Self-pulsing Research Articles

Demonstration of high-frequency self-pulsing oscillations in an active silicon micro-ring cavity

We experimentally investigated the self-pulsing (SP) oscillations induced by the thermo-optic, free carrier, and Kerr nonlinear effects in integrated active silicon microring resonators. We demonstrate high frequency self-pulsing oscillations (up to 30 MHz) by applying a few millivolts of reverse bias voltage to the PIN junction of the active cavity. We illustrate that the shape of those oscillations (i.e., frequency and duty cycle) can be controlled by adjusting the CW input power and applying a reverse bias voltage to the PIN junction for carrier removal. This controlling is important for synchronizing the cavity which is crucial for neural network applications. Furthermore, we utilize a mathematical model for visualizing the stability regions by numerically studying coupled mode theory in silicon microcavity under different conditions.

Terahertz quantum cascade laser under optical feedback: effects of laser self-pulsations on self-mixing signals.

In this article, we explore the interplay between the self-pulsations (SPs) and self-mixing (SM) signals generated in terahertz (THz) quantum cascade lasers (QCLs) under optical feedback. We find that optical feedback dynamics in a THz QCL, namely, SPs, modulate the conventional SM interference fringes in a laser feedback interferometry system. The phenomenon of fringe loss in the SM signal - well known in interband diode lasers - was also observed along with pronounced SPs. With an increasing optical feedback strength, SM interference fringes transition from regular fringes at weak feedback (C ≤ 1) to fringes modulated by SPs under moderate feedback (1 < C ≤ 4.6), and then [under strong feedback (C > 4.6)] to a SM waveform with reduced number of fringes modulated by SP, until eventually (under even greater feedback) all the fringes are lost and only SPs are left visible. The transition route described above was identified in simulation when the SM fringes are created either by a moving target or a current modulation of the THz QCL. This SM signal transition route was successfully validated experimentally in a pulsed mode THz QCL with SM fringes created by current modulation during the pulse. The effects of SP dynamics in laser feedback interferometric system investigated in this work not only provides a further understanding of nonlinear dynamics in a THz QCL but also helps to understand the SM waveforms generated in a THz QCLs when they are used for various sensing and imaging applications.

Terahertz imaging with self-pulsations in quantum cascade lasers under optical feedback

The phenomenon of self-pulsation (SP) in terahertz (THz) quantum cascade lasers (QCLs) due to optical feedback was reported recently. In this Letter, we propose a THz imaging modality using the SP phenomenon in a THz QCL. We explore the theoretical oscillation properties of the SP scheme and demonstrate its suitability to perform imaging experimentally. The SP imaging scheme operates in self-detection mode, eliminating the need for an external detector. Moreover, the scheme requires only a fixed current, meaning that one can avoid many of the pitfalls associated with high temperature operation of THz QCLs, including frequency chirp and mode hops caused by sweeping the laser current. This also means that one is free to locate the operating point at the maximum power, to produce the desired beam profile or for highest spectral purity, depending on the application. The SP imaging modality proposed in this work can be translated directly to high operating temperature THz QCLs.

Experimental observations of thermo-optical bistability and self-pulsation in silicon microring resonators

Optical bistability and self-pulsation (SP) in silicon microring resonators (MRRs) are experimentally observed. The waveforms and frequencies of SP can be controlled by changing input light power and its wavelength, and the region of SP can be modulated readily by applying a reverse voltage on the PN junction embedded in the MRR. These phenomena are theoretically studied by adopting the coupled-mode theory and linear stability analysis method for differential equations, with theoretical results fitting well with the experimental ones.

