Continuous-wave Regime Research Articles

Influence of gaseous laser in continuous-wave and pulse regimes on biomaterial characteristics

Aromatic plants and laser beam wavelength in the red range of visible spectra (623.8 nm), most commonly used for treating plant species, were selected. As there is not much information in the references, it is necessary to set a scale with qualitative and partially quantitative evaluations of the results. Specimens of thyme seed (Thymus vulgaris) were selected as aromatic plants for studying the effect of low-power lasers on plant development, and continuous He-Ne laser, with irradiation times in the range of minutes.

Reverse saturation absorption of CaWO4 and Ce3+ substituted CaWO4 growth on rGO sheets an 2D/3D nanocomposite for smart filtering optical radiation

The CaWO4, CaWO4/rGO and Ce3+:CaWO4/rGO nanocomposites are synthesized and subjected to hydrothermal treatment. The optical filtering behavior of materials was explored using reliable Z-scan analysis. It utilizes the focused Gaussian beam of He:Ne laser (632.8 nm) in a continuous wave regime. Closed aperture Z-scan analysis of the synthesized samples exhibits negative refraction evident from the pre-focal peak to post-focal valley. The explicit third order nonlinear parameters such as kerr nonlinearityΔn = 5.6 x10-7 (W/cm2), two photon absorption β = 2.3 x10-4 (W/cm) and optical susceptibility χ3 = 10.5 x10-8 esu were obtained for the Ce3+:CaWO4/rGO (5 wt%) nanocomposites. Ce3+:CaWO4 micro dumbbells grown on the sheets of rGO is observed using HRSEM and HRTEM images. The C 1 s spectra recorded from XPS displays the area of sp2 hybridized (CC) graphitic like structure increases which enhances the optical nonlinearities. Id/Ig ratio increases by increasing the addition of rGO signifies the reduction of oxygenated functional groups in the nanocomposite evidenced by Raman spectra analysis. The bandgap calculated from the Tauc-plot relation is found to be around 3.0 eV and it decreases to 2.9 eV when the wt.% of rGO is increased.

A photonic integrated continuous-travelling-wave parametric amplifier.

The ability to amplify optical signals is of pivotal importance across science and technology typically using rare-earth-doped fibres or gain media based on III-V semiconductors. A different physical process to amplify optical signals is to use the Kerr nonlinearity of optical fibres through parametric interactions1,2. Pioneering work demonstrated continuous-wave net-gain travelling-wave parametric amplification in fibres3, enabling, for example, phase-sensitive (that is, noiseless) amplification4, link span increase5, signal regeneration and nonlinear phase noise mitigation6. Despite great progress7-15, allphotonic integrated circuit-based demonstrations of net parametric gain have necessitated pulsed lasers, limiting their practical use. Until now, only bulk micromachined periodically poled lithium niobate (PPLN) waveguide chips have achieved continuous-wave gain16,17, yet their integration with silicon-wafer-based photonic circuits has not been shown. Here we demonstrate a photonic-integrated-circuit-based travelling-wave optical parametric amplifier with net signal gain in the continuous-wave regime. Using ultralow-loss, dispersion-engineered, metre-long, Si3N4 photonic integrated circuits18 on a silicon chip of dimensions 5 × 5 mm2, we achieve a continuous parametric gain of 12 dB that exceeds both the on-chip optical propagation loss and fibre-chip-fibre coupling losses in the telecommunication C band. Our work demonstrates the potential of photonic-integrated-circuit-based parametric amplifiers that have lithographically controlled gain spectrum, compact footprint, resilience to optical feedback and quantum-limited performance, and can operate in the wavelength ranges from visible to mid-infrared and outside conventional rare-earth amplification bands.

