Single-lobed Far-field Pattern Research Articles

Surface emitting quantum cascade lasers operating in continuous-wave mode above 70 °C at λ ∼ 4.6 μm

We demonstrate surface emitting distributed feedback (DFB) quantum cascade lasers for very high temperature continuous-wave (cw) operation at λ ∼ 4.6 μm. A second-order DFB grating beneath the waveguide provides efficient vertical outcoupling. cw operation is reported up to a temperature of 75 °C. Total output power of 105 mW is obtained with a record low threshold current density of 0.85 kA/cm2 at 10 °C. Single-lobed far-field radiation pattern with a low divergence angle of about 0.17° × 18.7° is achieved. Robust single-mode emission with a side-mode suppression ratio about 30 dB is continuously tunable by the heat sink temperature and injection current.

Stable single-mode operation of surface-emitting terahertz lasers with graded photonic heterostructure resonators

Graded photonic heterostructures (GPH) can be regarded as energy wells for photons. We show that judicious engineering of such photonic wells, obtained by tailoring the grading and the slit width of the GPH resonator, allows one to ensure spectrally single-mode emission on the fundamental symmetric mode in the whole lasing dynamical range of terahertz quantum cascade lasers. Furthermore, the radiative character of the symmetric mode leads to single-mode emission with mW output power in continuous-wave operation, as well as to single-lobed far-field beam patterns. A careful combination of theoretical analysis and experimental observations reveals that the results stem from interplay between mode competition and spatial hole burning effects.

Far-Field Patterns of Plasmon Waveguide Interband Cascade Lasers

The far-field patterns of InAs-based plasmon-waveguide interband cascade lasers are experimentally and theoretically investigated. It is shown that different ridge waveguide structures and metal contact configurations lead to distinctive far-field patterns. With the commonly adopted center metal contact on the top of the ridge waveguide, an undesirable double-lobe far-field pattern along the lateral direction is exhibited, while with two metal contacts near the edges of the ridge or a broad-area metal contact on the mesa structure, the more desirable single lobe far-field pattern is observed. These characteristic far-field patterns and their implications are discussed with the aid of a theoretical model which is in good agreement with our experimental results.

Design of surface emitting distributed feedback quantum cascade laser with single-lobe far-field pattern and high outcoupling efficiency

A 7.8-μm surface emitting second-order distributed feedback quantum cascade laser (DFB QCL) structure with metallized surface grating is studied. The modal property of this structure is described by utilizing coupled-mode theory where the coupling coefficients are derived from exact Floquet–Bloch solutions of infinite periodic structure. Based on this theory, the influence of waveguide structure and grating topography as well as device length on the laser performance is numerically investigated. The optimized surface emitting second-order DFB QCL structure design exhibits a high surface outcoupling efficiency of 22% and a low threshold gain of 10 cm−1. Using a π phase-shift in the centre of the grating, a high-quality single-lobe far-field radiation pattern is obtained.

Subwavelength Nanopatch Cavities for Semiconductor Plasmon Lasers

We propose and analyze a family of nanoscale cavities for electrically pumped surface-emitting semiconductor lasers that use surface plasmons to provide optical mode confinement in cavities which have dimensions in the 100-300-nm range. The proposed laser cavities are in many ways nanoscale optical versions of micropatch antennas that are commonly used at microwave/RF frequencies. Surface plasmons are not only used for mode confinement but also for output beam shaping to realize single-lobe far-field radiation patterns with narrow beam waists from subwavelength size cavities. We identify the cavity modes with the largest quality factors and modal gain, and show that in the near-IR wavelength range (1.0-1.6 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mu</i> m) cavity losses (including surface plasmon losses) can be compensated by the strong mode confinement in the gain region provided by the surface plasmons themselves and the required material threshold gain values can be smaller than 700 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> .

