the following descriptions of the preferred embodiments are provided to understand the features and the structures of the present invention . please refer to fig1 , which is a system diagram according to an embodiment of the present invention . as shown in the figure , the present invention is an optical fiber system and method for carrying both catv and ethernet signal , comprising a transmitting part 1 , a receiving part 2 and an optical channel 3 , where the optical channel 3 , such as a glass fiber , a plastic fiber or a free space , is a channel for transmitting signals . according to the embodiment , in a first transmitter 11 of the transmitting part 1 , signals of a first signal source 111 are transmitted to a low - pass filter 112 for transmission to be transmitted to a power combiner 13 . in a second transmitter 12 of the transmitting part 1 , signals of a second signal source 121 are up - converted to an rf ( radio frequency ) band by adding an rf carrier 124 of 1 . 8 ghz ( giga - hertz ) or 2 . 4 ghz through a mixer 122 for transmission . then , the signals of the second signal source 121 are transmitted to a band - pass filter 123 . in order to avoid overlapping the bandwidth of the digital signals and the analog signals , the signals of the second signal source 121 are kept within a fixed rf band , and the higher / lower band power outside 0 . 75 times of the bit - rate is removed . and then , the signals are transmitted to the power combiner 13 . the power combiner 13 together with an amplifier 14 for transmission and an electrical - to - optical converter 15 can be assembled to form a combiner amplifier 10 . the signals of the first signal source 111 and the signals of the second signal source 121 are combined by the power combiner 13 to form mixed electrical signals . then , the level and the power of the mixed electrical signals are modulated by an amplifier 14 for transmission . and , the mixed electrical signals are converted into optical signals by an electrical - to - optical converter 15 , and then are transmitted through an optical channel . in the receiving part , the optical signals are received from the optical channel into an optical - to - electrical converter . the receiver converter 20 comprises an optical - to - electrical converter 21 and a power divider 22 . the optical signals are converted into the original mixed electrical signals of the transmitting part by the optical - to - electrical converter 21 . and , by the power divider , the mixed electrical signals are divided into signals of a third signal source 231 and signals of a fourth signal source 241 to be transmitted to a first receiver 23 and a second receiver 24 . the signals of the third signal source 231 are transmitted to a low - pass filter 232 for receipt , to avoid the interference between the analog signals and the digital signals . then , the signals of the third signal source 231 are transmitted to a signal amplifier 233 to be outputted as a first signal output . at the same time , in order to avoid the interference between the digital signals and the analog signals , the signals of the fourth signal source 241 are transmitted to a high - pass filter 242 . the high - pass filter 242 is connected to a mixer 243 for receipt so that the signals of the fourth signal source 241 are down - converted with a down - converter carrier 247 of 1 . 8 ghz or 2 . 4 ghz to form enveloped signals . then , the enveloped signals are un - enveloped into non - enveloped signals through a low - pass filter 244 for un - enveloping . and then , the non - enveloped signals are amplified by a limiting amplifier 245 to be outputted as a second signal output 246 . the receiving part can be a receiving node of a passive optical network . as long as the optical receiving power allows , there can be as many receiving nodes as can be . please further refer to fig2 through fig3 b , which , according to an embodiment of the present invention , are a spectrum diagram of digital / ethernet signals at a transmitter and up - converted spectrum diagrams of digital / ethernet signals before and after filtering . as shown in the figures , the analog signals for the present invention have a working frequency between 50 mhz ( mega - hertz ) and 850 mhz , where each channel requires 6 mhz . and the digital signals for the present invention are transmitted under a circuit bandwidth of 10 mb / s ( mega - bit per second ) to 10 gb / s ( giga - bit per second ). take 622 mb / s for example . the bandwidth where a digital signal occupies is from that of a dc ( direct currency ) to infinity . in order to be isolated from the analog signals , the digital signals are directly up - converted by the mixer for transmission 122 to be shifted to a certain carrier of a frequency of such as 1 . 8 ghz or 2 . 4 ghz , and so the frequency of the digital signals are extended in a range from the frequency of the certain carrier to infinity along a symmetric sinc function . and , the mixer 122 for transmission can be made of a wlan ( wireless local area network ) circuit chip with a bandwidth of 1 . 8 ghz , 2 . 4 ghz or 5 . 2 ghz . because the digital signals is directly converted by the mixer 122 for transmission and its bit - rate is higher than 100 mb / s , the mixer 122 for transmission can be made of a wideband active mixer , a diode mixer or an rf switch which has a characteristic of wide band . and , because the power density of the signal spectrum under 0 . 75 times of the bit - rate occupies 95 %, a band - pass filter 123 is used with only a limited distortion to the digital signals . therein , a carrier is used as the center frequency of the band - pass filter 123 , while the frequency of the band - pass filter 123 is extended to the higher and lower parts each having an amount of 0 . 75 times of the bit - rate , which means 466 . 