Continuous-wave Regime Research Articles

High-slope-efficiency 2.06 μm Ho: YLF laser in-band pumped by a fiber-coupled broadband diode.

We first demonstrate the laser performance of a compact 2.06 μm Ho: YLF laser resonantly pumped by a broadband fiber-coupled diode. In continuous-wave (CW) operation, maximum output power of 1.63 W, corresponding to a slope efficiency of 89.2%, was obtained with a near diffraction-limited beam quality. In actively Q-switched operation, maximum pulse energy of 1.1 mJ was achieved at the repetition frequency of 100 Hz. The minimum pulse duration was 43 ns. The performance in both the CW and Q-switched regimes indicates that the current fiber-coupled diode in-band pumped Ho: YLF laser has great potential in certain conditions that require several watts of output power or several millijoules of short pulse energy.

Resonantly pumped actively mode-locked Ho:YAG ceramic laser at 2122.1 nm.

We discuss what we believe is the first continuous-wave mode-locked Ho:YAG ceramic laser. We produced a mode-locked pulse using an acousto-optic modulator. We used a 1.91 μm Tm-fiber laser as the pump source. At the incident pump power of 11.4 W, we achieved the maximum output power of 1.84 W at 2122.1 nm in a continuous-wave mode-locked regime. We obtained a short-duration pulse of 241.5 ps at a repetition frequency of 82.15 MHz and achieved the beam quality factor M² of 1.2. In addition, the maximum single pulse energy was 22.4 nJ.

Análisis de regímenes de onda continua y pulsado sobre nuevos esquemas láser basados en fibra óptica

This paper presents the experimental results obtained in the development of novel optical fiber lasers architectures operating in continuous-wave and pulsed regimes. The first laser shown is an actively Q-switched operating at 1565 nm with a double-clad fiber doped with Er 3+ /Yb 3+ , pulse widths in the order of hundreds of ns, repetition rate from 200 kHz to 310 kHz, and average power up to 2.5 W. The second architecture is a passive mode-locked laser with higher harmonics generation. Output characteristics are; center wavelengths 1563 nm, noise-like pulses with durations around 80 ns, harmonics evolution up to the order 1270, and repetition rates to 259 MHz. The results presented seek to provide a comparison between two types of optical fiber lasers of great technological interest.

Peculiarities of Optimizing the Subsystems of a Continuous-Wave Gyrotron with a Generation Frequency of 0.26 THz at the Fundamental Cyclotron Resonance

We present the results of developing the main units of a gyrotron operated in the continuous-wave regime with a generation frequency of 0.26 THz. To improve selection of the operating mode in a oversized electrodynamic system, the gyrotron works at the fundamental cyclotron harmonic, which anticipates the use of a cryomagnet with a maximum magnetic field of 10 T, which does not require filling with liquid helium. The results of optimizing the electron-optical system, the cavity, and the quasi-optical converter of the output radiation are presented, and the control system, which is developed for the gyrotron setup, is described.

Automated Microwave Complex on the Basis of a Continuous-Wave Gyrotron with an Operating Frequency of 263 GHz and an Output Power of 1 kW

We study experimentally the automated microwave complex for microwave spectroscopy and diagnostics of various media, which was developed at the Institute of Applied Physics of the Russian Academy of Sciences in cooperation with GYCOM Ltd. on the basis of a gyrotron with a frequency of 263 GHz and operated at the first gyrofrequency harmonic. In the process of the experiments, a controllable output power of 0.1–1 kW was achieved with an efficiency of up to 17% in the continuous-wave generation regime. The measured radiation spectrum with a relative width of about 10 −6 and the frequency values measured at various parameters of the device are presented. The results of measuring the parameters of the wave beam, which was formed by a built-in quasioptical converter, as well as the data obtained by measuring the heat loss in the cavity and the vacuum output window are analyzed.

