Continuous-wave Regime Research Articles

Design of a 3.7 GHz TE10 – TE30 mode converter using stepped impedance transformer for CW applications

This paper presents the design and simulation of a novel TE10 to TE30 mode converter operating at 3.7 GHz and is subjected to high RF power of 500 kW in Continuous Wave (CW) regime. The design incorporates a stepped impedance transformer as opposed to the conventional capacitive button to improve the return loss and power handling capacity of the converter. A rigorous RF, breakdown and multiphysics analysis including thermal, fluid dynamics and stress physics is performed on the mode converter to ascertain its viability and usefulness as a power dividing network in the Passive Active Multijunction launcher for Lower Hybrid Current Drive systems of tokamaks. The proposed mode converter exhibits a return loss of ∼50 dB, mode conversion efficiency of ∼97% and a peak E-field of 685 kV/m is obtained as compared to that of 4.2 MV/m obtained in the conventional mode converter. Thus, the power handling capacity of the proposed mode converter design is found to be 97.3% higher than the conventional mode converter.

Exploring the Use of Ramsey-CPT Spectroscopy for a Microcell-Based Atomic Clock.

We investigate the application of Ramsey spectroscopy for the development of a microcell atomic clock based on coherent population trapping (CPT). The dependence of the central Ramsey-CPT fringe properties on key experimental parameters is first studied for optimization of the clock's short-term frequency stability. The sensitivity of the clock frequency to light-shift effects is then studied. In comparison with the continuous-wave (CW) regime case, the sensitivity of the clock frequency to laser power variations is reduced by a factor up to 14 and 40.3 for dark times of 150 and [Formula: see text], respectively, at the expense of intensity 3.75 times higher for short-term stability optimization. The dependence of the clock frequency on the microwave power is also reduced in the Ramsey case. We demonstrate that the Ramsey-CPT interrogation improves the clock Allan deviation for averaging times higher than 100 s. With a dark time of [Formula: see text], a clock fractional frequency stability of 3.8 ×10-12 at 104 s is obtained, in comparison with the level of 8×10-11 obtained in the standard CW case, in similar environmental conditions. These results demonstrate that Ramsey-based interrogation protocols might be an attractive approach for the development of chip-scale atomic clocks (CSACs) with enhanced mid- and long-term stability.

Selection of parameters in nanosecond pulsed wave laser micro-welding

The digital control of the latest nanosecond pulsed wave (PW) fibre lasers allows very high flexibility in controlling the application of the total energy to a workpiece, which brings several advantages to the joining process. By choosing different pulse shapes in different spatial profiles, it is possible to apply low energy per pulse with high precision and accuracy resulting in lower heat input. Since the energy of each pulse is insufficient to generate melting, these lasers operate at very high pulse repetition frequencies near continuous wave (CW) regime. Nevertheless, the peak powers of PW lasers are much higher than CW. In this research, the effect of peak power, pulse energy, pulse width, pulse repetition frequency and duty cycle has been studied. The experimental work was conducted in bead on plate of austenitic stainless steel to investigate the effect of laser on the weld geometry, i.e. depth of penetration and width. An empirical model, previously established for CW mode, which enables the achievement of a particular penetration depth independent of the beam diameter, was redesigned and tested for PW mode. The “pulse power factor model” allows the laser user to select a weld profile that meets certain quality and productivity requirements independent of the laser system. It was shown that identical depth of penetration but different weld metal profile can be obtained for a specific beam diameter for a range of different system parameters by keeping a constant trade-off between pulse power factor and interaction time.

Experimental Demonstration of Gyrotron Frequency Stabilization by Resonant Reflection

This letter presents the results of an experimental and theoretical study of the gyrotron frequency stabilization by a partial reflection of its radiation from an external high-Q resonator. The experiment was carried out with a gyrotron operating in a continuous wave regime at a frequency of 28 GHz at the second cyclotron harmonic. The use of an external quasi-optical two-mirror resonator as a reflector made it possible to reduce the variation of the radiation frequency upon modulation of the guiding magnetic field by order of magnitude to 0.8 GHz/T. The experimental results are in good agreement with the calculations.

2090 nm 200 W Peak Power 50 ns Pulsed PM Ho-Doped Fiber Amplifier Pumped at 1860 nm

We report the design and performance of a 2 μm dual-stage polarization maintaining (PM) holmium-doped fiber amplifier (HDFA) operating in either continuous-wave (CW) or pulsed regime. The amplifier is pumped at 1860 nm, which offers a better efficiency than a 1940 nm pump. In CW regime, we measured more than 1 W of output power at both 2051 nm and 2090 nm signal wavelengths. In pulsed mode, we tested the amplifier over a broad range of pulse widths (50-500 ns) and pulse repetition frequencies (PRFs) (50-1500 kHz). The PM HDFA delivered up to 200 W of peak power and 10 μJ of pulse energy with an excellent stability, while integrated inside a compact module.

