Theory and simulation of subwavelength high contrast gratings and their applications in vertical-cavity surface-emitting laser devices

This work intends to fully explore the qualities and applications of subwavelength gratings. Subwavelength gratings are diffraction gratings with physical dimensions less than the wavelength of incident light. It has been found that by tailoring specific dimension parameters, a number of different reflection profiles can be attained by these structures including high reflectivity or low reflectivity with broad and narrow spectral responses. In the course of this thesis the physical basis for this phenomenon will be presented as well as a mathematical derivation. After discussion of the mechanics of the reflection behavior, the methods used in modeling subwavelength gratings and designing them for specific functions will be explored. Following this, the fundamentals of vertical-cavity surface-emitting lasers (VCSELs) will be discussed, and the applications of subwavelength gratings when used with these lasers will follow. Several devices, both theoretical proposals and fabricated examples, will be presented in addition to the available performance measurements. Finally, the fabrication challenges that restrict subwavelength gratings from adoption as standard components in VCSEL design will be considered with regard to ongoing fabrication research

Electrical Parasitic Bandwidth Limitations of Oxide-Free Lithographic Vertical-Cavity Surface-Emitting Lasers

Nowadays, Vertical-Cavity Surface-Emitting Lasers (VCSELs) are the most popular optical sources in short-reach data communications. In the commercial oxide VCSEL technology, an oxide aperture is created inside resonant cavity in realizing good mode and current confinement, however, high electrical resistance comes along with forming the oxide aperture and the electrical parasitic bandwidth becomes the main limitation in modulation speed. In this report, electrical bandwidths of oxide-free lithographic VCSELs have been studied along with their general lasing properties. Due to the new ways of fabricating the aperture, record low resistances have been achieved in oxide-free lithographic VCSELs with various sizes, while high slope efficiencies and high output powers have been maintained. High speed simulation has been performed showing the very low differential resistances will benefit much to the electrical parasitic bandwidths, and are expected to produce higher modulation speed. A bottom emitting structure has been proposed and analyzed, showing reduction in both mirror resistance and capacitance will further improve the modulation speed. The total 3-dB modulation bandwidth is expected to be 50-80 GHz, much higher than the bandwidth reached in existing oxide VCSELs. Lithographic VCSELs also show superior lasing characteristics, including record low thermal resistance and record high output power. The maximum power exceeds 19 mW in a 6 µm device and over 50 % power conversion efficiency has been achieved. A maximum single mode operation power of 5 mW has been observed from a 1 µm diameter VCSEL. High temperature stress testing has been performed showing lithographic VCSELs can operate more reliably than oxide VCSELs under extreme operating conditions. Lithographic VCSEL with low electrical resistance, single-mode operation, high efficiency, and high power will be a strong candidate as the optical source in high speed data communications, as well as other applications such as high power VCSEL arrays and optical sensing

All epitaxial mode and current confined semiconductor laser using selective fermi level pinning

textThis dissertation presents a new lithographically defined approach to form self-aligned mode- and current-confined all-epitaxial GaAs-based VCSELs and quantum dot lasers. The mode confinement mechanism by intracavity phase shifting mesa is verified by a specially designed mode confined VCSEL, which shows the lasing peaks only on the phase shifting mesa with relatively low threshold current density. The current-confining mechanism, which is self aligned with mode confining phase shifting mesa using selective Fermi-level pinning is verified with the test structure with the same active layers in the cavity by substantially increasing the turn on voltage of the region outside mesa. The mode- and current-confined VCSEL also shows improved slope efficiency and lasing spectrum which indicate the only lasing on the mesa region throughout the entire operation range. All-epitaxial, fully planarized mode- and current-confined VCSELs, which have only single phase shifting mesa and surrounding current blocking regions are successfully demonstrated with considerably improved efficiency and threshold current density. As an important application of this technology, all- epitaxial index- and current-confined quantum dot laser is demonstrated. The performances of quantum dot laser shows ground state lasing and stable operation up to high input current of 1.5 A. Extracted waveguide loss indicates additional loss is coming from the current blocking hetero-interfaces which can be reduced by optimization of regrowth condition.Electrical and Computer Engineerin

