CW Oscillation Research Articles

Passively Q-switched erbium fiber laser with a thin crystalline film of bismuth telluride used as a saturable absorber

Performance of a passively Q-switched erbium fiber laser with a narrow bandgap Bi2Te3 semiconductor thin film used as a saturable absorber was studied experimentally. In order to obtain repetitively-pulsed lasing, a Q-switcher was fabricated as a section of the standard SMF-28 optical fiber with a short (∼4 mm in length) chemically etched out tapered section laterally covered with the bismuth telluride film. This Q-switcher was installed in a specially designed circuit of a ring-type erbium fiber laser pumped by a laser diode at 975 nm wavelength. The fiber diameter in the middle of the tapered section was 10-15 μm. Two types of taper coverage were studied: Bi2Te3 flakes suspended in isopropanol and a ZnTe/Bi2Te3 uniform polycrystalline thin-film heterostructure synthesized on a lateral surface of a tapered fiber section via MOCVD. In the first case, we observed laser oscillation at a wavelength of 1565 nm as a train of 7-26 μs pulses with a 86-50 μs repetition period at pump powers varying from 26 to 75 mW. In the second case, CW oscillation in the circuit appeared at a pump power of 30 mW while conversion to the repetitively-pulsed mode with a kilohertz repetition rate occurred at pump powers within the range of 34-40 mW. The results obtained indicate the feasibility of effective interaction of the evanescent field with a bismuth telluride film used as a saturable absorber. This interaction is sufficient to achieve a stable repetitively-pulsed lasing in the circuits of passively Q-switched fiber lasers.

Fabrication of kW-level chirped and tilted fiber Bragg gratings and filtering of stimulated Raman scattering in high-power CW oscillators

Suppression of stimulated Raman scattering (SRS) by means of chirped and tilted fiber Bragg gratings (CTFBGs) has become a key topic. However, research on high-power systems is still lacking due to two problems. Firstly, after the inscription, there are a large number of hydroxyl compounds and hydrogen molecules in CTFBGs that cause significant heating due to their strong infrared absorption. Secondly, CTFBGs can couple Stokes light from the core to the cladding and the coating, which causes serious heating in the coating of the CTFBG. Aimed at overcoming these bottlenecks, a process that combines constant-low-temperature and variable-high-temperature annealing is used to reduce the thermal slope of the CTFBG. Also, a segmented-corrosion cladding power stripping technology is used on the CTFBG to remove the Stokes light which is coupled to the cladding, which solves the problem of overheating in the coating of the CTFBG. Thereby, a CTFBG with both a kilowatt-level power-carrying load and the ability to suppress SRS in a fiber laser has been developed. Further, we establish a kW-level CW oscillator to test the CTFBG. Experimental results demonstrate that the power-carrying load of the CTFBG is close to 1 kW, the thermal slope is lower than $0.015\,^{\circ }\text{C}/\text{W}$, and the SRS suppression ratio is nearly 23 dB.

CW molecular iodine laser pumped with a low power DPSSL

Cw oscillation of molecular iodine on many lines in the range of 557–802 nm pumped with a low power common diode pumped and frequency doubled solid state laser DPSSL is reported. The DPSSL is temperature stabilized, operates in single frequency and can be tuned by about 2 nm at 532 nm. Operation conditions of this simple and low cost iodine ring laser will be described and possible applications will be discussed.

Characterization of tunable low power Yb-doped fiber CW oscillators

A continuous-wave fiber laser characterization based on a Findlay–Clay analysis is proposed. With this procedure, commonly used for solid-state lasers, information about saturation power and reabsorption loss is obtained. The knowledge of these parameters is commonly sufficient for a number of fiber oscillator design issues, but it also allows estimating values for fabrication dependent parameters, such as absorption and emission cross-sections, doping concentrations and dopant effective diffusion areas. As an application example, a single-mode low-power oscillator employing standard and well-known fiber components has been studied, providing values in good agreement with previously reported spectroscopical measurements.

Photonic bandgap fibre oscillators and amplifiers

In the search for ever higher output power or energy from fibre oscillators or amplifiers a nowadays mature technology relies on enlarging the fibre mode area. Broadening of the core diameter, all other things being equal, inevitably yields a multimode fibre, thereby dramatically limiting the device usefulness. Various strategies have been deployed to design and manufacture single transverse mode fibre oscillators and amplifiers, among which making use of the so-called photonic bandgap effect to restrict the modal population seems promising. Helped by efficient and reliable numerical tools the design of large mode area singlemode photonic bandgap fibres is presented. Two fibres with 20-μm and 40-μm core diameter, both of them heavily doped with Yb 3+ ions, have been fabricated by the widespread modified chemical vapour deposition process and are shown to behave properly when used as the core element of either continuous wave oscillators or femtosecond amplifiers. Good output beam quality (M 2 parameter spanning from 1.12 to 1.5 for the set of fibres studied) and high slope efficiency of 80% in cw oscillation regime are demonstrated. Furthermore the 40-μm core diameter fibre is shown to be resilient to tight bending down to 7.5-cm radius. The stack-and-draw process makes it easy to tailor the outer cladding so that a large numerical aperture can be reached. Subsequently, from this air-clad fibre, 500 fs 47 W pulses at 35 MHz are obtained from a two-stage chirped pulse amplification system.

