James A. Lott

99 fJ/(bit

We present extremely energy-efficient oxide-confined 850-nm single-mode vertical-cavity surface-emitting lasers (VCSELs) for optical interconnects. Error-free transmission at 17 Gb/s across 1 km of multimode optical fiber is achieved with an ultra-low energy-to-data ratio of 99 fJ/bit, corresponding to a record-low energy-to-data-distance ratio of 99 fJ/(bit ·km). This performance is achieved without changing any of the driving parameters up to 55 °C. To date our VCSELs are the most energy-efficient directly modulated light-sources for data transmission across all distances up to 1 km of multimode optical fiber.

\cdot

Presents a semi-analytic full-vector method for calculating the spatial profile, optical confinement factor resonant frequency, absorption loss, and mirror loss of lasing modes in cylindrically symmetric microcavity vertical-cavity surface-emitting lasers (VCSELs). It can be shown that this method gives the best separable approximation for the electric and magnetic vector potentials. Our technique can model the entire VCSEL structure and can treat complex media. We apply the method to etched-post and oxide-apertured VCSELs designed for 980-nm emission and find a blueshift in cavity resonance as the cavity radius shrinks. We also find a minimum optical cavity radius below which radially bound lasing modes cannot be supported. This radius depends on the device geometry and lies between 0.5 and 1 /spl mu/m for the devices studied. Once this model is augmented to include diffraction losses-the dominant loss mechanism for conventional small aperture lasers-it will provide a complete picture of lasing eigenmodes in microcavity VCSELs.

km) Energy to Data-Distance Ratio at 17 Gb/s Across 1 km of Multimode Optical Fiber With 850-nm Single-Mode VCSELs

The design, fabrication, and performance of the presently most energy-efficient oxide-confined 850 nm vertical-cavity surface-emitting lasers (VCSELs) for optical interconnects are presented. We employ a novel current spreading layer to reduce differential resistance. Compared to our previous designs, a higher indium content is used in the InGaAs quantum wells to increase the differential gain at low injected current densities. The influence of the oxide aperture diameter on the energy efficiency is determined by comparing the key performance parameters for a batch of VCSELs produced on the same epitaxial wafer, but with varying aperture diameters from 2.5 to 9.0 μm. The static light output power-current-voltage characteristics, small-signal modulation response, and large signal performance of our VCSELs are investigated in detail. The parameters important for energy-efficient operation are analyzed including threshold current, differential quantum efficiency, and differential resistance. We observe that our single-mode VCSELs are more energy efficient than our multimode VCSELs, although our multimode VCSELs typically exhibit a larger maximum static wallplug efficiency. Error-free (defined as a bit error ratio <;1 × 10 -12) data transmission at 25 Gb/s with a record-low dissipated heat energy of only 56 fJ/bit is achieved using a single-mode VCSEL with an oxide aperture diameter of 3.5 μm.

Analysis of microcavity VCSEL lasing modes using a full-vector weighted index method

New oxide-confined 980-nm vertical-cavity surface-emitting lasers (VCSELs) with record temperature-stable small-signal bandwidths of 25.6 to 23.0 GHz at 25 to 85 °C are designed, fabricated, and characterized. Technology-based device parameters essential for system-level models of VCSEL-based short-reach and ultrashort-reach optical interconnects are extracted. These parameters include key intrinsic figures-of-merit, including the -3-dB modulation bandwidth, the bandwidth-to-electrical power ratio, and device input impedance, all as functions of temperature, oxide-aperture diameter, and desired range of bias current or current density. Further, the M-factor, relating the intrinsic VCSEL bandwidth to the error-free bit rate for a given external systems configuration and application, is introduced. Our present 980-nm VCSEL technology is capable of 40 Gb/s operation at 85 °C at a simultaneously low current density of 10 kA/cm2 with an energy of only 100 fJ per bit.

Energy Efficiency of Directly Modulated Oxide-Confined High Bit Rate 850-nm VCSELs for Optical Interconnects

With the separation of optical and current apertures, photonic crystal vertical-cavity surface-emitting lasers can reach a 3-dB small-signal modulation bandwidth of > 18 GHz while lasing in the fundamental mode. Because of reduced chromatic dispersion, such devices enable error-free transmission over 1-km OM4 multimode fiber at a data rate of 25 Gb/s and operating at a current density of 5.4 kA/cm2. This can potentially lead to a laser source that is useful for rack-to-rack transmissions in large data centers and potentially long device lifetime.

