A HIGH-POWER FREQUENCY-STABILIZED TUNABLE TWO-FREQUENCYDIODE LASER SYSTEM FOR GENERATION OF COHERENT

TERAHERTZ-WAVE BY PHOTOMIXING

Shuji Matsuura and Geoffrey A. BlakeDivision of Geological and Planetary Sciences, California Institute of Technology, Pasadena. CA 91125

E-mail: matsuura@gps.caltech.edu

Pin ChenDivision of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena CA 91125

J. C. Pearson and Herbert M. PickettJet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109

Abstract

A tunable two-frequemcy high-power diodelaser system at 850 nm for terahertz (Thz)-wave generation by photomixing in low-temperature-grown GaAs photo-conductors hasbeen developed. The difference frequency isobtained through a three laser system, wheretwo lasers are locked to different orders of aFabry-Perot cavity and a third is offset-lockedto the first, The difference-frequency signal isgenerated by the offset laser and the othercavity-locked laser. The spectral purity of thebeat note is better than 1 MHz. The maximumoutput power of —500 mW was obtained byusing the master oscillator power amplifier(MOPA) technique, simultaneous injection oftwo seed frequencies with a singlesemiconductor optical amplifier. Here wereport the generation of THz waves andspectroscopy of acetonitrile as proof ofconcept.

1. Introduction

The difference-frequency generation, or thefrequency down-conversion, by photomixingusing nonlinear optical materials andphotoconductors has been long investigated asa promising technique to develop widelytunable coherent sources in the terahertz (THz)region. Photomixing in low-temperature-grown (LTG) GaAs photoconductors with

planer antennas is the most attractive method interms of conversion efficiency 1 -3 , and LTG-GaAs photornixer sources have already beenapplied to laboratory spectroscopy by severalauthors4-6 . However, the frequency stabilityand calibration of these sources have not beensufficient for high-resolution spectroscopicapplications. The frequency control andcalibration of pump laser system is the keyissues for the next generation of such THzsources.

Diode-laser-based systems are suitable forspace-borne fiber-coupled instruments becauseof their compactness, low power consumption,and long lifetime.23 Wide frequency tunabilityof diode lasers is also an important advantagefor applications in the Tliz-wave generation.The advances in frequency stabilization ofdiode lasers locked to cavity modes allows usto define the frequency of the THz-waveprecisely. We have developed a fiber-coupled,tunable, two-frequency diode laser system at850 nm using such frequency-stabilizationscheme.

The THz-wave output power generated byphotomixing with a photoconductor has aquadratic dependence on the photocurrentoscillation induced by the pump lasers. Up tonow, the THz-wave output power generatedwith LTG-GaAs photomixers have been limitedto 0.1-1 1.tW level corresponding to the ac-photocurrent of —0.1 mA, dc-photocurrent of—1 mA.6,7 The photocurrent is proportional tothe pump laser power, but conventional type

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photomixers cannot handle the incident powerover -50 mW because of thermal failure.'Large dimension devices allow high-powerlaser input and provide much higherphotocurrent by avoiding the thermal problem.In order to drive these devices, the developmentof high-power (>>50 mW) tunable diode laseris required. Unfortunately, the output power ofour fiber-coupled diode laser system was notsufficient for this purpose.

The master oscillator power amplifier(MOPA) technique solve the dilemma betweennarrow linewidth and high power. The masterlaser with narrow linewidth is injected into thepower amplifier, and the laser power isamplified while preserving the spectral purity.Although the MOPA has been normally usedfor single-frequency operation, simultaneoustwo-frequency injection seeding to a singlepower amplifier is achievable8 . The two-frequency MOPA operation has an advantage

in spatial overlap of the two frequency beams,which is essential to achieve an efficientphotomixing.

We developed a high-power, narrow-linewidth, tunable two-frequency diode lasersystem at 850 nm based on the two-frequencyMOPA with the fiber-coupled tunable masterlaser and a single optical amplifier. In thispaper, design and performance of this systemand its application to the THz-wave generationand spectroscopy are presented.

2. Laser system design andperformance

2-1. Frequency-stabilized tunable diode laser

The fiber-coupled tunable two-frequencydiode laser system to synthesize a precisedifference-frequency consists of three external

Fig. 1: Schematic diagram of the laser system and experimental setup of the spectroscopy. DL, DBR diode laser, PR,partial reflector, AP, anamorphic prism pair; CH, optical isolator; X/2, half-wave plate; A, attenuator; HL, hyper-hemispherical lens; PD, photodetector.

