Linearly polarized supercontinuum laser source has linear polarization characteristics, which provides a new dimension for the research and application of supercontinuum. In recent years, many linearly polarized supercontinuum laser sources have been reported, and some achievements have been made in high birefringence realizing, high coherence and low noise, power increasing, and spectrum broadening. However, there are still some factors that limit the development of linearly polarized supercontinuum laser sources, and it is meaningful to make a summary and propose prospects for linearly polarized supercontinuum laser sources.

In existing reports on linearly polarized supercontinuum, the phenomenon of polarization extinction ratio (PER) degradation of linearly polarized supercontinuum has been found. However, due to the inability of the existing method to reflect the polarization states at each wavelength of the linearly polarized supercontinuum, the reason for PER degradation cannot be explained. We introduce two PER measurement methods suitable for broadband linearly polarized supercontinuum laser sources.

We review the research results of linearly polarized supercontinuum fiber sources, mainly focusing on high PER realization, output power increasing, and spectrum broadening. The measurement methods of PER are introduced, especially two methods for linearly polarized supercontinuum laser sources.

The research progress of linearly polarized supercontinuum is mainly listed below.

1) The design of polarization-maintaining photonic crystal fiber (PM-PCF). Traditional polarization-maintaining fiber realizes birefringence through stress rods. The birefringence coefficient is usually on the order of 10^-4. PM-PCF can achieve birefringence through the irregular arrangement of air holes. The birefringence coefficient of optimally designed PM-PCF reaches the order of 10^-3 at the wavelength of above 1000 nm, resulting in a linearly polarized supercontinuum laser with PER of 21.2 dB.

2) In terms of coherence. Linearly polarized supercontinuum laser source generated in all-normal dispersion PM-PCF, which is essentially based on coherent nonlinear effects such as self-phase modulation and optical wave breaking, has a high degree of coherence. The coherence of linearly polarized supercontinuum lasers generated in all-normal dispersion PM-PCF is greater than 0.7 in the wavelength range of 650 nm.

3) Output power increasing. The output power of linearly polarized supercontinuum fiber lasers generated in PM-PCF is limited by the small core of the PM-PCF and its splice with the output fiber of the pump laser. By optimizing the splicing parameters, the coupling efficiency between PM-PCF and pump laser reaches 90%, and the output power of linearly polarized supercontinuum generated in PM-PCF reaches 93 W. Linearly polarized supercontinuum laser directly generated in polarization maintaining fiber amplifier is more suitable for output power increasing due to its extremely low loss of fiber splicing. The highest output power of linearly polarized supercontinuum reaches 322.5 W.

4) Spectrum broadening. The short wavelength edge of linearly polarized supercontinuum generated in PM-PCF reaches 300 nm, and the long wavelength edge reaches 2300 nm. The spectrum of linearly polarized supercontinuum generated in PM-ZBLAN covers 350-4500 nm. The spectrum of linearly polarized supercontinuum generated in chalcogenide rib waveguide covers 2-10 μm.

It can be expected that in the near future, with the output power of the polarizing amplifier increasing and the use of large core nonlinear fiber, the output power of linearly polarized supercontinuum generated in polarization-maintaining fiber amplifier can exceed kilowatts. With the coupling efficiency between PM-PCF and pump laser increasing and better thermal management, the output power of the linearly polarized supercontinuum generated in PM-PCF is expected to exceed 200 W, and the spectrum is expected to reach the short wavelength and long wavelength limits of silica fiber.

The PER of a linearly polarized laser is usually measured by a rotating polarizer. This method is suitable for single-wavelength fiber lasers but cannot be used to measure the PER of each wavelength of a wide spectrum laser source such as supercontinuum. Two measurement methods suitable for the PER measurement of linearly polarized supercontinuum are introduced. By using these two methods, we can study the inherent mechanism of linearly polarized supercontinuum laser sources.