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Abstract
Coherent pulse synthesis in the mid-infrared (mid-IR) domain is of great interest to achieve broadband sources from parent pulses, motivated by the advantages of optical frequency properties for molecular spectroscopy and quantum dynamics. We demonstrate a simple mid-IR coherent synthesizer based on two high-repetition-rate optical parametric amplifiers (OPAs) at nJ-level pump energy. The relative carrier envelope phase between the two OPAs was passively stable for a shared continuous wave (CW) quantum cascade laser (QCL) seed. Lastly, we synthesized mid-IR pulses with a duration of 105 fs ranging from 3.4 to 4.0 µm. The scheme demonstrated the potential to obtain broader mid-IR sources by coherent synthesis from multiple CW QCL-seeded OPAs.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The generation of high-repetition-rate ultrafast mid-infrared (mid-IR) pulses has made great progress over recent decades, owing to their potential in various applications, including highly sensitive molecular spectroscopy [1,2], strong-field physics [3,4], and quantum dynamics [5,6]. In the mid-IR spectral region, the absorption strength of gas molecules is usually 103 times higher than that in the near-infrared (near-IR) region. Moreover, the driving laser sources at longer wavelengths have the potential to obtain higher-order harmonic generation and perform medical diagnostics without causing damage [7,8]. Many proof-of-principle sources have been demonstrated to obtain mid-IR pulses, including Tm-doped fiber lasers centered at 2 um [9], quantum cascade lasers (QCLs) [10], laser systems based on nonlinear frequency conversion. The QCLs can directly emit mid-IR pulses, but usually with a duration of picosecond level [11,12]. The nonlinear frequency conversion is the main method to obtain mid-IR pulses, including difference frequency generation (DFG) [13], optical parametric oscillators (OPOs) [14], and four-wave mixing (FWM) in the formation of filament [15]. Generally, multiple frequency conversion and power amplification processes are needed to obtain broadband mid-IR sources [16,17]. Recently, supercontinuum technique in mid-IR domain has been developed using various mediums, such as mid-IR high nonlinear fibers and well-designed waveguide structures [18–20], which reduces the requirement of pulse energy and the complexity of the systems. However, the severe degradation of noise characteristics mainly from nonlinear amplification dynamics of the input pulse fluctuations limits the application in some fields requiring high beam quality and frequency stability, typically such as nonlinear physics and mid-IR dual comb spectroscopy.
In order to further achieve transform limited (TL) pulses, precise phase control and broadband spectral coverage are needed. To overcome the restriction between the spectral bandwidth and the difficulty of dispersion control for a single laser source, a coherent synthesizer can be used to stitch the spectra inherited from parent pulses. The ultimate aim is to obtain coherent sources with different frequency regions, which share the same repetition frequency (fr) and carrier envelope phase (CEP) frequency (fcep) [21]. In recent decades, phase control and stabilization techniques have been implemented to eliminate fluctuations in the repetition rate and CEP for mode-locked lasers [22,23]. However, active control of coherent synthesis has been developed, mainly in the near-IR region [24,25]. Kärtner et al. achieved coherent synthesis between a near-IR optical parametric chirped pulse amplifier (OPCPA) and a mid-IR OPCPA, in which an actively CEP-stable octave-spanning Ti:sapphire oscillator served as the seed [26]. Significantly, intra-pulse DFG can generate the mid-IR idler with zero-offset CEP directly [27], in which the complex CEP locking process is avoided. Based on this mechanism, Liu et al. and Hong et al. achieved coherent synthesis for different mid-IR spectral regions, respectively [28,29]. Generally, these complex schemes based on multiple frequency conversion limit the repetition rate of sub-hundred-kHz systems and require the pump sources with a single pulse energy at the µJ-level. Moreover, the complex instrument setups also increase the maintenance cost and the accumulative noise in multiple nonlinear processes, which results in the degradation of the mid-IR sources.
Passive coherence has been demonstrated in continuous wave (CW)-seeded OPAs [30]. A pulsed and amplified signal with the same repetition rate as the pump source inherits the CW seed frequency teeth. Based on this mechanism, we obtained passive coherent near-IR dual-comb spectroscopy, while the common near-IR CW seed was used in the OPAs [31]. Compared to the OPAs seeded by parametric fluorescence or white-light continuum (WLC) radiation, the CW seed provided better stability in the nonlinear process [32,33]. Recently, we achieved femtosecond mid-IR pulses from a mid-IR CW-seeded OPA at nJ-level pump energy [34]. The bandwidth of the generated mid-IR pulses was limited by the phase-match condition. On the other hand, coherent synthesis between CW-seed OPAs has been demonstrated in the near-IR region, which overcame the limitations of single CW-seeded OPA sources [35].
