stable, continuously tunable operation of a diode-pumped doubly resonant optical parametric...

1264 OPTICS LETTERS / Vol. 25, No. 17 / September 1, 2000

Stable, continuously tunable operation of a diode-pumpeddoubly resonant optical parametric oscillator

Angus J. Henderson, Pamela M. Roper, Lawrence A. Borschowa, and Roy D. Mead

Aculight Corporation, 11805 North Creek Parkway South, Bothell, Washington 98011

Received April 24, 2000

We demonstrate, for what is the first time to our knowledge, long-term stable, continuously tunable operationof a doubly resonant optical parametric oscillator (OPO) pumped by a single-stripe diode laser without theuse of an external semiconductor amplifier. The OPO is based on periodically poled lithium niobate andis pumped by a 150-mW distributed Bragg grating diode laser. 18-mW total output power is generated at1.3- and 2.3-mm wavelengths. A cavity-length servo system allows continuous signal tuning of 17 GHz andidler tuning of 10 GHz, limited only by the range of a piezoelectric cavity mirror mount. OPO tuning isdemonstrated from 1.1 to 1.4 mm and from 2.2 to 3.7 mm © 2000 Optical Society of America

OCIS codes: 190.4970, 140.3480, 130.3730.

Spectroscopic applications of lasers are currently sig-nificantly limited by the lack of practical mid-infrared,narrow-bandwidth, continuous-wave (cw) sources.Semiconductor lasers whose bandwidth is controlled byinternal or external gratings offer ideal operating char-acteristics for such applications. Single-frequencysemiconductor lasers have been developed that addressselected wavelength regions up to 2 mm. However,above 2 mm, available sources such as quantum-cascade, II–VI, and lead-salt semiconductor lasers stillrequire liquid nitrogen temperatures for cw operation.Cw OPO’s offer an alternative means of generatingsingle-frequency cw mid-infrared output without cryo-genic operation. The recent development of high-gainnonlinear materials such as periodically poled lithiumniobate (PPLN) and of high-quality diode-pumpedsolid-state lasers has produced significant advances,including cw singly resonant operation from 1.5 to4.5 mm.1 In parallel with these developments, single-frequency near-infrared semiconductor lasers withhigh beam quality have become available. The use ofthese devices as pump lasers for cw OPO’s offers sig-nificant advantages over alternative sources. First,direct diode pumping removes one stage of energyconversion from the process. Second, diode lasers canbe highly compact, robust, and cheap in significantquantity. Finally, the operating characteristics ofdiode lasers (narrow bandwidth, wide continuoustuning, and fast response of the output wavelength todrive current) that make them attractive as spectro-scopic sources can potentially be replicated in the midinfrared by frequency conversion in OPO’s.

Diode-pumped OPO’s have been demonstrated by anumber of groups. These OPO’s fall into four cate-gories: singly resonant oscillators (SRO’s),2 pump-enhanced singly resonant oscillators (PESRO’s),3

triply resonant oscillators (TRO’s), and doubly reso-nant oscillators (DRO’s). Operation of devices in thefirst two categories requires that an external semicon-ductor amplifier be used to raise the available pumppower to the level required for attaining oscillationthreshold. Such arrangements add significant com-plexity to the overall system and are not commerciallyavailable. SRO’s and PESRO’s offer straightforward

0146-9592/00/171264-03$15.00/0

coarse-tuning characteristics but require intracavityetalons for continuous tuning beyond one free spectralrange. Although DRO’s (and TRO’s) require muchless pump power to reach oscillation threshold thando the PESRO and SRO configurations, they havelong been thought to be unstable and diff icult totune. Recently the use of continuously tunable pumpsources allowed stable, continuously tunable opera-tion of DRO’s to be achieved.4 Pumping of a DROby low-power monolithic semiconductor lasers wasdemonstrated with a Fabry–Perot device.5 However,the unstable frequency characteristics of this pumpsource made it impossible to stabilize the OPO beyonda matter of seconds.

