A comprehensive finite element physics model is constructed to analyze the thermal effects of a thin disk medium. Based on this physical model, the temperature and stress distribution inside the medium are obtained. According to the corresponding theory, the internal optical path difference (OPD) and thermal focal length of the medium can be calculated when subjected to thermal load. According to the numerical model of the thermal effect, which has been verified by experiments conducted on commercial thin disk purchased from Dausinger + Giesen GmbH (DG), the study investigated the influence of various factors on the thermal effect of the thin disk medium. Additionally, a thin disk medium based on photoadhesion technology has been developed. The thermal focal length of the thin disk medium has been compensated by adjusting the surface shape in advance to match the desired focal length, which is significantly higher than that of the uncompensated medium. Below a pump density of 1.27 kW/cm2, the thermal focal length of the self-developed thin disk medium is comparable to that of the DG's thin disk medium.
In order to accurately analyze and evaluate the output beam quality of large-aperture laser amplifiers, we established a new method of simulating thermal effects which can analyze the distributed thermal effects in the direction of optical transmission. In this method, gain medium would be divided to several segments. Thermal effects and beam propagation in every segment are calculated and cascaded to the next one in the direction of optical transmission until the end of gain medium rod. To achieve propagation of aperture-level distance in these segments, a close-range propagation algorithm based on angular spectrum is proposed and its correctness is verified by comparing with theoretical results obtained by Fresnel diffraction. Though this new method, simulations of the thermal effects in simplified amplifiers with two gain medium rods that compensate each other at different apertures were carried out. The results show that residuals of depolarization loss increase with aperture. When the diameter is up to 35mm, the maximum depolarization loss is 98.8%. This new method of simulating thermal effects lays a solid foundation for the further research on large-aperture highrepetition- rate laser systems.
The thin-disk laser is promising in high average and peak power laser systems. Due to its high surface-to-volume ratio, the thin-disk laser has many advantages: low thermal lens, low brightness constraints for pump diodes, average and peak power scaling by increasing the beam cross section and negligible nonlinearities. In this letter, we report on our latest development of key technologies of the thin-disk laser, including the thin-disk crystal module, multi-pass pump scheme, thermal management and large-mode resonator technology. The small diameter thin-disk module with a 24 pump passes system and an efficient cooling module is realized. The optical path difference of the thin-disk module is measured. The key large-mode resonator technology is developed in an Nd:glass large-mode rod regenerative amplifier which delivers 125 mJ pulses at 1053 nm.
A preliminary concept design of a diode pump solid-state Nd: glass laser amplifier based on helium-cooled multi-slab geometry is presented. The laser amplifier is designed to obtain an output of 100 J/10 Hz. The gain slabs applied in the laser amplifier can lead to low thermally induced wavefront distortion, which are based on heating the edge by the cladding layer. In addition, we also develop a comprehensive numerical model for study effect of amplifier spontaneous emission on the stored energy and thermo-optic effects in the Nd: glass laser amplifier. The results of simulations show that energy storage efficiency of 48% and the average volume density of the stored energy greater than 0.55 J/cm3 are obtained for the laser amplifier. The wavefront distortion can be decreased to 0.78 λ for a gain slab in the laser amplifier.
We demonstrated a broadband Nd: glass laser amplifier with controllable gain bandwidth in experiment, which is expected to be used in high energy low coherent laser amplifier system. A birefringent crystal for spectral filtering in regenerative laser amplifier was designed and proposed, and the parameters of birefringent crystal was optimized in simulation. The birefringent crystals with different bandwidths were designed by this method. The laser amplification output with different gain bandwidths of 6~10 nm was realized, which was also verified by experiments. This broadband Nd: glass laser amplifier is expected to be used in large scale broadband laser amplification system, and provides important technical support for a new type of broadband tunable nanosecond high-power laser facility.
