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1.3 μm Quantum Dot Laser in coupled-cavity-injection-grating design with bandwidth of 20 GHz under direct modulation
Optics Express, Vol. 16, Issue 8, pp. 5596-5601 (2008)
http://dx.doi.org/10.1364/OE.16.005596
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– Abstract
1. Introduction
2. Design
3. Results
4. Conclusion
– References and links
– Abstract
1. Introduction
2. Design
3. Results
4. Conclusion
– References and links
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Abstract
Using a multi section laser in coupled cavity injection grating design based on 1.3 µm InGaAs/GaAs quantum dot (QD) active region we were able to enhance the 3 dB modulation bandwidth well beyond the inherent material modulation bandwidth. The material bandwidth was determined by measurements on distributed feedback (DFB) devices to approximately 8 GHz. The special multisectional design allows interaction between the lasing mode and a second mode used as catalyst and enables a high resonance frequency of the device. Based on active QD material this approach allowed us to reach a cut off frequency of 20 GHz in the small signal response of the device.
© 2008 Optical Society of America
1. Introduction
The constantly growing data transmission rates in all fields of telecommunication
cause a strong demand for single mode laser devices with optimized modulation
properties and nevertheless low manufacturing costs. The potential advantages of QD
lasers compared to quantum well (QW) based devices for these applications at a
wavelength of 1.3 µm are a high temperature stability combined with a low
laser threshold and an improved feedback resistance. The option of uncooled and
isolator-free operation can significantly reduce future packaging costs as well as
power consumption. However compared to their QW counterparts, the bandwidth of
lasers based on QD active material seems to be limited by higher gain compression [1] and carrier capture in the wetting layer [2, 3]. Conventional QD lasers on GaAs at 1.3 µm so far
reach cut off frequencies of 8.1 GHz [4] with related damping limits around 10 GHz [4, 5]. Lasers using a tunnel injection approach were able to
extend the modulation bandwidth at 1.3 µm to 11 GHz [6]. Due to the high temperature stability of QDs, temperature
insensitive 10 Gbit/s operation of a distributed feedback (DFB) laser over 21 km
single mode fibre could be realized in a temperature range from 25°C to
70°C [7]. Also data transmission was demonstrated using a QD DFB
laser at constant operation parameters in the temperature range from 25°C
up to 85°C over a distance of 8 km [5].
H. Su and L. F. Lester, “Dynamic properties of quantum dot distributed feedback lasers: high speed, linewidth and chirp,” J. Phys. D: Appl. Phys. 38, 2112–2118 (2005). [CrossRef]
J. Urayama, T. B. Norris, H. Jiang, J. Singh, and P. Bhattacharya, “Temperature-dependent carrier dynamics in self-assembled InGaAs quantum dots,” Appl. Phys. Lett. 80, 2162–2164 (2002). [CrossRef]
D. R. Matthews, H. D. Summers, P. M. Smowtown, and M. Hopkinson, “Experimental investigation of the effect of wetting-layer states on the gain-current characteristic of quantum-dot lasers,” Appl. Phys. Lett. 81, 4904–4906 (2002). [CrossRef]
M. Sugawara, N. Hatori, M. Ishida, H. Ebe, Y. Arakawa, T. Akiyama, K. Otsubo, T. Yamamoto, and Y. Nakata, “Recent progress in self-assembled quantum-dot optical devices for optical telecommunication: temperature-insensitive 10 Gbs-1 directly modulated lasers and 40 Gbs-1 signalregenerative amplifiers,” J. Phys. D: Appl. Phys. 38, 2126–2134 (2005). [CrossRef]
M. Sugawara, N. Hatori, M. Ishida, H. Ebe, Y. Arakawa, T. Akiyama, K. Otsubo, T. Yamamoto, and Y. Nakata, “Recent progress in self-assembled quantum-dot optical devices for optical telecommunication: temperature-insensitive 10 Gbs-1 directly modulated lasers and 40 Gbs-1 signalregenerative amplifiers,” J. Phys. D: Appl. Phys. 38, 2126–2134 (2005). [CrossRef]
B. Dagens, M. Fischer, F. Gerschütz, J. Koeth, I. Krestnikov, A. Kovsh, O. Bertran-Pardo, O. Le Gouezigou, and D. Make, “Uncooled isolator-free directly modulated quantum dot laser 10 Gb/s transmission at 1.3 µm with constant operation parameters,” European Conference on Optical Communication , Th4.5.7. (2006).
