{"id":17640,"date":"2021-04-13T23:46:36","date_gmt":"2021-04-14T03:46:36","guid":{"rendered":"https:\/\/iridian.com.cn\/?page_id=17640"},"modified":"2021-05-05T16:45:28","modified_gmt":"2021-05-05T20:45:28","slug":"large-format-narrow-bandpass-filters-a-uniformity-challenge-dup","status":"publish","type":"post","link":"https:\/\/iridian.com.cn\/en\/learning_center\/large-format-narrow-bandpass-filters-a-uniformity-challenge-dup\/","title":{"rendered":"Large Format Narrow Bandpass Filters – A Uniformity Challenge"},"content":{"rendered":"

1\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Introduction<\/h2>\n

Large format (>100 mm diameter), narrow bandpass filters (NBPF) are required in many fields. Applications requiring a large field of view drive the need for large collection optics, however high wavelength selectivity provided by narrow, flat-top bandpass optical filtering is also required to facilitate specific and selective analysis of a phenomena or substance of interest.\u00a0 The diversity of applications of these optics include earth observation remote sensing (whether satellite imaging or UAV mounted LIDAR), astronomical\/solar imaging, UV micro-lithography tools, and live animal biological (fluorescence) imaging.<\/p>\n

Different applications can have different demands on an optical filter\u2019s wavelength selectivity \u2013 the characteristic of the filter defined by range and level of wavelengths transmitted by the filter (the passband) and the range and level of wavelengths rejected by the filter (the blocking band) \u2013 in order to optimize the signal to noise ratio at the detector of a system.\u00a0 Filters need to be designed and manufactured to achieve the desired wavelength selectivity over the entire clear aperture (CA) of the optic and over the entire range of operating parameters including angle of incidence (AOI) and cone angle, and environmental conditions of use.<\/p>\n

In multi-layer, dielectric, thin-film filters, variations in angles of incidence and operating temperature can produce a shift in center wavelength (CWL) \u2013 increasing AOI causes the filter profile to shift to shorter wavelengths; increasing operating temperature typically causes a shift in the filter profile to longer wavelengths.\u00a0 These wavelength shifts need to be accounted for in the filter design and manufacturing processes and contribute to an increased requirement for steep transitions from passbands to blocking bands.<\/p>\n

Each of these requirements for additional design \u201cmargin\u201d can make the successful manufacture of large NBPF a particular challenge.\u00a0\u00a0 To achieve this both superior precision (uniformity and control of filter shape) and superior accuracy (control of center wavelength positioning) are required.<\/p>\n

Additionally, many applications, particularly those in remote environments (e.g. deployed in satellite instrumentation), require that the filters maintain their spectral performance characteristics over many years. This requires a very robust filter design and manufacturing strategy along with careful selection and treatment of materials used in production.<\/p>\n

Iridian has addressed each of these challenges and has recently demonstrated the successful design and manufacture of an environmentally stable 1.72\u00a0nm-wide (FWHM) NBPF centered near 780\u00a0nm to within 20\u00a0pm of the target over an operating clear aperture >125\u00a0mm in diameter.<\/p>\n

2\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0How Much Better is this?<\/h2>\n

Excellent uniformity control of spectral performance is desirable in all applications of optical filter manufacturing as it increases the coated area that meets specifications.\u00a0 In the case of small format filters, such as \u00bd\u201d to 1\u201d spectroscopy filters or very small (~1.5\u00a0x\u00a01.5\u00a0mm2<\/sup>) filters for fiber-optic telecommunications systems, a broad spectral uniformity can help increase the number of parts yielded from a coating run and therefore reduce the cost per filter.\u00a0 In the case of very large optics (>100\u00a0mm in diameter) the requirement for highly uniform coatings becomes critical to the functionality that can be achieved as uniformity governs the size of clear aperture and\/or degree of wavelength selectivity that can be delivered.<\/p>\n

