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A theoretical and experimental investigation for wavelength switchable TmYAG laser modulated by Tm:YAG crystal length
Optics & Laser Technology 68 (2015) 18–22
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
A theoretical and experimental investigation for wavelength switchable TmYAG laser modulated by Tm:YAG crystal length Caili Wang, Yanxiong Niu n, Wenwen Liu, Haisha Niu, Bing Xu, Da Man School of Instrument Science and Optoelectronic Engineering, Beihang University, Beijing 100191, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 June 2014 Received in revised form 1 November 2014 Accepted 3 November 2014
Based on the analysis of the quasi-three-level side pumped Tm:YAG laser system, the oscillating conditions of this laser are predicted from the point of pump threshold. The laser oscillation at 2.02 mm with larger stimulated emission sections is suppressed when the crystal length is larger than 85 mm with a 5% output coupling and then an efficient side-diode-pumped rod Tm:YAG laser operating at 2.07 mm is realized. Experimental results are reported from single-module and two-modules Tm:YAG laser and compared with the predictions obtained from the model. The experimental results show that the center wavelength of the Tm:YAG laser is shifted to 2.07 μm when the Tm:YAG crystal length is switched from 69 mm to 138 mm, which is in well agreement with the predictions obtained from the model. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Lasers Quasi-three-level Wavelength switchable
1. Introduction In recent years, a lot of effort has been made on 2 mm laser systems for applications in many fields, such as radar systems, remote sensing, optical frequency conversion and medical applications [1–3]. In respect of the doping ions, singly doped thulium (Tm) and holmium (Ho), as well as co-doped Tm:Ho systems, have been investigated for this purpose [4–6]. Tm3 þ -doped materials possess an advantage over Ho3 þ -doped materials in that they can be directly pumped by the commercial 785 or 808 nm high-power diode arrays (LDs), while Ho3 þ -doped materials are usually pumped by another laser at 2 mm, usually a Tm laser [7,8]. Compared with Tm–Ho codoped crystals, the Tm-doped crystals show better performance, attributed to the reduced up-conversion mechanisms and the longer upper laser level lifetimes [9]. Among various host matrices, YAG crystal possesses the advantages of large heat conductivity and high mechanical strength, which allows high-power operation with reduced risk of fracture [10,11]. Thus, Tm:YAG material is promising material for obtaining high power lasers around 2 mm. In the Tm3 þ doped YAG crystal, the laser radiation around 2 mm takes place between the lower Stark level of 3F4 and the higher Stark level of 3H6, which include 610 cm 1 and 730 cm 1. When the ions transition from lowest Stark sub-level in 3F4 at 5556 cm 1 to 3H6 level, it results in the possible laser emissions at 2.02 μm and 2.07 μm. High power Tm:YAG lasers near 2.02 μm with larger stimulated emission sections have been reported by several groups. Honea et al. reported an 115 W continuous wave (cw) Tm:YAG laser
n
Corresponding author. E-mail address: [email protected] (Y. Niu).
http://dx.doi.org/10.1016/j.optlastec.2014.11.002 0030-3992/& 2014 Elsevier Ltd. All rights reserved.
at 2.01 μm with diode laser (LD) end-pumped structure [12]. Lai et al. achieved an 150 W cw LD side-pumped rod Tm:YAG laser at 2.02 mm under 10 1C with the mixture of water and glycerol as the coolant [13]. Cao et al. reported an cw 200 W output power in Tm: YAG laser at 2.02 mm under 8 1C water-cool environment in 2011 [14]. The rod Tm:YAG laser near 2.07 μm with smaller stimulated emission sections has been very little published. With Ti:sapphire laser pumping, Stoneman et al. reported a tunable Tm:YAG laser from 1.9 to 2.16 mm and the output powers at 2.07 mm were only 280 mW [15]. Recently, we reported wavelength switchable highpower LD side pumped rod Tm:YAG laser around 2 mm, which operated at either 2.07 or 2.02 mm depending on the transmission of output coupler, and the output power at 2.07 mm was up to 115 W [16]. Later, we reported a high-power LD side-pumped rod Tm:YAG laser at 2.07 mm with output power of 267 W [17]. Meanwhile, in some special applications such as free-space optical communication, laser lidar, long-range measurements, the laser output is required to operate at high average powers as well as minimal atmospheric attenuation. The longer wavelength near 2.07 μm has relatively higher atmospheric transmission comparing to the shorter wavelength near 2.02 μm [18,19]. Additionally, for frequency downconversion into the mid-infrared region, the longer wavelength near 2.07 μm is more suitable for pumping ZnGeP2 (ZGP) for their less absorption loss in the ZGP crystal [20,21]. The laser can be tuned without the use of a tuning element in the laser cavity, which has been reported by several groups. Esser et al. reported that the Tm:GdVO4 laser can be efficiently operated with multi-watt output at the short wavelength of 1818 nm simply by appropriate selection of the reflectivity of the output coupler [22]. Recently, we reported a theoretical model predictions and
