Surface-emitting Distributed Feedback Research Articles

Experiment demonstration of high speed 1.3 µm grating assisted surface-emitting DFB lasers.

Surface emitting lasers are attractive light sources for silicon integrated photonic circuits. High speed direct operation is of great importance for these lasers in high capacity and low cost on-chip communication system. Here, we demonstrate a 1.3 µm surface emitting ridge-waveguide distributed feedback (DFB) laser with second order grating and λ/4 phase shift grating, which can achieve a 24 Gb/s operation over a wide temperature. The fabricated lasers can achieve low threshold current as 6.8 mA, and 12.5 mA at 20, and 70°C, respectively. Stable single mode operation has been observed with high side mode suppression ratio (SMSR) > 40 dB at all temperatures (20-70 °C). Meanwhile, the surface emitting optical power can reach 1.7 mW at high temperature as 70 °C. 3 dB bandwidth of small signal response is 21 GHz and 12 GHz at 20 °C and 70 °C respectively. The far-field divergence angle of surface emitting beam is 13.4°×20.2° of 10 µm length second order grating coupler. The proposed laser may have great advantages of single mode, high speed modulation and good temperature tolerance. In addition, compared with conventional DFB lasers, the surface emitting DFB laser has no additional manufacturing process, which is simple to fabricate and easy to integrate with silicon platform.

Narrow-linewidth surface-emitting distributed feedback semiconductor lasers with low threshold current

Surface emitting distributed feedback (SE-DFB) semiconductor laser has become one of study hotspots due to its narrow linewidth and high power. Herein, a SE-DFB semiconductor laser is proposed based on second-order Bragg grating for feedback and out-coupling. The grating is fabricated on the confinement layer, which can realize the laser emission from the substrate and avoid the secondary epitaxial growth. The asymmetric waveguide structure is introduced into the device to increase the probability of photon and grating interaction while reducing the threshold current. The results show that when the current is 3A, the output laser power from the substrate achieves 1.22 W. In the meantime, the threshold current, voltage and slope efficiency are 300 mA, 1.73 V and 0.45 W/A, respectively. The center wavelength is 980.1 nm and the linewidth is 0.84 nm. This work has important implications for low threshold current narrow linewidth SE-DFB semiconductor lasers.

Narrow-linewidth surface-emitting distributed feedback laser emitting from P-surface

In order to obtain semiconductor lasers with narrow linewidth and low far-field divergence angle semiconductor lasers, a surface emitting distributed feedback (SE-DFB) semiconductor laser emitting from P-surface is researched theoretically and experimentally in this paper. The device is obtained with wavelength of 976.2 nm, linewidth of 0.94 nm, far-field divergence angle of 2.6∘×6.1∘. Meanwhile the device is simple to fabricate and easy to integrate. The research proposes a novel method for obtaining narrow linewidth SE-DFB semiconductor lasers.

High-intensity and low-divergence THz laser with 1D autofocusing symmetric Airy beams

Single-mode THz quantum-cascade lasers (QCLs) have been realized using a wide variety of techniques to obtain a combination of large power output, good beam quality with single-lobed beams, and low far-field divergence. Beam shaping using external components has not been previously exploited due to limited commercial availability of THz optical components and also the accompanying large loss from most THz optical materials. Here, we demonstrate that excellent beam characteristics could be obtained for a THz QCL by integration of a surface-emitting distributed-feedback (DFB) QCL with a simple lens within the vacuum chamber of a cryocooler. Plano-convex lenses are made from inexpensive plastic balls and integrated in proximity with a 3.4THz DFB QCL. With appropriately chosen lens parameters, dual-lobed Airy beams are generated that autofocus into a high-intensity single-lobed beam with large focusing efficiency. A simple and general method to generate one-dimensional autofocusing Airy beams is thus demonstrated that is applicable at any wavelength. THz laser beams with high peak intensity (57mW/mm2 in a spot-size of 1mm2) or low-divergence (1.4∘×1.8∘ for a beam with 118mW peak power) are realized at 62K in a compact electrically operated Stirling cooler. A high brightness of 5.4×106Wsr-1m-2 is estimated for the focused beams by measuring the beam propagation ratios (M2 parameters).

