Waveguide type photoswitch with a short coupling region

Abstract: A waveguide type photoswitch has a substrate (30) on which a plurality of waveguide paths (32, 34) are provided in close proximity to each other. Electrodes (42, 44, 46) are arranged along the waveguide paths (32, 34) in a coupling region (40) of the paths (32, 34). The distance between the waveguide paths (32, 34) is greater at the center of the coupling region (40) than at opposite ends of the region (40), so that the coupling coefficient can be reduced at the central part of the coupling region (40) and the coupling region (40) itself can be shortened. The electrodes (42, 44, 46) each is shorter than the coupling region (40) to allow the operating voltage to be noticeably lowered and the operation speed to be increased.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a photoswitch of the type electrically switching over the direction in which an optical signal is propagated through a waveguide path. Description of the Prior Art A waveguide type photoswitch has been proposed in various forms in, for example, JOURNAL OF LIGHTWAVE TECHNOLOGY, LT-4 [9], pp. 1324-1327, 1986-1989 (hereinafter referred to as document 1), Nishihara et al "Optical Integrated Circuit", Ist Edition, Ohm-Sha, pp. 304-307, Feburary 25, 1985, (hereinafter referred to as document 2), and Japanese Patent Application No. 295872/1987 (JP-A-1-136 131) (hereinafter referred to as document 3). FIG. 1 shows a specific structure of a conventional waveguide type photoswitch adopting a directional coupler system, as disclosed in the document 1. The photoswitch has a substrate 12 exhibiting an electrooptical effect, a pair of waveguide paths 14 and 16 formed on the substrate 12 in close proximity to each other, and electrodes 22 and 24 provided in a coupling region 20 which couples waveguide paths 14 and 16. In the coupling region 20, the refractive indexes n₀ of the waveguide paths 14 and 16 are controlled by an operating voltage vo which is applied between the electrodes 22 and 24. While the coupling region 20 has a length l , the electrodes 22 and 24 each has a length l e which is smaller than the length l . The length l of the coupling region 20 is selected such that while the operating voltage vo is not applied, light entering the waveguide path 14 or 16 is outputted from the waveguide path 16 or 14, i. e. in a cross condition. The operating voltage vo applied between the electrodes 22 and 24 destroys the resonance condition of the two waveguide paths 14 and 16. Then, light inputted to the waveguide path 14 or 16 is outputted from the same one 14 or 16, i. e. in a bar condition. Assuming that the length l e of the individual electrodes 22 and 24 is equal to the length l of the coupling region 20 ( l = l e ), then the light entering the waveguide path 14 or 16 can be fully outputted from the same waveguide path 14 or 16 if a difference Δn₀ between the refractive indexes of the waveguide paths 14 and 16 satisfies following equation: where λ is the wavelength of light in vacuum. The refractive index difference Δ n₀ mentioned above is proportional to the operating voltage vo, while the electrode capacitance decreases with the decrease of l e of the individual electrodes 22 and 24. It follows that the smaller the left member, 2 πΔ n₀ l e/ λ , of the Eq. (1), i. e. , the smaller the value of Δ n₀ l e, the lower the operating voltage vo is. This is successful in reducing the electrode capacitance and thereby enhancing high-speed operation. When the electrodes 22 and 24 each has a length l e which is shorter than the length l of the coupling region 20 ( l > l e), the waveguide paths 14 and 16 will be brought into a bar condition if the refractive index difference Δn₀ has a particular value expressed as: As the value of l e/ l included in the right member of the Eq. (2) is sequentially decreased toward zero, the result of the Eq. (2) sequentially approaches π and may, therefore, be made smaller than the result of the Eq. (1). More specifically, the product Δ n₀ l e is smaller in the Eq. (2) than in the Eq. (1). It has been customary to provide electrodes each having a small length l e in the coupling region 20 so as to lower the operating voltage vo and to thereby enhance high-speed operation. A problem with the prior art photoswitch having the above structure is as follows. Assuming that the refractive index difference Δ n₀ is constant, a decrease in the length l e of the individual electrodes 22 and 24 results in an increase in the length l of the coupling region 20. Specifically, the refractive index difference Δ n₀ and the length l of the coupling region 20 are related as: In the above Eq. (3), the