Monolithic high-power tunable laser at ECOC

Bookham Technology will present a paper at next week's ECOC 02 describing a novel monolithic high-power tunable laser.

Bookham Technology will present a paper at next week's ECOC 02 describing a novel monolithic high-power tunable laser that sets a new benchmark for power uniformity and side-mode suppression ratio (SMSR) for C-band DWDM optical communications.

A breakthrough new tuning mechanism allows each of 80 optical channels to have an SMSR in the region of 50dB, while maintaining power uniformity within 3dB over the full tuning range.

"This is not just a new laser device, it is an entirely new tuning mechanism, and it solves several problems in DWDM systems", says Giacinto Busico, Senior Engineer, Bookham Technology, and lead author of the paper.

"Suppressing side modes is crucial in DWDM because they are a source of cross-channel noise, and high-power lasers make the problem worse in long-haul DWDM systems.

Until now it has been very difficult to combine a broad tuning range with both high power and a good SMSR".

Bookham's new design is a monolithic InP-based distributed Bragg reflector (DBR) laser with four main sections.

The innovative front and rear grating sections are the key to the new tuning mechanism and are unique to the Bookham design.

The rear grating is a multiple-phase shift design that provides a comb-like wavelength filter in reflection.

It provides seven uniform, equally spaced peaks, with very low out of band responses.

At the front of the laser, the device employs a multigrating structure that, when activated electrically, selects one of the supermode reflection peaks created by the rear phase grating reflector.

The front section consists of a series of short gratings of different pitches, addressed by individual contacts.

The supermode selected depends on which of the front contacts receives current.

The current levels involved are typically 5mA, much lower than in existing designs, and the current is injected only into a short length of grating.

This fact is responsible for the laser's excellent power output and uniformity: optical loss resulting from injected carriers in the grating section is very low and the light emitted in the gain section of the laser accordingly reaches the facet with very little absorption.

The low current levels also result in better thermal stability when the device is tuned and open up potential for fast wavelength switching applications.

The front-grating mechanism thus digitally selects a broad range of wavelengths and operates as a coarse tuning mechanism across the C-band.

Quasicontinuous tuning in each sub-band is achieved by varying the current into the rear grating and fine tuning to the ITU grid point is accomplished using the phase section.

A feedback loop, using the rear current as a single control parameter to tune the device, can be used to track the front reflector together with the rear comb response.

The device is designed to provide precise access to 80 ITU channels with 50GHz spacing.

The laser design contains several unique features that optimise the SMSR.

The rear grating reflector, with its very uniform reflection peaks and absence of sidemodes, is an important element.

Furthermore, it is possible to optimise the front grating's spectral response to maximise the SMSR by applying a current profile to one or more of the contacts.

Alternatively, power uniformity can be improved even further at the cost of a slightly lower SMSR.

This gives system designers valuable flexibility in adjusting the devices characteristics to their exact requirements.

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