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Case at 1550 nm SSMF-to-PD

Dans le document The DART-Europe E-theses Portal (Page 127-130)

The top side of the taper (cone) structure is designed to have larger diameter with respect to the core of the optical fiber to easily collect the light beam. However, the lower side is designed to fit with the size of the photodiode. As a result, the SU-8 conical-shape taper guides the light and reduces the mode size from the fiber to the photodetector. This taper structure reduces fiber-to-air /air-to-fiber and air-to-photodiode reflections as it is made directly in contact with the fiber and the photodiode.

For this case the optimization of the structure has been performed exploiting a RSoftR commercial simulator. In particular, the simulation has been performed in order to maximize the coupling efficiency between a SSMF with a core diameter of 9µm, operating at 1.55 µm wavelength, and a 10µm diameter photodiode. One has to consider, although the fiber core diameter is less than the active area of the photodiode, the optical beam emitted from this core at 1.55 µm has mode field diameter of 10.6 µm [31] which is larger than the optical opening of the photodiode. In the simulation the taper is considered touching the photodiode and far from the fiber of 10µm. This distance is set to better fit the experimental measurements. The simulations have indicated that it is possible to obtain a coupling efficiency greater than 90%

for resin thickness, TH, greater than 35µm for a top side cone diameters, TWF, greater than 15µm. For the same configuration, the Full Width Half Maximum (FWHM) (in µm) is also extracted via RSoftR simulation showing that it can be greater than 20µm for TWF greater than 20µm and TH of 40 µm.

To validate the proposed concept, the conical-shaped taper structure is fabricated on glass wafer with diaphragm approach to be tested with a wide active area optical power meter detector (PIN PD), as illustrated in Figure 4.3b.

The diaphragm structure is fabricated based on the processes shown in Figure 4.3a. A 4 inches glass wafer of 500µm thickness is sputtered with a 300 nm layer of aluminum. Then, photolithography process followed by chemical etching is performed to etch the deposited

metal so as to create an optical opening of the diaphragm (as in Figure 4.3a - G-3).

Figure 4.3: a) Technological manufacturing process illustration for the fabrication of the proposed taper structure over a glass wafer: G-1: clean glass wafer, G-2: the glass wafer after metal sputtering, G-3: the glass wafer after metal etching to create an optical opening of the diaphragm, G-4: the taper structure fabricated on the glass wafer. b) The schematic profile view of the proposed diaphragm structure.

The fabricated taper and butt coupling structures are tested with a G.652 single mode optical fiber excited by a 1550 nm laser by the test bench shown in Figure 4.4 considering the switch (S) in case (A) in which the climatic chamber represented is switched off.

Optical Probe

Figure 4.4: Experimental setup for testing in case of simple butt-coupling and using the polymer-based structure.

The schematic figure of the butt-coupling (reference) and taper structure along with the optical fiber are shown as inset. The end tip of the fiber is cleaved and placed vertically to the structure. Its position is controlled by a nano-positioner to scan over the surface of the cone with an accuracy of 20 nm. A 10µm air gap between the fiber and the taper is considered to scan the fiber over the top of the device under test. The optical power meter head is placed

beneath the structure under test as shown in the schematic picture in Figure 4.4. A SNOM is then performed by moving the fiber on the top of the taper. The same mapping is performed for the reference structure that is not covered by the polymer (without taper structure). The coupling efficiency is then experimentally extracted in both cases and thus compared as shown in Figure 4.5. The fitting between the experimental and simulation results are also presented in the same figure.

A coupling efficiency of 86% and 94% is measured for butt coupling and the taper structure respectively. This peak value is obtained when the fiber is exactly aligned with the taper structure (i.e. X position equal to 0µm). An improvement of 8% is measured compared to the butt coupling structure. This improvement is due to the fact that the upper cone diameter of the taper is wide enough to collect all the light coming from the standard single mode fiber while the conical polymer taper structure is good enough to confine the light into the smaller diameter of the cone. This coupling ratio could increase if the fiber comes into contact with the structure.

-20 -10 0 10 20

X Position ( m) 0

20 40 60 80 100

Coupling Efficiency (%)

ICON Technology Butt Coulpling Simulation Butt Coupling Simulation ICON technology

Figure 4.5: Experimental and simulated results of coupling efficiency versus optical fiber position for butt coupling and conical optical taper structure with an expected photodetector diameter of 10µm. The air gap between the fiber and the optical taper or reference structure is about 10 µm. The simulation structure size: TH=100 µm; TWF=25 µm and TWD=10 µm

A very good agreement between measurement and simulation is shown in Figure 4.5 for both butt and taper coupling structures. For this simulation, a Gaussian beam with mode field diameter of 10.6µm is considered according to the standard single mode G.652 fiber. We also considered the reflection effect at the injection of light (i.e. the glass for butt coupling structure and the SU-8 for the taper structure). The 1 dB alignment tolerance of ±9µm is measured for the polymer-based structure while 1 dB alignment tolerance of the butt coupling is±3.5µm which is less than a factor of 2.57 compared to the taper structure case.

Dans le document The DART-Europe E-theses Portal (Page 127-130)