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Tunable high-power blue external cavity semiconductor laser
Optics and Laser Technology 94 (2017) 1–5
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Optics and Laser Technology journal homepage: www.elsevier.com/locate/jolt
Full length article
Tunable high-power blue external cavity semiconductor laser Ding Ding a, Xueqin Lv a,⇑, Xinyi Chen a, Fei Wang a, Jiangyong Zhang b, Kaijun Che b a b
Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, China Department of Electronic Engineering, Xiamen University, Xiamen 361005, China
a r t i c l e
i n f o
Article history: Received 19 January 2017 Accepted 5 March 2017
Keywords: High-power blue laser Littrow-type external cavity Wide tuning bandwidth Narrow linewidth
a b s t r a c t A commercially available high-power GaN-based blue laser diode has been operated in a simple Littrowtype external cavity (EC). Two kinds of EC configurations with the grating lines perpendicular (A configuration) and parallel (B configuration) to the p-n junction are evaluated. Good performance has been demonstrated for the EC laser with B configuration due to the better mode selection effect induced by the narrow feedback wavelength range from the grating. Under an injection current of 1100 mA, the spectral linewidth is narrowed significantly down to 0.1 nm from 1 nm (the free-running width), with a good wavelength-locking behavior and a higher than 35 dB-amplified spontaneous emission suppression ratio. Moreover, a tuning bandwidth of 3.6 nm from 443.9 nm to 447.5 nm is realized with output power of 1.24 W and EC coupling efficiency of 80% at the central wavelength. The grating-coupled blue EC laser with narrow spectral linewidth, flexible wavelength tunability, and high output power shows potential applications in atom cooling and trapping, high-resolution spectroscopy, second harmonic generation, and high-capacity holographic data storage. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Along with the advance in material growth and device process technology, GaN-based edge-emitting semiconductor lasers are under active development. Nowadays, GaN-based violet, blue, and green multi-mode laser diodes (LDs) with the output power greater than 1 W have been commercially available [1]. However, the performance of LDs is limited by the poor mode-selection characteristic of Fabry-perot (FP) laser structure and the flat gain profile of semiconductor material, which result in the broad lasing linewidth and untunable wavelength. If tunable single mode operation can be achieved, such high-power lasers could be very useful for a variety of applications such as atom cooling and trapping [2], high-resolution spectroscopy [3], second harmonic generation [4], and high-capacity holographic data storage [5]. One possibility to shape the laser emission behavior is to change the chip design and append frequency selective components directly during the LD manufacturing process. With this technology, light emission with tunable wavelength, narrow linewidth, and high output power has been commercially achieved for GaAs-based Distributed feedback (DFB) [6] and distributed-Bragg-reflection [7] LDs. However, the fabrication technique of such GaN-based LDs is too complex to be widely commercially available. Nichia corporation ⇑ Corresponding author. E-mail address: [email protected] (X. Lv). http://dx.doi.org/10.1016/j.optlastec.2017.03.015 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
reported continuous-wave (CW) operation of first-order AlInGaN 405 nm DFB LD on the free-standing GaN substrate [8]. The single longitudinal mode emission can be maintained up to an output power of 60 mW. However, the wavelength tuning control was not mentioned and the cost of material was very high. As far as we know, Nichia company is the only one who has reported CW operation of GaN-based DFB laser. Alternatively, optical feedback achieved by adding an externalcavity (EC) element such as diffraction grating, can not only enhance the wavelength tunability, but also reduce the intrinsic linewidth. Correspondingly, much effort has been devoted to developing a grating-coupled EC laser [5,9–16]. Lonsdale et al. have reported a 398 nm-EC laser with a maximum tuning bandwidth of 6.3 nm. However the output power is limited by the low-power LD [10]. Hildebrandt et al. have fabricated an EC laser with an antireflection-coated GaN-based LD. A total tuning range of 4 nm and an optical output power of 30 mW were realized [11]. Tanaka et al. has reported an EC laser with a wavelength of 405 nm and an output power of 80 mW for holographic data storage [5]. Recently, Ruhnke et al. demonstrated a 400 mW EC laser at 445 nm based on a commercially available high-power LD [16]. Then they used this home-made EC laser as pump source for single-pass second harmonic generation at 222.5 nm [4]. Our research team has been working on GaN-based EC laser since 2013. The performance of a violet (405 nm) EC laser with the injection current below and just above the free-running lasing
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threshold has been investigated [14]. Although there have been some reports on GaN-based grating-coupled EC lasers, the research is mainly focused on the low-power ones, while the high-power EC laser needs more studies. Generally, in order to obtain high power output, a gain device with wide stripe width and high tolerable working current is needed. In this situation, optimization of the EC configuration is very important for improving the mode selection effect. Besides, the working current is generally far beyond the free-running lasing threshold, and the competition between the inner FP cavity resonance and EC resonance makes the tuning properties become more complex. In this paper, we fabricate a high-power grating-coupled EC laser by employing a commercially available high-power GaNbased blue LD. Two kinds of Littrow configurations with the grating lines perpendicular (A configuration) and parallel (B configuration) to the p-n junction are constructed and the properties are investigated in detail. By using B configuration, 1.24 W-output power, 3.6 nm-wavelength tuning range, 0.1 nm spectral linewidth, and higher than 35 dB-amplified spontaneous emission suppression ratio are realized simultaneously under an injection current of 1100 mA. The improved performance should be more beneficial to its applications.
