Broadband optical frequency combs (OFCs) with stabilized phases and femtosecond durations are attractive tools for rapid high-resolution nonlinear spectroscopy, including coherent Raman spectral imaging [1], multidimensional coherent spectroscopy [2,3], and chiral optical activity spectroscopy [4]. In particular, OFCs with wide spectral coverage are far more competitive regarding high-sensitivity measurements for the simultaneous detection of multiple absorption bands. In addition, dual-comb spectroscopy completely removing mechanical scanning also has high-speed measurement capabilities, which can characterize complex substance dynamics of optical chemical, optical physics, and optical biology [5–9]. However, directly generating broadband light is a significant challenge due to the lack of a broadband gain medium and limited amplification bandwidth. Thus, laser sources are usually adopted with the coherent optical frequencies in local spectral regions. In addition, OFCs with a wide spectral coverage from the ultraviolet (UV) to the mid-infrared (MIR) region are desirable for simultaneous parallel spectral analysis. Over the past two decades, nonlinear frequency conversion technologies have been developed to generate broadband spectra. One approach is supercontinuum generation (SCG) based on third-order nonlinearity (χ(3)) in cubic nonlinear media such as gas-filled photonic crystal fiber (PCF) [10] and non-centrosymmetric crystalline waveguides [11]. The main effects of χ(3) causing spectral broadening include self-phase modulation (SPM), four-wave mixing (FWM), high-order soliton fission, and dispersive wave generation [12–15]. Typically, the SCG light sources at UV, visible (VIS), and MIR regions have been demonstrated using ZBLAN solid-core PCF [16], gas-filled PCF [10], and Si/Si3N4 waveguides [17,18], respectively. Even with recent advances, the weak nature of χ(3) remains constrained to the conversion efficiency and broadening spectral range. Therefore, the SCG usually requires a high-power pump laser to drive spectral expanding.

Compared with χ(3), quadratic nonlinear effects (χ(2)) could provide specific wavelength conversion with lower pump power, but the spectral range is still limited by the phase-matching bandwidth. Through manipulation of various χ(2) effects, many investigations and substantial progress have been made to realize several-octave lights [10,19–23]. Among them, periodically poled lithium niobate (PPLN, LiNbO3) crystals with broad transparent spectral range and strong nonlinearities play significant roles in broadband chip-level laser device functionalities [19,24]. Moreover, LiNbO3 possesses large χ(2) nonlinear coefficients [25] like the other popular materials of GaAs [26] and GaP [27], supporting various χ(2) effects of difference frequency generation (DFG) [28,29], optical parametric oscillators (OPOs) [30], sum-frequency generation (SFG) [31], and high-order harmonic generation (HHG) [32]. Therefore, lasers based on multi-PPLN chips have significant potential for integrated broadband chip devices. On the other hand, the PPLN exhibits excellent characteristics for processing chirps and waveguide structures, which help to maintain broadband phase-matching spectra and high conversion efficiency. Importantly, the combs showing well-preserved coherence without any apparent degradation were verified in Refs. [22,33], regardless of the highly complex nonlinear process. However, the hybrid χ(2) parametric processes would result in inconsistent carrier envelope phase offset frequencies (f0) during nonlinear frequency conversion. For example, HHG generates multiple harmonics with different offset frequencies m×f0 where m represents the harmonic orders. Frequency aliasing also occurs in dual-comb spectroscopy if the obtained harmonics and fundamental waves overlap in the spectrum. To remove these overlaps in the spectra, we can employ offset-free OFCs (f0=0Hz) as the driving sources, where the zero setting of f0 can be realized via DFG [34,35] and using an f-2f interferometer [36]. These offset-free frequency combs with broadband spectral coverage in the UV-to-MIR region are urgently desired in precise measurement fields of high-precision frequency metrology and high-resolution broadband spectroscopy, and it is of value to further explore the compact chip-based schemes.

