Single Longitudinal Mode Pulses Research Articles

Self-focusing and stimulated Brillouin scattering effect of low-temporal coherence light and corresponding damage characteristics in fused silica

Laser-induced damage thresholds (LIDTs) of input/exit surfaces and filamentation of fused silica under low-temporal coherence light (LTCL) and a corresponding damage mechanism are investigated. By comparing self-focusing effects of fused silica for each incident laser, the lower LIDT of filamentation damage under LTCL irradiation is mainly attributed to stronger self-focusing than traditional single longitudinal mode (SLM) pulse lasers. Meanwhile, differences in LIDTs for input/exit surfaces by LTCL and SLM pulse laser irradiation are attributed to self-focusing effects and backward stimulated Brillouin scattering. Finally, influences of fused silica thickness and incident laser polarization on LIDT are demonstrated. The research contributes to exploring safe boundaries for fused silica application in high-power LTCL devices.

High-average-power single-frequency pulse optical parametric oscillator based on pulse-integrated seed-injection automatic locking.

Near-infrared nanosecond (ns) single-longitudinal-mode (SLM) pulse light generated from an optical parametric oscillator (OPO) is an important source in nonlinear optics and high-precision spectral analysis. In this Letter, a stable SLM near-infrared ns pulse light source generated from the OPO is presented, which is achieved by developing a seed-injection automatic locking technique based on a pulse-integrated photodetector (PIPD). Depending on the PIPD, the peak power of the pulse light detected by the photodiode is converted to the average power by integrating several pulses. As a result, the detector saturation is thoroughly eliminated, and the interference signal including the resonance point between seed and pulse lights can easily be attained by scanning the resonator length. On this basis, a microcontroller unit (MCU) is employed to realize automatic locking by looking for the minimum value of the interference signal. Finally, a SLM 824 nm pulse light source with an output power of 20.5 W and a linewidth of 51.42 MHz is obtained. The presented method can pave the way to implement a low-cost and compact high-average-power SLM pulse OPO.

Low-temporal coherence light self-focusing effect by spatial resolved method

The low-temporal coherence light (LTCL) has received extensive attention in the research of inertial confinement fusion due to its physical properties of instantaneous broadband. Recent reports demonstrated that the LTCL has significant suppression effects on laser plasma instability. However, the temporal spike structures of the LTCL will not only induce the amplification of the self-focusing effect, but also make its small-scale self-focusing characteristics and corresponding damage mechanism more complicated. Exploring the self-focusing characteristics of the LTCL will provide an important information for improving the output power of the LTCL. In this work, we design a more accurate test method for comparing the nonlinear self-focusing effects of different lasers, and compare the self-focusing effect of LTCL with single longitudinal mode (SLM) pulse. In the experiments, fused silica is tightly focused by a short focal length lens to avoid damaging the input surface. A spatially resolved test method is designed to measure the nonlinear <i>I</i>×<i>L</i> value (where <i>I</i> is the incident intensity, <i>L</i> is the distance from the head of filamentation damage to the input surface), which is accumulated from the input surface to the head of filamentation damage. The results show that the nonlinear <i>I</i>×<i>L</i> value obtained by the spatially resolved method is lower than by the traditional test method, since the energy loss caused by incident surface damage and backward stimulated Brillouin scattering (SBS) has been resolved. Furthermore, the nonlinear <i>I</i>×<i>L</i> values of the SLM pulse and the LTCL are also compared by the traditional test method and spatially resolved method. The test results show that due to the temporal spike structure, the LTCL has a lower nonlinear <i>I</i>×<i>L</i> value than the SLM pulse. The SBS effect and the different damage characteristics of the input surface are also analyzed. This study provides a more accurate test method for better analyzing the self-focusing effect of LTCL and laser pulses with different characteristics, and hence presenting a reference for designing high-power devices of low-temporal coherence light.

Damage characteristics of fused silica under low-temporal coherence light

Abstract The damage characteristics of fused silica were investigated under low-temporal coherence light (LTCL). It was found that the laser-induced damage threshold (LIDT) of fused silica for the LTCL was lower than that of the single longitudinal mode pulse laser, and for the LTCLs, the LIDTs decrease with the increasing of laser bandwidth, which is not consistent with the temporal spike intensity. This is due to the nonlinear self-focusing effect and multi-pulse accumulation effect. The specific reasons were analyzed based on theoretical simulation and experimental study. This research work is helpful and of great significance for the construction of high-power LTCL devices.

Laser-induced damage of fused silica optics at 355 nm due to backward stimulated Brillouin scattering: experimental and theoretical results.

