Efficient idler broadening via oppositely dual-chirped difference frequency generation Download: 510次
1 Introduction
The development of mid-infrared (IR) ultrafast pulsed lasers of high intensity, short pulse duration and tunable carrier wavelength at
Previously, Zhang et al. reported an OPA scheme termed dual-chirped optical parametric amplification (DC-OPA)[13, 14], where both the pump and signal waves are temporally stretched. The prolonged laser pulses may relieve the influence of pulse slipping owing to the nonuniform group velocities. In addition, the broadband spectrum of the pump wave may even be utilized to fulfill the broadband phase-matching (PM) amplification. Compared to its chirp-free counterpart, it provides an additional degree of freedom to regulate the spectrum of the generated idler wave[15]. Specifically, when the chirp signs of the pump and the signal waves are opposite, a remarkably broadened idler wave may be generated. Similar nonlinear techniques were successfully applied for the broadband second-harmonic generation and frequency tripling, or the generation of narrow-bandwidth picosecond pulses[16–18]. However, broadband PM is always the prerequisite for efficient energy boosting and circumvention of the GVM-associated gain-narrowing effect. Evidently, to satisfy the broadband PM condition, the instantaneous wave vectors should be properly arranged into pairs to constantly meet the PM conditions[15, 19].
In this study, specific to an oppositely dual-chirped DFG scheme, we derive the precondition to realize broadband frequency conversion. Then, to select the proper nonlinear crystals for various parametric processes, we systematically analyze the group-velocity relationship and the gain spectra of different nonlinear crystals at different wavelengths. Thereinto, the periodically poled lithium niobate (PPLN) crystal that satisfies an inactive Type-II (eo-o) quasi-phase-matching (QPM) condition exhibits the required negative correlation coefficient of the group velocity and the expected broadband gain characteristics across an ultra-broad idler spectral region
Excellent experimental works have been reported on the generation of few-cycle mid-IR pulses. Resorting to a traditional noncollinear PM geometry or a well-designed aperiodic QPM crystal, broadband mid-IR pulsed lasers are directly generated as the idler of a difference frequency generator[20, 21]. However, a subsequent angular dispersion correction is generally necessary for the noncollinear DFG design, and an aperiodic QPM crystal is not convenient for realizing the idler wavelength tunability for the latter method. As presented, an oppositely dual-chirped DFG is capable of generating a spectrum-broadened idler seeding in combination with the broadband PM circumstances. It also allows for using a longer crystal and a lower intensity to achieve efficient energy transfer, which is beneficial to limit the negative
2 Broadband phase-matching condition
In a standard ultrafast OPA/DFG system, the pump and signal pulses are both nearly transform-limited. Nevertheless, prolonged pulse duration is advantageous to relaxing the ubiquitous pulse slipping, reducing the input power density to avoid unwanted disturbing phenomena and potentially providing the PM environment across a broad spectrum.
Fig. 1. (a) Instantaneous angular frequencies and the bandwidth of each interacting wave (, , ) of the oppositely dual-chirped DFG scheme; (b) sketch of the ideal broadband PM condition.
A simple qualitative picture of such a parametric process is given in Figure
Assuming that the pulsed lasers have been sufficiently stretched and the pulse envelope slipping is negligible, individual temporal slices (i.e., individual spectral components) of the pump and the signal waves are arranged in pairs. The instantaneous wave-vector mismatch of
3 Selection of the nonlinear crystal
Generally, the group velocity and the resulting correlation coefficient of
On one hand, a negative
Fig. 2. The wavelength-dependent in various commonly used nonlinear crystals under different PM conditions. A well-developed Ti:sapphire ultrafast pulsed laser is employed as the pump laser. The special cases where the generated idler wave is absorbable by the employed nonlinear crystal are also included (dashed lines), in consideration of its other potential applications[27, 28]. The shadowed area indicates the desired values of , which may realize the optimum idler broadening.
