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We introduce small-signal modelling on both relative intensity noise (RIN) and frequency noise (FN) as well as the associated linewidth of the semiconductor laser. Influence of gain suppression on the frequency characteristics of the RIN and FN spectra and the corresponding damping rate and frequency of relaxation oscillations are elucidated. Also, we investigate the dependence of laser noise on the mathematical form of nonlinear gain using three common formulas of optical gain over a wide range of injection current. We show variations of the levels of both RIN and FN with the form of optical gain in the regime of resonance frequency as a result of variation of damping rate and resonance frequency. The tolerances in predicting the RIN and FN levels are 1% and 14%, respectively, which corresponds to the tolerance of linewidth of 21.6% at low frequency of 10 MHz and current of five times its threshold value, while they are 39% and 21.6% at the relaxation frequency. [ABSTRACT FROM AUTHOR]

The capability of optical reference cavities to resist ambient thermal fluctuations is significant in ultrastable laser systems especially applying in transportable system and space station. To reduce the deterioration of long-term laser frequency stability induced by temperature variation, the thermal response of cavities is investigated. The simulation and analysis indicate that the use of multiple enclosing thermal shields with a low emissivity allows a larger thermal time constant. The quantitative functional relationships between the thermal time constant and the heat capacity and surface emissivity of the passive thermal shields are firstly obtained using FEA simulation. Based on these results, a temperature-insensitive cavity system is designed with a thermal time constant of approximately 680 h. According to the designed system, two different methods are used to evaluate the thermal performance. And mathematical expressions for the cavity temperature over time are obtained. The calculated thermal time constant is validated by the simulated results. The thermal sensitivity of the designed system is 4.1 × 10−11 and 1.3 × 10−5 when temperature variation period is 1 h and 24 h, respectively. The fractional length instability of the cavities induced by temperature fluctuation is 8.2 × 10−16 over an average time of 1 day, which can compete theoretically with fountain clocks. [ABSTRACT FROM AUTHOR]

The present paper evidences the existence of an analytical equation expressing the different physical parameters of a passively Q-switched laser. This equation is obtained by studying the laser linear stability and allows computing the saturable absorbents concentration. This concentration optimizes the laser peak power. The analytical equation is validated by comparing the obtained results with that of the numerical simulation as well as that obtained by other authors. A good agreement has been observed in these comparisons. [ABSTRACT FROM AUTHOR]

A phenomenon of modal instability is investigated theoretically in some high power solid-state lasers with an unstable cavity. This modal instability is caused by a thermo-optic-thermo feedback coupled with an asymmetrical laser oscillation inside some unstable cavities. Numerical investigations indicate that this instability can appear in all lasers with Nd 3+ -doped disk scheme, Yb 3+ -doped disk scheme and direct-liquid-cooled slab scheme, but the physical mechanisms of optic-thermo feedback in the three schemes are completely different. It is shown that the modal instability can appear within a special power range only in the Nd- or Yb-doped disk lasers due to certain saturation phenomenon, but exhibits a power threshold in the direct-liquid-cooled slab lasers. Therefore different strategies to mitigate the modal instability in different lasers are suggested. Finally, the presence of transverse modal instability in the Nd:YAG thin-disk laser is confirmed qualitatively by an experiment. [ABSTRACT FROM AUTHOR]