Forward pump pulses with nanosecond duration are able to generate an acoustic wave via electrostriction through a few centimeters of bulk silica. Part of the incident energy is then scattered back on this sound wave, creating a backward Stokes pulse. This phenomenon known as stimulated Brillouin scattering (SBS) might induce first energy-loss, variable change of the temporal waveform depending on the location in the spatial profile making accurate metrology impossible, and moreover it might also initiate front surface damage making the optics unusable. Experiments performed on thick fused silica optics at 355 nm with single longitudinal mode pulses allowed us to detect, observe and quantify these backward pulses. Experimental results are first compared to theoretical calculations in order to strengthen our confidence in metrology. On this basis a phase-modulator has been implemented on the continuous-wave seeders of the lasers leading to pulses with a wide spectrum that suppress SBS and do not exhibit temporal overshoots that also reduce Kerr effects. The developed set-ups are used to check the reduction of the backward stimulated Brillouin scattering and they allow measuring with accuracy the rear surface damage of thick fused silica optics.

A Single-Longitudinal-Mode Passively Q-Switched Nd:YVO4 Laser Using Black Phosphorus Saturable Absorber**Supported by West Young Scholar Foundation of the Chinese Academy of Sciences under Grant No XAB2015B27.

A black phosphorus (BP) saturable absorber (SA) solution with different concentrations (1.0 and 0.5 mg/ml) is fabricated with the liquid-phase exfoliation method. By using the BP-SA, a compact diode-pumped passively Q-switched Nd:YVO4 laser is demonstrated. One reflecting Bragg gratings is used as the output coupler for mode selection. By inserting those BP-SA solutions in the laser cavity, the maximum single longitude mode, Q-switched output powers of 126 mW at 692.5 kHz and 149 mW at 630.3 kHz are achieved at the pump power of 8.0 W, corresponding to the pulse durations of 144 ns and 196 ns, respectively. Moreover, longitudinal-mode characteristics of Q-switched output laser in different optical cavity lengths based on two-kind BP-SA solution concentrations are investigated. Our results show that BP-SA could also be developed as an effective SA for the Q-switched, single longitudinal mode pulse laser.

The effect of the depth of single longitudinal mode modulation in Q-switching pre-Pr3+:YLF laser

The single longitudinal mode (SLM) can be obtained under the condition of Q-switching pre-lase. In this paper, the model of Q-switching pre-lase is firstly established. Taking the Pr3+:YLF laser as an example, the process of Q-switching pre-lase is simulated and optimized, then the optimized parameters and best output characteristics under different depth of SLM modulation are obtained. Comparing with the normal Q-switching laser, the SLM pulse energy can reach to 79.29%, the pulse width exceeds 16.45% and the depth of SLM modulation get to be 20. The results show that the Q-switching pre-lase output characteristics can be effected obviously by the SLM modulation depth, and the pulse energy and pulse width can be close to the normal Q-switching laser as long as the depth of SLM modulation is optimized.

Influence of longitudinal mode beating on laser-induced damage in fused silica

In our study, the laser-induced damage densities on a fused silica surface produced by multiple longitudinal mode (MLM) pulses are found to be higher than those produced by single longitudinal mode pulses at 1064 nm. This behavior is explained by the enhancement of the three-photon absorption due to the intensity spikes related to longitudinal mode beating. At 355 nm, the absorption is linear and an opposite behavior occurs. It can be explained with the help of a process involving thermomechanics coupled with the fine time structure of MLM pulses, leading to the possible annealing of part of the absorbent defects.

Single mode oscillation of TEA CO 2 laser with an interference resonator

An effective single-longitudinal mode (SLM) pulse TEA CO 2 laser operation was demonstrated using a Michelson’s type resonator with a tilting Fabry–Perot etalon. A modified numerical model of the interference resonator was investigated for designing the laser. The experimentally measured values were found to have good agreement with the numerical model. A pulse width of about 90 ns and the maximum pulse energy of about 300 mJ were achieved at 10.59 μm in SLM and TEM 00 mode. The reliability of producing SLM pulses was 100% and there was no damage on the etalon. By turning the interference resonator, the SLM output was tuned 44 lines of the CO 2 spectrum.

