Note: Descriptions are shown in the official language in which they were submitted.
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Free space optical communications
The present invention relates to optical communications, and more particularly
relates to high bandwidth
free space optical communications.
The increased need for high bandwidth (high data rate) communication links
induced by the recent growth
of the Internet and mobile communications has led to renewed interest in free
space optical communica-
tion (Whipple, "Free space communications connects", Photonics at work,
Ootober 1999). In free space
optical communications the data are transmitted through a communication link
between a transmitting
station and a receiving station by a laser beam preferably having a wavelength
of about 1550 nm without
using a physical medium such as an optical fibre or the like. Depending on the
weather conditions, com-
munications links over a distance of several kilometres with a bandwidth of up
to 2.5Gbits per second
have been demonstrated (P.F. Szajowski et al., "Key elements of high speed WTM
terrestrial free space
optical communication systems", SPIE paper no. 3932-01). Such free space
optical telecommunications
links are especially useful for connecting facilities having high data
transmission needs with one another,
such as banks and universities in metropolitan areas. Another possible
application is the high bandwidth
live broadcasting of sports events, where an optical free space communication
link can be set up tempo-
rarily at low cost.
In order to avoid health risks associated with laser radiation, the laser
power has to be low (a few milli-
watts) and the beam diameter must be large (about several tens of
centimetres). To establish an optical
free space communication link, the optical signal therefore has to be coupled
out of an optical fibre net-
work and directed with a transmission telescope over the desired distance
directly to the receiving tele-
scope where the received beam has to be concentrated and coupled into another
optical network.
Various aspec"ts of optical free space communication systems have been
described. For example, EP-A-
1,152,555 discloses electroforming replication techniques for the fabrication
of optical mirror elements for
high bandwidth free space optical communication. In addition, EP-A-1,172,949
discloses a free space
optical communication system comprising a first unit having a first
transmitter and a first receiver and a
second unit having a second receiver corresponding to the first transmitter
and a second transmitter cor-
responding to the first receiver, wherein the first and second transmitter and
the first and second receiver
comprise a reflective optical telescope, and optical fibre positioned in the
focal region of the reflective opti-
cal telescope, and a positioning unit for moving the optical fibre in the
direction of the optical axis of the
telescope and within a plane perpendicular thereto. The unit is mounted on a
tip-tilt positioning system
electronically controlled (gimbal). This system provides an optical tracking
function allowing a stable, se-
cure, and high-bandwidth optical communication link.
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US-B-6,411,414 discloses an optical wireless link using wavelength division
multiplexing. And EP-A-
0,977,070 discloses an optical telescope with a shared (Tx/Rx) optical path in
an optical communications
terminal; hcwever, a separate link is provided, taking the beacon signal from
the secondary (dichroic)
mirror.
Furthermore, a problem with available free space optical communication systems
is a lack of power or
redundancy in the signalling, making communications more vulnerable to
atmospheric conditions. Also,
the systems advanced heretofore also tend to involve complex optical
arrangements for handling signals.
There is a need for optical communications terminals and communications
systems that overcome the
aforementioned problems and provide an improved performance. There is a need
for terminals having
optical systems of reduced complexity and component weight, so as to greater
facilitate usage in diverse
environments (e.g. aircraft- or satellite-borne, as well as ground-based).
The present invention provides an optical communications terminal, comprising:
an optical telescope; a
transmitter unit including at least one transmitter coupled to source of
optical signals; a receiver unit for
receiving optical signals; an optical system defining a transmit optical path
between the optical telescope
and the transmitter unit, and defining a receive optical path between the
optical telescope and the receiver
unit; and a beacon detector for detecting beacon optical signals received at
the optical telescope; charac-
terised in that a beacon optical path between the optical telescope and the
beacon detector comprises at
least a portion of said transmit optical path and/or said receive optical
path.
Preferably, the transmitter unit, receiver unit and beacon detector are
disposed at or adjacent the focal
plane of the optical telescope.
In one embodiment: the system, the optical system includes a relay lens and a
first mirror, and the optical
path between said first mirror and the optical telescope is common to the
transmit optical path, the receive
optical path and the beacon optical path. The optical system may include a
beamsplitter between the first
mirror and the receiver unit, the beamsplitter, in use, passing receiver
optical signals along the transmit
optical path to the receiver unit and reflecting beacon optical signals along
the beacon optical path to the
beacon.
Preferably, the transmitter unit includes a plurality of transmitters.
Preferably, for the or each transmitter an aperture is provided in the first
mirror, a separate transmit optical
path thereby being provided from the or each transmitter to the optical
telescope via a respective aperture.
Preferably, the or each transmitter comprises the terminating portion of a
single mode optical fibre, a col-
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limating lens preferably being provided at said terminating portion in a
respective transmit optical path. In
the case of a plurality of transmitters, each transmitter may be fed by the
same optical signal, or may be
fed by a different optical signal. In one embodiment, there are three
transmitters.
Preferably, the beacon optical path includes a second focussing lens between
said beamsplitter and the
beacon detector. Preferably, the beacon optical path includes a filter system
between said second focus-
sing lens and the beam detector, the filter system preferably including, in
sequence, a filter passing a first
predetermined frequency and a neutral density filter. The first predetermined
frequency is, for example,
830nm.
