Laser Scheme Research Articles

Efficient Hybrid Integration of Long-Wavelength VCSELs on Silicon Photonic Circuits

An efficient integration scheme for a vertical cavity surface emission laser (VCSEL) on a silicon photonic circuit is introduced. A grating coupler fabricated on a tilted silicon membrane is used to couple the vertical emission of the VCSEL to the in-plane silicon waveguide with a high coupling efficiency and a low reflection back to the laser. Single-mode and single-polarization VCSELs at 1310nm wavelength band are flip-chip bonded on the silicon photonic chip. A coupling efficiency of -5.8dB is measured at the laser wavelength and a high optical power of 0.66mW is coupled to the silicon waveguide. Data transmission up to 10Gb/s with direct current modulation is realized using the integrated VCSEL. All the structures in the present coupling scheme are massively producible, and the hybrid integration of VCSELs relies on a standard high-accuracy flip-chip process with passive alignment.

Growth of large aperture LBO crystal applied in high power OPCPA schemes

LBO crystal up to 192 × 130 × 89 mm3 in size and 2327 g in weight has been grown by TSSG method and cut into an optical element with a size of 100 mm × 100 mm × 17 mm for critically phase-matched type I (θ = 90°, Ø = 13.85°). Near phase-matching direction growth was introduced to promote yield of the final large aperture LBO elements greatly from the limited mass of crystal. Take into consideration of crystallographic symmetry, the linear thermal expansion coefficients and crystal growth habit, LBO grown along [1 0 0] direction was proved to be the best choice to produce large element which meet the demands of high power OPCPA schemes. The grown LBO crystal possesses high optical quality, and shows no evidence that crystal growth modifications have detrimental influence on the crystal property. The laser-induced damage threshold (LIDT) at λ = 1064 nm and τ = 5 ns is 42 J/cm2 for 1-on-1 type tests. Absorption at 1064 nm has been found to be 10–20 ppm/cm at 1064 nm). Especially, the optical homogeneity reached 1.9 × 10−7 cm−1 in full aperture for the optical elements mentioned above, to the best of our knowledge, the highest optical homogeneity in borate crystals. This extremely high optical homogeneity, large aperture optical element of LBO is more favorable in the high power laser schemes.

Compact Ho:YLF-pumped ZnGeP2-based optical parametric amplifiers tunable in the molecular fingerprint regime.

We report on a compact mid-infrared laser architecture, comprising a chain of $ {\rm ZnGeP}_2 $ZnGeP2-based optical parametric amplifiers (OPAs), which afford a higher energy yield ($ \mathbin{\lower.3ex\hbox{$\buildrel \lt \over{\smash{\scriptstyle\sim}\vphantom{_x}}$}} 60\;\unicode{x00B5} {\rm J} $∼x<60µJ at 1 kHz) compared to most conventional OPA gain media transparent in the 2-8-µm wavelength range. Specifically, our OPA scheme allows ready tunability in the molecular fingerprint regime and is tailored for strong-field excitation and coherent control of both stretch and bend (or torsional) vibrational modes in molecules. The OPAs are pumped and directly seeded (via supercontinuum generation) by a 2-µm, 3-ps Ho:YLF regenerative amplifier. The compressibility of the OPA output is demonstrated by a representative measurement of the near-Gaussian temporal profile of a dispersion-compensated 105-fs idler pulse at a central wavelength of 5.1 µm, corresponding to ${\sim}6 $∼6 optical cycles. Detailed numerical simulations closely corroborate the experimental measurements, providing a benchmark and a platform to further explore the parameter space for future design, optimization, and implementation of high-energy, ultrafast, mid-infrared laser schemes.

Design, specifications, and first beam measurements of the compact linear accelerator for research and applications front end

The compact linear accelerator for research and applications (CLARA) is an ultrabright electron beam test facility being developed at STFC Daresbury Laboratory. The ultimate aim of CLARA is to test advanced free electron laser (FEL) schemes that can later be implemented on existing and future short-wavelength FELs. In addition, CLARA is a unique facility to provide a high-quality electron beam to test novel concepts and ideas in a wide range of disciplines and to function as a technology demonstrator for a future United Kingdom x-ray FEL facility. CLARA is being built in three phases; the first phase, or front end (FE), comprises an S-band rf photoinjector, a linac, and an S-bend merging with the existing versatile electron linear accelerator beam line; the second phase will complete the acceleration to full beam energy of 250 MeV and also incorporate a separate beam line for use of electrons at 250 MeV; and the third phase will include the FEL section. The CLARA FE was commissioned during 2018, and the facility was later made available for user experiments. Significant advancements have been made in developing high-level software and a simulation framework for start-to-end simulations. The high-level software has been successfully used for unmanned rf conditioning and for characterization of the electron beam. This paper describes the design of the CLARA FE, performance of technical systems, high-level software developments, preliminary results of measured beam parameters, and plans for improvements and upgrades. © 2020 authors. Published by the American Physical Society.

