WO2007045882A2

WO2007045882A2 - Improved optical excursion device

Info

Publication number: WO2007045882A2
Application number: PCT/GB2006/003895
Authority: WO
Inventors: Paul Blair; James R. Bonar; Michael W. Street
Original assignee: Ovisor Technologies Limited
Priority date: 2005-10-19
Filing date: 2006-10-19
Publication date: 2007-04-26
Also published as: GB0521255D0; WO2007045882A3

Abstract

An optical excursion device (15) comprising a channel waveguide (2) having a channel optical axis (7), a first mirror (9) and second mirror (11) is described. The first (9) and second (11) mirrors tend angles relative to the channel optical axis (7) that exhibit substantially equal magnitudes and orientations. This mirror arrangement results in the coupling of an optical signal reflected out of an input section of the channel waveguide (2), and thereafter back into an output section of the channel waveguide (2), not being critically dependent on the absolute angle between the mirrors, (9) and (11), and the channel optical axis (7). A further advantage is that the first (9) and second (11) mirrors can be located upon a common surface. As a result the manufacturing tolerance levels of the optical excursion device (15) are significantly reduced when compared to those known in the art.

Description

Improved Optical Excursion Device

This invention relates to the field of optical routing and in particular to an improved optical excursion device that enables out of plane routing of an optical signal propagating within a waveguide.

Optical technologies are becoming ever more pervasive in the modern world, for example intelligent next-generation optical telecommunication networks are evolving towards mesh, topologies with flexible nodes that are remotely and automatically controlled for the dynamic allocation of bandwidth; research and development of next-generation optical interconnects is accelerating to alleviate the "electronics bottleneck" ; optical technologies are being more widely deployed for a host of sensing applications including chemical and biological sensing; and optoelectronics are continuing to find applications in the automotive, avionics and consumer electronics industries. It is against this background, that an enhancement in any basic optical function such as optical routing should be viewed. In many applications, optical processing is most efficiently performed in a planar lightwave circuit (PLC) that can support a high density of disparate and/or integrated optical waveguide devices. In the present application the term optical processing includes, but is not limited to, alteration or modulation of the optical field amplitude or phase; WDM MUX or DEMUX; optical routing, branching, switching or coupling; optical tap- detection for power monitoring and channel OSNR monitoring; and wavelength conversion. Light is commonly edge-coupled in and/or out of the PLC, alternatively light may be surface-coupled in and/or out of the PLC via prism-coupling, grating-coupling or via re-direction mirrors. Although edge-coupling is generally more efficient than surface-coupling, in some cases the application will dictate that surface-coupling is the only viable option. Alternatively, light that is edge- coupled in the PLC may be required to be subsequently surface coupled or vice versa.

It is known to those skilled in the art that prism- coupling tends to be relatively inefficient, is sensitive to the individual waveguide layer refractive indices, is cumbersome and does not lend itself easily to integration. Grating-coupling tends to be relatively inefficient and is sensitive to the individual waveguide layer refractive indices, the polarisation, temperature and the wavelength. In contrast, surface-coupling, employing out-of-plane re-direction mirrors is, in theory, efficient and temperature, polarisation and wavelength-insensitive. The fabrication of out-of-plane re-direction mirrors can however be challenging depending on the particular technology involved. There are many examples in the prior art that involve surface-coupling from an optical source such as a vertical cavity surface emitting laser (VCSEL) or a surface emitting light emitting diode (LED) into a PLC backplane and from the PLC backplane onto a photodetector, such as a photodiode or a CCD or CMOS sensor array. Note that here an optical source is defined as any device that converts electrical power into optical power while a photodetector is defined as any device that converts optical power into electrical power.

