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Stable 811.53 nm diode laser pump source for optically pumped metastable Ar laser
Optics & Laser Technology 84 (2016) 48–52
Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Stable 811.53 nm diode laser pump source for optically pumped metastable Ar laser Jun Gao a, Duluo Zuo a,b,n, Jun Zhao a, Bin Li a, Anlan Yu a, Xinbing Wang a,b a b
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 January 2016 Accepted 20 April 2016
A stable external cavity diode laser coupled with volume Bragg grating for metastable argon atoms pumping is presented. The measured maximum output power of the continuous wave is 6.5 W when the spectral width (FWHM) is less than 21 pm around 811.53 nm and the power efficiency is 68%. The tuning range of the emission wavelength is bigger than 270 pm. The calculated deviation in relative absorption efficiency caused by the fluctuations of wavelength and power is less than 4%. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Optically pumped gas laser Laser diode Narrow spectral width Volume Bragg grating
1. Introduction A new style of optically pumped gas laser with active medium based on rare gas atoms in metastable states produced by gas discharge was proposed and demonstrated by Han and Heaven from Emory University in 2012 [1]. The optically pumped metastable rare gas laser (OPRGL) has attracted considerable interests for its potential in generating high power laser with good beam quality and atmospheric transmittance [2–7]. Different metastable rare gas atoms (Ne*, Ar*, Kr* and Xe*) as laser media have been examined [1,6,8]. Among all of the rare gases, argon is the cheapest one and is conducive to large-scale application. As the absorption line is an atomic line, the first step for the high-power optically pumped metastable Ar laser (OPMAL) is to realize a high power pump source with narrow spectral width emitting around 811.53 nm. Diode laser of high electro-optical efficiency, high power and small size, as pump source for diode-pumped alkali laser (DPAL), has enabled alkali vapor lasers to exhibit excellent properties [9]. Presumably, the development of OPRGL will move towards to the direction of diode-pumped metastable rare gas laser and the feasibility of diode pumped metastable rare gas laser was confirmed in 2013 [3]. The challenge for laser diode (LD) to be the pump source of OPMAL is to match the narrow absorption linewidth of metastable argon atoms with buffer gas He at atmospheric pressure or a little higher. Absorption linewidth (FWHM) of roughly n Corresponding author at: Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China. E-mail address: [email protected] (D. Zuo).
http://dx.doi.org/10.1016/j.optlastec.2016.04.012 0030-3992/& 2016 Elsevier Ltd. All rights reserved.
15.6 pm was obtained for OPMAL with buffer gas He at 300 K and 105 Pa from the pressure broadening coefficients reported by Mikheyev [7], which is much narrower than the spectral width of free running commercial LD. To ensure efficient absorption of pump laser and high electro-optical efficiency of OPMAL, the spectral width of LD must be narrowed. Nonetheless, pump spectral width less than absorption linewidth is not mandatory and pump sources with spectral width up to 2 or 3 times larger than the absorption linewidth can still yield a very high absorption efficiency [10]. In addition, the drift of the wavelength with temperature and current should also be controlled. Wavelength stabilized LDs with narrow spectral width can be realized by volume Bragg Gratings (VBGs) as output couplers in external cavities and have been used in pumping of gas lasers including singlet delta oxygen laser and alkali vapor laser in the recent years. An maximum output power of 13.5 W with the spectral width narrowed to 13 pm for oxygen molecule pumping was demonstrated by employing a laser diode bar and a 25% reflectivity VBG with bandwidth of about 13 pm [11]. A large bandwidth (30 pm) VBG with higher reflectivity (70%) and a laser diode bar were used by Gourevitch et al. and a narrow spectral width of about 20 pm for Rb vapor pumping was obtained [12]. Both of the cavities mentioned above were in classical beam-collimated configuration and the laser diodes were anti-reflection coated with very low reflectivity (R ¼0.5%). However, Classical external-cavity laser diode (ECLD) is more sensitive to mechanical instabilities which is against long term reliable operation. For a better stability, a configuration called “cat's eye” has been shown and proved to be very effective in ECLD [13–15]. Extended-cavity tapered lasers with VBG in focus configuration designed by Lucas-Leclin et al. [16] has demonstrated a
J. Gao et al. / Optics & Laser Technology 84 (2016) 48–52
wavelength-stabilized output with narrow linewidth less than 20 pm and studied the performance in bandwidth and threshold current under two different configurations. In this paper, for the purpose of getting a stable LD pump source with narrow spectral width for OPMAL, we employed the method of external cavity based on a VBG in “cat's eye” configuration. Several parameters are measured and discussed including the spectral width, the power characteristic, the thermal tuning and the stabilities of laser output as well as their influence on absorption efficiency.
