Abstract:
A Raman amplifier applicable to a wavelength division multiplexing optical transmission system is proposed. The Raman amplifier, which can make good use of a wavelength range, comprises an optical fiber for Raman amplification and a pump light introducing means. The zero-dispersion wavelength of the optical fiber and the wavelength of the pump light are nearly equal. The wavelength of pump light is preferably in the range where the absolute magnitude of the chromatic dispersion of the optical fiber is less than 0.3 ps.nm 31 1 .km −1 . A range near a zero-dispersion wavelength cannot be utilized as a signal range because waveform deterioration of signal light occurs due to four wave mixing. This range can be utilized effectively as a wavelength range of exciting light for Raman amplification.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a Raman amplifier that is used in an optical communication system and that amplifies signal light in an optical fiber.  
           [0003]    2. Related Background Arts  
           [0004]    When light is incident on an optical fiber, the light is scattered by the vibration of glass structure of the optical fiber, thereby yielding scattering components on the longer wavelength side and the shorter wavelength side relative to the wavelength of the incident light. The scattering component on the longer wavelength side is called a Stokes line, and the scattering component on the shorter wavelength side is called an anti-Stokes line. The power of the Stokes line is stronger than that of the anti-Stokes line. If signal light having the same wavelength as the Stokes line is incident on an optical fiber at the same time as Stokes line occurs, stimulated Raman scattering is generated. The signal light is thereby Raman-amplified while propagating through the optical fiber. That is, the light performs as pump light for Raman amplification. This phenomenon is used for obtaining a Raman amplifier of discrete type or distributed-constant type. A rare-earth-doped optical fiber amplifier is suitable for amplifying light having a wavelength that corresponds to the energy level, whereas a Raman amplifier is characterized in that it can Amplify signal light having any wavelength by selecting a suitable wavelength of pump light for Raman amplification.  
           [0005]    In the case of an optical fiber made of silica glass as a main component, the transmission loss becomes the smallest around the 1.55 μm wavelength. Also, the strongest stimulated emission occurs at the wavelength that is 14 THz (about 100 nm) away on the longer wavelength side relative to the pump light.  
           [0006]    On the other hand, to avoid the waveform degradation of signal light pulses due to four-wave mixing which is one kind of nonlinear optical phenomenon, the wavelength of signal light and the zero dispersion wavelength of an optical fiber must not overlap each other. Moreover, to allow signal light to be received at an acceptable S/N ratio at the receiving end, and also to avoid the waveform degradation of signal light due to four-wave mixing or cross-phase modulation which is one kind of nonlinear optical phenomenon, the wavelength of signal light and the wavelength of pump light for Raman amplification must not overlap each other.  
           [0007]    Under such restrictions an optical communication system proposed at present that uses a Raman amplifier employs the 1.55 μm band as the spectrum band for signal light, using an optical fiber having a finite chromatic dispersion of a few ps.nm −1 .km −1  at the 1.55 μm wavelength (i.e. non zero dispersion shifted optical fiber). It also employs pump light for Raman amplification near the 1.45 μm wavelength. The zero dispersion wavelength of the non zero dispersion shifted optical fiber is near the 1.50 μm wavelength.  
         SUMMARY OF THE INVENTION  
         [0008]    An object of the present invention is to provide a Raman amplifier where the spectrum band can be used efficiently and that is applicable to a Wavelength Division Multiplexing (WDM) optical communication system.  
           [0009]    In order to achieve this and other objects, a Raman amplifier according to the present invention comprises an optical fiber allowing signal light to be amplified during propagation therethrough and a pump light supplying means for supplying pump light to the optical fiber. The zero dispersion wavelength of the optical fiber is nearly the same as the wavelength of the pump light. Preferably, the wavelength of the pump light is in the wavelength range where the absolute magnitude of the chromatic dispersion of the optical fiber is equal to or less than 0.3 ps.nm −1 .km −1 .  
           [0010]    The pump light may include a plurality of wavelength components. The optical fiber may have a plurality of zero dispersion wavelengths and may be supplied with pump light having wavelengths that correspond to their respective zero dispersion wavelengths. Also optical fiber may have the absolute magnitude of dispersion slope of 0.01 ps.nm −2 .km −1  or more but not exceeding 0.1 ps.nm −2 km −1  at the wavelength of the pump light.  
