WO2005116707A1 - Improved variable optical attenuator - Google Patents

Abstract

A single channel variable optical attenuator (1) is described that comprising an input fibre (4), an output fibre (5), an optical shutter (8) and an optical tap (9-13) The optical tap is located within an optical channel (3) defined by the input (4) and output fibres (5) and provides a means of closed-loop control for the optical shutter (8). The variable optical attenuator (1) allows for input or output power monitoring of the device that does not exhibit the performance penalties associated with devices known in the art. Furthermore, the design of the variable optical attenuator (1) makes it readily extendable to form an arrayed configuration so as to provide a multi channel variable optical attenuator (15) that retains many of the design advantages of the variable optical attenuator (1).

Description

Improved Variable Optical Attenuator

This invention relates to the field of variable optical attenuators . More particularly the invention relates to the field of variable optical attenuators that comprise a tap-detector to actively monitor and control the power input or output of the attenuator.

Fibre optic, electrically controlled, variable optical attenuators (VOAs) are ubiquitous in the field of optical telecommunications networks. They are employed for a variety of functions including: • pre-emphasis at the transmitter end of a dense wavelength division multiplexing (DWDM) system prior to optical amplification; • channel balancing at add/drop nodes where optical signals arrive independently from different points in the network; and • gain control which requires the VOA to simultaneously attenuate multiple wavelengths between erbium-doped fibre amplifier (EDFA) gain stages for optimisation of power levels throughout the network.

VOAs are also employed in transponder applications so as to increase the dynamic range of the receiver, to control laser launch power and to block the output of a tuneable laser during tuning.

The design of VOAs known in the art comprise intrinsically discrete devices that rely upon the use of graded index (GRIN) lenses i.e. one GRIN lens per channel . Multichannel devices are then assembled on a per channel basis and subsequently repackaged together in a secondary housing.

In a so called "folded" configuration the GRIN lens is employed to colli ate an optical signal from an input fibre onto either a mirror or grating that then acts to reflect the optical signal back through the GRIN lens. The optical signal is then refocused by the GRIN lens into an output fibre. Such VOAs allow for control of the attenuation of the optical field by precise adjustment of either the angle of tilt of the mirror or the phase of the grating so as to change the direction of the reflected beam. However, since the attenuation mechanism lies in the controlled coupling of the optical signal into the output fibre it is not possible with these configurations to "tap-off" a portion of the output optical signal before it enters the output fibre. Essentially, the output power cannot be monitored prior to the output fibre as the attenuation of the optical signal takes place within the fibre. An alternative VOA design known in the prior art is the so-called "inline" configuration. Here a first GRIN lens collimates the optical signal before a second GRIN lens focuses the beam into an inline output fibre. Attenuation within such VOAs is typically achieved by electrically controlled phase retardation methods using liquid crystal or electro-optic ceramic materials that again rely on controlled coupling into the output fibre. As such these devices suffer from the same disadvantages as described above in relation to "folded" VOAs.

Alternative technologies for the attenuation of an optical signal currently employed within the art include: • thermo-optic waveguides, • moving suspended silicon waveguides actuated by micro-electro-mechanical systems (MEMS) ; • MEMS actuated shutters for occlusion of a free-space optical beam.

For all of the above applications closed-loop control of the VOA requires power monitoring that is typically achieved by fusion splicing a fibre tap-detector onto the output fibre. However applications exist that require tap-detectors to be inserted on the input side of the VOA e.g. at the receiver end of a DWDM system where feedback from an input photodiode allows for input power level monitoring when the VOA is in a protection or dark mode.

To date the prior-art VOA tap detector (VOATD) solutions are complex and expensive, often incorporating fibre or planar lightwave circuit (P C) taps. For example, for polarisation independent operation within the field of voltage-controlled phase retarders, attenuation relies upon expensive polarisation beam displacement elements to separate the beam into two beams with different polarisation and then to recombine these beams within the output fibre. The attenuation is therefore only realised once light is coupled into the output fibre so that the tap-detector must necessarily follow the output fibre. An example of such a complex system is described in US Patent No. US 6,181,846.

Prior art fibre-taps are relatively large and require unwieldy fibre-management solutions. In addition P C taps also require a degree of fibre-management to fusion splice them into the system, and generally have associated insertion loss (IL) , wavelength dependent loss (WDL) and polarisation dependent loss (PDL) penalties. Furthermore, the hybrid integration of photodiodes onto a PLC, so as to form a PLC tap-detector array, has the additional disadvantage that it is a non-trivial process.

Examples of these PLC tap-detector array solutions include: • the integration of micromirrors for the redirection of light out of the PLC plane; • active alignment and bonding of the photodiode to the PLC chip edge; and • selective etching of the clad to enable evanescent coupling to the photodiode.

All of the above power-monitoring solutions however require complex, and therefore expensive, manufacturing and assembly techniques.

