We have developed an InAs/InP quantum dot (QD) gain material using a double cap growth procedure and GaP sublayer to tune QDs into the L-band. By using it, a passive L-band mode-locked laser with pulse duration of 445 fs at the repetition rate of 46 GHz was demonstrated. The 3-dB linewidth of the RF spectrum is less than 100 KHz. The lasing threshold injection current is 24 mA with an external differential quantum efficiency of 22% and an average output power of 27 mW. The relationship between pulse duration and 3-dB spectral bandwidth as a function of injection current was investigated.
© 2009 OSA
Ultra-large-capacity optical transmission systems are vital to the network infrastructure of the 21st century, supporting various emerging information systems such as internet, mobile communication systems, digital CATV and intelligent transport systems. Optical networking systems based on wavelength-division multiplexing (WDM) technology  are considered the most effective means of boosting the transmission capacity, and now commercial ones are available, mostly in the C-band with a wavelength range of 1530 nm–1560 nm. In an effort to increase system bandwidth adding capabilities at longer wavelengths, such as in the L-band with a wavelength range of 1570 nm–1600 nm, is strongly desirable, thereby increasing the network capacity and flexibility .
Monolithic semiconductor mode-locked lasers (MLLs) are of great interest for optical communications due to their compact nature, mechanical stability and robustness, high repetition rates and low timing jitter. As a result, monolithic MLLs have been extensively studied in bulk and quantum well (QW) semiconductor materials for many years . However, typically these sources only generate picosecond (ps) pulses . Recently, quantum dot (QD) MLLs have received attention , their inherent properties, such as very broad spectral gain bandwidths [5,6], better temperature stability , ultra-low threshold current density , and much faster carrier dynamics  are expected to lead to improved performance. The first demonstration of a QD MLL was by Huang et al  in 2001 using a two-section InAs/GaAs-based QD gain material. More recently, QD MLLs have been reported using InAs/InP-based QDs operating at the important telecom wavelengths around 1.5 µm [11–15]. To date, most existing research work on QD MLLs has focused on the C-band wavelength range. In order to increase network capacity and flexibility, QD MLLs operating at the L-band are very attractive laser sources for large-capacity optical communications. We report here for the first time femtosecond (fs) pulse generation in the L-band in a Fabry-Perot (F-P) semiconductor laser.
2. InAs/InP-based QD samples, experimental set-up, results and discussions
The InAs/InP QD laser samples used in this study were grown by chemical beam epitaxy (CBE) on exactly (100) oriented n-type InP substrates. The undoped active region of the QD sample consisted of five stacked layers of InAs QDs with In0.816Ga0.184As0.392P0.608 (1.15Q) barriers. The QDs were tuned to operate in the L-band using a QD double cap growth procedure and a GaP sublayer [16,17]. In the double cap process the dots are partially capped with a thin layer of barrier material, followed by a 30 second growth interruption and then complete capping. The thickness of the partial cap controls the height of the dots, and hence their emission wavelength. It also helps to narrow the height distribution of the dots, and therefore narrow the FWHM of the gain spectrum. Growing the dots on a thin GaP layer allows a high dot density to be obtained and improved layer uniformity when stacking multiple layers of dots, providing maximum gain. Further details can be found in ref. 17. This active layer was embedded in a 355 nm thick 1.15Q waveguiding core, providing both carrier and optical confinement. An average dot density of approximately 3.5×1010 cm-2 per layer was obtained. The waveguiding core was surrounded by p-doped (top) and n-doped (bottom) layers of InP and capped with a heavily doped thin InGaAs layer to facilitate the fabrication of low resistance Ohmic contacts. The sample was fabricated into single lateral mode ridge waveguide lasers with a ridge width of 3 µm, and then cleaved to form a Fabry-Perot (F-P) laser cavity. One facet had a broadband high reflectivity (HR) coating and the other was left as-cleaved and was used as the output facet. This was coupled to an anti-reflection coated lensed fiber followed by an L-band optical isolator to reduce any back-reflection to the laser. The laser was driven with a DC injection current, and tested on a heat sink maintained at 18°C. The performance of the QD MLL was characterized using an optical spectrum analyzer (Ando AQ6317B), an optical autocorrelator (Femtochrome Research Inc FR-103HS), an electronic spectrum analyzer (HP70004A) with RF receiver (HP70909A), a 100-GHz bandwidth photodetector (U2t Photonics XPDV4120R), an RF harmonic mixer (HP 11970V), a digital phosphor oscilloscope (Tektronix TDS3054B), a delayed self-heterodyne interferometer (Advantest Q7332 and R3361A) and a power meter (Newport 840).
Figure 1(a) shows the evolution of the PL of the QDs as a function of the thickness of the QD partial cap layer. It can be seen that the emission wavelength range can be tuned from the C- to L-band by only changing the overgrowth cap layer thickness. It is interesting to see how the properties of the core layer observed above influences the properties of the working devices. Figure 1(b) shows their corresponding lasing wavelength spectra for devices with a ridge 1 mm long, 3 µm wide with as-cleaved facets at an injection current of 60 mA. As expected, the lasing wavelength tracks the emission wavelength of the core material, with a small offset to shorter wavelength. The exact wavelength at which the lasing peak occurs is related to the details of the gain and losses in the device and will change with cavity length due to varying facet losses  as well as drive current, and therefore the observed offset is not surprising.
Figure 2(a) and (b) show schematics of the L-band InAs/InP QD MLL and its typical lasing spectrum, respectively. This device was 930 µm long (with one facet HR coated), had a central lasing wavelength of 1585 nm and 3-dB spectral bandwidth of approximately 13 nm, suggesting the capability to obtain femtosecond pulses. The average output power and the 3-dB spectral bandwidth against injection current for this laser are shown in Fig. 3. The lasing threshold current was 24 mA with a slope efficiency of 0.175 mW/mA. The lasing threshold current density per QD layer was less than 172 A/cm2 and the external differential quantum efficiency around 1585 nm was up to 22%. The average output power was 27 mW at an injection current of 180 mA.
The 3-dB bandwidth of the amplified spontaneous emission (ASE) from the QD gain material before lasing was 85 nm and 46 nm for injection currents of 10 mA and 16 mA respectively. When the lasing threshold current of 24 mA was reached the 3-dB bandwidth had decreased to 3.8 nm, as shown in Fig. 3. Above threshold the spectral bandwidth increases with injection current because of the contribution of the inhomogeneous broadening of the QDs due to statistically distributed sizes, geometries and compositions. At the same time, the homogeneous broadening of the QDs becomes larger with increasing injection current, and hence an increase of internal-cavity optical intensity, due to power broadening effects. When this homogeneous broadening is large enough for mode competition between adjacent longitudinal modes to become dominant, compared to inhomogeneous broadening of the QDs, the spectral bandwidth of the QD MLL will only increase slowly. In other words, if the intra-cavity laser intensity is sufficient, the above mentioned homogeneous power broadening process would be the dominant factor in controlling the spectral bandwidth of the QD MLL. We would then observe an almost constant 3-dB lasing bandwidth for the QD MLL, as is shown in Fig. 3 for injection currents above 100 mA.
Modelocking action of the laser was confirmed by measuring its RF spectra as shown in Fig. 4, which has a clear resonance at 46 GHz with a 3-dB linewidth of less than 100 KHz and a signal to noise ratio (SNR) of greater than 20 dB. Both intensity noise and timing jitter (or phase noise) of MLLs can broaden its RF spectral linewidth. Figure 4 showed that 46 GHz QD MLL has very narrow linewidth of less than 100 KHz, which means our fs pulse trains has low timing jitter. So the QD MLL offers a promising solution to high-bit-rate clock recovery modules for DPSK signals with ultra-low timing jitter and transparency to the RZ and NRZ formats . In the experiment we also used a delayed self-heterodyne interferometer and an RF spectrum analyzer to measure the optical longitudinal mode beating linewidth, which was less than 20 KHz as shown in Fig. 5, which is the resolution-limit given by the current 5 km delay optical fiber. Such a small mode beating linewidth clearly demonstrates that the phase fluctuations of these longitudinal modes from our QD F-P cavity are largely synchronized or correlated, as expected in a strong phase-mode-locked laser.
Pulse duration measurements were made with a self-referenced intensity autocorrelator based on second harmonic generation (SHG) as shown in Fig. 6(a). The inset in Fig. 6(a) shows a long scan pulse train autocorrelation signal exhibiting the 21.74 ps periodic time of the emitted pulse train, corresponding to the repetition rate of 46 GHz and the free spectral range (FSR) of 0.385 nm at the central wavelength of 1585 nm. The autocorrelation pulse width was measured to be 629 fs. Fitting the experimental data using a Gaussian pulse shape provides a conversion factor of 0.707, resulting in a real pulse width Δτ of 445 fs at the output of the laser, without any external pulse compression scheme. Figure 6(a) and its inset shown some small DC background which was mostly coming from the optical autocorrelator because we have to use the high sensitivity of the photomultiplier tube detector with the 10-fs time resolution in order to obtain the strong enough SHG signals. If we subtract these DC background, the extinction ratio of pulses in time domain is larger than 10 dB. Figure 6(b) shows the relationship of pulse duration and time-bandwidth product (TBP) as a function of injection current. Both the pulse duration and the TBP remained almost constant around 445 fs and 0.69 when the injection current was over 100 mA. The TBP of 0.69 is larger than the transform-limited Gaussian-shaped pulses of 0.44. It indicates that there is some residual frequency chirp being present in the fs pulses. Because the nonlinear phenomenon of SPM in MLLs could cause a considerable spectral broadening and distortion of fs pulses, in the experiment optical pulses or 3-dB bandwidths did not change much as shown in Fig. 6 (b). That means the SPM-based frequency chirping is not dominant in the L-band QD MLLs.
We would like briefly to summarize the proposed QD MLL working principle. Due to statistically distributed QD size, geometry and composition, electrically pumped self-assembled QDs have highly imhomogeneously-broadened ASE spectra [5,6]. This means that lasing can occur over a wide wavelength range where intracavity gain is larger than the total optical loss. Since inhomogeneous gain dramatically suppresses mode competition, stable QD multi-wavelength lasers (QD MWL) have been obtained in such QD F-P cavities with tens or hundreds of longitudinal lasing modes [19–21]. Due to the intrinsic linear intracavity dispersion from the QD waveguide and facet mirror coatings, QD MWLs have a small, but non-zero, dispersion in longitudinal mode spacing. When the injection current, and hence intracavity optical intensity, becomes large enough this non-equal mode spacing can be compensated by intracavity nonlinear dispersion effects related to the interaction of QD excitons with the intracavity laser field. The longitudinal mode spacing then becomes equal over a broad wavelength range. In this case four-wave mixing (FWM) is dramatically enhanced within the QD F-P cavity . If the spectral bandwidth is broad enough, tens or even hundreds of longitudinal modes would lase and their phases lock together through FWM. These phase-locked lasing modes lead initially to random intensity spikes in the time domain, and subsequently to a periodic pulse train due to Kerr-lens effects based on self-focusing . If the nonlinear index n2 of the QD active material is positive, the center part of the beam transverse profile, where the intensity is higher, experiences a larger refractive index relative to the edges. The resulting nonlinear dielectric waveguide increases the beam confinement near its center and hence narrows the beam diameter to an extent proportional to the optical power. As a result, a smaller beam diameter, in turn, leads to a decreased mode interaction with the QD waveguide interface, which is a major source of waveguide losses. The net result is a decrease in the optical losses with increasing intensity, and it becomes favorable for the laser to emit ultrashort pulses. Eventually a train of ultrashort pulses with a repetition rate corresponding to the cavity round-trip time is generated.
We have experimentally developed a passive single-section L-band QD MLL with 445 fs optical pulses at a repetition rate of 46 GHz without any external pulse compression scheme. The measured 3-dB RF linewidth was less than 100 KHz, which indicated that stable and ultra-low timing jitter femtosecond pulses were generated. The threshold injection current was less than 24 mA corresponding to a lasing threshold current per QD layer of 172 A/cm2.
References and links
1. K. M. Sivalingam and S. Subramaniam, “Optical WDM Networks — Principles and Practice,” 3–374 (Springer, Berlin Heidelberg, 2000).
2. T. Sakamoto, J. Kani, M. Jinno, S. Aisawa, M. Fukui, M. Yamada, and K. Oguchi, “Wide wavelength band (1535–1560 nm and 1574–1600 nm), 28x10Gbit/s WDM transmission over 320km dispersion-shifted fibre,” Electron. Lett. 34(4), 292–294 (1998). [CrossRef]
3. L.A. Jiang, E.P. Ippen, and H. Yokoyama, “Semiconductor mode-locked lasers as pulse sources for high bit rate data transmission,” in Book Series of Ultrahigh-Speed Optical Transmission Technology3, 21–51 (Springer Berlin Heidelberg, 2007). [CrossRef]
4. E. U. Rafailov, M. A. Cataluna, and W. Sibbett, “Mode-locked quantum-dot lasers,” Nat. Photonics 1(7), 395–401 (2007). [CrossRef]
5. Z. G. Lu, J. R. Liu, S. Raymond, P. J. Poole, P. J. Barrios, G. Pakulski, D. Poitras, F. G. Sun, S. Taebi, and T. J. Hall, “Ultra-broadband quantum-dot semiconductor optical amplifier and its applications,” The Proceedings of the Optical Fiber Communication Conference, Anaheim, CA, USA, paper JThA33 (25–29 March 2007).
6. R. Brenot, F. Lelarge, O. Legouezigou, F. Pommereau, F. Poingt, L. Legouezigou, E. Derouin, O. Drisse, B. Rousseau, F. Martin, and G. H. Duan, “Quantum dots semiconductor optical amplifier with 3-dB bandwidth of up to 120 nm in semi-cooled operation,” The Proceedings of the Optical Fiber Communication Conference, San Diego, CA, USA, paper OTuC1 (24–28 February 2008).
7. Y. Tanaka, M. Ishida, Y. Maeda, T. Akiyama, T. Yamamoto, H. Z. Song, M. Yamaguchi, Y. Nakata, K. Nishi, M. Sugawara, and Y. Arakawa, “High-speed and temperature-insensitive operation in 1.3-µm InAs/GaAs high-density quantum dot lasers,” The Proceedings of the Optical Fiber Communication Conference, San Diego, CA, USA, paper OWJ1 (24–26 March 2009).
8. G. T. Liu, A. Stintz, H. Li, K. J. Malloy, and L. F. Lester, “Extremely low room-temperature threshold current density diode lasers using lnAs dots in In0.05Ga0.85As quantum well,” Electron. Lett. 35(14), 1163–1165 (1999). [CrossRef]
9. A. J. Zilkie, J. Meier, P. W. E. Smith, M. Mojahedi, J. S. Aitchison, P. J. Poole, C. N. Allen, P. J. Barrios, and D. Poitras, “Femtosecond gain and index dynamics in an InAs/InGaAsP quantum dot amplifier operating at 1.55 microm,” Opt. Express 14(23), 11453–11459 (2006). [CrossRef] [PubMed]
10. X. D. Huang, A. Stintz, H. Li, F. Lester, J. L. Cheng, and K. J. Malloy, “Passive mode-locking in 1.3 µm two-section InAs quantum dot lasers,” Appl. Phys. Lett. 78(19), 2825–2827 (2001). [CrossRef]
11. J. Renaudier, R. Brenot, B. Dagens, F. Lelarge, B. Rousseau, F. Poingt, O. Legouezigou, F. Pommereau, A. Accard, P. Gallion, and G. H. Duan, “45 GHz self-pulsation with narrow linewidth in quantum dot Fabry-Perot semiconductor lasers at 1.5 µm,” Electron. Lett. 41(18), 1007–1008 (2005). [CrossRef]
12. M. J. R. Heck, E. A. J. M. Bente, B. Smalbrugge, Y. S. Oei, M. K. Smit, S. Anantathanasarn, and R. Nötzel, “Observation of Q-switching and mode-locking in two-section InAs/InP (100) quantum dot lasers around 1.55 mum,” Opt. Express 15(25), 16292–16301 (2007). [CrossRef] [PubMed]
13. Z. G. Lu, J. R. Liu, S. Raymond, P. J. Poole, P. J. Barrios, and D. Poitras, “312-fs pulse generation from a passive C-band InAs/InP quantum dot mode-locked laser,” Opt. Express 16(14), 10835–10840 (2008). [CrossRef] [PubMed]
14. Z. G. Lu, J. R. Liu, S. Raymond, P. J. Poole, P. J. Barrios, and D. Poitras“Femtosecond pulse generation in a C-band quantum dot laser,” The Proceedings of SPIE: Optoelectronic Materials and Devices III (edited by Yi Luo, Jens Koyama, Fumio Buus, and Yu-Hwa Lo), 7135, 71352L-1-7 (2008).
15. X. F. Tang, J. C. Cartledge, A. Shen, A. Akrout, and G. H. Duan, “Low-timing-jitter all-optical clock recovery for 40 Gbits/s RZ-DPSK and NRZ-DPSK signals using a passively mode-locked quantum-dot Fabry-Perot semiconductor laser,” Opt. Lett. 34(7), 899–901 (2009). [CrossRef] [PubMed]
16. P. J. Poole, R. L. Williams, J. Lefebvre, and S. Moisa, “Using As/P exchange processes to modify InAs/InP quantum dots,” J. Cryst. Growth 257(1–2), 89–96 (2003). [CrossRef]
17. P. J. Poole, K. Kaminska, P. Barrios, Z. G. Lu, and J. R. Liu, “Growth of InAs/InP-based quantum dots for 1.55 µm laser applications,” J. Cryst. Growth 311(6), 1482–1486 (2009). [CrossRef]
18. C. NÌ, “Allen, P.J. Poole, P. Marshall, J. Fraser, S. Raymond, and S. Fafard, “InAs self-assembled quantum dot lasers grown on (100) InP,” Appl. Phys. Lett.80, 3629–3631 (2002). [CrossRef]
19. J. R. Liu, Z. G. Lu, S. Raymond, P. J. Poole, P. J. Barrios, G. Pakulski, D. Poitras, G. Z. Xiao, and Z. Y. Zhang, “Uniform 90-channel multiwavelength InAs/InGaAsP quantum dot laser,” Electron. Lett. 43(8), 458–460 (2007). [CrossRef]
20. J. R. Liu, Z. G. Lu, S. Raymond, P. J. Poole, P. J. Barrios, and D. Poitras, “1.6-µm multi-wavelength emission of an InAs-InGaAsP quantum-dot laser,” IEEE Photon. Technol. Lett. 20(2), 81–83 (2008). [CrossRef]
21. Z. G. Lu, J. R. Liu, P. J. Poole, S. Raymond, P. J. Barrios, D. Poitras, G. Pakulski, X. P. Zhang, K. Hinzer, and T. J. Hall, “Low noise InAs/InP quantum dot C-band monolithic multiwavelength lasers for WDM-PONs,” The Proceedings of the Optical Fiber Communication Conference, San Diego, CA, USA, paper JWA27 (24–26 March 2009).
22. C. Gosset, K. Merghem, A. Martinez, G. Moreau, P. Patriarche, G. Aubin, A. Ramdane, J. Landreau, and F. Lelarge, “Subpicosecond pulse genearation at 134 GHz using a quantum-dash-based Fabry-Perot laser emitting at 1.56 µm,” Appl. Phys. Lett. 88(24), 241105 (2006). [CrossRef]