Stable generation of cylindrical vector beams with an all-fiber laser using polarization-maintaining and ring-core fibers

All-fiber hectowatt radially polarized laser system

A high-power all-fiber radially polarized laser system is demonstrated, in which an integrated nanograting mode convertor (S-wave plate) is used for the generation of radially polarized beam. Experimentally, a 1-W radially polarized beam was used as the seed laser, whose mode purity and mode extinction ratio (MER) were 96.5% and 98.3%, respectively. A single-stage few-mode fiber amplifier was employed to boost the 1-W seed laser to an average power of 113.2 W, when the pump power was 160 W. The corresponding slope efficiency and beam quality factor (M2) were approximately 72% and 2.3%, respectively. Moreover, the mode purity and MER of the amplified radially polarized laser were measured to be 95.7% and 97%, respectively. To the best of our knowledge, this is the highest output power from an all-fiber radially polarized laser system without obvious degradations of the mode purity and MER.

Coherent beam combining of cylindrical vector beams for power scaling

Coherent beam combining (CBC) of cylindrical vector beams (CVBs) based on an active phase controller is proposed and demonstrated. Experimentally, two pieces of spatially variant wave plates (S-wave plate) were employed as vector mode convertors for two individual 1064-nm fiber amplifier channels. When the system was in a closed loop, a combined output power of 10 W and a CBC efficiency of higher than 94% were achieved, for both TM01 (radially polarized) mode and TE01 (azimuthally polarized) mode cases. Moreover, the laser system showed a high vector mode purity, which was measured to be ∼97.4% (TM01) and ∼97.3% (TE01). To the best of knowledge, this could possibly be the first demonstration of CBC of CVBs, which paves the way for power scaling of CVBs.

Broadband generation of multiple high-order OAM modes in ring-core fibers using multi-pitch chirped long-period fiber gratings

A multi-order broadband mode converter in a ring-core fiber (RCF) using a multi-pitch chirped long-period fiber grating (LPFG), where multiple pitches were introduced in each chirp to further increase the bandwidth, is proposed and demonstrated. The grating parameters were optimized both theoretically and experimentally to achieve broadband mode generation of OAM ± 2 and OAM ± 3 modes by increasing the number of chirps and pitches. The mode conversion efficiency is higher than 90% with a broadband of 57 nm from 1456 nm to 1513 nm and 51 nm from 1573 nm to 1624 nm, corresponding to the second-order OAM mode and third-order OAM mode, respectively. Additionally, the insertion loss is less than 0.8 dB, and the purity is over 90%. The demonstrated mode converter has successfully achieved simultaneous generation of multi-order broadband OAM modes in a RCF for the first time, which has promising potential for application in OAM mode-division multiplexing systems.

High-power cylindrical vector beam fiber laser based on an all-polarization-maintaining structure

We propose and demonstrate an all-polarization-maintaining (PM) high-power cylindrical vector beam (CVB) fiber laser based on the principle of mode superposition. The non-degenerated LPy 11a is generated from the oscillator with the maximum power of 11.9W, whose slope efficiency is 24.4%. Then the stable single TE₀₁ vector beam is achieved by the superposition of LPy 11a and LPx 11b in an all-PM architecture, its output power is 3.1W and mode purity of 91.2%. Due to the all-PM architecture, our configuration is free of adjusting polarization controller (PC) and reliable during long-term operation. This laser could be used as a high-power CVBs source for a wide range of applications towards scientific research and industrial field.

Wideband, large mode field and single vector mode transmission in a 37-cell hollow-core photonic bandgap fiber

Stable generation and propagation of ultrafast high-order mode beams has become an important research direction. A core diameter not more than 10 times the wavelength is regarded as the upper limit for single mode transmission. However, a high-power laser requires a core diameter 20 to 40 times the wavelength to achieve high-power and stable output, which exceeds the design limit of the traditional fiber. In this paper, a novel 37-cell hollow core photonic bandgap fiber (HC-PBF) that only supports pure TE₀₁ mode over a bandwidth of 50 nm with the lowest loss of 0.127 dB/km is proposed. The HC-PBF has a core diameter of more than 40 μm. Single mode guidance is achieved by adjusting the lattice size in a particular of the cladding. The best single mode performance with a loss ratio as high as 150,000 between TE₀₁ mode and other modes with minimum loss is obtained. The fiber also has low bend-loss and thus can be coiled to a small bend radius of 1 cm having 1.6 dB/km bend loss. The tunability of the single-mode window and the manufacturing feasibility of the proposed fiber are also discussed.