Femtosecond (fs) fiber oscillators are a standard technology used to generate fs laser pulses. They offer a compact design, outstanding performance, and are cost-effective to use. However, the operating wavelengths for these oscillators are located primarily in the infrared (IR) region. Extending their reach to the visible (VIS) range, so they can be used in a variety of applications that require VIS light sources, has been a longstanding goal of laser science. In an advancement for ultrafast laser technology, researchers at Xiamen University developed a VIS-wavelength, 635-nm, soliton, mode-locked fs fiber oscillator and amplifier. They constructed the oscillator on a phase-biased nonlinear amplifying loop mirror (PB-NALM). VIS fs fiber oscillator and amplifier. Schematic includes an inset and a photograph. Courtesy of J. Zou, Q. Ruan, et al. The soliton, mode-locked fs fiber oscillator and amplifier system provides the ability to source fs laser pulses from VIS fiber lasers, marking a potential breakthrough in ultrafast laser technology. The laser source developed by the Xiamen team could be used in applications ranging from advanced materials processing to biomedicine to nonlinear optics. The VIS, mode-locked fs fiber oscillator is based on a high-gain praseodymium-doped (Pr3+)-doped fluoride fiber and uses a pair of custom, high-efficiency, high-groove-density diffraction gratings for dispersion management. The oscillator achieves highly stable, self-starting mode locking by building on the PB-NALM. The PB-NALM eliminates the need for long intracavity fibers to accumulate phase shifts. This enables flexible tuning, helps extend the life of the device, and allows intracavity dispersion to be managed in a larger parameter space from normal to anomalous dispersion regimes. The self-starting, mode-locking capability provided by the PB-NALM allows the fs fiber oscillator to yield laser pulses with a central wavelength of 635.5 nm, a widest three-decibel (3-dB) bandwidth of 5.4 nm, a minimum pulse duration of 196 fs, and a repetition rate of 53.957 megahertz (MHz). The fs fiber laser uses a figure-nine cavity configuration. By manipulating the intracavity dispersion and polarization, the researchers could observe multiple mode-locking states in the oscillator, including conventional solitons, dispersion-managed solitons, dissipative solitons, and bound-state solitons. To further enhance laser performance, the team built a VIS chirped-pulse amplification (CPA) system alongside the oscillator. This enhancement resulted in an average output power of more than one watt (1W) and a pulse energy of 19.55 nanojoules (nJ), while maintaining a compressed pulse duration of 230 fs. Typical characteristics of the VIS fs fiber oscillator. (a): Optical spectra of mode-locking and continuous wave operations. (b): Oscilloscope trace of the pulse train (inset: a screenshot of oscilloscope trace). (c): Autocorrelation trace of the output pulses. (d): RF spectrum at the fundamental frequency (inset: a broadband RF spectrum and 3 GHz span). Courtesy of J. Zou, Q. Ruan, et al. Due to the exceptional heat dissipation of the fibers, the laser system operates with excellent long-time stability, characterized by a low power deviation of less than 0.3% and negligible wavelength drift. The laser architecture with dispersion management capabilities could serve as a testbed to explore complex, ultrafast soliton dynamics in the VIS-wavelength region, and could pave the way for miniaturized VIS fiber fs laser sources that have a broad spectral range, narrow pulse width, and high power level. “Our result represents a concrete step toward high-power femtosecond fiber lasers covering the visible spectral region and could have important applications in industrial processing, biomedicine, and scientific research,” professor Zhengqian Luo said. Although VIS fs lasers based on the nonlinear frequency conversion of Ti:sapphire fs oscillators or near-infrared (NIR) ultrafast lasers are well-developed, they are also expensive and limited in footprint and efficiency. The fiber fs mode-locked oscillator used for the IR wavelengths is now available for the VIS spectrum. The researchers anticipate that their new scheme for high-performance, VIS fs fiber laser pulse generation will make VIS fs fiber lasers suitable for a range of applications, such as special material precision processing, biomedicine, underwater detection, and optical atomic clocks. The research was published in Advanced Photonics Nexus (www.doi.org/10.1117/1.APN.3.2.026004).
