Laser Scheme Research Articles

Frequency agile photonic integrated external cavity laser

Recent advances in the development of ultra-low loss silicon nitride integrated photonic circuits have heralded a new generation of integrated lasers capable of reaching fiber laser coherence. However, these devices are presently based on self-injection locking of distributed feedback laser diodes, increasing both the cost and requiring tuning of laser setpoints for their operation. In contrast, turn-key legacy laser systems use reflective semiconductor optical amplifiers (RSOAs). While this scheme has been utilized for integrated photonics-based lasers, so far, no cost-effective RSOA-based integrated lasers exist that are low noise and simultaneously feature fast, mode-hop-free, and linear frequency tuning as required for frequency modulated continuous wave (FMCW) LiDAR or for laser locking in frequency metrology. Here we overcome this challenge and demonstrate a RSOA-based, frequency agile integrated laser, that can be tuned with high speed, with high linearity at low power. This is achieved using monolithic integration of piezoelectrical actuators on ultra-low loss silicon nitride photonic integrated circuits in a Vernier filter-based laser scheme. The laser operates at 1550 nm, features a 6 mW output power and a 400 Hz intrinsic laser linewidth, and allows ultrafast wavelength switching within 7 ns rise time and 75 nW power consumption. In addition, we demonstrate the suitability for FMCW LiDAR by showing laser frequency tuning over 1.5 GHz at 100 kHz triangular chirp rate with a nonlinearity of 0.25% after linearization and use the source for measuring a target scene 10 m away with a 8.5 cm distance resolution.

Environmentally robust all-polarization-maintaining random fiber laser of a cylindrical vector beam.

Random lasers with vector modes have garnered widespread attention due to their unique polarization and phase properties and the absence of cavity-defined longitudinal modes. In this work, we propose and demonstrate an all-polarization-maintaining (PM) random fiber laser (RFL) based on a half-open cavity that can simultaneously generate linearly polarized fundamental modes and higher-order modes. Then a cylindrical vector beam (CVB) of azimuthal polarization is generated by the mode superposition method with mode purity exceeding 93.4%. The mode conversion process is realized with a homemade long-period fiber grating and a fiber Bragg grating based on a PM few-mode (FM) fiber. The proposed compact and simple all-PM random laser is capable of generating a CVB without polarization controllers; thus, the environmental stability and self-starting ability will be greatly improved. This laser scheme will be beneficial for the design of vector mode random lasers and is expected to find diverse applications in modal division multiplexing systems, time-domain ghost imaging, and secure communication.

Frequency-locked Si3N4 microring for Doppler frequency shift detection

In this paper, we propose to use a homemade all-fiber Si3N4 microring as the frequency discriminator for Doppler frequency shift detection. The full width at half maximum of the microring is 361.83 MHz, which covers the dynamic range of ± 70 m/s for wind measurement applications. By introducing a time-division multiplexing method, we have achieved the frequency locking of the microring to the central frequency of the laser source, which effectively stabilizes the measurement accuracy against perturbations such as temperature fluctuations. By alternatively upshifting and downshifting the pulses, a dual-frequency lasing scheme has been designed to realize the double-edge technique for frequency shift detection. A commercial single-photon detector with 25% quantum efficiency and approximately 1300 Hz dark count rate is used to detect the backscattered signals, which circumvents the need for bulky and expensive superconducting single-photon detectors. The proposed system is validated through an outdoor wind speed detection experiment using a reference anemometer. The experiment results demonstrate the feasibility of using microring as the frequency discriminator and that the precise frequency locking control is able to improve the measurement accuracy to the state-of-the-art level under the influence of perturbations, which highlights the potential for highly integrated direct detection Doppler wind lidar design.

Limitation of single-repumping schemes for laser cooling of Sr atoms

We explore the efficacy of two single-repumping schemes, 5s5pP23−5p2P23 (481nm) and 5s5pP23−5s5dD23 (497nm), for a magneto-optical trap (MOT) of Sr atoms. We reveal that the enhancement in the MOT lifetime is limited to 26.9(2) for any single-repumping scheme. Our investigation indicates that the primary decay path from the 5s5pP11 state to the 5s5pP03 state proceeds via the 5s4dD13 state, rather than through the upper states accessed by the single-repumping lasers. We estimate that the branching ratio for the 5s5pP11→5s4dD13→5s5pP03 decay path is 1:3.9(1.5)×106 and the decay rate for the transition from the 5s5pP11 state to the 5s4dD13 state is 83(32)s−1. This outcome underscores the limitation on atom number in the MOT for long loading times (≳1s) when employing a single-repumping scheme. These findings will contribute to the construction of field-deployable optical lattice clocks. Published by the American Physical Society 2024

Study of complex instability in a Thulium-doped figure-eight fiber laser including a polarization-imbalanced NOLM with no polarization control.