C. Wang et al. / Optics & Laser Technology 68 (2015) 18–22
2. Theoretical analysis In the quasi-three level laser system the emission wavelength is determined not only by the laser resonator loss, but also by the amount of reabsorption loss from the ground state 3H6 (proportional to Tm concentration and crystal length). Based on the analysis for a side pumped Tm:YAG quasi-three level system, the absorbed pump power required to achieve oscillation threshold for different transitions can be expressed as [24,25] π hvp w2 þ w2 P th;i ¼ 4η f s;þ fi σ τp Li þ T i þ 2N0a;i σ i l ð Þ i i b i a;i i ¼ 0; 1
ð1Þ
where hvp is the pump photon energy, wp is the pump-beam waist and ws;i is the laser-beam waist for i transition, Li is the intrinsic cavity loss for i transition, T i is the transmittance of the output coupler for i transition, ηi is the quantum efficiency for i transition, f a;i is the fractional population in the lower laser level for i transition, f b is the fractional population in the upper laser level, σ i is the reabsorption cross section for i transition, τi is the lifetime of the upper manifold for i transition, N 0a;i is the population density in the lower laser level, and l is the length of the laser crystal. i ¼ 0 is for the 2.02 μm mode and i ¼ 1 is for the 2.07 μm mode. Owing to the same upper level with close wavelengths of the two oscillating modes in the same cavity, ws;1 , L1 , T 1 and τ1 can be considered to be equal to ws;0 , L0 , T 0 and τ0 , respectively. ws;1 ¼ ws;0 ¼ ws , L1 ¼ L0 ¼ L, T 1 ¼ T 0 ¼ T and τ1 ¼ τ0 ¼ τ: Therefore, the thresholds can be expressed as π hvp ðw2 þ w2 Þ P th;i ¼ 4τη σ fs þ fp L þ T þ 2N 0a;i σ i l bÞ i i ð a;i i ¼ 0; 1
ð2Þ
The laser was designed for side pumping with the pump beam and the laser mode of equal size ðwp ¼ ws ¼ wÞ in the crystal. The expression for the absorbed threshold power can then easily be converted to an expression of the absorbed threshold power density as follows:
P th_density;
hvp L þ T þ 2N 0a;i σ i l P th ¼ i¼ π w2 2τηi σ i f a;i þf b
ð3Þ
In order to suppress the oscillating at about 2.02 μm, the threshold for 2.07 μm laser should be lower, P th;1 o P th;0 , which
means that hvp L þ T þ 2N 0a;1 σ 1 l hvp L þ T þ 2N 0a;0 σ 0 l o ¼ P th_density;0 P th_density;1 ¼ 2τη1 σ 1 f a;1 þ f b 2τη0 σ 0 f a;0 þ f b
ð4Þ In this model, we consider that, in equilibrium, the population distribution among the various fine structure sublevels follows the Boltzmann distribution. The rod temperature is also assumed to be uniform, with 281 K used as the value for comparison to the experiment data, which correspond to cooled operation temperature set by de-ionized water around the rod Tm:YAG. Boltzmann occupation factors are calculated for the upper and lower laser levels to be f a;1 ¼ 0:0075, f a;0 ¼ 0:0138, f b ¼ 0:485. So, the ratio of the fractional population in the lower and upper laser level for i transition f a;i =f b ⪡1. If it is assumed that there are no cavity losses, then the threshold condition is f b N 1 ¼ f a;i N 0 , so the population of 3 F4 level is small compared to the total doping level, that is, the population of 3H6 ground level is not significantly reduced [25]. The value of the population density in the lower laser level can be obtained to be N 0a;i ¼ f a;i N 0 , N 0 is the concentration level of Tm3 þ , which is taken here as 4:8 1020 cm 3 for the 3.5% doping levels in Tm:YAG crystal. The reabsorption cross section for i transition is assumed to be equal to emission cross section, which can be deduced from the fluorescence spectrum measured in the experiment for Tm:YAG crystal, as shown in Fig. 1. It can be seen that the peak emission is at 2.02 μm, and the ratio of the emission cross section for the 2.07 μm and 2.02 μm mode is about σ 1 : σ 0 ¼ 1 : 2:8. The value of the emission cross section for the 2.02 μm mode is about σ 0 ¼ 2 10 21 cm2 [26], and the value of emission cross section for the 2.07 μm mode is about σ 1 ¼ 0:7 10 21 cm2. The quantum efficiency for the 2.07 μm and 2.02 μm mode is η1 ¼ 785=2070 nm ¼ 37:9%, η0 ¼ 785=2020 nm ¼ 38:9%, respectively. The lifetime of the upper manifold is τ ¼ 11 ms, and the pump wavelength is 785 nm. Taking the above known data into Eq. (3), and it is assumed that there is no intrinsic cavity loss, we can plot the absorbed threshold power density against the Tm:YAG crystal length for 2.07 μm and 2.02 μm mode with a 5% transmission output coupler, as shown in Fig. 2. It can be seen from Fig. 2 that the Tm:YAG crystal length for the Tm:YAG laser should larger than 85 mm, under this condition, the oscillation at 2.02 μm can be suppressed. Because of the intrinsic cavity loss in the Tm:YAG laser, the calculated value of the Tm:YAG crystal length for wavelength switching may be a little smaller than actual value. The dependence of the laser wavelength on the length of the Tm:YAG crystal is attributed to reabsorption loss, the longer laser crystal length, and the larger reabsorption loss. The reabsorption loss is the dominant term in determining threshold given in Eq. (3) when the laser resonator loss is small compared with the reabsorption loss, which correspond to Tm:YAG crystal length larger than 85 mm. Under this condition, the 2.07 μm mode with
