A 1.3-μm, 4 × 25-Gbit/s, EADFB laser array module with large output power and low driving voltage is developed for 100GbE. A novel rear grating DFB laser is introduced to increase the output power of the laser while keeping the single mode lasing, which is desirable for a monolithic integration. Also, InGaAlAs-based electroabsorption modulators make very-low-driving-voltage operation possible due to their steep extinction curves. With the module, very clear 25-Gbit/s eye openings are obtained for four wavelengths with the driving voltage of only 0.5 V while securing the dynamic extinction ratio required by the system. These results indicate that the presented module is a promising candidate for energy-efficient future 100GbE transmitter.
© 2011 Optical Society of America
The abrupt increase in the demand for huge data capacity for data communication systems requires higher and higher bit rates for local area networks (LANs). 100Gbit Ethernet (100GbE) was just standardized in 2010  to cope with the demand, and related optical technologies have been intensively studied. On the transmitter side, four 25-Gbit/s (25G) light sources are required with their optical multiplexer (MUX) in the 1.3-μm band for single-mode-fiber (SMF) based 100GbE (100GBASE-LR4 and –ER4), and a lot of work has been done on 25G light sources in discrete form for the first generation, the so-called centum form-factor pluggable (CFP) transceiver. Among them, electroabsorption modulators (EAM) integrated with DFB lasers (EADFB lasers) are promising candidates due to their clear waveform and large extinction ratio [2–6]. One of the problems with the CFP transceiver is its size, since all the components [four 25G light sources, the MUX, and others] are placed separately.
For future 100GbE transceivers, size reduction is indispensable. A monolithically integrated transmitter chip, in which four 25G EADFB lasers and a 4 × 1 multi-mode interference (MMI) optical MUX are integrated on one chip, has recently been developed . However, the module output power is not large enough, because a normal λ/4 wavelength shift structure was used for the DFB lasers and the 4 × 1 MMI MUX has 6-dB intrinsic coupling loss.
Another issue for future 100GbE transceivers is power consumption. Since the package will be smaller than that for the CFP transceiver, the power consumption has to be reduced while ensuring that the performance of the module is maintained. Currently, the power consumed by 25G electrical laser/modulator drivers is relatively large. When a EADFB laser is used as the light source, the power consumption of the drivers is partly determined by the voltage swing required to operate the EAMs. So far, the reported driving voltages of 25G EAMs are over 2 V [2–7]. By lowering these values, there is a possibility of a dramatic reduction of the driver power consumption, or even driverless operation may be possible.
In this work, we developed a 1.3-μm, 4 × 25G EADFB laser array module for the next-generation of 100GbE transceiver. A novel rear grating structure increases the output power while keeping the single mode lasing. By placing the active region without a grating at the front facet side and increasing the ratio in the cavity, the output power from the front facet is increased due to the amplification of the optical field in the cavity. Further, a newly designed EAM structure reduces the driving voltage while securing the dynamic extinction ratio (DER) required by the system. With the device, ultralow-driving-voltage operation (0.5 V) of the EADFB laser array is achieved for the first time and clear eye-openings are obtained after 10-km single-mode fiber (SMF) transmission.
2. Chip design
Figure 1 shows a micrograph of our fabricated EADFB laser array chip. The layout, waveguide structures, and the fabrication process are basically the same as in  and . A shallow-ridge waveguide buried with benzocyclobutene (BCB) is used for the EADFB laser section to obtain a large E/O bandwidth, while a deep-ridge waveguide is used for the MUX region due to its strong confinement of light. Metal-organic vapour-phase epitaxy (MOVPE) was used for crystal growth and a butt-joint technique was used to connect the DFB lasers, EAMs, monitor photodiodes (PDs), and MUX region. The layer structure of PDs is the same as that of DFB lasers. The front and rear facets of the chip were coated with AR films. The length of the EAMs and monitor PDs is 150 μm. The width and length of the MMI are 20 and 250 μm, respectively. The measured loss of the entire passive region, including the shallow- and deep-ridge waveguide interface, is around 8 dB.
Since 100GbE standards specify a minimum limit of average output power, Pave, for the transmitter and since future 100GbE transceivers will be very small (allowable electrical power consumption is small), increasing the optical output power of the transmitter is critically important to satisfy the specifications and reduce the electrical power consumption. Conventionally, to increase the output power of a DFB laser, a uniform grating and anti- and high-reflective (AR and HR) coatings for the front and rear facets are employed . However, the AR/HR laser suffers from multi-mode lasing. Because the facet phase of a DFB laser is not controllable, there is always the probability of multi-mode lasing. For the monolithically integrated light source for 100GbE, this is not acceptable, since four lasers are integrated on one chip. If the lasing state of one of the lasers is multi-mode, the chip cannot be used and the yield becomes significantly low. It is well known that λ/4-wavelength-shift DFB lasers with AR/AR coatings [Fig. 2 , left] are useful for stabilizing the lasing mode, and they were employed in  and . However, the output power is relatively low because the optical field is strongly confined in the cavity and the output powers emitted from the front and rear facets are similar. Here, to increase the output power of the DFB laser while keeping the single mode lasing, we introduce a novel rear grating structure as shown in right part of Fig. 2. The structure is composed of two sections: one is a conventional λ/4 wavelength shifted grating placed at the rear facet side and the other is just an active region without a grating placed at the front facet side. Both facets are coated with AR films. By placing the active region in front of the conventional phase-shifted DFB laser, the lasing mode is amplified in the active region and the longitudinal optical distribution becomes asymmetric, resulting in large output power from the front facet.