Multibistability and self-pulsation in nonlinear high-Qsilicon microring resonators considering thermo-optical effect

Optical bistability (BI) and self-pulsation (SP) in high-$Q$ silicon microring resonators (MRRs) induced by thermo-optical (TO) effect and other nonlinear effects are theoretically studied with coupled mode theory and linear stability analysis method. It is found that the boundaries for both BI and SP are mainly restricted by two counteracting effects: free carrier dispersion effect and TO effect. If the refractive index changes of a MRR caused by these two effects are on the same order of magnitude, the output power will exhibit much more complicated dependence on the input power and wavelength, namely, input-power-dependent multi-BI and multi-SP regions will exist at certain input wavelength range. The controllability of multi-BI and multi-SP phenomena by the input power and input wavelength could be very useful in all-optical nonlinear devices.

Bistability and self-pulsation phenomena in silicon microring resonators based on nonlinear optical effects

Bistability (BS) and self-pulsation (SP) phenomena in silicon microring resonators (MRR) with intense CW light injection are studied. Several nonlinear optical effects including Kerr effect, two-photon absorption, free carrier absorption and free carrier dispersion are taken into account. The threshold optical intensity of BS and SP is derived from the coupled mode theory and a linear stability analysis method. The influences of MRR's parameters (carrier lifetime, linear loss and radius) and light injection conditions (input power, wavelength detuning) on the characteristics of SP (modulation depth and oscillating frequency) are analyzed and discussed. It is shown that, SP occurs only if the carrier lifetime ranges from several ps to several-hundred ps and the input light intensity is higher than 10⁶W/cm². The modulation depth of SP can be as large as 8dB and the associated oscillating frequency is in the range from several GHz to beyond 10 GHz.

Short-pulse-width self-pulsed Tm^3+-doped silica fiber lasers

Influence of fiber length on the self-pulsing (SP) characteristics of 2&#x2009;&#x3BC;m Tm3+-doped silica fiber lasers are investigated experimentally and theoretically. In SP operation, the laser pulse width narrows and the repetition rate increases with the decrease of fiber length. By shortening the fiber length to 2&#x2009;m and angle-polishing the output fiber-end facet, SP pulse width less than 100&#x2009;ns is achieved, comparable to that from conventional actively Q-switched Tm3+ fiber lasers. Theoretically simulated SP characteristics of the 2&#x2009;&#x3BC;m Tm3+-doped fiber laser show good agreement with the experimental results. Further narrowing the pulse width in SP fiber lasers and their potential applications are also discussed.

Model and Characteristics of Self-Pulsing in ${\rm Tm}^{3+}$-Doped Silica Fiber Lasers

In this paper, a theoretical model, taking into account the phonon-assisted excited-state absorption process ( <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> → <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ), has been constructed to explain the self-pulsing (SP) phenomenon in the Tm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped fiber lasers. Both numerical simulation and experimental investigation on the SP characteristics of ~2-μ.m Tm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped fiber lasers have been carried out. The numerical simulation shows agreement with the experimental results. Furthermore, the impacts of output coupling, pump intensity, and Tm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -ion doping concentration on the SP characteristics of the Tm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -doped fiber lasers have been studied explicitly, and optimization of the SP features has been discussed. In addition, the potential applications of SP in fiber lasers or solid-state lasers have also been proposed.

양 단면 반사율과 위상 조정 영역의 위상이 Self-Pulsation 주파수가 THz 대역인 다중 영역 Complex-Coupled DFB 레이저의 수율 특성에 미치는 영향

