Single-Pass Laser Frequency Conversion to 780.2 Nm and 852.3 Nm Based on Ppmgo:LN Bulk Crystals and Diode-Laser-Seeded Fiber Ampliﬁers

applied sciences

Article Single-Pass Laser Frequency Conversion to 780.2 nm and 852.3 nm Based on PPMgO:LN Bulk Crystals and Diode-Laser-Seeded Fiber Ampliﬁers

Kong Zhang 1, Jun He 1,2 and Junmin Wang 1,2,*

1 State Key Laboratory of Quantum Optics and Quantum Optics Devices, and Institute of Opto-Electronics, Shanxi University, Taiyuan 030006, China; [email protected] (K.Z.); [email protected] (J.H.) 2 Collaborative Innovation Center of Extreme Optics of the Ministry of Education and Shanxi Province, Shanxi University, Taiyuan 030006, China * Correspondence: [email protected]

Received: 16 October 2019; Accepted: 15 November 2019; Published: 17 November 2019

Abstract: We report the preparation of a 780.2 nm and 852.3 nm laser device based on single-pass periodically poled magnesium-oxide-doped lithium niobate (PPMgO:LN) bulk crystals and diode-laser-seeded fiber amplifiers. First, a single-frequency continuously tunable 780.2 nm laser of more than 600 mW from second-harmonic generation (SHG) by a 1560.5 nm laser can be achieved. Then, a 250 mW light at 852.3 nm is generated and achieves an overall conversion efficiency of 4.1% from sum-frequency generation (SFG) by mixing the 1560.5 nm and 1878.0 nm lasers. The continuously tunable range of 780.2 nm and 852.3 nm are at least 6.8 GHz and 9.2 GHz. By employing this laser system, we can conveniently perform laser cooling, trapping and manipulating both rubidium (Rb) and cesium (Cs) atoms simultaneously. This system has promising applications in a cold atoms Rb-Cs two-component interferemeter and in the formation of the RbCs dimer by the photoassociation of cold Rb and Cs atoms confined in a magneto-optical trap.

Keywords: ﬁber ampliﬁer; single-pass sum-frequency generation; single-pass second-harmonic generation; rubidium D2 line (780.2 nm), cesium D2 line (852.3 nm)

1. Introduction The special structure of alkali metal atoms is the foundation of precision spectra, laser cooling and trapping of atoms, atom interferometers, atomic frequency standards, etc. Amongst these atoms, rubidium (Rb) and cesium (Cs) have been studied in the greatest detail. The 5S1/2–5P3/2 transition (D2 line) of rubidium atoms corresponds to 780 nm, and the 6S1/2–6P3/2 transition (D2 line) of cesium atoms corresponds to 852 nm. Therefore, the 780 nm and 852 nm lasers are important. High-power single-frequency 780 nm and 852 nm lasers can be employed for laser cooling and trapping [1–4], atomic coherent control [5,6], atomic interferometers [7,8], and quantum frequency standards [9,10]. In addition, 1.5 µm squeezed light field and quantum-entangled light fields can be prepared from an optical parametric oscillator or optical parametric amplifier pumped by a single-frequency 780 nm laser and has important applications in continuous-variable quantum communication [11], gravitational wave detection [12] and so on. The common means to produce high-power 780 nm and 852 nm laser beams are semiconductor lasers with tapered amplifiers, a master-oscillator power amplifier (MOPA) and Ti:sapphire lasers. However, these systems can only operate in a relatively quiet and clean laboratory environment. In this context, the disadvantages of semiconductor MOPA systems and Ti:sapphire lasers, such as high cost, large size, and sensitivity to vibration and temperature fluctuations, are obvious.

Appl. Sci. 2019, 9, 4942; doi:10.3390/app9224942 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, 4942 2 of 9

On the other hand, telecom-band lasers have attracted increasing attention in recent years for their extensive and important applications, such as integrated optical devices, all-optical networks, multiwavelength channels [13] and so on. The 1560 nm erbium-doped fiber laser (EDFL) and erbium-doped fiber amplifier (EDFA) are more stable and durable and have been commercialized. These systems can provide a more powerful output than solid-state lasers, and a large number of experiments have been carried out with this system. They even achieved frequency doubling at 780 nm [14] in the plane. In 2015 [15], our group also used fiber lasers and amplifiers to produce a 637.2 nm red laser from single-pass sum-frequency generation (SFG) with two infrared lasers at 1560.5 nm and 1076.9 nm in periodically poled magnesium-oxide-doped lithium niobate (PPMgO:LN) bulk crystal. Compared with the previous structures, a system that was composed of fiber lasers and amplifiers, and quasi-phase-matched (QPM) non-linear materials, is more remarkable. Therefore, it can be applied in many fields. First, a 780 nm laser can be produced from second-harmonic generation (SHG) with this system, and we have performed these experiments previously [16–18]. The 852 nm laser can be produced from SFG, and there are many research groups working on these experiments. In 2017 [19], Diboune et al. reported mulyi-line operation of all fiber laser system at 780 nm and 852 nm based on waveguides, and they used this system for interferometry of cesium and rubidium atoms. In 2018 [20], Antoni-Micollier et al. demonstrated a watt-level narrow-linewidth fibered laser source at 852 nm by a periodically poled lithium niobate waveguide. The lasers are confined to a very small space in the waveguide, which makes the waveguide a good match with the lasers; however, there will be dispersion and incomplete matching in the bulk crystal. Nonetheless, the bulk crystal can hold more power input, which means that we can obtain a higher power output with a bulk crystal using a high-power injection. In our experiment, the 780 nm and 852 nm lasers are produced from single-pass SHG and SFG with the 1560 nm and 1878 nm lasers in the PPMgO:LN bulk crystal. Two fundamental lasers are boosted by the corresponding fiber amplifiers, an EDFA for the 1560 nm laser and a thulium-doped fiber amplifier (TDFA) for the 1878 nm laser. The system is particularly robust with single-pass configuration and diode-laser-seeded fiber amplifiers. Therefore, it can work in harsh environments outside a laboratory. It can also produce both 780 nm and 852 nm lasers, so we could conveniently perform laser cooling and trapping, as well as manipulating both Rb and Cs atoms simultaneously, it is useful in the Rb-Cs biatomic interferometer and cold RbCs dimer experiments. Most importantly, the spots produced by this system have good beam quality and a wide tuning range, which make it a scheme that is more promising for application. Considering all of the advantages, the biatomic interferometer and cold RbCs dimer experiments can be performed in unstable environments, such as an airborne system and a system in the space station.