This study proposes a Thulium-doped figure-eight (F8) fiber laser design to be used as a platform to explore complex dynamics in passively mode-locked fiber laser systems. The proposed scheme incorporates a polarization-imbalanced nonlinear optical loop mirror (PI-NOLM) without a polarizer in the ring section. The absence of a polarizer prevents controlling the polarization state at the PI-NOLM input, which in turn allows the PI-NOLM switching power to vary dynamically during the laser operation. This leads to an intermittent noise-like pulse (NLP) operation, which is affected by complex instability in the form of non-periodic Q-switched-like behavior. Using real-time time-domain mapping techniques, we get a deep insight into the laser intricate dynamics, which involves not only NLPs, but also solitons and quasi-continuous-wave components with their subtle interactions. This study highlights the importance of the PI-NOLM in F8 laser schemes for the study and understanding of complex dynamics in Thulium-doped fiber lasers, which have been less extensively studied compared to other rare-earth-doped lasers. Our findings will contribute to the optimization and design of advanced laser systems for various scientific and industrial applications.

Multi-Wavelength Narrow-Spacing Laser Frequency Stabilization Technology Based on Fabry-Perot Etalon

Classical frequency-stabilized lasers have achieved high-frequency stability and reproducibility; however, their extensive wavelength spacing limits their utility in various scenarios. This study introduces a novel frequency-stabilized laser scheme that integrates a Fabry-Perot etalon (FPE) with digital control technology and wavelength modulation techniques. The FPE, characterized by multiple transmission peaks at minimal frequency intervals, provides stable frequency references for different lasers, thereby enhancing the system’s flexibility and adaptability. An error signal is derived from the first-order differentiation of the FPE’s transmission curve. A 180° phase difference was observed in the feedback output signal when the laser’s central frequency diverged from the reference, determining that the direction of the frequency control was accordingly determined.Employing feedback control, the laser’s output frequency is stabilized at the transmission peak frequency of the FPE. Experimental results demonstrate that this stabilization scheme effectively locks the laser’s output wavelength to different transmission peak frequencies of the FPE, achieving 25 GHz wavelength spacing. The frequency stability is improved by two orders of magnitude on a second-level timescale, maintained within hundreds of kHz, equating to a frequency stability level of 10−10.

Cover Picture: Contrib. Plasma Phys. 09/2024

Mylar/Au thin foil (a), irradiated target in different places with different laser shots (b), scheme (c), and photo (d) of the used experimental setup. Fig. 1 of the paper by L. Torrisi et al. https://doi.org/10.1002/ctpp.202300166image

Multi-Fresnel-Lens Pumping Approach for Simultaneous Emission of Seven TEM00-Mode Beams with 3.73% Conversion Efficiency

TEM00-mode operation is a requirement in many laser-based applications due to the small divergence and high-power density of the emitted laser beam. A solar laser scheme was designed and numerically studied with the goal of increasing the solar-to-laser power conversion efficiency in the TEM00-mode operation. The collection and primary concentration of sunlight was performed via twelve sets of folding mirrors and Fresnel lenses, toward a laser head composed of a fused silica torus volume and seven Ce:Nd:YAG rods, in a side-pump configuration. With this scheme, TEM00-mode laser power totaling 212.39 W could potentially be produced from seven beams, with six of them being 32.60 W each and with Mx2 = 1.00, My2 = 1.01 quality factors. Notably, 35.40 W/m2 collection efficiency and, most importantly, 3.73% solar-to-laser power conversion efficiency were numerically achieved. The latter efficiency value represents a 1.81-time improvement over the experimental record, established with a prototype that had a single Ce:Nd:YAG rod in an end-side pump configuration.

