Technical principle diagram of mode division control realized by mode MUX/DEMUX. Credit: Dr. Hu Guijun
From precision machining to advanced microscopy, the demand for higher-power, ultrafast lasers continues to grow. Traditionally, researchers have relied on single-mode fibers to build these lasers, but they face a fundamental physical limit on energy output. To break through this bottleneck, we have turned to multimode fibers, which can carry many light modes—essentially different shapes of light—at once, a technique known as spatiotemporal mode-locking (STML).
However, getting these different modes to work together in harmony has been a significant challenge. In our latest research, published in Optics Letters, we have developed a new technique that allows us to precisely and independently control each of these transverse modes, leading to a dramatic boost in laser power and versatility.
The core problem we faced is known as intermodal dispersion. In a multimode fiber, different light modes travel at slightly different speeds. This velocity mismatch causes the laser pulses to spread out and separate in time and space, preventing the formation of stable, high-power pulses. Previous STML techniques typically used a method called spatial filtering to compensate for this dispersion, but this approach limits the number of modes that can be locked together, thereby capping the potential power enhancement.
To solve this, we proposed a transverse modes division control technique. Our approach is straightforward: We use a device called a mode multiplexer/demultiplexer (MUX/DEMUX) to separate the mixed beam inside the multimode fiber into individual channels, one for each mode. Once separated, we can manage the dispersion (i.e., the travel delay) for each mode independently by adding precise lengths of compensating fiber to each channel.
After optimizing each mode, we recombine them with a multiplexer into a single, powerful, and coherent beam. This method theoretically allows us to lock any number of modes, maximizing the energy potential of the fiber.
We implemented our technique in a figure-eight, Yb-doped, all-fiber, spatiotemporal, mode-locked laser. The experimental results were highly encouraging. By locking four transverse modes (LP01, LP11, LP21, and LP02) simultaneously, we achieved dissipative soliton pulses with 15 nJ of energy at a repetition rate of 14.49 MHz.
Crucially, we demonstrated that the output power scales with the number of participating modes. When four modes were locked simultaneously, the laser’s slope efficiency—a measure of how efficiently it converts pump power to output power—reached 7.9%, which is more than double the 3.79% efficiency of single-mode operation.
Furthermore, our technique offers unprecedented beam-shaping capabilities. By dynamically selecting the combination of modes involved in the mode-locking, we successfully generated a quasi-flat-top beam with a uniform intensity profile. This specialized beam achieved an average output power of 150 mW and a single pulse energy of 10.4 nJ at a pump power of 3 W. Our laser also demonstrated excellent long-term stability, with minimal center-frequency drift after 12 hours of continuous operation.
In conclusion, we have developed and experimentally validated a new control technique that overcomes the core power-scalability bottleneck in STML fiber lasers. By independently controlling the dispersion of each transverse mode, our scheme provides a viable path to synchronizing any number of modes and maximizing energy extraction.
We believe this universal framework for multi-mode spatiotemporal dynamics control paves the way for the next generation of ultrafast light sources, promising impactful applications in precision fabrication, nonlinear microscopy, and attosecond science.
This story is part of Science X Dialog, where researchers can report findings from their published research articles. Visit this page for information about Science X Dialog and how to participate.
More information:
Wenqi Ma et al, High-power spatiotemporal mode-locked Yb-doped fiber laser based on transverse modes division control, Optics Letters (2025). DOI: 10.1364/OL.568362
Dr. Hu Guijun is a professor, doctoral supervisor, Tang Aoqing Distinguished Professor at Jilin University, project leader of the National Key R&D Program, and project leader of the National Natural Science Foundation of China’s major scientific research instrument development project. He has been a visiting scholar and senior research scholar at the Korea Institute of Science and Technology, the Optical Center of the University of Central Florida, and Bangor University, U.K. He has been engaged in research in optical communications and optoelectronic devices.
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New laser technique boosts power by individually controlling light modes (2025, August 26)
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