Optical path design of rectangular pulsed lasers

Optical path design of rectangular pulsed lasers

Overview of Optical path design

A passive mode-locked dual-wavelength dissipative soliton resonant thulium-doped fiber laser based on a nonlinear fiber ring mirror structure.

2. Optical path description

The dual-wavelength dissipative soliton resonant thulium-doped fiber laser adopts an “8″ shaped cavity structure design (Figure 1).

The left part is the main unidirectional loop, while the right part is a nonlinear optical fiber loop mirror structure. The left unidirectional loop includes a bundle splitter, a 2.7m thulium-doped optical fiber (SM-TDF-10P130-HE), and a 2 μm band optical fiber coupler with a coupling coefficient of 90:10. One polarization-dependent Isolator (PDI), two Polarization Controllers (Polarization controllers: PC), a 0.41m Polarization-maintenance Fiber (PMF). The nonlinear fiber optic ring mirror structure on the right is achieved by coupling the light from the left unidirectional loop to the nonlinear fiber optic ring mirror on the right through a 2×2 structure optical coupler with a coefficient of 90:10. The nonlinear optical fiber ring mirror structure on the right includes a 75-meter-long optical fiber (SMF-28e) and a polarization controller. A 75-meter single-mode optical fiber is used to enhance the nonlinear effect. Here, a 90:10 optical fiber coupler is employed to increase the nonlinear phase difference between clockwise and counterclockwise propagation. The total length of this dual-wavelength structure is 89.5 meters. In this experimental setup, the pump light first passes through a beam combiner to reach the gain medium thulium-doped optical fiber. After the thulium-doped optical fiber, a 90:10 coupler is connected to circulate 90% of the energy within the cavity and send 10% of the energy out of the cavity. At the same time, a birefringent Lyot filter is composed of a polarization-maintaining optical fiber located between two polarization controllers and a polarizer, which plays a role in filtering spectral wavelengths.

3. Background knowledge

At present, there are two basic methods for increasing the pulse energy of pulsed lasers. One approach is to directly reduce nonlinear effects, including lowering the peak power of pulses through various methods, such as using dispersion management for stretched pulses, giant chirped oscillators, and beam-splitting pulsed lasers, etc. Another approach is to seek new mechanisms that can tolerate more nonlinear phase accumulation, such as self-similarity and rectangular pulses. The above-mentioned method can successfully amplify the pulse energy of the pulsed laser to tens of nanojoules. Dissipative soliton resonance (Dissipative soliton resonance: DSR) is a rectangular impulse formation mechanism first proposed by N. Akhmediev et al. in 2008. The characteristic of dissipative soliton resonance pulses is that, while keeping the amplitude constant, the pulse width and energy of the non-wave splitting rectangular pulse increase monotonically with the increase of the pump power. This, to a certain extent, breaks through the limitation of the traditional soliton theory on single-pulse energy. Dissipative soliton resonance can be achieved by constructing saturated absorption and reverse saturated absorption, such as the nonlinear polarization rotation effect (NPR) and the nonlinear fiber ring mirror effect (NOLM). Most reports on the generation of dissipative soliton resonance pulses are based on these two mode-locking mechanisms.