Modifying single-crystal silicon and trimming silicon microring devices by femtosecond laser irradiation.

Single-crystal silicon (c-Si) is a vital component of photonic devices and has obvious advantages. Moreover, femtosecond-pulsed laser interactions with matter have been widely applied in micro/nanoscale processing. In this paper, we report the modification mechanisms of c-Si induced by a femtosecond laser (350 fs, 520 nm) at different pulse fluences, along with the mechanism of this technique to trim the phase error of c-Si-based devices. In this study, several distinct types of final micro/nanostructures, such as amorphization and ablation, were analyzed. The near-surface morphology was characterized using optical microscopy, scanning electron microscopy, and atomic force microscopy. The main physical modification processes were further analyzed using a two-temperature model. By employing Raman spectroscopy, we demonstrated that a higher laser fluence significantly contributes to the formation of more amorphous silicon components. The thickness of the amorphous layer was almost uniform (approximately 30 nm) at different induced fluences, as determined using transmission electron microscopy. From the ellipsometry measurements, we demonstrated that the refractive index increases for amorphization while the ablation decreases. In addition, we investigated the ability of the femtosecond laser to modify the effective index of c-Si microring waveguides by either amorphization or ablation. Both blue and red shifts of resonance spectra were achieved in the microring devices, resulting in double-direction trimming. Our results provide further insight into the femtosecond laser modification mechanism of c-Si and may be a practical method for dealing with the fabrication errors of c-Si-based photonic devices.

Reduction of relative intensity noise in a diamond Raman laser.

The relative intensity noise (RIN) characteristics of a continuous-wave diamond Raman laser are investigated for the first time. The results reveal the parasitic stimulated Brillouin scattering (SBS) that usually occurred with higher-order spatial modes in the diamond Raman resonator is a pivotal factor impacting the Raman longitudinal modes and deteriorating the RIN level. The diamond Raman laser automatically switches to single-longitudinal-mode operation and the RIN level is significantly decreased in the frequency range of 200 Hz to 1 MHz after the parasitic SBS is effectively suppressed through inserting a spatial aperture or a χ(2) nonlinear crystal into the cavity. Due to the introduction of additional nonlinear loss to the high intensity Raman fluctuations and the non-lasing spontaneous Raman modes, the χ(2) nonlinear crystal enables better performance in the RIN-level reduction compared to the spatial aperture which can only achieve SBS inhibition. The RIN reduction routes are well suited for various crystalline Raman media to achieve high power and low intensity noise laser at different wavelengths.

Preliminary analysis of global column-averaged CO2 concentration data from the spaceborne aerosol and carbon dioxide detection lidar onboard AEMS.

In contrast to the passive remote sensing of global CO2 column concentrations (XCO2), active remote sensing with a lidar enables continuous XCO2 measurements throughout the entire atmosphere in daytime and nighttime. The lidar could penetrate most cirrus and is almost unaffected by aerosols. Atmospheric environment monitoring satellite (AEMS, also named DQ-1) aerosol and carbon dioxide detection Lidar (ACDL) is a novel spaceborne lidar that implements a 1572 nm integrated path differential absorption (IPDA) method to measure the global XCO2 for the first time. In this study, special methods have been developed for ACDL data processing and XCO2 retrieval. The CO2 measurement data products of ACDL, including the differential absorption optical depth between the online and offline wavelengths, the integral weighting function, and XCO2, are presented. The results of XCO2 measurements over the period from 1st June 2022 to 30th June 2022 (first month data of ACDL) are analyzed to demonstrate the measurement capabilities of the spaceborne ACDL system.

Airborne atmospheric carbon dioxide measurement using 1.5 µm laser double-pulse IPDA lidar over a desert area.

