Under proper injection of an incoming optical signal to be format-converted, a semiconductor laser can be driven at period-one dynamics due to the dynamical competition between the injection-imposed laser oscillation and the injection-shifted cavity resonance. Equally separated spectral components therefore emerge, the intensity and frequency of which strongly depend on the intensity and frequency of the incoming optical signal. Optical modulation format conversion between amplitude-shift keying (ASK) and frequency-shift keying (FSK) can thus be achieved by applying such a mechanism. The conversion depends solely on the property of the incoming optical signal for a given laser and therefore only a typical semiconductor laser is necessary as the key conversion unit. Due to the unique underlying mechanism, both ASK-to-FSK and FSK-to-ASK conversions can be achieved using the same system. The bit-error ratio at 10 Gb/s is observed down to
External-cavity lasers are usually used for chaos encryption in optical chaos-based communication systems. The external-cavity round-trip time (the time delay in the laser dynamics) is often regarded as an additional key to encode messages, which is a critical security parameter. The feasibility of identifying the time delay has been a crucial issue in chaotic optical communication. Some researchers propose that the time delay can be hidden by modulating the value of feedback strength or increasing the number of feedback cavities. In this paper, we experimentally and numerically demonstrate that the time delay signatures cannot be concealed in optical feedback semiconductor lasers. Whether single or double optical feedback, the time delay signatures can all be identified by the power spectrum analysis method. Furthermore, adjusting the feedback strength, the pumping current and the time-delay value, we find that the extraction of the time delay signatures still cannot be influenced.
Gold nanoparticles embedded in an optical gain material, particularly in a water solution of Rhodamine 6G, used in dye lasers can both increase and damp dye flourescence, thus changing the laser output intensity. Simultaneously, such nanoparticles influence the gain material's resistance against photobleaching. In this paper, we report our study on the impact of the
We report on the design for an electroabsorption-modulated laser (EML) that can provide a high immunity to the optical signal reflected from its output facet without any fabrication complexity and performance compromise. In particular, we theoretically investigate the effects of residual facet reflection on the static and dynamic performances of the high-speed EMLs. For this analysis, an accurate and consistent time-domain transfer-matrix-based laser model is developed. Based on this model, we perform the steady-state and large-signal analyses for the EML with finite facet reflectivities. The simulation result indicates that the EML with a high-gain compression factor is capable of reducing the power fluctuation and facet reflection-induced chirp under large-signal modualtion. Thus, it is desirable to design for increasing the damping of a distributed feedback (DFB) laser to realize the EML with a high immunity to this facet reflection.
This report reveals that diffusion of hydrogen induces gradual degradation in InGaN-based laser diodes (LDs). The increase in nonradiative recombination centers (NRCs) in the LDs has been attributed to diffusion-related phenomena. Factors other than NRCs, such as the threshold carrier density
We discuss the spatial and spectral intensity distribution of emission in optical gain media under stripe excitation in the case where the intensity reaches saturation level. It is found that modes propagating along the stripe in different directions are spatially separated if they affect each other due to saturation. The investigation includes the effects of wavelength-dependent inhomogenities, such as localized losses and reflective perturbations. Even relatively small distortions of this kind are found to cause considerable spatial and spectral redistribution of the intensity compared to an ideal disorder-free medium. Our results indicate that a simple ansatz may describe some mechanisms that lead to the formation of random laser action that is commonly observed in high gain media, such as organic semiconductors. Furthermore, consequential difficulties of gain measurements in such media using stripe excitation experiments are highlighted.
We present a modified version of the multisection delayed differential equation model, for quantum dot passively mode-locked (ML) lasers when competition between ground state (GS) and excited state (ES) ML takes place. The model takes into account the difference in the group velocity of GS and ES fields. Sole GS, sole ES, and dual-state lasing and ML have been studied. The results are verified with time domain traveling wave simulations and compared, when possible, with experimental results. These tests confirmed the reliability of the model. We found that, in two-section ML lasers, GS lasing and mode locking are always more easily established. For instance, GS lasing can be either self-starting or induced by the initial lasing from the higher energy ES. On the contrary, GS lasing tends to inhibit, to a certain extent, the onset of ES lasing, especially at low injection current and low reverse voltage. Moreover, ES shows less potential to achieve stable ML than GS. Based on these findings, we propose proper theoretical explanation of the achieved lasing and ML regimes in realized devices.
A compact and tunable Erbium-doped fiber laser is demonstrated using a highly concentrated Erbium-doped fiber in conjunction with a microfiber Mach–Zehnder interferometer (MMZI) structure for the first time. A stable laser output is achieved at 1531.7 nm region with a peak power of
An innovative optical scheme to generate software-defined phase-modulated radio frequency (RF) pulses with carrier frequency agility from a mode-locked laser (MLL) is proposed. The technique exploits a direct digital synthesizer and a Mach–Zehnder modulator to apply an intermediate frequency modulation to the MLL's modes. The heterodyne detection of the optical signal allows the generation of amplitude- and phase-modulated RF carriers with very high phase stability, suitable for coherent radar applications. Further, a single MLL can be used to generate carriers simultaneously at different frequencies, enabling frequency hopping or multifunctional radars, with no need to increase the complexity of the transmitter. Results show chirped and Barker-coded pulses at around 10 or 40 GHz in a single setup, without any performance degradation while increasing the carrier frequency. The proposed technique allows the practical realization of compressed pulses for coherent radars over a wide carrier frequency range, allowing the development of software-defined radar systems with improved functionalities.
The modulation response of quantum-dot (QD) lasers is studied. Based on a set of four rate equations, a new analytical modulation transfer function is developed via a small-signal analysis. The transfer function can clearly describe the impacts of the wetting layer and the excited states: finite carrier capture and carrier relaxation times as well as the Pauli blocking limits the modulation bandwidth. The definitions of the resonance frequency and the damping factor of QD lasers are also improved. From the analysis, it is demonstrated that carrier escape from the ground state to the excited states leads to a nonzero resonance frequency at low bias powers associated to a strong damping factor.