Laser Central Wavelength Research Articles

Mode conversion via reflected stepped phase plate in relativistic systems

The mode conversion efficiency (CE) of the relativistic Laguerre–Gaussian (LG) laser is researched in detail within the context of current petawatt laser facilities. The topological charge, radial integer, laser central wavelength, laser bandwidth, and the design of reflective phase plate are integrated into a unified equation in theory. It is found that the vortex laser mode can be expanded as a series of LG modes, with calculations indicating that the LG10 mode predominates, constituting ∼78% of the total mode distribution. Our analysis reveals that mode CE tends toward a saturation value as the number of steps of the reflective phase plate increases. The 32-step phase plate utilized in relativistic systems is fine enough to obtain a higher CE for LG10 mode lasers, which is also verified in three-dimensional particle-in-cell simulations. This research holds promise for optimizing the design of reflective phase plates to enhance the conversion efficiency of intense LG lasers, thereby facilitating broader applications in intense vortex laser technologies.

Investigating wavelength dependence of near-threshold harmonics with different atoms and laser intensities: from analysis of an expanded quantum trajectory Monte Carlo model.

We present an investigation of the wavelength dependence of near-threshold harmonics (NTH) in different atoms and laser intensities. Similar to the previous research [Phys. Rev. Lett.112, 233001 (2014)10.1103/PhysRevLett.112.233001], using the three-dimensional time-dependent Schrödinger equation, two distinct types of NTH can be identified: one exhibiting a noticeable energy shift correlating with the laser center wavelength(frequency) (NTH 1), and another with a fixed photon energy (NTH 2). To gain deeper insights, we develop an expanded quantum trajectory Monte Carlo model, including the Rydberg state contributions, and identify three quantum trajectories: TRA 1, 2, and 3. TRA 1 and TRA 3 generate NTH 1 and NTH 2, respectively, while TRA 2 serves as a transitional trajectory capable of generating both types simultaneously. In varying the atomic species and laser intensities, TRA 1 behaves similarly to continuous states, while TRA 3 resembles Rydberg states, revealing the connections between the quantum trajectories and excited states. This research significantly enhances our understanding of NTH generation and offers valuable insights for investigating the structure and ultrafast dynamics of Rydberg states using NTH in future experiments.

2.1–2.2 μm wavelength-tunable cascade gain modulation Raman fiber laser

We present a theoretical and experimental analysis of a mid-infrared wavelength-tunable cascade gain modulation Raman fiber laser. Our simulation model incorporates the Generalized Nonlinear Schrödinger Equation with the fiber Bragg grating transmission functions. Utilizing the pulse tracking method, we investigate and analyze the impact of pump pulse width and Raman gain fiber length on the first and second order Raman laser thresholds. Additionally, we exhibit the evolution process of the pulse and optical spectrum at a constant pump pulse energy. Furthermore, we present the output pulse width and energy, and the Raman laser center wavelength as a function of the pump pulse energy. Subsequently, using the optimal theoretical values of pump pulse width and Raman gain fiber length, we experimentally achieve the gain modulation Raman fiber laser. With a 1990 nm laser pumping, the Raman laser can produce a 2172 nm laser with a pulse width of 1.25 µs and a single pulse energy of approximately 400 nJ. When tuning the pump laser wavelength from 1950 nm to 2030 nm, the gain modulation Raman laser wavelength can vary from 2130 nm to 2215 nm. Importantly, our experimental results closely align with the theoretical predictions. This study provides valuable insights for the investigation of gain modulation Raman fiber lasers.

Implementation and simulation of direct Doppler wind measurement technology under regime of multi-longitudinal mode laser

