Diffraction-limited Power Research Articles

Experimental study on mode instability in high power all-fiber master oscillator power amplifer fiber lasers

Mode instability (MI) is an abrupt mode change when the average output power increases above a certain threshold power, which results in degrading beam quality and restricting the enhancement of power output for diffraction-limited high power fiber laser. The experimental study on MI in all-fiberized master oscillator power amplifer (MOPA) is presented in detail for the first time. It is revealed that MI in all-fiberized MOPA results in reducing the slope efficiency. The theoretical threshold power of MI for 20/400 step-index large-mode-area double-cladding Yb-doped fiber is about 1 kW and MI is rooted mainly from the thermal effect.

Mode-converters for rectangular-core fiber amplifiers to achieve diffraction-limited power scaling

A rectangular-core (ribbon) fiber that guides and amplifies a single higher-order-mode (HOM) can potentially scale to much higher average powers than what is possible in traditional circular-core large-mode-area fibers. Such an amplifier would require mode-conversion at the input to enable interfacing with seed sources that typically output TEM(00) mode radiation and at the output to generate diffraction-limited radiation for end-user applications. We present the first simulation and experimental results of a mode conversion technique that uses two diffractive-optic-elements in conjugate Fourier planes to convert a diffraction limited TEM(00) mode to the HOM of a ribbon fiber. Mode-conversion-efficiency is approximately 84% and can theoretically approach 100%. We also demonstrate a mode-converter system that converts a single HOM of a ribbon fiber back to a diffraction-limited TEM(00) mode. Conversion efficiency is a record 80.5%.

GaSb-based tapered diode lasers at 1.93 μm with 1.5-W nearly diffraction-limited power

High-power high-brightness 1.93-/spl mu/m wavelength (AlGaIn)(AsSb) tapered diode lasers with a narrow vertical waveguide design are reported for the first time. A nearly diffraction-limited continuous-wave output power of 1.5 W together with a remarkable low fast axis divergence of 43/spl deg/ full-width at half-maximum have been demonstrated. The maximum brightness amounts to 32 MW/cm/sup 2/sr.

How to launch 1 W into single-mode fiber from a single 1.48-μm flared resonator

The operation of 1.48-/spl mu/m flared resonators is thoroughly studied, both experimentally and theoretically: the accurate determination of threshold condition as a function of geometrical and material parameters, the study of emission spectra and astigmatism variations as a function of optical power level allow us to better understand the may these devices operate. The origin of modal distortion is then analyzed, and an original solution is proposed to increase the single-transverse-mode power at high injection level: it is shown that implanting the multiple-quantum-well active layer with protons efficiently enhances the filtering capability of the overall structure, and particularly that of the ridge waveguide, by bringing additional lateral absorption losses. The explanation of the filtering mechanism is successfully confirmed by simulations using the beam-propagation method. This technique finally allowed more than 1.3 W of continuous wave (CW) diffraction-limited power at 6 A. Low-modal-gain structures were then realized to reduce modal optical absorption in the implanted structures with a view to maintaining a high external efficiency and a reduced vertical divergence. Finally, a three-lens coupling system was designed and the effects of optical feedback minimized so as to obtain a very high coupling efficiency: with an improved laser design, 1.12 W of CW power were then coupled into single-mode fiber at 6.6 A, which represents 65% of the power emitted by the laser chip.

Single transverse-mode filtering utilizing ion implantation: application to 1.48-μm unstable-cavity lasers

We demonstrate the relevance of ion implantation of the multiple quantum-well active layer in unstable-cavity lasers as a means of efficiently filtering the parasitic higher order waves by introducing additional propagation loss within the cavity. Several H/sup +/ implantation schemes are proposed and a comparison is successfully made of experiment to a beam propagation method (BPM) model on the basis of modal behavior. The work finally resulted in improved single transverse-mode behavior of those lasers: more than 1.3 W CW of diffraction-limited power at 1.48 /spl mu/m was then obtained utilizing a two-step implantation process.

