Fig. 1. Experimental setup for the nonlinear pulse compression experiments comprising a thulium-doped fiber laser, an amplifier, and a double-stage compressor. WDM, wavelength division multiplexer; ISO, isolator; M1–M3, mirrors; Col, collimator; QWP, quarter-wave plate; HWP, half-wave plate; PBS, polarization beam splitter; OC, output coupler; SMF, single-mode fiber; TSF, thulium-doped gain fiber; UHNA4, ultrahigh numerical aperture fiber; HNLF, highly nonlinear fiber; DC-TDF, double-clad thulium-doped fiber.

Fig. 2. (a) Spectra of the seed (blue line) and the amplifier (red line). (b) Measured (blue line) and sechfitting (red line) AC traces in the time domain. (c) Fundamental repetition rate of 199.74 MHz measured with a spectrum analyzer. (d) Measured pulse train with an oscilloscope.

Fig. 3. (a) Measured spectra at points A and B. (b) Retrieved AC trace and phase distribution in the first nonlinear compression stage. (c) Interference autocorrelation of output at point B. (d) Transformed-limited and retrieved pulses at point B.

Fig. 4. (a) Output power stability measured over a 2-h period. (b) Center wavelength (λc) and 10 dB spectral bandwidth (λb) evolution for 2 h. The inset shows the monitored spectra evolution spanning from 1200 to 2400 nm at the same time.

Fig. 5. (a) Simulation of the first nonlinear compression stage in the spectral (a) and time (b) domains.

Fig. 6. Simulation of pulse propagation in the second nonlinear compression stage. (a) Spectral evolution ranging from 1200 to 2800 nm. (b) Temporal evolution in the same fiber. (c) Output spectrum of the simulation at the end of the HNLF. (d) Output pulse shape in the time domain.