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HIGH-POWER SINGLE MODE
FIBRE COUPLING©2021 Novanta Corporation. All rights reserved.

High-Power Single Mode Fibre CouplingAuthors: Dr. Christian Knothe from Schaefter+Kirchhoff GmbH; Konstantinos Saroglou, Matt
Fitton, Simon Novelli and Dr. Gregor Klatt from Novanta

Abstract
High-power Single-Mode (SM) fibre coupling of continuous wave (cw) lasers in the visible range is shown at different
wavelengths with coupling efficiencies as high as 80%. Whilst this value is easily achievable when laser light is coupled
into multimode fibres, for single-mode fibres, 80% efficiency is close to the theoretical limit, and presents a number ofsignificant challenges, especially at powers higher than a few 100 mW. This paper describes work undertaken by Novanta
where a 2.4 W output from a SM fibre with 3 W input is achieved at 532nm.

Overview
In 2009, Charles K. Kao received the Nobel Prize in Physics for his ground-breaking theoretical work on single-mode
optical fibres, performed in the 1960s at STL in Harlow, UK. The small core diameter of only a few µm makes Single-Modeand also Polarization-Maintaining fibre coupling (SM/PM fibre coupling) much more challenging than simple multimodefibre coupling. The main difference between both fibre types is the area or volume which is really guiding the light. In a
SM fibre, only one single transverse mode, the ‘fundamental transverse mode’ typically labeled as the TEM00 mode, canpropagate in form of a Gaussian beam. In contrast, in a multimode fibre, many modes exist and propagate simultaneously.
Crucial parameters for SM fibre coupling are the Numerical Aperture (NA), the Mode Field Diameter (MFD) and the cutoff wavelength (λ0). While the NA describes the ability of a fibre to gather light, the MFD is the diameter of the Gaussian
intensity profile within the fibre. Only light above the cut-off wavelength is guided in single Gaussian mode through thefibre.
During this development, Novanta worked closely with Schäfter + Kirchhoff (Hamburg, Germany); a well-known supplierof advanced fibre optics who provided a series of wavelength matched fibres and fibre couplers optimized to Novanta’s
gem and opus lasers. Both the gem and opus are high power lasers available across the visible and infrared spectrum,
delivering perfect beam quality in a very small form factor. For safe laser operation, the fibre coupler can be directly
attached to the laser head, eliminating any open, free space beam. Furthermore, no optical isolator is necessary forstable laser operation. Figure 1 illustrates a basic configuration, consisting of a gem laser at 532 nm coupled into a SM
fibre with a 1 m fibre cable. The gem laser was set to 1.25 W, resulting in an output of 1 W output from the fibre. Thisachieved coupling efficiency of 80% is close to the theoretically achievable results of such a high-power configuration,with a ceiling of 85% achieved with low power lasers in the 10 mW range. The reflectivity losses from the fibre end facets
are 4% each, so the additional losses cause by all other imperfections only amount to around 10%.

Fig. 1: SM-fibre coupled gem 532 laser with a 1 m fibre.
The fibre output is directed to the thermal head of a power meter.

©2021 Novanta Corporation. All rights reserved.

High-Power Single Mode Fibre Coupling

Many applications especially in the field of microscopy require an almost perfect beam with TEM00 beam profile for
either sophisticated beam manipulation or tight focusing. Multimode fibre-coupled systems do not provide a solution,
since after the fibre no TEM00 beam exists. Figure 2 shows the beam profile after the fibre output from the Novanta SM
fibre coupling. The already near-perfect beam from the laser with a M2 < 1.2 (M2 < 1.1 for 532 nm systems) is further
improved by the fibre as it makes the laser beam even more symmetrical and removes residual astigmatism. In effect, aSM fibre acts as a mode filter even for input beams with inferior beam quality.

Fig. 2: Measured and filtered beam profile from the fibre output showing an almost “perfect” beam.
The red ring indicates the 1/e2 beam diameter.