Very high power 1.48 µm semiconductor lasers

1.48 µm semiconductor diode laser structures with very high CW output power levels have been developed. Over 3.2 W from broad area lasers, record power of 410 mW from narrow stripe single mode lasers and record 1.8 W diffraction limited power from flared lasers have been obtained.

GaInAsSb-AlGaAsSb tapered lasers emitting at 2.05 μm with 0.6-W diffraction-limited power

Tapered lasers fabricated from a GaInAsSb-AlGaAsSb single-quantum-well structure are reported. The laser structure, grown by molecular beam epitaxy, has broad-stripe pulsed threshold current densities as low as 50 A/cm/sup 2/ at room temperature. Tapered lasers have exhibited diffraction-limited continuous-wave output power up to 600 mW.

400 mW continuous-wave diffraction limited flaredunstable resonatorlaser diode at 635 nm

400 mw of continuous-wave (CW) diffraction-limited power is demonstrated for the first time from a flared unstable resonator semiconductor laser at a wavelength of 635 nm. More than 75% of the total power is contained in the central lobe of the far field.

1.26 W CW diffraction-limited InGaAsP flared amplifierat 780 nm

Flared amplifiers fabricated with InGaAsP alloys have produced 1.26 W CW at a wavelength of 780 nm in a single-lobed, diffraction-limited far field pattern. Under quasi-CW conditions, 2.4 W of diffraction-limited power has been obtained. This represents the highest diffraction-limited power reported at 780 nm and the highest diffraction-limited power reported in the InGaAsP material system.

Three-core ARROW-type diode laser: novel high-power, single-mode device, and effective master oscillator for flared antiguided MOPA's

A novel type of strong-index-guided laser structure for achieving coherent, high-power (/spl sim/1 W) reliable operation is proposed, analyzed, and demonstrated for the first time. The three-core antiresonant-reflective-optical waveguide (ARROW) laser combines the relatively large fabrication tolerances of single-core ARROW-type devices with the excellent intermodal-discrimination properties of small-element-number (<10) resonant antiguided arrays (i.e., resonant optical-waveguide (ROW) devices.) Thus, the three-core ARROW device should prove a more practical device to fabricate than large element (>10) ROW arrays, which are known to have tight fabrication tolerances. For single-spatial-mode selectivity the device relies both on ARROW-type (lateral) reflectors and interelement loss. The effect of interelement loss on the resonant in-phase mode and the unwanted nonresonant modes is calculated, and the results are used to design optimal structures. For structures of index step, /spl Delta/n, of 0.1; element/interelement width ratio of 5; 500 cm/sup -1/ interelement-loss coefficient; and 7.5-/spl mu/m-wide individual elements, intermodal discrimination /spl ges/5 cm/sup -1/ is obtained over a relatively large range in /spl Delta/n: /spl sim/0.01. Above-threshold analysis shows that, due to strong index guiding, the structures are virtually immune to gain spatial hole burning. The central-lobe energy content is 65-70%, and the emitting aperture is 25 /spl mu/m, which should provide 1 W CW reliable single-mode power from devices with oxide-free facets. Preliminary experimental results are obtained from Al-free 0.98-/spl mu/m emitting structures: diffraction-limited beam operation up to 30/spl times/ threshold and 0.55 W peak pulsed output power. Finally it is shown that three-core ARROW devices used as integrated master oscillators (MO's) for flared antiguided master-oscillator power amplifiers (MOPA's), provide uniform MOPA near-field intensity profiles, thus opening the way to achieving 3-5 W CW diffraction-limited power from stable, flared MOPA's.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Kink power in weakly index guided semiconductor lasers

A periodic dependence of kink power on laser length is observed and explained. Weakly index guided high power stripe lasers in the AlGaAs, InGaAlP, and InGaAs–AlGaAs material systems are studied and oscillation periods of 100–350 μm are found. Relative kink power differences exceeding a factor of 4 are observed. Facet coatings lead to differences in the oscillation amplitude but not in the oscillation period. The observations indicate that phase-locked fundamental and first-order modes exist at certain preferred laser lengths. This general model fully explains the oscillatory behavior of the kink power and the correlated changes in lateral far field distributions at the front and rear mirrors. It is concluded that the optimum diffraction limited power output can be obtained by choosing the proper laser length.

