Abstract: An electro-optical device for controlling an optical signal in response to an electrical signal includes a substrate having an optical path in which at least two waveguide arms diverge from an input portion. Typically, the waveguide arms include parallel regions and include convergence to an output portion. An electrical transmission line having a desired electrical impedance is positioned relative to the substrate for selectively changing the indices of refraction of the waveguide arms. The transmission line is a multi-branch design, with the branches in a one-to-one correspondence with the waveguide arms, so that each signal branch is aligned relative to a corresponding waveguide arm to couple an index-affecting electrical field to the corresponding waveguide arm. Preferably, the waveguide arms have opposite poling polarities, with each poling polarity being perpendicular to the direction of light propagation through the waveguide arms.

Abstract: Integrated optical devices which utilize a magnetostrictively, electrostrictively or photostrictively induced stress to alter the optical properties of one or more waveguides in the device are disclosed. The integrated optical devices consist of at least one pair of optical waveguides preferably fabricated in a cladding material formed on a substrate. A stress applying material, which may be a magnetostrictive, electrostrictive or photostrictive material, is affixed to the upper surface of the cladding material near at least one of the optical waveguides. When the appropriate magnetic, electric or photonic field is applied to the stress applying material, a dimensional change tends to be induced in the stress applying material. The constrained state of the stress applying material, however, caused by its adhesion to the cladding material, causes regions of tensile and compressive stress, as well as any associated strains, to be created in the integrated optical device.

Abstract: A method of bonding glass-based optical elements comprising the steps of positioning a first glass-based optical element relative to a second glass-based optical element, applying a glass-based bonding compound about the first and second optical elements, and applying sufficient localized heat to the glass-based bonding compound to cause the glass-based bonding compound to soften and fuse with the optical elements.

Abstract: A Y-junction optical switch includes an input branch and two output branches optically coupled to the input branch at a junction. A single one of the output branches has an electrode. The two output branches are designed to have dissimilar optical refractive indices under a non-biasing condition.

Abstract: A first terminal is connected to a second terminal by means of a first waveguide having a first radius of curvature. The first terminal (or second terminal) is connected to a third terminal by means of a second waveguide (or a third waveguide) having a second radius of curvature which is grater than the first radius of curvature. Incident light supplied from the first terminal is branched between the first waveguide and the second waveguide, and the light entering the first waveguide undergoes the greater attenuation since this waveguide has the smaller radius of curvature. Hence, the light component passing through the first waveguide has a difference in output from the component passing through the second waveguide that is sufficiently great to provide a large branching ratio.

Abstract: A method of producing passive and/or active optical polymer components (10) having vertical coupling structures (45), wherein at least one structure (22) for receiving a light waveguide (40; 46), and at least two fiber guide structures (14) and at least two adjusting structures (36) are produced on a substrate plate (32) and on a lid plate (34) such that both the plate and the lid plate possess fiber guide structures (14) with which the adjusting structures (36) located opposite in the lid plate or substrate, plate respectively are associated after assembly, and wherein the light waveguide structures (22) of the substrate plate (32) and the lid plate (34) which connect the fiber guide structures (14) extend parallel to each other in at least one region (45) such that the optical fields in the respective light waveguides (40) can be coupled with one another.

Abstract: A wavelength conversion element includes a substrate made of LiTaO.sub.3 of non-linear medium, a three-dimensional waveguide formed in the substrate and a second harmonic generation portion for defining a change in refractive index in the waveguide with a period .LAMBDA. and generating a second harmonic of a fundamental wave introduced into the wave guide, where .LAMBDA.=2 m.pi./{.beta.(2.omega.)-2.beta.(.omega.)}, m is a natural number, .omega. is a frequency of fundamental harmonic, .beta.(.omega.) is a propagation constant of the fundamental wave, and .beta.(2.omega.) is a propagation constant of second harmonic.

Abstract: A polarization splitter comprises an optical waveguide formed on a substrate and having an S-shaped curve portion, for permitting propagation of two linearly polarized lights orthogonal to each other therethrough, and an anisotropic optical waveguide branching from the beginning of the S-shaped curve portion, into which is guided one of the two linearly polarized lights alone.

