Fiber optic rotary joint

A Fibre optic Rotary Joint is equivalent to an electrical slip ring but in the domain of optics and is mostly used as a co-existing element along with slip rings. Fibre optic Rotary Joint (FORJs) are available in single and multi-channel forms and have a special characteristic of having unlimited reach.

A Fibre Optic Rotary Joint (FORJ) is a device allowing a light or optic signal to be transferred across the interface between a stationary support structure and its continuously rotating platform. The FORJ applications have multiplied with the increasing adoption of fiber optic joints for communication transmission.

FORJ’s are used in a plethora of applications like radar pedestals, wind turbines, as well as electro-optic sensors, have implemented fiber optic rotary joints to take care of optical signals simultaneously with slip rings to handle electrical power and signals.

FORJs can be divided into categories basis mode of operation:
• Single or multi-pass
• Passive or active.

Passive FORJs is responsible for transferring an optical signal between the rotating and stationary structure without the support of electronic processing with the help of elements like filters and lenses that can be used to interpret the optical signal.

Active FORJs uses the principle of electronics to process the signal to positively influence rotor to stator transmission properties and involves electrical/ optical conversions, amplification, etc.

An example of the implementation of the active FORJ can be the medical CT scanner which can be used to carry high-speed image data from the rotating X-ray detectors to the stationary data processors.

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Process of working of FORJs

Single Pass FORJ

Single-pass FORJs are rotary joints where a single fiber enters the FORJ from one of the sides of the rotating assemblage making the optical signals combined between them as one fiber rotates relative to the other. It follows the basic principle of magnifying the diameter of the coupled light by the use of lenses, bundles, or large diameter fiber.

However, the most common approach used here is the usage of lenses to extend the optical beam leading to minimizing the effects of mechanical misalignment.

Implementation: Single-pass FORJs are used primarily for digital data transmission and has been commercialized using the general design philosophy of multimode fiber. FORJs also follows the rule of directly combining the light across the gap without lenses. This is possible by the significantly greater core size of the Plastic Optical Fibre (approx.1.0 mm) which makes tolerances at all combining sites, including connectors, less critical. The success of Plastic Optical Fibre FORJ technology has its use in industrial applications, with the basic limitation being difficult to operate over shorter distances.

In such kinds, anti-reflective coatings are used on both the ends of the GRIN lenses to reduce reflections and remarkably improve the returned loss of the FORJ simultaneously improving the insertion loss to a lesser degree.

Without anti-reflective coatings, the reflectance at a glass

(refractive index, n_gl =1.5) to air (refractive index, n_air =1) interface normal to the beam path is
R = ((ngl – nair)/(ngl + nair))2 = 0.04
If all of this reflected light is coupled back into the input fiber the return loss (RL) is approximately 14 dB where
RL = -10 x log10 (R)

Multi-Pass FORJ

In these cases, multiple fibers enter the FORJ from either side of the rotating interface. In such kinds, the optical signals are coupled between special single selected pairs of fibers across the rotating interface enabling a multi-pass FORJ to spread multiple independent data streams all across the interface. This is common even when the optical multiplexing is not used.

A multi-pass FORJs are usually more complicated than single-pass FORJs because the specifics are required to provide rotary alignment of multiple fibers. Multi-pass FORJs are available in a variety of configurations and these kinds are usually physically larger than single-pass FORJs because of the existence of an increased number of fibers pairs and its complex structure.

Mirror-based Cell Approach

The mirror-based cell design is another way and has found its place in FORJ applications. This design is specifically designed to be useful in the marine environment where it is frequently required to fluid fill the FORJ in order to compensate for the sealing pressure.

In such configuration, a number of optical fibers enter the FORJ on one side of the rotating interface and, light from one fiber is reflected towards the rotation axis along with a short distance by using mirrors that are attached to a rotor rotating at the same speed in the same direction as the direction of the variety of fibers that entered the FORJ.

Within each cell, there is a third mirror that is held stationary to the cell and maintains the function of reflecting the beam of light located on the rotational axis of the FORJ towards the fiber aligned at 90 degrees to the rotation axis.

The light from a fiber that is combined across the rotary interface in a cell away from the variety of optical fibers that entered the FORJ must pass through the rotors of all individual cells closer to the variety of fibers.

Another important law is that each cell must also match its rotation, speed, and direction which can be achieved by clubbing the rotors to each other and to the variety of optical fibers using a gear train.

The mirrors that are attached to each of the cells are held in place by technique’s which specifically allows the light from fibers clubbed across the rotary interface in cells far from the variety of fibers to pass unimpeded. The distance between the two corresponding lenses attached with each pass increases as the cell-attached with the pass is located further from the variety of fibers.

