Raman and Brillouin Nonlinearities in High Power Fiber Lasers: Novel Effects and Applications

Abstract

High power fiber lasers due to their immense utility in both industry and research divisions, have seen a dramatic level of power scaling in the last two decades. This is due to the superior beam quality, thermal handling and compact nature inherently offered by all-fiber based systems as compared to other forms of laser systems. As of now, the rare earth doped Ytterbium (Yb), Erbium (Er) and Thulium/Holmium (Th-Hm) fiber lasers have shown remarkable levels of power with Yb outperforming the rest by order of magnitude levels. However as the power in the tightly confined fiber modes scale up, fiber nonlinearity sets into action. Nonlinear effects such as self-phase modulation, modulation instability (MI), supercontinuum (SC) generation, etc alters the spectral and temporal properties of lasers. Similarly nonlinear effects based on photon-phonon interactions, i.e. Raman and Brillouin scattering as well as thermal Rayleigh scattering also hinders or enhances the features of a laser depending upon the application. More recently stimulated Raman scattering (SRS) has evolved to be the only known power scalable technology for generating new wavelength bands, otherwise nonexistent in the rare earth platform. Stimulated Brillouin scattering (SBS) and stimulated thermal Rayleigh scattering (STRS) on the other hand, have been the primary bottleneck in power scaling of narrow linewidth high power lasers. This thesis consisting of two main parts dealing with some new applications and novel effects of SRS and SBS. In the first, we exploit the properties of SRS in silica fiber medium to generate highly spectrally flat SC lasers and subsequently explore their utility in different frontiers. In the second, we demonstrate a widely linewidth tunable high power fiber amplifier which generates an SBS limited kW class narrow linewidth fiber laser showing a previously unseen unique visible light generation phenomena.

Raman fiber lasers (RFL) aided by SRS have very recently been established as the only power scalable route to filling in the gaps between Yb, Er and Th emission windows. Similarly spectrally broad lasers, SC lasers too allow a path to spectrally populate these otherwise inaccessible gaps, though not yet known to be power scalable. We identified the shortcomings in bandwidth, conversion efficiency, power scalability and flatness in the existing SC modules and propose utilizing Raman fiber lasers to simultaneously mitigate all of them. Thus we demonstrated a CW RFL with 24 W power (highest at the time of publishing) tunable across the L-band and subsequently generated a 700 nm broad, 35 W SC with ~40% conversion efficiency. The SC had a remarkable flatness of ~5 dB across at least 400 nm which is highest ever reported so far in all fiber and CW format.

Flat SC generation was followed by exploring the spectral and temporal properties of such lasers. Femtosecond pulse shaping technology enables one to generate light sources with user defined amplitude, phase and polarization in the ultrafast regime. However due to power handling limitations of the components, this technology lacks a bridge with the modern high power fiber lasers. We propose and demonstrate a scalable design for a high power Fourier shaper capable of handling 20 W of CW laser power with simultaneously covering over 450 nm bandwidth between 1-1.5 micron band. Our design implements several modifications from conventional designs to conform with the demands of high power fiber laser technology. We believe that the ability to shape high power SC or RFL sources will potentially aid controlled Raman conversion to increase spectral purity and many more applications. Further, in the experiments concerning the temporal properties of SCs, we proposed a new method of characterizing high speed photodetectors in a spectrally resolved manner. This is enabled by the stochastic pulse nature of CW SCs while simultaneously having broad optical bandwidth. Our method also gives an insight to how the RF source spectra of CW SC varies across the optical bandwidth as well as its evolution from a single wavelength laser through SRS, MI regime etc. In the second part of the thesis, we demonstrate a high power tunable narrow linewidth laser source at 1064 nm. This was achieved by stepwise amplification of a 1064 nm polarization maintained (PM) DBR (Distributed Bragg reflector) laser, externally phase modulated by a power and bandwidth tunable noise source. This resulted in a tunable linewidth, SBS limited at 10 GHz linewidth laser delivering more than 500 W CW power. This laser operating in the NIR (1064 nm), showed some surprising visible light flashes at a fusion splice point when operated in the SBS regime. Subsequent experimental analysis showed that at the onset of SBS, the backward propagating SBS pulses in the core of the fiber also undergo SRS. This generates multiple Stoke orders of 1064 nm and they eventually undergo Cherenkov type phase matching to the second harmonics of the respective Stoke orders in the cladding of the fiber. Thus any part of the laser with the cladding exposed dissipated the visible light. The analysis was also validated by numerical simulations carried out conforming to the fiber parameters. We believe any laser system with high enough power irrespective of the temporal dynamics could possibly trigger this effect.