Structural, thermal, dielectric, nonlinear optical properties and DFT investigations of a novel material 2-(6-chloropyridin-3-yl)-N'-(2,3-dihydro-1,4-benzodioxin-6-ylmethylidene)acetohydrazide for optoelectronic applications

A new organic nonlinear optical (NLO) material 2-(6-chloropyridin-3-yl)-N'-(2,3-dihydro-1,4-benzodioxin-6-ylmethylidene)acetohydrazide (CDA) has been synthesized by reflux method. Single crystals were grown by slow evaporation technique and the crystal structure was elucidated by single crystal X-ray diffraction method. Density functional theory (DFT) calculations at B3LYP/6–31+ + G(d, p) basis set was used to predict the molecular geometry and were carried out further to comprehend the electronic structure, vibrational spectra, natural bonding orbitals (NBO), frontier molecular orbitals (FMO) and molecular electrostatic potential (MEP). An optical transparency at the cut-off wavelength of 355 nm was determined by UV–Vis–NIR spectroscopy. Thermal behavior of CDA was studied by TGA/DTA analysis. The dielectric constant (e), dielectric loss (tan δ) and AC conductivity as a function of frequency and temperature was studied. Second Harmonic Generation (SHG) efficiency of the CDA was determined using Kurtz and Perry powder technique and was 0.5 times greater than that of the KDP crystal. The third-order nonlinear optical properties were investigated in solution by Z-scan technique using a continuous wave (CW) DPSS laser at the wavelength of 532 nm. The title compound exhibited significant two-photon absorption (β = 2.228 × 10–4 cm W−1), nonlinear refraction (n2 = 1.095 × 10–8 cm2 W−1) and optical limiting (OL threshold = 2.511 × 103 W cm−2) under the CW regime. The nonlinear optical parameters were calculated using time-dependent Hartree–Fock (TDHF) method. The overall obtained results suggested that the studied CDA molecule could be a potential NLO material for frequency generator, optical limiters and optical switching applications.

Efficient continuous wave and acousto-optical Q-switched Tm:Lu2O3 laser pumped by the laser diode at 1.7 μm

A resonantly-pumped continuous wave and acousto-optical (AO) Q-switched Tm:Lu2O3 laser was demonstrated in this paper. In the continuous-wave regime, with absorbed pump power of 15.4 W, the Tm:Lu2O3 laser delivered a maximum output power of 6.1 W and a slope efficiency of 66.4% with respect to the absorbed pump power. With a pulse repetition frequency of 10 kHz, the AO Q-switched Tm:Lu2O3 laser produced maximum average output power of 3.87 W. At the pulse repetition frequency of 5 kHz, the AO Q-switched Tm:Lu2O3 laser reached the maximum pulse energy of 0.74 mJ and the minimum pulse width of 46 ns, resulting in a peak power of 16.1 kW.

High-power narrow spectrum GaSb-based DBR lasers emitting near 2.1 µm

Stable high-power narrow-linewidth operation of the 2.05-2.1 µm GaSb-based diode lasers was achieved by utilizing the sixth-order surface-etched distributed Bragg reflector (DBR) mirrors. The DBR multimode devices with 100 µm wide ridge waveguides generated ∼850mW in the continuous wave (CW) regime at 20°C. The device CW output power was limited by thermal rollover. The laser emission spectrum was defined by Bragg reflector reflectivity at all operating currents in a wide temperature range. The devices operated at DBR line with detuning from gain peak exceeding 10 meV.

High-power actively Q-switched Ho-doped gadolinium tantalate laser.

In this paper, we present the acousto-optical (AO) Q-switched performance of a holmium (Ho):gadolinium tantalate (GdTaO4) (Ho:GTO) laser pumped by a thulium (Tm)-fiber laser emitting at 1.94 µm. In the efficient continuous wave (CW) regime, a maximum output power of 30.5 W at 2068.8 nm was achieved, corresponding to a slope efficiency of 74.9% with respect to the absorbed pump power. In the Q-switching regime, pulse energies of 2.4 mJ, 1.2 mJ, and 0.9 mJ were obtained with pulse repetition frequencies of 10 kHz, 20 kHz, and 30 kHz, respectively. The minimum pulse widths were 18 ns, 23 ns, and 26 ns, corresponding to peak powers of approximately 133.3 kW, 52.2 kW, and 34.6 kW, respectively.

