Abstract:
The invention provides a control signal transmission method for an optical transmission system by which a control signal having a high signal to noise ratio is transmitted without providing any deterioration in characteristic to the information signal. The control signal transmission method for an optical transmission system is characterized in that, upon transmission of a control signal of the optical transmission system, an optical signal modulated with an information signal is polarization modulated with the control signal, and the polarization state of the optical signal is varied with the control signal to obtain a modulated optical signal and the control signal is transmitted with the modulated optical signal. The present invention also provides a control signal reception method for an optical transmission system, which is characterized in that a modulated optical signal produced by polarization modulating an optical signal modulated with an information signal with a control signal and varying the polarization state of the modulation optical signal with the control signal is received, and the polarization modulated control signal is extracted by means of a polarizer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a long distance optical transmission system in which an erbium-doped fiber amplifier is used, and more particularly to a control signal transmission method and apparatus for a long distance optical transmission system. 
     2. Description of the Relates Art 
     Conventionally, in a long distance optical transmission system in which an erbium-doped fiber amplifier is used, it is necessary for a terminal equipment to transmit a control signal to a repeater equipment in order to perform various controls and supervision of the repeater equipment. A transmission method for a control signal is based on optical intensity modulation of an optical signal modulated with an information signal further with the control signal. FIG. 1 shows an example of a terminal equipment. In the terminal equipment shown, laser light generated from optical source (LD)  1  is modulated in accordance with an information signal by optical modulator  4  and is further modulated in accordance with a control signal generated from control signal generating circuit  5  by optical intensity modulator  23 . Optical amplifier  10  is used to amplify an optical output level which exhibits a drop. FIG. 2 shows an example of a repeater equipment. An optical signal applied to the repeater equipment is amplified by erbium-doped fiber amplifier (EDF)  12  and branched by optical coupler (CPL)  15 . One of the two branched optical signals is generated as an output signal while the other branched optical signal is applied to control circuit  21  after it passes through band pass filter (BPF)  20 . Optical signals and electric signals of several components are illustrated in FIG.  3 . In particular, with regard to the terminal equipment, (A) of FIG. 3 shows an optical output waveform of a modulated information signal of optical modulator  4 , (B) shows an output waveform of a control signal of intensity modulation driving circuit  22 , and (K) shows an optical output waveform of optical intensity modulator  23 , and with regard to the repeater equipment, (L) of FIG. 3 shows an output waveform of optical-electric converter (O/E converter)  19 , and (M) shows a waveform of a control signal from BPF  20 . 
     The prior art described above has the following problems. The prior art has a problem in that, if the modulation degree of a control signal is raised in order to amplitude modulate an information signal with a control signal as seen in FIG. 3, this causes deterioration of the information signal. 
     The prior art has another problem in that, in order to suppress deterioration of an information signal, the modulation degree must be set low, and consequently, the amplitude of a control signal received by the repeater equipment becomes so small that a sufficient signal to noise ratio of the control signal cannot be obtained. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method and apparatus by which a control signal having a high signal to noise ratio is transmitted without providing any deterioration in characteristic to the transmission signal by modulating a polarization state of an optical signal with the control signal and transmitting the modulated optical signal. 
     According to an aspect of the present invention, there is provided a control signal transmission method for an optical transmission system, characterized in that an optical signal modulated with an information signal is polarization modulated with a control signal, and the polarization state of the polarization modulation optical signal is varied in synchronism with the control signal to obtain a modulated optical signal and the control signal is transmitted with the modulated optical signal. 
     According to another aspect of the present invention, there is provided a control signal reception method for an optical transmission system, characterized in that a modulated optical signal produced by polarization modulating an optical signal modulated with an information signal with a control signal and varying the polarization state of the polarization modulated optical signal in synchronism with the control signal is received, and the polarization modulated control signal is extracted by means of a polarizer. 
     According to a further aspect of the present invention, there is provided a control signal transmission apparatus for a transmission terminal or a repeater equipment of an optical transmission system which multiplexes a modulated optical signal with a control signal, characterized in that it comprises a first polarization modulator for polarization modulating an optical multiplex signal generated from an optical coupler, which multiplexes a plurality of optical signals modulated with information signals, with a control signal, and a second polarization modulator for varying the polarization state of the polarization modulated optical signals in synchronism with the control signal and transmitting resulting modulated optical signals. 
