Australian Antarctic lidar facility

The Australian Antarctic lidar facility

A.R. Klekociuk1, P.S. Argall2'3, R.J. Morris1, P. Yates1, A. Fleming1, R.A. Vincent2, I.M. Reid2, P.A. Greet1'4, and D.J. Murphy1 1Auroral and Space Physics, Antarctic Division, Kingston, Tasmania 7050. Australia 2Department of Physics and Mathematical Physics, University of Adelaide, Adelaide, South Australia 5005. Australia

3Department of Physics, University of Western Ontario, London, Ontario N6A 3K7. Canada 4lnstitute for Antarctic and Southern Ocean Studies, University of Tasmania, Hobart, Tasmania 7001. Australia ABSTRACT

A high spectral resolution lidar, under development by the Australian Antarctic Division and the University of Adelaide, is described. This instrument will be stationed at Davis, Antarctica (68.6° 5, 78.0° E) from early

1996 for the long-term measurement of atmospheric parameters as a function of altitude from the lower stratosphere to the mesopause. The siting of the lidar will allow for data comparison with existing optical, radar and balloon-borne atmospheric studies. Research utilising the multi-instrument database will be aimed at assessing climatic variability and coupling processes throughout the atmosphere. The lidar transmitter consists of a commercial injection-seeded pulsed Nd:YAG laser coupled to a altazimuth mounted Cassegrain telescope with a 1 metre diameter primary mirror. The laser emits at a wavelength of 532 nm with an average power of 30 W. The telescope also serves as the collecting optics for the receiving system. The lidar is switched between transmit and receive modes by a high speed rotating shutter system.

The detection system consists of a dual scanning Fabry Perot Spectrometer (FPS) followed by a cooled photomultiplier operated in 'photon counting' mode. The received signal is integrated as a function of equivalent range over a bandpass that may be either fixed or scanned in the wavelength domain.

Performance simulations for the fixed bandpass operating mode are discussed. These indicate that useful

measurements of density and inferred temperature should be achievable for the mesopause region, particularly at night and during twilight. In addition, detection of clouds in the mesosphere during the day appears feasible. 1 . INTRODUCTION

The lidar technique provides a means of inferring basic parameters (such as density, temperature and wind velocity) of the bulk atmosphere and its constituent species as a function of altitude1'2. This is commonly achieved by directing a beam of high intensity pulsed laser radiation into the atmosphere and measuring the temporal and spectral characteristics of the radiation backscattered by gases and aerosols.

Advances in high power solid-state laser technology during the past decade, combined with concerns and uncertainties relating to global atmospheric change, have encouraged the development of lidar systems capable of probing the upper atmosphere. Recent modelling of the chemistry and dynamics of the middle atmosphere (10-100 km altitude) suggests that the first and most readily detectable changes in atmospheric temperature associated with increasing concentrations of anthropogenic 'greenhouse' gas concentrations will be manifest as a cooling of the stratosphere and mesosphere3'4. These predictions are supported by recent observations. For example, a trend in the stratosphere of approximately -0.3 K yr1 has been observed in 624 ISPIE Vol. 2266

O-8194-1590-1/941$6.OO

satellite and lidar data5, while temperatures and densities in the mesopause region, as determined from sodium lidar observations, have also exhibited a cooling trend6. It is clear that high precision long term monitoring of middle atmosphere temperatures are required not only to assess these trends, but to more fully understand the complex influence of natural and anthropogenic sources on the climate of this region.

The polar mesopause (near 90 km altitude7) is a region that may be particularly sensitive in the context of global atmospheric change. The frequency, extent and intensity of noctilucent clouds (NLCs) has apparently increased during the past century. These clouds form at altitudes of -'82 km during the summer at high latitudes. Decreasing mesopause temperatures and increased oxidisation of the 'greenhouse' gas methane8'9 have been postulated as catalysts for NLC formation. Lidar and radar techniques are currently being used in the study NLCs and related phenomena and to quantify parameters in the mesopause region. To date, most of this effort has been concentrated in the northern hemisphere. Accompanying 'greenhouse'-induced temperature changes in the middle atmosphere will be the redistribution

of constituent species and alteration of atmospheric densities and scale heights. This will influence the propagation and dissipation of the various naturally occurring wave processes in the region, specifically gravity waves, planetary waves and tides. These waves are only crudely modelled in existing global circulation models. However, they are known to exert a substantial influence on thermal and constituent structure and general circulation throughout the atmosphere. Lidars are currently playing an important role in probing wave processes over a large range of spatial and temporal scales.

