Combined Laser Beam Research Articles

Modeling the optical field in off-axis integrated-cavity-output spectroscopy using the decentered Gaussian beam model.

Off-axis integrated-cavity-output spectroscopy (OA-ICOS) is widely used in trace gas detection and isotopic analysis for its enhanced sensitivity as well as robustness against optical instability. However, its sensitivity is ultimately limited by the spurious coupling noise formed in the cavity, and much of the design and optimization process relies on empirical iterations while quantitative analysis is lacking. In this paper, we develop a method to model the optical field in OA-ICOS based on the decentered Gaussian beam model, which is a generalization for large tilting angles as compared with previously developed models. From the optical field, the cavity transmission spectrum for different cavity configurations or input beam conditions can be calculated, and the fringe noise level can be derived. Results show that an optimum combination of input laser beam and off-axis alignment exists to fully suppress the interference fringes. Factors affecting the fringe noise level, including a mismatch between the input beam and the cavity, optical alignment conditions, and deviation from the re-entrant condition, are studied thoroughly. The developed method can serve to guide the design and optimization of OA-ICOS systems.

Two-dimensional network of atomtronic qubits

Through a combination of laser beams, we engineer a two-dimensional optical lattice of Mexican hat potentials able to host atoms in its ring-shaped wells. When tunneling can be ignored (at high laser intensities), we show that a well-defined qubit can be associated with the states of the atoms trapped in each of the rings. Each of these two-level systems can be manipulated by a suitable configuration of Raman laser beams imprinting a synthetic flux onto each Mexican hat cell of the lattice. Overall, we believe that the system has the potential to form a scalable architecture for atomtronic flux qubits.

A plasma amplifier to combine multiple beams at NIF

Combining laser beams in a plasma is enabled by seeded stimulated Brillouin scattering which allows cross-beam energy transfer (CBET) to occur and re-distributes the energy between beams that cross with different incident angles and small differences in wavelength [Kirkwood et al. Phys. Plasmas 4, 1800 (1997)]. Indirect-drive implosions at the National Ignition Facility (NIF) [Haynam et al. Appl. Opt. 46, 3276–3303 (2007)] have controlled drive symmetry by using plasma amplifiers to transfer energy between beams [Kirkwood et al., Plasma Phys. Controlled Fusion 55, 103001 (2013); Lindl et al., Phys. Plasmas 21, 020501 (2014); and Hurricane et al. Nature 506, 343–348 (2014)]. In this work, we show that the existing models are well enough validated by experiments to allow a design of a plasma beam combiner that, once optimized, is expected to produce a pulse of light in a single beam with the energy greatly enhanced over existing sources. The scheme combines up to 61 NIF beams with 120 kJ of available energy into a single f/20 beam with a 1 ns pulse duration and a 351 nm wavelength by both resonant and off-resonance CBET. Initial experiments are also described that have already succeeded in producing a 4 kJ, 1 ns pulse in a single beam by combination of up to eight incident pump beams containing <1.1 kJ/beam, which are maintained near resonance for CBET in a plasma that is formed by 60 pre-heating beams [Kirkwood et al., Nat. Phys. 14, 80 (2018)].

Theoretical and experimental research of tiling error compensation method based on a small-size mirror for large-aperture tiled-grating compressors

Tiled-grating techniques have been adopted in many multi-kilojoule petawatt laser facilities to meet the size requirements for compression gratings. To realize coherent tiling, we propose a new simple method of tiling error compensation for large-aperture tiled-grating compressors. In this method, errors are controlled by adjusting a small-size mirror. We theoretically analyze that this method improves errors tolerances many times higher than that of traditional compensation method and approaches the limit of adjustment accuracy. An experiment also has been carried out to demonstrate the feasibility. The method significantly reduces the difficulty of tiling, increases the stability of large-aperture tiled-grating compressor system and could be applied to coherent combination of large-aperture short laser beams.

基于光学零差偏振探测和锁相的合束激光偏振控制

基于光学零差偏振探测和锁相的合束激光偏振控制

Polarization control of combined laser beams based on optical homodyne polarization detection and phase locking

Polarization control of combined laser beams based on optical homodyne polarization detection and phase locking

Single-mode millijoule fiber laser system with high pulse shaping ability

The coherent amplification network (CAN) aims at developing a laser system based on the coherent combination of multiple laser beams, which are produced through a network of high beam quality optical fiber amplifiers. For the front-end sources of high power laser facilities, single-mode linearly polarized beams in the nanosecond time scale are required. According to these two requirements, a millijoule polarized fiber laser system was studied. Using polarization maintaining Ytterbium-fiber laser system as the seed, 10-μm core diameter Yb-doped fiber amplifier as the first-stage power amplifier and 40-μm core diameter polarizing photonic crystal fiber (PCF) as the second-stage power amplifier, the all-fiber laser system output 1.06-mJ energy at 10 ns with diffraction limited mode quality. Using 85-μm core diameter rod-type PCF as the third-stage power amplifier, 2.5-mJ energy at 10-ns pulse width was obtained with better than 500:1 peak-to-foot pulse shaping ability and single-mode beam quality. The energy fluctuation of the system was 1.3% (rms) with 1-mJ output in an hour. This fiber laser system has the advantages of high beam quality, versatile pulse shaping ability, good stability, small volume, and free of maintenance, which can be used in many applications.

