ANATAC Meeting Line Length Correction Status 2004-Apr-23

In November, Tests and theory indicated that phase change occured when the two lightwave polarizations were not fully aligned when the fiber (antenna) moves Further tests indicated that: Misalignment of the polarizations can be added by either fiber or other components Effect of fiber optic circulators on polarization alignment is large Effect of 25km of fiber is fairly small*

“Optical Circulator” - Polarization Independent Input Type From US Patent #4,650,289 Mar 17, 1987 A  B  C  D Any path length difference on the split-path causes a wavelength dependent polarization change

Polarization Requirement Depends on relative polarization alignment Misalignment must be minimized Alignment must be done at the central building, before the two lightwaves are combined Fiber introduces a misalignment that is unavoidable in the present baseline Maximum angular misalignment introduced was measured for 25 km benchtop spool, about equivalent to 15 deg of arc on the Poincare sphere, or 97% alignment. This is enough to cause a 3% amplitude reduction (compared to perfect alignment) The phase change induced (with no circulators) was below 0.5 deg at 20 GHz, the limit of the measurement (about 100 fsec) Since the spec is 12.2 fsec, we have not retired the risk, we need to measure to better precision A reason to be optimistic: the phase change was unmeasurable for a 300 degree rotation of a fairly crude azimuth mock wrap. This is a much worse case than for ALMA, which will have a carefully engineered wrap and will in general make much smaller angular moves.

PD Fiber Stretcher OFS PD Reference out: 27 to 142 GHz Faraday Mirror 25 MHz Harmonic Mixer ~9 to 11 GHz  50 MHz Loop Filter Slave (tunable) laser Master (stabilized) Laser PBS PD  50 MHz Loop Filter Line Length Corrector (64 ea.) Laser Synthesizer (4 ea.) At Each Antenna PD = photodetector PBS = polarization beam splitter OFS = optical frequency shifter New Block Diagram with Optical Circulators Removed optical signals electrical signals up to 14 km of fiber Master laser stability: ~1e-11 short term (>30km coherence) ~1e-10 long term Not shown: optical signal distribution and switching. Separate photodetector for each band, output in waveguide nm — nm

References [1] L. Ma, P. Jungner, J. Ye, J.L. Hall,, “Delivering the same Optical Frequency at two places: accurate cancellation of phase noise introduced by an optical fiber or other time-varying path,” Optics Letters, Vol. 19, No. 21, Nov. 1, 1994, pp [2] M. Martinelli, “A Universal Compensator for Polarization Changes Induced By Birefringence on a Retracing Beam,” Optics Communication, Vol. 72, No. 6, 15 Aug 1989, pp

Prototype Master laser was delivered and accepted Coherence was adequate to “close the loop” on a 5 km spool of fiber. Longer lengths of 10km and 15 km have now also been successfully tested Some initial tests were conducted on a 5km length of fiber, transmitting a 20 GHz beatnote

Test Setup for measurement of phase drift between the near and far end of the fiber. The fiber length is actively corrected by means of an optical interferometer servoed off of the round-trip phase of a highly stable and coherent master laser.

Phase Drift of 18.6 GHz signal going through 5 km of fiber. The fiber length correction is "ON" for 660 seconds and then is turned "OFF".

Same test as previous figure, showing a closeup of the measured residual phase with the correction "ON"

Phase Drift of 18.6 GHz signal going through 5 km of fiber. The fiber length correction is "ON" for 400 seconds and then is turned "OFF". This plot also shows the correction voltage to the fiber stretcher, which has a scale of 165 microns per volt. (Right vertical scale is “volts”) Figure 5 - Zoom of corrected portion of Figure 4.

10 km

Plot of Phase Drift of 18.6 GHz signal going through 0 km of fiber. The fiber length correction is "ON" for 800 seconds and then is turned "OFF". The fiber stretcher voltage has a scale of 165 microns per volt.

The preliminary experiments have successfully demonstrated the transmission of a 18 GHz reference over 5 km with a residual phase fluctuation lower than 0.22 degrees RMS (33 fs RMS) over 10 sec. These short term fluctuations are at the limit of our measuring system. Further measurements are planned at higher frequency in order to further reduce the measured RMS phase residual. Over a longer period of time, 600 sec, the total peak-to-peak drift was less than 100 fsec. However, this was similar or only slightly larger than what was measured for a zero-length of fiber, so we conclude that part of the source of the drift is in the test setup. Additional drift may be due to the polarization or birefringent effect of the coarse fiber stretcher.

This is the first demonstration of the ALMA line length correction technique to instrument-limited accuracy. The main remaining objectives are: To prove the technique to greater accuracy To prove the technique with a moving fiber wrap To prove the technique in a field-system, such as the prototype antennas, esp. with buried fiber rather than a spool To improve the master laser coherence so that up to 18 km of fiber can be stabilized and better immunity to environmental perturbations results. Other subsystem issues: Polarization maintaining components, or polarization alignment servo on the source end Minimization of polarization dispersion of the other compoents and the fiber, esp. fiber isolators required at the antenna end