3D Penetrating Waveguide Array
A waveguide array for optical neural stimulation is reported in Biomedical Optics Express. Abaya, et al.  micromachined and characterized an array with > 1 mm needle-shaped waveguides to facilitate three-dimensional deep-tissue light access with capabilities for simultaneous spatiotemporal modulation with different wavelengths. The array can be adapted to a wide variety of illumination systems (e.g., microscope, μLED arrays, optical fibers, collimated beams, spatial light modulators) to allow its application in a broad range of optogenetic and infrared neural stimulation experiments. Nearly all modes of optical excitation (e.g., visible, infrared, multi-photon excitation) can be achieved with the penetration depth determined by the waveguide length and not by wavelength; the output beam profile is controlled by other waveguide geometrical features. The waveguide array size, and waveguide length, width and tip angle may be varied independently from wafer to wafer to control the stimulation area, depth of access, output beam spot size and output beam divergence, respectively. The waveguide length and tip angle does not affect the normalized power out of each waveguide tip, but the width needs to be larger than the optical source aperture for maximum power coupling in order to maintain the same transmission efficiency of ~71% in air. This value is expected to increase when the array is implanted in tissue.
Multiwaveguide Implantable Probe
In a recent issue of Optics Letters, Zorzos et al.  describe a new type of probe capable of delivering different wavelengths from different points along the same shank. The probe looks just like a conventional silicon probe used for multiunit recordings, except that it is only used to deliver light through an array of several independent waveguides (here 12) running parallel to each other. These waveguides are made of a core of silicon oxynitride (20 microns wide and 9 microns thick) coated with a thin silica cladding and an additional reflective layer of aluminum. Each waveguide is terminated at a desired position along the shank using an aluminum coated corner mirror which reflects light perpendicular to the probe axis.
The Boyden lab recently developed a linear probe comprising a set of integrated microwaveguides running in parallel to each other, microfabricated on a single substrate and capable of delivering light independently to multiple brain targets along the probe axis. In a paper published in Optics Letters, they now extend this design to the case of 3-dimensional light delivery to a set of targets distributed throughout the brain, by first fabricating waveguide combs containing many linear probes parallel to one another, then aligning multiple combs in a custom engineered baseplate for coupling to a digital micromirror device (DMD) for arbitrary light patterning. Each waveguide is about 10 microns x 10 microns wide, very small, and thus hundreds can be packed in the space occupied by a typical optical fiber. In addition, a 1024 x 768 pixel DMD, with a laser coupled, could easily enable the control of up to ~1000 points in 3-D space (even pooling many DMD mirrors, e.g. 800, per waveguide to increase power). The power out the end of the waveguide can be up to ~150 mW/mm^2, comparable to the highest powers used in optogenetics.
- Zorzos2010 pmid=21165114
- Abaya2012 T.V.F. Abaya, S. Blair, P. Tathireddy, L. Rieth, and F. Solzbacher, "A 3D glass optrode array for optical neural stimulation," Biomed. Opt. Express 3, 3087-3104 (2012)
- White Paper by Annik Yalnizyan-Carson et al. "Patterned Illumination Systems for Optogenetics".
- White Paper: "All-Optical Deep Brain Imaging & Stimulation Tools for In Vivo Neuroscience" has provided an overview of the existing all-optical imaging and stimulation tools, and compared the advantages and disadvantages of these techniques.