Laser Cavity Modes Research Articles

Vibration-Induced Sweeping Operation in Fiber Lasers

A new vibration-based mechanism of optical frequency/wavelength sweeping in fiber lasers induced by optical path length modulation of a laser cavity section is proposed. The mechanism is implemented for an erbium-doped fiber ring laser with a saturable absorber. Without the vibrations, the laser generates a single longitudinal mode (SLM) radiation. We show experimentally for the first time that mechanical vibrations of the laser cavity section can lead to mode dynamics in both frequency and time domains. The possibility of obtaining various mode dynamics, such as vibration-induced sweeping in a wavelength range of up to 2.2 nm or SLM generation with periodic mode hopping between two fixed longitudinal modes depending on the pump wavelength, is experimentally shown. In this vibration-based approach, the interval between changes in the laser cavity modes has good stability, because it directly relates to the vibration period.

Light squeezing enhancement by coupling nonlinear optical cavities

In this paper, we explore the squeezing effect generated by two coupled optical cavities. Each cavity contains a second-order nonlinear material and coherently pumped by a laser. Our results show that light intensity is strongly improved due to the presence of the nonlinearities and mainly depends on the detunings between external laser frequencies and cavity modes. More interestingly, the proposed scheme could enhance light squeezing for moderate coupling between cavities : the squeezing generated by one cavity is enhanced by the other one. For resonant interaction, highest squeezing effect is obtained near resonance. When fields are non resonant, squeezing increases near resonance of the considered cavity, but decreases for large detunings relative to the second cavity. Further, when the dissipation rate of the second cavity is smaller than the first, the squeezing could be improved, attaining nearly the perfect squeezing. While the temperature elevation has a negative impact overall on the nonclassical light, squeezing shows an appreciable resistance against thermal baths for appropriate parameter sets.

Improved two-mode dynamic model for external-cavity quantum cascade lasers under strong optical feedback

In this paper, we propose an improved model for describing the dynamical behavior and mode selection mechanism of an external-cavity quantum-cascade laser (EC-QCL) when subject to strong optical feedback. Theoretical analysis including the dynamical effective reflectivity model, two-mode rate-equation model and Lang-Kobayashi equations are employed to carry out the laser output characteristics under the multiple feedback effects. A main aspect of the model is that the excess mirror loss caused by the EC effective reflectivity is incorporated into the laser dynamical equations as a nonlinear dynamical variable. Because of accounting the multiple round-trip feedback terms in the EC effective reflectivity, the analysis is valid for the arbitrary feedback regimes from weak to strong levels. Besides the stationary characteristics, the model accurately predicts the dynamical competition between the fundamental laser-cavity mode and the desired EC mode that reflected strongly by external diffraction grating. The results are in well agreement with theoretical and experimental data reported earlier for the EC-QCLs.

Bi-frequency operation in a membrane external-cavity surface-emitting laser.

We report on the achievement of continuous wave bi-frequency operation in a membrane external-cavity surface-emitting laser (MECSEL), which is optically pumped with up to 4 W of 808 nm pump light. The presence of spatially specific loss of the intra-cavity high reflectivity mirror allows loss to be controlled on certain transverse cavity modes. The regions of spatially specific loss are defined through the removal of Bragg layers from the surface of the cavity high reflectivity mirror in the form of crosshair patterns with undamaged central regions, which are created using a laser ablation system incorporating a digital micromirror device (DMD). By aligning the laser cavity mode with the geometric centre of the loss patterns, the laser simultaneously operated on two Hermite-Gaussian spatial modes: the fundamental HG00 and the higher order HG11 mode. We demonstrate bi-frequency operation over a range of pump powers and sizes of spatial loss features, with a wavelength separation of approximately 5 nm centred at 1005 nm.

Multiplexed optical fiber cell temperature sensing system with high sensitivity and accuracy.

