Integrated Optics Laser

[Ermengardi Atkisson]

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The structures we fabricate are several voxel sizes large in all three dimensions. Therefore, we cannot write the whole 3D device (e.g., a 3D bridge waveguide) using a single DLW trajectory. Instead, we decompose the volume of the 3D device to be written into slices less than a voxel size apart from each other. These slices are then filled up using a rectangular spiral pattern. By writing the spiral patterns slice by slice using DLW, we finally obtain the designed 3D structure.

In order to transfer light from planar waveguides to arbitrary 3D components, efficient coupling between the nanophotonic circuit and a 3D waveguide is crucial. Low insertion loss can be achieved by employing inverted tapers which are conveniently used for coupling nanophotonic waveguides to optical fibers in order to overcome the large insertion loss due to modal size mismatch. Such an approach is ideally suited for interconnecting planar and 3D waveguides written by DLW. Therefore, the transmission characteristics of a 3D bridge waveguide are studied.

where α is the coupling efficiency of a grating coupler and t is the insertion loss per facet of the bridge waveguide. The factor is due to the Y-splitter. As the transmission measured on port 1 is

With access to 3D form shaping on a submicron-scale, waveguide geometries that cannot be achieved by traditional nanofabrication techniques can readily be produced. This additional degree of freedom is of particular interest for achieving control of the polarization of propagating optical modes on a chip, which is non-trivial with planar architectures.

Using 3D waveguides to achieve polarization rotation of a propagating mode, we employ DLW to twist the waveguide along the propagation direction. Such devices can be integrated into planar circuits using the polymer-inverted tapers described above. The twist, however, induces coupling among all waveguide modes. Thus, in order to optimize polarization rotation of a given mode, two properties of the rotator can be tuned. First, the number of guided modes should be ideally one and the propagation constants of the remaining modes should differ from each other as much as possible. The latter can be achieved by employing a rectangular waveguide with a large aspect ratio. Second, for a given cross-section of the waveguide, increasing the twist length will improve polarization rotation.31

Measurement of polarization rotator (a) Close-up of the rotator between the feeding waveguide and the drop waveguide. η is the rotation efficiency of the rotator. (b) Light is coupled into the chip on port 2, where a TM grating coupler excites the TM-like waveguide mode. One half of the light is guided towards reference port 1, the other half passes the polarization rotator. The TE-like portion of the light is efficiently coupled out through a TE grating coupler on port 3. Port 4 is used as reference port in an inverted experiment, where initially the TE-like mode is excited through port 3. (c) Plot showing the results for the forward experiment, i.e., the relative power leaving the rotator in the rotated polarization state (black curve; normalized to the total power leaving the rotator), and the power loss induced by the polarization rotator (red curve). See text for details. TE, transverse electric; TM, transverse magnetic.

In addition, we repeat the experiment on the same device, but use port 3 (TE) as input and port 2 (TM) and port 4 (TE) as outputs this time. Ignoring propagation losses in the planar waveguides, the following transmittances (relative to the laser output power) can be measured:

where tTM is the transmittance for the TM-like mode through the polarization rotator and γTM is the fraction of a TM-like mode that is transmitted through the Y-splitter in backward direction. This can be measured in the same way as γTE, but may have a different value. The coupling efficiency of the TM grating coupler in this nanophotonic circuit is calculated separately from the transmission from port 2 to port 1 there (), as it may be slightly different from the one obtained in the first circuit due to slightly different alignment of the fiber array. Finally, the ratio between the polarization-rotated light intensity (in the TE-like mode) and the total intensity in the drop waveguide (TE-like+TM-like mode) after the rotator qTE can be calculated:

where T is the transmission relative to the laser output power and α is the coupling efficiency of the grating coupler. Subscripts denote the type of grating coupler (either TE or TM) and superscripts indicate the waveguide mode (also either TE or TM) that is coupled in or out of the chip by the grating. In the second step, we minimize transmission by only changing the input polarization. Then we get for the transmission through the nanophotonic circuit

To get the suppression for the TM grating coupler, an analogous measurement is carried out. Only the TE grating couplers are replaced by TM gratings and thus maximum transmission is obtained for the TM mode being excited by the coupler.

Further applications of interest that could prospectively be investigated include on-chip coupling to ultra-high Q resonators like microspheres59 or convenient on-chip access to 3D photonic crystals.22,23 Also, the approach could be extended to the visible regime. For the silicon nitride devices, this is readily done by adjusting grating coupler period and waveguide geometry. For the polymer structures, however, resolution is limited by the DLW voxel size. Thus, in order to fabricate monomode waveguides for visible light, usage of more advanced 3D lithography techniques like stimulated-emission-depletion DLW60 could be required.

We would like to thank Christian Koos and Nicole Lindenmann at KIT for valuable discussions. We acknowledges support by DFG grant PE 1832/1-1 and PE 1832/2-1 and the Helmholtz Society through grant HIRG-0005. We also acknowledge support by the Deutsche Forschungsgemeinschaft (DFG) and the State of Baden-Wrttemberg through the DFG-Center for Functional Nanostructures (CFN) within subproject A1.04. We further want to thank Silvia Diewald and Stefan Khn from the CFN Nanostructure Service Laboratory for assistance with the electron-beam exposition of the samples.

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Direct laser writing is a popular scheme for constructing three-dimensional integrated optical structures. Martin Schumann and co-workers from the Karlsruhe Institute of Technology and the Institute of Nanotechnology in Germany used two-photon polymerization to create three-dimensional polymer objects such as bridge waveguides, a twisted-waveguide polarization rotator and free-standing disk resonators. The structures, which would be difficult or impossible to construct using planar lithography, were successfully integrated with silicon optical chips featuring silicon nitride waveguides that guide light in the 1,550 nm telecommunications wavelength window. The researchers say that their approach could also be used to provide convenient access to three-dimensional photonic crystals. An advanced form of this approach that exploits higher resolutions would allow the construction of structures that are compatible with visible wavelengths.

The MatchBox laser control software is used for checking preset parameters, such as the max. laser diode current, target temperatures for laser internal diode, laser body, and actual measured values of these parameters, as well as the load percentage of the Peltier elements. This is very helpful for laser diode age tracking and other troubleshooting actions.

The laser unit is designed as an integration-ready electro-optics unit, which can be connected to a control mainboard and power supply of an instrument. This way the laser/detector unit provides unprecedented compactness and functionality. Thus mass production of multi-wavelength instruments becomes much easier and faster.

MatchBox series lasers feature a Universal Asynchronous Receiver/Transmitter (UART) controller interface. The pinout of MatchBox Combiner is an expanded version of the standard MatchBox pin-out. The lower row of pins is the same as for single-wavelength lasers, while the second (upper) row is added for individual TTL modulation of each laser diode installed in the combiner.

A 'Break-out-Box' is used for converting UART into a USB protocol. The converter chip for USB is 'Silicon Labs CP2102'. Additionally, the break-out-box of the multi-wavelength MatchBox laser is equipped with the necessary circuit for communication and PD type (power delivery) power supplies.

Prerequisites for such integration are a microcontroller mainboard supporting UART communication, 12V or 9V at 1.5 A (Power delivery) power supply, and 2 to 4 TTL channels for an individual on/off digital modulation of each laser diode.

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