2.1\u00a0\u00a0\u00a0\u00a0\u00a0 Precision and accuracy<\/h3>\n

Typical high-end commercial filters demonstrate a uniformity over >100\u00a0mm of ~0.1-0.2% of the center wavelength.\u00a0 The filter described here is coated to within a uniformity of <0.02% of the center wavelength over a clear aperture diameter of >125\u00a0mm \u2013 nearly an order of magnitude improvement in uniformity over the existing high-end benchmark.<\/p>\n

\"\"<\/p>\n

With any very narrow band filter uniformity and filter shape are not sufficient to ensure excellent functional performance.\u00a0 Offset in center wavelength targeting can result in loss of signal if the peak of the NBPF is not aligned with the spectral line of interest- be it a laser wavelength or absorption peak of an analyte of interest.\u00a0 Typical optical filter manufacturing can target small (<1\u201d) filters to within \u00b1100\u00a0pm of the target CWL.\u00a0 The large NBPF created in this work was manufactured to match the target CWL to within \u00b110\u00a0pm (averaged over the clear aperture).<\/p>\n

\"\"<\/p>\n\n\n\n\n\n\n
Change in CWL (uniformity)<\/td>\n0.094nm<\/td>\n\u0394 \u2264 0.013%<\/td>\n<\/tr>\n
Change in Bandwidth (FWHM)<\/td>\n1.705nm \u2013 1.735nm<\/td>\n\u0394 \u2264 1.8%<\/td>\n<\/tr>\n
CWL targeting<\/td>\n\u00b110 pm<\/td>\n\u0394 \u2264 0.003%<\/td>\n<\/tr>\n
Change in peak Transmittance<\/td>\n98.70% \u2013 99.52%<\/td>\n\u0394 \u2264 0.82%<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

2.2\u00a0\u00a0\u00a0\u00a0\u00a0 Durability, Reliability, and Stability<\/h3>\n

Having achieved a remarkable performance level is of no use if the filter cannot survive processing or if it degrades in operational environmental use conditions.\u00a0 The coating process (energetic magnetron sputtering) and materials used inherently produce robust and reliable coatings.\u00a0 However, in pushing the deposition conditions and annealing processes we were unsure that we would achieve our typical reliability and durability for this filter.<\/p>\n

Nonetheless, the filter successfully passed a series of prescribed environmental durability tests such as mild abrasion (50 strokes cheesecloth at 5N force), solubility (acetone and ethanol dip for 5 minutes), humidity (55 \u00b0C and 95% RH for 48 hours), multiple adhesion (rapid tape removal), and thermal vacuum cycling (30 cycles between \u00b190 \u00b0C with 30 min dwell time at each extreme). Additionally, the filter survived without change post-coating sizing (coring) which is a very harsh test for adhesion and filter robustness (it will never experience any similar abuse in use).<\/p>\n

Lifetime stability (minimization of change in performance over time and temperature) was also a critical parameter for this filter.\u00a0 Thermal aging testing (damp heat) models indicates that this filter would shift by <10\u00a0pm in CWL if used at 85 \u00b0C for 10 years.\u00a0 Additionally, the stability in performance between use in air and in vacuum was tested as other technologies such as evaporative coating can exhibit shifts between air and vacuum.\u00a0 No measurable change in CWL between air and vacuum was detected to within the measurement accuracy of the test instruments (\u00b120\u00a0pm).<\/p>\n

3\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Design and Manufacturing Considerations<\/h2>\n

To produce filters with a steep, narrow bandpass at a fixed center wavelength over a large clear aperture the deposited film quality has to be homogenous across the surface and have low scatter\/haze to maximize signal to noise.\u00a0 Additionally, uniformity stability is needed throughout the coating run to maintain the desired filter shape.\u00a0 To produce filters with these characteristics careful consideration was necessary with respect to coating target and substrate positions and relative motions, gas flow distribution, in situ temperature control, real-time in situ single wavelength monitoring (SWM), and stability of the coating process.<\/p>\n\n\n\n\n\n\n\n
Parameter<\/td>\nInfluenced by:<\/td>\n<\/tr>\n
Film homogeneity<\/td>\nTemperature control, target and substrate position and motion<\/td>\n<\/tr>\n
Thickness homogeneity<\/td>\nTarget and substrate position and motion<\/td>\n<\/tr>\n
Scatter\/haze\/transmission loss<\/td>\nGas flow, temperature control, target and substrate position and motion<\/td>\n<\/tr>\n
Filter band shape<\/td>\nSingle wavelength monitoring, coating process stability<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