Management of the high-order mode content in large (40 μm) core photonic bandgap Bragg fiber laser

Very large-mode-area Yb(3+)-doped single-mode photonic bandgap (PBG) Bragg fiber oscillators are considered. The transverse hole-burning effect is numerically modeled, which helps properly design the PBG cladding and the Yb(3+)-doped region for the high-order mode content to be carefully controlled. A ratio of the Yb(3+)-doped region diameter to the overall core diameter of 40% allows for single-mode emission, even for small spool diameters of 15 cm. Such a fiber was manufactured and subsequently used as the core element of a cw oscillator. Very good beam quality parameter M(2)=1.12 and slope efficiency of 80% were measured. Insensitivity to bending, exemplified by the absence of temporal drift of the beam, was demonstrated for curvature diameter as small as 15 cm.

Multi-wavelength diode-pumped Nd:LGGG picosecond laser

Laser operation near 1.06 μm by a diode-pumped Nd:(LuxGd1−x)3Ga5O12 (Nd:LGGG) disordered crystal has been investigated. Cw oscillation, with a slope efficiency as high as 61% and 230 mW output power, was achieved with 400 mW absorbed power from a 1-W laser diode. Stable passive mode locking with single- or multi-banded spectrum was obtained with a semiconductor saturable absorber mirror (SAM) and a single-prism, dispersion-compensated cavity. Fourier limited pulses with duration ≈3–9 ps and output power ≈40 mW were generated at three well-defined laser transitions in the range 1062–1067 nm.

High power cw and mode-locked oscillators based on Yb:KYW multi-crystal resonators

We report on a high power diode-pumped laser using multiple bulk Yb:KY(WO(4))(2) (KYW) crystals in a resonator optimised for this operation. From a dual-crystal resonator we obtain more than 24 W of cw-power in a TEM(00) mode limited by the available pump power. We also present results for semiconductor saturable absorber mirror (SESAM) mode-locking in the soliton as well as positive dispersion regime with average output powers of 14.6 W and 17 W respectively.

2D time-domain electromagnetic macroscopic numerical modelling on parallel computer: application to the mm-wave silicon DIMPATT diode

A new time-domain two-dimensional (2D) electromagnetic (EM) physical modelling of non-linear distributed semiconductor devices has been developed. It is based on a numerical procedure which solves in a self consistent manner both Maxwell’s equations and a macroscopic transport model based on the drift-diffusion approximation. The software can be run on a parallel computer. It is based on finite-difference time-domain (FDTD) explicit schemes associated to the domain decomposition method. The millimetre-wave travelling-wave IMPATT diode or Distributed IMPact ionisation and Avalanche and Transit Time (DIMPATT) diode is the non linear test structure chosen to validate the model. RF simulations under amplification and CW oscillation operating modes have been performed. The results presented in this paper consist on the one hand of results that can be qualitatively compared to previously published theoretical works and on the other hand of features especially pointed out thanks to the new electromagnetic physical model capabilities.

High-power oscillator of continuous electromagnetic radiation with a frequency of 300 GHz

We consider a general concept of construction, the possible versions, and specific features of a gyrotron, whose output power in CW oscillation regime can reach a few kilowatts at a frequency of 300 GHz. The gyrotron is designed for work in a high-frequency facility in combination with a “dry” cryomagnet, which ensures a magnetic field of up to 12 T, required for the gyrotron operation. The basic results of numerical simulation and optimization of the electron gun, the resonant cavity, and other subsystems of the gyrotron are presented. The designs used for the gyrotron development are justified. Preliminary experiments showed the efficiency of the pilotproduction gyrotron with an output power of about 2 kW, which is record-breaking in this frequency range.

High-efficiency laser-diodes-pumped microthickness Yb:Y3Al5O12 slab laser

A high-efficiency and high-power Yb:Y3Al5O12 oscillator was developed based on the microthickness slab structure with high aspect ratio to obtain one-dimensional temperature gradient and to reduce the temperature increase in the crystal. Laser output power of 257W was obtained in cw oscillation with 42% optical conversion efficiency at a pump power of 608W.

A Method to Produce Short Pulse on a VHF-Plasma Excitation Slab-Type CO2 Laser.