For the reasons already mentioned above, tap-detectors are not easily integrated with thermo-optic polymer or silicon PLC approaches. Additional limitations of these particular technologies are that generally non- interferometric waveguide VOAs are only available as bright state devices and exhibit inherently high power consumption. Furthermore, these systems also suffer from relatively high associated fibre-chip coupling and propagation losses.

More recently, there has been a shift in emphasis within the field of VOA technology, the emphasis increasingly relating to cost and footprint reduction. This change in emphasis has been accompanied by a drive to develop intelligent multichannel VOA modules that integrate VOAs with tap-detectors and electronics for closed loop control.

As described above, existing VOA multichannel arrays are currently produced by secondary packaging of intrinsically discrete single-channel VOAs. Thus, the prior-art VOA solutions are not naturally extendable to the manufacture of arrays that would exhibit associated advantages of economies of scale. A further consequence of the design of these devices is that they do not lend themselves easily to miniaturisation. Thus, when multiple VOA operations are required the known apparatus quickly becomes expensive and cumbersome to produce.

It is therefore an object of an aspect of the present invention to provide a variable optical attenuator that comprises a tap-detector for input or output power monitoring of the device that does not exhibit the performance penalties associated with those devices known in the prior art . A further object of an aspect of the present invention is to provide a variable optical attenuator that comprises a tap-detector for input or output power monitoring of the device that is readily extendable to form an arrayed configuration of devices.

For clarity purposes the following definitions employed by those skilled in the art are also employed throughout the present specification. When a VOA operates in a totally blocking mode it is referred to as being in a dark state. Alternatively, when the VOA operates in a totally transmissive mode it is referred to as being in a bright state.

Summary of Invention

According to a first aspect of the present invention there is provided a variable optical attenuator comprising one or more input fibres, one or more output fibres, one or more optical shutters and an optical tap wherein the optical tap is located within one or more optical channels defined between the one or more input and output fibres and provides a means of closed-loop control for the one or more optical shutters .

Most preferably the one or more input fibres and output fibres are arranged in a folded configuration within the variable optical attenuator.

Preferably the optical tap comprises a first lens, one or more photodetectors and a reflective coating located between the first lens and the one or more photodetectors . Most preferably the reflective coating comprises a transmission coefficient such that a portion of an optical signal within one or more of the input fibres is incident on one or more of the photodetectors .

Optionally the first lens comprises one or more microlenses located on a first surface of a microlens substrate.

Preferably the reflective coating is located on a second surface of the microlens substrate. Alternatively the reflective coating is located on a surface defined by the one or more photodetectors .

Optionally a plane of an endface of the one or more input and output fibres defines a fibre plane that coincides with a first focal plane of the one or more microlenses.

Alternatively, a plane of an endface of the one or more input and output fibres defines a fibre plane that coincides with a plane located at twice the focal length of the one or more microlenses .

Most preferably the microlens substrate has a predetermined thickness so that the reflective coating coincides with a second focal plane of the one or more microlenses. Alternatively when the reflective coating is located on a surface defined by the one or more photodetectors, the microlens substrate has a predetermined thickness so that the reflective coating on the one or more photodetectors coincides with a second focal plane of the one or more microlenses. Alternatively the microlens substrate has a predetermined thickness so that the reflective coating coincides with a plane located at twice the focal length of the one or more microlenses. Alternatively when the reflective coating is located on a surface defined by the one or more photodetectors, the microlens substrate has a predetermined thickness so that the reflective coating on the one or more photodetectors coincides with a plane located at twice the focal length of the one or more microlenses.

Preferably the one or more microlenses are independently located and exhibit a circular cross-section.

Alternatively the one or more microlenses comprise a truncated microlens .

Most preferably, the one or more microlenses exhibit a specification selected to minimise a collimated beam diameter at the one or more photodetectors .

Alternatively the optical tap further comprises a second lens located between the reflective coating and the one or more photodetectors in order to minimise the collimated beam diameter at the one or more photodetectors

In this alternative embodiment the first lens optionally comprises a first graded refractive index lens. Preferably the second lens also comprises a second graded refractive index lens. In this alternative embodiment the second lens alternatively comprises one or more microlenses located on a first surface of a microlens substrate.

Most preferably the one or more photodetectors comprise a means for providing an electrical feedback signal that is dependent on the power of an optical signal incident on the photodetector to the one or more optical shutters .

Preferably the one or more optical shutters are located between the one or more input fibres and the first lens so that the one or more photodetectors measure output powers of one or more of the optical channels. Alternatively the one or more optical shutters are located between the one or more output fibres and the first lens so that one or more photodetectors measure input powers of one or more of the optical channels .

Optionally the one or more optical shutters comprise a profiled leading edge so as to minimise crosstalk between the optical channels.

Preferably the one or more fibres are housed within a fibre ferrule .

Optionally the fibre ferrule locates the one or more input fibres and the one or more output fibres within a one dimensional array. Alternatively the fibre ferrule locates the one or more input fibres and the one or more output fibres within a two dimensional array.

Most preferably a fibre ferrule exit surface is angled so that a surface normal of the exit surface is not parallel to the optical axes of the one or more input and output fibres .

