We show that laser-target interaction physics demands the shortest laser pulse of which hardware is capable (but not less than 100 ps) in the ORION ground-based laser concept. We compare two leading ways to achieve such pulses - SRS/SBS cascade compression and grating compression - with the standard MOPA approach, and conclude that the first of these is most robust. However, the state of the art in laser devices will require a year or two to implement these ideas. We present a pulse format and beam footprint protocol which will solve the conflict between relativistic lookahead and beam tilt and should permit all-laser active acquisition and tracking in ORION

Phipps, Claude R.

English

NASA Technical Reports Server

NASA-CR-201960
ORION Low Cost Laser Analysis
prepared for
Dr. Jonathan Campbell, PS-02
NASA George C. Marshall Spaceflight Center
under Order No. H-27250-D
issued August 14, 1996
by
Photonic Associates
1621 Calle Torreon
Santa Fe, NM 87501
Dr. Claude R. Phipps, president
Claude R. Phipps
https://ntrs.nasa.gov/search.jsp?R=19960047506 2020-06-16T03:23:35+00:00Z
Photonic Associates i
,excellence in_hotonics |
Table of Contents
page 2
1: Motivation for short pulse generation in ORION ............................. 3
2: Methods of short pulse generation compared ................................... 6
3: Key issues and problems for Phase II ................................................... 8
4: Recommendations ................................................................................. 11
Photonic Associates i
page 3
Section 1: Motivation for short pulse generation in ORION
1.1 SRS requires short pulses
In work performed during Phase I of NASA's ORION program, it was shown that very
short pulse 1.06-_tm wavelength lasers can penetrate the lower atmosphere at very high
peak pulse intensity without being converted to other wavelengths and scattered in
unintended directions Stimulated Raman Scattering (SRS). This gives an advantage to very
short laser pulses, because it is necessary to ignite a plasma on the debris target for efficient
generation of de-orbiting thrust, and the energy required to do that is less for very short
pulses.
Figure 1 shows a desirable short pulse ORION operating point on the left, above the "1E-10"
(100-ns pulse width) label on the horizontal axis. In the illustration, a near-term, short
range (600 km) system is shown, for which 450J at 100ps is equivalent in effect on the target
to 6,600 J at 40 ns.
Maneuvering Room for the ORION System limited by SRS,
STRS, n2 and other effects
3E+10
1 E+I 0
vertical propagation from 6km re MSL
with 1.061_m
E
O
1E+9
1E+8
d 27 cm
1E+4.
1E+3.
1E-11 1E-10 1E-9 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 1E-2 1E-1
Laser pulse duration (s)
Figure I
1E+0
I Photonic Associates _
page 4
It is clear that the 6.6kJ laser would be more difficult and expensive to build, perhaps by as
much as afactor of 10 based on construction cost of the LLNL "BEAMLET" laser.
The fact that short pulses defeat SRS is shown by upward curvature the line labeled "SRS"
for pulses shorter than 10 ns.
1.2. Target physics says short pulse lasers require less energy
As we showed in §0 of ORION Phase I work, the required laser parameters on the ground
can be connected to the target intensity required to form plasma and obtain optimum
coupling, particularly to relate laser pulse energy W to mirror diameter Db:
Ca [ kz ]
W = _-LGj21_ (_ [1]
In this expression, C = 2.3E4 is a constant derived from optimum target coupling
c_ = 0.45 is an exponent derived from optimum target couling
"_ is laser pulse width
T is atmospheric transmission (0.85 for a vertical path)
S is Strehl ratio (1/N 2 in § 0) = 0.5
a = 4/n
;_ is laser wavelength in cm
and z is range to target in cm.
As a reminder, this expression arises from two sources: These are a) the pulsewidth
dependence of intensity for optimum coupling to a target, a feature which determines o_ on
a strictly empirical basis, and b) the effect of diffraction over propagation distance z in
determining the laser spot size on target, given the emitted beam diameter Db, and
therefore the intensity on target given a pulse energy W.
