Detection of waveform change due to chromatic dispersion

Abstract: Waveform degradation due to chromatic dispersion of an optical fiber is detected and compensated for. A waveform detector detects a change in waveform, based on the ratio between the powers of a plurality of frequency components or on peak or duty detection, and the frequency bandwidth of an equalizing amplifier or the chromatic dispersion of an optical fiber is controlled based on the result of the detection.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for detecting waveform degradation due to chromatic dispersion of an optical fiber in an optical transmission system. 2. Description of the Related Art In optical transmission systems, as transmission speeds increase, waveform degradation due to chromatic dispersion of optical fibers becomes a practical concern, and techniques for compensating for such waveform degradation become necessary. In the prior art, a compensating technique has been known that involves inserting a dispersion-compensating fiber having a dispersion value opposite in sign to the dispersion value of a transmission line. This prior art technique has had the following problems. The effect of optical fiber dispersion changes depending on temperature, transmission distance, characteristics of the fiber installed, etc., but, since practical systems do not have means for detecting the degree of dispersion-induced waveform degradation during system operation, it is difficult with the prior art to make optimum settings individually according to the system characteristics while the system is in service. Dispersion-compensating fiber has such shortcomings as high cost, large size, and high insertion loss. Further, controlling the frequency bandwidth of an equalizing amplifier circuit in an optical receiver in order to compensate for waveform degradation creates the following problems. In ultra-high speed regions of about 10 Gb/s, electronic circuits must be implemented as integrated circuits, but resistors and capacitors capable of being varied over a wide range are difficult to implement on an IC. Increasing the operating bandwidth is difficult because of the bandwidth limiting by the bandwidth control circuit itself. In EP-A-684 709 there is described an optical communications system using adjustable dispersion compensating fibers to compensate for dispersion in system fibers. The amount of dispersion introduced by the dispersion compensating fibers is varied, depending upon the amount of compensation required. A dispersion compensating unit optimises the dispersion compensation based upon an operating parameter such as eye margin or bit error rate (BER). SUMMARY OF THE INVENTION Accordingly, it is an of the present invention to provide a method and apparatus for detecting a waveform change caused by optical fiber chromatic dispersion. According to one aspect of the present invention, there is provided a method of detecting a signal waveform change caused by chromatic dispersion of an optical transmission line, comprising the steps of: converting an optical signal, received via the optical transmission line, into an electrical, signal; detecting signal powers of the electrical signal at a plurality of frequencies including a frequency substantially sensitive to the effect of the chromatic dispersion of the optical transmission line and a frequency substantially insensitive to the ef fect of the chromatic dispersion of the optical transmission line; and detecting the signal waveform change caused by the chromatic dispersion of the optical transmission line on the basis of the ratio between the signal powers at the plurality of frequencies. According to another aspect of the present invention, there is provided an apparatus for detecting a signal waveform change caused by chromatic dispersion of an optical transmission line,comprising: a photodetector for converting an optical signal, received via the optical transmission line, into an electrical signal; power detection means for detecting signal powers of the electrical signal at a plurality of frequencies including a frequency substantially sensitive to the effect of the chromatic dispersion of the optical transmission line and a frequency substantially insensitive to the effect of the chromatic dispersion of the optical transmission line; and a waveform change detection circuit for detecting the signal waveform change caused by the chromatic dispersion of the optical transmission line on the basis of the ratio between the signal powers at the plurality of frequencies. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagram showing the basic configuration of the present invention; Figure 2 is a diagram for explaining the relationship between a waveform change due to optical fiber chromatic dispersion and a frequency component ratio; Figure 3 is a diagram showing a first example of an apparatus for detecting a waveform change on the basis of the frequency component ratio, according to the present invention; and Figure 4 is a diagram showing a second example of the detection based on the frequency component ratio. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows the basic configuration of an apparatus, according to the present invention, for detecting a waveform change caused by chromatic dispersion, and for compensating for the waveform change thus detected. In Figure 1, an optical signal received from an optical transmitter 10 through an optical fiber 12 is converted by a photodetector 14 into an electrical signal, and equalized and amplified by an equalizing amplifier circuit 16. A waveform change detector 18 detects a waveform change caused by chromatic dispersion, as described in detail below, and compensates for the waveform change by controlling either the frequency characteristic of the equalizing amplifier 16 or the amount of chromatic dispersion of the optical fiber 12 in accordance with the result of the detection. As shown in Figure 2, a change occurs in the received waveform because of chromatic dispersion of an optical fiber, with the characteristic that when the waveform becomes a peaked waveform due to pulse compression, the higher frequency component increases (part (a)), and when the waveform becomes rounded due to pulse spreading, the higher frequency component decreases (part (b)). By detecting the power of the higher frequency component, the change in the waveform can be detected. At this time, the powers of at least two frequency components, a low-frequency component insensitive to the effect of chromatic dispersion and a high-frequency component sensitive to that effect, should be detected and the ratio between them should be calculated so that the waveform change can be distinguished from the change in the input optical power itself. In the present invention concerning the waveform change detection in the waveform change detector 18, the powers of the above-mentioned at least two frequency components are detected and the ratio between them is calculated. The low-frequency component insensitive to the effect of chromatic dispersion refers to the frequencies lower than 1 GHz, for example, when the bit rate is 40 GHz, and the high-frequency component sensitive to it refers to the frequencies between 30 GHz to 40 GHz, for example. Figure 3 shows one example of the waveform change detection apparatus according to the present invention. The output of the equalizing amplifier circuit 16 is supplied in parallel to the inputs of a plurality of band-pass filters 20 whose pass band center frequencies are f 1 , f 2 , ... f n , respectively; then, the powers of their outputs are detected by respective power detectors 22, and the ratio between them is calculated by a power ratio calculator 24. Figure 4 shows a second example of the waveform change detection apparatus according to the present invention. The output of the equalizing amplifier circuit 16 is supplied as an input to a band-pass filter 26 whose pass band center frequency is variable, and whose output power is detected by a power detector 28. The center frequency of the band-pass filter 2 6 is swept by a f requency sweeper 3 0 , and from the output of the power detector 28 representing powers detected at a plurality of frequencies, a power ratio calculator 32 calculates the power ratio between the plurality of frequency components.

