Atmospheric CO2 measurements with a 2 μm airborne laser absorption spectrometer employing coherent detection

Feasibility study of a total precipitable water IPDA lidar from a solar-powered stratospheric aircraft

Repetitive, high spatial resolution measurements of water vapor are highly desirable for a range of critical applications, including quantitative forecasts of wildfire risk forecasting, extreme weather, drought implicated in mass refugee dislocation, and air quality. A point design for an integrated path differential absorption (IPDA) light detection and ranging (lidar) for column precipitable water vapor (PWV) intended for high-altitude long-endurance (HALE) uncrewed aerial systems (UASs) is described and analyzed. A novel, to the best of our knowledge, all-semiconductor source utilizing an intensity-modulated continuous wave approach to ranging is proposed, which facilitates reductions in weight, power, and size. Analytic and Monte Carlo calculations suggest that high spatial resolution (<10m) or high precision (<1%) may be obtained.

Performance Optimization of Holmium Doped Fiber Amplifiers for Optical Communication Applications in 2–2.15 μm Wavelength Range

In this paper, we address the performance optimization of Holmium doped fiber amplifier (HDFA) for optical communications in 2–2.15 μm wavelength range based on a single in-band forward pump source. The performance of the HDFA is analyzed with the help of theoretical simulations by considering an optimized length of Holmium doped fiber (HDF), doping concentration of Ho3+, and pump power. The impact of signal wavelength and power on gain, amplified spontaneous emission (ASE) noise, and noise figure (NF) of the amplifier is investigated. Furthermore, we investigate the variations in the gain of the amplifier, its output power, and NF by varying the power and wavelength of the pump source. After optimizing the parameters of the amplifier, the peak gain observed is around 56.5 dB, the 3 dB saturated output power obtained is 33.3 dBm, and the output power is 3 W at signal wavelength of 2.0321 μm for HDF having an optimized length of 12 m and pump power of 3.5 W. Minimum NF of around 8.2 dB is observed at 2.0321 μm for signal power of −5 dBm. The impact of ion-ion interaction on the performance of HDFA is also investigated. A reduction of 24.2 dB and 0.051 W is observed in peak gain and output power of HDFA, respectively by considering the ion-ion interaction.

2 μm passively mode-locked thulium-doped fiber lasers with Ta2AlC-deposited tapered and side-polished fibers

In this work, mode-locked thulium-doped fiber lasers operating in the 2 µm wavelength region were demonstrated using tantalum aluminum carbide (Ta2AlC)-based saturable absorbers (SAs) utilizing the evanescent wave interaction. The Ta2AlC MAX Phase was prepared by dissolving the Ta2AlC powder in isopropyl alcohol and then deposited onto three different evanescent field-based devices, which were the tapered fiber, side-polished fiber, and arc-shaped fiber. Flame-brushing and wheel-polishing techniques were used to fabricate the tapered and arc-shaped fibers, respectively, while the side-polished fiber was purchased commercially. All three SA devices generated stable mode-locked pulses at center wavelengths of 1937, 1931, and 1929 nm for the tapered, side-polished, and arc-shaped fibers. The frequency of the mode-locked pulses was 10.73 MHz for the tapered fiber, 9.58 MHz for the side-polished fiber, and 10.16 MHz for the arc-shaped fiber. The measured pulse widths were 1.678, 1.734, and 1.817 ps for each of the three SA devices. The long-term stability of the mode-locked lasers was tested for each configuration over a 2-h duration. The lasers also showed little to no fluctuations in the center wavelengths and the peak optical intensities, demonstrating a reliable, ultrafast laser system.

2 µm lasing from Tm3+-doped PbO-PbF2-Bi2O3-Ga2O3 glass microspheres

Whispering gallery mode (WGM) lasers at ∼2µm are demonstrated in PbO-PbF₂-Bi₂O₃-Ga₂O₃ (PBG) heavy metal oxyfluoride glass microspheres. A 793 nm diode laser is used to pump the PBG microsphere and achieve single-mode and multimode WGM lasing. The fluorescence spectra of Tm³⁺-doped PBG glasses are measured under 793 nm diode laser pumping. The maximum absorption and emission cross sections of Tm³⁺:³F₄→³H₆ are calculated to be 8.23×10^-21 and 4.42×10^-20cm², respectively. The experimental results indicate that these PBG glass microspheres could become an important photonic component for infrared laser applications.

