GB2219099A

GB2219099A - Tunable Fabry-Perot filter

Info

Publication number: GB2219099A
Application number: GB8812650A
Authority: GB
Inventors: Peter William Ross
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1988-05-27
Filing date: 1988-05-27
Publication date: 1989-11-29
Also published as: GB8812650D0

Abstract

Tunable Fabry-Perot filters are used in a variety of optical application, e.g. in the cavity of a laser. To provide such a filter a layer of twisted nematic liquid crystal 3 is placed between the two reflectors 1,2 of a Fabry-Perot device. The resulting twisted nematic liquid crystal cell is driven above its threshold level. This enables tuning of the filter by the changes in refractive index thus produced. <IMAGE>

Description

TUNABLE FABRY-PEROT FILTER The present invention relates to optical filters, and especially to such filters which use tunable Fabry-Perot interferometers.

Such tunable Fabry-Perot filters are needed in a wide range of electro-optic applications, the most important including laser cavities, wavelength demultiplexers for fibre-optic transmission systems, and wavelength rejection for military systems.

According to the present invention, there is provided an optical filter arrangement, which includes a Fabry-Perot device through which the light to be filtered passes, a liquid crystal element within the Fabry-Perot device with its transparent plates between and substantially parallel to the reflectors of the Fabry-Perot device, and connections to the liquid crystal element via which a voltage is applied to suitably set the characteristics of the filter.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a liquid crystal device usable in a Fabry-Perot filter, three conditions of the liquid crystal structure being indicated at (a), (b) and (c).

Figure 2 shows how such a liquid crystal device is used in a simple Fabry-Perot cavity.

Figure 3 shows another application of the liquid crystal device, this time in a laser cavity.

Figure 4 shows schematically a wavelength demultiplexer using a liquid crystal Fabry-Perot filter according to the invention.

Figure 5 shows schematically a wavelength rejection system, using a reflective Fabry-Perot filter according to the invention.

Such a filter may be tuned by making a gap into a liquid crystal cell, and using the variation of apparent refractive index with drive voltage. For many of the applications of such filters, the response of the filter should be independent of the incoming state of polarisation of the incoming light. This is usually difficult to achieve for liquid crystal systems because of their strong anisotropic nature.

To overcome these difficulties, a conventional twisted nematic structure driven at a voltage higher than is normally used for display purposes can provide a polarisation-insensitive filter.

A liquid crystal device of the twisted nematic type uses a thin (e.g. 6-12 micron) layer of nematic liquid crystal material between two glass plates unidirectionally rubbed to align liquid crystal molecules, the rubbing directions on the two plates being orthogonal. This results in a twisted molecular structure which rotates the electric field of plane polarisation in the absence of an electric field, and when a voltage above a threshold is applied ceases to rotate plane polarised light. As used in a display the cell is between polarisers with their optical axes parallel or crossed, so that light transmission or extinction is effected by switching the applied voltage on or off, or vice versa.Small amounts, e.g. 1% of a cholesteric material may be added to the liquid crystal material, also small amounts of dichroic or pleochroic dye may be added, when one polariser may be dispensed with.

The conventional twisted nematic structure, as briefly described above, is used in the present application as a filter, with a small tilt bias to prevent reverse tilt effects. There is also a small addition of cholesteric dopant to prevent reverse twist effects. The surface alignment directions, as defined by the above-mentioned rubbing direction, should be accurately at right angles. With no voltage applied the structure inside the liquid crystal material is as shown in (a), on the drawing. The light entering the device passes vertically downwards through the liquid crystal material. The effective refractive index is highly dependent on the polarisation of the incoming beam.Use of a polarising filter is undesirable since it would entail a 3dB loss of intensity, and may also cause loss of the signal if the incoming beam is linearly polarised along the absorption axis of the polariser.

To remove the polarisation sensitivity referred to above, the cell is used in the so-called saturation region of the electro-optic characteristic. The internal structure at the low-voltage end of saturation are shown at (b), and at a higher voltage at (c), in the drawing. Thus at voltages of typically two to ten times the threshold voltage, the central majority of the liquid crystal structure has the director parallel with the electric field. However, there always remains a surface region where the director distorts to conform with the surface azimuth and tilt angles. The structures of these regions is symmetric with the centre of the cell, but are at right angles on the surface, so the effective refractive index is the same for all states of incoming polarisation for normally-incident beams of light.

According to the basic theory of liquid crystal devices, the tilt angle has approximately a negative exponential relationship with distance into the cell, and the exponential factor is proportional to the applied field. For a 10 micron thick cell the surface layer at the start of the saturation region may be about 2 microns at each edge, reducing to a fraction of a micron at the highest voltages usually applied to such cells. This is indicated schematically at (b) and (c) for the upper-most layer. The calculated tuning range of the filter is thus approximately the product of the birefringence and the effective change in the surface layers, i.e. approximately .2 x 1 microns, which gives 200nm.

Such a range is large enough for wavelength demultiplexer applications, and with materials of higher birefringence in thicker than normal cells, e.g. .27 in 20 micron cells, a tuning range across the visible band should be achieved.