Terahertz radiation emission from three-color laser-induced air plasma

Based on the photocurrent model, terahertz (THz) radiation generation in three-color (fundamental ($$\omega $$), second-harmonic ($$2\omega $$), and third-harmonic ($$3\omega $$)) laser-induced air plasma is studied using particle in cell simulation. Our simulation results show that the amplitude of THz radiation in the three-color laser scheme is several times larger than the amplitude of THz radiation in the two-color laser scheme (fundamental ($$\omega $$), second harmonic ($$2\omega $$)). In addition, the effect of the phase difference between the first and second harmonics $$\left( \varphi _{2} \right) $$ and the phase difference between the fundamental and third harmonics $$\left( \varphi _{3} \right) $$ on THz generation is investigated in order to obtain the optimum conditions. It is indicated that the effect of $$\varphi _{3}$$ on THz radiation strongly depends on $$\varphi _{2}$$ in the three-color scheme and it is found that the THz radiation is maximum in the case with $$\varphi _{2}=\pi / 6$$ and $$\varphi _{3}= \pi / 6$$. The influence of laser wavelength and pulse duration on THz generation is also studied. Our results reveal that the THz radiation increases proportionally to the square of the laser wavelength. Moreover, it is shown that THz radiation increases by increasing pulse duration.

Superradiant amplification in a chirped-tapered x-ray free-electron laser

The chirp-taper free-electron laser (FEL) scheme has been proposed to generate subfemtosecond broadband pulses using laser-modulated electron bunches in x-ray FELs. In this paper, we extend the chirp-taper concept to the postsaturation superradiant regime by combining an energy-modulated beam with a sinelike undulator taper. We show that subfemtosecond duration pulses with peak powers approaching a terawatt may be obtained with typical x-ray FEL parameters such as those at the Linac Coherent Light Source. The method shown here can produce a stable source of attosecond-scale, high-power x-ray pulses suitable for probing electronic dynamics within molecules.

Utilizing C-band erbium fiber and saturable absorber for broadband and continuous wavelength tunability laser with single-mode oscillation

In the paper, we report and exhibit a C-band erbium-doped fiber (EDF) based gain medium laser together with a 5 m unpumped EDF saturable absorber (SA) to attain broadly wavelength-selectable range and single-longitudinal-mode operation. Here, the whole tunability range of 1521.0 to 1601.0 nm (C plus L bands) can be accomplished by the designed EDF laser scheme. The output power and optical signal-to-noise ratio of the EDF laser are detected between −10.9 and 2.3 dBm and 33.1 and 45.1 dB/0.06 nm, respectively. In addition, the achievable SLM operation range runs from 1521.0 to 1581.0 nm due to the absorption limit of the EDF SA. The detected output stability and Lorentzian linewidth are also experimentally considered and significantly discussed.

Study of gain efficiency in quasi-distributed amplification systems

We propose the mathematical model of quasi-distributed gain in fiber laser systems with cavity dumping. Particularly, we consider in detail the case of a small number of periodic cells in such lasers. The study of the signal gain in Yb-doped active fiber includes the experimental measurement of its characteristics and the theoretical approximations based on experimental results. Theoretical analysis of laser schemes with quasi-distributed gain considered here enables optimization of the output pulse characteristics for various numbers of active fiber cells.

Active region designs and mounting schemes of THz quantum-cascade laser

THz quantum cascade-lasers (QCLs) are the best sources for generating efficient THz radiations, which are fabricated by the semiconductor heterostructure. The layers of GaAs quantum wells and AlGaAs barriers are formed with different thicknesses to design the heterostructure. In the present paper, we review different designs of THz QCLs active regions which are followed by a comparison between epitaxy-up and down processing techniques of the fabricated wafers. The present review should be helpful for the scientific community working in the scientific area of the THz radiation field and, in particular, for the researchers interested to switch from the epitaxy-up processing of the wafers to epitaxy-down mounting to make possible an increase of the operating temperature of the THz QCLs in the continuous wave operation mode.

