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

Abstract

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