Division-mode multiplexing (MDM) is based on the excitation and propagation of multiple spatial optical modes as individual/separate/independent data channels in a common physical transmission medium, which aims to increase the transmission capacity [8]. OAM modes constructed from HEl,mmodes rotate in the same direction as the rotation (spin-orbit aligned modes) and OAM modes constructed from EHl,mmodes rotate in the opposite direction to the rotation (anti-aligned spin-orbit modes) .
OAM-MDM through optical fibers
Conventional fibers
Few mode fibers (FMFs) with classical refractive index profile (stepped/graded), were used to transmit OAM modes. The transmission of OAM modes over FMF required a MIMO DSP in combination with coherent detection to equalize the intermodal crosstalk.
OAM specialty fibers
Taking advantage of the high refractive index contrast (air/silica), the fabricated fiber supports the transmission of 36 OAM states [53]. Figure 9 shows the refractive index of ACF. Where a, n1, n2, n3 are the core radius, the refractive index at the core-cladding interfaces, the refractive index at the core center, and the cladding refractive index, respectively.
Photonic crystal fibers
On the other hand, the internal smooth behavior of HTAN-FMF, even with an external abrupt variation, guarantees the improvement of the obtained OAM mode purities (≥99.9%), leading to intrinsic crosstalk of at least 30 dB during propagation. In addition, the results obtained in terms of chromatic dispersion (max CD = 60 ps/(km.nm)), differential group delay (max DGD = 55 ps/m) and bending insensitivity show that the HTAN-FMF are a viable candidate for improving of transmission capacity and spectral efficiency in next generation OAM Mode Division Multiplexing (OAM-MDM) systems [78].
Conclusion
A compact trench-assisted multi-orbital angular momentum multi-ring fiber for ultra-high density space division multiplexing (19 rings22 modes). A design strategy of the circular photonic crystal fiber supporting good quality transmission in orbital angular momentum mode.
TOP 1%
Introduction
- Cladding less evanescent based optical fiber sensors
- Tapered optical fiber sensor
- Interferometers
- Grating based optical fiber sensor
The optical signal propagates in optical fiber by obeying the principle of total internal reflection (TIR) with very low losses, and the first working model of optical fiber was proposed in 1965 [1]. By dressing smaller optical fiber sensors, the interaction of optical signal with the environment can be improved by bending it in a U-shape [11]. The slits in the optical fiber structure lead to the modulation of the propagating optical signal.
Biochemical measurands in healthcare
- Chemical optical Fiber sensors
Although, the optical fiber sensors for the detection of organic compounds are not very sensitive [44]. An EW-based fiber optic sensor has been forwarded for the detection of gas exhaled from human skin [ 45 ]. The optical fiber sensors were also used for the diagnosis of humidity, which is one of the important factors in the case of critical conditions [46].
Characterization and analysis process of optical fiber biosensors The different geometries of optical fiber sensors should need to be characterized
- Optical fiber sensor geometry
- Nanoparticles
- Biomolecules
- Sensing analysis
SEM image of a SERS probe of a pointed optical fiber sensor structure: (A) pointed optical fiber and (B) nanoparticle distribution along the fiber [54]. Analysis of samples of target biomolecules can be performed by preparing them in different basic pH solutions. Repeatability is another important factor for analyzing the performance of any fiber optic sensor.
Conclusions
The stability of any fiber optic biosensor can be evaluated by measuring the base solution through a sensor probe more than 10 times. Reusability is another important parameter to analyze the performance of fiber optic sensor. The highest specificity of any fiber optic sensor can be achieved by functionalizing the sensor head with the appropriate enzyme, which is oxidized only in the presence of the target bio-samples.
Photonics materials and their characteristics for AI
- New Investigative Materials for AI
It has been used as a disorder-driven metal-insulator transition in crystalline void-rich Ge-Sb-Te phase change materials [7]. While numerous integrations are possible with the Ge-Sb-Te alloy, the new material GST467[6] revealed by CAMEO (Closed-Loop Autonomous System for Materials Exploration and Optimization) is best suited for phase-change applications. CAMEO found the best Ge-Sb-Te alloy with the greatest difference in "optical contrast" [6].
AI for photonics
GST467 has also found applications in photonic switching devices that can be used to control the direction of light in given circuitry. It is therefore very clear that the use of modern computational techniques such as AI can be used to improve the rate of discovery of these new photonic materials and vice versa. The desired optical response can be obtained by adjusting the initial design and performing several simulations until the outcome is achieved.
Proposed dual core PCF design integrated with AI
With the help of this setup, the proposed dual-core PCF can be used with artificial intelligence for better and improved results. As the analyte interacts, there are variations in terms of minimum and maximum values that can be observed and displayed using a computer. The dataset of blood serum, ethanol and water for this case study is selected as the input to be passed through the setup and the results obtained have been optimized using AI.
