Enabling Spectral Diversity in Fiber Laser Systems: Architectures and Applications
Nonlinear effects occur due to the modification of optical properties of a material medium by the presence of high intensity light such as lasers, resulting in the generation of new frequency components. The availability of low-loss silica fibers has not only led to a revolution in the field of optical communications but also the beginning of a new field of nonlinear fiber optics. Optical fibers have become the nonlinear medium of choice as they can confine high power light over long lengths and provide efficient thermal management. Additionally, fiber laser systems are rugged and compact due to efficient fusion splicing technology and the absence of external optics for alignment. Different applications require fiber lasers having different operating wavelengths, bandwidth, coherence, and/or power. While lasers are typically described as monochromatic or single-frequency sources, it is possible to create lasers with diverse spectral features. Broadband sources such as supercontinuum (SC) or optical frequency comb (OFC) can be generated using nonlinear effects. Depending on the regime of operation, various nonlinear effects become predominant. For example, in case of continuous-wave (cw) anomalously pumped SC, modulation instability (MI) seeded spectral broadening along with the combined effects of Four-Wave Mixing (FWM) and Stimulated Raman Scattering (SRS) results in a broad spectrum. For OFCs, bandwidth scaling is done using FWM. In narrow linewidth systems, Stimulated Brillouin Scattering (SBS) manifests as a threshold phenomenon limiting power scaling. Here, the intensity of scattered light grows exponentially when the incident light exceeds a certain value of threshold power. It was identified that in the aforementioned cases, poor spectral shape and control imposed limitations on system performance and use. In this thesis, we propose simple and easy-to-implement solutions to overcome these shortcomings. Thereby, opening up avenues for these diverse (broadband to narrowband) sources to be used in a wide range of applications. High power cw pumped SC sources have large bandwidth, high average power, and equalized spectra. At low powers, these sources exhibit poor spectral flatness and high sensitivity to the position of pump-wavelength with respect to the zero-dispersion of the nonlinear fiber. Obtaining a pump wavelength agile SC at lower, easily manageable optical powers is attractive for many applications such as imaging, spectroscopy, characterization, test and measurement. But current techniques require the use of complex system design with multiple pump lasers and/or significantly longer lengths of fibers. In this work, a two-stage broadening architecture (to increase pump incoherence) is proposed to improve the spectral flatness and successfully demonstrate for the first time, a cw pump wavelength flexible, low power (80 mW to 550 mW) SC with spectra spanning from 1300 nm to 2000 nm and a 10 dB bandwidth of over 400 nm. The resulting SC generation module is self-standing and uncorrelated with the specifications of the C-band pump source. This offers excellent practical benefits and makes for an ideal source for broadband low power applications. In addition to large bandwidth, OFCs have high temporal coherence and are widely used in Radio frequency (RF) photonics, optical arbitrary waveform generation, ultra-short pulse generation and optical communications. Direct bandwidth scaling of high-repetition-rate electro-optic (EO) frequency combs is limited by the power handling capability of modulators used. Significant bandwidth can be achieved by using nonlinear spectral broadening (in fibers) of a single laser comb, but conventional techniques produce an OFC with uneven spectra and hence limited flatness. Current techniques to improve the OFC profile require the use of complex architectures or optimization algorithms. We identified that by using tailored optical feedback to filter out an optimal set of comb lines, enhanced FWM and power redistribution is achieved, resulting in a smooth, bandwidth scaled OFC. The optimal result demonstrated is a 25 GHz frequency comb spanning 2.5 THz bandwidth in the C-band with ~0.5 W power and 100 usable sub-carriers (i.e., a ten-fold increment compared to the initial EO comb). The effective linewidth of an equalized OFC is greater than its discrete narrow linewidth sub-carriers. Since the enhancement in SBS threshold depends largely on the spectral width, SBS is suppressed in bandwidth scaled systems such as OFCs but becomes a limiting factor in power scaling of narrow linewidth systems. High power narrow linewidth fiber lasers have garnered significant interest over the last decade for their widespread use in industrial, scientific, and directed energy applications. SBS is the lowest threshold nonlinear effect in narrow linewidth cw fiber lasers and occurs as high peak power pulses propagating in the backward direction. These pulses cause catastrophic damage to the system and must be mitigated. While several SBS mitigation techniques exist, optical linewidth broadening through external phase modulation using white noise source (WNS) has become the preferred method for SBS suppression due to its simplicity and continuous tuning of linewidth through noise power and bandwidth control. But it was observed that the enhancement obtained experimentally with WNS was lower than the theoretical estimate. We attribute this to the non-ideal line shape of the WNS broadened laser where the signal tail overlaps with Brillouin gain spectrum causing increased SBS seeding. We anticipated that with better line shape control, the SBS seeding would be greatly minimized resulting in higher output powers. In this work, we synthesize a line shape with faster roll-off and improved flatness by incorporating dual sinusoid and noise modulation (DSNM). The effectiveness of the proposed technique is demonstrated experimentally using an in-house built multi-stage Ytterbium (Yb) doped fiber amplifier by optimizing the pumping scheme, gain fiber lengths, splices, and thermal management techniques to enable power scaling of a 30 mW narrow linewidth seed at 1064 nm to over 1 kW. We compare the results with that of pure WNS modulation at similar linewidths and demonstrate over 2.36X enhancement in SBS limited output power at ~7.3 GHz and >1 kW SBS unlimited output power at ~10.4 GHz in a fully polarization-maintained system. This makes for an excellent source in beam combining applications and remote detection.