2. Experiments Fig. 1 shows the schematic diagram of the constructed two kinds of Littrow EC configurations. For A configuration, the grating lines are perpendicular to the p-n junction of gain device. While for B configuration, the gain device is rotated by 90° and the grating lines are parallel to the p-n junction. Both of the EC lasers consist of four optical components: a gain device, a beam collimator, an external grating, and a plane mirror. The gain device under investigation is a commercially available high-power GaN-based LD emitting at around 445 nm (Nichia, NDB7875-E). The stripe width is estimated to be 15 mm according to the observation under a microscope. The output beam has a farfield divergence of 25.5° full width at half maximum (FWHM) perpendicular to the p-n junction (fast axis), and 3.5° parallel to the pn junction (slow axis). When in use it was mounted on a thermoelectrically cooled and temperature controlled plate with a fixed temperature of 20 ± 0.3 °C. Under continuous-wave injection, the device has a threshold current of 130 mA and a maximum output power of 1.6 W. To operate safely below damage threshold, we limited the operation current to be less than 1100 mA with a maximum free-running power of 1.56 W. In the EC laser, the radiation emitted on the fast axis of gain device is nearly collimated by using an aspherical lens with a numerical aperture of 0.5 and a focal length of 8 mm (Thorlabs, 352240-A). Then the collimated light beam with the size of
7 mm in this direction hit a 2400 grooves/mm-grating under the Littrow angle, where the first order of diffraction is reflected into itself. By rotating the grating, the EC resonance wavelength can be selected. The light diffracted in the zeroth order is used for outcoupling. The length of the EC laser is about 18 cm. In order to avoid the alteration of the output beam direction during the tuning process, a beam-correction mirror is applied. For the used two kinds of EC configurations, the grating presents different diffraction efficiency due to its polarizationsensitivity. It is known that the light emitted from the LD is polarized in the plane of p-n junction. For A configuration, the grating shows a much higher first order diffraction efficiency of 36% and a much lower zeroth order diffraction efficiency of 44%, while for B configuration, the diffraction efficiency is only 11.5% in the first order and is 76% in the zeroth order.
3. Results and discussion Before performing EC tuning experiments, the properties of the free-running gain device are characterized. The normalized emission spectra of the device at four different injection currents are depicted in Fig. 2. Obviously, the lasing peak red-shifts with increasing current. Such behavior can be attributed to the decrease of the band-gap energy induced by the increasing temperature under the higher injection level. Besides, a significant broadening of the lasing spectrum with increasing current can also be observed. The FWHMs are 0.52, 1.01, and 1.08 nm for the injection current of 300, 700, and 1100 mA, respectively. This is related to the broadened gain spectrum induced by the band-filling effect. A high-resolution spectral measurement result shows that the longitudinal-mode interval is 0.025 nm. This results in a FP cavity length of around 1.6 mm. Then, two kinds of grating-coupled EC lasers are constructed and compared. Fig. 3 shows a comparison of light output power versus injection current (P-I) curves among the free-running gain device and the EC lasers tuned at 445 nm measured from the zeroth order of the grating. For the EC lasers with A and B configurations, the threshold current (Ith) is reduced to 95 mA, compared with that of free-running gain device (130 mA). Besides, the slope efficiency is extracted according to the P-I curve above its threshold and it decreases from 1.625 W/A to 0.655 and 1.280 W/A. It is well known that for conventional edge-emitting semiconductor lasers, an increase in the facet reflectivity will reduce the output loss, and thus the Ith and slope efficiency. Similarly, as an extension of the FP cavity, the grating-coupled EC laser can also be equivalent to a two-mirror cavity by combining the first-order diffraction of the grating with the reflection of laser end facet [17]. The additional diffraction of the external grating increases the effective reflectance of output end, leading to the reduction in the Ith and
Fig. 1. Schematic diagram of two kinds of EC configurations: (A) grating lines are perpendicular to the p-n junction, (B) grating lines are parallel to the p-n junction.