Cascaded multi-dithering technique using PZT modulators for high control bandwidth in coherent laser beam combining

A cascaded multi-dithering (CMD) technique using piezoelectric ceramic transducer (PZT) tubes as phase modulators is proposed as a tool for obtaining high control bandwidth in coherent laser beam combination. To prove its validity, eight coherent fiber beam elements were combined using the CMD technique with PZT tubes. As a result, residual phase error was recorded to be λ/54 at 100 Hz control bandwidth, which is comparable to that of a four laser beam combination in the previous experiment. To our knowledge, this is the first case to date of combining eight laser beam elements using PZT tubes as phase modulators and achieving such good results.

Multi-photon absorption enhancement by dual-wavelength double-pulse laser irradiation for efficient dicing of sapphire wafers

The evidence of multi-photon absorption enhancement by the dual-wavelength double-pulse laser irradiation in transparent sapphire was demonstrated experimentally and explained theoretically for the first time. Two collinearly combined laser beams with the wavelengths of 1064 nm and 355 nm, inter-pulse delay of 0.1 ns, and pulse duration of 10 ps were used to induce intra-volume modifications in sapphire. The theoretical prediction of using a particular orientation angle of 15 degrees of the half-wave plate for the most efficient absorption of laser irradiation is in good agreement with the experimental data. The new innovative effect of multi-photon absorption enhancement by dual-wavelength double-pulse irradiation allowed utilisation of the laser energy up to four times more efficiently for initiation of internal modifications in sapphire. The new absorption enhancement effect has been used for efficient intra-volume dicing and singulation of transparent sapphire wafers. The dicing speed of 150 mm/s was achieved for the 430 μm thick sapphire wafer by using the laser power of 6.8 W at the repetition rate of 100 kHz. This method opens new opportunities for the manufacturers of the GaN-based light-emitting diodes by fast and precise separation of sapphire substrates.

Note: Lossless laser beam combiner employing a high-speed rotating half-wave plate.

We have developed a laser beam combiner employing a high-speed rotating half-wave plate based on the specific requirements of the Thomson scattering measurement systems in the ITER. The polarization extinction ratio of the output beam may exceed 1000 and was maintained for more than 1 h via feedback control of the half-wave plate rotation speed. The pointing fluctuations introduced by rotating the half-wave plate were in the order of microradians. The high-speed rotating half-wave plate provides a lossless means of combining laser beams together with stable beam pointing.

Efficient phase-locking of 37 fiber amplifiers by phase-intensity mapping in an optimization loop.

Coherent combination of laser beams from 37 fiber amplifiers in a tiled aperture configuration has been achieved thanks to an innovative iterative process. The high efficiency as well as the speed of the phase control demonstrated the relevance of the method for phase locking of a large array of fiber lasers.

Spectral Bandwidth Narrowing of High Brightness Spectral Beam Combining Diode Laser

We introduce a 5x demagnifying telescope system into an external cavity spectral beam combining and realize a narrow spectrum diode laser combining output by using a nine-emitter 970 nm diode laser mini-bar. With an external feedback mirror of 10% reflectivity, this experiment yields 44-W combined laser power at the operating current of 70 A. The measured M2 values of the combined laser beam are 12.2 and 2.8 for the slow and the fast axis, respectively. The spectral bandwidth of the combined laser is about 2.7 nm, which is nearly five times narrower than the output spectral bandwidth of 12.1 nm without the 5x demagnifying telescope. To our knowledge, this is the narrowest spectral bandwidth obtained from the spectral beam combining scheme with a short focal length transform lens.

Coherent Combination Characteristics of Laser Array with Hexagonal Ring Distribution

Coherent combination of laser beam is an important and challenging area of high power laser science, and many proof-of-principle experiments have been demonstrated. In this paper, we presented a mathematical model and derived the far field intensity distribution of a 2-D hexagonal ring distribution laser array using Frauhofer diffraction theory. The effects of fill factor and phase errors on far field intensity patterns are numerically analyzed and simulated. The result shows that nonunity fill factor causes the side lobe, the fill factor is smaller, the central lobe angular width will be much narrower and the power in side lobes will be larger. To make sure the side lobe intensity less than 1/e2 central peak power, the fill factor should be larger than 64%. Strehl ratio is also referred as evaluation parameter of the effect of phase errors on far field intensity. To limit the degradation of the Strehl ratio caused by phase errors to 0.8, the rms phase errors must be limited to ～􀟨⁄4 or better.

