The mid-infrared (MIR) laser lies within the atmospheric window and the molecular fingerprint region [1,2]. Due to the unique value of this wavelength in spectroscopy, medical treatment, and other fields, the spectrum of the MIR laser has become a critical parameter [3–5]. For instance, a widely tunable MIR laser offers potential for gas component detection [6–8]. Multi-wavelength MIR lasers have the capability to obtain terahertz radiation through difference frequency generation [9–11]. Broadband MIR lasers are beneficial in enhancing the precision of Fourier transform spectroscopy [12–14]. Overall, spectral programming of the MIR laser is of paramount significance and highly desirable.

The optical parametric oscillator (OPO), owing to its distinct advantage of wide-range tuning and implementing customized output spectrum, has emerged as one of the important means for realizing MIR lasers [11,15,16]. It does not rely on energy level transition radiation but rather achieves frequency down-conversion through quadratic [χ(2)] nonlinearities in special crystals. The MIR OPO is capable of converting a 1-μm pump light into two parametric lights, located at short-wave infrared (SWIR) and MIR wavelengths, respectively. Therefore, the output MIR laser can effectively inherit the spectral characteristics of the pump light, thereby enabling the transference of well-established spectral programming techniques from the 1 μm to the MIR region.

Fortunately, the coordinated action of resonant cavity design and functional fiber-based components has propelled the spectral programming capacity of the 1-μm fiber laser to achieve an unparalleled level [17–19]. Over the past decades, significant advancements have been made in the spectral configuring of fiber laser pumped OPOs, partially accomplishing wide tuning, multi-wavelength, and broadband operation [11,14,20]. However, no study has documented an individual OPO that fulfills all these spectral properties, primarily attributable to the absence of spectral programmable pump sources. Recently, through the refinement of the laser cavity structure and the strategic manipulation of gain competition, we have developed a high-power, high linearly polarized fiber laser with adaptable spectral characteristics at 1 μm, which shows encouraging potential as a pump source for OPOs [21,22].

In this paper, by using the above-mentioned homemade fiber laser for the pump, we demonstrated a spectral programmable, continuous-wave MIR OPO. The idler power could be up to 5.53 W, and the idler quantity could be switched from one to three. The single idler wave could be finely tuned over a spectral range of 528 nm, and the interval between two idlers could be adjusted for 470 nm with controllable intensity. Furthermore, the implementation of pre-shaped broadband multi-peak pump spectra resulted in a broadband-flattened parametric gain, thus enabling the tuning of idlers with a 10 dB bandwidth ranging from 20 to 125 nm.

The working principle of the spectral programmable MIR OPO is schematically shown in Fig. 1(a). The fiber laser pump source enables fine-grained control over spectral parameters such as the wavelength position, number, interval, linewidth, intensity, and envelope. Such a spectral customizing capability allows pre-modulation of the pump spectrum shape, thereby compensating for the parametric gain variance of different wavelengths during the nonlinear frequency conversion. Building on this, the OPO we acquired supports simultaneous wide tuning, multi-wavelength, and broadband operation within a single experimental setup.

Figure 1(b) depicts the schematic of the experimental configuration. The spectral tunable fiber pump source was configured in an all-polarization-maintaining ring cavity. Two 976-nm laser diodes (LDs) were used to pump a piece of polarization-maintaining Yb-doped fiber by a (2+1)×1 combiner. The polarization-maintaining Yb-doped fiber was 5 m long, with a core/cladding diameter of 10/125 μm, a core NA of ∼0.075, and an average cladding absorption of ∼4dB/m at 976 nm. A 99/1 output coupler working on fast-axis-blocked mode was employed, extracting 99% power for OPO pumping and enabling 1% to be injected into the feedback loop. Unidirectional operation was guaranteed by an optical circulator. After that, an acousto-optic tunable filter (AOTF) was applied as the wavelength selection device, which was driven by the RF signals ordered from a laptop. An isolator was connected after the ring cavity to protect the pump source from undesirable backward light damage. All the free ports were cleaved at an angle of 8° to avoid backward reflection.