2. Experimental Setup The experimental scheme is shown in Figure1. The first master-oscillator power fiber amplifier (MOPFA) consists of a compact distributed feedback (DFB) diode laser at 1560.5 nm and a 10 W EDFA, and another MOPFA consists of a compact DFB at 1878.0 nm and a 2 W TDFA. The optical isolator is used to restrain the laser feedback, thus ensuring the stability of EDFA and TDFA. The half-wave plate (λ/2) and the polarization beam splitter (PBS) cube are used to control the power of the 1560.5 nm and 1878.0 nm lasers, while also transforming the polarization of the fundamental lasers to the s polarization to meet the requirement of the sum-frequency process. Then, a 50 mm-long PPMgO:LN crystal (HC Photonics; Taiwan; the thickness is 0.5 mm; poling period of 23.4 µm; type 0 matching; both ends of the crystal have flat surfaces with anti-reflection coatings for the fundamental and sum-frequency laser, and the residual reflectivity R < 0.2%) is used as the SFG crystal. Additionally, the crystal is placed in a homemade oven, which is made of red copper and stabilized at a precise temperature using a temperature controller (Newport Corp., California, America, Model 350B). We can achieve optimized phase matching by adjusting the temperature of the crystal. Matching is an important factor Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 10 important factor in a single-pass configuration [21]. We choose f = 100 mm and f = 75 mm with anti- reflection coating for both fundamental and sum-frequency lasers as the matching lens. After the laser passes through the crystal, a 75 mm lens is used to collimate the output laser. Subsequently, the 852.3 nm laser is divided into two beams by PBS: one beam passes through Cs atomic vapor cells and another beam passes through the Fabry–Perot cavity to monitor its frequency tuning range. Appl. Sci. 2019, 9, 4942 3 of 9 Before the SFG, we divide the 1560.5 nm laser into two beams: one beam for the above experiment and another beam for SHG. Single-pass cascaded 25 mm-long PPMgO:LN crystals (HC Photonics;in a single-pass Taiwan; configuration the thickness [21 ].is We1.0 mm; choose poling f = 100 period mm of and 19.48 f = 75μm; mm type with 0 matching anti-reflection; both ends coating of thefor bothcrystals fundamental have flat surfaces and sum-frequency with anti-reflection lasers as coatings the matching for the lens. fundamental After the laser and passesdoubled through laser; andthe crystal,the residual a 75 reflectivity mm lens is R used < 0.2%) to collimate are used as the the output SHG laser.crystals. Subsequently, We chose f = the 50 mm 852.3 with nm an laser anti- is reflectiondivided into coating two beamsfor both by fundamental PBS: one beam and passes second-harmonic through Cs atomiclasers as vapor matching cells and lensse. another The beamwaist passesspot radius through was the~28 Fabry–Perotμm, which is cavity very close to monitor to the value its frequency determined tuning by range.the optimum B–K focus factor ξ = 2.84 [22] 780.2 nm SHG SHG Dump Lens s PPMgO:LN Lens Lens PPMgO:LN Lens

Dump PBS DM s λ/2 λ/2 PBS IO Dump 50:50 BS 1560.5 nm DFB Diode Laser EDFA

PBS IO Dump 1878.0 nm TDFA DFB Diode Laser s SFG λ/2 DM LensPPMgO:LN Lens s

Dump M DM

Confocal F-P Cavity PD M 852.3 nm s

λ/2 PBS λ/4 NDF

M M Cs cell PD .

