October 1, 2009 / Vol. 34, No. 19 / OPTICS LETTERS 2897

High-efficiency mid-infrared optical parametricoscillator based on PPMgO:CLN

Yuefeng Peng,* Weimin Wang, Xingbin Wei, and Deming LiInstitute of Applied Electronics, China Academy of Engineering Physics, P.O. Box 919-1013,

Mianyang Sichuan 621900, China*Corresponding author: qiaopyf@yahoo.com.cn

Received June 24, 2009; revised August 18, 2009; accepted August 22, 2009;posted August 27, 2009 (Doc. ID 113160); published September 17, 2009

The experimental results of a high-efficiency mid-IR laser are presented on a quasi-phase-matched single-resonated optical parametric oscillator in PPMgO:CLN pumped by a 1064 nm laser of an elliptical beam.The pump source was an acousto-optical Q-switched cw-diode-side-pumped Nd:YAG laser. The beam polar-ization matched the e–ee interaction in PPMgO:CLN. When the crystal was operated at 110°C and the pumppower was 104 W with a repetition rate of 7 kHz, average output powers of 16.7 W at 3.84 µm and 46 W at1.47 µm were obtained. The slope efficiency of the 3.84 µm laser with respect to the pump laser was 19.1%.The M2 factors of the 3.84 µm laser were 2.03 and 5.89 in the parellel and perpendicular directions,respectively. © 2009 Optical Society of America

OCIS codes: 140.3070, 140.3580, 140.3600, 190.4970.

Mid-IR lasers in the 3–5 µm wavelength region havemany applications, such as military countermea-sures, remote monitoring of the special environment,spectrum, and so on [1,2]. Because of their high rep-etition rate, high stability, and compact configura-tion, mid-IR solid-state lasers play an important rolein the field of mid-IR countermeasures. NorthopGrumman’s Viper laser is a small, lightweight multi-band laser for IR countermeasure applications thatcovers the wavelength ranges of 1–3 µm, 3–5 µm, and8–12 µm [3]. At present, there are mainly threemethods to obtain solid-state mid-IR lasers. The firstis laser diode technology; M. Troccoli et al. have donemuch work on mid-IR quantum-cascade lasers [4].The second is pumping metal-ion-doped crystal tech-nology, Voranov et al. achieved 142 mJ output energywith the tuning range of 4.0–4.17 µm from a Fe:ZnSelaser pumped by the Er:YAG laser [5]. The third isthe optical parametric oscillator (OPO) [6–9]; Budniet al. reported a ZGP-OPO that was pumped by a 1.9µm Ho:YAG laser and achieved �4.2 W mid-IR out-put (3.8 and 4.65 µm) [8]. Mason et al. achieved 3.7 Wmid-IR output (3.7–4.0 µm) through the polarizationcombination technology of two periodically poledlithium niobate (PPLN) OPOs pumped by a 1.047 µmNd:YLF laser [9]. OPOs can offer a unique combina-tion of high peak power, good beam quality, widewavelength tunability, and power scalability.

Quasi phase matching (QPM) can utilize the larg-est nonlinear coefficient of crystals. In theory, QPM isable to achieve phase matching in the entire trans-mission range of crystals. LiNbO3 is a typical nega-tive uniaxial crystal with the transmission range of330–5000 nm. Of all its second-order nonlinear polar-ization tensors, d33 has the largest nonlinear opticalcoefficient of 27.4 pm/V, which is 7.5 times largerthan d31 of the birefringent phase matching com-monly used. The d33 of PPLN is utilized along withadvantages of high gain, low threshold, and high ef-ficiency. Periodically poled MgO-doped LiNbO3 crys-

tal (PPMgO:LN) is the most universally used ferro-

0146-9592/09/192897-3/$15.00 ©

electric material for quasi phase matching an OPO,since the doping MgO can significantly enhance thePPLN crystal’s photorefractive damage thresholdand effectively reduce its coercive field. Chen et al.achieved over 10 W output at 3 µm from a singlyresonant MgO-doped PPLN OPO pumped by a 50 Wytterbium fiber laser [10]. Hirano et al. reported ahigh-average-power OPO based on a 1 mm thicknessMgO-doped PPLN; the total output power was 57 Wfor both a 2.02 µm and a 2.25 µm laser [11].

In this Letter, an average output power of 62.7 Wwas obtained on a quasi-phase-matched single-resonated OPO in PPMgO:CLN (periodically poled 5mol.% MgO-doped congruent LiNbO3 crystal)pumped by a 1064 nm laser of an elliptical beam,with 16.7 W at the mid-IR wavelength of 3.84 µm and46 W at the near-IR wavelength of 1.47 µm.

The experimental setup is a single-resonated OPO,pumped by a Nd:YAG 1064 nm laser, shown in Fig. 1.The 1064 nm laser cavity was formed by flat mirrorM1 �R�99.5%@1064 nm�, output coupler flat mirrorM2 �R=50%@1064 nm�, two cw-diode-side-pumpedNd:YAG modules, an acousto-optical Q-switch, a 90°rotator of quartz, and a 1064 nm laser polarizer. The1064 nm laser with high power and good beam qual-ity was obtained by optimizing the laser parameters.With the repetition rate of 7 kHz, a 1064 nm laserwith an average output power of 110 W and the beamquality M2 factor of less than 2 was obtained. Thebeam polarization matched the e–ee interaction inPPMgO:CLN, thus the maximal nonlinear coefficientd33�27.4 pm/V� is available and walk-off of the beamscan be avoided. The PPMgO:CLN OPO was formed

Fig. 1. Schematic of the experimental setup.

