GB2370109A

GB2370109A - Optical wavelength meter

Info

Publication number: GB2370109A
Application number: GB0017405A
Authority: GB
Inventors: Tom Parker; Mahmoud Farhadiroushan
Original assignee: Sensornet Ltd
Current assignee: Sensornet Ltd
Priority date: 2000-07-14
Filing date: 2000-07-14
Publication date: 2002-06-19
Also published as: GB0017405D0

Abstract

Non-linear, stimulated, Brillouin scattering in an optical fibre (111) is used to accurately determine the wavelength of light. The Brillouin shift is inversely proportional to the optical wavelength, hence its determination yields the wavelength. Light from a test source (101) is split (at 109) into two optical waves that counter-propagate through the fibre (111). Light in one direction is modulated (at 113) with a known frequency difference from the test source. The modulated light interacts with the unmodulated light travelling in the opposite direction, and the light leaving the optical fibre is detected (at 117). The modulation frequency is varied, and when the modulation frequency equals the Brillouin shift frequency, the interaction will be maximised. The modulation frequency that gives the maximum Brlllouin signal equals the Brillouin shift and so yields the wavelength of the test source (101). Reference sources (102) of several wavelengths may be connected by a switch (105) for calibration.

Description

Wavelength Meter The present invention relates to an apparatus for the measurement of the wavelength of optical waves and, in particular, for the highly accurate determination of optical wavelengths.

There is a requirement in the field of optical communications for optically pure sources of light for wavelength division multiplexing. As the number of multiplexed channels increase down a single fibre, it is important to obtain the wavelength measurement with increasing accuracy so that the source spectra do not overlap. The accurate wavelength measurement of these sources is essential.

The current measurement of wavelength to high accuracy uses expensive and often large interferometers. A recent advance uses spontaneous Brillouin scattering for wavelength measurement1. This promises a cheaper method for measuring wavelength to high accuracy, and could be used in a portable instrument. The spontaneous Brillouin system, however, requires the extremely accurate calibration of one of its components: the Fabry-Perot (FP) interferometer. This calibration is lengthy, needs to be completed frequently and is difficult to achieve to the accuracy required unless a FP of exemplary quality is used.

The present invention uses the non-linear, stimulated, Brillouin scattering within an optical fibre to accurately determine the wavelength of light. The Brillouin shift is inversely proportional to the optical wavelength, hence its determination yields the optical wavelength.

When light propagates through an optical fibre, a small amount is backscattered with a frequency that differs from that of the source light by an amount known as the Brillouin shift. In this process, the Brillouin light is said to be spontaneously generated, and the power level is very low. This is the technique used in the prior art.

If, however, in addition to the source light, light with a frequency that differs from that of the source light by the Brillouin shift is sent into the fibre from the other direction (such that the new light and the source light counter-propagate) then the Brillouin light is greatly amplified. This is the stimulated Brillouin scattering used in the present invention. Here the Brillouin light will typically have a large power, but only when the counter-propagating light is of the correct frequency. This means that the Brillouin power is far greater at resonance than off resonance, giving an excellent means for determining the resonance position, and hence the Brillouin shift.

If, then, the frequency difference between the source and shifted light is known, and this difference can be varied, then by monitoring the power of the counterpropagating light, the Brillouin shift is the frequency difference that produces the greatest detected signal. In the present invention, light with a controlled, known frequency difference is generated by modulating a portion of the source light at an accurately known frequency. This converts some of the light to new frequencies that are shifted from the source light by an amount equal to the modulation frequency, hence producing the required shifted light. A modulation frequency with the required accuracy can be supplied by a tuneable frequency synthesiser. The Brillouin shift can then be determined directly by measuring the frequency applied to the modulator that produces the maximum detected signal. The Brillouin shift is inversely proportional to the source wavelength, hence determining the Brillouin shift yields the source wavelength.

