Enhanced Four-wave mixing fibre lasers for multiphoton microscopy
File(s)
Author(s)
Bartosz, Krawczyk
Type
Thesis
Abstract
Endoscopic assessment of potentially cancerous tissue still depends on biopsy and
ex vivo pathology, delaying diagnosis and increasing both patient anxiety and the
risk of incomplete resection. Real-time, in vivo imaging would therefore offer clear
clinical benefit. Two-photon microscopy (TPM) is attractive for this purpose, but its
standard excitation source, the femtosecond Ti:Sapphire laser, is bulky, alignment-
sensitive and difficult to deploy in portable or endoscopic systems.
This thesis develops a fibre-integrated picosecond source near 800 nm for TPM
based on pump-amplified four-wave mixing (PAFWM) in a custom Yb-doped photonic-
crystal fibre (PCF). In this scheme, ytterbium gain replenishes the pump during four-
wave mixing, sustaining power along the fibre, broadening the phase-matching band-
width and lowering the conversion threshold. A theoretical model based on coupled-
amplitude equations predicts an approximately twofold increase in anti-Stokes con-
version efficiency over conventional FWM, and these predictions are supported ex-
perimentally using both 5 ps and 15 ps master-oscillator power-amplifier systems.
In both cases, the Yb-doped PCF produced substantially stronger anti-Stokes out-
put than an equivalent undoped fibre, and the 15 ps system was translated into a
portable TPM source.
The laser was integrated into a commercial TPM platform, where a saturation-
limited model of two-photon excitation fluorescence predicted an optimal repetition
rate of approximately 5.9 MHz. This optimum was confirmed experimentally using
Convallaria majalis, and imaging performance was comparable to that of a flagship
Ti:Sapphire system.
These results establish PAFWM in Yb-doped PCF as a compact and practical
route towards portable and endoscopic TPM, with picosecond pulse durations that
avoid dispersion-compensation requirements in fibre delivery.
ex vivo pathology, delaying diagnosis and increasing both patient anxiety and the
risk of incomplete resection. Real-time, in vivo imaging would therefore offer clear
clinical benefit. Two-photon microscopy (TPM) is attractive for this purpose, but its
standard excitation source, the femtosecond Ti:Sapphire laser, is bulky, alignment-
sensitive and difficult to deploy in portable or endoscopic systems.
This thesis develops a fibre-integrated picosecond source near 800 nm for TPM
based on pump-amplified four-wave mixing (PAFWM) in a custom Yb-doped photonic-
crystal fibre (PCF). In this scheme, ytterbium gain replenishes the pump during four-
wave mixing, sustaining power along the fibre, broadening the phase-matching band-
width and lowering the conversion threshold. A theoretical model based on coupled-
amplitude equations predicts an approximately twofold increase in anti-Stokes con-
version efficiency over conventional FWM, and these predictions are supported ex-
perimentally using both 5 ps and 15 ps master-oscillator power-amplifier systems.
In both cases, the Yb-doped PCF produced substantially stronger anti-Stokes out-
put than an equivalent undoped fibre, and the 15 ps system was translated into a
portable TPM source.
The laser was integrated into a commercial TPM platform, where a saturation-
limited model of two-photon excitation fluorescence predicted an optimal repetition
rate of approximately 5.9 MHz. This optimum was confirmed experimentally using
Convallaria majalis, and imaging performance was comparable to that of a flagship
Ti:Sapphire system.
These results establish PAFWM in Yb-doped PCF as a compact and practical
route towards portable and endoscopic TPM, with picosecond pulse durations that
avoid dispersion-compensation requirements in fibre delivery.
Version
Open Access
Date Issued
2025-06-23
Date Awarded
2026-05-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Runcorn, Timothy
Publisher Department
Department of Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)