WO2012167310A1

WO2012167310A1 - Optical system and method for pulse repetition rate multiplication of a laser source

Info

Publication number: WO2012167310A1
Application number: PCT/AU2012/000648
Authority: WO
Inventors: David Francis KIELPINSKI; Omri Gat
Original assignee: Griffith University; Yissum Research Development Company Of The Hebrew University Of Jerusalem Ltd.
Priority date: 2011-06-07
Filing date: 2012-06-07
Publication date: 2012-12-13

Abstract

Disclosed herein is an optical system and method for improved pulse repetition rate multiplication of a pulsed laser source using a master and slave resonator, where the slave repetition rate is set to a rational harmonic of the master repetition rate. The optical system comprises an optical source (master) arranged to generate optical pulses, an optical resonator (slave) having an optical input arranged for receiving the optical pulses, and also having an optical output. The optical resonator also contains a saturable absorber and gain medium. The system can furthermore comprise an active frequency modulating element.

Description

OPTICAL SYSTEM AND METHOD FOR PULSE REPETITION RATE MULTIPLICATION OF A LASER SOURCE

Technical Field

The disclosure herein generally relates to an optical system, and particularly but not exclusively to an optical system that produces optical pulses.

Background

Ultrafast optical pulses are useful for many applications. Example applications include but are not limited to metrology, communications, optical frequency counting and synthesis, broadband spectroscopy, and LIDA .

For some applications sources of ultrafast pulses may need to exhibit at least one of low optical phase noise, stable repetition rate, application specific pulse duration, low pulse-to-pulse parameter variations, be insensitive to environmental fluctuations, and be low cost. Some combinations of these qualities may be very difficult to achieve simultaneously. Summary

Disclosed herein is an optical system. The optical system comprises an optical source arranged to generate optical pulses. The optical system comprises an optical resonator having an optical input arranged for receiving the optical pulses, and also having an optical output. The optical resonator has a component arranged to cause loss to a light within the resonator when in use, the loss decreasing as a power of the light increases. The optical5 resonator has an optical gain medium arranged to cause gain to the light within the optical resonator when in use . An embodiment of the system may be described as having an optical resonator having active components. The resonator may be described as a filter that filters the optical output of the optical source. The optical pulses emitted by the optical resonator may have more desirable properties than optical pulses emitted by the optical •source. In an embodiment, at least one of the optical resonator and the optical source are arranged to give a ratio of free spectral range of the optical resonator and repetition rate of the optical pulses a value which is a predefined rational number. The repetition rate may not be an integer multiple of the free spectral range of the optical resonator. The optical system may be arranged such that gain competition between frequency components of the pulses within the optical gain medium compensates for a deviation of the ratio from the predefined rational number.

In an embodiment, the system is arranged to set each of at least two transmission frequencies of the optical resonator to a respective frequency component of the pulses. The optical system may be arranged such that in use gain competition between a plurality of frequency components of the pulses within the gain medium

compensates for a deviation between each of the at lea3t two transmission frequencies of the optical resonator and their respective frequency component of the pulses.

An embodiment is arranged to produce a plurality of emitted optical pulses at the output having a repetition frequency that is a least common multiple of a free spectral ran'ge of the optical resonator and a repetition rate of the optical pulses. In an embodiment, the optical system comprises a controller arranged to control a frequency relationship between the optical pulses and the optical resonator. The optical system may comprise a resonator length stabiliser arranged to stabilise the length of the optical resonator. The optical system may comprise a frequency modulator arranged to modulate at least one of the frequency of the . pulses from the optical source and the light within the optical resonator.

In an embodiment, the optical source comprises a mode locked laser. The mode locked laser may comprise a passively mode locked laser. In an embodiment, the component arranged to cause loss may comprise a mode locking element. The mode locking element may comprise a saturable absorber. The saturable absorber may comprise a semiconductor saturable absorber. The saturable absorber may comprise graphene based materials.

In an embodiment, at least one of the optical source and the resonator comprises optical fibre. Disclosed herein ΪΞ a method for configuring an optical system having an optical resonator arranged to receive optical pulses from an optical source. The method comprises the step of defining a value that a ratio of a free spectral range of the optical resonator and a repetition rate of the optical pulses should be, the value being a rational number. The method comprises the step of arranging the system to give the ratio the value.

