GB2354597A

GB2354597A - Ferroelectric acousto-optic devices

Info

Publication number: GB2354597A
Application number: GB9922403A
Authority: GB
Inventors: Christopher Noel Pannell; Nadhum Zayer; Harald Gnewuch
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-09-23
Filing date: 1999-09-23
Publication date: 2001-03-28
Also published as: GB9922403D0

Abstract

An acousto-optic tuneable filter comprises a ferroelectric crystal in which domain inverted regions R1, R2 have been created by electric field poling. Preferably, the spacing of the domain inverted regions varies along the length of the crystal. The domain walls are not normal to the x-face of the crystal. Acoustic waves may be generated in the crystal by applying rf power to electrodes situated on one or other of the faces of the crystal. Once generated the acoustic waves cause coupling between polarisation eigenstates of the crystal. The polarisation state of an input light beam is altered by the acoustic wave such that certain frequencies of can be separated from the rest by polarisation optics.

Description

1 2354597 Acousto-optic devices made from domain inverted ferroelectric

crystals

Introduction and current state of the art

In the field of optical science and technology there is frequently a need for separating some subset of the optical frequency spectrum of an optical signal from the rest of the signal. Such a process is called filtering, and it is of importance for example in the field of optical telecommunications, where several different communications channels are present on the same optical fibre link. The channels are separated by spacing the centre wavelengths of each channel far enough apart in terms of optical wavelength so that interference between adjacent channels is negligible. An optical filter can then be used to select a particular channel and reject all the others.

Optical filters may be fixed or tuneable, and many schemes already exist for both. Of the currently existing tuneable filters, one method to realise such a device is by means of the interaction of an acoustic wave produced by a suitable acoustic transducer with the optical signal in a suitably chosen and prepared crystal. The application of radio frequency (RF) power to the transducer causes the generation of acoustic waves necessary to the functioning of the device hence this generic class of device is called an acousto-optical tuneable filter or an AOTF.

Several different configurations for such devices exist, they typically consist of a crystal, referred to hereafter as the interaction crystal, such as tellurium dioxide which has been suitably cut with respect to its natural crystal axes, and polished to a high quality optical finish. An acoustic transducer, typically of another material such as lithium niobate is then attached to one face of the interaction crystal, typically by means of a thin layer of epoxy, anaerobic adhesive or by thermo-compression bonding using indium metal, this transducer is then lapped and polished down to a thickness determined by the design of the device. Prior to transducer bonding, one or more acoustic matching layers may be needed between the transducer and the interaction crystal, these are typically formed by thermal evaporation of layers of various metals having specific thickness onto the interaction crystal under vacuum. These matching layers may be needed in order to render the acoustic transducer capable of delivering sufficient acoustic power to the interaction crystal over a wide enough electrical bandwidth to ensure that the device has a large enough optical tuning range. Ignoring the effects of optical dispersion and assuming a substantially single frequency RF input to the acoustic transducer, the optical frequency selected is proportional to the RIF drive frequency and the optical tuning range of the AOTF is proportional to the tuning range of the RF transducer.

Typically, an electrical matching circuit is placed between the RF power source and the acoustic transducer of the AOTF. The purpose of the matching circuit is to ensure that the electrical impedance at the output terminals of the transducer is suitably transformed so that the impedance presented to the source of RF power is close to its design characteristic output impedance, typically 50 Ohms, over the whole design tuning range of the AOTF. This ensures that RIF power can be efficiently coupled from RF source to the transducer efficiently over the whole design bandwidth of the AOTF. This matching circuit is typically built on a small electrical circuit board and placed in a package along with the interaction crystal.

The technique of apodisation may be used in the construction of an AOTF. This involves using a means of causing the acoustic wave intensity to vary in a specific way with position in the interaction crystal. The object of apodisation is to modify the passband of the AOTF in such a way that it is optimised for some application, for example it is frequently advantageous to maximise rejection of optical signals outside the currently selected passband.

From the previous description of current state of the art AOTFs, it may be appreciated that several different materials and processes are used in their construction, the interaction crystal and acoustic transducer must be lapped and polished, the acoustic transducer must be bonded to the interaction crystal, and frequently an acoustic matching layer or layers must be placed between the interaction crystal and acoustic transducer in order to achieve a sufficiently wide electrical tuning range for the AOTF. The mechanical differences between the interaction crystal and acoustic transducer, in particular the differences in specific acoustic impedances, necessitate the use of acoustic matching layers. The purpose of these layers is to match the specific acoustic impedances of the different materials over a broad range of RF frequencies and thereby ensure broad tuning range.

