Showing papers by "P. St. J. Russell published in 2001"

White-light frequency comb generation with a diode-pumped Cr:LiSAF laser

Abstract: We have created a broad spectrum spanning more than an optical octave by launching femtosecond pulses from a battery operated Cr:LiSAF laser into a photonic crystal fiber. Despite the massive broadening in the fiber, the comb structure of the spectrum is preserved, and this frequency comb is perfectly suited for applications in optical frequency metrology.

Absolute frequency measurement of iodine lines with a femtosecond optical synthesizer

Abstract: We have used a single laser femtosecond optical frequency synthesizer together with a widely tunable Nd:YAG laser to measure the absolute frequency of several absorption lines in molecular iodine around 532 nm. The use of two different repetition frequencies allows us to determine the number of modes used for the frequency measurement unambiguously. The lines also provide data for the determination of improved ro-vibrational constants of the iodine molecule.

Fundamental-mode cutoff in a photonic crystal fiber with a depressed-index core.

Abstract: We report a photonic crystal fiber with a depressed-index core doped with fluorine The effective index of the cladding matches that of the core at an antiguiding wavelength, below which the fiber does not guide light at all

Vertical-cavity surface-emitting resonances in photonic crystal films

Abstract: It is shown that thin films of dielectric, etched through with a suitably chosen lattice of holes, can support surface-emitting vertical resonances with very-high-cavity Q factors (∼105 in the case of AlxGaAs1-x on oxidized AlyGaAs1-y). A Bloch-wave expansion is used to develop a complete vector-field analysis of these resonances and to reveal their underlying physics. Since they do not require multilayer mirrors, such resonators are a practical and simple replacement for conventional vertical-cavity surface-emitting laser structures. Other applications include wavelength-division-multiplexing components and highly sensitive gas detectors.

Acoustic modes of a dual-core square-lattice photonic crystal fiber preform

Abstract: We report for the first time the observation of acoustic defect modes in a dual-core two-dimensional square-lattice photonic crystal fiber preform. This result shows coupling between the two acoustics defect modes and provides the basis for developing acousto-optic devices based on photonic crystal fibers.

Room-temperature photoluminescence from erbium-doped multilayer porous silicon microcavity

Abstract: A multilayer microcavity was electrochemically etched on a silicon substrate and doped with erbium. Thermal processing under a nitrogen atmosphere optically activated the erbium-ions. Photo-pumping yielded room temperature emission around 1.54 μm from the erbium-doped samples with the emitted light strongly modified by the microcavity structure. Emission spectra with a peak at 1.536 μm had a full width at half maximum of ∼6 nm.

Microstructured Silica as an Optical-Fiber Material

Abstract: Conventional optical fibers are fabricated by creating a preform from two different glasses and drawing the preform down at an elevated temperature to form a fiber. A waveguide core is created in the preform by embedding a glass with a higher refractive index within a lower-index “cladding” material. Over the last few years, researchers at several laboratories have demonstrated very different forms of optical-fiber waveguides by using a drawing process to produce two-dimensionally microstructured materials in the form of fine “photoniccrystal fibers” (PCFs). One such waveguide is represented schematically in Figure 1. It consists of a silica fiber with a regular pattern of tiny airholes that run down the entire length. The optical properties of the microstructured silica cladding material enable the formation of guided waves in the pure silica core.

Spectral superbroadening of subnanojoule Cr:Forsterite femtosecond laser pulses in a tapered fiber

Abstract: Spectral superbroadening of subnanojoule femtosecond Cr:forsterite laser pulses is demonstrated for the first time in experiments with a tapered fiber. Coupling 40-fs 0.6-nJ pulses of 1.25-μm Cr:forsterite laser radiation into a tapered fiber with a taper waist diameter of about 2 μm and a taper waist length of 90 mm, we observed the spectra spanning more than two octaves at the output of the fiber. These experimental results open new horizons for the creation of compact femtosecond systems based on Cr:forsterite lasers and tapered fibers for optical metrology and biomedical applications.

Wavelength-tunable soliton generation in the 1400 -1600 nm region using an Yb fiber laser

Abstract: Soliton pulses tunable in the 1400 - 1600 nm region are generated in standard telecom fiber seeded with a continuum produced in a fiber taper using an Yb fiber laser. Here we demonstrate supercontinuum generation in the 500 nm - 1600 nm wavelength region using a compact ultrafast Yb fiber laser as a systems-compatible pump source for the first time. Moreover, by injecting the supercontinuum into a length of standard telecommunications fiber, we can induce Raman-soliton generation to produce near bandwidth-limited pulses tunable in the 1400 - 1600 nm wavelength region. The required pulse energy for supercontinuum and Raman-soliton generation is only of the order of 1 nJ, which demonstrates that this technique is compatible with fiber sources operating at GHz repetition rates.

