Self-Pulsating Amplified Feedback Laser Based on a Loss-Coupled DFB Laser

Monolithic self-pulsating semiconductor lasers called amplified feedback lasers (AFLs) can generate high-frequency self-pulsations according to the concept of a single-mode laser with shortly delayed optical feedback, which consist of a distributed-feedback (DFB) laser, a phase control, and an amplifier section. Since mode degeneracy of the DFB section, which should operate as a single-mode laser, affects the self-pulsation, single-mode characteristics of the DFB section are critical for the self-pulsation. The effect of a complex coupling in the DFB section on the self-pulsation is numerically analyzed to reveal that the complex coupling provides a wide operation range for the self-pulsation. Also, self-pulsating AFLs based on a loss-coupled DFB laser are experimentally demonstrated to verify the self-pulsation characteristics and the capability for all-optical clock recovery.     

Self-pulsation in multisection laser diodes with a DFB reflector

A novel self-pulsation regime is observed in multisection laser diodes which consist of a loss-coupled distributed-feedback (DFB) section, a phase control section, and gain sections, where 10-GHz self-pulsation due to compound cavity mode beating has been reported with the DFB section operated as a single-mode laser. When the DFB section is below threshold current, the devices give the self-pulsation in a very wide operating range. We attribute the pulsation to passive mode-locking and also confirm that this structure is applicable to 40-GHz operation.     

Optical generation and sideband injection locking of tunable 11-120GHz microwave/millimetre signals

It is shown that two-section gain-coupled DFB lasers with large section lengths and weak distributed feedback coupling exhibit a self-pulsation tuning range greater than reported previously. The phase noise of a sideband injection locked self-pulsation is measured and the jitter introduced by the self-pulsing laser found to be negligible     

Dispersive self-Q-switching in self-pulsating DFB lasers

Self-pulsations reproducibly achieved in newly developed lasers with two distributed feedback sections and with an additional phase tuning section are investigated. The existence of the dispersive self-Q-switching mechanism for generating the high-frequency self-pulsations is verified experimentally for the first time. This effect is clearly distinguished from other possible self-pulsation mechanisms by detecting the single-mode type of the self-pulsation and the operation of one section near the transparency current density using it as a reflector with dispersive feedback. The operating conditions for generating this self-pulsation type are analyzed. It is revealed that the required critical detuning of the Bragg wavelengths of the two DFB sections is achieved by a combination of electronic wavelength tuning and current-induced heating. The previous reproducibility problems of self-pulsations in two-section DFB lasers operated at, in principle, suited current conditions are discussed, and the essential role of an electrical phase-control section for achieving reproducible device properties is pointed out. Furthermore, it is demonstrated that phase tuning can be used for extending the self-pulsation regime and for optimizing the frequency stability of the self-pulsation. Improved performance of the devices applied as optical clocks thus can be expected     

Controllable self-pulsations in multi-section DFB lasers with anintegrated phase-tuning section

1.55-μm InGaAsP-InP multi-section DFB lasers with an integrated phase tuning section have been fabricated. It is shown for the first time that the self-pulsation can be electrically switched on and off by adjusting the phase current. Reproducible self-pulsation characteristics from device to device are achieved in this way     

三区DFB激光器高速自脉动的研究

全光信号再生技术是超高速大容量全光网络中的核心技术,其中全光时钟提取是全光再生技术的关键,基于多区DFB激光器件自脉动进行时钟提取是最佳选择方案。基于双区DFB激光器件自脉动研究的基础上,对三区DFB激光器件的自脉动特性进行了讨论和数值模拟分析,并对提高自脉动频率的方案进行了研究。     

High-frequency pulsations in DFB lasers with amplified feedback

We describe the basic ideas behind the concept of distributed feedback (DFB) lasers with short optical feedback for the generation of high-frequency self-pulsations and show the theoretical background describing realized devices. It is predicted by theory that the self-pulsation frequency increases with increasing feedback strength. To provide evidence for this, we propose a novel device design which employs an amplifier section in the integrated feedback cavity of a DFB laser. We present results from numerical simulations and experiments. It has been shown experimentally that a continuous tuning of the self-pulsation frequency from 12 to 45 GHz can be adjusted via the control of the feedback strength. The numerical simulations, which are in good accordance with experimental investigations, give an explanation for a self-stabilizing effect of the self-pulsations due to the additional carrier dynamic in the integrated feedback cavity.     

