Youen Jiang

Theoretical and experimental research on cryogenic Yb:YAG regenerative amplifier

Based on the theory of quasi-three-level rate equations modified by amplified spontaneous emission, the stored energy density and the small signal gain of the cryogenic Yb:YAG regenerative amplifier for a given geometry for pulsed pumping in three dimensions are theoretically studied using the Monte Carlo simulation. The present model provides a straightforward procedure to design the Yb:YAG parameters and the optical coupling system for optimization when running at cryogenic temperature. A fiber-coupled laser diode end-pumped cryogenic Yb:YAG regenerative amplifier running at 1 030 nm is demonstrated with a maximum output energy 10.2 mJ at a repetition rate of 10 Hz. A very good agreement between the experiments and the theoretical model is achieved.

Suppression of FM-to-AM modulation by polarizing fiber front end for high-power lasers.

FM-to-AM modulation is an important effect in the front end of high-power lasers that influences the temporal profile. Various methods have been implemented in standard-fiber and polarization-maintaining (PM)-fiber front ends to suppress the FM-to-AM modulation. To analyze the modulation in the front end, a theoretical model is established and detailed simulations carried out that show that the polarizing (PZ) fiber, whose fast axis has a large loss, can successfully suppress the modulation. Moreover, the stability of the FM-to-AM modulation can be improved, which is important for the front end to obtain a stable output. To verify the model, a PZ fiber front end is constructed experimentally. The FM-to-AM modulation, without any compensation, is less than 4%, whereas that of the PM fiber front end with the same structure is nearly 20%. The stability of the FM-to-AM modulation depth is analyzed experimentally and the peak-to-peak and standard deviation (SD) are 2% and 0.38%, respectively, over 3 h. The experimental results agree with the simulation results and both prove that the PZ fiber front end can successfully suppress the FM-to-AM conversion. The PZ fiber front end is a promising alternative for improving the performance of the front end in high-power laser facilities.

Laser pulse spectral shaping based on electro-optic modulation

A new spectrum shaping method, based on electro-optic modulation, to alleviate gain narrowing in chirped pulse amplification (CPA) system, is described and numerically simulated. Near-Fourier transform-limited seed laser pulse is chirped linearly through optical stretcher. Then the chirped laser pulse is coupled into integrated waveguide electro-optic modulator driven by an aperture-coupled-stripline (ACSL) electrical-waveform generator, and the pulse shape and amplitude are shaped in time domain. Because of the direct relationship between frequency interval and time interval of the linearly chirped pulse, the laser pulse spectrum is shaped correspondingly. Spectrum-shaping examples are modeled numerically to determine the spectral resolution of this technique. The phase error introduced in this method is also discussed.

Real-time characterization of FM-AM modulation in a high-power laser facility using an RF-photonics system and a denoising algorithm

FM-AM modulation of high-power lasers significantly affects laser performance. Therefore, precise measurement of the FM-AM modulation depth is necessary. The subsequent FM-AM modulation generated by group velocity dispersion when the laser pulse propagates through a fiber affects the measurement accuracy. In order to eliminate this effect, a waveform-acquisition module is proposed that converts a broad-spectrum pulse of 1053 nm to a narrow-spectrum pulse of 1550 nm, without affecting the waveform. In addition, a signal-processing algorithm based on the orthogonal matching pursuit method is implemented to remove the sampling noise from the waveform. In this way, the signal-to-noise ratio of the measurement can be readily improved. Both theoretical and experimental results indicate that the proposed FM-AM modulation detection system is effective and economical. It can measure the FM-AM modulation depth precisely, and therefore shows considerable promise for future applications in high-power lasers.

