In this paper, we review the recent progress in the optical signal processing based on the nonlinearity of semiconductor optical amplifiers (SOAs). The four important optical signal processing functional blocks in optical switching are presented, i.e., optical wavelength conversion, optical regeneration, optical logic, and optical format conversion. We present a brief overview of optical wavelength conversion, and focus on various schemes to suppress the slow gain recovery of the SOA and improve the operating speed of the SOA-based optical switches. Optical regeneration including re-amplification, re-shaping and re-timing is also presented. Optical clock recovery that is essential for optical regeneration is reviewed. We also report the recent advances in optical logic and optical format conversion, respectively. After reviewing the four important optical signal processing functional blocks, the review concludes with the future research directions and photonic integration.
Slow light in planar photonic structures has attracted for some years an increasing interest due to amazing physical effects it allows or reinforces and to the degrees of freedom it raises for designing new optical functions. Controlling light group velocity is achieved through the use of periodical optical media obtained by nano-structuration of semiconductor wafers at the scale of light wavelength: the so-called photonic crystals. This article reviews present achievements realized in the field of slow light photonic bandgap structures, including the physical principles of slow light to the description of the most advanced integrated optical devices relying on it. Challenges and current hot topics related to slow light are discussed to highlight the balance between the advantages and drawbacks of using slow waves in integrated photonic structures. Then, future trends are described, which is focused on the use of slow wave slot waveguides for non-linear optics and bio-photonic applications.
Microwave photonic filters have been characterized by low loss, light weight, broad bandwidth, good tunability, and immunity to electromagnetic interference, and these filters can overcome inherent electronic limitations. Fiber-based filters are inherently compatible with fiber-optic microwave systems and can provide connectivity with built-in signal conditioning. This review paper presents developments of microwave photonic filters based on semiconductor optical amplifier in the last few years. Challenges in system implementation for practical application are also discussed.
A novel method is demonstrated to tunably compensate dispersion effect in phase modulated radio over fiber (RoF) links using an optical carrier Brillouin processing technique, which is based on stimulated Brillouin scattering (SBS) to control the phase shift of optical carrier in the modulated lightwave signal. Since this phase shift can be dynamically tuned, frequency response can be tunably improved. Both simulation and experimental results show that a uniform frequency response ranging from 1–12 GHz with a fluctuation of less than±1 dB can be obtained by an optimal phase shift on the optical carrier.
This paper experimentally investigated slow light effect in cascaded silicon-on-insulator (SOI) microring resonators. Double channel and single channel side-coupled integrated spaced sequence of resonators (SCISSOR) devices were fabricated with electron beam lithography and dry etching technology. The delay performances of the SCISSOR devices were demonstrated using non-return-to-zero (NRZ) and return-to-zero (RZ) signals at different bit rates. Group delays and bandwidths of cascaded microrings are significantly enhanced compared with single microring.
In this paper, we describe the impact of quadrature imbalance (QI) in the presence of frequency offset in an optical coherent offset quadrature phase shift keying (OQPSK) receiver. Arbitrary conjugate misalignment was realized in a 2×4 90° optical hybrid, and the ellipse correction (EC) method of quadrature imbalance was applied in our simulation. In the case of transmission, the EC method can significantly improve the system performance.
Pulse sources based on lithium niobate modulators are very attractive for optical time division multiplexing (OTDM) transmission systems because the modulators are now commercially available, qualified for system use, and can operate up to very high speeds and over a wide wavelength range. In this paper, we describe the principles of operation and performance of the pulse source based on lithium niobate modulators. The pulse source is based on a Mach-Zehnder intensity modulator (IM) and two phase modulators (PMs). The continuous-wave (CW) light is modulated in an IM and then strongly phase modulated in two cascaded PMs. The chirped pulses are subsequently compressed to desired width using dispersion compensation technology. This method has the advantage of acquiring larger chirp using normal PM rather than that special designed PM of very low
Based on spin-flip model (SFM), the impacts of mismatched intrinsic parameter on leader-laggard chaos synchronization between two mutually coupled vertical-cavity surface-emitting lasers (VCSELs) have been investigated numerically. Results show that, for two VCSELs with identical intrinsic parameter, the switching point of leader-laggard caused by continually varying frequency detuning or injection rate detuning is located at zero frequency detuning or zero injection rate detuning, which indicates that the VCSEL with higher frequency or subject to lower injection level plays a leader role. However, for two VCSELs with mismatched intrinsic parameter, the switched point of leader-laggard will deviate from zero frequency detuning or zero injection rate. Therefore, compared with the results obtained under matched intrinsic parameter, the opposite results have been observed in the range between zero detuning and switching point. Additionally, the offsets of switching point induced by different intrinsic parameters are different, and the influence of line-width enhancement factor is found to be the most significant.