2. Experimental setup Fig. 1 shows the schematic of the experimental setup. The VBG external cavity laser diode (VBG-ECLD) consists of a commercial available c-mount bare LD (Axcel Photonics, Inc.), an aspheric lens f1 (f¼ 2.75 mm, Thorlabs, Inc.), a cylindrical lens f2 (f¼15 mm, Thorlabs, Inc.) and a VBG (OptiGrate Corp.). The LD is a single emitter diode producing the maximum output power of 9.6 W when the current is 9.5 A at 25 °C. Regarding this LD, the front facet is covered with standard coating, R¼2.5%, while the back facet is covered with highly-reflecting coating of reflectivity, R495%. The emitting size of the chip is about 1 400 mm2 with a 2 mm cavity length. The aspheric lens f1 is used to collect the laser. For compensating the astigmatism between the fast and slow axis, the cylindrical lens f2 is added to reduce the beam size along the slow axis. The VBG is used as the frequency feedback element and the output coupler (OC). The diffraction efficiency of the OC is about 70% for plane wave fronts at the resonant wavelength of 811.5 nm at 22 °C. The spectral selectivity is about 50 pm. All light-passing surfaces on lens and VBG are AR coated with left reflection less than 0.5%. A home-made copper mount is used to support the VBG. Two separated thermoelectric coolers (TECs) are used to control the temperatures of VBG mount and LD heat sink with stabilities of 0.01 K. A high resolution spectrograph with resolution of 13 pm near 812 nm (modified from THR1500 of Jobin yvon with a Hamamatsu CCD array detector S10420-1106) is used for spectrum measurement. The laser beam is diffused by a frosted glass plate before entering the spectrograph for the full utilization of the resolving capability. The power is measured by a power meter (UP19K-30H-H5-D0, Gentec-EO) placed in the position of the attenuator.
49
running mode and external cavity operation. Fig. 2 shows the spectra at the status of free running and VBG external cavity when the injection current of LD is set to be 9.48 A close to the maximum operating current. The temperatures of LD and VBG are set as 25 °C and 17 °C respectively. As shown in Fig. 2 below, The reading of the LD spectral width (FWHM) is narrowed from 1.7 nm in free running mode to less than 21 pm in VBG external cavity though the gain at central wavelength of VBG is very small. It is noted that the intrinsic cavity modes cannot be suppressed efficiently when the injection current of LD is aligned between 2 A and 8 A at the operating temperature of 25 °C as shown in Fig. 3. With the driving current rising, the ratio of the intensities of intrinsic modes and VBG-locked modes decreases gradually which means a transfer of power from intrinsic cavity to the external one. When the LD temperature is increased to be 31.5 °C at which the center wavelength of free running is red-shifted to 811.53 nm at 9.48 A, the current range of incomplete suppression narrows to be an interval from 2 A to 4 A. This phenomenon could be explained by the decrease of mismatch between the peak position of gain and the central wavelength of VBG when the LD temperature increases. Lower reflectivity of front facet means stronger feedback of VBG into the intrinsic cavity and better suppression effect. After all, reflectivity of 2.5% at the LD emitting facet is much bigger than 0.5% or even lower used in other spectral narrowing experiments with gratings [11,12,17]. As is explained by Hjelme and Mickelson [18], the spectral width of external cavity laser diode is roughly proportional to the reflectivity of LD emitting face. Reducing the reflectivity of the emitting surface is conducive to the elimination of intrinsic cavity modes and narrower spectral width.
3. Results and discussions Fig. 2. High resolution spectrum of free running and VBG narrowed.
3.1. Spectral narrowing Narrow spectral width is our primary goal during the experiments. The laser spectral characteristic is studied under both free
LD
Plane Attenuator mirror R1
Cylindrical lens f 2
Fast axis Aspheric lens f1
VBG
PC
Plane mirror R2
Monochromator Convergent Frosted Plane glass mirror R3 lens f3 Fig. 1. Schematic diagram of the experimental setup for spectral narrowing.
