August 1, 2014 / Vol. 39, No. 15 / OPTICS LETTERS
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All-optical switching with a dual-state, single-section quantum dot laser via optical injection Boguslaw Tykalewicz,1,2,* David Goulding,1,2 Stephen P. Hegarty,2 Guillaume Huyet,1,2,3 Diarmuid Byrne,4 Richard Phelan,4 and Bryan Kelleher1,2 1
Centre for Advanced Photonics and Process Analysis (CAPPA), Cork Institute of Technology, Cork, Ireland 2
3
Tyndall National Institute, National University of Ireland, University College, Cork, Ireland National Research University of Information Technologies, Mechanics and Optics, Saint Petersburg, Russia 4
Eblana Photonics, Unit 32 Trinity Technology and Enterprise Campus, Pearse St., Dublin 2, Ireland *Corresponding author: [email protected] Received May 21, 2014; revised June 27, 2014; accepted July 4, 2014; posted July 7, 2014 (Doc. ID 212502); published July 31, 2014
An all-optical switching mechanism via optical injection of an InAs/GaAs quantum dot laser is presented. Relative state suppression in excess of 40 dB is achieved, and experimental switching times of the order of a few hundred picoseconds are demonstrated. © 2014 Optical Society of America OCIS codes: (250.5590) Quantum-well, -wire and -dot devices; (250.6715) Switching; (250.3750) Optical logic devices; (140.3520) Lasers, injection-locked. http://dx.doi.org/10.1364/OL.39.004607
Over the past few years, the topic of optical injection [1] has attracted immense interest from both the scientific and engineering communities due to an abundance of rich dynamical regimes in the system and the array of potential applications of the technique. For example, from a nonlinear dynamics standpoint optically injected lasers have demonstrated multistability [2], chaos [3], and excitability [4,5], thereby providing an opportunity to gain further insight into fundamental dynamical processes using a relatively straightforward configuration. From the point of view of modern day optical engineering applications, there are many features that make optical injection an attractive technique, including a significant reduction in the laser linewidth [6], improvements in the relative intensity noise [7], and a large enhancement in the laser modulation bandwidth [8]. Self-assembled InAs quantum dots (QDs) grown on a GaAs substrate have been successfully utilized in many optoelectronic applications. An important property of QD-based semiconductor lasers is the high damping of the relaxation oscillations (ROs) [9]. It has been shown that this is one of the main reasons for the improved performance of QD devices over both bulk and quantumwell-based devices in several dynamical configurations, ranging from optical feedback to mutual coupling [5,10–13]. A key result for this work is that in comparison to other device types, a relatively broad region of stable locking can be obtained using QD lasers even for low optical injection powers. QDs display inhomogeneously broadened discrete energy states that can lead to the interesting property of lasing at clearly separated wavelength bands. For example, one can obtain simultaneous lasing from the ground state (GS) and the excited state (ES). The specific behavior depends on several control parameters, such as cavity length, injected current, and temperature of operation, and was previously attributed to incomplete clamping of the ES population at the GS threshold [14,15]. Control of switching between states is a very interesting topic and could be of interest for both fundamental science and applications [16]. 0146-9592/14/154607-04$15.00/0
All-optical switching is of great interest in a variety of technological applications and has previously been demonstrated using, for example, vertical-cavity surfaceemitting lasers [17,18] and two-color edge-emitting lasers [19]. In QD lasers electrical switching between GS and ES emission has been demonstrated using two-section devices [20,21]. This Letter presents an all-optical switching mechanism between the GS and the ES of a singlesection QD laser. Dynamic switching between the states is also shown with very fast switching times of the order of several hundreds of picoseconds obtained. The device used in the experiment was an InAs/GaAs discrete mode QD laser [22]. The length of the device and ridge waveguide width were 0.6 mm and 2.0 μm, respectively. The threshold current of the laser was approximately 33 mA. The laser was designed to operate single mode in the GS by selective placement of reflective elements along the laser cavity. The positioning of these elements provides an enhancement of a specific Fabry– Perot mode in the GS. In order to simplify nomenclature, we refer to this mode as the preferential mode. The devices were not designed to provide enhancement of a specific mode at the ES wavelength and thus the device operates multimode in the ES. Figure 1 shows the lightcurrent (LI) curve and optical spectrum at three different currents corresponding to three different regimes of operation. In the first regime, as can be seen in the top panel of Fig. 1(b), the device lases at the preferential longitudinal mode in the GS with a typical side mode suppression ratio in excess of 50 dB. As the injection current to the device was increased, the device underwent a second threshold (at approximately twice the GS threshold value) and lasing from both the GS and the ES was observed. The optical spectrum of the device in this regime of operation is shown in the middle panel of Fig. 1(b). As described above, the emission from the GS was dominated by the preferential mode while the emission from the ES was multimode. At a typical operating condition of 20°C, the GS and ES lasing wavelengths were approximately 1300 and 1215 nm, respectively. As the current was further increased (above 80 mA) the device finally © 2014 Optical Society of America
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Fig. 1. (a) LI characteristics of the SL. Different lasing regimes are separated by vertical dashed lines. (b) SL spectral evolution for three different bias currents, (i) 50 mA, (ii) 70 mA, and (iii) 100 mA.
