Klein Johnson

Advances in Red VCSEL Technology

Red VCSELs offer the benefits of improved performance and lower power consumption for medical and industrial sensing, faster printing and scanning, and lower cost, higher speed interconnects based upon plastic optical fiber (POF). However, materials challenges make it more difficult to achieve the desired performance than at the well-developed wavelength of 850 nm. This paper will describe the state of the art of red VCSEL performance and the results of development efforts to achieve improved output power and a broader temperature range of operation. It will also provide examples of the applications of red VCSELs and the benefits they offer. In addition, the packaging flexibility offered by VCSELs, and some examples of non-Hermetic package demonstrations will be discussed. Some of the red VCSEL performance demonstrations include output power of 14 mW CW at room temperature, a record maximum temperature of C for CW operation at an emission wavelength of 689 nm, time to 1% failure at room temperature of approximately 200,000 hours, lifetime in a C, 85% humidity environment in excess of 3500 hours, digital data rate of 3 Gbps, and peak pulsed array power of greater than 100 mW.

Push-Pull Modulation of a Composite-Resonator Vertical-Cavity Laser

The two coupled optical cavities within a vertical-cavity surface-emitting laser have the unique ability to modulate the spatial distribution of the longitudinal optical mode, without changing the total photon density in the laser cavities, by simultaneously directly modulating the two optical cavities exactly out-of-phase. A rate-equation analysis predicts that this condition, which we term push-pull modulation, exhibits a superior modulation response than that of conventional direct modulation. The push-pull modulation can enable high-speed operation with low power consumption, as a large modulation bandwidth can be achieved independent of the total photon density and/or the injection dc current. Experimental evidence of spatially changing the longitudinal mode is presented, and push-pull modulation at 2.5 Gb/s is demonstrated for the first time.

Cost Effective Optoelectronic Packaging for Multichip Modules and Backplane Level Optical Interconnects

Optical backplanes are of increasing interest for commercial and military avionic processors, and for commercial supercomputers. Projected interconnection density limits of electrical interconnects are rapidly becoming a bottleneck, preventing optimal exploitation of electronic processor capability. A potential obstacle to the commercial development of optoelectronic interconnect components for backplane-based systems is the small market for such specialized technology. In order to ensure that a cost effective solution is available for backplane based systems, commonality with a higher volume application is required. We describe optical packaging techniques for board level waveguides and multichip modules which exploit materials, processes and equipment already in widespread use in the electronics industry, and which can also be applied to a wide range of optoelectronic modules for local area network and telecommunications applications. Rugged polyetherimide waveguides with losses of 0.24 dB/cm have been integrated with conventional circuit board materials, and optoelectronic die have been packaged in a multichip module process using equipment normally used for purely electronic packaging. Practical optical interfaces and connectors have been demonstrated for board-to-backplane and board-to-multichip module applications, and offer increased pincount over their electrical counterparts while retaining compatibility with existing electrical connector alignment and fabrication tolerances.

High output power 670nm VCSELs

The VCSELs investigated in this work are based upon AlGaAs semiconductor mirrors with AlGaInP based active regions. Single mode optical output powers at 670nm in excess of 2.8mW at a 20C ambient temperature have been achieved, with single mode output powers in excess of 1mW at 60C and 0.5mW at 70C. Single mode devices continue to lase up to temperatures in excess of 75C. Record output powers in excess of 11.5mW have been achieved in 673nm multi-mode devices at 20C, with power conversion efficiencies as high as 22.9%. The VCSELs are linearly polarized and are stable over a wide range of drive currents, with a typical SM beam divergence of approximately 7.5 degrees full-width half maximum at 5mA. The paper will provide additional detail regarding the performance characteristics of these devices.

Chip-scale integration of VCSEL, photodetector, and microlens arrays

New generations of network application continue to demand component solutions of higher speed (10 Gbps and beyond) with good reliability and affordability. The potential of VCSEL arrays technology in meeting those demands has long been recognized and is being actively explored by many in the field. In this paper we present a few examples of VCSEL array based technology developments including monolithic integration of VCSELs and photodetector (PD) in both 1D and 2D arrays, and their hybrid integration with micro-lens array and electronic integrated circuits for optical interconnects. As we explore to achieve more functionality through integration, we also emphasize on the merits of usability of the electrical and optical interfaces of the new components, and producibility and manufacturability aspects of the new technologies.

