COUPLING ARRANGEMENT FOR THIN-FILM OPTICAL DEVICES

• US 3,610,727 A
• Filed: 10/09/1969
• Issued: 10/05/1971
• Est. Priority Date: 10/09/1969
• Status: Expired due to Term

• Current Assignee: Reinhard Ulrich
• Original Assignee: Reinhard Ulrich
• Inventors: Ulrich, Reinhard
• Primary Examiner(s): Corbin, John K.
• Application Number:
  US04/865,051
• Time in Patent Office: 726 Days
• Field of Search: 350/96WG
• US Class Current: 372/108
• CPC Class Codes: G02B 6/34 utilising prism or grating G02B6/293 takes precedence

Abstract

A coupling arrangement for thin-film optical devices is disclosed which couples a laser beam into or out of a thin-film light guide with high efficiency. The coupling between the thinfilm guide and its bulk substrate is provided by an auxiliary film of low refractive index whose thickness is smoothly increased throughout the coupling region so that, for input coupling, the competing output coupling effect is minimized. In a preferred embodiment, the thickness of the thin-film guide itself is also increased beyond the coupling region. This permits then the thickness of the auxiliary coupling film to be tapered off, eventually to zero thickness, in the region of increased guide thickness.

Claims

1. A light-wave coupling arrangement of the type in which a prism is disposed in proximity to one of the two major opposite surfaces of a body of optically transparent material to provide coupling of light between the prism and the body by frustrated internal reflection, the separation between said prism and said body being of the order of the wavelength of said light, said body having an index of refraction exceeding the ones of the materials bounding said body on said opposite surfaces and having a thickness between said opposite surfaces comparable to the wavelength of said light, said arrangement being characterized in that the separation g(x) of said body and said prism varies for a first distance along the path of said light to enable phasematched cumulative coupling of said light into said body, and the thickness of said body between said opposite surfaces varies to proportion the relative propagation constant Beta of said body to provide said phase-matched cumulative coupling throughout said first distance along said path.

2. A light-wave coupling arrangement of the type claimed in claim 1 in which the body of optically transparent material is proportioned to have a constant relative phase constant, Beta , throughout the first distance, where is the index of refraction prevailing in the separation between the body and the prism, and n3 is the index of refraction of the prism.

3. A light-wave coupling arrangement of the types claimed in claim 1 in which the prism is disposed in proximity to the one of the two major opposite surfaces of a body of optically transparent guide material and is separated therefrom by a layer of transparent gap material with a thickness of the order of the wavelength of said light to provide coupling of said light from the prism to the body by frustrated internal reflection, said arrangement being characterized in that the thickness of said layer of transparent gap material is increased throughout said distance in the direction of propagation to enable said cumulative coupling.

4. A light-wave coupling arrangement according to claim 3 in which the thickness, go(x), of the gap material adjacent the region of frustrated internal reflection is increased according to the relationship or to an approximation thereof, where lambda is the light wavelength, m is the relative propagation constant of the light in the mth mode of propagation in the adjacent body, n2 is the index of refraction of the transparent gap material, a is

5. A light-wave coupling arrangement according to claim 4 in which g(x) is smoothly decreased with increasing x past the region of frustrated internal reflection while the relative phase constant Beta of the light in the body is smoothly increased for values of increasing x past said region until reaching a value sufficient to prevent substantial reverse coupling, g(x) going eventually and smoothly to zero.

6. A light-wave coupling arrangement according to claim 5 including an output coupling portion which is a mirror image of the aforesaid portion of the arrangement about an intermediate value of x.

7. A light-wave coupling arrangement according to claim 5 including a plurality of output coupling portions, at least one of which includes a portion of the gap material and a portion of the guide material having contours which are approximately mirror images of the contours of the aforesaid input coupling portion of the arrangement about an intermediate value of x for each of said input coupling portion and said one output coupling portion, said output coupling portion having lengths and thicknesses of said gap material and said guide material effective to couple out less than all of the energy propagating in said guide in the vicinity of said output coupling portion.

8. A light-wave coupling arrangement according to claim 7 in which a second output coupling portion is substantially similar to said first output coupling portion.

9. A light-wave coupling arrangement of the type claimed in claim 1 adapted to provide output coupling from the body to the prism in that the separation g(x) and the thickness w(x) of the body are varied to provide a desired radiation pattern from said body.

10. A light-wave coupling arrangement of the type claimed in claim 9 in which the separation g(x) varies to yield a net decrease from start to end of the first distance in the direction of propagation.

11. A light-wave coupling arrangement of the type claimed in claim 10 in which the body of optically transparent material is proportioned to have a constant relative phase constant, Beta , throughout the first distance, where for TE polarization, and for TM polarization, where n2 is the index of refraction prevailing in the separation between the body and the prism, and n3 is the index of refraction of the prism.

12. A light-wave coupling arrangement of the type claimed in claim 1 adapted to provide output coupling from the body to the prism in that the thickness w(x) of the body is decreased from a first value preventing said coupling during a second distance along the path preceding the first distance in the direction of propagation, and the separation g(x) is first increased for said second distance preceding said first distance to inhibit said coupling and is thereafter decreased during said first distance to enable said coupling.

13. A light-wave coupling arrangement of the type in which a coupling means is disposed in proximity to one of the two major opposite surfaces of a body of optically transparent material to provide coupling of light between the coupling means and the body by the evanescent field of said light, the separation between said coupling means and said body being of the order of the wavelength of said light, said body having an index of refraction exceeding the ones of the materials bounding said body on said oPposite surfaces and having a thickness between said opposite surfaces comparable to the wavelength of said light, said arrangement being characterized in that the separation g(x) of said body and said coupling means varies for a first distance along the path of said light to enable phase-matched cumulative coupling of said light into said body, and the thickness of said body between said opposite surfaces varies to proportion the relative propagation constant Beta of said body to provide said phase-matched cumulative coupling throughout said first distance along said path.