Figure 1. Schematic diagram of the single-pass sum-frequencysum-frequency generation (S (SFG)FG) system; 1560.5 nm and 1878.0 nm lasers are frequency summed in PPMgO PPMgO:LN:LN to produce 852.3 nm laser. Temperature controllers for three PPMgO:LN crystalscrystals are not shown in this figure figure.. DFB: DFB: distributed distributed feedback diode laser; EDFA: erbium-doped fiber fiber amplifier; amplifier; TDFA: TDFA: thulium-doped fiber fiber amplifier; amplifier; OI: OI: optical optical isolator; isolator; λ/2: half-wave plate; λ/4: quarter-wave plate; BS: beam splitter plate; PBS: polarization beam splitter λ/2: half-wave plate; λ/4: quarter-wave plate; BS: beam splitter plate; PBS: polarization beam splitter cube; DM: dichromatic mirror; NDF: neutral density filters; s: s polarization. cube; DM: dichromatic mirror; NDF: neutral density filters; s: s polarization. Before the SFG, we divide the 1560.5 nm laser into two beams: one beam for the above experiment 3. Experimental Results and Discussion and another beam for SHG. Single-pass cascaded 25 mm-long PPMgO:LN crystals (HC Photonics; Taiwan; the thickness is 1.0 mm; poling period of 19.48 µm; type 0 matching; both ends of the crystals 3.1. Single-Pass Second-Harmonic Generation (SHG) to 780.2 nm have flat surfaces with anti-reflection coatings for the fundamental and doubled laser; and the residual reflectivityFirst, on R < the0.2%) basis are usedof our as pr theevious SHG crystals.experiment, We chose we find f = 50 th mme optimized with an anti-reflection phase matching coating by adjustingfor both fundamental the temperature and second-harmonicof the PPMgO:LN lasers crystal as in matching a single-pass lensse. SHG The experiment. waist spot radius When wasthe ~28powerµm, of which the fundamental is very close wave to the 1560.5 value determinednm is 5.27 W, by the optimumbest phase B–K matching focus factor of theξ PPMgO:LN-= 2.84 [22] crystal is found at a temperature of 81.8 °C. The results are shown in Figure 2. 3. Experimental Results and Discussion

3.1. Single-Pass Second-Harmonic Generation (SHG) to 780.2 nm First, on the basis of our previous experiment, we find the optimized phase matching by adjusting the temperature of the PPMgO:LN crystal in a single-pass SHG experiment. When the power of the fundamental wave 1560.5 nm is 5.27 W, the best phase matching of the PPMgO:LN-crystal is found at a temperature of 81.8 ◦C. The results are shown in Figure2. The single-pass one PPMgO:LN crystal can produce 334 mW for the 780.2 nm laser, and the efficiency is 6.3%; single-pass two PPMgO:LN crystals can produce 634 mW for the 780.2 nm laser, and the efficiency is 12.0%. These results are shown in Figure3. If we increase the injection, an additional 780.2 nm laser can be produced. The transverse beam quality of the 780.2 nm laser is evaluated by using the beam quality factor parameter (M2) in the two orthogonal transverse directions X and Y. Figure4 shows the 1 /e2 beam radius versus the axial position Z after passing through a plano-convex lens with a focal length of 2 2 80 mm. Fitting of the experimental data gives MX = 1.05 and MY = 1.11. Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 10

400 Single-pass SHG with one Optimal Temp~81.1℃ PPMgO:LN bulk crystal [HC Photonics] 300 Size: 25.0×3.4×1.0 mm3; Poling period: Λ = 19.48 μm; [email protected] nm: 5.27 W Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 10 Appl. Sci. 2019, 9, 4942 200 4 of 9

ΔT=4℃

100400 Single-pass SHG with one Optimal Temp~81.1℃ PPMgO:LN bulk crystal [HC Photonics] SHG output power @ 780.2 nm(mW) 0300 Size: 25.0×3.4×1.0 mm3; Poling period: Λ = 19.48 μm; [email protected] 70 nm: 755.27 W 80 85 90 95 200 o

Temperature of PPMgO:LN SHG crystalΔT=4℃( C)

Figure 2. Temperature dependence100 of 780.2 nm laser power. Circles represent the experimental data. The optimal quasi-phase-matched temperature is 81.1 °C.

SHG output power @ 780.2 nm(mW) 0 The single-pass one PPMgO:LN crystal can produce 334 mW for the 780.2 nm laser, and the efficiency is 6.3%; single-pass65 two PPMgO:LN 70 75 crystals 80 can 85 produce 90 634 mW 95 for the 780.2 nm laser, o and the efficiency is 12.0%. TheseTemperature results are of PPMgO:LN shown SHGin Figurecrystal( C 3.) If we increase the injection, an additional 780.2 nm laser can be produced. Figure 2. Temperature dependence of 780.2 nm laser power. Circles represent the experimental data. Figure 2. Temperature dependence of 780.2 nm laser power. Circles represent the experimental data. The optimal quasi-phase-matched temperature is 81.1 ◦C. The optimal quasi-phase-matched temperature is 81.1 °C. 1.0 20 Crystals #A + #B Crystals #A + #B Single-pass SHG with 25 mm-long P780 ~ 634 mW Single-pass SHG with 25 mm-long η ~ 12.0 % The PPMgO:LN single-pass crystals one PPMgO:LN crystal can produce 334 mW for the 780.2 nm SHGlaser, and the 0.8 η ~ 12.0 % PPMgO:LN crystals Poling Peoriod: Λ = 19.48 μm; SHG P780 ~ 634 mW 15 Poling Peoriod: Λ = 19.48 μm; efficiency is 6.3%; single-pass o two PPMgO:LN crystals can produce 634 mW for the 780.2 nm laser, Temp. of crystal #A: 81.1 C; Temp. of crystal #A: 81.1 oC; o Temp. of crystal #B: 79.6 C Temp. of crystal #B: 79.6 oC and 0.6the efficiency is 12.0%. These results are shown in Figure 3. If we increase the injection, an

