WO2002075380A2

WO2002075380A2 - Optical channel monitor

Info

Publication number: WO2002075380A2
Application number: PCT/GB2002/001278
Authority: WO
Inventors: Stephen William Roberts; Daniel John Kitcher
Original assignee: Bookham Technology Plc
Priority date: 2001-03-19
Filing date: 2002-03-18
Publication date: 2002-09-26
Also published as: GB0106784D0; AU2002241123A1; WO2002075380A3

Abstract

An integrated planar optical waveguide device (11) having a plurality of separate optical input paths (13,14,15), an optical dispersive device, for example an arrayed waveguide grating (12), having a plurality of inputs arranged to receive light respectively from said input paths, and a plurality of light-detecting devices (23) at the output end of the optical dispersive device (12) for receiving a respective portion of the dispersed output of the optical dispersive device, each of said input paths including a serially located variable light attenuator (18,19,20) operable selectively to permit or prevent transmission of light from the input path to the respective input of the device.

Description

OPTICAL CHANNEL MONITOR

The invention relates to optical channel monitors and demultiplexers and methods of monitoring and demultiplexing selected wavelength channels. The invention is applicable to integrated circuit waveguide devices.

Demultiplexing optical channel monitors are known in which light to be demultiplexed enters the device through a single optical fibre. If the light to be monitored may come from one of a plurality of input fibres, then some routing switches may be needed to select light from one of the fibres for entry into a single input of the demultiplexer.

It is an object of the present invention to provide an improved optical channel monitor and method of monitoring channels that enables the channels of multiplexed signals from one of a plurality of sources to be monitored without the need for routing switches, and the losses associated with such switches.

The invention provides an integrated planar optical waveguide device having a plurality of separate optical input paths, an optical dispersive device having a plurality of inputs arranged to receive light respectively from said input paths, and a plurality of light-detecting devices at the output end of the optical dispersive device for receiving a respective portion of the dispersed output of the optical dispersive device, each of said input paths including a respective serially located variable light attenuator operable selectively to permit or prevent transmission of light from the input path to the respective input of the dispersive device.

In one embodiment, a preset light attenuator is provided serially in one of said input paths in addition to a variable light attenuator. In one embodiment, a plurality of optical output paths are provdied, each output path arranged to receive a respective output channel and direct it to a respective one of the array of light-detecting devices.

In one embodiment, the inputs have a mutual lateral separation so that the dispersive device provides demultiplexed channels which are mutually offset and have a lateral position depending on which input is used to form the demultiplexed output, the change in lateral position of the demultiplexed channels resulting from two different adjacent inputs being equal to the lateral separation of adjacent output paths, and the number of output detectors equals one less than the sum of the number of inputs plus the number of channels to be demultiplexed from one input.

The invention also provides a method of monitoring demultiplexed wavelength channels obtained from one of a plurality of optical input signals, which method comprises inputting separate light signals simultaneously into a plurality of separate light input paths, each including a variable serial light attenuator and each optically coupled to a respective input of a wavelength-dispersive device, operating said attenuators to transmit light from only one selected input path to its respective input of the wavelength dispersive device, and measuring the optical power of dispersed portions of the output of the wavelength dispersive device.

In one embodiment, the attenuators in said input paths are sequentially varied to permit light from each of the plurality of input paths to be separately and sequentially monitored. Preferably, the variation of the attenuators is controlled to maintain a constant level of aggregate power consumption during sequential monitoring of light in each of the input paths. In one embodiment, light is input from a plurality of sources of different light intensities and attenuation is effected in at least one input path associated with a high light input intensity by a preset attenuator in addition to a variable light attenuator.

The invention also provides provides an integrated planar optical waveguide device having a plurality of separate optical input paths, an optical dispersive device having a plurality of inputs arranged to receive light respectively from said input paths and for dispersing light from a selected one of said optical inputs into a plurality of spatially separated output channels, and a plurality of optical output paths each arranged to receive a respective one of said output channels and each optically coupled to an output detector for detecting light in said output path, each of said input paths including a respective serially located variable light attenuator operable selectively to permit or prevent transmission of light from the input path to the respective input of the dispersive device.

The invention further provides a method of monitoring demultiplexed wavelength channels obtained from one of a plurality of optical input signals, which method comprises inputting separate light signals simultaneously into a plurality of separate light input paths, each including a variable serial light attenuator and each optically coupled to a respective demultiplexer input, operating said attenuators to transmit light from only one selected input path to its respective demultiplexer input , demultiplexing light from the demultiplexer input to form a plurality of spatially separated wavelength selected output channels and detecting light values in each of said output channels.

Some embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a plan view of one embodiment of the invention, Figure 2 is a cross section through an input waveguide of the device of Figure 1 , and

Figure 3 shows schematically an alternative construction for an input waveguide attenuator for use in the device of Figure 1.

The example of Figure 1 shows an integrated planar optical waveguide device which in this example is an integrated semiconductor optical chip device 11. On chip is a wavelength dispersive device in the form of a multiplexer/demultiplexer 12 having three light input channels 13, 14 and 15 together with a plurality of light output channels 17. The multiplexer/demultiplexer 12 may comprise a known structure of multiplexer or demultiplexer or light branching device as already known in the field of semiconductor waveguide devices. The waveguides formed on the chip 11 may comprise ridge waveguides of known type and may for example have the structure shown in US Patent 5757986.

