Modulation of high-quality internal multifoci based on modified three-dimensional Fourier transform.

We report an efficient method to generate arbitrary three-dimensional (3D) parallel multifoci inside a material. Taking into account the numerical aperture of the objective lens and the refractive index of the material, the Ewald cap was modified with a longer radius, then the whole 3D intensity distribution inside the material could be calculated using only a single Fourier transform (FT). By introducing the adaptive weight coefficient, the uniformity of the 3D multifoci improves from 81.3% to 98.9%. By adjusting the axial resolution of the Ewald cap, the uniformity of the axial multifoci improves from 85.9% to 99.7%. In the experiment, we have realized one-dimensional (1D), 2D, and 3D structures inside the fused silica, which are in excellent agreement with the simulation results. The experimental results of the "H-U-S-T" structure demonstrate that customized arbitrary intensity distribution inside the material can be realized.

Coordinate-transformation iteration algorithm for phase modulation of arbitrary axial light distribution.

Axial light distribution modulation is widely applied in optical tweezers, hard-brittle material cutting, multilayer laser direct writing, etc. To generate arbitrary axial light distribution, the coordinate-transformation iteration (CTI) algorithm is presented. The CTI algorithm unifies equations in low and high numerical aperture (NA) scenarios, using the same iterative algorithm to produce phase computer-generated holograms. In a low NA scenario, twin-foci, flattop, and sin2 distributions have been achieved. In high NA scenarios, multirings, multifoci, and needle-like distributions have been realized in simulation with specific polarized incident beams. Since the CTI algorithm is inherently an efficient one-dimensional phase retrieval algorithm that is not limited by NA, this method has the potential to become a well-received solution for axial light distribution modulation.

Stealth dicing of 1-mm-thick glass with aberration-free axial multi-focus beams.

Laser stealth dicing can realize material separation with negligible surface damage, but severe aberrations in thick materials degrade processing quality. This Letter presents a nonlinear point-to-point transformation method combined with spherical aberration compensation to achieve aberration-free axial multi-focus beams. The focus peak intensity increases 7 times at a depth of 0.5 mm after spherical aberration compensation, and reaches 44 times at 3.5 mm. Spherical aberration compensation experiments showed that the width of the heat-affected zone remains almost unchanged at different depths inside the glass, and stealth dicing experiments for 1-mm-thick glass demonstrated that aberration-free 1-focus, 2-foci, and 3-foci stealth dicing can be successfully realized.

Helical drilling of carbon fiber reinforced polymer by a femtosecond laser.

Carbon fiber reinforced polymer (CFRP) has been widely used in the aerospace industry and other fields, but its processing is still subject to many restrictions. A femtosecond laser helical drilling device is proposed. It can realize beam deflection and translation through a scanning galvanometer and a pair of lenses, respectively, and finally obtain the effect of beam oblique focusing. With the combination of a spiral scanning path and focus feed, the non-taper and high-quality drilling of materials can be realized. CFRP materials were processed through this method, and the influences of process parameters on the heat affected zone and machining taper were studied. The distribution and influence degree of the heat affected zone and the formation mechanism of the machining taper were analyzed. Finally, a high-quality straight hole processing effect of CFRP with up to a 9.7: 1 aspect ratio, less than 15 µm heat affected zone outside the hole, and no delamination in the hole was realized. This new drilling method is expected to further expand CFRP applications.

Modeling of a picosecond CO2 amplifier with electrical and optical pumping.

As a long-wavelength laser with strong energy storage capacity and large scale of amplification, the CO2 laser is considered an effective amplifier in the next-generation picosecond terawatt infrared laser system, but the pulse splitting effect caused by its discrete gain spectrum limits its behavior. In this paper, we have developed a specific model of a CO2 amplifier, which is optically and electrically pumped at the same time. The model is based on gas discharge that is combined with photon absorption, temperature, and wave equation. The proposed hybrid pumped CO2 amplifier scheme can increase the gain proportion of the sequence band transition (0002-1001, 0003-1002, etc.) from 12% to nearly 50% of the regular band and broaden the bandwidth of each line by over 15.8% by the overlap of the sequence band and regular band. The relative energy concentration of the first subpulse can be increased by up to 190% when the amplification factor reaches 103. The study on the model of a picosecond CO2 amplifier with electrical and optical pumping may contribute to the amplification of an ultrafast mid-infrared pulse to the terawatt or higher region.

Continuous wavelength tuning of nondegenerate femtosecond doubly resonant optical parametric oscillators.

We report an approach for continuous tuning of the central wavelengths of a femtosecond nondegenerate doubly resonant optical parametric oscillator (DRO). Key in this scheme is the insertion of a wavelength-selective element into the DRO cavity-length-locking system, which allows selected narrow-band emission from a DRO to be detected and enables the DRO cavity length to be locked away from the peak of its cavity resonances. In a preliminary experiment, we demonstrated a femtosecond nondegenerate DRO with its central-wavelength continuously-tunable over each of its cavity resonances and achieved a wavelength-tuning range of 1910-2070 nm for the signal wave and 2140-2340 nm for the idler wave. This is, to the best of our knowledge, the first demonstration of the continuous-wavelength-tuning capability of DRO systems.

Modeling of scaling of a diode longitudinally pumped metastable rare gas with a master oscillator power amplifier.

