There are many potential applications of visible, red (650nm - 690nm) vertical cavity surface emitting lasers (VCSELs)
including high speed (Gb) communications using plastic optical fiber (POF), laser mouse sensors, metrology, position
sensing. Uncertainty regarding the reliability of red VCSELs has long been perceived as the most significant roadblock
to their commercialization. In this paper we will present data on red VCSELs optimized for performance and reliability
that will allow exploitation of this class of VCSEL in a wide range of high volume consumer, communication and
VCSELs operating at ~665nm have been fabricated on 4" GaAs substrates using MOCVD as the growth process and
using standard VCSEL processing technology. The active region is AlGaInP-based and the DBR mirrors are made from
AlGaAs. Threshold currents are typically less than 2mA, the devices operate up to >60C and the light output is polarized
in a stable, linear characteristic over all normal operating conditions. The 3dB modulation bandwidth of the devices is in
excess of 3GHz and we have demonstrated the operation of a transceiver module operating at 1.25Gb/s over both SI-POF
Ageing experiments carried out using a matrix of current and temperature stress conditions allows us to estimate that the
time to failure of 1% of devices (TT1%F) is over 200,000h for reasonable use conditions - making these red VCSELs
ready for commercial exploitation in a variety of consumer-type applications. Experiments using appropriate pulsed
driving conditions have resulted in operation of 665nm VCSELs at a temperature of 85°C whilst still offering powers
useable for eye-safe free space and POF communications.
We have developed fiber optic transceivers (FOT) based on leadframe and plastic molding technology for large core POF and PCS fiber systems. The leadframe style package exhibits a compact small size of 9.7mm x 6.2mm x 3.6mm. For PCS fiber system, the VCSEL based fiber optic transceivers have an operating wavelength at 850nm. The transmitter has an internal control circuit to stabilize the optical output of VCSEL and an integrated coupling optics to provide loose alignment tolerances of ±70μm in lateral axis for a working distance of 500μm to PCS optical fibers. The optical coupling power from the transmitter to the PCS fiber has a variation within 1dB over the wide ambient temperature range from -40oC to +105oC. Eye diagrams of the transceivers at 50Mbps are wide open over the normal operating temperature range from -40oC to +105oC. With a pair of VCSEL transmitter and MSM-PD receiver we achieve 1.25Gb/s transmission over 10m of PCS fiber. In a POF transmission system of 40m, over an operating temperature range from -40oC to +85oC, the fiber output power variation with the green LED transmitter is 1.5dB less than that with a similar red LED transmitter. The eye-diagrams of the green LED transmitter at 20Mbps are captured at back-to-back transmission and after 40m POF transmission.
We have developed ultra small form factor plastic package fiber optic transceivers which are designed for large core fiber system
(PCS-fiber, PF-POF). The VCSEL based fiber optic transmitter exhibits an output power stability within 0.5dB over an operating temperature from -40°C to +105°C. The fiber optic receiver consists of a GaAs MSM-photodetector with a large active area and high bandwidth. Giga bit transmission over a 10m PCS-fiber is demonstrated with the transceivers. The modules are applicable to automotive, consumer electronics, home networking, and datacom applications.
We have developed VCSEL based fiber optics transceivers for PCS fiber systems. The PCS fiber has a core diameter of 200μm. The relatively large diameter enables the usage of low cost optical connectors for the fiber link and provides wide alignment tolerances. The measured lateral and longitudinal 3dB coupling tolerances are ±100μm and 500μm, respectively. The VCSEL is integrated with an electronics driver chip and some passive electronics components on a leadframe structure before plastic encapsulation. The hybrid integration on the leadframe enables batch processing to increase throughput and lower manufacturing cost. No full-hermetic sealing is required for the VCSEL chosen. The variation of optical output power is less than 0.2dB from -40°C to 105°C. Eye diagrams show wide open eyes at a data rate of 500Mbit/s at wide temperature range up to 105°C. The technology can go up to data rates in the Gbit/s range, but this is currently not required for the target applications. The module is reliable over 1000 temperature cycles from -40°C to 125°C. For the receiver side we developed high speed MSM photodetectors. The large area MSM photodetectors relax the coupling alignment tolerance to the core of the optical fiber for 80μm and 4000μm in lateral axis and longitudinal axis, respectively. The MSM photodetector is capable of data rates of 3.2Gb/s. At this high speed the sensitivity is better than -18dBm for the MSM photodetector co-packaged with a suitable transimpedance amplifier (TIA).