Surface-emitting distributed feedback terahertz quantum-cascade lasers in metal-metal waveguides

Single-mode surface-emitting distributed feedback terahertz quantum-cascade lasers operating around 2.9 THz are developed in metal-metal waveguides. A combination of techniques including precise control of phase of reflection at the facets, and use of metal on the sidewalls to eliminate higher-order lateral modes allow robust single-mode operation over a range of approximately 0.35 THz. Single-lobed far-field radiation pattern is obtained using a pi phase-shift in center of the second-order Bragg grating. A grating device operating at 2.93 THz lased up to 149 K in pulsed mode and a temperature tuning of 19.7 GHz was observed from 5 K to 147 K. The same device lased up to 78 K in continuous-wave (cw) mode emitting more than 6 mW of cw power at 5 K. In general, maximum temperature of pulsed operation for grating devices was within a few Kelvin of that of multi-mode Fabry-Perot ridge lasers.

Single high-order transverse mode surface-emitting laser with controlled far-field pattern

Single transverse mode vertical-cavity surface-emitting lasers (VCSELs) with high output power are important for high-speed data links, optical recording, laser processing, and so on. We demonstrate a single high-order transverse mode VCSEL with narrow trenches formed on its top surface by using a focused ion beam. The formation of four straight cross trenches resulted in the selection of a stable LP/sub 41/ mode oscillation. A large far-field angle of the LP/sub 41/ mode, which causes a low coupling efficiency even with a multimode fiber, was drastically reduced by loading a phase shift layer. A single-lobe far-field pattern with low scattering loss was realized by loading a spatial phase shift of SiO/sub 2/. We evaluated a coupling efficiency with a single-mode fiber including its alignment tolerance. The controlled far-field pattern enables lens-free direct coupling with a single-mode fiber even for large active-area VCSELs.

Surface-emitting, single-lobe operation from second-order distributed-reflector lasers with central grating phaseshift

Surface-emitting diode lasers containing a second-order Bragg grating with a central phaseshift, Δφ, of values around π are found to fundamentally favor lasing in a mode of symmetric near-field amplitude profile, which in turn provides emission in a single-lobed beam orthonormal to the surface. Devices of 500 μm long distributed-feedback (DFB) active region (λ=0.98 μm) and 500 μm long distributed feedback reflector passive regions provide, for Δφ=π, a surface-emitted beam pattern with 88% central-lobe power content, and external differential quantum efficiency, ηD, of 51%. Since the guided field is antisymmetric to start with, and a central π phaseshift causes two grating-outcoupled beams to be out-of-phase with each other, the net result is a single-lobed far-field pattern. The guided-field peak-to-valley (intensity) ratio, R, in the active (i.e., DFB) region is only 2, which insures single-mode operation to high powers, since the intermodal discrimination is high (⩾100 cm−1). Over a wide range in Δφ:60°; ηD remains high (&amp;gt;50%) and the degree of guided-field uniformity remains low (R&amp;lt;2).

1.1-W continuous-wave 1480-nm semiconductor lasers with distributed electrodes for mode shaping

Diffraction-limited high-power devices may suffer from self-focusing effects due to nonuniform gain saturation. In this letter, we propose the concept of the distributed electrode, which allows one to improve the modal behavior of these lasers and to reduce spatial-hole burning effects by preferentially localizing current injection in the center of the structure, therefore shaping the optical mode. Utilizing this concept, we have realized unstable cavity lasers exhibiting single-lobe far-field patterns. We report the first realization of flared unstable cavity lasers emitting at 1480 mm with maximum output powers up to 1.1-W continuous-wave and external efficiencies as high as 0.45 W/A.

