SignificanceCarbon dioxide (CO2) is one of the most significant anthropogenic greenhouse gases, and its concentration is closely linked to global climate change. Since the industrial revolution, the extensive combustion of fossil fuels and changes in land use have led to a continuous rise in atmospheric CO2 levels, triggering a series of environmental issues such as global warming, increased frequency of extreme weather events, glacier retreat, and sea-level rise. CO2 plays a critical role in radiative forcing within the climate system and profoundly influences the balance of the carbon cycle, ecosystem stability, and the sustainable development of human society. Therefore, comprehensive research on the observation, simulation, and underlying mechanisms of CO2 dynamics is a central component of global climate change studies. With the global push for carbon peaking and carbon neutrality, there is an urgent demand from both the scientific community and policymakers for high-precision, high-resolution CO2 observations to evaluate carbon source-sink patterns, verify emission reduction outcomes, and improve climate models. Therefore, achieving high-precision CO2 observations has become particularly important.ProgressThe integral path differential absorption (IPDA) lidar technology has unique advantages in global CO2 monitoring, making it an important tool in climate change research. Unlike passive remote sensing technologies that rely on sunlight, IPDA lidar uses its own laser pulses as the light source, enabling high-precision observations in all weather conditions and at any time, making it particularly suitable for nighttime and high-latitude gas monitoring. This allows IPDA technology to overcome limitations of passive remote sensing in these conditions. Furthermore, IPDA is less affected by cloud cover and aerosols, showing strong resistance to interference, and can still provide reliable measurement data under complex meteorological conditions.The principle of IPDA is based on measuring the absorption characteristics of gas molecules to laser signals, allowing precise inversion of gas concentrations by analyzing the attenuation of the laser signal as it passes through the atmosphere. This technology offers high spatial resolution and large coverage, making it particularly suitable for satellite platforms, enabling global CO2 monitoring. IPDA also has a high signal-to-noise ratio, allowing it to maintain high precision in long-range detection, significantly improving measurement accuracy.China has made significant progress in this field, successfully developing lidar systems based on IPDA technology. In 2022, China successfully launched the atmospheric environment monitoring satellite (DQ-1), marking an important step for China in global CO2 monitoring technology.Conclusions and ProspectsThe DQ-1 satellite offers significant promise for enhancing global CO2 monitoring capabilities. Trends indicate the integration of artificial intelligence with satellite data to improve carbon flux detection. Ongoing advancements in active remote sensing technology, along with improvements in data processing and fusion techniques, will enhance the accuracy and spatial resolution of CO2 observations. These developments will drive progress in global carbon monitoring, enabling more precise tracking of emissions and supporting efforts to mitigate climate change.

ObjectiveWith the rapid and diversified development of the national economy, the current domestic atmospheric environment remains complex. So lidar is in high demand as a critical tool for monitoring air pollution and greenhouse gases. In atmospheric remote sensing lidar system, high-spectral-resolution lidar (HSRL) and differential absorption lidar (DIAL) play significant roles in fields such as atmospheric composition detection and environmental monitoring due to their high precision and high sensitivity. A high-power, narrow-linewidth, and frequency tunable 1064 nm continuous-wave (CW) single-frequency laser can be used as the light source for HSRL via pulse modulation and power amplification. It can also generate short-wave and mid-infrared lasers through nonlinear optical frequency conversion techniques such as optical parametric oscillators (OPOs), providing DIAL with rich wavelength options. This paper aims to develop a broadband, mode-hop-free, tunable CW single-frequency Nd∶YVO4 laser based on a unidirectional figure-eight ring cavity structure. In the future, this