M. Wegener

Ultrafast gain dynamics in 1.5 μm multiple quantum well optical amplifiers

Following gain saturation by a short pulse, the gain recovery process in multiple quantum well optical amplifiers includes contributions from carriers occupying states with energies above the depleted states within the well, from carriers stored in the barrier layers, which serve as carrier reservoirs, and from conventional Auger recombination. Gain recovery caused by carriers from the reservoirs is rapid (7 ps) and can be made to dominate the slower (Auger) recovery process.

Carrier capture times in 1.5 μm multiple quantum well optical amplifiers

The carrier capture times in multiple quantum well semiconductor amplifiers of different structures are studied under high plasma density conditions. Fast (<1 ps), slow (≳150 ps), and intermediate time constants (2–7 ps) are identified in InGaAs quantum well structures. The intermediate time constant is attributed to carrier diffusion in the cladding layers and identified as the carrier capture time. Short capture times can be achieved by proper design of the device structure.

Large refractive index changes in tunable-electron-density InGaAs/InAlAs quantum wells

Measurements of electrorefraction and electroabsorption in a multiple-quantum-well waveguide structure in which each InGaAs quantum well is provided with an individual electron reservoir are presented. External bias transfers electrons into the wells, thus quenching the absorption and producing a refractive index change at wavelengths below the bandedge which is linear in the applied voltage. It is shown that in this type of structure both the change in refractive index per applied field and the ratio of optical phase to intensity modulation can be significantly enhanced over those found in the quantum confined Stark effect.<<ETX>>

Electroabsorption and refraction by electron transfer in asymmetric modulation‐doped multiple quantum well structures

We present a novel heterostructure that exhibits large electroabsorption and refraction. The structure is periodic with a stackable building block, thus it allows large contrast and waveguide operation. The mechanism used is the quenching of absorption produced by transfer of electrons from a reservoir into a quantum well. We demonstrate the principle by presenting differential absorption and refraction spectra on a ten‐period device.

Optical waveguide intensity modulators based on a tunable electron density multiple quantum well structure

With a recently developed semiconductor heterostructure it has become possible to tune continuously the electron density in multiple quantum wells. Here we demonstrate the first electro‐optic waveguide intensity modulators based on this concept. We achieve a 22 dB on/off ratio for 9 V applied at 1.54 μm wavelength in a rib waveguide electroabsorption modulator. Electrorefractive devices include a waveguide Mach–Zehnder interferometer with an active length 650 μm operating at 1.58 μm wavelength with 5.4 V half‐wave voltage. We show that the operating voltage can be further reduced by operating the Mach–Zehnder modulators in push‐pull configuration.

Multi‐gigahertz‐bandwidth intensity modulators using tunable‐electron‐density multiple quantum well waveguides

We report the first measurement of modulation bandwidth in electron transfer quantum well modulators. A device with 1 pF capacitance provides ≳10 dB optical modulation depth at 1.537 μm wavelength with a 3 dB electrical bandwidth of 5.7 GHz. Optical pump‐probe measurements indicate that the fundamental response time is determined by the voltage‐dependent speed of carrier escape from the well.

InGaAs-InAlAs quantum well intersecting waveguide switch operating at 1.55 mu m

The first electrooptic waveguide switch based on voltage-controlled transfer of electrons into multiple quantum wells is demonstrated. This mechanism is named barrier, reservoir, and quantum well electron transfer (BRAQWET) In GaAs-InAlAs quantum wells it provides a large modulation of refractive index at 1.55 mu m with multigigahertz switching capability. The BRAQWET X-switch is free from heating and speed limitations associated with current injection. Beam propagation method (BPM) calculations suggest that the crosstalk performance of this preliminary device demonstration can be significantly enhanced by changes in waveguide geometry.<<ETX>>

Gain and gain saturation spectra in 1.5 μm multiple quantum well optical amplifiers

We describe the wavelength dependence of small‐signal picosecond pulse energy gain and cw power gain, as well as saturation energy and saturation output power spectra, in 1.5 μm multiple quantum well optical amplifiers of different lengths and under various drive conditions. The present devices have gain spectra with bandwidths (3 dB) that can exceed 1000 A. Saturation output energies and powers increase with wavelength and are as large as 5 pJ and ∼40 mW, respectively.

Novel modulator structure permitting synchronous band filling of multiple quantum wells and extremely large phase shifts

The authors have demonstrated synchronous band filling of multiple quantum wells using a novel blockaded reservoir and quantum-well electron transfer structure (BRAQWETS). The resulting electroabsorptive response per quantum well is not only stronger than that produced by the quantum confined Stark effect (QCSE) but is also free of induced absorption below the bandgap. The measured electrorefractive response shows that the maximum phase shift that can be achieved in one absorption length is one order of magnitude larger than what is possible with QCSE. Furthermore, the dependence of induced refractive index change on the applied voltage is essentially linear. Design criteria for very-high-performance modulators with picosecond intrinsic speed are also discussed. It is concluded that BRAQWETS provides a novel basis for high-performance electroabsorption modulators as well as low-loss phase modulators and interferometric amplitude modulators and switches. With suitable engineering, it will also be possible to construct multi-quantum-well lasers and detectors using BRAQWETS.<<ETX>>

High quality GaInAs/AlGaInAs/AlInAs heterostructures on Si ion implanted semi-insulating InP substrates for novel high performance optical modulators

The wide range of conduction-band discontinuity available in the GaInAs/AlGaInAs/AlInAs material system has enabled us to demonstrate high sensitivity, high speed optical modulators and switches based on the band filling effect in the novel blockaded reservoir and quantum well electron transfer structure (BRAQWETS). As phase modulators, these devices also offer linear response and very low attendant absorption modulation. High quality samples have been obtained on Si+ implanted InP:Fe substrates which will permit predefinition of conduction patterns for optoelectronic integration. Experimental results and design principles are presented.