3.1

Instrument with one grating spectrometer, used in the 1st diffraction order

Grating optimization:

As mentioned before a monoblazed grating cannot cover efficiently the whole spectral band [400,2500nm] with diffraction order #1: diffraction efficiency is 100% for the blazed wavelength, but drops on the 2 edges of the spectral band. The goal is to modify the grating blazed profile in order to balance efficiency all over the spectral band, at the expense of the peak value. More specifically we put more weight on the higher wavelengths, for which scene luminance is lower than for the VIS wavelengths.

This python optimization led to the profile shown below, and the related diffraction efficiency for the first diffraction orders:

Figure 4.

Multiblazed grating profile optimized for the [400,2500nm] spectral band, diffraction efficiency for this multiblazed grating. The five wavelengths used for the multiblazed profile are: [623, 623, 883, 2965, 3504] nm

The simplified scalar model is very convenient to quickly optimize the grating profile. A more detailed model, such as the one used by software PC grate, is more realistic, and takes the polarization into account.

Figure below shows the two models fit correctly, and that the scalar approximation is valid as wavelength is smaller than grating period. At shorter wavelengths the two models do not agree well and further investigation shall be made in order to optimize the diffraction efficiency in this region. Ref [4] shows such a grating is possible.

Figure 5.

Multiblazed grating profile optimized for the [400,2500nm] spectral band: comparison of scalar model and PC grate model

Table 1.

The instrument key parameters are calculated from SNR budget and taking into account the grating efficiency:

(*) EPD = Entrance Pupil Diameter

Optical design:

In order to achieve the UV SNR specification a large pupil (diameter 350mm) has to be accomodated.

This leads to a very large telescope, whereas the spectrometer volume is much smaller, whatever the type of design: Dyson spectrometer or Offner spectrometer.

In order to optimize the telescope volume the best choice is to design a 3 mirrors telescope, with freeform mirrors.

In the case of the Offner spectrometer, the volume of the spectrometer is optimized with 2 freeform mirrors. The convex grating is ruled on a spherical surface: choosing a freeform grating marginally reduces the global volume of the instrument, and keeping a spherical grating is best for manufacturability.

As for the Dyson spectrometer a spherical grating permits a quite compact spectrometer. Aspherizing it does not help in reducing notably the spectrometer volume, which is, again, negligible vs. telescope volume.

Figure 6.

Telescope for the first instrument option: 3 freeform mirrors, volume = 100 liters

Table 2.

Offner spectrometer and Dyson spectrometer comparison for the first instrument option

An OSF (Order Sorting Filters) will be placed just before the FPA, in order to decrease the level of straylight due to neighbor grating diffraction orders.

The Dyson shall be folded before the FPA, in order to accommodate the detector.

The figure below gives a view of the telescope and the Offner spectrometer:

Figure 7.

instrument for the first instrument option

3.2

Instrument with one grating spectrometer, using two diffraction orders

Grating optimization

The previous design volume is quite big: its large entrance pupil compensates the low grating diffraction efficiency.

As waveband covers more than 2 decades it is possible to dedicate shorter wavelengths to grating diffraction order 2, and longer ones to diffraction order 1. The waveband is therefore split into two sub wavebands, imaged on two dedicated detectors. The limit between the 2 wavebands is fixed at 1000nm, so that a silicon detector is able to image lower wavelengths. With this option we get better grating diffraction efficiency:

Figure 8.

Multiblazed grating profile optimized for the [400,2500nm] spectral band. [400,1000nm] spectral band uses diffraction order number 2, [1000,2500nm] spectral band uses diffraction order number 1. On the right: Diffraction efficiency for this multiblazed grating. The five wavelengths used for the multiblazed profile are: [807, 829, 1565, 1869, 1974]nm

As diffraction efficiency gets better in the SWIR waveband, it is possible to work with a smaller entrance pupil instrument: entrance pupil diameter is now 300mm:

Table 3.

The instrument key parameters are calculated from SNR budget and taking into account the grating efficiency:

In this case, pupil size is well balanced between UV and SWIR SNR needs.