Photonic Crystal Surface Emitting Diode Lasers with λ near 2 µm

Epitaxially regrown electrically pumped photonic crystal surface emitting lasers (PCSELs) operating near 2 µm were designed and fabricated within a III-V-Sb material system. A high-index-contrast photonic crystal layer was incorporated into the laser heterostructures by air-pocket-retaining epitaxial regrowth. Transmission electron microscopy studies confirmed uniform and continuous AlGaAsSb initial growth over the nano-patterned GaSb surface, followed by the development of the air-pockets. The PCSEL threshold current density had a minimal value of ~170 A/cm2 in the 160–180 K temperature range when the QW gain spectrum aligned with the Γ2 band edge of the photonic crystal. The devices operated in a continuous wave regime at 160 K. The divergence and polarization of the multimode laser beam emitted from the 200 µm × 200 µm PCSEL aperture were controlled by filamentation.

Electro-optically Q-switching of dual-diode-pumped Ho-doped lutetium vanadate laser

In this paper we report on a continuous-wave and electro-optically (EO) Q-switched Ho-doped lutetium vanadate (Ho:LuVO4) laser dual end-pumped by two laser diodes at 1.94 µm. Under continuous-wave regime, a maximum output power of 18.8 W at 2058.1 nm was achieved with absorbed pump power of 44.7 W, corresponding to a slope efficiency of 54.1 % with respect to the absorbed pump power. By using a langasite (La3Ga5SiO14, LGS) crystal as the EO Q-switch, at a pulse repetition frequency of 3 kHz, the EO Q-switched Ho:LuVO4 laser produced a maximum pulse energy of 3.2 mJ and a minimum pulse width of 8.9 ns, resulting in a peak power of 358.8 kW.

Self-starting high-order mode oscillation fiber laser.

In this paper, we proposed and demonstrated two kinds of all few-mode fiber lasers with self-starting high-order mode (HOM) oscillation. The fundamental mode can be completely suppressed by using a bandpass filter with a few-mode fiber pigtail. In the continuous-wave (CW) regime, the fiber laser directly oscillates in HOM with a signal-to-noise ratio as high as 70dB, and the slope efficiency is up to 46%. The self-starting HOM mode-locked pulse can be easily achieved by employing a saturable absorber. The HOM oscillation pulsed fiber laser stably operates at 1063.72nm with 3dB of 0.05nm, which can deliver cylindrical vector beams with a high mode purity of over 98%. To our knowledge, this is the first demonstration for self-starting HOM direct oscillation in stable CW and pulsed operation states without additional adjustment. This compact and stable HOM fiber laser with a simple structure can have important applications in materials processing, optical trapping, and spatiotemporal nonlinear optics. Moreover, this work may offer a promising approach to realizing high-power fiber laser with arbitrary HOMs stable output.

Upconversion pumping of lasers via excited-state absorption: an analytical rate-equation formulation

Coupled power-gain equations were used to model lasers where optical pumping occurs as a result of a weak ground-state absorption followed by strong pump excited-state absorption (ESA). Inspired by the gain dynamics and energy-level structure of upconversion-pumped 2.3 µm T m 3 + lasers and 3.4 µm E r 3 + lasers, the model assumes that population in the lower laser level relaxes via a fast nonradiative decay to an intermediate storage level, which typically has a considerably slower decay rate than that of the upper laser level. After obtaining a description of the rate equation model, the continuous-wave regime was investigated by deriving analytical expressions for the power-dependent absorption, threshold pump power for lasing, output laser power, and laser slope efficiency. Results indicate that a slow decay rate of the lower level is necessary, so that sufficient population buildup can lead to enhanced nonlinear absorption via ESA. Furthermore, the asymptotic slope efficiency of the ESA pumped laser is similar to that of the ideal four-level laser, where the linear absorption is replaced by the effective nonlinear absorption.

SESAM mode-locked Tm:Y2O3 ceramic laser.

We demonstrate a widely tunable and passively mode-locked Tm:Y2O3 ceramic laser in-band pumped by a 1627-nm Raman fiber laser. A tuning range of 318 nm, from 1833 to 2151 nm, is obtained in the continuous-wave regime. The SESAM mode-locked laser produces Fourier-transform-limited pulses as short as 75 fs at ∼ 2.06 µm with an average output power of 0.26 W at 86.3 MHz. For longer pulse durations of 178 fs, an average power of 0.59 W is achieved with a laser efficiency of 29%. This is, to the best of our knowledge, the first mode-locked Tm:Y2O3 laser in the femtosecond regime. The spectroscopic properties and laser performance confirm that Tm:Y2O3 transparent ceramics are a promising gain material for ultrafast lasers at 2 µm.

