GB2389897A - Optical power monitoring devices - Google Patents
Optical power monitoring devices Download PDFInfo
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- GB2389897A GB2389897A GB0214317A GB0214317A GB2389897A GB 2389897 A GB2389897 A GB 2389897A GB 0214317 A GB0214317 A GB 0214317A GB 0214317 A GB0214317 A GB 0214317A GB 2389897 A GB2389897 A GB 2389897A
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- 230000003287 optical effect Effects 0.000 title claims abstract description 104
- 238000012806 monitoring device Methods 0.000 title claims abstract description 24
- 238000012544 monitoring process Methods 0.000 claims abstract description 27
- 230000010287 polarization Effects 0.000 claims description 25
- 238000005070 sampling Methods 0.000 claims description 23
- 238000001514 detection method Methods 0.000 claims description 5
- 230000000694 effects Effects 0.000 claims description 3
- 239000012212 insulator Substances 0.000 claims description 3
- 239000000758 substrate Substances 0.000 claims description 2
- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 description 8
- 230000001419 dependent effect Effects 0.000 description 7
- 238000004519 manufacturing process Methods 0.000 description 7
- 238000010079 rubber tapping Methods 0.000 description 6
- 230000001939 inductive effect Effects 0.000 description 4
- 239000000377 silicon dioxide Substances 0.000 description 4
- 238000005253 cladding Methods 0.000 description 3
- 238000010586 diagram Methods 0.000 description 3
- 238000000034 method Methods 0.000 description 3
- 238000012545 processing Methods 0.000 description 3
- 235000012239 silicon dioxide Nutrition 0.000 description 3
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 2
- 230000002238 attenuated effect Effects 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 230000035945 sensitivity Effects 0.000 description 2
- 229910052710 silicon Inorganic materials 0.000 description 2
- 239000010703 silicon Substances 0.000 description 2
- 230000007423 decrease Effects 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 238000009792 diffusion process Methods 0.000 description 1
- 239000002019 doping agent Substances 0.000 description 1
- 238000005530 etching Methods 0.000 description 1
- 230000002452 interceptive effect Effects 0.000 description 1
- 238000005468 ion implantation Methods 0.000 description 1
- 238000001465 metallisation Methods 0.000 description 1
- 238000010561 standard procedure Methods 0.000 description 1
- 238000012360 testing method Methods 0.000 description 1
- 238000009966 trimming Methods 0.000 description 1
Classifications
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
- H04B10/075—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal
- H04B10/079—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal using measurements of the data signal
- H04B10/0795—Performance monitoring; Measurement of transmission parameters
- H04B10/07955—Monitoring or measuring power
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
- G01J1/00—Photometry, e.g. photographic exposure meter
- G01J1/10—Photometry, e.g. photographic exposure meter by comparison with reference light or electric value provisionally void
- G01J1/20—Photometry, e.g. photographic exposure meter by comparison with reference light or electric value provisionally void intensity of the measured or reference value being varied to equalise their effects at the detectors, e.g. by varying incidence angle
- G01J1/22—Photometry, e.g. photographic exposure meter by comparison with reference light or electric value provisionally void intensity of the measured or reference value being varied to equalise their effects at the detectors, e.g. by varying incidence angle using a variable element in the light-path, e.g. filter, polarising means
- G01J1/24—Photometry, e.g. photographic exposure meter by comparison with reference light or electric value provisionally void intensity of the measured or reference value being varied to equalise their effects at the detectors, e.g. by varying incidence angle using a variable element in the light-path, e.g. filter, polarising means using electric radiation detectors
- G01J1/26—Photometry, e.g. photographic exposure meter by comparison with reference light or electric value provisionally void intensity of the measured or reference value being varied to equalise their effects at the detectors, e.g. by varying incidence angle using a variable element in the light-path, e.g. filter, polarising means using electric radiation detectors adapted for automatic variation of the measured or reference value
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
- H04B10/075—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal
- H04B10/077—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using an in-service signal using a supervisory or additional signal
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B2006/12083—Constructional arrangements
- G02B2006/12097—Ridge, rib or the like
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- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- General Physics & Mathematics (AREA)
- Spectroscopy & Molecular Physics (AREA)
- Optical Integrated Circuits (AREA)
Abstract
An optical power monitoring device is provided for monitoring the power of an optical signal supplied along a waveguide 2 substantially independently of the polarisation of the optical signal. At least one tap-off coupler 5, 10 is provided for purpose of monitoring the power of the optical signal with a view to providing respective signals of different optical modes within two intermediate waveguides 6, 14 incorporating respective attenuators 11 and 12. The optical mode signal in at least one of the waveguides 6, 14 is controlled by the attenuators in such a way that the sum of the mode signals detected by a single photodiode 7 produces an electrical output signal which is substantially independent of the polarisation of the optical signal monitored. Such an arrangement enables the desired low polarisation-dependence monitoring signal to be obtained using phototonics of relaxed specification, and in particular using only one monitoring photodiode.
