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Abstract
Femtosecond laser direct writing is widely used to create waveguide circuits for optical processing in applications including communications, astrophotonics, simulation and quantum information processing. The properties of these waveguide circuits can be sensitive to the fabrication conditions, meaning that noticeable variability can be present in nominally identical manufactured components. One potential solution to this problem is the use of device trimming, whereby additional laser fabrication is applied to optimise the optical properties of a device based upon measurement feedback. We show how this approach can be used in the manufacture of directional couplers by overwriting the laser-written structure to alter the coupling ratios.
Highlights
Integrated optical circuits are used for the control and processing of optical signals in a range of applications including tunable lasers [1], transmitters [2] and integrated receivers [3]
The splitting ratio of directional couplers was determined with free-space coupling of a diode laser beam at 785 nm, which was focused with a lens onto the input port of a DC to couple light into one of the waveguides
The shape of the propagating mode supported by each of the straight waveguides was characterized as a function of the representative net fluence RNF during writing, remembering that there is a linear increase of the RNF with each overwriting scan as described in Eq (4)
Summary
Integrated optical circuits are used for the control and processing of optical signals in a range of applications including tunable lasers [1], transmitters [2] and integrated receivers [3]. For optimal performance of the waveguide circuits, the two outputs of the DC should have designed splitting ratios and phase differences between each other, for which a precisely controllable and highly repeatable fabrication process is required [14]. This may be implemented through an external stimulus to the glass surface to induce thermo-optical [20] or strain-optic [21] adaptive tuning of the refractive index and achieve specified output ratios This approach is only applicable to near-surface waveguides and not suitable for complex three-dimensional waveguide structures. Tuning may be achieved with the FLDW fabrication method itself, either through the introduction of additional stresses in the glass [17] or by overwriting one of the waveguides, which allows modification of its refractive index [22] and the splitting ratio and phase difference of a DC [23].
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