Narrow-linewidth operation of a compactTHz-wave parametric generator system

Atsushi Satoa,b,*, Kazuhiro Imaia, Kodo Kawasea, Hiroaki Minamidea,Satoshi Wadac, Hiromasa Itoa,d

a Photodynamics Research Center, RIKEN, 519-1399 Aramaki Aoba, Sendai 980-0845, Japanb Tohoku Institute of Technology, 35-1 Yagiyama Kasumi-cho, Taihaku-ku, Sendai 982-8577, Japan

c Advanced Engineering Center, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japand Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

Received 19 February 2002; received in revised form 30 April 2002; accepted 2 May 2002

Abstract

We developed a compact THz-wave parametric generator (TPG) system with a diode-pumped Nd:YAG laser that

was specially designed for pumping the TPG. The pump laser produced an output energy of 11 mJ in a single Q-

switched pulse with a pulse width of 6 ns. Using MgO :LiNbO3 crystals, a THz-wave energy of 1.7 pJ was obtained in

unseeded operation, and a broad THz-wave spectrum (1.4–2.2 THz) was then observed. Furthermore, a narrow THz-

wave linewidth of less than 33 GHz was achieved by injection seeding of the idler wave. The THz-wave energy was

increased by a factor of 3 compared to the unseeded output. � 2002 Elsevier Science B.V. All rights reserved.

PACS: 42.65.-k; 42.65.Y; 42.55.Xi

1. Introduction

THz-radiation sources are of considerable top-ical interest for spectroscopy, communication,imaging, etc. Several methods have been suggestedfor generating THz radiation. One of the morecommon THz-radiation sources is a photocon-ductive device or a bulk-semiconductor emitterpumped by a femtosecond-laser, which can gen-

erate an ultrashort THz pulse with a broad spec-tral bandwidth [1–5]. In addition, widely tunablegeneration of ultrabroadband THz radiation wasachieved by phase-matched optical rectification offemtosecond laser pulses in nonlinear opticalcrystals [6]. Recently, time-domain spectroscopy inthe THz-frequency region was demonstrated usingsuch broad-bandwidth THz-radiation sources [7–9].

Some applications place severe demands ofnarrow-linewidth and wide tunability on a THz-radiation source. Current p-type Ge lasers cangenerate a narrow-linewidth THz-wave in the fre-quency range from 1.5 to 4.2 THz, although the

15 June 2002

Optics Communications 207 (2002) 353–359

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*Corresponding author. Tel.: +81-22-229-1151x362; fax:

+81-22-228-2452.

E-mail address: atsushi@tohtech.ac.jp (A. Sato).

0030-4018/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S0030-4018 (02 )01558-4

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system needs to be cooled by liquid helium [10–13].In contrast, a THz-wave parametric generator(TPG) based on optical parametric processes in anonlinear crystal can be operated at room tem-perature [14–17]. This method simultaneouslygenerates a signal wave in the THz frequency re-gion and an infrared idler wave (�1070 nm) usingnoncollinear phase matching conditions. The typ-ical THz-wave linewidth of the TPG exceeds 500GHz. In previous works, narrow-linewidth oper-ations were achieved in a THz-wave parametricoscillator (TPO), which had a resonator for theidler wave. Continuous tunability from 1 to 3 THzwas obtained by slightly changing the angle be-tween the pump and idler waves, and the typicallinewidth of the TPO was approximately 50 GHz.Moreover, further narrow-linewidth operation(Dm � 200 MHz) was accomplished with injectionseeding of both the pump and idler waves. In orderto realize mode-hop-free tuning, however, thelength of the idler-wave resonator had to be ac-tively controlled using a piezoelectric actuator anda feedback circuit, and this complicated systemdesign.

An injection-seeded TPG that has no resonatoris a promising tunable source with a narrow-line-width, because the absence of a longitudinal modeoffers the potential advantage of mode-hop-freetuning. We achieved a Fourier-transform-limitednarrow-linewidth (�100 MHz) and wide tunability(0.7–2.4 THz) by injection seeding both the pumpand idler waves [15,16]. In spite of excellent per-formance, the conventional TPG system wasunsuitable for practical use, since using a flash-lamp-pumped Q-switched laser to pump the TPGresulted in a large system that required a high-voltage power supply and had a shorter opera-tional lifetime. No one has sought to produce apractical TPG system until now, although a com-pact, complete TPG system using a diode-pumped,solid-state laser (DPSSL) as a pump source is ofgreat interest for future THz research.

