WO2001075503A2

WO2001075503A2 - Variable optical attenuator

Info

Publication number: WO2001075503A2
Application number: PCT/GB2001/001559
Authority: WO
Inventors: Adrian Janssen
Original assignee: Nortel Networks Limited
Priority date: 2000-04-05
Filing date: 2001-04-04
Publication date: 2001-10-11
Also published as: AU4668901A; WO2001075503A3

Abstract

A micro-mechanical attenuator comprising a straight-edged occulting shutter moved across into the small gap between the ends of two co-aligned single mode optical fibres, or the gap between two beam-expanding lens-terminated optical fibres, is found to introduce a measure of polarisation sensitivity that may be unacceptably large for certain applications, whether that shutter is electrically conductive or not. (Different phenomena are involved in the two instances.) By appropriate shaping of the leading edge of the shutter so that it presents a mirror symmetry of orthogonally oriented edge portions to the beam that it occults, this polarisation sensitivity can be substantially eliminated.

Description

VARIABLE OPTICAL ATTENUATOR

FIELD OF THE INVENTION

This invention relates to electrically controlled variable optical attenuators (VOAs), particularly, though not necessarily exclusively, VOAs for use in optical telecommunications systems.

BACKGROUND TO THE INVENTION

For many applications, a VOA will be required to meet a certain measure of spectral flatness over a defined spectral range - i.e. at any given setting of the VOA, the attenuation is required to vary by not more than a certain defined amount over the whole of the defined spectral range. Additionally, the VOA may be required to meet a certain measure of polarisation state insensitivity, polarisation dependent loss (PDL) - i.e. at any given setting of the VOA, and at any given wavelength within the defined spectral range, the state of polarisation (SOP) that affords the maximum attenuation, and the SOP that affords the minimum attenuation, shall afford attenuation values that differ by not more than a certain defined amount.

The specification of United Kingdom Patent Application GB 2,187,858 A describes an electrically controllable VOA constituted by a tandem arrangement of two 4-port 3dB fused fibre couplers in Mach Zehnder configuration. Such a device is depicted schematically in Figure 1 in which a first 4-port 3dB single mode fused fibre coupler 10 with ports 10a, 10b, 10c and 10d, and a coupling region 10e, is optically coupled by means of two lengths 12 and 13 of single mode fibre with a second 4-port 3dB single mode fused fibre coupler 11 having ports 11a, 11b, 11c and 11d, and a coupling region 11e. The lengths 12 and 13 of single mode optical fibre, which extend respectively between ports 10b and 11a, and between ports 10c and 11d, constitute the two interference arms of the Mach Zehnder configuration. Accordingly, if the optical path lengths of these interference arms 12 and 13 are equal, then all light launched into port 10a of the configuration emerges from port 11c, and similarly all light launched into port 10d emerges from port 11b. (The optical path length of any stretch of waveguide in which light propagates is the product of its physical length with the effective refractive index of light propagating in the guide.) If the^~ two arms are of unequal optical path length, then the light that is launched into port 10a is shared between ports 11 b and 11 c in a ratio determined by the difference in phase introduced by the difference in optical path length. For any given wavelength, increasing the optical path length difference will cause the proportion of the light reaching port 11c from port 10a to vary according to a raised cosine characteristic. If the power from port 10a that emerges by way of port 11 c is absorbed or otherwise disposed of, the optical coupling between port 10a and port 11b can be viewed in terms of the configuration acting as an optical attenuator. By the introduction into one of the interference arms of some form of electrically biased optical path length adjuster 14, the coupling between port 10a and port 11b can now be viewed in terms of the configuration acting as an electrically controllable optical attenuator. In the case of the specific configuration described in GB 2 187 858 A, the adjuster 14 is an electrostrictive adjuster which- changes the optical path length by physical stretching of one of the interference arm fibres.

Since the value of attenuation provided by the device is determined by the phase difference introduced by the difference in optical path length of the two interference arms, the value of attenuation provided by the configuration is inevitably wavelength dependent. The specification of United States Patent 5,956,437 describes a way of ameliorating such wavelength dependence by the use of two Mach Zehnders optically in series. A further source of wavelength dependent attenuation arises from the fact that the coupling strength of, and hence the division of optical power by, the (nominally) 3dB couplers 10 and 11 is wavelength dependent. The coupling strength is also state of polarisation (SOP) dependent, and in consequence optical waveguide type Mach Zehnder type VOAs are also liable to exhibit polarisation dependent loss (PDL).

