- Email: [email protected]
Widely tunable, high-energy, mid-infrared (2.2–4.8 µm) laser based on a multi-grating MgO:PPLN optical parametric oscillator
Infrared Physics and Technology 104 (2020) 103121
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Widely tunable, high-energy, mid-infrared (2.2–4.8 µm) laser based on a multi-grating MgO:PPLN optical parametric oscillator Niu Sujiana,b, Palidan Aierkena, Mairihaba Ababaikea,b, Wang Shutonga,b, Taximaiti Yusufua,b,c,
T ⁎
a
School of Physics and Electronic Engineering, Xinjiang Normal University, Urumqi, Xinjiang 830054, China Laboratory of Novel Light Source and Micro/Nano-Optics, Xinjiang Normal University, Urumqi, Xinjiang 830054, China c Key Laboratory of Mineral Luminescent Material and Microstructure of Xinjiang, Urumqi, Xinjiang 830054, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nonlinear optics Parametric oscillators and amplifiers Mid-infrared Quasi-phase matching
We designed a compact, widely tunable, high-energy, stable, mid-infrared optical parametric oscillator pumped by a conventional Q-switched Nd:YAG nano-second laser. The idler wavelengths were tuned from 2.2 to 4.8 μm through gratings and temperature control of the multi-grating MgO-doped periodically poled lithium niobite (PPLN) crystal. A maximum idler output energy of 2.15 mJ was obtained at a pump energy of 21 mJ, which corresponded to an optical-optical efficiency in excess of 10%. A broad-spectrum bandwidth of 21 nm of idler radiation in the mid-infrared wavelength range was considered the result of the large grating period of the PPLN crystal.
1. Introduction Widely tunable, high-energy, stable, compact, high beam quality, mid-infrared 2–5 μm light sources based on optical parametric oscillator (OPO) and optical parametric amplifier (OPA) systems, known as the fingerprint region, are of considerable importance in applications including remote sensing, atmospheric monitoring, spectroscopy analysis, and photoelectric detection surveys [1–5]. In particular, it is desirable to utilize high-energy, mid-infrared light sources with large wavelength tunability for highly sensitive and selective photoacoustic trace-gas sensing, in which most molecules have strong vibrational transitions [6,7]. At present, the technologies available that can achieve the desired laser output in the widely tunable and highly-energized mid-infrared region of 3–5 μm are primarily quantum and inter band cascade lasers (QCLs) and OPOs. QCLs have the advantages of a low threshold, high quantum efficiency, narrow line width, etc., and can achieve integration and miniaturization [8]. However, QCL sources have some significant problems with low output energy and poor beam quality, while still being relatively expensive. Although OPO technology has been around for a long time, it is still an excellent light source choice for the widely tunable mid-infrared region. It provides selectivity owing to its large wavelength tunability, high energy, increased beam quality, and compact, cost-effective devices for the generation of mid-infrared light in the 2–5 μm spectral range. There are many nonlinear crystals that generate mid-infrared light,
⁎
including potassium titanyle arsenate (KTA) [9,10], rubidium titanyle phosphate (RTP) [11], periodically poled LiTaO3 (PPLT) [12,13], periodically poled KTiOPO4 (PPKTP) [14], MgO: periodically poled LiNbO3 (PPLN) [15,16], etc. Among these crystals, MgO:PPLN has attracted the most attention for such versatile properties as high nonlinearity (deff of approximately 17 pm/V), a wide optical transparency range (0.5–5 µm), and long interaction lengths. In addition, it is insusceptible to photorefractive damage, which is an inherent advantage of quasiphase-matching (QPM). Several preliminary studies concerning tunable mid-infrared 2–5 μm light source generation based on OPOs (i.e., continuous-wave single-frequency singly-resonant mid-infrared OPO based on MgO:PPLN) have adopted a four-mirror bow-tie ring cavity configuration where a single-frequency idler output power higher than 1 W at 3.68 μm was obtained [17]. A miniaturized, idler-resonant, all-fiber, laser-pumped OPO based on MgO:PPLN generated a maximum midinfrared power of 5.84 W at 3.76 μm with a conversion efficiency greater than 14.0% [18]. In addition, tuning a fan-out grating crystal achieved wavelength-tunable OPO outputs from 2.5 to 3.6 μm for the idler beam, based on a fan-out grating MgO:PPLN crystal OPO pumped at 1.064 μm [19]. However, in all these approaches, using the multigrating quasi-phase matching MgO:PPLN crystals [20–22] to achieve a wide tuning range of the laser resulted in disadvantages that included low energy, low conversion efficiency, and unstable beam quality of the idler that exceeded 3.5 μm. When the mid-infrared wavelength exceeded 3.5 μm, the absorption of the MgO:PPLN crystal was high. In
Corresponding author. E-mail address: [email protected] (T. Yusufu).
https://doi.org/10.1016/j.infrared.2019.103121 Received 15 August 2019; Received in revised form 7 November 2019; Accepted 11 November 2019 Available online 12 November 2019 1350-4495/ © 2019 Elsevier B.V. All rights reserved.
Infrared Physics and Technology 104 (2020) 103121
S. Niu, et al.
mirrors (M1 and M2), which were made of CaF2. The input cavity mirror, M1, was used to obtain a high reflectivity for 1.4–1.6 μm and 3.1–4.2 μm and a high transmission for the 1.064 μm pump beam, while the output mirror M2 had a high reflectivity for 1.4–1.6 μm and a high transmission for 3.1–4.2 μm, in order to ensure a high intensity singly resonant signal beam. The resonant length was fixed at 110 mm. A simple compact cavity was used in the OPO module, where the planeparallel cavity configuration allowed the development of a compact mid-infrared laser source by utilizing minimal cavity elements (i.e., two flat mirrors).
