GB2192454A

GB2192454A - System for measuring optical coatings

Info

Publication number: GB2192454A
Application number: GB08714264A
Authority: GB
Inventors: Mark Burton Holbrook
Original assignee: Intellemetrics Ltd
Current assignee: Intellemetrics Ltd
Priority date: 1986-07-01
Filing date: 1987-06-18
Publication date: 1988-01-13
Also published as: GB8714264D0

Abstract

Optical characteristics of optical coatings are monitored by measuring variations in a beam of light that has encountered said coatings. A beam of light is split into a measurement beam which encounters the coatings and a reference beam which does not encounter the coating, the reference beam and measurement beam being compared in a single detector. Preferably the measurement is performed during formation of the coating in a vacuum chamber. As shown, light from source 5 is filtered, chopped and separated at 8 into measuring beam 9 which is reflected from the coating on test object 15, and reference beam 21 which is passed along optical fibre 23. The beams are recombined and fed via spatial or rotating aperture filter 25 to single detector 19, the outputs of which are processed by phase sensitive detector 20 and sample and hold circuits 26. Alternatively the reference beam may pass through the vacuum chamber 18. In another embodiment, the two beams are differently modulated and, after recombination are passed through a scanning monochromator before electronic filtering to separate the beams. This embodiment enables noise correction across different wavelengths. <IMAGE>

Description

SPECIFICATION System for measuring optical coatings The present invention relates to a system for measuring optical coatings.

It is common practice to measuretheoptical characteristics of an optical coating whilst it is being deposited on an optical device. The measurement technique involves measuring the intensity modulation of a beam of light, such modulation arising from the optical properties ofthefilm that is gradually being deposited. The measurement may be made on either a reflected beam or a transmitted beam, and taken either from an optical device or substrate or else from individual films or groups of coatings deposited on separate pieces of material commonly known as witness plates.

This is a sensitive measurement and thus generally the incoming beam is modulated and the signal processing technique of phase sensitive detection is then used to improve the signal to noise ratio. Afurther known improvement to the technique is to detect changes in the light source intensity so that they may be removed from the measurement by the process known as comparison. This is achieved by splitting the beam from the light source to obtain a reference light beam and measuring the reference light beam using an additional detector. However, this introduces the problem of noise in the additional detector and also the duplication of components which increase the cost of the monitoring equipment.

According to the present invention there is provided a system for measuring optical coatings by monitoring variations in a beam of light that has encountered the said coatings, the beam having been split, one part of which being a measurement beam encounters the said coatings, a second part of which being a reference beam, wherein said measurement beam and said reference beam are measured using a single detector.

The reference beam may pass through an optical fibrewhilstthe measurement beam passes through a vacuum chamber of a coating unit. The reference beam may also travel within the vacuum chamber, beam splitting means being either within our without the chamber.

The beams may be intermittent and alteratively received by the single detector, or the signals may be received simultaneously with the beams being indentified by differences in their modulation.

The detector may either be a simple light power sensitive device combined with a single optical wavelength selective unit or it may consist of a single or multiplicity of light power sensitive devices combined with fixed or continuously scanning wavelength selective elements.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure lisa schematic illustration of a hitherto employed system for measuring optical coatings; Figure2 is a schematic illustration of one embodiment of a system for measuring optical coatings according to the invention; Figure 3 is a schematic illustration of a second embodiment of a system for measuring optical coatings; and Figure 4is a schematic illustration of a third embodiment of a system for measuring optical coatings.

