GB2439102A

GB2439102A - Optical fibre delay device with orthogonal polarisation axes

Info

Publication number: GB2439102A
Application number: GB0612005A
Authority: GB
Inventors: Philip Anthony Davies; Nathan Joseph Gomes; Pengbo Shen
Original assignee: University of Kent at Canterbury
Current assignee: University of Kent at Canterbury
Priority date: 2006-06-16
Filing date: 2006-06-16
Publication date: 2007-12-19
Also published as: GB0612005D0; WO2007144663A1

Abstract

The device uses matched lengths 20,22 of optical fibre which are connected at 24 such that their polarization axes are orthogonal to each other. In this way, the invention is arranged to compensate for differential changes in optical signal polarization and delay caused by variation in temperature and/or stress. 24 may be a polarization rotator device or spliced polarization maintaining (PM) fibre.

Description

OPTICAL DELAY DEVICE

Field of the invention

The invention relates to the field of fibre-optics and more particularly to improvements in or relating to optical delay devices, for example for implementing optical delay lines or for interconnections in polarization maintaining systems.

Background

Fibre stretchers, variable optical delay lines incorporating fibre stretchers, and fixed optical fibre delay lines are used in many applications. Such applications include sensors and interferometers for measurement, medical, communication, and imaging applications and in line length correction schemes.

Conventional fibre delay lines use single mode (SM) fibre. Many single-pass applications require such delay lines to preserve the polarization axis, as well as to maintain a stable output polarization over time, even during the stretching of the fibre and temperature variations.

In round-trip applications, where the optical signal is reflected such that it travels in a return or backwards' direction, the propagation delay in the forward and backward directions must also be controlled.

In an effort to meet these requirements, fibre stretchers with an oval structure and using polarization maintaining (PM) fibre have been developed.

However, such arrangements still demonstrate polarization fluctuations in their output. These fluctuations are associated with variations in temperature and length of the fibre and the extinction ratio at the fibre input. Consequently, uncertainty (or noise) is created in such systems, and this is particularly problematic in interferometer applications and systems where a polarization sensitive device follows the fibre stretcher.

In fibre length correction applications, where a round-trip signal is fed back into the fibre stretcher, there is a difference in the delays for the light in the forward and backward direction due to the birefringence of the PM fibre (even when the round-trip light is held stably in the two polarization states). Furthermore, length or temperature changes in the fibre will result in this differential delay changing.

Therefore, locking the round trip delay does not result in either a stable forward delay or a stable backward delay in the system.

Accurate polarization alignment at the input to the axis of the PM fibre is known to help with the problem of polarization fluctuation, and it can keep the polarization fluctuation within a small range. More specifically, the polarization state is typically represented on a sphere (a Poincaré sphere), and any fibre disturbance causes the polarisation state to rotate in small circle on this sphere. It has been shown that a more accurate polarisation alignment with the axis of a PM fibre (i.e. higher values of the extinction ratio) reduces the size of the circle bout which the polarisation state rotates.

However, the rate of procession around this circle is independent of extinction ratio. Therefore, accurate polarization alignment does not overcome the second problem of the differential delay in the forward and backward passes.

Furthermore, a high extinction ratio value is often difficult to achieve.

Such problems are due to the existence of a birefringence changes in the optical fibre. When the optical fibre is under mechanical stress or suffers temperature changes, the birefringence will alter slightly. For example, temperature changes cause stress in the fibre due to differing expansion ratios between the fibre and its protective coating.

Typically, the differential change can be as high as 1% of the amount of stretching or elongation that the fibre undergoes. For a stretching range of 1 mm this is enough to cause the output polarization to rotate through several cycles.

Thus, there is a need to improve the performance of fibre stretchers incorporating optical delay lines. In the respect, it is desired to minimise the effects of the differential change in birefringence

Summary of the Invention

According to an aspect of the invention, there is provided an optical delay device comprising: first and second sections of optical fibre of substantially equal length arranged such that an optical signal may pass through the optical fibre between an input and an output of the device and there is an orthogonal polarization axis change half way between the input and output.