value of the left member increases as the value of l e/ l included in the right member is reduced toward zero. More specifically, assuming that the refractive index difference Δ n₀ is constant, the length l of the coupling region 20 increases with the decrease in the length l e of the electrodes 22 and 24. This is acceptable so long as the waveguide type photoswitch is used alone. However, when such a photoswitch is integrated in a plurality of stages on a semiconductor substrate, for example, the coupling region 20 having a greater length would undesirably add to the required area of the substrate. The document 2 teaches a waveguide type photoswitch elaborated to eliminate the above-discussed problem particular to the photoswitch of the document 1. The switch of the document 3 has two waveguide paths arranged side by side on a substrate which exhibits an electrooptical effect. Electrodes are disposed in the coupling region of the two waveguide paths. The distance between the waveguide paths sequentially increases from opposite ends toward the center of the coupling region. In such a switch configuration, the coupling coefficient of the two waveguide paths decreases with the increase in the distance between the waveguide paths. Hence, reducing the coupling coefficient in the position where the two waveguide paths are greatly spaced from each other allows the length of the coupling region to be reduced and the operating voltage to be lowered. Such an approach, however, is not fully satisfactory because it cannot lower the operating voltage beyond a certain level or increase the operating speed to a sufficient degree. The switch disclosed in document 3 is of the type defined in the first part of claim 1. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a waveguide type photoswitch which reduces the length of the coupling region of waveguide paths. It is another object of the present invention to provide a waveguide type photoswitch which is operable with a far lower operating voltage than the prior art. It is yet another object of the present invention to provide a waveguide type photoswitch which is operable at a sufficiently high speed. These objects are solved, according to the invention, with a waveguide type photoswitch comprising the technical features of claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The objects and features of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings in which: FIG. 1 is a perspective view of a prior art waveguide type photoswitch; FIG. 2 is a perspective view of a waveguide type photoswitch embodying the present invention; FIG. 3 is a graph representative of a product Δβ L characteristic particular to the switch shown in FIG. 2; FIG. 4 is a graph showing a product Δβ Le characteristic also particular to the switch of FIG. 2; FIG. 5 is a graph showing a product Δβ Le characteristic of the illustrative embodiment which holds in a condition of L = Le ; and FIG. 6 is a perspective view showing an alternative embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 2, a waveguide type photoswitch embodying the present invention is shown. Also shown in the figure is a parameter τ representative of a distance as measured from the center of a coupling region 40 which will be described. In FIG. 2, the photoswitch has a substrate 30 implemented with a lithium niobate (LiNbO₃) monocrystal which has an electrooptical effect. Two waveguide paths 32 and 34 are formed on the substrate 30 in close proximity to each other. The waveguide path 32 has an input port 32A and output port 32B, while the waveguide path 34 has an input port 34A and an output port 34B. In the coupling region 40 of the waveguides 32 and 34, the distance between the waveguide paths 32 and 34 is reduced at opposite ends of the coupling region 40 in order to couple optically between the waveguide paths 32 and 34, and it is sequentially increased toward the center of the coupling region 40. Electrodes 42 and 44 are positioned in the coupling region 40 to extend along the outer side end of the waveguide paths 32 and 34, respectively. An electrode 46 is also located in the coupling region 40 between the inner side end of the waveguide paths 32 and 34. The coupling region 40 has a length L while the electrodes 42, 44 and 46 each has a length Le which is smaller than the length L. The length L of the coupling region 40 is selected such that while no voltage is applied between the electrodes 42, 44 an 46, light entered the input port 32A or 34A is emitted via the output port 34B or 32B in a cross condition. In the above configuration, assume that the electrode 46 is connected to ground, and a positive voltage +V or a negative voltage -V