Referring to Figure 1 ofthe drawings, there is illustrated schematically a system hitherto employed for measuring optical coatings. The prior art system comprises an input system 1 and an output system 2 linked bya processing station 3wherein,under vacuum, coating is deposited and measured. In the input system 1, a beam of light4 is emitted from a light sourceS, passes through an optical filter6 and a rotating chopper7 before being splitbya beam splitter 8. One beam 9 ofthe split beam passes through a window 10 into a vacuum chamber 11 of the processing station 3. The other beam 12 terminates in a reference detector 13 which detects changes in light source intensity.In the vacuum chamber 11 an evaporation source 14supplies coatings to a substrate held in a test glass changer 15, the beam of light 9 being reflected from a first angled mirror 16 onto the substrate such that the beam 9 is reflected onto a second angled mirror 17 and subsequently leaves the vacuum chamber 11 through a window 18 to the output system 2. The beam 9 is then monitored by a detector 19 and compared with the signal from detector 13 both of which are passed through phase sensitive detectors 20,20'. In such a system, however, the different noise in the separate detectors 13 and 19 is not eliminated and results in inaccuracies.

Referring now to Figures 2, 3and4ofthe drawings, embodiments of the present invention are described wherein some parts of the systems are the same or closely similar to pa rts of the h itherto employed system of Figure 1, these parts being given, therefore, the same reference numerals. In the embodiment of Figure 2, one beam 9 of the split beam 4 travels through the processing station 3 as before, but the second or reference beam 21 is no longer passed to a detecting device but instead passes through a lens 22 to be carried byan optical fibre 23, by-passing the processing station 3,to another lens 24, which is positioned in the output system 2.Hence, both beams 9,21 nowenterthe output system 2 so they are incident on a rotating aperture or spatial filter 25, such that first beam 9 and then beam 21 are incident on the detectors 19,20, detector 19 converting into light into a signal, such as voltage, and the phase sensitive detector 20 improving the signal to noise ratio. The signalsthen enter a sample and hold device 26 which compares the signals generated by beams 9 and 21 such that the noise in the detecting devices of the system may be eliminated.

Referring now to the system of Figure 3 for measuring optical coatings according to the invention, in which the beam splitter 8 and recombining angled component 27 are within the vacuum chamber 11. The reference beam 21 therefore travels within the the vacuum chamber 11 and mechanical noise within the vacuum chamber may be taken out, in addition to reducing noise in the detectors and light source. In afurtherembodiment (not shown) the reference beam travels a path closely similar to the other beam, the reference beam being reflected off an angled optical component which is shielded from the evaporation source.

Referring now to the alternative system of Figure 4, the spatial filter 25 ofthe systems of Figures 2 and 3 is removed and the beams 9, 21 kept distinct in the output system 2 by modulation differences introduced to the two beams 9, 21 by modulators, 28 and 29 respectively, placed in the beam paths. The beams 9, 21 are recombined and compared, as before, in the output system 2. The reference beam 21 and measuring beam 9 are recombined and rendered as one monitoring signal 30 by a scanning monochromator31 , a subsequent electrnnicfilter32 being used to separate a measuring signal 33 and a reference signal 34 from the one monitoring signal 30. A su bsequentreference circuit 35 compares the two signals 31, 32. The system of Figure 4 enables correction for noise across different wavelengths both in the light source 5and in the detector ofthe scanning monochromator31.

Modifications and improvements may be incorporated without departing from the scope ofthe invention.

Claims

1 Asystemformeasuring opticalcoatingsby monitoring variations in a beam of lightthat has encountered the said coatings, the beam having been split, one partofwhich being a measurement beam encounters the said coatings, a secondpart of which being a reference beam, wherein said measurement beam and said reference beam are measured using a single detector.

2. A system as claimed in Claim 1, wherein said coating is located within a vacuum chamber of a coating unit and said beam is split outside the vacuum chamber, the reference beam passes to the single detectorthrough an optical fibre.

3. A system as claimed in Claim 1, wherein the reference beam also travels within the vacuum chamber.

4. A method of measuring optical coatings by measuring variations in a beam oflight that has encountered said coatings comprising splitting the beamtoform one partwhich isa measurement beam and encounters said coatings, and a second partwhich is a reference beam, and comparingsaid measurement beam and said reference beam in a single detector.

5. A system for measuring optical coatings substantially as herein before described with reference to Figures 2 to 4 of the accompanying drawings.

6. A method for measuring optical coatings substantially as hereinbefore described with reference to Figures 2 to 4 of the accompanying drawings.