           [0011]    The Raman amplifier may be wound in a coil form and may be stored within a station. In this case, preferably the optical fiber has an effective area equal to or less than 20 μm 2 , transmission loss equal to or less than 1 dB/km at the wavelength of the signal light, polarization mode dispersion equal to or less than 0.2 ps/km ½  at the wavelength of the signal light, and a length equal to or less than 5 km.  
           [0012]    The Raman amplifier may be installed as an optical transmission line between stations. In this case, preferably the optical fiber has an effective area equal to or more than 45 μm 2 , transmission loss of 0.3 dB/km or less at the wavelength of the signal light, polarization mode dispersion equal to or less than 0.2 ps.km −½  at the wavelength of the signal light, and a length equal to or more than 10 km.  
           [0013]    The above and further objects and novel features of the invention will be more fully clarified from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a schematic diagram of a Raman amplifier  1  according to an embodiment of the present invention.  
         [0015]    [0015]FIG. 2 is a schematic diagram of a Raman amplifier  2  according to an embodiment of the present invention.  
         [0016]    [0016]FIG. 3 shows the relationship between the chromatic dispersion characteristics of optical fibers  11  and  21 , signal light spectrum band, and a pump light wavelength in a first preferable example according to an embodiment of the present invention.  
         [0017]    [0017]FIG. 4 shows the relationship between the chromatic dispersion characteristics of optical fibers  11  and  21 , signal light spectrum band, and a pump light wavelength in a second preferable example according to an embodiment of the present invention.  
         [0018]    [0018]FIG. 5 shows the relationship between the chromatic dispersion characteristics of optical fibers  11  and  21 , signal light spectrum band, and a pump light wavelength in a third preferable example according to an embodiment of the present invention.  
         [0019]    [0019]FIG. 6 shows the relationship between the chromatic dispersion characteristics of optical fibers  11  and  21 , signal light spectrum band, and a pump light wavelength in a fourth preferable example according to an embodiment of the present invention.  
         [0020]    [0020]FIG. 7 shows the relationship between the chromatic dispersion characteristics of optical fibers  11  and  21 , signal light spectrum band, and a pump light wavelength in a referential example according to an embodiment of the present invention.  
         [0021]    [0021]FIG. 8 shows the relationship between the chromatic dispersion characteristics of optical fibers  11  and  21 , signal light spectrum band, and a pump light wavelength in a fifth preferable example according to an embodiment of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]    In the following, preferred embodiments of the present invention will be explained in detail with reference to the accompanying drawings. To facilitate the comprehension of the explanation, the same reference numerals denote the same parts, where possible, throughout the drawings, and a repeated explanation will be omitted. The dimensions in the drawings are partly exaggerated and do not always correspond to actual ratios of dimensions.  
         [0023]    An embodiment of the present invention will be explained mainly with respect to the composition of a Raman amplifier. FIG. 1 is a schematic diagram of a Raman amplifier  1 . The Raman amplifier  1  is equipped with a pump light supplying means  14  and an optical fiber  11  for Raman amplification that is coiled in a module. The pump light supplying means  14  consists of a multiplexer/demultiplexer  12  and a pump light source  13 . The pump light for Raman amplification that has been output from the pump light source  13  is supplied to the optical fiber  11  via the multiplexer/demultiplexer  12 . The signal light incident on an input end  1   a  is Raman-amplified while propagating through the optical fiber  11 . The Raman-amplified signal light is discharged from an output end  1   b  via the multiplexer/demultiplexer  12 . That is, the Raman amplifier  1  is a Raman amplifier of discrete type that Raman-amplifies signal light propagating through the optical fiber  11  coiled in a module. A fiber optic coupler, an interference filter, or a planar waveguide can be used as the multiplexer/demultiplexer  12 .  
