Fiber Lasers & Amplifiers
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High power fiber lasers and amplifiers have been extensively studied in the past few decades and a phenomenal increase in the peak (MW) as well as average power levels (kW) have been demonstrated. However, the power scaling of such fiber lasers is limited by several factors including fiber non-linear effects, occurrence of self pulsing and thermal mode instability. Our research is focused on developing high power CW and pulsed fiber laser sources. Master Oscillator Power Amplifier (MOPA) is usually a reliable and scalable approach, where a low power highly stable laser source (CW or pulsed) output is amplified using multiple amplifier stages to achieve the desired power levels. Q-switch fiber lasers is used to generate high power peak pulses by modulating the loss of laser cavity. Mode lock fiber laser is used to generate ultra-short (ps) pulsed by phase locking the longitudinal modes of the cavity.
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Reference:
Y. Panbiharwala, AV Harish, D. Venkitesh, J. Nilsson, B. Srinivasan, Investigation of temporal dynamics due to stimulated Brillouin scattering using statistical correlation in a narrow-linewidth CW high power fiber amplifier, Optics Exp. 26, 33409-417 (2018)
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Y. Panbiharwala, D. Venkitesh, and B. Srinivasan,"Mitigation of Self-Pulsing in High Power Pulsed Fiber Lasers", Invited Article, Kiran (2014)
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Thermal mode Instability
The large-mode area (LMA) double-clad fibers are used to increase the surface area of the fiber, thereby increase the threshold for non-linear effects such as SBS and SRS. However, the LMA fibers have been reported to exhibit an instability in the transverse modes of the fiber beyond a certain threshold average power, where the power from fundamental mode starts transferring from FM to HOM and vise-versa. The instability or exchange in power among the transverse modes does not reflect as a power fluctuation at the output. However it severly degrades the beam quality. The present work is towards the design of a multi-stage MOPA with a high average power to achieve power levels at which the thermal mode instability threshold have been reported before. The design of the high power amplifier is presented which includes four amplification stages where the parameters at each stage is optimised using RP-Fiber Power software such that the output of the final stage is > 500W.
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Reference:
A. Ghosh, Y. Panbiharwala, D. Venkitesh, A. Prabhakar, B. Srinivasan, "Design and Development of an Experimental Testbed for the Study of Thermal Mode Instability in High-Power Fiber Lasers", National Laser Symposium, Bhubaneswar (2016).
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Q-switched lasers
High energy pulses with short durations find applications in material processing, laser based ranging and distributed sensing. Q-switching is a commonly used technique to generate such laser pulses by introducing a suitable loss-modulator in the laser cavity. The goals of this research work include the development of a complete simulation model of Q-switched fiber ring laser, experimental implementation of Q-switched laser in 1550 nm and 1064 nm wavelength range, and validation of simulation model against experimental results.
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Reference:
Manas Srivastava, Deepa Venkitesh, and Balaji Srinivasan, "Effects of Wavelength Filtering on Pulse Dynamics in a Tunable, Actively Q-Switched Fiber Laser", Optics and Laser Technology, Vol. 98, 2018, pp 190-197.
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Mode-locked lasers
Mode-locking technique is used to generate ultra-short pulses (ps-fs regime) at higher repetition rates (>MHz) for applications in communications. Mode-locking is realized by modulating the loss of the laser cavity at a repetition rate equal to the cavity round-trip frequency, thus phase-locking the longitudinal modes of the cavity. When a phase relationship is established between the modes, the output of the cavity is a pulse train of short pulse-widths and the spectrum of the output is a comb of frequencies.
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Reference:
A.Bekal, K.Vijayan, B.Srinivasan, Characterization of regenerative stabilized actively mode-locked fiber laser incorporating a saturated amplifier in feed-back chain, Optics & Laser Technology 67, 98-106 (2015)
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A. Bekal and B.Srinivasan, Numerical simulation of a dispersion-managed active harmonically mode-locked fiber laser using a spectral double-grid technique,JOSA B 30, 1373-1381 (2013)
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