GB2376293A - Determining polarisation dependent power loss in an optical device - Google Patents
Determining polarisation dependent power loss in an optical device Download PDFInfo
- Publication number
- GB2376293A GB2376293A GB0113510A GB0113510A GB2376293A GB 2376293 A GB2376293 A GB 2376293A GB 0113510 A GB0113510 A GB 0113510A GB 0113510 A GB0113510 A GB 0113510A GB 2376293 A GB2376293 A GB 2376293A
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- power
- polarisation
- output signal
- signal
- component
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
- G01M11/00—Testing of optical apparatus; Testing structures by optical methods not otherwise provided for
- G01M11/30—Testing of optical devices, constituted by fibre optics or optical waveguides
- G01M11/33—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face
- G01M11/337—Testing of optical devices, constituted by fibre optics or optical waveguides with a light emitter being disposed at one fibre or waveguide end-face, and a light receiver at the other end-face by measuring polarization dependent loss [PDL]
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- Physics & Mathematics (AREA)
- Optics & Photonics (AREA)
- Chemical & Material Sciences (AREA)
- Analytical Chemistry (AREA)
- General Physics & Mathematics (AREA)
- Optical Transform (AREA)
Abstract
To determine a polarisation power loss in an optical device such as a demultiplexer (20), a signal of known input power such as a wavelength multiplexed signal is applied to the device, and the power levels of an output from the device are measured at each of two mutually orthogonal polarisations. The total output power from the device and the output polarisation are determined from the measured power levels, and the power loss at the determined polarisation angle is determined from the known input power and the total output power. The input signal may be adjusted to a plurality of different polarisation angles using a polarisation controller (22) connected to the optical device by a polarisation maintaining optical fibre (24). The input may be demultiplexed by the optical device into three output channels ( g 1, g 2, g 3) directed to respective polarisation sensitive measuring devices (26).
Description
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OPTIC SYSTEM The present invention relates to optic devices, in particular to a method of compensating for polarisation dependent effects in an optic device, to a method of operating such an optic device, and to an optic system for use in such methods.
Some optic devices exhibit polarisation dependent effects such as polarisation dependent loss and polarisation dependent frequency. For example, it has been observed in the case of an integrated demultiplexer comprising an array waveguide grating (AWG) for receiving a wavelength-multiplexed input signal and selectively directing each of its component channels into a respective output waveguide, the power of each component channel in the respective output may depend on the polarisation state of the respective optical signal at the input end. In particular, the loss exhibited through the AWG may be dependent on the input polarisation state. Also, there may be a frequency shift in the signal guided by each waveguide in the array depending on its polarisation state, the so called polarisation dependent frequency which is in fact measured as a frequency shift (Af).
Notwithstanding these problems, there is a requirement to report the input powers for each optical channel, based on measurements of the output power. It is therefore necessary to provide some way of compensating for these polarisation dependent effects.
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According to a first aspect of the invention there is provided: a method of determining a polarisation dependent power loss in an optic device, the method comprising applying a signal of known input power into the optic device; measuring the power levels of an output signal from the device at each of two mutually orthogonal polarisations; determining from the measured power levels a total output power from the device and an output polarisation angle; determining the total output power and the known input power a power loss at the determined polarisation angle.
Another aspect of the invention provides a method of operating an optic device including the steps of (a) introducing an input signal into the optic device, (b) measuring the power of an output signal from the device, (c) determining the polarisation angle of the output signal; and (d) calculating the power of the input signal from the power measured for the output signal in step (b) by applying a power loss correction factor from a pre-determined power loss vs. polarisation angle characteristic.
A further aspect of the invention provides an optic system including a dispersive optic device having an input end for receiving an input signal and an output end at which an output signal is produced; means for measuring the power levels of the output signal at each of two mutually orthogonal polarisations; means for
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determining from the measured power levels a total output power from the device and an output polarisation angle.
A still further aspect of the invention provides a method of determining the power loss-polarisation angle characteristic for an optic device, the method including the steps of : (a) introducing an input signal of known power into the optic device, (b) measuring the power of an output signal from the device, (c) measuring the polarisation angle of the output signal; (d) calculating the power loss from the power of the input signal and the power measured for the output signal ; (e) changing the polarisation angle of the input signal with time, and repeating steps (b) to (d) with each change of polarisation angle of the input signal to determine the power loss for a plurality of input polarisation angles with reference to the respective output polarisation angle.