D. Ding et al. / Optics and Laser Technology 94 (2017) 1–5
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Fig. 2. Spectral characteristics of the free-running gain device at four different injection currents.
Fig. 4. Lasing spectra of the two kinds of EC lasers with constant grating angle.
Fig. 3. Light output power versus injection current for the free-running gain device and the EC lasers.
slope efficiency finally. At an injection current of 1100 mA, the optical powers are 1.559 W, 0.655 W, and 1.242 W for the freerunning gain device, and the EC lasers with A and B configurations respectively. The latters correspond to the efficiencies of 42% and 80% with respect to the free-running device. Thus for the gratingcoupled EC lasers with the light diffracted in the zeroth order as output, the higher zeroth order diffraction efficiency conduces to achieving the higher output power. The spectral characteristics of the two kinds of EC lasers are then investigated. Fig. 4 shows the typical lasing spectra at different injection currents when the grating is fixed to a constant angle. It can be seen that for A configuration in which the wavelength selection takes place along the emitter window (15 mm), the lasing spectra presents a multi-longitudinal-mode structure. At 1100 mA, the FWHM is 0.28 nm. For B configuration in which the wavelength selection takes place along the thickness of p-n junction (1 mm), the lasing linewidth for the injection current of 340 mA is narrowed down to 0.02 nm which is the spectral resolution of the spectrometer. With increasing current, the linewidth broadens to 0.10 nm at 1100 mA due to the enhanced light feedback from the external grating. Meanwhile the peak wavelength exhibits a good wavelength-locking behavior with only 0.02 nm shifting towards the long wavelength side. This is probably related to the high temperature induced red shift of the material gain spectrum caused by the higher injection level which has been identified
by the free-running lasing spectra as shown in Fig. 2. Besides, the high temperature induced thermal expansion will lead to a slight deformation of the device. Both of them will result in an influence on the spectral stability of EC laser. It is concluded that the better mode selection effect can be obtained by using the B configuration. This can be explained by the different feedback wavelength ranges from the grating. For the Littrow-type EC laser, the emitter size contribution to the feedh back wavelength range dk can be given by the formula: dk ¼ kw cot , f where w is the emitter size, h is the angle between the optical axis and the grating normal, and f is the focal length of the fast axis collimation. It seems that dk is proportional to the emitter size w. According to the formula, the dks are estimated to be 1.32 nm and 0.09 nm at the lasing wavelength of 445 nm for A and B configurations with emitter size of 15 mm and 1 mm respectively. Therefore, for A configuration the obtained large feedback wavelength range dk from the grating results in the broad multi-peak spectral structure. Moreover, under this configuration the continuous wavelength tuning is difficult to achieve by rotating the angle of the grating due to the poor mode selection effect. Thus B configuration is adopted in our following tuning experiments. The wavelength tunability is then evaluated for the EC laser with B configuration by tuning the incidence angle of the grating. Fig. 5(a)–(d) shows the tunable lasing spectra under varieties of bias conditions: the injection current from 130 mA to 1100 mA with normalized intensity for clarity. The EC lasing is realized around the free-running lasing wavelength. Especially, when the EC laser is tuned at the center of the gain peak, the amplified spontaneous emission suppression ratio (ASESR) is higher than 35 dB. However, the ASESR is reduced to 10–20 dB at the edged wavelengths due to the appearance of the FP resonance peak. The lower ASESR at these wavelengths reduces the output power within the narrowband emission of the EC laser. For example, at 1100 mA for the EC wavelengths of 443.9 nm and 447.5 nm, the output power decreases from the measured 1230 mW and 1202 mW to the estimated 712 mW and 906 mW respectively by the spectral area integral procedure.
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D. Ding et al. / Optics and Laser Technology 94 (2017) 1–5
Fig. 5. Tuning spectra of the EC laser with B configuration at different injection currents.