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cavity diode lasers as depicted in Figure 1.Two of these lasers (#1 and #2) are locked todifferent longitudinal modes of a Fabry-Perot(FP) cavity with a free spectral range (FSR) of3 GHz. The third laser (#3) is locked to one ofthe cavity-locked lasers (#2) with a 3-6 GHztunable offset, which can be continuouslyswept over the FSR. The difference frequencybetween the #1 and #3 is precisely determinedby the sum of integral multiples of the FSR andthe offset frequency. All the components areconnected with single-mode polarization-maintained fiber optics.

Each external cavity laser assembly consistsof an 852 nm distributed Bragg-reflector(DBR) semiconductor laser diode (SDL5722),an f=4 nun collimating lens, a 20% partialreflector mounted on piezoelectric transducer(PZT), an anamorfic prism pair, a 60-dBoptical isolator, an f=4 mm focusing lens, anda FC-connecterized fiber mount. Thesecomponents were assembled in an aluminumrail assembly as shown in Figure 1. Thetemperature of the diode laser is controlled by athermo-electric (TE) cooler with an accuracybetter than 1 mK. The injection current issupplied by a low noise (7 11A/4Hz) currentdriver. The partial reflector and the DBR in thelaser chip constitute an external cavity withFSR of -3 GHz. The optical feedback inducedby the external cavity narrows the linewidth of-500 kHz, while typical linewidth of DBRlasers is several megahertz. The laserfrequency was continuously tunable within theFSR of the external laser cavity by changingthe cavity length. The continuous tunablerange could be expanded to 5 GHz, whichcorresponds to the voltage limit applied to thePZT (±I5 V), by tracking the laser temperatureto maximize the laser gain at the external cavitymode frequency.

To improve the long-term frequencystability, the laser frequency was locked tolongitudal modes of the ULE (Ultra LowExpansion: a=-2x10- 10 °C- 1 ) FP-cavity with afinesse of 750 and FSR of 3 GHz. The FP-cavity is installed in a sealed box filled with drynitrogen to prevent from refractive indexchange caused by variations in surroundingpressure. To lock the laser frequency to theFP-cavity mode, we used the Pound-Drever-Hall method. The FM sidebands of the laserare generated at -100 MHz with a fiber-coupledelectro-optic phase modulator (EOM), and the

modulated signal is injected into the FP-cavity.The reflected signal from the FP-cavity isdetected and fed back to the PZT of the lasercavity. The feedback loop bandwidth waslimited to 500 Hz by acoustic resonance of thePZT. The dc-drift of the frequency caused bythermal expansion of the external laser cavity iscanceled by tracking the P'ZT valtage with theintegrated error signal through the computercontrol system. In the laboratory environmentthe drift was so large that the feedback voltageto the PZT reaches to the 15-V limit in a fewminutes. The drift was considerably reducedby putting the whole laser system on atemperature- controlled thick aluminum plate,and the all-day-long cavity-lock of the laserfrequency was achieved.

Combined output power of the #1 and #3lasers from the final 3-0 fiber coupler wasapproximately 30 mW, while the output powerof each DBR laser was originally 150 mW.The total power loss of the present systemconsisted of the transmission loss in the free-space optics in the laser assembly (1.5dB), theinsertion loss to the fiber (4dB), and the loss infiber directional couplers and at fiberconnectors (1.5dB).

2-2. Two-frequency MOPA system

The two-frequency MOPA system wasconstructed using a single traveling-wave 850nm semiconductor tapered optical amplifier,which was a component of a commercialexternal-cavity single-mode laser (SDL8630).The two-frequency fiber-coupled output beamfrom the tunable laser system, a masteroscillator, is launched into the free-spacethrough a collimating lens. The circular beampassed through an optical isolator and a half-wave plate for fine adjustment of thepolarization, and is reshaped to elliptical beamby an anamorphic prism pair to match thespatial mode to the amplifier facet spot. Afterappropriate attenuation, the beam is injectedinto the optical amplifier chip through an f=8mm focusing lens. The output beam from theamplifier is spatially filtered and collimated to-3-mm size Gaussian beam by a lensassembly. After passing through an attenuatorand a beam divider for monitoring thespectrum, the beam is focused on thephotomixer.

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Fig. 2: The output power of the laser system (circles)and the intensity ratio between the two frequencycomponents (squares) as a function of the differencefrequency.

