Optical communications system

Abstract: In an optical communication system, a transmitted signal (1) is modulated with a subcarrier signal having a frequency higher than the frequency of said transmitted signal on the transmission side, the modulated signal light emitted to a transmission channel. On the reception side (2), the received signal is filtered and decoded by means of a band-pass filter (23) having a center frequency approximately equal to the frequency of the subcarrier signal of the received signal. The above arrangement makes it possible to achieve an arrangement free from the influence of deterioration in the degree of modulation due to spontaneous emission generated from an optical amplifier disposed midway along the transmission channel. The optical amplifier (3) may be an optical differential gain amplifier which has the differential-gain optical input/output characteristics of outputting spontaneous emission of low intensity according to a digital signal value "0" and output light of high intensity according to a digital signal value "1". The arrangement enables optical amplification of signal light without the risk of outputting high-level spontaneous emission due to direct current excitation. It is therefore possible to provide received signals of high quality by eliminating any deterioration in S/N ratio and any deterioration of the degree of modulation of signal light due to spontaneous emission from the optical amplifier disposed midway along the transmission channel.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to an optical communication system and, more particularly, to an optical communication system in which an optical amplifier is disposed midway along a transmission channel or at one end thereof. The present invention also relates to an optical communication system which provides optical communication through an optical communication channel provided with at least one optical differential gain amplifier. Description of the Related Art In an optical communication system of the type which utilizes an optical fiber as an optical transmission channel or, optical spatial propagation, as an optical transmission channel, the attenuation of an optical signal over the transmission channel leads to a deterioration in the signal/noise ratio (S/N ratio) of the optical receiver. The result is deterioration in the quality of the received signal and impairment of the reliability of the optical transmission itself. For this reason, the attenuation of the optical signal presents significant problems. To cope with the above problems, one method is considered. In this method, an optical fiber whose attenuation of optical signals is reduced is used as an optical transmission channel, and an optical amplifier is disposed midway along the optical fiber so as to amplify and retransmit the signal light. Fig. 6 shows a conventional type of optical communication system provided with such an optical amplifier. As shown in Fig. 6, the conventional optical communication system is arranged so that a transmitted digital signal 5 is input to a laser driving circuit 12. A semiconductor laser 13 is turned on or off in accordance with whether the value of the transmitted digital signal 5 is a "1" or a "0". A laser light signal, which is emitted when the semiconductor laser 13 is on, is transmitted to an optical amplifier 43 over an optical fiber 41. The optical amplifier 43 amplifies the light intensity which has been attenuated due to transmission over the optical fiber 41, and assures the required light intensity. The light output from the optical amplifier 43 is transmitted over an optical fiber 42 and is incident on an optical detector 21. The incident light is converted by the optical detector 21 into an electrical signal corresponding to the incident light intensity. The received light signal is amplified and delivered as a received signal 6 by an amplifier 22. In Fig. 6, there are also shown examples of transmission signal waveforms. However, the optical amplifier 43 used in the aforesaid conventional optical communication system entails a number of problems. For example, the optical amplifier 43 not only effects optical amplification of signal light but also outputs high-level spontaneous emission due to direct current excitation. The result is a deterioration in the S/N ratio or the quenching ratio due to spontaneous emission and it has, therefore, been impossible to realize satisfactory transmission quality. More specifically, the signal output from the optical amplifier 43 encounters the following problems: 1) The degree of optical modulation (on-off ratio) deteriorates with respect to the input light. 2) So-called beat noise occurs due to interference between the input signal light and the high-level spontaneous emission due to direct current excitation, and this leads to a deterioration in the S/N ratio. SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide an optical communication system which is capable of providing received signals of improved quality by eliminating any deterioration in S/N ratio and any deterioration of the degree of modulation of signal light due to spontaneous emission from an optical amplifier. According to the invention this object is accomplished by an optical communication system, comprising: optical output means for emitting an optical output corresponding to a digital output signal; receiving means for receiving said optical output emitted from said optical output means; an optical communication channel made of an optical transmission channel for transmitting said optical output emitted from said optical output means to said receiving means; said optical communication system being characterized in that at least one optical bistable element is disposed along