Description of the Durban LIDAR systems
3.3 Location of the LIDAR systems
five st oreys high. This location was chosen to avoid stray light s from the sur ro unding city light s and the Durban harbour from int erfering with the return LIDAR signal. All the equipments of the old LIDAR, viz. receiver and transmit ter, are housed in a cont ainer which has a one metre diameter circular lid opening on the receiver side . The transmit ter chambe r is well air-condit ioned. The new LIDAR system is found in a Wendy house which is attached to the container of the old LIDAR. Fig. (3.5) shows a schematic view ofthe set-up ofthe new LIDAR system inside the Wendy house.
Figure 3.5: Schematic view of the interior of the wendy house showing the set-up of the new L1DAR.The vertical green box on the left is the electronic acquisition system; the laser beam can be seen projected vertically into the atmosphere after passing through the Galilean telescope.
3.3.1 The transmitter
The transmitter consists of a pulsed Nd:YAG (Neodymium:Yttri um Alu- minium Garnet)laser. Thefundament alwavelengthAo= 1064nmis frequen cy- doubled using a pot assium dihydrogen phosphate (KDP) crystal to produce the emission wavelength Ae
=
532 nm. The main character ist ics of the laserare summarised in table (3.2).
Model GCR-150 (Spectra Physics)
Repetition rate 10Hz
Energy per pulse (Q-switch mode) 650 mJ for AO
=
1064 nm 300 mJ for Ae=
532 nm Pulse width R::: 6-7 ns for Ae=
532 nmR::: 8-9 ns for AO
=
1064 nm Beam divergence (FWHM) before beam expansion R::: 0.7 mradBeam divergence after beam expansion R::: 0.07 mrad Diameter of beam before expansion R::: 8 mm
Diameter of beam after expansion R::: 80 mm Table 3.2: Characteristics of the laser for the new LIDAR.
The emission wavelength Ae has been selected so that it does not corre- spond to any transition characteristic of any constituent of the atmosphere (absorption or resonance). Two pairs of dichroic mirrors are used at emis- sion to separate the second harmonic from the fundamental wavelength. The dichroic mirrors have coatings which have refractive indices lower for AO but higher for Ae . The mirrors allow 96
%
reflection of Ae • AO is absorbed by a beam trap after the frequency doubling by the KDP crystal.3.3.2 The emitter
The new LIDAR system at Durban operates with two acquisition channels, referred to channel A and channel B.Channel A allows vertical temperature and relative density profiles to be measured in the stratosphere and lower mesosphere (from 10 km to 60 km). Channel B allows measurement in the
troposphere and lower stratosphere (from 10 km to 45 km) of aerosol profiles.
A correction has been done on the temperature and relative density profiles on the altitude overlap of the two channels.
The laser pulses are transmitted vertically into the atmosphere after pass- ing through a system of mirrors and a Galilean telescope.
Let
e
be the divergence of the laser beam (diameter d) before entering the Galilean telescope and ( be the divergence of the beam (diameter D) at the exit of the telescope.e
ande'
are related to D and d by Lagrange invariance:(3.7)
, ( ) D 0.70
Using the values for
e
ande
from table 3.2,d =
0.07=
10.This factor 10 beam-expansion reduces the divergence of the emitted beam by the same amount and also increases the laser-atmosphere surface interaction. Typically, for the new LIDAR ( = 0.070 mrad.
3.3.3 The receivers
Channel A
In the field of view of the mirrors of channel A, the backscattered photons from different heights of the atmosphere are received by two parabolic mir- rors (diameter 4>mirror = 445 mm). The mirrors are held inside two long tubes, as shown in fig.(3.6), which shield them from luminous interference.