CONTINUOUS-WAVE MICROCHIP LASER GENERATION OF Tm:KLu(WO4)2 AND Tm:KY(WO4)2 CRYSTALS

Diode-pumped solid-state lasers are attractive for a variety of practical applications in many fields of human activity due to their high efficiency, compactness, and long durability. For applications in remote sensing lasers emitting in the spectral range of about 2 microns are required. Materials doped with trivalent thulium ions are promising active media emitting in this spectral range. Potassium rare-earth tungstates are attractive materials among Tm-doped crystals due to their suitable characteristics, such as high values of absorption and stimulated emission cross sections, incignificant concentration quenching of luminescence, well-proven technology of the high quality crystals growth. The purpose of this paper was to compare lasing properties of lasers based on potassium lutetium and potassium yttrium tungstate crystals doped with thulium ions in continuous-wave regime. Experiments were carried out with a diode pumping in microchip cavity configuration. The maximum power of laser radiation at 1947 nm of 1010 mW was obtained with Tm:KY(WO 4 ) 2 crystal with the slope efficiency with respect to the absorbed pump power of 51 %. When Tm:KLu(WO 4 ) 2 crystal was utilized an output power of 910 mW at 1968 nm wavelength with the slope efficiency of 38 % was obtained. With Tm:KLu(WO 4 ) 2 laser a tuning range over 160 nm range was realized with a prism inserted into the laser cavity.

A Hybrid Single-Mode Laser Based on Slotted Silicon Waveguides

An InGaAsP–Si hybrid single-mode laser based on etched slots in silicon waveguides was demonstrated operating at 1543 nm. The InGaAsP gain structure was bonded onto a patterned silicon-on-insulator wafer by selective area metal bonding method. The mode-selection mechanism based on a slotted silicon waveguide was applied, in which the parameters were designed using the simulation tool cavity modeling framework. The III–V lasers employed buried ridge stripe structure. The whole fabrication process only needs standard photolithography and inductively coupled plasma etching technology, which reduces cost for ease in technology transfer. At room temperature, a single mode of 1543-nm wavelength at a threshold current of 21 mA with a maximum output power of 1.9 mW in continuous-wave regime was obtained. The side mode suppression ratio was larger than 35 dB. The simplicity and flexibility of the fabrication process and a low cost make the slotted hybrid laser a promising light source.

Actively mode-locked Ho:LuAG laser at 2.1 μm

Actively mode-locked Ho:LuAG laser pumped by a 1.91-μm Tm3+ fiber laser was demonstrated. To our best knowledge, it was the first time to report an actively mode-locked Ho:LuAG laser. The maximum output power of 2.7 W at 2010.4 nm in continuous-wave mode-locked regime was obtained at the maximum incident pump power of 11.4 W. The mode-locked pulse width of 333.4 ps was obtained at a pulse repetition frequency of 82.48 MHz with the same incident pump power of 11.4 W, corresponding to single-pulse energy of 32.7 nJ. Furthermore, the M 2 factor of the Ho:LuAG laser was calculated to be 1.1.

Design of a Second Harmonic Double-Beam Continuous Wave Gyrotron with Operating Frequency of 0.79 THz

This paper presents the most essential steps of a design study of a novel second harmonic gyrotron operating in CW (continuous wave) regime at a frequency of 0.79 THz and an output power of 1–100 W. It is based on a novel idea for suppression of the parasitic modes using a double-beam electron-optical system (EOS). It includes a triode magnetron injection gun (MIG), which forms two high-quality helical electron beams (HEB). Different schemes, namely one with two generating beams and another with one generating and one absorbing beam, have been investigated and compared. It has been shown that the scheme with two generating beams is more advantageous since it allows an effective suppression of the parasitic modes and a stable single-mode operation at the second harmonic resonance. A MIG which is appropriate for the realization of the latter scheme has been optimized using numerical codes for computer-aided design (CAD). It forms beams with practically equal pitch factors and moderate velocity spread. The construction of the gun is not sensitive to small misalignments and shifts of the electrodes and the magnetic field. Among the most promising characteristics of the presented design are an improved mode selection and a stable single-mode generation at currents that are two to three times higher than the currents in the single-beam (i.e., conventional) gyrotrons.

Power and efficiency scaling of diode pumped Cr:LiSAF lasers: 770-1110 nm tuning range and frequency doubling to 387-463 nm.

