WO1996041403A1

WO1996041403A1 - Multi-gigahertz frequency-modulated vertical-cavity surface emitting laser

Info

Publication number: WO1996041403A1
Application number: PCT/US1996/007752
Authority: WO
Inventors: Robert A. Morgan
Original assignee: Honeywell Inc.
Priority date: 1995-06-07
Filing date: 1996-05-28
Publication date: 1996-12-19
Also published as: DE69610499D1; DE69610499T2; US5574738A; EP0830718A1; JPH11511292A; EP0830718B1; CA2221308A1

Abstract

A GHz-range frequency-modulated laser based on manufacturable vertical cavity surface emitting lasers (VCSELs) and arrays. The present invention exploits a saturable absorber contained within the VCSELs distributed Bragg reflector which may itself be adjusted during fabrication or in operation. Under controllable operating conditions, the saturable absorber, strategically sized and placed, will force the VCSEL to self-pulsate (in the GHz-regime) at rates related to the local intensity, absorption, lifetime, and carrier density of the saturable absorber. These conditions can be controlled in real time in one of three ways; first, by adjusting the injection current into the VCSEL itself; second, for a fixed VCSEL bias and the use of a third terminal, by modifying the carrier density within the saturable absorber via additional current injection; or third, the saturable absorber can be reverse-biased by simultaneously altering its absorption and carrier lifetime and thus carrier density. Additionally, the frequency response can be controlled in the fabrication process by affecting the location of the saturable absorber, the mirror design and cavity Q, and the laser size (and hence threshold current). One can easily multiplex numerous GHz channels into a 100 plus GHz transmitter in a cost-effective manner. Application of the saturable absorber for self-pulsation provides for a decoherence of the VCSEL light output to eliminate modal noise in data communication systems, or for use as a local oscillator in an RF or other system. A plurality of VCSELs may be formed into an array having various frequencies, intensities, phases or other properties. The VCSELs may form a phased array, for instance.

Description

MULTI-GIGAHERTZ FREQUENCY-MODULATED VERTICAL-CAVITY

SURFACE EMITTING LASER

PACKGRQUND

This invention relates to the field of semiconductor lasers, and particularly

relates to vertical cavity surface emitting lasers. More particularly, the invention relate

to self-pulsing vertical cavity surface emitting lasers (VCSELs).

Conventional semiconductor lasers have found widespread use in modern

technology as the light source of choice for various devices, e.g., communications

systems, compact disc players, and so on. The typical semiconductor laser is a double

heterostructure with a narrow bandgap, high refractive index layer surrounded on

opposed major surfaces by wide bandgap, low refractive index layers. The low bandga

layer is termed the "active layer", and the bandgap and refractive index differences serv

to confine both charge carriers and optical energy to the active layer or region. Opposi

ends of the active layer have mirror facets which form the laser cavity. The cladding

layers have opposite conductivity types and when current is passed through the

structure, electrons and holes combine in the active layer to generate light.

Several types of surface emitting lasers have been developed. One such laser o

special promise is termed a "vertical cavity surface emitting laser" (VCSEL). (See, for example, "Surface-emitting microlasers for photonic switching and interchip

connections," Optical Engineering. 29, pp. 210-214, March 1990, for a description of

this laser. For other examples, note U.S. patent 5,115,442, by Yong H. Lee et al., issu

May 19, 1992, and entitled "Top-emitting surface emitting laser structures," which is

hereby incorporated by reference, and U.S. patent application serial number 08/175,01

by Mary K. Hibbs-Brenner, allowed, issue fee sent March 20, 1995, and entitled

"Integrated laser power monitor," which is hereby incorporated by reference. Also, se "Top-surface-emitting GaAs four-quantum-well lasers emitting at 0.85 μm," Electronic

Letters. 26, pp. 710-711, May 24, 1990.) The laser described has an active region with

bulk or one or more quantum well layers. The quantum well layers are interleaved wit

barrier layers. On opposite sides of the active region are mirror stacks which are form

by interleaved semiconductor layers having properties, such that each layer is typically

quarter wavelength thick at the wavelength (in the medium) of interest thereby formin

the mirrors for the laser cavity. There are opposite conductivity type regions on

opposite sides of the active region, and the laser is turned on and off by varying the

current through the active region. However, a technique for digitally turning the laser

on and off, varying the intensity of the emitted radiation from a vertical cavity surface

emitting laser by voltage, with fixed injected current, is desirable. Such control is

available with a three terminal voltage-controlled VCSEL described in U.S. patent

