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Spectroscopic Methods
Published in Somenath Mitra, Pradyot Patnaik, Barbara B. Kebbekus, Environmental Chemical Analysis, 2018
Somenath Mitra, Pradyot Patnaik, Barbara B. Kebbekus
When a filter is not adequate, for instance when it is desired to make measurements at discrete wavelengths over a span of wavelengths, one must use a monochromator. This is a device that takes the incident polychromatic radiation and spreads it out in space. Different wavelengths of light are thus directed to different points. A prism or a grating designed to disperse the middle area of the spectrum, is normally used. Of course, this method is not suitable for very short wavelength radiation, which would interact with the material of the prism or grating, or for very long wavelengths, in the radio and microwave regions. Through the ultraviolet, visible, and infrared, the grating is the most frequently used dispersing element. It consists of a base material which is either reflective or transparent. This is ruled with closely spaced grooves that serve to diffract the light. Gratings are often made of aluminum or of plastic coated with aluminum. These can be formed from a master grating in a pressing process, which makes them quite inexpensive.
Design and Operation of Chemical Phosphorus Removal Facilities
Published in Richard Sedlak, Phosphorus and Nitrogen Removal from Municipal Wastewater, 2018
Chemical resistant grating should be installed in chemical containment areas. The grating can keep the operator raised above spilled corrosive and hazardous chemicals and keep slippery polymers off of operator walking surfaces. An elevated grating platform allows installation of chemical piping below grade instead of overhead which would pose a greater potential for exposure to chemicals due to leakage. FRP grating with a nonskid surface is suitable for the chemicals used.
An overview of HPDLC films and their applications
Published in Liquid Crystals, 2022
Katariya-Jain Anuja, Rajendra R. Deshmukh
From the Equation 1, it is clear that the wavelength of reflecting light is proportional to the grating period. It means that the wavelength of light can be selected by tuning grating period. Alternatively, if the grating period is fix, then by varying RI, the wavelength of light can be shifted. This is the basic concept behind the creation of pressure sensor. The grating period of HPDLC film is lowered by applying pressure in a perpendicular direction of the film, i.e. in parallel direction to the grating vector. Zharkova et al. prepared and studied HPDLC pressure sensor by doping LC by porphyrin platinum complex (PtOEP) [53]. They found that introduction of PtOEP enhances the pressure sensitivity of the sensor by around 1.8 times. Opto-mechanical properties of reflecting HPDLC was employed for optical strain gauges. On increasing the strain on reflective HPDLC films spectral blue shift was observed in reflected wave [54]. The proposed reason for this is Poisson contraction of the material and elongation in LC droplet on application of strain. Even the change in the composition of HPDLC can change the elasticity of the film . Pressure sensors are used for early glaucoma detection. Li et al. prepared continuous, multiple exposure diffraction grating (CMEDG) holographically, by exposing PDLC film point by point to/under, two interfering beams, multiple times (18 repeated exposure/non-exposure cycles). The diffraction pattern obtained was clear and depends on probing position. As the probing position deviates from centre, the diffraction pattern changes, hence CMEDG can be used to record the vibrational sensitivity of laser beams [55].
Compact cross-dispersion device based on a prism and a plane transmission grating
Published in Journal of Modern Optics, 2018
We define the plane of incidence to be the plane that is made up of the incident light ray and the grating normal. The normal plane of the grating is the plane that is perpendicular to the grating grooves. Because ϕ is the angle between z′-axis and x-axis in the x–z plane, ϕ denotes the angle between the plane of incidence and the normal plane of the grating, which can be changed by rotating the transmission grating in its own plane.
A vertical cavity surface emitting laser for CPT atomic clock
Published in Journal of Modern Optics, 2022
Ming Li, Qiuhua Wang, Yaobin Li, Pingping Qiu, Hongling Meng, Yiyang Xie, Qiang Kan
The two-dimensional finite-difference time-domain (FDTD) method [21] was used to optimize the structural parameters of the directly-etched surface grating. Periodic boundary conditions were used to simulate the effect of an infinite grating on the polarization control. The reflectivity of the two polarization states at different duty cycles and etching depths was calculated, with the grating period set as 700 nm. In fact, the polarization state preferentially resonates along the crystal direction, but the simulation software could not define the material crystal direction, and therefore, only one grating orientation was set. The relationship between reflectivity and mirror loss can be expressed as: where αm is the mirror loss of the upper Bragg mirror, L is the effective cavity length, and R1 and R2 are the reflectivities of the upper and lower DBR, respectively. The corresponding reflectivity of the two polarizations under different grating parameters is obtained through simulation. Substituting the reflectivity into Equation 1 can obtain the mirror loss of the two polarization states. According to the mirror loss difference between the polarizations, a more competitive grating etching scheme can be selected. Figure 2 depicts the trend of polarization-dependent mirror loss change, corresponding to gratings with different etching depths and duty cycles. The ‘Orthogonal’ and ‘Parallel’ lines shown in this figure refer to perpendicular and parallel directions to the direction of the grating strips, respectively. As shown in Figure 2(a), when the grating period was 700 nm and the duty cycle was 0.5, the etching depth ranged from zero to 60 nm. As the etching depth increased, the difference in loss between the two polarization states increased gradually, reaching 10 cm−1 at an etching depth of 38 nm. The directly-etched grating etching depth in the range of 38 nm to 60 nm can make the VCSEL form a single-stable polarization, while the inverted grating needs to control the etching depth error within 5 nm. This grating has a considerable tolerance in the process, which greatly reduces the difficulty of grating fabrication. Figure 2(b) shows that the grating duty cycle can reach a loss difference of more than 10 cm−1 ranging from 0.3 to 0.9, for an etching depth and a period were 60 nm and 700 nm, respectively.