<sec>Single-longitudinal-mode (SLM) direct Doppler wind lidar (DDWL) requires complex techniques of the seed injection, high precision frequency stability, and frequency locking to provide an output of the stable frequency SLM laser, resulting in a complex structure of DDWL. To reduce the technical difficulty and structural complexity of the excitation light source of DDWL, a multi-longitudinal mode (MLM) DDWL is proposed. In the MLM DDWL, a free-running MLM laser is directly used as an excitation light source and quadri-channel Mach-Zender interferometer (QMZI) with four periodic outputs is adopted as a spectral discriminator.</sec><sec>Firstly, for the typical Nd:YAG pulsed laser, the scattering spectra of atmospheric elastic echo excited by the MLM laser are analyzed which are coincident with the longitudinal modes of the MLM laser. The peaks of atmospheric elastic echo scattering spectra excited by the MLM laser overlap with each other. The overlapping degree is affected by the laser radiation linewidth, laser optical resonator length, laser center wavelength, and type of scattering particles. In addition, the scattering spectra of atmospheric elastic echo excited by each longitudinal mode of the MLM laser have the Doppler frequency shift introduced by atmospheric wind. Therefore, it is necessary to select an optical interferometer with the periodic transmittance curve as the spectral discriminator of MLM DDWL.</sec><sec>Subsequently, a QMZI is designed as the spectral discriminator to achieve high-precision measurement for the Doppler frequency shift of scattering spectra of atmospheric elastic echo excited by the MLM laser. The designed QMZI has four periodic output channels and the phase difference between adjacent channels is π/2. The mathematical model of the transmittance function of the QMZI is established. The effective transmittance of the QMZI for atmospheric elastic echo scattering spectrum excited by the MLM laser is analyzed based on the partial coherence theory of quasi-monochromatic light interference and the polarization effect of light. On this basis, the data inversion algorithm of MLM DDWL is constructed.</sec><sec>Finally, the simulation experiments of wind measurement are carried out. The QMZI simulation model is built by the non-sequential mode of Zemax optical simulation software. The atmospheric elastic echo scattering spectra excited by the MLM laser are configured by the SPCD files of Zemax optical simulation software under different theoretical wind speeds ranging from –50 to 50 m/s, laser optical resonator lengths (<i>L</i> = 30 mm and 300 mm), and laser center wavelengths (<i>λ</i> = 1064, 532, and 355 nm). The SPCD files are fed to the QMZI simulation model as input signals. At the same time, the ray tracing on input signal is performed based on the principle of Monte Carlo simulation s, and the output signals of the four channels of the QMZI simulation model are recorded to retrieve the atmospheric wind information. The simulation results show that the proposed MLM DDWL can achieve high-precision measurement of atmospheric wind information. With the laser optical resonator length of 300 mm and laser center wavelengths <i>λ</i> = 1064 nm, <i>λ</i> = 532 nm, <i>λ</i> = 355 nm, the maximum detectable wind speeds of MLM DDWL are about 50, 30, and 20 m/s, and the wind measurement errors can be controlled within 2.5, 3.0, and 4.0 m/s, respectively. When the center wavelength of each laser is 532 nm, and the lengths of laser optical resonator are 30 mm and 300 mm, then the maximum detectable wind speeds of MLM DDWL are about 50 m/s and 30 m/s, and the wind measurement errors can be controlled within 2.0 m/s and 3.0 m/s, respectively. Therefore, the longer the laser center wavelength and the shorter the laser optical resonator length, the larger the wind measurement range will be and the smaller the wind measurement error.</sec>

DEVELOPMENT OF METHANE CARBON ISOTOPE BASED ON MID-INFRARED ABSORPTION SPECTRUM TECHNOLOGY APPLIED IN LOGGING PLATFORM

To meet the requirements of rapid measurement of oil and gas methane isotopes in offshore logging platforms, a methane carbon isotope measurement system based on mid-infrared TDLAS technology was proposed in this paper. The measuring system adopted a mid-infrared compact-linear optical structure, including an interband cascade laser (central wavelength at 2963.25 cm-1), long optical path multi-pass cell (effective optical path is 24 m) and mercury cadmium telluride detector (response wavelength is from 2 μm to 5 μm). Combining two 13CH4/12CH4 absorption lines, 2963.25 cm-1 and 2963.34 cm-1, with a high precision temperature control system using an integral separation PID control method, a temperature fluctuation within 100 mK can be achieved. In the experiment, CH4 gases with five different concentrations were configured to calibrate the measurement system, and the response linearity was more than 99%. When the integration time is 121s, the precision of isotope measurement was as low as 0.83 ‰. The average isotopic abundance of methane gas measured by the system was -10.2 ‰, and the error fluctuation was about ±0.5 ‰. It was verified that the system can rapidly measure oil and gas methane isotopes.

Study on the Technique for Wavelength Modulation Spectroscopy with Narrow Band Interference Filter Based External Cavity Diode Laser

In this study, an improved tunable external cavity diode laser (ECDL) is proposed. It is on the basis of narrow band interference filter, and its theoretical research is carried out as the light source of tunable diode laser absorption spectrum-wavelength modulation spectrum (TDLAS-WMS) system. The proposed model can use piezoelectric ceramic actuator to change and modulate its output central wavelength so that the laser central wavelength could periodically scan the absorption spectral band of the desired gas. Compared with the tunable monolithic diode laser, the narrow band interference filter based tunable ECDL might be able to accurately sense the type and concentration of a large variety of gases. Therefore, it can be used in the TDLAS-WMS system for high precision gas sensing and is expected to replace the tunable monolithic diode laser.