As shown in Figure
Intrinsically, oppositely dual-chirped DFG is capable of generating the conspicuously broader mid-IR pulsed seeding than the driving pulsed lasers, which is the premise of generating the high-intensity pulsed laser. However, only if the broadband PM condition is satisfied can the subsequent effective energy transfer be realized. Provided that an appropriate nonlinear crystal material is selected and the pump and signal waves are properly pre-chirped, broadband frequency conversion is achievable over an extremely broad spectral range.
Table 1. Nonlinear optical parameters for the 5% doped MgO:PPLN and the YCOB crystal at (, , ).
|
4 Gain characteristics of the Type-II PPLN crystal
According to the crystal-selection strategy presented in the previous section, the Type-II PPLN crystal shows an attractive application to generate high-intensity pulsed laser at
We take a conventional 790 nm Ti:sapphire pulsed laser as the pump laser, for which
Fig. 3. (a), (b) The calculated conversion efficiency and idler spectral bandwidth versus the ratio of in a weak-conversion situation . (c), (d) The gain in the spectrum is introduced to evaluate the performance of such a difference frequency generator as the spectrum broader.
The conversion efficiency and the idler spectral bandwidth versus the ratio of
Both the obtained idler spectrum and the conversion efficiency exhibit similar trends as a function of the ratio of
Further, we chose another nonlinear crystal with a positive group-velocity coefficient
In conclusion, the oppositely dual-chirped DFG scheme may provide the simultaneous achievement of higher conversion efficiency, broadened idler spectrum and potentially shorter pulse duration after compression if the broadband PM condition is satisfied.
5 Tunable mid-IR spectrum broader
When the constant chirped pump wave is utilized, a broader signal wave is advantageous to the generation of a broader idler wave. To evaluate the performance of such a difference frequency generator as the spectrum broader, we introduce the gain in spectrum, defined as the ratio between the bandwidth of the obtained idler wave
Note that
As a conclusion, the broadband phase-matched oppositely dual-chirped DFG shows an extraordinary ability to make the most of the pump energy and spectral components of the interacting pump and signal waves achieve efficient idler broadening. However, the initial linear chirps
Fig. 4. Schematic of a tunable mid-IR spectrum broader based on the oppositely dual-chirped DFG scheme. BS, beam splitter; DM, dichroic mirror; DL, delay line; and , lenses; C, sapphire plate.
As shown in Figure
Fig. 5. The calculated conversion efficiency and TL pulse duration of the generated idler wave as a function of the signal wavelength, which is continuously tuned from 980 nm to 1100 nm and from 1100 nm to 1500 nm, respectively. Note that, the TL pulse duration of the initialized 790 nm pump laser is . Solid line, a Type-II PPLN crystal is employed and the is optimized for various signal wavelengths; dashed line, the ideal cases, where the material dispersions are absent; dotted line, the comparison results based on a conventional Type-0 PPLN crystal. (a) ; (b) ; (c) ; (d) .
Assuming a 10 mm Type-II PPLN crystal is employed, the initial
Figures
6 Conclusion
Oppositely dual-chirped DFG is advantageous for obtaining a spectrum-broadened idler seeding compared with the incident pump and signal waves. However, the normally more rigorous broadband PM condition hampers its application in the generation of an ultrafast mid-IR pulsed laser. In association with the fundamental PM equations and the chirp characteristics of such DFG schemes, we derived the intrinsic requirement of the broadband frequency conversion on both the group-velocity correlation and the chirp parameters, and used this as a strategy to choose the appropriate nonlinear crystals for various parametric processes. Although most of the birefringence bulk crystals can only provide the required material dispersions in extremely limited spectral regions, the PPLN crystal that satisfies an inactive Type-II (eo-o) QPM condition has a relatively stable negative
[2]
[5]
[6]
[7]
[8]
[9]
[10]
[13]
[14]
[15]
[16]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
Article Outline
Haizhe Zhong, Bin Hu, Saisai Hu, Shengying Dai, Ying Li, Dianyuan Fan. Efficient idler broadening via oppositely dual-chirped difference frequency generation[J]. High Power Laser Science and Engineering, 2020, 8(2): 02000e27.