Free-space Interferometer Research Articles

Computational analysis of a scalable optically homogeneous free-space interferometer.

This study describes a scalable optically homogeneous free-space interferometer. Computationally modeled as an unbroken block of fused silica, the interferometer's six-sided design is simple and intuitive, exploiting total internal reflection and refraction to split and recombine a collimated input beam. During propagation, one portion of the split beam remains within the substrate to act as a reference beam. The second portion of the split beam is exposed to the surrounding environment, enabling real-world environment characterization in real time. Validation of the interferometer concept is performed using numerical and analytical techniques. Based on its scalability and robustness, the proposed interferometer design is primed for applications in atmospheric sensing, passive chemical detection, and spaceborne technologies.

Free-space interferometer design for optical frequency dissemination and out-of-loop characterization below the 10−21-level

For improving the performance of optical frequency dissemination and the resolution of its out-of-loop (OOL) characterization, we investigate a compact free-space interferometer design in which a monolithic assembly forms the reference arm. Two interferometer designs are realized, and their environmental sensitivity is analyzed based on the properties of the materials involved. We elucidate that in these designs the temperature sensitivities of the out-of-loop signal paths are greater than for the reference arm. As the estimated temperature-variation-induced frequency transfer errors are observed to be the relevant limitation, the out-of-loop characterization signal can be regarded as a trustworthy upper limit of the frequency transfer error to a remote place. We demonstrate a fractional frequency transfer uncertainty and OOL characterization resolution of ≤2.7×10−21 over many measurement runs. With a value of (0.23±1.07)×10−22 the weighted mean offset is significantly below the best reported results so far.

Multi-beam OCT imaging based on an integrated, free-space interferometer

While it is a common practice to increase the speed of swept-source optical coherence tomography (OCT) systems by using a high-speed source, this approach may not always be optimal. Parallelization in the form of multiple imaging beams is an alternative approach, but scalable and low-loss multi-beam OCT architectures are needed to capitalize on its advantages. In this study, we demonstrate an eight-beam OCT system using an interferometer architecture comprising planar lightwave circuits (PLC) splitters, V-groove assemblies (VGA), and optical ribbon fibers. We achieved an excess loss and heterodyne efficiency on each channel that was close to that of single-beam systems. In vivo structural imaging of a human finger and OCT angiography imaging of a mouse ear was performed to demonstrate the imaging performance of the system. This work provides further evidence supporting multi-beam architectures as a viable strategy for increasing OCT imaging speed.

A Free-Space Interferometer for Phase-Delay Measurements in Integrated Optical Devices in Degenerate Pump-and-Probe Experiments

The authors report on a free-space interferometer for simultaneous phase and transmittance measurements of an optical signal in an integrated photonic circuit during degenerate pump-and-probe experiments. Differentiation of the weak probe signal from the strong pump relies on a lock-in amplifier. The interferometer shows high flexibility in terms of operating wavelengths, tested devices, and optical powers; it works with any integrated optical device provided with optical input–output channels; it can operate both with high input power (up to 100 mW at the sample input port) and low output power ( $ at the detection stage). Measuring both the transmittance and the phase of the optical signal allows using the phasor representation in the analysis. A dynamic operating mode allows to perform power and/or wavelength scan of the input signal, obtaining transmittance and phase spectra of the tested device. Remote control of the system and insulation from the external environment brings to more stable operating conditions. Characterization of the system properties with photonic-integrated structures (waveguide and ring resonator) is reported, with observed phase stability as high as 0.25°/min and average phase noise below 1°.

Three-Path Atom Interferometry with Large Momentum Separation.

We demonstrate the scale up of a symmetric three-path contrast interferometer to large momentum separation. The observed phase stability at separation of 112 photon recoil momenta exceeds the performance of earlier free-space interferometers. In addition to the symmetric interferometer geometry and Bose-Einstein condensate source, the robust scalability of our approach relies on the suppression of undesired diffraction phases through a careful choice of atom optics parameters. The interferometer phase evolution is quadratic with number of recoils, reaching a rate as high as 7×10^{7} rad/s. We discuss the applicability of our method towards a new measurement of the fine-structure constant and a test of QED.

White light polarization sensitive optical coherence tomography for sub-micron axial resolution and spectroscopic contrast in the murine retina.

A white light polarization sensitive optical coherence tomography system has been developed, using a supercontinuum laser as the light source. By detecting backscattered light from 400 – 700 nm, an axial resolution of 1.0 µm in air was achieved. The system consists of a free-space interferometer and two homemade spectrometers that detect orthogonal polarization states. Following system specifications, images of a healthy murine retina as acquired by this non-contact system are presented, showing high resolution reflectivity images as well as spectroscopic and polarization sensitive contrast. Additional images of the very-low-density-lipoprotein-receptor (VLDLR) knockout mouse model were acquired. The high resolution allows the detection of small lesions in the retina.

Two-Particle Four-Mode Interferometer for Atoms.

We present a free-space interferometer to observe two-particle interference of a pair of atoms with entangled momenta. The source of atom pairs is a Bose-Einstein condensate subject to a dynamical instability, and the interferometer is realized using Bragg diffraction on optical lattices, in the spirit of our recent Hong-Ou-Mandel experiment. We report on an observation ruling out the possibility of a purely mixed state at the input of the interferometer. We explain how our current setup can be extended to enable a test of a Bell inequality on momentum observables.

