- Email: [email protected]
1.55 μm distributed feedback (DFB) lasers subject to strong 1.4 μm optical injection
Optics Communications 285 (2012) 5313–5318
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
1.55 mm distributed feedback (DFB) lasers subject to strong 1.4 mm optical injection S. Mazzucato n, M.J. Adams, N. Balkan School of Computer Science and Electronic Engineering, University of Essex, CO4 3SQ, Colchester, UK
a r t i c l e i n f o
abstract
Article history: Received 9 May 2012 Received in revised form 17 July 2012 Accepted 18 July 2012 Available online 2 August 2012
An experimental analysis of the influence of optical injection at 1.4 mm wavelength into two different commercial 1.55 mm DFB lasers is reported. The results demonstrate the strong dependence of the DFB behaviour on the injection parameters. Complete mode suppression or signal amplification can be obtained by varying the excitation wavelength and/or intensity, suggesting that these devices could be operated as logic ports or signal amplifiers, according to the injected signal. & 2012 Elsevier B.V. All rights reserved.
Keywords: Optical injection DFB Mode suppression
1. Introduction There has been extensive work on injection-locking effect in DFB lasers [1,2,3], and an injection-locking map under strong optical injection was recently reported [4].The dynamics of DFB lasers using optical modulation has also been studied, emphasising the effect of optical injection on the intrinsic behaviour of these devices [5,6]. While these conventional approaches rely on resonant excitation (the injected optical signal has its wavelength very close to a DFB mode), an all-optical 1310 to 1550 nm wavelength-conversion technique based on optical pumping of a DFB laser operated below threshold has been reported [7], thus demonstrating that the laser output can also be altered when working in non-resonant conditions. The present contribution describes the behaviour of two commercial 1.55 mm DFB lasers driven at and above threshold again under weak and strong optical non-resonant injection, but in the 1.4 mm wavelength region, where high pump semiconductor lasers are commercially available. To the best of our knowledge, this is the first investigation of the effect of optical injection at 1.4 on 1.55 mm DFB lasers, as a function of exciting wavelength and power, at different operating currents and temperature.
2. Experimental details Fig. 1 shows the experimental setup. The output beam of the 1.4 mm emitting semiconductor laser (master laser, ML) was directed to an optical circulator and then split through a 10/90 n
Corresponding author. Tel.: þ44 1206 87 2286. E-mail addresses: [email protected], [email protected] (S. Mazzucato).
0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.07.059
coupler. The 90% output signal was injected into the DFB laser (slave laser, SL) through the isolator, while the remaining 10% was used to monitor the input power (Pin). The returning power from the SL via the coupler was directed through the circulator to an optical spectrum analyser (OSA). Stabilised temperature-current controllers have been used to drive both the ML and the SL. The ML was kept at a fixed temperature of 20 1C, while the temperature of the SL was changed between 15 and 45 1C. Two commercial DFB lasers (Lasertron QLM5S790 and HP LSC2210, here denoted as SLL and SLH, respectively) have been used as slave lasers, operated at currents near and above threshold (22 mA for SLL and 32 mA for SLH, at T¼ 20 1C). At the same temperature, the central emission wavelength was 1533 and 1539 nm, for SLL and SLH, respectively. Three similar FITEL lasers, peaking at excitation wavelength lexc ¼1426, 1460 and 1490 nm, have been used as MLs. The injected power was changed by varying the applied current from 0 to 500 mA, giving a maximum power of 110 mW at the SL.
3. Results 3.1. Effects of optical injection on lasing mode amplitude and wavelength Starting from the analysis of the SLH results, it was found that all three injected MLs affect this DFB laser in a similar way. Fig. 2 shows the SLH dominant mode amplitude and wavelength plotted as functions of Pin, for a bias current equal to 35 mA and at T¼20 1C. Going from low to high excitation intensity, the main mode amplitude first remains constant, then decreases, and finally
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S. Mazzucato et al. / Optics Communications 285 (2012) 5313–5318
circulator
10/90 coupler
1.4 μm ML
1.55 μm SL
Pin
Temp – Curr controller
Temp – Curr controller
power meter OSA
Fig. 1. Experimental setup for 1.4 mm injection into the 1.55 mm DFB lasers.
-10
wavelength (nm)
amplitude (dBm)
1540.0 -20
-30 1426 nm 1460 nm 1490 nm
-40
1539.6 1426 nm 1460 nm 1490 nm
1539.2
-50 -30
-20 -10 Pin (dBm)
0
10
-30
-20 -10 Pin (dBm)
0
10
Fig. 2. Comparison of amplitude (a) and wavelength (b) vs. Pin plots for the SLH driven at 35 mA, when injected with the three different MLs.
