WO2002099396A1

WO2002099396A1 - Optical system

Info

Publication number: WO2002099396A1
Application number: PCT/GB2002/002417
Authority: WO
Inventors: Joseph Alan Barnard
Original assignee: Bookham Technology Plc
Priority date: 2001-06-04
Filing date: 2002-05-23
Publication date: 2002-12-12
Also published as: GB0113510D0; GB2376293A

Abstract

A method of determining a polarisation dependent power loss in an optic device, the method comprising applying a signal of know 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 from the total output power and the know input power a power loss at the determined polarisation angle.

Description

OPTICAL 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 die respective optical signal at d e 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 (Δf).

Notwitiistanding these problems, there is a requirement to report die input powers

for each optical channel, based on measurements of die output power. It is therefore necessary to provide some way of compensating for these polarisation dependent

effects. According to a first aspect of d e invention there is provided: a mediod of

determining a polarisation dependent power loss in an optic device, d e mediod

comprising applying a signal of known input power into die optic device; measuring

die power levels of an output signal from die device at each of two mutually

ordiogonal polarisations; determining from die measured power levels a total output power from the device and an output polarisation angle; determining die total output

power and die known input power a power loss at the determined polarisation angle.

Anotiier aspect of d e invention provides a method of operating an optic device including the steps of (a) introducing an input signal into d e optic device, (b) measuring the power of an output signal from die device, (c) determining die

polarisation angle of die output signal; and (d) calculating die power of die input

signal from d e power measured for die output signal in step (b) by applying a power

loss correction factor from a pre-determined power loss vs. polarisation angle

characteristic.

A furdier aspect of d e 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 die power levels of die

output signal at each of two mutually ordiogonal polarisations; means for determining from die measured power levels a total output power from d e device

and an output polarisation angle.

A still furtiier aspect of the invention provides a method of determining die power

loss-polarisation angle characteristic for an optic device, die mediod including the

steps of: (a) introducing an input signal of known power into die optic device, (b) measuring die power of an output signal from die device, (c) measuring the polarisation angle of die output signal; (d) calculating die power loss from the power

of the input signal and die power measured for die output signal; (e) changing die

polarisation angle of die input signal with time, and repeating steps (b) to (d) witii each change of polarisation angle of the input signal to determine d e power loss for

a plurality of input polarisation angles with reference to die respective output polarisation angle.

The determination of die power levels at each of two mutually orthogonal

polarisations can be carried out by splitting die output signal into respective

horizontal and vertical polarisations and measuring die output power levels of each of

d e horizontally and vertically polarised signals. Alternatively, a filter can be used to

selectively transmit horizontally polarised light to a light detecting element to check die power level, and subsequentiy only vertically polarised light to die same light

detecting element to measure the power level. A particular example of an optic device is an integrated dispersive optic device such

as an array waveguide grating.

Embodiments of d e present invention will now be described liereunder, by way of

example only, witii reference to the accompanying drawings, in which: -

Figure 1 is a schematic view 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 die power loss-

polarisation angle characteristic of an optic device;

Figure 3 is a graph showing how the power loss tiirough an optic device may vary with polarisation angle;

Figure 4 is a schematic view of an optic system according to a first embodiment of

die present invention;

Figures 5 to 9 are schematic views of measuring devices for use in die methods and

optic system of die present invention;

Figure 10 is a schematic plan view of an optic system according to an embodiment of die present invention; and

Figure 11 is a schematic block diagram of signal processing circuitry.

Witii reference to Figure 1, a demultiplexer exhibiting polarisation dependent effects

separates a wavelengd -multiplexed input signal into its component channels at

wavelengths λl, λ2 and λ3. It is an effect of such demultiplexer tiiat die power of each of d e channels at die output side is less ti an die driving power of d e respective

component channel at the input side, and it is known tiiat one of die factors affecting

die degree of power loss is d e polarisation angle of die respective component channel at die input side.

As a first step in compensating for such polarisation dependent effects tiiere is now described a technique for determining die power-loss vs. polarisation angle

characteristic of die device.

