OPTICAL SWITCH

Abstract: An optical switch has a semiconductor waveguide region (2) formed on a substrate (1), with a cladding region (3) on a projection of the waveguide region (2). By controlling the fields applied to electrodes (4, 5, 6) on the substrate (1), the waveguide region (2), and the cladding region (3), the injection and extraction of carriers into and from the wave­ guide region can be controlled. It is found that the carriers can be removed rapidly through the electrodes (4, 5, 6) and this causes the switch to turn off very rapidly.

Description: The present invention relates to an optical switch using carrier injection so as to utilize the change in refractive index and/or absorption coefficient caused thereby. Although the word "optical" is used, and reference is made to switching "light", the present invention is applicable generally to switching electromagnetic radiation, and not merely radiation in the visible wavelengths. One known type of optical switch using the carrier injection is the total internal reflection type (TIR) optical switch. This type of optical switch uses a change in the refractive index to alter its output port. This type of optical switch has been described in the Collected Papers S7-4 for the 1985 National Meeting of Semiconductor Material Department of Institute of Electronic Communication Engineer of Japan, INOUE et al., and in European Patent Application No. EP-A-147195 (MATSUMURA et al). Another known type of optical switch is the loss modulation type optical switch. This type of optical switch uses a change in absorption coefficient to enable it to act as an intensity modulator and/or an ON/OFF switch. A description of this type of optical switch is found in the Collected Papers 926 for the 1985 Combined National Meeting of Institute of Electronic Communication Engineers of Japan, YAMAGUCHI et al. These optical switches utilize a large refractive index change and/or absorption coefficient change caused by the injection of carriers, and have the advantages of small size and high extinction ratio. In these optical switches, however, the carriers injected into a switching region, to cause alteration of output port and/or intensity modulation and/or ON/OFF switching have an extinction speed determined by the recombination lifetime when the optical switches are to be turned OFF, i.e. when the injection of the carriers is stopped. Consequently, the response speed of the switches is limited to the order of several nanoseconds to several tens of nanoseconds. Thus the known optical switches have the problem that no consideration has been given to increasing the response speed (switching speed) of the optical switch, and hence it is difficult to make the switching speed less than several nanoseconds. The present invention seeks to improve significantly the switching speed of a carrier injection type optical switch, and thus permit the production of an element small in size and with a high extinction ratio, and therefore suitable for use in an optical integrated circuit. According to the present invention, there is provided an optical switch comprising: an optical junction region including a semiconductor substrate region a semiconductor cladding region having the same conductivity type as that of the substrate region, and a semiconductor waveguide region having a conductivity type opposite to that of the substrate region and being between the semiconductor substrate region and the semiconductor cladding region; an input waveguide and an output waveguide optically connected to each other via the optical junction region; and a first and a second electrode for applying to the semiconductor waveguide region an electric field causing the injection of carriers into the semiconductor waveguide region so as to change the intensity or the direction of propagation of light passing through the optical junction region; wherein the optical switch has a third electrode for applying to the waveguide region an electric field causing the extraction of the injected carriers from the semiconductor waveguide region so that the extraction of the injected carriers brings a rapid change in a refractive index or absorption coefficient in the semiconductor waveguide region. Embodiments of the invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: Figs. 1A and 1B show a total reflection type optical switch being a first embodiment of the present invention, Fig. 1A being a plan view and Fig. 1B being a cross-sectional view along the line B-B' in Fig. 1A; Figs. 2A and 2B show a second embodiment of the present invention, Fig. 2A being a plan view and Fig. 2B being a cross-sectional view along the line B-B' in Fig. 2A; Fig. 3 shows a cross-sectional view of a third embodiment of the present invention; Fig. 4 is a diagram of a circuit for evaluation of the characteristics of an optical switch according to the present invention; Fig. 5 shows a cross-sectional view of a fourth embodiment of the present invention; Figs. 6A and 6B show a fifth embodiment of the present invention, Fig. 6A being a plan view and Fig. 6B being a cross-sectional view along the line B-B' in Fig. 6A; and Figs. 7A and 7B show a sixth embodiment of the present invention, Fig. 7A being a plan view and Fig. 7B being a cross-sectional view along the line B-B' in Fig. 7A. First the principle of operation of an optical switch according to the