Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-27-7-9725
Timestamp: 2019-04-20 04:57:59+00:00

Document:
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.
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Fig. 1 Experimental setup of high-power cascaded random Raman fiber laser with near complete wavelength conversion.
Fig. 2 (a) In-band power ratio vs wavelength for the different architectures. (b) In-band power vs wavelength in different architectures.
Fig. 3 Output spectrum at all the Stokes orders (left column: Red and Pink – with filtering, right column: Blue and Cyan – without filtering) of ASE source pumped CRFL (a); FBG based source pumped CRFL (b).
Fig. 4 In-band ratio evolution of all Raman Stokes signals with respect to output power for ASE and FBG based pump cases combined with and without filtering. DFB – distributed feedback.
Fig. 5 (a) Ratio of the width of output power range with spectral purity >90% to the mean of this range for all the Raman Stokes orders with ASE and FBG pumping with and without filtering; (b)Linewidths of different Raman Stokes orders at maximum spectral purity for ASE pump combined with and without filtering.

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