Optical frequency division multiplexing transmission system

Abstract: An optical frequency division multiplexing transmission system for sending a large volume of information by a single optical fiber is disclosed. Each of an optical frequency division multiplexing transmitter equipment and an optical frequency division multiplexing receiver equipment has an optical filter (4, 8) having a periodic optical transmission characteristic therein. Each optical filter is stabilized with an absolutely-stabilized standard optical frequency. Each optical signal frequency is stabilized to a periodic transmission characteristic of the optical filter arranged in the optical frequency division multiplexing transmitter equipment, and each local frequency to a periodic transmission characteristic of the optical filter arranged in the optical frequency division multiplexing receiver equipment. Even in the case where no optical signal is applied to the optical frequency division multiplexing receiver equipment due to such causes as a fault of the transmission optical source or the breakage of an optical fiber, the local optical source frequency continues to be controlled in stable fashion.

Description: BACKGROUND OF THE INVENTION The present invention relates to an optical transmission system utilizing the optical frequency division multiplexing technology for sending a large volume of information by means of a single optical fiber. An optical frequency division multiplexing transmission system (the frequency division multiplexing is hereinafter referred to as "FDM") normally comprises an optical FDM transmitter equipment including a plurality of transmission optical sources (in the number of N, where N is a natural number of 1 or more), an optical fiber for transmitting an optical FDM signal and an optical FDM receiver including a plurality (a number N) of heterodyne optical receivers. The frequency interval of a transmission optical source is normally stabilized at a predetermined value of Dop. Each heterodyne optical receiver (the n-th one, for example, where n is a natural number satisfying the relation 1 ≦ n ≦ N) receives only an optical signal outputted from a corresponding (n-th) optical source on transmitter side. A conventional optical FDM transmission system is described, for example in IEEE Photonics Technology Letters, Vol. 2, No. 12, pp. 914-916. In the conventional optical FDM transmission system, the frequency of a local optical source (hereinafter referred to as "the local frequency") is controlled in such a manner as to follow the variations in the central frequency of a received optical signal (hereinafter referred to as "the optical signal frequency"), thereby stabilizing the difference frequency (hereinafter referred as "the intermediate frequency") at a predetermined value f IF . Conventional systems in which the local frequency is controlled in such a manner as to maintain a predetermined intermediate frequency pose the problem that a failure to apply an optical signal to an optical FDM receiver due to the fault of a transmission optical source or breakage of an optical fiber makes it impossible to control the local frequency. It is known to solve this problem by arranging an optical filter having a periodic optical transmission characteristic at each of an optical FDM transmitter and a receiver, stabilizing each optical filter at an absolutely-stabilized optical standard frequency, and by stabilizing each local frequency at the periodic transmission characteristic of an optical filter arranged at the optical FDM receiver. An optical FDM transmitter equipment with the features included in the first part of claim 1 is known from EP-A-O 298 598. This system uses a Fabry-Perot resonator as an essential element of the multiplexing equipment. A Fabry-Perot resonator requires comparatively complex stabilisation means. The use of a Mach-Zehnder type optical filter as a wavelength multiplexer is known from JP-A-2-87841. It is an object of the present invention to meet the problem set forth above with a smaller number of optical elements required for stabilisation. The solution according to the invention is set forth in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a diagram of an optical FDM transmitter equipment somewhat similar to that shown in EP-A-0 298 598 referred to above. Fig. 2 is a diagram showing the configuration of an optical FDM receiver equipment. Figs. 3A and 3B show an arrangement of the optical signal, the optical frequency of local optical signal and the transmission characteristics of a Fabry-Perot resonator. Figs. 4A to 4E are a diagram showing signal waveforms produced at various parts in a transmission optical fiber stabilization equipment. Figs. 5A to 5E are a diagram showing signal waveforms produced at various parts in a channel-1 heterodyne optical receiver. Fig. 6 is a diagram showing a configuration of an embodiment of an optical FDM transmitter equipment according to the invention. Fig. 7 is a diagram showing a configuration of an embodiment in which a frequency stabilization system according to the present invention is applied to an optical frequency division multiplexing network. Figs. 8A, 8B, to 13A, 13B are diagrams showing configurations of optical frequency regulators and optical transmitter/receivers of an optical frequency stabilization system shown in Fig. 7. DETAILED DESCRIPTION In Figs. 1 and 2, an optical FDM transmitter system comprises an optical FDM transmitter equipment (Fig. 1) inputted with a number N of information signals (channel 1 to N signals) for outputting an optical FDM signal, an optical fiber (designated by 1 in Fig. 1) for transmitting an optical FDM signal and an optical FDM receiver equipment (Fig. 2) inputted with an optical FDM signal for outputting signals of channels 1 to N. A modulation system considered for use with each optical signal includes a frequency shift keying, amplitude shift keying and a phase shift keying. 