WO2002086574A2

WO2002086574A2 - Optical signal transmitter module with mems shutter as variable optical attenuator

Info

Publication number: WO2002086574A2
Application number: PCT/GB2002/001799
Authority: WO
Inventors: Anthony Alan Needham; Andrew Cannon Carter; Peter John Duthie; Colin Edge
Original assignee: Bookham Technology Plc
Priority date: 2001-04-21
Filing date: 2002-04-18
Publication date: 2002-10-31
Also published as: GB2374680A; AU2002251306A1; GB0109839D0; WO2002086574A3

Abstract

An optical transmission module (10) for connection to an optical fibre (17) includes a laser (11), a waveguide (13) and at least one lens (15) mounted in a holder (23) for focusing light passing from the waveguide to at least one output for the optical fibre, the light beam diverging between the waveguide and adjacent lens and being subsequently focused into an optical fibre output, wherein a MEMS shutter assembly (21,121) is located in a diverging or converging portion of the light beam for attenuation of the optical output from the module.

Description

Optical Signal Transmitter Module Field

This invention relates to optical signal transmitter modules of the type used in telecommunication networks.

Background of the Invention

When telecommunications networks are being initialised or reconfigured, it is beneficial to be able to attenuate the output of optical modules until the modules have been stabilised at the required wavelength and power levels.

Solid state variable optical attenuators (VOAs) are known which are used to balance the optical signals but are not capable of extinction levels greater that 20dB. High levels of attenuation (greater than 50dB) are obtainable using MEMS (microelectromechanical systems) devices and external MEMS optical switches and VOAs are commonly used but introduce losses which have to be compensated for by an increase in the strength of the optical signal, typically by use of a more powerful laser or additional amplification. Typical MEMS VOAs are disclosed in "A MEMS Variable Attenuator" by Woods, Dhuler & Hill, Proceedings of International Conference on Optical MEMS, Kauai, Hawaii, Aug 2000 ppl21-122, Recent Developments in micromachined Silicon, by D.F.Moore & Dr R Sy s, Electronics and Communication Engineering Journal, Dec 1999 pp 261-290, and " High Speed Micro-Electectromechanical Light Modulation Arrays" by Perregaux, Weiss, Kloeck, Vuilliomenet & Thiebaud, 1977 International Conference on Solid State Sensors & Actuators, Digest of Technical Papers p 71-74 vol 1 . A micromechanical optical switch which operates in a similar manner to the VOAs described above is disclosed in " Micro-Opto-Mechanical 2x2 Switch for Single Mode Fibers Based on Plasma-Etched Silicon Mirror and Electrostatic Actuation" by Marxer & de Rooij , Journal of Lightwave Technology Jan 1999, Vol 17.

MEMS devices have an achievable displacement which is very small typically upto 50 microns and a typical light beam used in optical transmissions has a width/diameter of about 1.0 mm. A problem therefor arises as to the incorporation of MEMS VOAs within the optical module without interfering with the normal signal.

The present invention provides a solution to the use of MEMS VOAs with shutters integrally provided within an optical transmitter module.

Statements of Invention

According to the present invention there is provided an optical transmission module for connection to an optical fibre and including a laser, a waveguide and at least one lens for focusing light passing from the waveguide to the optical fibre, the light beam diverging between the waveguide and the adjacent lens and being subsequently focused into an optical fibre output, wherein a MEMS shutter assembly is located in a diverging or a converging portion of the light beam for attenuation of the optical output from the module .

Such a shutter assembly can preferably provide a VOA function through a range from 0 to in excess of 50 dB attenuation.

Preferably the shutter assembly is located between the waveguide and adjacent lens. More preferably, the shutter assembly is located immediately after the waveguide, with the shutter located as close to the waveguide as possible, preferably within about 200 microns of the waveguide. The shutter preferably therefore operates in already available space and introduces zero loss when open. The shutter assembly preferably comprises a single silicon chip with a shutter located on the chip and operated by a MEMS actuator also located on the chip and operable to move the shutter across the light beam as it emerges from the waveguide.

The chip preferably has an aperture therein of between 20- 50 microns in diameter and the shutter and actuator are preferably located on the side of the aperture away from the waveguide.

