Optical path design of the 66-femtosecond mode-locked laser
This 66-femtosecond mode-locked laser is a linear cavity all-polarization-maintaining ytterbium-doped fiber laser with a non-reciprocal phase shifter. It achieves 147 MHz fundamental frequency mode-locking. By adjusting the distance between the gratings, a spectral width of 39.8 nm and a pulse width of 66 fs after external compression are obtained. At high pump power, second-order and third-order harmonic mode-locking with repetition frequencies of 294.1 MHz and 442.3 MHz are achieved.
Optical path description:
The resonator consists of the spatial optical parts on both sides and the polarization-maintaining fiber part in the middle. The left spatial part includes a total reflection mirror (M1), a λ/8 wave plate (EWP), and a Faraday rotator (FR). The combination of EWP and FR can be used as a non-reciprocal phase shifter, providing non-reciprocal phase bias, thereby enhancing the self-starting ability. The fiber part consists of a custom wavelength division multiplexing – collimator (WDM-Collimator) integrated device, 62 cm ytterbium-doped polarization-maintaining fiber (Yb401-PM, CORACTIVE), and an optical fiber collimator (Col). The gain fiber is pumped by a single-mode 976 nm laser diode (LD) with a maximum pump power of 1.4 W. The right spatial part consists of a half-wave plate (HWP), a polarization beam splitter (PBS), a grating pair (LightSmyth T-1000-1040-3212-94), and a total reflection mirror (M2). The transmission grating pair with a line density of 1000 lines/mm provides intra-cavity dispersion compensation. The distance between the two gratings can be adjusted by a stage. The free space length from the collimator to the two reflection mirrors on both sides is 5.5 cm and 6.5 cm respectively. The laser outputs pulses in a linearly polarized manner from the PBS.
Working principle:
The initial normalized pulse transmitted through the intracavity loop starts from PBS and is transmitted to M1. Initially, the HWP will decompose the pulse into two orthogonal components, and then enter the polarization-preserving optical fiber and propagate along the fast and slow axes. The intensity ratio of the pulses along the two orthogonal axes is determined by the rotation angle (θh) of the HWP. During propagation within the optical fiber, due to nonlinear effects, the asymmetric intensity of the orthogonal polarized pulses will cause intensity-related nonlinear phase shifts. The end mirror M1 enables the orthogonal pulses to pass through the phase shifter twice and return to the polarization-preserving optical fiber. The orthogonal pulses acquire a π/2 non-reciprocal phase shift and exchange the propagation optical axis. The group velocity mismatch between the orthogonal polarized pulses leads to the compensation of the deviation effect. Finally, the pulse accumulates different nonlinear phase shifts and undergoes interference at PBS. As a polarizer, PBS allows the appropriate polarization state pulses to pass through, while the rest is reflected out of the cavity. This process plays the role of an artificial saturable absorber in this linear cavity optical laser. When the grating pair distance is further reduced to 3.2 mm, the left edge of the spectrum becomes significantly steeper. At this time, the cavity net dispersion is positive, and the maximum single-pulse energy of 3.57 nJ is obtained. The pulse self-correlation trace obtained by external compression of the pulse with the widest 39.8 nm spectral width is fitted by a Gaussian function, which is 66 fs.
Post time: Feb-25-2026