Description
" Optical Power Monitoring Devices" The present invention relates to
optical power monitoring devices for monitoring the power of an optical signal substantially independently of the polarisation of the optical signal.
The accurate monitoring of optical power provides a difficult challenge for integrated optical systems because of the need to provide waveguide taps having only a small tapping fraction and low polarisation dependence, in order to minimise the polarisation dependence of the monitoring signal. Typically optical power monitoring devices comprise a tap-off coupler and a photodiode for detecting the tapped-off fraction of the optical signal. However both the tap-off coupler and the optics of the photodiode exhibit birefringent properties, in that they provide a refractive index for light of one polarisation mode, for example transverse electric (TE) polarised light, which differs from the refractive index for light of another polarisation mode, for example transverse magnetic (TM) polarised light. Such birefringence, which may be affected by ambient conditions such as temperature and mechanical stress, results in the detected optical power varying with changes in the polarisation of the optical signal to be monitored, even if the optical power of the signal itself remains constant.
"Integrated Optical Polarisation Analyser on Planar Lightwave Circuit", T. Saida et al., Electronics Letters, Vol.35, No.22, p.l948, 1999 discloses an integrated optical polarisation analyser consisting of a polarisation splitter, a polarisation converter and a 3X3 MM1 coupler. The polarisation splitter has a balanced Mach-Zehnder interferometer configuration with a stress-inducing layer on one of its arms for applying a phase difference between the polarisation components, and a heater on its other arm for adjusting the phase bias. In such an arrangement an optical signal injected into the device is divided into TE and TM components by the polarisation splitter, and half of each component is extracted by way of 3 dB couplers to monitor the amplitude of the TE and TM components. The other half of the TM component is converted to TE light by the polarisation converter, and supplied to the MMI coupler with the other half of the TE component. The interference signals obtained at the three output ports of the NIMI
/ coupler differ in phase and can be used to determine the input polarisation state.
However such a device is complex and does not provide a direct indication of the optical power.
"Birefringence Control of Silica Waveguides on Si and its Application to a Polarisation-Bcam Splitter/Switch", M. Okuno et al.' IEEE Journal of Lightwave Technology, Vol.12, No.4, April 1994 discloses a polarisation beam splitter in the form of a Mach-Zehnder interferometer consisting of two 3 dB directional couplers linked by two waveguide arms of the same length with a stress-inducing layer and a phase shifter in the form of a heater being applied to one of the arms, and a phase shifter in the form of a stress-inducing layer being applied to the other arm. In operation the waveguide birefringence and phase states of the device are accurately controlled by lasers trimming the stress-inducing layers, with the output polarisation states being switched by controlling the phase shifter.
GB 2211956A discloses an optical switch comprising a pair of single mode optical waveguides coupled together by couplers to form a Mach-Zehnder interferometer, with a polarisation dependent phase shifter being constituted by a piezoelectric squeezer on one arm and a polarisationindependent phase shifter being provided on the other arm. Such a device may be used as a switchable polarisation splitter. It is an object of the invention to provide an optical power monitoring device having reduced polarisation dependence which may be formed in a straightforward manner, during fabrication of an integrated device based on SOI technology, for example.
