PHOTODETECTOR ASSEMBLY

The present invention relates to a photodetector assembly, and in particular, but not exclusively, to a photodetector assembly for use in a hearing device and system but which may also be used in other medical and non-medical applications . The present invention also relates to a hearing aid device and to a hearing aid system including a photodetector assembly. Additionally, the present invention relates to a hearing aid device and to a hearing aid system including alignment means .

Sensorineural deafness is by far the most common type of Hearing loss . Deafness affects 9 million people in the United Kingdom alone, of which over 80% have sensorineural deafness ( source Defeating Deafness , United Kingdom) . Causes include congenital, bacterial, high intensity noise and, especially, the ageing process , with a significant proportion of those affected being over 60 years old. Hearing impairment is the third most common chronic problem affecting the ageing population - and one of the least diagnosed . There is also an increased prevalence in some sections of the younger age group, due to exposure to loud noise .

There are currently no effective means of repairing the cochlea or the nervous pathways to the brain . For many patients hearing can be restored adequately by sufficient amplification of sound with a hearing aid. Conventional hearing aids have a number of problems including acoustic feedback (because the microphone is very close to the

speaker) and inadequate sound quality and discomfort due to occlusion of the ear canal . Many are also undesirable from the social point of view in that the appearance of wearing a hearing aid can cause users to feel that they are seen to be handicapped . The alternative is an implantable device . Middle ear implants provide mechanical amplification by vibrating the ossicular chain . They are intended for patients with moderate to severe sensorineural hearing loss who still have residual hearing . They could potentially benefit up to 50% of all people with hearing loss .

Applicant' s International patent publication no . WO 03/063542 discloses a hearing aid system which comprises an external ear canal module and an implant which is surgically positioned on the ossicular chain in the middle ear . The ear canal module includes a power source, a microphone and associated electronic circuitry for transducing sound into an electrical signal which is in turn converted and transmitted by a light emitting diode or other light emitting device, such as a laser diode, as a modulated light signal, through the patient' s ear drum and towards the implant . The implant includes a photodetector arrangement with appropriate electronic circuitry and an actuator mounted on the ossicular chain . In use, the modulated light signal is transmitted by the ear canal module towards the photodetector arrangement of the implant which generates an electrical signal to drive the actuator and thus vibrate the ossicular chain .

It should be noted, however, that any misalignment between the ear canal module and the implant , and specifically

between the axis of the modulated light signal and the photodetector arrangement, can impair the performance of the hearing aid system, as discussed below .

The photodetector arrangement typically includes four photosensors, such as photodiodes , connected in series . Consequently, the total voltage generated by the photodetector arrangement will be the sum of the individual voltages generated by each photosensor . In one particular system the photodetector arrangement must generate a signal of around 1.4 volts . Using silicon photodiodes without amplification, each photosensor may be selected to generate, for example, 0.4 volts . However, it is essential that the photodetector arrangement is also capable of generating a sufficient current to provide adequate power to the implant to drive the actuator . It is well known in the art that the current output from a photosensor, such as a photodiode, varies in accordance with the level of light energy impinging upon it . Specifically, the current increases as impinging light energy increases . Accordingly, where a low level of light energy impinges on one of the photosensors of the series connected photosensors in the photodetector assembly, for example, due to misalignment of the light emitter of the ear canal module with the photodetector assembly of the implant, then the current generated by the photodetector assembly will be reduced due to the misalignment . The short-circuit current, and therefore the maximum current available, is limited by the photosensor which is exposed to the least light energy .

Misalignment of the hearing aid system may occur during normal use, or following removal and replacement of the ear canal module, for example at night, for cleaning or for battery replacement or the like . Furthermore, it can be difficult to achieve alignment with hearing aid implants of the type described, which may present problems related to insertion of the ear canal module into the ear canal, or problems related , to implants which have become misaligned during use .

It is among the obj ects of aspects of the present invention to obviate or mitigate at least one of the aforementioned and other disadvantages so that the photodetector assembly has sufficient power to drive the actuator and its associated electronic circuitry.