Impact of the Oxide-Aperture Diameter on the Energy Efficiency, Bandwidth, and Temperature Stability of 980-nm VCSELs

Highly temperature stable, high bit rate oxide-confined vertical-cavity surface-emitting lasers (VCSELs) emitting at 980 nm are presented. Error-free data transmission at 38 Gb/s at 25 °C, 45 °C, 65 °C, and 85 °C is achieved without any change of working point and modulation condition. Static and high-speed properties are analyzed experimentally and theoretically. We numerically investigate the temperature dependence of the differential gain of our quantum well (QW) active region design to explain why a -15-nm QW gain-to-etalon wavelength offset facilitates our 980-nm VCSELs to show simultaneously high bit rate, temperature stability, and energy efficiency. Our VCSELs operate error-free at 42 and 38 Gb/s at 25 °C and 85 °C, respectively, with very low power consumption. Record low 175 fJ of dissipated heat per bit is achieved for 35-Gb/s error-free transmission at room temperature and 177 fJ/bit for 38-Gb/s error-free transmission at 85 °C. Such VCSELs are especially well suited for very-short-reach (<;1 m) optical interconnects in high-performance computers and board-to-board and chip-to-chip integrated photonics.

Error-Free Transmission Over 1-km OM4 Multimode Fiber at 25 Gb/s Using a Single Mode Photonic Crystal Vertical-Cavity Surface-Emitting Laser

Principles of energy-efficient high speed operation of oxide-confined VCSELs are presented. Trade-offs between oxideaperture diameter, current-density, and energy consumption per bit are demonstrated and discussed. Record energyefficient error-free data transmission up to 40 Gb/s, across up to 1000 m of multimode optical fiber and at up to 85 °C is reviewed.

Impact of the Quantum Well Gain-to-Cavity Etalon Wavelength Offset on the High Temperature Performance of High Bit Rate 980-nm VCSELs

Multimode 850 nm vertical cavity surface-emitting lasers (VCSELs) suitable for high bit rate operation are studied. VCSELs with oxide aperture diameters of 5–7 µm show a high −3 dB modulation bandwidth (~20 GHz) and D-factor (~ 8 GHz mA−1/2). To allow low capacitance a multiple layer oxide-confined aperture design was applied. Eye diagrams are clearly open up to 35 Gbit s−1 at the temperature of 25 °C. Using 35 µm diameter PIN photodiodes and 6 µm oxide aperture diameter VCSELs error-free 25 Gbit s−1 (defined as a bit error ration of ≤1 × 10−12) optical fiber communication links were tested over 100 m of standard OM3 multimode optical fibers at 25 °C and 85 °C. The received optical power for error-free operation was below −4 dBm at both temperatures. A VCSEL reliability study at 95 °C was performed at the high current densities (~18 kA cm−2) needed for error-free 25 Gbit s−1 operation at elevated temperatures. After 6000 h a slight increase (less than 5%) of the output optical power at a constant current was observed and most likely due to an ohmic contact burn in effect within the first 2000 h of the study. The results clearly indicate that 25 Gbit s−1 850 nm oxide-confined VCSELs with a complex AlGaO multilayer aperture design and with step-graded Al-compositions have the potential for reliable operation.

Energy-efficient oxide-confined high-speed VCSELs for optical interconnects

We report a new full vector finite element model for analyzing the optical properties of azimuthally symmetric oxide-apertured vertical-cavity surface emitting lasers (VCSELs). Our model allows for quasi-exact calculation of the lasing mode blueshift, threshold gain, and field profile. Through a detailed analysis of a sample VCSEL, we ascertain the physical effects which determine diffractive or parasitic mode loss. They are: 1) the background density of parasitic modes and 2) the coupling strength between the lasing mode and the parasitic mode continuum. The coupling strength is in turn determined by the relative alignment between the lasing and parasitic mode propagation vectors and the lasing mode penetration into the oxide region. This analysis improves our understanding of the optical physics of apertured VCSELs and should enable the next leap down in lasing threshold.

Reliability performance of 25 Gbit s−1 850 nm vertical-cavity surface-emitting lasers

Highly energy-efficient oxide-confined 850-nm singlemode vertical-cavity surface-emitting lasers (VCSELs) for optical interconnects are presented. Error-free (defined as a bit error ratio <; 1 × 10-12) data transmission at 17 and 25 Gb/s across 100 m of multimode optical fiber is achieved with a low dissipated heat energy of only 69 and 99 fJ/bit, respectively. At 17 and 25 Gb/s, the transmission distance is increased to 1000 and 600 m, respectively. To date, our VCSELs are the most energy-efficient directly modulated lasers for data transmission across distances up to 1 km of multimode optical fiber.