Free Space Laser Communications
Dr. James Jet Propulsion California Institute
Lesh Laboratory of Technology
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[Outline
• Fundamentals • Spacecraft • Ground • Simplified • Recent • Future
of Presentation
]
Technology Reception Systems
Link Calculation
Demonstrations Demonstrations
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Fundamentals
Free Space Propagation • Electromagnetic beams diverge at rates at least as fast as L/d (Diffraction-limit)
_, is the wavelength d is the diameter of the radiation of the transmitting aperture
• RF wavelengths • Optical
usually
in the cm-m range
wavelengths
are in the l.tm range
• The more wavelengths across the aperture, the more narrow the beam divergence
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Deep Space Communications Beam S _read
Voyager (X-Band) at Saturn Optical
(3_lm SIC Anl_ma)
i
at Saturn
(10 cm Telescope)
Era.fit
_\,/"
,,,,
_
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2
Optical
Advantage
Relative
IBased on a Pluto FB
Examplel*
to Ka-Band
I
13 dB -Data Rate Increase (4.9kbp, v= 27ObN)
t
65 dB
J __+--4
26 dB -Smaller SIC Aperture dB -less Transmiltad 7 dB -LowerTransmittar
(10 cmvs2.om) (l_)wv_ 2.7w1
Power Required Efficiency
(s'/, _2s%)
-&
_ "1---
2 dB -Lower System Efficlencles (24% v140 %) 3 dB Almospheric Loss 10 dB - Smaller Ground Station (t0
rn
vs 34 m)
' 5
_P "Same T4 Input Electrical Power
I Comm Li_
.... "1 [
Nomograp h I
leo 140
I'......
114)
!!!
i T4
:!
.o!
6
3
Equivalent
I0( IC 1 0.1 0.01
Fundamentals dB/km Loss for
Free Space
[
40000
150000000
1. 5E +09
--e-- 1 cm XL 1 cm Roy ,_ 0.001 O.O001 rei [-*,-lO_n xt, loom 10cm XI, tmRcv lOcm Xt, lore Rcv Rcv
• _t._.
1E.,_ IE-07 IE-O0 1E-09 IE-10
[...m- lm Xl. lOm I_v
/
d Unk_, km
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1A
A11
7
I Good
•Good News/Bad News Fundamentals
News: beams are more narrow transmitted energy
]
- Optical
- Concentrate Bad News:
on target RCVR
- Optical - Narrow
beams
are more narrow pointed receiver
8
beams must be more precisely signal from intended
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- Must track beacon
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I Spacecraft
Technology
]
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Optical
• • • •
Communications Demonstrator Simplified Optical Design
(OCD) ]
Uses only one steering mh.rorand one detector away for all beam control functions Eliminates many beam relay optics and need for large optical bench All optics are localed on telescope body Piber-coupled laser Iransmiuex signal removes laser heat from optics
DmlC_ON
OF
Tmolwr
lO_.
l
\
--
_r
Optical CommunicationsAssy (TOA) Demonstrator Telescope Optical
]
Ta
I I
Optical
CommunicationSTerminal Demonstrator
i
la
12
6
i,
OCD with Electronic
Assy
Telescope Optics Assembly (TOA) on gimbal Control Electronics and Enclosta'e
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I]
7
(With
Imager)
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15
I ACLAIM
Breadboard
Terminal
ACLAIM
i
- Over_ _memdons: (4" x 4" x 8") - Built fi_m COTS perts -_ cametm/optcomm - Pitt of micr_p6cecntft Im_adlx_rd
2-axis Steenng Mirror
APS Detector Array
(?.36x _6)
Fiber Coupled
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8
X2000 Program Optical Comm Subsystem
i
MultiFunction
Uses:
• Optical Comm (uplink and downlink) • High-Resolmion Imaging
• Science Images • Olxical (Image-based) Navigalion
• Laser Altimetereception R • Uplink Ranging Reception • Downlink Ranging Transmission
I -.1_
Communications
Characteristics:
ii k m
* Beacon Laser Tracking out to 1 AU • Earth-linage Tracking Beyond I AU • Redundant Critical Components
• Lasers,Detectors, Steerin| Mirrors, El_tronics
• > I00kbps(daytime eception)* r • >300 kbps (nighttime reception)* • Mass < 13kg • Powe_ < 38W
T4 " FromEas_a _ a 10-i I'l,am.lmam 17 II_at_
ms I ! i
JPL
• DESIGNED MODULATED, LASER • GOAL: • ACHIEVED: • • USES THREE SOURCE
2-WATT
& DI:MONSTRATED SOUD STARE
LASER
A GREEN AT 50 Kllz WATTS C'W AT
DEVELOPMENT
2 W OF GREEN PULSE 35 RATE (117 WAll'S
INFRARED WAVELENGTH) IO-WATT FIBER-COUPLED AS PUMP COMPANIES
N.. x
m_ SCH1rMATIC DIAGRAM OF THE SET-UP
DIODE-LASER-BARS
SEVERAL COMMERCIAl INTERESTED IN DESIGN
:ii
!
