Femtosecond-Laser-Written Composite Grating Pushes Raman-Suppressed Fiber Laser to 9 Kilowatts

The team, led by Hao Li, Rong Zhao and colleagues with Jinbao Chen and Zefeng Wang, reports in Frontiers of Optoelectronics that the device achieves the highest power to date, by their account, for a fiber oscillator in which stimulated Raman scattering is suppressed with a chirped and tilted fiber Bragg grating.

Stimulated Raman scattering, or SRS, is the central obstacle standing between high-power fiber oscillators and even greater output. In an oscillator, signal light traveling through tens of meters of doped fiber gradually transfers energy into longer-wavelength Raman light through a nonlinear scattering process. That conversion drains power and efficiency from the laser, distorts the spectrum, and can even damage the system: when material being processed reflects Raman light back into the fiber, the backward-traveling Raman light further amplifies the scattering. Because oscillators are favored over amplifier chains in high-end manufacturing for their compact structure, simple control logic and strong anti-reflection behavior, finding robust ways to suppress SRS inside the oscillator cavity itself has become a priority.

One established approach is to use a chirped and tilted fiber Bragg grating, or CTFBG, as an in-fiber spectral filter. Unlike a conventional mirror grating, which couples forward core light into backward core light, a CTFBG couples forward core light into backward cladding modes, stripping out light at the Raman wavelength while letting the signal pass. Compared with long-period fiber gratings, CTFBGs are relatively insensitive to temperature and stress, giving them better spectral stability in a hot, vibrating laser head. The catch has been fabrication: most CTFBGs and cavity mirror gratings are still made with ultraviolet laser phase-mask technology, which requires hydrogen loading and high-temperature annealing of the fiber. If annealing is incomplete, residual hydrogen and hydroxyl groups left in the grating absorb near-infrared light and generate heat, limiting how much power the grating can survive. The reported maximum handling power of UV-written mirror gratings and CTFBGs stands at 8 kilowatts and 3.4 kilowatts, respectively.

Femtosecond laser inscription sidesteps those problems. Because the ultrafast pulses do not depend on the fiber’s photosensitivity, gratings can be written directly into ordinary fiber without hydrogen loading, shortening fabrication time and eliminating the hydrogen- and hydroxyl-related heating. The refractive index changes induced by femtosecond lasers are also stable and heat-resistant. Previously reported handling powers for femtosecond-written mirror gratings and CTFBGs are 10 kilowatts and 4 kilowatts, respectively, but the team argues that the CTFBG figure needed to be raised to truly test SRS suppression in a high-power oscillator.

The new work also tackles a structural weakness. In an earlier demonstration, the group reached 3.5 kilowatts by splicing a CTFBG onto one side of a low-reflectivity fiber Bragg grating inside the resonator cavity. That splice point was a damage risk that could cause oscillator failure. In the present study, the researchers instead inscribed both the low-reflectivity grating and the CTFBG on the same 30-micrometer-core, 250-micrometer-cladding fiber, forming a single composite grating with no added splice. The low-reflectivity grating acts as one cavity mirror at the signal wavelength of about 1080 nanometers, while the chirped and tilted grating filters Raman light at about 1135 nanometers.

The fabrication itself is a study in precision. Using a 515-nanometer femtosecond laser with 290-femtosecond pulses and a 1-kilohertz repetition rate, focused through a cylindrical lens and a phase mask to a waist of roughly 3.7 micrometers, the team scanned the entire 30-micrometer core. For the mirror grating, the fiber stayed fixed while a galvanometer scanned the beam perpendicular to the fiber axis, producing vertical grating planes. For the CTFBG, the beam path stayed fixed while a translation stage moved the fiber obliquely, creating tilted planes. The Raman filter is actually a cascade of three CTFBGs with tilt angles of 7.4, 6.6 and 5.4 degrees over a total length of 40 millimeters, a design that widens the suppression band. The resulting filter is centered at 1135 nanometers with an 18-nanometer 3-dB bandwidth, matching the first-order Raman wavelength of the 1080-nanometer signal.

