2. FADOF* and its application in ASTROD and ASTROD I Albrecht Rüdiger Albert-Einstein-Institut Hannover albrecht.ruediger@aei.mpg.de Third International ASTROD Symposium, Beijing, 14 – 16 July, 2006

3. FADOF* and its application in ASTROD and ASTROD I Albrecht Rüdiger Albert-Einstein-Institut Hannover albrecht.ruediger@aei.mpg.de Third International ASTROD Symposium, Beijing, 14 – 16 July, 2006 * Faraday Anomalous Dispersion Optical Filter

4. Contents 1 ASTROD (I) orbits 2Need for filtering 3Conventional filters 4FADOF scheme 5Selectivity 6challenges 7Outlook

5. The ASTROD orbits For the relativistic measurements the orbits are chosen such that the two spacecraft are simultaneously behind the Sun (seen from Earth) Sunlight will shine directly into the optics of the spacecraft

6. 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

7. The ASTROD orbits relative to the near-Earth spacecraft exhibit non-monotonic changes in distances

8. 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)

9. 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

10. Conventional filtering schemes 1Geometric 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

11. Conventional filtering schemes 1Geometric 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

12. The LATOR coronagraph For 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

13. ASTROD study team 2006.07. 14. ASTROD & ASTROD I: overview and progress Schematic Diagram of the ASTROD I Spacecraft (Bao’s charging simulation) Thermal Control Black Surface FEEP Power Unit Pulse Laser CW LasersClock Optical Comb Optical Cavity FEEP TIPO Electronics Telescope

14. Thermal Control Black Surface FEEP Power Unit Pulse Laser CW LasersClock Optical Comb Optical Cavity FEEP TIPO Electronics Telescope Sketch of ASTROD I spacecraft, with optics on the right

15. 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

16. Conventional filtering schemes 2Dielectric 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

17. Conventional filtering schemes 2Dielectric 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

18. Conventional filtering schemes 3Heterodyne 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

19. FADOF Faraday 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

20. FADOF principle Of incident beam only one polarization passes P; beam of favored wavelength changes polarization, can pass the analyzer A on way to detector

21. 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

22. 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

23. 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

24. The Challenges What 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

25. Line-of-sight velocity between near-Earth and inner-orbit spacecraft Peak velocities are of order 20 km/s, both ways

26. Line-of-sight velocity between near-Earth and outer-orbit spacecraft Peak velocities are of order 20 km/s, both ways

27. 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

28. Other “minor” obstacles The 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

29. Conclusion The 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.

30. 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.

31. Thank you