Beam-shaping in laser-based powder bed fusion of metals: A computational analysis of point-ring intensity profiles

Laser-based powder bed fusion has the potential to enable manufacturing of local material properties and locally varying alloys. One promising approach under research is shaping of the applied laser beam. The current commercial standard using a Gaussian laser beam profile is associated with a high energy intensity peak and strong gradients. A potential alternative beam shape is the point-ring profile where laser energy is distributed between a central spot and an outer ring. We conduct a numerical study of beam shape impact on melt-pool shapes, surface temperatures, evaporation and flow dynamics of single-melt-tracks. To that end, we apply a Weakly Compressible Smoothed Particle Hydrodynamics solver coupled with a ray-tracing laser scheme. A point-ring profile is compared to a standard Gaussian profile. The profiles are studied at two different beam sizes to find the influence of beam size on the shape effect. We find good agreement between numerical results and experimental data for both narrow and deep, as well as wide and shallow melt-tracks. The simulation results show that beam size and thus energy density influence the effect beam-shaping has on the melt-pool dynamics. Both the distribution of evaporation and surface tension forces over the melt-pool surface are found to depend on the used beam shape. The study highlights the importance of distinguishing between size and shape effects when investigating laser beam shapes. Both energy density and its distribution need to be tuned for the desired melt-pool behavior.

In-band pumped, Q-switched thulium-doped fiber laser system delivering 140 W and 7 mJ pulse energy.

We report on a highly efficient, in-band pumped, Q-switched, Tm-doped, rod-type master oscillator power amplifier (MOPA) system delivering up to 140 W average output power and 7 mJ pulse energy with a slope efficiency of 77% at 20 kHz repetition rate. The amplifier is pumped with Raman-shifted fiber lasers centered at 1692 nm. This in-band pump scheme for Tm-doped fiber lasers can significantly mitigate their quantum defect-related heat load limitations. At the same time, this pump wavelength yields a similar amount of storable and extractable energy to the state-of-the-art pumping at 793 nm. This approach has allowed for the development of highly efficient Tm-doped fiber laser systems combining a high average power and a high output pulse energy.

Analyzing the effect of doping concentration in split-well resonant-phonon terahertz quantum cascade lasers.

The effect of doping concentration on the temperature performance of the novel split-well resonant-phonon (SWRP) terahertz quantum-cascade laser (THz QCL) scheme supporting a clean 4-level system design was analyzed using non-equilibrium Green's functions (NEGF) calculations. Experimental research showed that increasing the doping concentration in these designs led to better results compared to the split-well direct-phonon (SWDP) design, which has a larger overlap between its active laser states and the doping profile. However, further improvement in the temperature performance was expected, which led us to assume there was an increased gain and line broadening when increasing the doping concentration despite the reduced overlap between the doped region and the active laser states. Through simulations based on NEGF calculations we were able to study the contribution of the different scattering mechanisms on the performance of these devices. We concluded that the main mechanism affecting the lasers' temperature performance is electron-electron (e-e) scattering, which largely contributes to gain and line broadening. Interestingly, this scattering mechanism is independent of the doping location, making efforts to reduce overlap between the doped region and the active laser states less effective. Optimization of the e-e scattering thus could be reached only by fine tuning of the doping density in the devices. By uncovering the subtle relationship between doping density and e-e scattering strength, our study not only provides a comprehensive understanding of the underlying physics but also offers a strategic pathway for overcoming current limitations. This work is significant not only for its implications on specific devices but also for its potential to drive advancements in the entire THz QCL field, demonstrating the crucial role of e-e scattering in limiting temperature performance and providing essential knowledge for pushing THz QCLs to new temperature heights.

Two-component intra-cavity photoacoustic spectroscopy sensor using Y-shaped photoacoustic cell with two resonant cavities

In this paper, an intra-cavity photoacoustic spectroscopy (PAS) gas sensing system based on a 3D printed Y-shaped PAS cell with two longitudinal resonant cavities is designed for two-component detection. The sensor embeds the Y-shaped photoacoustic cell in the resonator of fiber ring laser for the intra-cavity gas detection, improving the laser power in the gas detection. The two resonant cavities of the Y-type PAS cell are formed into C-band and the L-band lasers, guaranteeing the laser stable output of C-band and the L-band. Comparing with the conventional dual-wavelength laser, the dual-wavelength laser scheme proposed in the paper eliminates the model competition and possesses much higher laser stability. A dedicated wavelength modulator is used to control the fiber bragger grating (FBG) to generate the stress change, and then scan the absorption peaks of the corresponding gases. The second harmonic (2f) PAS signal is used to calculate the gas concentration. The acetylene (C2H2) and carbon dioxide (CO2) were detected based on the frequency division multiplexing (FDM) technique using two resonant cavities. A microphone is used to simultaneously detect the photoacoustic signals of two resonance cavities. A demodulation frequency with a frequency intervals of 100 Hz ensures there is no cross-interference in demodulation of signals from each resonant cavity. The trace detection of C2H2 and CO2 was realized at normal temperature and pressure. The PAS signal intensity was further improved by optimizing the modulation amplitude and incident laser power. The minimum detection limits (MDLs) and the linear fit R-squared values (R2) of C2H2 and CO2 were 529 ppbv, 0.99971, 282 ppmv and 0.99991, respectively. It indicates that the two-component gas sensing system in this paper has good linearity and high sensitivity. It provides a valuable trace gas detection device for multi-component gas detection.

