Adaptive optics (AO) is widely used in the fields of ground-based telescopes, biological imaging, human eye aberration correction and laser communication to correct wavefront distortion. One of the key components of an AO system is a wavefront sensor (WFS). We demonstrate a 1×19 photonic lantern to be used as a single-aperture wavefront WFS. By detecting the power and the phase difference of 19 fundamental mode output at the single mode end of the photonic lantern, the lowest 19 order Zernike coefficients of the wavefront to be measured is determined. Simulation results show that when the wavefront distortion RMS< 1.5 π, the residual RMS between the reconstructed wavefront and the wavefront to be measured is always lower than 3.5 × 10−3 π, which provides a reliable technical means for realizing high-speed high accuracy and perfect common path wavefront sensing in AO system.
Adaptive mode control system based on photonic lanterns has been made great progress in achieving stable fundamental mode in large mode area fiber, but there are still difficulties in generating specific spatial mode, such as orbital angular momentum mode. In this paper, purity of the target mode which is obtained by optical correlation filtering method based on computational holograms is chosen to be the evaluation function of the stochastic parallel gradient descent algorithm. The schematic diagram of the adaptive mode control system is improved, which consists of the photonic lantern, phase modulators, optical correlation filtering, detector, controller, etc. A simulation model combining optical correlation filtering method based on computational hologram and stochastic parallel gradient descent algorithm is established, and the numerical simulation results show that the orbital angular momentum mode with topological charge of ±1 and ± 2 is realized, and the corresponding mode purity is above 95% by using this adaptive mode control method based on a 5×1 photonic lantern.
A high-power linearly-polarized all-fiber single-frequency amplifier at 1064 nm based on tandem corepumping is demonstrated by adopting large-mode-area (LMA) fiber with core/cladding diameter of 20/130 μm. The output performance of the amplifier dependence on the input signal power has been investigated, and the results indicate that enhancing the injection signal power is advantageous in mitigating the amplified simultaneous emission (ASE) and increasing slope efficiency. A maximum output power of 252 W with corresponding slope efficiency of 85% is achieved with injection signal power of 7 W. At the highest output power status, a polarization extinction ratio (PER) of 17 dB and a beam quality of 1.15 are obtained respectively. In addition, by virtue of LMA fiber and tandem core-pumping, the amplifier exhibits good performance on thermal load, which in turn facilitate the maintenance of frequency noise and linewidth. To the best of our knowledge, this is the highest output power of single-frequency all-fiber amplifier based on core-pumping scheme.
We propose a method to measure the mode coefficient in a few-mode fiber with OAM modes. In this method, the eigenmodes are calculated through the optical fiber parameters, including LP modes and orbital angular momentum (OAM) modes. Using a single spatial light modulator (SLM) to load multi-channel computer-generated hologram (CGH), multiple matched filters can be generated at the same time to realize the real-time and accurate measurement of each mode coefficient (mode content and relative phase) and reconstruct the optical field. We validate the method on multimode beams, and the accuracy of mode coefficients measurement is up to 99%. It can be used as adaptive control coefficient to provide solutions for the expansion of photon lantern and other technologies.
Single trench fiber (STF) is one kind of promising novel fibers. In this paper, we design and fabricate a piece of ytterbium-doped STF. The core diameter of the homemade STF is 30 μm and the cladding diameter is 250 μm. Based on this self-developed STF, we have constructed an all-fiberized fiber amplifier that is operating under a continuous-wave regime at 1070 nm wavelength. The maximum output power of the system reaches 1.5 kW, which, to the best of our knowledge, is the highest output power of STF-based laser systems. The M2 is measured to be 1.65 at 1.36 kW and 1.92 at the highest output power respectively. The slope efficiency of amplification system is 68%. The performance of the system can be further enhanced by optimizing the fiber design and system structure.
A single-polarization single-frequency (SPSF) 1030 nm distributed feedback (DFB) fiber laser with mode switchable output is achieved. The DFB fiber laser is realized based on a 5 cm long π-phase shifted fiber Bragg grating (PS-FBG). A maximum output power of 84 mW with single-polarization operation is achieved. The polarization extinction ratio (PER) is around 17 dB and the linewidth is 18 kHz. The slope efficiency is 13% and the spectrum at the highest power shows an excellent optical signal to noise ratio of about 75 dB. Moreover, LP11 mode single-frequency lasing is achieved by adopting an acoustically-induced fiber grating (AIFG). The mode purity is estimated to be higher than 96%. These results, to the best of our knowledge, show the highest output power among the reports that achieve single-frequency 1030 nm DFB laser output with single-polarization operation. Furthermore, it is the first time to realize transverse mode switchable single-frequency fiber laser based on an AIFG. The high order mode single-frequency 1030 nm fiber laser has much potential to find applications in multiplexing system.