Sub-150 fs dispersion-managed soliton generation from an all-fiber Tm-doped laser with BP-SA

We demonstrate an all-fiber, thulium-doped, mode-locked laser using a black phosphorus (BP) saturable absorber (SA). The BP-SA, exhibiting strong nonlinear response, is fabricated by inkjet printing. The oscillator generates self-starting 139 fs dispersion-managed soliton pulses centered at 1859nm with 55.6 nm spectral bandwidth. This is the shortest pulse duration and widest spectral bandwidth achieved directly from an all-fiber thulium-doped fiber laser mode-locked with a nanomaterial saturable absorber to date. Our findings demonstrate the applicability of BP for femtosecond pulse generation at 2 µm spectral region.

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.

Optical Energy Variability Induced by Speckle: The Cases of MERLIN and CHARM-F IPDA Lidar

In the context of the FrenchGerman space lidar mission MERLIN (MEthane Remote LIdar missioN) dedicated to the determination of the atmospheric methane content, an end-to-end mission simulator is being developed. In order to check whether the instrument design meets the performance requirements, simulations have to count all the sources of noise on the measurements like the optical energy variability induced by speckle. Speckle is due to interference as the lidar beam is quasi monochromatic. Speckle contribution to the error budget has to be estimated but also simulated. In this paper, the speckle theory is revisited and applied to MERLIN lidar and also to the DLR (Deutsches Zentrum für Luft und Raumfahrt) demonstrator lidar CHARM-F. Results show: on the signal path, speckle noise depends mainly on the size of the illuminated area on ground; on the solar flux, speckle is fully negligible both because of the pixel size and the optical filter spectral width; on the energy monitoring path a decorrelation mechanism is needed to reduce speckle noise on averaged data. Speckle noises for MERLIN and CHARM-F can be simulated by Gaussian noises with only one random draw by shot separately for energy monitoring and signal paths.

25 W 2 μm broadband polarization-maintaining hybrid Ho- and Tm-doped fiber amplifier

We report the design, evaluation, and performance of a polarization-maintaining (PM) fiber amplifier with a CW output power of >25  W at 2051 nm and a high input signal dynamic range of 34 dB at 25 W. To improve both the output power and dynamic range performance of previous amplifiers, we propose a PM hybrid design with a single-clad Ho-doped preamplifier (HDFA) followed by a double-clad Tm-doped power amplifier (TDFA). The role of the Ho-doped fiber preamplifier is to provide large input signal dynamic range, low noise figure, and moderate output power over an operating bandwidth from ≈2-2.1  μm. The role of the Tm-doped fiber power amplifier is to offer power amplification with good efficiency, taking full advantage of 2-for-1 ion-ion interactions, with the possibility of scaling up the output power to values much higher than 25 W. Our hybrid Ho-Tm-doped design provides a PM fiber amplifier with a combination of large input signal dynamic range, low noise figure, broad operating bandwidth, and high output power. Simulations of the hybrid HDFA/TDFA performance agree relatively well with experimental data.

On acceleration sensitivity of 2 μm whispering gallery mode-based semiconductor self-injection locked laser

While whispering gallery mode resonators are well known for their low acceleration sensitivity, there has not been much published experimental research on the subject. We performed environmental sensitivity tests of a 2 μm semiconductor distributed feedback (DFB) laser, self-injection locked to a high-Q crystalline whispering gallery mode resonator. Measured acceleration sensitivity of the laser is below 5×10^-11  g^-1 in the 1-200 Hz frequency bandwidth and thermal sensitivity does not exceed 12  MHz/°C. The laser's frequency noise is below 50  Hz/Hz^1/2 at 10 Hz, reaching 0.4  Hz/Hz^1/2 at 400 kHz. The instantaneous linewidth of the laser is improved by nearly 4 orders of magnitude compared to the free-running DFB laser and is measured to be 50 Hz at 0.1 ms measurement time. The Allan deviation of the laser frequency is on the order of 10^-9 from 1 to 1000 s. All these features make the laser attractive for metrology applications involving low-noise 2 μm seed lasers.