The invention - description

We, the named co-inventors, declare the following invention which relates to an optical device and in particular to an optical filter which is tuneable over a predetermined range of optical frequencies, by means of one or more radio frequencies applied to an acoustic transducer array which forms part of the device.

The features of the invention described in this section will become readily apparent from the following detailed description of preferred embodiments of the invention, when considered in conjunction with the accompanying drawings.

In direct contrast to state of the art devices, the present invention uses a single piece of crystalline ferroelectric material such as lithium niobate, a portion of which functions both as the interaction medium and as the acoustic transducer, so that the interaction medium and acoustic transducer are formed from a contiguous piece of crystal material, and there is therefore no need for a bond or acoustic matching layers between acoustic transducer and interaction medium as in state of the art devices. This results in a device that is much simpler in construction than state of the art devices, and the absence of the bond and matching layers allows greater electrical tuning ranges to be achieved. One such embodiment of such a device is shown in figure 1.

The transducer region covering the extent Lg in figure l(a) is made up of pairs of inverted and non-inverted domains, each pair constituting one period. The period A may be deliberately varied over the length Lg from the value A, to A2 as shown in figure l (a) for the purpose of determining the allowable tuning range of the AOTF.

The area of the electrodes used to supply the RF power to the device is determined by the lateral dimensions denoted by W in figure l(a) and LE in figure l(b). This embodiment was designed to produce bulk longitudinal acoustic waves travelling colinearly with the x-direction. For the purpose of controlling acoustic reflections in the device the ends of the crystal may be polished at an angle a to the z-axis as shown in figure l(a). The optical beam passing through the crystal may be aligned in such a way to also be co-linear with the x-axis.

Figure l(c) shows schematically the arrangement for the operation of the embodiment. F is the AOTF crystal containing a region in which the transducer region is formed by domain inversion, regions denoted E are the metallic electrodes. The input light is incident on the AOTF crystal linearly polarised parallel to one of the crystal axes which is ensured by a linear polarising element P. An RF signal is set to a desired frequency by the RF generator and amplified to a desired power by the RF amplifier. The RF signal is applied to the electrodes and excites the desired acoustic waves in the crystal.

Our prototype device had the following specification:

Material: lithium niobate, prototype device made from a 0.5mm thick z-cut wafer.

RF frequency tuning range 300-38OMHz.

Optical tuning range 1300 - 1600 nanometres Length of the device, L = 20mm Width in the z-direction, W = 0.5mm Thickness of device in y-direction, d = 0.67mm Transducer length, Lg = lomm Electrode length, L, = 14mm Wedge angle a = 10 degrees Observed optical passband width 2.6 run @ 1550 mn, 1.57nm @ 1319 nm Operating power for prototype -2W for 50% efficiency.

On passing through the device, the acoustic and optical fields interact in such a way that for a specific RF frequency applied to the device, only a narrow band of optical frequencies centred around a specific optical frequency has its polarisation changed in such a way that it can pass through the linear polarising element, or analyser, denoted by A, which is set to be orthogonal to the linear polariser P. All other optical frequencies are blocked out by A. With the device switched off (no RF power applied), no light will pass from input to output because the polariser P and analyser A are set to pass orthogonal optical linear polarisation states. A specific RF power applied to the acoustic transducer ensures that a maximum optical signal passes through A.

Figure 2 shows the view down the z-axis of one embodiment of the device in which the crystal, F, with its transducer region covered by a metallic top electrode TE, is bonded to a substrate S of material having high thermal conductivity in order to minimise the appearance of thermal gradients along the length of the AOTF. The bonding layer is referred to as BE and may be metallic in which case it can function as another electrode. In order for the AOTF passband width to be as narrow as possible, as required by some applications within the area of optical telecommunications it is important that the dissipation of RF power in or adjacent to the crystal does not cause temperature gradients along the crystal, the invention therefore provides for means of bonding the crystal to another medium of high thermal conductivity to minimise these effects. The passband width of the AOTF which forms the subject of this invention may be calculated by a person skilled in the art from a knowledge of the physical dimensions of the device, the optical properties of the interaction crystal and the optical wavelength range over which it is designed to work. This theoretical passband width will not be attained unless particular care is taken to minimise the thermal gradients. If this is not done, the performance will be degraded in that the passband will be broader than would otherwise be the case, because thermal gradients will cause fluctuations in the local birefringence of the interaction crystal. For example, the theoretical optical passband width (full width at half maximum) for our prototype device of length 20 mm is 1.15 nm. rather than the experimentally observed 1.57 @ 1319nm, and 1.67nm rather than the observed 2.6nm @ 1550run.

