Dispersion mitigation using cascaded FBGs with hybrid apodization profiles

The apodization function plays an important role in the behavior of the mean dispersion, group delay and significant reduction of sidelobe level in reflection spectrum. On the other hand, the grating length represents as one of the critical parameters in contributing to a high performance fiber Bragg grating compensator. In this paper, we have investigated and compared the performances of various apodization functions in linearly chirped apodized fiber Bragg grating. Six different apodization profiles have been studied including their effects on the performances of the dispersion compensator. Comparison reveals that the sinc profile results best performance for chromatic dispersion compensation. Simulations have been performed for sinc profile with different grating lengths. The simulated results were then analyzed by calculating the reflection spectrum. It is observed that the reflection spectrum becomes constant at a grating length of 10 cm.

Drawing-tower inscription of apodized fiber Bragg grating arrays using a rotated focusing cylindrical lens

We simulated chirped and apodized fiber Bragg gratings (fiber gratings with modulation of the amplitude and with modulation of the spacing) the same as uniform fiber gratings. Results of our simulations are presented. Apodization of fiber Bragg gratings (FBG) has many advantages in comparison to other types of FBGs. Main improvement is suppressing of the side lobes in the grating spectral properties. The difference of side lobes suppression for uniform and Gaussian apodized grating is more than 110dB. On the basis of these results we designed a special fiber Bragg grating with apodization especially for using to stabilize semiconductor lasers for high precision laser applications with central wavelength 760nm. We present application of methods for calculation of parameters of apodized fiber Bragg gratings (FBG). We used combination of methods based on layered dielectric media (LDM) and the transfer matrix. On the contrary to the other calculation techniques the LDM method is based on sequence of thin films of dielectric media assembled in the direction of wave propagation. The combination of the LDM method and the transfer matrix method can be used to the calculation of arbitrary fiber gratings with high precision. For this designed apodized FBG we calculated the phase mask to manufacture by interference patterns. The phase mask and the fixation bottom of the fiber were made by e-beam lithography to achieve highly precise stability during manufacturing. We present the set-up of this system for writing FBG by pulsed UV laser with wavelength 266nm. Measurement of commercially available FBG with comparison to our calculated FBG is presented.

<p>The transmission of high power inside the optical fiber, produce amplitude noise, phase noise and other transmission impairments that degrade the performance of optical communication system. The signal regeneration techniques are used to mitigate these nonlinear impairments in the electrical or in the optical domain. All-optical signal regeneration techniques are one of the solutions to mitigate these nonlinear transmission impairments in the optical domain without converting the signal from optical to electrical domain. The existing techniques are not capable enough to attain the Bit Error Rate (BER) less than 10<sup>-10</sup> with the power penalty less than – 9dBm. In this paper, a new all-optical signal regeneration technique is developed that mitigate amplitude and phase noises in the optical domain. The new optical signal regeneration technique is developed by combining the two existing technique one is 3R (Reshaping, Reamplification and Retiming) regeneration and other is Phase Sensitive Amplification (PSA). The 10Gb/s Differential Phase shift Keying (DPSK) noisy transmission system is used to verify the features of developed technique. The developed technique successfully mitigates the nonlinear impairments from the noisy DPSK system with significant improvement in BER at low power penalty with the additional feature of high Q-factor and an eye open response for the regenerated signal. It is determined that BER of 10<sup>-12</sup> is achieved at the power penalty of -14 dBm with Q-factor of 42 and an eye opened response. The developed technique in the DPSK system is realized using commercial software package Optisystem. The designed technique will be helpful to enhance the performance existing high-speed optical communication by achieving the minimum BER at low power penalty.</p>

A linearly scaling optical apodization method for laser beam power profile modulation is induced. Changing an amplitude mask position between the beam-focusing cylindrical lens and the fiber can linearly scale the vertical apodization profile; this, in turn, enables the optimization of the grating apodization. By using this method, an apodized fiber Bragg grating (FBG) that is designed as an uncooled pump laser diode stabilizer with the specifications of 4.5% reflectivity, 1.6 nm bandwidth, and the side-lobe suppression ratio (SLSR) of 30 dB was UV-imprinted with a tiny Gaussian amplitude mask. Using the same apodization method, a nonchirped-type FBG edge filter was written with a very smooth spectrum. The grating specifications of 0.5-dB attenuation bandwidth of 3.6 nm and 96.5% reflectivity were achieved. The half-spectrum ascending bandwidth in the longer wavelength side is 2.2 nm with 27-dB SLSR. The linearly scaling apodization method is highly beneficial for optimizing the apodization process of UV-writing shorter FBGs with strong apodization.