Pulsing mechanism based on power adiabatic evolution of pump in Tm-doped fiber laser

We prove an alternative pulsing mechanism based on the power adiabatic evolution of a pump in a Tm-doped fiber laser. A pulsed laser technique, unlike Q-switching and gain switching, is explored under the equilibrium between the stimulated emission and absorption. After the laser reaches a continuous wave (CW) steady state, the temporal fluctuation of pump power is called power adiabatic evolution if it does not break the balance between the stimulated emission and the absorption. Under pump power adiabatic evolution the population densities in the upper and lower laser levels get clamped to their steady-state values, and the temporal shape of the output laser is identical to that of the pump. Based on the power adiabatic evolution of the pump a laser pulse can therefore be generated by a pulsed pump which is added to a CW pump. Moreover, the temporal profile, duration and peak power of the laser pulse can all be precisely controlled. The power adiabatic evolution of the pump can be achieved in the two pumping schemes of the Tm-doped fiber laser, provided that the duration of the pump pulse is broader than a certain value. The study on the pulsing mechanism highlights an alternative pulsed technique possessing several advantages over Q-switching and gain switching.

High-quality 5 GeV electron bunches with resonant multi-pulse ionization injection

The production of high-quality electron bunches in laser wakefield acceleration relies on the possibility of injecting ultra-low emittance bunches in the plasma wave. A new bunch injection scheme (resonant multi-pulse ionization, ReMPI) has been conceived and studied, in which electrons extracted by ionization are trapped by a large-amplitude plasma wave driven by a train of resonant ultrashort pulses. Such a train of pulses can be obtained in a very efficient, compact and stable way, by phase manipulation in the laser front-end. The ReMPI injection scheme relies on currently available laser technology and is being considered for the implementation of future compact x-ray free electron laser schemes. Simulations show that high-quality electron bunches with an energy of up to 5 GeV and a peak current exceeding 2 kA, with normalized emittance of below 0.1 mm × mrad and a slice energy spread of below 0.1%, can be obtained with a single stage.

Reliability of stable fiber Bragg grating sensor system for monitoring temperature and strain individually

In the investigation, the fiber Bragg grating-based sensor system is proposed and demonstrated to monitor temperature and strain individually. In order to obtain a stable and accurate sensing signal, an erbium-doped fiber (EDF) ring laser scheme with a 2 m long unpumped EDF as the saturable absorber is also proposed. Hence, the observed stabilization of output power and sensing wavelength can be less than 0.1 dB and 0.02 nm, and 0.3 dB and 0.02 nm in the sensor system, respectively, in the back-to-back state and 25 km fiber link. Hence, this sensor system can improve the output stability of the sensing signal. In addition, the temperature- and strain-induced wavelength shifts can be operated in the range of 20 °C–70 °C and 0–37 500 µε, respectively.

Deep Laser Cooling and Efficient Magnetic Compression of Molecules

We introduce a scheme for deep laser cooling of molecules based on robust dark states at zero velocity. By simulating this scheme, we show it to be a widely applicable method that can reach the recoil limit or below. We demonstrate and characterize the method experimentally, reaching a temperature of 5.4(7) μK. We solve a general problem of measuring low temperatures for large clouds by rotating the phase-space distribution and then directly imaging the complete velocity distribution. Using the same phase-space rotation method, we rapidly compress the cloud. Applying the cooling method a second time, we compress both the position and velocity distributions.

The stripe-shaped lens laser scheme with Z-shaped optical path

A new stripe-shaped lens laser scheme is proposed. The Nd3+:glass concave-convex lens with a diameter of 110 mm and thickness of 10 mm is processed, which the curvature radius of concave surface is 10,000 mm and the curvature radius of convex surface is 9800 mm. The reflectivity at the rest of the two surfaces is 99.9%, except that the transmission is 15% at the central circular area of the concave having a diameter of 8 mm to 1.06 µm light wave. The stripe-shaped area of the lens is pumped with a pulsed Xenon flashlamp which the diameter is 8 mm, and the energy of 521 mJ single transverse mode at 1.06 µm laser is obtained from the central regions of the concave surface, and the feasibility of the lens laser scheme is verified.

FEL performance achieved at PAL-XFEL using a three-chicane bunch compression scheme.

PAL-XFEL utilizes a three-chicane bunch compression (3-BC) scheme (the very first of its kind in operation) for free-electron laser (FEL) operation. The addition of a third bunch compressor allows for more effective mitigation of coherent synchrotron radiation during bunch compression and an increased flexibility of system configuration. Start-to-end simulations of the effects of radiofrequency jitter on the electron beam performance show that using the 3-BC scheme leads to better performance compared with the two-chicane bunch compression scheme. Together with the high performance of the linac radiofrequency system, it enables reliable operation of PAL-XFEL with unprecedented stability in terms of arrival timing, pointing and intensity; an arrival timing jitter of better than 15 fs, a transverse position jitter of smaller than 10% of the photon beam size, and an FEL intensity jitter of smaller than 5% are consistently achieved.

