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Testing unit for laser rangefinder
Eduard V. Kuvaldin*, Alexandr G. Ershov**, Vitaly F. Zakharenkov, Vadim M. Polyakov,
Ludmila N. Arhipova
S.I. Vavilov State Optical Institute, The Russian Federation State Scientific and Manufacturing
Corporation, 12, Birgevaya line, Sankt-Peterburg, Russia, 199034
ABSTRACT
Four beams laser rangefinder with possibility to determinate an angle between the main axis of the space vehicle and the
normal to the plate ground was designed. The testing unit for fixing optical axis and measuring main characteristics of
receiving module is described. The unit allows to measure the focal plane position of main lens, detector chanel
detectivity, level of nonaxis sun scattering light, transmittance of optical system and dynamic range of receiving channel.
Keywords: optical measurement, detectivity, calibration, alignment, adjusting
INTRODUCTION
Four beams laser rangefinder with λ=1.064 μm, pulse energy 36 mJ and pulse duration of 10 ns consists of laser
transmitter module (LTM) and receiving module (RM) with possibility to determinate an angle between the main axis of
the space vehicle Fobos-Grunt [1,2,3] and the normal to the plate ground of Phobos was designed. The non-mechanical
beam direction switching is realized in the LTM. The switching is performed by an optical commuter based on liquid
crystal cells [4]. The twist-neumatic liquid crystal cells in combination with a system of polarizers allows to switch laser
pulses between four channels with switching rate 1/250 s. The laser is based on master oscillator with power amplifier
scheme. RM consists of main lens, fiber-optic coupler (FOC), projection lens with long pass and narrow bandwidth
filters and electronic module with a sensitive area of 3 mm diameter avalanche photodiode (APD). Rangefinder has three
kinds of technical parameters: 1. Range of performance. 2. Tolerance of measuring of range (out of this paper). 3.
Tolerance of adjusting four laser beams to main axes of space vehicle. It is a real problem to calibrate the rangefinder in
a reference conditions because of a large scattering laser radiation in an atmosphere at a long range. Therefore the
proving of range of performance was based on a measurement procedure of threshold value of RM detectivity.
Uncertainty of adjusting four laser beams to main axes of space vehicle consist of uncertainty of adjusting an optical axe
of RM to main axes of space vehicle, uncertainty of adjusting central fiber of FOC RM to the optical axe and uncertainty
of adjusting laser beams of LTM to all of four fibers of FOC RM. The uncertainty of adjusting an optical axe of RM to
main axes of space vehicle was determinate as ±45 arcsec and will be described in another paper. The testing unit for
alignment and adjusting RM and LTM with uncertainty of ± 41 arcsec and with ability to measure threshold value of
RM detectivity was designed. The unit permits to measure the focal plane position of RM main lens with uncertainty of
±10 μm, RM detectivity with uncertainty of ±0.5·10-16 J, level of nonaxis background scattering light, transmittance of
optical system and dynamic range of RM. The unit contains a collimator lens 4 called Apomars-7 (Fig.1) with the focal
length about 1800 mm and aperture 400 mm, light emitting diode LED 1 with wavelenght of radiation about 1060 nm,
Edmund Optics® Laser Line™ filter 2, glass absorber 3, and RM main lens 5 with field diaphragm 6, etalon photodiode 7,
LTM 8.
*ekuvaldin@yandex.ru, phone 7 812 323-8030; fax: 7 812 234-9419
**ers@npkgoi.ru , phone 7 812 323-8030
Sixth International Symposium on Precision Engineering Measurements and Instrumentation, edited by Jiubin Tan,
Xianfang Wen, Proc. of SPIE Vol. 7544, 754457 · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.885857
Proc. of SPIE Vol. 7544 754457-1
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Figure 1. Layout of the testing unit.
The testing unit is mounted on an optical table about 6 m long (Fig.2) with a passive vibration isolated mount. Optical
rails are mounted on the tables: one is for the carrier of LED and another is for six degrees of freedom high load capacity
rangefinder positioner.
Figure 2. Recent stage of the testing unit.