displayed a complete suppression of the GS emission and exhibited lasing in the ES only, as evidenced in the bottom panel of Fig. 1(b). Because of the higher degeneracy of the ES all available carriers contributed to ES lasing only and consequently the GS was quenched [15]. While the principal aim was to characterize the dynamic injection scenario, static optical injection of the SL was first examined. The ML used in the experiment was an external cavity laser and was tunable in steps of 0.1 pm. Light from the ML, operating in continuous wave (CW), was coupled via an optical circulator (with an isolation greater than 40 dB) to the SL. The SL output was then coupled (again via the optical circulator) to an optical spectrum analyzer. In the measurements described below the bias current of the SL was set to 84 mA. At this injection current the device was lasing in the ES, with an ES to GS suppression ratio in excess of 30 dB; see for example the free-running optical spectra of the ES and GS plotted in Fig. 2. When subjected to external optical injection at the preferential lasing mode of the GS emission, the ES emission was suppressed by ∼40 dB in comparison to the free-running value of the ES emission (see Fig. 2). On the other hand, the GS now lased at the preferential mode with an overall side mode suppression ratio in excess of 45 dB. The power from the ML at the facet of the SL was estimated to be ∼150 μW. As can also be seen in Fig. 2, the difference in wavelength between the peak of the ES emission and the preferential lasing mode of the GS was ∼85 nm, which corresponds to a frequency difference of ∼16 THz.
Fig. 2. ES (left) and GS (right) optical spectra for the freerunning SL (black) and the SL under external optical injection (bright/red). In both cases the injection current of the SL was 84 mA, while the power of the ML was approximately 150 μW.
It is worth noting that suppression of the ES could also be achieved by injecting the ML into a GS mode of the SL other than the preferential mode. However, the injection power required to achieve a similar level of ES suppression had to be significantly higher. Using the experimental setup depicted in Fig. 3, the dynamical behavior of both the GS and ES under pulsed external optical injection of the preferential mode of the GS emission was examined. The CW output of the ML was connected to a LiNbO3 Mach–Zehnder modulator (MZM) biased at minimum transmission and driven by an RF pulse generator with a repetition rate of 1 MHz and pulse width of 100 ns, thereby generating a series of square optical pulses. The output light from the MZM was passed through a 90∕10 fiber beam splitter, with 10% of the light going directly to a photodiode connected to a high-speed oscilloscope for real-time monitoring of the optical pulses injected into the SL. The remaining 90% of the light was connected to port 1 of the polarizationmaintaining (PM) circulator and was injected into the SL via lensed fiber. Since the lensed fiber was not PM, a polarization controller was used in order to ensure that the polarization states of the ML and SL were identical. The output of the SL, collected at port 3 of the circulator, was connected to a 50∕50 fiber beam splitter. Tunable optical filters were placed in each output arm of the splitter in order to separate the signals from the two different lasing states. In one case, the output arm of the splitter was guided through a fiber optical tunable bandpass filter, with the central wavelength set to the ground-state emission of the SL (∼1300 nm). The other output arm of the splitter was directed via a pair of collimating lenses through a free-space optical bandpass filter (OBPF), with the central wavelength set to the excited-state emission of the SL (∼1215 nm). The outputs of both tunable optical filters were then connected to a high-speed real-time oscilloscope in order to analyze the dynamical behavior of the SL under external optical injection independently at both the GS and ES. Figure 4(a) shows a time trace of the output intensities for both the GS (dashed blue line) and ES (solid red line). The figure clearly shows the switch-on of the GS at ∼100 ns concurrent with a complete switch-off of the ES emission. The duration of GS lasing was, of course, governed by the pulse width of the ML injection. Once the injection into the GS was switched off, the laser returned to its ES free-running operation. Due to excessive losses through the free-space OBPF the measured ES intensity was much lower than that of the GS. As a result, the plot of the ES measured emission intensity in Fig. 4 is rescaled for clarity. Figure 4(b) shows the switch-on of the GS and the switch-off of the ES, while the inset shows the rise time of a typical, representative injected pulse for comparison. Taking a 50 μs time trace, in which approximately 50 pulses were acquired, it was found that the average rise time (taken as the 10%–90% step transition) of the injected pulse was ∼100 ps. The average GS rise time and ES fall time were about 2–3 times longer than the injected pulse, with the GS switch-on being slightly faster than the ES switch-off. Nonetheless, both transitions are extremely quick.
