Nonlinear femtosecond pulse propagation in an all-solid photonic bandgap fiber

Nonlinear femtosecond pulse propagation in an all-solid photonic bandgap fiber is experimentally and numerically investigated. Guiding light in such fiber occurs via two mechanisms: photonic bandgap in the central silica core or total internal reflection in the germanium doped inclusions. By properly combining spectral filtering, dispersion tailoring and pump coupling into the fiber modes, we experimentally demonstrate efficient supercontinuum generation with controllable spectral bandwidth

Wavelength extension in speciality fibres

Since the invention of the laser and its first application, there has been an almost continuous stream of new applications - many of which require specific laser sources. These applications often require a laser source with a specific power, pulse duration, energy and wavelength. In some cases these demands are easily met, whilst in others they have proven rather more difficult to achieve. Traditionally, wavelength versatility has been limited to the regions for which rare earth or gas gain media are available. These lasers themselves can be used to generate other wavelengths through the nonlinear processes of second and third harmonic generation, as well as sum frequency generation. Despite all of this, there still exists a significant section of the visible and infrared spectrum for which no convenient sources exist. This thesis is concerned with the development of sources in these regions along with broadband sources covering significant portions of the spectrum by themselves. These new wavelengths are generated in a variety of speciality fibres using either nonlinear processes or new gain media doped into standard silica fibres. Three types of speciality fibre are used: low concentration bismuth doped fibre which provides gain in the 1.0-1.4 μm region; photonic crystal fibres; and very high (75%) concentration germanium fibres to generate a laser source at 2.1 μm based upon stimulated Raman scattering. Photonic crystal fibres provide high nonlinearities and controllable dispersion which enables the generation of broadband supercontinuum sources based upon the interaction of many nonlinear effects. Each source will be described in depth, with particular attention given to the underlying physics that gives rise to the source. Previous and current limitations will be examined and an outlook of the future development of such sources will be discussed

Controlling nonlinear optics with dispersion in photonic crystal fibres

Nonlinear optics enables the manipulation of the spectral and temporal features of light. We used the tailorable guidance properties of photonic crystal fibres to control and enhance nonlinear processeswith the aim of improving nonlinearity based optical sources. We utilised modern, high power, Ytterbium fibre lasers to pump either single photonic crystal fibres or a cascade of fibres with differing properties. Further extension of our control was realised with specifically tapered photonic crystal fibres which allowed for a continuous change in the fibre characteristics along their length. The majority of our work was concerned with supercontinuum generation. For continuous wave pumping we developed a statistical model of the distribution of soliton energies arising from modulational instability and used it to understand the optimum dispersion for efficient continuum expansion. A two-fold increase in spectral width was demonstrated, along with studies of the noise properties and pump bandwidth dependence of the continuum. For picosecond pumping we found that the supercontinuum bandwidth was limited by the four wave mixing phase-matching available in a single fibre. A technique to overcome this by using a cascade of fibres with different dispersion profiles was developed. Further improvement was achieved by using novel tapered PCFs to continuously extend the phase-matching. Analysis of this case showed that a key role was played by soliton trapping of dispersive waves and that our tapers strongly enhanced this effect. We demonstrated supercontinua spanning 0.34-2.4 ¹mwith an unprecedented spectral power; up to 5 mW/nm. The use of long, dispersion decreasing photonic crystal fibres enabled us to demonstrate adiabatic soliton compression at 1.06 ¹m. From a survey of fibre structures we found that working around the second zero dispersion wavelength was optimal as this allows for decreasing dispersion without decreasing the nonlinearity. We achieved compression ratios of over 15

Optics and Quantum Electronics

Contains table of contents on Section 3 and reports on nineteen research projects.Defense Advanced Research Projects Agency Grant F49620-96-0126Joint Services Electronics Program Grant DAAH04-95-1-0038National Science Foundation Grant ECS 94-23737U.S. Air Force - Office of Scientific Research Contract F49620-95-1-0221U.S. Navy - Office of Naval Research Grant N00014-95-1-0715Defense Advanced Research Projects Agency/National Center for Integrated Photonics TechnologyMultidisciplinary Research InitiativeU.S. Air Force - Office of Scientific ResearchNational Science Foundation/MRSECU.S. Navy - Office of Naval Research (MFEL) Contract N00014-91-J-1956National Institutes of Health Grant R01-EY11289U.S. Navy - Office of Naval Research (MFEL) Contract N00014-94-0717Defense Advanced Research Projects Agency Contract N66001-96-C-863

Control of high harmonic generation by manipulation of field parameters

?High harmonic generation is a well established technique to investigate the structure and the inner dynamics of atoms and molecules. This thesis describes how the generating field parameters can be manipulated to extend the limits imposed on the technique by the use of traditional laser sources. In this field, with traditional source we mean high intensity, linearly polarised laser pulses at 800 nm. The first parameter to be investigated is the wavelength λ of the generating beam. The unfavourable scaling of the high harmonic yield with λ seems to suggest that high harmonic spectroscopy of atoms and molecules should be restricted to the wavelengths that obviate this problem, and that therefore shorter wavelength should be used. But longer wavelengths, in the mid infrared, present a great advantage respect to shorter ones. The maximum harmonic order that we can obtain is proportional to the ionisation potential of the target and to the wavelength times the intensity of the beam, so a higher number of harmonic can be produced with a longer wavelength than with short, the intensity being equal. This becomes incredibly valuable when the specie under investigation is a molecule with low ionisation potential. To produce high harmonics, a linearly polarised beam is required. If ellipticity is introduced in the beam, the harmonic signal quickly fades out, as non-linearly polarisation in monochromatic beams switches off the mechanism at the basis of high harmonic generation. This is not true if the polarisation of the beam is changed through the introduction of an additional laser beam, perpendicularly polarised respect to the fundamental. In this thesis the additional degree of freedom that this second field implies is investigated by combining the fundamental with its second harmonic and by controlling the relative delay of the two with sub-cycle precision. The key result is that the addition of the second harmonic gives access to the control of the harmonic amplitude and to the time at which the high harmonics are emitted, by simply controlling the relative phase between the two pulses