Optics letters最新文献

This work presents a new configuration to generate vector beams, with shaped intensity and polarization distributions, based on two liquid-crystal spatial light modulators (SLM). The first device is used in a scalar mode to shape an input linearly polarized beam with a phase-only computer-generated hologram. Then, the Fourier transform is optically formed onto the second SLM, which operates as a pixelated retarder to spatially modify the state of polarization. The proposed optical architecture allows shaping the amplitude profile, while easily generating the cylindrically polarized vector beam pattern. Experimental results demonstrate the versatility of the approach.

X-ray dark-field imaging visualizes scattering from sample microstructure and has found application in medical and security contexts. While most x-ray dark-field imaging techniques rely on masks, gratings, or crystals, recent work on the Fokker-Planck model of diffusive imaging has enabled dark-field imaging in the propagation-based geometry. Images captured at multiple propagation distances or x-ray energies can be used to reconstruct dark-field from propagation-based images but have previously required multiple exposures. Here, we show single-exposure dark-field imaging by exploiting the harmonic content in a monochromatized synchrotron beam and utilizing an energy-discriminating photon-counting detector to capture dual-energy propagation-based images. The method is validated by filming time-varying samples, showing the advantage of the dark-field contrast in analyzing dynamic evolution. We measure and adjust for the impact of detector charge-sharing on the images. This work opens the way for dynamic dark-field x-ray imaging without the need for a high-stability setup and precision optics.

Compact and low-threshold III-V semiconductor lasers are considered to be promising light sources for the silicon photonics platform, as they could offer a small footprint and low energy consumption. However, the significant lattice mismatch between III-V materials and silicon poses a fundamental challenge for the monolithic integration of such lasers on a silicon substrate. Using aspect ratio trapping and nano-ridge engineering, it has been shown this challenge can be overcome. However, thus far, only devices with cavity lengths of several hundred micrometers have shown a laser operation. Here, we show what we believe to be a novel approach whereby an amorphous silicon grating is deposited on the sidewalls of the nano-ridge, allowing for much stronger feedback and much shorter cavity lengths. Based on this approach, we achieved lasing with a threshold density of 9.9 kW/cm² under pulsed optical pumping, for a device with a cavity length as small as ∼16 µm. The side-mode suppression ratio and linewidth of the laser reach 24 dB and 1.25 nm under 25 kW/cm². This laser not only demonstrates the high quality of the epitaxial material but also establishes a novel route to realize an ultra-compact electrically driven light source for future high-density and massively scalable silicon photonic integrated circuits.

A watt-level multi-gigahertz (GHz) repetition-rate femtosecond optical parametric oscillator (OPO) synchronously pumped by a GHz-repetition-rate Kerr-lens mode-locked Yb:CYA laser is demonstrated in this letter. The output signal had a wavelength tuning range from 1.41 to 1.64 μm, a maximum average power of 1.58 W, a minimum pulse duration of 139 fs, and a repetition rate of 1 GHz, while the mid-infrared idler had a wavelength tuning range from 2.8 to 4.0 μm, a maximum average power of 1 W, a minimum pulse duration of 103 fs, and a repetition rate of 1 GHz. Through increasing the cavity length of the OPO by a unit fraction of the cavity length of the pump source, the output signal with repetition rates of 2, 3, and 4 GHz was obtained, respectively, and the output power of each was above 1 W. This work represents the highest, to the best of our knowledge, power of GHz femtosecond OPOs.

We propose a method for generating extremely short (few- and even subcycle) ultraviolet (UV) pulses with the use of three-color ionizing fields. We demonstrate that low-order combination frequencies can form a supercontinuum up to the fifth harmonic of the fundamental field. This effect is achieved by using three-color fields with two weak components detuned from half of the frequency of the intense fundamental field, which can be obtained from an optical parametric generator. Our calculations based on the solution of the time-dependent Schrödinger equation for the helium atom show that using a three-color near-infrared ionizing field with a duration of 25 fs can result in generating an extremely short UV pulse with a central wavelength of about 300 nm and a full width at half maximum of the intensity of about 0.9 fs.