Here, the loss of the fiber due to insertion increases, as a result, leveling up to the increase in the lens to lens distance usually limiting the number of fibers that pass through to ten or less.
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    Functionality of Fibre optic Rotary Joint

    The functionality of a Fibre Optic Rotary Joint (FORJ) revolves around its ability to transmit optical signals across a rotating interface without signal degradation. Here’s a detailed look at how a FORJ functions:

    1. Optical Signal Transmission: The primary function of a FORJ is to allow the passage of optical signals (light) between a stationary and a rotating component. This is essential in applications where cables cannot be twisted or where continuous rotation is required.
    2. Core Components:
      • Rotary Interface: This is the part of the FORJ that allows for the rotation. It typically consists of a stationary part and a rotating part.
      • Optical Fibers: These carry the light signals. The FORJ ensures that the alignment of these fibers remains intact despite rotation.
      • Lenses and Mirrors: Some FORJs use lenses or mirrors to guide the light from the stationary fiber to the rotating fiber.
    3. Alignment: Maintaining precise alignment of the optical fibers is critical. Any misalignment can result in signal loss or degradation. FORJs are designed to maintain this alignment even at high rotational speeds.
    4. Single vs. Multi-Channel:
      • Single-Channel FORJ: Transmits one optical signal at a time.
      • Multi-Channel FORJ: Capable of transmitting multiple optical signals simultaneously. This is achieved through multiple optical paths within the same joint.
    5. Rotational Capability: FORJs can handle continuous rotation, which is essential for applications like wind turbines, medical imaging systems, and robotic arms where the system components need to rotate freely without hindrance.
    6. Low Signal Loss: FORJs are designed to ensure minimal signal loss. High-quality materials and precision engineering help in achieving low insertion loss and high return loss, ensuring that the integrity of the optical signal is preserved.
    7. Durability and Reliability: FORJs are built to withstand harsh environments, including extreme temperatures, moisture, dust, and vibrations. This makes them suitable for use in demanding industrial and military applications.
    8. Bidirectional Transmission: Many FORJs allow for bidirectional transmission, meaning that optical signals can travel in both directions across the joint.

    Use Case Scenarios

    • Telecommunications: In systems where antennas or other components need to rotate while maintaining a high-speed data link.
    • Medical Devices: In MRI machines and other diagnostic equipment that require rotating parts while maintaining a stable data connection.
    • Robotics: In robotic arms and machinery that require continuous rotation and data transmission.
    • Military: In radar systems, surveillance cameras, and other equipment that require 360-degree rotation.

    A Fibre Optic Rotary Joint ensures that optical signals can be transmitted seamlessly across rotating parts of a system, maintaining high data integrity and allowing for continuous rotation without the risk of cable tangling or signal loss.

    slip rings in Robotics

    Types of Fibre optic Rotary Joint

    Fibre Optic Rotary Joints (FORJs) come in various types, each designed to meet specific application needs and technical requirements. Here are the primary types of FORJs:

    Single-Channel FORJs

    • Description: These FORJs transmit a single optical signal across the rotating interface.
    • Applications: Ideal for applications requiring the transmission of one optical channel, such as simple sensor data links or communication systems.
    • Advantages: Simpler design, lower cost, and easier to maintain.

    Multi-Channel FORJs

    • Description: These FORJs can handle multiple optical signals simultaneously, using multiple optical fibers or wavelengths.
    • Applications: Suitable for complex systems that need to transmit multiple data streams, such as advanced communication networks, medical imaging systems, and industrial automation.
    • Advantages: Higher data throughput and the ability to support multiple devices or sensors.

    Single-Mode (SM) FORJs

    • Description: Designed for single-mode optical fibers, which have a small core diameter and are used for high-precision and long-distance data transmission.
    • Applications: Telecommunications, long-haul data transmission, and applications requiring high bandwidth and low signal loss.
    • Advantages: Low insertion loss, high return loss, and the ability to transmit over long distances with minimal signal degradation.

    Multi-Mode (MM) FORJs

    • Description: Designed for multi-mode optical fibers, which have a larger core diameter and are used for shorter distance data transmission with higher power.
    • Applications: Data centers, local area networks (LANs), and industrial applications.
    • Advantages: Can transmit more power and handle higher data rates over short distances.

