Influence of upconversion and excited state absorption on the performance of an erbium-ytterbium doped DFB fiber laser

Abstract: Cooperative up-conversion and excited state absorption are the main limiting factors of short cavity erbium-ytterbium doped fiber lasers like DFB fiber lasers. In this report we quantify the influence of these detrimental effect by numerical modelling. The results of our analysis demonstrate that cooperative upconversion account for almost 10% in reduction of the laser output power. In addition, laser threshold and slope efficiency of the laser are also strongly influenced by cooperative upconversion and excited state absorption

Optimization criteria and design of few-mode erbium-doped fibers for cladding-pumped amplifiers

We propose a novel optimization method that combines two design criteria to reduce the differential modal gain (DMG) in few-mode cladding-pumped erbium-doped fiber amplifiers (FM-EDFAs). In addition to the standard criterion that considers the mode intensity and dopant profile overlap, we introduce a second criterion that ensures that all doped regions have the same saturation behavior. With these two criteria, we define a figure-of-merit (FOM) that allows the design of MM-EDFAs with low DMG without high computational cost. We illustrate this method with the design of six-mode erbium-doped fibers (EDFs) for amplification over the C-Band targeting designs that are compatible with standard fabrication processes. The fibers have either a step-index or a staircase refractive index profile (RIP), with two ring-shaped erbium-doped regions in the core. With a staircase RIP, a fiber length of 29 m and 20 W of pump power injected in the cladding, our best design leads to a minimum gain of 22.6 dB while maintaining a DMGmax under 0.18 dB. We further show that the FOM optimization achieves a robust design with low DMG over a wide range of variations in signal power, pump power and fiber length

Design and development of an all-optical active Q-switched Erbium-doped fibre ring laser

M.Phil.This dissertation describes the design and experimental realization of an all-optical active Q-switched Erbium-doped fibre ring laser. The aim of this research is to propose an approach of Q-switching mechanism for a fibre laser. The Q-switch device combines a fibre Bragg grating and a tunable fibre Fabry-Perot filter. The Q-switching principle is based on dynamic spectral overlapping of two filters, namely FBG based filter and tunable F-P filter. When the spectra overlap, the filter system has the maximum transparency, the laser cavity has minimal losses and it can release the stored power in the form of the giant impulse. A series of experiments are performed to optimize the all-optical active Q-switched Erbium-doped ring laser system in term of output peak power and time duration of laser pulses. Two different Erbium-doped fibres having different Erbium ion concentration are used in this experimental investigation. The first fibre, with an Erbium ion concentration of 2200 ppm and pump absorption of 23.4 at 980 nm is referred to as “high concentration” and the second with an Erbium ion concentration of 960 ppm and pump absorption of 12.4 at 980 nm is referred to as “low concentration” To optimize the Q-switched fibre laser system, different parameters were investigated such as the length of the Erbium-doped fibre, the output coupling ratio, the repetition rate of pulses and the concentration of the Erbium Doped Fibres. The achieved output laser pulse characteristics, peak power and time duration, were 580 mW and 13 μs respectively, at 1 kHz of repetition rate. These characteristics were obtained using a length of 3.5 m “low concentration” Erbium-doped fibre in a ring laser cavity; the output coupling is 90 %, for a pump power of 80 mW. Employing this all-optical Q-switching approach, a simple, robust all-optical active Q-switched Erbium-doped laser is demonstrated

Development, characterisation and analysis of an active Q-switched fiber laser based on the modulation of a fiber Fabry-Perot tunable filter

D.Phil. (Electrical Engineering)Abstract: The field of fiber lasers and fiber optic devices has experienced a sustained rapid growth. In particular, all-fiber Q-switched lasers offer inherent advantages of relatively low cost, compact design, light weight, low maintenance, and increased robustness and simplicity over other fiber laser systems. In this thesis, a design of a new Q-switching approach in all-fiber based laser is proposed. The Q-switching principle is based on dynamic spectral overlapping of two filters, namely fiber Bragg grating based filter and fiber Fabry-Perot tunable filter. When the spectra overlap, the filter system has the maximum transparency, the laser cavity has minimal losses and it can release the stored power in the form of the giant pulse. Using this Q-switching technique, experimental construction of an all-fiber active Q-switched Erbium-doped fiber lasers is successfully demonstrated in both ring cavity and linear cavity fiber laser configurations. The output peak power of 9.7 W and time duration of 500 ns are obtained at 1 kHz of repetition rate for ring cavity Q-switched Erbium-doped fiber laser; and 5.6 W output peak power with a 450 ns pulse time duration are achieved for linear cavity Q-switched Erbium-doped fiber laser at the same repetition rate. The repetition rate of the pulses can be continuously varied from a single shot to a few kHz..

Influence of UpconversiExcited state absorption on the performance of an Erbium-Ytterbium doped DFB Fiber Laser

Abstract: Cooperative up-conversion and excited state absorption are the main limiting factors of short cavity erbium-ytterbium doped fiber lasers like DFB fiber lasers. In this report we quantify the influence of these detrimental effect by numerical modelling. The results of our analysis demonstrate that cooperative upconversion account for almost 10% in reduction of the laser output power. In addition, laser threshold and slope efficiency of the laser are also strongly influenced by cooperative upconversion and excited state absorption

Machine learning implementation for unambiguous refractive index measurement using a self-referenced fiber refractometer

The implementation of a machine learning algorithm for measuring refractive index of liquid samples using Fresnel reflection at the tip of a fiber is proposed in order to overcome the measurement ambiguity between samples having refractive index values below and above the effective refractive index of the fiber fundamental mode. This is the first time that a machine learning algorithm is implemented in a fiber refractometer. The algorithm, used for pattern classification, is the Support Vector Machine (SVM). The sensing head is formed by two-cascaded cavities that generate an interference pattern that changes each time the fiber is immersed in a different sample. The changes in the interference pattern are classified by the proposed algorithm, which extends the sensing range and eliminates any ambiguity in the obtained RI values. The proposed system is also self-referenced, and therefore it is unaffected by any intensity change of the optical source. A theoretical model and experimental results are presented in detail to demonstrate the effectiveness of the proposed system