-10
wavelength (nm)
amplitude (dBm)
1540.0 -20
-30
-40
-50 -30
35mA 37mA 39mA 41mA
1539.5 35mA 37mA 39mA 41mA
1539.0 -20
-10 Pin (dBm)
0
10
-30
-20
-10 Pin (dBm)
0
10
Fig. 3. Examples of change in SLH peak amplitude (left) and wavelength (right) as functions of Pin for different DFB bias currents from 35 to 41 mA for an excitation wavelength of 1426 nm.
increases above its initial (no injection) value. This U-shape behaviour is most pronounced when lexc ¼1426 nm and it is shown in Fig. 3 for a range of bias currents. Here at around Pin ¼2 dBm the lasing mode completely disappears, with a mode reduction MR of up to 35 dB observed in the current range of our investigation. The trend observed with the mode amplitude is reflected also in the plots of peak wavelength versus Pin given in Figs. 2 and 3. At low-to-medium input power the SLH emission blue-shifts, whereas it red-shifts at higher power. There is a good correlation between the trend of the amplitude and wavelength curves, both shown in Fig. 3 for various bias currents. Note that the
input power at which the mode is cancelled (termed P0) increases with increasing current, as well as the mode enhancement value ME (defined as the ratio of amplitude to its initial value) at higher Pin. Conversely, the U-feature narrows with increasing current. Regarding the wavelength change, a mode jump to the longerwavelength DFB resonance is observed in the region of P0. Fig. 4 presents the spectral evolution of the SLH when driven at 35 mA current as a function of Pin. The change in the mode amplitude and wavelength is evident. Spectra have been vertically shifted for clarity. Note the mode cancellation and jump in wavelength (from a1 to a2) as mentioned above. Similar plots
S. Mazzucato et al. / Optics Communications 285 (2012) 5313–5318
have been observed at all bias currents. While the amplitude of the main mode changes drastically, the side modes are less affected in intensity, but move similar to the main mode in wavelength. In the case when the SLH is operated below threshold, the ML injection at high power acts as an optical pump so that the SL lases with ME of more than 40 dB. A slightly different analysis of Fig. 4 can be made considering the evolution of all modes around the lasing one in SLH at 1426 nm injection wavelength. The change in wavelength and amplitude of these modes is plotted in Fig. 5, which expands Fig. 2 in terms of single mode evolutions. The side modes a3 and a4, as well as a2, first blue-shift and then red-shift with increasing injected power. On the contrary, the mode a1 blue-shifts only, however it seems to change direction in proximity to P0. The effect of lexc ¼1460 nm on SLH is similar in terms of a U-shaped characteristic with input power, but the main mode does not disappear and the minimum position P0 occurs at higher pump power, when compared with 1426 nm excitation wavelength. The mode enhancement ME is also reduced. An even lower effect on the DFB mode amplitude occurs at lexc ¼1490 nm. With this excitation wavelength the peak position red-shifts only. In order to further investigate the effect of 1.4 mm injection on 1.55 mm DFB lasers, the SLH was replaced by the SLL, and was driven under the same experimental conditions. A different
a2 a3
a1
200
evolution of the DFB spectrum as a function of injected power was observed when changing the excitation wavelength. Fig. 6 shows the spectra of SLL driven at 25 mA current at different Pin when lexc ¼1490 nm, at T¼20 1C. In this case very strong mode attenuation (up to 33 dB in the investigated incident power range) was observed, together with a red-shift of the mode wavelength. No jump mode was present and there is no recovery of the laser mode after disappearing. Note that also the longer wavelength side-mode redshifts with increasing Pin, while its amplitude remains quite constant. Again, the same trends were observed by changing the SLL bias current from 20 to 27 mA. When changing lexc, a different behaviour was observed. Fig. 7 summarises these effects at different lexc, by plotting the SLL mode amplitude and wavelength vs. Pin. Compared to 1490 nm excitation, at 1426 and 1460 nm, no reduction in the amplitude but an ME up to 5 dB was observed at high input power. Again, this enhancement was sufficient to make the SLL lasing when driven below threshold and pumped at high Pin. Regarding the wavelength trend, at 1460 nm the mode red-shifts with Pin while at 1426 nm it first blue-shifts, and then red-shifts. Differing from what was observed with SLH, the same U-shaped trend was not observed in the amplitude curve. An increase in the biasing current leads to the expected increase of MR at 1490 nm, and a reduction of ME at 1426 and 1460 nm.
a4 50 relative amplitude (dBm) - shifted
relative amplitude (dBm) - shifted
Pin
5315
150 100 50 0
Pin
0
-50
-50 1538
1540
1542
1532
wavelength (nm) Fig. 4. SLH spectra taken with increasing injection power at lexc ¼ 1426 nm, and DFB driving current equal to 35 mA. Individual spectra are vertically shifted for clarity.
a4
wavelength (nm)
amplitude (dBm)
1542
-20
-40
1536
Fig. 6. SLL spectra taken with increasing injection power at lexc ¼ 1490 nm, and DFB driving current equal to 25 mA. Individual spectra are vertically shifted for clarity.
-10
-30
1534 wavelength (nm)
a1 a2 a3 a4
a3 1541 a2 1540
-50 1539
a1
-60 -10
0
Pin (dBm)
10
-10
0
10
Pin (dBm)
Fig. 5. Change in amplitude and wavelength for the different modes of SLH for lexc ¼ 1426 nm. Labels a1, a2, a3 and a4 refer to the spectral peaks in Fig. 4.
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S. Mazzucato et al. / Optics Communications 285 (2012) 5313–5318
-10 1426nm 1460nm 1490nm
-20
wavelength (nm)
amplitude (dBm)
1534.0
-30
1426nm 1460nm 1490nm
-40
1533.9
1533.8
1533.7
-50 -30
-20
-10
0
-30
10
-20
-10
0
10
Pin (dBm)
Pin (dBm)
Fig. 7. Comparison of amplitude and wavelength vs. Pin plots for the SLL driven at 25 mA, when injected with the three different MLs.
