Source: https://www.osapublishing.org/josab/abstract.cfm?URI=josab-36-4-966
Timestamp: 2019-04-20 06:28:44+00:00

Document:
Laser-written waveguides created inside transparent materials are important components for integrated optics. Here, we demonstrate that subsurface modifications induced by nanosecond pulses can be used to fabricate tubular-shaped “in-chip” or buried waveguides inside silicon. We first demonstrate single-line modifications, which are characterized to yield a refractive index depression of ≈2×10−4 compared to that of the unmodified crystal. Combining these in a circular geometry, we realized 2.9-mm-long, 30-μm core-diameter waveguides inside the wafer. The waveguides operate in a single-mode regime at a wavelength of 1300 nm. We use near- and far-field imaging to confirm waveguiding and for optical index characterization. The waveguide loss is estimated from scattering experiments as 2.2 dB/cm.
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Fig. 1. (a) Pulsed laser that operates at 1.55 μm, along with an infrared (IR) imaging system form the experimental setup. The ns laser is used for in-chip microstructuring and photonic device fabrication, while the IR transmission microscope is used for characterization. MOPA, master-oscillator power amplifier; HWP, half-wave plate; PBS, polarizing beam splitter. (b) IR transmission microscope image of an in-chip single-line modification. The laser enters from the polished side surface (x−y plane) and propagates along the z axis. The sample is scanned along the optical axis to form long rod-like modifications.
Fig. 2. (a) Illustration of the longitudinal-writing scheme to form depressed-cladding waveguides deep inside Si. We first form concentric circles at a specific depth by iteratively rotating the sample. (b) Same procedure used at different depths along z. (c) IR transmission microscope image showing the circular cross-section from a waveguide array, positioned 500 μm from the top surface. (d) IR image of the top view of a single waveguide, where rod-like structures similar to that shown in Fig. 1(b) are used to form the cladding. (e) FDTD simulation showing the mode field profile.
Fig. 3. (a) Far-field image of light, used as a control experiment, is acquired after passing through unmodified silicon. (b) Far-field image from the exit port of a waveguide. (c) Near-field image of the optical mode from the waveguide output port. (d) Far-field intensity profile data measured from the waveguide output (blue crosses) and the corresponding control experiment (black circles). The blue solid curve is a double Gaussian fit to the waveguide output profile. The black solid curve is a Gaussian fit to control. (e) The waveguide loss is characterized with scattering data (orange circles), filtered with a 10-pixel moving average. The blue curve shows the fitted linear curve to the logarithmic plot.

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