In the present invention, it is preferable to calibrate the Brillouin shift of the test source wavelength by measuring the Brillouin shift of a reference source (of known wavelength) in addition to measuring that of the test source. Any difference in the Brillouin shift between the two sources is then due to a wavelength difference (that can thus be obtained). Higher accuracy may be obtained by calibrating the fibre by measuring the Brillouin shifts of sources with a range of wavelengths. This procedure measures, and so allows correction for, the small variations in the change in the Brillouin shift from its expected inverse proportional relationship with wavelength due to chromatic dispersion. The present invention may perform this calibration by using a tuneable source, or a range of lasers with various, known wavelengths. Higher accuracy still may be obtained by introducing an extra pulsing means into the system to pulse the light that propagates against the modulated light. This yields a measurement of dispersion at all points along the fibre. This extra data allows better calibration as the Brillouin signal grows non-linearly such that some regions of the fibre, with some dispersion, contribute more to the signal than others with a different dispersion.

Hence, by varying the modulation frequency that produces the amplified shifted light and monitoring the detected power, the Brillouin shift can be accurately determined and from this the source wavelength can be measured. Using the same technique with a different laser, or lasers, with known wavelength allows the fibre to be calibrated.

This present invention may be used as a preferred method for obtaining the Brillouin shift as a necessary step to obtaining extremely high accuracy wavelength measurement as described in the prior art. It is found that the present invention yields a more robust, quicker, easier and more accurate method for determining the wavelength from the Brillouin shift than that described in the prior art. This is because the stimulated Brillouin peaks have a high power, there is a non-linear gain mechanism that only produces a large signal when the Brillouin frequency is found and there is no need for calibrating an interferometer to determine the Brillouin frequency.

According to the present invention there is provided an apparatus for measuring the optical wavelength of a source from measurements of the Brillouin frequency shift of light from that source, which apparatus comprises at least one optical fibre which provides a scattering medium, a modulating means to generate modulation side bands to stimulate the non-linear interaction and a means for converting the optical signal into an electrical signal which may comprise a single detector or an array of detectors.

Light from a test source, the wavelength of which is to be measured, is transmitted down the optical fibre to a splitting means that splits the light into two optical waves that counter-propagate along a length of optical fibre. A proportion of the light is modulated using a modulation means that generates side bands with a known frequency difference from the test source. The modulated light travelling in onedirection and the unmodulated light travelling in the other will interact. When the modulation frequency equals the Brillouin shift frequency (which is dependent on the fibre and laser light wavelength) then the interaction will be maximised and the optical power travelling in the direction of the modulated light will be maximised.

Determining the modulation frequency that yields the maximum detected signal from light travelling in the direction of the modulated light yields the Brillouin shift and hence the wavelength of the light emitted by the source.

The fibre is calibrated by measuring the Brillouin shift of a laser with a known wavelength. More accurate calibrations are achieved by calibrating with a number of lasers of known wavelength, or a tuneable laser, to determine the average dispersion of the fibre. By pulsing the light that propagates against the modulated light, and using a number of lasers of known wavelength or a tuneable laser, a still more accurate calibration is made by measuring the distribution of the dispersion values along the fibre.

An embodiment of this invention uses the technique disclosed in the prior art to determine the wavelength to extremely high accuracy. Here the present invention is used to measure the wavelength of a test source from its Brillouin shift. A reference source is used (with a known wavelength) to calibrate the measured wavelength of the test source. Extremely high accuracy is obtained by comparing the test wavelength with the known wavelength of the reference signal using an interferometer. The difference in the interference orders between the two signals is determined from the measurement of the Brillouin shift, as described here and the extremely high accuracy is obtained from the interferometer reading.

The optical fibre may be singlemode, or multimode and may have tailored doping levels, or many different doping layers, to provide an enhanced interaction.

The optical fibre may be polarisation maintaining to provide an enhanced interaction.

The optical fibre may comprise lengths of different fibres to provide multiple Brillouin peaks.

The splitting ratio of the splitting means, length of fibre before and after the modulator, modulator extinction ratio, modulator loss, modulator transmission and loss of the fibre may be chosen to optimise the contrast between the backscattered power when the modulation frequency is at (or near to) and far from the Brillouin shift frequency.

The splitting ratio of the splitting means, modulator extinction ratio and modulator transmission may be controlled to optimise the contrast between the detected optical power when the modulation frequency is at, or near to, and far from the Brillouin shift frequency.

The test and reference source light may be attenuated or amplified to optimise the contrast between the optical power when the modulation frequency is at, or near to, and far from the Brillouin shift frequency.

The modulator may be operated around its zero transmission point rather than the more usual quadrature point such that the carrier (unmodulated) light is suppressed and such that the transmission of the modulator can be controlled.

