SwePub - sökning: WFRF:(Haglund Åsa 1976 )

Numrering	Referens	Omslagsbild	Hitta
1.	Haglund, Erik, 1985, et al. (författare) 850 nm datacom VCSELs for higher-speed and longer-reach transmission 2013 Ingår i: European VCSEL Day 2013. Konferensbidrag (refereegranskat)abstract The 850 nm GaAs-based VCSEL is already the dominating technology for transmitters in optical interconnects up to 100 m in datacenters, thanks to low-cost fabrication, excellent high-speed properties at low currents and the existence of high-speed OM4 multimode fiber optimized for this particular wavelength. Future datacenters will require faster and more energy-efficient VCSELs to increase the overall bandwidth and reduce the power consumption of the datacenter network. In addition, longer-reach interconnects exceeding 1 km will also be required as datacenters grow into large multi-building complexes.By optimizing the doping profiles of the DBRs to reduce resistance, using a short (½-λ) cavity to improve longitudinal optical confinement and optimizing the photon lifetime for optimal damping, we obtained a record-high small-signal modulation bandwidth of 28 GHz for a ~4 µm oxide aperture VCSEL. A 7 µm oxide aperture VCSEL (~27 GHz bandwidth) enabled error-free transmission (bit-error-rate 300 m), the large spectral width of VCSELs leads to severe signal degradation by fiber dispersion. We have investigated two methods of fabricating low-spectral width quasi-single mode VCSELs to mitigate this problem. By using a small oxide aperture of ~3 µm, error-free transmission was achieved at 22 Gbit/s over 1.1 km of OM4 fiber. An alternative approach is to use an integrated mode filter in the form of a shallow surface relief to reduce the spectral width of the VCSEL. The mode filter allows for the use of a larger oxide aperture and thereby enables a lower resistance and operation at a lower current density. A 5 µm oxide aperture VCSEL with a mode filter enabled error-free transmission at 25 Gbit/s over 500 m of OM4 fiber.
2.	Haglund, Erik, 1985, et al. (författare) Quasi-single mode VCSELs for longer-reach multimode fiber optical interconnects 2014 Ingår i: Summers School on Optical Interconnects, 3-6 June 2014, Thessaloniki. Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract Longer-reach (>300 m) optical interconnects are needed asdatacenters grow ever larger. Today the reach of 850 nmVCSEL-based optical interconnects is mainly limited by fiber dispersion. By reducing the spectral width of the VCSEL, the effects of fiber dispersion may be reduced, effectively increasing the error-free transmission distance.
3.	Kumari, Sulakshna, et al. (författare) Integration of GaAs-based VCSEL array on SiN platform with HCG reflectors for WDM applications 2015 Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. - 9781628414622 ; 9372, s. Art. no. 93720U- Konferensbidrag (refereegranskat)abstract We present a GaAs-based VCSEL structure, BCB bonded to a Si3N4 waveguide circuit, where one DBR is substituted by a free-standing Si3N4 high-contrast-grating (HCG) reflector realized in the Si3N4 waveguide layer. This design enables solutions for on-chip spectroscopic sensing, and the dense integration of 850-nm WDM data communication transmitters where individual channel wavelengths are set by varying the HCG parameters. RCWA shows that a 300nm-thick Si3N4 HCG with 800nm period and 40% duty cycle reflects strongly (
4.	Gustavsson, Johan, 1974, et al. (författare) High speed 850nm VCSELs for >40Gb/s transmission 2013 Ingår i: 2013 Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference, OFC/NFOEC 2013. - 9781479904570 ; , s. OTh4H.4- Konferensbidrag (refereegranskat)abstract VCSELs capable of direct modulation exceeding 40Gb/s are needed in next generation optical interconnect standards. Using a refined high-speed design we demonstrate GaAs-based VCSELs having record-high 28GHz modulation bandwidth, and that operate error-free at 47Gb/s.
5.	Gustavsson, Johan, 1974, et al. (författare) High-speed, high-temperature VCSELs for optical interconnects 2013 Ingår i: 2013 IEEE Photonics Society Summer Topical Meeting Series, PSSTMS 2013. - 9781467350600 ; , s. 7-8 Konferensbidrag (refereegranskat)abstract Directly modulated VCSELs operating at 40Gb/s are required for next generation optical interconnect standards. Using a refined high-speed design we demonstrate GaAs-based VCSELs that operate error-free at 47Gb/s at 25°C, and 40Gb/s at 85°C.
6.	Haglund, Erik, 1985, et al. (författare) 25 Gbit/s transmission over 500 m multimode fibre using 850 nm VCSEL with integrated mode filter 2012 Ingår i: Electronics Letters. - : Institution of Engineering and Technology (IET). - 1350-911X .- 0013-5194. ; 48:9, s. 517-518 Tidskriftsartikel (refereegranskat)abstract An integrated mode filter in the form of a shallow surface relief was used to reduce the spectral width of a high-speed 850 nm vertical-cavity surface-emitting laser (VCSEL). The mode filter reduced the RMS spectral width from 0.9 to 0.3 nm for a VCSEL with an oxide aperture as large as 5 mu m. Because of reduced effects of chromatic and modal fibre dispersion, the mode filter significantly increases the maximum error-free (bit error rate
7.	Haglund, Erik, 1985, et al. (författare) Demonstration of post-growth wavelength control of VCSELs using high-contrast gratings 2015 Ingår i: HP Laboratories Technical Report. ; 70 Tidskriftsartikel (refereegranskat)abstract We demonstrate post-growth wavelength setting of vertical-cavity surface-emitting lasers (VCSELs) using high-contrast gratings (HCGs). By fabricating HCGs with different duty-cycle and period, the HCG reflection phase can be varied, in effect giving different optical cavity lengths for HCGVCSELs with different grating parameters. This enables fabrication of monolithic multi-wavelength HCG-VCSEL arrays for wavelength-division multiplexing (WDM). The GaAs HCG is suspended in air by selective removal of an InGaP sacrificial layer. Electrically injected 980-nm HCGVCSELs with sub-mA threshold currents indicate high reflectivity from the GaAs HCGs. Lasing over a wavelength span of 15 nm was achieved, enabling a 4-channel WDM array with 5 nm channel spacing. Device design, fabrication and experimental proof-of-concept are presented.