Oblique beams interference for mode selection in multimode silicon waveguides

Here, we propose to use the interference pattern which arises from the superposition of two coherent free space tilted beams at the input facet of an optical waveguide to excite selectively a given optical mode. By tuning the period of the interference fringes, it is possible to select the excited mode in the waveguide by maximizing the overlap integral with the modal optical field. Our setup is based on a free space interferometer that is theoretically capable of selectively exciting higher order modes in a micron-sized waveguide with an average cross-talk of 37 dB and a mode selectivity higher than 90%. The system is easily reconfigurable and can be straightforwardly integrated on a chip to enhance miniaturization, compactness, and stability.

A monolithic micro-optical interferometer deep etched into fused silica

For free-space micro-optical systems, the alignment of the components is still a challenging task in manufacturing. Alternatively, a monolithic integration can overcome this problem, but especially for in-plane optical elements in the visible wavelength range, the optical surfaces have to fulfill critical demands. Here, we show a fabrication process that allows the deep-reactive ion etching (RIE) of fused silica with high optical quality. We achieve vertical sidewalls with etch depths of about 100μm with an arithmetic mean roughness of about 7.2nm. By using this process, a new in-plane monolithic, free-space interferometer is demonstrated that reaches a resolution of 20nm with our current setup.

Spatially continuous distributed fiber optic sensing using optical carrier based microwave interferometry.

This paper reports a spatially continuous distributed fiber optic sensing technique using optical carrier based microwave interferometry (OCMI), in which many optical interferometers with the same or different optical path differences are interrogated in the microwave domain and their locations can be unambiguously determined. The concept is demonstrated using cascaded weak optical reflectors along a single optical fiber, where any two arbitrary reflectors are paired to define a low-finesse Fabry-Perot interferometer. While spatially continuous (i.e., no dark zone), fully distributed strain measurement was used as an example to demonstrate the capability, the proposed concept may also be implemented on other types of waveguide or free-space interferometers and used for distributed measurement of various physical, chemical and biological quantities.

Ramsey interferometry with an atom laser

We present results on a free-space atom interferometer operating on the first order magnetically insensitive |F = 1,mF = 0) --> |F = 2,mF = 0) ground state transition of Bose-condensed (87)Rb atoms. A pulsed atom laser is output-coupled from a Bose-Einstein condensate and propagates through a sequence of two internal state beam splitters, realized via coherent Raman transitions between the two interfering states. We observe Ramsey fringes with a visibility close to 100% and determine the current and the potentially achievable interferometric phase sensitivity. This system is well suited to testing recent proposals for generating and detecting squeezed atomic states.

Experimental Realization of Maximum Confidence Quantum State Discrimination for the Extraction of Quantum Information

We present the first experimental demonstration of the maximum confidence measurement strategy for quantum state discrimination. Applying this strategy to an arbitrary set of states assigns to each input state a measurement outcome which, when realized, gives the highest possible confidence that the state was indeed present. The theoretically optimal measurement for discriminating between three equiprobable symmetric qubit states is implemented in a polarization-based free-space interferometer. The maximum confidence in the measurement result is 2/3. This is the first explicit demonstration that an improvement in the confidence over the optimal minimum error measurement is possible for linearly dependent states.

Experimental demonstration of optimal unambiguous state discrimination

We present a full demonstration of unambiguous state discrimination between nonorthogonal quantum states. Using a free-space interferometer we have realized the optimum quantum measurement scheme for two nonorthogonal states of light, known as the Ivanovic-Dieks-Peres (IDP) measurement. We have gained access to all three possible outcomes of this measurement. All aspects of this generalized measurement scheme, including its superiority over a standard von Neumann measurement, have been demonstrated within 1.5% of the IDP predictions.

Late-Time Source of Atomic Light in the Helium Afterglow

The time-dependent behavior of the light emitted from a pulsed helium afterglow at 3 Torr has been investigated with an improved spectrometric system of sufficient sensitivity-bandwidth ratio to resolve the behavior of the atomic light over a 50-msec time interval. The relationship between the atomic light and the time-resolved electron density, as measured with a 36-GHz free-space microwave interferometer, was found to indicate the importance of a persistent source of ionization at late times. Absorption measurements of time-dependent behavior of both the atomic $2^{3}S$ and molecular $2^{3}\ensuremath{\Sigma}$ metastable species were found to strongly favor mutual ionization of two metastable molecules as being the most important source of late-time ionization at 3 Torr.

Optimization of an X-band Interferometer for Plasma Diagnostics†

A 9 GHZ (3 cm) microwave free-space interferometer for measuring electron density decay over the range 2 × 1011−1010 cm−3 in a pulsed discharge afterglow is described. By using initial 90° unbalancing (instead of the conventional 180° balance) considerable improvement in sensitivity was obtained. In addition, amplitude modulation of the microwave probing signal enabled instantaneous power fluctuations to be recorded accurately as a function of time during the afterglow. Significant errors are shown to arise if the effects-of finite plasma boundaries are ignored.