Zhu Xiao Lei. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2001. / Includes bibliographical references (p. 116-121). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.

Tong Yu Chung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1997. / Includes bibliographical references. / Abstract --- p.i / Acknowledgments --- p.ii / Table of Contents --- p.iii / Chapter (1) --- Introduction --- p.1 / Chapter 1.1 --- Background --- p.1 / Chapter 1.2 --- Overview of the Thesis --- p.2 / References --- p.4 / Chapter (2) --- Review of Ultrashort Pulse Generation and Pulsewidth Measurement --- p.5 / Chapter 2.1 --- Introduction --- p.5 / Chapter 2.2 --- Q-switching --- p.5 / Chapter 2.3 --- Gain-switching --- p.8 / Chapter 2.4 --- Mode-locking --- p.11 / Chapter 2.4.1 --- Active mode-locking --- p.12 / Chapter 2.4.2 --- Passive mode-locking --- p.13 / Chapter 2.5 --- Optical Pulse Compression --- p.15 / Chapter 2.6 --- Pulsewidth Detection Methods --- p.18 / Chapter 2.6.1 --- Streak camera --- p.18 / Chapter 2.6.2 --- Photodetector and sampling oscilloscope --- p.20 / Chapter 2.6.3 --- Nonlinear autocorrelator --- p.21 / Chapter 2.6.4 --- Other techniques --- p.24 / References --- p.25 / Chapter (3) --- Erbium Doped Fiber Amplifier and Active Mode-locking --- p.28 / Chapter 3.1 --- Introduction --- p.28 / Chapter 3.2 --- Erbium Doped Fiber Amplifier --- p.28 / Chapter 3.2.1 --- Background --- p.28 / Chapter 3.2.2 --- Experiment --- p.31 / Chapter 3.3 --- Additive Pulse Mode-locking --- p.35 / Chapter 3.4 --- Active Mode-locking --- p.37 / Chapter 3.4.1 --- Background --- p.37 / Chapter 3.4.2 --- Experiment and result --- p.38 / Chapter 3.4.3 --- Discussion --- p.43 / Chapter 3.5 --- Chapter Summary --- p.46 / References --- p.46 / Chapter (4) --- Passive Mode-locking of Erbium Doped Fiber Laser --- p.49 / Chapter 4.1 --- Introduction --- p.49 / Chapter 4.2 --- Background --- p.49 / Chapter 4.3 --- Experimental Setup --- p.51 / Chapter 4.4 --- Initialing Mode-locking --- p.54 / Chapter 4.5 --- Experimental Result --- p.55 / Chapter 4.5.1 --- Real time pulse train --- p.55 / Chapter 4.5.2 --- Autocorrelation trace --- p.57 / Chapter 4.5.3 --- RF spectrum --- p.58 / Chapter 4.5.4 --- Optical spectrum --- p.59 / Chapter 4.5.5 --- Time-bandwidth product --- p.60 / Chapter 4.5.6 --- Output power --- p.61 / Chapter 4.6 --- Discussion --- p.63 / Chapter 4.6.1 --- Linear pulse broadening --- p.63 / Chapter 4.6.2 --- Cavity oscillation --- p.65 / Chapter 4.6.3 --- Pump power hysteresis --- p.66 / Chapter 4.6.4 --- Sideband generation --- p.67 / Chapter 4.6.5 --- Spectral distortion --- p.68 / Chapter 4.7 --- Chapter Summary --- p.71 / References --- p.72 / Chapter (5) --- Application of Ultrashort Optical Pulses from Figure Eight Laser --- p.74 / Chapter 5.1 --- Introduction --- p.74 / Chapter 5.2 --- Dispersion Measurement --- p.74 / Chapter 5.2.1 --- Introduction --- p.74 / Chapter 5.2.2 --- Background --- p.75 / Chapter 5.2.3 --- Experiment and result --- p.76 / Chapter 5.2.4 --- Discussion and conclusion --- p.80 / Chapter 5.3 --- Time Domain Spectral Estimation --- p.82 / Chapter 5.3.1 --- Introduction --- p.82 / Chapter 5.3.2 --- Background --- p.82 / Chapter 5.3.3 --- Experiment and result --- p.83 / Chapter 5.3.4 --- Discussion --- p.88 / Chapter 5.4 --- Ultrashort Pulse Amplification --- p.89 / Chapter 5.4.1 --- Introduction --- p.89 / Chapter 5.4.2 --- Background --- p.89 / Chapter 5.4.3 --- Experiment and result --- p.92 / Chapter 5.4.4 --- Discussion and conclusion --- p.95 / References --- p.96 / Chapter (6) --- Picosecond Pulse Generation from Semiconductor Laser Diodes --- p.99 / Chapter 6.1 --- Introduction --- p.99 / Chapter 6.2 --- Gain-switching --- p.99 / Chapter 6.2.1 --- Experiment using commercial laser diodes --- p.99 / Chapter 6.2.2 --- Repetition rate multiplication --- p.102 / Chapter 6.2.3 --- Pulse compression with HDSF --- p.107 / Chapter 6.2.4 --- Fiber loop compressor --- p.110 / Chapter 6.3 --- Active or Hybrid Mode-locking --- p.112 / Chapter 6.3.1 --- Introduction --- p.112 / Chapter 6.3.2 --- Laser structure --- p.113 / Chapter 6.3.3 --- Experiment and result --- p.113 / Chapter 6.3.4 --- Discussion and conclusion --- p.116 / Chapter 6.4 --- Amplifier Modulation --- p.117 / Chapter 6.4.1 --- Introduction --- p.117 / Chapter 6.4.2 --- Experiment and result --- p.118 / Chapter 6.5 --- Wavelength Tuning --- p.120 / Chapter 6.5.1 --- Introduction --- p.120 / Chapter 6.5.2 --- Experiment and result --- p.121 / Chapter 6.5.3 --- Conclusion --- p.123 / Chapter 6.6 --- Chapter Summary --- p.124 / References --- p.124 / Chapter (7) --- Conclusion --- p.126 / Chapter 7.1 --- Summary of the Research --- p.126 / Chapter 7.1.1 --- Fiber lasers --- p.126 / Chapter 7.1.2 --- Diode lasers --- p.128 / Chapter 7.2 --- Further Study --- p.129 / Appendix I Project Instrumentation --- p.A-l / Appendix II Curve Fitting Program for the SHG Autocorrelation Trace --- p.A-8 / Appendix III Experiment Setup of Figure Eight Laser --- p.A-12 / "Appendix IV Curve Fitting Program for Determination of Second Order Dispersion, dD/dλ" --- p.A-14 / Appendix V 1.3 μm two sections DFB/TA Laser Diode Chips --- p.A-17 / Appendix VI Publication List --- p.A-l9