Research of asymmetric waveguide on surface emitting distributed feedback semiconductor lasers

Surface emitting distributed feedback (SE-DFB) semiconductor lasers with second-order grating have become one of research hotspots due to be in terms of high beam quality, narrow linewidth and so on. The diffraction and feedback of the second-order grating are utilized to achieve wavelength stabilization and surface emitting. In the paper, SE-DFB laser with asymmetric waveguide is designed, and the effect of waveguide structures on slope efficiency is discussed. The results show that the slope efficiency of SE-DFB laser with P-asymmetric waveguide is 1.04 W/A which enhances 0.12 W/A than the device with symmetric waveguide. It provides an idea for SE-DFB laser research.

High power surface emitting terahertz laser with hybrid second- and fourth-order Bragg gratings

A surface-emitting distributed feedback (DFB) laser with second-order gratings typically excites an antisymmetric mode that has low radiative efficiency and a double-lobed far-field beam. The radiative efficiency could be increased by using curved and chirped gratings for infrared diode lasers, plasmon-assisted mode selection for mid-infrared quantum cascade lasers (QCLs), and graded photonic structures for terahertz QCLs. Here, we demonstrate a new hybrid grating scheme that uses a superposition of second and fourth-order Bragg gratings that excite a symmetric mode with much greater radiative efficiency. The scheme is implemented for terahertz QCLs with metallic waveguides. Peak power output of 170 mW with a slope-efficiency of 993 mW A−1 is detected with robust single-mode single-lobed emission for a 3.4 THz QCL operating at 62 K. The hybrid grating scheme is arguably simpler to implement than aforementioned DFB schemes and could be used to increase power output for surface-emitting DFB lasers at any wavelength.

Quantum-Dot Surface Emitting Distributed Feedback Lasers Using Indium–Tin–Oxide as Top Claddings

We demonstrate InAs/InGaAs quantum-dot surface emitting distributed feedback (SE-DFB) lasers, using an indium–tin-oxide layer as top cladding, which eliminates the regrowth process used in conventional DFB lasers to greatly simplify laser process. The lasing peak near 1.30 $\mu \text{m}$ realized using the grating period of 375 nm exhibited a 0.13-nm linewidth and a low-temperature red-shift rate of 0.1 nm/K. The laser had a threshold current density of 210 A/cm2 and a characteristic temperature of 94.6 K. The surface emitting laser light had the divergence angles of less than 1° and 8 $^{\circ }\sim 9^{\circ }$ along and perpendicular to the grating direction, respectively.

Monolithically Lens-Integrated Photonic Device Arrays for Compact Optical Transceivers

Monolithically lens-integrated surface-emitting distributed-feedback (DFB) laser arrays and p–i–n photodiode arrays were developed. A combination of dry etching and wet chemical etching was used to fabricate a monolithic InP lens. The lens profile within ±10 µm from the lens center was estimated to be parabolic shape, which is ideal for the collimation of a Gaussian beam. A fabricated lens-integrated surface-emitting DFB laser exhibited a circular, narrow far-field pattern with full-width at half maximums of 3.9×3.5°. An efficient direct coupling of the lens-integrated DFB laser to a single-mode fiber with a coupling efficiency of -3 dB was confirmed. A four-channel lens-integrated surface-emitting laser array and a lens-integrated p–i–n photodiode array were used to fabricate a very compact 100 Gbps (25 Gbps × 4 channel) optical transceiver with 14×9 mm2 footprint.

