OPTICAL WAVEGUIDE SWITCH

Abstract: An optical waveguide switch in which at least two optical waveguides intersect at a predetermined angle to each other to provide, on both sides of the intersecting region, incoming side optical waveguides (1, 2) for receiving incident light and outgoing side optical waveguides (1′, 2′) for outputting guided light and in which the incident light is input into one of the incoming side optical waveguides and is output from a desired one of the outgoing side optical waveguides. An optically non-linear material (3) whose refractive index undergoes a substantial variation, depending on the intensity of light, is disposed in the intersecting region and a loop (4) is provided in association with at least one of the incoming side optical waveguides (1), for essentially branching the guided light in said outgoing side optical waveguide (1′) for feedback to said at least one incoming side optical waveguide (1).

Description: The present invention relates to an optical switch for switching the propagation path of an optical signal and, more particularly, to an optical waveguide switch which utilizes light as control energy for triggering its switching operation. With the development of optical fibre communications the optical switching technology for switching an optical signal to a desired channel is now attracting attention. It is considered, in particular, that as an optical line becomes increasingly common in the subscriber's system, optical switches will be needed in vast numbers, and their miniaturization and sophistication are desired. There is also a demand for development of an integrated optical switching element for adaptation to submarine switching in a submarine optical fiber transmission system or satellite-loaded switch which is limited in space and required to be highly reliable. To meet with such requirements, mainly two types of optical switches are now under study. The one has an arrangement in which an optical signal is once converted to an electrical signal (an opto-electro conversion) and is switched by an electronic circuit as in the past, thereafter being re-converted to an optical signal (an electro-opto conversion). As a result of the recent progress of the OEIC (Opto-Electronic Integrated Circuit) technology, a four-by-four optical switch, though in a hybrid structure, has been reported. This O/E/O type optical switch which involves the conversion of an optical signal to electric form and then back to optical form is readily acceptable as a miniaturized version of a conventional optical switching device formed by discrete elements. On the other hand, however, it is considered that because of a large number of elements used this optical switch has the defects of complexity in its manufacture, low reliability and an increase in cross-talk by electromagnetic interference in the electronic circuit during high-speed operation. In addition, it is difficult, at present, to obtain an ultra high-speed electronic circuit capable of satisfactory operations at high frequencies above several giga-Hz. The other optical switch is one that switches an optical signal without conversion to an electric signal, and this switch is termed an O/O type. However, since these optical switches rely on electric energy such as a voltage and a current for switching the optical signal, they have to perform the O/E (opto-electro) conversion of a control signal superimposed on the optical signal. WO-A-84/03363 discloses another form of waveguide coupler in which two waveguides are arranged in parallel at a co-axial connection region incorporating a multiple quantum well which has a variable index of refraction according to its internal illumination. For one value of the index of refraction light is transmitted by multiple reflection along the length of the connection region, so as to leave the waveguide coupler on the same waveguide as it entered, and for another value of the index of refraction, so as to leave the coupler on the other waveguide. An object of the present invention is to provide an optical waveguide switch which permits the switching of an optical signal without the necessity of its opto-electro conversion. According to the present invention there is provided an optical waveguide switch comprising a first and a second optical waveguide, each of which has an intermediate region between an input portion and an output portion, the waveguides being optically coupled to each other through an optically nonlinear material whose refractive index is a function of the intensity of light incident onto said material, said optically nonlinear material being arranged between said intermediate regions of the waveguides such that light supplied to the input portion of one of the waveguides is coupled into the output portion of the other one of said optical waveguides when the intensity of the light is insufficient to effect a change in the refractive index of said nonlinear material, and that light supplied to the input portion of one of said waveguides is reflected into the output portion of this waveguide