Photonic lanterns can control beam modes by tuning phase, amplitude and polarization of multiple beam inputs, providing new methods to overcome mode instability in high-power large-mode area fiber lasers. The initial amplitude input of photonic lanterns are usually preset to be equal or random. The beam mode is then adaptively controlled by evaluating the beam output profile. This method can easily realize stable light output, but it sometimes runs into local optimum solutions. In this work, we propose a novel method to preset the initial amplitudes to realize better beam mode control. Based on the waveguide characteristics of the photonic lantern structure, we inversely work out the initial amplitude requirement of the input channels for controlling a certain mode using its transmission matrix. Taking 3×1 photonic lantern as an example, our simulation results show that this method can realize any mode control of LP01, LP11e, LP11o with their mode power ratios all above 99%, better than those preset methods to be equal or random. Our simulation method can be utilized in beam control experiments to achieve better beam control in photonic lantern-based fiber lasers
In this study, we design and fabricate a novel type of active fiber——double-tapered double-clad fiber (DT-DCF). Based on this self-developed DT-DCF, we have constructed an all-fiberized fiber amplifier that is operating under a continuous-wave (CW) regime at 1080 nm wavelength. The maximum output power of the system reaches 4 kW, which, to the best of our knowledge, is the highest output power of tapered fiber-based laser systems. The amplifier exhibits near-single-mode beam quality (M2=1.33) at the highest output power with a slope efficiency of 83%. Our result successfully verifies the potential of power scalability of DT-DCF, and the performance of our system can be further enhanced by fiber design optimization.
Taking all the pulse instantaneous power into consideration, we investigate the pulse interference dynamics of dissipative soliton (DS) and dissipative soliton resonance (DSR) through a discrete model based on nonlinear optical loop mirror (NOLM). We numerically and experimentally acquire that the reflection and transmission spectra of the NOLM under DS and DSR regimes show significant wavelength selectivity which results from the intensity-dependent nonlinearity. The wavelength selectivity phenomenon may be helpful for in-depth understanding on different operation dynamics of saturable absorber in passive mode-locked lasers.
In the past decade, two-dimensional (2D) materials have attracted increasing attention due to their energy band structure, optical properties and excellent performance in ultrafast photonics and nonlinear optics. As a kind of new 2D ternary layered material, NiPS3 can exhibit more novel electrical, optical and magnetic properties compared with those unary and binary 2D layered materials because of higher chemical diversity and structural complexity. In this work, we demonstrated a passively Q-switched operation based on few-layer NiPS3 as a saturable absorber (SA) in an erbium-doped fiber (EDF) laser. And the Q-switched output was achieved when the pump power at 40~105 mW. To our best of knowledge, it is the first time to utilize the NiPS3 as a SA in pulse laser generation.
We numerically present the generation of four types of pulses using nonlinear Schrödinger equations in an all-normal-dispersion dual-pump mode-locked fiber laser based on a nonlinear amplifying loop mirror (NALM). Transition state (TS) between dissipative soliton (DS) and dissipative soliton resonance (DSR), typical DSR, DS, noise-like pulse (NLP), and nonmode-locked state are investigated in a two-dimensional pump power space. Different from previous research, we first find that the pulse peak power and width vary dependently and nonmonotonously under asymmetric coupling ratios in the NALM. Under different coupler ratios and amplifier powers, the shift and distortion of simulated NALM transmittivity curve which are caused by the pump power intercoupling and the amplifier saturation are demonstrated. The ratio of incident power and the saturable power of NALM (when the ratio is >2, 1 ∼2, ∼1, <1) directly determines the pulse states (NLP, DSR, unstable, DS, or TS) in the cavity. Overall, the utilization of NALM could effectively enlarge the saturable power as well as the transmittivity, which provide a road to increase the DSR peak power.
We numerically present the generation of five types of pulse states using nonlinear Schrödinger equations in an all-normal-dispersion (ANDi) mode-locked fiber laser based on a nonlinear amplifying loop mirror (NALM). Using a two-dimensional pump power space of the two amplifiers, we investigate the pulse characteristics among typical dissipative soliton resonance (DSR), dissipative soliton (DS), transition state (TS) between DS and DSR, noise-like pulse (NLP) and unstable pulsation state. Different from previous research results, we firstly find that asymmetric coupling ratio of the NALM causes dependent and non- monotonous variation of the pulse peak power and width of DSR. The location of incident power and the saturable power of NALM (when the former is 2 times higher, higher, equal, lower than the later) determine the pulse states (NLP, DSR, unstable, DS or TS) in the cavity. The pump-power intercoupling-induced frequency-selective NALM transmission is demonstrated by the simulated pulse evolution over round-trips.