addition, with high power pumping, the MgO:PPLN crystal generated strong thermal lensing effects [18], which degraded the beam quality of the non-resonant idler output. Therefore, an appropriate pump laser and OPO cavity must be carefully designed and selected. Compared to the intra-cavity OPO [23,24], an extra-cavity OPO has the advantages of a relatively simpler structure, lower threshold, and higher conversion efficiency. Therefore, we chose a singly resonant plane-parallel cavity configuration owing to its high operational stability, beam quality, and conversion efficiency. This simple linear cavity configuration, which will enable the generation of non-resonant idler output, should allow the development of an ultra-compact mid-infrared source by utilizing minimal cavity elements (i.e., only two flat mirrors). In this work, we demonstrated a compact, widely tunable, millijoule-level, high beam quality, mid-infrared laser by singly resonant optical parametric oscillator using a multi-grating MgO:PPLN crystal with grating periods from 26 to 31 μm with a step of 1 μm. We obtained a pulse frequency 3.4 μm idler laser that reached an output energy of 2.15 mJ under a pumping energy of 21 mJ. Wide wavelength tuning was achieved, with a tuning range of 2.2–4.8 μm, by employing MgO:PPLN grating periods of 26–31 μm and controlling the crystal temperature from 25 to 200 °C. Idler output in the mid-infrared wavelength range with a broad-spectrum bandwidth of 21 nm was considered to be a result of the large grating period of the PPLN crystal.
2.2. Experimental results The spatial forms of the pump and idler outputs are shown in Fig. 2. Fig. 2(a) shows the spatial form of the pump beam with the Gaussian spatial form measured by a conventional charge coupled device (CCD) camera. The spatial forms of the idler output were imaged by a pyroelectric camera at the various crystal grating periods from 26−31 μm with a step of 1 μm at a constant crystal temperature of 25 °C. The results of the idler output exhibited a single-peak Gaussian spatial transverse electromagnetic (TEM) mode profile across all grating periods as shown in Fig. 2(b)-(g). The beam propagation factor, M2, defined by ISO standard, is 4σ (the second moment) of the 3.4 μm idler output, and was measured to be 1.4. The idler output energy, as a function of the pump energy, was measured at a grating period of 30 μm with an MgO:PPLN crystal temperature of 25 °C, which corresponded to an idler wavelength of 3.4 μm as shown in Fig. 3. As the pump energy grew, the idler output energy increased linearly. The maximum idler output energy of 2.15 mJ was obtained at a pump energy of 21 mJ and corresponded to an optical-optical efficiency of more than 10%. In the present experiment, the maximum power density at the maximum pump level was 107 Mw/ cm2, which was lower than the approximately 150 Mw/cm2 damage threshold of the MgO:PPLN crystal, Further improvement of the opticaloptical efficiency from the fundamental pump to the mid-infrared idler output of this system could be achieved by modifying the high-energy damage threshold mirrors, as well as the focusing optics for the pump beam. The mid-infrared idler output also exhibited a temporal energystability of less than 1% of the root mean square (rms) during an observation time of more than eight hours. In OPOs, the parametric gain is determined by the spatial amplitude overlapping efficiency of the pump, and the signal (idler) modes in the nonlinear crystal. In our system, the singly resonant high-Q cavity used for the signal output enforced the generation of the idler output with high energy in the mid-infrared region. We also investigated the wavelength tunability range of the midinfrared idler outputs by controlling the grating periods and temperature of the MgO:PPLN crystal. The six grating periods of the crystal were forwarded to the pump beam by controlling the mechanical stage, to ensure the pump beam passed through each grating period, and heating the nonlinear crystal from 25 °C to 200 °C each grating period.
2. Results and discussions 2.1. Experimental setup A schematic of the widely tunable, mid-infrared MgO:PPLN OPO experimental setup is shown in Fig. 1. It primarily consisted of a pumping system and OPO module. The pumping system contained a conventional Q-switched Nd:YAG laser (Lotis, LS-2136; pulse duration: 25 ns; wavelength: 1.064 μm; PRF: 50 Hz). This laser offered a simple method to obtain linearly polarized and relatively high-peak-energy pulses for the efficient pumping of a PPLN OPO, which had a maximum output energy of 21 mJ. We used a half-wave plate to modify the pump polarization to align with the crystallography of the MgO:PPLN crystal to improve the resonant conversion efficiency. Next, we utilized a lens to reduce the beam size of the pump laser at the waist radius of 1 mm and injected it into the OPO module. The nonlinear medium used in the OPO was 5 mol% MgO-doped PPLN with a type-o (e → e + e) phase matching for a pumping wavelength of 1.064 μm. The MgO:PPLN crystal contained six domain grating periods from 26 to 31 μm with a step of 1 μm and had a length of 40 mm, width of 10 mm, and thickness of 2 mm. In order to improve the output energy and the conversion efficiency, both end faces of the MgO:PPLN crystal were polished and antireflection-coated at a pump wavelength of 1.064 μm, signal wavelength range of 1.34–2 μm, and idler wave length range of 2.5–5.5 μm. The MgO:PPLN crystal was carefully positioned in an oven that controlled the crystal operating temperature from 25 to 200 °C with a precision of 0.1 °C. The single resonant cavity consisted of two flat
Fig. 1. Experimental setup of the MgO:PPLN OPO. 2
Infrared Physics and Technology 104 (2020) 103121
S. Niu, et al.
Fig. 2. Spatial forms of the (a) 1.064 μm pump, (b)–(g) idler output with crystal grating periods of 26–31 μm with a step of 1 μm at a constant crystal temperature of 25 °C.
Fig. 4(a) shows the experimental and simulated tuning curves for the idler outputs at the grating periods from 26 to 31 μm with a step of 1 μm. The idler wavelength was tuned from 2.2 to 4.8 μm, and the measured tuning curve matched better with that of the calculated value according to the Sellmeier equation. However, when the crystal utilized a period of 31 μm, especially in the high temperature portion, the experimental values differed very little from the theoretical values. This was due to the increasing thermal expansion effect of the crystal during the heating process. To the best of our knowledge, this was the widest tuning range generated using the OPO method at this wavelength range. Throughout the idler wavelength tuning range from 2.2 to 4.8 μm, we also measured the output energy of the idler output versus the wavelength at the incident pump energy of 21 mJ as shown in Fig. 4(b). A maximum idler output energy of 2.15 mJ at 3.4 μm was obtained at the crystal grating period of 30 μm and a crystal temperature of 25 °C, which corresponded to an optical-optical conversion efficiency in excess of 10%. The signal output energy was significantly low (< .15 mJ) in the entire tuning range of 2.06–1.36 μm owing to extremely low out-coupling from the high-Q cavity for the signal output. The output energies of the idler output wavelength ranges of 2.2–3.1 μm and 4.2–4.8 μm were limited to 0.55–1.5 mJ and 1.1–0.23 mJ, respectively, even at the maximum pumping level, which corresponded to optical efficiencies of 2.6–7.1% and 5.2–1.1%,
Fig. 3. Mid-infrared idler output energy as a function of the pump energy at an idler wavelength of 3.4 μm in the crystal grating period of 30 μm with a temperature of 25 °C.