Laser frequency stabilization and spectroscopy at 2051 nm using a compact CO2-filled Kagome hollow core fiber gas-cell system

We describe a compact, all fiber, frequency stabilized diode laser system at 2051 nm using CO₂ gas-filled Kagome Hollow Core Fiber (HCF), capable of tuning continuously over four transitions in ¹²C¹⁶O₂: R(24), R(26), R(28), and R(30). This laser system has been designed for use in future space-based atmospheric monitoring using differential absorption lidar (DIAL). The fully spliced Kagome HCF gas cell is filled to 2 kPa CO₂ partial pressure and we compare the observed CO₂ lineshape features with those calculated using HITRAN, to quantify the properties of the CO₂-filled fiber cell. In this first demonstration of Kagome HCF used in a fully sealed gas cell configuration for spectroscopy at 2 µm, we characterize the frequency stability of the locked system by beat frequency comparison against a reference laser. Results are presented for the laser locked to the center of the ¹²C¹⁶O₂ R(30) transition, with frequency stability of ∼40 kHz or better at 1 s, and a frequency reproducibility at the 0.4-MHz level over a period of > 1 month. For DIAL applications, we also demonstrate two methods of stabilizing the laser frequency ~3 GHz from this line. Furthermore, no pressure degradation was observed during the ~15-month period in which frequency stability measurements were acquired.

Energy calibration of integrated path differential absorption lidars

The stringent requirements for energy reference measurement represent a challenging task for integrated path differential absorption lidars to measure greenhouse gas columns from satellite or aircraft. The coherence of the lidar transmitter gives rise to speckle effects that have to be considered for accurate monitoring of the energy ratio of outgoing on- and off-line pulses. Detailed investigations have been performed on various measurement concepts potentially suited for deployment within future satellite missions.

Atmospheric boundary layer CO2 remote sensing with a direct detection LIDAR instrument based on a widely tunable optical parametric source

We report on the capability of a direct detection differential absorption lidar (DIAL) for range resolved and integrated path (IPDIAL) remote sensing of CO₂ in the atmospheric boundary layer (ABL). The laser source is an amplified nested cavity optical parametric oscillator (NesCOPO) emitting approximately 8 mJ at the two measurement wavelengths selected near 2050 nm. Direct detection atmospheric measurements are taken from the ground using a 30 Hz frequency switching between emitted wavelengths. Results show that comparable precision measurements are achieved in DIAL and IPDIAL modes (not better than a few ppm) on high SNR targets such as near range ABL aerosol and clouds, respectively. Instrumental limitations are analyzed and degradation due to cloud scattering variability is discussed to explain observed DIAL and IPDIAL limitations.

CHARM-F-a new airborne integrated-path differential-absorption lidar for carbon dioxide and methane observations: measurement performance and quantification of strong point source emissions

The integrated-path differential-absorption lidar CHARM-F (CO₂ and CH₄ Remote Monitoring-Flugzeug) was developed for the simultaneous measurement of the greenhouse gases CO₂ and CH₄ onboard the German research aircraft HALO (High Altitude and Long Range Research Aircraft). The purpose is to derive the weighted, column-averaged dry-air mixing ratios of the two gases with high precision and accuracy between aircraft and ground or cloud tops. This paper presents the first measurements, performed in the spring of 2015, and shows performance analyses as well as the methodology for the quantification of strong point sources applied on example cases. A measurement precision of below 0.5% for 20 km averages was found. However, individual measurements still show deviations of the absolute mixing ratios compared to corresponding data from in situ profiles. The detailed analysis of the methane point source emission rate yields plausible results (26±3  m³/min or 9.2±1.15  kt CH₄ yr^-1), which is in good agreement with reported numbers. In terms of CO₂, a power plant emission could be identified and analyzed.

Microresonator stabilized 2 μm distributed-feedback GaSb-based diode laser

We report on the stabilization of a high-power distributed feedback (DFB) semiconductor laser operating at 2.05 μm wavelength, using a crystalline whispering gallery mode microresonator. The laser's frequency noise is measured to be below 100  Hz/Hz^1/2 at Fourier frequencies ranging from 10 Hz to 1 MHz. The instantaneous linewidth of the laser is improved by four orders of magnitude compared with the free-running DFB laser, and is measured to be 15 Hz at 0.1 ms measurement time. The integral linewidth approaches 100 Hz. The stabilized DFB laser is integrated with a polarization maintaining output fiber and an integrated optical isolator.

