Aperiodically poled lithium niobate compresses and frequency-doubles ultrafast pulses
A monolithic device based on aperiodically poled lithium niobate (LiNbO3) simultaneously compresses and frequency-doubles ultrafast pulses. Stanford University (CA) researchers and others from IMRA America (Ann Arbor, MI) poled a sample of LiNbO3 such that it acts as a chirped grating for quasi-phase-matching (QPM). The device creates an effect analogous to group-velocity dispersion caused by the interaction of group-velocity mismatch between the fundamental and second-harmonic pulse dispersion--intrinsic to the nonlinear material--and the spatial localization of certain frequency components in the second-harmonic conversion caused by the QPM grating. Duration of the fundamental frequency pulse remains unchanged, but the second-harmonic pulses are either stretched or compressed.
Local QPM periods in the LiNbO3 sample varied linearly from 18.2 to 19.8 µm. In CLEO `97 paper #CPD6, the researchers describe generation of nearly transform-limited 110-fs pulses at 780 nm from the 17-fs, 1560-nm output of an externally chirped, erbium-doped fiber laser. With an all-fiber chirped-pulse-amplification system, the device produced u¥to 70 mW of 780-nm output from 200 mW of 1560-nm input, thus demonstrating that high conversion efficiencies and powers can be obtained with chirped-QPM pulse compressors.