Dieter Wiedenmann

Design and analysis of single-mode oxidized VCSELs for high-speed optical interconnects

Oxide-confined vertical-cavity surface-emitting laser diodes (VCSELs) are fabricated for applications in high-performance optical interconnects. Both 980-nm as well as 850-nm wavelength devices in one- and two-dimensional arrays are investigated. Noise properties of single- and multimode devices under different operation conditions are relative intensity noise of single-mode devices can be as low as -150 dB/Hz at output powers of about 1 mW and feedback levels up to -30 dB. Data rates up to 12.5 Gb/s with bit error rates below 10/sup -11/ are achieved with VCSELs showing stable single-mode emission at large-signal modulation, combined with modulation bandwidths exceeding 10 GHz. Arrays with 4/spl times/8 elements flip-chip mounted on Si CMOS driver chips ready for use in parallel data transmission systems are presented.

Noise characteristics of 850 nm single-mode vertical cavity surface emitting lasers

We have measured the noise characteristics of single-mode oxide confined surface emitting vertical cavity laser diodes. For pumping levels of more than 3.5 times threshold current, the relative intensity noise is below −150 dB/Hz up to 5 GHz at output powers near 1 mW. For various frequencies, we observe sub-Poissonian noise.

High-bit-rate data transmission with short-wavelength oxidized VCSELs, toward bias-free operation

Efficient oxide-confined vertical-cavity surface-emitting laser diodes are investigated as light sources for optical interconnection. Data rates of 2.5 Gb/s with bit-error rates (BERs) smaller than 10/sup -11/ are routinely achieved with 980-nm as well as 850-nm short-wavelength devices for transmission over several hundred meters of graded index multimode fiber. A bias-free modulation scheme is applied to vertical-cavity lasers for the first time and 1-Gb/s data links are established in both wavelength regimes. Successful demonstration of data transmission with rates up to 10 Gb/s with proton-implanted devices is briefly reviewed.

High volume production of single-mode VCSELs

Up to now applications for singlemode VCSELs were in low volume and high prized applications like tunable diode laser absorption spectroscopy (TDLAS, [1,2]) or optical interferometers. Typical volumes for these applications are in the range of thousands of pcs per year, with pricing levels of several 100 USD/pcs. New applications for singlemode VCSELs in consumer markets require manufacturing in very high volumes and at very low cost. Examples are laser-based optical mouse sensors, optical encoders, and rubidium atomic clocks for GPS systems [3,4]. U-L-M photonics presents manufacturing aspects, device performance and reliability data for these devices. The first part of the paper is dealing with high volume manufacturing of 850 nm singlemode VCSEL chips with very high efficiency and low operation current. Special processing technologies have been developed to achieve yields on 3 inch wafers of more than 90%. Wafer qualification procedures are discussed as well. The second part of the paper covers high volume packaging in TO and SMT type packages where very high packaging yields must be achieved. In the last part of the paper reliability issues are discussed, focused on the very high susceptibility of these devices to electrostatic discharge.

Volume production of polarization controlled single-mode VCSELs

Over the past 3 years laser based tracking systems for optical PC mice have outnumbered the traditional VCSEL market datacom by far. Whereas VCSEL for datacom in the 850 nm regime emit in multipe transverse modes, all laser based tracking systems demand for single-mode operation which require advanced manufacturing technology. Next generation tracking systems even require single-polarization characteristics in order to avoid unwanted movement of the pointer due to polarization flips. High volume manufacturing and optimized production methods are crucial for achieving the addressed technical and commercial targets of this consumer market. The resulting ideal laser source which emits single-mode and single-polarization at low cost is also a promising platform for further applications like tuneable diode laser absorption spectroscopy (TDLAS) or miniature atomic clocks when adapted to the according wavelengths.

GaAs VCSEL's at /spl lambda/=780 and 835 nm for short-distance 2.5-Gb/s plastic optical fiber data links

Selectively oxidized GaAs vertical-cavity surface-emitting lasers for /spl lambda/=780- and 835-nm emission wavelength and 120-/spl mu/m-core diameter step index plastic optical fiber (POF) are investigated for short distance interconnects. 2.5-Gb/s pseudorandom data transmission over up to 2.5 m of plastic fiber is demonstrated with a bit-error rate (BER) of better than 10/sup -11/. Furthermore, bias-free data transmission at 2.5 Gb/s over 1-m fiber length again at a BER of better than 10/sup -11/ is reported.