The pump beam was input to the OPO through an optical collimator and then focused into the resonant cavity by focusing lens L1. The OPO cavity was built in a signal bow-tie ring singly resonant configuration, consisting of two plano-concave mirrors M1 and M2, two plane mirrors M3 and M4, and a MgO-doped periodically poled lithium niobate (MgO: PPLN) crystal (50mm×10mm×1mm). The crystal was periodically poled with a grating period of Λ=31μm. The output coupling mirror M2 was highly transmissive for the pump at 1.0–1.1 μm and idler at 2.5–4.1 μm, and partially transmissive (T∼10%) for the signal at 1.4–1.7 μm. The other three mirrors, M3, M5, and M6, had high transmission for the pump and idler, and high reflectivity for the signal. Following M2, collimating lens L2 was placed to ensure the output beam was parallel for measurement. The dichroic mirror, M5, was positioned to separate the residual pump, signal, and idler beams. The total power of the former two mirrors was recorded by a power meter and the signal spectra were monitored by a near-infrared OSA. A sampling mirror, M6, was set behind M5, enabling ∼95% power of the idler wave to transmit to a power meter meanwhile sampling ∼5% of the idler to an MIR OSA.

First, we investigated the single-wavelength output and wide-wavelength tuning performance of the proposed OPO in this section. Considering that the maximum tolerant power of the optical isolator connected behind the pump source was 50.0 W level and the loss induced by the isolator, the pump power injected into the OPO was merely scaled to an average level of ∼37.0W for safety.

The tunable idler spectra are depicted in Fig. 2(a). A wide tuning range of 528 nm (2852–3380 nm) was achieved by adjusting the pump within a range of 40 nm (1040–1080 nm), as plotted in Fig. 2(b). The corresponding tunable spectra of the signal beam at the SWIR region are shown in the Appendix A. Figure 2(c) shows the simulated and experimental parametric wavelengths with the corresponding pump wavelengths (more simulation details can be found in the Appendix A). The experimental and simulation results demonstrate strong concordance, indicating that a minor adjustment in the pump wavelength can result in a substantial alteration in the MIR output wavelength. A wider tuning range was not permitted because the boosting amplified spontaneous emission of the pump source, which may expose the system to risks under high power operation [22]. In addition, the longest operating wavelength of the optical spectrum analyzer (OSA) we used was limited to 3400 nm, causing a low optical signal-to-noise ratio at that region.

Figure 2(d) illustrates the idler power as a function of the pump power at different central wavelengths, and it displays a general trend that laser thresholds of bordered wavelengths (e.g., 2852 nm and 3380 nm) are higher than those located at the central part of tuning range. Maximum single-wavelength output power was achieved at 3028 nm. In that case, with the elevation of the pump power, after it was beyond 15.0 W, the idler power increased rapidly and reached a peak value of 5.53 W with 37.4 W pump power, leading to a pump-to-idler conversion efficiency of 14.7%. Further analysis including the spectral evolutionary process, conversion efficiency, and power stability can be found in the Appendix A.

Interval tunability under dual-wavelength operation was studied. As shown in Fig. 3(a), the idler interval could dynamically zoom in a scale of 470 nm (53–523 nm). Figure 3(b) illustrates the corresponding dual-wavelength pump spectra. When the pump wavelengths were separated by 5 nm, 10 nm, 20 nm, 30 nm, and 40 nm, the idler wavelengths were accordingly spaced by 53 nm, 125 nm, 255 nm, 385 nm, and 523 nm. The utmost interval was decided by the tuning range of the pump source, while the minimal interval was confined by the peak-to-valley intensity difference. In our perspective, once this difference dropped to less than 10 dB, the two laser peaks became indistinguishable.

Experimentally, we spread the two wavelengths from the middle of the tuning range to both sides. In such a scenario, the output power gradually decreased as the wavelength interval increased. When the wavelength interval was 53 nm, the output power was 3.49 W, whereas when the interval increased to 523 nm, the output power dropped to 0.65 W. This phenomenon can be ascribed to the difference in threshold values of the optical parameter oscillating process at different wavelengths, which has been introduced above. Taking the dual-wavelength output of 2850 nm and 3373 nm as examples, when the total pump power of 1040 nm and 1080 nm reached 37.5 W, the power could be divided into about a half for each wavelength, i.e., 18.75 W. According to Fig. 2(c), it was not significantly higher than the laser threshold of idler emission. When two idler wavelengths were located at 3.1 μm, the situation became fairly different because those laser thresholds were lower, and the power of the two idlers had been developed to a considerable extent under maximum pump power.