The realisation during the 1980's that stratospheric ozone above Antarctica undergoes an alarming annual depletion that is linked to anthropogenic activity has resulted in a vigorous surge of interest in the physics of the atmosphere at high latitudes. Increasingly sophisticated Antarctic-based lidars have been commissioned at McMurdo, South Pole, Dumont D'urville and Syowa10. Australia is currently developing and testing an innovative lidar system that will deployed at its Davis station (68.6°S, 78.O°E) during the 1995/96 austral

summer. This project is a collaboration between the Auroral and Space Physics (ASP) group of the Australian Antarctic Division, and the Department of Physics and Mathematical Physics of the University of Adelaide. The power-aperture product of the lidar will be approximately 24 Wm2, placing it amongst the most sensitive of systems operating in Antarctica.

Davis is an important site in the context of investigating atmospheric dynamics. It is geographically isolated from other sites of atmospheric research. During the spring, the decaying stratospheric polar vortex, a feature which is of significance in the study of ozone depletion, passes overhead. During the day-time, energetic particles from the magnetospheric cusp region precipitate into the upper atmosphere providing a source energy input. In addition, the plasma motion in the thermosphere is strongly influenced by the cross-polar current.

The ASP group, in collaboration with several Australian universities and government organisations, is currently establishing a suite of radar, optical, and balloon-borne research instruments at Davis which will

probe various overlapping regions of the atmosphere, from the ground to the rmt . Research

utilising the multi-instrument database is aimed at assessing climatic variability and coupling processes throughout the atmosphere. The lidar project is an integral aspect of this effort. 2. THE LIDAR SYSTEM

During the early 1980's, the late Dr. Fred Jacka of the former Mawson Institute for Antarctic Research (MIAR) at the University of Adelaide initiated the development of the present lidar system. Dr. Jacka recognised that a high spectral resolution lidar capable of directly measuring winds and temperatures in the middle atmosphere during both day and night would be a powerful tool for studying atmospheric dynamics, particularly in the Antarctic region.

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The desire to obtain day-time measurements necessitated restriction of the bandwidth and field of view of the lidar receiver in order to minimise the detection of background light. The incorporation of a Fabry-Perot Spectrometer (FPS) in the detection system provided a logical means of achieving a narrow bandwidth and at the same time allowing measurement of the Doppler shift and temperature broadening imposed on the spectral profile of the laser line in the backscattering process. In order to restrict the field of view and at the same time provide a significant collecting area, high quality receiving optics with a large aperture are required. For the estimation of wind velocities, Doppler measurements are required at a number of 'off-zenith' positions. To ensure that the atmospheric volume under study is identical to that being illuminated, it was decided to use common transmitting and receiving optics.

The main operating parameters of the lidar are provided in Table 1, while the optical configuration is shown schematically in Fig. 1. Laser pulses are transmitted to the sky via a series of optical elements culminating in a Cassegrain telescope. The altazirnuth mounting of the telescope permits pointing of the laser beam to a maximum zenith angle of 45° over the full range of azimuth angles. The receiver consists of a dual etalon scanning FPS with a cooled photomultiplier detector. Switching between the transmit and receive modes is achieved by a high speed mechanical shutter system.

Telescope f/lO 1025 mm aluminium 2m parabolic 200mm aluminium

Telescope focal ratio

Piypr workidiameter

Primary mirror focal length pi:imary mirror shape Secondaiymirror worldngdiam eter Secondary mirror focal length Secondary mirror shape Field of view Effective collector area

f1=338.33 mm, f2= 1 9 1 6.67 mm hyperbolic 0. 15 mrad

0.74 m2

Laser 532 nm

Operating wavelength Energy per pulse Repetition frequency Pulse length Beam divergence Linewidth

0.6 J 50Hz 5 ns 0.5 mrad

0.9 pm

Detection_System

Bandwidth of interference filter Transmission coefficient of interference filter Etalon airy function finesse Etalon defect function finesse Etalon transmission coefficient

worldndiameter Nominal photomultiplier quantum efficiency Photomültiplier dark count

300 pm (2-pole)

0.4 50 29

03 single, 0.09 dual 5Omm'Homosil' 0.14 30 counts/s

Table 1. Main operating parameters of the lidar system.