We have made simulations for a 3*1 photonic lantern and the latter experiments were carried out to test characters of the lantern we designed. The three core lantern with 0.004 rad taper angle has a 15 mm taper length which begins with a 125 μm outer diameter and ends with a 10.4 μm outer diameter(core diameter to outer diameter ratio is 2/25), while the NA of single mode fiber is 0.08 and for 25/400 μm core/cladding diameter few mode fiber it is 0.1. In order to obtain preferable LP01 mode shape, we used a phase control system to lock the phase of each channel staying consistent. The consequence demonstrated that the whole phase distribution of single mode fibers was controlled to an extent. Compared to amplitude, photonic lantern is more sensitive to phase fluctuation. We made simulations using Rsoft to test different variables of light propagating in 3*1 photonic lantern, and the results were in agreement with the conclusion. Thus, coherent beam combination based on dithering phase locked technique was studied systematically and we applied it to 3*1 photonic lantern. As a result, the beam received by light spot analyzer showed a stable mode shape but still existing some differences compared to expected LP01 mode or LP11 mode, which put forward reflection about the basic theory inside photonic lanterns (the beam propagating through tapered region didn’t make sense if using coherent beam combination calculation). At last, we attempted to explore the beam propagation through the lantern and to give any better evaluation function in phase control algorithm so as to improve the phase locked process.
In this work, we establish a unified temporal-spatial model to study the temporal-spectral dynamics of Ytterbium-doped fiber lasers (YDFLs). Different from the previously reported theories, this model is capable of obtaining the temporal property of YDFLs from the relaxation oscillation region to the relative stable region in the time window of millisecond scale with the time resolution of sub-nanosecond scale. The simulation results reveal that the three temporal instabilities, i.e. sustained self-pulsing (SSP), self-mode locking (SML) and turbulence-like pulsing (TLP), coexist in a multilongitudinal mode YDFL. When the output power evolution of the YDFL is observed in different time scales, different temporal instability phenomena dominate in the observation. Along with the increase of pump power, a single regular SML pulse would gradually break up into irregular TLP pulses and the life of the SSP pulse envelop would decrease. Besides, the spectral evolution property of the YDFLs at different pump powers are given and significant spectral broadening is observed.
The photonic lantern is an all-fiber-based linear optical element that couples light efficiently and evolves modes functionally between a set of single-mode waveguide and a multimode waveguide. Our study is based on different core photonic lanterns, which are fabricated by using the ‘ferrule technique’ method, with the single-mode array in a certain geometric arrangement. The process of the light propagation through the lanterns is simulated and the experiments according to the simulation results are conducted. The mode combining and evolution in the 3-core lanterns is simulated to study the necessary conditions for achieving the fundamental mode with good beam quality in large-mode area (LMA) fiber. Appropriate input (the amplitude ratio of each channel is 1:1:1, the phase and polarization state are the same) is injected at the single-mode (SM) end with some random disturbance on amplitude and polarization (the relative change is 20%). The M2 factor at the multimode (MM) end has a standard deviation of 0.0001 orders of magnitude. However, the M2 factor varies from 1.05 to 2 or even more with the changes of the phase difference at the SM end. Thus, the adaptive optics (AO) technique is used in our experiment, which can adaptively determine the appropriate phase to be applied to the input fibers. In addition, the modes behavior in 6- and 7-core photonic lanterns is simulated to obtain the ideal input (including the amplitude, phase and polarization of each SM channel) for achieving the six lowest order modes output. And these two kinds of photonic lanterns are compared from the aspects of drawing difficulty, mode coupling efficiency, transmission loss, and application feasibility in module multiplexing.
In the pulsed fiber amplifiers with repetition frequency of several tens kHz, amplified spontaneous emission (ASE) is easy to build up because of the low repetition frequency and weak pulse signal. The ASE rises the difficulty to amplify the weak pulse signal effectively. We have demonstrated an all-fiber preamplifier stage structure to amplify a 40 kHz, 10 ns bandwidth (FWHM) weak pulse signal (299 μW) with center wavelength of 1062 nm. Compared synchronous pulse pump with continuous wave(CW) pump, the results indicate that synchronous pulse pump shows the better capability of increasing the output power than CW pump. In the condition of the same pump power, the output power of synchronous pulse pump is twice as high as CW pump. In order to suppress ASE, a longer gain fiber is utilized to reabsorb the ASE in which the wavelength is shorter than 1062nm. We amplified weak pulse signal via 0.8 m and 2.1 m gain fiber in synchronous pulse pump experiments respectively, and more ASE in the output spectra are observed in the 0.8 m gain fiber system. Due to the weaker ASE and consequent capability of higher pump power, the 2.1 m gain fiber is capable to achieve higher output power than shorter fiber. The output power of 2.1 m gain fiber case is limited by pump power.
