Jiang Zongfu

Performance analysis of multiplexed phase computer-generated hologram for modal wavefront sensing

We investigate the performance and capability of a holographic modal wavefront sensor (HMWS) that is based on a multiplexed phase computer-generated hologram (MPCGH). The theoretical treatments of the HMWS are presented with scalar diffraction approximations and Fourier analysis. Several MPCGHs have been designed with different linear carrier frequencies, by using of the multiplexed coding scheme we have proposed, and by coding some common Zernike modes. The numerical simulation is carried out to investigate the performance of the HMWS to detect particular aberration mode(s), by considering the effect of different carrier frequency selections and the capability of coding a large number of modes. The results exhibit the expected characteristics of a corresponding symmetric spot pair, and indicate that the wavefront distorted by a particular Zernike mode(s) can be retrieved immediately through solving the amplitude of each mode coded in MPCGHs through the response curves of the HMWS.

Modal wavefront sensor based on binary phase-only multiplexed computer-generated hologram

We propose an approach for implementing a modal wavefront sensor using a binary phase-only multiplexed computer-generated hologram (BPMCGH). To simplify the coding and fabricating processes, a model based on tilt plane reference waves and an effective coding scheme for BPMCGH have been developed. The necessary number of subholograms to be recorded or coded is significantly reduced, from two or even more to just one per aberration mode, accordingly. The numerical and experimental demonstration results are presented and discussed and show that this approach is convenient for producing a BPMCGH and efficient for sensing the aberration modes.

High-diffractive-efficiency defocus grating for wavefront curvature sensing

The optimum phase defocus grating for wavefront curvature sensing is proposed. It features an equidistantly quantized, binary-phase-step defocus grating with a phase-step height of pi. The diffractive efficiency of the phase defocus grating is theoretically computed. The optical transfer function is obtained. The optimum phase defocus grating is fabricated. The high diffractive efficiencies of the +/-1 diffraction orders are verified experimentally, the average values of which are 38.08% and 40.36%, respectively.

Coherent Beam Combination of Two Polarization Maintaining Ytterbium Fibre Amplifiers

We investigate coherent beam combination of fibre laser beams by phase locking. Phase noise of a polarization maintaining ytterbium fibre amplifier is inspected with a fibre interferometer. In a feed back control loop, two fibre polarization maintaining ytterbium amplifiers are phase locked and coherent combined when the phase noise is properly controlled by a LiNO3 phase modulator.

Active compensation of low-order aberrations with reflective beam shaper

Abstract. A reflective beam shaper is designed for the purpose of compensation of the low-order aberrations in high-power slab lasers. A proportional-integral-derivative (PID) control algorithm is used to control the optical parameters of a beam shaper for active compensation of low-order aberrations. Simulations of the PID algorithm show that different combinations of defocus, 0-deg astigmatism, and 45-deg astigmatism, which are the main contributors of beam aberrations in slab lasers, can be well compensated by variation of distance and rotation angle of mirrors. For a beam with large wave aberrations [peak-to-valley (PtV)=87.7λ, root-mean-square (RMS)=19λ], the adjustment of distance between mirrors is less than 100 mm, and the rotation angle about the z-axis is <3.2  deg, and the wavefront distortion is reduced to a level (PtV=0.50λ, RMS=0.09λ) that can be further corrected with one deformable mirror. The effectiveness and performance of low-order aberration compensation with the reflective beam shaper are also verified by experiments.

Analysing the structure of the optical path length of a supersonic mixing layer by using wavelet methods

The nano-particle-based planar laser scattering (NPLS) technique is used to measure the density distribution in the supersonic mixing layer of the convective Mach number 0.12, and the optical path difference (OPL), which is quite crucial for the study of aero-optics, is obtained by post processing. Based on the high spatiotemporal resolutions of the NPLS, the structure of the OPL is analysed using wavelet methods. The coherent structures of the OPL are extracted using three methods, including the methods of thresholding the coefficients of the orthogonal wavelet transform and the wavelet packet transform, and preserving a number of wavelet packet coefficients with the largest amplitudes determined by the entropy dimension. Their performances are compared, and the method using the wavelet packet is the best. Based on the viewpoint of multifractals, we study the OPL by the wavelet transform maxima method (WTMM), and the result indicates that its scaling behaviour is evident.

Specific mode output from multimode fiber oscillators by designing rare earth doping profiles

Based on rate equations, a theoretical model of a fiber oscillator with a multimode gain fiber was built. We studied the effect of the rare earth doping profile in the core on the output characteristics of the multimode fiber oscillator. The results indicated that a pure fundamental mode can be obtained by partly doping the core of the large mode area (LMA) ytterbium doped fiber (YDF) in the fiber laser. Furthermore, a sole specific high-order mode can also be implemented by tailoring the rare earth doping profile according to the simulations. The mode coupling effect was also taken into account in the model. In spite of the mode coupling effect, the specific mode was able to dominate in the output of the fiber laser by utilizing the designed LMA YDF.

Frequency analysis of wavefront curvature sensing: optimum propagation distance and multi-z wavefront curvature sensing

In this paper we determine the optimum propagation distance between measurement planes and the plane of the lens in a wavefront curvature sensor with the diffraction optics approach. From the diffraction viewpoint, the measured wavefront aberration can be decomposed into Fourier harmonics at various frequencies. The curvature signal produced by a single harmonic is analyzed with the wave propagation transfer function approach, which is the frequency analysis of wavefront curvature sensing. The intensity of the curvature signal is a sine function of the product of the propagation distance and the squared frequency. To maximize the curvature signal, the optimum propagation distance is proposed as one quarter of the Talbot length at the critical frequency (average power point at which the power spectrum density is the average power spectrum density). Following the determination of the propagation distance, the intensity of the curvature signal varies sinusoidally with the squared frequencies, vanishing at some higher frequency bands just like a comb filter. To cover these insensitive bands, wavefront curvature sensing with dual propagation distances or with multi-propagation distances is proposed.

Temporal evolution of the optical path difference of a supersonic turbulent boundary layer

The density distribution of a supersonic turbulent boundary layer is measured with the nanoparticle-based planar laser scattering technique, and the temporal evolution of its optical path difference (OPD) in a short time interval is characterized by proper orthogonal decomposition (POD). Based on the advantage of POD in capturing the energy of a signal, a temporal evolution model is suggested for the POD coefficients of the OPD. In this model, the first few coefficients vary linearly with time, and the others are modeled by Gaussian statistics. As an application, this method is used to compute the short-exposure optical transfer function.

Evolution of modes in double-clad Raman fiber amplifier

Stimulated Raman scattering in a double cladding optical fiber is studied with a continuous wave laser used as a pump source. Under various launch conditions, pump modes are differently excited. Considering the mode coupling effect among the pump modes, the evolution of the power in the Stokes modes is studied. The results show that the scattered waves (the Stokes waves) in the fiber core with 9-μm diameter and 0.14 NA could propagate predominantly in the fundamental mode of the fiber by carefully adjusting the pump light launching conditions.