基于亮度的激光光束合成系统性能

基于亮度的激光光束合成系统性能

Spectral Beam Combining of Fiber Lasers by Using Reflecting Volume Bragg Gratings**Supported by the National Natural Science Foundation of China under Grant Nos 11474257 and 61605183.

By employing three reflecting volume Bragg gratings, a near-infrared 4-channel spectral-beam-combining system is demonstrated to present 720 W combined power with a combining efficiency of 94.7%. The combined laser beam is near-diffraction-limited with a beam factor M2 ∼ 1.54. During this 4-channel beam-combining process, no special active cooling measures are used to evaluate the volume Bragg gratings as combining elements are under the higher power laser operation. Thermal expansion and period distortion are verified in a 2 kW 2-channel beam-combining process, and the heat issue in the transmission case is found to be more remarkable than that in the diffraction case. Transmitted and diffracted beams experience wave-front aberrations with different degrees, thus leading to distinct beam deterioration.

Long-term stable coherent beam combination of independent femtosecond Yb-fiber lasers.

We demonstrate coherent beam combination between independent femtosecond Yb-fiber lasers by using the active phase locking of relative pulse timing and the carrier envelope phase based on a balanced optical cross-correlator and extracavity acoustic optical frequency shifter, respectively. The broadband quantum noise of femtosecond fiber lasers is suppressed via precise cavity dispersion control, instead of complicated high-bandwidth phase-locked loop design. Because of reduced quantum noise and a simplified phase-locked loop, stable phase locking that lasts for 1 hour has been obtained, as verified via both spectral interferometry and far-field beam interferometry. The approach can be applied to coherent pulse synthesis, as well as to remote frequency comb connection, allowing a practical all-fiber configuration.

Generation of blue light by sum-frequency generation of a spectrally combined broad-area diode laser array.

We present the first demonstration of a spectrally beam combined diode laser array with subsequent sum-frequency generation (SFG). The combined beam of the diode laser array with 19 emitters has the same beam quality as a single emitter, and the wavelength of each emitter is different. The blue light is generated by sum-frequency mixing of pairs of emitters in the diode laser array. About 93 mW of blue light power is produced using a PPLN crystal. Compared with the SFG of two emitters, this approach can increase the number of lasers participating in nonlinear frequency conversion. Thus, it can enhance the available power.

Fabrication of various shaped tungsten micro pin arrays using micro carving technology

This paper describes a state of the art in micro-structuring high strength metallic materials. Tungsten micro pin arrays in a variety of shapes are fabricated using a micro carving technology, which combines laser beam machining and electrochemical etching processes. First, micro pin arrays were rough-shaped by laser beam machining along a pre-defined scanning path to control their structural shape. The micro pin array in this stage had near-conical shape of structures due to a recast layer. Next, the genuine shape of micro pin arrays came to the surface via electrochemical etching process to elute the recast layer into electrolyte. Quantitative elemental analysis with energy-dispersive spectroscopy (EDS) was implemented to characterize the formation of recast layer on a micro pin structure after the laser beam machining process. The atomic percentage EDS maps indicated that higher percentage of tungsten was detected on the core micro pin structure, whereas relatively large percentage of oxygen was found on the recast layer (O 9%, W 91% in the center area, and O 53%, W 47% in the outer area).

Design and performance test of a two-axis fast steering mirror driven by piezoelectric actuators

A novel design of a two-axis fast steering mirror (FSM) with piezoelectric actuators is proposed for incoherent laser beam combination. The mechanical performance of the FSM is tested. The results show that the tilting range of the mirror is about 4 mrad, and the 1st-order resonance frequency is about 250 Hz. A self-designed grating encoder is taken as the sensor, which ensures the optimal precision of 10 μrad. The novel mechanical design can meet the requirement of engineering in incoherent laser beam combination.

Tracking performance of optical phase locking loop with frequency discrimination and control subloop

Optical phase locking is a key technique in homodyne coherent optical communication, coherent optical detection, and active coherent laser beam combination. In these applications, environmental temperature variation and mechanical vibration would affect the accuracy of phase locking, or even cause losing lock. These disturbances are generally equivalent to introducing phase jitter, phase step, frequency ramp, and frequency step in the loop. A frequency discrimination and control subloop is introduced to improve the frequency acquisition, and the tracking performance is studied experimentally. The loop can track phase step in 0.2 ms, and precisely track ±π/2 sine phase jitter for jittering frequency lower than 1 kHz. For frequency ramp, the residual phase error is unaffected for ramping rates slower than 40 MHz/s. The frequency discrimination and control subloop makes the loop lock quickly under a frequency step larger than the pull-in frequency. The mean tracking time is 31 ms for a 1 MHz frequency step. The maximum trackable frequency step is around 160 MHz. Continuous or step variation of phase and frequency could be tracked by the loop with the frequency discrimination and control subloop.