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Fig. 3. Setup of the pulsed injection experiment. For detailed explanation see text. ML, master laser; SL, slave laser; MZM, Mach– Zehnder modulator; PC, polarization controller; GS, ground-state pass filter; ES, excited-state pass filter; OSC, oscilloscope.
The switch-off of the GS emission and the concurrent switch-on of the ES emission are plotted in Fig. 4(c), while again the inset shows a typical injected pulse over the same time scale. The average switch-off time of the injected pulse was found to be ∼300 ps. While the average GS fall time (∼700 ps) was found to be longer than average GS rise time, it is still significantly quicker than any previously reported optical switching times. The ES switch-on transition, on the other hand, displays a pronounced ring, showing an extremely quick initial response of the ES to the injected pulse. This is followed by a longer “leveling off” time before the steady-state ES emission is attained, which is achieved within 1 ns of the injected pulse switch-off. From an examination of the switching transitions observed, it is clear that the high RO damping of QD lasers plays a significant role in achieving these extremely fast switching times. In conclusion, an all-optical switching mechanism via optical injection into the ground state of a single-section quantum dot laser when the free-running emission is from the excited state (ES) has been presented. Continuous wave injection reveals that a complete suppression of the ES is easily obtained. Furthermore, pulsed injection results show rapid switching transitions
Fig. 4. Filtered GS and ES outputs of the SL under pulsed optical injection. (a) Shows two consecutive pulses; (b) shows an example of the switch-on (off) of the GS (ES); (c) shows an example of the switch-off (on) of the GS (ES). The insets show typical examples of the injected pulses. GS, ground state; ES, excited state. The injection current of the SL was 84 mA.
between the ground and excited states. Transition times of several hundreds of picoseconds were obtained, suggesting that QD lasers are excellent candidates for use in all-optical switching applications. The authors would like to express their thanks to E. A. Viktorov for valuable discussions, and S. Porto of the Photonic Systems Group for the use of equipment. This work was conducted under the framework of the INSPIRE Structured Ph.D. Programme, funded by the Irish Government’s Programme for Research in Third Level Institutions, Cycle 5, National Development Plan 2007– 2013 with the assistance of the European Regional Development Fund. The authors also gratefully acknowledge the support of Science Foundation Ireland under contract 11/PI/1152. References 1. R. Lang, IEEE J. Quantum Electron. 18, 976 (1982). 2. L. A. Lugiato, in Progress in Optics, E. Wolf, ed. (North-Holland, 1984), Chap. 2, Vol. 21, pp. 69–216. 3. T. B. Simpson, J. M. Liu, A. Gavrielides, V. Kovanis, and P. M. Alsing, Phys. Rev. A 51, 4181 (1995). 4. S. Wieczorek, B. Krauskopf, and D. Lenstra, Phys. Rev. Lett. 88, 063901 (2002). 5. D. Goulding, S. P. Hegarty, O. Rasskazov, S. Melnik, M. Hartnett, G. Greene, J. G. McInerney, D. Rachinskii, and G. Huyet, Phys. Rev. Lett. 98, 153903 (2007). 6. F. Mogensen, H. Olesen, and G. Jacobsen, Electron. Lett. 21, 696 (1985). 7. T. B. Simpson, J. M. Liu, and A. Gavrielides, IEEE Photon. Technol. Lett. 7, 709 (1995). 8. O. Lidoyne, P. B. Gallion, and D. Erasme, IEEE J. Quantum Electron. 27, 344 (1991). 9. K. Lüdge, M. J. P. Bormann, E. Malić, P. Hövel, M. Kuntz, D. Bimberg, A. Knorr, and E. Schöll, Phys. Rev. B 78, 035316 (2008). 10. D. O’Brien, S. P. Hegarty, G. Huyet, and A. V. Uskov, Opt. Lett. 29, 1072 (2004). 