present invention will be described referring to the total internal reflection type optical switch illustrated in Figs. 1A and B. As illustrated in Figs. 1A and 1B, electrodes 4,5 and 6 are provided in close contact with or in the vicinity of semiconductor regions 1,2 and 3 respectively (being a substrate region 1, a wave-guide region 2, and a cladding region 3). A circuit (not shown) applies a potential difference between the electrodes 4 and 5 and/or between the electrodes 5 and 6 in the period after injection of carriers ceases. In this way, carriers can be extracted at a very high speed from the semiconductor region 2 through which light waves are guided. In other words, the carriers can be extracted in accordance with a transit time τ t determined by the diffusion of the carriers. The transit time τ t is given by the following equation, in which the distance of transfer of the carriers through the semiconductor region 2 is denoted by W and a diffusion coefficient by D: It is known that the diffusion coefficient is about 80 to 130 cm²/sec when the carriers are electrons and the semiconductor region 2 is of e.g. GaAs, GaAℓAs, InP, InGaAs or InGaAsP. When W = 1 µm, then τ t = 39 to 63 ps, which means that the carriers can be extracted at a very high speed. This rapid change in the number of carriers in the region 2 causes a correspondingly rapid change in the refractive index and/or the absorption coefficient in the switching region. Therefore, the light reflectivity of the total internal reflection type optical switch shown in Figs. 1A and 1B changes sharply, and thus a high-speed switching operation can be obtained. The time τ required for this switching operation is expressed generally by a value about 2.3 times larger than the value of τ t , and thus high-speed operation of about 90 to 145 ps may be achieved. As stated above, the change in the amount of carriers in the region 2 results in the change in the refractive index and/or the absorption coefficient. It is clear, therefore, that the principle of the present invention may be applied to any optical switch which is operated by the injection of carriers, such as a directional coupler type switch, an interferometer type switch such as the Mach-Zehnder interferometer, a cavity type switch, a loss modulation type switch, a cut-off type switch, and a variety of others, in addition to the total internal reflection type optical switch of Figs. 1A and 1B. Examples of the present invention will now be described. Example 1 The first example is of the embodiment of the invention shown in Figs. 1A and 1B. First, Zn diffusion regions 7 were formed in parts of the surface layer of an n-type InP substrate 1 having a carrier density of 2 x 10¹⁸ cm⁻³. Then a p-InGaAsP layer (optical waveguide layer) 2, an n-InP layer (cladding layer) 3 and an n-InGaAsP layer (cap layer) 13 were formed sequentially by using liquid-phase epitaxy. Next, Zn was diffused into parts of the InP layer 3 and the cap layer 13 to form current blocking regions 8. Then, parts of the layers 13, 3 and 2 were removed by using a phosphoric acid etchant, to form projecting ridge waveguide 9 and 10, as illustrated in Figs. 1A and B. Finally, rhombic electrodes 5 of Au-Zn and electrodes 4 and 6 of Au-Ge-Ni respectively were formed by using evaporation, and thereby a 2 x 2 terminal optical switch was produced. The optical switching characteristic of the optical switch thus formed was examined by applying a beam of light of wavelength 1.55 µm to the light incident end 14 of the optical switch and by measuring the intensity of the light emitted from the light emitting ends 16 and 17 thereof. With electric connection arranged as shown in Fig. 4, an electric signal of a pulse width 2 ns, a pulse rise time of 0.1 ns and a pulse amplitude 5 Vp-p was applied by using an oscillator 19. The effect was that beams of light emitted from the light emitting ends 16 and 17 of the switch were opposite in polarity to each other and had the same waveform as that of the electric signal. The delay in response was 0.5 ns or less in terms of the delay in rise time and fall time. Example 2 The optical switch of this second example has a structure shown in Figs. 2A and B, which is generally similar to that of the embodiment of Figs. 1A and 1B, was manufactured according to a similar method. The differences between the embodiment of Figs. 1A and 1B, and the embodiment of Figs. 2A and 2B is that a strip-like cladding region 3 was formed directly on the waveguide region 2, so that it was not necessary to etch the layers to form the waveguides. The current blocking regions 8 were omitted, and the electrodes 5 on the waveguide region 2 were strip-like, rather than rhomboidal. The same results as in the first example were obtained. Example 3 A 1 x 2 terminal optical switch having a Y-branch structure but otherwise similar to that of the second embodiment was manufactured according to a method similar to that described above. The same results were obtained. Example 4 The same results were obtained by modifying the second embodiment so that it had an electrode structure as shown in the embodiment of Figs. 6A and 6B. In this embodiment, an insulating layer 30 was provided on the waveguide region 2 adjacent the cladding region 3 so that an electrode 6 was in contact only with the cladding region 3 being invisible behind it and not shown in the figure and