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Abstract
A continuous-wave mid-infrared radiation from difference frequency generation by mixing a continuous-wave Ti: sapphire laser and a continuous-wave YAG laser in a 15 mm long BaGa4Se7 crystal is demonstrated for the first time. The tunable range from 3.15 to 7.92 μm was achieved by rotating the crystal to fulfill the type I phase-matching condition. A maximum DFG power of 1.41 μW was obtained at 5 μm. Meanwhile the experimental DFG power conversion efficiency was 20.2 μW/W2, with a length-normalized slope efficiency of 15.5 μW/cmW2. The conversion efficiency decreases rapidly from 50 μW/cmW2 at 3.15 μm to 1 μW/cmW2 at 7.92 μm. The wavelength acceptance bandwidth and the angular acceptance bandwidth were measured to be 16.4 cm−1 and 44′ for DFG at 5.1 μm, respectively.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The continuous-wave (CW) mid-infrared (MIR) laser sources tuning in the range from 3 to 20 μm, especially from 3 to 5 μm and 8 to12 μm atmospheric windows, are very important for spectroscopy application such as remote sensing, spectrum analysis, environmental monitoring, biomedical diagnostics, molecular astrophysics and etc [1]. Among common methods of generating CW MIR laser radiation, difference frequency generation (DFG) is an attractive approach because of its precise wavelength resolution, narrow linewidth, widely and rapidly continuous wavelength tunability and room temperature operation. Generally, for the DFG process, the nonlinear optical crystals (NLO) are expected to have a wide range of transparent regions, large nonlinear coefficients and high laser damage thresholds. Up to now, a large number of oxide and non-oxide NLO crystals have been investigated and some of which have even been commercialized. However, the two-photon or multi-photon absorption starting from about 4 μm causes oxide crystals to have a transmittance range of no more than 5 μm in the MIR region, which makes such crystals can hardly be used to generate MIR radiation with wavelengths longer than 4 μm. Therefore, the non-oxide crystals with wide transparent regions is necessary for the longer wavelength generation, such as AgGaS2/Se2, ZnGeP2, LiInS2/Se2, CdSiP2, and BaGa4S7 [2–7]. Among these crystals, although AgGaS2/Se2 and ZnGeP2 possess large NLO coefficients and wide transparent regions, AgGaS2/Se2 have low laser damage thresholds and ZnGeP2 exhibits strong two photo absorption of conventional ~1μm pumping source. As for the other non-oxide crystals mentioned above, each has its own advantages and disadvantages. For example, LiInS2/Se2 has a high laser damage threshold, but the nonlinear coefficient is relatively small and the laser loss is still high; CdSiP2 shows a strong nonlinear effect, but its transmission wavelength is limited to 10 μm; BaGa4S7 has a wider bandgap, but the nonlinear coefficient is two times smaller than that of AgGaS2/Se2. In addition, the crystals that have been investigated for generating MIR lasers also have Ag3AsS3, HgGa2S4, CdxHg1-xGa2S4, LiNbO3 and 4H-SiC [8–14]. However, there is still an urgent need for new NLO crystals with wide transparent regions, large nonlinear coefficients and high laser damage threshold at the same time to generate MIR laser radiation.
Recently, a newly developed NLO crystal BaGa4Se7 (BGSe) was reported to be a promising material for generating MIR radiation [15]. The crystal is monoclinic (m-point group, space group Pc) and positive biaxial crystal with a = 7.6252(15) Å, b = 6.5114(13) Å, c = 14.702(4) Å, β = 121.24(2)°, Z = 2 and the angle Ω = 45.6° between the optical axes and the z-principal dielectric axis. It has high laser induced damage threshold owing to its relatively wide bandgap of 2.64 eV, which allows the use of well-developed and commercial near infrared lasers, such as Nd: YAG and Ti: Sapphire lasers, as pump and signal sources for DFG without two-photo absorption. In addition, it is transparent from 0.47 to 18 μm and can be grown into large size single crystals by Bridgman-Stockbarger technique. Particularly, its sufficient birefringence to phase-matched (PM) to its entire transparent range and relatively large NLO coefficients furtherly make the BGSe crystal a very promising alternative for practical application in the MIR region [16].
