1645 nm coherent Doppler wind lidar with a single-frequency Er:YAG laser

Single-frequency orbital angular momentum switchable modes from a microchip laser

We demonstrate the direct generation of single-frequency switchable orbital angular momentum (OAM) modes in a 1 µm wavelength range using a Nd:YVO4 microchip laser. The 808 nm laser diode pump beam is shaped into annular through an axicon associated with a lens. By adjusting the diameter and power of the annular pump beam, various OAM modes with different mode volumes can oscillate inside the Nd:YVO4 microchip. Moreover, a single-frequency output is also available due to the short cavity of the microchip. In the proof-of-principle experiment, single-frequency twofold multiplexed OAM modes | ± 1> and | ± 2> are generated, with experimentally measured fidelity higher than 96%. This work presents a compact and versatile single-frequency OAM source and will inspire multiple advanced scenarios ranging from classical to quantum photonics.

Demonstration on the performance enhancement of the 1.645 µm coherent Doppler lidar for long-range wind measurements with modification of laser transmitter and optical antenna

We have developed and experimentally investigated a long-range 1.645 µm coherent Doppler wind lidar (CDWL) system. A compact 1.645 µm single-frequency Er:YAG laser is utilized as the laser transmitter. The impact of laser transmitter parameters on wind detection was assessed using the figure of merit (FOM) concept. To enhance the measurement efficiency, the influence of wave aberrations on the heterodyne efficiency was analyzed. A Galilean telescope with an optical aperture of 100 mm is designed as the optical antenna based on the analysis. The line of sight (LOS) detection range exceeds 30.42 km with a data rate of 1 Hz at an elevation angle of 3.5°. To evaluate the effectiveness of the CDWL, comparison experiments were conducted between the 1.645 µm CDWL and a calibrated 1.55 µm CDWL, revealing a correlation coefficient of 0.9816 for the whole detection path in the wind velocity measurement.

Pulse Accumulation Approach Based on Signal Phase Estimation for Doppler Wind Lidar

Coherent Doppler wind lidar (CDWL) uses transmitted laser pulses to measure wind velocity distribution. However, the echo signal of CDWL is easily affected by atmospheric turbulence, which can decrease the signal-to-noise ratio (SNR) of lidar. To improve the SNR, this paper proposes a pulse accumulation method based on the cross-correlation function to estimate the phase of the signal. Compared with incoherent pulse accumulation, the proposed method significantly enhances the correlation between signals from different periods to obtain high SNR gains that arise from pulse accumulation. Using simulation, the study evaluates the effectiveness of this phase estimation method and its robustness against noise in algorithms which analyze Doppler frequency shifts. Furthermore, a CDWL is developed for measuring the speed of an indoor motor turntable and the outdoor atmospheric wind field. The phase estimation method yielded SNR gains of 28.18 dB and 32.03 dB for accumulation numbers of 500 and 1500, respectively. The implementation of this method in motor turntable speed measurements demonstrated a significant reduction in speed error-averaging 9.18% lower than that of incoherent accumulation lidar systems. In experiments that measure atmospheric wind fields, the linear fit curve slope between the measured wind speed and the wind speed measured via a commercial wind-measuring lidar can be reduced from 1.146 to 1.093.

Single-frequency fiber laser at 1627 nm based on a self-designed Er-doped hybridized glass fiber

Aiming at applications like expanding usable wave band of optical telecommunication and preparing Sr optical lattice clocks, a 1627 nm single-frequency fiber laser (SFFL) is demonstrated based on a 7-m-long self-designed Er-doped hybridized glass fiber (EDHF) and a linear cavity configuration with a loop mirror filter (LMF). By inserting a 10-m-long unpumped commercial Er-doped fiber as a dynamic Bragg grating into the LMF, a stable single-longitudinal-mode (SLM) laser with an output power of about 10 mW is obtained. The optical signal-to-noise ratio (OSNR) of SFFL is over 50 dB, and the linewidth is about 3.7 kHz. The measured relative intensity noise (RIN) is less than -140 dB/Hz at frequencies of over 0.5 MHz, and a power variation in 1 h is less than ±0.26%. To our best knowledge, it is the first demonstration of a SFFL operating at the U-band. This 1627 nm SFFL can provide advanced light source technology support for many cutting-edge applications.

Sub-kHz linewidth 1.6-µm single-frequency fiber laser based on a heavily erbium-doped silica fiber

We present a single-frequency erbium-doped fiber laser operated at 1608.8 nm using a homemade, heavily erbium-doped silica fiber as gain medium. The laser configuration is based on a ring cavity, which is combined with a fiber saturable absorber to achieve single-frequency operation. The measured laser linewidth is less than 447 Hz and the optical signal-to-noise ratio exceeds 70 dB. The laser exhibits an excellent stability, without any instance of mode-hopping during 1-hour observing. The fluctuations in both wavelength and power were measured to be 0.002 nm and less than 0.09 dB in a 45-minutes period. The laser produces over 14 mW of output power with a slope efficiency of 5.3%, which, to the best of our knowledge, is currently the highest power directly obtained from a single-frequency cavity based on an erbium-doped silica fiber above 1.6 µm.

