Semiconductor lasers subject to optical feedback usually produce rich nonlinear dynamics, including periodic oscillations, quasi-periodic oscillations, and chaotic oscillations. However, quantum cascade lasers (QCLs) are highly stable against normal optical feedback, owing to the intersubband transition with ultrashort carrier lifetime and the near zero linewidth broadening factor. This work numerically shows that tilted optical feedback from a misaligned reflection mirror triggers the generation of nonlinear dynamics from QCLs, which is in good agreement with our previous experimental observations. The physical mechanism is attributed to the non-degeneration of the odd-order round-trip feedback path with the even-order ones.
Gas sensing based on modulation spectroscopy requires sinusoidal modulation of the laser sources. This work proposes a modulation scheme for quantum cascade lasers, using the period-one (P1) oscillations. The P1 oscillations are introduced by the tilted optical feedback. Although the optical linewidth of the laser is around 15.0 MHz, the beat-note electrical linewidth of the modulation is less than 2.0 kHz, which suggests that the optical sidebands induced by the P1 oscillations are highly coherent with the main optical mode. In addition, the modulation frequency can be simply tunned by adjusting the feedback length, and the modulation depth of the optical signal is in the range of 1.0 % to 3.0 %. In contrast to the direct modulation scheme and the external modulation scheme, the proposed P1 modulation method does not require any radio-frequency electronics.
This work theoretically investigates the optical noise characteristics of mutually-coupled quantum cascade lasers, which is achieved through the small-signal analysis of a set of rate equations with Langevin noise sources. It is shown that the stable locking range of the mutually-coupled lasers is on the order of several GHz. Within the stable locking range, the inphase mutual injection hardly changes the relative intensity noise of the lasers. In contrast, the frequency noise and the spectral linewidth of the coupled lasers can be reduced by about 10 dB.
The spectral linewidth of quantum cascade lasers (QCL) is a few megahertz, while the intrinsic linewidth is only a few hundred hertz. This work proposes to employ strong optical feedback to narrow the linewidth of QCLs without any phase control. It is found that strong optical feedback always reduces the linewidth for any feedback phase. In addition, we experimentally show that the spectral linewidth of the measured QCL is reduced from 7.6 MHz down to 107 kHz by optical feedback with a feedback ratio of -3.8 dB. The corresponding frequency noise below 100 kHz Fourier frequency is reduced by about 40 dB.
In this work, we theoretically investigate the relative intensity noise (RIN) properties of quantum dot (QD) lasers through a rate equation model including the Langevin noises and the contribution from the off resonance energy levels. It is shown that the carrier noise significantly enhances the RIN which can be further reduced by properly controlling the energy separation between the first excited and the ground states. In addition, simulations also unveil that the RIN of QD lasers is rather temperature independent which is of prime importance for the development of power efficient light sources. Overall, these results indicate that QD lasers are excellent candidates for the realization of ultra-low noise oscillators hence being advantageous for fiber optics communication networks, short reach optical interconnects and integrated photonics systems.
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