This application is based upon and claims priority from Korean Patent Application No. 2001-85837, filed Dec. 27, 2001 in the Korean Intellectual Property Office (KIPO), the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical signal receiving apparatus for restoring an optical signal transmitted from an optical transmission system, to the original electric signal, and more particularly, to an optical signal receiving apparatus having an optimum receiving performance regardless of a change in the power of an input optical signal, and a method using the same.
2. Description of the Related Art
FIG. 1 is a block diagram schematically illustrating the structure of a conventional optical signal receiving apparatus. Referring to FIG. 1, the conventional apparatus includes a photodiode 10, an amplifier 12, a limiting amplifier 14, and a clock and data recovery unit 16.
Referring to FIG. 1, the photodiode 10 converts an optical signal INopt into an electric signal through photoelectric conversion. The amplitude of the converted electric signal is feeble and thus amplified by the amplifier 12. However, since the amplitude of the amplified electric signal depends on the power of the original optical signal INopt, the electric signal is not suitable for a signal input to the clock and data recovery unit 16 that requires a signal of a regular amplitude. Thus, the limiting amplifier 14 regularly maintains the amplitude of a signal amplified by the amplifier 12 regardless of a change in the power of an input optical signal INopt, and the clock and data recovery unit 16 restores a clock signal CLOCK and data DATA from a signal output from the limiting amplifier 14.
FIG. 2 is a graph of stochastic distributions of a signal with respect to space(0) and mark(1) when an optical signal is converted into an electric signal. FIG. 3 is a graph of stochastic distributions of a signal with respect to space and mark when bit noise increases due to an increase in the power of an input optical signal.
The graph of FIG. 2 reveals that a probability density function PDF(1) of a signal for mark and a probability density function PDF(0) of a signal for space show regular stochastic distributions. From the graph of FIG. 2, the average and standard deviation of a signal for mark are V1 and "sgr"1, and the average and standard deviation of a signal for space are V0 and "sgr"0, respectively. If a voltage of a crossing point of these probability density functions PDF(1) and PDF(0) is set as a reference voltage Vopt for a limiting amplifier, it is possible to obtain a minimum ratio of bit error when distinguishing between mark and space on the basis of the reference voltage.
In conventional optical signal receiving apparatuses, a reference point for distinguishing between mark and space is fixed to be one reference voltage Vopt, e.g., 0V. In case that the power of an optical signal is weak, thermal noise and shot noise predominate, and thus, the noise distribution of the signal for mark is almost the same as that of the signal for space. As a result, the voltage of crossing point of the probability density functions PDF(1) and PDF(0) is located at the center of V0 and V1, e.g., 0V. Adversely, if the power of the optical signal is strong, bit noise predominates, and thus, the noise of the signal for mark is larger than that of the signal for space. As a result, as shown in FIG. 3, the voltage of crossing point of the probability density functions PDF(1) and PDF(0) moves toward the signal for space. In conclusion, in case that an input signal has high power, the reference voltage Vopt, which is a reference point in obtaining a minimum ratio of bit error when distinguishing between space and mark, must move toward the signal for space, as shown in FIG. 3. Information on various noises is described in the article xe2x80x9cLightwave Systems with Optical Amplifiersxe2x80x9d, introduced by N. A. Olsson IEEE J. Light Technol., vol. 7, No. 7, pp.1071-1081, 1989.
Conventional optical signal receiving apparatuses are, however, disadvantageous in that the reference voltage Vopt, which is used to regenerate data, is fixed irrespective of a change in the power of an optical signal, which makes it impossible to realize optical signal receiving apparatus having an optimum receiving performance regardless of a change in the power of an optical signal.
To solve the above and related problem, it is an object of the present invention to provide an optical signal receiving apparatus having an optimum receiving performance that is not affected by a change in the power of an input optical signal, the apparatus detecting a change in the power of an input optical signal and controlling a reference voltage for a limiting amplifier used in distinguishing between space and mark, according to the detected power, and a method using the apparatus.
To achieve one aspect of the above object, there is provided an optical signal receiving apparatus including an optical coupler for dividing an input optical signal at a predetermined ratio to produce first and second divided signals; a photoelectric converter for converting the first divided signal into an electric signal; an amplifier for amplifying the electric signal within a predetermined range centering on a reference voltage; a reference voltage controlling unit for detecting the power of the second divided signal, predicting the power of the first divided signal, and controlling the amplitude of the reference voltage in accordance with the predicted power; and a clock and data recovery unit for recovering a clock signal and data from a signal output from the amplifier.
To achieve another aspect of the above object, there is provided a method of receiving an optical signal, including (a) measuring the power of an input optical signal and producing a reference voltage corresponding to the measured power; (b) converting the input optical signal into an electric signal to produce the electric signal; (c) amplifying the electric signal to a value within a predetermined range centering on the reference voltage produced in (a); and (d) recovering a clock signal and data from the signal amplified in (c).