To achieve the CWL accuracy of within \u00b110pm of the target CWL it was necessary to develop a thermal annealing setup that would ensure uniform annealing over the entire surface of this large, thick filter (large thermal mass).\u00a0 Since this filter design shifts to longer wavelengths with annealing it was necessary to deliberately target a shorter CWL out of the coater to allow for the CWL of the filter to be iteratively pushed to slightly longer wavelengths with each anneal cycle.\u00a0\u00a0 Additionally, the annealing process was necessary to stabilize the filter against further thermally induced wavelength shifts with use in its operating environment.<\/p>\n

The ability to accurately characterize the spectral performance is critical both as a feedback loop into the annealing processes above and to ensure that final product meets all customer specifications.\u00a0 Measurement uncertainties in AOI, degree of collimation (cone angle) of interrogating beams and analyzing optics, and filter temperature while under test each contribute to the over-design margin needed to guarantee spectral compliance.\u00a0 By minimizing the uncertainties in these aspects of the measurement system it was possible to minimize the over-design needed to \u201cpad\u201d the specs and maximize the over-design margin allowed for variability in the other manufacturing processes (annealing and coating).\u00a0 Lastly to ensure that the spectral characteristics were met over the entire 125\u00a0mm clear aperture a custom-built measurement set-up providing x-y spectral mapping of the part with a fully collimated beam path (collimated launch and catch optics) was employed.<\/p>\n

4\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Conclusion<\/h2>\n

By employing and optimizing the design and manufacturing techniques described above Iridian successfully produced an environmentally stable narrow (1.72nm FWHM) band-pass filter centered to within \u00b110pm of the target CWL (~780nm) over a clear aperture of 125\u00a0mm in diameter.\u00a0 This filter demonstrates Iridian\u2019s ability to produce state-of-the-art performance large NBPF to a uniformity variation of <0.02%.<\/p>\n

The above mentioned NBPF has been developed under a contract with Leonardo S.p.a for the Lightning Imager Instrument, in the frame of the ESA program Meteosat Third Generation (MTG), with THALES ALENIA SPACE France as prime contractor.<\/p>\n","protected":false},"excerpt":{"rendered":"

1\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 Introduction Large format (>100 mm diameter), narrow bandpass filters (NBPF) are required in many fields. Applications requiring a large field of view drive the need for large collection optics, however high wavelength selectivity provided by narrow, flat-top bandpass optical filtering is also required to facilitate specific and selective analysis of a phenomena or substance […]<\/p>\n","protected":false},"author":159,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"_et_pb_use_builder":"","_et_pb_old_content":"","_et_gb_content_width":"","footnotes":""},"categories":[2205,2211],"tags":[],"class_list":["post-17640","post","type-post","status-publish","format-standard","hentry","category-learning_center","category-tech_notes"],"acf":[],"_links":{"self":[{"href":"https:\/\/iridian.com.cn\/en\/wp-json\/wp\/v2\/posts\/17640","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/iridian.com.cn\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/iridian.com.cn\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/iridian.com.cn\/en\/wp-json\/wp\/v2\/users\/159"}],"replies":[{"embeddable":true,"href":"https:\/\/iridian.com.cn\/en\/wp-json\/wp\/v2\/comments?post=17640"}],"version-history":[{"count":3,"href":"https:\/\/iridian.com.cn\/en\/wp-json\/wp\/v2\/posts\/17640\/revisions"}],"predecessor-version":[{"id":17912,"href":"https:\/\/iridian.com.cn\/en\/wp-json\/wp\/v2\/posts\/17640\/revisions\/17912"}],"wp:attachment":[{"href":"https:\/\/iridian.com.cn\/en\/wp-json\/wp\/v2\/media?parent=17640"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/iridian.com.cn\/en\/wp-json\/wp\/v2\/categories?post=17640"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/iridian.com.cn\/en\/wp-json\/wp\/v2\/tags?post=17640"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}