A multiplexed fiber laser sensing system for cell temperature is proposed. To the best of the authors' knowledge, this is the first multilongitudinal mode (MLM) optical fiber laser sensor array designed for cell temperature sensing. A two-channel cell temperature sensing system with high sensitivity and real-time sensing capability is achieved. The temperature change of human hepatoellular carcinomas (HepG2) cells under the influence of exogenous chemical aflatoxin B1 (AFB1) can be monitored in real time. A fiber laser cavity consists of a pair of fiber Bragg gratings (FBGs) with matched central wavelengths and a piece of erbium-doped fiber (EDF). The static FBG is utilized for design of fiber laser cavity and laser modes selection. In comparison, the sensing FBG is used for cell temperature sensing. The sensing FBG has a length of 10mm and a diameter of . Beat frequency signals (BFS) are generated by MLM lasers after optical-to-electrical conversion at a photodetector. Frequency change of a BFS is closely related to the reflected wavelength change of the sensing FBG. Through frequency division multiplexing, two fiber laser cavities are designed in the sensing system for two-channel temperature sensing. Frequency shift of a BFS that represents temperature change of cells can be automatically recorded in seconds. A two-channel cell temperature sensing system is designed with high sensitivities of 101.62 and , respectively. The temperature change of HepG2 cells under the influence of exogenous chemical AFB1 is monitored in real time. The proposed system has the advantages of simple structure, high sensitivity, and two-channel sensing capability. Our study provides a simple and effective method to design a fiber laser sensor system without complex demodulation techniques and expensive optical components.

Electrically driven single microwire-based single-mode microlaser

Engineering the lasing-mode oscillations effectively within a laser cavity is a relatively updated attentive study and perplexing issue in the field of laser physics and applications. Herein, we report a realization of electrically driven single-mode microlaser, which is composed of gallium incorporated zinc oxide microwire (ZnO:Ga MW) with platinum nanoparticles (PtNPs, d ~ 130 nm) covering, a magnesium oxide (MgO) nanofilm, a Pt nanofilm, and a p-type GaN substrate. The laser cavity modes could resonate following the whispering-gallery mode (WGM) among the six side surfaces by total internal reflection, and the single-mode lasing wavelength is centered at 390.5 nm with a linewidth of about 0.18 nm. The cavity quality factor Q is evaluated to about 2169. In the laser structure, the usage of Pt and MgO buffer layers can be utilized to engineer the band alignment of ZnO:Ga/GaN heterojunction, optimize the p-n junction quality and increase the current injection. Thus, the well-designed device structure can seamlessly unite the electron-hole recombination region, the gain medium, and optical microresonator into the PtNPs@ZnO:Ga wire perfectly. Such a single MW microlaser is essentially single-mode regardless of the gain spectral bandwidth. To study the single-mode operation, PtNPs working as superabsorber can engineering the multimode lasing actions of ZnO:Ga MWs even if their dimensions are typically much larger than that of lasing wavelength. Our findings can provide a straightforward and effective scheme to develop single-mode microlaser devices based on one-dimensional wire semiconductors.

Intra-cavity dark pulse generation through synchronized sum-frequency mixing

A Nd:YVO4 laser operating at 1064 nm generating a stable mode-locked train of 10 ps-long dark pulses with a 211 MHz repetition rate is presented. The mode-locking relies on a periodic loss modulation produced by intra-cavity sum-frequency mixing with a synchronous bright-pulse train from a mode-locked femtosecond Yb:KYW laser at 1040 nm. A modulation depth of 90% was achieved for the dark pulses, confirmed by cross-correlation measurements. The ultrafast loss modulation injects power into the Nd:YVO4 laser cavity modes beyond the laser gain bandwidth. At proper laser cavity length, the detuning interaction of these modes with the lasing modes leads to the generation of periodic ultrafast transients at frequencies above 1.5 THz.

