Double-Peaked Mid-Infrared Generation Based on Intracavity Difference Frequency Generation

Abstract

It was reported that a double-peaked mid-infrared laser was generated based on an intracavity difference frequency generation (DFG). The double-peaked pump source was achieved by suppressing the intensity at the central wavelength of the pump source. The double-peaked mid-infrared generation had a double-peaked spacing of 23–37 nm, and the full width at half-peak (FWHM) of the peaks was up to 30 nm. It was demonstrated that a tilted Bragg fiber grating (TFBG) with a specific transmission spectral depth and bandwidth can generate a double-peaked mid-infrared laser with controllable double-peaked spacing. It was the first generation of a double-peaked mid-infrared laser based on a near-infrared pumping spectral modulation. Furthermore, the double-peaked mid-infrared generation was tunable, and the FWHMs of the peaks were controllable by an intracavity DFG. It had a high potential for the monitoring and absorption of gas molecules by differential absorption lidar (DIAL).

1. Introduction

The 3–5 μm mid-infrared laser covers the absorption peaks of many atoms and molecules. It can detect and quantify molecules effectively [1] and with significant application prospects in physical analysis [2,3], environmental management [4,5,6], and infrared countermeasures [7].

Mid-infrared lasers mainly include rare-earth, ion-doped, solid-state, or fiber lasers; gas lasers; quantum cascade lasers (QCLs); and nonlinear frequency conversion techniques [8,9,10,11,12]. The nonlinear frequency conversion-based mid-infrared optical parametric oscillator (OPO) utilizes a nonlinear crystal to generate a near-infrared and a mid-infrared beam [13,14,15]. The near-infrared laser is called the signal beam, and the mid-infrared generation is called the idler beam. The advantage of a nonlinear frequency conversion is to realize the tunable wavelength and conversion efficiency improvement of the mid-infrared generation. It has become one of the essential research directions on mid-infrared generation in recent years [16]. In 2018, Yichen Liu et al. [17] demonstrated a high-power CW mid-infrared OPO. This module weighs only 2.5 kg and has a maximum output power of 10.4 W at 2.7 μm. Meanwhile, the nonlinear frequency conversion and the fiber laser technology make the system optimized in structure and significantly improve output performance [18,19].

DIAL is one of the essential techniques for detecting the distribution of trace gases and other meteorological elements. DIAL utilizes a transmitter unit to generate two laser beams at different wavelengths and a receiver unit to receive the backscattered echoes of the two laser beams. The critical information, such as the gas concentration, is calculated based on the different absorption coefficients of the gas molecules by the two laser beams that are located at different wavelengths. DIAL is based on the absorption spectrum of the gas to be measured. Although the structure is more complex and requires high stability and tunability of the laser, the detection sensitivity is high. It is one of the significant development directions in the current gas analysis field, and it is widely used internationally [20,21]. The OPO is also widely used in DIAL as a mid-infrared generation with a wide range of tuning [22]. In 2020, Yu Gong et al. [23] used a solid-state OPO as lidar to emit laser pulses tunable within 2.5–4 μm with a repetition frequency of 500 Hz. The monitoring of NO₂ and SO₂ was performed with high efficiency.

Moreover, a broadband, multi-wavelength, or multi-peaked laser emission significantly contributes to the development of DIAL [24,25]. V Pencheva et al. [26] developed a broadband DIAL for methane monitoring. Zehou Yang et al. [27] used a multi-wavelength DIAL (MW-DIAL) to simultaneously measure multiple pollutants in the atmosphere, which improved the accuracy of the technique and achieved high efficiency. Since the spectrum of most gas molecules has high absorption lines in the mid-infrared band, it is essential to apply the tunability and compact structure of the OPO to DIAL [28,29,30].

