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Wavelength tuning of InAs quantum dot laser by micromirror device
Journal of Crystal Growth 425 (2015) 373–375
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Wavelength tuning of InAs quantum dot laser by micromirror device J.Y. Yan a, Q. Gong a,n, C.Z. Kang a, H.X. Xu a, C.F. Cao a, Y.Y. Li a, S.M. Wang a,1, H.L. Wang b a State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, People's Republic of China b Department of Physics, Qufu Normal University, Qufu 273165, People's Republic of China
art ic l e i nf o
a b s t r a c t
Available online 17 March 2015
We report on the InAs quantum dot (QD) external cavity laser (ECL) using a digital mirror device (DMD) as the key component for wavelength tuning. The InAs QD laser diode was grown by gas source molecular-beam epitaxy, which had a broad gain profile. Single mode operation was achieved with the side mode suppression ratio of 21 dB when the optical feedback was provided by a mirror pattern consisting of 9 micromirrors. Moreover, two-color lasing was demonstrated with two laser lines having frequency difference in the THz range. The incorporation of DMD in the ECL enables great flexibility and many unique features, such as high tuning speed independent of the tuning step, two-color or multicolor lasing, and adjustable intensity for individual laser lines. & 2015 Elsevier B.V. All rights reserved.
Keywords: A3. Molecular beam epitaxy B1. Gallium compounds B2. Semiconducting Gallium Arsenide B3. Laser diode
1. Introduction A unique feature of quantum dot (QD) lasers is that their gain profile is much broader than the conventional quantum well (QW) lasers, which is extremely desirable for realizing broadband tunable external cavity lasers (ECLs) [1,2]. In classical ECL configurations, i.e., Littman and Littrow, the function of wavelength tuning is achieved by mechanically rotating the grating or the cavity mirror. Thus, a broad wavelength tuning range needs significant rotation movement of the grating or mirror, which may drastically degrade the reliability and reproducibility of the whole system. Recently, a concept of Fourier transform ECL (FTECL) was developed by Breede et al. [3,4] to tune QW lasers for the purposes of fast tuning and multiple-color lasing. In this paper, we propose that the FTECL concept can be utilized to perfectly solve the problem mentioned above by using a digital micromirror device (DMD) to choose the optical feedback for the QD laser. Each micromirror can be electronically turned on or off in order to adjust the optical feedback. In this way, the movement of mirror/grating in the Littman/Littrow configuration can be completely replaced by control of the micromirrors at desired locations, resulting in good reliability and reproducibility of the broadband tunable QD ECL. Regarding the laser diode for tunable laser, broad gain profile is one of the very desirable properties. Growth of self-assembled QDs in Stranski–Krastanov (SK) mode has been demonstrated as a very n
Corresponding author. E-mail address: [email protected] (Q. Gong). 1 Also at Department of Microtechnology, Chalmers University of Technology, 41296 Göteborg, Sweden. http://dx.doi.org/10.1016/j.jcrysgro.2015.03.011 0022-0248/& 2015 Elsevier B.V. All rights reserved.
efficient way to obtain high density of zero-dimensional nanostructures [5]. In addition, these densely packed QDs have very good optical properties and can be used as active material for high performance QD lasers [6–9]. Due to the basic nature of the SK growth mode, the QD assembly always has relative broad size distribution, leading to a gain profile much wider than that of QWs. This feature is very useful for broadly tunable lasers. Previously, we studied external cavity tunable InAs/GaAs and InAs/InP [2] QD lasers with broad tunable range in Littrow configuration. In this work, we modified the setup by using a DMD to generate optical feedback, which brings unique features and flexibility to the ECL system.