Distributed Kerr Lens Mode-Locked Yb:YAG Thin-Disk Oscillator

Ultrafast laser oscillators are indispensable tools for diverse applications in scientific research and industry. When the phases of the longitudinal laser cavity modes are locked, pulses as short as a few femtoseconds can be generated. As most high-power oscillators are based on narrow-bandwidth materials, the achievable duration for high-power output is usually limited. Here, we present a distributed Kerr lens mode-locked Yb:YAG thin-disk oscillator which generates sub-50 fs pulses with spectral widths far broader than the emission bandwidth of the gain medium at full width at half maximum. Simulations were also carried out, indicating good qualitative agreement with the experimental results. Our proof-of-concept study shows that this new mode-locking technique is pulse energy and average power scalable and applicable to other types of gain media, which may lead to new records in the generation of ultrashort pulses.

Synchronized multi-wavelength soliton fiber laser via intracavity group delay modulation

Locking of longitudinal modes in laser cavities is the common path to generate ultrashort pulses. In traditional multi-wavelength mode-locked lasers, the group velocities rely on lasing wavelengths due to the chromatic dispersion, yielding multiple trains of independently evolved pulses. Here, we show that mode-locked solitons at different wavelengths can be synchronized inside the cavity by engineering the intracavity group delay with a programmable pulse shaper. Frequency-resolved measurements fully retrieve the fine temporal structure of pulses, validating the direct generation of synchronized ultrafast lasers from two to five wavelengths with sub-pulse repetition-rate up to ~1.26 THz. Simulation results well reproduce and interpret the key experimental phenomena, and indicate that the saturable absorption effect automatically synchronize multi-wavelength solitons in despite of the small residual group delay difference. These results demonstrate an effective approach to create synchronized complex-structure solitons, and offer an effective platform to study the evolution dynamics of nonlinear wavepackets.

Optical feedback-induced dynamics and nonclassical photon statistics of semiconductor microcavity laser

We study a coherently driven semiconductor laser cavity containing a single quantum dot(QD) (as gain medium) with optical feedback under Markovian approximation. We have obtained coupled operator equations for the model Hamiltonian using standard input-output formalism of cavity-QED and have found that these equations do not have any finite steady state solutions. We have also used an exact numerical framework based on Matlab platform qotoolbox, to compute the temporal dynamics of the mean excitation number of laser cavity mode under high feedback coupling regime. We have further studied the photon correlations of both the cavity mode as well as external feedback mode to feedback identify the laser parameters and coupling strength that give the nonclassical sub-Poissonian photon statistics. This work is useful for coherent control of photon statistics and photon correlations in the semiconductor laser with optical feedback.

Linear cavity tapered fiber sensor using amplified phase-shift cavity ring-down spectroscopy

Phase-shift cavity ring-down spectroscopy (PS-CRDS) enables the measurement of minute fluctuations in optical loss encountered by an intensity modulated laser beam via lock-in demodulation of the accumulated signal phase as the laser beam propagates through the cavity. A linear fiber cavity containing two highly reflective fiber Bragg gratings (FBGs) and a tapered fiber is a favorable cavity geometry for sensor applications due to its compact nature and wide availability of its components at telecom wavelengths. However, due to the optical absorption of water, usage of telecom wavelengths for sensing of aqueous solutions degrades the sensitivity. This problem can be worked around via an optical amplifier placed inside the fiber cavity, compensating for intrinsic cavity losses. In this work, we demonstrate amplified PS-CRDS using such a linear cavity tapered fiber sensor utilizing an optical amplifier that enables the use of thinner tapered fibers, thus achieving higher sensitivities. We also employ continuous laser wavelength sweeps and analyze peak-to-peak PSs of individual cavity modes instead of using laser-cavity mode locking with the Pound–Drever–Hall technique. This enables further simplification of the experimental arrangement without compromising sensor performance. To test the performance of the reported sensor, solutions of varying sucrose concentrations in deionized water were measured systematically by tracking the average peak-to-peak PS ⟨ ϕ pp ⟩ of the cavity modes. Tapered fibers with waist diameters ( D t a p e r ) around 2.2 µm were used during the experiment and limit of detection (LOD) values were measured down to 2.7 µM, corresponding to 1.01 × 10 − 7 R I U . The reported LOD values can be further improved by mechanical stabilization and thermal control of the device by the use of FBGs with higher reflectivity, by applying automatic optical gain control, and by spectral filtering to remove errors caused by amplified spontaneous emission. The presented concentration sensing device can be suitable for developing compact and highly sensitive, label-free optofluidic sensors.