In an embodiment, the method further comprises arranging the system to^' emit optical pulses having a repetition frequency that is a least common multiple of a free spectral range of the resonator and the repetition rate of the optical pulses .

In an embodiment, the method comprises arranging the system such that the repetition rate is not an integer multiple of the free spectral range of the resonator.

An optical system of the second aspect of the invention may be in accordance with an optical system of the first aspect of the invention.

Disclosed herein is an optical system. The optical system comprises an optical source arranged to generate optical pulses. The optical resonator has an optical input arranged for receiving the optical pulses, and also has an optical output. At least one of the optical resonator and the optical source are arranged to give a ratio of free spectral range of the resonator and

repetition rate of the optical pulses a value that is a predefined rational number.

Disclosed herein is an optical system. The optical source is arranged to generate optical pulses. The optical system comprises an optical resonator having an optical input arranged for receiving the optical pulses, and also has an optical output. At least one of the optical source and the optical resonator are arranged such that the repetition rate of the optical pulses is not an integer multiple of the free spectral range of the optical resonator.

Disclosed herein is an optical system. The optical system comprises an optical source arranged to generate optical pulses. At least one of the optical source and the optical resonator are each arranged such that in use at least two transmission frequencies of the optical resonator are each in accordance with a respective frequency component of thp pulses.

Disclosed herein is an optical system. The optical system comprises an optical source arranged to generate optical pulses. The optical system comprises an optical resonator having an optical input arranged^' for- receiving the optical pulses, and also having an optical output. At least one of the optical source and the optical resonator are arranged to give optical pulses at the output a repetition frequency that is a least common multiple of a free spectral range of the resonator and the repetition rate of the optical pulses. Disclosed herein is a method for generating optical output . The method comprising the step of launching optical pulses into an optical resonator. The method comprises the step of causing loss to a light within the optical resonator, the loss decreasing as the power of the light increases. The method comprises the step of causing gain to the light within the optical resonator.

In an embodiment, the method comprises the steps of: defining a value that a free spectral range of the resonator divided by a repetition rate of the optical pulses should be, the value being a rational number; and causing a ratio of the free spectral range and the repetition rate to be equal to the value. It should be noted that any of the various features of each of the above aspects of the invention, and of the various features of the embodiments described below, can be combined as suitable and desired. Brief description of the Figures

In order to achieve a better understanding of the nature of embodiments, an optical system and a method for configuring an optical system will now be described, by way of example only, with reference to the accompanying figures in which:

Figure 1 shows a schematic representation of an embodiment of an optical system;

Figure 2 is an optical spectrum from the output of the optical system of Figure 1;

Figure 3 shows the result of a measurement of the repetition rate by spectrum analysis of an output

photodiode signal of the system of Figure 1;

Figure 4 shows a measurement of the optical frequency noise added by a resonator of the system of Figure 1; and

Figure 5 shows a measurement of the repetition rate frequency noise of the system of Figure 1.· Detailed Description of embodiments

Figure 1 shows a schematic representation of an embodiment of an optical system generally indicated by the numeral 10. The system 10 has an optical source 12 arranged to generate optical pulses. The optical source 12 may be any suitable optical source such as an electro- optic pulse generator, a passively mode locked laser, an actively mode locked laser, or a source using soliton generation in an optical fibre or waveguide. In this embodiment the optical source comprises a passively mode locked erbium-doped fibre laser manufactured by Precision Photonics, and an erbium doped fibre amplifier. The optical output of the passively mode locked erbium-doped fibre laser is amplified to an average power of between 10-20mW by the erbium doped fibre amplifier. It will be appreciated that the average power in other embodiments may be either above or below this range according to system configuration.

The pulses leaving the output optical fibre 14 of the optical source 12 then enter an optical circulator 16, with ports 18, 20, and 24 coupled to the laser 12. The optical circulator 16 is arranged such that light that enters the port 18 leaves port 20, and light that enters port 20 leaves port 22 which is in communication with optical fibre 24. Generally, any suitable commercially available circulator may be used.

