1. Introduction

With the rapid growth of data communication services, such as data storage service and cloud computing, there is an urgent and strong demand for more network capacity. These so-called “datacom” services cover distances ranging from a few meters to some tens of kilometers. For relatively long-distance transmission, optical data links are promising and have been intensively studied. For example, in 100-Gbit (100G) Ethernet (100GbE) [1] standardized in 2010, two standards, LR4 and ER4, are defined for transmission distances larger than 100 m. However, since it was difficult to achieve 100-Gbit/s by cost-effective on-off keying format, 25G × 4, a multi-lane LAN-WDM system was employed. For the transmitter side, this means that four 25G light sources and their optical multiplexer (MUX) are required and various light sources have been reported [2–6], including a compact 25G × 4 monolithically integrated light source [7–9].

For future data communication beyond 100G, such as 400G and 1T, the use of a multi-lane system as in the 100GbE to fulfill the system’s transmission capacity requirement is almost unavoidable. At the same time, increasing the data rate of serial lanes is also a very important for reducing the number of lanes. Since the volume of the transceiver is usually limited, a large number of lanes make it impossible to place the related devices in a limited area. Therefore, for beyond-100G optical communications, it is one of the important challenges to make a compact multi-lane transmitter, in which the data rate of each lane is over 25G.

In this paper, we developed the first ultracompact optical transmitter subassembly (TOSA) for long-distance, beyond-100G data communication systems. A small monolithically integrated transmitter chip, consisting of four high-speed electroabsorption modulators integrated with DFB lasers (EADFB lasers) and a 4 × 1 optical mulitiplexer based on multimode interference waveguide (MMI), and three-dimensional interconnection boards make the size of TOSA extremely small with the size of 1.82 cc. By using the module, we achieved 40-Gbit/s × 4 operation with clear eye openings in 10-km single-mode fibre (SMF) transmission. Electrical crosstalk under the multi-lane operation is also evaluated, and the error-free transmission is achieved with the power penalty to the discrete operation of 1 dB. These results show the first feasibility of beyond-100G optical transmitter for long-distance datacom.

2. Chip and module design

For datacom transmitters, directly modulated lasers (DMLs) and EADFB lasers have been intensively developed. DMLs are easy to fabricate and their cost is low and the operation of DMLs at the modulation speed of over 40-Gbit/s was reported at 1.3-μm range [10–12]. However, if we consider the fabrication of a compact multi-lane optical transmitter module for small transceivers, such as CFP4, it is not easy to assemble discrete light source chips and the optical multiplexer in the module with low cost. Therefore, monolithic integration of these components on one chip is almost essential. In that case, the monolithic integration of multi-lane high-speed DMLs is difficult because the usable wavelength range for high-speed modulation covered by single active layer is usually narrow. Here, we use EADFB lasers for the beyond-100G transmitter since the modulation speed, the extinction ratio, operating temperature flexibility, and the clearness of eye diagrams of discrete EADFB lasers are better than those of DMLs. In addition, a well-designed EADFB laser can cover a wide wavelength range (~20 nm) with a single layer structure.

An InGaAlAs-based tensile-strained quantum well (QW) is used for the EAM because it has large absorption characteristics and a small hole-pileup effect due to its large conduction and small valence band offset, which is favorable for obtaining a large extinction ratio and faster modulation response [13]. Shallow ridge waveguides buried in low-index benzocyclobutene (BCB) are used in the EADFB laser section to obtain large E/O bandwidth. Since the dielectric constant of BCB (around 2.5) is significantly lower than that of semiconductor (around 10), the parasitic capacitance under the bonding pad for EAM can be reduced. Therefore, the ridge waveguide structure has the superiority in terms of E/O bandwidth than buried structure.

Figure 1(a) shows a micrograph of the fabricated chip. Four EADFB lasers and monitor photodiodes (MPDs) and their MMI-MUX are monolithically integrated on one chip. The fabrication method is basically the same as in [7–9]. A butt-joint regrowth technique is utilized to connect core layers of DFB lasers, EA modulators (EAMs), and passive waveguides including MUX. The length of the DFB laser, EAM, and MPD are 400, 150, and 150 μm, respectively. The distance between two EAMs is 600 μm. The total chip width and length are 2.4 and 3.3 mm, respectively.