In this thesis, novel multi-section laser diodes based on quantum-dot material are designed and investigated which exhibit a number of advantages such as low threshold current density; temperature-insensitivity and suppress carrier diffusion due to discrete nature of density of state of quantum-dots. The spectral versatility in the range of 1.1 µm – 1.3 µm wavelengths is demonstrated through novel mode-locking regimes such as dual-wavelength mode-locking, wavelength bistability and broad tunability. Moreover, broad pulse repetition rate tuning using an external cavity configuration is presented. A high peak power of 17.7 W was generated from the quantum-dot laser as a result of the tapered geometry of the gain section of the laser has led to successful application of such device for two-photon imaging. Dual-wavelength mode-locking is demonstrated via ground (?=1180 nm) and excited (?=1263 nm) spectral bands with optical pulses from both states simultaneously in the 5-layer quantum-dot two-section diode laser. The widest spectral separation of 83 nm between the modes was achieved in a dual-wavelength mode-locked non-vibronic laser. Power and wavelength bistability are achieved in a mode-locked multi-section laser which active region incorporates non-identical QD layers grown by molecular beam epitaxy. As a result the wavelength can be electronically controlled between 1245 nm and 1290 nm by applying different voltages to the saturable absorber. Mode-locked or continuous-wave regimes are observed for both wavelengths over a 260 mA – 330 mA current ranges with average power up to 28 mW and 31 mW, respectively. In mode-locked regime, a repetition rate of 10 GHz of optical pulses as short as 4 ps is observed. Noticeable hysteresis of average power for different bias conditions is also demonstrated. The wavelength and power bistability in QD lasers are potentially suitable for flip-flop memory application. In addition, a unique mode-locked regime at expense of the reverse bias with 50 nm wavelength tuning range from 1245 nm to 1290 nm is also presented. Broad repetition rate tunability is shown from quantum-dot external cavity mode-locked 1.27 µm laser. The repetition rate from record low of 191 MHz to 1 GHz from fundamental mode-locking was achieved. Harmonic mode-locking allows further to increase tuning up to 6.8 GHz (34th-order harmonic) from 200 MHz fundamental mode-locking. High peak power of 1.5 W can be generated directly from two-section 4 mm long laser with bent waveguide at angle of 7° at 1.14 GHz repetition rate without the use of any pulse compression and optical amplifier. Stable mode-locking with an average power up to 60 mW, corresponding to 25 pJ pulse energy is also obtained at a repetition frequency of 2.4 GHz. The minimum time-bandwidth product of 1.01 is obtained with the pulse duration of 8.4 ps. Novel tapered quantum-dot lasers with a gain-guided geometry operating in a passively mode-locked regime have been investigated, using structures that incorporated either 5 or 10 quantum dot layers. The peak power of 3.6 W is achieved with pulse duration of 3.2 ps. Furthermore, the record peak power of 17.7 W and transform limited pulses of 672 fs were achieved with optimized structure. The generation of picosecond pulses with high average power of up to 209 mW was demonstrated, corresponding to 14.2 pJ pulse energy. The improved optical parameters of the tapered laser enable to achieve nonlinear images of fluorescent beads. Thus it is for the first time that QD based compact monolithic device enables to image biological samples using two-photon microscopy imaging technique.