Preferably a surface normal of the reflective coating is substantially parallel to surface normal of the exit surface. This orientation provides an optimum for the optical configuration of the VOA because the optical field occupies a smaller cylindrical volume about the optical axes of the one or more input and output fibres . In particular this allows for minimising the optical volume with respect to the lens diameter.

Preferably an anti-reflection coating is located on one or more of the elements comprising the set of the surface of the fibre ferrule, the first lens and the second lens.

Preferably the one or more optical shutters comprise an opaque shutter the position of which is controlled by a micro electromechanical actuator.

Preferably a micro electromechanical actuator locates the opaque shutters within a one dimensional array. Alternatively a micro electromechanical actuator locates the opaque shutters within a two dimensional array.

Brief Description of Drawings

Aspects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the following drawings in which:

FIGURE 1 presents a: a) plan view; and b) side view; of a single channel Variable Optical Attenuator (VOA) in accordance with an aspect of the present invention;

FIGURE 2 presents a: a) plan view; and b) side view; of a dual fibre ferrule of the single channel VOA of Figure 1;

FIGURE 3 presents a: a) plan view; and b) side view; of a multichannel VOA in accordance with an aspect of the present invention;

FIGURE 4 presents a schematic representation of a MEMS actuator array, as viewed along XX looking towards a fibre v-groove array (FVA) of the multichannel VOA of Figure 3 ;

FIGURE 5 presents a schematic representation of a two dimensional fibre array employed within an alternative embodiment of the multichannel VOA;

FIGURE 6 presents a schematic representation of a one dimensional array of truncated microlenses employed within an alternative embodiment of the multichannel VOA;

FIGURE 7 presents a: a) plan view; and b) side view; of an alternative embodiment of a single channel Variable Optical Attenuator (VOA) .

FIGURE 8 presents a schematic representation of a two dimensional array of truncated microlenses employed within an alternative embodiment of the multichannel VOA;

FIGURE 9 presents a plan view of a further alternative embodiment of the single channel VOA; and

FIGURE 10 presents a plan view of a yet further alternative embodiment of the single channel VOA.

For consistency and clarity purposes the various features of the described Variable Optical Attenuators (VOAs) are referred to by the same reference numerals throughout the specification. Where appropriate, those reference numerals employed to describe the features common to alternative embodiments of the VOAs are also maintained within the specific description.

Detailed Description

A plan view and a side view of a single channel VOA 1 in accordance with an aspect of the present invention is shown in Figure 1(a) and 1(b), respectively. Moving from left to right the single channel VOA 1 can be seen to comprise a dual fibre ferrule 2 that contains two optical fibres that in combination form an optical channel 3.

In order to aid the understanding of the operation of the single channel VOA 1 a plan view and side view of the dual fibre ferrule 2 is shown in Figure 2 (a) and 2 (b) , respectively. The first optical fibre is employed as an input fibre 4 for the single channel VOA 1 while the second acts as an output fibre 5. The ferrule fibre exit face 6 is polished at a small angle so that the surface normal is no longer parallel to the optical axes of the fibres, 4 and 5, so reducing the effects of back reflections, as described below. Added to the polished facet 6 is an anti-reflection (AR) coating that acts to minimise transmission losses.

Following the dual fibre ferrule 2 is a MEMS actuator 7 the function of which is to contain and propel an optical shutter 8. The plane of the MEMS actuator 7 is substantially parallel to the polished ferrule fibre exit face 6. With no drive voltage applied to the MEMS actuator 7 the optical shutter 8 is closed and so blocks the input path of the single channel VOA 1 so rendering it in a dark state in this embodiment. The optical shutter 8 can be controllably withdrawn from the input path of the optical channel 3 so as to vary the optical signal power of the single channel VOA 1.

In this embodiment the optical shutter plane is equidistant between the dual fibre ferrule 2 and the next component of the single channel VOA 1, namely a refractive microlens 9 mounted on a microlens substrate 10. Typically the microlens 9 exhibits a focal length of around 0.5mm. The plane of the microlens substrate 10 is again substantially parallel to the polished ferrule fibre exit face 6. An AR coating is added to the front surface of the microlens 9 so as to minimise transmission losses. Onto the rear surface of the microlens substrate 10 is added a reflective coating 11 which exhibits pre- specified reflection and transmission properties. The reflective coating 11 acts to fold the optical system of the single channel VOA 1 so providing for a compact design. Importantly, the reflective coating 11 also allows for the input fibre 4 and the output fibre 5 to be located on the same side of the single channel VOA 1.

The next component in the single channel VOA 1 is a photodetector 12. The reflective coating 11 on the rear of the microlens substrate 10 has a small transmission coefficient and so allows a portion of the attenuated signal to be incident onto the photodetector 12. The photodetector 12 then acts to generate an electrical signal that is dependent on the power of the incident optical signal. A photodetector electrical connection 13 is then employed to relay the generated electrical signal to the MEMS actuator 7 so as to provide a control signal for the optical shutter 8.