not in contact with the waveguide region 2. Example 5 This example will be described with reference to Fig. 3. First a channel 11 was formed on the surface of an n-type InP substrate 1 having a carrier density of 2 x 10¹⁸ cm⁻³ by a conventional etching method. This formed optical waveguides corresponding to the optical waveguides 9 and 10 in the first embodiment. Next, Zn was diffused into part of the surface of the substrate 1 to form current confining layers 7. Subsequently, a p-InGaAsP layer 2 in which a wave-length corresponding to the band gap energy was 1.5 µm, an n-InP layer 3, and a cap layer 13 of InGaAsP, were formed sequentially by liquid-phase epitaxy. Then, Zn was diffused into parts of the n-InP layer 3 and the cap layer 13 to form a current channel 12 (a current confining layer in relation to injected current). Lastly Cr-Au was evaporated to provide electrodes 5 and 6, and Au-Ge-Ni was evaporated to provide an electrode 4, the optical switch thus being completed. The optical switch thus formed showed the same operation characteristics as in the first embodiment. The optical waveguide of this example is called a channel stripe waveguide. With embodiment of Fig. 3, the same results were obtained either by using the Y-branch structure of the third example or by using the electrode structure shown in Figs. 6A and 6B. Example 6 This example will be described by using Fig. 5, which shows an optical switch in which an optical waveguide having a Buried Hetero-junction (BH) structure is formed on an insulating substrate. An n⁺-InP layer 21 was grown on an insulating InP substrate 100 by liquid-phase epitaxy. Then Zn was diffused into parts of this layer, and thereby a current confining layer 7 was formed. Next, a p-InGaAsP layer (buffer layer) 25, a p-InGaAsP waveguide layer 26, an n-InGaAsP layer (buffer layer) 27, an n-InP layer (cladding layer) 28 and an n-InGaAsP layer (cap layer) 13 were grown sequentially in a crystallizing manner by liquid-phase epitaxy. Subsequently mesa etching was used to form the layers 7, 25, 26, 27 and 28 with a mesa structure respectively by using a Br-methanol etchant. A p-InGaAsP layer 22 and an n-InP layer 23 were then grown so that they buried said mesa structures. Subsequently, Zn was diffused to form a current confining layer 24, selective etching was applied to expose parts in which electrodes were to be formed, electrodes 4, 5 and 6 were formed therein by evaporation, and thus the optical switch was completed. This embodiment operates in a similar way to the first embodiment, but is easier to apply to integration, because all the electrodes of the optical switch of this embodiment are formed on one surface only of the substrate. In this embodiment, the same results were obtained either by using a Y-branch structure as in the third example, or by using the electrode structure shown in Figs. 6A and B. Because injected carriers are used to change the refractive index to operate the optical switch, in the six examples described above, the present invention is applicable to any optical switch construction, such as a directional coupler type, an interferometer type, or a cavity type in which change in refractive index may be used, in addition to the total internal reflection type of switch. Example 7 This example will be described with reference to Figs. 7A and 7B. Manufacture of this switch was made by the same method as in the case of the first example. The optical switch of this example is different from that of the first example, in that it has only one waveguide 9 and that the width of the electrode 6, i.e. its dimension in the B-B' direction, is about the same as the width of the waveguide 9. When light was applied to the light incidence end 14 of this optical switch and the intensity of the light emitted from the light emitting end 16 thereof was measured in the same way as in the first example, the optical signal was modulated in accordance with the waveform of the signal applied to the electrode 6, while the response speed characteristic thereof was the same with those in the first, second, and sixth examples. In this seventh example, in which a strip loaded waveguide was employed, the same result was obtained also when a waveguide of a ridge type, a channel strip type or a BH type having the same waveguide structure as in the first, second, fifth and sixth examples was employed. In the seven examples described above, the same results were obtained when the conductivity type of each semiconductor region was reversed. Moreover, the same result was obtained when semiconductors of GaAs-GaAℓAs or Si were employed, and furthermore, the present invention could be adapted also when the junctions between semiconductor regions were homogeneous. In the examples described above, current confining layers 7, 8, 12 and 24 were provided but satisfactory results could also be obtained with a construction in which part or the whole of such layers was removed. Although the efficiency of switching is reduced slightly due to occasional diffusion of a current, there is the advantage that manufacture is facilitated. The present invention permits the production of an optical switch which is small in size and has a high extinction ratio and a remarkably high response speed, thus facilitating the construction of a large capacity optical transmission-communication system or an optical exchange system.

