This paper presents a detailed description about the new transmitting system of the Modular Transportable Laser Ranging Systems MTLRS-1/2. A simplified theory of the Self Filtering Unstable Resonator (SFUR) is explained. Laser design details are discussed concerning the extreme environmental conditions in which these mobile systems are operating. Details are given concerning the new avalanche START detector. The new SFUR laser and START detector are necessary parts in order to bring both mobile systems towards 1 cm ranging accuracy

Beek, Wiard

Hessels, U.

Motz, R.

Offierski, Jacek W.

Sperber, Peter

Vanes, C.

English

NASA Technical Reports Server

N94"15612
The new MTLRS Transmitting System
P. SPERBER, O. HESSELS, R. MOTZ
INSTITOT FOR ANGEWANDTE GEOD;_,S1E
FUNDAMENTALSTATION WETTZELL
D-8493 K6TZTING
GERMANY
W. BEEK, J.W. OFFIERSKI, C. VAN ES
DELFT UNIVERSITY OF TECHNOLOGY
KOOTWI/K OBSERVATORY FOR SATELLITE GEODESY
POSTBOX 581, 7300 AN APELDOORN
THE NETHERLANDS
Abstract
This paper presents a detailed description about the new transmitting system of the Modular
Transportable Laser Ranging Systems MTLRS-1/2. A simplified theory of the Self Filtering
Unstable Resonator (SFUR) is explaned. Laser design details are discussed concerning the
extreme environmental conditions in which these mobile systems are operating. Details are
given concerning the new avalanche START detector. The new SFUR laser and START
detector are necessary parts in order to bring both mobile systems towards 1 cm ranging
accuracy.
1. Introduction
Since 1984 the two European Modular Transportable Laser Ranging Systems MTLRS-1,
operated by the Institute for Applied Geodesy (IfAG, Germany), and MTLRS-2, operated by
the Delft University of Technology (DUT, The Netherlands), have supplied the international
network with laser ranging data from sites in Europe, CIS (former USSR) and North
America. To improve the ranging accuracy from 5 cm to the 1 cm level, both systems
carded out a major upgrade in 1991. The main part of this upgrade was the exchange of the
transmitting system consisting of the Laser and the Start Detector (The German MTLRS-1
system also upgraded the receiver package [1].
2. The Laser
The accuracy and the number of returns produced by a satellite laser ranging system is
mainly influenced by the duration and energy of the emitted laser pulses. To improve these
two points, the original Nd:YAP laser (370 ps FWHM pulse-duration, 10 mJ pulse-energy at
539 nm) was exchanged for a new Nd:YAG laser (30 ps FWHM pulse-duration, 30 mJ
pulse-energy at 532 nm). To realize this high demands with a simple and reliable optical
configuration, a new resonator setup, the Self Filtering Unstable Resonator (SFUR) [2,3]
was used. Together with a simple optical setup, this resonator has the advantage to be less
sensitive to optical adjustments and operates with a very homogeneous spatial pulse-shape,
minimizing the danger of destroying optical components at high resonator energy output
levels.
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https://ntrs.nasa.gov/search.jsp?R=19940011139 2020-06-16T19:12:22+00:00Z
2.1 Simplified SFUR Theory [4]
Fig. 1 shows the principle of the SFUR resonator.
S.F.U.R. RESONATOR
M2 aperture Fth
L_ II -- i
R2 d
Ii!
/..2
LO
F
R1
i
L1
M1
l°
Fig. 1
Two mirrors MI and M z are forming an instable, confocal resonator:
Rl + R2 =2 "L o
RI and R2 are the (concave) radii of the mirrors and L o the optical' resonator length. This
resonator configuration generates a spot in the resonator, which usually destroys optical
elements. If a small pinhole is placed in this focal point this problem disappears and some
major advantages can be seen. The diameter d of the aperture has to be chosen in a way,
that only the central lobe of the Airy pattern (generated by mirror M2) propagates to M r.
d = 42.44 .f. x
(fi focal length of 1t42, X: laser wavelength)
