Printed circuit board waveguide

[0002] This invention relates to a printed circuit board and more particularly to such a board comprising a material through which optical energy can be transmitted.

[0003] Printed circuit boards are commonly used as a support for a variety of electrical components as is well known. It is also well known that laser diodes are amongst the components secured to a printed circuit board.

[0004] Laser diodes are constructed from a plurality of suitable layers on a substrate. The layers form a quantum well which defines an emitting facet for light energy. The quantum well is sandwiched between waveguiding layers which, in turn, are sandwiched between cladding layers as is well understood.

[0005] Printed circuit boards which include lasers also typically require sensors which are responsive to laser emissions. Such sensors have to be placed in an optical path of the laser output. Consequently, constraints are placed on the position of the sensors and applications for photosensitive circuits, thus, are limited.

[0006] In accordance with the principles of this invention, the circuit board itself is constructed of a material which transmits light energy and thus functions as a waveguide. Further, reflective coatings are added to the opposite faces of the circuit board to enhance the waveguide properties. Light directed at the edge of the circuit board may be sensed by a sensor located anywhere on the face of the board at a window opened in the coating there. Constraints on the placement of sensors are relaxed.

[0007] FIGS. 1, 2 and 3 are schematic illustrations of prior art circuit boards with sensors attached thereto.

[0008] FIGS. 4A, 5A, and 6A are schematic illustrations of alternative circuit boards in accordance with the principles of this invention;

[0009] FIGS. 4B, 5B, and 6B are schematic illustrations of side views of the circuit boards of FIGS. 4A, 5A, and 6A respectively; and

[0010] FIGS. 5C and 6C are enlarged views of portions of the side views of FIGS. 5B and 6B respectively.

[0011] FIG. 1 shows a photosensitive circuit attached to a prior art circuit board made of opaque material. The circuit board is designated 1 and the photo sensitive circuit is designated 2. The circuit is responsive to light, designated 4, represented by the downward directed arrow in the figure. The sensor is connected to utilization circuitry (not shown) via conductor traces represented by lines 6 and 7.

[0012] FIG. 2 shows an alternative prior art arrangement where the sensor is placed at the edge of a circuit board. Specifically, FIG. 2 shows a circuit board 10 with a photosensor 12 coupled to the edge of the board as shown and responsive to light represented by arrow 14. The sensor is connected to utilization circuits (not shown) via conductor traces 15 and 16.

[0013] FIG. 3 shows an alternative prior art construction using an opaque circuit board 31 with a photosensor 32 attached to the surface of the board over a hole 33 in the board. The sensor is responsive to light directed upwards, as viewed, through hole 33 at the sensor. The light is represented by arrow 34 and the sensor, in this case, is mounted face down.

[0014] FIG. 4A shows a printed circuit board 46 of a transparent material, illustratively Kapton, in accordance with the principles of this invention. A photosensor 50 is coupled to the edge of the circuit board. Light directed into the opposite edge of the board, as represented by arrow 54, is guided to the sensor by the board itself.

[0015] FIG. 4B shows a schematic side view of board 46 with sensor 50 secured by top and bottom ribbons 61 of solder or epoxy cement. Light entering edge 62 of the board is transmitted by the board if the light is incident at an angle Ø₂less than the critical angle of reflection characteristic of the board. Light incident at an angel Ø₁greater than that critical angle, exits the board as indicated by arrow 63.

[0016] FIG. 5A shows a printed circuit board with reflective coatings on its surfaces. The board is designated 65 in FIG. 5A and has a transparent, illustratively rigid center layer 66 with reflective coatings 68 and 69 sandwiching layer 66. Light directed from the left, as viewed, at the edge of the board is transmitted to the opposite edge. Layer 66 corresponds to a prior art circuit board in structure but comprises a transparent material according to the principles of this invention.

[0017] FIG. 5B is a schematic side view of the embodiment of FIG. 5A. Light, represented by arrow 74, incident at an angle Ø₂and greater angle Ø₁, exits at the right edge of the board to impinge upon a sensor (not shown) coupled to that (exit) edge. FIG. 5C shows enlarged the portion of FIG. 5B encompassed by circle 75. FIG. 5C indicates the angles Ø₁and Ø₂within layer 66.

[0018] FIGS. 6A, 6B, and 6C illustrate an embodiment analogous to that shown in FIGS. 5A, 5B, and 5C with a sensor positioned at a window in one of the reflective coatings rather than at the edge of the circuit board. Specifically, FIG. 6A shows a circuit board 80 with a transparent layer 81 and first and second reflective coatings 83 and 84. Coating 83 has a window 86 opened in it. Photosensor 87 is positioned, face down, at the window and is responsive to light directed at an edge of the board as indicated by arrow 94. The light is shown emitted from a laser diode 95 under the control of controller 96. Photosensor 87 is connected to a utilization circuit 97.

[0019] FIG. 6B shows the ray traces for light within layer 81, reflecting back and forth between reflective coatings. Light incident at angle Ø₁exits through hole 86. Light incident at angle Ø₂exits at the right edge of the board as shown. FIG. 6C shows enlarged the portion of the board encompassed by circle 100 in FIG. 6B illustrating the angles Ø₁and Ø₂as well as the ray traces therein.

[0020] The sensor is secured by solder or epoxy cement as indicated at 105.

[0021] The circuit board in accordance with the present invention may be rigid or flexible and may be made of Kapton or any other transparent, electrically insulating material such as polyimide, polyester, acrylic and propylene. In one reduction to practice, a board had a thickness of about 0.020 inch with antireflective coatings of copper, gold, magnesium fluoride, silver or aluminum 0.000050 inch thick. But the thicknesses may vary as do those of prior art boards.