11. S. P. Hegarty, D. Goulding, B. Kelleher, G. Huyet, M. T. Todaro, A. Salhi, A. Passaseo, and M. De Vittorio, Opt. Lett. 32, 3245 (2007). 12. T. Erneux, E. A. Viktorov, B. Kelleher, D. Goulding, S. P. Hegarty, and G. Huyet, Opt. Lett. 35, 937 (2010). 13. B. Kelleher, D. Goulding, S. P. Hegarty, G. Huyet, C. Ding-Yi, A. Martinez, A. Lemaître, A. Ramdane, M. Fischer, F. Gerschütz, and J. Koeth, Opt. Lett. 34, 440 (2009). 14. A. Markus, J. X. Chen, C. Paranthoën, A. Fiore, C. Platz, and O. Gauthier-Lafaye, Appl. Phys. Lett. 82, 1818 (2003). 15. M. Abusaa, J. Danckaert, E. A. Viktorov, and T. Erneux, Phys. Rev. A 87, 063827 (2013).
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16. I. White, R. Penty, M. Webster, Y. J. Chai, A. Wonfor, and S. Shahkooh, IEEE Commun. Mag. 40(9), 74 (2002). 17. M. Sciamanna and K. Panajotov, Phys. Rev. A 73, 023811 (2006). 18. A. Hurtado, I. D. Henning, and M. J. Adams, IEEE J. Sel. Top. Quantum Electron. 14, 911 (2008). 19. P. Heinricht, N. Brandonisio, S. O’Brien, and S. Osborne, IEEE Photon. Technol. Lett. 23, 513 (2011).
20. A. Markus, M. Rosetti, V. Cagliari, D. Chek-Al-Kar, J. X. Chen, A. Fiore, and R. Scollo, J. Appl. Phys. 100, 113104 (2006). 21. H.-Y. Wang, H.-C. Cheng, S.-D. Lin, and C.-P. Lee, Appl. Phys. Lett. 90, 081112 (2007). 22. C. Herbert, D. Jones, A. Kaszubowska-Anandarajah, B. Kelly, M. Rensing, J. O’Carroll, R. Phelan, P. Anandarajah, P. Perry, L. P. Barry, and J. O’Gorman, IET Optoelectron. 3, 1 (2009).
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All-optical switching with a dual-state, single-section quantum dot laser via optical injection Boguslaw Tykalewicz,1,2,* David Goulding,1,2 Stephen P. Hegarty,2 Guillaume Huyet,1,2,3 Diarmuid Byrne,4 Richard Phelan,4 and Bryan Kelleher1,2 1
Centre for Advanced Photonics and Process Analysis (CAPPA), Cork Institute of Technology, Cork, Ireland 2
3
Tyndall National Institute, National University of Ireland, University College, Cork, Ireland National Research University of Information Technologies, Mechanics and Optics, Saint Petersburg, Russia 4
Eblana Photonics, Unit 32 Trinity Technology and Enterprise Campus, Pearse St., Dublin 2, Ireland *Corresponding author: [email protected] Received May 21, 2014; revised June 27, 2014; accepted July 4, 2014; posted July 7, 2014 (Doc. ID 212502); published July 31, 2014
An all-optical switching mechanism via optical injection of an InAs/GaAs quantum dot laser is presented. Relative state suppression in excess of 40 dB is achieved, and experimental switching times of the order of a few hundred picoseconds are demonstrated. © 2014 Optical Society of America OCIS codes: (250.5590) Quantum-well, -wire and -dot devices; (250.6715) Switching; (250.3750) Optical logic devices; (140.3520) Lasers, injection-locked. http://dx.doi.org/10.1364/OL.39.004607
Over the past few years, the topic of optical injection [1] has attracted immense interest from both the scientific and engineering communities due to an abundance of rich dynamical regimes in the system and the array of potential applications of the technique. For example, from a nonlinear dynamics standpoint optically injected lasers have demonstrated multistability [2], chaos [3], and excitability [4,5], thereby providing an opportunity to gain further insight into fundamental dynamical processes using a relatively straightforward configuration. From the point of view of modern day optical engineering applications, there are many features that make optical injection an attractive technique, including a significant reduction in the laser linewidth [6], improvements in the relative intensity noise [7], and a large enhancement in the laser modulation bandwidth [8]. Self-assembled InAs quantum dots (QDs) grown on a GaAs substrate have been successfully utilized in many optoelectronic applications. An