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Focal-spot wandering is the main cause for the major power loss in free-space optical communications. Thus,mitigating it is a primary requirement for the successful performance improvement. In order to prove this prerequisite, an experimental set-up using 155-Mbps data transmission is built for the link range of 0.5 km at an altitude of 15.25 m. In the experiment, the receiver is equiped with a control system to stabilize the received optical propagation at the detector plane which is called as focal-spot wandering mitigation control so as to couple the power in bucket perfectly to the photodetector. The performance improvements due to mitigating focal-spot wandering are regressively investigated in terms of various quality assessment key parameters. Maximum radial distance of 0.25 mm, maximum effective scintillation index of 0.17, optical signal-to-noise ratio of 9 dB, minimum eye-opening of ±0.37 V, minimum eye-height of ±0.51 V, controlled bit-error-rate of 6.45 × 10−9 to 7.09 × 10−8 and the li...

In the last decade, the coherent technology of narrow linewidth optical system has contributed to achieving a new generation called high speed optical communication. This technology has improved the capacity of transmitting long-haul fiber optic transmission systems. State of the art in the market is the bit rate up to 100 Gbps per channel, where in one fiber optic can be traversed by 80 channels or 8 Tbps per fiber. The main key to high speed transmission is narrowing the linewidth laser that will increase laser coherency. Narrowing Linewidth laser is also widely used for power laser cutting applications, because narrowing linewidth results in reducing chromatic aberration that narrowing of the focal diameter, such that laser cutting becomes more effective. Several techniques have been known previously such as the use of a quantum well structure, distributed Bragg feedback and the use of an external cavity resonator that can obviously narrow the semiconductor laser linewidth. In this paper, we make a new theoretical analysis to narrow the semiconductor laser linewidth by controlling laser current injection. The analysis is performed based on the Schawlow-Townes equation and its new revision. We prove that by increasing current injection causes narrowing of the linewidth. The analysis and simulations are performed using Optiwave software.

We propose an optical spatial filter (OSF) method to suppress beam wander and spatial noise effects. Signal from random displacements of the focus spot around the optical axis within the constricted area is collected. This method advantageously suppresses fluctuations in signal intensity. The OSF consists of a pinhole and cone reflector. The pinhole produces Fresnel diffraction on the focus spot. The cone reflector provides directed reflectance onto the pinhole for random focus spot displacements due to beam wander. The calculations of signal power are based on fluctuations of signal intensity that are minimized by the circular aperture function of the pinhole and the cosine of the reflectance angle from the cone reflector. The method is applied to free-space optical communications at a wavelength of 1.55 μm with an atmospheric chamber to provide optical propagation media. Based on calculations, the beam wander angles that can be received by the OSF are from 14.0° to 28.0°. Moreover, based on experiment, the OSF with a pinhole diameter of 20.0 μm and cone reflector diameter of 1.5 mm produces signal power of −15.3 dBm. Both calculations and experiment show that the OSF enhances the received signal power in the presence of turbulence.

The optical propagation of free-space optical (FSO) communications under the influence of atmospheric turbulence exhibits deterioration of signal spectral. By this nature, fiber-detection method that is implemented in the receiver system frequently, cannot overcome the turbulence effects. In order to improve the fiber-detection method, an optical spatial filter (OSF) that is composed of cone reflector, pinhole and multi-mode fiber is proposed as a detection method. It aims to increase intensity of signal spectral and signal power by suppressing temporal fluctuation. Thus performance improvement of FSO can be achieved by the OSF. The measurements of signal spectral from the OSF are compared to fiber-detection method. The OSF is intalled in rear focus spot of receiver lens before photodetector. The experiment results show significant improvement in suppressing temporal fluctuation in signal spectral from the OSF. (SNR) increases that are 36.5 dB, 37.5 dB, 37.7 dB, and 38.2 dB for D P1 , D P2 , D P3 , and D P4 , respectively. (BER) are produced more lower that are 10−9 to 10−12 by OSF. Those performances improvement is achieved higher by the OSF compared to fiber-detection method. The outcome of this work is recovering signal spectral that is deteriorated caused by turbulence effects modulation into optical propagation. Hence the OSF can be implemented for integrating FSO with optical fiber communication system in order to enhance bit-error-rate and signal-to-noise ratio performances.

On this preliminary work we employ distributed raman amplifier with pumping schemes are forward- & backward pumping on SMF. The model of simulation and analysis used are multiple-pump interactions on CWDM band. We used four level configuration of pumps on wavelengths; 1435 nm, 1445 nm, 1455 nm and 1465 nm with the range of tuneable power levels are 300 mWatt - 400 mWatt each ones. The average results of Stokes gain measured at L = 0 km and L = 40 km are 17.875 B and -3.5 dB respectively while the noise figure is at the range of 3 dB - 2.5 dB. The maximum length of fiber are reach at L = ~50 km. Two factors found that could decreasing the performances of distributed Raman amplifier are amplified spontaneous scattering (ASS) and Rayleigh backscattering effects. Finally the flattening gains for C band are reached successfully.