laser will be integrated as a core component into mobile-platform lidar systems, providing robust support for air pollution and greenhouse gases monitoring.MethodsIn this work, we develope a mode-hop-free, continuously tunable single-frequency laser at 1064 nm, which features a compact structure, high output power, and a broad frequency tuning range. The figure-eight ring cavity consists of four mirrors (M1?M4). M1 and M4 also serve as the pump light input and laser output couplers, respectively. The laser is pumped by a fiber-coupled laser diode. The gain medium is an Nd∶YVO4 crystal with natural birefringence. Compared to the Nd∶YAG crystal, Nd∶YVO4 has a higher absorption coefficient, a broader absorption bandwidth, and can directly generate linearly polarized lasers. To ensure single-longitudinal-mode operation, an optical unidirectional device composed of a terbium gallium garnet (TGG) crystal and a half-wave plate (HWP) is added into the cavity to eliminate the spatial hole burning. To achieve high power output, an etalon with a free spectral range (FSR) much larger than the cavity’s FSR is incorporated to narrow the effective gain bandwidth and suppress competing longitudinal modes. Intracavity insertion of a noncritically phase-matched LiB3O5 (LBO) crystal enables 532 nm laser output, which coincides with the absorption line of iodine molecules, facilitating long-term frequency stabilization. Finally, through coordinated control of the laser crystal temperature and piezoelectric ceramics (PZT)-driven cavity mirror, continuous mode-hop-free frequency tuning over a range of 14 GHz is achieved.Results and DiscussionsExperimental results demonstrate that the laser designed in this work exhibits excellent performance in all aspects. Under a pump power of 25 W, the maximum output powers of the fundamental-wave and second-harmonic-wave laser reach 6.5 W and 2.1 W, with slope efficiencies of 26.12% and 8.36%, respectively (Fig. 4). The power stabilities over 4 h are 0.23% and 0.31% (Fig. 5). Within 1.35 h, the fundamental-wave power fluctuation is approximately ±0.238 pm (±63.1 MHz) (Fig. 7). The PZT tuning coefficient of the fundamental-wave is about -0.58 pm/V (-153.53 MHz/V) (Fig. 8), while the temperature tuning coefficient is about -26.89 pm/℃ (-7.12 GHz/℃) (Fig. 9). By employing the coordinated control of crystal temperature and cavity length, mode-hop-free wavelength tuning over a range of 53.16 pm (14.08 GHz) is achieved (Fig. 10). Compared to the method of coordinated control of the etalon angle and PZT-driven mirror, the wavelength tuning approach proposed in this paper eliminates the need for complex mechanical and electronic control systems. Although the experimental results align well with theoretical predictions, the current frequency tuning method requires further refinement. Due to the inherent thermal inertia of crystal temperature adjustment, the tuning speed must remain moderate, and nonlinearity is observed during the initial and final stages of frequency tuning (Fig. 10). Additionally, hysteresis effects in the PZT actuator introduce deviations in wavelength tuning linearity. These issues will be systematically addressed in future work.ConclusionsThis paper presents a continuous-wave single-frequency Nd∶YVO4 laser based on a figure-eight ring cavity, achieving 6.5 W of fundamental-wave and 2.1 W of second-harmonic wave laser output, with power stabilities of 0.23% and 0.31% over 4 h, respectively. Continuous wavelength tuning across a 14 GHz range is achieved by driving a PZT mounted cavity mirror to adjust the resonator length while synchronizing with laser mode variations induced by crystal temperature tuning. This laser overcomes the limitations of conventional cavity length control methods, which only allow continuous tuning within a single longitudinal mode. And, in contrast to approaches that require coordinated control of the etalon angle and cavity length, our design eliminates the need for complex control systems involving galvanometer actuators and phase-locked electronics. However, its tuning range and speed are still inferior to what can be achieved with etalon angle control, and further improvements are needed for the linearity of the control algorithm. Finally, the wavelength tuning performance is experimentally verified by scanning the absorption spectrum of iodine molecules. Potential applications of this laser include lidar systems or serving as a pump source for mid-infrared OPOs.

