Abstract:
An interferometric device for measuring optical thin film parameters such as refractive index, thickness and absorption uses phase conjugate mirrors in place of standard mirrors. The optical thin film for which the refractive index, thickness and absorption are determined acts as a beam-splitter in the interferometer.

Description:
BACKGROUND OF THE INVENTION 
     Thin films are vital to the operation of many integrated optic and optoelectronic devices. Thin film ion-conductive coatings are currently employed within electrochromic devices and solid state storage batteries as described, for example, in U.S. Pat. No. 4,832,463. These devices basically consist of a number of transparent optical films deposited on a transparent substrate such that the properties of the optical films such as thickness, index of refraction and absorption must be carefully controlled. One method for determining such optical properties is to employ the optical film sample as one beam-splitter within an optical interferometer as described in the paper entitled &#34;Optical Parameters of Partially Transmitting Thin Films&#34; by J. Shamir and P. Graff, published in the Applied Optics, December 1985, Volume 14, No. 12. The use of the optical film sample as one of the beam-splitters, however, requires manual beam alignment procedures that are difficult to achieve. Since the accuracy of the measurements strongly depends on the uniformity of the interference patterns developed, non-uniformity of the optical film sample or of the other optical components could lead to ambiguous results. Phase conjugate mirrors as one means of correcting phase distortions in an optical laser cavity is described within U.S. Pat. No. 4,529,273. The use of phase conjugate mirrors within other such optical devices is described within U.S. Pat. No. 4,280,764; 4,718,749 and 4,773,719. When the electrochromic devices described in the aforementioned U.S. Patents are considered for commercial utilization, such precision manual alignment would render interferometric evaluation extremely difficult and economically infeasible. One object of the instant invention is to provide apparatus which automatically provides beam alignment while compensating for any non-uniformity of both the optical films and the accompanying optical components. 
     SUMMARY OF THE INVENTION 
     The invention comprises an optical film interferometric measuring device in which the optical film to be measured is employed as one of the beam-splitters. The use of phase conjugate mirrors within the interferometer automatically aligns the transmitted and reflected light beams while compensating for any non-uniformity within the optical components or the optical film, per se. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic representation of an optical film used as a beam-splitter combining input beams interferometrically in accordance with the prior art; 
     FIG. 2 is a diagrammatic representation of a thin film interferometer employing the thin film depicted in FIG. 1; and 
     FIG. 3 is a diagrammatic representation of one embodiment of the phase conjugate interferometer in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Before describing the Preferred Embodiment, it is helpful to review the use of a sample 10 consisting of an optical film 8 of thickness t deposited on a transparent substrate 9 such as plastic, glass or crystal, for example, as shown in FIG. 1. The sample being employed as a beam-splitter within a modified mach-zender interferometer 11 (FIG. 2). The index of refraction n 1 , is that of air and the index of refraction n 2  is that of the substrate 9. The thickness t of the optical film is small with respect to that of the substrate and is determined by interferometric techniques as described within the aforementioned paper to J. Shamir and P. Graff. E 1  is a beam of light incident on one side of the sample which results in a transmitted component E 12  and a reflected component E 11 . E 2  is a beam of light coherent with beam E, incident on the opposite side which results in a transmitted component E 21  and a reflected component E 22  as indicated. 
     In the simplified interferometer 11 depicted in FIG. 2, a laser 12 is used to provide the initial beam of incident light which is reflected off a first mirror 13 onto the first beam-splitter 14. The reflected component of the beam is directed to and reflected from translatable mirror 15. This reflection is transmitted through beam-splitter 14 towards the sample 10 and constitutes the incident beam E 1  described earlier. The component of the original beam reflected from mirror 13 and transmitted through beam-splitter 14 is reflected off a pair of mirrors 16, 17 onto the opposite side of sample 10 and constitutes the incident beam E 2  described earlier. The transmitted beams E 12 , E 21 , must be precisely superimposed on the reflected beams E 22 , E 11  respectively and thereafter received upon light detecting devices 18, 19 wherein their intensities are determined as a function of the position of the translatable mirror 15. It is within this arrangement that the mirrors 15, 16, 17 and beam-splitters 14, 10 must be precisely aligned such that the transmitted and reflected light beams become exactly superimposed. 
     In the phase conjugate interferometer 26 depicted in FIG. 3, a similar laser 12 provides an incident beam of coherent light directly to a first beam-splitter 22. A portion A 1  of the transmitted component reflected from the sample 10, is reflected back on itself from the phase conjugate mirror 20 and returns to the sample 10 thereby constituting the incident beam E 1  to the sample 10. A second portion A 2  of the transmitted component transmitted through the sample 10, and reflected from a translatable mirror 23, is reflected back on itself from the phase conjugate mirror 21, and is reflected again from the translatable mirror 23 returning to the sample 10 thereby constituting the incident beam E 2  to the sample 10. As in FIG. 2, incident beam E 1  produces beams E 11  and E 21  while incident beam E 2  produces beams E 22  and E 21 . The intensity of the sum of beams E 22  and E 12  is measured by detector 25 and the intensity of the sum of beams E 11  and E 21  is measured by detector 24. The phase of beam E 2  relative to beam E 1  must be varied during the measurement process. In the embodiment shown in FIG. 3 this phase variation is accomplished by moving the translatable mirror 23 which could, for example be a piezomirror. A piezomirror, for purposes of this disclosure, consists of a reflective mirror mounted on a crystal having a thickness dependent upon an applied voltage to translate the mirror in a controllable fashion. Other embodiments may use alternative phase shifters such as electro-optic modulators in place of translatable mirror 23 for beam E 2 . The optical properties are calculated on the basis of measurements from detectors 24, 25 by a computer contained within the controller 29. For some applications, the second phase conjugate mirror 21 can be eliminated and the angle of the piezomirror 23 is adjusted to reflect the modulated beam A 2  directly to phase conjugate mirror 20 as indicated in phantom. 
     The phase conjugate interferometer of the invention can be used as a real time monitor in a thin film deposition process whereby the optical properties of the thin film can be measured during deposition for exact control over the thickness, refractive index and absorption properties of the deposited film. It is anticipated that a feedback control system could be employed within thin film deposition apparatus that could employ thermal evaporation as well as ion-deposition such as described within U.S. patent application Ser. No. 405,271 filed Sept. 11, 1989 entitled &#34;Ion-Beam Based Deposition of Coatings for Electrochromic Devices&#34;, which Application is incorporated herein for purposes of reference. The temperature of the evaporation source and the ion voltages can be automatically controlled to produce the best electrochromic devices heretofore attainable. In this arrangement, the sample 10 is enclosed within a transparent deposition chamber 27 which includes an ion beam and thermal deposition source 28. A controller 29 is connected with the detectors 24, 25 over conductors 30, 31 and is connected with the deposition source 28 by means of the feedback conductor 32.