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WHY 9 CUTS A GOOD FIGURE
Technology Primer

figure 9® stands for Menlo Systems proprietary fiber laser design1, the name derived from thecharacteristic shape of the cavity scheme employed. This primer explains the basic principles behind the technology - and the unique advantages it offers.
Mode locking
In simple terms, mode locking can be considered as a prerequisite to generate femtosecond laser
pulses. It refers to designing a laser cavity with operational conditions such that the cavity modes
will reliably self-organize into a stable train of ultrashort pulses, with as little of unwanted background and noise as possible.
In femtosecond fiber lasers, this has been commonly achieved by employing so-called intrinsic
saturable absorbers - materials whose absorption drops at high optical intensities. However, apartfrom potential degradation over time, intrinsic effects related to their slow relaxation might cause
excessive noise floors2 – which is detrimental to precision applications like optical frequency combs.
This has driven our search for a superior alternative: A fast mode-locking scheme based on effects
other than absorption.
Fiber loop mirrors
For an intuitive picture, we recall a fiber coupler splitting a light pulse arriving at one of its input
ports into two equal portions (Figure 1a). Joining the output ports by a fiber loop, the two portions
will propagate in opposite directions and interfere again at the splitter (Fig. 1b). In the simplest case,this will result in the original pulse reflected back to where it came from – the fiber loop acts like amirror (Fig. 1c).
The reflectivity of the loop mirror changes if the two counterpropagating pulses, for some reason,acquire a phase shift relative to each other before interfering. The phase shift will affect the interference conditions such that a certain fraction of the original input pulse will be transmitted into thesecond input port, rather than being reflected (Fig. 1d). Now, if the ratio of transmission to reflection
can be designed to depend on the intensity of the original input pulse, the fiber loop may act as afast artificial saturable absorber.

Figure 1: A fiber loop mirror

Nonlinear amplifying loop mirrors
An implementation of the above principle, shown in the upper part of Figure 2, is commonly referred
to as nonlinear amplifying loop mirror (NALM). It relies on an asymmetric arrangement of an amplifying doped fiber section and a long section of undoped fiber.
The principle of the NALM relies on the phase shift between pulses that are first amplified (and thenpropagate in the long fiber) and pulses travelling the opposite way around: The former experience astronger intensity-dependent nonlinearity during the round trip. For an increasing nonlinear phasedifference, hence input pulse intensity, the transmission of the NALM increases, very much like asaturable absorber. As shown in Figure 2, mode locked laser operation can thus be achieved in a
so-called figure-of-eight laser. A major drawback of this configuration, however, is the difficulty toachieve self-starting of mode locked operation in polarization-maintaining fibers.

Figure 2: Figure-of-eight laser

WHY 9 CUTS A GOOD FIGURE
Technology Primerffigure 9®
figure 9® builds up on the NALM based concept to advance its practicability and functional versatility to yet another level. The approach relies on operating the NALM in reflective mode, rather thanin transmission, and allows to implement polarization-maintaining fiber components3.
As shown in Figure 9, part of the cavity scheme is straightened into a linear arm, while a nonreciprocal phase shifter in the NALM part biases the phase acquired by counterpropagating pulse
pairs. The amount of phase bias is thereby carefully optimized for self-starting mode locked operation at low intensities, and, on the other hand, for highly efficient reflection into the linear arm at
favorable operational conditions.
Thanks to its unique features, figure 9® has become the back bone of our femtosecond fiber laser
portfolio, combining unprecedented performance with functional versatility:■ Robust.

Robust self-start of mode locked operation, in a layout offering theenvironmental insensitivity of polarization maintaining fibers.

Figure 9: figure 9®

■ Versatile.

Straight-forward integration of intracavity actuators for repetition ratetuning,synchronization, and carrier envelope phase (CEP) stabilization.

■ Low noise.

Exceptionally low intrinsic phase noise, surpassing the performance of fiber lasers
mode locked by intrinsic saturable absorbers.

■ Cost-effective.

Excellent cost-performance ratio and low cost of ownership through reduced
complexity, compactness, and use of standard telecom components.

At Menlo Systems, we are particularly proud that figure 9® femtosecond fiber lasers have also
become the working horses in our state-of-the-art optical frequency combs and Terahertz instruments.
[1] Patents: EP 2637265 A1, CN 103311780 A, US 8873601 B2[2] Excess carrier-envelope phase noise generation in saturable absorbers N. Raabe et. al., Optics Lett. (2017), Vol. 42,No. 6, 1068
[3] All Polarization-maintaining fiber laser architecture for robust femtosecond pulse generation W. Hänsel et. al., Appl.
Phys. B (2017) 123:41

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