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We demonstrate a compact 100 Gbit/s DP-QPSK receiver module that is only 18 mm (W) x 16 mm (D) x 2.8 mm (H). The module size is reduced by using a ball grid array (BGA) package with three-dimensional assembly technology and by applying a heterogeneous integrated PLC. Error-free DP-QPSK signal demodulation is successfully demonstrated.
© 2014 Optical Society of America
1. Introduction
The rapid increase in data traffic has led to an urgent need for a high-capacity photonic network. To increase the signal bit rate in the network, advanced modulation formats are commonly used such as dual-polarization quadrature phase shift keying (DP-QPSK) and quadrature amplitude modulation (QAM) with a digital signal processor (DSP). Such advanced modulation formats require complicated transmitters and receivers, and they tend to become large. However, it is considered important to reduce the size and power consumption of network equipment and components.
Several studies have described the downsizing of coherent receivers [1–3] including Si photonics receivers, InP based monolithic receivers, and heterogeneously photo-diode integrated silica-based PLC receivers. Among these lightwave circuit technologies, silica-based PLCs for a coherent demodulator perform relatively well over the C- and L-bands. They provide 1) a low propagation loss of less than 0.05 dB/cm, 2) a polarization extinction ratio of above 20 dB, and 3) small phase-difference deviations of the I and Q channels, which are positioned at an orthogonal angle of less than 3 degrees [4].
At ECOC 2012, we reported a one-chip DP-QPSK PLC demodulator [5]. The photodiodes (PDs) are integrated on the top surface of the PLC edge where micro-mirrors are monolithically employed with heterogeneous technology.
In this paper, we propose a 3-dimensionally integrated 100G DP-QPSK compact receiver module integrated with our one-chip PLC demodulator. Two transimpedance amplifiers (TIAs) and eight DC block capacitors each connected to equal-length RF transmission lines are installed in the receiver module. This technique enables the size of the receiver module to be greatly reduced without any deterioration in performance.
2. Concept model of compact optical coherent receiver module
Figure 1 shows our newly developed 100G DP-QPSK optical coherent receiver module. As shown in this figure, we introduce a resin-based ball grid array (BGA) package.
A conventional receiver module uses a package compatible with surface mounting technology (SMT) as shown in Fig. 2(a). The SMT package has many finely shaped RF and DC electrical lead pins for soldering. It also has four screw holes for securing the package to a printed circuit board (PCB). Thus, the assembly technique used on the PCB of a conventional receiver module inevitably requires some redundant space. This space means that the RF transmission lines connecting a DSP and a receiver module must be relatively long. In general, a shorter RF transmission line interconnecting different RF ICs is highly desirable for maintaining optimum performance in terms of signal integrity.
On the other hand, the DSP has small solder balls, which are used for fixing the BGA package in place as well as for connecting to the RF transmission lines on the PCB. Our new coherent receiver concept is the same as that behind the BGA package of the DSP. The new BGA package means that our module can be placed adjacent to a DSP without requiring extended RF transmission lines. This new design enables us to shorten the transmission lines and obtain good signal integrity as shown in Fig. 2(b).
Figure 3 shows the basic concept for a compact optical module. All the components, namely indium phosphide transimpedance amplifiers, a PLC, DC-block capacitors, and equal-length RF transmission lines, are embedded in a small BGA package by using three-dimensional assembly technology. On the bottom of the BGA package, there are many solder balls for forming an RF interface as found on a DSP package. Thus, this module can be placed adjacent to a DSP without requiring extended RF transmission lines. With our concept module we can easily achieve size reduction and signal integrity improvement simultaneously.
To establish a compact optical module, all the components are directly mounted on or embedded in a BGA made of resin-based material. Generally, the temperature expansion coefficient of resin is larger than that of semiconductors. Thus, the direct mounting of semiconductor devices has been considered difficult. Figure 4 shows the temperature expansion coefficient characteristics of various printed circuit board materials made from resins. We selected a low CTE resin material, which can be used for various applications such as server CPU packages and RF-LSI packages. This low CTE resin material is suitable for the 3D assembly of BGA packages of various components made from indium phosphide, silicon germanium, silicon, or silicon dioxide.
However, the resin used as a material for the BGA package poses a problem if the BGA package is utilized as an optical base plate. This material has natural plasticity. Thus, optical devices such as lenses and PDs cannot be precisely aligned on the resin-based BGA package. To overcome this problem, we introduce a silica-based PLC with heterogeneously integrated PDs. This PLC enables optical coupling between waveguides and PDs without any optical alignment or lenses.
The eight PDs are integrated on the edge of the top surface of the PLC. And each individual PD is electrically wire bonded to TIAs, which are placed on the top surface of the BGA package.
Amplified electrical signals from the TIAs are transmitted to equal-length RF transmission lines and DC block capacitors. These RF components are embedded in the BGA package three-dimensionally. Then the electrical signals are finally output at solder balls, which are placed on the bottom surface of the BGA package. The receiver module is successfully downsized. As shown in Fig. 5, the total size of the receiver is 18 mm (W) x 16 mm (D) x 2.8 mm (H), which is the smallest coherent receiver module yet reported.
Figure 6(a) and 6(b), respectively, show the measured transmission and reflection coefficients of the BGA package soldered on an evaluation board. This evaluation board has eight co-planar transmission lines. The transmission lines embedded in the BGA package and the co-planar transmission lines on the evaluation board are both about 20 mm long. The measured insertion and return losses are about 4.6 and 15 dB, respectively, at a frequency of around 35 GHz. These two graphs show that the BGA package performs sufficiently well for actual 100G DP-QPSK receiver applications.
3. Characteristics of receiver module
Figure 7(a) and 7(b) show the small signal O/E response and return loss of the receiver, respectively. From these two graphs, we can confirm that the 3-dB bandwidth is about 24 GHz and that the return loss exceeds 10 dB up to 25 GHz. These features are also suitable for 100G DP-QPSK receivers.
Figure 8 shows measurement systems for evaluating Q values and constellations. We evaluate the DP-QPSK receiver performance with the BGA package as regards the optical signal noise ratio (OSNR) dependence of the Q value under the following conditions: 1) the input optical signal is modulated with a pseudorandom binary sequence (PRBS) of 215-1. 2) The signal bit rate generated from a pulse pattern generator (PPG) is 128 Gbps (32GB x 4ch). 3) The local oscillator power and signal power are set at 15.6 and 5 dBm, respectively.
Figure 9 shows the OSNR dependence of the Q value. The measured Q values exceed the Q-limit of 6.4 dB at an OSNR of 16 dB or more, which means error-free operation is achieved after the EFC [6]. Figure 10 shows the constellations observed after digital signal processing.
5. Conclusion
We propose a compact optical coherent receiver module. By introducing a BGA package and a heterogeneous integrated PLC into a 100 Gbps DP-QPSK receiver, the size is reduced to 18 mm(W) x 16 mm (D) x 2.8 mm (H) without any degradation in receiver performance. Error-free 100G DP-QPSK signal demodulation is successfully demonstrated.
Acknowledgments
We thank all the members of NTT Laboratories, especially S. Tsunashima, S. Aozasa, Y. Hashizume, T. Hashimoto, H. Nosaka, T. Itoh, and M. Yoneyama, for their collaboration.
References and links
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