High-speed transmission limitations due to Polarization Mode Dispersion

Bit error rate (BER) performances of DPSK are researched for space uplink optical communication system. Considering further phase fluctuation effect based on the traditional atmospheric turbulence model, BER performance versus transmission power, zenith angle, frequency deviation, divergence angle and transmission diameter are discussed for different wavelengths. Simulations, especially, indicate that considering the phase fluctuation effect, BER performance is sensitive for low frequency deviation. It also confirms that wavelength of 1550nm on communication system is still a good choice for the better BER performance. This work is helpful for enhancing transmission quality and the design of space optical communication system.

In this paper, characterization of the fiber Bragg grating (FBG) based on grating profile and apodization type has done. The FBG parameters considered here are grating length, grating profile and apodization type. The characteristics of FBG include reflectivity, full width at half maximum (FWHM) and side lobe power. The FBGs having high reflectivity, low FWHM and side lobe power are required for the sensing applications. A GratingMOD simulation tool based on a coupled mode theory and a transfer matrix method is used for the characterization of FBG. The effect of FBG parameters, grating length, grating profile and apodization on the characteristics of FBG is studied. The FBG parameters, grating length, grating profile and apodization type are optimized for the sensing applications. The FBG having grating length, 5 mm, sinusoidal profile and apodization type is optimized.In this paper, characterization of the fiber Bragg grating (FBG) based on grating profile and apodization type has done. The FBG parameters considered here are grating length, grating profile and apodization type. The characteristics of FBG include reflectivity, full width at half maximum (FWHM) and side lobe power. The FBGs having high reflectivity, low FWHM and side lobe power are required for the sensing applications. A GratingMOD simulation tool based on a coupled mode theory and a transfer matrix method is used for the characterization of FBG. The effect of FBG parameters, grating length, grating profile and apodization on the characteristics of FBG is studied. The FBG parameters, grating length, grating profile and apodization type are optimized for the sensing applications. The FBG having grating length, 5 mm, sinusoidal profile and apodization type is optimized.

The upper transmission range of the wavelength division multiplexing passive optical network (WDM-PON) can be restricted by chromatic dispersion (CD). This paper contains the investigation of till 16-channel WDM-PON system with efficient CD pre-compensation and post-compensation methods. It is shown that CD offset has a marginal role for guaranteed optical link downstream performance and maximum length of high-speed WDM-PON system. The results show that employment of extra 7 km long dispersion compensating fiber (DCF) placed in central office (pre-compensation configuration) enhances the WDM-PON network link length by 19.3% (from 57 km up to 68 km), but if extra 2 km long DCF fiber is used in remote terminal (post-compensation configuration) link length can be extended up to 5.3% (from 57 km to 60 km). It was found that usage of fiber Bragg grating (FBG) in central office (pre-compensation configuration) improves network reach up to 26.3% (from 57 km up to 72 km), but using this FBG for CD post-compensation in remote terminal network reach can be improved by 15.8% or 9 km in length - from 57 km to 66 km. Basis on the results authors recommend to use FBG as the best solution for CD compensation in pre-compensation configuration (before SMF line) in future high speed long-reach DWDM-PON systems.

Fiber Bragg gratings (FBGs) technology has demonstrated its suitability for many applications in recent fiber technologies. Sensing application is one of the main applications of FBGs. In this work, we present a comprehensive investigation for using apodized FBGs in sensing applications. Different evaluation parameters such as, reflectivity, sidelobes, and full-width-half-maximum (FWHM) are tested in order to determine the most proper apodization profile for sensors. According to our study, the Blackman apodization gives the best profile that can be used in sensing applications. The reflectivity of Blackman apodization is nearly unity with minimum sidelobes level, −60.3 dB, and narrow FWHM. The length of Blackman apodized FBG is 0.33 cm and Δn = 14.4 × 10□4 and maximum reflectivity is 99.44%.

