Dominic F. Murphy

Dispersion-insensitive measurement of thickness and group refractive index by low-coherence interferometry

We describe a low-coherence interferometric technique for simultaneous measurement of geometric thickness and group refractive index of highly dispersive samples. The technique is immune to the dispersion-induced asymmetry of the interferograms, thus overcoming limitations associated with some other low-coherence approaches to this simultaneous measurement. We use the experimental configuration of a tandem interferometer, with the samples to be characterized placed in an air gap in one arm of the measurement interferometer. Unambiguous, dispersion-insensitive measurements of critical group-delay imbalances in the measurement interferometer are determined from the optical frequency dependence of interferogram phases, by means of dispersive Fourier transform spectrometry. Sample thickness and group refractive index are calculated from these group delays. A thickness measurement precision of 0.2 microm and group index measurement accuracy of 5 parts in 10(5) across a wavelength range of 150 nm have been achieved for BK7 and fused-silica glass samples in the thickness range 2000 to 6000 microm.

Interferometric interrogation of in-fiber Bragg grating sensors without mechanical path length scanning

We report the interrogation of a fiber Bragg grating (FBG) using an interferometer with a tilted mirror such that the optical path difference is a function of position on an array detector. Absolute measurements of mean resonant wavelength from the phase of the analytic signal of the spatial interferogram are determined, and a technique based on using a reference laser to compensate for performance degrading effects otherwise associated with spatially scanned interferometers is introduced. These measurements are not critically dependent on the accurate location of zero phase position. We have applied the technique to the absolute measurement of temperature-induced shifts in the grating resonant wavelength. A resolution of 0.025 nm for a spatially scanned optical path delay of only 200 /spl mu/m was achieved. The technique has the potential for higher resolutions and for multiplexing.

Multicore optical fibres for astrophotonics

We report progress towards multimode (MM) fibre filters for suppressing the OH emission that hinders ground-based observation of the early Universe. Fibre Bragg gratings (FBGs) can filter these narrow spectral lines in single-mode (SM) fibres [1]. Implementing them in MM fibres well-matched to astronomical instruments requires transitions between the MM fibre and several SM fibres [2]. Such hand-crafted “photonic lanterns” require many identical FBGs to be made and spliced in place. Instead we are pursuing the idea in multicore (MC) fibres, Fig. 1(a). The FBG is written at once in all the SM cores. The fibre is jacketed with low-index glass and tapered to form the core and cladding of a MM fibre, giving a monolithic FBG filter with conventional MM ports. Such a device can stand alone, or be incorporated into more elaborate instruments [3, 4].

Statically scanned single and tandem low-coherence interferometers

Statically scanned single and tandem Michelson interferometer configurations are compared for the remote measurement of thermally induced group delay change in optically dispersive glass samples. A broadband tungsten filament bulb was used to illuminate the single interferometer, and a much narrower spectral bandwidth superluminescent diode (SLD) was used to illuminate the tandem interferometer. For a BK7 glass sample, measurements of thermally induced group delay changes were made with <0.5 fs root mean square error for optical path delay (OPD) scan lengths of only 260 µm when applied to low-coherence interferograms with signal-to-noise ratios as low as 6.5 dB. These results demonstrate the power of dispersive Fourier transform spectrometry (DFTS) applied to noisy, dispersion distorted low-coherence interferograms captured with non-mechanical, short path length scans. Further, following experimentally observed source bandwidth-induced measurement resolution limitations between the different illuminating sources, simulations were performed to examine this feature.

Second generation OH suppression filters using multicore fibers

Ground based near-infrared observations have long been plagued by poor sensitivity when compared to visible observations as a result of the bright narrow line emission from atmospheric OH molecules. The GNOSIS instrument recently commissioned at the Australian Astronomical Observatory uses Photonic Lanterns in combination with individually printed single mode fibre Bragg gratings to filter out the brightest OH-emission lines between 1.47 and 1.70μm. GNOSIS, reported in a separate paper in this conference, demonstrates excellent OH-suppression, providing very “clean” filtering of the lines. It represents a major step forward in the goal to improve the sensitivity of ground based near-infrared observation to that possible at visible wavelengths, however, the filter units are relatively bulky and costly to produce. The 2nd generation fibre OH-Suppression filters based on multicore fibres are currently under development. The development aims to produce high quality, cost effective, compact and robust OH-Suppression units in a single optical fibre with numerous isolated single mode cores that replicate the function and performance of the current generation of “conventional” photonic lantern based devices. In this paper we present the early results from the multicore fibre development and multicore fibre Bragg grating imprinting process.

Large core photonic microcells for coherent optics and laser metrology

A photonic microcell (PMC) is a length of gas-filled hollow core-photonic crystal fiber (HC-PCF) which is hermetically sealed at both ends by splicing to standard single mode fiber. We describe advances in the fabrication technique of PMCs which enable large core Kagome-lattice HC-PCFs to be integrated into PMC form. The modified fabrication technique uses fiber-tapering to accommodate the large dimensions of the fiber and enables low loss splices with single mode fiber by reducing mode field mismatch. Splice losses as low as 0.6 dB are achieved between 1-cell defect Kagome HC-PCF and single mode fiber. Relative to the previously reported PMCs, which were based on photonic bandgap HC-PCF, the present Kagome HC-PCF based PMC provides broad optical transmission, surface mode-free guidance and larger core at the cost of slightly increased fiber attenuation (~0.2 dB/m). Therefore, the integration of this fiber into PMC form opens up new applications for PMC-based devices. The advantage of the large core dimensions and surface mode free guidance for quantum optics in gas-filled HC-PCF are demonstrated by generation of narrow sub-Doppler features in an acetylenefilled large core PMC.

Non-Mechanically Scanned DFTS

Non-mechanically scanned dispersive Fourier transform spectrometry (DFTS) is reported for dispersion-insensitive measurements of thermally-induced change in dispersive group delay; optical path scan lengths of 260 microns yield 0.5fs resolution for a dispersive optical sample.

A statically scanned tandem interferometer

A static tandem Michelson interferometer configuration is reported for remote measurements of group delay change in a thermally modulated, optically dispersive BK7 glass sample. Using a superluminescent diode (SLD) to illuminate the interferometer, low-coherence measurement interferograms with signal-to-noise ratios as low as 16 dB were captured and subsequently processed using dispersive Fourier transform spectrometry (DFTS). Measurements of thermallyinduced delay change were made with < 2 fs root mean square error for optical path delay scans lengths of only 260 μm.

Optical Wavefront Interferometry - evolution, challenges and opportunities

Optical wavefront interferometry has evolved into increasingly compact and portable forms with optimizations and novel developments from multi-component, bulk-optic configurations to miniaturized multi-core fiber forms; each presents its own advantages, challenges and opportunities.

Smart, portable, miniature, static, broadband, optical interferometers and measurements

D Solar Cells (DSCs) and Organic Solar Cells (OSCs) represent very promising photovoltaic technologies able to reach power conversion efficiencies of 15% and 11% respectively. A DSC is essentially an electrochemical cell based on a thin nanoporous titanium dioxide layer (10-15 μm) where a monolayer of molecular dye (generally Ru-based) is chemisorbed. This hybrid organic-inorganic structure is dip in a liquid electrolyte where a redox couple is present. The whole system is sandwiched between two conductive glasses and encapsulated by a sealant. On the other hand the active layer of an OPV is a blend of two mixed organic materials, a donor and an acceptor semiconductor, conventionally P3HT and PCBM, respectively. The active blend is closed between two contacts with different work functions to collect free charge carriers. We have developed a model based on finite element to describe both devices within drift-diffusion and Poisson equations. The application of finite element method allows to solve the model over a general domain in 1D, 2D and 3D. This opens interesting new perspectives for the analysis and optimization of DSCs and OSCs. The model is implemented within the multiscale Tiber CAD simulation tool. The fine tuning of the light absorption, transport parameters and the geometry of the active layer, allows us to define a consistent parameterization of the simulator which is then used as a predictive tool to calculate maps of efficiency for different working conditions and different fabrication geometries (Tandem configuration).M laser sources (3-14 mm) which have wide spectral coverage and high output power are attractive for many applications. This spectral range contains unique absorption fingerprints of most molecules, including toxins, explosives, and nerve agents. Infrared spectroscopy can also be used to detect important biomarkers, which can be used for medical diagnostics by means of breath analysis. Many groups are actively trying to build systems that can cover the entire range of interest. To realize this type of extreme range requires both a broad gain bandwidth and a dynamic wavelength selection. The quantum cascade laser has demonstrated the ability to achieve very large spectral bandwidths via band structure engineering. Our group is currently developing heterogeneous core technology for broadband gain coverage in a single device. Broadband tuning, at least commercially, is realized with external cavity devices. These are fairly complicated optomechanical devices, however, which come with inherent size, speed, and stability limitations. As an alternative, our group is developing multisection laser geometries with wide electrical tuning (hundreds of cm-1). These devices are roughly the same size as a traditional quantum cascade laser, but tuning is accomplished without any external optical components. This talk will describe our current research efforts and potential for both broadband spectral coverage and broadband electrical tuning. The goal is to produce a broadband mid-infrared source which is smaller, lighter, more robust, and less expensive than what is currently available.W developed within the modified effective mass method the theory of an exciton formed from spatially separated electron and hole (the hole is in the semiconductor spherical quantum dot (QD)) volume and the electron is localized at the outer spherical surface of the QD-matrix interface. The effect of significantly increasing the binding energy of an exciton in a nanosystem containing ZnSe QDs in comparison with the binding energy of an exciton in a ZnSe single crystal (by a contribution factor of 73) was detected. It was shown that nanosystems consisting of ZnSe QDs grown in a borosilicate glass matrix can be used as the active region of semiconductor QD lasers. We developed the theory of a biexciton formed from spatially separated electrons and holes (the hole is in QD volume, and the electron is localized at the outer surface of the QD-matrix interface) in a nanosystem that consists of ZnSe QDs synthesized in a glassy matrix. It is shown that the major contribution to the biexciton binding energy is made by the energy of the exchange interaction of electrons with holes and this contribution is much more substantial that the contribution of the energy of Coulomb interaction between the electrons and holes. It is established that the spectral shift of the peak in such QDs is due to quantum confinement of the energy of the biexciton ground state.T Photo Injector Test facility at DESY Zeuthen site, (PITZ) develops and characterizes high brightness electron sources for short wavelength Free-Electron Laser (FEL) facilities like FLASH and the European XFEL. An optimization of the photo injector is only possible with high resolution diagnostics. Therefore, a variety of beam diagnostics is being developed and implemented. The complex optical system at PITZ is supporting diagnostic techniques for measuring the electron bunch length and the longitudinal phase space using a streak camera.A superluminal ring laser (SRL) is a laser in which the group velocity of light far exceeds, by a factor as large as a million, the vacuum speed of light, without violating special relativity or causality. This behavior results from the so-called fast light effect, corresponding to critically tuned anomalous dispersion. The anomalous dispersion is produced by adding inside the ring cavity an auxiliary medium that produces a narrow dip in the broad-band gain profile produced by the main gain medium. In an SRL, the change in the laser frequency, as a function of a change in the cavity length, is enhanced by a factor of nearly a million, when compared to that of a conventional ring laser. This effect can be used for enhancing the sensitivity of many devices, including a gyroscope, and accelerometer, and a fiber optic sensor. It can also be configured to realize a novel type of gravitational wave detector. We have realized an SRL using a buffer gas loaded Rb vapor cell, pumped on the D1 line, as the main gain medium, and an additional Rb vapor cell for producing the anomalous dispersion via Raman induced pump depletion. In this talk, the author will describe the behavior of this SRL, and outline the various applications we are pursuing, including rotation sensing and accelerometry for inertial navigation under GPS-denied conditions, and gravitational wave detection. He will also describe a fiber-optic based SRL, employing dual peaked Brillouin gain, for ultra-sensitive, general purpose fiber optic sensing.Optical interferometry is a well-known, highly regarded and powerful measurement technique that is used in many forms to perform a very wide range of measurements. Applications of interferometry range from source characterisation to material characterisation; from OCT to photonic sensing; and from space weather monitoring to gravitational wave detection efforts. In this work, we report this powerful measurement tool in its form as a PIE instrument that is based on the Youngs interferometer configuration. We report several metrology and FT-spectroscopy measurements using this miniature, portable, static, broadband interferometer configuration that includes both optical fiber circuits and optical waveguide circuits. Using these simple and powerful interferometer configurations, we report sub-nanometer spectral resolutions, picometer detection of wavelength changes, high resolution measurements of group velocity dispersion and attosecond measurements of delay change.