FR2756995A1 - DISPERSION COMPENSATION OPTICAL CIRCUIT AND OPTICAL TRANSMISSION COMMUNICATION SYSTEM USING THE SAME - Google Patents

Abstract

An optical signal from a second access point (212) of an optical circulator (21) for guiding from a first access point (211) to a second access point (212) and the second access point (212) to a third access point (213), is inputted into a WDM coupler (3). The input optical signal is divided by the WDM coupler (3) into signals having different respective wavelengths, which are then outputted from a plurality of output access points. A light reflector is optically connected to at least one of the output access points of the WDM coupler (3) by a high dispersion medium.

Description

DISPERSION COMPENSATION OPTICAL CIRCUIT AND
COMMUNICATION SYSTEM WITH OPTICAL TRANSMISSION
USING IT
FIELD OF THE INVENTION
The present invention relates to an optical circuit for wavelength division multiplex optical communication in which a plurality of light signals having a plurality of different respective wavelengths propagate through an optical transmission line to carry out communication, and in particular an optical dispersion compensation circuit capable of homogenizing the quantity of dispersion for each signal wavelength and an optical transmission communication system using it.

BACKGROUND OF THE INVENTION
The development of a wavelength division multiplex optical communication system as a means for realizing a high capacity optical communication system has been vigorously carried out, in which a plurality of light signals having different respective wavelengths are propagate through an optical transmission line to increase the amount of information transmitted by the optical transmission line. In addition, a method for improving the transmission capacity by high speed modulation has also been studied.

The combined use of wavelength division multiplex transmission and high speed transmission is most effective for achieving high capacity communication. However, this is disadvantageous in that the dispersion of the transmission line degrades the waveform after transmission, making it impossible, on the reception side, to accurately receive information from the transmitted signals. In addition, the amount of dispersion is different for each wavelength. Consequently, even if the dispersion for a certain signal wavelength is optimized, degradation by dispersion is created in the other wavelengths of the signals.

 Conventional dispersion compensation optical circuits will be explained.

 (1) An optical dispersion compensation circuit in which a dispersion compensation fiber is arranged in the path of a transmission line, in particular just after a transmitter, just before a receiver or before and after a repeater.

 (2) An optical dispersion compensation circuit in which a circulator is used in two stages and a dispersion compensation fiber is used between the two stages.

(3) An optical dispersion compensating circuit comprising a circulator, a dispersion compensating fiber and a light reflector, and in which back and forth in the dispersion compensating fiber is provided to provide a desired amount of dispersion [see Nishimura et al., "Hacho Bunsan Koshoki (Wavelength Dispersion
Compensator) "and Japanese Patent Laid-Open No. 276160/1994].

 The first three optical dispersion compensation circuits mentioned above have the following drawbacks.

 (1) For the first and third optical dispersion compensation circuits, dispersion compensation which manages to process each signal wavelength is impossible, and thus these optical dispersion compensation circuits are not suitable for transmission at WDM (wavelength division multiplexing) using a wide band wavelength range. In addition, the wavelength range should be limited to a considerably narrow band to allow optical dispersion compensation circuits to successfully process WDM transmission.

 (2) The second optical dispersion compensation circuit suffers from drawbacks, such as complicated construction, large size and high cost. In proportion to the number of signal wavelengths, two circulators, two wavelength filters and an optical dispersion compensation circuit must be provided for each increase in the number of wavelengths.

 In addition, the following problems exist, associated with the level of transmission between channels. In WDM transmission, a difference in transmission level between the channels is created, due to the dependence of the loss of the transmission line on the wavelength. Since the difference in transmission level degrades the quality of the transmission, necessitating minimizing the level difference between the wavelength signals, in conventional dispersion compensation optical circuits, it is impossible to perform individual regulation of the difference in level.

SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an optical dispersion compensation circuit for wavelength multiplexing having a simple construction and an optical transmission communication system using it.

 According to the first characteristic of the invention, an optical dispersion compensation circuit comprises: an optical circulator intended to guide an optical signal from a first access point to a second access point and from the second access point to a third access point; a WDM coupler (wavelength division multiplexing) having an input access point connected to the second access point, said WDM coupler being provided for splitting the optical signal, applied as input by the access point input, into a plurality of signals having respective respective wavelengths and outputting the divided signals, respectively by a plurality of output access points; and a light reflector optically connected at least to one of the output access points of the WDM coupler through a dispersion medium.

According to the second characteristic of the invention, an optical dispersion compensation circuit comprises: a first WDM coupler intended to divide an optical signal, applied as input by a first access point, into a plurality of signals having lengths of respective respective waves and outputting the divided signals, respectively by a plurality of output access points; and a second WDM coupler having the same function as the first WDM coupler, the second WDM coupler being provided for outputting the divided optical signals having different wavelengths, applied as input by a plurality of access points of input, from an output access point, the plurality of output access points in the first WDM coupler and the plurality of input access points in the second coupler
WDM being mutually optically connected by a respective high dispersion medium.

 According to the third characteristic of the invention, an optical transmission communication system comprises: a fiber optic transmission line and at least one optical dispersion compensation circuit disposed on the path of the fiber optic transmission line, the circuit dispersion compensation optics comprising: an optical circulator for guiding an optical signal from a first access point to a second access point and from the second access point to a third access point; a WDM coupler having an input access point connected to the second access point, said WDM coupler being provided for dividing the optical signal applied as input by the input access point, into a plurality of signals having respective respective wavelengths and outputting the divided signals respectively from a plurality of output access points; and a light reflector optically connected at least to one of the output access points of the WDM coupler by a dispersion medium.

According to the fourth characteristic of the invention, an optical transmission communication system comprises: a fiber optic transmission line and at least one optical dispersion compensation circuit disposed on the path of the fiber optic transmission line,
the optical dispersion compensation circuit comprising: a first WDM coupler intended to divide an optical signal, applied as input by a first access point, into a plurality of signals having respective respective wavelengths and to output the signals divided, respectively, by a plurality of egress access points; and a second WDM coupler having the same function as the first WDM coupler, the second WDM coupler being provided for outputting the divided optical signals having different wavelengths, applied as input by a plurality of access points of input, by an output access point, the plurality of output access points in the first WDM coupler and the plurality of input access points in the second coupler
WDM being mutually optically connected by a respective medium of high dispersion.

BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be explained in more detail in conjunction with the accompanying drawings, in which
Figure 1 is a block diagram showing a first conventional dispersion compensation optical circuit
Figure 2 is a block diagram showing a second conventional optical dispersion compensation circuit
Figure 3 is a block diagram showing a third conventional dispersion compensation optical circuit
FIG. 4 is a block diagram showing a first preferred embodiment of the optical dispersion compensation circuit according to the invention
FIG. 5 is a block diagram showing a second preferred embodiment of the optical dispersion compensation circuit according to the invention;
FIG. 6 is a block diagram showing a third preferred embodiment of the optical dispersion compensation circuit according to the invention;
FIG. 7 is a block diagram showing a fourth preferred embodiment of the optical dispersion compensation circuit according to the invention;
FIG. 8 is a block diagram in which the optical dispersion compensation circuit according to a preferred embodiment of the invention is illustrated in more detail
Figure 9 is a block diagram in which the optical dispersion compensation circuit according to another preferred embodiment of the invention is illustrated in more detail.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
Before explaining the optical dispersion compensation circuit and the optical transmission communication system using it in the preferred embodiments according to the invention, the first three above-mentioned optical dispersion compensation circuits will be explained.

 Figure 1 shows a first conventional dispersion compensation optical circuit. The light signals having different wavelengths from four respective light sources 41, 42, 43 and 44 are multiplexed by means of a multiplexer 5 and sent to a transmission line 11. A repeater 6 is inserted in the transmission line 11. A demultiplexer 7 is connected to the end of the transmission line 11, and the multiplexed light signal from the transmission line 11 is divided into a plurality of light signals having wavelengths respective ones, which are then transmitted to respective receivers. In the transmission system, a plurality of dispersion compensating fibers (DCF) are inserted on the path of the transmission line 11. The value of the dispersion compensated by the dispersion compensating fibers DCF depends on the wavelength , and at a given time, the compensation produces certain differences in the amount of dispersion between or among the wavelengths of the signals.

