E. Zeeb

Tunable extremely low threshold vertical-cavity laser diodes

Submilliampere-threshold wavelength-tunable three-terminal vertical-cavity laser diodes have been fabricated by proton implantation and wet chemical etching. Laser diodes of 8- mu m active diameter exhibit record low threshold currents of 650 mu A and emit upto 170- mu W output power under CW conditions and 0.4 mW under pulsed excitation. Slightly larger devices of about 12- mu m diameter emit up to 1.85 mW in a single mode under pulsed conditions. The emission wavelength is about 970 nm and matched to the spectral gain of the three active InGaAs quantum wells. Individual elements in a two-dimensional array can be continuously tuned over 2.2 nm by applying separate tuning currents of 70 mu A to the corresponding top mesa electrodes. Tuning and laser currents are controlled independently.<<ETX>>

Butt-coupling efficiency of VCSELs into multimode fibers

We report a detailed study on butt coupling efficiencies of vertical-cavity surface-emitting lasers (VCSELs) to standard graded index multimode silica fibers. Coupling efficiency strongly depends on active laser diameter as well as index guiding and transverse mode spectrum of the laser. For typical active laser diameters of 16-20-/spl mu/m coupling efficiencies of about 90% are obtained with weakly index guided proton-implanted vertical-cavity lasers (VCLs) whereas stronger index guided oxidized lasers show considerably lower coupling efficiencies between 75% and 55%, depending on driving currents.

Optimization of planar Be-doped InGaAs VCSEL's with two-sided output

Threshold current, output power, wall-plug efficiency and operating voltage of epitaxially grown InGaAs-AlGaAs planar vertical-cavity surface-emitting lasers (VCSELs) are strongly influenced by the electrical and optical properties of the p-doped Bragg reflector. Here we study in some detail the dependence of output behavior on the composition, interface grading, and modulation doping of the Be-doped AlGaAs-GaAs Bragg reflector. Using optimized p-doped mirrors VCSELs with low threshold current densities of 300 A/cm/sup 2/, low driving voltages of 1.6 V and high wall-plug efficiencies of 17.6% are obtained. Transverse single-mode emitting devices show a record low emission linewidth of 30 MHz and a linewidth-power product of 2.2 mW/spl middot/MHz.<<ETX>>

Planar proton implanted VCSEL's and fiber-coupled 2-D VCSEL arrays

Efficient planar proton implanted InGaAs-GaAs MQW vertical-cavity surface-emitting laser diodes (VCSELs) and 2-D arrays are fabricated using molecular beam epitaxy and p-type beryllium doping. Using single-step grading and modulation /spl delta/-doping in the p-type AlGaAs-GaAs Bragg reflectors top surface emitting devices with a maximum wall-plug efficiency of 17.6% and a threshold voltage of 1.8 V are demonstrated. Transverse mode behavior is well described in terms of Laguerre-Gaussian functions. Independently addressable 10/spl times/10 arrays of 12 /spl mu/m diameter VCSELs exhibit electrical 3-dB modulation bandwidths up to 6 GHz. Light from the array is simultaneously launched into a 10/spl times/10 multimode fiber bundle with coupling efficiency above 70% applying a simple butt-coupling technique. >

High speed performance of 2-D vertical-cavity laser diode arrays

We have fabricated and tested 10/spl times/10 independently addressable vertical-cavity surface-emitting laser diode arrays. Arrays with 55 /spl mu/m active diameter devices show an average threshold current density of 590 A/cm/sup 2/ and an excellent homogeneity of the output characteristics over the full array size with maximum CW output powers of 12 mW. Broad area laser diodes with active diameters of 75 /spl mu/m reach output powers of 18 mW for CW operation and 180 mW under pulsed conditions. Small-signal modulation bandwidths are beyond 10 and 8 GHz for the 55 and 75 /spl mu/m devices, respectively.<<ETX>>

