FASL for WDM applications

 

Frequency Agile Semiconductor Lasers
for WDM applications

Tin Cheng, Ju Bin Song, Cyril Renaud

Support

Agilent, Corning, Intense Photonics and EPSRC

 

Partners

Bristol University, Glasgow University, Imperial College London, Southampton University, Strathclyde University

 

Description

 

This project forms part under the EPSRC funded project entitled as “Physical Layer High Speed Optoelectronics for Tomorrow’s Optical Networks” (PHOTON). The PHOTON project involves collaboration of six Universities including UCL. It is strongly supported by leading industrial companies. Its aim is to develop a suitable technology for producing ultra-compact uncooled transceivers operating at bit rates of over 100 Gbit/s in the short to medium term and in excess of 1Tb/s in the longer term.

 

Our Laboratory is particularly involved in the development of tunable semiconductor lasers for applications in Wavelength Division Multiplexing (WDM) systems.

 

Developments of tunable sources based on the carrier injection effect (CIE) have been difficult because of problems associated with

(a) complex, multiple terminal control requirements to select the required emission frequency,

(b) irreproducible tuning behaviour even from adjacent devices on the same chip,

(c) concern over ageing drift in characterised devices.

 

CIE produces irreproducible tuning behaviour since with increasing current a red-shift response is present due to thermal effects, as well as a blue-shift response due to plasma and band-filling effects. The final response is highly non-uniform with modulation frequency and dependent on device mounting details.

 

 

Preliminary Results

 

At UCL, we have investigated the use of quantum confined Stark effect (QCSE) to change the refractive index required as an alternative tuning mechanism. We have successfully fabricated a two-section QCSE tuned GaAs/AlGaAs lasers with very wide and uniform tuning response (<±3dB between 30kHz–6GHz). The tuning response is found to be independent of laser bias current, and therefore output power, and is reproducible from laser to laser.

 

 

Future Work

 

Our aim is to extend our previous work on the GaAs/AlGaAs system and to develop tunable lasers based on the InP/InGaAsP material system for operation in the 1.55 µm range.

 

 

Figure 1 shows a schematic diagram of the tunable DBR laser structure for 1.55µm operation. Initial estimates show that tuning range of a few nm is possible with a wavelength offset of 70nm between zero bias e1-hh1 exciton peaks in the active and tuning regions and a maximum applied tuning voltage of 3V. For larger tuning ranges required, we will use two approaches.

 

Firstly, CIE is used in the Bragg section to give coarse wavelength selection, whilst QCSE is used in the phase section for chirp compensation and rapid fine tuning.

 

Secondly, the use of sample grating technique in conjunction with QCSE tuning will be investigated. Estimates indicate that tuning ranges of up to 30nm should be possible. In order implement the desired device, we will use the quantum well intermixing technique in collaboration with the University of Glasgow. We will fabricate the device based on the air-bridged ridge guide approach developed at UCL to give low parasitic capacitance lasers capable of picosecond tuning response. The ultimate aim is to realize reproducible tunable lasers capable of chirp compensated operation, with tuning ranges up to 30nm and tuning speeds <100ps.