WO1987005125A1

WO1987005125A1 - Voltage tunable optical beam switching device

Info

Publication number: WO1987005125A1
Application number: PCT/US1987/000105
Authority: WO
Inventors: Gregory L. Tangonan
Original assignee: Hughes Aircraft Company
Priority date: 1986-02-18
Filing date: 1987-01-15
Publication date: 1987-08-27
Also published as: EP0257059A1; IL81259A0; JPS63502533A

Abstract

A liquid crystal electro-optical switching device utilizing a liquid crystal cell (10) wherein one electrode (5) has a grating pattern (11), and the other electrode is a planar layer. The device acts as a voltage tunable phase grating, whose grating strength is controlled by an external field applied to the electrodes. The switch provides one-dimensional 1 * N splitting with even outputs, and two-dimensional 1 * N switching in a crossed grating configuration.

Description

VOLTAGE TUNABLE OPTICAL BEAM SWITCHING DEVICE

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to optical information transmission systems and, more particularly, to optical phase gratings and arrangements for optically switching and splitting information utilizing liquid crystal cells.

2. Description of Related Art Liquid crystal (LC) cells have been typically used in a wide range of image display applications and more recently have been increasingly used in real-time optical signal processing applications. U.S. Patent No. 4,389,096, Hori et al., "Image Display Apparatus of Liquid Crystal Valve Projection Type", issued on June 21, 1983, discloses a liquid crystal light valve apparatus comprising a liquid crystal layer having dielectric and optical anisotropy which forms part of an electro- optic element, and a Schlieren optical system. An electric field having a spatial intensity distribution corresponding to input image signals is applied to the liquid crystal so that a spatially phase-modulated phase diffraction grating is formed in the liquid crystal layer. The diffracted light forms an enlarged image suitable for projection on a screen. In one of the embodiments disclosed therein, in Col. 5, one of the electrodes on one of the substrates is constructed to have a stripe-type electrode structure at a predeter¬ mined spatial period. An alternative construction wherein an electric field is applied to the liquid crystal layer through a high resistance layer of periodical shape on the electrode attached over the whole surface of the substrate is also disclosed. A two-dimensional embodiment uses a light mask and three gratings arranged in different directions as means for forming plural types of diffraction gratings in the liquid crystal layer, on the writing light side of the photoconductive layer. As in other typical image display applications, the Hori et al. invention uses a photoconductive layer and is directed towards accurately reproducing an input image. However, optical switch applications require high efficiency beam control to ensure that the controllable diffracted orders contain most of the incident power. For star coupler applications, the transmitted power intensity in the different diffracted orders must be very nearly equal, with variation being less than _+ 10%. Thus the variation of the diffracted intensity of the different orders as a function of the voltage or wavelength is an important consideration for optical switch applications, a consideration not typically present in the case of image display applications.

As an example of an LC signal processing application, see U.S. Patent No. 4,351,589, "Method and Apparatus for Optical Computing and Logic processing by Mapping of Input Optical Intensity into Position of an Optical Image" assigned to Hughes Aircraft Company. U.S. Patent No. 4,351,589 discloses an implementation utilizing an LC light valve operating in a mode in which a locally variable phase grating is produced. The variable grating mode (VGM) operation of a liquid crystal device useful for optical processing is discussed by B.H. Soffer et al. in "Variable Grating Mode Liquid Crystal Device for Optical Processing," SPIE, Vol. 218, Devices and Systems for Optical Signal Processing, pp. 81-86, 1980. In VGM operation, a variable phase grating is formed in the liquid crystal cell. The grating period depends upon the applied voltage and therefore can be changed by varying the voltage applied to the LC cell, or in response to an optical signal by adding a photoconductive layer to the cell. In the latter case, each input signal component will generate a local grating structure with a particular spatial frequency which depends on the intensity level of that component. Thus, in VGM operation, the resultant grating period is neither fixed nor uniform throughout the cell.