when the intensity of the light is sufficient to effect a change in the refractive index of said nonlinear material, said optical waveguide switch being characterised in that the input portions of the waveguides are aligned with the output portions of the waveguides so as to form an "X"-shaped waveguide arrangement, and in that a feedback waveguide is provided which connects the output portion and the input portion of one of said waveguides so as to provide a positive feedback of a part of the light reflected into the output portion back to said nonlinear material in order to enhance the change effected in the refractive index of said material. Positive feedback by means of the feedback waveguide enables the optical signal to be switched intact. Embodiments of the present invention will now be described in detail, by way of example, in comparison with the prior art and with reference to the accompanying drawings, in which: Figs. 1 and 2 show a known optical switch utilizing an electric control signal; Fig. 3 is a diagram explanatory of the principle of the optical switching element of the present invention; Figs. 4A and 4B show wavelength characteristics of the absorption coefficient and refractive index of an optically non-linear material; Figs. 5A and 5B illustrate embodiments of the optical waveguide switch of the present invention; Fig. 6 is a timing chart showing the operation of the embodiments depicted in Figs. 5A and 5B; Figs. 7A and 7B shows the relationship between input light and output light in the embodiments depicted in Figs. 5A and 5B; and Fig. 8 illustrates a further embodiment of the optical waveguide switch of the present invention. To make differences between prior art and the present invention clear, prior art will be described first. Figs. 1 and 2 show, in elevation, known optical switches. The optical switch depicted in Fig. 1 employs directional couplers. By applying a voltage as a control signal to a switching section the refractive indices are varied so that the phase matching condition between two waveguides is satisfied or not satisfied for switching incident light between them. The optical switch in Fig. 2 employs a current as a control signal. By injecting a current into the intersecting portion of two waveguides centrally thereof their refractive indices are varied with the resulting carrier density, whereby incident light is totally reflected and switched from one waveguide to the other. These optical switches do not involve the aforementioned O/E and E/O conversions of the optical signal, and hence have advantages in that they are simple in structure and capable of switching without impairing the wide band property of the optical signal. However, since these optical switches rely on electric energy such as a voltage and a current, for switching the optical signal, they have to perform the O/E conversion of a control signal superimposed on the optical signal, and accordingly they are inevitably provided with a photo-detector and circuit elements therefor. With reference to the accompanying drawings, embodiments of the present invention will hereinafter be described in detail. A description will be given first of the principle of operation of the optical switch according to the present invention. Fig. 3 shows the basic structure of an optical waveguide switch, explanatory of the principle of operation of the present invention. Waveguides 1, 1' and 2, 2' are provided so that they form an X-letter shape with each other. Reference numerals 1 and 2 indicate incoming waveguides and 1' and 2' outgoing waveguides. At the centre of their intersection an optically non-linear material 3 (such as, for example, a quantum well structure or quantum wire structure of semiconductor) is disposed. The reason why the refractive index of such a non-linear material 3 varies is explained as follows: The optically non-linear material 3 has an absorption spectrum based on the resonance of the levels of atoms or molecules forming it, as shown in Fig. 4A, and its absorption coefficient is related to the refractive index by the Kramers-Kronig relations, as shown in Fig. 4B. When light applied to the non-linear material is weak, it shows a characteristic such as indicated by the solid line, but when a material of a large non-linearity constant is irradiated by light of a certain intensity, its absorption is saturated as indicated by the broken line and accordingly the dispersion characteristic of the refractive index also varies as indicated by the broken line. That is, the refractive index decreases at wavelengths longer than the centre wavelength λ₀ of the absorption spectrum and increases at shorter wavelengths. The non-linear material 3 of such characteristics is disposed at an appropriate angle ϑ to the waveguide 1, as depicted in Fig. 3, and light is applied to the non-linear material through the waveguide 1 as indicated by P ex or directly from the outside as indicated by P' ex . When the incident light has a longer wavelength than the centre wavelength λ₀, the refractive index of the non-linear material lowers sharply