Preferably, the receiver unit includes one receiver for receiving optical
signals at a second predetermined
frequency, different to said first predetermined frequency, said second
predetermined frequency prefera-
bly being 1550 nm. T he receiver may comprise a terminating portion of a
multimode optical fibre.
In accordance with another aspect of the invention there is provided an
optical communications terminal,
comprising: an optical telescope; a transmitter unit coupled to source of
optical signals; a receiver unit for
receiving optical signals; an optical system defining a transmit optical path
between the optical telescope
and the transmitter unit, and defining a receive optical path between the
optical telescope and the trans-
mitter unit; and characterised in that the transmitter unit comprises a
plurality of transmitters, each trans-
mitter being coupled to a respective source of optical signals.
In accordance with another aspect of the invention there is provided optical
free space communications
system, comprising: a first optical communications terminal, the first optical
communications terminal be-
ing a terminal according to any of claims 1 to 30 of the appended claims; and
a second optical communi-
cations terminal, the second optical communications terminal being a terminal
according to any of claims
1 to 30 of the appended claims; wherein the first optical communications
terminal and the second optical
communications terminal are arranged whereby, in use, the transmitter unit of
the first optical communica-
tions terminal may transmit said optical signals to the receiver unit of the
second optical communications
terminal and the transmitter unit of the second optical communications
terminal may transmit said optical
signals to the receiver unit of the first optical communications terminal.
An advantage of the present invention is that the same optical system that is
used to send and receive
high data rate optical signals is also used simultaneously by beacon optical
signals for pointing, acquisi-
tion and tracking purposes.
Another advantage is that by disposing a greater proportion of the hardware in
or near the focal plane,
good optical alignment of the Tx and Rx beams can be attained and maintained.
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A further advantage is that the use of multiple transmitters and multiple air
paths enables a greater total
power of signal to be employed. If several identical signal beams are sent,
there is less susceptibility to
error; and if several different signal beams are sent, the total data rate is
higher.
Embodiments of the invention will now be described, by way of example, with
reference to the accompa-
nying drawings. In the following, various embodiments are described, including
an optical communications
terminal in accordance with one aspect of the invention and adapted to be
mounted on the ground (here-
after "ground demonstrator"). The drawings are briefly described as follows.
Figure 1 is a schematic diagram of a free space optical communication system
in accordance with one
aspect of the invention.
Figure 2 is a hardware tree for the ground demonstrator in accordance with one
aspect of the invention.
Figure 3(a) is a schematic view of the optical configuration of the ground
demonstrator.
Figure 3(b) shows transmission at 1550 nm in the ground demonstrator: optical
layout (only one of the 3
beams is shown).
Figure 3(c) shows beam shape (1550 nm) at the Rx telescope (only one of the 3
beams is shown).
Figure 3(d) shows reception at 1550 nm: optical layout (only one of the 3
beams is shown).
Figure 3(e) shows reception at 1550 nm: spot diagram (only one of the 3 beams
is shown).
Figure 3(f) shows Transmission of the beacon at 830 nm: optical layout.
Figure 3(g) shows beam shape (beacon at 830 nm) at the Rx telescope.
Figure 3(h) shows reception of the beacon at 830 nm: optical layout.
Figure 3(i) shows Reception of the beacon at 830 nm: spot diagram.
Figure 4 shows the positions of the optical components (only one Tx is shown),
(a) in longitudinal cross-
section, and (b) in rear view.
Figure 5 shows the Optical layout of the R-C telescope.
Figure 6 shows the Telescope Assembly, (a) in longitudinal cross-section, and
(b) in front view..
Figure 7 shows the Hardware Tree for the Indoor Units for the ground
demonstrator.
Figure 8 shows the functional block diagram of the Transmitter for the ground
demonstrator.
Figure 9 shows the transmitter unit indoor cabling for the ground
demonstrator.
Figure 10 shows the functional block diagram of the Receiver for the ground
demonstrator.
Figure 11 shows bit error rate (BER) as a function of the extinction ratio at -
25 dB peak received power
for the ground demonstrator.
Figure 12 shows BER as a function of the peak received power at 8.2 extinction
ratio for the ground dem-
onstrator.
Throughout this description, like numerals are used to denote like elements.
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1. Introduction
There has been developed a low-cost lightweight terminal designed for Free
Space Optics (FSO) com-
munication, for example between collocated spacecrafts in geostationary orbit.
Based on the use of the
lightweight mirrors produced by Media Lario s.r.L's proprietary electroformed
replication technology, the
terminal presents the following advantages:
~ simple design with minimum number of components
~ compact and light mass system, based on advantages of Nickel replicated
mirrors
~ large field of view in the focal plane of the telescope
easy access to focal plane for tracking and communication purposes
~ uniform power distribution inside the transmitted (Tx) bearn; minimum losses
~ high coupling in reception of Rx beam in Rx multi-mode fibre optics
~ possibility to use gimbals systems for Pointing, Acquisition and Tracking
without the necessity to
include fast tracking devices
~ symmetrical system to allow the link between any couple of terminals of a
given constellation
The optical communications terminal or ground demonstrator described herein is
based on an architec-
tural design, where appropriate using commercial components with the purpose
of implementing the func-
tion of the architecture of the optical head at a low cost and therefore at a
low risk. This is a necessary
step in the development of a~low cost lightweight Inter-Satellite link (ISL)
terminal.