 Figure 2 shows a second conventional dispersion compensation optical circuit. The light signals having different wavelengths coming from four respective light sources 41 to 44 are multiplexed by means of a multiplexer 5, and the multiplexed light signal is applied as input by a transmission line 11 in a point access 211 of a first circulator 21, and output by an access point 212. A first wavelength filter 41 ′, permeable to a specific wavelength and reflecting the other wavelengths , is connected to the access point 212. Consequently, the multiplexed wavelength signal, having been applied as input to the access point 212 of the circulator 21, and outputted by the access point 213, comprises light signals having different wavelengths, for example, three light signals having different wavelengths, having been multiplexed.

The wavelength-multiplexed signal is applied at the input to an access point 221 of a second circulator 22, and finally delivered at the output by an access point 223. A second wavelength filter 42 ′, having substantially the same transmission property as the first wavelength filter 41 ', is connected to the access point 222 of the second circulator 22. An access point 412 of the first wavelength filter 41' and a access point 422 of the second wavelength filter 42 ′ are connected to each other by a dispersion compensation fiber
DCF. The length of the dispersion compensating fiber DCF is fixed so that the amount of dispersion of the signal wavelength of the transmission line can be compensated.

 The above construction can produce an optical dispersion compensating circuit able to process a wavelength. To be able to process four wavelengths, four optical dispersion compensation circuits of the above type must be connected in tandem.

Figure 3 shows a third conventional dispersion compensation optical circuit. A light reflector 71 is connected to a circulator 21 by a DCF dispersion compensation fiber. A multiplexed light signal from a multiplexer 5 is transmitted through the circulator 21 and switches back and forth through a dispersion compensation fiber
DCF so as to halve the distance between the circulator 21 and the light reflector 71.

 However, in the above construction, since the wavelengths of the signals, having been multiplexed by the multiplexer 5, are compensated at a given instant in the same manner as described above in relation to FIG. 1, a certain difference in amount of dispersion is created between, or among the wavelengths of the signals.

 Next, the optical dispersion compensation circuit of the first preferred embodiment according to the invention will be explained.

 FIG. 4 shows an optical dispersion compensation circuit according to the first preferred embodiment of the invention.

Optical signals having different respective wavelengths, having been multiplexed and transmitted by the transmission line 11, are applied as input to a first access point 211 of an optical circulator 21, then applied as input to a second access point 212. The second access point 212 is connected to an access point 31 of a coupler
WDM 3. The multiplexed optical signal, having been applied as input to the access point 31 of a WDM 3 coupler, is divided into wavelengths (X1, X2,., Xx) (x being an integer equal to 2 or more), then applied as input to the respective access points 321 to 32x (x being an integer equal to 2 or more) in the WDM coupler 3. Light reflectors 71 to 7x are respectively connected to the access points 321 to 32x by high dispersion media 61 to 6x. Conventional dispersion compensation fiber can be used as 61 to 6x high dispersion media.

 The light signals output by the WDM coupler 3 reach the light reflectors 71 to 7x by the highly dispersed media 61 to 6x. The optical signals reflected by the light reflectors 71 to 7x are applied as input by the high dispersion media 61 to 6x and the access points 321 to 32x in the WDM coupler 3, where the optical signals are multiplexed. The multiplexed optical signal is applied as input by the second access point 212 in the optical circulator 21, then applied as input by a third access point 213 in a transmission line 12.

 The high dispersion media 61 to 6x are arranged separately, so that the amount of dispersion is respectively regulated by the high dispersion media 61 to 6x for each wavelength, making it possible to homogenize the amount of dispersion of the transmission signals output from the third access point 213 of the optical circulator 21 for each wavelength.

 FIG. 5 is an optical dispersion compensation circuit according to the second preferred embodiment of the present invention. In this optical dispersion compensation circuit, a variable light reflector, having the function of regulating the reflectance, is used as light reflectors 71 to 7x.

 FIG. 6 shows the third preferred embodiment of the optical dispersion compensation circuit according to the present invention. In this optical dispersion compensation circuit, variable light attenuators 81 to 8x are inserted between the high dispersion media 61 to 6x and the light reflectors 71 to 7x. The high dispersion media 61 to 6x are respectively provided for the wavelengths so that the amount of dispersion can be compensated for each wavelength. According to this construction, the regulation of the variable light attenuators 81 to 8x allows a difference in transmission level between or among the channels to be eliminated, that is to say the transmission level to be homogenized.

 Variable light attenuators 81 to 8x can be inserted between the WDM 3 coupler and high dispersion media 61 to 6x.

 FIG. 7 shows the fourth preferred embodiment of the optical dispersion compensation circuit according to the present invention. In this optical dispersion compensation circuit, an optical signal multiplexed in wavelength, transmitted by a transmission line 11, is applied as input to an access point 31 of a first WDM coupler 3 where the optical signal multiplexed in wavelength is demultiplexed into a plurality of optical signals having different respective wavelengths, output by the access points 321 to 32x. The demultiplexed optical signals are applied as an input to the respective access points 921 to 92x of a second WDM coupler 9, respectively connected to the access points 321 to 32x by dispersion compensation fibers DCF. The demultiplexed optical signals applied as input to the second WDM coupler 9 are again multiplexed and output by an access point 91. The amount of dispersion can be regulated by the dispersion compensation fibers DCF for each wavelength , making it possible to standardize the amount of dispersion of the transmission signals output from the access point 91 of the second WDM coupler 9.

 The preferred embodiments of the present invention will be explained in more detail.

 FIG. 8 shows a block diagram in which the optical dispersion compensation circuit according to a preferred embodiment of the invention is illustrated in more detail.

 In this optical dispersion compensation circuit, the optical signals having different wavelengths respectively output from four light sources 41 to 44, are multiplexed by means of a multiplexer 5, and the multiplexed optical signal is applied in input to a transmission line 11. The wavelengths of the optical signals coming from the light sources 41 to 44 are respectively 1555 nm, 1557 nm, 1560 nm and 1562 nm.

The multiplexed optical signal applied as input to the transmission line 11 passes through an optical circulator 21 and is applied as input to a coupler
WDM 3. The optical circulator 21 operates by guiding the multiplexed optical signal from a first access point 211 to a second access point 212 and from the second access point 212 to a third access point 213.

 The WDM 3 coupler uses a type of quartz waveguide, and four access points 321, 322, 323 and 324, delivering respectively wavelengths of 1555 nm, 1557 nm, 1560 nm and 1562 nm , connected to it. Thus, the multiplexed light transmitted by the transmission line 11 is demultiplexed by the WDM coupler 3 into optical signals having wavelengths of 1555 nm, 1557 nm, 1560 nm and 1562 nm, respectively, then output from the points d '' access 321, 322, 323 and 324 of the WDM 3 coupler.

 Variable light attenuators 81, 83 and 84 are connected to the access points 321, 322, 323 and 324 of the WDM coupler 3 before or after the highly dispersed media 61, 63 and 64. In addition, light reflectors 71, 72, 73 and 74, constituted by a metallic film and having a reflectance of 99.99%, are connected to the variable light attenuators 81, 83 and 84, on their output sides.

 In this preferred embodiment, a dispersion compensating fiber was used as the high dispersion medium 61, 63 and 64. The light reflectors 71, 73 and 74 were of a fiber network type, the amount of reflection being fixed, and the reflector 72 was such that a metallic film was used as the reflecting sheet and the amount of reflection could vary by varying the angle of the reflection sheet.

 The high dispersion medium can also be of the modulated grating type, and the light reflector can comprise a fiber and a dielectric layer or a metallic layer, deposited on the end face of the fiber.

 Figure 9 shows a block diagram in which the optical dispersion compensation circuit according to another preferred embodiment of the present invention is illustrated in more detail. In this optical dispersion compensation circuit, a type of AWG quartz waveguide was used for each of the first and second WDM couplers 3 and 9. DCF dispersion compensation fibers corresponding respectively to the wavelengths of 1555 nm, 1557 nm, 1560 nm and 1562 nm and a variable light attenuator 81, are connected to four access points 321, 322, 323 and 324 of the first WDM 3 coupler, the outputs of which are multiplexed using the second WDM coupler 9, and the multiplexed optical signal is output from the access point 91.

 As described above, the optical dispersion compensation circuit according to the present invention is advantageous in that the construction is simple, the amount of dispersion can be controlled for each wavelength, and the compensation for transmission loss by a medium of high dispersion can be carried out for each wavelength, making it possible to homogenize the level of transmission.

 The invention has been described in detail with particular reference to preferred embodiments, but it should be understood that variations and modifications can be made within the scope of the present invention as presented in the appended claims.