The BGSe was first discovered in 2010 [15] and successfully grown by Bridgman-Stockbarger method in 2011 [17]. Its phase-matching properties, Sellmeier equations (S-E) and NLO coefficients have been intensively discussed to a certain degree at different wavelength bands by the comparison of theoretical and experimental results [18–23]. Until now, the application of this crystal in optical parametric oscillator (OPO), optical parametric amplifier (OPA) and DFG for frequency down conversion has been reported in many papers [24–28]. However, among these reports, the original seed sources used to produce the MIR laser are all pulse lasers, such as pulse Nd: YAG laser with different pulse duration and repetition rate. To the best of our knowledge, there has been no report of using CW lasers as seed sources to generate CW MIR lasers through a frequency conversion process, especially for difference frequency process. In this letter, A widely tunable CW MIR difference frequency radiation with BGSe crystal, using CW Ti: sapphire laser and CW YAG laser as pump and signal source respectively, is first demonstrated. The tuning performance and power conversion efficiency of the DFG were extensively investigated.
2. Experimental setup
Figure 1 shows the experimental configuration of the DFG system. A CW Ti:sapphire laser (M Squared: SolsTis-SRX) with the tunable range from 730 to 970 nm, the specification nominal linewidth of 50 KHz and the maximum output power of 1.5 W at 850 nm was used as the pump laser. The signal source is a diode-pumped single frequency CW Nd:YAG laser (Innolight: Mephister 1000) with 1 W output at 1064 nm. A beam splitter was used to split the pump beam into two beams, one of which (~5%) was directed to wavemeter with a optical fiber to monitor the pump wavelength, and the other part is spatially combined with the signal beam by a dichroic mirror (high transmission near 1064 nm). Then, the overlapped beams were collinearly focused with a lens of 10 cm focal length into a BGSe. A mechanical chopper, with chopping frequency of 1200 Hz for a better signal-to-noise ratio of detector, was placed in front of the Ti:sapphire laser to chop the pump laser. The generated DFG radiation from the crystal was collimated by another 10 cm focal length CaF2 lens onto a liquid-nitrogen-cooled HgCdTe detector (Hamamatsu: P2748-40). The HgCdTe detector, biased at 15 mA, has a peak detectivity of 4.0 × 1010 cm Hz1/2/W and a detector noise of 2.5 × 10−12 W/Hz1/2 at 1200 Hz. A 1 mm thick uncoated Germanium filter with transparent range from 2 to 14 μm was used to block the unconverted residual pump and signal beams. The output electrical signal of detector, after being amplified by a low-noise preamplifier, was fed to a lock-in amplifier for demodulation, then acquired by a 16-bit data-acquisition (DAQ) card (NI: 6353) and recorded by a laptop. For positive biaxial crystals, the type I PM condition can be expressed as:
also known as momentum and energy conservation respectively. The subscript o stands for ordinary wave and the e stands for extraordinary one. λp, λs and λi are the input pump, signal and corresponding idler wavelength respectively with the relation of λp<λs<λi. Therefore, to satisfy the PM, two half-wave plates (HWP) were used to adjust the polarizations of seed beams and the PM angle θPM could be achieved by rotating a rotation stage with an accuracy of ± 1.2′, on which the crystal was mounted.
Fig. 1 Schematic diagram of the experimental setup used for DFG. HWP, half-wave plate; M, mirror; BS, beam splitter; L, lens; OF, optical fiber; DM, dichroic mirror; GF, germanium filter; D, detector; PA, preamplifier; DAQ, data acquisition card.
3. Results and discussion
The BGSe crystal used in the present work was grown by Bridgman-Stockbarger method and had the dimensions of 10 mm × 10 mm × 15 mm (W × H × L). It was cut in the x-z plane at θ = 67°, φ = 0° for type I (o-e→e) PM DFG with the 10 mm edge parallel to the y axis, to ensure large effective nonlinearity deff, and polished in both end-faces without coatings, as shown of the insert figure in Fig. 2. The transmittance of the crystal to pump and signal light is measured, as shown in Fig. 2. The transmittance is about 63% for signal at 1064 nm, and from 51% to 60% for pump within its tuning range measured on the 15 mm bulk, which is close to the theoretical limit of (1-R)2 with Fresnel equation R = (n-1)2/(n + 1)2 (n: refractive index) and the reported refractive index of BGSe [20]. These transmittances, on the one hand, shows the good quality of the crystal and on the other hand, it guarantees adequate power of seed sources for the DFG with mutual and commercial visible (V) and near-infrared (NIR) lasers.
Fig. 2 The transmittance of BGSe in the tuning range of pump measured on the 15 mm bulk in this work. The insert shows a photograph of BGSe.