Controllable nonreciprocal phonon laser in a hybrid photonic molecule based on directional quantum squeezing.

Here, a scheme for a controllable nonreciprocal phonon laser is proposed in a hybrid photonic molecule system consisting of a whispering-gallery mode (WGM) optomechanical resonator and a χ(2)-nonlinear WGM resonator, by directionally quantum squeezing one of two coupled resonator modes. The directional quantum squeezing results in a chiral photon interaction between the resonators and a frequency shift of the squeezed resonator mode with respect to the unsqueezed bare mode. We show that the directional quantum squeezing can modify the effective optomechanical coupling in the optomechanical resonator, and analyze the impacts of driving direction and squeezing extent on the phonon laser action in detail. Our analytical and numerical results indicate that the controllable nonreciprocal phonon laser action can be effectively realized in this system. The proposed scheme uses an all-optical and chip-compatible approach without spinning resonators, which may be more beneficial for integrating and packaging of the system on a chip. Our proposal may provide a new route to realize integratable phonon devices for on-chip nonreciprocal phonon manipulations, which may be used in chiral quantum acoustics, topological phononics, and acoustical information processing.

Review of molecular dynamics simulations in laser-based micro/nano-fabrication.

Laser technology is integral to the advancement of micro/nano-fabrication. While laser machining offers numerous advantages over alternative micro/nano-fabrication techniques, several challenges and bottlenecks continue to impede its large-scale industrial implementation. In response to these constraints, the molecular dynamics (MD) method has emerged as a formidable tool for optimizing the process parameters of laser micro/nano-fabrication and investigating alternative laser-based micro/nano-fabrication techniques. In this review, the application of MD in laser-based micro/nano-fabrication is comprehensively examined, including numerical simulations of short-pulse, long-pulse, continued laser and hybrid laser machining. The corresponding MD simulation schemes for lasers with different pulse widths are outlined. The mechanisms of laser-material interactions across diverse processing scenarios and the complete process of laser-based micro/nano-fabrication are also elucidated. Furthermore, the prevailing challenges in this domain and potential solutions are discussed, with future research directions being charted based on current knowledge and technological advancements.

Type-I intermittency route to chaos in passively Q-switched Tm:YLF laser emitting at 2.3 μm

We report on an original chaotic dynamic behaviour of a passively Q-switched 2.3-μm Thulium laser operating on the 3H4 → 3H5 transition. The experiment employs a Tm:LiYF4 laser crystal within various laser cavity configurations, involving optional additional cascade laser operation on the 3F4 → 3H6 transition at 1.9 μm. The saturable absorber employed is Cr2+:ZnSe, which is exclusively saturated by the 2.3 μm laser emission. A precise analysis of the Q-switching dynamics shows a pronounced inclination of the cascade laser scheme towards chaotic operation. To investigate the origins of chaos, we originally monitor the metastable 3F4 level population using cascade laser operation at 1.9 μm, which proves to be a crucial underlying parameter for explaining the observed instabilities. This analysis allows for explaining the specific dynamics of the Q-switched 2.3 μm Tm-laser intrinsically linked to Tm3+-doped materials. A very atypical route to chaos for a Q-switched laser is demonstrated involving type I intermittencies. The obtained results are of great interest for studying the premises of chaos in pulsed solid-state lasers.

Optimized laser photoionization scheme of separation of complex isotopes in the separator devices