below the free-running lasing threshold, the output power is limited to be a dozen milliwatts. To obtain high output power, the injection current should be enhanced. By increasing the injection current from the free-running lasing threshold to 900 mA, the tuning range decreases gradually from 7.2 nm to 2.6 nm due to the lasing interference of inner cavity. A further increase in injection level causes an extension of the tuning range to the longer wavelength side owing to the combined contributions of thermal effect and band-filling effect. Besides, it can also be seen that the output power increases monotonously from several milliwatts to 1242 mW with the increasing injection current. Under a certain injection current, the estimated output power decreases by 54–90% at the edged tuning wavelengths due to the appearance of FP lasing peak. However, at the central wavelength the output power is not affected by the FP interference. The above results demonstrate that high-power blue EC laser with narrow spectral linewidth and flexible wavelength tunability has been obtained. For the further practical applications, another aspect worth noting is that accurate temperature control of the gain device is important to maintain the power and spectral stability of EC laser. The precision of the temperature controller used in this paper is plus or minus 0.3°. However, under high injection current, the gain device will produce a lot of heat which cannot dissipate quickly, therefore the true temperature change of the gain device will be relatively large. The temperature rise of the gain device will not only cause a redshift of the gain spectrum, but also lead to a slight deformation of the device due to the thermal expansion. Both of them will result in an influence on the stability of EC laser. So how to control the temperature of gain device accurately is a technical problem to be solved. Besides, in our paper the wavelength tuning was realized with a manual control of the diffraction grating movement. In order to facilitate the application, the wavelength change needs to be controlled more accurately and the electric tuning is necessary. To solve this problem, the mechanical structure such as sine mechanism or worm and gear can be designed.
4. Summary
Fig. 6. Wavelength tuning range and output power of EC laser with B configuration under different injection currents. The solid and dashed lines represent the measured and estimated output power respectively.
Fig. 6 shows the actual wavelength tuning range and the output power of the EC laser with B configuration achieved under per different injection current. The measured and estimated output power are marked by the solid and dashed lines respectively. The highest wavelength tuning range of 7.2 nm from 440.3 nm to 447.5 nm is obtained with the injection current of 130 mA, which is just at the threshold current of the free-running gain device. Although the tuning range is large when the injection current is
In conclusion, we fabricated a high-power grating-coupled EC laser based on a commercially available high-power GaN LD. Benefiting from the applied EC placement configuration in which the wavelength selection takes place along the 1 mm-emission aperture, high-power EC laser with narrow linewidth and broad wavelength tuning range is demonstrated. The spectral measurement results show that the lasing linewidth is about 0.1 nm at 1100 mA, the ASESR is higher than 35 dB at the central wavelength, and the peak wavelength is well locked under different injection currents. Moreover, under the injection current of 1100 mA, a tuning bandwidth of 3.6 nm from 443.9 nm to 447.5 nm is realized by simply changing the grating rotation angle. The output power can reach up to 1.24 W and the EC coupling efficiency is 80%. This type of blue EC laser will be especially suitable for atom cooling and trapping, high-resolution spectroscopy, second harmonic generation, and high-capacity holographic data storage.
Acknowledgements This work has been supported by the National Natural Science Foundation of China (61306087 and 61574119), Natural Science Foundation of Fujian Province of China (2017J01120 and 2015J05130), and Fundamental Research Funds for the Central Universities (20720150084).
D. Ding et al. / Optics and Laser Technology 94 (2017) 1–5
References [1] http://www.nichia.co.jp/. [2] I. Courtillot, A. Quessada, R.P. Kovacich, J.J. Zondy, A. Landragin, A. Clairon, P. Lemonde, Efficient cooling and trapping of strontium atoms, Opt. Lett. 28 (6) (2003) 468–470. [3] J. Hult, I.S. Burns, C.F. Kaminski, Measurements of the indium hyperfine structure in an atmospheric-pressure flame by use of diode-laser-induced fluorescence, Opt. Lett. 29 (8) (2004) 827–829. [4] N. Ruhnke, A. Müller, B. Eppich, R. Güther, M. Maiwald, B. Sumpf, G. Erbert, G. Tränkle, Single-pass UV generation at 222.5 nm based on high-power GaN external cavity diode laser, Opt. Lett. 40 (9) (2015) 2127–2129. [5] T. Tanaka, K. Takahashi, K. Sako, R. Kasegawa, M. Toishi, K. Watanabe, D. Samuels, M. Takeya, Littrow-type external-cavity blue laser for holographic data storage, Appl. Opt. 46 (17) (2007) 3583–3592. [6] http://nanoplus.com/. [7] http://www.photodigm.com/. [8] S. Masui, K. Tsukayama, T. Yanamoto, T. Kozaki, S. Nagahama, T. Mukai, CW operation of the first-order AlInGaN 405 nm distributed feedback laser diodes, Jpn. J. Appl. Phys. 45 (46) (2006) L1223–L1225.