Ground-based, integrated path differential absorption LIDAR measurement of CO2, CH4, and H2O near 1.6 μm

A ground-based, integrated path, differential absorption light detection and ranging (IPDA LIDAR) system is described and characterized for a series of nighttime studies of CO₂, CH₄, and H₂O. The transmitter is based on an actively stabilized, continuous-wave, single-frequency external-cavity diode laser (ECDL) operating from 1.60 to 1.65 μm. The fixed frequency output of the ECDL is microwave sideband tuned using an electro-optical phase modulator driven by an arbitrary waveform generator and filtered using a confocal cavity to generate a sequence of 123 frequencies separated by 300 MHz. The scan sequence of single sideband frequencies of 600 ns duration covers a 37 GHz region at a spectral scan rate of 10 kHz (100 μs per scan). Simultaneously, an eye-safe backscatter LIDAR system at 1.064 μm is used to monitor the atmospheric boundary layer. IPDA LIDAR measurements of the CO₂ and CH₄ dry air mixing ratios are presented in comparison with those from a commercial cavity ring-down (CRD) instrument. Differences between the IPDA LIDAR and CRD concentrations in several cases appear to be well correlated with the atmospheric aerosol structure from the backscatter LIDAR measurements. IPDA LIDAR dry air mixing ratios of CO₂ and CH₄ are determined with fit uncertainties of 2.8 μmol/mol (ppm) for CO₂ and 22 nmol/mol (ppb) for CH₄ over 30 s measurement periods. For longer averaging times (up to 1200 s), improvements in these detection limits by up to 3-fold are estimated from Allan variance analyses. Two sources of systematic error are identified and methods to remove them are discussed, including speckle interference from wavelength decorrelation and the seed power dependence of amplified spontaneous emission. Accuracies in the dry air retrievals of CO₂ and CH₄ in a 30 s measurement period are estimated at 4 μmol/mol (1% of ambient levels) and 50 nmol/mol (3%), respectively.

Lidar reflectance from snow at 2.05 μm wavelength as measured by the JPL Airborne Laser Absorption Spectrometer

We report airborne measurements of lidar directional reflectance (backscatter) from land surfaces at a wavelength in the 2.05 μm CO₂ absorption band, with emphasis on snow-covered surfaces in various natural environments. Lidar backscatter measurements using this instrument provide insight into the capabilities of lidar for both airborne and future global-scale CO₂ measurements from low Earth orbit pertinent to the NASA Active Sensing of CO₂ Emissions over Nights, Days, and Seasons mission. Lidar measurement capability is particularly useful when the use of solar scattering spectroscopy is not feasible for high-accuracy atmospheric CO₂ measurements. Consequently, performance in high-latitude and winter season environments is an emphasis. Snow-covered surfaces are known to be dark in the CO₂ band spectral regions. The quantitative backscatter data from these field measurements help to elucidate the range of backscatter values that can be expected in natural environments.

Thulium/holmium-doped fiber laser passively mode locked by black phosphorus nanoplatelets-based saturable absorber

By coupling black phosphorus (BP) nanoplatelets (NPs) with a fiber-taper evanescent light field, a saturable absorber (SA) based on the BP NPs has been successfully fabricated and used in a thulium/holmium-doped fiber laser as the mode locker. The SA had a modulation depth of ∼9.8% measured at 1.93 μm. A stable mode-locking operation at 1898 nm was achieved with a pulse width of 1.58 ps and a fundamental mode-lock repetition rate of 19.2 MHz. By increasing the pump intensity, phenomena of multi-pulsing operations, including harmonic mode-locked states and soliton bunches, were obtained in the experiment, showing that the BP NPs possess an ultrafast optical response time. This work suggests that the BP NPs-based SA is potentially useful for ultrashort, pulsed laser operations in the eye-safe region of 2 μm.

Mode-locked, 1.94-μm, all-fiberized laser using WS₂ based evanescent field interaction

We demonstrate the use of an all-fiberized, mode-locked 1.94 μm laser with a saturable absorption device based on a tungsten disulfide (WS2)-deposited side-polished fiber. The WS2 particles were prepared via liquid phase exfoliation (LPE) without centrifugation. A series of measurements including Raman spectroscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) revealed that the prepared particles had thick nanostructures of more than 5 layers. The prepared saturable absorption device used the evanescent field interaction mechanism between the oscillating beam and WS2 particles and its modulation depth was measured to be ~10.9% at a wavelength of 1925 nm. Incorporating the WS2-based saturable absorption device into a thulium-holmium co-doped fiber ring cavity, stable mode-locked pulses with a temporal width of ~1.3 ps at a repetition rate of 34.8 MHz were readily obtained at a wavelength of 1941 nm. The results of this experiment confirm that WS2 can be used as an effective broadband saturable absorption material that is suitable to passively generate pulses at 2 μm wavelengths.

