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Abstract
A high efficiency, continuous-wave, narrow linewidth, pump-enhanced optical parametric oscillator (OPO) at 3.8 µm was demonstrated, which was pumped by a 1064 nm fiber laser with a linewidth of 18 kHz. The low frequency modulation locking technique was employed to stabilize the output power. The wavelengths of signal and idler were 1475.5 nm and 3819.9 nm at 25 °C, respectively. The pump-enhanced structure was applied, leading to a maximum quantum efficiency of over 60% with pump power of 3 W. The maximum output power of idler light is 1.8 W with a linewidth of 363 kHz. The excellent tuning performance of the OPO was also demonstrated. In order to avoid mode-splitting and decrease of pump enhancing factor due to feedback light in the cavity, the crystal was placed obliquely to the pump beam and the maximum output power was increased by 19%. At the maximum output power of idler light, the M2 factors in the x and y directions were 1.30 and 1.33, respectively.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
3-5 µm mid-infrared (MIR) band is the well-known fingerprint region [1], and is located in the atmosphere transport windows, in addition, kHz and MHz level narrow linewidth MIR lasers have better precision and sensitivity for free space optical communication [2], remote sensing [3], trace gases detection [4,5], biological diagnosis [6], Kerr optical frequency combs (OFCs) [7] and so on. Especially for MIR Kerr OFCs, narrow linewidth tunable MIR lasers satisfy the conditions for being the pump of the MIR Kerr OFCs, which are the exceptional broadband coherent light sources, and attract great attention and revolutionize the optical frequency metrology and spectroscopy [8–10]. Continuous-wave (CW) optical parametric oscillators (OPOs), an effective way to generate MIR coherent radiation with narrow linewidth, have the advantages of high power, simple structure, high stability, excellent tunability and simplicity to adjust. Therefore, the CW and narrow-linewidth MIR OPOs at 3-5 µm are useful and meaningful.
Singly-resonant OPO (SRO) is the most common type of OPOs, but limited by the nonlinear coefficient of the optical materials, so the oscillation threshold of SRO is very high, like a few watts or even more than 10 watts [11–17], even if the PPLN crystals used have high effective nonlinear coefficient, so it is not conducive for integration and miniaturized pump of MIR Kerr OFCs, which need sub-watt level high efficiency MIR narrow linewidth tunable laser. For example, Zeil et al. reported a CW OPO at 3.4 µm and a maximum power of 11 W with a pump power of 40 W and quantum efficiency of 88%, which has a threshold of 4.4 W [11]. Tan et al. have reported a narrow linewidth OPO at 3.77 µm with a threshold of approximately 10 W [14]. Meanwhile, the corresponding conversion efficiencies of above CW OPOs were generally low with low pump power (for instance, below 5 W), for example, the quantum efficiency of the OPO reported by Zeil was about 27% and the OPO reported by Tan could not generate idler light with the same pump power. To adapt to different application scenarios, it is important to achieve miniaturization in the industry, manufacturing and other fields, which means low pump power with high quantum efficiency. The pump-enhanced SRO (PE-SRO) has the advantages of SRO and can also effectively reduce the threshold. Turnbull et al. employed a dual-cavity configuration and an input coupler of 95% reflectivity for pump to allow independent control of resonant pump and signal lights to get a maximum idler power of 16 mW at 2.71-3.26 µm or 4.07-5.26 µm with a maximum pump of 750 mW [18]. Lindsay et al. reported a maximum 4 mW idler output power at 2.58-3.44 µm with PE-SRO and a 95% reflectivity input coupler for pump, which was pumped by an extended-cavity diode laser of 62 mW [19]. Stothard et al. demonstrated a PE-SRO with the ring cavity and a 96% reflectivity input coupler for pump, which had a low threshold of 300 mW and generated a single frequency idler, whose power was 40 mW with a pump power of 625 mW and a quantum efficiency at 25% [20]. Thomas et al. developed a dual bow-tie ring cavity with an input coupler of 90% reflectivity for pump providing a maximum idler power at 3-4 µm of 130 mW with a pump power of 850 mW at 48% quantum efficiency to apply in photo-thermal interferometric trace ethane detection [21]. They chose the input couplers with reflectivity of over 90%, so that the thresholds were low, but they did not achieve efficient energy conversion [22], resulting in low quantum efficiency and low output idler power.