According to the present invention there is provided an optical power monitoring device for monitoring the power of an optical signal substantially independently of the polarisation of the optical signal, the device comprising waveguide means for receiving the optical signal to be monitored, sampling means for sampling the optical signal for the purpose of monitoring the power of the optical signal, two mode
waveguides for receiving respective output signals of different optical modes from the sampling means, photodetector means for detecting the optical mode signals from the mode wavcguides and for supplying an electrical output signal indicative of the power of the optical signal, and control means for controlling the optical mode signal in at least one of the mode waveguides in order to reduce the polarisation dependence of the electrical output signal.
Such an arrangement is advantageous since the device is capable of providing an accurate monitor signal which is substantially independent of the polarisation state of the light in the original waveguide. Furthermore such performance can be obtained using optical components of relaxed tolerance, thus decreasing the manufacturing cost relative to alternative arrangements requiring higher tolerance components.
Additionally the device can be fabricated on a single chip utilising silicon-on-insulator (SOI) technology, meaning that the device can be produced at a relatively low unit cost.
It should be understood that the sampling means may be such as to receive substantially the whole of the optical signal from the waveguide means although it is preferred that the sampling means is such as to receive only a proportion of the optical signal from the waveguide means with the remainder of the optical signal being supplied for further optical processing.
It should be stressed that the polarisation modes are not necessarily pure transverse electric (TE) and transverse magnetic (TM) modes, but may instead be two distinct linear combinations of these modes.
In one embodiment in accordance with the invention the photodetector means comprises a single photodiode for monitoring the optical mode signals from both of the mode waveguides. The use of a single photodiode is advantageous in that it decreases the cost and complexity of the device. In this case the control means may provide for detection of the two optical mode signals by the photodiode at the same time, the mode waveguides being arranged relative to the photodiode such that the intensities of the optical mode signals add together at the photodiode.
( Alternatively the control means may provide for detection of the two optical mode signals by the photodiode sequentially. This provides the same advantages inherent in the use of only a single photodiode whilst avoiding the possible inaccuracies caused by overlap of the mode signals in the preceding embodiment.
Furthermore the sampling means may comprise a tap-off coupler for sampling the optical signal, and polarisation splitting means for receiving the sampled optical signal from the tap-off coupler and for supplying respective output signals of different optical modes to the two mode waveguides. In this case the polarisation splitting means may comprise a Mach-Zehnder-based polarisation splitter, although other types of polarisation splitting means may also be used.
Alternatively the sampling means may comprise two tap-off couplers for sampling the optical signal at two spaced positions along the waveguide means and for supplying respective output signals of different optical modes to the two mode waveguides. In this case, instead of using a polarisation splitter, a polarisation rotator may provided in the waveguide means between said two spaced positions to effect rotation of the polarisation of the optical signal between the two sampling steps performed by the tap-off couplers. This provides two tapped-off signals of different polarisation modes without requiring a polarisation splitter.
The sampling means preferably comprises at least one evanescent coupler.
Alternatively the sampling means may comprise at least one Y branch or other suitable form of tap-off coupler. The proportion of the optical signal which is tapped off may be very small so that a large part of the optical signal is transmitted to an output of the waveguide means without being significantly attenuated by the presence of the monitoring device, so that the monitoring device is largely unobtrusive. However it is also possible within the scope of the invention for all of the optical signal to be passed to the photodetector means for monitoring purposes, or to be otherwise absorbed within the monitoring device, so that none of the optical signal supplied to the waveguide means is transmitted beyond the monitoring device.
( s The control means advantageously comprises attenuating means for attenuating the optical mode signal in at least one of the mode waveguides in dependence on an electrical control signal in order to reduce the polarisation dependence of the electrical output signal. More particularly the control means may comprise respective attenuating means in the two mode waveguides for attenuating the optical mode signals in the two mode waveguides in dependence on electrical control signals in order to reduce the polarization dependence of the electrical output signal. The use of such attenuating means enables the relative intensities Pi and P2 of the two polarization modes to be adjusted so that the desired low polarization dependent monitor signal is obtained.
The attenuating means preferably comprises at least one variable optical attenuator (V()A), and may comprise a respective diode in the vicinity of each of the mode waveguides. Moreover the attenuating means in one of the mode waveguides may be electrically corrected in anti-paralle1 with the attenuating means in the other of the mode waveguides. Such an arrangement can be used to economise on the electrical connections required.