2. Experimental setup The QD lasers were grown on n-type GaAs (100) substrates by gas source molecular-beam epitaxy (GSMBE). In the GSMBE system, pure metal gallium, indium, and aluminum are used to provide III element flux, while AH3 gas is introduced into a high temperature injector and thermally decomposed there at 1000 1C. After oxide desorption, a 500 nm-thick Si-doped GaAs buffer layer was grown, followed by a 1500 nm-thick Si-doped Al0:3 Ga0:7 As layer for the optical confinement. The active region consisted of five-stacked InAs QD layers embedded in a 160 nm-thick Al0:15 Ga0:85 As layer. GaAs spacers (40-nm-thick) were inserted into the adjacent QD layers. Finally, the structure was finished by growing 1500 nm-thick Be-doped Al0:3 Ga0:7 As cladding layer and 200 nm-thick Be-doped GaAs contact layer. Each QD layer was formed by deposition of nominally 2.2 monolayers (MLs) InAs with growth rate of 0.12 ML/s.
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J.Y. Yan et al. / Journal of Crystal Growth 425 (2015) 373–375
The background pressure in the growth chamber was 1:5 10 5 Torr during growth. Ridge waveguide laser diodes were fabricated with strip width of 6 μm. Both facets of the laser diode were left as-cleaved without coating. The LD chips were soldered on copper heat sinks by thin indium films. The QD laser diode was mounted in a FTECL setup illustrated in Fig. 1. Firstly, the laser beam was collimated by a lens, and then diffracted by a grating. A second lens was used to focus the diffracted light, resulting in a series of focused light spots, where the DMD was placed. Note that the beam only contains light of Fabry–Perot mode existing in the internal cavity of laser diode. Therefore on the focal plane of the second lens there are a series of laser spots corresponding to diffracted beams with discrete wavelengths. The beam size for a single mode is estimated as about 20 μm. The DMD was controlled to reflect the light either back to the laser diode or just to the free space. In the former case an external cavity is formed between the laser facet and the DMD, leading to optical feedback to the laser diode, i.e., reduction of mirror loss of the whole system. The DMD consists of an array of micromirrors with dimensions of 13:68 13:68 μm2 . Each micromirror can be electronically controlled to rotate 7121, corresponding to an “on” and “off” state. The micromirrors with “on” state will generate optical feedback to the laser diode, while that with “off” state will not. Therefore, we can choose the mirror patterns corresponding to one or more focused laser spots on the focal plane to reflect the laser beam back and generate optical feedback to the QD laser diode.
3. Results and discussions
activating different columns of micromirrors. For example, single mode lasing at 1013.1 nm was realized by activating the single column of micromirrors with column number of L1. On the other hand, lasing occurred at 1010.2 nm when the active column was at number L4. Note that the tuning speed of the system was mainly limited by the operation speed of DMD, e.g., in the kHz range. It is worthy to note that such a high tuning speed is independent of the wavelength tuning gap, because just one command is needed to change the mirror pattern whatever the pattern is. So this system has an advantage over the conventional Littrow or Littman configuration, where large tuning gap usually needs more time to gain enough rotation angle of the grating or mirror.
3.2. Optimization of the mirror pattern In order to obtain the optical feedback as large as possible, we optimized the reflecting mirror pattern. A pattern consisted of 9 micromirrors was found to fit the focused laser spot very well, resulting in high quality lasing spectrum in single mode, as shown in Fig. 3. The side mode suppression ratio was measured as 21 dB, when the feedback was provided by the optimized pattern shown in the inset of Fig. 3. Adding more active mirrors to the optimized mirror pattern did not improve the laser performance, indicating that the focused laser spot was completely covered by the mirror pattern and maximum optical feedback was achieved. The output power was measured as a few mW, depending on the injection current.