Intensity (arb. units)
experimental results for a wavelength switchable Tm:YAG laser modulated by the transmission of the output coupler [23]. In this paper, we report the results of numerical modeling of the performance of wavelength switchable LD side pumped rod Tm: YAG laser around 2 mm, which is analyzed from the point of pump threshold with taking into account reabsorption loss. The laser oscillation at 2.02 mm with larger stimulated emission sections is suppressed when the Tm:YAG crystal length is larger than 85 mm with a 5% output coupling, then an efficient LD side pumped Tm: YAG laser operating at 2.07 mm is realized. Experimental results are reported from single-module and two-modules Tm:YAG laser for comparing with the predictions obtained from the model. The central wavelength of the single-module Tm:YAG laser with Tm: YAG crystal length of 69 mm is located at 2.02 μm at maximum output power of 69.7 W, while the central wavelength of twomodules Tm:YAG laser with the total Tm:YAG crystal length of 138 mm is located at 2.07 μm at maximum output power of 115 W.
19
1.0 0.8 0.6 0.4 0.2 0.01.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3
Wavelength (µm) Fig. 1. Fluorescence spectrum for Tm:YAG crystal.
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C. Wang et al. / Optics & Laser Technology 68 (2015) 18–22
much lower reabsorption loss would oscillate first. The central wavelength of the Tm:YAG laser is determined by stimulated emission cross when the Tm:YAG crystal length is smaller than or equal to 85 mm, and then the laser oscillation at 2.02 mm with larger stimulated emission sections is superior.
3. Experiment and results Experiments are performed to confirm the prediction of the model. The LD side pumped laser module consists of a Tm:YAG rod, a jet cooling sleeve, five cw diode laser arrays with the total diode-pumped power of 1200 W and five pump light reflectors. The rod Tm:YAG (doped with 3.5% Tm) used in our experiment is 4 mm in diameter and 69 mm in length, and each end-face of the rod is bonded with an 18 mm un-doped YAG, which is reported of our former work in Ref. [16]. The rod Tm:YAG is cooled to 8 1C by de-ionized water. The performance of single-module Tm:YAG laser with Tm:YAG crystal length of 69 mm is measured using a symmetrical cavity configuration, which contains rear mirror and output coupler mirror, as shown in Fig. 3. The rear mirror M1 is coated with reflectivity R499.5% at 2 μm, and output coupler mirror M2 is coated with transmission of 5% around 2 μm, which has the same value for
Pth-density (W/mm2)
110 100
P
(2.02 µm)
P
(2.07 µm)
th-density
90
th-density
80 70 60 50 40 0
20
40
60
80
100
120
2.02 mm and 2.07 mm. L1¼L2¼ 25 mm is the distance between the mirror and the laser modular. The two-modules Tm:YAG laser with Tm:YAG crystal length of 138 mm comprises two identical LD side pumped laser modules, a rear mirror M1 and an output coupler M2, as shown in Fig. 4. The rear mirror M1 is coated with reflectivity R499.5% at 2 μm, and output coupler mirror M2 is coated with transmission of 5% around 2 μm, which is the same as the single-module Tm:YAG laser experiment as discussed above. L1¼ L3¼25 mm is the distance between the mirror and the laser modular, L2¼50 mm is the distance between two laser modules. In order to design an optimum laser cavity, we measure the thermal focus length of each laser module as a function of LD pump power. The typical result is shown in Fig. 5. The thermal focus length fT decreases with the increase of LD pump power, and fT is 109 mm at pump power of 594 W. The cavity parameters are optimized by computer simulation using ABCD propagation matrix formula for the fundamental mode. Figs. 6 and 7 show the simulation results for the calculated fundamental mode beam radius at the center of laser rod of laser module as a function of thermal focal length in the single-module and twomodules Tm:YAG laser. The parameters used for the calculation are L1¼L2¼25 cm in the single-module Tm:YAG laser system, which operates in the stable region under LD pump power of 594 W. While the parameters used for the calculation are L1¼ L3¼25 cm and L2¼50 cm in the two-modules Tm:YAG laser system, and the twomodules Tm:YAG laser system also operates in the stable region under total LD pump power of 1188 W, which corresponds to a thermal focus length around 109 mm according to Fig. 5. The output power of the single-module and two-modules Tm: YAG laser is measured with a power meter (Ophir F300A-SH), as shown in Figs. 8 and 9. The single-module Tm:YAG laser delivers a output power of 69.7 W under the pump power of 594 W at a cooling temperature of 8 1C, and the slope efficiency is about 20.9%. The maximum output power of the two-modules Tm:YAG laser is up to 115 W under pump power of 1188 W at a cooling
140
The crystal length (mm) Fig. 2. The calculated Tm:YAG laser absorbed threshold power density as a function of the crystal length at 2.02 and 2.07 μm.