In this paper, we report a broadband offset-free frequency comb covering a wide spectral range using a three-stage cascaded PPLN chain. The offset-free MIR comb of 2.5–5 μm was generated via intra-pulse DFG (IPDFG) in a chirped PPLN waveguide. Then a chirped PPLN crystal was employed to perform OPA without bandwidth loss, acquiring a 230 mW broadband MIR comb of 2.8–5 μm. Driven by this broadband offset-free MIR pump comb, up to the 10th harmonic (354–369 nm) was generated in a well-designed chirped PPLN waveguide based on cascaded χ(2) HHG effects. The output spectra covered the wide UV-to-MIR range, and particularly the output of broadband frequency combs in the visible region (H5–H8) is obtained without any pump wavelength tuning. It is believed that this broadband comb has the potential as a compact dual-comb source for exploring parallel broadband measurements of semiconductors, biomacromolecules, and gas molecules.

Figure 1 presents the schematic of the proposed compact broadband frequency comb with a three-stage cascaded PPLN chain. A near-infrared Er-fiber comb source, with the f0 and fr phase-locked to a standard reference [35], was delivered to three PPLN chips in sequence, which were designed for DFG, OPA, and HHG, respectively. For the seed comb, we optimized the pump power, cavity net dispersion, and environmental isolation to generate a low-noise pulse train with a flatter spectrum around 1.5 μm. The amplified seed comb was then delivered to a highly nonlinear fiber (HNLF) for octave SCG. Subsequently, the different spectral components of the supercontinuum simultaneously served as the pump and signal of the IPDFG, directly achieving an offset-free MIR comb in the first PPLN chip. Moreover, the total lengths of the passive fibers were optimized to control the nonlinear spectral broadening and achieve a spatiotemporally aligned pump for IPDFG. A Ge plate was added to separate the MIR light from the nonconverted NIR light. In the MIR OPA, the separated NIR light was amplified in the YDFA and served as a pump light. The DFG-based MIR pulses filtered by an AR-coated germanium plate were employed as the signal light, which was combined with an NIR pump light using a Ge window. In addition, a tunable delay was used to synchronize the pump with the signal. The combined beam of the signal light and pump light was focused onto a PPLN crystal using a 75 mm calcium fluoride (CaF2) lens. It is important that a suitable telescope system should be used to control the beam size before the combination, which contributes to improving the amplification efficiency of the OPA.

After the OPA, a 25 mm CaF2 lens was used to collimate the diffuse MIR light behind the PPLN crystal. Finally, the amplified MIR comb was delivered to the third PPLN waveguide for HHG generation, simultaneously outputting 1st–10th harmonic combs covering the UV-to-MIR region. Notably, adjusting the pulse duration of the MIR pump light can improve the conversion efficiency of HHG. The emitted HHG was collimated using an aluminum-coated off-axis parabolic mirror and was dispersed using a prism. Continuous visible light can be easily observed on white paper and infrared detector cards. As expected, this system, based on the structure of the three-stage cascaded PPLN chain, has the potential to miniaturize broadband OFC devices. In addition, all the PPLN crystals were finely designed for broadband high-efficiency nonlinear frequency conversions. The first PPLN waveguide has designed longitudinal chirped periods in the range of 20.3–30.6 μm with the width of 15 μm and length of 25 mm, supporting 2.5–5 μm MIR DFG. Two types of PPLN were designed for the second OPA. One was a 25 mm fan-out PPLN crystal with crosswise-poled periods of 29.5–31.5 μm, which was designed to scale up the power of the signal as much as possible. The other one was a 25 mm chirped PPLN crystal with axial-poled periods of 24.2–32.6 μm, which was designed to maximally increase the bandwidth of OPA. The third waveguide was a 25 mm PPLN crystal with axial-poled periods of 21.5–30.0 μm.

