Appropriate micro-optical tools are required to exploit the key advantages of optogenetics in neuroscience, i.e. optical
stimulation and inhibition of neural tissue at high spatial as well as temporal resolutions, providing cell specificity and
the opportunity to simultaneously record electrophysiological signals. Besides the need for minimally invasive probes
mandatory for a reduced tissue damage, highly flexible or wireless interfaces are demanded for experiments with freely
behaving animals. Both these technical system requirements are achieved by integrating miniaturized waveguides for
light transmission combined with bare laser diode (LD) chips integrated directly into neural probes.
This paper describes a system concept using integrated, side emitting LD chips directly coupled to miniaturized
waveguides implemented on silicon (Si) substrates. It details the fabrication, assembly, and optical as well as electrical
characterization of waveguides (WG) made from the hybrid polymer Ormorcere. The WGs were photolithographically
patterned to have a cross-section of 20x15 μm2. Bare LD chips are flip-chip bonded to electroplated gold (Au) pads with
±5 μm accuracy relative to the WG facets. Transmitted radiant fluxes for blue (430 nm, (Al,In)GaN) and red (650 nm,
AlGaInP) LDs are measured to be 150 μW (ID = 35 mA, 5% duty cycle) and 4.35 μW (ID = 225 mA, 0.5% duty cycle),
respectively. This corresponds to an efficiency of the coupled and transmitted light of 44% for the red LDs. Long term
measurements for 24 h using these systems with red LDs showed a decrease of the radiant flux of about 4% caused by
LD aging at stable WG transmission properties. WGs immersed into Ringer’s solution showed no significant change of
their optical transmission properties after four weeks of exposure to the ionic solution.
The fabrication and the design of a new fiber connector for up to 16 single- or multimode fibers is presented. The connector features the following essential advantages: low cost fabrication by micro injection molding, easy assembly due to elastic alignment structures made possible using LIGA technology and bonding by UV-curing adhesive, and a hermaphroditic connector design in order to avoid damage of the precision part of the ferrule. The mean insertion loss is 0.35 dB with multimode fibers and as it turned out from first experiments 1.16 dB with singlemode fibers.
An ultra-miniaturized sensor head for absolute measurement which has a size of less than 5 mm cube is realized. The sensor works by means of the optical triangulation. To achieve the optical function, the sensor consists of a three layer polymer waveguide patterned by X-ray lithography, an incoupling fiber and two detection fibers and micro cylindrical lenses which are also fabricated by X-ray lithography. The fibers and the cylindrical lenses are placed into the waveguide without any active alignment by using waveguide walls as guiding and fixing structures. The ratio of intensity detected in the two detection fibers is a measure for the distance between the object and the sensor. The detection range of the sensor is 1 to 6 mm between the sensor and an object with diffusively reflecting surface.
The LIGA process is used to fabricate micro-optical benches which allow to mount hybridically active and passive optical components with very high precision and without active alignment. Moreover, also micro mechanical structures like electro-mechanical actuators are fabricated on the same substrate. To avoid any lateral misalignment al fixing structures in the optical bench are produced in the lithography step. Due to the high precision of x-ray lithography lateral tolerances are in the range of 0.1 to 0.2 micrometers depending on thermal distortions. Thus, optical losses for these components are rather small. The potential of the free space concept based on LIGA technology for the fabrication of devices for optical telecommunication has been demonstrated by a bi-directional transceiver module as well as an optical bypass. In the case of the optical bypass element, a movable mirror is fabricated on the substrate together with the fixing elements. This movable mirror is the end face of an electro-static actuator which allows to move the mirror into the collimated light beam between two fibers and thus, change the direction of the light. For the first prototypes the losses in the beam without mirror are about 1.7 dB, whereas the losses in the deflected beam are about 4.5 dB.