With reference to Figure 2, die wavelength-multiplexed input signal is introduced into a polarisation controller 22, which can be used to controllably vary die polarisation

angle of each of die component channels of die input signal. The polarisation controller is serially connected to the input of die optic device 20 via a polarisation

maintaining (PM) fibre 24, which ensures that die polarisation of die signal into the

optic device 20 is determined only by the polarisation controller 22. The input signal having a polarisation angle set by die polarisation controller is demultiplexed by die

optic device 20 into tiiree output signals at die tiiree channel wavelengti s λl, λ2 and

λ3. Each of die output signals is directed to a respective measuring device 26

(described later) for measuring die polarisation angle and power of die respective

output signal. The power of each component channel at the input side of the optic

device is known, and die power loss for each component channel can be calculated from die known input power and the power of die output signal measured at die measuring device 26. The power loss is recorded for each output signal together witii die respective output polarisation angle measured by die respective measuring device.

Next, d e polarisation controller is adjusted to change die polarisation angle at the

input side of die optic device 20, and the above described steps are repeated for a

plurality of different input polarisation angles to determine for each component

channel die power loss for a number of different input polarisation angles witii reference to die respective output polarisation angle.

In this way die power loss-polarisation angle characteristic over a range of different input polarisation angles for each wavelengtii can be determined. The characteristic

for a particular component channel may be plotted as shown in Figure 3a, from

which die power loss for a particular output polarisation angle θ 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 die form of a look up table (LUT).

In later use of die optic device 20, an input signal is introduced into die device as

shown in Figure 4, and each of the output signals at respective wavelengtiis λl, λ2

and λ3 are directed to a respective measuring device 26 for measuring die output

power and polarisation angle of the respective output signal. These parameters are supplied to a processor 29 where die input power of each of the component

channels at wavelengtiis λl, λ2 and λ3 may tiien be precisely determined by

correcting the output power measured at die measuring device in accordance with die

power loss determined in the calibration step for die particular output polarisation

angle measured by die measuring device 26. For example, witii reference to Figure 3a,

if die output polarisation angle is measured to be 150^", the input power is equal to die

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 die principal upon which the following measuring devices are

based. That is, die measuring devices each rely on separation of the output signal

into horizontally and vertically polarized parts. The power level of die horizontally

polarised part Ph is measured, as is the power level of the vertically polarised part P_v.

From these measurements, die polarisation angle β can be determined, together with

the total power P_tot by normal geometrical calculations. That is:

θ = arctan (P_v/Ph)

D

Where γ_v, yh are the measured photodiode currents in the vertical and horizontal

directions respectively and D is the photodiode sensitivity in Amps per watt. Witii reference to Figure 5, one type of measuring device 26 includes a mode- independent power splitter 50 for equally splitting die power of die respective output

signal into two parts. The first part is directed to a vertical polariser 52, and die 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 die electrical outputs from the two

photodiodes 56, 58.

Witii reference to Figure 6, another type of measuring device 26 includes a liquid crystal polarisation rotator 60 for receiving die respective output signal. Behind die

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

die 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 tiirough the LC

may be rotated by 90° when the voltage applied across the electrodes is switched between two voltage levels. The component power of die output signal in ordiogonal

transverse directions can be successively measured by rotating die polarisation of the

respective output signal using die liquid crystal rotator. For example, if a horizontal

polariser sheet 68 is used, tiien at a first voltage level across the liquid crystal at which die polarisation of d e light is not rotated, only die horizontal component of d e

output signal is transmitted through to the photodiode upon which a first electrical signal is produced. Then at a second voltage level at which die polarisation of die

light is rotated by 90", only the vertical component of the output signal is transmitted dirough to the photodiode, upon which a second electrical signal is produced. The

polarisation angle and power of die respective output signal may be calculated from

die two electrical signals.

Witii reference to Figure 1 , a tiiird type of measuring device 26 includes a sandwich

photodiode structure 72 of the type described in US5767507. A first part A 74 of die photodiode absorbs die component of die respective output signal in a first transverse direction, and a second part B 76 absorbs the component of die output

signal in an ordiogonal transverse direction. The polarisation angle and power of the

respective output signal may be calculated from die voltages developed across the

two parts A and B.