In this work, the spectral broadening and the stimulated Raman scattering (SRS) properties of fiber oscillators with different temporal and spectral characteristics were analyzed theoretically. We propose a spectral model for the amplification process in fiber amplifiers and a qualitative model for fiber oscillators through the superposition of multi-longitudinal modes. It is revealed that there are there are clearly linear and nonlinear broadening regions along with the power scaling in the MOPA structure, while the growth rate and the SRS threshold are all closely related to the temporal and spectral characteristics of the fiber oscillator. The preliminary analysis of the simulation results shows that both the noise floor and the temporal fluctuations in fiber oscillators will significantly impact the SRS threshold of the MOPA structure, while only the temporal fluctuations in fiber oscillators would impact the spectral broadening properties. The developed model represents a powerful tool for the optimization of a fiber oscillator through its structural parameters.
We demonstrated photodarkening induced core laser leakage phenomenon in the long term operation of a 3 kW copumping fiber amplifier. Output power perturbation of 4~6 minutes period was observed in the 6 hours maximum power operation. Despite the perturbation, the maximum power also drops from 3 kW to 2.65 kW due to the leakage of the core laser. This indicates that the co-pumping scheme may not be suitable to achieve high power fiber laser with long term output stability. We also provide a solution by adopting the counter-pumping scheme that shows very stable output power in the 1-hour maximum power operation test.
Modern teaching opinions require research-based teaching mode in research universities. It is an advanced teaching mode enabling students to explore science and technology autonomously. It has a lot of advantages comparing with the conventional knowledge-oriented teaching mode. In this paper, we show some understanding of research-based teaching mode. And based on the course features of Geometric optics and optical instruments and characteristics of students, we explore the research-based teaching mode in the part of Practical Optical Systems.
We demonstrated a new phenomenon, namely, thermally induced core laser leakage in single trench fiber (STF), for the first time. The STF provides very high loss and power delocalization of higher order mode (HOM) and maintain the effective single mode operation. However these properties are chartered only under low power situations. In this paper we established a 976nm directly pumped high power co-pumping fiber amplifier based on the STF. The maximum output power was 1022W with a slope efficiency of 76%. Further increase the pump power will leads to the output power decrease. Meanwhile a micro second Level noise like power fluctuation was observed. No resonance frequency was observed in frequency domain indicating the mode instability is not triggered. We believe that it is the thermally induced waveguide index profile change due to the excessively heat load in the front section of STF that leads to the failure of HOM suppression and the power of FM was coupled into the HOM. However the heat load in the rear section of STF was relatively low and the HOM leaked into the cladding due to the bending loss. We provide a mitigating method by pumping with pump light of smaller absorption. A maximum power of 1330W was achieved without power decrease via pumping the STF with 905nm and 976nm pump light (same amplifier). To our best knowledge, this is the first demonstration of thermally induced core laser leakage in STF and the pertinent results can provide significant reference for future optimization.
We report the results of our recent experimental investigation of the modulation frequency detuning effect on the output pulse dynamics in a pulse modulated actively mode-locked ytterbium doped fiber laser. The experimental study shows the existence of five different mode-locking states that mainly depend on the modulation frequency detuning, which are: (a) amplitude-even harmonic/fundamental mode-locking, (b) Q-switched harmonic/fundamental mode-locking, (c) sinusoidal wave modulation mode, (d) pulses bundle state, and (e) noise-like state. A detailed experimental characterization of the output pulses dynamics in each operating mode is presented.
A supercontinuum source with larger than 200 W average power covering 1 μm to beyond 1.7 μm is constructed in a
four-stage master-oscillator power-amplifier (MOPA) configuration. The process of power amplification and spectral
evolution during supercontinuum generation and the influence of pulse duration on this process are investigated. For the
signal light in normal dispersion region, Raman effect plays dominant role in the first phase and the power transfer from
amplified signal light to long wavelength region can be substantially achieved. When the spectrum is extended across the
zero dispersion wavelength, soliton effect becomes the main effect for spectral broadening and the power transfer
efficiency is lower to make the signal light a high peak. With similar average output power, narrower seed pulse leads to
higher peak power and can induce Stokes waves earlier which leaves longer fiber for soliton propagating. Thus the
spectrum can be shifted to longer wavelength. However, the signal light becomes a higher spike. In a word, narrower
seed pulse leads to higher signal peak and wider SC source.
