Raman amplifier, optical repeater, and raman amplification method

Abstract

A Raman amplifier according to the present invention comprises a plurality of pumping means using semiconductor lasers of Fabry-Perot, DFB, or DBR type or MOPAs, and pumping lights outputted from the pumping means have different central wavelengths, and interval between the adjacent central wavelength is greater than 6 nm and smaller than 35 nm. An optical repeater according to the present invention comprises the above-mentioned Raman amplifier and adapted to compensate loss in an optical fiber transmission line by the Raman amplifier. In a Raman amplification method according to the present invention, the shorter the central wavelength of the pumping light the higher light power of said pumping light. In the Raman amplifier according to the present invention, when a certain pumping wavelength is defined as a first channel, and second to n-th channels are defined to be arranged with an interval of about 1 THz toward a longer wavelength side, the pumping lights having wavelengths corresponding to the first to n-th channels are multiplexed, and an pumping light having a wavelength spaced apart from the n-th channel by 2 THz or more toward the longer wavelength side is combined with the multiplexed light, thereby forming the pumping light source. The pumping lights having wavelengths corresponding to the channels other than (n-1)-th and (n-2)-th channels may be multiplexed, thereby forming the pumping light source. The pumping lights having wavelengths corresponding to the channels other than (n-2)-th and (n-3)-th channels may be multiplexed, thereby forming the pumping light source.

1 . A method for optically amplifying a wavelength division multiplex optical signal in an optical fiber configured to be a Raman amplifying medium, comprising steps of:
 generating a plurality of pump lights;   injecting the plurality of pump lights to an optical fiber configured to be a Raman amplifying medium while the wavelength division multiplex optical signal propagates though the optical fiber; and   controlling respective levels of the plurality of pump lights to adjust at least one of a wavelength-dependent loss characteristic of the optical fiber and a wavelength-dependent noise characteristic of the optical fiber.   
   
   
       2 . The method of  claim 1 , wherein:
 the optical fiber comprises at least one of a single mode fiber and a dispersion compensating fiber, DCF, having a predetermined non-linearity characteristic;   the plurality of pumping light sources comprise four semiconductor lasers with fiber Bragg gratings, FBG, a polarization beam combiner, PBC, and a WDM coupler; and   the four semiconductor lasers are configured to generate pumping light at different wavelengths.   
   
   
       3 . The method of  claim 1 , further comprising a step of:
 combining the pumping lights via a Mach-Zehnder interferometer wave combiner.   
   
   
       4 . The method of  claim 3 , further comprising steps of:
 selectively applying a subset of the pumping lights to the amplification medium; and   selectively setting a uniform input power to each of the subset of pumping lights, wherein   the subset of the pumping lights comprise a number of wavelengths from a short wavelength side of a pumping light band greater than another number of wavelengths at a long wavelength side of a pumping light band, and   the step of selectively applying a subset of the pumping lights is controlled to provide a substantially uniform gain.   
   
   
       5 . A method for repeating a wavelength division multiplex optical signal, comprising steps of:
 providing a pump light from a plurality of pumping light sources to an optical fiber configured to be a Raman amplifying medium and to receive the wavelength division multiplex optical signal from an optical transmission fiber; and   amplifying the optical signal with at least one of the plurality of pump light sources and at least one EDFA connected in series with the optical fiber, wherein   said amplifying step includes wavelength-dependent amplification.   
   
   
       6 . The method of  claim 5 , wherein:
 the optical fiber comprises at least one of a single mode fiber, SMF, and a dispersion compensation fiber.   
   
   
       7 . The method of  claim 5 , wherein:
 at least one of the plurality of pumping light sources and the at least one EDFA are configured to impart an adjustable gain on the optical signal.   
   
   
       8 . The method of  claim 7 , further comprising a step of:
 setting a gain of at least one of the plurality of pumping light sources and the at least one EDFA so as to at least partially offset a wavelength dependency of an EDFA gain.   
   
   
       9 . The method of  claim 8 , further comprising steps of:
 detecting a monitor signal; and   controlling a signal generating circuit connected between the optical fiber and the plurality of pumping light so as to control a gain of the optical fiber.   
   
   
       10 . The method of  claim 9 , wherein:
 the controlling step is configured to maintain a pre-determined difference between an input level and an output level of a plurality of WDM signals.   
   
   
       11 . The method of  claim 5 , further comprising steps of:
 controlling a gain of a first EDFA and a second EDFA of the at least one EDFA to produce at least one of a constant optical amplifier gain control and a constant optical amplifier output control; and   independently controlling a gain of the plurality of pumping light sources to reduce an inter-channel optical repeater output signal deviation.   
   
   
       12 . The method of  claim 5 , further comprising a step of:
 pumping a residual pumping light to the input optical transmission fiber through at least one of an input and an output side of the optical fiber.   
   
   
       13 . The method of  claim 5 , further comprising:
 pumping a residual pumping light input to the at least one EDFA.   
   
   
       14 . The method of  claim 6 , wherein:
 the plurality of pumping light sources is configured to compensate for a loss of signal in at least one of the dispersion compensation fiber and the optical transmission fiber.   
   
   
       15 . The method of  claim 5 , wherein:
 the plurality of pumping light sources is configured to suppress a gain fluctuation of the EDFA and to maintain a repeater gain flatness.   
   
   
       16 . A Raman amplifying medium comprising:
 an optical fiber configured to guide therein a WDM optical signal having a signal bandwidth of at least 20 nm, wherein   said optical fiber is configured to receive WDM pump light having a predetermined pump bandwidth and to produce an amplification characteristic of a predetermined gain having less than 1 dB of ripple across an amplification bandwidth that is at least 20 nm and that overlaps said signal bandwidth, said pump light comprising light from a plurality of semiconductor lasers that produce multimode optical outputs having respective center wavelengths separated from one another in an inclusive range of 6 nm through 35 nm.   
   
   
       17 . The Raman amplifying medium of  claim 16 , wherein:
 the optical fiber comprises at least one of a single mode fiber and a dispersion compensation fiber.   
   
   
       18 . The Raman amplifying medium of  claim 16 , wherein:
 the optical fiber comprises a dispersion compensation fiber having a dispersion property of less than −20 ps/nm per 1 km.   
   
   
       19 . The Raman amplifying medium of  claim 16 , wherein:
 the optical fiber is configured to have a non linear index n 2  of refraction equal to or greater than 3.5×10 −20  m 2 /W.