Types of fiber lasers: MOPA Pulsed Fiber Lasers

As we discussed in a previous blog article, there are many different types of fiber lasers, with many possibilities and options to customize the light beam to the target application. Different laser powers and output characteristics are needed depending on what process is needed and on what materials. Pulsed lasers are a particularly important category of fiber lasers, in which the output emits bursts of light instead of a continuous stream.

There are several ways to design a fiber laser that will generate pulses. In this article, we will explore one of these technologies: MOPA pulsed fiber lasers. We’ll see how they work, their main advantages, applications, and the fibers Coractive offers to build these lasers—including our line of phosphosilicate ytterbium doped double-clad fiber

How MOPA fiber lasers work

The acronym MOPA stands for Master Oscillator – Power Amplifier. In short, instead of the same laser cavity or stage generating both the pulsing features and laser amplification up to the final output power, very low power pulses are generated by a seed laser (the Master Oscillator) that are then amplified in a subsequent stage (the Power Amplifier). Figure 1 shows a typical schematic of a MOPA pulsed laser.

Figure 1: Typical MOPA pulsed laser configuration

The seed pulsed laser can be a diode laser or a lower power fiber laser, usually built using single clad fibers. The seed’s role is to generate the desired pulses, but not to reach the final power level. For that, the Power Amplifier stage will be built to significantly increase the power amplitude of the incoming pulses. The amplification is usually rendered using double clad fibers, enabling pumping by high-power multimode diodes. In some cases, this amplification can be achieved over multiple stages.

Advantages and limitations

The main advantages of using a MOPA architecture for a pulsed fiber laser are:

Precise control over the pulse characteristics: the original pulses are generated by a lower power laser, which is easier to modulate and to modify the pulse shapes.
Versatility and scalability: the same seed laser can also drive systems with many different power levels, requiring only the amplifier stage to be changed.
Cost effectiveness: MOPA laser designs can be much simpler than other pulsed laser types, especially if the application needed requires only a single pulse shape and repetition rate.

There is one major limitation with this laser architecture: Reaching very short pulse lengths can be challenging, partly because the original pulse will broaden during the amplification. This technology is very well suited to nanosecond level pulses, but less effective for picosecond and femtosecond lasers. Since shorter pulses result in higher peak power for the same energy level, MOPA lasers cannot reach the absolute highest peak powers.

Main applications for MOPA pulsed laser

Surface treatment and processing: Exposing a surface to laser pulses can change its chemical proprieties, create specific textures, help promote adhesion to specific compounds or prepare it for the coating of other materials.

Marking: Pulsed lasers can be used to inscribe marking on many different materials, from metal pieces to even food items. An interesting feature is that, depending on the power level, different colors can be achieved on the same material. The flexibility of MOPA fiber lasers offers a big advantage for this application.

Cleaning: As discussed in a previous blog article, laser cleaning is an application with enormous potential. Many of the lasers used in cleaning are pulsed but with longer pulse durations. MOPA fiber lasers enable highly efficient and cost-effective cleaning systems. A more compact design is also available, with the hand-held system becoming increasingly popular.

Other applications include peeling or stripping coatings and paint with lasers, drilling holes in metals and even sheet metal cutting and welding, a field where continuous lasers are more often used than pulsed lasers.

Which fibers are used in MOPA lasers?

Most industrial lasers operate in the 1 µm wavelength range, with many different fiber options. Ytterbium double clad fibers are very well suited to the power amplifier stage of theses lasers. Depending on the applications, different fiber core sizes are needed to balance beam quality and achievable maximum power.

Coractive’s phosphosilicate fiber product line is optimized for superior performance in MOPA designs, featuring critical parameters that differentiate them from standard fibers.

Higher absorption – phosphosilicate glass fibers feature higher cladding absorption than standard aluminosilicate glass fibers, over the whole 900-1000 µm pump wavelength range. This high absorption allows shorter fiber length to be used, raising the power threshold before nonlinear effects are observed.

Figure 2: Typical example of cladding absorption spectrum of standard versus phosphosilicate, for similar fiber dimensions

Higher saturation energy: phosphosilicate glass fibers have approximately twice the saturation energy of standard fibers. This leads to lower pulse deformation and contributes to the higher nonlinear effect threshold.
Stable long-term output: the chemical composition of the core is optimized to operate at full capacity without photodarkening.

The demand for increasingly higher-powered MOPA fiber lasers will continue to rise with the constant development of new industrial processes and for improvement in the speed and scale of current applications. Coractive can help you choose the right fiber for your needs and help design optimal MOPA fiber lasers. Don’t hesitate to ask a member of our team about it!

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