Multi-wavelength oscillating and transverse mode switching in an all few-mode fiber laser based on polarization manipulation

We have proved the polarization dependence of the fiber Bragg grating (FBG), and demonstrated an all few-mode fiber laser that can realize multi-wavelength oscillation and switchable transverse modes simultaneously based on the polarization dependence of FBG. In this fiber laser, single-, dual- and triple-wavelength oscillation can be realized by merely adjusting the polarization controller inside the cavity. Three output transverse modes including fundamental mode, second-order mode, and hybrid modes can be switched as the laser operates at three different single-wavelengths correspondingly. In addition, azimuthally and radially polarized beams can be easily generated. This all few-mode fiber laser scheme may play a more significant role in wavelength division multiplexed (WDM) systems, mode division multiplexed (MDM) systems, sensing, and so on.

Proposal and numerical studies of a terahertz Cherenkov free electron laser driven by a non-ultra relativistic bunched electron beam

Corrugated and dielectric structures have been widely used for producing terahertz radiation source. Recently, a novel scheme of sub-terahertz free electron laser (FEL) from a metallic pipe with corrugated walls driven by a non-ultra-relativistic ( < 10 MeV) picosecond electron beam was proposed and analyzed by using the Vlasov–Maxwell equations which can only provide crude estimates of the expected parameters of the FEL. It offers a promising candidate for compact accelerator-based high power terahertz source which can be realized with relatively low energy and low peak-current electron beams. In this paper, we propose a terahertz Cherenkov free electron laser scheme by using the cylindrical dielectric loaded waveguide (DLW) instead, and the new device scheme is studied numerically using both one-dimensional (1D) numerical model and particle in cell (PIC) simulations. The 1D model is developed in specialty for the longitudinal beam dynamics and electromagnetic simulations of the new Cherenkov FEL scheme. The basic idea of the approach is to use the analytic expressions of the longitudinal mode fields steady-state solution using Green function) to compute the wakefields at each time step using the macro particle currents as sources, while the Lorentz equations of the particles can be integrated numerically. The transitive effects in electromagnetic field (related to the group velocity of the wakefield) are modeled only approximately even in 1D case. It allows fast and accurate time-dependent calculation of the interaction between the wakefields and the electron beam. Good agreement is obtained comparing with particle in cell (PIC) simulation results.

Excitation of Copper Vapor Lasers by Storage Capacitor Direct Discharge via High-Speed Photothyristors

The possibility of using a “pulsed fiber laser–photothyristor” optocoupler as a switch in the excitation schemes of copper vapor lasers (CVLs) has been investigated. It has been shown that such a switch has a nanosecond performance and is able to form monopolar and alternating current pulses through CVLs with a power of up to 10 MW and a repetition rate of tens of kilohertz when an electrical efficiency of the excitation circuit is more than 95%. A simple but very accurate model of a photothyristor is proposed, which can be used in full-scale CVL modeling programs.

Ponderomotive and resonant effects in the acceleration of particles by electromagnetic modes

In the present analysis, we study the dynamics of charged particles under the action of slowly modulated electromagnetic carrier waves. With the use of a high-frequency laser mode along with a modulated static magnetic wiggler, we show that the ensuing total field effectively acts as a slowly modulated high-frequency beat-wave field typical of inverse free-electron laser schemes. This effective resulting field is capable of accelerating particles in much the same way as space-charge wake fields do in plasma accelerators, with the advantage of being more stable than plasma related methods. Acceleration occurs as particles transition from ponderomotive to resonant regimes, so we develop the ponderomotive formalism needed to examine this problem. The ponderomotive formalism includes terms that, although not discussed in the usual applications of the approximation, are nevertheless of crucial importance in the vicinity of resonant capture. The role of these terms is also briefly discussed in the context of generic laser-plasma interactions.

Revealing the Presence of Potential Crossings in Diatomics Induced by Quantum Cavity Radiation.

We propose an experiment to find evidence of the formation of light-induced crossings provoked by cavity quantum radiation on simple molecules by using state-of-the-art optical cavities, molecular beams, pump-probe laser schemes, and velocity mapping detectors for fragmentation. The procedure is based on prompt excitation and subsequent dissociation in a three-state scheme of a polar diatomic molecule, with two ^{1}Σ states (ground and first excited) coupled first by the UV pump laser and then by the cavity radiation, and a third fully dissociative state ^{1}Π coupled through the delayed UV/V probe laser. The observed enhancement of photodissociation yields in the ^{1}Π channel at given time delays between the pump and probe lasers unambiguously indicates the formation of a light-induced crossing between the two ^{1}Σ field-dressed potential energy curves of the molecule. Also, the production of cavity photons out of the vacuum field state via nonadiabatic effects represents a showcase of a molecular dynamical Casimir effect. To simulate the experiment outcome, we perform abinitio coherent quantum dynamics of the molecule LiF subject to external lasers and quantum cavity interactions in the strong coupling regime, using a product grid representation of the total polaritonic wave function for both vibrational and photon degrees of freedom.