Comparison of AlGaInAs/InP semiconductor lasers (λ = 1450 – 1500 nm) with ultra-narrow and strongly asymmetric waveguides

Semiconductor lasers based on AlGaInAs/InP heterostructures with ultra-narrow and asymmetric waveguides are comparatively studied. It is shown that the use of these waveguides with a simultaneous increase in the quantum well depth makes it possible to increase output powers. Such lasers based on both strongly asymmetric and ultra-narrow waveguides with a stripe contact width of 100 μm demonstrate an output power of 5 W (at pump currents of 11.5 and 14 A, respectively) in a continuous-wave regime at room temperature and a wavelength of 1450 – 1500 nm.

Correlation technique using of-the-shelf CW DFB laser for interrogation of fiber optic sensors

The paper presents a simple fiber-optic sensor technique based on methods of correlation optical time domain reflectometry. A correlation reflectometry technique can measure distribution of reflection coefficient along the optical fiber by calculating the correlation function between a probe optical signal (reference) and the signal returned back due to reflections or/and back-scattering from the fiber under the test. To obtain the best sensor performance, the probe signal power should be a truly random function of time. As an optical source we use a free-running DFB laser diode operating in a continuous wave regime without any external modulation. To generate the probe test signal, laser light is passed through an interferometer with an optical path difference much longer than the coherence length of the laser light. The light intensity at the interferometer output has a truly random fluctuations and its auto correlation function is suitable for correlation optical reflectometry. We present results of experimental verification of the techniques in different sensor configurations. Multipoint sensor using very low reflective fiber Bragg gratings with reflectivity of 0.01% printed in a long SMF-28 optical fiber was demonstrated.

Semiconductor AlGaInAs/InP lasers (λ = 1450 – 1500 nm) with a strongly asymmetric waveguide

Semiconductor lasers based on AlGaInAs/InP heterostructures with a strongly asymmetric waveguide are studied. It is shown that the use of such a waveguide simultaneously with an increased quantum well energy depth provides conditions for increasing the output laser power. The semiconductor AlGaInAs/InP lasers based on a strongly asymmetric waveguide with a stripe contact width of 100 μm demonstrated an output optical power of 5 W (pump current 11.5 A) in a continuous-wave regime and 19 W (100 A) in a pulsed regime (100 ns, 1 kHz) at a wavelength of 1450 – 1500 nm at room temperature. The obtained data are compared with the output characteristics of lasers based on a symmetric waveguide.

Performance of subcarrier-wave quantum key distribution in the presence of spontaneous Raman scattering noise generated by classical DWDM channels

In this paper, we study the performance of the subcarrier-wave quantum key distribution system (SCW QKD) in the presence of spontaneous Raman scattering (SpRS) noise generated by classical channels of the dense wavelength division multiplexing (DWDM) network within a single-mode optical fiber. We present the mathematical model for evaluation of the quantum bit error rate and secure key generation rate with the SpRS noise taken into account. We consider two regimes of the SCW QKD system: the continuous wave regime, which uses a continuous wave laser, and the pulsed regime. For these regimes, performance of the system is analyzed depending on receiver sensitivity of classical DWDM. It is found that the pulsed regime outperforms the continuous wave regime in both the secure key generation rate and maximum achievable distance.

Multifunctional Optical Crystals for All-Solid-State Raman Lasers

In the past few decades, the multifunctional optical crystals for all-solid-state Raman lasers have been widely studied by many scholars due to their compactness, convenience and excellent performance. In this review, we briefly show two kinds of multifunctional Raman crystals: self-Raman (laser and Raman effects) crystals and self-frequency-doubled Raman (frequency-doubling and Raman effects) crystals. We firstly introduce the properties of the self-Raman laser crystals, including vanadate, tungstate, molybdate and silicate doped with rare earth ions, as well as self-frequency-doubled Raman crystals, including KTiOAsO4 (KTA) and BaTeMo2O9 (BTM). Additionally, the domestic and international progress in research on multifunctional Raman crystals is summarized in the continuous wave, passively Q-switched, actively Q-switched and mode-locked regimes. Finally, we present the bottleneck in multifunctional Raman crystals and the outlook for future development. Through this review, we contribute to a general understanding of multifunctional Raman crystals.

Femtosecond laser direct writing of depressed cladding waveguides in Nd:YAG with "ear-like" structures: fabrication and laser generation.