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Frequency [MHz]

Fig. 3: The spectrum of the beat signal between thetwo frequency components before amplification (upper)and after amplification (lower).

Under the conditions of the temperature of22 *C and the injection current of 1.9 A, theoptical amplifier provides the saturated outputpower of —500 mW for the input power ofapproximately 4 mW. Since we set the inputpower to —10 mW, the amplifier was operatedunder highly saturated conditions. Therefore,the output power is insensitive to the inputpower and the frequency.

2-3. Spectral properties

In Figure 2 the amplified output power as afunction of the frequency difference between #1and #3 lasers is shown by the circles.According to the highly saturated condition, theoutput power was constant within 5% over theentire difference frequency range.

The intensity ratio between the twofrequency components in the amplifier output,1 3 /1 1 , measured with a scanning FP-spectrometer is also plotted in Figure 2 as afunction of the difference frequency. Theintensity ratio was close to unity over a widerange of difference frequencies from —10 GHzto 1.3 THz. Unbalanced amplification occurredat difference frequencies lower than 10 GHzdue to the two-frequency interaction driven bythe refractive index change induced by thecarrier density modulation at the differencefrequency7. The lower frequency limit of thewell-balanced two-frequency amplification isdetermined by the carrier lifetime of the

amplifier.Since the amplifier used in this system is a

component of a commercial external-cavitysingle-mode laser, an amplifier chip facet wasanti-reflection coated (R .1%) but the otherfacet was not coated (R=30%). Small variationin the output power and the intensity balanceseen in Figure 2 are caused by the chip modewith a spacing of —15 GHz. Therefore, thesefrequency dependence can be considerablyreduced by AR-coating both of the amplifierchip facets.

The spectral purity of the beat signal at thedifference frequency was measured with the25-GHz bandwidth photodetector. Figure 3represents the beat signal spectrum at 12.6 GHzfor both the amplified output and the masterlaser. The linewidth of the beat signal wasapproximately 1 MHz and completely preservedthrough the amplification process.

3. Generation of THz-wave andapplication to spectroscopy

3-1. Experimental setup

The laser system described above wasused for the 'THz-wave generation with a LTG-GaAs photomixer, and the absorptionspectroscopy of acetnitrile (CH 3CN) wascarried out using this THz source. Theexperimental setup of the spectroscopy isshown in Figure 1. The two-frequency output

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Fig. 4: The measured rotational spectrum of acetnitrile.

beam from the MOPA is attenuated and focusedon the LTG-GaAs photomixier. The THz-wave output beam is collimated with acombination of a hyper-hemispherical lens puton a backside of the photornixer and a Teflonlens. The collimated beam passes through a 8-cm long 1-inch diameter gas cell withpolyethylene windows. The transmitted beamwas weakly focused with a Teflon lens and fedinto a 4.2-K InSb hot-electron bolometer.

3-2. Generation of THz-wave by photomixing

The photomixer used in this experiment wasfabricated at Minnesota University. The LTG-GaAs wafer was grown on a 0.5-mm.-thicksemi-insulating GaAs substrate, and a planerlog-spiral antenna with 1-gm -wideinterdesitated electrodes and 1-p.m-wide gaps ina 10x10 1=2 active area was etched on thewafer. The bandwidth of the photomixerdetermined by a combination of the carrier

lifetime of the LTG-GaAs and the RC timeconstant of the electrode is estimated to beapproximately 300 GHz. The dc bias voltagewas applied to the electrode by a constantcurrent supply of —0.5 mA for the laser powerof 30 mW.

Before carrying out the spectroscopy, theTHz-wave power was measured- without thegas cell. The pump laser beam was modulatedat 400 Hz by a mechanical chopper, and theoutput signal of the InSb detector wasmeasured. The x100-amplified detector signalat 300 GHz was —100 mV that corresponds tothe radiation power of —1 gW.

3-3. Spectroscopy

The tone-burst FM spectroscopy method9was used to obtain the spectrum. The injectioncurrent of the #1 cavity-locked laser of themaster laser system was modulated at 2 MHzabove the cavity-lock loop bandwidth, and the

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modulation input was switched on and off at a10-kHz rate. The detected signal wasdemodulated by a lock-in amplifier at 10-kHzswitching rate. The spectrum was obtained bysweeping the offset frequency. The lock-insignal traces the second derivative of theabsorption spectrum.

