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FADOF*and its application in ASTROD
and ASTROD I
Albrecht Rüdiger
Albert-Einstein-Institut Hannover
Third International ASTROD Symposium, Beijing, 14 – 16 July, 2006
FADOF*and its application in ASTROD
and ASTROD I
Albrecht Rüdiger
Albert-Einstein-Institut Hannover
Third International ASTROD Symposium, Beijing, 14 – 16 July, 2006
* Faraday Anomalous Dispersion Optical Filter
Contents
1 ASTROD (I) orbits
2 Need for filtering
3 Conventional filters
4 FADOF scheme
5 Selectivity
6 challenges
7 Outlook
The ASTROD orbitsFor the relativistic measurementsthe orbits are chosen such that the two spacecraft are simultaneously behind the Sun (seen from Earth)
Sunlight willshine directlyinto the opticsof the spacecraft
In good company:In the inspiring talk on LATOR, by Slava Turyshev we have heard about the excellent suppression that can be achieved with sufficient effort
My talk will therefore limit itself to the implications on the design of ASTROD and/or ASTROD I when applying some of these schemes
The ASTROD orbitsrelative to the near-Earth spacecraft
exhibit non-monotonic changes in distances
The ASTROD orbits
The communication (laser beams) between the spacecraft is in the ecliptic plane, and for the relativistic measurements even close to the line of sight to the sun (brown ----). Sunlight enters the sensitive optics
But also for the gravitational-wave measurements (blue ----), a shielding of sunlight has to be taken care of (unlike LISA)
The need for filtering
The laser light coming from a distant spacecraft (up to 2 AU away) is of the order of 100 fW (at wavelengths of near infrared).The sunlight, on the other hand, is of order 100 W (visible, IR), 15 powers of ten higher (!)
Great care must be taken to reduce the incoming sunlight.
That is partly to avoid heating of the optics; more importantly, however, to protect the sensitive optics and keep the photo diodes within their (limited) dynamic range
To distinguish the laser light information from the sunlight irradiation, a reduction factor of 15 powers of ten would be desirable
All statements are only rough order-of-magnitude approximations
Conventional filtering schemes1 Geometric shielding (coronagraph) sunlight sufficiently away from the line of sight between the spacecraft can be easily shielded using some type of coronagraph Coronagraph poses no problem in the GW case of the two spacecraft not in line with the sun
Conventional filtering schemes1 Geometric shielding (coronagraph) sunlight sufficiently away from the line of sight between the spacecraft can be shielded using some type of coronagraph This scheme is not so easy to apply in the case of the two spacecraft close to being behind the sun
The LATOR coronagraphFor LATOR, talk by Slava Turyshev, a very effective coronagraph has been designed, allowing several orders of magnitude reduction.
Similar schemes will have to be applied in ASTROD (I).A very efficient coronagraph is to be employed in LATOR (talk by Slava Turyshev)Several orders of magnitude sunlight reduction
The LATOR coronagraph
2006.07. 14. ASTROD & ASTROD I: overview and progress ASTROD study team
Schematic Diagram of the ASTROD I Schematic Diagram of the ASTROD I Spacecraft (Bao’s charging simulation)Spacecraft (Bao’s charging simulation)
Thermal Control
Black Surface
Black Surface
FEEP
Power Unit
Power Unit
Pulse Laser
CW LasersClockOptical Comb
Optical Cavity
FEEP
TIPO
Electronics
Telescope
Thermal Control
Black Surface
Black Surface
FEEP
Power Unit
Power Unit
Pulse Laser
CW LasersClockOptical Comb
Optical Cavity
FEEP
TIPO
Electronics
Telescope
Sketch of ASTROD I spacecraft, with optics on the right
Orientation of coronagraph
The orientation of the sun with respect to the line-of-sight to the other spacecraft will vary strongly during orbit, and most drastically just when closest to the sun
Some scheme must be devised to align the coronagraph according to this change in orientation.
Just at the time of the sensitive relativistic measurements, this (perhaps noisy) maneuver has to be made
As Slava Turyshev said, various schemes are under study
Conventional filtering schemes2 Dielectric optical filters multi-layer coatings can provide narrow-band filters, such as 10 nm or even 1 nm width. For, say, the 1064 nm line of the Nd:YAG laser, this would correspond to a filter of up to 1:1000.
Transmission is bound to be temperature dependent (thermal expansion, change of refractive index): thus narrow transmission bandwidth is ruled out as the transmission line would move by heating due to changing incident angles from the sun
Filter factor of 1:100 seems more realistic
Conventional filtering schemes2 Dielectric optical filters Where to place the dielectric filter(s) ?
