High-power fiber lasers have attracted much research interest and been widely employed in tremendous fields for its compact structure, high conversion efficiency and robust operation. However, quantum defect (QD), which is one of the important heat sources in the gain fiber, could result in serious thermal effects (e.g. thermal lens effect, thermal mode instability), additional noise, and even operation security threating of high-power laser system. In hence, continuous efforts have been focused on reducing the QD. Raman fiber laser (RFL) has shown outstanding performance on thermal management and been rapidly developed and widely investigated in recent years. Generally, the QD of RFL is around 5%, which is half of common laser diode (LD)-pumped Yb-doped fiber laser. Employing tens-meters or even km-level passive fiber, RFL can effectively relieve the density of the thermal deposition. Besides, the characteristic of non-photon-darkening enables RFL to maintain stable operation. Accordingly, RFL is a promising alternative on both high-power and low-QD fiber lasers. Up to hundreds watt-level high-power RFLs in all-fiber format have been demonstrated by several independent groups. By adopting a main oscillator power amplifier structure, up to 3 kW RFL has been demonstrated. Usually, the high-power fiber lasers aforementioned have QDs of about 4~5% based on the 13.2 THz frequency shift. To be noted, challenge in power scaling of RFL has also been revealed and observed recently, where further decreasing the QD of RFL is one of the promising solutions for power scaling.
Moreover, it has been reported that, there is a gain peak with <4 THz frequency shift called boson peak in phosphorus-doped fiber (PDF), which may enable RFL to further reduce the QD. Recently, up to hundred-watt level core-pumped RFL with less than 1% QD has been demonstrated. The results have indicated that RFL gained by the boson peak of PDF has the ability to realize low-QD output. Nevertheless, it is difficult to enhance the pump density for power scaling in conventional core-pumped RFL due to the limitations of power handling capacity and small mode area of the core in a single-mode fiber. One of the solutions to this challenge is cladding pumping. The pump light is guided in the cladding of the fiber, which can significantly release the requirement on the pump brightness. Since the first demonstration of cladding-pumped Raman regime, there has been growing research interest, and significant advances on high-power RFL have been successfully achieved. Whereas on the thermal management, the QDs of cladding-pumped RFL reported so far were almost >4%, and the realization of further reduced QD has been dilatory owing to weak gain intensity with tiny frequency shift of passive fiber and low pump intensity in the fiber cladding, which follows the intense gain competition and eliminates the short signal wavelength. As the QD of a cladding-pumped RFL could be further decreased, it is a promising solution for high-power fiber laser because it combines the advantages of RFL, cladding pumping and low heat production.
The work presented by Xiaoya Ma, Jiangming Xu, Jun Ye, Pu Zhou et al. from National University of Defense Technology proposed and demonstrated a cladding-pumped RFL with QD <1%. The research results are published in High Power Laser Science and Engineering, Vol. 10, Issue 2 (Xiaoya Ma, Jiangming Xu, Jun Ye, Yang Zhang, Liangjin Huang, Tianfu Yao, Jinyong Leng, Zhiyong Pan, Pu Zhou. Cladding-pumped Raman fiber laser with 0.78% quantum defect enabled by phosphorus-doped fiber[J]. High Power Laser Science and Engineering, 2022, 10(2): 020000e8).
Figure Caption: Structure and output characteristics of the low-QD RFL. (a) Schematic of the RFL; (b) evolutions of signal power with different QD
The cladding-pumped RFL demonstrated in this work presents a QD of less than 1 % and a maximal output power of dozens watts. Firstly, the contrast experiment indicated that cladding-pumped RFL had higher maximal signal power and larger residual pump power in comparing with core-pumped RFL. Furthermore, by utilizing the Raman gain of boson peak in a phosphorus-doped fiber to enable the cladding pump, the QD is reduced to as low as 0.78% with a 23.7 W output power. Additionally, the output power of low QD RFL can be scaled to 47.7 W with a QD of 1.29%.
To our knowledge, this is the lowest QD ever reported in cladding-pumped RFL. In future work, properties investigation and performances scalability of high power low QD RFL will be explored. This work not only offers a preliminary platform for the realization of high-power low-QD fiber laser, but also proves low-QD fiber laser's great potential in power scaling.
