Continuous-wave diode-laser injection-seeded bbb ... - OSA Publishing

March 1, 1995 / Vol. 20, No. 5 / OPTICS LETTERS

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Continuous-wave diode-laser injection-seeded b -barium borate optical parametric oscillator: a reliable source for spectroscopic studies P. Bourdon, M. P´ealat, and V. I. Fabelinsky* Office National d’Etudes et de Recherches A´erospatiales, B.P. 72, Chatillon ˆ Cedex, France Received November 2, 1994 A b-barium borate optical parametric oscillator pumped by the third harmonic of a pulsed Nd:YAG laser and seeded by a cw diode laser is described. It operates on a single longitudinal mode of its cavity, with a linewidth lower than 500 MHz (0.017 cm21 ). The conversion efficiency of the device is as great as 20% on signal output alone. The seeded tuning range is limited by the diode’s tunability.

The wide tunability of optical parametric oscillators (OPO’s) is of great interest for spectroscopic studies. In the 1980’s the reliability and the efficiency of OPO’s were increased by improvement in existing nonlinear materials and by the use of new crystals such as b-barium borate (BBO).1,2 Recently it was demonstrated that spectroscopic studies such as photoacoustic absorption spectroscopy and coherent anti-Stokes Raman spectroscopy (CARS) could benefit from the use of an OPO.3,4 However, for high-resolution spectroscopic studies the broad natural linewidth of the OPO (1 nm or even more) is a major limitation, and spectral narrowing of the OPO is necessary. Singlemode operation of the OPO can be achieved by use of a wavelength-selective cavity.5 These techniques are widely used on narrow-bandwidth dye lasers. Often OPO’s are low gain sources compared with dye lasers; therefore their single-mode operation through injection seeding 6 – 8 can be performed with the advantage of avoiding any loss in efficiency. Here we present a 355-nm-pumped OPO operating in the visible and near-infrared ranges. It is injection seeded by a cw diode laser driving a single longitudinal mode of the cavity. The OPO was designed to be the Stokes source for scanning CARS spectroscopy or for either of the two tunable sources with highfrequency stability required for dual-line CARS.9 It has been operated near 608 nm (seeding near 850 nm) and near 680 nm (seeding at 680 nm), corresponding to the Stokes frequencies of N2 and H2 lines, respectively. The tuning range of the seeded OPO is limited by that of the seeder (typically 20 nm). The OPO configuration is shown in Fig. 1. A 12 mm 3 7 mm 3 5 mm BBO crystal is placed inside a 4 cm-long cavity (cavity optical length, 5 cm). The crystal is cut for type I phase matching (u ­ 30±, f ­ 0±); the faces are uncoated. The apparatus is mounted upon a rotatable stage (1023 -deg accuracy). The pump radiation is the third harmonic of a Quantel YG781-30 Nd:YAG laser. The laser operates at a 30-Hz repetition rate. It is injection seeded and emits a single longitudinal mode. To produce good beam quality, the beam does not fully cover the 6-mm-diameter Nd:YAG rods. The energy at 355 nm is 60 mJ per 9-ns pulse in a 4-mm (diameter at 1ye2 ) 0146-9592/95/050474-03$6.00/0

Gaussian beam. It is sent inside the crystal by a turning mirror placed at 45± within the cavity. It has high reflectivity at 355 nm and low reflectivity in the visible and near-infrared ranges. The residual pump beam after the crystal is split off by an identical turning mirror. The cavity mirrors, M1 and M2, are flat. In a collinear phase-matching configuration, even just a few percent reflectivity on each mirror is enough to operate the OPO in a doubly resonant scheme. Then, for singly resonant operation, it was preferred to turn the pump beam off the cavity axis by approximately half a degree and to compel the idler beam to walk off, according to a noncollinear phasematching scheme. Two pairs of mirrors were tested. In the first arrangement, mirror M1’s reflectivity is 99% from 440 to 700 nm, and that of M2 is approximately 65% from 400 to 1000 nm. The OPO is critically phase matched, and a 20± rotation of the crystal covers the entire tuning range from 400 to 710 nm for the signal wave and from 710 to 2000 nm for the idler

Fig. 1. Schematic of the seeded OPO. YAG, a Quantel YG 781-30 laser with an unstable Gaussian cavity. The 90± rotator (R) permits compensation for the stress-induced birefringence of the oscillator rod (O) in the amplifier rod (A). I, seeder; Q, Q switch; 4, quarter-wave plate; S, second-harmonic generator; T, third-harmonic generator; HS, harmonic separator; H, 4-mm-diameter hole; 2, half-wave plate; P, polarizer; LD, laser diode SDL 5410; FI, Faraday isolator; BS, beam splitter; PZT, piezoelectric transducer.  1995 Optical Society of America

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OPTICS LETTERS / Vol. 20, No. 5 / March 1, 1995

Fig. 2. Evolution of signal output energy versus wavelength. From 450 to 600 nm the conversion efficiency is approximately 7.7%. It decreases with mean reflectivity of the cavity mirrors. The pump energy is 60 mJ per pulse.