Performance of smoothing by spectral dispersion combined with distribute phase plate on SG-II

Experimental performance of one-dimensional (1D) smoothing by spectral dispersion (SSD) combined with distributed phase plate (DPP) on the ninth beam of SG-II is presented. Without the application of SSD, normalized focal-spot non-uniformity of an 85% energy concentration is about 60%. Then, spectral bandwidth of the 3-ns, 1053-nm laser pulse is broadened to 0.3 nm (as 270GHz in 3ω) by a 3-GHz modulator and a 10-GHz modulator integrated in the front-end system. Spectral dispersion of 236 μrad/Å is achieved by a Littrow-configuration, 1480-l/mm grating placed between the Φ40mm faraday isolator and the third Φ40mm rod-amplifier. By using such SSD, normalized non-uniformity with the same energy concentration is decreased to 16%. A scheme of spatial power spectral density (PSD) in different directions is adopted to analyze the intensity distribution of the far-field irradiation. Based on the spatial PSD analysis, theoretical predictions of spectral peak caused by SSD’s color cycles is in excellent agreement with the experimental result. With double-frequency modulation, the amplitude of the spectral peak is reduced by ~10dB. The temporal waveform of the 1ω laser is measured. Waveform distortion criterion defining the frequency modulation to amplitude modulation conversion (FM-to-AM) is about 6% with 1ω laser energy of ~1.8kJ.

Experimental investigation on beam smoothing by combined spectral dispersion and lens array technology

The experimental performance of beam smoothing by combined one-dimensional (1D) spectral dispersion and lens array (LA) technology is presented, as applied in the ninth beam of SG-II. Using 3\omega spectral dispersion with a bandwidth of 270 GHz and a line dispersion that is 24.9 times the beams diffraction-limited width decreases the focal spot non-uniformity of 80% energy concentration from 46% to 17%. The multiple-beam interference properties of the LA are theoretically and experimentally validated by spatial power spectral density analysis. Peak-spectra suppression ratios of 20 and 10 dB are achieved in the dispersion and orthogonal directions, respectively.

Microwave resonant electro-optic bulk phase modulator for two-dimensional smoothing by spectral dispersion in SG-II

A special-velocity-matched electro-optic (EO) phase modulator employing a microwave resonant design in lithium niobate is presented. Both the microwave property and phase-modulation performances of the 3.25-GHz modulator agree well with the numerical simulations. Using this modulator in a single-pass configuration with 1-kW microwave drive power, the spectral bandwidth of a 1 053-nm, 3-ns pulse-length laser is broadened to 0.13 nm. With a clear aperture of 5×5 (mm), the modulator is suited for two-dimensional smoothing by spectral dispersion in high power laser systems.

Tunable compensation of GVD-induced FM–AM conversion in the front end of high-power lasers

Group velocity dispersion (GVD) is one of the main factors leading to frequency modulation (FM) to amplitude modulation (AM) conversion in the front end of high-power lasers. In order to compensate the FM-AM modulation, the influence of GVD, which is mainly induced by the phase filter effect, is theoretically investigated. Based on the theoretical analysis, a high-precision, high-stability, tunable GVD compensatory using gratings is designed and experimentally demonstrated. The results indicate that the compensator can be implemented in high-power laser facilities to compensate the GVD of fiber with a length between 200-500 m when the bandwidth of a phase-modulated laser is 0.34 nm or 0.58 nm and the central wavelength is in the range of 1052.3217-1053.6008 nm. Due to the linear relationship between the dispersion and the spacing distance of the gratings, the compensator can easily achieve closed-loop feedback controlling. The proposed GVD compensator promises significant applications in large laser facilities, especially in the future polarizing fiber front end of high-power lasers.

High precision long-term stable fiber-based optical synchronization system

A fiber-based，high precision long-term stable time synchronization system for multi-channel laser pulses is presented，using fiber pulse stacker combined with high-speed optical-electrical conversion and electronics processing technology. This scheme is used to synchronize two individual lasers including a mode-lock laser and a time shaping pulse laser system. The relative timing jitter between two laser pulses achieved with this system is 970 fs (rms) in five minutes and 3.5 ps (rms) in five hours. The synchronization system is low cost and can work at over several tens of MHz repetition rate.

Compensation system for FM-to-AM effects in high-power laser system

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.