1426nm 1490nm
50
1426nm 1490nm
30
40
Ith (mA)
Ith (mA)
45
35
25
20 30 25
15 SLH
20 -30
-20
-10 Pin (dBm)
0
10
-30
SLL -20
-10 Pin (dBm)
0
10
Fig. 8. Threshold current I th versus injected power for SLH (left) and SLL (right) at 1426 and 1490 nm excitation wavelength.
3.2. Effect of optical injection on threshold current Fig. 8 shows that the evolution of mode amplitude presented in Figs. 2 and 7 is directly related to the threshold current dependence upon pump power. In this figure, the threshold current Ith of both SLs is plotted for 1426 and 1490 nm lexc, at various Pin values. Data have been taken by obtaining Ith from light–current curves under different Pin. Optical injection forces the threshold to change, as the gain required for lasing conditions varies. As already seen in Fig. 2a, also here the maximum value of Ith for SLH injected at 1426 nm occurs for Pin ¼ P0. 3.3. Temperature dependence Further investigation of the SLs behaviour was carried out by varying the operating temperature between 15 and 45 1C. In order to compare the different temperature results, the SL current was changed above threshold in order to get the same laser emission. Before analysing the effect of temperature for the optical injection on SLs, the shift of resonant wavelength of the solitary DFB (i.e., without injection) with temperature and current was considered. Starting with the current dependence, no change in wavelength position was observed. On the contrary the wavelength change with temperature was 0.076 nm/1C for SLH and 0.077 nm/1C for SLL. These values, obtained driving the SLs above threshold at
I¼1.1 Ith, agree with those commonly reported in the literature [8,9]. No mode jump was observed. The laser threshold current temperature dependence gives a characteristic temperature T0 in the range of 50 K for both SLs. With relation to the mode amplitude vs. input power curves presented in Figs. 2 and 7 for T¼20 1C, they do not generally change in shape with temperature, but some clear trends in the minimum position P0 and wavelength shifts are observed. The temperature acts in the same way as the driving current for P0 and the magnitude of the wavelength shift. An increase in temperature leads to an increase in P0 and in the wavelength red-shift, while the blue-shift extent decreases. For SLH at lexc ¼1426 nm for example, Fig. 9 shows the change in P0 with temperature and driving current. Regarding MR and ME, and in general the mode amplitude vs. Pin curves, it has been observed that a change in temperature translates the curve horizontally, as observed with P0. This is shown in Fig. 10 for SLH at lexc ¼ 1426 nm. Therefore, MR and ME are almost independent of temperature, just appearing at different Pin.
4. Discussion In view of the observed results, a common explanation of the SLs behaviour under optical injection is not feasible, but each case
3.0
2.5
2.5
2.0
P0 (dBm)
P0 (dBm)
S. Mazzucato et al. / Optics Communications 285 (2012) 5313–5318
2.0
5317
1.5
1.5 1.0 1.0 15
20
25
30
35
Temperature
40
45
(°C)
32
34 36 38 Bias current (mA)
40
Fig. 9. P0 versus temperature (left) and bias current (right) for SLH at 1426 nm exciting wavelength.
amplitude (dBm)
-10 -20 -30 -40 -50 -15
15°C 25°C 35°C 45°C -10
-5 0 Pin(dBm)
5
10
Fig. 10. SLH peak amplitude vs. Pin for different temperatures from T¼15 to 45 1C, at 1426 nm excitation wavelength.
should be treated separately. However, a general trend for the mode wavelength is visible, as all modes red-shift, especially at higher input power. In same cases this shift follows a previous blue-shift of the modes, indicating that two different phenomena are involved in the overall process. At low injection power the blue-shift of the modes is similar to the one observed when changing the bias current, corresponding to the change of effective refractive index with carrier concentration. MR is often observed, together with an increase in the threshold value. However, at higher input power, the modes shift towards higher wavelengths, and ME is present. It is well known that optical pumping affects the DFB behaviour through carrier injection into the active region and heating [10]. Carrier injection into other layers of the DFB structure is quite unlikely, since the layers should be transparent to the 1.4 mm pump wavelength. Therefore all the effect of the pump laser on the DFB will affect the active region only. In general, DFB laser emission wavelength is determined by the Bragg condition l ¼2neffD/m where neff is the effective refractive index, D is the period and m the order of the DFB grating. For DFB lasers with no phase-shift [11], the lasing spectrum is governed by the simultaneous presence of two modes, positioned symmetrically on each side of the Bragg wavelength. Good discrimination of single-mode devices was achieved by choosing the right combination of anti-reflected or cleaved facets [12]. The use of a phase-shifted grating allowed better discrimination of the modes and single-mode operation at
the Bragg frequency. Considering the manufacturing period of the investigated devices, it is very likely that they each have a uniform corrugation pattern. Despite the lack of information about the DFB structure and material composition, it is clear that any change in the effective refractive index will affect the operating wavelength. While the effect of carrier injection into the active region is to reduce the effective refractive index [13], and hence to shift the cavity resonances to shorter wavelengths, on the contrary the effect of heating is to increase the effective refractive index, hence to shift the cavity resonances to longer wavelengths. This is also supported by the change in the threshold current with optical pumping. In addition the energy bandgap of the active region decreases with temperature, so that the wavelength of the peak gain shifts to longer wavelengths. It is common to observe a cavity resonance shift to lower wavelengths as injected current is increased below threshold, and to longer wavelengths as current is increased above threshold [14]. Detailed calculations have been reported on the detuning of the emission wavelength in terms of injection current and thermal effect as determined by the position of the lasing mode inside the DFB grating reflectivity spectrum [15]. Therefore, it is reasonable to expect a change in the operating point of the laser, together with its output power, as the result of a competition between the resonance wavelengths shifting to longer values by heating or to shorter values by carrier injection. Regarding the thermal influence, it is known that the laser performance deteriorates rapidly as temperature increases. Additionally, the change of amplitude gain becomes complicated since the gain always depends on the injection current. However, the change in optical gain due to the variation of injected carrier is more significant than that due to changes in temperature [16]. At the same time, the shift in wavelength of the DFB lasers follows only the temperature dependence of the material refractive index [17]. The presented results for wavelength versus injected power show the competition between heating and carrier injection effects with increasing pump power. Keating et al. [18] investigated the temperature dependence of optical (and electrical) modulation of a 1550 nm DFB pumped with a 1300 nm laser, but they could probably not induce significant variations in the threshold and wavelength because of their limited ML power. Indeed, we tried repeating our investigation with a 1.3 mm tunable laser as ML, with 6 mW maximum power output. As expected, extremely little variation in the DFB wavelength and amplitude was observed.
