1) Field of the Invention
The present invention relates to a transmission characteristics evaluation system, and more particularly to a transmission characteristics evaluation system which is suitable, for example, when used in evaluating the characteristics of an optical communication transmission system.
2) Description of the Related Art
It is known in the art that, together with the communication performance of light that is emitted by a laser diode (LD) used in an optical communication system, the signal conversion performances of an electric/optical (E/O) conversion module for converting an electric signal into an optical signal and an optical/electric (O/E) conversion module for converting an optical signal into an electric signal are dependent on the dispersion characteristics of an optical fiber constituting a transmission path.
Particularly, the dispersion tolerance, which is a transmission characteristic of the optical fiber constituting the aforementioned transmission path relative to the magnitude of the amount of dispersion, is a critical index value in evaluating the performance quality of an optical transmitting apparatus provided with an LD or an E/O module or an optical receiving apparatus provided with an O/E module described above.
FIG. 13 is a block diagram illustrating a conventional measurement system for measuring the dispersion tolerance. In this measurement system 200 shown in FIG. 13, an optical transmitting apparatus 210 and an optical receiving apparatus 220 are connected with a dummy fiber 230 which is assumed to be an optical fiber constituting the transmission path in a real optical communication system.
Here, the optical transmitting apparatus 210 is provided with a pattern pulse generator (PPG) 204 together with an E/O module 203 comprising a light source 201 and a modulator 202, whereby an electric signal of a specific pulse pattern generated by the pattern pulse generator 204 of the E/O module 203 is converted into an optical pulse signal by the modulator 202 and is transmitted to the optical receiving apparatus 220 via the aforementioned dummy fiber 230.
Further, the optical receiving apparatus 220 is provided with a receiving amplifier 221, an O/E module 222, and a BER tester (Bit Error Rate tester) 223, whereby the optical pulse signal transmitted via the dummy fiber 230 (and amplified by the receiving amplifier 221) is converted into an electric signal by the O/E module 222, and is output to the BER tester 223 as a received electric signal.
The BER tester 223 performs error detection on the received electric signal relative to the electric signal of the pulse pattern generated by the pulse pattern generator 204 of the optical transmitting apparatus 210. When the dispersion value (fiber length) of the dummy fiber 230 used here is changed, the error rate detected by the BER tester 223 changes. By using this change in the error rate relative to the change in the fiber length, the dispersion tolerance of the optical element, the optical module, and the measurement system can be measured.
In other words, with this error detection value obtained from the BER tester 223, the communication quality of the E/O module 210 and the O/E module 220 relative to the dispersion value of the dummy fiber 230 can be evaluated. In addition, by increasing or decreasing the dispersion value of this dummy fiber 230, the communication quality of the E/O module 210 and the O/E module 220 can be measured as a dispersion tolerance.
Here, in measuring the error detection value by increasing or decreasing the dispersion value of the dummy fiber 230 in the above-described measurement system 200, it is necessary to connect a dummy fiber 230 having a different length newly to the optical transmitting apparatus 210 and the optical receiving apparatus 220 each time the error detection value is measured with the dummy fiber 230.
Recently, there is known a measurement technique that uses a variable dispersion compensator of a Fiber Bragg Grating (FBG) type (optical network simulator manufactured by JDS-Uniphese (US), TeraXion (Canada), or SPIRENT Co., Ltd.) instead of this dummy fiber. These techniques eliminate the need for replacing the dummy fiber 230 in order to increase or decrease the dispersion value of the transmission path, such as in the above-described measurement system 200.
On the other hand, in recent years, in evaluating the transmission characteristics of an optical transmitting apparatus provided with an LD or an E/O module or an optical receiving apparatus provided with an O/E module described above, the insertion loss gradient tolerance, which is the amount of change in the insertion loss of the optical fiber constituting the aforementioned transmission path in accordance with the wavelength of the transmitted light, is considered as one of the index values.
As a technique for measuring this insertion loss gradient tolerance, a variable wavelength bandpass filter module having a bandwidth about ten times larger than the original bandwidth for use is interposed between the optical transmitting apparatus 210 and the optical receiving apparatus 220 such as described above and, by shifting the center wavelength of this filter module, a gradient is given to the distribution of the insertion loss relative to the wavelength of the used transmittance band, and the signal error rate is measured with the BER tester 223.
Namely, the insertion loss gradient tolerance is measured by measuring the signal error rate that changes in accordance with the degree of gradient of the distribution of the insertion loss relative to the wavelength of the used transmittance band.
Here, as a technique related to the present invention, a technique disclosed in the following patent document 1 is known.                [Patent Document 1] Japanese Patent Application Laid-open No. 2002-258207        
However, in the case of measuring an index value for evaluating the performance quality of an optical module such as an optical transmitting apparatus provided with an LD and an E/O module or an optical receiving apparatus provided with an O/E module, a technique is demanded having a high measurement precision with reduced number of working steps for the measurement while widening the measurement range as compared with such a conventional technique.
For example, the aforementioned conventional technique for measuring the dispersion tolerance raises a problem in that, in order to perform evaluation of multiple channels, i.e. in order to evaluate the dispersion tolerance of each wavelength in transmitting a wavelength multiplex light, a variable dispersion compensator interposed between the optical transmitting apparatus and the optical receiving apparatus must be prepared separately in accordance with the wavelength of the object of measurement. Moreover, the dispersion value cannot be changed arbitrarily from a positive dispersion value to a negative dispersion value, so that the dispersion tolerance for evaluating the transmission performance of the optical module cannot be measured in a sufficient range.
Also, the aforementioned conventional technique for the measurement of insertion loss gradient tolerance raises a problem in that the insertion loss gradient tolerance cannot be measured commonly in the aforementioned measurement system for measuring the dispersion tolerance. Moreover, when the center wavelength of the bandpass filter module is shifted, a change occurs not only in the value of insertion loss but also in the dispersion value within the band. This makes it difficult to evaluate correctly whether the error detection value detected by the BER tester 223 is due to the change in the dispersion value or due to the change in the value of insertion loss.