Enabling Optical Communication from Avionics to Social Media

Joe Bos
Lead Optical Engineer
Product Development, R&D

Q.  What do the F-35 JSF, Netflix, cloud computing and Facebook all have in common (besides counting their net worth in billions of dollars)?

A.  They all rely on high-bandwidth, fast data communication realized through optical fiber technology

Our voracious appetite for high-speed data and increasing bandwidth is seemingly insatiable and will continue to drive telecommunications innovation.  

It doesn’t matter what sector of the industry you are in- component manufacture, integration, system design, or test and measurement like us- this means tighter specifications on insertion loss (IL), return loss (RL), length, chromatic dispersion (CD), polarization mode dispersion (PMD), polarization dependent loss (PDL)… and a host of other two and three letter acronyms.

In this entry we will discuss the easy ones- IL, RL, and length measurements- what they are, what they mean, and how to measure them.  We will leave the topics of polarization and dispersion for a future conversation. 


Of course everyone knows what length is, but its significance lies beyond the point that when you order optical spools, or patch cords, you expect them to be the correct length.  In parallel fiber communication, length differences across a fiber bundle, or ribbon can lead to bit errors.  This measurement is typically referred to as “skew” and is measured in units of optical delay, e.g. femtoseconds.  Skew is especially important as data transfer rates increase and clock cycles get shorter.  Skew Whitepaper

Return Loss

Optical return loss (RL) is a measure of reflected power.  By definition it is the ratio of the reflected power to the input power, and is typically expressed in dB.  A MEMs mirror, a damaged connector or a botched AR coating each exhibit some amount of RL.  In the case of a mirror, high RL is intended, but often we desire the lowest possible RL, especially as DWDM architectures are increasingly deployed.  Reflections in a DWDM network can lead to wavelength instability in the laser source.  Further, accurately identifying and measuring RL events in a device, module or fiber link can help identify design and manufacturing short-falls before installation.  Learn More  

 Insertion Loss

Insertion loss (IL) is a measure of the light lost through an optical event, or device in transmission.  More precisely, it is the ratio of the transmitted power to the input power and is typically expressed in dB.  Minimizing and budgeting for IL is paramount, since the data we send is often encoded in the amplitude of light pulses- no light implies no data.

So, how do we measure RL, IL and length?

In long haul networks (greater than 1 km) a source and power meter, or OTDR, is typically used to measure RL, IL and length, however OTDR is not suitable for networks found in avionics, shipboard applications or data centers.  The problem is, OTDR provides length resolution on the order of meters- which is good enough for a 50 km run of optical fiber- but events in short-haul networks occur on a scale of millimeters to meters.  For these applications, OFDR-based reflectometers provide the best solutions for locating and measuring RL, IL and Length.

Learn More

Reflected amplitude versus length down a fiber assembly. IL, RLand length are all contained in this single one-sided measurement.

Okay, but specifically for components and modules?

We recently developed an instrument specifically for IL, RL and length measurements in components and modules- the OBR 5T-50.  Luna’s OBR 5T-50 is a simple to use, low-cost reflectometer for fast and accurate distributed IL, RL and length measurements.  With 19.6 micron spatial resolution it’s a great tool for locating faults, aligning optics, and verifying designs of precision optical cables and components, including fiber based and PLC based waveguide devices.

Learn More data sheet

Reflective interfaces inside a 5 mm long photodiode assembly, measured with Luna’s OBR 5T-50. The RL events marked by the yellow and red cursors are 421 microns apart. The measurement two-point resolution is 19.6 microns.

Will you be at OFC?

You can find us in booth #3619 with several demonstrations on hand.  Be sure to visit and try out an OBR 5T-50 or just stop by and talk with an engineer – we’re happy to discuss your application and help ensure your success.

Oh, and one last thing- we are also introducing at OFC our newest OSA-like spectral analysis instrument- the SRM 5T-50, which boasts IL, GD and PDL measurements over the entire C band at a measurement rate of 11.9 Hz.  With better than 1 pm spectral resolution, and NIST- traceable spectral and length accuracy- it is an indispensible tool for DWDM manufacturing and test applications- you have to see it to believe it! 

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