AlGaAs high-power laser-diodes (LDs) capableof generating a peak power exceeding 1 kW in aquasi-continuous-wave (QCW) mode are nowcommercially available; consequently, a high-en-ergy Q-switched DPSSL that is comparable withflashlamp-pumped lasers can now be realized.

Recently, we developed a compact DPSSL forTPG pumping, and we have demonstrated atabletop TPG system with a broad linewidth (>1THz) [17], although narrow-linewidth operation ina compact TPG system has not been achieved.Here, we describe the design and performance of acompact, injection-seeded TPG system. To ourknowledge, this is the smallest complete TPGsystem to date.

2. Design of the TPG system

A TPG using a LiNbO3 crystal typically re-quires a pump intensity greater than100 MW=cm2 to generate a THz-wave [14–17].The maximum pump intensity should be less than500 MW=cm2 in order to avoid optical damage tothe LiNbO3 crystal. A pump intensity of300 MW=cm2 is deemed practical, because it willensure reliable operation, even if other smallproblems such as thermal lensing and self-focusingoccur. A Q-switched Nd:YAG laser is suitable forpumping a TPG because of its efficiency and re-liability. In addition to laser performance, acompact design is also important for a practicalsystem. Therefore, we developed a diode-pumped,Q-switched Nd:YAG laser with a simple side-pumping geometry. Fig. 1(a) shows a cross-sectionof the pump chamber, which consists of aNd:YAG rod, a fused-silica flow tube for water-cooling, two QCW-LDs mounted on copper heat-sinks, and a ceramic diffuse reflector with two slitsfor side pumping. Each LD was capable of oper-ating with a 1-kW peak power at a center wave-length of 808 nm. They were operated at a pulserepetition frequency of 20 Hz, which was limitedby the duty cycle of the LDs. The laser rod was 3mm in diameter, 65 mm long, and encased in afused-silica flow tube. The temperature of thecooling water was maintained at 20 �C. The op-tical resonator had a cavity length of 190 mm, andwas folded using a polarizing beam splitter (PBS)in order to produce an output beam that waslinearly polarized along the z-axis of the LiNbO3

crystals. In Q-switched mode operation, a KD*PPockels cell Q-switch was operated at a voltage ofk=4.

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For the pump laser used in a previous TPGsystem, the Q-switched output energy was 12 mJ,and the pulse width was then 12 ns. Although thepump beam was collimated and focused onto theincident surface of the LiNbO3 crystal using beam-shaping optics, the available pump intensity was atmost 200 MW=cm2 [17]. By contrast, the presentpump laser produced an output energy of 11 mJ ina single Q-switched pulse with a pulse width of 6ns. A relatively short pulse width is advantageousfor reducing the threshold of TPG, since a highpump intensity leads to a high parametric gain.Using the knife-edge method, the 1=e2 radii of thelaser beam in the horizontal and vertical directionswere measured to be 680 and 620 lm, respectively.In addition, the beam quality factors (M2

x and M2y )

in both directions were determined to be 2.5 and2.1, respectively. Consequently, a pump intensityof up to 280 MW=cm2 is available in the improved

pump source, even without focusing optics. Thismeans that we can expect enhanced THz-waveoutput in smaller and simpler TPG systems.

Fig. 2 shows a schematic diagram of a compactTPG system with an injection seeder for the idlerwave. The TPG system included the pump source,two nonlinear crystals, and a single-longitudinal-mode Yb-doped fiber laser operating at a wave-length of 1:070 lm. 65-mm-long LiNbO3 andMgO :LiNbO3 crystals were used as nonlinearcrystals, and both ends of each crystal were anti-reflection-coated for a wavelength of 1:064 lm.These crystals were mounted in aluminum holderswithout cooling. We used a cascade arrangementof two crystals, since this was optimal for gener-ating THz-waves efficiently in preliminary experi-ments that examined several differentarrangements. In this system, the LiNbO3 crystalwas used only as an idler-wave generator; that is,THz waves were not extracted from this crystal.After the idler wave was generated from theLiNbO3 crystal, it was incident to theMgO:LiNbO3 crystal, and was enhanced duringpropagation due to the higher gain in the LiNbO3

crystal doped with an optimum MgO concentra-tion (5 mol%). As a result, efficient THz-wavegeneration occurred, according to the noncollinearphase-matching condition. Since the MgO:LiNbO3 crystal has a large absorption coefficientin the THz-frequency region (>10 cm�1) [12], thepumped region in the crystal must be close to anoutput-side surface (y-surface). Hence, the dis-tance between the y-surface and the center of thepump beam was precisely adjusted to obtain themaximum THz-wave output in our experiment. Inaddition, total internal reflection of the THz-wave

Fig. 2. Schematic diagram of a compact, injection-seeded TPG

system.