important property of QD-based semiconductor lasers is the high damping of the relaxation oscillations (ROs) [9]. It has been shown that this is one of the main reasons for the improved performance of QD devices over both bulk and quantumwell-based devices in several dynamical configurations, ranging from optical feedback to mutual coupling [5,10–13]. A key result for this work is that in comparison to other device types, a relatively broad region of stable locking can be obtained using QD lasers even for low optical injection powers. QDs display inhomogeneously broadened discrete energy states that can lead to the interesting property of lasing at clearly separated wavelength bands. For example, one can obtain simultaneous lasing from the ground state (GS) and the excited state (ES). The specific behavior depends on several control parameters, such as cavity length, injected current, and temperature of operation, and was previously attributed to incomplete clamping of the ES population at the GS threshold [14,15]. Control of switching between states is a very interesting topic and could be of interest for both fundamental science and applications [16]. 0146-9592/14/154607-04$15.00/0
All-optical switching is of great interest in a variety of technological applications and has previously been demonstrated using, for example, vertical-cavity surfaceemitting lasers [17,18] and two-color edge-emitting lasers [19]. In QD lasers electrical switching between GS and ES emission has been demonstrated using two-section devices [20,21]. This Letter presents an all-optical switching mechanism between the GS and the ES of a singlesection QD laser. Dynamic switching between the states is also shown with very fast switching times of the order of several hundreds of picoseconds obtained. The device used in the experiment was an InAs/GaAs discrete mode QD laser [22]. The length of the device and ridge waveguide width were 0.6 mm and 2.0 μm, respectively. The threshold current of the laser was approximately 33 mA. The laser was designed to operate single mode in the GS by selective placement of reflective elements along the laser cavity. The positioning of these elements provides an enhancement of a specific Fabry– Perot mode in the GS. In order to simplify nomenclature, we refer to this mode as the preferential mode. The devices were not designed to provide enhancement of a specific mode at the ES wavelength and thus the device operates multimode in the ES. Figure 1 shows the lightcurrent (LI) curve and optical spectrum at three different currents corresponding to three different regimes of operation. In the first regime, as can be seen in the top panel of Fig. 1(b), the device lases at the preferential longitudinal mode in the GS with a typical side mode suppression ratio in excess of 50 dB. As the injection current to the device was increased, the device underwent a second threshold (at approximately twice the GS threshold value) and lasing from both the GS and the ES was observed. The optical spectrum of the device in this regime of operation is shown in the middle panel of Fig. 1(b). As described above, the emission from the GS was dominated by the preferential mode while the emission from the ES was multimode. At a typical operating condition of 20°C, the GS and ES lasing wavelengths were approximately 1300 and 1215 nm, respectively. As the current was further increased (above 80 mA) the device finally © 2014 Optical Society of America
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OPTICS LETTERS / Vol. 39, No. 15 / August 1, 2014
Fig. 1. (a) LI characteristics of the SL. Different lasing regimes are separated by vertical dashed lines. (b) SL spectral evolution for three different bias currents, (i) 50 mA, (ii) 70 mA, and (iii) 100 mA.