Under dual-wavelength mode, we further analyzed the intensity tunable capacity. Via giving appropriate radio frequency (RF) amplitude allocation to the AOTF, the intensity of the two idler peaks could be tuned flexibly with each other, which was sharply distinct from the previous report [11]. In particular, we chose the 3028 nm and 3283 nm dual-wavelength pairing as an example, in which the gap of intensity was regulated to 3 dB and 10 dB mutually. The output MIR laser power maintained a level of more than 2.12 W in all cases. Figures 4(a)–4(d) present the idler output spectra in detail, and Figs. 4(e)–4(h) show their corresponding pump spectra with the power ratio remarked. It should be noted that in this study, the power ratio was calculated by integrating the concerned part of the spectra and then making a ratio with the summation. Because the pumping power obtained by the two idler beams exactly lay in their rapidly increasing regions [see Fig. 2(c)], a tiny discrepancy in the pump spectra might lead to a great difference in the idler spectra, as shown in Fig. 4.

Moreover, triple-wavelength idlers, taking 3071 nm, 3166 nm, and 3257 nm for instance, were achieved, as shown in Fig. 5(a). The output power was 0.72 W, and the intensity difference was controlled within 3 dB. Its corresponding pump spectrum is depicted in Fig. 5(b). The multi-wavelength spectral stability and total power stability were also monitored (see in Appendix A).

On this basis, we exhibit the triple-wavelength spectral custom-tailored characteristics. Downhill-shape, basin-shape, uphill-shape, and peak-shape spectra are demonstrated in Figs. 6(a)–6(d). Figures 6(e)–6(h) show their corresponding pump spectra.

In this section, we demonstrate the MIR broadband spectral tuning capacity of the OPO. By expanding the channel numbers of the AOTF from 1 to 5 and carefully varying the amplitude of each channel, the 10 dB bandwidth of the pump wave could be tuned from 3 to 14 nm. In contrast to the comparatively uniform spectral profile of the super-fluorescent light source, the pump spectrum in this case exhibits a distinct distributed multi-peak feature. After the nonlinear frequency conversion process, the 10 dB bandwidth of the idler emission could be tuned from 20 to 125 nm. With the broadening bandwidth, the idler output power declined and reached the lowest value of 2.98 W with a bandwidth of 125 nm. Both the broadband idler spectra and their corresponding pump spectra are plotted in Figs. 7(a) and 7(b). The central wavelength of the broadband idler and pump wave was 3190 nm and 1062 nm, respectively. In particular, pump beams with bandwidths of 3 nm, 7 nm, 10 nm, 12 nm, and 14 nm consequently generated idlers with bandwidths of 20 nm, 50 nm, 65 nm, 95 nm, and 125 nm. To some extent, this OPO acted like a spectral bandwidth transformer as well as an expander.

Additionally, we noted that the bandwidth of the signal light was also wide. Taking the scenario with the maximum bandwidth for instance, the signal light spectrum covered approximately the range of 1590–1600 nm, as shown in Fig. 7(c). This differed from previous reports [23] on continuous-wave, broadband OPO, where the signal light had a narrow linewidth. Based on the broadband and multi-peak pump spectrum shape, we simulated the gain spectra of the signal and idler light in this situation, with the results shown in Figs. 7(c) and 7(d). We believe that the broadband width was caused by the superposition of multiple independent OPO processes. The specially designed pump spectrum balanced the fluctuations in the parametric gain spectrum, thereby achieving a broadband flattened output.

Moreover, it could be noticed that as the bandwidth broadened, the spectral power density generally decreased. The idler intensity with the bandwidth of 125 nm was approximately 10 dB lower than that of the 20 nm one. Almost the same difference was present between the broadest and narrowest ones in the pump spectra. Further bandwidth expansion was limited by the available channels, whose augment might not ensure the safety of the AOTF.

In conclusion, we have proposed and realized spectrum reconfigurable, continuous-wave, MIR generation via a homemade spectral tunable fiber oscillator pumped OPO. During single-wavelength operation, the highest idler output power of 5.53 W could be obtained at 3028 nm, resulting in a pump-to-idler conversion efficiency of 14.7%. The central wavelength could be tuned in the range of 2852–3380 nm. When operating in dual-wavelength mode, the wavelength interval could be regulated in a range of up to 470 nm, and the intensity of each wavelength could be flexibly adjusted. Triple-wavelength idler emission was achieved, and the envelope of the spectral intensity was programmable. In addition, leveraging the parametric gain flattened mechanism induced by a multi-dimensionally designable pump shape, broadband MIR output with 10 dB bandwidth tunable from 20 to 125 nm was realized. This system is thought to hold significant promise for a range of applications, such as spectroscopy, lidar, and terahertz generation.

Acknowledgment. Junrui Liang thanks Mi Yang, Xiran Zhu, Peng Wang, Kaifeng Wang, and Bin Zhang for their help in this work.