details of embodiments of the present invention will now be described with reference to the drawings . first , the principle of the invention will be explained , with reference to fig3 . in fig3 , the input lightwave has a frequency f 0 . a narrow bandpass filter 1 transmits the input lightwave of frequency f 0 and reflects light having a slightly different frequency than that of the input lightwave . the optical modulator is an intensity modulator that can modulate lightwaves traveling to the left or right with the same modulation frequency f m . a filter 2 is a narrow bandpass filter having a different pass - band center to that of the filter 1 , and has a smaller free spectral range ( fsr ) than the filter 1 . the input lightwave of frequency f 0 passes through the filter 1 and is modulated , generating the sidebands shown in fig3 ( b ). for the sake of simplicity , modulation is assumed to be linear . it is also assumed that the original carrier wave disappears in the modulation and that only a group of first - order sidebands is generated . the first - order sidebands that do not correspond to the transmission spectrums of the narrow bandpass filter 2 are reflected back through the intensity modulator , in the course of which the sidebands undergo modulation that gives them the spectrum shown in fig3 ( c ). components corresponding to the carrier wave are transmitted by the narrow bandpass filter 1 , so just the sideband is reflected , as shown in fig3 ( d ), and is followed by modulation , resulting in the spectrum of fig3 ( e ). this modulation gives rise to first - order and third - order sidebands . when the wavelength of a third - order high - frequency sideband shown in fig3 ( g ) corresponds to the transmission spectral bands of the narrow bandpass filter 2 , it is transmitted by the filter 2 . in this way , a lightwave input from the narrow bandpass filter 2 is output with a frequency that has been tripled by the high - frequency signal . also , a sideband can be matched to a transmission spectrum of the narrow bandpass filter 2 by changing the frequency of the high - frequency signal . conversely , this means that the frequency of the high - frequency signal and the requisite sideband order can be clarified by specifying the position from the center frequency of the transmission spectrum . the above explanation has been made with reference to an optical modulator that is an intensity modulator , but the same effect can be obtained with a phase modulator . for the purpose of the present invention , a preferred modulator is a traveling - wave type modulator . with a traveling - wave type modulator , a lightwave traveling in either direction can be modulated with the same characteristics by inputting the modulation signal via the electrode provided at each end . fig4 shows a test means for demonstrating the principle of the invention . by means of reflection by a fiber grating ( fbg ) 1 and a fiber grating ( fbg ) 2 , a lightwave input to the optical modulator is repeatedly reflected back and forth to obtain high - order sidebands . the source laser is a semiconductor laser with a wavelength of 1550 nm and an output of 10 mw ; the isolator is a commercial one manufactured by newport corporation . the gratings were made by 3m company , and are described , for example , in reference 3 ( toru inoue , “ development trends in grating technology ,” c - 3 - 67 , conference 2000 of the institute of electronics , information and communications engineers , pp . 246 – 247 ). the optical modulator is a traveling - wave type manufactured by sumitomo - osaka cement co ., ltd . that can be operated by a high - frequency signal input of up to 40 ghz . using these components , it was possible to obtain a − 32 dbm sideband output 210 ghz from the carrier wave by inputting a 30 - ghz , 27 . 8 dbm modulation signal . fig1 shows a preferred embodiment of the optical frequency converter of the present invention . the optical frequency converter comprises a single mode laser source 1 that emits light at a preset frequency ( laser frequency f ld = 200 . 033 thz ), an isolator 2 to suppress the effect of light traveling back , a polarization controller 3 , a fabry - perot filter 4 ( with a transmission spectrum of 200 . 033 thz and a fsr of 300 ghz ) forming a first reflecting means , an optical phase modulator 5 for performing the modulation with a modulation signal and generating sidebands , a fabry - perot filter 6 ( with a transmission spectrum of 200 . 000 thz and a fsr of 50 ghz ) forming a second reflecting means , a splitter 7 , an amplifier 8 , and a high - frequency signal source 9 . the frequency of the high - frequency signal source 9 can be changed , changing modulation frequency , and constitutes the above sideband selection means . a lightwave from the single mode laser source 1 passes through the fabry - perot filter 4 and is phase - modulated by the optical phase modulator 5 . phase modulation produces high - order sidebands . fig5 shows the relationship between sidebands , modulation frequency and the transmission spectrum of the fabry - perot filter 6 . a modulation frequency of 17 ghz results in a first - order sideband of frequency 200 . 050 ghz . since that is within the transmission spectrum of the fabry - perot filter 6 , it can pass through the filter 6 , but the second - order sideband having a frequency of 200 . 067 ghz cannot pass through the fabry - perot filter 6 , and neither can the third - order sideband having a frequency of 200 . 084 ghz . a modulation frequency of 33 . 5 ghz results in a first - order sideband of frequency 200 . 0665 ghz . since that is not within the transmission spectrum of the fabry - perot filter 6 , it cannot pass through the filter 6 , but the second - order sideband having a frequency of 200 . 100 ghz can pass through the fabry - perot filter 6 . the third - order sideband of 200 . 1335 ghz also cannot pass through the fabry - perot filter 6 . other lightwaves can pass through the fabry - perot filter 6 . table 1 shows the relationship between the modulation frequency that produces this light , and the sideband order . thus , when the frequency of the single mode laser source 1 is fixed , lightwave frequencies can be instantaneously switched by selecting a modulation frequency and sideband order that match a transmission spectrum of the fabry - perot filter 6 . with respect to the positive integers n and k , taking k as sideband order , n as the order from the center frequency of the transmission spectrum of the fabry - perot filter 6 , f ld as the frequency of the single mode laser source 1 , f fp as the center spectral frequency of the fabry - perot filter 6 , with f fsr denoting the fsr frequency and f m modulation frequency , the following relationships are obtained . f ld + k × f m = f fp + n × f fsr , or f ld − k × f m = f fp + n × f fsr , or f ld + k × f m = f fp − n × f fsr , or so , each value can be set in accordance with these relationships . values thus obtained are stored in controller 10 and referred to , if necessary , to set k and modulation frequency f m for a given n . with respect to the high - speed switching of modulation frequency f m , there are existing high - frequency oscillators capable of switching within 10 to 20 ns , and these can be used to realize the high - speed switching of lightwave frequencies . an advantage of the present invention is that it is not necessary to prepare a high - frequency signal able to cover the lightwave frequency range , since the object can be attained using a high - frequency signal with about one - fourth the range . fig2 shows another preferred embodiment of the optical frequency converter of this invention . this optical frequency converter comprises a single mode laser source 1 , an optical isolator 2 , a polarization controller 3 , a fabry - perot filter 4 ( with a transmission spectrum of 200 . 