An interferometer, or other optical filter means, may be used to view the detected signal to better resolve the Brillouin light from the elastically scattered, or transmitted, light at the source wavelength.

The reference source may be a single mode or multimode coherent source with known wavelength and it may be a gas laser or a solid-state laser.

The returning optical signals, may be amplified and/or attenuated, using an optical amplifier means or an optical fibre attenuator means.

Optical isolators may be used to prevent instability of the laser sources.

Optical isolators may be used in the loop so that only light travelling around the loop in one direction will be measured.

Reference lasers of different, known, wavelengths, or a tuneable laser, may be used to determine the chromatic dispersion of the fibre for accurate calibration.

A pulsing means, such as a second modulator, may be used to pulse the light travelling in one or both directions to determine the local chromatic dispersion at all points along the fibre for accurate calibration.

The wavelength of the source light may be determined by measuring the frequency of the Brillouin shift and comparing this to the Brillouin shift produced with a laser of a known wavelength.

The wavelength determined from the Brillouin shift may be used to determine the number of free spectral ranges separating a test source and a reference source viewed in an interferometer scan, and so allow extremely high accuracy wavelength measurement.

The modulator may be an electro-optic modulator, an integrated optic, fibre optic or bulk device with high frequency response.

The optical signals are converted to electrical signals and are fed into a processor and recorded in the desired form.

A processor controls the operation of the components of the system.

The invention is described with reference to the accompanying drawing in which :- Figure 1 is a diagram of an embodiment of the present invention, in which the wavelength of a test source is measured from its Brillouin shift and a reference source is used for calibration. Replacing the test and/or reference lasers with a tuneable laser, or a number of lasers, of known, different, wavelengths tested one after the other, allows the system to be calibrated by determining the effective average chromatic dispersion of the fibre.

Figure 2 is a diagram of an embodiment of the present invention, in which the wavelength of the test source is determined using the Brillouin shift and an interferometer.

Figure 3 is a diagram of an embodiment of the present invention, in which accurate calibration is obtained by using a pulsing means to pulse the unmodulated light such that the local chromatic dispersion at all points in the fibre is determined. The embodiment measures the wavelength of the test source using the Brillouin shift.

Figure 4 is a diagram of an embodiment of the present invention, in which accurate calibration is obtained by using a pulsing means to pulse the unmodulated light such that the local chromatic dispersion at all points in the fibre is determined. The embodiment measures the wavelength of the test source using the Brillouin shift and an interferometer.

Referring to Figure 1, a test source (101) and a reference source (102) are each connected by optical fibres to isolators (103 & 104), which prevent instabilities in the lasers, and then to an optical switch (105) which selects which source is connected to the rest of the instrument. Either, or both, of the lasers (101 & 102) may be a tuneable laser, or a number of lasers, of known, different, wavelengths tested one after the other, to calibrate the system by determining the effective average chromatic dispersion of the fibre. An optical amplifier and/or attenuator unit (106) may be used to control the amount of optical power entering the system. The optical power may be monitored by using an optical tap (107), which may be an optical fibre coupler, and a photodetector (108) in order to optimise the optical amplification and/or attenuation of the amplifier and/or attenuation unit and/or the splitting ratio of the splitting means

(109) and/or the modulator (113) transmission. The photodetector (108) may have a variable gain. The transmitted light passes to a splitting means (109), such as an optical fibre coupler, which splits the light into two paths. The splitting ratio of the splitting means (109) may be controlled to adjust the proportion of light directed towards the two optical paths or the splitting ratio may be fixed. Light in one path may be directed through a polarisation controller (110), which helps to control the non-linear interaction, and is sent into a length of fibre (111). Light in the other path may also pass through a polarisation controller (112), which alters the polarisation of the light to match that which is optimally required by the modulator (113). The modulator may have a controlled transmission. The modulator modulates the light.

Light from the modulator then may pass through an isolator (114) that ensures that the unmodulated light transmitted down the first path does not return to the detector.

