Mode Selection In Lasers Research Articles

Stable and Tunable PT-Symmetric Single-Longitudinal-Mode Fiber Laser Using a Nonreciprocal Sagnac Loop

A stable and tunable parity-time (PT) symmetric single-longitudinal-mode fiber laser using a nonreciprocal Sagnac loop is proposed, which is experimentally and numerically demonstrated. The nonreciprocal Sagnac loop only including a 3-dB optical coupler (OC) and a polarization controller (PC) is incorporated into a fiber ring cavity, in which nonreciprocal light transmission between the frequency-degenerate clockwise (CW) and counterclockwise (CCW) resonator modes is induced to suppress multiple-longitudinal-mode oscillation of the laser. The two light paths along the CW and CCW directions in the Sagnac loop are respectively defined as the gain and loss loop of the PT-symmetric laser, due to a controllable birefringence induced by the PC. By controlling the polarization states of the two light waves, the PT symmetry is broken when the gain coefficient is larger than the coupling coefficient, single-mode lasing is thus achieved. Experimental results show that the optical signal to noise ratio (OSNR) of the single-mode lasing at 1550 nm is 43.0 dB, proving the great superiority of PT symmetry for lasing mode selection. By tuning an optical tunable band-pass filter incorporated into the fiber cavity, the wavelength of output single-mode lasing varies from 1530 to 1560 nm, and their 3-dB Lorentzian linewidth varies within a range from 529 to 687 Hz. During a 30-min observation period, the variations of wavelength drift and OSNR of the single-mode lasing are less than 6 pm and 2.42 dB, respectively. The advantages of the proposed scheme are obvious, which include simple structure, low cost, simple operation, and good stability.

Unidirectional mode selection in bistable quantum cascade ring lasers.

Ideal ring resonators are characterized by travelling-wave counter-propagating modes, but in practice travelling waves can only be realized under unidirectional operation, which has proved elusive. Here, we have designed and fabricated a monolithic quantum cascade ring laser coupled to an active waveguide that allows for robust, deterministic and controllable unidirectional operation. Spontaneous emission injection through the active waveguide enables dynamical switching between the clockwise and counterclockwise states of the ring laser with as little as 1.6% modulation of the electrical input. We show that this behavior stems from a perturbation in the bistable dynamics of the ring laser. In addition to switching and bistability, our novel coupler design for quantum cascade ring lasers offers an efficient mechanism for outcoupling and light detection.

Hybrid integrated external cavity laser with a 172-nm tuning range

Chip-scale lasers with a wide tunable range and a narrow linewidth have rich applications in optical communications, sensing, and light detection and ranging systems. External cavity lasers (ECLs) have the advantage of a wide tuning range and a narrow linewidth compared with their counterparts such as distributed feedback lasers and distributed Bragg reflector lasers because the passive filter separated from the gain section can be widely tuned and the low-loss external cavity extends the photon lifetime. However, so far, the tuning range achieved by the chip-scale ECLs is in the range of 80–120 nm, smaller than that of the bulk free-space ECL (typically >160 nm). The bottleneck in reaching an ultrabroad tuning range for chip-scale ECLs lies in the unsuppressed side modes of the external cavity. Here, we demonstrate a hybrid-integrated ECL consisting of a broadband III–V reflective semiconductor optical amplifier and a low-loss Si3N4 wavelength-selective reflector. A record 172-nm lasing wavelength tuning range together with a more than 40 dB side-mode suppression ratio is achieved beyond the free spectral range of the Vernier ring filter due to the enhanced lasing mode selection from a tunable Sagnac loop reflector. The Si3N4 platform enables a low-loss external cavity, facilitating laser linewidth reduction below 4 kHz over the full tuning range. The on-chip output power reaches 26.7 mW at a wavelength of 1550 nm.

Mode engineering of semiconductor lasers with vertical periodic layered structures

High-power and low-divergence semiconductor lasers are useful in light detection and ranging, as well as material processing and so on. In this paper, an efficient approach is presented to explain the physics of mode selection in semiconductor lasers with vertical periodic layers for high power and low vertical beam divergence. The transmission matrix method is used to obtain the allowed band and forbidden band of the vertical periodic layered structure, and the band diagram of the layered structure of the semiconductor laser. The fundamental mode of the layered structure of the semiconductor laser lies in the forbidden band of the vertical periodic layered structure. Tapered lasers based on the designed layered structure are fabricated and can achieve an output power of 2.45 W. The measured vertical divergence angle of the tapered laser is below 10°, consistent well with the calculated result of 9.5°.