An integrated path differential absorption (IPDA) lidar can accurately measure regional C O 2 weighted column average concentrations (X C O 2), which are crucial for understanding the carbon cycle in climate change studies. To verify the performance and data inversion methods of space-borne IPDA lidar, in July 2021, we conducted an airborne lidar validation experiment in Dunhuang, Gansu Province, China. An aircraft was equipped with a lidar system developed to measure X C O 2 and an in situ greenhouse gas analyzer (GGA). To minimize measurement errors, energy monitoring was optimized. The system bias error of the DAOD was determined by changing the laser output mode from the off/on to the on/on mode. The X C O 2 inversion results obtained through comparing the schemes of averaging signals before "log (logarithm)" and averaging after "log" indicate that the former performs better. The IPDA lidar measured X C O 2 over the validation site at 405.57 ppm, and both the IPDA lidar and GGA measured sudden changes in the C O 2 concentration. The assimilation data showed a similar trend according to the altitude to the data measured by the in situ instrument. A comparison of the mean X C O 2 derived from the GGA results and assimilation data with the IPDA lidar measurements showed biases of 0.80 and 1.12 ppm, respectively.

Dual wavelength laser Doppler anemometer for simultaneous velocity and particulate size distribution measurements in submarine environments.

An in-situ laser Doppler current probe (LDCP) for the simultaneous measurements of the micro-scale subsurface current speed and the characterizations of micron particles is dedicated in this paper. The LDCP performs as an extension sensor for the state-of-the-art laser Doppler anemometry (LDA). The all-fiber LDCP utilized a compact dual wavelength (491 nm and 532 nm) diode pumped solid state laser as the light source to achieve the simultaneous measurements of the two components of the current speed. Besides its ability for the measurements of the current speed, the LDCP is also capable of obtaining the equivalent spherical size distribution of the suspended particles within small size range. The micro-scale measurement volume formed by two intersecting coherent laser beams makes it possible to accurately estimate the size distribution of the micron suspended particles with high temporal and spatial resolution. With its deployment during the field campaign at Yellow Sea, the LDCP has been experimentally demonstrated as an effective instrument to capture the micro-scale subsurface ocean current speed. The algorithm for retrieving the size distribution of the small suspended particles (2∼7.5µm) has been developed and validated. The combined LDCP system could be applied to the continuous long-term observations of plankton community structure, ocean water optical parameter over a wide range, and useful to elucidate the processes and interactions of the carbon cycles in the upper ocean.

Efficient single-frequency 972†nm Yb-doped fiber amplifier with core pumping and elevated temperature.

In this work, we present a monolithic single-frequency, single-mode and polarization maintaining Yb-doped fiber (YDF) amplifier delivering up to 6.9 W at 972 nm with a high efficiency of 53.6%. Core pumping at 915 nm and elevated temperature of 300 °C were applied to suppress the unwanted 977 nm and 1030 nm ASE in YDF, so as to improve the 972 nm laser efficiency. In addition, the amplifier was further used to generate a single-frequency 486 nm blue laser with 590 mW of output power by single-pass frequency doubling.

Robust algorithm for precise XCO2 retrieval using single observation of IPDA LIDAR.

CO2 column-weighted dry-air mixing ratio (XCO2) products with high precision and spatial resolution are essential for inverting CO2 fluxes and promoting our understanding of global climate change. Compared with passive remote sensing methods, IPDA LIDAR, as an active remote sensing technique, offers many advantages in measuring XCO2. However, a significant random error in IPDA LIDAR measurements causes XCO2 values calculated directly from LIDAR signals to be unqualified as the final XCO2 products. Hence, we propose an efficient particle filter-based inversion of CO2 for single observation (EPICSO) algorithm to precisely retrieve the XCO2 of every LIDAR observation while preserving the high spatial resolution of LIDAR measurements. The EPICSO algorithm adopts the sliding average results as the first estimate of the local XCO2; subsequently, it estimates the difference between two adjacent XCO2 points and calculates the posterior probability of XCO2 based on particle filter theory. To evaluate the performance of the EPICSO algorithm numerically, we perform an EPICSO to process pseudo-observation data. The simulation results show that the results retrieved by the EPICSO algorithm satisfy the required high precision and that the algorithm is robust to a significant amount of random errors. In addition, we utilize LIDAR observation data from actual experiments in Hebei, China, to validate the performance of the EPICSO algorithm. The results retrieved by the EPICSO algorithm are more consistent with the actual local XCO2 than those of the conventional method, indicating that the EPICSO algorithm is efficient and practical for retrieving XCO2 with high precision and spatial resolution.

