Far-field Beam Divergence Research Articles

Measuring far-field beam divergence angle of supercontinuum fiber sources

We measured the divergence angles of supercontinuum fiber sources from various types of silica fibers in the spectral range from 0.5 to 2.3[Formula: see text][Formula: see text]m, covering almost the entire range of supercontinuum that can be generated from silica fibers. The divergence angle of the supercontinuum directly output from the photonic crystal fibers (PCFs) increases monotonously with the increase of the wavelength. The divergence angle of the supercontinuum after transmitting through single-mode fibers, including HI1060, SMF-28 and 10/130 fibers, increases firstly and decreases then with the increase of the wavelength. The turning point is different for the three kinds of fibers. The beam parameter product (BPP) increases monotonously with wavelength for both the PCF and the single-mode fibers. Theoretical calculations are also made for the mentioned four types of fibers. The divergence angle of supercontinuum after transmitting through multimode fibers with 50, 100 and 200[Formula: see text][Formula: see text]m core diameters is also measured, which varies in a certain range without obvious rules depending on the wavelength.

Millimeter-long metamaterial surface-emitting antenna in the silicon photonics platform.

Integrated optical antennas are key components for on-chip light detection and ranging technology (LIDAR). In order to achieve a highly collimated far field with reduced beam divergence, antenna lengths on the order of several millimeters are required. In the high-index contrast silicon photonics platform, achieving such long antennas typically demands weakly modulated gratings with lithographic minimum feature sizes below 10 nm. Here, we experimentally demonstrate a new, to the best of our knowledge, strategy to make long antennas in silicon waveguides using a metamaterial subwavelength grating (SWG) waveguide core loaded with a lateral periodic array of radiative elements. The mode field confinement is controlled by the SWG duty cycle, and the delocalized propagating mode overlaps with the periodic perturbations. With this arrangement, weak antenna radiation strength can be achieved while maintaining a minimum feature size as large as 80 nm. Using this strategy, we experimentally demonstrate a 2-millimeter-long, single-etched subwavelength-engineered optical antenna on a conventional 220 nm SOI platform, presenting a measured far-field beam divergence of 0.1° and a wavelength scanning sensitivity of 0.13°/nm.

High pulsed power VCSEL arrays with polymer microlenses formed by photoacid diffusion.

We demonstrate millimeters-long VCSEL linear arrays with SU-8 epoxy-based microlenses that are directly patterned and cross-linked on the output apertures by a simple, photoacid-diffusion-aided photolithography technique. The linear arrays are capable of delivering >7 W of peak pulsed output power. By exploiting the photoacid diffusion effect, it is possible to produce a range of microlens structures with height and radius of curvature ranging from approximately ten to tens of microns. Simulation and experimental results show that the far-field beam divergence can be reduced by a factor of up to 7 in VCSELs integrated with optimal microlens dimensions.

Hyperspectral imaging for the spectral measurement of far-field beam divergence angle and beam uniformity of a supercontinuum laser.

A new novel method, hyperspectral imaging (HSI), is presented in this work to measure the beam divergence angle and beam profile uniformity of supercontinuum lasers. The obtained results of divergence angles are consistent with theoretically calculated values. The uniformity of different-size projected Gaussian beams was measured through referencing the data sets provided by HSI camera under the wavelength variation. HSI, compared with traditional methods, is much faster and capable of providing critical reference to supercontinuum output parameters measurements and practical application in far-field situation.

微弧度量级远场发散角光束发射系统的设计与实现

微弧度量级远场发散角光束发射系统的设计与实现

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.

Suppression of far-field blooming in high-power broad-area diode lasers by optimizing gain distribution

Far-field blooming, a serious far-field dependence on driving current, affects the stability of beam quality and applications of broad-area (BA) diode lasers. In this Letter, the lateral ridge waveguide (LRW) is introduced to BA lasers by a simple and cost-effective approach to control the far-field stability and beam divergence. The influences of LRW length on output power, near- and far-field, are investigated and it is found that the optimized LRW length is able to improve both the far-field blooming and output power. The mechanism behind this is analyzed and a 0.13°/A dependence of lateral divergence angle on the injection current is achieved.

The design and analysis of sub-wavelength metal-grating large-aperture VCSELs

The polarization control of vertical-cavity surface-emitting lasers with high power emission has been demonstrated by using metal grating to import non-isotropic gain. The grating with a period of 186 nm and a duty ratio of 0.5 was fabricated on the GaAs-cap layer to provide additional reflectance for H-polarization. The pairs of P-distributed Bragg reflectors were reduced to realize the maximum difference threshold gain of two orthogonal polarizations. A polarization ratio of 3 with an output power of 860 mW at continuous-wave operation was demonstrated at room temperature, and the high-order modes were suppressed. As a result, the far-field beam divergence was suppressed to 12°.