Abstract: Optical waveguide isolator (121) for monolithic integration with semiconductor light emitting diodes such as Fabry-Perot or ring laser diodes. The present optical isolator (121), with optical input port (95) and output pod (96), comprises a branching waveguide coupler (56). This branching waveguide coupler (56) has a waveguide stem (60) splitted at one end into two waveguide branches (57, 58) such that a light wave fed via said input pod (95) into a first of these branches (58), is guided via the waveguide stem (60) and the output pod (96) out of the device. A light wave fed to the isolator's output pod (96) is guided along the stem (60) and coupled into the second waveguide branch (57).

Abstract: The present invention relates to methods and apparatus for wavelength division multiplexing (WDM) using arrays of optical filtering elements to create a desired wavelength passband in an optical filter. In one aspect of the invention, an optical demultiplexer is formed using an input waveguide, such as a planar waveguide or an optical fiber, coupled to plural output waveguides. Each output waveguide includes a wavelength selective configuration of optical filtering elements formed within a contiguous portion of the waveguide forming an optical channel-selective filter having spectral regions having an optical transmission passband and spectral regions of low transmissivity. Exemplary optical filtering elements are Bragg gratings formed into an optical filter which transmits a characteristic wavelength band.

Abstract: An optical waveguide device formed by a material showing anisotropy for refraction index and an optical instrument using the same device. The device comprises an electro-optic single crystal substrate on which are formed a first core portion higher in refractive index than the substrate with respect to both ordinary and extraordinary rays and a second core portion higher in refractive index than the substrate with respect to extraordinary rays pnly whereby the first core portion forms a single-moded waveguide for ordinary rays, and the overlapping region of the first and second core portions forms a double-moded waveguide for extraordinary rays. This optical waveguide device is used in place of a pinhole elementin an optical system of a confocal scanning optical microscopeso as to guide illuminating light to an object to be detected and to guide the reflected light from the object into a detector.

Abstract: We disclose a new optical TE-TM mode splitter on lithium niobate. The splitter is fabricated by using an asymmetric Y-junction structure which is composed of a straight waveguide, and two branch waveguides. The straight waveguide is the input one that can guide randomly-polarized light (i.e. both TE and TM modes). The two branch waveguides are the output ones, and one of them can guide only the TE mode, the other can guide only the TM mode. Because the output waveguides can individually guide the TE and TM modes, the input modes are then split by the branch waveguides. The input waveguide is fabricated by diffusing titanium into lithium niobate. The two output waveguides are made by nickel indiffusion and magnesium-oxide induced lithium outdiffusion or proton exchanged techniques.

Abstract: An integrated optical, polarization-manipulating device, comprises a first waveguiding section (A) with a first guide (21), a second waveguiding section (E) with a second and a third optically decoupled and physically separated guide (28, 29) and an intermediary waveguiding section (C) for an adiabatic coupling between the first and the second sections (A, E). The intermediary section (C) comprises a polarization-sensitive asymmetric Y-branching device, provided with two mutually diverging intermediary guides (25, 27) coupled to the guides (28, 29) of the second section. At least one of the two intermediary guides (25) has a channel structure fragmented in the longitudinal direction. Coupling sections (B, D) for adiabatic couplings between guides of successive sections are provided, on the respective coupling sides, with the fragmented waveguide of a corresponding fragmented structure.

Abstract: An optical waveguide device has a casing, a base member housed in the casing, at least one input optical fiber cable and at least one output optical fiber cable which are introduced into the casing, an optical waveguide substrate mounted on the base member and connected to respective ends of the input and output optical fiber cables for transmitting a light signal from the input optical fiber cable to the output optical fiber cable, support bases and holders fixed to the base member for holding end portions of the input and output optical fiber cables on the base member, and a bushing or bushings in the casing for introducing the input and output optical fiber cables into the casing. The optical waveguide substrate has an optical waveguide branched into a plurality of waveguide paths, and input and output optical fiber cables are coupled to each other through the optical waveguide.

Abstract: Optical logic gates are designed using inverse-scattering theory, resulting in devices which are intrinsic and passive, having high extinction ratios and large fan out. The Boolean operation is modeled by a pair of reflection coefficients, corresponding to the `1` and `0` states of the device. When inverse scattering theory is applied, each reflection coefficient yields a unique refractive index profile. The multivalued nature of the refractive index profile is achieved by introducing nonlinearity in the core region of a thin-film waveguide, which acts as the basic structure of the devices.