Biased and bias-free multi-Gb/s data links using GaAs VCSELs and 1300-nm single-mode fiber

Selectively oxidized single-mode GaAs vertical-cavity surface-emitting lasers are investigated for biased 3-Gb/s and bias-free 1-Gb/s data links. Bit-error rates of better than 10/sup -11/ for pseudorandom data transmission over 4.3 km of standard 1300-nm single-mode fiber are demonstrated. A simple mode filter is used to suppress intermodal dispersion. The requirements of the gigabit ethernet are fulfilled even for bias-free operation.

Proton implanted VCSEL's for 3 Gb/s data links

Transverse single-mode and multimode intensity modulated butt-coupled InGaAs vertical cavity surface emitting lasers (VCSEL)s are investigated as a light source for optical fiber communication systems. Data transmission at 3 Gb/s with a bit error rate (BER) of less than 10/sup -11/ is reported for both 4.3 km of standard fiber, as well as 0.5 km of multimode graded-index fiber, 10-/spl mu/m active diameter single-mode VCSELs are shown to have lower mode competition noise requiring 3 dB and 6 dB less power at the front end receiver at a BER of 10/sup -11/ compared to 19-/spl mu/m and 50-/spl mu/m active diameter devices, respectively. In data transmission with multimode VCSELs, the dispersion penalty is lower than for single-mode sources since the noise at the receiver is mainly determined by transmitter-mode competition noise.<<ETX>>

Commercial VCSELs reach 0.1-W cw output power

Following the success in fiber based DataCom, VCSELs start to conquer additional market shares in a variety of other applications like free space optics (FSO), lighting, printing, and sensing. U-L-M photonics presents a new family of commercial high power VCSELs emitting powers of up to 50 mW cw at RT based on top-emitting technology. The devices are available at 850 nm emission wavelength. All devices can be operated passively cooled and provide modulation bandwidths of up to 1 GHz. Wallplug efficiencies are in excess of 25 %. Even higher output power of 250 mW cw from a 80 μm active diameter bottom-emitting VCSEL operating at 980 nm has already been obtained although just beeing passively cooled. Further power up scaling is achieved by arrangement of multiple VCSELs in 2D arrays. For the first time we demonstrate cw output power of 10 Watt cw at RT from compact monolithic VCSEL module of 14 mm2 chip area. Transfer of the technology to other wavelengths, e.g. 808 nm and 945 nm, is presented, too, and shows perspectives towards homogeneous optical pumping of solid state lasers. Almost identical device performance levels can be presented for the entire wavelength span. All discussed results are based on highest quality epitaxy optimized for maximum intrinsic efficiency and differential slope efficiency. Oxide confinement is used for current constriction that provides most efficient electrical pumping of the active area. In combination with advanced mounting techniques all mentioned aspects sum up to allow for cost effective VCSEL products in the medium and high power laser regime. The circular output beam in addition to simple heat sinking offers attractive solutions for advanced system integration.

Fabrication and performance of tuneable single-mode VCSELs emitting in the 750- to 1000-nm range

The growing demand on low cost high spectral purity laser sources at specific wavelengths for applications like tuneable diode laser absorption spectroscopy (TDLAS) and optical pumping of atomic clocks can be met by sophisticated single-mode VCSELs in the 760 to 980 nm wavelength range. Equipped with micro thermo electrical cooler (TEC) and thermistor inside a small standard TO46 package, the resulting wavelength tuning range is larger than +/- 2.5 nm. U-L-M photonics presents manufacturing aspects, device performance and reliability data on tuneable single-mode VCSELs at 760, 780, 794, 852, and 948 nm lately introduced to the market. According applications are O2 sensing, Rb pumping, Cs pumping, and moisture sensing, respectively. The first part of the paper dealing with manufacturing aspects focuses on control of resonance wavelength during epitaxial growth and process control during selective oxidation for current confinement. Acceptable resonance wavelength tolerance is as small as +/- 1nm and typical aperture size of oxide confined single-mode VCSELs is 3 μm with only few hundred nm tolerance. Both of these major production steps significantly contribute to yield on wafer values. Key performance data for the presented single-mode VCSELs are: >0.5 mW of optical output power, >30 dB side mode suppression ratio, and extrapolated 10E7 h MTTF at room temperature based on several millions of real test hours. Finally, appropriate fiber coupling solutions will be presented and discussed.