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Titre: A Raman laser system for multi-wavelength satellite laser ranging
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Science in China Ser. G Physics, Mechanics & Astronomy2004 Vol.47 No.737--743
A Raman laser system for multi-wavelength
ellite laser ranging
H U J ngfu
, YANG Fumin
, K. H a m a l , I. P r o c h a z k a
& J. B l a z e j
1. Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China;
2. United Center for Astro-geodynamics Research, Chinese Academy of Sciences, Shanghai 200030,
3. Czech Technical University in Prague, Czech
Correspondence should be addressed to Yang Fumin (email: email@example.com )
Received July 12, 2004
In orderto develop a laser system for multi-wavelength satellite laser ranging,
the joint group of the Shanghai Astronomical Observatory and the Czech Technical
University has studied the conversion efficiency of the Raman-shifting beam and its
spatial characteristics. We adopted a 0.53 IJm laser with pulse width of 35 ps and peak
energy of 35 m J, the second harmonic of a Nd:YAG actively and passively mode-locked
laser, to pump a one-meter-long Raman tube which is full of H with high pressure at the
Prague-based laboratory. We get the first Stokes laser (0.68 IJm): 7 mJ (single pulse) with
beam divergence of 40 '~ (arcsecond) and spatial wobbling of less than 4" ; and the first
anti-Stokes laser (0.43 IJm): 2 mJ (single pulse) with divergence of 56" and spatial
wobbling of less than 4" . The emitting beam from the Raman cell also includes 0.53 IJm:
10 mJ (single pulse) with divergence of 40 '~ and spatial wobbling of less than 7"
According to the radar link equation and based on the above obtained multi-wavelength's
energy, we can estimate the detection probabilities for three colors respectively. It is
shown by the result that the developed multi-wavelength Raman laser system has the
capability of satellite ranging. The Raman laser system will be installed at the laser station
in Shanghai Astronomical Observatory to research the multi-wavelength satellite laser
Keywords: multi-wavelength satellite laser ranging, Raman laser, atmosphere correction, detection probability.
In the space techniques, satellite laser ranging (SLR) is the most accurate one with
single-shot measurements and its accuracy has reached sub-centimeter level. For some
satellites with well distributed retro-reflectors (Starlette, Stella, ERS-2, Lageos-1, 2 and
Topex/Poseidon), the single-shot ranging precision of 7 - - 8 m m was obtained at the
Shanghai Astronomical Observatorym. With the development of astro-geodynamics reCopyrightby Science in China Press 2004
Science in China Ser. G
Physics, Mechanics & Astronomy2004 Vol.47 No.6 694--701
search and monitoring of crustal movements, the millimeter accuracy of satellite laser
ranging system is required ca. Now the correction of atmosphere refraction is one of the
most important accuracy-limiting factors, due to the inaccuracy of atmospheric models.
The multi-wavelength satellite laser ranging does not depend on the models, which can
use the differential flight time of different colors from satellites to correct the atmos-phere delay ~, and it is of high accuracy. The experiments have been performed in
sev-eral countries: Grasse station in France, Czech Technical University, Graz station in
Austria, NASA/GSFC in U.S.A., Zimmerwald station in Switzerland and Matera station
in Italy etc. The multi-wavelength laser system is the key technique for the Multiwavelength satellite laser ranging. The optical sources available for multi-wavelength
satellite laser ranging include~a: Nd:YAG at 1.06 gin, doubled Nd:YAG (0.53 gm) and
third harmonic at 0.35 gm, or the frequency-drifting laser: first Stokes at 0.68 gm and
the first anti-Stokes at 0.43 gm in the H pumped by 0.53 gm laser. The titaniumz
sapphire laser at 0.84 ~tm together with the second harmonic wavelength at 0.42 gm is
also available, but the laser system is expensive. The Nd:YAG together with Raman
la-ser system is a good choice for the multi-wavelength SLR for ranging satellites with
the Single Photon Avalanche Diodes (SPAD). The Czech Technical University and Graz
station carried out multi-color ranging with 0.68 gm/0.53 gm/0.43 gm in Graz success-fully. Among the three wavelengths the pair of 0.68 gm/0.43 gin has the biggest
disper-sion, so this pair has the merit of high accuracy of atmosphere correction under
the same timing precision. But the single pulse energy in Graz was only 0.2 mJ at 0.43
gm ~a. It was too difficult to get the returns from satellites, even using small divergence
In our experiment, we studied the beam's spatial characteristics and conversion
ef-ficiency of the first Stokes and first anti-Stokes laser in hyperbaric H 2 pumped with
Nd:YAG laser at 0.53 ~tm, and analyzed the optimal output condition for first Stokes,
and in addition three-color outputs with stronger energies are obtained. According to the
characteristics at Shanghai SLR station, we estimate the detection probability for Lageos
at about 3 9 % - - 9 9 % for the three-color lasers. It can meet the need of satellite ranging.