VCSEL and Integration Techniques for Wavelength-Multiplexed Optical Interconnects

GaAs-based vertical-cavity surface-emitting lasers (VCSELs) are dominating short-reach optical interconnects (OIs) due to their high modulation speed, low power consumption, circular output beam and low fabrication cost. Such OIs provide the high bandwidth connectivity needed for interconnecting servers and switches in data centers. With the rapidly increasing use of Internet-based applications and services, higher bandwidth connectivity and higher aggregate capacity VCSEL-based OIs are needed. Until now, this has been achieved mostly through an increase of the lane rate by higher speed VCSELs and higher order modulation formats. Furthermore, spatial-division-multiplexing has proven effective for increasing the aggregate capacity. Much higher capacity can be achieved by multiple wavelengths per fiber, known as wavelength-divisionmultiplexing (WDM). Moreover, smaller footprint and higher bandwidth density WDM transceivers can be built using monolithic multi-wavelength VCSEL arrays with densely spaced VCSELs. This requires a VCSEL technology where the wavelength of individual VCSELs can be precisely set in a post-epitaxial growth fabrication process and a photonic integrated circuit (PIC) for multiplexing and fiber coupling. Flip-chip integration over grating couplers (GCs) is considered for interfacing VCSELs with waveguides on the PIC. In this thesis, an intra-cavity phase tuning technique is demonstrated for setting the resonance wavelength of VCSELs in a monolithic array with an accuracy in spacing of <1 nm. Uniform performance over the array is achieved by spectral matching and balancing of mirror reflectances, optical confinement factor and optical gain. Single transverse and polarization mode VCSELs, as required for flip-chip integration over GCs, with a record output power of 6 mW are also demonstrated.Finally, an investigation of angled flip-chip integration of a VCSEL over a GC on a silicon photonic integrated circuit (Si-PIC) is presented. Dependencies of coupling efficiency and optical feedback on flip-chip angle and size of the VCSEL die are studied using numerical FDTD simulations. Moreover, flip-chip integration of a VCSEL over a GC on a Si-PIC is experimentally demonstrated. The insertion loss from the VCSEL at the input GC to a singlemode fiber, multimode fiber or flip-chip integrated photodetector over the output GC was measured and quantified. The latter forms an on-PIC optical link

High Speed VCSELs and VCSEL Arrays for Single and Multicore Fiber Interconnects

Our recent work on high speed 850 nm VCSELs and VCSEL arrays is reviewed. With a modulation bandwidth approaching 30 GHz, our VCSELs have enabled transmitters and links operating at data rates in excess of 70 Gbps (at IBM) and transmission over onboard polymer waveguides at 40 Gbps ( at University of Cambridge). VCSELs with an integrated mode filter for single mode emission have enabled transmission at 25 Gbps over > 1 km of multimode fiber and a speed-distance product of 40 Gbps . km. Dense VCSEL arrays for multicore fiber interconnects have demonstrated 240 Gbps aggregate capacity with excellent uniformity and low crosstalk between the 40 Gbps channels

III-V-on-silicon photonic devices for optical communication and sensing

In the paper, we review our work on heterogeneous III-V-on-silicon photonic components and circuits for applications in optical communication and sensing. We elaborate on the integration strategy and describe a broad range of devices realized on this platform covering a wavelength range from 850 nm to 3.85 μm

Contact Geometrical Study for Top Emitting 980 nm InGaAs/GaAsP Vertical-Cavity Surface Emitting Lasers