     According to a still further aspect of the present invention, there is provided a control signal transmission apparatus for a reception terminal or a repeater equipment of an optical transmission system which multiplexes a modulated optical signal with a control signal, characterized in that it comprises a polarizer which applies a received modulated optical signal and extracts a polarization modulated control signal from the received modulated optical signal, and the optical signal obtained by the polarizer is converted into an electric signal and the control signal is generated through a band-pass filter. 
     The first advantage of the present invention is that a control signal can be superposed and transmitted together with an information signal without any characteristic deterioration of the information signal. This is because, since the polarization state of an optical signal is modulated with the control signal, the intensity of the optical signal is not varied. 
     The second advantage of the present invention is that a repeater equipment can receive a control signal of a high signal to noise ratio by means of a simple circuit formed by adding only a polarizer to a conventional circuit. This originates from the control of the present invention that the polarization modulation state of the optical signal is made coincide with the polarization surface of the polarizer by the second polarization modulator so that the output amplitude of the control signal may be maximized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a construction of a control signal transmission apparatus of a transmitter of a terminal or repeater equipment of a conventional optical transmission system; 
     FIG. 2 is a block diagram showing a construction of a control signal transmission apparatus of a receiver of a terminal or repeater equipment of the conventional optical transmission system; 
     FIG. 3 is a waveform diagram showing waveforms of several components for explaining operation of the terminal or repeater equipment of the conventional optical transmission system; 
     FIG. 4 is a system diagram of a long distance optical transmission system to which a control signal transmission apparatus for an optical transmission system of the present invention is applied; 
     FIG. 5 is a block diagram showing a construction of a control signal transmission apparatus of a transmitter of a transmission terminal or repeater equipment of the optical transmission system of the present invention; 
     FIG. 6 is a block diagram showing a construction of a control signal transmission apparatus of a receiver of a reception terminal or repeater equipment of the optical transmission system of the present invention; 
     FIG. 7 is a waveform diagram showing waveforms of several components for explaining operation of the control signal transmission apparatus of FIG. 5; 
     FIG. 8 is a waveform diagram showing waveforms of several components for explaining operation of the control signal transmission apparatus of FIG. 6; 
     FIG. 9 is a system diagram of another long distance optical transmission system to which the control signal transmission apparatus for an optical transmission system of the present invention is applied; 
     FIG. 10 is a block diagram showing a construction of a control signal transmission apparatus of a first another embodiment of a transmitter of a transmission terminal or repeater equipment of the optical transmission system of the present invention; 
     FIG. 11 is a block diagram showing a construction of a control signal transmission apparatus of a second another embodiment of a transmitter of a transmission terminal or repeater equipment of the optical transmission system of the present invention; and 
     FIG. 12 is a block diagram showing a construction of a control signal transmission apparatus of a receiver of a reception terminal or repeater equipment of a second another embodiment corresponding to the control signal transmission apparatus of FIG.  11 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of a control signal transmission method and apparatus for an optical transmission system of the present invention are described in detail with reference to the drawings. 
     FIG. 4 is a system diagram of a long distance optical transmission system to which a control signal transmission apparatus for an optical transmission system of the present invention is applied; FIG. 5 is a block diagram showing a construction of a control signal transmission apparatus of a transmitter of a transmission terminal or repeater equipment of the optical transmission system of the present invention; and FIG. 6 is a block diagram showing a construction of a control signal transmission apparatus of a receiver of a reception terminal or repeater equipment of the optical transmission system of the present invention. 
     Referring to FIG. 4, the optical transmission system includes n repeater equipments  300  to  600  interposed between terminal  100  and another terminal  200 . Terminal  100  includes optical transmitter  101  and optical receiver  102 ; terminal  200  includes optical transmitter  201  and optical receiver  202 , and each of repeater equipments  300  to  600  includes ascending and descending repeaters  301  and  302 , . . . , or repeaters  601  and  602 . 
