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Optical Loss: Principles and Applications
Published in John G. Webster, Halit Eren, Measurement, Instrumentation, and Sensors Handbook, 2017
M.M. Rad, Halit Eren, Martin Maier
Depending on the role of the fiber, fiber-optic sensors can generally be classified into two groups: intrinsic (or hybrid) fiber sensors and extrinsic fiber sensors (or all-fiber sensors). In intrinsic fiber sensors, fiber is utilized as the sensing element, where the propagating light is modulated by the environmental effects, whereas in extrinsic fiber sensors, the signals are relayed via fiber from a remote sensor (can also be fiber optics) to the optoelectronics, which process the signals. In its simplest form, a fiber-optic sensor is an intrinsic type, whose operation is based on intensity modulation. Spectral, interferometric, and distributed (multiplexing) mechanisms may also be employed for sensing application. Fluorescent-based fiber sensors are used in medical application and chemical sensing; fiber-optic gyros are used in navigation systems (most types of vehicles such as Boeing 777), pointing and tracking of satellite antennas.
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
At the optical transmitter end, one of the most popular forms of modulation is intensity modulation (IM), wherein the waveform of the information is used to modulate the instantaneous power of the transmitted energy at the desired wavelength. In this technique, the electrical data signal is used as the input of the transmitter driver. At the receiver end, the information is recovered (down-converted from an optical into an electrical signal) using a technique called direct detection (DD), in which the photodetector generates an electrical signal according to the instantaneous power of the received optical signal. This transmission-detection technique, which has been widely discussed by Barry and Kahn [21, 47], presents a fundamental characteristic: in an IM/DD channel, the input power at the receiver cannot be negative.
Photodetectors and Receiver Architectures
Published in Hamid Hemmati, Near-Earth Laser Communications, 2020
Peter Winzer, Klaus Kudielka, Werner Klaus
Since photodetectors are typically polarization-independent square-law detectors with respect to the optical field, they are fundamentally insensitive to any phase or polarization information contained in eā(t). Thus, from a detection point of view, optical intensity modulation is the most straightforward way to establish an optical communication link. Intensity modulation can be analog, e.g., with many radio-frequency subcarriers sharing a common, intensity-modulated optical channel, or digital, either with two (on/off keying, OOK) or more intensity levels (e.g., M-ary pulse amplitude modulation, M-PAM), or even with information encoded on the arrival time of an optical pulse within the bit slot (pulse-position modulation, PPM). More sophisticated modulation formats (e.g., [differential] phase shift keying, (D)PSK, frequency shift keying, FSK, quadrature amplitude modulation, QAM, or polarization shift keying, Pol-SK), which also use the signal's phase or polarization to encode information, have to employ special optical preprocessing in order to make the phase or polarization information accessible to square-law photodetection [7]. Such preprocessing, which is described in more detail in Section 7.4, can be accomplished by means of a local laser source (coherent receiver) or by means of appropriate optical filtering (e.g., an optical delay line to demodulate DPSK, or a polarization beam splitter to demodulate Pol-SK).
Performance Analysis of Radio Over Fiber System Employing Photonics Antenna and Different Modulation Schemes
Published in IETE Journal of Research, 2023
The coming generations of wireless networks require more diminished cells than that of the current generation. For such applications, the photonic antenna and Radio over fiber (RoF) technique are the major technologies. RoF is a promising technology that converges wireless and wired network (hybrid network/communication) to provide a better solution for the increasing demand of bandwidth in communication systems [3]. RoF system seems to be a promising solution and will be widely used for communication and applications, such as digital audio and video broadcasting standards, Wi-Fi networks etc. It is a technology in which modulation of light with RF signal takes place and the final modulated signal is transferred through the fiber for cellular access transmission [4]. Optical fiber is considered as the best transmission medium because of its competitive cost, low loss and larger bandwidth. Photonic antenna is basically used to replace and solve the problem of the transceiver that converts baseband optical gigabit Ethernet signal to baseband electrical signal which is very expensive. The antenna has been used to transmit radio signals directly obtained through optical fiber and radiate the RF wave using RF antenna which is integrated with optoelectronics. This method of transmitting RF signal over optical fiber is called intensity modulation with direct detection. This is because the RF signal is used to directly modulate the laser diode in the Central Station (CS) and at the radio access units (RAU) site, the PIN photo detector recovers the transferred radio signals by direct detection.
Approximate analytical expressions for pulse modulation response of quantum cascade lasers
Published in Journal of Modern Optics, 2018
K. S. C. Yong, M. K. Haldar, J. F. Webb
In near-infrared interband laser, the turn-on delay with the laser biased below threshold occurs due to the carrier number rising to the threshold value. There is no stimulated emission during this period [17,18]. The rise is determined by the electron lifetime which is of the order of nanoseconds [18]. In a QCL on the other hand, the difference in the electron numbers almost immediately reaches above the threshold value. Stimulated emission starts immediately. The delay is mainly caused by the slow rise of the photon number from a small value. Consequently, the turn-on delay for a QCL is of the order of a few tens of picoseconds which is much lower than the turn-on time of interband lasers. Fast digital intensity modulation of QCL is possible even with the laser biased below threshold. On the other hand, the interband laser has to be biased slightly above threshold for fast digital intensity modulation. This gives rise to a power penalty [23] in an intensity modulation direct detection communication system. Rise and fall times of QCL are in the order of few tens of picoseconds which is similar to the times reported in interband lasers [22]. Similar to interband lasers [18], fall time is greater than rise time when QCLs are biased at above threshold and it becomes the switching speed limiting factor in high bit-rate systems.
Efficient coding techniques for flip-OFDM in IM/DD systems
Published in International Journal of Electronics Letters, 2019
Muhammed Abd El-Aziz Eltoukhi, Mohammed Abd-Elnaby, Sami A. El-Dolil, Fathi E. Abd El-Samie
Optical wireless systems can be implemented using Intensity Modulation with Direct Detection (IM/DD) due to its reduced cost and complexity. In intensity modulation (IM), the transmitted optical power is changed according to some characteristics of the modulating signal (Elgala, Mesleh, & Haas, 2011; Wu & Bar-Ness, 2015). The detection of intensity modulated signals is usually performed with photodiodes. Hence, only real and positive signals can be transmitted.