GB2114313A - Device for lateral transfer of information in optically non-linear media - Google Patents

Abstract

The invention relates to a device for transfer of optically stored information, in an optically non-linear medium, between laterally disposed memory elements (element 1, element 2, element 3) which are illuminated by at least one light source (1A-1F). In the device, the light intensity, polarization state, wavelength and/or phase position of the light is or are adapted to be influenced differently during the propagation of the light to and/or through two adjacent memory elements, by providing each memory element with one or more light influencing means 10, e.g. screens, polarizers, filters, phase shift layers and/or electrodes, and that said medium is arranged to be subjected to light which, on at least one of the limiting surfaces (6, 9) of the medium is not perpendicularly incident, in order to obtain an optical influence between adjacent memory elements. As shown the device preferably comprises a Fabry-Perot resonator. (With Figs. 3a and 4a) <IMAGE>

Description

SPECIFICATION Device for lateral transfer of information in optically non-linear media BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for transfer of optically stored information in an optically non-linear medium between laterally positioned memory elements, which are illuminated by at least one light source.

Throughout this specification the term "light" should be taken to include electromagnetic radiation in the infrared and/or ultraviolet bands and not just radiation in the visible band of the spectrum 2. Description of the Prior Art In U.S. Patent Application S.N. 363,598 (filed on March 30th, 1982, in the names of Adolfsson, Brogardh and Ovrén and assigned with the present application to a common assignee), the entire disclosure of which is incorporated herein by reference, there is described a device for transforming information in electrical form into optical form or vice versa.Optical information is transferred, addressed, or sensed in memory elements with the aid of circuits with electro-optical feedback, these circuits preferably being integrated on a disc typically, for example, for implementation of a display, a digitizer, an optical memory, or an opto-processor. The circuits with electro-optical feedback include optical modulators, the light transmission of which is dependent on an electric feedback signal. With present day technology, liquid crystals are generally employed as modulators because of their low power consumption and high degree of modulation. However, for many future applications, such as fast optoprocessors and memories, liquid crystals will be too slow. Furthermore liquid crystals will be unsuitable if, in the future, time constants down towards 1 ,lbS are achieved.

Electro, optical modulators of Fabry-Perot type have greater possibilities when it comes to speed, the switching times of these modulators approaching 1 ns (see e.g. P.W. Smith and E.H. Turner: "A bistable Fabry-Perot resonator", Applied Physics Letters, Vol. 30, No.

6, March 1 5, 1977). With these modulators, the devices described in the above mentioned patent application can be implemented, but problems will arise because the time constants of the electric circuits are limited.

The present invention aims to provide a solution to the above-mentioned problems by using, instead of electro-optical feedback, "opto-optical" feedback, whereby the speedlimiting electrical stage disappears.

SUMMARY OF THE INVENTION According to the present invention there is provided a device for transfer of information in an optically non-linear medium, between spaced apart memory elements which are illuminated in light source means, wherein an optical property (e.g. light intensity, polarization state, wavelength and/or phase position) of the light from said light source means is adapted to be influenced differently during propagation of the light to and/or through two memory elements positioned next to each other, by providing each memory element with light influencing means (e.g. one or more screens, polarizers, filters, phase shift layers and/or electrodes), and said medium is arranged to be subjected to light which, on at least one of the limiting surfaces of the medium, is not perpendicularly incident, for obtaining an optical influence between adjacently positioned memory elements.

If the interaction between light and electrons in a material is of such a nature that an increased light intensity in one light intensity range gives a non-linear change of the light absorption or the refractive index, a strong non-linear relationship can be obtained between output and input light if part of the output light from said material is fed back into the material.

In 1976 it was found (H.M. Gibbs et al: Phys. Rev. Lett. 36, 11 35) that, given special conditions, this optical feeback could give a bistable behaviour, i.e. a memory function.

Gibbs et al utilized a Fabry-Perot interferometer to obtain an optical feedback coupling, and Na vapour was used as an optically nonlinear medium. Since then bistability using Fabry-Perot resonators has been obtained with several different types of optically non-linear materials, for example CO2, SF6, Kerr fluids, ruby, InSb and GaAs. The primary non-liner effect is due to changes in the refractive index, which for the wavelength in question changes the optical path length and therefore the resonance conditions in the Fabry-Perot interferometer.

