AN OPTICAL SWITCH AND A TIME DIVISION OPTICAL DEMULTIPLEXER USING SUCH SWITCH

Abstract: A high speed optical switch and a time division demultiplexer using such switch. The optical switch comprises a length of linear material (12) including an outer surface on a portion of which is formed a layer of nonlinear material (11) to form a nonlinear interface (10) at the boundary of the two materials. An input data light beam (14) is propagated in the linear material with a predetermined intensity and angle of incidence on the nonlinear interface to, by itself, cause total reflection of the input beam. A control light beam (11) is also selectively energized and directed at the nonlinear interface with an intensity and angle of incidence to cause a portion of the input data beam to be formed into a self-focused channel or beam propagating in the nonlinear material. The self-focused beam can then be detected at an edge of the nonlinear material. By disposing a number of such switches in the path of a repetitively reflected data beam, an optical time division demultiplexer is provided.

Description: AN OPTICAL SWITCH AND A TIME DIVISION OPTICAL DEMULTIPLEXER USING SUCH SWITCH 5 Background of the Invention 1. Field of the Invention The present invention relates to an optical switch and the use of such switch to form a time division optical demultiplexer. ,10 2. Description of the Prior Art Optical switching devices have been of considerable interest for use in laser and optical communication systems to enable light beams to be switched along various paths and for performing multiplexing and 15 demultiplexing functions. An optical switching device is disclosed in U. S. Patent 4, 190,811, ' February 26, 1980. There, apparatus is described wherein a signal beam from a first laser is directed at a surface of a semiconductor at Brewster's angle and is transmitted through the 20 semiconductor to a first utilization device. A control beam of sufficiently high intensity from a second laser is also selectively directed at the semiconductor surface and when such control beam is present, free carriers are created in the semiconductor to cause total reflection of 25 the signal beam. The article "Optical Bistability at a Nonlinear Interface" by P. W. Smith et al in Applied Physics Letters, Vol. 35, No. 11, December, 1, 1979 at pages 846-848 describes an optical element based on the intensity 30 dependent reflectivity of an interface between a linear and a nonlinear medium. A low intensity beam with an angle of incidence less than the critical angle is directed at the interface and is totally reflected. However, at some threshold input intensity a sudden switch in state occurs 35 to produce both a reflected beam portion and a beam portion which is transmitted in the nonlinear medium. The problems remaining in the prior art are to provide a time division optical demultiplexer and an optical switch having, among other things, characteristics suitable for such in such a demultiplexer. Summary of the Invention An optical switch comprises a layer of nonlinear material and layer of linear material forming a nonlinear interface at the boundary of the two materials, and a selectively energizable control light beam directed at the nonlinear interface. An input data beam propagating in the linear material with a predetermined intensity and angle of incidence when impinging the nonlinear interface is totally reflected in the absence of the control light beam and a portion thereof is formed into a self-focused beam in the nonlinear material in the presence of the control light beam. By disposing a number of such switches in the path of a repetitively reflected data beam, an optical time division demultiplxer is provided. Brief Description of the Drawings FIG. 1 illustrates a high speed optical switching device including a nonlinear interface in accordance with the present invention; FIG. 2 illustrates a high speed optical time division demultiplexer in accordance with the present invention; and FIG. 3 illustrates a typical time division multiplexed signal for four channels for use with the arrangement of FIG. 2. Detailed Description A nonlinear interface is described as an interface or boundary between two transparent dielectric materials where one is a layer of a linear material and the other is a layer of a nonlinear material which has a light intensity-dependent refractive index also defined as an optical Kerr effect, see, for example, A. E. Kaplan, "Hysteresis Reflection and Refraction by a Nonlinear Boundary - A New Class of Effects in Nonlinear Optics" in JETP Letters, Vol. 24, No. 1, July 5, 1976 at pages 114- 119, and "Theory of Hysteresis Reflection and Refraction of Light by a Boundary of a Nonlinear Medium" in Soviet Physics - JETP, Vol. 45, No. 5, May 1979 at pages 896-905, wherein the linear material is defined as comprising a refractive index n n and the nonlinear material comprises a refractive index ' where Δn nA = n 2 I I being the intensity in the nonlinear material and Δn. is a field-independent increment to the dielectric constant. It is also known that under appropriate conditions, a beam incident on such interface will be totally reflected if its intensity is below a threshold value, and that for intensities above the threshold value a significant portion of the beam is transmitted into the nonlinear medium