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Tunable diode pumped Rb laser with single longitudinal and transverse mode operation
Optics Communications 357 (2015) 67–70
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Tunable diode pumped Rb laser with single longitudinal and transverse mode operation Yunfei Li a,b, Weihong Hua a,b, Zining Yang a,b,n, Hongyan Wang a,b, Xiaojun Xu a,b a b
College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, China Interdisciplinary Center of Quantum Information, National University of Defense Technology, Changsha 410073, China
art ic l e i nf o
a b s t r a c t
Article history: Received 24 April 2015 Received in revised form 25 July 2015 Accepted 28 July 2015
In this paper, a tunable single frequency Rb laser with TEM00 mode is realized. The linewidth is less than 1 GHz, with tuning range of 5 GHz, and central wavelength drift of less than 95 MHz (0.2 pm). The laser power is 1.4 W, with power fluctuation of less than 2%. Such kinds of tunable DPALs could be potential in applications for laser spectroscopy and quantum optics etc. & 2015 Elsevier B.V. All rights reserved.
Keywords: DPAL Rubidium Etalon Diode pumped
1. Introduction As a hybrid kind of optically pumped gaseous laser, diode pumped alkali lasers (DPALs) show great potential for extremely high power operation due to their many advantages, such as the high quantum efficiency, the feasibility of electrically driven diode pumping, the intrinsic capacity of high efficient convective thermal management, and the lightweight and compact configuration etc. Till now, a great number of DPALs have demonstrated high efficiency and good beam quality [1–9]. A Cesium DPAL with 1 kW output power and 48% optical conversion efficiency has been demonstrated in 2012 [10]. However, in power scaling of DPALs, some challenges also exist, such as the damage of optics windows and films which are exposed to alkali vapors [11]. To solve such problems, new technologies were adopted, for example by using the sapphire window with AR microstructures instead of traditional AR films [12] etc. DPALs not only have shown potential for high power operation, but also are suitable in applications such as spin exchange optical pumping (SEOP), laser spectroscopy and quantum optics etc., which require a narrow linewidth tunable laser source. Zhdanov et al. have realized a tunable single frequency cesium laser with output power of 80 mW and tuning n Corresponding author at: College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, China. E-mail address: [email protected] (Z. Yang).
http://dx.doi.org/10.1016/j.optcom.2015.07.078 0030-4018/& 2015 Elsevier B.V. All rights reserved.
range of about 14 GHz [13]. In this paper, we present our experimental results of a tunable Rb vapor laser at 795 nm operating in single longitude and TEM00 transverse mode.
2. Experimental setup of a tunable Rb DPAL A schematic of the experimental setup is shown in Fig. 1. The pump source is a volume Bragg grating (VBG) coupled laser diode array (LDA). The central wavelength is tuned to 780.2 nm (Rb D2line) and the line-width is narrowed to 0.2 nm (FWHM) [14,15]. A half-wave plate (λ/2) and a polarization beam splitter (PBS) are used to adjust the incident pump power. The pump light is focused into the Rb cell by using a spherical lens. The pump intensity at the focal point in the cell is about 2.3 kW/cm2. The Rb cell contains buffer gases of 200 Torr ethane and 300 Torr helium (room temperature). It is placed in an oven and heated to 120 °C. To avoid the thermal effect in such static Rb cell, the pump light was chopped to decrease the average power level. Under this condition, the absorption line-width which is collisionally broadened by buffer gases is Δν0 ¼ 10 GHz (FWHM) [16,17]. The resonator consists of a concave mirror (R¼99%) as back reflector, and a plane mirror (R¼ 70%) as output coupler. The optical length of the resonator is L ¼40.96 cm, which determines an adjacent longitudinal mode separation of Δν ¼365 MHz (Fig. 2).
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Y. Li et al. / Optics Communications 357 (2015) 67–70
An etalon with a free spectral range (FSR) of ΔνFSR ¼ 21 GHz and a finesse of F¼ 30 was used, inclined at an angle θ to the resonator axis (Fig. 1). The etalon’s transmitted linewidth is
existence of beat signal indicates that the laser is in multimode operation. And when the etalon is in place, the absence of beat signal indicates a single longitudinal mode operation.
Fig. 1. Schematic of the tunable Rb DPAL. The yellow line represents the pump light, and the red line represents the laser light (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
Fig. 2. Longitudinal mode selection by using a transmission Fabry–Perot etalon The gain linewidth Δν0 is 10 GHz, the FP transmitted linewidth Δνc is 700 MHz, the adjacent longitude mode separation Δν is 365 MHz.
