张扬, 何俊鸿, 柯延钊, 郭艺东, 梁峻锐, 马小雅, 叶俊, 许将明, 冷进勇, 周朴. 大频移范围光纤拉曼增益谱的测量[J]. 红外与激光工程, 2024, 53(5): 20240041. DOI: 10.3788/IRLA20240041
引用本文: 张扬, 何俊鸿, 柯延钊, 郭艺东, 梁峻锐, 马小雅, 叶俊, 许将明, 冷进勇, 周朴. 大频移范围光纤拉曼增益谱的测量[J]. 红外与激光工程, 2024, 53(5): 20240041. DOI: 10.3788/IRLA20240041
Zhang Yang, He Junhong, Ke Yanzhao, Guo Yidong, Liang Junrui, Ma Xiaoya, Ye Jun, Xu Jiangming, Leng Jinyong, Zhou Pu. Fiber Raman gain spectrum measurement over broad frequency shift range[J]. Infrared and Laser Engineering, 2024, 53(5): 20240041. DOI: 10.3788/IRLA20240041
Citation: Zhang Yang, He Junhong, Ke Yanzhao, Guo Yidong, Liang Junrui, Ma Xiaoya, Ye Jun, Xu Jiangming, Leng Jinyong, Zhou Pu. Fiber Raman gain spectrum measurement over broad frequency shift range[J]. Infrared and Laser Engineering, 2024, 53(5): 20240041. DOI: 10.3788/IRLA20240041

大频移范围光纤拉曼增益谱的测量

Fiber Raman gain spectrum measurement over broad frequency shift range

  • 摘要: 光纤拉曼增益谱的测量对光纤激光器的设计与优化至关重要。当前光纤拉曼增益谱的测量主要是基于小信号增益法,但该方法测试时间较长,且可测量的频移范围受限于信号源的波长调谐范围,难以获得光纤在大频移范围内的拉曼增益谱。为此,提出了一种利用光纤的自发拉曼散射谱来反演光纤在大频移范围内拉曼增益谱的方法。利用该方法测量了两款光纤——掺磷光纤与掺锗光纤在1~42 THz频移范围内的拉曼增益谱。在1.6~22 THz频移范围内,基于该方法测得的拉曼增益系数与基于小信号增益法测得的结果基本一致。该工作为测量光纤在大频移范围内的拉曼增益谱提供了一种解决方案。

     

    Abstract:
      Objective  With the rapid increase of the output power of fiber lasers, the stimulated Raman scattering effect in optical fibers has also attracted more and more attention. On the one hand, it is one of the main limiting factors for the further increase in the power of current high-power fiber lasers. On the other hand, it can be used as a new way for laser generation, which is expected to achieve both high-power and wide-band laser output. Current Raman fiber lasers are mainly based on low-loss quartz fibers. In order to optimize the performance of a Raman fiber laser, the researchers doped the quartz fiber with different elements to change its Raman response characteristics. For example, the Raman gain coefficient of optical fibers can be increased by doping germanium, and the Raman peak with a frequency shift of about 40 THz can be introduced by doping phosphorus elements to achieve wavelength conversion with a large frequency shift. Different doping components and doping concentrations will change the Raman gain spectrum of the fiber, and the measurement of Raman gain spectrum of the fiber is of great significance for the design of Raman fiber lasers. Currently, the measurement of the Raman gain spectrum of optical fiber is mainly based on the small signal method, which has a long testing time, and the measurable frequency shift range is limited by the wavelength tuning range of the seed laser, so it is difficult to obtain the Raman gain spectrum of the fiber over broad frequency shift range.
      Methods  To measure Raman gain spectrum of the fiber over broad frequency shift range, a new method which derive the Raman gain spectrum of optical fiber from its spontaneous Raman scattering spectrum is proposed. Firstly, the backward Raman scattering spectrum is measured by the experimental setup (Fig.7). The pump source is a ytterbium-doped fiber laser operating at 1018.4 nm. Two bandpass filters are spliced after the pump source to remove the background noise. The filtered pump is coupled into the test fiber through a circulator. And the backward Raman scattered light is transported into the optical spectrum analyzer through the P2-P3 passage of the circulator. The backward Raman scattering spectrum of the test fiber can be obtained by subtracting the transmission spectrum of the P2-P3 passage (Fig.2(b)) from the output spectrum from P3 port of the circulator. Secondly, the Raman output powers under different Raman gains is calculated using the power balanced model. From the measured spontaneous Raman scattering spectrum and the calculated Raman output powers at different Raman gain coefficients, the Raman gain spectrum of the test fiber can be obtained.
      Results and Discussions  The simulated Raman output powers at different Raman gain coefficients is shown (Fig.5(b)). The output spectrum from P3 port of the circulator is shown in Fig.8(a). From the data above, the Raman gain spectra of a phosphorus-doped fiber and germanium-doped fiber over a broad frequency shift range of 1-42 THz are obtained. The measured results are shown (Fig.8(b)). To validify the accuracy of this method, the measured Raman gain coefficients are compared to that measured by the traditional small signal amplification method. In the frequency shift range of 1.6-22 THz, the results agree well with the Raman gain data measured by the traditional small signal amplification method.
      Conclusions  A new method to derive the Raman gain spectrum of optical fiber from the spontaneous Raman scattering spectrum is proposed. Using this method, the continuous Raman gain spectra of a phosphorus-doped fiber and an undoped silicon-based fiber in the range of 1-42 THz are obtained. In the range of 1.6-22 THz, the Raman gain coefficients obtained by this method agrees well with the results of the small signal amplification method. This work provides a convenient and accurate method for measuring the continuous fiber Raman gain spectrum over broad frequency shift range.

     

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