Nonlinear optical microscopy (NLOM) utilizing femtosecond laser pulses is well suited for imaging living tissues. This work reports on the design and development of an optical fiber based multispectral NLOM developed around a laser generating broadband sub-10-fs pulses. An all-mirror dispersion-compensation setup is used to correct for quadratic and cubic phase distortions induced within the NLOM. Mouse tail tendon was used to characterize sub-10-fs pulses by interferometric autocorrelation. This is an effective method for characterizing dispersion from the optical system, immersion medium, and wet biological sample. The generation of very short autocorrelations demonstrates the ability to compensate for phase distortions within the imaging system and efficient second-harmonic upconversion of the ultrashort pulse spectrum within collagen. Reconstruction of ultrashort pulses at the focal plane of the objective allows the excitation of multiple fluorescent probes simultaneously. Multiple fluorescent probe excitation and spectral discrimination is demonstrated using mixtures of fluorescent dye solutions and an in-vitro angiogenesis model containing human umbilical vein endothelial cells (HUVEC’s) expressing multiple fluorescent proteins. Sub-10-fs pulses can be propagated through polarization-maintaining single mode fiber (PMF) for use in NLOM. We demonstrate delivery of near transform-limited, 1 nJ pulses from a Ti:Al2O3 oscillator via PMF to the NLOM focal plane while maintaining 120 nm of bandwidth. Negative group delay dispersion (GDD) introduced to pre-compensate normal dispersion of the optical fiber and microscope optics ensured linear pulse propagation through the PMF. Nonlinear excitation of multiple fluorophores simultaneously and polarization sensitive NLOM imaging using second harmonic generation in collagen was demonstrated using PMF delivered pulses. Two-photon excited fluorescence spectra and second harmonic images taken with and without the fiber indicates that the fiber based system is capable of generating optical signals that are within a factor of two to three of our traditional NLOM.

The ultrafast optical regime is defined, as it applies to laser pulses, along with a brief introduction to pulse generation and characterisation technologies. A more extensive description of our particular amplified pulse generation and SPIDER characterisation systems follows. Data verifying the correct operation of the characterisation system is presented and interpreted. Our laser system is then characterised in two different configurations. In each case, the data describing the system is presented and analyzed. Conclusions are made regarding the performance of both the characterisation and laser systems, along with suggested improvements for each.