displayed a complete suppression of the GS emission and exhibited lasing in the ES only, as evidenced in the bottom panel of Fig. 1(b). Because of the higher degeneracy of the ES all available carriers contributed to ES lasing only and consequently the GS was quenched [15]. While the principal aim was to characterize the dynamic injection scenario, static optical injection of the SL was first examined. The ML used in the experiment was an external cavity laser and was tunable in steps of 0.1 pm. Light from the ML, operating in continuous wave (CW), was coupled via an optical circulator (with an isolation greater than 40 dB) to the SL. The SL output was then coupled (again via the optical circulator) to an optical spectrum analyzer. In the measurements described below the bias current of the SL was set to 84 mA. At this injection current the device was lasing in the ES, with an ES to GS suppression ratio in excess of 30 dB; see for example the free-running optical spectra of the ES and GS plotted in Fig. 2. When subjected to external optical injection at the preferential lasing mode of the GS emission, the ES emission was suppressed by ∼40 dB in comparison to the free-running value of the ES emission (see Fig. 2). On the other hand, the GS now lased at the preferential mode with an overall side mode suppression ratio in excess of 45 dB. The power from the ML at the facet of the SL was estimated to be ∼150 μW. As can also be seen in Fig. 2, the difference in wavelength between the peak of the ES emission and the preferential lasing mode of the GS was ∼85 nm, which corresponds to a frequency difference of ∼16 THz.
Fig. 2. ES (left) and GS (right) optical spectra for the freerunning SL (black) and the SL under external optical injection (bright/red). In both cases the injection current of the SL was 84 mA, while the power of the ML was approximately 150 μW.
It is worth noting that suppression of the ES could also be achieved by injecting the ML into a GS mode of the SL other than the preferential mode. However, the injection power required to achieve a similar level of ES suppression had to be significantly higher. Using the experimental setup depicted in Fig. 3, the dynamical behavior of both the GS and ES under pulsed external optical injection of the preferential mode of the GS emission was examined. The CW output of the ML was connected to a LiNbO3 Mach–Zehnder modulator (MZM) biased at minimum transmission and driven by an RF pulse generator with a repetition rate of 1 MHz and pulse width of 100 ns, thereby generating a series of square optical pulses. The output light from the MZM was passed through a 90∕10 fiber beam splitter, with 10% of the light going directly to a photodiode connected to a high-speed oscilloscope for real-time monitoring of the optical pulses injected into the SL. The remaining 90% of the light was connected to port 1 of the polarizationmaintaining (PM) circulator and was injected into the SL via lensed fiber. Since the lensed fiber was not PM, a polarization controller was used in order to ensure that the polarization states of the ML and SL were identical. The output of the SL, collected at port 3 of the circulator, was connected to a 50∕50 fiber beam splitter. Tunable optical filters were placed in each output arm of the splitter in order to separate the signals from the two different lasing states. In one case, the output arm of the splitter was guided through a fiber optical tunable bandpass filter, with the central wavelength set to the ground-state emission of the SL (∼1300 nm). The other output arm of the splitter was directed via a pair of collimating lenses through a free-space optical bandpass filter (OBPF), with the central wavelength set to the excited-state emission of the SL (∼1215 nm). The outputs of both tunable optical filters were then connected to a high-speed real-time oscilloscope in order to analyze the dynamical behavior of the SL under external optical injection independently at both the GS and ES. Figure 4(a) shows a time trace of the output intensities for both the GS (dashed blue line) and ES (solid red line). The figure clearly shows the switch-on of the GS at ∼100 ns concurrent with a complete switch-off of the ES emission. The duration of GS lasing was, of course, governed by the pulse width of the ML injection. Once the injection into the GS was switched off, the laser returned to its ES free-running operation. Due to excessive losses through the free-space OBPF the measured ES intensity was much lower than that of the GS. As a result, the plot of the ES measured emission intensity in Fig. 4 is rescaled for clarity. Figure 4(b) shows the switch-on of the GS and the switch-off of the ES, while the inset shows the rise time of a typical, representative injected pulse for comparison. Taking a 50 μs time trace, in which approximately 50 pulses were acquired, it was found that the average rise time (taken as the 10%–90% step transition) of the injected pulse was ∼100 ps. The average GS rise time and ES fall time were about 2–3 times longer than the injected pulse, with the GS switch-on being slightly faster than the ES switch-off. Nonetheless, both transitions are extremely quick.
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Fig. 3. Setup of the pulsed injection experiment. For detailed explanation see text. ML, master laser; SL, slave laser; MZM, Mach– Zehnder modulator; PC, polarization controller; GS, ground-state pass filter; ES, excited-state pass filter; OSC, oscilloscope.
