Light tuning inspired by heat expanded grating
Tom Shelley reports on how a simple observation of using heat to tune a fibre Bragg grating helped lead to the design of a sophisticated full band tunable laser
A new wavelength filter has been derived from the observation that a strong reflective response could be induced in a chirped fibre grating by the simple application of local heat. This led onto a laser design in which local carrier injection is used as the analogue of heat, in a very compact monolithic design.
In order to make maximum use of the information carrying capacity of optical fibres, it is necessary to work at a large number of different wavelengths at the same time. Up to around 100 channels each 50GHz apart are used in the C band, potentially as many in the L band and vendors are working on 25GHz spacing to double the capacity again.
In order to interface successfully with these channels, it is desirable to have lasers that can be quickly and precisely tuned to any wavelength or wavelengths of interest.
Not surprisingly, there are a significant number of methods that have been developed and patented to do this, but that described by Professor Andy Carter of Bookham, at a recent meeting of the Institute of Physics on "Commercialising Research for Opto Technologies" we thought to be particularly ingenious.
Some of the other methods that he mentioned included having a whole array of tuned lasers on an integrated device and switching between them (Fujitsu, Santur). Of other ideas described in articles available on the Internet, we might mention the use of strong magnetic fields (University of Michigan), acoustically tuning optical amplifiers (University of Paderborn) and stretch tuning optical fibres using piezoelectric actuators acted on by up to 500V (NASA).
None are as elegant as the Bookham method. Professor Carter revealed that feasibility in initial experiments was established by applying heat from the tip of a soldering iron. These observations led to the design of the key front grating filter in the DSDBR laser, which selects the frequency band for the laser. Heat is not used in the actual device, but the simple analogue of slight current injection, which changes the effective length of the laser locally.
The finished device is 1.7mm long and made in Indium Phosphide. It has five main sections. The central section provides the necessary optical gain within the laser cavity, and the phase section enables continuous tuning by adjusting the optical cavity length. The rear section has an electron beam written phase grating reflector that provides a sharp and flat comb reflectance response. It is the front linearly chirped Bragg grating that provides the key to the tuning mechanism. When activated, it selects one of the supermode reflection peaks created by the rear phase grating reflector acting as a comb filter, thereby tuning the laser output.
Sub sections of the front filter are addressed by individual contacts. The supermode selected depends on which contact receives current, rather than on the size of the current.
Output power is controlled by the semiconductor amplifier current. When free running, with no tuning current, powers greater than 100mW are possible. Typically, 200mA in the gain section and 150mA in the amplifier provides 70mW ex facet power. Test modules have fibre coupled powers of +16dBm on all C-band channels. Optical bandwidth (tuning range) is around 50nm. More than 2500 devices are fabricated at the same time on a single wafer.
The tuning method is potentially applicable to a wide range of sensing devices as well as communication lasers, and demonstrates the latest example of the company's ingenuity in making novel integrated electro optic devices.
* Laser can be precisely tuned over a wide range by applying currents to appropriate sections of a linearly chirped fibre Bragg grating
* The device is monolithic, so it can be fabricated using industry standard laser processes. Bonding and packaging is also industry standard, albeit with more connections needed to the grating sections
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