It is also appreciated by those skilled in the art that some optical processing functions are readily performed using PLC technology, while other functions are more efficiently performed in free-space. Accordingly, when the optimum solution for a given application involves both PLC and free-space optical processing, there is a need to efficiently couple light from the source into the PLC for PLC processing, re-direct the light out-of-plane for free-space optical processing, and subsequently re- direct the light back into-plane for further PLC processing before finally coupling the light back out of the PLC for detection. Such an optical path may be described by the term "source-air-PLC-air-PLC-air- detector". In general, the light is neither detected nor re-generated during the out-of-plane "PLC-air-PLC" excursion into free-space i.e. no optical-electrical- optical (OEO) conversion takes place. Note however that light in the "PLC-air-PLC" excursion path can be partially tapped in free-space using e.g. a partially reflective mirror, and subsequently detected for feedback requirements, etc. Light may be added in free space using a combiner, etc. Regardless of the exact details of the optical path, for some high-end optical applications (especially in telecommunications) , the insertion loss (IL) of the "PLC- air-PLC" out-of-plane excursion is absolutely critical and the integrated waveguides are often only single-mode. In addition, for some applications (especially in telecommunications) other optical performance parameters associated with the "PLC-air-PLC" out-of-plane excursion may be of paramount importance such as polarisation dependent loss (PDL) , wavelength dependent loss (WDL) , return loss (RL) and cross-talk (XT). An ideal out-of- plane excursion technology therefore would exhibit the following ideal attributes: it is based on re-direction mirrors, it is manufacturable, it is compatible with single-mode and multi-mode waveguides, and it ideally has optimal IL, PDL, WDL, XT and RL performance.

To assist understanding of the present invention Figure 1 presents side and plan view cross-sections of a prior art optical excursion device 1, based on downward total internal reflection (TIR) from a monolithically- integrated re-direction mirror that is integrated with a channel waveguide 2. In particular, the optical excursion device 1 comprises a substrate 3, a lower clad layer 4, a waveguide core layer 5, and an upper clad layer 6.

The waveguide core layer 5 is patterned so as to form a rib of width ^Mw" . Depending on the formation process, the upper clad layer 6 can either be non-conformal resulting in a substantially flat upper clad surface profile, or it can be conformal with the result that a feature is produced in the upper clad surface profile that substantially conforms in width and height to the waveguide core layer 5. It is well known in the art that for efficient coupling from/to the channel waveguide 2 to/from a second optical waveguide with circular symmetry (such as an optical fibre) , or coupling to the channel waveguide 2 from a source emitting a beam with circular symmetry, that all layer indices and thicknesses, and the waveguide core layer 5 width must all be carefully controlled to generate an optical waveguide mode field that is well matched to that existing in the second waveguide or that emitted by the source. To ensure good mode-matching and therefore efficient coupling, the waveguide mode must be substantially symmetric in the vertical plane through a channel optical axis 7. This demands the presence of the ^' upper clad layer 6 of sufficient thickness while the requirement for a lower clad layer 4 of sufficient thickness depends on the substrate index.

It can be seen that the waveguide core layer 5 terminates at a first substantially vertical facet 8 that is displaced with respect to a first out-of-plane re- direction mirror facet 9 and a second substantially vertical facet 10 that is displaced with respect to a second out-of-plane re-direction mirror facet 11. The re-direction mirror facets 9 and 11 fully intersect the upper clad layer 6 and at least partially intersect the lower clad layer 4 so as to provide for efficient re- direction. The displacement of the waveguide core facets 8 and 10 with respect to the mirror facets 9 and 11 respectively, eliminates any wavefront distortion associated with propagation of the beam up or down through the waveguide core layer 5, while the presence of the waveguide facets 8 and 10 in the optical path has only a marginal effect on the device return loss for waveguide index contrasts up to 1.5%. To suppress back reflections at higher index contrasts, the normal of the waveguide facets 8 and 10 can be angled at 8° with respect to the channel optical axis 7 when viewed in plan. The mirror facets 9 and 11 are then also correspondingly rotated to achieve reflection in a direction that is substantially normal to the channel optical axis 7. For substantially smooth mirrors, TIR at the mirror facets 9 and 11 results in zero theoretical IL, PDL and WDL penalties on reflection.