Linewidth enhancement factor of vertical-cavity surface-emitting laser diodes

The linewidth enhancement factor /spl alpha/ characterizes the performance of semiconductor lasers, such as spectral linewidth broadening for CW operation and wavelength chirping under high-frequency modulation. We have for the first time determined the /spl alpha/ factor of vertical-cavity surface-emitting lasers from the measured linewidth-power product and more precisely from the frequency to amplitude modulation ratio at high-frequency current modulation. Both methods provide the same /spl alpha/ factor of 3.7. The minimum linewidth was as narrow as 70 MHz and the linewidth-power product was 5.4 MHz/spl middot/mW neglecting the residual linewidth.<<ETX>>

Potential barriers and current‐voltage characteristics of p‐doped graded AlAs‐GaAs heterojunctions

Detailed calculations are presented of the potential shape of graded AlAs‐GaAs heterojunctions and of the current‐voltage characteristics across the heterobarriers using a variational wave‐function model. Tunneling currents are taken into account and it is shown that common models based on thermionic emission are not applicable for low temperatures, high applied voltages, and heavily doped semiconductors. The resistivity of the junction is shown to be strongly temperature dependent. The recently reported low resistivities of graded‐gap mirrors in vertical cavity laser diodes can be explained with a simple variational wave‐function model.

Characterization of two-sided output vertical-cavity laser diodes by external optical feedback modulation

We have fabricated 5.6 nm wavelength tunable vertical-cavity laser diodes with record low CW threshold currents of 650 /spl mu/A. Laser output power depends on mirror reflectivities as well as intrinsic loss and effective internal efficiency which gives the conversion of carriers into photons of the lasing mode. To optimize laser mirror design for maximum output power it is important to know the loss and efficiency values. With in-plane lasers these intrinsic device parameters are commonly determined by measuring the threshold currents of equally processed devices of different cavity lengths. Obviously, this is impossible for vertical-cavity lasers even if in-plane lasers are made of epitaxial multilayers grown for vertical-cavity lasers due to different waveguiding mechanism. Therefore we have modulated the effective reflectivity of the vertical-cavity laser Bragg mirrors by external reflection to obtain the intrinsic parameters. We applied the technique for the first time to vertical-cavity laser diodes with two-sided light output but it can be applied to devices with one-sided output as well.<<ETX>>

Vertical-cavity surface-emitting laser diodes for short distance optical fiber networks

We have fabricated wavelength tunable vertical-cavity laser diodes in 2D arrays by molecular beam epitaxy, proton implantation, and wet chemical etching. Record low threshold currents of 650 /spl mu/A for continuous wave and 600 /spl mu/A for pulsed operation are obtained for devices with 8 /spl mu/m active diameter. Output power is up to 170 /spl mu/W CW and 400 /spl mu/W pulsed. The emission is single longitudinal and transversal mode at about 970 nm wavelength. Three terminals for each laser diode supply two separate currents for the independent control of output power and emission wavelength of individual elements in a 2D laser array. A record wide continuous wavelength tuning range of 8.2 nm is achieved in a slightly larger device with tuning currents of just 2.5 mA. Highly efficient and alignment tolerant coupling to single-mode optical fibers is demonstrated with quasi-planar vertical-cavity laser diodes which provide a two-sided light output. Direct contact butt coupling to flat cut single-mode optical fibers of 4.5 /spl mu/m diameter core results in coupling efficiencies over 90%. The lateral alignment tolerances defined by a -3 dB coupling efficiency decrease are as large as 5.6 /spl mu/m. Maximum power coupled into a 9 /spl mu/m diameter core fiber is 0.75 mW for a non-heat sinked laser diode.<<ETX>>

Hybrid integrated VCSEL-fiber module for efficient mode-locking

We have fabricated a simple VCSEL-fiber module for stable short pulse generation. The hybrid integration of amplifier and external cavity sections avoids the use of gratings or Fabry Perot etalons for frequency filtering and eases mechanical stability problems. The compact device generates 22 ps mode-locked pulses at a repetition rate of 821 MHz and launches the pulse sequence with timing jitter below 1 ps into a butt-coupled output fiber.