Δνc ¼700 MHz. The parameters satisfies the relation of ΔνFSR 4 1/2Δν0 and Δν 41/2Δνc, which theoretically determines the single longitudinal mode operation of the laser [18].
3. Results and analysis Measurements of the laser’s linewidth and tuning range are conducted by using a FP scanning interferometer with a FSR of 10 GHz. Fig. 3 shows the measured longitudinal mode characteristics with (a) and without (b) etalon. We can see that, with the etalon, the laser operates in a single longitudinal mode with linewidth of less than 1 GHz (FWHM), and when the etalon is removed, the laser operates in multiple modes with the envelope’s linewidth is 5 GHz. A much more precise measurement of the linewidth of single longitudinal mode needs a FP scanning interferometer with much smaller FSR and high finesse. For a further verification of the single longitudinal mode operation, the laser is detected by photodiode to observe the beat note signal. Fig. 4 shows the results in oscilloscope, when there is no etalon in the cavity (a) and when it is in place (b). The frequency of the beat signal was about 357 MHZ, which is in agreement with the longitudinal mode separation ( 365 MHz). So the
The laser wavelength can be tuned by rotating the etalon. Fig. 5 shows the tuning range. Each curve (the envelope) represents a different mode frequency which corresponds to a different etalon angle. The central wavelength of the Rb laser could be tuned from 794.980 nm to 794.990 nm by rotating the etalon, which corresponds to a 5 GHz tuning range. It could be seen that, the tuning range ( 5 GHz) is smaller than the atomic emission spectral width (D1 line), which is 10 GHz (FWHM) that broadened by the buffer gases. The reason of the relatively small tuning range is mainly due to the large loss of the resonator, which has a single-pass loss as high as 16% mainly because of the uncoated alkali cell windows. The pump intensity is also relatively small, which is 2.3 kW/cm2 at the pump focus. So the laser operates near the threshold. For the three-level optically pumped laser, it is not reliable to estimate the gain by using the small signal gain, because the lasing process will dramatically enhance the pump absorption, which will be much larger than the small signal pump absorption without lasing. For this reason, we estimate the steady state gain coefficient of 0.31 cm 1 (for all the realized tuning frequencies), under which condition the gain is equal to the loss. For a further tuning frequency, due to the decrease of stimulated emission cross section s21 (λ), the lasing process will not occur. For a further increase of the tuning range, the resonator loss should be
Y. Li et al. / Optics Communications 357 (2015) 67–70
69
Fig. 3. The signal of output laser detected by the interferometer when the etalon was in the cavity (a) and when it was removed (b).
Fig. 4. Beat signal detection of the laser by using photodiode and oscilloscope.
Fig. 5. Wavelength tuning of the single frequency Rb laser.
reduced by using AR windows and the pump intensity should be enhanced. Larger buffer gas pressure will be benefit for the broadening of the atomic emission spectral width as well as the tuning range, but also increase the requirement of higher pump intensity, which should be balanced in laser design.
Fig. 6 shows the stabilities of the averaged laser power (a) and central wavelength (b). The power drift is less than 2%, and the central wavelength drift is 95 MHz. The laser operates in a TEM00 mode, see Fig. 7. Which is also required in many applications. The M2 value of the laser beam was
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Y. Li et al. / Optics Communications 357 (2015) 67–70
4. Conclusion In this paper, a tunable single frequency Rb laser with TEM00 mode is realized. The linewidth of the output laser is less than 1 GHz, the tuning range is about 5 GHz, and the central wavelength drift is 95 MHz(0.2 pm). The laser power is 1.4 W with power fluctuation of less than 2%. In the next step, by using FP interferometer with proper FSR and finesse, we will measure the linewidth of the output laser much more precisely. And we will further improve the power and wavelength stability by an optimal design. This method provides a way to realize high power single frequency lasers in IR region. For example, by increasing the buffer gas pressure and pump intensity, the tuning range could be further improved, and by adopting active thermal management, the averaged power could be largely increased. The tunable DPAL could be potential in applications such as SEOP, laser spectroscopy and quantum optics etc.
Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 11272343).
Reference
Fig. 6. Power and spectrum stability measurement.
Fig. 7. Transverse mode characteristics of the output laser.
1.305 in horizontal direction and 1.389 in vertical direction. The beam profiler and M2 factor both suggest a TEM00 mode operation.
