US 20060027737 A1
A system and method for monitoring all the characteristic parameters of a DWDM communication system is implemented with two variants. Firstly, this is achieved by means of a specific grating spectrometer displaying a high 5 resolution and a high-speed sampling of the measured values, and secondly by the application of an opto-electronic cross correlator as a purely electronic solution. The grating spectrometer is expediently a particular system in a mixed array according to Ebert and Fastie, wherein the light to be measured passes four times through the grating in a specific manner. The opto-10 electronic cross correlator can mix the working light with a reference light tunable in terms of its frequency to form an electrical low-frequency signal that is analyzed in a high-impedance operation.
20. A system for monitoring the performance of a DWDM multi-wavelength system comprising:
means for converting a portion of an optical signal from the DWDM multi-wavelength system at a particular wavelength to an electrical signal;
wherein the converting means comprises
an optical unit having the optical signal as an input and the particular wavelength portion of the optical signal as an output; and
means for processing the electrical signal to determine the performance of the DWDM multi-wavelength system at the particular wavelength and for controlling the converting means so that each particular wavelength of the DWDM multi-wavelength system is processed.
21. The system as recited in
22. The system as recited in
an optical unit having the optical signal as an input and the particular wavelength portion as an output; and
a photodetector having the particular wavelength portion as an input and the electrical signal as an output.
23. The system as recited in
24. The system as recited in
25. The system as recited in
a movable grating having a wavelength range that covers a measurement range for the DWDM multi-wavelength system;
an imaging element for reflecting the optical signal; and
a beam deflection system mounted such that the optical signal incident on the imaging element and the optical signal exiting from the imaging element are essentially symmetrical, the movement of the movable grating selecting the particular wavelength portion, and the optical signal being subjected to multiple passes between the movable grating and the image element.
26. The system as recited in
27. The system as recited in
28. The system as recited in
29. The system as recited in
a high precision light source for generating a focused beam;
a reflecting surface rigidly coupled to the movable grating upon which the focused beam impinges; and
a position sensor for receiving the focused beam reflected from the reflecting surface to determine the angular position.
30. The system as recited in
31. The system as recited in
an incremental scale that influences the intensity of the reflected focused beam as a function of the point on the incremental scale upon which the reflected focused beam impinges; and
a detector for detecting an intensity of light from the incremental scale, the intensity being a measure of the angular position.
32. The system as recited in
means for mixing the optical signal with a tunable reference optical signal to produce a combined optical signal; and
a photodetector for converting the combined optical signal to the electrical signal.
This application is a continuation of U.S. application Ser. No. 09/806,704, filed Jun. 27, 2001, which National Stage entry of International Application No. PCT/EP99/07340, filed Oct. 5, 1999, which claims priority from German Application No. DE 19845701.4 filed Oct. 6, 1998, all of which are incorporated by reference herein in their entirety.
The present invention relates to optical monitoring, and more particularly to a system and method of monitoring the performance of dense wavelength division multiplexing optical communication services.
In densely packed WDM systems (dense WDM, DWDM) messages are communicated by light signals at different wave lengths via a single fiber only. Each wave length is the carrier of an information signal. All channels are within the wave length range from presently roughly 1,520 nm to 1,565 nm. The inter-channel separation amounts to a few nanometers or some hundreds of picometers, respectively. For standardization of these telecommunication systems, the international ITU-T Working Group has recommended the wave lengths (corresponding to the channels) to be used with an inter-channel separation of 100 GHz (≈0.8 nm) as standard. The ongoing development of these DWDM systems aims at the extension of the utilizable wave length range up to 1,610 nm for example.
Systems for the continuous monitoring of all characteristic parameters with the possibility of signal regeneration or improvement are required at many sites of this communication system. The most important parameters include the wave length and the capacity of all channels, the monitoring of the line width and the wave length drift of the lasers as well as the signal-to-noise ratio in each communication channel. Typical specification requirements for monitoring are:
Fundamentally different methods are suitable for monitoring purposes, which are employed in conventional optical spectrum analyzers.
Tunable narrow-band filters are used for wave length selection in the filtering technique. Acousto-optical filters (e.g. those produced by Wandel & Goltermann) or piezo-electrically controlled micro filters (e.g. those from the Queensgate company) or tunable fiber Bragg gratings (e.g. those from ElectroPhotonics Corp.) are applied, which can be tuned directly via an electrical parameter.
The filtering technique is not only restricted to the optical filtering operation but it may also be performed at the electrical signal level after a preceding conversion into electronic signals. With electronic filtering, the optical signal is mixed with an optical reference signal in a non-linear optical component while the differential frequencies are analyzed on an electronic spectral analyzer (Hewlett Packard Co.).