Simultaneous generation of spectrally distinct third harmonic in photonic crystal fibers

Abstract: Summary form only given. Recently, photonic crystal fibers have attracted considerable interest for their unique structure and optical properties. These fibers contain an ordered array of air holes which form a low-index cladding around a solid silica core. Other examples include fibers with hollow cores where light is guided by a photonic bandgap effect. Understanding the propagation of fs pulses in these fibers, and in particular the origin of nonlinear effects such as continuum generation, is important for optimizing their applications. In our experiments, we employ a 95 cm segment of fiber having a 2.5 micron silica core suspended in air by a web of sub-micron silica strand with a cladding diameter of 90 microns. 170 fs pulses from an optical parametric oscillator at a wavelength of 1550 nm of variable average power are coupled into the fiber. The output from the fiber is then analyzed spectrally and temporally and the fundamental pulses are sent to a frequency-resolved optical gating apparatus.

Ultrashort pulse propagation in highly birefringent photonic crystal fibers

Abstract: Summary form only given. Supercontinuum generation in "photonic crystal" fibers results from a relatively small core size and anomalous group velocity dispersion." Although the polarization sensitivity of this process has been previously noted it has not been investigated in polarization maintaining Hi-Bi fiber. We have fabricated photonic crystal fibers with an intentionally high birefringence by stacking capillaries and rods as previously reported. Typically, our fibers had a core dimension in the range of 1-4 /spl mu/m and polarization-mode beat lengths of substantially less than 1 mm - less than that previously reported in conventional fibers.

Optical measurement of trapped acoustic mode at defect in square-lattice photonic crystal fiber preform

Abstract: Summary form only given. Photonic crystal fibers (PCFs) are all-silica fibers in which the cladding consists of a two-dimensional array of microscopic air holes running along their length. The central hole of the structure is missing, leaving a solid silica core to guide the light. The periodic nature of the cladding makes also possible the existence of acoustic band gaps. Moreover, the core can also act as a defect of the phononic crystal and so elastic defect modes can be confined in this region. We present the first experimental demonstration of an acoustic defect mode trapped at the solid core in a square-lattice PCF preform.

Supercontinuum generation in fused fibre couplers

Abstract: Summary form only given. Supercontinuum (SC) light with a 400-1600 nm spectrum has been generated in optical fibres using unamplified femtosecond Ti:sapphire lasers. The fibre can be a special microstructure (photonic crystal) fibre with a core surrounded by large air holes, or it can be a standard telecommunications fibre that is tapered-narrowed by heating and stretching. In both cases, the waveguide is essentially silica (diameter /spl sim/2 /spl mu/m) surrounded by air. The small diameter and large refractive index step give high intensities and desirable dispersion characteristics. We demonstrate SC generation in fused couplers made by tapering two fibres together.

Ultrashort pulse propagation in highly birefringent photonic crystal fibers

Abstract: Summary form only given. Supercontinuum generation in "photonic crystal" fibers results from a relatively small core size and anomalous group velocity dispersion. Although the polarization sensitivity of this process has been previously noted it has not been investigated in polarization maintaining highly birefringent fiber. We have fabricated photonic crystal fibers with an intentionally high birefringence by stacking capillaries and rods. Typically, our fibers had a core dimension in the range of 1-4 /spl mu/m and polarization-mode beat lengths of substantially less than 1 mm-less than that previously reported in conventional fibers. The variation with wavelength of the group index and the dispersion of the two polarization eigenmodes in the single-mode fiber is shown. A Ti:sapphire laser was used to launch pulses of different energies (875 nm, 76 MHz repetition frequency, 200 fs FWHM pulse duration) into the fiber in different polarization states, and the output was analyzed. It is worth noting that the polarization-mode walk-off length for these pulses in this fiber is around 10.4 mm, while the group-index-matched wavelengths for 875 nm excitation are 575 nm and 665 nm. Furthermore, one can readily find wavelengths where the dispersion of the two modes eigenmodes is opposite in sign.

Strain-induced phase-matching and tunability of acoustic gratings in fibers

Abstract: We study the effect of strain on the properties of a one-dimensional acoustic grating: a tapered optical fiber with a periodically varying diameter. Its spectral response has a strong frequency stop band for flexural waves. Changing the tension of the fiber changes the wave velocity, and so modifies the acoustic response of the grating. The resulting tunability of the stop band over a wide frequency range is demonstrated experimentally. Furthermore, the stop band can almost disappear completely due to a strain-induced phase-matching phenomenon. Such structures are suitable as frequency-selective acoustic mirrors in acousto-optic fiber devices.

1-D acoustic cavity in optical fibers using two acoustic Bragg gratings

Abstract: We report the fabrication and characterization of a one-dimensional acoustic cavity made on a tapered single-mode optical fiber with two acoustic mirrors. The mirrors are acoustic Bragg gratings formed by modulating the fiber diameter periodically. The cavity exhibits resonances with quality factors of around 300. Higher Q factors are achievable by increasing the number of periods in the gratings. A simple transfer matrix model successfully simulates the acoustic properties of the cavity. This type of acoustic cavity is of interest for its potential applications in acoustooptic components and the development of devices based on stimulated Brillouin scattering.

Using photonic crystals as optical fibers

Abstract: By designing and fabricating optical fibers using photonic crystal materials as cladding, one can create fibers with previously unimaginable waveguiding properties. The development of photonic crystal fiber substantially broadens the possible applications for fiber optics.