Fibre Laser are a special class of Lasers differing from the “classic design”. The optical resonator consists of an optical fibre which can be coiled to a drum providing extreme long amplification lengths. Within this experiment the students are introduced to the basics of optical pumping with subsequent application by means of an Erbium doped optical fibre (EDF). This type of fibre is commonly used as amplifier in long distance telecommunication as so called EDFA (Erbium doped Fibre Amplifier). Due to its particular properties the EDF is a promising candidate also as laser source for telecommunication and remote sensing. The eye safe radiation makes such a device also useful for long range finding applications.
This experiment allows to study the EDF in a linear as well as ring configuration. As pump source a diode laser emitting around 300 mW at a wavelength of 980 nm is applied. Via a wavelength division multiplexer (WDM) the pump light is coupled to the EDF. The other end of the EDF is connected to a beam expander which delivers a parallel light bundle which is coupled after a distance of 100 mm back into a patch fibre.
To close the ring this fibre is connected to the WDM. In common applications this “free space” propagating part is not used. However, for experimental purposes a variety of demonstrations can be done. By means of a thin glass plate - used as output coupler - a small fraction of the ring laser radiation is coupled out for further analysis. Herewith also counter propagating ring modes can be verified. Additional optical components can be inserted like passive q-switch, birefringent tuner etc. With the provided photodetector and the modulator the time response like spiking, life time of excited states can be studied.
Fibre Linear Laser
Fibre Ring Laser
The pump laser diode has a so called single mode fibre pigtail, terminated with a fibre connector and is directly connected to the EDF fibre. The light inside the fibre is partially reflected at each end face due to Fresnel reflections. Despite the low reflectivity of 4% laser oscillation is built up between the two end faces of the EDF due to the high gain. A collimator with short focal length forms an almost parallel beam and an InGaAs photodetector is used to detect the emission of the fibre. The interference filter is mounted to an adjustment holder and passes only the 1.55 µm radiation. With its front face reflectivity of about 10% it serves also as an external cavity mirror to study the effect of a coupled optical cavity. The pump laser diode is connected to a controller with adjustable injection current and modulation. This allows to study the amplified spontaneous emission (ASE) below and above the laser threshold. By using the modulation of the injection current the lifetime of the excited state as well as impressive laser spiking is displayed and measured on an oscilloscope.
In the ring laser setup, the pump radiation is injected by a wavelength division multiplexer (WDM) into the ring structure which consists out of the EDF, a 4 port fibre coupler and an optical diode or just a fibre patch cable. The optical diode is used to force the ring laser either to the clockwise (cw) or counter clockwise (CCW) operation. Exchanging the optical diode against a fibre the ring laser oscillates in both directions. The 4 port fibre coupler is used to extract about 10% of the internal power of the cw and ccw ring laser mode for measurement purposes. Including the optical diode into the ring structures only cw or ccw oscillation occurs. By using an electronic spectrum analyser the beat frequency of the longitudinal ring resonator modes is measured and set into relation to the resonator length for the 2, 4 or 8 m long EDF.