and the incident light from the waveguide 1 is totally reflected and is switched to the waveguide 1'. Namely, the optical path is switched by light irradiation. In this instance, if the wavelength of the incident light P ex (P' ex ) is selected close to the wavelength λ₀, then the light can be switched at a lower intensity. Even if the wavelength somewhat deviates from that λ₀, the same effect could be produced by increasing the light intensity. Now, a detailed description will be given of an embodiment which utilizes the principle of operation of the optical waveguide switch according to the present invention. Figs. 5A and 5B illustrate an embodiment of the optical waveguide switch of the present invention, in which guided light, effective in practical applications, is used as irradiation light (hereinafter referred to as excitation light). In Fig. 5A reference numerals 1 and 2 indicate incoming side waveguides, 1' and 2' outgoing side reflection and transmission waveguides, 3 a non-linear material, 4 a feedback waveguide, 5 an optical amplifier and 6 a semi-conductor material other than the non-linear material 3. The incoming and outgoing waveguides 1 and 1' and the feedback waveguide 4 are formed to constitute a loop as shown. Although in this embodiment the optical feedback amplifier 5 facilititating an optical feed back is provided on the feedback waveguide 4 so that it may operate as a transmitting type optical amplifier upon injection of current, this optical amplifier need not always be provided and only the positive feedback loop will suffice in practical applications. To facilitate a better understanding of this optical switching element, Fig. 6 shows timing charts of excited light P ex , reflected light P o , reflected signal light P so , transmitted signal light P' so , the reflectivity R at the interface between the non-linear material 3 and the surrounding semiconductor material 6 and an injection current I al to the optical amplifier 5. When the excited light P ex of a longer wavelength than the absorption centre wavelength λ₀ is incident to the incoming side waveguide 1, it is absorbed by the non-linear material 3, in consequence of which the absorption coefficient of the non-linear material 3 decreases as shown in Fig. 4 and its refractive index also lowers correspondingly. Since the refractive indices of the non-linear material 3 and the surrounding material 6 are selected such that they are equal to each other in the absence of the excitation light P ex , the reflectivity at the interface between them is zero. When a refractive index difference has occurred upon incidence of light P ex , the reflectivity R starts to increase and a portion of the light P ex is reflected toward the outgoing side waveguide 1'. Further, a portion of the reflected light P o is branched into the feedback waveguide 4, in which it is amplified by the optical amplifier 5 and the amplified light is led again to the incoming side waveguide 1, thereafter being absorbed by the non-linear material 3. This process further promotes the reduction of the absorption coefficient of the non-linear material 3, and consequently the reflected light P o also increases. That is to say, by the presence of the feedback waveguide 4 a positive feedback is caused in the absorption and refractive index variation of the non-linear material 3 and the abovementioned process is repeated, by which the feedback effect is markedly accelerated, reducing the refractive index to such an extent that total reflection will ultimately occur. In this case, by selecting the gain of the optical amplifier 5 larger than a certain value, the total reflection can be maintained even if the excitation light P ex is cut off. Furthermore, the reflectivity R is restored to zero by decreasing the current I al which is injected into the optical amplifier 5. Accordingly, by applying the signal light P s in the period during which the excitation light P ex and the injected current I al are both increased, the signal light P s is totally reflected and emitted through the outgoing side waveguide 1'. On the other hand, in a second interval in which no excitation light P ex is applied, there will not occur the abovementioned process which gives rise to total reflection and the signal light P s incident through the incoming side waveguide 1 is transmitted intact throughout the switch and is output through the outgoing side waveguide 2'. In other words, the signal light P s can be switched to the waveguide 1' or 2', depending on the presence or absence of the excitation light P ex . Incidentally, the component P o ascribable to the excitation light P ex is superimposed on the output from the waveguide 1' but it can be eliminated subsequently by an electric filter because each interval is longer than the time slot of the signal light P s . Fig. 5B shows a case where the feedback waveguide 4 is provided corresponding to each of the incoming side waveguides 1 and 2. The use of two feedback waveguides permits enhancement of the reflectivity of the non-linear optical material 3 even if the