H. A. Abdul-Rashid

WIDE-BAND HYBRID AMPLIFIER OPERATING IN S-BAND REGION

Wide-band hybrid amplifier is theoretically proposed using a series configuration of Thulium-doped fiber amplifier (TDFA) and fiber Raman amplifier (FRA), which using the similar type of pump laser. The operating wavelength of this amplifier covers the bandwidth of entire short wavelength band (S-band) region by combining the gain spectrum of TDFA and FRA. The theoretical gain varies from 20 to 24dB within a wavelength region from 1460 to 1525nm and which is in a good agreement with the experimental result. The development of reliable high-power diode lasers in the 1420nm wavelength range will make this type of wide-band hybrid amplifier an interesting candidate for S-band optical telecommunication systems.

Cyclic prefix reduction for 20.48 Gb/s direct-detection optical OFDM transmission over 2560 km of SSMF

Optical Orthogonal Frequency Division Multiplexing (O-OFDM) provides major advantages in mitigating Group-Velocity Dispersion (GVD) in Single Mode Fiber (SMF). Unfortunately, when the uncompensated long-haul transmission ranges become very large, substantial dispersion is accumulated. Owing to the large accumulated dispersion, the Cyclic Prefix (CP) duration will occupy a substantial fraction of the OFDM frame. This effect sets some limitations on the overall throughput and the spectral efficiency. Moreover, the transmission is inefficient because of the energy wastage contained within the CP. In the case where the CP length is shorter than the Channel Impulse Response (CIR), energy wastage is reduced but the system performance is limited by the Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI). To overcome this problem, a Time Domain Equalizer (TEQ) immediately after the channel is proposed. It can cancel the residual ISI and ICI caused by both the GVD and the CP length being shorter than the CIR. The simulation results show that, using BER of 10−3 as a reference, the system performance improves by 2.69thinspacedB while considering 6-feedforward/5-feedback-weight Decision Feedback TEQ (DF-TEQ) and 2.03 dB while considering 10-tap Least Mean Square TEQ (LMS-TEQ) even though the CP length is shorter than the CIR. This method reduces size of the CP, and consequently the performance of the system will be improved. Copyright

Gain and noise figure improvements in a shorter wavelength region of EDFA using a macrobending approach

Gain and noise figure improvements are demonstrated in a shorter wavelength region of a double-pass EDFA using a macrobending approach. The EDF is wound in a small radius to suppress the amplified spontaneous emission at the longer wavelength in order to achieve a high amplification in the shorter wavelength. Gain enhancements of about 12–14 dB are obtained with macrobending at the wavelength region between 1480 and 1530 nm. The macrobending also reduces the noise figure of the EDFA at wavelengths shorter than 1525 nm with a maximum improvement of 25 dB.

Thermoluminescence characteristics of flat optical fiber in radiation dosimetry under different electron irradiation conditions

Thermoluminescence (TL) flat optical fibers (FF) have been proposed as radiation sensor in medical dosimetry for both diagnostic and radiotherapy applications. A flat optical fiber with nominal dimensions of (3.226 × 3.417 × 0.980) mm3 contains pure silica SiO2 was selected for this research. The FF was annealed at 400°C for 1 h before irradiated. Kinetic parameters and dosimetric glow curve of TL response were studied in FF with respect to electron irradiation of 6 MeV, 15 MeV and 21 MeV using linear accelerator (LINAC) in the dose range of 2.0-10.0 Gy. The TL response was read using a TLD reader Harshaw Model 3500. The Time-Temperature-Profile (TTP) of the reader used includes; initial preheat temperature of 80°C, maximum readout temperature is 400°C and the heating rate of 30°Cs-1. The proposed FF shows excellent linear radiation response behavior within the clinical relevant dose range for all of these energies, good reproducibility, independence of radiation energy, independence of dose rate and exhibits a very low thermal fading. From these results, the proposed FF can be used as radiation dosimeter and favorably compares with the widely used of LiF:MgTi dosimeter in medical radiotherapy application.

Thermoluminescence characteristics of Ge-doped optical fibers with different dimensions for radiation dosimetry

Important thermoluminescence (TL) properties of five (5) different core sizes Ge-doped optical fibers have been studied to develop new TL material with better response. These are drawn from same preform applying different speed and tension during drawing phase to produce Ge-doped optical fibers with five (5) different core sizes. The results of the investigations are also compared with most commonly used standard TLD-100 chips (LiF:Mg,Ti) and commercial multimode Ge-doped optical fiber (Yangtze Optical Fiber, China). Scanning Electron Microscope (SEM) and EDX analysis of the fibers are also performed to map Ge distribution across the deposited region. Standard Gamma radiation source in Secondary Standard Dosimetry Lab (SSDL) was used for irradiation covering dose range from 1Gy to 10Gy. The essential dosimetric parameters that have been studied are TL linearity, reproducibility and fading. Prior to irradiation all samples ∼0.5cm length are annealed at temperature of 400°C for 1h period to standardize their sensitivities and background. Standard TLD-100 chips are also annealed for 1h at 400°C and subsequently 2h at 100°C to yield the highest sensitivity. TL responses of these fibers show linearity over a wide gamma radiation dose that is an important property for radiation dosimetry. Among all fibers used in this study, 100μm core diameter fiber provides highest response that is 2.6 times than that of smallest core (20μm core) optical fiber. These fiber-samples demonstrate better response than commercial multi-mode optical fiber and also provide low degree of fading about 20% over a period of fifteen days for gamma radiation. Effective atomic number (Zeff) is found in the range (13.25-13.69) which is higher than soft tissue (7.5) however within the range of human-bone (11.6-13.8). All the fibers can also be re-used several times as a detector after annealing. TL properties of the Ge-doped optical fibers indicate promising applications in ionizing radiation dosimetry.

