Single-lobed Far-field Pattern Research Articles

High continuous-wave power surface emitting terahertz lasers integrated with multimode interferometer

Distributed feedback (DFB) surface emitting semiconductor laser realizes power extraction by periodic photonic heterostructure. A direct method to increase the output power is scaling up the area of active region, however, the length of the device is restricted by the coupling strength of the DFB grating. Herein we develop a high-power single-mode surface emitting scheme by monolithically integrating second-order DFB terahertz quantum cascade laser and a multimode interferometer, which can significantly enhance the optical power by increasing photon density and altering field distribution in the grating region. Application of this concept leads to high continuous-wave (CW) surface emitting power of 55 mW, with single-lobed far-field pattern and single-mode operation at about 3.9 THz, which is among the best reported results. Such an integrated scheme can be combined with any one-dimensional photonic heterostructure, which has great potential in further improving the pulsed and CW output power of single-mode semiconductor lasers.

High-power distributed feedback lasers based on InP corrugated sidewalls at λ∼2 μm

We report a high-power single-mode InP-based 2 μm distributed feedback (DFB) laser with a second-order buried grating and corrugated sidewalls. A second-order semiconductor grating is used for in-plane feedback and vertical out-coupling. The corrugated sidewalls are used to eliminate higher-order transverse modes. For the DFB laser with a 2 mm long cavity and 15 μm wide ridge, the maximum continuous-wave edge-emitting and surface-emitting single-mode powers at 300 K are up to 81 and 42 mW, respectively. A single-lobed far-field radiation pattern with a low divergence angle of approximately 8.6° is achieved by a device with a ridge width of 15 μm. The single-longitudinal-mode emission wavelength of the fabricated laser can be adjusted from 2003.8 nm at 288 K to 2006.9 nm at 313 K without any mode hopping. Robust single-mode emission with a side-mode suppression ratio of 30 dB is achieved under all injection currents and temperature conditions.

Electrically driven supersymmetric semiconductor laser arrays with single-lobe far-field patterns.

Semiconductor laser arrays based on the third-order supersymmetric (SUSY) transformation are proposed to increase the mode discrimination between fundamental supermode and high-order supermodes. The distance between the edge waveguide of the main array and that of the superpartners is optimized. Then, the electric field distributions of different modes are also calculated, which show that, except for the fundamental supermode, the high-order supermodes penetrate deeper into the superpartner arrays, which accounts for the increased loss of high-order supermodes. The fabricated third-order SUSY laser array can emit light with a single-lobe far-field pattern under an injection current of 70 mA, which is a promising candidate for optical couplings between lasers and optical elements.

Room temperature surface emission on large-area photonic crystal quantum cascade lasers

We design and fabricate large-area (1.1 mm × 1.1 mm) photonic crystal quantum cascade lasers, enabling single-mode (wavelength ∼ 8.5 µm) surface emission at room temperature, with a maximum peak power up to 176 mW. The beam divergence is <1∘ and with no side-lobes. Moreover, by introducing asymmetry into the photonic crystal pillar shape, a single-lobed far-field pattern is realized. The photonic band structure is measured with high spectral (0.72 cm−1) and angular (0.1°) resolution by using the photonic crystal quantum cascade laser itself as a detector.

Single-transverse-mode broadband InAs quantum dot superluminescent light emitting diodes by parity-time symmetry.

Parity-time (PT) symmetry breaking in counterintuitive gain/loss coupled waveguide designs is numerically and theoretically investigated. The PT symmetry mode selection conditions are determined theoretically. Single-transverse-mode broadband InAs quantum dot (QD) superluminescent light emitting diodes (SLEDs) are fabricated and characterized; the PT symmetric broad-area SLEDs contain laterally coupled gain and loss PT- symmetric waveguides. Single-transverse-mode operation is achieved by parity-time symmetry breaking. The broadband SLEDs exhibit a uniform Gaussian-like emission spectrum with the 3-dB bandwidth of 110 nm. Far-field characteristics of the coupled waveguide SLEDs exhibit a single-lobe far-field pattern when the gain and loss waveguides are biased at the injection current of 600 mA and 60 mA, respectively.

Photonic Crystal Diode Laser Array Integrated With a Phase Shifter for Narrow Beam Divergence

A narrow far-field edge-emitting photonic crystal diode laser array integrated with a phase shifter emitting in the 980-nm range has been designed and fabricated. A phase shifter structure is integrated on the chip to convert the out-of-phase mode to the in-phase mode. The real effective-refractive-index difference produced by the phase shifter is measured according to the comparison of the experiment and the simulation results. The effects of phase shifters with different lengths on the near and far fields are investigated. With the phase and the amplitude modulation of the phase shifter, the out-of-phase mode is successfully converted to the in-phase mode. Thus, a single-lobe far-field pattern is achieved in the lateral direction. The far-field divergence angles of the lateral and vertical directions are 1.5° and 12°, respectively. Such a device is promising for narrow far-field emission with a high output power.

