Edwin Jan Klein - Universiteit Twente

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Microring-resonator building blocks
on a 50 µm wide multi mode fiber. The light is then measured using a HP 81521B
photo-detector.
Aside from the measurements with the tunable laser this setup was also used in
combination with a Sensors Unlimited 320 MX-1.7 RT infrared camera. The camera
provides unique information that cannot be obtained through other means, e.g. by
measuring the different ports of a device. Figure 6.16a and 6.16b for instance, show
an infrared image of a cross coupled microring resonator. The orientation and heater
placement in these images is similar to that of the resonator shown in Figure 6.3b. In
Figure 6.16a the resonator is off-resonance. Most of the light propagates past the
resonator to the through port and only a minor fraction of the light is coupled into the
drop port.
In In
Through
Drop
Through
Drop
Figure 6.16. Infrared image (inverted) of a vertically coupled resonator in the off a) and onresonance
b) state.
In Figure 6.16b where the resonator is in the on-resonance state the reverse can be
seen. Most of the light propagates through the resonator to the drop port and only a
minor fraction of the light is present in the through port. Of special interest in this
image is the “scatter” point that appears near the input (pointed to by the diagonal
arrow). This point is only seen when the resonator is in resonance and is caused by
light that radiates out of the resonator and then hits the port waveguide where it
scatters to all directions. Another scatter point, seen in both images, is located at the
crossing between the two waveguides.
Although the lens setup is highly useful for the measurement of devices that would
otherwise be difficult to characterize, the adjustment of the lenses to off-axis port
waveguides, i.e. waveguides that are not in the same line from input to output, can be
quite difficult. Also it is not possible to measure an output port that is on the same
side as the input. In these cases or when devices are relatively easy to measure then it
is useful to use a fiber setup as shown in Figure 6.17a. This setup can be equipped
with single fibers or fiber arrays which are relatively easy to align to port waveguides,
thereby speeding up the measurements. Also, the fiber arrays make it possible to
measure ports that are on the same side of a device. Where possible small core fibers
have been used to coupled light into the devices in order to reduce the fiber-chip
coupling loss. The setup, of which a photograph is shown in Figure 6.17b, uses a
HeNe laser for the alignment of the fibers to the port waveguides. Two laser sources
have been used in this setup, depending on the resolution required. The previously
mentioned tunable laser source in combination with the power meter is used for slow
but high resolution measurements down to 1 pm. The Erbium Doped Fiber Amplifier
with Amplified Spontaneous Emission Source (EDFA ASE) in combination with an
Optical Spectrum Analyzer (OSA) on the other hand is used for near real-time
measurements with a resolution down to 100 pm. On the input port of the device a