An alternative form of VOA is briefly described and illustrated in a paper by C. R. Giles, entitled 'Lightwave Micromachines'. 24th European Conference on Optical Communication, September 20-24 1998 Madrid, Spain, Volume 1 Regular and Invited Papers pp 249-51. The VOA of this paper uses a micro- electro-mechanical system (MEMS) device to operate, via a capacitor plate, a lever carrying a straight-edged occulting (obstructing) metallic shutter to move it across the end face of an optical fibre.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a form of VOA which avoids the wavelength and SOP dependence properties of optical waveguide Mach Zehnder based VOAs that arise from their use of 3dB waveguide couplers.

A VOA according to the present invention operates to vary the attenuation it provides by the movement of an occulting shutter so as to obstruct proportionately more, or less, of a beam of light transmitted from one waveguide to another along a transmission path that includes a gap in which the shutter is located.

The gap in which the shutter is located may, in principle, be a gap extending between the spaced planar ends of a pair of co-aligned optical waveguides.

By choosing an air gap of not more than 10μm, the loss arising from beam divergence in the VOA can be kept to less than 0.04dB. A greater gap can be tolerated by lensing the adjacent ends of the two waveguides, or by the use of collimating lenses, for instance graded index lenses, to terminate the ends of these waveguides. In each instance, the wavelength dependence of beam divergence is relatively small, typically being small enough to be negligible over wavelength ranges in current use in optically amplified transmission systems. Upon a superficial view, one might suppose that the use of a shutter to occult a beam should have no PDL implications, but in practice this is found not to be the case. It is postulated that in the case of an electrically conductive (metal) shutter, the polarisation dependence arises through differences in the way the E and H fields of the incident light, according to their orientations with respect to the shutter edge, induce current flow in the shutter adjacent that edge, and hence re-radiate forward and backward waves. In particular it is surmised that, when the E field is parallel to the edge, it induces an electron current along that edge which cause the re-radiation of forward and backward waves. The forward wave will be in antiphase with the incident radiation, and hence will provide additional attenuation, at least some of this additional attenuation of forward propagating optical power being occasioned by the power propagating away from the shutter in the backward wave. (The underlying basis for this effect is believed to be akin to the polarisation effect of a wire grid, as for instance described by E. Hecht in chapter 8 of the book Optics' at page 279 (2nd edition) published by Addison-Wesley Publishing Company.) Correspondingly, it is surmised that, when the E field is perpendicular to the edge, the efficiency of the associated H field in inducing the re-radiation of forward and backward waves is impaired by the fact that this H field is parallel to the edge and hence the circulating electron flow induced by it is cut by the edge. In the case of an electrically non-conductive (dielectric) shutter the polarisation dependence is postulated to arise from the effects of differential field penetration at the dielectric interface. A treatment of the basis underlying this effect, known as the Goos-Haenchen Shift, is for instance described by H. C. Casey, Jr. and M. B. Panish in chapter 2 of the book Ηeterostructure Lasers Part A, Fundamental Principles' (publ: Academic Press) at pages 64 to 66. Irrespective of whatever physical phenomena are in fact responsible for the PDL effects of dielectric and electrically conductive occulting shutters, it has been experimentally found that a variety of different shutter materials, including silicon, ceramics and gold-metal alloys all exhibit similar magnitude PDL effects. The magnitude was found to correspond, in the case of a straight-edged occulting shutter, to an effective perpendicular shift of the position of that edge by approximately one wavelength of the incident light when the polarisation state of that light is switched between the SOP aligned with that edge and the SOP perpendicular to that edge. It follows therefore that the magnitude of the PDL can be reduced by increasing the diameter of the beam being occulted, but this requires a corresponding increase in the distance the shutter is required to travel to occult the beam, which in turn has implications concerning the type of mechanism that can be conveniently employed to effect that travel. For instance, the length of travel that a particular micro-electro-mechanical systems (MEMS) device may be able to provide may be limited to about 100μm. If the beam that it is required to occult is assumed to have a Gaussian amplitude distribution, then for a straight-edged shutter to be able to provide attenuation values in the range from 0.1 dB to 60dB, would require the shutter to be capable of moving through approximately 3.5 beam radii (beam radius defined as distance between points of 1/e² of peak intensity). Under these conditions, the MEMS device affording 100μm travel is capable of handling beam radii no greater than about 25μm.

An object of the present invention is to provide a profile of occulting shutter that, for a given diameter of beam, will afford a reduced measure of PDL.

According to the present invention, there is provided a variable optical attenuator having an input waveguide optically coupled with an output waveguide via an optical path that includes a part where light that is coupled between the input and output waveguides extends in a beam across a gap in which is positioned an occluding shutter moveable substantially linearly in a direction across the beam to vary the extent to which the shutter occults the beam, which shutter is provided with a beam-occluding edge divided into a plurality of lengths each aligned substantially at right angles to its immediate neighbour, and wherein the beam-occluding edge has mirror symmetry about an axis that passes through the centre of the beam and that is aligned with the direction of substantially linear movement of the shutter across the beam.