Emission characteristics of surface second-order metal grating distributed feedback semiconductor lasers

To obtain high-power semiconductor lasers with stable operation in a single longitudinal mode and improve the characteristics of the output beam, an end-emitting surface second-order metal grating distributed feedback (DFB) laser emitting at around 940 nm is fabricated. The characteristics of the uncoated devices with and without gratings are tested under room temperature continuous-wave conditions without any temperature-control device and compared. The devices with gratings achieve high powers of up to 385 mW/facet and a small lateral far-field angle of 2.7° at 1.5 A, have only 4.13 nm/A wavelength-shift, and 0.09 nm spectral linewidth at 600 mA, and operate in a stable longitudinal mode. Devices without gratings operate in multimode, with a larger lateral far-field angle (7.3°) and spectral linewidth (1.3 nm), although with higher output powers. Because of the integration of second-order metal gratings and their very high coupling capability, the output beam quality is improved greatly, the lasing wavelength is stable and varies slowly with changes in injection current, while the spectrum is narrowed dramatically, and the far-field angles are greatly reduced. This opens the way for the realization of watt-scale power broad-stripe (>100 μm) surface second-order metal grating end and surface-emitting DFB lasers and arrays with single frequency, single mode operation and high output beam quality.

Phase-locked arrays of surface-emitting terahertz quantum-cascade lasers

We report the demonstration of phase-locked arrays of surface-emitting distributed-feedback (DFB) terahertz quantum-cascade lasers with single-mode operations. Carefully designed “phase sector” locks several surface-emitting DFB laser ridges in-phase, creating tighter beam-patterns along the phased-array direction with full width at half maximum (FWHM)≈10°. In addition, the phase sector can be individually biased to provide a mechanism of frequency tuning through gain-induced optical index change, without significantly affecting the output power levels. A tuning range of 1.5 GHz around 3.9 THz was achieved. This fine tunability could be utilized to frequency- or phase-lock the DFB array to an external reference.

Modal characteristics of terahertz surface-emitting distributed-feedback lasers with a second-order concentric-circular metal grating

A theoretical model is developed to study the modal characteristics of a second-order concentric-circular metal grating surface-emitting distributed-feedback (DFB) laser operating at terahertz regime. A series of high-order diffracted fields, which can be expressed as a Floquet–Bloch expansion of Hankel functions, is assumed to be generated from the concentric-circular metal grating. The resonant frequencies and transverse profiles of all the diffracted fields can be deduced from the related eigenequations established through the boundary conditions of the interfaces of the metal-dielectric-metal waveguide. The results show that the interference of the diffracted cylindrical waves can form two types of resonant modes, namely, quasisymmetric and quasiantisymmetric modes. Surface radiation is excited mainly by the influence of quasisymmetric modes, which exhibit constructive interference with the grating geometry. Furthermore, the resultant intensities of the diffracted waves decay exponentially from the center of the circular grating, indicating that the proposed grating geometry has the potential to realize surface terahertz radiation with excellent beam quality. The influence of grating duty cycle on the resonant conditions and transverse distributions of the diffracted fields are also investigated.

Design and analysis of surface emitting distributed feedback laser for radio over fiber (ROF)

This research is focused on design and analysis of semiconductor laser for ROF application. The alternative and advanced laser is designed for high performance operation and ease in manufacturing; that is a surface emitting distributed feedback (SEDFB) laser. Optimisation is performed at the wavelength of 1.55 μm using the horizontal-cavity structure. This device contains an active region by means of multi-quantum well (MQWs) structure and a separated confinement heterostructure (SCH) in order to optimise its performance. The simulation results in a ridge SEDFB structure producing a single-mode, 1550-nm output operation with optical confinement factor of 0.186, and far-field angle divergence of 450. The transverse edge loss is 0.031 cm-1. An optimum grating duty cycle is in the interval of 0.5-0.6, with a surface loss of 24cm-1. The resulting laser design has a single-lobe pattern with a differential quantum efficiency of 41.3%