The following description is of a ground demonstration terminal designed for
communication at 2.5 Gbit/s
between ground stations at a relative distance of 1.1 km. Only minor
modifications, simplifications and
improvements are needed compared to the terminal design for the ISL scenario.
The terminal configura-
tion for the ground demonstrator uses "multi-beam transmission" (three Tx
beams) for the compensation
of atmospheric scintillation. Additionally, some optical bench components in
the focal area of the tele-
scopes have been adapted for use with Ritchey-Chretien telescopes available
from Media Lario; for this
purpose three additional lenses have are used in order to extract the focus
and make it accessible to ac-
commodate the Rx fiber optics, the Tx fiber optics and the CCD camera. For the
usage of the ground
demonstrator under standard atmospheric environment and nominal operational
conditions (ground appli-
cation) with the same main technical solutions and concepts relative to the
optical components and to the
telecom equipment as for the Inter-Satellite link scenario, the tracking
system may be simplified: it may be
constituted by simple manual positioners to guarantee correct pointing and
tracking only for the short peri-
ods during the optical verifications.
2. System Architecture
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2.1 Overall Configuration
Referring to Fig. 1, two terminals, one transmitter 102 and one receiver 104,
consist each respectively of
two subsystems, i.e. the Outdoor Unit 1 06, 108 (composed by the Optical Head
114, 116 and the Pedes-
tal) and the Indoor Unit 110, 112 (the Transmitter Indoor Unit for the
Transmitter Terminal and the Re-
ceiver Indoor Unit for the Receiver Terminal), as discussed further
hereinafter. The Optical Head 114, 116
is identical both for the Transmitter Terminal 102 and the Receiver Terminal
104; it comprises the Tele-
scope, which is mounted on the Pedestal that provides manual gimbals movement
for the alignment to the
counter terminal. The Optical Bench includes all the components in the focal
plane of the R-C telescope.
The Optical Head 114, 116 is a compact assembly; it is suitably installed on
an exposed site providing the
necessary field of view with the remote terminal, without obscuration.
The Transmitter Indoor Unit 110 and the Receiver Indoor Unit 112 are connected
respectively to the
Transmitter and to the Receiver Optical Heads 114, 116, respectively.
The Indoor Units 110, 112 include all the electronics and the optoelectronics
circuits and devices required
to supply the required power, to convert the RF signals into the optical ones
and vice versa and to drive
the lasers.
2.2 Functional Description
Fig. 2 is a hardware tree for the ground demonstrator in accordance with one
aspect of the invention.
The Optical Head (114, 116) is the core of the free-space connection between
two terminals 102, 104. It is
constituted mainly by a Ritchey-Chretien telescope and by the opto-mechanical
components to transmit
and receive the optical signals from the Tx fiber optics to the Rx fiber
optics.
The Transmitter Indoor Units (110) and the Receiver Indoor Units (112)
supervise the operation of the
Terminal and manages the control communication. The Indoor Units 110, 112
interface all the electronic
sub-systems through a dedicated communication bus.
The main sub-systems of the Indoor Units (110, 112) are the Receiver Control
Electronics and the Trans-
mitter Control Electronics; they supervise the operation of Tx and Rx modules
respectively by managing
the required power, the enabling and the control signals and by monitoring
their operational parameters in
order to detect faults and failures. The transmitter control electronics also
supervises the operation of the
optical amplifier.
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Pointing and acquisition are monitored by the CCD detector/camera 206 (in the
Optical Head 114, 116)
and the electronics required for its operation (included in the Receiver
Indoor Unit); its goal is the determi-
nation of the signal power and centroid co-ordinates of the signal collected
by the CCD camera 206.
The pointing is performed through the gimbals manual mechanism (not shown) of
the Pedestal 118, 120
on which the optical head 114, 116 is mounted, based on the maintenance of the
signal received by the
CCD camera 206 on a reference position set under laboratory conditions.
The acquisition is performed automatically once the pointing has been
performed, being the transmitter
and the receiver optical axis of the terminal set parallel under laboratory
conditions.
Apart from the previously standard terminal functional operation, if needed
the optical components can be
moved from their positions so that typical experimental tests will be set with
the goal to test the optical
performance and the characteristics of the terminal, its stability and its
degree of optimisation.
2.3 Interfaces
The Terminal (102, 104) possesses the following interfaces:
~ Optical interface
~ RF interface
~ Power supply and grounding
~ Mechanical mounting
And only the first of these will be discussed, for brevity.
Optical Interface
The optical design of the terminal 102, 104 has been performed assuming that
no protective optical glass
will be present in front of the terminal.
The optical constraint is that the lines of view (200 mm diameter for the 1550
nm radiation and 9 mm for
the 830 nm beacon radiation, plus divergence) between the two connected
terminals must be maintained
free from mechanical obstructions.
3. Optical Head (102, 104)
3.1 Overview on the Optical Head Configuration
Figure 3(a) is a schematic view of the optical configuration of the ground
demonstrator. The Optical Head
configuration (the same for the Transmitter Terminal 102 and for the Receiver
Terminal 104) is shown.
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The optical head 114, 116 includes a telescope generally designated 300 in
which the incoming and out-
going beams are reflected by primary mirror 302 and secondary mirror 304.
