Wavelength tuning and spectral properties of distributed feedback diode lasers with a short external optical cavity

We report on the wavelength tuning and spectral properties of distributed f eedback (DFB) diode lasers operated with a plane external cavity (XC) mirro r positioned as close as possible to the diode-laser front facet. These las ers generate single-frequency near IR radiation at wavelengths of 1392, 158 0, 1602, and 1653 nm. A piezoelectric variation of the XC length provided c ontinuous single-frequency tuning to as high as 19 GHz. A further benefit o f XC DFB lasers is a residual amplitude modulation per gigahertz tuning of less than 10(-3). The XC feedback also suppresses residual side-mode oscill ations to less than 60 dB. The laser's total intensity noise is close to th e shot noise limit. The laser linewidth (measured in a beat note experiment ) is less than 90 kHz within an acquisition time of 40 ms. The advantageous properties of XC DFB lasers for molecular spectroscopy are demonstrated by recording R(3) 2 nu (3) overtone spectra of methane by single-scan single- pass absorption or frequency-modulation spectroscopy. (C) 2001 Optical Soci ety of America.