Experiment of stimulated Raman scattering
The experimental scheme of Raman shifting is shown in fig. 1. The oscillator in the
dotted scheme is Nd:YAG actively and passively mode-locked laser. In fig. 1, 1 is concave mirror and dye cell, 2 is AOM (frequency at 57.3 MHz), 3 is Nd:YAG rod, the end
faces of which are tilted at Brewster's angle to choose polarization mode, 4 is pinhole to
select single transverse mode, 5 is Fabry-Perot output mirror, and the output beam is
single transverse mode with 35 ps at 1.06 gm. P1 and P2 are polarizers. SL is electro-optical single pulse selector, which selects a single pulse from the pulse trains. Then
the selected pulse passes through the beam expanders, two amplifiers and frequency
doubler (KDP crystal), and the laser output at 0.53 gm with maximum energy 35 mJ is
Copyrightby Sciencein China Press 2004
A Raman laser system for mulfi-wavdength
satellite laser ranging
obtained. M3 and M4 are high-reflection for 0.53 fun and high-transmission for 1.06 lam.
Being reflected by M3 and M4, the green light is focused into the center of the Raman
cell. At the end of the Raman cell, we obtain the conversion efficiency by measuring the
energy of shifting laser with filter and energy meter, and also research the far field
fea-ture of the output laser by surveying its spatial energy distribution with a concave
mirror M7 whose radius of curvature is 5 m and a CCD camera located at the focal plane.
In our experiment, the Raman cell is made of a stainless steel tube with inner diameter of
3 cm, and the effective length is 100 cm. Both ends of the tube are coupled with two
confocal quartz lenses with focal length of 50 cm, and are sealed with quartz Brewster
Scheme of Raman laser conversion.
Result and discussions
1.2.1 Study of Raman conversion efficiency. When laser beam focuses into the
Ra-man cell being full of H with high pressure and the beam intensity is greater than
the threshold, the weak spontaneously scattered Stokes light will be amplified and Raman laser will be generated. The pulse width of Raman laser is a little narrower than that
of pumping laserm, and it is useful for SLR. Fig. 2 shows the conversion efficiency of
first-order Raman laser (0.68 fun) versus H 2 pressure with various pumping energies,
and we can see that as the pressure of H increases, the conversion efficiency of first
Stokes becomes greater with lower pumping energy (5--10 m J), and when the pumping
energy is raised to 20 m J, the conversion efficiency of first Stokes (0.68 lam) increases
first, then decreases with the pressure increasing. The reason is the stimulated Raman
gain coefficient becomes bigger as the H pressure increases, and the output of
first-order Raman laser increases. When the output reaches a certain value, the
first-order Raman laser, as a pumping source, will cause second-order Raman shifting or
even high-order Raman laser. This procedure will consume the energy of first-order
Raman laser. Meanwhile, as the dispersion is serious in high-pressure gas, the four-wave
Science in China Set. G Physics, Mechanics & Astronomy 2004 Vol.47 No.6 694--701
mixing process is suppressed because of phase mismatching, and the outputs of the
high-order Stokes and anti-Stokes will be limited. So higher pressure gas is not always
good for getting high conversion efficiency of first-order Raman laser. It is shown in the
experiment that the output of first Stokes is subjected to H 2 pressure and pumping energy,
and there exists gain saturation phenomenon. To choose the optimal pressure under our
experimental condition, we study the conversion efficiency versus pumping energy with
various pressure values in fig. 3. When the pumping energy is 10 mJ or higher, the curve
is flat for 20 bar. If the pumping energy reaches 25 m J, the conversion efficiency
de-creases obviously, which is also shown in fig. 2. Referring to the curve for 15 bar, the
conversion efficiency of first-order Raman laser enhances with pumping energy
in-creasing. So it is reasonable to set the pressure at 18 bar.
Conversion effidency of first Raman laser versus H 2 pressure.
Fig. 4 shows the conversion efficiency of first Stokes and first anti-Stokes versus
pumping energy when the H pressure is 18 bar. The curves become flat when the
pumping energy exceeds 5 mJ especially for the first anti-Stokes. This is because the
Raman cell's ends are sealed with quartz Brewster windows, which reduced the loss of
pumping light. We do not choose high pressure for taking care of the stronger output
energy of first anti-Stokes. The energies at various wavelengths are measured as follows:
7 mJ at 0.68 pm, 10 mJ at 0.53 pm, 2 mJ at 0.43 pm. The typical energy about 10 mJper
pulse can be used for SLR, so three wavelengths obtained have the capability of satellite
1.2.2 Divergence of laser beam. When a beam of laser passes through a reflective
mirror whose focal length is f and converges to a spot, the divergence of the beam can be
estimated through measuring the spot diameter by the formula 0=d/f. In the formula, f
Copyright by Science in China Press 2004
A Raman laser system for multi-wavelength satellite laser ranging
Fig. 3. Conversionefficiencyof first Raman laser versus pumping energy.