1. METHODOLOGY, DATA AND ANALYSIS
The operation of adjusting four laser beams to main axes of space vehicle can be divided in three steps. The first step is
to adjust the optical axe of objective RM to main axes of space vehicle with the uncertainty of Δ1= ±46 arcsec. The
second is to adjust the central fiber of FOC RM to the optical axe. The third is to adjust four laser beams of LTM to four
fibers of FOC RM. The second and third are described below. First of all the optical axes of collimator and the objective
RM ought to be centered. It was done by an autocollimator with the function of refocusing by centering the
autocollimating points of all optical surfaces of both objectives. A red LED was placed at the place of 7 (Fig.1) and in
place of 1 was an intermediate image of autocollimator. So it can observe an image of central fiber and all of
autocollimating points. The uncertainty of this method (Δ2) can be defined as
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2
25,0
2
⋅⋅⋅
±=Δ VSf (2)
The scaling factor of autocollimator Sf=0.00015 rad, the maximal magnification of two autocollimating points
V=20.5x0.5=0.7 get Δ2=±8 arcsec. Others points may be neglected. The main layout of the third step can be got by
locating in place 1 (Fig.3) 1600x1200 Silicon CCD Camera Gras 20 and replacing LED with Edmund Optics® Laser
Line™ filter 5 after FOC 4 for back illumination. The radiation of main laser 6 must be decresed drastically over 108
times with a system of filters 7.
Figure 3. Layout of the adjusting procedure (2 and 3 are as 4 and 5 at Fig.1).
The image of one of fibers and the spot of one beams of LTM together can be observed at 1.064 μm wavelength. The
third step can be divided into three stages: 1. The fiber №1 (central) is centered with a spot of beam №1; 2. All of four
beams are adjusted parallel in the disassembled commuter at 0.55 μm wavelength (Fig.4). 3. Laser beams №2,3,4 are
adjusted in the fully assembled LTM with external edges with the same fibers of FOC.
Figure 4. Assembled (left) and disassembled (right) commuter. Front and back view.
The beam distribution of the image of one of fibers and the spot of the laser energy before adjusting (uncentered) and
after (centered) are shown at Fig.5.
1 2 3 4 5
7
6
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Figure 5. The beam distribution of the image of one of fibers and the spot of the laser energy before adjusting (uncentered) and
after (centered).
The uncertainty of the third step Δ3 can be formulated as
2
2
31.1 AΔ+Θ⋅±=Δ Σ (3)
2'
2
'
12
2
12
2
12
12
2
12
2
12
12 )( f
f
A
Y
Y
A
X
X
A
AΔ⋅
∂
∂
+Δ⋅
∂
∂
+Δ⋅
∂
∂
=Δ (4)
Where ΘΣ
2= Θ1
2+ Θ2
2 – is the total systematic uncertainty, and A12 – is the angle distance between №1 and №2 of the
fibers
'
2
12
2
12
12 f
YX
A+
= (5)
f’=1798 mm – is the focal distance of Apomars-7, X12 – is the linear distance between the images of №1 and №2 of
fibers (axe X) in the focal plane of Apomars-7, Y12 – is the linear distance between images of №1 and №2 of fibers (axe
Y) in the focal plane of Apomars-7. Similarly it can get A13 and A14 . Θ1=1 arcsec – is the systematic uncertainty of
adjusting sensitive array of Silicon CCD Camera Gras 20 in the focal plane of Apomars-7, Θ2=20 arcsec is the
systematic uncertainty as a result of deviation of the maximum spot profile.
2
12
2
12
'
1212
12
12
12
YXf
XX
X
X
A
+⋅
Δ⋅
=Δ⋅
∂
∂;
2
12
2
12
'
1212
12
12
12
YXf
YY
Y
Y
A
+⋅
Δ⋅
=Δ⋅
∂
∂; (6)
2'
2
12
2
12
'
'
'
12
)( f
YXf
f
f
A+⋅Δ
=Δ⋅
∂
∂
Where ΔX12 – the uncertainty of the linear distance between images of №1 and №2 of fibers (axe X), ΔY12 – the
uncertainty of the linear distance between images of №1 and №2 of fibers (axe Y), Δf’=2 mm – the uncertainty of the
focal distance of Apomars-7. Is assumed that ΔX12= ΔY12=ΔX=0.3 mm, X12=Y12= 22.25 then
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sec_36_000176.025.222103.0
1798
1
)()(
1262
2
12
2
12
2
'
'
2
'arcradYX
f
f
X
f
A==⋅⋅+=+⋅
Δ
+Δ=Δ −
Therefore Δ3=±1.1·(1+400+1296)0.5=±41 arcsec, and ΔΣ=±(Δ1
2+Δ2
2+Δ3
2)0.5=±(452+82+412)0.5=±61 arcsec=±1 arcmin.
