KEYWORDS: Orthogonal frequency division multiplexing, Adaptive optics, Receivers, Signal to noise ratio, Modulation, Interference (communication), Transmitters, Monte Carlo methods, Quadrature amplitude modulation, Optical engineering
A dual stream asymmetrically clipped optical (DSACO)-orthogonal frequency division multiplexing (OFDM) with intensity modulation/direct detection receiver is proposed that combines the ACO-OFDM, which is generated from individual streams of odd and even frequencies in a single frame without loss of information. The clipping noise, which is its absolute value, is subtracted from even subcarriers of DSACO-OFDM signal and further delayed in time domain to recover the ACO-OFDM data of the odd stream. The proposed DSACO has the same spectral efficiency of DC-biased optical OFDM (DCO-OFDM). It performs better in terms of optical signal-to-noise ratio (OSNR) and peak-to-average power ratio (PAPR) when compared to ASCO-OFDM and Lowery’s layered/enhanced ACO-OFDM (L/E ACO-OFDM). For a bit error rate of 10 − 3, DSACO shows around 1.9, 3.4, and 7.6 dB improvements of OSNR over two-layered L/E ACO-OFDM, three-layered L/E ACO-OFDM, and ASCO-OFDM for 1024-quadrature amplitude modulation constellations, respectively. The complimentary cumulative distributive function of PAPR for DSACO achieves 0.27 dB lower PAPR than three-layered L/E ACO-OFDM system and 1.17 dB lower PAPR than two-layered L/E ACO-OFDM and ASCO-OFDM.
This paper presents an experimental investigation of a triple lasing wavelength of fiber laser employing bidirectional SOA incorporated with a tapered fiber which operates in the L-band region. This ring configuration comprises of a bidirectional semiconductor optical amplifier and a tapered fiber. Based on the results, a single mode tapered fiber in the configuration has the ability to enhance the wavelength performance in terms of the optical signal to noise ratio and its stability. This tapered optical fiber has a diameter waist of 12 μm, length waist of 10 mm and 5 mm for its uptaper length and downtaper length. Semiconductor optical amplifier injected current varies for 130 mA, 210 mA, and 300 mA to observe fiber laser performance in term of average output power and optical signal to noise ratio. This fiber laser can lase three wavelengths at 1576 nm, 1586 nm and 1596 nm. These triple wavelengths output exhibits an excellent performance by having the average optical signal to noise ratio of 44 dB, and average peak power is -13 dbm. Furthermore, fiber laser employing bidirectional SOA incorporated with a tapered fiber has efficient performance lasing wavelength stability over 60 minutes.
Linewidth optimization of a fiber grating Fabry–Perot (FGFP) laser is performed numerically. In addition to the external optical feedback (OFB), the effect of temperature, injection current, cavity volume, gain compression factor, and external cavity parameters [i.e., coupling coefficient (C o ) and external cavity length (L ext )] on linewidth characteristics are investigated. The effects of external OFB and temperature on linewidth characteristics are calculated according to their effect on threshold carrier density (N th ). The temperature dependence (TD) of linewidth characteristics is calculated according to the TD of laser parameters instead of the well-known Pankove relationship. Results show that the optimum external cavity length (L ext ) is 3.1 cm and the optimum range of operating temperature is within ±2°C from the fiber Bragg grating (FBG) reference temperature (T o ). In addition, the antireflection (AR) coating reflectivity value of 1×10 −2 is sufficient for the laser to operate at narrow linewidth and low fabrication complexity. The linewidth can be reduced either by increasing the laser injection current or the strength of external OFB level.
Realization of two-bit all-optical analog-to-digital conversion for an analog signal sampled by a femtosecond soliton sequence is investigated. Two approaches are suggested. The first one is based on filtering the broadened soliton spectrum after evolution over half of the soliton period in a standard single-mode fiber. In the second approach, the pulse is temporally sampled at the specified times after propagating through one soliton period. The sampled soliton sequence must be amplified to achieve an initial peak power of between 0 and 75 kW for the first method and between 0 and 66 kW for the second method. The soliton pulse-width is 50 fs. Based on the resulted peak power, the "0" or "1" bit is generated with reference to the threshold values. Subsequently, the digital gray code is produced at the outputs. The effect of inaccuracy in filter frequency and fiber length are also studied in this paper. The first method is sensitive to variations in the filter frequency, whereas the second method is affected by the fiber length inaccuracy.
We demonstrate a method of generating a multiwavelength Brillouin/Erbium fiber laser in a ring cavity, with stimulated
Brillouin scattering as a mirror. All Four generated Stokes lines have peak powers above 0 dBm with equal spacing of 10
GHz (0.08 nm) at 130 mW pump power from a 1480 nm laser diode. Our technique suppresses other potential modes to
circulate in the laser cavity, thus the self-lasing modes are eliminated. The tuning range over 39 nm from 1527 nm to
1566 nm was successfully demonstrated, which is only limited by the amplification bandwidth of the erbium gain.
All-optical gain-control Raman fiber amplifier (RFA) based on ring cavity technique using a pair of circulators is investigated experimentally. A pair of 1465nm laser diodes is used as the pump source to the RFA which provides a gain peak at around 1565 nm. The analyses of gain and noise figure are carried out using lasing wavelengths of 1550 and 1565 nm. Gain and noise figure variations are 0.4 and 0.5 dB respectively. We observed strong influence of pump-signal relative intensity noise transfer that results in noise figure penalty. The analyses are also carried out on hybrid configuration of RFA and EDFA and the characteristics of the dynamic range of the configurations are being compared.
For 2 x 2 fused coupler, all power launched into first fiber is able to couple its whole power to second fiber after fusion. When fabricating monolithic fused coupler, degree of coupling (i.e. total power transfer from one fiber to another fiber) depends on all fibers put together in closed contact. For 3 x 3 coupler, the arrangements of fibers are normally equilateral triangle and linear array, each of which has its own advantages and applications. There are also other factors that determine the spectral characteristics of 3 x 3 fused coupler besides the arrangements of fibers. One of the factors is twist, specifically known as intertwined twist used in this work. In an equilateral triangle arrangement of fibers placed on a conventional Coupler Workstation, there is never total power transfer from the launched fiber to the non-launched fibers. The intertwined twist also modifies the equilateral triangle arrangement at fusion region and this causes more power to couple to one non-launched fiber then the other non-launched fiber. However, it is possible to get as close as 33 ± 3% splitting ratio among the three output ports by manipulating intertwined twist in the newly triangular arrangement of fibers.