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Fundamental Concepts
Published in Andrew Sarangan, Optical Thin Film Design, 2020
Besides antireflection and high-reflection filters, there are many other types of optical filters, such as long-pass, short-pass, bandpass, and line-pass filters. Long-pass filters transmit wavelengths longer than a certain value (known as the cut-on wavelength) and reflect shorter wavelengths. Short-pass filters do the opposite – they transmit everything shorter than the cut-off wavelength and reflect longer wavelengths. Bandpass filters transmit a band of wavelength and reflect everything outside that range. Line-pass filters are similar to bandpass filters, except their transmission band is much narrower, often coinciding with common laser wavelengths. An example is shown in Figure 1.3. All of these filters can be synthesized by combining the antireflection concepts with high-reflection concepts such that the reflection and transmission spectra exhibit both of these characteristics at different wavelength ranges. For example, a long-pass filter utilizes a high reflector designed for short wavelengths, and an antireflector designed for longer wavelengths, such that the two bands are adjacent to each other without interfering with each other. A bandpass filter utilizes an antireflection region with two high reflecting regions on either side of the antireflection region.
Optics Components and Electronic Equipment
Published in Vadim Backman, Adam Wax, Hao F. Zhang, A Laboratory Manual in Biophotonics, 2018
Vadim Backman, Adam Wax, Hao F. Zhang
Optical filters are used to selectively transmit certain wavelengths of incident light while blocking the rest. These filters can be divided into two groups: colored filters, which transmit light of the wavelength corresponding to the filter's color; and neutral density filters, which nonselectively block a portion of the incident light. Absorption filters also exist, which absorb selective wavelengths of light while transmitting the rest. The colored filters used in optics, however, are bandpass filters; they allow only a certain range of wavelength through while stopping the rest of the light; that is, a red-colored filter will allow only a wavelength of 610 nm through. The range of the bandpass could be as general as ±8 nm or as precise as ±0.2 nm. Note that the central wavelength of bandpass filters does not need to fall within the visible spectrum. The term colored filter simply refers to a specific subset of bandpass filter that selectively allows wavelength in the visible spectrum; there exist bandpass filters for wavelengths of 340 nm (ultraviolet) to 4.75 μm (infrared).
Modulation, Coding, and Multiple Access
Published in Roberto Ramirez-Iniguez, Sevia M. Idrus, Ziran Sun, Optical Wireless Communications, 2008
Roberto Ramirez-Iniguez, Sevia M. Idrus, Ziran Sun
Multiple access techniques refer to the way in which the IR medium can be shared among a number of users. These techniques define the transmission capacity allocated to different users when a number of them are trying to access the system. This is particularly relevant in indoor systems where the same device may be required to exchange information with a number of other units, as compared to outdoor systems that tend to operate in a point-to-point fashion. The current section refers to physical-layer multiplexing techniques, which are based on some of most important characteristics of the IR medium. These characteristics include [21]: Optical filters can be used to discriminate unwanted wavelengths and to allow only the relevant ones to reach the detector.It is possible to use the same wavelength in different rooms (in a building or a house) due to the fact that IR radiation does not penetrate walls.Intensity modulation (IM) favors the use of waveforms with short duty cycles because they improve power efficiency. IM also favors the use of time division multiple access (TDMA).It is possible to achieve high angular resolution (in an angle-diversity receiver) due to the fact that the wavelengths corresponding to the IR part of the spectrum are short, which is important for space division multiplexing.
Modeling nanomaterial physical properties: theory and simulation
Published in International Journal of Smart and Nano Materials, 2019
Tanujjal Bora, Adrien Dousse, Kunal Sharma, Kaushik Sarma, Alexander Baev, G. Louis Hornyak, Guatam Dasgupta
Optical filters are devices which allow transmission of selective wavelengths based on their design. They are commonly used in various optical and opto-electronic devices. FDTD simulation was used by Hosseini et al. [102] for the design of a low pass optical filter based on a plasmonic nanostrip waveguide capable of proving sub-wavelength confinement. The simulated filter showed flat low pass filter with 3 dB attenuation at the optical communication wavelength (λ = 1.55 µm). Another FDTD simulation was reported by Lee et al. [103] for simulating transmission characteristics of transparent conducting electrodes composed of silver or aluminum grids with widths from 25 nm to 5 μm.