Abstract
Cascaded Raman fiber lasers based on random distributed feedback (RDFB) are proven to be wavelength agile, enabling high powers outside rare-earth doped emission windows. In these systems, by simply adjusting the input pump power and wavelength, high-power lasers can be achieved at any wavelength within the transmission window of optical fibers. However, there are two primary limitations associated with these systems, which in turn limits further power scaling and applicability. Firstly, the degree of wavelength conversion or spectral purity (percentage of output power in the desired wavelength band) that can be achieved is limited. This is attributed to intensity noise transfer of input pump source to Raman Stokes orders, which causes incomplete power transfer reducing the spectral purity. Secondly, the output power range over which the high degree of wavelength conversion is maintained is limited. This is due to unwanted Raman conversion to the next Stokes order with increasing power. Here, we demonstrate a high-power, cascaded Raman fiber laser with near complete wavelength conversion over a wide wavelength and power range. We achieve this by culmination of two recent developments in this field. We utilize our recently proposed filtered feedback mechanism to terminate Raman conversion at arbitrary wavelengths, and we use the recently demonstrated technique (by J Dong and associates) of low-intensity noise pump sources (Fiber ASE sources) to achieve high-purity Raman conversion. Pump-limited output powers >34W and wavelength conversions >97% (highest till date) were achieved over a broad - 1.1μm to 1.5μm tuning range. In addition, high spectral purity (>90%) was maintained over a broad output power range (>15%), indicating the robustness of this laser against input power variations.
Highlights
High-power fiber lasers have gained significant attention over the last decade for industrial, defense and medical applications
Intensity noise properties of Fiber Bragg gratings (FBGs) based lasers was compared with fiber based amplified spontaneous emission (ASE) sources and having found them more temporally stable, were used as pump source and a spectral purity of >90% has been achieved up to 8th order Raman Stokes of 1691.6nm. the maximum power achieved in this system is ~7W at 1691.6nm
In-band power ratios of >97% were achieved for all the Stokes wavelengths till 1480nm when ASE pump combined with filtering mechanism was used
Summary
High-power fiber lasers have gained significant attention over the last decade for industrial, defense and medical applications. Cascaded Raman fiber lasers based on Random distributed feedback (RDFB) have been proposed which offer grating-free solution for wavelength conversion [6,7] In these systems, by adjusting the input pump power and wavelength, high-power, ultra-widely tunable lasers have been demonstrated [8,9]. The reason attributed to the limited output power range over which high spectral purity can be maintained is the unwanted Raman conversion to the higher order Stokes signal, with increasing power. We demonstrated a novel filtered feedback mechanism to terminate cascaded Raman conversion at any arbitrary wavelength band enabling a high-power, ultra-widely tunable, cascaded Raman fiber laser [16]. Pumplimited output powers of >34W and wavelength conversions >97% were achieved over broad output power variations and is tunable over the entire 1.1μm to 1.5μm wavelength range
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