GB2127987A

GB2127987A - Integrated optic devices

Info

Publication number: GB2127987A
Application number: GB08227756A
Authority: GB
Inventors: John Moroz
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1982-09-29
Filing date: 1982-09-29
Publication date: 1984-04-18
Also published as: GB2127987B

Abstract

Alignment of an optical beam (not shown) with a waveguide (2) of an integrated optic device (6) is facilitated by mounting the device on an alignment unit (5) adapted to adjust the position of the waveguide (2) relative to the beam in, for example, the z and theta x directions. The alignment unit comprises two spaced metal bars (7, 8) separated by thermally insulating material (11) whose temperatures, and thus their physical lengths, are adjustable by means of respective thermoelectric heat pumps (9, 10). The optical power coupled to the waveguide may be monitored and employed in an electronic feedback loop (Figure 3 - not shown) in order to dynamically control the temperatures (T1, T2) of the bars (7, 8) and thus the alignment of the beam and waveguide in the z and theta x directions, for optimum optical throughput of the waveguide. A laser printer may employ this alignment method for aligning a laser beam modulated with the information to be printed with the waveguide of a surface acoustic wave device acting as a laser beam deflector. Acoustic waves generated in the waveguide are employed to scan the laser beam over a photosensitive surface moving transversely to the scan direction. <IMAGE>

Description

SPECIFICATION Integrated optic devices This invention relates to integrated optic devices and in particularto light beam/device alignment forthe control of device optical throughput, and to the use of such alignment techniques in information transfer methods and apparatus.

According to one aspect of the present invention there is provided a method of transferring information comprising modulating a light beam with the information, applying the modulated light beam to a surface acoustic wave device having transducers for causing surface waves in response to an electrical signal applied thereto, applying a deflection control signal to the transducers so as to scan the light beam, and applying the scanning modulated light beam to a surface whilst causing relative movement between the surface and the light beam transversely of the scan direction, said surface being sensitive to record or display the information thereon, wherein the light beam is aligned with a waveguide of the surface acoustic wave device and the alignment is controlled by controlling the temperature of atemperature dependent element of or associated with the waveguide and/or the source.

According to another aspect of the present invention there is provided an apparatus fortransferring information including a light beam source; means for modulating the light beam with the information; a surface acoustic wave device having a waveguide and transducers for causing surface waves in the waveguide; means for coupling the modulated light beam to the waveguide; means for generating a light beam deflection control signal to be applied to the transducers in use of the apparatus whereby to scan the coupled light beam; means to couple the scanned light beam to a light sensitive surface for recordal or display of the information thereon, means for causing relative movement between the light sensitive surface and the beam transverse to the direction of scanning; and means for mounting the light source or the surface acoustic wave device, or both, including one or more elements whose physical dimensions are temperature dependent whereby the relative position between the device and the light source is adjustable for alignment of the light beam with the waveguide, the or each element being provided with respective temperature controlling means.

According to a further aspect of the present invention there is provided an integrated optic arrangement comprising a surface acoustic wave device having a waveguide and transducers for causing surface waves in response to an electrical signal applied thereto, and device mounting means including one or more elements whose physical dimensions are temperature dependent whereby the relative position between the device and an optical beam to be coupled to the waveguide is adjustable for alignment purposes, the or each element being provided with respective temperature controlling means.

According to yet another aspect of the present invention there is provided a method of aligning a light beam and a waveguide of an integrated optic device to which the light beam is to be coupled, including the steps of mounting the integrated optic device ora source forthe light beam, or both, on one or more elements whose physical dimensions are temperature-dependent, and adjusting the temperature of the respective element or elements whereby to obtain the appropriate relative positions of the light beam and the waveguide for alignment thereof.

According to a still further aspect of the present invention there is provided an integrated optic arrangement comprising an integrated optic device and mounting means therefor, the mounting means including one or more elements whose physical dimensions are temperature dependent whereby the relative position between the device and an optical beam to be coupled to a waveguide thereof is adjustable for alignment purposes, the or each element being provided with respective temperature controlling means.

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figures la and ib show respectively and schematically a side view and an end (front) view of edge coupling to an integrated optic device; Figure 2 shows one embodiment of alignment unit for an integrated optic device; Figure 3 shows an embodiment of an electronic feedback loop; Figures 4a and 4b show, schematically, side views of two further edge-coupling arrangements, and Figure 5 indicates possible control directions.