5 mhz for each higher or lower part to form a total bandwidth of 933 mhz in 622 mb / s ; and while the deduction of the band - pass and band - stop is the greater the better . in this way , the high frequency part of the digital signals can be removed ; besides , the digital signals can be distinguished from the analog signals . please further refer to fig4 a through fig6 b , which , according to an embodiment of the present invention , are views showing a wilkinson power divider as a passive power divider / combiner , a 90 ° hybrid as a passive power divider / combiner , a coupler as a passive power divider / combiner , a mixed spectrum of catv signals and ethernet signals , the saturation of an optoelectronic converter and the linearity of an optoelectronic converter . as shown in the figures , the transmitting part 1 uses a passive power combiner or an active wideband analog adder as a power combiner 13 to combine separated digital signals and analog signals into mixed electrical signals . and , an amplifier 14 for transmission is deposited in the transmitting part 1 to adjust the level and the power of the mixed electrical signals . the amplifier 14 for transmission can be a constant gain amplifier or an auto gain control ( agc ) amplifier . then , the mixed electrical signals are transmitted to an electrical - to - optical converter to be converted to optical signals of a single wavelength and then are transmitted through the optical channel . because the mixed electrical signals include analog signals , a circuit element with high linearity and low noise is required . the circuit element can be an externally modulated eo ( electro - optic ) modulator or a directly modulated laser driver together with a semiconductor laser diode so that the ctb ( composite triple beat ) and the cso ( composite second - order ) of the channels , which are between 50 mhz and 900 mhz can be above the standard required 62 db and its noise rate , can be above 48 db . meanwhile , by controlling the outputted optical power to be around 0 dbm , the nonlinear effect of the optical fiber under high optical power , and the insufficient signal power of the receiving part 2 caused by low optical power , both can be prevented . the optical signals of a single wavelength are received from the optical channel 3 into the receiving part 2 to be converted into electrical signals by the optical - to - electrical converter . the optical - to - electrical converter 21 comprises an optical diode ( such as a pin - type diode , an avalanche photodiode or a metal - semiconductor - metal diode ) and a transimpedance amplifier to amplify converted electrical signals . the optical - to - electrical converter 21 is operated under a carrier with a bandwidth of 1 . 8 ghz , whose bit - rate is 622 mb / s ; and , the valid power spectrum of the signals ranges from 1 . 2 ghz to 2 . 4 ghz . the proper range of the wavelength for the operation of the optical diode in the optical - to - electrical converter 21 is decided according the emission wavelength of the transmitting part . and , the transimpedance amplifier of the optical - to - electrical converter 21 have to use a circuit having high input saturation current and high linearity so that the distortion of the analog signals can be minimized . after the mixed electrical signals are outputted from the transimpedance amplifier , through a passive power divider or a power divider 22 which is identical to that of the transmitting part yet is reversely used , the mixed electrical signals are divided up to be transmitted in two paths ; and , one path is to transmit digital signals , and the other one , analog signals . the analog signals are transmitted to a low - pass filter 232 for receipt whose 3 db cutoff frequency is is 900 mhz so that the interference from the digital signals can be avoided . then , the low - pass filter 232 for receipt is connected to a signal amplifier 233 so that the analog signals are amplified , and are transmitted to a first signal output 234 . the signal amplifier 233 whose bandwidth is between 50 mhz to 850 mhz is to compensate the signal loss made by the optical - to - electrical converter 21 , the power divider 22 and the low - pass filter for receipt 232 . the passive power combiner or the power divider is made on a printed circuit board according to the transmission line theory . by using a carrier obtained by up / down - converted digital signals as the center frequency , signals can be combined or divided by the combiner / divider depending on its direction . and , the combiner / divider can be a wilkinson power divider , a 90 ° hybrid or a coupler . please further refer to fig7 a through fig9 b , which , according to an embodiment of the present invention , are spectrum diagrams of down - converted digital / ethernet signals before and after filtering , oscillograms of down - converted digital / ethernet signals before and after un - enveloping , a frame view of a stepping low - pass filter and a frame view of a coupling band - pass filter . as shown in the figures , the digital signals are transmitted to a high - pass filter 242 whose 3 db cutoff frequency is 900 mhz so that the interference from the analog signals can be isolated . the digital signals are then directly down - converted with a down - converter carrier 247 of 1 . 8 ghz or 2 . 4 ghz through a mixer 243 for receipt to obtain enveloped signals with a mixed frequency . and then , the digital signals are transmitted to a low - pass filter 244 for un - enveloping whose cutoff frequency is the same to the bit - rate of the digital signals as 3 db so that the original non - enveloped digital signals of the transmitting part which are not up - converted can be obtained through the un - enveloping . after obtaining the non - pocketed digital signals , the limiting amplifier is connected to amplify the digital signals to a level which the digital signal process circuit of the back - end allows . the filters of the present invention , such as the filters for transmission / receipt , the band - pass filter , the high - pass filter and the filter for un - enveloping , can be made of smds ( surface mounted device ), stepping / coupling transmission lines or mixing transmission lines made of smd . nevertheless , the signals for the first and the second signal transmitters can be analog / catv signals under a working frequency between 50 mhz and 850 mhz or digital / ethernet signals with a bandwidth above 10 mb / s ; and , the signals from the first and the second signal outputs can be analog / catv signals or digital / ethernet signals . the preferred embodiments herein disclosed are not intended to unnecessarily limit the scope of the invention . therefore , simple modifications or variations belonging to the equivalent of the scope of the claims and the instructions disclosed herein for a patent are all within the scope of the present invention .