Single longitudinal mode pulse from a TEA CO2 laser by using a three-mirror resonator with a Fabry–Pérot etalon

We present a single longitudinal mode (SLM) TEA CO2 laser oscillation by using a three-mirror resonator with a Fabry–Perot etalon. The etalon was inserted in the optical path taken out from the main resonator of the CO2 laser for protecting the etalon from damage on the surface. A modified numerical model of the three- mirror resonator was investigated for design the laser. SLM pulse from the TEA CO2 laser was achieved, and the experimentally measure values were found to have good agreement with the numerical model. The maximum pulse energy of reliable SLM emission is obtained in excess of 200 mJ at 9.57 μm. The reliability of producing SLM pulses was higher than 90%, and there was no damage on the etalon

Double-pumped phase conjugation in inverted Nd:YAG

An experimental study of double-pumped phase conjugation (DPPC) in a gain medium (Nd:YAG) is presented. The input–output energy and phase conjugate reflectivity of the system was experimentally characterized, with a maximum reflectivity of R∼160 observed. A self-starting DPPC laser was also produced by use of a feedback mirror, and spatial and temporal outputs are presented. The DPPC laser produced a TEM00 spatial mode output of 113 mJ in a 20-ns single-longitudinal mode pulse at a repetition rate of 10 Hz.

Generation of 2.5-ps light pulses with 15-nm wavelength tunability at 1.3 mu m by a self-seeded gain-switched semiconductor laser

The generation of 2.5-ps pulses by a Fabry-Perot semiconductor laser with 15-nm wavelength tunability is demonstrated. A commercial laser is switched to single longitudinal mode pulse emission by self-seeding, using an external grating. The sidemode suppression ratio is better than 15 dB, with a peak value of 21 dB. The linearly red chirped pulses are compressed from 21-30 ps to 2.5-3.5 ps in a dispersion-shifted fiber. The simple experimental setup consists of a fiber coupler, a grating, and the dispersion-shifted fiber.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Comparative performances of stable-and unstable-resonator TEA CO2 lasers with cw injection

The performances of stable- and unstable-resonator TEA CO2 lasers with cw injection are experimentally compared. Two types of measurements are reported. First, locking band-widths are measured for a wide range of cw injected powers. It is shown that 5-μs 1-J single longitudinal mode pulses can be obtained with unstable resonator lasers for injected powers as low as 200 nW. Second, heterodyne measurements of the intrapulse TEA laser frequency are presented. The use of unstable resonators is experimentally proven to be the most efficient way to achieve reproducible high-power chirpless single-mode pulses at 10μm.

Single-longitudinal-mode operation of a TEA CO_2 laser with a modified Fabry-Perot interferometer

An interferometric device consisting of two metallic mirrors and a dielectric beam splitter of low reflectivity is used to reduce the spectral emission of a CO2 TEA laser to a single longitudinal mode (SLM). The finesse of the interferometer can be varied continuously by turning the polarization of the laser beam or in steps by using different beam splitters. The resonant reflectivity of the device was found to vary from 20 to 90% depending on transverse mode matching and alignment. With a low reflectivity beam splitter, the reliability of producing SLM pulses reached 100%. As the beam splitter reflectivity was increased, the interferometer finesse decreased resulting in a low reliability of producing SLM pulses. The maximum free spectral range which gave SLM reliably was 5 GHz. We also replaced one of the mirrors of the interferometer by a grating, and we observed SLM output on forty lines of the CO2 spectrum with a reproducibility of 80% or better. We discuss how the device could be used for intracavity pulse selection.

Generation of single longitudinal mode pulses in passively Q-switched lasers via passive pre-lasing

Greatly improved control over the production of single longitudinal mode (SLM) laser pulses is obtained by applying pre-lase techniques to passively Q-switched oscillators. Passive but controlled pre-lasing is obtained by forming a sub-cavity within the main Q-switching cavity. Use of pre-lasing and a single solid low-reflectivity etalon resulted in 96% SLM production over 5 h periods during which the oscillator fired intermittently several thousand times in a non-stabilized temperature environment and without any active feed-back.

Multiple-photon absorption of a CO 2-pumped CF 4 laser radiation in UF 6

Pressure, fluence and intensity dependent measurements of the multiple-photon absorption (MPA) of 16-μm CF 4 laser pulses in UF 6 are reported. Pumping of the CF 4 gas by single longitudinal mode pulses from an injection controlled TEA-CO 2 laser yielded up to 40 mJ pulses of 16-μm laser radiation which were used in the present study. The pressure dependent results at constant fluence (〈φ〉), show that at UF 6 pressures ( P) lower than 0.5 Torr the average number of photons absorbed per molecule (〈 n〉) increases with P and that at higher pressures 〈 n〉 levels off. This indicates that, similar to what is found in SF 6, rotational hole filling takes place in UF 6 at low pressures. The fluence dependent results show that at a given P and for 〈φ〉 smaller than about 5 mJ/cm 2, 〈 n〉 increases linearly with 〈φ〉, indicating single-photon absorption, and that at higher values of fluence 〈 n〉 is proportional to 〈φ〉 1.6, which is typical of MPA processes. The use of laser pulses of two different durations (300 ns and 600 ns at FWHM) for the excitation shows that the results of 〈 n〉 versus P and 〈φ〉 are independent of the length of these pulses.