Thus, the invention provides an optical delay device for use in compensated fibre assemblies which can minimise the effects of the differential change in birefringence. These compensated assemblies can be incorporated into optical interconnections or optical delay lines, and variable optical delay lines.

Such arrangements provide a number of advantages. Firstly, they preserve the output polarization axis therefore eliminating the need for a polarization controller, and enabling direct connection to a PM system. Secondly, they can maintain a stable polarization output under temperature change and stretching.

Thirdly, they can maintain an equal forward and backward propagation delay, or a fixed difference in forward and backward propagation delay.

Thus, when compared to conventional fibre stretchers, and delay lines incorporating such fibre stretchers, the performance in terms of the output polarization and the forward and backward propagation delays is less sensitive to stretching in the fibre stretcher, temperature variations, and the alignment of the polarization of the input light to the axis of the PM fibre.

For optical interconnection purposes, the invention provides improved temperature stability in terms of both the polarization and the polarization mode dispersion.

With matched lengths of PM fibre to compensate for the birefringence and the birefringence change, the rate of polarization rotation is much reduced. A factor of 10,000 improvement can be achieved, depending on the balance of the structure. If unbalanced, electrical compensation may be applied.

If the output polarization must be maintained in the same axis as the input relative to the polarization axes of the PM fibre, a short PM fibre pigtail can be spliced at 90 degree rotation to the output of the fibre.

In line length correction applications, the compensated birefringence change in the fibre stretcher not only provides high levels of polarization stability against temperature and stretching. Furthermore, it also removes differential changes of the forward transmission and backward transmission delays. This allows the control of the round-trip delay to be equivalent to control of the forward transmission.

According to another aspect of the invention, there is provided a method for compensating differential birefringence changes in optical fibre, the method comprising the step of passing an optical signal through first and second sections of optical fibre of substantially equal length between an input and an output, wherein there is an orthogonal polarization axis change half way between the input and output.

Brief Description of the Drawings

Examples of the invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 is an illustration of apparatus according to an embodiment of the invention; Figure 2 shows a modification of the embodiment of Figurel; Figure 3 is an illustration of apparatus according to a further embodiment of the invention; and Figure 4 shows an optical device according to an embodiment of the invention.

Like reference numerals denote like elements.

Detailed Description

The invention relates to the implementation of a optical delay device using substantially matched lengths of orthogonally spliced fibre sections to eliminate or reduce effects caused by changes in birefringence (due to temperature or stress, for example); or the use of a polarization rotation mirror in a reflection configuration to overcome such birefringence changes in a fibre stretcher.

In this way, the invention compensates for differential changes in polarization and delay caused by changes in optical fibre birefringence with temperature and/or stress.

The invention can be implemented with off the shelf components, and the following are the key components used in preferred configurations of the invention: Piezo-electric Transducer (PZT) fibre stretchers 2; Polarisation Maintaining (PM) optical fibre 4; Single Mode (SM) optical fibre 6; an optical circulator, a polarization beam splitter or an optical coupler 8; a polarization rotation mirror 10; and a polarisation maintaining fibre pigtail 12.

All of these components are standard, readily available optical/fibre-optic components.

Referring to Figure 1, a first example of a fibre stretcher according to the invention comprises a PZT 2 around which is wound a length of PM fibre 4. The PM fibre 4 is cleaved/broken at its midpoint to define first 4a and second 4b sections of the PM fibre 4 of substantially equal length wound around the PZT 2.

The sections of PM fibre 4 are wound such that there is sufficient fibre free from the stretcher around the midpoint break to perform a suitable connection operation (for example, cleaving and splicing). The "free" lengths of fibre around the midpoint are connected together such that the polarisation axes in the first 4a and second 4b fibre sections are orthogonal to each other (i.e. the fast-axis of the first fibre section 4a is aligned to the slow-axis of the second fibre section 4b, and vice-versa). In this way, light propagating in one polarisation state though the first fibre section 4a propagates in an orthogonal polarisation state in the second fibre section 4b.

