1 January 1999

Ž .Optics Communications 159 1999 74–79

A nanosecond optical parametric generatorramplifierseeded by an external cavity diode laser

Sheng Wu a,), Vadym A. Kapinus a, Geoffrey A. Blake b

a DiÕision of Chemistry and Chemical Engineering, California Institute of Technology, 127-72, Pasadena, CA 91125, USAb DiÕision of Geological and Planetary Science, California Institute of Technology, 150-21, Pasadena, CA 91125, USA

Received 24 September 1998; accepted 3 November 1998

Abstract

Ž .We report on the development of an injection-seeded nanosecond optical parametric generatorramplifier ns-OPGrOPAthat combines relatively low thresholds with good conversion efficiencies. Because there is no cavity whose opticalproperties must be actively controlled to match the injected radiation field, external seeding over wide ranges by CW single

Ž . Ž .longitudinal mode SLM lasers is straightforward. The seeded ns-OPGrOPA spectral resolution 650"150 MHz is closeŽ .to the pump-limited bandwidth, and the output beam has a small angular divergence ;0.5 mrad at 628 nm . Other

Ž . Ž .properties of the seeded ns-OPGrOPA, such as the efficiency )30% , the threshold for external seeding ;1–3 mW , andŽ y1.the locking range )20–30 cm , are also characterized. The continuous scanning and narrow spectral bandwidth of the

OPGrOPA is verified in photo-acoustic absorption experiments on the third O–H stretching overtone of water near 815 nm.q 1999 Elsevier Science B.V. All rights reserved.

Keywords: Nanosecond OPG; OPA; Injection seeding; SLM diode laser; Photo-acoustic spectra

1. Introduction

Over the past several years, there has been tremendousprogress in nonlinear optical devices, particularly in the

Ž .area of optical parametric oscillators OPOs , amplifiersŽ . Ž .OPAs , and generators OPGs . This progress has beendriven in part by the development of novel nonlinearoptical crystals, but equally important are advances in highpeak power all-solid-state pump lasers and new opticaldesigns which utilize the unique properties of the paramet-

w x Ž .ric process 1–5 . For laser pulses of nanosecond nsduration, one of the important goals in developing thesenonlinear optical devices is to realize compact, widelytunable coherent light sources with narrow bandwidth thatcan be used in a variety of laboratory, in situ, and remote

) Corresponding author. E-mail: sheng@its.caltech.edu

sensing applications. Such studies often require spectralresolution approaching the transform limit, and while it ispossible to design very high-resolution free-running OPOcavities, they tend to be inefficient and difficult to tune

w xover long ranges 6,7 .Injection-seeding OPOs with high-resolution lasers form

an alternative approach to high spectral resolution, as hasw xbeen demonstrated in a wide variety of designs 8–14 ,

Ž . w xwhile OPGs based on picosecond ps 15,16 and fem-Ž . w xtosecond fs 17–19 pump lasers have been designed by

several groups. Commercial ps- and fs-OPGs are nowavailable from several manufacturers, but ns-OPGs havenot been investigated to date because the nonlinear opticaldrive provided by ns pump lasers is typically very low, andone therefore expects that ns-OPG thresholds will be highand their efficiencies low. Here, we propose and character-ize a novel OPG that is pumped by a ns Nd:YAG laser and

Ž .seeded by an external single longitudinal mode SLM CWlaser. This new design combines high efficiency and nar-

0030-4018r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0030-4018 98 00609-9

( )S. Wu et al.rOptics Communications 159 1999 74–79 75

row bandwidth with wide tunability and low angular diver-gence.

OPGs, as compared to OPOs, are especially preferredfor external seeding by narrow bandwidth light sourcessuch as CW SLM lasers for two reasons. First, an OPG hasno cavity; therefore, there is no need to actively control thecavity length to match the frequency of the seeding laserw x10 . Closed-loop servo controls for frequency calibrationand tuning need, therefore, are applied only to the seedlaser, which greatly simplifies scanning of the OPG wave-length. Second, OPGs can operate in a single-pass fashion,and therefore there is no feedback from the OPG backtoward the external seeding laser. This is important forCW SLM lasers, because even small amounts of feedbackcan disturb SLM operation, and under certain circum-stances can even damage the gain material. The latter isparticularly important when using diode lasers as seeding

w xsources 13,14 .