3.3. Two-color lasing
3.1. Tuning by a column of micromirrors The wavelength tuning of the system was tested by using a single column of micromirrors for feedback. As shown in Fig. 2, single mode lasing at different wavelengths was observed by
Single mode operation was obtained by activating one mirror pattern as mentioned above. If more mirror patterns were added, more laser lines would appear, given that each mirror pattern covers the corresponding focused laser spot in space. For example, twocolor lasing was obtained by just activating two mirror patterns, as shown in Fig. 4. The separated mirror patterns are shown in the inset, while the distance between two patterns determines the wavelength difference between the two lasing lines. The diffraction capability of grating and the focal length of the lens together define the dependence of the wavelength difference on the distance between the active patterns. It is worth noting that the wavelength difference of the two-color lasing shown Fig. 4 is 3.56 nm, i.e., 1.04 THz difference in frequency. Therefore, this system is able to generate two laser lines with frequency difference in the THz range, which might be used in THz generation by the technique of frequency mixing. Moreover, by activating proper mirror patterns, multiple lasing lines with desirable wavelengths can be obtained.
Fig. 1. Schematic illustration of the FTECL setup with a InAs QD laser, a grating, two lenses, and a DMD.
L3
Intensity (arb. units)
L1 L2 L4
1009
1010
1011
1012
1013
Wavelength (nm) Fig. 2. Lasing spectra of the InAs QD ECL with optical feedback provided by activating single column of micromirrors with different column numbers.
Fig. 3. Lasing spectrum of the InAs QD ECL with the optical feedback provided by the optimized mirror pattern consisting of 9 micromirrors, which are shown in the inset.
J.Y. Yan et al. / Journal of Crystal Growth 425 (2015) 373–375
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was proved by the results shown in Fig. 5. We started with a twocolor lasing case, where one laser line had much higher intensity than the other, as shown in Fig. 5(a). Then the active mirror patterns were changed by increasing the optical feedback for the low intensity laser line, and, decreasing the optical feedback for the high intensity laser line, as shown in the insets. Indeed, the relative intensities of the two laser lines became more close, as shown in Fig. 5(b). The original low intensity laser line was enhanced in intensity, while the high intensity one was suppressed. Therefore, fine regulation of the strength of optical feedback is very effective to adjust the intensity of an individual laser line. This method might be very important to balance the intensity of two or even more lasing lines when the system is working in multi-color mode. Fig. 4. Lasing spectrum of the QD ECL when there were two active mirror patterns. The spectrum consisted of two lasing lines with a wavelength difference determined by the distance between the mirror patterns.
4. Conclusion We have investigated the InAs QD ECL using a DMD as the key component for wavelength tuning. The system has fast tuning speed, and, in particular, the tuning speed is intrinsically independent of the wavelength tuning gap. High quality of single mode spectrum was achieved by using the optimized mirror pattern consisting of 9 micromirrors. Moreover, it is quite convenient to realize two-color or multicolor operation by just activating two or more mirror patterns. The wavelength difference between the laser lines can be adjusted by the distance between the mirror patterns. Two laser lines with frequency difference in the THz range were demonstrated. In addition, the intensities of the laser lines can be adjusted by fine regulation of the mirror patterns when the system is working in the multi-color mode. We have shown that this system has much more flexibility than the conventional Littrow and Littman configurations.
Acknowledgments This work was supported by the National Natural Foundation of China (Grant nos. 61176065 and 61321492) and the National Basic Research Program of China (973 project) (Grant no. 2013CB632800). References
Fig. 5. (a) Lasing spectrum of the InAs QD ECL working in two-color mode, where one laser line was much stronger than the other. (b) Lasing spectrum after modifying the active mirror patterns. The insets show the active mirror patterns corresponding to the spectra.