excitation light P ex is low in intensity. The above is a case where the gain of the optical amplifier 5, that is, the injection current I al . is relatively large. Fig. 7A shows the relationship between the excitation light P ex and the output light P o from the waveguide 1'. As will be seen from Fig. 7A, once the excitation light P ex is applied, the output light P o continues to be provided (the total reflection state), and when to stop this state, the current I al must be cut off. On the other hand, when the current I al is relatively small, the output light P o shows such bistability as depicted in Fig. 7B. In this instance, each output P o can be switched from its high level (the total reflection state) to its low level (the zero reflection state) by holding the excitation light P ex at the centre P exb of the bistable loop and adding thereto incremental or decremental pulses as shown. In other words, the switching can be achieved with the excitation light P ex alone, and a direct current can be used as the current I al . The abovedescribed embodiment employs, as the feedback waveguide 4, a simple Y-shaped structure for coupling and branching the incoming and outgoing side waveguides 1 and 1'. Fig. 8 illustrates another embodiment in which the feedback waveguide 4 utilizes a Bragg reflection type branching filter (7) having wavelength selectivity, for branching to the outgoing side. Generally speaking, the more the wavelength λ ex of the excitation light P ex approaches the absorption centre wavelength λ₀ shown in Figs. 4A and 4B, the more the absorption of light is saturated and the insertion loss for the signal light P s is small when its wavelength λ s is longer than the absorption centre wavelength λ₀ to some extent. Accordingly, in case of a system in which the wavelengths of the excitation light P ex and the signal light P s are adopted, so that λ ex ≃ λ₀ and λ s > λ ex , the Bragg reflection type branching filter 7 is formed so that its Bragg wavelength may be equal to λ ex . By this, only the signal light P so can be output from the outgoing side waveguide 1' and the insertion loss for the signal light P s can also be decreased. Also in this embodiment, the feedback waveguide 4 can be used for each of the incoming side waveguides 1 and 2. The waveguides can be formed by ridge, rib and buried structures, using semiconductors such as the In GaAsP and the AlGaAs crystal systems. As will be appreciated from the foregoing description, in the embodiments of optical waveguide switch of the present invention, the optically non-linear material 3 is used in its switching section to provide the zero reflection or total reflection state, depending on the intensity of light, and at least one feedback waveguide 4 is provided for controlling the optically non-linear material 3 to permit switching of an optical signal even in case of a feeble excitation light. Accordingly, embodiments of the present invention provide a wide band optical switching element of a simple device structure with no opto-electro transducer and hence is of great practical utility.

Claims: An optical waveguide switch comprising a first and a second optical waveguide, each of which has an intermediate region between an input portion (1,2) and an output portion (1',2'), the waveguides being optically coupled to each other through an optically nonlinear material (3) whose refractive index is a function of the intensity of light incident onto said material (3), said optically nonlinear material (3) being arranged between said intermediate regions of the waveguides such that light supplied to the input portion of one of the waveguides is coupled into the output portion of the other one of said optical waveguides when the intensity of the light is insufficient to effect a change in the refractive index of said nonlinear material, and that light supplied to the input portion of one of said waveguides is reflected into the output portion of this waveguide when the intensity of the light is sufficient to effect a change in the refractive index of said nonlinear material, said optical waveguide switch being characterised in that the input portions of the waveguides are aligned with the output portions of the waveguides so as to form an "X"-shaped waveguide arrangement, and in that a feedback waveguide (4) is provided which connects the output portion and the input portion of one of said waveguides so as to provide a positive feedback of a part of the light reflected into the output portion back to said nonlinear material (3) in order to enhance the change effected in the refractive index of said material. An optical waveguide switch according to claim 1, wherein said feedback waveguide (4) is provided with an optical amplifier (5). An optical waveguide switch according to claim 1 or 2 wherein said feedback waveguide (4) is provided with a Bragg reflection type branching filter (7).

Inventors: MATSUSHIMA, YUICHI, SAKAI, KAZUO, UTAKA, KATSUYUKI
Applicants: KOKUSAI DENSHIN DENWA KABUSHIKI KAISHA
Application year: 1988
Main classification: G02B6
IPC codes: G02F1, G02B6