4.5 Combination
4.5.2 Optical Path Delay
JouFLUcombines two beams pair-wise fromCHARA; soJouFLUcan take either beams 3 & 4 or beams 5 & 6. To simplify discussion, beam 3 or 4 is referred to as beam A and either beam 5 or 6 is beam B. TheCHARAbeams have a differential offset prior to reaching the FLUORtable, such thatA−B =−284 mm. Placement of the M0 mirrors forJouFLUalters the optical path,A−B = 140 mm + 76 mm. Internal to theJouFLUoptical table, the
placement of the stages and optics changed the pathA−B = 170 + 98 mm.
It is important to match the amount of dispersion as much as possible as a mismatch results in a reduced visibility measurement. ForJouFLU, the primary issue is chromatic dispersion
and is the result of inhomogeneities within the fibers. Chromatic dispersion affects the phase of the Fourier transform of the fringe. Large amounts of chromatic dispersion can arise in relatively short lengths of optical fiber (Coudé du Foresto et al. 2001). To compensate for this, when MONA was assembled the differential chromatic dispersion was measured and the lengths of the fiber arms of the combiner were adjusted. As a result of these fiber length adjustments to equalize dispersion in the two arms of the combiner, there is a differential offset internal to MONA of,A−B =−200 mm. The final path length result is:
A−B =−284 + 140 + 76 + 170 + 98−200 = 0 OPD.
A change inOPDis needed for theFTSmode due to the reflection of the input beam from the beam splitter to theFTSmirror of140mm. So forFTSmode,
A−B = 170 + 98−200−140 =−72 mm. This is compensated for by moving theOPD Stat stage by36mm.
4.5.2.1 OPD ScanandOPD Stat
There are twoOPDstages: OPD Scan, a dynamic scanning stage that modulates theOPD and generates fringes within MONA andOPD Stat, an adjustable static stage to correct residualOPD. These stages each carry a pair of mirrors in a dihedral arrangement. Movement of one of theOPDstages along the axis of the beam results in a change in the path length of twice the amount the stage moved.
The scanning stage meets rigid requirements as to linear velocity stability over its full range of travel. This stage was tested for such stability while in Meudon and achieves≈1%error in its
velocity at 110µm/s (SeeFigure 4.6). In addition, further custom tuning was performed by a Newport technician. The stage is actuated by a linear DC motor and has 50 mm of travel. The greater range of travel for this new stage greatly surpasses the 200µm of theFLUOR piezoelectric dither mirror. The increased range is necessary for the use of theFTSmode. During normal observation mode and while collecting fringes at 100 Hz, the scanning stage travels at 105µm/s (half the optical path velocity due to double pass) over a range of 150 µm. ForFTSmode, the stage travel range must be 10 times this. The exact velocity of theOPD Scanstage is determined by theNICMOScamera readout frequency. Fringes are temporally modulated and scanned at a rate of five samples per fringe (2.5 times Nyquist). This rate was chosen based on experienced learned with the CLASSIC beam combiner; five samples per fringe produces data that can be well calibrated; more than five samples per fringe does not improve the data quality. So, theOPD Scanstage velocity is determined by
Velscan =
Rcamera·λ0
2·Nsample
(4.4)
where, in practice,Rcamera= 500Hz,Nsample = 5,λ0 is the central wavelength, and the factor of 2 is due to the double pass of the beam on the stage.
While collecting fringes at 100 HzNICMOSreads out at 500 Hz (2 ms). The Newport XPS Motion Controller (XPS) has been programmed to send a signal to theJouFLUcontrol computer to report when theOPD Scan stage is moving at a constant velocity. However, this method of triggering the camera was found to be unnecessary and goes unused in order to
decrease the overhead on theJouFLUcomputer. Instead, the motion of theOPD Scan stage and the readout of the camera are synchronized by a fixed time delay. The acceleration period ofOPD Scanis constant, and has been measured, for a given rate so the data recording sequence is triggered only after that delay. This ensures that the fringes are only recorded under the constant velocity situation and not when the stage is accelerating.
Figure 4.6: This plot shows the results of testing theOPD Scanstage’s motion. Plotted is the stage position (dashed line) and the stage velocity for a scan of 250-µm at 110-µm/s (solid line). This shows the rapid acceleration period and velocity stability of the stage.
The parameters duringOPD Scancomponent qualification testing:
scan length = 0.500mm
absolute value of maximum velocity = 0.116mm/s
mean velocity = 0.109mm/s
set velocity = 0.110mm/s
mean velocity rms error = 1.33%.
The second stage,OPD Stat, is a static stage and does not move during data acquisition. OPD Statdoes not have the strict velocity requirements ofOPD Scan, but it does need longer
travel. OPD Statcorrects for the offset created by introduction of theFTSbeam splitter. Approximately 4 cm of stage difference are introduced inFTSmode due to the separation of the two beam paths (seeFigure 4.3).