The determination of the power levels at each of two mutually orthogonal polarisations can be carried out by splitting the output signal into respective horizontal and vertical polarisations and measuring the output power levels of each of the horizontally and vertically polarised signals. Alternatively, a filter can be used to selectively transmit horizontally polarised light to a light detecting element to check the power level, and subsequently only vertically polarised light to the same light detecting element to measure the power level.
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A particular example of an optic device is an integrated dispersive optic device such as an array waveguide grating.
Embodiments of the present invention will now be described hereunder, by way of example only, with reference to the accompanying drawings, in which :Figure 1 is a schematic vtew of an optic device that processes a single input signal into a plurality of output signals ; Figure 2 is a schematic view of an optic system for determining the power losspolarisation angle characteristic of an optic device; Figure 3 is a graph showing how the power loss through an optic device may vary with polarisation angle; Figure 4 is a schematic view of an optic system according to a first embodiment of the present invention; Figures 5 to 9 are schematic views of measuring devices for use in the methods and optic system of the present invention; Figure 10 is a schematic plan view of an optic system according to an embodiment of the present invention; and Figure 11 is a schematic block diagram of signal processing circuitry.
With reference to Figure 1, a demultiplexer exhibiting polarisation dependent effects separates a wavelength-multiplexed input signal into its component channels at wavelengths À1, À2 and À3. It is an effect of such demultiplexer that the power of
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each of the channels at the output side is less than the driving power of the respective component channel at the input side, and it is known that one of the factors affecting the degree of power loss is the polarisation angle of the respective component channel at the input side.
As a first step in compensating for such polarisation dependent effects there is now described a technique for determining the power-loss vs. polarisation angle characteristic of the device.
With reference to Figure 2, the wavelength-multiplexed input signal is introduced into a polarisation controller 22, which can be used to controllably vary the polarisation angle of each of the component channels of the input signal. The polarisation controller is serially connected to the input of the optic device 20 via a polarisation maintaining (PM) fibre 24, which ensures that the polarisation of the signal into the optic device 20 is determined only by the polarisation controller 22. The input signal having a polarisation angle set by the polarisation controller is demultiplexed by the optic device 20 into three output signals at the three channel wavelengths "A 1, "A2 and "A3. Each of the output signals is directed to a respective measuring device 26 (described later) for measuring the polarisation angle and power of the respective output signal. The power of each component channel at the input side of the optic device is known, and the power loss for each component channel can be calculated from the known input power and the power of the output signal measured at the
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measuring device 26. The power loss is recorded for each output signal together with the respective output polarisation angle measured by the respective measuring device.
Next, the polarisation controller is adjusted to change the polarisation angle at the input side of the optic device 20, and the above described steps are repeated for a plurality of different input polarisation angles to determine for each component channel the power loss for a number of different input polarisation angles with reference to the respective output polarisation angle.
In this way the power loss-polarisation angle characteristic over a range of different input polarisation angles for each wavelength can be determined. The characteristic for a particular component channel may be plotted as shown in Figure 3a, from which the power loss for a particular output polarisation angle 8 over a range of 180 may be easily determined.
The power loss vs. polarisation angle characteristics is stored in a memory (Figure 4) for example in the form of a look up table (LUT).
In later use of the optic device 20, an input signal is introduced into the device as shown in Figure 4, and each of the output signals at respective wavelengths X 2 and À3 are directed to a respective measuring device 26 for measuring the output power and polarisation angle of the respective output signal. These parameters are
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supplied to a processor 29 where the input power of each of the component channels at wavelengths Al, À2 and À3 may then be precisely determined by correcting the output power measured at the measuring device in accordance with the power loss determined in the calibration step for the particular output polarisation angle measured by the measuring device 26. For example, with reference to Figure 3a, if the output polarisation angle is measured to be 150", the input power is equal to the measured output power plus 0.5dB.
The measuring device may take a number of different forms, examples of which are shown in Figures 5 to 9.
Figure 3b illustrates the principal upon which the following measuring devices are based. That is, the measuring devices each rely on separation of the output signal into horizontally and vertically polarized parts. The power level of the horizontally polarised part Ph is measured, as is the power level of the vertically polarised part Pv.