According .to a first aspect of the present invention, there is provided a photodetector assembly for use in a hearing aid device, said photodetector assembly comprising a plurality of photodetector units connected in parallel, each photodetector unit comprising at least one photosensor .

Conveniently, the photodetector assembly is adapted to convert light energy into electrical energy which may be utilised to power a hearing aid device or, indeed, any other suitable electrical apparatus .

Advantageously, the photodetector assembly is adapted to generate a minimum required voltage suitable for use to operate a hearing aid device, such as an implanted hearing

aid device . As the photodetector units are connected in parallel, the voltage across each photodetector unit will be the same for similar individual units and will, accordingly, be equal to the voltage generated by the assembly. Each photodetector unit may comprise a single photosensor adapted to generate the required voltage .

Alternatively, and in a preferred embodiment, each photodetector unit comprises a plurality of photosensors preferably connected in series such that the voltage generated by each photodetector unit is the sum of the individual voltages generated by each photosensor in the unit .

Coupling the photodetector units together in parallel advantageously assists to permit the photodetector assembly

^" to generate a level of current required to provide sufficient power to operate a hearing aid device . For example, where one photodetector unit is exposed to a reduced level of light energy, relative to the remaining photodetector units, this photodetector unit will generate a reduced local current . However, as the photodetector units are connected in parallel the total current generated will be the sum of the current generated in the individual units . Accordingly, the effect of the local reduced current is minimised .

Advantageously, where each photodetector unit comprises a plurality of photosensors connected in series, this in combination with the parallel connection arrangement of the photodetector units , permits the photodetector assembly of

the present invention to generate an optimised voltage and current to operate a hearing device .

The at least one photosensor may be a photo-sensitive diode, a laser diode, photovoltaic cell or other type of photoreceiver such as a monolithic dual j unction or triple junction p-n tandem cell, the individual p-n junctions being connected in series the selection of which would readily be made by a person of skill in the art to convert light energy to electrical energy.

Preferably, the photodetector units are arranged in a cluster. In a preferred embodiment the photodetector units are arranged in a honeycomb pattern . Other preferred cluster arrangements include square, rectangular, and substantially circular arrangements . Compact cluster arrangements advantageously optimise the use of space, which in some applications may be limited, such as in a hearing device required to be implanted into the ear of a patient . Furthermore, compact arrangements of photodetector units permit the use of space to be optimised to minimise the inactive area of the cluster, that is the area that is not photosensitive . It is to be understood that any other suitable cluster arrangement may be utilised.

Preferably, the photodetector assembly is adapted for use in a hearing aid system comprising an active implant for location within the auditory system of a patient, such as the middle ear, and an ear canal module for location within the external ear canal of the patient .

Advantageously, the photodetector assembly may be coupled to an actuator of the active implant such that, when in use, the photodetector assembly may convert light energy transmitted by the ear canal module into electrical energy to be used to drive an actuator of the active implant in the required manner . Conveniently, the active implant may be of the type which comprises an actuator coupled to a structure or position of the auditory system, such as the ossicular chain, wherein the actuator is caused to vibrate by the electrical energy generated by the photodetector assembly.

Accordingly, in use, the ear canal module may receive and convert sound to an electrical signal, and subsequently to a modulated light signal, which is transmitted towards the active implant comprising the photodetector assembly such that the modulated light signal may provide the required signal and energy to drive the actuator and provide a power source to any electronic circuitry associated with the_ actuator .

Preferably, the photodetector assembly further comprises a reflector adapted to reflect at least a portion of a light signal directed towards a surface of the photodetector assembly back towards a light sensor located such that adj ustments to the position of the light emitter can be made for alignment purposes . The reflector advantageously may be utilised to ensure that a modulated light signal transmitted towards the photodetector assembly is suitably aligned therewith to ensure that the photodetector assembly is exposed to a sufficient and optimum level of light energy to

generate the required voltage and current output required for the specific application .