According to the above-mentioned U.S. Patent Application S.N. 363,598 employing circuits with electro-optical feedback, an optoelectric interconnection of the circuits with electro-optical feedback enables a lateral information transfer to be performed, which is necessary for reading in and out informationand for spatial opto-processing. The present invention thus relates to devices for achieving a corresponding information transfer by "opto-optical" interconnection of the elements with opto-optical feedback. In the same way as in the case with circuits with electro-optical feedback, new techniques must be introduced to obtain a transfer of information which is well-defined in time and space.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described, by way of example, with reference to the accom panying drawings, in which: Figures la and ib show graphically two examples of the relationship between incoming and outgoing light for a Fabry-Perot resonator containing different, non-linear optical media; Figure 2 shows schematically a lay-out for measuring the incoming and outgoing light characteristics represented graphically in Figs.

laand 1b; Figures 3a and 3b are views from the side and above, respectively, of a device according to the invention incorporating a Fabry-Perot resonator and provided with ten light sources, one or more of these light sources being used to enable information to be transferred laterally in different directions within the Fabry-Perot resonator; Figures 4a-4c show schematically three different embodiments of Fabry-Perot resonator, for use in the device shown in Figs. 3a and 3b, in which the information is transferred by switching the light sources between different light levels in a specified sequence; Figures 5a and 5b show schematically two embodiments illustrating how information is fed electrically into and out of a Fabry-Perot resonator in a device according to the invention;; Figures 6a-6c show schematically how an electrode pattern is employed in a Fabry-Perot resonator of a device according to the invention to clock the transmission of information between different memory elements; Figure 7 is a schematic view of a Fabry Perot resonator of a device according to the invention in which memory elements of the resonator are provided with polarization filters; Figure 8 is a schematic view of a Fabry Perot resonator of a device according to the invention, the resonator having in front thereof a Fabry-Perot filter; and Figure 9 is a schematic view of a Fabry Perot resonator of a device according to the invention provided with phase-displacement filters.

DESCRIPTION OF THE PREFERRED EMBODI MENTS Figs. 1 a and 1 b show graphically, for Fabry-Perot resonators provided with two different, non-linear optical media, the relationship between the intensity lin of incident light fed into, and the intensity of lout of light passing out of, the resonators. Curves of the type shown in Fig. 1 a have been measured, among other things, in ruby resonators (see, for example, T.N.C. Venkatesan and S.L. McCall: "Optical bistability and differential gain between 85 and 296 K in a Fabry-Perot containing ruby", Applied Physics Letters, Vol.

30, No. 6, March 15, 1977). Curves of the type shown in Fig. 1 b have been obtained when measuring on InSb (see, for example, David A B. Miller et at "optical Bistabilitv in Semi-conductors", IEEE Journal of Quantum Electronics, Vol. QE-1 7, No. 3, March 1981).

Fig 2 shows a layout for measurement, on a laboratory scale, of the relationships between lin and lOUt. In this layout a laser 1 emits a parallel bundle of rays towards the Fabry Perot resonator (FP) 4, which is housed in a cryostat 2 having two optical windows 3a and 3b. The light lout which passes through the Fabry-Perot resonator 4, is captured by the photo-detector 5.

For obtaining suitable light pulse sequences in a device (see Figs 3a and 3b) according to the invention, several laser light sources lA-I L may be used. With the device according to Figs. 3a and 3b, FP (4) can be subjected to time-multiplexed wave fronts having different angles of incidence, the significance of which will be clear from the following figures.

The main principle of the invention will be described in the following description, with reference to Fig. 4a with the aid of Fig. 1 a.

The Fabry-Perot resonator proper consists of a pair of spaced-apart, parallel semi-transparent mirrors 7 and 8, between which a non-linear optical medium is situated. On either side of the non-linear optical medium contained between the semi-transparent mirrors 7 and 8, there are two additional parallel semi-transparent mirrors 6 and 9, which are coated with a screen pattern 10 which may be absorbing or reflecting. The screens of the screen pattern 10 on the mirror 6 have the function of blocking light from A and B, and the screens of the screen pattern 10 on the mirror 9 have the function of blocking light from D and E.

With the screen pattern 10 shown in the drawing, an optical division of the Fabry-Perot resonator into a number of memory elements is obtained.

Now, let it be assumed that for memory element 1 (see Fig. 4a), lin = 12 (see Fig. 1 a) and 1out = ION (i.e. state B), for memory elements 2, 4, 6 etc. on mirror 9, lin = 13 out#1OFF' and for the other memory elements 3, 5, 7 etc. on mirror 6, lin = 12 and 1Out = 1OFF (i.e; state A). Now, if 1D is increased, because of its angle of incidence light will be switched to the underlying (as viewed in Fig. 4a) FP region, which is illuminated by B. The light intensity lin to this "underlying" FP region can therefore be caused to reach the value 14 (Fig.

