Fiberoptic Rotary Joints


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Connecting an optical fiber to an unrestrained freely moving animal raises the same issue as connecting wires: as the fiber twists it tends to exert an elastic force in the direction opposite to the twisting. This increases the chances of breaking the fiber and might impair the movements of the animal. The twisting effect can be minimized by using long (> 100 cm) and thin fibers. Alternatively, one can add a fiberoptic rotary joint (FoRJ) to allow free rotations of the fiber with minimum torque applied. Doric Lenses, Mightex, Prizmatix and Thorlabs are manufacturing FoRJs suited to experiments on freely moving animals. These joints usually use high quality ball bearings for minimal friction during rotation. Some FoRJs such as Doric's ones integrate a pair of aspherical lenses for fiberoptic beam collimation and focusing. Doric Lenses also incorporate additionnal components into its FoRJs for intensity and/or wavelength division, electrical transmission and fluid delivery.

As a similar alternative, Plexon Inc. offers an electrical commutator, to which miniaturized LEDs can be mounted. These compact fiber-coupled LEDs rotate along with the animal, preventing cable tangling without the need for a traditional FoRJ. (Also see diagrams at the end of Plexon's optogenetics product guide.)

Overview of commercially available FoRJs

Thorlabs' single rotary joint
Thorlabs' single rotary joint.
Prizmatix's' single rotary joint.
Prizmatix's' single rotary joint.
Doric Lenses' single rotary joint.
Doric Lenses single rotary joint connected to two patchcords through FC/PC connectors.
Doric Lenses' 1x4 wavelength and intensity division rotary joint.
Bottom view of Doric Lenses' 1x4 wavelength and intensity division FoRJ. This FoRJ allows simultaneous delivery of blue light to 2 brain regions and yellow light to 2 other brain regions.

Doric Lenses' FoRJs

Examples of FRJs manufactured by Doric Lenses. From left to right: (1) single FRJ (FC/PC to FC/PC), (2) intensity division FRJ 1x4 (FC/PC to mini SMA), (3) blue-yellow wavelength division FRJ 1x2 (FC/PC to FC/PC) and (4) combined blue-yellow wavelength and intensity division FRJ 1x4 (FC/PC to FC/PC).

Single Rotary Joint (1x1 FoRJ)

The simplest version of these FoRJs provides a 1x1 transmission of light (1 optical fiber to 1 optical fiber). Doric Lenses announces a coupling efficiency (transmission) of 85% and less than 0.2 dB of variation in power transmission during rotations. This FoRJ works fine with fibers up to 1 mm and NA=0.50.

Intensity Division

Intensity division (= beam splitting) can be performed at the level of the FoRJ by incorporating small prisms. Available beam splitting FRJs provide 1x2 or 1x4 transmission, allowing to simultaneously illuminate 2 to 4 brain regions using the same light source.

Wavelength Division

FoRJs can also incorporate small dichroic mirrors to perform wavelength division (= separation). For example, two wavelengths (say blue and yellow) can be combined into a unique fiber and then split into two separate fibers by the FRJ, allowing to simultaneously deliver one color to one brain region and the other color to another. Wavelength division can be combined with intensity division.

Prizmatix's Rotary Joint

Prizmatix's Rotary Joints combine excellent light transmission with extremely low torque.

A high NA (0.63) large core 1000um polymer (POF) fiber emerges from the LED to the Rotary Joint, from which on the other side is coupled one or more smaller core fibers (500um - 200um) to the cannula. Single or Y-shaped fibers available to stimulate two areas of the brain simultaneously with no losses.

Fiberoptic Rotary Joint + Electrical Commutator

Tucker-Davis' motorized commutator
Tucker-Davis' motorized commutator has a through hole that can accept an optical fiber and can be used in tandem with a FoRJ.
Doric Lenses' electrical rotary joint.
Doric Lenses' electrical rotary joint.

Tucker Davis' ACO32 commutator

Motorized commutators are used routinely for chronic recordings of neuronal activity. These commutators start rotating when a certain torque has accumulated in the wire bundle, until the torque is reduced or cancelled. Tucker-Davis Technologies is a historical manufacturer of acquisition systems and accessories for chronic multiunit recordings. Their motorized commutator was recently modified to include a through hole which can accept an optical fiber, allowing to use the commutator in tandem with a passive FoRJ.

Doric Lenses' Electrical Rotary Joint

Doric's passive electrical rotary joint (ERJ) has a 7.5 mm through hole allowing to pass an optical fiber patchcord with a miniature fiberoptic connector. It also comes with accessories for attaching a 1x1 or 1x2 FoRJ and using it in tandem with the ERJ. The ERJ has a torque of 0.9 mNm and 1.8 mNm for 6 and 12 electrical contacts respectively. Dimensions are: 46 mm OD and 60 mm length. HARWIN M80-82612 connectors are used for electrical connection.

Electrical commutators for mounting miniaturized LED sources that rotate with the animal

Diagram showing Plexon's miniaturized fiber-coupled LEDs, mounted on electrical commutators. This figure was taken from a white paper overview of Plexon's optogenetics products.