ObjectiveThe accurate detection of near-space (20?100 km) atmospheric density profiles plays a pivotal role in spacecraft orbit optimization, aircraft thermodynamic performance evaluation, and space environment monitoring. Nevertheless, conventional optical remote sensing techniques are limited by constraints such as low light flux, inadequate spectral resolution, and coarse vertical sampling intervals, which pose significant challenges to the detection of fine-scale structures associated with key dynamic processes. Aiming at the demand for space-borne high-precision detection of near-space atmospheric density profiles, optical design and performance verification of hyperspectral atmospheric density profile imager (HDI) are conducted. Using single frame limb observation image by HDI, high-spatial-resolution hyperspectral information of oxygen A-band absorption spectrum and airglow radiation spectrum can be acquired over an atmospheric vertical height range of approximately 100 km, while simultaneously inverting atmospheric density profiles and temperature profiles.MethodsBased on the principle of spatial heterodyne interferometric imaging spectroscopy (SHIS), the HDI optical system is designed. It can meet the high-precision detection requirements for near-space atmospheric density profiles, which include high signal-to-noise ratio (greater than 100 under typical spectral radiance), hyperspectral resolution (better than 0.04 nm), and fine vertical sampling interval (better than 0.1 km).To meet the technical index requirements of HDI, relationships between basic parameters of each functional component and residual polarization sensitivity characteristics of HDI are analyzed, and the optimized design of the optical system is completed. The optical system includes a front component with orthogonal heterogeneous optical field modulation, a spatial heterodyne interferometer, and a re-imaging component. The front component consists of a front cylindrical lens and a collimator lens. The front cylindrical lens group has a focal length of 534.5 mm, a spectral field of view of ±1.85°, and corresponds to a horizontal atmospheric width of 160 km. The front collimator lens group has a focal length of 461.6 mm, a spatial field of view of ±1.185°, and corresponds to a vertical atmospheric height range of 102 km. Consequently, the sampling resolution interval of HDI in the spatial dimension is approximately 0.1 km.In spatial heterodyne interferometric spectrometers, there are various polarization-sensitive components such as diffraction gratings, mirrors, and beam-splitting prisms. When the optical system itself exhibits residual polarization sensitivity, it not only impairs the radiometric measurement accuracy of the spectrometer but also leads to image separation in imaging spectrometers. The HDI polarization response model and measurement platform are established. Test results indicate that by incorporating a quartz wedge-type depolarizer between the field diaphragm of the front cylindrical lens group and the mirror in the HDI optical system, and adjusting the depolarizer to ensure the maximum image separation occurs in the spectral dimension, the influence of the depolarizer on the spatial dimension imaging quality and reconstructed spectral accuracy of HDI can be minimized.Results and DiscussionsGround tests and calibration results confirm that HDI meets all design specifications. Spectral characterization shows an operational range of 756.8?771.4 nm, which fully covers the primary absorption region of the oxygen A-band (759?769 nm). Calibration with tunable laser and wave-meter yields a measured spectral resolution of 0.0394 nm, which is in close agreement with theoretical predictions. Under typical entrance pupil radiance, during limb observation at an orbital altitude of 520 km, with spatial binning of 20 pixel in the atmospheric vertical direction (corresponding to a vertical profile spatial resolution of 2 km), the average spectral signal-to-noise ratio (SNR) is 149. However, due to the detector heat sink cooling temperature of -16.2 ℃, which is slightly higher than the designed value of -20 ℃, the dark current noise of HDI in orbit has increased marginally. Consequently, the average spectral SNR of HDI is slightly lower than 149, but it still satisfies the requirement (≥100).ConclusionsThe development of HDI makes a significant advancement in space-borne hyperspectral detection of near-space atmospheric parameters. These technical achievements not only provide critical data support for current aerospace atmospheric monitoring missions but also lay a foundation for long-term global atmospheric density profile remote sensing.To meet the requirements of data applications, a variable-scale data binning processing method will be adopted in the future to retrieve high-precision atmospheric density profile data products. Additionally, to accurately assess the on-orbit spectral resolution of HDI and its capability for airglow spectrum in the middle and upper atmospheres, it is planned to reconfigure the on-orbit operating parameters of HDI. This reconfiguration aims to detect and analyze the hyperspectral characteristics of atmospheric tracers at altitudes above 80 km, while also conducting research on the retrieval of atmospheric temperature profiles in the near-space region (80?110 km).