Summary of characteristics of beamed power radiation patterns and side lobe levels for the lunar solar power system

The lunar solar power system (LSP) is a system for collecting solar radiant power on the lunar surface, converting the power to microwaves, and transmitting multiple power beams directly (or indirectly using orbital reflectors) to Earth stations. The microwave transmission systems for LSP direct beams consist of phased arrays of randomly placed reflector antennas distributed over large diameter equivalent apertures near the lunar limbs, each aperture generating multiple radiating wave fronts of different frequencies converging toward equivalent focal points beyond the Earth surface receiver planes. Unlike most versions of the space solar power system (SPS), the radiation patterns are governed by radiative near field or Fresnel diffraction constraints and provide a more uniform intensity over most of the receiver ground area and more rapid attenuation beyond the beam collection zone than is possible with Fraunhofer diffraction limited power beams. Statistical theories for radiation patterns of random arrays and phase or amplitude errors of reflector antennas have been described. These are combined to predict mean and peak side lobe power levels. Central beam power distributions are calculated from transmitting wavefront curvature and amplitude distributions. Parametric values for single power beams and mature global power distribution systems are presented for beaming power to receivers on Earth.

Intracavity phase correction of an external Talbot cavity laser with the use of liquid crystals

Arrays of GaAlAs emitters have been diffractively coupled by using an external Talbot cavity to support a single spatial mode; however, element-to-element phase differences distort the desired spatial mode. To enable element-to-element phase correction, we incorporated a 20-element array of tunable liquid-crystal phase shifters into a 20-element GaAlAs external Talbot cavity laser. Using the tunable phase shifters, we corrected spatial mode distortions, which resulted in 663 mW of nearly diffraction-limited power.

Laser beam propagation in turbulent conditions

Diffraction-limited low power laser beams at 1.06 and 3.8 microm were focused by a 91-cm diam aperture onto a receiver located 10.5 km away on a low slant angle path over a desert basin. The analysis of beam areas and turbulence levels was used to determine a new scaling law for tilt-corrected turbulence-induced beam spread.

High power, nearly diffraction-limited output from a semiconductor laser with an unstable resonator

The authors describe an on-the-chip design of a nearly diffraction-limited broad-area semiconductor diode laser. The device achieved single lateral mode operation as unstable resonators with magnifications between two and three. The unstable resonators were realized by focused ion beam (FIB) micromachining a diverging mirror at one of the outcoupling facets. The modeling efforts agree well with experimental data and show that an optimum device design exists in which stable nearly diffraction-limited operation is predicted for up to six times threshold. This unstable resonator design has achieved, experimentally, the highest diffraction limited power and best external differential efficiency ever reported for any broad-area device with a curved facet.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Phase-front measurements of an injection-locked AlGaAs laser-diode array

The phase-front quality of the primary spatial lobe emitted from an injection-locked gain-guided AlGaAs laser diode array is measured by using an equal-path, phase-shifting Mach-Zehnder interferometer. Root-mean-square phase errors of 0.037 +/- 0.003 wave (lambda/27) are measured for the single spatial lobe, which contained 240-mW cw output power in a single longitudinal mode. This phase-front quality corresponds to a Strehl ratio of S = 0.947, which results in a 0.23-dB power loss from the single lobe's ideal diffraction-limited power. These values are comparable with those measured for single-stripe index-guided AlGaAs lasers.

A simple beam expander for frequency narrowing of dye lasers

Design considerations and performance of a prism beam expander are presented. Using a prism beam expander and holographic grating, a dye laser pumped by a nitrogen laser has given 15 kW of diffraction limited power in 0.1–0.2 cm−1 linewidth. Addition of a single etalon gave a single frequency output of 10 kW in a linewidth of less than 0.01 cm−1.