Single Mode (SM) Fiber Coupling Benefits
SM fibre coupling makes the utilization of lasers much easier; it enables “plug & play” laser light without the need for
mirror realignment or other guiding optics. Furthermore, a fibre coupled system is also highly advantageous from a laser
safety perspective. Beside these obvious advantages, SM fibre coupled systems also help when beam pointing variations
caused by temperature changes are an issue.

Fig. 3: Coupling efficiency of a gem laser at 561 nm over the output power at constant temperature.
©2023 Novanta Corporation. All rights reserved.

High-Power Single Mode Fibre Coupling

The gem and opus laser family offer exceptional beam pointing stability which results in consistent fibre coupling efficiency
across a large portion of the power curve and across the operating temperature range (see Fig. 3). Delivering the lightthrough a SM fibre further improves beam pointing stability by eliminating any minor drift with changing temperaturesor power levels since the fibre coupler with the attached fibre will follow the movement in any direction as it is subjectedto the same external variation.

Fig. 4: Long term coupling efficiency over 45 hours at constant temperature.

In another test, long-term coupling efficiency over an extended 45-hour period was measured at constant temperature(see Fig. 4). No fluctuations were visible, demonstrating that the laser and fibre coupler arrangement is also extremely
mechanically stable. Mechanical stability was further proven in a 30-minute vibration test, where the laser was subjected
to a maximum acceleration of 4.9 g RMS in one direction. The coupling efficiency was measured before and after the testwith no change in performance being observed.

Fig. 5: gem laser with attached fibre on a vibration test plate.

Finally, the effect on the coupling efficiency with changing
laser temperature was investigated. In this arrangement,
the gem laser was placed on a temperature-controlled plate
while the laser was operating in constant power mode. The
output power from the fibre was compared against theinternal power monitor reading each time after the laser
temperature had settled. By comparing the two powers, boththe internal reading from the laser and the external readingfrom the fibre, the coupling efficiency could be determinedeach time for each new temperature. The fitted results areshown in figure 6 and show that over the 16 degrees Celsius
range observed, the coupling efficiency remained above
75% and was repeatable across successive runs and always
returned to its peak efficiency of 81% at the originally aligned
temperature. This supports the statement that the laser
coupling is highly mechanically stable and repeatable.

©2021 Novanta Corporation. All rights reserved.

High-Power Single Mode Fibre Coupling

High-power single-mode fibre coupling enables solutions in many optical applications. In super-resolution microscopy
for example, SM fibre-coupled laser sub-systems in the multi-Watt regime are now possible, thanks to the excellent
optical engineering and beam quality now available. Taking these advances further, the fast modulation of the output
power with the help of Acousto-Optic-Modulators (AOM) or Acousto-Optic Tunable Filters (AOTF) is also possible, as
fibre coupled devices achieve much higher switching speeds. This is important in the field of quantum sensing or, more
precisely, in the excitation of nitrogen-vacancy centres (NV-centres) in diamond. On the other hand, applications that arevery sensitive to beam pointing variations benefit from efficient SM fibre coupling since any minor drift with changing
temperatures or power levels is eliminated due to the fact the fibre coupler with the attached fibre will follow the movein any direction.
Novanta is now able to offer a robust solution to the previously challenging task of achieving high efficiency fibre couplingof high-power lasers. This solution is now available across all Novanta’s DPSS laser wavelengths (473, 532, 561, 640, 660,671, 1064 nm) where up to 3 W power can be coupled to single-mode or polarization maintaining fibres with a guaranteed
coupling efficiency exceeding 70%.

Fig. 6: Long term coupling efficiency over 45 hours at constant temperature.

Novanta Benefits
Novanta is uniquely positioned to solve even the most complex challenges for OEMs, system integrators, and end-use
customers seeking to advance their manufacturing processes with high precision laser systems. With some of the most wellknown brands in the industry and in-country application and service support, Novanta delivers reliable, precise, and durable
components and sub-systems. For more information, visit our website. www.novantaphotonics.com.

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