Lidar Transmitter Research Articles

A proposed all-solid-state transportable narrow-band sodium lidar for mesopause region temperature and horizontal wind measurements

An all-solid-state narrowband sodium lidar transmitter based on proven technologies is proposed. These proven technologies include: (i) the sum frequency generation of 589 nm coherent radiation with pulsed solid-state Nd:YAG lasers at 1064 and 1319 nm, developed for laser-guided star research and used in lidar measurements of mesopause-region temperatures by Shinshu University, (ii) the Doppler-free spectroscopic seed control developed by the Colorado State University and employed in a lidar system with hybrid solid-state and dye technologies for mesopause-region temperature and horizontal wind measurements, and (iii) the low-power continuous wave, tunable light around 589 nm via sum frequency generation developed by the University of Nevada. The proper combination of these mature technologies, deployed along with sodium-vapor Faraday filters, also developed by Colorado State University, will lead to a new all-solid-state sodium fluorescence lidar, capable of measuring mesopause-region temperatures and horizontal winds on a 24 h continuous basis, weather permitting. The integration of these proven solid-state technologies will make this proposed lidar transportable and suitable for remote operation in harsh environments. PACS Nos.: 42.68.Wt, 42.79.Qx, 92.60.–e, 07.20.Dt, 42.79.Qx

Polarization-discrimination technique to maximize the lidar signal-to-noise ratio for daylight operations

The impact and potential of a polarization-selection technique to reduce the sky background signal for linearly polarized monostatic elastic backscatter lidar measurements are examined. Taking advantage of naturally occurring polarization properties in scattered skylight, we devised a polarization-discrimination technique in which both the lidar transmitter and the receiver track and minimize detected sky background noise while maintaining maximum lidar signal throughput. Lidar elastic backscatter measurements, carried out continuously during daylight hours at 532 nm, show as much as a factor of square root 10 improvement in the signal-to-noise ratio (SNR) over conventional unpolarized schemes. For vertically pointing lidars, the largest improvements are limited to the early morning and late afternoon hours, while for lidars scanning azimuthally and in elevation at angles other than vertical, significant improvements are achievable over more extended time periods with the specific times and improvement factors depending on the specific angle between the lidar and the solar axes. The resulting diurnal variations in SNR improvement sometimes show an asymmetry with the solar angle that analysis indicates can be attributed to changes in observed relative humidity that modifies the underlying aerosol microphysics and observed optical depth.

Development of a SNR parameterization scheme for general lidar assessment

We present a general methodology for evaluating the capabilities of a general lidar system encompassing both backscatter (elastic and Raman lidar) and topographic targets. By introducing a well-defined atmospheric reference medium and by individually examining and decomposing the contributions of lidar system parameters including lidar transmitter power, field of view, receiver noise, atmospheric conditions, and sky background on the signal-to-noise ratio, we obtain a simple dimensionless parameterization of the lidar system. Using this parameterization, numerical simulations are carried out to determine achievable lidar performance including operation range, minimum detectable gas concentration, and so on.

Suppression of first order stimulated Raman scattering in erbium-doped fiber laser based LIDAR transmitters through induced bending loss

A model is presented for suppression of first-order stimulated Raman scattering (SRS) in pulsed Er-doped fiber amplifiers with wavelength-selective induced bending loss. The model estimates the effectiveness of SRS suppression through fiber coiling in a pulsed amplifier, considering both the single- and dual-clad fiber cases. The basic approach is to fit analytical expressions to the evolution of a quasi-CW signal in the amplifier. Then, assuming that SRS does not deplete the signal, exact analytical expressions for the propagation of the Stokes wave are determined, from which the SRS threshold is estimated. Finally, example calculations are presented, and the model is used to determine a fiber geometry for the suppression of SRS through fiber coiling.

Multi-component chemical analysis of gas mixtures using a continuously tuneable lidar system

Differential absorption lidar (DIAL) measurements are usually made on single compounds by alternately switching the wavelength between on and off a resonance line. The selection of more than two wavelengths is a mathematical necessity for simultaneous measurement of multiple species or for resolving interference effects between a compound of interest and a background gas such as water vapour or carbon dioxide. This is especially true in the mid-IR region, where many hydrocarbon compounds have important spectral features. We present a method for remote measurement of gas mixtures in the mid-IR region based on a newly developed fast-switching, frequency-agile optical parametric oscillator lidar transmitter. A multivariate statistical procedure has also been applied for this system, which combines a genetic algorithm for wavelength selection with a partial least squares method for identifying individual compounds from their combined absorption spectrum. A calibration transfer is performed for compounds of interest using reference spectra from an absorption spectra database. Both indoor absorption cell measurements and outdoor remote range resolved measurements of hydrocarbon mixtures were performed to explore the performance of the method.

