Optical spectrum analyzer

Details Optical spectrum analyzer Optical spectrum analyzer

2002-01-24

2004-11-23

Gutierrez, Diego (Department: 2859)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C250S227180, C385S010000, C356S032000, C398S084000, C398S085000

Reexamination Certificate

active

06822217

ABSTRACT:

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,
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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 −20 dB, and is most preferably greater than −30 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 bandwi

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