It is to be understood that, in all embodiments of the present invention, each photosensor or plurality of photosensors is provided with ancillary electronics , comprising at least demodulator means in the form of demodulation circuitry, in order to demodulate a received modulated light signal thereby providing means for the actuator to be properly driven . Demodulator means may comprise active electronic devices but preferably comprise passive resistor-based demodulation and possibly inductor- based demodulation neither of which require a power supply.

It may also be envisaged that in some designs a voltage bias may be advantageously introduced to photodetectors or it may be desirable to introduce a degree of amplification or other form of electronic signal conditioning . In such an embodiment at least a part of the light signal is utilised, particularly those components of the light signal which lie outside the audio frequencies , which are otherwise ignored or removed by filtering .

According to a second aspect of the present invention, there is provided a hearing aid device comprising a photodetector assembly, said photodetector assembly comprising a plurality of photodetector units connected in parallel, each photodetector unit comprising at least one photosensor .

Advantageously, in use, the photodetector assembly converts

light energy to electrical energy which may then be supplied to the hearing aid device for operation thereof in the required manner .

Advantageously, the hearing aid device may be a hearing aid implant which may be located within the auditory system of a patient, such as in the middle or inner ear . Preferably, the hearing aid device of said hearing aid implant comprises an actuator mounted on the ossicular chain of a patient, and a photodetector assembly wherein said photodetector assembly converts light energy to electrical energy to be used to drive said actuator to vibrate the ossicular chain .

Preferably, the hearing aid device forms part of a hearing aid system which comprises an ear canal module having a transmitter module adapted to transmit a modulated light signal towards the hearing aid device, wherein the photodetector assembly converts light energy into electrical energy to power the hearing aid device . In this arrangement the hearing aid device may be an active implant mounted on the ossicular chain of a patient' s middle ear and the ear canal module may be retrievably mounted in the patient' s external ear canal directed towards the implanted hearing aid device . The transmitter module of the ear canal module may, therefore, transmit appropriate modulated light signals towards the implanted device through the patient' s ear drum

(tympanum) .

Each photodetector unit may comprise a single photosensor adapted to generate the required voltage . Alternatively, and

in preferred embodiments , each photodetector unit comprises a plurality of photosensors preferably connected in series such that the voltage generated by each photodetector unit is the sum of the individual voltages generated by each photosensor in the unit .

Advantageously, by connecting each photodetector unit in parallel, the total current generated by the photodetector assembly will be the sum of the currents generated in the individual photodetector units . Accordingly, if the local current produced in one photodetector unit is significantly reduced due to reduced exposure to light energy, then the effect of this reduced local current on the overall assembly will be minimised.

Preferably, the photodetector assembly further comprises a reflector adapted to reflect at least a portion of a light signal directed towards the photodetector assembly of the hearing aid device . The reflected light signal may be detected by a suitable detector arrangement and advantageously utilised to ensure that a light signal transmitted towards the hearing aid device, for example from a transmitter module in an ear canal module, is suitably aligned therewith to ensure that the photodetector assembly is exposed to a sufficient level of light energy to generate the required voltage and current output for operation of the device .

Preferably, the photodetector units of the hearing aid device are arranged in a cluster . In a preferred embodiment the

photodetector units are arranged in a honeycomb pattern. This compact arrangement advantageously optimises the use of limited space . Compact arrangements of photodetector units also minimise the inactive area of the cluster, that is the area that is not photosensitive . Other preferred cluster arrangements include square, rectangular, and substantially circular arrangements . It is to be understood that any other suitable cluster arrangement may be utilised.

According to a third aspect of the present invention, there is provided a hearing aid system comprising : an ear canal module having a transmitter module and provided with a light source for transmitting a light signal which may be a modulated ^' light signal ; and a hearing aid device in the form of an active implant comprising an actuator, a photodetector assembly for converting light energy from a modulated light signal into electrical energy to be supplied to said active implant, and associated electrical circuitry including demodulator means said photodetector assembly comprising a plurality of photodetector units connected in parallel, each photodetector unit comprising at least one photosensor .