Hybrid integrated Si3N4 external cavity laser with high power and narrow linewidth.

We have designed and fabricated a hybrid integrated laser source with full C-band wavelength tunability and high-power output. The external cavity laser is composed of a gain chip and a dual micro-ring narrowband filter integrated on the silicon nitride photonic chip to achieve a wavelength tuning range of 55 nm and a SMSR higher than 50 dB. Through the integration of the semiconductor optical amplifier in the miniaturized package, the laser exhibits an output power of 220 mW and linewidth narrower than 8 kHz over the full C-band. Such a high-power, narrow-linewidth laser diode with a compact and low-cost design could be applied whenever coherence and interferometric resolutions are needed, such as silicon optical coherent transceiver module for space laser communication, light detection and ranging (LiDAR).

All-fiber laser system for all-optical 87Rb Bose Einstein condensate to space application.

In the development of the Cold Atom Physics Research Rack (CAPR) on board the Chinese Space Station, the laser system plays a critical role in preparing the all-optical 87 R b Bose-Einstein condensates (BECs). An all-fiber laser system has been developed for CAPR to provide the required optical fields for atom interaction and to maintain the beam pointing in long-term operation. The laser system integrates a 780 nm fiber laser system and an all-fiber optical control module for sub-Doppler cooling, as well as an all-fiber 1064 nm laser system for evaporative cooling. The high-power, single-frequency 780 nm lasers are achieved through rare-Earth doped fiber amplification, fiber frequency-doubling, and frequency stabilization technology. The all-fiber optical control module divides the output of the 780 nm laser system into 15 channels and regulates them for cooling, trapping, and probing atoms. Moreover, the power consistency of each pair of cooling beams is ensured by three power tracking modules, which is a prerequisite for maintaining stable MOT and molasses. A high-power, compact, controlled-flexible, and highly stable l064 nm all-fiber laser system employing two-stage ytterbium-doped fiber amplifier (YDFA) technology has been designed for evaporative cooling in the optical dipole trap (ODT). Finally, an all-optical 87 R b BEC is realized with this all-fiber laser system, which provides an alternative solution for trapping and manipulating ultra-cold atoms in challenging environmental conditions.

A Linearly Polarized Wavelength-Tunable Q-Switched Fiber Laser with a Narrow Spectral Bandwidth of 112 MHz.

A tunable and narrow-bandwidth Q-switched ytterbium-doped fiber (YDF) laser is investigated in this paper. The non-pumped YDF acts as a saturable absorber and, together with a Sagnac loop mirror, provides a dynamic spectral-filtering grating to achieve a narrow-linewidth Q-switched output. By adjusting an etalon-based tunable fiber filter, a tunable wavelength from 1027 nm to 1033 nm is obtained. When the pump power is 1.75 W, the Q-switched laser pulses with a pulse energy of 10.45 nJ, and a repetition frequency of 11.98 kHz and spectral linewidth of 112 MHz are obtained. This work paves the way for the generation narrow-linewidth Q-switched lasers with tunable wavelengths in conventional ytterbium, erbium, and thulium fiber bands to address critical applications such as coherent detection, biomedicine, and nonlinear frequency conversion.

M6A methylation-mediated elevation of SM22α inhibits the proliferation and migration of vascular smooth muscle cells and ameliorates intimal hyperplasia in type 2 diabetes mellitus.

