Feedback-induced line broadening in cw channel-substrate planar laser diodes

Abstract

The effect of optical feedback on the spectral characteristics of channel-substrate planar single-mode laser diodes operating at room temperature is reported. The impact on the performance of interferometric sensor systems using such sources is discussed. The linewidth for the free-running laser at 10-mW output power was determined to be less than 5 MHz at room temperature. Broadening on the order of 40 times the intrinsic linewidth was observed for 0.1% feedback and increased with increasing feedback. The presence of self-oscillation modes was observed at 0.04% feedback. Satellite modes symmetrically located with respect to the primary mode appeared in the spectrum for feedback greater than 0.04%. These satellite modes are attributed to self-oscillation in the laser induced by feedback. As the feedback was increased, the satellite mode spectrum began to overlap that of the primary mode, reducing the effective coherence length from 60 m (for the single-mode linewidth) to less than a few centimeters.

Bistability and low-frequency fluctuations in semiconductor lasers with optical feedback: a theoretical analysis

“…From (4) we find where the total photon number I ( r ) is expressed by the integrated field power spectrum 7 …”

“…For a laser diode coupled to a single-mode fiber the Rayleigh backscatter-induced line narrowing can result in linewidths in the subhertz region [6]. Higher feedback levels may, however, result in laser line broadening to a width of several GHz [4], [7]- [lo]. Also, a kink in the light-current characteristic is observed near the threshold of the solitary laser [lo]- [13].…”

“…From a level (approximately) given by the feedback the intensity shows a randomly occurring sudden drop to a low value followed by a stepwise buildup to the original level. .The steplength is equal to the roundtrip time 7 in the external cavity, and the buildup requires about 10 steps. In this paper we present a theoretical investigation of these phenomena related to higher feedback levels (greater than -30 dB).…”

<title>Low-frequency intensity fluctuations in external cavity semiconductor lasers</title>

“…where #c = Kext/Td = rext(l _ 1r212)/rdr2), and dq5/dt = (0 -) + (vga/2)(g -at) -,cJ[S(t-rext)/S(t)Jsin(wrext + 6), (18) with c5 = q! (t) -q!(trext).…”

Self-mixing type of phase-locked laser diode interferometer

“…It has also been applied to linewidth reduction 5 and broadening. 6 Self-mixing can be easily realized by feeding back the laser beam from the external reflector to the LD. The LD incorporates a photodiode ͑PD͒ for monitoring optical power.…”