additional 780.2 nm laser can be produced. 10

0.4

5 0.2 1.0 20 (b) (a) Crystal Crystals #A #A + #B Single-passSHG Efficeincy (%) Crystals Crystal #A + #B#A Single-pass SHG with 25 mm-long P 780 P 780 ~ 334 ~ 634 mW mW Single-pass SHG with 25 mm-long η η SHGOutput Power @ 780.2 nm (W) ~ 12.0 ~ %6.3 % PPMgO:LN crystals SHG SHG 0.8 η η ~ 6.3 ~ 12.0 % % PPMgO:LN crystals 0.0 Λ μ SHGSHG 0 P780 P ~ 634 ~ 334mW mW Poling Peoriod: = 19.48 m; Poling Peoriod: Λ = 19.48 μm; 780 o 15 Temp. of crystal #A: 81.1 C; Temp. of crystal #A: 81.1 oC; 0123456o 0123456 Temp. of crystal #B: 79.6 C Temp. of crystal #B: 79.6 oC 0.6 Input Power @ 1560.5 nm (W) Input Power @ 1560.50 nm (W) 10 0.4

(a) 5 (b) 0.2 (b) (a) Crystal #A Single-passSHG Efficeincy (%) Crystal #A P780 ~ 334 mW η Appl.FigureFigure Sci.SHGOutput Power @ 780.2 nm (W) 3.2019 3.Experimental Experimental, 9, x FOR PEER data REVIEWdata for for single-pass single-pass second-harmonic second-harmonic generation generation (SHG) (SHG) output outputSHG ( a( ~)a 6.3) and %and the the5 of 10 η SHG ~ 6.3 % P ~ 334 mW frequencyfrequency0.0 doubling doubling e ﬃefficiencyciency (b ().b). The The squares squares represent represent the the case0 case using using one one PPMgO:LN PPMgO:LN bulk780 bulk crystal, crystal, 0123456 0123456 andand the the circles circles represent representInput Power the @ the 1560.5 case case nm using (W) using two two cascaded cascaded PPMgO:LN PPMgO:LN bulk bulkInput crystals. crystals. Power @ 1560.50 nm (W)

The transverse beam quality of the 780.2 nm laser is evaluated by using the beam quality factor 2 (a) (b) parameter (M ) in the two orthogonal transverse directions X and Y. Figure 4 shows the 1/e2 beam radius versusFigure 3.the Experimental axial position data Z for after single-pass passing second-harmonic through a plano-convex generation (SHG)lens with output a focal (a) and length the of 80 mm. Fittingfrequency of the doubling experimental efficiency data (b). Thegives squares 2represent = 1.05 andthe case using2 = one1.11. PPMgO:LN bulk crystal, and the circles represent the case using twoM cascadedX PPMgO:LNM bulkY crystals.

The transverse beam quality of the 780.2 nm laser is evaluated by using the beam quality factor 2 parameter (M ) in the two orthogonal transverse directions X and Y. Figure 4 shows the 1/e2 beam radius versus the axial position Z after passing through a plano-convex lens with a focal length of 80 mm. Fitting of the experimental data gives 2 = 1.05 and 2 = 1.11. M X MY

Figure 4. Figure 4. Beam quality factor M2of the SHG laser at 780.2 nm. Typical values for the horizontal 2 2 2 and verticalFigure 4. directions Beam quality are factorMX = 1.05M of and theM SHGY = 1.11.laser at Inset 780.2 shows nm. Typical the typical values intensity for the horizontal proﬁles of and the SHGvertical laser beam directions spot. are 2 = 1.05 and 2 = 1.11. Inset shows the typical intensity profiles of the M X M Y SHG laser beam spot.

We evaluate the frequency tunability of the 780.2 nm output by measuring the absorption spectra of Rb atomic vapor cells, while the 1560.5 nm fundamental laser frequency is scanned linearly. Figure 5 shows the Doppler-broadened absorption spectra of the 5S1/2–5P3/2 transition (D2 line) for 87Rb and 85Rb atoms. These spectra indicate that the continuously tunable range of the doubled laser at 780.2 nm is at least 6.8 GHz.

1.0 6.834 GHz

0.8

87Rb 0.6 87 Rb 85Rb 0.4

85Rb

Transmission Signal (Arb. Unit) (Arb. Signal Transmission 0.2 3.036 GHz 0.0 -2-1012345678 Frequency Detuning (GHz)

Figure 5. Doppler-broadened absorption spectra of 5S1/2–5P3/2 transition (D2 line) of 87Rb and 85Rb atoms by using SHG laser at 780.2 nm, while the 1560.5 nm fundamental-wave laser is frequency- scanned linearly. Clearly the continuously tunable range of the 780.2 nm laser frequency is more than 6.8 GHz.

