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Basic Optical Systems and Simple Photographic Lenses
Published in Daniel Malacara-Hernández, Zacarías Malacara-Hernández, Handbook of OPTICAL DESIGN, 2017
Daniel Malacara-Hernández, Zacarías Malacara-Hernández
It is frequently necessary to produce a well-collimated beam of laser light. A normal telescope objective with a large f-number may be used, but an important requirement is that the focal length be short. An f-number as low as possible is convenient. This imposes the need for an extremely good spherical aberration correction, with a low zonal aberration. As described by Korones and Hopkins (1959), the zonal aberration may be reduced by any of four methods: Choosing the proper glassesUsing an air spaceIntroducing an aspherical surfaceSplitting the positive lens into two
Optical Sources and Detectors
Published in Z. Ghassemlooy, W. Popoola, S. Rajbhandari, ®, 2017
Z. Ghassemlooy, W. Popoola, S. Rajbhandari
There are a number of light sources and photodetectors that could be for OWC systems. The most commonly used light sources used are the incoherent sources-light emitting diodes (LEDs) and coherent sources—laser diodes (LD). LEDs are mainly used for indoor applications. However, for short link (e.g., up to a kilometre) and moderate data rates, it is also possible to use LEDs in place of LDs. Lasers, because of their highly directional beam profile, are mostly employed for outdoor applications. Particularly for long transmission links, it is crucial to direct the energy of the information to be transmitted precisely in the form of a well-collimated laser beam. This is to limit the often still very large channel power loss between the transmitter and the receiver. In order to limit the beam divergence, ideally, one should use a diffraction-limited light source together with a relatively large high-quality optical telescope. At the receiving end, it is also advantageous to use a high- directionality telescope not only to collect as much of the transmitted power as possible but also to reduce the background ambient light, which introduces noise and thus reduces the performance of the link. As for detectors, both the PIN and the APD photodetectors could readily be used. This chapter discusses the types of lights sources, their structures and their optical characteristics. The process of optical direct detection as well as coherent detection is also covered in this chapter. Different types of noise encountered in optical detection will be introduced and the statistics of the optical detection process is also discussed.
Optical Sources and Detectors
Published in Z. Ghassemlooy, W. Popoola, S. Rajbhandari, Optical Wireless Communications, 2019
Z. Ghassemlooy, W. Popoola, S. Rajbhandari
There are a number of light sources and photodetectors (PDs) that could be used in OWC systems. The most commonly used light sources are the incoherent light-emitting diodes (LEDs) and coherent laser diodes (LDs). LEDs are mainly used for indoor OWC applications. However, for short links (e.g., up to a kilometer) and moderate data rates, it is also possible to use LEDs in place of LDs. LDs are monochromatic, coherent, and directional, and therefore they are mostly employed for outdoor applications. Particularly for long transmission links, it is crucial to direct the energy of the information to be transmitted precisely in the form of a well-collimated light beam to withstand atmospheric conditions, which can result in high attenuation. In order to limit the beam divergence, ideally, one should use diffraction-limited light sources together with relatively large high-quality optical telescopes. At the receiving end, it is also advantageous to use a high-directionality telescope not only to collect as much of the transmitted power as possible but also to reduce the background ambient light, which will introduce additional noise and thus deteriorate the link performance. As for PDs, both the PIN and the avalanche photodiode (APD) could readily be used, though the latter is costlier. This chapter discusses the types of light sources, their structures, and their optical characteristics. The process of optical direct detection, as well coherent detection, is also covered in this chapter. Different types of noise sources encountered in optical detection will be introduced, and the statistics of the optical detection process is also discussed.
A Comprehensive Survey on the Detection of Diabetic Retinopathy
Published in IETE Journal of Research, 2022
Spectral-domain OCT (SDOCT) consists of a super-luminescent diode that generates light of 840 nm wavelength and a 150 nm bandwidth. A spectrometer is fixed to compute the interference spectra of reflected light from the tissue and light from a stationary reference arm [20]. The imaging speed of SD-OCT is 50 times greater than that of TD-OCT providing more excellent images per unit area. The source of low coherence light of SD-OCT is identical to that of TD OCT. However, the stationary mirror is used rather than using moving reference mirror for the signal acquisition. The diffraction grating separates the interference pattern into frequency components. A charge-coupled device detects the separated frequency components, shown in Figure 3 [17]. The function of a collimator is to align the beam of light in a different direction, making it parallel or collimated.