Impact of the effective refractive index in AlGaN-based mid-UV laser structures on waveguiding

The effective refractive index in optically-pumped 265 nm AlGaN-based lasers is assessed from the spacing of the longitudinal cavity modes in short laser cavities. It is found that the effective refractive index is significantly higher than the value estimated from the Sellmeier equation (n = 2.5) and reaches values of 2.9 and 3.2 for structures with 3 and 15 quantum wells, respectively. These results indicate that the Sellmeier equation underestimates the effective refractive index in AlGaN-based laser structures and that a different approach is needed for successful mid-UV laser modeling and design.

Modulation Properties of an Extended Cavity Diode Laser and Dynamic Mode Splitting

We have found a fine structure in the frequency dependence of modulation efficiency of a diode laser (DL) with a strong selective optical feedback. The FM response of an extended cavity diode laser (ECDL) to the microwave current modulation has two peaks whose frequencies are close to the intermode frequency of the laser longitudinal modes. The frequency splitting of the peaks depends on the power of laser radiation and on the difference between the frequency of operating ECDL mode and the frequency of the closest DL mode. We have observed similar splitting in AM noise spectra with the same type of dependencies on power and aforementioned frequency detuning. The peculiarity of the AM noise spectrum has been observed and explained previously by nonlinear interaction of fields in the active region of a DL that results in the frequency splitting of the modes. This dynamic splitting of the laser cavity modes explains the specific features of the modulation characteristics.

Influence of the pump scheme on the output power and the intensity noise of a single-frequency continuous-wave laser.

The influence of the pump scheme on the intensity noise of the single-frequency continuous-wave (CW) laser is investigated in this paper, which is implemented in a single-frequency CW Nd:YVO4 1064 nm laser by comparing the traditional 808 nm pumping scheme (TPS) to the direct 888 nm pumping scheme (DPS). Under the conditions that the lasers with TPS and DPS have the same cavity structure and the cavity mirrors, as well as the same operation state including the thermal lens of the laser crystals and the mode-matching between the pump laser mode and the laser cavity mode at the laser crystals, the output power of the laser with DPS is up-to 32.0 W, which is far higher than that of 21.1 W for the laser with TPS. However, the intensity noise of the DPS laser including resonant relaxation oscillation (RRO) frequency of 809 kHz, RRO peak amplitude of 31.6 dB/Hz above the shot noise level (SNL) and the SNL cutoff frequency of 4.2 MHz, respectively, is also higher than that of 606 kHz, 20.4 dB/Hz and 2.4 MHz for the TPS laser. After further analyses, we find that the laser crystal with high doping concentration and long optical length is employed for DPS laser in order to improve the pump laser absorption efficiency, which can simultaneously increase the dipole coupling between the active atoms and the laser cavity, and then results in a high RRO frequency with a large amplitude peak as well as a high SNL cutoff frequency of the laser.

Hermite-Gaussian mode detection via convolution neural networks.

Hermite-Gaussian (HG) laser modes are a complete set of solutions to the free-space paraxial wave equation in Cartesian coordinates and represent a close approximation to physically realizable laser cavity modes. Additionally, HG modes can be mode-multiplexed to significantly increase the information capacity of optical communication systems due to their orthogonality. Because cavity tuning and optical communication applications benefit from a machine vision determination of HG modes, convolution neural networks were implemented to detect the lowest 21 unique HG modes with an accuracy greater than 99%. As the effectiveness of a CNN is dependent on the diversity of its training data, extensive simulated and experimental data sets were created for training, validation, and testing.

