A new generation of phase modulators for high-power fiber lasers

High-power fiber lasers for defense and industry

Demand for high power fiber lasers has increased tremendously in the last 10 years. These lasers are used today in an increasing variety of fields such as industrial machines (marking, welding, cutting), LIDAR systems, intense laser systems (inertial fusion, plasma), and recently in optical systems that can deliver very high power for defense applications. With the growing presence of drones on the battlefield, the development of offensive laser applications, such as anti-structure laser weapons, have been accelerated. The fiber lasers are the ideal candidate of laser Directed Energy Weapons (DEW) in the field.

Until recently, chemical lasers or free-electron lasers, then solid-state lasers, were mainly considered as Directed Energy Weapons (DEW). However, these technologies are particularly expensive and complex to implement. On the opposite, high power fiber lasers offer a new alternative thanks to their compactness, lower mass allowing easier deployment, lower production and operating costs, higher reliability over time, resistance to severe and even extreme environments, etc. Thus, they are perfectly suited to operational environments, for example embarked on moving vehicles or ships.

Indeed, phasing multiple fiber sources of several kilowatt by optical combination can result in laser architectures delivering powers of up to tens of kilowatts today. Ongoing works are carried out to expend this power to several 100 kW which is the estimated power required to disable a drone. These laser architectures, developed for the defense market, can also be used for industrial cutting laser, and future applications when a new optical modulation technique will be available.

Gaining power with combined laser while dealing with the non-linear Brillouin effects

Gaining power with several combined laser beams can be done by superimposing lasers of distinct wavelengths in a uniform laser output. It can be done using the Coherent combination of laser beams technique (CBC), where the combination of several lasers by real-time control of their relative phases allows to permanently maintain constructive interferences and thus guarantee maximum power efficiency during the combination (iXblue NIR-MPX-LN-01 modulator and its matching amplifier DR-VE-0.1-MO are both ideal for high-power coherent beam combining applications). And it can also be done with the Spectral Beam Combination technique (SBC) where different laser beams emitting a continuous signal, centered on distinct wavelengths, are superimposed by adaptive optics. The result of the laser system is a uniform intensity distribution and an optical signal with a power proportional to the number of laser beams combined.

However, the maximum transmitted optical power in each fiber amplifier is severely constrained by the non-linear Brillouin effect. This effect, also called Stimulated Brillouin Scattering (SBS) can be mitigated by broadening the laser linewidth (to spread the spectral power density some GHz around the central wavelength) by the mean of an electrooptical phase modulator.

To create these side bands, three different RF source can be combined with the electro-optical phase modulator. A sinusoidal electric signal, a "white noise" or a telecom "PRBS – Pseudo- Random-Bit-Sequence". With this technique, optical output higher than a kilowatt continuous signal can be reached, spectrally modulated, and combined with other fiber amplifiers to obtain the final and expected laser power level.

Spectral combination

Spectrum broadening set-up, based on electro-optical phase modulator mean

iXblue modulation solution for high frequency range, insensitive to pyroelectric and photorefractive effects

Gaining in modulation efficiency in a highpower fiber laser, while lowering the electrical power consumption, means lowering the driving voltage (Vπ) of the phase modulator. iXblue has developed key building blocks to
master the production of modulators. They benefit from the technological mastery of the "Annealed Proton Exchange" (APE) waveguide manufacturing process, the same process deployed for modulators integrated in iXblue gyroscopes.

The NIR-MPZ series proves an excellent stability in temperature (< 0,3 dB), equivalent to the best X-cut generation.

A generation of iXblue phase modulators is now widely used and represents a commercial success: the NIR-MPX-LN (Near InfraRed – Modulator Phase with X-cut, on LiNbO₃ fiber), also proposed with their matching RF amplifier. iXblue components have certainly the best output performance and have proven the best behavior: low fluctuations and very good stability despite wide temperature variations (products have been tested from -40 to 85 °C and proved over 10 years’ time, in CW mode) and high CW optical power to 300 mW.

A new generation of modulators is emerging in iXblue portfolio in early 2022, dedicated to the high frequency range (up to 40 GHz). For lower RF power consumption, an efficient RF to optical transmission is required, while maintaining stability even under important outdoor temperature variations. With the NIR-MPZ-LN-20, based on a "Z-cut*" LiNbO3 crystal orientation structure, iXblue has developed a novel generation of phase modulators. They gain 40 % on Vπ efficiency, and as an accomplishment reach the same optical stability versus temperature variation and high optical input power than in the "X-cut" modulation design which is considered as the reference.

Spectral Beam Combination applied with such NIR-MPZ-LN-20 new generation of phase modulators are reaching large bandwidth up to 40 GHz and low driving voltage of 3,5 V, together with high optical input power up to 300 mW, high RF input power, with still high optical stability whatever the optical and temperature operating condition.

The efficiency of the modulation is proven with this new generation of Z-cut, while the stability in severe external conditions and optical power handling is perfectly maintained. iXblue proves its ability to solve technical challenges thanks to its expertise and experience over different markets. iXblue portfolio covers now the whole frequency range with solutions of high stability:

• NIR-MPX-LN-0,1 for up to 3 GHz;
• NIR-MPX-LN-10 for up to 10 GHz;
• NIR-MPZ-LN-20 for up to 40 GHz.

The plan is now to offer an ultra-low Vπ, 10 GHz version in the near future, by using the same Z-cut generation of phase modulators. Every technical step reached, and mastered, paves the way for the development of future new products.

With its large experience in the field of electrooptic manufacturing, iXblue has the capacity to industrialize a new idea or a new technology, into a process, to master it and then deliver it as a final product, in hundreds or thousands of samples. Today, iXblue is now working on the complete integration of different components into packaged product.