3.2. Single-Pass Sum-Frequency Generation (SFG) to 852.3 nm Then, in a single-pass SFG experiment, we find the optimized phase matching by adjusting the temperature of the PPMgO:LN crystal. When the power of fundamental-wave 1560.5 nm and 1878.0 Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 10

Figure 4. Beam quality factor M 2 of the SHG laser at 780.2 nm. Typical values for the horizontal and vertical directions are 2 = 1.05 and 2 = 1.11. Inset shows the typical intensity profiles of the M X M Y SHG laser beam spot. Appl. Sci. 2019, 9, 4942 5 of 9 We evaluate the frequency tunability of the 780.2 nm output by measuring the absorption spectra of Rb atomic vapor cells, while the 1560.5 nm fundamental laser frequency is scanned linearly. We evaluate the frequency tunability of the 780.2 nm output by measuring the absorption spectra Figure 5 shows the Doppler-broadened absorption spectra of the 5S1/2–5P3/2 transition (D2 line) for of Rb atomic vapor cells, while the 1560.5 nm fundamental laser frequency is scanned linearly. Figure5 87Rb and 85Rb atoms. These spectra indicate that the continuously tunable range of the doubled laser shows the Doppler-broadened absorption spectra of the 5S –5P transition (D line) for 87Rb and at 780.2 nm is at least 6.8 GHz. 1/2 3/2 2 85Rb atoms. These spectra indicate that the continuously tunable range of the doubled laser at 780.2 nm is at least 6.8 GHz.

1.0 6.834 GHz

0.8

87Rb 0.6 87 Rb 85Rb 0.4

85Rb

Transmission Signal (Arb. Unit) (Arb. Signal Transmission 0.2 3.036 GHz 0.0 -2-1012345678 Frequency Detuning (GHz)

87 85 Figure 5. Doppler-broadened absorption spectra of 5S1/2–5P3/2 transition (D2 line) of Rb and Rb atomsFigure by using 5. Doppler-broadened SHG laser at 780.2 nm,absorption while the spectra 1560.5 nmof fundamental-wave5S1/2–5P3/2 transition laser (D2 is line) frequency-scanned of 87Rb and 85Rb linearly.atoms Clearlyby using the SHG continuously laser at 780.2 tunable nm, range while of the the 1560.5 780.2 nm nm laser fundamental-wave frequency is more laser than is 6.8frequency- GHz. scanned linearly. Clearly the continuously tunable range of the 780.2 nm laser frequency is more than 3.2.Appl. Single-Pass Sci.6.8 2019 GHz., 9, x Sum-Frequency FOR PEER REVIEW Generation (SFG) to 852.3 nm 6 of 10 Then, in a single-pass SFG experiment, we ﬁnd the optimized phase matching by adjusting nm lasers are both 1.0 W, the best phase matching of the PPMgO:LN-crystal is achieved at a the3.2. temperature Single-Pass ofSum-Frequency the PPMgO:LN Generation crystal. (SFG) When to the852.3 power nm of fundamental-wave 1560.5 nm and temperature of 80.6 °C. The results are shown in Figure 6. 1878.0 nmThen, lasers in a aresingle-pass both 1.0 W,SFG the experiment, best phase we matching find the of optimized the PPMgO:LN-crystal phase matching is by achieved adjusting at athe temperaturetemperature of 80.6of the◦C. PPMgO:LN The results crys aretal. shown When in the Figure power6. of fundamental-wave 1560.5 nm and 1878.0

70 Single-pass SFG with MgO:PPLN bulk crystal Optimal Temp ~ 80.6 oC 60 [HC Photonics] Size: 50.0×4.6×0.5 mm3; Λ μ 50 Poling period: = 23.4 m; Focusing lens: f = 75 mm; [email protected] nm: 1.0 W; 40 [email protected] nm: 1.0 W

ΔT~2.6oC 30

10 SFG output Power @ 852.3 nm (mW)

0 75 76 77 78 79 80 81 82 83 84 85 86 Temperature of PPMgO:LN SFG crystal (oC)

Figure 6. Temperature dependence of 852.3 nm laser power. Circles represent the experimental data.

