Abstract
We have recently introduced a new semiconductor laser design which is based on an extreme, 99%, reduction of the laser mode absorption losses. In previous reports, we showed that 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 was shown to lead 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 alpha 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. Here, we show that 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 Hz^2/Hz (linewidth of 0.4 kHz) for frequencies beyond the relaxation resonance frequency 680 MHz. This development is of key importance in the development of semiconductor lasers with higher coherence, particularly in the context of integrated photonics with a small laser footprint without requiring any sort of external cavity.
Highlights
The semiconductor laser (SCL) has become and, very likely, will continue to be, in the foreseeable future, the linchpin of optoelectronics[1,2,3,4,5]
The high-coherence Silicon/III–V laser is based on a Silicon waveguide to which a III–V epitaxially grown stack is bonded separated by a thin SiO2 layer (Fig. 2a)
We showed that the strategy of quantum noise control, i.e., control of the spontaneous emission rate, can decrease the relaxation resonance frequency fR of semiconductor lasers to a few hundred MHz, resulting in the suppression of the phase noise induced by the amplitude-phase coupling and reduction in the frequency noise PSD by an additional order of magnitude
Summary
The semiconductor laser (SCL) has become and, very likely, will continue to be, in the foreseeable future, the linchpin of optoelectronics[1,2,3,4,5]. A few major obstacles, remain before the promise of CMOS-like Photonic Integrated Circuits (PICs) can be realized Chief among these problems is: low-coherence, the dependence on external isolators to reduce optical feedback, and the coherence-reducing amplitude-phase coupling. We provide theoretical arguments and experimental evidence that the reduced interaction of the excited electrons in the SCL with the quantum-mandated zero-point field of the laser mode in our laser design leads, to a practical elimination of the amplitude-phase coupling at frequencies above that the relaxation resonance[10,11]. We take an ab-initio look at the relationships between some of the key attributes of the semiconductor laser; the current modulation response, phase-amplitude coupling, relaxation resonance, and the frequency noise (or phase noise). We start with the set of coupled equations for the number of photons nl in the lasing mode of the laser resonator and for the number of inverted electrons ne in the active regions (Equation 15.5-113), dne dt
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