Abstract: We describe a technique for producing a flat passband in a wavelength multiplexer. We obtain this by combining two output waveguides, and by optimizing their parameters so as to produce a maximally flat passband with minimal loss. The optimum parameters are obtained, to a good approximation, by choosing each output waveguide so that its mode approximately matches the input waveguide mode (see page 9 on) and by choosing the spacing of the two waveguides so that the multiplexer transmission coefficient at the edges of the passband is approximately equal to the value at the center of the passband. We also include, in the multiplexer, additional waveguides that allow the center wavelength of the passband to be varied in steps of S/Q, where S is the channel spacing and .delta. is an integer exceeding the number of channels (multiple control input waveguides). As a result, we obtain a unique arrangement, suitable for realization in integrated form using waveguide arrays.

Abstract: An optical connector (2) includes housing halves (4A) and (4B) having Y-shaped contours (6A, 6B, 8A, 8B) in the abutting faces of the housing halves (4A, 4B). A Y-shaped member (30) is inserted into the housing halves and sandwiched between the two housing halves. An optically transparent gel (20) is inserted into the housing adjacent to the end faces of the Y-piece, such that upon insertion of the optical fibers into the openings created by the Y-shaped contours, the unfinished fiber ends are in optical connection with the gel (20) and split into separate signals.

Abstract: Optical waveguide devices for interconnecting optical fibers and use in integrated optical systems. The invention includes a substantially dry method for making the devices and intermediate elements.

Abstract: An optical demultiplexing/multiplexing device made with a few stable components by easy fabrication processes, which provides a flat response over the specified frequency range.

Abstract: A wavelength-division-multiplexed transmitter has a single modulator modulating the output of an array of individually actuable semiconductor lasers integrated onto the same substrate as the modulator. An optical combiner integrated onto the same substrate between each of the lasers and the single modulator combines the outputs of the lasers. One or more of the lasers are activated as desired for wavelength division multiplexing.

Abstract: In a wavelength division multiplexer suitable for integrated optical systems, to multiplex a succession of waves ranked according to their wavelength two diffraction gratings are formed at the edges of a common guide area. One focuses odd ranked waves onto a first entry of an output guide. The other focuses even ranked waves onto a second entry of the same output guide.

Abstract: A bidirectional light transmission system comprises a transmitting station having a light source for intensity-modulating a transmission signal, a first light branching portion for branching intensity-modulated light of the light source in a predetermined ratio and a first light receiving portion, and a receiving station having a second light branching portion, a light phase-modulating portion, a light reflecting portion and a second light receiving portion.

Abstract: An electro-optical modulator system obtained by linking a modulator incorporating a Y-junction coupler having a length Ll(101-108) to a passive coupler (104,105) whose length L2 in relation to the coupling length LC2 is determined so as to cancel substantially the third order distortions of the modulator. The resulting modulator has all of its even distortions cancelled and its uneven distortion is substantially zero. Furthermore, the rate of decay can potentially be zero.

Abstract: A semiconductor device is operated as an optical filter. The semiconductor device has a substrate and a monolithically integrated branched waveguide structure disposed above the substrate, portions of the waveguide structure being divided into a plurality of regions by troughs, one of the regions being a branching region. Light is radiated into the waveguide structure at an end face of one of the regions and currents that are smaller than respective laser threshold currents of the regions flow through the regions, including the branching region, perpendicular to a propagation direction of light through the waveguide.

Abstract: A broadband optical fiber coupler comprising at least three continuous optical fibers fused at a region of coupling, at least one of the continuous optical fibers being dissimilar to others of the continuous optical fibers in the region of coupling, the region of coupling being of a length and the dissimilarity being of a degree to provide broadband response over a predetermined range of wavelengths. A related method for making a broadband optical fiber coupler is also disclosed.

Abstract: An integrated optical receiver/transmitter (transceiver) 8 has continuous wave (cw) light 16 that enters a Mach-Zehnder optical modulator 10 controlled by a modulator control circuit 12 that provides modulated light 30 along a waveguide 32 to an optical coupler 34 which couples a predetermined portion of the light 30 to a waveguide 44 as light 42 which then exits the transceiver from a port 46. Receive light 90 is accepted at the port 46 and travels along the waveguide 44 to the coupler 34 which couples a predetermined amount of the light 90 to a waveguide 70 as light 92 which is detected by a waveguide-integrated photodetector 72. The photodetector 72 provides a current signal on a line 74 to a receiver circuit 76 which provides a voltage signal indicative of the light 92. Alternatively, the coupler may be passive, thereby not requiring the coupler control circuit, or no coupler at all may be employed and two fibers used for communications with the transceiver 8.