Optical design:

Telescope:

Spectrometer:

The two useful grating orders are superimposed over spectral range and split before focal planes using a Dichroic Beam Splitter (DBS). The DBS reflects wavelengths shorter than 1000nm, and transmits wavelengths larger than 1000nm. The two sub wavebands are then imaged on a dedicated detector.

The spectrometer is an Offner spectrometer. Indeed it would be very difficult to accommodate the DBS in a Dyson spectrometer:

Figure 9.

spectrometer for the second instrument option: 2 freeform mirrors, spherical convex grating with period 35μm. Separation VIS/SWIR with a dichroic plate. Spectrometer volume is 1.5 liters

The spectral resolution is 10 nm over SWIR band and 5 nm over VNIR band. VNIR SNR is thus computed using binning over two spectral samples. It may be interesting to have access to this improved spectral resolution of 5 nm, even with degraded SNR.

The figure below gives a view of the telescope and the Offner spectrometer:

Figure 10.

telescope + spectrometer for the second instrument option

3.3

Instrument with two grating spectrometers

To further improve compactness, and thus grating diffraction efficiency another option is to use two spectrometers, with two gratings: one spectrometer dedicated to VIS ([400-1000]nm), and one spectrometer dedicated to SWIR wavelengths ([1000-2500]nm). It is possible to drastically improve gratings’ diffraction efficiency:

Grating optimization:

Grating for VIS spectrometer ([400-1000]nm):

A monoblazed grating calculated the wavelength 580nm gives:

Figure 11.

Diffraction efficiency with scalar model of a monoblazed grating over the [400-1000nm] spectral band. Blazed wavelength is 580 nm.

A few trials showed that it is not possible to get a better efficiency with the use of a multiblazed grating. Besides it is better to use a classical monoblazed grating if possible, for cost reasons.

Grating for SWIR spectrometer ([1000-2500]nm):

Multiblazed profile is necessary so that grating diffraction efficiency gets higher for wavelengths with the smallest luminance (the longer the wavelength, the larger the diffraction efficiency shall be).

Multiblazed grating profile, and the related diffraction efficiency is given in the plots below:

Figure 12.

Multiblazed grating profile optimized for the [1000,2500nm] spectral band. On the right: Diffraction efficiency for this multiblazed grating. The five wavelengths used for the multiblazed profile are: [911, 1579, 1590, 2784, 3953] nm

Optical design:

Splitting can be implemented at slit level, with spectral splitting using a Dichroic Beam Splitter or with spatial splitting using two slits in the telescope focal plane. The « in-the-field » splitting increases telescope ALT field of view, but saves a dichroic beam splitter and its related drop of transmission. Polarization sensitivity is also improved: this option is preferred.

Table 4.

The instrument key parameters are calculated from SNR budget and taking into account the grating efficiency:

With these data and some margin, instrument dimensioning gives Entrance Pupil Diameter of 236mm to reach the SNR specification at 2200nm.

The telescope design is similar to the 2 cases described before. The volume is optimized, thanks to a smaller entrance pupil size.

Telescope volume is 50 liters

The gap between the two slits is set to 1.5 mm in order to ease separation of the 2 sub wavebands.

The spectrometers are Dyson spectrometers: as we need two such spectrometers the Dyson has the advantage to offer the smallest volume, and the smallest number of optical components.

Each Dyson spectrometer includes a silica plano-convex lens, with an aspherical convex face, and a concave grating.

Table 5.

VIS and SWIR Dyson spectrometers characteristics:

The SWIR spectrometer tends to be longer than the VIS spectrometer, due to the larger waveband it covers. Figure below depicts the optical layout of the whole instrument for this concept:

Figure 13.

instrument for the third option: one telescope and two Dyson spectrometers for the VIS & SWIR sub spectral bands. On the right: zoom on the spectrometers

The 2 wavebands are split by using small prisms after the slit plane. The SWIR path is thus folded, in order to optimize the instrument global volume:

OSF (Order Sorting Filters) are implemented in front of each detector.