Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical access network system, particularly to an optical access system for dual service network. 
     2. Description of the Related Art 
     Fiber-optic communication is a communication method based on optical signal and fibers which is also a kind of cable communication. Light can be used to carry information after being modulated where it features a superior transmission capacity and a high information security. Since 1980s, the fiber-optic communication system plays a very important role in the digital world. In principle, the outgoing information is transferred from the sender side to the transmitter, and the signal modulates the carrier wave, which functions as the transmission medium of information, and then the modulated carrier wave is sent to the recipient side in a distant place, and the receiver demodulates the modulated carrier wave to obtain the original information. 
     Since the development for wireless and cable network has grown rapidly for the past few years, the demand for higher transmission speed and bandwidth has also increased. Current, the best solution would be to use optical fiber as a transmission carrier between wireless and cable network. As a result, many dual-service optical communication related inventions have been proposed lately, however, those newly proposed inventions still have many obstacles to overcome with. For example, in U.S. Pat. No. 7,127,176, the proposed invention utilizes optical fiber as radio frequency (RF) and uses semiconductor optical amplifier and electro-absorption electro-optic modulator at its base station. However, optical fiber dispersion will decrease signal strength for RF over long distance, create a restriction on its practical applications. Additionally, in the paper “Optical Fiber Communication Conference” released by GK. Chang on 2006, in order to solve the optical fiber dispersion issue among long distance transmission, Chang proposed using Double Sideband Suppressed Carrier Modulation. Unfortunately, this kind of modulation method can only transmit amplitude shift modulation signal, it also required at least two electro-optic modulators which will greatly increase manufacturing cost. To overcome the abovementioned problem, the present invention proposes an optical access system for dual service network which can effectively overcome the problem of RF signal strength being reduced by optical-fiber dispersion over long distance transmission. 
     SUMMARY OF THE INVENTION 
     The primary objective of the present invention is to provide an optical access system for dual service network, which can effectively solve the problem of RF signal strength reduced by optical fiber dispersion without using additional electro-optic modulator. 
     Another objective of the present invention is to provide an optical access system for dual service network that does not require remote nodes (receiver side) to use any optical filter to discern on-off keying (OOK) signal from cable network and RF signal from wireless network. The present invention can also apply to wavelength-division multiplexing (WDM) system. 
     In order to realize the objectives mentioned above, the optical access system for dual service network of the present invention mainly comprises an optical modulation device which is used to receive OOK signal from cable network and RF signal from wireless network, the optical modulation device then modulates the OOK signal and the RF signal to an optical signal and send out an output optical signal. Lastly, the output optical signal is being delivered to an optical receiving device through an optical fiber transmission channel, and the optical receiving device can access the OOK signal and RF signal from the output optical signal. 
     Below, the embodiments are described in detail in cooperation with the attached drawings to make easily understood the objectives, technical contents, characteristics and accomplishments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically showing the architecture of the present invention; 
         FIG. 2  is diagram schematically showing the architecture of the present invention; 
         FIG. 3  is a diagram schematically showing the architecture of experimental system of the present invention; 
         FIG. 4  is a spectra diagram showing the output optical signal of the present invention after passing an optical filter; 
         FIG. 5  is an eye diagram showing the output optical signal of present invention after received by an photo detector; and 
         FIG. 6  is a diagram showing BER curves of BB OOK and RF BPSK BTB signal after transmitting over 25 km. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Since the development for wireless and cable network has grown rapidly, the demand for higher transmission speed and bandwidth has also increased relatively. The present invention integrates both wireless and cable network communication system to enhance the signal transmission efficiency and also at the same time reduces the system complexity. The present invention can apply on integrated optical communication network, wireless communication network and wavelength-division multiplexing (WDM) system. 
     Please refer to  FIG. 1  for a diagram schematically showing the architecture of the present invention where dash line represents traveling path for electrical signal and thick line represents traveling path for optical signal. The present invention comprises an optical modulation device  10  which is used to receive on-off keying (OOK) signal from cable network and radio frequency (RF) signal from wireless network, the optical modulation device  10  then modulates the OOK signal and the RF signal to an optical signal and generates an output optical signal. In addition, the RF signal is in phase shift keying (PSK) format such as binary phase shift keying (BPSK), quadrature phase shift keying (QPSK) or octal phase shift keying (OPSK) and the source of optical signal is laser. The modulated output optical signal is then transmitted to an optical splitter  20  via an optical fiber transmission channel, and received by an optical receiving device  22 . The optical receiving device  22  can access the OOK signal and RF signal from the output optical signal. 