Fig. 3. Schematic of the single-module Tm:YAG laser around 2 mm.
Fig. 5. The thermal focus length of a single laser module versus diodepump power.
Fig. 4. Schematic of the two-modules Tm:YAG laser around 2 mm.
C. Wang et al. / Optics & Laser Technology 68 (2015) 18–22
Fig. 6. Stability-zone calculations for the fundamental mode beam radius at the center of laser rod of laser module as a function of thermal focal length in the single-module Tm:YAG laser.
Fig. 7. Stability-zone calculations for the fundamental mode beam radius at the center of laser rod of laser module 2 as a function of thermal focal length in twomodules Tm:YAG laser.
21
Fig. 10. Spectra of single-module and the two-modules Tm:YAG laser. The central wavelength is 2.02 μm and 2.07 μm, respectively.
temperature of 8 1C, and the slope efficiency is about 22.5%, which is reported in Ref. [16]. The emission wavelengths of the single-module and two-modules Tm:YAG laser are recorded with a spectrometer (NIRQuest256-2.5, Ocean Optics), as shown in Fig. 10. The central wavelength of the single-module Tm:YAG laser with Tm:YAG crystal length of 69 mm is located at 2.02 μm at maximum output power of 69.7 W, while the central wavelength of the two-modules Tm:YAG laser with the total Tm:YAG crystal length of 138 mm is located at 2.07 μm at maximum output power of 115 W. Experimental results from single-module and two-modules Tm:YAG laser are cited to verify the theoretical calculations, which show that the oscillation at 2.02 μm can be suppressed when the Tm:YAG crystal length is larger than 85 mm with a 5% transmission output coupler.
4. Conclusions
Fig. 8. Output power of the single-module Tm:YAG laser versus diode pumping power.
In this paper, we demonstrate an analysis for the performance of a wavelength switchable LD side-pumped rod Tm:YAG laser around 2 mm, whose central wavelength is switchable between 2.07 and 2.02 mm by means of different Tm:YAG crystal lengths. The oscillating conditions of this laser are theoretically analyzed from the point of pump threshold, and the laser oscillation at 2.02 mm with larger stimulated emission sections is suppressed by selecting appropriate Tm:YAG crystal length, then an efficient LD side pumped Tm:YAG laser operating at 2.07 mm is realized. Experimental results are reported from the LD side pumped Tm: YAG laser and compared with the predictions obtained from the model. The central wavelength of the single-module Tm:YAG laser with Tm:YAG crystal length of 69 mm was located at 2.02 μm at maximum output power of 69.7 W, while the central wavelength of two-modules Tm:YAG laser with the total Tm:YAG crystal length of 138 mm was located at 2.07 μm at maximum output power of 115 W. Experiments confirmed the accuracy of the model, and the model was able to accurately predict the performance of wavelength switchable side-diode-pumped Tm:YAG laser around 2 mm.
Acknowledgments
Fig. 9. Output power of the two-modules Tm:YAG laser versus diode pumping power.
This work was supported by the Innovation Foundation of BUAA for PhD Graduates. This experiment work was carried out at the Research Center for Laser Physics and Technology, Key Lab of Functional Crystal and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. The authors thank Shifeng Du for technical assistance and Dafu Cui and Qinjun Peng for helpful discussions.