From these measurements, the polarisation angle ss can be determined, together with the total power Prar by normal geometrical calculations. That is:
8 = arctan (Pv/Ph)
Ptor = (Pv+ Ph) =yv +yh D Where y.., yi, are the measured photodiode currents in the vertical and horizontal directions respectively and D is the photodiode sensitivity in Amps per watt.
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With reference to Figure 5, one type of measuring device 26 includes a modeindependent power splitter 50 for equally splitting the power of the respective output signal into two parts. The first part is directed to a vertical polariser 52, and the second part is directed to a horizontal polariser 54. Each polariser is serially connected to a respective photodiode 56,58. The polarisation angle and power of the respective output signal can be calculated from the electrical outputs from the two photodiodes 56,58.
With reference to Figure 6, another type of measuring device 26 includes a liquid crystal polarisation rotator 60 for receiving the respective output signal. Behind the liquid crystal polarisation rotator 60 is a polariser sheet 68 for selectively transmitting light of a specific polarisation to a photodiode 70 such a pin diode positioned behind the polariser sheet 68. The polariser sheet may, for example, be a vertical or horizontal polariser. The liquid crystal polarisation rotator comprises a liquid crystal layer 64 sandwiched between transparent front and rear electrodes 62,64, which may be made from glass coated with InSnO. The polarisation of the light through the LC may be rotated by 90 when the voltage applied across the electrodes is switched between two voltage levels. The component power of the output signal in orthogonal transverse directions can be successively measured by rotating the polarisation of the respective output signal using the liquid crystal rotator. For example, if a horizontal polariser sheet 68 is used, then at a first voltage level across the liquid crystal at which
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the polarisation of the light is not rotated, only the horizontal component of the output signal is transmitted through to the photodiode upon which a first electrical signal is produced. Then at a second voltage level at which the polarisation of the light is rotated by 90 , only the vertical component of the output signal is transmitted through to the photodiode, upon which a second electrical signal is produced. The polarisation angle and power of the respective output signal may be calculated from the two electrical signals.
With reference to Figure 7, a third type of measuring device 26 includes a sandwich photodiode structure 72 of the type described in US5767507. A first part A 74 of the photodiode absorbs the component of the respective output signal in a first transverse direction, and a second part B 76 absorbs the component of the output signal in an orthogonal transverse direction. The polarisation angle and power of the respective output signal may be calculated from the voltages developed across the two parts A and B.
With reference to Figure 8, a fourth type of measuring device 26 includes a dichroic prism optical assembly 80 which separates the respective output signal into a horizontally polarised part and a vertically polarised part and directs each part to a respective photodiode 82,84. The polarisation angle and power of the respective output signal may be calculated from the electric signals from the two photodiodes 82,84.
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With reference to Figure 9, a fifth type of measuring device includes a modeindependent beam power splitter 90 for splitting the respective output signal into two component parts of equal power. A first part is directed to a vertical polariser 92 which only transmits the vertical component of the signal to a photodiode 96 to measure the power of the vertical component. A second part is directed to a horizontal polariser 94 which only transmits the horizontal component of the signal to a second photodiode 98 to measure the power of the horizontal component. The polarisation angle and the power can be measured from the electrical signals produced at the two photodiodes 96,98.
The methods of the present invention have particular application to integrated optical devices such as integrated demultiplexers based on array waveguide gratings of the type shown in Figure 10 including a integrated semiconductor chip 100 (such as a silicon-on-insulator chip) having defined therein an array waveguide grating 104 optically connected to an input waveguide 102 and an array of output waveguides 106 input waveguide via free propagation regions 112,114. In one embodiment of the optic system of the present invention, each output waveguide 106 is provided with an integrated polarisation beam splitter of the type described in co-pending GB patent application no. 0026415.0, whose content is incorporated herein by reference. Each polarisation beam splitter splits the respective output signal into its respective horizontal and vertical components and directs each component to a respective one
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of an array of photodiodes 110 positioned at an edge of the chip 100. The polarisation angle and power of each output signal can be calculated from the electrical signals produced at the photodiodes 110 at which the horizontal and vertical components are respectively received.
Although the methods described above are applications of the present invention to optic devices having a plurality of outputs at different wavelengths, the method of the present invention also has application to optic devices having a single output.