Beam Properties of Visible Proton-Implanted Photonic Crystal VCSELs

We investigate the optical properties of proton-implanted photonic crystal (PhC) vertical-cavity surface-emitting lasers (VCSELs) emitting in the visible spectrum. The fabricated lasers have a threshold current of 1.3 mA and single-mode output power greater than 1 mW at room temperature. The incorporation of a PhC into the top facet of a proton-implanted VCSEL results in a stable single-fundamental-mode operation with a side-mode suppression ratio larger than 30 dB and a constant beam divergence independent of injection current level or ambient temperature. Using an index-step optical fiber model, we compare the effects of different hole etching depths to variations in output beam divergence. By varying the design and etching depth of the hole pattern, the lasers can either be optimized for low beam divergence or low threshold currents. The controllable refractive index guidance effect from the PhC allows for precise engineering of the optical properties of these visible VCSELs for consumer and imaging applications.

Computed radiography imaging based on high-density 670-nm VCSEL arrays

Although vertical cavity surface emitting lasers (VCSELs) have traditionally found their place in high-speed communication links, the recent advancements of VCSELs emitting in the visible spectrum has sparked interest for new applications in scanning and imaging. Compared to other lasers, VCSELs have many advantageous characteristics including compact size, low power requirements, low cost, and high reliability. VCSELs also offer the unique ability to be fabricated in one- or two-dimensional arrays, making it possible for multiple VCSELs on a single chip to perform the same function as a mechanically scanned beam. One such application is computed radiography (CR), which provides an efficient solution for digitizing and electronically storing x-ray images. In this work, we demonstrate a 1-inch prototype CR scanner based on high-density VCSEL arrays. The device is capable of generating very fast scans with no moving parts, and has the potential to increase throughput, stability, and image quality. In this paper, we discuss the design and performance of this scanner and demonstrate X-ray image acquisition with resolution exceeding 5 line pairs per millimeter (lp/mm).

Progress in extended wavelength VCSEL technology

Vixar has been developing VCSELs at both shorter (680nm) and longer (1850nm) wavelengths. This paper reports on advances in technology at both of these wavelengths. 680nm VCSELs based upon the AlGaAs/AlGaInP materials system were designed and fabricated for high speed operation for plastic optical fiber (POF) based links for industrial, automotive and consumer applications. High speed testing was performed in a “back-to-back” configuration over short lengths of glass fiber, over 42 meters of POF, with and without I.C. drivers and preamps, and over temperature. Performance to 90°C, 10 Gbps and over 40 meters of plastic optical fiber has been demonstrated. Reliability testing has been performed over a range of temperatures and currents. Preliminary results predict a TT1% failure of at least 240,000 hours at 40°C and an average current modulation of 4mA. In addition, the VCSELs survive 1000 hours at 85% humidity 85°C in a non-hermetic package. 1850nm InP based VCSELs are being developed for optical neurostimulation. The goals are to optimize the output power and power conversion efficiency. 7mW of DC output power has been demonstrated at room temperature, as well as a power conversion efficiency of 12%. Devices operate to 85°C. Over 70mW of pulsed power has been achieved from a 35 VCSEL array, with a pulse width of 10μsec.

Record high temperature high output power red VCSELs

Red VCSELs are of interest for medical and industrial sensing, printing, scanning, and consumer electronics applications. This paper will describe the optimization of red VCSEL design to achieve improved output power and a broader temperature range of operation. We will also discuss alternative packaging approaches and in particular will describe non-hermetic packages and performance of the VCSELs in a humid environment. Record output power of 14mW CW and a record maximum temperature of operation of 105°C have been achieved at an emission wavelength of 680nm. The achievement is the result of attention to many details including resonance cavitygain peak offset, material choices, current and mode confinement approaches, and metal aperture design. We have also demonstrated lifetimes >1000 hours for non-hermetic packages in an 85% humidity environment. A chip on board approach has been used to create a large scale linear array of VCSELs for a scanning application.

Development of VCSELs for optical nerve stimulation

Neural stimulation using infrared optical pulses has numerous potential advantages over traditional electrical stimulation, including improved spatial precision and no stimulation artifact. However, realization of optical stimulation in neural prostheses will require a compact and efficient optical source. One attractive candidate is the vertical cavity surface emitting laser. This paper presents the first report of VCSELs developed specifically for neurostimulation applications. The target emission wavelength is 1860 nm, a favorable wavelength for stimulating neural tissues. Continuous wave operation is achieved at room temperature, with maximum output power of 2.9 mW. The maximum lasing temperature observed is 60° C. Further development is underway to achieve power levels necessary to trigger activation thresholds.