1. Optical FDM transmitter equipment An optical FDM transmitter equipment comprises an optical transmission filter stabilizer 2, a number N of optical transmitters 3-1 to 3-N (optical transmitters for channels 1 to N), an optical multiplexer for multiplexing optical signals outputted from the N optical transmitters and outputting an optical FDM signal, and an optical coupler for dividing an optical FDM signal into two parts. (1) Stabilizer for transmission optical filter First, the configuration of this device will be described. The component parts of the transmission optical filter stabilization equipment 2 include a standard optical source for an optical frequency signal for outputting an absolutely-stabilized optical standard frequency F 0 , an optical frequency modulator for subjecting the light outputted from an optical source to frequency modulation at a frequency of f 0 , an optical coupler for multiplexing a frequency-modulated light with an optical FDM signal and outputting it to a transmission Fabry-Perot resonator (with a transmission characteristic shown by the dashed line in Fig. 3A), a photo-diode for converting the light outputted from the transmission Fabry-Perot resonator into an electrical signal and producing a detected signal, a bandpass filter (designated by BPF in the diagram with a central frequency f 0 ) for extracting a signal component of the frequency f 0 from the detected signal, a multiplier for multiplying the extracted signal with a signal (frequency f 0 ) from an oscillator, a low-pass filter (designated by LPF in the diagram with a cut-off frequency of f LPF << f 0 ) for extracting an error signal from a multiplied signal, a temperature control circuit for controlling the temperature of a transmission Fabry-Perot resonator by the error signal. An optical frequency standard source may be realized, for example, by a configuration described in IEEE Photonics Technology Letters, Vol. 1, No. 6, pp. 140-141). An optical frequency modulator, on the other hand, is realizable by using lithium niobate or the like having a photo-electric effect. Also, a transmission FP resonator is realized by using, for example, such a material as synthesized quartz or BK-7. These materials have an expansion coefficient not zero against temperature changes, and therefore are capable of controlling the length of a resonator by changing the temperature thereof. Now, the operating principle will be explained. Fig. 4 shows signal waveforms produced at various parts in the case where the transmission FP resonator is stabilized by use of the light of frequency F 0 outputted from a standard optical source. Fig. 4A shows a voltage signal of frequency f 0 applied to an optical frequency modulator. As a result of application of this voltage, an optical signal (central frequency F 0 , frequency shift ±ΔF, ΔF << Dop/2) subjected to frequency modulation is produced as an output of the optical frequency modulator (Fig. 4B). When this optical signal is applied to a transmission FP resonator having a transmission characteristic as shown in Fig. 3A and the output thereof is detected by a photo-diode, a signal as shown in Fig. 4C is produced as a detected signal. This waveform is obtained because the transmission FP resonator has a peak transmission coefficient at the frequency F 0 and the optical power outputted from the transmission FP resonator becomes maximum each time the optical frequency of the frequency modulated signal coincides with F 0 . In the process, the frequency of variations in the detected signal becomes 2fo, and therefore this detected signal is removed by a bandpass filter. As a result, the error signal produced from a low-pass filter is zero, so that the temperature control circuit maintains the temperature of the transmission FP resonator at a constant value. More specifically, the frequency associated with a peak transmission coefficient of the transmission FP resonator continues to be stabilized at F 0 . When the transmission characteristic of the transmission FP resonator undergoes such a change as to have a peak transmission coefficient at a frequency F 0 + ΔF, however, the detected signal assumes a waveform as shown in Fig. 4D. This detected signal has a frequency of f 0 and passes through a bandpass filter. Multiplication of a signal (frequency f 0 ) from an oscillator produces a positive signal with a frequency of almost zero (almost direct current). When this signal is applied to a low-pass filter, a positive error signal is outputted. This error signal causes the temperature control circuit to operate in such a manner as to restore the frequency associated with a peak transmission coefficient of the transmission FP resonator from F 0 + ΔF to F 0 . In the case where the transmission coefficient of the transmission FP resonator undergoes such a change as to have a peak transmission coefficient at a frequency F 0 - ΔF, on the other hand, the detected signal assumes a waveform (with frequency f 0 but displaced by 180 degree as compared with the waveform in Fig. 4D) as shown in Fig. 4E, thereby producing a negative error signal. As a result, the temperature control circuit operates in the direction opposite to Fig. 4D, and the frequency associated with a peak transmission coefficient of the transmission FP resonator is restored from F 0 - ΔF to F 0 . In this way, the transmission characteristic of the transmission FP resonator is controlled to have a peak at a frequency F 0 by the transmission Fabry-Perot resonator stabilization equipment shown in Fig. 1. Although an optical FDM signal is inputted to the transmission FP resonator in addition to an optical standard frequency fo, the effect of the optical FDM signal can be removed by a bandpass filter (central frequency f 0 ) if the frequency f 0 is sufficiently large as compared with the transmission rate of each optical signal constituting an optical FDM signal. Specifically, a signal similar to the one shown in Fig. 4 is obtained even when an optical FDM signal is inputted to the transmission FP resonator. (2) Optical transmitter An example of configuration of a system for stabilizing the frequency of each transmission optical source for the above-mentioned transmission FP resonator is shown as an optical transmitter in Fig. 1. The optical transmitter includes a transmission optical source, an adder and an optical signal frequency