5,056,098, by Philip J. Anthony et al., and issued October 8, 1991, which is hereby

incorporated by reference. For several reasons, it is desirable to use surface emitting devices. For exam

surface emitting devices can be fabricated in arrays with relative ease while edge

emitting devices can not be as easily fabricated. An array of lasers can be fabricated

growing the desired layers on a substrate and then patterning the layers to form the

array. Individual lasers may be separately connected with appropriate contacts. Suc

arrays are potentially useful in such diverse applications as, for example, image

processing inter-chip communications, i.e., optical interconnects, and so forth. Sec

typical edge-emitter lasers are turned on and off by varying the current flow throug

device. This requires a relatively large change in the current through the device wh is undesirable; the surface emitting laser, described below, requires lower drive current,

and thus the change of current to switch the VCSEL need not be large.

High-yield, high performance VCSELs have been demonstrated, and expedited

in commercialization. There have been demonstrated breakthroughs in record

performance and flexibility exploiting variation of this VCSEL platform.

Top-surface-emitting AlGaAs-based VCSELs are producible in a manner

analogous to semiconductor integrated circuits, and are amenable to low-cost high-

volume manufacture and integration with existing electronics technology platforms.

Moreover, VCSEL uniformity and reproducibility have been demonstrated using a

standard, unmodified commercially available metal organic vapor phase epitaxy

(MOVPE) chamber and molecular beam epitaxy (MBE) giving very high device yields.

The flexibility of this technology was exploited for lateral mode engineering

including spatially filtered hybrid semiconductor/dielectric DBR VCSELs for single-

mode emission with stable wavelengths and current. At the other extreme, a "quasi-

incoherent" (multi-wavelength) VCSELs have been demonstrated with properties that

alleviate modal noise in multi-mode fibers to overcome mode selective loss, especially

in data communication applications, or analogously noisy speckle patterns in imaging

applications.

SUMMARY OF THE INVENTION

The present invention is a controllable frequency-modulated, producible, vertic cavity surface emitting laser (VCSEL) and array. The invention uses a saturable

absorber (SA) contained within the VCSEL's distributed Bragg reflector (DBR), or spacer (as described in "Self-pulsations in vertical-cavity surface-emitting lasers," by D

Nugent et al., Electronics Letters. 31, pp.43-44, January 5, 1995). Under appropriate operating conditions, the saturable absorber, strategically placed, forces the VCSEL to

self-pulsate (in the GHz-regime) at rates related to the local intensity, absorption,

lifetime, and carrier density at the saturable absorber. This characteristic is exploited a

a technique to frequency modulate a VCSEL. These operating conditions can be

controlled in real time by adjusting the injection current into the VCSEL itself, or by

using a third terminal to modify the carrier density within the saturable absorber with

additional current injection for a fixed VCSEL bias, or by reverse-biasing the saturabl

absorber thereby simultaneously altering the absorber's absorption and earner lifetime

and hence carrier density. Additionally, the center frequency of oscillation can be

determined by the material, location and thickness of the saturable absorber, the mirro

design, cavity Q and structure, and the laser size (and hence threshold current).

Furthermore, by extending VCSELs of this type into arrays, one can easily multiplex

numerous multi-GHz channels into a 100 plus GHz frequency-modulated transmitter a cost-effective manner. Each of an array of individual VCSEL elements can have a

particular modulation frequency range determined during fabrication and be further

tuned during operation. The total operating range across the array can be increased b

varying each element structure, and subsequently tuning it. VCSELs, arranged in an

array, having differing sizes of apertures or other characteristics may be adjusted so to form a phased array, with certain VCSELs turned on and others turned off, or a variation of frequency to achieve a certain direction of radiation by the resultant pha

array, or a phased-antenna array controller. VCSELs permit the construction of hig

effective, long range phased arrays functioning at high frequencies using low power

with low cost, unsophisticated electronics, in contrast to radio frequency (RF) or

millimeter (MM) based phased arrays. An additional advantage may be determined by self-pulsation of the VCSEL

which effectively decoheres the laser light output, wherein each individual pulse is

reasonably coherent but the pulses together make the output laser light beam appear

incoherent by averaging over the many pulses during the detection integration period,

even though the laser beam itself may be in a coherent single transverse and longitudin

mode. Such decoherence can be used to reduce modal noise in a multi-moded fiber

medium to overcome mode selective noise penalties. A two terminal version of the

VCSEL is suited for decoherence purposes as a fixed VCSEL bias would be used for a

predetermined frequency of self pulsation. A predetermined injected current would

result in VCSEL emitting light pulsating at a certain frequency. The output of a VCSE

may be coupled into and conveyed by either a single mode or multi-mode medium or into a "free-space" lensed system. A controlled frequency modulated VCSEL may als

serve as a low cost, low power optical local oscillator within numerous radio-frequenc

(RF) systems.