1.3 Laser COSt can be developed from the pulse energy dictated by target physics
Also in Phase I, we developed a cost model as follows:
Where W = laser energy in joules from Eq. [1],
4
we have C c = 1.1 _ C i [2]
i=1
with the following cost elements:
Laser heada: C1 = $1.02E6*W °'45 [2a]
Power supplyb: C2 = $3.2E4*(fW/1000) 0.85 [2b]
Cooling gas flow loopb: C3 = $6.8E4*(fW/1000)°'88"(f/1000) °'°83 [2c]
System integrationb: C4 = $6.0E4*(fW/1000) 0.256 [2d]
a Source: C. Phipps study of the Lawrence Livermore (LLNL) Nova-Athena-NIF (National Ignition Facility)
construction and engineering design sequence, plus recent input from Lloyd Hackel at LLNL regarding 100-J, 30Hz,
10-ns laser system 5e has built for anilluminator at Starfire Optical Range.
b Source: J. P. Reilly
Photonic Associates i
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assuming a laser electrical to light efficiency of 4%.
Eqns. [1] and [2] can be put together, if we assume a mirror diameter of 6 m and _, = 1.06_m,
to give a plot of estimated laser cost vs. pulsewidth selected. (Figure 2). This Figure
dramatically summarizes the motivation.
ORLON Laser Cost vs.
Pulsewidth
2E+9-
1E+9-
6-meter mirror
3000 km
z = 1500 km
= 750 km _-
z = 375 km
1E+7
1E-10 1E-9 1E-8 1E-7 1E-6 1E-5
Laser pulsewidth (s)
Figure 2
Photonic Associates
page 6
Alternatively, Figure 3 shows how pusher pulse energy varies with pusher pulse duration
in order to ignite a plasma and optimally couple to the debris target, using the same
assumptions as in Figure 2. Slant range to the target is the parameter for the 4 trends
plotted. It will be seen that the ultimate system with 100ps duration requires only 6kJ per
pusher pulse even at 3000 km, and just 100 J per pulse for demo ranges of order 375 kin.
Midterm systems, limited as they are by currently available technology, will have to use 5 ns
pulses and generate as much as 37kJ for the maximum range and about 550J for the demo.
1E+8-
1E+7"
-- W(375km)
.... W(F50km)
m,. W(1500km)
,-,..--- W(3000km)
1E+3-
1E+2-
1E-10 1E-9 1 E-8 1E-7 1E-6 1E-5 1E-4
Laser pulsewidth (s)
Figure 3
This is why we have discussed 30kJ lasers at 5ns for the past several months, rather than the
smaller lasers we were hoping for in earlier reports.
2: Methods of short pulse generation compared
Having identified that 100-ps pulses are desirable, it remains to find efficient, low-cost ways
of obtaining them from conventional solid state lasers which more readily operate in the
20-40-ns regime. In this section, we will compare the three principal methods of obtaining
100-ps pulses which have been demonstrated in the literature. Much of this work has
occurred at the Lawrence Livermore Laboratory and at the Russian Vavilov Institute.
Experts at either laboratory are capable of developing the actual hardware.
Photonic Associates
excellence !n photonics _ page7
Two of these methods feature clever schemes to deliberately compress a longer pulse. Of
these, one involves the use of a holographic grating pair to passively compress a so-called
"chirped" large bandwidth pulse of about 10 ns duration. The second compression method
uses the physics of Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering
(SRS) or both in a cascade to provide passive compression. The third method is the "brute
force" approach: make a very short oscillator pulse, and amplify it in an amplifier of
adequate bandwidth.
2.1. Holographic Gratings
The beauty of this technique is its nearly perfect energy convesion efficiency (in principle).
However, the difficulty is that these gratings are limited to about 1 meter in transverse
dimension by current technology. Therefore, the chirped input pulse to the grating must
then be not much longer than 3 ns, due to the finite speed of light.
Neodymium laser = -_-_
10-ns pulse duration -_ _ | "_'_:'Z2Z_ratintr--"_
,with chirped outp_
_ "bluer" portion bends less,
_ _ _,,I/ follows longer path
[_ holo grating -] "redder" portion bends more,
follows shorter path
Output ._.]]._
Figure 4
Holographic Grating approach to pulse shortening
That is to say, since the speed of light is 0.3 m/ns, a pulse that is 20ns long will require a 6
meter grating. Conversely, 3 ns pulsewidth, which matches the maximum size of gratings
that can be made using current techniques, is too short to get good extraction and high
beam quality at the same time from a neodymium system at the present time.