This study presents the hydrodynamic simulation for high energy large-size Nd:YAG liquid-cooled laser amplifier. Based on hydrodynamic equations, the heat transfer coefficient as a function of cooling fluid velocity is derived. The velocity of cooling fluid could be chosen as 0.8m/s by considering critical Reynolds numbers for laminar and turbulent flows. In order to assess the uniformity of the cooling fluid, a four-channel fluid model is established. The uniformity of four D2O fluid channel in gain region are 98.5%, 98.7%, 98.5%, 98.6%, respectively. Besides, A simple Nd:YAG heat transfer model is built for assessing the cooling capacity of fluid, which is based on the calculation of heat transfer coefficients equation. The temperature differences of central and marginal gain medium planes is 3.451K and 1.951K, respectively, which is close to 3.255K and 1.778K calculated in the heat-fluid-solid coupling model. Based on the heat fluid-solid coupling model, the total wavefront aberration is 0.266λ cooled by D2O. Finally, another cooling fluid FC770 has been compared with D2O. In terms of fluid uniformity, the uniformity of four FC770 fluid channels in the gain region are 98.5%, 98.7%, 98.5%, 98.6%, respectively, which is close to D2O. In terms of heat effect, the heat transfer coefficients of D2O is larger than FC770. And total wavefront aberration of Nd:YAG cooled by FC770 is 0.840λ, which is larger than D2O. Eventually, the thermally induced wavefront aberrations of D2O and FC770 are 0.0475λ and 0.6092λ, respectively. The calculated results showed that D2O is a better cooling fluid than FC770.
We demonstrated the arbitrary pulse shaping for a high power, joules class, and multi-pass ring Nd: glass laser amplifier system with nanosecond pulses based on the direct calculation method. While the square pulse with pulse energy 0.9mj and FWHM (full width at half maximum) pulse width 6ns, a 0.9J at 1Hz high power laser pulse energy is generated, which has the ability to change the waveform arbitrarily based on the full fiber front end. The laser amplifier system consists of three parts: full fiber seeder, diode pumped Nd: glass regenerative amplifier, and multi-pass ring amplifier. With the help of direct calculation method based on the input-output model, the input pulse shape has been calculated and the output pulse shape has been compared to the target pulse shape, showing that the simulation and experiment is consistent. Some other interesting pulse shapes have been produced with pre-compensated inject pulse based on the calculation which shows great potential to be applied in high power laser amplifier system with a desired pulse shape.
In this work, we evaluated the feasibility of sapphire conductive cooling for short-pulse Ti:Sapphire (Ti:sa) laser amplifiers which suffers from thermal issues under high-repetition-rate and high-energy operation. Numerical heat transfer simulations of 100-TW class sapphire face-cooled Ti:sa gain modules operated around 300 W average powers are presented. The distributions of temperature, stress, strain, and birefringence in liquid cooled sapphire/Ti:sa/sapphire assembly are calculated by a finite element analysis. Based on these data, the thermal induced wave front distortions and depolarization are investigated for different repetition rates. We determine that sapphire face cooling concept holds a promise of achieving higher energies and repetition-rates in Ti:Sa amplifiers.
This paper introduces the recent development of our integrated optical addressed spatial light modulator and its applications in the high power laser systems. It can be used to convert the incident beam into uniform beam for high energy effiency, or it can realize special distribution to meet the requirements of physical experiment. The optical addressing method can avoid the problem of the black matrix effect of the electric addressing device. Its transmittance for 1053nm light is about 85% and the aperture of our device has reached 22mm× 22mm. As a transmissive device, it can be inserted into the system without affecting the original optical path. The applications of the device in the three laser systems are introduced in detail in this paper.
In the SGII-Up laser facility, this device demonstrates its ability to shape the output laser beam of the fundamental frequency when the output energy reaches about 2000J. Meanwhile, there’s no change in the time waveform and far field distribution. This means that it can effectively improve the capacity of the maximum output energy.
In the 1J1Hz Nd-glass laser system, this device has been used to improve the uniformity of the output beam. As a result, the PV value reduces from 1.4 to 1.2, which means the beam quality has been improved effectively.
In the 9th beam of SGII laser facility, the device has been used to meet the requirements of sampling the probe light. As the transmittance distribution of the laser beam can be adjusted, the sampling spot can be realized in real time. As a result, it’s easy to make the sampled spot meet the requirements of physics experiment.
In the high-power laser facility, frequency modulation to amplitude modulation (FM-to-AM) effects has seriously affected the power balance between beams and restricted the laser flux levels of safe operation in the system. For FM-to- AM effects produced by gain-narrowing effects, according to the amplifier gain-narrowing function model, after simulating and analyzing the properties of FM-to-AM effects, a corresponding compensation function is designed. Using sinusoidal compensation function, with the use of a birefringent crystal and liquid crystal modulator, adjusting the crystal angle in the range of 45 °, the center wavelength could be reduced in the magnitude of the range from 0 to 30dBm. By changing the voltage of the liquid crystal, the center wavelength could be adjusted within 1051.5-1054.5nm freely. For the regenerative amplifier with the gain of 70dB and input center wavelength of 1053nm and bandwidth of 0.7nm, the output FM-to-AM magnitude could be controlled within ~11% by this compensation system.
