Xesús Prieto-Blanco

Analytical design of an Offner imaging spectrometer

We present the analytical design of an imaging spectrometer based on the three-concentric-mirror (Offner) configuration. The approach presented allows for the rapid design of this class of system. Likewise, high-optical-quality spectrometers are obtained without the use of aberration-corrected gratings, even for high speeds. Our approach is based on the calculation of both the meridional and the sagittal images of an off-axis object point. Thus, the meridional and sagittal curves are obtained in the whole spectral range. Making these curves tangent to each other for a given wavelength results in a significant decrease in astigmatism, which is the dominant residual aberration.RMS spot radii less than 5 mm are obtained for speeds as high as f/2.5 and a wavelength range of 0.4-1.0 microm. A design example is presented using a free interactive optical design tool.

The Offner imaging spectrometer in quadrature

This is a proposal and description of a new configuration for an Offner imaging spectrometer based on the theory of aberrations of off-plane classical-ruled spherical diffraction gratings. This new spectrometer comprises a concave mirror used in double reflection and a convex reflection grating operating in quadrature, in a concentric layout. A very simple procedure obtains designs that are anastigmatic for a given point on the entrance slit and a given wavelength. Specific examples show that the performance of this type of system improves the performance of analogous conventional in-plane systems, when compactness and/or high spectral resolution is of fundamental importance.

Off-plane anastigmatic imaging in Offner spectrometers

In this paper, the imaging performance of an Offner concentric imaging spectrometer is analyzed when the spectrometer entrance slit is disposed arbitrarily on the plane that is parallel to the grating grooves and contains the common center of curvature. Astigmatism-corrected designs are obtained for off-plane incidence on the grating if one point on the slit is located on the Rowland circle of the primary mirror. In this case, the combined system of primary mirror plus diffraction grating provides two astigmatic line images oriented parallel and orthogonal to the plane of diffraction, with the former located on the same plane as the slit. Consequently, these images can be brought to a single focus on this plane by the tertiary mirror if its radius of curvature is chosen properly. In addition, coma aberration is simultaneously removed. These results can be applied to the design of two-mirror or three-mirror spectrometers, generalizing the concept of the best imaging circle and providing solutions to get anastigmatic imaging for two object points and two wavelengths.

Ion-exchanged glass binary phase plates for mode-division multiplexing.

Significant efforts are being made to increase optical network capacity in response to ever-growing data traffic. One promising candidate is mode-division multiplexing (MDM) in few-mode fibers. A fundamental element for MDM is a modal transformer. Modal transformation can be implemented in a free-space basis by using multiregion phase plates. In this work, we show that efficient monolithic binary phase plates can be fabricated by ion exchange in glass and used for MDM tasks. We present an optical characterization method of such plates, which is based on a combination of the inverse Wentzel-Kramers-Brillouin (IWKB) method and Mach-Zehnder interferometric techniques. The IWKB method allows us to design and characterize the phase plates in an easy and fast way, whereas interferometry gives us a precise measurement of the phase step. Far-field optical intensities are measured, and a high-quality mode transformation is confirmed.

Design of Dyson imaging spectrometers based on the Rowland circle concept

We aim to show that Dyson imaging spectrometers can be easily designed by applying the concept of the Rowland circle to refracting surfaces. This allows us to conceive an analytical procedure that is based on the removal of astigmatism at two wavelengths. Following this procedure, high-optical-quality spectrometers can be designed even for high speeds. Root-mean-square spot radii less than 2.5 μm are obtained for speeds as high as f/1.5, slit lengths of 15 mm, and wavelength ranges of 0.4-1.7 μm. Design examples are presented for classical Dyson spectrometers in which the detector is glued to the glass plane surface and for spectrometers with an air gap between this surface and the image plane.

Two-wavelength anastigmatic Dyson imaging spectrometers

High-quality Dyson imaging spectrometers are designed by applying a telecentric condition for off-axis image points. By imposing this condition for two different wavelengths, designs presenting low aberrations for the whole spectral range of the system are obtained. A UV-TO-NIR fast design (f/1.5) exhibiting excellent optical performance is presented.

Imaging with classical spherical diffraction gratings: the quadrature configuration

We review the theory of spherical diffraction gratings with regard to their imaging properties in off-plane arrangements. Our study is restricted to gratings with equally spaced grooves, and it is focused on the quadrature configuration, where the incident and diffraction planes are orthogonal to each other. We identify regions of low astigmatism and propose some monochromator mounts.

Pupil aberrations in Offner spectrometers

The light path function (LPF) of an Offner spectrometer is presented. The evaluation of the LPF of this spectrometer enables its imaging properties to be studied for arbitrary object and image positions, while avoiding the more complicated analysis of intermediate images generated by the diffraction grating, which is often involved. A power series expansion of the LPF on the grating coordinates directly determines pupil aberrations of the generated spectrum and facilitates the search for configurations with small low-order aberrations. This analysis not only confirms the possibility of reducing low-order aberrations in Rowland-type mounts, namely astigmatism and coma, as predicted in previous studies, but also proves that all third-order terms in the series expansion of the aberration function can be canceled at the image of the design point and for the corresponding design wavelength, when the design point is located on a plane orthogonal to the optical axis. Furthermore, fourth-order terms are computed and shown to represent the most relevant contribution to image blurring. Third- and fourth-order aberrations are also evaluated for Rowland mounts with the design point located outside the aforementioned plane. The study described in this manuscript is not restricted to small angles of incidence, and, therefore, it goes beyond Seidel and Buchdahl aberrations.

Spatial mode multiplexing/demultiplexing by Gouy phase interferometry.

We present a theoretical study about spatial mode multiplexing/demultiplexing (mux/demux) without theoretical losses by means of interferometry with selective control of the Gouy phase of optical beams, that is, Gouy phase interferometry (GPI). Different Gouy phase values can be obtained by inserting appropriate optical systems at each arm of an interferometer. Thus, spatial mode mux/demux operations of strategic interest in optical communications with few-mode optical fibers are implemented by means of constructive interference and regardless of the parity and separability of the optical beams. Consequently, unachievable mux/demux by interferometry based on image inversion methods becomes possible with GPI. This kind of operation can also be interesting for optical sensors, optical metrology, image processing, and so on.

Interferometric space-mode multiplexing based on binary phase plates and refractive phase shifters

A Mach-Zehnder interferometer (MZI) that includes in an arm either a reflective image inverter or a Gouy phase shifter (RGPS) can (de)multiplex many types of modes of a few mode fiber without fundamental loss. The use of RGPSs in combination with binary phase plates for multiplexing purposes is studied for the first time, showing that the particular RGPS that shifts π the odd modes only multiplexes accurately low order modes. To overcome such a restriction, we present a new exact refractive image inverter, more compact and flexible than its reflective counterpart. Moreover, we show that these interferometers remove or reduce the crosstalk that the binary phase plates could introduce between the multiplexed modes. Finally, an experimental analysis of a MZI with both an approximated and an exact refractive image inverter is presented for the case of a bimodal multiplexing. Likewise, it is proven experimentally that a RGPS that shifts π/2 demultiplexes two odd modes which can not be achieved by any image inverter.