is applied between the electrodes 42 and 44. Then, an electric field is developed in the horizontal direction of the substrate 40 to destroy the resonance condition of the two waveguides 32 and 34. As a result, light entering the photoswitch via the input port 32A or 34A leaves it straightforward via the associated output port 32B or 34B in a bar condition. Hence, when the voltage V of predetermined polarity applied between the electrodes 42, 44 and 46 is turned on and off, the direction of light being propagated through the waveguide paths 32 and 34 will be successfully switched over between the cross and bar conditions. The operation of the photoswitch constructed as shown in FIG. 2 may be represented by an equation, as follows: where ρ is the complex amplitude ratio, τ is the parameter representative of the distance as measured from the center of the coupling region 40, Δβ is the difference in propagation constant between the waveguide paths 32 and 34, and κ ( τ ) is the coupling coefficient. In the above Eq. (4), Δβ is equal to 2 πΔ n/ λ , where Δ n and λ are the difference in refractive index electrically induced between the waveguide paths 32 and 34 and the waveform of light in vacuum, respectively. The value of κ ( π ) L included in the Eq. (4) may be represented by a ratio R of the minimum coupling coefficient κ min to the maximum coupling coefficient κ max , with no regard to the formation of the function. It is to be noted that the minimum and maximum coupling coefficients κ min and κ max are respectively the values measured at the center and opposite ends of the coupling region 40. The structure and characteristics of the photoswitch of FIG. 2 in which the distance d between the waveguide paths 32 and 34 is sequentially increased from opposite ends to the center of the coupling region 40 will be analyzed by using the Eq. (4). The coupling coefficient κ ( τ ) is produced by: κ ( τ ) = κ ₀ exp (- γ d) Eq. (5) where κ ₀ and γ are constants. The differential Eq. (4) is solved as follows: where Z is the propagation distance ( Z = L τ ). The value of L obtained by solving the Eq. (7) for m = 0 is the coupling length L₀ as represented by: where R is the ratio κ min / κ max The length L of the coupling region 40 is selected to be an odd multiple of the coupling length L₀. In the illustrative embodiment, L is selected to be equal to L₀. The value of κ ( τ ) L is produced by: Regarding Δβ , it has a value only when the absolute value of τ is smaller than the ratio Le/L. The results of analysis are represented by curves in FIGS. 3, 4 and 5. Specifically, FIG. 3 shows a product Δβ L characteristic and indicates the values of Δβ L/ π and Le/L on its ordinate and abscissa, respectively. In FIG. 3, a curve R=1 indicates a case wherein the distance between the waveguide paths 32 and 34 is not increased toward the center of the coupling region 40. In this case, sequentially reducing the length Le of the individual electrodes 42, 44 and 46 while maintaining the voltage (i. e. Δβ ) constant causes the length L of the coupling region 40 to increase. On the other hand, when the distance between the waveguide paths 32 and 34 is increased, the length L can be maintained small even when the electrode length Le is reduced with the voltage remaining the same, as represented by curves R= 0. 5 and R=0. 1. FIG. 4 shows a product Δβ Le characteristics and indicates the values of β Le/ π and Le/L on its ordinate and abscissa, respectively. As FIG. 4 indicates, the value of Δβ Le/ π is smaller when the distance between the waveguide paths 32 and 34 is increased (curves R=0. 5 and R=0. 1) than when it is not increased (curve R=1). As FIG. 4 also indicates, the value of Δβ Le/ π further decreases when the length Le of the individual electrodes 42, 44 and 46 is reduced. FIG. 5 shows a curve representative of a product Δβ L characteristic under the condition of L = Le . In FIG. 5, the ordinate and abscissa indicate Δβ L/ π ( L = Le ) and R, respectively. As shown, when the length Le of the individual electrodes 42, 44 and 46 is equal to the length L of the coupling region 40, it is possible to reduce the product Δβ L by increasing the distance between the waveguide paths 32 and 34 (R = 0. 1 to 0. 