By way of example, the connection may be performed either through rotating the polarization axes of the two sections of PM fibre such that they are orthogonal to each other and splicing the sections together, or by using a polarization rotator connected between the two sections of PM fibre.

It will be appreciated that this arrangement provides a single PZT stretcher which is wound with two sections of identical PM fibre of equal length, wherein the two sections are spliced together with a 90 degree rotation between their polarization axes. In this preferred embodiment, the two sections of fibre originate from a single length of PM fibre 4. However, in an alternative embodiment, two separate lengths of PM fibre may be wound around a PZT 2 in different winding processes and then spliced together such that their polarization axes are orthogonal to each other.

An optical signal launched into an input end 14 of the first fibre section 4a, with its polarisation aligned with the polarisation axis of the first fibre section 4a, propagates through the fibre until it reaches the opposite end of the first fibre section 4a (the point at which the first fibre section 4a is spliced to the second fibre section 4b).

Due to changes in the birefringence of the PM fibre 4 caused by temperature or stress variations, the optical signal may have experienced polarisation changes (retardation) and delay as it propagated through the first fibre section 4a. However, due to the orthogonal splicing arrangement of the matched first 4a and second 4b fibre sections, light propagating in one polarisation state though the first fibre section 4a is launched into the second fibre section 4b and propagates in an orthogonal polarisation state. In this way, propagation of the optical signal through the second fibre section 4b introduces equal and opposite polarisation changes to those introduced in the first fibre section 4b. In other words, the second fibre section 4b compensates for the differential changes and delay caused by the first fibre section 4a. The compensated optical signal is then output at an output end 16 of the second fibre section.

Figure 2 shows an alternative embodiment of the invention using two matched fibre stretchers 2a and 2b. In the context of this description, reference to matched fibre stretchers will be appreciated as denoting fibre stretchers with similar, and preferably equal, transducer responses.

In this regard, it is noted that matched PZT transducer responses may be difficult to achieve practically. Where this is the case, electrical compensation can be utilized to provide unequal drive to the transducers in order that the stretching applied to each fibre is equal. It is preferable that the length of fibre in each section is equal for optimized temperature stability.

Equal lengths of similar PM fibre are wound onto each PZT. Thus, the first PZT stretcher 2a is wound with a first section of PM fibre 4a, and the second PZT stretcher 2b is wound with a second section of PM fibre 4b. The first 4a and second 4b fibre sections are then connected such that their polarization axes are orthogonal. As explained above, light propagating in one polarization state through the first fibre section 4a is coupled into the second fibre section 4b in which it propagates in an orthogonal state to that of the first fibre section 4a and compensates for differential changes and delay introduced by the first fibre section 4a.

Either of the two methods of PM fibre section connection detailed for the embodiment of Figure 1 can be used.

To maintain the output polarization in the same axis as the input relative to the polarization axes of the PM fibre, a short PM fibre pigtail 12 is spliced at 90 degree rotation to the output end 16 of the second fibre section 4b.

Referring to Figure 3, an alternative embodiment of the invention is arranged such that it has a return path.

A length of single mode (SM) optical fibre 6 is wound around a PZT 2. An optical transmitting device 8 is arranged at a first end of the optical fibre 6 such that light may be launched into the first end of the fibre 6 through an input port of the optical transmitting device 8 and light may be output from the first end of the fibre 6 through an output port of the optical transmitting device 8.

A suitable optical transmitting device 8 may be an optical circulator, a polarization beam splitter or an optical coupler. Other suitable optical transmitting devices will be apparent to the skilled reader.