Tunable single-mode operation of a high-power laser-diode array by use of an external cavity with a grating and a photorefractive phase-conjugate mirror

We demonstrate a technique for tunable single-mode operation of high-power laser-diode arrays. A spatially filtered part of the far field from a gain-guided GaAlAs laser-diode array is directed to a diffractive grating and is coupled to a phase-conjugating self-pumped barium titanate crystal. The interaction among the grating, the dynamic gratings in the photorefractive crystal, and the spatial filter forces the laser-diode array with poor spatial and temporal coherence to oscillate in a single spatial and single longitudinal mode. At a drive current of two times the threshold, the array operates in a single-lobed far-field pattern that is only 1.5 times the diffraction limit. The bandwidth of the enhanced output beam is measured to be less than 0.03 nm, and the coherence length of the output is increased 45 times, to at least 16 mm. Once single-mode operation has been obtained, one can continuously tune the frequency over a range of 5 nm around a center wavelength of 811 nm by tilting the grating. More than 50% of the radiated energy of the free-running laser is coupled out of the system. The significantly enhanced coherence properties of the output may lead to a number of new or improved applications in areas such as second-harmonic generation, coupling into single-mode fiber, and pumping of solid-state lasers.

High-energy (59 pJ) and low-jitter (250 fs) picosecond pulses from gain-switching of a tapered-stripe laser diode via resonant driving

High-energy (59 pJ), low-jitter (250 fs), high-extinction ratio (800), and short (14.5 ps FWHM) pulses were generated by gain-switching of a tapered stripe gain-guided laser diode (LD) via resonant driving. These characteristics are attributed to a stable single-mode near-field pattern, a single-lobe far-field pattern, and multilongitudinal modes associated with the tapered stripe LD under high driving current pulses. These pulses from a comb generator were enhanced mainly from reflection at the LD-transmission line impedance mismatch; the line length was adjusted for resonance with the reflection from the step recovery diode, further increasing the driving current.

Numerical procedure for the lateral-mode analysis of broad-area semiconductor lasers with an external cavity

A numerical procedure for the investigation of the lateral modes of semiconductor lasers with an external cavity is described. The propagation of the optical field inside the semiconductor laser is carried out via a standard beam-propagation scheme, while the method of coordinate scaling with the generalized Huygens-Fresnel integral allows for a computationally efficient propagation of the field in the external cavity. Using an eigenfunction solver based on the Prony method, it is shown that a simple external-cavity configuration comprising a broad area laser, a collimation lens, and a uniform end reflector can exhibit a complex modal behavior. Compared to a solitary broad area laser, optimized external cavities can yield enhanced modal discriminations, resulting in a single-lateral-mode operation with output power in excess of 100 mW along with a clean, single-lobed far-field pattern. The spatial-filtering action of the broad-area laser is highlighted, and we also discuss of the role played by the wavefront curvature of the beam incident upon the broad-area laser for achieving an efficient suppression of the higher order modes along with minimum increase of the threshold current. >

Stable singlemode operation of 0.98 µmGaInAs/GaInAsP/GaInP buried ridge stripelaser with AlGaInP current blocking layer

By using a AlGaInP current blocking layer lattice-matched to GaAs, a GaInAs/GaInAsP/GaInP strained single quantum well (SSQW) buried ridge stripe laser emitting at 0.98 µm wavelength region was fabricated and successful operation was achieved for the first time. Single-longitudinal-mode operation with single-lobed far-field patterns was obtained at up to an output power of 50 mW for the uncoated laser (4 × 1000 µm2) under continuous-wave (CW) operation at room temperature. In addition, CW operation beyond 50 mW output was obtained at up to 100°C for the same device.

Achieving broad-area laser diodes with high output power and single-lobed far-field patterns in the lateral direction by loading a modal reflector

A modal reflector, which consists of a high-reflectivity region surrounded by low-reflectivity regions, is added to the front facet of two types of broad-area laser diodes (LD's) to control the lateral modes. One type of LD is the self-aligned laser with a bent active layer (SBA LD) that has a real index-guiding mechanism. The other is a planar-stripe LD that consists of a Zn-diffused region to confine the current flow and has a gain-guiding mechanism. For the SBA LD's with a modal reflector, stable single-lobed far-field patterns (FFP's) are obtained at up to 0.3 and 0.4 W output powers in continuous wave (CW) operation and pulsed operation, respectively. In addition, for planar-stripe LD's with a modal reflector, stable single-lobed FFP's are obtained at up to 0.4 W in CW operation. The lateral modes inside the cavity are analyzed by utilizing a slit model and FFP's are calculated. Good agreement is found between experimental and calculated FFP's for a large Fresnel number. >

