Chaohong Huang

Stable and spacing-adjustable multiwavelength Raman fiber laser based on mixed- cascaded phosphosilicate fiber Raman linear cavity

A novel multiwavelength Raman fiber laser based on the mixed-cascaded Stokes effects of phosphosilicate fiber is proposed and demonstrated experimentally. By using stimulated Raman scattering of both P(2)O(5) and SiO(2) along 1 km phosphosilicate fiber pumped with a 1064 nm double-clad fiber laser, the mixed-cascaded Raman linear cavity is formed by a pair of fiber Bragg gratings at 1239 nm, a polarization-maintaining fiber (PMF) Sagnac loop filter, and a conventional optical loop mirror. Up to 15-wavelength stable oscillations around 1320 nm are obtained with a wavelength spacing of 0.44 nm and power nonuniformity of less than 4 dB. By changing the length of the PMF in the Sagnac loop filter from 10 to 5.5 m, the wavelength spacing is adjustable from 0.44 to 0.8 nm. The extinction ratio of the laser is more than 30 dB. Excellent stability is also observed with a peak power fluctuation of less than 0.8 dB in 1 h.

Fast and noninterpolating method for subpixel displacement analysis of digital speckle images using phase shifts of spatial frequency spectra

A fast noninterpolation method for calculating displacement of digital speckle images with subpixel precision was introduced. In this method, the precise displacement is obtained from phase shifts of spatial frequency spectra of two digital speckle images instead of digital correlation calculation. First, digital speckle images before and after displacement are windowed and fast Fourier transform is performed. Then, phase shifts of different spatial frequencies are linearly fitted in spectral space using the least square method, and a coarse displacement value is directly calculated according to the phase shift theorem of Fourier transform. By a window technique and iterative procedure, the influence of finite image size on the accuracy of the results is eliminated, and the accurate displacement is obtained finally. It is significant that the method obtains the subpixel-precision displacement without any interpolation operations. The test results show that the method has high computing efficiency, high precision, and good robustness to low image quality.

A C+L-band erbium-doped fiber ASE source using dual-forward pumping configuration

We present a C+L-band ASE fiber source using double-pass dual-forward pumping configuration. The proposed ASE source offers a high pumping efficiency of 24.6%, output power of 52 mW and a wide bandwidth of about 80 nm.

Explicit solution for Raman fiber laser using Lambert W function

In this paper, an approximate explicit solution for the first-order Raman fiber laser is obtained by using Lambert W function. Good agreement between the explicit solution and numerical simulation is demonstrated. Furthermore, the optimal design of Raman fiber laser is discussed using the proposed solution. The optimal values of fiber length, reflectivity of output fiber Bragg grating and power transfer efficiency are obtained under different pump power. There exists a certain tolerance of the optimal parameters, in which the output power decreases only slightly. The optimal fiber length and reflectivity of output FBG decrease with increasing pump power.

Optimal Design of Cascaded Raman Fiber Lasers Using an Improved Analytic Method

An approximate explicit solution for Raman fiber lasers with arbitrary cascades is obtained. The solution is deduced without using depleted pump approximation. The proposed analytical solution shows excellent agreement with numerical simulation. Furthermore, optimal design of the cascaded Raman fiber lasers is carried out analytically. The optimal values of fiber length, reflectivity of output fiber Bragg grating and power conversion efficiency are obtained under different pump power. The optimal fiber length and reflectivity of output FBG decrease with increasing pump power. There exists a certain tolerance of the optimal cavity parameters, in which the conversion efficiency decreases only slightly. In addition, by the proposed solution one can readily determine whether pump power is depleted.

Application of Lambert W function to Raman fiber laser

With the assistance of Lambert W function, we derive an explicit expression for output power of Raman fiber laser without using the depleted-pump approximation. Moreover, the optimal fiber length and FBG reflectivity are obtained analytically.

Characteristics of the dual-forward pumped L-band ASE fiber source

We present a two-stage dual-forward pumped configuration to achieve a wavelength stable L-band amplified spontaneous emission (ASE) source. The effects of EDF length and pump power arrangements on the characteristics of L-band ASE spectrum, output power, and mean wavelength are investigated. The results show that not only the pumping conversion efficiency can be improved largely but also the pump power independent mean wavelength operation can be achieved by optimizing the fiber length ratio and pump ratio of the dual-forward pumped configuration as comparing to that of the conventional double-pass forward configuration.

Design and optimization of a high-power L-band ASE fiber source

Two kinds of configurations of L-band amplified spontaneous emission (ASE) source with two-stage EDF and dual forward pumps are suggested for generating a high power L-band ASE spectrum. The characteristics are theoretically compared in terms of the output power, pumping conversion efficiency, bandwidth, and mean wavelength stability for these two configurations. The EDF length and pump power allotment are also optimized. An L-band ASE source of 76mW output power with about 36.2% pumping conversion efficiency are experimentally obtained.

High power LD-end-pumped Nd:YVO4 laser as a pump source for raman fiber laser

A LD-single-end-pumped Nd:YVO4 CW all-solid-state laser with maximum output power 12.5W at 1064nm was designed to pump the cascaded phosphosilicate fiber Raman laser. The Nd:YVO4 laser is lower in cost than Yb-doped cladding fiber laser which is usually used as a pump for Raman fiber laser. However, it is inefficient to couple pump beam into single mode fiber (SMF). The coupling efficiency from pump beam to SMF is largely affected by the beam quality. Thus, a high coupling efficiency requires maintaining lasers operation in the TEM00 mode while scaling the power. The beam quality and maximum output power of the Nd:YVO4 laser is restricted by the thermal lens and fracture within the gain medium under high pump intensity conditions. The thermal effects was decreased by using a 3×3× 10mm3 Nd:YVO4 crystal with a low neodymium-doped concentration (0.3at%). Furthermore, a plane-plane resonant cavity with large mode volume and a large pump size was also used to reduce the effects. The Nd:YVO4 laser with maximum output power 12.5W and M2<1.2 was obtained. More power than 4W was injected into cascaded Raman cavity at the maximum pump power and about 1W coherent second-order Stokes radiation at 1484nm was achieved.

Design optimization of the first-order P-doped fiber Raman lasers using an explicit approach

Theoretical design optimization of the first-order P-doped fiber Raman laser (FRL) by an explicit approach was investigated. The authors derived an explicit expression for the output power of the laser without using the depleted-pump approximation. The proposed solution shows excellent agreement with numerical simulation. According to the explicit solution, one can clearly know the effects of fiber length, reflectivity of output fiber Bragg grating (FBG), Raman gain and loss of the P-doped fiber and extra losses on the output power. The solution also present a criteria by which one can determine whether the depleted-pump approximation is valid or not. It is very fast and convenient to optimize the output power of the FRL using the proposed explicit solution. The optimal values of fiber length, reflectivity of output FBG and conversion efficiency are obtained under different pump power. While increasing pump power, the optimal fiber length and reflectivity of output FBG decrease and the optimal conversion efficiency increase. There exists a certain tolerance of the optimal parameters, in which the conversion efficiency decreases only slightly. The results provide us an intuitive physical understanding to the laser and are instructive to experimental design of the laser.