Consequences of Quantum Noise Control for the Relaxation Resonance Frequency and Phase Noise in Heterogeneous Silicon/III-V Lasers

Abstract We have recently introduced a new semiconductor laser design which is based on an extreme, 99%, reduction of the laser mode absorption losses. This was achieved by a laser mode design which confines the great majority of the modal energy (> 99%) in a low-loss Silicon guiding layer rather than in highly-doped, thus lossy, III-V p+ and n+ layers, which is the case with traditional III-V lasers. The resulting reduced electron-field interaction leads directly to a commensurate reduction of the spontaneous emission rate by the excited conduction band electrons into the laser mode and thus to a reduction of the frequency noise spectral density of the laser field often characterized by the Schawlow-Townes linewidth. In this paper, we demonstrate theoretically and present experimental evidence of yet another major beneficial consequence of the new laser design: a near total elimination of the contribution of amplitude-phase coupling (the Henry α parameter) to the frequency noise at “high” frequencies. This is due to an order of magnitude lowering of the relaxation resonance frequency of the laser. The practical elimination of this coupling enables yet another order of magnitude reduction of the frequency noise at high frequencies, resulting in a quantum-limited frequency noise spectral density of 130 Hz2/Hz (linewidth of 0.4 kHz) for frequencies beyond 680 MHz. This development is of key importance in the drive to semiconductor lasers with higher coherence, particularly in the context of integrated photonics with a small laser footprint.