1645-nm single-frequency vortex laser from an Er:YAG nonplanar ring oscillator

A 1645-nm single-frequency vortex beam with narrow linewidth from an Er:YAG nonplanar ring oscillator (NPRO) using an annular pump beam is demonstrated. The pump beam from a 1532-nm fiber laser is shaped to an annular beam by an axicon. The Er:YAG NPRO generates a 1.96-W single-frequency vortex beam under a pump power of 13 W. The linewidth of the 1645-nm vortex laser is measured as 6 kHz. This work provides a convenient way of single-frequency vortex beam generation.

Extending laser wavelengths to 1630 nm in centimeter-scale Er-phosphate fiber

The spectral bandwidth of Er-doped fibers limits their lasing wavelength at longer wave band. Here, to the best of our knowledge, we report a broad emission band (1420‒1680 nm) of Er³⁺ and demonstrate for the first time an Er-phosphate fiber, which supports laser oscillation at the extended wavelengths of 1627 nm and 1630 nm, with the output powers and slope efficiencies of 44 mW/12.5% and 16.5 mW/5.6%, respectively, pumped at 1480 nm. To the best of our knowledge, these are the highest output powers and slope efficiencies at 1627 nm and 1630 nm from an Er³⁺-doped all-fiber configuration.

Optimization of laser stabilization via self-injection locking to a whispering-gallery-mode microresonator: experimental study

Self-injection locking of a diode laser to a high-quality-factor microresonator is widely used for frequency stabilization and linewidth narrowing. We constructed several microresonator-based laser sources with measured instantaneous linewidths of 1 Hz and used them for investigation and implementation of the self-injection locking effect. We studied analytically and experimentally the dependence of the stabilization coefficient on tunable parameters such as locking phase and coupling rate. It was shown that precise control of the locking phase allows fine-tuning of the generated frequency from the stabilized laser diode. We also showed that it is possible for such laser sources to realize fast continuous and linear frequency modulation by injection current tuning inside the self-injection locking regime. We conceptually demonstrate coherent frequency-modulated continuous wave LIDAR over a distance of 10 km using such a microresonator-stabilized laser diode in the frequency-chirping regime and measure velocities as low as sub-micrometer per second in the unmodulated case. These results could be of interest to cutting-edge technology applications such as space debris monitoring and long-range object classification, high-resolution spectroscopy, and others.

Coherent Doppler wind lidar signal denoising adopting variational mode decomposition based on honey badger algorithm

Coherent Doppler wind lidar (CDWL) is used to measure wind velocity distribution by using laser pulses. However, the echo signal is easily affected by atmospheric turbulence, which could decrease the effective detection range of CDWL. In this paper, a variation modal decomposition based on honey badger algorithm (VMD-HBA) is proposed and demonstrated. Compared with conventional VMD-based methods, the proposed method utilizes a newly developed HBA to obtain the optimal VMD parameters by iterating the spectrum fitness function. In addition, the Correlation Euclidean distance is applied to identify the relevant mode and used to reconstruct the signal. The simulation results show that the denoising performance of VMD-HBA is superior to other available denoising methods. Experimentally, this combined method was successfully realized to process the actual lidar echo signal. Under harsh detection conditions, the effective detection range of the homemade CDWL system is extended from 13.41 km to 20.61 km.

Injection-seeded 10 kHz repetition rate Er:YAG solid-state laser with single-frequency pulse energy more than 1 mJ

We report a single-frequency Q-switched Er:YAG all-solid-state laser with a pulse repetition rate of up to 10 kHz. The single-frequency feature is ensured by injecting the seed laser into a Q-switched ring cavity, and the pulse repetition rate is increased by combing the Pound-Drever-Hall method and optical feedback. Peak power of 4.12 kW with an average pulse energy of 1.35 mJ single-frequency 1645 nm laser pulses is achieved at a pulse repetition rate of 10 kHz, which matches an average power of 13.5 W.

Advances on Solid-State Vortex Laser

Vortex beams (VBs) are structured beams with helical wavefronts carrying orbital angular momentum (OAM) and they have been widely used in lots of domains, such as optical data-transmission, optical tweezer, quantum entanglement, and super-resolution imaging. The ability to generate vortex beams with favorable performance is of great significance for these advanced applications. Compared with extra-cavity schemes, such as spatial light modulation, mode conversion, and others which transform other modes into vortex modes, solid-state vortex lasers can output vortex beams directly and show advantages including a compact structure, high robustness, easy to integrate, and low cost. In this review, we summarize intra-cavity generation approaches to vortex beams in solid-state lasers. Our work on 1.6μm eye-safe vector vortex lasers is also introduced.

High-power 1640 nm Er:Y2O3 ceramic laser at room temperature

We report on the high-power operation of an Er:Y₂O₃ ceramic laser at approximately 1.6 µm using a low-scattering-loss, 0.25 at. % Er³⁺-doped ceramic sample fabricated in-house via a co-precipitation process. The laser is in-band pumped by an Er, Yb fiber laser at 1535.6 nm and generates 10.2 W of continuous-wave (CW) output power at 1640.4 nm with a slope efficiency of 25% with respect to the absorbed pump power. To the best of our knowledge, this is the first demonstration of an approximately 1.6 µm Er:Y₂O₃ laser at room temperature. The prospects for further scaling of the output power and lasing efficiency via low Er³⁺ doping and reduced energy-transfer upconversion are discussed.

Active alignment of receiving beam for coaxial optics in wind sensing coherent Doppler lidar using feedback control based on the processing of heterodyne-detected signal

We have developed an active alignment of receiving beam (AARB) function for coaxial optics in wind sensing coherent Doppler lidar using feedback control based on the heterodyne-detected signal processing of backscattered light from the aerosols. The proposed method needs only the simple alignment components and contributes to the robustness for the coherent lidars with the high-power laser transmitter under the risky condition of misalignment, for example, in the airborne application. The concept, design, and evaluation results of the alignment precision are shown. The effect of the AARB is demonstrated for both cases of the hard target and soft target (i.e., wind sensing). To the best of our knowledge, this is the first demonstration of the AARB concept for the wind sensing coherent lidar.

Smoothed accumulated spectra based wDSWF method for real-time wind vector estimation of pulsed coherent Doppler lidar

Wind vector estimation method with high accuracy in the low signal-to-noise ratio region improves the performance of pulsed coherent Doppler lidar. The key to improving accuracy is to process the incorrect radial wind estimates or the distorted power spectra better. The smoothed accumulated spectra based weighted sine wave fitting method proposed here minimizes the effects of bad radial wind estimates by considering both signal intensity and wind spatial continuity. Leveraging spatial continuity from smoothed accumulated spectra, the weight coefficients and real-time wind vector profiles can be quickly determined with non-looped operations. Simulations and field experiments showed that the proposed method provides comparable or even slightly better quality and more available wind vector estimates than the filtered sine wave fitting method.

Wind lidar signal denoising method based on singular value decomposition and variational mode decomposition

A denoising method based on singular value decomposition (SVD) and variational mode decomposition (VMD) is proposed for wind lidar. Utilizing the covariance matrix based lidar signal simulation model, the performance of VMD, SVD, and VMD-SVD is evaluated. The results show that the VMD-SVD method is of better performance, and the output signal-to-noise ratio (SNR) is about 12 dB at the input SNR of -9dB. The actual lidar signals processing is performed with this combined denoising method, and the detection range and wind speed at pulse accumulation numbers of 50,100, and 300 are compared. We set the wind speed resulting from noisy signal with pulse accumulation number of 300 as the reference wind speed, and the mean value and standard deviation of wind differences are analyzed. The results show that the denoising method can not only increase the detection range while ensuring the accuracy of wind speed estimation but also achieve the same detection distance with fewer pulse accumulations, thereby improving the temporal resolution. For the pulse accumulation number of 50, the detection range is extended to 24 km from 18.45 km, and the standard deviation of speed difference is 0.88 m/s; for the same detection range, the temporal resolution is increased by about 6 times.

Sub-kHz linewidth, hybrid III-V/silicon wavelength-tunable laser diode operating at the application-rich 1647-1690 nm

The wavelength region about of 1650 nm enables pervasive applications. Some instances include methane spectroscopy, free-space/fiber communications, LIDAR, gas sensing (i.e. C₂H₂, C₂H₄, C₃H₈), surgery and medical diagnostics. In this work, through the hybrid integration between an III-V optical amplifier and an extended, low-loss wavelength tunable silicon Vernier cavity, we report for the first time, a III-V/silicon hybrid wavelength-tunable laser covering the application-rich wavelength region of 1647-1690 nm. Room-temperature continuous wave operation is achieved with an output power of up to 31.1 mW, corresponding to a maximum side-mode suppression ratio of 46.01 dB. The laser is ultra-coherent, with an estimated linewidth of 0.7 kHz, characterized by integrating a 35 km-long recirculating fiber loop into the delayed self-heterodyne interferometer setup. The laser linewidth is amongst the lowest in hybrid/heterogeneous III-V/silicon lasers.