Fig. 3. Emission spectrum of VBG-ECLD for different driving current from 2 A to 8 A. Temperatures of LD and VBG are stabilized at 25 °C and 17 °C, separately.
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J. Gao et al. / Optics & Laser Technology 84 (2016) 48–52
Fig. 4. The P-I curves of LD under the free running mode (red) and VBG external cavity (blue). LD and VBG are set at 25 °C and 17 °C, separately. The losses caused by VBG are denoted in green. Lines have been drawn as a guide for the eyes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.2. Output characteristics Fig. 4 shows P-I characteristics of LD in VBG external cavity and free running mode. The maximum output power of VBG-ECLD reaches 6.5 W under the driving current of 9.48 A at which the LD output power is 9.55 W in free running mode. The current threshold and slope efficiency decrease from 1.42 A to 1.25 A and 1.196 W/A to 0.770 W/A respectively. Additional reflection from VBG results in the decrease of current threshold as compared to the free-running mode. The power efficiency of ECLD is only about 68%. One possible reason is that the laser is partially blocked by the VBG mount due to its small aperture thus the ultimate power efficiency and slope efficiency is affected. Attempt to increase the slope efficiency is taken by increasing the LD operating temperature to be 31.5 °C at which the free running center wavelength is red-shifted to 811.53 nm at 9.48 A to reduce the mismatch between the peak wavelength of free running LD and the central wavelength of VBG. The free running slope efficiency decreases to 1.185 W/A while the slope efficiency of VBG laser diode increases to 0.787 W/A. However, the maximum output power of external cavity drops to 6.32 W with threshold current changing from 1.25 A to 1.38 A. 3.3. Wavelength tuning The absorption wavelength of Ar* may be shifted from 811.53 nm with the addition of buffer gas, hence it is essential and important for tuning of pumping laser emission to precisely overlap with the absorption line. Oscillating wavelength of VBG-ECLD depends on the feedback wavelength of VBG, which can be finely controlled by temperature. Fig. 5 shows the tuning results, in which the emission wavelength of VBG-ECLD is tuned over a range of 270 pm by heating the Bragg grating with FWHM fluctuation less than 3 pm. A tuning rate of 7.87 pm/°C is measured by recording the VBG's temperature and the monochromator's reading. 3.4. Wavelength and power stabilities The laser spectrum stability is very important to pump efficiently when the pump linewidth is narrowed to 21 pm. Long-term stabilities data of oscillating wavelength and output power were measured every 200 s during our two 6-hour tests, and the experimental results are shown in Fig. 6. Between the two tests, the laser was off from 9:30 pm to 8:30 am next morning. Laser spectrum data were obtained with spectral calibration argon lamp on and the first data
Fig. 5. The spectral tuning of ECLD at I ¼9.4 A and operating temperature at 25 °C. Range of VBG temperature is from 20 °C to 50 °C.