Preferably, the transmitter module of the ear canal module is adapted to be retrievably mounted in a patient' s external ear canal and said active implant is adapted to be located within the patient' s middle or inner ear such that said transmitter module may transmit a modulated light signal through said patient' s ear drum and towards said photodetector assembly of said active implant .

Advantageously, the transmitter module comprises a power source, a microphone and electronic circuitry, preferably incorporating a suitable sound processor, adapted to convert detected sound into an electrical signal used to modulate a light signal provided by the said light source contained within said ear canal module . Beneficially, the modulated light signal may be sufficient to provide the sound information to said active implant and also to provide the required power thereto . Accordingly, said active implant may advantageously not require a separate i,nternal power source .

The light source may be, for example, a light emitting diode

(LED) or a laser diode and the modulated light signal may be visible light or more preferably near-infrared energy or low wavelength infrared energy. It has been shown that infra-red energy can penetrate over 15 mm of tissue at frequencies up to 30 KHz .

Preferably, the active implant of the hearing aid system comprises an actuator coupled to a photodetector assembly of the present invention and adapted to be mounted on a portion of a patient' s auditory system, wherein said actuator is caused to transmit energy derived from signals relating to sound directly to the auditory system. Said actuator maybe adapted to be located in the middle ear, such as on the ossicular chain, for example, it may be located on the incus long process or at the incudostapedial j oint or it may be disposed about the stapes . The actual design of said actuator will be determined according to the location selected, an important aim being to reduce acoustic feedback.

Alternatively, said actuator may be adapted to be positioned in the inner ear, for example in the promontory, where coupling may be direct, via fenestration which is a surgical technique to create a window in the inner ear in order to contact the inner ear fluid directly, or said actuator may be secured using an external anchoring support .

Said actuator may also be secured by methods such as cementing, grafting or mechanical means , for example screws or barbs . Said actuator could also be osseointegrated with the ossicular chain .

In embodiments of the present invention a hearing implant may operate using, for example, an actuator operating by vibration, wherein the frequency of vibration and thus the sound information communicated to the auditory system is contained in the modulated light signal transmitted by the transmitter module . Alternatively, a hearing implant may operate electrically by transmitting electrical signals , the frequency and form of said electrical signals corresponding to the sound information contained in the modulated light signal .

Advantageously, in the middle ear, actuation is preferably by mechanical vibration of the ossicular chain, or more specifically individual bones thereof, or the round window between the middle and inner ear . If an actuator is located within the inner ear then actuation may be carried out mechanically, for example, -by direct or indirect vibration of the perilymph fluid in the inner ear, or the transmission

of electrical signals may be made to an electrode or electrode array coupled, for example, to the cochlea .

Advantageously, an actuator may comprise an element of piezoelectric material such as lead zirconate titanate ( PZT) , or lead lanthanum zirconate titanate ( PLZT) . This is desirable because such materials are magnetic resonance imaging (MRI ) compatible, as well as being efficient transducers . Additionally, more than one element of piezoelectric material may be provided in a desired configuration . Mechanical vibrations may also be achieved using, for example, one or more disks of piezoceramic material in conjunction with a flexible diaphragm of, for example, stainless steel, titanium, or aluminium.

Advantageously, the photodetector assembly of the active implant is adapted to generate a minimum required voltage suitable for use to operate said active implant . As the photodetector units are connected in parallel the voltage across each photodetector unit will be the same and will accordingly be equal to the voltage generated by the photodetector assembly. Each photodetector unit may comprise a single photosensor adapted to generate the ^• required voltage .

Alternatively, and in a preferred embodiment, each photodetector unit comprises a plurality of photosensors preferably connected in series such that the voltage generated by each photodetector unit is the sum of the individual voltages generated by each photosensor in the

unit .

In a further preferred embodiment, a photodetector unit comprises monolithic tandem cells , which in this case are multi-junction photosensors stacked one above the other . The top, thinner layer collects some of the light, the remaining photons passing through to the next layers . The output voltages from the individual j unctions are added. A preferred embodiment is a monolithic dual-junction arrangement using, typically, Gallium Arsenide junctions .