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S. Mazzucato et al. / Optics Communications 285 (2012) 5313–5318
From the results presented, it is clear that at high injection power, lexc is important in determining the break-point between these heating and carrier injection effects. There are changes in the output power with injected power, probably due to the shift of resonance and consequent change of the operating point. Gain peak and resonance might also shift in different directions with heating, maybe depending on which side of the gain peak the resonance lays. There are situations where the DFB mode amplitude only increases with optical pump, so the carrier density is locked at threshold and any additional input energy goes into optical output. This has been observed for SLL at lexc ¼1426 and 1460 nm. On the other side, for SLL at lexc ¼1490 nm where the mode steadily decreases, and even worse for SLH, where an initial drop in power is followed by an increase in the mode amplitude (and a mode jump for lexc ¼1426 nm), the physical explanation would require at least the knowledge of the device structure. With our data we can just speculate on the reason for the change in the mode amplitude at low input power, since the carrier concentration is expected to be locked at threshold with all extra input energy being converted into lasing output. Possible causes can be related to (1) spatial hole-burning, so that the carrier concentration in some spatial region increases and causes a decrease of refractive index and hence of lasing wavelength, along with possibly increased loss and/or a shift of gain spectrum; and (2) carrier bottleneck due to saturation of states in QWs, thus leading to build-up of carriers in a reservoir region and hence refractive index changes, etc., as in (1). Spatial hole burning effects have been investigated by Rabinovich and Feldman [19], showing that the reduced suppression of the higher-order longitudinal modes of DFB lasers is very strong for cases of index coupling. Hole burning and mode hops have been modelled and investigated by Whiteaway et al. [20] and later by Zhang and Carroll [21] for l/4 phase-shifted DFB lasers. Finally, for a clearer insight and physical understanding, in future it would be desirable to repeat these experiments using injection of short optical pulses (and/or short pulse current modulation), so reducing considerably the optically-induced heating effects.
5. Conclusion In this paper measured effects of the injection of 1.4 mm beam into two commercial 1.55 mm operating DFB lasers are presented. Despite the lack of information about the DFB structure and layer composition, it is clear that different behaviours occur depending upon the injection wavelength and power. Mode attenuation
and/or enhancement can be observed. Repetition of this experiment with known-structure DFB lasers to investigate the processes leading to the observed behaviours is desirable, but is beyond the scope of the present work. Such measurements could lead to the design of devices operating, for example, as ON–OFF logic ports or signal amplifiers, according to the injected signal.
Acknowledgements We acknowledge the EPSRC grant EP/G023972/1 for financial support and the collaboration within the COST Action MP0805 entitled ‘‘Novel Gain Materials and Devices Based on III-V-N Compounds’’.