There are several techniques to generate a broadband or multi-peaked mid-infrared OPO. The first technique is to utilize nonlinear crystals with unique structures, such as cascaded or double-periodic, to simultaneously fit the phase-matching conditions for two or more pump beams to initiate and generate a multi-wavelength mid-infrared laser [31]. In 2020, Xi Feng et al. [32] generated a mid-infrared laser with instantaneous bandwidth covering 2.8–3.9 μm by cascading crystals. The multi-peaked idler beams generated by multiple crystals were also simulated. The second technique is designed in the cavity structure. The cascaded OPO or intracavity DFG is designed to convert the signal beam or the idler beam from the OPO into a secondary nonlinear frequency, generating multiple wavelengths of the mid-infrared laser [33]. In 2022, C. p. Bauer et al. [34] designed a single-cavity dual-comb OPO pumped by a dual-comb solid-state laser, generating an idler beam with a dual-peaked spacing of 29 nm and FWHMs of 134 and 130 nm, respectively. The third technique is nonlinear frequency conversion with pulsed pumping sources. The pulsed sources that are improved by the technique effectively change the optical properties of atomic systems [35,36]. At the same time, pulsed pump sources for OPO systems are also excellent at generating broadband mid-infrared lasers [37,38,39].

In this paper, the CW pump source is an ASE fiber source, and the TFBG is chosen as the spectral modulation device, which generates a double-peaked pump source. TFBGs are essential in gaining suppression and wavelength selection as excellent passive fiber devices. Previous applications have mainly suppressed nonlinear effects that impact laser performance [40]. There is little research that suppresses the gain extremum point. In this research, the modulation property of TFBG on the spectrum is utilized to modulate the pumping spectrum, which generates a double-peaked pumping source. In terms of nonlinear frequency conversion, the intracavity DFG technique is utilized for frequency conversion of the double-peaked pump source [41,42].

It is reported that a double-peaked pumping source is achieved by TFBG modulation, and intracavity DFG generates a tunable double-peaked mid-infrared laser. By modulating the TFBG bandwidth and transmission spectrum depth, it is essential to generate mid-IR lasers with a different spacing of peaks. The double-peaked mid-infrared generation is located at 3688–3786 nm. The spacing of the two peaks is 23–37 nm, and the FWHM of each peak is 12–30 nm. The double-peaked mid-infrared generation is 0.18 W, and the pump-idler conversion efficiency is 8.5%. The double-peaked mid-infrared generation with adjustable central wavelength, controllable double-peaked spacing, and controllable FWHM has excellent potential for DIAL application. The research also demonstrates the feasibility of modulating the mid-infrared laser spectrum by modulating the pump spectrum. It has essential applications in the investigation of nonlinear frequency conversion.

2. Experimental Setup and Methods

In the research of dual-wavelength mid-infrared OPOs, the current theory of multi-optical parametric oscillation is very mature and can generate wide-tuned, dual-wavelength, mid-infrared lasers. However, the development of dual-wavelength mid-infrared OPOs is limited by the shortage of pumping bandwidths and phase-matching bandwidths, which makes it challenging to generate a dual-peaked mid-infrared laser with a wide FWHM. In this paper, a scheme to generate a dual-peaked mid-infrared laser by a parametric conversion technique is considered for the demand of DIAL at 3–5 μm. Assume that the phase mismatch during the covariance conversion is 0. At this time, if two parametric beams, λ₁ and λ₂, are injected simultaneously into the OPO cavity, the two parametric beams satisfy the conservation equation in the OPO cavity.

$$\frac{1}{{\lambda }_{p1}}-\frac{1}{{\lambda }_{s1}}=\frac{1}{{\lambda }_{i1}}$$

(1)

$$\frac{1}{{\lambda }_{p2}}-\frac{1}{{\lambda }_{s2}}=\frac{1}{{\lambda }_{i2}}$$

(2)

λ_p, λ_s, and λ_i represent the pump, signal, and idler beam, respectively. Assuming that the two idler beams, λ_i1 and λ_i2, satisfy the parametric transition under phase-matching, two hypotheses exist for the case of generating a double-peaked idler beam.

The first hypothesis states that the two injected beams are the signal beams λ_s1 and λ_s2, the pump beam is located at the same wavelength, and the combined beam is λ_p* by tuning the spacing Δλ_s of the two signal beams to regulate the peak spacing Δλ_i of the idler beam. Equations (1) and (2) can be associated as follows.

$$\Delta {\lambda }_i=-\frac{{\lambda }_{p\ast }{}^2}{\left({\lambda }_{s11}-{\lambda }_{p\ast }\right)\left({\lambda }_{s12}-{\lambda }_{p\ast }\right)}\Delta {\lambda }_s$$

(3)

There is a severe paradox in Equation (3). The transformation relationship between the signal spacing Δλ_s and the idler frequency spacing Δλ_i is inversely proportional; therefore, injecting two signal beams is not feasible.

