High-power diode lasers (HDL’s) are used for optical pumping and material processing due to their wavelength selectability and high electrical-to-optical efficiency. However, owing to their spectrally and spatially erratic emission, 9xx-band broad area lasers have spectral widths from 2-6 nm at peak operating power and spectra that drift 0.3-0.4 nm per Kelvin. These characteristics can become problematic when strong interactions with a material’s narrow absorption peak are desired (e.g. pumping ytterbium-doped fiber lasers). Selective laser feedback is well known to be able to modify the spectral behavior of a laser; for HDL’s, two such prominent techniques are the inclusion of a distributed Bragg reflector on the diode or using a volume Bragg reflector in an external cavity configuration. While a distributed Bragg reflector that is monolithically integrated with the diode does not need to be aligned to narrow an HDL’s spectral output, such devices still suffer from wavelength instability due to temperature and, therefore, operating power. Alternatively, volume Bragg gratings are more robust to temperature changes but must be carefully aligned and tend to have scattering losses that reduce the overall output power efficiency.
Researchers have invented an apparatus including a high-power broad-area semiconductor laser source configured to emit light in a pattern extending along the emission axis, and a single-mode core—incorporating a Bragg grating (FBG) and embedded in a core of a dual-clad fiber—configured to spectrally selectively reflect back light from a sub-aperture portion of the emitted light to the HDL source. The single-mode core having the FBG is off-axis in comparison to the central axis of the double-clad fiber and allows for frequency stabilization of the broad area laser diode output, improving its performance as, for example, a pump laser for a doped fiber amplifier.
The coupling between the single-mode FBG cores and the laser(s) results in >10x enhanced spectral purity (narrowed emission bandwidth), which is highly desirable for pumping high-power fiber lasers. It also results in spectral selectivity, enhanced spectral stability with respect to changes in temperature, improved laser efficiency, and even improved beam quality of the laser output under certain circumstances. The invention can be implemented in a package compatible with current manufacturing techniques and could be a significant enhancement for the ~1 billion dollar/year high-power fiber laser industry.