There has been recent interest in diode pumped metastable rare gas lasers (DPRGLs) and their scaling to higher powers, due to the advantages of excellent beam quality and high quantum efficiency. In this paper, a cw diode pumped rare gas amplifier (DPRGA) with single-pass longitudinally pumped configuration is studied theoretically based on master oscillator and power amplifier (MOPA). A five-level kinetic model of DPRGAs is first established. Then, the influences of gain medium density, pump and seed laser intensities and gain length on DPRGA performance are simulated and analyzed. The results of numerical simulation agree well with those of Rawlins et al.'s experiment. With the best set of working parameters, the amplification factor reaches 22.18 dB, at pump intensity of 50 kW/cm2 and seed laser intensity of 100 W/cm2. Parameter optimization is helpful for design of a relatively high-power DPRGL system.

Laguerre-Gaussian mode expansion for arbitrary optical fields using a subspace projection method.

Laguerre-Gaussian (LG) mode decomposition has found applications in various optics fields. However, numerical LG mode expansion for arbitrary field is still a problem, since the physical dimension of LG modes would vary with three parameters-the beam waist width w, the radial index p, and the azimuthal index m, which make it difficult to determine the optimal value of w and the truncation order on p. Here a general method of LG mode expansion for an arbitrary field is developed. It is found that the local frequency distribution of the LG function consists of two parts, the quasi-periodic part and the chirped part. The effective space-bandwidth product of the LG function is defined as the product of the spatial width and frequency-domain width of the quasi-periodic part. Then, based on this space-bandwidth product definition, the criteria for determining the beam waist parameter and the truncation order of LG series expansion are given. The scheme is tested for the representation of off-axis Hermite-Gaussian mode, which shows that our method is of high accuracy.

Two-stage excitation model of diode pumped rare gas atoms lasers.

Diode pumped rare gas atoms lasers (DPRGLs) are potential candidates of the high-energy lasers, due to the advantages of high laser power and high optical conversion efficiency. In this paper, a two-stage excitation model of DPRGLs is established including gas discharge excitation and semiconductor laser pump to study energy loss mechanism and obtain total efficiency. The results of numerical simulation agree well with those of Rawlins et al.'s experiment. Through parameter optimization, the total efficiency and optical conversion efficiency reach 51.5% and 62.7% respectively, at pump intensity of 50 kW/cm2 and reduced electric field of 8 Td. Parameter optimization of two-stage excitation lasers is theoretically studied, which is significant for the DPRGLs design with high total efficiency.

Nonuniform fast Fourier transform method for numerical diffraction simulation on tilted planes.

The method, based on the rotation of the angular spectrum in the frequency domain, is generally used for the diffraction simulation between the tilted planes. Due to the rotation of the angular spectrum, the interval between the sampling points in the Fourier domain is not even. For the conventional fast Fourier transform (FFT)-based methods, a spectrum interpolation is needed to get the approximate sampling value on the equidistant sampling points. However, due to the numerical error caused by the spectrum interpolation, the calculation accuracy degrades very quickly as the rotation angle increases. Here, the diffraction propagation between the tilted planes is transformed into a problem about the discrete Fourier transform on the uneven sampling points, which can be evaluated effectively and precisely through the nonuniform fast Fourier transform method (NUFFT). The most important advantage of this method is that the conventional spectrum interpolation is avoided and the high calculation accuracy can be guaranteed for different rotation angles, even when the rotation angle is close to π/2. Also, its calculation efficiency is comparable with that of the conventional FFT-based methods. Numerical examples as well as a discussion about the calculation accuracy and the sampling method are presented.

Research on beam characteristics in a large-Fresnel-number unstable-waveguide hybrid resonator with parabolic mirrors.

Large-Fresnel-number unstable-waveguide hybrid resonators employing spherical resonator mirrors suffer from spherical aberration, which adversely affects beam quality and alignment sensitivity. In this paper, we present experimental and numerical wave-optics simulations of the beam characteristics of a negative-branch hybrid resonator having parabolic mirrors with a large equivalent Fresnel number in the unstable direction. These results are compared with a resonator using spherical mirrors. Using parabolic mirrors, the output beam has a smaller beam spot size and higher power density at the focal plane. We found that the power extraction efficiency is 3.5% higher when compared with a resonator using spherical mirrors as the cavity length was varied between -1 and 1 mm from the ideal confocal resonator. In addition, the power extraction efficiency is 5.6% higher for mirror tilt angles varied between -6 and 6 mrad. Furthermore, the output propagating field is similar to a converging wave for a spherical mirror resonator and the output beam direction deviates 3.5 mrad from the optical axis. The simulation results are in good agreement with the experimental results.

Beam shaping characteristics of an unstable-waveguide hybrid resonator.

The unstable-waveguide hybrid resonator emits a rectangular, simple astigmatic beam with a large number of high-spatial-frequency oscillations in the unstable direction. To equalize the beam quality, in this paper, a beam shaping system with a spatial filter for the hybrid resonator was investigated by numerical simulation and experimental method. The high-frequency components and fundamental mode of the output beam of the hybrid resonator in the unstable direction are separated by a focus lens. The high-frequency components of the beam are eliminated by the following spatial filter. A nearly Gaussian-shaped beam with approximately equal beam propagation factor M² in the two orthogonal directions was obtained. The effects of the width of the spatial filter on the beam quality, power loss, and intensity distribution of the shaped beam were investigated. The M² factor in the unstable direction is changed from 1.6 to 1.1 by optimum design. The power loss is only 9.5%. The simulation results are in good agreement with the experimental results.