In high power solid state lasers, thermal lens effect always give rise to the multi-modes oscillation in the resonator. The beam quality will deteriorate with the increase of output power. In this paper, an intra-cavity beam shaper is introduced to actively compensate the thermal lens in the laser resonator. One round trip ABCD matrix of the resonator with an intra-cavity beam shaper and thermal lens is calculated. The design parameters with wide stable zone are concluded through the ABCD matrix. The mode size and stability diagram of the resonator are calculated under different focal length of the thermal lens. The relationship between the adjustment of the intra-cavity beam shaper and the mode size under different thermal lenses are concluded, and general method to actively control the modes contents by adjusting the intra-cavity beam shaper is introduced. The effectiveness and performance of active mode control with the intra-cavity beam shaper are verified by simulations of the output modes of resonators. It shows that the M2 factor is well maintained below 1.6 even the focal length of the thermal lens changes from 5m to 0.5m.
High-power laser propagation through the inner optical path will produce a significant thermal effect on the beam-splitter mirror which will cause phase aberrations. Based on the three-dimensional transient heat conduction equation and the thermal elastic stress-strain equation, a simulation model of reflector mirror was built with three-dimensional finite element method (FEM). The temperature increase and thermal displacements of two kinds of mirror substrates (Al2O3 crystal and spinel) were especially investigated with different laser intensity, output duration and absorption coefficient. The effects of mirror thermal distortion on laser beam phase aberrations were also evaluated on both reflection and transmission directions. The experiments of high–power laser propagation through two kind materials of beam-splitter mirrors samples with diameters of 50mm and thicknesses of 5mm were carried out to measure the thermal effects induced by the absorbed laser energy. Both two kinds mirror samples were deposited the same film layer of same reflectance. A high power semiconductor laser was expanded to a beam of 35mm diameter, and double Shack-Hartmann wavefront sensors were used to detect both reflection and transmission thermal distortions of the mirror samples. The measurements showed that reflection aberrations of spinel mirror sample were larger than those of Al2O3 crystal mirror sample while its transmission aberrations were slightly less than Al2O3 crystal mirror sample. The results of experiments and simulations showed a very good consistency.
In this paper we discuss a new method to detect low-order aberration with large peak-valley value. This method also depends on wavefront slope measurements but only need measurements of 6 spots, which means that only 6 pieces of lens are used in detective process, and the mathematical algorithm involved in the calculation process is different from zonal or modal estimation used in Shack–Hartmann Wavefront Sensor. To evaluate the accuracy of this method we simulate this optical measurement process by using the Zemax simulation software and Matlab calculation software. Simulation results show that the reconstructed errors of Zernike aberration coefficients are higher for a larger peakvalley (PV) value of wavefront distortions. The maximal errors of aberration coefficients can be keep lower than 1% for aberrations with different combinations of defocus, astigmatism at 0° ,astigmatism at 45° and some high-order terms.. The new measurement method can be used to direct measure low-order aberrations for laser beam with large transverse area and do not need beam contracting system.
Influence function is one of the most important parameters for the correction of aberrations with deformable mirrors. When deformable mirrors are loaded with laser irradiation, the influence function will be affected by the heat-induced thermoelastic deformation, which would lower the aberration correction ability and effectiveness. On occasions when they are applied in laser systems, the influence of laser-induced thermoelastic deformation is significant, it would cause a considerable change in the influence function. We have analyzed the changes in the surface shape of a 37-element deformable mirror caused by laser induced thermoelastic deformation, and calculated the changes of influence function. The dependence of influence function on the arrangement of actuators, material properties of the actuators and the base and the heat flux is analyzed, with the actuator layout being square and hexagonal, different material properties and the heat flux varied from 0.005W/mm2 to 0.01W/mm2, using the finite element method. The results indicate that all of the previous factors will affect the influence function. Thus, in order to reduce the change of influence function caused by laser induced surface shape, the material and parameters of the deformable mirror should be chosen carefully.
Laser-induced static surface shape changes of deformable mirrors will cause difficulties for beam control of the laser system. The overall peak and valley (PV) value of the deformable mirror (DM) will reach the scale of micrometer when irradiated by high power lasers. We have investigated changes in the static surface shape of a 37-element DM caused by laser-induced thermoelastic deformation. It is found that the laser-induced profile change of the mirror shows a high-order characteristic. In this paper the finite element method is used to analyze the surface shape of the DM when it is irradiated by high power lasers. The surface shape is fitted using the Zernike polynomials and the influence functions to see the characteristic of it and the DM’s ability to compensate it. The fitting results show that the lower-order aberrations can be corrected by the DM itself with the cost of a reduced ability of aberration correction, as the available amount of stroke will decrease. In addition, changes of the influence functions are simulated and the modified influence functions are calculated. Furthermore, the performances of the DM in three different situations are simulated to reveal the impact of thermoelastic deformation on the DM’s aberration correcting ability.
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.
A new method to calculating the wavefront of slap laser is studied in this paper.