The second hypothesis states that the two injected parametric beams are the pump beam λ_p21 and λ_p22, and the signal beam λ_s* is located at the same wavelength. Couplings (1) and (2) are as follows.

$$\Delta {\lambda }_{i2}=\frac{{\lambda }_{s\ast }{}^2}{\left({\lambda }_{s\ast }-{\lambda }_{p21}\right)\left({\lambda }_{s\ast }-{\lambda }_{p22}\right)}\Delta {\lambda }_p$$

(4)

In Equation (4), the pump bimodal spacing Δλ_p is proportional to the transformation of the idler beam bimodal spacing Δλ_i, and the scheme is theoretically feasible. When the OPO cavity is simultaneously injected into two beams with a small central wavelength spacing or a double-peaked pump source in the pumping bandwidth, a double-peaked mid-infrared laser can be generated. However, generating a double-peaked mid-infrared laser in a short wavelength range with two pump beams injected into the OPO is challenging due to increased competition. For this reason, it is necessary to explore a double-peaked pump source with more stable power after particular spectral modulation and unique technical means to make the double peaks of the pump beam simultaneously with a signal beam to generate nonlinear frequency conversion.

The ASE source is the pump source in the experimental platform due to its excellent spectral flatness and stable power. The pump source and the spectrum were analyzed for the generation of the double-peaked pump source.

Figure 1a shows the experimental setup of the ASE fiber source, which is a typical forward pumping structure with a one-stage amplification system. As shown in Figure 1b,c, the ASE fiber source has a narrowing bandwidth and wavelength shift during the power amplification process. By analyzing the spectra, if special spectral modulation techniques are used, the intensity at the central wavelength of the ASE source is reduced before its power amplification. Because of the different absorption coefficients of Yb particles in the PM YDF, the spectrum after power amplification may generate a flat or multi-peaked spectrum due to the modulation depth and bandwidth difference. When the modulation reaches a specific depth, a double-peaked pump source will be generated.

A tilted fiber Bragg grating (TFBG) was chosen as the spectral modulation device after comparing various fiber passive devices for gain suppression. With the maturity of fiber grating inscription technology in recent years, TFBG has become an indispensable passive device for many laser systems due to its excellent gain flatness and nonlinear suppression. It provided suitable conditions for realizing broadband pumping sources in this paper.

Figure 2a shows the experimental setup of the TFBG-modulated ASE fiber source. By analyzing the ASE source spectrum and power, the transmission spectrum depth of TFBG is adjusted to 16 dB, and the central wavelength is adjusted to 1070 nm. The customized TFBG transmission spectrum is shown in Figure 2c, and the generated ASE source spectrum is shown in Figure 2d, with a double-peaked spectrum and a lower intensity in the short wavelength direction compared to the long wavelength direction. The TFBG-modulated double-peaked ASE source has the advantage of generating two nonlinear frequency conversions and a bandwidth of both peaks, which ensures that the two idler beams can be combined into a double-peaked idler beam.

The OPO cavity must also satisfy the signal beam at the same wavelength to generate a double-peaked mid-infrared laser. Among the existing PPLN OPOs, few experimental platforms satisfy the above conditions simultaneously. However, a scheme still exists to meet the experimental requirements: the intracavity difference frequency generation under the combined signal beam. The intracavity DFG can generate a high-conversion-efficiency dual-wavelength mid-infrared laser and combine with a Raman laser to generate a high-power mid-infrared laser, which has unique advantages and prospects in nonlinear frequency conversion technology. Moreover, the intracavity DFG has only one signal beam involved in the parametric conversion process, which is also very consistent with the double-peaked mid-infrared laser generation model.