     In the control signal transmission apparatus of one system in the optical transmission system of the present invention, for example, a control signal transmission apparatus of FIG. 5 is provided in optical transmitter  101  of terminal  100 , and a control signal transmission apparatus of FIG. 6 is provided in repeater  301  of repeater equipment  300  or optical receiver  202  of terminal  200 . 
     Referring to FIG. 5, the control signal transmission apparatus provided in optical transmitter  101  of terminal  100  includes optical source (LD)  1  for generating an optical signal, optical modulation driving circuit  3  for generating a driving signal for modulating the optical signal in accordance with information signal  2 , optical modulator  4  for modulating the optical signal with the driving signal, control signal generating circuit  5  for generating control signals, polarization modulator driving circuit  6  for modulating the polarization surface of the modulated optical signal in accordance with control signal  51  generated from control signal generating circuit  5 , polarization modulator  7 , polarization modulator driving circuit  8  for modulating the polarization surface of the optical signal with control signal  52  synchronized with control signal  51 , polarization modulator  9 , and optical amplifier  10  which amplifies the optical signal level, which has been dropped by polarization modulator  7  and polarization modulator  9 , and generates the amplified optical signal. 
     Referring to FIG. 6, the control signal transmission apparatus provided in repeater  301  of repeater equipment  300  or optical receiver  202  of terminal  200  includes optical isolator (ISO)  11  and erbium-doped fiber amplifier (EDF)  12  for amplifying an optical signal transmitted from a terminal equipment, wavelength division multiplex coupler (WDM)  13  for coupling the amplified optical signal and pumping light, pumping LD  16 , pumping LD driving circuit  18 , optical isolator (ISO)  14 , optical coupler (CPL)  15 , polarizer  17  for receiving a branched optical signal and extracting only an optical component of the control signal which coincides with the polarization surface thereof, optical-electric converter (O/E converter)  19  for converting the optical signal into an electric signal, band pass filter (BPF)  20 , and control circuit  21  for generating a control signal. 
     Erbium-doped fiber amplifier (EDF)  12 , for example, an erbium-doped fiber, amplifies an optical signal applied thereto with pumping light from pumping optical source (pumping LD)  16  having passed through wavelength division multiplex coupler (WDM)  13 . 
     Now, operation of the control signal transmission apparatus provided in optical transmitter  101  of terminal  100  of the optical transmission system of the present invention is described with reference to FIGS. 5 and 7. 
     An optical signal generated from optical source (LD)  1  is first modulated in accordance with an information signal by optical modulation driving circuit  3  and optical modulator  4 . Then, the polarization surface of the optical signal is modulated by polarization modulator driving circuit  6  and polarization modulator  7  in accordance with control signal  51  generated from control signal generating circuit  5 . Further, the polarization surface of the optical signal is modulated by polarization modulator driving circuit  8  and polarization modulator  9  with control signal  52  synchronized with control signal  51  and having a longer period. Finally, the optical signal level which has been dropped by the modulators in the stages is amplified by optical amplifier  10 , and a resulting optical signal is generated. 
     Referring to FIG. 7, (A) shows an optical output waveform of optical modulator  4  from which an optical signal modulated with an information signal is generated, and (B) shows a waveform of control signal  51  which shows an example wherein the control signal which is formed from one control instruction is transmitted three times like s 1 , s 2  and s 3 . (C) shows a waveform of control signal output  52  wherein a control signal is generated three times in synchronism with control signals s 1 , s 2  and s 3 . (D) shows an optical output waveform of polarization modulator  7  whose polarization state exhibits that it is modulated in same polarization modulation state (a) all three times in accordance with control signal  51 . In this instance, the optical intensity does not exhibit any variation. (E) shows an optical output waveform of polarization modulator  9  whose polarization state exhibits that it is modulated while polarization modulation state (b) varies like b 1 , b 2  and b 3  in accordance with control signal  52 . 
     Next, operation of the control signal transmission apparatus which is provided in repeater  301  of repeater equipment  300  or optical receiver  202  of terminal  200  of the optical transmission system of the present invention is described with reference to FIGS. 6 and 8. 
