Broadband supercontinuum generation in specialty fibers

• Main Author: Yemineni, Sivasankara Rao
• Other Authors: Arokiaswami Alphones
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/89263; http://hdl.handle.net/10220/48038
• Institution: Nanyang Technological University

Broadband supercontinuum (SC) generation is of great scientific and technical interest, and emerged as one of the best broadband light sources in a range of applications including optical communication, precision spectroscopy, optical coherence tomography (OCT), optical frequency metrology, hyperspectral imaging, light detection and ranging (LIDAR) and, chemical and remote sensing. Hence the broadband SC generation is of strong motivation. Supercontinuum (SC), is a process of generating broadband optical spectrum by launching optical pulses into a nonlinear medium, continuous interaction of laser pulses with nonlinear optical medium leads to the emission of a wider optical spectrum with a bandwidth many times greater than the bandwidth of the launched pulses. The key nonlinear physical phenomena involved in this processes are stimulated Raman scattering (SRS), modulation instability (MI), cross-phase modulation (XPM), self-phase modulation (SPM), four-wave mixing (FWM) and soliton related dynamics. The SC generation compromises of two parts: a seed laser and a nonlinear medium. The seed laser can generate femtosecond (short) pulses, picosecond or nanosecond (long) pulses or even continuous-wave (CW) laser. The nonlinear media such as normal single-mode-fibre (SMF) or specialty fibres such as highly nonlinear fibre (HNLF), highly nonlinear photonic crystal fibre (PCF), tapered fibres and soft-glass fibres like ZBLAN (ZrF4-BaF2-LaF3-AlF3-NaF). In this project, the carbon-nanotube (CNT) based saturable absorber (SA) is used for the realization of passively mode-locked femtosecond erbium-doped fibre laser (EDFL) as a seed to the specialty fibres, to generate the broadband SC generation and also to study the different aspects of spectral broadening phenomena inside each specialty fibre. Therefore, this thesis focuses on the generation of CNT-SA-based passively mode-locked femtosecond EDFL and generation of broadband SC spectrum from specialty fibre. The CNT-SA based passively mode-locked EDFL has achieved a pulse width of 620 fs with a pulse repetition rate of 18 MHz at a center wavelength of 1565 nm with a 3-dB bandwidth of 5 nm. The output of the mode-locked laser pulse has pulse peak power of 18.5 W and average power of 0.21 mW respectively at the stable mode-locked condition. Throughout this project, the CNT-SA based passively mode-locked femtosecond EDFL is used as seed laser to broadband SC spectrum from all the specialty fibres. For efficient activation of nonlinear phenomena inside each specialty fibre and, also to study the variation of spectral broadening with respect to the variation in input pulse power, the pulse power is further boosted through the amplification using a pulsed erbium-doped fibre amplifier (EDFA). The spectral broadening inside a 60-meter-long PCF with respect to the variation in input pulse power is observed. The input power to the PCF is varied from 0 dBm to 20 dBm and achieved a maximum SC spectrum bandwidth of 1080 nm from the output of PCF spanning from 1090 nm to 2170 nm. In the next step, the spectral broadening inside the HNLF is studied with respect to the variation in input pulse power from 0 dBm to 20 dBm and obtained a maximum SC spectrum bandwidth of 1400 nm covering from 1060 nm to 2460 nm. In addition to input pulse power variation, here the length of HNLF is also varied and studied the effect of the length variation on spectral broadening and SC spectrum bandwidth. The effect of tapering also observed by applying tapering ratio of two on a short length of HNLF. In order to overcome the limitations of silica-based fibres and to extend the SC spectrum further towards mid-infrared (mid-IR) side, soft-glass ZBLAN fibre is considered. The input pulse power to a 25-meter-long ZBLAN fibre is varied from 0 dBm to 25 dBm. A maximum SC bandwidth of 2100 nm spectrum extending from 1100 nm to 3200 nm is observed from the output of ZBLAN fibre.