Analysis and design of combined distributed-feedback/Fabry-Perotstructures for surface-emitting semiconductor lasers

We propose combined distributed-feedback/Fabry-Perot (DFB/FP) structures for surface-emitting semiconductor lasers. The analysis is based on coupled-wave equations modified for surface-emitting lasers. The proposed structures, which exhibit enhanced resonance due to a matching between the gain and field distributions resulting in a reduced threshold compared with simple FP structures, are formed by placing the DFB structure between two DBR mirrors of an FP resonant cavity and introducing phase layers between the DFB region and the mirrors. It was found that the periodic-gain structures are a special case of the combined DFB/FP structures in which the index coupling effect is assumed to be negligible due to a small fill factor or a small refractive-index difference. The effect of complex (gain and index) coupling on the design and the threshold characteristics of the structures is clearly illustrated. Some important design considerations that were neglected in the previous papers are addressed     

Self-pulsation in two-section DFB semiconductor lasers and itssynchronization to an external signal

A model of self-pulsation in two-section distributed feedback (DFB) lasers without a saturable absorber is developed by using generalized rate equations. The introduction of an effective differential gain in our model allows us to take into consideration both material and structural effects. The self-pulsation conditions are derived from a linear stability analysis. A mechanism based on a negative effective differential gain is proposed to explain the origin of self-pulsation in such lasers. By considering an injected optical signal, the optical synchronization of self-pulsating lasers is studied using nonlinear simulations. This leads to the determination of some locking-range properties, which are then compared to experimental and analytical results     

An Ultralow Noise and Narrow Linewidth λ/4-Shifted DFB Er-Doped Fiber Laser With a Ring Cavity Configuration

We report a polarization-maintaining lambda/4-shifted distributed feedback (DFB) Er-doped fiber laser with a ring cavity configuration. The ring cavity suppressed the self-pulsation of the stand-alone Er-doped DFB fiber laser. The laser with a 57-m-long ring cavity achieved single-longitudinal-mode operation, a linewidth as narrow as 6 kHz, and relaxation-oscillation-free noise characteristics.     

Distinction between multimoded and singlemoded self-pulsations-inDFB lasers

DFB lasers with split contacts are shown, by large signal dynamic modelling, to self-pulsate at gigabit frequencies. Two different self-pulsation schemes are discussed: where the laser switches between the higher and lower stop band modes, and where the laser pulsates around a single mode. The second scheme can yield self-pulsation frequencies beyond 20 GHz. Comparisons are made with experimental results     

High-power single-mode operation in DFB and FP lasers usingdiffused quantum-well structure

Distributed feedback (DFB) and Fabry-Perot (FP) semiconductor lasers with step and periodic interdiffusion quantum-well structures are proposed for high-power single-longitudinal-mode operation. It is shown that the phase-adjustment region formed by the diffusion step (i.e., step change in optical gain and refractive index) counteracts the influence of spatial hole burning, especially for DFB lasers with large coupling-length products biased at high injection current. Furthermore, it is found that with careful design of the diffusion grating (i.e., grating period and amount of diffusion extent) of FP lasers, side-mode suppression ratio can be enhanced and threshold current density can be minimized to a satisfied level     

Mode stability of a two-wavelength Fabry-Perot/distributed-feedbacklaser

A theory based on a Fokker-Planck equation has been developed to analyze the two-mode operation of a Fabry-Perot/distributed-feedback (FP/DFB) laser. Simultaneous oscillation of the DFB mode and the FP mode near the gain peak with negligible mode power fluctuation can be achieved if the DFB mode is detuned sufficiently from the gain peak. Calculated results for the design of stable two-wavelength lasers are presented     

Time-resolving wavelength chirp with Fabry-Perot etalons andgratings: a theoretical approach

Basic dynamic properties of Fabry-Perot (FP) etalons and gratings are investigated for a square-wave shaped input pulse with a linearly changing wavelength. It is shown that for the chirp rates present in distributed feedback (DFB) laser pulses a simultaneous good resolution in the wavelength and time domain is attainable with an etalon due to its small FWHM time-bandwidth product of 0.11. This is not possible with the grating, which has a time∞bandwidth product of 0.89. A simulation of time resolving the dynamic chirp of a DFB laser pulse is presented. The result is compared to the actually calculated incident wavelength variation. The dynamic aspects of FP etalons and gratings for time resolving the wavelength chirp in laser pulses are given     

Measurement of mode partition in DFB lasers

Some DFB (distributed feedback) laser diodes have a satellite mode beside a main DFB mode even if FP modes are suppressed. In this paper, the mode partition noise is presented for several DFB lasers operating in multilongitudinal modes. The results show that under modulation at 140 Mbit/s, the mode partition coefficient k²of multimode DFB lasers is very small and at most 0.02 while that of FP lasers biased at the threshold level is 0.03 to 0.12. The numerical evaluation of the mode partition effect in two-mode DFB lasers suggests that a 20- dB suppression of the satellite mode power is enough to achieve a repeater spacing of over 100 km in the 280 Mbit/s fiber-optic transmission system with less than 0.1-dB power penalty.     