The transmitted beams are supplied by three SM fibre optic cables 306
terminated by three collimation
lenses 308. The three beams pass through a hole mirror 310 having three
respective holes (not shown),
and pass through a relay lens 312 to the secondary mirror 304. The beams are
thence reflected via pri-
mary mirror 302 on the outgoing transmission path from the optical head.
The incoming beam, via primary mirror 302 and secondary mirror 306, passes
through relay lens 312 to
the hole mirror 310. The hole mirror 312 reflects the beam transversely to a
beamsplitter 314 that sepa-
rates 830nm and 1550nm light radiation. At the beamsplitter 314, the 1550nm
light beam passes directly
though, is focused by focusing lens 316 onto receiver multi-mode fibre optics
318 that is mounted on a x-
y-z positioner (not shown).
Also at the beamsplitter 314, the 830nm light beam is reflected at right
angles to the 1550nm beam, and
after focusing by second focusing lens 320, passes though, in succession, a
830nm filter 322 and a neu-
tral density filter 324 and is received at a receiver CCD 206.
Item 328 denotes Tx single mode fibre optics (for the beacon at 830nm), and
item 330 a simple objective
lens for focusing the beacon laser beam.
The following apply:
~ Each telescope 300 is used as transmitter and receiver at the same time.
~ In the focal plane of the telescope the series of optical components allow
simultaneously to
transmit and to receive optical signals at the identical wavelength of A =
1550 nm at a data rate of
2.5 Gbit/s.
~ One beacon (light beam) at a wavelength of 830 nm is used for pointing and
acquisition purposes.
Its divergence is always maintained as large as 3.0 mrad. It is transmitted
through a separate
simple lens with useful optical diameter of 9 mm.
~ In the focal plane of the R-C telescope 300 the Rx signals at 830 nm and
1550 nm are separated
by the beamsplitter 314 and directed to the CCD 206 and to the Rx multi-mode
fiber optics 318
respectively. An additional mirror 310 with three small holes separates
mechanically the Rx sec-
tion (CCD and Rx multi-mode fiber optics 50/125 Vim) from the Tx laser beams
(full beam diver-
gence = 190 brad a~ 1/e2 power angle; wavelength = 1550 nm; power of 1 mW out
of each of the
three Tx single-mode fiber optics) assuring optical isolation.
~ The utilization of three transmitters reduces greatly the fluctuations of
the intensity of the Rx beam
caused by the turbulence of the atmosphere.
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~ Three achromatic doublets 312, 316, 320 (diameter 25.4 mm) are used to
extract the focus from
the vertex of the primary mirror 302 to the area in the back part of the
telescope 300 where the
optical components can be accommodated.
~ The optical components are mounted on translation and rotation stages (not
shown) to allow their
correct fixation and alignment.
3.2 Optical Design
This will be described below with reference to the various optical components.
In Figs 3(b) to 3(i) are
shown various (Tx and Rx) ray pays and spot diagrams for the telescope 300
according to one embodi-
ment.
Figure 3(b) shows transmission at 1550 nm in the ground demonstrator: optical
layout (only one of the 3
beams is shown). Figure 3(c) shows beam shape (1550 nm) at the Rx telescope
(only one of the 3 beams
is shown). Figure 3(d) shows reception at 1550 nm: optical layout (only one of
the 3 beams is shown).
Figure 3(e) shows reception at 1550 nm: spot diagram (only one of the 3 beams
is shown).
Figure 3(f) shows Transmission of the beacon at 830 nm: optical layout. Figure
3(g) shows beam shape
(beacon at 830 nm) at the Rx telescope. Figure 3(h) shows reception of the
beacon at 830 nm: optical
layout. Figure 3(i) shows Reception of the beacon at 830 nm: spot diagram.
3.3 Telescope Focal Plane
3.3.1 The Optical Components
3.3.1.1 Position of the Optical Components
The position of the optical components is presented in Fig. 4.
The following should be noted:
~ The focus of the 830 nm beacon is focalised shifted (Ox= +55 pm; ~y= +55 Nm
) with respect to
the centre of the CCD. His is due to the fact that the optical axis of the Tx
beacon is shifted (Ox=
+123.7 mm; ~y= +123.7 mm) with respect to the optical axis of the Tx Ritchey-
Chretien telescope
300.
~ The fiber optics of the Tx beacon is 0.841 mm in intrafocal position to
increase the divergence of
the Tx beam; back focal length of the beacon lens is 46.641 mm at the
reference wavelength of
830 nm.
~ The focal plane where the Rx fiber optics is placed (18.0 mm from the
filter) is the plane when a
collimated beam is collected by the Rx telescope.
~ The three arms of the spider do not intercept radiation of the three Tx
beams that are placed at 60
deg with respect to the beams.
Figure 5 shows the Optical layout of the R-C telescope.
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3.3.1.2 The R-C Telescope
The telescope is a Ritchey-Chretien reflector (cf. Figs 2 and 4) designed to
have reduced dimensions, a
large field of view and the possibility to accommodate the needed optical
components in its focal plane.
