The success of the optical communication would not have been perfect without the invention of optical amplifier. A lot of effort has been invested in reducing the losses in optical fibres. However the residual losses limiting a maximum distance of around 80 km for a single mode fibre before the signal becomes to weak for detection. In principle the weak light could be amplified by an electronic amplifier and fed again into the fibre. However, this foils the extraordinary high bandwidth of optical fibre and a pure optically working amplifier is required. The concept of optical amplification is part of each laser and optical amplification is a well established technology. The genius underlying concept is the combination with an optical fibre and amplifier in one piece which has been realized in the erbium doped fibre amplifier (EDFA). The EDFA consists of an optical fibre which is doped with a defined concentration of erbium atoms. By means of a coupler the light of a pump source is fed into the fibre exciting the erbium atoms which are acting now as amplifier. The pump wavelength is typically 980 nm and the amplification takes place around 1.500 nm, the same range as the optical communication signal. Due to the coherence of the amplification process the amplified stream of photons are indistinguishable with respect to the incoming ones. What a great idea!
This experiment is designed as open frame and each component can be touched and viewed to enhance the understanding of the EDFA concept. It starts with the pump laser diode emitting a wavelength of 980 nm. The pump radiation is coupled via a dichroic beam splitter plate and an adjustable microscope objective into the erbium doped fibre (EDF). The EDF has a length pf about 16 metre and is coiled up on a drum. The fibre ends are kept in ceramic ferrules allowing the easy cleaning and re-polishing if required. As signal source a laser diode emitting at 1.550 nm is used. Its radiation passes the same beam splitter plate and is also launched into the EDF. At the output end of the EDFA either a SiPIN for the detection of the 980 nm radiation and an InGaAs detector for the detection of the 1.550 nm radiation respectively. A variety of measurements are carried out like the characterisation of the two diode lasers. The injection current of each laser can be set independently by two controller. In a next experiment the 980 nm radiation is coupled into the EDF and the created fluorescence is detected and monitored on an oscilloscope. The controller allows the modulated operation of the diode laser in such a way that the fluorescence decay the excited erbium atoms are displayed and the life time determined. By a further increase of the power of the pump diode laser the EDFA turns into a fibre laser which dynamic behaviour like distinctive spiking. Finally the 1.550 nm radiation is fed into the EDFA and the gain is measured as function of the pump power.
The experiment uses two laser diodes, one emits a wavelength of 980 nm with a power of 300 mW and serves as pump source. The other emits a wavelength of 1550 nm with lower power around 5 mW and serves as signal source. Both diode laser are fibre coupled and are connected via single mode patch cables to the fibre coupler. The pump as well as the signal wave enter the Erbium doped fibre and the signals leaving the fibre are detected by a InGaAs photodetector. In order to detect only the 1550 nm radiation a laser line or interference filter is placed in front of the photodetector. Each diode laser has its own controller to set the individual injection current for the measurement of the EDFA as function of the pump and the signal power.
In such case, the laser diode is directly coupled to the photodiode by means of fibre patch cable and the photocurrent is converted by means of the provided junction box into a linear voltage.