Coupling the photodetector units of the hearing aid system together in parallel advantageously assists to permit the photodetector assembly to generate a level of current required to provide sufficient power to operate the active implant . For example, where one photodetector unit is exposed to a reduced level of light energy, relative to the remaining photodetector units , there will be a resulting reduction in local current . However, as the photodetector units are connected in parallel then the total current generated will be the sum of the current generated in the individual units . Accordingly, the effect of the local reduced current is minimised.

Advantageously, where each photodetector unit comprises a plurality of photosensors connected in series , this in combination with the parallel connection arrangement of the photodetector units , permits the photodetector assembly of the present invention to generate an optimised voltage and current to operate the active implant .

The at least one photosensor may be a photo-sensitive diode, a laser diode, photovoltaic cell or other type of photoreceiver such as a monolithic dual junction or triple junction p-n tandem cell, the individual p-n junctions being connected in series the selection of which would readily be made by a person of skill in the art to convert light energy to electrical energy .

Preferably, the photodetector units of the hearing aid device are arranged in a cluster . In a preferred embodiment the photodetector units are arranged in a honeycomb pattern . This compact arrangement advantageously optimises the use of limited space . Compact arrangements of photodetector units also minimise the inactive area of the cluster, that is the area that is not photosensitive . Other preferred cluster arrangements include square, rectangular, and substantially circular arrangements . It is to be understood that any other suitable cluster arrangement may be utilised.

Preferably, the photodetector assembly further comprises a reflector adapted to reflect at least a portion of the modulated light signal . The reflector may comprise any suitable reflective material . The reflector advantageously may be utilised to ensure that the light signal is suitably aligned with the active implant, specifically the photodetector assembly thereof, to ensure that the assembly is exposed to a sufficient and optimum level of light energy to generate the required voltage and current output required for operation of said active implant . Preferably also, the transmitter module of the ear canal module comprises a

detector for detecting the light signal reflected by the reflector . Advantageously, the detector of the transmitter module may communicate with a feedback system which provides information relating to the alignment of the modulated light signal with the active implant . The feedback system may comprise an audible alarm or any other suitable feed back mechanism, such as vibrating means or the like, which may indicate, for example, when misalignment has occurred. The feedback system may be used to assist the surgeon, audiologist or other practitioner or the patient when refitting the ear canal module .

According to a fourth aspect of the present invention, there is provided a hearing aid device comprising : a photodetector assembly for converting light energy from a modulated light signal directed towards said hearing aid device to electrical energy to power said hearing aid device; and a reflector mounted on said photodetector assembly, said reflector adapted to reflect at least a portion of said modulated light signal, wherein the reflected modulated light signal is utilised to permit alignment of said modulated light signal with said photodetector assembly.

According to a fifth aspect of the present invention, there is provided a hearing aid system comprising: an ear canal module comprising a transmitter module and having a light source for transmitting a modulated light signal; an active implant having a photodetector assembly for

converting light energy from said modulated light signal into electrical energy to be supplied to said active implant; and a reflector mounted on said active implant, said reflector adapted to reflect at least a portion of said modulated light signal back towards said transmitter module said reflector comprising any suitable reflective material .

Advantageously, the reflector assists to ensure that the active implant, and specifically the photodetector assembly thereof is exposed to a sufficient and optimum level of light energy to generate the required voltage and current output required for operation of said active implant .

Preferably, a transmitter module comprises a detector for detecting a light signal reflected by a reflector . Advantageously, said detector of said transmitter module may communicate with a feedback system which provides information relating to the alignment of said light signal with the implant . Said feedback system may comprise an audible alarm or the like which may indicate, for example, when misalignment has occurred or when proper alignment has been achieved.

According to a sixth aspect of the present invention, there is provided a medical device comprising a photodetector assembly, or a plurality of said photodetector assemblies each said photodetector assembly comprising a plurality of photodetector units connected in parallel, each photodetector unit comprising at least one photosensor said medical device for powering implants and said photodetector being

implantable up to 15 mm subcutaneously .

