MOSAIC is a concept for a multi-object spectrograph for the Extremely Large Telescope (ELT). It is planned to cover the wavelength range from 460 nm to 1800 nm with 5 visible spectrographs and 5 near-infrared spectrographs. The ELT is far from diffraction limited in the visible wavelength range. Rather than developing a large and complex AO system, it was decided that the instrument will be seeing limited in the visible. Spot sizes are therefore about 2.8 mm in diameter in the ELT focal plane, and need to be sampled by multiple fibers with large core diameter. As a result, large optics is required to achieve the science requirements on spectral resolution, bandwidth and multiplex. We work in close collaboration with manufacturers to design an instrument that is feasible and meets the scientific requirements.
The unique properties of diffraction gratings make the optical design of diffraction spectrometers a complex problem. This Spotlight connects optical design and diffraction grating fabrication to address issues of diffraction efficiency, tolerance analysis, and optimization techniques. It further explores performance testing of gratings and modification of classical spectrometers using modern design and fabrication techniques.
An original optical design for two-step recording of holographic diffraction gratings is presented. It combines the merits of the two previously developed methods. On the one hand, due to the two-step recording one can use wide possibilities for the aberrations compensation. On the other hand, one can record both the grating objective and the final grating in spatially incoherent light. This simplifies the technology of the holographic recording. Previously, one must use spatially coherent light for at least one of two recording steps. Our method does not require the use of additional optics. Both surfaces of the grating objective blank have a spherical form. The blank of the final grating can be plane concave, which is usual for diffraction gratings. Since the blaze angle of the grating recorded in the counterpropagating beams is determined by the recording geometry, and the efficiency of the grating depends on the polarization of the incident light, the transmission calculations taking into consideration the polarization are also provided. The transmission of the system is optimal for the s-polarized light, additional increasing of the transmission can be achieved by use of an immersion grating objective.
Natural fluorescence is a very weak signal, which represents only a very small fraction of the light emanating from the surface. The only method to detect natural ground fluorescence is to observe in the Fraunhofer lines of the solar spectrum where the otherwise much stronger reflectance background is significantly reduced. Ideally, would a Fraunhofer line be completely dark, the fluorescence would introduce some light at the line position visible on a black background. The spectrometer is calculated for four Fraunhofer lines, any combination of which can be used for the measurements. The first concave pre - dispersing grating focuses the selected lines to the entrance slits. The second part of the device consists of two concave diffraction gratings, one for the blue channel, and one for the red channel. These gratings focus light to the same detector array. No other optical elements are necessary. Spectrometer shows diffraction limited image quality. All the gratings surfaces have spherical form. This spectrometer has small dimensions (about 350 x 250 x 70 mm) and can be attractive for the space applications. Several modifications of the spectrometer and some aspects of its diffraction gratings fabrication and their diffraction efficiency are discussed.
The original optical design for two steps recording of the holographic diffraction gratings is presented. It gives the combination of the merits of two previously developed methods in one. From one hand, due to recording in two steps, we can use wide possibilities of the aberrations compensation. From another hand, we can record both the gratings - objective and the final grating in spatially incoherent light. This simplifies the technology of the holographic recording. Previously per limit one of the two steps required using of the spatially coherent light. Method does not require using of the additional optics. Both surfaces of the grating - objective blank have spherical form. The blank of the final grating can be plane - concave, which is usual for diffraction gratings. Since the blaze angle of the grating, recorded in the counterpropagating beams is determined by the recording geometry, and the efficiency of the grating depends on the polarization of the incident light, the transmission calculations with taking into consideration the polarization also have been provided. The transmission of the system is optimal for the S polarized light, the addition increasing of the transmission can be achieved by using of the immersion grating - objective.
In present work the wave aberration is determined for the general case of the concave diffraction grating monochromator using of the direction cosines of the diffracted ray. These direction cosines are found from the ray tracing through the monochromator. The ray tracing includes the holographic grating recording system and the optical system of the monochromator as well. The results of the optimization of the wave aberration and of the light path function are compared for the example of diffraction monochromator.
A spherical concave diffraction grating was chosen as the dispersing element for a number of spectroscopic deices. More recent application of these grating is designing of multiplexers/demultiplexers for wavelength routed optical networks. Concave grating acts as the focusing element and can be the sole optical element of a device, which simplifies its adjustment and increases the transmittance. However, it possesses aberrations. Conventional method of concave diffraction grating recording using interference of two spherical waves formed using dividing an depending of laser beam, gives possibility to minimize three main types of aberrations, the defocusing, the meridional coma and the first order astigmatism. For the wide range of spectrometers these gratings can be used with rather good results. However, if we want to design spectrometer with increased aperture, wide spectral region or extremely high resolving power, we have to take into consideration per limit more than two aberrations - the sagittal coma and the spherical aberration. We also have this problem in designing of wavelength routers, where aberration geometric size of image should be not more than the optical fiber diameter. We can resolve this problem using aspheric wavefront recording systems. Since refraction optics is not good for holographic recording because of scattering, this system can include mirrors or other diffraction gratings. In present work different recording systems are discussed from the point of view of geometric theory of grating and from the point of view of reality of experimental installing and using of these systems as well.
The double monochromator is a system of two single monochromators, in which the exit slit of the first monochromator is the entrance slit of the second one. We can use the double monochromator if we want to decrease the scattering light level or increase the dispersion of the spectral device. In the adding dispersion double monochromator all aberrations add. The traditional way of optimization of the double monochromator parameters is the two concave diffraction gratings, in which the second single monochromator particularly compensates the defocusing, the meridional coma and the first order astigmatism of the first one. Calculation of this monochromator have been done using numerical methods. More compete optimization of the optical mounting of spectroscopic device consists of two diffraction gratings can be done using light path function consideration. Geometric theory of two - steps recorded grating have been developed for holographic gratings recording. In this paper this theory is expanded for the case of double monochromator. The developed theory gives possibility for more complete aberration compensation, which allows to increase the spectral and the spatial resolution as well. Improvement of spectral and the spatial resolution is actual for different types of spectroscopic deices, for example, for the plasma diagnostic spectrometers.
