Abstract

Phase-shifted fiber Bragg gratings are intrinsically ultra-narrow bandpass filters. If inserted inside the cavity of a fiber ring laser, a phase-shifted grating behaves as a selector of the longitudinal modes, narrowing the natural linewidth of the laser, which can work even in single-longitudinal-mode regime, as Chen et al. demonstrated recently [1]. Compared to other schemes to fabricate ultra-narrow linewidth lasers (DFB or DBR), such a ring configuration permits longer cavities, hence higher gains with low doping concentrations, preventing self-pulsation. In this paper, a technique to change the grating phase-shift dinamically is incorporated to the ring laser scheme of [1] (where the phase-shift is permanent). This increases the versatility of the laser significantly, since it can work either in a wavelength-tunable cw regime or in a Q-switched pulsed regime, adding pulsed light-based applications (as OTDR, or microprocessing of materials) to the list of potential uses of this type of narrow linewidth laser source. The use of chirped FBGs permits to overcome the limitations imposed by the narrow spectrum of uniform FBGs used in [1]. First, unwanted laser oscillation at high pumping levels due to sidelobes of the external grating is avoided thanks to the wide stop-band of CFBG2 (FWHM = 0.89 nm, reflectance = 99.9 %). Consequently, higher pumping powers can be used and around 30 dB higher output power can be obtained. Second, the bandwidth of the external grating (CFBG1, FWHM = 0.27 nm, reflectance = 46 %) avoids the need of tuning its central wavelength to compensate environmental temperature changes.

© 2007 IEEE