Photonic Associates i
2.2. SBS/SRS Cascade
page 8
input
SBS medium
e.g. benzene
Figure 5
SBS/SRS Cascade as a pulse shortening mechanism
The advantage of this technique is its simplicity and totally passive operation in shortening
a long input pulse, by as much as a factor of 10. The pulse reshaping that results is due to
strong saturation of the input or pump wave by the leading edge of the counterpropagating
output or "Stokes" wave. A similar diagram describes the behavior of a reverse SRS pulse
compressor. SBS and SRS units can be used in cascade to obtain certain desirable effects.
Total compression ratios of about 100 have been obtained, just about what we require in the
ORION system (efficient conversion to 100 ps pulses which couple efficiently to the debris
target, from cheap, relatively low energy 10 ns inputs).
The problem with this technique is: low efficiency. Compression ratios of 100 go along with
energy efficiency which may be as low as 2%, unacceptable for ORION. Such a low efficiency
would actually make it less expensive to build and use a higher energy long pulse laser.
2.3. Amplfiying a short oscillator pulse
100-ps oscillator
I neodymium amplifier 1J output
Figure 6
The "brute force" way to get short pulses
PhotonicexcellenceAsso iates,,,.,,,.,÷,,,,,.,,.,
' _ page 9
This "brute-force" approach is deceptively simple. Unfortunately, it depends, at the present
time, on unattainable combinations of laser parameters. The problem is that the brightness
integral or "B-integral" which determines beam breakup due to nonlinear refraction in the
glass host of a solid state laser system is the same integral, apart from constants, that
determines energy extraction efficiency. And, for 100-ps pulses, the result is that efficient
extraction is not yet possible for 100ps pulses if high beam quality is also required.
The consequence of all this is: At this time, the best choice is a 5 ns pulse for ORION, not
shorter, if that pulse is generated by a solid state laser system. We must accept the cost
penalty implied by our approximate cost model (Figure 2) until technology catches up. The
situation may well improve in the next year or two as efforts at solid state laser R&D labs
proceed.
3: Key issues and problems for Phase II
The key problem for future resolution in ORION Phase II is as follows.
3.1 Problem Statement
The lowest order wavefront distorition for a laser beam propagating through turbulence in
the atmosphere is an average tilt, which results in a pointing error. A target illumination
laser, or sunlight, solves this problem in principle by actively illuminating the target. The
tilt in the wavefrnt is measured by focusing the light returned from the debris as collected
by the entire telescope aperture onto a sensor and measuring the displacement of the focal
spot.
However, in the worst case, during the delay between launching the illumination laser
beam and capturing the return signal, the target may have traveled well outside the cone
including the column of air for which the adaptive optics system is set up to correct
aberrations.
In that case, successive pulses of the laser produce well formed focal spots all of which miss
the target.
3.2 Is there a problem?
For a particle in a circular orbit, if the local zenith angle is 0z and the geocentric angle
between the local Earth radius and the particle Earth radius is 0E, the relativistic lead angle
is determined by target velocity v, the speed of light c and these angles according to
0_ = 2v/c cos (0z-0E) • [3]
The maximum value (for a particle directly overhead in fairly low orbit) is about 50pxad (10
arc seconds) for ORION targets.
The relation for the tilt error as a function of the lead angle isc:
Ctilt = 0.6htKo(X seC0z/{D7/S[ro(_,o)] 5/6} [4]
Assuming a turbulence layer at 5 km, a lead angle 0_ of 50 _trad, a telescope diameter of 6 m
and a value of r o = 20 cm, the tilt angle is 75 nrads. The diffraction angle, assuming perfect
c H. Friedman in C. R. Phipps, et al. Laser and Particle Beams 14 no. 1 p. 28 (1996)
page 10
higher order correction is K/D = 180 nrads. Thus the tilt angle is considerably less than the
diffraction angle and the loss factor is given by:
Loss factor - (_./D) 2 / [ (_,/D) 2 + rYtilt 2] [5]
= 85%
This figure should be interpreted as a transmission to the target, with maximum value of
1.0 (good). The loss factor can be interpreted as a 180 nrad spot jittering pulse to pulse by an
amount of 75 nrad.
In principle, there should not be a severe problem from nonisoplanaticism due to
relativistic lead angle in ORION.