TheFigure optimal 6. Temperature quasi-phase-matched dependence temperature of 852.3 nm is laser 80.6 ◦poweC. r. Circles represent the experimental data. The optimal quasi-phase-matched temperature is 80.6 °C. The results of SFG output power and eﬃciency are shown in Figure7. The power of the 1878.0 nm and 1560.5The results nm lasers of SFG are 1.77output W and power 4.94 and W, respectively;efficiency are 276 shown mW ofin the Figure 852.3 7. nm The laser power can beof the obtained, 1878.0 andnm theand corresponding 1560.5 nm lasers optical–optical are 1.77 W and conversion 4.94 W, re eﬃspectively;ciency is 4.1%.276 mW Fitting of the to the852.3 data nm of laser the linearcan be obtained, and the corresponding optical–optical conversion efficiency is 4.1%. Fitting to the data of the linear region yields a nonlinear conversion efficiency of 0.62% (W cm) −1. The SFG nonlinear conversion efficiency can be written as [23] p 8ω 3d 2 ()()1−δ 2 1−γ 2  η = 3 = 0 eff h()()μ,ζ sin 2 πΔ πε 4  ()+δγ  p1 p2l 0c n0n3  1  where pi (i = 1, 2, 3) represent the laser power of 1560.5, 1878.0, and 852.3 nm, respectively. no = ( n1 ω ω ω ω + n2 )/2, 0 = ( 1 + 2 )/2, i are the frequency of the corresponding laser, and deff is the effective δ = − ω ()ω +ω γ = − ()+ ()μ ζ nonlinear coefficient. 1 2 1 / 1 2 and 1 2n1 / n1 n2 , h , indicates the B– K focusing factor, which is related to the focusing parameterζ . The last term is the correct for the crystal’s non-ideal grating duty cycle. In addition, the spatial mode matching of the two fundamental beams within the crystal is the most important factor. The optimal SFG can be obtained when the confocal parameters of these two fundamental beams are equal and the focusing parameter ξ = 2.84. We also use an f = 100 mm lens to match the crystal, and the corresponding optical–optical conversion efficiency is 3.7%. From the experimental results, we can see that when the lens is 75 mm, it can yield a high-power 852.3 nm laser. According to the optimal B–K focus factor ξ = 2.84, the optimal waist spot radii of 1560.5 nm and 1878.0 nm lasers are 45.3 μm and 49.7 μm, respectively. When using a matching lens with f = 75 mm, the waist spot radii of 1560.5 nm and 1878.0 nm lasers are 41.6 μm and 46.5 μm, respectively. The radii for f = 75 mm are closer to the optimal waist spot radius than those for f = 100 mm. In a single-pass sum-frequency experiment, the matching lens is not the only factor that influences efficiency, but the overlapping of the two fundamental-wave lasers in the crystal is also important. Although an achromatic doublet can make 1560.5 nm and 1878.0 nm lasers at one point in the crystal, but it is not easy to customize. Therefore, we will also study the method of separate focusing to make the fundamental waves match well, including size and position. Appl. Sci. 2019, 9, 4942 6 of 9

1 region yields a nonlinear conversion eﬃciency of 0.62% (W cm) − . The SFG nonlinear conversion eﬃciency can be written as [23]   8ω3d2 2 2 p3 0 e f f  1 δ 1 γ  η = =  − − h(µ, ζ) sin2(π∆) 4   p1p2l πε0c n0n3  (1 + δγ) 

where pi(i = 1, 2, 3) represent the laser power of 1560.5, 1878.0, and 852.3 nm, respectively. no = (n1+n2)/2, ω0 = (ω1+ω2)/2,ωi are the frequency of the corresponding laser, and de f f is the eﬀective nonlinear coeﬃcient. δ = 1 2ω /(ω + ω2) and γ = 1 2n /(n + n2),h(µ, ζ) indicates the B–K focusing factor, Appl. Sci. 2019, 9, x FOR− PEER1 REVIEW1 − 1 1 7 of 10 which is related to the focusing parameter ζ. The last term is the correct for the crystal’s non-ideal grating duty cycle.

350 Input Power @ 1878 nm： 1.77 W 4 4.1% 300 276 mW

250 3

200

2 -1 150 η=0.62%(W cm) Single-pass SFG with 50-mm-long PPMgO:LN Crystal [HC Photonics] 100 1 Poling Period: Λ = 23.4 μm;

Single-pass SFG Efficiency (%) Temp. of Crystal: 80.6 oC; 50

Input coupling lens: f = 75 mm PowerSFG Output @ 852.3 nm (mW)

0 0 0 1000 2000 3000 4000 5000 6000 Input Power @ 1560.5 nm (mW)

Figure 7. Experimental data for single-pass SFG using a PPMgO:LN crystal with matching lens f Figure= 75 7. mm. Experimental The solid data squares for single-p representass output SFG using power a PPMgO:LN of 852.3 nm, crystal while with the matching circles represent lens f = 75 the mm.corresponding The solid SFGsquares conversion represent eﬃ ciency.output power of 852.3 nm, while the circles represent the corresponding SFG conversion efficiency. In addition, the spatial mode matching of the two fundamental beams within the crystal is the mostThe important transverse factor. beam The quality optimal of 852.3 SFG can nm be laser obtained is evaluated when the by confocalusing the parameters beam quality of these factor two fundamental2 beams are equal and the focusing parameter ξ = 2.84. We also use an f = 100 mm lens parameter (M ) in the two orthogonal transverse directions X and Y. Figure 8 shows the 1/e2 beam to match the crystal, and the corresponding optical–optical conversion eﬃciency is 3.7%. From the radius versus the axial position Z using a plano-convex lens with a focal length of 50 mm. Fitting of experimental results, we can see that when the lens is 75 mm, it can yield a high-power 852.3 nm laser. the experimental data gives 2 = 1.07 and 2 = 1.13. According to the optimal B–KM X focus factor ξM=Y2.84, the optimal waist spot radii of 1560.5 nm and 1878.0 nm lasers are 45.3 µm and 49.7 µm, respectively. When using a matching lens with f = 75 mm, the waist spot radii of 1560.5 nm and 1878.0 nm lasers are 41.6 µm and 46.5 µm, respectively. The radii for f = 75 mm are closer120 to the optimal waist spot radius than those for f = 100 mm. In a single-pass