The shutter may be operated to be normally open or normally closed, and preferably is fail-safe in the open position with no power applied to the shutter assembly. In an alternative embodiment the shutter may be fail safe in the closed position. The shutter may be provided with a suitable reflective coating to prevent the transmission of light.

In one embodiment the shutter assembly is formed as a sub- assembly with the adjacent lens. As an alternative the shutter assembly may be mounted above and at the end of the waveguide. In both cases it may be mounted at a slight angle to the light beam to reduce back reflections.

In yet another embodiment, the shutter sub-assembly comprises at least two lenses formed into a telescopic arrangement and the shutter chip is located centrally between the two lenses where the beam diameter is substantially at a minimum.

Modules may include a plurality of waveguides and outputs, with a single MEMS assembly having a plurality of shutters and mechanisms thereon, one for each respective waveguide and output, the shutter mechanisms preferably being independently operable.

Description of Drawings

The invention will be described by way of example only and with reference to the accompanying drawings in which : Fig. 1 is a schematic plan view of a typical transmitter module in an open condition, Fig. 2 is a view of a modulator chip with MEMS shutter located between the modulator chip and output lens in the module shown in

Fig 1, Fig. 3 is an isometric view of the shutter in

Fig 2, Fig. 4 is side view of a second shutter arrangement,

Fig. 5 is an isometric view of the shutter shown in

Fig. 4, and Fig. 6 shows a third embodiment of the present invention.

Detailed description of the Invention

With reference to Fig.l, there is shown a typical* optical transmitter module 10 having a laser section 11 generating an optical signal which passes through a modulation section 12 which can produce pulses of different intensity light within its waveguide 13. The signal then passes through a VOA section 14 and is then directed to an output section 18 which includes a first lens 15 mounted in a lens holder 23 which directs the optical path to a window 16 in the output which connects to an fibre optics cable 17. The light beam between the lens 15 and window 16 in the output section is about 1mm in width/diameter. The light beam width where it exits the waveguide 13 is small (about 5 microns) but diverges rapidly and a MEMS shutter assembly 21 according to the present invention is placed as close to the waveguide 13 as is possible and preferably within 200 microns.

With reference to Figs. 2 & 3, a shutter assembly 21, is mounted on the face of the first lens holder 23 to form a sub-assembly 24. During the module build the sub-assembly 24 is placed accurately in the light path using active alignment with the waveguide 13 and laser section 11 already in place.

The shutter assembly 21 is shown in detail in Fig. 3. The MEMS shutter assembly 21 has a silicon chip 22 with an aperture 25 therein having a large diameter D (20-50 microns) compared with the waveguide 13 to reduce alignment and interference problems. There is substantially zero loss when the shutter 28 is open. The shutter 28, is located on the side of aperture away from the waveguide 13 and is as close as possible to where the light beam exits the waveguide 13 typically within 100-200 microns. The shutter 28 operated by a MEMS actuator device represented by the hand 30. The actuator mechanism 30 moves the shutter across the aperture when activated. The shutter 28 and actuator 30 are covered by a protective cover 29 which also provides a flat surface for bonding to the lens holder 23 as is shown in Fig. 2.. The cover 29 also has an aperture 25A therein which aligns with the chip aperture 25. The chip 22 is provided with minimal packing/support 26 to facilitate its location in the confined space adjacent the end of the waveguide .

The actuator mechanism 30 can be based on any suitable MEMS technique and possibly of the type described in the prior art disclosed in the preamble. The shutter 28 itself should be about 20% larger in area than the aperture 25 to minimise backscatter. The shutter and actuator mechanism will be typically made from silicon or polysilicon which is transparent at 1500nm, which is close to the telecommunication C-Band, and therefore the shutter is coated in a suitable coating material to avoid transmission of the output light. Suitable coatings include a reflective coating such as gold which may be coated on either side of the shutter and the quality of the reflective coating must be sufficient to avoid thermal problems due to excessive absorption of the light. The shutter 28 will preferably be operated with the respective shutter open when no power is applied. This is beneficial during assembly as no power is required to the chip 22 when actively aligning the shutter assembly 21, 121. Furthermore in normal use no power is required to maintain the shutter in an open condition.