Claims: A method of detecting a signal waveform change caused by chromatic dispersion of an optical transmission line (12), comprising the steps of: (a) converting (14) an optical signal, received via said optical transmission line, into an electrical signal; (b) detecting (18;22;28) signal powers of said electrical signal at a plurality of frequencies (f i , f 2 , ...f n ) including a frequency substantially sensitive to the effect of the chromatic dispersion of said optical transmission line and a frequency substantially insensitive to the effect of the chromatic dispersion of said optical transmission line; and (c) detecting (18; 24; 32) the signal waveform change caused by the chromatic dispersion of said optical transmission line on the basis of the ratio between the signal powers at said plurality of frequencies. A method according to claim 1, wherein said step (b) includes the'substeps of: (i) inputting said electrical signal in parallel to a plurality of band-pass filters (20) whose pass band center frequencies are respectively chosen equal to said plurality of frequencies; and (ii) detecting (22) respective output powers of said plurality of band-pass filters. A method according to claim 1, wherein said step (b) includes the substeps of: (i) inputting said electrical signal to a band-pass filter (26) whose pass band center frequency is variable; (ii) detecting (28) an output power of said band-pass filter; and (iii) sweeping (30) the center frequency of said band-pass filter over a range including said plurality of frequencies. An apparatus for detecting a signal waveform change caused by chromatic dispersion of an optical transmission line (12), comprising: a photodetector (14) for converting an optical signal, received via said optical transmission line, into an electrical signal; power detection means (18; 22; 28) for detecting signal powers of said electrical signal at a plurality of frequencies (f i , f 2 , ...f n ) including a frequency substantially sensitive to the effect of the chromatic dispersion of said optical transmission line and a frequency substantially insensitive to the effect of the chromatic dispersion of said optical transmission line; and a waveform change detection circuit (18; 24; 32) for detecting the signal waveform change caused by the chromatic dispersion of said optical transmission line on the basis of the ratio between the signal powers at said plurality of frequencies. An apparatus according to claim 4, wherein said power detection means includes: a plurality of band-pass filters (20) whose pass band center frequencies are respectively chosen equal to said plurality of frequencies, and to which said electrical signal is input in parallel; and a plurality of power detectors (22) for detecting respective output powers of said plurality of band-pass filters. An apparatus according to claim 4, wherein said power detection means includes: a band-pass filter (26) whose pass band center frequency is variable, and to which said electrical signal is input; a power detector (28) for detecting an output power of said band-pass filter; and a frequency sweeping circuit (30) for sweeping the center frequency of said band-pass filter (26) over a range including said plurality of frequencies.

Inventors: IHARA TAKESHI, OIKAWA YOICHI, IHARA, TAKESHI, OIKAWA, YOICHI, Ihara, Takeshi, c/o Fujitsu Limited, Oikawa, Yoichi, c/o Fujitsu H. Digital Techn. Ltd.
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1998
Main classification: H04B10
IPC codes: H04B10