WO2001018920A1

WO2001018920A1 - Laser cavity

Info

Publication number: WO2001018920A1
Application number: PCT/GB2000/003411
Authority: WO
Inventors: Andrew James Murray; Helen Frances Gleeson
Original assignee: The Victoria University Of Manchester
Priority date: 1999-09-06
Filing date: 2000-09-06
Publication date: 2001-03-15
Also published as: GB9920939D0; AU7021500A

Abstract

A laser cavity having a liquid crystal intracavity wavelength tuning element, wherein the wavelength tuning element comprises a liquid crystal birefringent filter and means for selectively controlling the direction of the average optic axis of the liquid crystal, the average optic axis of the liquid crystal remaining substantially in a plane defined by sides of the element.

Description

Laser Cavity

The present invention relates to a laser cavity.

The use of angled birefringent filters, also referred to as Lyot filters, as tuning elements for dye lasers is well known. A birefringent filter comprises one or more birefringent slabs oriented at Brewster's angle within a laser cavity, each slab having an optic axis cut parallel to its face. Due to the birefringent nature of the filter, light which is linearly polarised when incident upon the filter will in general emerge with elliptic polarisation. However, at a particular wavelength determined by the birefringent properties of the filter, the polarisation of the light will be unchanged (the filter acts as a whole wave plate). The light which emerges with elliptical polarisation will suffer losses upon reflection from other surfaces which are also oriented at Brewster's angle elsewhere in the laser cavity. In contrast to this, the light which emerges with an unchanged linear polarisation will not suffer loses at those surfaces. The laser will preferentially oscillate at the wavelength of light which provides the least losses, and thus will oscillate at the specific wavelength for which the filter acts a whole wave plate.

Rotation of the angled birefringent filter about its axis will move the optic axis of the filter, and will therefore adjust the wavelength of the laser. This is the conventional manner in which birefringent filters are used to tune a laser. The optic axis of the filter remains parallel to the face of the filter during rotation of the filter.

Electro-optically tuneable birefringent filters are available which are capable of tuning the wavelength of a laser without rotation of the filter. These filters comprise an electro-optic crystal such as KDP or ADP. A voltage is applied to the crystal, thereby providing a small rotation of the optic axis of the crystal, and as a result providing wavelength tuning. Electro-optically tuneable birefringent filters typically require the application of relatively large voltages to induce a suitably large change in the orientation of the optic axis. The electro-optic effect is sufficiently small that a crystal must be relatively long (several millimetres) to allow tuning. This in turn requires compensation techniques to compensate for beam deviation (for example, two crystals can be used).

It is known to tune a laser cavity using a variable diffraction grating or a tuneable Fabry-Perot filter incorporating liquid crystal. However, such arrangements usually only provide tuning over narrow wavelength ranges, for example less than one nanometer. A wavelength filter comprising an intra cavity electrically tuneable Fabry-Perot filter using nematic liquid crystal has been described by Wacogne, B, Goedgebure J P, Onokhov, A P, Tmilin, M 'Wavelength tuning of a semiconductor laser using nematic liquid crystals' IEEE Journal of Quantum Electronics, 29(4) 1015- 17 1993.

US Patent Number 5,218,610 describes a liquid crystal laser tuning device used in conjunction with a polariser to tune the oscillation wavelength of a laser. The device comprises a layer of nematic liquid crystal sandwiched between electrodes. The nematic liquid crystal is arranged such that when no voltage is applied to the electrodes, the molecules of the liquid crystal are all oriented parallel to the plane of oscillation of a laser mode (i.e. horizontal for a TEM₀₀ mode). This provides a first refractive index for horizontally polarised light and a second lesser refractive index for vertically polarised light (the average optic axis of the liquid crystal lies within the plane of the device). The polarisation of light which passes through the layer of liquid crystal is thus wavelength dependent. When a voltage is applied to the liquid crystal, the molecules of the liquid crystal tend to reorient such that they lie parallel to the direction of propagation of the laser beam (the average optic axis has a significant component out of the plane of the device). This reduces the refractive index of the device parallel to the plane of oscillation of the laser mode, thereby modifying the wavelength dependency of the polarisation of light which passes through the layer of liquid crystal. This change is used in conjunction with the polariser to tune the wavelength of oscillation of the laser.

The laser tuning device described in US Patent Number 5,218,610 suffers from the disadvantage that its operation is slow, tuning between wavelengths requires a time period of the order of milliseconds. A further disadvantage of the laser tuning device described in US Patent Number 5,218,610 is that the average optic axis of the device does not remain within the plane of the device, but instead has a substantial component out of the plane of the device.