[1] W.F. Krupke, R.J. Beach, V.K. Kanz, et al., Opt. Lett. 28 (23) (2003) 2336. [2] R.J. Beach, W.F. Krupke, V.K. Kanz, S.A. Payne, M.A. Dubinskii, L.D.J. Merkle, Opt. Soc. Am. B 21 (12) (2004) 2151. [3] B. Zhdanov, R.J. Knize, Opt. Lett. 32 (15) (2007) 2167. [4] B.V. Zhdanov, R.J. Knize, Opt. Eng. 52 (2) (2013) 021010. [5] K. Waichman, B.D. Barmashenko, S. Rosenwaks, J. Opt. Sci. Am. B 31 (11) (2014) 2628. [6] Z. Li, R. Tan, W. Huang, et al., Opt. Eng. 53 (11) (2014) 116113. [7] Z. Yang, H. Wang, Q. Lu, et al., J. Opt. Soc. Am. B 28 (6) (2011) 1353. [8] Y. Zheng, M. Niigaki, H. Miyajima, et al., Appl. Phys. Express 2 (3) (2009) 032501. [9] J. Zweiback, A. Komashko, Int. Soc. Opt. Photonics (2011) 791509. [10] A.V. Bogachev, S.G. Garanin, A.M. Dudov, V.A. Yeroshenko, S.M. Kulikov, G. T. Mikaelian, V.A. Panarin, V.O. Pautov, A.V. Rus, S.A. Sukharev, Quantum Electron 42 (2) (2012) 95. [11] Lindsay O.’Brien Quarrie, Opt. Mater. 35 (5) (2013) 843. [12] Hobbs, Douglas S. SPIE Defense, Security, and Sensing. International Society for Optics and Photonics, 2009. [13] B.V. Zhdanov, F. Kontur, S. Phipps, et al., Opt. Commun. 280 (1) (2007) 161. [14] Z. Li, R. Tan, C. Xu, et al., Quantum Electron. 43 (2) (2013) 147. [15] Y. Zi-Ning, W. Hong-Yan, L. Qi-Sheng, et al., Chin. Phys. Lett. 28 (10) (2011) 104202. [16] M.D. Rotondaro, G.P. Perram, J. Quant. Spectrosc. Radiat. Transf. 57 (4) (1997) 497. [17] N.D. Zameroski, G.D. Hager, W. Rudolph, et al., J. Quant. Spectrosc. Radiat. Transf. 112 (1) (2011) 59. [18] OrazioSvelto, Principles of Lasers. New York, Springer, p. 292, 2010.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Tunable diode pumped Rb laser with single longitudinal and transverse mode operation Yunfei Li a,b, Weihong Hua a,b, Zining Yang a,b,n, Hongyan Wang a,b, Xiaojun Xu a,b a b
College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, China Interdisciplinary Center of Quantum Information, National University of Defense Technology, Changsha 410073, China
art ic l e i nf o
a b s t r a c t
Article history: Received 24 April 2015 Received in revised form 25 July 2015 Accepted 28 July 2015
In this paper, a tunable single frequency Rb laser with TEM00 mode is realized. The linewidth is less than 1 GHz, with tuning range of 5 GHz, and central wavelength drift of less than 95 MHz (0.2 pm). The laser power is 1.4 W, with power fluctuation of less than 2%. Such kinds of tunable DPALs could be potential in applications for laser spectroscopy and quantum optics etc. & 2015 Elsevier B.V. All rights reserved.
Keywords: DPAL Rubidium Etalon Diode pumped
1. Introduction As a hybrid kind of optically pumped gaseous laser, diode pumped alkali lasers (DPALs) show great potential for extremely high power operation due to their many advantages, such as the high quantum efficiency, the feasibility of electrically driven diode pumping, the intrinsic capacity of high efficient convective thermal management, and the lightweight and compact configuration etc. Till now, a great number of DPALs have demonstrated high efficiency and good beam quality [1–9]. A Cesium DPAL with 1 kW output power and 48% optical conversion efficiency has been demonstrated in 2012 [10]. However, in power scaling of DPALs, some challenges also exist, such as the damage of optics windows and films which are exposed to alkali vapors [11]. To solve such problems, new technologies were adopted, for example by using the sapphire window with AR microstructures instead of traditional AR films [12] etc. DPALs not only have shown potential for high power operation, but also are suitable in applications such as spin exchange optical pumping (SEOP), laser spectroscopy and quantum optics etc., which require a narrow linewidth tunable laser source. Zhdanov et al. have realized a tunable single frequency cesium laser with output power of 80 mW and tuning n Corresponding author at: College of Optoelectronic Science and Engineering, National University of Defense Technology, Changsha 410073, China. E-mail address: [email protected] (Z. Yang).
http://dx.doi.org/10.1016/j.optcom.2015.07.078 0030-4018/& 2015 Elsevier B.V. All rights reserved.
range of about 14 GHz [13]. In this paper, we present our experimental results of a tunable Rb vapor laser at 795 nm operating in single longitude and TEM00 transverse mode.