Figure 1 also provides detail of optical signal paths 14 employed within the single channel VOA 1. As can be seen the optical signal from the input fibre 4 exits the fibre via the ferrule fibre exit face 6. Transmission loss and backscatter experienced by the optical signal are minimised due to the AR coating located on, and the angle of, the fibre exit face 6. At the exit face 6 the optical signal is refracted and then allowed to propagate freely to the shutter plane. At the shutter plane the optical signal is wholly passed, partially occluded or totally blocked according to the position of the optical shutter 8, which is dependant upon the electrical control signal transmitted to the MEMS actuator 7. In particular, with no electrical control signal the optical signal is totally blocked. An attenuated optical signal can then propagate freely between shutter plane and the microlens 9. The optical signal is collimated by the microlens 9 so that the total optical system from input fibre 4 to output fibre 5 is symmetric about the rear of the microlens substrate 10. The AR coating on the front of the microlens 9 minimises transmission loss and suppresses reflections. The reflective coating 11 on the rear of the microlens substrate 10 effectively folds the optical system so that the attenuated optical signal has an unimpeded path back through the microlens 9, which then couples the optical signal into the output fibre 5.

As the reflective coating 11 is partially transmissive a small portion of the attenuated signal is transmitted through the coating 11. This portion is incident onto the photodetector 12. In this way optical power monitoring of the attenuated signal is achieved.

Thus, a closed-loop control system is achieved that takes the photocurrent signal from the photodetector 12 as an input to make precise attenuation adjustments so as to maintain the optical power output of the single channel VOA 1 at a constant level. This control can be achieved even in the presence of input optical power or attenuation fluctuations within the system e.g. due to temperature etc. Thus, the single channel VOA 1 achieves, within a compact space, the functionality of attenuation and power monitoring for closed loop control.

An alternative embodiment of the present invention shall now be described. In particular, Figure 3(a) and 3(b) present a plan view and side view of a multichannel VOA 15, in accordance with an aspect of the present invention, respectively. The multichannel VOA 15 comprises a fibre v-groove array (FVA) 16 that is essentially a one-dimensional, regular array of fibres. Adjacent fibres within the FVA 16 act as the input 4 and output fibres 5 so forming an array of optical channels 3. As can be seen the optical channels 3 are adjacently disposed therefore the input 4 and output fibres 5 are interlaced. In Figure 3 eight optical channels 3 are presented, hence there are a total of sixteen fibres.

The FVA exit face 17 is polished at a small angle such that the surface normal is not parallel to the optical axes of the fibres and so back reflections are again reduced. Added to the polished facet is an AR coating so as to minimise transmission loss. The collection of input fibres and the collection of output fibres can be formed into separate fibre ribbons.

Following the FVA 16 is a MEMS actuator array 18 that contains and propels a plurality of optical shutters 8. In practice there are as many optical shutters 8 as there are optical channels 3. The plane of the MEMS actuator array 18 is substantially parallel to FVA fibre exit face 17. Each independently driven optical shutter 8 can be withdrawn from the input path of the associated optical channel 3 so as to vary the associated optical signal power. With no drive voltages applied to the MEMS actuator array 18 the optical shutters 8 are all in a dark state and so block the input paths of the multichannel VOA 15.

Figure 4 presents a schematic representation of the MEMS_. actuator array 18 as viewed along XX looking towards the FVA 16. The MEMS actuator array 18 can be seen to comprise an array of independently addressable electrostatic comb drive actuators 19 each carrying an opaque optical shutter 8. Each electrostatic comb drive actuator 19 further comprises an electrical contact pad 20 the function of which is to receive an electrical signal so as to provide a means for controlling the position of the associated optical shutter 8 along the direction of the Y axis. As shown, successive actuators 19 for adjacent optical channels 3 are positioned in opposition so as to maximise the use of the actuator length and thereby minimise actuation voltage, since it is known to those skilled in the art that the actuation voltage is inversely proportional to comb drive length.

In a similar manner to the previous embodiment the shutter plane is located equidistant between FVA 16 and a lens array 21 that is mounted on a lens array substrate 22. The lens array comprises an array of close-packed refractive microlenses 9 (each of typical focal length of 0.5 mm) . Each microlens 9 provides the coupling mechanism between the input fibre 4 and the associated output fibre 5 of an optical channel 3. In practice there are as many microlenses 9 as there are optical channels 3. The plane of the lens array substrate 22 is substantially parallel to the polished FVA exit face 17 while an AR coating is again added to the front surface of the lens array so as to minimise transmission losses. The reflective coating 11 is again added onto the back surface of the lens array substrate 22.

The next component in the multichannel VOA 15 is a photodetector array 23 that comprises a plurality of photodetectors 12. There are as many photodetectors 12 as there are optical channels 3 within the multichannel VOA 15. The small transmission coefficient of the reflective coating 11 on the rear of the lens array substrate 22 allows a portion of the attenuated optical signals to be incident onto an associated photodetector 12 of the photodetector array 23. Each photodetector 12 then acts to generate an electrical signal that is dependent on the power of the incident optical signals. Photodetector electrical connections 13 are then employed to relay the generated electrical signals to the MEMS actuator array 18 so as to provide feedback control to the associated optical shutters 8.