1 a). Since the other memory elements have out = IOFF, a lower optical coefficient of coupling will be obtained to the underlying IB- illuminated element, so these elements will not reach the value 14. When the second memory element 2 counting from above has achieved lin = 14 1D is again reduced to a low value (1D = 0), whereby lOUt= ION and element 2 thus attains state B, if lin is now given the value 12. Thereafter, 1E is reduced for a short Deriod so that the element 1 has l; > = Ii, whereby lout = 1OFF and thus element 1 reaches the state A.In this manner, a transfer of information between two adjacent memory elements has been carried out, which thus has required a switching of the light source D and thereafter a switching of the light sources B and E (see Fig. 3a).

In a corresponding manner, information can be transferred from the memory element 2 to the memory element 3 by first increasing 16 somewhat, then switching A to a sufficiently great 1A for lin to the element 3 to reach 14, and then switching B so that lin to the element 2 reaches Ii or a lower value. It should be pointed out that lin to elements 1, 3 etc.

consists both of 1E1 and of the part of 1E3 which is reflected in 6, and that lio to-elements 2, 4 etc. consists both of 1B1 #nd the part of 1,3 reflected in 9. When 1D is added, lin to elements 2, 4 etc. will also include the reflections of 1D falling towards 7 and 8, and when 1A is added, lin to elements 3, 5 etc. will also include the reflections of 1A falling towards 7 and 8.The strength of the coupling between the elements, caused by 1D and IA is dependent on the reflectances of 6-9, the A' d the angles of incidence of 1D and I and characteristic of the non-linear medium, chosen wavelengths, etc. and can be optimized with regard to the speed and reliability of the information transfer. The speed is substantially determined by the time of recombination of the non-linear medium, which causes a transfer from state A to state B for a memory element to be performed often more rapidly (factor 10-100) than a transfer from B to A.

This gives a memory function which enables the light sources E and B, in addition to the light sources A and D, to be pulsed with the rest light intensity = 0, which places lower demands on the light sources.

In addition, pulsed light sources give greater tolerances during information transfer.

Assume the same initial position as during the cycle of the information transfer described above. Now, light source E is first extinguished. Because of the recombination time, element 1 will be in resonance for a period Tr after 16 has received the value 0, whereas elements 3, 5, 7 etc. will not be in resonance.

If, during the time trt an 1D pulse is generated, this may have an intensity of up to 13 without elements 3, 5, 7 etc. leaving the state lout = IOFF. Since the element 1 is still in resonance, light corresponding to ION will be switched over to element 2, which receives a large switching light margin without elements 3, 5, 7 being brought to the state B by ID Different types of optical structures can be conceived to implement the principle of information transfer as described above. Thus, Fig.

4b shows a structure in which the mirrors 6 and 9 have been inclined to make it possible to utilize the same light rays IB and 16 both for holding information and for transferring information. This has the advantage of requiring fewer light sources while at the same time giving smaller margins for a reliable operation, and in addition information can only be transferred in one direction.

For a Fabry-Perot resonator, the transmitted electro-magnetic field is directly related to the internal field by the boundary conditions in the initial mirror, so loot in Figs. 1a, 1 b can, in principle, be replaced by 1intern without the shown relationships being changed

where E is the field and T the transmittance).

This makes it possible to use a simple Fabry Perot structure with screens 10 according to Fig. 4c, which simplifies the production technique in relation to the structures according to Figs. 4a and 4b.

Information can be fed into the FP resonator 4 either optically or electrically. Fig. 5a shows an example of electric information input. In a demultiplexer 11 the input data is divided into a number of parallel channels, which are connected via leads 12 to transparent electrodes 13 and 14 on either side of the first memory element in a row of memory elements with screens 10. By mutual electrooptical influence on the refractive index of the Fabry-Perot resonator, the first element can receive a resonance for the light wavelength used. According to the method previously described, the information is optically blocked from memory element to memory element and when the information reaches the last element in a row, it is optically sensed by a detector 15 which is connected to a multiplexer 16 for generation of output data.In Fig. 5b, two switched light rays on either side of the FP resonator 4 are required. If three switched rays per side and the screen pattern according to Fig. 5b are used, the information can be transferred both downwards and to the right, whereby serial information can be made parallel and vice versa.

