Observation of spectral power exchange in fiber Bragg grating stabilized 980 nm pump lasers

External wavelength stabilization of high power 980 nm pump lasers is observed to cause an exchange of power between amplified spontaneous emission (ASE) and stimulated emission (SE) over the operating range of the device. This effect is difficult to detect in the fiber coupled power versus drive current (L-I) characteristic because the variations in ASE and SE tend to compensate one another, resulting in a nearly linear relationship between output power and drive current. This phenomenon becomes evident when the output is separated spectrally into ASE and SE components, and is also evident in the rear facet monitor signal. The exchange between ASE and SE is attributed to an alteration of the gain/feedback relationship caused by a shift of the laser diode cavity modes relative to the Bragg grating wavelength as the drive current is changed.