Port 20 of the circulator 16 is coupled to the end of a fibre 26 which is connected to an optical resonator generally indicated by the rectangle 30 in dashing. The resonator 30 may be described as acting as a filter of the optical pulses.

The optical pulses emitted by the optical resonator 30 may have more desirable properties than optical pulses emitted by the optical source. For example, compared to those emitted from the optical source, optical pulses emitted by the resonator may have at least one of lower optical phase noise, lower pulse-to-pulse parameter variations, be relatively insensitive to environmental fluctuations, and have a more stable repetition rate.

Furthermore, the pulse duration may be altered to suit a particular application. The embodiment of figure 1, for example, may have a lower cost than another pulse source, such as a mode locked laser, with similar optical pulse characteristics.

Light resonates within the resonator. The light reflects off a reflector in the form of gold mirror 32. The light passes through a collimator in the form of a collimating lens 34 mounted on a screw driven translation stages. Any suitable collimator may be used, for example a compound collimating lens. The light passes through optical fibre 38, optical fused fibre coupler 28, optical fibre 40, an optical amplifier in the form of an erbium doped fibre amplifier 42 that in use provides optical gain, fibre 44, optionally another collimator in the form of a collimating lens 46 mounted on another translation stage 48, and a mode locking element, specifically a saturable absorber in the form of a semiconducting saturable absorbing mirror 50. The mode locking element causes loss to light within the resonator when in use, the loss decreasing as the power of the light increases. The fibre 44 may alternatively be butted against the

semiconductor saturable absorber and the other collimator 46 omitted.

In this but not in all embodiments, the erbium doped fibre amplifier 42 has 20 cm of LIEKKI ErllO-4/125 fibre, pumped through a 980/1550 nm wavelength-division .

multiplexer by a 400 mW laser diode operating at 976 nm. The various parameters of the amplifier may be widely varied in accordance with the configuration of each respective embodiment. For example, a longer or shorter length of erbium doped optical fibre may be used. The free-space distance between the gold mirror 32 and the end of the optical fibre 38 is 3 to 10 cm, although lower or higher values may be used in other embodiments. The resonator of this embodiment has .an optical length of around 200 cm, although it will be appreciated that other values greater or less than this may result in a readily useable resonator. The coupler 28, in this but not in all embodiments, is a four-port coupler having an 80/20 split between coupled ports, but^' any suitable coupler may be used, such as one with a 90/10 split. The coupler may be a planar waveguide coupler, or a fibre waveguide coupler, for example. Generally any suitable coupler may be used. The collimator may be a lOx microscope objective or any other suitable single or compound lens, Fresnel zone plate or lens, as appropriate. The translation stages may be.

omitted. In other embodiments piezoelectric or otherwise motorised translation units are used instead of stages. The gold, mirror may be replaced with a dielectric mirror. Generally any suitable reflecting structure of sufficient frequency bandwidth may be used. The saturable absorber may be replaced with any component that causes loss to a light within the resonator when in use, the loss

decreasing as the power of the light increases, such as a dye based saturable absorber or graphene based saturable absorber. These components may reshape and shorten pulses circulating in the resonator, and may increase robustness against pulse parameter mismatch between the optical source and the resonator. The absorber is, at least in this embodiment, passive and thus not affected by

electronic noise. A system such as that shown in Figure 1 may support 500 fs pulses with a bandwidth of around 5 nm within the telecommunications C-band, for example.

Port 18 acts as an optical input for the resonator and port 22 as an optical output for the resonator. Any suitable means of coupling light into and out of the resonator may be employed, however, such as partially reflective mirrors and/or frustrated total internal reflection. In the embodiment of Figure 1, the optical pulses received by port 20 are guided by fibre 26 into a coupler 28.

The pulses from the optical source may be chirped (an increasing or decreasing - generally but not necessarily quadratically - phase modulation along the pulse) to match the resonator. The amount of chirp required may be determined by trial and error, by, for example,

progressively adding dispersive elements such as optical fibres, to vary the chirp, between the optical source and the resonator while monitoring the optical spectrum of the output of the resonator. The chirp may be selected by maximising the period of ripples in the optical spectrum of the output .