Multiphoton induced photoreduction in BaFCl: Sm3+ nanocrystals under femtosecond laser field

Photoinduced valence state conversion in lanthanide-doped solid materials can find significant applications in areas like optical storage, opto-electronic devices and has been attracting great research efforts. Here, we experimentally demonstrate that trivalent samarium ions contained in the BaFCl nanocrystals can be efficiently reduced to divalent ones under 800 nm fs laser field. The dependences of the luminescence intensity of the photoinduced divalent ions on the femtosecond laser power, central wavelength and polarization are investigated in detail, respectively. Theoretical fits of the experimental observations indicate that a resonance-mediated (2 + 2) four-photon absorption is most probably involved in the multiphoton induced photoreduction process.

Methane, Ethylene, and Ethane Detection by Differential Helmholtz Resonance Spectroscopy Using a 3345 nm Mid-Infrared Tunable Diode Laser Source

Gas detection based on photoacoustic spectroscopy (PAS) has attracted extensive attention due to its high sensitivity and large range of linearity. Herein, to achieve the simultaneous detection of the light carbon gases (CH4, C2H4, and C2H6), a gas detection system was constructed using a single mid-infrared tunable diode laser (central wavelength 3345 nm) source based on differential Helmholtz resonance spectroscopy (DHRS). The detection parameters (driving current, modulation depth, phase angle, etc.) were optimized under ambient pressure using the 1f demodulation method. With an integration time of 2 s, the detection limit reached 98.8 ppb, 252 ppb, and 33 ppb for methane, ethylene, and ethane, respectively. The validation test of the three-component mixture shows that the cross-interference can be effectively reduced by multiwavelength linear regression, and single wavelength linear regression causes large errors in the quantification of methane and ethylene.

Realization of thermally-near-unstable resonator thin-disk laser and compensation of air wedge effect

This paper reports on a Yb: YAG thin-disk laser based on a thermally-near-unstable resonator. The cavity length of the resonator is 3 m, which is 3.5 m less than the cavity length of a stable resonator, simplifying the oscillator structure and reducing the optical loss of the resonator cavity. The disk laser based on a thermally-near-unstable resonator achieves a laser output power of 38.3 W with beam quality approaching the diffraction limit. The laser center wavelength is 1.03 μm. The resonator becomes misaligned during the increase in pump power due to air convection, reducing laser output power. In this paper, one end mirror of the thermally-near-unstable resonator is replaced with a retro-reflector to compensate for the misalignment of the resonator. The laser output power of the resonator with retro-reflector increased to 81 W. The beam qualityM2 factors in the × and y directions are 1.03 and 1.14, respectively. retro-reflector significantly improves the operational stability of the laser.

Integrated optical system design for large-field-of-view and broad-spectrum laser warning.

We introduce an integrated large-field-of-view optical system, including a diffraction grating and a multiband narrowband filter, for laser detection. Simulation of the system in the ZEMAX optical design software package indicated that it was capable of detecting laser radiation in the visible light to near-infrared spectral range (500-1700 nm) from azimuth angles up to 120°, and pitch angles up to 96°. In practical experiments conducted at typical laser wavelengths (532 nm, 1064 nm, 1550 nm), the angle error in the full field of view was better than 0.5°, while the laser center wavelength detection accuracy was better than 7 nm, confirming the system's ability to identify the angle of incidence and wavelength of an incoming laser beam.

Generation of nanosecond 2.65 μm pulses and pulse narrowing in fiber-bulk Ho3+:YAG laser pumped gain-switched Cr2+:CdSe laser

The generation of mid-infrared pulsed lasers was achieved in a Ho3+:YAG laser pumped gain-switched Cr2+:CdSe laser system with the shortest pulse duration of 4.15 ns. With a pump pulse duration of 50 ns and pump power of 2.7 W, the gain-switched Cr2+:CdSe laser achieved over 10 times pulse narrowing, obtaining the maximum peak power of 5.7 kW. The optical-to-optical conversion efficiency was 3.7%, which could be further improved with better crystal surface polishing quality. The laser central wavelength was measured to be 2.65 μm with a bandwidth (FWHM) of 50 nm. In addition, the parameter optimization for suppressing the pulse tail was discussed, while the long cavity and high output transmissivity contributed to obtaining the single-peak pulses.

Long-pulse 4.4-4.6μm laser oscillations of Fe2+ ions in a Zn1-xMnxSe (x = 0.3) crystal pumped by a 1940nm Tm fiber laser through Cr2+ → Fe2+ energy transfer.

