Ultrafast fiber laser technology generates femtosecond or picosecond-level ultrashort laser pulses, offering advantages such as high beam quality, exceptional stability, and compact structure. It finds extensive applications in precision processing, biomedical research, spectroscopy, and communications. Traditional ultrafast fiber lasers employ rare-earth-doped fibers as the gain medium, utilizing the multi-level energy structure of rare-earth ions to achieve stimulated emission. However, due to the fixed energy level spacing and limited spectral width of rare-earth ion transitions, laser output is confined to discrete spectral ranges, significantly limiting the application scope of ultrafast fiber lasers. Extending the output wavelength of ultrafast fiber lasers beyond the range covered by ion transitions is not only a natural progression in ultrafast technology development but also addresses practical demands across scientific research, medical applications, defense, and other fields.
Ultrafast Raman fiber lasers represent an effective method for generating laser pulses at specific wavelengths. Current mainstream techniques for generating ultrafast Raman lasers include mode-locking, synchronous pumping, and nonlinear optical gain modulation (NOGM). Mode-locking typically employs continuous-wave pumping, requiring tens to hundreds of meters of fiber to achieve sufficient Raman gain, resulting in resonators with significant dispersion and nonlinearity. Synchronous pumping utilizes pulsed pumping, effectively shortening the resonator length. However, it necessitates synchronization between the pump pulse and the Raman pulse, increasing system complexity. Both techniques rely on fiber resonator structures, limiting the output Raman pulse energy to the nJ range. In contrast, NOGM technology employs a single-frequency seed-injected Raman fiber amplifier configuration to generate high-energy Raman laser pulses. Currently, Raman pulses produced using this technique reach the hundreds of nJ range. Optimizing the system architecture to generate higher-energy Raman pulses is a key research focus.
Rare-Earth Raman Hybrid Amplifier
A joint research team comprising Professor Zhou Jiaqi from the Aerospace Laser Technology and Systems Department at SIOM, Chinese Academy of Sciences, and Professor Feng Yan from the Shanghai Advanced Research Institute of USTC, combined NOGM technology with ytterbium-doped fiber amplifiers. By leveraging the hybrid amplification mechanism of rare-earth ions and stimulated Raman scattering (SRS), achieving ultra-fast Raman laser output at the 1121 nm wavelength with microfocalization capability, where the pulse width can be compressed to 589 fs.
In a typical NOGM system, a single-frequency continuous laser serves as the seed source, amplified and shaped within a single fiber; an ultrafast laser acts as the pump source, providing nonlinear optical gain via SRS. Amplification occurs only in the temporal overlap region between the single-frequency continuous laser and the pump laser, ultimately converting it into a Raman pulse synchronized with the pump laser. In conventional NOGM systems, the pump pulse energy amplification unit and nonlinear optical frequency conversion unit are separated: high-power wavelength division multiplexers are required to couple high-energy pump pulses with single-frequency continuous laser seeds; moreover, fusion splicing of active and passive fibers under high-power conditions poses risks to long-term system stability. The research team developed a novel ultrafast fiber laser rare-earth Raman hybrid amplifier. By utilizing ytterbium-doped fiber to simultaneously provide rare-earth gain and Raman gain, it can generate Raman pulses with single-pulse energies in the microjoule range. As shown in Figure 1, a gain-switching diode generates 1065 nm pulsed laser with a pulse width of 18.3 ps, set to a repetition rate of 10 MHz, serving as the system's pump source. A narrow-linewidth 1121 nm semiconductor single-frequency continuous laser acts as the seed source, simultaneously input into the ytterbium-doped fiber amplifier.
Figure 1 Schematic of the ultrafast fiber laser rare-earth Raman hybrid amplifier system
As shown in Figure 2(a)-(d), the single-pulse energy of the 1121 nm Raman pulse can be amplified to ~1 μJ, with the pulse width compressed to 589 fs. The maximum Raman conversion efficiency reaches 69.9%, and the pulse repetition rate signal-to-noise ratio achieves 81.1 dB. Without single-frequency continuous laser injection, the characteristics of the 1121 nm Raman pulse are shown in (e)-(h). Under these conditions, the generated Raman pulse exhibits near-noise-like characteristics, with unstable pulse sequence intensity and a reduced repetition-rate signal-to-noise ratio of 67.4 dB. These experimental results confirm the feasibility of rare-earth Raman hybrid amplification NOGM and the necessity of single-frequency seed injection.
Figure 2 Raman pulse laser characteristics with (a)-(d) single-frequency continuous laser injection and (e)-(h) without single-frequency continuous laser injection
Simultaneously, numerical simulations modeled the pulse evolution under conditions of a 60 ps pump pulse width and a Raman fiber core diameter of 14.5 μm, as depicted in Figure 3. Results indicate that Raman pulse outputs in the 10 μJ range can be achieved by employing wider pump pulses and larger-core-diameter Raman fibers.
Figure 3 Simulation results for a pump pulse width of 60 ps and a fiber core diameter of approximately 14.5 μm
This study demonstrates a novel ultrafast fiber laser ytterbium-Raman hybrid amplifier, achieving ~1 μJ of 1121 nm Raman laser output with a pulse width compressible to 589 fs. Further numerical simulations reveal that utilizing a pump laser with a broader pulse width and fiber with a larger core diameter could potentially achieve femtosecond Raman pulse outputs in the 10 μJ range, which represents a key focus for subsequent research. This femtosecond Raman fiber laser, capable of generating high-energy pulses at specific wavelengths, offers promising light source technology support for applications such as material processing and biomedical imaging.