8.	Haglund, Erik, 1985, et al. (författare) Demonstration of post-growth wavelength setting of VCSELs using high-contrast gratings 2016 Ingår i: Optics Express. - : The Optical Society. - 1094-4087 .- 1094-4087. ; 24:3, s. 1999-2005 Tidskriftsartikel (refereegranskat)abstract We demonstrate, for the first time, post-growth wavelength setting of electrically-injected vertical-cavity surface-emitting lasers (VCSELs) by using high-contrast gratings (HCGs) with different grating parameters. By fabricating HCGs with different duty cycle and period, the HCG reflection phase can be varied, in effect giving different optical cavity lengths for HCG-VCSELs with different grating parameters. This enables fabrication of monolithic multi-wavelength HCG-VCSEL arrays for wavelength-division multiplexing (WDM). The GaAs HCG is suspended in air by removing a sacrificial layer of InGaP. Electrically-injected 980-nm HCG-VCSELs with sub-mA threshold currents indicate high reflectivity from the GaAs HCGs. Lasing over a wavelength span of 15 nm was achieved, enabling a 4-channel WDM array with 5 nm channel spacing. A large wavelength setting span was enabled by an air-coupled cavity design and the use of only the HCG as top mirror.
9.	Haglund, Erik, 1985, et al. (författare) GaAs High-Contrast Gratings with InGaP Sacrificial Layer for Multi-Wavelength VCSEL Arrays 2016 Ingår i: Conference Digest - IEEE International Semiconductor Laser Conference. - 0899-9406. - 9784885523069 ; TuD2, s. Article n0 7765746- Konferensbidrag (refereegranskat)abstract We report on highly reflective suspended GaAs high-contrast gratings (HCGs) using an InGaP sacrificial layer. A high reflectivity approaching 100% was observed both in direct reflectivity measurement and by low threshold currents in fabricated multi-wavelength HCG-VCSEL arrays.
10.	Haglund, Erik, 1985, et al. (författare) High-contrast gratings for WDM VCSEL arrays 2014 Ingår i: Optics & Photonics in Sweden, 11-12 Nov. 2014, Göteborg. Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract Vertical-cavity surface-emitting lasers (VCSELs) have become the workhorse of short-reach optical interconnects in datacenters and supercomputers. The last few years have seen an impressive increase in VCSEL modulation bandwidth, enabling record-high single-channel data rates exceeding 60 Gbit/s [1]. In addition to higher single-channel rates, interconnect capacity may be enhanced by employing multiplexing techniques such coarse wavelength division multiplexing (WDM). WDM VCSEL arrays can be designed using a high-contrast grating (HCG) as top mirror instead of a distributed Bragg reflector (DBR) [2]. The HCG consists of a subwavelength grating of high refractive index material (GaAs) surrounded by low refractive index material (air), see figure 1. The result is a thin, broad-band and highly reflective mirror. The reflection from the HCG has a varying phase depending on grating geometry. This can be used to set the HCG-VCSEL wavelength in a post-growth process by fabricating gratings with different period and duty-cycle. A first proof of concept has been realized and HCG-VCSELs showing resonances covering a span exceeding 20 nm have been demonstrated. Figure 1: Top: schematic figure of HCG-VCSEL array. Left: Top and cross-sectional SEM image of HCG. Right: Simulated and experimental HCG-VCSEL resonance wavelength for different duty cycles and periods (p) measured by electroluminescence. References[1] D. Kuchta et al., “64Gb/s Transmission over 57m MMF using an NRZ Modulated 850nm VCSEL,” OFC 2014[2] V. Karagodsky et al., “Monolithically integrated multi-wavelength VCSEL arrays using high-contrast gratings,” Opt. Express, 18(2), 2010
11.	Haglund, Erik, 1985, et al. (författare) High-speed lasers for optical interconnects 2012 Ingår i: Swedish Optics and Photonics Days 2012. Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract In the era of cloud computing, with services such as video streaming, social networking, and online storage and file sharing, the demand for online data processing and storage capacity is growing rapidly. These services are hosted in huge data centers that need not only fast servers but also fast communication between servers. Because copper cables have high attenuation at high frequencies, the most promising solution is to use fiber optical links (called optical interconnects) to connect different parts of the data center. Today, gallium arsenide-based vertical-cavity surface-emitting lasers (VCSELs) emitting at 850 nm are the standard light source in transmitters used in commercially available optical interconnects, operating at up to 14 Gbit/s with a link length of up to 300 m. These lasers have the advantages of low power consumption, fast direct modulation at low currents, and low-cost manufacturing. To keep up with the demand of increasing optical interconnect capacity, the Photonics Laboratory at Chalmers university of Technology is conducting research into improving the speed, reach and capacity of VCSELs for optical interconnects.
12.	Haglund, Erik, 1985, et al. (författare) Low Spectral Width High-Speed VCSELs 2011 Ingår i: International Nano-Optoelectronic Workshop (iNOW). Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract The transmission distance of high-speed multimode 850 nm VCSELs is currently limited by modal dispersion. A design and process to fabricate high-speed VCSELs with a reduced spectral width using a surface relief technique are presented.
13.	Haglund, Erik, 1985, et al. (författare) Mode-filtered semiconductor lasers enable longer-reach optical interconnects 2012 Ingår i: SPIE Newsroom. - : SPIE-Intl Soc Optical Eng. - 1818-2259. Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract An integrated mode filter significantly decreases the spectral width of conventional short-wavelength vertical-cavity surface-emitting lasers, promising longer optical interconnects for future data centers.
