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Optical transmitters and receivers
Published in Matthew N. O. Sadiku, Optical and Wireless Communications, 2018
The role of the multiplexer is to transmit two or more channels simultaneously over the same fiber. The two popular multiplexing schemes used in fiber optics are time-division multiplexing (TDM) and wavelength-division multiplexing (WDM). TDM is the process of combining many channels of information within a single transmission channel by assigning each channel a different time slot. TDM is rarely used in fiber optics; it is more common in telephone (voice) systems. WDM is a scheme that combines several channels into a single fiber by assigning each channel a different wavelength. WDM is specific to fiber optics. As shown in Figure 2.7, several optical sources are transmitted at different wavelengths. The various wavelengths are injected into a single optical fiber. The light is filtered into separate wavelengths and converted to their respective electrical signals by the receiver.
Data Communication
Published in Sunit Kumar Sen, Fieldbus and Networking in Process Automation, 2017
In synchronous TDM, data from different sources are divided into fixed time slots, in which a slot may contain a single bit, a byte of data, or a predefined amount of data. As shown in Figure 1.12, data from the first source is sent in the first time slot, followed by data from the second source in the second time slot. This is continued until data from the last source is sent. Then the system repeats itself. Thus, in the first four time slots, data Ax from source 1, data Bx from source 2, no data from source 3, and data Dx from source 4 are fed into the multiplexer. In this sequence, the next set of data from the four sources are sent, following the same logic. It should be noted here that even if some source does not have any data to be sent at any given instant of time, because of preallocation, that time is simply wasted. For Figure 1.12, four time slots are wasted—one for source 2, two for source 3, and one for source 4. In a particular case when a channel does not have any data to be sent for a considerable time, underutilization of the channel takes place, leading to a less efficient system. Statistical TDM addresses this problem by skipping slot allocation for a source if it does not have any data in that particular time slot.
Optical CDMA Network Architectures and Applications
Published in Paul R. Prucnal, Optical Code Division Multiple Access, 2018
While subcarrier modulation is a relatively simple means for carrying RF analog channels, it has several shortcomings. Since all of the subcarrier channels share the same optical carrier, the power per channel is reduced as additional channels are added. Nonlinearities and intermodulation distortion also occur when multiple subcarriers are mixed and modulated onto the optical carrier. TDM and WDM based multiplexing techniques offer greater scalability than subcarrier modulation but are more complex and expensive to deploy. In addition, TDM schemes require synchronization among the individual media channels, which is difficult to achieve without electronic processing.
Depth-enhanced 2D/3D switchable integral imaging display by using n-layer focusing control units
Published in Liquid Crystals, 2022
Qiang Li, Fei-Yan Zhong, Huan Deng, Wei He
Many methods have been proposed to improve the 3D performance of 2D/3D switchable display. In terms of resolution, some methods have been proposed by using additional lens array [1] and different light-emitting diode arrays [2]. In terms of viewing angle, some methods have been proposed by using partial masking lens arrays [3], curved electroluminescent film [4], polarization selective scattering film [5], and aspherical liquid crystal lens array [6]. Kim et al. used electrically movable pinhole array to improve both the resolution and the viewing angle [7]. In addition, 2D/3D mixed or hybrid displays based on time-division multiplexing technology [8], space-division multiplexing technology [9], and polarization division multiplexing technology [10] were proposed. However, depth of field (DOF), one of the most direct 3D display features for the viewers to experience depth cues, has rarely been studied in 2D/3D switchable displays.
Success Journey of Coherent PM-QPSK Technique with Its Variants: A Survey
Published in IETE Technical Review, 2020
Divya Sharma, Y. K. Prajapati, R. Tripathi
QPSK offers four phases with a theoretical bandwidth efficiency of 2 b/s/Hz by encoding two bits per symbol, which enhances data rate, reduces bandwidth requirement and minimizes BER [15]. However, optical fiber has a transmission capacity up to 100 THz, in the low loss region (1530–1620 nm). But a single channel symbol rate ranges from 10 to 40 GBaud, due to constrain of electronic components in transmitter & receiver circuit. Hence, for the sake of efficient fiber bandwidth utilization, several channels can be multiplexed to share the same channel [16]. Multiplexing techniques include WDM, polarization multiplexing (PM), subcarrier multiplexing (SCM), optical time division multiplexing (OTDM), orthogonal frequency division multiplexing (OFDM), SDM, optical code division multiplexing (OCDM); electronic time division multiplexing (ETDM) etc. as emerging research frontier [16]. Each modulation and multiplexing technique has its own advantages, but we prefer QPSK modulation and PM technique in this context [17]. In the PM technique, information is transmitted using two orthogonal polarization modes supported by single-mode fiber (SMF), which also doubles the channel transmission capacity.
Full-color computational holographic near-eye display
Published in Journal of Information Display, 2019
Seyedmahdi Kazempourradi, Erdem Ulusoy, Hakan Urey
As the last set of experiments, we captured a full-color hologram using the same setup illustrated in Figure 7(a). The effective eye-box in this experiment is 3 mm × 3 mm (reduced due to wavelength scaling) and the diagonal FOV is 16.7 degrees. A time-division multiplexing method is used to reconstruct the full-color holograms. Red, green and blue lasers are coupled into a single mode fiber. We used an Arduino to sequentially switch the lasers on and off in synchrony with the corresponding trigger signal from the SLM. The individual images of each red, green and blue holograms are captured using a 5 Mega-pixels color JAI camera. We combined the captured images into a single RGB image using MATLAB, as shown in Figure 11. The optical reconstruction of CGHs is based on the diffraction theory and the use of different wavelengths results in chromatic aberration. Without considering the correct compensation factors the reconstructed holographic images do not overlap to form a full-color hologram. One of the main sources of the chromatic aberration is neglecting the change of refractive indices of lenses with respect to the wavelength, which can be compensated using the thick lens approximation in the CGH computation algorithm. Figure 11(a,b) demonstrates the results when we do not use the thick lens approximations in our paraxial CGH computation. As evident in the magnified views, the colors are separated. In Figure 11(a) the camera is focused at 25 cm, wherein Figure 11(b) the focus of the camera is set to 100 cm.