Stuart D. Jackson

Theoretical modeling of Tm-doped silica fiber lasers

The theoretically determined slope efficiency and threshold pump power for continuous wave (CW) operation of a Tm-doped silica fiber laser are presented. The associated rate equations are solved using standard techniques and, in conjunction with the published and our measured spectroscopic parameters as input, the model was used to examine the fiber laser output for a variety of fiber and pump configurations. After good agreement was achieved between the model calculations and the published experimental measurements, the model was used to examine the relative performance of the fiber laser when the pump wavelength was varied over the /sup 3/F/sub 4/, /sup 3/H/sub 5/, and /sup 3/H/sub 4/ absorption bands of Tm/sup 3+/. The calculated maximum slope efficiencies were determined to be /spl sim/40, /spl sim/57, and /spl sim/84%, respectively, for each of the /sup 3/F/sub 4/, /sup 3/H/sub 5/, and /sup 3/H/sub 4/ absorption band pump schemes and the threshold pump power over the range of pump schemes was determined to vary by only 28%. The model was further used to analyze the fiber laser output when the fiber length, Tm/sup 3+/ concentration and /sup 3/H/sub 4/ energy level lifetime were varied and the consequences on the operation of the fiber laser with these variations are discussed.

High-power diode-cladding-pumped Tm-doped silica fiber laser

The operation of a diode-pumped Tm-doped silica fiber laser that uses the cladding-pumping arrangement to produce high-power cw output at wavelengths near 2 microm is reported. We obtained a maximum output power of 5.4 W at a slope efficiency of 31% with respect to the launched pump power at a total optical-to-optical efficiency of 22%. The fiber-laser output wavelength was tuned between 1.880 and 2.033 microm by adjustment of the fiber length, with >4W of power obtainable from 1.94 to 2.01 microm. Self-pulsations detected in the output from the double-clad fiber laser may indicate the presence of ion-clustering effects.

Dynamics of the output of heavily Tm-doped double-clad silica fiber lasers

The relaxation oscillations that are present in the output of a high-power free-running diode-pumped Tm-doped silica fiber laser are characterized and modeled. The laser operates on the xa03H4→3H6 quasi-three-level transition and operates efficiently from ∼1.9 to ∼2.0 µm. From the measurements of the xa03F4→3H4 fluorescence after 1.98-µm pumping of a separate, heavily Tm-doped silica fiber, it is established that the xa03H4,xa03H4→3H6,xa03F4 upconversion process is significant in heavily Tm-doped silica fibers. This process causes the saturable absorption that is indicated by the presence of the relaxation oscillations. A detailed theoretical model that describes the ion-pair dynamics relevant to the Tm-doped silica system is presented. Equations for the steady-state intracavity photon density and for the steady-state population densities of the isolated and paired ions are derived. A linear stability analysis and numerical analysis are also carried out. It is established that, for large emission-to-absorption cross-section ratios that are relevant to Tm-doped silica and for pump rates for which stable output is predicted, the oscillations are weakly damped before the steady state is reached. Best agreement between the experimental and model results is achieved when all the Tm3+ ions within the present fiber are considered paired.

10 mJ total output from a gain-switched Tm-doped fibre laser

A gain-switched cladding-pumped Tm-doped silica fibre laser operating at a wavelength of approximately 2-μm with moderate output energy is reported. When pumped with the output from a gain-switched Ti:sapphire laser, a maximum total output energy of 10.1 mJ per pump pulse is produced at a slope efficiency of 30% (with respect to launched pump energy) and, at an overall optical-to-optical efficiency of 17%. For relatively short fibre lengths, saturation of the output energy is observed with an increase in launched pump energy because of pump-induced bleaching of the pump transition. The laser has been continuously tuned using an intracavity prism over a range of 109 nm, from 1909 nm to 2018 nm.