If outside of telescope, glass plate of 30 cm diameter would be needed: floppy; low resonant frequency
If after telescope, small diameters would be sufficient, but extreme concentration of light intensity (up to 100 W/cm², extreme heating)
Application of dielectric filters is required, but the attainable suppression may be limited
Conventional filtering schemes3 Heterodyne signal extraction The laser signal is mixed with the field of a local laser: Only a certain bandwidth of beat frequency is accepted
Both the received frequency (due to Doppler shift as well as due to instability of laser frequency)
and the local oscillator (due to limited stability) will undergo certain (partly unforeseen) changes.
Thus, the heterodyning cannot be made narrow-band, at best perhaps 30 GHz, which brings a further reduction of noise by, say, 1 in 10^4.
Maybe better, if local oscillator takes Doppler into account
FADOFFaraday Anomalous Dispersion Optical Filter
This state of affairs necessitates a further drastic reduction, and it can be provided by the so-called
A gas cell, of proper temperature, under proper magnetic field, can rotate the polarization of a beam of a particular frequency so that it the can be separated from other frequencies, in a very narrow-band fashion
This scheme has become standard technology, it has been applied to several problems, including a number of space missions
FADOF principleOf incident beam only one polarization passes P;
beam of favored wavelength changes polarization,can pass the analyzer A on way to detector
more specifically
FADOF in ASTROD
50% of incoming sunlight has wrong polarization, is deflected; of remaining 50% very little is in the (narrow) FADOF bandwidth, so almost all of it is also deflected, only small portion (laser light) passes
For ASTROD application:narrow transmission band, typically several GHz, corresponds to 1 in 10^5 for FADOF alone, so with coronagraph, dielectric filter, heterodyning: perhaps 1 in 10^14 in all, quite close to the goal
After first dielectric filtering, and after the FADOF, the solar light power is now low enough to use additional, extremely narrow dielectric filters, if need may be. No thermal effects to be feared
That should provide sufficient sunlight reduction
FADOF wavelengths
FADOF filters cannot be had at just any laser wavelength, in particular not at the workhorse wavelength 1.064 µm
At certain frequencies, such as the doubled Nd:YAG, i.e. at 532 µm, an excited-state FADOF of Rb atoms is being operated at Darmstadt University (Thomas Walther)
The choice of the ASTROD laser must be made with the availability of a corresponding FADOF line in mind
The ChallengesWhat now appears as a convenient state of affairs is marred by some technical problems, the most important of which is Doppler shift
Doppler shift is due to the line-of-sight velocity between the spacecraft (or in ASTROD I: from Earth)
For ASTROD, Li Guangyu of PMO has supplied the following line-of-sight velocities:
which we inspect in more detail in the next slides
Line-of-sight velocity between near-Earth and inner-orbit spacecraft
Peak velocities are of order 20 km/s, both ways
Line-of-sight velocity between near-Earth and outer-orbit spacecraft
Peak velocities are of order 20 km/s, both ways
Doppler shift due to line-of-sight velocity
With l.o.s. velocities of up to 20 km/s at certain epochs, for wavelength 1.064 µm one would get a Doppler shift of as much as 20 GHz, which is much wider than the FADOF transmission band
That appears to be the main problem: how to conciliate the FADOF scheme with Doppler shifts Several schemes could work around that problem:
Attempt to pull the FADOF band according to Doppler shift (e.g., with temperature, magnetic field) but rather complex, not suited for space mission
Attempt to pull laser frequency according to Doppler shift (up to 0.01 %), appears within possibility
Other “minor” obstaclesThe FADOF cell requires elevated temperatures (say, 130 °C), not desirable close to test mass good thermal shielding required
The FADOF cell requires high magnetic fields (say, 250 gauss), not desirable close to test mass good magnetic shielding required
Thus, the implementation of the FADOF scheme will lead to some rather serious design constraints, for the location of FADOF with respect to test mass, as well as for additional shielding schemes.
Closer investigation of these implications is required
ConclusionThe implementation filtering using the FADOF scheme appears to be mandatory, and suitable lasers for existing FADOFs need to be found.
The Doppler problem must be led to a solution, possibly by pulling the laser frequency according to the quite well predictable Doppler shifts
For reducing the required installation space (“footprint” on OB), efficient ways of shielding FADOF’s heat and magnetic field must be designed.
However, no insurmountable problems seem in sight.
Acknowledgment
Professor Thomas Walther of Darmstadt University has been very cooperative with suggestions, and I plan to follow his invitation to Darmstadt for further discussion of the FADOF matter
Thomas Walther expressed his interest in this application of the FADOF scheme
Very much I feel indebted to Slava Turyshev for his explanation of some of the intricacies of the coronagraph and FADOF applications.
Thank you
Thank you
