Mode Competition Effects Research Articles

Tunable ultra-narrow linewidth linear-cavity fiber lasers assisted by distributed external feedback

We proposed and demonstrated a widely tunable and ultra-narrow linewidth fiber laser scheme by exploiting an original Fabry-Perot laser main cavity and a distributed external feedback structure. This was feasible with the aid of a feedback signal with distributed characteristics which is injected into the main cavity to induce an intense mode competition effect. Eventually, a single frequency fiber laser with a side mode suppression ratio of 69 dB, an output linewidth of 570 Hz, a white frequency noise of 110 Hz2/Hz at the frequency offset that is beyond 2 × 105 Hz, and a relative intensity noise less than −140 dB/Hz is successfully obtained under normal conditions. Also, the high spectral purity performance of the laser can be maintained under different pump powers for each wavelength channel. This work presents a promising method to obtain tunable ultra-narrow linewidth lasers.

Active Differential Fiber Coupled Plasmon Waveguide Resonance Sensor Based on the Mode Competition Effect

We proposed an active differential intensity (DI) fiber coupled plasmon waveguide resonance (CPWR) sensor based on the mode competition effect in the C-band, aiming to enhance the sensitivity. The sensing head is a fiber probe with a sensing layer of ITO/Au/ITO/TiO2 film, enabling the excitation of CPWR in the C-band. The narrow CPWR spectrum allows DI interrogation by tracking the intensity of light at two wavelengths. The fiber probe is inserted into a dual-wavelength fiber laser to adjust the intra-cavity loss at the two lasing wavelengths. By using the differential modulation of the reflectivity from the refractive index variations, the mode competition effect is triggered. The powers at two lasing wavelengths change oppositely, enlarging the power difference and then resulting in enhanced sensitivity. The average sensitivity is up to 5702 dB/RIUs, which is 10 times higher than that of conventional DI interrogation. The sensitivity enhancement mechanism based on the mode competition effect provides a new technical approach for enhancing the sensitivity of DI SPR sensors.

Single-Pulse Measurement of Orbital Angular Momentum Generated by Microring Lasers.

Optical beams with helical phase fronts carry orbital angular momentum (OAM). To exploit this property in integrated photonics, micrometer-scale devices that generate beams with well-defined OAM are needed. Consequently, lasers based on microring resonators decorated with azimuthal grating elements have been investigated. However, future development of such devices requires better methods to determine their OAM, as current approaches are challenging to implement and interpret. If a simple and more sensitive technique were available, OAM microring lasers could be better understood and further improved. In particular, despite most devices being pulsed, their OAM output has been assumed to be constant. OAM fluctuations, which are detrimental for applications, need to be quantified. Here, we fabricate quantum-dot microring lasers and demonstrate a simple measurement method that can straightforwardly determine the magnitude and sign of the OAM down to the level of individual laser pulses. We exploit a Fourier microscope with a cylindrical lens and then investigate three types of microring lasers: with circular symmetry, with "blazed" grating elements, and with unidirectional rotational modes. Our results confirm that previous measurement techniques obscured key details about the OAM generation. For example, while time-averaged OAM from our unidirectional laser is very similar to our blazed grating device, single-pulse measurements show that detrimental effects of mode competition are almost entirely suppressed in the former. Nevertheless, even in this case, the OAM output exhibits shot-to-shot fluctuations. Thus, our approach reveals important details in the underlying device operation that can aid in the improvement of micrometer-scale sources with pure OAM output.

Richtmyer–Meshkov instability on two-dimensional multi-mode interfaces

Shock-tube experiments on eight kinds of two-dimensional multi-mode air–SF $_6$ interface with controllable initial conditions are performed to examine the dependence of perturbation growth on initial spectra. We deduce and demonstrate experimentally that the amplitude development of each mode is influenced by the mode-competition effect from quasi-linear stages. It is confirmed that the mode-competition effect is closely related to initial spectra, including the wavenumber, the phase and the initial amplitude of constituent modes. By considering both the mode-competition effect and the high-order harmonics effect, a nonlinear model is established based on initial spectra to predict the amplitude growth of each individual mode. The nonlinear model is validated by the present experiments and data in the literature by considering diverse initial spectra, shock intensities and density ratios. Moreover, the nonlinear model is successfully extended based on the superposition principle to predict the growths of the total perturbation width and the bubble/spike width from quasi-linear to nonlinear stages.

