28 February 2009 Light-matter interaction processes behind intracavity mode-locking devices
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Non-interacting Boson-like properties of light beams imply that superposed light beams by themselves cannot re-organize or re-distribute their energies either in spatial or in the time domain. Yet, we explain short pulse generation by lasers with intra-cavity devices as due to phase locking of the longitudinal modes of the laser cavity, irrespective of whether the lasing material has homogeneously or in-homogeneously broadened spectral characteristics. Most short pulse generating "mode locked" lasers use homogeneously broadened gain media that always tend to run in a single frequency at the gain line center that has the highest gain under CW condition. Can a passive intra-cavity saturable absorber or a Kerr medium switch the spectral characteristics of a homogeneously broadened gain medium into an in-homogeneously broadened gain medium to make the laser run in multiple longitudinal modes? We believe that the lasing medium runs in a single mode (frequency) at the center of the gain medium and the intra-cavity saturable absorber or the Kerr medium simply plays the role of fast time gating (switching). This implies that "transform limited" "mode-locked" laser pulses, in reality, contain only a single carrier frequency. We will present the appropriate mathematical representation for the spectral analysis of such "mode-locked" pulses. We will also discuss models for the physical process that give rise to the generation of short (nanosecond class) pulses even in the absence of multiple longitudinal modes and then use the concepts for generating shorter (picosecond and femtosecond) pulses.
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Chandrasekhar Roychoudhuri, Chandrasekhar Roychoudhuri, Narasimha S. Prasad, Narasimha S. Prasad, } "Light-matter interaction processes behind intracavity mode-locking devices", Proc. SPIE 7193, Solid State Lasers XVIII: Technology and Devices, 71931Q (28 February 2009); doi: 10.1117/12.814868; https://doi.org/10.1117/12.814868

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