1. Field of Invention
The present invention relates to a lightwave transmission system. More particularly, the present invention relates to a light wave source designed to simultaneously suppress stimulated brillouin scattering (SBS) effects and composite second-order (CSO) distortions, which is induced by self phase modulation and external phase modulation, in a long-distance transmission system.
2. Description of Related Art
The recent trend of distributing broadcasting multi-channel amplitude-modulated (AM) cable TV signals over a distance greater than about 50 Km usually utilizes an optical fiber transmission system operating in the 1550 nm wavelength region. These systems usually use an optical transmitter which is composed of a high-power 1550 nm laser diode, a LiNbO3-based external modulator, and a booster erbium-doped fiber amplifier (EDFA) whose output power can exceed 20 dBm. It is well-know that although the optical power from this kind of transmitter can be increased almost indefinitely by using a high-power booster EDFA, the maximum optical power which can be launched into a single-mode fiber is limited by stimulated Brillouin scattering (SBS). The SBS effect causes reflection of input optical waves by the refractive-index grating, which is formed by acoustic waves travelling at a velocity of about 6 km/sec in the fiber. When the injected light is strong, the index grating is reasonantly enhanced by the input optical wave, resulting in more reflections. There have been several inventions proposed to solve this problem. Form example, the U.S. Pat. Nos. 5,420,868, 5,566,381, and 5,828,477 have proposed some solutions, in which the U.S. Pat. No. 5,828,477 allows an optical power of 17 dBm to be launched into a broadband optical fiber distribution system. However, the transmission distance can only be up to 50 Km. In the current stage, there is no commercial optical transmitter that can be used to deliever the multi-channel AM cable TV (CATV) signals over a distance greater than 65 Km. Ideally, for a launched transmitter power of 17 dBm with a typical optical receiver sensitivity of 0 dBm and an optical fiber transmission loss of 0.2-0.22 dB/km, the maximum transmission distance should be around (17xe2x88x920)/0.22xcx9c77 km or (17xe2x88x920)/0.22xcx9c85 km. Unfortunately, it can transmit by only 50 km at the limit range.
A repeaterless system in an optical transmission system usually means that only one EDFA is used for long-distance transmission. Currently, a repeaterless system can only transmit a limited distance of 50-65 km due to combined effects of self-phase modulation (SPM) and external-phase modulation (EPM). The SPM effect results from the launched power dependent-refractive index, which causes the phase of the optical field to vary as a function of distance. The EPM effect results from the light beam travelling through an external phase modulator. The combined effects of SPM and EPM cause usually composite second-order (CSO) distortions in a long-distance 1550 nm AM-CATV system. The SPM and EPM effects cause PM (phase modulation)-to-AM (amplitude modulation) conversion due to optical fiber dispersions, where the AM components are not desired. As a result, large CSO distortions occur after a long transmission distance. This negative effect becomes more serious as the EPM modulation depth becomes higher.
In the optical transmission system, the above SBS and CSO effects are two main problems necessarily to be solve. Conventionally, the SBS effect is suppressed by broadening the optical spectrum as wide as possible. There is no consideration about the damage to the external modulator due to large microwave driving power. The conventional manner only suppresses the SBS effect but cannot suppress the CSO distortions resulting from PM-to-AM conversion for a distance longer than about 65 km.
It is at least an objective that the invention provides an optical fiber transmitter to achieve a long transmission distance greater than about 75 km by suppression both the SBS effect and the CSO effects at an optimized condition, which is adjustable depending on the desired transmission distance.
The present invention provide an optical fiber transmitter, which includes a frequency modulator generating a first tone microwave with frequency f1 coupled to a laser source, and a phase modulator generating a second tone microwave with microwave with frequency f2 coupled to an external phase modulator, so as to broaden the modulated light spectrum. The first tone microwave with frequency f1 is set to be about greater than 2 to 3 times of the highest frequency content in the modulating signals. The second tone microwave with frequency f2 is designed to be about close to a harmonic frequency of f1 and no less than three times of f1. The first tone microwave with frequency f1 and a second frequency f2 are inputted to two power amplifiers, respectively, and then are respectively inputted to a laser source and an external phase modulator. These two power amplifiers are controlled by an microprocessor to automatically adjust a desired phase modulation index at 1.4 for the optical fiber transmitter with a repeaterless design or at an desired index depending on the desired transmission distance in a repeater design. The external phase modulator is coupled to a pare of fiber amplifiers for the repeaterless design or several pairs of fiber amplifiers coupled in series for the repeater design. The last pair of the fiber amplifiers are coupled to optical receivers.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.