 

Fig. 1 (a) Micrograph of the fabricated chip, (b) schematic of three-dimensional circuit board, and (c) photograph of the fabricated TOSA.

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In our 4 times 25-Gbit/s EADFB laser array chip, there are a lot of electrodes (four RF lines for EAMs, and many DC lines for DFB lasers and MPDs). Since the size of TOSA is usually limited by the transceiver size, it is necessary to install multiple RF signal lines in limited volume. Also, to miniaturize the size of the module, it is almost essential to gather all the electrical contacts at the rear end of the module body. However, if all the electrical lines are placed on the same circuit board, the distance between RF signal lines becomes small, leading to large electrical crosstalk.

To reduce electrical crosstalk, there are two effective ways. One is to shorten the length of bonding wires between EAM and RF signal line. By reducing the length of bonding wire, the coupling length between multiple RF signals through bonding wires can be shortened and the electrical crosstalk can be suppressed. Also, reduced bonding wire length leads to increased E/O bandwidth. The other is to widen the distance between two RF signal lines. By separating two RF signal lines physically, the interaction between two RF signal becomes weak and the electrical crosstalk is small.

Here, we used three-dimensional (3-D) circuit board supported by thickness-adjusted spacer as shown in Fig. 1(b) to incorporate above two ways. The thickness-adjusted spacer enables us to minimize the vertical distance between the chip and the RF circuit board since we can adjust the thickness of spacer according to the thickness of the chip. The vertical distance can be shortened by 0.15 mm compared with that of previously reported bridge type RF circuit board, where the RF circuit board was supported by the package [8]. Also, one more important effect of using 3-D circuit board is that we can separate the circuit board of 4 RF lines and many DC lines as shown in Fig. 1(b). By separating them, one circuit board can be used for RF lines only, leading to wider spacing between two RF lines, and therefore, reduced electrical crosstalk. Here, the distance between two RF signal lines is 600 μm, which is limited by the width of TOSA.

Figure 1(c) is a photograph of the fabricated TOSA. Since all the electrical lines are gathered at the rear-end of the TOSA, we can use an FPC for all the electrical interface, reducing the volume of the TOSA significantly compared with that of the conventional butterfly module [8], and it is 8 mm × 35 mm × 6.5 mm.

3. Module performance

Figure 2 shows the module output power as a function of injection current. All the measurements in this work were done at room temperature. The lasing wavelengths of the four lasers are 1293, 1297, 1302, and 1307 nm. From the shorter wavelength side, we labeled the lanes 0 to 3 for each wavelength. For DFB lasers, we used a novel rear-grating structure [9] to increase their output power. In this laser, an active region without grating is placed in front of phase-shifted grating. The lasing mode is amplified in the active region and large output power is obtained. We put 100-μm active region in front of 300-μm grating region (Total length of laser is 400 μm). The phase shift is inserted in the center of the grating region. The threshold currents are 20 mA and very uniform. Around 100 mA, output powers larger than 1.5 mW are obtained under 8-dB MUX loss. Single-mode lasings are obtained for all four lasers and the side-mode-suppression ratios are over 50 dB. Figure 3 shows the static extinction ratio of the EAMs. The injection currents to the DFB lasers are 70 mA. Very smooth and steep extinction curves, which is specific for InGaAlAs quantum wells [14,15], are obtained for all four lanes, indicating a 14-nm wavelength range can be covered by a single EAM structure.

 

Fig. 2 TOSA output power as a function of injection current.

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Fig. 3 Static extinction ratios for four EA modulators.