)

sum-frequency experiment,m 110 the matching lens is not the only factor that inﬂuences eﬃciency, but the μ overlapping of the two( 100 fundamental-wave lasers in the crystal is also important. Although an achromatic doublet can make90 1560.5 nm and 1878.0 nm lasers at one point in the crystal, but it is

not easy to customize. Therefore,80 we will also study the method of separate focusing to make the

fundamental waves match70 well, including size and position. -nm beam spot spot beam -nm The transverse beam60 quality of 852.3 nm laser is evaluated by using the beam quality factor 2 2 parameter (M ) in the two50 orthogonal transverse directions X and Y. Figure8 shows the 1 /e beam

radius versus the axial position40 Z using a plano-convex lens with a focal length of 50 mm. Fitting of the experimental data gives M2 = 1.07 and M2 = 1.13. 30 X Y __ M2 = 1.07 radius of 852.3 X 2 20 .... M2 = 1.13 1/e Y 10 024681012 Z Position (mm)

Figure 8. Beam quality factor M 2 of the SFG laser beam at 852.3 nm. Typical values for the horizontal and vertical directions are 2 = 1.07 and 2 = 1.13. Inset shows the typical intensity M X MY profiles of the SFG laser beam spot.

When we fix the 1878.0 nm laser and scan the 1560.5 nm laser, we can obtain cesium absorption spectra by Cs atomic vapor cells. Then, we analyze the transmitted signal of a confocal Fabry–Perot cavity with a free spectral range (FSR) of 750 MHz while the 852.3 nm laser frequency is linearly scanned. Figure 9 shows a typical result, where the 852 nm laser can be tuned across more than 12 FSRs, which means that the continuously tunable range is at least 9.2 GHz. The scanning range is Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 10

350 Input Power @ 1878 nm： 1.77 W 4 4.1% 300 276 mW

250 3

200

2 -1 150 η=0.62%(W cm) Single-pass SFG with 50-mm-long PPMgO:LN Crystal [HC Photonics] 100 1 Poling Period: Λ = 23.4 μm;

Single-pass SFG Efficiency (%) Temp. of Crystal: 80.6 oC; 50

Input coupling lens: f = 75 mm PowerSFG Output @ 852.3 nm (mW)

0 0 0 1000 2000 3000 4000 5000 6000 Input Power @ 1560.5 nm (mW)

Figure 7. Experimental data for single-pass SFG using a PPMgO:LN crystal with matching lens f = 75 mm. The solid squares represent output power of 852.3 nm, while the circles represent the corresponding SFG conversion efficiency.

The transverse beam quality of 852.3 nm laser is evaluated by using the beam quality factor 2 parameter (M ) in the two orthogonal transverse directions X and Y. Figure 8 shows the 1/e2 beam radius versus the axial position Z using a plano-convex lens with a focal length of 50 mm. Fitting of the experimental data gives 2 = 1.07 and 2 = 1.13. M X M Y Appl. Sci. 2019, 9, 4942 7 of 9

120

)

m 110 μ ( 100

70 -nm beam spot spot beam -nm 60

40 30 __ M2 = 1.07 radius of 852.3 X 2 20 .... M2 = 1.13 1/e Y 10 024681012 Z Position (mm)

Figure 8. Beam quality factor M2 of the SFG laser beam at 852.3 nm. Typical values for the horizontal 2 2 2 andFigure vertical 8. Beam directions quality are MfactorX = 1.07 M and ofM theY = SFG1.13. laser Inset beam shows at the 852.3 typical nm. intensity Typical profilesvalues offor the the SFGhorizontal laser beam and spot. vertical directions are 2 = 1.07 and 2 = 1.13. Inset shows the typical intensity M X MY Whenprofiles we of fix the the SFG 1878.0 laser nmbeam laser spot. and scan the 1560.5 nm laser, we can obtain cesium absorption spectra by Cs atomic vapor cells. Then, we analyze the transmitted signal of a confocal Fabry–Perot Appl. Sci.When 2019 , we9, x fixFOR the PEER 1878.0 REVIEW nm laser and scan the 1560.5 nm laser, we can obtain cesium absorption8 of 10 cavity with a free spectral range (FSR) of 750 MHz while the 852.3 nm laser frequency is linearly spectra by Cs atomic vapor cells. Then, we analyze the transmitted signal of a confocal Fabry–Perot scanned. Figure9 shows a typical result, where the 852 nm laser can be tuned across more than 12 FSRs, mainlycavity withlimited a free by thespectral DFB. rangeIf the scanning(FSR) of 750range MHz of thewhile fundamental the 852.3 nmfrequency laser frequency laser is sufficiently is linearly which means that the continuously tunable range is at least 9.2 GHz. The scanning range is mainly large,scanned. the Figuresum-frequency 9 shows a852.3 typical nm result, laser canwhere also the be 852scanned nm laser following. can be Thistuned is across another more advantage than 12 limited by the DFB. If the scanning range of the fundamental frequency laser is sufficiently large, comparedFSRs, which with means cavity-enhanced that the continuously scheme. tunable range is at least 9.2 GHz. The scanning range is the sum-frequency 852.3 nm laser can also be scanned following. This is another advantage compared with cavity-enhanced scheme.