In this paper, several applications in all-optical signal processing based on a semiconductor optical amplifier (SOA) and variable delayed interferometers (DIs) have been experimentally demonstrated. Wavelength converter based on a nonlinear polarization switch (NPS) and a DI is proposed and presented for the wavelength conversion of nonreturn-to-zero (NRZ) signals. An all-optical nonreturn-to-zero to return-to-zero (NRZ-to-RZ) format converter with tunable duty cycles is achieved by the DI with variable delays. The 40 Gb/s reconfigurable optical OR/NOR gate in a single SOA, followed a tunable optical bandpass filter (OBF) and a DI, optical 2R regeneration using an SOA-DI are investigated. It is found that this combinative realization of filters has been endowed with great flexibility and quality for 40 Gb/s optical logic and 2R regeneration.
By introducing gain/loss perturbation into periodic refractive index modulation, the unified coupled-mode equations and their close-form analytical solutions of complex long-period-grating-assisted-coupler (LPGAC) are deduced. Characteristics of power coupling are investigated and results show that unidirectional nonreciprocal signal transferring can be achieved by matching gain/loss with the refractive index modulation. Utilizing the feature of complex LPGAC, a novel optical signal buffer is proposed and its structure is examined in detail.
We propose and simulate a simple scheme of all-optical format conversion from non-return-to-zero differential phase-shift keying (NRZ-DPSK) to return-to-zero differential phase-shift keying (RZ-DPSK) by using phase modulators and detuning filters. The operation principle is theoretically analyzed and simulated by exploiting spectra, temporal waveform and eye diagram with commerical optical design software VPI Transmission Maker 8.5. The use of electrical clock recovery and linear phase modulation in the conversion scheme may be potiental in practise use.
A 10 Gb/s all-optical clock extraction based on magnetically controllable fiber optical parametric oscillator (MC-FOPO) is demonstrated. The operation properties of the magnetic control unit, composed of a solenoid and a magneto-optic crystal of high Verdet constant, are experimentally investigated. By adjusting the drive current of the solenoid, the magneto-optic crystal unit may serve as a tunable optical fiber delay line with polarization control to some extent. The experimental results show that the MC-FOPO is capable of repetitively magnetic tunability desirable for all-optical clock recovery.
In this paper, we have calculated the band structure of strained quantum well (QW) semiconductor optical amplifiers (SOAs) by using plane wave expansion method (PWEM) and finite difference method (FDM), respectively. The difference between these two numerical methods is presented. First, the solution of Schr?dinger’s equation in a conduction band for parabolic potential well is used to check the validity and accuracy of these two numerical methods. For the PWEM, its stability and computational speed are investigated as a function of the number of plane waves and the period of QW. For FDM, effects of mesh size and QW width on its accuracy and calculation time are discussed. Finally, we find that the computational speed of FDM generally is faster than that of PWEM. However, the PWEM is more efficient than the FDM when wider SOAs are needed to be calculated. Therefore, to obtain high accuracy and efficient numerical solutions for band structures, numerical methods should be selected depending on required accuracy, device structure and further applications.
We report a hybrid two-step approach for fabricating silica micro/nanofibers with different diameters (the minimum one down to 180 nm). Due to tapering and etching techniques introduced to this approach, the time is reduced from hundreds of minutes to several minutes to manufacture silica nanofibers by etching and the complexity of tapering mechanical system is brought down, because this approach has the ability to control the micro/nanofiber diameter on a nanometer-scale. Uniform nanofibers with losses as low as 0.05 dB/mm at 1.55 μm wavelength are obtained suggesting the advantage of the hybrid approach to build up micro/nanofiber-based devices, especially in locally changing the structure of micro/nanofiber.
Direct bonded periodically poled MgO doped lithium niobate (PPMgLN) ridge waveguide is a new wavelength converter with high conversion efficiency. The optical field distribution of the ridge waveguide is simulated by employing finite-difference method (FDM), the mode overlap of propagated waves in the ridge waveguide is calculated and the relationship between the overlap coefficient and the waveguide structure sizes is also investigated. The overlap coefficient to difference frequency generation (DFG) process conversion efficiency calculation is firstly introduced.