An integrated optic device basically comprises a substrate 1 (Figure 1 a), typically lithium niobate (LiNbO3), having a surface layer 2 of a higher refractive index than the substrate 1. The surface layer 2 is produced by diffusion or ion implantation techniques and may comprise a planar (as shown), or channel, optical waveguide for an optical beam coupled thereinto. A technique for coupling light into integrated optic devices which is becoming more commonly used is "edge coupling". In this technique light is coupled into a waveguide simply by directing the light at the edge of the waveguide. Light from a laser 3 is, for example, focussed onto the waveguide 2 by means of a lens 4.

A particular edge coupling configuration is shown in Figures 1 a and 1 b, which show side and front views, respectively. In this configuration critical adjustment is required of the linear position Z and the angular position (3x of the waveguide of the integrated optic device. The present invention proposes to achieve these critical adjustments by means of a temperature sensitive and temperature controlled alignment unit. One embodiment of alignment unit 5 is illustrated in Figure 2 and the integrated optic device 6 is mounted thereon for adjustment in the Z and ()x directions. The device 6 is shown in an end (front) view as in Figure 1 b.The alignment unit consists of two bars of metal 7 and 8, for example aluminium, which are spaced apart in the width direction of the waveguide, are thermally insulated from and serve to support the device. The temperature, and hence the dimensions, of the bars 7 and 8 is controlled by respective thermoelectric heat pumps 9 and 10 which are only illustrated schematically. The bar7and its respective heat pump 9 are separated from the bar 8 and its heat pump 10 by thermal insulation material 11. In addition, the bars 7 and 8 are insulated from a heat sink 1 for the heat pumps by insulation 13, and have respective insulation 14 and 15 at least between them and the device in order to prevent the device temperature changing.

The bars 7 and 8 may be such that they both have the same length L at a first temperature such as ambient.

When the temperature of both bars is raised by AT they will both expand vertically by an amount A1, where Al Lx AT, > being the thermal expansion coefficient of the bar material. Thus the waveguide will be raised in the Z direction by an amount Al.

In the case of two aluminium bars 150 mm long, whose temperature can be changed by t 1000 from ambient with a resolution of 0.1 C (coefficient of expansion for aluminium being 22.9 x 10-6 per degree C), a Z movement range of 68 microns with a resolution of 0.4 microns is obtained. This performance is well suited to the device configuration shown in Figure 1 a.

The angular control Ex is achieved by maintaining the two bars 7 and 8 at different temperatures T1 and T2. The rotation A < 3x thus achieved is given by ASx = sin-1 ((Lx/d) (T1 -T2)), where d is approximately the separation of the two bars.

The present invention thus allows accurate alignment to be achieved inexpensively, in comparison with the hitherto conventionally employed very high accuracy mechanical fabrication and adjustment techniques, and also facilitates dynamic electronic control of the optical throughput of the device. For example, if it is desired to maximise 12, the light level in the waveguide, then light whose intensity 13 is proportional to 12 could be drawn from the waveguide by some convenient means of output coupling and monitored with a photodiode. By means of an electronic feedback loop the optical power 12 may be maximised by varying the temperatures T1 and T2 of the two bars 7 and 8 to maximise 13. Since two variables T1 and T2 must be optimised a microprocessor control of the feedback loop would be applicable.

A typical electronic feedback loop is illustrated schematically in Figure 3. The basic operation of the arrangement is as follows. Light power 13 is monitored by photodiode 16, amplified at 17, and the z position and angular position Ox of the integrated optic device are controlled via bar temperature controllers 24 and 25 to temperatures T1 and T2. The system alternately scans the z position and the angular Ox position, monitoring 13 as it does so, for different values of T, and T2.From the information thus gathered, the circuitry, comprised by sample and hold circuit 18, microprocessor 20 with its peripherals and controls 21, and associated converters 19,22 and 23, determines the z and Ox position giving the desired value of 13 and hence 12 and then adjusts the lengths of bars 7 and 8 accordingly, via temperature controllers 24 and 25.

The facility of dynamic control is particularly useful in view of the fact that both gas and solid state lasers have output optical beams that can change in direction, intensity and wavelength with time.

There are a variety of different configurations with which the alignment unit of the present invention may be employed. As illustrated schematically in Figures 4a and 4b the light from laser3' may be edge coupled directly into a waveguide 2, or light emergent from an optical fibre 3" may be edge coupled into a waveguide 2. A lens may be arranged between fibre 3" and waveguide 2. In addition the alignment unit of the present invention may also be used with integrated optic devices where the light is coupled into the waveguide via a prism or grating coupler in order to control the coupling angle and thus control the optical throughput of the device.

Whereas the alignment unit described above serves to control alignment in the z and Ox directions, fine control may be required in several or even all of the possible, y, z, Ox, Oy and Oz (Figure 5) directions, requiring appropriate design of the alignment unit.

