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

Abstract

A two-channel fiber optical rotary joint has been invented in which optical signals can be transmitted simultaneously along two optical passes through a single mechanical rotational interface for both single mode fiber and multi mode fiber. The first channel of light path consists of a pair of micro-collimators, or a pair of fibers, co-axially fixed in 2 holders respectively. The light signal from one of micro-collimator, or fiber can be directly coupled into another micro-collimator, or fiber. The second channel of light path is off-axis arranged, including a pair of conventional collimators, a pair of first reflecting surfaces and a pair of second reflecting surfaces. The light signal emitted from one of the said conventional collimator will be reflected by one of the said first reflecting surface, one of the said second reflecting surface, another said second reflecting surface, and another said first reflecting surface, then coupled into another said conventional collimator. An index matching fluid is filled in the housing for lubrication and pressure compensation purposes.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is related to two channel fiber optic rotary joint in the field of optical 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 source on either rotor side or stator side and a photo detector on another side. 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 optical 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. 4,725,116.

In U.S. Pat. No. 5,588,077, the two optical 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 optical 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 optical fiber transmitted coaxially of the stationary and rotary side, transfer across the rotational plane between the two components being accomplished by opposing centrally located optical lenses. A second channel transmitted through a second optical 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 optical 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. 4,725,116 a two-channel and multi-channel FORJ. Within the joint reflecting reflecting surface are used to redirect off-axis optical 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.

SUMMARY OF THE INVENTION

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

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.

A further objective of the preset invention is to reduce the insertion loss and increase return loss and to allow the rotary joint to work at any ambient pressure by filling index-matching fluid.

An even further objective is to incorporate a pre-loaded precision ceramic ball bearing to achieve reduced optical losses by improving concentricity and long-lasting precision between the rotational and stationary elements of optic rotary joint.

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG. 2 is a cross section view of second basic embodiment of the invention.

FIG. 3 is an enlarged view of micro-rotational interface in FIG. 2.

FIG. 4 illustrates a full embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a basic design of a two-channel fiber optical rotational interface. The holder 13 and 14 can be rotatable relative to each other. The axis of the rotation is the geometrical axis of the holder 13 and 14. The first channel of light pass consists of a micro-collimator 11 which is fixed on the axis of holder 13, and another micro-collimator 12 which is fixed on the axis of holder 14. The diameter of the micro-collimator is about 0.125 mm. Just like a single channel FORJ, the two micro-collimators are closely arranged opposite at their end section and the parallel ray of light emitted from one micro-collimator is easily transmitted to another micro-collimator through the rotational interface. 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 13 and 14 respectively, with their own axis parallel to the rotational axis. The angled surface 21 on holder 13 and the angled surface 22 on holder 14 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 13 and 14 rotate relatively. Thus a two-channel passive FORJ is obtained. Because the diameter of the conventional collimator can be larger than 1.8 mm, the collimated beam of the micro collimator can be much smaller than that of the conventional collimator so that the lower loss and the lower cross-talk between the channels could be assured.

FIG. 2 shows another embodiment of the invention. It's very similar to FIG. 1. Instead of utilizing micro-collimators, a pair of optical fibers are used. The holder 133 and 144 have a through central hole at the diameter of 0.126 mm, slightly larger than the diameter of fiber 111 and 122 which is 0.125 mm. Fiber 111 should be longer than fiber 122 so that it could get into the central hole of holder 144. The end surfaces of fiber 111 and 122 are oppositely positioned inside the central hole 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 off-axis channel is exactly the same as in FIG. 1. The insertion loss for the second channel can be controlled smaller than 6 dB, the cross talk can be 50 dB, because the fiber diameter is much smaller than the light core size of second channel.

The mechanical details of a full embodiment of the present invention is shown in FIG. 4. The two collimator assemblies in FIG. 1, or FIG. 2 now are secured in the central holes of rotor 31 and stator 32 respectively by adhesive bonding. A couple of ceramic ball bearings 33 and 34 are used to allow the rotor and stator to rotate relatively each other with higher rigidity, extended bearing life and more precision. A shaft seal 36 between stator 32 and rotor 31 is mounted on the shaft 40 of rotor 31 and the boring hole of seal holder 35, which is threaded with stator 32. The ceramic ball bearings, 33 and 34, are separated by an annular spacer 37 and preloaded by a seal cover 35 and wave spring 43. The preload force should be accurately calculated to ensure the concentricity and long-lasting static and dynamic precision between the rotor 31 and stator 32.