         [0024]    [0024]FIG. 2 is a schematic diagram of a Raman amplifier  2 . The Raman amplifier  2  is made of an optical fiber  21  for transmitting signal light that is installed at a repeater section between a repeater station (or a transmitting station)  3  and a repeater station (or a receiving station)  4 . A pump light supplying means  24  is provided within the station  4 . The pump light supplying means  24  comprises a multiplexer/demultiplexer  22  and a pump light source  23 . The pump light for Raman amplification that has been output from the pump light source  23  is supplied to the optical fiber  21  via the multiplexer/demultiplexer  22 . While propagating through the optical fiber  21 , the signal light transmitted from the station  3  suffers from transmission loss peculiar to the optical fiber  21 , and the transmission loss is compensated by Raman-amplification in the optical fiber  21 . Therefore, the signal light reaches the station  4  with sufficient power and is received by it or is transmitted to the downstream via the multiplexer/demultiplexer  22 .  
         [0025]    That is, the Raman amplifier  2  is a Raman amplifier of distributed constant type that Raman-amplifies signal light propagating through the optical fiber  21  installed as an optical transmission line at a repeater section between the stations  3  and  4 . A fiber optic coupler, an interference filter, or a planar waveguide can be used as the multiplexer/demultiplexer  22 .  
         [0026]    Next, the relationship between the chromatic dispersion characteristics of the optical fibers  11  and  21 , signal light wavelength, and pump light wavelength for Raman amplification in the Raman amplifiers  1  and  2  will be explained referring to FIGS.  3  to  8 , respectively. The abscissa of FIGS.  3  to  8  represents wavelength and the ordinate shows dispersion. A curve represents the chromatic dispersion characteristics of the optical fibers  11  and  21 , a double-pointed arrow shows signal light spectrum band, and an upward-pointed arrow shows pump light wavelength for Raman amplification.  
         [0027]    In the case of FIG. 3, the pump light wavelength for Raman amplification and the wavelength where the chromatic dispersion of the optical fibers  11  and  21  becomes zero (hereinafter, zero dispersion wavelength) are nearly the same in the Raman amplifiers  1  and  2 . Preferably the wavelength of the pump light for Raman amplification exists in a wavelength range where the absolute magnitude of the chromatic dispersion of the optical fibers  11  and  21  is equal to or less than 0.3 ps.nm −1 .km −1 . In the case of FIG. 3, the wavelength of the pump light for Raman amplification is 1450 nm. Also, the signal light spectrum band includes the 1550 nm wavelength and it is about 100 nm longer than the pump light wavelength for Raman amplification. The vicinity of the zero dispersion wavelength of the optical fibers  11  and  21  can be used as a pump light wavelength for Raman amplification but cannot be used as a signal light spectrum band because the waveform degradation of signal light occurs there due to four-wave mixing.  
         [0028]    In an optical transmission system using a conventional Raman amplifier, it has been impossible to employ the 1.50 μm band in addition to the 1.55 μm band as the spectrum band of signal light because four-wave mixing easily causes the waveform degradation of signal light since the dispersion of the optical fiber is zero near 1.50 μm, even if Raman amplification can be done. On the other hand, in the case of a Raman amplifier according to the present invention, it is possible to use a spectrum band efficiently to increase the transmission capacity by employing the vicinities of 1.40 μm and 1.45 μm as the wavelength of pump light for Raman amplification while using the 150 μm band and the 1.55 μm band as the spectrum band of signal light, whereby allowing Raman amplification to be done without causing the waveform degradation of signal light due to four-wave mixing.  
         [0029]    Also, as in the case shown in FIG. 4, the pump light for Raman amplification preferably includes a plurality of wavelength components. One of the wavelength components of the pump light for Raman amplification nearly corresponds to the zero dispersion wavelength of the optical fibers  11  and  21 . Preferably each wavelength component of the pump light for Raman amplification exists in the wavelength range where the absolute magnitude of the chromatic dispersion of the optical fibers  11  and  21  is equal to or less than 0.3 ps.nm −1 .km −1 . In the case of FIG. 4, each wavelength component of the pump light for Raman amplification is 1450 nm or around the vicinity thereof. The signal light spectrum band includes the 1550 nm wavelength and it is about 100 nm longer than the pump light wavelength for Raman amplification.  
         [0030]    In this case also, the spectrum band can be used efficiently as in the case shown in FIG. 3. The signal light spectrum band can be broadened more than in the case shown in FIG. 3 by suitably setting the power of the respective wavelength components of the pump light for Raman amplification. Furthermore, the gain spectrum can be made flat in a wide band. The supply of pump light for Raman amplification including such a plurality of wavelength components may be achieved using a plurality of semiconductor laser sources having different output wavelengths such that the light output from each semiconductor laser source is multiplexed into pump light for Raman amplification to be supplied to the optical fibers  11  and  21 .  