Light from the isolator may pass through another polarisation controller (115), again to help control the non-linear interaction, and passes into the length of fibre (111) where the stimulated Brillouin interaction takes place. The light then returns to the splitting means (109) and may be sent to an attenuator and/or amplifier unit (116) and then is detected by a photodetector (117) that may have a variable gain. A processing unit (118) analyses the signal from the signal photodetector (117) and controls the modulator's (113) frequency, and controls the optical switch (105). The processing unit may also analyse the signal from the tap photodetector (108), and/or control the gain of the tap detector, and/or control the gain of the signal photodetector (117), and/or control the modulator's (113) extinction, and/or transmission, and/or control the splitting ratio of the splitting means (109), and/or control the amount of amplification and/or attenuation from either or both of the amplification and/or attenuation units (106 & 116), and/or control the settings of any of the polarisation controllers (110, 112 & 115).

In use, the Brillouin shift of the test and reference sources (101 & 102) are each determined using stimulated Brillouin scattering in the length of optical fibre (111).

Comparison of the Brillouin shifts obtained from the two sources (101 & 102) gives the difference in the wavelength between the test and reference sources (101 & 102).

If the wavelengths of the lasers are known, then the same method is used to calibrate the fibre by determining the effective average dispersion of the fibre.

Referring to Figure 2, components labelled (201-218), respectively, are named the same, and perform the same purpose, as those labelled (101-118), respectively, in Figure 1. Following the splitting means (209), light in one path may pass through a polarisation controller (212), which performs the same function as component (112) in Figure 1. The modulator (213) either modulates the light to determine the Brillouin shift, or to calibrate the interferometer (222) (which may be a Fabry-Perot interferometer), or it transmits unmodulated light for measuring the source frequency directly in the interferometer. Light from the isolator (214) passes to a switch (220), which acts with another switch (219) to selects one of two paths. Either the light travels thorough a short length of fibre (221) to the switch (219) so that there is no interaction between it and the counter-propagating light, and to reduce losses, or it travels through a longer length of fibre (211), where there is an interaction with the counter-propagating light, and may pass through one or two polarisation controllers (210 & 215) which may help enhance the stimulated Brillouin interaction. In this second case, the light returns to the splitting means (209) and then to the switch (219).

The light exiting the switch (219) may be sent to an attenuator'and/or amplifier unit (216) before passing to the interferometer (222) for spectral analysis. The light selected by the interferometer is then detected by a photodetector (217) that may have a variable gain. A processing unit (218), in addition to fulfilling the tasks of component (118) in Figure 1, controls the optical switches (219 & 220) and may also control the frequency scan rate and/or centring and/or frequency scan range of the interferometer (222).

In use, the Brillouin shift of the test and reference sources (201 & 202) are each determined using stimulated Brillouin scattering in the long length of optical fibre (211). The interferometer (222) is here used to separate the Brillouin light from the elastically scattered light and from the unmodulated light. The short length of fibre (221) is used to allow calibration of the interferometer (222) using modulated light from either the test or reference source (201 & 202). The short length of fibre (221) is also used to compare the relative frequencies of the test and reference sources (201 & 202) as seen in the interferometer (222) scan.

Referring to Figure 3, components labelled 301-318, respectively, are named the same, and perform the same function, as those labelled 101-118, respectively, in Figure 1. Following the splitting means (309), light in one path may be directed through a polarisation controller (323), which may optimise the polarisation state of the light for the pulsing means (324), and is passed to a pulsing means (324), such as a optical modulator. The pulsing means (324) pulses the light when the embodiment is calibrating the fibre (311), by measuring the distributed dispersion, and passes through continuous light when the embodiment is measuring the wavelength of the test or reference sources (301 & 302). The light exiting the pulsing means (324) may be passed through a polarisation controller (310), which helps to control the nonlinear interaction within the fibre length (311) and is passed to a splitting means (325), such as an optical fibre coupler. Light from the splitting means is passed to the optical fibre length (311). The counter-propagating, modulated light, passes to the splitting means (325) and may be sent to an attenuator and/or amplifier unit (316) and then is detected by a photodetector (317) that may have a variable gain. A processing unit (318) performs the same function as component (118) in Figure 1 and additionally controls the pulsing of the pulsing means (324) and may also control the splitting ratio of the splitting means (325), and/or control the settings of the polarisation controller (323).

In use, the Brillouin shift of the test and reference sources (301 & 302) are each determined using stimulated Brillouin scattering in the length of optical fibre (311).