The focal position of FOC was found by the method of maximum concentration of energy in a pinhole of 30 μm. The
uncertainty of this method is about ±10 μm. The correction of -0.2 mm for vacuum and heat was made. The size of the
pinhole can be changed from 0,02 mm to 0,5 mm. It allows to calculate the energy concentration in the focal plane of
main lens of RM by the measuring detector signal with the use of different pinholes. The position of the pinhole when
the maximum signal comes from the detector indicates a focal plane position of the main lens. When we use such a small
pinhole our optical system suffers losses of the order 1000 by a big divergency LED source . To compensate these losses
it is necessary to have a large dynamic range of measuring system. The pulse regime of LED and a special designed
detector circuit was used for this purpose. This measuring system has linear dynamic range more than 105. It permits to
measure transmittance of RM optical system. The transmittance in this case can be measured as the ratio signal behind
the field diaphragm and further to signal behind another diaphragm with the same LED source and beam divergence,
equal the ratio of field diaphragm diameter to focus of the main lens of RM. Other diaphragm may be of greater diameter
and need the divergence which is determined by distance from the diaphragm to LED source.
The pulse flash lamp was used for measuring the level of nonaxis sun scattering light. First of all the distance from RM
main lens to flash lamp was defined in order to have necessary illuminance at the lens. Then the signal in RM circuit was
measured. In this case the signal was too small so it was necessary to increase illuminance to measure it.
The measuring of the threshold detectivity was conducted in two stages. First of all the driver for LED was designed and
assembled. The main function of the driver was to provide a large dynamic range about 10000 of pulse energy by
switching time duration of pulse current. The energy must be in dynamic range of APD RM. The detectivity of receiving
module can be determined in terms of energy. The threshold energy is the energy with the ratio signal to noise equal 1.
So we must measure the noise signal and the signal with the well-known energy in focal plane of the main lens. For
example, if we want to have the level energy of 1·10-16 J in the rectangular pulse with the duration of 1·10-8 s, we should
have power in the pulse equal 1·10-8 W. The maximum energy can be 10-11 J with the pulse duration of 1·10-3 s and flat
pulse response of the detector. Such energy can be measured by the silicon diode energy meter. First of all we have
measure the pulse response of APD detector. It was flat from 10 ns to direct current. So it is possible to measure energy
of 10-3 s duration pulse and then to calculate the energy that correspond 10-8 s duration, which is 10-16 J. If the pulse
response of detector used in the rangefinder is not flat we should form the short duration rectangular pulse with the same
power till 10 ns. The maximum pulse duration, which can be used, is determined by the bandwidth of the rangefinder
detector amplifier. The energy of this pulse can be calculated if it has quite good rise and fall time of pulse. If it is not,
we could not use the direct calculation from the power to energy, and must take into account the spectral response of the
detector amplifier. It is rather difficult task, as rise and fall time of LED is not good enough. So we can not precisely
calculate energy of such pulse of radiation. It is necessary to measure the ratio of energy of shortest pulse, which can be
received from this LED source to the energy of pulse with proper rise and fall time, which energy can be calculated from
measurement. This ratio must be taken in account during finish energy calculation.
The responsivity of the silicon detector should be measured. Usually it is made by comparing with the standard
photodiode responsivity [5]. It can be a self calibration trap based detector or another type of a standard photodiode. Its
responsivity should be well-known in visible region of spectra. Therefore relative spectral response of the above
mentioned silicon detector is measured and then both detectors are comparing in the visible region of spectra. The
responsivity of the silicon diode at the laser wavelength is calculated as a ratio of responsivity on the laser wavelength
and a wavelength of comparison. If we know the responsivity of the silicon diode S in A/W, it can be easily transformed
into the responsivity S in V/J by formula S(V/J)=S(A/W)/C , where C is the capacitance of the condenser in the silicon
photodiode circuit. It is necessary to note that all detectors and readout device should work in linear regime and in a
proper range of the measurement. It was measured that the optical system had losses approximately 103. So we should
have energy of 1·10-8 J from LED source. It is less than the upper limit of the detector linearity and it can be measured by
the silicon energy meter, but the output voltage will be too small on the load capacitance of detector. So it is necessary to
have an additional amplifier. The amplifier must be of the narrow bandwidth and low frequents from 100 to 10000 Hz. It
is a simple amplifier and its gain can be easily measured. There may be two ways of measuring procedure. The first one
is as described above with an additional amplifier. The second one is by increasing power from LED source, then
measuring the energy in the focal plane of the optical system and after all inserting a glass filter behind LED source. The
glass filter transmittance can be measured in the same measuring unit, because its transmittance is 0.03...0.05. We have
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used the second way of measuring in our testing unit because of less errors. A common error of measuring consists of the
calibration error of silicon detector ±3%, the error of measuring of glass filter transmittance ±3%, the error of energy
readout in focal plate of detector ±2%, the error of measuring capacitance of ±0.1%, the error of measuring noise voltage
of ±3%.