The integrated optic device referred to above may be employed, for example, in a solid state deflector for a laser printer or copier. A laser printer is a high speed printer which basically comprises means to convert the characters etc. of an input text to be printed to a suitable serial data stream form, a deflection control signal, for example a line scan voltage ramp, being generated in synchronism with the serial data stream; a laser beam source; laser modulation drive circuits which convert the logic pulses in the serial data stream to suitable voltage and current levels for either driving a modulator for a CW (continuous wave) laser or driving the modulation of a semiconductor laser directly; a line scan laser deflector driven in accordance with the line scan voltage ramp referred to above, for example; and optics to focus the deflected laser beam to write on a photo-sensitive surface, such as a rotating selenium drum, to produce an electrostatic pattern of the text character, for example.

The drum picks up powdered ink on the electrostatic pattern and deposits it on to plain paper. The ink is set in e.g. a pressure process to produce the printed copy.

The laser deflector may be formed by a surface acoustic wave device which comprises an integrated optic device to which a laser beam is coupled by a method as described above, that is a lithium niobate substrate on which a surface layer of a higher refractive index than the substrate is produced. The surface layer comprises an optical waveguide for the laser beam which is coupled to and from this layer by, for example, separate prismatic coupling assemblies which, together with appropriate optics, interface to the laser source and the photosensitive surface, respectively. Surface transducers are arranged to generate acoustic waves in the surface layer waveguide, causing the refractive index of the layer to be modulated and so produce a diffraction grating.

Optimisation of the drive power to these transducers and therefore the depth of index modulation achieved, together with the interaction length of the grating order, which can then be scanned by controlling the drive frequency and hence the grating period. The grating may comprise a linearly controlled deflection grating or, alternatively, a travelling chirp grating. Whereas edge coupling has been referred to, the deflector may alternatively employ grating coupling or prism coupling for coupling the laser beam into the waveguide.

Various aspects and variations of such a laser printer are disclosed on our co-pending Application No.

(Serial No. ) (J. S. Heeks - R. E. Cooke 37-5) Application No. (Serial No. ) (R. E. Cooke -6) and Application No. (Serial No. ) (R. E.

Cooke -7).

Claims

1. A method of transferring information comprising modulating a light beam with the information, applying the modulated light beam to a surface acoustic wave device having transducers for causing surface waves in response to an electrical signal applied thereto, applying a deflection control signal to the transducers so as to scan the light beam, and applying the scanning modulated light beam to a surface whilst causing relative movement between the surface and the light beam transversely of the scan direction, said surface being sensitive to record or display the information thereon, wherein the light beam is aligned with a waveguide of the surface acoustic wave device and the alignment is controlled by controlling the temperature of a temperature dependent element of or associated with the waveguide and/or the source.

2. A method as claimed in claim 1, wherein the surface acoustic wave device is mounted on one or more of said elements, comprising an alignment unit, whereby to be movable relative to a light beam source which is mounted without provision for movement, and wherein the temperature of the element or elements is controlled whereby to optimise the optical throughput of the device.

3. A method as claimed in claim 2, wherein the surface acoustic wave device is mounted on two spaced apart elements of the alignment unit and wherein the temperatures of the elements are controlled separately whereby to adjust the elevation and angle of the waveguide relative to the optical beam.

4. A method as claimed in claim 3, wherein the temperatures of the elements are controlled dynamically by means of an electronic feedback loop circuit.

5. A method as claimed in any one of claims 2 to 4, wherein the temperature of each element may be adjusted by means of a respective thermoelectric heat pump, and wherein the elements are thermally insulated from one another.

6. A method as claimed in any one of the preceding claims, wherein the optical beam is provided by a gas or solid state laser.

7. A method as claimed in any one of the preceding claims wherein the optical beam is coupled to the waveguide via a prism coupler.

8. A method as claimed in claim 4 or any one of claims 5 to 7 as appendent to claim 4, wherein the feedback loop serves to monitor the power of the optical beam coupled into the waveguide, or a beam proportional thereto, and control the temperature of the element or elements accordingly whereby to optimise the monitored power.

9. Apparatus for transferring information including a light beam source; means for modulating the light beam with the information; a surface acoustic wave device having a waveguide and transducers for causing surface waves in the waveguide; means for coupling the modulated light beam to the waveguide; means for generating a light beam deflection control signal to be applied to the transducers in use of the apparatus whereby to scan the coupled light beam; means to couple the scanned light beam to a light sensitive surface for recordal or display of the information thereon, means causing relative movement between the light sensitive surface and the beam transverse to the direction of scanning; and means for mounting the light source or the surface acoustic wave device, or both, including one or more elements whose physical dimensions are temperaturedependentwherebythe relative position between the device and the light source is adjustable for alignment of the light beam with the waveguide, the or each element being provided with respective temperature controlling means.

