Optical fiber communication system and methods having a reflective optical network unit

An optical fiber communication system is provided, including a central office and an optical network unit. The central office generates a first downstream signal and a second downstream signal according to a radio frequency signal and a baseband signal, respectively. The optical network unit is coupled to the central office to receive the first downstream signal and the second downstream signal through a first fiber and a second fiber different from the first fiber, respectively, such that the optical network unit only modulates the second downstream signal to generate an upstream signal and then delivers the upstream signal to the central office through the first fiber, thereby decreasing signal Rayleigh backscatter noise.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of Taiwan Patent Application No. 100122129, filed on Jun. 24, 2011, the entirety of which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical fiber communication systems, and in particular relates to an optical fiber communication system having a reflective optical network unit.

2. Description of the Related Art

FIG. 1Ais a diagram of an optical fiber communication system. As shown inFIG. 1A, the optical fiber communication system100is a wavelength-division multiplexing-passive optical network (WDM-PON) system. The plurality of the laser sources L1, L2. . . Ln in the central office (CO)110(or called head-end) respectively generate light sources having different wavelengths λ1, λ2, . . . λn. The light sources are integrated into an optical carrier DS by the optical multiplexer111of the array wave guide (AWG). The multiplexer111is coupled to an optical circulator112. The first terminal of the optical circulator112receives the optical carrier DS and delivers the optical carrier DS to the fiber113coupled to the second terminal of the optical circulator112. The third terminal of the optical circulator112is coupled to the optical demultiplexer114of the central office110. The optical demultiplexer114delivers the upstream signal US to a corresponding receiver115by the fiber113and the optical circulator112.

In addition, the optical carrier DS is separated by an optical demultiplexer121into user devices (e.g., user device122). A reflective optical network unit (RONU)120having at least one reflective modulator123is established, in which the reflective modulator123reuses the optical carrier DS and delivers the upstream signal US of the client to the receiver115of the central office110.

However, the cross-sectional area of the fiber becomes an ellipse shape due to manufacturing process or procedures for establishing an optical fiber communication system. Thus an optical fiber communication system with loop back network structures may more easily generate interference noise from Rayleigh backscatter (RB) effect, which especially affects the transmission of the upstream data. In a nutshell, the portions of the light signal or the radio frequency signal delivered in fibers are constantly reflected by the fibers. Finally, the reflected signals delivered to the receiver115of the central office110become Rayleigh backscatter noise.

FIG. 1Bis another diagram of the optical fiber communication system ofFIG. 1A. As shown inFIG. 1B, carrier Rayleigh backscattering CRB and signal Rayleigh backscattering SRB are two main types of Rayleigh backscatter noises. Carrier Rayleigh backscattering CRB is generated by the optical carrier DS, and signal Rayleigh backscattering SRB is generated by the upstream signal US. The carrier Rayleigh backscattering CRB is generated by the process where the optical carrier DS is delivered from the central office110to the optical demultiplexer121. The signal Rayleigh backscattering SRB is generated by the process where the Rayleigh backscattering RB, generated when the optical carrier DS is delivered from the central office110to the optical demultiplexer121, is modulated again by the reflective modulator123of the reflective optical network unit120and then delivered to the receiver115of the central office110.

Therefore, there is a need for an optical fiber communication system to decrease the carrier Rayleigh backscattering CRB and the signal Rayleigh backscattering SRB for improving a signal transmission.

BRIEF SUMMARY OF THE INVENTION

In light of the previously described problems, the invention provides an embodiment of an optical fiber communication system, including a central office and an optical network unit. The central office generates a first downstream signal and a second downstream signal according to a radio frequency signal and a baseband signal, respectively. The optical network unit is coupled to the central office to receive the first downstream signal and the second downstream signal through a first fiber and a second fiber different from the first fiber, respectively, such that the optical network unit only modulates the second downstream signal to generate an upstream signal and then delivers the upstream signal to the central office through the first fiber, thereby decreasing signal Rayleigh backscatter noise.