3.4. Intensity adjustment of the laser lines In the case of two-color lasing discussed above, the relative intensities of the two laser lines are mainly determined by the optical gain and loss, which vary as a function of the wavelength. Therefore, the intensity of the individual lasing line should be very sensitive to the optical feedback provided by the active mirror pattern. In other words, the active mirror pattern reduces the mirror loss of the external cavity, thus, enhances the optical feedback. The concept
[1] P.M. Varangis, H. Li, G.T. Liu, T.C. Newell, A. Stintz, B. Fuchs, K.J. Malloy, L. F. Lester, Low-threshold quantum dot lasers with 201 nm tuning range, Electron. Lett. 36 (18) (2000) 1544–1545. [2] P. Chen, Q. Gong, C.F. Cao, S.G. Li, Y. Wang, Q.B. Liu, L. Yue, Y.G. Zhang, S.L. Feng, C.H. Ma, H.L. Wang, High performance external cavity InAs/InP quantum dot lasers, Appl. Phys. Lett. 98 (12) (2011) 121102. [3] M. Breede, S. Hoffmann, J. Zimmermann, J. Struckmeier, M. Hofmann, T. KleineOstmann, P. Knobloch, M. Koch, J.P. Meyn, M. Matus, S.W. Koch, J.V. Moloney, Fourier-transform external cavity lasers, Opt. Commun. 207 (2002) 261–271. [4] M. Breede, C. Kasseck, C. Brenner, N.C. Gerhardt, M. Hofmann, R. Höfling, Micromirror device controlled tunable diode laser, Opt. Lett. 43 (8) (2007) 456–457. [5] D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaars, P.M. Petroff, Direct formation of quantum-sized dots from uniform coherent islands of InGaAs on GaAs surfaces, Appl. Phys. Lett. 63 (23) (1993) 3203–3205. [6] D.L. Huffaker, G. Park, Z. Zou, O.B. Shchekin, D.G. Deppe, 1.3 μm roomtemperature GaAs-based quantum-dot laser, Appl. Phys. Lett. 73 (18) (1998) 2564–2566. [7] M. Grundmann, The present status of quantum dot lasers, Physica E 5 (2000) 167–184. [8] Z. Mi, P. Bhattacharya, S. Fathpour, High-speed 1.3 μm tunnel injection quantum-dot lasers, Appl. Phys. Lett. 86 (15) (2005) 153109. [9] C.S. Lee, W. Guo, D. Basu, P. Bhattacharya, High performance tunnel injection quantum dot comb laser, Appl. Phys. Lett. 96 (10) (2010) 101107.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Wavelength tuning of InAs quantum dot laser by micromirror device J.Y. Yan a, Q. Gong a,n, C.Z. Kang a, H.X. Xu a, C.F. Cao a, Y.Y. Li a, S.M. Wang a,1, H.L. Wang b a State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, People's Republic of China b Department of Physics, Qufu Normal University, Qufu 273165, People's Republic of China
art ic l e i nf o
a b s t r a c t
Available online 17 March 2015
We report on the InAs quantum dot (QD) external cavity laser (ECL) using a digital mirror device (DMD) as the key component for wavelength tuning. The InAs QD laser diode was grown by gas source molecular-beam epitaxy, which had a broad gain profile. Single mode operation was achieved with the side mode suppression ratio of 21 dB when the optical feedback was provided by a mirror pattern consisting of 9 micromirrors. Moreover, two-color lasing was demonstrated with two laser lines having frequency difference in the THz range. The incorporation of DMD in the ECL enables great flexibility and many unique features, such as high tuning speed independent of the tuning step, two-color or multicolor lasing, and adjustable intensity for individual laser lines. & 2015 Elsevier B.V. All rights reserved.
