Broadband optical supercontinuum generation using low-cost multimode 975-nm pump lasers

Broadband laser sources are very attractive for optical communications and technologies requiring low-coherence laser sources. The supercontinuum (SC) is generated by ultra-short pulsed or continuous wave (CW) lasers that are injected into fibers with high nonlinearity. Mode-locked lasers and Raman fiber lasers are generally used in the two pumping regimes. Pumping with 975-nm multimode laser diode is considered a low cost technology in the CW regime. However, it has not been well investigated. This thesis studies the SC generation process using low-cost 975-nm multimode laser diodes, a piece of Erbium/Ytterbium co-doped fiber (EYDF) and two units of highly nonlinear fibers (HNLFs) with different dispersion properties around 1550nm wavelengths. Three broadband and high optical power SCs extending to 2000nm wavelength are successfully generated, one in ring and two in single-line structure. To our knowledge, they are the broadest SCs using this low cost technology. In addition, one of the two SCs in the single-line structure even covers a wavelength range from 1200nm to more than 2000nm by getting rid of the band limit of splitter. Moreover, the key role of four-wave mixing (FWM) in the broadness and flatness of CW pumped SC are demonstrated. The demonstration is more evident than previous works and convergence is obtained in the two pumping regimes on the importance of FWM in SC generation. Our designs realize fiber laser sources with high power, broad bandwidth and reduced cost

Investigation of Single-Section InAs/InP Quantum Dot Mode-Locked Lasers

The study of mode-locking in generating short pulses began in the 1960s. Since then, the advances have been remarkable over almost 50 years and some of the mode-locked lasers (MLLs) have been commercialized. Short pulses from sub-picoseconds to femtoseconds have been successfully demonstrated from crystal and fiber based lasers. The diverse applications of MLLs have been pushing the development of MLLs in high bit rate transmission, optical time division multiplexed transmission, optical clock recovery, ultrafast signal processing and frequency comb, etc. Semiconductor lasers have advantages of simplicity, compactness and high efficiency. They have attracted interests in the application of optical communications. Until now, semiconductor MLLs are mainly based on bulk and quantum well (QW) structures. More recently, quantum dot (QD) based MLLs have attracted more and more attentions. The main characteristic of QD is the delta-function-like density of states with electrons confined in all three dimensions. It is promising in ultrashort and ultrafast pulse generations as a result of inhomogeneous gain broadening, broad gain bandwidth and fast carrier dynamics. In passive mode-locking, a two-section structure is usually used. A saturable absorber section is essential in the lasing cavity to initiate and shape pulses, which is also the case in almost all QD MLLs. However, without the absorber, passive mode-locking can also be achieved in single-section QD cavity, which has not been well studied yet. This thesis focuses on investigating single-section InAs/InP QD MLLs. It aims at improving the laser performance by both experimental and theoretical analyses. The following works have been done in this thesis. Firstly, as an important parameter of all semiconductor lasers, the linewidth enhancement factor is measured using two methods: the Hakki-Pauli method that is used for a laser below threshold and injection-locking technique for a laser above threshold. The results from the two methods agree with each other, and it is found that the linewidth enhancement factor of our QD lasers is much smaller than that of QW based lasers. Secondly, the time-domain travelling-wave model is used to investigate the single-section QD MLLs. By introducing an equivalent saturable absorber, the pulse generation and evolution are successfully simulated. Furthermore, this model is improved by including the effects of group-velocity dispersion (GVD) and self-phase modulation. It is found that the GVD effect plays an important role in the pulse width evolution of our mode-locked lasers. The improved model can be widely extended to other types of semiconductor lasers and amplifiers. Thirdly, high-repetition-rate pulse trains of up to 1 THz are generated from a QD laser combined with fiber-Bragg-grating (FBG) external cavities. The QD laser is used for multi-mode gain and several specific modes are selected by the FBGs. The pulse train is measured by using the time-domain autocorrelator, and the repetition rate is in agreement with the frequency spacing of the FBGs. Finally, tunable terahertz beat waves of up to 2.1 THz are generated also using FBG external cavities. This method may find applications for generating microwave, millimeter wave and terahertz wave