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 is an illustration of a planar, current-guided, GaAs/AlGaAs top surfac

emitting vertical cavity laser.

Figure 2 is a schematic of a VCSEL with an absorber.

Figure 3 is a graph of pulsation frequency versus drive current for various

positions of a saturable absorber in a VCSEL.

Figure 4 is a cross-section of a three terminal frequency modulated VCSEL.

Figure 5 are graphs for various voltage-current levels or light power and self-

pulsation frequency versus drive current for a three terminal VCSEL. Figure 6 is a plot of the absorption coefficient of a quantum well region versus

the applied (reverse) voltage.

Figures 7a and 7b show waveforms of VCSEL self-pulsation and modulation.

Figure 8a is an optical field distribution of a VCSEL.

Figure 8b shows light intensity versus position near the active region of a

VCSEL.

Figure 9 is a graph of typical light power-current- voltage (L-I-V) characteristic of a state-of-the-art GaAs/AlGaAs VCSEL.

Figure 10 shows a hybrid-DBR spatial-filtered VCSEL having a dielectric

mirror, with spatial filtering for single TEM₀o-mode control for improved performanc

and potential fabrication advantages.

Figure 11 is a diagram of a modulator and the three terminal VCSEL for providing frequency-modulated self-pulsations.

Figure 12 reveals an array of VCSELs having various sized and/or structured

VCSEL apertures.

Figure 13 is a cross section of an array of VCSELs having a Q-graded coatin

DESCRIPTION OF THE EMBODIMENTS

In figure 1 is a diagram of a two terminal VCSEL 10. Formed on an n+ galli

arsenide (GaAs) substrate 14 is an n- contact 12. As indicated, substrate 14 is doped

with impurities of a first type (i.e., n type). An n- mirror stack 16 is formed on subs

14. Formed on stack 16 is a spacer 18. Spacer 18 has a bottom confinement layer 2

formed on stack 16, an active region 22 formed on layer 20 and a top confinement l

24 formed on active region 22. A p- mirror stack 26 is formed on top confinement l

24. A p- metal layer 28 is formed on stack 26. The emission region may have a passivation layer 30. Isolation region 29 restricts the area of the current flow 27 throug

the active region. Region 29 can be formed by deep H+ ion implantation or by other

known techniques.

Spacer 18 may contain quantum wells disposed between minor stacks 16 and

26. Stacks 16 and 26 are distributed Bragg reflector stacks. Quantum well active regio

22 has alternating layers of aluminum gallium arsenide (AlGaAs) barrier layers and

GaAs well layers. Stacks 16 and 26 have periodic layers of doped AlGaAs and

aluminum arsenide (AlAs). The AlGaAs of stack 16 is doped with the same type of

impurity as substrate 14 (i.e., n type), and the AlGaAs of stack 26 is doped with the other kind of impurity (i.e., p type).

Contact layers 12 and 28 are ohmic contacts that allow appropriate electrical

biasing of laser diode 10. When laser diode is forward biased with a more positive

voltage on contact 28 than on contact 12, active region 22 emits light 32 which passes

through stack 26.

There may be a saturable absorber, such as layer 25, composed of GaAs, for

example, to absorb light at wavelengths (λ) less than 870 nm or composed of In_xGa_].

_xAs quantum wells (such as 80 angstroms in thickness and wherein x may be 0.2 as an

example) to absorb light at wavelengths less than one micron. Layer 25 can be of a λ/4

order in thickness but need not be such. Layer 25 can be situated anywhere in the stack

of device 10. Instead, for instance, layer 31 may be the saturable absorber. Placement of the saturable absorber at a position in the cavity within layers 16 through 26 is

influenced by the confinement factor. The saturable absorber may also be placed withi

spacer regions 20 or 24. For example, saturable absorber layer 25 is indicated to be a

kth distance 90 of k one-quarter wavelengths from active region 22, as illustrated in figure 2 which is a schematic of a portion of VCSEL 10. Active region 22 is illustrated with a thickness 94 and absorber 25 with a thickness 96. The thickness should be

chosen in conjunction with the rest of the structure to maintain an adequate optically

thick cavity. Figure 3 shows the results of pulsation frequency versus drive current for

various (k) SA positions of λ/4 thickness for a VCSEL of a particular design but not

necessarily VCSEL 10 of figure 1. Ibid.. D. Nugent et al. The drive current for VCSE

10 would be applied via contacts 28 and 12. These figures are noted again below. Not

that pulsation frequency may be tuned by injection current and influenced by design

(i.e., absorber position).