14.	Haglund, Erik, 1985, et al. (författare) Multi-wavelength VCSEL arrays using high-contrast gratings 2017 Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. - 9781510606678 ; 10113 Konferensbidrag (refereegranskat)abstract The use of a high-contrast grating (HCG) as the top mirror in a vertical-cavity surface-emitting laser (VCSEL) allows for setting the resonance wavelength by the grating parameters in a post-epitaxial growth fabrication process. Using this technique, we demonstrate electrically driven multi-wavelength VCSEL arrays at ∼980 nm wavelength. The VCSELs are GaAs-based and the suspended GaAs HCGs were fabricated using electron-beam lithography, dry etching and selective removal of an InGaP sacrificial layer. The air-coupled cavity design enabled 4-channel arrays with 5 nm wavelength spacing and sub-mA threshold currents thanks to the high HCG reflectance.
15.	Haglund, Erik, 1985, et al. (författare) Reducing the spectral width of high speed oxide confined VCSELs using an integrated mode filter 2012 Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. - 9780819489197 ; 8276, s. Article Number: 82760L- Konferensbidrag (refereegranskat)abstract We have reduced the spectral width of high speed oxide confined 850 nm VCSELs using a shallow surface relief for suppression of higher order transverse modes. The surface relief acts as a mode filter by introducing a spatially varying and therefore mode selective loss. The VCSEL employs multiple oxide layers for reduced capacitance which leads to a strong index guiding and a large spectral width in the absence of a mode filter. With an appropriate choice of surface relief parameters, the RMS spectral width for a 5 jam oxide aperture VCSEL is reduced from 0.6 to 0.3 nm. The small signal modulation bandwidth is 19 GHz. Due to reduced effects of chromatic and modal fiber dispersion, the maximum error-free (bit-error-rate
16.	Haglund, Erik, 1985, et al. (författare) Wavelength Control of VCSELs using High-Contrast Gratings 2017 Ingår i: VII Workshop on Physics and Technology of Semiconductor Lasers. Konferensbidrag (refereegranskat)abstract The vertical-cavity surface-emitting laser (VCSEL) is a well-established light source for sensing and short-reach optical links. The surface emission allows wafer-scale testing enabling low-cost manufacturing, while the VCSELs’ small modal volume leads to low power consumption, high-speed modulation at small currents, and small footprint [1]. Conventional VCSELs consist of an active region sandwiched between two distributed Bragg reflectors (DBRs). Replacing the top DBR with a high-contrast grating reflector offers unique possibilities to engineer and control VCSEL emission wavelength and modal properties [2,3]. A high-contrast grating (HCG) is typically formed by bars of high refractive index suspended in air. HCGs with certain grating parameters (duty cycle, period, and thickness) can function as ultra-thin reflectors with close to 100% reflectivity [4]. Besides the reflectivity, the grating parameters also influence the reflection phase. This enables fabrication of multi-wavelength VCSEL arrays by fabricating HCG-VCSELs with different grating parameters. In order to utilize the extraordinary properties of the HCG, the VCSEL mode must be sensitive to the HCG, which leads to complicated cavity configurations with coupled cavity effects and low optical confinement. This talk will summarize experimental work performed at Chalmers University of Technology in collaboration with Hewlett Packard Enterprise. The design of HCGs and HCG-VCSELs will be presented as well as experimental results from 980 nm HCG-VCSELs and demonstration of post-growth wavelength setting for wavelength-division multiplexing (WDM) VCSEL arrays [5].References[1]Larsson, “Advances in VCSELs for communication and sensing,” IEEE J. Sel. Top. Quantum Electron. 17(6), 1552-1567 (2011).[2]V. Karagodsky, et al., ”Monolithically integrated multi-wavelength VCSEL arrays using high-contrast gratings”, Opt. Express 18(2), 694-699 (2010).[3]S. Inoue, et al., “Highly angular dependent high-contrast grating mirrors and its application for transverse-mode control of VCSELs”, Jpn. J. Appl. Phys. 53, 090306 (2014).[4]C. J. Chang-Hasnain et al., “High-contrast gratings for integrated optoelectronics”, Adv. Opt. Photon. 4, 379-, (2012).[5]E. Haglund, et al., “Demonstration of post-growth wavelength-setting of VCSELs using high-contrast gratings”, Opt, Express 24(3), 1999-2005 (2016).AcknowledgementThis work was been supported by Hewlett Packard Enterprise (HPE), the Swedish Foundation for Strategic Research (SSF) and the Swedish Research Council (VR). The epitaxial material was provided by IQE Europe.
17.	Kögel, Benjamin, 1979, et al. (författare) Integrated MEMS-tunable VCSELs for reconfigurable optical interconnects 2012 Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. - 9780819489197 ; 8276, s. Article Number: 82760Q- Konferensbidrag (refereegranskat)abstract A simple and low-cost technology for tunable vertical-cavity surface-emitting lasers (VCSELs) with curved movable micromirror is presented. The micro-electro-mechanical system (MEMS) is integrated with the active optical component (so-called half-VCSEL) by means of surface-micromachining, using a reflown photoresist droplet as sacrificial layer. The technology is demonstrated for electrically pumped, short-wavelength (850nm) tunable VCSEts. Fabricated devices with 10 mu m oxide aperture are singlemode with sidemode suppression >35 dB, tunable over 24 mit with output power up to 0.5 mW, and have a beam divergence angle
18.	Kögel, Benjamin, 1979, et al. (författare) Integrated MEMS-Tunable VCSELs Using a Self-Aligned Reflow Process 2012 Ingår i: IEEE Journal of Quantum Electronics. - 0018-9197 .- 1558-1713. ; 48:2, s. 144-152 Tidskriftsartikel (refereegranskat)abstract A simple microelectromechanical systems technology for wafer-scale integration of tunable vertical-cavity surface-emitting lasers (VCSELs) is presented. The key element is a self-aligned reflow process to form photoresist droplets, which serve as sacrificial layer and preform for a curved micromirror. Using a 3-D electromagnetic model, the half-symmetric cavity is optimized for singlemode emission. The technology is demonstrated for electrically pumped, short-wavelength (850 nm) tunable VCSELs, but is transferable to other wavelengths and material systems. Fabricated devices with 10 mu m large current aperture are singlemode and tunable over 24 nm. An improved high-speed design with reduced parasitic capacitance enables direct modulation with 3dB-bandwidths up to 6 GHz and data transmission at 5Gbit/s. Small signal analysis shows that the intrinsic parameters (resonance frequency and damping) are wavelength dependent through the differential gain.