Abnormal proliferation of vascular smooth muscle cells (VSMCs) induced by insulin resistance facilitates intimal hyperplasia of type 2 diabetes mellitus (T2DM) and N6-methyladenosine (m6A) methylation modification mediates the VSMC proliferation. This study aimed to reveal the m6A methylation modification regulatory mechanism. In this study, m6A demethylase FTO was elevated in insulin-treated VSMCs and T2DM mice with intimal injury. Functionally, FTO knockdown elevated m6A methylation level and further restrained VSMC proliferation and migration induced by insulin. Mechanistically, FTO knockdown elevated Smooth muscle 22 alpha (SM22α) expression and m6A-binding protein IGF2BP2 enhanced SM22α mRNA stability by recognizing and binding to m6A methylation modified mRNA. In vivo studies confirmed that the elevated m6A modification level of SM22α mRNA mitigated intimal hyperplasia in T2DM mice. Conclusively, m6A methylation-mediated elevation of SM22α restrained VSMC proliferation and migration and ameliorated intimal hyperplasia in T2DM.

Calibration experiments based on a CO2 absorption cell for the 1.57-µm spaceborne IPDA LIDAR.

The spaceborne IPDA LIDAR has the potential to measure the global atmosphere CO2 column concentrations with high accuracy. For this kind of LIDAR, system calibration experiments in the laboratory are of high importance. In this study, a specially-customized CO2 absorption cell is employed to simulate the CO2 column absorption of the spaceborne platform. Then calibration experiments are constructed for the receiving system and the entire LIDAR system. The absorption of several different XCO2 concentrations from 400 to 415 ppm in the atmosphere is equivalent to that of the absorption cell charged with different pressures of pure CO2. Under the zero pressure of the absorption cell, the calculated equivalent column average concentration (XCO2) is 12.53 ppm, which acts as system bias. In the calibration experiments, the absolute errors are all less than 1 ppm. And the standard deviations (STDs) are less than 1.1 ppm (148-shot averaging) and 0.8 ppm (296-shot averaging) for receiving system and less than 1.2 ppm and 0.9 ppm for the IPDA LIDAR system. All the results of different average times are close to each other and less than 1 ppm, which proves the high accuracy of the IPDA LIDAR system. In addition, the XCO2 concentrations Allan deviation of 0.25 ppm and 0.35 ppm at 100 s shows that the receiving system and IPDA LIDAR system function with long-term stability. Using a CO2 absorption cell as a standard calibration device in the laboratory validates the measurement accuracy and stability of the spaceborne IPDA LIDAR prototype. Furthermore, the proposed absorption cell may serve as a standard calibration device for related atmosphere trace gases sounding research.

Simulation and Design of Circular Scanning Airborne Geiger Mode Lidar for High-Resolution Topographic Mapping.

Over the last two decades, Geiger-mode lidar (GML) systems have been developing rapidly in defense and commercial applications, demonstrating high point density and great collection efficiency. We presented a circular scanning GML system simulation model for performance prediction and developed a GML system for civilian mapping. The lidar system used an eye-safe fiber laser at 1545 nm coupled with a 64 × 64 pixels photon-counting detector array. A real-time data compression algorithm was implanted to reduce half of the data transmission rate and storage space compared to the uncompressing situation. The GML system can operate at aircraft above-ground levels (AGLs) between 0.35 km and 3 km, and at speeds in excess of 220 km/h. The initial flight tests indicate that the GML system can operate day and night with an area coverage of 366 km2/h. The standard deviations of the relative altimetric accuracy and the relative planimetric accuracy are 0.131 m and 0.152 m, respectively. The findings presented in this article guide the implementation of designing an airborne GML system and the data compression method.

Widely-tunable single-frequency diamond Raman laser.