01). This successfully shortens the length L of the coupling region 40, thereby reducing required operating voltage. Specifically, in a system having two waveguide paths arranged side by side, two different charactertistic modes exist. The difference in propagation constant between the two characteristic modes is expressed as: 2√ κ ² + δ ² Eq. (10) where δ is equal to Δβ /2, and κ is the coupling coefficient. The phase difference  between the two characteristic modes is produced by:  = (2√ κ ² + δ ² ) L Eq. (11) The above system is caused into a cross condition when the phase difference  is π and into a bar state when it is 2 π . Assuming a simplified model of the photoswitch shown in FIG. 2, the coupling constant κ is zero at and in the vicinity of the center of the coupling region 40 and can be provided with a sufficiently large value at opposite ends of the coupling region 40, compared to the value δ . When the propagation constant difference Δβ is zero, the phase difference  is produced by: 2 κ (L - Le) = π Eq. (12) On the other hand, when the propagation constant difference Δβ is not zero, the phase difference  is determined as: 2κ(L - Le) + 2 δ Le = 2 π Eq. (13) From the Eqs. (11) and (12), there is obtained: Δβ Le = π Eq. (14) As the Eq. (14) indicates, the product Δβ Le becomes as small as π . Such a decrease in Δβ allows the operating voltage to be lowered, while the decrease in Le allows the electrode capacitance to be reduced. Assuming that the coupling coefficient at the center of the coupling region 40 is κ p , then the product Δβ L is expressed as: As the value of κ p decreases toward zero, the produce Δβ L sequentially decreases. Therefore, assuming that the voltage (i. e. Δβ ) is constant, the length L of the coupling region 40 decreases as the distance between the waveguide paths 32 and 34 increases (κ p decreases toward zero). The embodiment constructed and operated as described above has various advantages as enumerated below. (1) Since the distance between the waveguide paths 32 and 34 gradually increases from opposite ends toward the center of the coupling region 40, the length L of the coupling region 40 is reduced without the operating voltage V being increased. The switch, therefore, can be formed on a semiconductor substrate with a small area. (2) The electrodes 42, 44 and 46 each has a length Le which is smaller than length L of the coupling region 40, so that the operating voltage can be reduced to a significant degree. This reduces the electrode capacitance and thereby enhances high-speed operation. For example, assuming that the ratio Le/L is nearly equal to 0. 7 and the ratio R is smaller than 0. 1, then the illustrative embodiment reduces the required operating voltage by more than 30 percent, compared to the conventional photoswitch shown in FIG. 1, for the same length of coupling region. (3) The lower operating voltage V and the lower electrode capacitance improve the switching characteristic of the photoswitch to such an extent that in a bar condition the ratio of light propagated crosswise is greater than 30 decibels. (4) The electrodes 42 to 46 do not overlap with the waveguide paths 32 and 34. In such a unique configuration, an electric field is developed in the horizontal direction of the substrate 30 while an electrooptical effect is developed in the polarization in the vertical direction, whereby the photoswitch is provided with stable operating characteristics. FIG. 6 shows an alternative embodiment of the present invention. In the figures, the same components are designated by like reference numerals, and redundant description will be avoided for simplicity. The embodiment of FIG. 6 differs from that of FIG. 2 concerning the configuration of the three electrodes 42, 44 and 46. Although electrodes 42B, 44B and 46B shown in FIG. 6 are substantially identical in configuration with the electrodes 42, 44 and 46 of FIG. 2, each of them is made up of three segments, as illustrated. Specifically, electrodes 42A, 44A and 46A and electrodes 42C, 44C and 46C are positioned on the substrate 30 at opposite sides of the electrodes 42B, 44B and 46B, respectiely. When a bias voltage is applied between the electrodes 42A, 44A and 46A and electrodes 42C, 44C and 46C, an electric field is developed to set up a cross condition. A bar condition is set up when a positive drive voltage +V or a negative drive voltage -V is applied between the electrodes 42B and 44B. With the embodiment of FIG. 6, it is possible to achieve a cross condition by a predetermined bias voltage close to but not equal to zero, for example. Hence, the switch of FIG. 6 is capable of setting up a cross condition more accurately than the switch of FIG. 2 by having the bias voltage finely adjusted, thereby further improving the ratio of light propagated crosswise in a bar condition. In summary, it will be seen that the present invention provides a waveguide type photoswitch which is operable with a lower drive voltage and, therefore, with a relatively short coupling region. This unprecedented advantage is derived from a unique configuration wherein the distance between nearby waveguides is gradually