Fig. 4. Conversionefficiencyof first Stokes and first anti-Stokes versus pumping energy.
is focal length (2.5 m) o f the reflective mirror, and d is diameter o f the spot. The
spot can be imaged on a CCD. In our experiment, one pixel on the C C D corresponds to
0.1 m m and 8" in space. The C C D is exposed at 0.1 s time interval, and the remits are
recorded with a computer and are shown in table 1.
Table 1 Divergenceand wobbling of three-color-output(unit: pixel)
Red (0.68 I~m)Oreen
5 • 5"~,/~,, 0,3.,,- 0.5
+/~,, 0.3 -: 0,g
Blue (0.43 pro)
+/-,.0.4 -. 0,4
Science in China Ser. G Physics, Mechanics & Astronomy 2004 Vol.47 No.6 694--701
The wobbling of the spots of Stokes and anti-Stokes near the threshold is serious
and cannot be adopted in satellite ranging. When the pumping energy is above 20
per-cent of the threshold, the wobbling decreases obviously. There are three-color lasers
in-cluding 0.53 gm, 0.68 gm, 0.43 gm from the Raman cell and with different divergences and wobblings. The multi-wavelength satellite laser ranging requires the laser
beams with narrow divergence and small spatial wobbling, so three beam expanders are
intro-duced to three-color beams respectively, and then are combined and aligned onto
the transmitting optics.
Estimation of capability of the multi-wavelength satellite laser ranging system
For satellite laser ranging, the average photoelectrons produced by the returned
signals from satellites on the photo-sensitive-area of the detector depend on the radar
A~. " 4
" K t . K~. . T z .~7.cz
~:2. R 4 ..~/ . ~ :
In this equation N is the average photoelectrons produced from satellites; E is the
laser pulse energy; S shows the photons per Joule; A~ is the effective area of the
retro-reflector on satellite, 257 cm for Lageos; A,.is the effective area of our receiving
telescope, 0.251 m ; Ktis the efficiency of transmitting system, about 0.60, and K,. is
the effi-ciency of receiving system, 0.60; T2 is the atmosphere transmission for
round-trip and depends on wavelengths; 8 is the quantum efficiency of photon-detector,
and has the slight difference for SPAD for three wavelengths; a is an attenuating factor,
including the influence of retro-reflector efficiency, atmospheric seeing and turbulence
etc., set at 13 dB for Lageos; and R is the distance of satellite, 8000 km for Lageos. For
our system the divergence of laser beam et may be set at 10" , and 0s is the divergence
of the retro-reflector on satellite, 16.9"
for Lageos. The signal detection probability of
the pho-ton-detector conforms to the Poisson probability distribution. W~aen the number
of the average photoelectrons is N, we can use the formula
- N ~ 9 e-N /m!
to estimate the probability P(m, N) for getting m photoelectrons. So we can calculate the probability for getting one photoelectron at least:
1 - e -iv .
Table 2 shows the estimation of the detection probabilities and return rates using
three different wavelengths for Lageos ranging.
In the above calculation, we assumed that the laser beam hits the satellite without
Copyrightby Sciencein China Press 2004
A Raman laser system for multi-wavelength satellite laser ranging
t r a c k i n g errors, and n e g l e c t e d the i n f l u e n c e o f laser b e a m w o b b l i n g b e c a u s e the w o b b l i n g is m u c h s m a l l e r t h a n d i v e r g e n c e angle.
Table 2 Estimation of the detection probabilities and return rates using three wavelengths for Lageos ranging
laser s y s t e m d e v e l o p e d b y the j o i n t g r o u p o f the S h a n g h a i A s t r o -
n o m i - c a l O b s e r v a t o r y and the C z e c h T e c h n i c a l U n i v e r s i t y f o r t h e m u l t i - w a v e l e n g t h satellite laser r a n g i n g c a n p r o d u c e t h r e e - w a v e l e n g t h lasers. T h e p u l s e e n e r g y c a n m e e t the
n e e d o f t h e satellite r a n g i n g . It has b e e n s h o w n f r o m the t h e o r e t i c a l a n a l y s i s that the
lowest detection probability reaches about 39%.
We wish to thank Dr. Jean Gaignebet, OCA Observatory, France, for presenting the excel-lent Ramml cell to us, which was developed by himself. We also appreciate the helpful discussion and help from
our colleague, Chen Wanzhen. This research is supported by the National Natural Science Foundation of China
(Grant Nos. 10373022, 10173018).
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