Modal instabilities in high power fiber laser oscillators.

Transverse mode competition and instabilities in high-power fiber oscillators have been studied experimentally by monitoring the dynamic power exchanges and characteristic frequencies of the transmitted fundamental mode (FM) and scattered high-order modes (HOMs) of the fiber laser cavity under CW and pulsed pumping. The FM and HOM power evolution indicates the presence of two competing effective laser cavities which result in rich output dynamics and full chaotic operation. The thermal and inversion related contributions to the observed instabilities have been identified by monitoring the associated characteristic instability frequencies under pulsed pumping. It is shown that in the transient regime, both inversion and thermal effects contribute successively to the observed power instabilities. Increasing the pump power leads to full chaotic response through an interplay between transverse and longitudinal mode instabilities.

Channelized amplification of RF signal based on actively mode locked fiber laser

A new approach for implementing channelized radio frequency (RF) signals amplification based on actively mode locked laser (MLL) is proposed and demonstrated. The channelized amplification (CA) is based on the mode-locking gain of the fiber ring laser while the frequency of RF signal applied to Mach–Zehnder modulator (MZM) in the fiber laser matching the fundamental or harmonic of the laser cavity mode. The experimental results show that high RF regenerative gain with a maximum 44 dB and high Q factor normally greater than 105 can be achieved with this approach of CA. By electrically tuning the fiber delay line incorporated in the ring laser so as to scan the center frequency of the channelized amplifier, spectrum recognition of quadrature phase shift keying (QPSK) modulated electro-magnetic signal with low power density is successfully implemented. The limit of input power and linear operating dynamic range is also measured and discussed.

Large astigmatic laser cavity modes and astigmatic compensation

In this paper, it was experimentally verified that anisotropic (astigmatic) Hermite–Gaussian modes can be generated from a hemi-cylindrical cavity and can be transformed into Laguerre–Gaussian modes by using an extra-cavity plano-convex cylindrical lens which can modify different beam divergence angle in the x–z and y–z planes. By controlling the off-axis pumping and the angle of the extra-cavity cylindrical lens, a variation in laser patterns was observed. Moreover, several additional relationships under different conditions have been analyzed. All experimental results corresponding to the theoretical analysis, and the role of the hemi-cylindrical cavity can be completely understood by comparing the theoretical and experimental results.

RF phase noise filtering properties of an actively mode‐locked laser

AbstractIn this article, we are presenting the experimental results and theoretical analysis of radio frequency (RF) phase noise filtering properties of a high Q cavity actively driven mode‐locked laser. The experimental results show that, the phase noise of a 10 GHz RF tone can be filtered by at least 10 dB at 200 kHz offset frequency. A theoretical analysis on the limits of phase noise filtering due to the spontaneous emission and the frequency mismatch between the laser cavity modes and the synthesizer frequency is also included.

Real-time physical random bit generation at Gbps based on random fiber lasers.

We demonstrate for the first time, to the best of our knowledge, a high-speed physical random bit generator at gigabits/second without the time-delay signature based on chaotic power fluctuations of a random fiber laser. The random fiber laser is configured using a ring structure with semiconductor optical amplifiers as the optical gain and a fiber random grating as the random feedback medium. The long-length ring cavity provides a large number of densely spaced cavity modes that co-exist within the gain bandwidth tailored by the optical filter. Unlike fixed cavity modes in conventional fiber ring lasers, these modes are less phase-correlated due to the random distributed feedback from the fiber random grating fabricated by a plane-by-plane inscription technique without phase control, leading to intensified lasing output fluctuations with high-dimensional chaotic dynamic natures without specific cavity features, when the random fiber laser is operated slightly above the lasing threshold. Random bit generation at a bit rate up to ∼1.6 Gbps is achieved with verified randomness through standard tests. Different from the previously reported schemes requiring offline post-processing procedures, this physical random number generator can function in real time, offering high-quality randomness and security assurance for pertinent applications.