The ability of the AOTF which forms the subject of this invention to reject optical signals outside the desired passband is, in common with currently existing AOTFs, determined by the mathematical form of the passband function. The ability of any AOTF to reject out of band optical signals may be improved by reducing the intensity of the side-lobes of the passband function, this process being known as apodisation. In the device which forms the subject of this invention, apodisation may be obtained by introducing Ru-ther inverted domains in the crystal in such a way at to cause the acoustic waves generated to be scattered or attenuated in a precise way as they travel along the device. In figure 2, extra domain inverted regions R1 and R2 may be present. The purpose of these regions is to control the acoustic intensity by reflecting and scattering the acoustic power in a particular manner.

Figure 3 shows three possible configurations of the AOTF. In figure 3a, the AOTF crystal is angle polished at both ends for the purpose of controlling the acoustic reflections at both ends. It is possible with this configuration to operate the device in transmission, with the optical beam incident for example along the x-axis from the left, and after passing through the device, exiting on the right.

Figure 3b shows another possible configuration in which the end face X, is normal to the x-axis, and the other end face X2 is angle polished for controlling acoustic reflection. If the end face Xi is made reflective, the optical beam incident via surface X2 will pass twice along the length of the device in the x-direction before finally exiting again at surface X2. The acoustic field generated by the transducer is reflected by the surface X, so that it passes back along the x-axis and is not lost. Thus the interaction length of the AOTF is doubled relative to the device of figure 3a. This is advantageous in that the power requirement for the device is lowered and the optical passband of the filter is reduced, i.e. the filter is made more selective. It is not essential for the operation of this device that the transducer is situated at the end of the ferroelectric crystal which is polished so as to reflect the optical and acoustic waves back along their own path.

Figure 3c shows another configuration in which both the end-faces of the AOTF crystal X, and X2 are polished so as to be normal to the x-axis. This has the effect of turning the AOTF crystal into a resonant cavity, with closely spaced acoustic standing wave resonances. If the RY generator is tuned to one of these resonances, the effect will be to cause an increase in the intensity of the acoustic wave in the AOTF crystal, in turn causing an increase in efficiency, i.e. the AOTF will require less electrical power for the same total output optical power. If one of the end faces is made optically reflective, the device can be operated in reflection in the same way as the device of figure 3b, offering still greater efficiency and selectivity.

An alternative method of forming an acoustic reflector is to use distributed reflectors made by domain inversion of the ferroelectric crystal, in which the period A is equal to one half of the wavelength of the acoustic wave it is desired to reflect.

The acoustic transducer is formed in the ferroelectric material by means of inversion of the natural spontaneous polarisation axis of the ferroelectric induced by an electric field or other means, so as to cause well-defined regions of the ferroelectric material to reverse its natural spontaneous polarisation axis. This process, hereafter called domain inversion has the effect of making the crystal mechanically inhomogeneous in a predictable way and in particular of reversing the sign of the piezoelectric tensor components in the inverted regions. After domain inversion, the regions of the crystal

6 still sharing a common natural polarisation axis are hereafter called domains. Before domain inversion, the crystal as obtained from the manufacturer would normally be in the form of a single domain. The process of domain inversion of selected regions of the original crystal, usually available in the forin of a flat plate or "wafer" may be effected for example, by the application of an electric field which is greater than the coercive field of the material. This field is applied until an amount of electrical charge has flowed through the circuit corresponding to the inversion of all the desired domains. The areas to be inverted are defined using a photolithographic technique which leaves an insulating layer of photoresist on the surface of the crystal wafer in places where it is desired not to cause inversion to occur. Such methods are well known by those skilled in the art, and the process of domain inversion by the method described here is not part of the claims of this invention.