The VGM operation of nematic liquid crystals has been utilized to achieve 1 * 3 or active splitting devices as reported by G. L. Tangonan in "Variable-Grating-Mode Liquid Crystals for Fiber-Optic Applications," Electronics Letters, Vol. 21, No. 16, pp. 701-2, August 1985. The

(*) in "1 * 3" is used in the article to denote one input - port communicating the same signal to 3 output ports, with each output port receiving equal intensity as distinguished from the standard nomenclature "N x N" which is used to denote transmission of N signals from N input ports to N output ports. The star * denotes transmission of signals from one input port to N output ports (1 * N) or N input ports to N output ports (N * N) with each output port receiving equal intensity. Draw- backs of VGM LC devices for fiber optic app *_lications include the lack of sufficient efficiency and problems due to grating imperfections at high fields. Moreover, reported results indicate that VGM light valves require dc operating voltages. The rapidly developing field of optical communi¬ cations and the growing interest in fiber optic communi¬ cations has created a need for switches which can optimally distribute optically transmitted information into different channels. In the past, electrical switching arrangements have been typically used. Optical signals are first converted into electric signals which are switched to new paths. The electric signals are then used to modulate the output of semiconductor lasers, whereby they are reconverted to optical signals which are transmitted by the optical fibers. A major drawback of this type of electrical switching is that the effective communication bandwidth is limited. U.S. Patent No. 4,516,837, issued on May 14, 1985 to Soref et al. discloses an "Electro-optical Switch for Unpolarized Optical Signals" which uses a polarizing beam splitter cube and a reflector to separate an arbitrarily polarized incident light beam into polarized components which propagate along parallel paths. A polarization rotator is positioned in the path of the reflected component to rotate the plane of polarization of the light beam component to be coplanar with that of the undeviated light beam in the parallel path. The two beams are simultaneously or individually deflected by selectively activating the electrodes of a liquid crystal nematic reflector/transmitter array confined between prismatic bodies to emerge at one or more of a plurality of desired outputs. As discussed in Col. 8, lines 26-64, the "individual segmented optically transparent electrodes" are preferably disposed in two parallel rows, with one optically transparent electrode in opposition to the individual segmented electrodes, and a single, uniform liquid crystal film confined between the electrode layers. This embodiment provides an array of four parallel LC cells for independent switching of either beam,,.making possible eight outputs. Star couplers and bidirectional couplers utilizing binary phase transmission gratings are discussed by U. Killat et al. in "Binary Phase Gratings for Couplers Used in Fiber-Optic Communications," Fiber and Integrated Optics, Vol. 3, Nos. 2-3, pp. 221-235, 1980. The concepts presented in this article are extended further by Killat et al. in "Binary Phase Gratings for Star Couplers with High Splitting Ratio," Fiber and Integrated Optics, Vol. 4, No. 2, pp. 159-167, 1982. In these articles, the authors discuss how binary phase trans- mission gratings exhibiting a certain number of central diffraction orders of equal intensity can be used to construct one-to-N (that is, 1 x N) and N-to-N (that is, N x N) couplers. These devices are passive phase grating devices. Passive gratings are suited for use in applications requiring fixed or invariant splitting.

However, with active gratings, the capability of actively controlling the degree of splitting, which is unavailable with passive gratings, permits the dynamic reduction of the number of.active star terminals in a data distri- bution network,- when the data bus is overloaded.

Therefore active switching devices are particularly suited for use in data distribution networks. A holographic optical switch which deflects light beams using a diffraction grating recorded by holographic means in a light-sensitive crystal, preferably bismuth silicon oxide, to connect two N x N matrices of optical fibers, is disclosed in U.S. Patent No. 4,543,662, entitled "Optical Beam Switching Device and Telephone Exchange Comprising a Device of This Kind." Although this optical switch overcomes the bandwidth limitations of the electrical switches because it is holographic rather than electrical, once the grating is holographi- cally recorded in the crystal, the device acts as a passive optical switch. The orientation and spatial frequency of the grating can thereafter be changed only be erasing and rewriting the grating. Therefore, erasure of the existing grating and recording of a new grating will have to be done each time a change in- the switching pattern is desired. This would necessitate using several lasers for hologram formation, if a plurality of beams are controlled. Moreover, the holographic switch performs only N x N switching and, in a variety of telecommunications, signal processing and data communications areas, 1 * N switching is necessary.

SUMMARY OF THE INVENTION

The present invention provides an LC optical switch which utilizes an LC cell in which a layer of liquid crystal material is sandwiched between two plates of transpar nt material such as glass. The plates are coated with a transparent conductive electrode layer, and a grating pattern is formed on one of the plates of the LC cell. The device acts as a voltage tunable phase grating, whose grating strength is controlled by an externally applied field. An advantage of the present invention is the provision of an optical switch for 1 * N splitting with even outputs.

Another advantage of the present invention is the utilization of liquid crystal cells to provide one- dimensional and two-dimensional switching for 1 * N coupling of light to fibers.