The waveguide facets 8 and 10 can be seen to lie in the back focal plane of a bulk optic focusing lens 12 while a folding mirror 13 lies in the front focal plane. Focusing lens 12 is disposed with its optical axis 14 located in the plane of symmetry between the mirror facets 9 and 11. A principal ray optical path is represented by the solid black arrows for re-direction mirror angles of 45° and -45°, measured with respect to the channel optical axis 7.

An essential aspect of the described configuration is that the mirror facets 9 and 11 have opposing orientations: mirror facets 9 and 11 subtending angles of equal magnitude (i.e. 45°) but of opposite sign with respect to the channel • optical axis 7. However, depending on the exact processes used to fabricate the re-direction mirrors facets 9 and 11, different mirrors facets on the same chip may be subject to different imperfections such as absolute angle error, uniformity of mirror angle across the mirror facet (intra-mirror angle uniformity) , uniformity of mirror angle between different mirror facets on the same chip (inter-mirror angle uniformity), surface roughness etc. In practice, it is found that the most likely error for an optimised micromachining process resides in the absolute magnitude of the mirror angle rather than in the intra-mirror or inter-mirror angle uniformity.

Unfortunately the optical excursion device 1 of Figure 1 is found to be highly intolerant to the absolute re- direction mirror facet angles. This is illustrated by Figure 2 which presents a schematic version of the device of Figure 1. In particular three re-direction mirror facet angles are shown along with the corresponding principal ray paths, namely 45° (solid line), >45° (dashed line) and <45° (dotted line) . Clearly, with the folding mirror 13 not tilted (i.e. its normal lies along the focusing lens optical axis 14) and the focusing lens optical axis 14 in the plane of symmetry between the mirror facets 9 and 11, the principal rays corresponding to re-direction mirror facet angles >45° (dashed line) and <45° (dotted line) , are not reflected parallel to the channel optical axis 7 after reflection from the second mirror facet 11 i.e. light is not efficiently coupled back into the waveguide core layer 5 when the re- direction mirror angle departs from 45°. Furthermore, it will be obvious to one skilled in the art there is no combination of folding mirror 13 tilt and offset, and focusing lens 12 offset with respect to the plane of symmetry between the mirror facets 9 and 11 that will lead to efficient coupling at the second mirror facet 11 when the re-direction mirror angle differs from 45°. This has a dramatic effect on the manufacturing yield for this process as it results in a very narrow range of acceptable mirror facet angles . It will also be obvious to those skilled in the art that, in addition to the specific optical excursion device configuration of Figures 1 and 2 there exist many more optical configurations comprising lenses and mirrors capable of folding the optical path such that light re- directed by the first mirror facet 9 is focused onto the second mirror facet 11. What all of these configurations have in common however, is that the re-coupling efficiency (or equivalently the out-of-plane excursion IL) will be critically-dependent on the absolute value of the re-direction mirror facet angles.

It is therefore an object of an aspect of the present invention to provide an optical excursion device that exhibits an inherent tolerance to an absolute value of the angles of the re-direction mirror facets.

Summary of Invention According to a first aspect of the present invention there is provided an optical excursion device comprising at least one channel waveguide having a channel optical axis; a first mirror that tends a first angle relative to the channel optic axis and which is suitable for reflecting an optical signal out of an input section of the channel waveguide; and a second mirror that tends a second angle relative to the channel optic axis and which is suitable for reflecting an optical signal into an output section of the channel waveguide, wherein the first and second angles have substantially equal magnitudes and orientations.

Employing first and second mirrors arranged such that their relative angle with the channel optical axis have equal magnitudes and orientations, has the particular advantage that the coupling of the optical signal reflected out of the input section of the channel waveguide back into the output section of the channel waveguide is not critically dependent on the absolute angle between the mirrors and the channel optical axis. Therefore, the tolerance required in producing the optical excursion device of the present invention is significantly reduced when compared to those known in the art.

Most preferably the first and second mirrors are located upon a common surface. Alternatively the first and second mirrors are spatially separated along the channel optical axis .