양 단면 반사율과 단면 회절격자의 위상, 위상 조정 영역의 위상이 모드 비팅에 의한 SP(self-pulsation) 주파수가 THz 대역인 다중 영역 CC(complex-coupled) DFB 레이저의 SP 동작 특성에 미치는 영향을 수율 관점에서 살펴보았다. 양 단면 회절격자의 위상을 0으로 고정시킨 다중 영역 CC DFB 레이저에서 같은 결합세기와 CR(coupling-ratio)을 가지는 경우 양 단면 반사율이 커질수록, 같은 결합 세기와 단면 반사율을 가지는 경우 CR이 작을수록, SP 주파수 변화폭이 큼을 볼 수 있다. 또한 같은 CR과 단면 반사율을 가지는 경우는 결합세기가 큰 경우가 SP 주파수 변화폭이 작음을 볼 수 있다. 결합세기가 3인 경우 같은 단면 반사율에서 CR이 커질수록 (같은CR에서 단면 반사율이 커질수록) 수율이 80% 이상 되는 위상 조정 영역의 위상 범위가 감소함을 볼 수 있다. 결합세기가 4인 경우 같은 단면 반사율에서 CR이 커질수록(같은 CR에서 단면 반사율이 커질수록) 수율이 80% 이상 되는 위상 조정 영역의 위상 범위가 증가함을 볼 수 있다. We investigate the effect in terms of yield of the reflectivity of both facets and of the phase of a phase tuning section on the self-pulsation (SP) characteristics of multisection complex-coupled (CC) DFB lasers with self-pulsation frequency of the THz region. When the grating phases on both facets of a multisection CC DFB laser are fixed as 0, the variation of SP frequency increases as the reflectivity of both facets increases, while that of SP frequency decreases as the coupling ratio (CR) and the coupling strength increase. For the coupling strength of 3, the range of the phase of a phase tuning section with yields greater than 80% decreases as the CR and the reflectivity of both facets increases. For the coupling strength of 4, the range of the phase of a phase tuning section with yields greater than 80% increases as the CR and the reflectivity of both facets increases.

Detailed Theoretical Investigation on Enhanced Nondegenerate Four-Wave Mixing in Passive Mode-Locked Semiconductor Lasers

The passive mode-locked semiconductor laser (PMLL) can realize terahertz (THz) four-wave mixing (FWM) with high conversion efficiency and no need for pump input. Theoretical investigation on its FWM characteristics has been carried out by using the traveling wave and carrier rate equations combined with nonlinear dynamics of carrier density pulsation (CDP), carrier heating (CH), and spectral-hole burning (SHB). The stable mode-locked conjugate pulse with short-duration high-repetition frequency can be obtained by the PMLL. The pedestal-free conjugate pulse has narrower width and larger amplitude jitter compared with the pump wave. Smaller absorption recovery time is helpful to extend the stable conjugate output regime. There exists a down-limit for absorption recovery time to avoid self-pulsation (SP) instability. The larger injection current is required for smaller absorption recovery time to reach the given conversion efficiency. The larger conversion efficiency and narrower conjugate pulse can be obtained by proper design of distributed Bragg reflector (DBR) coupling strength. The DBR bandwidth must be designed to be within signal-pump detuning to avoid resonance-induced conjugate pulse distortion. The FWM in PMLL has considerable potential such as demultiplexing in optical time-division multiplexing systems.

양 단면 반사율과 위상 조정 영역의 위상이 다중 영역 Complex-Coupled DFB 레이저의 수율 특성에 미치는 영향

양 단면 반사율과 단면 회절격자의 위상, 위상 조정 영역의 위상이 두 개의 DFB 영역과, 가운데 위상 조정 영역을 가진 다중 영역 CC(complex-coupled) DFB 레이저의 SP(self-pulsation) 동작 특성에 미치는 영향을 수율 관점에서 살펴보았다. 결합세기와 CR(coupling ratio)이 작을수록 양 단면과 다른 DFB 영역에서 반사되는 필드가 모드의 특성에 미치는 영향이 증가하여 수율이 감소한다. 결합세기와 CR에 상관없이 양 단면 반사율이 증가할수록 최대 수율과 수율이 60% 이상 되는 위상 조정 영역의 위상 범위가 감소함을 볼 수 있었다. 같은 결합세기에서는 CR이 0.2인 경우가 CR이 0.1인 경우보다 복소 결합 효과가 커 최대 수율과 수율이 60%이상 되는 위상 조정 영역의 위상 범위가 큼을 볼 수 있었다. 결합세기가 3이고 CR이 0.2인 경우가 DFB 영역의 발진 모드가 외부의 영향을 적게 받고 SHB(spatial hole burning) 효과도 크지 않아 양 단면 반사율의 변화와 위상 조정 영역의 위상 변화에 따른 수율 특성이 가장 좋음을 볼 수 있었다. The effect of the reflectivity of both facets and the phase of a phase tuning section on the self-pulsation (SP) characteristics of multisection complex-coupled (CC) DFB lasers is investigated in terms of yield. The lasers are composed of two CC DFB sections and a phase tuning section between them. As the coupling strength and the coupling ratio (CR) decrease, the effect of the reflected fields from both facets and the other DFB section on the mode characteristics of one DFB section increases, so that the yield decreases. As the facet reflectivity increases, the maximum yield and the range of the phase of a phase tuning section with yield more than 60% decrease independent of the coupling strength and CR. The yield characteristics of CR=0.2 are better than those of CR=0.1 with the same coupling strength due to the larger complex coupling effect. The case with <TEX>${\mid}{\kappa}L{\mid}=3$</TEX> and CR=0.2 shows best yield characteristics among the cases considered in this work.