We propose a novel method for producing a short pulse from a VHF-discharge-excitation slab-type CO2 laser using a halved confocal of an unstable concave and convex resonator. This method utilizes a full reflection concave mirror with a function of variable curvature, which can be controlled using a piezo electric device (PZT). Generally, slab-type lasers are modulated directly by varying pulse voltage; however, there is a fluctuation of plasma at the transition of the kHz pulsation. Consequently, the repetitive frequency is less than 10 kHz. On the other hand, the pulse oscillation with our method has a short pulse width of 300 ns and a maximum repetitive frequency of about 58 kHz. The peak power at the pulse oscillation is about 12 times as high as that of the CW oscillation.

Quelques propriétés optiques du laser tunnel

The cw oscillation of a tunnel laser is demonstrated. The high losses encountered in tunneling systems are here compensated for by a high gain amplifying medium for barrier widths between 0 and λ/2. For a first particular angle of incidence, the TM frequency remains invariant when the total length of the laser is varied. For a higher value of the incidence angle, the frequency associated with the TE polarization increases while the frequency associated with the TM polarization undergoes a negative variation when the total length of the laser is reduced.

Self modulation phenomena in an external-cavity semiconductor laser

We report the observed self-modulation phenomena in an external-cavity semiconductor laser, in which a slant ridge-loading waveguide was formed for the gain medium. A region beside the ridge at a cleaved end of the waveguide served as a saturable absorber for self-modulation. Under different conditions of injection current and cavity alignment, we observed three laser operation regimes, including pure cw oscillation, constant-amplitude pulse-like signal on the top of a cw component, and amplitude-modulated pulse-like signal coexistent with a cw component. The amplitude modulation phenomenon is different from the previously reported concurrence of mode-locking and self-pulsation. A model was proposed to explain the observed results.

High spectral purity CW oscillation and pulse generationin optoelectronic microwave oscillator

A tunable optoelectronic oscillator generating a microwave output signal at 1 GHz with an extremely low phase noise is presented. The oscillator was also operated as a pulse generator, making use of the inherent nonlinearity of an intracavity electro-optic modulator. Pulses of 1-2 ns in duration at repetition rates of 20-200 MHz were generated.

Room-temperature CW operation of II–VI lasers grown on ZnSe substrates cleaned with hydrogen plasma

Hydrogen plasma cleaning was applied for ZnSe homoepitaxial growth along with a novel wet etching technique for ZnSe substrates. The dependencies of etch-pit density (EPD) on RF power and cleaning temperature were investigated; for RF power below 250 W, the EPD is suppressed to about 1 × 10 5/cm 2, and the optimum temperature range is between 260 and 280°C. The EPD of ZnSe film grown on ZnSe substrate cleaned at 260°C and 220 W for 20 min is 2.7 × 10 4/cm 2. SCH II–VI laser diodes (LDs) containing ZnMgSSe cladding layers were fabricated on semi-insulating ZnSe substrates cleaned under the optimum conditions. LDs with HR coating on both facets demonstrate CW oscillation at room temperature. Typical threshold current and wavelength are 84 mA and 517 nm, respectively.

Periodic pulse oscillation in an intracavity-doubled Nd:YVO_4 laser

We report periodic pulse oscillation in an intracavity-doubled Nd:YVO(4) laser. The pulse oscillation is caused by alternate oscillation between two modes. We investigated improvement of green-light power with the pulse oscillation in a Nd:YVO(4) laser. The pulse oscillation gave a factor of approximately 2 higher average green-light power than that for cw oscillation at the same pumping power.

Comparison of Tm-sensitized Ho:Yag and Ho:YLF crystals for a laser-pumped 2 μm CW oscillator

We report on a comparison of YAG and YLF crystals for Tm,Ho based CW laser systems operating at 2 μm. As pump source we have used both a Ti:Al 2O 3 laser and an arry of diode lasers in order to quantitatively assess the relevance of the pump beam spatial features on the 2 μm laser performance. The results are compared with the predictions of a computer simulation using a rate equation model.

Frequency measurements of 9- and 10-μm N/sub 2/O laser transitions

We have measured frequencies of N/sub 2/O transitions by heterodyning sub-Doppler fluorescence-stabilized N/sub 2/O laser radiation with that from a reference CO/sub 2/ laser. A high-resolution cavity incorporates a ribbed tube and a highly reflective grating, permitting the CW oscillation of both the 10/sup 0/0-02/sup 0/0 9-/spl mu/m and the 10/sup 0/0-00/sup 0/1 10-/spl mu/m regular bands. This is the first sub-Doppler frequency measurement of the 9-/spl mu/m band. The accuracy in the determination of the rotational constants for both bands has been improved by an order of magnitude, and calculated transition frequencies are presented.

Wave-front reconstruction with a Fourier hologram in a phase-conjugating mirror oscillator

A laser profile consisting of 113 coupled Gaussian waves is obtained in a cw oscillator by use of a BaTiO(3) crystal as both an amplifying medium and a phase-conjugating mirror. Mutual locking of Gaussian waves is performed by a pair of spatial f ilters. One of them is a high-resolution holographic component; the other is an opaque screen with identical circular apertures.