Preferably the channel waveguide has a folded configuration such that the output section of the channel waveguide is located adjacent to the input section of the channel waveguide .

Preferably the optical excursion device further comprises a focusing lens and a folding mirror wherein the focusing lens and the folding mirror are arranged so as to direct the optical signal reflected out of the input section of the channel waveguide towards .the second mirror .

Preferably an optical axis of the focusing lens lies in a plane of symmetry of the first and second mirrors .

Optionally a normal of the folding mirror is collinear with the optical axis of the focusing lens.

Alternatively, the optical excursion device comprises a beam steering optical element located between the first and second mirrors . Such an arrangement provides an alternative means for routing an optical signal out of the optical excursion device.

Optionally the optical excursion device further comprises a first collimating lens located between the first mirror and the focusing lens, the first collimating lens being arranged so as to focus the optical signal reflected out of the input section of the channel waveguide towards the focusing lens.

Preferably the first collimating lens is arranged so as to collimate the optical signal reflected out of the input section of the channel waveguide.

Optionally the optical excursion device further comprises a second collimating lens located between the second mirror and the focusing lens, the second collimating lens being arranged so as to focus the optical signal transmitted by the focusing lens onto the second mirror.

Preferably the at least one channel waveguide comprises a waveguide core layer located between a first clad layer and a substrate.

Optionally the at least one channel waveguide further comprises a second clad layer located between the waveguide core layer and the substrate.

Preferably the first mirror comprises an integrated facet located within the input section of the channel waveguide . Preferably the second mirror comprises an integrated facet located within the output section of the channel waveguide .

Preferably the waveguide core layer terminates at a facet having a normal substantially parallel to the channel optical axis, the facet being displaced along the channel optical axis with respect to the common surface of the first and second mirrors .

Optionally common surface of the first and second mirrors comprises a coating that is suitable for reflecting the optical signal.

Optionally a surface from which the optical field exits and/or enters the device comprises an anti-reflection coating.

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) side and (b) plan view cross- sections . of a prior art optical excursion device wherein a principal ray path for a signal propagating through the device is represented by the arrows;

Figure 2 presents a schematic representation of the optical excursion device of Figure 1 wherein the principal ray paths are presented for three separate mirror angles, namely <45°, 45° and 45°, with respect to the channel optical axis;

Figure 3 presents a schematic representation of an optical excursion device in accordance with an aspect of the present invention wherein the principal ray paths are presented for three separate mirror angles, namely <45°, 45° and 45°, with respect to the channel optical axis;

Figure 4 presents a schematic representation of an alternative embodiment of the optical excursion device of Figure 3 ;

Figure 5 presents a schematic representation of a further alternative embodiment of the optical excursion device of Figure 3; and

Figure 6 presents six side-view cross-section, and one plan-view cross-section, of monolithically- integrated out-of-plane optical excursion devices .

For consistency and clarity purposes the various features of the described optical excursion devices 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 optical excursion devices are also maintained within the specific description. Detailed Description Figure 3 presents an embodiment of an optical excursion device 15 in accordance with an aspect of the present invention. The optical excursion device 15 is again based on downward total internal reflection (TIR) from a monolithically-integrated first re-direction mirror facet 9 that is integrated within a channel waveguide having a channel optical axis 7. Further components of the optical excursion device 15 include: a single bulk optic focusing lens 12; a folding mirror 13; and a second re- direction mirror facet 11.

It should be noted that the focusing lens 12 is located such that the plane where the optical signal exits and enters the waveguide core layer 5 coincides with the back focal plane of focusing lens 12. For a channel waveguide 2 of the type shown in Figure 1, this means that the distance from the waveguide facets, 8 and 10, to the focusing lens 12 corresponds to the focal length of the lens 12. This is also the case for the channel waveguide 2 of the type shown in Figure 6 (described in further detail below) , although it should be noted that in these configurations the end of the waveguide core layer 5 also coincides with the re-direction mirror facets 9 and 11. In a similar manner the folding mirror 13 is located in the front focal plane of the focusing lens 12.