3.3 When there is, how can we combat it?
Answer: simply by prefacing the high energy pusher pulse with a rapid (lkHz) sequence of
low energy pulses, while continuously reducing the laser footprint in space from the
presumed 50-m diameter handoff circle from the radar or other acquisition system down to
the 0.5-m diameter required for pushing on the target.
Conditions: 1-cm target at 1500 km slant range, 0.28 Bond albedo.
100 photons received by 6-m diameter collector
33 k,l, 1Hz Pulse Sequence
PusherFine A/T
Pulse
Next
Sequence
d I lms*_ Is fl
'fo_,a.e_ II FI .. II
pro_,_ _Jt]l I_t
,, + ++÷-- I I
spot size d s (m) 50 25 10 5 2 1 0.5
W(J) 520 130 21 5.2 0.83 0.21 3.052 3(}qpulse energy
pulses 4 4 4 4 4 4 4
total energy Wtot (J ) 2080 520 84 21 3.3 0.8 0.2 _Total energy for fine A/T: 2.7kJ(9%)
Figure 7
Smart beam dither solves 10ok-ahead/tilt AO conflict
Photonic Associates
page 11
The procedure is very similar to what many modern flash cameras do prior to taking a
picture.
As shown in Figure 7, only the first prepulse has very much energy, because it has to
illuminate a 50-m diameter circle with enough flux to get a 100-photon return from a target
as small as lcm. Successive pulses are much smaller, and the whole sequence uses only 9%
of the total laser energy, including the final pusher pulse. Of course, the smaller pulses
could be designed to return 10,000 photons rather than 100 photons without measurably
changing the energy budget, for greater acquisition speed and reliability.
4: Recommendations
We recommend that 5ns be the ORION laser pulsewidth, for any system designed within
the next 2 years.
We recommend that 100ps be the goal for future systems.
We recommend that this pulsewidth be achieved by an appropriate combination of the best
procedures available at the time.
However, we predict that better pulse shortening will be available when this decision has to
be made, and that the choice of how best to get 100ps pulses will be much more clear at that
time than it is now.
We recommend that all-optical acquisition and tracking be employed. The cost of a radar in
ORION is prohibitive, unless it is absolutely required. Cheap, sunlight-assisted wide field of
view boresight telescopes can help with initial acquisition.
PART '_3--FORMS
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4. I_TL; AND $UITITLt
ORLON Low Cost Laser Analysis
|. AUTHOAtS)
Dr. Claude Phipps
7, PElUORMIN50I_G,_NIZXl'ION NAMI_S) AND AOO/IISSI|S} "
Photonic Associates
1 621 Calle Torreon
Santa Fe, NM 87501
t, SPC)NSOmN_/MONITOmaMO AGeN¢+ _IAMI($j' AND AO'C}_E._SC[S} "
Marshall Space Flight Center, Code PS02
MSFC, AL 35812
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J. RIP_RT TYI)n '_,ND OAI'JI| ¢OViR|+
FINAL REPORT 1 JUL - 15 JUL 96
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H-27250D
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REPORT NUMIIR
PA- NAS - 3
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AOIN_Y _|PORT NUMSill
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_:1. AllITR_CT (Ml+imUffl 2_ wo_J ......
We show that laser-target interaction physics demands the shortest
laser pulse of whicfi-I_ardware is ca-pable (but not less than 100 ps)
in the ORION ground-based laser concept. We compare two leading
ways to achieve such pulses - SRS/SBS cascade compression and
grating compression - with the standard MOPA approach, and conclu<
that the first of these is most robust. However, the state of the art
in laser devices will require a year or two to implement these ideas.
We present a pulse format and beam footprint protocol which will
solve the conflict between relativistic Iookahead and beam tilt and
should.permit all.l, aser active acquisition and trackinq in ORLON.
SUI_|CT T_RMS IS. NUM_JR OF PA(]||
117.
UNCLASSIFIED UNCLASSIFIED
_SN 7_40-01.2B0.S._00
Solid state lasers SBS SRS
Laser plasma interaction
SI'CUIqlI'Y O.ASSItlCIkTION 1i. SIC_ITY-CLA$'$1F[¢ATION
OP RIPOAT OF ?HIS PAOI
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OP A IISTAAC'I"
UNCLASSIFIED UNLIMITED
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53.301.298
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ORION Low Cost Laser Analysis

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