said optical communication channel; whereby optical communication is provided through said optical communication channel. The optical bistable element can be an optical differential gain amplifier having the differential-gain type optical input/output characteristic of providing spontaneous emission of extremely reduced intensity according to a digital signal value "0" and output light of extremely increased intensity according to a digital signal value "1". In the above arrangement, the optical differential gain amplifier is an oscillation type differential gain amplifier whose optical threshold is set to an intermediate approximate intensity between the values "1" and "0" of optical digital signals. The arrangement outputs a weak spontaneous emission in response to an optical digital signal denoting a "0" and high-intensity laser light in response to an optical digital signal denoting a "1", thereby eliminating any deterioration in S/N ratio and quenching ratio due to spontaneous emission, as with a progressive type of optical amplifier, and providing improved transmission quality. The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a conceptual diagram of an example of an optical communication system; Fig. 2 is a diagram showing the waveforms of an electrical signal and an optical signal, taken at the points A-E of Fig. 1; Fig. 3 is a conceptual diagram of an optical communication system according to a first embodiment of the present invention; Fig. 4 is a conceptual diagram of an optical communication system according to a second embodiment of the present invention; Fig. 5 is a conceptual diagram showing in detail the structure of a bistable element according to the second embodiment of the present invention; and Fig. 6 is a conceptual diagram showing a conventional type of optical communication system. DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Fig. 1 is a block diagram of an example of an optical communication. The illustrated example essentially comprises an optical transmitter 1, an optical receiver 2, an optical amplifier 3 and optical fibers 41 and 42 serving as optical transmission channels. A transmitted signal and a received signal are denoted by 5 and 6, respectively. The optical transmitter 1 includes a mixer 10, a local oscillator 11 for generating a subcarrier signal as a predetermined high-frequency signal, a laser driving circuit 12 for turning on or off a semiconductor laser 13 in accordance with whether the input signal is a "1" or a "0", and the semiconductor laser 13. The light emitted from the semiconductor laser 13 is made incident upon the optical fiber 41 and transmitted to the optical amplifier 3 therethrough. The optical receiver 2 comprises the following major elements: an optical detector 21 made from, for example, a PIN type photodiode for converting the incident light from the optical fiber 42 into a corresponding electrical signal; an amplifier 22 for amplifying the electrical signal from the optical detector 21; a band-pass filter 23 having a center frequency equal to the frequency of the subcarrier signal generated from the local oscillator 11; and a detector 24. The traveling wave semiconductor amplifier disclosed in Japanese Patent Publication No. 57/6715 may be suitably used for the optical amplifier 3 in the example. The operation of the example having the aforesaid arrangement will be explained below with reference to Fig. 2. Fig. 2 is a conceptual diagram showing the waveforms of the electrical or optical signals provided at the points shown at A, B, C, D and E in Fig. 1. The transmitted signal 5 provided at point A, which is a binary signal denoting "1" or "0" according to the data transmitted, is input to the optical transmitter 1 in the form shown in, for example, part A of Fig. 2. The mixer 10 mixes the transmitted signal 5 with the subcarrier signal of a predetermined frequency (high frequency) which is generated from the local oscillator 11, thereby providing the modulated signal shown in part B of Fig. 2. This modulated signal is input to the laser driving circuit 12. The laser driving circuit 12 turns on or off the semiconductor laser 13 in accordance with the modulated signal, thereby modulating an optical signal output. As is known, since the semiconductor laser 13 emits light only when a forward voltage is applied, the optical output from the semiconductor laser 13 is as shown in part C of Fig. 2. The light emitted from the semiconductor laser 13 propagates down the optical fiber 41 to the optical amplifier 3. The optical amplifier 3 amplifies the incident light to light having a predetermined intensity and outputs it to the optical fiber 42. The light propagates over the optical fiber 42 to the optical detector 21 of the optical receiver 2. The light incident on the optical detector 21 is converted into an electrical signal corresponding to the intensity of the light received by the detector 21. After the electrical signal has been amplified, it is transmitted to the band-pass filter 23. The band-pass filter 23 is, as described above, a band-pass filter having a predetermined frequency band centered around the modulation frequency used on the transmitter side (or frequency of the subcarrier signal generated by the local oscillator 11). The band-pass filter 23 eliminates unwanted signal components such as noise components other than the subcarrier signal. The subcarrier signal alone is transmitted to the detector 24 for the purpose of envelope detection, and is decoded into the original frequency signal. The decoded signal is output as the received signal 6 shown in part E of Fig. 2 which is approximately similar to the transmitted signal 5. In the example, spontaneous emission is still combined with the output from the optical amplifier 