Comparison of the Brillouin shifts obtained from the two lasers (301 & 302) gives the difference in the wavelength between the test and reference sources (301 & 302) If the wavelengths of the lasers are known, then the same method is used to calibrate the fibre by determining the chromatic dispersion distribution along the length of fibre (311). This distributed measurement is achieved by recording the amplitude of the signal detected by the signal photodetector (317) versus the time-of-flight down the optical fibre (311) of the pulses produced by the pulsing means (324).

Referring to Figure 4, components labelled (401-418), respectively, are named the same, and perform the same function, as those labelled (101-118), respectively, in Figure 1. The components labelled (419-422), respectively, are named the same, and perform the same function, as those labelled (219-222), respectively, in Figure 2. The components labelled (423-425), respectively, are named the same, and perform the same function, as those labelled (323-325), respectively, in Figure 3.

In use, the Brillouin shift of the test and reference sources (401 & 402) are each determined using stimulated Brillouin scattering in the length of optical fibre (411).

The interferometer (422) is here used to separate the Brillouin light from the elastically scattered light and from the unmodulated light. The short length of fibre (421) is used to allow calibration of the interferometer (422) using modulated light from either the test or reference source (401 & 402). The short length of fibre (421) is also used to compare the relative frequencies of the test and reference sources (401 & 402) as seen in the interferometer (422) scan. Comparison of the Brillouin shifts obtained from the two sources (401 & 402) gives the difference in the wavelength between the test and reference sources (401 & 402). If the wavelengths of the lasers are known, then the same method is used to calibrate the fibre by determining the chromatic dispersion distribution along the length of fibre (411)