Continuous-wave and pulsed operation of Ho:YLF laser end-pumped by Tm:fiber laser

Continuous-wave and pulsed operation of a Ho:YLF laser was demonstrated. A Tm:fiber laser with 70 W output power was used for the pump source. In this experimental setup, the end-pumped gain medium with 0.5% and 1% Ho concentration in the optical resonators were used. In the continuous-wave regime, 14.2 and 12.8 W linearly polarized power with a concave-concave resonator for 1.0% and 0.5% Ho dopes were extracted, respectively. These amounts correspond to slope efficiencies of 51.9% and 71.3% and optical-to-optical efficiencies of 39.4% and 54.7% for absorbed pump power, respectively. To avoid optical damage, a U-shaped concave-concave resonator was installed, and the resulting spot size in the resonator was about 1.31 mm. The spot size changes about 10% along the total physical length of the resonator. In the pulsed operation regime, for 0.5% Ho concentration with 20 ms pump pulse duration and 10 Hz repetition rates, the free-running output energy was 172 mJ with 16 ms pulse duration. In addition, the absorbed pump energy was about 485 mJ, corresponding to the slope and the optical-to-optical efficiencies of 14.8% and 12.3%, respectively. Also, 27 mJ pulse energy with 29.7 ns duration was achieved for Q-switched operation. Finally, the angle of output beam divergence was measured at approximately 1.1 mrad, and the laser emission wavelength was 2.0568 µm with 0.0065 µm line width.

Noise investigation of CW and mode-locked harmonic cavity nanolasers.

We theoretically investigate the noise properties of harmonic cavity nanolasers by introducing a model of coupled equations of evolution of the modes, taking spontaneous emission into account. This model is used to predict the noise among the nanolaser Hermite-Gaussian modes, both in continuous wave and mode-locked regimes. In the first case, the laser noise is described in terms of noise modes, thus illustrating the role of the laser dynamics. In the latter case, this leads to the calculation of the fluctuations of the pulse train parameters. The influence of the different laser parameters, including the amount of saturated absorption and the Henry factors, on the noise of the mode-locked regime is discussed in details.

Generation of cylindrical vector beams in a linear cavity mode-locked fiber laser based on nonlinear multimode interference.

In this paper, a linear cavity mode-locked pulsed fiber laser generating cylindrical vector beams (CVBs) is proposed and demonstrated based on a nonlinear multimode interference. A homemade long-period fiber grating with a broad bandwidth of 121 nm is used as a mode converter inside the cavity. The saturable absorber was formed by single-mode fiber-graded index multimode fiber-single mode fiber (SMF-GIMF-SMF) structure. By controlling the pump power, the operation states are switchable among continuous-wave, Q-switched mode-locked (QML), and mode-locked regimes. The repetition rate of the QML CVB pulse envelope varies from 57.4 kHz to 102.7 kHz at the pump range of 118 to 285 mW. When increasing pump power to 380 mW, mode-locked CVB pulse repetition rate of 3.592 MHz, and pulse duration of 4.62 ns are achieved. In addition, the maximum single-pulse envelope energy can reach 510 nJ, and 142 mW average-power CVBs with a slope efficiency of as high as 20.2% can be obtained. Moreover, azimuthally and radially polarized beams can be obtained with mode purity over 95% in different operating regimes. The proposed fiber laser has a simple structure, and the operation is controllable in both temporal and spatial domains, which presents a flexible pulsed CVB source for application of laser processing, time or mode division multiplexing system, and spatiotemporal nonlinear optics.