3, and a significant influence is exerted over the output from the optical detector 21. As a result, the output from the optical amplifier 22 deteriorates in the degree of modulation and S/N ratio with respect to the transmitted signal 5. However, the electrical frequency spectrum of the spontaneous emission from the optical amplifier 3 is in the form in which white noise is combined with direct current, and even after the optical signal has been converted into an electrical signal, a similar form of frequency spectrum remains. Accordingly, a substantial portion of the noise signal power due to spontaneous emission does not pass through the band-pass filter 23. In contrast, the optical signal transmitted from the optical transmitter 1 is subjected to modulation using the subcarrier signal from the local oscillator 11, and the center frequency of the spectrum is coincident with the center frequency of the band-pass filter 23. Thus, this modulated signal power can substantially pass through the band-pass filter 23. Accordingly, the output signal from the band-pass filter 23 is of improved quality which is free from the influence of spontaneous emission as shown in part D of Fig. 2. Therefore, the signal obtained by envelope-detection of the signal of part D approximately coincides with the input signal of the optical transmitter 1 as shown in part E of Fig. 2, whereby output signals which excel in the degree of modulation and S/N ratio can be obtained. For example, although the optical fiber communication system example, although the optical fiber communication system using an optical fiber as its optical transmission channel has been explained in connection with Fig. 1, the arrangement can also be applied to spatial propagation optical communications. In this case, it suffices to provide an optical amplifier at at least either the output of the optical transmitter or the input of the optical receiver. The arrangement of Fig. 1 is shown as a unilateral communication system in which an optical transmitter is disposed on one side of a transmission channel with an optical receiver disposed on the other side of the transmission channel. The arrangement can also be applied to a bilateral communication system which has an optical transmitter and an optical receiver on each end of the transmission channel so that an optical signal is separated and mixed at each end of the transmission channel. In addition, although the example has been explained with reference to the communication between two points, the arrangement can also be effectively applied to a so-called optical communications network in which communication channels are provided among three points or more to realize communications between arbitrary two points or more by means of exchange or the like. The example has been explained with reference to the optical communication system in which the optical transmitter is provided with only one light source. However, it is apparent that the arrangement can be applied to a so-called wavelength-multiplex optical communication system in which the optical transmitter is provided with a synthesizer and a plurality of light sources having different wavelengths, while the optical receiver is provided with a wavelength divider, so that a plurality of signals are communicated at the same time. It will be appreciated from the foregoing that, according to the example, there is provided an optical communication system employing an optical amplifier, which system can achieve improved transmission quality by eliminating any deterioration in S/N ratio and any deterioration of the degree of modulation due to spontaneous emission from the optical amplifier by means of a simplified arrangement. This arrangement is constructed to multiplex an input signal with a subcarrier signal and transmit the modulated signal, filter the received signal by means of a band-pass filter having a center frequency approximately equal to the frequency of the subcarrier signal, and detects the envelope of the filtered signal. The foregoing is an explanation of the example in which any deterioration in S/N ratio and any deterioration of the degree of modulation due to spontaneous emission from the optical amplifier is eliminated by means of the simplified arrangement which is constructed to multiplex an input signal with a subcarrier signal and transmit the modulated signal, filter the received signal by means of a band-pass filter having a center frequency approximately equal to the frequency of the subcarrier signal, and detect the envelope of the filtered signal. However, the arrangement for eliminating any deterioration in S/N ratio and any deterioration of the degree of modulation due to spontaneous emission from the optical amplifier is not limited to the above-described one. For example, it is also possible to achieve improved transmission quality by means of an arrangement which outputs weak spontaneous emission in response to an optical digital signal denoting a "0" and high-intensity laser light in response to an optical digital signal denoting a "1", thereby eliminating any deterioration in S/N ratio and quenching ratio due to spontaneous emission, as with a progressive type of optical amplifier. A first embodiment of the present invention having the aforesaid arrangement will be explained below in detail. Fig. 3 is a block diagram which represents the features of the first embodiment of the present invention in the best mode. The illustrated first embodiment essentially comprises an optical transmitter 100, an optical receiver 200, an optical differential gain amplifier 103 having optical input/output characteristics of the differential gain transfer type, and optical fibers 141 and 142 serving as optical transmission channels. The optical