Preferably the profile of the beam-occluding edge of the shutter is re-entrant at the axis of mirror symmetry.

A convenient form of actuating device for moving the shutter is a MEMS device. Such a device may employ, for instance, Joule heating to effect movement. Alternative forms of actuating device include motor-driven drives, for instance using a worm drive, and piezo-electric forms of actuating mechanism.

Other features and advantages of the invention will be readily apparent from the following description of preferred embodiments of the invention, from the drawings and from the claims. -

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 (to which previous reference has been made) depicts a schematic representation of a prior art Mach Zehnder configuration of VOA,

Figures 2a, 2b and 2c are schematic representations of one design of occulting shutter in different positions relative to a light beam so as to present different values of attenuation of that beam,

Figures 3a, 3b and 3c are corresponding schematic representations in respect of an alternative design of occulting shutter,

Figures 4a and 4b are further schematic representations of the two shutters of Figures 2a, 2b, 2c, 3a, 3b and 3c illustrating the relative effects of their misalignment with respect to the light beams that they occult,

Figure 6 is a schematic representation of a MEMS device operated VOA employing the shutter of Figures 2a, 2b and 2c, and

Figure 7 is a schematic representation of some of the constructional details of the MEMS device of Figure 6.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A simple form of occulting shutter in accordance with the teachings of the present invention is schematically depicted at 20 in Figures 2a, 2b and 2c, which depict the shutter occulting respectively a small proportion of a light beam 21 so as to provide a relatively low value of optical attenuation of the beam, an intermediate proportion of the light beam, and a high proportion so as to provide a relatively high value of optical attenuation of the beam. The shutter has a beam-occulting edge constituted by portions of two leading edges 22a and 22b of the shutter that are at right-angles to each other, each being inclined at 45° to the direction of motion of the shutter, which is indicated by double ended arrow 23. The two edges 22a and 22b meet at an acute angle point of the shutter, which the direction of motion of the shutter causes to move along an axis that passes through the centre of the beam 21.

Because of the mirror symmetry and orthogonality of the beam-occulting portions of the shutter leading edges 22a and 22b in relation to light beam 21 , the PDL effect introduced by the beam-occulting portion of the shutter leading edge 22a is substantially compensated by that introduced by the beam- occulting portion of the shutter leading edge 22b.

Figures 3a, 3b and 3c schematically depict corresponding shutter positions in respect to light beam 21 , but in respect of an alternative design of occulting shutter 30 constructed in accordance with the teachings of the present invention. This shutter 30 similarly has a beam-occulting edge constituted by portions of two leading edges 32a and 32b of the shutter that are at right- angles to each other, each being inclined at 45° to the direction of motion of the shutter, which is indicated by double ended arrow 33. The two edges 32a and 32b have a re-entrant form, meeting at a reflex angle point of the shutter, which the direction of motion of the shutter causes to move along an axis that passes through the centre of the beam 21.

The re-entrant form of the shutter 30 of Figures 3a, 3b and 3c may be preferred to the form of shutter 20 of Figures 2a, 2b and 2c because, as can be seen from Figures 4a and 4b, the PDL suppression performance of the reentrant form of shutter is the form less susceptible to a displacement misalignment 'd' of the mirror symmetry axis of the beam-occulting shutter edge with respect to the centre of the beam 21.

The beam-occulting shutter edges of the shutters of Figures 2a, 2b, 2c, 3a, 3b, and 3c both have the form composed of two lengths aligned at right angles to each other, but more complex forms, having more than two such lengths are possible, providing always that mirror-symmetry is preserved. An example of one of these more complex form is schematically depicted at 50 in • i

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Figure 5. The leading edge of this shutter includes portions 56a and 56b which are not at right-angles to each other. This is perfectly acceptable because these portions are so far from the symmetry axis that they do not form any effective part of the beam-occulting portion of the leading edge. This also means that, though these portions 56a and 56b of the leading edge happen to have mirror-symmetry, they do not need to.