The Design of >2000-nm, ∼100-MHz Ultrafast Tm-Doped Fiber Soliton Laser Source

We firstly propose a roadmap for thedesign of high repetition rate (∼100 MHz) Tm-doped ultrafast fiber laser with an emission wavelength above 2000 nm. This roadmap is confirmed by both theoretical simulation and experimental verification. The designed Tm-doped fiber oscillator not only operates in continuous wave regime with a broad tunable wavelength range of ∼100 nm, but also in mode-locking regime with an emission wavelength >2000 nm (up to 2020 nm). A 470-fs laser pulse with a pulse repetition rate of ∼96 MHz and the maximum average power of 50 mW are achieved at the output wavelength of 2007 nm, matching well with the theoretical prediction. In a further step, the seed laser pulse is amplified to 3.3 W, accompanied by a Raman frequency shift of the output wavelength to 2346 nm. The homogeneous intense broadband spectrum with a bandwidth of 240-nm centered at 2200 nm is also observed.

Fully Integrated, 80 GHz Bandwidth, 1.3μ m InAs/InGaAs CW-PW Quantum Dot Passively Colliding-Pulse Mode-Locked (CPM) Lasers for IR Sensing Application

Integrated quantum dot passively mode-locked lasers: IQDMLLs open opportunities for IR sensing. Therefore, here, we investigated structure of quantum dot passively mode-locked lasers (QDMLLs) with colliding-pulse mode-locked (CPM) method in order to reinforce the performance of QDMLLs for ultrahigh-bit-rate compared to self-colliding pulse mode-locking (SCPM) method. The proposed structure is numerically based on finite difference traveling wave (FDTW) method. By considering bias current of 80 mA, which is the bias applied to the gain section, the laser is at its maximum output power. Finally, by comparing the QDMLLs with self-colliding pulse mode-locking (SCPM) method, it is shown that due to the grating caused by the collision of the two counter-propagating pulses in the saturable absorber, the laser bit rate in the structure of CPM is twice the bit rate in structure of SCPM somehow bit rate from 40 GHz in SCPM method increased to 80 GHz in CPM method. Also, grating coupling factor (GCF) changes due to the proposed structure has been shown, so that in the range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1cm^{ - 1} \le \kappa \le 5cm^{ - 1}$ </tex-math></inline-formula> the laser output power is pulsed wave (PW), while after this range the laser output power changes from a pulsed regime to a continuous wave (CW) regime.

Diode-pumped SESAM mode-locked Yb:(Y,Gd)AlO3 laser.

We report on the continuous-wave (CW) and mode-locked (ML) laser performance of an Yb3+-doped yttrium-gadolinium orthoaluminate crystal, Yb:(Y,Gd)AlO3. Pumping by a single-transverse-mode fiber-coupled 976 nm InGaAs laser diode, the maximum output power in the CW regime amounted to 429 mW at 1041.8 nm corresponding to a slope efficiency of 51.1% and a continuous wavelength tuning across 84 nm (1011-1095 nm) was achieved. The self-starting ML operation of the Yb:(Y,Gd)AlO3 laser was stabilized by a semiconductor saturable absorber mirror. Soliton pulses as short as 43 fs were generated at 1052.3 nm with an average output power of 103 mW and a pulse repetition rate of ∼70.8 MHz. To the best of our knowledge, our result represents the first report on the passively mode-locked operation of a Yb:(Y,Gd)AlO3 laser, and the shortest pulse duration ever achieved from any Yb3+-doped orthorhombic perovskite aluminate crystals.

Passively Q‐switched operation of Yb,Ho:LuVO4 laser with a semiconductor saturable absorber mirrors

AbstractA continuous‐wave (CW) and passively Q‐switched (PQS) Yb,Ho:LuVO4 laser pumped by a 978.6‐nm laser diode (LD) is demonstrated. The maximum output power of 131 mW at 2059.78 nm in CW regime is achieved with a pump power of 3.01 W, corresponding to a slope efficiency of 10.74%. Under PQS mode operation, an average output power of 45 mW and a shortest pulse width of 100 ns were acquired by a semiconductor saturable absorber mirrors, and for all we know, this is the first report about the PQS Yb,Ho:LuVO4 laser. In addition, a maximum energy per pulse of 12.6 nJ with a pulse repetition frequency of 7.692 MHz was obtained, corresponding to a peak power of 58.5 mW. The beam quality factor was measured to be = 1.10 and = 1.10 from the PQS Yb,Ho:LuVO4 laser.

