In an optical communication system, if a transmission speed reaches as high a speed as 40 Gb/s or more, a light pulse width of a transmission signal becomes as narrow as several picoseconds. Thus, waveform distortion accompanying slight wavelength dispersion (Chromatic Dispersion) or polarization-mode dispersion (Polarization-Mode Dispersion) of an optical fiber considerably deteriorates transmission characteristics. In addition, as is well known, a dispersion value of a transmission fiber varies with time along with a change in temperature or environment. This slight change influences transmission characteristics (for example, see NAKAZAWA Masataka and HIROOKA Toshihiko, “Recent Progress and Future Prospects for High-Speed Optical Transmission Technology Using an Ultrashort Optical Pulse Train”, IEICE Trans., Vol. J89-B, No. 11, pp. 2067-2081).
FIG. 11 illustrates a light receiving unit of a wavelength division multiplexing (Wavelength Division Multiplexing: WDM) transmission system using a chromatic dispersion compensation or polarization-mode dispersion compensation technique. In FIG. 11, a light receiving unit 100 divides WDM light subjected to simultaneous amplification in an optical preamplifier 101 on a wavelength basis by use of a separator 102. Light signals CH1, CH2, . . . , CHn of different wavelengths output from the separator 102 are supplied and input to receiving modules 103_1, 103_2, . . . , 103—n. On optical paths of the receiving modules 103_1, 103_2, . . . , 103—n, functional components 111 such as a tunable dispersion compensator (Tunable Dispersion Compensator: TDC) and a polarization-mode dispersion compensator (Polarization-Mode Dispersion Compensator: PMDC) are provided. The individual functional components 111 perform appropriate dispersion compensation on received light.
In the case of adopting the functional components 111 such as the TDC and the PMDC as above, if a power level of received light reduces due to a light loss in each functional component 111, a bit error rate (Bit Error Rate: BER) increases in a receiver 112 and a regenerator 113. To suppress the increase in BER, the receiving modules 103_1, 103_2, . . . , 103—n corresponding to each wavelength need to add an optical amplifier 114 at its previous stage together with the aforementioned functional components 111 and let the amplifier to function to compensate for the light loss of the functional components 111. However, the adoption of the optical amplifier 114 might cause the following problem: a waveform of received light is deteriorated due to noise light such as amplified spontaneous emission (Amplified Spontaneous Emission: ASM) generated at the time of amplifying a light signal with the optical amplifier 114.
Here, a difference between a WDM optical amplifier for simultaneously amplifying multiple wavelengths and a single-waveform optical amplifier is described. In the WDM transmission system, many WDM optical amplifiers for simultaneously amplifying multiple wavelengths (for example, the optical amplifier 101 in FIG. 11) are provided on an optical path through which WDM light propagates, in addition to a single-waveform optical amplifier disposed at a previous stage to the receiver corresponding to each wavelength as above. As illustrated in FIGS. 12A to 12C, noise light such as ASE generated in the WDM optical amplifiers (see in the FIG. 12A) passes through the separator 102 of the light receiving unit 100. Thus, only noise components in a band of each light signal transmit the separator 102 and sent to the receiver 112 corresponding to each signal wavelength, while noise components outside the band is blocked by the separator 102 (see in the FIG. 12B). Thus, noise light generated in the WDM optical amplifier has less effect on reception characteristics of light signals of different characteristics, and the effect is often negligible.
On the other hand, the single-wavelength optical amplifier 114 is provided on each optical path for propagating a light signal of a corresponding wavelength divided by the separator 102. Therefore, noise light generated in the optical amplifier 114 at a wide wavelength band directly enters the receiver 112 (see in the FIG. 12C). This reduces a rate of power of a light signal of single wavelength to the total power of noise light, which causes an increase in BER.
One of the known conventional techniques for suppressing deterioration of reception characteristics due to noise light generated in a single-wavelength optical amplifier provides, for example, an optical filter 115 on an optical path between the optical amplifier 114 and the receiver 112 as illustrated in FIG. 13 to filter out light outside a band of a received light signal by use of the optical filer 115 (for example, see Japanese Laid-open Patent Publication No. 8-321805 and Japanese Laid-open Patent Publication No. 2004-179799).
Further, another problem of a light receiving device applied to the above WDM transmission system is a transmission error caused by rapid change in power level of received light. To elaborate, if a transmission speed reaches 40 Gb/s or more, a light pulse width of a transmission signal becomes as narrow as several picoseconds (ps). A receiver designed for such high-speed light signals is originally insufficient in transmission margin (margin) and in addition, is more likely to cause a transmission error if such situations that a detection point for a reception level cannot be followed well and a reception power exceeds an allowable level occur at the same time.
The rapid change in power level of a received light would easily occur due to the following factors: a connector is inserted or removed due to human error and an optical fiber is exposed to any external stimulus (for example, a stress accompanying a touch on the optical fiber or a change in wired environment). In addition, as is also known, in the case where the number of wavelengths of WDM light rapidly changes as well as in the case where a power of a received light changes, a phenomenon that a power level of a residual light signal changes occurs, which causes a transmission error (for example, see Japanese Laid-open Patent Publication No. 2006-295113). It is also considered that the rapid change in the number of wavelengths easily occurs due to dynamic re-setup of an optical transmission path, insertion/removal of a connector caused by human error, and disconnection of an optical fiber.