μ μ μ μ μ μ
Fig. 4. (a) Wavelength-tuning curve for the idler outputs at the grating periods from 26 to 31 μm with a step of 1 μm. The spots and curves indicate the experimental and simulated values. (b) Idler output energies as a function of the idler wavelength at a pump energy of 21 mJ. 3
Infrared Physics and Technology 104 (2020) 103121
S. Niu, et al.
Fig. 5. Measured spectrum of the (a) pump wavelength centered at 1064 nm and (b) mid-infrared idler output wavelength centered at 3071 nm in the 30 μm crystal grating period with a crystal temperature of 200 °C.
expansion of the crystal and its refractive index behaviour with temperature [18]. The phase-matching bandwidth is usually defined as the bandwidth in which the phase mismatch (as accumulated over the entire length of the device) varies by 2.7831 rad. The estimated value is influenced by the crystal period, Λ ; crystal length, L; mid-infrared wavelength, λi ; and temperature, T [25]. Additionally, the phasematching bandwidth increases with an increase in the crystal period of Λ . Therefore, the broadband idler radiation in the mid-infrared wavelength range can be considered to be a result of the large grating period of the PPLN crystal. It is worth noting that such a broad-spectrum bandwidth is necessary for the simultaneous observation of absorption lines [26], which will be studied in a forthcoming paper.
respectively. The low output energy in this region was attributed to the lack of an antireflection film coating in this band of the output coupler. Additionally, the rapidly increased absorption of the PPLN crystal and lower photon energy at longer wavelengths resulted in difficulty obtaining a high-energy, high-efficiency mid-infrared output beyond 3.4 μm. Further improvement of the optical-optical conversion efficiency of this OPO system would be possible by modifying the parameters of the output mirror to increase the resonant efficiency of the outputs. The spectral characteristics of the pump and idler outputs were recorded using Ocean Optics (HR4000CG-UV-NIR, resolution in the wavelength range 200–1100 nm: 0.75–1 nm) and a high-performance scanning monochromator (SpectraPro HRS-500, grating: 300 g/mm, aperture size: 50 µm, resolution in the wavelength range 3000–5000 nm: 0.35–0.4 nm), respectively. The spectral band width (FWHM) of the idler output was also studied at the 30 μm grating period with a MgO:PPLN crystal temperature of 200 °C. The pump beam had a spectral band width (FWHM) of Δλ p ~ 0.65 nm (~5.7 cm−1) centered at λp = 1064 nm, as shown in Fig. 5(a). In contrast, the midinfrared idler output had a large spectral band width (FWHM) of Δλi ~ 21 nm (~22.3 cm−1) centered at λi = 3071 nm. As shown in Fig. 5(b), which clearly showed the generation of broadband idler radiation in the mid-infrared wavelength range. This broadband idler output was originated by the relatively wide phase matching bandwidth of the MgO:PPLN crystal.
3. Conclusions We have demonstrated a widely tunable, compact, milli-joule-level, mid-infrared laser formed by a 1 µm conventional Q-switched Nd:YAG nano-second laser-pumped quasi-phase matching MgO-doped multigrating periodically poled lithium niobate (MgO:PPLN) optical parametric oscillator. The compact singly resonant cavity configuration, in which the maximum mid-infrared 3.4 µm idler output energy reached 2.15 mJ at a pump energy of 21 mJ, corresponded to an optical-optical conversion efficiency in excess of 10% that was fundamental to the achieved mid-infrared idler photon conversion efficiency of 32%. The wavelength tunability of the mid-infrared output was also achieved by controlling the grating periods and temperature of the MgO:PPLN crystal. The system achieved a wide tuning range of 2.2–4.8 µm laser output with high beam quality. The generated milli-joule level, midinfrared laser output with wide wavelength tunability opens up a new generation of applications in molecular spectroscopy, organic material processing, and atmospheric sensing.
2.3. Theoretical considerations Parametric nonlinear interactions lead to an efficient exchange of energy only when phase matching is achieved. Phase matching bandwidth is the width of an optical frequency range in which a nonlinear interaction can be efficient along the direction of propagation. Due to chromatic dispersion, phase matching can only be achieved in a limited bandwidth that is related to the group velocity mismatch of the interacting waves. According to the QPM technique, both energy and momentum conservation can be fulfilled simultaneously. The phase mis2π match is given by Δκ = κp − κs − κi − Λ , where Λ is the domain grating period, κp , κs , and κi are the wave numbers of the pump, signal, np, s, i and idler beam, respectively, and are expressed as κp, s, i = 2π λ . The
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments
p, s, i
resulting
Δκ = 2π
(
momentum np λp
−
ns λs
−
ni λi
−
conservation 1 Λ
)
equation
is
given
as
This work is supported the National Natural Science Fund Foundation of China (Grant Nos. 11664041), the Natural Science Fund Foundation of Xinjiang Uygur Autonomous Region (Grant Nos. 2016D01B047), the Xinjiang Uygur Autonomous Region Graduate Research and Innovation Project, and the Foundation of Xinjiang
. The QPM condition can be achieved by
using special values of κp , κs , and κi and a domain grating period, Λ . In order to obtain the proper QPM condition, which can be calculated using a Sellmeier equation, one must take in to account the thermal 4
Infrared Physics and Technology 104 (2020) 103121
S. Niu, et al.
Normal University Key Laboratory of mineral luminescent material and microstructure of Xinjiang, Xinjiang, China (Grant Nos. KWFG1805).