In this work, using an input coupler with appropriate reflectivity for pump could effectively improve the maximum power of idler laser to 1.8 W, which was the maximum output power of the PE-SRO at 3-5 µm, and ensured the maximum quantum efficiency of over 60% and the minimum quantum efficiency of near 50%. The low frequency modulation locking technique [23] was employed to stabilize the output power. With the bow-tie ring cavity, the linewidth of signal light was 345 kHz, the corresponding linewidth of idler light was calculated to be less than 363 kHz. Meanwhile, the PE-SRO has good wavelength-tuning performance and beam quality, and it can be an excellent pump of OFC.
2. Experimental setup
The experimental setup of the PE-SRO system is depicted in Fig. 1. The system was pumped by a CW 1064 nm Yb-doped fiber laser, which provided the single transversal mode, linearly polarized output, with a good beam quality (M2 < 1.2). The corresponding linewidth is less than 18 kHz. Both the pump and signal light were resonated in the bow-tie ring cavity designed to provide the pump enhancement and the down-converted waves. The nonlinear crystal was a MgO-doped periodical poled LiNbO3 (MgO:PPLN) with dimensions of 50 mm (length) $\times$ 3 mm (width) ${\times} $ 1 mm (thickness), with a grating period of 29.5 µm. Both crystal faces are coated with high transmission for pump, signal and idler lights. The crystal was wrapped by the indium foil and clamped by a home-made oven with a temperature stability of 0.01 °C.
Fig. 1. Experimental setup of the PE-SRO. PD, photodiode; PZT, Piezo-electric Transducer; Lock-in, lock in amplifier; PID, Proportion Integration Differentiation circuits; HVA, high voltage amplifier.
The cavity was formed by two concave mirrors (M1 & M2) and two plane mirrors (M3 & M4). The spherical radius of concave mirrors is 100 mm and the total cavity length was about 542 mm. The pump laser was carefully injected into the center of the MgO:PPLN crystal with a 1/e2 beam radius of about 36 µm, then the beam radius of signal was calculated to be about 41 µm based on ABCD method. The variation of pump beam radius as a function of the distance from the center of the crystal is shown in Fig. 2. In the design, the confocal parameter (ξ) defined as ξ = L / (k ω02) was 3.04, where L is the crystal length, k is the propagation vector and ω0 is the beam waist [17]. The confocal parameter was slightly larger than 2.84, which could achieve the maximum quantum efficiency [24], to pursue the high quantum efficiency and a low threshold. In order to achieve efficient energy conversion, the reflectivity of the input coupler should be 40% - 65% [22], so the input coupler M1 was coated with the reflectivity of 60% at 1064 nm, and the high reflectivity at 1.5 µm. M2 was coated with an anti-reflectivity at 3.8 µm, and the high reflectivity at 1.5 µm and 1064 nm. Meanwhile, M3 was coated for the high reflective surface at 1064 nm and 1.5 µm, while the output coupler M4 was coated with the reflectivity of 98.5% at 1.5 µm and the high reflectivity at 1064 nm. The cavity finesses of pump and signal were about 15 and 273 [25], respectively. All mirrors were coated with the incident angle of 5 degrees.
Typically, nonlinear crystals are placed vertically to the pump beam in a pump-enhanced cavity, which is able to introduce mode-splitting and decrease of pump enhancement factor because of the Bragg reflectivity and surface reflectivity of PPLN crystal [26,27]. The Bragg reflection was caused by the small index discontinuity at the boundaries of the domain walls. This phenomenon was also observed in an injection-locked Ti:Sapphire laser [26] and a doubly resonant OPO [28]. Therefore, the PPLN crystal was oblique at about 2 degrees from the pump beam in our experiment setup.