Figure 11 is a schematic block diagram of circuitry arranged to receive the signals from the photodiodes (for example 56 and 58 in Figure 5) and to generate the polarisation angle 0 and total power Ptot. The signal from each photodiode 56,58 is supplied to a respective amplifier 57,59 and from there to a respective A/D converter 61,63. Each A/D converter converts the analogue electrical signal into a digital output representing the power level of each of the vertically and horizontally polarized components, denoted Xv and Xh accordingly. These signals are processed by respective correction blocks 65,67 which are designed to take into account errors in the photodiodes themselves. The process carried out in correction block 65 and 67 generates corrected power signals yv, yh respectively where: yv = axv + bxv + c, and where a similar equation applies for yh. a, b and c are calibration coefficients for the respective photodiode. Their determination is not described herein, but it will be noted that they can be
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determined according to any suitable technique. One such technique is described in our UK patent application no. 0113103. 6.
The outputs yv, yh are supplied to a processing block 69 which generates the polarisation angle 0 and total power Pilot as has already been described.
Claims (18)
1. A method of determining a polarisation dependent power loss in an optic device, the method comprising: applying a signal of known input power into the optic device ; measuring the power levels of an output signal from the device at each of two mutually orthogonal polarisations; determining from the measured power levels a total output power from the device and an output polarisation angle; determining the total output power and the known input power a power loss at the determined polarisation angle.
2. A method according to claim 1 when repeated for a plurality of input signals having differing polarisation states to generate a power loss is polarisation angle characteristic for the device.
3. A method according to claim 1 or 2 including the step of splitting the output signal into a first component signal polarised in a first direction and a second component signal polarised in a second direction orthogonal to the first direction, and directing each of the first and second component signals to a respective power detecting element for separate measurement of the power levels thereof.
<Desc/Clms Page number 14>
4. A method according to claim 1 or 2 including the step of filtering the output signal to selectively transmit a component polarised m a first direction to a light detecting element to measure the power level thereof ; and subsequently filtering the output signal to selectively transmit a component polarised in a second direction orthogonal to the first direction to the same light detecting element to measure the power level thereof.
5. A method according to claim 1 wherein the optic device is an integrated dispersive optic device.
6. A method of operating an optic device including the steps of (a) introducing an input signal into the optic device, (b) measuring the power of an output signal from the device, (c) determining the polarisation angle of the output signal; and (d) calculating the power of the input signal from the power measured for the output signal in step (b) by applying a power loss correction factor from a pre-determined power loss vs. polarisation angle characteristic.
7. A method according to claim 6 wherein the power loss vs. polarisation angle characteristic is determined according to the method defined in any of claims 1 to 5.
8. A method according to claim 6 or 7 wherein steps (b) and (c) are effected by measuring the component of the power of the output signal at a first polarisation;
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measuring the component of the power of the output signal at a second polarisation orthogonal to the first polarisation: and calculating the polarisation angle and the total output power from the component powers at the first and second polarisations.
9. A method according to claim 8 including the step of splitting the output signal into a first component signal polarised in a first direction and a second component signal polarised in a second direction orthogonal to the first direction, and directing each of the first and second component signals to a respective power detecting element for separate measurement of the power thereof.
10. A method according to claim 8 including the step of filtering the output signal to selectively transmit a component polarised in a first direction to a light detecting element to measure the power thereof ; and subsequently filtering the output signal to selectively transmit a component polarised in a second direction orthogonal to the first direction to the same light detecting element to measure the power thereof.
11. An optic system including a dispersive optic device having an input end for receiving an input signal and an output end at which an output signal is produced; means for measuring the power levels of the output signal at each of two mutually orthogonal polarisation ; means for determining from the measured power levels a total output power from the device and an output polarisation angle.
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12. A system according to claim 11 including a first element for selectively directing a component of the power of the output signal at a first polarisation to a first light detecting element, and a second element for selectively directing a component of the power of the output signal at a second polarisation orthogonal to the first polarisation to a second light detecting element.
13. A system according to claim 12 wherein the first and second elements are constituted by a polarisation beam splitter.
14. A system according to claim 13 wherein the polarisation beam splitter is integrated together with the integrated optic device.
15. A system according to claim 11 including a power splitter for splitting the respective output signal into first and second signals of equal power, a filter for selectively transmitting the component of the power of the first signal in a first transverse direction to a first light detecting element, and a filter for selectively transmitting the component of the power of the second signal in a second transverse direction orthogonal to the first direction to a second light detecting element.
16. A system according to any of claims 11 to 15 which further comprises means for determining from the total output power and the known input power a power loss at a predetermined polarisation angle.
