Download The phase change induced

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.
New Block Diagram with Optical Circulators Removed
Line Length Corrector (64 ea.)
50 MHz
Master laser stability:
~1e-11 short term (>30km coherence)
~1e-10 long term

Slave
(tunable)
laser
Separate photodetector for
each band, output in waveguide.
PD
Laser Synthesizer (4 ea.)
Master
(stabilized)
Laser
At Each Antenna
Loop
Filter
1556.21 nm
Fiber
Stretcher
PBS
1556.4—1557.4 nm
Loop
Filter
PD
PD
up to
14 km
of fiber
Harmonic
Mixer

~9 to 11 GHz
OFS
Reference out:
27 to 142 GHz
Faraday
Mirror
25 MHz
Not shown: optical signal
distribution and switching.
50 MHz
optical signals
electrical signals
PD = photodetector
PBS = polarization beam splitter
OFS = optical frequency shifter
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. 17771779
[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. 341-344
•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
Collimator 2
10%
coupler
Optical
Amplifier
Slow fiber
stretcher
P
Polarization
controller
10dB
1:2 coupler
s
s
Fast fiber
stretcher
Driver
Polarization
monitor
Driver
Inegrator
New Focus
Tunable
Slave Laser
3dB 1:2
coupler
Collimator 3
Collimator 1
Optical
Frequency
Shifter
25 MHZ
2X2
Coupler
Faraday
mirror
DICOS Master
Laser
5km of SMF
fiber spool
P
PBS
Polarization
Controller
1:2
RF
splitter
1:2
Coupler
PID
Controller
50 Mhz
signal
generator
Near End
Photomixer
Far End
Photomixer
Mixer 2
Mixer 1
Labview
PLL
near
end
18.5 Ghz
RF Synthesizer
100 Mhz
crystal
5 dBm
PC
RF
AMP
Vector
Voltmeter

near -  far
GPIB
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”)
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