According to a seventh aspect of the present invention, there is provided a medical device for use in ophthalmology comprising a photodetector assembly, or a plurality of said photodetector assemblies each said photodetector assembly comprising a plurality of photodetector units connected in parallel , each photodetector unit comprising at least one photosensor said medical device being in the form of a retinal implant provided with transcorneal light telemetry means used in conjunction with said photodetector assembly or photodetector assemblies forming part of a complex retinal array .

According to an eighth aspect of the present invention, there are provided devices employing photodetector assemblies according to the present invention for applications such as micro-surveillance devices .

These and other aspects of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a diagrammatic representation of a hearing aid system according to an embodiment of an aspect of the present invention, shown in position;

Figure 2 is a block diagram identifying components of the hearing aid system of Figure 1 ;

Figure 3a is a diagrammatic representation of a photodetector assembly in accordance with an aspect of the present invention;

Figure 3b is a diagrammatic representation of a photodetector unit of the photodetector assembly shown in ^' Figure 3a;

Figures 4a and 4b are diagrammatic representations of a photodetector unit having photosensor units clustered in an array in accordance with an alternative embodiment of the present invention;

Figure 5 is a diagrammatic representation of a photodetector assembly having photodetector units clustered in a honeycomb pattern and provided with reflector means ;

Figure 6 is a schematic representation of a photodetector assembly according to the present invention showing a mirror and a detector used for the purpose of alignment of a transmitter module with the photodetector assembly; and

Figures 7a, 7b and 7c are greatly enlarged schematic representations of photodetector units comprising silicon photodiode photosensors and gallium arsenide photosensors connected in series in configurations used in preferred embodiments of the present invention .

Reference is first made to Fig . 1 in which there is shown a hearing aid system according to an embodiment • of an aspect of the present invention, shown located within the ear 10 of

a patient . The hearing aid system, generally indicated by reference numeral 1, comprises an active implant. 12 which is surgically located within the middle ear 14 at the incudostapedial j oint which, in use, oscillates the stapes to provide sound information to the auditory system of the patient . It should be noted, however, that active implant 12 may be located elsewhere, for example in the inner ear in the promontory. Hearing aid system 1 further comprises an ear canal module 16 which is retrievably mounted within the external ear canal 18. Ear canal module 16 has a channel 20 defined therethrough in order to prevent complete occlusion of external ear canal 18.

As may be seen by brief reference to Fig. 2, transmitter module 17 of ear canal module 16 comprises a microphone 28 and associated electronic circuitry 30 which includes a sound processor (omitted for clarity) . In use, microphone 28 of transmitter module 17 detects sound and converts this into a modulated light signal 22 which is transmitted through the patient' s eardrum 24 towards active implant 12 by an LED or laser diode 26. Modulated light signal 22 provides the required sound information and energy to operate implant 12 , as discussed below .

Further reference is now made to Fig . 2 in which there is shown a block diagram representation of the various components of hearing aid system 1 of Fig. 1. Transmitter module 17 comprises microphone 28 and associated electronic circuitry 30 which includes a sound processor (omitted for clarity) for transducing sound detected by microphone 28 into

an electrical signal which is , in turn, converted into and transmitted as modulated light signal 22 by LED or laser diode 26 which preferably emits near-infrared energy or low wavelength infrared energy. Power for transmitter module 17 is provided by a battery 32. Modulated light signal 22 passes through eardrum (tympanum) 24 and is detected by a photodetector assembly 34 of active implant 12. Photodetector assembly 34 converts modulated light signal 22 into an electrical signal for driving or oscillating an actuator 36 made of piezoelectric material forming part of active implant 12 which, by being coupled to the stapes bone, acts to transmit sound information into the auditory system.

