Wavefront Calibration Research Articles

Formation and propagation characteristics of pseudo-wavefronts of optical polarization states signal inside OPGW under lightning strikes

Lightning strikes on Optical Fiber Composite Overhead Ground Wires (OPGW) result in a phenomenon where the current traveling wave along the OPGW’s outer stranding propagates faster than the optical signals inside the fiber. This leads to the emergence of pseudo-wavefronts ahead of the optical polarization state signal after lightning strikes OPGW, introducing errors in lightning point localization based on the optical polarization state. This paper investigates the formation and propagation characteristics of pseudo-wavefronts. The study employs theoretical analysis and simulation to examine the process of optical polarization state formation during OPGW lightning strikes. Additionally, it conducts comparative analyses between laboratory-simulated OPGW lightning strikes and field measurements obtained from OPGW lightning monitoring systems. The results affirm the existence of the proposed pseudo-wavefront phenomenon, which substantially impacts the precision of lightning point localization. Notably, the duration of pseudo-wavefront effects increases with greater distance from the lightning point and higher lightning current amplitudes. These findings raise questions regarding the suitability of traditional wavefront calibration methods based on the traveling wave approach. This research contributes valuable insights for the practical implementation of lightning point localization methodologies relying on OPGW self-sensing technology.

High-precision spherical wavefront calibration method for shack-hartmann wavefront sensor

High-precision spherical wavefront calibration method for shack-hartmann wavefront sensor

Accuracy characterization of Shack–Hartmann sensor with residual error removal in spherical wavefront calibration

Accuracy characterization of Shack–Hartmann sensor with residual error removal in spherical wavefront calibration

Virtual wavefront calibration method based on ray tracing for alignment error compensation in the interferometric bearing ball measurement system

The accuracy of laser interferometry for measuring bearing balls is limited because it is challenging to realize null positioning of the ball being assessed. Even a slight alignment error causes the reflected wavefront to carry an uncertain aberration, which results in the surface error being obscured by the wavefront aberration. To effectively separate the surface error from the reflected wavefront, a virtual wavefront calibration method of ray tracing for alignment error compensation in the interferometric bearing ball measurement system is proposed. A virtual wavefront measurement model with variable alignment vector of the system is established based on the ray tracing principle and coordinate transform theory. According to the virtual wavefront, a calibration process is achieved in the virtual model with a known regularization and optimization method; as a result, no accurate adjustment mechanism is required to adjust the position of the bearing ball in the actual measurement system. Simulation and experiment results indicate that the proposed method is effective, and the repeatability of this measurement system is better than λ/40 peak–valley value. The final results may promote the application of this method in other fields of optical measurement.

Design and development of Shack–Hartmann wavefront sensor-based testing of high-resolution optical system for earth observation

The deformation of the optical surfaces of a large aperture high-resolution space-borne optical system induced by earth’s gravity on the ground, which is not present during in-orbit operations, necessitates the evaluation of its performance in terms of wavefront error at various stages of development of the earth observation system. A direct method of evaluation for an optical system at an integrated electro-optical module based on a Shack–Hartmann wavefront sensor (SH WFS) is proposed. Design and analysis of the wavefront sensor that are tailored to meet the requirements of the high-resolution optical system are described. We show that the procedure followed for the development of the SH WFS not only addresses the parameters of the wavefront sensor that are critical to its performance, but also aides in the wavefront sensor alignment and calibration. The performance of the developed SH WFS is demonstrated by testing a simulated telescope which is <italic<in situ</italic< verified in a test configuration using a standard Fizeau interferometer; a close match of the coefficients of Zernike modes between them is established.

Absolutely Testing of Off-Axis Aspherical Mirror Using Dichotomy Computer-Generated Holograms

A method for absolutely testing off-axis aspherical mirror is presented. A dichotomy CGH including two zones on one substrate is designed. One sub aperture generates the spherical wave front that is used for calibrating the pattern errors of the hologram; the other sub aperture reconstructs the aspherical wave front to test the off-axis aspherical mirror. By means of known absolute interferometric methods, the aberrations of the calibration wave front can be obtained. These aberrations can be transferred to the null testing wave front. The testing method is explained and the dichotomy CGH is designed. Finally, a simulation result supports this absolutely testing method.

A New High Contrast Imaging Program at Palomar Observatory

ABSTRACTWe describe a new instrument that forms the core of a long-term high contrast imaging program at the 200 inch (5 m) Hale Telescope at Palomar Observatory. The primary scientific thrust is to obtain images and low-resolution spectroscopy of brown dwarfs and young exoplanets of several Jupiter masses in the vicinity of stars within 50 pc of the Sun. The instrument is a microlens-based integral field spectrograph integrated with a diffraction-limited, apodized-pupil Lyot coronagraph. The entire combination is mounted behind the Palomar adaptive optics (AO) system. The spectrograph obtains imaging in 23 channels across the J and H bands (1.06–1.78 μm). The image plane of our spectrograph is subdivided by a 200 × 200 element microlens array with a plate scale of 19.2 mas per microlens, critically sampling the diffraction-limited point-spread function at 1.06 μm. In addition to obtaining spectra, this wavelength resolution allows suppression of the chromatically dependent speckle noise, which we describe. In addition, we have recently installed a novel internal wave front calibration system that will provide continuous updates to the AO system every 0.5–1.0 minutes by sensing the wave front within the coronagraph. The Palomar AO system is undergoing an upgrade to a much higher order AO system (PALM-3000): a 3388-actuator tweeter deformable mirror working together with the existing 241-actuator mirror. This system, the highest-resolution AO corrector of its kind, will allow correction with subapertures as small as 8.1 cm at the telescope pupil using natural guide stars. The coronagraph alone has achieved an initial dynamic range in the H band of 2 × 10-4 at 1″, without speckle noise suppression. We demonstrate that spectral speckle suppression provides a factor of 10–20 improvement over this, bringing our current contrast at 1″ to ∼2 × 10-5. This system is the first of a new generation of apodized-pupil coronagraphs combined with high-order adaptive optics and integral field spectrographs (e.g., GPI, SPHERE, HiCIAO), and we anticipate that this instrument will make a lasting contribution to high-contrast imaging in the Northern Hemisphere for years.

Calibration of residual speckle in a nulling coronagraph

The most challenging aspect of the direct detection of planets around nearby stars is the huge brightness difference between planet and star associated to the very small angular separation of the two objects. This article describes the operation of a post coronagraph wavefront calibration system for measuring the residual speckle pattern, making use of the coherence of starlight. For a ground based AO coronagraph, the calibration interferometer potentially offers photon limited planet detection instead of atmospheric speckle limited detection. In a space based coronagraph, the use of the calibration system reduces the stability requirements of the optical system by several orders of magnitude. To cite this article: M. Shao, C. R. Physique 8 (2007).

Twin-CGHs for absolute calibration in wavefront testing interferometry

A novel technique for absolute calibration the wavefront reconstructed by computer generated holograms (CGHs) is presented. It requires a specially designed multiplexed CGH (Twin-CGH), which reconstructs two wavefronts with the same symmetry. One of those is used for calibration of the CGH by means of known absolute interferometric methods. The patterns of both phase functions are simultaneously structured. The aberrations of the calibration wavefront can be transferred to those of the second wavefront. The testing method is explained and two different encoding methods are discussed. For different encoded Twin-Fresnel zone plates we experimentally demonstrate this novel calibration method.