Broadband Tunable PM Holmium Doped Ring Fiber Laser for the 2000-2100nm Band

Introduction

High-power fiber lasers and fiber amplifiers operating in the eye-safe 2-21μm wavelength band have significant applications in various emerging fields, including LiDAR, optical communications, medical treatments, gravitational wave detection, and spectral sensing. This spectral range aligns with the emission bands of holmium-doped fiber (HDF) and thulium-doped fiber (TDF). The experimental setup consists of an all-fiber, polarization-maintaining (PM) holmium-doped tunable ring laser (HTRL), operating within the 2000-2100nm band. It is amplified by a single-stage holmium-doped fiber amplifier (HDFA) and integrated into a compact package (200 × 150 × 43 mm³) [1].

Experimental Setup

Figure 1 illustrates the complete laser system architecture, comprising the ring laser and amplifier stage.

The PM tunable fiber laser architecture consists of the HTRL and a single-stage HDFA. The HTRL is powered by an 1860nm pump fiber laser, capable of delivering up to 6.7W of output power. This pump laser is a TDF linear cavity laser with a linewidth of 0.2nm and an optical signal-to-noise ratio (OSNR) exceeding 50dB/1nm. The system employs a 1567nm Er/Yb-doped linear cavity fiber laser as the pump source, providing up to 11.5W of optical power. The pump fiber laser is split using a 30/70 passive coupler (C1) to enable backward pumping of both the ring laser and amplifier, utilizing fiber wavelength-division multiplexers (WDM1 and WDM2), respectively.

Figure 2 provides a detailed configuration of the ring laser without the amplifier stage.

Both the ring laser and amplifier utilize two meters of PM HDF (F1 and F2) with an 8μm core diameter, sourced from Exail (IXF-HDF-PM-8-125). WDM1 combines the pump and signal wavelengths to enable backward pumping of the active fiber (F1). The narrowband electro-tunable wavelength filter (ETWF) features a 3dB bandwidth of 500MHz, covering a 100nm range, with a tuning resolution of 0.1nm and a tuning speed of 2.1nm/s.

Ring Laser Performance

Figure 3 illustrates (a) the relationship between output power and pump power at 2050nm, and (b) the variation of output power with wavelength at a constant pump power.

The measured insertion losses for various devices are as follows: WDM1 exhibits losses of 1dB at 2050nm and 1.1dB at 1860nm; Iso1, Iso2, and Iso3 have losses of 0.85dB, 0.87dB, and 1dB, respectively; and the excess loss of C1 is 0.26dB. The output power increases linearly with pump power, yielding an optical-to-optical power conversion efficiency of 18% with a threshold of 0.47W. At a constant pump power of 1.41W, the output power exhibits a parabolic dependence on the set wavelength, reaching a maximum of 140mW at λset=2080nm.

Figure 4 presents (a) the output spectra at different wavelengths and (b) the OSNR measurement results.

OSNR measurements are based on two different ratios: (1) OSNRASE is the ratio of the signal wavelength output power to the peak ASE power. (2) OSNRsignal is the ratio of the output power to the ASE level at the signal wavelength. The OSNRsignal remains above 59dB over a 100nm bandwidth, while OSNRASE has a minimum value of 54dB at 2000nm, increasing to 59dB at longer wavelengths.

Amplified Signal Performance

Figure 5 shows the variation of output power with pump power at different operating wavelengths.

The output signal power demonstrates a linear dependence on pump power across all tested wavelengths, with slope efficiencies ranging from 32% to 37%. The peak output power reaches 32.3dBm (equivalent to 1.7W) at 2050nm, constrained by the available 1860nm pump power of 5.0W (6.5W before the 70/30 splitter).

Figure 6 displays the amplified spectra at different operating wavelengths.

Across the entire tunable range, the ASE contribution remains at least 40dB below the peak power. The OSNR after amplification exceeds 42dB/0.05nm, the polarization extinction ratio (PER) is greater than 19.6dB, and the power stability remains within ±1.2% over two hours.

Conclusion

The system achieves an average output power of 1.6W within the 2000-2100nm range, with an OSNR exceeding 42dB/0.05nm, a PER above 19.6dB, and power stability better than ±1.2% over two hours. The spectral linewidth is measured at 530MHz. The system's performance is primarily limited by the available pump power, and increasing pump power could further enhance performance. This tunable laser system is well-suited for testing 2μm-band devices and serving as a seed source for high-power amplifiers.

References

[1] J. A. Zepeda-Galvez, W. Walasik, R. E. Tench, A. Amavigan, and J. M. Delavaux, "Broadband tunable PM holmium doped ring fiber laser for the 2000–2100 nm band," Optical Fiber Technology, vol. 90, p. 104105, 2025, doi: 10.1016/j.yofte.2024.104105.