C.E. Zah

Multiwavelength DFB laser array transmitters for ONTC reconfigurable optical network testbed

We discuss the design, fabrication, and performance of experimental multiwavelength laser array transmitters that have been used in the reconfigurable optical network testbed for the Optical Network Technology Consortium (ONTC). The experimental four-node multiwavelength network testbed is SONET/ATM compatible. It has employed multiwavelength DFB laser arrays with 4 nm wavelength spacing for the first time. The testbed has demonstrated that multiwavelength DFB laser arrays are indeed practical and reproducible. For the DFB laser arrays used in such a network the precise wavelength spacing in the array and the absolute wavelength control are the most challenging tasks. We have obtained wavelength accuracy better than /spl plusmn/0.35 nm for all lasers, with some registered to better than /spl plusmn/0.2 nm. We have also studied the array yield of our devices and used wavelength redundancy to improve the array yield. Coupling efficiencies between -2.1 to -4.5 dB for each wavelength channel have been obtained. It is achieved by using specially designed lensed fiber arrays placed on a silicon V-grooved substrate to exactly match the laser spacing. The transmitter consisted of a multichip module containing a DFB laser array, an eight-channel driver array based on GaAs ICs, and associated RF circuitry.

Nearly ideal InP/In0.53Ga0.47As heterojunction regrowth on chemically prepared In0.53Ga0.47As surfaces

When III‐V growth is interrupted for processing, in the ambient laboratory environment for example, regrown heterojunction quality has been rather disappointing in comparison to uninterrupted epitaxial growth. We have conducted a search for surface chemical preparations on In0.53Ga0.47As which would produce the highest‐quality InP/In0.53Ga0.47As regrown heterojunction interface, as measured by surface recombination velocity (SRV). After an extensive survey, we have found that dilute bromine‐based etching solutions are best for preparing a free In0.53Ga0.47As surface for subsequent InP regrowth. The resulting InP/In0.53Ga0.47As interfacial SRV is ≲20 cm/s, comparable to heterojunctions grown without any interruption at all.

Measurement of nonradiative Auger and radiative recombination rates in strained‐layer quantum‐well systems

To understand the feasibility of using strain to reduce the nonradiative Auger recombination rate of 1.5 μm semiconductor lasers, we have directly measured the radiative and nonradiative Auger recombination rates in strained‐layer InGaAs/AlGaInAs quantum‐well systems, using time‐resolved photoluminescence measurements. We find that the Auger recombination rate can be reduced in either biaxial compressively or tensilely strained quantum‐well structures. A longer radiative carrier lifetime is observed for the tensile‐strained materials. The effect of strain and quantum confinement on the carrier lifetime is discussed.

Very low threshold current density 1.5 μm GaInAs/AlGaInAs graded‐index separate‐confinement‐heterostructure strained quantum well laser diodes grown by organometallic chemical vapor deposition

Very low threshold current densities of 200 and 400 A/cm2 were obtained in 1.5 μm GaInAs/AlGaInAs tensile and compressive strained‐layer quantum well laser diodes (SL‐QW LDs), grown by organometallic chemical vapor deposition, with continuously graded‐index separate‐confinement‐heterostructure. The differential quantum efficiency of SL‐QW LDs showed less sensitive to temperature in contrast to that of a lattice matched QW LD. This is attributed to the decrease of intervalence band absorption due to the strain‐induced reduction in the valence band density of state. The polarization of output power for a tensile SL‐QW LD showed transverse magnetic (TM) mode, while that for a lattice matched and a compressive SL‐QW LDs showed transverse electric (TE) mode.

Analysis of origin of nonlinear gain in 1.5 μm semiconductor active layers by highly nondegenerate four‐wave mixing

The origin of the nonlinear gain effect in 1.5 μm semiconductor active layers is investigated by using highly nondegenerate four‐wave mixing, where the pump‐probe detuning is extended up to 2 THz. From the signal intensity measured as a function of the detuning frequency we find that both the spectral hole burning and the dynamic carrier heating contribute to the four‐wave mixing. The dynamic carrier heating, however, creates the index grating rather than the gain grating, and hence, the spectral hole burning is the main origin of the nonlinear gain effect.

Gain compression in tensile‐strained 1.55 μm quantum well lasers operating at first and second quantized states

Gain compression coefficients in tensile‐strained 1.55 μm single quantum well lasers are measured using an optical injection method. Lasers operating in the first and second quantized states are used. An explicit linear dependence of nonlinear gain on the differential gain is obtained from these measurements. These results are quantitatively compared to a recently proposed model involving carrier transport in and out of the quantum well.

Orientation‐dependent doping in organometallic chemical vapor deposition on nonplanar InP substrates: Application to double‐heterostructure lasers and lateral p‐n junction arrays

In this letter we show that the orientation dependence of dopant incorporation can be used to obtain lateral patterning of doping by growing on nonplanar substrates. Specifically, organometallic chemical vapor deposition has been used to obtain lateral p‐n junction arrays and selective deposition of alternating p‐n layers of InP. The latter technique has been used to grow double‐heterostructure lasers with current confinement layers in a single step.

Low‐threshold (≤ 92 A/cm2) 1.6 μm strained‐layer single quantum well laser diodes optically pumped by a 0.8 μm laser diode

To explore the ultimate threshold current limit in long‐wavelength semiconductor lasers, InxGa1−xAs/InP strained‐layer single quantum well laser diodes were studied for the first time by optically pumping with a 0.8 μm laser diode. Low‐threshold (≤92 A/cm2) cw operation was obtained and the lasing wavelength (1.62 μm) corresponding to the transition from the first quantization state of a 25 A In0.8Ga0.2As well was observed. By taking the carrier collection efficiency (≤77%) into account, the actual threshold current density could be as low as 70 A/cm2.

All-optical image transmission through a single-mode fiber.

All-optical transmission of two-dimensional images through a single single-mode fiber with a two-dimensional multicolor surface-emitting laser array and a volume hologram is described. A preliminary experimental result to prove the concept of the idea is demonstrated by transmitting a simple 2 x 2 image over a distance of 1 mile (1.61 km).

Effect of operating electric power on the dynamic behavior of quantum well vertical‐cavity surface‐emitting lasers

We investigated the effect of high operating voltage and series resistance on the dynamic behavior of strained InGaAs/GaAs quantum well vertical‐cavity surface‐emitting lasers (VCSELs). A large wavelength chirp in the lasing spectrum is observed for the lasers with high voltage/resistance even under low‐duty‐cycle pulse operation due to resistive heating close to the laser junction. Using an optimized laser design, VCSELs with 2.6 V threshold voltage and 40 Ω resistance are achieved. We believe this is the lowest threshold voltage and resistance reported to date for a 20 μm VCSEL with as‐grown mirrors. The wavelength chirp is reduced by nearly two orders of magnitude for these improved lasers.