10. Apparatus as claimed in claim 9, wherein the temperature controlling means comprise thermoelectric heat pumps.

11. Apparatus as claimed in claim 9 or claim 10, wherein the surface acoustic wave device is mounted on said elements, which elements comprise metallic bars one end of each of which is mounted in a heat insulating manner to the device and the other end of is mounted in a heat-insulating manner to a heat sink for the temperature controlling means.

12. Apparatus as claimed in any one of claims 9 to 11, including means for controlling the temperature of the elements separately and dynamically by means of an electronic feedback loop circuit whereby to optimise the optical throughput of the device.

13. An integrated optic arrangement comprising a surface acoustic wave device having a waveguide and transducers for causing surface waves in response to an electrical signal applied thereto, and device mounting means including one or more elements whose physical dimensions are temperature dependent whereby the relative position between the device and an optical beam to be coupled to the waveguide is adjustable for alignment purposes, the or each element being provided with respective temperature controlling means.

14. An arrangement as claimed in claim 13, wherein the temperature controlling means comprise thermoelectric heat pumps.

15. An arrangement as claimed in claim 13 or claim 14, wherein the elements comprise metallic bars one end of each of which is mounted in a heat-insulating manner to the device and the other end of which is mounted in a heat insulating manner to a heat sink for the temperature controlling means.

16. An arrangement as claimed in any one of claims 13 to 15, further including electronic feedback loop means whereby to control the temperature of the elements dynamically and optimise the optical throughput of the device.

17. A method of aligning a light beam and a waveguide of an integrated optic device to which the light beam is to be coupled, including the steps of mounting the integrated optic device or a source for the light beam, or both, on one or more elements whose physical dimensions are temperature-dependent, and adjusting the temperature of the respective element or elements whereby to obtain the appropriate relative positions of the light beam and the waveguide for alignment thereof.

18. A method as claimed in claim 17, wherein the integrated optic device is mounted on one or more of said elements, comprising part of an alignment unit, whereby to be movable relative to a light beam source which is mounted without provision for movement, and wherein the temperature of the element or elements is controlled whereby to optimise the optical throughput of the device.

19. A method as claimed in claim 18, wherein the integrated optic device is mounted on two spaced apart elements of the alignment unit and wherein the temperatures of the elements are controlled separately whereby to adjust the elevation and angle of the waveguide relative to the optical beam.

20. A method as claimed in claim 19, wherein the temperatures of the elements are controlled dynamically by means of an electronic feedback loop circuit.

21. A method as claimed in any one of claims 18 to 20, wherein the temperature of each element may be adjusted by means of a respective thermoelectric heat pump, and wherein the elements are thermally insulated from one another.

22. A method as claimed in any one of claims 17 to 21, wherein the optical beam is provided by a gas or solid state laser.

23. A method as claimed in any one of claims 17 to 22, wherein the optical beam is edge coupled to the waveguide.

24. A method as claimed in any one of claims 17 to 22, wherein the optical beam is coupled to the waveguide via a prism or grating coupler.

25. A method as claimed in any one of claims 17 to 24, wherein the waveguide is a planar or channel waveguide.

26. A method as claimed in claim 20 or any one of claims 21 to 25 as appendantto claim 4, wherein the feedback loop circuit serves to monitor the power of the optical beam coupled into the waveguide, or a beam proportional thereto, and control the temperature of the element or elements accordingly whereby to optimise the monitored power.

27. A method of aligning a light beam and a waveguide of an integrated optic device to which the light beam is to be coupled substantially as herein described with reference to the accompanying drawings.

28. An integrated optic arrangement comprising an integrated optic device and mounting means therefor, the mounting means including one or more elements whose physical dimensions are temperature dependent whereby the relative position between the device and an optical beam to be coupled to a waveguide thereof is adjustable for alignment purposes, the or each element being provided with respective temperature controlling means.

29. An arrangement as claimed in claim 28, wherein the temperature controlling means comprise thermoelectric heat pumps.

30. An arrangement as claimed in claim 28 or claim 29, wherein the elements comprise metallic bars one end of each of which is mounted in a heat-insulating manner to the device and the other end of each of which is mounted in a heat-insulating mannerto a heat sink forthe temperature controlling means.

31. An arrangement as claimed in claim 30, wherein the bars are of aluminium.

32. An integrated optic arrangement, comprising an integrated optic device and mounting means therefor, substantially as herein described with reference to and as illustrated in Figure 2, with or without reference to Figure 3, of the accompanying drawings.

33. Apparatus for transferring information including an integrated optic arrangement as claimed in claim 32.