One may use acoustic shear waves rather than longitudinal waves to build an AOTF, again these waves may be generated using an acoustic transducer formed by domain inversion in a crystal of ferroelectric material such as lithium niobate. With reference to figure 4, shear waves are generated using the electric field produced by using two parallel electrodes on the x-faces (diagram 4a) or alternatively one on the x-face and one or more on the z-faces (diagram 4b). In both cases the shear wave propagates colinearly with the x-axis. This latter configuration ensures that the shear wave is launched into the crystal without encountering the electrode. In figure 4, the electrodes are denoted by the symbol E, and the orientation of the crystal axes is shown.

In all the preceding description and examples, the walls between adjacent domains were plane and in the z-y plane, i.e. normal to the x-axis. Under these circumstances the acoustic wave group velocity and phase velocity are parallel. It is frequently desired in acousto-optics to be able to engineer a situation in which this is no longer the case, and so-called walk-off occurs. This may be done with the domain inverted ferroelectric devices described here, by making the domain walls lie in another plane other than normal to the x-axis, preferred crystal directions are at 60 degrees and 120 degrees to this direction. The walk-off may for example be used as yet another method to control the degree to which acoustic and optical fields overlap on progressing through the device, i.e. another method of producing apodisation.

Claims

WHAT WE CLAIM IS:-

I A device comprising a crystal of ferroelectric material suitably treated by electric field poling to cause domain inversion to occur in selected regions of the crystal for the purpose of generating and controlling acoustic waves in the crystal.

2 An acoustic transducer comprised of a number of domain inverted regions in a ferroelectric crystal created by electric field poling, the periods and duty cycles of which vary with position in the crystal in a manner designed to influence the input electrical impedance of the device so as to increase the band of electrical frequencies over which the device is capable of generating acoustic waves efficiently compared to the case where all the periods are of uniform size and duty cycle.

3 An acousto-optic, device in which acoustic waves generated in one or more transducer regions formed in a crystal of ferroelectric material according to claim 2 are allowed to interact with a beam of optical radiation inside the ferroelectric material with the purpose of causing polarisation coupling to occur in one or more narrow bands of optical frequencies. Polarising optics are then used to allow these selected bands of frequencies to pass, while blocking the rest, so that a filtering action will occur. The output of the device consists of a filtered version of the input band of frequencies.

4 An acousto-optic device in which longitudinal acoustic waves generated in one or more transducer regions formed in a crystal of ferroelectric material according to claim 2 are allowed to interact with a beam of optical radiation inside the ferroelectric. material with the purpose of causing polarisation coupling to occur in one or more narrow bands of optical frequencies. Polarising optics are then used to allow these selected bands of frequencies to pass, while blocking the rest, so that a filtering action will occur. The output of the device consists of a filtered version of the input band of optical frequencies.

An acousto-optic device in accordance with any of the claims 1-4 in which the ferroelectric material is lithium niobate.

6 An acousto-optic device in which shear acoustic waves are generated by the use of an electrode system comprising of a pair of parallel electrodes situated on the x-faces of a ferroelectric crystal which has been treated so as to domain invert at least a portion of it in the manner of claim 2.

7 An acousto-optic device in which shear acoustic waves are generated by the use of an electrode system comprising of an electrode situated on the x-face and one or more electrodes situated on the z-faces of a ferroelectric crystal which has been treated so as to domain invert at least a portion of it in the manner of claim 2.

8 An acousto-optic tuneable filter (AOTF) in which shear acoustic waves are generated by the use of an electrode system comprising of a pair of parallel electrodes situated on the x-faces of a ferroelectric crystal which has been treated so as to domain invert at least a portion of it in the manner of claim 2, the scheme is represented in figure 4a.

1;? 9 An acousto-optic tuneable filter (AOTF) in which shear acoustic waves are generated by the use of an electrode system comprising of an electrode situated on the x-face and one or more electrodes situated on the z-faces of a ferroelectric crystal which has been treated so as to domain invert at least a portion of it in the manner of claim 2, such a scheme being represented by figure 4b.

An acousto-optic tuneable filter (AOTF) in which shear acoustic waves are generated by the use of an electrode system comprising of two pairs of electrodes situated on the z-faces of a ferroelectric crystal which has been treated so as to domain invert at least a portion of it in the manner of claim 2. The second pair of electrodes on the z-faces are electrically connected and replaces the single electrode on the xface of claim 9. The RF power is applied between the two pairs of electrodes.