Analysis of the spatial intensity distribution of a CW laser light through tissue-like turbid media
Published in Journal of Modern Optics, 2021
Yuhu Ren, Jimo Jian, Jing Wang, Cunguang Zhu, Wei Xia
The spatial intensity distributions of the CW laser light through highly turbid media with different scattering coefficients and anisotropy factors are shown in Figure 3. In Figure 3(a), the turbid media have a specific anisotropy factor g = 0.9, and an increasing scattering coefficient from 11 to 19 at intervals of 2. In Figure 3(b), the turbid media have a specific scattering coefficient μs = 15, and an increasing anisotropy factor from 0.1 to 0.9 at an interval of 0.2. In these two cases, the absorption coefficients of the turbid media are ignored. It can be seen from Figure 3(a) that the FWHM of the normalized intensity of the light spatial intensity distribution increases with an increase of the scattering coefficient. For example, the FWHM of the spatial intensity distribution of incident laser through turbid medium with μs = 15 and μs = 19 are 68 and 90 pixels, respectively. This phenomenon can be attributed to the scattering effect arising from multiple scattering photons. Due to the scattering effect, the collimated incident light becomes diffused light deviating from the optical axis and thus the FWHM of the transmitted light increases compared with that of the incident light. The number of multiple scattering photons grows with an increase of the scattering coefficient, resulting in the further rise of the FWHM of the transmitted light. From Figure 3(b), we see that the FWHM of the spatial intensity distributions of the CW laser light through turbid media does not increase monotonously with an increase of the anisotropy factor. The FWHM of the spatial intensity distribution does not nearly vary with an increase of the anisotropy factor from g = 0.1 to g = 0.7. Because the spatial intensity distributions of the CW laser light through turbid media depends on the scattering phase function of a microsphere in turbid media with a specific scattering coefficient.
Digital Holographic Interferometry for Temperature Measurement of Flame without Phase Unwrapping
Published in IETE Journal of Research, 2023
A schematic diagram of the experimental setup used for recording digital hologram is shown in Figure 3. A light beam emitted from a 17 mW He–Ne Laser (632.8 nm) source is spatially filtered using a spatial filtering arrangement comprising of 40× microscopic objective and a 5 µm pinhole. The expanded light is collimated with help of a collimator. The collimated beam is divided into two parts by a circular beam splitter. The reflected part of the beam constitutes the reference beam whereas the transmitted part acts as the object beam. The object beam after being scattered from the diffuser plate is directed towards the CCD camera. The diffuser plate is placed at a distance of 57.7 cm from the CCD screen. A diffuser plate is used for uniform illumination for a larger range of temperature field measurement as the surface area of CCD is small. A candle is kept behind a diffuser plate as shown in Figure 3 such that the light passes just above the root of the candle flame. The object and reference beam undergo different paths to ultimately interfere on the CCD sensor. The contrast of the interferogram can be optimized by assuring that the intensity in both arms of the interferometer is almost equal. A pair of holograms is recorded one without flame and another with the flame for four single flames (A, B, C, and D) as well as for four coalesced candle flames (AA, BB, CC, and DD) using Pike F-145C CCD camera with a pixel size of 6.45 µm × 6.45 µm. Holograms are reconstructed in the MATLAB environment. The experiment is conducted at an initial room temperature of 298 K and an atmospheric pressure of 1 atm. Four different categories of candles are used with different diameters of wick varying from 0.60 mm to 1.60 mm which are mentioned in Table 1. Length of the wick is kept the same i.e. 7 mm for all the flames. Candlewick is covered with a thin coating of liquefied wax during the burning process. This ensures that the mass transfer process in the wick exceeds the evaporation process and prevents burning of wick. Candle is enflamed for a short duration and wick thickness remains constant during the recording work.