Optical component testing in a rapidly evolving 5G world

Technical Features

Backgrounder
Ever since low loss optical fiber was introduced in telecom networks, fiber-based components have been evolving to address new challenges. The examples are many–from couplers allowing easy signal redirection to multiplexers combining several optical signals with low insertion loss.
These devices called for innovation in the test & measurement (T&M) industry to measure power and wavelength with the help of lasers. At the same time, phenomena such as chromatic dispersion, return loss or polarization dependent loss (PDL), became more important parameters to consider as the optical telecom market started to grow.

New challenges
Nowadays, testing passive components has become a routine task that typically involves measuring devices over a limited number of parameters to reduce costs. Some characteristics, such as chromatic dispersion, may not be as problematic as they used to when it comes to impacting overall network performance. Others like PDL are now more critical. And there’s more; new optical characterization challenges are popping up with the bandwidth boom and the advent of 5G. Tests on 5G devices are increasingly stringent and need to be performed reliably, putting more pressure on existing T&M systems. The arrival of compact components based on integrated photonics also adds to the need for T&M methods to evolve.

Measuring loss of passive components
To take reliable loss measurements efficiently for today`s complex components, the best option is the swept laser technique. In this setup, a continuously tunable laser is used to scan the wavelength across the spectral range of interest. Operating jointly with the laser, a component tester is then used to record the optical power of each output port of the device under test.
Lasers have an obvious advantage: the high optical power offered by these sources makes it possible to measure the characteristic loss of devices over a much larger dynamic range than techniques using a broadband source. This accessible dynamic range is further enhanced when using tunable lasers with large signal to source spontaneous emission ratio (SSSER), where high insertion loss can readily be measured (Figure 1).

Fig 1. Spectrum of an optical filter acquired with a laser with high SSSER (green) and low SSSER (red). Transfer function is the additive inverse of insertion loss.

The component tester is a specialized instrument that has very short test times and excellent accuracy: it records the test wavelength and output power simultaneously as the laser is being swept, as opposed to a stepped wavelength measurement where the laser is moved one step at a time. The technique allows wavelength resolution of the order of the picometer, sometimes even smaller, with a full spectrum acquired within a few seconds. A larger spectral range, important when characterizing components spanning all telecom bands, can also be tested using several tunable lasers, each covering a portion of the total spectrum under test and concatenating the insertion loss or return loss result into a single spectrum. It is also the only method to provide picometer-resolution PDL spectra within a reasonable timescale.

Figure 2 shows a typical setup for the swept laser technique and the resulting spectrum when measuring four output ports of a WDM demultiplexer. In this configuration, each laser will successively perform a sweep, the component testing unit taking care of switching between lasers, recording wavelength and the output power on all four ports. The results are then displayed with 1 pm sampling resolution and a 5 pm resolution for PDL measurements.

Fig 2. Typical Setup for a component characterization over the full telecom range. Bottom right: Spectrum of WDM demultiplexer obtained with an EXFO CT440 component tester.

Photonic integrated circuits (PIC) and the future
Integrated photonics is set to be the next disruptive technology, particularly for the development of 5G across the globe. Some key developments of the technology are also underway in medical, sensing or military applications. In the same way as the electronic processors, photonic integrated circuits are imprinted onto wafers, before being cut into individual chip. To reduce operating costs, PIC manufacturers need to characterize each chip directly on the wafer. Advanced passive optical components now include opto-electronic or electro-optic functions that also need to be tested. Some component testers can test chips both optically and electrically across a spectral range, drawing a more complete picture of the component characteristics.

As the telecom landscape continues its transformation, optical T&M vendors and PIC manufacturers are already working together to provide fast and reliable characterization setups, where the results can then be analyzed to sort defective chips, to better understand fabrication tolerances and improve yield or to record those characteristics in a database to improve simulation and PIC design.

Fig 3. Photonic integrated circuit parametric testing directly on wafer. Optical and electronic characterization can be performed by the component tester.

François Couny
Product Line Manager, EXFO

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