where it propagates in the form of a self-focused channel. In accordance with the present invention, it has been found that the generation of a self-focused beam in the nonlinear material forming a nonlinear interface can be controlled by a control light beam with an intensity and power that are both lower than the intensity and power of the data light beam being controlled. More particularly, it has been found that a data light beam propagating in a linear medium and impinging a nonlinear interface with an intensity below the threshold value for partial transfer into the nonlinear material can be made to couple power into a self-focused beam in the nonlinear material if the nonlinear interface is also illuminated by a separate light control beam. FIG. 1 illustrates a high speed optical switching device (e.g., operable with sub-picosecond switching times) in accordance with the present invention where a nonlinear interface 10 is formed at the boundary between a layer of a nonlinear material 11 and a layer of a linear material 12. A data light beam, designated by a ray 14, propagating in linear material 12 impinges nonlinear interface 10 with an angle of incidence i^ and an intensity below a predetermined threshold level. Therefore, by itself, data beam 14 will be totally reflected and continue to propagate in linear material 12 to the output thereof. However, in the presence of a control light beam 16 radiated from a selectively energizable control light beam source 17, that was enabled by a timing signal, in a direction to impinge nonlinear interface 10 with an angle of incidence i 2 and a predetermined sufficient intensity, the data beam is caused to penetrate into the nonlinear medium. Therefore, in the presence of a selectively energized control light beam 16, a portion of the data beam 14 will be coupled into a self- focused beam 18 that propagates down the layer of nonlinear material 11 while any remaining portion will be reflected for continued propagation in linear material 12. The self-focused beam 18 can then be directed down any desired path or, as shown in FIG. 1, can be detected by a detector 19 for conversion to an electrical signal at the output thereof. Exemplary parameters for obtaining the operation illustrated in FIG. 1 are: (a) a linear medium 12 with a refractive index n n =1.5; (b) a nonlinear medium 11 with a refractive index n=n Q +0.02+0.01091 where I is the intensity of the light in the nonlinear medium; (c) an input two- dimensional beam having a power of 12.6 units, a Gaussian beam radius of 10λ_, a peak intensity of unity and an angle of incidence i-^ on the nonlinear interface 10 of 85 degrees; and (d) a control beam 16 comprising a uniform intensity of 0.1 units, a width of 50 wavelengths, aligned such that it intersects the nonlinear interface at the point on the interface 10 where the axis of the input beam intersects it, and a power of 5 units at an angle of incidence i 2 normal to the nonlinear interface 10. A resultant self-focused beam 18 is produced in the presence of control beam 16 comprising a power of 7 units which propagates at an angle to the nonlinear interface 10 of approximately 88.8 degrees. Other parameters can be used. For example, data beam 14 could comprise a lower peak intensity or different angle of incidence with interface 10 and, in turn, control beam 16 could comprise a higher intensity or different angle of incidence for the combination of beams to generat a self-focused beam 18 in the nonlinear medium. There are not currently available simple analytical expressions from which one can calculate the required characteristics of th control beam to cause switching of a given input beam. However, with the numerical simulation techniques exist which make it possible to calculate the behavior for any given control beam, see, for example, "Reflection of a Gaussian Beam at a Nonlinear Interface" by W. J. Tomlinson et al in Applied Optics, Vol. 21, No. 11, June 1, 1982. By carrying out such calculations for various control beam parameters, one can determine the optimum control beam for any given input beam. An optical time division demultiplexer in accordance with the present invention, using the switching means of FIG. 1, is shown in FIG. 2. There, an input data light beam 14 is directed into a length of linear material 12 in a predetermined manner to propagate therein with a predetermined intensity below a predetermined threshold value and to impinge the boundary thereof at a predetermined angle of incidence i* j _ along the length of the linear material. The input data signal 14 for use with the demultiplexer of FIG. 2, in the preferred embodiment, is a time division multiplexed signal wherein separate channels are interleaved in time as shown in FIG. 3 for the special case of four time division multiplexed channels. Such type signal is well known in the art and essentially comprises N channels, i.e., N=4, which are transmitted in a predetermined sequence during a frame period, which sequence is generally repeated in a similar manner in subsequent sequential frame periods. In the arrangement of FIG. 2, a series of four nonlinear interfaces 10- j^ -lo^, similar to the one of FIG. 1, are formed along the surface of the length of a linear material 12 by forming four separate layer sections of nonlinear material- 11^-11^ at predetermined locations. A