A polarization rotator mirror 10 is arranged at a second end of the SM fibre 6 such that light output from the second end of the fibre 6 is reflected back into the second end of the fibre 6 with a polarisation angle which is orthogonal to that of the light output from the second end of the fibre 6. Thus, return light is reflected back in the orthogonal polarisation state by the polarisation rotator mirror 10.

By way of example, the polarisation rotator mirror 10 may be a Faraday Rotator Mirror. However, suitable alternatives will be obvious to the skilled reader.

Light is launched into the SM fibre 6 in a particular polarization with the aid of the optical transmitting device 8. The light propagates through the SM fibre 6 wound around the PZT 2 until it is reflected back in the orthogonal polarization by the mirror 10. The return light is coupled into the output port of the optical transmitting deice 8. Similarly to above, because the light has propagated in orthogonal axes, the round trip polarization and propagation delay are insensitive to the changes in birefringence of the fibre.

The skilled reader will appreciate that, in this embodiment, the two orthogonally connected sections of optical fibre are, in fact, realised from the same physical piece of fibre 6, with the polarization rotating mirror 10 being used to excite orthogonal polarization axes in transmission and reflection.

It is noted that the arrangement of Figure 3 uses SM fibre 6. In this case, the output polarization is stable regardless of the input polarization, and is not sensitive to stretching or temperature change. Such a configuration can also be inserted into PM systems if the output polarization from the SM fibre is aligned to the PM fibre principal axis. The invention, therefore, allows the use of SM fibre based stretchers in PM fibre systems, for example, without exciting polarization rotations.

It will be appreciated that the arrangement can also use PM fibre on the stretcher. This provides no performance advantage to the use of SM fibre, but may ease alignment into an external PM fibre system.

The invention may be applied in a range of applications that use fibre stretchers and benefit from the use of PM fibre. These applications may include, but are in no way limited to: PM/SM fibre-based line-length correction The use of PM fibre components in line-length correction schemes for highly accurate reference signal distribution is becoming very attractive.

Polarization fluctuations coupled with the polarization mode dispersion (differential group delay) of the fibre distribution network will cause fluctuations in the phase of the delivered signal. While PM fibre in the line length correction unit may be attractive for reasons of compatibility with other components in the reference signal generation/distribution units (in order to closely maintain the transmitted state of polarization), its use will lead to the differential birefringence changes which in turn lead to uncorrected errors in the path length (round-trip time) stabilisation. This invention will minimize such errors.

PM/SM fibre-based scanning/tracking interferometric sensors Fibre stretchers can be used in scanning or tracking interferometer-based sensors or polarimetric measurement-based sensors. Scanning/tracking interferometer-based sensors using Fibre Bragg Gratings (FBG5) for example have been demonstrated, while interest has also been shown in sensors made from FBGs in PM fibre. This invention will permit the merging of these techniques while minimising errors due to birefringence changes. Scanning versions of already demonstrated polarimetric temperature and strain sensors might be similarly achievable.

There are two main approaches described above for implementing the optical fibre stretcher, but they share the same concept, and thus both include a piezo-electric transducer; optical fibre around the transducer, with an optical signal passing through the fibre between an input and an output of the device, with an orthogonal polarization axis change half way between the input and output.

The general concept is not limited to optical devices comprising a fiber stretcher. Instead, the invention extends to the concept of compensating optical fibre by using matched lengths orthogonal fibre sections.

This general concept is illustrated in Figure 4, which shows an optical device according to an embodiment of the invention.

First 20 and second 22 sections of optical fibre of substantially equal length are arranged such that an optical signal may be passed through the optical fibre between an input IN and an output OUT of the device. Half way between the input IN and the output OUT there is provided a polarisation rotator device 24. The polarisation rotator device couples the light propagating in one polarization state through the first fibre section 20 into the second fibre section 22 so that it propagates in an orthogonal state to that of the first fibre section 20. In this way, there is provided an orthogonal polarization axis change half way between the input and output.

