Patent Application: US-53671300-A

Abstract:
the present invention provides a method for generating four - wave mixing to obtain idler light with high efficiency , in which the range of lengths of an optical fiber is appropriately set , and probe light and pumping light , having different frequencies , are launched into the optical fiber . when the nonlinear coefficient of the optical fiber , the loss per unit distance , and the wavelength and intensity of the probe light and pumping light are set to certain values , the idler light conversion efficiency at the output end of the optical fiber is a periodic function with respect to optical fiber length having a maximal value and a minimal value . the maximum length of the optical fiber to be used to obtain four - wave mixing is set to be equal to or less than the length lmax which is given by adding the length of the optical fiber lm , at which the idler light conversion efficiency takes on the first maximal value in the aforementioned periodic function and distance δl or 10 % of lm .

Description:
now , the present invention will be specifically explained with reference to each of the embodiments below . embodiment 1 shows a form of a wavelength converter for implementing the present invention . fig1 is a conceptual view showing a conceivable wavelength converter . pumping light launched into a highly nonlinear optical fiber 3 from a pumping light source 1 is combined with a light signal 5 by an optical combiner 2 . this combined light generates idler light in a highly nonlinear optical fiber 3 of which length is optimized . the wavelength of the signal light is converted by the fpm interaction into this idler light . light 6 on the frequency axis of which all of the signal light , the pumping light , and the idler light are present is emitted from the output end of an optical fiber 3 . accordingly , a wavelength converter can be implemented by filtering the light 6 to obtain only the idler light 7 using an optical filter 4 . incidentally , the idler light 7 and the light signal 5 are phase conjugated , and thus this device serves also as a generator of an optical phase , conjugation beam of light . embodiment 2 shows a form of an optical parametric amplifier for implementing the present invention . fig2 is a conceptual view showing a conceivable optical parametric amplifier . pumping light launched from a pumping light source 1 is combined with a light signal 5 by an optical combiner 2 . this combined light generates idler light in a highly nonlinear optical fiber 3 of which length is optimized , while the intensity of the signal light itself is being amplified by the parametric amplification effect . light 6 on the frequency axis of which all of the amplified signal light , the pumping light , and the idler light are present is emitted from the output end of the optical fiber 3 . accordingly , an optical parametric amplifier can be implemented by filtering the light 6 to obtain only the amplified signal light 9 using an optical filter 8 . embodiment 3 shows a form of an optical circuit to be used in an optical signal regeneration circuit for implementing the present invention . as described in literature [ 12 ], the optical regeneration circuit outputs an optical signal of which waveform is shaped with respect to an input signal light or extracts optical time division multiplexed signals . the optical circuit that constitutes the optical signal regeneration circuit shown in fig2 of the literature [ 12 ] comprises , as shown in the specification of the literature [ 12 ], a highly nonlinear medium for generating four wave mixing , an optical filter for allowing only the idler light to pass therethrough , and a pumping light source . therefore , the construction is the same as that of embodiment 1 . in the literature [ 12 ], a semiconductor amplifier was used as the highly nonlinear medium . by constituting the optical signal regeneration circuit shown in the literature [ 12 ] using the optical circuit of the configuration of fig1 a fiber optical signal regeneration circuit can be implemented . embodiment 4 shows a form of a multi - wavelength optical source for implementing the present invention . fig3 is a conceptual view showing the multi - wavelength optical source . pumping light launched from a pumping light source 1 is combined with ia light signal 5 by an optical multiplexer 2 . this multiplexed light generates multiple idler light beams in a highly nonlinear optical fiber 3 of which length is optimized . light 11 on the frequency axis of which all of the signal light , the pumping light , and a multiple of beams of idler light are present is emitted from the output end of an optical fiber . accordingly , a multi - wavelength optical source can be implemented by filtering the light 11 to obtain only light 12 of desired wavelengths using a narrow - band optical filter 10 . in embodiment 5 , based on the results obtained through actual experiments , the implementation of a broadband wavelength converter with a half width of the half maximum of 20 nm is discussed . a wavelength conversion experiment is discussed in which the length of a fiber is changed . a broadband wavelength conversion was performed in the system as shown in embodiment 1 using a highly nonlinear optical fiber . fig1 - 14 show the results of the wavelength conversion experiment in which the length of a fiber was varied . the light source employed a continuous - wave light source . in each case , the pumping light input power is limited by sbs . thus , this provided 10 . 0 dbm at 24 . 5 km , 18 . 5 dbm at 1 . 2 km , and 20 . 0 dbm at 0 . 2 km . as can be seen from the drawings , the shorter the lengths of the fiber , the greater the conversion efficiency and frequency bandwidth become . at the same time , the results of numerical calculation performed with corresponding parameters are shown with solid lines in fig1 - 14 . the horizontal axis represents wavelength of the idler wavelength offset by the wavelength of the pumping light , while the vertical axis represents the conversion efficiency in the wavelength conversion . the calculation parameters were selected along the experiments . in addition , all losses other than those unique to the fiber such as splicing loss , which were generated in the actual system , were eliminated at the time of calculation . what is not notable in fig1 - 14 is that the results of the numerical calculation do not agree with the experimental results at distances 24 . 