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Lasers for Optical Interconnects
Published in Lukas Chrostowski, Krzysztof Iniewski, High-Speed Photonics Interconnects, 2017
Γgth is the threshold optical gain of the mode and αi. is the average internal loss in the cavity, as determined by the waveguide loss and other losses in the cavity other than the losses due to light escaping through the mirrors. r1 and r2 are the electric field reflectivities of the front and back mirror (which can be replaced by different coupling loss coefficients for ring lasers or other structures). Note that the terms gth and αi are in terms of optical power and should be divided by 2 in the earlier electric field equation, but since a full round trip in the cavity includes two passes through the gain material, another factor of 2 in front of the length cancels this. This equation shows that the lasing threshold occurs when the gain in the laser becomes equal to all of the loss contributions such as light escaping through the mirrors or scattering in the waveguide. The previous equation can also be expressed with the following equation: () Γgth=αi+αm
Introduction to lasers and optical amplifiers
Published in John P. Dakin, Robert G. W. Brown, Handbook of Optoelectronics, 2017
William S. Wong, Chien-Jen Chen, Yan Sun
Since light is circulating inside the cavity, the phase of the reflected light, after a complete round trip, needs to match that of the original light so that a constructive interference or a standing wave can form. This condition is satisfied only at certain frequencies and these resonant frequencies are referred to as the longitudinal (or axial) modes of the laser cavity. For a cavity with a fixed length, the longitudinal modes are separated by a constant frequency that is equal to the inverse of the cavity round trip time. Within the gain bandwidth of the gain medium of a laser, there may exist multiple longitudinal modes. The lasing threshold is reached when the gain medium is pumped, in which the total round-trip gain of the mode (closest to the gain peak) equals the total round-trip loss. When the threshold is exceeded, the laser oscillation starts in this axial mode. Depending on the property of the gain medium, when the pump power increases, a homogeneously broadened gain medium, which maintains the same gain shape when saturated, can sustain one stable lasing axial mode only, while an inhomogeneous broadened medium can have multiple modes coexisting in a laser.
Primer on Photonics
Published in Paul R. Prucnal, Bhavin J. Shastri, Malvin Carl Teich, Neuromorphic Photonics, 2017
Paul R. Prucnal, Bhavin J. Shastri, Malvin Carl Teich
The steady state behavior of G and I are shown in Fig. 3.16. There is a key change in behavior where the transcritical bifurcation occurs. This is called the lasing threshold, which is an important quantity for every laser. Below threshold, no optical intensity builds up through stimulated emission (although spontaneous emission was neglected in this analysis). The pump energy proportionally energizes the gain medium. Above threshold, the gain carrier concentration locks to its threshold value, and increases in pump power are 100% converted into optical intensity. This analysis was a basic starting point to enter into the rich area of laser dynamics, which will be a central topic of Chapters 5 and 6.
Dependence of random lasing properties on the gain volume in dye doped nematic liquid crystals
Published in Liquid Crystals, 2021
Fengfeng Yao, Yanbo Pei, Chunfeng Hou, Xiudong Sun
Besides, the reason that Fabry-Perot like lasing mode was easily obtained in low threshold value in this study is discussed preliminarily. We speculate that the orientational order parameter of nematic LCs is one of the crucial parameters determining which lasing mode has lower lasing threshold. The reason is that the reflections from the LC-substrate interfaces provides feedbacks for the Fabry-Perot like lasing mode and light scattering induced by the nematic director fluctuations provides feedbacks for the random lasing mode. And if the order parameter is small, i.e. the orientation of the rod-like LC molecules is far from uniformity, the significant orientation fluctuation leads to strong scattering and destructs the Fabry-Perot cavity formed by the two LC-substrate interfaces. For example, in the article [7], the order parameter of the nematic LC was 0.2 obtained by measuring polarised fluorescence, resulting in random lasing mode. That is, in this case, random lasing mode has lower lasing threshold. In our work the order parameter was about 0.6 also obtained by measuring polarised fluorescence. Larger value of order parameter means less spatial fluctuation of the molecule orientation, leading to less light scattering. As a result, the feedback from interfaces reflections plays predominant role and the Fabry-Perot like lasing mode has lower lasing threshold.