Porous waveguide facilitated low divergence quantum cascade laser

A quantum cascade laser with a porous waveguide structure emitting at 4.5 μm is reported. A branchlike porous structure filled with metal material was fabricated on both sides of the laser ridge by an electrochemical etching process. In contrast to the common ridge waveguide laser, devices with a porous structure give rather better beam quality. Utilizing this porous structure as a high-order mode absorber, the device exhibited fundamental transverse mode emission with a nearly diffraction limited far-field beam divergence angle of 4.9°.

High power and good beam quality of two-dimensional VCSEL array with integrated GaAs microlens array

High power and good beam quality of two-dimensional bottom-emitting vertical-cavity surface-emitting laser array with GaAs microlens on the substrate is achieved. Uniform and matched convex microlens is directly fabricated by one-step diffusion-limited wet-etching techniques on the emitting windows. The maximum output power is above 1 W at continuous-wave operation at room temperature, and the far-field beam divergence is below 6.6° at a current of 4 A. These properties between microlens-integrated and conventional device at different operating current are demonstrated.

Small-divergence singlemode emitting tapered distributed-feedback quantum cascade lasers

A tapered distributed feedback quantum cascade laser emitting at λ∼8.1 µm is reported. Utilising a tapered waveguide structure with a surface metal grating, the device exhibited singlemode operation over the temperature range of 100 to 214 K, with sidemode suppression ratio >20 dB and a nearly diffraction limited far-field beam divergence angle of 5.4°.

Fabrication of self-aligned ridge waveguide lasers with spot-size converter for quantum well intermixed superluminescent diodes

We fabricated InGaAs/GaAs multiple quantum well ridge waveguide lasers with a spot-size converter (SSC) in a single epitaxial growth step by using conventional lithography and reactive ion etching. The SSC was based on the laterally tapered rib adiabatic waveguide to expand the mode size. The narrow tapered ridge top of ∼0.7 µm was opened by a self-aligned method associated with benzocyclobutene planarization and overall etch-back processes. For an uncoated 1.5 mm long cavity, the far-field beam divergence angle from the SSC facet was about 15° (parallel) and 27° (perpendicular) at 20 °C under continuous-wave (CW) operation, indicating a significant reduction compared to that from the laser facet (i.e., 32° × 41°). The device exhibited a maximum output power of 62 mW/facet with a threshold current of 73 mA and a slope efficiency of 0.34 mW mA−1. A selective intermixing technique using impurity-free vacancy diffusion was applied to the SSC laser structure to achieve a superluminescent light source via an optimized anti-reflectivity coating. The CW output power up to 1.4 mW at 20 °C was observed and the spectral bandwidth was approximately 24 nm.

A high-power 975 nm tilted cavity laser with a 0.13 nm K−1 thermal shift of the lasing wavelength

We address design and performance issues of a 975 nm range tilted cavity laser (TCL) based on GaAs/GaAlAs waveguide and GaInAs quantum wells as an active medium. The all-epitaxial design of a TCL includes a cavity and a multilayer interference reflector (MIR). The cavity and the MIR are constructed as follows. The cavity confines one tilted optical mode, and the wavelength of this mode as a function of the tilt angle follows a first dispersion curve. The reflectivity spectrum of the MIR for tilted incidence of light has a well-pronounced stopband, and the stopband reflectivity maximum follows a second dispersion curve. The two dispersion curves intersect at one optimum tilt angle and one optimum wavelength λ0. At the wavelength λ0 the leakage loss of the optical mode through the MIR to the substrate has a minimum, which favors lasing at this optimum wavelength. The thermal shift of λ0 is governed by the shift of the intersection point. As the photon energy corresponding to the operation wavelength of the laser 975 nm is close to the resonance with the electronic energy gap in GaAs, the refractive index temperature coefficient dn/dT of GaAlAs strongly increases at the GaAs side. This gives a tool to control the thermal shift of λ0 by replacing some layers of the cavity by a superlattice containing narrow insertions of GaAs. Such a superlattice may have the same averaged refractive index at room temperature, but significantly different values of dn/dT, thus affecting the thermal shift of λ0. Within this approach broad-area (100 µm) devices have been fabricated showing a high temperature stability of the lasing wavelength (0.13 nm K−1), low threshold current density (300 A cm−2), a high power operation (>7 W in pulsed mode and >1.5 W in continuous wave mode), a high spectral stability at high output power, and a narrow vertical far-field beam divergence (FWHM ∼ 20°).

High-performance 640-nm-range GaInP-AlGaInP lasers based on the longitudinal photonic bandgap crystal with narrow vertical beam divergence

We address the design and performance issues of 640-nm range GaInP-AlGaInP laser diodes based on a longitudinal photonic bandgap crystal (PBC). The all-epitaxial design is based on selective filtering of high-order modes and allows extending of the fundamental mode over a PBC waveguide achieving very large vertical modal spot size. At the same time the robustness of the narrow far-field vertical beam divergence is remarkably high with respect to layer thickness variations. Optimal design ensures that all high-order optical modes show high absolute values of leakage loss (> 30 cm/sup -1/), which are order (orders) of magnitude higher than the leakage loss for the fundamental mode. This PBC-induced resonant tunneling effect for high-order modes leads to preferential excitation of the fundamental mode and the high-order modes are not excited even at the highest injection current densities. Broad-area (100 /spl mu/m) devices show vertical beam divergence of 8' (full-width at half-maximum) and lateral beam divergence of 7'-8'. The far-field pattern is circular shaped and stable upon an increase in injection current. Differential quantum efficiency is as high as 85 %. Maximum pulsed total optical output power is 20 W for 100-tim-wide stripe lasers with uncoated fac.