Chirped coupled ridge waveguide quantum cascade laser arrays with stable single-lobe far-field patterns

Power scaling in a broad area quantum cascade laser (QCL) tends to deteriorate beam quality with the emission of a multiple-lobe far-field pattern. In this paper, we demonstrate a coupled ridge waveguide QCL array consisting of five elements with chirped geometry. In-phase mode operation is secured by managing supermode loss with properly designed geometries of ridges. A single-lobe lateral far-field with a near diffraction limited beam pattern was obtained in the whole current dynamic range. The devices were fabricated with the wet and dry etching method. The regrowth technique of the InP:Fe insulation layer and InP:Si waveguide layer was employed. Such a structure has the potential to optimize the beam quality of the recently reported high-power broad-area QCL with a reduced cascade number.

Broadband monolithic extractor for metal-metal waveguide based terahertz quantum cascade laser frequency combs

We present a monolithic solution to extract efficiently light from terahertz quantum cascade lasers with metal-metal waveguides suitable for broadband frequency comb applications. The design is optimized for a bandwidth of 400 GHz around a center frequency of 2.5 THz. A five-fold increase in total output power is observed compared to standard metal-metal waveguides. The extractor features a single-lobed far-field pattern and increases the frequency comb dynamical range to cover more than 50% of the laser dynamic range. Frequency comb operation up to a spectral bandwidth of 670 GHz is achieved.

Interview

Dr Jun-ichi Hashimoto is a researcher at the Transmission Devices Laboratory, Sumitomo Electric Industries Ltd., Japan. Here he tells us about his research and new Electronics Letters paper, ‘Low power-consumption mid-infrared distributed feedback quantum cascade laser for gas-sensing application’ page 549. Dr Jun-ichi Hashimoto Since the late 1980s, our laboratory has been engaged in developing semiconductor devices in the optical communication field of the near infrared region (1.3–1.55 µm). We have manufactured various kinds of commercial products including laser diodes, detectors, and modulators. Recently we started developing optical devices in the mid-infrared (MIR) region. We consider a quantum cascade laser (QCL) to be a future key light source in the MIR region, and we are now focusing our attention on its development. Our developing technologies cover all fields including device design, epitaxial growth, fabrication process, and evaluation system. A QCL is a new semiconductor laser enabling an oscillation in the MIR region. It has a structure similar to a conventional pn-junction laser diode, but the core region (emission region) structure is totally different. The core region of QCL consists of several tens of unit structures stacked together, and each unit structure has an active region serving as a light-emitting region, and an injector region to transport carriers (electrons) into the next active region. Both are made of super-lattice structures. In the core region, the radiative transition occurs between the sub-bands of the conduction band of the active region, which makes it possible to oscillate in the MIR wavelength region. We developed a low power consumption distributed feedback quantum cascade laser (DFB-QCL) applicable to a light source of a portable gas sensor. To reduce its power consumption significantly, we introduced our original high-gain vertical-transition core structure, a buried-hetero waveguiding structure with a high thermal conductivity and a low optical loss, a buried-type grating favourable for a large coupling-coefficient, and a high-reflective Au-coating on the rear facet. As a result, we succeeded in realising a DFB-QCL with a low threshold power-consumption (0.93W) at 20°C. In addition, this QCL demonstrated a sufficient continuous wave (CW) output power (≈20mW), a good singlemode property (single mode suppression ratio>25 dB), and a single-lobed far-field patterns (FFPs) at 20°C. We think this DFB-QCL can be used as a light source of a portable gas sensor. The QCL we developed in this study has low power consumption (∼1W) as mentioned earlier, so it is especially promising as a light source for portable QCL gas sensors. These QCL gas sensors have compact, fast (<1s) and high-sensitive (ppb∼ppt) characteristics which could not be satisfied simultaneously with the conventional MIR gas sensors. We expect these to open up new markets in various fields in the future, such as process and/or emission gas control in factories, environmental gas sensing, breath analysis for medical diagnosis and military applications. Developing our original high-gain vertical-transition core structure proved most challenging. In order to achieve high net optical gain in the core structure design, it is important to increase the carrier transition probability between the sub-bands, as well as reducing the optical loss due to the non-radiative recombination caused predominantly by longitudinal optical phonon (LO phonon) scattering. To address this issue, our structure has a high optical gain due to the vertical-transition of carriers between the sub-bands in the conduction band of the active region. At the same time, the LO-phonon scattering is supressed effectively by optimising the coupled quantum well structure of the active region. We predict that a further power consumption reduction of QCL, such as below 0.5 W, is needed to realise cheaper and smaller mass-marketable gas sensors for consumer use. Therefore, finding a way to reduce the power consumption more drastically seems a first priority. Expanding the wavelength tuning range of QCL is another important issue to improve the detection accuracy of the gas sensor and to increase the kinds of target gas which can be detected simultaneously with one gas sensor. We would like to study the integration of QCL and the related devices striving towards better performance and a novel function such as a wider wavelength tuning range, a higher output power, a narrower beam shape, a light emission in vertical direction, and so on. We are also interested in other wavelength regions having promising markets like MIR region, and Terahertz region is one of the candidates.