Above-Threshold Spectrum of the Radiation Field in Surface-Emitting DFB Lasers

This paper addresses the above-threshold analysis of the amplified spontaneous emission spectrum of the radiation field in surface-emitting distributed feedback (DFB) lasers with a second-order grating. To the best of the authors' knowledge, this paper is the first report in this regard. The analysis takes advantage of the time- and frequency-domain approaches. The essence of this method is obtaining the spectrum of the laser power using the fields in the frequency domain, whereas the above-threshold carrier distribution inside the cavity is obtained by the finite-difference time-domain approach. The use of this approach can be justified by the fact that the wave equations in the time and frequency domains are related by the Fourier transform. Applying this method to the surface-emitting DFB lasers with a second-order grating, the authors will demonstrate that excitation of the radiation field by the interference between the counter-propagating waves inside the cavity provides additional filtering mechanism, which makes the side-mode suppression ratio of the power emitted from the surface different than that from the edge. More importantly, it is shown that the interesting features of a properly designed quarter-wave phase-shifted surface-emitting DFB lasers with a second-order grating can be exploited in the design of transmitters for optical communications

Amplified-spontaneous-emission spectrum of the radiation field in surface-emitting DFB lasers

In this paper, the authors calculate the amplified-spontaneous-emission spectrum of the radiation field in surface-emitting distributed feedback (DFB) lasers. The response of the laser cavity to the Langevin noise source in the frequency domain is obtained using the newly developed Green's functions for the slowly varying amplitudes of the guided waves. The authors show that the power spectra from the surface and the edge are different, and this discrepancy is due to excitation of the radiation field by the interference between the counter-propagating waves inside the cavity. This feature can be properly exploited in the design of surface-emitting DFB lasers for optical communications.

Spontaneous emission model of surface-emitting DFB semiconductor lasers

A general spontaneous emission model is developed for surface-emitting (SE) distributed feedback (DFB) semiconductor lasers. The frequency distribution of spontaneous emission noise below lasing threshold and the spontaneous emission rate in lasing operation are formulated by using a transfer matrix method combined with the Green's function method. The effective linewidth enhancement factor is obtained from this model in terms of the elements of the transfer matrix. By way of example, the author applies the formulation to a standard SE DFB laser, and a SE lambda /4-shifted DFB laser with a distributed Bragg reflector (DBR) mirror. In particular, the author analyzes the below-threshold spectrum, the threshold current density, the differential quantum efficiency, and the spectral linewidth of these lasers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Theoretical analysis of the spectral linewidth of a surface-emitting DFB semiconductor laser

Theoretical analysis of the spectral linewidth is presented for a surface-emitting distributed feedback (DFB) semiconductor laser which consists of alternating active and passive layers. The transfer-matrix method is used for the analysis. The spontaneous emission rate is calculated taking into account the effect of nonuniform longitudinal field distribution in the laser cavity.

Analysis of surface-emitting lambda /4-shifted DFB lasers with a distributed Bragg reflector

Threshold current density and differential quantum efficiency are analyzed for a surface emitting (SE) lambda /4-shifted distributed feedback (DFB) laser consisting of alternating active and passive layers with a distributed Bragg reflector (DBR). It is shown that the threshold current density can be reduced by using the DFB action induced by alternating active and passive layers in the active region of the SE DFB laser structure, as compared to SE DBR lasers with a homogeneous active region. It is also shown that the differential quantum efficiency of the DFB laser with a DBR can be higher than that of conventional DBR lasers without increasing the threshold current density. >

Transfer-matrix formulation of spontaneous emission noise of DFB semiconductor lasers

A unified formulation of the spontaneous emission noise in semiconductor DFB (distributed feedback) lasers is presented by using a transfer-matrix approach. Analytical expressions for the noise power per unit frequency bandwidth below threshold and the spontaneous emission rate into the lasing mode are obtained based on the Green's function method. Three DFB laser structures are analyzed: (1) a standard DFB structure with facet reflectivities, (2) a multisection DFB structure composed of n sections which models a phase-shifted DFB laser and a multielectrode (tunable) DFB laser, and (3) a periodic layered DFB structure which models a surface-emitting DFB laser. It is shown that the spontaneous emission noise of a complicated DFB laser structure can be calculated easily by the transfer matrix of each section of the structure and its derivative to frequency. >