An effective approach to determining parameters of optimal schemes of the method of laser selective photoionization of heavy isotopes, in particular, lanthanides (Gd atoms as the example) with ionization at the final stage by a pulsed electric field, autoionization through the narrow autoionization resonances and due to collisions is developed for separation of heavy isotopes in the separator devices. On the basis of the theory of optimal control and previously developed theoretical quantum approaches (the energy formalism and the relativistic many-body perturbation theory with the Dirac-Kohn-Sham-Sturm zeroth approximation for computing the parameters of elementary atomic processes such as excitation, ionization amplitudes, cross-sections, energies, widths of autoionization resonances etc.) an optimized scheme for the separation of Gd isotopes by the method of 3-stage laser photoionization with ionization at the final stage by a pulsed electric field, ionization through narrow autoionization resonances is presented. For the first time, there are theoretically calculated parameters of the narrow autoionization resonances for Gd in a sufficiently weak electric field, which are in physically reasonable agreement with data of the known experiment by Letokhov et al. Narrow-width autoionization resonances in Gd (in general in spectra of any lanthanide & actinide atom) have a relatively long lifetime, correspondingly, their excitation, ionization cross sections have the same order as the excitation one at the initial stage of separation scheme. Therefore, the use of these long-lived states can ensure the optimal implementation of a laser scheme for the separation of heavy isotopes. The obtained results for Gd with using models of optimal governing confirm the perspective of construction of the optimized schemes of the laser photoionization method with ionization at the final stage by a pulsed electric field, autoionization and allow determining the optimal parameters of the separation scheme, including diagram of atomic transitions, shape of laser pulses, etc. It is hoped that this work will also provide new opportunities for further development of laser chemistry of lanthanides, as well as actinides, taking into account new physics of autoionization resonances in their spectra.

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.

Spectral dynamics of pseudo-distributed erbium gain in Raman random fiber laser

This work explores the spectral dynamics of pseudo-distributed erbium gain in a Raman-based open cavity hybrid random fiber laser (RFL) and compares it to two other schemes; without erbium-doped fiber (EDF) and with EDF integrated mid-cavity. The pseudo-distribution of erbium gain is simulated by splicing short segments of EDF in between single-mode fiber (SMF) spools. The three schemes have similar total SMF lengths of 78 km, and a total of 30 m EDF if any. It was established that the laser wavelength shifts significantly from 1555.936 nm to 1565.904 nm simply by the insertion of EDF mid-cavity, while the distribution of EDF segments along the cavity allowed the generation of dual-wavelength lasing at 1559.964 and 1565.236 nm. The distribution of EDF also generated a higher maximum output power of 485.60 mW compared to the schemes with lumped EDF and without EDF of 409.17 mW and 352.02 mW output powers, respectively. However, the increment came at the expense of a longer instability event from homogenous gain broadening of SBS induced Stokes and a delayed onset of laser threshold condition. We discuss the formations of these different laser schemes and its dynamics permitted solely due to the uniqueness of random fiber lasers. The scheme presents an interesting opportunity in the development of hybrid random fiber lasers, and the analysis of this structure offers insights for better designed dual-wavelength random fiber lasers and the role that rare-earth Erbium dopant plays in the formation of laser in a Raman-based fiber cavity.

High-Q lasing via all-dielectric Bloch-surface-wave platform

Controlling the propagation and emission of light via Bloch surface waves (BSWs) has held promise in the field of on-chip nanophotonics. BSW-based optical devices are being widely investigated to develop on-chip integration systems. However, a coherent light source that is based on the stimulated emission of a BSW mode has yet to be developed. Here, we demonstrate lasers based on a guided BSW mode sustained by a gain-medium guiding structure microfabricated on the top of a BSW platform. A long-range propagation length of the BSW mode and a high-quality lasing emission of the BSW mode are achieved. The BSW lasers possess a lasing threshold of 6.7 μJ/mm2 and a very narrow linewidth reaching a full width at half maximum as small as 0.019 nm. Moreover, the proposed lasing scheme exhibits high sensitivity to environmental changes suggesting the applicability of the proposed BSW lasers in ultra-sensitive devices.

Cascade laser optimization for 3H4 → 3H5 and 3F4 → 3H6 sequent transitions in Tm3+-doped materials.

We study a cascade laser scheme involving the 3H4 → 3H5 and 3F4 → 3H6 consecutive transitions in Tm3+-doped materials as a promising technique to favor laser emission at 2.3 µm. We examine the conditions in terms of the Tm3+ doping levels for which the cascade laser is beneficial or not. For this, Tm:LiYF4 lasers based on crystals with several doping levels in the range of 2.5 - 6 at.% with and without cascade laser are studied. For low doping of 2.5 at.% Tm3+, adding the laser emission at 1.9 µm allows to double the output power at 2.3 µm, whereas for high doping of 6 at.%, allowing the laser to operate at 1.9 µm totally suppresses the laser emission at 2.3 µm. An analytical model is developed and confronted with experimental results to predict this doping-dependent phenomenon and forecast the potential benefits. This study of cascade laser emission on the 3H4→ 3H5 and 3F4→ 3H6 transitions versus the Tm3+ doping level is finally extended to other well-known Tm3+-doped laser materials.