Active mode-locking optoelectronic oscillator.

The optoelectronic oscillator (OEO) has been widely investigated to generate ultra-pure single-frequency microwave signals. In this study, we propose and experimentally demonstrate an active mode-locking optoelectronic oscillator (AML-OEO), which can generate broadband microwave frequency comb (MFC) signals. An additional intensity modulator is inserted into the OEO as an active mode-locking device for loss modulation to realize phase-locking between adjacent oscillation modes. Through the active mode-locking technique, steady multi-mode oscillation is achieved, which is difficult to realize in a conventional OEO due to the mode-competition effect. By tuning the frequency of the active modulation signal (AMS), both fundamental and harmonic AML-OEOs can be established. In the experiments, MFC signals with a frequency spacing of 195 kHz and 50.115/100.035 MHz are generated with fundamental and harmonic AML-OEOs.

Multi-Wavelength SOA Fiber Laser With Ultra-Narrow Wavelength Spacing Based on NPR Effect

A multi-wavelength semiconductor optical amplifier (SOA) fiber laser with ultra-narrow wavelength spacing is experimentally demonstrated in this paper. The SOA is used as the primary gain medium due to its inhomogeneous broadening and nonlinear polarization rotation (NPR) effect that allows to achieve stable multi-wavelength output with ultra-narrow wavelength spacing. The combination of PC (polarization controller)-SOA-PC-polarizer and nonlinear optical loop mirror (NOLM) serve as the intensity equalizer for inducing IDL to suppress mode competition effect furtherly, while the NOLM mainly achieves power equalization and improve SMSR in this laser. The Lyot-Sagnac loop mirror is designed to realize different channel spacing, when the length of PMF is 71 m, stable multi-wavelength output has been achieved at room temperature. The number of lasing lines is up to 184 with wavelength spacing of 0.08 nm within 10 dB bandwidth from 1549.75 nm to 1664.25 nm, and all peaks experience some minor power fluctuation, and the maximum value is about 5 dB. The side mode suppression ratio (SMSR) is about up to 22 dB. Moreover, the maximum value of optical signal-to-noise ratios (OSNR) is 10.7 dB.

Mode rolling effects in nitride laser diodes

Due to their small separation of longitudinal modes, Fabry-Pérot type laser diodes show rich mode competition effects. We present streak camera measurements of two nitride laser diodes with different cavity lengths and model them using a fully dynamic model based on the semiconductor Bloch equations, obtaining good agreement. Both theory and experiment show that the different mode spacing has a large influence on the interactions between longitudinal modes. In contrast to rate-equation type models, our approach includes the detailed density distribution as well as the derivation of the relevant parameters, e.g. broadening, from standard material quantities, thus setting a milestone on the way towards a fully predictive laser model.

Stable multiwavelength semiconductor amplifier fiber laser based on nonlinear fiber loop mirror

A stable multiwavelength semiconductor amplifier (SOA) fiber laser using a nonlinear fiber loop mirror (NOLM) and a Lyot–Sagnac loop mirror is presented and experimentally demonstrated. The Lyot–Sagnac loop mirror serves as a wavelength selection device, and the wavelength spacing is controlled by the length of polarization-maintained fiber (PMF). The NOLM serves as a power equalizer to achieve power equalization and improve stability. Moreover, the NOLM can also suppress the mode competition effect within homogenous broadening linewidth of SOA. Multiwavelength output with power equality has been achieved at room temperature, with no obvious power fluctuation. The number of lasing lines is 31 within the 10-dB bandwidth, and the wavelength spacing is 0.63 nm. In addition, the average optical signal-to-noise ratio is up to 25.15 dB. Finally, narrower wavelength spacing is obtained by increasing the length of the PMF.