Millisecond-pulse laser operation of Fe2+ ions at 78K is demonstrated in the Zn1-xMnxSe:Fe2+,Cr2+ (x=0.3) crystal under a Tm fiber 1940nm laser pumping through a Cr2+→Fe2+ energy transfer process for the first time, to the best of our knowledge. The laser slope efficiency was 1% with respect to absorbed pumping energy at 1940nm. The laser central wavelength shift from 4450nm at 78K up to 4510nm at 110K was observed. Tunability from 4350 up to 4670nm at 78K was achieved using an intracavity tuning element.

High repetition-rate passively mode-locking Tm-doped fiber laser based on narrow bandwidth filtering

Abstract We propose and demonstrate a high repetition-rate passively mode-locking Tm-doped fiber laser. Mode-locking operation is achieved by using a conventional nonlinear polarization rotation (NPR) structure. Fundamental frequency soliton laser is obtained with the repetition-rate of 5.05 MHz and the pulse width of 54.7 ps. 3-dB bandwidth and signal and noise ratio (SNR) are 3.1 nm and 53.03 dB, respectively. In order to improve repetition-rate of the mode-locking pulse, a narrow band filter is adopted. In this case, maximum repetition-rate of the output pulse can be up to 7.75 GHz (1534 orders). Pulse width and 3-dB bandwidth of mode-locking laser are compressed to 67.2 ps and 0.2 nm, respectively, and SNR is 19.71 dB. In addition, the laser center wavelength can be tuned from 1882 nm to 1925 nm by adjusting the filter.

Room temperature Fe2+:Cd1-xMnxTe laser generating at 5.4-6 μm.

Six-μm laser generation at room temperature was achieved from Fe2+-doped Cd1-xMnxTe solid-solution active media for the first time, to the best of our knowledge. Laser properties of Fe2+:Cd1-xMnxTe crystals with various concentrations of manganese Mn (x=0.1, 0.52, 0.56, 0.68, and 0.76) were investigated. The increase of Mn content in these crystals was shown to result in an almost similar long-wavelength shift of absorption, fluorescence, and laser output spectra of about ∼60 nm per each 10% of Mn. Laser generation was achieved in all crystals with maximum output energies up to 30μJ (for x=0.52). The laser central oscillation wavelength is significantly influenced by Mn concentration and varies in the range from 5400nm up to 6000nm.

Passively mode-locked 354–1200 fs and continuous-wave 1044–1081 nm tunable operation of diode-pumped Nd,La:SrF2 laser

The fundamental spectroscopic properties of four Nd,La:SrF2 crystals with different and concentrations are described. The laser generation was successfully achieved with all crystals; the best results with the at.% , 8 at.% : are presented. The laser output wavelength tuning properties were investigated, and were found to have continuous tuning range over 37 nm from 1044 to 1081 nm. Two different saturable absorbers were succesfully tested for the passively mode-locked regime, and the intracavity dispersion compensation influence was studied. In the optimal set-up, a continuously mode-locked train of pulses as short as 354 fs was generated at the repetition rate of MHz. The laser central wavelength was ∼1059 nm and the overall output power reached 300 mW in two beams with absorbed pump power of less than W.

A 95-nm-wide Tunable Two-Mode Vertical External Cavity Surface-Emitting Laser

We report on a wide tunability of an optically pumped semiconductor vertical external cavity surface-emitting laser with a unique two-mode resonant microcavity. Using only high-reflectivity mirrors in the resonator and controlling the active region temperature by readjustment of pump power and heat extraction, we have demonstrated a record 95-nm wavelength-tuning range. Laser center wavelength was 985 nm.

Characteristics of optical parametric oscillator synchronously pumped by Yb:KGW laser and based on periodically poled potassium titanyl phosphate crystal

We present experimental data and numerical simulation results obtained during investigation of synchronously pumped optical parametric oscillator (SPOPO) pumped by femtosecond Yb:KGW laser (central wavelength at 1033 nm). The nonlinear medium for parametric generation was periodically poled potassium titanyl phosphate crystal (PPKTP). Maximum parametric light conversion efficiency from pump power to signal power was more than 37.5% at λs=1530 nm wavelength, whereas the achieved signal wave continuous tuning range was from 1470 nm to 1970 nm with signal pulse durations ranging from 91 fs to roughly 280 fs. We demonstrated wavelength tuning by changing cavity length and PPKTP crystal grating period and also discussed net cavity group delay dispersion (GDD) influence on SPOPO output radiation characteristics. The achieved high pump to signal conversion efficiency and easy wavelength tuning make this device a very promising alternative to Ti:sapphire based SPOPOs as a source of continuously tunable femtosecond laser radiation in the near and mid-IR range.