Abstract: An integrated optical coupler comprises a substrate with fiber-aligning grooves and waveguiding channels and a covering with complementary grooves and, optionally, channels. These couplers can be made from reproducible masters and electroplated molds made from those masters.

Abstract: A single silica optical fiber (4) is formed in a Sagnac interferometer (2) by forming a fused, dichroic optical fiber coupler (8) having four ports (10, 12, 14 and 16). A 1.3 .mu.m optical data signal is coupled to the port (10) via a dichroic optical fiber coupler (18) which combines it with a 1.53 .mu.m c-w probe signal from laser (16). The coupler (8) splits the 1.53 .mu.m probe signal into two equal intensity, counter-propagating portions and couples the 1.3 .mu.m data signal so it propagates in one direction round the loop (6). The loop (6) exhibits a non-linearity such that the data signal provides a relative phase shift of the counter-propagating c-w probe portions causing a portion of the 1.53 .mu.m signal is switched to port (12). This 1.53 .mu.m switched output provides tapping of the data signal without affecting its intensity.

Abstract: The present invention relates to a new tunable filter which, when used as an intracavity mode selecting filter in a semiconductor laser structure, provides a large tuning range. The disclosed filter is, in one embodiment, a two branch asymmetric Y-branch tunable filter where the ends of the two branches terminate at a common plane. In another embodiment, the invention is realized by using a cascade of two or more Y-branches to place several asymmetrical Y-branch filters in parallel.

Abstract: The invention provides a wavelength division multiplexer and a wavelength division demultiplexer by combining a wavelength-selective mode converter (3) with a wavelength-independent mode splitter (5). The demultiplexer comprises a monomodal input channel (1), an adapter (2) to the bimodal input of a 100%TX.sub.00 -TY.sub.01 (.lambda..sub.1) mode converter (3), a bimodal junction channel (4), and a mode splitter (5) having a first output channel (6) and a second output channel (7), both monomodal, the first output channel (6) having a higher propagation constant than the second. A .lambda..sub.1 component, which propagates with a polarization TX (TE or TM) in an input signal S.sub.i1 (.lambda..sub.1, .lambda..sub.2, .lambda..sub.3 . . . ), will, after conversion in the converter (3), propagate in the intermediate channel (4), in the first-order guided mode having the polarization TY (TE or TM), and subsequently leave the splitter via output channel (7).

Abstract: Arrays of blocking optical switches formed from optically amplifying waveguides are used to transmit or block one or more input beams to one or more selected optical channels. Passive optical waveguides couple the optical input to the amplifying waveguides. Optical splitters monolithically formed with passive optical waveguides are used to route the optical signal from a single fiber to the array of amplifying waveguides.

Abstract: The invention relates to an integrated optical device which comprises an adjustable Mach-Zehnder interferometer. The waveguide structure of the device is fabricated on the basis of polable unpoled material, the material in one (2) of the branches (2, 3) of the MZ interferometer being poled in a certain area (7). In the case of maximum poling in advance, the degree of poling of the poled material can subsequently be adjusted under conditions of accelerated thermal relaxation. Advantage: the device can first be fabricated, as far as its structure is concerned, using fabrication techniques which are standard for the selected material without additional attention to correct sizing, after which the interferometer section can then be adjusted in a simple manner to the optical pathlength differences required for the specific function of the device. As an example, a passive polarisation splitter is described.

Abstract: Distortion in an optical transmission system is significantly reduced by the use of two modulatable optical sources with non-ideal transfer functions of electric signal to light. The output of the first modulator is sampled and then delayed using an optical delay line. The sampled signal is compared with the input signal to generate an electrical error signal. This error signal is then used to modulate the second optical source. The output of this second modulatable optical source is then combined with the delayed output from the first optical source in a four-port optical coupler in such a way that the departures from linearity (which form the error signal) are removed on both outputs of the coupler, giving two optical outputs which possess a linear transfer function of electric signal to light through the entire system. The first optical source is chosen to have high power and low noise. The second source is chosen to be low power, moderate noise.