Fig. 1. Pump laser design for a TPG system: (a) cross-section of

the pump chamber; (b) resonator configuration for a Q-swit-

ched Nd:YAG laser.

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on the y-surface occurs, due to the large refractiveindex (n � 5:2) in this frequency region. In orderto achieve efficient THz-wave output-couplingwithout this problem, we used an arrayed Si-prismcoupler (n � 3:4) [18], which was placed on theoutput-side surface of the MgO:LiNbO3 crystal.The y-surface of the MgO:LiNbO3 crystal waspolished over its entire length, since such a pol-ished surface allows minimization of the couplinggap at the interface with the arrayed Si-prismcoupler. The unseeded TPG system is compact(400 � 160 mm2). When an idler seeder is added tothe TPG system, the whole system is only400 � 250 mm2. This is significantly smaller thanthe size of conventional TPG systems pumped by aflashlamp-pumped Nd:YAG laser (1450 � 700mm2) [15,16].

3. Results and discussion

The input–output characteristics of the TPGsystem in unseeded operation were obtained underdifferent pumping conditions. Fig. 3 shows theidler-wave energy as a function of the pump en-ergy. Since the pump energy was changed by usingan attenuator consisting of a half-wave plate and aPBS, pulse widths were fixed during each experi-ment. After the second-harmonic output (k ¼ 532

nm) from the MgO:LiNbO3 crystals was blockedusing a green-cut filter, the net idler-wave energywas measured using a pyroelectric detector. With6-ns pulse pumping, the maximum idler-wave en-ergy reached 1.4 mJ, which was 3.4 times higherthan that with 12-ns pumping. Note that no opti-cal elements were used to focus the pump beam inthe case of 6-ns pumping, whereas the previouspumping system used beam-shaping optics, asmentioned above. In addition, the shorter cavitydesign of the Q-switched laser is also advantageousfor constructing a compact TPG system.

THz-wave energies were measured under thesame conditions as in Fig. 3. Fig. 4 compares theTHz-wave energies for the two pumping condi-tions. A 4.2-K Si-bolometer was used to measurethe THz-wave energy. With 6-ns pulse pumping, aTHz-wave energy of 1.7 pJ was achieved with apump energy of 11 mJ; the THz-wave energy in-creased by a factor of 5.7. The higher pump in-tensity increased the parametric gain for both theTHz- and idler-waves, so increases in both outputenergies were due to the higher gain with intensepumping. Moreover, the parametric gain for theTHz-wave in our method is approximately pro-portional to that for the idler-wave. Therefore, weexpected an improvement equivalent to that inFig. 3, but the results of the experiments differed.The reason for the greater improvement in THz-

Fig. 3. Comparison of the idler-wave energies of an unseeded

TPG system under different pumping conditions.

Fig. 4. THz-wave energy versus incident pump energy under

the same conditions as in Fig. 3.

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wave energy might be that the distance betweenthe pump beam path and the Si-prism coupler wasoptimally adjusted in this measurement. The pulsewidth of the idler wave, measured using a digitiz-ing oscilloscope, was 4 ns (FWHM). Therefore, thepeak THz-wave power was estimated to be425 lW. No saturation of the idler-wave or THz-wave energy was observed during these experi-ments.