Figure 3 also presents the optical signal path for the first optical channel 3 of the multichannel VOA 15. The optical path traced follows a similar path to that described in detail in relation to the single channel VOA 1. In this way optical power monitoring of the attenuated optical signal of the first optical channel 3 is achieved. Therefore, it will be readily apparent to those skilled in the art that the multichannel VOA 15 provides a means for increasing the capacity of the single channel VOA so providing means for multiple VOA operations to take place within a single compact device.

In an alternative embodiment of the multichannel VOA (not shown) a two dimensional FVA 24, see Figure 5, and a one dimensional lens array 25, see Figure 6, are employed so as to allow further miniaturisation of the device. In this particular embodiment the input 4 and output fibres 5 are no longer required to be interlaced in a one dimensional array but instead can be formed into separate upper and lower ribbons. Furthermore, only a portion of the aperture defined by each microlens 9 is required to be employed, therefore a packing advantage is gained by truncating the lenses as shown, or otherwise as dictated by the specific optical configuration.

Both the single channel VOA 1 and the multichannel VOA 15 employ optical system that are of a so-called folded 4f arrangement. A folded 4f arrangement means that the fibre plane lies in the front focal plane of the microlens 9 and the reflective coating 11 lies in the back focal plane of the microlens 9. This is achieved by determining the thickness of microlens substrate 10 as a function of the material refractive index. Importantly this optical arrangement leads to one-to-one imaging between the input 4 and output fibres 5 and hence theoretically a zero coupling loss. The solution also provides both a means of minimising the collimated beam size so as to maximise coupling at the photodetector 12 and a means of minimising the required displacement of the optical shutters 8 so as to achieve maximum attenuation. Minimising the collimated beam at the photodetector has the advantage that photodetectors with smaller active areas can be used, hence lower dark currents result, which improves the dynamic range.

In an alternative embodiment of the VOA (not shown) the reflective coating 11 is located on the front surface of the photodetector 12. The folded 4f arrangement is achieved by determining the thickness of the microlens substrate 10, plus an optional air gap, as a function of the material refractive index. The air gap can then be exploited to reduce the overall device length.

A further alternative embodiment of a single channel VOA lb is presented in plan view and side view within Figure 7(a) and 7 (b) , respectively. In this embodiment the single channel VOA lb is in a so called folded 8f arrangement. A folded 8f arrangement means that instead of the fibre plane being coplanar with the front focal plane of the microlens 9 and the reflective coating 11 being coplanar with the back focal plane of the microlens 9, these planes are in fact positioned at a distance of twice the focal length of the microlens 9b in front of, and behind, the plane of the microlens 9b, respectively. This arrangement is achieved through the microlens 9b being designed to comprise a double lens system, as shown in Figure 7 (a) . Employing such an arrangement provides for one to one imaging (unitary magnification) between the input 4 and the output fibres 5 and hence, in theory, a zero coupling loss . A further advantage of the folded 8f arrangement is that since it provides an optical signal path 14b that is focussed at the reflective coating 11 a small active area photodetector 12b can be employed with a lower associated dark current.

It will be apparent to those skilled in the art that the single channel VOA lb can be extended to a multichannel VOA design in a similar manner to the previously described for the single channel VOA 1. For example the single channel VOA lb of Figure 7 can be converted to a multichannel VOA by replacing: 1) the dual fibre ferrule 2 with the two dimensional FVA 24 of Figure 5; 2) the MEMS actuator 7 with the MEMS actuator array 18 of Figure 3; and 3) the microlens 9b with the lens array 25b, presented in Figure 8. As can be seen from Figure 8 the lens array 25b is similar in design to the lens array presented in Figure 6 in that it comprises a plurality of lenses that have been truncated in the X- direction. In addition however, the lenses of Figure 8 have additionally been truncated in the Y direction so as to offer additional miniaturisation to this element .

The relative orientations of both the polished ferrule fibre exit face 6 and the FVA fibre exit face 17 relative to the optical axes are shown in Figures 1, 3 and 7. In Figures 2 a simplified schematic in plan and side elevation of the dual fibre ferrule 2 is also presented. This schematic representation is also equivalent to the single channel of the FVA 16 and the following should be read in that context.

As shown the surface normal of the polished facets 6 and 17, and therefore the refraction axis, lies in the YZ- plane, whereas reflections from the reflective coating 11 lie principally in the XZ-plane. This orientation provides an optimum for the optical configuration because the field occupies a smaller cylindrical volume about the Z-axis than is the case if the normal to the polished facets was in the XZ-plane. i.e. when the optical axis of the refraction and reflection paths are both in the XZ- plane. This allows for minimising the optical volume with respect to the lens diameter.