!
PICTURE
or
TI IE SET-UP T4
OU'|'PU'I'
POWER
VS.
INPLrr
POW1ER 18
LTES laser -
is • high
optical
quality terminals
Instrument (LCT's)
that
characterizes
the
performance optical output
of
communications
Measures beam divergence, acquislUon and hacking power, and BERs of LCTs up to 1.4 Gbps data rates Appropdato exchange of beamsplltters to extend from 0.6 S_nno 2 I_m _ and detectors
performance,
allows spectral operating
range
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iGround Reception
Systems
i
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I0
[1-m Optical
• •
Comm R+D Facility]
Optical Comm Telescope Laboratory
(OCTL)
Located at YPL's Table Mountain Facility
2.4 kln (7400 ft) elevation
• l-m diameterperture a • Fast(Ea.qh-orbit) tracking mount • Completion atendof2000
Atmospheric
0.9-
Transmission
Clear Weather
O.II-
0.?-
0.6-
0.5-
04-
0.)-
0.2-
O.I
o.o o.4 o16
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11
IAtmospheric
Visibility
Data 1.
•
Vlslblllty Cumulative
¢=mlnVell mw..Jlml
Dlstrlbutlon
•
AVM _
at 6oidstone,
CA
02)ebiree IRO
•
AVM O_rv=tofy at Table Mtn, CA
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I Deep Space Reception
• • •
_I \\
Station
apertule
I
lO-m collection
Photon bucket (non-diffractionlimited) Segmented
li.I
primary minor
t _ai|
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12
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Simplified
(Signal
• • • • • Calculate Calculate Calculate
Link Calculation
Level at Receiver)
0 =_d
transmit beam divergence, spot diame_r,
Z, at target R meters away using Z=R* 0 spot (n22/4) (D-_eceiver diameter) (receiver area)
area of illuminated
Area of receiver = r,.D2/4 i_ropagation
loss (I 0 is fraction of signal iatercepled
relative to total spot area = D2/Z 2 • Received • power P, (Watts) = P,*_*T.*Tt**T_. - I", = Atmospheric Transmission - Tin= Receive Optics Thruput (photons/sec) hv= 2e-19
_. (in nni_om)
Pt= Transmi_.d power Tw= Transmit Optics Thmput signal rate = Pr/0av)
Received
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13
Simplified
(Background
Background
-
Link Calculation
Level at Receiver)
Effects
Point source interference signals produce a background flux rate over the receive aperture and over a spectral bandwidth (Warts/rr_*nm) if in the detector field-of-view Distributed sources (e.g. daylight) provide a background flux rate over the receive aperture over the entire field-of-view of the receiver (Watts/m2*nm*Sr) Background signals we limited and by d_tector FPV (in St) by narrow band filters of BW (in rim) flux level*Receiver
-
-
Received background power (Pb) = background area*f'tlter BW (*FOV if extended source) Background Noise rate = Pb/(hv)
(in photons/sec)
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Simplified Link Calculation (Detection Performance)
Signal Detection type of detector, levels)
Receiver Inexpensive State-of-the-Art Low Receiver Receiver Rate Rcvr
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]
(performance depends on coding, and background
Type
Sensitivity > 100 photons/bit ~ 10-20 photons/bit
Background/Low
< 1 photons/bit
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14
Comparison
• Optical - Need - But, • Weather links
of Optical and RF Links ]
compared to RF links basis differences
are often
to use a common optical affects
comparison
and RF have some RF and optical weather spatial
fundamental
systems fades
differently infrequently reception from
RF links experience
- Optical must consider the start. • Need enables to develop comparison an optical
diversity
5nk design
methodology for uniqueness
that of
with RF but allows
the two technologies
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Atmospheric
Vlslbmty Monitoring
Dalm
_,R INIOnol
_-_
(_ - atmospbed¢ attenuation;
Zenith AttonuaUon
Aa - atmnustlon
(dB)
Pa - IXOb(attenumHon < o0 moclele 30
uncertainty;
Nots: a must be sdJusled for opemtlomll wmvelengllh billed on known (I.OWTRAN) (If different from rnemsurKl wavelengths), 0ncl for elevation tangle T4
15