Keywords: A3. Molecular beam epitaxy B1. Gallium compounds B2. Semiconducting Gallium Arsenide B3. Laser diode
1. Introduction A unique feature of quantum dot (QD) lasers is that their gain profile is much broader than the conventional quantum well (QW) lasers, which is extremely desirable for realizing broadband tunable external cavity lasers (ECLs) [1,2]. In classical ECL configurations, i.e., Littman and Littrow, the function of wavelength tuning is achieved by mechanically rotating the grating or the cavity mirror. Thus, a broad wavelength tuning range needs significant rotation movement of the grating or mirror, which may drastically degrade the reliability and reproducibility of the whole system. Recently, a concept of Fourier transform ECL (FTECL) was developed by Breede et al. [3,4] to tune QW lasers for the purposes of fast tuning and multiple-color lasing. In this paper, we propose that the FTECL concept can be utilized to perfectly solve the problem mentioned above by using a digital micromirror device (DMD) to choose the optical feedback for the QD laser. Each micromirror can be electronically turned on or off in order to adjust the optical feedback. In this way, the movement of mirror/grating in the Littman/Littrow configuration can be completely replaced by control of the micromirrors at desired locations, resulting in good reliability and reproducibility of the broadband tunable QD ECL. Regarding the laser diode for tunable laser, broad gain profile is one of the very desirable properties. Growth of self-assembled QDs in Stranski–Krastanov (SK) mode has been demonstrated as a very n
Corresponding author. E-mail address: [email protected] (Q. Gong). 1 Also at Department of Microtechnology, Chalmers University of Technology, 41296 Göteborg, Sweden. http://dx.doi.org/10.1016/j.jcrysgro.2015.03.011 0022-0248/& 2015 Elsevier B.V. All rights reserved.
efficient way to obtain high density of zero-dimensional nanostructures [5]. In addition, these densely packed QDs have very good optical properties and can be used as active material for high performance QD lasers [6–9]. Due to the basic nature of the SK growth mode, the QD assembly always has relative broad size distribution, leading to a gain profile much wider than that of QWs. This feature is very useful for broadly tunable lasers. Previously, we studied external cavity tunable InAs/GaAs and InAs/InP [2] QD lasers with broad tunable range in Littrow configuration. In this work, we modified the setup by using a DMD to generate optical feedback, which brings unique features and flexibility to the ECL system.
2. Experimental setup The QD lasers were grown on n-type GaAs (100) substrates by gas source molecular-beam epitaxy (GSMBE). In the GSMBE system, pure metal gallium, indium, and aluminum are used to provide III element flux, while AH3 gas is introduced into a high temperature injector and thermally decomposed there at 1000 1C. After oxide desorption, a 500 nm-thick Si-doped GaAs buffer layer was grown, followed by a 1500 nm-thick Si-doped Al0:3 Ga0:7 As layer for the optical confinement. The active region consisted of five-stacked InAs QD layers embedded in a 160 nm-thick Al0:15 Ga0:85 As layer. GaAs spacers (40-nm-thick) were inserted into the adjacent QD layers. Finally, the structure was finished by growing 1500 nm-thick Be-doped Al0:3 Ga0:7 As cladding layer and 200 nm-thick Be-doped GaAs contact layer. Each QD layer was formed by deposition of nominally 2.2 monolayers (MLs) InAs with growth rate of 0.12 ML/s.
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J.Y. Yan et al. / Journal of Crystal Growth 425 (2015) 373–375
The background pressure in the growth chamber was 1:5 10 5 Torr during growth. Ridge waveguide laser diodes were fabricated with strip width of 6 μm. Both facets of the laser diode were left as-cleaved without coating. The LD chips were soldered on copper heat sinks by thin indium films. The QD laser diode was mounted in a FTECL setup illustrated in Fig. 1. Firstly, the laser beam was collimated by a lens, and then diffracted by a grating. A second lens was used to focus the diffracted light, resulting in a series of focused light spots, where the DMD was placed. Note that the beam only contains light of Fabry–Perot mode existing in the internal cavity of laser diode. Therefore on the focal plane of the second lens there are a series of laser spots corresponding to diffracted beams with discrete wavelengths. The beam size for a single mode is estimated as about 20 μm. The DMD was controlled to reflect the light either back to the laser diode or just to the free space. In the former case an external cavity is formed between the laser facet and the DMD, leading to optical feedback to the laser diode, i.e., reduction of mirror loss of the whole system. The DMD consists of an array of micromirrors with dimensions of 13:68 13:68 μm2 . Each micromirror can be electronically controlled to rotate 7121, corresponding to an “on” and “off” state. The micromirrors with “on” state will generate optical feedback to the laser diode, while that with “off” state will not. Therefore, we can choose the mirror patterns corresponding to one or more focused laser spots on the focal plane to reflect the laser beam back and generate optical feedback to the QD laser diode.
