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The generation of deep ultraviolet ultrashort pulses is a key technology required in many fields of physics. Generally, deep ultraviolet light sources based on harmonic generation have limitations such as the inability to directly tune the wavelength, low energy conversion efficiency, and can only generate pulses with a pulse width similar to the driving pulse source. To this end, this article proposes a deep ultraviolet ultrashort pulse generation method based on dispersion wave generation. By adjusting the air pressure and air pressure gradient in the hollow photonic crystal fiber to adjust the phase matching conditions for dispersion wave generation, it has experimentally achieved a tunable, narrow pulse width and deep ultraviolet pulse close to the transformation limit, and used numerical simulation to complete the theoretical interpretation. The experimental device is divided into three parts: (a) pre-compression part (b) deep ultraviolet generation part (c) pulse characterization part as shown in Figure 1.
Figure 1 Experimental device [1]
In Figure 1, the seed source is a titanium sapphire laser, BS is a beam splitter, BPF is a bandpass filter to obtain a narrow-band near-infrared window pulse, CM is a chirped mirror, HWP is a half-wave plate, TFP is a thin film polarizer, PCF uses anti-resonant kagome PCF, 7.5 cm long, 33 μm core diameter, filled with argon gas, and BBO is barium metaborate crystal.
(a) The main body of the pre-compression part consists of a hollow fiber with a length of 1 m and a pressure of 0-250 mbar, a compressor composed of a pair of chirped mirrors, and an attenuator composed of a pair of thin film polarizers. This part broadens and compresses the input pulse spectrum with a center wavelength of 790 nm and a pulse width of 30 fs to 13.5 fs to obtain the driving pulse required for the deep ultraviolet generation part and the reference pulse required for the pulse characterization part. This compression part is not required, but the quality of the soliton self-squeezing in the deep UV generation part can be optimized to reduce the complex structure in the final dispersive wave spectrum.
(b) The deep ultraviolet generation part is the core of the entire system. After spatial filtering, the pulse output from the pre-compression part is focused and coupled into the anti-resonance Kagome fiber to experience the process of high-order soliton self-compression and dispersion wave generation. One end of the Kagome optical fiber is an air chamber with adjustable air pressure, and the other end is a vacuum chamber, thereby forming the required negative air pressure gradient.
(c) The pulse characterization part is used to measure the energy, pulse width and other information of the output pulse, revealing that this method can obtain pulses narrower than 4 fs and close to the transformation limit with an efficiency of 3%. Because BBO is opaque in bands shorter than 190 nm, this part was implemented by DFG XFROG and the rPIE method was applied. This method does not require phase information of the reference pulse and can measure pulses shorter than the reference pulse. By adjusting the gas chamber pressure to 6 bar, 7.5 bar and 12.5 bar respectively, the center wavelengths of 225 nm, 250 nm and 300 nm can be obtained respectively, and the pulse width is 3 fs acting on deep ultraviolet ultra-short pulses, as shown in Figure 2.
Figure 2 Deep UV pulse obtained by adjusting air pressure [2]
The black line in Figure 2 represents the spectrum and time domain pulse of the DFG XFROG method after rPIE processing, the purple line represents the phase at different wavelengths, and the red dotted line represents the UV pulse spectrum obtained by direct measurement. It can be seen from the figure that the spectra obtained by the XFROG method and direct measurement are relatively consistent. The phase distribution of the 250 nm and 300 nm ultraviolet pulses is gentle, and the pulses are close to the transformation limit. The 225 nm ultraviolet pulse has more long-wavelength components and has a higher delay. In the theoretical part, the article simulates the evolution process of pulses in Kagome fiber with a certain pressure gradient, revealing two main reasons for the generation of narrow pulse width and transformation-limited dispersion waves:
Figure 3 Theoretical simulation of 225nm ultraviolet generation [3]
Figure 3 is the simulation result obtained by applying 6 bar air pressure at the end of the PCF gas chamber. The input pulse energy is 1.15 μJ, the PCF is 7.5 cm long, 33 μm in core diameter, and filled with argon. Figure 2(a) is the spectrum (black line) and phase (purple line) of the output dispersion wave. Figure 2(b) is the simulation result of pulse transmission along the PCF in the frequency domain. The green line is the zero dispersion wavelength. Figure 2(c) is the simulation result in the time domain. The small picture shows the ultraviolet pulse filtered by the bandpass filter, and the green line is the pulse center.
On the one hand, changes in air pressure directly lead to changes in the dispersion zero point of Kagome fiber, changing the phase matching conditions for dispersion wave generation. As the pulse propagates in the optical fiber, the gas pressure gradually decreases, and the wavelength of the dispersion wave corresponding to the phase matching also gradually becomes shorter. Therefore, a very broad dispersive wave spectrum is formed, which provides a prerequisite for a very small pulse width at the conversion limit.
On the other hand, the reduction of air pressure makes the dispersion in the deep ultraviolet region smaller and smaller. The dispersion wave generated in the second half of the fiber will not be rapidly broadened due to the larger dispersion, thus maintaining the pulse width close to the conversion limit.
In short, this article uses a hollow photonic crystal fiber with a gradient of air pressure and a gradient of dispersion zero point to achieve the generation of dispersive waves with pulse widths of several femtoseconds and close to the limit of transformation. A powerful tunable deep ultraviolet light source has been invented, enabling ultrafast measurements with unprecedented spectral range and temporal resolution.
References
[1] Christian Brahms, Dane R. Austin, Francesco Tani, at el. Direct characterization of tuneable few-femtosecond dispersive-wave pulses in the deep UV. Optics Letters, 2019, 44(4):731~734.
[Copyright Statement] This article is reproduced from the "Guangbo Chang" public account,
has been authorized by the public account. If you need to reprint again, please scan the QR code and contact the person in charge of the official account.