The mode, generated by the central maximum of the Airy pattern has a smooth, nearly
Gaussian shape, with zero intensity at radius ro where
R 1
ro = - M • d (M = - --)
2 R 2
If the parameters of the resonator are correct, the mode is exactly filling the whole volume
of the active medium. Therefore nearly all the energy stored in the rod is converted into the
laser pulse. The outcoupled pulse-energy is much higher compared to usual resonator
configurations. The aperture (typical diameter d< 1 mm) gives a strong spatial filtering at
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each round trip smoothing the energy distribution in the beam and avoiding hot spots. In an
exact analysis of the SFUR configuration the influence of the thermal lens (/'_: focal length)
in the laser rod has to be taken into account.
Lt " /-'2
L°=LI + 1"2 fth
g! =
Lo L_
Lo LI
g2 = 1 P_ fa
g = 2 "gl "g2 - 1
M =g - _/g2 - 1
(L 1 and L 2 are the distances of MI and M2 to the virtual position of the thermal lens). The
optimum position Lp (distance from M9 of the aperture, the diameter d of the aperture and
diameter D of the laser beam are given by:
1 _ 1 + _/g2 - 1
L v 1_ 2 "gl "Lo
d = _ 4.88 'Lv "_. (1- L---e-)P_
D = 1.5 .M.d.
"/
. m . --
2 -"i" -
E
2.2 Technical Realization
The laser system consists of four separate units, the laser head, the power supply with
electronics, an external dye-cooling unit and the control panel.
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A schematic drawing of the laser head is shown in Fig. 2. To save place, the SFUR
Resonator Ms - M2 is folded over a prism PR. Picosecond pulses are generated with an
acousto-optic modulator AML and a bleachable dye (Kodak 9740 in 1,2 Dichloroethane)
0aML). The intense 30 ps long pulse
is cavity dumped by the pockelscell
PC and two polarizers PO. Because
of the high energy of the output
pulse of a SFUR resonator a single
stage amplification (AMPL) is
sufficient to get 100 mJ per pulse in
the infrared. The second Harmonic
Generator SHG converts about 40%
of the energy into green light at 532
nm. The two wavelengths are separ-
ated by the dichroic mirror M,. The
excessive use of diagnostic diodes
and motor driven micrometers
allows the fine adjustment of the
laser without opening the dust
LASER HEAD OPTICAL CONFIGURATION
E
E
O
O
,, PO, ,, OUTI.
1PR i ="_=_ 1 I"
I D I
ooo.i
t
PCT
,SD
1200 mm
- MOTOR MIKES
- POCKELS CELL TRIGGER
- START DETECTOR
cover. Even the spatial shape of the Fig. 2
pulse is monitored by a CCD - Camera. For maximum mechanical stability, the whole laser
head is built on a temperature controlled, reinforced invar plate.
t_9
tq
.<
O
Fig. 3
POWER SUPPLY CONFIGURATION
HIGH
COOUNG ELECTRONICS VOLTAGE
SMPS I]
]
] ,1L2
20 20
] uF I.IF
[3 i
1100 m m
0
0
3
3
Fig. 3 shows an inside picture of the power supply. The power supply and control elec-
tronics are also built in an isolated, temperature stabilized housing. The laser switches
automatically between 380 V/50 Hz and 460V/60 Hz input, and the 3 phase power con-
sumption is 5 KVA. Two independent control panels for the laser can be used; one for the
Cabin, in use during ranging, and another one in the Cart for maintenance purposes.
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Fig. 4 shows the laser temperature control system. The telescope together with the laser are
built in the Cart which is located in full sunshine in the middle of the concrete platform
(Pad). During satellite
tracking this Cart is open and
because of extreme environ-
mental conditions (-20°C to
+40°C), considerable atten-
tion is given to the stability
and reliability of the laser
(komacel-isolation of the head
and power supply, excellent
mechanical stability of all
optical components). The heat
generated by the flashtubes in
the secondary cooliag system
is exchanged to the primary
cooling system by means of an