F. Gerschütz, M. Fischer, J. Koeth, M. Chacinski, R. Schatz, O. Kjebon, A. Kovsh, A. Krestnikov, and A. Forchel, “Temperature insensitive 1.3 µm InGaAs/GaAs quantum dot distributed feedback lasers for 10 Gbit/s transmission over 21 km,” Electron. Lett. 42, 1457–1458 (2006). [CrossRef]
B. Dagens, M. Fischer, F. Gerschütz, J. Koeth, I. Krestnikov, A. Kovsh, O. Bertran-Pardo, O. Le Gouezigou, and D. Make, “Uncooled isolator-free directly modulated quantum dot laser 10 Gb/s transmission at 1.3 µm with constant operation parameters,” European Conference on Optical Communication , Th4.5.7. (2006).
We present for the first time high bandwidth QD devices at 1.3 µm based on
multi section laser concept in coupled-cavity-injection-grating design. The special
design of a CCIG laser allows the utilization of higher resonances like the
photon-photon-resonance (PPR) in the resonator [8–13]. Using this approach we were able to achieve a
high modulation bandwidth of 20 GHz for a directly modulated laser based on QD
active material for the first time. The demonstrated cut off frequency of 20 GHz
under small signal modulation is much higher than the intrinsic relaxation
frequency
G. Morthier, R. Schatz, and O. Kjebon “Extended modulation bandwidth of DBR and external cavity lasers by utilizing a cavity resonance for equalization,” IEEE J. Quantum Electron. 36, 1468–1475 (2000). [CrossRef]
2. Design
The design of the QD structure as well as corresponding device processing steps have
already been described elsewhere [7].
F. Gerschütz, M. Fischer, J. Koeth, M. Chacinski, R. Schatz, O. Kjebon, A. Kovsh, A. Krestnikov, and A. Forchel, “Temperature insensitive 1.3 µm InGaAs/GaAs quantum dot distributed feedback lasers for 10 Gbit/s transmission over 21 km,” Electron. Lett. 42, 1457–1458 (2006). [CrossRef]
The presented CCIG laser consists of three electrically separated sections, each with
a different length. To ensure an appropriate isolation of each section, breaks in
the metal contacts were used. In addition the highly p-doped cap layer was removed
between the different sections by etching with a depth of 300 nm, resulting in a
resistance of ~1.5 kΩ between the sections.
Our laser was designed as a coupled-cavity-injection-grating laser (Fig. 1) comprising three sections with a total length of 2.4
mm. The relatively long cavity of 2.4 mm allows the frequency of the PPR to be
adequately separated from the first resonance. Furthermore the position of the PPR
can be tuned within a range of ~2 GHz by modifying the operation parameters,
especially by current injection in the grating section.
The gain section has a length of 450 µm and is used to pump the laser
beyond threshold. For bandwidth measurements the modulation signal is superimposed
to this section. The grating section has a length of 1000 µm. To avoid
strong coupling of the electro-magnetic field to the grating and hence to avoid
strong damping, an appropriate etch depth of the ridge waveguide structure and a
duty cycle of 1:1 for the grating is selected. The current injection in this section
allows to control the coupling strength between the gain and the phase section. The
phase section with a length of 950 µm is used to control and to tune the
phase matching. Current injection in this section results in a change of the
refractive index causing a slightly different optical cavity length. This allows for
an exact adjustment of the optimum resonance position and hence the resonance level.