References [1] R. Hui, A. D’Ottavi, A. Mecozzi, P. Spano, IEEE Journal of Quantum Electronics 27 (1991) 1688. [2] F. Ke´fe´lian, P. Gallion, IEEE Journal of Quantum Electronics 44 (2008) 547. [3] C. Bornholdt, B. Sartorius, S. Schelhase, M. Mohrle, S. Bauer, Electronics Letters 36 (2000) 327. [4] N.M. Al-Hosiny, R. El-Agmy, M.M. Abd El-Raheem, M.J. Adams, Optics Communications 283 (2010) 579. [5] C.B. Su, J. Eom, C.H. Lange, C.B. Kim, R.B. Lauer, W.C. Rideout, J.S. LaCourse, IEEE Journal of Quantum Electronics 28 (1992) 118. [6] D.C. Lin, H.H. Liao, J.W. Liaw, Y.K. Tu, Electronics Letters 29 (1993) 1223. [7] D.N. Maywar, Y. Nakano, G.P. Agrawal, IEEE Photonics Technology Letters 12 (2000) 858. [8] M. Nakamura, K. Aiki, J. Umeda, Applied Physics Letters 27 (1975) 403. [9] S. Akiba, K. Utaka, K. Sakai, Y. Matsushima, Japanese Journal of Applied Physics 21 (1982) 1736. [10] R. Hofmann, V. Wagner, H.-P. Gauggel, F. Adler, P. Ernst, H. Bolay, A. Sohmer, F. Scholz, H.C. Schweizer, IEEE Journal of Selected Topics in Quantum Electronics 3 (1997) 456. [11] K. Sekartedjo, N. Eda, K. Furuya, Y. Suematsu, F. Koyama, T. Tanbun-Erk, Electronics Letters 20 (1984) 80. [12] C.A.F. Fernandes, Microwave and Optical Technology Letters 48 (2006) 205. [13] B.R. Bennett, R.A. Soref, J.A. Del Alamo, IEEE Journal of Quantum Electronics 26 (1990) 113. ¨ ¨ [14] B. Sartorius, M. Mohrle, S. Reichenbacher, H. Preier, H.-J. Wunsche, U. Bandelow, IEEE Journal of Quantum Electronics 33 (1997) 211. ¨ [15] M. Radziunas, H.-J. Wunsche, B. Sartorius, O. Brox, D. Hoffmann, K.R. Schneider, D. Marcenac, IEEE Journal of Quantum Electronics 36 (2000) 1026. [16] Y. Kotaki, H. Ishikawa, IEE Proceedings-Optoelectronic 138 (1991) 171–177. [17] R. G. Hunsperger, ‘‘Distributed-Feedback Lasers’’, in Integrated Optics: Theory and Technology, sixth ed., Springer, 319. [18] T. Keating, X. Jin, S.L. Chuang, K. Hess, IEEE Journal of Quantum Electronics 35 (1999) 1526. [19] W.S. Rabinovich, B.J. Feldman, IEEE Journal of Quantum Electronics 25 (1989) 20. [20] J.E.A. Whiteaway, G.H.B. Thompson, A.J. Collar, C.J. Armistead, IEEE Journal of Quantum Electronics 25 (1989) 1261. [21] L.M. Zhang, J.E. Carroll, IEEE Journal of Quantum Electronics 28 (1992) 604.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
1.55 mm distributed feedback (DFB) lasers subject to strong 1.4 mm optical injection S. Mazzucato n, M.J. Adams, N. Balkan School of Computer Science and Electronic Engineering, University of Essex, CO4 3SQ, Colchester, UK
a r t i c l e i n f o
abstract
Article history: Received 9 May 2012 Received in revised form 17 July 2012 Accepted 18 July 2012 Available online 2 August 2012
An experimental analysis of the influence of optical injection at 1.4 mm wavelength into two different commercial 1.55 mm DFB lasers is reported. The results demonstrate the strong dependence of the DFB behaviour on the injection parameters. Complete mode suppression or signal amplification can be obtained by varying the excitation wavelength and/or intensity, suggesting that these devices could be operated as logic ports or signal amplifiers, according to the injected signal. & 2012 Elsevier B.V. All rights reserved.
Keywords: Optical injection DFB Mode suppression
1. Introduction There has been extensive work on injection-locking effect in DFB lasers [1,2,3], and an injection-locking map under strong optical injection was recently reported [4].The dynamics of DFB lasers using optical modulation has also been studied, emphasising the effect of optical injection on the intrinsic behaviour of these devices [5,6]. While these conventional approaches rely on resonant excitation (the injected optical signal has its wavelength very close to a DFB mode), an all-optical 1310 to 1550 nm wavelength-conversion technique based on optical pumping of a DFB laser operated below threshold has been reported [7], thus demonstrating that the laser output can also be altered when working in non-resonant conditions. The present contribution describes the behaviour of two commercial 1.55 mm DFB lasers driven at and above threshold again under weak and strong optical non-resonant injection, but in the 1.4 mm wavelength region, where high pump semiconductor lasers are commercially available. To the best of our knowledge, this is the first investigation of the effect of optical injection at 1.4 on 1.55 mm DFB lasers, as a function of exciting wavelength and power, at different operating currents and temperature.
2. Experimental details Fig. 1 shows the experimental setup. The output beam of the 1.4 mm emitting semiconductor laser (master laser, ML) was directed to an optical circulator and then split through a 10/90 n
Corresponding author. Tel.: þ44 1206 87 2286. E-mail addresses: [email protected], [email protected] (S. Mazzucato).
0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.07.059
coupler. The 90% output signal was injected into the DFB laser (slave laser, SL) through the isolator, while the remaining 10% was used to monitor the input power (Pin). The returning power from the SL via the coupler was directed through the circulator to an optical spectrum analyser (OSA). Stabilised temperature-current controllers have been used to drive both the ML and the SL. The ML was kept at a fixed temperature of 20 1C, while the temperature of the SL was changed between 15 and 45 1C. Two commercial DFB lasers (Lasertron QLM5S790 and HP LSC2210, here denoted as SLL and SLH, respectively) have been used as slave lasers, operated at currents near and above threshold (22 mA for SLL and 32 mA for SLH, at T¼ 20 1C). At the same temperature, the central emission wavelength was 1533 and 1539 nm, for SLL and SLH, respectively. Three similar FITEL lasers, peaking at excitation wavelength lexc ¼1426, 1460 and 1490 nm, have been used as MLs. The injected power was changed by varying the applied current from 0 to 500 mA, giving a maximum power of 110 mW at the SL.