(2017) 072701. [14] S. Chaitanya Kumar, S. Parsa, M. Ebrahim-Zadeh, “Fiber-laser-based, greenpumped, picosecond optical parametric oscillator using fan-out grating PPKTP, Opt. Lett. 41 (1) (2016) 52. [15] M.K. Shukla, R. Das, High-power single-frequency source in the mid-infrared using a singly resonant optical parametric oscillator pumped by Yb-fiber laser, Ieee J. Selected Topics Quantum Electron. 24 (5) (2018). [16] Y.J. Yu, X.Y. Chen, J. Zhao, et al., High-repetition-rate tunable mid-infrared optical parametric oscillator based on MgO: periodically poled lithium niobate, Opt. Eng. 53 (6) (2014). [17] Z. Jiaqun, C. Ping, X. Feng, et al., Watt-level continuous-wave single-frequency midinfrared optical parametric oscillator based on MgO:PPLN at 3.68 μm, Appl. Sci 8 (8) (2018) 1345. [18] Y. He, F. Chen, D. Yu, et al., Improved conversion efficiency and beam quality of miniaturized mid-infrared idler-resonant MgO: PPLN optical parametric oscillator pumped by all-fiber laser, Infrared Phys. Technol. 95 (2018) 12–18. [19] I.H. Bae, S.D. Lim, et al., Development of a Mid-infrared CW optical parametric oscillator based on fan-out grating MgO:PPLN pumped at 1064 nm, Curr. Opt. Photon. 3 (1) (2019) 33–39. [20] T.D. Wang, S.T. Lin, Y.Y. Lin, et al., Forward and backward terahertz-wave difference-frequency generations from periodically poled lithium niobate, Opt. Express 16 (9) (2008) 6471. [21] N. Dixit, R. Mahendra, O.P. Naraniya, et al., High repetition rate mid-infrared generation with singly resonant optical parametric oscillator using multi-grating periodically poled MgO:LiNbO3, Opt. Laser Technol. 42 (1) (2010) 18–22. [22] S.D. Liu, Z.W. Wang, B.T. Zhang, et al., Wildly tunable, high-efficiency MgO:PPLN Mid-IR optical parametric oscillator pumped by a Yb-fiber laser, Chin. Phys. Lett. 31 (2) (2014) 024204. [23] D.J.M. Stothard, M. Ebrahimzadeh, M.H. Dunn, Low-pump-threshold continuouswave singly resonant optical parametric oscillator, Opt. Lett. 23 (24) (1999) 1895–1897. [24] H.Y. Zhu, J.H. Guo, Y.M. Duan, Efficient 1.7 μm light source based on KTA-OPO derived by Nd:YVO4 self-Raman laser, Opt. Lett. 43 (2) (2018) 3445–4348. [25] D.H. Jundt, Temperature-dependent Sellmeier equation for the index of refraction, ne, in congruent lithium niobate, Opt. Lett. 22 (20) (1997) 1553. [26] Z. Cao, X. Gao, W. Chen, et al., Study of quasi-phase matching wavelength acceptance bandwidth for periodically poled LiNbO3 crystal-based difference-frequency generation, Opt. Lasers Eng. 47 (5) (2009) 589–593.
References [1] B.M. Walsh, H.R. Lee, N.P. Barnes, Mid infrared lasers for remote sensing applications, J. Lumin. 169 (2016) 400–405. [2] G.V. Basum, D. Halmer, P. Hering, et al., Parts per trillion sensitivity for ethane in air with an optical parametric oscillator cavity leak-out spectrometer, Opt. Lett. 29 (8) (2004) 797–799. [3] P. Weibring, H. Edner, S. Svanberg, Versatile mobile lidar system for environmental monitoring, Appl. Opt. 42 (18) (2003) 3583–3594. [4] Y. Zhang, F.R. Wang, Y.H. Zhao, et al., Experiment research on ellipsoidal structure methane using the absorption characteristics of 3.31 μm mid-infrared spectroscopy, Infrared Phys. Technol. 55 (4) (2012). [5] M. Yan, P.L. Luo, K. Iwakuni, et al., Mid-infrared dual-comb spectroscopy with electro-optic modulators, Light Sci. Appl. 6 (10) (2016) e17076. [6] M.W. Sigrist, R. Bartlome, D. Marinov, et al., Trace gas monitoring with infrared laser-based detection schemes, Appl. Phys. B: Lasers Opt. 90 (2) (2008) 289–300. [7] T. Tomberg, M. Vainio, T. Hieta, et al., Sub-parts-per-trillion level sensitivity in trace gas detection by cantilever-enhanced photo-acoustic spectroscopy, Sci. Rep. 8 (1) (2018). [8] N. Bandyopadhyay, Y. Bai, B. Gokden, et al., Watt level performance of quantum cascade lasers in room temperature continuous wave operation at 3.76 μm, Appl. Phys. Lett. 97 (13) (2010) 101105. [9] F. Bai, Q. Wang, Z. Liu, et al., Theoretical and experimental studies on output characteristics of an intracavity KTA OPO, Opt. Express 20 (2) (2012) 807. [10] C. Dong, H.Y. Xu, X.M. Wang, et al., High energy picosecond mid-infrared optical parametric amplifier based on KTiOAsO4 crystal, Infrared Phys. Technol. 97 (2019) 440–443. [11] Y.M. Duan, H.Y. Zhu, Y.L. Ye, et al., Efficient RTP-based OPO intracavity pumped by an acousto-optic Q-switched Nd:YVO4 laser, Opt. Lett. 39 (5) (2014) 1314. [12] H. Hatano, M. Watanabe, K. Kitamura, et al., Mid IR pulsed light source for laser ultrasonic testing of carbon-fiber-reinforced plastic, J. Opt. (2015) 094011. [13] H. Hatano, R. Slater, S. Takekawa, et al., Optimization of mid-IR generation from a periodically poled MgO doped stoichiometric lithium tantalate optical parametric oscillator with intracavity difference frequency mixing, Jpn. J. Appl. Phys. 56
5
Contents lists available at ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
Widely tunable, high-energy, mid-infrared (2.2–4.8 µm) laser based on a multi-grating MgO:PPLN optical parametric oscillator Niu Sujiana,b, Palidan Aierkena, Mairihaba Ababaikea,b, Wang Shutonga,b, Taximaiti Yusufua,b,c,
T ⁎
a
School of Physics and Electronic Engineering, Xinjiang Normal University, Urumqi, Xinjiang 830054, China Laboratory of Novel Light Source and Micro/Nano-Optics, Xinjiang Normal University, Urumqi, Xinjiang 830054, China c Key Laboratory of Mineral Luminescent Material and Microstructure of Xinjiang, Urumqi, Xinjiang 830054, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Nonlinear optics Parametric oscillators and amplifiers Mid-infrared Quasi-phase matching
We designed a compact, widely tunable, high-energy, stable, mid-infrared optical parametric oscillator pumped by a conventional Q-switched Nd:YAG nano-second laser. The idler wavelengths were tuned from 2.2 to 4.8 μm through gratings and temperature control of the multi-grating MgO-doped periodically poled lithium niobite (PPLN) crystal. A maximum idler output energy of 2.15 mJ was obtained at a pump energy of 21 mJ, which corresponded to an optical-optical efficiency in excess of 10%. A broad-spectrum bandwidth of 21 nm of idler radiation in the mid-infrared wavelength range was considered the result of the large grating period of the PPLN crystal.