이득 분산이 다중 영역 복소 결합 DFB 레이저의 Self-Pulsation 특성에 미치는 효과

이득 분산이 두 개의 복소 결합 DFB 영역과 위상조정 영역으로 구성되는 다중 영역 복소 결합 DFB 레이저에서 발진하는 두 모드의 비팅에 의한 self-pulsation (SP) 주파수와 출력 파워의 변조지수에 미치는 영향에 대하여 살펴보았다. 능동 영역의 이득이 최대가 되는 파장을 두 DFB 영역의 발진 모드의 파장 중심 또는 브래그 파장 중심에 위치시킨 경우가 특정 DFB 영역의 발진 모드의 파장 또는 브래그 파장에 위치시킨 경우에 비하여 두 DFB 영역의 브래그 파장 차이, <TEX>${\Dalta}{\lambda}_{B}$</TEX>, 증가에 따른 두 발진 모드가 받는 이득 차이가 적게 발생하여 최대 SP 주파수가 크고, 변조지수 특성도 좋음을 볼 수 있었다. 또한 능동 영역의 이득이 최대가 되는 파장을 두 DFB 영역의 브래그 파장 중심에 위치시키는 경우가 발진 모드의 파장 중심에 위치시키는 경우에 비하여 더 큰 최대 SP 주파수를 얻을 수 있었고 변조지수 특성도 더 우수함을 볼 수 있었다. The effect of gain dispersion on the self-pulsation (SP) characteristics due to the mode beating of two modes emitted in a multisection DFB laser composed of two complex-coupled DFB sections and a phase control section is investigated. When the peak wavelength, <TEX>${\lambda}_{p}$</TEX>, of the gain spectrum of the DFB section is positioned in the center of the lasing wavelengths or the Bragg wavelengths of the two DFB sections, the maximum SP frequencies are higher and the modulation index has better characteristics compared to those cases for <TEX>${\lambda}_{p}$</TEX> fixed at the lasing wavelength or Bragg wavelength of one DFB section, when the difference between the Bragg wavelengths of the two DFB sections, <TEX>${\Dalta}{\lambda}_{B}$</TEX>, is varied. When <TEX>${\lambda}_{p}$</TEX> is positioned in the renter of the Bragg wavelengths of the two DFB sections, the maximum SP frequency is higher and of the modulation index has better characteristics compared to those of the case for <TEX>${\lambda}_{p}$</TEX> positioned in the center of the lasing wavelengths of the two DFB sections.

Dependence of self-pulsation characteristics of multisection index-coupled distributed feedback lasers on section lengths

We investigate the effect of section lengths on the self-pulsation (SP) characteristics of a multisection index-coupled distributed feedback (DFB) laser that is composed of two spectrally detuned DFB sections and a phase-tuning section between them. Compound-cavity modes in multisection DFB lasers can be obtained by the concept of Fabry-Perot cavity modes. The simple equation for a CS number, which represents the number of compound-cavity modes within the stop-band width of a DFB section, is derived. By using the CS number, we were able to predict the SP characteristics such as abrupt changes of SP frequency due to mode hopping. Stable SP characteristics without abrupt changes of SP frequency for the variation of the spectral detuning and the phase shift in a phase-tuning section were obtained in cases with small CS numbers, which can be obtained by adjustment of section lengths.