SignificanceIn a stable climate system, there is a fundamental balance between the solar radiation energy reaching the Earth and the reflected shortwave solar radiation and emitted longwave radiation energy leaving the Earth. The transfer of radiative energy within the Earth-atmosphere-ocean system is a complex dynamic process. Remote sensing of Earth’s radiation budget (ERB) from space is the most effective way to monitor the climate system’s radiative energy balance, verify progress toward global carbon sinks and carbon neutrality, and track large-scale climate changes. Achieving these goals requires precise, continuous, quantitative remote sensing of incoming solar radiation, reflected shortwave radiation, and emitted longwave radiation from multiple locations outside Earth’s atmosphere. Human activities can significantly alter surface albedo and atmospheric composition. Changes in surface albedo directly affect the absorption of shortwave radiation, while the increase in atmospheric greenhouse gases enhances the greenhouse effect, hindering the escape of longwave radiation energy. Most of this energy remains trapped in the atmosphere and hydrosphere, disrupting Earth’s radiative energy balance. The longwave and shortwave radiation measurements help quantify Earth’s radiation energy imbalance (EEI), with an average of 0.50?1.00 W/m2 from 2005 to 2025, derived from satellite-based observations and ocean heat content measurements, and are associated with unprecedented warming in the 21st century. This imbalance, driven by increased greenhouse gases trapping longwave radiation, results in over 90% of excess heat being absorbed by oceans, exacerbating sea-level rise and extreme weather events. By continuously improving the accuracy of instrument observations and calibration, and optimizing algorithms for multi-source data analysis and fusion, the future direction of Earth’s radiation budget remote sensing lies in advancing spatiotemporally continuous monitoring of the true magnitude of Earth’s radiation energy imbalance from outside the atmosphere. Furthermore, ERB data shed light on the roles of clouds and aerosols. Clouds can cool the planet by reflecting sunlight or warm it by trapping heat, with their net effect depending on type and structure. Aerosols act as cloud condensation nuclei, influencing precipitation and radiation scattering. In essence, space measurements validate models, detect trends, and inform policy on climate mitigation. As of 2025, ongoing data from CERES and emerging satellites underscore ERB’s role in addressing global warming, emphasizing the need for sustained orbital monitoring to safeguard Earth’s habitability.ProgressThe Clouds and the Earth’s Radiant Energy System (CERES) offers the longest continuous record of outgoing longwave and reflected shortwave radiation. However, achieving complete global spatial sampling of Earth’s diurnal cycle remains a challenge. This typically requires combining geostationary satellite data, which interpolates the 24-h cycle, with polar-orbiting satellite data to derive a global Earth’s radiation energy imbalance. To address these limitations, the Space Science Center at the Institute for Advanced Study, Shenzhen University, is leading a pathfinder ERB experiment that will monitor outgoing longwave and reflected shortwave radiation using the Chang’e-7 lunar orbiter. The experiment will test the feasibility of monitoring Earth’s radiation budget from the Moon. If successful, Moon-based ERB measurements could provide a stable, long-term, highly accurate platform for global climate monitoring, overcoming traditional satellite limitations and greatly improving our understanding of Earth’s radiation energy imbalance. The Moon-based ERB (MERB) experiment is an international collaboration, with Dr. Mustapha Meftah and Dr. Alain Sarkissian from UVSQ/LATMOS contributing to instrument validation, calibration, characterization, and scientific data analysis. The MERB flight model is being ground-calibrated in a traceable radiometry laboratory at Shenzhen University. A 2.2-m-diameter, 2.0-m-long vacuum chamber is being set up within a Class 1000 cleanroom. This chamber integrates a Sun simulator, a deep-space background radiation blackbody, a terrestrial longwave radiation blackbody, and an integrating sphere light source with eight distinct wavelength bands. The spectral and radiometric responses of the total and longwave channels are calibrated against these traceable radiation targets. The MERB qualification model has successfully passed all environmental and vibration tests. The flight model has been manufactured and integrated into the Chang’e-7 orbiter, which is scheduled for launch in 2026.Conclusions and ProspectsEarth’s radiation energy imbalance is a crucial climate variable that constrains the global rate of climate change. While polar-orbiting and geostationary satellites remain the primary platforms for monitoring Earth’s radiation energy imbalance, a MERB experiment provides a valuable complement. Leveraging a unique and stable vantage point from Earth’s natural satellite, the Moon, this experiment greatly enhances Earth’s radiation energy imbalance monitoring, not only in tracking global trends but also in determining its absolute value with greater accuracy.