Theoretical and experimental angle tuning performance of type I BGSe DFG was investigated as shown in Fig. 3. The tuning curve was measured under fixing signal beam wavelength at 1064.49 nm and pump beam tuning from 795.72 nm to 938.30 nm. The theoretical PM curve shown in Fig. 3 was calculated with the S-E reported by Boursier et al. in 2016 [20] and newly reported by Kato et al in 2017 [21]. It can be seen that the PM angle range is from 47.48° to 81.14° corresponding the idler wavelength tuning range from 3.15 to 7.92 μm. Within idler wavelength less than 5 μm, the experimental PM angles are in excellent agreement with the theoretical results calculated with Boursier et al.′s S-E. However, for the idler wavelength larger than 5 μm, the deviation between them becomes larger with the increase of idler wavelength. The reason might be that the cutting angle of BGSe had an measurement error and/or the S-E used for the theoretical calculation was not precise enough. Obviously, there is a big deviation throughout the tuning range between the experimental data and the calculated result with Kato et al′s S-E, which is basically similar to the result of DFG exhibited by Fig. 3 in reference [21]. Regrettably, we failed to find out the reasons for the difference. However, we noted that Boursier et al.′s S-E was obtained by simultaneously fitting of second harmonic generation (SHG) and DFG experimental data, but Kato et al′s S-E was derived without the fit of DFG, which may explain the agreement between our experimental data and the theoretical result calculated with Boursier et al.′s S-E. In fact, there is still no definite and extensively accepted S-E for BGSe, and the dispersion property of the crystal are being investigated including its temperature-dependent characteristics [22,23].
Fig. 3 Type I internal PM angles of the DFG with BGSe versus angle tuning. The black solid line and blue dash line are the theoretical curves calculated with the S-E from [20] and [21] respectively. The red closed circles are experimental data.
In order to estimate the CW DFG output power, an analysis was performed based on reference [29] with Gaussian input beams assumed. In this case, the maximum conversion efficiency η for the DFG process can be expressed as:
where the subscripts p, s and i correspond to the pump, signal and idler beams, respectively; P is beam power; l is crystal length; ω represents the angular frequency; deff is the effective nonlinear coefficient; k refers to the wave vector; c is the speed of light in vacuum; ε0 is the free-space permittivity; n is the refractive index; h(μ, ξ) is the focusing parameter, which is a function of μ = ks/kp and the ratio ξ = l/b; as a function of Gaussian beam waist ω0, b is called the confocal parameter with the expression of b = 2πω02n/λ. The generated DFG power at 5.0 μm in the BGSe crystal are plotted as a function of the input pumping power shown in Fig. 4. The pump wavelength, signal wavelength and the PM angle were 877.06 nm, 1064.49 nm and 57.56°, respectively. A linearly variable neutral density filter was used to vary the input pump laser power from 64 mW (PD (Power density): 10.40 kW/cm2) to 260 mW (PD: 42.25 kW/cm2), and the signal laser power was fixed at 364 mW (PD: 45.28 kW/cm2). These values shown in Fig. 5 were corrected for optical losses resulting from the Fresnel effect, BGSe crystal, germanium filter, CaF2 lens and the HgCdTe detector. The DFG power increased linearly with the increase of the product of two incident beams, which is consistent with the theoretical prediction with the Eq. (3). A maximum DFG power of 1.41 μW was obtained when the pump laser power was 260 mW. The experimental DFG power conversion efficiency was derived from the slope of a linear fit to the experimental data, and a value of 20.23 μW/W2 was found, with a length-normalized slope efficiency of 13.49 μW/cmW2. For the type I PM of BGSe, the effective nonlinear coefficient can be expressed as: deff = d16sin2θ + d23cos2θ, so to obtain a theoretical value of the power conversion efficiency, the d16 and d23 of the second-order susceptibility tensor must be known. So far, however, the information on d16 and d23 in the report is very limited, and even whether they are the same sign or the opposite sign is also in dispute [27]. Therefore, the theoretical value of conversion efficiency cannot be given.Fig. 4 DFG power as a function of the product of two incident beams. The red solid line is a least-squares linear fit to the experimental data (closed circles). The pump and signal wavelengths were fixed at 877.06 nm and 1064.49 nm, which corresponds to the DFG wavelength ~5.0 μm and PM angle ~57.56°.
Fig. 5 Experimental conversion efficiency versus DFG wavelength from 3.15 to 7.92 μm. A beam quality picture of the idler generated at normal incidence is inserted into the figure.