Observations of Dicke narrowing and speed dependence in air-broadened CO₂ lineshapes near 2.06 μm

Frequency-stabilized cavity ring-down spectroscopy was used to study CO2 lineshapes in the (20013) ← (00001) band centered near 2.06 μm. Two rovibrational transitions were chosen for this study to measure non-Voigt collisional effects for air-broadened lines over the pressure range of 7 kPa-28 kPa. Lineshape analysis for both lines revealed evidence of simultaneous Dicke (collisional) narrowing and speed-dependent effects that would introduce biases exceeding 2% in the retrieved air-broadening parameters if not incorporated in the modeling of CO2 lineshapes. Additionally, correlations between velocity- and phase/state changing collisions greatly reduced the observed Dicke narrowing effect. As a result, it was concluded that the most appropriate line profile for modeling CO2 lineshapes near 2.06 μm was the correlated speed-dependent Nelkin-Ghatak profile, which includes all of the physical effects mentioned above and leads to a consistent set of line shape parameters that are linear with gas pressure.

Differential absorption lidar for volcanic CO(2) sensing tested in an unstable atmosphere

Motivated by the need for an extremely durable and portable instrument to quantify volcanic CO(2) we have produced a corresponding differential absorption lidar (DIAL). It was tested on a volcano (Vulcano, Italy), sensing a non-uniform volcanic CO(2) signal under turbulent atmospheric conditions. The measured CO(2) mixing ratio trend agrees qualitatively well but quantitatively poorly with a reference CO(2) measurement. The disagreement is not in line with the precision of the DIAL determined under conditions that largely exclude atmospheric effects. We show evidence that the disagreement is mainly due to atmospheric turbulence. We conclude that excluding noise associated with atmospheric turbulence, as commonly done in precision analysis of DIAL instruments, may largely underestimate the error of measured CO(2) concentrations in turbulent atmospheric conditions. Implications for volcanic CO(2) sensing with DIAL are outlined.

Error reduction in retrievals of atmospheric species from symmetrically measured lidar sounding absorption spectra

We report new methods for retrieving atmospheric constituents from symmetrically-measured lidar-sounding absorption spectra. The forward model accounts for laser line-center frequency noise and broadened line-shape, and is essentially linearized by linking estimated optical-depths to the mixing ratios. Errors from the spectral distortion and laser frequency drift are substantially reduced by averaging optical-depths at each pair of symmetric wavelength channels. Retrieval errors from measurement noise and model bias are analyzed parametrically and numerically for multiple atmospheric layers, to provide deeper insight. Errors from surface height and reflectance variations are reduced to tolerable levels by "averaging before log" with pulse-by-pulse ranging knowledge incorporated.

A femtosecond pulse fiber laser at 1935 nm using a bulk-structured Bi2Te3 topological insulator

We experimentally demonstrate a femtosecond mode-locked, all-fiberized laser that operates in the 2 μm region and that incorporates a saturable absorber based on a bulk-structured bismuth telluride (Bi(2)Te(3)) topological insulator (TI). Our fiberized saturable absorber was prepared by depositing a mechanically exfoliated, ~30 μm-thick Bi(2)Te(3) TI layer on a side-polished optical fiber platform. The bulk crystalline structure of the prepared Bi(2)Te(3) layer was confirmed by Raman and X-ray photoelectron spectroscopy measurements. The modulation depth of the prepared saturable absorber was measured to be ~20.6%. Using the saturable absorber, it is shown that stable, ultrafast pulses with a temporal width of ~795 fs could readily be generated at a wavelength of 1935 nm from a thulium/holmium co-doped fiber ring cavity. This experimental demonstration confirms that bulk structured, TI-based saturable absorbers can readily be used as an ultra-fast mode-locker for 2 μm lasers.

A resonantly-pumped tunable Q-switched Ho:YAG ceramic laser with diffraction-limit beam quality

We demonstrated a Q-switched Ho:YAG ceramic laser operating at 2097 nm. The Ho:YAG ceramic laser was resonantly pumped by a Tm:YLF laser at 1908 nm. The laser performance with two Ho-doping concentrations of Ho:YAG ceramics in a U-shaped resonator was studied. Different pump spots were investigated to obtain high extract efficiency. The wavelength of Ho:YAG ceramic laser was tuned from 2090.70 nm to 2098.10 nm. The Q-switched pulse energy were 9.6 mJ at a pulse repetition frequency (PRF) of 200 Hz and 10.2 mJ at a PRF of 100 Hz, respectively. The beam quality M(2) factors were measured to be less than 1.1 in both directions.