033 thz and a fsr of 300 ghz ), an optical phase modulator 5 , a variable optical delay line 11 that can be used to externally control the length of the optical path , a fabry - perot filter 6 ( with a transmission spectrum of 200 . 000 thz and a fsr of 50 ghz ), a splitter 7 , an amplifier 8 , and a high - frequency signal source 9 . the variable optical delay line 11 can be constituted by a conventional means , such as a prism or reflector , that is used to change the free - space optical path , or by using an optical fiber that is heated to employ thermal expansion to change the length of the optical path , or the length of the optical fiber can be mechanically changed by using a piezoelectric element or a magnetostrictor . the effect of the variable optical delay line 11 is that it optimizes the optical output intensity by adjusting the length of the optical path between the fabry - perot filter 4 and the fabry - perot filter 6 . in the course of obtaining high - order sidebands by repeatedly reflecting the lightwave input to the optical modulator back and forth between the fabry - perot filters 4 and 6 , the intensity of the optical output depends on the phase of the light at the point of reflection . since this light phase depends on the phase of the light from the single mode laser source 1 , the modulation frequency and the length of the optical path , the length of the optical path is adjusted to optimize the intensity of the output lightwave . the variable optical delay line 11 is controlled by the controller 10 , being switched to match the switching of the lightwave frequency . thus , in the case of this embodiment , with respect to a given n , in addition to the aforementioned switching of k and f m , variable optical delay line 11 conditions are also changed . as described in the foregoing , the variable optical delay line 11 is used to adjust the light phase to optimize the intensity of the output light by adjusting the optical path length . however , this can also be achieved by using a bias generator 12 controlled by the controller 10 to apply a bias voltage to the optical phase modulator 5 to thereby adjust the phase . the advantage of using a bias voltage to optimize the output intensity is the short response time . an advantage of using the variable optical delay line 11 for the adjustment is that it can be used in high - noise - level environments . this embodiment uses a selector 13 to select the adjustment means . to ensure stable operation under changing ambient temperature conditions , it is desirable to be able to externally control the transmission spectrum characteristics of the narrow bandpass filter 2 , via such means as voltage , current , temperature , magnetic field or electro - magnetic waves . this can be done by using the fabry - perot etalon type variable - wavelength filter in a cavity filled with dispersion type polymer liquid crystal described , for example , in reference 4 ( jp - a hei 11 - 95184 ). reference 5 ( shimotsu and four others , “ subcarrier generation by integrated type ln phase modulator ,” c - 3 - 20 , conference 2000 of the institute of electronics , information and communications engineers , p . 199 ) describes a modulator that attenuates the carrier wave , leaving sidebands , which modulator can be used instead of the aforementioned modulator . the optical modulator can also be a semiconductor - based absorption type , a mach - zehnder type intensity modulator that uses a material having an electro - optical effect , or a phase modulator having an electro - optical effect . as long as the optical amplifier used in this means is located between the narrow bandpass filters 1 and 2 , its placement has no particular significance , with the same effect being forthcoming even if it and the variable optical delay line 11 exchange places . each of the above embodiments has been described as using a fabry - perot filter as the narrow bandpass filters 1 and 2 . however , the filters do not have to be fabry - perot filters , it being also possible to use fiber bragg gratings , for example , to form the narrow bandpass filters . an advantage of using a fiber bragg grating is that it enables the optical path to be configured inside the optical fiber , which makes it possible to prevent loss of signal intensity during optical signal input / output outside the fiber . another merit is that by changing the fiber bragg grating structure , it is possible to form a filter having a different period to that of a fabry - perot filter , in which the transmission band periods are equally spaced . the present invention thus configured using the means described in the foregoing , provides the following effects . the optical frequency converter configuration that includes means for modulating light of predetermined frequency with a modulation signal to obtain a group of sidebands thereof , means for selecting sidebands from among the group of sidebands , and means for changing frequency of the modulation signal and selecting a predetermined sideband makes it possible to instantaneously switch optical signal frequencies by selecting the high - frequency signal frequency and sideband order . also , the use of repeated modulations makes it possible to instantaneously switch optical signal frequencies with a smaller modulation signal . also , using reflecting means to configure a folded optical path makes it possible to perform multiplex modulation . moreover , since repeated modulation operations can be performed with a single modulator , the number of modulators used can be decreased . also , the second reflecting means having a plurality of transmission bands that is used for sideband selection is an existing , well - known optical component , facilitating the configuration of an optical frequency converter that can switch optical frequencies . the fact that the laser source , the reflecting means used to form the narrow bandpass filters and the optical modulator are also existing , well - known means also makes it readily possible to realize an optical frequency converter that can switch lightwave frequencies . in addition , in the seventh invention , the inclusion of means for adjusting the length of the optical path makes it possible to obtain output light of optimum intensity .