In this Letter we show that long-term stable, con-tinuously tunable output can be generated in a doublyresonant OPO pumped by a single-emitter diode laserwhose operating frequency is stabilized by an internalgrating structure. A significant advantage of theDRO is that we can tune the device continuouslythroughout the gain bandwidth of the OPO simplyby locking the cavity length of the OPO to the peakof a single-idler double resonance. By this methodthe wide continuous tunability of the semiconductorlaser can be reproduced by the OPO without theneed for intracavity frequency-control componentssuch as etalons. Continuous tuning of SRO’s6 andPESRO’s7 was demonstrated by use of intracavityetalons and electronic cavity-length control, and thediode-pumped SRO described in Ref. 6 has been usedin spectroscopic measurement. The use of PESRO’s8

and DRO’s9 pumped by diode-pumped solid-statelasers in spectroscopy has also been demonstrated.We demonstrate here, for what is the f irst time to ourknowledge, spectroscopic measurement of absorptionfeatures by use of a DRO pumped directly by a diodelaser. We believe that the 17-GHz continuous tuningdemonstrated by this diode-pumped OPO is the widestcontinuous tuning demonstrated by any DRO.

The configuration of the OPO system is illus-trated in Fig. 1. The pump source for the OPOis a 150-mW, single-frequency, 852-nm-wavelengthdistributed Bragg grating diode laser (Spectra DiodeLabs SDL-5722-H1). This device emits in a single

© 2000 Optical Society of America

September 1, 2000 / Vol. 25, No. 17 / OPTICS LETTERS 1265

Fig. 1. Schematic diagram of the diode-pumped OPO con-figuration. PZT, piezoelectric transducer.

transverse mode with a measured beam quality M2

value of 1.3. The output of this source is collimatedby an aspheric lens and focused through a 60-dB iso-lator. On recollimation, the beam is circularized bythe use of an anamorphic prism pair. A retardationplate returns the polarization to vertical before thebeam is focused to a 34-mm waist at the center ofthe nonlinear crystal in the OPO cavity. The OPOcould be configured as either a single-pass-pumpedor a double-pass-pumped doubly resonant OPO, withboth the signal and the idler waves resonant. Themajority of our work (and all measurements describedin this Letter) was performed in double-pass devicesas described below.

The OPO cavity is designed to minimize the oscil-lation threshold by use of a Boyd–Kleinman focusingparameter, j � 1. The 34-mm waist of the pump beamis mode matched to a 36-mm-long linear cavity formedby two 15-mm radius-of-curvature calcium f luoridemirrors and a 19-mm-long PPLN crystal. The OPOcrystal is antiref lection coated at the signal andidler wavelengths. OPO input mirror M1 is highlytransmitting at the pump wavelength and highlyref lective at the signal and idler wavelengths. Signaland idler ref lectivities of 99.7% are used to minimizethe threshold, and ref lectivities of 99.0% allow outputcoupling from this mirror. Rear mirror M2 is highlyref lective at the pump, signal, and idler wavelengths.Mirror curvatures and separations are selected thatprovide a cavity that is mode matched to the pumpwaist.

PPLN crystals with periods from 23.0 to 23.45 mmare used. The OPO crystal is mounted in atemperature-controlled oven operated from 130 to180 ±C, with short-term temperature stability of63 mK. Three separate sets of mirror and crystalcoatings are used to provide nearly complete wave-length coverage from 2.2 to 3.7 mm (Fig. 2) at the idlerwavelength (1.1 to 1.4 mm at the signal wavelength).Note that all data reported here, other than thecoarse-tuning data shown in Fig. 2, were recordedin an OPO with wavelengths centered on 1.34 and2.34 mm. Oscillation thresholds as low as 5 mWare observed under optimum alignment, with highlyref lecting mirrors, and thresholds as low as 17 mWare observed with 1% output coupling.

Total signal and idler power output of 18 mW areobserved for a pump power of 89 mW incident upon the

input mirror of the OPO. We measure 4 mW of idlerpower, using a germanium filter to cut out the pumpand the signal. Scanning of the pump frequency (bymodulation of the diode current) or the OPO cavitylength (by piezoelectric transducer voltage modulation)reveals the longitudinal-mode structure within clus-ters at a fixed PPLN temperature. The clusters showa monotonic variation of OPO wavelength as theseparameters are varied, with adjacent output peakscorresponding to adjacent longitudinal-mode pairs.The separation of longitudinal modes is �1.5 nm incavity length and 22 MHz in pump frequency. Thepump frequency varies by 0.7 GHz�mA with currentand by 1.4 MHz�mK with diode temperature. Withthese sensitivities, frequency- and amplitude-stableOPO output is attained with a cavity servo-controlcircuit with a response bandwidth of 50 kHz. Thiscircuit applies a dither to the diode current at afrequency of 500 kHz and generates an error signalfrom changes in OPO signal amplitude as measuredby a photodetector. The high-frequency error signalis fed back to the diode current, and the low-frequencycomponent is applied to a piezoelectric transducer onthe rear mirror of the OPO cavity. In this way theOPO maintains amplitude-stable (Fig. 3) operation ona single signal–idler-mode pair. Operation withoutmode hops for as long as 90 min has been observed.This locking scheme also allows the frequency of theOPO to be continuously tuned. The pump source has