Channel A allows backscattered photons to be received from 12 km to 60 km. After collection by the telescope the photons are transmitted by optical
Figure 3.6: Schematic viewof the receivers of channel A and B.
fibres (diamet er rPfibre
=
1 mm) placed at the focal point ofeach mirror, to the dete ction box which contains a collimator, an int erference filter and a photomultiplier tube (PMT). The int erferencefilter cent red on Ae=
532 nm and bandwidth 6.A=
1 nm is placed between the arrival point of the fibre and the PMT.Channel B
As shown in fig.(3.6) , channel B consists ofa smaller mirror (rPmirror = 200 mm) used to receive backscat t ered photons from the lower layers of the at- mosph ere. Mounted bistatically, the mirror can receive signa ls from 10 km to 45 km. As with channel A, the mirror of cha nne l B is held inside a
tube which shields it from luminous interference. Table (3.3) summarises the characteristicsof the receiver systems.
Receivers Channel Specifications
Telescope (x2) A Parabolic mirrors: <Pmirror
=
445 mmFocal length, fA
=
2000 mm Telescope (x1) B Parabolic mirror: <Pmirror=
200 mmFocal length, fB
=
1000 mmOptical fibres (x2) A <Pfibr e
=
1 mmOptical fibre (x1) B <Pfibr e
=
1 mmInterference filter (x1) A Centred on Ae
=
532 nmBandwidth Llb
=
1 nmInterferen ce filter (x1) B Cent red on Ae
=
532 nmBandwidth Llb
=
1 nm Table 3.3: Characterist ics of the receiver system.3.3.4 The Detector and Data acquisition system
Type HamamatsuR 1477
Maximum Voltage 900 V
Gain 107
Quantum efficiency at 532 nm 17
%
Cathode sensitivity 72.9/-LAjW
Rising time 2.2 ns
Transit time 22 ns
Anode dark current typical 2 nA max 5 nA
Table 3.4: Specifications of the photomultiplier of the new LIDAR.
The main characterist ics of the R 1477 PMT are summarised in table (3.4). The PMT (Hamamatsu R 1477) of the new LIDAR syst em is a pho- todet ector of high sensitivity. The R 1477is equipped with a multialkaline
photocathode (Na, K, Sb, Cs) of high sensitivity and has 9 dynodes. The spectral response covers the range from 185 nm to 900 nm with a maximum at 450 nm wavelength. The R 1477 has a high gain and a low dark current.
The new LIDAR system uses a PMT which is contained in a Peltier effect cooling system (model C659-S) which reduces the dark current by lowering the cathode temperature and maintaining it between -15 QC and -20 QC. The Peltier cooling system has a built-in water cooling system.
The signals detected by the PMTs are amplified by pre-amplifiers which are mounted directly above the PMTs. The signal is reconstructed for each channel in terms of the number of photon counts per ue. Each electronic bin of the acquisition system corresponds to an integration time of 1 /-is.
Hence the vertical resolution of the LIDAR is 150 m. The acquisition system stores the data in counts every microsecond for a duration of 1024 /-is (1024 electronic bins). Each bin corresponds to a vertical resolution of 150 m.
The maximum theoretical height attainable by the LIDAR is 153.6 km. The vertical resolution can be degraded by grouping the bins.
An electronic shutter is applied on the PMT of channel A which prevents saturation due to strong returns from the lower layers of the atmosphere.
The electronic shutter operates as follows: a reverse voltage is applied to the second dynode or simultaneously to all the dynodes of the PMT for a time
~t which prevents acceleration of the photoelectrons. A certain amount of time is necessary before the PMT stabilises to its normal working state (100 /-is which correspond to a vertical height of 15 km). Recent improvements in shutter function have reduced this transition period from 100 /-is (15 km) to 10 /-is (1.5 km) by simultaneously applying a weaker reverse voltage on
several dynodes of the PMT. By using an electronic shutter, the LIDAR signal is improved. The saturation of the photomultiplier for LIDARreturns from the lower atmosphere is reduced.
The electronic shutter for channel A was set to ~t = 60us, which corre- sponds to a vertical resolution of 9.0 km. Backscattered photons of the lower atmosphere (0 ::; z ::; 9 km) are not received by the PMT. Figs. (3.7)-(3.8) show this effect where the curves represent two acquisitions, one done with- out electronic shutter (June 2 1999) and the other with electronic shutter set at ~t = 60 f.1S (June 8 1999).