Following the experiments in unseeded opera-tions, the TPG system was injection seeded using asingle-longitudinal-mode Yb-doped fiber laser.Fig. 5 shows the THz-wave energies from the in-jection-seeded TPG system as a function of seederpower for the idler wave. The seeder power wasvaried from 0 to 350 mW, and injection seedingwas observed to have an effect on the THz-waveenergy even when the seeder power was less than 2mW. The THz-wave energy increased with in-creasing seeder power, but there was only a slightincrease for seeder powers greater than 20 mW.This can be explained as follows: since the injec-tion seeder defines the idler wavelength, the gainbandwidth for the idler wave becomes narrower,leading to a higher parametric gain at that wave-length. Once the TPG system is completely injec-tion seeded, however, an increase in the seederpower no longer enhances the output energy, sincethe injection seeder merely plays a role in nar-

rowing the gain bandwidth. A maximum THz-wave energy of 5 pJ (peak power � 1:25 mW) wasachieved with a seeder power of 350 mW, and anidler-wave energy of 2 mJ was then obtained.These results are approximately three times greaterthan those in the unseeded TPG system. In thisexperiment, the laser beam of the seeder was notfocused, in order to realize a simple, compactsystem; a focused beam will allow us to use aninjection seeder with lower power, in the order of afew milliwatts.

Fig. 6 shows the TPG idler-wave spectra forvarious seeder powers. The upper horizontal axis inthis figure is the corresponding THz wavelength,which was calculated using the energy conservationlaw with a pump wavelength of 1:064 lm and theidler wavelengths shown on the lower horizontalaxis. The idler-wave spectrum was measured withan optical spectrum analyzer, which had a resolu-tion of 0.2 nm. The pump energy was fixed at 11mJ. In unseeded operation, the idler-wave spec-trum was observed over the wavelength rangeof 1.0694–1:0723 lm, which corresponds to a

Fig. 5. THz-wave energy of an injection-seeded TPG system as

a function of seeder power for the idler wave. Fig. 6. Idler-wave spectra of unseeded and seeded TPGs.

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THz-wavelength range of 134–212 lm (1.4–2.2THz). This linewidth is somewhat broader than atypical value obtained using a single LiNbO3

crystal [14], because the cascaded arrangement oftwo crystals led to a higher gain enough to generateTHz waves over the broader spectral bandwidth.As the seeder power increased from 0 to 2 mW, theintensity of the idler-wave near the wavelength ofan emission peak decreased slightly, and a newpeak near the seeder wavelength of 1:070 lm ap-peared in the idler-wave spectrum. For a maximumseeder power of 350 mW, THz-wave generationdominated at a wavelength of 1:070 lm, ratherthan at the peak wavelength of the unseeded TPGspectrum. The Yb-doped fiber laser used as a see-der in our experiments is not tunable. Hence, in-jection seeding has not been performed at otherwavelengths. However, the fact that the TPG sys-tem was successfully injection seeded at a wave-length that was not the center wavelength of theunseeded idler-wave spectrum implies that tunableoperation over the emission wavelength region ofthe unseeded TPG or broader would be possibleusing a tunable seeder source.

The THz-wave linewidth and wavelength weremeasured using a scanning Fabry–Perot etalon,which consisted of two metal-mesh plates, placedbetween the Si-prism coupler and the Si-bolome-ter. The results are shown in Fig. 7. The etalon

spacing was varied using a precise actuator. ATHz-wavelength of 190 lm was obtained from thedisplacement between the two periods. The twopeaks in the inset of Fig. 7 are separated by a 69-GHz free spectral range. The THz-wave linewidthwas measured to be less than 33 GHz, which was1/24 times narrower than that in unseeded opera-tion. At present, the THz-wave linewidth is limitedby the linewidth of the pump laser, since the idlerseeder has a sufficiently narrow-linewidth (�1MHz). Even a transform-limited narrow-linewidthoperation should be feasible using a single-longi-tudinal-mode Q-switched laser as a pump source.

4. Conclusions

We developed a compact, complete TPG systemthat included a diode-pumped, Q-switched Nd:YAG laser as the pump source. Using an injection-seeding technique, the THz-wave energy was threetimes higher (5 pJ, peak power �1.25 mW) and thelinewidth was 1/24 narrower (33 GHz). It shouldbe emphasized that the TPG system presented hereis the smallest such system reported, and its per-formance remains excellent.

Acknowledgements

The authors thank J. Shikata of the ResearchInstitute of Electrical Communications, TohokuUniversity for his useful advice and discussion. Wealso thank C. Takyu for his excellent work coatingthe crystal surfaces, T. Shoji for superbly polishingthe crystals, and A. Kudo for excellent technicalsupport. This work was supported in part by aGrant-in Aid for Developmental Scientific Re-search (No. 12555105) from the Ministry of Edu-cation, Science, and Culture of Japan.

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