Nonlinear dynamics in directly modulated multiple-quantum-welllaser diodes

A theoretical and experimental analysis of the nonlinear dynamics of Fabry-Perot (FP) and distributed feedback (DFB) multiple-quantum-well (MQW) laser diodes is presented. The analysis is performed under single-tone and two-tone direct modulation. In the FP laser, we observe period doubling and in the DFB laser both period doubling and period tripling are identified. Period doubling is found over a wide range of modulation frequencies in both lasers. The reason for this wide modulation frequency range is attributed to the large relaxation frequencies found in MQW laser diodes. The spontaneous emission factor is measured for both FP and DFB lasers. The dependencies of period doubling on output power and RF input power level are also analyzed. The nonlinear dynamics of the laser are found to be enhanced when modulated under two-tone modulation. Numerical simulations carried out show good agreement with the measured results     

Widely frequency tunable amplified feedback laser as 20 GHz optical microwave source

Optical microwave sources are required in optical signal processing. Amplified feedback laser (AFL) which can generate high frequency self-pulsation due to compound cavity modes beating are used as optical microwave sources. In this paper, we fabricate a four-section AFL consisted of a different distribute feedback (DFB) section, a phase control section, an amplifier section, and a transparent section. This AFL generate continuously tunable microwave in the range 19.87–26.30 GHz with 3 dB linewidth about 3 MHz. Microwave with narrow linewidth is obtained by injecting quarter frequency modulated light experimentally.     

Electrically tunable dual-wavelength switching in a mutually injection-locked erbium-doped fiber ring laser and distributed-feedback laser diode

Wavelength switching in a mutually injection-locked erbium-doped fiber ring laser and distributed-feedback laser diode (DFBLD) is experimentally demonstrated. By adjusting the power and the polarization state of injected light, three stable oscillation regimes were observed: Fabry-Pe/spl acute/rot (FP) mode oscillation, main distributed-feedback (DFB) mode oscillation, and dual-wavelength oscillation. Wavelength switching of 0.8 nm from one of the FP oscillation modes to the main DFB mode of the DFBLD is accomplished when the bias current to the DFBLD is varied from 9 to 14 mA. We also show that these wavelengths can be tuned as much as 2.48 nm when the temperature of the DFBLD is changed by 20/spl deg/C.     

Condition for short pulse generation in ultrahigh frequency mode-locking of semiconductor lasers

It is shown that although it is possible to obtain mode-locking without self-pulsation when certain criteria are satisfied, the shortest pulses are almost always generated at or close to the onset of self-pulsation. Thus, the amplitude of the optical pulse train is modulated by the (relatively) low-frequency envelop of a few gigahertz under this condition. This observation was obtained by simultaneously measuring the pulsewidth using an autocorrelator and monitoring the optical intensity using a high-speed photodiode and a microwave spectrum analyzer. It is concluded that while it is possible to generate picosecond optical pulses in ultrahigh-frequency mode-locking of quantum-well lasers, very short pulses ( to 1 ps) are almost always accompanied by self-pulsation which is manifested as low-frequency (gigahertz) envelope modulation of the optical pulse train.<>     

Modeling self-pulsating DFB lasers with an integrated phase tuningsection

A theoretical model of a self-pulsating three-section DFB laser with an integrated phase tuning section is established. It is based on traveling wave equations and the standard carrier rate equations. Parameters of an existing device are used for applying the model. Key conditions and characteristics of self-pulsations (SPs) are modeled and compared with experimental results. The important role of phase tuning for turning on the SP is pointed out. The dependence of the SP regime on the detuning between the Bragg wavelengths in the laser and reflector is determined and the essential role of phase-readjustment is identified. Frequency tuning via the laser currents, as well as the pulse shape at various frequencies, is investigated. This allows us to identify the mechanism for frequency tuning. The model turns out to be a good tool to improve our knowledge of the self-pulsation effect and to design optimized devices