was taken after the laser had run for 2 h. In our whole test, the wavelength deviation is about 7 pm and the maximum fluctuation in the output power is below 1%. There are four factors that will lead to wavelength instability: the temperature fluctuations of LD and VBG, the current fluctuation of LD, the misalignment of the external cavity and the cavity length change due to the temperature variation [19]. For our system, the driving current fluctuation is 70.001 A. The temperature fluctuations of LD and VBG are both 0.01 K. The main reason of fluctuation in wavelength may be the misalignment of the external cavity caused by mechanical instability. More stable mechanical design will help to increase the stabilities of laser system. The influence of wavelength deviation on relative absorption efficiency shown in Fig. 7 is calculated based on equations described in [5]. In the calculation, we assume that: (1) the spectral width of pump laser is 21 pm, (2) and the absorption line shape is Lorentzian with FWHM of 15.6 pm. The relative absorption efficiency is defined as follows:
ηabs _ rel =
ηabs (λ ) ηabs ( λ 0 )
Where ηabs is the absorption efficiency of pump laser, λ 0 and λ represent the central wavelengths of absorption line and pump laser, respectively. As shown in Fig. 7, ηabs is sensitive to wavelength deviation. When the wavelength deviation reaches to 77.0 pm and 76.5 pm for Lorentzian profile and Gaussian profile, respectively, decrease in ηabs _ rel will be up to 10%. For our laser system, the fluctuations in wavelength and power displayed in Fig. 6 mean that the deviation in ηabs _ rel is less than 4%. By exerting stabilization methods on intensity and wavelength, much smaller deviation should be possible. Han et al. [3] has reported a diode laser pump source in their diode-pumped metastable Ar laser. In their design, the fast axis of
J. Gao et al. / Optics & Laser Technology 84 (2016) 48–52
51
Fig. 6. Long-term stability of peak wavelength (a) and power (b). Data were measured every 200 s
4. Conclusions In summary, by employing the external cavity consisting of a VBG with wider bandwidth (FWHM¼50 pm) and a LD with considerable front facet reflectivity (R¼ 2.5%) compared with that reported in the literatures, we obtain the similar performance in spectral width (less than 21 pm) though focusing laser inside the grating will lead to an increase in bandwidth of VBG [16]. The tunability is over 270 pm, which could cover the Ar* absorption bands. The maximum CW output power of 6.5 W is sufficient for our following pumping research. Considering the cost of VBG with bigger size, these obtained experimental results will benefit the development of higher-power laser diode pumping source for OPMAL by power scaling such as injection locking described by Liu etc. [20] and related pumping research on OPRGL as well. Fig. 7. Calculation between relative absorption efficiency and wavelength deviations.
LD was collimated by a cylindrical lens with divergence close to diffraction limits. The divergence in slow axis was utilized for angle selectivity. A maximum power of about 8 W was obtained with spectral width narrower than 10 GHz. Compared with the above reported pump source, a different way is employed in coupling and angle selectivity which is less sensitive to mechanical instability for our system. Roughly higher power efficiency and equivalent spectral width are obtained. Further comparison is infeasible for lack of detailed information such as rated power of LD and spectral width of VBG.
Acknowledgments The authors thank Mr. Zhe Li for his assistance in Zemax simulation.
References [1] J.D. Han, M.C. Heaven, Gain and lasing of optically pumped metastable rare gas atoms, Opt. Lett. 37 (2012) 2157–2159, http://dx.doi.org/10.1364/OL.37.002157. [2] A.V. Demyanov, I.V. Kochetov, P.A. Mikheyev, Kinetic study of a cw optically pumped laser with metastable rare gas atoms produced in an electric
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Contents lists available at ScienceDirect
Optics & Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Stable 811.53 nm diode laser pump source for optically pumped metastable Ar laser Jun Gao a, Duluo Zuo a,b,n, Jun Zhao a, Bin Li a, Anlan Yu a, Xinbing Wang a,b a b
Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 January 2016 Accepted 20 April 2016
A stable external cavity diode laser coupled with volume Bragg grating for metastable argon atoms pumping is presented. The measured maximum output power of the continuous wave is 6.5 W when the spectral width (FWHM) is less than 21 pm around 811.53 nm and the power efficiency is 68%. The tuning range of the emission wavelength is bigger than 270 pm. The calculated deviation in relative absorption efficiency caused by the fluctuations of wavelength and power is less than 4%. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Optically pumped gas laser Laser diode Narrow spectral width Volume Bragg grating
1. Introduction A new style of optically pumped gas laser with active medium based on rare gas atoms in metastable states produced by gas discharge was proposed and demonstrated by Han and Heaven from Emory University in 2012 [1]. The optically pumped metastable rare gas laser (OPRGL) has attracted considerable interests for its potential in generating high power laser with good beam quality and atmospheric transmittance [2–7]. Different metastable rare gas atoms (Ne*, Ar*, Kr* and Xe*) as laser media have been examined [1,6,8]. Among all of the rare gases, argon is the cheapest one and is conducive to large-scale application. As the absorption line is an atomic line, the first step for the high-power optically pumped metastable Ar laser (OPMAL) is to realize a high power pump source with narrow spectral width emitting around 811.53 nm. Diode laser of high electro-optical efficiency, high power and small size, as pump source for diode-pumped alkali laser (DPAL), has enabled alkali vapor lasers to exhibit excellent properties [9]. Presumably, the development of OPRGL will move towards to the direction of diode-pumped metastable rare gas laser and the feasibility of diode pumped metastable rare gas laser was confirmed in 2013 [3]. The challenge for laser diode (LD) to be the pump source of OPMAL is to match the narrow absorption linewidth of metastable argon atoms with buffer gas He at atmospheric pressure or a little higher. Absorption linewidth (FWHM) of roughly n Corresponding author at: Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China. E-mail address: [email protected] (D. Zuo).
http://dx.doi.org/10.1016/j.optlastec.2016.04.012 0030-3992/& 2016 Elsevier Ltd. All rights reserved.