An index matching fluid could be used to fill in the space of stator. The shaft seal 36 and o-ring 41 are utilized to seal the assembly. One function of the index matching fluid is for the lubrication between ceramic ball bearings and the “micro bearing”. Another function of index matching fluid is for pressure compensating purposes. The whole space inside the stator 32 could be used as the pressure compensation chamber. The shaft 40 on rotor 31 is designed long enough to allow the shaft seal 36 to slide axially like a pressure compensation piston when the ambient pressure is not balanced with the pressure inside the stator 32.

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.

Claims

1. A fiber optic rotary joint for optic signal transmissions comprising: A pair of relatively rotatable members: a rotor and a stator; A rotor is mounted in said stator to rotate relatively thereto through a pair of ceramic ball bearings; A first fiber optical collimator assembly being secured in the central hole of one of said rotatable member; A second fiber optical collimator assembly being secured in the central hole of another said rotatable member; A shaft seal, a seal holder and o-ring means to seal the collimator assembly of the said stator and said rotor to form a sealed space with the said first fiber optical collimator assembly and second fiber optical collimator assembly.

2. For fiber optical rotary joint of claim 1 wherein one each of said first fiber optical collimator assembly and said second fiber optical collimator assembly including a holder with an angled end surface; a micro-collimator secured in the central hole of said holder; a second conventional collimator secured in the off-axis hole of the said holder through a hollow collimator housing with its own axis parallel to the rotational axis; a first reflecting surface located on the front end of said hollow collimator housing at a specific angle with the axis of said conventional collimator and parallel to the said angled end surface of said holder; a second reflecting surface being formed on the said angled end surface of said holder.

3. For fiber optical rotary joint of claim 1 wherein one each of said first fiber optical collimator assembly and said second fiber optical collimator assembly including a holder with an angled end surface; a conventional collimator secured in the off-axis hole of the said holder through a hollow collimator housing with its own axis parallel to the rotational axis; a first reflecting surface located on the front end of said hollow collimator housing at a specific angle with the axis of said conventional collimator and parallel to the said angled end surface of said holder; a second reflecting surface being formed on the said angled end surface of said holder; a first fiber secured in the central hole of said first holder and protruded out of said central hole of said first holder; a second fiber secured in the central hole of said second holder and recessed inside said central hole of said second holder.

4. For fiber optical rotary joint of claim 1 and 2 wherein a first channel of light path including said micro-collimators co-axially fixed in said holders respectively; light signal from one of said micro-collimator directly coupled into another micro-collimator; a second channel of light path including said conventional collimators and said first reflecting surface and said second reflecting surface; light signal emitted from one of the said conventional collimator will be reflected by one of the said first reflecting surface, one of the said second reflecting surface, another said second reflecting surface, and another said first reflecting surface, then coupled into another said conventional collimator.

5. For fiber optical rotary joint of claim 1 and 3 wherein a first channel of light path including said first fiber and said second fiber; said first fiber should be long enough to protrude into the central hole of said second holder; the end surfaces of said first fiber and said second fiber being oppositely positioned inside the said central hole of said second holder on the rotational axis but separated by a very small clearance; the diameter of central hole of said holders should be slightly larger than the diameter of said fibers; light signal from one of said fibers directly coupled into another said fiber; a second channel of light path including said conventional collimators and said first reflecting surface and said second reflecting surface; light signal emitted from one of the said conventional collimator will be reflected by one of the said first reflecting surface, one of the said second reflecting surface, another said second reflecting surface, and another said first reflecting surface, then coupled into another said conventional collimator.

6. For fiber optical rotary joint of claim 2 and 4 wherein the diameter of said micro-collimator should be much smaller than the diameter of said conventional collimator.

7. For fiber optical rotary joint of claim 3 and 5 wherein the diameter of said fibers should be much smaller than the diameter of said conventional collimator.