1. Introduction Widely tunable, high-energy, stable, compact, high beam quality, mid-infrared 2–5 μm light sources based on optical parametric oscillator (OPO) and optical parametric amplifier (OPA) systems, known as the fingerprint region, are of considerable importance in applications including remote sensing, atmospheric monitoring, spectroscopy analysis, and photoelectric detection surveys [1–5]. In particular, it is desirable to utilize high-energy, mid-infrared light sources with large wavelength tunability for highly sensitive and selective photoacoustic trace-gas sensing, in which most molecules have strong vibrational transitions [6,7]. At present, the technologies available that can achieve the desired laser output in the widely tunable and highly-energized mid-infrared region of 3–5 μm are primarily quantum and inter band cascade lasers (QCLs) and OPOs. QCLs have the advantages of a low threshold, high quantum efficiency, narrow line width, etc., and can achieve integration and miniaturization [8]. However, QCL sources have some significant problems with low output energy and poor beam quality, while still being relatively expensive. Although OPO technology has been around for a long time, it is still an excellent light source choice for the widely tunable mid-infrared region. It provides selectivity owing to its large wavelength tunability, high energy, increased beam quality, and compact, cost-effective devices for the generation of mid-infrared light in the 2–5 μm spectral range. There are many nonlinear crystals that generate mid-infrared light,
⁎
including potassium titanyle arsenate (KTA) [9,10], rubidium titanyle phosphate (RTP) [11], periodically poled LiTaO3 (PPLT) [12,13], periodically poled KTiOPO4 (PPKTP) [14], MgO: periodically poled LiNbO3 (PPLN) [15,16], etc. Among these crystals, MgO:PPLN has attracted the most attention for such versatile properties as high nonlinearity (deff of approximately 17 pm/V), a wide optical transparency range (0.5–5 µm), and long interaction lengths. In addition, it is insusceptible to photorefractive damage, which is an inherent advantage of quasiphase-matching (QPM). Several preliminary studies concerning tunable mid-infrared 2–5 μm light source generation based on OPOs (i.e., continuous-wave single-frequency singly-resonant mid-infrared OPO based on MgO:PPLN) have adopted a four-mirror bow-tie ring cavity configuration where a single-frequency idler output power higher than 1 W at 3.68 μm was obtained [17]. A miniaturized, idler-resonant, all-fiber, laser-pumped OPO based on MgO:PPLN generated a maximum midinfrared power of 5.84 W at 3.76 μm with a conversion efficiency greater than 14.0% [18]. In addition, tuning a fan-out grating crystal achieved wavelength-tunable OPO outputs from 2.5 to 3.6 μm for the idler beam, based on a fan-out grating MgO:PPLN crystal OPO pumped at 1.064 μm [19]. However, in all these approaches, using the multigrating quasi-phase matching MgO:PPLN crystals [20–22] to achieve a wide tuning range of the laser resulted in disadvantages that included low energy, low conversion efficiency, and unstable beam quality of the idler that exceeded 3.5 μm. When the mid-infrared wavelength exceeded 3.5 μm, the absorption of the MgO:PPLN crystal was high. In
Corresponding author. E-mail address: [email protected] (T. Yusufu).
https://doi.org/10.1016/j.infrared.2019.103121 Received 15 August 2019; Received in revised form 7 November 2019; Accepted 11 November 2019 Available online 12 November 2019 1350-4495/ © 2019 Elsevier B.V. All rights reserved.
Infrared Physics and Technology 104 (2020) 103121
S. Niu, et al.
mirrors (M1 and M2), which were made of CaF2. The input cavity mirror, M1, was used to obtain a high reflectivity for 1.4–1.6 μm and 3.1–4.2 μm and a high transmission for the 1.064 μm pump beam, while the output mirror M2 had a high reflectivity for 1.4–1.6 μm and a high transmission for 3.1–4.2 μm, in order to ensure a high intensity singly resonant signal beam. The resonant length was fixed at 110 mm. A simple compact cavity was used in the OPO module, where the planeparallel cavity configuration allowed the development of a compact mid-infrared laser source by utilizing minimal cavity elements (i.e., two flat mirrors).
