Two-channel, dual-mode, fiber optic rotary joint

1. Field of the Invention

The invention is related to two channel fiber optic rotary joint in the field of optic transmission through a mechanical rotational interface.

2. Description of Related Art

The Fiber optic Rotary Joint (FORJ) is the opto-mechanical device which allows uninterrupted transmission of an optical signal in a fiber guide through a rotational interface to a stationary apparatus. FORJ can be categorized as active and passive. An active FORJ consists of a light on either rotor side or stator side and a photo detector on another side and the signal transmission is limited to unidirectional. The disadvantage of active FORJ is requirement for electrical power. The passive FORJ is intended to transfer optical signals from fiber to fiber without any electronic or electrical units. The use of FORJ can be widely found in missile guidance systems, robotic systems, remotely operated vehicles (ROVs), oil drilling systems, sensing systems, and many other field applications where a twist-free fiber cable is essential. Combined with electrical slip rings or fluid rotary joints, FORJs add a new dimension to traditional rotary joints. As fiber optic technology advances, more and more traditional slip ring users will benefit from FORJs in their new fiber systems. This issue can be solved relatively easy if only a single channel is to be transmitted because it can be transmitted by keeping alignment between the optical axis and mechanical rotational axis. However, the transmission results in difficulties when it is desired to transmit two channels separately from each other through a single rotation interface.

A couple of prior inventions of two channel fiber optic rotary joint are described in the following patents: U.S. Pat. No. 5,588,077, U.S. Pat. No. 4,842,355, and U.S. Pat. No. 47,251,116.

In U.S. Pat. No. 5,588,077, the two optic fiber channels are arranged in-line along the same rotational axis. Isolation of one channel from the other is achieved through a novel application of gradient index rod lenses of suitable pitch. A pair of lenses is arranged adjacent each other on each side of the rotational interface and a second pair of axially aligned lenses is arranged outboard of the first pair. An optic signal from one of the outboard lenses can be directed to one of the other lenses depending on the pitch selection. The drawback of this design is that the losses due to crosstalk and overlap of the signal paths would be pretty significant.

Gold, et al designed another two channel FORJ in U.S. Pat. No. 4,842,355. A first channel signal is delivered to an optic fiber transmitted coaxially of the stationary and rotary side, transfer across the rotational plane between the two components being accomplished by opposing centrally located optic lenses. A second channel transmitted through a second optic fiber is delivered to a lens system which converts the light into a cylinder of light coaxial with the first channel and which surrounds the optic management for the first channel. Second channel thus are converted into coaxial hollow cylinders of light. These cylinders of light are transmitted between facing lens systems in the rotary and stationary sides of the apparatus. But the facing lens systems are very difficult to be fabricated.

Spencer, et al shows in U.S. Pat. No. 47,251,116 a two-channel and multi-channel FORJ. Within the joint reflecting surface are used to redirect off-axis optic signals onto the joint axis, with relative rotation occurring while the signals are on-axis. A rotating member for each channel has a reflecting surface for reflecting the on-axis signal portion off-axis to a receptor fiber. Alignment between the rotating member and the receptor fiber, as well as drive for the rotating member, is provided by a pair of magnets of opposite polarity, one being secured to the rotating member and the other being secured to the rotor. But it could be very difficult for the magnetic interaction to accurately ensure the synchronous rotation of the rotor and the rotating member. The size of the magnetic element and the adjustment of the reflecting surface also increase the size of the invented embodiment.

The first object of the present invention is to utilize the conventional fibers, collimators, and reflecting surfaces to realize a two-pass fiber optic rotary joints which can simultaneously transmit optical signals through a single mechanical rotational interface with a very low-profile and compact structure.

Another object of the present invention to minimize the need for maintaining precise axial alignment between the rotating and non-rotating elements of a two channel fiber optic rotary joint so that it could be used in any harsh environments such as temperature change, vibration and shock.

FIG. 1 shows a cross sectional view of the basic embodiment of the invention.

FIG. 2 is the cross section view of the holder for the collimator assembly.

FIG. 3 is an enlarged view of micro-rotational interface in FIG. 1 .

FIG. 1 illustrates a basic design of a two-channel fiber optical rotational interface. The holder 133 and 144 can be rotatable relative to each other. The axis of the rotation is the geometrical axis 101 , 201 of the holder 133 and 144 (shown in FIG. 2 ). The first channel of light pass consists of a optic fiber 111 which is fixed on the axis 101 of holder 133 , and another optic fiber 122 which is fixed on the axis 201 of holder 144 . The diameter of the optic fiber is about 0.125 mm. The holder 133 and 144 have a through central hole 103 , 203 at the diameter of 0.126 mm, slightly larger than the diameter of fiber 111 and 122 . Fiber 111 should be longer than fiber 122 so that it could get into the central hole 103 of holder 144 . The end surfaces of fiber 111 and 122 are oppositely positioned inside the central hole 103 of holder 144 on the rotational axis but separated by a clearance about 0.5 um. An enlarged view inside of central hole of holder 144 is shown in FIG. 3 . When the holder 133 and 144 rotates relatively each other, the fiber 111 can be rotating inside the central hole of holder 144 relative to fiber 122 . Thus forms a micro rotational interface, or “micro bearing”. The micro bearing is able to compensate the mechanical alignment error of the two fibers so that the alignment task would be just focused on second channel.

The second channel of light pass includes conventional collimator 15 and 16 , which are fitted inside of collimator housing 17 and 18 respectively, and reflecting surface 19 and 20 . The collimator housing 17 and 18 are off-set on the holder 133 and 144 respectively, with their own axis 102 , 202 parallel to the rotational axis 101 , 201 . The angled surface 21 on holder 133 and the angled surface 22 on holder 144 are coated optical reflecting surface. The reflecting surface 19 and 20 are supposed to be adhered to the angled surface of collimator housing 17 and 18 and parallel to the optical surface 21 and 22 respectively. When the light beam emitted from one of the collimator 15 , it will be reflected by the reflecting surface 19 , 21 , 22 and 20 , then get into another collimator 16 . Vise versa. The second light pass can be remained when the holders 133 and 144 rotate relatively. Thus a two-channel passive FORJ is obtained. The beam diameter of the conventional collimator can be significantly larger than fiber diameter to reduce loss due to the presence of center fiber. Extremely low cross-talk between the channels could be ensured by the fiber cladding.

Although the present invention has been described in several particular embodiments of an FORJ, it is expected that additional embodiments and modification will be apparent without departing from the spirit of the invention.