Hong Shu

More on analyzing the reflection of a laser beam by a deformed highly reflective volume Bragg grating using iteration of the beam propagation method

A further extension of the iteration method for beam propagation calculation is presented that can be applied for volume Bragg gratings (VBGs) with extremely large grating strength. A reformulation of the beam propagation formulation is presented for analyzing the reflection of a laser beam by a deformed VBG. These methods will be shown to be very accurate and efficient. A VBG with generic z-dependent distortion has been analyzed using these methods.

Three-dimensional computer model for simulating realistic solid-state lasers

We developed an accurate and fast three-dimensional computer model for simulating realistic solid-state laser systems. An iteration of the beam propagation calculation was developed to account for the counterpropagation of the laser beams in the saturated gain medium and eventually obtain the converged solution for the output beam. An analytic method was devised to account for the curved cavity mirror and the surface deformation of the gain medium induced by the temperature gradient due to pump absorption. The temperature gradient induced thermal lensing and stress birefringence is also properly included in the model calculation. This model has been validated and shown to be very accurate and efficient for modeling three-dimensional laser systems in a personal computer.

Modeling the reflection of a laser beam by a deformed highly reflective volume bragg grating

A model is presented for analyzing volume Bragg gratings having large diffractive strength that may become distorted upon the passage of high-power laser light. One result of this analysis is that very little distortion in a volume Bragg grating can greatly reduce the reflectivity. This is a critical issue in the design of systems in which these devices are used to combine high-power laser beams.

Think Outside the Fiber: Imaging Amplifier for Space-Multiplexed Optical Transmission

This paper proposes a simple and practical method to amplify signals for space-multiplexed optical transmission. In this amplification technique, the output facet of the multicore or multimode fiber is mapped back to the same type of fiber after passing through an imaging and bulk amplifying region. Simulations are carried out for a seven-core multicore fiber with the signal lasers amplified by a bulk erbium-ytterbium-doped phosphate glass amplifier. Amplifier gain of ~20 dB is achieved at an input power of 6 mW for each individual core with an optical power conversion efficiency of 32.5%. The proposed amplifier technique does not have a core or mode count limit for multicore and multimode fibers.

Numerical modeling of alkali vapor lasers

Detailed numerical analyses are presented of a continuous wave (cw), single spatial mode alkali vapor laser pumped by a diffraction-limited Ti: Sapphire laser. These analyses provide insight into the operation of alkali vapor lasers to aid in the development of high power, diode laser pumped alkali vapor lasers. It is demonstrated that in the laser considered the laser spatial pattern is significantly changed after each pass through the gain medium, and the laser spatial pattern in steady state operation is also very different from that of the passive cavity mode. According to the calculation, lasing significantly improves the pump absorption efficiency and changes the absorbed pump distribution. The effect of varying the transverse size of the pumped region is also analyzed and an optimum pump beam waist radius is demonstrated. In addition, the shift of the pump beam waist location is also studied. The computation method and its convergence behavior are also described in detail.

Split step solution in the iteration of the beam propagation method for analyzing Bragg gratings.

The split step method is applied to the iteration of the beam propagation method for analyzing the reflection of a laser beam by a volume Bragg grating. The application of the split step method is made possible by a way to properly treat the grating coupling terms in the paraxial wave equations. This method is demonstrated to be accurate in addition to efficient and robust. After this modification, the iteration of the beam propagation method is suitable for analyzing finite beams in volume Bragg gratings, for which the grating strength might be large. It is also suitable for analyzing Bragg gratings with nonuniform grating structures.

A computer model for simulating real laser systems by solving partial differential Eq.s in FEMLAB

We developed a computer model for simulating real solid state laser systems, by solving the paraxial wave Eq.s in the multi-physics modeling software FEMLAB. The reflection of the laser on the curved cavity mirror is calculated by an analytical method. This model was verified to be able to give very accurate results, by applying it to empty stable and unstable resonators, and a face pumped Yb:YAG disk stable resonator laser.

Analysis and Optimization of the Numerical Calculation in the Slowly Decaying Imaginary Distance Beam Propagation Method

The detailed analyses of different numerical schemes are presented for solving the governing equation of the slowly decaying imaginary distance beam propagation method (SD-ID-BPM), including their convergence speed and stability. It will be demonstrated that the fully implicit scheme has great advantages over the Crank-Nicholson scheme to obtain the eigenmodes using SD-ID-BPM. The converged solution of the eigenmodes can be obtained tremendously faster using the fully implicit scheme than using the Crank-Nicholson scheme with similarly good accuracy. In addition the fully implicit scheme will be shown to be easier to implement, more reliable and robust than the Crank-Nicholson scheme. The use of the fully implicit scheme in the SD-ID-BPM results in great improvements of the modeling capability of this method. It will also be demonstrated that both the fully implicit and Crank-Nicholson schemes are numerically stable.

Quartic form of the slowly decaying imaginary distance beam propagation method

The governing equation of the slowly decaying imaginary distance beam propagation method (SD-ID-BPM) is further modified, for calculating the eigenmodes in optical fibers and waveguides. Its convergence is analyzed in detail and compared to the earlier version of SD-ID-BPM and other methods. It is demonstrated that the method described here can converge to the same desired accuracy within fewer propagation steps than the earlier version of SD-ID-BPM and other methods. Since the governing equation of the SD-ID-BPM is a partial differential equation with higher order derivatives, it might be interesting if the discretization in the transverse x-y plane is performed by applying the numerical techniques for partial differential equations with higher order derivatives.

Modeling a diode pumped Nd: YAG rod laser

We evaluate the performance potential of a diode pumped Nd: YAG rod laser by finding the absorbed pump distribution using ASAP, pump induced thermal lensing, gain medium surface distortion and stresses using FEMLAB and depolarization losses using MATLAB. Beam propagation in the optically distorted Nd:YAG rod and the free space part of the cavity, and the output laser beam were determined with a computational scheme we developed which employs the beam propagation method combined with sparse matrix technology. We propose a special cavity design that can select the spatial eigen mode shape of the laser and simultaneously compensate for pump induced thermal lensing, gain medium surface distortion and birefringence. The converged solutions calculated this special cavity design give both high extraction efficiency and good output beam quality. Sensitivity of the output beam to mirror tilt, thermal induced mirror distortion, and errors in the cavity length or the optical distortions in the rod were also calculated.