U-Shaped Relative Intensity Noise Characteristics of High Spectral Purity Higher Order Cascaded Random Raman Fiber Laser

Cascaded Raman fiber lasers based on random distributed feedback (RDFB) governed by Rayleigh backscattering (RB) in optical fibers are called as cascaded random Raman fiber lasers (CRRFLs). They are proven to be simple, wavelength agile and power scalable systems that are used in many applications [1]. Increasing the spectral purity (or in-band power percentage), defined as the percentage of output power in the desired Stokes wavelength band, has been the subject of recent interest in these systems [2]–[4]. It was shown that lower the relative intensity noise (RIN) of the pump source used for cascaded Raman conversion, higher the spectral purity of the CRRFLs [4]. However, the spectral purity of CRRFLs can be increased by just using (at the output of CRRFLs) a simple spectral band-pass filter centered at the desired Stokes wavelength band. But no research is available on how the RIN properties of such CRRFLs would vary. Here, we experimentally measured the RIN of spectrally band-pass filtered CRRFL, up to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$7^{\text{th}}$</tex> Stokes orders (1580nm), pumped with l064nm laser. Our results show that each Stokes order has a U-shaped RIN variation where, the minimum RIN (bottom of U) is achieved only when the spectral purity (with no spectral band-pass filter at the output of CRRFL) is maximum.