Training a Multi-Layer Photonic Spiking Neural Network With Modified Supervised Learning Algorithm Based on Photonic STDP

We propose a framework for hardware architecture and learning algorithm co-design of multi-layer photonic spiking neural network (SNN). The vertical-cavity surface-emitting laser with an embedded saturable absorber (VCSEL-SA) which contains two polarization-resolved modes is employed as a spiking neuron. The connection between two identical polarization modes is considered as the excitatory synapse, whereas the connection between two orthogonal polarization modes is regarded as the inhibitory synapse. The physical model of the photonic spiking neuron is derived based on the combination of spin-flip model and Yamada model. The photonic spike timing dependent plasticity (STDP) is applied to design a hardware-friendly biologically plausible supervised learning algorithm for a multi-layer photonic SNN. Thanks to the polarization mode competition effect in the VCSEL-SA, the proposed neuromorphic network is capable of solving the classical XOR problem. The effect of physical parameters of photonic neuron on the training convergence is also considered. We further extend the multi-layer photonic SNN to realize other logic learning tasks. To the best of our knowledge, such a modified supervised learning algorithm dedicated for a multi-layer photonic SNN has not yet been reported, which is interesting for spiking learning of neuromorphic photonics.

Optoelectronic parametric oscillator

Oscillators are one of the key elements in various applications as a signal source to generate periodic oscillations. Among them, an optical parametric oscillator (OPO) is a driven harmonic oscillator based on parametric frequency conversion in an optical cavity, which has been widely investigated as a coherent light source with an extremely wide wavelength tuning range. However, steady oscillation in an OPO is confined by the cavity delay, which leads to difficulty in frequency tuning, and the frequency tuning is discrete with the minimum tuning step determined by the cavity delay. Here, we propose and demonstrate a counterpart of an OPO in the optoelectronic domain, i.e., an optoelectronic parametric oscillator (OEPO) based on parametric frequency conversion in an optoelectronic cavity to generate microwave signals. Owing to the unique energy-transition process in the optoelectronic cavity, the phase evolution in the OEPO is not linear, leading to steady single-mode oscillation or multimode oscillation that is not bounded by the cavity delay. Furthermore, the multimode oscillation in the OEPO is stable and easy to realize owing to the phase control of the parametric frequency-conversion process in the optoelectronic cavity, while stable multimode oscillation is difficult to achieve in conventional oscillators such as an optoelectronic oscillator (OEO) or an OPO due to the mode-hopping and mode-competition effect. The proposed OEPO has great potential in applications such as microwave signal generation, oscillator-based computation, and radio-frequency phase-stable transfer.

Theoretical Modeling of Multi-Channel Intracavity Spectroscopy Technology Based on Mode Competition in Er-Doped Fiber Ring Laser Cavity.

An analytical model for analyzing multi-channel intracavity spectroscopy technology (ICST) is established based on rate equations of Er-doped fiber laser. With the consideration of the amplified spontaneous emission, how the mode competition influences the iterative process for a stable output is analyzed. From the perspective of iterative times, the sensitivity-enhanced mechanism of the ICST is explained. Moreover, the theoretical modeling is employed to analyze the role that the mode-competition effect plays in switching the sensing channel automatically. It is demonstrated that, owing to the mode-competition effect in the laser cavity, the modulation of the cavity loss can be used to tune the sensing channel automatically. Furthermore, our proposed theoretical modeling is verified using a multi-channel ICST sensing system. It is indicated that the calculated estimates agree well with those data from the experimental absorption spectra. The principle will play a significant role in realizing the multiplexing of ICST.

Analysis of the time domain characteristics of tapered semiconductor lasers

We use traveling wave coupling theory to investigate the time domain characteristics of tapered semiconductor lasers with DBR gratings. We analyze the influence of the length of second order gratings on the power and spectrum of output light, and optimizing the length of gratings, in order to reduce the mode competition effect in the device, and obtain the high power output light wave with good longitudinal mode characteristics.