19.	Kögel, Benjamin, 1979, et al. (författare) Integrated Tunable VCSELs With Simple MEMS Technology 2010 Ingår i: Semiconductor Laser Conference (ISLC), 2010 22nd IEEE International. - 0899-9406. - 9781424456833 Konferensbidrag (refereegranskat)abstract A simple MEMS technology for wafer-scale fabrication of tunable VCSELs is presented. Reflown photo-resist droplets serve as preform for making curved movable micro-mirrors. First devices show a tuning range of 15 nm with mW-output power.
20.	Kögel, Benjamin, 1979, et al. (författare) Singlemode tunable VCSELs with integrated MEMS technology 2011 Ingår i: European Conference on Laser and Electro-Optics (CLEO/Europe). Konferensbidrag (refereegranskat)abstract A simple MEMS technology for wafer-scale integration of short-wavelength tunable VCSELs is presented. Using a 3D model the half-symmetric cavity is optimized for singlemode emission from 10 μm large apertures over 12 nm tuning range.
21.	Larsson, Anders, 1957, et al. (författare) High-speed tunable and fixed-wavelength VCSELs for short-reach optical links and interconnects 2012 Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. - 9780819489197 ; 8276, s. Article Number: 82760H- Konferensbidrag (refereegranskat)abstract This paper presents a review of recent work on high speed tunable and fixed wavelength vertical cavity surface emitting lasers (VCSELs) at Chalmers University of Technology. All VCSELs are GaAs-based, employ an oxide aperture for current and/or optical confinement, and emit around 850 nm. With proper active region and cavity designs, and techniques for reducing capacitance and thermal impedance, our fixed wavelength VCSELs have reached a modulation bandwidth of 23 GHz, which has enabled error-free 40 Gbps back-to-back transmission and 35 Gbps transmission over 100 m of multimode fiber. A MEMS-technology for wafer scale integration of tunable high speed VCSELs has also been developed. A tuning range of 24 nm and a modulation bandwidth of 6 GHz have been achieved, enabling error-free back-to-back transmission at 5 Gbps.
22.	Larsson, Anders, 1957, et al. (författare) High speed VCSELs for optical interconnects 2012 Ingår i: Conference Proceedings - International Conference on Indium Phosphide and Related Materials. - 1092-8669. - 9781467317252 ; , s. 269-272 Konferensbidrag (refereegranskat)abstract This paper presents an overview of our recent work on high speed, oxide confined, 850 nm vertical cavity surface emitting lasers (VCSELs). With proper active region and cavity designs, and techniques for reducing capacitance and thermal impedance, we have reached a modulation bandwidth of 23 GHz and demonstrated 40 Gbps transmission. Using an integrated mode filter for reducing the spectral width we have extended the reach on multimode fiber at 25 Gbps from 100 to 500 m. Improved link capacity was also demonstrated using a more spectrally efficient multi-level modulation format (4-PAM). Finally, a MEMS-technology for wafer scale integration of tunable high speed VCSELs was developed, enabling a tuning range of 24 nm, a 6 GHz modulation bandwidth, and 5 Gbps transmission.
23.	Westbergh, Petter, 1981, et al. (författare) High speed VCSELs for short reach communication 2011 Ingår i: European Semiconductor Laser Worshop 2011. Konferensbidrag (övrigt vetenskapligt/konstnärligt)
24.	Adolfsson, Göran, 1981, et al. (författare) Realization of spectrally engineered semiconductor Fabry-Perot lasers with narrow geometrical tolerances 2011 Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 109:9, s. 093112- Tidskriftsartikel (refereegranskat)abstract Spectrally engineered semiconductor Fabry-Perot laser resonators are designed to enhance the optical feedback for selected longitudinal modes, which thereby require less gain for lasing. This is achieved by introducing refractive index perturbations along the length of the resonator. However,the physical realization of these resonators is a challenge because of very narrow tolerances; in particular the need for precise positioning of the end facets of the resonator in relation to the perturbations, and the excess propagation loss associated with the perturbations, has been a majorconcern. We report on a method to achieve high-quality end facet mirrors enabling precise positioning relative to the perturbations, the latter which are realized as lateral corrugations of the waveguide. Measurements show that the mirror quality is comparable to that of cleaved mirrorsand that the additional loss introduced by the perturbations adds
25.	Apaydin, Dogukan, 1991, et al. (författare) Optically Pumped UVC Photonic Crystal Surface-Emitting Laser 2023 Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract Photonic crystal surface-emitting lasers (PCSELs) are a new type of semiconductor lasers that offer a high optical output power while maintaining single-mode operation and a low divergence angle. Such devices rely upon the in-plane optical feedback from a two-dimensional photonic crystal and feature out-of-plane emission of the modes with zero group velocity at the photonic band edges by diffraction. Since the demonstration of the first PCSEL [1], the concept has been implemented in standard semiconductor materials with the demonstration of highperforming infrared and blue-emitting lasers [2, 3]. Extending the laser operation to shorter emission wavelengths would be a major breakthrough as such lasers would be of high interest for disinfection, material processing, curing, and medical treatments.