λ λ 1.2 1~ 852.356 nm 2~ 852.335 nm 9.192 GHz 1.0 1)

Cs 6S1/2 (F=3) 0.8 - 6P3/2 (F'=2,3,4)

Cs 6S1/2 (F=4) 0.6 - 6P3/2 (F'=3,4,5)

0.4 100-mm-long CFP cavity FSR = 0.75 GHz 2) 0.2 Transmission Signal (Arb. Unit) Transmission Signal

0.0 -1012345678910 Frequency Detuning (GHz)

Figure 9. The trace (1) shows the D2 line absorption spectra of 6S1/2–6P3/2 transition of cesium atoms by using SFG laser at 852.3 nm, while the 1560.5 nm fundamental-wave laser is frequency-scanned Figure 9. The trace (1) shows the D2 line absorption spectra of 6S1/2–6P3/2 transition of cesium atoms linearly,by using but SFG the laser 1878.0 at nm 852.3 fundamental-wave nm, while the 1560.5 laser’s nm frequency fundamental-wave is ﬁxed. Frequency laser is scannedfrequency-scanned 852.3 nm laserlinearly, beam but is monitored the 1878.0 usingnm fundamental-wave a confocal Fabry–Perot laser’ cavitys frequency with freeis fixed. spectral Frequency range (FSR) scanned of 0.75 852.3 GHz nm aslaser shown beam by theis monitored trace (2). Clearlyusing a the confocal continuously Fabry–Pero tunablet cavity range with of the free 852.3 spectral nm laser’s range frequency(FSR) of 0.75 is moreGHz than as shown 9.2 GHz. by the trace (2). Clearly the continuously tunable range of the 852.3 nm laser’s 4. Conclusionsfrequency is more than 9.2 GHz.

4. ConclusionsWe have implemented a simple, robust and efficient scheme that combines single-pass SHG at 780.2 nm with single-pass SFG at 852.3 nm based on diode-laser-seeded fiber amplifiers. A power of more thanWe have 630 mWimplemented of the 780 a nm simple, single-frequency robust and effici continuous-waveent scheme that laser combines is demonstrated single-pass through SHG at 780.2 nm with single-pass SFG at 852.3 nm based on diode-laser-seeded fiber amplifiers. A power of more than 630 mW of the 780 nm single-frequency continuous-wave laser is demonstrated through two PPMgO:LN cascaded bulk crystals, and the SHG efficiency obtained is 12.0%. At the same time, a power of more than 270 mW of the 852 nm single-frequency continuous-wave laser is reported with the PPMgO:LN bulk crystal, and the SFG efficiency is 4.1%. In addition, the continuously tunable ranges of the 780 nm and 852 nm laser beams are at least 6.8 GHz and 9.2 GHz. Although, the size of our device is large, its mechnical stability can be further improved by careful design with a much more compact structure. Most importantly, the output lasers have a high beam quality, which cannot be addressed by using a diode laser combined with a semiconductor tapered amplifier. In addition, we can confine Rb atoms by using an 852 nm optical dipole trap on the basis of a 780 nm magneto-optical trap. Previously, we required 852 nm and 780 nm diode lasers to perform such an experiment, but our current system can produce both lasers at high power. Finally, the system has great potential for application in the formation of RbCs dimers via photoassociation of cold Rb and Cs atoms and an Rb–Cs two-component atomic interferemeter.

Author Contributions: The experiment and data were completed by kong Zhang. Jun He undertaked guidance during the experiment. And Junmin Wang’s contributions included coordination, guidance and analysis in the experiment.

Funding: This research was funded by [the National Key R&D Program of China] grant number [2017YFA0304502], [the National Natural Science Foundation of China] grant number [11774210 and 61875111], and [the Shanxi Provincial 1331 Project for Key Subject Construction] grant number [201542033]. Appl. Sci. 2019, 9, 4942 8 of 9 two PPMgO:LN cascaded bulk crystals, and the SHG efficiency obtained is 12.0%. At the same time, a power of more than 270 mW of the 852 nm single-frequency continuous-wave laser is reported with the PPMgO:LN bulk crystal, and the SFG efficiency is 4.1%. In addition, the continuously tunable ranges of the 780 nm and 852 nm laser beams are at least 6.8 GHz and 9.2 GHz. Although, the size of our device is large, its mechnical stability can be further improved by careful design with a much more compact structure. Most importantly, the output lasers have a high beam quality, which cannot be addressed by using a diode laser combined with a semiconductor tapered amplifier. In addition, we can confine Rb atoms by using an 852 nm optical dipole trap on the basis of a 780 nm magneto-optical trap. Previously, we required 852 nm and 780 nm diode lasers to perform such an experiment, but our current system can produce both lasers at high power. Finally, the system has great potential for application in the formation of RbCs dimers via photoassociation of cold Rb and Cs atoms and an Rb–Cs two-component atomic interferemeter.

Author Contributions: The experiment and data were completed by K.Z. J.H. undertaked guidance during the experiment. And J.W. contributions included coordination, guidance and analysis in the experiment. Funding: This research was funded by [the National Key R&D Program of China] grant number [2017YFA0304502], [the National Natural Science Foundation of China] grant number [11774210 and 61875111], and [the Shanxi Provincial 1331 Project for Key Subject Construction] grant number [201542033]. Conﬂicts of Interest: There is no conﬂict of interest in this article.

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