In the case of the single channel VOA 1 this allows for the lens diameter to be minimised in comparison to alternative configurations and hence leads to a more compact solution. Likewise, in the case of the multichannel VOA 15 where the lens diameter is of the order of the channel spacing this configuration minimises field clipping at the lens aperture. Furthermore, the multichannel VOA 15 provides for a reduction in optical signal crosstalk by ensuring that the field of optical channel N does not coincide with the lenses of channels N-l or N+l due to field clipping at the lens aperture. In this way the fibre spacing within the FVA and hence the channel spacing is regular, with the lens diameter of the order of the channel spacing.

It will be appreciated by those skilled in the art that the actuation direction within the above embodiments could be rotated through 90 degrees, so as to move the optical shutters in a direction substantially parallel to the X-axis . The length of the comb drive actuators 19 would then be vertical with respect to figure 4 and so could be interposed between the channels .

When the two dimensional FVA 24 is employed the comb drive actuators 19 can be either formed on the same side (not opposed as in Figure 4) or rotated through 90 degrees with respect to Figure 4 and so can be interposed between the optical channels 3. One versed in the art would appreciate that the optimum orientation is a function of the actuation principal and the use of the comb drive actuators 19 to explain actuator orientation is merely illustrative.

An alternative embodiment of the single channel VOA 26 shall now be described with reference to Figure 9. In this embodiment the dual fibre ferrule 2, the MEMS actuator 7, the reflective coating 11, the photodetector 12b and the photodetector electrical connection 13 perform the same function as previously described in detail above. However, in this embodiment the MEMS actuator 7 is followed by a first GRIN lens 27 that exhibits a typical pitch of -0.21. The front surface of the first GRIN lens 28 is polished at a small angle such that the surface normal is not parallel to the axis of the cylindrical optic and so the effects of back reflections are reduced.

An AR coating to minimise transmission losses is added to the polished front surface of the first GRIN lens 27 while the reflective coating 11 is added to the rear surface of the first GRIN lens 27. Following the first GRIN lens 27 is located a second GRIN lens 29 of a typical pitch of -0.25. An AR coating to minimise transmission losses is also added to the polished front and rear surfaces of the second GRIN lens 29.

The optical signal path within the single channel VOA 26 is also shown in Figure 9 and follows a similar route to that described in detail above. However, in this embodiment the optical signal is collimated within the first GRIN lens 27 so that the total optical system from input fibre to output fibre is symmetric about the rear of the first GRIN lens 27. As previously described, a small portion of the attenuated optical signal is transmitted through the reflective coating 11 and so can pass through the second GRIN lens 29 whose function is to optimally focus the optical signal onto the photodetector 12b. As shown in Figure 9, the photodetector 12b is offset so as to maximise the photocurrent. A closed loop circuit is established as before between the photodetector 12b and the optical shutter 8. The reason for employing the second GRIN lens 29 is as follows. Employing just the first GRIN lens 27 leads to a large beam diameter at the reflective coating 11. This is due to the fact that in general there is a substantial difference between the working distance and the effective focal length (EFL) for a GRIN lens. For example, the ratio of the collimated beam diameter at the reflective coating 11 to the typical diameter of the active area of the photodetector is 5.4 for the above described embodiment. This calculation assumes that the distance in free space region from the fibre plane to the lens plane is 0.5 mm, whereas the EFL is closer to 2 mm. Therefore, if the photodetector were to be placed directly after the reflective coating 11, overfilling of the active area would occur.

Overfilling the active area is an inefficient use of the tapped percentage of the attenuated power, especially at high attenuation levels. Furthermore, for a back illuminated photodetector an aperture is typically formed on the back surface. As the aperture is usually formed in gold then overfilling of this aperture can lead to spurious reflection back into the photodetector 12b, the input fibre 4, or the output fibres 5. By employing the second GRIN lens 29 this ratio is dramatically reduced to -0.14 through the refocusing of the collimated beam onto the photodetector 12b. In this way coupling to the active area of the photodetector 12b is maximised, which enhances the dynamic range of the device in closed loop operation, as well as suppressing any undesired optical signal paths.

In an alternative embodiment (not shown) the effects of back reflection are reduced by having the rear surface, instead of the front surface 28, of the first GRIN lens 27 polished at a small angle such that the surface normal is not parallel to the axis of the cylindrical optic. In a further alternative embodiment (not shown) the effects of back reflection are reduced by having both the front or rear surfaces of the second grin lens 29 polished in a similar manner to that described above in relation to the first GRIN lens 27.

A yet further alternative embodiment of the single channel VOA 30 shall now be described with reference to Figure 10, which presents a plan view of the single channel VOA 30. Single channel VOA 30 differs from the single channel VOA 1 presented in Figure 1 in that a second microlens 9 and microlens substrate 10 is located between the reflective coating 11 and the photodetector 12b. The function of the second microlens 9 is to focus the collimated optical signal path 14 down onto the photodetector 12b so as to again exploit the previously described advantages of a having to detect a signal of reduced area i.e. a small active area photodetector 12b can be employed with a lower associated dark current.

