Single-frequency fiber lasers operating in the 1 μm wavelength regime have become pivotal for scientific and industrial applications, including gravitational wave detection, coherent lidar system, and optical atomic clock, where low intensity noise and high output power are paramount. Traditional pumping schemes for Yb-doped fiber amplifiers (YDFAs) relying on 976 nm laser diodes (LDs) suffer from inherent limitations, including large quantum defect causing excessive thermal loading in the gain fiber and pumping-induced intensity fluctuations degrading the amplified laser stability. These limitations hinder their applicability in precision interferometry and frequency metrology, where even minor intensity fluctuations will propagate into systematic errors. Recent studies have explored tandem pumping strategy to mitigate these issues. By employing a pumping wavelength closer to the signal wavelength (e.g., 1018 nm pumping for 1064 nm amplification), quantum defect is minimized, and thermal deposition and excessive amplified spontaneous emission (ASE) are reduced. However, the relative intensity noise (RIN) characteristics of such tandem-pumped YDFAs, especially under high-power operation, are scarcely investigated. This study aims to analyze the RIN transmission and suppression mechanisms in a single-frequency YDFA at 1064 nm with maximum output power of 4 W, which is tandem-pumped by a single-frequency high-power laser at 1018 nm. Furthermore, in the same YDFA system, we explore the RIN performance under tandem pumping against that under conventional 976 nm LD pumping to quantify the RIN suppression in the frequency band of 10 Hz?10 MHz. This work provides a feasible method for designing low-RIN, high-power fiber amplifiers suitable for quantum technology and precision optical systems.
The single-frequency 1064 nm fiber laser consists of a semiconductor distributed feedback (DFB) laser at 1064 nm with output power of 23 mW and a one-stage YDFA with gain fiber length of 4 m. The tandem pumping laser at 1018 nm is formed with a single-frequency semiconductor DFB seed laser and two-stage YDFAs. For the second YDFA at 1018 nm, a 27 W 976 nm LD is used as the pumping source and the active fiber is a 1.5 m long Yb-doped double-cladding fiber with core and cladding diameters of 15 μm and 130 μm. The maximum output power of the single-frequency 1018 nm laser reaches about 10 W. Two band-pass fiber filters at 1018 nm with bandwidth of 2 nm are spliced with the two ends of the first YDFA to remove the ASE generated from the seed and the first amplifier. The 1018 nm tandem pumping laser is coupled into the 1064 nm YDFA through a 1018/1064 nm wavelength division multiplexer (WDM). For comparison, an LD at 976 nm with maximum power of 27 W is employed as conventional pumping scheme for the 1064 nm YDFA. Both the 1018 nm tandem pumping and the 976 nm LD pumping can amplify the 1064 nm seed laser to 4 W power. The RIN measurements are conducted using a photodetector (150 MHz bandwidth) and a spectrum analyzer (26.5 GHz bandwidth).
At the output power of 4 W of the 1064 nm YDFA, the RIN level of the 1064 nm laser pumped using a single-frequency 1018 nm laser is below -125 dBc/Hz in the full range of 200 Hz?10 MHz, and the RIN level of the tandem-pumped YDFA output is reduced by 15 dB compared to that pumped using the 976 nm LD in the frequency range from 100 Hz to 100 kHz [Fig. 6(a)]. Notably, in the frequency range of 200 Hz?50 kHz, the RIN level of the tandem-pumped laser is even lower than that of the seed laser. The better performance of the RIN suppression of the tandem pumping scheme is attributed to the high spectral purity and the low thermal loss. The spectra of the 1064 nm seed laser, the amplified 1064 nm laser pumped by 976 nm LD, and the amplified 1064 nm laser pumped by the 1018 nm single-frequency laser are investigated and shown in Fig. 6(b). The result reveals that the spectral proportion of the ASE is 0.31% for tandem-pumping and 0.58% for 976 nm LD pumping.
This study validates the effectiveness of tandem pumping for RIN suppression in single-frequency YDFAs. By utilizing a single frequency pumping laser at 1018 nm, the ASE and quantum defect are reduced in the 1064 nm YDFA, enabling a 15 dB RIN reduction in the frequency band of 100 Hz?100 kHz. While low-frequency (<100 Hz) noise remains terrible due to the excessive thermal and ambient noises, this passive RIN reduce strategy using tandem pumping offers a cost-effective alternative to complex active stabilization systems.