11 An acousto-optic device according to any of the claims of 3-4 or of 67 in which the ferroelectric crystal is bonded using a metallic bond layer such as indium. solder or thermo-compression indium bonding, or a non metallic bond such as epoxy, to a substrate which has a high thermal conductivity with the object of minimising the degrading effects of local thermal gradients on the optical performance.

12 An acousto-optic tuneable filter (AOTF) according to any of the claims of 3-4 or of claims 8-9 in which the ferroelectric crystal is bonded using a metallic bond such as indiurn solder or thenno-compression indium. bonding, or a non metallic bond such as epoxy, to a substrate which has a high thermal conductivity with the object of keeping the passband as narrow as possible by minimising local thermal gradients.

13 An acousto-optic tuneable filter (AOTF) according to any of the claims 3-4 or of 8-9, in which ferroelectric domain reversal is used to create acoustic scattering and partially reflective structures for the purpose of manipulating the variation in strength with position of the acoustic wave in the device. This variation will be pre-set when the device is built and is also called apodisation, and has the purpose of increasing the ability of the AOTF to reject unwanted optical signals, i.e. those falling outside the desired optical passband.

14 An acousto-optic tuneable filter (AOTF) made from a crystal of ferroelectric material in which ferroelectric domain reversal of a least a portion of the crystal for the purpose of forming one or more acoustic transducers is carried out in such a manner that the domain walls are no longer non-nal to the x-axis The use of ferroelectric domain reversal when forming the acoustic transducer in such a manner that the domain walls are no longer normal to the x-axis, this for the purposes of apodising the AOTF.

16 An acousto-optic device utilising a ferroelectric crystal containing one or more acoustic transducers of substantially non-uniform period formed by ferroelectric domain inversion of a portion of the crystal according to claim 2 and in which the acoustic wave and the optical wave are caused to reflect from one end of the device back along their original path inside the device in order to increase the optical interaction length. Such a scheme is illustrated in figure 3b, with waves reflecting off the surface labelled X,.

9 17 An acousto-optic tuneable filter (AOTF) utilising a ferroelectric crystal containing one or more acoustic transducers of substantially nonuniform period formed by ferroelectric domain inversion of a portion of the crystal according to claim 2 and with or without the apodisation of claim 13 in which the acoustic wave and the optical wave are caused to reflect from one end of the device back along their original path inside the device in order to increase the optical interaction length. Such a scheme is illustrated in figure 3b, with waves described here reflecting off the surface labelled X1.

18 An acousto-optic device utilising a ferroelectric crystal containing one or more acoustic transducers of substantially non-uniform period according to claim 2 formed by ferroelectric domain inversion of a portion of the crystal and in which the acoustic wave is caused to reflect back along its original path in the crystal from two parallel surfaces at the ends of the device such that a resonant acoustic cavity is formed. Such a scheme is illustrated in figure 3c, with the extent of the acoustic cavity defined by the perpendicular distance between acoustically reflecting surfaces labelled X, and X2.

19 An acousto-optic tuneable filter (AOTF) utilising a ferroelectric crystal containing one or more acoustic transducers of substantially nonuniform period according to claim 2 formed by ferroelectric domain inversion of a portion of the crystal and in which the acoustic wave is caused to reflect from two parallel surfaces at the ends of the device such that a resonant acoustic cavity is formed. Such a scheme is illustrated in figure 3c, with the extent of the acoustic cavity defined by the perpendicular distance between acoustically reflecting surfaces labelled X, and X2. The surfaces defining the ends of the resonant acoustic cavity are transparent to the optical signal.

An acousto-optic device as described in claim 19 except that one of the end surfaces of the resonant acoustic cavity is made optically reflective in addition to acoustically reflective, so that the optical power retraces its original path inside the device and makes a double pass of the device. The optical reflection at the other end of the device corresponding to the plane at which the optical radiation enters the ferroelectric crystal may be reduced by using antireflection coating. Such a device is shown in figure 3c, with one Of (Xl,X2) optically reflective and the other having its optical reflection minimised, while both X, and X2 are acoustically reflective.

21 An acousto-optic device in which one or more acoustic reflections are effected by distributed acoustic reflectors composed of domain inverted regions of the ferroelectric crystal in which the period of the distributed reflector A is equal to one half of the acoustic wavelength it is desiredto reflect, and in which reflection over a band of acoustic frequencies may be effected by varying the period A in the reflecting structure 22 Acousto-optic devices as laid out in claims 16-21 in which the ferroelectric material is lithium niobate.