Design and characterization of 1.3-μm AlGaInAs-InP multiple-quantum-well lasers

A comprehensive design method for long wavelength strained quantum-well lasers is applied to design uncooled multiple-quantum-well AlGaInAs-InP 1.3-/spl mu/m lasers for communication systems. The method includes multiband effective mass theory and electromagnetic waveguide theory. The resulting AlGaInAs-InP laser has a threshold current of 12.5 mA at 25/spl deg/C, with a slope efficiency of 0.43 W/A, at 77 K or greater characteristic temperature, a 38/spl deg/ perpendicular far-field beam divergence, and will operate at temperatures in excess of 100/spl deg/C.

Mid-infrared angled-grating distributed feedback laser

We report near-diffraction-limited output from an angled-grating distributed feedback type-II W laser emitting near 3.4 μm. For pulsed optical pumping of a 50-μm-wide stripe at 78 K, the far-field beam divergence angle was only 1.4°. The slope efficiency was 64% of that for a conventional Fabry–Pérot laser on the same bar. However, the spectral linewidth decreased by only a factor of 2. The beam quality was substantially better than that for the Fabry–Pérot laser out to stripe widths of at least 800 μm.

High power asymmetrical InAsSb/InAsSbP/AlAsSb double heterostructure lasers emitting at 3.4 μm

Midinfrared lasers with an asymmetrical InPAsSb/InAsSb/AlAsSb double heterostructure are reported. Using the asymmetrical double heterostructure, p- and n-cladding layers are separately optimized; high energy-gap AlAsSb (Eg≈1.5 eV) for the p-type cladding layer to reduce the leakage current, and thus to increase To, and low energy-gap InPAsSb (Eg≈0.5 eV) for the n-cladding layer to have low turn-on voltage. 100-μm-width broad-area lasers with 1000 μm cavity length exhibited peak output powers of 1.88 W in pulse and 350 mW in continuous wave modes per two facets at T=80 K with To of 54 K and turn-on voltage of 0.36 V. Maximum peak output powers up to 6.7 W were obtained from a laser bar of total aperture of 400 μm width and cavity length of 1000 μm, with a differential efficiency of 34% and far-field beam divergence narrower than 40° at 80 K.

Room-temperature lasing operation of GaInNAs-GaAs single-quantum-well laser diodes

We have succeeded in demonstrating continuous-wave (CW) operation of GaInNAs-GaAs single-quantum-well (SQW) laser diodes at room temperature (RT). The threshold current density was about 1.4 kA/cm/sup 2/, and the operating wavelength was approximately 1.18 /spl mu/m for a broad-stripe geometry. Evenly spaced multiple longitudinal modes were clearly observed in the lasing spectrum. The full-angle-half-power far-field beam divergence measured parallel and perpendicular to the junction plane was 4.5/spl deg/ and 45/spl deg/, respectively. A high characteristic temperature (T/sub 0/) of 126 K under CW operation and a small wavelength shift per ambient temperature change of 0.48 nm//spl deg/C under pulsed operation were obtained. These experimental results indicate the applicability of GaInNAs to long-wavelength laser diodes with excellent high-temperature performance.

Two-mirror beam-shaping technique for high-power diode bars

A simple beam-shaping technique is described whereby the output from a highly asymmetric non-diffractionlimited laser source, such as a high-power diode bar, can be reconfigured with nearly equal beam-quality factors in orthogonal planes and without a significant reduction in brightness. Using this technique, in conjunction with conventional lenses, we have focused the output from a 20-W cw diode bar to a very intense, nearly circular spot with a 1/e(2) beam diameter of ~100 microm and a far-field beam divergence (half-angle) of ~0.2 rad.

High Power InAsSb/InAsSbP Laser Diodes Emitting at 3 ∼ 5 μm Range

ABSTRACTWe report metalorganic chemical vapor deposition-grown double heterostructure and multiple quantum well InAsSb/InAsSbP laser diodes emitting at 3 to 4 μm and light emitting diodes up to 5 μm. Maximum output power up to 1 W (from two facets) with differential efficiency above 70 % up to 150 K was obtained from a MQW laser with stripe width of 100 μm and cavity length of 700 μm for emitting wavelength of 3.6 μm at 90 K. Maximum operating temperature up to 220 K with threshold current density of 40 A/cm2 at 78 K were achieved from the double-heterostructure lasers emitting at 3.2 μm. The far-field beam divergence as narrow as 24° was achieved with the use of higher energy gap barrier layers, i.e., lower effective refractive index, in MQW active region. We also discuss the effect of composition-fluctuation in the InAsSb active region on the gain and threshold current of the lasers.