3. Results and discussions
activating different columns of micromirrors. For example, single mode lasing at 1013.1 nm was realized by activating the single column of micromirrors with column number of L1. On the other hand, lasing occurred at 1010.2 nm when the active column was at number L4. Note that the tuning speed of the system was mainly limited by the operation speed of DMD, e.g., in the kHz range. It is worthy to note that such a high tuning speed is independent of the wavelength tuning gap, because just one command is needed to change the mirror pattern whatever the pattern is. So this system has an advantage over the conventional Littrow or Littman configuration, where large tuning gap usually needs more time to gain enough rotation angle of the grating or mirror.
3.2. Optimization of the mirror pattern In order to obtain the optical feedback as large as possible, we optimized the reflecting mirror pattern. A pattern consisted of 9 micromirrors was found to fit the focused laser spot very well, resulting in high quality lasing spectrum in single mode, as shown in Fig. 3. The side mode suppression ratio was measured as 21 dB, when the feedback was provided by the optimized pattern shown in the inset of Fig. 3. Adding more active mirrors to the optimized mirror pattern did not improve the laser performance, indicating that the focused laser spot was completely covered by the mirror pattern and maximum optical feedback was achieved. The output power was measured as a few mW, depending on the injection current.
3.3. Two-color lasing
3.1. Tuning by a column of micromirrors The wavelength tuning of the system was tested by using a single column of micromirrors for feedback. As shown in Fig. 2, single mode lasing at different wavelengths was observed by
Single mode operation was obtained by activating one mirror pattern as mentioned above. If more mirror patterns were added, more laser lines would appear, given that each mirror pattern covers the corresponding focused laser spot in space. For example, twocolor lasing was obtained by just activating two mirror patterns, as shown in Fig. 4. The separated mirror patterns are shown in the inset, while the distance between two patterns determines the wavelength difference between the two lasing lines. The diffraction capability of grating and the focal length of the lens together define the dependence of the wavelength difference on the distance between the active patterns. It is worth noting that the wavelength difference of the two-color lasing shown Fig. 4 is 3.56 nm, i.e., 1.04 THz difference in frequency. Therefore, this system is able to generate two laser lines with frequency difference in the THz range, which might be used in THz generation by the technique of frequency mixing. Moreover, by activating proper mirror patterns, multiple lasing lines with desirable wavelengths can be obtained.
Fig. 1. Schematic illustration of the FTECL setup with a InAs QD laser, a grating, two lenses, and a DMD.
L3
Intensity (arb. units)
L1 L2 L4
1009
1010
1011
1012
1013
Wavelength (nm) Fig. 2. Lasing spectra of the InAs QD ECL with optical feedback provided by activating single column of micromirrors with different column numbers.
Fig. 3. Lasing spectrum of the InAs QD ECL with the optical feedback provided by the optimized mirror pattern consisting of 9 micromirrors, which are shown in the inset.
J.Y. Yan et al. / Journal of Crystal Growth 425 (2015) 373–375
375
was proved by the results shown in Fig. 5. We started with a twocolor lasing case, where one laser line had much higher intensity than the other, as shown in Fig. 5(a). Then the active mirror patterns were changed by increasing the optical feedback for the low intensity laser line, and, decreasing the optical feedback for the high intensity laser line, as shown in the insets. Indeed, the relative intensities of the two laser lines became more close, as shown in Fig. 5(b). The original low intensity laser line was enhanced in intensity, while the high intensity one was suppressed. Therefore, fine regulation of the strength of optical feedback is very effective to adjust the intensity of an individual laser line. This method might be very important to balance the intensity of two or even more lasing lines when the system is working in multi-color mode. Fig. 4. Lasing spectrum of the QD ECL when there were two active mirror patterns. The spectrum consisted of two lasing lines with a wavelength difference determined by the distance between the mirror patterns.