increased from opposite ends toward the center of the coupling region. In addition, since the photoswitch is provided with electrodes each being shorter than the coupling region, the drive voltage is noticeably reduced and high-speed operation is enhanced. While the present invention has been described with reference to the particular illustrative embodiments, it is not to be restricted by the embodiments but only by the appended claims. For example, the electrodes 42 and 44 shown in FIG. 2 may be configured to overlap the waveguide paths 32 and 34, respectively. Likewise, the intermediate electrode 46 may be configured to overlap the waveguide paths 32 and 34. Although such a configuration will generate an electric field not only in the horizontal direction but also in the vertical direction, the advantages of the embodiment of FIG. 2 are insured only if the crystalline orientation of the substrate 30, for example, is selected adequately. The electrode 46 intervening between the electrodes 42 and 44 as shown in FIG. 2 may be omitted in which case the electrodes 42 and 44 will be extended to overlap the waveguides 32 and 34, respectively. In this configuration, a bar condition will be set up when a positive or a negative drive voltage is applied to the electrode 44 or 42 with the electrode 42 or 44 being connected to ground, while a cross condition will be set up when such a drive voltage is not applied. In any of the illustrative embodiments, the waveguide paths 32 and 34 may each be provided with an arcuate or similar shape other than the angled shape shown and described. Then, the electrodes may also be provided with a particular configuration complementary to the waveguide paths. The substrate 30 may be implemented as a monocrystalline substrate of LiTaO₃ or similar compound, a substrate of InGaAsP, GaAlAs or similar compound semiconductor, etc.

Claims: A waveguide type photoswitch comprising: a substrate (30); a plurality of waveguide paths (32, 34) formed on said substrate (30) side-by-side and having a coupling region (40), said waveguide paths (32, 34) being optically coupled in said coupling region (40), said coupling region (40) having opposite coupling region ends, a centre between said coupling region ends and a length measured from one of said coupling region ends to the other, a distance between said waveguide paths measured at said centre being greater than a distance between said waveguide paths measured at said coupling region ends; and a plurality of electrodes (42, 44, 46) extending along said waveguide paths in a portion of said coupling region (40), said electrodes each having opposite electrode ends, each of said electrode ends being located adjacent to a respective one of said coupling region ends, characterised in that each of said electrodes (42, 44, 46) has a length , measured from one of said electrode ends to the other, which is less than the length of said coupling region (40), and in that the electrode ends of each of said electrodes (42, 44, 46) are spaced equidistantly from the respective adjacent coupling region ends. A waveguide type photoswitch in accordance with claim 1, wherein said waveguide paths (32, 34) comprise two waveguide paths, said electrodes (42, 44, 46) being positioned at the outside of said waveguide paths (32, 34), and between said waveguide paths (32, 34). A waveguide type photoswitch in accordance with claim 2, wherein said electrodes (42, 44, 46) positioned at the outside of and intermediate between said waveguide paths (32, 34) each comprises three segments arranged along said waveguide paths, outermost one of said segments of each of said electrodes (42, 44, 46) being used as bias electrodes. A waveguide type photoswitch in accordance with claim 2, wherein said electrodes (42, 44, 46) individually partly overlap said waveguide paths 32, 34). A waveguide type photoswitch in accordance with claim 3, wherein said electrodes (42, 44, 46) individually partly overlap said wave guide paths (32, 34). A waveguide type photoswitch in accordance with claim 4, wherein said electrodes (42, 44, 46) extend continuously in a centre part thereof adjacent to said centre of said coupling region (40). A waveguide type photoswitch in accordance with claim 5, wherein said waveguide paths (32, 34) diverge from each other continuously from each of said coupling region ends to said centre.

Inventors: ASABAYASHI ISSEI OKI ELECTRIC, OKAYAMA HIDEAKI OKI ELECTRIC I, ASABAYASHI, ISSEI. OKI ELECTRIC INDUSTRY CO., LTD., OKAYAMA, HIDEAKI, OKI ELECTRIC INDUSTRY CO., LTD.
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY CO., LTD.
Application year: 1990
Main classification: G02F1
IPC codes: G02B6, G02F1