It will be appreciated that the inclusion of a polarisation rotator device 24 to provide an orthogonal polarization axis change is not essential. For example, in a similar fashion to the apparatus of Figure 1, the fibre sections may instead be formed from PM fibre and spliced together such that the polarization axes of the two fibre sections are orthogonal to each other.

The invention reduces the retardation rate, therefore reducing the total polarization state change (because the polarisation state will not have moved far around the circle on the sphere representing the polarisation state). In this way, there is provided a device and method for compensating differential birefringence changes in optical fibre. This approach can for example be used for temperature compensation of the fibre characteristics.

Although particular embodiments of the invention have been described above, various modifications will be apparent to those skilled in the art.

Claims

CLAIMS

1. An optical delay device comprising: first and second sections of optical fibre of substantially equal length arranged such that an optical signal may pass through the optical fibre between an input and an output of the device and there is an orthogonal polarization axis change half way between the input and output.

2. The device of claim 1, further comprising a stretcher for stretching the optical fibre.

3. The device of claim 2, wherein the stretcher comprises a piezo-electric transducer and the first and second sections of optical fibre are wound around the piezo-electric transducer.

4. The device of claim 2, wherein the stretcher comprises a first and second piezo-electric transducers having substantially the same transducer response characteristics, either intrinsically or through the use of electrical drive equalisation, and wherein substantially equal lengths of the first and second sections of optical fibre are wound around the first and second piezo-electric transducers, respectively.

5. The device of any preceding claim, wherein the first and second sections of optical fibre are formed from the same optical fibre.

6. The device of any preceding claim, further comprising a polarisation maintaining fibre pigtail connected to the output of the device and arranged to rotate the polarisation of light incident to the pigtail by 90 .

7. The device of any preceding claim wherein the first and second optical fibre sections are connected together.

8. The device of claim 7 wherein the first and second optical fibre sections are connected together by splicing the sections together.

9. The device of claim 7 wherein the first and second optical fibre sections are connected together via a polarization rotator.

10. The device of claim 5, further comprising: an optical transmitting device arranged at a first end of the optical fibre such that an optical signal may be launched into the first end of the optical fibre via an input port of the optical transmitting device and an optical signal may be output from the first end of the optical fibre via an output port of the optical transmitting device; and a polarization rotator mirror arranged at a second end of the optical fibre such that an optical signal output from the second end of the optical fibre is reflected back into the second end of the fibre with a polarisation angle which is substantially orthogonal to that of the signal output from the second end of the fibre.

11. The device of claim 10, wherein the polarization rotator mirror is a Faraday rotator mirror.

12. The device of claim 10 or 11, wherein the optical transmitting device is an optical circulator.

13. The device of claim 10 or 11, wherein the optical transmitting device is an optical beam splitter.

14. The device of claim 10 or 11, wherein the optical transmitting device is an optical coupler.

15. The device of any preceding claim, wherein the optical fibre is polarisation maintaining fibre.

16. The device of any of claims I to 14, wherein the optical fibre is single mode fibre.

17. An optical system comprising an optical delay device according to any preceding claim.

18. A method for compensating differential birefringence changes in optical fibre, the method comprising the step of passing an optical signal through first and second sections of optical fibre of substantially equal length between an input and an output, wherein there is an orthogonal polarization axis change half way between the input and output.

19. The method of claim 18 further comprising the step of stretching the optical fibre.

20. The method of claim 19, wherein a first piezo-electric transducer is arranged to undertake the step of stretching the optical fibre and the first and second sections of optical fibre are wound around the first piezo-electric transducer.

21. The method of any of claims 18 to 20, wherein the first and second sections of optical fibre are formed from the same optical fibre.

22. The method of any of claims 18 to 21, wherein the first and second optical fibre sections are connected together.

23. The method of any of claims 18 to 22, wherein the optical fibre is polarisation maintaining fibre.

24. The method of any of claims 18 to 22, wherein the optical fibre is single mode fibre.