Spectroscopically sensitive laser transmitter to the ISS

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Spectroscopically sensitive laser transmitter to
the ISS

• Mikko Väänänen, Observatory, 2.10.2002

• Abstract The aim is to propose a new space
  instrument to be
• realised on board the ISS. SSLT is a multipurpose
  instrument that has
• applications in the field of fundamental space
  physics, and quite
• surprisingly, a big commercial potential. The
  SSLT can be used
• to study the dynamics of least attenuating paths
  in the Earths
• atmosphere, and
• 2) for broadband data transmission.
• No doubt also other significant applications will
  be found in the future.

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Theoretical background
Electromagnetic propagation has been a
fundamental subject of physics and astronomy for
centuries. There are several separate physical
factors that affect the propagation of photons in
a medium the main factors being 1) Line
absorption by molecules and aerosols
spectrally dependent, strong for molecules, weak
for aerosols. 2) Thermal emission by molecules
and aerosols, this can be calculated using the
Planck distribution at the actual gas
temperature 3) Scattering into or out of beam by
molecules and aerosols, spatially and wavelength
dependent 4) Redirection by refractive effects
caused by inhomogeneities in atmospheric density
refractive index this may be in steady state
or turbulent.
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Other factors affecting EM propagation

• Polarisation state changes
• Wavelength changes by Raman effect
• Other effects such as halos, glories, rainbows
  etc.

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WHAT is the SSLT?
SSLT is the general adaptive photon source. This
instrument can provide a measurement beam at any
wavelength to derive such optical properties as
transmission, radiance, optical depth or the
like. It is optically adaptive, which means that
the dynamics of many optical phenomena can be
studied with extremely high time resolution. In
addition to functioning as a measurement instrumen
t, SSLT, can be used for broadband communication
between the ISS and the Earth
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Overall arrangement
earth station for communication
portable receivers
transmitter
ISS
Earth
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Signal flow -Transmission
  1) Feed a signal to the cable network, for
instance with a signal generator.   2) Generated
pulse train enters SONET transceiver and is
modulated according to e.g. SONET standard. A
current or a voltage signal proceeds.   3)
Current or voltage drives the semiconductor
lasers on the chip and laser/maser light is
emitted. All of the lasers may be on at once
only some of them, or just the most penetrating
or the penetrating few are selected.
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Signal flow -Optics
  4) The light generated from the lasers is
diffracted to form several redundant beams.
Redundancy may also be achieved across
wavelengths.  5) The laser/maser beams are
expanded with the telescopic beam expander in
order to reduce divergence. The beam is emitted
to the receiver.  6) A portion of the transmitted
beam is reflected back by the retroreflector.  7)
laser/maser transmission is controlled according
to retroreflection detector readings. The beam is
optimised.  8) The received beam is compressed
and focused to the APD.  
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Signal flow -Reception
9) APD converts the optical signal to an
electrical one.   10) The electrical signal is
received from the APD by the SONET receiver.
SONET transceiver converts the information to a
favourable interface desired by the cable
network, WLAN, or backhaul network. If the
communication is to be continued in SONET,
interface adaptation is restricted to a bare
minimum.   11) Signal enters the terminal
network we can view it with an oscilloscope.
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JHMV100402 Optimized Link Unit
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RECEIVER
TRANSMITTER
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telecom network
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telecom network
SONET transmitter, power control, laser choice
etc.
retroreflection detector(s)
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Retroreflector
SONET receiver
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ray multiplication and beam expansion
beam receiver and telescopic beam compression
unit
Avalanche photodiode (APD)
laser chip unit
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3
5
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laser at ?2
Retroreflector
telescopic beam expander unit
laser at ?1
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? for example, 1400-1800nm , selected
wavelenghts
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Transmitter chip details
The required precision is accomplished by
designing the chips to a certain sharp
wavelength. They are then bonded to a Peltier
element. The temperature of the Peltier element
is modified to tune the specific chip 5-10 nm.
Some tuning may also be realised by controlling
the cavity length and the optical properties of
the reflective materials such as mirrors.
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Receiver details
Two alternatives 1) Monochromator with an array
of detectors at specified wavelenghts 2)
Dispersive element (grating/prism) APD array
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What type of data can SSLT experiments produce?
Photometrical environmental data. Transmission,
radiance, optical depth, optical properties of
the atmosphere at any location. Remote sensing
data of molecules and substances in the
atmosphere.
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WHY ISS?

• Close to Earth with good coverage from the
  equatorial plane
• Provides a good platform for upgrades and further
  tests, this is a fast paced ongoing development
• For maximum multitasking and functionality, the
  instrument should be manually manageable.
• ISS is Open for Business

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Remote sensing application
Based on various techniques nearly all of the
molecules, such as Ozone, Carbondioxide etc. can
be probed in the atmosphere by using light. We
need expert advice on the preferable
characteristics of the device for remote sensing.

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Least attenuating paths
We have a hypothesis that the atmosphere, or any
gas in fact, has least attenuating paths,
where Transmission is at maximum Attenuation is
at minimum The least attenuating paths are
undoubtedly very dynamic, due to the complexity
of the atmosphere. The existence and
characteristics of LAPS is a fundamental physical
issue of importance
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Commercial potential in broadband telecom
The possibility of terrestrial wireless laser
links, using least attenuating paths ? replacing
optical fiber with a reliable wireless solution
is clearly a multibillion euro proposition with
global impact. Terrestrial communication using
the SSLT on the ISS? A specialised, low value
proposition
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Femto SSLT
The same instrument could have a femtosecond
laser version. This would be used to study the
extreme short time scale dynamics of the
atmosphere and the reported better gas
penetration of the molecules. ? The next
generation in broadband laser links
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Other missions of relevance environmental
satellite Terra
The instruments onboard TERRA are ASTER stands
for Advanced Spaceborne Thermal Emission and
Reflection Radiometer. It measures
high-resolution images of the Earth in fourteen
different wavelengths ranging from visible to
thermal infrared. CERES stands for Clouds and
the Earth's Radiant Energy System. The two CERES
instruments onboard TERRA measure the Earth's
total radiation budget and provide data that
helps scientists to estimate the role of clouds
in radiative fluxes throughout the atmosphere.
MISR stands for Multi-angle Imaging
Spectro-Radiometer and has cameras pointed at
nine different angles to measure sunlight
scattered in different directions. MISR records
three visible and one infrared band.MOPITT
stands for Measurement of Pollution in the
Troposphere and it measures emitted and
reflected radiance in three spectral bands. This
measures carbon monoxide and methane properties
in the lower atmosphere. MODIS stands for
Moderate-Resolution Imaging Spectroradiometer
andmeasures light in 36 frequency bands at three
different resolutions.
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CONCLUSIONS
SSLT is a MULTIDISCIPLINARY mission with great
scientific and economic potential for the entire
mankind. We are making an AO (Announcement of
Opportunity) to ESA. The instrument lacks
funding, and we are looking for ways to fund the
project.
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