stabilizing circuit. The optical transmitter for each channel has a similar configuration. The transmission optical source in a channel-n optical transmitter is called a transmission optical source (n). In this configuration, an optical FDM signal is inputted through the above-mentioned optical coupler to the transmission FP resonator. The correspondence between a frequency associated with a peak transmission coefficient of the transmission FP resonator and each optical signal frequency is shown in Fig. 3A. A detected signal outputted from a photo-diode is distributed among the optical signal frequency stabilizing circuits in respective channel optical transmitters by being divided accordingly. A detected signal contains the information on frequency fluctuations of all transmission optical sources. As a result, in the channel-1 optical transmitter, for example, a channel-1 signal is multiplied by the detected signal to extract only the information on frequency fluctuations (correlation signal) of the channel-1 transmission optical source. The correlation signal applied to a low-pass filter (LPF in Fig. 1, with a cut-off frequency of f LPF << transmission rate f 0 of each signal) to remove high-frequency components, thus producing an error signal. The error signal is added to a channel-1 signal and a bias signal and is injected into the transmission optical source (1) in such a polarity that the optical frequency of the transmission optical source (1) is stabilized at a frequency F 0 + Dop associated with a peak transmission coefficient of the transmission FP resonator. A similar operation stabilizes the transmission optical sources (2) to (N) of other channels at a peak transmission coefficient of corresponding transmission FP resonators. Specifically, the channel-1, channel-2, ..., channel-N optical signal frequencies are stabilized at F 0 + Dop, F 0 + 2Dop, ...., F 0 + NDop, respectively. This stabilization method is described, for example, in Electronics Letters, Vol. 23, No. 14, pp. 750-752. 2. Optical FDM receiver equipment An example of configuration of equipment at receiving end is shown in Fig. 2. An optical FDM receiver equipment includes a receiving optical filter, a receiving optical filter stabilization equipment (designated by numeral 6 in Fig. 2), a number N of heterodyne optical receivers (channel-1 to channel-N heterodyne optical receivers) (designated by numeral 7 in Fig. 2), and an optical demultiplexer for dividing the optical DM signal into a number N of heterodyne optical receivers. (1) Receiving optical filter stabilization equipment The operation of a receiving optical filter stabilization equipment is identical to that of a transmission Fabry-Perot resonator stabilization equipment in the optical FDM transmitter equipment. An optical multiplexer is added to the transmission Fabry-Perot resonator stabilization equipment because the local optical signal outputted from each heterodyne optical receiver is multiplexed and applied to the receiving Fabry-Perot resonator. A Mach-Zehnder optical filter or a star coupler may be used as an optical multiplexer. (2) Heterodyne optical receiver First, the configuration of this receiver will be explained with reference to a channel-1 heterodyne optical receiver. A channel-1 heterodyne optical receiver includes a local signal optical source for outputting a local optical signal of F 0 + Dop + f IF in optical frequency, an optical coupler for dividing a local optical signal, a polarization diversity optical receiver for receiving an optical signal and producing a base band signal by use of one output of the optical coupler, a decision and regeneration circuit for discriminating and regenerating a channel-1 signal from the signal outputted from the polarization diversity optical receiver, and a local frequency stabilization circuit for producing an error signal from a detected signal obtained in the receiving Fabry-Perot resonator stabilization equipment. The local frequency stabilization circuit, in turn, includes a bandpass filter (BPF with a central frequency f 1 ) for extracting the components of frequency f 1 from the detected signal, an oscillator (with frequency f 1 identical to the central frequency of the bandpass filter) for outputting a sinusoidal wave signal, a multiplier for multiplying an extracted signal with a sinusoidal signal, a low-pass filter (LPF with a cut-off frequency of f LPF << f 1 ) for extracting an error signal from a multiplied signal, and an adder for adding an error signal, a sinusoidal signal and a bias signal to each other. The heterodyne optical receiver for each channel has a similar configuration, although the central frequencies of the oscillator and the bandpass filter are f 1 , f 2 , ....., f N and are different for different channels 1, 2, ....., N respectively (f 1 ≠ f 2 ≠ ..... ≠ f N ≠ f 0 ). Also, the local optical source in a channel-n heterodyne optical receiver is called a local optical source (n). The above-mentioned polarization diversity optical receiver is realized with a configuration described in IEEE, Journal of Lightwave Technology, Vol. LT-5, No. 2, pp. 274-276. Now, the operating principle of this equipment will be explained. A heterodyne optical receiver is such that a local optical source is subjected to frequency modulation by a minute sinusoidal wave current in order to stabilize the optical frequencies of local optical sources ((1) to (N)) of channels 1, 2, ...., N to peak transmission coefficient frequencies (F 0 + Dop + f IF , F 0 + Dop + 2f IF , ...., F 0 + Dop + Nf IF ) corresponding to the receiving FP resonators, respectively. Modulation frequencies f 1 , f 2 , ...., f N (f 1 ≠ f 2 ≠ .... ≠ f N ≠ f 0 ) are allocated to channels 1, 2, ...., N, respectively. The stabilizing operation will be explained with reference to the channel-1 heterodyne optical receiver. Signal waveforms produced at various parts are shown in Fig. 5. Fig. 5A shows a waveform of a modulation current. As a result of modulation, an optical signal subjected to frequency modulation around a frequency F 0 + Dop + f IF is produced (Fig. 5B). This optical signal is applied to a receiving FP resonator