3. Results 3.1. Effects of optical injection on lasing mode amplitude and wavelength Starting from the analysis of the SLH results, it was found that all three injected MLs affect this DFB laser in a similar way. Fig. 2 shows the SLH dominant mode amplitude and wavelength plotted as functions of Pin, for a bias current equal to 35 mA and at T¼20 1C. Going from low to high excitation intensity, the main mode amplitude first remains constant, then decreases, and finally
5314
S. Mazzucato et al. / Optics Communications 285 (2012) 5313–5318
circulator
10/90 coupler
1.4 μm ML
1.55 μm SL
Pin
Temp – Curr controller
Temp – Curr controller
power meter OSA
Fig. 1. Experimental setup for 1.4 mm injection into the 1.55 mm DFB lasers.
-10
wavelength (nm)
amplitude (dBm)
1540.0 -20
-30 1426 nm 1460 nm 1490 nm
-40
1539.6 1426 nm 1460 nm 1490 nm
1539.2
-50 -30
-20 -10 Pin (dBm)
0
10
-30
-20 -10 Pin (dBm)
0
10
Fig. 2. Comparison of amplitude (a) and wavelength (b) vs. Pin plots for the SLH driven at 35 mA, when injected with the three different MLs.
-10
wavelength (nm)
amplitude (dBm)
1540.0 -20
-30
-40
-50 -30
35mA 37mA 39mA 41mA
1539.5 35mA 37mA 39mA 41mA
1539.0 -20
-10 Pin (dBm)
0
10
-30
-20
-10 Pin (dBm)
0
10
Fig. 3. Examples of change in SLH peak amplitude (left) and wavelength (right) as functions of Pin for different DFB bias currents from 35 to 41 mA for an excitation wavelength of 1426 nm.
increases above its initial (no injection) value. This U-shape behaviour is most pronounced when lexc ¼1426 nm and it is shown in Fig. 3 for a range of bias currents. Here at around Pin ¼2 dBm the lasing mode completely disappears, with a mode reduction MR of up to 35 dB observed in the current range of our investigation. The trend observed with the mode amplitude is reflected also in the plots of peak wavelength versus Pin given in Figs. 2 and 3. At low-to-medium input power the SLH emission blue-shifts, whereas it red-shifts at higher power. There is a good correlation between the trend of the amplitude and wavelength curves, both shown in Fig. 3 for various bias currents. Note that the
input power at which the mode is cancelled (termed P0) increases with increasing current, as well as the mode enhancement value ME (defined as the ratio of amplitude to its initial value) at higher Pin. Conversely, the U-feature narrows with increasing current. Regarding the wavelength change, a mode jump to the longerwavelength DFB resonance is observed in the region of P0. Fig. 4 presents the spectral evolution of the SLH when driven at 35 mA current as a function of Pin. The change in the mode amplitude and wavelength is evident. Spectra have been vertically shifted for clarity. Note the mode cancellation and jump in wavelength (from a1 to a2) as mentioned above. Similar plots
S. Mazzucato et al. / Optics Communications 285 (2012) 5313–5318
have been observed at all bias currents. While the amplitude of the main mode changes drastically, the side modes are less affected in intensity, but move similar to the main mode in wavelength. In the case when the SLH is operated below threshold, the ML injection at high power acts as an optical pump so that the SL lases with ME of more than 40 dB. A slightly different analysis of Fig. 4 can be made considering the evolution of all modes around the lasing one in SLH at 1426 nm injection wavelength. The change in wavelength and amplitude of these modes is plotted in Fig. 5, which expands Fig. 2 in terms of single mode evolutions. The side modes a3 and a4, as well as a2, first blue-shift and then red-shift with increasing injected power. On the contrary, the mode a1 blue-shifts only, however it seems to change direction in proximity to P0. The effect of lexc ¼1460 nm on SLH is similar in terms of a U-shaped characteristic with input power, but the main mode does not disappear and the minimum position P0 occurs at higher pump power, when compared with 1426 nm excitation wavelength. The mode enhancement ME is also reduced. An even lower effect on the DFB mode amplitude occurs at lexc ¼1490 nm. With this excitation wavelength the peak position red-shifts only. In order to further investigate the effect of 1.4 mm injection on 1.55 mm DFB lasers, the SLH was replaced by the SLL, and was driven under the same experimental conditions. A different
a2 a3
a1
200
evolution of the DFB spectrum as a function of injected power was observed when changing the excitation wavelength. Fig. 6 shows the spectra of SLL driven at 25 mA current at different Pin when lexc ¼1490 nm, at T¼20 1C. In this case very strong mode attenuation (up to 33 dB in the investigated incident power range) was observed, together with a red-shift of the mode wavelength. No jump mode was present and there is no recovery of the laser mode after disappearing. Note that also the longer wavelength side-mode redshifts with increasing Pin, while its amplitude remains quite constant. Again, the same trends were observed by changing the SLL bias current from 20 to 27 mA. When changing lexc, a different behaviour was observed. Fig. 7 summarises these effects at different lexc, by plotting the SLL mode amplitude and wavelength vs. Pin. Compared to 1490 nm excitation, at 1426 and 1460 nm, no reduction in the amplitude but an ME up to 5 dB was observed at high input power. Again, this enhancement was sufficient to make the SLL lasing when driven below threshold and pumped at high Pin. Regarding the wavelength trend, at 1460 nm the mode red-shifts with Pin while at 1426 nm it first blue-shifts, and then red-shifts. Differing from what was observed with SLH, the same U-shaped trend was not observed in the amplitude curve. An increase in the biasing current leads to the expected increase of MR at 1490 nm, and a reduction of ME at 1426 and 1460 nm.
a4 50 relative amplitude (dBm) - shifted
relative amplitude (dBm) - shifted
Pin
5315
150 100 50 0
Pin
0
-50
-50 1538
1540
1542
1532
wavelength (nm) Fig. 4. SLH spectra taken with increasing injection power at lexc ¼ 1426 nm, and DFB driving current equal to 35 mA. Individual spectra are vertically shifted for clarity.
a4
wavelength (nm)
amplitude (dBm)
1542
-20
-40
1536
Fig. 6. SLL spectra taken with increasing injection power at lexc ¼ 1490 nm, and DFB driving current equal to 25 mA. Individual spectra are vertically shifted for clarity.
-10
-30
1534 wavelength (nm)
a1 a2 a3 a4
a3 1541 a2 1540
-50 1539
a1
-60 -10
0
Pin (dBm)
10
-10
0
10
Pin (dBm)
Fig. 5. Change in amplitude and wavelength for the different modes of SLH for lexc ¼ 1426 nm. Labels a1, a2, a3 and a4 refer to the spectral peaks in Fig. 4.
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-10 1426nm 1460nm 1490nm
-20
wavelength (nm)
amplitude (dBm)
1534.0
-30
1426nm 1460nm 1490nm
-40
1533.9
1533.8
1533.7
-50 -30
-20
-10
0
-30
10
-20
-10
0
10
Pin (dBm)
Pin (dBm)
Fig. 7. Comparison of amplitude and wavelength vs. Pin plots for the SLL driven at 25 mA, when injected with the three different MLs.
1426nm 1490nm
50
1426nm 1490nm
30
40
Ith (mA)
Ith (mA)
45
35
25
20 30 25
15 SLH
20 -30
-20
-10 Pin (dBm)
0
10
-30
SLL -20
-10 Pin (dBm)
0
10
Fig. 8. Threshold current I th versus injected power for SLH (left) and SLL (right) at 1426 and 1490 nm excitation wavelength.
3.2. Effect of optical injection on threshold current Fig. 8 shows that the evolution of mode amplitude presented in Figs. 2 and 7 is directly related to the threshold current dependence upon pump power. In this figure, the threshold current Ith of both SLs is plotted for 1426 and 1490 nm lexc, at various Pin values. Data have been taken by obtaining Ith from light–current curves under different Pin. Optical injection forces the threshold to change, as the gain required for lasing conditions varies. As already seen in Fig. 2a, also here the maximum value of Ith for SLH injected at 1426 nm occurs for Pin ¼ P0. 3.3. Temperature dependence Further investigation of the SLs behaviour was carried out by varying the operating temperature between 15 and 45 1C. In order to compare the different temperature results, the SL current was changed above threshold in order to get the same laser emission. Before analysing the effect of temperature for the optical injection on SLs, the shift of resonant wavelength of the solitary DFB (i.e., without injection) with temperature and current was considered. Starting with the current dependence, no change in wavelength position was observed. On the contrary the wavelength change with temperature was 0.076 nm/1C for SLH and 0.077 nm/1C for SLL. These values, obtained driving the SLs above threshold at
I¼1.1 Ith, agree with those commonly reported in the literature [8,9]. No mode jump was observed. The laser threshold current temperature dependence gives a characteristic temperature T0 in the range of 50 K for both SLs. With relation to the mode amplitude vs. input power curves presented in Figs. 2 and 7 for T¼20 1C, they do not generally change in shape with temperature, but some clear trends in the minimum position P0 and wavelength shifts are observed. The temperature acts in the same way as the driving current for P0 and the magnitude of the wavelength shift. An increase in temperature leads to an increase in P0 and in the wavelength red-shift, while the blue-shift extent decreases. For SLH at lexc ¼1426 nm for example, Fig. 9 shows the change in P0 with temperature and driving current. Regarding MR and ME, and in general the mode amplitude vs. Pin curves, it has been observed that a change in temperature translates the curve horizontally, as observed with P0. This is shown in Fig. 10 for SLH at lexc ¼ 1426 nm. Therefore, MR and ME are almost independent of temperature, just appearing at different Pin.
4. Discussion In view of the observed results, a common explanation of the SLs behaviour under optical injection is not feasible, but each case
3.0
2.5
2.5
2.0
P0 (dBm)
P0 (dBm)
S. Mazzucato et al. / Optics Communications 285 (2012) 5313–5318
2.0
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1.5
1.5 1.0 1.0 15
20
25
30
35
Temperature
40
45
(°C)
32
34 36 38 Bias current (mA)
40
Fig. 9. P0 versus temperature (left) and bias current (right) for SLH at 1426 nm exciting wavelength.
amplitude (dBm)
-10 -20 -30 -40 -50 -15
15°C 25°C 35°C 45°C -10
-5 0 Pin(dBm)
5
10
Fig. 10. SLH peak amplitude vs. Pin for different temperatures from T¼15 to 45 1C, at 1426 nm excitation wavelength.
should be treated separately. However, a general trend for the mode wavelength is visible, as all modes red-shift, especially at higher input power. In same cases this shift follows a previous blue-shift of the modes, indicating that two different phenomena are involved in the overall process. At low injection power the blue-shift of the modes is similar to the one observed when changing the bias current, corresponding to the change of effective refractive index with carrier concentration. MR is often observed, together with an increase in the threshold value. However, at higher input power, the modes shift towards higher wavelengths, and ME is present. It is well known that optical pumping affects the DFB behaviour through carrier injection into the active region and heating [10]. Carrier injection into other layers of the DFB structure is quite unlikely, since the layers should be transparent to the 1.4 mm pump wavelength. Therefore all the effect of the pump laser on the DFB will affect the active region only. In general, DFB laser emission wavelength is determined by the Bragg condition l ¼2neffD/m where neff is the effective refractive index, D is the period and m the order of the DFB grating. For DFB lasers with no phase-shift [11], the lasing spectrum is governed by the simultaneous presence of two modes, positioned symmetrically on each side of the Bragg wavelength. Good discrimination of single-mode devices was achieved by choosing the right combination of anti-reflected or cleaved facets [12]. The use of a phase-shifted grating allowed better discrimination of the modes and single-mode operation at
the Bragg frequency. Considering the manufacturing period of the investigated devices, it is very likely that they each have a uniform corrugation pattern. Despite the lack of information about the DFB structure and material composition, it is clear that any change in the effective refractive index will affect the operating wavelength. While the effect of carrier injection into the active region is to reduce the effective refractive index [13], and hence to shift the cavity resonances to shorter wavelengths, on the contrary the effect of heating is to increase the effective refractive index, hence to shift the cavity resonances to longer wavelengths. This is also supported by the change in the threshold current with optical pumping. In addition the energy bandgap of the active region decreases with temperature, so that the wavelength of the peak gain shifts to longer wavelengths. It is common to observe a cavity resonance shift to lower wavelengths as injected current is increased below threshold, and to longer wavelengths as current is increased above threshold [14]. Detailed calculations have been reported on the detuning of the emission wavelength in terms of injection current and thermal effect as determined by the position of the lasing mode inside the DFB grating reflectivity spectrum [15]. Therefore, it is reasonable to expect a change in the operating point of the laser, together with its output power, as the result of a competition between the resonance wavelengths shifting to longer values by heating or to shorter values by carrier injection. Regarding the thermal influence, it is known that the laser performance deteriorates rapidly as temperature increases. Additionally, the change of amplitude gain becomes complicated since the gain always depends on the injection current. However, the change in optical gain due to the variation of injected carrier is more significant than that due to changes in temperature [16]. At the same time, the shift in wavelength of the DFB lasers follows only the temperature dependence of the material refractive index [17]. The presented results for wavelength versus injected power show the competition between heating and carrier injection effects with increasing pump power. Keating et al. [18] investigated the temperature dependence of optical (and electrical) modulation of a 1550 nm DFB pumped with a 1300 nm laser, but they could probably not induce significant variations in the threshold and wavelength because of their limited ML power. Indeed, we tried repeating our investigation with a 1.3 mm tunable laser as ML, with 6 mW maximum power output. As expected, extremely little variation in the DFB wavelength and amplitude was observed.
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From the results presented, it is clear that at high injection power, lexc is important in determining the break-point between these heating and carrier injection effects. There are changes in the output power with injected power, probably due to the shift of resonance and consequent change of the operating point. Gain peak and resonance might also shift in different directions with heating, maybe depending on which side of the gain peak the resonance lays. There are situations where the DFB mode amplitude only increases with optical pump, so the carrier density is locked at threshold and any additional input energy goes into optical output. This has been observed for SLL at lexc ¼1426 and 1460 nm. On the other side, for SLL at lexc ¼1490 nm where the mode steadily decreases, and even worse for SLH, where an initial drop in power is followed by an increase in the mode amplitude (and a mode jump for lexc ¼1426 nm), the physical explanation would require at least the knowledge of the device structure. With our data we can just speculate on the reason for the change in the mode amplitude at low input power, since the carrier concentration is expected to be locked at threshold with all extra input energy being converted into lasing output. Possible causes can be related to (1) spatial hole-burning, so that the carrier concentration in some spatial region increases and causes a decrease of refractive index and hence of lasing wavelength, along with possibly increased loss and/or a shift of gain spectrum; and (2) carrier bottleneck due to saturation of states in QWs, thus leading to build-up of carriers in a reservoir region and hence refractive index changes, etc., as in (1). Spatial hole burning effects have been investigated by Rabinovich and Feldman [19], showing that the reduced suppression of the higher-order longitudinal modes of DFB lasers is very strong for cases of index coupling. Hole burning and mode hops have been modelled and investigated by Whiteaway et al. [20] and later by Zhang and Carroll [21] for l/4 phase-shifted DFB lasers. Finally, for a clearer insight and physical understanding, in future it would be desirable to repeat these experiments using injection of short optical pulses (and/or short pulse current modulation), so reducing considerably the optically-induced heating effects.
5. Conclusion In this paper measured effects of the injection of 1.4 mm beam into two commercial 1.55 mm operating DFB lasers are presented. Despite the lack of information about the DFB structure and layer composition, it is clear that different behaviours occur depending upon the injection wavelength and power. Mode attenuation
and/or enhancement can be observed. Repetition of this experiment with known-structure DFB lasers to investigate the processes leading to the observed behaviours is desirable, but is beyond the scope of the present work. Such measurements could lead to the design of devices operating, for example, as ON–OFF logic ports or signal amplifiers, according to the injected signal.
Acknowledgements We acknowledge the EPSRC grant EP/G023972/1 for financial support and the collaboration within the COST Action MP0805 entitled ‘‘Novel Gain Materials and Devices Based on III-V-N Compounds’’.
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