We report a diamond Raman laser that is continuously-tunable across the range from 590 nm to 625 nm producing continuous wave output with up to 8 W. The system is based on an all-fiber and tunable (1020-1072 nm) Yb-doped pump laser with a spectral linewidth of 25 GHz that is Raman-shifted and frequency doubled in a cavity containing diamond and a lithium triborate second harmonic crystal. Despite the broad pump spectrum, single frequency output is obtained across the tuning range 590-615 nm. The results reveal a practical approach to obtain tunable high-power single-frequency laser in a wavelength region not well served by other laser technologies.

Inter-satellite laser-ranging based on intradyne coherent detection.

With the development of laser communication networking, laser-ranging technology is becoming more and more applicable. In this paper, high-accuracy ranging is implemented based on intradyne coherent detection at a communication rate of 1048.576 Mbps. The ranging accuracy is affected by clock phase calculation error and code loop track error. Parallel clock phase difference calculation, frame head correlation, and ranging ambiguity region handle are combined with the ranging calibration method, realizing millimeter-level corrected distance measurement. Dynamic range measurement above 1 m is proven to be continuous through the ranging ambiguity region handle. In addition, high-precision clock frequency deviation between two asynchronous terminals can be obtained through derivation of one-way distance at static ranging or by derivative of distance difference at bidirectional ranging. The methods proposed in this paper are verified by inter-satellite laser-ranging on orbit, and the results are analyzed.

Denoising the space-borne high-spectral-resolution lidar signal with block-matching and 3D filtering.

The constituents and structures of the atmosphere directly or indirectly affect the radiative energy budget of the Earth; thus, there is an urgent need to measure these components. Space-borne lidar is a powerful instrument for depicting the global atmosphere. Several space-borne lidars with spectral discrimination filters are proposed and even currently being developed, including the Chinese Aerosol-Cloud High-Spectral-Resolution Lidar (ACHSRL) onboard the Aerosol Carbon Detection Lidar satellite. However, the long distance from the satellite to the atmosphere near the Earth surface weakens the signal strength and debilitates the detection accuracy of space-borne lidar. Furthermore, due to absorption of Rayleigh scattering when it passes through the spectral discrimination filter, the signal-to-noise ratio in the molecular channel decreases. The traditional denoising method is to average the echo signals both vertically and horizontally, but the high speed of the satellite (7.5 km/s) and the varying atmosphere structure will blur detected layer features. A novel method to reduce the signal noise level of ACHSRL is proposed in this paper. A state-of-the-art algorithm for imaging denoising, block matching 3D filtering (BM3D), is employed. As ACHSRL has not been launched, a simulation study is performed. In the simulation experiment, we connect adjacent lidar signal profiles into one 2D matrix and treat it as an image. Unlike the existing lidar denoising algorithm which uses neighboring profiles to smooth, BM3D performs frequency domain transformation of the signal image and then searches for a similar patch in a given block to conduct collaborative filtering. This algorithm not only achieves denoising, but also preserves aerosol/cloud feature details. After denoising by BM3D, the peak signal-to-noise ratios of echo signals in all channels are improved and the retrieval accuracy of particulate optical properties is also refined, especially for the retrieval of the extinction coefficient.

Five-channel fiber-based laser Doppler vibrometer for underwater acoustic field measurement.

In this paper, a 1550 nm five-channel all-fiber homodyne laser Doppler vibrometer with high sensitivity and good signal probing probability is presented. Under the anechoic tank, standing on an airborne platform above the water surface 3 m away, the calibration experiments of the designed system are conducted. The minimum detectable sound pressure level is up to 101.73 dB re 1 µPa at 10 kHz under the hydrostatic water surface condition, and the time distribution of the final outputs are consistent with that of the underwater sound transducer. For the hydrodynamic detection capability, with the help of a 1064 nm high-pulse-energy laser whose pulse energy is 6J, pulse duration is about 8 ns, and repetition rate is 1 Hz, the system performance is tested in Qiandao Lake. And the signal probing probability of the whole sensing system is up to 59.77%.

Compact dual-wavelength blue-green laser for airborne ocean detection lidar.

