![\"SNR=Psigmal\/Pnoise\"](\"https:\/\/www.iridian.ca\/wp-content\/uploads\/2022\/11\/CodeCogsEqn.svg\")
<\/a><\/p>\nIn an environment where the signal power \u201cis what it is\u201d (i.e. it can\u2019t be controlled or increased) the path to improving this ratio, and thus the ability to better discriminate the desired information from the undesired background, is to reduce the power of the noise \u2013 less background.<\/p>\n
Optical filters provide the wavelength selectivity that delivers \u201cmore signal, less background<\/em><\/strong>\u201d to these, and many more, photonics applications.<\/p>\n <\/p>\n One of the best examples of the role optical filters play in delivering \u201cmore signal, less background\u201d is Raman spectroscopy<\/a>.\u00a0 In Raman spectroscopy the inelastic scattering of photons that have lost, or in fewer cases gained, energy through interaction with vibrational modes of the sample of interest, provides a unique spectral fingerprint that identifies the molecules present. This non-destructive molecular analysis technique is extremely useful, but challenging to implement since only approximately one in a million photons incident on a molecule will experience Raman scattering and have the associated shift in wavelength whereas the remaining elastically scattered (Rayleigh) photons will still be present in the detection path at the same wavelength as the incident light.\u00a0 The challenge lies in extracting the spectral information (signal) of that one photon from the overwhelming large background signal.\u00a0 Fortunately, high performing wavelength selective optical filters are up to the challenge.<\/p>\n <\/p>\n In the detector path of most Raman systems, you will find an optical edge pass or notch filter.\u00a0 As passive components, optical filters can do nothing to boost the weak Raman signal although they can be designed and manufactured to minimize any loss in the Raman signal region (at a longer wavelength than the incident light).\u00a0 Their primary function, however, is to suppress the background noise at the wavelength of the incident light and they achieve this by providing broad and deep spectral blocking of the laser source, often to six or more orders of magnitude, enabling the system to achieve more signal, less background.<\/em><\/strong><\/p>\n <\/p>\n The ability to manage and control a system is dependent on the ability to monitor and measure changes in that system over time, thus providing a feedback loop to the control system.\u00a0 In the field of photonics, realization of this principle is seen in applications as diverse as machine vision, and its ability to analyze samples under test, to remote sensing <\/a>\u00a0and \u00a0earth observation<\/a> (EO) which provide satellite imaging of environmental phenomena, natural disasters such as floods and fire, and the effects of agriculture and forestry practices.<\/p>\n <\/p>\n Each of these sensing\/imaging applications relies on the spectral difference in transmission, reflection, and absorption of the materials or phenomena (such as gases) under investigation.<\/p>\n In the remote sensing example of non-dispersive infrared (NDIR) gas detection systems, a background measurement of mid-wave infrared (MWIR) light is made and then compared to the measurement in the presence of a gas of interest.\u00a0 Narrow band optical filtering<\/a> is necessary to ensure that only light from the absorption band corresponding to the gas of interest is present at the detector — more signal, less background<\/em><\/strong>.<\/span><\/p>\n EO systems consist of a camera\/detector seeking to image at a specific wavelength band (or bands in the case of multi-spectral imaging (MSI)) corresponding to the reflectivity or emission associated with the science line of interest.\u00a0 This could be a single oxygen triplet at 777.4nm associated with the presence of lightning or broader spectral bands in the visible\/near-infrared associated with different terrestrial objects such as water, rocks, or vegetation (such as ESA\u2019s so called \u201cSENTINEL-2\u201d bands<\/a>).\u00a0 <\/p>\n Dragonfly does this by using a camera array to look at the image formed by narrow emission lines given off by the diffuse, low surface brightness gas forming the intergalactic medium.\u00a0 To do so it relies on individual very narrow bandwidth (<1nm) optical filters<\/a> positioned in front of each camera lens to selectively resolve these bands from the background atmospheric and \u201cnight-sky\u201d lines.\u00a0 These filters provide much more precise, and angle tunable, wavelength selectivity to each element of Dragonfly\u2019s compound \u201ceye\u201d than conventional astronomical filters providing more signal, less background <\/em><\/strong><\/span>to this revolutionary telescope.<\/p>\n Each of the examples above has used \u201csignal\u201d to mean the image or spectral line associated with a specific phenomenon of interest.\u00a0 Optical filters for communication applications, including terrestrial telecom and datacom and extra-terrestrial satcom, literally facilitate transport and routing of information carrying communication signals.<\/p>\n In terrestrial fiber-optic based communication systems, including long-haul\/metro telecom<\/a> networks, and inside the massive cloud data server systems (datacom<\/a>) packing as much information onto a single fiber at the same time enables the bandwidth and speed necessary to support the massive amounts of communication data transported \u2013 whether for phone, internet, and video streaming by individual users or increasingly communications by and between inanimate devices making up the internet of things.<\/p>\n Wavelength division multiplexing (WDM) associates a given communication transport signal with a given wavelength band on the same fiber at the same time.\u00a0 To accomplish WDM, optical filters are often used to add, drop, and select a specific wavelength signal band of interest from the other signal bands continuing down the network to another user destination or information node.\u00a0 This selectivity can be defined by how many channels are selected without having to skip neighbouring channels to the band of interest \u2013 that is a 4 skip 0 filter<\/a> transmits four signal bands and is steep enough to reflect the next channel band without loss. More selective and steeper filters result in more signal, less background<\/em><\/strong><\/span> in the network by providing high signal and low-cross talk between signal bands.<\/p>\n 100 G DWDM 4s0 band splitter in ADD\/Drop module<\/em><\/p>\n The same principles of WDM apply in the rapidly growing market of satcom<\/a> where free-space optical communications connect constellations of satellites in orbit forming an orbiting network above.\u00a0 The optical intersatellite links (OISL) rely on WDM filters to select and direct the signal carrying optical bands from the lasers and to the corresponding detectors albeit after travelling through space between the individual satellites at distances of 1000s of kilometers.<\/p>\nOne in a million! — Spectroscopy<\/span><\/h3>\n
<\/a><\/p>\n<\/a><\/p>\nYou cannot manage what you do not measure — Remote Sensing<\/span><\/h3>\n
<\/a><\/p>\n<\/a><\/p>\n<\/a>\u00a0In all cases, the desire is to achieve high contrast – more signal, less background<\/em><\/strong><\/span> – between the light detected in the spectral band of interest and the background, including ambient light, solar background, and the light from other neighbouring spectral bands in MSI systems. This can be achieved with individual high contrast bandpass filters or a series of filters arranged as a multi-zone filter array providing the wavelength selectivity needed.<\/p>\n\u201cA long time ago, in a galaxy far, far away\u201d — Astronomy<\/span><\/h3>\n
<\/a>Rather than looking down from the skies as shown with EO, astronomy directs our attention into the great beyond.\u00a0 Searching for pattern and meaning in the night skies can be traced back to many ancient cultures and has given us the named constellations we are all familiar with.\u00a0 The Dragonfly Telephoto Array<\/a> continues this tradition by probing the skies to attempt to answer one of the biggest astrophysics questions of days; what is the nature and distribution of dark matter?<\/p>\n<\/a><\/p>\n<\/h3>\n
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\u201cMr. Watson \u2013 come here\u201d — Communications<\/span><\/h3>\n
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