It will be apparent to those skilled in the art that the single channel VOA 30 can be extended to a multichannel VOA design in a similar manner to the previously described for the single channel VOA 1. For example the single channel VOA 30 of Figure 10 can be converted to a multichannel VOA by replacing: 1) the dual fibre ferrule 2 with the two dimensional FVA 16 of Figure 3; 2 ) the MEMS actuator 7 with the MEMS actuator array 18 of Figure 3; and 3) the microlenses 9 with the lens arrays 21b, of Figure 3.

Alternative arrangements that can be employed within any of the aforementioned embodiments include: • Any of the lenses described within the above embodiments can be replaced by a lens based on diffraction, refraction, ion-implantation or otherwise. Similarly the lenses can be made from any suitable material (e.g. Si0 , Si, etc.). • The actuator (or array of actuators) being formed from any controllable actuation mechanism of an appropriate scale e.g., any MEMS actuator based on an electrostatic, thermal, piezoelectric, magnetic principal; • In the disclosed embodiments all of the VOAs are in the dark state when no drive voltage is applied. However, the devices can be readily adapted so that with no drive voltage applied all of the VOAs are in a bright state or a partially attenuated state. Alternatively, in the case of the array configuration optical channels can be in any combination of dark, light or partially attenuated states depending on the specific configuration of the channels within the array; • In the disclosed embodiments the optical shutters interact with the input signal, therefore the photodetector measures output power. However, the devices can be readily adapted so that the optical shutters interact with the output signal (i.e. the signal after the reflection) and therefore the photodetector measures the input power. Alternatively, in the case of the array configuration the optical shutters could be arranged so as to interact with either the input or output signals depending on the specific configuration of the channels within the array; and • In the disclosed embodiments the optical shutter plane is equidistant between fibre plane and the first optical surface after the shutter. In an alternative embodiment the shutter plane can be located anywhere within the free space gap between fibre plane and the first optical surface after the shutter. • The shutters are described as being opaque. It will be appreciated by those skilled in the art that partially transparent shutters may also be employed. Furthermore, the shutters are not restricted to any particular design or shape. For example, shutters with profiled leading edges may be employed so as to minimise crosstalk between optical channels through the control of the scattered light.

The VOA of the present invention provides significant advantages over those devices known in the prior art . In the first instance a VOA is described that comprises a tap-detector for input or output power monitoring of the device that does not exhibit the performance penalties associated with those devices known in the prior art. Furthermore, a VOA is described that is readily extendable to form an arrayed configuration of devices. This avoids both the re-packaged single channel VOA approach and the requirement to splice together fibre- based power taps and fibre-pigtailed photodetectors after each VOA. The discrete component count is therefore significantly reduced and the assembly is therefore more efficient and produced at a much lower cost. As the photodetector array, the microlens array and the MEMS actuator array are all substantially planar this allows for passive alignment and epoxy bonding to form a vertical stack using relatively simple inexpensive techniques. The photodetector array, the microlens array and the MEMS actuator array assembly can then be aligned in a single active step with the FVA. Compared with the dual fibre ferrule-VOA-GRIN configuration used in the prior art this results in the elimination of one active alignment step per channel and therefore provides further significant cost savings.

The design of the VOAs has the added advantage that they allow for the incorporation of fibre ribbon inputs and outputs. As a result fibre management is significantly simplified and the requirement for multiple single fibre I/O splices is replaced to one input ribbon splice and to one output ribbon splice.

A yet further advantage resides in the fact that the employment of the microlens array and the photodetector array results in a lower production cost per channel than multiple discrete GRIN lenses. Similarly, since the power tap is readily implemented as a robust thin film coating it is also reduces the production costs per channel when compared with multiple discrete fibre-optic tap couplers .

The employment of a partially transmissive coating on the rear side of the microlens substrate has the further advantage that it results in improved cross-talk performance at the photodetector. Any stray light which originates from channel i must first pass through the plane of the partially transmission coating and is necessarily therefore attenuated in transmission before being incident on the channel j photodetector.

The foregoing description of the invention has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise form disclosed. The described embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilise the invention in various embodiments and with various modifications as are suited to the particular use contemplated. Therefore, further modifications or improvements may be incorporated without departing from the scope of the invention herein intended.

Claims

Claims

1) A variable optical attenuator comprising one or more input fibres, one or more output fibres, one or more optical shutters and an optical tap wherein the optical tap is located within one or more optical channels defined between the one or more input and output fibres and provides a means of closed-loop control for the one or more optical shutters .

2) A variable optical attenuator as claimed in claim 1 wherein the one or more input fibres and output fibres are arranged in a folded configuration within the variable optical attenuator .

3) A variable optical attenuator as claimed in claim 1 or claim 2 wherein the optical tap comprises a first lens, one or more photodetectors and a reflective coating located between the first lens and the one or more photodetectors .

4) A variable optical attenuator as claimed in Claim 3 wherein the reflective coating comprises a transmission coefficient such that a .portion of an optical signal within one or more of the input fibres is incident on one or more of the photodetectors .