Pseudo-random noise-continuous-wave laser radar for surface and cloud measurements

Laser radar (lidar) application may require an instrument with compact size, long life of the components, low consumption and eye-safety. One possibility to achieve these features is to use a continuous-wave (cw) diode laser as lidar transmitter. A practical way to perform range-resolved measurements with a cw laser diode is the pseudo-random noise (PRN) modulation. This paper presents a compact PRN-cw lidar, using a 370-mW cw diode laser and an APD as detector. Daytime measurements of cloud base and topographic surface are demonstrated with the PRN-cw lidar technique, where the range detection exceeds 2km. The detection of the topographic surface is performed with integration time of some tens of milliseconds during daytime and some tens of microseconds during night-time.

Development and testing of a frequency-agile optical parametric oscillator system for differential absorption lidar

An all-solid-state fast-tuning lidar transmitter for range- and temporally resolved atmospheric gas concentration measurements has been developed and thoroughly tested. The instrument is based on a commercial optical parametric oscillator (OPO) laser system, which has been redesigned with piezoelectric transducers mounted on the wavelength-tuning mirror and on the crystal angle tuning element in the OPO. Piezoelectric transducers similarly control a frequency-mixing stage and doubling stage, which have been incorporated to extend system capabilities to the mid-IR and UV regions. The construction allows the system to be tuned to any wavelength, in any order, in the range of the piezoelectric transducers on a shot-to-shot basis. This extends the measurement capabilities far beyond the two-wavelength differential absorption lidar method and enables simultaneous measurements of several gases. The system performance in terms of wavelength, linewidth, and power stability is monitored in real time by an étalon-based wave meter and gas cells. The tests showed that the system was able to produce radiation in the 220–4300-nm-wavelength region, with an average linewidth better than 0.2 cm−1 and a shot-to-shot tunability up to 160 cm−1 within 20 ms. The utility of real-time linewidth and wavelength measurements is demonstrated by the ability to identify occasional poor quality laser shots and disregard these measurements. Also, absorption cell measurements of methane and mercury demonstrate the performance in obtaining stable wavelength and linewidth during rapid scans in the mid-IR and UV regions.

Compact high-pulse-energy ultraviolet laser source for ozone lidar measurements.

An all solid-state Ti:sapphire laser differential absorption lidar transmitter was developed. This all-solid-state laser provides a compact, robust, and highly reliable laser transmitter for potential application in differential absorption lidar measurements of atmospheric ozone. Two compact, high-energy-pulsed, and injection-seeded Ti:sapphire lasers operating at a pulse repetition frequency of 30 Hz and wavelengths of 867 and 900 nm, with M2 of 1.3, have been experimentally demonstrated and their properties compared with model results. The output pulse energy was 115 mJ at 867 nm and 105 mJ at 900 nm, with a slope efficiency of 40% and 32%, respectively. At these energies, the beam quality was good enough so that we were able to achieve 30 mJ of ultraviolet laser output at 289 and 300 nm after frequency tripling with two lithium triborate nonlinear crystals.

Compact solid-state dye polymer laser for ozone lidar applications

A solid-state dye laser for potential application as an ozone lidar transmitter is described. The solid-state dye laser material is made from a polymer host polymethyl methacrylate (PMMA) injected with a pyrromethene laser dye PM-597. The laser oscillator cavity is end-pumped by a Q-switched Nd:YAG laser at 532 nm. Broadband and narrowband laser oscillator cavities were constructed and tested. For the broadband oscillator, slope efficiency as high as 79% was achieved. A narrowband laser oscillator was constructed to generate radiation at 578 and 600 nm with 390-pm linewidth. This output was frequency doubled to 289 or 300 nm using a beta-barium borate (BBO) crystal. The oscillator cavity generated a maximum energy of 11 mJ at 578 nm when pumped by 100 mJ, 532 nm, at 10-Hz repetition rate. Maximum output energy of 380 μJ was achieved at a wavelength of 289 nm.

