Dr. Boudewijn Docter
Opto-Electronic Devices Group, Eindhoven University of Technology, The Netherlands
|
|
|
Figure 1: Schematic cross-section of the different waveguide types |
InP-based materials are historically the material of choice for laser devices used in fiber optic communication systems. It is therefore also the most interesting material to use for complex Photonic Integrated Circuits, but to create these circuits the fabrication process is much more complex than for the creation of single laser devices.
The fabrication process must be capable of producing high-performance amplifiers and lasers, but also needs to facilitate a large variety of passive waveguide circuits (filters, splitters, phase modulators, etc.) As it is not feasible to develop a custom fabrication process for each circuit, we have developed what we call a generic integration technology in which all of these functionalities can be realized [1].
 |
| Figure 2: Side-view of a deeply etched broadband mirror. After etching the gaps are filled by a polymer |
The generic integration technology allows the designer to build basic active and passive components using both shallowly etched waveguides (for low loss interconnects and efficient amplifiers) and deeply etched waveguides (for smaller bend radii and efficient phase shifter sections). A cross-section of the different waveguide types is shown in figure 1. Recently we have upgraded the fabrication process so that it is possible to etch narrow slots into the deeply etched waveguides. This allows the realization of deeply etched broadband mirrors in the passive and active waveguides.
|
|
|
Figure 3: Calculated reflectivity (solid) and transmission (dashed) spectra of a 3-period deeply-etched DBR mirror. |
A schematic side-view of such a mirror is shown in figure 2 and the resulting reflection/ transmission spectra are plotted in figure 3. For the mirrors it is very important that the dimensions of the gaps are accurately defined, since the reflection from all interfaces must be in phase with each other. In this specific design there is approximately ¾ of the wavelength (λ = 1.55 μm in vacuum) between each interface. Also the etch depth and the profile inside the narrow slots is very important. A small deviation in sidewall angle can reduce the reflectivity significantly.
These structures are fabricated using an Oxford Instruments Plasmalab System100 ICP. The deeply etched structures are etched using a Cl2: Ar:H2 chemistry. This process provides a straight and smooth etching profile, also in the narrow mirror slots. However, the etch rate is quite high (~2 μm/min) and is therefore not so suitable for the shallowly etched structures, since the etch depth should be accurately controlled. Therefore we use a more gentle process using cycles of CH4:H2 plasma alternated with an O2 descum step.
 |
 |
 |
| Figure 4: SEM images of shallow-etched waveguide (left), deeply-etched waveguides (middle) and deeply-etched mirror (right).mirror. |
The results of the different processing steps are shown in the SEM images in figure 4. The broadband mirrors can be used in a large variety of devices. One of the first applications that was realized to demonstrate the capabilities of the new fabrication process is a novel ultra-fast tunable laser [2]. This new device can switch between a set of discrete wavelengths within just a few nano-seconds – 100 times faster than today’s commercially available tunable lasers – which makes it very interesting in a variety of optical telecommunication and fiber sensing applications.
- [1] M.K. Smit, R. Baets, M. Wale, “InP-based Photonic Integration: Learning from CMOS”, 35th European Conference on Optical Communication (ECOC) 2009
- [2] B. Docter, J. Pozo, S. Beri, I.V. Ermakov, F.Karouta, J. Danckaert, M.K. Smit, “Discretely Tunable Laser Based on Filtered Feedback for Telecommunication Applications”, Journal of Selected Topics in Quantum Electronics, accepted for publication, DOI 10.1109/JSTQE.2009.2038072, Nov. 2009