We report high average power THz emission from a GaAs-based large-area photoconductive emitter, excited by a frequency-doubled, commercial Yb-laser amplifier without any pulse compression. The LAE is pumped at 11.4 W of green average power (515 nm) and 310 fs pulse duration at 400 kHz repetition rate. We obtain a maximum THz power of 6.7 mW with a spectrum extending up to 3 THz. Using electro-optical sampling (EOS) for detection, we measure a high peak dynamic range of 107 dB in a measurement time of 70 s. This is the highest power THz source so far, to the best of our knowledge, demonstrated with photoconductive emitters, based on a simple and robust, commercially available Yb-laser.

We propose an eye-box extension method for augmented reality (AR) glasses utilizing a dual-holographic optical element (dual-HOE) and a micro-OLED (µOLED) light source with a broadband spectrum. This scheme leverages the strong chromatic dispersion of HOE to significantly extend the eye-box without compromising AR quality. The proportional relationship between µOLED spectral bandwidth and eye-box size is analyzed theoretically, indicating that a broader spectrum µOLED provides a wider eye-box. Experimental results using a prototype demonstrate eye-box expansion up to 8 mm for µOLED with a 60 nm spectral bandwidth.

Fiber Bragg gratings (FBGs) are widely used as sensors for temperature, strain, and vibration measurement. However, current FBG demodulation methods face issues with stability, size, and cost. In this study, we proposed a silicon-on-insulator (SOI) chip to demodulate FBGs based on random speckles. A 20-mm-long coiled multimode silicon waveguide was designed to generate the speckle pattern, which was then compressed into 8 single-mode outputs. The architecture similarity between the convolutional neural network (CNN), and the proposed SOI chip was discussed. A multilayer perceptron (MLP) network was applied to regress the speckle data for prediction. The demonstrated experiments indicated that a standard deviation of 0.0414°C was achieved in the single FBG demodulation. Furthermore, we also explored the capability of demodulating multiple FBGs. This speckle-based SOI chip provides a highly stable, compact, and lightweight solution in a FBG sensing system.

We present a high-power-seed difference-frequency generator (DFG) system using two signal pulse trains with the same power level from two optical parametric oscillators (OPOs). Compared to the conventional signal-idler DFG system, our scheme can avoid the efficiency drop of long-wave infrared (LWIR) pulses at longer wavelengths due to the power drop of the OPO at a near-degeneracy wavelength. Moreover, neither the pump nor the seed waves of our DFG would be affected by the atmospheric absorption. Experimentally, femtosecond pulses tunable over 5-20 μm, with up to 62 mW power and 23% quantum conversion efficiency (QCE), are obtained at a repetition rate of ∼82 MHz. Over 20 mW power and nearly 20% QCE can be obtained at a central wavelength of 13 μm. This represents the highest value at such a high repetition rate. Our scheme represents the first, to the best of our knowledge, experimental demonstration of high-repetition-rate DFG systems with the seed and pump waves having the same power level.

Thin-disk oscillators have made significant progress in high power and pulse energy generation, but their ability to achieve high repetition rates is limited, primarily due to the structure of the multi-pass pumping cavity and the requirement for large beam spot sizes within the cavity. In this Letter, we employed an asymmetric cavity structure and used a thin-disk crystal as an end mirror, successfully achieving repetition rates of 432 MHz and 520 MHz in Kerr-lens mode-locked Yb:YAG thin-disk oscillators, with corresponding output powers of 33 W and 36 W and pulse durations of 253 fs and 276 fs, respectively. These results represent the highest, to the best of our knowledge, repetition rates reported for mode-locked thin-disk oscillators to date. Achieving a repetition rate of 1 GHz appears feasible by integrating custom components within the disk pumping module.