White-light channeled imaging polarimeter using Savart plates and a polarization Sagnac interferometer.

A Stokes white-light channeled imaging polarimeter using Savart plates and a polarization Sagnac interferometer (IPSPPSI) is presented, which provides an effective solution to the problem of channel aliasing in broadband polarimeters. The expression for the light intensity distribution and a method to reconstruct polarization information are derived, and an example design for an IPSPPSI is given. The results reveal that a complete measurement of the Stokes parameters in broad band can be achieved with a snapshot on a single detector. The use of dispersive elements like gratings suppresses broadband carrier frequency dispersion so the channels in the frequency domain do not affect each other, ensuring the integrity of information coupled across the channels. Furthermore, the IPSPPSI has a compact structure and does not employ moving parts or require image registration. It shows great application potential in remote sensing, biological detection, and other fields.

Convex blazed grating of high diffraction efficiency fabricated by swing ion-beam etching method.

A swing ion-beam etching method to fabricate convex blazed gratings used in shortwave infrared hyperspectral imaging spectrometers is presented. This method solves the consistency problem of blaze angles by swing etching through the meridian direction of the gratings. The mathematical relationship of the curvature, aperture, and diffraction efficiency of convex gratings is studied to demonstrate the limitation of conventional translational lithography and the necessity of swing etching. A geometric model is built to analyze the influence of swinging speed and beam slit width on groove evolution. Convex gratings with a 45.5 gr/mm groove density, 67 mm aperture, 156.88 mm radius of curvature, and 2.2° blaze angle have been fabricated and measured where the peak and average diffraction efficiency in the shortwave infrared band reach 90% and 70%, respectively. Experimental results validate that high-efficiency convex gratings of small blaze angle and high groove consistency can be produced by swing etching, which satisfy the requirements for high spectral resolution and miniaturization of imaging spectrometers.

Broadband transmission Raman measurements using a field-widened spatial heterodyne Raman spectrometer with mosaic grating structure.

A field-widened spatial heterodyne Raman spectrometer with a mosaic grating structure is developed for the simultaneous sensitivity enhancement and broadband transmission Raman measurements. We optimize the etendue to maximize the signals collected from the samples by using field-widening prisms and employ two mosaic gratings to achieve broadband operation, covering 5638 cm-1 with 2.865 cm-1 spectral resolution. The signal-to-noise ratios are improved by a factor of more than 11 and show a good stability and fair repeatability. We investigate the effects of the sample thickness and outer layer depth and observe liquids, solids, mixed targets, and anti-Stokes shifts. The instrument exhibits good performance for wide-field, high-resolution broadband transmission Raman measurements.

Removal of all mosaic grating errors in a single-interferometer system by a phase-difference reference window.

An interference method to remove all mosaic grating errors using a phase-difference reference window without compensation for coupled mosaic errors or analyzing the far-field diffraction intensity patterns is proposed. The reference window is achieved by adding a small-aperture prism; therefore, the incident light on the mosaic gratings contains two different angles of incidence and optical paths, eliminating the uncertainty of longitudinal mosaic error in the interferometer. The mathematical model of the method is established and optical arrangement is presented. Interference fringes with the reference window and alignment process are simulated and the accuracy of mosaic errors are analyzed. The experimental and simulation results show that our method can eliminate longitudinal mosaic error and all other mosaic grating errors. The accuracy of angle mosaic errors can be less than 1 µrad and position mosaic errors can be to the nanometer level. Compared with the far-field diffraction intensity patterns method, our method reduces the complexity of the optical structure and ensures measurement accuracy.

Using the compensating effect for Rayleigh anomalies to design a type of broadband mid-IR grating.

We suggest and numerically demonstrate a specific design method of broadband mid-IR echelette grating. The method, which applies the compensating of the efficiency blaze in TM-plane polarization to Rayleigh anomalies, is called compensating effect method. This idea allows the grating designer to realize fully the considerable advantages of this type of echelette grating with an apex angle 90 degrees and a blaze angle 19.47 degrees , especially, it can be fabricated easily with conventional (holographic and ruled) techniques. Its diffraction efficiency in unpolarized light has a minimum value of 40%. The cases of Littrow mount and off-Littrow mount are both discussed.