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     

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.     

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.     

Electrically switchable self-pulsations in integratable multisection DFB-lasers

An integratable version of a self-pulsating AR-coated multisection DFB-laser with a phase-tuning section integrated between two DFB sections is presented. It is shown that the phase current can act as an electrical switch for turning ON and OFF the self-pulsations. In addition, the frequency stability can be improved by properly adjusting the phase current. The frequency of the self-pulsations is electrically tunable over more than one octave continuously.     

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     

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.     

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.     

Widely frequency-tunable amplified feedback lasers for 10-GHz optical pulsation

Monolithic amplified feedback semiconductor lasers are demonstrated as a new solution to 10-GHz optical pulsation, where self-pulsations are generated according to the concept of a single-mode laser with shortly delayed optical feedback. They consist of a loss-coupled distributed feedback section operating as a single-mode laser and an integrated feedback cavity including a phase control, an amplifier, and a transparency section. The pulsation frequency is continuously tunable in the range of 7-11.5 GHz with an extinction ratio above 6.5 dB, which indicates that precise control of a cavity length is not needed.     

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     

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     

Optical feedback on self-pulsating semiconductor lasers

Weak optical-feedback effects on the statistical properties of self-pulsations in narrow-stripe semiconductor lasers are analyzed using Lang-Kobayashi-type equations. The self-pulsation features are compared with the characteristics of excited relaxation oscillations. We determine the operating regime in which the randomizing effect of spontaneous-emission noise destroys pulse coherence. In this regime, only phase-insensitive effects of optical feedback are possible, and optimum jitter reduction is achieved with delay times of the order of an integer-odd multiple of the free-running pulsation period. In the high-pump operating regime, interpulse coherence is retained and the optical-feedback phase is shown to be instrumental for pulse-jitter control. Our results show that for cavity lengths up to 10 cm, variations on the order of half an optical wavelength induce jitter variations of one order of magnitude     

Gigahertz self-pulsation in 1.5 μm wavelength multisection DFBlasers

Self-pulsation in InGaAsP/InP multisection distributed feedback (DFB) lasers was generated reproducibly by adjusting appropriate injection conditions. Frequencies of up to some gigahertz were achieved. It was demonstrated that-in contrast to Fabry-Perot (FP) elements-no selective treatment of one section is required for creating the self-pulsation. It is concluded that the self-pulsation in DFB elements is of a different type than in FP elements     

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     

Self-pulsations in strongly coupled asymmetric external cavitysemiconductor lasers

Self-pulsations in asymmetric external cavity semiconductor lasers are studied experimentally and are analyzed using improved rate equations which include multiple reflections. These equations are valid for arbitrary levels of coherent external optical feedback. The dependence of self-pulsation frequencies on injection current, external mirror tilt angle and reflectivity, and external cavity length is explained by small-signal analysis of the rate equations. By numerical integration of the rate equations, self-pulsations are demonstrated theoretically and resonant enhancement of intensity noise is shown to occur when the self-pulsation frequency is an integer fraction of the external cavity resonance frequency     

Simple and Accurate Optical Frequency Domain Ranging Using Off-the-Shelf DFB Lasers Subject to Frequency-Shifted Optical Feedback

It is demonstrated that accurate distance measurement may be effected using a commercially available distributed feedback (DFB) semiconductor laser subject to frequency-shifted optical feedback. A simple experimental arrangement is employed wherein a chirped frequency comb is generated in an external cavity DFB laser using an intracavity acoustic-optic modulator. The frequency comb generates a beat frequency which is proportional to the path difference of a target and reference arm. An estimate of the accuracy and resolution of the measurement is given.     

Design rules for a low-chirp integrated DFB laser with an electroabsorption modulator

Feedback from the front facet of an integrated DFB laser with an electroabsorption modulator generates additional chirp to the single modulator. The reduction of the sensitivity to that feedback is in tradeoff relation with the external differential quantum efficiency and the single-mode yield. We introduce a figure-of-merit for the feedback sensitivity. It is obtained from the modeling of the small signal frequency modulation (FM). It indicates design rules for low-chirp and high-efficiency devices.     

Distributed feedback modes in a partially filled ring cavity

We have studied the threshold longitudinal modes of a structure composed of a ring cavity partially filled with a distributed feedback (DFB) medium. An eigenvalue equation and dispersion relation for the propagation constant, threshold gain, and mode location are derived and numerically solved for a number of cases. We show that the incommensurate nature of the unfilled ring cavity eigenfrequencies and DFB eigenfrequencies can lead to an enhancement of the frequency selectivity of the distributed feedback laser. The analysis is extended to consider finite linewidth and dispersion.     

Impact of gain dispersion on the spatio-temporal dynamics ofmultisection lasers

We present a refined model for multi-section lasers, introducing an additional equation for material polarization in the well-known traveling wave model. We investigate the polarization-induced changes in the spectral properties of the optical waveguide. Finally, we show the relevance of this model for a more realistic simulation of the complicated dynamical behavior of multi-section distributed feedback (DFB) lasers, such as fast self-pulsations, multi-stability, and hysteresis effects due to mode competition     

Self-consistent Simulation of self-pulsating two-section gain-coupled DFB lasers

The role of cavity conditions in the dynamics of two-section gain-coupled distributed feedback (DFB) lasers is investigated using a self-consistent model. Self-sustained pulsation (SSP) exists only for devices with strongly coupled DFB gratings. As the coupling strength increases, multiple SSP regimes are developed. The SSP frequency tuning range increases as cavity length decreases. The frequency and modulation index predicted by the model agree well with experimental results. The facet condition of each section is found to affect SSP differently because of the asymmetrical behavior of the modes responsible for SSP.     

Transient and optoelectronic feedback-sustained pulsation of laserdiodes at 1300 nm

We report the observation of self-sustained pulsation and transient self-pulsation in laser diodes at 1300 nm and the effects of optoelectronic feedback on the pulsations. Transient self-pulsation has a lifetime of a few minutes with frequencies up to 7 GHz. The linewidth of self-pulsation is on the order of 0.5 GHz. With optoelectronic feedback, the transient self-pulsation can be sustained and the linewidth significantly reduced to about 20 kHz. The center frequency of feedback-sustained pulsation is dependent on the passband of the bandpass filter in the feedback loop. The feedback sustained pulsation can be frequency modulated for applications in subcarrier multiplexed optical networks such as cable TV distribution and antenna remoting