For a better understanding of the present invention and to show how the same may be carried into effect, various embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure I is a block diagram of a prior art monitoring device;
Figures 2, 3 and 4 are block diagrams of three embodiments of monitoring device in accordance with the invention; Figures 5 and 6 show examples of electrical connections which may be used in embodiments of the invention; and Figures 7 and 8 are schematic diagrams illustrating the fabrication of the device.
( The following description will be given with reference to optical power
monitoring devices for monitoring the power of an optical signal supplied along a waveguide by tapping off and measuring a small fraction of the light passing along the waveguide. it is desirable that such devices should provide an accurate monitor signal which is substantially independent of the polarisation state of the light supplied to the waveguide. Previously the production of a device providing a monitor signal of the required accuracy has required exacting requirements to be met in terms of tight manufacturing control of the tap and monitor interface phototonics. However the phototonics tolerances can be relaxed in the case of the devices in accordance with the invention to be described below in that two measurements are made of light intensities with different polarization modes, for example TE and TM modes or two distinct linear combinations PI and P2 of such modes.
However the invention is also applicable to other types of monitoring device, including devices in which the whole of the optical signal is supplied for the purpose of monitonng, and none of the signal is transmitted beyond the monitoring device, rather than only a small proportion of the monitoring signal being tapped off for monitoring purposes as in the described embodiments. For example, such a monitoring device could also be used as an optical channel monitor based on an arrayed waveguide (AWG) or other dispersive element. In this case the device could be arranged to monitor the optical channel signals by tapping off small fractions of those signals, and the device could include attenuators controlled in dependence on the monitored signals to compensate for the polarization dependence not only of the components of the monitoring device itself, but also of the AWG or other dispersive element.
Figure I shows a known monitoring device comprising a through waveguide 2 to which an optical signal is supplied at an input end 3 of the waveguide and from which an output signal is supplied for further processing from an output end 4 of the waveguide. A tap-off coupler 5, which may be in the form of an evanescent coupler, is provided for tapping off a small proportion (for example 5%) of the optical signal from the through waveguide 2 to a monitoring waveguide 6 and for conducting the tapped-off portion of the optical signal to a photodiode 7. The photodiode 7 provides an electrical
output signal indicative of the intensity of the tapped-off signal, and hence also indicative of the power of the optical signal supplied to the input end 3 of the through waveguide 2.
However this known arrangement suffers from the disadvantage that the magnitude of the monitor signal supplied by the photodiode 7 is dependent on the polarization state of the light supplied to the input end 3 of the through waveguide 2 due to the polarization dependence of both the tapoff coupler 5 and the optical interface of the photodiode 7. Tight manufacturing control of the tap and monitor interface phototonics is necessary if such polarisation dependence of the monitor signal is to be minimised. Figure 2 shows a first embodiment of an optical power monitoring device in accordance with the invention, with like parts being denoted by the same reference numerals as in Figure 1. In this case a further tap-off coupler 10 is provided, in addition to the tap-off coupler 5, for tapping off a further portion of the optical signal supplied to the through waveguide 2. In order to provide the desired low polarization dependence of the monitor signal supplied by the photodiode 7, it is necessary to detect two distinct linear combinations, denoted by Pl and P2, of the TE and TM intensities, and to this end the output signals from the tap-off couplers 5 and 10 are supplied to respective VOAs 11 and 12 which are supplied with electrical control signals in order to attenuate at least one of the optical signals to obtain the required combinations Pl and P2. Such attenuation may be preset during the set up and calibration procedure in order to maximise the intensity of the monitor signal from the photodiode 7, or may be controlled continuously by a feed-back control signal from processing electronics dependent on the monitor signal.
Such an arrangement is most effective if the two tap-off couplers 5 and 10 have opposing polarization dependence, since this minimises the required attenuation. The output signals Pl and P2 from the VOAs 11 and 12 are both incident on the same photodiode 7, with the interface being such that the two signal beams do not overlap at the photodiode 7 so that the intensities of the beams are added for the purposes of
( detection by the photodiode 7 (rather than interfering with one another) . Accordingly the electrical monitor signal from the photodiode 7 will be substantially polarization independent. Such an arrangement is suitable for low polarisation-dependence monitoring where the polarization dependent imperfections in the signals supplied to the the two waveguides 6 and 14 are random or oppose one another in the waveguides G and 14.