A three terminal version of a vertical cavity surface emitting laser is shown in

sectional view in figure 4, in contrast to the two terminal VCSEL of figure 1. The

frequency of the self pulsations of VCSEL 60 light 70 may be modulated or tuned wit

a drive power of varying amplitude between terminals 52 and 50. The saturable absorber may be situated anywhere between terminals 50 and 52. This inexpensive, l

power device 60 has a very large frequency modulation bandwidth. The application o

cunent across terminals 48 and 50 of VCSEL 60 can be constant, but tuned to give th

right center self-pulsation frequency and/or light output. This configuration would

result in minimal amplitude modulation of the VCSEL 60 light 70, as opposed to a t

terminal cunent-injected frequency-modulated VCSEL. Typically, the three terminal device 60 has a fixed constant current between

terminals 48 and 50 resulting in a particular voltage-current (VI) (reverse or forward

biased) being applied between those terminals. In figure 5, three levels 74, 76 and 7

VI₅₀_₅2 (voltage-current) across terminals (50 and 52) are illustrated with light powe

and the self-pulsation frequency f_sp versus the drive current through terminals 48 an 50. The drive power is used to modulate f_sp. VI 78 is greater than VI 76 which is

greater than VI 74. As one can note, under these conditions for a given f_sp, the drive

current is less for a greater VI, as indicated by the dashed lines.

As will be appreciated by those skilled in the art, some elements which are not

essential to an understanding of the invention are either not depicted or described in

detail. For example, only a single laser is illustrated in Figure 4, although it will be

readily noted that an arcay of lasers typically may be present. Shown are substrate 34,

regions 36 and 46 having a first conductivity type, active region 38, regions 40 and 42

having a second conductivity type, with saturable absorption region 44 having either or

neither conductivity type depending on design and operating conditions. Region 44

may comprise any number of bulk materials or one or more quantum wells, being

normally absorbing at the lasing wavelength. Regions 36, 40, 42 and 46 comprise

mirrors which are depicted as interference minors. Region 36 comprises a first minor.

Only several layers are shown for reasons of clarity. Appropriate regions of different

conductivity types will be readily selected by those skilled in the art. Regions 40, 42,

44 and 46 form a second distributed minor with a cavity Q and hence an oscillation

frequency controllable via power applied to the saturable absorber through contacts 50

and 52. The active region typically comprises one or more quantum well regions whic

are interleaved with barrier layers, i.e., layers having a bandgap greater than the bandg

of the quantum well region. However, the use of bulk semiconductors instead is not

precluded. There are first, second, and third electrical contacts 48, 50, and 52, to regio

36, region 40, and layer 46, respectively. Contact 48 may be physically made to substrate 34 if the substrate is conducting and not semi-insulating. Isolation region 54

restricts the area of the current flow through the active region to the area generally und region 46. Isolation region 54 can be formed by, e.g., deep ion implantation. Other

forms of current and optical confinement may be utilized. The portions of regions 36

and 40 having first and second conductivity types, form means for injecting carriers int the active region. The first and second interference minors further comprise a plurality

of interleaved first and second semiconductor layers with each layer having

characteristics such that it is typically a quarter wavelength thick at the medium

wavelength of interest thereby forming the respective interference minor. The

individual layers of the active region and the interference minors are not described wit

particularity as those skilled in the art know the structure of these elements.

In the embodiment, substrate 34 is conducting or semi-insulat g GaAs, and

regions 36, 40, 42 and 46 comprise alternating layers of AlAs and AlGaAs, as an

example, with properties as described in the previous paragraph. The active region m

comprise one or multiple GaAs (or, e.g., In_xGa_1-xAs) quantum wells interleaved with

AlGaAs barrier layers. Saturable absorption (SA) region 44 is optically coupled to

region 40, i.e., the absorption due to the SA is within the distributed minor

incorporating regions 40, 42, 44 and 46. Region 46 comprises interference minor lay

of, e.g., AlAs and AlGaAs, and has a first conductivity type. Those skilled in the art

will readily select appropriate layer thicknesses and these parameters need not be described in detail. The use of other semiconductors is contemplated and appropriat

choices will readily be made by those skilled in the art. For instance, other Group II

semiconductors may be used.