internal water to water heat
exchanger connected to an
external water flow with
temperatures between 5-16°C.
TEMPERATURE CONTROL SYSTEM
_It
\ ELEC'FRONK_ _J
Fig. 4
The laser was first tested under field conditions during the campaign of MTLRS-1 in CIS
(Common wealth of Independant States - the former USSR). The experiences were excellent.
The specifications of the laser are summarized in Fig. 5.
LASER SPECIFICATIONS
Wavelength
Pulse energy
Pulse width
Rep. rate
Divergence
Mode locking
SHG
Beam diameter
Environment
Power
Voltage
532 nm
30 mJ
30 ps
20 pps (max)
0.4 mrad
active + passive
KD* P
7 mm
- 20 ° C to +40 ° C
5 kVA
380/460 - 5Nph.
Fig. 5
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3. Start Detector
To start the time interval counter with high accuracy, the optical output signal of the laser
has to be converted with minimum jitter to an electrical pulse, which is used in the start
channel of the counter. Until 1990 this problem was solved in the MTLRS in the conven-
tional way: a part .of the green pulse was sent to a fast photo diode which generates an
electrical pulse with an amplitude corresponding to the energy of the laser pulse which is
fluctuating from shot to shot. These amplitude fluctuations were compensated by a constant
fraction discriminator. Changes in the temporal pulse shape and strong energy fluctuations in
this setup caused errors in the start-channel of the counter.
To minimize these errors, an optically triggered avalanche diode [5,6] was integrated into
the system. This diode delivers an output signal with a constant amplitude independent on
the energy of the laser pulse. The output signal is directly used as startsignal for the counter,
a constant fraction generator is superfluous. Furthermore, to introduce high stability and the
possibility to vary the pulse energy during tracking, the start diode is placed before the
amplifier (behind mirror M3 in Fig. 2) to detect the infrared light. Fig. 6 shows the output
signal of the start diode which has a risetime of 300 - 400 ps and the jitter between optical
and electrical pulse is less than 20 ps.
+ 7.8 V--
I.--
0V --
START DETECTOR OUTPUT SIGNAL
\
\
_ ,,.,.,,.._.__
5n_d_
Fig. 6
In order to get stable results from the avalanche start detector under field conditions, the
fibre together with the electronics have to be built inside the laser head where the tempera-
ture of the optical bench is stabilized at 25°C.
4. Summary
The upgrade of the transmitting part of MTLRS-1/2 formes the basis to improve the ranging
accuracy of the systems significantly and makes them again some of the most advanced SLR
systems worldwide.
First tracking' results [7] show, that the system's single shot accuracy is about 1 cm after the
upgrade of the receiver package which is already completed in MTLRS-1.
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References:
[1] The new MTLRS-1 Receiving System, 1992 by P. Sperber, U. Hessels, R. Motz, in
this Proceedings.
[2] Paper 1989 by A. Bianchi, A. Ferrario, P.G. Gobbi, C. Malvicini, G.C. Reali, SPIE
701, p. 132.
[3] Paper 1987 by K. Hamal, V. Hraskova, H. Jelinkova, CLEO'85 Proc.e_ings p.82,
WE4.
[41 Paper 1989 by C. Malvicini, A. Ferrario, G. Gabetta, G.P. Banff, G.C. Reali,
Proceedings of the seventh international Workshop on Laser Ranging Instrumentation
p. 181.
[5] Paper 1990 Subcentimeter Single Photon Ranging All Solid State Detection Package,
by I. Prochazka, K. Hamal, B. Sopko.
ial Paper 1989 by I. Prochazka, K. Hamal, B. Sopko, Proceedings of the seventh
international Workshop on Laser Ranging Instrumentation p. 219.
[71 Results of the MTLRS-1 Upgrade by P. Sperber, L. Amberg, L. Beyer, G. Blenski,
W. Etling, U. Hessels, R. Motz, in this Proceedings.
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The new MTLRS transmitting system

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