The active control technology is required by the PE-SRO, since the equivalent length of the cavity is changed by the instability of the environment, so that the oscillation power of the pump will be changed. The low frequency modulation locking technique was used to stable the equivalent cavity length to make the pump light resonated, which was changed by the Piezo-electric Transducer (PZT), so that the pump resonating in the cavity is stabilized at the maximum power [23]. The lock in amplifier (Lock-in, SRS, SR830) provided the high voltage amplifier (HVA) with a sinusoidal signal for scanning the cavity length, then demodulated the error signal from the 1064 nm cavity transmission signal detected by the photodiode (PD), which was set behand the M3. The Proportion Integration Differentiation circuits (PID, New Focus, LB1005) modified the error signal and sent it to the HVA to lock the cavity length in order to ensure the intracavity pump resonant power at maximum.
3. Results and discussion
Figure 3 shows the variation of the idler power and quantum efficiency ηq = (λi Pi) / (λp Pp) as a function of the incident pump power with the crystal placed vertically or obliquely in PE-SRO and vertically in SRO, which has the approximate confocal parameter and cavity length as the PE-SRO. Each point represents an average of 10-minutes result. The oscillation threshold was about 1 W in PE-SRO with the crystal placed obliquely. When the pump power was 3 W, 0.5 W of idler was generated, corresponding to a quantum efficiency of more than 61%. Moreover, the quantum efficiencies were almost over 50%. The oscillation threshold was about 8 W in the SRO, which had the approximate cavity length, confocal parameter, mirrors coated for signal and idler with the PE-SRO. Compared with the PE-SRO and the SRO, the results indicated that the PE-SRO could dramatically reduce the pump threshold, and much higher quantum efficiency was achieved by the PE-SRO than the SRO, with the low pump power. The pump enhancement factor was calculated as 7 [25] with the pump power under the threshold. The ratio of threshold reduction matched the pump enhancement factor, which confirmed the effect of pump enhancement. Furthermore, there was also pump enhancement with the crystal placed vertically. The maximum quantum efficiency was 50.2%, with the pump power of 4.9 W and the idler power of 0.7 W. Although this performance was much better than SRO, it could still be improved by just placing the crystal obliquely and not changed other parameters.
Fig. 3. Variation of idler power and quantum efficiency as a function of incident pump power in PE-SRO and SRO.
The low frequency modulation locking technique was used in our PE-SRO to ensure the stability of pump resonant power in the cavity. A kHz level sinusoidal signal was used to scan the cavity length, and the cavity transmission mode signal obtained was shown in Fig. 4, which was input to the Lock-in for demodulation to get the error signal. The error signal was an approximate odd function, whose high slope part cross the zero was required by PID. Then the sinusoidal signal reduced, and the PID would stabilize the error signal at zero, so that the pump power could be kept at maximum. Since the general noise brought by external interference was mostly low frequency noise, the sinusoidal signal frequency like kHz magnitude was enough. However, PE-SRO has a common problem that the cavity mode signal is truncated with the high pump power so that the error signal cannot be identified, because the pump light is converted to parametric light after exceeding the pump threshold acutely. Low frequency modulation locking technique can increase scan voltage so that the cavity mode fluctuates periodically at the top in order to stabilize the idler power. Figure 5(a) showed that the average power of idler was 1.51 W with a standard deviation of 0.61% in more than 10 mins with the crystal placed vertically to the pump beam. The corresponding pump power was 15.2 W with a standard deviation of 1.08%. Figure 5(b) showed the idler power at 1.8 W in 10 mins with the crystal placed obliquely to the pump beam, which was 19% higher than that of the crystal placed vertically. This is by far the largest power of PE-SRO in the 3-5 µm waveband. The percentage of the standard deviation for idler power was 1.9% and that of the pump is 1.2%. The standard deviation of idler power in Fig. 5(b) was higher than that in Fig. 5(a), because the idler power was higher. In addition, higher cavity enhancement factor would also have a small impact. Certainly, in the case of low pump power, it has more stable performance. Therefore, our PE-SRO can provide stability 0-1.8 W, high quantum efficiency idler light. Compared with the common Pound-Drever-Hall (PDH) technology [20,21,29–32], the low frequency modulation locking technique does not need the electro-optic modulator and the phase shifter, and has low requirements on the performance of the devices, which is convenient for miniaturization and integration.