Tunable ultraviolet optical parametric oscillator for differential absorption lidar measurements of tropospheric ozone

A pulsed optical parametric oscillator (OPO) with intracavity sum frequency mixing was developed generating energies of up to 16 mJ in the 281–293 nm wavelength range. Both OPO process and sum frequency mixing are pumped by the harmonics of a single, medium-sized Nd:YAG laser. The system is characterized by a high overall efficiency (∼4% conversion from 1064 nm to the UV), a very compact set-up and stable and reliable operation. This system was successfully employed to measure tropospheric ozone using the differential absorption lidar (DIAL) technique and shows much promise as a lidar transmitter in airborne case studies as well as in unattended lidar systems for long-term monitoring. An unattended ozone profiling system could already be successfully realized.

Optical circulator for an in-line-type compact lidar.

The application of an optical circulator is demonstrated for an in-line-type lidar. The lidar's transmitter and receiver are installed in a telescope. The optical circulator of interest here can separate the transmitting laser beam and the echo lights on the same optical axis. It can also divide the echo lights simultaneously into orthogonally polarized components. An insertion loss of 2.2 dB and isolation of >60 dB for the developed optical circulator are obtained in a laser-transmitting situation. This optical circulator makes it possible to measure the polarization ratio caused by cloud phases with a narrow field of view in an in-line-type lidar operation.

2-µm Doppler lidar transmitter with high frequency stability and low chirp

A coherent Doppler lidar system was frequency stabilized in a master-slave configuration by a phase-modulation technique. The short-term frequency stability, ~0.2 MHz rms, was maintained in a vibrational environment on a ship during a field campaign in the tropical Pacific Ocean. The long-term frequency stability was <2.6 kHz/h. Thus, in many applications, shot-to-shot frequency correction can be disregarded, which will result in increased speed and simplicity of the data-acquisition system. A frequency chirp could not be detected. These properties permit Doppler wind measurements with high efficiency and duty cycles to be made, even on airborne and spaceborne platforms.

ABLE: Development of an Airborne Lidar

Abstract The acronym ABLE (Airborne Lidar Experiment) identifies a project to develop and fly an optical radar on a stratospheric platform for studies related to atmospheric radiation and composition. The prototype, ABLE 1, has been successfully flown on board the M55 Geophysica aircraft in the Arctic campaign of December 1996–January 1997 to observe stratospheric clouds and aerosol. The lidar, which runs automatically, has been installed in the unpressurized bay of the aircraft where the temperature approaches the low values of external air. The lidar transmitter is based on a Nd:YAG laser, with second and third harmonic outputs. The receiver consists of a 0.3-m Cassegrain telescope and several detection channels to look at different wavelengths and polarizations. A fluid circulation unit connected to the aircraft provides heating control. The instrument can point to the zenith or to the nadir. In the past campaign only λ = 532 nm was utilized: observations were carried out at two polarizations, pointing...

Using the spectral asymmetry of TEA CO_2 laser pulses to determine the Doppler-shift sign in coherent lidars with low frequency stability

We develop a method for determinating the relative positions of the lidar transmitter (LT) and the local oscillator (LO) frequencies in Doppler CO2 lidars. It uses the weak spectral asymmetry of TEA CO2 laser pulses, defined by a number of secondary peaks at the high-frequency side of the main spectrum peak. Depending on the sign of the beat frequency, these peaks may appear in the demodulated spectrum at either the high- or the low-frequency side. Each laser pulse spectrum is compared with reference spectra with two types of asymmetry, with the cross-correlation coefficients used as criteria. The performance of the method at different values of signal-to-noise ratio is analyzed numerically. The method is also applied to raw data from the lidar reference channel and demonstrates good performance at noise levels lower than the secondary peaks in the pulse spectrum or at a signal-to-noise ratio of > or = 20 dB. Application of the pulse spectrum asymmetry for lidar frequency stabilization is analyzed. Lidar operation without frequency stabilization is considered as well. The method offers a simple Doppler lidar hardware for the creation of low-cost coherent lidars, velocimeters-rangefinders, etc.