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[14] Cao D, Peng Q, Du S, Xu J, Guo Y, Yang J, et al. A 200 W diode-side-pumped CW 2 μm Tm:YAG laser with water cooling at 8 1C. Appl Phys B 2011;103:83–8. [15] Stoneman RC, Esterowitz L. Efficient, broadly tunable, laser-pumped Tm:YAG and Tm:YSGG cw lasers. Opt Lett 1990;15:486–8. [16] Wang CL, Du SF, Niu YX, Wang ZC, Zhang C, Bian Q, et al. Wavelength switchable high-power diode-side-pumped rod Tm:YAG Laser around 2 mm. Opt Exp 2013;21:7156–61. [17] Wang CL, Niu YX, Du SF, Zhang C, Wang ZC, Li FQ, et al. High-power diodeside-pumped rod Tm:YAG laser at 2.07 mm. Appl Opt 2013;52:7494–7. [18] Henderson SW, Suni PJM, Hale CP, Hannon SM, Magee JR, Bruns DL, et al. Coherent laser radar at 2 mm using solid-state lasers. IEEE Trans Geosci Remote Sens 1993;31:4–7. [19] Yao BQ, Chen F, Zhang CH, Wang Q, Wu CT, Duan XM. Room temperature single-frequency output from a diode-pumped Tm,Ho:YAP laser. Opt Lett 2011;36:1554–6. [20] Petrov V, Tanaka Y, Suzuki T. Parametric generation of 1-ps pulses between 5 and 11 mm with a ZnGeP2 crystal. J Quantum Electron 1997;33:1749–55. [21] Nieuwenhuis AF, Lee CJ, van der Slot PJM, Groß P, Boller KJ. Mid-infrared ZGP optical parametric oscillator directly pumped by a lamp-pumped, Q-switched Cr,Tm,Ho:YAG laser. SPIE 2013;455 (645518-2). [22] Esser MJD, Preussler D, Bernhardi EH, Bollig C, Posewang M. Diode-end-pumped Tm:GdVO4 laser operating at 1818 and 1915 nm. Appl Phys B 2009;97:351–6. [23] Niu YX, Wang CL, Liu WW, Niu HS, Xu B, Man D. Theoretical model predictions and experimental results for a wavelength switchable Tm:YAG laser. Appl Opt 2014;53:4359–62. [24] Taira T, Tulloch WM, Byer RL. Modeling of quasi-three-level lasers and operation of cw Yb:YAG lasers. Appl Opt 1997;36:1867–74. [25] Fan TY, Byer RL. Modeling and CW operation of a quasi-three-level 946 nm Nd:YAG laser. J Quantum Electron 1987;23:605–12. [26] Koechner W. Solid-state laser engineering. (Sixth Revised and Updated Edition). Berlin: Springer; 2006 (Chap.).
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
A theoretical and experimental investigation for wavelength switchable TmYAG laser modulated by Tm:YAG crystal length Caili Wang, Yanxiong Niu n, Wenwen Liu, Haisha Niu, Bing Xu, Da Man School of Instrument Science and Optoelectronic Engineering, Beihang University, Beijing 100191, China
art ic l e i nf o
a b s t r a c t
Article history: Received 22 June 2014 Received in revised form 1 November 2014 Accepted 3 November 2014
Based on the analysis of the quasi-three-level side pumped Tm:YAG laser system, the oscillating conditions of this laser are predicted from the point of pump threshold. The laser oscillation at 2.02 mm with larger stimulated emission sections is suppressed when the crystal length is larger than 85 mm with a 5% output coupling and then an efficient side-diode-pumped rod Tm:YAG laser operating at 2.07 mm is realized. Experimental results are reported from single-module and two-modules Tm:YAG laser and compared with the predictions obtained from the model. The experimental results show that the center wavelength of the Tm:YAG laser is shifted to 2.07 μm when the Tm:YAG crystal length is switched from 69 mm to 138 mm, which is in well agreement with the predictions obtained from the model. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Lasers Quasi-three-level Wavelength switchable
1. Introduction In recent years, a lot of effort has been made on 2 mm laser systems for applications in many fields, such as radar systems, remote sensing, optical frequency conversion and medical applications [1–3]. In respect of the doping ions, singly doped thulium (Tm) and holmium (Ho), as well as co-doped Tm:Ho systems, have been investigated for this purpose [4–6]. Tm3 þ -doped materials possess an advantage over Ho3 þ -doped materials in that they can be directly pumped by the commercial 785 or 808 nm high-power diode arrays (LDs), while Ho3 þ -doped materials are usually pumped by another laser at 2 mm, usually a Tm laser [7,8]. Compared with Tm–Ho codoped crystals, the Tm-doped crystals show better performance, attributed to the reduced up-conversion mechanisms and the longer upper laser level lifetimes [9]. Among various host matrices, YAG crystal possesses the advantages of large heat conductivity and high mechanical strength, which allows high-power operation with reduced risk of fracture [10,11]. Thus, Tm:YAG material is promising material for obtaining high power lasers around 2 mm. In the Tm3 þ doped YAG crystal, the laser radiation around 2 mm takes place between the lower Stark level of 3F4 and the higher Stark level of 3H6, which include 610 cm 1 and 730 cm 1. When the ions transition from lowest Stark sub-level in 3F4 at 5556 cm 1 to 3H6 level, it results in the possible laser emissions at 2.02 μm and 2.07 μm. High power Tm:YAG lasers near 2.02 μm with larger stimulated emission sections have been reported by several groups. Honea et al. reported an 115 W continuous wave (cw) Tm:YAG laser
n
Corresponding author. E-mail address: [email protected] (Y. Niu).
http://dx.doi.org/10.1016/j.optlastec.2014.11.002 0030-3992/& 2014 Elsevier Ltd. All rights reserved.