Coupled Ridge Waveguide Substrate-Emitting DFB Quantum Cascade Laser Arrays

A coupled ridge waveguide substrate-emitting quantum cascade laser array operating at $\lambda \sim ~7\mu $ m was presented. Fifteen elements were integrated in parallel to get high peak power and improved beam quality. The device was operating at fundamental supermode with a single-lobed far-field pattern. A narrow divergence angle of 2.9° in the ridge-width direction and 0.36° in the cavity-length direction were obtained by carefullydesigning the coupled ridge waveguide structure.Single-mode emission was observed by employing a buried second-orderdistributed-feedback grating. The peak power of the device was more than 2 W in pulsed operation at room temperature. This letter showed great potential to enlarge the output power of the surface-emitting devices while maintaining good beam quality.

Far-field analysis of third-order distributed feedback terahertz quantum cascade lasers

The single lobe far-field patterns produced from terahertz quantum cascade lasers (QCLs) are greatly demanded for various applications, such as imaging, data transmission, etc. However, for a ridge waveguide terahertz QCL, the far-field beam divergence is large due to the fact that the waveguide aperture is far smaller than the terahertz wavelength. This is the case typically for double-metal waveguide terahertz QCL which emits terahertz photons in almost every direction in the space. Even for a single plasmon waveguide terahertz QCL, the divergence angle is as large as 30 in both horizontal and vertical direction. Here, in this work we design and fabricate a double metal third-order distributed feedback terahertz QCL emitting around 4.3 THz, and investigate the characteristics of the longitudinal and transverse modes. This work aims to achieve high beam quality for terahertz QCL by exploiting the third-order distributed feedback geometry, and in the meantime to achieve single longitudinal mode operation. The electromagnetic field distribution in the waveguide is modelled by employing a finite element method. The mode selection mechanism is studied by using the eigen frequency analysis, and the far-field beam is simulated by applying the near-field to far-field Fourier transform technique. The QCL active region used in this work is based on the resonant-phonon design, which is grown by a molecular beam epitaxy (MBE) system on a semi-insulating GaAs (100) substrate. The wafer bonding and traditional semiconductor device fabrication technology, i.e., optical lithography, electron beam evaporation, lift-off, wet and dry etching, are used to process the MBE-growth wafer into the third-order distributed feedback geometry with double-metal waveguides. By carefully designing the grating structures and optimizing the fabrication process, we achieve third-order distributed feedback terahertz QCL with quasi-single-longitudinal mode operation and single lobe far-field beam pattern with low beam divergence in both vertical and horizontal directions. The effect of grating duty cycle on the far-field beam divergence is systematically studied theoretically and experimentally. By the simulation, we finally achieve the divergence angle of 1213 for a third-order distributed feedback laser with a grating duty cycle of 12% that results in an effective refractive index close to 3. The experimental results show good agreement with the simulation. There is still room to further reduce the beam divergence of third-order distributed feedback terahertz QCL by improve the accuracy of the simulation and the fabrication.

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.

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.

Development of Low Power Consumption DFB Quantum Cascade Lasers

We report on the development of $\lambda \sim 4.97~\mu \text{m}$ continuous-wave operation of substrate emitting distributed feedback quantum cascade lasers. A low threshold power consumption of 1.27 W is obtained at 20 °C, along with an ultralow threshold current density of 0.81 kA/cm2 by depositing the high reflection coatings on both facets. Single-lobed far-field radiation pattern with a low divergence angle of about 0.24° $\times $ 20.34° is obtained. Reliable single-mode emission with a side-mode suppression ratio about 25 dB is observed among the working temperature range of 10 °C–30 °C.

