Wide Infrared Bandpass Filter
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This coating is for light at normal incidence on a germanium substrate (index 4). The requirements are:
A. Transmittance99% for wavelengths 3300-5000 nm
C. Transmittance Second, starting with a single thin layer, the needle/tunneling method was used to create a 17-layer short-wave pass filter using requirements B and C. The performance is shown below.
http://www.sspectra.com/designs/shortpass1.gif
Third, experience shows that it is best to place the short-wave pass filter closest to the substrate and then to append the long-wave pass filter. (The needle/tunneling method "discovers" this fact when the brute-force approach is used.) Before optimizing, the performance is:
http://www.sspectra.com/designs/bandpass1.gif
After optimizing this design, the final performance is:
http://www.sspectra.com/designs/bandpass2.gif
Here are the designs, with the first layer closest to the substrate and thickness given in nm. With a little more work, it may be possible to eliminated some thin layers.
Short Long Bandpass
ZnS 83.39 97.33
Ge 48.25 48.60
ZnS756.31 761.47
Ge 403.51 412.85
ZnS710.33 720.06
Ge 377.93 382.28
ZnS696.05 705.03
Ge 370.17 370.42
ZnS696.91 709.26
Ge 365.05 358.23
ZnS705.00 718.52
Ge 361.44 353.08
ZnS719.03 724.86
Ge 360.53 360.01
ZnS751.56 710.47
Ge 328.61 398.52
ZnS369.01 158.71 564.95
Ge 124.38 40.79
ZnS 260.14 224.72
Ge 78.55 125.31
ZnS 149.33 133.58
Ge 102.25 98.28
ZnS 235.97 268.21
Ge 145.53 138.25
ZnS 279.07 238.01
Ge 128.80 125.48
ZnS 196.52 232.65
Ge 67.11 68.54
ZnS 159.26 168.55
Ge 132.16 150.14
ZnS 271.50 254.28
Ge 144.32 125.25
ZnS 281.93 307.19
Ge 149.33 165.16
ZnS 278.65 256.22
Ge 140.79 133.04
ZnS 271.93 289.60
Ge 147.16 147.63
ZnS 276.89 266.04
Ge 138.39 134.34
ZnS 271.52 265.60
Ge 152.12 156.86
ZnS 291.92 294.15
Ge 135.69 123.17
ZnS 249.93 250.12
Ge 166.98 178.96
ZnS 553.73 528.64
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