Ultrafast nonlinear optical technology - 36 GHz PRF, broadband mid infrared frequency comb

Coherent mid infrared light source with wide spectrum can simultaneously perform absorption calibration for multiple molecules, thus accelerating the detection process, such as element detection in cultural heritage protection and calibration of cancer markers in biomedical applications, and can also be used to study the singular properties of materials such as topological phase and superconductivity in two-dimensional materials.

This issue introduces the work of Scott A. Diddams Group of the National Institute of Standards and Technology of the United States, a high repetition rate broad spectrum mid infrared light source based on the intra pulse autodyne scheme.

In 2021, based on the intra pulse self difference frequency of erbium-doped fiber laser, the research group will generate an optical frequency [1] with a wavelength covering six octaves. On this basis, the same research group will develop a mid infrared optical frequency comb with repetition frequency up to 1GHz [2].

Fig. 1 Experimental device diagram [2]

The experimental device is shown in Figure 1. The seed pulse with a repetition rate of 1GHz, a pulse width of 300fs, a power of 60mW and a spectral bandwidth of 13nm has a spectral broadening of 39nm after pre amplification. The pre amplified pulse is amplified by CPA to obtain a pulse with an average power of 3.6W and a pulse width of 120fs, and the corresponding pulse energy is 3.6nJ. In order to meet different needs, the research group selected two kinds of highly nonlinear optical fibers, positive dispersion and negative dispersion, to broaden the spectrum and compress the pulse at least periodically, and finally focused on different nonlinear crystals for intra pulse self difference frequency to obtain broadband mid infrared pulses.

Fig. 2 Widening and compression results of positive dispersion high nonlinear optical fiber [2]
The spectrum broadening and pulse compression results produced by using positive dispersion and highly nonlinear fiber are shown in Figure 2. The pulse spectrum is broadened to 1.3-1.7 through a 21cm long optical fiber μ m. Then, the pulse is compressed to 22fs by using massive fused quartz that provides negative dispersion.

Figure 3 7-13 μ M middle infrared pulse generation [2]
The compressed pulse is focused into two kinds of nonlinear crystals, 560 μ M CSP and 1mm OP － GaP. The output spectrum is shown in Figure 3, where the mid infrared spectrum generated in CSP ranges from 7.5 to 13 μ m. The mid infrared spectrum produced in OP － GaP ranges from 7 to 14 μ m. The mid infrared pulse power generated by both is less than 100 μ W。

Fig. 4 Widening and compression results of negative dispersion high nonlinear optical fiber [2]
To generate a wavelength range of 7 μ The mid infrared pulse below m requires further broadening of the near-infrared spectrum. In order to overcome the broadening stagnation caused by the light wave splitting effect in the positive dispersion optical fiber, the author uses a negative dispersion, highly nonlinear fiber to obtain a wider broadening spectrum, and uses the soliton self compression effect to directly output short period pulses. The author uses a 3.6 cm long negative dispersion high nonlinear fiber to broaden the spectrum to 1-2.2 μ m. The pulse is self compressed to 8.1fs, and the result is shown in Figure 4.

Fig. 5 3 － 4.7 μ M middle infrared pulse generation [2]
The pulse with few periods is focused into two PPLNs to realize the self difference frequency within the pulse. By changing the period of the crystal, 3 － 4.7 can be obtained μ M tunable mid infrared pulse output, the result is shown in Figure 5. The maximum average power of the infrared pulse in the first five cycle PPLN output is 4.5mW, and the maximum average power of the infrared pulse in the second 16 cycle PPLN output is 6.2mW.

Figure 6 GHz Comb Lock [2]
In order to lock the optical comb, the author compares two schemes: Scheme 1 uses the cascading second-order nonlinear effect of nonlinear crystals to obtain the fceo of 3.5 um beat signal (Fig. 6a), and uses it as a feedback signal to lock the near-infrared optical comb (Fig. 6c). In Scheme 2, the near-infrared optical comb is directly locked to a narrow linewidth reference light source (Fig. 6b). The integrated phase noise in these two cases is 1.5 rad and 85 mrad respectively. Obviously, locking to the reference source is better, but the first scheme can also meet many application requirements.

This work reports for the first time the mid infrared optical comb with a repetition rate of up to 1GHz and a wavelength range of 3-13 μ m, laying a good foundation for the application of GHz mid infrared dual optical comb spectroscopy.

reference:
［1］ Lesko， D．， Timmers， H．， Xing， S．， Kowligy， A．， Lind， A． J．， ＆ Diddams， S． A． （2021）． A six－octave optical frequency comb from a scalable few－cycle erbium fibre laser． Nature Photonics， 15（4）， 281－286．

［2］ Hoghooghi， N．， Xing， S．， Chang， P．， Lesko， D．， Lind， A．， Rieker， G．， ＆ Diddams， S． （2022）． Broadband 1－GHz mid－infrared frequency comb． Light： Science ＆ Applications， 11（1）， 1－7．