         [0031]    It is also preferable that the optical fibers  11  and  21  have a plurality of zero dispersion wavelengths as in the cases shown in FIGS. 5 and 6, and that pump light for Raman amplification having the wavelengths nearly corresponding to the respective zero dispersion wavelengths be supplied to the optical fibers  11  and  21 . In the example shown in FIG. 5, the optical fibers  11  and  21  have two zero dispersion wavelengths (1450 nm and 1610 nm), and pump light for Raman amplification having wavelengths λ 1  and λ 2  nearly corresponding to the respective zero dispersion wavelengths is supplied to the optical fibers  11  and  21 . The signal light spectrum band Λ 1  includes the 1550 nm wavelength and it is about 100 nm longer than the pump light wavelength λ 1  for Raman amplification. The signal light spectrum band Λ 2  includes the 1710 nm wavelength and it is about 100 nm longer than the pump light wavelength λ 2  for Raman amplification.  
         [0032]    In the example shown in FIG. 6, the optical fibers  11  and  21  have three zero dispersion wavelengths (1335 nm, 1465 nm and 1600 nm), and pump light for Raman amplification having wavelengths λ 1 , λ 2  and λ 3  nearly corresponding to the respective zero dispersion wavelengths is supplied to the optical fibers  11  and  21 . The signal light spectrum band Λ 1  includes the 1435 nm wavelength and it is about 100 nm longer than the pump light wavelength λ 1  for Raman amplification. The signal light spectrum band Λ 2  includes the 1565 nm wavelength and it is about 100 nm longer than the pump light wavelength λ 2  for Raman amplification. The signal light spectrum band Λ 3  includes the 1700 nm wavelength and it is about 100 nm longer than the pump light wavelength λ 3  for Raman amplification.  
         [0033]    The spectrum band can be efficiently used in the cases shown in FIGS. 5 and 6 as well. In the cases shown in FIGS. 3 and 4, the dispersion compensation to be made at a terminal station is large because the chromatic dispersion of the optical fibers  11  and  21  in the signal light spectrum band is about 5 ps.nm −1 .km −1  to 10 ps.nm −1 .km −1 . On the other hand, in the cases shown in FIGS. 5 and 6, the absolute magnitude of the chromatic dispersion of optical fibers  11  and  21  is small in a wide spectrum band, and the absolute magnitude of the chromatic dispersion of the optical fibers  11  and  21  in the signal light spectrum band may be about 0.5 ps.nm −1 .km −1  to 2 ps.nm −1 .km −1 , the dispersion compensation to be made at a terminal station is comparatively small. This is also advantageous because the cost needed for dispersion compensation can be relatively reduced. Moreover, the examples shown in FIGS. 5 and 6 are effective for the performance of soliton transmission where the optical fibers  11  and  21  are required to have an extraordinary dispersion of about +0.5 ps.nm −1 .km −1  in the signal light spectrum band.  
         [0034]    When the optical fibers  11  and  21  have a plurality of zero dispersion wavelengths, the following point should be noted. That is, if pump light for Raman amplification having a wavelength that nearly corresponds to the zero dispersion wavelength of the shorter wavelength side is supplied to the optical fibers  11  and  21  when the wavelength interval between one zero dispersion wavelength and another zero dispersion wavelength is about 100 nm as shown in FIG. 7, the signal light spectrum band where optical amplification can be done by the pump light for Raman amplification includes other zero dispersion wavelength on the longer wavelength side. As a result, the waveform degradation of the signal light tends to occur due to four-wave mixing or cross-phase modulation. Therefore, the wavelength interval between one zero dispersion wavelength and another zero dispersion wavelength should not be about 100 nm.  
