Carlos Montero-Orille

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.

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.

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.

A windowing/pushbroom hyperspectral imager

We show that any pushbroom hyperspectral imager can be converted into a windowing one by only attaching in front of it a simple dispersive element. The resulting device displays the same spectral resolution than the former one. To test this idea we have built a pushbroom imaging spectrograph in our laboratory. A plane transmission grating has been used as the dispersive element. We point out the main characteristics of these devices and we show some illustrative results.

Chromatic characterization of ion-exchanged glass binary phase plates for mode-division multiplexing.

Mode-division multiplexing (MDM) in few-mode fibers is regarded as a promising candidate to increase optical network capacity. A fundamental element for MDM is a modal transformer to LP modes which can be implemented in a free-space basis by using multiregion phase plates, that is, LP plates. Likewise, several wavelengths have to be used due to wavelength multiplexing purposes, optical amplification tasks, and so on. In this work we show that efficient monolithic binary phase plates for different wavelengths can be fabricated by ion-exchange in glass and used for MDM tasks. We introduce an optical characterization method of the chromatic properties of such phase plates which combines the inverse Wentzel-Kramers-Brillouin (IWKB) together with Mach-Zehnder and Michelson-based interferometric techniques. The interferometric method provides a measurement of the phase step for several wavelengths, which characterizes the chromatic properties of the phase plate. Consequently, it is shown that the IWKB method allows us to design and characterize the phase plates in an easy and fast way.