It is an object of the present invention to provide a laser cavity having an electro- optically tuneable birefringent filter which overcomes or mitigates at least some of the above disadvantages.

According to a first aspect of the present invention there is provided a laser cavity having a liquid crystal intracavity wavelength tuning element, wherein the wavelength tuning element comprises a liquid crystal birefringent filter and means for selectively controlling the direction of the average optic axis of the liquid crystal, the average optic axis of the liquid crystal remaining substantially in a plane defined by sides of the element.

The use of a liquid crystal as a birefringent filter allows tuning over a wide range of wavelengths, for example tens of nanometers. The average optic axis of the wavelength tuning element remains in the plane of the tuning element, in a manner analogous to a conventional (i.e. non-liquid crystal) birefringent filter.

Preferably, the liquid crystal birefringent filter is substantially oriented at Brewster's angle with respect to the direction of propagation of light within the laser cavity. Preferably, the liquid crystal birefringent filter comprises a layer of liquid crystal sandwiched between alignment layers, which in turn are sandwiched between transparent electrodes.

Preferably, the transparent electrodes are coated on glass substrates.

The liquid crystal may be a ferroelectric liquid crystal. Ferroelectric liquid crystals are advantageous because they provide very fast switching between birefringent states (of the order of lOμs)

The ferroelectric liquid crystal may be provided in a layer which is sufficiently thin that the helicoidal structure of the liquid crystal is suppressed. Alternatively, the ferroelectric liquid crystal is provided in a layer which is sufficiently thick that the helicoidal structure of the liquid crystal is retained.

The liquid crystal may be an electroclinic liquid crystal, an antiferroelectric liquid crystal, or a ferrielectric liquid crystal.

According to a second aspect of the present invention there is provided a method of tuning the wavelength of light emitted by a laser, the method comprising modifying the voltage applied to a liquid crystal birefringent filter provided within the cavity.

A specific embodiment of the invention will now be described by way of example only with reference to the accompanying figures, in which:

Figure 1 is a schematic illustration of a laser cavity according to the invention;

Figure 2 is a schematic illustration of a ferroelectric liquid crystal birefringent filter; and

Figure 3 is a schematic illustration of the structure of a ferroelectric liquid crystal. The laser cavity illustrated in Figure 1 comprises a dye jet 1 (the dye is Rhodamine 6G), a high reflector 2, a folding mirror 3, and an output coupler 4. The laser cavity is further provided with a ferroelectric liquid crystal birefringent filter 5. The laser is excited using a pump beam from an Argon Ion laser (not shown), which is coupled to the laser cavity using a pump mirror 7. The wavelength at which the laser oscillates is determined by a voltage applied to the ferroelectric liquid crystal birefringent filter 5.

The birefringent filter 5 is shown in more detail in Figure 2. The entire structure is sandwiched between two parallel glass substrates 8. Inner surfaces of the glass substrates 8 are coated with a transparent conductive layer of Indium Tin Oxide 9. Inner surfaces of the conductive layers 9 are provided with an alignment layer 10 of rubbed polymer. A thin layer of ferroelectric liquid crystal 11 is held between the alignment layers 10. The liquid crystal is SCE13, manufactured by Merck Ltd.

It will be understood that the Indium Tin Oxide 9 may be replaced by any other suitable material, and similarly, the rubbed polymer may be replaced by any other suitable material.

The optimum thickness of the ferroelectric liquid crystal layer 11 depends on the required filter characteristics as well as the type of liquid crystal used, but is preferably between lμm and 20 μm. The liquid crystal layer may be as much as 50μm thick. In this case the ferroelectric liquid crystal layer is 5μm. The alignment layers 10 are typically less than O. lμm thick, and the conductive layers 9 are typically less than O.lμm thick. The glass substrates 8 are typically 1mm thick. The ferroelectric liquid crystal filter shown in Figure 2 typically provides a birefringence of as much as 0.2.

Figure 3 shows schematically a ferroelectric liquid crystal to which a voltage has been applied (thereby generating an electric field which aligns the molecules comprising the ferroelectric liquid crystal). The ferroelectric liquid crystal has a symmetry axis 12, and may be in one of two states. When it is in the first state, it has an optic axis 13a which is tilted by an angle θ away from the symmetry axis 12. The size of the tilt angle θ is determined by the temperature of the ferroelectric liquid crystal as well as the material structure. The actual molecular orientation of the liquid crystal and therefore its optical properties are complex. The birefringent filter is constructed such that the optic axis 13a lies as close as possible to the plane of the glass substrates 8. This form of ferroelectric is effectively a Surface Stabilised Feπoelectric Liquid Crystal (SSFLC).