Geometrical contacts of a double mesa structure with 16 rows ×15 columns arrays of top emitting GaAs based 980 nm vertical cavity surface emitting lasers (VCSELs) are fabricated and characterized. In this paper, 5 strained In0.22Ga0.78As/Ga0.9AsP0.1 quantum wells (QWs) within λ/2 thick cavity have been employed. The top and the bottom epitaxially grown mirrors are based on the linear graded Al0.9Ga0.1As/GaAs distributed Bragg reflectors (DBRs) with 20.5 and 37 periods, respectively. Static parameters including threshold currents, rollover currents, maximum optical output power and wall-plug efficiency are extracted from light out power-current-voltage (LIV) of VCSELs with fixed oxide aperture diameter of ∅~ 6 μm and various mesa2 diameters. In addition, spectral emission for 980 nm VCSELs of oxide aperture between ∅~ 6 and 19 μm and with fixed ∅~ 6 μm and different bias currents are analyzed. The highest optical output power of around 33 dBm is observed at bias current of 0.8 mA for short−reach optical interconnect applications

Intrinsic Modulation Response Modeling and Analysis for Lithographic Vertical-Cavity Surface-Emitting Lasers

Vertical-cavity surface-emitting lasers (VCSELs) have been greatly improved and successfully commercialized over the past few decades owing to their ability to provide both mode and current confinement that enables low energy consumption, high efficiency and high modulation speed. However, further improvement of oxide VCSELs is limited by the nature of the oxide aperture because of self-heating, internal strain and difficulties in precise size control. In this dissertation, VCSELs using lithographic approach are demonstrated to overcome the limitations of oxide VCSELs, in which an intra-cavity phase shifting mesa is applied to define the device size and provide optical mode and electrical current confinement instead of an oxide aperture. A newly developed model of intrinsic modulation response is proposed and analyzed to focus on the thermal limit of the modulation speed of VCSELs. The results show that both the temperature dependent differential gain and stimulated emission rate impact laser speed and the stimulated emission rate dominates the speed limit. Thermal limits of modulation response are compared for oxide and lithographic VCSELs for various sizes. The results predict that the intrinsic modulation response can be significantly increased by using lithographic VCSELs due to low thermal resistance and reduced mode volume while maintaining high efficiency. The intrinsic bandwidth could exceed 100 GHz for a 2-?m-diameter lithographic VCSEL. Combined with low electrical parasitics, it is expected to produce over 100 Gb/s data rate from a single directly modulated laser. VCSELs designed for high speed are discussed and their characteristics are demonstrated

Lithographic Vertical-cavity Surface-emitting Lasers

Remarkable improvements in vertical-cavity surface-emitting lasers (VCSELs) have been made by the introduction of mode- and current-confining oxide optical aperture now used commercially. However, the oxide aperture blocks heat flow inside the device, causing a larger thermal resistance, and the internal strain caused by the oxide can degrade device reliability, also the diffusion process used for the oxide formation can limit device uniformity and scalability. Oxide-free lithographic VCSELs are introduced to overcome these device limitations, with both the mode and current confined within the lithographically defined intracavity mesa, scaling and mass production of small size device could be possible. The 3 μm diameter lithographic VCSEL shows a threshold current of 260 μA, differential quantum efficiency of 60% and maximum output power density of 65 kW/cm2 , and shows single-mode singlepolarization operation with side-mode-suppression-ratio over 25 dB at output power up to 1 mW. The device also shows reliable operation during 1000 hours stress test with high injection current density of 142 kA/cm2 . The lithographic VCSELs have much lower thermal resistance than oxide-confined VCSELs due to elimination of the oxide aperture. The improved thermal property allows the device to have wide operating temperature range of up to 190 °C heat sink temperature, high output power density especially in small device, high rollover current density and high rollover cavity temperature. Research is still underway to reduce the operating voltage of lithographic VCSELs for high wall plug efficiency, and the voltage of 6 µm device at injection current density of 10 kA/cm2 is reduces to 1.83 V with optimized mesa and DBR mirror iv structure. The lithographic VCSELS are promising to become the next generation VCSEL technology