Atmospheric attenuation (ct) is a continuous distribution rang_tg from low values (clear conditions) to very high values (due to clouds) Cloud outages impact "Station Availability" - Mitigated by station diversity Need to define what "outage" means Recomngndation - Use AVM data to define atmospheric model - Select a value of tx and the corresponding value of (Pa) • Po - Probability that attenuation < tz • Must be corrected for wavelength and elevation angle - Approximag the AVM distribution by two states • < ct means clear (but with some attenuation) • > (z means (totally) obscured by clouds - Pa determines station availability; a is nominal link attenuation and Act is weather attenuation uno_rtainty (when available)
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Link Analysis Using Weather Model
r
• Analyze link using -_ (dB) for atmospheric transmission +/- Aog2 as the favorable and adverse tolerances
and
• Design link Initially for a "Link Summary" of 0 dB margin using nominal parameter values and calculate the favorable (+at) and adverse (-02) uncertainties
• Calculate "Recommended adverse link uncertainty • Redo link design Link Margin" based (i.e. margin = 2_ z) link margin on the
with a nominal Link Margin" data as a basis
equal
to the
"Recommended - Uses visibility
for link loss and link loss
uncertainty - Provides margin a formal basis for establishing T4 value of link 32
16
Unl
I:)_lan
Conlrol Table
Psn.neter Nominal Fav Adv
Transmit laser power Transmit aperture dia
XXX
o.,
FFF
,. ,
AAA
,.,
Abnosphedc
Trans. (dB)
®
0t
Link Summery (0 (lIB Margin) Recommended Margin (dB)
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•
Link Availability
• Optical reception • Assume weather • Define all ground a station stations systems assume
Analysis]
spatially-diverse are in independent km) if it can and Station" removed
ceils (separated
by few hundred
as a "Candidate when atmosphere elevation
see spacecraft degrees) • Define station a station
above some minimum
angle (say 20 if it is a candidate (i.e. atmospheric 3,
as "Available" T,
and it has clear weather < (_)
attenuation
17
Link Availability
If N stations probability are "Candidate that m of them
Analysis
Stations", are "Available"
(Cont)
then is the
PN(m) =
(N)
m PN(m)
(Pa) m (I"P0_) N'm one of the N staUons
and the probability that at least is able to receive the link is
PN = E
m,,1
=1- (1-P_) N
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Link Availability
Next, consider total time Let N 1 be the number beginning stations N 1 (at the of this change _me, with
Analysis
stations number pass
(Cont)
support at the of candidate duration )ass.
•
i
"pass".
(T) of spacecraft
of candidate and let the time over the to N K at the
T
from
beginning)
,I N2 N1
end of the
NK
• Let the
t_ _,_ h
.,.
t3 _-
_
tx stations be tl
36
corresponding
times
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of N_ candidate
18
b"
Link Availability
Analysis
(Cont)
]
Then, the daily "Expected Data Volume" (EDV) for the link considered above, with the weather station configuration being considered
K
returned and
is
EDV = R _
i-1
tiPNi
where
"R"
is the data
rate in the link design
control
table
RECOMMENDATION
: Use EDV for RFIOptical comparisons
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19
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4¢)
20
I
Ground-Orbit
GOLD
beam upltnk mitiptai
Lasercom
Multiple-beam
eflreetl of 81mtlpherk
Demo
Transmission
Ktalilatlen
(GOLD)
aml beam wander
i
I
Multiple -
Beams am propagated
through different
Itmolpherk:
coherent cells length
Each beam is delayed relative tothe
other by greater than laser's cohenmce
'_'--'_
f
""_
If
'4""
0.6-m Transmitter Telescope T4 41
I Future Demonstrations
I
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21
Demonstration
from ISS
• OCD = (_Jc_
Cmv_munr.lu(n_
Dem_msu-_r
• PN Data dumped to ground at 1 Gbps when over ground site • Ground transmits beacon laser to ISS • ISS Terminal uses beacon to point downlink • Station optical comm terminal can also dump other science data to ground • Can demonstrate space-to-space optical comm if second optica] terminal on Shuttle
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[Location
of Flight Terminal
]
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22
FOCAL Demonstration
|
Flight in 2001
45
Shuttle Link to Ground
1.6 Gbps
|
I
100 10 1050 1 7 5.2 60 1.6 21.3 mW
cm
Transmit Transmit Link Receive
Laser
Power Dia. (Space) range) Dia. (Ground) (space-ground)
Telescope (Slant Telescope
Range
km
m
Atmospheric Losses System Losses Detector Efficiency Data Rate Link Margin
dB dB % Gbps dB
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