26.	Apaydin, Dogukan, 1991, et al. (författare) UVC photonic crystal surface-emitting lasers with low-divergent far-fields 2023 Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract Photonic crystal surface-emitting lasers (PCSEL) emitting in the ultraviolet (UV) C spectral range are exciting devices due to their low divergence and single-mode emission capable of high output powers as already demonstrated in the infrared [1] and blue spectral range [2]. This is due to their unique design, which incorporates a photonic crystal leading to a large optical gain area. PCSELs are based upon in-plane feedback from the photonic crystal and out-of-plane emission by the diffraction of the modes with zero group velocity at the photonic band edges. We recently demonstrated, to the best of our knowledge, the first UVC PCSEL with an emission at 279 nm. The device structure consists of 3 x 2 nm AlGaN quantum wells (QW) in a 60 nm Al0.70Ga0.30N waveguide and AlN cladding layers. The 140x140 μm large photonic crystal is dry etched into the top AlN cladding layer with a hexagonal lattice consisting of circular holes with a lattice constant of 140 nm and an etch depth of 65 nm, leaving 65 nm between the bottom of the photonic crystals and the first quantum well. Lasing in these PCSELs was achieved by resonant pumping of the QWs by a 266 nm pulsed laser with a spot size of 82 μm at room temperature. The devices exhibit threshold pump power densities from 25 down to 13 MW/cm2 showing a spectral narrowing down to 25 pm. Far-field patterns and band structures were investigated for a range of filling factors (fraction of the surface that is etched) between 10% to 26%, and the far-fields contain emission bands that were not yet reported in PCSELs at longer wavelengths. Changing the filling factor affects the photonic crystal band structure and thereby the optical mode at the Γ-point that will reach threshold first. This feature enables us to intentionally select the lasing mode with the desired far-field pattern. By a proper choice of filling factor, the intensity in the angular emission bands is diminished, resulting in a far-field with a narrow beam divergence of <1°.
27.	Baveja, Prashant, 1985, et al. (författare) Impact of device parameters on thermal performance of high speed oxide confined 850 nm VCSELs 2012 Ingår i: IEEE Journal of Quantum Electronics. - 0018-9197 .- 1558-1713. ; 48:1, s. 17-26 Tidskriftsartikel (refereegranskat)abstract We study the impact of device parameters, such asinner-aperture diameter and cavity photon lifetime, on thermal rollover mechanisms in 850-nm, oxide-confined, vertical-cavity surface-emitting lasers (VCSELs) designed for high-speed operation. We perform measurements on four different VCSELs of different designs and use our empirical thermal model for calculating the power dissipated with increasing bias currents through various physical processes such as absorption within the cavity, carrier thermalization, carrier leakage, spontaneous carrier recombination, and Joule heating. When reducing the topmirror reflectivity to reduce internal optical absorption loss we find an increase of power dissipation due to carrier leakage. There is therefore a trade-ff between the powers dissipated owing to optical absorption and carrier leakage in the sense that overcompensating for optical absorption enhances carrier leakage (and vice versa). We further find that carrier leakage places the ultimate limit on the thermal performance for this entire class ofdevices. Our analysis yields useful design optimization strategies for mitigating the impact of carrier leakage and should thereby prove useful for the performance enhancement of 850-nm, highspeed, oxide-confined VCSELs.
28.	Baveja, P. P., et al. (författare) Assessment of VCSEL thermal rollover mechanisms from measurements and empirical modeling 2011 Ingår i: Optics Express. - 1094-4087 .- 1094-4087. ; 19:16, s. 15490-15505 Tidskriftsartikel (refereegranskat)abstract We use an empirical model together with experimental measurements for studying mechanisms contributing to thermal rollover in vertical-cavity surface-emitting lasers (VCSELs). The model is based on extraction of the temperature dependence of threshold current, internal quantum efficiency, internal optical loss, series resistance and thermal impedance from measurements of output power, voltage and lasing wavelength as a function of bias current over an ambient temperature range of 15-100 degrees C. We apply the model to an oxide-confined, 850-nm VCSEL, fabricated with a 9-mu m inner-aperture diameter and optimized for highspeed operation, and show for this specific device that power dissipation due to linear power dissipation (sum total of optical absorption, carrier thermalization, carrier leakage and spontaneous carrier recombination) exceeds power dissipation across the series resistance (quadratic power dissipation) at any ambient temperature and bias current. We further show that the dominant contributors to self-heating for this particular VCSEL are quadratic power dissipation, internal optical loss, and carrier leakage. A rapid reduction of the internal quantum efficiency at high bias currents (resulting in high temperatures) is identified as being the major cause of thermal rollover. Our method is applicable to any VCSEL and is useful for identifying the mechanisms limiting the thermal performance of the device and to formulate design strategies to ameliorate them.
29.	Baveja, P. P., et al. (författare) Impact of photon lifetime on thermal rollover in 850-nm high-speed VCSELs 2012 Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. - 9780819489197 ; 8276 Konferensbidrag (refereegranskat)abstract We present an empirical thermal model for VCSELs based on extraction of temperature dependence of macroscopic VCSEL parameters from CW measurements. We apply our model to two, oxide-confined, 850-nm VCSELs, fabricated with a 9-mu m inner-aperture diameter and optimized for high-speed operation. We demonstrate that for both these devices, the power dissipation due to linear heat sources dominates the total self-heating. We further show that reducing photon lifetime down to 2 ps drastically reduces absorption heating and improves device static performance by delaying the onset of thermal rollover. The new thermal model can identify the mechanisms limiting the thermal performance and help in formulating the design strategies to ameliorate them.