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With the module, we performed a transmission experiment on SMF using 41.1ig. 4: 41.25Gnt on SMF using 25ion ratio of the EAMs. Very smooth and steep extinction curves are obtained for all four lanes. 25-Gbit/s, non-return to zero (NRZ), 2⁷-1 pseudo-random bit stream (PRBS) signal. The bias currents of the DFB lasers are 70 mA, and the driving voltage of the EAMs is 2.5 V_pp. The average output powers are −3.9, −3.3, −2.7, and −2.5 dBm and the dynamic extinction ratios are 8.1, 8.1, 7.8, and 7.4 dB for lane0, 1, 2, and 3, respectively. Figure 4 shows the 41.25-Gbit/s eye diagrams for back-to-back (BTB) and after 10- and 40-km transmission. Very clear eye openings are obtained for all four lanes up to 10-km transmission. For 40-km transmission, although the eyes become noisy, they are still open. These degradations at 40-km transmission are mainly comes from the loss of the SMF since 1.3-μm band is used, where the loss is larger than that for 1.55-μm band. Similar degradations can be seen at 25-Gbit/s operation [7]. It should be noted that the degradation of eye diagrams for lane0 (1293 nm) is larger than that of lane3 (1307 nm). This is because that the chromatic dispersion of SMF is negative for lane0 while it is almost zero for lane3.

 

Fig. 4 41.25-Gbit/s eye diagrams for BTB, after 10- and 40-km transmissions under discrete operation.

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For high-speed multi-lane transmitters, the effect of electrical crosstalk on optical eye diagrams is very important because the degradation of the diagrams becomes severe for high-frequency modulation. Therefore, we did a multi-lane operation of the module to investigate the effect of crosstalk. The first row of Fig. 5 shows the 41.25-Gbit/s eye diagram of lane0 for BTB. The figure in first column is the eye diagram when only the lane0 EADFB laser is operated. The other three diagrams in the first row show them when each of the other EADFB lasers is operated simultaneously with the lane0 laser. The eye diagrams are slightly degraded due to the electrical crosstalk, especially when two adjacent EADFB lasers are operated simultaneously [in this case, lane0 and 1; the placement of each laser is shown in Fig. 1(a)]. However, the eye diagrams are still clear and open, indicating that multi-lane 41.25-Gbit/s operation is possible. The second row of Fig. 5 shows the results for lane1. In this case, since the lane1 laser is sandwiched between the lane0 and 2 ones, the degradation is large when lane0 and 2 lasers are operated with the lane1 laser.

 

Fig. 5 41.25-Gbit/s, BTB eye diagrams when two EADFB lasers are simultaneously operated: 1st row, lane0, and 2nd row, lane1.

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Figures 6 show the bit error rate for four EADFB lasers for BTB configuration. In this measurement, due to our experimental set-up limitation, 41.25-Gbit/s signal was inputted to the measured lane, and 20.625-Gbit/s signals for other three lanes to see the effect of electrical crosstalk. These all four signals are decorrelated. For discrete operation (four DFB lasers and only one EA modulator are operated), error-free operation is achieved with the receiver sensitivity of about −7.5 and −6.5 dBm for discrete and multi-lane operations at the bit error rate of 10⁻¹². For multi-lane operation (four DFB lasers and four EA modulators are operated simultaneously), although the power penalty is about 1 dB, error-free operation is still possible for all four lanes. This power penalty is possibly reduced by widening the distance between two RF signal lines more (currently, 600 μm). However, it increases the volume of the module simultaneously. Therefore, there is a tradeoff between the electrical crosstalk and the size of the module. One possible way to reduce the electrical crosstalk is to use a flip chip bonding technique. Since the flip chip bonding does not need bonding wires, there is a possibility to reduce electrical crosstalk significantly.

 

Fig. 6 Bit error rate for BTB configuration for (a) lane0, (b) lane1, (c) lane2, and (d) lane3 for discrete and multi-lane operation.

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4. Conclusion

We developed the first TOSA for long-distance, beyond-100G data communications. The transmitter chip is based on an ultrasmall monolithically integrated light source, and this small chip makes it possible to realize a CFP4-class TOSA. Using the TOSA, we achieved 40-Gbit/s × 4 operation with the clear eye openings in up to 10-km SMF transmission. The effect of electrical crosstalk was also evaluated and it was found that the degradation of the eye diagrams is very small and the error-free operation was achieved under multi-lane operation. These results indicate that the TOSA developed here is promising for future large-capacity data links.