    Hybrid FORJs

    • Description: Combine optical signal transmission with electrical signal transmission. These joints can transmit both types of signals across the rotating interface.
    • Applications: Suitable for applications requiring both power and data transfer, such as robotic systems, surveillance systems, and advanced sensor arrays.
    • Advantages: Versatility in handling multiple types of signals and the ability to support complex systems.

    Multi-Pass FORJs

    • Description: These FORJs use multiple optical paths or passes within the joint to increase the number of channels or improve signal quality.
    • Applications: High-performance systems requiring redundancy or enhanced signal integrity.
    • Advantages: Enhanced reliability, higher channel capacity, and improved signal quality.

    Custom FORJs

    • Description: Tailored to specific customer requirements, custom FORJs can be designed to meet unique technical specifications or environmental conditions.
    • Applications: Specialized industrial, military, or scientific applications where standard FORJs may not suffice.
    • Advantages: Customized to meet precise needs, offering optimal performance for specific applications.

    Key Considerations in Choosing a FORJ

    • Number of Channels: Determine the number of optical signals that need to be transmitted.
    • Mode Type: Choose between single-mode or multi-mode based on the required distance and data rate.
    • Environmental Conditions: Consider the operating environment, including temperature, humidity, and potential exposure to contaminants.
    • Rotational Speed: Ensure the FORJ can handle the required rotational speed without compromising signal integrity.
    • Integration with Other Systems: For hybrid FORJs, consider the need for combining optical and electrical signal transmission.

    Each type of FORJ is designed to meet specific requirements, ensuring reliable and efficient data transmission across rotating interfaces in a wide range of applications.

    Applications of Fibre optic Rotary Joint

    Fibre Optic Rotary Joints (FORJs) are crucial components in various applications that require reliable data transmission across rotating interfaces. Here are some key applications of FORJs:


    • Radar Systems: FORJs are used in radar systems to transmit data between rotating antennas and stationary control units.
    • Satellite Communication: Ensures seamless data transmission between rotating satellite dishes and stationary ground equipment.

    Medical Equipment

    • MRI Machines: FORJs are used to transmit data from rotating parts of the MRI scanner to stationary control systems.
    • Endoscopy: In medical endoscopes, FORJs enable the transmission of high-quality images from the rotating end of the endoscope to the display system.

    Slip ring application-medical equipment

    Industrial Automation

    • Robotic Arms: FORJs facilitate the transfer of data and signals in robotic arms that require continuous rotation.
    • Manufacturing Machines: Used in automated manufacturing systems to ensure data integrity between rotating parts and control units.

    Aerospace and Defense

    • Gyroscopes: In advanced navigation systems, FORJs ensure the transmission of data from spinning gyroscopes.
    • Surveillance Systems: Employed in military surveillance systems to transmit data from rotating cameras or sensors to control centers.

    Wind Turbines

    • Data Transmission: FORJs enable the transfer of data from the rotating blades of wind turbines to stationary monitoring systems.

    Wind turbines slipring

    Marine Applications

    • Submarines and Ships: Used in periscopes and other rotating sensor systems on submarines and ships.
    • Underwater Robots: Ensure data transmission in remotely operated underwater vehicles (ROVs) with rotating parts.

    Broadcasting and Entertainment

    • 360-Degree Cameras: FORJs are used in 360-degree cameras to transmit video data from the rotating camera to the recording or broadcasting system.
    • Stage Lighting: In complex stage lighting systems, FORJs allow for the control of rotating lights and their data transmission.

    Entertainment Equipments

    Oil and Gas

    • Drilling Systems: FORJs are employed in oil and gas drilling systems to transmit data from rotating drill bits to monitoring systems.
    • Pipeline Inspection: Used in pipeline inspection robots that need to rotate and transmit data to the surface.

    Scientific Research

    • Telescopes: FORJs are used in telescopes with rotating components to transmit data to analysis systems.
    • Particle Accelerators: Employed in particle accelerators to ensure the transmission of data from rotating parts.

    Security and Surveillance

    • CCTV Systems: In CCTV systems with rotating cameras, FORJs enable continuous data transmission.
    • Radar Systems: Used in security radar systems to transmit data from rotating antennas to monitoring stations.


    • Railway Systems: FORJs are used in railway signaling systems to ensure data transmission from rotating parts of the signaling equipment.
    • Automotive: In advanced automotive systems, FORJs enable data transmission between rotating parts and control units.

    Fibre Optic Rotary Joints are vital for maintaining high-speed, reliable data transmission in applications where parts rotate continuously. They are essential in industries ranging from telecommunications and medical equipment to aerospace, defense, and industrial automation, ensuring seamless communication and operational efficiency in complex systems.

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