30.	Bengtsson, Jörgen, 1968, et al. (författare) Diffraction loss in long-wavelength buried tunnel junction VCSELs analyzed with a hybrid coupled-cavity transfer-matrix model 2008 Ingår i: Optics Express. ; 16:25, s. 20789-20802 Tidskriftsartikel (refereegranskat)
31.	Bergmann, Michael Alexander, 1989, et al. (författare) Electrochemical etching of AlGaN for the realization of thin-film devices 2019 Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 115:18, s. 182103- Tidskriftsartikel (refereegranskat)abstract Heterogeneously integrated AlGaN epitaxial layers will be essential for future optical and electrical devices like thin-film flip-chip ultraviolet (UV) light-emitting diodes, UV vertical-cavity surface-emitting lasers, and high-electron mobility transistors on efficient heat sinks. Such AlGaN-membranes will also enable flexible and micromechanical devices. However, to develop a method to separate the AlGaN-device membranes from the substrate has proven to be challenging, in particular, for high-quality device materials, which require the use of a lattice-matched AlGaN sacrificial layer. We demonstrate an electrochemical etching method by which it is possible to achieve complete lateral etching of an AlGaN sacrificial layer with up to 50% Al-content. The influence of etching voltage and the Al-content of the sacrificial layer on the etching process is investigated. The etched N-polar surface shows the same macroscopic topography as that of the as-grown epitaxial structure, and the root-mean square roughness is 3.5 nm for 1 µm x 1 µm scan areas. Separated device layers have a well-defined thickness and smooth etched surfaces. Transferred multi-quantum-well structures were fabricated and investigated by time-resolved photoluminescence measurements. The quantum wells showed no sign of degradation caused by the thin-film process.
32.	Bergmann, Michael Alexander, 1989, et al. (författare) Increased Light Extraction of Thin-Film Flip-Chip UVB LEDs by Surface Texturing 2023 Ingår i: ACS Photonics. - : American Chemical Society (ACS). - 2330-4022. ; 10:2, s. 368-373 Tidskriftsartikel (refereegranskat)abstract Ultraviolet light-emitting diodes (LEDs) suffer from a low wall-plug efficiency, which is to a large extent limited by the poor light extraction efficiency (LEE). A thin-film flip-chip (TFFC) design with a roughened N-polar AlGaN surface can substantially improve this. We here demonstrate an enabling technology to realize TFFC LEDs emitting in the UVB range (280-320 nm), which includes standard LED processing in combination with electrochemical etching to remove the substrate. The integration of the electrochemical etching is achieved by epitaxial sacrificial and etch block layers in combination with encapsulation of the LED. The LEE was enhanced by around 25% when the N-polar AlGaN side of the TFFC LEDs was chemically roughened, reaching an external quantum efficiency of 2.25%. By further optimizing the surface structure, our ray-tracing simulations predict a higher LEE from the TFFC LEDs than flip-chip LEDs and a resulting higher wall-plug efficiency.
33.	Bergmann, Michael Alexander, 1989, et al. (författare) Thin-film flip-chip UVB LEDs realized by electrochemical etching 2020 Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 116:12, s. 121101- Tidskriftsartikel (refereegranskat)abstract We demonstrate a thin-film flip-chip (TFFC) light-emitting diode (LED) emitting in the ultraviolet B (UVB) at 311 nm, where substrate removal has been achieved by electrochemical etching of a sacrificial Al0.37Ga0.63N layer. The electroluminescence spectrum of the TFFC LED corresponds well to the as-grown LED structure, showing no sign of degradation of structural and optical properties by electrochemical etching. This is achieved by a proper epitaxial design of the sacrificial layer and the etch stop layers in relation to the LED structure and the electrochemical etch conditions. Enabling a TFFC UV LED is an important step toward improving the light extraction efficiency that limits the power conversion efficiency in AlGaN-based LEDs.
34.	Bergmann, Michael Alexander, 1989, et al. (författare) Thin-film flip-chip UVB LEDs realized by electrochemical etching 2022 Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract We will give an overview of different concepts to increase the light extraction efficiency (LEE) of ultraviolet (UV) light-emitting diodes (LEDs) with a focus on thin-film flip-chip (TFFC) devices. Optical simulations show that a TFFC design can greatly improve the LEE with a transparent p-side, reflective contacts, and optimized surface roughening. We will demonstrate UVB-emitting TFFC LEDs based on our fabrication platform for AlGaN thin films with high aluminum content. The fabrication is compatible with a standard LED process and uses substrate removal based on selective electrochemical etching as the key enabling technology.
35.	Bergmann, Michael Alexander, 1989, et al. (författare) Thin-film UVB LEDs enabled by electrochemical etching of AlGaN 2018 Konferensbidrag (övrigt vetenskapligt/konstnärligt)
36.	Bergmann, Michael Alexander, 1989, et al. (författare) Towards ultraviolet and blue microcavity lasers 2018 Ingår i: Northen Optics and Photonics conference. - 9789163964886 ; 2018 Konferensbidrag (refereegranskat)abstract The development of III-nitride-based (Al,Ga,In(N)) microcavity lasers is a challenging task. Significant progress in recent years has resulted in realizations of electrically pumped devices with optical output power in the mW-range and with threshold current densities below 20 kA/cm2. However, to become practical, the lifetime and power conversion efficiency of these devices must be improved. Among the challenges are achieving transverse optical mode confinement, highreflectivity mirrors and control over the resonator length. We will highlight our theoretical work on transverse optical mode confinement, emphasising the overwhelming risk of ending up with an optically anti-guided cavity, and its consequences such as very high optical losses that easily could double the threshold gain for lasing. We will show some anti-guided cavities with reasonable threshold gain and built-in modal discrimination. However, all anti-guided cavities are very sensitive to temperature effects and small structural changes in the cavity caused by fabrication imperfections. We have explored electrically conductive distributed Bragg reflectors (DBRs) in both AlN/GaN and ZnO/GaN. The AlN/GaN DBRs were grown with different strain-compensating interlayers, and the DBR without interlayers had the lowest vertical resistivity with a specific series resistance of 0.044 cmfor eight DBRpairs. In the ZnO/GaN DBR, the measured resistance was dominated by lateral and contact contributions, setting a lower measurable limit of ~10 for three DBR-pairs. Numerical simulations show the importance of having in-plane strained layers in the ZnO/GaN DBR, since that leads to cancellation of the spontaneous and piezoelectric polarization. This results in a dramatically reduced vertical resistance, potentially three orders of magnitude lower than what could be measured. cm An alternative to an epitaxially grown DBR is a dielectric DBR, which offers high reflectivity over a broader wavelength range, relaxing the requirements on resonator length control. To deposit a dielectric DBR on the bottom side of the cavity, the sample must first be bonded to a carrier wafer before the substrate can be removed. We used thermocompression gold-gold bonding to successfully bond the laser structure to a Si carrier wafer. The subsequent substrate removal is a challenging process due to the chemical inertness of the III-nitride-based materials. A doping-dependent electrochemical etch technique was used, which allows for the selective removal of a sacrificial (n-doped) layer between the cavity and the substrate. This resulted in nm-precise cavity lift-off with a low root-mean-square surface roughness down to 0.3 nm. Thus, the process is suitable for the fabrication of high-quality optical devices such as microcavity lasers. In addition, the technique offers a new alternative to create III-nitridebased optical resonators, mechanical resonators, thin film LEDs and transistors.
