The invention relates to an optical spectrum analyzer, particularly, but not exclusively, an optical spectrum analyser for use with a multi-channel optical system.
In this specification a multi-channel optical system means a multi-channel optical telecommunications system, a multi-channel cable television system, an in-fibre Bragg grating sensor system including a plurality of Bragg gratings, or any other optical system generating a plurality of optical signals of different wavelengths.
In multi-channel optical telecommunication systems, 16 or more optical signals travel in each single-mode optical fibre and in cable television system 4 or more optical signals travel in each fibre. Each optical signal is generated by a different laser light source and is of a different wavelength. Each wavelength corresponds to a channel and information is encoded on each channel. There is a requirement that the 16 different wavelengths should be monitored, in order to detect a failure of any laser or a change in the wavelength of any channel. The wavelengths of the telecommunications channels are set by an international standard for channels, known as the ITU grid.
Current technology uses an optical spectrum analyser (OSA) having an optical tuning element in the form of a bulk optic diffraction grating. Light exiting from an optical fibre is focused onto the diffraction grating and the light reflected by the diffraction grating is focused onto an optical detector. As the diffraction grating is rotated, the wavelength of the light which it reflects to the detector gradually changes, and the spectrum of the light is scanned and measured.
There are several disadvantages to using this type of OSA The diffraction grating is subject to mechanical shock and damage, and so the OSA is limited in its ruggedness and tolerance of mechanical vibration. The bulk optic diffraction grating is limited in its accuracy and resolution by its mechanical movement. The position of the diffraction grating is likely to drift with time and may be affected by mechanical backlash, therefore the OSA must be regularly calibrated. The light must be extracted from the optical fibre for measurement, requiring accurate focusing of the light onto the diffraction grating and the optical detector. All of these factors result in a measuring instrument which is limited in its performance, is expensive, and is not well suited for field monitoring of optical systems.
According to one aspect of the present invention there is provided an optical spectrum analyser comprising: a length of optical fibre for receiving an input optical signal; a tuneable optical filter in optical communication with the input fibre, the tuneable optical filter including a first in-fibre Bragg grating inscribed in a first section of fibre, and means operable to apply a variable axial force to the first section of fibre, to thereby time the peak wavelength of the grating over a desired wavelength range; and optical detection means operable to detect an optical signal selected by the tuneable optical filter.
The tuneable optical filter preferably includes first and second in-fibre Bragg gratings inscribed in first and second sections of fibre respectively, the spectra of the gratings having different peak wavelengths. The said means is preferably operable to apply a variable axial force to one or each of the first and second sections of fibre.
The optical spectrum analyser is preferably for use with a multi-channel optical system The channels are preferably substantially equally spaced In wavelength space.
Preferably the peak wavelengths of the gratings are tuneable over different wavelength ranges, the ranges preferably being of substantially the same spectral width The wavelength tuning ranges preferably substantially abut or overlap in wavelength space. Preferably, the combined wavelength tuning range of the two gratings extends from approximately 1530 nanometers to approximately 1560 nanometers. Alternatively, the combined wavelength tuning range of the two gratings may extend from approximately 1580 nanometers to approximately 1620 nanometers.
Preferably the full width half maximum spectral bandwidth of the or each grating is between 0.05 nanometers and 0.5 nanometers, and is most preferably between 0.05 nanometers and 0.2 nanometers. The side-lobe suppression ratio of the or each grating is preferably greater than xe2x88x9220 dB, and is most preferably greater than xe2x88x9230 dB.
The axial force is preferably strain.
The peak wavelength of each grating, when unstrained, is preferably less than the wavelengths of the optical channels present within the respective wavelength tuning ranges of the gratings.
An optical signal selected by the tuneable optical filter is preferably reflected by only one grating.
The optical spectrum analyser preferably further comprises an optical fibre signal routing means, for routing an input optical signal from the input fibre to the tuneable optical filter.
The optical fibre signal routing means is preferably a first optical fibre coupler, one leg on one side of the coupler being communicatively connected to the input fibre and one leg on the other side of the coupler being communicatively connected to the tuneable optical filter. An optical isolator is preferably provided between the input fibre and the one leg on one side of the coupler.
