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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 8 — Apr. 14, 2008
  • pp: 5596–5601
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1.3 µm Quantum Dot Laser in coupled-cavity-injection-grating design with bandwidth of 20 GHz under direct modulation

F. Gerschütz, M. Fischer, J. Koeth, I. Krestnikov, A. Kovsh, C. Schilling, W. Kaiser, S. Höfling, and A. Forchel  »View Author Affiliations


Optics Express, Vol. 16, Issue 8, pp. 5596-5601 (2008)
http://dx.doi.org/10.1364/OE.16.005596


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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

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

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]

]. 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

2. Design

The design of the QD structure as well as corresponding device processing steps have already been described elsewhere [7

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.

Fig. 1. Schematic illustration of a CCIG laser design with the three electrically separated sections.

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.

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

5. 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).

]. 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

12. 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

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]

, 12

12. 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]

]. 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

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]

].

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

1.

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]

2.

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]

3.

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]

4.

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]

5.

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).

6.

S. Fathpour, Z. Mi, and P. Bhattacharya, “High-speed quantum dot lasers,” J. Appl. Phys. 38, 2103 (2005).

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]

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]

9.

U. Feiste, “Optimization of modulation bandwidth of DBR lasers with detuned Bragg reflectors,” IEEE J. Quantum Electron. 34, 2371–2379 (1998). [CrossRef]

10.

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]

11.

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]

12.

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]

13.

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

  1. 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]
  2. 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]
  3. 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]
  4. 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]
  5. 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).
  6. S. Fathpour, Z. Mi, and P. Bhattacharya, "High-speed quantum dot lasers," J. Appl. Phys. 38, 2103 (2005).
  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]
  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]
  9. U. Feiste, "Optimization of modulation bandwidth of DBR lasers with detuned Bragg reflectors," IEEE J. Quantum Electron. 34, 2371-2379 (1998). [CrossRef]
  10. 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]
  11. 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]
  12. 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]
  13. 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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