ATLAS

 

All-optical wavelength conversion using multiple quantum well saturable absorbers

 

E.P. Burr, M. I. Pantouvaki and A.J. Seeds

 

Project Framework

 

This work is carried out within the ATLAS Project of the EU IST program. The ATLAS project investigates high capacity wavelength division multiplexed (WDM) transmission of distances over 500km to 1000km. Wavelength routeing using all-optical wavelength converters (AOWCs) is considered as a means of enhancing the flexibility of the WDM network.

 

 

Description of work

 

UCL's role within the ATLAS project is the study and development of a novel AOWC, based on optical switching in multiple quantum well (MQW) devices, suitable for deployment within WDM network with 40Gb/s channel capacity. The AOWC is an all-optical switch, controlled by a data stream at the input wavelength, which modulates a CW signal at the output wavelength. Ultra-fast, high contrast switching is required for successful operation.

 

Our work has focused on devices fabricated from InGaAsP quaternary-quaternary structures grown by MOVPE at the EPSRC III-V Semiconductor Central Facility at the University of Sheffield. Optical switching based on several non-linear mechanisms, including electric field screening in biasable p-i-n devices and, latterly, excitonic absorption bleaching (EAB) has been considered. We have demonstrated that, by incorporating an MQW which exhibits a moderate absorption change due to EAB in an anti-resonant Fabry-Perot (AFP) cavity, we can produce an all-optical switch with 10 dB contrast ratio.

 

The speed of such a switch is governed by the lifetime of photo-generated free carriers in the quantum wells, which is typically a few nanoseconds in MOVPE-grown III-V MQWs, considerable longer than the picosecond switching times required for 40Gb/s operation.

 

We have used pump-probe measurements to show that the free carrier lifetime can be reduced to 5ps in MQWs implanted with 4MeV nitrogen ions. Ion-implantation was carried out at the University of Surrey Ion Beam Centre. During their passage through the MQW layer such ions damage the lattice structure resulting in the creation of energy levels within the 'forbidden' gap between the conduction and valence bands. These energy levels allow rapid recombination of free carriers.

 

 

Results

 

The experimental result below shows the output signal in an initial demonstration of wavelength conversion using an ion-implanted AFP cavity AOWC. A CW signal at 1534nm has been switched by a 2ps pulses at 1530nm; the plot shows the signal at the output wavelength, as observed using a sampling oscilloscope. A 9dB contrast ratio is apparent, despite broadening of the observed pulse due to the 20GHz bandwidth of the oscilloscope used.

 

Future Work

 

Development of this AOWC is continuing, with the aim of demonstrating wavelength conversion in the transmission test-bed which being developed by other partners in the ATLAS project.