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[9] R.S. Conroy, J.J. Hewett, G.P.T. Lancaster, W. Sibbett, J.W. Allen, K. Dholakia, Characterisation of an extended cavity violet diode laser, Opt. Commun. 175 (2000) 185–188. [10] D.J. Lonsdale, A.P. Willis, T.A. King, Extended tuning and single-mode operation of an anti-reflection-coated InGaN violet laser diode in a Littrow cavity, Meas. Sci. Technol. 13 (2002) 488–493. [11] L. Hildebrandt, R. Knispel, S. Stry, J.R. Sacher, F. Schael, Antireflection-coated blue GaN laser diodes in an external cavity and Doppler-free indium absorption spectroscopy, Appl. Opt. 42 (12) (2003) 2110–2118. [12] I.S. Burns, J. Hult, C.F. Kaminski, Spectroscopic use of a novel blue diode laser in a wavelength region around 450 nm, Appl. Phys. B 79 (2004) 491–495. [13] J. Hult, I.S. Burns, C.F. Kaminski, Wide-bandwidth mode-hop-free tuning of extended-cavity GaN diode lasers, Appl. Opt. 44 (18) (2005) 3675–3685. [14] X.Q. Lv, S.W. Chen, J.Y. Zhang, L.Y. Ying, B.P. Zhang, Tuning properties of external cavity violet semiconductor laser, Chin. Phys. Lett. 30 (7) (2013), 074204-1–074204-4. [15] X. Zeng, D.L. Boiko, 1/f noise in external-cavity InGaN diode laser at 420 nm wavelength for atomic spectroscopy, Opt. Lett. 39 (6) (2014) 1685–1688. [16] N. Ruhnke, A. Müller, B. Eppich, M. Maiwald, B. Sumpf, G. Erbert, G. Tränkle, 400 mW external cavity diode laser with narrowband emission at 445 nm, Opt. Lett. 39 (13) (2014) 3794–3797. [17] C. Ye, Tunable External Cavity Diode Lasers, World Scientific Publishing, 2004.
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/jolt
Full length article
Tunable high-power blue external cavity semiconductor laser Ding Ding a, Xueqin Lv a,⇑, Xinyi Chen a, Fei Wang a, Jiangyong Zhang b, Kaijun Che b a b
Pen-Tung Sah Institute of Micro-Nano Science and Technology, Xiamen University, Xiamen 361005, China Department of Electronic Engineering, Xiamen University, Xiamen 361005, China
a r t i c l e
i n f o
Article history: Received 19 January 2017 Accepted 5 March 2017
Keywords: High-power blue laser Littrow-type external cavity Wide tuning bandwidth Narrow linewidth
a b s t r a c t A commercially available high-power GaN-based blue laser diode has been operated in a simple Littrowtype external cavity (EC). Two kinds of EC configurations with the grating lines perpendicular (A configuration) and parallel (B configuration) to the p-n junction are evaluated. Good performance has been demonstrated for the EC laser with B configuration due to the better mode selection effect induced by the narrow feedback wavelength range from the grating. Under an injection current of 1100 mA, the spectral linewidth is narrowed significantly down to 0.1 nm from 1 nm (the free-running width), with a good wavelength-locking behavior and a higher than 35 dB-amplified spontaneous emission suppression ratio. Moreover, a tuning bandwidth of 3.6 nm from 443.9 nm to 447.5 nm is realized with output power of 1.24 W and EC coupling efficiency of 80% at the central wavelength. The grating-coupled blue EC laser with narrow spectral linewidth, flexible wavelength tunability, and high output power shows potential applications in atom cooling and trapping, high-resolution spectroscopy, second harmonic generation, and high-capacity holographic data storage. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Along with the advance in material growth and device process technology, GaN-based edge-emitting semiconductor lasers are under active development. Nowadays, GaN-based violet, blue, and green multi-mode laser diodes (LDs) with the output power greater than 1 W have been commercially available [1]. However, the performance of LDs is limited by the poor mode-selection characteristic of Fabry-perot (FP) laser structure and the flat gain profile of semiconductor material, which result in the broad lasing linewidth and untunable wavelength. If tunable single mode operation can be achieved, such high-power lasers could be very useful for a variety of applications such as atom cooling and trapping [2], high-resolution spectroscopy [3], second harmonic generation [4], and high-capacity holographic data storage [5]. One possibility to shape the laser emission behavior is to change the chip design and append frequency selective components directly during the LD manufacturing process. With this technology, light emission with tunable wavelength, narrow linewidth, and high output power has been commercially achieved for GaAs-based Distributed feedback (DFB) [6] and distributed-Bragg-reflection [7] LDs. However, the fabrication technique of such GaN-based LDs is too complex to be widely commercially available. Nichia corporation ⇑ Corresponding author. E-mail address: [email protected] (X. Lv). http://dx.doi.org/10.1016/j.optlastec.2017.03.015 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.