In the embodiment of Figure 1, the length of the cavity may be adjusted and/or the repetition rate of the pulses from the optical source may be adjusted so that the free spectral range of the resonator divided by the repetition rate of the optical pulses from the optical source gives a value that concords with a rational number, or one of a set of rational numbers. This concordance may be described as the vernier condition, and represented mathematically as

^Vp _P _.

v_L q

where V_F is the free spectral range of the resonator, v_L is the repetition frequency of the optical pulses, and p and q are each an integer. It may be desirable to achieve a ratio as close as possible to the above condition, although it practice it may be difficult to obtain it exactly. Generally, p and q may each have a value between 1 and 100, although any suitable values may be used. A rational number may be determined according to a desired output of the system before its operation., The system may then be arranged such that the vernier condition is sufficiently satisfied for the predetermined rational number. The rational number may be - and generally is - determined in accordance with a desired operation of system prior to its operation. The numbers p and q may be chosen such that p/q is an integer.

The ratio — may deviate from the selected rational

V_L

number. The optical system may however exhibit gain competition between frequency components of the pulses in the resonator to ameliorate such a deviation. This may result in observed low added phase noise and high

tolerance of the device to misalignment as compared to resonators not having active components and/or power- dependent loss.

The frequency components of the optical pulses may bi regularly spaced apart in frequency, forming a so-called frequency comb. The frequency comb may be given by v = mv_t+v_LQ, where m is an integer.

The resonator may also have transmission frequencies that are regularly spaced apart. The transmission frequencies may be given by V = nv_F +v_F0, where n is an integer.

In the embodiment of Figure 1, the optical system may be arranged such that at least two transmission

frequencies of the resonator each concord with a

respective frequency component of the pulses. This may be described as the comb alignment condition, and may be represented by the equation

np

vpo = v_L0 +—v_L

where V_F0 is the lowest theoretical transmission frequency of the resonator and V_L0 is one of the frequency components of the optical pulses. It may be desirable to achieve a value as close as possible to the above condition, although in practice it may be difficult to obtain it exactly.

^■ he the system, or at least one component part thereof, is arranged to a condition or result, it should be understood that the system is arranged with the purpose and intent of achieving the condition or result, even if the condition or result is not exactly satisfied. Similarly, when the resonator and/or optical source are arranged to a condition or result, it should be understood that the arrangement is made with the purpose and intent of achieving the condition or result, even if not exactly satisfied.

The system may deviate f om the comb alignment conditio . The optical system may, however, exhibit gain competition between frequency components of the pulses that ameliorate a discord between each of the at least two transmission frequencies of the resonator and their .

respective frequency component of the pulses.

When the vernier condition and the comb alignment condition are both approximately met, optical pulses may be emitted from the output having a repetition frequency that concords with a least common multiple of a free spectral range of the resonator and the repetition rate of the optical pulses.

Generally, but not necessarily, the repetition rate is . not an integer multiple of the free spectral range of the resonator.

The optical system 10 may include a controller arranged to control a frequency relationship between the optical pulses and the resonator, such as those defined by the vernier condition and the comb alignment condition. The controller may take the form of an active resonator length stabiliser arranged to stabilise the length of the resonator, having for example a piezoelectric driver. The controller may alternatively or additionally comprise a frequency modulator arranged to modulate the frequency of the pulses from the optical source (or within the

resonator) . The active resonator length stabiliser and the frequency modulator may be used simultaneously. The controller may be driven in accordance with an optical signal detected by detector 52 and used by a Pound-Drever Hall apparatus as disclosed by ED Black, Am J. Phys 69, 79 (2001) . Figure 2 is an optical power spectrum from the output of the optical system of Figure 1. The -3 dB bandwidth is approximately A nm. A second order autocorrelation trace was also obtained to verify that short pulses were actually formed. A distinct autocorrelation peak of approx. 1 ps width was obtained, corresponding to approx. 700 fs pulse duration.

Figure 3 shows the power spectrum of a signal from a photodiode that detects the optical output of the

embodiment of figure 1. The output repetition rate lies near 442 MHz. The small other peaks arise from the slight variation of pulse energy at a frequency equal to the free spectral range of the active filter. This so-called supermode noise is suppressed by 33 dB, indicating that the pulse-to-pulse energy is constant at the level of a few parts in 10⁴.