addition, with high power pumping, the MgO:PPLN crystal generated strong thermal lensing effects [18], which degraded the beam quality of the non-resonant idler output. Therefore, an appropriate pump laser and OPO cavity must be carefully designed and selected. Compared to the intra-cavity OPO [23,24], an extra-cavity OPO has the advantages of a relatively simpler structure, lower threshold, and higher conversion efficiency. Therefore, we chose a singly resonant plane-parallel cavity configuration owing to its high operational stability, beam quality, and conversion efficiency. This simple linear cavity configuration, which will enable the generation of non-resonant idler output, should allow the development of an ultra-compact mid-infrared source by utilizing minimal cavity elements (i.e., only two flat mirrors). In this work, we demonstrated a compact, widely tunable, millijoule-level, high beam quality, mid-infrared laser by singly resonant optical parametric oscillator using a multi-grating MgO:PPLN crystal with grating periods from 26 to 31 μm with a step of 1 μm. We obtained a pulse frequency 3.4 μm idler laser that reached an output energy of 2.15 mJ under a pumping energy of 21 mJ. Wide wavelength tuning was achieved, with a tuning range of 2.2–4.8 μm, by employing MgO:PPLN grating periods of 26–31 μm and controlling the crystal temperature from 25 to 200 °C. Idler output in the mid-infrared wavelength range with a broad-spectrum bandwidth of 21 nm was considered to be a result of the large grating period of the PPLN crystal.
2.2. Experimental results The spatial forms of the pump and idler outputs are shown in Fig. 2. Fig. 2(a) shows the spatial form of the pump beam with the Gaussian spatial form measured by a conventional charge coupled device (CCD) camera. The spatial forms of the idler output were imaged by a pyroelectric camera at the various crystal grating periods from 26−31 μm with a step of 1 μm at a constant crystal temperature of 25 °C. The results of the idler output exhibited a single-peak Gaussian spatial transverse electromagnetic (TEM) mode profile across all grating periods as shown in Fig. 2(b)-(g). The beam propagation factor, M2, defined by ISO standard, is 4σ (the second moment) of the 3.4 μm idler output, and was measured to be 1.4. The idler output energy, as a function of the pump energy, was measured at a grating period of 30 μm with an MgO:PPLN crystal temperature of 25 °C, which corresponded to an idler wavelength of 3.4 μm as shown in Fig. 3. As the pump energy grew, the idler output energy increased linearly. The maximum idler output energy of 2.15 mJ was obtained at a pump energy of 21 mJ and corresponded to an optical-optical efficiency of more than 10%. In the present experiment, the maximum power density at the maximum pump level was 107 Mw/ cm2, which was lower than the approximately 150 Mw/cm2 damage threshold of the MgO:PPLN crystal, Further improvement of the opticaloptical efficiency from the fundamental pump to the mid-infrared idler output of this system could be achieved by modifying the high-energy damage threshold mirrors, as well as the focusing optics for the pump beam. The mid-infrared idler output also exhibited a temporal energystability of less than 1% of the root mean square (rms) during an observation time of more than eight hours. In OPOs, the parametric gain is determined by the spatial amplitude overlapping efficiency of the pump, and the signal (idler) modes in the nonlinear crystal. In our system, the singly resonant high-Q cavity used for the signal output enforced the generation of the idler output with high energy in the mid-infrared region. We also investigated the wavelength tunability range of the midinfrared idler outputs by controlling the grating periods and temperature of the MgO:PPLN crystal. The six grating periods of the crystal were forwarded to the pump beam by controlling the mechanical stage, to ensure the pump beam passed through each grating period, and heating the nonlinear crystal from 25 °C to 200 °C each grating period.
2. Results and discussions 2.1. Experimental setup A schematic of the widely tunable, mid-infrared MgO:PPLN OPO experimental setup is shown in Fig. 1. It primarily consisted of a pumping system and OPO module. The pumping system contained a conventional Q-switched Nd:YAG laser (Lotis, LS-2136; pulse duration: 25 ns; wavelength: 1.064 μm; PRF: 50 Hz). This laser offered a simple method to obtain linearly polarized and relatively high-peak-energy pulses for the efficient pumping of a PPLN OPO, which had a maximum output energy of 21 mJ. We used a half-wave plate to modify the pump polarization to align with the crystallography of the MgO:PPLN crystal to improve the resonant conversion efficiency. Next, we utilized a lens to reduce the beam size of the pump laser at the waist radius of 1 mm and injected it into the OPO module. The nonlinear medium used in the OPO was 5 mol% MgO-doped PPLN with a type-o (e → e + e) phase matching for a pumping wavelength of 1.064 μm. The MgO:PPLN crystal contained six domain grating periods from 26 to 31 μm with a step of 1 μm and had a length of 40 mm, width of 10 mm, and thickness of 2 mm. In order to improve the output energy and the conversion efficiency, both end faces of the MgO:PPLN crystal were polished and antireflection-coated at a pump wavelength of 1.064 μm, signal wavelength range of 1.34–2 μm, and idler wave length range of 2.5–5.5 μm. The MgO:PPLN crystal was carefully positioned in an oven that controlled the crystal operating temperature from 25 to 200 °C with a precision of 0.1 °C. The single resonant cavity consisted of two flat
Fig. 1. Experimental setup of the MgO:PPLN OPO. 2
Infrared Physics and Technology 104 (2020) 103121
S. Niu, et al.
Fig. 2. Spatial forms of the (a) 1.064 μm pump, (b)–(g) idler output with crystal grating periods of 26–31 μm with a step of 1 μm at a constant crystal temperature of 25 °C.
Fig. 4(a) shows the experimental and simulated tuning curves for the idler outputs at the grating periods from 26 to 31 μm with a step of 1 μm. The idler wavelength was tuned from 2.2 to 4.8 μm, and the measured tuning curve matched better with that of the calculated value according to the Sellmeier equation. However, when the crystal utilized a period of 31 μm, especially in the high temperature portion, the experimental values differed very little from the theoretical values. This was due to the increasing thermal expansion effect of the crystal during the heating process. To the best of our knowledge, this was the widest tuning range generated using the OPO method at this wavelength range. Throughout the idler wavelength tuning range from 2.2 to 4.8 μm, we also measured the output energy of the idler output versus the wavelength at the incident pump energy of 21 mJ as shown in Fig. 4(b). A maximum idler output energy of 2.15 mJ at 3.4 μm was obtained at the crystal grating period of 30 μm and a crystal temperature of 25 °C, which corresponded to an optical-optical conversion efficiency in excess of 10%. The signal output energy was significantly low (< .15 mJ) in the entire tuning range of 2.06–1.36 μm owing to extremely low out-coupling from the high-Q cavity for the signal output. The output energies of the idler output wavelength ranges of 2.2–3.1 μm and 4.2–4.8 μm were limited to 0.55–1.5 mJ and 1.1–0.23 mJ, respectively, even at the maximum pumping level, which corresponded to optical efficiencies of 2.6–7.1% and 5.2–1.1%,
Fig. 3. Mid-infrared idler output energy as a function of the pump energy at an idler wavelength of 3.4 μm in the crystal grating period of 30 μm with a temperature of 25 °C.