2. Experimental setup of a tunable Rb DPAL A schematic of the experimental setup is shown in Fig. 1. The pump source is a volume Bragg grating (VBG) coupled laser diode array (LDA). The central wavelength is tuned to 780.2 nm (Rb D2line) and the line-width is narrowed to 0.2 nm (FWHM) [14,15]. A half-wave plate (λ/2) and a polarization beam splitter (PBS) are used to adjust the incident pump power. The pump light is focused into the Rb cell by using a spherical lens. The pump intensity at the focal point in the cell is about 2.3 kW/cm2. The Rb cell contains buffer gases of 200 Torr ethane and 300 Torr helium (room temperature). It is placed in an oven and heated to 120 °C. To avoid the thermal effect in such static Rb cell, the pump light was chopped to decrease the average power level. Under this condition, the absorption line-width which is collisionally broadened by buffer gases is Δν0 ¼ 10 GHz (FWHM) [16,17]. The resonator consists of a concave mirror (R¼99%) as back reflector, and a plane mirror (R¼ 70%) as output coupler. The optical length of the resonator is L ¼40.96 cm, which determines an adjacent longitudinal mode separation of Δν ¼365 MHz (Fig. 2).
68
Y. Li et al. / Optics Communications 357 (2015) 67–70
An etalon with a free spectral range (FSR) of ΔνFSR ¼ 21 GHz and a finesse of F¼ 30 was used, inclined at an angle θ to the resonator axis (Fig. 1). The etalon’s transmitted linewidth is
existence of beat signal indicates that the laser is in multimode operation. And when the etalon is in place, the absence of beat signal indicates a single longitudinal mode operation.
Fig. 1. Schematic of the tunable Rb DPAL. The yellow line represents the pump light, and the red line represents the laser light (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
Fig. 2. Longitudinal mode selection by using a transmission Fabry–Perot etalon The gain linewidth Δν0 is 10 GHz, the FP transmitted linewidth Δνc is 700 MHz, the adjacent longitude mode separation Δν is 365 MHz.
Δνc ¼700 MHz. The parameters satisfies the relation of ΔνFSR 4 1/2Δν0 and Δν 41/2Δνc, which theoretically determines the single longitudinal mode operation of the laser [18].
3. Results and analysis Measurements of the laser’s linewidth and tuning range are conducted by using a FP scanning interferometer with a FSR of 10 GHz. Fig. 3 shows the measured longitudinal mode characteristics with (a) and without (b) etalon. We can see that, with the etalon, the laser operates in a single longitudinal mode with linewidth of less than 1 GHz (FWHM), and when the etalon is removed, the laser operates in multiple modes with the envelope’s linewidth is 5 GHz. A much more precise measurement of the linewidth of single longitudinal mode needs a FP scanning interferometer with much smaller FSR and high finesse. For a further verification of the single longitudinal mode operation, the laser is detected by photodiode to observe the beat note signal. Fig. 4 shows the results in oscilloscope, when there is no etalon in the cavity (a) and when it is in place (b). The frequency of the beat signal was about 357 MHZ, which is in agreement with the longitudinal mode separation ( 365 MHz). So the
The laser wavelength can be tuned by rotating the etalon. Fig. 5 shows the tuning range. Each curve (the envelope) represents a different mode frequency which corresponds to a different etalon angle. The central wavelength of the Rb laser could be tuned from 794.980 nm to 794.990 nm by rotating the etalon, which corresponds to a 5 GHz tuning range. It could be seen that, the tuning range ( 5 GHz) is smaller than the atomic emission spectral width (D1 line), which is 10 GHz (FWHM) that broadened by the buffer gases. The reason of the relatively small tuning range is mainly due to the large loss of the resonator, which has a single-pass loss as high as 16% mainly because of the uncoated alkali cell windows. The pump intensity is also relatively small, which is 2.3 kW/cm2 at the pump focus. So the laser operates near the threshold. For the three-level optically pumped laser, it is not reliable to estimate the gain by using the small signal gain, because the lasing process will dramatically enhance the pump absorption, which will be much larger than the small signal pump absorption without lasing. For this reason, we estimate the steady state gain coefficient of 0.31 cm 1 (for all the realized tuning frequencies), under which condition the gain is equal to the loss. For a further tuning frequency, due to the decrease of stimulated emission cross section s21 (λ), the lasing process will not occur. For a further increase of the tuning range, the resonator loss should be
Y. Li et al. / Optics Communications 357 (2015) 67–70
69
Fig. 3. The signal of output laser detected by the interferometer when the etalon was in the cavity (a) and when it was removed (b).
