Fiber based mode locked fiber laser using Kerr effect

This dissertation reports on the research to design and build a pulsed fiber laser with the Er doped fiber based on a new mode locking technique. The numerical simulations begin by launching an optical wave in a fiber which will be amplified during propagation. The device to mode-lock the waves is outside the fiber, but connecting to fibers at both ends; it is a nonlinear optical material that can reshape the beam as it propagates using a nonlinear change of the refractive index, which is called a Kerr effect. The device is made with a nonlinear material sandwiched between two fiber ends; it takes an optical field from one end of the fiber and propagates it to the other fiber end. In between the two ends, a nonlinear medium will be used to balance the diffraction through Kerr effect (which can lead to Self-focusing of the optical beam). With the second fiber end working as a soft aperture, the combination of the self-focusing effect through the nonlinear medium and the aperture will act as an intensity dependent coupling loss; this effect is referred to as a fast saturable absorber which means that higher intensity corresponds to higher coupling efficiency and thus the cavity modes will be gradually phase locked together to form pulses. The saturable absorber action is calculated using different nonlinear mediums ( CS 2 , As 2 S 2 and As 40 Se 60 ) and the fibers used are assumed to be of the same size.Whole cavity simulation is then conducted using the proposed SA design and the pulse energy produced from the laser cavity is generally below 1 nJ. In those simulations the pulse peak power is weak and the saturable absorber action is not strong.Experiments are designed to test the mode locking idea with the chalcogenide glass plate ( As 40 Se 60 ). Firstly, a mode locked laser is constructed from a ring fiber laser cavity with an Er doped fiber as the gain fiber. Three modes from this cavity are routinely generated. Two modes have pulse durations of 220 fs and 160 fs with spectral width of about 30 nm and 40 nm, respectively. Mode 3 is more interesting since it covers a huge spectrum range (1490 to 1640 nm) and the pulse duration is estimated to be about 40 fs from the transform limited pulse calculated from the spectrum, which could be the shortest pulse ever reported from an Er doped fiber laser. Further efforts are needed to better dechirp the pulse to verify the transform limited calculation. Due to the weak saturable absorber action from the original design, we use a telescope design to test our SA idea in experiment; the ChG As 40 Se 60 plate is placed inside a telescope which is then inserted into the laser cavity explained in previous paragraph. The telescope design is used to focus the pulse so that higher level of nonlinearity is induced. Then an iris is placed behind the glass plate to create a transmission discrimination mechanism against different powers (Kerr…