A further advantage of the present invention is that it achieves the splitting in a single stage. Yet another advantage of the present invention is the potential for use in active star networks and power manifolding.

BRIEF DESCRIPTION OF THE DRAWINGS These and other advantages and features of the invention will become more fully apparent from the following detailed description and the accompanying drawings wherein liκe referenced characters refer to like parts throughout and in which: FIG. la is a sectional view of the LC cell of the present invention;

FIG. lb is an exploded perspective view of component parts of the cell shown in FIG. la;

FIG. lc is a schematic diagram of the switching arrangement of the present invention;

FIGS. 2-5 show the various diffraction orders resulting when different operating voltages are applied to the device of the present invention;

FIG. 6a is a schematic diagram of the arrangement for two-dimensional switching utilizing the present invention; and

FIG. 6b is a display resulting from the two- dimensional switching achieved using the arrangement shown in FIG. 6a. DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. la is a sectional view showing the basic structure of the LC cell 10 of the present invention. Two transparent, smooth plates 1 and 3, of a material such a~ glass, preferably with a thickness of the order of 1 mm, are coated with a transparent, conductive material such as indium tin oxide (ITO) to provide conductive electrode surfaces 5 and 7. Homogeneous alignment ot" the liquid crystal molecules in the quiescent state is induced by conventional methods such as rubbing or ion bombardment etch-alignment. In a homogeneously aligned LC, the quiescent state alignment is parallel to the boundary surface. The LC material 9 is sandwiched in a thin layer, as is typically done, between plates 1 and 3. Liquid crystal layers of thickness-on the order of 6 μm are used in conventional cells to provide efficient electro-optic modulation and reasonably lov drive voltages (<10 V).

A grating pattern, as shown in FIG. lb, is then formed in ITO layer 5 by photolithographically forming a mask thereon and then removing the unmasked ITO portions using conventional ion beam sputtering. Other suitable conventional techniques can be used to form the grating pattern. The electrode 5 of the present invention is thus arranged in a typical grating pattern 11, as a series of parallel lines of uniform and identical widths with uniform spacing or gaps between them. The width of the grating lines and the grating period selected will depend on individual system require- ments and the following generally applicable design consideration. The thicker the LC layer, the lower is the operating voltage required. Additionally, if the lines 11 are made extremely narrow (for example, 5 μm) and the gaps between them of .equal width or greater, the effect of the electrical field they exert may result only in insufficient local rearrangement of the molecular alignment in an LC layer 9 on the order of 6-12 μ thick. However, if the lines are, for example, 25 μm wide and the gaps between them are of equal or greater width, the effect of the electric fields exerted by the relatively wider electrode lines will be more efficiently localized, resulting in rearrangement of molecular alignment over a larger area. Hence, the design considerations of voltage magnitude, electric field efficiency, area of domains wherein rearrangement of molecular alignment occurs, and the tradeoff between operating voltage and electric field efficiency in effecting molecular orientation, will influence the determination of the dimensions of the grating pattern in which the electrodes are arranged.

The LC cell 10 of the present invention can be made by any of the techniques commonly used in the liquid crystal electro-optic device art. The primary difference between the LC cell 10 of the present invention and the prior electro-optic liquid crystalline devices is the patterning of the electrode structure to provide a voltage-tunable phase grating suitable for electro- optic switching. Any suitable nematic liquid crystalline material can be used as LC layer 9. Since the bire¬ fringence, B, of the liquid crystal can be very large, for example B = 0.2, and the LC cell thickness is typically, as indicated earlier, less than 12 μm, strong electroptic interactions occυr even at low operating voltages. The LC cell described above is driven by conventional external circuitry in a known manner. An LC cell in accordance with the present invention was constructed -for test purposes. Referring to FIG. la, the experimental cell 10 was constructed using two glass plates 1 and 3, each 3 mm thick, and each coated with 700A thick transparent and conductive ITO layers 5 and 7. After rubbing the coated glass plates 1 and 3 to uniformly and homogeneously align the liquid crystal molecules, a 12 μ thick layer of a commercially available nematic LC mixture (BDH E7), supplied by BDH Chemicals, Ltd. Poole, Dorset, England, was sandwiched between the two glass plates 1 and 3 which were separated by a Mylar spacer. Patterning of ITO layer 5 was then accomplished by photolithographically forming a mask thereon, and removing the ITO left exposed by the mask using ion beam sputtering. A grating pattern 11 with lines 25 μ wide with 25 μm spacing between them was formed.