The method is based on the ray trace theory of geometrical optics. By using the Zemax
simulation software and Matlab calculation software, the wavefront of rectangular beam in
beam reshaping system is reconstructed. Firstly, with the x- and y-slope measurement of
reshaping beam the direction cosine of wavefront can be calculated. Then, the inverse beam
path of beam reshaping system is built by using Zemax simulation software and the direction
cosine of rectangular beam can be given, too. Finally, Southwell zonal model is used to
reconstruct the wavefront of rectangular beam in computer simulation. Once the wavefront is
received, the aberration of laser can be eliminated by using the proper configuration of beam
reshaping system. It is shown that this method to reconstruct the wavefront of rectangular
beam can evidently reduce the negative influence of additional aberration induced by beam
reshaping system.
We present a new method to calculate wavefront pre-compensation of the thermal deformation aberrations based on the finite element method (FEM) and Zernike polynomials. The thermal deformation aberrations of a plat circular Si mirror are theoretically analyzed in detail. Model of the beam path with 4 reflective mirrors and a uniform incident laser source is established. With the above model, performances of the outgoing laser with and without wavefront pre-compensation are calculated, respectively. The results show that the Strehl ratio of the outgoing laser beam is increased from 0.13 to 0.66 with wavefront pre-compensation using the new method. The influence of Fresnel number on the ability of wavefront pre-compensation was also studied. The value of SR increases to 0.83 as the Fresnel number is 257. The ability of wavefront pre-compensation is limited when the Fresnel number is small.
Compensation of low order aberrations is essential for high power solid state slab laser. With the increase of output
power, the peak-to-valley of wavefront distortion increase to dozens of micrometer. It’s difficult to control the wavefront
with deformable mirrors which always has limited stroke(<20μm). In this paper, a reflective beam shaping system is
designed to shaping the beam spot from rectangular to squarer. The beam shaping system consists of two x-oriented
cylindrical mirrors and two y-oriented cylindrical mirrors. Simulations of PID control algorithm for actively
compensating of low-order aberrations with reflective beam shaping system are presented. It shows that different
combinations of defocus, 0o astigmatism and 45° astigmatism, which is the main contributor of beam aberrations in slab
laser, can be well compensated by adjustment of distance and rotation angle of mirrors. And the convergence is fast when
the control error signal is set to a suitable combination of low order Zernike coefficients. For beam with wave aberrations
(PtV=82.6λ, RMS=18.2λ, Z4=23.6, Z5=7.1, Z6=19.6), the adjustment of distance between mirrors is below 100mm, and the rotation angle about z-axis is below 2 degree. The wavefront aberrations are decreased to a low level (PV=0.16λ,
RMS=0.04λ) which can be easily corrected later with DM.
Based on the generalized nonlinear Schrödinger equation, the femtosecond chirped-pulse propagation and supercontinuum (SC) generation in all-normal dispersion photonic crystal fibers (ANDi PCFs) are numerically investigated. The simulation results show that pre-chirp of the input pulse has a significant effect on the pulse evolution in ANDi PCFs. In the initial stage, up-chirped pulse broadens drastically, non-chirped pulse increases normally, and the spectrum of down-chirped pulse narrows rapidly through self-phase modulation (SPM). The ANDi PCF with smaller absolute value of dispersion (|D|) at the maximum dispersion wavelength (MDW) compresses the spectrum more than that with larger |D| but needs longer fiber as SPM dominate over GVD in it. As ANDi PCF is long enough, the bandwidth of the spectrum depends on the pre-chirp parameter at a fixed peak power. Bigger chirp result in broader spectrum for either up-chirped or down-chirped pulse. Increasing peak power can also enhance the bandwidth of the spectrum, but it is not such useful with largely chirped pulse.
The dispersion characteristics of Arsenic Trisulfide
As2S3 chalcogenide glass were analyzed and a kind of
As2S3 microstructured optical fiber was designed to shift the zero dispersion wavelength from longer than 4 μm to
around 2 μm. The supercontinuum generation in the designed fiber with selected pump wavelengths and pulse durations
were simulated through numerically solving the Generalized Nonlinear Schrödinger Equation. The results reveal that the
broadest supercontinuum covering 1.1 μm to 5.5 μm can be obtained with pump laser with duration 150 fs centered at
2.0 μm which is slightly longer than the first zero dispersion wavelength. When the pump wavelength is around the zero
dispersion wavelength apparent dispersive waves which are significant for spectral broadening can be observed.
A simple way to approach the problem of counteracting the phase aberrations induced by thermal deformations in a beam
control system is advanced and the influence of wavefront pre-compensation on correcting the phase shifts is proved by
experiments. The thermal deformations of a Si reflector irradiated by a circle-sharp laser beam are simulated, and the
phase aberrations of emitted laser are calculated. In order to overcome the effects of phase aberrations, the conjugation
wavefront pre-compensation of incident laser is studied theoretically and experimentally. There are three works in this
paper. Firstly, the thermal deformation of a mirror was simulated by using ANSYS Mechanical APDL application.