5 km and 1 . 2 km . various reasons are conceivable for this fact . among them , a variation in wavelength dispersion in the longitudinal direction can be thought to be the most probable reason . the results of the calculation gradually come closer to the experimental results as the fiber becomes shorter in length to cause the dispersion to become less . for the purpose of understanding this fact more schematically , with the same parameters used in the calculation shown in fig1 - 14 , the process was calculated where the idler light , which the signal light set at a position 23 nm apart from the pumping light was converted to and which was thus generated , and grew in the longitudinal direction . fig1 - 17 show the results . in fig1 - 17 , a wavelength conversion was considered in which the signal light - was set 20 nm apart from the pumping light . it can be seen in the most preferable result shown in fig1 that the operation is carried out near the first peak among the series of peaks generated in the longitudinal direction . in fig1 , the effect of the loss becomes dominant and thus wavelength conversion can be no longer desired . in fig1 , wavelength conversion takes place near the third peak in the longitudinal direction . the same conversion efficiency can be obtained even with a fiber 200 m in length . if so , a fiber of a length of 200 m would provide less variations in dispersion in the longitudinal direction and less mismatching of the state of polarization between the pumping light and the signal light caused through propagation by birefringence . from this point , realizing a more desirable wavelength conversion was expected . in fact , the results of fig1 and 17 proved that this expectation was correct . under the prospect according to fig1 , the fiber was cut to a length of 200 m . this caused the sbs threshold value to increase and made it possible to allow pumping light to be launched into the fiber up to 20 dbm . the frequency bandwidth reached 2 . 6 times or more than that of the system shown in fig1 . this shows that the length of a fiber is preferably shorter for the same conversion efficiency . moreover , as can be seen in fig1 , a more highly efficient wavelength converter could be obtained by performing wavelength conversion in a system in which the length of fiber is set so as to be shorter than that of a fiber for the first peak . however , if pumping light of an output power below 20 dbm is used as continuous light , as can be seen from fig1 , making the length of a fiber 200 m or less would only result in a decreased conversion efficiency . in this sense , the lower limit obviously exists even in making the length of the fiber shorter . the value can be readily determined by numerical calculation . incidentally , the aforementioned method can be applied as it is to the design of opa if a discussion is made with the conversion efficiency of the vertical axis in fig1 - 14 and fig1 - 17 replaced by the gain of a signal light . fig1 shows the measured result of the idler light conversion efficiency at a fiber length of 24 . 5 km . ( the solid line shows the calculated result . a half width of the half maximum 0 . 85 nm .) fig1 shows the measured result of the idler light conversion efficiency at a fiber length of 1 . 2 km . ( the solid line shows the calculated result . a half width of the half maximum 8 . 7 nm .) fig1 shows the measured result of the idler light conversion efficiency at a fiber length of 0 . 2 km . ( the solid line shows the calculated result . a half width of the half maximum 23 nm .) under these circumstances , the calculated result and the experimental result coincide with each other . they do agree with each other though parameters were chosen as such . fig1 shows the calculated result of the idler light conversion efficiency . loss becomes a dominant for a fiber 24 . 5 km in length ( which is denoted by ). the calculation was carried out under a pumping light intensity of 10 . 0 dbm ( corresponding to fig1 ). fig1 shows the calculated result of the idler light conversion efficiency . the length of the fiber is 1 . 2 km ( which is denoted by ) and corresponds to around the third peak . the calculation was carried out under the power of the pumping light of 18 . 5 dbm ( corresponding to fig1 ). fig1 shows the calculated result of the idler light conversion efficiency . the length of the fiber is 0 . 2 km ( which is denoted by ) and corresponds to around the first peak . the calculation was carried out under the intensity of the pumping light of 20 . 0 dbm ( corresponding to fig1 ). now , the implementation of a broadband wavelength converter with a bandwidth of 30 nm is considered . as an example , here , it is considered to perform wavelength conversion from the shorter wavelength side to the longer wavelength side across the pumping light . when an optical network is constructed in a transmission system employing the wavelength division multiplexing ( wdm ), it is necessary to convert optical signals multiplexed over a broad wavelength bandwidth simultaneously [ 4 - 7 ]. discussion is made regarding the broadband simultaneous wavelength conversion at this time using numerical calculation . it is considered to set the intensity of pumping light to 20 dbm with the fiber being 200 m in length . it is to be understood that table 3 . 