The focusing lens 12 is further arranged such that the focusing lens optical axis 14 lies in the plane of symmetry between the two re-direction mirrors, 9 and 11, while the folding mirror 13 is not tilted i.e. its normal is collinear with the focusing lens optical axis 14. Critically it should be noted that the first and second mirror facets 9 and 11 are orientated such that their angle relative to the channel optical axis 7 exhibits the same sign and magnitude. In particular, Figure 3 presents three different absolute re-direction mirror facet angles along with their corresponding principal rays: 45° (solid line) , >45° (dashed line) and <45° (dotted line) . Clearly it can be seen that for re- direction mirror facet angles of >45° (dashed line) and <45° (dotted line) , the principal rays remain parallel to the channel optical axis 7 after reflection from the second mirror facet 11 i.e. light is efficiently coupled back into the waveguide core layer 5 even when the re- direction mirror angle departs from 45°. The re-directed rays then exit the optical excursion device 15 on the same side of the device on which the light is initially coupled into the waveguide core 5.

Alternatively, a beam steering optical component 100, such as a reflective surface (shown as a dashed line component in Figures 3, 4 and 5) can be located between the first and second mirror facets, 9 and 11, so as to optionally provide the means for routing the redirected rays out of the optical excursion device 15 for subsequent signal processing.

Furthermore, it will also be apparent to those skilled in the art that for re-direction mirror facet angles that depart from 45°, there are many combinations of folding mirror 13 tilts and offsets and focusing lens 12 offsets that will also result in efficient coupling, independent of the particular angles of the re-direction mirrors facets 9 and 11. Figure 4 presents a schematic of an alternative embodiment of the optical excursion device 16 that is also tolerant to the absolute angle of the re-direction mirrors facets 9 and 11. As can be seen the optical excursion device 16 comprises two independent collimating lenses, 17a and 17b, one associated with each of the re- direction mirrors facets 9 and 11. As with the previously described embodiment the optical excursion device 16 further comprises a focusing lens 12, and a folding mirror 13. In this embodiment the plane of the waveguide facets 8 and 10 lie in the back focal plane of the collimating lenses, 17a and 17b respectively, while the back focal plane of the focusing lens 12 substantially coincides with the front focal plane of collimating lenses 17a and 17b. The folding mirror 13 lies in the front focal plane of focusing lens 12. It should also be noted that the collimating lenses 17a and 17b are located directly below re-direction mirrors facets 9 and 11 such that their optical axes 18a and 18b run through the points of incidence of the principal rays on the mirror facets 9 and 11. The focusing lens optical axis 14 is located in the plane of symmetry between the re-direction mirrors facets 9 and 11.

In this configuration, light is re-directed by the first mirror facet 9, is collimated by lens 17a and focused by lens 12 onto folding mirror 13. In the return path, the light traverses lens 12 once again and is focused by lens 17b onto the second re-direction mirror facet 11. In Figure 4 only one re-direction mirror angle of >45° (dashed line) is shown with the corresponding principal ray path. Folding mirror 13 is shown to be tilted (i.e. its normal 19 is not collinear with the focusing lens optical axis 14) such that light emanating from the first re-direction mirror facet 9 is imaged back onto the second re-direction mirror facet 11. Clearly, for re- direction mirror facet angles of >45° (dashed line) , the principal ray emerges parallel to the channel optical axis 7 after reflection from the second mirror facet 11 i.e. light is efficiently coupled back into the waveguide core layer 5 when the re-direction mirror angles departs from 45°.

It will be apparent that a different folding mirror angle will result in efficient re-coupling for a re-direction mirror angle of <45° (not explicitly shown) . Furthermore, it will be obvious to one skilled in the art that there are many combinations of folding mirror 13 tilts and offsets, collimating lens 17a and 17b offsets and focusing lens 12 offsets that will result in efficient coupling when the re-direction mirror facet angles differs from 45°. One such example is presented in the optical excursion device 20 of Figure 5. In this embodiment the re-direction mirror facet angles are >45°, the folding mirror 13 is not tilted while both collimating lens optical axes, 18a and 18b, and the focusing lens optical axis 14 are offset with respect to the plane of symmetry between the two re-direction mirror facets 9 and 11.

