848 OPTICS LETTERS / Vol. 29, No. 8 / April 15, 2004

Highly efficient narrow-line generation by difference-frequencymixing of a green pump and the Stokes

seed in RbTiOPO4 crystals: excitation of 943-nm emission

Sergey Kuznetsov, Guerman Pasmanik, Alexander Shilov, and Larissa Tiour

Passat, Ltd., 401 Magnetic Drive, Unit 43, Toronto, Ontario M3J 3H9, Canada

Received November 7, 2003

An efficient and compact scheme for diode-pumped Nd:YLF laser wavelength conversion to 943 nm wasdemonstrated by use of difference-frequency mixing and stimulated Raman scattering. We believe that thisis the highest conversion efficiency from the laser fundamental wavelength reported to date. It is shown thatRbTiOPO4 crystals are capable of providing highly efficient frequency mixing as a nonlinear medium. © 2004Optical Society of America

OCIS codes: 140.3480, 290.5860, 290.5830, 190.4410.

The 940–943-nm spectral range is considered oneof the most promising for active remote sensing ofatmospheric water vapor.1,2 During the past fewyears several approaches for generation of thesewavelengths were proposed and intensively studied.These approaches were based on the second har-monic of a Nd:YAG or a Nd:YLF laser pumping anoptical parametric oscillator3 or a Ti:sapphire slaveamplifier4 seeded by an external-cavity diode laser.Nonconventional Nd-doped crystals emitting directlyin the spectral range near 943 nm are also underconsideration.5,6 However, these approaches havemet with the problem of achieving output frequencystabilization with an accuracy of �1 GHz or better.Moreover, these schemes are quite bulky, and thebest conversion efficiency from a green pump to the940–943-nm range of which we are aware did notexceed 23%,4 which is insufficient for airborne andspaceborne applications.

In this Letter we present the results of an experi-mental investigation of a new approach to achievinghigh eff iciency and a narrow spectral line output inthe spectral range 940–943 nm in a simple and com-pact scheme. This approach is based on difference-frequency mixing (DFM) of the second harmonic of aNd:YLF laser with frequency 2v (pump wave) and aseed emitted by the same laser and then shifted to theStokes frequency v 2 V by stimulated Raman scatter-ing (SRS) in crystals such as Ba�NO3�2, KGd�WO4�2,KYb�WO4�2, and CaCO3. As a result of DFM, theidler wave at frequency 2v 2 �v 2 V� � v 1 V hasan anti-Stokes shift with respect to the laser’s fun-damental wavelength. Below we present the resultsof a study of this approach, in which we employ aBa�NO3�2 crystal as the SRS medium. The Ramanshift in Ba�NO3�2 is V�2pc � 1047.4 cm21, resultingin 943-nm output of the anti-Stokes wavelength.

A schematic conf iguration of a Nd:YLF laser experi-mental setup is presented in the top part of Fig. 1.The passively Q-switched master oscillator (MO), witha 50-mm cavity length, consists of a 4-mm-diameter,10-mm-long, a-cut Nd:YLF rod (coated for high ref lec-tivity at 1047 nm and high transmission at 792 nm

0146-9592/04/080848-03$15.00/0

on one side and antiref lection coated at 1047 nm onthe other side), a Cr:YAG saturable absorber with 40%initial transparency, and an output coupler with 60%ref lectivity. The Nd:YLF crystal was longitudinallypumped by one quasi-cw laser diode bar at 792 nm.The MO, operated at a 25-Hz pulse repetition rate, de-livered pulses with 750-mJ energy and 2.3-ns durationin TEM00 transverse and single longitudinal modeswith 0.5% pulse-to-pulse energy stability.

For shortening of the MO output pulse we used aphase-conjugation–pulse-compression mirror based onstimulated Brillouin scattering (SBS). To exceed theSBS threshold ��1 mJ�, the MO output was pream-plified in Amplifier 1 and focused by lens L1 into a

Fig. 1. Laser and frequency converter setup. See text fordetails.