We report significant average power and efficiency scaling of diode-pumped Cr:LiSAF lasers in continuous-wave (cw), cw frequency-doubled, and mode-locked regimes. Four single-emitter broad-area laser diodes around 660 nm were used as the pump source, which provided a total pump power of 7.2 W. To minimize thermal effects, a 20 mm long Cr:LiSAF sample with a relatively low Cr-concentration (0.8%) was used as the gain medium. In cw laser experiments, 2.4 W of output power, a slope efficiency of 50%, and a tuning range covering the 770-1110 nm region were achieved. Intracavity frequency doubling with beta-barium borate (BBO) crystals generated up to 1160 mW of blue power and a record tuning range in the 387-463 nm region. When mode locked with a saturable absorber mirror, the laser produced 195 fs pulses with 580 mW of average power around 820 nm at a 100.3 MHz repetition rate. The optical-to-optical conversion efficiency of the system was 33% in cw, 16% in cw frequency-doubled, and 8% in cw mode-locked regimes.

Room temperature continuous wave operation of injection quantum dot microdisk lasers

We present results of characterization of injection microdisk lasers with InAs/InGaAs self-organized quantum dot active region grown on a GaAs substrate. The minimal microdisk diameter is 15 μm. The microlasers operate in continuous wave regime at room temperature without external cooling. Lasing wavelength is around 1.26... 1.27 μm, minimal threshold current is 1.6 mA (threshold current density 900 A/cm2). Specific thermal impedance is estimated to be 5·0-3 °C·cm2/W.

Low temperature phase barium borate: A new optical limiter in continuous wave and nano pulsed regime

Low temperature phase barium borate was synthesized by hydrothermal method. XRD analysis confirms the formation of γ-BBO or hydrated barium polyborate (Ba3B6O9(OH)6) which crystallizes in monoclinic system in the P2/c space group. The molecular structure analysis shows the presence of dominant BO4 unit and the hydrated nature of material. γ-BBO exhibits sharp absorption edge at 202nm and highly transparency in the UV–Visible–NIR region. The peak at 347nm in the emission spectrum is due to the presence of self-trapped exciton. The third order nonlinear optical properties and limiting behavior of low temperature barium borate in both pulsed and continuous wave regime were studied. The effective 2PA absorption coefficient of γ-BBO under ns pulse excitation is estimated to be 0.38×10−10m/W. The nonlinear absorption coefficient, refractive index and optical susceptibility of the material in cw regime were found to be in the order of 10−5mW−1, 10−12m2W−1, 10−6esu respectively. In both regimes, low temperature phase barium borate exhibits better optical limiting properties than high temperature phase β-BBO.

Watt-level, continuous-wave bismuth-doped all-fiber laser operating at 17 μm

A watt-level all-fiber laser radiating at the wavelength of 1.7 μm in a continuous-wave regime was developed by using bismuth-doped high-germania optical fiber. A maximum slope efficiency of 33% with respect to the launched pump power was achieved. The dependencies of the slope efficiencies of bismuth-doped fiber laser versus the length of active fiber and reflectivity of the output mirror were obtained.

Experimental study on the actively Q-switched performance of Ho : LuVO4laser

A compact continuous-wave and actively Q-switched Ho : LuVO 4 laser pumped by a 1.94 μ m Tm:YAP laser is demonstrated. The maximum output power of 6.7 W at 2058.13 nm in continuous-wave regime is obtained at an absorbed pump power of 17.8 W, corresponding to a slope efficiency of 48.4%. For Q-switched operation, average output power of 6.65 W and pulse width of 22.3 ns are obtained with a repetition frequency of 20 kHz, corresponding to a central wavelength of 2058.13 nm. In addition, different Q-switched pulses at repetition frequencies of 20, 30, 40, 50, and 60 kHz are investigated comparatively.

Stokes and anti-Stokes Raman spectra of the high-energy C-C stretching modes in graphene and diamond

The Stokes and the anti-Stokes Raman spectra from the high energy carbon–carbon stretching modes in graphene and diamond are studied, with a focus on their intensity dependence on the excitation laser power. For diamond, the experiment was performed on a 50 μm slice of chemical vapour deposition grown single crystal using a 785 nm laser, in both the continuous wave (CW) and pulsed mode-locked (ML—130 fs pulses) excitation regimes. For graphene, the experiments were performed on suspended samples, including single layer, bilayer with AB-staking, twisted bi-layer and a sample with 4–6 layers, using λ = 532 and 633 nm CW lasers. The Stokes and anti-Stokes intensities vary largely from sample to sample, and the anti-Stokes/Stokes intensity ratio shows different excitation laser power behaviours. These differences can be explained on the basis of laser heating or correlated Stokes–anti-Stokes scattering phenomena.