Secondly, on the base that the phase shifts has been calculated, the complex amplitude distribution of the outgoing laser
was calculated by using a space ray method of geometrical optics and light diffraction theory. Finally, the influence of
wavefront pre-compensation on emitted laser had studied experimentally with a scaled model in laboratory. The results
show that the uneven distribution of phase shifts which is induced by inhomogenous thermal deformations greatly
distorts the wavefront of the emitted laser. The beam quality of emitted laser has been ameliorated when a conjugation
phase is been placed on the incident beam.
The phase information of the thin heated air cylinder transection was acquired with a self-collimated interferometer
and line CCD camera at the frame rate of 27kHz , based on the presumption of axial symmetry ,using addition algebraic
reconstruction algorithm , 2-dimension index-of-refraction distribution of the heated air in the transection was acquired
,and the 2-dimension temperature distribution was calculated, the temperature shift was observed, the central
temperature of the air cylinder changed between 330K and 430K. The results show the method could be used to the
measurement of near axial symmetry aero-optical medium.
For a semi-conductive thin film and substrate system, it is difficult to obtain the geometric thickness and optical constants directly from reflectance value, if the materials of interest have finite thickness, absorption and dispersion. To simplify this, the model of Forouhi-Bloomer (F-B) is adopted to express reflectance merely with wavelength and several parameters which have definite values. Then the genetic algorithm (GA) is introduced to optimize and fit these parameters on data of reflection index in detected wave ranges. By comparing with a traditional optimization method the fitting error of GA is smaller. Finally, a sample with a thin film of amorphous Si coated on a crystal Si is analyzed. Thus, a novel method is presented to characterize the optical property of the film coated on a semi-conductive substrate, merely basing on the reflection index data.
Laser beam heat the air on the optic path, Beam-deflection optical tomography is a non-intrusive method to measure the 2-dimension temperature distribution in the transect. By means of linear Hartmann Sensor at the rate of 27kHz, the optic path was heated by a 2.7μm HF laser, continuous and high time resolution gradients of optic phase were obtained. the result of analysing and calculation showed the temperament shift in the heated beam path was not higher than 50K when the HF laser power was 9W. The experiment showed that it is a practical non-intrusive temperature shift measurement method for a small area aero-optical medium.
By means of Hartmann Sensor at the rate of 419 Hz, a series of dynamic degraded wave front of collimated laser transport perpendicularly across a heated air flow was measured. The airflow was generated using a vertical-placed nozzle. An integral result of the airflow density distribution was educed. Both span-wise direction and stream-wise direction structure convection were observed. Some pilot study of flow field visualization was made.
The optically active, turbulent flow field was investigated to simulate a low speed two-dimensional heated jet. Dynamic spatio-temporal index-of-refraction distribution and velocity profile of the turbulent flow field were obtained by applying computational fluid dynamic methods, the corresponding optical path difference numerical results were educed by optical computational methods. Some relationships between the flow large-scale structure and corresponding optical path differences are inferred.
Thermal blooming is one of the most important factors, which make the beam diffused and cause the decline of laser intensity on the object. This paper presents a method to reduce thermal blooming of high-energy laser (HEL) in the beam tube and compares two models describing the thermal blooming. Helium has the qualities of low absorption coefficient, high thermal conductivity and low index of refraction, and can be used as the medium through which HEL beam propagates. The condensations of thermal conduction model and acoustic wave model are evaluated respectively. Results show the method is practicable, and two models' applicable ranges are got.
The free-vortex aerodynamic windows (abbr. FADW) can take the place of the conventional solid windows for high energy lasers to solve the problems caused by the intrinsic fractional absorption of the laser energy. Usually the procedure in designing FADW includes two central processes. The first is to design the asymmetric supersonic laval nozzle using the method of characteristics and the method of simple wave under the ideal conditions, and the second is to design the diffuser. The method of designing the diffuser of FADW is based on the experimental method mostly, but the diffuser's properties influences the performance ofthe overall FADW system deadly, so an experimental prototype of FADW whose structure and layout is adjustable has been designed and manufactured to optimize the design of the diffuser for FADW system. This paper reports the design methods of the FADW system and discusses the experimental results. The results show that this prototype of the FADW has good running properties, and FADW whose structure and layout is adjustable can be used to optimize the FADW design perfectly.
KEYWORDS: Aerodynamics, Interferometry, Optical testing, High power lasers, Crystals, Laser crystals, Laser optics, Laser resonators, Fluctuations and noise, Laser energy
The concept of the free-vortex aerodynamic windows (FADW) is one of the best effective methods taking the place of the conventional beam output crystal windows of high power laser. An experimental model of the supersonic free-vortex aerodynamic window has been designed and made in our latest study. This paper studies the aero-optical phenomena of the FADWs supersonic jet to realize the laser optical quality degradation caused by the free- vortex jet, and to find out the relation of output laser beam quality through the FADW to the structure of the FADWs shearing interferometry. At last the structure of FADWs jet has been studied by means of shearography.