μ μ μ μ μ μ
Fig. 4. (a) Wavelength-tuning curve for the idler outputs at the grating periods from 26 to 31 μm with a step of 1 μm. The spots and curves indicate the experimental and simulated values. (b) Idler output energies as a function of the idler wavelength at a pump energy of 21 mJ. 3
Infrared Physics and Technology 104 (2020) 103121
S. Niu, et al.
Fig. 5. Measured spectrum of the (a) pump wavelength centered at 1064 nm and (b) mid-infrared idler output wavelength centered at 3071 nm in the 30 μm crystal grating period with a crystal temperature of 200 °C.
expansion of the crystal and its refractive index behaviour with temperature [18]. The phase-matching bandwidth is usually defined as the bandwidth in which the phase mismatch (as accumulated over the entire length of the device) varies by 2.7831 rad. The estimated value is influenced by the crystal period, Λ ; crystal length, L; mid-infrared wavelength, λi ; and temperature, T [25]. Additionally, the phasematching bandwidth increases with an increase in the crystal period of Λ . Therefore, the broadband idler radiation in the mid-infrared wavelength range can be considered to be a result of the large grating period of the PPLN crystal. It is worth noting that such a broad-spectrum bandwidth is necessary for the simultaneous observation of absorption lines [26], which will be studied in a forthcoming paper.
respectively. The low output energy in this region was attributed to the lack of an antireflection film coating in this band of the output coupler. Additionally, the rapidly increased absorption of the PPLN crystal and lower photon energy at longer wavelengths resulted in difficulty obtaining a high-energy, high-efficiency mid-infrared output beyond 3.4 μm. Further improvement of the optical-optical conversion efficiency of this OPO system would be possible by modifying the parameters of the output mirror to increase the resonant efficiency of the outputs. The spectral characteristics of the pump and idler outputs were recorded using Ocean Optics (HR4000CG-UV-NIR, resolution in the wavelength range 200–1100 nm: 0.75–1 nm) and a high-performance scanning monochromator (SpectraPro HRS-500, grating: 300 g/mm, aperture size: 50 µm, resolution in the wavelength range 3000–5000 nm: 0.35–0.4 nm), respectively. The spectral band width (FWHM) of the idler output was also studied at the 30 μm grating period with a MgO:PPLN crystal temperature of 200 °C. The pump beam had a spectral band width (FWHM) of Δλ p ~ 0.65 nm (~5.7 cm−1) centered at λp = 1064 nm, as shown in Fig. 5(a). In contrast, the midinfrared idler output had a large spectral band width (FWHM) of Δλi ~ 21 nm (~22.3 cm−1) centered at λi = 3071 nm. As shown in Fig. 5(b), which clearly showed the generation of broadband idler radiation in the mid-infrared wavelength range. This broadband idler output was originated by the relatively wide phase matching bandwidth of the MgO:PPLN crystal.
3. Conclusions We have demonstrated a widely tunable, compact, milli-joule-level, mid-infrared laser formed by a 1 µm conventional Q-switched Nd:YAG nano-second laser-pumped quasi-phase matching MgO-doped multigrating periodically poled lithium niobate (MgO:PPLN) optical parametric oscillator. The compact singly resonant cavity configuration, in which the maximum mid-infrared 3.4 µm idler output energy reached 2.15 mJ at a pump energy of 21 mJ, corresponded to an optical-optical conversion efficiency in excess of 10% that was fundamental to the achieved mid-infrared idler photon conversion efficiency of 32%. The wavelength tunability of the mid-infrared output was also achieved by controlling the grating periods and temperature of the MgO:PPLN crystal. The system achieved a wide tuning range of 2.2–4.8 µm laser output with high beam quality. The generated milli-joule level, midinfrared laser output with wide wavelength tunability opens up a new generation of applications in molecular spectroscopy, organic material processing, and atmospheric sensing.
2.3. Theoretical considerations Parametric nonlinear interactions lead to an efficient exchange of energy only when phase matching is achieved. Phase matching bandwidth is the width of an optical frequency range in which a nonlinear interaction can be efficient along the direction of propagation. Due to chromatic dispersion, phase matching can only be achieved in a limited bandwidth that is related to the group velocity mismatch of the interacting waves. According to the QPM technique, both energy and momentum conservation can be fulfilled simultaneously. The phase mis2π match is given by Δκ = κp − κs − κi − Λ , where Λ is the domain grating period, κp , κs , and κi are the wave numbers of the pump, signal, np, s, i and idler beam, respectively, and are expressed as κp, s, i = 2π λ . The
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments
p, s, i
resulting
Δκ = 2π
(
momentum np λp
−
ns λs
−
ni λi
−
conservation 1 Λ
)
equation
is
given
as
This work is supported the National Natural Science Fund Foundation of China (Grant Nos. 11664041), the Natural Science Fund Foundation of Xinjiang Uygur Autonomous Region (Grant Nos. 2016D01B047), the Xinjiang Uygur Autonomous Region Graduate Research and Innovation Project, and the Foundation of Xinjiang
. The QPM condition can be achieved by
using special values of κp , κs , and κi and a domain grating period, Λ . In order to obtain the proper QPM condition, which can be calculated using a Sellmeier equation, one must take in to account the thermal 4
Infrared Physics and Technology 104 (2020) 103121
S. Niu, et al.
Normal University Key Laboratory of mineral luminescent material and microstructure of Xinjiang, Xinjiang, China (Grant Nos. KWFG1805).
