H. Ghafouri-Shiraz

Modified quadratic congruence codes for fiber Bragg-grating-based spectral-amplitude-coding optical CDMA systems

We have constructed a series of new code families for the spectral-amplitude-coding optical code-division multiple-access (CDMA) system, and proposed new transmitter and receiver structures based on tunable chirped fiber Bragg gratings (FBGs). The proposed system has been analyzed by taking into account the effects of phase-induced intensity noise, shot noise, and thermal noise. We have also compared the performance of this system with that of a former system where a Hadamard code is used. It has been shown that the new code families can suppress the intensity noise effectively and improve the system performance significantly. When the effective power is large (i.e., >-10 dBm), the intensity noise is the main factor that limits the system performance. When the effective power is not sufficiently large, thermal and shot noise sources become the main limiting factors and the effect of thermal noise is much larger than that of shot noise.

New code families for fiber-Bragg-grating-based spectral-amplitude-coding optical CDMA systems

A series of new code families are constructed for spectral-amplitude-coding optical code division multiple access (CDMA) systems. Then structures of both the transmitter and the receiver in such a system are also proposed based on tunable chirped fiber Bragg gratings. Our analysis shows that the proposed new code families can suppress the intensity noise effectively and, hence, improve the overall system performance.

Performance analysis of optical spectral-amplitude-coding CDMA systems using a super-fluorescent fiber source

It has been proposed that the arrived photon count obeys negative binomial (NB) distribution when a thermal optical source is used. In this letter, we have analyzed the bit-error rate (BER) of an optical spectral-amplitude-coding CDMA system using NB distribution. It has been shown that when the number of simultaneous users is small, the resultant BER is much lower than that calculated using Gaussian approximation. When the number of simultaneous users is large (for example N>19 when the effective power is -27 dBm), both NE distribution and Gaussian approximation give almost the same value for the BER of the system.

Analysis of combined ball lens and conically lensed fiber scheme to improve the coupling efficiency and misalignment tolerance between laser diodes and single mode fibers

Conical microlenses fabricated at the end of single-mode fibers can increase the coupling efficiency between fibers and laser diodes but it has the disadvantage of having stringent sub-micron lateral alignment tolerances. In this paper, we present a theoretical analysis of a coupling scheme, which involves a combination of both ball lens and conically lensed fiber configuration. In the first part of the coupling efficiency and misalignment tolerances analysis, we have ignored the effect of aberration from the ball lens, and also the effect of reflection. We found that by using the proposed scheme, the coupling efficiency improved compared to using a single conically lensed fiber scheme, i. e. from 38.8 % to 89.1 % for a laser diode operating at λ = 1.55 pm and from 22.7 % to 87.4 % for a laser diode operating at λ = 1.3 μm. However, when lens aberration is taken into account in the analysis, it reduces the coupling efficiency from the ideal case of 89.1 % to 59.5% for a laser diode operating at λ = 1.55 pm and from 87.4% to 40.4% for a laser diode operating at λ = 1.3 pm. We also found in the analysis that the 1-dB longitudinal and lateral misalignments of the two lens coupling scheme improved compared to the single conically lensed fiber scheme.

Tunable optical narrowband filter using a gain-coupled phase-shift-controlled distributed feedback laser

In this paper, we have proposed and analyzed a new tunable optical narrowband filter using gain-coupled phase-shift-controlled distributed feedback (GC-PSC-DFB) laser diode. Coupled-mode equations are solved by using the transfer matrix method (TMM). The GC-PSC-DFB filters offer a stable single-mode bandpass output, similar to that obtained with phase-shifted index-coupled structures. However, GC structures do not suffer from the severe longitudinal spatial hole burning (SHB) that occurs in high-coupling quarter-wave-shifted DFB filters. This SHB can cause multimoded behavior for high-input signal power. Various filter parameters such as wavelength tuning range, side-mode suppression ratio (SMSR), and channel gain deviation have been investigated and discussed. Our results show that the GC DFB structures offer a wider tuning range of 23.3 /spl Aring/ compared with the similar index-coupled DFB structures with nearly steady bandwidth of 12 GHz, while maintaining 41.8-dB constant gain.

A simple technique to determine the far field intensity pattern of laser diodes and its influence on coupling efficiency

Both the far field intensity pattern (FFIP) of a laser diode and the fiber electric field profile play an important role in determining the coupling efficiency between laser diodes (LDs) and optical fibers. In this paper, we have introduced a novel model for the FFIP of laser diodes, which is based on the non-linear regression technique. The advantage of this new model compared to that of previous FFIP models is that, the later requires laser diode parameters such as the thickness, effective index and relative refractive index to generate the theoretical far field intensity pattern. Manufacturers of commercial laser diodes normally do not provide these parameters and thus researchers have been using Gaussian approximation in the analysis of FFIP. In this work, we would show that the proposed new model reflects a more accurate representation of the far field intensity pattern compared to the Gaussian model, thus enabling a better estimation of the spot size of the LD. Based on this model and the actual electric field distribution of the single-mode fiber we have analysed the coupling efficiency between a LD and a conically lensed fiber. The obtained results are in better agreement with the experiment, for example we have obtained 56% as the maximum theoretical value for the coupling efficiency using this new proposed model which is very close to the experimental result of 55%).

Analysis of coupling coefficient in a laterally tapered waveguide distributed feedback laser using ray optics technique

Based on ray optics technique we have analyzed the first-order backward coupling coefficient in a laterally tapered distributed feedback (DFB) laser diode. These tapered structures are designed to realize narrow beam divergence, low threshold current and high longitudinal mode selectivity. The coupling coefficient, κ, for three different laterally tapered waveguide distributed feedback laser diodes have been studied. It has been found that without changing the grating period and the material properties, the coupling coefficient can be altered by modifying the taper profile and waveguide dimensions while satisfying the adiabatic single-mode condition.