Optimization of the 1050nm Pump Power and Fiber Length in Single-Pass and Double-Pass Thulium Doped Fiber Amplifiers

The pump power and thulium-doped fiber (TDF) length for both single-pass and double-pass Thulium-Doped Fiber Amplifiers (TDFA) are theoretically optimized by solving differential equations. The 1050 nm pump is used to provide both ground-state and excitedstate absorptions for amplification in the S-band region. The TDFA is saturated at a shorter length with a higher gain value as the operating pump power increases. The double-pass TDFA allows double propagation of the test signal in the gain medium, which increases the effective TDF length and thus improves the gain of the TDFA compared to the single-pass configuration. Therefore, a small signal gain improvement of approximately 15 dB is obtained in the 1465 nm region. However, a noise figure penalty of approximately 1 dB is also obtained in this wavelength region. The theoretical result is in agreement with the experimental result.

Optimization of gain flattened C-band EDFA using macro-bending

Optimization of gain flattened C-band erbium-doped fiber amplifier (EDFA) using a macrobending method with an improved gain flatness and bandwidth is demonstrated. The optimization for gain flatness and bandwidth was achieved by varying the bending radius and the length of the doped fiber. In the optimized condition, the gain saturation effect as well as the energy transfer from shorter wavelengths to longer wavelengths resulted in a flattened and broadened gain profile in the C-band region. The amplifier was optimized to a 9 m long erbium-doped fiber (EDF) with erbium ion concentration of 1100 ppm and bending radius of 6.5 mm. The gain variation of the EDFA is obtained within ±1 dB over 25 nm bandwidth of C-band region.

Effect of transverse distribution profile of thulium on the performance of thulium-doped fibre amplifiers

We study numerical models for different transverse thulium distribution profiles (TTDPs) characterising the fibres used in thulium-doped fibre amplifiers (TDFAs). Our models consider the overlap factor and the absorption/emission dynamics. Basing on the radial TTDP function of the form nT (r) = nT,max exp[-(| r-δ |/θ) β], we show that the TDFA gain increases with increasing β parameter and decreases as the θ parameter increases from 1 up to 3 μm, due to the overlap factor which affects the absorption and emission dynamics of the TDFA. The overlap factor increases with increasing β and decreases with increasing θ value. Finally, the noise figure increases as θ does so, due to suppression of the amplified spontaneous emission.

Performance analysis of Least Mean Square Time-Domain Equalizer for 20.48 Gb/s direct-detection optical OFDM transmission over 2560 km of SMF

Optical Orthogonal Frequency Division Multiplexing (O-OFDM) provides major advantages in mitigating Group-Velocity Dispersion (GVD) in Single Mode Fiber (SMF). Unfortunately, when the uncompensated long-haul transmission ranges become very large, substantial dispersion is accumulated. Due to the large accumulated dispersion, the Cyclic Prefix (CP) duration will occupy a substantial fraction of the OFDM frame. This effect sets some limitations on the overall throughput and the spectral efficiency. Moreover, the transmission is inefficient because of the energy wastage contained within the CP. In the case where the Channel Impulse Response (CIR) is larger than the CP, the system performance is limited by the Inter Symbol Interference (ISI) and Inter Carrier Interference (ICI). In this paper, we propose a Least Mean Square Time Domain Equalizer (LMS-TEQ) immediately after the channel. It can cancel the residual ISI and ICI caused by both the GVD and the CP length being shorter than the CIR. Our simulation results show that, using BER of 10−3 as a reference, the system performance improves by 2.71 dB while considering an 8-tap LMS-TEQ, GVD, Gaussian Noise and in the case where the CP length is shorter than the CIR. This method reduces size of the CP, and consequently the performance of the system will be improved.

Effects of an auxiliary pump on the performance of TDFA

An efficient fluoride-based thulium-doped fiber amplifier (TDFA) is theoretically demonstrated using a dual pumping scheme. Differential equations are solved directly in the theoretical analysis. An auxiliary pump at 1560 nm is used for ground-state absorption to enhance the excited-state absorption provided by the main pump of 1050 nm and, thus, to improve the gain and noise figure of the TDFA. A gain improvement of more than 10 dB is obtained at the 1470-nm region with the use of a 1560-nm pump at 20 mW. A small signal gain as high as 30 dB is obtained at this region with 100 mW of a 1050-nm pump and 20 mW of a 1560-nm pump using a 20-m of thulium-doped fiber. The corresponding noise figure is obtained at lower than 5 dB.