Real-Time Imaging of Gap Progress during and after Composite Polymerization

The aims of this study were to observe the behavior of composite and formation of gaps during and immediately after light polymerization using swept source optical coherence tomography (OCT) and to compare the interfacial integrity of adhesives in cavities through 3-dimensional (3D) image analysis. Forty tapered cylindrical cavities (4-mm diameter, 2-mm depth) were prepared in bovine incisors and restored using Bond Force (BF), Scotchbond Universal Adhesive (SBU), OptiBond XTR (XTR), or Clearfil SE Bond 2 (SE2), followed by Estelite Flow Quick flowable composite. Real-time imaging was performed at the center of restoration by the OCT system (laser center wavelength: 1,330 nm; frequency: 30 KHz) during and up to 10 min after light curing. The 3D scanning was performed 0, 1, 3, 5, and 10 min after light curing. The percentages of sealed enamel and dentin interface area (E%, D%) were calculated using Amira software. In real-time videos, the initial gaps appeared as a bright scattered area mainly on dentin floor and rapidly progressed along the cavity floor. The timing, rate, and extent of gap formation were different among the specimens. From 3D visualization, gap progress could be seen on both enamel and dentin even after irradiation; furthermore, typical toroidal gap patterns appeared at the dentin floor of BF and SBU. XTR and SE2 showed nearly perfect sealing performance on the dentin floor up to the 10 min that images were recorded. From quantitative analysis, SE2 and XTR showed significantly higher E% and D% than other groups. SBU showed the smallest E% and BF showed a significantly smaller D% than other groups (P < 0.05). In conclusion, real-time observation of composite placement and 3D quantification of interfacial gaps were implemented within the experimental limitations. Interfacial gap formation during polymerization of the composite depended on the adhesive system used. The formed gaps continued to propagate after composite light curing finished.

Cavitation-Free Continuous-Wave Laser Ablation from a Solid Target to Synthesize Low-Size-Dispersed Gold Nanoparticles.

Continuous wave (CW) radiation from a Yb-fiber laser (central wavelength 1064 nm, power 1-200 W) was used to initiate ablation of a gold target in deionized water and to synthesize bare (unprotected) gold nanoparticles. We show that the formed nanoparticles present a single low-size-dispersed population with a mean size of the order of 10 nm, which contrasts with previously reported data on dual populations of nanoparticles formed during pulsed laser ablation in liquids. The lack of a second population of nanoparticles is explained by the absence of cavitation-related mechanism of material ablation, which typically takes place under pulsed laser action on a solid target in liquid ambience, and this supposition is confirmed by plume visualization tests. We also observe a gradual growth of mean nanoparticle size from 8-10 nm to 20-25 nm under the increase of laser power for 532 nm pumping wavelength, whereas for 1064 nm pumping wavelength the mean size 8-10 nm is independent of radiation power. The growth of the nanoparticles observed for 532 nm wavelength is attributed to the enhanced target melting and splashing followed by additional heating due to an efficient excitation of plasmons over gold nanoparticles. Bare, low-size-dispersed gold nanoparticles are of importance for a variety of applications, including biomedicine, catalysis, and photovoltaics. The use of CW radiation for nanomaterial production promises to improve the cost efficiency of this technology.

Periodic structure formation and surface morphology evolution of glassy carbon surfaces applying 35-fs–200-ps laser pulses

In this work laser-induced periodic structures with lateral dimensions smaller than the central wavelength of the laser were studied on glassy carbon as a function of laser pulse duration. To generate diverse pulse durations titanium–sapphire (Ti:S) laser (center wavelength 800 nm, pulse durations: 35 fs–200 ps) and a dye–KrF excimer laser system (248 nm, pulse durations: 280 fs, 2.1 ps) were used. In the case of Ti:S laser treatment comparing the central part of the laser-treated areas a striking difference is observed between the femtoseconds and picoseconds treatments. Ripple structure generated with short pulse durations can be characterized with periodic length significantly smaller than the laser wavelength (between 120 and 165 nm). At higher pulse durations the structure has a higher periodic length (between 780 and 800 nm), which is comparable to the wavelength. In case of the excimer laser treatment the different pulse durations produced similar surface structures with different periodic length and different orientation. One of the structures was parallel with the polarization of the laser light and has a higher periodic length (~335 nm), and the other was perpendicular with smaller periodic length (~78–80 nm). The possible mechanisms of structure formation will be outlined and discussed in the frame of our experimental results.