Each of the input channels 13, 14 and 15 comprises a ridge waveguide including a serially located variable attenuator 18, 19 and 20. Channel 15 also includes a fixed serial attenuator 29. The waveguides 13, 14 and 15 are connected at the edge of the chip to a connector 21 in which each waveguide is connected to a respective optical fibre in a multichannel ribbon fibre 22. The output waveguides 17 comprise a plurality of laterally spaced output ridge waveguides connected at the edge of the chip to respective detectors in a multichannel detector array 23. In this way the detector array 23 can give an individual indication of the light received in each of the output channels from the channel monitor. All three input waveguides 13, 14 and 15 are optically connected to the array 12 through a free propagation region 25 and the output waveguides 17 are similarly connected to the output of the array 12 by a similar free propagation region 26. The structure of the attenuators on the chip is generally as shown in Figure 2 and this will be described in further detail below. However each ridge waveguide comprises a upstanding ridge formed by trenches etched on either side of the ridge and the waveguide terminates at positions where the trenches terminate. The free propagation regions 25 and 26 comprise regions of silicon on insulator in which no trenches have been formed thereby providing a uniform slab thickness for transmission of optical signals from the ends of the input waveguides into the dispersive array 12 and similarly transmit light from the output end of the array 12 through the slab into the ends of the waveguides forming the output channels 17. The three input wave guides 13, 14 and 15 provide three laterally separated inputs to the demultiplexer 12 and any one of the inputs may be selected to input light for demultiplexing.

The attenuators 18, 19 and 20 are provided in respective input waveguides so as to control whether light is transmitted or not through that input waveguide into the dispersive array 12. In use, light is directed from the ribbon fibre 22 simultaneously into each of the respective input waveguides 13, 14 and 15. Each of the attenuators 18 and 19 is variable progressively to vary transmission between a zero value and a maximum transmission value. In order to select which input is directed into the dispersive array 12, one of the attenuators is adjusted to permit maximum transmission while the other two are set to prevent any light transmission. The light input through the three input channels can be sequentially monitored by varying simultaneously the degree of attenuation on any two of the attenuators 18 to 20. As one input is progressively switched off, another input can be progressively switched on so as to change from a first selected input to a second selected input. During this selection operation the aggregate power consumption by the two attenuators may be maintained constant. Attenuators used in semiconductor optical monitors generate heat depending on the degree of attenuation to be achieved. By arranging the simultaneous operation of two attenuators in order to change input channels, the aggregate degree of attenuation and thereby the aggregate amount of heat generated, can be maintained at a substantially constant level while changing the input from one channel to another. In this way variation heat dissipation is avoided and the requirement for fast active temperature control can be reduced.

By using transmission control on a plurality of side by side inputs to the demultiplexer, it is possible to avoid the need for a 1 x n selector switch. Such switches normally require movable light directing elements which are difficult to manufacture particularly for use with semiconductor optical chips. Furthermore, the attenuators of the present embodiment have lower optical losses as they can achieve better performance at both zero transmission and at full transmission than movable selector switches.

The structure of the attenuators 18, 19 and 20 may be of the type shown in Figure 2 or alternatively the type shown in Figure 3.

Figure 2 illustrates a PIN diode construction of the type shown in our UK Patent Application No 0019771.5.

The structure of Figure 2 comprises a silicon crystal layer 50 separated from a silicon substrate 51 by an insulating layer 52 formed in this example of silicon dioxide. Two trenches 53 and 54 are formed in the silicon 50 so as to leave an upstanding ridge waveguide 55. Regions 56, 57 on either side of the trenches have electrical conductors 58 and 59 extending along their length and over the trenches in order to allow an electrical potential to be supplied from a voltage source 60. The voltage is applied across the conducting strips 57 and 58 which in turn biases the PIN diode to vary the light transmission along the rib waveguide 55 between the input marked I and the output marked O. The silicon in the rib waveguide 55 is only lightly doped whereas the regions 61 and 62 below the trenches 53 and 54 are heavily doped. One of these regions is heavily doped with p type dopant and the other heavily doped with n type dopant. The doped regions are shown by the dotted lines in Figure 2. The operation of such a device to cause attenuation is already known and has been described in our UK Patent Application 0019771.5. Again the amount of heat generated by operating the attenuator depends on the amount of attenuation required.

The alternative shown in Figure 3 is a known construction of a Mach Zehnder interferometer in which a ridge waveguide 35 in one of the input channels is arranged to couple with a further ridge waveguide 36 having two closely coupled regions 37 and 38 such that an output 39 represents an output from the attenuator. The waveguide which is indicated by 36 includes a heated region 40 in which heat is applied to the waveguide region by passing current through an input 41 and out through an output 42. The operation of such Mach Zehnder interferometers is well known and will not be described further. However heat is generated in dependence on the amount of attenuation required by the interferometer.

