3.1

SRW requirements

In Table 2, the optical performance requirements for SRW are listed in terms of transmittance at the scientific (lightning) band and the SRW Total Energy Transmitted (TET_SRW), Reflected (TER_SRW) and Absorbed (TEA_SRW) values corresponding to the transmitted, reflected and absorbed energy in the 200 to 2000 nm spectral range are calculated as follows:

Table 2

SRW optical performance and ghost minimization requirements. All requirements must be met up to 16.3° AOI, except the polarization sensitivity that is limited to 5.1°.

where S(λ) is the Planck distribution for an equivalent black body at 5780°K.

The SRW filter must additionally be designed to minimize the ghost generation in the 777.41 – 777.75 nm range for all angles between 0 and 16.3°, as well as minimizing the polarization sensitivity (Table 2). The ghost generation (Ghost1), defined by Eq. 4, is caused by internal reflections between the filter surfaces where R_A and R_B are the average reflection coefficients of the A and B filter surfaces.

The polarization sensitivity (PS), is defined as follows (same equation for the NBF). :

where T_P(λ) and T_S(λ) are the filter transmittance for the p- and s-polarization at any given λ.

It is critical that the SRW can survive temperatures from -100°C to +150°C and meet all optical performance specifications in Table 2 over the -10°C to +70°C temperature range, in vacuum.

3.2

NBF requirements

The NBF, placed between the SRW and the first lens, performs the spectral discrimination of the lightning pulse from the Earth background radiance. It is supported by the lens mounting system so its operative thermal range is the same as the rest of the optical system.

Table 3 shows the NBF optical performance and ghosting requirements. The transmittance in the scientific band (T_NBF) is defined as follows:

Table 3

NBF optical performance, ghosting and polarization sensitivity requirements. All parameters must be met up to 5.1° AOI.

where λ₁ = 777.41 nm, λ₂ = 777.63 nm, λ₃ = 777.75 nm.

The equivalent bandwidth (EB_NBF) is defined as follows:

where the spectral integral bounds have been made sufficiently wide to allow the optimization of the coating design regardless of the proposed pass band shape.

The ideal NBF bandwidth would be no larger than the scientific bandwidth itself, i.e., ~0.34 nm. However, the actual bandwidth is specified ~1.9 nm to guarantee the required transmittance over the operating AOI (0° to 5.1°) range and the temperature range (+20°C to +40°C), which can spectral shift the NBF and also considering coating manufacturing errors (coating thickness non-uniformity and CWL centering), and lifetime aging.

The out-of-band rejection requirements for the combined performance of the fully coated SRW and NBF (including substrate contributions) are specified in Table 4. The two requirements are: (1) the mean out-of-band transmittance to minimize the background signal out-of-band in the detector sensitivity range and (2) the maximum out-of-band transmittance to avoid any significant pulse spectral radiance contribution out of the required emission lines.

Table 4

Combined out of band transmittance requirements

The Combined Out-of-Band Mean Transmittance is calculated as follows:

4.1

SRW design considerations

The SRW multilayer coating must reflect light over a wide wavelength range and is therefore relatively thick. Based on the required optical performance values in Table 2, the total metric thickness of the SRW filter design was ~40 μm. In addition, a two-sided design approach, which balances the coating thickness between the two surfaces, was adopted for the SRW filter to ensure compliance for the surface figure. The drawback to this balanced design approach is that it increases the risk of ghosted images.

4.2

NBF design considerations

The NBF Side A design is based on a classical multi-cavity Fabry Perot structure, where each cavity consists of two ‘reflectance’ stacks sandwiching a ‘spacer’ layer [5]. The reflectance stack consists of alternating low and high index materials, each a quarter-wave optical thickness referenced to the bandpass wavelength. The spacer layer typically consists of low or high index layers which are a multiple of a half-wave optical thickness, again referenced to the bandpass wavelength. The use of quarter-wave or half-wave optical thickness layers allows the NBF to be deposited using single wavelength monitoring (i.e., extrema monitoring). The NBF Side B design is an anti-reflection (AR) coating to maximize the NBF passband transmittance.

