Broadband mid-infrared (2-20 ) light sources have extremely important applications in the field of molecular spectroscopy, and have promoted the development of gas analysis, biomedical diagnosis and other fields. The article in this issue introduces a new type of mid-infrared light source. The author uses a thulium-doped fiber laser to drive three-channel nonlinear wavelength conversion to realize the output wavelength seamlessly covering 1.33 to 18 . far surpasses existing synchrotron radiation sources.

Figure 1(a) is the device diagram of the light source. The seed pulse comes from an erbium-doped fiber oscillator operating at 1.55 and a repetition rate of 100 MHz. After reducing the repetition frequency, the soliton self-frequency shift is used to generate a femtosecond pulse with a center wavelength near 1.96. The chirped pulse is amplified in the amplifier, and the spectra and pulses shown in Figure 1(b) and Figure 1(c) are obtained after grating pair compression, and the pulse width is 254fs.

Figure 1 Layout diagram of fiber optic CPA and three-channel output, CPA spectrum and pulse shape [1]

The compressed pulses are injected into 3 parallel channels. Channel 1 uses the soliton self-squeezing phenomenon in photonic crystal fibers to achieve wavelength conversion. The length of the photonic crystal fiber is 2.3cm, and it is anomalous dispersion when the wavelength is greater than 1.3. When the pulse propagates in this fiber, the soliton self-compression can be realized. The best spectral broadening occurs when the incident power is 7 W, and the output pulse is close to the transformation limit pulse, and the output power is about 4.5 W. If the input power continues to increase, the spectrum will be disturbed and the fiber will be damaged. Figure 2(a) and Figure 2(b) are the measured shape and spectrum of the output pulse, the pulse width is 13 fs, the main peak energy accounts for more than 40%, and the wavelength covers 1.4-2.4.

Figure 2 The output pulse and spectrum of the soliton after self-compression [1]

Channel 2 uses fluoride fiber to generate supercontinuum for wavelength conversion. Figure 3(a) shows the evolution of the spectrum of a 20nJ, 250fs pulse in a 12cm ZBLAN fiber. In the early stage of evolution, the spectral broadening mainly comes from the self-phase modulation effect. After about 10 cm of propagation, the self-compression of the pulse leads to an increase in the peak power of the pulse. Other effects other than self-phase modulation are more significant, and the spectrum rapidly broadens to 4.5. After many tests, the authors determined that the optimal fiber length was 12 cm. Figure 3(b) shows the spectra measured by three spectrometers operating in different wavelength ranges, with a total power of 1.07 W, and after filtering with a long-pass filter, the power at wavelengths greater than 2.4 exceeds 216 mW.

Figure 3 (a) Spectral evolution (b) Supercontinuum [1]

Channel 3 uses the difference frequency within the pulse to realize wavelength conversion. First, the pulse passes through a 5.6cm photonic crystal fiber for pulse self-compression and spectral broadening, and then the beam is focused into a GaSe crystal with a thickness of 1 mm to generate long-wave mid-infrared spectrum through the difference frequency within the pulse. The wavelength range of the output spectrum is 5.3-18, the output power is 500mW, and the total efficiency is about 2%.

Figure 4 The overall spectrum produced by the three channels [1]

Figure 4 shows the total spectrum produced by the three channels, which is comparable in brightness to a synchrotron radiation source, but is several orders of magnitude less expensive and therefore more practical. This infrared broadband spectral system based on thulium-doped fiber amplifiers only uses optical fibers and nonlinear crystals to achieve spectral broadening. Comb, time-resolved, and field-resolved studies provide a reliable light source.