     In an embodiment of a repeater equipment which employs erbium-doped fiber amplifier (EDF)  12 , an optical signal transmitted from a terminal equipment is generated after it passes through optical isolator  11 , erbium-doped fiber amplifier (EDF)  12 , wavelength division multiplex coupler (WDM)  13 , optical isolator  14  and optical coupler (CPL)  15 . Erbium-doped fiber amplifier  12 , for example, an erbium doped fiber, amplifies the applied optical signal with pumping light from pumping optical source (pumping LD)  16 . Wavelength division multiplex coupler (WDM)  13  couples the optical signal and the pumping light. The optical signal branched by optical isolator (ISO)  14  is applied to polarizer  17 , by which only an optical component of the polarization modulated optical signal coincident with the polarization surface of polarizer  17  is extracted with a control signal. Then, the optical signal is converted into an electric signal by optical-electric converter  19 , and the electric signal is applied to control circuit  21  after it passes through band pass filter  20 . 
     Referring to FIG. 8, (F) shows an output waveform of optical isolator  14  when an optical signal transmitted from a terminal equipment and branched by optical isolator  14  is generated in such polarization modulation states as c 1 , c 2  and c 3  from optical isolator  14  because the polarization state of the optical signal is varied by the transmission line. It is assumed that c 1  exhibits a state wherein the optical signal does not coincide with the polarization plane of polarizer  17 ; c 2  exhibits another state wherein the optical signal coincides well with the polarization plane of polarizer  17 ; and c 3  exhibits an intermediate state between the two stages. Since the polarization state of the optical transmission signal in the transmission line always varies slowly, the states of c 1  to c 3  always exhibit a very slow variation with respect to time. The term “slow” signifies that the rate of the variation is low with respect to the rate of the control signal. (G) shows an optical output waveform of polarizer  17  whose amplitude is minimum in polarization state c 1 , maximum in polarization state c 2 , and medium in polarization state c 3 . (H) shows an output waveform of band pass filter  20  which generates a signal from which noise components other than a control signal frequency component are removed. The noise components in this instance are the information signal and spontaneous emission noise (ASE) of the optical amplifier. (J) shows an output waveform of control circuit  21  which receives the optical signal regularly only in polarization state d 2  in which the band pass filter output exhibits its maximum, but does not receive it in the other polarization states d 1 , d 3  because the amplitude is small. 
     In this manner, in the control signal transmission apparatus provided in repeater  301  of the repeater equipment, since the polarization state of an optical signal in the transmission line varies slowly, there is the possibility that the polarization state of the optical signal may come out of coincidence with the polarization surface of the polarizer, which disables the reception of the optical signal. In order to eliminate this, polarization modulator  9  has a function of varying the polarization state for each control signal block like b 1 , b 2 , b 3  as seen from polarization state (b) of FIG. 7 to produce a polarization state coincident with the polarization surface of polarizer  17 . 
     From an actually conducted examination, a result that the polarization surface of an optical signal and the polarization surface of a polarizer can be made coincide with each other by varying the polarization state approximately three to six times has been obtained. 
     Next, another embodiment of the present invention is described with reference to the drawings. 
     FIG. 9 is a system diagram of another long distance optical transmission system to which the control signal transmission apparatus for an optical transmission system of the present invention is applied. The present optical transmission system includes optical coupler equipment  1100  which branches an optical signal midway of a transmission line so as to be distributed to a plurality of terminal equipments. The control signal transmission apparatus of the present invention can naturally be used in a terminal equipment and a repeater equipment of an optical transmission system which includes such optical coupler equipment  1100  as just described. 
     FIG. 10 is a block diagram showing a construction of a control signal transmission apparatus of a first another embodiment of a transmitter of a transmission terminal or repeater equipment of the optical transmission system of the present invention. 
     The control signal transmission apparatus of the first another embodiment includes n optical transmitters  31  to  3 n each including optical source (LD)  1  for generating an optical signal, optical modulation driving circuit  3  for generating a driving signal for modulating the optical signal in accordance with information signal  2 , and optical modulator  4  for modulating the optical signal with the driving signal, optical coupler  22  for optically wavelength multiplexing the n optical transmitters, control signal generating circuit  5  for generating control signals, polarization modulator driving circuit  6  for modulating the polarization surface of the modulated optical signal in accordance with control signal  51  generated from control signal generating circuit  5 , polarization modulator  7 , polarization modulator driving circuit  8  for modulating the polarization surface of the optical signal with control signal  52  synchronized with control signal  51  and having a longer period, polarization modulator  9 , and optical amplifier  10  for amplifying the optical signal level which has been dropped by polarization modulator  7  and polarization modulator  9  and generating the optical signal of the amplified level. 