The laser was mounted on a copper heat-sink and no facet coatings were applied. The
device was operated cw and in small signal modulation mode.
3. Results
Figure 2 shows the emission spectrum of the CCIG device. For
this measurement the three sections were biased separately. The gain section was
pumped by a current of 38 mA, the grating section was pumped by 175 mA and the phase
section by 170 mA. This current combination was - amongst others - also used for the
measurement presented under small signal modulation in the following. As can be
seen, the device emits at a wavelength of 1302.6 nm with a side mode suppression
ratio (SMSR) of about 50 dB. In addition a total output power of 32 mW was measured
for this combination of currents.
Fig. 2. Emission spectrum of the CCIG device under cw operation and at
25°C. The injected currents are: 38 mA (gain) – 175 mA
(grating) – 170 mA (phase). Single mode emission at a wavelength of 1302.6 nm
with SMSR of ~50 dB is observed.
In addition, the output power characteristic of the device was studied for
independent current injection into the sections (Fig. 3). While the current in the gain section was kept
constant at 50 mA, the grating current and the phase current were each varied from
100 mA to 200 mA. As can be seen from the map when looking along lines of constant
current in one section while increasing the current in the other section, the output
power does not continuously increase. This behavior is characteristic for CCIG
devices but has not been described in detail yet. It may be explained by a change in
the refractive index caused by the increasing current in only one section and by the
thereby resulting effects like reflection and non ideal phase matching.
Fig. 3. Output power of the front facet f a QD CCIG device as a function of grating
current and phase current under cw operation at 25°C. The gain
current is kept constant at 50 mA for this measurement.
The most interesting property of the fabricated QD CCIG device is its capability of
enhancing the modulation bandwidth beyond the limits related to the base material
properties of the underlying QD material [11]. In commonly used telecommunication lasers the
electron-photon-resonance (EPR) is limiting the modulation bandwidth by the
interaction speed between the electronic states and the electromagnetic field, i.e.
the relaxation time of the excited electrons. But by using a higher order resonance
like the PPR, which is related to the roundtrip time of the photons in the laser
cavity, it is possible to extend the cutoff frequency beyond usual limitations.
Although the position of the PPR is mainly fixed by the device length, a fine tuning
is possible by changing the current injection in the phase section. Because of the
relation between the round trip time of the photons in the cavity and the PPR a
relatively long device is needed to take advantage of the PPR. A necessary
prerequisite for the PPR is the existence of a weak second mode used as a catalyst.
The second mode has to be placed on the longer wavelength side of the main mode in a
distance equal to the position of the PPR. Since the position of the PPR occurs at
~15 GHz, the distance of the main mode and the side mode has also to be 15 GHz,
corresponding to a wavelength difference of ~0.1 nm.
L. Bach, W. Kaiser, J. P. Reithmaier, A. Forchel, T. W. Berg, and B. Tromborg, “Enhanced directmodulated bandwidth of 37 GHz by a multi-section laser with a coupled-cavity-injection-grating design,” Electron. Lett. 39, 1592–1593 (2003). [CrossRef]
In Fig. 4 several small signal curves for different
combinations of currents in the sections are shown. For these measurements the
sections were separately electrically contacted and the small signal modulation
signal was superimposed to the cw current in the gain section. For optimized
operation parameters a maximum 3 dB frequency of 20 GHz is achieved. This maximum
cut off frequency was measured at the same currents as the spectrum in Fig. 3. At ~2–5 GHz (Fig. 4(a)) and ~2.5 GHz (Fig. 4(b)) the first resonance caused by interaction of the
electrons and the electromagnetic field can be observed. The observed cut off
frequency of 20 GHz is much higher than the maximum bandwidth we achieved using DFB
lasers based on identical QD material, which show a typical bandwidth around 8 GHz
under small signal direct modulation and were also investigated under 10 Gbit/s
operation [5]. The behavior of the second resonance can be tuned,
depending on the injected current, from a relatively strong peak (Fig. 4(b)) to a flat response (Fig. 4(a)). With increasing phase current the PPR becomes
flat and the bandwidth is extended while shifting to higher frequencies when the
grating current is increased. This second resonance peak is caused by the
interaction of the main mode and a weak side mode on its long wavelength side. The
position and shape of the second resonance are influenced by the different
combinations of current in the sections. The strength of damping of the PPR is
controlled by the phase matching of the front facet and the back facet by current
injection in the phase section [12].
B. Dagens, M. Fischer, F. Gerschütz, J. Koeth, I. Krestnikov, A. Kovsh, O. Bertran-Pardo, O. Le Gouezigou, and D. Make, “Uncooled isolator-free directly modulated quantum dot laser 10 Gb/s transmission at 1.3 µm with constant operation parameters,” European Conference on Optical Communication , Th4.5.7. (2006).
W. Kaiser, L. Bach, J. P. Reithmaier, and A. Forchel, “High speed coupled cavity injection grating lasers with tailored modulation transfer function,” IEEE Photon. Technol. Lett. 16, 1997 (2004). [CrossRef]
Fig. 4. (a) Small signal response of a QD CCIG device for various grating currents at
25°C. The currents of the phase and gain section are kept
constant during the measurement whereas the grating current is varied.
Fig. 4. (b) Small signal response of a QD CCIG device for various phase currents at
25°C. The currents in the grating and gain section are kept
constant during the measurement.
While the phase section controls the height of the second resonance peak (Fig. 4(a)), its position is controlled by the grating
section [8, 12]. As can be seen in Fig. 4(b) only a small change in the grating current of 1
mA is necessary to shift the position of the peak. Hence it is not only possible to
control the strength of the second resonance but even to control its position. These
possibilities allow tailoring of the modulation transfer function for different
applications like large signal transmission with high transmission rates or local
oscillator applications [8].
G. Morthier, R. Schatz, and O. Kjebon “Extended modulation bandwidth of DBR and external cavity lasers by utilizing a cavity resonance for equalization,” IEEE J. Quantum Electron. 36, 1468–1475 (2000). [CrossRef]
W. Kaiser, L. Bach, J. P. Reithmaier, and A. Forchel, “High speed coupled cavity injection grating lasers with tailored modulation transfer function,” IEEE Photon. Technol. Lett. 16, 1997 (2004). [CrossRef]
G. Morthier, R. Schatz, and O. Kjebon “Extended modulation bandwidth of DBR and external cavity lasers by utilizing a cavity resonance for equalization,” IEEE J. Quantum Electron. 36, 1468–1475 (2000). [CrossRef]
4. Conclusion
We have fabricated a 1.3 µm InGaAs/GaAs QD laser in CCIG design with
laterally coupled grating structure. Single mode emission with SMSR around 50 dB is
demonstrated. The output power of the device exceeds 30 mW for currents
<300 mA. We observe a record high bandwidth of 20 GHz under small signal
modulation, demonstrating the potential of the concept for a bandwidth extension of
QD based lasers under direct modulation.
Acknowledgments
The authors would like to thank B. Hubert, C. König and S. Ehrke for
expert technical assistance. The work was supported by the European project ZODIAC
(project number 017140).
References and links
H. Su and L. F. Lester, “Dynamic properties of quantum dot distributed feedback lasers: high speed, linewidth and chirp,” J. Phys. D: Appl. Phys. 38, 2112–2118 (2005). [CrossRef] | |
J. Urayama, T. B. Norris, H. Jiang, J. Singh, and P. Bhattacharya, “Temperature-dependent carrier dynamics in self-assembled InGaAs quantum dots,” Appl. Phys. Lett. 80, 2162–2164 (2002). [CrossRef] | |
D. R. Matthews, H. D. Summers, P. M. Smowtown, and M. Hopkinson, “Experimental investigation of the effect of wetting-layer states on the gain-current characteristic of quantum-dot lasers,” Appl. Phys. Lett. 81, 4904–4906 (2002). [CrossRef] | |
M. Sugawara, N. Hatori, M. Ishida, H. Ebe, Y. Arakawa, T. Akiyama, K. Otsubo, T. Yamamoto, and Y. Nakata, “Recent progress in self-assembled quantum-dot optical devices for optical telecommunication: temperature-insensitive 10 Gbs-1 directly modulated lasers and 40 Gbs-1 signalregenerative amplifiers,” J. Phys. D: Appl. Phys. 38, 2126–2134 (2005). [CrossRef] | |
B. Dagens, M. Fischer, F. Gerschütz, J. Koeth, I. Krestnikov, A. Kovsh, O. Bertran-Pardo, O. Le Gouezigou, and D. Make, “Uncooled isolator-free directly modulated quantum dot laser 10 Gb/s transmission at 1.3 µm with constant operation parameters,” European Conference on Optical Communication , Th4.5.7. (2006). | |
S. Fathpour, Z. Mi, and P. Bhattacharya, “High-speed quantum dot lasers,” J. Appl. Phys. 38, 2103 (2005). | |
F. Gerschütz, M. Fischer, J. Koeth, M. Chacinski, R. Schatz, O. Kjebon, A. Kovsh, A. Krestnikov, and A. Forchel, “Temperature insensitive 1.3 µm InGaAs/GaAs quantum dot distributed feedback lasers for 10 Gbit/s transmission over 21 km,” Electron. Lett. 42, 1457–1458 (2006). [CrossRef] | |
G. Morthier, R. Schatz, and O. Kjebon “Extended modulation bandwidth of DBR and external cavity lasers by utilizing a cavity resonance for equalization,” IEEE J. Quantum Electron. 36, 1468–1475 (2000). [CrossRef] | |
U. Feiste, “Optimization of modulation bandwidth of DBR lasers with detuned Bragg reflectors,” IEEE J. Quantum Electron. 34, 2371–2379 (1998). [CrossRef] | |
O. Kjebon, R. Schatz, S. Lourdudoss, S. Nilsson, B. Stalnacke, and L. Backbom, “30 GHz direct modulation bandwidth in detuned loaded InGaAsP DBR lasers at 1.55 µm,” Electron. Lett. 33, 488–489 (1997). [CrossRef] | |
L. Bach, W. Kaiser, J. P. Reithmaier, A. Forchel, T. W. Berg, and B. Tromborg, “Enhanced directmodulated bandwidth of 37 GHz by a multi-section laser with a coupled-cavity-injection-grating design,” Electron. Lett. 39, 1592–1593 (2003). [CrossRef] | |
W. Kaiser, L. Bach, J. P. Reithmaier, and A. Forchel, “High speed coupled cavity injection grating lasers with tailored modulation transfer function,” IEEE Photon. Technol. Lett. 16, 1997 (2004). [CrossRef] | |
M. Radziunas, A. Glitzky, U. Bandelow, M. Wolfrum, U. Troppenz, J. Kreissl, and W. Rehbein, “Improving the modulation bandwidth in semiconductor lasers by passive feedback,” IEEE J. Sel. Top. Quantum Electron. 13, 136–142 (2007). [CrossRef] |
OCIS Codes
(140.5960) Lasers and laser optics : Semiconductor lasers
(250.5590) Optoelectronics : Quantum-well, -wire and -dot devices
ToC Category:
Lasers and Laser Optics
History
Original Manuscript: January 15, 2008
Revised Manuscript: March 8, 2008
Manuscript Accepted: March 20, 2008
Published: April 7, 2008
Citation
F. Gerschütz, M. Fischer, J. Koeth, I. Krestnikov, A. Kovsh, C. Schilling, W. Kaiser, S. Höfling, and A. Forchel, "1.3 μm Quantum Dot Laser in coupled-cavity-injection-grating design with bandwidth of 20 GHz under direct modulation," Opt. Express 16, 5596-5601 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-8-5596
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References
- H. Su and L. F. Lester, "Dynamic properties of quantum dot distributed feedback lasers: high speed, linewidth and chirp," J. Phys. D: Appl. Phys. 38, 2112-2118 (2005). [CrossRef]
- J. Urayama, T. B. Norris, H. Jiang, J. Singh, and P. Bhattacharya, "Temperature-dependent carrier dynamics in self-assembled InGaAs quantum dots," Appl. Phys. Lett. 80, 2162-2164 (2002). [CrossRef]
- D. R. Matthews, H. D. Summers, P. M. Smowtown, and M. Hopkinson, "Experimental investigation of the effect of wetting-layer states on the gain-current characteristic of quantum-dot lasers," Appl. Phys. Lett. 81, 4904-4906 (2002). [CrossRef]
- M. Sugawara, N. Hatori, M. Ishida, H. Ebe, Y. Arakawa, T. Akiyama, K. Otsubo, T. Yamamoto, and Y. Nakata, "Recent progress in self-assembled quantum-dot optical devices for optical telecommunication: temperature-insensitive 10 Gbs-1 directly modulated lasers and 40 Gbs-1 signal-regenerative amplifiers," J. Phys. D: Appl. Phys. 38, 2126-2134 (2005). [CrossRef]
- B. Dagens, M. Fischer, F. Gerschütz, J. Koeth, I. Krestnikov, A. Kovsh, O. Bertran-Pardo, O. Le Gouezigou, and D. Make, "Uncooled isolator-free directly modulated quantum dot laser 10 Gb/s transmission at 1.3 µm with constant operation parameters," European Conference on Optical Communication, Th4.5.7. (2006).
- S. Fathpour, Z. Mi, and P. Bhattacharya, "High-speed quantum dot lasers," J. Appl. Phys. 38, 2103 (2005).
- F. Gerschütz, M. Fischer, J. Koeth, M. Chacinski, R. Schatz, O. Kjebon, A. Kovsh, A. Krestnikov, and A. Forchel, "Temperature insensitive 1.3 µm InGaAs/GaAs quantum dot distributed feedback lasers for 10 Gbit/s transmission over 21 km," Electron. Lett. 42, 1457-1458 (2006). [CrossRef]
- G. Morthier, R. Schatz, and O. Kjebon "Extended modulation bandwidth of DBR and external cavity lasers by utilizing a cavity resonance for equalization," IEEE J. Quantum Electron. 36, 1468-1475 (2000). [CrossRef]
- U. Feiste, "Optimization of modulation bandwidth of DBR lasers with detuned Bragg reflectors," IEEE J. Quantum Electron. 34, 2371-2379 (1998). [CrossRef]
- O. Kjebon, R. Schatz, S. Lourdudoss, S. Nilsson, B. Stalnacke, and L. Backbom, "30 GHz direct modulation bandwidth in detuned loaded InGaAsP DBR lasers at 1.55 µm," Electron. Lett. 33, 488-489 (1997). [CrossRef]
- L. Bach, W. Kaiser, J. P. Reithmaier, A. Forchel, T. W. Berg, and B. Tromborg, "Enhanced direct-modulated bandwidth of 37 GHz by a multi-section laser with a coupled-cavity-injection-grating design," Electron. Lett. 39, 1592-1593 (2003). [CrossRef]
- W. Kaiser, L. Bach, J. P. Reithmaier, and A. Forchel, "High speed coupled cavity injection grating lasers with tailored modulation transfer function," IEEE Photon. Technol. Lett. 16, 1997 (2004). [CrossRef]
- M. Radziunas, A. Glitzky, U. Bandelow, M. Wolfrum, U. Troppenz, J. Kreissl, and W. Rehbein, "Improving the modulation bandwidth in semiconductor lasers by passive feedback," IEEE J. Sel. Top. Quantum Electron. 13, 136-142 (2007). [CrossRef]
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