Continuous‐wave lasing at 100°C in 1.3 µm quantum dot microdisk diode laser

A 31 µm in diameter microdisk laser with an InAs/InGaAs quantum dot active region has been tested in the continuous-wave regime at elevated temperatures. Lasing is achieved up to 100°C with a threshold current of 13.8 mA. The emission spectrum demonstrates single-mode lasing at 1304 nm with a side mode suppression ratio of 24 dB and a dominant mode linewidth of 35 pm.

Continuous wave room temperature external ring cavity quantum cascade laser

An external ring cavity quantum cascade laser operating at ∼5.2 μm wavelength in a continuous-wave regime at the temperature of 15 °C is demonstrated. Out-coupled continuous-wave optical powers of up to 23 mW are observed for light of one propagation direction with an estimated total intra-cavity optical power flux in excess of 340 mW. The uni-directional regime characterized by the intensity ratio of more than 60 for the light propagating in the opposite directions was achieved. A single emission peak wavelength tuning range of 90 cm−1 is realized by the incorporation of a diffraction grating into the cavity.

Polythiophene derivative functionalized with disperse red 1 chromophore: Its third-order nonlinear optical properties through Z-scan technique under continuous and femtosecond irradiation

A copolymer of 3-hexylthiophene and thiophene functionalized with disperse red 1, poly(3-HT-co-TDR1), was synthesized. Chemical structure, molecular weight distribution, optical and thermal properties of this copolymer were characterized by NMR, FT-IR, UV–vis, GPC and DSC–TGA. An optical nonlinear analysis by Z-scan method was also performed for both continuous wave (CW) and pulsed laser pumping. In the CW regime the nonlinearities were evaluated in solid films, and a negative nonlinear refractive index in the range 2.7–4.1×10−4cm2/W was obtained. These values are notoriously high and allowed to observe self-defocusing effects at very low laser intensities: below 1mW. Further, nonlinear self-phase modulation patterns, during laser irradiation, were also observed. In the pulsed excitation the nonlinear response was evaluated in solution resulting in large two-photon absorption cross section of 5725 GM for the whole copolymer chain and with a value of 232 GM per repeated monomeric unit.

The Effect of Laser Wavelength and Concentration on Thermal Nonlinear Refractive Index of Graphene Suspensions

In this work, the nonlinear refractive properties of graphene suspension at different concentration levels are investigated by means of Z-scan technique with continuous wave low power laser beams at 532 nm and 635 nm. Results show that the dispersion of graphene in water exhibits negative nonlinear refractive index in continuous wave regime, which is mainly attributed to thermal nonlinear effect. We find that the absorption coefficient and nonlinear refractive index of the samples increase with increasing the concentration level of the graphene. Moreover, it is obtained that the nonlinear refractive index of the samples at different concentration levels depends on the wavelength of laser beam.

Characterization of the lasing properties of a 5%Yb doped Lu₂SiO₅ crystal along its three principal dielectric axes.

The laser performance of a 5% Yb doped Lu2SiO5 (Yb:LSO) has been investigated in quasi continuous-wave pumping regime along the three principal dielectric axes of the crystal, to obtain a complete characterization of its laser properties. The comparison among the obtained results for differently polarized lasers, in term of relative slope efficiency and absolute efficiency, allows the exploitability of different orientations of the material in order to be determined to obtain efficient laser sources. The laser slope efficiency and the energy conversion efficiency were similar for emission polarized along the three indicatrix axes, with noticeable maximum values of slope efficiency around 90% for polarization along the Y and Z axes. Tunable laser action has been obtained in the range 990 nm - 1084 nm, with sizeable differences in the shape of the tuning curve for polarization along the X, Y and Z axes. In particular, the tuning for polarization along the Z axis is relatively flat and uniform in the range 1023 nm - 1083 nm.