The calibration was made in the focal plane of objective RM with power Pc=1.6·10-7 W. The power at the entrance pupil
of objective RM have to divide calibrated power by the transmittance of objective τ=0.8. The signal of RM (Fig.6) shows
oscilloscope snapshot. The pulse duration is about 10 μsec, amplitude of pulse is about 18 mV and it is constant for any
duration of the pulse.
Figure 6. Snapshot of oscilloscope.
The simple operation follows threshold level of RM (Dt):
Rt
TpPc
Dt ⋅
⋅
=
τ
(7)
The assumed time duration of real laser pulse is Tp=10 nsec, the relative threshold of RM is Rt=3, get Dt=1.0·10-15 J.
The threshold range which determines the maximum rangefinder measuring distance L can be formulated as
Dt
PDp
LL
ρ
⋅
=2 (10)
Where Dp – entrance pupil diameter of RM objective, PL – the energy of main laser of LTM, ρ – reflectance of Phobos.
If Dp=16 sm, PL=36 mJ, ρ=0.05, the L will be107 km. The uncertainty of L can be formulated as
2
2
2
2
2
2
2
2
1.1 Dt
Dt
LL
P
P
L
Dp
Dp
L
LL
L
Δ⋅
∂
∂
+Δ⋅
∂
∂
+Δ⋅
∂
∂
+Δ⋅
∂
∂
⋅=Δ
ρ
ρ
(11)
If we assume that ∆ρ=0.01, ∆Dp=0.1 sm, ΔDt =5.4·10-17 J and ∆PL=2 mJ, than
kmsmL 1210115)1029()10107()1030()107( 424242424 =⋅=⋅+⋅+⋅+⋅=Δ
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The uncertainty of reflectance of Phobos at λ=1.064 μm contributes the main uncertainty in the range determination
L=107±12 km, therefore it must be decreased more then three times not to be a determinable. To determinate the
dynamic range it is necessary to increase input signal. First of all the neutral absorber removes and signal increases
proportionally. The second step is to increase power of LED and it follows to increase the dynamic range to 400 without
overloading. This means that the minimum range of 5 km for L will be reached normally. Further increasing of power of
LED was not available without the destruction of LED.
2. CONCLUSIONS
All the adjustments and calibrations were reached by the testing unit for laser rangefinder. The uncertainties of adjusting
four laser beams to main axes of space vehicle are about ±1 arcmin. The threshold range is 107±12 km and the dynamic
range 107… 5 km. It is clear that the achievement of the smaller uncertainty of range caused by smaller uncertainty of
the Phobos reflectance at wavelength λ=1.064 μm. If it is necessary it may be measured more precisely by another space
instrument.
ACKNOWLEDGMENTS
We gratitude the colleagues who helped us to create this unique testing unit and provide all the test operations. We
express appreciation to Dr. Leonid N. Soms, Prof. Alexandr G. Murzin and Mr. Anatolii Y. Evseev. Also we want to
thank Professor Zbigniew Raszewski and Doctor Edward Nowinowski-Kruszelnicki for their collaboration in developing
liquid crystall cells for the rangefinder.
REFERENCES
1. http://www.laspace.ru/rus/phobos_ship.php
2. http://www.ilph.ru/index.php?page=hor
3. http://www.youtube.com/watch?v=u2zLR4bLFL0
4. V. Polyakov, V. Pokrovsky, S. Studentsov, L. Soms, M. Tomilin, High accuracy TN optical commutator of laser
radiation for application in space navigation. ALT09 Advanced Laser Technologies 2009, book of abstracts, p. 166
5. E. Kuvaldin: Counting Method for Measuring and Linearity Checking Photometry Devices Measurement science
review. http://www.measurement.sk/2010/S1/p.html
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