© 2004 Optical Society of America

April 15, 2004 / Vol. 29, No. 8 / OPTICS LETTERS 849

40-cm-long SBS cell filled with highly purified SiCl4.By choosing the proper focal length of lens L1, therequired duration of SBS backref lected pulses couldbe obtained. Thus, for an L1 focal length comparablewith or longer than half of the MO pulse length inthe SBS medium, the pulse-compression conditioncould be satisfied, and the SBS pulses shortened to350 ps. For shorter focal lengths the longer SBSpulses were ref lected, so by changing the L1 focallength we could vary the laser output pulse durationover the 1–0.35-ns range to provide the optimumpower density for further DFM. The SBS ref lectivitywas close to 40% at 3.8-mJ pump-pulse energy andafter a backward pass through Amplifier 1 the pulseenergy was equal to 3.4 mJ with 6% pulse-to-pulseenergy stability. By use of a Faraday rotator anda l�2 retardation plate, amplif ied and shortenedpulses were passed through polarizer P1 and directedto four-pass Amplif ier 2 that consisted of an a-cut,6-mm-diameter, 20-mm-long Nd:YLF rod with a 1%Nd concentration. The rod was end pumped by thelaser diode array with 650-W optical peak power(after fast-axis collimation) at 805-nm wavelengthand 300-ms pump-pulse duration. This wavelengthcorresponds to the sideband of a Nd:YLF absorptionline with an �0.25-mm21 absorption coefficient forp polarization. After shaping and symmetrizationoptics the diode pump was focused within the Nd:YLFrod to a spot with a diameter of 2.5 mm. The pulseenergy was boosted to 16 mJ after the f irst two passesand to 32.5 mJ after four passes (the light-to-lightconversion efficiency of Amplifier 2 is �17%) witha pulse-to-pulse energy stability of �3% and beam-quality parameter M2 , 1.5.

The laser output was directed to the frequency con-verter depicted at the bottom of Fig. 1. The frequencyconverter consists of three main parts: a second-harmonic generator (SHG) to provide a pump beamat 523 nm for DFM, a Raman crystal employed forgeneration of a seed pulse at 1176 nm, and a DFMemployed for mixing of 523- and 1176-nm wavelengthsto generate a 943-nm output. Polarizer P2 dividesthe 1047-nm laser output into two beams. One beamwas directed to the Raman crystal, and the other wasused for SHG. A half-wave plate, l�2, placed beforepolarizer P2 rotates the polarization by an arbitraryangle to split the laser energy between the Ramanand the SHG arms in a variable ratio to optimize thefrequency-converter energy efficiency.

The first beam passed through polarizer P2 andwas then ref lected by spectral splitter SS1 towardan 84-mm-long Ba�NO3�2 crystal to excite backwardSRS. Positive, L3, and negative, L4, lenses reducedthe beam diameter to 1 mm before the Raman crystalto increase the pump intensity. In this experiment acollimated pump-beam geometry was used. The rearmirror, M, with high ref lectivity for both the pump(1047-nm) and the Stokes (1176-nm) wavelengths,was employed to reduce the SRS threshold and toincrease the conversion efficiency. First, this mirrorref lected the forward SRS emission in the backwarddirection and doubled the effective length of Ramaninteraction. Second, the pump ref lection doubled the

pump intensity, since the Stokes wave interacted withboth counterpropagating pump beams. This geome-try enables one to achieve damage-free conversion atlower f luences but at higher conversion eff icienciescorresponding to relatively high energy of the pumpbeams. This scheme has the advantage of using thenarrow SRS line in Ba�NO3�2 crystal to make pos-sible DFM seeding with a narrow-line ��0.06-cm21�Stokes wave. When it is mixed with a 523-nm pump,the narrow line of a seed provides a high spectralpurity of 943-nm output (within 2 GHz). Furthernarrowing of a seed wave linewidth may provide aneven narrower 943-nm output (the result of 300-MHz-linewidth Raman seed excitation was reported inRef. 7).

In the scheme presented in Fig. 1 the SRS thres-hold was equal to 0.4 mJ and the conversion efficiencyreached 40% at 1.85-mJ pump energy. The smallpump-beam diameter in the SRS crystal generated asingle-transverse-mode Stokes beam with a divergenceangle close to the diffraction limit. This Stokes waveat 1176 nm with energy up to 0.75 mJ passed throughspectral splitter SS1 and was used as a seed for DFM.

The second beam contained most of the energy andis ref lected from polarizer P2 and passed through avariable-delay line to overlap the pump and the seedpulses in the DFM crystal in time. The SHG occurredin a 16-mm-long KTP crystal that was chosen becauseof its high nonlinearity, large acceptance angle, andreasonable damage threshold (�1 GW�cm2 for a 1-ns,1064-nm pulse). A doubling efficiency of 68% wasachieved at a maximum pump-pulse energy of 30 mJin a beam with a 2.3-mm diameter.