Discussion
With the above suggested design, the following tested have been done with Dual core Silica PCF. Depending on the refractive index of blood serum, the light intensity is modulated and detected at the other end of the PCF [16]. The properties of the dual-core photonic crystal fiber (PCF) sensor are studied using the finite element method (FEM), and the structure is improved according to the numerical simulation results.
Conclusions
The complexity of both approaches is related to the non-standard way of describing the modulation of the nonlinear method for the internal (direct) structure and the use of a specific Mach-Zehnder modulator for the first stage of external modulation. The purpose of the presentation is to discuss the main features of OEO as a low noise generator. The important role of choosing a coherent laser for OEO with a small spectral linewidth is shown.
Introduction. The opto-electronic oscillator structure
Structure diagrams of the optoelectronic oscillator (a) with the direct modulation by current and (b) with external Mach-Zehnder modulator. It consists of the multiplier "*", two optical channels with different delays, and the delay cell defined by the delay in the optical fiber. The functional diagram illustrates the principles of the correlator method and the frequency discriminator method in OEO with the MZ modulator and in the circuit with direct amplitude modulation at suppression of the one harmonic. a) Diagrams (1–4) of optical frequency selection; b) L = the laser,Т1MandТ2M, = delay lines have delay times in channels,“+”= (addition),“х”= (multiplication),“*”= (conjugate operation),“Ð.
OEO with direct modulation and OEO with the Mach Zehnder modulator
OEO construction and its operation principle
If the excitation conditions are met, the laser generates optical oscillations which pass from its output to the MZ, and then pass via two optical channels with. The laser is the pump source for the radio frequency network (Figure 1b) closed in a loop and formed by a modulator, an optical fiber, a photodetector, a. As a result of oscillation processes, the spectra are formed with fluctuations of different nature, but the spectral linewidth of radio frequency oscillations is defined by parameters of two oscillating systems: the laser and the radio frequency oscillator.
Problem statement
Such emission sources are quantum laser diodes (QWs) and fiber optic lasers with polarizers at their outputs.
Laser in OEO
The optical oscillation frequencyν0L generated by quantum-dimensional laser diodes in the autonomous steady state can be found (if the excitation condition is fulfilled) based on the solution of the phase balance equations for stable optical intensity oscillations. in the optical resonator and in the laser active element. The main assumption for using the semiclassical equations is that the carrier lifetime at the upper operating level and the time constant T0F of the laser optical filter (OF) are much larger than the polarization relaxation time T2. In (3), T2 is the polarization time constant of excited particles at a higher energy level, T1 is the lifetime of excited particles at the upper energy level.
Compact fiber optic delay line in OEO
It becomes possible to reduce the thickness of the optical shell and reduce the required volume. Therefore, by reducing the critical bending radius when winding the optical fiber and the cladding diameter, it is possible to potentially significantly reduce the maximum dimensions of the FODL OEO. The width of the spectral line of the laser after passing through the optical fiber in the FOLD does not change.
OEO differential equations
Dynamics of transients in OEO DM
Based on the mentioned OEO differential equations. 5) an analogue model of the OEO, shown in Figure 1a, was constructed. And their level depends on the value of the DC pump laser current. The setup time of the laser oscillations is 0.8 ns (or from 0 to 40 points in Figure 6a and b).
OEO DM system of the laser emission
The steady-state oscillation frequency is close to the natural frequency of the electronic filter (F) and was about 10 GHz. When considering the hard-excited OEO DM mode, one should note the more complex dynamics and picture of the phase plane in the transition mode. At the same time, the duration of the OEO DM oscillation transition process changes significantly.
Laser phase noise and OEO phase noise
Positive feedback is incorporated into the DE system, taking into account the. photodetection of optical radiation, radio frequency selectivity and nonlinear amplification on a nonlinear amplifier. New in the analysis of the OEO DM operation is that the Lotka-Volterra laser differential equations for the optical field intensity, inverted population and optical phase with positive selective feedback can be reduced with a delayed argument to a single van der Pol differential equation for the pump electric current. For a stable single-frequency mode of OOO DM generation, the following conditions must be met: a two-fold exceedance of the electron filter time constant (F) relative to the electron relaxation time constant in the active layer of the laser.
OEO as the EMF correlator
OEO phase noise
It should be noted that with an optical fiber length of 2 km, a uniform attenuation of the phase noise of the laser is achieved in the offset range of 1...50 kHz. It has been shown that at OF length further reduction of the OEO phase noise is possible using a PLL (phase locked loop) system. The calculation results are in good agreement with the experimental dependences of the OEO phase noise PSD that can be found in [14–16].
Experimental investigations
Conclusion
For stable operation of the OEO, the laser coherence time and the delay time in the optical fiber must be balanced. The relatively simple expressions for phase noise PSD of the radio frequency generation in optoelectronic generator in single-side carrier mode with an explanation of the laser phase noise. The value of the OEO power spectral density is proportional to the spectral linewidth of the laser optical emission.