         [0035]    On the other hand, when the wavelength interval between one zero dispersion wavelength and another zero dispersion wavelength is about 50 nm to 80 nm as shown in FIG. 8, the above-mentioned problem does not occur. In the example shown in FIG. 8, the optical fibers  11  and  21  have two zero dispersion wavelengths (1450 nm and 1510 nm), and pump light for Raman amplification having wavelengths λ 1  and λ 2  that nearly correspond to the respective zero dispersion wavelengths is supplied to the optical fibers  11  and  21 . A signal light spectrum band Λ 1  includes the 1550 nm wavelength and it is about 100 nm longer than the pump light wavelength λ 1  for Raman amplification. A signal light spectrum band Λ 2  includes the 1610 nm wavelength and it is about 100 nm longer than the pump light wavelength λ 2  for Raman amplification. Neither of the signal light spectrum bands Λ 1  and Λ 2  include the zero dispersion wavelength of the optical fibers  11  and  21 . Therefore, the waveform degradation of signal light due to four-wave mixing or cross-phase modulation is suppressed.  
         [0036]    Moreover, the example shown in FIG. 8 is advantageous in the following respect. That is, the pump light for Raman amplification having the wavelength λ 2  exists in the band where Raman scattering is caused by the pump light for Raman amplification having the wavelength λ 1 , and thereby the pump light for Raman amplification having the wavelength λ 2  is Raman-amplified. As a result, increased gain of the optical amplification is obtained in the signal light spectrum bandΛ 2 .  
         [0037]    Next, a preferable constitution of the optical fibers  11  and  21  in the Raman amplifiers  1  and  2  will be explained. The refractive index profile of the optical fibers  11  and  21  is optional, but their preferable properties are as follows.  
         [0038]    The optical fiber  11  used for the Raman amplifier  1  of discrete type, which is coiled into a module and stored within a station, preferably has an effective area equal to or less than 20 μm 2 , transmission loss of 1 dB/km or less at a signal light wavelength, polarization mode dispersion equal to or less than 0.2 ps.km −½  at a signal light wavelength, and a length equal to or less than 5 km.  
         [0039]    The optical fiber  11  is suitable for storage in a coil form because the length is equal to or less than 5 km. Since the optical fiber  11  can enhance the power density of pump light for Raman amplification because the effective area is equal to or less than 20 μm 2 , gain of optical amplification can be sufficiently obtained even if the length is equal to or less than 5 km. Since the polarization mode dispersion of the optical fiber  11  at a signal light wavelength is equal to or less than 0.2 ps.km −½ , the transmission loss characteristics of signal light are excellent even if the bit rate is 10 Gb/s or more.  
         [0040]    On the other hand, the optical fiber  21  used for the Raman amplifier  2  of distributed type, which is installed as a transmission line between stations, preferably has an effective area equal to or more than 45 μm 2 , transmission loss of 0.3 dB/km or less at a signal light wavelength, polarization mode dispersion at a signal light wavelength equal to or less than 0.2 ps.km −½ , and a length equal to or more than 10 km.  
         [0041]    The optical fiber  21  can obtain sufficient gain of optical amplification as a whole although the gain of optical amplification at a unit length is small because it has a long length sufficient for installation at a repeater section and because it has an effective area equal to or more than 45 μm 2 . In the optical fiber  21 , the occurrence of nonlinear optical phenomenon such as self-phase modulation is suppressed because the effective area is equal to or more than 45 μm 2 . The overall transmission loss of the optical fiber  21  is small notwithstanding that it has a long length sufficient for installation at a repeater section because the transmission loss at a signal light wavelength is equal to or less than of 0.3 dB/km. The optical fiber  21  has excellent transmission loss characteristics of signal light even if the bit rate is 10 Gb/s or more because the polarization mode dispersion at a signal light wavelength is equal to or less than 0.2 ps.km −½ .  
         [0042]    Also, the optical fibers  11  and  21  preferably have the absolute magnitude of dispersion slope equal to or more than 0.01 ps.nm −2 .km −1  but not more than 0.1 ps.nm −2 .km −1  at the wavelength of pump light for Raman amplification. Then, the absolute magnitude of the chromatic dispersion of the optical fibers  11  and  21  becomes equal to or more than 0.5 ps.nm −1 .km −1  but not more than 10 ps.nm −1 .km −1  in a signal light spectrum band including the wavelength and it is about 100 nm longer than a pump light wavelength for Raman amplification. Since the absolute magnitude of the chromatic dispersion of the optical fibers  11  and  21  is within such range at the signal light spectrum band, the waveform degradation of signal light due to four-wave mixing is suppressed.