A photodetector assembly in accordance with an embodiment of the present invention for use in active implant 12 will now be described with reference to Figs . 3a, 3b, 4a and 4b . Referring initially to Fig . 3a, there is shown a diagrammatic representation of a photodetector assembly, generally indicated by reference numeral 134. Photodetector assembly 134 comprises a cluster in the form of an array 40 of photodetector units 40a arranged in a honeycomb pattern. This arrangement advantageously optimises the use of space within and on active implant 12.

The photodetector units 40a of array 40 are electrically connected in parallel, which is diagrammatically represented in Fig . 3b . Coupling the photodetector units 40a of array 40 together in parallel advantageously assists to permit photodetector assembly 134 to generate a level of current

required to provide sufficient power to operate active implant 12 in order to vibrate actuator 36. For example, where one photodetector unit 40a of array 40 is exposed to a reduced level of light energy, relative to the remaining photodetector units 40a of array 40 , the current generated in that photodetector unit 40a will consequently decrease, resulting in a reduced local current in the respective branch 42. However, as the photodetector units 40a of array 40 are connected in parallel then the total current generated will be the sum of the current generated in the individual units . Accordingly, the effect of the local reduced current is minimised.

Each photodetector unit 40a of array 40 of photodetector assembly 134 comprises four photosensors , such as photodiodes

44 , as shown diagrammatically in Fig . 4a . Photodiodes 44 are electrically connected in ■ series , which is diagrammatically represented in Fig . 4b, such that the voltage generated by each photodetector unit 40a of array 40 is the sum of the individual voltages generated by each photodiode 44 in array 40. Advantageously, where each photodetector unit 40a of array 40 comprises a plurality of photodiodes 44 , connected in series , this in combination with the parallel connection arrangement of the photodetector units 40a of array 40 permits photodetector assembly 134 to generate an optimised voltage and current to operate active implant 12.

An alternative photodetector assembly 234 is shown in Fig . 5. Assembly 234 is substantially identical to assembly 134

shown in Fig . 3a with the exception that a reflector, such as a mirror or reflective material 46, is provided which is centrally mounted on a surface of an array 240 of photodetector units 240a . Referring additionally to Fig. 6, mirror 46 is adapted to reflect a portion of a modulated light signal 222 back towards a transmitter module 216, which is substantially similar to transmitter module 17 of Fig . 1. Transmitter module 216 comprises a detector 48 for detecting the reflected modulated light signal 50 which is used to enable proper alignment of transmitter module 216 with photodetector assembly 234. Detector 48 is provided with indicator means (omitted for clarity) for indicating when misalignment has occurred or when alignment has been achieved preferably comprising audible alarm means or vibrating means .

Fig . 7a, is a greatly enlarged schematic representation of a closely packed four-cell cluster photodetector unit 340a comprising four square silicon photodiode photosensors 344 used in certain preferred embodiments of the present invention connected in series and forming a larger square in a single encapsulation of only slightly more than twice the dimension of an individual photosensor and with a high proportion of its area being actively photo-sensitive .

, Fig . 7b, is a greatly enlarged schematic representation of an alternative closely packed two-cell cluster photodetector unit 440a comprising two substantially semi-circular gallium arsenide photosensors 444 used in certain preferred embodiments of the present invention connected in series forming a substantially circular unit .

Fig . 7c, is a greatly enlarged schematic representation of a preferred embodiment of a photodetector unit 540a comprising a dual-junction arrangement using gallium arsenide photosensors 544 in the form of a closely packed substantially circular two-cell cluster . Photosensors 544 comprise monolithic dual j unction p-n tandem cells , one above the other, the individual p-n junctions being connected in series . The top, thinner, layer collects some of the light, the remaining photons passing through to the next layer . The output voltages from the individual junctions are added. Monolithic triple junction, tandem cells , similarly arranged, are also preferred but are not illustrated separately

Compact cluster arrangements , such as those hereinbefore described advantageously optimise the use of space, which in many applications , such as those relating to middle and inner ear surgery, is limited. These compact arrangements of photodetector units also minimise the inactive non- photosensitive area of the cluster .

