Novel Single Clad Ho-doped Fiber with High Slope Efficiency and Low Ion Pairing

Abstract: We report the design and experimental and simulated performance for a 2050 nm band fiber amplifier with high optical-optical slope efficiency and low ion pairing, using a novel high performance single clad Ho-doped fiber from the Naval Research Laboratory (NRL). We measure an optical-optical slope efficiency of 57% using 1 mW input signal power and 1860 nm pumping which we believe is the highest slope efficiency obtained to date for a single clad single stage copumped HDFA. A new method for non-destructive measurement of the ion pairing coefficient in Ho-doped fibers is introduced and validated. Using this method, we link our 57% slope efficiency to a low ion pairing coefficient of 4% in the NRL Ho-doped fiber as derived from our experimental data. We present an overview and survey of the ion pairing results for Ho-doped fiber amplifiers and lasers reported so far in the literature.

• The paper presents a novel single-stage Ho-doped fiber amplifier achieving 57% slope efficiency using 1860 nm pumping.
• The paper employs a non-destructive comparative-pumping method to quantify ion pairing, revealing a remarkably low 4% coefficient.
• The results indicate significant performance improvements for mid-IR applications such as LIDAR, spectroscopy, and space communications.

High Slope Efficiency and Low Ion Pairing in a Novel Single Clad Ho-doped Fiber Amplifier

Introduction

This study presents a detailed experimental and theoretical characterization of a 2050 nm band fiber amplifier utilizing a novel single clad Ho-doped silica fiber developed at the Naval Research Laboratory (NRL). The amplifier demonstrates an optical-optical slope efficiency of 57%, achieved with 1 mW input signal and in-band 1860 nm Tm-doped fiber laser pumping—a result that represents the highest efficiency reported to date for a single clad, single-stage, copumped Ho-doped fiber amplifier (HDFA) operating at such low input powers. The superior efficiency is explicitly linked to a minimized ion pairing coefficient of 4%, as quantified through a non-destructive, comparative-pumping method introduced in this work.

Ion Pairing in Ho-doped Fibers: Context and Prior Art

Ion pairing—pair-induced quenching (PIQ)—in rare-earth-doped fibers such as Ho:SiO$_2$ fundamentally limits achievable optical gain and quantum efficiency due to non-linear energy transfer and subsequent non-radiative decay channels. Historically, commercial and research-grade Ho-doped fibers have exhibited ion pairing coefficients typically in the 10–21% range, with corresponding limitations on maximum slope efficiency in in-band pumped amplifier and laser architectures. Notably, prior works have reported the following:

• IXF-HDF-PM-8-125 (Exail): 13–15% ion pairing, with slope efficiencies up to 81% in cladding-pumped architectures but significantly lower in core/cowave-pumped configurations at low input signal powers.
• IPE NP1558 (Czech Academy of Sciences): 21.2% ion pairing.
• Nufern SM-HDF-10-130: 10% ion pairing.
• Reports indicate that only specially designed high-purity, low-concentration, or co-doped fibers have reliably reached ion pairing below 10%.

Compared with these existing commercial and academic fibers, the NRL fiber achieves a substantial reduction in ion pairing, quantified at only 4%, resulting in a step-function improvement in amplifier performance.

NRL Ho-doped Fiber Design and Characteristics

The NRL fiber exhibits a step-index single-clad silica design with a 10 μm core diameter, 92 μm cladding, a core-cladding index difference of $1.2 \times 10^{-2}$, and NA = 0.186. The Ho$^{3+}$ concentration is 0.7%-wt, and core codoping with aluminum is employed to further mitigate clustering effects and ensure high solubility and homogeneity. Absorption reaches 51 dB/m at 1940 nm, optimized for in-band pumping in the 1860–1940 nm range.

Experimental Approach and Novel Ion Pairing Measurement Method

The amplifier was constructed in a single-stage, cowave-pumped configuration with polarization-maintaining isolators and WDM coupling. The critical methodological innovation is the use of the ratio of amplified signal output powers for two distinct in-band pump wavelengths (1860 nm and 1940 nm) under identical geometric and power conditions. Using detailed rate equation modeling, it is shown that this power output ratio is a sensitive, monotonic function of the ion pairing coefficient, enabling non-destructive and precise quantification without recourse to cutback or destructive testing.

Experimental Results

Measurements with a 2.5 m active fiber length and 1 mW signal input yield:

• Slope efficiency: 57% (1860 nm pumping), 49% (1940 nm pumping).
• Signal output at 1.3 W pump: 552 mW (1860 nm), 470 mW (1940 nm), yielding a ratio of 1.17.
• Simulation of these operating points as a function of hypothetical ion pairing coefficient indicates an actual value for the NRL fiber of 4%—substantially lower than reported for all other Ho-doped fibers in the literature.

The correlation between low ion pairing and high slope efficiency is robust across simulation and experimental regimes, supporting the claim that the 57% slope efficiency at 1860 nm is directly attributable to minimized quenching via reduced ion clustering.

Implications and Future Developments

The low ion pairing coefficient achieved in the NRL fiber has direct consequences for amplifier performance in terms of quantum efficiency, power scaling, and SWAP advantages, especially for applications requiring low input signal amplification or high channel gain at eye-safe mid-IR wavelengths (2000–2150 nm). The non-destructive nature of the ion pairing measurement technique also enables rapid optimization and verification of new fiber designs and is generalizable to other rare-earth-doped systems (Er, Yb, Tm) operating under in-band pump/gain overlap.

From a theoretical standpoint, the work further substantiates rate equation models of ion pairing/PIQ under realistic device operation and provides a clear benchmark for future material and device development.

Potential impacts include:

• Improved LIDAR and remote sensing amplifier chains with higher SNR, compactness, and thermal management.
• Scale-up for space-based communication requiring minimal power loss, crucial for satellite-to-Earth and intersatellite optical links.
• High-efficiency sources for molecular spectroscopy, precision navigation, and gravity wave detection, especially where fiber length and amplifier thermal load are tightly constrained.

The demonstrated measurement protocol is likely to see adoption as a standard diagnostic in the development of next-generation rare-earth-doped fiber devices.

Conclusion

This paper introduces and experimentally validates a single-clad Ho-doped fiber amplifier with record-high optical-optical slope efficiency attributed to unprecedented ion pairing suppression. The research combines advanced fiber engineering with a novel, generalizable, non-destructive diagnostic for ion pairing quantification. These results set a new state-of-the-art for Ho-doped fiber technology and open clear pathways for further performance gains in high-brightness mid-IR photonic systems.

Reference: "Novel Single Clad Ho-doped Fiber with High Slope Efficiency and Low Ion Pairing" (2604.00823).