The homemade Er-fiber comb directly emitted a 118.8 MHz femtosecond pulse train as a seed laser, which was scaled up to 350 mW in a single-mode polarization-maintaining fiber amplifier, bidirectionally pumped by two 1 W, 980 nm semiconductor laser diodes. As shown in Fig. 2(a), the amplified seed spectrum covering 1.5–1.7 μm is broadened to 0.9–2.2 μm in HNLF utilizing SCG (χ(3)). Subsequently, a broadband IPDFG process with a high efficiency was achieved in the first chirped PPLN waveguide, successfully obtaining offset-free MIR light with a spectrum of 2.5–5 μm. In addition, the cascaded sum-frequency process resulted in a new spectral component of 0.4–1.0 μm. As depicted in Fig. 2(b), the spectrum emitted from the IPDFG pulses is in the range of 0.4–5.0 μm, simultaneously including the nonconverted injected SCG spectrum. To avoid the appearance of f0 overlaps in broadband OFC spectra for realizing the offset-free comb, the 2.8–5.0 μm MIR components coming from IPDFG were completely filtered out, which served as signal light in the broadband OPA. After passing through the bandpass filter and the Ge plates, the 0.5 mW MIR signal light was directly focused into the second PPLN chip while the 1 μm NIR pump light was amplified to 3.1 W, serving as the pump light of the OPAs.

The offset-free MIR comb was scaled up using a broadband OPA system. As shown in Fig. 3(a), the 3.1 W pump light exhibits a spectrum from 1025 to 1075 nm. Meanwhile, we calculated the signal profiles at different pump wavelengths according to the quasi-phase matching (QPM) conditions, as described in Fig. 3(b), and the periods in 24.2–32.6 μm supported the QPM for the 2.8–5.0 μm signal. Two separate OPAs were performed experimentally using fan-out crosswise-poled PPLN crystal (periods: 29.5–31.5 μm, OPA1) and axial-poled CPPLN crystal (periods: 24.2–32.6 μm, OPA2). Moreover, we compared the effects of the two single-stage OPAs on HHG by changing the PPLN types. In detail, the OPA1 scaled the pulses at 3.0–3.8 μm with the higher efficiency while the OPA2 amplified the MIR comb with the ultrawide spectral bandwidth of 2.8–5.0 μm. Thereafter, we obtained the OPA results using a fast Fourier transform analyzer (Bristol 771B), and the spectra of the signal and the MIR OPAs are displayed in Figs. 3(c), 3(d), and 3(e), respectively. As expected, with the 0.5 mW signal light injected in the OPA1 system, the MIR light was scaled up to 360 mW with relatively high conversion efficiency, but the bandwidth was reduced to 0.8 μm with the spectral range of 3.0–3.8 μm. The main reason for bandwidth loss was the lack of poled periods to satisfy the phase-matching condition. Regarding OPA2, the spectrum of the MIR laser was extended to 2.8–5 μm without any bandwidth loss, but at the expense of conversion efficiency. The pulse energy and average power decreased to 1.9 nJ and 230 mW, respectively. After the Ge plates and molded aspherical lens, the average power of the MIR light was 183 mW.

The average powers and conversion efficiencies of OPA1 and OPA2 are depicted in Fig. 4. At the pump power of 3.1 W, the conversion efficiencies of OPA1 and OPA2 were separately 11.6% and 7.4%, presenting an expected condition that the efficiency of OPA1 is higher than the one of OPA2. As shown in Fig. 4(b), the growth rate of the conversion efficiency increases with increasing pump power when the pump power is below 2 W. However, when the pump power exceeds 2 W, the growth rate decreases. The main reason is that the pulse expands in the temporal domain with increasing pump power; thus, the peak power is reduced to lower the nonlinearity of χ(2) for OPA. Finally, we separately focused these two OPA MIR combs to the third PPLN chip (Axial-poled periods: 21.5–30.0 μm). Up to the 10th-order harmonics were generated from the chirped PPLN waveguide owing to the high χ(2) of PPLN. With a similar coupling efficiency of 60%, the laser powers coupled to the HHG waveguide were 172 and 110 mW for OPA1 and OPA2, respectively. To improve the efficiency of OPA and HHG, we compressed the duration of the signal pulses using the Ge wedges with positive group velocity dispersion (GVD). Experimentally, we adjusted the thickness of the Ge wedges to compensate for the negative GVD, which was mainly caused by the dispersion of LiNbO3 [37]. Therefore, a high efficiency can be ensured via fine tuning of the GVD of the pump pulses.