A two-dimensional program is applied to simulate the chemical dynamic process,gas dynamic process and lasing process of a combustion-driven CW HF overtone chemical lasers.Some important parameters in the cavity are obtained.The calculated results include averaged pressure and temperature,zero power gain coefficient, laser spectrum, output power, chemical efficiency and overtone efficiency.Based on the numerical simulation,the influence of entering gas flow parameter,nozzle structure and nozzle array length on the performance of HF overtone lasers are investigated separately.Because of small zero power gain coefficient of each HF overtone spectral line,the saturation degree of gain medium in the cavity is the most important parameter to influence the overtone efficiency of HF lasers.
A two-dimensional program was applied to simulate the chemical dynamic process, gas dynamic process and lasing process of a combustion-driven CW HF overtone chemical lasers. Some important parameters in the cavity were obtained. The calculated results included HF molecule concentration on each vibration energy level while lasing, averaged pressure and temperature, zero power gain coefficient of each spectral line, laser spectrum, the averaged laser intensity, output power, chemical efficiency and the length of lasing zone.
The characters of combustion driven cw deuterium fluoride (DF) chemical laser with C2H4/NF3 reactant were numerically investigated. The numerical simulation was carried out using compressibility scaling method--a finite difference technique for the numerical integration of the steady and unsteady Navier-stokes equations for reactive flow. The small signal gain and the flow field were calculated. The numerical results shown that active zone length of the cw DF chemical laser with C2H4/NF3 is very long, which is about 6 cm, and the average cavity pressure is about 7 torr as the combustion pressure is about 1.5 atm. These results shown that the DF chemical laser with C2H4/NF3 is suitable for high cavity pressure performance.
The performance change of photovoltaic (PV) indium stibium (InSb) detector illuminated by continuous wave (CW) chemical oxygen iodine laser (COIL) with different power density was measured and the damage threshold interval of the detector was achieved to be 26(0.89S) approximately 113(1.4S)W/cm2. Using the integrating sphere technology, the thermal coupling coefficient of InSb crystal material to CW COIL was measured to be 0.664 +/- 0.004. Compared with the theoretical result, the thermal coupling coefficient measured was a little bigger due to the effect of the surface defect of InSb material. The principle and damage mechanism of PV InSb detector was reviewed. The temperature rise and performance change of PV InSb detector illuminated by CW COIL were calculated with 1D thermal model. The calculated results was consistent with the experimental results.
The He is usually used as diluent in combustion cw hydrogen/deuterium fluoride (HF/DF) chemical lasers. Sometimes, the N2 is also used as diluent in cw HF/DF chemical laser, because N2 is more cheaper than the He. It was thought that the efficiency of cw HF/DF laser with N2 diluent was lower than that of HF/DF laser with He diluent since the molecular weight of N2 is heavier than that of He. In this paper, the performance of cw DF chemical lasers with He and N2 diluent was investigated. The small signal gain, power spectral distributions, cavity pressure, power and efficiency of DF lasers with these two diluents have been calculated using a finite-difference technique for the numerical integration of the steady and unsteady Navier-Stokes equation with reactive flow (compressibility scaling method-CSM), which was developed in the chemical gas dynamic laboratory of the state institute of applied chemistry in Russia. The numerical results show that the efficiency of DF lasers with N2 diluent is not lower than that of the lasers with He diluent.
Different geometry nozzles used in CW hydrogen fluoride chemical laser are investigated. Four geometry parameters, including throat width; area ratio; axis length and base width are considered. The flow properties, laser outcoupling power and small signal gain (SSG) of a Fabry-Perot resonator are calculated. The results show that when throat width and area ratio increase, the power and SSG peak will decrease in varying degree; they also varied when axis length changed; larger base width is related to lower cavity pressure, and smaller base width is related higher cavity pressure.
The optical limiting behavior has been first observed for C60 solution in toluene and in polymethyl methacrylate by Lee W. Tutt et al and A. Kost et al respectively. In order to make the possible use of the optical limiting in practice, we have studied the optical limiting of Q- switched Nd:YAG radiation at 532 nm using the C60 in KBr as a solid host and the C60 thin film on optical glass. The experimental results for the C60 in KBr show that the threshold power of the optical depends on the concentrations of the C60 in KBr, and the solid KBr-C60 is induced optical damaged as the input fluence reach to 0.7 J/cm2. The experiment results for the C60 thin film on optical glass show that the optical limiting behavior is not obvious, and the C60 film is be induced optical damaged when the input fluence is greater than 0.55 J/cm2.
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