reported continuous-wave (CW) operation of first-order AlInGaN 405 nm DFB LD on the free-standing GaN substrate [8]. The single longitudinal mode emission can be maintained up to an output power of 60 mW. However, the wavelength tuning control was not mentioned and the cost of material was very high. As far as we know, Nichia company is the only one who has reported CW operation of GaN-based DFB laser. Alternatively, optical feedback achieved by adding an externalcavity (EC) element such as diffraction grating, can not only enhance the wavelength tunability, but also reduce the intrinsic linewidth. Correspondingly, much effort has been devoted to developing a grating-coupled EC laser [5,9–16]. Lonsdale et al. have reported a 398 nm-EC laser with a maximum tuning bandwidth of 6.3 nm. However the output power is limited by the low-power LD [10]. Hildebrandt et al. have fabricated an EC laser with an antireflection-coated GaN-based LD. A total tuning range of 4 nm and an optical output power of 30 mW were realized [11]. Tanaka et al. has reported an EC laser with a wavelength of 405 nm and an output power of 80 mW for holographic data storage [5]. Recently, Ruhnke et al. demonstrated a 400 mW EC laser at 445 nm based on a commercially available high-power LD [16]. Then they used this home-made EC laser as pump source for single-pass second harmonic generation at 222.5 nm [4]. Our research team has been working on GaN-based EC laser since 2013. The performance of a violet (405 nm) EC laser with the injection current below and just above the free-running lasing
2
D. Ding et al. / Optics and Laser Technology 94 (2017) 1–5
threshold has been investigated [14]. Although there have been some reports on GaN-based grating-coupled EC lasers, the research is mainly focused on the low-power ones, while the high-power EC laser needs more studies. Generally, in order to obtain high power output, a gain device with wide stripe width and high tolerable working current is needed. In this situation, optimization of the EC configuration is very important for improving the mode selection effect. Besides, the working current is generally far beyond the free-running lasing threshold, and the competition between the inner FP cavity resonance and EC resonance makes the tuning properties become more complex. In this paper, we fabricate a high-power grating-coupled EC laser by employing a commercially available high-power GaNbased blue LD. Two kinds of Littrow configurations with the grating lines perpendicular (A configuration) and parallel (B configuration) to the p-n junction are constructed and the properties are investigated in detail. By using B configuration, 1.24 W-output power, 3.6 nm-wavelength tuning range, 0.1 nm spectral linewidth, and higher than 35 dB-amplified spontaneous emission suppression ratio are realized simultaneously under an injection current of 1100 mA. The improved performance should be more beneficial to its applications.
2. Experiments Fig. 1 shows the schematic diagram of the constructed two kinds of Littrow EC configurations. For A configuration, the grating lines are perpendicular to the p-n junction of gain device. While for B configuration, the gain device is rotated by 90° and the grating lines are parallel to the p-n junction. Both of the EC lasers consist of four optical components: a gain device, a beam collimator, an external grating, and a plane mirror. The gain device under investigation is a commercially available high-power GaN-based LD emitting at around 445 nm (Nichia, NDB7875-E). The stripe width is estimated to be 15 mm according to the observation under a microscope. The output beam has a farfield divergence of 25.5° full width at half maximum (FWHM) perpendicular to the p-n junction (fast axis), and 3.5° parallel to the pn junction (slow axis). When in use it was mounted on a thermoelectrically cooled and temperature controlled plate with a fixed temperature of 20 ± 0.3 °C. Under continuous-wave injection, the device has a threshold current of 130 mA and a maximum output power of 1.6 W. To operate safely below damage threshold, we limited the operation current to be less than 1100 mA with a maximum free-running power of 1.56 W. In the EC laser, the radiation emitted on the fast axis of gain device is nearly collimated by using an aspherical lens with a numerical aperture of 0.5 and a focal length of 8 mm (Thorlabs, 352240-A). Then the collimated light beam with the size of
7 mm in this direction hit a 2400 grooves/mm-grating under the Littrow angle, where the first order of diffraction is reflected into itself. By rotating the grating, the EC resonance wavelength can be selected. The light diffracted in the zeroth order is used for outcoupling. The length of the EC laser is about 18 cm. In order to avoid the alteration of the output beam direction during the tuning process, a beam-correction mirror is applied. For the used two kinds of EC configurations, the grating presents different diffraction efficiency due to its polarizationsensitivity. It is known that the light emitted from the LD is polarized in the plane of p-n junction. For A configuration, the grating shows a much higher first order diffraction efficiency of 36% and a much lower zeroth order diffraction efficiency of 44%, while for B configuration, the diffraction efficiency is only 11.5% in the first order and is 76% in the zeroth order.