2. Experimental

As Fig. 1 shows, our OPG consists of four BBOcrystals, arranged in two pairs. Crystals 1 and 2 form theOPG stage, and their optical axes are arranged in such away that the two crystals counter-rotate against each otheras the wavelength is changed. Crystals 3 and 4 alsocounter-rotate against each other and form an OPA stagewhich amplifies the OPG output. This widely usedcounter-rotating design both cancels the ‘walk-off’ effectin nonlinear optical crystals and dramatically reduces theoutput beam position shifts that are generated when the

w xcrystal angle is tuned 2 . The signal and idler generated inthe OPG stage pass through a pump reflecting dielectricmirror, and are then directed through a long pass filterwhich absorbs the signal output of the OPG. Therefore, theOPA stage is seeded only by the idler beam. This idler-only

Table 1Free-running OPGrOPA characteristics

Wavelength Parametric Efficiency at Bandwidthy1 Ž .signalridler threshold 120 mJ pulse nm

y1Ž . Ž . Ž .nm mJ pulse signalqidler

435r1921 33 35% ;0.2550r999 31 37% ;0.5630r812 32 35% ;2

OPA design maintains a small angular divergence for theoverall OPGrOPA output, as shown in test results de-scribed below.

ŽPump pulses are provided by the third harmonic 355.nm output from a Coherent Infinity 40-100 pulsed

Nd:YAG laser. The laser delivers pulses of 2.5 ns width at355 nm, with 0–160 mJ of pulse energy at repetition ratesfrom 0 to 100 Hz. The beam is circular, approximately 5mm in diameter, and has a flat-top intensity profile. Asnoted above, the OPGrOPA gain media consists of fourtype I BBO crystals, cut at us308 and coated with aprotective AR coating. Crystals 1 and 2 have a length of10 mm each, while crystals 3 and 4 are 17 mm in length.The pump beam is directed into the OPGrOPA layoutwithout prior collimation or focusing.

We first carried out tests to characterize the free-run-ning OPGrOPA’s tuning range, parametric threshold, andefficiency. Table 1 lists the parametric thresholds andconversion efficiencies at three different wavelengths. TheOPGrOPA setup is capable of tuning from 410 to 2400nm without gaps, and the efficiency and parametric thresh-old are rather constant over the signal tuning range from

Ž .435 to 700 nm idler from 1920 to 720 nm . The efficiencydrops and the threshold rises steeply when the OPGrOPAis tuned to signal wavelengths below 435 nm. This is dueprimarily to infrared absorption by the BBO material at

Fig. 1. An outline of the diode laser injection-seeded ns-OPGrOPA. The pump steering mirrors are standard 355 nm dielectric Nd:YAGŽmirrors, and combine high reflectivity at the pump wavelength with good transmission of the signal and idler beams. The filter Schott

.RG710 selectively absorbs only the signal radiation, leading to seeding of the OPA stage by the idler output of the OPG.

( )S. Wu et al.rOptics Communications 159 1999 74–7976

Ž . ŽFig. 2. A graphical illustration of the divergence properties of signal- and idler-seeded ns-OPGrOPAs. a When signal radiation 410–710. Ž . Ž .nm is used to seed the OPA, much larger divergences are obtained for the idler beam, whereas if b the idler 710–2500 nm is used as the

seed, the overall divergence, especially that of the signal beam, is markedly reduced.

idler wavelengths greater than 2 mm. The bandwidth of thew xOPGrOPA is very close to that of a type I OPO 2 , and

Ž .can be as small as ;0.1 nm at deep blue 430 nm signalwavelengths, but rises quickly to several nm as degeneracyŽ .710 nm is approached.

At a signal wavelength of 630 nm and with the long-passfilter removed, we measure an angular divergence of over6 mrad for the signal beam – the output of the OPGrOPAis expanding very fast. With the long pass filter installed,the signal beam angular divergence is reduced to roughly 2mrad, much smaller than that without the filter. The effi-ciencies with and without the filter are nearly identical.This clearly demonstrates the importance of using only theidler as the seed for the final amplifier, and can beunderstood graphically using the diagrams shown in Fig. 2.