37.	Capmany, Jose, et al. (författare) Introduction to the JSTQE Special Issue on Semiconductor Lasers 2022 Ingår i: IEEE Journal of Selected Topics in Quantum Electronics. - 1558-4542 .- 1077-260X. ; 28:1, s. 1-3 Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
38.	Cardinali, G., et al. (författare) Low-Threshold AlGaN-based UVB VCSELs enabled by post-growth cavity detuning 2022 Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 121:10 Tidskriftsartikel (refereegranskat)abstract The performance of vertical-cavity surface-emitting lasers (VCSELs) is strongly dependent on the spectral detuning between the gain peak and the resonance wavelength. Here, we use angle-resolved photoluminescence spectroscopy to investigate the emission properties of AlGaN-based VCSELs emitting in the ultraviolet-B spectral range with different detuning between the photoluminescence peak of the quantum-wells and the resonance wavelength. Accurate setting of the cavity length, and thereby the resonance wavelength, is accomplished by using doping-selective electrochemical etching of AlGaN sacrificial layers for substrate removal combined with deposition of dielectric spacer layers. By matching the resonance wavelength to the quantum-wells photoluminescence peak, a threshold power density of 0.4 MW/cm2 was achieved, and this was possible only for smooth etched surfaces with a root mean square roughness below 2 nm. These results demonstrate the importance of accurate cavity length control and surface smoothness to achieve low-Threshold AlGaN-based ultraviolet VCSELs.
39.	Cardinali, Giulia, et al. (författare) Single-mode lasing in optically pumped UVB VCSELs with circular relief structures 2022 Ingår i: International Workshop on Nitride semiconductors (IWN). Konferensbidrag (refereegranskat)
40.	Chang, Tsu Chi, et al. (författare) Electrically Injected GaN-Based Vertical-Cavity Surface-Emitting Lasers with TiO2 High-Index-Contrast Grating Reflectors 2020 Ingår i: ACS Photonics. - : American Chemical Society (ACS). - 2330-4022. ; 7:4, s. 861-866 Tidskriftsartikel (refereegranskat)abstract We demonstrate the first electrically injected GaN-based vertical-cavity surface-emitting lasers (VCSELs) with a TiO2 high-index-contrast grating (HCG) as the top mirror. Replacing the top distributed Bragg reflector (DBR) with an HCG offers substantial thickness reduction, polarization-pinning, and setting of the resonance wavelength by the grating parameters. Conventional HCGs are usually suspended in the low refractive index material, such as air, in order to create the largest refractive index contrast. However, the mechanical stability of such structures can be questioned and creating free-hanging GaN-membrane on top of GaN is problematic. We have therefore fabricated TiO2-HCGs resting directly on GaN without an air-gap. No DBR layers are used below the HCG to boost the reflectivity. A VCSEL with an aperture diameter of 10 μm shows a threshold current of 25 mA under pulsed operation at room temperature. The lasing modes locate around 400 nm and are transversely electrically -polarized with a line width of 0.5 nm. The full-width half-maximum beam divergence is 10°. This demonstration of a TiO2-HCG VCSEL offers a new route to achieve polarization pinning and could also allow additional benefits such as postgrowth setting of the resonance wavelength.
41.	Chang, Tsu-Chi, et al. (författare) GaN-based vertical-cavity surface-emitting laser incorporating a TiO2 high-index-contrast grating 2020 Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 11280 Konferensbidrag (refereegranskat)abstract We demonstrate the first electrically injected GaN-based VCSEL with a TiO2 high-contrast grating (HCG) as the top mirror. The TiO2-HCG rested directly on the n-GaN without an airgap for mechanical stability. A VCSEL with an aperture diameter of 10 mu m had a threshold current of 25 mA under pulsed operation at room temperature. Multiple longitudinal modes coexist around 400 nm, each TM-polarized with a linewidth of 0.5 nm (spectral resolution limited). This first demonstration of a TiO2-HCG VCSEL offers a new route to achieve polarization pinning and could also allow additional benefits such as post-growth setting of resonance wavelength.
42.	Chang, Tsu-Chi, 1990, et al. (författare) GaN vertical-cavity surface-emitting laser with a high-contrast grating reflector 2018 Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. ; 10542 Konferensbidrag (refereegranskat)abstract We report a GaN-based VCSEL with a high-contrast grating (HCG) as the top mirror. The HCG consisted of TiO2 and rested directly on the n-GaN without an airgap or the use of any DBR layers to boost the reflectivity. The full VCSEL structure was optically pumped at room temperature and showed a lasing threshold of approximately 0.69MW/cm2 and a lasing wavelength at 369.1 nm. This first demonstration of lasing in a HCG GaN-based VCSEL opens up the possibility to explore all the potential benefits of HCGs in the blue and ultraviolet spectral regime.
