A novel high-power all-fiberized flexible spectral filter for high power linearly-polarized Raman fiber laser

Yb-doped mode-locked fiber laser with a hybrid structure of NPR and a Lyot filter as the saturable absorber

Passively mode-locked fiber lasers based on a nonlinear polarization rotation (NPR) have attracted much attention due to their ability to generate short pulses with wide spectra and high peak power. However, environmental perturbations can easily cause the lasers to lose the mode-locked state and make it a challenge for practical application. The aim of this research is to improve the laser stability by inserting a Lyot filter into the mode-locked laser cavity. The experimental results indicate that the mode-locked state can be maintained when the radius of the fiber loop is changed from 7.5 to 1.5 cm, while the signal-to-noise ratio of the fundamental frequency remains almost the same. The tunability of the output power can be achieved by adding a half-wave plate (HWP) in the laser cavity without changing the pump power, while the mode-locked state remains stable. By adjusting the angle of the HWP2, the output power can be adjusted from 3.36 to 66.5 mW at repetition rate of 29.7 MHz.

Tailoring the spectrum and spatial mode of Yb-doped random fiber laser

In this paper, we make a comprehensive study on tailoring the spectrum and transverse mode of random fiber lasers (RFLs). By simply temperature tuning, the mode gain profile of RFL can be flexibly and precisely manipulated. The spectrum of laser output can be easily tailored in single-wavelength, dual-wavelength, and three-wavelength, respectively. Meanwhile, the operating transverse mode is also optional among LP₀₁ mode, LP₁₁ mode, and hybrid mode. The slope efficiency of 17.9% and 27.3% are obtained for LP₁₁ mode and LP₀₁ mode operation, respectively. Besides, the coherence control can be confirmed by making speckle contrast measurements. This high-efficiency RFL with the customizable spectrum and spatial mode would have unique applications in wavelength or mode division multiplexing systems, speckle-free imaging, secure communication, and information encryption.

Hundred-watt level linearly polarized visible supercontinuum generation

An all-fiber linearly polarized supercontinuum (SC) laser source with 93 W average output power and spectrum ranging from 520 nm to 2300 nm is experimentally demonstrated. The linearly-polarized SC is generated in a piece of 2.6 m long polarization-maintaining photonic crystal fiber (PM-PCF), pumped by a polarization-maintaining picosecond Yb-doped master oscillator power amplifier (PM-MOPA). The source exhibits a flat spectrum from 600 nm to 1880 nm at -10 dB level except for the residual pump peak. A new method is proposed to measure the polarization extinction ratio (PER) of each single wavelength of the broadband supercontinuum at a high-power level, resulting in larger than 16 dB PER from 900 nm to 1600 nm and larger than 15 dB PER from 540 nm to 650 nm. To our knowledge, this is the first demonstration of hundred-watt level linearly polarized visible SC and the first demonstration of PER measurement of each single wavelength within such a wide spectrum range.

High-power, cascaded random Raman fiber laser with near complete conversion over wide wavelength and power tuning

Cascaded Raman fiber lasers based on random distributed feedback (RDFB) are proven to be wavelength agile, enabling high powers outside rare-earth doped emission windows. In these systems, by simply adjusting the input pump power and wavelength, high-power lasers can be achieved at any wavelength within the transmission window of optical fibers. However, there are two primary limitations associated with these systems, which in turn limits further power scaling and applicability. Firstly, the degree of wavelength conversion or spectral purity (percentage of output power in the desired wavelength band) that can be achieved is limited. This is attributed to intensity noise transfer of input pump source to Raman Stokes orders, which causes incomplete power transfer reducing the spectral purity. Secondly, the output power range over which the high degree of wavelength conversion is maintained is limited. This is due to unwanted Raman conversion to the next Stokes order with increasing power. Here, we demonstrate a high-power, cascaded Raman fiber laser with near complete wavelength conversion over a wide wavelength and power range. We achieve this by culmination of two recent developments in this field. We utilize our recently proposed filtered feedback mechanism to terminate Raman conversion at arbitrary wavelengths, and we use the recently demonstrated technique (by J Dong and associates) of low-intensity noise pump sources (Fiber ASE sources) to achieve high-purity Raman conversion. Pump-limited output powers >34W and wavelength conversions >97% (highest till date) were achieved over a broad - 1.1μm to 1.5μm tuning range. In addition, high spectral purity (>90%) was maintained over a broad output power range (>15%), indicating the robustness of this laser against input power variations.

High-power, widely wavelength tunable, grating-free Raman fiber laser based on filtered feedback

The cascaded Raman fiber laser is a proven technology that provides wavelength agile high-power fiber lasers outside the rare-earth emission windows. However, conventional cascaded Raman fiber lasers lack wavelength agility due to the use of fixed wavelength fiber Bragg gratings. Recently, proposed cascaded Raman fiber lasers based on random distributed feedback have provided a grating-free solution enabling wavelength agility. With these lasers, wide wavelength tunability has been achieved. However, there are still limitations in scaling output power while maintaining high spectral purity of wavelength conversion. Spectral purity is characterized by the in-band power ratio, which is the ratio of the output power in the required wavelength to the total power. The origin of this limitation arises from the inability to efficiently terminate the Raman cascade at a specific wavelength with increasing power. In this Letter, we propose a novel filtered distributed feedback mechanism to terminate the Raman cascade at any desired wavelength, enabling power scaling with high spectral purity. Output power up to 28 W has been achieved with >85% in-band power ratio and >400  nm tuning range from 1118 to 1535 nm.

Power scalability of linearly polarized random fiber laser through polarization-rotation-based Raman gain manipulation

Random fiber laser based on Raman gain and random distributed feedback has drawn great attention in recent years. One of the most widely-studied fields is to improve the optical efficiency and the output power. However, the power scaling of a random fiber laser is instinctively restricted by the high order Stokes generation. In this manuscript, we propose a simple yet effective method, which employs a homemade all-fiber Lyot filter to manipulate the polarization dependent Raman gain, thus increasing the threshold of the 2nd-order Stokes wave and enhancing the maximum output power of the linearly polarized random fiber laser. Through reliable theoretical analysis, we optimize the design of the wavelength dependent Lyot filter. Moreover, the performance of the filter and the power scaling capability of the linearly polarized random fiber laser are investigated in detail. A proof-of-principle experiment is carried out by inserting the homemade Lyot filter into a half-opened random fiber laser. The experimental results indicate that the 2nd-order Stokes wave can be effectively suppressed, and the maximum output power of the 1st-order Stokes wave is significantly increased with a range of ~50% (from 43.6 to 63.2 W).