Phillip St. J. Russell

Endlessly single-mode photonic crystal fiber

We made an all-silica optical fiber by embedding a central core in a two-dimensional photonic crystal with a micrometer-spaced hexagonal array of air holes. An effective-index model confirms that such a fiber can be single mode for any wavelength. Its useful single-mode range within the transparency window of silica, although wide, is ultimately bounded by a bend-loss edge at short wavelengths as well as at long wavelengths.

Group-velocity dispersion in photonic crystal fibers

The dispersion properties of photonic crystal fibers are calculated by expression of the modal field as a sum of localized orthogonal functions. Even simple designs of these fibers can yield zero dispersion at wavelengths shorter than 1.27 mum when the fibers are single mode, or a large normal dispersion that is suitable for dispersion compensation at 1.55 mum.

Supercontinuum generation in photonic crystal fibers and optical fiber tapers: a novel light source

Broadband continua extending from 400 to 1600 nm are generated in photonic crystal fibers and in tapered conventional optical fibers. The continuum is generated in the fundamental fiber mode. Femtosecond pulses from an unamplified Ti:sapphire laser with energies up to 4 nJ are used, and the resultant spectra from several photonic crystal fibers and taper structures are compared and analyzed.

Spectral shaping of supercontinuum in a cobweb photonic-crystal fiber with sub-20-fs pulses

Multiple approaches to generate a smooth, powerful, and stable supercontinuum in cobweb photonic-crystal fibers were undertaken by use of 18-fs pulses. These approaches include utilization of incident pulses with various chirp, power, and polarization states, as well as fibers with different lengths and core sizes. For long fibers (tens of centimeters) the supercontinuum contains a finely modulated structure that can be smoothed when the oscillator is in a regime of relaxation oscillations. Short fibers provide a supercontinuum free of gaps. By optimization of these parameters supercontinua exceeding one octave with modulations of less than 10 dB have been generated.

Improved hollow-core photonic crystal fiber design for delivery of nanosecond pulses in laser micromachining applications

We report the delivery of high-energy nanosecond pulses (approximately 65 ns pulse width) from a high-repetition-rate (up to 100 kHz) Q-switched Nd:YAG laser through the fundamental mode of a hollow-core photonic crystal fiber (HC-PCF) at 1064 nm. The guided mode in the HC-PCF has a low overlap with the glass, allowing delivery of pulses with energies above those attainable with other fibers. Energies greater than 0.5 mJ were delivered in a single spatial mode through the hollow-core fiber, providing the pulse energy and high beam quality required for micromachining of metals. Practical micromachining of a metal sheet by fiber delivery has been demonstrated.

Directional coupling in a twin core photonic crystal fiber using heat treatment

Summary form only given. Multicore fibers are optical fibers with two or more cores. The cores can be coupled or uncoupled depending on the properties of the fiber. Photonic crystal fiber (PCF) is an undoped all silica fiber with an array of air holes that run along its length. This novel type of optical fiber has some remarkable waveguiding properties that can be characterized by the size of the air holes and the distance between them. PCF is fabricated using a proven stack and draw technique, due to this unique fabrication process the inclusion of multiple cores, anywhere within the two-dimensional hexagonal lattice is significantly simpler than the fabrication of conventional step index fibers. In this paper we demonstrate an evanescent field coupler by selective heat-treating a region of a previously uncoupled twin core PCF.

Large mode area photonic crystal fibre laser

Summary form only given. Fibre lasers have attracted much interest in recent years. Ytterbium-doped silica fibres in particular are capable of high efficiency and may be pumped directly by diode lasers at 915 or 980 nm. However at high power the intensity within the core of an optical fibre becomes very large and this can give rise to optical nonlinearity and physical damage. High power lasers based on step-index fibres have sought to use the largest mode areas possible. Photonic crystal fibre (PCF) is an optical fibre with an ordered array of microscopic air-holes running along its length. It has been shown that a pure silica PCF may be strictly single mode at all wavelengths or conversely may have a core of arbitrarily large diameter whilst remaining single mode (20 /spl mu/m is readily experimentally achievable at 633 nm). By introducing Yb/sup 3+/-doped silica into a PCF we recently reported the first PCF laser. In this paper we report the first implementation of a large mode area PCF laser, with a core diameter of 15 /spl mu/m.