having a transmission characteristic as shown by the dashed line in Fig. 3A, and the output thereof is detected by a photo-diode, thus producing a detected signal as shown in Fig. 5C. This waveform is produced because the receiving FP resonator has a peak transmission coefficient at a frequency F 0 + Dop + f IF , and the optical power outputted from the receiving FP resonator becomes maximum each time the optical frequency of the frequency-modulated signal coincides with F 0 + Dop + f IF . At the same time, the frequency of detected signal variations becomes 2f 1 . This detected signal is therefore removed by a bandpass filter. As a result, the error signal outputted from the low-pass filter becomes zero and the local optical source (1) is maintained at a constant frequency. When the frequency of the local optical source (1) undergoes a variation and the central frequency thereof becomes F 0 + Dop + f IF - ΔF, however, the detected signal assumes a waveform as shown in Fig. 5D. This detected signal has a frequency f 1 and passes through a bandpass filter, and by multiplication with a signal (f 1 ) from an oscillator, generates a positive signal with a frequency of almost zero (direct current). As a consequence, a positive error signal is produced from a low-pass filter. This error signal restores the frequency of the local optical source (1) to F 0 + Dop + f IF . When the frequency of the local optical source (1) undergoes a change and the central frequency becomes F 0 + DFop + f IF + ΔF, on the other hand, the detected signal assumes a waveform shown in Fig. 5E thereby to produce a negative error signal. The frequency of the local optical source (1) is thus restored from F 0 + Dop + f IF + ΔF to F 0 + Dop + f IF . Although the receiving FP resonator is supplied with lights from other local optical sources (2) to (N) in addition to the light from the local optical source (1), the fact that these lights are subjected to frequency modulation with different frequencies makes it possible to extract only the information on fluctuations of related local optical source frequencies through a bandpass filter (BPF) of each heterodyne optical receiver. More specifically, even when other lights are inputted to the receiving FP resonator, a signal waveform similar to the one shown in Fig. 5 is capable of being produced. As seen from above, the optical frequencies of the local optical sources of channels 1, 2, ....., N of a heterodyne optical receiver are stabilized to F 0 + Dop + f IF , F 0 + 2Dop + f IF ...., F 0 + NDop + f IF , respectively. 3. Optical FDM transmission system As will be understood from the foregoing explanation, according to an optical FDM transmission system having a configuration as shown in Fig. 1, when a channel-n (1 ≦ n ≦ N) optical signal is received by a channel-n heterodyne optical receiver, the intermediate frequency of a resulting signal is always fIF. In the process, the optical signal frequency and the local frequency are stabilized independently of each other, and therefore the advantage results that the local frequency continues to be controlled in stable manner even when no optical signal is applied to a heterodyne detector. An embodiment of a transmission optical filter stabilization equipment according to the invention, used for direct modulation of an optical source, is shown in Fig. 6. Also, an optical filter at the sending end may be used as an optical multiplexer in an optical FDM transmitter equipment at the same time. This sharing is realized by using an optical filter of Mach-Zehnder type at the sending end. An example of configuration of the optical FDM transmitter equipment in the case of shared use is shown in Fig. 6. The transmission optical filter stabilization equipment (designated by 15 in Fig. 6) and the optical transmitters are configured the same way as in Fig. 1. In spite of this, the optical coupler in the transmission optical filter stabilization equipment is eliminated, and the output light from each optical transmitter is applied to an optical filter of Mach-Zehnder type (designated by 16 in Fig. 6). The optical FDM signal outputted from an optical filter is divided into two branches, one being sent to an optical fiber and the other to a photo-diode in the transmission filter stabilization equipment thereby to stabilize each optical signal frequency on the same principle as in Fig. 1. According to this configuration, the optical FDM transmitter equipment is simplified as compared with Fig. 1. In similar fashion, the optical FDM receiver equipment can be simplified as compared with Fig. 2 by being configured in such a manner that a multiplexer for multiplexing a plurality of local optical signals is used as an optical filter at the receiving end at the same time. As will be seen from the foregoing description, according to the present invention, an optical signal frequency and a local frequency are controlled (stabilized and tuned) independently of each other at an optical FDM transmitter equipment and an optical receiver equipment respectively, and therefore the local frequency can continue to be controlled in stable manner even when an optical signal fails to be applied to an optical FDM receiver equipment due to such causes as the breakage of an optical fiber or a fault of the transmission light source. Fig. 7 shows a configuration of an embodiment in which a frequency stabilization system according to the present invention is applied to an optical network of optical frequency division multiplexing type. A plurality of optical transmitter/receivers 1401, 1402, 1403 and an optical frequency regulator 1404 are connected in loop by an optical fiber 1405. Each of the optical transmitter/receivers 1401, 1402, 1403 has a laser providing an optical source of a specific frequency. Although the embodiment shown in Fig. 7 represents a case in which a single optical frequency regulator 1404 is included in the network, a plurality of optical frequency regulators may be constructed in the network. Since the optical network according to this embodiment is of an optical frequency division multiplexing type, the frequencies of optical signals outputted from the