3. Results and discussion Before performing EC tuning experiments, the properties of the free-running gain device are characterized. The normalized emission spectra of the device at four different injection currents are depicted in Fig. 2. Obviously, the lasing peak red-shifts with increasing current. Such behavior can be attributed to the decrease of the band-gap energy induced by the increasing temperature under the higher injection level. Besides, a significant broadening of the lasing spectrum with increasing current can also be observed. The FWHMs are 0.52, 1.01, and 1.08 nm for the injection current of 300, 700, and 1100 mA, respectively. This is related to the broadened gain spectrum induced by the band-filling effect. A high-resolution spectral measurement result shows that the longitudinal-mode interval is 0.025 nm. This results in a FP cavity length of around 1.6 mm. Then, two kinds of grating-coupled EC lasers are constructed and compared. Fig. 3 shows a comparison of light output power versus injection current (P-I) curves among the free-running gain device and the EC lasers tuned at 445 nm measured from the zeroth order of the grating. For the EC lasers with A and B configurations, the threshold current (Ith) is reduced to 95 mA, compared with that of free-running gain device (130 mA). Besides, the slope efficiency is extracted according to the P-I curve above its threshold and it decreases from 1.625 W/A to 0.655 and 1.280 W/A. It is well known that for conventional edge-emitting semiconductor lasers, an increase in the facet reflectivity will reduce the output loss, and thus the Ith and slope efficiency. Similarly, as an extension of the FP cavity, the grating-coupled EC laser can also be equivalent to a two-mirror cavity by combining the first-order diffraction of the grating with the reflection of laser end facet [17]. The additional diffraction of the external grating increases the effective reflectance of output end, leading to the reduction in the Ith and
Fig. 1. Schematic diagram of two kinds of EC configurations: (A) grating lines are perpendicular to the p-n junction, (B) grating lines are parallel to the p-n junction.
D. Ding et al. / Optics and Laser Technology 94 (2017) 1–5
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Fig. 2. Spectral characteristics of the free-running gain device at four different injection currents.
Fig. 4. Lasing spectra of the two kinds of EC lasers with constant grating angle.
Fig. 3. Light output power versus injection current for the free-running gain device and the EC lasers.
slope efficiency finally. At an injection current of 1100 mA, the optical powers are 1.559 W, 0.655 W, and 1.242 W for the freerunning gain device, and the EC lasers with A and B configurations respectively. The latters correspond to the efficiencies of 42% and 80% with respect to the free-running device. Thus for the gratingcoupled EC lasers with the light diffracted in the zeroth order as output, the higher zeroth order diffraction efficiency conduces to achieving the higher output power. The spectral characteristics of the two kinds of EC lasers are then investigated. Fig. 4 shows the typical lasing spectra at different injection currents when the grating is fixed to a constant angle. It can be seen that for A configuration in which the wavelength selection takes place along the emitter window (15 mm), the lasing spectra presents a multi-longitudinal-mode structure. At 1100 mA, the FWHM is 0.28 nm. For B configuration in which the wavelength selection takes place along the thickness of p-n junction (1 mm), the lasing linewidth for the injection current of 340 mA is narrowed down to 0.02 nm which is the spectral resolution of the spectrometer. With increasing current, the linewidth broadens to 0.10 nm at 1100 mA due to the enhanced light feedback from the external grating. Meanwhile the peak wavelength exhibits a good wavelength-locking behavior with only 0.02 nm shifting towards the long wavelength side. This is probably related to the high temperature induced red shift of the material gain spectrum caused by the higher injection level which has been identified
by the free-running lasing spectra as shown in Fig. 2. Besides, the high temperature induced thermal expansion will lead to a slight deformation of the device. Both of them will result in an influence on the spectral stability of EC laser. It is concluded that the better mode selection effect can be obtained by using the B configuration. This can be explained by the different feedback wavelength ranges from the grating. For the Littrow-type EC laser, the emitter size contribution to the feedh back wavelength range dk can be given by the formula: dk ¼ kw cot , f where w is the emitter size, h is the angle between the optical axis and the grating normal, and f is the focal length of the fast axis collimation. It seems that dk is proportional to the emitter size w. According to the formula, the dks are estimated to be 1.32 nm and 0.09 nm at the lasing wavelength of 445 nm for A and B configurations with emitter size of 15 mm and 1 mm respectively. Therefore, for A configuration the obtained large feedback wavelength range dk from the grating results in the broad multi-peak spectral structure. Moreover, under this configuration the continuous wavelength tuning is difficult to achieve by rotating the angle of the grating due to the poor mode selection effect. Thus B configuration is adopted in our following tuning experiments. The wavelength tunability is then evaluated for the EC laser with B configuration by tuning the incidence angle of the grating. Fig. 5(a)–(d) shows the tunable lasing spectra under varieties of bias conditions: the injection current from 130 mA to 1100 mA with normalized intensity for clarity. The EC lasing is realized around the free-running lasing wavelength. Especially, when the EC laser is tuned at the center of the gain peak, the amplified spontaneous emission suppression ratio (ASESR) is higher than 35 dB. However, the ASESR is reduced to 10–20 dB at the edged wavelengths due to the appearance of the FP resonance peak. The lower ASESR at these wavelengths reduces the output power within the narrowband emission of the EC laser. For example, at 1100 mA for the EC wavelengths of 443.9 nm and 447.5 nm, the output power decreases from the measured 1230 mW and 1202 mW to the estimated 712 mW and 906 mW respectively by the spectral area integral procedure.