2.1 Telescope The optical configuration of the telescope provides a conveniently compact physical arrangement to minimise the moment of inertia when the structure is tilted off vertical and rotated about the zenith. Both the primary

626 ISPIE Vol. 2266

SECONDARY MIRROR

7/,

Axis of Tilt

PRIMARY MIRROR

P1

of Telescope

Mirror Shutter

M2

Blanking

Shutter

High Resolution Etalon Laser

Low Resolution Etaion

Nd : YAG Laser Wavelength 532nm PRF 50Hz Pulse Length Sne

Average Power 30W Beam Divergence O.5mflad

D91

MI

Positions

Duel Scanning Fabry-Perot Spectrometer

SI Filter

LC. PJD-176

Fig. 1. Schematic diagram showing the optical configuration of the lidar. The FPS is

shown with the switching mirror arrangement configured for dual etalon mode.

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and secondary mirrors are manufactured from aluminium alloy. This provided a significant saving in weight (and cost) that simplified the structure and mechanical drive system of the telescope. The surfaces of both mirrors were initially machined to their desired shape. The figure of the primary mirror was mechanically tested, and errors resulting from the machining process were reduced by hand grinding. After initial polishing, the primary mirror was tested for the presence of axisymmetric and asymmetric errors

using an indirect optical test. The standard Foucault and Ronchi tests that are normally used in the manufacture of high quality mirrors were found to be unsuitable in this application owing to the low fnumber of the mirror and to the difficulty in interpreting the resulting shadowgrams in the presence of significant asymmetric errors. The testing method developed involved placing opaque masks on the mirror and determining the approximate volume of the focal region resulting from the reflection (from exposed mirror zones) of a point source placed near the focal point. Two types of mask were used which exposed either a thin annulus of the mirror or two diametrically opposed small circular regions. Several masks of each type having different radial aperture positions were used in the characterisation of the errors across the mirror. The refiguring process proceeded until the shape errors as revealed by the volume of the focal region were regarded as being small enough to permit useful night-time broadband lidar measurements. A focal point test was also developed for the secondary mirror12. This verified that the anticipated accuracy of machining had been met.

A 175 im thick coating of electroless nickel was applied to each mirror after polishing. The coating is extremely resilient to scratching and has a reflectivity of approximately 90% at the laser wavelength. This coating was applied in a bath heated to 90°C. Unfortunately during the coating process the two halves of the

primary mirror debonded. The reassembly process required the mechanical warping of the structure to compensate for the stresses relieved in the heating process. Despite these problems, successful broadband night-time observations been made, demonstrating the basic feasibility of using metal mirrors in a high gain lidar12. In order to maximise the number of photons incident on the telescope that reach the detector, the field of view of the receiver must be at least twice the divergence of the transmitted beam. In the present case, the beam divergence is limited by the quality of the mirror surfaces. Numerical simulations indicate that an adequate day-time performance will be achieved if the slope of the mirror surfaces deviates by at most 2.5x105 rad. The compliance of the mirrors with this tolerance has yet to be demonstrated. Further quantitative testing of

the mirrors is to be undertaken using a purpose-built high precision (5 im rms) measuring machine, and optical techniques (possibly involving interferometry). Additional performance uncertainties centre on the stability of the surface at low temperatures and under the varying gravitational loads imposed by tilting in zenith angle. Active control of the mirror temperature may be required.

2.2 Optical Relay System

The optical configuration of the lidar was designed for use with the 5 10 nm emission from a copper vapour 13 The effective maximum altitude range of the system was limited to about 60 km by the average (CuV) power level of the laser (.-6 W maximum) and its 2 kHz pulse repetition frequency (PRF). In order to extend the range of the instrument to the more scientifically interesting mesopause region, a higher power commercial Nd:YAG laser has been acquired. This laser produces an average power of 30 W at 532 nm (the second harmonic of the principle Nd:YAG emission) with a PRF of 50 Hz. The Nd:YAG laser is frequency stabilised by the 'injection seeding' process, and has a bandwidth of -O.85 pm. Drift in the central pm rms over periods of hours for the expected wavelength of the laser is expected to be less than temperature variation of the operating environment. The stability and narrowness of the laser linewidth in comparison to that of the CuV laser is of particular advantage for high spectral resolution measurements of temperature and wind velocity.