15.6 pm was obtained for OPMAL with buffer gas He at 300 K and 105 Pa from the pressure broadening coefficients reported by Mikheyev [7], which is much narrower than the spectral width of free running commercial LD. To ensure efficient absorption of pump laser and high electro-optical efficiency of OPMAL, the spectral width of LD must be narrowed. Nonetheless, pump spectral width less than absorption linewidth is not mandatory and pump sources with spectral width up to 2 or 3 times larger than the absorption linewidth can still yield a very high absorption efficiency [10]. In addition, the drift of the wavelength with temperature and current should also be controlled. Wavelength stabilized LDs with narrow spectral width can be realized by volume Bragg Gratings (VBGs) as output couplers in external cavities and have been used in pumping of gas lasers including singlet delta oxygen laser and alkali vapor laser in the recent years. An maximum output power of 13.5 W with the spectral width narrowed to 13 pm for oxygen molecule pumping was demonstrated by employing a laser diode bar and a 25% reflectivity VBG with bandwidth of about 13 pm [11]. A large bandwidth (30 pm) VBG with higher reflectivity (70%) and a laser diode bar were used by Gourevitch et al. and a narrow spectral width of about 20 pm for Rb vapor pumping was obtained [12]. Both of the cavities mentioned above were in classical beam-collimated configuration and the laser diodes were anti-reflection coated with very low reflectivity (R ¼0.5%). However, Classical external-cavity laser diode (ECLD) is more sensitive to mechanical instabilities which is against long term reliable operation. For a better stability, a configuration called “cat's eye” has been shown and proved to be very effective in ECLD [13–15]. Extended-cavity tapered lasers with VBG in focus configuration designed by Lucas-Leclin et al. [16] has demonstrated a
J. Gao et al. / Optics & Laser Technology 84 (2016) 48–52
wavelength-stabilized output with narrow linewidth less than 20 pm and studied the performance in bandwidth and threshold current under two different configurations. In this paper, for the purpose of getting a stable LD pump source with narrow spectral width for OPMAL, we employed the method of external cavity based on a VBG in “cat's eye” configuration. Several parameters are measured and discussed including the spectral width, the power characteristic, the thermal tuning and the stabilities of laser output as well as their influence on absorption efficiency.
2. Experimental setup Fig. 1 shows the schematic of the experimental setup. The VBG external cavity laser diode (VBG-ECLD) consists of a commercial available c-mount bare LD (Axcel Photonics, Inc.), an aspheric lens f1 (f¼ 2.75 mm, Thorlabs, Inc.), a cylindrical lens f2 (f¼15 mm, Thorlabs, Inc.) and a VBG (OptiGrate Corp.). The LD is a single emitter diode producing the maximum output power of 9.6 W when the current is 9.5 A at 25 °C. Regarding this LD, the front facet is covered with standard coating, R¼2.5%, while the back facet is covered with highly-reflecting coating of reflectivity, R495%. The emitting size of the chip is about 1 400 mm2 with a 2 mm cavity length. The aspheric lens f1 is used to collect the laser. For compensating the astigmatism between the fast and slow axis, the cylindrical lens f2 is added to reduce the beam size along the slow axis. The VBG is used as the frequency feedback element and the output coupler (OC). The diffraction efficiency of the OC is about 70% for plane wave fronts at the resonant wavelength of 811.5 nm at 22 °C. The spectral selectivity is about 50 pm. All light-passing surfaces on lens and VBG are AR coated with left reflection less than 0.5%. A home-made copper mount is used to support the VBG. Two separated thermoelectric coolers (TECs) are used to control the temperatures of VBG mount and LD heat sink with stabilities of 0.01 K. A high resolution spectrograph with resolution of 13 pm near 812 nm (modified from THR1500 of Jobin yvon with a Hamamatsu CCD array detector S10420-1106) is used for spectrum measurement. The laser beam is diffused by a frosted glass plate before entering the spectrograph for the full utilization of the resolving capability. The power is measured by a power meter (UP19K-30H-H5-D0, Gentec-EO) placed in the position of the attenuator.
