The Fiberoptic Rotary Joint (FORJ) is the optical equivalent of the electrical slip ring. It allows uninterrupted transmission of an optical signal while rotating along the fiber axis. The FORJ is widely used in missile guidance systems, robotic systems, remotely operated vehicles (ROVs), oil drilling systems, sensing systems,
medical devices (OCTs), broadcasting and many other field applications where a twist-free fiber cable is essential.
A quick patent search produces hundreds of inventions around two-channel and
multiple-channel fiberoptic rotary joints. Commercially,
however, only a limited number of design principals are
implemented. At Princetel, we use our
proprietary technologies to facilitate both the dual-port (2-channel) and multiport
(multi-channel) FORJs. We offer our customers an exciting new
alternative with better performance.
A FORJ’s only function is to provide connection between two or multiple fiber cables
while allowing free rotation between them. Therefore, it is critically
important to ensure that the device has low insertion loss, small insertion loss variation
(wow), and high return loss
(a measure that characterizes the amount of reflection a FORJ generates).
Insertion Loss: 3-dB insertion loss is equivalent to 50% transmission. Most
commonly used media converters offer
overall optical budget greater than 15 dB. However, there can be a number of
fiber connections and components between the converter
pair, each gives rise to loses. Therefore, it is
essential to maintain a relatively low insertion loss
through fiber optic rotary joints. Typical insertion
loss of Princetel's single-channel FORJ is 0.5 dB and 3
dB for dual and multiple channels.
Insertion Loss Variation (wow):
It is natural to experience some loss variation as the FORJ rotates due to changing
coupling conditions. However, if this variation reaches a certain level, signal-to-noise ratio degrades.
If wow is critical in your application, select
Princetel's single-channel FORJs which feature less than
+/-0.25 dB wow. A rotary joint's wow is also an
important indicator on the device's overall condition.
It should always be the first thing to check when things
go wrong. For example, if a FORJ suddenly shows
significantly higher wow, it is almost certain that the
FORJ is faulty. Otherwise, the most likely cause is
somewhere else, such as faulty connectors or broken
Return Loss: All laser sources, especially DFB (distributed feedback) lasers, are sensitive to optical reflection,
which causes spectral fluctuation and subsequently, power jitter. Return loss is a measure of the amount of
reflection accruing in an optical system. A -45 dB reflection is equivalent to 45 dB return loss.
While return loss as high
as 60 dB can be achieved with singlemode fibers even for multi-channel FORJs, the typical figure for multimode is around 40 dB.
40 dB return loss is a common requirement in the industry for
singlemode passive components such as a FORJ to ensure normal system operation.
All Princetel's FORJs satisfy this requirement. 50 dB or
even 60 dB figures can be provided as standard options.
Single channel vs. multiple channels
The single channel FORJ has a very simple mechanical structure which allows the device to be compact, high speed,
highly reliable, and long lasting. A single singlemode fiber can carry tens if
not hundreds channels of electronic data channels. The cost of an n-channel
multiplexer and a single channel FORJ
combined can be much lower than that of an n-channel FORJ with no multiplexer.
For those who definitely need two-channel FORJs Princetel offers a coaxial design to maintain a compact
package and good performance at a reasonable cost. And there is no “dead spot” at any rotation angle.
A much more complex mechanical structure is necessary to facilitate the multi-pass (multi-channel) FORJs. The
device also requires careful optical alignment to maintain good performance in all three key areas mentioned
above. Poor alignment or inadequate optical design will not only compromise the
key optical performance, but also create
potential new problems such as high wavelength dependent loss, high PDL (polarization dependent loss), high PMD
(polarization mode dispersion), and high crosstalk (see further discussion
below). Princetel’s proprietary multi-channel design allows
channel count as high as 12 as standard and dozens as custom solutions.
Wavelength dependent loss is a measure of uniformity of the insertion loss vs. wavelength since multiple
wavelengths, such as 850, 1310, and 1550 nm, are common. If the optical system contains a highly dispersive element, wavelength dependent loss can be an issue. 0.5-1.0 dB insertion
loss difference between 1310 nm and 1550 nm is generally acceptable.
Similar to wavelength dependent loss, polarization
dependent loss is a measure of uniformity of insertion
vs. polarization. The input polarization randomly changes in a typical fiber
optic system. Less than 0.1 dB is considered the
industry standard for passive components. An optical
component with a non-normal
incidence angle is known to create PDL at elevated level.
Polarization mode dispersion is a measure of pulse stretching due to the travel speed difference of the two
orthogonal polarization modes. Any natural or induced birefringence will cause PMD in the system.
0.1 pico-second (ps) or less is a common requirement for passive components.
PMD is usually not a consideration unless the signal
bandwidth is gigabit or higher.
Crosstalk measures the amount of signal crossing between adjacent channels. The
higher the figure, the lower the crossing. For example,
60 dB crosstalk represents crossing of 1 part per
million to adjacent channels. Crosstalk requirement
differs from system to system. In general, the higher
the system optical budget, the higher the crosstalk
figure should be. All Princetel's dual and multiple
channel FORJs offer greater than 60 dB crosstalk as
Singlemode vs. multimode
The much smaller fiber core (9 microns) in singlemode fibers
maintains optical beams in their purest form, a mathematically perfect Gaussian profile.
Singlemode fiber's bandwidth capacity is far larger than
that of multimode fibers. Its bandwidth is also much
less dependent on fiber length and can be expanded
easily. Overall, multimode systems still offer the cost
advantage over singlemode. However, the gap is narrowing
as the popularity of singlemode grows.
The history of passive fiber optic components has proved that size does matter. In fact, size reduction has
long been one of the driving forces for innovation. Most FORJs are used with
electrical slip rings, RF rotary joints, or fluid rotary
union. Occupying the center of the system, FORJ's
diameter often drives the overall size of the system.
Smaller FORJ usually means lower cost for the entire
Pigtails vs. receptacles
A FORJ can be packaged with either pigtails or receptacles on both ends of the device.
The pigtailed version typically has more repeatable optical performance since it eliminates the
uncertainty in connector joints. Faulty connectors in
pigtailed version can be easily re-conditioned or
re-terminated while similar defect in receptacle version
might result in irreversible damage.
We recommend the pigtailed version to most of our customer for those reasons. However, for applications that do not
allow fixed pigtails on the device, we do offer both FC and ST format receptacles in our R series FORJs.
We can even
mix and match pigtail and receptacle in one device to create custom configurations.
Almost all standard FORJs available on the market do not
have center bores. Those that do are not truly passive
FORJs. In other words, the output fiber is replaced with
photo detector. It is much more challenging to design a
passive FORJ with center bore. Princetel has a few
different designs of passive FORJs with center bores
targeting a variety of potential applications. Please
provide the following key parameters in your inquiry.
Number of fibers
Limited rotation or infinite rotation in each run
Overall max dimension
FORJs have been used widely in underwater applications where water tightness and pressure equalization are important.
Fluid filling not only makes pressure compensation
possible but also provides lubrication and index
matching. Princetel offers both fluid filled and fluid
free models to satisfy different needs.
All our FORJ products can be custom made to meet specific requirements. Call us if you do not see an immediate
solution on our website. You may be surprised on how much we can make your job easier.
Last updated: 2009.07.18