Fiber random laser enables power enhancement and spectral domain expansion
summary
Combining the unique advantages of fiber waveguide and random laser, fiber random laser has rapidly developed into an important way to obtain high time-domain stability, high spatial brightness, low coherence and high power laser beams in recent years. Different from the traditional fixed-cavity long fiber laser with point feedback mechanism, the fiber random laser opens up a new way for the output power increase and spectral expansion of high-performance fiber lasers. In this paper, we summarize and review the latest research progress of fiber random lasers to enable the generation of high-power and new-wavelength fiber lasers with their time-domain stability and wavelength flexibility, introduce the current practical application of fiber random lasers with their high brightness and low coherence characteristics, and look forward to their future development directions and possible challenges.
abstract
significance
Random fiber lasers (RFLs) use distributed feedback in optical fibers to form resonators of random length. They combine the low coherence of random lasers with the high brightness of fiber lasers. As a result, RFL has been widely used in areas such as environmental sensing and optical communications.
Since RFLs are proposed using Rayleigh backscatter as a feedback mechanism to propose all-fiber integration, RFLs have attracted a great deal of attention in terms of power scaling and wavelength expansion of fiber lasers. Rayleigh backscattering occurs in silicon fibers due to disordered fluctuations in the refractive index, which provides a randomly distributed feedback to the RFL. Since the output of an RFL is the sum of resonators of random length, RFLs do not have a self-pulsing effect, unlike conventional fiber lasers generated in a fixed cavity. Amplifying RFL through the Master Oscillator Power Amplification (MOPA) configuration can suppress spectral broadening, as the peak power from the RFL seed is not enhanced in the process. A control experiment was performed in a 10 kW class MOPA system using RFL and a fixed-cavity fiber laser as seeds. The results show an inhibitory effect.
Rayleigh backscatter provides broadband reflection in optical fibers and can replace the reflective components in wavelength-tunable fiber laser systems and supercontinuum (SC) sources. As a result, RFLs can easily achieve wavelength tunability using a single-frequency selection component. By utilizing cascaded Raman scattering, the operating wavelength of the RFL can cover the transmission band of the silicon fiber. The study of 1.1‒2.0 μm RFL demonstrates the flexibility of its output wavelength. In addition, modulation instabilities provide broadband gain for optical waves close to the zero dispersion wavelength of the fiber. This type of light wave can be produced by cascaded Raman scattering in RFL. In addition, Rayleigh backscatter enhances the nonlinear effect because it increases the effective length. As a result, RFLs are an excellent choice for SC sources. The feedback provided by the fiber itself, without the need for additional optics, guarantees a strong high-power processing capability; As a result, RFLs can be a good platform for high-power SC generation.
progress
In the second part, the generation method of high-power RFL is introduced, and the 100-watt to kilowatt-class RFL oscillator is summarized. In addition, a MOPA configuration seed for RFLs was introduced to further expand its functionality. Suppression of stimulated Raman scattering (SRS) facilitates 10 kW amplification of RFL. In the third part, the flexibility of operating wavelengths achieved by the RFL configuration is reviewed, and the wavelength-tunable RFLs obtained by rare earth ions are summarized. Rayleigh backscatter broadband feedback simplifies the structure of wavelength tuning. Using cascaded Raman scattering, the Random Raman Fiber Laser (RRFL) was introduced. Amplification configurations with ytterbium and Raman scattering mixed gain are used to achieve high-power RRFLs. In the fourth part, the use of RFL to generate SCs in optical fibers is reviewed. Both semi-open and fully open cavity RFLs can be used to achieve SC output, and their spectral range can cover the conveyor belt of silicon fibers. By combining Rayleigh backscatter and CW pumping, the average power generated by the SC in the RFL was scaled from the 100 watt class to the 3 kW level. In the fifth part, the practical application of RFL is discussed. Due to its high brightness and low coherence, RFL enables speckle-free imaging and is compatible with fiber-integrated imaging systems.
Conclusions and prospects
RFL paves the way for power scaling and wavelength scaling of high-performance fiber lasers. Their temporal stability helps to suppress spectral broadening during power amplification. The broadband feedback of Rayleigh backscatter and the gain of the cascaded Raman effect make them suitable for wavelength expansion of fiber lasers. Rayleigh backscatter not only simplifies the structure and enables fiber lasers operating over a wide spectral range, but also improves the power handling capabilities of the reflective components. Due to its high brightness and low coherence, RFL has been widely used in fields such as fiber optic imaging and inertial confinement fusion.