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Babinet Soleil Compensator
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The Babinet Soleil Compensator is constructed from two opposed birefringent crystal wedges with a compensating crystal block in optical contact with the smaller wedge. Both wedges are cut with the optic axis parallel to their long edges, and the compensating block has its axis at right angles. In operation, the large wedge is translated across the smaller, thus presenting a variable path length difference to an optical beam passing through the instrument. The compensating block ensures that this difference is uniform across the aperture.
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Theory
The compensator is aligned so that its axis (instrument base) lies at 45° to the polarization plane of the input beam. This beam can be considered to be resolved into two components lying parallel to the crystal fast and slow axes. On emerging from the compensator, each component now has a differential phase retardation dependent upon the optical path length difference through the crystal. Adjusting this path length difference by translating the large wedge varies the retardation in direct proportion. The wedge angle is calculated such that full scale movement (up or down) from centre (micrometer 0.500") introduces a half wave retardation (plus or minus) at the longest design wavelength. The compensator then functions as a variable zero order retardation plate.
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Operation
The compensator is oriented so that its base lies at 45° to the polarisation direction of the input beam. The compensator is factory calibrated to introduce zero phase shift at a micrometer reading of 0·5". At this setting the polarisation state of an input beam is unaffected by passing through the compensator. The calibration graph in the operator's handbook is a plot of the micrometer setting required to introduce either a positive or negative half wave phase shift. Intermediate phase retardations can be set up by simply setting the micrometer to the equivalent proportion of half wave retardation. The instrument is completely linear over its whole range.
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User Calibration
This procedure enables the user to accurately calibrate the instrument at his operating wavelength. The compensator is set between a crossed polarizer/analyser pair, with its base at 45° to the polarisation direction and input face normal to the beam (checked by return reflections). The analyser is then rotated through 90° for maximum transmission. The micrometer is adjusted either way to extinguish the beam. This reading corresponds to the half wave value. Intermediate retardation states are set by proportion.
Custom Compensator
When enquiring about a custom special, simply specify the aperture and wavelength range of operation. Contact us at sales@halbo.com, and we will send you a quotation by return.
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BSC100 Babinet Soleil Compensator
This compensator, using synthetic crystal quartz elements, is designed for use over the wavelength range 190nm - 1600nm, covering the most popular laser wavelengths from Nd:YAG 4th harmonic @ 266nm to GaInAsP @ 1550nm. The compensator is factory preset for zero retardation at the micrometer range centre (0·500"), and we supply a calibration curve for the calculation of retardation vs micrometer setting at any wavelength within 190nm - 1600nm.
BSC100F Babinet Soleil Compensator
This compensator, using magnesium fluoride elements, is designed for use over the wavelength range 120nm - 7·5μm, covering the vacuum ultra violet through to the mid infrared band. The compensator is factory preset for zero retardation at the micrometer range centre (0.500"), and we supply a calibration curve for the calculation of retardation vs micrometer setting at any wavelength within 120nm - 7·5μm.
Specifications
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Model #
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Material
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Spectral Range (nm)
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Aperture
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Retardation range
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Calibration accuracy
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BSC100
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Quartz
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190-1600
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10
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+/-180°
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+/-1°
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BSC100F
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MgF2
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120-7500
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10
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+/-180°
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+/-1°
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