1. Introduction
In recent years, the amount of information transmitted
through communication networks is increasing
by leaps and bounds. Gigabit Ethernet is applied to the
edge of access networks as well as local area networks
(LANs). With the spread of Gigabit Ethernet, 10 Gigabit
Ethernet (10GbE) is beginning to be applied to the core
networks, such as LAN backbones, metropolitan area
networks (MANs) and wide area networks (WANs) to
satisfy the bandwidth demand and to realize the end-toend
seamless Ethernet services.
Optical transceiver is a key component of 10GbE
devices such as switches and routers. To economically
increase the throughput and port density of the network
equipments, it is preferable that 10GbE optical transceivers
are low cost, small size and have low power consumption.
In addition, it is also desirable that they are
easy to use in terms of installation, maintenance and
upgrade.
The X2 multi-source agreement (MSA) is specified
in order to meet such demands on optical transceivers.
The X2 MSA defines a small form-factor 10 Gb/s pluggable
optical transceiver that supports IEEE802.3ae
10GbE optical interface and 4-lane 3.125 Gb/s electrical
interface called 10 Gigabit Attachment Unit Interface
(XAUI).
The authors have developed an X2 optical transceiver,
SDX4101LR, for 10 km transmission over standard
single mode fiber. To realize small form factor and
low power consumption without compromising optical
performance, the authors have developed a new DFB
laser diode (DFB-LD), a new transmitter optical subassembly
(TOSA), a new PIN photodiode and a new
receiver optical sub-assembly (ROSA) as well as a new
10Gb/s laser driving circuitry and a new optical receiving
circuitry. The SDX4101LR transceiver has been
designed to show improved performance also in terms
of electromagnetic interference (EMI) so that radiation
emitted by electrical circuits carrying 10Gb/s signals is suppressed to the minimum. The rest of this paper
describes the design of the SDX4101LR X2 transceiver
and its internal sub-assembly in detail.
2. Transceiver design
2-1 X2 transceiver specifications and configuration
The principal specifications and the block diagram
of X2 transceiver the authors have developed are shown
in Table 1 and in Fig. 1, respectively. This transceiver
consists of four units; an optical transmitter unit, an
optical receiver unit, a controller unit and a LAN-PHY
unit that manages the physical layer (PHY) of a
10GBASE-R.
Electrical interface of this transceiver is a 3.125Gb/s
x 4-lane parallel interface called XAUI, and parallel signals
are converted to 10.3125Gbps serial signals at the
LAN-PHY unit. Because the signal speed of XAUI is less than 10Gb/s, there are some advantages that makes
developing host systems easy. The first advantage is that
the LSI of MAC-Layer with XAUI interface is easy to
design and the second advantage is that XAUI signals
can be traced over 50 cm even on the general-purpose
printed circuit board (PCB) materials such as FR-4.
A micro controller system shown in Fig. 2 has been
implemented in the transceiver. The controller unit
controls both bias current and modulation current of
the laser diode to maintain stable optical output power
and extinction ratio. The controller system also has a
function of digital optical monitoring (DOM), which
allows host systems to supervise the transceiver status,
such as optical output power and receive power, via
MDIO serialcommunications.
2-2 Optical transmitter unit
The optical transmitter consists of a laser driver IC
and a transmitter optical subassembly (TOSA) shown in
Photo 1 which includes a directly modulated DFB-LD.
The most important parameters when a semiconductor
laser is directly modulated at high speed are relaxation
oscillation frequency (fr) and cut-off frequency. The
authors developed a new DFB-LD having high fr and
low capacitance for 10Gb/s application.
The active layer of the DFB-LD is based on the InGaAsP strained multi quantum well (MQW) structure,
the high reliability of which has already been verified. In
order to suppress the decrease in the relaxation oscillation
frequency of the DFB-LD especially at high temperatures,
the design of the MQW structure was optimized.
Typical fr performance of the newly developed TOSA is
reported in Fig. 3, showing fr more than 10 GHz in a
wide temperature range from -20 to 75 deg. C.
Regarding improvement of cut-off frequency, it is
important to achieve low parasitic capacitance of the LD
and to minimize the frequency response degradation
caused by the package and assembly. A low dielectric
constant polymer layer is placed under the anode electrode
pad to reduce the parasitic capacitance of the LD,
as shown in Photo 2.
A low cost, small TO-header having 3.8 mm diameter
is applied to the newly developed TOSA. Usually
TOSA using TO-header does not have enough frequency
bandwidth for 10 GHz NRZ signal operation due to
parasitic inductance caused by the bonding wire
between LD and package.
The authors have reduced parasitic inductance by
more than 50% by applying new LD heat sink and minimizing
the length of bonding wire.
Owing to the low capacitance and low parasitic inductance
assembly of the DFB-LD, the cut-off frequency of the
TOSA has been improved. Figure 4 shows the small signal frequency response of the TOSA. Enough frequency
bandwidth is obtained for 10 GHz NRZ signal operation.
In order to reduce electrical reflection between LD
driver and LD chip, the authors implemented a back termination
resistor on the output stage of the LD driver
and a damping resistor into the TOSA. Moreover, low
supply voltage and low power dissipation is achieved in
the optical transmitter unit by using the AC coupling and
differential LD drive techniques. Figure 5 shows the simulated simulated
optical eye diagrams for when all of the methods
described above are applied to the optical transmitter.
The diagrams confirm the appropriateness of the design. |