(2017) 072701. [14] S. Chaitanya Kumar, S. Parsa, M. Ebrahim-Zadeh, “Fiber-laser-based, greenpumped, picosecond optical parametric oscillator using fan-out grating PPKTP, Opt. Lett. 41 (1) (2016) 52. [15] M.K. Shukla, R. Das, High-power single-frequency source in the mid-infrared using a singly resonant optical parametric oscillator pumped by Yb-fiber laser, Ieee J. Selected Topics Quantum Electron. 24 (5) (2018). [16] Y.J. Yu, X.Y. Chen, J. Zhao, et al., High-repetition-rate tunable mid-infrared optical parametric oscillator based on MgO: periodically poled lithium niobate, Opt. Eng. 53 (6) (2014). [17] Z. Jiaqun, C. Ping, X. Feng, et al., Watt-level continuous-wave single-frequency midinfrared optical parametric oscillator based on MgO:PPLN at 3.68 μm, Appl. Sci 8 (8) (2018) 1345. [18] Y. He, F. Chen, D. Yu, et al., Improved conversion efficiency and beam quality of miniaturized mid-infrared idler-resonant MgO: PPLN optical parametric oscillator pumped by all-fiber laser, Infrared Phys. Technol. 95 (2018) 12–18. [19] I.H. Bae, S.D. Lim, et al., Development of a Mid-infrared CW optical parametric oscillator based on fan-out grating MgO:PPLN pumped at 1064 nm, Curr. Opt. Photon. 3 (1) (2019) 33–39. [20] T.D. Wang, S.T. Lin, Y.Y. Lin, et al., Forward and backward terahertz-wave difference-frequency generations from periodically poled lithium niobate, Opt. Express 16 (9) (2008) 6471. [21] N. Dixit, R. Mahendra, O.P. Naraniya, et al., High repetition rate mid-infrared generation with singly resonant optical parametric oscillator using multi-grating periodically poled MgO:LiNbO3, Opt. Laser Technol. 42 (1) (2010) 18–22. [22] S.D. Liu, Z.W. Wang, B.T. Zhang, et al., Wildly tunable, high-efficiency MgO:PPLN Mid-IR optical parametric oscillator pumped by a Yb-fiber laser, Chin. Phys. Lett. 31 (2) (2014) 024204. [23] D.J.M. Stothard, M. Ebrahimzadeh, M.H. Dunn, Low-pump-threshold continuouswave singly resonant optical parametric oscillator, Opt. Lett. 23 (24) (1999) 1895–1897. [24] H.Y. Zhu, J.H. Guo, Y.M. Duan, Efficient 1.7 μm light source based on KTA-OPO derived by Nd:YVO4 self-Raman laser, Opt. Lett. 43 (2) (2018) 3445–4348. [25] D.H. Jundt, Temperature-dependent Sellmeier equation for the index of refraction, ne, in congruent lithium niobate, Opt. Lett. 22 (20) (1997) 1553. [26] Z. Cao, X. Gao, W. Chen, et al., Study of quasi-phase matching wavelength acceptance bandwidth for periodically poled LiNbO3 crystal-based difference-frequency generation, Opt. Lasers Eng. 47 (5) (2009) 589–593.
References [1] B.M. Walsh, H.R. Lee, N.P. Barnes, Mid infrared lasers for remote sensing applications, J. Lumin. 169 (2016) 400–405. [2] G.V. Basum, D. Halmer, P. Hering, et al., Parts per trillion sensitivity for ethane in air with an optical parametric oscillator cavity leak-out spectrometer, Opt. Lett. 29 (8) (2004) 797–799. [3] P. Weibring, H. Edner, S. Svanberg, Versatile mobile lidar system for environmental monitoring, Appl. Opt. 42 (18) (2003) 3583–3594. [4] Y. Zhang, F.R. Wang, Y.H. Zhao, et al., Experiment research on ellipsoidal structure methane using the absorption characteristics of 3.31 μm mid-infrared spectroscopy, Infrared Phys. Technol. 55 (4) (2012). [5] M. Yan, P.L. Luo, K. Iwakuni, et al., Mid-infrared dual-comb spectroscopy with electro-optic modulators, Light Sci. Appl. 6 (10) (2016) e17076. [6] M.W. Sigrist, R. Bartlome, D. Marinov, et al., Trace gas monitoring with infrared laser-based detection schemes, Appl. Phys. B: Lasers Opt. 90 (2) (2008) 289–300. [7] T. Tomberg, M. Vainio, T. Hieta, et al., Sub-parts-per-trillion level sensitivity in trace gas detection by cantilever-enhanced photo-acoustic spectroscopy, Sci. Rep. 8 (1) (2018). [8] N. Bandyopadhyay, Y. Bai, B. Gokden, et al., Watt level performance of quantum cascade lasers in room temperature continuous wave operation at 3.76 μm, Appl. Phys. Lett. 97 (13) (2010) 101105. [9] F. Bai, Q. Wang, Z. Liu, et al., Theoretical and experimental studies on output characteristics of an intracavity KTA OPO, Opt. Express 20 (2) (2012) 807. [10] C. Dong, H.Y. Xu, X.M. Wang, et al., High energy picosecond mid-infrared optical parametric amplifier based on KTiOAsO4 crystal, Infrared Phys. Technol. 97 (2019) 440–443. [11] Y.M. Duan, H.Y. Zhu, Y.L. Ye, et al., Efficient RTP-based OPO intracavity pumped by an acousto-optic Q-switched Nd:YVO4 laser, Opt. Lett. 39 (5) (2014) 1314. [12] H. Hatano, M. Watanabe, K. Kitamura, et al., Mid IR pulsed light source for laser ultrasonic testing of carbon-fiber-reinforced plastic, J. Opt. (2015) 094011. [13] H. Hatano, R. Slater, S. Takekawa, et al., Optimization of mid-IR generation from a periodically poled MgO doped stoichiometric lithium tantalate optical parametric oscillator with intracavity difference frequency mixing, Jpn. J. Appl. Phys. 56
5