Conventional and well-known epitaxial growth techniques, such as molecula

beam epitaxy or metallo-organic chemical vapor deposition, may be used to grow th

layers described. After the layers have been grown, conventional patterning techniq are then used to form the individual lasers in the array. Electrical contacts to the

individual lasers are also fabricated. Those skilled in the art will readily select appropriate patterning and contacting techniques.

The frequency of oscillation of the self-pulsing light emitted from the device ca

be varied by controlling the properties of the SA region within the VCSEL structure.

An embodiment may use current or voltage alteration of bulk or quantum- well material

such as the quantum-confined Stark effect in quantum wells. This effect is well known

and understood by those skilled in the art; the effect is described in Chapter 4 entitled

"Nonlinear optical properties of semiconductor quantum wells," by D.S. Chemla et al.,

in Optical Nonlinearities and Instabilities in Semiconductors, pp. 339-347, (Academic

Press 1988). Basically, the exciton absorption depends on the magnitude of the electric

field in the quantum well. Figure 6 plots the absorption coefficient α of the quantum

well region vertically versus the applied voltage V horizontally for a wavelength longer

than that of the zero field exciton. In other words, in this figure, the absorption

coefficient α is plotted vertically versus the voltage V horizontally for an exemplary

quantum-confined Stark effect region useful in the present device. For device 60, a

voltage is between contacts 52 and 50, i.e., a reverse bias p-i-n junction, and an

increasing voltage corresponds to an increasing electric field and vice versa. Arbitrary

units are used for purposes of exposition. The initial voltage is V_b. The absorption

coefficient α can be varied a significant amount with a relatively small change in the

applied voltage. Similarly the field-dependent Franz-Keldysh effect can be used for an

SA composed of bulk material. The use of this characteristic is noted below. Moreove SA 44 of figure 4 may be forward biased to inject current into it also allowing frequenc

tuning. A vertical cavity surface emitting laser needs relatively large reflectivities in

both mirror stacks for lasing; typically, minor stack reflectivities should be 99 percent or greater. The S A region functions as a bias-dependent absorber, by appropriately

varying the bias, the laser pulsation can be frequency modulated at different rates. A

small voltage or cunent change may be used to vary the absorption or carrier density o

the SA and hence the frequency of the VCSEL self-pulsation. However, the magnitud

of the cunent supplied through contacts 48 and 50 of device 60 of figure 4, may remai

essentially constant as the laser is modulated. This simplifies the design of the power

supply (not shown) for the anay and minimizes any problems that might otherwise ari

due to the varying heat generated in the vertical cavity laser array, due to the varying

carrier density in the active region, and due to the resulting index changes in the optic

cavity.

The optical spatial field distribution of the surface emitting laser and the positi

of the saturable absorber are both engineering parameters. More or less absorption ca be obtained by placing the absorber near a node or antinode, respectively. More or le

absorption can also be obtained by increasing or decreasing the reflectivity of regions

and 36 of device 60, thereby coupling relatively less or more of the SA into the

distributed minor. Additionally, the material, thickness and/or design and number o the quantum wells, or of bulk materials, in SA 44 can be varied from that described herein. Those skilled in the art will readily select appropriate parameters.

The invention utilizes the saturable absorber in the optical cavity to achieve

high-speed bias-controlled tuning of the self oscillation or pulsation frequency of a

VCSEL or VCSEL array while maintaining constant current and hence minimizing

chirp, relaxation oscillations, and so forth. In Figure 4, terminal 50 represents the to (usually the p type) contact and terminal 48 represents the bottom contact (usually the n

type) contact. The bottom contact may be a common metalization on the bottom like

contact 12 as shown in Figure 1. Contact 52 represents a third connection which can be

used to either reverse bias or forward bias the saturable absorber layer which is

schematically illustrated by layer 44.

The incorporation of a saturable absorber can result in self pulsations of the lase

and is a common technique for "mode-locking" the laser to produce a train of laser

pulses whose period is proportional to the cavity length. Figure 7a reveals the nature o

a frequency-modulated self-pulsing waveform. Profile 62 is the frequency of the self-

pulsations of the VCSEL with respect to time. Profile 64 is the amplitude of the

modulating waveform of the power (VI) signal sent to the VCSEL. The overall

envelope 62 may of course be changed by the power (VI) between terminals 48 and 50

of VCSEL 60. The waveform of the self-pulsations switches between I_max and I_min

which are light "on" and light "off" conditions, respectively, of the pulsation. I_min may

be between zero amplitude and I_max, depending on the electrical and physical

characteristics of, and the power parameters to the VCSEL, and is ideally zero.