The pump (523-nm) and the seed (1176-nm) beamswere combined at spectra splitter SS2 and directedto a type II RbTiOPO4 (RTP) crystal for DFM. Thiscrystal was chosen because of its high nonlinearity�x � 2.14 3 1024 1�W1�2�, large acceptance angle (sev-eral times wider than KTP), and high damage thresh-old (�8 GW�cm2 for a 1-ns pulse at 1064 nm). Two orthree identical 25-mm-long, consecutively placed RTPcrystals were used in our experiments.

Because the RTP crystal has an absorption edge at350 nm, the two-photon absorption (TPA) of the pumpwave at 523 nm could significantly reduce the effi-ciency of DFM. To determine the maximum 523-nmbeam intensity acceptable for RTP pumping, we in-vestigated the absorption losses caused by the TPA intwo consecutive crystals with a total length of 50 mm.We measured the overall energy of the 523-nm beamand the energy of the beam that passed through a1.3-mm aperture after two crystals versus the energyof the 523-nm pulse at the crystal entrance. To avoidpump depletion due to DFM, we blocked the seed wave.The results are presented in Fig. 2. Fitting the mea-sured data to the theoretical results obtained from theequation dI �r�, t��dz � 2bI 2�r�, t�, which describesTPA, and taking into account beam profile and pulseshapes, we found the TPA coefficient b to be �6 3

10210 cm�W. By measuring the TPA nonlinearity wefound the pump-wave intensity, which allowed us toavoid significant TPA absorption in the RTP crystals.Since DFM eff iciency is determined by gain on the

850 OPTICS LETTERS / Vol. 29, No. 8 / April 15, 2004

Fig. 2. Pulse energy at 523 nm passed RTP crystals asa function of pulse input energy. �, energy of the wholebeam; �, energy of the beam’s central part. The thin linesare the output linearly depending on input approximationswithout TPA.

Fig. 3. DFM energy-conversion efficiency E943�E523 as afunction of gain g � 2x

pI �0, 0�L at different energies of

the seed wave at 1176 nm: 1–4, with two RTP crystalsemployed �L � 50 mm�. 1, E1176 � 0; 2, E1176 � 5 mJ;3, E1176 � 10 mJ; 4, E1176 � 750 mJ; 5, with three RTPcrystals employed �L � 75 mm�, E1176 � 750 mJ.

beam axis g � 2xpI �0, 0�L and TPA is determined by

the value of bI �0, 0�L, it is obvious that high conver-sion efficiency of DFM without TPA can be reached inrelatively long crystals. This is why in DFM experi-ments we used relatively low pump intensity and longnonlinear crystals (two or three crystals, 25 mm longeach).

The 523-nm pump had a 2.3-mm beam diameter anda 0.5-ns pulse duration. We measured the energy ofthe generated idler wave at 943 nm as a function of

pump-pulse energy varying from 6 to 15 mJ as well asfrom seed pulse energy variation. Figure 3 shows thepump-to-idler �523 ! 943 nm� energy-conversion effi-ciency as a function of gain g � 2x

pI �0, 0�L at dif-

ferent energies of the seed at 1176 nm. The resultspresented in Fig. 3 show that the conversion efficiencyincreases sharply at a relatively low pump intensitybut then becomes saturated. Also, there is no strongefficiency dependence on seed pulse energy.

The energy eff iciency that we obtained was 30%with three RTP crystals for conversion of 523–943 nm(curve 5 in Fig. 3). The maximum eff iciency of overallfrequency conversion (1047 ! 943-nm conversion)reached �18% and was limited by the eff icienciesof the Raman conversion, SHG, and DFM processes.Taking into account the diode-pumped laser perfor-mance, we can estimate the light-to-light efficiency�805 ! 943 nm� as 3.1%, which is, to the best of ourknowledge, the highest eff iciency of nearly 943-nmlight generation.

In conclusion, we have experimentally demonstratedthe new approach for 943-nm wavelength generation.The RTP crystal was proposed and proved as a non-linear crystal, providing high conversion eff iciency byDFM. The conversion efficiency of 30% for the DFMprocess �1047 ! 943 nm� and 18% for the whole fre-quency converter �1047 ! 943 nm� has been demon-strated. TPA of 523-nm wavelength in a RTP crystalhas been found and measured. In addition, the devel-oped technique makes possible the generation of new(anti-Stokes) wavelengths in the near-infrared regionwith high conversion efficiency via combinations of themost effective solid-state laser lines and different Ra-man crystals as a narrow-line seed source.

This work was supported by the CanadianSpace Agency (contract 9F028-024112/001-MTB). G.Pasmanik’s e-mail address is info@passatltd.com.

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