Emission wavelength selection and tuning in the [formula omitted] manifolds of a Nd:KGd(WO4)2 solid-state laser

We present a Nd:KGW laser with a wavelength selectable intracavity loss scheme comprising a notch filter and external-cavity feedback, which enables distinct continuous-wave laser oscillation regimes. Based on the spectroscopy of the 4F3/2 - 4I11/2 transition in Nd:KGW, we have achieved tunable laser oscillation at either 1067 nm, 1076 nm or 1085 nm corresponding to different spectral peaks of the emission cross-section. An adequate balance of the intracavity loss also allows for dual-wavelength operation at both 1067–1076 nm and 1076–1085 nm peaks, and the inclusion of an additional external reflector permits dual-wavelength oscillation in the vicinity of the main emission peak at 1067 nm. From the external-cavity laser theory perspective, in our approach, the tuning mechanism is treated as an effective intracavity loss instead of an effective reflectivity treatment. The key parameters for further improving the emission wavelength range and tunability are identified and discussed.

Generation of bimodal solitons in a sapphire whispering-gallery-mode maser at millikelvin temperatures

We present experimental observations of bimodal solitons in a solid state three-level maser cooled to millikelvin temperatures. The maser is built on a highly dilute $\textrm{Fe}^{3+}$ spin ensemble hosted by a high purity $\textrm{Al}_{2}\textrm{O}_{3}$ crystal constituting a high quality factor whispering-gallery-mode resonator. The maser is pumped through one of these modes, near 31 GHz, generating two signals near 12.04 GHz from two distinct modes, 8 MHz apart. The system demonstrates three regimes, namely, a continuous wave regime, a dense soliton regime and a sparse soliton regime. These results open new avenues for studying nonlinear wave phenomena using microwave systems as well as new applications of solitons in this part of the electromagnetic spectrum.

All-fiber [formula omitted]m band tunable continuous-wave and soliton mode-locked Ho-doped fiber laser

We report the design and performance of a 2.1μm tunable holmium-doped fiber laser (HDFL) in both continuous-wave (CW) and mode-locked regime. Nonlinear polarization rotation (NPR) mechanism is used as a saturable absorber to realize the evolution from CW to mode-locked state. Another function of NPR mechanism is to realize wavelength tuning as a wavelength selection device. The tunable single-wavelength CW laser can be tuned from 2039 nm to 2118 nm. In addition, conventional soliton operation is obtained with the wavelength range of 2049.2–2087.3 nm.

Polarized spectroscopy and SESAM mode-locking of Tm,Ho:CALGO.

A Tm,Ho:CALGO laser passively mode-locked by a GaSb-based SESAM generated pulses as short as 52 fs at a central wavelength of 2015 nm with a broad spectral bandwidth of 82 nm (full width at half maximum) owing to the combined gain profiles of both dopants for σ-polarized light. The average output power reached 376 mW at a repetition rate of 85.65 MHz. In the continuous-wave regime, the laser was power scaled up to 1.01 W at 2080.6 nm with a slope efficiency of 32.0%, a laser threshold of 155 mW and π-polarized emission. Polarized spectroscopic properties of Ho3+ ions in singly doped and codoped CALGO crystals were revisited to explain the observed laser performance.

Backward optical parametric oscillator threshold and linewidth studies

We carry out a theoretical investigation of the backward interaction optical parametric oscillator (OPO), in which one of the generated waves is counterpropagating with respect to the pump. We derive simple, self-consistent analytical formulas for the continuous-wave and pulsed regimes. While consistent with previous works on the continuous-wave regime, our study enables us to extend the analysis to the pulsed regime. In particular, we derive simple expressions of the oscillation build-up time, pulsed threshold, and efficiency, for the first time, to our knowledge. We also investigate the peculiar spectral features of the backward OPO when pumped with a narrow-linewidth pulsed radiation, in particular, the absence of tolerance to phase mismatch and the natural ability to emit two waves with a Fourier-transform-limited spectrum. A comparison with the conventional forward OPO is also carried out to emphasize the unique properties of the backward OPO.