optical transmitter/receivers 1401, 1402, 1403 are different from each other. The function of the optical frequency regulator 1404 is to stabilize the optical frequency of the optical sources of the optical transmitter/receivers 1401, 1402, 1403 to a regulated value in order to prevent the interference between the three optical signals. For this purpose, the optical frequency regulator 1404 internally determines three stable regulated optical frequencies different from each other. The optical frequency regulator 1404 receives a part of the optical signals from the optical transmitter/receivers 1401, 1402, 1403 connected by the optical fiber 1405, and determines the deviation between the frequencies of the three optical signals and corresponding regulated optical frequencies respectively. These values of deviation are carried on an optical signal (by subjecting an optical signal to frequency modulation with the deviation value, or otherwise) and are transmitted to the corresponding optical transmitter/receivers 1401, 1402, 1403 through the optical fiber 1405. The optical transmitter/receivers 1401, 1402, 1403 select deviation values associated therewith respectively from a plurality of deviation values contained in the optical signal received from the optical frequency regulator 1404, and correct the frequencies of the respective optical sources in such a manner as to obtain a deviation of 0 or a predetermined value. This series of operations stabilize the frequencies of the optical signals produced by the optical transmitter/receivers 1401, 1402, 1403 in conformity with or with a predetermined offset from the regulated optical frequency determined by the optical frequency regulator 1404. According to the present embodiment, each value of deviation may be transmitted to each optical transmitter/receiver by use of an electrical signal instead of an optical signal. In the case where an electrical signal is used for transmission, the optical frequency regulator 1404 and each of the optical transmitter/receivers 1401, 1402, 1403 may be connected only by electric wires apart from the configuration shown in Fig. 7. Also, the optical frequency regulator 1404 and the optical transmitter/ receivers 1401, 1402, 1403 may be coupled not in loop but in star. According to the present embodiment, the deviation between the value of a regulated optical frequency in the optical frequency regulator 1404 and the frequency of the optical signal outputted from each optical transmitter/receiver is determined at the optical frequency regulator 1404 and is transmitted to the optical transmitter/receivers 1401, 1402, 1403, which in turn correct the optical frequencies of their own optical sources accordingly. As an alternative, the system may comprise means for transmitting the optical signal itself of a regulated optical frequency internally determined by the optical frequency regulator 1404, directly to the corresponding optical transmitter/receivers 1401, 1402, 1403, so that the optical transmitter/receivers may detect the deviation between the optical signal thus sent and their own optical signals respectively and control their own optical frequencies in such a manner as to keep the deviation frequency at zero or a predetermined value. Although the embodiment shown in Fig. 7 includes a single transmission optical source in each optical transmitter/receiver, the optical frequencies of a plurality of transmission optical sources which may be provided in each optical transmitter/receiver may be regulated in exactly the same manner by means of a frequency regulator. Further, the optical frequency of a local optical source included in the receiving section of each optical transmitter/receiver, which is not described above, may be regulated by a frequency regulator in similar fashion. Explanation will be made about an embodiment of the optical transmitter/receivers 1401, 1402, 1403 and the optical frequency regulator 1404 applied to a case embodying a frequency stabilization system according to the present invention in the optical network shown in Fig. 7. Fig. 8 is a diagram showing a configuration of a first embodiment of the optical transmitter/receiver and the optical frequency regulator in the optical frequency stabilization system shown in Fig. 7. Fig. 8A shows a configuration of an optical transmitter/receiver and Fig. 8B an optical frequency regulator. According to this embodiment, the deviation between the actual optical frequency of an optical source of each of the optical transmitter/receivers 1401 to 1403 and that of each optical transmitter/receiver regulated by the optical frequency regulator is transmitted to each optical transmitter/receiver by use of an optical signal from an optical frequency regulator 1404. The laser 1509 in the optical transmitter 1502 of the optical transmitter/receiver has the optical frequency thereof regulated by the control circuit 1506 in such a manner as to be differentiated from the optical frequency of the laser of the remaining optical transmitter/receivers. Also, the laser 1509 is modulated by a data i1508 providing information to be transmitted, and is further subjected to fine frequency modulation by a signal 1507 of a frequency fi specific to the optical transmitter/receiver. The optical signal outputted from the laser 1509 is led to an optical network by the optical fiber 1501. The optical frequency regulator 1404 shown in Fig. 8B includes an optical resonator 1520 made up of a Fabry-Perot resonator having a periodic transmission characteristic on the optical frequency axis. According to the present embodiment, a plurality of frequencies associated with an optical signal which is transmitted through the optical resonator 1520 and has a maximum intensity are used as regulated optical frequencies of the optical transmitter/receivers 1401, 1402, 1403. As an alternative, frequencies associated with an optical signal intensity reflected on the optical resonator 1520 and having a minimum value may be employed as regulated optical frequencies. A part of the optical frequency division multiplexing signal sent from the optical transmitter/receivers 1401, 1402, 1403 by the optical