     The optical modulation device  10  further comprises two phase shifters  12  and  14 , a laser diode  16  and an electro-optic modulator (Mach-Zehnder modulator)  18 . The phase shifter  12  is responsible for receiving the RF signal and shifting the phase of the RF signal by 90 degrees and the phase shifter  14  is responsible for receiving the OOK signal and shifting the phase of the OOK Signal by 90 degrees also. The laser diode  16  is used to generate an optical signal. The electro-optic modulator  18  is responsible of receiving the original OOK signal and the OOK signal from phase shifter  14  at one electrical receiving end, and receiving original RF signal and the RF signal from phase shifter  12  at another electrical receiving end. In addition, an optical receiving end of electro-optic modulator  18  is responsible for receiving the optical signal and bias the electro-optic modulator  18  to a V π /2 position, so that it can effect the transmission characteristic of the optical signal, allowing optical signal to modulate with two signals received by the two electrical receiving ends (OOK and RF) and generate an output optical signal to an optical fiber transmission channel. 
     The optical receiving device  22  further comprises a low speed photo detector  28  and a high pass filter (HPF)  26 . The low speed photo detector  28  receives the output optical signal splitted from the optical splitter  20  and extracts the OOK signal which can be used by optical fiber cable network  32  applications such as fiber-to-the-home (FTTH) system. In addition, the present invention is compatible with existing optical receiving device at user ends, therefore, the users does not require any upgrade on their optical receiving devices. The high speed photo detector  24  receives the output optical signal splitted from the optical splitter  20  and output an electrical signal. The HPF  26  in combination with the high speed photo detector  24  can be used to receive the electrical signal and extract the RF signal, which can be used by wireless network  30  applications. 
     The following will describe the procedure of the present invention in detail. At first, the optical modulation device  10  receives RF and OOK signal, the RF signal will be received by the phase shifter  12  and having its phase shifted by 90 degrees and output to the electro-optic modulator  18 , similarly, the OOK signal will be received by the phase shifter  14  and having its phase shifted by 90 degrees also and output to the electro-optic modulator  18 . The electro-optic modulator  18  has to electrical receiving ends and one optical receiving end, one electrical receiving end will receive one shifted RF signal and one un-shifted OOK signal, the other electrical receiving end will receive one un-shifted RF signal and one shifted OOK signal. The optical receiving end receives an optical signal generated by a laser diode  16 , and when the electro-optic modulator  18 &#39;s bias is under V π /2 state, the electro-optic modulator  18  can effect the transmission characteristic of the optical signal, allowing optical signal to modulate with two signals received by the two electrical receiving ends (OOK and RF) and generate an output optical signal to an optical fiber transmission channel. After passing the optical fiber transmission channel, the output optical signal will be received by an optical splitter  20  and splitted into two signals. One of the two signals will be received by the low speed photo detector  28  and high speed photo detector  24  within the optical receiving device  22 , the high speed photo detector  24  combines with HPF  26  can allow low speed photo detector  28  to extract OOK signal, which can be used by the optical fiber cable network  32  applications. In addition, after the high speed photo detector  24  receives the output optical signal, it will output an electrical signal; HPF  26  will receive this electrical signal and extract the RF signal, which can be used by wireless network  30  applications. 
     Please refer to  FIG. 2  for another diagram schematically showing the architecture of the present invention where the optical receiving device  22  of the present invention is being substitute with alternating device. In this diagram, the optical receiving device  22  comprises a high speed photo detector  34 , a low speed photo detector  36  and a DC block  38  wherein the low speed photo detector  36  is combined with DC block  38 . The high speed photo detector  34  receives output optical signal splitted from the optical splitter  20  and extracts the RF signal from it, which can be used by wireless network  30  applications. Whereas the low speed photo detector  36  receives output optical signal splitted from the optical splitter  20  and generates an electrical signal, which allows DC block  38  to receive this electrical signal and extract the OOK signal from it, which can be further used by optical fiber cable network  32  applications. 