On‐Chip Hyperuniform Lasers for Controllable Transitions in Disordered Systems

AbstractDesigning light sources with controllable properties at the nanoscale is a main goal in research in photonics. Harnessing disorder opens many opportunities for reducing the footprints of laser devices, enabling physical phenomena and functionalities that are not observed in traditional structures. Controlling coherent light–matter interactions in systems based on randomness, however, is challenging especially if compared to traditional lasers. Here, how to overcome these issues by using semiconductor lasers created from stealthy hyperuniform structures is shown. An on‐chip InGaN hyperuniform laser is designed and experimentally demonstrated, a new type of disordered laser with controllable transitions—ranging from lasing curve slopes, thresholds, and linewidths— from the nonlinear interplay between randomness and hidden order created via hyperuniformity. Theory and experiments show that the addition of degrees of order stabilizes the lasing dynamics via mode competition effects, arising between weak light localizations of the hyperuniform structure. The properties of the laser are independent from the cavity size or the gain material, and show very little statistical fluctuations between different random samples possessing the same randomness. These results open to on‐chip lasers that combine the advantages of classical and random lasers into a single platform.

Discrete element models of fracture in tooth enamel: Failure mode competition and statistical effects

Capturing how cracks initiate and propagate in tooth enamel is difficult because of the complex three-dimensional microstructure of this material. In this work we use the discrete element method (DEM) to model fracture in idealized enamel structures where the enamel rod microstructure is explicitly represented with DEM elements. The model captures the interactions of a propagating crack with the mineral rods and their interfaces, including crack deflection and penetration into the rods. We used this model to assess the effect of relative strength and stiffness between the rods and the interfaces, of the decussation angle, and of statistical distributions of defects in the mineral rods. We show that higher strength rods (smaller flaws) promote interface crack deflection and branching which increases toughness through spreading of an inelastic region. Stiffer rods increase the load carried by the rod, which ultimately decreases crack resistance. Statistical variations in rod strength were shown to have an overall negative impact on the average crack resistance. In particular, for high coefficients of variation we observed substantial nucleation of ‘daughter’ cracks far from the main crack, which steer the main crack along the weakest trajectories and decrease overall toughness. This model and results provide insights to better interpret fracture experiments in human or bovine enamel, as well as better guidelines for the design of synthetic dental and bioinspired materials.

Fundamentals of the theory of self-injection locking of multi-frequency laser diode to high-Q optical microresonator

In this work we present a theoretical model describing a phenomenon of self-injection locking of multi-frequency laser to high-Q optical microresonator. Such process allows high-power single-frequency emission with sub-kHz linewidth from the compact multi-frequency diode laser. Elaborated theory takes into account fast optical feedback due to the Rayleigh scattering, mode competition effect and Bogatov asymmetric mode interaction.

Laser mode competition effects on output optical pulse high-frequency noise

High-frequency noise in an output optical pulse severely affects amplitude stability and pulse within the high-power acousto-optic Q-switched diode-pumped solid-state laser. In this study, we assessed the output optical pulse via fast Fourier transform (FFT) method under high pump power conditions to find that high-frequency noise in the output optical pulse is chiefly concentrated in the 0.25–1.8 GHz frequency band. High-frequency noise within 0.25–1 GHz and 1–1.8 GHz is mainly caused by high transverse and multi-longitudinal modes in the laser resonator. Our experimental and theoretical analysis results are in close accordance; both indicate that mode competition is the main factor affecting the output optical pulse high-frequency noise.

