Integrated Optics and Bragg Grating Technology
Integrated optical circuits enable processing of optical signals in an manner analogous to the familiar processing of electrical signals provided by conventional electrical integrated circuits i.e. ‘silicon chips’. Computing and telecommunications technology is moving from using electricity, to using light so as to achieve higher speeds and better performance. Consequently, processing of optical signals is becoming increasingly important. Conventionally integrated optical circuits have been very expensive, and take a long time to develop. This is due to the significant set-up costs and long turn-around times for each new design of circuit when fabricated using the conventional photolithography approach. Stratophase’s new Direct Grating Writing approach enables low cost production and fast prototyping of optical circuits, thus opening many markets to optical circuits that have not been viable in the past. Using this technology advanced components known as ‘Bragg gratings’ can be incorporated in the circuits, which enable sophisticated functionality. This is in addition to move conventional components found in integrated optical circuits.
The proprietary Direct Grating Writing process uses two focussed UV laser beams which are overlapped to give a small circular spot with an inherent linear interference pattern. This spot is positioned onto a photosensitive material and translated to ‘write' a waveguide structure. Modulating the laser beam as it is scanned across the photosensitive substrate enables the interference pattern in the laser writing spot to be imprinted on the waveguide thus producing a Bragg grating. This allows fabrication of any design of waveguide circuit or component, eg. splitters, combiners, bends, crossovers, wavelength separators etc all with Bragg gratings at any point in the structure.
The direct grating writing process is computer controlled, providing a straightforward method of producing complex waveguide circuits and Bragg gratings. The period of the Bragg grating, and hence the wavelength at which it operates can easily be controlled by changing the modulation of the laser beam as the Bragg grating is written. It is also possible arbitrarily to change the period and contrast of the grating within a single grating thus producing more sophisticated gratings known as chirped, apodised and phase shifted grating.
Each circuit is written
individually directly into the device
substrate in a single step without the need for photolithography masks
for the waveguides and phase masks for the Bragg grating. This makes
new designs significantly faster and economic to fabricate as there is
no need to design and procure a one off mask. Accuracy is also improved
as grating structures are written simultaneously with the waveguide
removing any alignment error between the separate Bragg and waveguide
masks often found in other processes.
The UV writing technology produces waveguides with ‘soft’ graded edges, unlike the photolithography approach which gives abrupt edges to the waveguides. These can cause loss of the optical signal and a degradation of performance. These same soft edges also result in better quality splitters and combiners, in addition to allowing waveguides to cross each other which simplifies circuit design.
The substrates used for passive device fabrication are industry standard silicon wafers with a germanium doped silica glass top layer. The waveguide circuit is fabricated in this glass layer which has a low optical loss for improved device performance. By using standard material, high quality precision substrates are available in large sizes and high volume. Many devices are normally fabricated on a single wafer before it is diced and the ends then polished to provide individual devices.
The direct writing process anables optical elements can be fabricated into previously processed devices. For example individual Bragg gratings can be written into customers' waveguides that have been previously produced by other techniques. This means complex precision devices incorporating passive and active devices, such as lasers or detectors, can be trimmed or tuned at the end of the production process for improved performance and higher yields.
If you would like further details or to discuss an application please contact Stratophase.
Example products that illustrate the use this technology are our sensing, telecommunication DWDM and CWDM products.