In this letter, we propose and demonstrate the potential of the combination of coherent synthesis techniques and mid-IR CW-seeded OPAs. A shared CW quantum cascade laser (QCL) served as the seed of two OPAs, and a common pump source at nJ-level energy was used. Then, the two generated mid-IR OPAs were synthesized coherently. Thus, a spectrum covering from 3.4. to 4.0 µm was obtained. The duration of the synthesized pulses was ∼ 105 fs, which was consistent with the simulations. By further introducing extra pulse compression devices, TL pulses with a duration of 88 fs can be achieved. In addition, the noise associated with the coherent synthesis was also analyzed.
2. Experimental setup and results
The experimental setup is illustrated in Fig. 1. The system consists of two mid-IR CW-seeded OPAs and a coherent synthesizer. A mode-locked ytterbium-doped fiber amplifier, which was centered at 1030 nm with a 10 nm full width at half maximum (FWHM), served as the pump source for the OPAs, delivering a 50 nJ, 160 fs pulse train at a repetition rate of 160 MHz. A CW QCL (tunable from 3.71 to 3.9 µm, Daylight, TLS-SK-41038-HHG, less than 5 MHz linewidth for an integration time of 100 ms) served as the seed of OPAs. Suitable telescope systems were used to match the beam sizes to improve the conversion efficiency before the pump pulses were combined with the CW signal. The output power from the pump source and CW seed was split equally into two branches. In each OPA branch, the combined beams were focused into a multiple-grating MgO-doped periodically poled lithium niobate (PPLN) crystal (tuning from 28 um to 31 um, stepped by 0.5 um) using a 150 mm focal length calcium fluoride (CaF2) lens. The mid-IR CW signal was pulsed and amplified via the OPA process. Then, the generated mid-IR pulses were collimated using another 150-mm-focal-length lens and filtered by a 5 mm thickness germanium (Ge) window with an anti-reflection (AR) coating (Thorlabs, WG91050-C9). Identical optical structures and mirrors were used for the other branch, but with the PPLN crystal tuned to a different period – the branch using a crystal period of 29.0 µm was denoted by ‘OPA 1’ while the branch using a period of 28.5 µm was denoted by ‘OPA 2.’ A delay line fixed on a translation stage was inserted into one branch to compensate for the relative time delay. When the time delay was close to zero, the two OPAs were coherently synthesized.
Fig. 1. Experimental setup comprising two CW-seeded OPAs and a combined coherent synthesizer. HWP, half-wave plate; PBS, polarizing beam splitter; BS, beam splitter; M, mirror; DM, dichroic mirror; L, lens; PPLN, periodically poled lithium niobate crystal; F, Ge windows; Delay, delay line.
The operating wavelength of the tunable CW QCL was set to be 3.71 µm. For each branch, the injecting power of the CW seed to the crystal was ∼50 mW, and the maximum injecting pump power was ∼ 2.85 W owing to the energy loss of the dichroic mirrors. The output power of mid-IR pulses from OPA 1 was measured to be 380 mW, while that from OPA 2 was 330 mW. The power can be improved further by increasing the pump power or using a double-pass scheme [32]. A broadband CaF2 beam splitter (Thorlabs, BSW511) was used to combine the two OPAs, and a variable density filter was inserted in one branch to adjust the energy ratio. The resulting power of two mid-IR pulse sequences injected into the coherent synthesizer was measured to be 100 mW of OPA 1 and 84 mW of OPA 2, respectively.