The spectra of the two series of high harmonics pumped by OPA1 and OPA2 are presented in Fig. 5. When measuring the harmonic spectra of the blue line (H4–H10), a colored bandpass filter was adopted (Thorlabs, FGS900-A, 350–700 nm) to simultaneously measure the entire UV–NIR spectrum (350–900 nm) because the power of H5–H10 was too low to be detected compared to H4. The H4 spectrum (829–909 nm) could still be observed with the spectrometer because the transmittance of the filter remained 1.23% at 900 nm. The H9 and H10 were filtered using a UV (245±200nm) bandpass plate, plotted as the purple line in Fig. 5(b). The highest harmonic at 354–369 nm is close to the transparency limitation of the LiNbO3. As shown in Fig. 5(a), we used the fan-out PPLN crystal to perform OPA1, but any harmonics are not continuous due to the 0.8-μm-limited bandwidth of the injected MIR light. The spectra of the harmonics were broadened accordingly when a wider 2.2-μm-bandwidth MIR light was injected from OPA2, as described in Fig. 5(b). The harmonic spectra of H3–H4 and H4–H7 were jointly connected in the ranges of 775–1316 nm and 440–750 nm, respectively. All harmonics emitted from the HHG inherit the coherence properties of the driving-offset MIR comb even though a complex nonlinear process occurred, which was verified in Ref. [33]. Therefore, this offset-free OFC source from UV to MIR with good coherence can be applied to dual-comb spectroscopy for the simultaneous characterization of several octaves of complex substances.

The total conversion efficiencies from the fundamental to higher-order harmonics (H2–H10) were calculated as 24.64% and 23.95% for OPA1 and OPA2, respectively. The corresponding harmonic powers are listed in Table 1. For the harmonics using OPA1, the output power was measured after applying certain bandpass filters. For the HHG using OPA2, a combination of filters and dichroic mirrors was employed to separate the harmonics. The bandwidth of H1–H8 pumped with OPA2 was too wide to be detected with a photodiode power sensor. Therefore, a thermal probe was used to evaluate the power owing to its relatively flat spectral response over a wide wavelength range. The dominant mechanism of HHG from the PPLN waveguide was the cascaded χ(2) processes, which was identified using an effective approach based on the single-mode nonlinear analytical envelope equation [22,38,39]. Furthermore, both the SFG and DFG processes are important in cascaded HHG, as concluded by Rutledge et al. [22], and are highly dependent on the QPM. However, there may be a 3rd-nonlinear SPM broadening effect, which is driven by a moderate peak power at long interaction lengths owing to the binding effect of the waveguide to light [40,41].Table 1.

Output Powers of the Generated Harmonic Components Using OPA1 and OPA2

In summary, we realized an offset-free frequency comb in the UV-to-MIR region based on a three-stage cascaded PPLN chip chain. The HHG was driven by an MIR pump pulse generated by a homebuilt OPA. We compared the effects of OPAs using two crystals and used a designed CPPLN to generate a 2.8–5 μm pump pulse with the pulse energy of 1.9 nJ at 118.8 MHz without bandwidth loss. A broadband HHG frequency comb up to the 10th harmonic was generated from the CPPLN waveguide owing to the wide bandwidth of the MIR pump, covering the UV-to-MIR range with a continuous visible spectrum. Due to the long interaction length of the waveguide, the HHG can be generated with a nanojoule pump pulse compared with a microjoule pulse in the crystal. The dominant HHG mechanisms from PPLN waveguide are cascaded χ(2) processes, including SHG and DFG. In addition, the HHG conversion efficiency can be further enhanced through optimization of the waveguide poling period and MIR pump power, supporting the valuable spectral characterization in two-dimensional material analysis, biofluorescence microscopy, and nonlinear amplitude-phase metrology.