Abstract: A wye-branching optical circuit including an input waveguide and at least two output waveguides, which are single mode waveguides or multi-mode waveguides in which only several modes are propagated. The waveguides are formed in a substrate, wherein a portion of the input waveguide is a low effective refractive index waveguide where higher order modes are cut-off, and the effective refractive index of a fundamental mode is lower than in the other waveguides formed in the substrate.

Abstract: An optical device includes a semiconductor laser monolithically integrated on a substrate having a branched cavity extending above a plane that is coplanar with a base surface of the substrate, and an adjustable optical power light source for radiating light into the cavity of the semiconductor laser thereby controlling the operation of the semiconductor laser optically.

Abstract: Controllable polarization transformer comprising a passive mode converter (4), a controllable phase shifter (5) and a controllable Mach-Zehnder interferometer (7). The mode converter (4) converts the first signal component of one polarization (TE or TM) of a light signal entering via an input (1) into a signal component of the other polarization without affecting the second signal component of the other polarization (TM or TE), the conversion taking place to another order of the guided mode. Phase control by means of the phase shifter (5) controls the intensity distribution of the light signal over the two branches (8, 9) of the interferometer (7). Optical wavelength control in the interferometer (7) controls the phase with which the signals through the branches come together in the output (2).

Abstract: A semiconductor active waveguide optical crossbar switch to allow optical signals to be routed to any number of ports with no net attenuation of signal strength. The optical crossbar switch is comprised of a network of optical waveguides wherein both lateral and transverse carrier confinement is provided. Redirection of the optical waveguide is accomplished by means of total internal reflectance turning mirrors. Furthermore, the optical waveguides are formed such that electrical current may be applied to the semiconductor structure to amplify the optical signal travelling therein. Electro-absorption switches formed from distinct metallic contacts overlying portions of the optical waveguides amplify the optical signal travelling through the waveguide underneath the switch if sufficient electrical stimulation is applied to the switch.

Abstract: An optical coupler capable of reducing the number of terminals of a star coupler per node and allowing easy expansion of a network configured by star couplers. A star coupler of the invention has a plurality of terminals and allows an optical signal to be inputted to and outputted from a single terminal. The star coupler also has such a transfer characteristic that an input signal to an initially inputted terminal is not distributed as an output signal from the initially inputted terminal but distributed as output signals to its terminals other than the initially inputted terminal.

Abstract: This invention discloses a method and apparatus for providing high speed optical tuning. With this invention, light is spatially routed through a tree of optical switches interconnected by waveguides to a selected fixed tuned optical filter in an array of such filters. Each of the optical switches can be controlled by a binary signal thereby permitting digitally controlled optical filtering.

Abstract: An optical grating being formed by a waveguide branching structure having a single infeed end face for infeeding an optical wave to be coupled into the grating, said branching structure is composed of a strip-like optical waveguide that proceeds from the infeed end face in a tree-like branching fashion and ends at the outfeed end faces, the distance between the infeed end face and each of the outfeed end faces can be the same or be different, as desired. The outfeed end faces can also be arranged to extend obliquely to the propagation direction of the optical wave being propagated in the grating.

Abstract: An optical coupler with an input and 2.sup.n outputs comprises a plurality of substantially coplanar waveguides, each optically coupling said input to one of said 2.sup.n coupler outputs, said waveguides comprising a plurality of waveguide sections connecting n stages of Y-junctions disposed along each waveguide path in a tree and branch arrangement, and further comprises a first junction (J.sub.21), having an axis not parallel to that of said coupler, which is connected to two second junctions (J.sub.31, J.sub.32) by curved waveguide sections without singularities, wherein any inflection point along any one of the waveguides, between the input and the nth stage of the junctions, is situated at one of the n junctions along the waveguide path. The coupler may be successfully applied to splitters or combiners and carried out in integrated optics, and a method for designing the mask used in the manufacture of the integrated optics coupler is also described.

Abstract: An optical coupler/splitter having an optical waveguide on the side of light incidence, two optical waveguides on the branched side, branched from the former optical waveguide and each having straight and curved sections, and a dielectric multi-layer film filter provided in a manner crossing the latter waveguides and common to them. The filter is so formed as to intersect with a straight section of one waveguide and with a curved section of the other, so that respective crossing angles between the filter and the branched optical waveguides differ from each other, thereby providing different reflection or transmission characteristics with respect to the branched optical waveguides. An optical signal having a variable wavelength and issued from a measuring apparatus based on the backscattering method is distinguished by the common filter of the coupler/splitter, making it possible to perform measurement of the losses of the branched optical waveguides independently without being affected by interference light.