43.	Chang, Tsu Chi, et al. (författare) Lasing Action in GaN-Based VCSELs with top High-Contrast Grating Reflectors 2018 Ingår i: Conference Digest - IEEE International Semiconductor Laser Conference. - 0899-9406. ; 2018-September, s. 191-192 Konferensbidrag (refereegranskat)abstract We report a GaN-based VCSEL with a high-contrast grating (HCG) as the top mirror. The HCG consisted of TiO2 and rested directly on the n-GaN without an airgap or the use of any DBR layers to boost the reflectivity. The full VCSEL structure was optically pumped at room temperature and showed a lasing threshold of approximately 0.99 MW/cm2 and a lasing wavelength at 369.8 nm. The demonstration of HCG GaN-based VCSEL opens up the possibility to explore all the potential benefits of HCGs in the blue and ultraviolet spectral regime.
44.	Ciers, Joachim, 1991, et al. (författare) Impact of polarization fields on electrochemical lift-off of GaN membranes 2021 Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract III-nitride membranes offer novel device designs in photonics, electronics and optomechanics. However, substrate removal often leads to a rough back surface, which degrades device performance. Here, we demonstrate GaN membranes with atomically smooth etched surfaces by electrochemical lift-off, through the implementation of a built-in polarization field in the sacrificial layer. This leads to a faster reduction in the sacrificial layer free carrier density during etching and thus an abrupter etch stop, reducing the root-mean-square roughness down to 0.4 nm over 5×5 µm2. These results open interesting perspectives on high-quality optical cavities and waveguides in the ultraviolet and visible.
45.	Ciers, Joachim, 1991, et al. (författare) Smooth GaN membranes by polarization-assisted electrochemical etching 2021 Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 118:6 Tidskriftsartikel (refereegranskat)abstract III-nitride membranes offer promising perspectives and improved device designs in photonics, electronics, and optomechanics. However, the removal of the growth substrate often leads to a rough membrane surface, which increases scattering losses in optical devices. In this work, we demonstrate membranes with etched surface roughness comparable to that of the as-grown epitaxial material, accomplished by the implementation of a properly designed built-in polarization field near the top of the sacrificial layer from an AlInN interlayer, which is polarization-mismatched to GaN. This leads to a steeper reduction in free carrier density during the electrochemical etching of the sacrificial layer, limiting the etching current and thus causing an abrupter etch stop. As a result, the root mean square roughness is reduced to 0.4nm over 5x5 mu m(2). These smooth membranes open attractive pathways for the fabrication of high-quality optical cavities and waveguides operating in the ultraviolet and visible spectral regions.
46.	Davani, Hooman A., et al. (författare) Polarization investigation of a tunable high-speed short-wavelength bulk-micromachined MEMS-VCSEL 2012 Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. - 9780819489197 ; 8276, s. Art. no. 82760T- Konferensbidrag (refereegranskat)abstract We report the investigation of the state of polarization (SOP) of a tunable vertical-cavity surface-emitting laser (VCSEL) operating near 850 nm with a mode-hop free single-mode tuning range of about 12 nm and an amplitude modulation bandwidth of about 5 GHz. In addition, the effect of a sub-wavelength grating on the device and its influence on the polarization stability and polarization switching has been investigated. The VCSEL with an integrated sub-wavelength grating shows a stable SOP with a polarization mode suppression ratio (PMSR) more than 35 dB during the tuning.
47.	Fuchs, Christian, et al. (författare) Spatio-temporal turn-on dynamics of grating relief VCSELs 2007 Ingår i: IEEE Journal of Quantum Electronics. - 0018-9197 .- 1558-1713. ; 43:11-12, s. 1227-1234 Tidskriftsartikel (refereegranskat)abstract Within a joint collaboration between modeling, technology and experiments, we investigate the polarization-resolved spatial emission and turn-on dynamics of oxide-confined vertical-cavity surface-emitting lasers with an integrated surface-relief grating. By applying a time-resolved imaging technique we demonstrate that the introduced high dichroism also maintains its influence dynamically. This leads to highly polarization-stable spatially fundamental Gaussian mode emission on 100-ps timescale. Finally, the achieved progress, but also the limits of this promising stabilization scheme are discussed.
48.	Gierl, Christian, et al. (författare) Tuneable VCSEL aiming for the application in interconnects and short haul systems 2011 Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 0277-786X .- 1996-756X. - 9780819484963 ; 7959 Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract Widely tunable vertical cavity surface emitting lasers (VCSEL) are of high interest for optical communications, gas spectroscopy and fiber-Bragg-grating measurements. In this paper we present tunable VCSEL operating at wavelength around 850 nm and 1550 nm with tuning ranges up to 20 nm and 76 nm respectively. The first versions of VCSEL operating at 1550 nm with 76 nm tuning range and an output power of 1.3mW were not designed for high speed modulation, but for applications where only stable continious tuning is essential (e.g. gas sensing). The next step was the design of non tunable VCSEL showing high speed modulation frequencies of 10 GHz with side mode supression ratios beyond 50 dB. The latest version of these devices show record output powers of 6.7mW at 20 °C and 3mW at 80 °C. The emphasis of our present and future work lies on the combination of both technologies. The tunable VCSEL operating in the 850 nm-region reaches a modulation bandwidth of 5.5GHz with an output power of 0.8mW.
49.	Grigoletto, Massimo, et al. (författare) Growth of UVB tunnel-junction LEDs: Impact of GaN interlayer thickness on morphology and electro-optical characteristics 2022 Ingår i: International Workshop on Nitride semiconductors (IWN). Konferensbidrag (refereegranskat)
50.	Gustavsson, Johan, 1974, et al. (författare) Dynamic behavior of fundamental-mode stabilized VCSELs using a shallow surface relief 2004 Ingår i: IEEE Journal Quantum Electronics. ; 40:6, s. 607-619 Tidskriftsartikel (refereegranskat)

Skapa referenser, mejla, bekava och länka

Länka till träfflistan

Träfflista för sökning "WFRF:(Haglund Åsa 1976 ) "

Avgränsa träffmängd

År