|Over the last decade, fiber lasers have gained significant traction for industrial, defense and medical applications. This is owing to their superior beam quality along with power scalability, compactness, reliability and robustness. The current fiber laser technology is based on rare-earth elements such as Ytterbium (Yb), Erbium (Er), Thulium (Tm), Holmium (Ho) etc. which are doped inside the core of an optical fiber which in turn acts as a gain medium. However, current fiber laser technology suffers from serious drawbacks such as limited wavelength coverage and power scalability. Wavelength coverage is limited to the emission spectra of different rare-earth dopants and power scalability is limited to only Yb emission window. For example, tens of Kilo Watts of continuous wave (CW) output power has already been demonstrated at 1μm wavelength region. This a rather serious limitation as there are numerous applications like LIDAR, Free space optical communications etc. which require high power fiber lasers outside the rare-earth emission bandwidths.
Among the several nonlinear frequency conversion techniques for extending the spectral coverage, stimulated Raman scattering (SRS) is proven to be more versatile as it doesn’t require any phase matching conditions to be satisfied. The nonlinear process of SRS converts a shorter wavelength high power pump laser to a long wavelength Stokes laser. This process can be cascaded to longer and longer wavelengths and such systems are called cascaded Raman fiber lasers. Currently, cascaded Raman fiber lasers is the only power scalable technology which offers wavelength diversity while spanning the spectrum.
However, conventional cascaded Raman fiber lasers are limited in terms of efficiency and reliability which stopped them from further power scaling. Even though enhanced efficiency and reliability has been demonstrated, but only at the cost of increased complexity of overall system.
In the 1st part of this thesis, I will be talking on the efforts that I have made in making cascaded Raman fiber lasers highly efficient and reliable with as simple architecture as possible. In this work, a new, all-passive architecture for high-efficiency cascaded Raman conversion is demonstrated. This architecture has been tested out with a fifth-order cascaded Raman converter from 1117nm to 1480nm with output power of ~64W and efficiency of 60%.
Even though the above system demonstrates the wavelength agility of cascaded Raman fiber lasers, the same system can’t be used if the input pump wavelength is changed. This is because of the use of fixed wavelength fiber Bragg grating sets (RIG and ROG) for wavelength conversion. These grating sets decide both output signal wavelength and the input pump wavelength thereby limiting the wavelength flexibility of these systems. However, Raman gain based on stimulated Raman scattering is available at any arbitrary wavelength inside the optical fiber. Therefore, one can always use a tunable-wavelength pump laser and achieve wavelength tunability at any arbitrary wavelength band with these systems. It necessitates to have tunable-wavelength pump fiber laser and wavelength independent (broadband) feedback mechanism. Hence, motived by the wavelength tunability of cascaded Raman fiber lasers, I have built a high power tunable wavelength Yb-doped fiber laser which has additional property of independent tuning of linewidth and output power. Independent tuning of wavelength (from 1050nm to 1100nm), linewidth (from 0.2nm to 1.4nm) and power (up to 130W) has been achieved in continuous-wave Ytterbium-doped fiber laser. This has been achieved with the help of simple architecture based on master oscillator power amplifier (MOPA) configuration. This system addresses the conventional linewidth vs power coupling relation of fiber laser by demonstrating independent tuning of linewidth and wavelength at a constant output power.
The above high-power, tunable wavelength, Yb-doped fiber lasers is the basic building block (pump source) for our advanced cascaded Raman fiber lasers. By utilizing this pump source, a simple architecture for high-power, fixed, and wavelength tunable, grating-free, cascaded Raman conversion between different wavelength bands has been achieved. The architecture is based on the recently proposed random distributed feedback Raman fiber lasers. Here, a module which converts the ytterbium band to the eye-safe 1.5μm region has been implemented. Pump-limited output powers of over 30W in fixed and continuously wavelength tunable configurations have been achieved.
Cascaded Raman fiber lasers requires termination of Raman cascade for power scaling in the desired wavelength band. In the previous system, termination of cascaded Raman conversion at 1.5μm wavelength band thereby achieving high power wavelength tuning over a bandwidth of one Raman Stokes (1440-1520nm) has been achieved. However, because of the use of specialty Raman filter fiber, termination of cascaded Raman conversion at any arbitrary wavelength is not possible. This is important to achieve power scaling at any arbitrary wavelength within the transmission window of optical fiber.
In final part, I demonstrate such a cascaded Raman fiber laser with both ultra-wide wavelength tunability and power scaling. Here, a novel filtered distributed feedback mechanism has been used to terminate the Raman cascade at desired wavelength and hence naturally achieve power scaling. Output powers of up to 28W has been achieved with >400nm tuning from 1118nm-1535nm, bridging the Ytterbium, Erbium emission bands.