Advanced Architectures for Cascaded Raman Fiber Lasers

Abstract

Fiber lasers have exhibited significant expansion in their diverse applications within the fields of communications, industrial operations, defence, and the medical sector. They require a rare earth doped element as their gain medium to absorb light and re-emit a coherent, intense laser beam. Although the fiber lasers are used for high-power amplifiers ranging from a few watts to kilowatts, the wavelength coverage is limited by the emission spectra of the rare-earth elements. The limitation of wavelength spanning in fiber lasers is overcome by cascaded Raman fiber lasers (CRFLs) are used. It utilizes stimulated Raman scattering (SRS) to produce multiple Raman Stokes orders, thereby generating wavelengths outside the emission spectrum band of rare-earth doped fiber lasers. The CRFL technology has been demonstrated to be suitable for developing high-power, scalable lasers with wavelength agility due to its versatile and compact fiber-based configuration. Despite all these advantages, the wavelength tunability of conventional CRFLs is constrained by the fixed wavelength of input/output highly reflective fiber Bragg gratings (FBGs). Hence, randomly distributed feedback (RDFB) on the CRFLs platform could improve the controllability of wavelength and enable the system to be more adaptable to different wavelengths. This configuration allows for efficient energy transfer and enables broad spectral coverage with high output power. However, there are some limitations to this, such as getting an efficiently desired wavelength spectrum, wavelength tunability reduced spectral purity, and line broadening of the output wavelengths. This thesis explores advanced architectures for CRFLs to overcome the tunability and spectral purity limitations. Previously, using RDFB Raman lasers, wavelength tunability was achieved using a tunable pump laser module. This module enhances the system complexity and increases the overall cost. Here, we proposed a system configuration to address these limitations by allowing the tunability of the output wavelengths using a fixed wavelength pump source. The proposed architecture also enables the tuning of the linewidth of the laser. Our proposed architecture incorporates a reflective Fourier spectral/pulse shaper as an advanced feedback mechanism. The shaper achieved a spectral resolution of 0.5 nm, and loss through the spectral shaper is less than 10 dB for a range of wavelengths from 1100 nm to 1250 nm. This configuration enables filtering out unwanted higher-order Raman Stokes lines, enhancing power in the desired output wavelength. The desired wavelength tunability and linewidth tunability are achieved by using the desired spatial mask patterns at the Fourier plane of the spectral shaper. We demonstrated a CRFL with wavelength tunability over three Raman Stokes orders with spectral purity of > 90% and in-band power of ~10 W. Further, the proposed architecture achieves linewidth tuning over an order of magnitude from ~0.5 nm to > 4 nm. Spectral purity, the power ratio in the desired output wavelength to the power in all other wavelengths, is a key performance measure for laser sources, and many applications require only a single wavelength with high spectral purity. The next part of the thesis proposes methods to enhance the spectral purity of Raman lasers. By analyzing the reasons for spectral purity degradation, we proposed a new architecture that achieves highly spectrally pure random distributed feedback CRFLs over six orders of Raman shifts. The proposed architecture used a narrow linewidth source as a seed with less intensity noise (-147 dBc/Hz from 9 kHz to 10 GHz). The seed is line-broadened by dual-phase modulation, both white noise source and sinusoid using phase modulators, and then amplified with Ytterbium amplifiers, and it is used for Raman conversion. At the output of the Raman fiber laser, we achieved up to 23 W power, tunable from the pump wavelength (1064 nm) all the way to 1480 nm. This approach yields high spectral purity, ~ 99%, over the entire range of Raman conversion. The last part of the thesis explores the measurement and analysis of relative intensity noise (RIN) in Raman fiber lasers. A Raman fiber laser's RIN refers to the fluctuations in the laser output power relative to the average power. Pump RIN influences RIN in CRFLs. We measured the RIN of the pump Raman Stokes orders for a narrow linewidth pumped Raman fiber laser and compared them with the conventional FBG-based pumped Raman fiber laser. The maximum RIN measured for a narrow linewidth pump is less, -128 dBc/Hz compared to a conventional fiber laser of -98 dBc/Hz. There is a 36 dB reduction in low-frequency RIN for phase modulated narrow linewidth pumped Raman Stokes compared to conventional pumped Raman fiber lasers. However, the high-frequency RIN (beyond a few GHz) is the same for both. In addition to the goal of reduction in intensity noise of Raman lasers, an added goal of the intensity noise studies is to investigate methods to create low linewidth Raman lasers with the goal of efficient harmonic conversion to the visible or mid-infrared regions. We investigated whether low-intensity noise Raman lasers have reduced linewidth due to lower self-phase modulation type line-broadening. However, this was not found to be the case. There are primarily two factors that can cause line-broadening; the first one is the inherently broad Raman gain spectrum, and the second is the nonlinear spectral broadening due to the intensity noise. In our case, though the low-frequency (< 1 GHz) RIN has been reduced, SPM effects would still be relevant with high-frequency RIN, which is the same as before. Further, linewidth broadening could arise from broadband spontaneous Raman scattering. In the future work section of the thesis, using the insights developed from these studies, we propose a new configuration which simultaneously uses the narrow linewidth feedback together with low intensity noise pumping to develop high-performance narrow linewidth CRFLs.