wave. With a pump energy of 60 mJ per pulse at 355 nm, the efficiency is 15% (signal energyypump energy) for a doubly resonant OPO (DROPO) and 8% for a singly resonant OPO (SROPO). For the singly resonant case, the efficiency follows the evolution of the mean reflectivity of the cavity mirrors: it decreases below 450 nm and between 650 and 710 nm (Fig. 2). The threshold is 17 MWycm2 for the DROPO and 25 MWycm2 for the SROPO. The signal pulse duration is 6 ns for the DROPO and 4.5 ns for the SROPO with 60 mJ per pulse at 355 nm. The beam profile reproduces that of the pump beam. The signal beam energy increases with the pump energy; the available pump level (i.e., 60 MWycm2 ) is not high enough to reach the saturation regime. The coupling mirror reflectivity of 65% is optimum for our 60-mJ pump beam. It should be adjusted properly for other pump levels. In the second arrangement, M1 and M2 are identical. The reflectivity is . 95% for the infrared idler beam from 730 nm to 890 nm and , 10% for the visible signal beam from 620 nm to 700 nm. In this case the energy is stored in the cavity on the idler beam. The energy generated in the signal beam leaves the cavity in the direction defined by the pump beam. We obtain 20% efficiency on the visible signal with 60 mJ per pulse in the ultraviolet. This cavity is the most efficient for a singly resonant OPO. Although there is no proof of a correlation between the cavity finesse and the linewidth of the OPO cavity modes, this cavity is expected to give rise to the narrowest mode linewidth. In both arrangements the natural linewidth of the OPO is broad (0.5–1 nm) and, because of the type I phase matching, it further increases, up to 10 nm near the degeneracy. The DROPO spectrum is modulated, whereas the SROPO spectrum is smooth (Fig. 3). The peaks of these modulations occur when the signal and the idler complementary frequencies simultaneously match the cavity modes. Inversely,

the holes occur when either the signal or the idler frequency does not coincide with a cavity mode. Then, for a DROPO, the gain varies greatly from one cavity mode to another, depending on its doubly or singly resonant characteristic. That mode competition prevents proper seeding of a DROPO. For single-mode operation we use a SROPO with noncollinear phase matching. The cw single-mode diode-laser seeder (SDL 5410) emitting near 805 nm is isolated from the OPO by a 30-dB Faraday isolator (Fig. 1). The seeder beam overlaps the idler beam in the cavity of the OPO. Cavity mirror M1 is now mounted upon piezoelectric translators for fine adjustment of the cavity length and to bring one mode of the OPO cavity onto the frequency of the seeder. Although it was first hoped that in the noncollinear phase-matching configuration there would be no reflected idler beam coming back to the diode, isolation of the seeder from the OPO appeared to be necessary. Indeed, a beam at the idler frequency is created inside the BBO crystal, sending back a weak self-aligned perturbating feedback to the diode. This phenomenon can be explained by a phase-conjugation process (Fig. 4). In fact, using interference filters and detection by a photomultiplier on the path between the OPO and the seeder confirm the presence of a weak phaseconjugated beam. Seeding occurs with diode power as low as 300 mW . It lowers the threshold to 17 MWycm2 without noticeable change in efficiency. Further, the signal buildup time decreases, which, in turn, increases the signal pulse duration to 6 ns.

Fig. 3. Single-shot signal spectra for a SROPO and a DROPO at 620 nm. The spectrograph resolution of 0.2 nm is not sufficient to separate the cavity modes.

Fig. 4. Idler phase-conjugate beam generation in the BBO crystal. The two counterpropagating signal waves kS and 2kS create an index grating in the crystal. The idler beam kI interacts with this grating, generating a phase-conjugate beam 2kI .

March 1, 1995 / Vol. 20, No. 5 / OPTICS LETTERS

Fig. 5. Transmission through a 5-GHz free-spectralrange Fabry – Perot analyzer. The analyzer’s cavity length is scanned over its free spectral range (FSR), showing the single-mode operation of the OPO and its linewidth (,500 MHz). This scan was taken over a period of 2 min.

Special attention has been paid to maintaining good long-term frequency stability of the seeded OPO. First, the stability of the lasers was measured with a Cluster Ltd. wavemeter: the seeded Nd:YAG pump laser frequency drift is , 50 MHzyh; for the temperature- and current-controlled diode laser, it is , 30 MHzyh. Second, the cavity of the OPO is built in a monolithic Invar structure, with compensation of the residual thermal expansion, so that its length is temperature independent. Once aligned and seeded, the OPO is single mode, as can be seen with a tunable Tecoptics FPI-25 Fabry–Perot analyzer (Fig. 5). Moreover, it keeps its single-mode characteristic for more than 3 h without any control of the cavity length. The linewidth is narrower than 500 MHz, compared with the 120-MHz Fourier transform for a 5-ns pulse duration and the 200-MHz cavity mode width. The tunability of the seeded OPO is now limited by the tuning range of the seeder (20 nm near 805 nm). With such a diode, continuous tuning without mode hops is impossible. For CARS experiments we use an external-cavity tunable diode with a wider continuous scanning range. Synchronous scanning of the OPO can be obtained by application of proportional voltages on the piezoelectric translators that control the OPO

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cavity length and the diode wavelength. For broad scanning, note that the noncollinear phase-matching arrangement yields a change in the idler beam direction as the OPO is tuned. However, it has been determined that seeding of the OPO is achieved over approximately 60 GHz without realignment of the seeder. This reduced tunability and the changing beam direction could be drawbacks for scanning CARS spectroscopy but not for dual-line CARS, for which no frequency scan is required. In conclusion, we have built a single-mode visible BBO OPO, seeded by a cw diode laser emitting a few milliwatts of power. The bandwidth of the system is less than 500 MHz, and the conversion efficiency is very high (20% signalypump) when the low level of pump energy used is taken into account. The stability of this source makes it available for practical spectroscopic use. The continuous scanning range of the system is limited by the scanning range of the seeder and by the fact that a noncollinear phase-matching arrangement is required for prevention of unwanted double resonance. The 60-GHz tuning range is, however, large enough for CARS thermometry or spectroscopic studies over short scanning ranges. This research was supported by Direction des Recherches Etudes et Techniques contract 89-001-277. *On leave from the General Physics Institute of the Academy of Sciences of the Republic of Russia, Vavilova Street 38, Moscow 117942, Russia.

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