Self pulsations up to 6 GHz have also been demonstrated in a VCSEL external

cavity configuration, utilizing the VCSELs geometrical polarization degeneracy. This

approach is also a way of producing GHz-range self-modulation whose frequency can

be modulated by varying the optical cavity length. "High Frequency Polarization Self- modulation in Vertical Cavity Surface Emitting Lasers," by S. Jiang et al., on pp. 3545 3547 in Appl. Phvs. Lett.. Vol. 63 (1993). Self pulsations within a long-wavelength

VCSEL were theoretically analyzed using the following coupled rate equations (Ibid..

Nugent et al.): dP/dt = r_ag_a(n_a - noa)P + r_sg_s(n_s - no_s)P - P/τ_pb + V_aβBn_a (1);

dn_a/dt = -T_ag_a(n_a -

+ -n_a/τ_a + I_a/eV_a (2); and

dn_s/dt = -r_sgs(n_s - no_s)PA _s + -n_s/τ_s + (I_s/eV_s) (3);

where we have added the third term in equation (3) to allow for additional modulation

of the SA.

In the equations, the subscripts "a" and "s" identify the active and saturable

absorber, respectively; the symbol 'T" refers to the photon confinement factor. The

third term I_s/eV_s is added in equation (3) to represent the case for current injection via

contacts 50 and 52 as shown in Figure 4. In the third equation, T_s and V_s are

determined or affected by material, structure and material placement; and g_s, n_s, no_s, τ

and I_s are changed by current-voltage (VI₅₀.5₂) across terminals 50 and 52, and the

device consequently can be tuned.

In the three above-disclosed equations, the respective terms mean: T_a is the

active region confinement factor; g_a is the active region differential gain; n_a is the act

region carrier density; 1 is the active region transparency density; P is the optical

power; T_s is the S A confinement factor; g_s is the S A differential gain; n_s is the S A

carrier density; no_s is the SA transparency density; τ_pt, is the cavity lifetime; V_a is th

active region volume; β is the spontaneous emission factor; B is the bimolecular

recombination coefficient; τ_a is the active region lifetime; I_a is the active region injection current; e is the electron charge; V_s is the SA volume; τ_s is the SA lifetime;

and I_s is the current in the SA.

Figure 3 graphically represents the calculations of pulsation frequency versus

drive cunent for long-wavelength laser parameters. Ibid. Nugent et al. The k-paramete

represents the location of the saturable absorber, assumed to be the kth quarter wave in

active spacer 18 of figure 1. The graph of figure 3 shows the self-sustained pulsation

frequency in GHz of a VCSEL, versus drive cunent I_a in milliamperes for various k's o

the saturable absorber. Curve 66 shows the maximum drive cunents and conesponding

frequencies for the respective k's for self-sustained pulsations. Curve 68 shows the

maximum drive currents against frequencies of damped relaxation oscillations,

determined from small signal analyses. Points 70 reveal the minimum drive currents fo the various k's. Ibid.. D. Nugent et al. The absolute cunent, frequency and k

dependence will vary in accordance with equations (l)-(3) for VCSEL 60.

The optical intensity of the light decreases roughly exponentially from the space

region as indicated in Figure 8a. This figure reveals the photon density distribution

within the VCSEL structure. Figure 8b shows a relationship of light intensity relative t

physical position in the VCSEL for a typical VCSEL with a one λ-thick cavity (a cavity

having one of other multiples of λ/2 is also possible).

The curves for device 60 of Figure 3 are generated using InP/InGaAsP based

materials near 1.55 microns. Typical light power-current-voltage (L-I-V) characteristic

of state-of-the art GaAs/ AlGaAs VCSELs are given in Figure 9. Figure 9 is a graph of

power in milliwatts, voltage in volts and wallplug efficiency in percent versus current i

milliamperes for a GaAs VCSEL having an implant diameter (g) of 20 microns, a window aperture (w) of 15 microns, a series resistance (R) of 31 ohms or less, and a

wavelength (λ) of 844 nm at room temperature (25 degrees C). Window aperture 93 and implant diameter 95 are illustrated in figures 1 and 10. The graph of figure 9

reveals the VCSEL to have a 1.6 volt (0.1 volt above bandgap) CW threshold voltage,

3 milliampere threshold current and a wallplug efficiency η of 22 percent. The power

proportional to the drive cunent. It is clear that an order of magnitude less drive curre

for a few milliamperes of power per element may be needed. This may also result in

higher speed at lower drive-power requirements as compared to the VCSEL modeled i

Figure 3.