In use the liquid crystal element is in the Fabry-Perot device with its two plates of glass or other suitable transparent material parallel with the reflectors of the Fabry-Perot device. Thus the characteristics of the filter can be varied by varying the voltage across the liquid crystal element. This is shown schematically in Figure 2, where we see the two reflectors 1, 2 of the Fabry-Perot device, with the liquid crystal 3 between them. If the Fabry-Perot device has metallic reflectors, then they could also be used as the electrodes for driving the liquid crystal cell. In that case the liquid crystal cell fills the gap between the two relectors, and is in fact a unitary structure with them.

Figure 3 shows the use of a liquid crystal cell of the non-reflective type in a laser cavity. Here we see the two reflectors 5, 6, each with a metallic reflector on its inner face, a liquid crystal cell 7, and the laser medium 8.

For the present purpose, examples of the materials which can be used for the liquid crystal material are as follows: Material n Threshold Voltage BDH E7 0.225 1.5 BDH E18 0.228 1.34 BDH E47 0.209 1.49 The main cirteria are high birefringence, and a fast speed of opration. In the above list the speed of operation increases as one descends the list. The region of interest for use in the present application is about twice or three times the threshold voltage. That is, the operating voltage for the cell is 3 volts or more.

Figure 4 shows schematically a system in which a Fabry-Perot filter according to the invention is used for wavelength demultiplexing. The light to be dealt with arrives via an optical fibre 10, from which is passes via a collimating lens 11 to the liquid crystal Fabry-Perot filter 12. Up to the filter, the light beam contains light of a number of different wavelengths.

The filter is set by a drive circuit 13, which applies a voltage appropriate to the wavelength of light to be passed by the filter. This is set by the input marked 'Wavelength Select Signal".

Light at the selected wavelength only, which may be modulated in accordance with the wanted signal, leaves the filter 12, and is focussed by a collimating lens 14 on to the end of another optical fibre 15. This applies the light to a signal detector 17, whose output represents the incoming signal. If desired, a feedback connection may be provided from the detector 16 to the drive circuit 13. This can be used, for instance, to adjust the drive circuit 13 so that it can take account of frequency drift by the incoming signal.

Figure 5 shows, again schematically, a wavelength rejection system using a reflective Fabry-Perot filter. Note that wavelength rejection is in effect the reverse of wavelength selection. The incident light beam arrives as indicated at, and contains information of interest, referred to as the scene, and has a superimposed single wavelength. This could be light of a frequency not relevant to the scene, but sent with, e.g. for control or signalling purposes.

This combined beam falls on a Fabry-Perot filter 21 according to the invention, and the wanted light is reflected, as shown at 22. The filter is sent to pass light at the unwanted wavelength, and this latter thus leaves the filter 21, passing to a detection circuit 23. The output of this detection circuit is used, via a control circuit 24 and a drive circuit 25 to maintain the voltage across the filter 21 to that appropriate to the wavelength to be removed.

To ensure that the correct frequency is rejected, the incoming beam can be wavelength searched under control of the control circuit 24 until a narrow pulse is detected. This pulse will be at the wavelength to be rejected, and in response to this pulse the Fabry-Perot detector is tuned to maximise the detection signal.

Claims

CLAIMS:

1. An optical filter arrangement, which includes a Fabry-Perot device through which the light to be filtered passes, a liquid crystal element within the Fabry-Perot device with its transparent plates between and substantially parallel to the reflectors of the Fabry-Perot device, and connections to the liquid crystal element via which a voltage is applied to suitably set the characteristics of the filter.

2. An arrangement as claimed in claim 1, in which the liquid crystal device is of the twisted nematic type wherein the liquid crystal material contains a small addition of a cholesteric dopant, and in which the device is biased to a higher voltage than used in a display, so that it operates in a saturation mode.

3. An optical filter arrangement, which includes a Fabry-Perot device through which the light to be filtered passes, a liquid crystal device of the twisted nematic type whose liquid crystal material contains a small addition of a cholesteric dopant, said device being arranged with its transparent plates between and substantially parallel to the reflectors of the Fabry-Perot device, and connection to the liquid crystal element via which a voltage is applied to suitably set the characteristics of the filter, said device being so biased by said voltage as to operate in a saturation mode.

4. An optical filter arrangement, substantially as described with reference to Figs. 1 and 2 of the accompanying drawing.

5. A laser arrangement in which the laser has a 'cavity defined by parallel reflectors between which is located a filter as claimed in claim 1, 2, 3 or 4, which filter is between one of the reflectors and the laser medium.

6. A wavelength demultiplexing arrangement, in which an optical fibre via which a signal to be handled is received directs the signal via lens means on to a filter as claimed in claim 1, 2, 3, or 4, in which a voltage is applied to the filter to set it to a condition wherein it passes a desired signal wavelength, and in which lens means directs the light leaving the filter via another optical fibre on to detecting means.

7. An arrangement as claimed in claim 6, and which includes a feedback connection from the detector means to drive means via which said voltage is applied to the filter.

8. A wavelength rejection arrangement, in which an incident optical wave which includes wanted light and light at an unwanted wavelength is applied to a filter as claimed in claim 1, 2, 3 or 4, and in which a voltage is applied to the filter which sets it to a condition in which it passes light of the unwanted wavelength but reflects the remainder of the light to a viewing portion.

9. An arrangement as claimed in claim 8, in which the unwanted light includes a short pulse defining its wavelength, in which the light passing through the filter falls on a detector arranged to scan the light so as to respond to said unwanted wavelength, and in which in response to said short pulse the detection means causes the voltage applied to the filter to be suitably adjusted.