation of optical devices therein compnsmg a block of glass having distinct regions doped with varying concentrations of one or more laser species, wherein the substrate is constructed by fusing together a plurality glass blocks having diffenng concentrations of the same or different dopants, the substrate further comprising a plurality of laser resonators fabncated therein, wherein the resonators are formed in regions of the substrate doped with different laser species so that lasing occurs within the resonators at different wavelengths.

30. A laser compnsmg: a glass substrate doped with a laser species; a waveguide withm the substrate forming a laser resonator cavity; a reflection grating in the substrate waveguide for providing feedback to the resonator cavity; means for tuning the laser by altenng the wavelength reflected by the grating; and means for pumping the laser by exciting the laser species of the substrate waveguide wherein the laser tuning means compnses: a cladding deposited on the reflection gratmg of the substrate waveguide, the claddmg being composed of an electro-optic polymer with a vanable index of refraction; and electrodes for applying an electrical potential across the gratmg cladding to vary the index of refraction m accordance therewith and thereby vary the wavelength of light reflected by the grating.

31. A laser comprising a glass substrate doped with a laser species; a waveguide withm the substrate forming a laser resonator cavity, a reflection grating in the substrate waveguide for providing feedback to the resonator cavity; means for tuning the laser by altering the wavelength reflected by the gratmg; and means for pumping the laser by exciting the laser species of the substrate waveguide wherem the laser tuning means comprises. one or more additional reflection gratings formed on the substrate waveguide, each gratmg having a claddmg composed of an electro-optic polymer with a vanable index of refraction deposited thereon; and electrodes for selectively applying an electrical potential across each gratmg claddmg to vary the index of refraction m accordance therewith and render the gratmg transparent or reflective at a wavelength corresponding to a longitudinal mode of the substrate waveguide laser cavity.

32 A laser comprising. a glass substrate doped with a laser species; a waveguide withm the substrate forming a laser resonator cavity; a reflection grating m the substrate waveguide for providing feedback to the resonator cavity; means for tuning the laser by altenng the wavelength reflected by the grating; and means for pumping the laser by exciting the laser species of the substrate waveguide wherem the laser tuning means compπses: electrodes and a resistive element for heating and thermally expanding the reflection gratmg of the substrate waveguide to alter the wavelength reflected by the grating

33. A laser compnsmg: a glass substrate doped with a laser species, a waveguide withm the substrate forming a laser resonator cavity; a reflection gratmg in the substrate waveguide for providing feedback to the resonator cavity; means for tuning the laser by altenng the wavelength reflected by the grating; and means for pumping the laser by exciting the laser species of the substrate waveguide wherein the laser tuning means compnses: electrodes for applying an electrical potential to a piezo-electnc layer applied to the reflection grating to thereby vary the wavelength of light reflected by the grating.

34. A laser comprising: a glass substrate doped with a laser species; a waveguide laser resonator cavity formed withm the substrate; a claddmg composed of an electro-optic polymer with an electncally vanable index of refraction deposited on the waveguide, electrodes for applying an electrical potential across the claddmg to vary the index of refraction in accordance therewith and thereby vary the effective refractive index of the waveguide cavity; and a pump laser for injecting pump light mto the substrate waveguide cavity.

35 A method for operating a waveguide laser comprising injecting pump light mto a waveguide laser resonator cavity wherem the laser cavity has a reflection gratmg at one end for providing optical feedback to the cavity and further wherem the grating is responsive to application of an external voltage by changing the wavelength of light reflected by the grating in accordance the applied voltage, the method further comprising tuning the laser by applying an electncal potential to the grating to select a wavelength reflected by the grating that corresponds to a longitudinal mode of the substrate waveguide cavity.

36. The method of claim 35 wherem the grating is coated with a claddmg composed of an electro- optic polymer having a vanable index of refraction such that application of a voltage to the claddmg changes the wavelength of light reflected by the grating.

37. The method of claim 35 wherein the grating is composed of an electro-optic polymer having a vanable index of refraction such that application of a voltage to the claddmg changes the wavelength of light reflected by the grating.

38. The method of claim 35 wherein the grating is coated with a piezo-electnc coating such that application of a voltage to the coating thereby varies the wavelength of light reflected by the gratmg.