As noted above, the saturable absorber's size (V_s) and placement may be

strategically designed parameters exploiting the optical field pattern of Figure 8a

affecting T_s, in order to control the oscillation frequency. n_s and g_s can also be affect

by material, doping and wavelength of operation in an effort to engineer the VCSEL

frequency response. To modulate the present VCSEL; the injection current (I.) into t

VCSEL can be varied; for a fixed VCSEL bias, using a third terminal (50 of Figure 4

the carrier density, n_s, within the saturable absorber can be modified via additional or

less current injection; or the saturable absorber can be reverse-biased, by simultaneo

altering its absorption (g_s and n_s) and carrier lifetime (τ_s) and thus modulating the

VCSEL self-oscillation frequency. In effect, the latter is the shifting of a given k- va

curve of Figure 3. Furthermore, a given modulation frequency can be controlled in t

fabrication process by location of the saturable absorber (T_s) and effects of the mirr

design and cavity Q (P, T_s). For example, Q can be altered by varying the epi-mirr design, or by exploiting a hybrid semiconductor/dielectric minor 72 as illustrated in

Figure 10. This approach may also have the additional advantage of controlling the

laser to single transverse mode emission stability over the entire operating range. T_s

and Q may also be altered by changing laser size (and hence threshold current), all of

which permit great design flexibility into a VCSEL GHz generator.

Figure 11 shows a configuration 63 which incorporates VCSEL 60 of figure 4.

Self-pulsing light 70 of VCSEL 60 is frequency modulated with amplitude signals

across terminals 50 and 48 from modulator driver 61. Modulator driver 61 receives

signals 69 that are to modulate the self-pulsating light 70 from signal source 67. Signal

69, having a digital and/or analog format, may originate as signals conveying data,

control information, communications and so forth.

The laser cavity frequency itself has been estimated to be in the tens of the GHz regime (at about 50 GHz from the modulation response of the VCSEL shown in Figure

10.). Naturally, individual VCSELs are inherently extendible to both one and two

dimensional arrays permitting easy multiplexing of numerous GHz channels into a 100

plus GHz transmitter in a cost-effective, high yield style. Individual elements operating

modulation frequency range can be controlled by fabrication and/or tuned in operation.

The total operating range across the array can be increased by varying each element

structure, and also subsequently tuned as discussed previously.

Figure 12 shows an array 80 of rows or columns of VCSELs having apertures o

various sizes. For instance, VCSELs 81, 82, 83 and 84 may have aperture sizes of 10,

20, 30 and 40 microns, and self-pulsation frequencies of 100, 70, 50, and 30 GHz,

respectively, for same drive cunent and electrical power applied to those VCSELs.

VCSEL 85 has the same aperture size as that of VCSEL 81 but a self-pulsation frequency of 120 GHz which is higher than that of VCSEL 81 for the same applied

drive current and electrical power. The only structural difference between VCSELs 81

and 85 is that VCSEL 85 has a dielectric mirror layer that increases the Q of its cavity thereby increasing its self-pulsation frequency. Increasing the drive current would

increase the self-pulsation frequencies of all the VCSELs. Typically, array 80 would

have two terminal VCSELs but could have three terminal VCSEL or a combination of

them. The VCSELs of anay 80 may be selectively turned on or off, or have their self-

pulsation frequencies individually varied or modulated. Such an anay would allow fo

greatly extended bandwidth. On the other hand, figure 12 may represent an array

wherein the phases of each of the VCSELs of anay 80 may be controlled relative to o

another via current or external delays to result in a VCSEL phased array.

Figure 13 reveals a cross section of an array of VCSELs having the same

structural and electrical characteristics. However, a coating 86 of material having a

varying thickness is applied to the VCSEL array. Coating 86 has the effect of alterin

the Q of the VCSEL relative to the thickness of the coating immediately or directly o

the respective VCSEL. For example, the greater the thickness of coating 86, the low

the Q of the respective VCSEL as shown by Q's 91 and 92 plotted in graph 89 and

positions 87 and 88 of the respective VCSELs.