628 ISPIE Vol. 2266

Initially, the new laser will operate with the existing optics in 'low-power' mode (by virtue of a polarising beam attenuator). Some design modifications are necessary to avoid gas breakdown and optical damage when operating at the highest pulse energies. This largely involves the incorporating Galilean collimating optics in order to remove focal points from the system. The change in operating wavelength will not significantly alter the performance of individual optical elements. Three high speed (24,000 rpm) shutters that are electrically synchronised with the firing of the laser form an integral part of the lidar. The 'blanking shutter' blocks the photomultiplier while the laser is firing to protect it from stray photons. The 'laser shutter' prevents any photons that may be emitted by the laser after each pulse from scattering along the optical path to the detection system. The provision of this shutter was essential for the operation of the lidar with the CuV laser, as the cavity of the laser produced a significant amount of

thermal radiation after each pulse. It is likely that the Nd:YAG laser will produce negligible 'off-pulse' emission to allow the removal of this shutter and simplification of the optical train prior to the 'mirror shutter' . The mirror shutter switches the system between transmit and receive modes. It consists of an aluminium disk with a highly reflective upper surface from which 5 identically shaped regions ('cut-outs')

have been symmetrically removed. The rotation of this shutter is synchronised so that each laser pulse passes through one of the cut-outs while the return signal is reflected by the upper surface to the detection system. The cut-outs are profiled so that backscattered light returned from ranges below 5 km passes back towards the laser (and is hence not analysed), while light corresponding to ranges between 5 km and 18 km is attenuated in a progressively decreasing manner in order to protect the photomultiplier from saturation and to

minimise the distortion to its statistical behaviour that is associated with high illumination levels. The mechanical dimensions of the mirror shutter are governed by the laser PRF and the desired maximum altitude range. The construction of a new shutter is required for use with the Nd:YAG laser. The optical elements D2 to D5 and the beam steering prisms P1 and P2 are manufactured from a type of fused silica known as Dynasil. All other elements are BK7ILF7 achromatic doublets. Dynasil was chosen to avoid fluorescence in common elements of the transmitter and receiver illuminated by the CuV laser. Initially, it was

found that more commonly used optical glasses (e.g. BK7) fluoresce with a decay time constant of a few hundred microseconds, which is on the order of the reLurn time from ranges of a few tens of kilometres. In the presence of fluorescence, the detectability of the backscattered signal, particularly for the longest ranges, is significantly degraded. The fluorescence performance of the current optical materials when illuminated by the highest energy pulses from the Nd:YAG laser is still to be determined. All optical elements have anti-reflection coatings. This is of particular importance in the vicinity of the laser, where reflected light has the potential to produce significant heating. It is anticipated that the beam quality of the Nd:YAG laser will be sufficiently uniform so as to avoid damage to the lenses in the near-field.

2.3 Detection System

Light received by the telescope is optically relayed to the detection system which may be operated in one of four modes by virtue of a 'switching mirror' arrangement. In 'total power' detection mode, the received light is passed through a narrow bandpass interference filter and detected by the photomultiplier. Pulses from the photomultiplier are integrated as a function of range; that is, as a function of time after the start of each transmitted laser pulse. This type of operation will be used almost exclusively for high time resolution density measurements at night when the contamination from background ('non-laser') light is sufficiently low to permit detectable returns from altitudes up to the mesopause.

The other three detection modes make use of the FPS for total power or spectral measurements of the received light as a function of range with different spectral resolutions. The basic design of the FPS incorporates many of the innovations developed MIAR during the design and construnction of two other systems for atmospheric research.14'15 For this FPS to successfully operate, very precise mechanical and

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thermal tolerances must be achieved. The FPS is currently undergoing assembly and testing. It is proposed to describe this instrument and its performance in detail in a future publication.

The upper and lower etalons of the FPS provide 'narrow' (high resolution) and 'wide' (low resolution) spectral bandpasses respectively, and both or either of the etalons may be introduced prior to the interference filter by the 'switching mirror' system. Normally, either the low resolution etalon and the dual etalon system will be used. For these configurations, the FWHM instrument response functions are expected to be 25pm and at best ''2 pm, respectively.