The first and second sections of fibre are preferably located within a grating length of optical fibre, the grating length of fibre being long compared to the lengths of said sections. The first and second sections of fibre, and hence the gratings, are preferably spatially separate within the grating length of fibre. Alternatively, the first and second sections of fibre may be the same section of fibre, the gratings being inscribed in the same section of fibre and thus being superimposed one upon the other.
The difference in the peak wavelengths of the gratings is preferably equal to the wavelength spacing of the optical channels multiplied by a numerical factor. The numerical factor is preferably equal to an integer plus a fraction of one such as one half or one third.
Preferably, the grating length of optical fibre is mounted on the means operable to apply a variable strain, to thereby enable a variable strain to be applied to the first and second sections of fibre, and hence to both gratings, at the same time.
The optical detection means is preferably communicatively connected to the second leg on the one side of the first coupler. The optical detection means preferably comprises a first photodetector.
The optical detection means may further comprise a second optical fibre coupler, one leg on one side of the second coupler being communicatively connected to the photodetector, and one leg on the second side of the second coupler being communicatively connected to the second leg on the one side of the first coupler. The optical detection means may further comprise a second photodetector communicatively connected to the second leg on the second side of the second coupler.
The optical detection means may further comprise a broadband, in-fibre optical filter communicatively connected between the first photodetector and the one leg on one side of the second coupler. The in-fibre optical filter is preferably a chirped in-fibre Bragg grating. The optical bandwidth of the chirped Bragg grating preferably substantially extends over the wavelength tuning range of one of the two gratings, such that the chirped Bragg grating reflects an optical signal reflected by the said one grating to the second photodetector and transmits an optical signal reflected by the other grating to the first photodetector.
Alternatively, the optical detection means may further comprise a broadband, in-fibre optical filter communicatively connected to the second leg on the one side of the second coupler, the optical filter reflecting an optical signal reflected by either grating to the second photodetector. Preferably, the reflectivity of the optical filter varies as a function of wavelength across the optical bandwidth of the optical filter. The reflectivity preferably varies linearly with wavelength across the optical bandwidth of the optical filter. The optical filter is preferably a chirped in-fibre Bragg grating. The optical filter may alternatively be an in-fibre sampled grating.
The optical spectrum analyser preferably further includes means operable to compare the output signals of the first and second photodetectors, to thereby determine the wavelength of the optical signal reflected from one of the gratings. The said means is preferably operable to calculate the ratio of the amplitude of the output signal of the first photodetector to the amplitude of the output signal of the second photodetector, the ratio being indicative of the wavelength of the signal reflected from one of the gratings.
Alternatively, the first and second sections of fibre may be provided in physically separate first and second grating lengths of optical fibre, the grating lengths of fibre being physically long compared to the said sections. Preferably, two optical detection means are provided, a first optical detection means being communicatively connected between one leg on the second side of the first coupler and the first grating length, and a second optical detection means being communicatively connected between the second leg on the second side of the first coupler and the second grating length
The detection means preferably each comprise a further optical coupler communicatively connected to the respective grating lengths, the respective legs on the first coupler and to a photodetector. Preferably, one leg on one side of the further optical coupler is communicatively connected to the respective one of the legs on the second side of the first coupler, the second leg on the one side of the further coupler is communicatively connected to the photodetector, and one leg on the second side of the further coupler is communicatively connected to the respective grating length.
Preferably, each of the grating lengths of fibre is mounted on a separate means operable to apply a variable strain to a respective one of the first and second sections of fibre. The said means are preferably operable to enable a variable strain to be applied to each of the first and second sections of fibre at either the same time or at different times.
The optical fibre signal routing means may alternatively comprise an optical fibre circulator.
The optical spectrum analyser preferably further comprises means operable to reduce the signal to noise ratio in the output signal of a or each photodetector. The said means preferably comprises phase-lock loop apparatus connected to the respective means for applying a variable strain and the said photodetector.
The tuneable optical filter may include more than two in-fibre Bragg gratings, each grating being inscribed in a respective section of fibre.