It will be seen that when using a plurality of either type of attenuator of Figure 2 or Figure 3 it is possible to avoid the use of a 1 x n selector switch. The degree of attenuation can be controlled to vary the transmission in the on state and thereby compensate for different input signal strengths. The attenuators described above can be readily made in silicon devices with effective variation over a full range of 0% to substantially 100% transmission. Furthermore the progressive adjustment of any two attenuators simultaneously on selected input channels maintains a constant heat dissipation requirement while the selection of input channel changes.

Although the variable attenuators 18, 19 and 20 may be used to effect some compensation for different input signal strengths, the device may be used in connection with input signals which have substantially different input strengths. This may for example result from connection of the device to a plurality of sources at very different distances from the device such that the light losses before reaching the device are substantially different. In such cases it may be desirable to include in one or more of the input channels 13, 14 or 15 a fixed attenuator so as to provide more effective compensation for the different signal input strengths. Such is indicated in Figure 1 with the fixed attenuator 29 illustrated in channel 15. The structure of the attenuator may be similar to that previously described with reference to Figures 2 and 3. The degree of attenuation does not however need to be variable as the selective switching effect will be achieved by the variable attenuator 20 connected serially with the fixed attenuator 29. The fixed attenuator 29 may be positioned serially upstream or downstream of the variable attenuator. In this way, the structure of the variable attenuators may be similar for each of the input channels with the different attenuation effect being achieved by the provision or absence of a fixed preset attenuator such as attenuator 29. The fixed attenuator may be formed by a P doped strip or by cuts in the waveguide or by PN doping with selected voltage across the PN junction.

The above examples provide an improved apparatus and method for monitoring the light signals located in any of the inputs in the ribbon fibre 22 without the requirement for more expensive selector switch construction or variable heat dissipation which arises with on/off switch operation.

The applicant draws attention to the fact that the present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof, without limitation to the scope of any definitions set out above. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

CLAIMS:

1. An integrated planar optical waveguide device having a plurality of separate optical input paths, an optical dispersive device having a plurality of inputs arranged to receive light respectively from said input paths, and a plurality of light-detecting devices at the output end of the optical dispersive device for receiving a respective portion of the dispersed output of the optical dispersive device, each of said input paths including a respective serially located variable light attenuator operable selectively to permit or prevent transmission of light from the input path to the respective input of the dispersive device.

2. An integrated planar optical waveguide device according to claim 1 in which a preset light attenuator is provided serially in one of said input paths in addition to a variable light attenuator.

3. An integrated planar optical waveguide device according to claim 1 or claim 2 further including a plurality of optical output paths, each output path arranged to receive a respective output channel and direct it to a respective one of the array of light-detecting devices.

4. An optical channel monitor device according to any of claims 1 to 3 in which said dispersive device comprises an optical waveguide array.

5. An optical channel monitor device according to any preceding claim in which each of said input paths comprises an optical waveguide device.

6. An optical channel monitor device according to any one of the preceding claims comprising an integrated semiconductor device.

7. An optical channel monitor device according to claim 6 comprising an integrated silicon on insulator waveguide device.

8. An optical channel monitor device according to claim 6 or claim 7 as dependent on claim 4 in which a planar optical coupler region is provided at each end of the waveguide array to couple the waveguide array to the input and output optical paths.

9. An optical channel monitor device according to any preceding claim in which each said attenuator comprises a lateral PIN diode.

10. An optical channel monitor device according to any preceding claim in which each said attenuator comprises a Mach Zehnder interferometer.

11. An optical channel monitor device according to claim 3 in which the inputs have a mutual lateral separation so that the dispersive device provides demultiplexed channels which are mutually offset and have a lateral position depending on which input is used to form the demultiplexed output, the change in lateral position of the demultiplexed channels resulting from two different adjacent inputs being equal to the lateral separation of adjacent output paths.

12. An optical channel monitor device according to claim 11 in which the number of output detectors equals one less than the sum of the number of inputs plus the number of channels to be demultiplexed from one input.

13. An optical channel monitor device according to any one of the preceding claims in which each input path is connected to a respective input optical fibre.

14. A method of monitoring demultiplexed wavelength channels obtained from one of a plurality of optical input signals, which method comprises inputting separate light signals simultaneously into a plurality of separate light input paths, each including a variable serial light attenuator and each optically coupled to a respective input of a wavelength-dispersive device, operating said attenuators to transmit light from only one selected input path to its respective input of the wavelength dispersive device, and measuring the optical power of dispersed portions of the output of the wavelength dispersive device.

15. A method according to claim 14 including sequentially varying the attenuators in said input paths to permit light from each of the plurality of input paths to be separately and sequentially monitored.

16. A method according to claim 15 in which the variation of the attenuators is controlled to maintain a constant level of aggregate attenuation during sequential monitoring of light in each of the input paths.

17. A method according to any one of claims 14 to 16 in which light is input from a plurality of sources of different light intensities and attenuation is effected in at least one input path associated with a high light input intensity by a preset attenuator in addition to a variable light attenuator.

18. An optical channel monitor device substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

19. A method of monitoring selected wavelength channels in a plurality of optical input signals which method is substantially as hereinbefore described with reference to the accompanying drawings.