However such an arrangement is not suitable for such monitoring in the case where similar polarization dependent imperfections are present in the signals supplied to both waveguides 6 and 14.; If the case is considered where the signals tapped off by the tap-off couplers 5 and 10 are of the same power, it can be demonstrated that the attenuation required to achieve polarisation-independent monitoring is given by the expression: Z = R(l - x)/(l - y) where the symbols in this expression have the following meaning: Z = attenuation (positive implies attenuation in the VOA l l, negative implies attenuation in the VOA 12) x = PDL of tap-off coupler 5 (ratio of TE fraction to TM fraction sampled) y = PDL of tap-off coupler 10 (ratio of TM fraction to TE fraction supplied) R = ratio of the TE fraction in the tap-off coupler 5 to the TM fraction in the tap-
off coupler 10 Figure 3 illustrates a further embodiment of monitoring device in accordance with the invention having the same general arrangement as the embodiment of Figure 2,
but in addition having a polarisation rotator 20 positioned in the through waveguide 2 intermediate the tap-offcouplers 5 and 10.
Such an arrangement can be used to provide low polarisation-dcpendence monitoring where the polarisation dependent imperfections in the signals supplied to the two waveguides 6 and 14 are similar, as in the case where the tap-off couplers 5 and 10 are designed to be identical so that the variation between the tap-off signals is small. In this case the polarisation rotator 20 is used to convert TE polarised light to TM polarised light and TM polarised light to TE polarised light, so as to ensure that the tapped-off signals in the waveguides 6 and 10 are dissimilar, so that relatively little attenuation is required to provide the required combinations P I and P2 for low polarisation-dependence monitoring.
A further embodiment of the invention shown in Figure 4 requires only one tap-
off coupler 5 for supplying a tapped-off optical signal along the waveguide 6 to a polarisation splitter 30 which splits the signals into two polarisation components supplied to respective waveguides 31 and 32 incorporating the VOAs 11 and 12. The polarisation splitter 30 may be a Mach-Zeknder-based polarisation splitter or some other type of polarisation splitter. In this case the tapped-off signal supplied to the waveguide 6 is split into two polarisation components TE and TM, or linear combinations of such components, which are then attenuated by the VOAs 11 and 12 in order to compensate for the PDL inherent in the tap-off coupler 5 and the photodiode interface, so that the sum of the light intensities detected by the photodiode 7 is substantially independent of the polarisation of the signal monitored.
The VOAs 11 and 12 are preferably formed by a pair of diodes 40 and 41 connected in anti-parallel as shown in Figure 5. Only a single pair of electrical terminals 42 and 43 connected to the two diodes 40 and 41 is required since, in operation, it will be necessary for only one of the diodes to be biased at a time in order to attenuate one of the signals, rather than both diodes being biased to attenuate both signals at the same time. It will be appreciated that the electrical biasing of each diode
( determines the degree of optical attenuation (if any) applied to the optical signal supplied along the associated waveguide.
Figure 6 shows the corresponding electrical connections where more than one pair of VOAs is required, for example for monitoring of multiple channels. In this case each pair of diodes 50, 51 may be connected at one end to a common rail 52 to save space on the chip, whereas the opposite ends of the diodes are connected to respective control terminals 53 by means of which the biasing of each pair of diodes may be independently controlled.
In operation of the device an overall test and calibration procedure is initially followed in which the polarization dependence of the output signal of the photodiode 7 is measured as a function of the attenuation applied to the VOAs 11 and 12, and in which the VOAs 1 1 and 12 are set to achieve a null if the polarization dependence of the output signal with respect to the optical signal either at the input end or at the output end of the through waveguide 2. In the null condition only one of the two VOAs 1 1 and 12 requires to be biased to attenuate the signal, and which of the two VOAs 11 and 12 requires to be biased will depend upon the particular linear combination of PI and P2 required which in turn depends upon the detailed properties of the tap-off couplers 5 and 10 (which will not necessarily be known in advance). If the anti-parallel connection arrangement of Figure 5 is used, the particular VOA to be activated can be selected according to the sign of the bias.