For this NBF filter, a complex modeling scheme was developed to track all optical performance requirements listed in Table 3 versus a wavelength shift (Figure 2). Min Tx, is the minimum transmittance in the pass band range and is an important consideration for the ghost requirement. Since absorption at the NBF CWL is minimal for the chosen substrate and material pair, the reflectance is, to first order, (1-T). The modelling indicates that the 0.45 nm NBF wavelength budget available requires relaxation of the polarization sensitivity requirement to ~0.04. Also, the ghost requirement will be met by ensuring the worst-case reflectance from the NBF Side A filter is less than 10% and the NBF Side B reflectance is less than 0.05%.

Figure 2

Wavelength tolerancing for the optical performance requirements of the NBF

5.1

SRW fabrication and performance

The uniformity requirements of the SRW are not as stringent compared to the NBF so that a deposition system capable of depositing on multiple SRW substrates was selected. With each deposition batch, a set of witness pieces were included to be used for later durability tests. Typically, the SRW Side A is deposited first followed by the SRW Side B.

After the completion of a SRW deposition run (Side A or Side B), the SRW filter is first inspected for surface defects such as stains, scratches and large digs to ensure that SRW coated surface passes the ISO 10110-7 surface imperfection specifications with a grade of 5/3x0.25; L2x0.02. Next, the SRW filter is spatially mapped across the surface by measuring the transmittance from 740 to 820 nm at normal incidence to provide a preliminary performance assessment and to evaluate the thickness uniformity. These normal incidence transmittance measurements can then be compared to modeled theory calculations to confirm the accuracy of the deposited multilayer coating and to ensure, indirectly, that the entire optical performance is met across the SRW clear aperture.

After both sides of the SRW filter are deposited, the SRW filter is then annealed at 225°C for several hours to accelerate the aging process and to ensure that its optical performance has a long-term (i.e., 15 years) stability based on the storage and survival temperature ranges.

The transmittance and reflectance of each SRW filter was also measured over a wavelength range from 200 to 10,000 nm at several angles of incidence between 0 and 16.3° and with s- and/or p-polarized light. Figure 3 shows the SRW transmittance and reflectance measurements over a wavelength range from 200 to 2,000 nm at an AOI of 16.3° made with a Agilent CARY 7000 spectrophotometer to validate design model. This angle was chosen as it is the largest angle required in the validation. For each SRW filter, measurements at other specified AOI’s were made to confirm the agreement with design. Figure 3 shows the measured (solid curves) and calculated (dashed curves) transmittance and reflectance for both s- and p-polarized light. The wavelength range is too large to clearly see the match (or mismatch) between the measured and calculated curves, hence the wavelength range subsets shown. The agreement is generally very good, except for the reflectance in some wavelength regions as a result of beam “walk-off”. What happens here is that the beam can “walk-off” to the edge of the filter due to multiple reflections between the two surfaces of the SRW substrate, a consequence of the two-sided design. Therefore, the light at some wavelengths is ‘guided’ along the two surfaces until it escapes at the edge of the filter – as a result this light is not captured by the detector giving rise to a discrepancy between the measured and theoretical curves (Figure 4).

Figure 3

SRW Measured (solid) and calculated (dashed) transmittance and reflectance curves at 16.3° for both s- and p-polarized light.

Figure 4

Top view of a witness under test in the spectrophotometer instrument. One can see the light escaping at the edge of the substrate after bouncing back and forth from the substrate center position where it entered.

Using various measurement curves and the corresponding calculated curves, the T_min, TET, TER, TES, AET, AER, AEA and polarization sensitivity of the filter at 0° and 6.5° and 16.3° were estimated. In Table 5 are shown the results of these calculations which demonstrates that all the SRW filter requirements were met. The calculations assumed a 4 nm spectral band width and 3° half-cone angles, corresponding to the measurement conditions.

Table 5

Comparison of the filter (full size) measured and calculated optical performance quantities for AOI of 0°, 6.5° and 16.3°. Measured values are shown in bold fonts.

The uniformity of the SRW filter was evaluated from the center wavelength of the bandpass estimated at each spatially measured location. The maximum deviation over the 126-mm diameter area is +0.32% and +0.00%, respectively (±0.16%). Applying these scaling factors to the layer thicknesses of the SRW multilayer model allows the estimation of the optical performance at the extreme cases; i.e., at 0° AOI for the largest positive deviation and 16.3° AOI for the largest negative deviation. Table 6 summarizes these calculations and shows that the SRW filter meets all requirements at the extremes of the uniformity variations, temperature, and aging, over a 126-mm diameter area (which is larger than the desired 120 mm diameter effective aperture).