The disclosure also provides a method for optical fiber communication. The method for optical fiber communication includes the steps of: generating a first downstream signal and a second downstream signal according to a radio frequency signal and a baseband signal, respectively; respectively outputting the first downstream signal and the second downstream signal through a first fiber and a second fiber to an optical network unit, wherein the first fiber is different from the second fiber; and only modulating the second downstream signal to generate an upstream signal to a central office through the first fiber, thereby decreasing signal Rayleigh backscatter noise.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2illustrates an embodiment of an optical fiber communication system. As shown inFIG. 2, the optical fiber communication system200includes a central office (CO)210and an optical network unit (ONU)220. For example, the central office210generates a first downstream signal DS1and a second downstream signal DS2according to a radio frequency signal (RF signal) and a baseband signal, respectively. The optical network unit220is coupled to the central office to receive the downstream signal DS1and the downstream signal DS2through a fiber F1and a fiber F2different from the fiber F1, respectively, such that the optical network unit220only modulates the downstream signal DS2to generate an upstream signal US and then delivers the upstream signal US to the central office210through the fiber F1, thereby decreasing signal Rayleigh backscatter noise.

In detail, the central office210comprises a light generating unit230, an electro-optical modulator M1, a phase modulator M2, an optical circulator OC1and an uplink receiving unit R1. For example, the light generating unit230outputs a light source signal, in which the light generating unit230comprises a light source CW, an optical coupler CP1and polarization controllers PC1and PC2. The optical coupler CP1outputs the light source signal to the electro-optical modulator M1and phase modulator M2, respectively.

The electro-optical modulator M1generates the downstream signal DS1according to the radio frequency signal, a sine wave signal and the light source signal. In detail, in this embodiment, the electro-optical modulator M1is a Mach-Zehnder modulator and modulates the light source signal to a double-sideband (DSB) signal with optical carrier suppression (OCS) according the radio frequency signal (e.g., 2.5 Gb/s) and the sine wave signal (e.g., 10 GHz), in which the frequency of the double-sideband signal is 20 GHz (i.e., downstream signal DS1). The downstream signal DS1and the upstream signal US are on-off keying (OOK) signals. The phase modulator M2modulates the light source signal to generate the downstream signal DS2according to the baseband signal (e.g., 10 Gb/s), in which the downstream signal DS2is a differential phase shift keying (DPSK) signal.

The optical circulator OC1outputs the downstream signal DS1and receives the upstream signal US. In detail, a first terminal of the optical circulator OC1is coupled to the electro-optical modulator M1to receive the downstream signal DS1. A second terminal of the optical circulator OC1is coupled to a third terminal of the optical circulator OC2of the optical network unit220to output the downstream signal DS1to the optical circulator OC2, and receive the upstream signal US from the optical network unit220. A third terminal of the optical circulator OC1is coupled to an uplink receiving unit R1to output the upstream signal US to the uplink receiving unit R1. In addition, the Erbium doped fiber amplifiers (EDFA) E1and E2amplify the downstream signals DS1and DS2, respectively.

In this embodiment, the optical network unit220is a reflective optical network unit. The optical network unit220includes a reflective modulator M3, an optical circulator OC2, a baseband receiving unit R2, an optical coupler CP2and a radio frequency receiving unit R3. In detail, the reflective modulator M3modulates the downstream signal DS2to generate the upstream signal US. In this embodiment, the reflective modulator M3is a reflective semiconductor optical amplifier (RSOA).

The optical circulator OC2receives the downstream signals DS1and DS2, respectively, and outputs the upstream signal US to the central office210. In detail, a first terminal of the optical circulator OC2is coupled to the fiber F2to receive the downstream signal DS2. A second terminal of the optical circulator OC2is coupled to the reflective modulator M3to output the downstream signal DS2to the reflective modulator M3and receive the upstream signal US. A third terminal of the optical circulator OC2is coupled to the fiber F1to receive the downstream signal DS1and output the upstream signal US to the central office210. A fourth terminal of the optical circulator OC2outputs the downstream signal DS1to the radio frequency receiving unit R3.