The orientation of the LC molecules was such that the optic axis, also referred to as the director, L, of the liquid crystal was parallel to the grating lines 11. The LC cell 10 was tested at HeNe (0.63 μ ) and GaAlAs (0.82 μm) wavelengths using the arrangement shown in FIG. lc. Polarization orientation of input light with respect to the grating lines was adjusted to obtain an even 1 * 5 split at a certain voltage for each wavelength. Initially, with the applied voltage, V = 0, the LC molelcules are homogeneously aligned parallel to the L direction, that is, the boundary surface. The applied voltage was typically a 10 KHz ac signal with a 0 to 20 V range. No analyzer was used at the output. Instead, to facilitate voltage adjustment, viewing and quantitative measurement, light transmitted by the LC cell 10 was focussed on a conventional reticon. The reticon output was displayed on an oscilloscope.- This arrangement permitted observation of 1 * N splitting and the voltage dependence of diffraction orders.

Referring now to FIG. 2, the oscilloscope display of the transmitted light output for an operating voltage of 3.5 V shows that the maximum intensity transmitted corresponds to the zero order and a weak tap state on the order of 0.9 dB exists at the two first orders on either side of the zero order.

The oscilloscope display of the light transmitted for an operating voltage of 4.0 V is shown in FIG. 3. Three maxima are clearly visible corresponding to the zero and first orders, with weak tap states in the second orders. The intensity of the diffracted light is thereby almost entirely confined to these three maxima and thus 1 * 3 power splitting is achieved with loss on the order of 1 dB.

As can be seen from FIG. 4, for an operating voltage of 4.2 V, five maxima are clearly visible corresponding to the zero, first and second orders, with weak tap states at the third orders. Thus, 1 * 5 splitting is achieved with loss on the order of only 0.7 dB or less.

The loss figures given above include the loss to thϋ unwanted orders. The experimental observations indicate that the splitting efficiency is high since the overall loss figure is only on the order of 1 dB and the LC grating generates diffraction orders of equal intensity. The latter is visually evident in FIGS. 3 and 4. The spread in output intensities was observed to be on the order of 1/50. Cell transmission is typically > 95%, neglecting reflections.

In the arrangement whose output is illustrated in FIG. 3, the LC cell 10 of the present invention generates N = 2 diffraction orders for each of the light beams originating from one input port. Therefore, 2N - 1 or 3 output ports will be necessary to collect all of these light beams. Similarly, corresponding to FIG. 4, wherein N = 3, five output ports will be necessary to collect the light transmitted by LC cell 10.

Referring to FIG. 5, -it can be seen that for an operating voltage of 4.75 V, there are first order nulls and weak tap states in the second, third and fourth orders.

In other cells, with different degrees of LC alignment 1 * 7 splitting was observed with +_ 20% variation in the output intensities.

It is to be understood that without departing from the spirit of the invention in using a grating electrode pattern, actual dimensions of the LC cell and the grating pattern, polarization orientation of input light, and output collection means can be varied in alternative embodiments. The experimental observations discussed herein are made by way of illustration only - and not of limitation. Couplers with higher splitting factors than those discussed above can be obtained by crossing two gratings at an angle. The concept of two-dimensional switching using crossed gratings is known. See, for example, the earlier-referenced article by Killat et al, "Binary Phase Gratings for Star Couplers with High Splitting Ratio," pp. 163-7. Killat et al. report results obtained by crossing two gratings at an angle of 60 or 90 degrees. They were able to achieve a 1 * 35 coupler with an overall loss on the order of 2.3 dB using graded index fibers with 50 μm core diameter, which to their knowledge was the highest splitting ratio achieved at that time.

The two-dimensional switching embodiment for the present invention is illustrated in FIG. 6a.

A conventional binary phase grating 12, such as that described by Killat et al. or other suitable phase gratings is crossed at an angle, preferably 60° or 90° relative to the voltage tunable phase grating LC cell 10 of the present invention. Without any voltage being applied to LC cell 10, only the binary phase grating 12 will perform passive switching. When a suitable voltage is applied to LC cell 10, the active grating is activated and two-dimensional splitting with a high splitting ratio is achieved, thereby permitting active addressing of a large number of output ports. For example, passive 1 * 3 splitting was achieved using a conventionally fabricated binary phase grating formed by etching a glass slide. The splitting was achieved by controlling the etch time until a suitable phase shift of j2f = 2 rad was attained, which is known to correspond to a splitting ratio of 1 * 3. The voltage tunable phase grating