at 2.01 μm with diode laser (LD) end-pumped structure [12]. Lai et al. achieved an 150 W cw LD side-pumped rod Tm:YAG laser at 2.02 mm under 10 1C with the mixture of water and glycerol as the coolant [13]. Cao et al. reported an cw 200 W output power in Tm: YAG laser at 2.02 mm under 8 1C water-cool environment in 2011 [14]. The rod Tm:YAG laser near 2.07 μm with smaller stimulated emission sections has been very little published. With Ti:sapphire laser pumping, Stoneman et al. reported a tunable Tm:YAG laser from 1.9 to 2.16 mm and the output powers at 2.07 mm were only 280 mW [15]. Recently, we reported wavelength switchable highpower LD side pumped rod Tm:YAG laser around 2 mm, which operated at either 2.07 or 2.02 mm depending on the transmission of output coupler, and the output power at 2.07 mm was up to 115 W [16]. Later, we reported a high-power LD side-pumped rod Tm:YAG laser at 2.07 mm with output power of 267 W [17]. Meanwhile, in some special applications such as free-space optical communication, laser lidar, long-range measurements, the laser output is required to operate at high average powers as well as minimal atmospheric attenuation. The longer wavelength near 2.07 μm has relatively higher atmospheric transmission comparing to the shorter wavelength near 2.02 μm [18,19]. Additionally, for frequency downconversion into the mid-infrared region, the longer wavelength near 2.07 μm is more suitable for pumping ZnGeP2 (ZGP) for their less absorption loss in the ZGP crystal [20,21]. The laser can be tuned without the use of a tuning element in the laser cavity, which has been reported by several groups. Esser et al. reported that the Tm:GdVO4 laser can be efficiently operated with multi-watt output at the short wavelength of 1818 nm simply by appropriate selection of the reflectivity of the output coupler [22]. Recently, we reported a theoretical model predictions and
C. Wang et al. / Optics & Laser Technology 68 (2015) 18–22
2. Theoretical analysis In the quasi-three level laser system the emission wavelength is determined not only by the laser resonator loss, but also by the amount of reabsorption loss from the ground state 3H6 (proportional to Tm concentration and crystal length). Based on the analysis for a side pumped Tm:YAG quasi-three level system, the absorbed pump power required to achieve oscillation threshold for different transitions can be expressed as [24,25] π hvp w2 þ w2 P th;i ¼ 4η f s;þ fi σ τp Li þ T i þ 2N0a;i σ i l ð Þ i i b i a;i i ¼ 0; 1
ð1Þ
where hvp is the pump photon energy, wp is the pump-beam waist and ws;i is the laser-beam waist for i transition, Li is the intrinsic cavity loss for i transition, T i is the transmittance of the output coupler for i transition, ηi is the quantum efficiency for i transition, f a;i is the fractional population in the lower laser level for i transition, f b is the fractional population in the upper laser level, σ i is the reabsorption cross section for i transition, τi is the lifetime of the upper manifold for i transition, N 0a;i is the population density in the lower laser level, and l is the length of the laser crystal. i ¼ 0 is for the 2.02 μm mode and i ¼ 1 is for the 2.07 μm mode. Owing to the same upper level with close wavelengths of the two oscillating modes in the same cavity, ws;1 , L1 , T 1 and τ1 can be considered to be equal to ws;0 , L0 , T 0 and τ0 , respectively. ws;1 ¼ ws;0 ¼ ws , L1 ¼ L0 ¼ L, T 1 ¼ T 0 ¼ T and τ1 ¼ τ0 ¼ τ: Therefore, the thresholds can be expressed as π hvp ðw2 þ w2 Þ P th;i ¼ 4τη σ fs þ fp L þ T þ 2N 0a;i σ i l bÞ i i ð a;i i ¼ 0; 1
ð2Þ
The laser was designed for side pumping with the pump beam and the laser mode of equal size ðwp ¼ ws ¼ wÞ in the crystal. The expression for the absorbed threshold power can then easily be converted to an expression of the absorbed threshold power density as follows:
P th_density;
hvp L þ T þ 2N 0a;i σ i l P th ¼ i¼ π w2 2τηi σ i f a;i þf b
ð3Þ
In order to suppress the oscillating at about 2.02 μm, the threshold for 2.07 μm laser should be lower, P th;1 o P th;0 , which
means that hvp L þ T þ 2N 0a;1 σ 1 l hvp L þ T þ 2N 0a;0 σ 0 l o ¼ P th_density;0 P th_density;1 ¼ 2τη1 σ 1 f a;1 þ f b 2τη0 σ 0 f a;0 þ f b
ð4Þ In this model, we consider that, in equilibrium, the population distribution among the various fine structure sublevels follows the Boltzmann distribution. The rod temperature is also assumed to be uniform, with 281 K used as the value for comparison to the experiment data, which correspond to cooled operation temperature set by de-ionized water around the rod Tm:YAG. Boltzmann occupation factors are calculated for the upper and lower laser levels to be f a;1 ¼ 0:0075, f a;0 ¼ 0:0138, f b ¼ 0:485. So, the ratio of the fractional population in the lower and upper laser level for i transition f a;i =f b ⪡1. If it is assumed that there are no cavity losses, then the threshold condition is f b N 1 ¼ f a;i N 0 , so the population of 3 F4 level is small compared to the total doping level, that is, the population of 3H6 ground level is not significantly reduced [25]. The value of the population density in the lower laser level can be obtained to be N 0a;i ¼ f a;i N 0 , N 