Claims

Claims 1. Apparatus for measuring the optical wavelength of a source from measurements of the inelastic scattering frequency shift of light from that source, which apparatus comprises at least one optical fibre which provides a scattering medium and a modulating means to generate modulation side bands to stimulate the non-linear interaction.
2. Apparatus as claimed in claim 1 in which the optical fibre is singlemode, or multimode and inelastic Brillouin scattering is utilised.
3 Apparatus as claimed in claims 2 in which the optical fibre has tailored doping levels to enhance the interaction.
4. Apparatus as claimed in claims 1-3 in which the optical fibre has a plurality of different doping layers to provide an enhanced interaction.
5. Apparatus as claimed in claims 1-4 in which the optical fibre is polarisation maintaining to provide an enhanced interaction.
6 Apparatus as claimed in claims 1-5 in which the optical fibre comprises lengths of different fibres to provide multiple Brillouin peaks.
7. Apparatus as claimed in claims 1-6 in which the loss and/or length of the fibre are/is chosen to optimise the contrast between the backscattered power when the modulation frequency is at (or near to) the Brillouin shift frequency and far from the Brillouin shift frequency, and to provide an adequate signal level for efficient detection.
8. Apparatus as claimed in claim 1 in which a splitting means is provided to allow the modulated and unmodulated light to counter-propagate in a loop.
9. Apparatus as claimed in claims 1 and 8 in which one or more optical isolators are provided in the loop so that only light travelling around the loop in one direction is detected
10. Apparatus as claimed in claim 8 in which the splitting ratio of the splitting means is tailored to optimise the contrast between the backscattered power when the modulation frequency is at (or near to) the Brillouin shift frequency and far from the Brillouin shift frequency, and to provide an adequate signal level for efficient detection.
11. Apparatus as claimed in claim 1 in which the modulating means is an electrooptic modulator, an integrated optic, fibre optic or bulk device with high frequency response.
12. Apparatus as claimed in claims 1 and 11 in which the modulator extinction ratio and/or loss are/is chosen to optimise the contrast between the backscattered power when the modulation frequency is at (or near to) the Brillouin shift frequency and far from the Brillouin shift frequency, and to provide an adequate signal level for efficient detection.
13. Apparatus as claimed in claims 1 and claims 11 to 12 in which there are means to operate the modulating means around its zero transmission point rather than the more usual quadrature point such that the carrier (unmodulated) light is suppressed and such that the transmission of the modulator is controlled.
14. Apparatus as claimed in claim 1 and claims 8 to 13 in which the splitting ratio of the splitting means and/or modulator extinction ratio and/or modulator loss are/is controlled to optimise the contrast between the detected optical power when the modulation frequency is at, or near to, the Brillouin shift frequency and far from the Brillouin shift frequency, and to provide an adequate signal level for efficient detection.
15. Apparatus as claimed in claim 1 in which there are means to attenuate or amplify the test and/or reference source light to optimise the contrast between the optical power when the modulation frequency is at, or near to, the Brillouin shift frequency and far from the Brillouin shift frequency, and to provide an adequate signal level for efficient detection.
16. Apparatus as claimed in claim 1 in which an interferometer, or other optical filter means, is provided to view the detected signal to better resolve the Brillouin light from the elastically scattered, or transmitted, light at the source wavelength.
17. Apparatus as claimed in claim 1 in which there is provided a reference source of light of known wavelength and the wavelength of the test source is compared with the wavelength of the reference source.
18. Apparatus as claimed in claim 1 and claim 17 in which the reference source is a single mode or multimode coherent source with known wavelength and is a gas laser or a solid-state laser.
19. Apparatus as claimed in claims 1 and claims 17 to 18 in which reference lasers of different, known, wavelengths, or a tuneable laser, are/is provided to determine the chromatic dispersion of the fibre and for accurate calibration.
20. Apparatus as claimed in claim 1, claim 8 and claims 17 to 19 in which a pulsing means, such as a second modulator, is provided to pulse the light travelling in one or both directions to determine the local chromatic dispersion at all points along the fibre and for accurate calibration.
21. Apparatus as claimed in claim 1 in which means are provided to amplify or attenuate the returning optical signal to provide efficient detection.
22. Apparatus as claimed in claim 1 in which one or more optical isolators are provided to prevent instability of the laser sources.
23 Apparatus as claimed in claim 1 in which a detection means is provided to convert the optical signals to electrical signals which are fed into a processor and recorded in the desired form.
24. Apparatus as claimed in claim 1 in which a processor is provided that controls the operation of the components of the system.
25. A method for measuring the optical wavelength of a source from measurements of the Brillouin frequency shift of light from that source, which method comprises interacting light from the source with light in the modulation side bands generated by modulating some of the source light to stimulate a non-linear interaction, with the interaction taking place in at least one optical fibre which provides a scattering medium.
26. A method as claimed in claim 25 in which the Brillouin shift from multiple peaks are used, the multiple peaks obtained by using lengths of different fibres.
27. A method as claimed in claims 25 in which the modulating means is operated around its zero transmission point rather than the more usual quadrature point such that the carrier (unmodulated) light is suppressed and such that the transmission of the modulator is controlled.
28. A method as claimed in claim 25 in which the splitting ratio of the splitting means and/or modulator extinction ratio and/or modulator loss are/is controlled to optimise the contrast between the detected optical power when the modulation frequency is at, or near to, the Brillouin shift frequency and far from the Brillouin shift frequency, and to provide an adequate signal level for efficient detection.
29. A method as claimed in claim 25 in which the test and reference source light is attenuated or amplified to optimise the contrast between the optical power when the modulation frequency is at, or near to, the Brillouin shift frequency and far from the Brillouin shift frequency, and to provide an adequate signal level for efficient detection.
30. A method as claimed in claim 25 in which an interferometer, or other optical filter means, is used to view the detected signal to better resolve the Brillouin light from the elastically scattered, or transmitted, light at the source wavelength.
31. A method as claimed in claim 25 in which there is provided a reference source of light of known wavelength and the wavelength of the test source is compared with the wavelength of the reference source.
32. A method as claimed in claim 25 in which reference lasers of different, known, wavelengths, or a tuneable laser, are/is used to determine the chromatic dispersion of the fibre for accurate calibration.
33. A method as claimed in claim 25 in which a pulsing means, such as a second modulator, is used to pulse the light travelling in one or both directions to determine the local chromatic dispersion at all points along the fibre for accurate calibration.
34. A method as claimed in claim 25 in which means are used to amplify or attenuate the returning optical signal to provide efficient detection.
35. A method as claimed in claim 25 in which a detection means is used to convert the optical signals to electrical signals which are fed into a processor and recorded in the desired form.
36. A method as claimed in claim 25 in which a processor is used to control the operation of the components of the system.
37. A method as claimed in claim 25 in which the wavelength determined from the Brillouin shift is used to determine the number of free spectral ranges separating a test source and a reference source viewed in an interferometer scan, and so allow extremely high accuracy wavelength measurement.