Phase Correlation and Linewidth Reduction of 40 GHz Self-Pulsation in Distributed Bragg Reflector Semiconductor Lasers

In this paper, self-pulsation (SP) in a distributed Bragg reflector (DBR) semiconductor laser without a saturable absorber is experimentally and theoretically investigated. Detailed experimental characterizations of the SP DBR laser are reported in the optical and radio-frequency domains. Phase correlation between the longitudinal modes selected by the DBR mirror has been experimentally demonstrated. A theoretical model based on coupled rate equations for three modes has been developed to study the time evolution of phases and amplitudes of the modes. The carrier density modulation, resulting from the beating between adjacent longitudinal modes generates four-wave mixing (FWM) and is responsible for mutual injection locking, leading to passive mode-locking. The calculated power spectral density of the frequency noise derived from the model is in agreement with experimental results and proves that the phases of the longitudinal modes are identically correlated through the FWM process in this type of SP lasers

양 단면 반사율과 위상 조정 영역의 위상이 다중 영역 Index-Coupled DFB 레이저의 수율 특성에 미치는 영향

양 단면 반사율과 단면 회절격자 위상, 위상 조정 영역의 위상이 두 개의 DFB 영역과, 가운데 위상 조정 영역을 가진 다중 영역 Index-Coupled DFB 레이저의 Self-Pulsation동작 특성에 미치는 영향을 수율 관점에서 살펴보았다. 결합세기가 작을수록 양 단면 반사율이 증가할 때 양 단면과 다른 DFB 영역에서 반사되는 필드가 모드의 특성에 미치는 영향이 커져 양 단면 회절격자 위상 변화에 따른 발진 모드의 호핑이 많이 발생하고 수율이 감소한다. 결합세기가 클수록 양 단면 반사율이 증가하여도 양 단면과 다른 DFB 영역에서 반사되는 필드가 모드의 특성에 미치는 영향이 작아 양 단면 회절격자 위상 변화에 따른 모드의 호핑은 적게 발생하나 Spatial Hole Burning 효과가 증가하여 수율이 감소하는 것을 볼 수 있었다. 결합세기에 상관없이 단면 반사율이 증가할수록 최대 수율과 수율이 40% 이상 되는 위상 조정 영역의 위상 범위가 감소함을 볼 수 있었다. 또한 결합세기가 클수록 위상 조정 영역의 위상 변화에 따른 수율의 변화가 커지고 단면 반사율의 변화에 따른 최대 수율을 주는 위상 조정 영역의 위상 값의 변화가 작음을 볼 수 있었다. 결합세기가 2와 3일 경우가 1.2와 4인 경우에 비하여 수율 특성이 좋음을 볼 수 있었다. We investigate the effect of the reflectivity of both facets and the phase of a phase tuning section on the self-pulsation (SP) characteristics of multisection index-coupled (IC) DFB lasers composed of two index-coupled DFB sections and a phase tuning section between them in terms of yield. In the case of weak coupling strength, as the reflectivity of both facets increases, the effect of reflected fields from both facets and the other DFB section on the mode characteristics of one DFB section increases. Thus the number of mode hoping increases and yield decreases for the variation of phases of both facets. In the case of strong coupling strength, as the reflectivity of both facets increases, the spatial hole burning effect increases, so that the yield decreases. The maximum yield and the range of the phase of a phase tuning section with yield more than 40% decrease as the facet reflectivity increases irrespective of coupling strength. As the coupling strength increases, the variation of yield for the variation of the phase of a phase tuning section increases and the variation of the phase of a phase tuning section with the maximum yield for the variation of the reflectivity of both facets decreases. The yield characteristics of the cases with the coupling strengths of 2 and 3 are better than those with the coupling strengths of 1.2 and 4.