0 is the concentration level of Tm3 þ , which is taken here as 4:8 1020 cm 3 for the 3.5% doping levels in Tm:YAG crystal. The reabsorption cross section for i transition is assumed to be equal to emission cross section, which can be deduced from the fluorescence spectrum measured in the experiment for Tm:YAG crystal, as shown in Fig. 1. It can be seen that the peak emission is at 2.02 μm, and the ratio of the emission cross section for the 2.07 μm and 2.02 μm mode is about σ 1 : σ 0 ¼ 1 : 2:8. The value of the emission cross section for the 2.02 μm mode is about σ 0 ¼ 2 10 21 cm2 [26], and the value of emission cross section for the 2.07 μm mode is about σ 1 ¼ 0:7 10 21 cm2. The quantum efficiency for the 2.07 μm and 2.02 μm mode is η1 ¼ 785=2070 nm ¼ 37:9%, η0 ¼ 785=2020 nm ¼ 38:9%, respectively. The lifetime of the upper manifold is τ ¼ 11 ms, and the pump wavelength is 785 nm. Taking the above known data into Eq. (3), and it is assumed that there is no intrinsic cavity loss, we can plot the absorbed threshold power density against the Tm:YAG crystal length for 2.07 μm and 2.02 μm mode with a 5% transmission output coupler, as shown in Fig. 2. It can be seen from Fig. 2 that the Tm:YAG crystal length for the Tm:YAG laser should larger than 85 mm, under this condition, the oscillation at 2.02 μm can be suppressed. Because of the intrinsic cavity loss in the Tm:YAG laser, the calculated value of the Tm:YAG crystal length for wavelength switching may be a little smaller than actual value. The dependence of the laser wavelength on the length of the Tm:YAG crystal is attributed to reabsorption loss, the longer laser crystal length, and the larger reabsorption loss. The reabsorption loss is the dominant term in determining threshold given in Eq. (3) when the laser resonator loss is small compared with the reabsorption loss, which correspond to Tm:YAG crystal length larger than 85 mm. Under this condition, the 2.07 μm mode with
Intensity (arb. units)
experimental results for a wavelength switchable Tm:YAG laser modulated by the transmission of the output coupler [23]. In this paper, we report the results of numerical modeling of the performance of wavelength switchable LD side pumped rod Tm: YAG laser around 2 mm, which is analyzed from the point of pump threshold with taking into account reabsorption loss. The laser oscillation at 2.02 mm with larger stimulated emission sections is suppressed when the Tm:YAG crystal length is larger than 85 mm with a 5% output coupling, then an efficient LD side pumped Tm: YAG laser operating at 2.07 mm is realized. Experimental results are reported from single-module and two-modules Tm:YAG laser for comparing with the predictions obtained from the model. The central wavelength of the single-module Tm:YAG laser with Tm: YAG crystal length of 69 mm is located at 2.02 μm at maximum output power of 69.7 W, while the central wavelength of twomodules Tm:YAG laser with the total Tm:YAG crystal length of 138 mm is located at 2.07 μm at maximum output power of 115 W.
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1.0 0.8 0.6 0.4 0.2 0.01.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3
Wavelength (µm) Fig. 1. Fluorescence spectrum for Tm:YAG crystal.
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much lower reabsorption loss would oscillate first. The central wavelength of the Tm:YAG laser is determined by stimulated emission cross when the Tm:YAG crystal length is smaller than or equal to 85 mm, and then the laser oscillation at 2.02 mm with larger stimulated emission sections is superior.
3. Experiment and results Experiments are performed to confirm the prediction of the model. The LD side pumped laser module consists of a Tm:YAG rod, a jet cooling sleeve, five cw diode laser arrays with the total diode-pumped power of 1200 W and five pump light reflectors. The rod Tm:YAG (doped with 3.5% Tm) used in our experiment is 4 mm in diameter and 69 mm in length, and each end-face of the rod is bonded with an 18 mm un-doped YAG, which is reported of our former work in Ref. [16]. The rod Tm:YAG is cooled to 8 1C by de-ionized water. The performance of single-module Tm:YAG laser with Tm:YAG crystal length of 69 mm is measured using a symmetrical cavity configuration, which contains rear mirror and output coupler mirror, as shown in Fig. 3. The rear mirror M1 is coated with reflectivity R499.5% at 2 μm, and output coupler mirror M2 is coated with transmission of 5% around 2 μm, which has the same value for
Pth-density (W/mm2)
110 100
P
(2.02 µm)
P
(2.07 µm)
th-density
90
th-density
80 70 60 50 40 0
20
40
60
80
100
120