The intracavity DFG utilizes a nonlinear frequency conversion process between a narrow-linewidth 1018 nm beam and a pumped beam in the OPO. By tuning the phase-matching state of the OPO cavity, the parametric conversion process of the two lasers in the OPO cavity is in the region where the signal beams are combined. The OPO and intracavity DFG processes are realized simultaneously at the signal-combining point of the two incident laser beams. A high-conversion-efficiency mid-infrared laser is generated. Figure 3 shows the experimental schematic diagram of the intracavity differential frequency of the double-peaked ASE fiber source. The first part is the pump source. The seed source, the spectral modulation device (TFBG), and the power amplification system are inserted with an ISO to prevent the return beam. The second part is the DFG connection system. The ASE fiber source and a narrow-linewidth high-power 1018 nm laser are injected into the wavelength division multiplexer (WDM).

Moreover, an optical collimator (OC) is utilized to inject the combining beam into the OPO system. The other output port of the WDM is connected to a spectrometer that monitors the spectrum of the ASE fiber source with the 1018 nm laser to prevent nonlinear effects from causing damage to the OPO. The third part is the intracavity DFG system. An anti-reflection (AR) focusing lens is utilized to focus the beams into the OPO cavity. The OPO cavity is a linear structure consisting of concave mirrors M₁ and M₂. The crystal inside the OPO cavity is a MgO: periodically poled lithium niobate (PPLN) with a size of 40 mm × 10 mm × 1 mm, placed between M₁ and M₂. The fan structure of the nonlinear crystal optimizes the OPO. Moreover, the chirp of the fan-shaped nonlinear crystal can provide sufficient phase-matching bandwidth for the nonlinear frequency conversion process of the broadband pumping source. The lens collimates the beam generated by the OPO cavity. M₃ and M₄ separate the remaining pump beam and signal beam in the output laser of the OPO cavity. M₅ is a germanium (Ge) sheet, which further absorbs the near-infrared laser from the mid-infrared laser. M₆ is a dichroic mirror that separates the OPO idler beam from the DFG idler beam.

The generated double-peaked mid-infrared laser is fitted with the difference frequency theory, assuming that the covariance is converted to a Gaussian beam [43].

$$\eta =\frac{P_i}{P_p}=\frac{4{\omega }_{{}_i}^2{d}_{eff}{}^2{k}_{s}L}{\pi c^3{\epsilon }_0{n}_p{n}_s{n}_i(k_p+k_s)}{P}_{s}h(\mu ,\xi )\sin c^2(\Delta kL/2)$$

(5)

In Equation (5), P is the beam power, L is the crystal length, and ω denotes the angular frequency. The d_eff is the effective nonlinear coefficient, k is the finger wave vector, c is the speed of light in vacuum, ε₀ is the free space permittivity, and n is the refractive index. The h(μ,ξ) is the focusing parameter, a constant value. The spectral data, such as the double peaks’ central wavelength, are eventually substituted to generate the simulated idler spectrum. As shown in Figure 4, the TFBG-modulated ASE fiber source can generate a double-peaked idler beam with the double-peaked spacing of 32 nm, and the FWHM of the primary peak is 36 nm through the intracavity DFG.

3. Results and Discussion

The 1018 nm laser is turned on and the power is increased to 43 W. The OPO process is generated in the cavity. After the signal beam is stabilized in the cavity, the ASE fiber source is turned on. The polarization period of MgO: PPLN is tuned, and two nonlinear frequency conversion processes are generated in the OPO cavity. In the experiment, it can be observed that the DFG process is generated in the OPO cavity when the pump source is first turned on. With the power of the ASE fiber source turned on to 2.1 W, the DFG idler power is stable, and the spectrum is shown in Figure 5a. The primary peak is 3695 nm, and the secondary peak is 3731 nm. Two peaks are spaced at 36 nm. Compared with the intensity of the 3695 nm peak, the intensity of the 3731 nm peak is lower. The FWHMs of the peaks at 3695 and 3731 nm are 30 and 24 nm, respectively. The polarization period is tuned at the phase-matched interval extreme values, as shown in Figure 5b. The primary peak is 3747 nm, and the secondary peak is 3770 nm. The FWHM of the two peaks is 25 and 20 nm, and the experimental results are consistent with the above simulation. In tuning the primary peak of the double-peaked mid-infrared generation from 3695 nm to 3747 nm, the intensity of the peaks is consistent, and the intensity at the long wavelength slightly decreases. Under the phase-matching condition, when the pump power is 2.1 W, the idler power generated by the DFG in the cavity is 0.18 W, and the pump-to-idler conversion efficiency is 8.5%.