49
running mode and external cavity operation. Fig. 2 shows the spectra at the status of free running and VBG external cavity when the injection current of LD is set to be 9.48 A close to the maximum operating current. The temperatures of LD and VBG are set as 25 °C and 17 °C respectively. As shown in Fig. 2 below, The reading of the LD spectral width (FWHM) is narrowed from 1.7 nm in free running mode to less than 21 pm in VBG external cavity though the gain at central wavelength of VBG is very small. It is noted that the intrinsic cavity modes cannot be suppressed efficiently when the injection current of LD is aligned between 2 A and 8 A at the operating temperature of 25 °C as shown in Fig. 3. With the driving current rising, the ratio of the intensities of intrinsic modes and VBG-locked modes decreases gradually which means a transfer of power from intrinsic cavity to the external one. When the LD temperature is increased to be 31.5 °C at which the center wavelength of free running is red-shifted to 811.53 nm at 9.48 A, the current range of incomplete suppression narrows to be an interval from 2 A to 4 A. This phenomenon could be explained by the decrease of mismatch between the peak position of gain and the central wavelength of VBG when the LD temperature increases. Lower reflectivity of front facet means stronger feedback of VBG into the intrinsic cavity and better suppression effect. After all, reflectivity of 2.5% at the LD emitting facet is much bigger than 0.5% or even lower used in other spectral narrowing experiments with gratings [11,12,17]. As is explained by Hjelme and Mickelson [18], the spectral width of external cavity laser diode is roughly proportional to the reflectivity of LD emitting face. Reducing the reflectivity of the emitting surface is conducive to the elimination of intrinsic cavity modes and narrower spectral width.
3. Results and discussions Fig. 2. High resolution spectrum of free running and VBG narrowed.
3.1. Spectral narrowing Narrow spectral width is our primary goal during the experiments. The laser spectral characteristic is studied under both free
LD
Plane Attenuator mirror R1
Cylindrical lens f 2
Fast axis Aspheric lens f1
VBG
PC
Plane mirror R2
Monochromator Convergent Frosted Plane glass mirror R3 lens f3 Fig. 1. Schematic diagram of the experimental setup for spectral narrowing.
Fig. 3. Emission spectrum of VBG-ECLD for different driving current from 2 A to 8 A. Temperatures of LD and VBG are stabilized at 25 °C and 17 °C, separately.
50
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Fig. 4. The P-I curves of LD under the free running mode (red) and VBG external cavity (blue). LD and VBG are set at 25 °C and 17 °C, separately. The losses caused by VBG are denoted in green. Lines have been drawn as a guide for the eyes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.2. Output characteristics Fig. 4 shows P-I characteristics of LD in VBG external cavity and free running mode. The maximum output power of VBG-ECLD reaches 6.5 W under the driving current of 9.48 A at which the LD output power is 9.55 W in free running mode. The current threshold and slope efficiency decrease from 1.42 A to 1.25 A and 1.196 W/A to 0.770 W/A respectively. Additional reflection from VBG results in the decrease of current threshold as compared to the free-running mode. The power efficiency of ECLD is only about 68%. One possible reason is that the laser is partially blocked by the VBG mount due to its small aperture thus the ultimate power efficiency and slope efficiency is affected. Attempt to increase the slope efficiency is taken by increasing the LD operating temperature to be 31.5 °C at which the free running center wavelength is red-shifted to 811.53 nm at 9.48 A to reduce the mismatch between the peak wavelength of free running LD and the central wavelength of VBG. The free running slope efficiency decreases to 1.185 W/A while the slope efficiency of VBG laser diode increases to 0.787 W/A. However, the maximum output power of external cavity drops to 6.32 W with threshold current changing from 1.25 A to 1.38 A. 3.3. Wavelength tuning The absorption wavelength of Ar* may be shifted from 811.53 nm with the addition of buffer gas, hence it is essential and important for tuning of pumping laser emission to precisely overlap with the absorption line. Oscillating wavelength of VBG-ECLD depends on the feedback wavelength of VBG, which can be finely controlled by temperature. Fig. 5 shows the tuning results, in which the emission wavelength of VBG-ECLD is tuned over a range of 270 pm by heating the Bragg grating with FWHM fluctuation less than 3 pm. A tuning rate of 7.87 pm/°C is measured by recording the VBG's temperature and the monochromator's reading. 3.4. Wavelength and power stabilities The laser spectrum stability is very important to pump efficiently when the pump linewidth is narrowed to 21 pm. Long-term stabilities data of oscillating wavelength and output power were measured every 200 s during our two 6-hour tests, and the experimental results are shown in Fig. 6. Between the two tests, the laser was off from 9:30 pm to 8:30 am next morning. Laser spectrum data were obtained with spectral calibration argon lamp on and the first data
Fig. 5. The spectral tuning of ECLD at I ¼9.4 A and operating temperature at 25 °C. Range of VBG temperature is from 20 °C to 50 °C.