A plurality of VCSELs may be formed into a phased array wherein the phase

relationships among the VCSEL outputs are controlled and the outputs can be selectively switched to generate radiator groupings for establishing a particular or desired radiated pattern for a certain direction at a particular frequency.

Variations of the embodiment described are contemplated. For example, the

controllable minor may use any voltage for a controllable effect. The light may be emitted through either the substrate at one end or the top minor at the other end. It will

also be understood that the term, "vertical," is used to mean perpendicular to the major

surfaces of the substrate. The means for injecting power can have first and second

conductivity types on opposite sides of the active region, either along the axis formed by

the first minor, active region and second minor, or along some other axis.

Claims

THE CLAIMS

1. A frequency modulated self-pulsing VCSEL comprising:

a VCSEL comprising:

a first mirror region having a first terminal;

a second minor region having a second terminal;

an active region situated between the first and second mirror regions; an a saturable absorber situated between the first and second terminals; an

a power source, connected to the first and second terminals of said VCSEL, for

providing variable power to said VCSEL to affect a pulsing of a light

output of said VCSEL, the pulsing having a frequency modulated by th

variable power from said power source.

2. The VCSEL of claim 1 having a third terminal connected to said VCSEL and

situated between the first and second terminal, the third terminal for receiving a signa

to effect frequency modulation of the frequency of the pulsing.

3. A frequency modulated self-pulsing VCSEL comprising:

a VCSEL having a saturable absorber, and first and second terminals; and

a variable power source, connected to the first and second terminals of said

VCSEL, for providing a variable injected power through the VCSEL

which results in a variable frequency of pulsing of a light output of s

VCSEL, in proportion to the variable injected power.

4. The VCSEL of claim 3 having a third terminal for receiving a signal having a

variable amplitude to effect frequency modulation of the frequency of the pulsing.

5. A VCSEL system comprising:

a plurality of VCSELs wherein each VCSEL of said plurality, comprises:

a first minor region having a first terminal;

a second minor region having a second terminal;

an active region situated between the first and second minor regions; and

a saturable absorber situated between the first and second terminals; and

wherein for certain magnitudes of power applied to the first and second

terminals, the VCSEL outputs self-pulsing light having certain

frequencies conesponding to the certain magnitudes of power

applied, respectively;

a signal source for providing a signal having an amplitude; and a modulation driver, connected to the first and second terminals of each VCSEL

and to said signal source, for providing the power applied having a

magnitude varied in accordance with the amplitude of the signal from

said signal source, to each VCSEL to affect the frequency of self-pulsing

light of each VCSEL, the frequency of the self-pulsing light modulated

in accordance with the signal from said signal source.

6. The VCSEL system of claim 5 wherein each VCSEL of said plurality of VCSELs

has a third terminal situated between the first and second terminal, the third terminal for receiving a second signal to effect frequency modulation of the frequency of self- pulsing light.

7. The VCSEL system of claim 5 wherein each VCSEL of said plurality of VCSELs has a frequency of self-pulsation that is different from a frequency of self-pulsation of other VCSELs of said plurality of VCSELs, for a certain magnitude of power applied t the first and second terminals of each VCSEL of said plurality of VCSELs.

8. The VCSEL system of claim 5 wherein said plurality of VCSELs is arranged in an

anay.

9. The VCSEL system of claim 8 wherein said array is a phased array wherein the phase relationships among VCSEL outputs are controlled and the outputs can be selectively switched to generate radiator groupings for establishing a selected radiated pattern for a certain direction at a particular frequency.

10. The VCSEL system of claim 5 wherein each VCSEL is a local oscillator.

11. The VCSEL system of claim 5 wherein at least one VCSEL has a noncoherent output.

12. The VCSEL system of claim 11 wherein the at least one VCSEL is for communications.

13. The VCSEL system of claim 12 wherein the at least one VCSEL is for reducing modal noise or speckle.

14. A VCSEL, for outputting a self-pulsing light having a frequency, comprising:

a first minor region having a first terminal;

a second minor region having a second terminal;

an active region situated between the first and second minor regions; and

a saturable absorber, situated between the first and second terminals, having a

third terminal; and

a power source connected to the first and second terminals of said VCSEL;

a signal source for providing a signal having an amplitude; and

a modulation driver, connected to the first and third terminals of said VCSEL

and to said signal source, for providing power having a magnitude varied

in accordance with the amplitude of the signal from said signal source, to

said VCSEL to affect the frequency of the self-pulsing light of said

VCSEL, the frequency of the self-pulsing light modulated in accordance with the signal from said signal source.