Fig. 4. Beat signal detection of the laser by using photodiode and oscilloscope.
Fig. 5. Wavelength tuning of the single frequency Rb laser.
reduced by using AR windows and the pump intensity should be enhanced. Larger buffer gas pressure will be benefit for the broadening of the atomic emission spectral width as well as the tuning range, but also increase the requirement of higher pump intensity, which should be balanced in laser design.
Fig. 6 shows the stabilities of the averaged laser power (a) and central wavelength (b). The power drift is less than 2%, and the central wavelength drift is 95 MHz. The laser operates in a TEM00 mode, see Fig. 7. Which is also required in many applications. The M2 value of the laser beam was
70
Y. Li et al. / Optics Communications 357 (2015) 67–70
4. Conclusion In this paper, a tunable single frequency Rb laser with TEM00 mode is realized. The linewidth of the output laser is less than 1 GHz, the tuning range is about 5 GHz, and the central wavelength drift is 95 MHz(0.2 pm). The laser power is 1.4 W with power fluctuation of less than 2%. In the next step, by using FP interferometer with proper FSR and finesse, we will measure the linewidth of the output laser much more precisely. And we will further improve the power and wavelength stability by an optimal design. This method provides a way to realize high power single frequency lasers in IR region. For example, by increasing the buffer gas pressure and pump intensity, the tuning range could be further improved, and by adopting active thermal management, the averaged power could be largely increased. The tunable DPAL could be potential in applications such as SEOP, laser spectroscopy and quantum optics etc.
Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 11272343).
Reference
Fig. 6. Power and spectrum stability measurement.
Fig. 7. Transverse mode characteristics of the output laser.
1.305 in horizontal direction and 1.389 in vertical direction. The beam profiler and M2 factor both suggest a TEM00 mode operation.
[1] W.F. Krupke, R.J. Beach, V.K. Kanz, et al., Opt. Lett. 28 (23) (2003) 2336. [2] R.J. Beach, W.F. Krupke, V.K. Kanz, S.A. Payne, M.A. Dubinskii, L.D.J. Merkle, Opt. Soc. Am. B 21 (12) (2004) 2151. [3] B. Zhdanov, R.J. Knize, Opt. Lett. 32 (15) (2007) 2167. [4] B.V. Zhdanov, R.J. Knize, Opt. Eng. 52 (2) (2013) 021010. [5] K. Waichman, B.D. Barmashenko, S. Rosenwaks, J. Opt. Sci. Am. B 31 (11) (2014) 2628. [6] Z. Li, R. Tan, W. Huang, et al., Opt. Eng. 53 (11) (2014) 116113. [7] Z. Yang, H. Wang, Q. Lu, et al., J. Opt. Soc. Am. B 28 (6) (2011) 1353. [8] Y. Zheng, M. Niigaki, H. Miyajima, et al., Appl. Phys. Express 2 (3) (2009) 032501. [9] J. Zweiback, A. Komashko, Int. Soc. Opt. Photonics (2011) 791509. [10] A.V. Bogachev, S.G. Garanin, A.M. Dudov, V.A. Yeroshenko, S.M. Kulikov, G. T. Mikaelian, V.A. Panarin, V.O. Pautov, A.V. Rus, S.A. Sukharev, Quantum Electron 42 (2) (2012) 95. [11] Lindsay O.’Brien Quarrie, Opt. Mater. 35 (5) (2013) 843. [12] Hobbs, Douglas S. SPIE Defense, Security, and Sensing. International Society for Optics and Photonics, 2009. [13] B.V. Zhdanov, F. Kontur, S. Phipps, et al., Opt. Commun. 280 (1) (2007) 161. [14] Z. Li, R. Tan, C. Xu, et al., Quantum Electron. 43 (2) (2013) 147. [15] Y. Zi-Ning, W. Hong-Yan, L. Qi-Sheng, et al., Chin. Phys. Lett. 28 (10) (2011) 104202. [16] M.D. Rotondaro, G.P. Perram, J. Quant. Spectrosc. Radiat. Transf. 57 (4) (1997) 497. [17] N.D. Zameroski, G.D. Hager, W. Rudolph, et al., J. Quant. Spectrosc. Radiat. Transf. 112 (1) (2011) 59. [18] OrazioSvelto, Principles of Lasers. New York, Springer, p. 292, 2010.