Wavelength-tunable performance is one of the important properties of lasers, because wavelength-tuning in the MIR band means covering more characteristic absorption spectrums. OPO with PPLN crystal has good wavelength-tuning performance. Since the reflectivity of the cavity mirrors for the signal light was high and the narrow linewidth pump light was single-longitudinal-mode oscillation [14], the signal light was also single-longitudinal-mode oscillation, and the Lorentz linewidth measured by the zero-beat heterodyne method was 345 kHz with the idler power of over 1.3 W. Figure 6 shows the variation of the wavelength of the signal light due to temperature changes. The wavelength of signal light was measured at 1475.5 nm (Yokogawa, AQ6370D) at 25 °C, and the wavelength of idler light was 3819.9 nm (Thorlabs, OSA 207C). The idler spectrum was shown in the inset of Fig. 6(a). The idler linewidth was no more than 363 kHz by calculation with the phase matching condition and law of conservation of energy [12]. Figure 6(a) shows the case with locked continuously and Fig. 6(b) shows the case with locked after temperature adjustment. When the signal laser mode was locked continuously, the tuning rate with temperature was 0.88 nm/°C, corresponding to the idler light as 5.8 nm/°C by calculation, because the equivalent cavity length changed along with the crystal temperature, leading to the movement of the signal cavity mode and the variation of output wavelength. The tuning rate with temperature of signal laser was 0.19 nm/°C with locked after temperature adjustment, and the rate of idler light was 1.2 nm/°C. The experimental and simulated results matched in an acceptable range. For example, at 25 °C, the measured signal wavelength was 1475.491 nm, and the simulated result was 1473.111 nm, so the deviation was only 0.16%. However, there was a little deviation between experimental and simulated results, which was caused by the slight deviations in actual and measured temperatures. In addition, there were also small differences between the theoretical and practical Sellmeier coefficients, resulting in the deviation. The combination of two methods can better obtain the required wavelength. The temperature was set at a room temperature to avoid a large volume cooling module, which was beneficial to miniaturization.
Fig. 6. The wavelength variation of the signal light with locked continuously (a), and with locked after temperature adjustment (b). Inset shows the spectrum of idler light at 25 °C.
When the idler light was operated at the maximum output power, the idler beam quality was measured by a camera (DataRay, WinCamD-IR-BB). The idler output was nearly in fundamental transverse mode, and the M2 factors in x and y directions were 1.20 and 1.28, respectively, as shown in Fig. 7(a), with the crystal placed vertically to the pump beam. The inset shows the beam profile of the idler. As shown in Fig. 7(b), when the crystal was obliquely placed to the pump beam, the M2 factors in x and y directions were 1.30 and 1.33, which did not deteriorate obviously.
Fig. 7. Beam quality measurement and fitting, with the crystal placed vertically (a), and obliquely (b).
4. Conclusion
In conclusion, a high efficiency, CW, narrow linewidth PE-SRO source with high stability was demonstrated. Based on the pump-enhanced bow-tie ring cavity and the low frequency modulation locking technique, the idler light had a maximum output power of 1.8 W. The wavelengths of signal and idler were 1475.5 nm and 3819.9 nm at 25 °C, respectively. Since the linewidth of pump laser was 18 kHz and the cavity finesse (${\sim} \; $273) of signal was high, the linewidth of signal was 345 kHz measured by zero-beat heterodyne method and the linewidth of idler was less than 363 kHz by calculation. Meanwhile, the PE-SRO has excellent wavelength tunability, the tuning rate with temperature was 5.8 nm/°C with locked continuously and 1.2 nm/°C with locked after temperature adjustment. By obliquely placing the crystal, the maximum output power was increased by 19%, and the maximum quantum efficiency was increased by 22%. In addition, because of bow-tie ring cavity and low thermal effects in crystal, the idler light has a nearly fundamental transverse mode, and the M2 factors in x and y directions were 1.30 and 1.33. In the future work, the PE-SRO will be miniaturized and used as the pump of an OFC.
Funding
National Natural Science Foundation of China (62075116, 62075117); Key research program of Shandong Province (2020JMRH0302); Natural Science Foundation of Shandong Province (ZR2019MF039, ZR2020MF114, ZR2022QF087); Founding for Qilu Young Scholars from Shandong University, China Postdoctoral Science Foundation (2021TQ0190); Postdoctoral Innovation Foundation of Shandong Province (SDCX-ZG-202202014).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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