The electro-optical effect, which in Figs. 5a and Sb is used for input of data to the FP resonator, can also be used for clocking the transmission of information between different memory elements on the FP resonator as is shown in Figs. 6a-6c. By energizing, in turn, the electrodes 13 relative to the electrodes 14 in Fig. 6a, the resonance conditions of the memory elements can be displaced so that their optical state according to Fig. 1 a or 1 b can be switched between A and B and vice versa. For the transmission of information between the memory elements, there can be used either directed rays, oblique mirrors or, as in Fig. 6a, an oblique electro-optically controlled FP plate 20 with a light-absorbing background 19.With the FP reflector accord ing to Fig. 6a, the coefficient of coupling between the memory elements can be controlled, which makes possible a separately pulsed coupling of memory elements. Fig. 6b shows the electrode pattern on the surfaces of the FP resonator 4. The electrodes 13 comprise at least two toothed structures each having a plurality (i.e. at least two) of parallel, spaced-apart, tooth-like parts joined together at one end, the tooth-like parts of the toothed structures being interdigitated. The electrodes 14 are in the form of spaced-apart parallel bands extending substantially perpendicularly to the tooth-like parts of the electrodes 13.

The electric information is read in, in parallel, at the top in Fig. 6b and is then clocked downwards with the aid of clock pulses +1 and t2 which are connected to every second electrode 13. If several light rays are used the information can be transferred both in the xand in the y-direction with the aid of crossed electrode patterns according to Fig. 6c. In Fig.

6c, the electrodes 14 comprise at least two toothed structures similar to the toothed structures of the electrodes 13, the tooth-like parts of the electrodes 14 extending substantially perpendicular to the tooth-like parts of the electrodes 1 3.

The function of the described storage and transfer of information is based on the feature that every second (or even better, every third) memory cell in some direction of the FP resonator surface can be excited (16' 16) and obliquely illuminated (IAT IC 1D etc.) independently of the other memory cells. In addition to using screens 10 and illumination from two directions, this lateral light multiplexing can also be performed with polarizers 21 a and 21 b according to Fig. 7 with different directions of polarization corresponding to the directions of polarization of the light sources Bt and Bo, respectively. If the non-linear effect in the FP resonator 4 is present in a certain wavelength range, wavelength multiplexing according to Fig. 8 can be used. The filter in front of the FP resonator 4 here consists of another FP resonator with a separate resonance wavelength for two adjacent memory elements. This can be obtained, for example with the aid of electrodes 24, which are energized, for example, via a pattern as in Fig.

6b. Finally there will be mentioned the possibility of phase addressing, whereby the FP resonator 4 is provided with a phase displacement pattern 25 (Fig. 9) for obtaining the different memory elements.

The devices according to the foregoing description can be varied in many ways within the scope of the following claims.

Claims (20)

1. A device for transfer of information, in an optically non-linear medium, between laterally positioned memory elements which are illuminated in light source means, wherein an optical property of the light from said light source means is adapted to be influenced differently during its propagation to and/or through two memory elements positioned next to each other, by providing each memory element with light influencing means, and said medium is arranged to be subjected to light which, on at least one of the limiting surfaces of the medium, is not perpendicularly incident, for obtaining an optical influence between adjacently positioned memory elements.

2. A device according to claim 1, in which the said light influencing means comprises at least one screen, at least one polarizer, at least one filter, at least one phase shift layer and/or at least one electrode.

3. A device according to claim 1, comprising a Fabry-Perot having a pair of spaced apart semi-transparent reflecting means between which the said medium is enclosed and at least one further reflecting means disposed, externally of said pair of semi-transparent reflecting means, at least one surface of at least one of said reflecting means being provided with pattern means to optically define said memory elements, said pattern means being defined by variations in transmittance, polarization direction and/or phase shift and/ or by the spectral dependence of said quantities.

4. A device according to claim 1, 2 or 3, wherein said medium is arranged to be driven at optical bistability with a longer time constant for a transfer from a state with a high light transmission (B) to a state with a low light transmission (A) than for a transfer in the opposite direction (A#B), and wherein at least one light source of said light source means is switched with time constants which correspond to or are smaller than the above-mentioned first time constant.

5. A device according to claim 3, wherein said light source means is adapted to generate at least one light wave front perpendicular to one or both of said pair of semi-transparent reflecting means and at least one light wave front which is not perpendicularly incident on said semi-transparent reflecting means, and the light intensity, the direction of polarization, the wavelength and/or the phase position of these wave fronts can be modulated.