A Novel Multisection Distributed Feedback Laser with VariedRidge Width for Self-Pulsation Generation

A novel ridge-waveguide multisection (MS) distributed feedback (DFB)laser, which consists of two identical DFB sections but different ridgewidths, is proposed to generate beating-type self-pulsations (SPs). Thespatiotemporal dynamic response of such a multisection DFB laser iscalculated based on a large-signal travelling-wave model. Self-pulsatingoutput at about 150 GHz is predicted, and evidences for the beatingmechanism of the SPs are provided. To the best of our knowledge, this isthe first report on SP generated by MS-DFB lasers with varied ridgewidth. Compared to other alternatives, such devices are much easier toimplement and also enjoy the advantages of lower cost and higher designfreedom.

매우 넓은 영역의 Self-Pulsation 주파수와 높은 변조 지수를 가자는 다중 영역 복소 결합 DFB 레이저

We analyze the self-pulsation(SP) characteristics due to mode beating of two modes emitted in a multi-section complex-coupled (CC) DFB laser composed of two DFB sections and a phase control section between them. SP frequency due to mode beating of the two modes is determined by the difference of grating periods in the two CC DFB regions. As the difference of grating periods in the two CC DFB regions increases, the SP frequency increases from very low frequency to the THz region. In the case of a mode which is not located in the stop band of the other DFB region, the mode propagates into the other DFB region without a high reflection, so that output powers emitted in a multi-section CC DFB laser have high modulation indexes due to the large interaction between the two modes.

Phase correlation between longitudinal modes in semiconductor self-pulsating DBR lasers

Phase correlation leading to self-pulsation (SP) in semiconductor distributed Bragg reflector (DBR) lasers is investigated experimentally and theoretically. Under proper biasing conditions, the laser oscillates with three main modes and we observe that each two-modes beating provides SP with identical spectral linewidth. Under the same operating conditions, the measured spectral linewidths of the beating modes are much larger than the linewidth of the self-pulsating signal. These results demonstrate the natural occurrence of passive mode-locking (PML) and phase correlation in semiconductor DBR lasers. A model based on multimode coupled-wave rate equations, including four-wave mixing (FWM), is developed to describe PML and SP in the gain region of the laser cavity. This model demonstrates that the existence of phase correlation between longitudinal modes is due to FWM.

Modeling of mode control and noise in self-pulsating phaseCOMB lasers

Self-pulsations (SPs) in phase-controlled mode beating lasers (PhaseCOMB) are very attractive for all-optical clock recovery at ultra-high bit rates. In this paper, we apply the comprehensive simulation tool Longitudinal Dynamics in Semiconductor Lasers, developed by us, for studying the SP features of PhaseCOMB lasers, considering the effects of spontaneous emission noise, longitudinal spatial hole burning, and gain dispersion. In particular, the importance of mode control for adjusting the PhaseCOMB operating conditions is pointed out. The simulation results are confirmed by measurements on fabricated devices.

Detuned grating multisection-RW-DFB lasers for high-speed optical signal processing

We present a detailed description and a first theoretical study of an improved concept for high-frequency self-pulsations (SPs) in multisection (MS)-DFB lasers with an integrated phase tuning section. The DFB wavelengths of the two DFB sections are spectrally detuned by nearly the stopband width using two gratings with different grating periods. If both DFB sections are operated at lasing conditions and an appropriate phase is chosen, we obtain beating-type SP with a frequency given by the spectral distance of two lasing modes. Good agreement between theory and experiment is obtained with respect to the role of the detuning, the role of the phase section, as well as the synchronization to external injected signals. The modeling shows a strong nonlinear coupling of the two involved modes via the carrier densities. This effect is important for the mutual coherence and for the observed locking of the beating oscillations to external signals. From the results of the calculations, we draw the conclusion that even higher SP frequencies can be obtained based on the new concept.