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The scientific research team has proposed a modeless Raman fiber laser using a traditional resonant cavity structure

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2023-08-15 15:00:52
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The pump source, gain material, and resonant cavity are the three elements that make up a laser. Due to the selective effect of the resonant cavity on the lasing frequency, multi longitudinal mode operation is one of the characteristics of fiber lasers based on traditional resonant cavity structures, manifested as periodic beat peaks in the radio frequency (RF) spectrum and periodic fluctuations in the intensity sequence in the time domain, and the frequency interval of the laser longitudinal mode is determined by the length of the resonant cavity.
 
However, the discrete multi longitudinal mode structure also poses challenges for some laser based applications. For example, in laser based optical sensing systems, the single frequency signal peak obtained through sensing elements such as phase-shifting gratings can only jump between discrete longitudinal modes and cannot achieve continuous frequency shift when it shifts with sensing parameters such as temperature and strain.

Therefore, the discrete longitudinal mode limits the maximum resolution of such optical sensors. In addition, in secure communication based on hardware encryption technology, the periodic fluctuations of the optical signal introduced by the resonant cavity feedback in the time domain can leak the length information of the laser cavity, reducing the security of secure optical communication.
 
In the generation of ultra fast random bit sequences based on laser intensity fluctuations, time period fluctuations can lead to signal repetition, thereby weakening the randomness of the generated sequence. Meanwhile, it seems impossible for multi longitudinal mode fiber lasers to achieve extremely low relative intensity noise similar to single frequency lasers.

Recently, Professor Shu Xuewen's team from the Wuhan National Center for Optoelectronics at Huazhong University of Science and Technology proposed a modeless Raman fiber laser (RFL). The laser adopts a traditional resonant cavity structure, but the output cavity mirror uses a fiber Bragg grating (FBG) with ultra-low reflectivity. Due to modulation instability, as the Stokes wave power inside the cavity increases, the longitudinal modes inside the cavity gradually widen, ultimately covering the longitudinal mode spacing, resulting in adjacent longitudinal modes overlapping each other and presenting a state of modeless operation. The relevant research results are titled "Modeless Raman fiber laser" and published in Optica, Vol. 10, Issue 8, 2023.

The structure of patternless RFL is shown in Figure 1. In order to avoid the transfer of relative intensity noise from the pump source to the Raman laser, the pump source uses an ASE light source built in the laboratory, with a central wavelength of 1540nm and a maximum output power of 10.3 W. The output grating adopts an ultra-low reflection FBG with a reflectivity of -27 dB.

The longitudinal mode of the laser can be reflected through the RF spectrum. Figure 2 (a) shows the RF spectra of Raman lasers experimentally measured at different laser output powers. At low power levels, there are significant characteristic peaks on the RF spectrum. But as the Stokes wave power increases, the periodic beat frequency peak related to the cavity length gradually widens and the height of the peak decreases. When the laser output power reaches 5.71 W, there are no longer distinguishable periodic beat peaks on the RF spectrum. This means that the discrete multiple longitudinal modes of the laser will gradually broaden with the increase of power, gradually covering the longitudinal mode spacing, and ultimately the longitudinal modes will completely overlap. In this case, RFL no longer has a discrete longitudinal mode structure like traditional lasers, but generates quasi continuous spectra similar to ASE light sources.

The research team used the generalized nonlinear Schr ö dinger equation to simulate the evolution of the optical field in Raman fiber lasers with laser output power, and obtained the RF spectrum of RFL as shown in Figure 2 (b). The simulation results show that the periodic beat frequency peak gradually disappears with the increase of power, which is basically consistent with the experimental measurement results.

Due to the generation of quasi continuous spectra by modeless Raman fiber lasers, their use in laser sensing systems can achieve continuous frequency modulation, significantly improving the resolution of optical sensors. At the same time, the disappearance of beat frequency peaks in the RF spectrum means that laser radiation no longer has temporal periodicity corresponding to the length of the resonant cavity, which has enormous application potential in secure communication, random sequence generation, and time-domain ghost imaging. Meanwhile, compared with traditional lasers, the modeless Raman fiber laser constructed has extremely low relative intensity noise. This research work has been supported by the National Key R&D Program (2018YFE0117400), the National Natural Science Foundation of China (62275093), and the EU H2020 MSCA RISE project.

Article link: https://doi.org/10.1364/OPTICA.488920


Figure 1. Schematic diagram of patternless RFL structure

Source: China Optical Journal Network
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