2.02 mm and 2.07 mm. L1¼L2¼ 25 mm is the distance between the mirror and the laser modular. The two-modules Tm:YAG laser with Tm:YAG crystal length of 138 mm comprises two identical LD side pumped laser modules, a rear mirror M1 and an output coupler M2, as shown in Fig. 4. The rear mirror M1 is coated with reflectivity R499.5% at 2 μm, and output coupler mirror M2 is coated with transmission of 5% around 2 μm, which is the same as the single-module Tm:YAG laser experiment as discussed above. L1¼ L3¼25 mm is the distance between the mirror and the laser modular, L2¼50 mm is the distance between two laser modules. In order to design an optimum laser cavity, we measure the thermal focus length of each laser module as a function of LD pump power. The typical result is shown in Fig. 5. The thermal focus length fT decreases with the increase of LD pump power, and fT is 109 mm at pump power of 594 W. The cavity parameters are optimized by computer simulation using ABCD propagation matrix formula for the fundamental mode. Figs. 6 and 7 show the simulation results for the calculated fundamental mode beam radius at the center of laser rod of laser module as a function of thermal focal length in the single-module and twomodules Tm:YAG laser. The parameters used for the calculation are L1¼L2¼25 cm in the single-module Tm:YAG laser system, which operates in the stable region under LD pump power of 594 W. While the parameters used for the calculation are L1¼ L3¼25 cm and L2¼50 cm in the two-modules Tm:YAG laser system, and the twomodules Tm:YAG laser system also operates in the stable region under total LD pump power of 1188 W, which corresponds to a thermal focus length around 109 mm according to Fig. 5. The output power of the single-module and two-modules Tm: YAG laser is measured with a power meter (Ophir F300A-SH), as shown in Figs. 8 and 9. The single-module Tm:YAG laser delivers a output power of 69.7 W under the pump power of 594 W at a cooling temperature of 8 1C, and the slope efficiency is about 20.9%. The maximum output power of the two-modules Tm:YAG laser is up to 115 W under pump power of 1188 W at a cooling
140
The crystal length (mm) Fig. 2. The calculated Tm:YAG laser absorbed threshold power density as a function of the crystal length at 2.02 and 2.07 μm.
Fig. 3. Schematic of the single-module Tm:YAG laser around 2 mm.
Fig. 5. The thermal focus length of a single laser module versus diodepump power.
Fig. 4. Schematic of the two-modules Tm:YAG laser around 2 mm.
C. Wang et al. / Optics & Laser Technology 68 (2015) 18–22
Fig. 6. Stability-zone calculations for the fundamental mode beam radius at the center of laser rod of laser module as a function of thermal focal length in the single-module Tm:YAG laser.
Fig. 7. Stability-zone calculations for the fundamental mode beam radius at the center of laser rod of laser module 2 as a function of thermal focal length in twomodules Tm:YAG laser.
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Fig. 10. Spectra of single-module and the two-modules Tm:YAG laser. The central wavelength is 2.02 μm and 2.07 μm, respectively.
temperature of 8 1C, and the slope efficiency is about 22.5%, which is reported in Ref. [16]. The emission wavelengths of the single-module and two-modules Tm:YAG laser are recorded with a spectrometer (NIRQuest256-2.5, Ocean Optics), as shown in Fig. 10. The central wavelength of the single-module Tm:YAG laser with Tm:YAG crystal length of 69 mm is located at 2.02 μm at maximum output power of 69.7 W, while the central wavelength of the two-modules Tm:YAG laser with the total Tm:YAG crystal length of 138 mm is located at 2.07 μm at maximum output power of 115 W. Experimental results from single-module and two-modules Tm:YAG laser are cited to verify the theoretical calculations, which show that the oscillation at 2.02 μm can be suppressed when the Tm:YAG crystal length is larger than 85 mm with a 5% transmission output coupler.
4. Conclusions
Fig. 8. Output power of the single-module Tm:YAG laser versus diode pumping power.
In this paper, we demonstrate an analysis for the performance of a wavelength switchable LD side-pumped rod Tm:YAG laser around 2 mm, whose central wavelength is switchable between 2.07 and 2.02 mm by means of different Tm:YAG crystal lengths. The oscillating conditions of this laser are theoretically analyzed from the point of pump threshold, and the laser oscillation at 2.02 mm with larger stimulated emission sections is suppressed by selecting appropriate Tm:YAG crystal length, then an efficient LD side pumped Tm:YAG laser operating at 2.07 mm is realized. Experimental results are reported from the LD side pumped Tm: YAG laser and compared with the predictions obtained from the model. The central wavelength of the single-module Tm:YAG laser with Tm:YAG crystal length of 69 mm was located at 2.02 μm at maximum output power of 69.7 W, while the central wavelength of two-modules Tm:YAG laser with the total Tm:YAG crystal length of 138 mm was located at 2.07 μm at maximum output power of 115 W. Experiments confirmed the accuracy of the model, and the model was able to accurately predict the performance of wavelength switchable side-diode-pumped Tm:YAG laser around 2 mm.
Acknowledgments
Fig. 9. Output power of the two-modules Tm:YAG laser versus diode pumping power.
This work was supported by the Innovation Foundation of BUAA for PhD Graduates. This experiment work was carried out at the Research Center for Laser Physics and Technology, Key Lab of Functional Crystal and Laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. The authors thank Shifeng Du for technical assistance and Dafu Cui and Qinjun Peng for helpful discussions.
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