Fang-Liang Yan

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.

Coupled ridge waveguide distributed feedback quantum cascade laser arrays

A coupled ridge waveguide quantum cascade laser (QCL) array consisting of fifteen elements with parallel integration was presented. In-phase fundamental mode operation in each element is secured by both the index-guided nature of the ridge and delicate loss management by properly designed geometries of the ridges and interspaces. Single-lobe lateral far-field with a nearly diffraction limited beam pattern was obtained. By incorporating a one-dimensional buried distributed feedback grating, the in-phase-operating coupled ridge waveguide QCL design provides an efficient solution to obtaining high output power and stable single longitudinal mode emission. The simplicity of this structure and fabrication process makes this approach attractive to many practical applications.

1.8-W Room Temperature Pulsed Operation of Substrate-Emitting Quantum Cascade Lasers

High-power broad area substrate-emitting distributed feedback (DFB) quantum cascade laser emitting at λ ~ 4.6 μm is reported. Proper second-order DFB grating coupling, low waveguide loss mechanism, and epilayer-down mounting bring about a high vertical radiative optical power. The maximum peak power is as high as 1.82 W at 20 ^°C and still 1.22 W at 50 ^°C operated in pulsed mode with a pulsed width of 1 μs and duty cycle of 0.2%. A single-lobed radiation far-field with a divergence angle of 7.9 ^° in the ridge-width direction and two-lobed radiation far-field with a divergence angle of 0.61 ^° in the cavity-length direction are achieved by depositing the high-reflection coatings on both facets. Robust single-mode emission is continuously tunable by the heat sink temperature up to 115 ^°C.

High efficiency, single-lobe surface-emitting DFB/DBR quantum cascade lasers.

We demonstrate a surface-emitting quantum cascade laser (QCL) based on second-order buried distributed feedback/distributed Bragg reflector (DFB/DBR) gratings for feedback and outcoupling. The grating fabricated beneath the waveguide was found to fundamentally favor lasing in symmetric mode either through analysis or experiment. Single-lobe far-field radiation pattern with full width at half maximum (FWHM) of 0.18° was obtained along the cavity-length direction. Besides, the buried DFB/DBR grating structure successfully provided an efficient vertical outcoupling mechanism with low optical losses, which manages to achieve a high surface outcouping efficiency of 46% in continuous-wave (CW) operation and 60% in pulsed operation at room temperature. Single-mode emission with a side-mode suppression ratio (SMSR) about 25 dB was continuously tunable by heat sink temperature or injection current. Our work contributes to the realization of high efficiency surface-emitting devices with high far-field beam quality that are significantly needed in many application fields.

Directional collimation of substrate emitting quantum cascade laser by nanopores arrays

We report a beam reshaping phenomena via nanopore array defined on the emission window of a substrate emitting quantum cascade lasers. A pronounced beam divergence reduction by a factor of 6 in the elongated beam direction is achieved. A collimated laser beam with small divergence (2.9° × 0.12°) is observed. These single-mode devices preserve good power and spectral performances at room temperature continuous wave operation. This beam reshaping design is simple to fabricate and able to be adapted for other wavelengths ranging from the visible to mid-infrared regimes, which would open up major opportunities for the control of beam divergence.

High-power phase-locked quantum cascade laser array emitting at λ ∼ 4.6 μm

A phase-locked quantum cascade laser(QCL) array consisting of one hundred elements that were integrated in parallel was achieved at λ ∼ 4.6 μm. The proposed Fraunhofer’s multiple slits diffraction model predicted and explained the far-field pattern of the phase-locked laser array. A single-lobed far-field pattern, attributed to the emission of an in-phase-like supermode, is obtained near the threshold (Ith). Even at 1.5 Ith, greater than 73.3% of the laser output power is concentrated in a low-divergence beam with an optical power of up to 40 W.

Very low threshold operation of quantum cascade lasers

A strain-compensated InP-based quantum cascade laser (QCL) structure emitting at 4.6 μm is demonstrated, based on a two-phonon resonant design and grown by solid-source molecular beam epitaxy (MBE). By optimizing the growth parameters, a very high quality heterostructure with the lowest threshold current densities ever reported for QCLs was fabricated. Threshold current densities as low as 0.47 kA/cm2 in pulsed operation and 0.56 kA/cm2 in continuous-wave (cw) operation at 293 K were achieved for this state-of-the-art QCL. A minimum power consumption of 3.65 W was measured for the QCL, uncooled, with a high-reflectivity (HR) coating on its rear facet.

Design and Fabrication of Six-Channel Complex-Coupled DFB Quantum Cascade Laser Arrays Based on a Sampled Grating

We designed and fabricated a six-channel complex-coupled distributed feedback (DFB) quantum cascade laser arrays based on a sampled Bragg grating. The six-channel DFB laser arrays exhibit a linear tuning range of 74 nm centered at a wavelength of 7.55 μm at room temperature. Robust single-mode emission with a side mode suppression ratio about 20 dB was observed, even at full power. The used sampled grating and reflectivity coating on the back facet lead to the peak output power varying from 55 to 82mW with a small difference in slope efficiency from 100 to 128mW/A.

Broad area quantum cascade lasers operating in pulsed mode above 100 ℃ at λ ~4.7 μ m

We demonstrate a broad area (400 μ m) high power quantum cascade laser (QCL). A total peak power of 62 W operating at room temperature is achieved at λ ~4.7 μ m. The temperature dependence of the peak power characteristic is given in the experiment, and also the temperature of the active zone is simulated by a finite-element-method (FEM). We find that the interface roughness of the active core has a great effect on the temperature of the active zone and can be enormously improved using the solid source molecular beam epitaxy (MBE) growth system.

Multi-wavelength surface emitting quantum cascade laser based on equivalent phase shift

A novel surface emitting distributed feedback quantum cascade laser emitting around λ ∼ 4.6 μm is demonstrated by employing an equivalent phase shift (EPS) of quarter-wave (λ/4). The EPS is fabricated through extending one sampling period by 50% in the center of a sampled Bragg grating. Single-lobed far-field radiation pattern with a low divergence angle of about 0.6° × 16.8° is obtained. Selective single-mode lasing with a mean side mode suppression ratio above 20 dB and wavelength coverage range of 72 nm is achieved simultaneously on a single wafer only by changing the sampling period.