Using ZnO nanosheets grown by electrodeposition in random lasers as scattering centers: the effects of sheet size and presence of mode competition

In this paper, a random laser is fabricated based on zinc oxide (ZnO) nanosheets, which are grown on a fluorine-doped tin oxide glass substrate using an electrodeposition technique. ZnO nanosheets act as scattering centers, while optical gain in the random laser is provided by Rhodamine 6G dye solution. It is found that there is only a single emission peak in the random laser spectrum. It is also observed that changing the electrolyte concentration used in electrodeposition leads to the generation of ZnO nanosheets with different sizes, thicknesses, and shapes. The effects of changing the sheet geometry on the random laser characteristics are studied. Our results reveal that by increasing the size of nanosheets and decreasing their thickness, first the lasing threshold decreases and then two peaks appear in the emission spectrum, which are called random laser modes. These effects arise from the dependence of optical absorption and scattering strength of ZnO nanosheets on their sizes and thicknesses. Because of mode competition and cross gain saturation effects between the excited modes, an increase in their threshold and a decrease in their intensity are observed. Furthermore, our results show that the double mode random lasing emission from ZnO nanosheets strongly depends on the region of the sample illuminated by the pump laser.

Development of a novel overmoded sub-terahertz inclined coaxial clinotron with asymmetric mode suppressed

Overmoded coaxial high frequency structures can improve the power capacity of vacuum electron devices (VEDs) and make the fabrication and assembly of the devices easy in the terahertz band, but they also lead to mode competition and deleterious effect on the beam-wave interaction. The numerical investigation on the cold cavity characteristics indicates that the dispersion curve of the quasi-TEM mode almost overlaps with the dispersion curve of the high-order quasi-TE11 mode, and the 3-D numerical simulation results also illustrate that due to the mode competition, the quasi-TEM cannot be excited steadily in the VEDs with traditional coaxial SWSs with the available current density. In order to effectively suppress the mode competition of overmoded coaxial SWSs, novel inclined coaxial slow wave structures (SWSs) are proposed in this paper. As the overmoded ratio of the SWSs reaches 9.7, the 3D particle-in-cell simulation results indicate that this novel device can successfully produce the terahertz wave with the output power of 571 W at the operating frequency of 0.338 THz, and the frequency spectrum of the generated signal is very pure. The field distributions in the inclined coaxial SWSs illustrate that there is no asymmetric mode excited in the proposed device.

Broadband terahertz-power extracting by using electron cyclotron maser

Terahertz applications urgently require high performance and room temperature terahertz sources. The gyrotron based on the principle of electron cyclotron maser is able to generate watt-to-megawatt level terahertz radiation, and becomes an exceptional role in the frontiers of energy, security and biomedicine. However, in normal conditions, a terahertz gyrotron could generate terahertz radiation with high efficiency on a single frequency or with low efficiency in a relatively narrow tuning band. Here a frequency tuning scheme for the terahertz gyrotron utilizing sequentially switching among several whispering-gallery modes is proposed to reach high performance with broadband, coherence and high power simultaneously. Such mode-switching gyrotron has the potential of generating broadband radiation with 100-GHz-level bandwidth. Even wider bandwidth is limited by the frequency-dependent effective electrical length of the cavity. Preliminary investigation applies a pre-bunched circuit to the single-mode wide-band tuning. Then, more broadband sweeping is produced by mode switching in great-range magnetic tuning. The effect of mode competition, as well as critical engineering techniques on frequency tuning is discussed to confirm the feasibility for the case close to reality. This multi-mode-switching scheme could make gyrotron a promising device towards bridging the so-called terahertz gap.

Mode Competition Effect on Frequency Locking of a Multimode Gyrotron by a Monochromatic External Signal

We study frequency locking of the gyrotron operating mode by a monochromatic external signal under conditions of competition of several modes. The calculations were performed for a megawatt-power 170-GHz gyrotron with the operating TE28.12 mode. Mode interaction of both equidistant and nonequidistant spectra is considered. Frequency detuning of the modes is comparable to the cyclotron resonance bandwidth. Influence of the external-signal power on the boundaries of the frequency locking zones is studied. The possibility to achieve a high orbital efficiency and broadband frequency tuning in the frequency-locking regime is considered.