1 gives fiber parameters . the pumping light is to be set to 1550 nm . here , as shown in embodiment 5 , the length is selected by solving equation 1 and thus showing the result of conversion efficiency in the longitudinal direction . at that time , design should be carried out using the wavelength that is farthest from the pumping light in the signal light bandwidth . fig1 shows the calculated result with a wavelength 20 nm apart from the pumping light without considering that . fig1 shows the calculated result of conversion efficiency when the signal light is 20 nm apart from the pumping light . in the figure , “” shows the case of 200 m . the foregoing discussion shows that the system may be constructed near the first peak . according to this discussion , the optimum length of fiber is given at about 400 m . therefore , to increase the pumping light can be considered , however , to shorten the fiber in length should not be considered . next , as can be found in the specification , fig1 shows the calculated result in the case of a signal light set to a point 30 nm apart from the pumping light . as can be seen in the figure , the fiber of a length of 200 m has already exceeded the first peak . in consideration of this fact , the guideline of the present invention allows a fiber shorter in length ( 100 m in the figure ) to be used , which would give the same value . the optimum fiber has a length of 150 m . that is , the result shown in fig1 indicates that the fiber should be made much shorter . to summarize the aforementioned results , fig2 shows the conversion efficiencies for a fiber of a length l = 200 m and a fiber of a length l = 150 m . fig1 shows the calculated result of the conversion efficiency with the signal light being 30 nm apart from the pumping light . in the figure , “” shows the case of 200 m . fig2 shows the calculated result of bandwidth spectrum by means of numerical calculation . the fiber of a length l = 150 m ( with a half width of the half maximum 30 . 0 nm ) shown by a solid line can provide a wider bandwidth than the fiber of a length l = 200 m ( with a half width of the half maximum 26 . 0 nm ) shown by a dashed line . as can be seen from fig2 , the fiber of a length l = 150 m can provide a bandwidth broader and a conversion efficiency flatter to wavelength than the fiber of a length l = 200 m . as such , to implement the wavelength conversion over a broad bandwidth of 30 nm , it is necessary to carry out a design by placing the wavelength is λs = λp − 30 nm of the signal light at the edge of the bandwidth for calculation . it is possible to employ the aforementioned discussion as it is in the design of opa . now , such a circumstance is considered in which the length of the fiber has to be adjusted due to the effect of pmd . at the time of carrying out wavelength conversion , the effect of pmd increases as the interval between the pumping light and the signal light is made larger . as has already been explained , it is possible to reduce the effect of pmd by applying the concept of the state of principal polarization . for this purpose , as shown in embodiment 5 , the length l of a fiber is determined only in consideration of fpm . then , the optical fiber of a length l 1 thus determined is evaluated in the same manner as is carried out in literature [ 22 ] for a conventional dispersion shifted optical fiber . thus , the bandwidth δλ psp of the state of principal polarization of a highly nonlinear fiber at hand is evaluated . at this time , the wavelength is made the same as that of the pumping light . then , the bandwidth δλ psp is compared with the bandwidth δλ wc of the wavelength conversion , which is considered to be employed as the specification . if δλ wc & lt ; δλ psp , the effect of pmd can be reduced only by making the polarization coincident with each other immediately before the pumping light and the signal light are incident on the fiber . then , the wavelength conversion is thought about with the length of the fiber remaining as it is . fig2 shows an embodiment of a short - pulse generator of the present invention . in the case where pumping light and probe light , which have different wavelengths but have substantially the same optical intensity , are allowed to be incident on a transmission line ( comb - like dispersion - profiled fiber [ 26 ]) in which a dispersion shifted optical fiber 3 and a single mode optical fiber 13 are alternately connected as shown in the figure , beat signals of frequencies produced corresponding to the wavelength interval cause multiple four - wave mixing to develop in the dispersion shifted optical fiber that is disposed as the first fiber and thus the spectrum becomes dispersed . here , the self phase modulation becomes dominant to cause frequency chirping to develop . letting the light of this dispersed spectrum into the single mode optical fiber causes the group velocity dispersion to become dominant , compressing the chirped pulse . by allowing the pulse to pass through many transmission lines comprising the dispersion shifted optical fibers and single mode optical fibers , the pulse repeats chirping and compression to be shaped into a short pulse in the shape of solutions , which had a sinusoidal waveform at the time of incidence . to implement the foregoing technique , it is critical to allow four - wave mixing to develop with efficiency in a dispersion shifted optical fiber . accordingly , the technique for producing the four - wave mixing according to the method described in the present invention can be used to implement a high - efficiency short - pulse generator . if δλ wc & gt ; δλ psp , the effect of pmd can be reduced by shortening the length of the fiber . in general , it is known that δλ psp simply reduces with respect to the fiber length [ 21 , 22 ]. accordingly , the condition δλ wc & lt ; δλ psp is implemented by shortening the length of the fiber . suppose that the length of the fiber , which is determined by placing the first priority on the pmd , is l 2 ( l 2 ≦ l 1 ). with this length of the fiber , the wavelength conversion is implemented . at this time , if the intensity of the pumping light can be varied , it is also possible to optimize the system again so as to compensate for the shortened fiber . t . yamamoto , t . imai and m . nakazawa , optical wavelength converting circuit , japanese laid - 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