EXPERIMENTAL FINDINGS

Preliminary work [2 ] has demonstrated that it is possible to transmit sufficient infrared energy across the human eardrum (tympanum) to power an actuator for use in a hearing implant . In these experiments the target voltage output from a photodetector unit was 1 V _p _ _p , in order to achieve a desired output of > 9OdB across a defined frequency range .

Experiments using a human tympanic membrane demonstrated a

fairly constant attenuation of about 50% of the infrared energy across the thinner parts of the organ over the whole useable near-infrared and infrared spectrum. On this basis the choice of wavelength for preferred embodiments of a hearing aid - system employing photodetector assembles according to the present invention should be based on the selection of the most efficient optoelectronic components and configurations to maximise output from the photosensor for a given input to the optical emitter .

A photodetector unit giving an output of 1 V _p _ _p may comprise a cluster of three or four series configured silicon photodiodes, or a cluster of two or three series connected gallium arsenide devices based on the properties of a single silicon device which produces about 0.4 V and on the properties of a single gallium arsenide device which produces about 0.7V. It is important in the hearing implant application referred to here and in other applications where the transmitted light is focussed on a small area, that the photosensors are close-packed. Larger or smaller output voltages may be obtained by connecting, more or fewer series connected photosensors as required.

Experimental work has . confirmed the above to be the case, using as an illustration, a closely packed four-cell cluster photodetector with four, square photosensors forming a larger square in a single encapsulation of only slightly more than twice the dimension of an individual photosensor and with a high proportion of its area being active (that is being comprised of active photosensor) as hereinbefore described

with reference to Fig 7a . This configuration lends itself to parallel arrangements of photodetector units to control the loss of current output due to misalignment of the infrared emitter and the photodetector unit .

Further experiments with a two-element in series gallium arsenide photosensor cluster (see Fig 7b) and a two-element in tandem gallium arsenide photosensor cluster ( see Fig 7c) have been shown to produce a suitable combination of voltage and current output to drive a preferred actuator for a hearing implant and to be capable of providing adequate bandwidth to transmit modulated signals in various modulation modes at frequencies many times higher than those of the audio frequency range required in a hearing implant . As for the previous arrangement using silicon photodiodes , for a given light output from the light source the driving current obtained is dependent upon the type of photodetector used.

It is to be understood that in all embodiments of the present invention each photosensor or plurality of photosensors , is provided with ancillary electronics , comprising at least demodulator means in the form of demodulation circuitry (omitted in the Figures for clarity) , in order to demodulate the received modulated light signal in order that the actuator may be properly driven .

Demodulator means may comprise active electronic devices but preferably comprise passive resistor-based demodulation and possibly inductor-based demodulation neither of which require a power supply.

It may also be envisaged that, in some designs, a reverse voltage bias may be advantageously introduced to photodetectors or it may be desirable to introduce a degree of amplification. In such an embodiment at least a part of the light signal is utilised, particularly those components of the light signal which lie outside the audio frequencies and which are otherwise ignored or removed by filtering.

The photodetector assembly substantially as hereinbefore described with respect to Figs . 3a, 3b, 4a and 4b , 5 and 6, may also be applied in other medical applications . By way of non-limiting example photodetector assemblies according to the present invention may be used for powering other implants with said photodetector assembly being implanted up to 15 mm subcutaneously.

In ophthalmology a retinal implant may be envisaged wherein transcorneal light telemetry is used in conj unction with photodetector assemblies according to the present invention forming part of a complex retinal array .

Furthermore, in applications outside medicine photodetector assemblies according to the present invention and substantially as hereinbefore described with respect to Figs . 3a, 3b, 4a and 4b , 5 and 6, might be employed in such applications as , by way of non-limiting example, micro- surveillance devices .

It is to be understood that the various embodiments described are merely exemplary of the present invention and that

various modifications may be made without departing from the scope of the invention . For example, the photodetector units of a photodetector assembly may be arranged in any suitable cluster arrangement which permits close packing of the units and optimised use of space . Additionally, each photodetector unit may comprise any suitable number of photosensors such as photodiodes , which will provide the required voltage output to operate the implant .