Four Common Problems With Fiber Optic Rotary Joints

Four Common Problems With Fiber Optic Rotary Joints

A slip ring is an electromechanical device that transmits electrical current between a stationary and rotating structure. A Fiber Optic Rotary Joint (FORJ) is the contactless optical equivalent of a slip ring, since it transmits an uninterrupted optical signal as it rotates. FORJs are widely used in many applications, including remotely operated vehicles (ROVs), robotic systems, Autonomous vehicles, medical lighting systems and military applications..

However, fiber optic rotary joints are also subject to a variety of problems that can weaken their optical signals or interrupt them completely. Signal loss is usually the most common of these issues, with other factors including the use of multi-channel FORJs, connectors and harsh environments.

1. Signal Loss

FORJs routinely experience signal losses from a variety of sources, which often accumulate from multiple causes. The transmitter must send a signal that’s much stronger than what is needed by the receiver to account for these losses, and the difference between these signal strengths is known as the FORJ’s optical budget.

The types of signal loss commonly affecting FORJs include the following:

  • Insertion loss
  • Insertion loss variation
  • Return loss
  • Wavelength dependent loss
  • Polarization dependent loss
  • Polarization mode dispersion

Insertion Loss

Insertion loss is a type of signal loss caused by the insertion of a device in the optical fiber. It can be calculated with the equation IL = 10 log 10 (Pt/Pr), where IL is the insertion loss in decibels, Pt is the power of the transmitted signal and Pr is the power of the received signal. This equation shows that an insertion loss of 3 dB means that about half the signal is lost between transmitter and receiver. Media converters typically have an optical budget above 15 dB, but each component between the transmitter and receiver contributes to the total insertion loss.

Insertion Loss Variation

The insertion loss in an FORJ varies as it rotates due to changes in the coupling conditions, which can reduce its signal-to-noise (S/N) ratio when the variation is sufficiently high. Applications that are particularly sensitive to insertion loss variation, commonly known as wow, may require an FORJ with a wow below +/-0.25 dB. Wow is also an important indicator of an FORJ’s general condition, since it will often increase sharply shortly before the FORJ fails completely.

Return Loss

Lasers are sensitive to optical reflection, especially the distributed feedback (DFB) lasers used in FORJs. Return loss is a measure of the reflected light in an optical system such that a return loss of x dB is equivalent to a reflection of –x dB. This system of measurement means that a high return loss is desirable since it indicates less reflection. A 40 dB return loss is a common standard for a single-mode FORJ, although they can achieve return losses as high as 60 dB. Multi-mode FORJs typically have a return loss of about 40 dB.

Wavelength Dependent Loss

Wavelength dependent loss (WDL) is a measure of the insertion loss’s uniformity for different wavelengths of light. This metric is important for FORJs that use multiple wavelengths, with 850, 1310 and 1550 nanometers (nm) being the most common. WDL becomes a more likely cause of failure when the FORJ contains a component that disperses light. A WDL between 0.5 and 1.0 dB for wavelengths between 1310 and 1550 nm is a common standard.

Polarization Dependent Loss

Polarization dependent loss (PDL) is similar to WDL, except that it measures the uniformity of insertion loss over polarization rather than wavelength. PDL is generally due to variations in the polarization of the input signal that normally occur in an FORJ. The most common cause of an elevated PDL is a component with an abnormal incidence angle, with 0.1 dB being the industry standard for PDL for passive FORJs.

Polarization Mode Dispersion

The speed of light varies slightly according to its polarization mode, which can cause polarization mode dispersion (PMD) in FORJs that use optical signals with different polarization. The common standard for PMD in passive FORJs is less than 0.1 picoseconds (ps). However, PMD isn’t normally a cause of failure for FORJs with signal bandwidths below one gigabit, which includes the great majority of models currently in use.

multi channel forj2. Multi-Channel FORJs

The mechanical structure of multi-channel FORJs is much more complex than that of single-channel FORJs, which adds additional causes of failure. Multi-channel FORJs have more components, making their alignment a particularly critical design consideration. Poor alignment of components causes various types of signal loss, especially WDL, PDL and PMD.

Crosstalk is also a new problem that arises with multi-channel FORJs. This phenomenon occurs when signals on adjacent channels interfere with each other, increasing their S/N ratio. Crosstalk is calculated from the signal’s strength with the equation S/N=10^(S/10), where S/N is the S/N ratio in dB, and S is the signal strength in dB. Thus, a crosstalk value of 60 dB means that the noise level of the signal is one part in one million.

3. Connectors

The connectors in an FORJ are a common cause of failure since they must join separate pieces of fiber together. The two most common types of connectors are receptacles and pigtails, with pigtails being preferable provided the application allows them. In cases where receptacles must be used, the FC and ST formats are the current industry standards. The primary advantage of pigtails is that they eliminate much of the uncertainty in a connector joint. Furthermore, faulty pigtails can be easily re-terminated, whereas a faulty receptacle typically requires the replacement of the entire connector.

4. Harsh Environments

FORJs that operate in harsh environments such as mines, oil rigs and ships require additional protections to avoid failures. This protection primarily takes the form of seals, which are rated according to the International Protection (IP) system. This system uses two digits such that the first digit indicates the FORJ’s protection from solid objects, and the second digit indicates its protection from water. The first digit in the IP system ranges from 0 (no protection) to 6 (dust tight). The second digit ranges from 0 (no protection) to 8 (continuous immersion in water below one meter).

Get a Quote for a FORJ to meet your application requirements.

Contact BGB Technology to learn more about our custom designed fiber optic rotary joints, slip rings and other joint solutions.

pneumatic slip ring

Sources

http://www.bgbtechnology.com/

http://www.freepatentsonline.com/y2012/0134622.html

http://www.dsmt.com/resources/ip-rating-chart/

 

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