Using the same method, the wavelength dependence of the DFG conversion efficiencies was investigated, as shown in Fig. 5. The DFG conversion efficiencies decreased rather quickly from 50 μW/cmW2 at 3.15 μm to 1 μW/cmW2 at 7.92 μm. According to the Eq. (3), This trend of change is mainly due to the increase of idler wavelength, the decrease of deff and nonoptimal overlap between pump and signal beams. Moreover, there is a slight enhancement at 3.81 μm, as indicated by the arrow in Fig. 5. Such phenomenon probably was caused by the idler beam reflection of the crystal surfaces under normal incidence, which makes the DFG quasi-doubly resonant [25]. Some irregular features might be related to air absorption.
Wavelength tunability is another important characteristic parameter for a laser source used in spectroscopy, in particular for the spectroscopic detection of multispecies or broadband absorbers. In present work, DFG-based wavelength tuning is achieved by scanning the wavelength of pump laser around 881.71 nm while fixing the signal laser at 1064.49 nm. In this case, the wavelength tuning is limited by phase mismatch effects. In Fig. 6, phase matched DFG wavelength acceptance bandwidth is presented for an external phase matching angle fixed at 56.71° and a bandwidth of about 16.40 cm−1 (FWHM) was observed with the DFG wavelength of 5.14 μm. Moreover, phase-matching sets a limit to the angle acceptance, which determines the maximum permissible divergence. In Fig. 7, the angular acceptance is depicted for the DFG at 5.10 μm with seed laser wavelengths fixed at 880.72 nm and 1064.49 nm, respectively. A value of about 44′ was observed. Theoretically, the DFG wavelength acceptance bandwidth and the angular acceptance bandwidth can be obtained by fitting wavelength tuning curve at fixed phase matching angle and angle tuning curve at fixed seed laser wavelengths with a plane-wave model, as described in Reference [30], respectively. However, these fitting processes cannot be performed due to the lack of exact nonlinear coefficient, beam or spot parameters and accurate S-E.
Fig. 6 Wavelength dependent PM characteristic of DFG process. The wavelength acceptance bandwidth of 16.40 cm−1 was observed at 5.14 μm.
Fig. 7 Phase matched angular acceptance bandwidth for type I PM DFG in BaGSe crystal. The angle acceptance bandwidth of 44′ was observed at 5.10 μm.
4. Conclusion
In summary, A CW MIR difference frequency generation in a BGSe crystal has been demonstrated. The DFG system use CW Ti: sapphire laser and CW YAG laser as pump and signal source respectively. The transmittance is about 63% for signal at 1064 nm, and from 51% to 60% for pump within its tuning range measured on the 15 mm bulk crystal. The tunable range from 3.15 to 7.92 μm was achieved by rotating the crystal to fulfil the type I PM condition. A maximum DFG power of 1.41 μW was obtained at 5 μm, when the pump laser power was 260 mW with signal laser power of 364 mW. The experimental DFG power conversion efficiency was derived from the slope of a linear fit to the experimental data, and a value of 20.23 μW/W2 was found, with a length-normalized slope efficiency of 13.49 μW/cmW2. The wavelength dependence of the DFG conversion efficiencies was that it decreased rather quickly from 50 μW/cmW2 at 3.15 μm to 1 μW/cmW2 at 7.92 μm. The experimentally measured wavelength acceptance bandwidth and angular acceptance bandwidth are 16.40 cm−1 and 44′ at around 5.1 μm, respectively. Compared with the reported pulse OPO and DFG based on the BGSe crystal, the output of CW MIR difference frequency radiation is the most important feature of the current work.
Potential improvements lie particularly in increasing the generated DFG power and in extending tuning range of DFG wavelength. The corresponding measures could be performed by (1) optimizing the Ti: sapphire laser power and coating antireflection film of pump laser to the incident end of crystal, (2) applying infrared antireflection coating to the output surface of crystal, (3) using an anamorphic optics to transform the elliptical beam profile of seed laser into a nearly Gaussian beam profile, and (4) optimizing spatial overlap of the laser beams. These approaches would significantly promote the present DFG radiation to be a compact, efficient, room-temperature, widely tunable, narrow-band source required in spectroscopic, monitoring and sensing applications.
Funding
Ministry of Science and Technology of the People's Republic of China National Key Research & Development Program of China (2017YFC0209700, 2016YFC0303900); National Natural Foundation of China (NSFC) (51472251, 41805014, 41730103); Innovation Fund of the Chinese Academy of Sciences (CXJJ-17-M164).
Acknowledgments
The authors thank the helpful discussion with Dr. Youbao Ni and Liusan Wang. Specially acknowledged is Professor Weidong Chen for experimental guidance to this work and critically reading the manuscript.
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