4. Conclusion We have investigated the InAs QD ECL using a DMD as the key component for wavelength tuning. The system has fast tuning speed, and, in particular, the tuning speed is intrinsically independent of the wavelength tuning gap. High quality of single mode spectrum was achieved by using the optimized mirror pattern consisting of 9 micromirrors. Moreover, it is quite convenient to realize two-color or multicolor operation by just activating two or more mirror patterns. The wavelength difference between the laser lines can be adjusted by the distance between the mirror patterns. Two laser lines with frequency difference in the THz range were demonstrated. In addition, the intensities of the laser lines can be adjusted by fine regulation of the mirror patterns when the system is working in the multi-color mode. We have shown that this system has much more flexibility than the conventional Littrow and Littman configurations.
Acknowledgments This work was supported by the National Natural Foundation of China (Grant nos. 61176065 and 61321492) and the National Basic Research Program of China (973 project) (Grant no. 2013CB632800). References
Fig. 5. (a) Lasing spectrum of the InAs QD ECL working in two-color mode, where one laser line was much stronger than the other. (b) Lasing spectrum after modifying the active mirror patterns. The insets show the active mirror patterns corresponding to the spectra.
3.4. Intensity adjustment of the laser lines In the case of two-color lasing discussed above, the relative intensities of the two laser lines are mainly determined by the optical gain and loss, which vary as a function of the wavelength. Therefore, the intensity of the individual lasing line should be very sensitive to the optical feedback provided by the active mirror pattern. In other words, the active mirror pattern reduces the mirror loss of the external cavity, thus, enhances the optical feedback. The concept
[1] P.M. Varangis, H. Li, G.T. Liu, T.C. Newell, A. Stintz, B. Fuchs, K.J. Malloy, L. F. Lester, Low-threshold quantum dot lasers with 201 nm tuning range, Electron. Lett. 36 (18) (2000) 1544–1545. [2] P. Chen, Q. Gong, C.F. Cao, S.G. Li, Y. Wang, Q.B. Liu, L. Yue, Y.G. Zhang, S.L. Feng, C.H. Ma, H.L. Wang, High performance external cavity InAs/InP quantum dot lasers, Appl. Phys. Lett. 98 (12) (2011) 121102. [3] M. Breede, S. Hoffmann, J. Zimmermann, J. Struckmeier, M. Hofmann, T. KleineOstmann, P. Knobloch, M. Koch, J.P. Meyn, M. Matus, S.W. Koch, J.V. Moloney, Fourier-transform external cavity lasers, Opt. Commun. 207 (2002) 261–271. [4] M. Breede, C. Kasseck, C. Brenner, N.C. Gerhardt, M. Hofmann, R. Höfling, Micromirror device controlled tunable diode laser, Opt. Lett. 43 (8) (2007) 456–457. [5] D. Leonard, M. Krishnamurthy, C.M. Reaves, S.P. Denbaars, P.M. Petroff, Direct formation of quantum-sized dots from uniform coherent islands of InGaAs on GaAs surfaces, Appl. Phys. Lett. 63 (23) (1993) 3203–3205. [6] D.L. Huffaker, G. Park, Z. Zou, O.B. Shchekin, D.G. Deppe, 1.3 μm roomtemperature GaAs-based quantum-dot laser, Appl. Phys. Lett. 73 (18) (1998) 2564–2566. [7] M. Grundmann, The present status of quantum dot lasers, Physica E 5 (2000) 167–184. [8] Z. Mi, P. Bhattacharya, S. Fathpour, High-speed 1.3 μm tunnel injection quantum-dot lasers, Appl. Phys. Lett. 86 (15) (2005) 153109. [9] C.S. Lee, W. Guo, D. Basu, P. Bhattacharya, High performance tunnel injection quantum dot comb laser, Appl. Phys. Lett. 96 (10) (2010) 101107.