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Illinois ECE graduate student develops new method to quantify QCE in optical fibers

11/8/2020

Joseph Park, Illinois ECE

Nanjie Yu
Nanjie Yu

Illinois ECE graduate student Nanjie Yu and his advisor Illinois ECE Assistant Professor Peter D Dragic recently developed a new method to quantify the quantum conversion efficiency (QCE) in optical fibers.  Their research was recently published in the IEEE Journal of Lightwave Technology.

The QCE in a fiber laser system is the fraction of photons that undergo a desired radiative process. Ideally, this number is 100%. In reality, however, non-radiative processes such as impurities absorption and quenching can lead to a reduction in the QCE value because part of energy from the pump photons eventually ends up as thermal energy.  A QCE value as close to one as possible is of particular importance.

To be more specific, with fiber laser power exceeding 10 kW per beamline, even a 99% QCE represents a significant additional thermal load (1% of the total power) on the fiber. QCE values are typically around 95% to 98% for common silica glasses used to make these fibers and can be much worse in developmental glasses. This has, until recently, been the main reason why laser cooling in silicate glasses has not been observed.  Given all these reasons, there is a need to carefully quantify the QCE, in an effort to optimize the glass compositions used in active laser fibers. 

Yu has developed a method that allows for a very high-precision measurement of the QCE value by making use of a second major source of heating in high power fiber laser systems: the quantum defect, or QD. This parameter describes the change in energy when a signal photon is generated from the pump photon. The former usually has lower energy (there are exceptions to this rule) and this energy difference is eventually converted to heat within the fiber.

Peter D Dragic
Peter D Dragic

The QD only depends on the wavelengths of the pump and signal photons, and therefore is a deterministic quantity. When pumping, any excess heat that is generated beyond the QD can be attributed to the QCE.  To quantify the QCE, Yu pumped the fiber in a vacuum situation and contactlessly measured the change in temperature of the fiber and compared that with theoretical heating expected from the QD alone.

"He used a very unique form of thermometry for his fiber temperature measurements. He used an interaction that takes place between hypersonic acoustic and optical waves in the fiber," said Dragic.

"Light waves reflect from, and are Doppler shifted by, these acoustic waves (by around 10 GHz). Since the Doppler frequency associated with these waves depends on temperature, Yu simply measured this frequency as a function of the pumping power to determine the temperature of the fiber as a function of the pumping power. Of course, this takes very careful calibration of the thermometer. Thermal processes represent one of the key limiting factors to further power scaling in high-power fiber lasers. Part of this thermal energy generation originates from the quantum defect, which can be partly managed through judicious selection of pumping and lasing wavelengths."
 
As the first high-resolution, non-contact experimental method to quantify QCE in Yb-doped fibers, their method can be built for testing platforms both in research labs or industry. Their proposed method will be particularly useful to give insight into the origins of nonradiative processes and give directions to the future design of fibers aiming at high-power applications and anti-Stokes fluorescence cooling. Furthermore, it can also be used as a sensitive non-contact for accurate temperature sensing in optical fiber research such as and anti-Stokes fluorescence cooling.
 

Dragic is affiliated with the HMNTL