Research on the frequency stabilization of pulsed differential absorbing lidar for CO2 detection based on matching algorithm

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.

We present a new method for accurate mid-infrared frequency measurements and stabilization to a near-infrared ultra-stable frequency reference, transmitted with a long-distance fibre link and continuously monitored against state-of-the-art atomic fountain clocks. We demonstrate the frequency stabilization of a mid-infrared laser with fractional stability better than 4×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-14</sup> at 1 s averaging. This new stabilization scheme gives us the ability to transfer frequency stability in the range of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-15</sup> or even better, currently accessible in the near-infrared or in the visible, to mid-infrared lasers in a wide frequency range.

We present a differential absorption lidar (DIAL) laser transmitter concept designed around a Nested Cavity Optical Parametric Oscillator (NesCOPO) based Master Oscillator Power Amplifier (MOPA). The spectral bands are located around 2051 nm for CO2 probing and 1982 nm for H216O and HD16O water vapor isotopes. This laser is aimed at being integrated into an airborne lidar, intended to demonstrate future spaceborne instrument characteristics: high-energy (several tens of mJ nanosecond pulses) and high optical frequency stability (less than a few hundreds of kHz long term drift). For integration and efficiency purposes, the proposed design is oriented toward the use of state-of-the-art high aperture periodically poled nonlinear materials. This approach is supported by numerical calculations and preliminary experimental validations, showing that it is possible to achieve energies in the 40–50 mJ range, reaching the requirement levels for spaceborne Integrated Path Differential Absorption (IPDA) measurements. We also propose a frequency referencing technique based on beat note measurement of the laser signal with a self-stabilized optical frequency comb, which is expected to enable frequency measurement precisions better than a few 100 kHz over tens of seconds integration time, and will then be used to feed the cavity locking of the NesCOPO.

The water vapor differential absorption lidar (DIAL) of the German Aerospace Research Establishment (DLR) was flown aboard the National Center for Atmospheric Research (NCAR) Electra research aircraft during the Boreal Ecosystem‐Atmosphere Study (BOREAS). The downward looking lidar system measured two‐dimensional fields of aerosol backscatter and water vapor mixing ratio in the convective boundary layer (CBL) and across the CBL top (zt). We show a case study of DIAL observations of vertical profiles of mean water vapor, water vapor variance, skewness, and integral scale in the CBL. In the entrainment zone (EZ) and down to about 0.3 zi the DIAL observations agree with in situ observations and mixed‐layer similarity theory. Below, the water vapor optical depth becomes large and the DIAL signal‐to‐noise ratio degrades. Knowing the water vapor surface flux and the convective velocity scale w* from in situ aircraft measurements, we derive entrainment fluxes by applying the mixed‐layer gradient (MLG) and mixed‐layer variance (MLV) methods to DIAL mixing ratio gradient and variance profiles. Entrainment flux estimates are sensitive to our estimate of zt. They are shown to be rather insensitive to the input surface flux and to the DIAL data spatial resolution within the investigated range. The estimates break down above about 0.9 zt as the flux‐gradient and flux‐variance relationships were developed to describe the large‐scale mixing in the mid‐CBL. The agreement with in situ entrainment flux estimations is within 30% for the MLV method. On a flight leg with significant mesoscale variability the entrainment flux turns out to be 70% higher than the in situ value. This is in good agreement with the fact that large‐eddy simulations (LES) of mean water vapor profiles and variances, upon which the MLG and MLV methods are based, do not include mesoscale variability. The additional water vapor variance from mesoscales may then lead to the overestimate of the flux. Deviations from the in situ observations may also be due to poor LES resolution of small‐scale mixing in the EZ, similarly coarse resolution of the DIAL data, or a capping inversion in the LES model (8 K) which is significantly stronger than the observed inversion (3–4 K).

Single frequency laser sources with high frequency stabilization serve as the backbone for the fields from advanced scientific experiments to industrial or commercial applications, thanks to the rapid development of laser frequency stabilization techniques based on optical or electrical feedback from an external reference. Despite the tremendous progress, it still remains an open question as to how to realize a laser source that can reach the high frequency stabilization and still remain sufficiently compact and portable for field use, with the increasing interest in applying these systems outside of the laboratory. Here, we describe a frequency stabilization DFB fiber laser with intracavity frequency reference mechanism, by utilizing the beat note of dual modes inside the laser cavity as the laser wavelength drift reference signal. As one of the best results for the laser frequency stabilization without the external frequency reference, the outcome of this work presents a way to achieve a stable laser source as an alternative to the laser-locked to absorption line or FP cavity design with a simpler, lower cost feature, offering promising prospects for versatile applications outside of the laboratory.