6. A device according to claim 4, wherein the light transmission state of one memory element is transmitted to the adjacent memory element by giving the light, which keeps the first-mentioned memory element in state B, a fast reduction in intensity, whereafter the light, which is not perpendicularly incident, is pulsed for a time corresponding to the abovementioned time constant.

7. A device according to claim 5, wherein the said pattern means is applied on two opposed reflecting sides of the Fabry-Perot resonator, said pattern means being so ar ranged that said light wave fronts are influenced differently on their way to a memory element in dependence on which of said reflecting sides the wave fronts are incident.

8. A device according to claim 7, wherein the Fabry-Perot resonator, on each of its reflecting sides, is provided with light-absorbing and/or light-reflecting regions, each memory element on one reflecting side being provided with such a region and adjacently positioned memory elements having said regions on the opposite reflecting side.

9. A device according to claim 8, wherein the light source means is arranged to illuminate the Fabry-Perot resonator from each side by means of perpendicularly incident light as well as by obliquely incident light, the perpendicularly incident light giving such an intensity that optical bistability is maintained, the state of a memory element being transmitted to an adjacently positioned memory element by giving the obliquely incident light a sufficiently high value for the adjacently positioned memory element to be triggered, and the direction of the surface of the Fabry-Perot resonator between the memory elements, between which the state shall be transmitted, being determined by the direction of the obliquely incident light.

10. A device according to claim 3, wherein said at least one further reflecting means is inclined relative to the pair of spaced-apart semi-transparent reflecting means of the Fabry-Perot resonator in order to generate wave fronts which are not perpendicularly incident on the latter semi-transparent reflecting means.

11. A device according to claim 10, wherein the said at least one further reflecting means consists of an electro-optically controlled Fabry-Perot resonator.

12. A device according to claim 3, wherein said pattern means consists of a polarizing layer applied to one of or both of the spaced-apart semi-transparent reflecting means of the Fabry-Perot resonator, the or each polarizing layer being divided into regions defining said memory elements, adjacently positioned regions have different directions of polarization, whereby for greatest selectivity a difference in polarization direction of 90' is chosen.

13. A device according to claim 3, wherein said pattern means is generated by one or two additional Fabry-Perot resonators on one or both sides of the first-mentioned Fabry-Perot resonator, the or each patterngenerating Fabry-Perot resonator being coated with electrically conducting and/or dielectrically optically phase-controlling thin film patterns on one side or on both sides in order to obtain regions with separate spectral transmission characteristics.

14. A device according to claim 1, wherein said pattern means consist of transparent, electrically conducting electrodes which can be controlled to generate differently large electric field strengths in adjacently positioned memory elements, said medium being electro-optically active at the wavelengths employed for the non-linear effect(s).

15. A device according to claim 3, wherein said pattern means consists of transparent, electrically conducting electrode means in the form of at least two electrode structures which each have spaced-apart and parallel tooth-like parts joined together at one end thereof, the tooth-like parts of the electrode structures being interdigitated, whereby adjacently positioned memory elements (in a direction perpendicular to said tooth-like parts) are connected to separate electrode structures so that separate electric voltages can be applied to two adjacent memory elements, and wherein said medum is electro-optically active at the wavelengths employed for the nonlinear effect(s).

16. A device according to claim 15, wherein said Fabry-Perot resonator is provided on one side with said interdigitated electrode structures and on the other side with spaced apart electrode bands extending perpendicular to the tooth-like parts.

17. A device acording to claim 15, wherein said Fabry-Perot resonator is provided on one side with said interdigitated electrode structures and on the other side with further electrode means comprising at least two further electrode structures which each have spaced-apart and parallel tooth-like parts joined together at one end thereof, the toothlike parts of the said further electrode structures being interdigitated and extending substantially perpendicular to the interdigitated tooth-like parts of the first-mentioned electrode structures.

18. A device according to claim 1, wherein one or more of said optical memory elements are supplied with electric information by means of electrodes placed thereon, said medium is electro-optical, and an electric voltage across each memory element can change the optically bistable state of said memory element.

19. A device according to claim 1, wherein said optically non-linear medium is placed in the ray path in an optical processor in order to carry out, in two dimensions, filtering and/or correlation of optical information, such as images, which exists in two dimensions.

20. A device for transfer of information, in an optically non-linear medium, between laterally positioned memory elements which are illuminated by light source means, the device being constructed and arranged substantially as herein described with reference to, and as illustrated in, Figs. 3a and 3b, or Figs. 3a and 3b as modified by any of Figs.

4a, 4b, 4c, 5a, 5b, 6a, 6b, 6c, 7, 8 or 9.