Abstract: A process for the production of optical couplers from polymers in which the coupler structure is produced by injection molding and the polymers employed are preferably amorphous polymers, in particular PMMA, polystyrenes, polycarbonates, polyolefins, polyesters, polysulfones, polyether sulfones, polyether imides, polyarylates, polyamides or polyester carbonates. The coupler element is solid and may be coated with a polymer which has a lower refractive index than the coupler, so that the light propagation takes place in the coupler itself.

Abstract: An optical connection between optical fibres of two optical cables includes a cassette in which is mounted a preformed component having a housing and discrete optical conductors which are so disposed in the housing that opposite end parts of each of the conductors, each of which end parts is formed of a length of optical fibre, protrude from the housing at an acute angle to one another lying in the range 0.degree. to 90.degree. and that at least a part of an optical signal entering any one of the discrete optical conductors along one of its protruding lengths of optical fibre will pass along a discrete optical path and the part of the optical signal will leave the optical conductor along another of its protruding lengths of optical fibre.

Abstract: An optical fibre probe for use with a spectrophotometer, for remote analysis of samples, includes a probe assembly having an outer tubular body member, a sleeve member, and a further tubular inner body member within that. Input and output optical fibres pass into the assembly from one end, and the fibres are terminated in ferrules secured within a ferrule plate which is mounted by means of grub screws at the end of the inner body member. Radiation emerging from the end face of the input optical fibre is reflected back within the probe to be focused on the end of the other fibre, after total internal reflection in the case of an ATR probe or after passing through a transparent sample in the case of a transmission probe. Lateral positioning of the ferrule plate and thus the ends of the optical fibres, across the optical axis, is achieved by differential tightening of the grub screws.

Abstract: In an optical coupler for dividing or combining lights of different wavelengths, at least one wavelength separating filter connector terminal is provided at least one light input and output terminals. The wavelength separating filter connector comprises first and second optical fibers which are in contact with each other by placing an optical wavelength separating filter film between the first and second optical fibers, so that an isolation is large between separated lights.

Abstract: An optical switch biasing flow of light between a main optical waveguide and one or the other of a pair of secondary waveguides extending from one axial end of the main waveguide to define a Y structure with the secondary waveguides symmetrically positioned with respect to the longitudinal axis of the main waveguide is provided. A resonant conductor having a pair of legs each of which is operatively associated with the main waveguide and its respective adjacent one of the secondary waveguides generates a resonant voltage that directs the flow of light between the main waveguide and the secondary waveguide associated with the leg having an appropriate voltage.

Abstract: A waveguide-type optical switch having a Y-branch structure composed of a Y-waveguide and branched waveguides provided with electrodes for electrically controlling the guide asynchronism thereof characterized in that the branched waveguides arranged at a converging angle of .theta..sub.1, and the Y-waveguide having a branching angle .theta..sub.2 (.theta..sub.2 >.theta..sub.1) connected to the branched waveguides. It is possible to shorten the length of the device of the Y-structured arrangement having the electrodes capable electrically of controlling the asynchronism of the branched waveguides.

Abstract: It has been determined that optical fibers possess a small amount of nonlinearity and, therefore, they are not exactly linear waveguides. This small amount of nonlinearity enables certain special pulse shapes to establish themselves and travel long distances without changing shape. These special pulse shapes are called solitons. This invention relates to a device for generating solitons by amplitude modulating an optical signal with separate in-phase electrical signals of different harmonically related frequencies. Specifically, an amplitude modulator such as a Y junction Mach-Zehnder interferometer has multiple sets of distributed electrodes. The interferometer has a set of electrodes for each electrical signal of a specific frequency. Thus, if three separate signals of harmonically related frequencies are used to amplitude modulate an optical signal, then the interferometer has three sets of electrodes located between the Y junctions of the Mach-Zehnder interferometer.

Abstract: An optical switch includes at least one input optical semi-conductor waveguide. Two output optical semiconductor waveguides are connected at a branch point to the input optical waveguide, and diverge from the branch point with a preset angle .theta. (degree) between them. A refractive index controlling portion is located on at least one of the output optical waveguides and away from the branch point. The refractive index controlling portion effects a light mode cut-off by electromagnetically causing a reduction of the refractive index of the associated output optical waveguide.