Frequency stabilization for external cavity diode laser has played an important role in physics research. Many laser frequency locking solutions have been proposed by researchers. Traditionally, the locking process was accomplished by analog system, which has fast feedback control response speed. However, analog system is susceptible to the effects of environment. In order to improve the automation level and reliability of the frequency stabilization system, we take a grating-feedback external cavity diode laser as the laser source and set up a digital frequency stabilization system based on National Instrument’s FPGA (NI FPGA). The system consists of a saturated absorption frequency stabilization of beam path, a differential photoelectric detector, a NI FPGA board and a host computer. Many functions, such as piezoelectric transducer (PZT) sweeping, atomic saturation absorption signal acquisition, signal peak identification, error signal obtaining and laser PZT voltage feedback controlling, are totally completed by LabVIEW FPGA program. Compared with the analog system, the system built by the logic gate circuits, performs stable and reliable. User interface programmed by LabVIEW is friendly. Besides, benefited from the characteristics of reconfiguration, the LabVIEW program is good at transplanting in other NI FPGA boards. Most of all, the system periodically checks the error signal. Once the abnormal error signal is detected, FPGA will restart frequency stabilization process without manual control. Through detecting the fluctuation of error signal of the atomic saturation absorption spectrum line in the frequency locking state, we can infer that the laser frequency stability can reach 1MHz.

The single-step Rydberg excitation of cesium atoms requires a 319 nm ultraviolet laser with a narrow laser linewidth, high frequency stability, and high output power. To meet these requirements, in this work, we construct a high-power, single-frequency UV laser system at this wavelength. In this system, the frequency stabilization of the 1560.492 nm seed laser is critical to the performance of the ultraviolet laser. We employ nonlinear frequency conversion technology, the 1560.492 nm laser is frequency-doubled to 780.246 nm via a single pass through a PPLN crystal, and function integration is realized based on the modular parameter adjustment interface provided by the PyRPL software. Subsequently, the 1560.492 nm laser is stabilized to the D2 hyperfine transition line of Rb-87 atoms using polarization spectroscopy (PS) and radio-frequency-modulated saturation absorption spectroscopy (RF-SAS). A comparative study of these two techniques shows that RF-SAS achieves superior stabilization performance, with the residual frequency fluctuation of the frequency-doubled laser being 1.07 MHz over 30 min. According to frequency doubling theory, the actual residual frequency fluctuation of the 1560.492 nm fundamental-frequency laser can be calculated as 0.535 MHz. Compared with our earlier scheme that utilized an ultra-low-expansion (ULE) optical cavity as a frequency reference, the present scheme eliminates the long-term drift induced by environmental factors. In contrast to frequency stabilization relying on discrete instruments, this integrated scheme significantly reduces the cost, simplifies the system architecture, saves space, and greatly enhances the flexibility and controllability of the system. It therefore provides a reliable and cost-effective solution to ensure the portability and practicability of high-performance UV laser sources. This high-precision frequency stabilization scheme directly guarantees the performance of the 319 nm UV laser, suppressing its linewidth below 10 kHz. Thus, it fully meets the stringent laser linewidth and frequency stability requirements for the single-step Rydberg excitation of cesium atoms and provides a reliable light source foundation for subsequent precision spectroscopic measurements.

The article based on the method of statistical frequency stabilization deals with the issues of increasing frequency stability and synchronization of the forming HF signals in a transmitting device of a localizer using a multichannel variant of construction. It was demonstrated that the available digital unit of frequency and phase correction allows easy application of the proposed method. Two main features of the localizer operation affecting frequency stability and phase synchronization of HF signals are noted. The first factor is determined by deviation of the present -HF signal frequency (on the measurement interval) from the average (i) frequency value in n-channel. The second one is related to average frequency variation of each of the forming HF signals and its deviation within the (i) value from the nominal value during localizer operation. On the basis of HF signals description in channels of the transmitting device of a localizer ratios are obtained determining optimal values in terms of the method of least square method as (i) deviations of the present frequency values from the average value as well as variations of average frequency values during localizer operation. The article considers the most significant, from an applicatory point of view, case of assessment covering only deviations of the present HF signals frequency values from the average value on the measurement interval. It is shown that application of the method of statistical frequency stabilization allows the transmitting device of a localizer including N channels of HF signals formation to increase frequency stability and HF signals phase synchronization times. That enables to improve accuracy of forming integrated and difference directivity diagrams and setting heading in the runway direction as well. Apart from that, on the basis of the received values of frequency parameters estimation and relative instability of the forming HF signals a decision can be made about the condition of the controlled parameter by the criterion STANDARD-DETERIORATION-ACCIDENT.

An electromagnetically induced transparency (EIT) signal is observed in a V-type energy level scheme in a cesium vapor cell at room temperature. The effects of frequency detuning and the intensity of the pump laser on the EIT signal have been investigated. The performance of the probe-diode laser system, which is frequency stabilized on the EIT signal by using electrical feedback, is explored. The first derivative of the EIT signal offers a steeper slope and better S/N ratio for laser frequency stabilization than that of the Doppler-free hyperfine and crossover resonances. A comparative study of the frequency stability of an external-cavity diode laser stabilized at EIT and at the crossover resonance is presented. The square root of the Allan variance (σ) vs. integration time (τ) plot shows about a tenfold improvement in the frequency stability of the EIT-locked laser (σ ∼ 2.043 × 10−13τ−1/2) over that of the crossover-locked laser under a short integration time (1–10 ms), whereas a twofold improvement is found under a long integration time (∼1 s).

Active frequency stabilization method is proposed and demonstrated in this paper. The variation of frequency is used as the feedback signal, and the system is composed of a closed current loop of the semiconductor laser. A DSP is used to realize the automatic frequency finding and frequency stabilization. Stable single mode output with a linewidth of less than 100 kHz was obtained. The frequency shift is within 15 MHz in 2 hours.

The measurement sensitivity of CO2 differential absorption LIDAR (DIAL) can be affected by a number of different processes. Two of these processes are atmospheric optical turbulence and reflective speckle. Atmospheric optical turbulence affects the beam distribution of energy and phase on target. The effects of this phenomenon include beam spreading, beam wander and scintillation which can result in increased shot-to-shot signal noise. In addition, reflective speckle alone has been shown to have a major impact on the sensitivity of CO2 DIAL. We have previously developed a Huygens-Fresnel wave optics propagation code to separately simulate the effects of these two processes. However, in real DIAL systems it is a combination of these phenomena, the interaction of atmospheric optical turbulence and reflective speckle, that influences the results. In this work, we briefly review a description of our model including the limitations along with a brief summary of previous simulations of individual effects. The performance of our modified code with respect to experimental measurements affected by atmospheric optical turbulence and reflective speckle is examined. The results of computer simulations are directly compared with lidar measurements and show good agreement. In addition, simulation studies have been performed to demonstrate the utility and limitations of our model. Examples presented include assessing the effects for different array sizes on model limitations and effects of varying propagation step sizes on intensity enhancements and intensity probability distributions in the receiver plane.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A method of measuring laser frequency stability is proposed by using the spectral-hole-burning technique. The power spectra of the measured laser can be recorded as a spectral hole, and the engraving time of the spectral hole is mapped into the frequency of the measured laser. Frequency fluctuation can be expressed by spectral hole frequency variation with different engraving times. By using the proposed method, the frequency stability of the external-cavity diode laser is measured to be 2.22 × 10−9 with an integration time of 20 ms. The frequency stability measurement resolution reaches 14 kHz and the repetition rate is 50 Hz. Compared to the conventional method, it avoids the need for a high stability reference laser source.

This study explores a new concept of Martian differential absorption Lidar operating at the 2‐μm CO2 absorption band for atmospheric CO2 and pressure observations. For the considered system, two or more closely spaced wavelengths are selected, so that environmental factors such as surface reflection, atmospheric scattering, and absorptions from other trace gases on the lidar return signals are very similar for all selected wavelengths, but the difference in CO2 absorption is substantial. Thus, Martian CO2 amount and air pressure could be retrieved from the measured CO2 differential absorption optical depth at the selected wavelengths. Simulations based on Mars's environmental conditions and technically feasible lidar systems show that lidar returns from the surface could have sufficient signal strengths that allow surface air pressure measurements with 1 Pa precision after a horizontally 5‐km averaging when dust optical depth is smaller than 1.5. Even when the dust optical depth increases to 3, the uncertainty in the air pressure and column CO2 amount measurements with a random error is still smaller than 1%. In the presence of moderate dust aerosol loads and with the optimal selection of offline and online wavelengths, atmospheric CO2 and pressure profiles could also be retrieved from the surface up to an ∼13‐km altitude with the random error smaller than 1% for a horizontal resolution of 100 km and a vertical resolution of 100 m during night or 400 m during day. These CO2 and pressure measurements would significantly improve weather and climate modeling and prediction.

&amp;lt;p&amp;gt;The Atmospheric Thermodynamic LidAr in Space (ATLAS) is a mission concept proposed to the European Space Agency in the frame of &amp;amp;#8220;Earth Explorer-11 Mission Ideas&amp;amp;#8221; Call by a team of researchers, with the aim to develop the first Raman Lidar in space capable to measure simultaneously atmospheric temperature and water vapour mixing ratio profiles with high temporal and spatial resolutions. Accurate measurements of these profiles are essential to understand water and energy cycles, as well as the prediction of extreme events, that nowadays still show huge deficiencies on all temporal and spatial scales (1). Such measurements would have a revolutionary impact on our understanding of the Earth system and would close the gap in our observational capabilities from the surface to the lower troposphere.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;The specifications of the different lidar sub-system, as well as the expected capability to provide measurements with high temporal and spatial resolution in the low and middle troposphere, have already been established with an analytical simulation model (2,3). These simulations considered different atmospheric models and conditions to estimate the statistical uncertainty on water vapour and temperature measurements. New studies have been now performed to estimate the performances along several dawn-dusk orbits. An end-to-end simulator has been developed and used to estimate the statistical and systematic uncertainties. The input data, comprehensive of thermodynamic and optical parameters, have been extracted from the GEOS-5 Nature Run and have been chosen to perform simulations with different solar zenith angles and therefore different background contributions. The model includes information on cloud fraction and optical thickness, so it was also possible to consider the performances in cloudy conditions. The simulations show promising results, both in clear and cloudy conditions and with different background contributions. A comprehensive study of the assessed performances will be presented at the conference.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;The simulated measurements obtained from the simulator will be also used as input observations in the Weather Research and Forecasting model (WRF). The aim is to estimate the impact of global measurements from a space-borne Raman Lidar in terms of skill-scores, obtained by the comparison of the weather forecast output with and without the assimilation of the simulated lidar data.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;1 - Wulfmeyer, Hardesty, Turner, Behrendt, Cadeddu, Di Girolamo, et al. A review of the remote sensing of lower tropospheric thermodynamic profiles and its indispensable role for the understanding and the simulation of water and energy cycles. Reviews of Geophysics. 2015; 53(3):819&amp;amp;#8211;95.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;2 - Di Girolamo, Behrendt, Wulfmeyer. Space-borne profiling of atmospheric thermodynamic variables with Raman lidar: performance simulations. Opt Express,OE. 2 aprile 2018; 26(7):8125&amp;amp;#8211;61.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;3 - Di Girolamo, Behrendt, Wulfmeyer. Spaceborne profiling of atmospheric temperature and particle extinction with pure rotational Raman lidar and of relative humidity in combination with differential absorption lidar: performance simulations. Appl Opt, AO. 10 aprile 2006; 45(11):2474&amp;amp;#8211;94.&amp;lt;/p&amp;gt;

Abstract. To validate products of the Greenhouse gases Observing SATellite (GOSAT), we observed vertical profiles of aerosols, thin cirrus clouds, and tropospheric ozone with a mobile-lidar system that consisted of a two-wavelength (532 and 1064 nm) polarization lidar and a tropospheric ozone differential absorption lidar (DIAL). We used these lidars to make continuous measurements over Saga (33.24° N, 130.29° E) during 20–31 March 2015. High ozone and high aerosol concentrations were observed almost simultaneously in the altitude range 0.5–1.5 km from 03:00 to 20:00 Japan Standard Time (JST) on 22 March 2015. The maximum ozone volume mixing ratio was ∼ 110 ppbv. The maxima of the aerosol extinction coefficient and optical depth at 532 nm were 1.2 km−1 and 2.1, respectively. Backward trajectory analysis and the simulations by the Model of Aerosol Species IN the Global AtmospheRe (MASINGAR) mk-2 and the Meteorological Research Institute Chemistry-Climate Model, version 2 (MRI-CCM2), indicated that mineral dust particles from the Gobi Desert and an air mass with high ozone and aerosol (mainly sulfate) concentrations that originated from the North China Plain could have been transported over the measurement site within about 2 days. These high ozone and aerosol concentrations impacted surface air quality substantially in the afternoon of 22 March 2015. After some modifications of its physical and chemical parameters, MRI-CCM2 approximately reproduced the high ozone volume mixing ratio. MASINGAR mk-2 successfully predicted high aerosol concentrations, but the predicted peak aerosol optical thickness was about one-third of the observed value.