Figure 4 is the result of a measurement of a signal from the photodiode showing the optical frequency noise- added by^■ the filter. This was obtained by optical

heterodyne detection of the filter output using the optical source as a local oscillator, followed by

• spectrum analysis of an output photodiode signal. The -3 dB width of the signal is consistent with the 1 kHz resolution limit of the spectrum analyser. Hence the additional optical frequency noise is less than

approximately 1 kHz .

Figure 5 shows a measurement of the repetition rate frequency noise near 442 MHz by spectrum analysis of an output photodiode signal. The -3 dB width of the signal is consistent with the 1 kHz resolution limit of the spectrum analyser. Again, the additional repetition rate noise is less than approximately 1 kHz.

The optical system may be operated detuned from the vernier condition by a small amount (of the order of a few parts per thousand) . The output of the resonator then may consist of a frequency comb, in one realised case having a repetition rate of 440 MHz. The stability Was similar to that in a regime where both conditions are met, but the stability of the optical phase was not determined.

Remarkably, the pulse-to-pulse variation, as measured in the beatnote spectrum, was limited to a few parts in 10^"5.

In another embodiment, the optical source and resonator are respective branches of a two-branch laser resonator, with the branches satisfying at least one of the vernier condition and the comb alignment condition. Each branch plays a role both as a source in its own right and as a filter for the other branch, even if the two branches share a single gain medium and a single passive saturable absorber.

Some embodiments of the system have applications in at least one of metrology, communications, optical frequency counting and synthesis, broadband spec roscopy, and LIDAR.

Now that embodiments of an optical system and a method for configuring an optical system have been described, it will be appreciated that some embodiments have some of the following advantages:

• low optical phase noise is achievable;

• a stable repetition rate is achievable;

• the pulse duration may be modified according to the applica ion;

• low pulse-to-pulse parameter variations are

achievable ; ^• the system may be insensitive to environmental fluctuations; and

^• the system may be low cost.

Some variations on the specific embodiments include:

^• The component arranged to cause loss to a light within the - resonator may be any suitable component that exhibits a loss that decreases as the power of the light increases, such as a a graphene based material or a dye based saturable absorber, and is not limited to a semiconductor saturable absorber as used in the above described embodiments. Other embodiments may have loss components that use additive pulse or nonlinear polarization-rotation effects.

• The described embodiments employ optical fibre and optical fibre amplifiers arranged for wavelengths in the telecommunications C-band frequencies. Other embodiments may use other bands. Some embodiments may use Ti: Sapphire wavelengths or any other suitable wavelengths, for example. Other embodiments may not be fibre based, but rather predominantly use bulk optics, semiconductor or other waveguides, for example. Generally any suitable optical components may be used.

It will be appreciated that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described.

The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

It is to be understood that, if any prior art is referred to herein, such reference does not constitute an admission that the prior art forms or formed a part of the common general knowledge in the art, in Australia or any other country. In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention. /

Claims

An optical system comprising:

an optical source arranged to generate optical pulses; and

an optical resonator having an optical input arranged for receiving the optical pulses, and also having an optical output;

wherein the optical resonator has a component arranged to cause loss to a light within the

resonator when in use, the loss decreasing as the power of the light increases, and the optical resonator has an optical gain medium arranged to cause gain to the light within the optical resonator when in use .

An optical system defined by claim 1 wherein at least one of the resonator and the optical source are arranged to give a ratio of free spectral range of the optical resonator and repetition rate of the optical pulses a value which is a predefined rational number.

An optical system defined by claim 2 wherein the repetition rate is not an integer multiple of the free spectral range of the optical resonator.

An optical system defined by either one of claim 2 and claim 3 arranged such that in use gain

competition between frequency components of the optical pulses within the optical gain medium compensates for a deviation of the ratio from the predefined rational number.

An optical system defined by any one of the preceding claims arranged to set each of at least two

transmission frequencies of the optical resonator to a respective frequency component of the optical pulses.

An optical system defined by claim 5 arranged such that gain competition between a plurality of

frequency components of the pulses within the gain medium compensates for a deviation between each of the at least two transmission frequencies of the optical resonator and their respective frequency component of the pulses.

An optical system defined by any one of the preceding claims arranged to produce a plurality of optical pulses at the output having a repetition frequency that is a least common multiple of a free spectral range of the optical resonator and a repetition rate of the optical pulses.