The telescope optical and dimensional characteristics are given below:
~ Optical configuration: Ritchey-Chretien
Primary mirror: Diameter = 200 mm (hole diameter = 20 mm)
Radius of curvature = 315.8 mm concave
Conic constant = 1.0667 (hyperbola)
~ Secondary mirror: Diameter = 52 mm
Radius of curvature = 110.8 mm convex
Conic constant = -4.573 (hyperbola)
~ Distance between mirrors = 120 mm
~ Distance between secondary mirror and focal plane = 120 mm (without
additional optical compo-
nents in the focal plane)
~ Effective focal length of the telescope = 500 mm
~ Effective numerical aperture = 0.2
~ Effective focal ratio = f/2.5
~ Coating of primary and secondary mirrors = gold
~ Reflectivity of the gold layer (at ~ = 1550 nm and ~ = 830 nm) ~ 98%
Figure 6 shows the Telescope Assembly, (a) in longitudinal cross-section, and
(b) in front view. The tele-
scope assembly is a compact unit, which can easily be handled without
significant risks. In order to mini-
mise the influence of the mechanical interface and environmental conditions,
the telescope is mounted to
the optical bench 600 by means of three stainless steel blades 602 distributed
at a distance of 120
around the outer edge of the telescope (300). The blades are attached to the
spider 604 on one side. The
blades are arranged such that the stiffness in tangential and longitudinal
direction of the mirror 302 is high
while the stiffness in the radial direction is low, thus allowing for nearly
unconstrained thermal expansion.
Returning to Figs 3(a) and 4, details of each of the optical components in the
illustrated embodiment will
be given.
3.3.1.3 Relay Lens and the Focusing Lenses
The Relay Lens 312 and Focusing Lenses 316, 320 are achromatic doublets
introduced in the optical
head to extract the focus of the Ritchey-Chretien telescope 300 from its inner
position to an outer position
to accommodate the components of the focal plane. These lenses are identical.
They have been designed
for this specific purpose; additionally the beam emerging from the Relay Lens
312 is collimated with ad-
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vantages during its integration and for its propagation through the
beamsplitter 314.
Availability: the Relay Lens 312 and the Focusing Lenses 316, 320 are
available from China Daheng Cor-
poration (China). The technical characteristics of this specific product are
the following:
~ Type: cemented achromatic doublet
~ Materials: LAKN22 and SFL6
~ Diameter: 25.4 mm +0.0 / -0.2 mm
~ Clear aperture: 23 mm
~ Radii: 25mm, 18 mm, 81.66 mm
~ Central thicknesses: 9 mm and 3 mm ~0.1 mm
~ Surface quality: 60-40
~ Focal length: 48 mm ~2%
~ Surtace figure: 1.5 ~, (vis)
~ Coating: AR at 1550 nm
~ Back focal length ~~-4550 nm - 31 mm.
3.3.1.4 The Hole Mirror
The Hole Mirror 310 has the purpose to reflect the Rx radiation at 830 nm and
1550 nm respectively to the
CCD 206 and to the Rx multi-mode fiber optics 318, while the Tx radiation is
transmitted through three
holes of 3 mm in diameter of the mirror 310 itself. The amount of Rx power
blocked by the holes is about
0.6 dB. The Hole Mirror 310 optical and dimensional characteristics are given
below:
~ Coating: gold
~ Diameter: 50 mm
~ Number of holes: 3
~ Holes diameter: 3 mm
Availability: the Hole Mirror is available from Gestione Silo Sr.l. (Italy).
3.3.1.5 The Beamsplitter 830/1550 nm
The Beamsplitter 314 has a coating on the 45~ facet so to reflect the 830 nm
received signal to the CCD
206, and to transmit the 1550 nm signal to the Rx fiber optics 318. The
Beamsplitter 314 is available (Part
Number: 47-7437) from Optarius (UK).
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3.3.1.6 Tx Collimation Lens
The Tx Collimation Lens 308 is a small lens placed just in front (3.644 mm) of
each of the three Tx
fiber optics to malee the signal more converging (full beam divergence = 190
brad @ 1/e2 power an-
gle). In this way the Tx beams, whose divergence is due mainly to diffraction
effects, have a very well
corrected Gaussian profile. Advantages of this configuration are that only
small areas (f~~ 12 mm
each) of the Ritchey-Chretien telescope 300 are used by the Tx beams (the
telescope has a Wave
Front Error WFE < ~ l4 P-V in any circular area with Qj = 20 mm) and the
obscuration of the secondary
mirror 304 is avoided.
Availability: the Tx Collimation Lens 308 is available (Code: A45-976) from
Edmund Scientific (USA).
3.3.1.7 Rx MM fiber optics
The Rx MM fiber optics 318 is a standard multi-mode fiber optics (Corning~
50/125) used for tele-
communication.
Availability: this fiber optics is manufactured by Corning (USA) and available
from LIGHTECH (Italy)
3.3.1.8 Tx SM fiber optics for 1550 nm
The Tx SM fiber optics 306 for the transmission of the 1550 nm signal is a
standard single-mode fiber
optics (Corning~ SMF-28TM) used for telecommunication.
Availability: this fiber optics 306 is manufactured by CORNING (USA) and
available from LIGHTECH
(Italy).
3.3.1.9 Tx Beacon Simple Lens
The beacon is transmitted through a simple objective lens placed outside the R-
C terminal. The diver-
gence of the beacon of 3.0 mrad (full beam divergence @ 1 /e2 power angle) is
obtained by positioning
of the fiber optics through which the beacon is emitted in intrafocal
position.
Availability: the Tx Beacon Simple Lens is available (Code: A45-486) from
Edmund Scientific (USA).
The technical characteristics of this specific product are the following:
3.3.1.10 Tx SM fiber optics for the beacon at 830 nm
The Tx fiber optics through which the beacon is emitted is a standard single-
mode fiber optics (M~
FS-SN-4224) used for telecommunication.
Availability: this fiber optics is manufactured by 3M (USA) and available from
LIGHTECH (Italy).
3.3.1.11 Filter 830 nm
[20040506] - 12 -
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The high sensitivity of the CCD camera 206 (~ -95 dBm/px at 830 nm) requires
avoiding as much as
possible the presence of background radiation. An IR band bass rejection
filter 322 has been therefore
selected to be placed in front of the CCD. Considering that:
~ the stability of the wavelength of the selected 830 nm Tx laser is X10 nm
~ the typical tolerance of the central wavelength of this is type of filters
is about t10 nm
~ the typical tolerance of the band pass (FWHM) of this is type of filters is
about t10 nm
the filter band pass has been selected with enough bandwidth (FWHM = 50 nm) to
cover the above
tolerances, but not too large to avoid radiation from background.
Availability: the Filter 830 nm is available (Part Number: 47-7436) from
Optarius (UK).
3.3.1.12 Neutral Densit Fy filter
The high sensitivity of the CCD camera 206 (=-95 dBm/px ~t 830 nm), the
background radiation of the
sky and the of the sky and the high intensity of the radiation of the beacon
require the utilisation of a
filter to reduce the intensity of the radiation collected by the CCD camera. A
neutral density filter 324
(in addition to the band pass filter 322 at 830 nm) is therefore placed in
front of the CCD 206.
Availability: the Natural Density Filter is available (Part Number: FSR-OD300)
from Newport (USA).
3.3.1.13 The CCD Camera
The selected CCD 206 has been chosen being available as off-the-shelf
equipment while being sensi-
tive at 830 nm wavelength (after removal of the internal IR cut filter).
Availability: the CCD Camera 206 (Model: XC-75CE (internal IR cut filter
removed)) is available from
Sony (USA).
3.3.2 The mechanical supports
3.3.2.1 Supports for the Relay lens
The relay lens 312 is mounted on:
~ Translating lens mount for 1" optics, manufactured by Thorlabs Inc. (USA),
code LM1XY/M,
catalogue 2003, page 117, that allows translation adjustments of +1 mm in x
and y.
~ Single axis steel translation stage, manufactured by Melles Griot (USA),
code 07TES502
side drive, catalogue 2003, page 28.6, that allows translation of +3 mm in z.
3.3.2.2 Sua!ports for the Hole Mirror
The Hole Mirror 319 is mounted on:
~ Lens mount for 2" optics, manufactured by Thorlabs Inc. (USA), code LMR2/M,
catalogue
2003, page 97.
~ Two single axis steel translation stages, manufactured by Melles Griot
(USA), code 07 TES
502 side drive, catalogue 2003, page 28.6, that allcws translation of t3 mm in
x and y.
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3.3.2.3 Supports for the Tx SM fiber optics for 1550 nm and for the
collimation lens
Each of the three Collimation Lenses 308 is mounted inside a cylindrical tube
connected to the corre-
sponding Tx fiber optics 306. These three systems are then inserted inside a
larger tube that is
mounted on: Gimbal mount for 1" optics, manufactured by Thorlabs Inc. (USA),
code GM100/M, cata-
logue 2003, page 84, that allows tip/tilt with resolution of about 25 arcsec.
3.3.2.4 Supports for the Beamsplitter 830/1550 nm
The Beamsplitter 314 is mounted on: Lens mount for 2" optics, manufactured by
Thorlabs Inc. (USA),
code LMR2/M, catalogue 2003, page 97.
3.3.2.5 Supports for the Focusing Lens of the CCD
The Focusing Lens 320 of the CCD 206 is mounted on: Translating lens mount for
1" optics, manufac-
tured by Thorlabs Inc. (USA), code LM1XY/M, catalogue 2003, page 117, that
allows translation ad-
justments of t1 mm in x and y.
3.3.2.6 Supports for the Focusing Lens of the Rx fiber optics
The Focusing Lens 316 of the Rx fiber optics 318 is mounted on: Translating
lens mount for 1" optics,
manufactured by Thorlabs Inc. (USA), code LM1XY/M, catalogue 2003, page 117,
that allows transla-
tion adjustments of t1 mm in x and y.
3.3.2.7 Supports for the Filter at 830 nm and the Neutral Density Filter
The filters 322, 324 are mounted on a holder connected to the CCD camera 206.
3.3.2.8 Supports for the CCD camera
The CCD camera 206 is mounted on: Single axis steel translation stage,
manufactured by Melles Griot
(USA), code 07TES502 side drive, catalogue 2003, page 28.6, that allows
translation of +3 mm in z.
3.3.2.9 Supports for the Rx fiber optics
The Rx fiber optics 318 is mounted on:
~ Fiber adapter, manufactured by Thorlabs (USA), code SM1 FC, catalogue 2003,
page 116.
~ Translation stage, manufactured by Thorlabs (USA), code ST1XY-S/M, catalogue
2003, page
123, that allows translation of 13.25 mm in x and y.
~ Single axis steel translation stage, manufactured by Melles Griot (USA),
code 07TES502 side
drive, catalogue 2003, page 28.6, that allows translation of t3 mm in z.
3.3.2.10 Supports for the Tx Beacon Simple Lens
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The Beacon Lens 320 is mounted inside a cylindrical tube attached to the
vertical plate of the optical
head 114, 116.
3.3.2.11 Supports for the Tx SM fiber optics of the beacon at 830 nm
The Tx fiber optics of the beacon is mounted on:
~ Fiber adapter, manufactured by Thorlabs (USA), code SM1 FC, cat. 2003, page
116.
~ Translation stage, manufactured by Thorlabs (USA), code ST1XY-S/M, catalogue
2003, page
123, that allows translation of 13.25 mm in X and Y.
~ Single axis steel translation stage, manufactured by Meiles Griot (USA),
code 07TES502 side
drive, cat. 2003, page 28.6, that allows translation of t3 mm in Z.
3.3.2.12 Supports for additional filters
Free space in front of the focusing lenses and in front of the Tx fiber optics
has been left to insert, if
needed, additional filters in case excessive radiation from external sources
not considered in the pre-
sent embodiment that could prevent the correct performance of the system.
4. Pedestal
The Pedestal 118, 120 is the support on which the Optical Head 114, 116,
respectively is mounted. It
is very stiff and heavy and provides therefore a stable support for the
optical head.
The pedestals 118, 120 provide an azimuth and elevation manual adjustment
capability so that the
terminal 102, 104 can be aligned with respect to the counter terminal 104,
102; the elevation and azi-
muth ranges are about 1200 mrad, quite large also to compensate possible
misalignment during the
initial installation of the terminals in the sites for the operational field
tests.
The interface between the pedestal and the optical head is the horizontal
aluminium plate of the Ped-
estal 118, 120 (with 6 holes fdJ 8.5 mm) and the base plate of the Optical
Head 114, 116 (with 6 holes
M8).
5. Indoor Unit
This section describes the design of the optoelectronic equipment of the
ground demonstrator. The
design is based on off-the-shelf components as far as possible.
The hardware tree of the optoelectronic equipment is shown in Fig. 7. It
mainly consists in three sec-
tions.
~ The transmitter unit 702 consists in tvuo RF splitters 704 that divide the
input clock and data
into two pairs of three identical signals that in turn are applied to three
transmitter lasers 706.
The unit also contains the beacon laser 708.
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~ The receiver unit 710 contains the receiver 712 that converts the input
optical signal into the
RF clock and data signals.
~ The CCD camera and the frame grabber.
5.1 Transmitter Unit
The block diagram of the transmitter unit 702 is shown in Fig. 8. The input
clock and data RF signals
are split by two passive devices 802, 804 in two pairs of three identical
signals that in turn are applied
to three transmitter lasers.
The transmitter unit 702 also contains the beacon laser. As an option an
external oft-the shelf optical
amplifier can be used on one channel. In this case the other two channels are
switched oft. The optical
amplifier is considered an instrument rather than part of the optoelectronic
equipment.
5.1.1 Radio Frequency Spiitter
Two passive identical RF 1:4 splitters (not shown), as are well known in the
art, are used to split the
clock and the data signals into four channels. One of the channels is not used
and terminated by a 50
S2 impedance.
The only main design challenge related to the splitter is the requirement to
reduce to a minimum the
relative phase shift of the si gnats in the different splitter arms.
5.1.2 Optical Transmitter
Each of the three optical transmitters 806, 808, 810 is made by an off-the-
shelf transmitter laser
mounted, by soldering, on a custom board.
5.1.2.1 Transmitter Laser
The laser transmitter 806, 808, 810 used is Photon Technology PT9552-6-10-AA-
FC. It is a complete
24 pins transmitter that converts the input RF clock and data signals into a
modulated 1550 nm laser
beam launched into a single mode fiber optics pigtail.
5.1.2.2 Transmitter Laser Board
On the transmitter board two switches may be used on pins #5 and #6 whereas
two output buffers on
pins #2 and #3 allow the possible readout of the laser bias and laser current.
5.1.3 Beacon Laser Subassembly
The beacon laser subassembly is made by an off-the-shelf laser mounted on an
off-the shelf driving
board.
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5.1.3.1 Beacon Laser
The beacon laser 708 used is PD-LD Inc.'s PL83 series.
5.1.3.2 Beacon Laser Driver
The driving of the beacon laser and the control of its output optical power is
accomplished by an off-
the-shelf driver 709 (see Fig. 7), model CCA by Roithner Lasertechnik. The
driver is available as a
mounted printed circuit.
5.1.3.3 Integration of the Beacon Laser Subassembly
The beacon laser diode is integrated on the driver by direct soldering of its
pins on the driver board.
A twin cable internal to the transmitter unit is soldered on the driver power
supply pin-through-holes
and connected to the 5 V power supply connector of the power supply unit (see
Section 5.1.4).
5.1.4 Power Supply
The power supply 711 accepts as input either 220 VAC or 12 VDC. The output
power supply is at 5
VDC, 10 W.
5,1.5 Case and Harness
The receiver telecommunication equipment is housed in a standard case for a
19" rack, 1 U. The
transmitter indoor unit internal cable connections are shown in Fig. 9.
5.1.6 External Interfaces
A summary of the transmitter unit interfaces is listed in Table 5.5.