901 nm Lithographic vertical‐cavity surface‐emitting laser with stable single‐lobed beam pattern

Data are presented on 901 nm lithographic vertical-cavity surface-emitting lasers (VCSELs) demonstrating high efficiency for small VCSEL sizes, and stable beam patterns. Power conversion efficiency >40% is obtained for VCSELs ranging in size from 6 to 2 µm diameter. The 2 µm diameter VCSELs produce output powers in excess of 6.5 mW, and produce single-lobed far-field radiation patterns over the full range of operation.

Design for high-power, single-lobe, grating-surface-emitting quantum cascade lasers enabled by plasmon-enhanced absorption of antisymmetric modes

Resonant coupling of the transverse-magnetic polarized (guided) optical mode of a quantum-cascade laser (QCL) to the antisymmetric surface-plasmon modes of 2nd-order distributed-feedback (DFB) metal/semiconductor gratings results in strong antisymmetric-mode absorption. In turn, lasing in the symmetric mode, that is, surface emission in a single-lobe far-field beam pattern, is strongly favored over controllable ranges in grating duty cycle and tooth height. By using core-region characteristics of a published 4.6 μm-emitting QCL, grating-coupled surface-emitting (SE) QCLs are analyzed and optimized for highly efficient single-lobe operation. For infinite-length devices, it is found that when the antisymmetric mode is resonantly absorbed, the symmetric mode has negligible absorption loss (∼0.1 cm−1) while still being efficiently outcoupled, through the substrate, by the DFB grating. For finite-length devices, 2nd-order distributed Bragg reflector (DBR) gratings are used on both sides of the DFB grating to prevent uncontrolled reflections from cleaved facets. Equations for the threshold-current density and the differential quantum efficiency of SE DFB/DBR QCLs are derived. For 7 mm-long, 8.0 μm-wide, 4.6 μm-emitting devices, with an Ag/InP grating of ∼39% duty cycle, and ∼0.22 μm tooth height, threshold currents as low as 0.45 A are projected. Based on experimentally obtained internal efficiency values from high-performance QCLs, slope efficiencies as high as 3.4 W/A are projected; thus, offering a solution for watt-range, single-lobe CW operation from SE, mid-infrared QCLs.

Broad area single mode operation of quantum cascade lasers by integrating porous waveguide and distributed feedback grating

Quantum cascade laser (QCL) emitting at 4.8 \(\upmu \)m with a single-longitudinal mode and small divergence was achieved by integrating porous (PS) waveguide structure and distributed feedback (DFB) grating. The PS waveguide is fabricated in highly doped InP cladding layer by electro-chemical etching process. The DFB grating was defined above the active region by holographic lithography technique. A fundamental transverse mode of single-lobed radiation far-field pattern with full width at half maximum of 7.8\(^{\circ }\) was obtained. Single-longitudinal mode emission with a high peak power of 1.2 W was achieved. Our work is useful for many potential applications of the QCL.

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.

Single-Mode Vertical-Cavity Surface-Emitting Laser Array With High Power and Narrow Far-Field Divergence Angle

We demonstrate novel structures of an 850-nm vertical-cavity surface-emitting laser (VCSEL) array for high output power, single-lobe far-field pattern, and narrow divergence angle. By using the Zn-diffusion process with proper sizes of oxide-current-confined and Zn-diffusion apertures, each unit of VCSEL in the demonstrated array is highly single-mode (side-mode suppression ratio 30 dB) with a narrow far-field divergence angle (9°- 10°) and high maximum single-mode output power ( ~ 6.3 mW). Due to the high uniformity of single-mode performance of each VCSEL unit, the 6 × 6 array exhibits an excellent lasing phenomenon, which includes single-lobe far-field pattern, weak in-phase coupling, narrowing of divergence angle (from 9° to 4°), and output power as high as around 104 mW. Furthermore, by measuring the bias-dependent output optical spectra in different positions of our array, the high similarity of these spectra indicates the excellent uniformity of our fabrication process for single-mode VCSEL.

High-brightness diode laser arrays integrated with a phase shifter designed for single-lobe far-field pattern

High-brightness, edge-emitting diode laser arrays integrated with a phase shifter have been designed and fabricated at a wavelength of about 910 nm. Stable out-of-phase mode is generated through coupling evanescently and converted to be nearly in-phase by the phase modulation from the phase shifter. With a very simple manufacture process, stable single-lobe far-field pattern is achieved in the slow axis when the continuous wave output power exceeds 460 mW/facet, and the divergence angle is only 2.7 times the diffraction-limited value. Such device shows a promising future for high-brightness application with low cost and easy fabrication.