Another variant is the grating monochromator technique wherein either the grating is rotated or the spatially resolved signal spectrum is sensed by means of a single photodiode, or the grating is stationary and a scanning deflection mirror is provided in front of the exit slit of the monochromator, or a mobile reflecting element varies the angle of incidence of the radiation on the grating (e.g. Photonetics company), or a stationary grating is used in combination with a line of photodiodes as detector unit (e.g. Yokogawa company).
In the interferometric technique, the spectrum is obtained from the 10 detector signal of a Michelson interferometer with variable optical paths, with application of the Fourier transform (e.g. Hewlett Packard company).
None of the aforementioned conventional systems is suitable to satisfy the high demands made on a monitoring module for a DWDM system in terms of resolution, measuring accuracy, ASE measurement and dynamic ratio, at the same time and in a suitable manner and to satisfy moreover the demands in terms of short measuring intervals, longevity and low space requirements as well as low-cost realization.
What is desired is a suitable measuring system that satisfies the demands on a DWDM monitoring system in terms of resolution, measuring accuracy, ASE measurement and dynamic ratio, short measuring intervals, ‘longevity and low space requirements as well as a low-cost production.
In accordance with the present invention this object is achieved with a system permitting two variants. This aim is firstly reached in accordance with the invention with a narrow-band tunable band-pass filter in the form of a specific grating spectrometer permitting a high resolution and a high-speed sampling of the measured values according to Variant 1, and secondly the solution according to the present invention is presented in a Variant 2 as a purely electronic solution using an opto-electronic cross correlator.
The objects, advantages and other novel features of the present invention are apparent from the following detailed description when read in conjunction with the appended claims and attached drawing.
An example of a system based on a multiple spectrograph is illustrated in
The problem to achieve a high resolution is solved, in accordance with
The angular position of the dispersing grating, that is decisive for assigning the measuring wave length, is determined by means of an auxiliary means, the position sensor, according to
For a general grating the fundamental equation
In the definition according to Ebert the basic equation (I) applies. The optical path of the beams is so designed that the most symmetrical optical path possible will be available with respect to the concave mirror. As in this case, too, the angles of incidence or exit are almost equal, the angular dispersion comes also under a similar magnitude order as in the definition according to Fastie. Due to the multiple passages—here quadruplicate, for instance—of the radiation through the dispersive element the overall dispersion and hence the resolution of the device is quadruplicated, too. On account of the utilization of mirror areas n symmetrical positions, the symmetrical optical path relative to the imaging concave mirror results in an extensive compensation of the imaging errors, particularly of astigmatism that leads to a substantial deterioration of resolution.
With a dielectric optical preliminary filter as band-pass element in the multiple optical paths any light of wavelengths beyond the DWDM range is suppressed. In such a case the filter is then passed only by the DWDM range, for instance, with a width of roughly 100 nm.
The detection of the entire spectrum is performed by a single radiation detector while the adjustment of the wave length to be detected is realized by rotating the grating about its vertical axis, which is performed both by motor drive means and by the configuration as spring-mass array with torsion bars, capable of oscillating.
Furthermore, the position of the grating is detected by a secondary laser with a very high precision. The focused beam of the secondary laser is directed onto a reflecting surface rigidly connected to the grating while the reflected beam is supplied to a position sensor including an incremental scale.
The influence which the incremental scale takes on the laser intensity is detected by the joining detector 46 and made accessible for analysis.
The Variant 2 according to
The two light signals are defined by the following two relationships:
It is apparent that the last term defines a current variable in time, that is dependent on the amplitudes of both radiations and on the difference of the light frequencies. When both frequencies are approaching each other a low-frequency signal is created with the maximum amplitude Imax=2 EMER. Moreover, the direction of polarization of both light sources is equally considered. In order to eliminate this dependence, it is possible, on the one hand, to render the reference light laser or the source of working light statistically variable in terms of its direction of polarization, or, on the other hand, to make two orthogonally polarized beams available, for instance, as reference light sources while the optical mixture is performed in two separate detectors with a subsequent logic operation in the signal processor. For another solution, for example, it is possible to switch the reference laser over in a time-sequential manner in the polarization plane while the subsequent measurements in succession are subjected to a logic operation in the signal processor.
By employment of the wave length calibrator 29 for wave length assignment the provision of wave length references is made possible in both variants. To this end known arrays such as absorption cells are suitable for this purpose, which contain gases displaying characteristic lines of absorption in the required wave length range. When such a cell is inserted into the optical path, for instance in the spectrometer, and when the system is exposed to wide-band illumination characteristic signal developments are created which permit a precise assignment of the wave lengths. Another possibility is the measurement of the reference laser wave length by means of an additional interferometer array. In such a system, one part of the light from the tunable reference light laser is passed on to an interferometer that is provided with a supplementary highly precise light source and in which the interference signals vary in time, which are generated when the reference light laser is tuned, serve to assign the wave length present in that moment.
The combination of the working light and the reference light can be realized in different manners.