ObjectiveAs the sophistication of national deep space exploration missions increases, the demand for high-precision exploration of extraterrestrial galaxies is increasing day by day. Taking the lunar exploration as an example, the lunar exploration project has planned specific tasks to acquire high-resolution three-dimensional (3D) terrain maps of the lunar surface. The high-precision optical stereo mapping camera is the key payload for achieving stereoscopic mapping of the lunar surface. In the deep space exploration domain, mapping cameras are required to withstand face far more rigorous challenges during both the launch and operational phases compared to Earth-orbiting satellites. As highly sensitive and complex precision aerospace equipment, extremely harsh conditions impose higher demands on the critical process of optical alignment for deep space exploration mapping cameras. To address these stringent conditions, adaptive responses are made in the structural and optical design of these cameras. Currently, there are only a few similar payloads successfully launched and operational worldwide, and their image resolutions are generally not high. Among them, the narrow angle camera carried by national aeronautics and space administration (NASA)’s lunar-reconnaissance-orbiter (LRO) is the one with the most comparable performance. Therefore, it is essential to study optical alignment processes of such cameras to ensure their excellent in-orbit performance after precise alignment.MethodsBased on these characteristics of deep space optical mapping cameras, which include high integration, high lightweight design, and multiple boundary condition constraints, the force, thermal, and optical simulation analyses of these key components for the system are carried out. A dual-camera co-referenced alignment and detection scheme based on computer-aided adjustment has been proposed. A simulation model for sub-components is established, with adhesive shrinkage stress and forced displacement stress as objective functions. This enables targeted thermal and mechanical analyses of the primary and secondary mirror assemblies (Fig. 3, Fig. 4). Based on these thermal-mechanical analysis results, these coupling risks between surface deformation of the primary/secondary mirrors and system misalignment are prioritized. An optical simulation model is further developed to analyze distinctions caused by surface deformation and misalignment of the primary and secondary mirror assemblies (Fig. 6, Fig. 7). In order to ensure the precise alignment of the optical axes of two cameras under constrained conditions, the control of lens optical axis pointing should be managed at the optical design and optical manufacturing stages. Specifically, the optical axes of each reflective mirror need to be precisely extracted. Taking the primary mirror as an example, a null-compensator-testing based self-aligning optical axis extraction method has been proposed and applied to the high-precision extraction for the optical axis of the reflector (Fig. 8). Through comprehensive full-chain simulations that take into system aberration, optical axis, and distortion, these key variables for controlling distortion are identified (Table 3). These key variables enable iterative adjustments to achieve distortion compensation.Results and DiscussionsUsing the aforementioned alignment methodology, strict control is implemented during the assembly of each reflector component. Surface shape test results for the primary and secondary mirrors before and after adhesive bonding are provided. Surface shape root mean square (RMS) of primary mirror remains stable at 0.02λ pre- and post-assembly (Fig. 9). Surface shape of secondary mirror RMS increases slightly from 0.013λ to 0.015λ post-bonding (Fig. 10). After initial system integration via the co-rotational alignment method, RMS of wavefront errors at three normalized fields are 0.106, 0.060, and 0.144, respectively (Fig. 12). Computer-aided alignment optimization further reduces these values to RMS of 0.087, 0.068, and 0.085 (Fig. 13), approaching the diffraction limit because of these residual high-order aberrations from mirror fabrication. After alignment, the integration and testing of lens assembly and detector assembly are carried out. Average modulation transfer function (MTF) across three fields exceeds 0.15 at the Nyquist frequency. The internal orientation element and distortion of the camera are measured by the precision-angle-measurement method. Focal length deviation is less than 3.5‰ of design specifications, and full-field distortion is less than 3.5 μm. Boresight alignment error relative to satellite reference axis measured by theodolites is 1.92′ (design target: 0°).ConclusionsWe propose a co-referenced alignment and detection scheme for the space mapping camera used in deep space exploration. Based on ensuring that key optical performance metrics of the camera meet requirements, it can rapidly achieve precise control of critical parameters for a stereo mapping camera such as camera boresight and distortion. By establishing thermal and mechanical simulation models for key components, and combining traditional computer-aided alignment method, the stress-free assembly of the secondary mirror is identified as a critical step in the alignment of a single-camera system. High-precision extraction of the optical axis for each reflective mirror is brought forward to the optical design and manufacturing stages, and a method based on null-compensator-testing for the optical axis is proposed and applied, thereby improving the initial alignment precision and the angle control precision between two cameras. A multivariable full-link simulation model is established to simultaneously meet these high-precision requirements for the angle between two cameras’ boresights and the system distortion. Ultimately, the alignment and testing of a spaceborne mapping camera for deep space exploration are successfully completed, with all performance metrics meeting required standards. This achievement addresses the challenging issue of optical alignment for lightweight, deep space cameras under multiple boundary constraints. It provides a technical foundation for the development of stereoscopic mapping cameras in future deep space exploration missions.