Mode-controlled random laser assisted by stimulated Raman scattering

In this study, a reproducible and stable mode random laser is fabricated from Rhodamine 6G dye-containing ZnO@C-N microstructures by using an approach based on the coupling of the stimulated Raman scattering effect and random lasing process. Under intense pump energy, the Raman peaks of Rhodamine 6G work as seeds, and multiple elastic scattering in the disordered medium will amplify them. On the other hand, ZnO@C-N porous microstructures with plenty of nanosheets and high surface area effectively act as scatterer centers in providing strong multiple elastic scattering. So that, the strong feedback from nanosheets of the individual microstructures, allows the system to operate at a low concentration of scatterers (as low as 2mg/ml). Moreover, the enhancement of Raman modes as a function of the pump pulse energies and scatterers concentrations is examined. The results show that the Raman modes after manifestation are stable and reproduce well from pulse-to-pulse pump energy variation. Our findings introduce a new approach to tune the wavelength of random lasers for real applications and also enable the laser mode selection.

Vortex generation from a fiber laser by a polarization rotation technique

Vortex beams have wide applications in optical tweezers, optical communications and so on. Long period fiber gratings, mode selective couplers, few mode fiber Bragg gratings or specially designed fibers are often used to generate vortex beams from fiber lasers. New methods of mode selection in fiber lasers are always attractive. When transverse modes propagate along a multimode fiber, the polarization state of each mode rotates differently due to birefringence. In this paper, a linear cavity fiber laser and a ring cavity fiber laser using a polarization rotation technique are proposed to select the output laser mode. Three waveplates and a polarizing beam splitter (PBS) are used as a mode loss controller inside the laser cavity. By rotating the waveplates inside the cavity, different laser modes can be selected and achieved from the laser cavity by the PBS. Experimentally, the fundamental LP01 mode, the LP11 mode, the LP21 mode and vortex beams with ±1 topological charges were achieved.

Dynamical tuning for single mode whispering gallery mode microcavity lasing

The dynamically modulated lasing mode is extremely significant for laser applications in optical interconnections, on-chip communications and trace detection. However, tunable micro/nano laser (especially for single mode laser) is still a challenge although enormous efforts have been devoted. The coexisting modes could be selected out through light field coupling between two adjacent cavities based on Vernier effect, which could provide a compact and suitable approach for single mode lasing operation in various kinds of compound microcavity. Herein, a spherical whispering gallery cavity with small size and low threshold was rolled along a hexagonal tapered microrod to adjust the lasing mode. The coupling mechanism was explored based on static and time-resolution photoluminescence measurements in situ. The tunable lasing mode mainly derives from resonant mode of microsphere according to carrier recombination analysis. Such a spectral control establishes to realize a lasing mode selection flexibly in real-time micromanipulation.

Corrections to “Mode Selection and Tuning Mechanisms in Coupled-Cavity Terahertz Quantum Cascade Lasers” [Jul/Aug 17 Art. no. 1200312

In [1], the affiliation for Andrew Grier was incorrect. The correct affiliation where his contribution was made is as follows: A. T. Grier was with the School of Electronic and Electrical Engineering, University of Leeds, LS2 9JT Leeds, U.K. (e-mail: atgrier4@gmail.com).

Optimization of an artificial neural network to estimate laser ablation efficiency

The use of fiber laser in surgery has many advantages due to its small size, efficiency, operation in continuous and pulse mode and easy coupling to fiber optics for various applications. Laser surgery requires very precise control on laser parameters, and imprecision can cause unacceptable damages during operation, particularly in neurosurgical applications. There have been many experimental studies looking to optimize laser parameters to control laser thermal therapy. In this study, we propose an artificial neural network method (ANN) to predict laser ablation damage as a function of laser parameters and temperature. The purpose of this study was to investigate the performance of artificial neural networks to predict the ablation efficiency of a 1940 nm thulium fiber laser on ovine brain tissue. Twelve experimental data were used to train an ANN model. Tissue type (cortical/subcortical), laser mode, laser power, laser energy, time, temperature and temperature change were used as inputs for the ANN and the ablation efficiency (ablated area/total thermally altered area) was the output of the model. Four untrained data were used to validate the ANN prediction ability after finding correlation between the ANN inputs and output. Experimental and predicted data were compared to find the accuracy of the model. Moreover, optimum laser mode (continuous/pulse) selection was also studied. Five different machine learning methods were used for laser mode selection, and the results were compared. Our results showed that prediction for the ablation efficiency of an ANN is lower than 15% and 87.5% classification accuracy was obtained for optimum laser mode selection.