A first terminal of the optical coupler CP2is coupled to the second terminal of the optical circulator OC2to receive the downstream signal DS2. A second terminal and a third terminal of the optical coupler CP2output the downstream signal DS2to the baseband receiving unit R2and the reflective modulator M3, respectively The baseband receiving unit R2receives the downstream signal DS2, in which the baseband receiving unit R2includes a delay interferometer DI demodulating the downstream signal DS2. The radio receiving unit R3is coupled to the fourth terminal of the optical circulator OC2to receive the downstream signal DS1.

Note that signals inputted to the first terminal of the optical circulator OC2are only outputted from the second terminal of the optical circulator OC2. Similarly, signals inputted to the second terminal of the optical circulator OC2are only outputted from the third terminal of the optical circulator OC2, and signals inputted to the third terminal of the optical circulator OC2are only outputted from the fourth terminal of the optical circulator OC2. In other words, signals inputted to the third terminal of the optical circulator OC2are not outputted from the first terminal, the second terminal or the third terminal of the optical circulator OC2. The optical circulator OC1has the same feature as the optical circulator OC2, thus, the illustration of the optical circulator OC1which is the same as the illustration of the optical circulator OC2is omitted for brevity. Therefore, the Rayleigh backscatter noise generated by the upstream signal can not be outputted to the reflective modulator M3through the second terminal of the optical circulator OC2, thereby preventing the generation of signal Rayleigh backscatter noise.

In general, by the use of two different fibers F1and F2respectively delivering the downstream signals DS1and DS2in this embodiment, Rayleigh backscatter noise generated by the upstream signal US can not be outputted to the reflective modulator M3through the second terminal of the optical circulator OC2. Therefore, the reflective modulator M3can not reflect the Rayleigh backscatter noise generated by the upstream signal US, thereby decreasing the generation of signal Rayleigh backscatter noise.

FIG. 3illustrates a waveform of the light source signal and the downstream signal DS1of the disclosure. As shown inFIG. 3, the light source signal is a continuous wave and the central wavelength of the light source signal is 1550 nm. The downstream signal DS1is a double-sideband (DSB) signal with optical carrier suppression (OCS) and the frequency of the downstream signal DS1is 20 GHz.

FIG. 4shows an embodiment of the relationship between power and the bit error rate (BER) of the upstream signal US. As shown inFIG. 4, the horizontal axis is the receiving power (The amount of the receiving power is indicated by the unit of dBm). The vertical axis is the bit error rate indicated by logarithm. The line with circular markers is the bit error rate of the upstream signal US in the optical network unit220. The line with triangular markers is the bit error rate of the upstream signal US which has delivered 20 km. Thus, the bit error rate has decreased along with increase of the received power measured by the receiver. In addition, the subgraph410is the eye diagram of the upstream signal US in the back to back transmission (i.e., in the optical network unit220). The subgraph420is the eye diagram of the upstream signal US which has delivered 20 km. The subgraph420shows that the center of the eye diagram is still very clear. It can be seen that this embodiment can decrease the generation of Rayleigh backscatter noise.

FIG. 5illustrates a sequence diagram of a method for optical fiber communication. As shown inFIG. 5, the method for optical fiber communication includes the following steps.

In step S51, the downstream signal DS1and the downstream signal DS2are generated according to the radio frequency signal and the baseband signal, respectively. In step S52, the downstream signals DS1and DS2are respectively outputted through the fibers F1and F2to the optical network unit220, wherein the fiber F1is different from the fiber F2. In step S53, only the downstream signal DS2is modulated to generate the upstream signal US to the central office210through the fiber F1, thereby decreasing signal Rayleigh backscatter noise.