threshold of the optical differential gain amplifier 103 is set to an appropriate intermediate light intensity between the values "1" and "0" of optical digital signals so that weak spontaneous emission is delivered in response to an optical digital signal denoting a "0" with high-intensity laser light delivered in response to an optical digital signal denoting a "1". A transmitted signal and a received signal are denoted by 105 and 106, respectively. The optical transmitter 100 includes a laser driving circuit 112 for turning on or off a semiconductor laser 113 in accordance with whether the transmitted signal 105 is a "1" or a "0", and the semiconductor laser 113. The light emitted from the semiconductor laser 113 is made incident upon the optical fiber 141 and transmitted to the optical differential gain amplifier 103. The optical receiver 200 comprises the following major elements: an optical detector 221 made from, for example, a PIN type photodiode for converting the incident light from the optical fiber 142 into a corresponding electrical signal; an amplifier 222 for amplifying the electrical signal from the optical detector 221; a low-pass filter 223; and a detector 224 for regenerating and shaping a waveform. The optical differential gain amplifier 103 employed in the first embodiment is arranged to provide optical input/output characteristics of the differential gain transfer type by utilizing adjustment of the injection current of a bistable laser, as described in "Bistability and Pulsations in Semiconductor Lasers with Inhomogenous Current Injection", reported by "C.HARDER" in " IEEE JOURNAL OF QUANTUM ELECTRONICS Vol. QE-18, No. 9" of 1982, pp. 1351-1361. The operation of the first embodiment having the aforesaid arrangement will be explained below. The transmitted signal 105 is input to the laser driving circuit 112 as a binary digital signal. The laser driving circuit 112 generates a current pulse signal proportional to whether the transmitted signal 105 is a"1" or a"0", and supplies it to the semiconductor laser 113. The semiconductor laser 113 outputs an optical digital signal corresponding to the transmitted signal 105, and the optical signal output is conducted to the optical differential gain amplifier 103 through the optical fiber 141. The optical differential gain amplifier 103 is adjusted in the following manner in order to provide optical input/output characteristics of the differential gain function type based on the above-described adjustment of the injection current of the bistable laser. The optical threshold of the optical differential gain amplifier 103 is set to an appropriate intermediate light intensity between the values "1" and "0" of optical digital signals (for example, a light intensity equal to approximately the middle between "1" and "0" represented by the optical digital signals of the received light) so that weak spontaneous emission is delivered in response to an optical digital signal denoting a "0" with high-intensity laser light delivered in response to an optical digital signal denoting a "1". With this arrangement, it is possible to amplify the optical signal without deteriorating the quenching ratio. The optical output from the optical differential gain amplifier 103 is conducted to the optical receiver 200 through the optical fiber 142. The optical signal, passed through the optical fiber 142, is input to the optical receiver 200. The incident light is converted by the optical detector 221 into an electrical signal corresponding to the incident light intensity and is then amplified by the amplifier 222. The low-pass filter 223 eliminates the signals contained in an unwanted band, and the output from the low-pass filter 223 is subjected to waveform shaping in the waveform regenerating and shaping circuit 224. The circuit 224 outputs the received signal 106 similar to the transmitted signal 105. In the first embodiment described above, the optical differential gain amplifier 103 is arranged as an oscillation type differential gain amplifier, and the optical threshold of this optical amplifier is set to an appropriate intermediate light intensity between the values "1" and "0" of optical digital signals. It is, therefore, possible to output weak spontaneous emission in response to an optical digital signal denoting a "0" and high-intensity laser light in response to an optical digital signal denoting a "1". Accordingly, it is possible to provide a highly reliable optical communication system which can effect optical amplification of signal light without the risk of outputting high-level spontaneous emission due to direct current excitation. It is, therefore, possible to accomplish an optical communication system having excellent reliability by performing optical communication utilizing the above optical amplifier disposed midway along an optical transmission channel. Although the first embodiment has been explained with reference to the example in which the differential gain optical amplifier is disposed midway along the optical fiber as an optical amplifier, the present invention is not limited to the above-described first embodiment, either. The present invention may utilize an arrangement employing a GaAs Fabri-Perot etalon as a second embodiment. Fig. 4 shows an arrangement in which the GaAs Fabri-Perot etalon is employed as a differential-gain type optical amplifier unit. The arrangement shown in Fig. 4 comprises a bistable element 307 of the etalon type made from, for example, GaAs, an optical mixer 308, and optical fibers 391-394. An input optical signal and an output optical signal are denoted by 300 and 301, respectively. Fig. 5 shows in detail the structure of the optical bistable element 307 used in the second embodiment. The optical bistable element 