All nonlinear optical crystals have a certain acceptanceangle, for type I BBO it is about the same at signal andidler wavelengths ranging from 410 to 2400 nm. Theangular divergence of the OPG output at both signal andidler wavelengths is dictated by this acceptance angle.Since the signal wave vector, K , is always larger thans

idler wave vector K , and since the K and K emergei s i

from the initial OPG stage with the same angular diver-gence, it is easy to understand that using the idler as theseed for the OPA stage will result in signal beams withmuch smaller beam divergence. This same principle alsoapplies to OPO design, and we have discovered that byresonating the idler part of the parametric output, muchsmaller angular OPO angular divergences can be achieved

Ž .compared to resonating the short wavelength signal beamw x5 .

The high efficiency of the OPGrOPA is a direct resultof the characteristics of our pump laser. The Infinity40-100 laser has a pulse width of only 2.5 ns, less thanhalf of the pulse width of typical flashlamp-pumped, Q-switched Nd:YAG lasers at 355 nm. Its beam size is alsorather small, and the Rayleigh distance is nearly 2 m,resulting in peak power densities in excess of 200 MWcmy2 at pump energies of roughly 150 mJ pulsey1. Thispower density is maintained over long distances, and isclose to the operational conditions of many ps OPGrOPA

w xsystems 15,16 .Despite the high efficiency and excellent beam quality

obtained, the broad spectral output of ns-OPGrOPA, espe-

cially at wavelengths near degeneracy, is useless to mostspectroscopy experiments. Injection seeding with a CW

Žsource was therefore tested using a diode laser EOSI.Model 2001 . When properly adjusted, the 810 nm diode

laser is capable of SLM operation for tuning ranges ofover 10 nm without mode hops. The output of the diodelaser passes through an anamorphic prism pair, an opticalisolator, and a mode-matching telescope before being di-rected to the OPGrOPA. Once the wavelength of thediode laser is matched with the OPGrOPA operatingwavelength, seeding is easily achieved. The seededOPGrOPA has a beam divergence of only 0.5 mrad, muchsmaller than the free-running divergence, and is attributedto the small angular divergence of the collimated diodelaser beam.

We next measured the locking range of the OPGrOPA.The idler output was fixed at ;815 nm, and the EOSI2001 laser was scanned through the OPGrOPA wave-length. At the same time the seeded pulse energy was

Žmeasured in the mid-field ;3 m in front of the final OPA.crystal with a 6-mm-diameter iris placed in front of an

average power thermopile. The results of this lockingrange test are presented in Fig. 3. From this, we see that

Fig. 3. OPGrOPA output versus seed laser frequency for a fixedBBO crystal orientation. The FWHM of the pulse energy enhance-ment by the seeding field is approximately 20–30 cmy1 .

( )S. Wu et al.rOptics Communications 159 1999 74–79 77

Fig. 4. Output pulse energy dependence of ns-OPGrOPA versusthe CW seed power.

continuous tuning of the diode laser wavelength is possibley1 Žfor a range of )20–30 cm at 815 nm idler wave-

.length, or 628 nm for the signal beam without having toadjust the crystal orientation.

We also measured the CW power required in order toseed the OPGrOPA. We increased the seeding powerfrom zero by increasing the diode current, at the same timemeasuring the seeded pulse energy in the mid-field, againwith a 6-mm iris placed in front of the power meter. Theresults of the seeding power experiment are presented inFig. 4. Clearly, seeding the OPGrOPA takes very littleCW input – less than 6 mW. In the current configuration,the polarization of the diode laser after leaving the isolatoris rotated by 458 from the OPGrOPA optical axis, leadingto a 3-dB insertion loss. Similar results are obtained whensignal radiation is used to seed the OPG, for example, witha HeNe laser at 632 nm. Thus, with properly configuredoptics any external seeding laser between 410 and 2400nm with even a few mW of power should be able to seedthe OPGrOPA robustly.

The bandwidth of the OPGrOPA output was measuredŽ .with a pulsed spectrum analyzer Burleigh PLSA 3500 .