Measuring the gratings posed its own challenge. In multimode fiber, high-order modes excited by mode-field mismatch can swamp the weak transmission dip of a low-reflectivity grating, whose fundamental-mode reflectivity of 6 percent corresponds to a transmission depth of only a few tenths of a decibel. The team therefore built a reference, or scale, grating system: a known high-reflectivity grating at a different wavelength is measured alongside the unknown one, and the reflectivity is calculated from the difference in reflection peak intensity. By this method the low-reflectivity grating was verified at 6 percent, centered at 1079.4 nanometers with a 2.1-nanometer 3-dB bandwidth, while the high-reflectivity mirror grating showed reflectivity above 99 percent, centered at 1079.3 nanometers with a 4-nanometer bandwidth.

The laser testbed used a 38-meter ytterbium-doped gain fiber with a 30-micrometer core, pumped purely backward through a (36+1)×1 combiner by dual-wavelength 969- and 982-nanometer laser diodes, a choice that helps raise the threshold for transverse mode instability, another power-limiting effect. Notably, no cladding light stripper was installed at either output end, allowing output power to keep climbing after the onset of transverse mode instability, at the cost of some beam quality. In the first configuration, with only the low-reflectivity grating, the oscillator reached a maximum of 8910 watts at 85.4 percent slope efficiency, limited by transverse mode instability, which first appeared at 8250 watts. Raman components became visible at 8.7 kilowatts, and by 8.9 kilowatts the spectrum had broadened to nearly 1200 nanometers.

With the composite grating installed, the picture changed markedly. The Raman suppression spectrum reached a depth of 16 dB, equivalent to 97.5 percent suppression, with an 86-nanometer 3-dB bandwidth. The Raman light ratio, the power fraction between 1120 and 1200 nanometers, fell from 4.5 percent to 0.2 percent at maximum output, an order-of-magnitude improvement in signal purity. Output power rose by 140 watts to 9050 watts at a slope efficiency of 83.4 percent, the modest drop reflecting the filter’s insertion loss. The transverse mode instability threshold also climbed from 8250 to 8700 watts, consistent with the understanding that SRS-induced heating helps trigger that instability, so suppressing SRS raises its threshold. Temperatures of all three gratings, monitored with a thermal camera under active cooling, stayed below 50 degrees Celsius even at maximum power.

The authors present the result as a record for fiber oscillators using CTFBG-based SRS suppression and as evidence that femtosecond-written gratings can outperform their UV-written counterparts in power handling, while the monolithic composite design removes the splice-point vulnerability of earlier schemes. They note that power growth in both configurations was ultimately limited by transverse mode instability, and that continuing to push power after its onset carries risks of pump-source damage through the combiner. The stated next goal is a fiber oscillator exceeding 10 kilowatts based on femtosecond-written composite gratings, with comprehensive suppression of nonlinear effects including both SRS and transverse mode instability. If achieved, such systems would strengthen the role of fiber oscillators in industrial processing and high-end manufacturing, where compact, efficient, multi-kilowatt beams are increasingly in demand.

Subject of Research: Suppression of stimulated Raman scattering in high-power fiber laser oscillators using a femtosecond-laser-written composite fiber Bragg grating

Article Title: Composite fiber Bragg grating written by femtosecond laser for Raman suppression in high-power fiber oscillators

Article References: Li, H., Zhao, R., Rao, B., Ye, X., Yang, B., Wang, M., Li, Z., Chen, Z., Wang, Z., & Chen, J. (2025). Composite fiber Bragg grating written by femtosecond laser for Raman suppression in high-power fiber oscillators. Frontiers of Optoelectronics, 18(1), Article 21. https://doi.org/10.1007/s12200-025-00165-3

Image Credits: AI Generated

DOI: 10.1007/s12200-025-00165-3

Keywords: fiber laser, fiber Bragg grating, femtosecond laser, stimulated Raman scattering, chirped tilted grating, high-power oscillator, transverse mode instability, ytterbium-doped fiber, Raman suppression, laser physics, photonics, 9 kW output