10 of the present invention, adjusted to perform 1 * 7 splitting, was oriented at an angle of 90 degrees relative to the passive grating 12. It was found that an operating voltage of 8V optimally resulted in 1 * 7 splitting with 1*5 dB intensity uniformity. It was also observed that devices of the present invention perform

1 * 7 splitting with less uniformity in output intensities than corresponding devices performing 1 * 3 or 1 * 5 splitting. When the voltage controlled phase grating 10 was operated at 4V, the 3 output spots, generated by the passive grating 12 were transformed to a two- dimensional 3 x 7 pattern, as shown in FIG. 6b. In effect, therefore a conversion of 1 * 3 splitting to 1 * 21 splitting was achieved. In this embodiment, a two-dimensional output fiber array can effectively be addressed by the combination of active and passive gratings. Note that different output states can be achieved by altering the voltage settings of the phase gratings 12 and 10, by altering the angle at which the gratings are crossed, by altering the splitting characteristics of the passive grating 12 used in this embodiment and by suitably varying the operating characteristics of LC cell 10.

In yet another embodiment, two dimensional splitting can be performed with two crossed gratings, both active, in essentially the same arrangement as discussed above.

The specific embodiments of the present invention shown and described herein are merely illustrative. Many modifications and variations of the present invention are possible in light of the above teachings and may be made therein without departing from the spirit and scope of the invention as set forth in the appended claims.

Claims

CLAIMSWhat is Claimed is:

1. A voltage-tunable optical beam switching device comprising: a liquid crystal cell having transparent electrodes arranged in a grating pattern and forming a voltage-tunable phase grating; and voltage applying means connected to said liquid crystal cell for applying different voltages to said liquid crystal cell through said electrodes, whereby an input optical beam from an input port is switched to at least one output port after passage through said voltage-tunable phase grating.

2. A device according to Claim 1 in which said liquid crystal cell comprises: two plates of flat, transparent material; liquid crystal material sandwiched between said plates; said plates being coated with a transparent, conductive electrode material; and wherein said conductive coatings on said plates are suitably processed to form transparent electrodes arranged in a grating pattern.

3. A device according to Claim 2 wherein said plates are made of glass.

4. A device according to Claim 2 wherein said liquid crystal is nematic.

5. A device according to Claim 2 wherein said transparent conductive coating is a composite oxide of indium and tin.

6. A device according to Claim 1 further including a display means connected to said liquid crystal cell for displaying the distribution of intensity amongst the difracted orders resulting from an input beam passing through said- voltage-tunable phase grating.

7. A device according to Claim 1 wherein said input port is an optical fiber.

8. A device according to Claim 1 wherein said at least one output port is at least one optical fiber.

9. An active optical switching arrangement for optically connecting one input port to a plurality of output ports comprising: a liquid crystal cell having transparent electrodes arranged in a grating pattern; means for inputting light of suitable polarization at a suitable angle with respect to the orientation of said grating pattern; means for applying a variable voltage to said electrodes; and means at said output ports for receiving the output of said liquid crystal cell.

10. An arrangement according to Claim 9 further including a display means connected to said liquid crystal cell at said output port.

11. A device according to Claim 9 wherein said input and output ports are optical fibers.

12. A two-dimensional voltage-tunable optical beam switching device comprising: a binary phase grating; said grating being crossed at an angle with respect to the optic axis of a liquid crystal cell; said liquid crystal cell having transparent electrodes arranged in a grating pattern; and voltage applying means connected to said liquid crystal cell for applying variable voltages to said liquid crystal cell through said electrodes, so as to form a voltage-tunable phase grating, whereby an optical beam from an input port is deflected in two dimensions by said crossed phase gratings, enabling optical connection of an input port with a plurality of output ports.

13. A two-dimensional voltage-tunable optical beam switching device comprising at least two liquid crystal cells; said cells having transparent electrodes arranged in a grating pattern and voltage applying means for applying variable voltages through said electrodes, thereby forming voltage-tunable phase gratings; said at least two voltage-tunable phase gratings being crossed such that the optic axis of each is at an angle to the other, whereby an optical beam from an input port is deflected in two dimensions by said crossed phase gratings, enabling optical connection of an input port with a plurality of output ports.

14. A multi-dimensional voltage-tunable optical beam switching device according to Claim 1, comprising at least two said liquid crystal cells in a crossed configuration.