Compact all-fiber pulsed coherent Doppler lidar system for wind sensing

Three-dimensional detection of CO2 and wind using a 1.57 µm coherent differential absorption lidar

A 1.57-µm coherent differential absorption lidar is demonstrated for measuring three-dimensional CO2 and wind fields simultaneously. The maximum detection range of CO2 is up to 6 km with a range resolution of 120 m and a time resolution of 1 min. A preliminary assessment of instrument performance is made with a 1-week continuous observation. The CO2 concentration over a column from 1920 to 2040 m is compared with the one measured by an optical cavity ring-down spectrometer placed on a 2 km-away meteorological tower. The concentration is strongly correlated with the in-situ spectrometer with a correlation coefficient and RMSE of 0.91 and 5.24 ppm. The measurement accuracy of CO2 is specified with a mean and standard deviation of 2.05 ppm and 7.18 ppm, respectively. The regional CO2 concentration and the three-dimensional wind fields are obtained through different scanning modes. Further analysis is conducted on vertical mixing and horizontal transport of CO2 by combining with the measured wind fields.

Theoretical performance of a 1.5-µm satellite-borne coherent Doppler wind lidar using a planar waveguide optical amplifier with a demonstrated figure of merit: simulation of signal detection probability, measurement precision, and bias

We report performance of a satellite-borne coherent Doppler wind lidar (SCDWL), which equips a planer waveguide amplifier (PWA) operating in a wavelength of 1.5 µm. The performance is defined by detection probability, measurement precision, and bias, and is characterized with a Doppler wind lidar (DWL) simulation that considers a realistic wind velocity profile, and instrumental and atmospheric parameters. Among the parameters, we carefully model those related to the PWA whose figure of merit has great impact on the performance of SCDWL and has shown rapid improvement in recent years. Moreover, we introduce three models for a backscattering coefficient (high, moderate, and low) to assess the influence from variation of atmospheric backscattering. Our simulation demonstrates that the SCDWL can work with reasonable performance for the target altitude of 6 km in the case of the high-backscattering model. The simulation also exhibits that the SCDWL can observe wind velocity at the altitude of 12 km if improved instrumental parameters or higher backscattering coefficients are considered. In addition, we reveal that non-uniform wind velocity distribution degrades the performance of the SCDWL and induces a bias between measured and real wind velocity.

1.2 W single-frequency Tm3+/Ho3+ co-doped fiber oscillator at 2050 nm

In this Letter, a watt-level single-frequency fiber oscillator at 2050 nm was demonstrated for the first time, to the best of our knowlegde, in a linear laser cavity with a piece of an un-pumped Tm3+/Ho3+ co-doped fiber serving as a saturable absorber. With delicate optimization of mode filtering effect of the dynamic gratings formed in the saturable absorber, a maximum single-frequency laser output power of 1.2 W was achieved under a total bidirectional pump power of 5.8 W at 1570 nm, and the corresponding optical efficiency is 20.7%. This is, to the best of our knowledge, the highest power of a single-frequency fiber oscillator at the wavelength above 2 µm.

Evaluation of a coherent 2-µm differential absorption lidar for water vapor and radial wind velocity measurements

The performance of a coherent 2-µm differential absorption lidar (DIAL) for simultaneously measuring water vapor (H₂O) and radial wind velocity was evaluated. For measuring H₂O, a wavelength locking technique was applied to the H₂O-DIAL system. The H₂O-DIAL system was evaluated under summer daytime conditions in Tokyo, Japan. H₂O-DIAL measurements were compared with measurements from radiosondes. The H₂O-DIAL-derived volumetric humidity values agreed with the radiosonde-derived values over the range from 11 to 20 g/m³ with a correlation coefficient of 0.81 and a root-mean-square difference of 1.46 g/m³. Comparisons between the H₂O-DIAL and the in-situ surface meteorological sensors demonstrated the simultaneous measurement of H₂O and radial wind velocity.