Figure 6 schematically depicts a VOA having an input optical fibre waveguide 60 optically coupled with an output optical fibre waveguide 61 via a pair of beam-expanding graded index lenses 62 and 63 secured in alignment in V-groove mounting blocks 64 and 65. The length, strength and spacing of the lenses 62 and 63 are chosen to provide substantially optimised optical coupling between the fibres 60 and 61 with the required measure of beam expansion. Under these conditions the light propagating across the gap between the lenses 62 and 63 extends in a beam 66 that is slightly waisted towards its mid-point. Near this mid-point is located the shutter 30 of Figures 3a, 3b and 3c secured to the moving part of a MEMS device schematically depicted at 67. In the specific instance of a MEMS device providing a linear translation of up to 100μm, the lenses 62 and 63 were designed to provide an approximately threefold beam expansion in order to provide a beam radius at the waist of the beam 66 of about 15μm. An example of such a MEMS device is that marketed by Cronos Integrated Microsystems of North Carolina.

Some details of construction of this MEMS structure 67 are depicted in Figure 7. The structure is formed by selective etching of a single crystal substrate 70, with the shutter 30 forming an integral part of that substrate projecting from a bowed I-beam which is itself also an integral part of the silicon substrate 70. The I-beam spans an associated generally rectangular-shaped well 71 in the substrate, has a pair of bowed webs 72 projecting from adjacent corners of the well, and has a bowed central stem 73 extending between the mid-points of the two webs 72. The shutter 30 projects outwardly from the mid-point of the curved face of the central stem 39 of the I-beam. Rectilinear movement of the shutter 30 is effected by Joule heating of the I-beam. Such heating causes the stem 73 and the webs 72 of the I-beam to lengthen and bow more strongly to take up the respective positions 73' and 72' indicated in broken outline, and thereby translate the shutter to the position 30', also indicated in broken outline. In Figure 7, the extent of the elongation of the stem and webs of the I-beam has, solely for the purpose of convenience of illustration, been greatly exaggerated. The actual movement of the shutter is not, as specifically represented, several shutter lengths long, but is less than a single shutter length long. Optionally, the well 71 may be provided with a 'back-stop' 74 positioned to preclude the possibility, through shock or otherwise, of the stem of the I-beam being ever able to assume the reverse curvature.

The shutter movement provided by the MEMS structure of Figures 6 and 7 is essentially linear, but in its place other types of MEMS device can be employed that provide a rotational movement. In these instances the distance separating the axis of rotation from the centre of the beam of light 66 must be large compared with the diameter of that beam so that the movement of the beam-occulting edge of the shutter 30 across the beam, though actually arcuate, is substantially (effectively) linear.

Claims

CLAIMS:

1. A variable optical attenuator having an input waveguide optically coupled with an output waveguide via an optical path that includes a part where light that is coupled between the input and output waveguides extends in a beam across a gap in which is positioned an occluding shutter moveable substantially linearly in a direction across the beam to vary the extent to which the shutter occludes the beam, which shutter is provided with a beam-occluding edge divided into a plurality of lengths each aligned substantially at right angles to its immediate neighbour, and wherein the beam-occluding edge has mirror symmetry about an axis that passes through the centre of the beam and that is aligned with the direction of substantially linear movement of the shutter across the beam.

2. A variable optical attenuator as claimed in claim 1 , wherein the profile of the beam-occluding edge of the shutter is re-entrant at the axis of mirror symmetry.

3. A variable optical attenuator as claimed in claim 1 , wherein the input and output waveguides terminate in respective beam-expanding lenses, and wherein the gap extends between these lenses.

4. A variable optical attenuator as claimed in claim 1 , wherein the shutter is integrally formed with, or mechanically coupled to, a MEMS device.

5. A variable optical attenuator as claimed in claim 4, wherein the MEMS device employs Joule heating to effect movement of the shutter.

6. A variable optical attenuator for attenuating an optical beam, the attenuator comprising a shutter arranged to be moveable across the optical path of the beam so as to block at least a proportion of the beam from being transmitted, the attenuating edge of the shutter being divided into a plurality of lengths, each substantially perpendicular to its immediate neighbour. i

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7. An attenuator as claimed in claim 6, wherein the attenuating edge has mirror symmetry about an axis that passes through the centre of the beam.

8. A method of attenuating an optical beam comprising inserting a shutter into the path of the beam so as to at least partially block the transmission of said beam along the path, the shutter having an attenuating edge divided into a plurality of lengths, each length substantially perpendicular to its immediate neighbour.

9. A method as claimed in claim 8, wherein the beam stopping edge has mirror symmetry about an axis that passes through the centre of the beam.

10. A shutter for an optical device comprising an attenuating edge divided into at least two lengths, each substantially perpendicular to its immediate neighbour.

11. A shutter as claimed in claim 10, wherein said edge is divided into at least three lengths, each substantially perpendicular to its immediate neighbours.

12. An optical system comprising a transmitter for transmitting an optical signal, a receiver for receiving said signal, and an attenuator as claimed in claim 6.