Claims (18)

 1. An optical dispersion compensation circuit comprising  an optical circulator (21) for guiding an optical signal from a first access point (211) to a second access point (212) and from the second access point (212) to a third access point access (213); a WDM coupler (3) (wavelength division multiplexing) having an input access point (31) connected to the second access point (212), said WDM coupler (3) being provided for dividing the optical signal, applied as input by the input access point, in a plurality of signals having different respective wavelengths and outputting the divided signals, respectively by a plurality of output access points; and a light reflector optically connected at least to one of the output access points of the WDM coupler (3) through a dispersion medium.  2. The optical dispersion compensation circuit according to claim 1, in which the WDM optical coupler (3) is an optical circulator (21).  3. The optical dispersion compensation circuit according to claim 1, wherein the WDM optical coupler (3) comprises a quartz waveguide.  4. The optical dispersion compensation circuit according to claim 3, in which the quartz waveguide is of the AWG (network of waveguides) type.  5. The optical dispersion compensation circuit according to claim 1, wherein the light reflector comprises a light attenuator and a light reflector.  6. The optical dispersion compensation circuit according to claim 5, wherein the light reflector comprises a fiber network.  7. The optical dispersion compensation circuit according to claim 5, wherein the light reflector comprises a dielectric film.  8. The optical dispersion compensation circuit according to claim 5, wherein the light reflector comprises a metallic film.  9. The optical dispersion compensation circuit according to claim 1, wherein the high dispersion medium is a dispersion compensation fiber.  10. The optical dispersion compensation circuit according to claim 1, in which the high dispersion medium is of the modulated lattice type.  11. An optical dispersion compensation circuit, comprising  a first WDM coupler (3) for dividing an optical signal, applied as input by a first access point (211), into a plurality of signals having different respective wavelengths and outputting the divided signals, respectively by a plurality of exit access points; and a second WDM coupler having the same function as the first WDM coupler (3), the second WDM coupler being provided for outputting the divided optical signals having different wavelengths, applied as input by a plurality of dots input access, from an output access point, the plurality of output access points in the first WDM coupler (3) and the plurality of input access points in the second WDM coupler being mutually connected optically by a respective high dispersion medium.  The optical dispersion compensation circuit according to claim 11, wherein at least one of the first and second WDM couplers comprises a quartz waveguide.  13. The optical dispersion compensation circuit according to claim 12, in which the quartz waveguide is of the AWG (waveguide grating) type.  14. The optical dispersion compensation circuit according to claim 11, wherein the high dispersion medium comprises a dispersion compensation fiber.  15. The optical dispersion compensation circuit according to claim 11, wherein the high dispersion medium is of the modulated lattice type.  16. The optical dispersion compensation circuit according to claim 11, wherein the high dispersion medium comprises a variable light attenuator.  17. An optical transmission communication system comprising  a fiber optic transmission line (11); and at least one optical dispersion compensation circuit arranged on the path of the optical fiber transmission line (11),  the optical dispersion compensation circuit comprising: an optical circulator (21) for guiding an optical signal from a first access point (211) to a second access point (212) and from the second access point (212) at a third access point (213); a WDM coupler (3) having an input access point (31) connected to the second access point (212), said WDM coupler (3) being provided for dividing the optical signal, applied as input by the point d input port (31) into a plurality of signals having different respective wavelengths and outputting the signals divided respectively by a plurality of output access points; and a light reflector optically connected at least to one of the output access points of the WDM coupler (3) by a dispersion medium.  18. An optical transmission communication system comprising  a fiber optic transmission line (11) and at least one optical dispersion compensation circuit disposed on the path of the fiber optic transmission line (11), the optical dispersion compensation circuit comprising: a first WDM coupler (3) for dividing an optical signal, applied as input by a first access point (211), into a plurality of signals having different respective wavelengths and outputting the divided signals, respectively from a plurality of exit access points; and a second WDM coupler having the same function as the first WDM coupler (3), the second WDM coupler being provided for outputting the divided optical signals having different wavelengths, applied as input by a plurality of dots input access, from an output access point, the plurality of output access points in the first WDM coupler (3) and the plurality of input access points in the second coupler WDM being mutually optically connected by respective media of high dispersion.