Frequency-agile kilohertz repetition-rate optical parametric oscillator based on periodically poled lithium niobate

We report kilohertz repetition-rate pulse-to-pulse wavelength tuning from 3.22 to 3.7 mum in a periodically poled lithium niobate (PPLN) optical parametric oscillator (OPO). Rapid tuning over 400 cm(-1) with random wavelength accessibility is achieved by rotation of the pump beam angle by no more than 24 mrad in the PPLN crystal by use of an acousto-optic beam deflector. Over the entire tuning range, a near-transform-limited OPO bandwidth can be obtained by means of injection seeding with a single-frequency 1.5-mum laser diode. The frequency agility, high repetition rate, and narrow bandwidth of this mid-IR PPLN OPO make it well suited as a lidar transmitter source.

Injection-seeded optical parametric oscillator for airborne water vapour DIAL

Any detailed understanding of the role of water vapour in the global climate system requires precise measurements in the atmosphere with high temporal and spatial resolution. A technique that is appropriate to deliver these data is airborne differential absorption lidar (DIAL) applied in the near-infrared spectral range. In order to accurately determine atmospheric water vapour concentrations by means of the DIAL technique tunability and spectral properties of the lidar transmitter are very important. It is demonstrated that the spectral and energy requirements are achieved using optical parametric oscillators (OPOs) that are line-narrowed by the technique of injection seeding with radiation from dye lasers or from laser diodes. When the OPO is unseeded its spectral width is broad enough to suffer only negligible absorption from the absorption lines. Therefore, the unseeded OPO radiation can be used for off-line measurement, which simplifies the DIAL set-up significantly. In addition, the laser diodes were employed to measure line parameters of a selected absorption line in the near-IR. Their precise determination is important in order to make accurate DIAL measurements.

125-mJ diode-pumped injection-seeded Ho:Tm:YLF laser

We describe a diode-pumped, room-temperature Ho:Tm:YLF power oscillator with an optical-to-optical efficiency of 0.03. A Q -switched output energy of as much as 125 mJ at 6 Hz with a pulse width of 170 ns was obtained. Single-frequency, nearly transform-limited operation of the laser was achieved by injection seeding. Laser performance as a function of laser rod temperature and pump intensity was also investigated. The high power and high beam quality of this laser make it well suited for use as a coherent wind lidar transmitter on a space platform.

Transverse-mode selection in apertured super-Gaussian resonators: an experimental and numerical investigation for a pulsed CO_2 Doppler lidar transmitter

A representative prototype of a high-energy, long-pulse, and narrow-bandwidth pulsed CO(2) laser suitable for a spaceborne Doppler wind lidar application has been developed. We obtained 10 J of output energy at greater than 8% efficiency in long, narrow-bandwidth, single-longitudinal, and transverse-mode pulses. We used a positive branch unstable resonator with a fourth-order super-Gaussian mirror as the output coupler. Experiments were carried out to assess the effect of intracavity hard apertures of different diameters that induce diffractive perturbation of the theoretical field and reduce the transverse-mode selectivity of the cavity. An upper limit to the choice of the mirror soft radius has been found, which allows optimization of the trade-off between laser efficiency and beam quality. We determined experimentally that a value of 0.75-0.8 for the ratio between the exp(-1) diameter of the beam intensity and the laser clear aperture gave a single-transverse-mode operation without significant loss of efficiency.

Optimization of coherent lidar performance with graded-reflectance transmitter resonator optics in the low equivalent Fresnel number regime

Using a diffractive eigenmode treatment to model the laser output we show that graded-reflectance resonator optics offer significant efficiency benefits over conventional hard-edge coupled unstable resonators in the context of coherent detection lidar applications. Extending previous research pertinent to the high equivalent Fresnel number regime, we have modeled the optimum performance of a notional super-Gaussian coupled cavity as a function of the key resonator parameters in the low equivalent Fresnel number (<3) regime. The findings from this study are applicable to the design of coherent lidar transmitters operated within this regime.

Using fibre gratings to stabilise laser diode wavelengthunder modulation for atmospheric lidar transmitters

The authors report on the optical spectrum of a high power (150 mW) laser diode under large signal intensity modulation with feedback from an external cavity fibre grating. Single longitudinal facet mode operation was maintained for 6 kHz 1% and 3% duty cycle pulsed and 1 Mbit/s psuedonoise code modulation with extinction ratios as small as 0.01.