Where reflected light might cause problems the mirrored shutter may be orientated at a slight angle and placed as far into the far-field as possible. In an alternative embodiment a plurality of shutter mechanisms 30 may be provided on chip 22 each capable of being independently operated using a MEMS device similar to that disclosed the applicants co-pending application GB0003186 to control the operation of a like number of parallel waveguides.

In use the module 10 will be hermetically sealed by a cover 31 (see Fig. 4 only) .

With reference to Figs 4 & 5, there is shown a second embodiment of the invention in which the shutter assembly 121 is mounted above and at the end of the waveguide 13. The shutter chip 122 is mounted on a support 126 and the shutter 128 is moved downwards by the MEMS actuator 130 to block the output light beam from the wave guide 13.

In this embodiment, the shutter mechanisms 130 will have a failsafe open retracted condition. This mode of operation will also avoid breakages to the mechanism shown in Figs. 4 & 5.

With reference to Fig. 6 there is shown a third embodiment of the invention for use with a transmission module in which the light beam from the waveguide 13 passes through a telescopic lens arrangement comprising two lenses 42 and 44. The light beam is focused in space centrally between the two lenses and the shutter assembly 222 is located substantially at the central point where the light beam is at a minimum. The lenses 42,44 and the shutter assembly 222 are formed into a sub-assembly 45 which is mounted in the transmitter using active alignment methods. The lens 44 collimates the light beam prior to it passing into the output optics 40 & 17.

The shutter assembly 222 may be similar to the assembly 21 disclosed with reference to Fig. 2.

The light emitting from the waveguide 13 may be first collimated by the use of an additional lens (not shown) placed before the sub assembly 45, and the final lens 44 in the sub-assembly may also be used to focus the light beam directly onto the output fibre.

It will be appreciated that any number of different lens combinations may be used with the proposed shutter assembly, which in accordance with the present invention is located a point where the light beam has a small diameter having been either converging to a focus point or diverging away from said focus point.

Claims

Claims.

1. An optical transmission module for connection to an optical fibre and including a laser, a waveguide and at least one lens for focusing light passing from the waveguide to at least one output for the optical fibre, the light beam diverging between the waveguide and adjacent lens and being subsequently focused into an optical fibre output, wherein a MEMS shutter assembly is located in a diverging or converging portion of the light beam for attenuation of the optical output from the module.

2. A module as claimed in claim 1 wherein the shutter assembly is located between the wave guide and the adjacent lens .

3. A module as claimed in Claim 1 or Claim 2 wherein the shutter assembly is located immediately after the wave guide and as close as possible thereto and within 200 microns .

- 4. A module as claimed in Claim 1, wherein the module comprised at least two lenses formed into a telescopic arrangement and the shutter assembly is located centrally between the two lenses where the beam diameter is substantially at a minimum.

5. A-module as claimed in any one of Claims 1 to 4 wherein the shutter assembly . comprises a chip with a shutter located on the chip and operated by a MEMS actuator also located on the chip and operable to move the shutter across the light beam.

6. A module as claimed in Claim 5 wherein the chip has an aperture therein of between 20-50 microns in^' diameter and the aperture is located within 200 microns of a point where the light beam is of minimum diameter.

7. A module as claimed in Claim 5 or Claim 6, wherein the shutter and actuator are located on the side of the aperture away from the focus point.

8. A module as claimed in any one of Claims 1 to 7 wherein the shutter operates with a fail-safe in the open position with no power applied to the shutter assembly.

9. A module as claimed in any one of Claims 1 to 7 wherein the shutter operates with a fail-safe in the closed position with no power applied to the shutter assembly.

10. A module as claimed in any one of Claims 1 to 9 wherein the shutter assembly is formed as a sub-assembly with the adjacent lens.

11. A module as claimed in any one of Claims 1 to 9 wherein the shutter assembly includes a shutter having a reflective coating thereon.

12. A module as claimed in Claim 11 wherein the shutter is mounted at a slight angle to the light beam.

13. A module as claimed in any one of Claims 1 to 12 and including a plurality of waveguides and outputs, with a single MEMS shutter assembly having a plurality of shutters and mechanisms thereon, one for each respective waveguide and output, the shutter mechanisms being independently operable.