Interface Type # Description
Optical
Transmitted Output 4 Single mode fibre, FC connector
data
Beacon Output 1 Single mode fibre, FC connector
Spare Output 1 Single mode fibre, FC connector
Electrical
Clock Input 1 Unbalanced, 50 S2, SMA connector
Date Input 1 Unbalanced, 50 S2, SMA connector
Control Output 1 D9 connector
Power supplyInput 1 230 VAC or 12 Voc
Mechnical
Case type - Rack 19", 1 U
Table 5.5. I ransmitter unit interfaces.
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5.2 Optical Amplifier
As an option an external off-the-shelf bench-top optical amplifier (i.e. the
optical EDFA, IPG Photonics
EAD-1-C) can be used on one of the output optical channels (see Fig. 8) _ In
this case the other two
channels are switched off. Since the transmitter has a peak power of 3 dBm,
the peak output of the
amplifier is at 33 dBm equivalent to 2W.
5.3 Receiver Unit
The block diagram of the receiver unit 710 is shown in Fig. 10. The input
optical signal 1002 is de-
modulated and the clock 1004 and data RF 1006 signals generated as output by
the optical receiver
712. Refer to Section 5.1.3 for the beacon laser subassembly description.
5.3.1 Optical Receiver
The optical receiver 712 is made by an off-the-shelf receiver mounted, by
soldering, on a custom
board.
5.3.1.1 Receiver
The optical receiver 712 used is Photon Technology PT0236-6-FC. This is a
complete receiver with
data retiming and clock recovery based on an InGaAs APD and supply with a 50
pm core multimode
fibre optics. Figure 11 shows bit error rate (BER) as a function of the
extinction ratio at -25 dB peak
received power for the receiver in the ground demonstrator. Figure 12 shows
BER as a function of the
peak received power at 8.2 extinction ratio for the receiver in the ground
demonstrator.
5.3.1.3 Receiver Board
On the receiver board, an output buffers on pin #23 allows the possible
readout of the average input
optical power.
5.3.2 Power Supply
The power supply 1008 accepts as input either 220 Vac or 12 VDC. The output
power supply is at 5
VDC, 10 W.
5.3.3 Case and Harness
The receiver telecommunication equipment is housed in a standard case for a
19" rack, 1 U. The re-
ceiver indoor unit internal cable connections are shown in Fig. 13.
5.3.4 External Interfaces
A summary of the receiver unit intertaces is listed in Table 5.9.
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Interface Type # Description
Optical
Received dataInput 1 50 ~Im core multi mode fibre, FC
connector
Beacon Output 1 Single mode fibre, FC connector
Spare Output 1 Single mode fibre, FC connector
Electrical
Clock Output 1 Unbalanced, 50 Ohm, SMA connector
Data Output 1 Unbalanced, 50 Ohm, SMA connector
Control Output 1 D9 connector
Power supply Input 1 230Vac or l2Vdc
Mechanical
Case type Rack 19", 1 U
Fable 5.9. Transmitter unit interfaces.
5.4 CCD Camera and Frame Grabber
The CCD camera and the frame grabber are used for detection of the beacon
signal. They have been
both selected from off-the-shelf devices.
5.4.1 The CCD Camera
The CCD camera suitably used is Sony's XC-75CE. The scanning is at 625 lines
operated at both 2:1
interlaced and non-interlaced mode.
5.4.2 The Frame Grabber
The frame grabber 714 (See Fig. 7) is suitably model IC-PCI-2.0 by Imaging
Technology Inc. plus the
AM-VS acquisition module by the same company.
6. Conclusions
The exemplary communications terminal has the following features:
~ A total number of 3 transmit beams is used to reduce the effects of the
scintillation of the at-
mosphere.
~ Relay lenses have been introduced in order to allow the utilization of the
desired R-C tele-
scope design.
~ Comrnercial off-the-shelf components are used, where appropriate, for the
electrical and elec-
tronics devices. It will be appreciated by persons skilled in the art that
other equivalent com-
ponents may be employed for space applications.
~ In the described embodiment, for the sake of illustration, a simple manual
pointing and track-
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ing system has been included to perform the optical communication tests at 2.5
Gbit/s; al-
though it will be appreciated that non-manual systems may also be used.
These features do not fundamentally modify the architecture and the
functionality of the terminal,
compared with the ISL embodiment. They improve the availability of the link
under ground environ-
mental conditions.
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ANNEX1
List of acronyms
AC Alternate Current
AD Applicable Document
APD Avalanche Photo Diode
AR Anti Reflection
BER Bit Error Rate
CCD Charge Coupled Device
DC Direct Current
EDFA Erbium Doped Fiber Amplifier
ESA European Space Agency
ESTEC (ESA) European Space Research and
Technology Centre
FO Fiber Optics
FSO Free Space Optics
FWHM Full Width Half Maximum
H/W Hardware
IR Infra Red
(SL Inter-satellite Link
ML Media Lario S.r.l.
MM Multi Mode
N.A. Numeric Aperture
NA Not Applicable
NRZ Non Return to Zero
OH Optical Head
OR Original
PC Personal Computer
P-V Peak to Valley
R-C Ritchey-Chretien
RD Reference Document
RF Radio Frequency
Rx Receiver
SM Single Mode
Tx Transmitter
TTL Transistor-Transistor Logic
WFE Wavefront Error
ZEMAX~ Focus Software Inc. Optical Analysis Package
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z