Effect of Ridge Width on the Lasing Characteristics of Triangular and Rectangular InAs/In0.53Ga0.47As Quantum Well Lasers

The lasing characteristics of InP-based InAs/In0.53Ga0.47As quantum well (QW) lasers with different ridge widths are investigated. Two groups of lasers are grown for comparison, one with active triangular QW regions and the other with rectangular QW regions. Their output powers, characteristic temperatures (T0), external differential quantum efficiencies (ηd) and junction temperatures (Tj) are analyzed and compared. The parameter of ridge width is found to play an important role in the performance of the lasers. In triangular QW lasers, by broadening the ridge width from 8 to 12\ua0μm, output power and ηd of the lasers are decreased for the temperature range of 100–320\ua0K due to heating effect. But by broadening the ridge width from 8 to 100\ua0μm in rectangular QW lasers, output power has about 3.5 time increase at 100\ua0K and ηd also has a little increase for temperatures from 100 to 180\ua0K due to much larger emission area and much faster heat dissipation. Tj, the real temperature of the active region, is also found to have accelerated increase at high injection current and heat sink temperature. Besides, compared to the rectangular QW laser of the same ridge width, the improved thermal performance of triangular QW laser is also demonstrated

Modeling of single-section quantum dot mode-locked lasers: impact of group velocity dispersion and self phase modulation

A short pulse train with pulsewidth < 1 ps was generated in a quantum dot mode-locked laser (QD MLL). Due to the short dispersion length, it is required to include group-velocity dispersion (GVD) in modeling pulse train generation and evolution from QD MLLs. On the other hand, Kerr effect is also required to consider due to high peak power density in the laser cavity, and its induced self-phase modulation (SPM) also contributes to the pulse evolution. In this paper, a time domain traveling wave model, including the effect of GVD and SPM, combined with rate equations, is established to model the pulse evolution in a single-section QD MLL. It is shown that the pulse evolution calculated by this model is in reasonable agreement with the experiments. The contribution to the pulse evolution by the GVD and SPM impact is discussed.Peer reviewed: YesNRC publication: Ye

A C -Band InAs/InP quantum dot semiconductor mode-locked laser emitting 403-GHz repetition rate pulses

A passive InAs/InP quantumdot (QD) semiconductor mode-locked laser (MLL) emitting 403-GHz repetition rate pulses with 268- and 563-fs pulse durations is demonstrated experimentally around 1.54 m. The QDMLL consists of a QD Fabry\u2013P\ue9rot laser and external cavities that include eight fiber Bragg gratings (FBGs). The mode-locking is realized by four-wave mixing in the QD waveguide between the longitudinal modes selected by the FBGs.Peer reviewed: YesNRC publication: Ye

Tunable terahertz beat signal generation from an InAs/InP quantum-dot mode-locked laser combined with external-cavity

Tunable terahertz beat signal generation is demonstrated by using a C-band InAs/InP quantum-dot (QD) mode-locked laser combined with external cavity of two fiber Bragg gratings (FBGs), where one of the FBGs is tunable in wavelength. Beat signals with ultra-high repetition rates quasi-continuously from 1 to 2.21 THz are observed between the two modes, which are phase-correlated due to intracavity four-wave mixing effect in the QD waveguide.Peer reviewed: YesNRC publication: Ye

InAs triangular quantum wells grown on InP/SiO2/Si heterogeneous substrate for mid-infrared emission

The properties of InAs/In0.53Ga0.37As triangular quantum wells (QWs) grown on an InP/SiO2/Si integrated substrate by ion-slicing technology are investigated. The material structure and growth quality are characterized by the X-ray diffraction (XRD) and transmission electron microscope measurements. The photoluminescence (PL) spectra at various temperatures are also analyzed. The PL peak wavelengths red-shift from 1.94 to 2.13 μm with the increase of temperature from 12.4 to 300 K. The experimental results of the QWs on InP/SiO2/Si substrate are found to be comparable with the performance of the same QWs grown on an InP substrate. The results are promising for future integration of Si with InP-based optical devices for the applications of light emission in mid-infrared wavelength range