Figure 4 presents the absorption spectrum ofCH 3CN J=16 rotational transitions near 312GHz. The lock-in amplifier signal taken by asingle frequency sweep with a rate of 2MHz/sec was plotted against the offsetfrequency between #2 and #3 lasers. The datasampling rate was 7 sample/sec whichcorresponds to the frequency resolution of 0.3MHz, while the time constant of the lock-inamplifier was 0.3 sec. The spectrum shows aK-structure of a symmetric-top molecule thatwas assigned to K=0-11 lines as depicted in thefigure. The K=0 and 1 lines separated by -6MHz are clearly resolved. The width of theabsorption lines was pressure-limited, whilethe gas pressure was 60 mTorr. To evaluatethe instrumental linewidth, furthermeasurements by changing the gas pressure arerequired.

The lock-in amplifier outputs for theabsorption lines were -100 mV, while thenoise amplitude was -5 1.Np-p, and thedetection limit of the absorption is estimated tobe -10-4. The spectrum of the C13-isotopomerof acetnitrile, which has the natural abundanceof -1%, was detected with a signal-to-noiseratio of -20 according to the detection limit.Unfortunately, the noise originated mainlyfrom an electronic pick-up at the A/D input, andthe detection limit was determined by neitherthe detector noise nor the source noise. Thedetection limit of 10-5 level can be achievedwith the present system by reducing this noise.Although the present measurement was carriedout at -300 GHz, the source intensity issufficiently high for the spectroscopy at higherfrequencies if only the detector is replaced tothat for high-frequency use; the output power at1 THz is estimated to be -10dB down.

For further spectroscopic measurementssuch as the search for unknown molecular linesand the use for astronomical observation, theabsolute frequency calibration of the system isnecessary. The frequency calibration is carriedout by measuring the FSR of the FP-cavity.Once the exact value of the FSR was obtained,the Tliz-wave frequency can be determined

with an accuracy of -10- 10, even if there existsa fluctuation of the cavity temperature of -1 *C,because of an extremely low thermal expansioncoefficient of the cavity material which isequivalent to the crystal used as a microwaveoscillator. The FSR was measured bydetecting the beat signal between the twocavity-locked lasers (#1 and #2). The accuracyof the measurement was -10 kHz, while theFSR is 3 GHz. Well-known strong molecularlines in the THz region such as rotationaltransitions of carbon monoxide (CO) are usablefor more accurate calibration. Frequencies ofsuch molecular lines in the THz range thatcorresponds to -300 times of the FSR areknown with an accuracy of -10- 7, and theresultant accuracy of the FSR measurement isexpected to be -3x10- 10 or -300 Hz. Thecavity calibration with CO lines is in theprocess.

4. Conclusions

A 850 nm tunable two-frequency diodelaser system based on the three-laser frequencycontrol scheme and the two-frequency MOPAtechnique was developed, and the output powerof 500 mW and the linewidth of <1 MHz wereachieved. The laser system was applied to theTHz-wave generation using the LTG-GaAsphotomixer, and the absorption spectroscopyof acetnitrile with this source wasdemonstrated. The present THz-wave sourcesystem is usable for actual spectroscopy, onlyif the frequency calibration is carried out. Thesystem will be useful not only for spectroscopybut also for the application in development oflocal oscillators for future space-borneheterodyne receivers.

The photomixer used in the presentexperiment provided the output power of -1IINV for the pump laser power of 30 mW.According to a straightforward calculation bythe quadratic dependence of the THz-wavepower on the pump laser power, the sub-mWlevel output power is achievable by using thepresent laser system with the maximum powerof 500 mW. In order to use such high-powerlaser effectively, the development of new typeof the photomixer with high thermal-damagethreshold such as the large dimension device isrequired.

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Since the gain bandwidth of thesemiconductor optical amplifier is over 10THz, the two-frequency MOPA system isapplicable to the difference-frequencygeneration of the mid-infrared radiation. Athigher frequencies, even at several THz, theoptical down-conversion method usingnonlinear optical materials lo or recentlydeveloped quantum-well devices could be moreefficient than the electro-optical conversionmethod using the photomixer. Furtherdevelopment of nonlinear optical materials andnovel devices with large x( 2) at the diode laserfrequency and the phase matching condition atthe THz frequency is expected.

We thank T. J. Crawford of Jet PropulsionLaboratory for his technical support. Thiswork was supported by the NationalAeronautics and Space Administration.

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