We have demonstrated a dual-wavelength blue-green laser for airborne ocean lidar based on an all-solid-state master oscillator power amplifier and nonlinear frequency conversion methods. A Q-switched pulsed laser with 10 mJ energy at 1064 nm was amplified to 108 mJ by a Nd:YAG amplifier side pumped by vertical-cavity surface-emitting laser modules. This fundamental laser was then frequency tripled to 355 nm wavelength by lithium triborate (LBO) crystals. With maximum pump energy of 43.5 mJ at 355 nm, 9.6 mJ of blue laser pulse at 486.1 nm was successfully obtained from an optical parametric oscillator unit using two beta-barium borate crystals. The energy of the residual 532 nm laser pulse was 10.6 mJ. Equipped with this laser system, an airborne blue-green lidar was developed, and ocean detection was carried out in the South China Sea, where an optical vertical profile at seawater depth of 94 m was obtained.

Sensitivity analysis and correction algorithms for atmospheric CO2 measurements with 1.57-µm airborne double-pulse IPDA LIDAR.

In this study, a 1.57-µm airborne double-pulse integrated-path differential absorption (IPDA) light detection and ranging (LIDAR) system was developed for CO2 measurements. This airborne IPDA LIDAR is integrated with a real-time frequency monitoring system, an integrated sensor for temperature, pressure, and humidity, an inertial navigation system, and a global positioning system. The random errors of the LIDAR system, which are caused by the signal noise, background noise, and detector noise, among other factors, are analyzed for different target reflectivities at a flight altitude of 8 km. After parametric optimization, the signal is unsaturated at high target reflectivity. Further, it can be detected at low target reflectivity by adjusting the detector gain. After the averaging of 148 shots, the relative random error (RRE) was 0.057% for a typical target reflectivity of 0.1 sr-1. Moreover, the systematic errors caused by the laser pulse energy, linewidth, spectral purity, and frequency drift, as well as the atmospheric parameters related to the flight experiments are also investigated. The relative system error (RSE) was 0.214% as determined based on an analysis of the systematic errors, which are primarily caused by the frequency drift. Two methods are proposed to reduce the RSE caused by the frequency drift. The first is the averaging of 148 shots, which can reduce the RSE to 0.096%. The other method involves calculating the integral weight function (IWF) using real-time frequency. However, this is a time-consuming and computationally intensive process. Hence, look-up tables for the absorption cross-section were created to overcome this issue, resulting in a decrease in the RSE to 0.096%. Using actual aircraft attitude angles, velocity, and position data from flight experiments, the relative errors (REs) in the IWF caused by the uncorrected integral path and Doppler shift were determined to be 0.273% and 0.479%, respectively. However, it was found that corrections to the integral path and Doppler shift based on accurate calculations of the IWF cause the airborne platform to turn in such a way that the REs are eliminated. Hence, this study confirms the validity of system parameters and provides a reference for other researchers who study similar IPDA LIDAR systems. Further, the sensitivity analysis of the airborne IPDA LIDAR system can provide a reference to future data inversions. Moreover, the proposed correction algorithms for the integral path and Doppler shift contribute to more accurate inversion results.

Multiple scattering effects on the return spectrum of oceanic high-spectral-resolution lidar.

The return spectrum of the oceanic high-spectral-resolution lidar (HSRL) is simulated with a semianalytic spectral Monte Carlo (MC) method. The results show that the spectrum is similar to the single scattering spectrum at the water surface but broadens with the depth due to multiple scattering. Therefore, if the non-spectral MC method that ignores the spectrum broadening is used, deviations will be introduced into the HSRL retrieval, e.g., the effective particulate 180° volume scattering function (backscatter) and lidar attenuation coefficient (attenuation). The simulation indicates that the backscatter and attenuation deviations are within 10% and 2%, respectively, when the HSRL discriminator is the iodine absorption cell, and are within 3% and 1%, respectively, when the discriminator is changed to the field-widened Michelson interferometer.