Quasiphasematched frequency doubling in a waveguide of a 1560 nm diode laser and locking to the rubidium D absorption lines

An external-cavity 1560-nm diode laser was frequency doubled in a 3-cm-long periodically poled LiNbO 3 waveguide doubler with 120% W 21 conversion efficiency. The 780-nm light was used to detect the D 2 transitions of Rb, and the laser frequency was locked to Doppler-broadened lines of Rb. Furthermore, the ϳ1 mW of second-harmonic power was sufficient for detecting the sub-Doppler lines of Rb, and the laser was locked to a 87 Rb crossover line. © 1996 Optical Society of America Lasers operating at f ixed and known frequencies near the 1550-nm transmission window of optical fibers are required for densely packed multiwavelength communication systems. 1 Such lasers may also be required for coherent optical communication systems to ease the acquisition and locking of a local oscillator laser to a transmitter laser and for achieving coldstart communication. 2 In addition, absolutely stabilized sources may be applicable to fiber-optic sensors and as frequency standards for high-resolution spectroscopy. Optical frequency standards can be realized by locking to atomic or molecular transitions. Molecular absorptions in the 1550-nm wavelength range, e.g., ammonia, 3 acetylene, 4,5 and hydrogen iodide, 6 are usually weak overtone or combination bands. Lasers at 1550 nm were locked to Doppler-broadened transitions of these molecules. 5 Atomic transitions that can be used as frequency references, e.g., transitions between excited states in noble gases (Ar, Kr, etc.) 2 and transitions between upper levels in Rb, 7 do not originate from the ground state. Hence additional excitation, electrical (with a discharge lamp 2 ) or optical (with another laser 7 ), is required for populating one of these upper levels. An alternative approach that may overcome the difficulties associated with frequency references near 1550 nm is second-harmonic generation (SHG) and locking to absorption lines near 780 nm. A thoroughly characterized reference at 780.25 nm is the atomicRb D 2 line. 8 This reference was already used to stabilize 1560-nm laser diodes with the internally generated second harmonic of diode lasers, 9 but the SHG power was only 2 pW. Recently bulk external SHG in KNbO 3 crystal with a second-harmonic power of 2.2 nW was employed for the same goal. 10 Locking to a Doppler-broadened line was possible, but the power level was not sufficient to saturate the absorption for locking to sub-Doppler lines. Frequency doubling in KNbO 3 was also used to lock to K at 770 nm, 11 with a second-harmonic power of 20 nW. Because the power levels of diode lasers near 1550 nm are quite low (typically a few milliwatts), higher-eff iciency frequency conversion is required for detection and locking to sub-Doppler lines as well as to improve the signal-to-noise ratio for locking to Dopplerbroadened lines. A technique that may achieve this goal is quasi-phase-matched 12 (QPM) frequency conversion in a waveguide. In QPM doubling, a periodic modulation of the material nonlinear coefficient compensates for the phase velocity mismatch between the fundamental and the second-harmonic waves. This technique permits the use of large nonlinear coefficients, e.g., d 33 , in LiNbO 3 that are not accessible by birefringent phase matching. In LiNbO 3 the improvement in conversion eff iciency compared with birefringent phase matching is ͑2d 33 ͞pd 31 ͒ 2 ϳ 20, where 2͞p is the QPM reduction factor and d 31 is the effective nonlinear coeff icient for birefringent phase matching. Further improvement in conversion efficiency is obtained by waveguide confinement. Furthermore, room-temperature operation, as well as relaxed temperature and wavelength tolerances, is possible. The use of QPM waveguides for optical frequency standards at the 1300-nm fiber-optic transmission window has already been demonstrated 13 : the second harmonic of a 1319-nm Nd:YAG laser was locked to I 2 transitions near 660 nm. We applied the technique of waveguide QPM frequency conversion for efficient single-pass doubling of a 1560-nm external-cavity diode laser. The second-harmonic power was sufficiently high that we could detect sub-Doppler lines, and the laser was locked to Doppler-broadened lines as well as to subDoppler lines of Rb near 780 nm. The experimental setup for locking to Doppler-broadened lines of Rb is shown i

Broadly tunable mid- infrared femtosecond optical parametric oscillator using all-solid-state-pumped periodically poled lithium niobate