Mode-locked pulse generation from an all-fiberized, Tm-Ho-codoped fiber laser incorporating a graphene oxide-deposited side-polished fiber

An in-depth experimental investigation was conducted into the use of a graphene oxide-based saturable absorber implemented on a side-polished fiber platform for femtosecond pulse generation in the 2 μm region. First, it was experimentally shown that an all-fiberized thulium-holmium (Tm-Ho)-codoped fiber ring laser with reduced cavity length can produce stable femtosecond pulses by incorporating a graphene oxide-deposited side-polished fiber. Second, the measurement accuracy issue in obtaining a precise pulse-width value by use of an autocorrelator together with a silica fiber-based 2 μm-band amplifier was investigated. It showed that the higher-order soliton compression effect caused by the combination of anomalous dispersion and Kerr nonlinearity can provide incorrect pulse-width information. Third, an experimental investigation into the precise role of the graphene oxide-deposited side-polished fiber was carried out to determine whether its polarization-dependent loss (PDL) can be a substantial contributor to mode-locking through nonlinear polarization rotation. By comparing its performance with that of a gold-deposited side-polished fiber, the PDL contribution to mode-locking was found to be insignificant, and the dominant mode-locking mechanism was shown to be saturable absorption due to mutual interaction between the evanescent field of the oscillated beam and the deposited graphene oxide particles.

Airborne measurements of CO2 column absorption and range using a pulsed direct-detection integrated path differential absorption lidar

We report on airborne CO(2) column absorption measurements made in 2009 with a pulsed direct-detection lidar operating at 1572.33 nm and utilizing the integrated path differential absorption technique. We demonstrated these at different altitudes from an aircraft in July and August in flights over four locations in the central and eastern United States. The results show clear CO(2) line shape and absorption signals, which follow the expected changes with aircraft altitude from 3 to 13 km. The lidar measurement statistics were also calculated for each flight as a function of altitude. The optical depth varied nearly linearly with altitude, consistent with calculations based on atmospheric models. The scatter in the optical depth measurements varied with aircraft altitude as expected, and the median measurement precisions for the column varied from 0.9 to 1.2 ppm. The altitude range with the lowest scatter was 8-10 km, and the majority of measurements for the column within it had precisions between 0.2 and 0.9 ppm.

Actively Q-switched, thulium-holmium-codoped fiber laser incorporating a silicon-based, variable-optical-attenuator-based Q switch

An actively Q-switched thulium-holmium-codoped fiber laser incorporating an Si-based variable optical attenuator (VOA) is experimentally demonstrated. It has been shown that an Si-based VOA with a response time of hundreds of nanoseconds can be used as a cost-effective 2 μm Q switch due to its extremely wide operating bandwidth from 1.5 to 2 μm, and low electrical power consumption. In our study, the laser's slope efficiency was measured to be ~17% at an operating wavelength of 1.89 μm. The repetition rate tuning range was from 20 to 80 kHz, which was limited by the optical damage threshold and the response time. The minimum temporal pulsewidth was measured to be ~184 ns at a modulation frequency of 20 kHz, and the corresponding maximum peak power was ~10 W.

Comparison of IPDA lidar receiver sensitivity for coherent detection and for direct detection using sine-wave and pulsed modulation

We use theoretical models to compare the receiver signal to noise ratio (SNR) vs. average rate of detected signal photons for an integrated path differential absorption (IPDA) lidar using coherent detection with continuous wave (CW) lasers and direct detection with sine-wave and pulse modulations. The results show the coherent IPDA lidar has high receiver gain and narrow bandwidth to overcome the effects of detector circuit noise and background light, but the actual receiver performance can be limited by the coherent mixing efficiency, speckle and other factors. For direct detection, using sine-wave modulation allows the use of a low peak power laser transmitter and synchronous detection. The pulse modulation technique requires higher laser peak powers but is more efficient than sine-wave modulation in terms of average detected signal photon rate required to achieve a given receiver SNR. We also conducted experiments for the direct detection cases and the results agreed well with theory.

Laser absorption spectrometer using frequency chirped intensity modulation at 1.57 μm wavelength for CO2 measurement

We have demonstrated the laser-absorption spectrometer system using frequency chirped intensity modulation at 1.57 μm wavelength for measurement of CO(2) concentration. Using this technique, backscattered laser radiation from different ranges can be discriminated in the frequency domain of the electrical signal. We have reported the discrimination of two signals from the targets with different ranges. It is shown that stable measurements with short time fluctuation corresponding to 4 ppm (rms) were obtained with 32 s measurement intervals. Furthermore, there is qualitative good agreement on, at least, the diurnal changes between the results of the laser absorption spectrometer system and the in-situCO(2) sensor.