Ž .The full width at half maximum FWHM of theOPGrOPA is found to be 650"150 MHz, as the spec-trum in Fig. 5 shows. The pump laser always operatesunder SLM conditions, and has a bandwidth of roughlytwice the transform limit, or -250 MHz, at 1064 nm. The

Fig. 5. Spectral bandwidth of the ns-OPGrOPA idler output as measured by a Burleigh PLSA 3500 pulsed spectrum analyzer.

( )S. Wu et al.rOptics Communications 159 1999 74–7978

Fig. 6. Photo-acoustic absorption spectrum of the third O–Hstretch overtone of water near 815 nm. The photo-acoustic cellcontained 1 Torr of water vapor in a 500 Torr bath of drynitrogen. The measured FWHM of the individual features is 0.07cmy1 , and is dominated by a combination of pressure and Dopplerbroadening.

Ž .pump laser therefore has a bandwidth FWHM of approxi-mately 600 MHz at 355 nm. The pulse width of theOPGrOPA output must be shorter than the pump pulse,and the bandwidth of the OPGrOPA should therefore besomewhat larger than the pump bandwidth at 355 nm, as isobserved.

Finally, we have recorded photo-acoustic spectra of thethird O–H stretching overtone of water near 815 nm todemonstrate the continuous tunability of the OPGrOPAand to check its bandwidth. A portion of the photo-acous-tic spectra of water taken with the idler output is presentedin Fig. 6. The individual absorption features in this spec-trum have a FWHM of 0.07 cmy1, a value that is muchlarger than the measured bandwidth of the OPGrOPAoutput on the pulsed spectrum analyzer, and which iscaused primarily by of a combination of pressure andDoppler broadening. At the conditions under which the

Žspectra were acquired 1 Torr of water vapor in 500 Torr.of nitrogen , the intrinsic line width of the water absorp-

tion features is of order 0.05 cmy1. Convolution of themolecular broadening with the total observed line widthyields an estimate of 600 MHz for the OPGrOPA linewidth, in agreement with the pulsed etalon measurements.We are currently conducting measurements in a collision

Ž .and Doppler-free environment i.e., a supersonic jet toverify the bandwidth of the OPGrOPA in well resolvedspectra of a variety of molecules and clusters.

3. Conclusions

We have demonstrated a high spectral resolution, injec-tion seeded ns-OPGrOPA based on type I phase matching

in BBO and pumped at 355 nm. The seeding source in thiscase is an SLM diode laser, but any CW source providinga few mW across the free-running tuning range of 410–2400 nm can be used. Efficiencies of approximately 30%Ž .signalq idler are achievable, with a spectral resolution of;600 MHz and an angular divergence of order 0.5 mrad.In the absence of crystal tracking, the 3 dB seeding rangeis 20–30 cmy1, but since there is no cavity to be slavedthe seeded tuning range is limited in principle only by thescanning properties of the CW SLM laser.

The design presented here should also be extendible toother pump and OPG wavelengths. For example, 532nm-pumping of KTP or KTA ns-OPGs, or 1064 nm pump-

Ž .ing of LiNbO and Periodically Polled LiNbO PPLN3 3

ns-OPGrOPAs should produce tunable near-IR pulses outto 4–5 mm. For both 532 nm- and 1064 nm-pumpedns-OPGrOPAs, seeding on the signal branch is possiblewith diode lasers or other widely tunable all solid-state

w xCW SLM lasers such as diode-pumped Cr:LiSAF 20 .Additional difference frequency stages could extend theinfrared coverage to 10–20 mm, while a number of dou-bling and sum frequency schemes could be used to gener-ate nearly transform-limited pulses in the UV and vacuum-UV. Such sources would find wide applications in avariety of laboratory and diagnostic applications.

Acknowledgements

ŽSupport from the NSF MRI program grant ATM-9724500, sponsored by the Atmospheric Chemistry pro-

.gram is gratefully acknowledged, as is the generous dona-tion of nonlinear optical materials and optics for thisresearch by Casix, Inc., and Coherent, Inc. Additionalsupport was also provided by the NASA ISR and UVrVISAstrophysics programs.

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