Optical fiber amplifier in optical wireless communication applications

Introduction Wireless optical communication with laser as the information carrier, is a not need any cable channel as a communication method. Compared with the microwave communication, wireless optical communication by use of laser high frequency, strong directivity ( confidentiality ), the available spectrum is wide, without the need to apply frequency license; compare with optical communication, optical wireless communication has the advantages of low cost, simple construction, fast. It combines the advantages of microwave communication and optical fiber communication, has become a new broadband wireless access way, got the wide attention of people. However, severe weather conditions, for wireless optical communication system spread signals generated attenuation effect. The air of the scattering particles, can make the light in space, time and angle to produce different degrees of deviation. Atmospheric particles may also absorb the laser energy, so that the signal power decrease in wireless optical communication system, optical fiber communication system of low loss propagation path has ceased to exist. Atmospheric environment changeful objectivity cannot change, to get better and faster transmission effect, in the atmospheric channel transmits optical signals to put forward higher requirements, in general, the power of the light signal can be obtained better transmission effect. With the development of optical fiber amplifier ( EDFA ) is developing rapidly, stable and reliable power source in various applications to meet the requirements of wireless optical communication. 1 EDFA principle and structure Erbium doped fiber amplifier ( EDFA ) has high gain, low noise, wide frequency band, high output power, low loss and polarization insensitive connection etc., directly on the optical signal is amplified, without the need for conversion into an electrical signal, can ensure the optical signal in the case of stable minimum distortion power amplifier. 1.1 EDFA principle In erbium doped fiber pumped enough strong pump, can be most in the ground state Er3+ ion pumping to the excited state, in the excited states of the Er3+ ion and rapidly without radiation transfer to a metastable state. As a result of Er3+ ions in a metastable state, longer service life, thus easily in a metastable state and the ground state is formed between the particle number inversion. When the signal photons through the erbium doped fiber, and is in a metastable state Er3+ ion interactions occur stimulated radiation effect, produce a large number of their identical photons, then through the erbium-doped optical fiber transmission of signal photons increases rapidly, generating signal amplifying function. Er3+ ion in a metastable state, in addition to the occurrence of stimulated emission and stimulated absorption, but also produce spontaneous radiation ( ASE ), it caused the noise of EDFA. 1.2 EDFA structure A typical EDFA structure mainly comprises an erbium-doped fiber ( EDF ), a pumping light source, coupler, isolator. Erbium doped fiber is the core part of the EDFA. It is quartz optical fiber as matrix, mixing in the core of solid laser material of erbium ions, in a few meters to tens of meters of erbium doped fiber, the interaction of light and matter is amplified, enhancement. The optical isolator is the role of inhibition of light reflection, to ensure that the amplifier stable work, it must be a low insertion loss, polarization independent, isolated over 40 dB of. 1.3 EDFA characteristics and performance index Gain characteristics of said amplifier, which is defined as the ratio of the output power and input power: Type : Pout, Pin denote respectively the amplifier output and the input end of the continuous signal power. Gain coefficient is defined from the pump light source input 1 mW pump power through the fiber amplifier to obtain the gain, the unit is dB / mW: Type: G0 is composed of pump intensity of small signal gain coefficient, as a result of gain saturation phenomenon, with increasing signal power, gain coefficient decreased; Is, Ps respectively for the saturation intensity and saturation power, is to show that the gain of material properties, and the doping coefficient, fluorescence time and transition section for. The gain and the gain coefficient of difference: the gain is mainly directed against the input signal, and the gain coefficient is mainly directed against the input pump character. In addition, gain and pumping conditions ( including the power of pump and pump wavelength), the primary pump wavelength was 980 nm and 1480 nm. Because of all the gain factor is different, and the gain must be in the whole fiber integral is obtained, so the characteristics can be used to by selection of the fiber length is relatively flat gain spectrum. 1.4 EDFA bandwidth The gain bandwidth of signal light obtained certain gain amplification wavelength region. In fact EDFA gain frequency change ratio theory is much more complicated, and also the matrix fiber and its doping related. In the EDFA gain spectrum width has reached hundreds of nanometers and gain spectrum is relatively flat. ED-FA gain spectrum in the range of 1525～1565 nm.