In the all of the embodiments shown in Figures 3 , Figure 4 or Figure 5 the described devices have been based on downward total internal reflection (TIR) from monolithically-integrated re-direction mirror facets _. that are integrated with, and spatially displaced along the channel optical axis 7 of, the channel waveguide 2. However, a significant advantage of employing mirror facets 9 and 11 having the same magnitude and orientation is that this allows for the design of a range of alternative monolithically-integrated optical excursion devices 15 that employ a single mirror, separate areas of which provide the function of mirror facets 9 and 11. By way of example only, Figure 6 (a) to (f) present six schematic representations of such monolithically- integrated out-of-plane optical excursion device 15. By way of further illustration a plan view of the device shown in figure 6 (a) is presented in Figure 6 (g) .

As can be seen the elements located within the optical excursion path are arranged such that light reflected from the first mirror facets 9 is redirected to the second mirror facets 11, which is located on a common surface with the first mirror facets 9. This redirected light can then be arranged to exit any surface of the devices 15, as shown by the various device exits 101, 102, 103 and 104.

Although, for clarity purposes the describe devices comprise only a single channel waveguide it will be readily appreciated by those skilled in the art that devices comprising multiple channel waveguides may similarly be produced within which first and second mirrors are employed that have substantially equal magnitudes and orientations .

In theory the best IL and PDL performance is obtained employing pure total internal reflection (TIR) as exhibited by the configurations shown in Figure 6 (a) and Figure 6 (b) for downward and upward re-direction, respectively. It is known that TIR requires that the angle of the mirror facet be constrained over a limited range. To remove this constraint a high reflection (HR) coating 21 may be either blanket-deposited for downward re-direction as shown in Figure 6(c) or deposited and patterned for upward re-direction as shown in Figure β(d) . The configurations of Figure 6(c) and Figure 6 (d) will however be subject to small IL and PDL penalties relative to the corresponding pure TIR configuration.

Alternatively, a waveguide-air-mirror facet configuration may be adopted as shown in Figure 6(e) for downward re- direction or as shown in Figure 6(f) for upward re- direction. Both of these waveguide-air-mirror facet configurations require a patterned HR coating 21. Furthermore, to achieve a given out-of-plane beam profile and direction with these waveguide-air-mirror facet re- direction configurations, the substantially vertical waveguide exit facet angle and the re-direction mirror facet angle are coupled and must be simultaneously optimised. In addition, to suppress back-reflections the normal of the waveguide exit facet is typically angled by 8° with respect to the channel optical axes when viewed in plan, and therefore when compared to the waveguide-mirror facet configurations, there is an additional IL and an additional PDL penalty associated with transmission through this facet.

For all the foregoing embodiments, an anti-reflection (AR) coating 22 may be optionally applied at any dielectric-air interface to reduce back-reflections and optimise IL. Furthermore, for efficient out-of-plane downward re-direction, the substrate should be substantially transparent at the wavelength of interest and, in addition, the beam diameter at the substrate underside will be a function of the substrate thickness. Also, it will be appreciated by one skilled in the art that the re-direction mirror facets may be profiled to simultaneously effect re-direction and beam focusing (or defocusing) in at least one dimension.

The various configurations of the optical excursion device described above offer several advantages over those described in the prior art. What all of these configurations have in common is the fact that the re- coupling efficiency (or equivalently the out-of-plane excursion IL) is no longer critically-dependent on the absolute value of the re-direction mirror facet angles. Furthermore, the described devices exhibit the following ideal attributes: they are monolithically-integrated and manufacturable in volume thus rendering them inexpensive to produce; only minimal phase-front distortion of the propagating fields is experienced; the devices exhibit low insertion losses, low polarization dependent losses, low wavelength dependent losses, optimal return losses and low channel-to-channel cross-talk for multiple waveguide configurations; and the devices are compatible with channel waveguides designed for efficient coupling via circularly symmetric waveguides such as optical fibres or from optical sources emitting beams with circular symmetry.

