Ultrafast fiber lasers have many advantages, such as excellent heat removal, high single-pass gain, compactness. Moreover, it has numerous applications in fundamental research and industry, particularly in the biomedicine area. The wavelength tunability of ultrafast laser is one of the most demanding features for multiphoton microscopy imaging. For instance, in two-photon fluorescence microscopy, most fluorophores can be excited by femtosecond pulses in the 800-1300 nm wavelength range. Techniques such as optical parametric oscillator (OPO), supercontinuum generation (SCG), and self-phase modulation enabled spectral selection (SESS) are commonly used to generate such kinds of lasers. Although OPO can generate ultrafast pulses with broad spectral and high pulse energy, it is quite expensive and not user-friendly. SCG can generate an octave-spanning spectrum; however, the pulse energy is far lower than 1 nJ. SESS has been developed recently to generate tunable femtosecond pulses. The core of this scheme is to generate discrete spectral side-lobes enabled by the self-phase modulation-dominated nonlinear effect. A series of theoretical calculations and experiments have shown that this scheme can overcome the wavelength tuning restrictions such as fiber dispersion, high-order soliton fission, Raman effect, etc. The χ(3) nonlinear effect is generally understood to be connected not only to the fiber material but also to the polarization state of the incident pulse. When the B-integral is constant in the nonlinear transmission process, both theory and experiments suggest that circularly polarized pulse transmission can enhance the energy of pulses by approximately 1.5 times.

In this study, we fabricated an ultrafast fiber laser with tunable wavelength and scalable energy using circularly polarized pulses. The system consists of the front-end driven laser (fiber CPA laser) and the following spectral broadening and selection unit (SESS). The central wavelength of a home-built ultrafast-driven laser is 1030 nm. The maximum average power is 8 W, and the repetition rate is 55 MHz. The SESS unit comprises photonic crystal fiber, launching power adjustment setup, and spectral selection filter. The half-wave plate and PBS are used to modify the input power, while a 1/4-wave plate regulates the polarization state of the input pulse. The filter or dichroic mirror is responsible for filtering out the appropriate spectral components to produce output with wavelength adjustable.

The SESS and CPA systems are the major parts of the high-energy wavelength-tunable ultrafast fiber laser (Fig. 4). As the input power increases, the broadening of the output spectrum based on linearly and circularly polarized pulses displays a proportional increase. The spectral broadening of circularly polarized pulses is less than that of linearly polarized pulses when the input power is the same in two polarization states. The rightmost side-lobe of the linearly polarized pulse shifts to 1200 nm when the input power is increased to 3.45 W, and the power of the circularly polarized laser increases to 4.9 W to reach a similar spectral broadening. The power ratio of the two is 1.42, which is consistent with the results obtained via theoretical simulation. Furthermore, we discovered that the spectral structure of circularly polarized pulses is more distinct with the same degree of spectral broadening (Fig. 2 and Fig. 5). We compared their spectra to validate the input power ratio of the linear and circular polarization pulses under the same spectral broadening condition. When the spectrum s rightmost lobe is shifted to 1100 nm, the power in the linear polarization state is 1.5 W, while the power in the circularly polarized state will increase to 2.1 W, implying that the ratio is approximately 1.4. The linearly polarized pulse s input power is 2.55 W and 3.45 W, respectively, when switching to 1150 nm and 1200 nm, while the circularly polarized pulse s power is 3.55 W and 4.9 W, corresponding to power ratios of 1.39 and 1.42, respectively. The practical results of the three cases are consistent with our theoretical model, showing that using the circularly polarized pulse can boost the output power by around 1.4 times (Fig. 6). Furthermore, the output spectrum in a circularly polarized state has deeper modulation and a clearer lobe structure than the linearly polarized pulses, which benefits the subsequent filtering process and increases the stability of output pulse. Finally, a study on elliptically polarized pulses has discovered that the broadened spectrum shows broader but weak lobes due to the occurrence of cross-phase modulation exceptionally in elliptical polarization states (Fig. 3 and Fig. 7).

This study introduces a circularly polarized ultrafast fiber laser with tunable wavelength and scalable energy. The ultrafast input pulses spectrum is strongly broadened in PCF by nonlinear effects dominated by self-phase modulation. Furthermore, we filter out the desired spectral side-lobes with an optical filter, which will be a promising method for generating the light source with high energy and exotic wavelength. The theoretical and experimental studies show that, with the same degree of spectral broadening, the input power of circularly polarized pulses is about 1.4 times higher than that of linear polarized pulses, and the corresponding filtered out energy can be increased by about 1.4 times. Simultaneously, the study shows that circularly polarized pulses tend to have a larger wavelength tuning range by increasing the input power. Finally, this study presentes a high-energy femtosecond laser with a wavelength tuning range between 930 and 1200 nm by the self-phase modulation enabled spectral broadening method. This laser can be a promising alternative for driving multiphoton microscopy to enable high penetration depth imaging of biomedical tissue.