To investigate the time and spectral evolution of the scheme, numerical simulations based on the experimental parameters were performed. The processes were divided into three parts: (i) the CW-seeded OPAs, (ii) the pulse compression of the Ge filter, and (iii) the coherent synthesis, as shown in Fig. 2. To simplify the computation, we solved the coupled three-wave time-dependent nonlinear propagation equations using a one-dimensional plane-wave model and a split-step method [34,36]. The peak power variation for the focus effect was also considered. A beam waist radius of ∼ 35 µm was used, and the focus was located at a distance of 10 mm from the front plane of the crystals. The time evolutions and pulse duration variations of the two mid-IR pulse sequences are shown in Fig. 2(a). The mid-IR CW signal was pulsed and amplified when the pump pulses overlapped with the seed in the OPA process. Then, the generated mid-IR pulses were broadened after the focus, owing to the negative dispersion of the PPLN crystal (−1151.4 fs2/mm at 3.71 µm). The negative group-velocity dispersion (GVD) of the CaF2 lens (−218 fs2/mm) was negligible. The durations of the output mid-IR pulses in the two OPAs were ∼ 218 and ∼ 320 fs, respectively. A Ge plate (+1168.8 fs2/mm) was used for the pulse compression since it compensates for the negative dispersion of the system. The pulse compression was simulated when the mid-IR pulses passed through the two Ge plates. The spectral evolutions and normalized intensity curves of the two OPAs are shown in Fig. 2(b). For OPA 1, the optimal length for obtaining the narrowest mid-IR pulse sequence was calculated to be ∼ 7.3 mm, while for OPA 2 it was ∼ 9.7 mm. The intensity of mid-IR pulses was extremely weak before the focus, with the result that the center of the power-increasing curves deviated slightly away from the focus, as shown by the purple lines in Fig. 2(b).
Fig. 2. (a) Simulated time evolutions and pulse duration variations of two mid-IR pulses in the scheme. The left y-axis label represents the time walk-off between the generated mid-IR pulses and the pump pulses. The portion to the left of the white dashed line represents the time evolution process in the CW-seeded OPA process, while the area to the right shows the pulse compression using a Ge plate. (b) Simulated spectral and normal intensity evolution of the two OPAs.
In the experiment, the spectra of the two OPAs and the synthesized pulses were measured using a fast Fourier transform analyzer (Bristol 771B), as shown in Fig. 3. The spectral FWHM values of the two OPA 1 and OPA 2 were 105 nm and 75 nm, respectively. The difference of spectral bandwidths can be owing to the variations of phase-matching bandwidths in different PPLN periods. To further improve the spectral bandwidth of single OPA, a well-designed chirped PPLN waveguide can be used [37]. The simulated results agree closely with the measured data. The resulting spectrum of synthesized mid-IR pulses spanning from 3.4 to 4.0 µm is shown in Fig. 3(c). The frequency components beyond 4000 nm are due to the saturation of the detector in mid-IR spectrum analyzer. The stitched spectrum is shown by the magenta curve. A home-made auto-correlator based on the two-photon absorption effect was used to measure the pulse duration of mid-IR pulse sequences before coherent synthesis, as shown in Fig. 4. The baselines in the interferometric autocorrelation curves are normalized to unity, as shown in Figs. 4(a) and 4(b). The contrast ratios are about 7:1, slightly lower than that of an ideal interferometric autocorrelation curve due to the unbalanced power injected to the two-photon detection (Thorlabs, PDA10D-EC) [38]. The corresponding intensity autocorrelation curves are represented by the blue lines in Figs. 4(c) and 4(d). In the experiment, a 5 mm thickness Ge window was used to filter out the mid-IR pulses in each OPA branch. The intensity autocorrelation signals agreed well with the simulation results at a 5 mm transmission length of the Ge plate. Thus, the intensity envelope and temporal phase of the two OPAs can be obtained from the simulations, as shown in Figs. 4(e) and 4(f). The pulse durations of OPA 1 and OPA 2 after the Ge filters were 145 fs and 260 fs, respectively. Additional pulse compression sections can be applied to achieve TL pulses, such as well-designed Ge or Si sequences.
Fig. 3. Measured and simulated spectra of (a) OPA 1 and (b) OPA 2. The red lines represent the simulated spectra after the crystals in Fig. 2(b). (c) Spectrum of combined mid-IR pulses (cyan line, shadow). The stitched spectrum calculated based on the measured spectra of OPA 1 and OPA 2 is shown by the magenta curve.
Fig. 4. (a, b) Measured interference autocorrelation curves of the two OPAs before coherent synthesis. (c, d) Measured and simulated results of the two OPAs. The blue lines represent the intensity autocorrelation curves of (a) and (b), respectively. The shadows show the simulated results. (e, f) The intensity envelope (dark red line) and temporal phase (red line) of the two simulated OPAs.
