Kailiang Duan

High-Power Coherent Beam Combining of Two Photonic Crystal Fiber Lasers

Coherent beam combining of two large mode area photonic crystal fiber (LMA PCF) lasers is studied experimentally by using a self-imaging technique. Steady interference strips with high visibility of 35% are observed in high-power operation, which demonstrates the obvious advantages of the LMA PCF over the conventional double-clad fiber in coherent beam combining. Experiments also show that the reflectivity of coupling output mirror affects the coherent output power. The maximum coherent output power of 80 W with approximate 90% combining efficiency is achieved when the pump power is 130 W and the reflectivity of coupling output mirror is 5%.

1.5 kW Incoherent Beam Combining of Four Fiber Lasers Using Adaptive Fiber-Optics Collimators

We demonstrate the incoherent beam combining (BC) of four high-power single-mode fiber lasers. Adaptive fiber-optics collimators (AFOC) of 0.5 kW-level are developed to correct the tip/tilt-type phase errors among beamlets. The maximum deflection angle of the AFOC is about 0.29 mrad. A stochastic parallel gradient descent algorithm is employed as the tip/tilt control strategy. The tip/tilt control makes four separate laser beams overlap with each other in the far-field, and the average of normalized power-in-bucket increases from 0.171 in open loop to 0.798 in closed loop when 1.5-kW total output power is achieved. The precise tracking ability and control accuracy of this BC system are investigated. Experimental results expose that thermal-blooming effect disturbs BC seriously when the total output power is up to kW-level, which could be mitigated by inducing air flow near the laser output.

Calibration of a portable laser 3-D scanner used by a robot and its use in measurement

A robot scanning system consisting of a portable laser 3-D scanner and an industrial robot is demonstrated. In this system, the scanner is precalibrated by the traditional nonlinear two-step approach. In addition, by using a criterion sphere as the calibration object, a new robot tool center point (TCP) calibration approach is proposed for calibrating the relation between the precalibrated laser 3-D scanner and the robot. In this approach, two different translational motions of the robot are first made to determine the rotation part, and then at least three different rotational motions are made to determine the translation part. During the process, the extrinsic camera parameters are not recalibrated for each robot motion, so the calibration errors brought by camera calibration can be decreased. Moreover, the calibration error due to robot positioning error can be decreased by making use of the differences of different robot positions in calculations. An experiment was performed on a portable laser 3-D scanner and an ABB IRB-4400 industrial robot to test the validity of the proposed calibration approach. The experimental results show that this approach is simple and accurate compared to the conventional robot TCP calibration approach.

Modeling and effects of ion pairs in high-concentration erbium-doped fiber lasers

According to the modeling of ion pairs in high-concentration erbium-doped fiber lasers, a closed form rate and power evolution equations containing isolated ions and ion pairs are constructed. Based on the equations, the effects of ion pairs on the output performance of high-concentration erbium-doped fiber lasers for low Q and high Q laser cavity in three different pump modes are analyzed numerically. The results show that with the increasing percentage of ion pairs, the output power and the slope efficiency decrease, and the threshold increases for each pump mode. Meanwhile, the backward pump mode is proved to be the best pump mode and the deleterious effects of ion pairs for low Q laser cavity is proved to be smaller than that for high Q laser cavity.

Theoretical analysis of the heat dissipation mechanism in Yb3+-doped double-clad fiber lasers

Based on the special structure of high-power Yb3+-doped double-clad fiber lasers, a heat dissipation model and the corresponding steady-state heat equations with consideration of both convective and radiative heat transfer are constructed. The temperature distributions along the radial and axial directions of the fiber for forward pump of 1000 W and two-end pump of 500 W each side are calculated numerically by using the steady-state rate equations and the heat equations. The results show that the temperature distribution for two-end pump mode is more even than that for forward pump mode and the maximum temperature in the fiber decreases by 139.8°C. Finally, a comparison between the temperature distributions with and without consideration of radiative heat transfer is performed, the results of which indicate that radiative heat transfer plays an important role in heat dissipation of high-power fiber lasers. Therefore, radiative heat transfer has to be considered in the thermal, stress, and thermo-optic effects of any type of high-power fiber lasers.

Nonparaxial propagation of vectorial hollow Gaussian beams diffracted at an annular aperture

Based on the vectorial Raleigh-Sommerfeld diffraction integral, an analytical propagation equation of vectorial nonparaxial hollow Gaussian beams (HGBs) through an annular aperture is derived. The corresponding closed-forms for a circular aperture, a circular black screen, and unapertured case are given as special cases of the general results. The typical numerical examples are given to illustrate our analytical results. It is shown that the f parameter plays an important role in determining the nonparaxiality of vectorial apertured HGBs

Spectral anomalies of chirped Gaussian pulses from an annular aperture in the turbulent atmosphere

The anomalous spectral behaviors of a chirped Gaussian pulsed beam diffracted by an annular aperture in the turbulent atmo- sphere are studied. On the basis of the extended Huygens-Fresnel prin- ciple and by introducing a hard aperture function into a finite sum of complex Gaussian functions, the approximate analytical expression for the spectral intensity of chirped Gaussian pulses diffracted by an annular aperture in the turbulent atmosphere is derived. The corresponding ex- pressions for a circular aperture, a circular black screen, and the unap- ertured case are given as special cases of the general results. The rela- tive spectral shift of diffracted chirped Gaussian pulses in the turbulent atmosphere versus the different values of the radius of the annular ap- erture and the different values of the position parameter are also studied and illustrated with numerical calculations. The results of this work have potential applications in information encoding and transmission in a tur- bulent atmosphere.

Numerical analysis of large mode area segmented cladding fibers

We analyze numerically the dependence of segmented cladding fibers (SCF) characters, including the effective V-number, the effective fundamental mode area and the permissible bending radius of fundamental mode, on the structure parameters of the fiber and the signal wavelength with the methods of radial effective index and transmission matrix. The results show that a single mode and large mode area fiber with an acceptable permissible bending radius can be obtained when we design SCF with optimized structure parameters. The effective fundamental area is as large as 1948.93μm2 with proper fiber parameters. It will benefit the fabrication of the SCF for applications in high power fiber lasers.

Output performance of gain guided and index antiguided fiber lasers pumped by different methods

The output performance of Yb3+ doped gain guided and index antiguided (GG-IAG) fiber lasers pumped by different methods are compared by numerically solving rate equations (REs) with additional leakage losses. Three pump models, including forward pump, backward pump and bidirectional pump are established combined with REs model. Calculation results show that the output powers and slope efficiencies of GG-IAG fiber from high to low under the same condition are obtained by bidirectional pump, backward pump and forward pump in sequence. The upper energy level particle population density N2(z) and thermal distribution along the fiber generated by bidirectional pump are more uniform than that by other two pump methods. It benefits the lasing of GG-IAG fiber most.

Theoretical analysis of light transmission in a perfectly circular double clad fiber using coupled mode method

Light propagation in a perfect circular double clad fiber (DCF) is numerically investigated by using the coupled mode method with translating the DCF into a single mode fiber (SMF) and an annular core fiber (ACF). The results show that for the coupling coefficients between the LP01 mode of the SMF and the guided modes (LP0n modes) of the ACF, the high-order mode has a larger coupling coefficient than the low-order mode for the ACF. By using the coupled mode equations the field distribution in the DCF core is calculated, the results show that the power shows a quasi-periodic distribution along the DCF core and its average period increases with increasing wavelength for different DCF parameters. The simulation results agree with the light ray propagation in the DCF.