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17. A system according to any of claims 11 to 16 which comprises a memory holding a power loss vs. polarisation angle characteristic and means for applying a correction factor based on said characteristic determined from a measured output power and unknown input power.
18. A method of determining the power loss-polarisation angle characteristic for an optic device, the method including the steps of : (a) introducing an input signal of known power into the optic device, (b) measuring the power of an output signal from the device, (c) measuring the polarisation angle of the output signal; (d) calculating the power loss from the power of the input signal and the power measured for the output signal; (e) changing the polarisation angle of the input signal with time, and repeating steps (b) to (d) with each change of polarisation angle of the input signal to determine the power loss for a plurality of input polarisation angles with reference to the respective output polarisation angle.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0113510A GB2376293A (en) | 2001-06-04 | 2001-06-04 | Determining polarisation dependent power loss in an optical device |
| PCT/GB2002/002417 WO2002099396A1 (en) | 2001-06-04 | 2002-05-23 | Optical system |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0113510A GB2376293A (en) | 2001-06-04 | 2001-06-04 | Determining polarisation dependent power loss in an optical device |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB0113510D0 GB0113510D0 (en) | 2001-07-25 |
| GB2376293A true GB2376293A (en) | 2002-12-11 |
Family
ID=9915830
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB0113510A Withdrawn GB2376293A (en) | 2001-06-04 | 2001-06-04 | Determining polarisation dependent power loss in an optical device |
Country Status (2)
| Country | Link |
|---|---|
| GB (1) | GB2376293A (en) |
| WO (1) | WO2002099396A1 (en) |
Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH02103434A (en) * | 1988-10-13 | 1990-04-16 | Fujikura Ltd | Method for measuring strain distribution in longitudinal direction of optical fiber cable and apparatus and optical fiber used in said method |
| JPH0498139A (en) * | 1990-08-17 | 1992-03-30 | Ando Electric Co Ltd | Polarization crosstalk measuring device for polarization plane conserving optical fiber |
| EP0553460A2 (en) * | 1992-01-31 | 1993-08-04 | Hewlett-Packard Company | Method and apparatus for measuring polarization mode dispersion in optical devices |
Family Cites Families (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5298972A (en) * | 1990-01-22 | 1994-03-29 | Hewlett-Packard Company | Method and apparatus for measuring polarization sensitivity of optical devices |
| JPH03218440A (en) * | 1990-01-24 | 1991-09-26 | Matsushita Electric Ind Co Ltd | Birefringence measuring device |
| JP3122697B2 (en) * | 1993-03-01 | 2001-01-09 | 株式会社日立製作所 | Polarization plane detector |
| US5619325A (en) * | 1995-04-04 | 1997-04-08 | Advantest Corporation | Optical system for ellipsometry utilizing a circularly polarized probe beam |
| IT1291413B1 (en) * | 1997-02-13 | 1999-01-11 | Andrea Galtarossa | REFLECTOMETRIC INSTRUMENT FOR THE MEASUREMENT OF BIRIFRANGENCE DISTRIBUTED IN SINGLE-MODE FIBER OPTICS |
| JPH1130582A (en) * | 1997-07-09 | 1999-02-02 | Nikon Corp | Instrument and method for measuring polarization characteristic |
| JP2000304656A (en) * | 1999-04-22 | 2000-11-02 | Nikon Corp | Optical characteristic measuring instrument |
-
2001
- 2001-06-04 GB GB0113510A patent/GB2376293A/en not_active Withdrawn
-
2002
- 2002-05-23 WO PCT/GB2002/002417 patent/WO2002099396A1/en not_active Application Discontinuation
Patent Citations (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH02103434A (en) * | 1988-10-13 | 1990-04-16 | Fujikura Ltd | Method for measuring strain distribution in longitudinal direction of optical fiber cable and apparatus and optical fiber used in said method |
| JPH0498139A (en) * | 1990-08-17 | 1992-03-30 | Ando Electric Co Ltd | Polarization crosstalk measuring device for polarization plane conserving optical fiber |
| EP0553460A2 (en) * | 1992-01-31 | 1993-08-04 | Hewlett-Packard Company | Method and apparatus for measuring polarization mode dispersion in optical devices |
Also Published As
| Publication number | Publication date |
|---|---|
| WO2002099396A1 (en) | 2002-12-12 |
| GB0113510D0 (en) | 2001-07-25 |
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| Date | Code | Title | Description |
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| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |