Fabry-Perot Optical Cavity Research Articles

Decomposition of aberrated or turbulent wavefronts into a spatial mode spectrum using optical cavities.

It is shown that an aberrated wavefront incident upon a Fabry-Perot optical cavity excites higher order spatial modes in the cavity and that the spectral width and distribution of these modes is indicative of the type and magnitude of the aberration. The cavities are purely passive, and therefore frequency content is limited to that provided by the original light source. To illustrate this concept, spatial mode decomposition and transmission spectrum calculation are simulated on an example cavity; the effects of various phase delays, in the form of two basic Seidel aberrations and a composite of Zernike polynomial terms, are shown using both Laguerre-Gaussian and plane wave incident beams. The aggregate spectral width of the cavity modes excited by the aberrations is seen to widen as the magnitude of the aberrations' phase delay increases.

The Emerging Field of Quantum Based Measurements for Pressure, Vacuum and Beyond

The future of pressure, vacuum and even temperature measurement will employ lasers, Fabry-Perot optical cavities, cold atom traps and lots of quantum physics. For pressure measurement of a gas, photons interact at the quantum level such that light travels at a slower speed in gas than it does in vacuum. For extreme vacuum measurements, cold atom traps will be used to detect single collisions between gas trapped cold atoms enabling the number density of the gas to be measured. For temperature measurement is performed using silicon photonics to detect the small changes in refractive index in micro machined siliconphontoic cavities coupled to optical fibers. For dynamic pressure, NIST is developing a method where the unique quantum mechanical characteristics of the molecules are themselves the standard for pressure, making it consistent with the quantum-SI. Our approach is to use independent molecular spectroscopy as a dynamic measurement of pressure, where the pressure and temperature is ascertained by measuring time-resolved pressure-broadened spectra of CO molecules. This paper briefly reviews the status of these projects currently underway at the NIST Thermodynamic Metrology Group.

Polarisation dynamics of a birefringent Fabry–Perot cavity

Optical Fabry-Perot cavities always show a non-degeneracy of two orthogonal polarisation states. This is due to the unavoidable birefringence of dielectric mirrors whose effects are extremely important in Fabry-Perot based high-accuracy polarimeters. We have developed and present here a theory of the polarisation state dynamics in a birefringent Fabry-Perot resonator, and we validate it through measurements performed with the polarimeter of the PVLAS experiment. The measurements are performed while a laser is frequency-locked to the cavity, and provide values for the finesse of the cavity and for the phase difference between the two orthogonal polarisation components introduced by the combination of the two cavity mirrors (equivalent wave-plate). The theoretical formulas and the experimental data agree well showing that the consequences of the mirror birefringence must be taken into account in this and in any other similar experiment.

Design and characterization of an integrated surface ion trap and micromirror optical cavity.

We have fabricated and characterized laser-ablated micromirrors on fused silica substrates for constructing stable Fabry-Perot optical cavities. We highlight several design features which allow these cavities to have lengths in the 250-300 μm range and be integrated directly with surface ion traps. We present a method to calculate the optical mode shape and losses of these micromirror cavities as functions of cavity length and mirror shape, and confirm that our simulation model is in good agreement with experimental measurements of the intracavity optical mode at a test wavelength of 780 nm. We have designed and tested a mechanical setup for dampening vibrations and stabilizing the cavity length, and explore applications for these cavities as efficient single-photon sources when combined with trapped Yb171+ ions.

The improvement of lasing without inversion in the presence of multi-photon transition in a three-level atom confined in an optical cavity in the steady-state regime

A three-level atom in \( \Lambda\) configuration trapped in a single-mode Fabry-Perot optical cavity is investigated. It is assumed that the transition between levels 2 and 3 in the atom is accomplished through the number of q photons. To solve the steady-state master equation for the atom-cavity system, the matrix continued-fraction method for applied physical quantities is used. The results show that by raising the number of transitions, the curves of the population inversion finally overlap with one another and the output mean photon number from the optical cavity rises and moreover the laser effect appears in the system. On the other hand, the outcomes reveal that by increasing the number of transitions despite the negativity of the population inversion, the number of output photons from the cavity grows and lasing interval becomes larger. Eventually the steps of the transformation from the three-level atom to two-level one in the case of multi-photon transition under several specific conditions have been studied. The obtained results of the simulations confirm the accuracy of the used approximations in the two-level pattern appropriately. In summary, the lasing process without population inversion along with the multi-photon transition in the three-level atom is discussed.

All-optical switching in a continuously operated and strongly coupled atom-cavity system

We experimentally demonstrate collective strong coupling, optical bi-stability (OB), and all-optical switching in a system consisting of ultracold 85Rb atoms, trapped in a dark magneto-optical trap (DMOT), and coupled to an optical Fabry-Perot cavity. The strong coupling is established by measuring the vacuum Rabi splitting (VRS) of a weak on-axis probe beam. The dependence of VRS on the probe beam power is measured, and bi-stability in the cavity transmission is observed. We demonstrate control over the transmission of the probe beam through the atom-cavity system using a free-space off-axis control beam and show that the cavity transmission can be switched on and off in micro-second timescales using micro-Watt control powers. The utility of the system as a tool for sensitive, in-situ and rapid measurements is envisaged.

Cavity-induced backaction in Purcell-enhanced photon emission of a single ion in an ultraviolet fiber cavity

We study the behavior of a single laser-driven trapped ion inside a microscopic optical Fabry-Perot cavity. In particular, we demonstrate a fiber Fabry-Perot cavity operating on the principal $S_{1/2}\to P_{1/2}$ electric dipole transition of an Yb$^+$ ion at $369\,$nm with an atom-ion coupling strength of $g=2\pi\times 67(1)\,$MHz. We employ the cavity to study the generation of single photons and observe cavity-induced back-action in the Purcell-enhanced emission of photons. Tuning of the amplitude and phase of the back-action allows us to enhance or suppress the total rate of photoemission from the ion-cavity system.

Schottky hot-electron photodetector by cavity-enhanced optical Tamm resonance

We propose a design of Schottky-junction hot-electron photodetector under purely planar configuration, which is composed by a front distributed Bragg reflector (DBR), a metal/semiconductor (Au/Si) Schottky junction, and a metallic rear reflector. With such a hybrid design, optical Tamm resonance (i.e., a surface state) can be excited near the DBR/Au interface and significantly enhanced due to the presence of the metallic cavity. The intense Tamm resonance shows a strong field localization to the incident photon energy, enabling a high hot-electron generation for sensitive photodetection. Finite-element and rigorous coupled-wave simulations verify that both optical Tamm state and Fabry-Perot cavity mode can be excited simultaneously, which exhibit a high tunability by tailoring either the DBR or the metallic cavity. With a good angular performance, the proposed design shows an optical absorption in the top thin Au layer over 89%, leading to a 30-fold enhancement in the photoresponsivity compared to that of the normal Au/Si Schottky system.

Negative ion source development for a photoneutralization based neutral beam system for future fusion reactors

In parallel to the developments dedicated to the ITER neutral beam (NB) system, CEA-IRFM with laboratories in France and Switzerland are studying the feasibility of a new generation of NB system able to provide heating and current drive for the future DEMOnstration fusion reactor. For the steady-state scenario, the NB system will have to provide a high NB power level with a high wall-plug efficiency (η ∼ 60%). Neutralization of the energetic negative ions by photodetachment (so called photoneutralization), if feasible, appears to be the ideal solution to meet these performances, in the sense that it could offer a high beam neutralization rate (>80%) and a wall-plug efficiency higher than 60%. The main challenge of this new injector concept is the achievement of a very high power photon flux which could be provided by 3 MW Fabry–Perot optical cavities implanted along the 1 MeV D− beam in the neutralizer stage. The beamline topology is tall and narrow to provide laminar ion beam sheets, which will be entirely illuminated by the intra-cavity photon beams propagating along the vertical axis. The paper describes the present R&D (experiments and modelling) addressing the development of a new ion source concept (Cybele source) which is based on a magnetized plasma column. Parametric studies of the source are performed using Langmuir probes in order to characterize and compare the plasma parameters in the source column with different plasma generators, such as filamented cathodes, radio-frequency driver and a helicon antenna specifically developed at SPC-EPFL satisfying the requirements for the Cybele (axial magnetic field of 10 mT, source operating pressure: 0.3 Pa in hydrogen or deuterium). The paper compares the performances of the three plasma generators. It is shown that the helicon plasma generator is a very promising candidate to provide an intense and uniform negative ion beam sheet.

High flux circularly polarized gamma beam factory: coupling a Fabry-Perot optical cavity with an electron storage ring.

We report and discuss high-flux generation of circularly polarized γ-rays by means of Compton scattering. The γ-ray beam results from the collision of an external-cavity-enhanced infrared laser beam and a low emittance relativistic electron beam. By operating a non-planar bow-tie high-finesse optical Fabry-Perot cavity coupled to a storage ring, we have recorded a flux of up to (3.5 ± 0.3) × 108 photons per second with a mean measured energy of 24 MeV. The γ-ray flux has been sustained for several hours. In particular, we were able to measure a record value of up to 400 γ-rays per collision in a full bandwidth. Moreover, the impact of Compton scattering on the electron beam dynamics could be observed resulting in a reduction of the electron beam lifetime correlated to the laser power stored in the Fabry-Perot cavity. We demonstrate that the electron beam lifetime provides an independent and consistent determination of the γ-ray flux. Furthermore, a reduction of the γ-ray flux due to intrabeam scattering has clearly been identified. These results, obtained on an accelerator test facility, warrant potential scaling and revealed both expected and yet unobserved effects. They set the baseline for further scaling of the future Compton sources under development around the world.

Electromagnetic Modelling of Fiber Sensors for Low-Cost and High Sensitivity Temperature Monitoring.

An accurate design of an innovative fiber optic temperature sensor is developed. The sensor is based on a cascade of three microstructured optical fibers (MOFs). In the first one a suitable cascade of long period gratings is designed into the core. A single mode intermediate and a rare-earth activated Fabry-Perot optical cavity are the other two sensor MOF sections. An exhaustive theoretic feasibility investigation is performed employing computer code. The complete set-up for temperature monitoring can be obtained by utilizing only a low cost pump diode laser at 980 nm wavelength and a commercial optical power detector. The simulated sensitivity S = 315.1 μW/°C and the operation range ΔT = 100 °C is good enough for actual applications.

KWISP: An ultra-sensitive force sensor for the Dark Energy sector

An ultra-sensitive opto-mechanical force sensor has been built and tested in the optics laboratory at INFN Trieste. Its application to experiments in the Dark Energy sector, such as those for Chameleon-type WISPs, is particularly attractive, as it enables a search for their direct coupling to matter. We present here the main characteristics and the absolute force calibration of the KWISP (Kinetic WISP detection) sensor. It is based on a thin Si3N4 micro-membrane placed inside a Fabry-Perot optical cavity. By monitoring the cavity characteristic frequencies it is possible to detect the tiny membrane displacements caused by an applied force. Far from the mechanical resonant frequency of the membrane, the measured force sensitivity is 5.0e-14 N/sqrt(Hz), corresponding to a displacement sensitivity of 2.5e-15 m/sqrt(Hz), while near resonance the sensitivity is 1.5e-14 N/sqrt(Hz), reaching the estimated thermal limit, or, in terms of displacement, 7.5e-16 N/sqrt(Hz). These displacement sensitivities are comparable to those that can be achieved by large interferometric gravitational wave detectors.

Control of zero-crossing temperature of coefficient of thermal expansion and reduction of mechanical residual stress for TiO2–SiO2 glass optical cavity

We developed a procedure for fabricating a Fabry–Perot optical cavity with less long-term length variation. We fabricated a homogenized TiO2–SiO2 ultralow-expansion (ULE) glass ingot from a commercial TiO2–SiO2 ULE glass ingot with striae. We also controlled the zero-crossing temperature of the coefficient of thermal expansion of the homogenized ingot to approximately 30 °C by heat treatment at 970 °C for 72 h to change the fictive temperature of the ingot. A 100-mm-long cavity spacer was carefully fabricated while reducing the mechanical residual stress introduced by machining. A cavity composed of the spacer and two mirrors of 39-layered Ta2O5/SiO2 films exhibited a high finesse of 420,000 at 729 nm and a high length stability of 13 fm/day after 500 days from the beginning of the measurement at a constant temperature of 29.78 °C.

Fiber Optic Fabry-Perot Current Sensor Integrated with Magnetic Fluid Using a Fiber Bragg Grating Demodulation.

An optical fiber current sensor based on Fabry-Perot interferometer using a fiber Bragg grating demodulation is proposed. Magnetic fluid is used as a sensitive medium in fiber optical Fabry-Perot (F-P) cavity for the optical characteristic of magnetic-controlled refractive index. A Fiber Bragg grating (FBG) is connected after the F-P interferometer which is used to reflect the optical power at the Bragg wavelength of the interference transmission spectrum. The corresponding reflective power of the FBG will change with different external current intensity, due to the shift on the interference spectrum of the F-P interferometer. The sensing probe has the advantages of convenient measurement for its demodulation, low cost and high current measurement accuracy on account of its sensing structure. Experimental results show that an optimal sensitivity of 0.8522 nw/A and measurement resolution of 0.001 A is obtained with a FBG at 1550 nm with 99% reflectivity.

Interference characteristics analysis of optical fiber Fabry-Perot cavity with graphene diaphragm

With regard to the design of optical fiber Fabry-Perot cavity using graphene as sensitive diaphragm,the deflection change under uniformly distributed loads in graphene film was analyzed by finite element method based on the large deflection elastic theory of circular film. The pressure-sensing mathematical model of optical fiber Fabry-Perot cavity with graphene diaphragm was established based on the working principle of Fabry-Perot interferometer. The effects of graphene film layer and incident light angle on the film reflectivity were obtained according to the refractive index characteristics of the film. Then the interference spectra change,caused by both the cavity length losses and the film deflection deformations under the pressure loads,were analyzed. The simulation results show that adding the film layer can increase the film reflectivity and further improve the optical interference performance; however,a decreasing effect on the deflection deformation is caused by the film layer with the increase of pressure load. Thus,an 8 layer graphene film can achieve a reflectivity of 0. 715% and an approximate theoretical sensitivity of 10 nm / k Pa for a 40 μm Fabry-Perot cavity length with a membrane diameter of 25 μm. It provides a theoretical basis for the design and fabrication of high-sensitivity fiber-tip pressure sensor with multiple-layer graphene diaphragm.

Optical control of resonant light transmission for an atom-cavity system

We demonstrate the manipulation of transmitted light through an optical Fabry-Perot cavity, built around a spectroscopy cell containing enriched rubidium vapor. Light resonant with the $^{87}$Rb D$_{2}$ ($F=2/F=1$) $\leftrightarrow F'$ manifold, is controlled by transverse intersection of the cavity mode by another resonant light beam. The cavity transmission can be suppressed or enhanced depending on the coupling of atomic states due to the intersecting beams. The extreme manifestation of cavity mode control is the precipitious destruction (negative logic switching) or buildup (positive logic switching) of the transmitted light intensity, on intersection of the transverse control beam with the cavity mode. Both the steady state and transient response are experimentally investigated. The mechanism behind the change in cavity transmission is discussed in brief.

Design and Optimization of Microbolometer Multilayer Optical Cavity

Microbolometers are the most widely used detectors in long-wave infrared uncooled thermal imagers. An optical cavity is required within a microbolometer structure to increase its optical absorption. In this work we present a detailed study on the design and optimization of a microbolometer optical cavity using Essential-Macleod package. In the simulations, the cavity is considered as thin film multi-layers that form cascaded Fabry-Perot optical cavities. In the design phase, the layers structures are selected including materials and initial thickness. The absorbing layers are chosen to be vanadium-pentoxide (V2O5) and titanium (Ti). In the optimization phase, the designed layer thicknesses are varied to maximize optical absorption within the absorbing layers. The simulations show that Ti layer absorption dominates over V2O5 layer. Also, the optimization proves that the air-gap cavity thickness is not simply quarter-wavelength because of the complex cascaded Fabry-Perot structure. The optimized air-gap thi...

Frequency splitting of polarization eigenmodes in microscopic Fabry–Perot cavities

We study the frequency splitting of the polarization eigenmodes of the fundamental transverse mode in CO2 laser-machined, high-finesse optical Fabry–Perot cavities and investigate the influence of the geometry of the cavity mirrors. Their highly reflective surfaces are typically not rotationally symmetric but have slightly different radii of curvature along two principal axes. We observe that the eccentricity of such elliptical mirrors lifts the degeneracy of the polarization eigenmodes. The impact of the eccentricity increases for smaller radii of curvature. A model derived from corrections to the paraxial resonator theory is in excellent agreement with the measurements, showing that geometric effects are the main source of the frequency splitting of polarization modes for the type of microscopic cavity studied here. By rotating one of the mirrors around the cavity axis, the splitting can be tuned. In the case of an identical differential phase shift per mirror, it can even be eliminated, despite a nonvanishing eccentricity of each mirror. We expect our results to have important implications for many experiments in cavity quantum electrodynamics, where Fabry–Perot cavities with small mode volumes are required.

Coherent Coupling between a Molecular Vibration and Fabry–Perot Optical Cavity to Give Hybridized States in the Strong Coupling Limit

The coherent coupling between an optical transition and a confined optical mode, when sufficiently strong, gives rise to a new pair of mixed modes separated in frequency by the vacuum Rabi splitting. Such systems have been widely investigated for electronic-state transitions such as molecular excitons coupled to surface-plasmons and optical microcavities. However, only very recently have vibrational transitions been considered. Here we experimentally investigate the coupling between a Fabry–Perot cavity and the carbonyl stretch at an infrared frequency near 1730 cm–1 in polymethyl methacrylate. As is requisite for the “strong coupling” regime, the measured vacuum-Rabi-splitting of 132 cm–1 is much larger than the full width of either the cavity resonance (34 cm–1) or the inhomogeneously broadened carbonyl-stretch absorption (24 cm–1). With the assistance of quantitative analysis using transfer-matrix methods, we provide evidence that the mixed-state resonances are relatively immune to inhomogeneous vibrat...

Underwater blast wave pressure sensor based on polymer film fiber Fabry-Perot cavity.

This paper describes the theoretical and experimental aspects of an optical underwater shock wave sensor based on a polymer film optical fiber Fabry-Perot cavity manufactured by vacuum deposition technology. The transduction mechanism of the sensor involves a normally incident acoustic stress wave that changes the thickness of the polymer film, thereby giving rise to a phase shift. This transient interferometric phase is interrogated by a three-phase-step algorithm. Theoretically, the sensor-acoustic-field interaction principle is analyzed, and the phase modulation sensitivity based on the theory of waves in the layered media is calculated. Experimentally, a static calibration test and a dynamic calibration test are conducted using a piston-type pressure calibration machine and a focusing-type electromagnetic shock wave. Results indicate that the repeatability, hysteresis, nonlinearity, and the overall measurement accuracy of the sensor within the full pressure range of 55MPa are 1.82%, 0.86%, 1.81%, and 4.49%, respectively. The dynamic response time is less than 0.767μs. Finally, three aspects that need further study for practical use are pointed out.