fiber 1501 is applied to the optical resonator 1520, and after being converted into an intensity signal, is detected by an optical detector 1521. The signal 1522 thus detected is synchronously detected with a signal 1524 having an inherent frequency fi (i=1, 2, 3) by a mixer 1523. This synchronous detection is effected by a timing control circuit 1525 in time division. An output signal 1526 from the mixer 1523 makes up a deviation between the frequency fi of an optical signal outputted from the i-th optical transmitter/receiver and the resonance frequency of the optical resonator 1520 corresponding to the optical frequency fi. As a result, the laser 1528 is modulated by the deviation signal 1526 through the control circuit 1527, and the deviation data is transmitted as an optical signal to each of the transmitter/receivers 1401, 1402, 1403 through the optical fiber 1501. In the process, for discrimination from the deviation data for the other optical transmitter/receivers, the laser 1528 is subjected to intensity modulation by a signal of a frequency Fi in advance. The optical transmitter/receiver receives a part of the optical signal holding the above-mentioned deviation data sent from the optical fiber 1501 and intensity-modulated by the signal of frequency Fi. The signal thus received is divided into two parts, one being sent to the optical receiver 1503, and the other to the optical transmitter 1502. The optical signal received by the optical transmitter 1502 is converted into an electrical signal by the optical detector 1504. This electrical signal contains a deviation signal of the transmitter/receiver. In order to discriminate this signal from the deviation data of the other transmitter/receivers, only a deviation signal subjected to intensity modulation at Fi is taken out by use of a bandpass filter 1505. On the basis of this deviation signal, the control circuit 1506 corrects the optical frequency of the laser 1509. The configuration shown in Fig. 8 causes the optical frequency of the laser 1509 of the transmitter 1502 of each optical transmitter/receiver to coincide with a given resonance frequency of the optical resonator 1520. The coincidence between the optical frequency of the laser of each optical i transmitter/receiver with adjacent resonant frequencies of the optical resonator 1520 makes it possible to secure equal channel intervals. Fig. 9 is a diagram showing a configuration of a second embodiment of the optical transmitter/receiver and the optical frequency regulator of the optical frequency stabilization system shown in Fig. 7. Fig. 9A shows a configuration of the optical transmitter/receiver, and the Fig. 9B that of the optical frequency regulator. According to this embodiment, in order to send the deviation value from the optical frequency regulator while discriminating the optical transmitter/receiver, the optical frequency regulator is synchronized with each optical transmitter/ receiver thereby to transmit an optical signal carrying the deviation data in time division. The sync signal resulting from the modulation of the laser 1628 with the signal 1629 from the timing control circuit 1625 shown in Fig. 9B is applied through the optical fiber 1601 to each optical transmitter/receiver. The optical detector 1604 shown in Fig. 9A converts an optical signal into an electrical signal, and the sync detector 1605 detects the sync signal 1629. The control circuit 1606, during its own transmission time, subjects the laser 1609 to fine frequency modulation with the signal 1607 of frequency f on the basis of a detected sync signal, and applies an optical signal to the optical fiber 1601. This optical signal is transmitted through the optical resonator 1620 as shown in Fig. 9B and is converted into an electrical signal. This electrical signal 1622 is subjected to synchronous detection by the signal 1624 of frequency f. The laser 1628 is modulated with a deviation signal 1626 and transmitted to the optical transmitter/receiver. In this configuration, there is only one laser in the optical transmitter/receiver which is frequency-modulated with frequency f at a given time point, and only the particular laser 1609 in the optical transmitter/receiver is corrected. Fig. 10 is a diagram showing a configuration of a third embodiment of the optical transmitter/receiver and the optical frequency regulator of the optical frequency stabilization system shown in Fig. 7. Fig. 10A shows a configuration of an optical transmitter/receiver, and Fig. 10B that of an optical frequency regulator. According to this embodiment, in order to discriminate the optical transmitter/receivers 1401 to 1403 in transmitting a deviation data from the optical frequency regulator 1404, a header specific to each optical transmitter/receiver is attached to the optical signal carrying the deviation value. Specifically, an optical signal outputted from the laser 1728 of the optical frequency regulator contains a header for discriminating each optical transmitter/receiver as well as the deviation value thereof. Each optical transmitter/receiver takes in only a deviation sent with an optical signal carrying a header specific to itself detected by a header detector 1705 from the output of the optical detector 1704 as shown in Fig. 10A, thereby correcting the optical frequency of the laser 1709. In the example under consideration, the wavelength band of an optical signal for transmitting the data i1708 is differentiated from that of an optical signal for transmitting the deviation. Specifically, the lasers 1709 and 1728 belong to wavelength bands different from each other. In the optical transmitter/receiver, the use of a demultiplexer 1710 makes it possible to isolate a received optical signal into the optical transmitter 1702 and the optical receiver 1703. In the configuration shown in Fig. 10, the signal f i 1707 of the control circuit 1706 and the signal 1708 of the data i correspond to the signal f i 1507 of the control circuit 1506 and the data i signal 1508 respectively, while the mixer 1723, the timing control circuit 1725, the signal 1726 and the control circuit 1727 correspond respectively to the mixer 1523, the