CW high power single-lobed far-field operation of long-cavity AlGaAs-GaAs single-quantum-well laser diodes grown by MOCVD

Data on long-cavity 100- mu m-wide broad-stripe laser diodes that lase with a barrow single-lobed far-field pattern in continuous room-temperature operation are presented. Diodes with a cavity length of 1250 mu m emit a power of 200 mW per facet into a 2.5 degrees lobe (full width at half maximum). Short-cavity devices (cavity length of 350 mu m) lase with a continuously increasing number of lateral modes right from threshold, and exhibit a far-field divergence that is over three times greater than that of 1250- mu m diodes. Explanations for the effect of increasing cavity length on the field patterns of these devices are proposed, based on the measured increase in injected carrier diffusion length in long-cavity diodes and the influence of thermal waveguiding and mirror losses on intermodel discrimination. >

High-power 980-nm AlGaAs/InGaAs strained quantum-well laser grown by OMVPE

High-power lattice-strained AlGaAs/InGaAs graded index separate-confinement heterostructure (GRINSCH) quantum-well lasers emitting at a 980-nm wavelength have been grown by organometallic vapor phase epitaxy (OMVPE) and fabricated with a self-aligned ridge-waveguide structure. Using a 3- mu m-wide and 750- mu m-long AR-HR coated laser, 30 mV of optical power was coupled into optical fibers with 28.6% efficiency. A dominating single-lobe far-field radiation pattern was obtained from a wedge-shaped ridge-waveguide laser for output power as high as 240 mW with a maximum output power of 310 mW. >

In-phase operation of high-power nonplanar periodic laser arrays

The transformation of nonplanar periodic laser array modes from weakly locked out-of-phase to locked in-phase operation is investigated. A comparison study of near-field and far-field patterns is made for devices with differing mesa widths and heights. Data are presented which show that the mesa height and width can be adjusted to force in-phase operation. An array of 19 elements shows an essentially single-lobed far-field pattern centered at 0° with full width at half maximum of 1.6°, to output powers of more than 500 mW/uncoated facet.

High-power diffraction-limited phase-locked GaAs/GaAlAs semiconductor laser array

Abstract High-power diffraction-limited phase-locked gallium-arsenide (GaAs)/ gallium-aluminum-arsenide (GaAlAs) semiconductor laser arrays have been fabricated using the LPE technique, the standard photolithographic technique, wet etching, and proton bombardment. The tailored gain-guided arrays are made by varying the width of the channels of the lasers while keeping the spacing between them constant. The array consists of six lasers. Its optical output power per facet is 300 mW at 2.71th with single-mode continuous wave (CW) operation and single lobe far-field pattern with full width at half maximum (FWHM) of 1.90°.

Terraced substrate inner stripe visible semiconductor laser and its arrays

Lasers emitting in the range of 7500-7800 AA, which have average current thresholds of 30-40 mA and maintain single transverse mode operation up to 20 mW, are reported. Life tests show that the lasers have almost no threshold change after 3000 h of aging at 4-mW CW operation. The terraced substrate inner stripe laser arrays have clean single-lobe far-field patterns with full width at half power of 6 degrees in CW operation and 2.4 degrees in pulsed operation. >

Trapezoidal channeled substrate inner stripe phase-locked semiconductor laser arrays

A novel type of trapezoidal channeled substrate inner stripe diode laser array grown by single-step liquid-phase epitaxy was designed and fabricated. A six-element array with evanescent wave coupling, as well as leaky wave coupling, shows a continuous-wave threshold current of 170 mA and a pulsed peak power over 675 mW per facet without a coating. A clean single-lobe far-field pattern of beam width 2.4° is obtained.