With reference to Figure 8, a fourdi type of measuring device 26 includes a dicl roic

prism optical assembly 80 which separates d e 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 die respective

output signal may be calculated from die electric signals from die two photodiodes

82, 84. With reference to Figure 9, a fifth type of measuring device includes a mode-

independent beam power splitter 90 for splitting die respective output signal into two

component parts of equal power. A first part is directed to a vertical polariser 92

which only transmits ti e vertical component of d e signal to a photodiode 96 to

measure die power of the vertical component. A second part is directed to a

horizontal polariser 94 which only transmits die horizontal component of die signal

to a second photodiode 98 to measure the power of die horizontal component. The

polarisation angle and die power can be measured from die electrical signals

produced at die two photodiodes 96, 98.

The metiiods of die 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 witii 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 of an array of photodiodes 110 positioned at an edge of die chip 100. The

polarisation angle and power of each output signal can be calculated from d e

electrical signals produced at die photodiodes 110 at which die horizontal and vertical

components are respectively received.

Aldiough die metiiods described above are applications of d e present invention to

optic devices having a plurality of outputs at different wavelengtiis, the mediod of die

present invention also has application to optic devices having a single output.

Figure 11 is a schematic block diagram of circuitry arranged to receive die signals

from the photodiodes (for example 56 and 58 in Figure 5) and to generate die

polarisation angle θ and total power P_tot- The signal from each photodiode 56, 58 is

supplied to a respective amplifier 57, 59 and from tiiere 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 x_v and x accordingly. These signals are processed by

respective correction blocks 65, 67 which are designed to take into account errors in

die photodiodes themselves. The process carried out in correction block 65 and 67

generates corrected power signals y_v, yii respectively where:

y_v = ax_v ² + bx_v + 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 tiiey can be determined according to any suitable technique. One such technique is described in

our UK patent application no. 0113103.6.

The outputs y_v, y are supplied to a processing block 69 which generates die polarisation angle θ and total power P_tot as has already been described.

Claims

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 die optic device;

measuring the power levels of an output signal from the device at each of two

mutually ordiogonal polarisations;

determining from the measured power levels a total output power from the

device and an output polarisation angle;

determining from d e total output power and tiie 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 die first and second component signals to a respective power

detecting element for separate measurement of die power levels tiiereof.

4. A method according to claim 1 or 2 including the step of filtering die output

signal to selectively transmit a component polarised in a first direction to a light

detecting element to measure the power level tiiereof; and subsequentiy filtering die output signal to selectively transmit a component polarised in a second direction

ordiogonal to the first direction to die same light detecting element to measure the

power level thereof.

5. A method according to claim 1 wherein die optic device is an integrated

dispersive optic device.

6. A mediod of operating an optic device including the steps of (a) introducing an input signal into tiie optic device, (b) measuring die power of an output signal

from tiie device, (c) determining die polarisation angle of die 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 die power loss vs. polarisation angle

characteristic is determined according to tiie metiiod defined in any of claims 1 to 5.

8. A metiiod 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; measuring the component of die power of die output signal at a second polarisation

ordiogonal to die first polarisation: and calculating die polarisation angle and die total

output power from d e component powers at tiie first and second polarisations.

9. A mediod according to claim 8 including d e step of splitting die output signal

into a first component signal polarised in a first direction and a second component

signal polarised in a second direction orthogonal to die 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 metiiod according to claim 8 including die step of filtering die output signal

to selectively transmit a component polarised in a first direction to a light detecting

element to measure die power tiiereof; and subsequentiy filtering die 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 e power levels of the output signal at each of two mutually

ordiogonal polarisation;

means for determining from the measured power levels a total output power

from the device and an output polarisation angle.

12. A system according to claim 11 including a first element for selectively

directing a component of the power of die 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 ordiogonal to

die first polarisation to a second light detecting element.

13. A system according to claim 12 wherein die first and second elements are

constituted by a polarisation beam splitter.

14. A system according to claim 13 wherein die polarisation beam splitter is

integrated together witii die 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 die power of die first signal in a first

transverse direction to a first light detecting element, and a filter for selectively

transmitting die 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 die known input power a power

loss at a predetermined polarisation angle.

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, die metiiod including the steps of: (a) introducing an input signal of

known power into die optic device, (b) measuring die power of an output signal from

tiie device, (c) measuring tiie polarisation angle of the output signal; (d) calculating

tiie power loss from the power of die input signal and die power measured for the

output signal; (e) changing the polarisation angle of die input signal witii time, and

repeating steps (b) to (d) with each change of polarisation angle of the input signal to

determine die power loss for a plurality of input polarisation angles with reference to

die respective output polarisation angle.