Single-frequency 45-mJ pulses from a MOPA system using an Er,Yb:glass planar waveguide amplifier and a large mode area Er-doped fiber amplifier

We demonstrate a master oscillator power amplifier (MOPA) system that emits single-frequency high-energy optical pulses at 1540 nm using an Er,Yb:glass planar waveguide amplifier and a large mode area Er-doped fiber amplifier. A double under-cladding and a 50-µm-thick core structure are employed for the planar waveguide amplifier to increase the output energy without degrading the beam quality. A pulse energy of 45.2 mJ with a peak power of 27 kW is generated at a pulse repetition rate of 150 Hz with a pulse duration of 1.7 µs. Moreover, the beam quality factor M² of the output beam is 1.84 at the highest pulse energy thanks to its waveguide structure.

Dust flow analysis by low coherence Doppler lidar

Visualization of dust flow and wind dynamics near the ground surface are essential for understanding the mixing and interaction between geosphere and atmosphere near the surface. Knowing the temporal dust flow is beneficial in dealing with air pollution and health issues. Dust flows near the ground surface are difficult to monitor because of their small temporal and spatial scale. In this study, we propose a low-coherence Doppler lidar (LCDL) for measuring dust flow near the ground with high temporal and spatial resolutions of 5 ms and 1 m, respectively. We demonstrate the performance of LCDL in laboratory experiments using flour and calcium carbonate particles released into the wind tunnel. LCDL experiment results show a good agreement with anemometer measurement in wind speeds ranging from 0 to 5 m/s. The LCDL technique can reveal dust's speed distribution, which is affected by mass and particle size. As a result, different speed distribution profiles can be used to determine dust type. The simulation results of dust flow coincide well with the experimental results.

Optical single-sideband modulation for a coherent Doppler lidar

The first, to the best of our knowledge, implementation of optical single-sideband modulation (OSSBM) in coherent Doppler lidar (CDL) is demonstrated for use with short pulse widths. It is shown that by providing more bandwidth through a higher intermediate frequency (IF), the OSSBM CDL addresses cross talk between the IF and baseband spectra, reducing spectral distortion. Ultimately, a 4-ns pulse width with a 1-GHz IF is achieved and the effectiveness of the OSSBM CDL is confirmed through velocimetry experiments. This represents a several fold improvement over current fiber-based CDL implementations.

Mitigation of amplified spontaneous emission noise for an all-fiber coaxial aerosol lidar with different single-photon detectors

For a coaxial single-photon lidar system, amplified spontaneous emission (ASE) noise from the fiber amplifier is inevitable. The ASE backscattering from specular reflection annihilates the far-field weak signal, resulting in low signal-to-noise ratio, short measurement distance, and even misidentification. We propose a method for calibrating and mitigating ASE noise in all-fiber coaxial aerosol lidar and demonstrate the method for a lidar system with different single-photon detectors (SPDs). The accuracy of the coaxial aerosol lidar is comparable to that of the biaxial one. We conducted an experiment using three different detectors, namely, InGaAs/InP SPD, up-conversion SPD, and superconducting nanowire SPD in the same coaxial lidar system. Compared with the biaxial system, the three different detectors we used have achieved more than 90% ASE noise suppression, the measured visibility percent errors of InGaAs/InP SPD data, up-conversion SPD data, and superconducting nanowire SPD data all within 20%, and the percent error within 10% are 99.47%, 100%, and 95.12%, respectively. Moreover, time-sharing optical switching allowed to obtain background noise with high accuracy.

Intelligent and compact coherent Doppler lidar with fiber-based configuration for robust wind sensing in various atmospheric and environmental conditions

The intelligent and compact coherent Doppler lidar (CDL) for wind sensing is demonstrated. The configuration is fiber-based. Several functions for the robust wind sensing in various atmospheric and environmental conditions are shown. The main feature of this CDL is the intelligent functions of the beam focusing, spectral accumulation, and window wiping. The supplemental functions of the robust noise floor reduction and motion compensation are also introduced. The effect of the above-mentioned main feature is demonstrated for the improvement of data availability. The evaluation results of the highly accurate wind velocity measurement are additionally shown.

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.

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.

Dual-wavelength locking technique for coherent 2-µm differential absorption lidar applications

A dual-wavelength locking technique for coherent 2-µm differential absorption lidar (DIAL) applications has been developed for simultaneously measuring water vapor (${{\rm{H}}_2}{\rm{O}}$) and radial wind velocity profiles. The two wavelengths for DIAL measurement were stabilized by sidebands of an electro-optic modulated laser, which was locked to the ${{\rm{CO}}_2}$ R30 absorption line. We selected wavelengths of 2050.550 and 2051.103 nm to minimize the effects of height-dependent temperature and pressure variations. The long-term wavelength stability of the two locked lasers of ${\lt}{0.2}\;{\rm{pm}}$, which corresponds to 14 MHz, was achieved. The DIAL instrument using the developed technique meets the requirement of measuring the ${{\rm{H}}_2}{\rm{O}}$ concentration with a systematic error ${\lt} {{5}}\%$ below an altitude of 5 km.

Inversion probability enhancement of all-fiber CDWL by noise modeling and robust fitting

Accurate power spectrum analysis of weak backscattered signals are the primary constraint in long-distance coherent Doppler wind lidar (CDWL) applications. To study the atmospheric boundary layer, an all-fiber CDWL with 300µJ pulse energy is developed. In principle, the coherent detection method can approach the quantum limit sensitivity if the noise in the photodetector output is dominated by the shot noise of the local oscillator. In practice, however, abnormal power spectra occur randomly, resulting in error estimation and low inversion probability. This phenomenon is theoretically analyzed and shown to be due to the leakage of a time-varying DC noise of the balanced detector. Thus, a correction algorithm with accurate noise modeling is proposed and demonstrated. The accuracy of radial velocity, carrier-to-noise ratio (CNR), and spectral width are improved. In wind profiling process, a robust sine-wave fitting algorithm with data quality control is adopted in the velocity-azimuth display (VAD) scanning detection. Finally, in 5-day continuous wind detection, the inversion probability is tremendously enhanced. As an example, it is increased from 8.6% to 52.1% at the height of 4 km.

Demonstration of the 1.53-µm coherent DIAL for simultaneous profiling of water vapor density and wind speed

The 1.53-µm coherent differential absorption lidar (DIAL) is demonstrated for the simultaneous profiling of water vapor (H₂O) density and wind speed. The optical setup is fiber-based. The wavelength locking circuit can achieve precise locking of 13.0 MHz by the combination of the line center locking to the hydrogen cyanide (HCN) absorption line and offset locking to the H₂O absorption wavelength. The measurable range for the simultaneous profiling is up to 1.2 km. The DIAL-measured H₂O density is compared with the one measured by an in-situ sensor. Qualitative good agreement is shown with the random error of 0.56 g/m³.

Fine gust front structure observed by coherent Doppler lidar at Lanzhou Airport (103°49$^{\prime}$'E, 36°03$^{\prime}$'N)

Coherent Doppler lidar (CDL) has long been used to automatically identify gust front-induced wind shear signatures from the velocity data, but rare attention has been given to the fine structure of wind gust fronts. In this work, a compact and robust CDL with high efficiency and accuracy is equipped at Lanzhou Airport (103°49$^{\prime}$^'E, 36°03$^{\prime}$^'N) to conduct interpretation of wind gust front structures by using high-resolution CDL data. Outflows of gust fronts could be detected reliably from radial velocities, spectral widths, as well as radial shears. For the case study presented here, photographs of the velocity and spectrum width capacitates gust front characteristics such as height, advance speed, and radial shear, as well as vertical structure to be displayed in minute detail. Besides, the quasi-continuous vertical wind reveals the potential turbulent mixing and vertical transport process during the gust front event, which makes CDL a very attractive and essential technique for future development of gust front automatic detection systems.

Wavelength selection and measurement error theoretical analysis on ground-based coherent differential absorption lidar using 1.53 µm wavelength for simultaneous vertical profiling of water vapor density and wind speed

A feasibility study of coherent differential absorption lidar is conducted using a 1.53-µm wavelength for simultaneously retrieving the water vapor density and wind speed profiles. We selected the ON and OFF wavelengths to be 1531.383 and 1531.555 nm, respectively, for minimizing the effect of the temperature change in the atmosphere. The systematic measurement error can be reduced to below 5% by stabilizing the ON wavelength from ${-64}$-64 to 102 MHz around the center of the water vapor absorption line. Analysis of the speckle and photon statistics errors reveal that the relative error of the water vapor density is less than 10% at the altitude from 0.1 to 1.7 km with the 100 m range resolution with 10 min data accumulation time. The simultaneous measurement of wind speed and direction can also be achieved by employing a conical scan mechanism.

Power scaling and spectral linewidth suppression of hybrid Brillouin/thulium fiber laser

In this paper, an ultra-narrow linewidth hybrid Brillouin/thulium fiber laser (BTFL) was demonstrated. By optimizing the output coupling, pump scheme, fiber length and Brillouin pump power for the linewidth narrowing, 344-mW output power with a narrow linewidth of 0.93 kHz was obtained from the BTFL, in which the linewidth of Stokes light was suppressed more than 43 times compared with the 40 kHz linewidth of the Brillouin pump. Besides, the influences of output coupling and pump scheme on the power and linewidth behavior of a single-frequency BTFL were also experimentally investigated, and there exists a performance balance among linewidth narrowing, output power and SBS threshold. The output coupling exerted a significant influence on the BTFL performance.

Development of an All-Fiber Coherent Doppler Lidar in the IAO SB RAS

Paper presents a model of the all-fiber pulsed coherent Doppler lidar (IAO-lidar) build in the IAO SB RAS. Here is described lidar design, the algorithm for processing of lidar signals and the software-hardware system that implements signal processing in real time, created with the use of open source software. The results of joint measurements of the radial velocity by the IAO-lidar and the HALO Photonics (Stream Line) lidar are given.

Simultaneous wind and rainfall detection by power spectrum analysis using a VAD scanning coherent Doppler lidar

Doppler wind lidar is an effective tool for wind detection with high temporal and spatial resolution. However, precise wind profile measurement under rainy conditions is a challenge, due to the interfering signals from raindrop reflections. In this work, a compact all-fiber coherent Doppler lidar (CDL) at working wavelength of 1.5 µm is applied for simultaneous wind and precipitation detection. The performance of the lidar is validated by comparison with the weather balloons. Thanks to the ability of precise spectrum measurement, both aerosol and rainfall signals can be detected by the CDL under rainy conditions. The spectrum width is used to identify the precipitation events, during which the two-peak Doppler spectrum is observed. The spectrum is fitted by a two-component Gaussian model and two velocities are obtained. By using the velocity-azimuth display (VAD) scanning technique, wind speed and rainfall speed are simultaneously retrieved. The false detection probability of wind speed in the rainy conditions is thus reduced.

110 mW single-frequency Yb:YAG crystal-derived silica fiber laser at 1064 nm

A single-frequency laser based on Yb:YAG crystal-derived silica fiber (YDSF) was demonstrated. The YDSF was fabricated by a molten-core method with a doping concentration of 4.8 wt. % for Yb₂O₃, whose gain coefficient and transmission loss were measured to be 1.7 dB/cm and 0.005 dB/cm, respectively. An over 110 mW stable single-frequency laser at 1064 nm was obtained based on a distributed Bragg reflector setup, showing a slope efficiency of 18.5%. The signal-to-noise ratio was ∼80  dB, and the frequency fluctuation was less than 20 MHz within 10 min.

1.55-μm high-peak, high-average-power laser amplifier using an Er,Yb:glass planar waveguide for wind sensing coherent Doppler lidar

We have developed a high-gain, high-peak-power laser amplifier at an eye-safe 1.55 μm wavelength using an Er,Yb:glass planar waveguide for wind sensing coherent Doppler lidars (CDLs). Our planar waveguide is free from stimulated Brillouin scattering and realizes high gain thanks to its multi-bounce optical-path configuration. A peak power of 5.5 kW with a pulse energy of 3.2 mJ is achieved at the repetition frequency of 4 kHz, which leads to an average power of 12.8 W. The gain is more than 23 dB. The wind sensing at more than 30 km is demonstrated with a CDL using the developed amplifier.

High-spectral-resolution Mie Doppler lidar based on a two-stage Fabry-Perot etalon for tropospheric wind and aerosol accurate measurement

A two-stage Fabry-Perot etalon (FPE)-based high-spectral-resolution (HSR) Mie Doppler lidar technology is proposed that is capable of simultaneously detecting tropospheric wind and aerosol optical properties with high precision. The lidar structure is designed, and the measurement principle is analyzed. A two-channel integrated FPE forming a two-stage FPE ensures the relative stability of the spectra. The HSR first-stage etalon can effectively suppress the contamination of Rayleigh signal. The transmission and reflection spectra of the second-stage etalon can form a double edge (DE) to measure wind speed. Two multimode polarization-insensitive optical circulators are used to achieve high-efficiency utilization for backscattering signals. The parameters of the two-stage FPE are optimized. According to the selected system parameters, the detection performance of the proposed lidar is simulated. Simulation results show that with 150 m range resolution and 1 min total accumulation time for the paired line-of-sight (LOS) measurement, within ±25  m/s LOS wind speed range, the nighttime and daytime LOS wind speed errors are below 0.48 m/s and 2.5 m/s, respectively, for a clear day, and below 0.58 m/s and 5.5 m/s, respectively, for a hazy day from 0.1 km to 8 km altitude; the backscatter ratio relative errors are below 2.7% up to 8 km for a clear day, and below 4.6% up to 5 km for a hazy day. Compared with the traditional dual-FPE-based DE Mie Doppler lidar, the wind speed accuracies are improved by 2.02-3.58 times for a clear day and 2.14-4.31 times for a hazy day.

Frequency-modulated multifunction lidar for anemometry, range finding, and velocimetry-1. Theory and signal processing

Original waveforms and optimized signal processing are proposed for frequency-modulated continuous-wave lidar for range finding, velocimetry, and laser anemometry. For range finding, the aim of this signal processing is to extend lidar range and reduce ambiguities. Moreover, the effect of moderate atmospheric turbulence on lidar efficiency is analyzed for infinite and finite targets, taking into account wind-induced bistatism. For laser anemometry, the aim is to measure air speed at the shortest distance farther than the rotor-induced turbulent volume around the helicopter and to avoid parasitic echoes coming from clouds or hard targets in the vicinity of a helicopter.

Wind ranging and velocimetry with low peak power and long-duration modulated laser

A novel approach to simultaneous wind ranging and velocimetry using low peak power, long-duration modulated laser pulse transmissions is proposed. Received signals backscattered by aerosol particles are processed by a multi-reference matched-filter (MRMF) which performs matched filter processing between the received signal and several reference signals in parallel and outputs a range-velocity profile of received power. Ranging and velocimetry are performed simultaneously by estimating received power, radial velocity, and velocity dispersion from a velocity profile at an arbitrary range in the range-velocity profile. The accuracies of the three estimates improve in proportion to the square root of pulse duration; that is, a 100 times longer pulse is equivalent to a 10-dB amplification.

Frequency- and intensity-noise suppression in Yb3+-doped single-frequency fiber laser by a passive optical-feedback loop

The frequency and intensity noise of an Yb³⁺-doped single-frequency distributed Bragg reflector (DBR) fiber laser are effectively reduced by a simple, passive optical-feedback loop (POFL), which consists of only two optical couplers. The feedback loop, which has resonance with the high reflective grating of the DBR laser and relative long optical path compared to the DBR cavity, results in narrower linewidth and lower relative intensity noise (RIN) in the feedback signal. The RIN of relaxation oscillation is reduced by 20dB from -99.9dB/Hz @ 993 kHz to -119.4dB/Hz @ 192 kHz, and the frequency noise was suppressed at frequencies higher than 1 kHz, with a maximum reduction of about 30 dB from 10 kHz to 100 kHz, which results in a spectral linewidth compression from 3.96 kHz to 540 Hz. Even after one fiber amplification stage, the noise did not increase significantly, and a spectral linewidth well below 1 kHz were also achieved at output power of 10W.

Wind turbine wake visualization and characteristics analysis by Doppler lidar

Wind power generation is growing fast as one of the most promising renewable energy sources that can serve as an alternative to fossil fuel-generated electricity. When the wind turbine generator (WTG) extracts power from the wind, the wake evolves and leads to a considerable reduction in the efficiency of the actual power generation. Furthermore, the wake effect can lead to the increase of turbulence induced fatigue loads that reduce the life time of WTGs. In this work, a pulsed coherent Doppler lidar (PCDL) has been developed and deployed to visualize wind turbine wakes and to characterize the geometry and dynamics of wakes. As compared with the commercial off-the-shelf coherent lidars, the PCDL in this work has higher updating rate of 4 Hz and variable physical spatial resolution from 15 to 60 m, which improves its capability to observation the instantaneous turbulent wind field. The wind speed estimation method from the arc scan technique was evaluated in comparison with wind mast measurements. Field experiments were performed to study the turbulent wind field in the vicinity of operating WTGs in the onshore and offshore wind parks from 2013 to 2015. Techniques based on a single and a dual Doppler lidar were employed for elucidating main features of turbine wakes, including wind velocity deficit, wake dimension, velocity profile, 2D wind vector with resolution of 10 m, turbulence dissipation rate and turbulence intensity under different conditions of surface roughness. The paper shows that the PCDL is a practical tool for wind energy research and will provide a significant basis for wind farm site selection, design and optimization.

Eye-safe single-frequency single-mode polarized all-fiber pulsed laser with peak power of 361 W

An all-fiber, single-frequency, single-mode linearly polarized, high peak power pulsed laser at 1540 nm for coherent Doppler wind lidar is demonstrated. A narrow-linewidth seed laser is pulse modulated by an acousto-optic modulator and then amplified by two-stage cascade amplifiers. An 0.8 m long erbium/ytterbium co-doped polarization-maintaining fiber with a core diameter of 10 μm is used as the gain fiber of a power amplifier, and longitudinally varied strains are applied on the gain fiber to realize approximately 3.4 times enhancement of the stimulated Brillouin scattering threshold. Peak power of 361 W pulse width of 200 ns at 10 kHz repetition rate is achieved with transform-limited linewidth and diffraction-limited beam quality. To the best of our knowledge, it is the highest peak power of an eye-safe, single-mode narrow-linewidth pulsed fiber laser based on 10 μm core diameter silica fiber.

Lidar-radar velocimetry using a pulse-to-pulse coherent rf-modulated Q-switched laser

An rf-modulated pulse train from a passively Q-switched Nd:YAG laser has been generated using an extra-cavity acousto-optic modulator. The rf modulation reproduces the spectral quality of the local oscillator. It leads to a high pulse-to-pulse phase coherence, i.e., phase memory, over thousands of pulses. The potentialities of this transmitter for lidar-radar are demonstrated by performing Doppler velocimetry on indoor moving targets. The experimental results are in good agreement with a model based on elementary signal processing theory. In particular, we show experimentally and theoretically that lidar-radar is a promising technique that allows discrimination between translation and rotation movements. Being independent of the laser internal dynamics, this scheme can be applied to any Q-switched laser.

Single-pulse measurement of wind velocities using an Er:Yb:glass coherent laser radar

Many wind-field mapping applications require range-resolved atmospheric velocity measurements at long range and/or with a temporal resolution sufficient to investigate turbulence. We argue that this capability can be achieved only by coherent laser radar systems that transmit energetic (>1 mJ) pulses. We describe such a system and describe single-pulse measurement of the range-resolved line-of-sight velocities, and show that the instrument-limited reproducibility of the measurements is 0.4 ms(-1).

Widely-tunable parametric short-wave infrared transmitter for CO2 trace detection

An all-fiber, tunable, short-wave infrared transmitter is demonstrated using efficient four-wave mixing in conventional L and O bands. To realize this source a highly-nonlinear fiber, exhibiting low bend loss over the short-wave infrared spectral band, is employed because of its advantageous properties as a nonlinear mixing medium. The transmitter was subsequently exploited to probe and detect trace levels of carbon dioxide in the 2051-nm spectral region where its beam properties, tunability, narrow linewidth, and stability all coalesce to permit this application. This work indicates this transmitter can serve as a robust source for sensing carbon dioxide and other trace gasses in the short-wave infrared spectral region and should therefore play an important role in future applications.

Statistical model for fading return signals in coherent lidars

A statistical model for the return signal in a coherent lidar is derived from the fundamental principles of atmospheric scattering and turbulent propagation. The model results in a three-parameter probability distribution for the coherent signal-to-noise ratio in the presence of atmospheric turbulence and affected by target speckle. We consider the effects of amplitude and phase fluctuations, in addition to local oscillator shot noise, for both passive receivers and those employing active modal compensation of wavefront phase distortion. We obtain exact expressions for statistical moments for lidar fading and evaluate the impact of various parameters, including the ratio of receiver aperture diameter to the wavefront coherence diameter, the speckle effective area, and the number of modes compensated.

Semianalytic pulsed coherent laser radar equation for coaxial and apertured systems using nearest Gaussian approximation

We present a semianalytic pulsed coherent laser radar (CLR) equation for coaxial and apertured systems. It combines the conventional CLR equation, numerical Fresnel integration (NFI), and nearest Gaussian approximation, using correction factors that correspond to beam truncation. The range dependence of the signal-to-noise ratio obtained by this semianalytic equation was found to agree well with the precise NFI solution for not only the focal range, but also the near-field range. Furthermore, the optimum beam truncation condition depending on the atmospheric refractive index structure constant is shown. The derived equation is useful for precisely predicting the CLR performance simply by its semianalytic expression.

Reduction of phase-induced intensity noise in a fiber-based coherent Doppler lidar using polarization control

Optimization of signal-to-noise ratio is an important aspect in the design of optical heterodyne detection systems such as a coherent Doppler lidar (CDL). In a CDL, optimal performance is achieved when the noise in the detector signal is dominated by local oscillator shot-noise. Most modern CDL systems are built using rugged and cost-efficient fiber optic components. Unfortunately, leakage signals such as residual reflections inherent within fiber components (e.g. circulator) can introduce phase-induced intensity noise (PIIN) to the Doppler spectrum in a CDL. Such excess noise may be a few orders of magnitude above the shot-noise level within the relevant CDL frequency bandwidth--corrupting the measurement of typically weak backscattered signals. In this study, observation of PIIN in a fiber-based CDL with a master-oscillator power-amplifier tapered semiconductor laser source is reported. Furthermore, we experimentally demonstrate what we believe is a newly proposed method using a simple polarization scheme to reduce PIIN by more than an order of magnitude.