2009 Optical Society of America

2898 OPTICS LETTERS / Vol. 34, No. 19 / October 1, 2009

by a 1 mm � 4 mm � 40 mm PPMgO:CLN, togetherwith flat mirrors M3 and M4. Mirror M3 was antire-flection (AR) coated with AR@1064 nm, R�90%@1.3–1.6 �m, and high-reflection (HR) coatedHR@3.6–4.0 µm, while M4 was coated with HR@1064nm, AR@1.3–1.6 µm and R=60%@3.6–4.0 �m. ThePPMgO:CLN crystal had a grating period of 29.2µm@25°C. Both end faces of the PPMgO:CLN wereAR coated with AR@1064 nm, 1.3–1.6 µm, and 3.6–4.0 µm. The PPMgO:CLN was staged in an oven withthe temperature range up to 200°C, which is conve-niently used to adjust and control the crystal operat-ing temperature. The temperature of the oven can becontrolled to a precision of 0.1°C. The coupled systemwas formed by two cylindrical lenses and a sphericallens. The size and shape of the pump beam can be ad-justed by the coupled system to match the profile ofPPMgO:CLN. The spot size of the elliptical pumpbeam is about 0.8 � 3.0 mm at the center of the crys-tal. Limited by the thickness of the crystal and thelow damage threshold of the mid-IR OPO, a high-power mid-IR laser can hardly be achieved with around pump beam; however, an elliptical pump beamcovers more area of the incident surface, which effi-ciently elevates the output power and reduces thedanger of optics damage. A high-power mid-IR laserwith a round beam is possible in further experimentbased on a 3-mm-thick PPMgO:CLN provided by HCPhotonics Corp. of Taiwan. With the increase of thecrystal’s thickness (3 mm in commerce and 5 mm inlaboratory [12]), an even-higher-power mid-IR laserwould be obtained by PPMgO:CLN–OPO.

When the crystal was operated at 110°C and thepump power was 104 W with a repetition rate of 7kHz, average output powers of 16.7 W at 3.84 µm and46 W at 1.47 µm were obtained. The slope efficiencyof the 3.84 µm laser with respect to the pump laser

Fig. 2. (Color online) Laser output power versus pumppower.

Fig. 3. (Color online) Mid-IR laser output spectrum.

was 19.1%. Laser output power versus pump power isshown in Fig. 2. The output power saturation of 3.84µm laser does not appear, so it is possible to obtainhigher output power with higher pump power. Themain problem encountered during the experiment isthe damage of the optical coat. In the early experi-ments, the damage threshold of the PPMgO:CLNcoat was as low as about 15 MW/cm2 for a 1 µm laserpeak power density at 7 kHz and a 150 ns pulse;there occurred repeated coat damage at the end faceof the crystal. The damage threshold was up to about30 MW/cm2 at the same testing condition by improv-ing the coating technology. The instances of coatdamage were reduced by optimizing thePPMgO:CLN–OPO parameters and intensity distri-bution of the pump beam and strictly limiting thepump-peak power density to under 12 MW/cm2. Theoutput spectrum of the mid-IR laser is shown in Fig.3; the central wavelength is 3.84 µm. Because of theresolution of the spectrometer, the actual linewidth ofthe 3.84 µm laser is narrower than that shown in Fig.3. The beam quality of the 3.8 µm laser was mea-sured by measuring the size of laser spot using theknife-edge method at a different location and hyper-bolic fitting the measured data. The beam quality M2

factors of the 3.84 µm laser were 2.03 and 5.89 in theparellel and perpendicular directions, respectively.The near-field intensity distribution of the 3.84 µmlaser beam is shown in Fig. 4. When the pump powerwas 104 W with the repetition rate of 7 kHz, themid-IR laser output power versus the temperature ofPPMgO:CLN is shown in Fig. 5. The mid-IR laseroutput power of the OPO varies with the tempera-ture of PPMgO:CLN, but the discrepancy is small.We think such a difference may be caused by the dif-ference of the mid-IR laser output wavelength, theerror of measurement, the discrepancy of mid-IRwavelength coat and so on. By adjusting the tem-perature of PPMgO:CLN crystal, the mid-IR laseroutput wavelength was measured. The wavelength

Fig. 4. (Color online) Near-field intensity distribution of3.84 µm laser beam.

tunability of 3.9–3.7 µm can be achieved by adjusting

October 1, 2009 / Vol. 34, No. 19 / OPTICS LETTERS 2899

the temperature of a 29.2 µm period PPMgO:CLNcrystal from 30°C to 200°C, which basically accordedwith the theoretic calculation by Gayer et al. [13].

In conclusion, a singly resonant PPMgO:CLN OPOhas been operated. An average output power of 62.7W was obtained, with 16.7 W at the mid-IR wave-length of 3.84 µm and 46 W at the near-IR wave-length of 1.47 µm. Future work will be focused on op-timizing the experimental setup to obtain bettersystem stability during long-term operation.

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Fig. 5. Laser output power versus temperature ofPPMgO:CLN.

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