In our scheme, the two OPAs were seeded by a shared CW QCL, with a common pump source. The CW signal was nonlinearly modulated and amplified when the seed overlapped with the pump pulses. Thus, the two OPAs had the same repetition rates as the pump source and shared the common CW frequency teeth. Thus, the mutual coherence between the two OPAs was achieved passively, which is of great importance for the generation of arbitrary optical waveforms [39]. A time delay line was used to adjust the relative time delay between the two OPAs to achieve coherent synthesis. The measured interference autocorrelation curve of the combined beam is shown in Fig. 5(a). The two simulated mid-IR pulse sequences in Fig. 4 were used to model the coherent synthesis process. The intensity autocorrelation curve displayed close agreement with the simulated result, as shown in Fig. 5(b). The intensity envelope and temporal phase of the simulated coherent synthesized pulses are shown in Fig. 5(c) with a pulse duration of 105 fs, which was slightly broader than the TL pulse of 88 fs owing to the residual GVD and high-order dispersion in the two OPAs. The results demonstrated the potential of the scheme to generate few-cycle mid-IR pulses by using multiple mid-IR CW-seeded OPAs and suitable pulse compressors.
Fig. 5. (a) Measured interference autocorrelation curve of combined pulses after coherent synthesis. (b) Measured and simulated results of coherent synthesis. The shadow shows the simulated results. (c) The intensity envelope (dark red line) and temporal phase (red line) of the simulated coherent synthesized results.
Compared to OPAs seeded by the parametric fluorescence and WLC radiation, the CW-seeded OPA has a better noise characteristic, owing to the stability provided by the CW seed. The noise from the CW QCL was negligible. In our previous work, the measured noise performance of the mid-IR CW-seeded OPA was close to the pump source [34]. Here, the relative CEP between the two OPAs was extremely stable, since both OPAs had the same carrier frequency and repetition rate. According to our observations, the synthesized mid-IR pulses maintained well stability for the duration of the measurement while the experimental setup was sealed. The stability is limited principally by the slow drifts of the separated optical path [35,38]. An active control can be introduced to achieve the long-term stability [25,40]. To evaluate the noise of the synthesized mid-IR pulses, we measured the noise characteristics of the fr of the synthesized pulses and the two OPAs using a commercial phase noise analyzer (Rohde & Schwarz), as shown in Fig. 6. To increase the stability of the system, the fr of the pump source was locked by controlling the intracavity piezoelectric transducer (PZT), which can suppress the low-frequency fluctuation originating from the environmental and temperature variations validly [4,41]. The noise peaks at ∼ 300 Hz were due to the driving frequency of the phase-locked circuit. The phase noise associated with fr represents the stability of pulse sequences, which can be understood as jitter in the time domain [42]. The repetition rates of the two OPAs were determined by the common pump source, which resulted in a similar noise performance of the two OPAs. As shown in Fig. 6(a), the integrated phase noises from 10 MHz to 1 Hz for OPA 1, OPA 2, and the synthetized pulses were calculated to be ∼ 4.13, 4.08, and 3.81 mrad, respectively. On the other hand, the intensity stability of an OPA is mainly disturbed by the intensity fluctuation of the pump source and the parametric fluorescence in the nonlinear process [30]. As shown in Fig. 6(b), the integrated relative intensity noises from 10 MHz to 1 Hz for OPA 1, OPA 2, and the synthetized pulses were 2.05%, 3.20%, and 1.94%, respectively. In summary, the noise of synthesized mid-IR pulses was close to the parent OPAs, which was significant for improving the beam quality for strong-field physics and mid-IR combs.
Fig. 6. (a) Phase and (b) intensity noise distributions of the two branches (red for OPA1, blue for OPA 2) and synthetic mid-IR pulses (Cyan).
3. Conclusion
We demonstrated the approach of mid-IR coherent synthesis based on two CW QCL-seeded OPAs. The two OPAs inherit the common CW frequency teeth and have the same repetition rate as the pump source at nJ-level energies, which enables mutual coherence between the two OPAs for coherent synthesis. In the end, a spectrum covering from 3.4 to 4.0 µm was obtained by stitching the spectra of the two OPAs, while the duration of the synthesized pulses was measured to be ∼ 105 fs. The experimental results showed close agreement with the simulations. When the two OPAs are compressed into their own TL durations, mid-IR pulses with a duration of 88 fs can be achieved. Furthermore, we analyzed the noise performance before and after coherent synthesis. Compared to the parent pulses, there was no obvious degradation for our synthesized pulses, which is significant for applications involving high-sensitivity mid-IR combs and quantum physics. This mid-IR coherent synthesizer design highlights the potential for generating broader bandwidth and few-cycle mid-IR sources via coherent synthesis from multiple CW-seeded OPAs or subsequent cascade OPA structures.
Funding
National Key Research and Development Program of China (2017YFF0206000, 2018YFA0306301); National Natural Science Foundation of China (11874153, 11904105).
Disclosures
The authors declare no conflicts of interest.
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