Fig. 2. Idler tuning measured in OPO’s by use of threedifferent OPO sets of cavity optics.

Fig. 3. 1.3-mm OPO signal amplitude as a function of timeover 30 min under cavity servo lock. The standard devia-tion (sdev) in amplitude is 0.2%.

1266 OPTICS LETTERS / Vol. 25, No. 17 / September 1, 2000

Fig. 4. 20-pm wavelength scan of 2.3-mm OPO idler wave-length through the R6 absorption line in carbon monox-ide, showing normalized transmitted amplitude throughthe cell.

a continuous tuning range of �50 GHz by currentand greater than 200 GHz with temperature. Hence,using a piezoelectric transducer with an appropri-ate range of stretch, we can potentially attain over100 GHz of continuous tuning of OPO frequencies.We have so far demonstrated 17- and 10-GHz continu-ous tuning of the signal and the idler (1.3- and 2.3-mmwavelength), respectively, by current tuning. Tuningwas monitored by use of a confocal Fabry–Perotinterferometer. Diode temperature variation allowed12-GHz signal tuning and 7-GHz idler tuning. Inboth cases the travel ranges of the piezoelectric trans-ducers were the limitation to tuning. Some residualvariation ��10%� in OPO amplitude was observedduring tuning. This variation is partly due to thevariation in diode current and hence input pumppower and partly to weak etalon effects at the pumpwavelength in the OPO cavity.

The same tuning capability allows the outputfrequency of the OPO to be modulated over a rangeof 300 MHz at rates up to 1 kHz. Modulation of theoutput frequency over several gigahertz at slowerrates was also demonstrated. Continuous tuning ofthe OPO output was utilized in a simple single-passspectroscopic measurement of absorption features incarbon monoxide at 2.3 mm. The OPO idler beamintensity transmitted by a 60-cm cell containingcarbon monoxide at 4-Torr pressure was measured ona photodector and normalized to the input intensity.Figure 4 shows the normalized transmitted signal asthe idler frequency was slowly ��10 s� current tunedover �1 GHz.

Confocal interferometer measurements of the OPOsignal linewidth indicated that the linewidth was lessthan 7 MHz. We believe that the linewidth is in factnarrower and that it should match that of the pumplaser, which was measured to be less than 3 MHz.

The OPO frequency stability was shown by use of thesame interferometer to be �20 MHz over a period of10 s. We believe that this frequency stability is deter-mined principally by the temperature stability of thediode laser �63 mK�. Frequency locking of the OPOoutput to molecular absorption features is expected tobe possible by use of the device’s modulation capability.

We have demonstrated, for what is believed to be thefirst time, pumping of a cw OPO by use of a distributedBragg grating diode laser. We believe that this is thefirst demonstration of stable, continuously tunable out-put from a diode-pumped cw OPO, using a diode laserwithout an external cavity or an external semiconduc-tor amplif ier as part of the pump scheme.

We tuned the single-frequency OPO output continu-ously over a signal range of 17 GHz and an idlerrange of 10 GHz by continuously varying the pumpfrequency. This capability was used to performspectroscopic measurements of absorption featuresof carbon monoxide. We also demonstrated OPOfrequency-modulation capability at modulation ratesup to 1 kHz. We believe that these capabilities,coupled with the compact size and room-temperatureoperation of the device, make the diode-pumpedDRO a highly practical source for applications in themid-infrared spectral region.

We acknowledge helpful discussions with JamesGord of the Propulsion Directorate of the U.S. Air ForceResearch Laboratory (AFRL) and with Pat Looney ofthe National Institute of Standards and Technology(NIST). This work was supported by the AFRLSensors Directorate under contract F33615-98-C-1220and by NIST under contract 50-DKNB-8-900122.A. J. Henderson’s e-mail address is angus@aculight.com.

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