1. Field of the Invention
The present invention relates to an optical modulating circuit and an optical modulating method which perform pulse position modulation.
2. Description of Related Art
Pulse Position Modulation (PPM) is a modulating method by which pulse signals are assigned to pieces of digital information “0” and “1”, and modulation of the pulse signals is performed by changing pulse positions from a reference position on a time axis regulated by a clock signal having a constant frequency corresponding.
A conventional modulating circuit and method using PPM will be described below with reference to FIG. 13 and FIGS. 14A, 14B, 14C, 14D, 14E, and 14F (for example, see document 1: M. L. Stevens et al., “A Novel Variable—Rate Pulse—Position Modulation System with Near Quantum Limited Performance”, Lasers and Electro-Optics Society 1999 12th Annual Meeting., Vol. 1, pp. 301-302, 1999). FIG. 13 is a block diagram of a conventional modulating circuit. FIGS. 14A, 14B, 14C, 14D, 14E, and 14F are time charts for explaining an operational principle of the conventional modulating circuit shown in FIG. 13. In FIGS. 14A, 14B, 14C, 14D, 14E, and 14F, time is plotted along the horizontal axes, and voltages of signals are plotted along the vertical axes. In FIGS. 14A, 14B, 14C, 14D, 14E, and 14F, reference positions on the time axis are represented by 0, Tbit, 2Tbit, 3Tbit, 4Tbit, 5Tbit, and 6Tbit, respectively, at time intervals equal to a data cycle Tbit.
The modulating circuit includes a multiplexer 93, a first phase shifter 91, a second phase shifter 95, and an exclusive OR gate 97.
A clock signal (indicated by an arrow S151 in FIG. 13) generated inside or outside the modulating circuit and having a cycle T2CLK (=2×Tbit) twice the data cycle Tbit, i.e., a frequency ½ data rate is divided into two signals, i.e., a first clock signal (indicated by an arrow S153 in FIG. 13) and a second clock signal (indicated by an arrow S155 in FIG. 13). The first clock signal S153 is transmitted to the multiplexer 93 (FIG. 14A). On the other hand, the second clock signal S155 is delayed by time w by means of the first phase shifter 91 and then transmitted to the multiplexer 93 (FIG. 14B).
The multiplexer 93 has a 3-input-1-output configuration. Of first to third input ports held by the multiplexer 93, the first input port and the second input port receives the first clock signal S153 and the second clock signal S155, respectively. A transmission electric signal (indicated by an arrow S141 in FIG. 13) is input to the third input port. The transmission electric signal S141 is a signal of an NRZ (Non-Return to Zero) format having a cycle of Tbit, i.e., an NRZ signal. The transmission electric signal S141 is set at a Low (L) level and a High (H) level in accordance with pieces of digital information “0” and “1”, respectively (FIG. 14C). When the state of the transmission electric signal S141 is at H level, the multiplexer 93 outputs a signal input from the first input port, i.e., the first clock signal S153 as a multiplexed signal (indicated by an arrow S161 in FIG. 13). On the other hand, when the state of the transmission electric signal S141 is at L level, the multiplexer 93 outputs the signal input from the second input port, i.e., the second clock signal S155 as a multiplexed signal S161 (FIG. 14D).
The multiplexed signal S161 is divided or branched into two signals, i.e., a first multiplexed signal (indicated by an arrow S163 in FIG. 13) and a second multiplexed signal (indicated by an arrow S165 in FIG. 13). The first multiplexed signal S163 is transmitted to the exclusive OR gate 97. On the other hand, the second multiplexed signal S165 delayed by time w by means of the second phase shifter 95 and then transmitted to the exclusive OR gate 97 (FIG. 14E).
The exclusive OR gate 97 outputs a signal at L level when both the first multiplexed signal S163 and the second multiplexed signal S165 are at H level or L level. The exclusive OR gate 97 outputs a signal at H level when one of the first multiplexed signal S163 and the second multiplexed signal S165 is at H level and the other is at L level. As a result, a PPM signal (indicated by an arrow S171 in FIG. 13) is obtained as an electric signal (FIG. 14F). In the PPM signal S171, a pulse corresponding to the digital information “1” rises at a reference position on the time axis, and a pulse corresponding to the digital information “0” rises at a position delayed by the time w from the reference position on the time axis.
The modulating circuit disclosed in document 1 converts the PPM signal S171 obtained as the electric signal into an optical signal and then transmits the optical signal.
In this case, as shown in FIG. 14F, in the PPM signal, a rising edge and a falling edge of a pulse are generated once each within time corresponding to one data cycle Tbit. For this reason, as a frequency band of circuit parts constituting a modulating circuit, a frequency twice or more a data rate is necessary. As a result, the data rate is limited to the frequency band of the circuit parts. When data of two or more bits is transmitted in one data cycle, the circuit parts require further high speed of response.
In order to solve the problem, when the present inventor according to this application concentratedly studied, the following fact was found. That is, an optical pulse signal in which optical pulses having different wavelengths of carrier waves are arranged at constant time intervals corresponding to transmission signal is generated, and the optical pulses having the different wavelengths of the carrier waves are delayed by different delay times, so that PPM can be realized by a modulating circuit having circuit parts having a frequency band equal to a data rate.
The invention has been made in consideration of the above problems, and accordingly, objects of the present invention are to provide an optical modulating circuit which is constituted by circuit parts having a frequency band equal to a data rate to increase the data rate in communication using PPM and realizes PPM, and an optical modulating method using the optical modulating circuit, respectively.