The switch-off of the GS emission and the concurrent switch-on of the ES emission are plotted in Fig. 4(c), while again the inset shows a typical injected pulse over the same time scale. The average switch-off time of the injected pulse was found to be ∼300 ps. While the average GS fall time (∼700 ps) was found to be longer than average GS rise time, it is still significantly quicker than any previously reported optical switching times. The ES switch-on transition, on the other hand, displays a pronounced ring, showing an extremely quick initial response of the ES to the injected pulse. This is followed by a longer “leveling off” time before the steady-state ES emission is attained, which is achieved within 1 ns of the injected pulse switch-off. From an examination of the switching transitions observed, it is clear that the high RO damping of QD lasers plays a significant role in achieving these extremely fast switching times. In conclusion, an all-optical switching mechanism via optical injection into the ground state of a single-section quantum dot laser when the free-running emission is from the excited state (ES) has been presented. Continuous wave injection reveals that a complete suppression of the ES is easily obtained. Furthermore, pulsed injection results show rapid switching transitions
Fig. 4. Filtered GS and ES outputs of the SL under pulsed optical injection. (a) Shows two consecutive pulses; (b) shows an example of the switch-on (off) of the GS (ES); (c) shows an example of the switch-off (on) of the GS (ES). The insets show typical examples of the injected pulses. GS, ground state; ES, excited state. The injection current of the SL was 84 mA.
between the ground and excited states. Transition times of several hundreds of picoseconds were obtained, suggesting that QD lasers are excellent candidates for use in all-optical switching applications. The authors would like to express their thanks to E. A. Viktorov for valuable discussions, and S. Porto of the Photonic Systems Group for the use of equipment. This work was conducted under the framework of the INSPIRE Structured Ph.D. Programme, funded by the Irish Government’s Programme for Research in Third Level Institutions, Cycle 5, National Development Plan 2007– 2013 with the assistance of the European Regional Development Fund. The authors also gratefully acknowledge the support of Science Foundation Ireland under contract 11/PI/1152. References 1. R. Lang, IEEE J. Quantum Electron. 18, 976 (1982). 2. L. A. Lugiato, in Progress in Optics, E. Wolf, ed. (North-Holland, 1984), Chap. 2, Vol. 21, pp. 69–216. 3. T. B. Simpson, J. M. Liu, A. Gavrielides, V. Kovanis, and P. M. Alsing, Phys. Rev. A 51, 4181 (1995). 4. S. Wieczorek, B. Krauskopf, and D. Lenstra, Phys. Rev. Lett. 88, 063901 (2002). 5. D. Goulding, S. P. Hegarty, O. Rasskazov, S. Melnik, M. Hartnett, G. Greene, J. G. McInerney, D. Rachinskii, and G. Huyet, Phys. Rev. Lett. 98, 153903 (2007). 6. F. Mogensen, H. Olesen, and G. Jacobsen, Electron. Lett. 21, 696 (1985). 7. T. B. Simpson, J. M. Liu, and A. Gavrielides, IEEE Photon. Technol. Lett. 7, 709 (1995). 8. O. Lidoyne, P. B. Gallion, and D. Erasme, IEEE J. Quantum Electron. 27, 344 (1991). 9. K. Lüdge, M. J. P. Bormann, E. Malić, P. Hövel, M. Kuntz, D. Bimberg, A. Knorr, and E. Schöll, Phys. Rev. B 78, 035316 (2008). 10. D. O’Brien, S. P. Hegarty, G. Huyet, and A. V. Uskov, Opt. Lett. 29, 1072 (2004). 11. S. P. Hegarty, D. Goulding, B. Kelleher, G. Huyet, M. T. Todaro, A. Salhi, A. Passaseo, and M. De Vittorio, Opt. Lett. 32, 3245 (2007). 12. T. Erneux, E. A. Viktorov, B. Kelleher, D. Goulding, S. P. Hegarty, and G. Huyet, Opt. Lett. 35, 937 (2010). 13. B. Kelleher, D. Goulding, S. P. Hegarty, G. Huyet, C. Ding-Yi, A. Martinez, A. Lemaître, A. Ramdane, M. Fischer, F. Gerschütz, and J. Koeth, Opt. Lett. 34, 440 (2009). 14. A. Markus, J. X. Chen, C. Paranthoën, A. Fiore, C. Platz, and O. Gauthier-Lafaye, Appl. Phys. Lett. 82, 1818 (2003). 15. M. Abusaa, J. Danckaert, E. A. Viktorov, and T. Erneux, Phys. Rev. A 87, 063827 (2013).
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