Claims: An optical switch comprising: an optical junction region including a semiconductor substrate region (1;21), a semiconductor cladding region (3;28) having the same conductivity type as that of the substrate region, and a semiconductor waveguide region (2;26) having a conductivity type opposite to that of the substrate region and being between the semiconductor substrate region and the semiconductor cladding region; an input waveguide (14,15) and an output (16,17) waveguide optically connected to each other via the optical junction region; and a first and a second electrode (4,6) for applying to the semiconductor waveguide region an electric field causing the injection of carriers into the semiconductor waveguide region so as to change the intensity or the direction of propagation of light passing through the optical junction region; characterised in that: the optical switch has a third electrode (5) for applying in combination with the first and second electrodes to the waveguide region an electric field causing the extraction of the injected carriers from the semiconductor waveguide region so that the extraction of the injected carriers brings a rapid change in a refractive index or absorption coefficient in the semiconductor waveguide region. An optical switch according to claim 1, wherein the injection of carriers into the waveguide region (2,26) causes a change in the direction of light output of the switch or the modulation of light intensity, from a first state to a second state and the extraction of the injected carriers from the waveguide region (2,26) causes a change in the direction of light output of the switch or the modulation of light intensity from the second state to the first state. An optical switch according to claim 1 or claim 2, wherein the substrate region (1,21), the waveguide region (2,26), and the cladding region (3,28) are of compound semiconductors. An optical switch according to any one of claims 1 to 3, wherein there is at least one current confining region (7) in at least one of the substrate region (1,21), the waveguide region (2,26) and the cladding region (3,28). An optical switch according to any one of the preceding claims having at least two output waveguides (9,10) defined by the waveguide region (2,26) and/or the cladding region (3,28), the second electrode (6) being on the cladding region (3,26) extending over the junction region, and the third electrode (5) being on the waveguide region (2,26) adjacent the junction region but not overlapping. An optical switch according to any of the preceding claims, wherein the injection of carriers in to the waveguide region causes a change of absorption coefficient from a first value to a second value for modulating the intensity of light in the waveguide region, and the extraction of the injected carriers from the waveguide region causes the change of absorption coefficient from the second value to the first value. An optical switch according to any one of the preceding claims wherein each of the first and second electrodes (4,6) is on the substrate region and the cladding region respectively, and the third electrode (5) is on the waveguide region. An optical switch according to any one of the previous claims wherein the optical switch is a total internal reflection type optical switch. An optical switch according to any one of the previous claims wherein the optical switch uses a strip loaded waveguide. An optical switch according to any one of the previous claims wherein the optical switch includes a Y-branch structure. An optical switch according to any one of the previous claims wherein the optical switch includes a channel stripe waveguide. An optical switch according to any one of the previous claims wherein the optical switch includes a buried heterojunction. An optical switch according to any one of the previous claims wherein the optical switch includes a single waveguide.

Inventors: INOUE, HIROAKI, MATSUMURA, HIROYOSHI, OKADA, YOSHITAKA, TADA, KUNIO
Applicants: HITACHI, LTD.
Application year: 1987
Main classification: G02F1
IPC codes: H01L27, G02F1