Visible light optical coherence tomography

We demonstrate for the first time optical coherence tomography (OCT) in the visible wavelength range with unprecedented sub-micrometer axial resolution, achieved by employing a photonic crystal fiber in combination with a sub-15fs Ti:sapphire laser (FEMTOLASERS). The shaped emission spectrum produced by the photonic crystal fiber ranges from 535 nm to 700 nm (centered at ~600 nm) resulting in ~0.9 micrometers axial OCT resolution in air corresponding to ~0.6 micrometers in biological tissue. Preliminary demonstration of the sub-micrometer resolution achieved with this visible light OCT setup is demonstrated on a 2.2 micrometers thick nitrocellulose membrane. The visible wavelength range not only enables extremely high axial resolution for OCT imaging, but also offers an attractive region for spectroscopic OCT.

Optical Frequency Measurement Using Chirped-Mirror-Dispersion-Controlled Mode-Locked Ti:Al2O3 Laser

An optical frequency measurement system based on an octave-spanning optical frequency comb generated by a chirped-mirror-dispersion-controlled mode-locked Ti:Al2O3 laser and a photonic-crystal fiber is developed. All of the modes of the octave-spanning optical frequency comb are frequency-stabilized to a microwave frequency standard, where the carrier-envelope offset frequency is phase-locked with self-referencing of the comb. We investigate the methods of controlling carrier-envelope offset frequency in a chirped-mirror-dispersion-controlled mode-locked laser. The rotation of a pair of chirped mirrors is useful for setting the bias of carrier-envelope offset frequency. Although our mode-locked laser has a low pulse-repetition frequency of 150 MHz, a high signal-to-noise ratio in beats results in the direct measurement of beat frequency with a laser to be measured using a frequency counter, and enables us to phase lock carrier-envelope offset frequency merely by using a mixer analogously without the need for a prescaler, with a servo bandwidth at approximately 500 kHz. The uncertainty of our optical frequency measurement system, besides the uncertainty of microwave reference frequency, is 4×10-14, and is limited by the uncertainty of the rf synthesizer used for phase locking and by that of the beat frequency measurement. Frequency measurements of an iodine-stabilized frequency-doubled Nd:YAG laser at 532 nm, an iodine-stabilized He–Ne laser at 633 nm and a rubidium two-photon-absorption stabilized extended-cavity laser diode at 778 nm are conducted. The results contributed to the revision of the practical realization of the metre adopted by the International Conference on Weights and Measures (CIPM) in 2001.

Frequency control of a chirped-mirror-dispersion-controlled mode-locked Ti:Al2O3 laser for comparison between microwave and optical frequencies

The use of an optical frequency comb generated by an ultrafast mode-locked laser has been realized as a promising method of the direct comparison between microwave and optical frequencies. We are currently investigating frequency control of a chirped-mirror-dispersion-controlled mode-locked Ti:Al2O3 laser. We stabilized the pulse repetition rate frep to a rf synthesizer locked to a cesium (Cs) clock to the Allan deviation of 4 X 10-12 in 1 s. We found that the position of the crystal, rotation of the chirped mirrors, and change of the pump-laser power can be used in controlling the carrier-envelope offset frequency fCEO. We extended the span of the comb to over one octave, i.e., from 530 nm to 1190 nm, at -20 dB using a photonic-crystal fiber made at the University of Bath. We are currently trying to measure the frequency of an iodine-stabilized Nd:YAG laser using a floating ruler of a f:2f frequency interval chain. We detected the self-referencing beat between the fundamental and second- harmonic frequencies of the comb, which will enable further precise comparison between microwave and optical frequencies through the control of the fCEO.