Low cost optical components enable new emerging applications for short distance interconnects. We developed novel VCSEL transceiver modules based on electronics-style plastic packages for large core PCS fiber systems. The transmission data rate is 500Mbps over a wide ambient temperature range from -40°C to 105°C. For lower data rates we developed green LED based transmitters for extended reach applications using POF. On the receiver side MSM photodetectors exhibit a high sensitivity of -18dBm at a data rate of 3.2Gbps. The large area MSM receivers are essential components for high speed large core optical fiber links.
Integration of active optical components typically serves five goals: enhanced performance, smaller space, lower power dissipation, higher reliability, and lower cost. We are manufacturing widely tunable laser diodes with an integrated high speed electro absorption modulator for metro and all-optical switching applications. The monolithic integration combines the functions of high power laser light generation, wavelength tuning over the entire C-band, and high speed signal modulation in a single chip. The laser section of the chip contains two sampled grating DBRs with a gain and a phase section between them. The emission wavelength is tuned by current injection into the waveguide layers of the DBR and phase sections. The laser light passes through an integrated optical amplifier before reaching the modulator section on the chip. The amplifier boosts the cw output power of
the laser and provides a convenient way of power leveling. The modulator is based on the Franz-Keldysh effect for a wide band of operation. The common waveguide through all sections minimizes optical coupling losses. The packaging of the monolithically integrated chip is much simpler compared to
a discrete or hybrid solution using a laser chip, an SOA, and an external modulator. Since only one optical fiber coupling is required, the overall packaging cost of the transmitter module is largely reduced. Error free transmission at 2.5Gbit/s over 200km of standard single mode fiber is obtained with less than 1dB of dispersion penalty.
The use of oxide confined VCSELs in datacom applications is demonstrated. The devices exhibit low threshold currents of approximately 3 mA and low electrical series resistance of about 50 (Omega) . The emission wavelength is in the 850 nm range. Life times of the devices are several million hours under normal operating conditions. VCSEL arrays are employed in a high performance parallel optical link called PAROLITM. This optical ink provides 12 parallel channels with a total bandwidth exceeding 12 Gbit/s. The VCSELs optimized for the parallel optical link show excellent threshold current uniformity between channels of < 50 (mu) A. The array life time drops compared to a single device, but is still larger than 1 million hours.
We discuss our measurements on thermal impedance and thermal crosstalk of etched-pillar vertical-cavity lasers and laser arrays. The average thermal conductivity of AlAs-GaAs Bragg reflectors is estimated to be 0.28 W/(cmK) and 0.35W/(cmK) for the transverse and lateral direction, respectively. Lasers with a Au-plated heat spreading layer exhibit a 50% lower thermal impedance compared to standard etched-pillar devices resulting in a significant increase of maximum output power. For an unmounted laser of 64 micrometer diameter we obtain an improvement in output power from 20 mW to 42 mW. The experimental results are compared with a simple analytical model showing the importance of heat sinking for maximizing the output power of vertical-cavity lasers.
We have fabricated wavelength tunable vertical-cavity laser diodes on n-GaAs substrates in 2D arrays by molecular beam epitaxy, proton implantation, and wet chemical etching. Record low threshold currents of 650 (mu) A for continuous wave and 600 (mu) A for pulsed operation are obtained for 8 micrometers active diameter devices. Output power is up to 170 (mu) W cw and 400 (mu) W pulsed. The Bragg reflectors consist of AlAs-GaAs stacks. The active region contains three strained In0.2Ga0.8As quantum wells. The emission wavelength is about 960 nm. Three terminals for each laser diode supply two separate currents for independent control of output power and emission wavelength of individual elements in the 2D array. A record wide continuous wavelength tuning range of 5.6 nm is achieved with tuning currents just below 1 mA. Highly efficient and alignment tolerant coupling to single-mode optical fibers is demonstrated with modified vertical-cavity laser diodes. Quasi planar devices providing two-sided light output have been fabricated with an inverted npn-doping profile. Direct contact butt coupling to flat cut single-mode optical fibers of 4.5 micrometers diameter core results in over 90% coupling efficiency. The lateral alignment tolerances defined by a -3 dB coupling efficiency decrease are as large as 5.6 micrometers . Maximum power in 9 micrometers diameter core fibers is 0.75 mW for a non-heat-sinked device with 8 micrometers alignment tolerances. The easy fiber attachment is applicable to 2D laser arrays and mass production. The large alignment tolerances will improve reliability and drastically reduce packaging costs making vertical-cavity lasers very attractive for short distance optical interconnects.