Low-loss depressed cladding waveguide architecture is highly attractive for improving the laser performance of waveguide lasers. We report on the design and fabrication of the "ear-like" waveguide structures formed by a set of parallel tracks in neodymium-doped yttrium aluminum garnet (Nd:YAG) crystal via femtosecond laser writing. The obtained "ear-like" waveguides are with more symmetric mode profiles and lower losses by systematically comparing the guiding properties of two kinds of normal cladding waveguide. Efficient waveguide lasers are realized based on the designed structure in both continuous wave and pulsed regimes. Combined the high-gain from cladding waveguide and special "ear-like" structure, a passively fundamentally Q-switched laser with the narrow pulse width and the high repetition rate has been obtained by using tin diselenide (SnSe2) as saturable absorber.

Увеличение оптической мощности микродисковых лазеров InGaAs/GaAs, перенесенных на кремниевую подложку методом термокомпрессии

The output power is studied under continuous-wave operation of microdisk lasers with InGaAs/GaAs quantum well-dots hybridly integrated with a silicon substrate with the epitaxial side down using the thermocompression bonding method. Owing a decrease in the thermal resistance and suppression of self-heating, an increase in the values of currents is observed at which the power is saturated and the lasing is quenched, as well as an increase in the peak power. In microdisks with a diameter of 19 µm, the highest output optical power in the continuous wave regime was 9.4 mW.

Continuous-wave and SESAM mode-locked operation of the Yb:Bi4Si3O12 laser.

We demonstrate a comprehensive characterization of the diode-pumped Yb:Bi4Si3O12 laser operating in the continuous-wave and soliton mode-locked regimes. Pumping with a 650 mW, single-transverse mode, fiber-coupled laser diode, a maximum continuous-wave output power amounted to 213 mW with a slope efficiency up to 57.6%. A broadband wavelength tuning range of more than 70 nm was achieved in CW regime with a fused silica prism. Applying a SESAM as mode locker, nearly transform-limited pulses as short as 113 fs were generated for a maximum average power of 53 mW and a pulse repetition rate of ∼106 MHz. To the best of our knowledge, this is the first report on passively mode-locked operation with the Yb:Bi4Si3O12 crystal.

AlGaInAs/InP semiconductor lasers with an ultra-narrow waveguide and an increased electron barrier

Semiconductor lasers based on AlGaInAs/InP heterostructures with an ultra-narrow waveguide and an increased electron barrier layer are developed. It is shown that the use of this waveguide in conjunction with profiled doping ensures a balance between internal optical losses and heat resistance. Additional use of strained wide-bandgap layers as blocking barriers limiting electron leakage from the active region makes it possible to increase the output power at the same pump current. The developed lasers with a stripe contact 100 μm wide demonstrate at room temperature an output optical power of 4.0 – 4.4 W (pump current 14 A) in a continuous-wave regime and 15 – 17 W (100 A) in a pulsed regime (100 ns, 1 kHz) at wavelengths of 1450 – 1500 nm.

Diode pumped cryogenic Yb:Lu3Al5O12 laser in continuous-wave and pulsed regime

We report on diode pumped cryogenic Yb: Lu3Al5O12 (LuAG) laser in the continuous-wave and pulsed regime. Under given experimental condition, we determined an optimum crystal temperature of 140 K for efficient laser operation. A maximum output power of 10.46 W with slope efficiency values of 48.6 and 67% with respect to incident and absorbed power respectively were achieved for an output coupler transmission of 40%. Yb:LuAG laser performed significantly better at 140 K with an output power 16 times and slope efficiency 7 times better than the values at 240 K. Self-starting passively Q-switched Yb:LuAG lasers were also characterized using Cr:YAG saturable absorbers with three different initial transmissions. A maximum pulse energy of 0.215 mJ and a peak power of 1.46 kW were obtained corresponding to the shortest pulse duration of 148 ns.

Graphene Capacitor-Based Electrical Switching of Mode-Locking in All-Fiberized Femtosecond Lasers.

Effective high-capacity data management necessitates the use of ultrafast fiber lasers with mode-locking-based femtosecond pulse generation. We suggest a simple but highly efficient structure of a graphene saturable absorber in the form of a graphene/poly(methyl methacrylate) (PMMA)/graphene capacitor and demonstrate the generation of ultrashort pulses by passive mode-locking in a fiber ring laser cavity, with simultaneous electrical switching (on/off) of the mode-locking operation. The voltage applied to the capacitor shifts the Fermi level of the graphene layers, thereby controlling their nonlinear light absorption, which is directly correlated with mode-locking. The flexible PMMA layer used for graphene transfer also acts as a dielectric layer to realize a very simple but effective capacitor structure. By employing the graphene capacitor on the polished surface of a D-shaped fiber, we demonstrate the switching of the mode-locking operation reversibly from the femtosecond pulse regime to a continuous wave regime of the ring laser with an extinction ratio of 70.4 dB.