We describe a high-repetition-rate femtosecond optical parametric oscillator (OPO) that was broadly tunable in the mid infrared. The all-solid-state-pumped OPO was based on quasi-phase matching in periodically poled lithium niobate. The idler was tunable from approximately 1.7 mm to beyond 5.4 mm, with maximum average power levels greater than 200 mW and more than 20 mW of average power at 5.4 mm. We used interferometric autocorrelation to characterize the mid-infrared idler pulses, which typically had durations of 125 fs. This OPO had a pumping threshold as low as 65 mW of average pump power, a maximum conversion efficiency of .35% into the near-infrared signal, a slope efficiency for the signal of approximately 60%, and a maximum pump depletion of more than 85%. © 1997 Optical Society of America Mid-IR femtosecond pulses have potential applications for the study of dynamics in a variety of materials. Vibrational relaxations in molecules, intersubband transitions in quantum wells, and materials for use in lasers and detectors operating in the 3-5-mm atmospheric transmission window can all be studied effectively with ultrashort mid-IR pulses. In this Letter we report on the generation of mid-IR pulses with an all-solid-state-pumped high-repetition-rate femtosecond (fs) optical parametric oscillator (OPO). The most-common techniques for the generation of high-repetition-rate mid-IR fs pulses are differencefrequency mixing and the use of a synchronously pumped OPO. Difference frequency of the signal and the idler output from Ti:sapphire-pumped OPO's produced fs pulses of between 2.5 and 5.5 mm (Ref. 1) but with maximum average power levels of only 500 mW . Mid-IR synchronously pumped fs OPO's based on KTiOPO 4 and its isomorphs 2 -5 were demonstrated, but they were limited to wavelengths shorter than ϳ4.1 mm. Average power levels greater than 20 mW at 5.2 mm were obtained with KNbO 3 . 6 The all-solid-state-pumped broadly tunable mid-IR fs OPO described in this Letter is based on quasi-phase matching 7 (QPM) in periodically poled lithium niobate (PPLN) and produced more than 20 mW of average power at 5.4 mm. The experimental arrangement was similar to that described in Ref. 8, except that a ring cavity was used instead of a linear cavity. We used a pumping geometry with a small noncollinear angle between the pump and the signal so that the long-wavelength idler beam did not have to be transmitted through any of the cavity optics and optical isolation of the pump laser was unnecessary. The PPLN OPO was synchronously pumped at a repetition rate of 81 MHz by a mode-locked fs Ti:sapphire laser powered by a diodepumped cw frequency-doubled Nd : YVO 4 laser (SpectraPhysics Millennia). The Ti:sapphire laser produced nearly transform-limited pulses of ϳ 90-fs duration, with average power levels of as much as 850 mW over the 790-815-nm wavelength range used in this experiment. We used two optics sets and two PPLN crystals, which were antiref lection coated on both sides with single layers of SiO 2 , in the OPO to cover the tuning range. In both cases the signal was resonated in a ring cavity consisting of two 15-cm radius-of-curvature mirrors, a f lat high ref lector, a f lat output coupler, and a four-prism (SF-14) sequence for dispersion compensation. Several different output couplers were used, with transmission varying from ϳ1% to ϳ9% over the tuning range of the OPO. In a noncollinear fs OPO both temporal walk-off owing to the group-velocity mismatch (GVM) of the interacting pulses and spatial walk-off owing to the noncollinearity of the interacting beams can limit the effective interaction length in the crystal. Near degeneracy 9 the GVM between the pump pulses and signal and idler pulses was of the order of 300 fs͞mm (signal-and-idler-leading pump), 10 resulting in a temporal walk-off length 11 of the order of 330 mm for ϳ100-fs pulse widths. Far from degeneracy, the GVM between pump and signal was reduced to as little as ϳ110 fs͞mm, but the GVM between signal and idler became as large as ϳ260 fs͞mm (signal-leading idler), resulting in a temporal walk-off length of ϳ900 mm between the pump and the signal and ϳ 380 mm between the signal and the idler. For the focusing parameters (ϳ35-mm signal and pump waist radius) and the noncollinear angle (varied from ϳ1.0 ± to ϳ1.6 ± between pump and signal measured internal to the crystal) used in this experiment, near degeneracy the spatial walk-off length of ϳ 550 mm between signal and idler was much larger than the temporal walk-off length and, therefore, did not limit the interaction length. When we were trying to reach long idler wavelengths, however, the increasingly large angle between signal and idler (as large as 9 ± measure