It is desirable to operate using a regime in which the co-efficients of PI and P2 do not differ markedly, so that the null condition is achieved with modest levels of attenuation. In the case of the embodiment of Figure 2, the ideal operating condition is achieved if the two tap-off couplers 5 and 10 have opposite PDL and equal tap-off fractions, whereas, in the case of the embodiment of Figure 4, the ideal operating condition is obtained if the tap-off coupler 5 is substantially polarization independent, and the polarization splitter achieves substantially perfect polarization splitting. In such ideal operating conditions a zero PDL monitor could be obtained with zero attenuation.
However, in practice, some applied attenuation is likely to be required to obtain substantially zero PDL monitoring As shown in Figure 7 the above-described devices in accordance with the invention are based on a passive waveguide section 60 formed in a silicon layer 61 formed on a silicon-on-insulator (SOI) substrate with the intermediary of a buried silicon dioxide layer 62 (having a thickness in the range of 0.05-10 m). The light is guided in the ridge structure, the buried layer 62 forming the lower cladding of the waveguide, with the upper cladding being provided by the air above the waveguide.
Alternatively an upper layer of silicon dioxide may be grown or deposited by standard techniques to form an upper cladding layer. The waveguide section 60 is fabricated by etching a ridge structure using standard micro-fabrication techniques, with ridge width w in the range of 0.5-l 0 m, etch depth dl in the range of 0.5-3 1lm and thickness d2 in the range of 0.5-lO'lm.
Figure 8 is a cross-section through the waveguide 60 in the region in which a VOA 64 is provided for attenuating the optical signal within the waveguide. In this case the VOA 64 comprises n- and p- type doped regions 65 and 6G formed on either side of the waveguide by ion implantation or by diffusion. Each doped region 65 or 66 typically has a width in the range of 1-50 tom, and the distance of each such region from the waveguide ridge is typically in the range of 0-30 m. Optionally trenches are etched in the silicon layer before or after introduction of the dopant to increase the
attenuation efficiency (attenuation achieved for a given diode current). Typically a dielectric layer, for example a silicon dioxide layer, is grown on top of such a structure with contact holes being etched in this layer above the n- and p- type regions 65 and 66, and metallisation is deposited on top of this layer and patterned in such a way as to form tracks and contacts for the electrical connections to the VOA.
It is possible to envisage other embodiments of optical power monitoring device in accordance with the invention, the object being to provide two variable monitoring signals having different polarisation components, corresponding either to TE and TM or different combinations of TE and TM. These signals can be obtained by making two
( measurements. As in the embodiments of Figures 2 and 3, this can be done by tapping off two monitor signals such that the polarization dependence of the two tapped-off signals is not identical. The intensity can then be reconstructed providing that the tap sensitivities are known. For example, suppose that a tapped signal A provides 5% of the TE intensity and 10% of the TM intensity, and that a tapped signal B has inverted sensitivity, providing 10% of the TE intensity and 5 /O of the TM intensity, it follows that the addition of the two signals A and B can be used to provide a monitor signal incorporating 15% of TE and 15% of TM, thus providing a monitor signal of 15% intensity, regardless of the polarization state of the signal monitored. In general, provided that the tapped signals A and B have different polarization components, there will be a co-efficient C such that the signal A plus C times the signal B provides an intensity which is substantially independent of polarization.
Claims (18)
1. An optical power monitoring device for monitoring the power of an optical signal substantially independently of the polarization of the optical signal, the device comprising waveguide means for receiving the optical signal to be monitored, sampling means for sampling the optical signal for the purpose of monitoring the power of the optical signal, two mode waveguides for receiving respective output signals of different optical modes from the sampling means, photodetector means for detecting the optical mode signals from the mode waveguides and for supplying an electrical output signal indicative of the power of the optical signal, and control means for controlling the optical mode signal in at least one of the mode waveguides in order to reduce the polarization dependence of the electrical output signal.