By changing the separation of one plate of an etalon relative to the other, the central wavelength of the bandpass is altered. In practice, a spectral scan is achieved by moving the top plate in precise discrete steps using piezoelectric stacks. High time resolution density measurements during the day will make use of the narrow bandpass afforded by either of the low resolution or dual etalon modes for a fixed central wavelength

(that is, for a constant plate separation). Scanning of the dual etalon system across a spectral emission of known characteristics (for example, the cw emission of the injection seeder) during the 'off pulse' phase of the main laser will be required to verify the stability of the instrument as the operating bandpass in this mode will comparable to that of the received signal. During spectral measurements, light from the injection seeder or 'delayed' light from the main laser will be used to accumulate the 'instrument profile' spectrum in tandem with the atmospheric measurements. This is essential for estimations of the true spectral profile imposed during the backscattering process. It is useful to note that a 200 K thermal broadening function at 532 nm has a FWHM of 'O.9 pm, which is very similar to the width of the laser line. In contrast, the doppler shift expected from a 50 ips1 wind is only 0.09 pm.

During the accumulation of a spectrum, the etalon plate separations will be fixed while the signal from a particular laser pulse is received. Prior to the following laser pulse, the separation will be adjusted by one step. Every 256 laser pulses, the plate separation will revert back to minimum value. In this way, a two dimensional array of data will be accumulated, representing the total number of counts as a function of range and wavelength. 3. 'TOTAL POWER' PERFORMANCE SIMULATIONS

Numerical simulation of the performance of the lidar in 'total power' mode have been made in order to determine the accuracy of density and inferred temperature measurements near the mesopause region, and the detectability of NLCs for a range of instrument configurations and observing conditions. The methods used 11,13,16,17 in this analysis are well The response for three fixed instrument bandwidths were considered. These were defined by the narrow band interference filter (300 pm), the filter in combination with the low resolution etalon (25 pm), and the

filter in combination with both etalons (3 pm). The free spectral range of the low resolution etalon was specified so that the transmission of the sidebands was 1 % of that for the main passband. The bandwidth of the dual etalon mode was specified so that the area under the convolution of the normalised instrument and backscatter profiles was 95% of that for the normalised backscatter profile alone. It should be noted that the etalon finesse values quoted in Table 1 are estimated from the anticipated reflection and transmission coefficients of the plates, and have not as yet been measured.

The total number of photons received as a function of range was calculated for each instrument configuration and particular values of the altitude resolution and integration time. The efficiencies of the mirrors and lenses in the instrument were assumed to be 0.90 and 0.97 respectively. The background sky brightness was also

specified as one of five levels corresponding to night glow, solar zenith angles of 970, 92°, and 90°, and daylight. 17 A radiance of 3x109 W m2 nm1 sr1 was assumed for the nightglow.18 The scattering expected for a weak NLC layer was diso incorporated. The cloud layer was assumed to have a centroid height of 82 km, and a thickness of 4 km. Calculations were also made of the standard error in atmospheric temperature 630 / SPIE Vol. 2266

recovered from the backscatter profile neglecting the influence of noctilucent cloud scattering.11'16 The relative error in the assumed starting pressure for the calculation of the temperature errors was set to 15%. An example of the results from a simulation for the low resolution mode with a 20 minute integration time and a 2 km vertical resolution is shown in Fig. 2.

Total Photon Counts 1

10

Temperature Error (K) Nightglow

Solar ZA =97 deg.

Solar ZA =90 deg.

Solar ZA =92 deg.

— - - —. - — Daylight

Fig. 2. Simulated performance of the lidar operating with the low resolution etalon (25 pm

bandwidth), a vertical resolution of 2 km, and an integration time of 20 minutes. The

atmospheric model assumed is the 1976 U.S. Standard Atmosphere. (a) Variation of photon counts with equivalent altitude for five background continuum levels. The contributions from a 4 km thick noctilucent cloud layer centred at 82 and having a backscatter coefficient of 1 .6x107

km sr1 is included. (b) Estimated standard error in the temperature of the bulk atmosphere inferred from the backscatter measurements as a function of equivalent altitude16. Scattering from the noctilucent cloud layer in (a) has been neglected.

Fig. 2 indicates that acceptably low (