Spectral purity enhancement in cascaded random Raman fiber lasers using unidirectional ring-cavity pumps: insights from RF-resolved relative intensity noise

We demonstrate that conventional multi-longitudinal-mode unidirectional ring-cavity ytterbium fiber lasers enable high spectral purity (>91%) up to the seventh Stokes order in cascaded random Raman fiber lasers (CRRFLs). This is comparable to that of CRRFLs pumped with filtered amplified spontaneous emission (ASE) Yb sources reported in the literature but with significantly reduced system complexity. To systematically understand this behavior, we experimentally study spectral purity and radio-frequency (RF) - resolved relative intensity noise (RIN) in CRRFLs pumped by three widely used conventional ytterbium-doped fiber laser architectures: a standalone fiber Bragg grating (FBG) - based bidirectional linear-cavity laser, a master oscillator power amplifier (MOPA) seeded with an FBG, and a unidirectional ring-cavity seed lasers. All pump sources were operated to provide comparable output power and optical bandwidth (∼2 nm), enabling a direct and fair comparison of their impact on CRRFL performance. Spectral purity and RIN evolution are analyzed up to the seventh Raman Stokes order. CRRFLs pumped by the unidirectional ring-cavity laser achieve spectral purity exceeding 91% across all Stokes orders, compared to ∼78% for FBG-based systems. This improvement persists even when the cavity free spectral ranges (FSRs) are matched (∼5 MHz), indicating that cavity mode spacing alone cannot explain the enhancement. Instead, the improved performance is strongly correlated with the lower high-frequency pump RIN of the unidirectional ring-cavity architecture. RF-resolved analysis across three frequency bands (10 kHz-160 kHz, 160 kHz-2 MHz, and 2 MHz-1 GHz) reveals a characteristic U-shaped dependence of RMS RIN on output power, with minimum noise occurring near the maximum spectral purity operating point. Small deviations in output power can lead to significant variations in RIN and consequently spectral purity. While all pump architectures exhibit comparable low-frequency RIN (∼0.04%), substantial differences emerge above ∼160 kHz. In this regime, FBG-based systems exhibit significantly higher RIN, which is efficiently transferred and amplified along the Raman cascade, leading to reduced spectral purity. In contrast, ring-cavity-pumped CRRFLs suppress high-frequency intensity fluctuations and maintain low RIN across all Stokes orders, even in the presence of longitudinal modes. These results establish that spectral purity in CRRFLs is predominantly governed by pump noise above ∼160 kHz, rather than being determined solely by longitudinal mode beating, and that this high-frequency regime is inherently difficult to suppress using conventional feedback-based RIN control techniques due to bandwidth limitations. Overall, this work demonstrates that unidirectional ring-cavity ytterbium fiber lasers provide a simple and cost-effective alternative to complex ASE-based pump architectures for achieving low-RIN, high-spectral-purity CRRFL operation.