An optical system defined by any one of the preceding claims comprising a controller arranged to control a frequency relationship between the optical pulses and the optical resonator.

An optical system defined by any one of the preceding claims comprising a resonator length stabiliser arranged to stabilise the length of the optical resonator.

An optical system defined by any one of the preceding claims comprising a frequency modulator arranged to modulate at least one of the frequency of the pulses from the optical source and the light within the optical resonator.

An optical system defined by any one of the preceding claims wherein the optical source comprises a mode locked laser. An optical system defined by claim 11 wherein the mode locked laser comprises a passively mode locked laser.

An optical system defined by any one of the preceding claims wherein the component arranged to cause loss to the light comprises a mode locking element.

An optical system defined by claim 13 wherein the mode locking element comprises a saturable absorber.

An optical system defined by claim 14 wherein the saturable absorber comprises at least one of a semiconductor saturable absorber and a graphene based material.

An optical system defined by any one of the preceding claims wherein at least one of the optical source and the resonator comprises optical fibre.

A method for configuring an optical system having an optical resonator arranged to receive optical pulses from an optical source, the method comprising:

defining a value that a ratio of a free spectral range of the optical resonator and a repetition rate of the optical pulses should be, the value being a rational number; and

arranging the system to give the ratio the value

A method defined by claim 17 further oomprising arranging the system to emit optical pulses having a 'repetition frequency that is a least common multiple of a free spectral range of the resonator and the repetition rate of the optical pulses. A method defined by either one of claim 17 and claim 18 comprising arranging the system such that the repetition rate is not an integer multiple of the free spectral range of the resonator.

A method defined by any one of the claims 17 to 19 wherein gain competition between frequency components of the optical pulses within the optical gain medium compensates for a deviation of the ratio from the rational number.

A method defined by any one of the claims 17 to 20 comprising the step of arranging the optical system to set each of at least two t ansmission frequencies of the optical resonator to a respective frequency component of the optical pulses.

A method defined by claim 21 comprising the step of arranging the system such that gain competition between a plurality of frequency components of the pulses within the gain medium compensates for a deviation between each of the at least two

transmission frequencies of the optical resonator and their respective frequency component of the optical pulses.

A method defined by any one of the claims 17 to 22 comprising the step of controlling a frequency relationship between the optical pulses and the optical resonator.

A method defined by any one of the claims 17 to 23 comprising the step of modulating at least one of the frequency of the pulses from the optical source and the light within the optical resonator. A method defined by any one of the claims 17 to 24 wherein the optical system is defined by any one of the claims 1 to 16.

A method for generating optical output, the method comprising the steps of:

launching optical pulses into an optical resonator;

causing loss to a light within the optical resonator, the loss decreasing as the powe of the light increases; and

causing gain to the light within the optical resonator.

A method defined by claim 26 comprising the steps of defining a value that a free spectral range of the resonator divided by repetition rate of the optical pulses should be, the value being a rational number; and

causing a ratio of the free spectral range and the repetition rate to be equal to the value.

An optical system comprising:

a optical source arranged to generate optical pulses;

wherein at least one of the optical resonator a: the optical source are arranged to give a ratio of free spectral range of the resonator and repetition rate of the optical pulses a value that is a

predefined rational number.

29. An optical system comprising:

a optical source arranged to generate optical pulses; an optical resonator having an optical input arranged for receiving the optical pulses, and also having an optical output;

wherein at least one of the optical source and the optical resonator are arranged such that the repetition rate of the optical pulses is not an integer multiple of the free spectral range of the optical resonator.

An optical system comprising:

a optical source arranged to generate optical pulses;

wherein at least one of the optical source and the optical resonator are each arranged such that in use at least two transmission frequencies of the optical resonator are each in accordance with a respective frequency component of the pulses.

An optical system, comprising:

a optical source arranged to generate optical pulses;

an optical resonator having an optical input arranged for receiving the optical pulses, and also having an optical . output ;

wherein at least one of the optical source and the resonator are arranged to give optical pulses at the output a repetition frequency that is a least common multiple of a free spectral range of the resonator and the repetition rate of the optical pulses.