In order to suppress occurrences of transmission errors accompanying such a rapid change in power level of received light, it is effective to apply high-speed automatic level control (Automatic Level Control: ALC) to an optical amplifier(s) in the light receiving device, for example. The ALC is generally performed such that an output light power of an optical amplifier is monitored by a photodetector, and feedback control is executed on a drive state of the optical amplifier (for example, an output power of an excitation light source) until the monitor value reaches a target output light power (for example, see Japanese Laid-open Patent Publication No. 8-248455).
Incidentally, the single-waveform optical amplifier disposed at a previous stage to each receiver for a corresponding wavelength of the WDM light should be prepared in a number corresponding to the number of wavelengths of WDM light. The configuration or control method of the individual optical amplifiers is desirably designed to be common to all wavelengths in a wide wavelengths of WDM light, not varying depending on a wavelength of an input light signal from the viewpoint of cost saving. In connection with the purpose for meeting such a demand, a light loss varies among the aforementioned functional components such as the TDC and the PMDC. In addition, under actual conditions, it is inevitable that performance varies among receivers ready for high-speed light signals that propagate at as high a speed as 40 Gb/s or more. Therefore, how to uniformly design optical amplifiers to enable compensation for these variations is important. However, only the use of the above conventional techniques in combination causes the following problems and can hardly meet the above demand.
In the case of inserting an optical filter for filtering out light outside a band of received light to an optical path between an optical amplifier and a receiver to suppress deterioration of reception characteristics due to noise light generated in the single-wavelength optical amplifier, a rate of power of a light signal of single wavelength to the total power of noise light increases, with the result that an effect of improving the reception characteristics is large. However, an insertion loss of the optical filter and variations thereof are added, which further complicates the design of the single-wavelength optical amplifier. Similar to the single-wavelength optical amplifier, the optical filter should be prepared in a number corresponding to the number of wavelengths of WDM light. A general-purpose variable optical filter costs high and influences a cost of the entire system. Even though the effect of improving reception characteristics is large, there is a real problem that the optical filter would not be applied easily.
In the case of applying high-speed automatic level control (ALC) to the single-wavelength optical amplifier to suppress occurrences of transmission errors due to rapid change in power level of received light, the ALC needs to compensate for not only a dynamic change in power of received light but a difference in power of received light due to static factors from the inside/outside of a receiving module. The compensation brings about a large change in gain of the optical amplifier. The static factors from the outside of the receiving module include a change in output level of an optical preamplifier for simultaneously amplifying WDM light at a previous stage to a separator and variations in insertion loss of the separator. On the other hand, the static factors from the inside of the receiving module include variations in target value of a level of optical input to each receiver (corresponding to a target control level of ALC in the single-wavelength optical amplifier), which accompany variations in light loss between the functional components such as the TDC or the PMDC or variations in performance between the receivers.
It is known that an amount of generated noise light and wavelength characteristics thereof largely change if a gain of the optical amplifier changes. This also means that if a gain varies among plural single-wavelength optical amplifiers disposed in a number corresponding to the number of wavelengths of WDM light, the optical amplifiers differ in generation status of noise light. FIG. 14 illustrates gain spectra of an Erbium-doped fiber amplifier (Erbium-Doped Fiber Amplifier: EDFA) in accordance with an inverted distribution ratio of Erbium ions, for example. As can be understood from FIG. 14, in this EDFA, an amount of generated noise light is relatively large on a short-wavelength side in a high-gain (high inverted distribution ratio) status while an amount of generated noise light is relatively large on a long-wavelength side in a low-gain (low inverted distribution ratio) status.
If a light loss of the functional components such as the TDC or the PMDC is large or a target value of a level of optical input to the receiver is set high, a high gain is required of the single-wavelength optical amplifier. In the above example of the EDFA, an amount of generated noise light becomes relatively large on a short-wavelength side. If a light signal of a short wavelength is amplified with the optical amplifier in this status, a rate of power of signal light to the total power of noise light in output light of the optical amplifier (input light in the receiver) is reduced. On the other hand, if a light loss of the functional components is small or a target value of a level of optical input to the receiver is set low, a gain required of the single-wavelength optical amplifier is low. In the above example of the EDFA, an amount of generated noise light becomes relatively large on a long-wavelength side. If a light signal of a long wavelength is amplified with the optical amplifier in this status, a rate of power of signal light to the total power of noise light in output light of the optical amplifier is reduced.
In other words, a rapid change in power level of received light can be compensated for by applying high-speed ALC to the optical amplifier. As for how a gain change resulting from the ALC influences a reception characteristic of a light signal of a corresponding wavelength in WDM light, it is necessary to comprehensively consider a difference in generation status of noise light between the wavelengths, which is caused by variations in light loss among the functional components such as the TDC or the PMDC and variations in performance among the receivers. Realistically, this brings about a problem of hindering uniform design for the single-wavelength optical amplifier.