The tuning performance of the intracavity DFG system is further measured to generate a double-peaked idler beam in the tuning of 3688–3786 nm. As shown in Figure 6, the primary peaks are at 3725 and 3753 nm. The secondary peak intensity decreases when the primary peak is located at 3725 nm and the secondary peak of the idler beam is at 3688 nm. The spacing of the two peaks is 37 nm. The FWHMs of the primary and secondary peaks are 24 and 16 nm, respectively. In tune with the long wavelength, the secondary peak in the long wavelength also gradually decreases, as shown in Figure 6b. The spacing of the two peaks is 33 nm. The FWHM of the primary and secondary peaks are 20 and 12 nm, respectively.

By analyzing the difference between Figure 5 and Figure 6, the idler beam in the phase-matching condition is a double-peaked spectrum with similar peak intensities. Moreover, the central wavelength of the mid-infrared spectrum is 3713–3758 nm. The primary peak is located at 3695–3747 nm. With further tuning of the polarization period, the intensity of the secondary peak in the short wavelength gradually decreases when tuning to the short wavelength. While tuning the crystal polarization period to the long wavelength, the secondary peak in the long wavelength gradually decreases. In the phase-mismatched condition, two peaks of the double-peaked mid-infrared generation have a high difference in intensity. The intracavity DFG is phase-mismatched in Figure 5a,b. In the phase-mismatching interval, it is necessary to substitute the phase-mismatched Δk when calculating the central wavelength and the FWHM of each peak in the phase-mismatched condition.

$$\Delta k=2\pi \left(\frac{{\mathrm{n}}_{\mathrm{p}}({\lambda }_p,\theta )}{{\lambda }_p}-\frac{{\mathrm{n}}_{\mathrm{s}}({\lambda }_s,\theta )}{{\lambda }_s}-\frac{{\mathrm{n}}_{\mathrm{i}}({\lambda }_i,\theta )}{{\lambda }_i}-\frac{1}{\mathrm{\Lambda }}\right)$$

(6)

where Λ is the polarization period of the crystal, and n_p, n_s, and n_i are the refractive indices of the pump beam, signal beam, and idler beam in the PPLN crystal, respectively. The signal beam generated by the 1018 nm laser is shifted as the tuning of the polarization period and the ∆k increases. The bandwidth of the broadband mid-infrared generation decreases with an increase in ∆k. This results in a shortening of each peak and a decrease in the intensity away from the phase-matching interval.

In summary, the double-peaked mid-infrared generation with a primary peak of 3695–3747 nm is in the phase-matching condition. The FWHM of each peak varies in the range of 20–30 nm, and the double-peaked intensities are similar. In contrast, the double-peaked idler beams generated outside this range are in a state of phase-mismatched. With the tuning of the polarization period, the central wavelength of the double-peaked mid-infrared generation gradually moves toward the long wavelength, and the FWHM of the spectrum decreases. Meanwhile, the FWHM of the primary and secondary peaks decreases, and the secondary peak in the long wavelength gradually lowers. The secondary peak decreases when the double-peaked mid-infrared generation is tuned in the short wavelength under phase-matching.

4. Conclusions

The TFBG was chosen to suppress the intensity at the central wavelength of the ASE fiber source to generate a double-peaked pump source. The intracavity DFG of the double-peaked pump source generated the tunable double-peaked mid-infrared laser. The primary peak of the mid-infrared generation was located at 3695–3747 nm. Under the phase-matching condition, the spacing of the two peaks was 23–37 nm, and the FWHM of each peak was 20–30 nm. The power of the double-peaked idler beam was 0.18 W. The pump-to-idler conversion efficiency was 8.5%. With the increase in phase-mismatching during the intracavity DFG, the FWHM of the double-peaked idler spectrum gradually decreased. The intensity of the secondary peak in the short wavelength decreased during the tuning in the short wavelength.

Similarly, the intensity of the long wavelength’s secondary peaks decreased during the extended wavelength tuning. This was demonstrated by modulating the pump spectrum and combining it with the intracavity DFG. The double-peaked mid-infrared laser was generated with a tunable central wavelength, controllable peak spacing, and controllable peak FWHM. It was shown to be promising in realizing the absorption and analysis of gas molecules by DIAL.