Spectral narrowing effect by quasi-phase continuous tuning in high-speed wavelength-swept light source

This paper reports on a technique to improve the coherence length of a high-speed wavelength swept laser. The wavelength swept laser comprises a pigtailed semiconductor optical amplifier and a wavelength-scanning filter in a fiber extended cavity configuration. The laser operates in the 1310 nm wavelength region. The tunable filter consists of a diffraction grating and polygon mirror scanner. Littrow arrangement of external cavity in a specific geometry realizes the quasi-phase continuous tuning over wavelength range emphasizing coherent amplification of cavity modes resulting in spectral narrowing of the instantaneous linewidth to about 0.06 nm. Improvement by a factor of two is confirmed in comparison with coherence length without using this technique. Peak power is 12 mW and wavelength swept range is 55 nm, from 1271 nm to 1326 nm. Measured coherence lengths of over 30 mm and 17 mm were achieved at scanning rates of 2.5 kHz and 20 kHz, respectively. Correlation of laser cavity parameters with spectral linewidth is also discussed by introducing the rate equations for multi-mode laser operation. Shorter cavity length is considered effective to further improve the coherence length in terms of shorter roundtrip time as well as higher mode suppression ratio because of higher carrier concentration on cavity modes around the filter center.