The ultrafast optical regime is defined, as it applies to laser pulses, along with a brief introduction to pulse generation and characterisation technologies. A more extensive description of our particular amplified pulse generation and SPIDER characterisation systems follows. Data verifying the correct operation of the characterisation system is presented and interpreted. Our laser system is then characterised in two different configurations. In each case, the data describing the system is presented and analyzed. Conclusions are made regarding the performance of both the characterisation and laser systems, along with suggested improvements for each.

The main contributions of this thesis to the field of ultrashort pulse measurement are a new set of experimental tools to measure the spatio-temporal fields of femtosecond pulses, and a new simplified formalism to describe such fields in the presence of distortions. More specifically, we developed an experimental technique based on scanning-wavelength digital holography and frequency-resolved optical gating that allows the complete measurement of the electric field E(x,y,t) of trains of identical femtosecond pulses. A related method, wavelength-multiplexed digital holography, is also introduced. It achieves a single-shot measurement of the three-dimensional field E(x,y,t) but at a reduced resolution using a simple experimental apparatus. Both methods can be used to measure various spatio-temporal distortions that often plague femtosecond laser systems, in particular amplified ones. Finally, to unambiguously and intuitively quantify such distortions, we introduce normalized correlation coefficients so that a common language can be used to describe the severity of these effects.

Nonlinear optical microscopy (NLOM) utilizing femtosecond laser pulses is well suited for imaging living tissues. This work reports on the design and development of an optical fiber based multispectral NLOM developed around a laser generating broadband sub-10-fs pulses. An all-mirror dispersion-compensation setup is used to correct for quadratic and cubic phase distortions induced within the NLOM. Mouse tail tendon was used to characterize sub-10-fs pulses by interferometric autocorrelation. This is an effective method for characterizing dispersion from the optical system, immersion medium, and wet biological sample. The generation of very short autocorrelations demonstrates the ability to compensate for phase distortions within the imaging system and efficient second-harmonic upconversion of the ultrashort pulse spectrum within collagen. Reconstruction of ultrashort pulses at the focal plane of the objective allows the excitation of multiple fluorescent probes simultaneously. Multiple fluorescent probe excitation and spectral discrimination is demonstrated using mixtures of fluorescent dye solutions and an in-vitro angiogenesis model containing human umbilical vein endothelial cells (HUVEC’s) expressing multiple fluorescent proteins. Sub-10-fs pulses can be propagated through polarization-maintaining single mode fiber (PMF) for use in NLOM. We demonstrate delivery of near transform-limited, 1 nJ pulses from a Ti:Al2O3 oscillator via PMF to the NLOM focal plane while maintaining 120 nm of bandwidth. Negative group delay dispersion (GDD) introduced to pre-compensate normal dispersion of the optical fiber and microscope optics ensured linear pulse propagation through the PMF. Nonlinear excitation of multiple fluorophores simultaneously and polarization sensitive NLOM imaging using second harmonic generation in collagen was demonstrated using PMF delivered pulses. Two-photon excited fluorescence spectra and second harmonic images taken with and without the fiber indicates that the fiber based system is capable of generating optical signals that are within a factor of two to three of our traditional NLOM.

We simulate ultrashort pulse propagation through water by numerical methods, which is a kind of optical communication research. Ultrashort pulses have been known to have non Beer-Lambert behavior, whereas continuous waves (CW) obey the Beer-Lambert law. People have expected that the ultrashort pulse loses less intensity for a given distance in water than CW which implies that the pulse can travel over longer distances. In order to understand this characteristic of the pulse, we model numerically its spectral and temporal evolution as a function of traveling distance through water. We achieve the pulse intensity attenuation with traveling distance, obtain the temporal envelope of the pulse and compare them with experimental data. This research proves that the spectral and temporal profile of a pulse can be predicted knowing only the intensity spectrum of the input pulse and the refractive index spectrum of water in the linear regime. The real feasibility and the advantage of using an ultrashort pulse as a communication carrier will also be discussed.

This thesis presents a compact femtosecond laser amplifier design for optical preamplifiers and power amplifiers consist of theoretical perspective, simulations to analyze and optimize beam performance. The propagation through optical media is simulated for every optical component such as mirrors and nonlinear crystal separately and suggested realignment of these components required increasing amplifier performance. Finally Gaussian beam propagation and aberration compensation has been conducted.