Femtosecond-pulse inscribed FBGs for mode selection in multimode fiber lasers

Abstract Reflection properties of a fiber Bragg grating (FBG) inscribed in a multimode fiber strongly depend on the transverse profile of FBG. Point-by-point FBG fabrication by IR femtosecond pulses with highly nonlinear refraction index variation as a function of intensity within the focal spot offers an opportunity to tailor the FBG transverse profile with high spatial resolution. In this paper we review our recent results on the femtosecond inscription of FBGs in multimode fibers, study of their spectral and mode-selective properties as well as applications for mode selection in multimode and multicore fiber lasers.

Lasing in III–V microdisk core–TiO2 shell lasers

Microdisk lasers having a III–V core coated with a TiO2 shell are experimentally studied under optical pumping. Initial core microdisk lasers with a 5–10 μm diameter comprising five layers of InAs/In0.15Ga0.85As quantum dots demonstrate room temperature lasing with excellent characteristics (threshold, quality factor) at the ground state optical transition in the 1.28–1.30 μm spectral range. Deposition of the TiO2 dielectric shell results in a decimation of the whispering gallery modes of the microdisk resonator and, for thicker (>100 nm) shells, in a blueshift of the lasing wavelength. Using numerical analysis, we demonstrate that coating a III–V core microdisk with a high-index shell causes strong penetration of the second and third radial modes into the shell, whereas the first radial mode remains nearly undisturbed. Though optical loss added by the TiO2 dielectric shell is low (it does not exceed 2 cm−1 even for a 250-nm-thick TiO2 layer), mode leakage to the TiO2 results in a decrease in the optical confinement factor and an increase in the threshold pump power. This effect is more pronounced in microlasers of the smallest diameter studied (5 μm). Thus, in addition to other applications, a TiO2 shell can be used for mode selection in microdisk lasers or for selective outcoupling of the emission to the waveguide structure, which requires proper adjustment of the TiO2 shell thickness and microdisk diameter.

Diffraction mode selection in planar lasers with Bragg resonators

Using the coupled-wave approach supplemented by the quasioptical approximation, we investigate the possibilities of diffraction mode selection with respect to the transverse index in planar distributed-feedback lasers. Quality factors and spatial structures of planar Bragg resonators with finite width and length of the corrugate area were found. Allowable values of the Fresnel parameter were determined at which the diffraction losses at the resonator edges lead to effective discrimination of modes with large numbers of transverse variations and thus provide conditions for the onset of a stationary single-mode laser generation regime.

Directional power distribution and mode selection in micro ring lasers by controlling the phase and strength of filtered optical feedback.

We discuss the design and characterization of a micro ring laser with on-chip filtered optical feedback. The laser and feedback section have been fabricated on a generic photonic integration platform using only standard building blocks. The filtering process in the feedback scheme is based on the reflection from a distributed Bragg reflector. We include several control pads in the feedback section which allows us to control separately the wavelength, the strength and the phase of the filtered feedback. By controlling the phase of the feedback, we can fine-tune the longitudinal mode selection and wavelength of the laser output, while changing the strength of the feedback allows us to control the power distribution between the two directions of the micro ring laser. Numerical simulations reproduce our experimental observations.

Mode Selection and Tuning Mechanisms in Coupled-Cavity Terahertz Quantum Cascade Lasers

We present a model for longitudinal mode competition in coupled-cavity (CC) terahertz (THz) quantum cascade lasers (QCLs) by using a scattering matrix method and multi-mode reduced rate equations (RREs). The dependence of the mode selection and tuning characteristics on various device parameters is systematically investigated, including the net waveguide loss, the optical length of the passive cavity, and the heat-sink temperature for different relationship between the active and passive cavity lengths. The changes in eigenmode frequencies due to variations of device parameter are calculated before solving the RREs. The mode selection and tuning results obtained from solving the nonlinear RREs could be well explained by the linear scattering matrix analysis. The mode tuning process simulated by the proposed model is compared with experimentally measured data, yielding good agreement. Comprehensive study of the influence of the key device parameters on the performance of CC THz QCLs provides potential design rules for single-mode operation with either wide frequency tunability or high stability.