The means operable to apply a variable strain preferably comprises a spaced pair of mandrels, the part of the grating length of fibre including the section or sections of fibre including one or more gratings being mountable therebetween. The mandrels are preferably shaped to avoid sharply bending an optical fibre wound therearound, and are most preferably substantially cylindrical in shape. The mandrels are preferably fabricated from a material which minimises the forces acting between the mandrel and the fibre coating without abrading or chemically altering the fibre coating. The mandrels may be fabricated from a self-lubricating material, such as graphite.
Preferably, a continuous groove is provided around the outer surface of each mandrel, for receiving the parts of the grating length of fibre on either side of the section or sections of fibre including one or more gratings. The groove preferably extends for a plurality of turns around the mandrel, to enable the said lengths of fibre to complete a sufficient number of turns around the mandrel to be held in place on the mandrel by means of frictional forces.
Preferably, the mandrels are movably mounted on a mounting member, and are most preferably rotatably mounted on the mounting member. Alternatively, one mandrel is rotatably mounted on the mounting member and the other mandrel is fixedly mounted on the mounting member. One mandrel is preferably mounted on a motor means operable to rotate the said mandrel.
The means operable to apply a variable strain preferably further comprises a elongate member, in the form of a metal beam, mounted on one end of the said other mandrel and extending to a stop member provided on the mounting member, rotation of the one mandrel exerting a pulling force on the fibre mounted between the mandrels, thereby causing rotation of the other mandrel until the elongate member abuts the stop member, further rotation of the other mandrel thereby being prevented, such that a further rotation of the one mandrel causes strain to be applied to the said fibre and the elongate member.
The means operable to apply a variable strain is preferably constructed so as to minimise the effects of thermal expansion, and is most preferably a thermalised.
An electrical strain gauge may be provided on the elongate member, the strain gauge being operable to measure the strain applied to the elongate member, to thereby enable the amount of strain applied to the section or sections of fibre including one or more gratings, and hence the wavelength of the or each grating in the tuneable optical filter, to be inferred.
The optical spectrum analyser may alternatively or additionally comprise optical calibration apparatus for calibrating the peak, wavelength of the or each grating in the tuneable optical filter.
The optical calibration apparatus preferably comprises: an optical fibre coupler; a first section of fibre having a first reference Bragg grating inscribed therein, the said section of fibre being communicatively connectable to one leg on one side of the calibration coupler; and an optical source communicatively connected to one leg on the second side of the calibration coupler.
The peak wavelength of the first reference grating preferably falls within the wavelength tuning range of one of the gratings in the tuneable optical filter. Preferably, a further reference grating is provided in a further section of fibre for each further grating in the tuneable filter, the peak wavelength of each further reference grating falling within the wavelength tuning range of the corresponding grating. The further section or sections of fibre are preferably each communicatively connectable to the calibration coupler in place of the first section of fibre, each further reference grating thereby replacing the first reference grating.
The peak wavelength of the or each reference grating is preferably known at a specified temperature, from independent calibration. Alternatively, the or each grating may be a thermalised.
The optical source is preferably a light emitting diode. The optical output spectrum of the light emitting diode preferably includes the peak wavelength of the or each reference grating.
The optical calibration apparatus is preferably connectable to the optical spectrum analyser between the input length of fibre and the tuneable optical filter, to thereby provide an alternative input signal to the optical spectrum analyser. Preferably, the second leg on one side of the calibration coupler is communicatively connectable to the input length of fibre and the second leg on the second side of the calibration coupler is communicatively connectable to the one leg on one side of the first coupler.
The optical calibration apparatus may alternatively comprise: an optical fibre coupler; a first section of fibre having a first reference grating inscribed therein, the said section of fibre being communicatively connectable to one leg on one side of the calibration coupler; first and second photodetectors communicatively connected to the second leg on one side and one leg on the second said of the calibration coupler respectively; and means operable to compare the output signals of the photodetectors.