Changing the polarity of the electric field across the ferroelectric liquid crystal will cause the liquid crystal to switch from the first state to the second state which has an optic axis 13b. The optic axis 13b is tilted by an angle θ to an opposite side of the symmetry axis 12. The optic axis of the liquid crystal remains within the plane of the glass substrates 8 irrespective of whether the liquid crystal is in the first state 13a or the second state 13b. In this sense, operation of the liquid crystal birefringent filter 5 is analogous to operation of a conventional (i.e. non-liquid crystal) birefringent filter. This is in contrast to the wavelength tuning element described in US Patent Number 5,218,610, wherein the optical axis does not stay in the plane of the wavelength tuning element, but instead has a substantial out of plane component.

The ferroelectric liquid crystal birefringent filter provides switching of the wavelength of oscillation of the laser shown in Figure 1. Measured wavelength switching characteristics of the ferroelectric liquid crystal birefringent filter are summarised in the following table.

In the first case shown in the table, the laser cavity shown in Figure 1 was tuned mechanically to 578.9nm using a quartz birefringent filter. The quartz birefringent filter was then replaced with the ferroelectric liquid crystal birefringent filter described in relation to Figure 2. A field of approximately 5V/μm was applied to the ferroelectric liquid crystal birefringent filter 5, and the resulting birefringence and orientation of the optic axis tuned the laser to a wavelength of 583.2nm. The polarisation of the field was reversed, modifying the optical properties of the filter 5 as described above, and thereby tuned the laser to 587.5nm.

The surface stabilised ferroelectric liquid crystal used is not capable of providing continuous wavelength tuning, but rather can only provide switching between two wavelengths. The wavelengths may be pre-selected to some extent by mechanically rotating the filter to a different orientation. This was done to provide switching between 565.8nm (green light) and 588. Onm (red light) as shown in the table.

The response time of the switching was around lOμs.

The wavelength switching described is given by way of example only, and it will be appreciated that switching between other wavelengths may be achieved. Placing the ferroelectric liquid crystal in the laser cavity caused the liquid crystal to heat and its properties therefore varied with time. It is not believed that this effect is a significant hindrance to the use of the liquid crystal birefringent filter in a laser.

The rapid switching between two wavelengths provided by the ferroelectric birefringent filter may be used in a variety of applications. For example, in known laser video projection apparatus, a separate laser is used to provide each of the red, green and blue (or yellow) beams of light required to generate a colour image. These lasers are expensive to manufacture. Furthermore, it is difficult to align and adjust the intensity of three laser beams with sufficient accuracy to provide a colour image. The ferroelectric birefringent filter allows the red and green lasers to be replaced with a single laser which rapidly switches between red and green in a manner which may be synchronised with the generation of the image. Further, by adopting a different laser dye, or a combination of laser dyes, other colour changes can be implemented. The ferroelectric liquid crystal birefringent filter may also be useful in spectroscopy applications where rapid switching between wavelengths is required.

The ferroelectric liquid crystal described above does not provide continuous wavelength tuning, but rather only provides switching between two wavelengths. Other liquid crystals may be used to continuously vary the wavelength of laser oscillation over a range of values. The vast majority of liquid crystals which exhibit ferroelectric phases are chiral, as are the bulk phases. In the above described SSFLC birefringent filter, the helicoidal structure of the liquid crystal is suppressed by making thin devices in which the surface forces prevent the helical structure from forming. However, the helicoidal structure of ferroelectric liquid crystals may be utilised, for example in a Deformed Helix Device which employs material of relatively high chirality. Within the device, the helical axis (and therefore the optic axis) lies parallel to the glass substrates and along the rubbing direction of the polymer alignment layer. Application of an electric field perpendicular to the helical axis causes the helix to deform continuously up to the point where all of the molecules e in one tilt direction with respect to the rubbing direction, as descnbed for the SSFLC device above The optic axis of this device may be continuously rotated by vaπation of the electric field, and therefore may be used to provide continuous tuning of a laser wavelength. The optic axis stays within the plane of the device dunng rotation. The Deformed Helix effect is described in the following papers: L Komitov, S T Lagerwall, B Stebler, G Andersson, K Flatischler, 2^nd International Top. Meeting on Optics of Liquid Crystals, Toπno, 1988; K Yoshino and Y Inuichi, Japan J Appl. Phys. 17, 597 (1978), B I Ostrovskiy, A Z Rabinovich, A S Somn and B A Strukov, Zh. Eksper Teoret. Fiz 74 1748 (1978).