As the development of the laser-driven technology, the next generation of laser-driven device sets higher requirement for the repetition frequency. The higher repetition gives rise to thermal deposition, which induces thermo-optical effect, elasto-optical effect and bulk displacement. The thermal efficient management is an important approach to dissolve the thermal deposition. The quasi uniform distribution of heat medium is realized by helium cooling Nd:glass slab and the control of edge temperature. In the case, wavefront distortion and depolarization losses is obtained in experiment. Results said that both of them are improved greatly. At the same time, the distribution of temperature, stress and strain and stress birefringence in Nd:glass are analyzed by using finite element numerical simulation method. And the calculation results show that the wavefront distortion and depolarization losses match with the experimental results very well.
Diode-pumped solid state laser system based on cryogenic Yb:YAG active-mirror scheme are presented with recent energy output. With improved optical design, 6.05J/1Hz pulse energy is achieved and a conceptual design with 30J output energy is theoretical simulated. The doubling efficiency of YCa4O (BO3)(YCOB) crystal is also discussed in this paper.
In order to broaden the spectrum of laser pulse and reduce the gain narrowing effect in Nd:glass regenerative amplifier to
realize the ambition of inhibiting amplitude and frequency modulation, proper quartz birefringence crystal plate is inserted
into the cavity. The influence factors of central wavelength, depth of modulation and range of modulation are obtained
theoretically. The width of the spectrum is broadened by controlling all the factors. Two kinds of thickness, 5mm and
6mm, are inserted into the regenerative amplifier cavity. The results of theoretical calculation and experiment both show
that the effect of spectrum widening is evident, which reduces the gain narrowing effect to some extent. The amplitude and
frequency modulation resulted from gain narrowing effect is inhibited when the central wavelength deflects. The simulated
results show that inhibited effect of amplitude and frequency modulation is remarkable. And the method is a potential
effective technique for amplitude and frequency modulation inhibition.
Pre-amplifier between the frontend and main power amplifier is the key unit of high power laser divers. The recent
progresses on the off-axis quadruple pass amplifier are presented, which include the beam path design, parasitic
oscillation research and experimental results. A single longitudinal mode, temporally shaped laser pulse with 5ns pulse
duration at 1053nm is injected into a Nd: Glass regenerative amplifier, which can provide a 12mJ energy output with
0.5% long term energy stability. The quadruple pass amplifier is designed as an off-axis pattern. With 1.3mJ energy
injection, amplified pulse with 16.5J can be achieved, and the measured output energy stability of the amplifier is 7.3%
(PV) at this output energy level, corresponding to a 21 shot result. The total gain of the amplifier is more than 10,000.
The parasitic oscillation was analyzed and discussed, and the parasitic mode and pencil beam are neither observed in the
experiment.
The spectrum characteristic of cryogenic has been investigated and the cryogenic
Yb:YAG amplification has been developing. As the temperature decreases, the
stimulated emission cross section increasing rapidly with the center wavelength
becoming short and the gain spectrum bandwidth narrowed. A diode-pumped
cryogenic Yb:YAG regenerative amplifier at 10Hz repetition rate has been carrying
out. Temperature of the Yb: YAG crystal has been controlled between 185K and 190K.
A ~100 pJ optical pulse with 10 ns time duration and 10 Hz repetition rate at 1030 nm
wavelength is inject into the regenerative amplifier. ~10.5 mJ output energy at 10 Hz
from the regenerative amplifier with a square-pulse distortion of ~1.5 and an
output-pulse-energy fluctuation of 7% was achieved.
In this paper, an optical Long-period fiber grating (LPG) fabricated by CO2 laser in side-hole single-mode fiber (SHLPG)
for cantilever deflection measurement was demonstrated according with LPG flexural character. resonance wavelength
of SHLPG (1547.3nm) has a total blue shift of approximately -0.83nm for deflection ranging from 0 to 40mm, and a red
shift of 0.74nm with deflection ranging from 0 to -40mm. The results indicated that the SHLPG owns higher deflection
sensitivity than that of normal LPG and fiber Bragg grating. The deflection sensitivity coefficient of SHLPG is
-0.202nm/mm. The resonance wavelength shift has a good linear with the deflection increasing. The correlative
coefficient is 0.9966. Factual deflection measuring can be got with high sensitivity through SHLPG easily.
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