2-3 Optical receiver unit
The optical receiver unit consists of a receiver optical
subassembly (ROSA) and a limiting amplifier. Photo
3 shows a photograph of the newly developed ROSA.
A high-speed PIN photodiode and a low noise transimpedance
amplifier (TIA) are integrated on a TOHeader
and in a coaxial package, forming a module
called TO-stem. Not only is this coaxial module small in
size and low in cost, but it also has high sensitivity and
wide bandwidth due to its optimized design.
In order to obtain wide frequency bandwidth under
low bias condition, the authors adopted a mesa structure
PIN photodiode that has a light receiving area of
30 um in diameter. Typical frequency response of this
PIN photodiode is reported in Fig. 6, showing a bandwidth
of more than 10 GHz at 1 V bias voltage. The
plots in the figure also show that the bandwidth of the
ROSA is pretty stable at bias voltages higher than 1 V.
The TO-header of the ROSA is made from an law
cost iron based material that has acceptable thermal conductivity.
The structure of the TO-header is optimized to
obtain stable frequency response by minimizing parasitic
inductance caused by the bonding wires around the TIA.
Figure 7 shows small signal frequency response of
the ROSA. Flat frequency response and wide bandwidth
are obtained for both transimpedance and group delay.
Figure 8 shows receiver sensitivity of the ROSA.
Using a PRBS 231-1 pattern signal, receiver sensitivity of -
20 dBm was achieved at room temperature at BER = 10-12
and bit rate = 10.7 Gb/s. The plots in the figure also
exhibit a small temperature dependency, showing temperature
deviation of less than 1 dB in a wide temperature
range from -20 to 120 deg. C.
2-4 Mechanical configuration
Figure 9 shows one example of host board using the
X2 transceiver. Usually mechanical design of pluggable
transceivers needs to satisfy contradictory parameters
such as compact size and good heat radiation, and pluggable
structure and electro magnetic interference (EMI)
suppression. The above requirements are more critical
to the mechanical design of 10Gbps transceivers than that of lower bit rate transceivers. This section describes
suppression of EMI radiation from the X2 transceiver.
High frequency noise generated by the transceiver
circuit, TOSA and ROSA is radiated to outside through
a tiny gap in the housing of the X2 transceiver. In order
to prevent the pluggable structure of the transceiver
from providing a path for noise to radiate out, latches
and springs comprising a lock release structure must be
placed away from electrical circuit. An EMI gasket is
placed between the front panel of the host board and
the face plate of the X2 transceiver so that good electrical
contact is provided. In order to achieve good EMI
shield at frequency range up to 40 GHz, this gasket was
carefully chosen by its material properties, such as elasticity
and conductivity, and size. Actual EMI measurements
as well as comparison of EMI shield characteristics
of several candidates were done so as to determine
suitable gasket design.
High frequency radiation noise coming out of SC
receptacle is difficult to deal with when noise frequency
is over 10 GHz. Because the noise is coupled with metal
parts of SC connector, such as spring, it is easily radiated
to outside at high level. As a solution for obtaining both
good high-frequency performance of optical subassembly
(OSA) and good EMI performance, the authors
introduced a new structure to the OSA between the TOheader
portion and the SC sleeve portion, which electrically
isolates the two portions from each other. The SC
sleeve portion is connected rigidly to the X2 housing to
have frame ground potential. Figure 10 shows the struc-ture described above. As a result of introducing this
structure, electrical circuit is completely shielded by the
housing and good EMI performance is achieved.
Electric field intensity distributions of the SC receptacle
area measuned by using near magnetic field observation
probe are reported in Fig. 11. It shows the high intensity
area does not extend to the SC receptacle section when
new structure is introduced. |