timing control circuit 1525, the signal 1526 and the control circuit 1527. Also, the optical resonator 1720 shown in Fig. 10B is controlled in such a manner as to resonate at the optical frequency of the standard light 1730. This control makes it possible to connect a plurality of optical networks having a plurality of optical transmitter/receivers to each other. Specifically, if a plurality of optical resonators having the same characteristic are controlled to resonate at the frequency of a standard light shared by them by an optical frequency regulator in an optical network, then the frequencies of optical signals outputted from the optical transmitter/receivers in a plurality of optical networks can be integrally controlled. For this purpose, the standard light 1730 is frequency-modulated in advance by use of the signal source 1731 of frequency f. The output signal from the optical detector 1721 is applied to the mixer 1732 with the signal of the signal source 1731 thereby to produce a signal for controlling the optical resonator 1720. This signal corresponds to a relative deviation of a resonance frequency of the optical resonator 1720 for the optical frequency of the standard light 1731. As a result, when this signal is negatively fed back to the optical resonator 1720, one of the resonance frequencies of the optical resonator 1720 comes to coincide with the optical frequency of the standard light 1730. Fig. 11 is a diagram showing a configuration according to a fourth embodiment of the optical transmitter/receiver and the optical frequency regulator of the optical frequency stabilization system shown in Fig. 7. Fig. 11A shows a configuration of the optical transmitter/receiver and Fig. 11B a configuration of the optical frequency regulator. According to the embodiment under consideration, a plurality of deviation values corresponding to a plurality of optical transmitter/receivers from the optical frequency regulator are transmitted to the respective optical transmitter/receivers by being carried on optical signals of different optical frequencies. Specifically, the optical frequency regulator has as many lasers 1828-1, ...., 1828-n with different optical frequencies as the optical transmitter/receivers as shown in Fig. 11B. The lasers 1828-1, ...., 1828-n are controlled respectively by the control circuits 1827-1, ...., 1827-n, which in turn are energized by the outputs of mixers 1823-1, ...., 1823-n supplied with the outputs of the optical detector 1821 and the frequency signals f 1 , ...., f n from the synchronous detection signal generator 1825. Each optical transmitter/receiver receives only an optical signal carrying its own deviation value by use of an optical filter 1811. According to the present embodiment, a number n of deviation values can be transmitted at a time. The optical fiber 1801, the optical transmitter 1802, the optical receiver 1803, the optical detector 1804, the control circuit 1806, the frequency signal fi1807, the data i1808, the laser 1809 and the demultiplexer 1810 of the optical transmitter/receiver correspond to the optical fiber 1701, the optical transmitter 1702, the optical receiver 1703, the optical detector 1704, the control circuit 1706, the frequency signal f i 1707, the data i1708, the laser 1709 and the demultiplexer 1710 in Fig. 10, respectively. Also, the functions and configurations of the optical resonator 1820, the optical detector 1821, the standard light 1830, the signal source 1831 and the mixer 1832 of the optical frequency regulator are identical to those of the optical resonator 1720, the optical detector 1721, the standard light 1730, the signal source 1731 and the mixer 1732 in Fig. 10, respectively. The embodiment of Fig. 11 may be configured of a single laser by integrating a plurality of lasers 1828-1, ...., 1828-n. Specifically, the illuminance frequency is changed by changing the temperature, current, etc. of the laser to send a deviation. In this case, a plurality of deviation values are transmitted in time division. Fig. 12 is a diagram showing a configuration of each part of the optical frequency stabilization system shown in Fig. 7 according to a fifth embodiment. Fig. 12A shows a configuration of an optical transmitter/receiver, and Fig. 12B that of an optical frequency regulator. According to this embodiment, optical signals of a plurality of regulated frequencies determined by the optical frequency regulator are transmitted to the respective optical transmitter/receivers, which correct the optical frequencies of their respective lasers in such a manner that the deviation between the respective received optical signals and the optical signals of the respective lasers assume zero or a predetermined value. This embodiment is such that in order to generate optical signals of a plurality of regulated frequencies by the optical frequency regulator, the temperature, current, etc. of the laser are changed. As will be naturally understood, according to the present embodiment, only an optical signal of a single regulated frequency can be transmitted at a time. A part of the optical signals of the laser 1928 in the optical frequency regulator is introduced to the optical resonator 1920. The optical frequency of the laser 1928 is swept by the temperature/current control circuit 1927. The light transmitted through the optical resonator 1920 is detected by the optical detector 1921. The signal 1922 thus detected is applied to the temperature/current control circuit 1927 and the timing control circuit 1925. Once the optical frequency of the laser 1928 coincides with the resonance frequency of the optical -resonator 1920, the signal transmitted through the optical resonator 1920 increases, thereby increasing the detected signal 1922. The sweeping of the laser 1928 is stopped by the timing control circuit 1925. The optical frequency of the laser 1928 at this time point is defined as the optical frequency of the i-th transmitter. This optical signal is subjected to intensity modulation with the signal 1927 of frequency Fi from the timing control circuit 1925 by an intensity modulator 1929 and sent out to the network by the optical fiber 