Active sub-Rayleigh alignment of parallel or antiparallel laser beams

High-precision method for simultaneously measuring the six-degree-of-freedom relative position and pose deformation of satellites

The high-precision measurement of the six degrees-of-freedom (6DoF) relative position and pose deformation of satellites on the ground in vacuum and high-/low-temperature environments plays a critical role in ensuring the on-orbit mapping accuracy of satellites. To meet the strict measurement requirements for a satellite of a high accuracy, high stability, and a miniaturized measurement system, this paper proposes a laser measurement method for simultaneously measuring 6DoF relative position and attitude. In particular, a miniaturized measurement system was developed and a measurement model was established. The problem of error crosstalk between the 6DoF relative position and pose measurements was solved by conducting a theoretical analysis and OpticStudio software simulation, and the measurement accuracy was improved. Laboratory experiments and field tests were then conducted. The experimental results revealed that the measurement accuracy of the developed system for the relative position and relative attitude reached 0.2 µm and 0.4", within the measurement ranges of 500 mm along the X axis, ±100 µm along Y and Z axes, and ±100", and the 24-h measurement stabilities were superior to 0.5 µm and 0.5", respectively, which meets the ground measurement requirements for the satellite. The developed system was successfully applied on site, and the 6Dof relative position and pose deformation of the satellite were obtained via a thermal load test. This novel measurement method and system provides an experimental means for satellite development, in addition to a method for the high-precision measurement of the relative 6DoF position and pose between two points.

Three-degrees-of-freedom laser interferometer based on differential wavefront sensing with wide angular measurement range

We present a three-degrees-of-freedom laser interferometer based on differential wavefront sensing with a wide angular measurement range. To obtain measurement signals with a high AC/DC ratio and improve the linearity of differential wavefront sensing in a wide range angular displacement measurement, a fiber bundle instead of a quadrant photodiode is applied to receive the interference light. Meanwhile, a decoupled algorithm with quadratic correction derived by the ray tracing method and kinematic analysis is detailed in the case of the wide range angular displacement. The simulation and experimental results prove that the laser interferometer with the proposed structure could realize hundreds of microradian range of angular displacement measurement, which are expanded in the differential wavefront sensing.

Active fiber-based retroreflector providing phase-retracing anti-parallel laser beams for precision spectroscopy

We present an active fiber-based retroreflector providing high quality phase-retracing anti-parallel Gaussian laser beams for precision spectroscopy of Doppler sensitive transitions. Our design is well-suited for a number of applications where implementing optical cavities is technically challenging and corner cubes fail to match the demanded requirements, most importantly retracing wavefronts and preservation of the laser polarization. To illustrate the performance of the system, we use it for spectroscopy of the 2S-4P transition in atomic hydrogen and demonstrate an average suppression of the first order Doppler shift to 4 parts in 10⁶ of the full collinear shift. This high degree of cancellation combined with our cryogenic source of hydrogen atoms in the metastable 2S state is sufficient to enable determinations of the Rydberg constant and the proton charge radius with competitive uncertainties. Advantages over the usual Doppler cancellation based on corner cube type retroreflectors are discussed as well as an alternative method using a high finesse cavity.

Common mode noise rejection properties of amplitude and phase noise in a heterodyne interferometer

High precision metrology systems based on heterodyne interferometry can measure the position and attitude of objects to accuracies of picometer and nanorad, respectively. A frequently found feature of the general system design is the subtraction of a reference phase from the phase of the position interferometer, which suppresses low frequency common mode amplitude and phase fluctuations occurring in volatile optical path sections shared by both the position and reference interferometer. Spectral components of the noise at frequencies around or higher than the heterodyne frequency, however, are generally transmitted into the measurement band and may limit the measurement accuracy. Detailed analytical calculations complemented with Monte Carlo simulations show that high frequency noise components may also be entirely suppressed, depending on the relative difference of measurement and reference phase, which may be exploited by corresponding design provisions. While these results are applicable to any heterodyne interferometer with certain design characteristics, specific calculations and related discussions are given for the example of the optical metrology system of the LISA Pathfinder mission to space.

Sub-Rayleigh optical vortex coronagraphy

We introduce a new optical vortex coronagraph (OVC) method to determine the angular distance between two sources when the separation is sub-Rayleigh. We have found a direct relationship between the position of the minima and the source angular separation. A priori knowledge about the location of the two sources is not required. The superresolution capabilities of an OVC, equipped with an ℓ = 2 N-step spiral phase plate in its optical path, were investigated numerically. The results of these investigations show that a fraction of the light, increasing with N, from the secondary source can be detected with a sub-Rayleigh resolution of at least 0.1 λ/D.

A heterodyne interferometer for angle metrology

We have developed a compact, high-resolution, angle measurement instrument based on a heterodyne interferometer. Common-path heterodyne interferometer metrology is used to measure displacements of a reflective target surface. In the interferometer set up, an optical mask is used to sample the laser beam reflecting back from four areas on a target surface. From the relative displacement measurements of the target surface areas, we can simultaneously determine angular rotations around two orthogonal axes in a plane perpendicular to the measurement beam propagation direction. The device is used in a testbed for a tracking telescope system where pitch and yaw angle measurements of a flat mirror are performed. Angle noise measurement of the device shows 0.1 nrad/square root of Hz at 1 Hz, at a working distance of 1 m. The operation range and nonlinearity of the device when used with a flat mirror is approximately +/-0.15 mrad, and 3 microrad rms, respectively.

Atom interferometry with up to 24-photon-momentum-transfer beam splitters

We present up to 24-photon Bragg diffraction as a beam splitter in light-pulse atom interferometers to achieve the largest splitting in momentum space so far. Relative to the 2-photon processes used in the most sensitive present interferometers, these large momentum transfer beam splitters increase the phase shift 12-fold for Mach-Zehnder (MZ) and 144-fold for Ramsey-Bordé (RB) geometries. We achieve a high visibility of the interference fringes (up to 52% for MZ or 36% for RB) and long pulse separation times that are possible only in atomic fountain setups. As the atom's internal state is not changed, important systematic effects can cancel.

Overcoming the rayleigh criterion limit with optical vortices

We experimentally and numerically tested the separability of two independent equally luminous monochromatic and white light sources at the diffraction limit, using optical vortices (OV). The diffraction pattern of one of the two sources crosses a fork hologram on its center generating the Laguerre-Gaussian (LG) transform of an Airy disk. The second source, crossing the fork hologram in positions different from the optical center, generates nonsymmetric LG patterns. We formulated a criterion, based on the asymmetric intensity distribution of the superposed LG patterns so created, to resolve the two sources at angular distances much below the Rayleigh criterion. Analogous experiments in white light allow angular resolutions which are still one order of magnitude below the Rayleigh criterion. The use of OVs might offer new applications for stellar separation in future space experiments.