High-resolution spectrometer was designed to resolve the fine structure of discharge emitted radiation near hydrogenous line 4648.8 A. The device consists of two concave diffraction gratings with 2700 grooves per mm and the radius of curvature 1000 mm. Optical mounting is calculated in such a way that the second grating compensates aberration of the first one for one wavelength. Using slight nonequidistancy of grooves we reduce aberrations in narrow region near this wavelength and achieved limit of resolution about 0.015 A for the spectral region of interest: 464.18- 465.15 nm. Diffraction gratings for this spectrometer have been produced mechanically in Sate Vavilov Optical Institute, St. Petersburg, Russia, by EA Yakovlev. Gratings show good spectral and energetic characteristics at previous laboratory tests and will be used in spectrometer, which is under mounting now. Area of these gratings is limited by mechanical way of production. The limit size is 50 by 50 mm. To detect weak signals it could be good to increase the are of gratings. To do it we try to calculate optical mounting of recording of these gratings holography. Since classical method of recording using homocentric beams which go to the grating blank from the same side of it does not provide aberration compensation conditions, we calculated recording mounting using recording in opposite directed beams.
The general theory of diffraction grating was developed in 1974. Although this theory is in wide use, not all the problems associated with the theory have been resolved. We began our work from the particular solution of the problem, which allows us to compensate four aberrations instead of three. For recording of the grating with the compensation of the four aberrations it is necessary to use beams from opposite sides of the blanks. One of the recording beams, going from the back side of the grating blank should be convergent. To form this convergent beam some objective should be used. This objective should not introduce additional aberrations into the system. We suggest to produce objective holographically, like another diffraction grating, made in traditional mounting of recording. This two-steps recording system has six free parameters instead of three in the traditional one. Theoretically it allows to compensate more than four aberrations. Another advantage of this method of recording is in improving of the energetic characteristics since grating have the triangular form of the grooves.
High - resolution spectrometer was designed to resolve the fine structure of discharge emitted radiation near hydrogenous line 4648.8 Å. To achieve high resolution with relatively big aperture and only spherical optics the new method of the optical mounting calculation was applied. As it was mentioned1, some aberrations of the first part of the double monochromator can be compensated by its second part. For present device we chose the Rowland circle geometry for one wavelength. This geometry has no defocusing and meridional coma aberrations. The sagittal coma and the first order astigmatism were compensated using double monochromator mounting. To reduce aberrations for other wavelength of the spectral region the slight nonequidistancy of the grooves of two concave diffraction gratings was introduced. The device consists of two concave diffraction gratings with 2700 grooves per mm and the radius of curvature 500 mm. The theoretical limit of resolution for this case is 0.015 Å. The aberration limit of resolution, calculated using the mathematical model of spectrometer is 0.006 - 0.03 Å for the spectral region 4641.8 - 4661.6 Å. Because of property of the double monochromator mounting to compensate the second order astigmatism aberration1, the entrance slit of spectrometer can be high, more than 10-20 mm. Then, it is possible to analyse a number of emitting points simultaneously.
The latest advances in the field of holographic gratings and spectral devices is in calculation, manufacture and use of these gratings for spectral devices. The general theory of diffraction grating was developed in 1974. Although this theory is in wide use, not all the problems associated with the theory have been resolved. Theoretical calculations show that this is possible using a more complicated mounting of recording the grating. For recording of the grating with the compensation of the four aberrations it is necessary to use beams from opposite sides of the blanks. To examine this method special mathematical model was found. It is based on the ray tracing calculation, but includes two steps recording and the refraction in the glass blank. In this work we represent a system of nonhomocentric recording, which doesn't include aspheric or refractive optics, mathematical model of this system, spectral devices, which can be produced with the gratings, recorded in our system and the results of the mathematical model experiments with concrete examples of those devices.
The concave diffraction grating is both the dispersive and the focusing element at the same time. It can be the only optical unit of monochromator or polychromator. Using the concave diffraction gratings with nonequidistant and curved grooves gives the possibility for correction of the aberrations in the useful region of spectrum and provides the devices with determined focal surfaces. To increase the height of the entrance slit of the spectroscopic device we have to eliminate the first and the second-order astigmatism aberrations. Consideration of this type of aberration is very important now in view of the new types of spectral devices using fiber optics and multielement detectors being developed. These new elements allow us to register the spectrum of extended objects or a number of spectrums simultaneously. For the case of the double monochromator we noticed, that the second-order astigmatism can be completely eliminated if the second part of the double monochromator is equivalent to its first part, but the ray tracing is inverse. The experiment on the mathematical model of the double monochromator confirms this idea. For the case of polychromator or CCD spectrometer we can compensate that aberrations using the illumination system, consists of the spherical mirror. The angle of incidence of the light to the mirror is calculated such a way, that the astigmatism of the grating is compensated by the astigmatism of the mirror.
The utilization of wide range spectrometers is a very important feature for the design of optical diagnostics. This paper describes an innovative approach, based on charged coupled device, which allows to analyze different spectral intervals with the same diffraction grating. The spectral interval is varied by changing the position of the entrance slit when the grating is stationary. The optical system can also include a spherical mirror. In this case the geometric position of the mirror is calculated aiming at compensating the first order astigmatism and the meridional coma of the grating. This device is planned to be used in Thomson scattering diagnostic of the TOKAMAK of Instituto Superior Tecnico, Lisbon (ISTTOK).