307 used in the second embodiment has a structure such as that described in, for example, Applied Physics Letters of 1987, Vol. 51, No. 13, pp. 94-96, the paper entitled "GaAs-AlAs monolithic microresonator arrays", by J.L.Jewell et al. The optical bistable element 307 comprises a DBR mirror 371 including a total of seven pairs of AlAs/GaAs elements, a GaAs light emitting element 372 having a thickness of 2 µm, a DBR mirror 373 including a total of nine pairs of AlAs/GaAs elements, and a GaAs substrate 374. The GaAs substrate 374 is provided with an opening a having a predetermined diameter so as to effect a light transmitting operation. In the second embodiment having the above-described structure, the input optical signal 300 and excitation light 302 are synchronously input to the optical bistable element 307. The optical bistable element 307 is set to exceed its optical threshold and exhibit high transmittance only when it receives the input optical signal 300 and the excitation light 302 at the same time. Accordingly, only when receiving a digital signal denoting a "1", the optical bistable element 307 outputs high-intensity light as the output optical signal 301 but, in the case of a digital signal denoting a "0", the element 307 does not provide the output optical signal 301. Accordingly, it is possible to provide a highly reliable optical communication system which can achieve optical amplification of signal light without the risk of outputting high-level spontaneous emission due to direct current excitation and without any deterioration in the quenching ratio or S/N ratio of the signal. It is, therefore, possible to accomplish an optical communication system having excellent reliability by performing optical communication utilizing the above optical amplifier disposed midway along an optical transmission channel. As is apparent from the foregoing, there is provided an optical communication system employing an optical amplifier disposed along an optical transmission channel, which system can achieve improved transmission quality by using a differential gain optical amplifier as the optical amplifier to eliminate any deterioration in S/N ratio and quenching ratio due to spontaneous emission which has otherwise been experienced with a travelling-wave type optical amplifier. While the present invention has been described with respect to what is presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, the present invention is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

Claims: An optical communication system, comprising: optical output means (1; 100) for emitting an optical output (300) corresponding to a digital output signal (5; 105); receiving means (2; 200) for receiving said optical output (300) emitted from said optical output means (1; 100); an optical communication channel (41, 42; 141, 142; 391, 393, 394) made of an optical transmission channel for transmitting said optical output (300) emitted from said optical output means (1; 100) to said receiving means (2; 200); said optical communication system being characterized in that at least one optical bistable element (307; 103) is disposed along said optical communication channel (41, 42; 141, 142; 391, 393, 394); whereby optical communication is provided through said optical communication channel (41, 42; 141, 142; 391, 393, 394). An optical communication system according to claim 1, characterized in that excitation light inputting means (302, 392) is disposed along said optical communication channel (41, 42; 141, 142; 391, 393, 394); said optical bistable element (307) is set to exceed its optical threshold and exhibit high transmittance when an said optical output (300) and excitation light are input to said optical bistable element (307) at the same time; and said excitation light inputting means (302, 392) is arranged to supply said excitation light to said optical bistable element (307) in synchronism with said optical output (300). An optical communication system according to claim 1, characterized in that said optical bistable element is a differential gain amplifier (103) disposed on said optical communication channel (141, 142), said optical differential gain amplifier (103) having the differential-gain type optical input/output characteristic of providing spontaneous emission of extremely reduced intensity according to a digital signal value "0" and output light of extremely increased intensity according to a digital signal value "1". An optical communication system according to claim 3, characterized in that said optical differential gain amplifier (103) has optical input/output characteristics of the differential gain transfer type which utilizes adjustment of the injection current of a bistable laser, the optical threshold of said optical differential gain amplifier (103) being set to an intermediate approximate intensity between the values "1" and "0" of optical digital signals. An optical communication system according to claim 1, characterized in that said optical bistable element (307) is of the étalon type composed of GaAs. An optical communication system according to claim 3 or 5, characterized in that said optical output means (1; 100) converts a modulated signal to a corresponding optical output (300) and emits said optical output (300) into said transmission channel by means of a semiconductor laser (13; 113). An optical communication system according to any of the preceding claims 1 to 6, characterized in that said optical output means (1; 100) is disposed on a transmission side; and said receiving means (2; 200) is disposed on a reception side.

Inventors: NAKAMURA KENJI, NITTA JUN, NAKAMURA, KENJI, NITTA, JUN
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1990
Main classification: H04B10
IPC codes: H04B10