The optical spectrum of the first reference grating preferably falls within the wavelength tuning range of one of the gratings in the tuneable optical filter. Preferably, a further reference grating is provided in a further section of fibre for each further grating in the tuneable filter, the optical spectrum of each further reference grating falling within the wavelength tuning range of the corresponding grating. The further section or sections of fibre are preferably each communicatively connectable to the calibration coupler in place of the first section of fibre, each further reference grating thereby replacing the first reference grating.
Preferably, the reflectivity of the or each reference grating varies as a function of wavelength across its spectral bandwidth, such that the intensity of an optical signal reflected by a reference grating is dependent on the wavelength of the optical signal. The or each reference grating preferably has a plurality of reflectivity peaks included within its spectral profile. The or each reference grating is preferably a sampled grating. The or each reference grating may alternatively be a moirxc3xa9 grating. The or each reference grating may further alternatively comprise an array of uniform period Bragg gratings.
Alternatively, the or each reference grating may be a chirped Bragg grating.
The or each reference grating is preferably a thermalised.
The optical calibration apparatus is preferably connectable to the optical spectrum analyser in place of the or a photodetector within the optical detection means, the first photodetector detecting a part of an optical input signal reflected by the tuneable optical filter and the second photodetector detecting the said part of the input signal reflected by the tuneable optical filter and the reference grating.
Preferably, the outputs of the photodetectors are connected to the means operable to compare the output signals. The said means is preferably operable to calculate the ratio of the output signal of the first photodetector to that of the second photodetector, the ratio being indicative of the wavelength of the detected optical signal.
The optical calibration apparatus may further alternatively comprise: an in-fibre wavelength division multiplexing (WDM) device, one leg on one side of the WDM device being communicatively connectable to the second leg on the second side of the first coupler; a first section of fibre having two in-fibre Bragg gratings inscribed therein and communicatively connected to one leg on the second side of the WDM device; an optical fibre coupler, one leg on one side of the coupler being communicatively connected to the second leg on the one side of the WDM device; a broadband optical source communicatively connected to one leg on the second side of the calibration coupler; a second section of fibre having a reference grating inscribed therein and communicatively connected at one end to the second leg on the second side of the calibration coupler; and a photodetector communicatively connected to the other end of the second section of fibre.
Preferably the peak wavelengths of the gratings are separated by more than their individual wavelength tuning ranges. The peak wavelength of the first grating is preferably within the wavelength tuning range of the tuneable optical filter, and is most preferably within the 1540 nanometers to 1560 nanometers wavelength range. The peak wavelength of the second grating is preferably within the 1290 nanometers to 1310 nanometers wavelength range.
The optical source is preferably a broadband light emitting diode. The optical spectrum of the light emitting diode preferably includes the peak wavelength of the second grating.
The photodetector is preferably operable to detect an optical signal reflected by the second grating.
Preferably, the optical spectrum of the reference grating includes a plurality of passbands such that only wavelengths of light which correspond to the wavelengths of the passbands are transmitted to the photodetector. The reference grating preferably comprises a sampled grating. The reference grating may alternatively comprise a moirxc3xa9 grating. The reference grating may alternatively comprise a chirped Bragg grating. The reference grating is preferably a thermalised.
The first, second, calibration and further optical fibre couplers are preferably each 50:50 2xc3x972 optical fibre couplers. Preferably, the ends of any unconnected legs on any of the couplers are terminated with an angled cleave, to thereby substantially reduce any optical reflections from the said ends. Alternatively, the ends of the said unconnected legs are terminated in an index matching compound, to thereby substantially reduce any optical reflections from the said ends.
The axial force may alternatively be compression.
The optical signal selected by the tuneable optical filter may alternatively be transmitted by one grating.
According to a further aspect of the present invention there is provided a tuneable optical filter comprising: first and second in-fibre Bragg gratings inscribed in first and second sections of optical fibre respectively, the spectra of the gratings having different peak wavelengths, and means operable to apply a variable axial force to one or each of the first and second sections of fibre, to thereby the peak wavelength of the or each grating over a desired wavelength range, wherein the wavelength tuning range of the first grating is adjacent in wavelength space to the wavelength tuning range of the second grating, such that the combined tuning range of the gratings is greater than the tuning range of one grating.