1901. After that, the sweeping of the optical frequency of the laser 1928 is started and continued until it comes to coincide with the next resonance frequency. In the i-th optical transmitter/receiver, on the other hand, the optical signal sent from the network is introduced partly to the optical receiver 1902 and partly to the optical transmitter 1902. The optical detector 1904 of the optical transmitter 1902 detects the laser 1928 and the optical signal from a related laser 1909. Upon receipt of an intensity-modulated optical signal with frequency Fi from the optical frequency regulator, the optical detector 1904 detects the deviation (beat) signal 1911 and the laser 1909. The optical frequency of the laser 1909 is corrected by the temperature/current control circuit 1908 in such a manner that the beat signal 1911 becomes zero or a predetermined value. A bandpass filter 1905 of a pass bandwidth Fi is prepared in order to prevent the erroneous operation at an optical frequency setting signal for the transmitters of the other optical transmitter/receivers. A signal is outputted only when the beat signal 1911 contains a component of frequency Fi, and the temperature/current control circuit 1908 is operated only in that case. According to the embodiment shown in Fig. 12, the optical frequency intensity-modulated with frequency Fi sent from the network should be made to coincide with the optical frequency of the laser 1909. This embodiment facilitates the system initiation and permits automatic start. Apart from Fig. 12 showing an embodiment in which the optical frequency regulator has only one laser, an optical signal of a regulated optical frequency can be transmitted to all the optical transmitter/receivers at the same time, if lasers as many as the optical transmitter/receivers are prepared. Fig. 13 is a diagram showing a configuration of each part of the optical frequency stabilization system shown in Fig. 7 according to a sixth embodiment. Fig. 13A shows a configuration of an optical transmitter/receiver, and Fig. 13B that of an optical frequency regulator. According to this embodiment, the intensity modulator 1929 and the signal 1927 of frequency Fi of the optical frequency regulator are eliminated from the configuration of the embodiment shown in Fig. 12, and the bandpass filter 1905 is replaced by a sync detector 2005 for detecting a sync signal 2011 in the optical transmitter/receiver. As in the case of Fig. 9, the optical frequency regulator is synchronized with each optical transmitter/receiver to send a regulated optical frequency to each optical transmitter/receiver, thus eliminating the intensity modulator from the optical frequency regulator. The optical fiber 2001, the optical transmitter 2002, the optical receiver 2003, the detector 2004, the temperature/current control circuit 2006, the data i signal 2008 and the laser 2009 of the optical transmitter/receiver correspond to the optical fiber 1901, the optical transmitter 1902, the optical receiver 1903, the detector 1904, the temperature/current control circuit 1906, the data i signal 1908 and the laser 1909, respectively. Also, the output signal 2022 of the detector 2021 of the optical frequency regulator, the timing control circuit 2025, the temperature/current control circuit 2027 and the laser 2028 correspond to the output signal 1922 of the detector 1921, the timing control circuit 1925, the temperature/current control circuit 1927 and the laser 1928 shown in Fig. 12, respectively. Further, the functions and configuration of the optical resonator 2020, the standard light 2030, the signal source 2031 and the mixer 2032 of the optical frequency regulator are similar to those of the optical resonator 1720, the standard light 1730, the signal source 1731 and the mixer 1732 shown in Fig. 10, respectively.

Claims: An optical frequency division multiplexing transmitter equipment comprising N optical sources (3-1...3-N) for converting information signals of N channels into respective optical signals, means (16) having a periodic optical transmission characteristic on a frequency axis, for optically multiplexing the optical signals from said N channels and outputting an optical frequency division multiplexing, hereinafter referred to as FDM, signal, an optical coupler for dividing said FDM signal into two parts, means (15) for stabilising the transmission characteristic of said optical multiplexing means (16), and N optical signal frequency stabilisation circuits (5) for stabilising the central frequencies of said N channel optical signals on the basis of the transmission characteristic of said multiplexing means (16), characterised in that said optical multiplexing means is constituted by an optical filter (16). The transmitter equipment of claim 1, wherein said optical filter (16) is a Mach-Zehnder type optical filter. The transmitter equipment of claim 1 or 2, wherein said means (15) for stabilising the transmission characteristic of the optical filter (16) includes a standard light source for supplying a stabilised standard optical frequency F 0 to said optical filter. The transmitter equipment of any preceding claim, wherein said means (15) for stabilising the transmission characteristic of the optical filter (16) includes a photo-diode for converting one part of the FDM signal output by said optical coupler into a detected electrical signal, a band-pass filter for extracting a signal component proximate to a frequency f 0 from said detected signal, an oscillator outputting a sinusoidal wave signal of the frequency f 0 , a multiplier for multiplying the extracted signal from said band-pass filter with the sinusoidal wave signal from said oscillator, a low-pass filter for producing an error signal by cutting-off frequencies f LPF << f 0 from the output signal of said multiplier, and a circuit for controlling the temperature of said optical filter (16) by said error signal.

Inventors: KUBOKI KATSUHIKO, SASAKI SHINYA, TSUSHIMA HIDEAKI, KUBOKI, KATSUHIKO, SASAKI, SHINYA, TSUSHIMA, HIDEAKI, KUBOKI, KATSUHIKO HITACHI DAINI KYOSHINRYO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1992
Main classification: H04B10
IPC codes: H04B10