Absorption and refractive index dynamics in waveguide semiconductor electroabsorbers

This thesis describes the optical characterization of waveguide semiconductor electroabsorbers with an INGaAsP multi-quantum well heterostructure. The investigations have focused on applying the electroabsorbers as electroabsorption modulators or as saturable absorbers. The components have been manufactures with the Danish research council sponsored SCOOP-programme (Semiconductor COmponents for Optical signal Processing) in collaboration with the Danish company GiGA-An Intel Company.

The focus of the SCOOP-programme is to develop new semiconductor components for optical signal processing in telecommunication systems.Both the amplitude and phase transfer functions of electroabsorption modulators as function of reverse bias and wavelength, are measured using a heterodyne detection technique.

With this information, the bias and wavelength dependent áH-parameter is calculated and so is the electroabsorption modulator response to a 10 Gb/s modulation of the bias. It is concluded that operation close to the absorption edge is advantageous both chirp-wise and with respect to lowering the drive voltage.

This however becomes at the expense of a higher insertion loss. A comparison between a component with 10 shallow quantum wells and a component with 5 deep quantum wells shows that the shallow 10 quantum wells component is preferable with respect to chirp, extinction ratio and potentially also the insertion loss.

Calculations of the refractive indes change confirms the measurements and show, that the fabricated electroabsorption modulators can generate high quality pulses for optical fiber transmission. The all-optical wavelength conversion and demultiplexing capabilities of the electroabsorbers, when operated as saturable absorbers, are investigated using femtosecond laser pulses in an amplitude and phase sensitive heterodyne pump-probe experiment.

It is shown that the absorption can be bleached effectively by optical generation of carriers. The absorption recovery is measured as a function of pump pulse energy and reverse bias applied to the component and it is shown that a 10 ps switching window with 9.6 dB of extinction ratio can be realized.

Teh sign of the refractive index change, induced by optical generation of carriers in the active region, is seen to depend both on the optical power and on the reverse bias applied to the saturable absorber. The trends of the observed refractive index dynamics are explained from a combination of band filling and field screening.

It is concluded, that for the right bias and wavelength it is possible to wavelength convert into negatively chirped pulses.