Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Reexamination Certificate
2001-11-27
2004-07-20
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
C356S073100, C356S303000, C356S330000, C398S034000, C398S070000, C398S089000, C398S107000, C367S064000, C359S323000, C359S245000, C359S251000
Reexamination Certificate
active
06765670
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates to a spectrometer module, a monitor module, a monitoring unit and a monitoring system for use in monitoring an optical network.
The invention further relates to a spectrometer device, for spectrometry purposes.
BACKGROUND ART
The use of optical signals for purposes as carrier of information and the like is currently increasing rapidly. Consequently, there is a current need for developing new methods and products for dealing with this kind of information.
One field of technology is fibre optical communication systems, that are used for transferring large amount of information over large distances. Such system comprise a plurality of different network elements, being interconnected to form a communication network. However, as these networks are becoming larger and larger, and including more and more sophisticated network components, the risk for network faults is increasing rapidly. Consequently, there is a need for surveillance systems, keeping track of the optical signals being transmitted in the network, and reporting when an error has occurred.
Furthermore, the communication speed in the optical networks, such as WDM networks are constantly increasing, resulting in the need of less capacity demanding communication protocols. Thereby, existing protocols such as SDH/SONET, having built-in monitoring functions need to be replaced or at least supplemented. This has lead to the development of systems monitoring data in the optical domain.
One such surveillance or monitoring system is described in the patent document U.S. Pat. No. 6,104,492, in which an apparatus and a method of operating an optical signal monitor for WDM networks is disclosed. Here, a temperature tuned filter is used for generating an optical spectrum monitor for multi-wave signals. The tuneable filter is used for selecting the wavelength that is to be detected and monitored.
However, this technique is rather expensive and also relatively slow as it is based on temperature effects. Other known means of measuring the power spectrum is by use of a diffraction grating, which spatially distributes the wavelengths to one or many detectors. The drawbacks of such technology, however, are that expensive detector arrays is required, that the diffraction efficiency might be rather poor, large demands for careful mechanical alignment and as a result less mechanical robustness.
The proposed means of monitoring the signal quality in WDM systems are usually limited to monitoring of the signal power as a function of wavelength, by use of some spectrometer technology, such as e.g. fibre Bragg gratings, such as for example described in the patent document WO 0102885. This spectrometer technology, however, suffers from the same drawbacks as the ones listed above. An alternative monitoring method is further described in “Quality monitoring of optical signals influenced by chromatic dispersion in a transmission fibre using averaged Q-factor evaluation” by I. Shake, H. Takahara, K. Uchiyama, and Y. Yamabayashi, IEEE Photonics Technology Letters, vol 13, pp385-387 (2001). This method, which uses an asynchronous sampling system, is however rather complex and expensive. Yet another proposed method for signal monitoring is described in the document “Optical Performance Monitoring in Reconfigurable WDM Optical Networks Using Subcarrier Multiplexing” by G. Rossi, T. E. Dimmick, and D. J. Blumenthal, Journal of Lightwave Technology, vol 18, pp. 1639-1648 (2000). Also this approach, which is based on a sub-carrier modulation of the signal, is complex and expensive.
SUMMARY OF THE INVENTION
Consequently, an object of this invention is to achieve a spectrometer module, a monitor module, a monitoring unit and a monitoring system for use in monitoring an optical network, overcoming the above-mentioned drawbacks with the prior art.
A further object is to achieve a way of measuring properties of an optical signal, such as power, state of polarisation and degree of polarisation versus wavelength in an efficient way.
Yet a further object is to achieve a spectrometry device, overcoming the drawbacks with the prior art.
Further objects of this invention are evident from the following description of the invention.
The above and other objects are wholly or partly achieved by a spectrometer module comprising:
an input, for receiving an incoming optical signal,
a variable differential group delay (DGD) element, for applying a variable birefringence retardation to said incoming optical signal,
a detector unit for detecting the power of a defined state of polarisation of a signal exiting said variable DGD element.
By using a variable DGD element in a spectrometer module, it is possible to perform measurements on an incoming optical light signal, for detecting parameters such as polarization state and degree of polarisation as a function of the wavelength of the incoming signal.
In accordance with a first embodiment of the invention, said variable DGD element is implemented spatially. Preferably, said variable DGD element comprises a plurality of laterally spaced sub-elements having different optical lengths, whereby different parts of the incoming optical signal is arranged to be transmitted through different sub-elements of the DGD element. Suitably, said incoming optical signal is arranged to have essentially the same width as said variable DGD element, thereby covering each of said laterally spaced sub elements. Thereby, the DGD element is optimally used. Moreover, said variable DGD element suitably comprises a plane incidence surface, being essentially orthogonal to the optical signal path, and a stepped exit surface.
Alternatively, said variable DGD element may comprise a birefringent element having a decreasing thickness in a direction being transverse to said incoming optical signal. Thereby different parts of the optical signal beam experiences different optical paths. Preferably, said detector unit comprises an array of detectors, and a lens that is placed between said variable DGD element and said detector unit, whereby said detector array is arranged in the Fourier focal plane of said lens. Furthermore, said incoming optical signal is suitably arranged to be slightly divergent.
According to a second embodiment of the invention, said variable DGD element is implemented temporally, by use of an electrical control signal that changes the DGD with time. Preferably, said variable DGD element is comprised by a birefringent system, being essentially sandwiched between a first and a second reflective element, whereby said incoming optical signal is arranged to be reflected between said reflective elements one or more times before outputted from said birefringent system. Suitably, said first and second reflective elements are constituted by a first and second mirror element, respectively. Alternatively, said first and second reflective elements are constituted by a first and a second retroreflector, respectively. By using the above reflections systems, the same birefringent element may by utilized many times.
In accordance with preferred embodiments, a polarizer is arranged between said variable DGD element and said detector unit, said polarizer not being aligned relative to the birefringence eigenaxes of said DGD element. Moreover, said incoming optical signal preferably has a polarisation so as to inject light in both birefringence eigenaxes of said variable DGD element.
Preferably, said variable DGD element is manufactured from an electro-optical birefringent material. Furthermore, said variable DGD element is suitably arranged between a first and a second electrode, said electrodes being arranged to generate an electric field over said variable DGD element. Thereby, a variable DGD element is achieved, being easy to control by means of an electronic control unit or the like. Alternatively, said variable DGD element may be connectable with an acusto-optic transducer, for achieving the above controllability.
Furthermore, said detector unit is preferably connectable with an electronic processing dev
Karlsson Magnus
Olsson Bengt-Erik
Sunnerud Henrik
Brown Khaled
Burns Doane Swecker & Mathis L.L.P.
Font Frank G.
Pro Forma Alfa
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