Optical waveguides – Directional optical modulation within an optical waveguide – Electro-optic
Patent
1990-08-20
1992-04-07
Gonzalez, Frank
Optical waveguides
Directional optical modulation within an optical waveguide
Electro-optic
385 21, 385 28, 385 41, 385 50, G02B 610
Patent
active
051034913
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a waveguide type optical circuit element using a directional coupler, and particularly to a waveguide type optical circuit for use in optical fiber communications, which has an excellent effect of shutting light in optical waveguides and has reduced polarity-dependence.
BACKGROUND ART
Conventionally, this type of waveguide type optical circuit element includes, as a principal part thereof, a directional coupler as schematically shown in FIG. 10. In this figure, a conventional waveguide type optical circuit element, such as a polarity-independent optical switch, has a lithium niobate (LiNbO.sub.3 hereinafter) substrate 1 defining two waveguides 2 and 3, parts of the waveguides 2 and 3 being close to each other to form a coupling section 6. When light of intensity PO enters one of the waveguides 2 from a left incident end surface, it varies depending on coupling length L of the coupling section 6 where the parts are close to each other, and lights of varied intensities PA, PB exit the respective waveguides 2 and 3 from a right exit end surface. The first length l for the exit lights of intensities PA, PB to be in the relationship PA/(PA+PB).apprxeq.0 is called a complete coupling length. FIG. 11(A) shows the relationship of exit lights PA, PB with the complete coupling length l. Generally, the complete coupling length l varies according to the polarization state of incident light, namely whether the TE mode or TM mode. Here TE mode refers to a polarization state in which electric field components are parallel to the substrate 1, and TM mode refers to a polarization state in which the electric field components are vertical to the substrate 1. It is to be noted that the two waveguides 2 and 3 have an identical structure.
The conventional directional coupler of the polarity-independent optical switch acting as a waveguide type optical circuit element includes a pair of electrodes (not shown) in the coupling section 6 to be the uniform .DELTA..beta. type, and is capable of switching incident light in accordance with the electro-optical effect produced by applying an electric field. This switching state is shown in FIG. 11(B) as a uniform .DELTA..beta. switching diagram. In this figure, state (bar state) indicates a state where incident light intensity PO corresponds to exit light intensity PA, with the other exit light intensity PB=0, and state (cross state) indicates a state where incident light intensity PO corresponds to exit light intensity PB, with the other exit light intensity PA=0. The state appears on a plurality of circular arcs when an electric field (.DELTA..beta..multidot.L/.pi.) is applied.
Next, FIG. 12 shows switching diagrams of both the TE and TM modes where an electric field is applied with the ratio L/l between coupling section length L and complete coupling length l is "1" (L/=1) for both the TM mode and TE mode. In this figure, as the electric field is applied to increase .DELTA..beta..multidot.L/.pi., the TE mode becomes state at .DELTA..beta..multidot.L/.pi..apprxeq.5.2 and, as the electric field is applied further to increase .DELTA..beta..multidot.L/.pi., the TM mode becomes state at .DELTA..beta..multidot.L/.pi..apprxeq.5.9. In this way, switching of incident light is made by changing both the TE and TM modes from state to state.
Another conventional polarity-independent optical switch comprising a waveguide type optical circuit element is described in ELECTRONICS LETTERS Oct. 8, 1987 Vol. 23, No. 21, pages 1167-1168, which is shown in FIG. 13. In this figure, the conventional polarity-independent optical switch has a LiNbO.sub.3 substrate 1 defining two waveguides 2 and 3, a pair of electrodes 4 and 5 arranged on the two waveguides 2 and 3, and another pair of electrodes 45 and 55 arranged outwardly of the pair of electrodes 4 and 5.
In the above construction, the condition for both the TE and TM modes to become state with the same voltage applied to the electrodes 4, 5, 45 and 55 is determined by electro-optical coefficients .gamma.13 and
REFERENCES:
patent: 4243295 (1981-01-01), Alferness
patent: 4291939 (1981-09-01), Giallorenzi et al.
patent: 4390236 (1983-06-01), Alferness
patent: 4679893 (1987-07-01), Ramer
patent: 4711515 (1987-12-01), Alferness
patent: 4865408 (1989-09-01), Korotky
patent: 4917449 (1990-04-01), Granestrand
patent: 4997245 (1991-03-01), DuPuy et al.
Gonzalez Frank
Shimadzu Corporation
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