Another continuously rotatable optic axis in a liquid crystal device occurs in the so called electroclinic device. The effect is a pretransitional one: just above the ferroelectric to smectic-A (non-ferroelectnc) phase transition. With no voltage the average arrangement of the molecules is such that the optic axis is parallel to the device symmetry direction. Applying a voltage induces a tilt of the molecules such that the optic axis rotates within the plane of the device. To a first approximation, the induced tilt is linear with voltage, and tilt angles of up to 25° are obtainable (higher values have been reported but are not common). This effect may also be used to provide continuous tuning of a laser wavelength The switching time in electroclinic devices is reported to be faster than in ferroelectπc devices by an order of magnitude.

Antiferroelectπc and ferπelectπc liquid crystals were discovered m 1989 and 1990 respectively. In both cases, tilted, electπcally switchable phases occur in highly chiral liquid crystals. These phases may also be used to provide continuous tuning of a laser wavelength These phases may offer switching with and without thresholds, depending on the mateπal and geometry

Other suitable liquid crystals may be used as a birefrmgent filter according to the invention. Details of liquid crystal properties are described in the following publications:

SUBSTITUTE SHEET ( RULE 26 i 'Liquid Crystals. Applications and Uses', Volumes 1-3, Edited by B Bahadur, World

Scientific 1990. ISBN 981-02-0110-9.

'Introduction to Liquid Crystals' P J Collins and M Hird. Taylor & Francis 1997,

ISBN: 0-7484-0643-3.

'Ferroelectric Liquid Crystals Principles, Properties & Applications' by J W Goodby et al. Gordon & Breach 1991 ISBN: 2-88124-282-0:

'Electrooptic Effects in Liquid Crystal Materials', L M Blinov and V G Chigrinov.

Springer Verlag, New York 1994. ISBN 0-387-94030-8,

'The Handbook of Liquid Crystals' Ed. D Demus and Goodby, Wiley - VCH 1998

ISBN 3-527-29491.

Claims

1. A laser cavity having a liquid crystal intracavity wavelength tuning element, wherein the wavelength tuning element comprises a liquid crystal birefringent filter and means for selectively controlling the direction of the average optic axis of the liquid crystal, the average optic axis of the liquid crystal remaining substantially in a plane defined by sides of the element.

2. A laser cavity according to claim 1, wherein the liquid crystal birefringent filter is substantially oriented at Brewster's angle with respect to the direction of propagation of light within the laser cavity.

3. A laser cavity according to claim 1 or 2, wherein the liquid crystal birefringent filter comprises a layer of liquid crystal sandwiched between alignment layers, which in turn are sandwiched between transparent electrodes.

4. A laser cavity according to claim 3, wherein the transparent electrodes are coated on glass substrates.

5. A laser cavity according to any preceding claim, wherein the liquid crystal is a ferroelectric liquid crystal.

6. A laser cavity according to claim 5, wherein the ferroelectric liquid crystal is provided in a layer which is sufficiently thin that the helicoidal structure of the liquid crystal is suppressed.

7. A laser cavity according to claim 5, wherein the ferroelectric liquid crystal is provided in a layer which is sufficiently thick that the helicoidal structure of the liquid crystal is retained.

8. A laser cavity according to any of claims 1 to 4, wherein the liquid crystal is an electroclinic liquid crystal.

9. A laser cavity according to any of claims 1 to 4, wherein the liquid crystal is an antiferroelectric liquid crystal.

10. A laser cavity according to any of claims 1 to 4, wherein the liquid crystal is a ferrielectric liquid crystal.

11. A laser tuning element comprising a liquid crystal birefringent filter as described in any of claims 1 to 10.

12. A method of tuning the wavelength of light emitted by a laser, the method comprising modifying the voltage applied to a liquid crystal birefringent filter provided within the cavity to selectively control the direction of the average optic axis of the liquid crystal, the average optic axis of the liquid crystal remaining substantially in a plane defined by sides of the birefringent filter.

13. A laser cavity substantially as hereinbefore described with reference to the accompanying figures.

14. A method of tuning the wavelength of light emitted by a laser, substantially as hereinbefore described with reference to the accompanying figures.