Example applications that have been found to benefit from the described out-of-plane excursion devices include, but are not limited to, those that require the re-directed light to interact with a sensor head, a sample, or a switching matrix located proximate to the PLC along the dimension normal to the plane of the PLC (the "third" dimension) .

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 as defined by the appended claims.

Claims

1) An optical excursion device comprising at least one channel waveguide having a channel optical axis; a first mirror that tends a first angle relative to the channel optic axis and which is suitable for reflecting an optical signal out of an input section of the channel waveguide; and a second mirror that tends a second angle relative to the channel optic axis and which is suitable for reflecting an optical signal into an output section of the channel waveguide, wherein the first and second angles have substantially equal magnitudes and orientations.

2) An optical excursion device as claimed in Claim 1 wherein the first and second mirrors are located upon a common surface.

3) An optical excursion device as claimed in Claim 1 wherein the first and second mirrors are spatially separated along the channel optical axis.

4) An optical excursion device as claimed in any of Claims 1 to Claim 3 wherein the channel waveguide has a folded configuration such that the output section of the channel waveguide is located adjacent to the input section of the channel waveguide.

5) An optical excursion device as claimed in any of the preceding claims wherein the optical excursion device further comprises a focusing lens and a folding mirror, the focusing lens and the folding mirror being arranged so as to direct the optical signal reflected out of the input section of the channel waveguide towards the second mirror.

6) An optical excursion device as claimed in Claim 5 wherein an optical axis of the focusing lens lies in a plane of symmetry of the first and second mirrors.

7) An optical excursion device as claimed in Claim 6 wherein a normal of the folding mirror is collinear with the optical axis of the focusing lens.

8) An optical excursion device as claimed in any of Claims 3 to 7 wherein the optical excursion device comprises a beam steering optical element located between the first and second mirrors.

9) An optical excursion device as claimed in any of Claims 5 to 8 wherein the optical excursion device further comprises a first collimating lens located between the first mirror and the focusing lens, the first collimating lens being arranged so as to focus the optical signal reflected out of the input section of the channel waveguide towards the focusing lens .

10) An optical excursion device as claimed in Claim 9 wherein the first collimating lens is arranged so as to collimate the optical signal reflected out of the input section of the channel waveguide.

11) An optical excursion device as claimed in any of Claims 5 to 10 wherein the optical excursion device further comprises a second collimating lens located between the second mirror and the focusing lens, the second collimating lens being arranged so as to focus the optical signal transmitted by the focusing lens onto the second mirror.

12) An optical excursion device as claimed in any of the preceding claims wherein the at least one channel waveguide comprises a waveguide core layer located between a first clad layer and a substrate.

13) An optical excursion device as claimed in Claim 12 wherein the at least one channel waveguide further comprises a second clad layer located between the - waveguide core layer and the substrate.

14) An optical excursion device as claimed in any of the preceding claims wherein the first mirror comprises an integrated facet located within the input section of the channel waveguide.

15) An optical excursion device as claimed in any of the preceding claims wherein the second mirror comprises an integrated facet located within the output section of the channel waveguide.

16) An optical excursion device as claimed in any of Claims 12 to 15 wherein the waveguide core layer terminates at a facet having, a normal substantially parallel to the channel optical axis, the facet being displaced along the channel optical axis with respect to the first and second mirrors.

17) An optical excursion device as claimed in any of the preceding claims wherein the first and second mirrors comprise a coating that is suitable for reflecting the optical signal.

18) An optical excursion device as claimed in any of the preceding claims wherein a surface from which the optical field exits the device comprises an anti- reflection coating.

19) An optical excursion device as claimed in any of the preceding claims wherein a surface from which the optical field enters the device comprises an anti- reflection coating.