5) A variable optical attenuator as claimed in any of the preceding claims wherein the first lens comprises one or more microlenses located on a first surface of a microlens substrate. 6) A variable optical attenuator as claimed in Claim 5 wherein the reflective coating is located on a second surface of the microlens substrate.

7) A variable optical attenuator as claimed in Claims 5 wherein the reflective coating is located on a surface defined by the one or more photodetectors .

8) A variable optical attenuator as claimed in any of claims 5 to 7 wherein a plane of an endface of the one or more input and output fibres defines a fibre plane that coincides with a first focal plane of the one or more microlenses.

9) A variable optical attenuator as claimed in Claim 8 wherein the microlens substrate has a predetermined thickness so that the reflective coating coincides with a second focal plane of the one or more microlenses .

10) A variable optical attenuator as claimed in Claims 8 wherein the reflective coating is located on a surface defined by the one or more photodetectors, the microlens substrate having a predetermined thickness so that the reflective coating on the one or more photodetectors coincides with a second focal plane of the one or more microlenses .

11) A variable optical attenuator as claimed in any of claims 5 to 7 wherein a plane of an endface of the one or more input and output fibres defines a fibre plane that coincides with a plane located at twice the focal length of the one or more microlenses . 12) A variable optical attenuator as claimed in Claim 11 wherein the microlens substrate has a predetermined thickness so that the reflective coating coincides with a plane located at twice the focal length of the one or more microlenses .

13) A variable optical attenuator as claimed in Claim 11 wherein the reflective coating is located on a surface defined by the one or more photodetectors, the microlens substrate has a predetermined thickness so that the reflective coating on the one or more photodetectors coincides with a plane located at twice the focal length of the one or more microlenses.

14) A variable optical attenuator as claimed in any of Claim 5 to 13 wherein the one or more microlenses are independently located and exhibit a circular cross- section.

15) A variable optical attenuator as claimed in any of Claim 5 to 13 wherein the one or more microlenses comprise a truncated microlens.

16) A variable optical attenuator as claimed in Claim 3 or Claim 4 wherein the first lens comprises a first graded refractive index lens .

17) A variable optical attenuator as claimed in any of Claim 3 to 16 wherein the optical tap further comprises a second lens located between the reflective coating and the one or more photodetectors . 18) A variable optical attenuator as claimed in Claim 17 wherein the second lens comprises a second graded refractive index lens .

19) A variable optical attenuator as claimed in Claim 17 wherein the second lens comprises one or more microlenses located on a first surface of a microlens substrate.

20) A variable optical attenuator as claimed in any of Claim 3 to 19 wherein the one or more photodetectors comprise a means for providing an electrical feedback signal that is dependent on the power of an optical signal incident on the photodetector to the one or more optical shutters .

21) A variable optical attenuator as claimed in any of Claim 3 to 20 wherein the one or more optical shutters are located between the one or more input fibres and the first lens so that the one or more photodetectors measure output powers of one or more of the optical channels .

22) A variable optical attenuator as claimed in any of Claim 3 to 20 wherein the one or more optical shutters are located between the one or more output fibres and the first lens so that one or more photodetectors measure input powers of one or more of the optical channels.

23) A variable optical attenuator as claimed in any of the preceding claims wherein the one or more optical shutters comprise a profiled leading edge so as to minimise crosstalk between the optical channels . 24) A variable optical attenuator as claimed in any of the preceding claims wherein the one or more fibres are housed within a fibre ferrule.

25) A variable optical attenuator as claimed in Claim 24 wherein the fibre ferrule locates the one or more input fibres and the one or more output fibres within a one dimensional array.

26) A variable optical attenuator as claimed in Claim 24 wherein the fibre ferrule locates the one or more input fibres and the one or more output fibres within a two dimensional array.

27) A variable optical attenuator as claimed in any of Claims 24 to 26 a fibre ferrule exit surface is angled so that a surface normal of the exit surface is not parallel to the optical axes of the one or more input and output fibres .

28) A variable optical attenuator as claimed in Claims 27 wherein a surface normal of the reflective coating is substantially parallel to surface normal of the exit surface.

29) A variable optical attenuator as claimed in any of Claims 3 to 28 wherein an anti-reflection coating is located on one or more surfaces of the first lens.

30) A variable optical attenuator as claimed in any of Claims 17 to 29 wherein an anti-reflection coating is located on one or more surfaces of the second lens . 31) A variable optical attenuator as claimed in any of Claims 24 to 30 wherein an anti-reflection coating is located on one or more surfaces of the fibre ferrule.

32) A variable optical attenuator as claimed in any of the preceding claims wherein the one or more optical shutters comprise an opaque shutter the position of which is controlled by a micro electromechanical actuator.

33) A variable optical attenuator as claimed in Claim 32 wherein a micro electromechanical actuator locates the opaque shutters within a one dimensional array.

34) A variable optical attenuator as claimed in Claim 32 wherein a micro electromechanical actuator locates the opaque shutters within a two dimensional array.