was taken after the laser had run for 2 h. In our whole test, the wavelength deviation is about 7 pm and the maximum fluctuation in the output power is below 1%. There are four factors that will lead to wavelength instability: the temperature fluctuations of LD and VBG, the current fluctuation of LD, the misalignment of the external cavity and the cavity length change due to the temperature variation [19]. For our system, the driving current fluctuation is 70.001 A. The temperature fluctuations of LD and VBG are both 0.01 K. The main reason of fluctuation in wavelength may be the misalignment of the external cavity caused by mechanical instability. More stable mechanical design will help to increase the stabilities of laser system. The influence of wavelength deviation on relative absorption efficiency shown in Fig. 7 is calculated based on equations described in [5]. In the calculation, we assume that: (1) the spectral width of pump laser is 21 pm, (2) and the absorption line shape is Lorentzian with FWHM of 15.6 pm. The relative absorption efficiency is defined as follows:
ηabs _ rel =
ηabs (λ ) ηabs ( λ 0 )
Where ηabs is the absorption efficiency of pump laser, λ 0 and λ represent the central wavelengths of absorption line and pump laser, respectively. As shown in Fig. 7, ηabs is sensitive to wavelength deviation. When the wavelength deviation reaches to 77.0 pm and 76.5 pm for Lorentzian profile and Gaussian profile, respectively, decrease in ηabs _ rel will be up to 10%. For our laser system, the fluctuations in wavelength and power displayed in Fig. 6 mean that the deviation in ηabs _ rel is less than 4%. By exerting stabilization methods on intensity and wavelength, much smaller deviation should be possible. Han et al. [3] has reported a diode laser pump source in their diode-pumped metastable Ar laser. In their design, the fast axis of
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Fig. 6. Long-term stability of peak wavelength (a) and power (b). Data were measured every 200 s
4. Conclusions In summary, by employing the external cavity consisting of a VBG with wider bandwidth (FWHM¼50 pm) and a LD with considerable front facet reflectivity (R¼ 2.5%) compared with that reported in the literatures, we obtain the similar performance in spectral width (less than 21 pm) though focusing laser inside the grating will lead to an increase in bandwidth of VBG [16]. The tunability is over 270 pm, which could cover the Ar* absorption bands. The maximum CW output power of 6.5 W is sufficient for our following pumping research. Considering the cost of VBG with bigger size, these obtained experimental results will benefit the development of higher-power laser diode pumping source for OPMAL by power scaling such as injection locking described by Liu etc. [20] and related pumping research on OPRGL as well. Fig. 7. Calculation between relative absorption efficiency and wavelength deviations.
LD was collimated by a cylindrical lens with divergence close to diffraction limits. The divergence in slow axis was utilized for angle selectivity. A maximum power of about 8 W was obtained with spectral width narrower than 10 GHz. Compared with the above reported pump source, a different way is employed in coupling and angle selectivity which is less sensitive to mechanical instability for our system. Roughly higher power efficiency and equivalent spectral width are obtained. Further comparison is infeasible for lack of detailed information such as rated power of LD and spectral width of VBG.
Acknowledgments The authors thank Mr. Zhe Li for his assistance in Zemax simulation.
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