     In order to proof the practicability of the present invention, please refer to  FIG. 3  for a diagram schematically showing the architecture of experimental system of the present invention. In this setup, it utilizes an OOK signal from 1.25 G (Gb/s) cable network and a PSK signal from 625M (Mb/s) wireless network as illustration. Within the experimental system, the present invention can further install an optical filter  40 , an optical amplifier  42  and an optical filter  44  in order between the electro-optic modulator  18  and optical fiber transmission channel. The optical filter  40  will receive the output optical signal from the electro-optic modulator  18  and output the output optical signal after adjusting the output optical signal&#39;s optical carrier power. As a result, when photo detect  46  and  48  receives the output optical signal, it will be in optimal condition. The optical amplifier  42  then receives and amplifies the output optical signal received from optical filter  40  and outputs the processed signal, allowing the signal strength of the output optical signal to attenuate slower within the optical fiber transmission channel. Lastly, the optical filter  44  receives the output optical signal from the optical amplifier  42  and filter out noises within the output optical signal, the resulting output optical signal is transmitted to the optical fiber transmission channel. 
     Please refer to  FIG. 4  for a spectra diagram showing the output optical signal of the present invention after passing an optical filter. The electro-optic modulator  18  generates an output optical signal with one optical carrier, an OOK modulated optical sideband, and one PSK modulated optical sideband, as shown in  FIG. 4 . After transmission of optical fibers, only one optical coupler  20  is needed to separate the optical power for wireless and wired application. For wireless application, high speed photo receiver and a suitable RF band pass filter is used to receive the RF PSK signals. The high speed photo receiver and the suitable RF band pass filter is a photo detector  48  and a band pass filter  52  respectively in  FIG. 3 . For Baseband (BB) wired application, the optical carrier and PSK optical sideband signal only contribute DC signals in BB. The DC terms can be easily removed using a DC block. The DC block is a DC block  50  in  FIG. 3 . Only OOK signals will be observed in BB. Therefore, the present invention does not need a narrow band optical filter to separate the OOK optical sidebands for different applications. 
     Please refer to  FIG. 3  and  FIG. 5  at the same time, the modulation method for the PSK signal is phase shift modulation and modulation method for OOK signal is amplitude modulation. In  FIG. 5 , it can be clearly seen that after applying phase shift modulation to the optical signal, the optical signal would not cause any signal at base band (BB) after the optical signal passed photo detector; therefore, only the signal from amplitude modulation can be observed. As a result, the present invention can discern different signals from different channels by using different frequency band photo detector; that is, the user does not require designing additional optical filters to discern signals from different channels. In addition, the electrical signal mentioned above is not limited to a single wavelength, which allows the present invention to apply on wavelength-division multiplexing systems. Furthermore, the OOK signal of the 1.25 Gb/s cable network can be extracted by low speed photo detector  46  alone, and the PSK signal of the 625 Mb/s wireless network can be extracted by band pass filter  52  (BPF) alone. 
     Please refer to  FIG. 3  and  FIG. 6  at the same time, the experimental system of  FIG. 3  uses Bit Error Rate Tester (BERT)  54  and  58  relatively to measure the bit error rate (BER) of the OOK signal of the 1.25 Gb/s cable network and the PSK signal of the 625 Mb/s wireless network after being reduced frequency by a mixer  60 . From  FIG. 6 , it can be seen that after both signal (OOK and PSK) passed the optical fiber transmission channel for over 25 kilometers, the power loss is lower than 0.5 dB, which proofs that the present invention can overcome the obstacle of RF signal strength reduction caused by the optical fiber dispersion over long distance transmission. 
     The present invention not only eliminates RF signal reduction caused by the optical fiber dispersion, it also allows user to discern RF signal from wireless network and OOK signal from cable network without using any optical filters. The embodiments described above are only to exemplify the present invention but not to limit the scope of the present invention. Therefore, any equivalent modification or variation according to the shapes, structures, characteristics and spirits discloses in the present invention is to be also included within the scope of the present invention.

Technology Category: h