A novel technique for longitudinal mode selection in Q-switched lasers

We revealed experimentally and explained the generation of a second giant pulse at the neighboring longitudinal mode in an Nd:YLF Q-switched laser. The delay between two pulses was about 100 ns. Based on this effect, a new method of longitudinal mode selection was implemented.

Fundamental transverse mode selection and self-stabilization in large optical cavity diode lasers under high injection current densities

It is shown that in high-power, large optical cavity laser diodes at high injection currents, the optical losses due to nonuniform carrier accumulation in the optical confinement layer can ensure the laser operation in the fundamental transverse mode. An experimental demonstration of switching from second order mode to fundamental mode in large optical cavity lasers with current and/or temperature increase is reported and explained, with the calculated values for the switching current and temperature in good agreement with the measurements. The results experimentally prove the nonuniform nature of carrier accumulation in the confinement layer and may aid laser design for optimizing the output.

Pump-controlled modal interactions in microdisk lasers

We demonstrate an effective control of nonlinear interactions of lasing modes in a semiconductor microdisk cavity by shaping the pump profile. A target mode is selected at the expense of its competing modes either by increasing their lasing thresholds or suppressing their power slopes above the lasing threshold. Despite of strong spatial overlap of the lasing modes at the disk boundary, adaptive pumping enables an efficient selection of any lasing mode to be the dominant one, leading to a switch of lasing frequency. The theoretical analysis illustrates both linear and nonlinear effects of selective pumping, and quantify their contributions to lasing mode selection. This work shows that adaptive pumping not only provides a powerful tool of controlling the nonlinear process in multimode lasers, but also enables the tuning of lasing characteristic after the lasers have been fabricated.

Active control of emission directionality of semiconductor microdisk lasers

We demonstrate lasing mode selection in nearly circular semiconductor microdisks by shaping the spatial profile of optical pump. Despite of strong mode overlap, adaptive pumping suppresses all lasing modes except the targeted one. Due to slight deformation of the cavity shape and boundary roughness, each lasing mode has distinct emission pattern. By selecting different mode to be the dominant lasing mode, we can switch both the lasing frequency and the output direction. Such tunability by external pump after the laser is fabricated enhances the functionality of semiconductor microcavity lasers.

Tailoring of random lasing characteristics in dye-doped nematic liquid crystals

Amplified spontaneous emission and random lasing are investigated in random systems with dye-doped nematic liquid crystals. And that temporal stability of random lasing is analyzed. The influence of pumping polarization as well as the multiple scattering and reflection between boundaries on the emission behavior and the formation of coherent feedback is investigated in detail. For freely suspended samples, certain emission wavelength can be obtained by changing the pump wavelength. This feature is useful in making wavelength-tunable lasers. Moreover, as the pumping thickness of wedge sample increases, the emission spectrum is red shifted and the average spacing of adjacent spikes decreases. This property can be applied in laser mode selection, i.e., the number of modes within certain wavelength range can be chosen.

蓝紫光氮化镓光子晶体面射型激光器

We experimentally and theoretically investigate the optical properties of GaN-based photonic crystal surface emitting lasers( PCSELs). We discuss the effects of lattice constant,boundary shape,and lattice type of photonic crystal( PC) on lasing characteristics. The PCSEL structures are fabricated using the metal-organic chemical vapor deposition,the electron beam lithography,and inductively coupled plasma reactive ion etching technique. The optical properties of PCSEL which include the diffraction pattern,emission spectrum,divergence angle and so on are measured by the angular-resolved photoluminescence system. Meanwhile,the plane wave expansion and multiple scattering method( MSM) are used to calculate the band diagram and threshold gain of PCSELs,respectively. The results reveal that the lattice constant plays an important role in selection of lasing mode. Despite the lasing wavelength and linewidth of circular and hexagonal shapes PCSELs are almost the same,the threshold excitation energy density of circular shape PCSEL is 0. 3 mJ /cm2 smaller than that ofhexagonal PCSEL. The PCSEL with honeycomb lattice shows the lower excitation energy density of1. 6 mJ /cm2 and divergence angle of 1. 3°. In case of square lattice,the excitation energy density is twice as large as that of honeycomb lattice. The overall results show that the single-mode emission,low divergence angle,and so on can be produced from GaN-based PCSEL. On the other hand,the numerical results calculated using the MSM are in good agreement with experimental results. The MSM could be a fast and cost-effective approach for predicting the lasing characteristic. We believe that these contributions provide guidance for the development of GaN-based PCSEL.