separate control light beam source 17^-17^ is directed at a separate associated one of the nonlinear interfaces for selectively providing a separate control light beam 16I-16Λ during the time when a predetermined time slot signal representative of the desired channel in data beam 14 is impinging the associated nonlinear interface. A separate detector 19^-19^ is provided at the output of the associated layer of the nonlinear material ll*-_-ll4 for detecting the presence of a self-focused beam including the desired channel signal and generating an electrical output signal representative of such channel signal. If it is assumed that input data signal 14 is in the form of a time division multiplexed signal of FIG. 3 and that the elements of nonlinear interfaces lO-^-lO^ are to receive channels 1-4, respectively, then the demultiplexing arrangement of FIG. 2 will typically operate in the following manner. With regard to nonlinear interface 10-^, input data beam 14 is incident thereon during each of time slot intervals 1-4 of each frame interval. However, since it is desired to receive only channel 1 with the elements associated with nonlinear interface 10^, a control light beam source 17^ is energized, with a timing signal that is synchronized with the input data stream 14, to form a control light beam 16*-_ pulse over the time period when the input data signal of channel 1 is incident on nonlinear interface 10^. During the period of time slot 1 when the signal of channel 1 is incident on nonlinear interface 10* j _ concurrent with control light beam 16^, a self-focused beam 18^, comprising the signal in channel 1, will propagate in nonlinear material 11, and be detected by light detector 19-^. Since nonlinear interface lO ^ is to receive only the channel 1 signal, control light beam source 17-^ will not be energized during the periods of each frame interval when the signals - 7 - in channels 2-4 in input data beam 14 are ^incident on nonlinear interface 10 j , so that input data beam 14 will be totally reflected back into linear material 12 to propagate towards nonlinear interfaces 10 2 -lθ4. In a similar manner, control light beam sources 17 2 -17 will be selectively energized during the intervals when the signals in channels 2-4, respectively, are incident on the respective nonlinear interfaces lO^lO^. As a result, each of channels 1-4 are received by separate detectors respectively, by the selective energizing of control light beam sources. 17-1-17 in the arrangement of FIG. 4. Various modifications are possible. For example, the control light beam pulses 16 in FIGS. 1 and 2 need not be of the same wavelength as the signals in input data beam 14 and, therefore, the control light beam signals can easily be separated from the input data channel signals by any suitable filtering means. Also, the data in the various channels 1-4 can be received by different ones of detectors 19ι-194 or more than one channel can be received at a detector by the proper energizing of an associated control light beam source.

Claims: Claims 1. An optical switch comprising: a transparent linear dielectric medium (12) capable of propagating light: a transparent nonlinear dielectric medium (11) which exhibits an optical Kerr effect forming a first nonlinear interface (10) with said linear medium, the nonlinear interface being capable of totally reflecting a first light beam (14) propagating through the linear medium, CHARACTERIZED BY a selectively energizable light source (17) for producing a second light beam (16) directed at the nonlinear interface for causing a portion (18) of the first light beam to propagate into said nonlinear medium. 2. A time division optical demultiplexer utilizing the switch according to claim 1 characterized in that said first light beam comprises a time division multiplexed signal including a plurality of separate channel information signals transmitted in a plurality of separate sequential time slot intervals of a frame period, and said selectively energizable light source is selectively energized during a time when a predetermined at least one of the plurality of channel information signals is incident on the nonlinear interface. 3. A time division optical demultiplexer according to claim 2 characterized by a second nonlinear interface (10 2 ) formed between a second (H 2 ) nonlinear dielectric medium and said linear dielectric medium and disposed in the path of the first light beam totally - reflected from said first interface (10_.), and a second selectively energizable light source (17,,) for selectively controlling the propagation of said first beam at said second interface. 4. A demultiplexer according to claims 2 or 3 characterized by a light detector (19) for detecting the presence of a light beam in said nonlinear medium. 5. A demultiplexer according to claims 2, 3 or 4 wherein the first light beam and the second light beam comprise signals of the same wavelength. 6. A demultiplexer according to claims 2, 3 or 4 wherein the first light beam and the second light beam comprise signals of a different wavelength. f OMP

Inventors: SMITH PETER WILLIAM, TOMLINSON III WALTER JOHN, SMITH, PETER, WILLIAM, TOMLINSON, III, WALTER, JOHN
Applicants: WESTERN ELECTRIC CO, WESTERN ELECTRIC COMPANY, INC.
Application year: 1983
Main classification: H04B9
IPC codes: G02B6, G02F1, H04B10, H04J14, H04Q3