Efficient gain-switched operation of a Tm-doped silica fiber laser

We present the results from experiments relating to a gain-switched Tm-doped silica fiber laser in which a gain-switched Nd:YAG laser is used to pump the /sup 3/H/sub 5/ energy level of the Tm/sup 3+/ dopant ion. This fiber laser configuration is the first example to our knowledge of a moderate energy gain-switched fiber laser which is pumped with a low-repetition-rate high-energy pulsed laser. For a near-optimized cavity, the gain-switched fiber laser produces a maximum pulse energy of 1.46 mJ at a maximum linear slope efficiency of 20% and a total optical-to-optical efficiency (with respect to the launched energy) of 19%. At low pump energies, the slope efficiency is approximately 40%, however, saturation of the output pulse energy is observed with the increase in the launched pump energy. We also present results from a numerical model that simulates /sup 3/H/sub 5/-band pumping and includes all of the known pump excited-state absorption (ESA) mechanisms and, in addition, four cross-relaxation mechanisms have also been included. The calculations establish that the pump ESA mechanism contributes only a small loss factor to the overall efficiency of the laser when the Tm-doped silica fiber laser is pumped at low pump energies, however, as the pump energy is increased, losses due to pump ESA limit the amount of output energy from the fiber laser. The loss mechanism is mainly attributed to pump ESA from the /sup 3/H/sub 4/ upper laser level to the combined /sup 3/F/sub 2,3/ energy level at low launched pump energies because of the large absorption cross section for this transition and the relatively long lifetime of the /sup 3/H/sub 4/ energy level. For harder pumping conditions, the majority of the excited state population resides in the /sup 1/G/sub 4/ level, inhibiting in some laser configurations gain-switching of the fiber laser until cessation of the pump pulse itself.

Laser-tissue interaction with a continuous wave 3-μm fibre laser: Preliminary studies with soft tissue

Lasers operating at wavelengths in the mid‐infrared region have become increasingly popular for applications in areas of surgery and medicine. Advances in fibre laser technology have introduced a highly efficient, compact, diode‐pumped source operating at around the 3‐μm wavelength. This study examines the effects of this recently developed laser on soft biological tissue.

Modelling of high-power diode-pumped erbium 3-µm fibre lasers

Abstract We present theoretical calculations that relate to the cw operation of a high-power Er3+, Pr3+:ZBLAN double-clad fibre laser. Using the measured energy-transfer, energy-transfer-upconversion and cross-relaxation parameters relevant to Er3+-doped and Er3+, Pr3+-codoped ZBLAN, we compare the theoretical calculations from the model with recent experimental measurements. The model is then used to analyse the important pump and interionic processes occurring in Er3+, Pr3+:ZBLAN fibre lasers. The theoretical results indicate that energy transfer from Er3+ to Pr3+ leads to a fast depletion of the lower laser level and, due to the cw threshold condition, to low population densities in both laser levels. Thus, ground-state bleaching, pump excited-state absorption and energy-transfer upconversion amongst the Er3+ ions are avoided.

Laser–tissue interaction with a high-power 2-μm fiber laser: Preliminary studies with soft tissue

Recent developments in fiber laser technology have introduced highly efficient, compact sources with high output beam quality. The first laser–tissue interaction studies with a high‐power 2‐μm fiber laser were conducted.

CW operation of a 1.064-/spl mu/m pumped Tm-Ho-doped silica fiber laser

The results from experiments relating to the CW operation of a Tm-Ho-doped silica fiber laser which is pumped with the fundamental output from a Nd:YAG laser are presented. The measured maximum output power from the fiber laser of 11 mW was generated at a slope efficiency of approximately 1.8% for a fiber length of 0.574 m and an output coupling of 10%. An output wavelength of 2170 nm (one of the longest lasing wavelengths to be achieved with the use of a silica host material) was also generated from the Tm-Ho-doped fiber laser when the fiber length was extended to 1.240 m and a 5% output coupling incorporated. The reduced efficiency and increased threshold for the Nd:YAG-pumped Tm-Ho-doped silica fiber laser when compared to previous reports of Ti:sapphire pumping is discussed in detail with the aid of a comprehensive numerical model. The numerical model solves the rate equations for the Tm-Ho-doped silica fiber laser system by taking into account the cross relaxation, energy transfer, and upconversion mechanisms, and it utilizes all published spectroscopic parameters relevant to Tm-Ho-doped silica and Tm-Ho:ZBLAN glass materials. It is established that the excited state absorption relevant to Nd:YAG pumping severely depletes the /sup 3/H/sub 4/ energy level of Tm/sup 3+/ and consequently hinders the energy transfer process to the /sup 5/I/sub 7/ energy level of Ho/sup 3+/. Optimum dopant concentrations are also established for both Nd:YAG and Ti:sapphire pump schemes.

Efficient high power operation of a Tm-doped silica fiber laser pumped at 1.319 μm

Efficient cw and Q-switched operation of a Tm-doped silica fiber laser, pumped with the 1.319-μm output from a Nd:YAG laser, is described. An unsaturated continuous wave maximum output power of 310 mW and a slope efficiency of 23% was obtained for a near optimised cavity. A relatively low measured absorption cross section in conjunction with excited state absorption (ESA) of the pump light results in a minimum threshold pump power of 0.9 W. Q-switching by use of a mechanical chopper resulted in 16 W/280 ns pulses at a repetition rate of 20 kHz.