     The plurality of optical transmitters  31  to  3 n of the present control signal transmission apparatus modulate optical signals generated from optical sources (LD)  1  and having different wavelengths from one another in accordance with respective information signals each by means of optical modulation driving circuit  3  and optical modulator  4 . The optical signals of the n optical transmitters are optically wavelength multiplexed by optical coupler  22 . Then, the polarization surface of the optical signal from optical coupler  22  is modulated by polarization modulator driving circuit  6  and polarization modulator  7  in accordance with control signal  51  generated from control signal generating circuit  5 . Further, the polarization plane of the optical signal is modulated by polarization modulator driving circuit  8  and polarization modulator  9  with control signal  52  synchronized with control signal  51  and having a longer period. Finally, the optical signal level which has been dropped by the modulators in the preceding stages is amplified by optical amplifier  10 , and the optical signal having the amplified level is generated from optical amplifier  10 . 
     In the control signal transmission apparatus which has optical coupler  22 , optical signals modulated with n information signals  2  of optical transmitters  31  to  3 n are coupled by optical coupler  22 . In the, control signal transmission method, n optical signals are multiplexed first, and then a resulting optical signal is polarization modulated with a control signal. 
     FIG. 11 is a block diagram showing a construction of a control signal transmission apparatus of a second another embodiment of a transmitter of a transmission terminal or repeater equipment of the optical transmission system of the present invention. 
     The present control signal transmission apparatus includes optical transmitters  41  to  4 n each including optical source (LD)  1  for generating an optical signal, optical modulation driving circuit  3  for generating a driving signal for modulating the optical signal in accordance with information signal  2 , optical modulator  4  for modulating the optical signal with the driving signal, control signal generating circuit  5  for generating control signals, polarization modulator driving circuit  6  and polarization modulator  7  for modulating the polarization surface of the modulated optical signal in accordance with control signal  51  generated from control signal generating circuit  5 , and polarization modulator driving circuit  8  and polarization modulator  9  for modulating the polarization surface of the optical signal with control signal  52  synchronized with control signal  51 , optical coupler  23  for optically wavelength multiplexing the n optical signals, and optical amplifier  10  for amplifying the optical signal in an attenuated state. 
     FIG. 12 is a block diagram showing a construction of a control signal transmission apparatus of a receiver of a reception terminal or repeater equipment of a second another embodiment corresponding to the control signal transmission apparatus of FIG.  11 . The apparatus is a repeater equipment corresponding to the terminal equipment of FIG.  11 . The present control signal transmission apparatus includes optical isolator (ISO)  11  and erbium-doped fiber amplifier (EDF)  12  for amplifying an optical signal transmitted from a terminal equipment, wavelength division multiplex coupler (WDM)  13  for coupling the amplified optical signal and pumping light, optical isolator (ISO)  14 , optical coupler (CPL)  15 , polarizer  17  for applying one of branched optical signals and extracting only n optical components of the control signal which have a polarization surface thereof, optical-electrical converter (O/E converter)  25  for applying the n optical signals, n band pass filters  20 ,  26  corresponding to the individual outputs from optical-electrical converter  25 , and n control circuits  21 ,  27  for generating the individual control signals. 
     Further, the present invention can be applied also where a control signal receiving circuit having a same construction as that of a repeater equipment is incorporated in a terminal equipment on the reception side, and also transmission of a control signal between terminal equipments is possible. 
     As a modulation method for a control signal in the present invention, in addition to amplitude modulation, also frequency modulation, phase modulation and so forth can be applied to a control signal transmission system, and the applicable modulation method is not limited to a particular modulation method. 
     The control signal transmission method and apparatus for an optical transmission system of the present invention do not give rise to characteristic deterioration of an information signal since it does not involve an intensity variation of an optical signal. Further, a repeater equipment can demodulate, by addition only of a polarizer, a control signal of a high signal to noise ratio when compared with a conventional repeater equipment.