2. A device according to claim 1, wherein the photodetector means comprises a single photodiode for monitoring the optical mode signals from both of the mode waveguides.
3. A device according to claim 2, wherein the control means provides for detection of the two optical mode signals by the photodiode at the same time, the mode waveguides being arranged relative to the photodiode such that the intensities of the optical mode signals add together at the photodiode.
4. A device according to claim I or 2, wherein the control means provides for detection of the two optical mode signals by the photodiode sequentially.
5. A device according to any one of claims I to 4, wherein the sampling means comprises a tap-off coupler for sampling the optical signal, and polarization splitting means for receiving the sampled optical signal from the tap-off coupler and for supplying respective output signals of different optical modes to the two mode waveguides.
6. A device according to claim 5, wherein the polarisation splitting means comprises a Mach-Zehnder-based polarisation splitter.
7. A device according to any one of claims I to 4, wherein the sampling means comprises two tap-off couplers for sampling the optical signal at two spaced positions along the waveguide means and for supplying respective output signals of different optical modes to the two mode waveguides.
8. A device according to claim 7, wherein a polarisation rotator is provided in the waveguide means between said two spaced positions to effect rotation of the polarisation of the optical signal between the two sampling steps performed by the tap-
off couplers.
9. A device according to any one of claims 1 to 8, wherein the sampling means comprises at least one evanescent coupler.
10. A device according to any one of claims I to 8, wherein the sampling means comprises at least one Y branch.
11. A device according to any one of claims I to 8, wherein the sampling means is arranged to receive substantially the whole of the optical signal from the waveguide means.
12. A device according to any preceding claim, wherein the control means comprises attenuating means for attenuating the optical mode signal in at least one of the mode waveguides in dependence on an electrical control signal in order to reduce the polarisation dependence of the electrical output signal.
13. A device according to claim 12, wherein the control means comprises respective attenuating means in the two mode waveguides for attenuating the optical mode signals in the two mode waveguides in dependence on electrical control signals in order to reduce the polarisation dependence of the electrical output signal.
14. A device according to claim 12 or 13, wherein the attenuating means comprises at least one variable optical attenuator (VOA).
15. A device according to claim 14, wherein the attenuating means comprises a respective diode in the vicinity of each of the mode waveguides.
16. A device according to claim 15, wherein the attenuating means in one of the mode waveguides is electrically connected in anti-parallel with the attenuating means in the other of the mode waveguides.
17. A device according to any preceding claim, wherein the device is integrally fabricated on a SO1 (silicon-on-insulator) substrate.
18. An optical power monitoring device for monitoring the power of an optical signal substantially independently of the polarization of the optical signal, the device being substantially as hereinbefore described with reference to the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0214317A GB2389897A (en) | 2002-06-21 | 2002-06-21 | Optical power monitoring devices |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0214317A GB2389897A (en) | 2002-06-21 | 2002-06-21 | Optical power monitoring devices |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB0214317D0 GB0214317D0 (en) | 2002-07-31 |
| GB2389897A true GB2389897A (en) | 2003-12-24 |
Family
ID=9939018
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB0214317A Withdrawn GB2389897A (en) | 2002-06-21 | 2002-06-21 | Optical power monitoring devices |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2389897A (en) |
Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0721117A1 (en) * | 1994-12-30 | 1996-07-10 | AT&T Corp. | Polarization and wavelength independent optical waveguide tap |
| US5731871A (en) * | 1995-12-12 | 1998-03-24 | Fujitsu Limited | Light quantity measuring device that can accurately measure quantity regardless of polarization of measured light |
-
2002
- 2002-06-21 GB GB0214317A patent/GB2389897A/en not_active Withdrawn
Patent Citations (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0721117A1 (en) * | 1994-12-30 | 1996-07-10 | AT&T Corp. | Polarization and wavelength independent optical waveguide tap |
| US5731871A (en) * | 1995-12-12 | 1998-03-24 | Fujitsu Limited | Light quantity measuring device that can accurately measure quantity regardless of polarization of measured light |
Also Published As
| Publication number | Publication date |
|---|---|
| GB0214317D0 (en) | 2002-07-31 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |