To open up a 1-µm waveband for photonic transport systems, we developed a hybrid and harmonically mode-locked semiconductor laser (MLL) that can transmit return-to-zero (RZ) optical signals at data rates on the order of gigabits per second. A single-mode hole-assisted fiber (HAF) was also developed for use as a 1-µm waveband signal transmission line. A stable optical pulse train with a repetition rate of 9.953 GHz, pulse width of 22 ps, and low timing jitter of 120 fs was obtained from a 1035-nm harmonically MLL. With these devices, we successfully demonstrated 1-µm waveband error-free transmission of a high-speed 10-Gbps RZ signal over a long distance of 7 km.
© 2008 Optical Society of America
Photonic transport in the C- and L-bands has been extensively employed on conventional photonic networks [1, 2]. Due to the accelerating demand for higher data transmission capacities, it is necessary to utilize alternate wavebands and develop methods to enhance the capacities of photonic networks. We have focused on the development of 1-µm waveband systems for photonic networks. We also considered that the 1-µm waveband system is a possible candidate, since the ytterbium-doped fiber amplifiers (YDFAs) that are currently available can be used as 1R repeaters . Additionally, high-performance semiconductor devices such as high-power lasers , quantum dot lasers [5-8], vertical-cavity surfaceemitting lasers (VCSEL) [9–11], semiconductor optical amplifiers (SOA), and silicon-material based photo detectors may also be employed in the 1-µm waveband by using a GaAs-based quantum well or dot structure.
To construct a 1-µm waveband photonic transport system, the most important photonic devices are a stable and high-speed (>1 Gbps) optical signal source and a long-distance single-mode optical fiber. For the optical signal source, we focus on a 1-µm waveband mode-locked semiconductor laser (MLLs), because MLLs are considered to be a simple and compact option for multi-GHz pulse sources. The 1-µm waveband MLL has been developed for a nonlinear-optic bio-imaging . For bio-imaging applications, the required repetition frequency of the MLL was up to 500 MHz. But, for telecommunication applications, we require a higher repetition frequency of at least 1 GHz. In this paper, we denote the characteristics of a fabricated InGaAs/GaAs semiconductor based external-cavity harmonically-MLL. In a view point of telecommunication applications, we characterized the MLL laser pulse including the pulse width, spectral width, rf-spectra, timing jitter, and wavelength tunability. Furthermore, we have also focusing on a hole-assisted fiber (HAF) that is a possible candidate for future 1-µm-waveband single-mode transmission lines: the HAF has a flexible and simple structure, low-fabrication cost, and highly controllable optical properties . Here, we discuss the possibility of realizing a 1-µm waveband photonic transport system with the above-mentioned devices.
2. Mode-locked semiconductor laser in the 1-µm waveband
A bi-sectional semiconductor laser diode has a useful and simple structure, appropriate for the fabrication of 1-µm-waveband MLLs. Additionally, it is expected that 1-µm-waveband MLLs might be applied not only to RZ optical signal sources but also to high-speed optical signal processing applications, including optical 3R repeaters [14, 15]. We fabricated a semiconductor laser chip comprising a strained InGaAs quantum well (QW) on GaAs (001). Figure 1(a) is a schematic of the fabricated laser. The triple-QW active layer and AlGaAs cladding layers are grown by solid-source molecular beam epitaxy (MBE). The measured photoluminescence peak wavelength is around 1020 nm. And, the laser wavelength is tuned to 1030 nm by controlling the composition.
Two sectional electrode regions -optical gain and saturable absorber (SA)- are fabricated on the ridge-waveguide laser structure shown in Fig. 1(a). The lengths of the optical gain and SA regions are 500 and 50 µm, respectively . One of cleaved facets of the laser chip is coated with an anti-reflection (AR) coat (reflectance: <0.3%) to construct an external cavity laser. Figure 1(b) shows a schematic set-up of the external-cavity hybrid MLL. A narrow-band optical filter (0.6 nm) is installed between the laser chip and external mirror to stabilize the pulse generation from the MLL. The cavity length, which is the length between the mirror and laser chip, is fixed for 1.0-GHz optical pulsation. Naturally, the repetition frequency can be controlled by adjusting the cavity length. An RF-input (V rf) is applied to the gain region for hybrid mode-locking operation. The optical output from the MLL is coupled to a 1-µm wavelength single-mode optical fiber. Figure 2 plots the gain current vs. the light output curves (I-L curves) for different SA bias voltages (V sa). For V sa=+2 V (forward bias), the threshold current is found to be as low as 16 mA. The threshold current increases and the optical output power decreases with an increase in the reverse bias voltage V sa. The I-L curves exhibit no bi-stability or hysteresis. It is well known that this feature is important in preventing sudden interruptions in the laser oscillations . The gain current and SA reverse bias voltages must be tuned to achieve stable mode-locking operation for the optical pulse train generation.
Figure 3(a) shows the optical spectrum of the hybrid MLL. The current supplied to the gain section is 33.8 mA with an RF power of 16.3 dBm at 1 GHz. The voltage applied to the SA section is -1.53 V. The center wavelength is 1032 nm. This emission wavelength can be selected by tuning the optical filter in the external cavity. It is confirmed that a 40-nm-wide wavelength tuning range between 1015 nm and 1055 nm is available with controlling the optical filter shown in Fig. 1(b). Pulses that were a few tens of picoseconds wide (typically, approx. 30 ps) are observed from this MLL using autocorrelation detection. This pulse width is considered to be influenced with the reflectance of the optical AR coat on the edge of the laser chip. Additionally, sub-pulse generation and self-pulsation are observed when the reflectance of the AR coat is increased by a few percent. Figure 3(b) shows a pulse train obtained from the hybrid MLL. A 1.0-GHz optical pulse train can be clearly observed by using a digital communication analyzer and a high-speed photodetector. It is confirmed that stable pulse train generation can be achieved with a complete setup, including optical feedback to the laser chip from the external mirror.
Figure 4 shows the typical RF signal spectrum of the optical pulse train obtained from a passive and hybrid MLL. In case of hybrid mode-locking by incorporating RF modulation at the gain section, the RF spectrum is sharpened as shown in Fig. 4. By using a hybrid mode-locking technique, it is observed that the timing jitter of the optical pulse from the MLL is drastically suppressed. Additionally, we also confirmed that higher-order RF signals were produced by the passive and hybrid MLLs. This confirmed that we had successfully developed a stable hybrid MLL optical source with attractive characteristics such as 1-µm-waveband operation, low timing jitter, and 1-GHz pulsation.
Optical communication at or above 10 Gbps in the 1.55-µm waveband has received considerable scientific attention. Therefore, it is considered that higher-frequency (>few GHz) optical pulse generation in the 1-µm waveband may be necessary in order to realize a useful optical communication network in this waveband. Harmonic mode-locking is considered to be a useful technique for the generation of high-frequency pulses such as high-speed RZ optical signals. An external-cavity harmonically MLL can be structured to act as a 1-µm waveband light source. The setup of a harmonically MLL is similar to that shown in Fig. 1. The fundamental laser cavity frequency is set at 1.244 GHz under passively mode-locking conditions. A 9.953-GHz RF input corresponding to the eighth harmonic of the fundamental frequency is applied to the gain section for harmonic mode-locking operation. Figure 5 shows the optical spectrum of the harmonically MLL. The current supplied to the gain section is 30.0 mA with an RF power of 25.0 dBm. The voltage applied to the SA section is -0.2 V. The center wavelength is 1035.4 nm and the spectral width is 0.05 nm. The 9.953-GHz-frequency mode is clearly observed in the optical spectra. The center wavelength can be tuned by using the optical filter. Therefore, the fabricated harmonically MLL possesses the characteristic of wavelength tunability (1015–1055 nm). An RZ pulse train with a repetition rate of 9.953 GHz and pulse width of 22 ps is observed. Other lower harmonic modes and the fundamental mode are suppressed by over 40 dB. Figure 6(a) shows the RF spectrum of an optical output from the harmonically MLL. The eighth harmonic frequency (9.953 GHz) and a higher-order mode (20 GHz in this case) are clearly observed. Figure 6(b) shows a single sideband (SSB) of the eighth harmonic peak. By using the harmonic mode-locking, the timing jitter of the optical pulse train is significantly suppressed down to approximately 120 fs from 100 Hz to 100 MHz. These results indicate that we have developed a stable and high-speed RZ optical signal source for a 10-Gbps (OC-192/STM-64) photonic transport system in the 1-µm waveband.
3. 10-Gbps photonic transmission in the 1-µm waveband
We have demonstrated that a 10-GHz RZ optical signal source in the 1-µm waveband can be obtained by using the proposed harmonically MLL. Additionally, an optical fiber transmission line must also be developed to complete a 1-µm photonic transport system. We focus on the development of a HAF, which possesses characteristics such as high flexibility, low fabrication cost, and highly controllable optical properties . The HAF is structured with cladding air-holes near the silica core, and its structure resembles that of a holy fiber (HF). Optical properties such as the cut-off frequency and dispersion were controlled by adjusting the size of the hole and the distance from the core. Thus, a long-distance and single-mode HAF was newly developed for 1-µm waveband transmission.
Figure 7 shows the experimental setup used for 1-µm-waveband, 10-Gbps transmission. The external-cavity harmonically MLL was used as the RZ signal source. The RZ pulse train was amplified by using a YDFA after 9.953-Gbps modulation. A pseudorandom binary bit sequence (PRBS) with a pattern length of 215-1 was transmitted for bit error rate (BER) detection. A 2-nm-width optical filter was used to reduce the amplitude spontaneous emission (ASE) noise from the YDFA. The fabricated HAF was used as the 1-µm-waveband single-mode transmission line. The zero-dispersion wavelength and dispersion value at 1030 nm were considered to be 1091 nm and -12.9 ps/nm/km, respectively. The input power to the transmission line was 10 dBm and the transmission loss was 1.9 dB/km. In the experiment, we used fibers of lengths 4.0 km and 6.9 km. The transmitted optical pulse was amplified again by a YDFA before the detection of the optical pulse by an opto-electric converter. Subsequently, an electrical clock data recovery (CDR) procedure was carried out. In Fig. 8, clear eye-openings are successfully observed before and after the 6.9-km transmission and also after the 6.9-km transmission with CDR. Figure 8(d) shows the measured BERs for the different transmission distances: error-free (<10-9) photonic transmission of the 10-Gbps RZ signal in the 1-µm waveband was successfully achieved over the 6.9 km.
Photonic transport in the 1-µm waveband was demonstrated using the proposed system, thus making the 1-µm waveband available as a new photonic waveband for telecommunication applications. We developed an external-cavity hybrid and harmonically mode-locked semiconductor laser (MLL) that can produce a RZ optical signal for multi-Gbps data transmission in the 1-µm waveband. A stable pulse train with a repetition rate of 9.953 GHz, pulse width of 22 ps, and low timing jitter of 120 fs was obtained from the 1035-nm harmonically MLL RZ optical signal source. Additionally, a hole-assisted fiber (HAF) was developed and used as a flexible single-mode transmission line for the 1-µm waveband. We successfully demonstrated error-free transmission of a 10-Gbps RZ optical signal over a long distance of 6.9 km using the developed stable harmonically MLL and HAF. From these results, it is expected that a high-functionality 1-µm waveband photonic transport system with characteristics such as a high speed, long distance, and simplicity will be realized by using useful key devices such as semiconductor-based simple RZ optical signal sources, flexible single-mode optical fibers, and highly efficient YDFAs.
The authors express their appreciation to the Japan Science and Technology Agency for the grant for manufacturing the semiconductor laser. We are deeply grateful to Dr. K. Mukasa and Dr. T. Yagi at FURUKAWA ELECTRIC CO. for producing the novel optical fiber. We would like to thank Dr. T. Kawanishi, Dr. I. Hosako, Dr. T. Itabe, and Dr. Y. Matsushima at NICT for their encouragement. We also thank the entire staff of the Photonic Device Laboratory (PDL) at NICT.
References and links
1. A. H. Gnauck, G. Charlet, P. Tran, P. Winzer, C. Doerr, J. Centanni, E. Burrows, T. Kawanishi, T. Sakamoto, and K. Higuma, “25.6-Tb/s C+L-Band Transmission of Polarization-Multiplexed RZ-DQPSK Signals,” in Proc. of OFC2007, Anaheim, CA, Postdeadline paper PDP19.
2. H. Masuda, A. Sano, T. Kobayashi, E. Yoshida, Y. Miyamoto, Y. Hibino, K. Hagimoto, T. Yamada, T. Furuta, and H. Fukuyama, “20.4-Tb/s (204×111 Gb/s) Transmission over 240 km Using Bandwidth-Maximized Hybrid Raman/EDFAs,” in Proc. of OFC2007, Anaheim, CA, Postdeadline paper PDP20.
3. R. Paschotta, J. Nilsson, A. C. Tropper, and D. C. Hanna, “Ytterbiumdoped fiber amplifiers,” IEEE J. Quantum Electron. 33, 1049–1056 (1997). [CrossRef]
4. Y. Tanguy, J. Muszalski, J. Houlihan, G. Huyet, E. J. Pearce, P. M. Smowton, and M. Hopkinson, “Mode formation in broad area quantum dot lasers at 1060 nm,” Opt. Comm. 235, 387–393 (2004). [CrossRef]
5. N. Yamamoto, K. Akahane, S. Gozu, and N. Ohtani, “Over 1.3 µm continuous-wave laser emission from InGaSb quantum-dot laser diode fabricated on GaAs substrates,” Appl. Phys. Lett. 86, 203118 (2005). [CrossRef]
6. N. Yamamoto, H. Sotobayashi, K. Akahane, and M. Tsuchiya, “Quantum-dot Fabry-Perot laser-diode with a 4-THz injection-seeding bandwidth for 1-µm optical-waveband WDM systems,” in Proc. of ISLC2008, Sorrento, Italy, P20.
7. P. M. Varangis, H. Li, G.T. Liu, T. C. Newell, A. Stintz, B. Fuchs, K. J. Malloy, and L. F. Lester, “Low-threshold quantum dot lasers with 201 nm tuning range,” Electron. Lett. 36, 1544–1545 (2000). [CrossRef]
8. K. Otsubo, N. Hatori, M. Ishida, S. Okumura, T. Akiyama, Y. Nakata, H. Ebe, M. Sugawara, and Y. Arakawa, “Temperature-Insensitive Eye-Opening under 10-Gb/s Modulation of 1.3-µm P-Doped Quantum-Dot Lasers without Current Adjustments,” Jpn. J. Appl. Phys. 43, L1124 (2004). [CrossRef]
9. A. Onomura, M. Arai, T. Kondo, A. Matsutani, T. Miyamoto, and F. Koyama, “Densely Integrated Multiple-Wavelength Vertical-Cavity Surface-Emitting Laser Array,” Jpn. J. Appl. Phys. 42, L529–L531 (2003). [CrossRef]
10. H. Hasegawa, Y. Oikawa, M. Yoshida, T. Hirooka, and M. Nakazawa, “10Gb/s transmission over 5 km at 850nm using single-mode photonic crystal fiber, single-mode VCSEL, and Si-APD,” IEICE Electron. Express 3, 109–114 (2006). [CrossRef]
11. K. Yashiki, N. Suzuki, K. Fukatsu, T. Anan, H. Hatakeyama, and M. Tsuji, “1.1-µm-range tunnel junction VCSELs with 27-GHz relaxation oscillation frequency,” in Proc. of OFC2007, Anaheim, CA, OMK-1.
12. H. Yokoyama, A. Sato, H. -C. Guo, K. Sato, M. Mure, and H. Tsubokawa, “Nonlinear-microscopy optical-pulse sources based on mode-locked semiconductor lasers,” Opt. Express (in press). [PubMed]
13. K. Mukasa, R. Miyabe, K. Imamura, K. Aiso, R. Sugizaki, and T. Yagi, “Hole assisted fibers (HAFs) and holey fibers (HFs) for short-wavelength applications,” Proc. SPIE 6769, 67690J-1 (2007).
14. H. Kurita, I. Ogura, and H. Yokoyama, “Ultrafast All-Optical Signal Processing with Mode-Locked Semiconductor Lasers,” IEICE Trans. Electron. E81-C, 129–139 (1998).
15. H. Yokoyama, “Highly reliable mode-locked semiconductor lasers,” IEICE Trans. Electron. E85-C, 27–36 (2002).