Figure 3 shows the chip output power of one of the DFB lasers included in EADFB laser array measured at room temperature. The lasing wavelength is 1.305 μm. The length of the DFB laser, L, is 500 μm. The results for a conventional DFB laser (blue) and rear grating lasers with Lactive:Lgrating = 1:4 (green) and Lactive:Lgrating = 2:3 (red) are shown. Here, Lgrating and Lactive are the lengths of the grating and active regions. For larger values of Lactive, although the threshold current is increased, the large increase in differential quantum efficiency makes the output power very large. At the injection current of 100 mA, the output power of the rear grating laser with Lactive:Lgrating = 2:3 is 1.6 times larger than that of the conventional laser.
As described in Introduction, the power consumption of transmitter part is partly determined by the electrical driver. To reduce it, it is effective to reduce the driving voltage of the EAMs. Here, we used InGaAlAs-based, tensile-strained quantum wells (QWs) to obtain a large extinction ratio (ER) and steep extinction curves [2,6–10]. Also, we made the total thickness of insulator region small to effectively bias the QW region with low voltage.
3. Module performance
The fabricated chip is packaged in a specially designed module . Figure 4 shows the module output power as a function of the injection current to the DFB lasers. All measurements for the module were done at 40 °C. From the shorter wavelength side, we label lanes 0 to 3 for each wavelength. The length of the DFB laser is 400 μm and the rear grating structure with Lactive:Lgrating = 1:3 was used. The output power at the injection current of 150 mA is over 2 mW for all the lanes. Figure 5 shows the lasing spectra of the rear grating structure at the injection current of 100 mA. Single-mode lasing with the side mode suppression ratio of over 50 dB is obtained for all lanes, showing the usefulness of the rear grating structure for DFB lasers used in monolithically integrated devices. Figure 6 shows the photocurrent of monitor PDs as a function of the injection current to the DFB lasers. Very uniform and linear curves are obtained, showing that the PDs, having the same layer structure with the DFB lasers, are useful as power monitors. Figure 7 shows the static extinction curves. The injection currents to the DFB lasers are 100 mA. Static ERs larger than 20 dB as well as steep extinction curves suitable for low-voltage operation are obtained for all lanes. Figure 8 shows the dynamic extinction ratio (DER) of the EAMs as a function of driving voltage (Vpp). The bias voltage to the modulator is set at a level where the cross point of the optical eye diagram becomes 50%. For all lanes, the DERs are larger than 4 dB, which is the minimum value for 10-km 100GbE (100GBASE-LR4) even with driving voltage of 0.5 V.
With the device, we performed a 10-km transmission experiment on SMF using 25.78125 Gbit/s, non-return to zero (NRZ), 231-1 pseudo-random bit stream (PRBS) signal. Figures 9 and 10 show the eye-diagrams for back-to-back (BTB) and after 10-km transmission with Vpp = 0.5, 1, and 2 V for all the lanes. The bias voltages to the modulator for Vpp = 0.5, 1, and 2 V are −1.08, −0.95, −0.93 V for lane0, −1.28, −1.14, and −1.06 V for lane1, −1.51, −1.43, and −1.26 V for lane2, and −1.74, −1.64, and −1.53 V for lane3. Corresponding DERs are 4.99, 8.11, and 10.78 dB for lane0, 5.05, 9.31, and 10.63 dB for lane1, 4.32, 6.97, and 9.79 dB for lane2, and 4.4, 6.92, and 9.88 dB for lane3. Clear eye openings are obtained for all driving voltages even after 10-km transmission.
We developed a 1.3-μm, 4 × 25G EADFB laser array module for the next-generation of 100GbE transceiver. A rear grating structure is newly introduced to increase the output power of a DFB laser while keeping the single mode lasing. Also, the EAM structure is designed to reduce the driving voltage, leading to the possibility of a dramatic reduction of the electrical driver power consumption, and, possibly, driverless operation. With the device, we achieved 25G, ultralow-driving-voltage operation (0.5 V) for the first time with clear eye openings after 10-km SMF transmission. These results show the applicability and usefulness of the device for future compact and low-power-consumption 100GbE transmitters.
References and links
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