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Fig. 5. Tuning spectra of the EC laser with B configuration at different injection currents.
below the free-running lasing threshold, the output power is limited to be a dozen milliwatts. To obtain high output power, the injection current should be enhanced. By increasing the injection current from the free-running lasing threshold to 900 mA, the tuning range decreases gradually from 7.2 nm to 2.6 nm due to the lasing interference of inner cavity. A further increase in injection level causes an extension of the tuning range to the longer wavelength side owing to the combined contributions of thermal effect and band-filling effect. Besides, it can also be seen that the output power increases monotonously from several milliwatts to 1242 mW with the increasing injection current. Under a certain injection current, the estimated output power decreases by 54–90% at the edged tuning wavelengths due to the appearance of FP lasing peak. However, at the central wavelength the output power is not affected by the FP interference. The above results demonstrate that high-power blue EC laser with narrow spectral linewidth and flexible wavelength tunability has been obtained. For the further practical applications, another aspect worth noting is that accurate temperature control of the gain device is important to maintain the power and spectral stability of EC laser. The precision of the temperature controller used in this paper is plus or minus 0.3°. However, under high injection current, the gain device will produce a lot of heat which cannot dissipate quickly, therefore the true temperature change of the gain device will be relatively large. The temperature rise of the gain device will not only cause a redshift of the gain spectrum, but also lead to a slight deformation of the device due to the thermal expansion. Both of them will result in an influence on the stability of EC laser. So how to control the temperature of gain device accurately is a technical problem to be solved. Besides, in our paper the wavelength tuning was realized with a manual control of the diffraction grating movement. In order to facilitate the application, the wavelength change needs to be controlled more accurately and the electric tuning is necessary. To solve this problem, the mechanical structure such as sine mechanism or worm and gear can be designed.
4. Summary
Fig. 6. Wavelength tuning range and output power of EC laser with B configuration under different injection currents. The solid and dashed lines represent the measured and estimated output power respectively.
Fig. 6 shows the actual wavelength tuning range and the output power of the EC laser with B configuration achieved under per different injection current. The measured and estimated output power are marked by the solid and dashed lines respectively. The highest wavelength tuning range of 7.2 nm from 440.3 nm to 447.5 nm is obtained with the injection current of 130 mA, which is just at the threshold current of the free-running gain device. Although the tuning range is large when the injection current is
In conclusion, we fabricated a high-power grating-coupled EC laser based on a commercially available high-power GaN LD. Benefiting from the applied EC placement configuration in which the wavelength selection takes place along the 1 mm-emission aperture, high-power EC laser with narrow linewidth and broad wavelength tuning range is demonstrated. The spectral measurement results show that the lasing linewidth is about 0.1 nm at 1100 mA, the ASESR is higher than 35 dB at the central wavelength, and the peak wavelength is well locked under different injection currents. Moreover, under the injection current of 1100 mA, a tuning bandwidth of 3.6 nm from 443.9 nm to 447.5 nm is realized by simply changing the grating rotation angle. The output power can reach up to 1.24 W and the EC coupling efficiency is 80%. This type of blue EC laser will be especially suitable for atom cooling and trapping, high-resolution spectroscopy, second harmonic generation, and high-capacity holographic data storage.
Acknowledgements This work has been supported by the National Natural Science Foundation of China (61306087 and 61574119), Natural Science Foundation of Fujian Province of China (2017J01120 and 2015J05130), and Fundamental Research Funds for the Central Universities (20720150084).
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