Table 6

Calculated optical performance quantities for the extremes of temperature and angle combinations, i.e. lowest temperatures (-20°C) with largest AOI (16.3°) and highest temperature (+80°C) with smallest AOI (0°).

5.2

NBF fabrication and performance

The Side B of the NBF filter is deposited on the same deposition platform used to deposit the SRW Side A and Side B coatings. As mentioned previously, the NBF Side B design is an AR coating and it does not have stringent uniformity requirements as it is intended to maximize the NBF passband transmittance.

The Side A of the NBF filters does have stringent uniformity requirements over the NBF effective aperture. In order to achieve this uniformity, a different deposition platform was selected that can coat only one NBF filter at a time (along with the required witness pieces for later durability tests). With the quarter-wave/half-wave design structure of the Fabry-Perot filter (with FWHM ~ 1.4 nm), it is possible to use a single wavelength extremum monitoring method which has a well-known automatic error compensation mechanism to allow over/under-shoots in the thickness of one layer to be compensated by the following layer [5,6]. The uniformity of the deposited coating was optimized by fine-tuning the deposition geometry of the deposition. Great care was taken when loading a NBF substrate into the deposition chamber to achieve a reproducible substrate placement relative to the targets and to minimize substrate wobble (which can affect azimuthal uniformity).

The optical monitoring was performed with a laser operating at a wavelength slightly lower than the required passband CWL at room temperature. This allowed the CWL of the NBF filter to be fine-tuned by annealing the filter in a controlled manner. This annealing step also improves the long-term stability as previously described for the SRW filter.

The transmittance measurements for the NBF are challenging because of the narrowness of the bandpass and instruments with superior spectral and angular resolution are needed to correctly measure the transmittance peak shape. Typically, instruments that can achieve such spectral and angular resolution are usually limited to normal incidence and unpolarized transmittance measurements. However, with these instruments it is possible to accurately “map” the entire filter area and assess the distribution of the optical performance over the filter effective aperture, and calculate tolerances based on ‘shift-width’ estimations (see below) for all other operating conditions. To verify these results, other measurements are performed at oblique AOI’s, over wider wavelength ranges and at different temperatures but only on a witness pieces or at a single location on the filter.

The normal angle transmittance measurements were made on the NBF filter using a 4 mm x 4 mm grid step pattern. From this, a subset of the above spatially mapped measured points is then used to evaluate the uniformity and the ‘shift - width’ analysis: i.e., those located at every 10 mm along radial lines separated by 30° each, resulting in a total of 69 points after eliminating overlapping measurement points near the center of radius.

Figure 5 presents the center wavelength (CWL) values obtained from the 4 mm x 4 mm grid step mapped points based on their radial distance. The CWL values were normalized to the value obtained at the center of the substrate (radius of 0 mm). The red line in the chart indicates the edge of the effective aperture (∅114 mm). Up to a radius of 57 mm, the effective aperture limit, the highest and lowest values are separated by 0.014%, so the NBF CWL uniformity is ±0.007% over the effective aperture.

Figure 5

NBF Filter - Normalized-CWL versus radial position distribution over the full effective area of the filter, post anneal-tuning and edging. The red line indicates the edge of the effective area.

To determine whether an NBF filter is passing or failing based on 0°-incidence measurements, the following three-step methodology (“shift-width” analysis) was carried out:

This process can be repeated for different values of T_NBF and EB_NBF to determine the best performance that the filter area can meet. It is important to note that any variability in the peak transmittance and bandwidth is considered when performing the shift width analysis over the entire area of the filter.

For the filter shown in Figure 6, over the 69-point grid, all points are simultaneously passing T_NBF (>87%) and E_BNBF (<1.94 nm). Note that Figure 6 shows all 69 curves with the two extremes, corresponding to the lowest and highest wavelength on the measured filter area, highlighted in red.

Figure 6

Transmittance curves for one QM NBF filter from the 69 points map with highest and lowest measured wavelength measured curve.

From this measurement it was determined that the maximum negative shift (Δλ_min) was -0.042 nm and the maximum positive shift width (Δλ_max) was +0.096 nm after correcting for angle tuning, lifetime aging and operating temperature from +20 to +40°C. Therefore, the filter area has a shift width of 0.138 nm. This shift width is a measure of the wavelength tolerance on the part. These values have an uncertainty of approximately ±0.010 nm.