Effects of source beam, link, and oceanic turbulence parameters on the scintillation index and bit error rate (BER) performance of cosine (cos) and cosine-hyperbolic (cosh) Gaussian light beams have been investigated in order to improve wireless optical communication link performance in oceanic turbulence. The Nikishov and Nikishov power spectrum of oceanic water and extended Huygens Fresnel principle were used in our evaluations; the results were obtained via MATLAB. The scintillation index and BER were examined versus oceanic turbulence parameters, which are the rate of dissipation of mean-square temperature, the ratio of temperature and salinity contributions to the refractive index spectrum, and the dissipation rate of kinetic energy per unit fluid mass of fluid. Further, the scintillation index and BER are investigated against the source size, propagation distance, and complex displacement parameters of cos- and cosh-Gaussian beams. This study aimed to select the suitable sinusoidal beam to be employed in order to increase the performance of underwater wireless optical communication systems operating in oceanic turbulence.

In this paper, a new kind of method is developed to analyse the bit-error rate for a free-space optical communication system, which is based on the three most important parameters of scintillation statistics, i.e. fraction of time of fade and surge, mean frequency of level crossings, and mean duration of fade and surge. By using this method, the relationship between the bit-error rate and scintillation is clearly identified. The tip–tilt error deteriorates the performance of a free-space optical communication system. Fine tracking ability of adaptive optics (through tip–tilt compensation) can overcome this difficulty. The fade and surge statistical parameters with tip–tilt compensation are evaluated for the plane wave model. The bit-error rate as a function of the scintillation statistical parameters (with tip–tilt compensation) is developed. The results show that the bit-error rate is reduced notably through tip–tilt compensation for free-space optical communication.

The performance of free space optical communication system using multi-input multi-output orthogonal frequency division multiplexing (MIMO-OFDM) with a modified receiver structure is evaluated in this paper referred to as MIMO-OFDM-FSO with the modified receiver (MIMO-OFDM-FSO-MR). Utilizing equal gain combining the analysis is done by taking closed-form expression of average bit error rate and throughput. The simulation is performed to estimate various performance parameters such as throughput and bit error rate (BER) for the proposed MIMO-OFDM-FSO-MR model under different turbulence and weather conditions. Simulation reveals that the proposed model exhibits significant BER performance and provides better throughput and spectral efficiency as compared with conventional FSO systems, OFDM-FSO systems, MIMO-FSO systems and Low-density Parity Check (LDPC) coded MIMO-OFDM-FSO systems.

We propose and demonstrate a multi-layer composite apodization technique using a femtosecond laser to expand the bandwidth of fiber Bragg gratings (FBGs) with maintained reflectivity and a good side lobe suppression ratio (SLSR). A theory of multi-layer composite apodization incorporating two apodization functions is developed. The calculation results confirm the expanding effect of bandwidth and a gentler group delay characteristic by the proposed technique. The composite apodization is achieved using amplitude modulation by a spatial light modulator and variation of the inscribing path by programming simultaneously. Multi-layer line-by-line inscription is employed to achieve approximate plane modulation. The repeated results reveal that the FBG inscribed using the proposed technique exhibits a wider bandwidth without severe degradation in the SLSR while maintaining similar reflectivity, which is approximately 17 times wider than that inscribed by point-by-point conventional apodization. The results of the calculation for the influence on the spectrum with different inscribing layers in the radial direction are in good agreement with the experimental findings, demonstrating that the composite AFBG at 4 layers achieves an optimal balance between bandwidth and SLSR. The length of the FBG with the widest bandwidth is only 0.6 mm. The technique has potential in compact optical devices.

An experimental investigation of the performance of an open-loop optical chaotic communication system has been carried out under different optical injection conditions. The most popular message encoding techniques have been considered, tested and compared at gigabit rates and for different levels of optical injection to the receiver, by bit-error-rate (BER) measurements. Different low-pass electrical filters have been implemented for the various message bit-rates tested, in order to employ the most appropriate one for message decoding. The best performance of all the examined encryption methods for messages of 1.5 Gb/s bit-rate emerged for the chaos modulation encryption method and was characterized by Q-factor values up to 5.1, after synchronizing in the strong injection regime, while the corresponding BER of the decoded message was as low as 7/spl middot/10/sup -5/. In order to confirm the secure encryption of the message, BER measurements are also performed for the chaotic carrier into which the message is encrypted.