Semiconductor Group 1 06.97
Application of Module for Bidirectional Optical Transmission
Bidirectional Transmission
In the communication networks the most transmission lines need bidirectional transmission.
In early telephon lines two wire solution has been used in order to avoid expensive four wire
lines. This progress is also possible on the field of optical transmission by using an optical
beam splitter in connection with transmitter and receiver. A simple dual-fibre bidirectional
communication link consists of two transmitters (each containing a laser), two optical fibres
and two receivers (each containing a photodetector). These optical-electronic components
usually come in four packages with individual connectors, or in transceivers with dual
connectors or pigtails. A simple single-fibre bidirectional link also requires two transmitters
with two lasers, and two photodetectors for two receivers. However it requires only one fibre
and only two transceiver assemblies, which significantly reduces materials and labour costs.
Realisation with discrete elements
A transceiver for bidirectional optical transmission can be realised with discrete elements
(Figure 1). In this case transmitter and receiver with pigtails are connected with a fiber-
coupler (coupling ports 2:1 or 2:2, coupling ratio wavelength independent or wave length
dependent for wavelength division multiplexing (WDM) - systems).
Figure 1
Realisation of bidirectional optical transmission with discrete elements
Realisation in a compact module
Siemens has realized this transceiver configuration in a compact module called a BIDI.
Figure 2 shows the principal function of the BIDI containing transmitter, receiver and the
beam splitter in one case.
Transmitter
Receiver
Coupler
2:1 or 2:2
3 dB wavelength independent
or wavelength division multiplexing
Application of Module for
Bidirectional Optical Transmission
Semiconductor Group 2
Figure 2 Principal function of BIDI
This module is especially suitable for separating the opposing signals at the ends of a link . It
replaces a discrete solution with a transmitter, receiver and coupler. Space savings, faster
installation and cost savings are the major customer benefits of the BIDI . In Table 1 the
advantages are summarized.
Table 1
Comparison of discrete elements with a compact transceiver module
Discrete Elements Compact Transceiver Module
Flexibility All pigtail components can be
used Pretested TO-cans with suitable
optical interface can be used
Near end optical
crosstalk attenuation very high limited by internal reflections in
the module, (with WDM
increasable by blocking filters)
Space consumption very high (fiber and splices) low
Fiber(s) to be coupled two one
Fiber outputs working
with one wavelength two possible with 2:2 coupler one
Cost Three components (transmitter +
receiver + coupler )+
Fiber(connector)
One module (low cost
approach)+ Fiber(connector)
Pin
Pout
Plaser
Ipd(Pin)
Ilaser
Imon
Application of Module for
Bidirectional Optical Transmission
Semiconductor Group 3
Design of Module
The basic devices are a laser diode and a photodiode, each in a TO package, plus the filter
in the beam path. A lens in the TO laser concentrates the light and enables it to be launched
in to the single-mode fiber of the module. In the same way the light from the fiber is focused
onto the small, light-sensitive area of the photodiode to produce a high photo current. The
mirror for coupling out the received signal is arranged in the beam so that the transmitter and
receiver are at right angles to each other. This means the greatest possible degree of
freedom in the layout of the electric circuit.
Figure 3
Compact realisation of the transceiver in one module
A decisive advantage of the module is its use of standard TO components. These devices,
produced in large quantities, are hermetically sealed and undergo thorough testing before
they are built in. This makes a very substantial contribution to the excellent reliability of the
module. The solid metal package of the module serves the same purpose. It allows the use
of modern laser welding techniques for reliable fixing of the different elements and the fiber
holder. In other words the entire configuration is highly stable.
With this bidirectional module the user profits from SIEMENS’ many years of engagement in
optical transmission technology. The first prototypes of a module for bidirectional optical
transmission were produced back in 1985 and were used successfully in pilot projects. After
investigating all the variations, it was decided to implement the beam separation in the
module in the form of a general-diffuse optical system.
For applications with different wavelengths the mirror is wavelength-dependent (WDM), but
for the same wavelength it is semitransparent (beam splitting). In both cases the mirror
allows the transmitted signal to pass and couples out the received signal by reflection.
However the big breakthrough in this technology came with a new development that works
using the same principle but allows cost-attractive fabrication through the use of standard
components.
Application of Module for
Bidirectional Optical Transmission
Semiconductor Group 4
Performance
Transmitter
At the transmitter end both 1300-nm and 1550-nm Fabry Perot laser diodes are available.
Both laser diodes deliver on the chip optical power up to 5 mW at temperatures up to 85 °C.
The usable transmitter bandwidth is more determined by the external circuitry than by the
GHz-speed of the chip.
The high power modules are especially designed for passive optical networks (PON). The
laser diodes couple up to 1 mW peak power into a single-mode fiber. A high splitting factor of
up to 32 is possible.
For point to point applications, where this high power is not needed, low power versions are
available.
With a redesign of the laserchip (SL-MQW-structure) it was possible to improve the
temperature behaviour of the lasers. So, fully complementary modules for both wavelength
can offered.
Receiver
The detector diodes exhibit sensitivities of at least 0.8 A/W and, through their low
capacitance and large bandwidth, can be used up to more than 1 Gbit/s.
Besides the PIN Photodiode Ge-Avalanche photodiodes can also be used in order to get
higher sensitivities.
To get a complete receiver the photodiode has to be combined with a transimpedance
amplifier (TIA).
New receivers with an integrated preamplifier chip near the photodiode in the TO-can are
under development. This PIN-TIA will be available in the BIDI and are described in another
application note.
Beam Splitter (3 dB splitter and low loss WDM-splitter)
In the one wavelength module, a 3 dB beam splitter is used for separating the transmitted
and received optical signals.
By design the maximum launchable power and the responsivity are reduced by the beam
splitter.
For 1300/1550 nm wavelength selection in WDM a dielectric optical filter is used. Figure 4
shows the wavelength allocation. By reason of the great difference between the used
wavelengths there are no problems with tolerances of the filters and the transmitters. The
filter can always used in the low loss transmission or reflection mode.
Application of Module for
Bidirectional Optical Transmission
Semiconductor Group 5
Figure 4
Wavelength allocation for two channels in 2nd and 3nd window
Systems using both windows (WDM 1300/1550 nm) are very robust concerning the
wavelength tolerances. 100 nm for each laser and 120 nm for the filter allows the design of
the modules as low cost components.
If the selectivity of the filter is not high enough, the PIN-photodiode can be provided with an
additional blocking filter in order to improve the crosstalk attenuation.
With these basic devices it is possible to design suitable bidirectional modules for different
applications.
Crosstalk (optical and electrical)
Crosstalk paths
Crosstalk is a very important parameter in specifying transceivers for bidirectional
transmission. The optical crosstalk chiefly takes two different paths. On the one hand, light
from the transmitter may reach the receiver directly due to scatter inside the module. (The
case is optimised to avoid internal reflections.) On the other hand light can be reflected from
joints in the connected transmission route (connectors or splices) and be coupled into the
receiver.
Electrical crosstalk has to be considered as well.
Necessary crosstalk attenuation
In the module a received optical power in the order of 100 nW contrasts a transmitted power
level about 10000 times higher (several mW). The difference is even greater, if the driving
current for the laser is compared with the minimum photocurrent. A ratio of about 120 dB can
be reached.
For calculating the needed crosstalk attenuation it is important to specify suitable reference
points. Equal values for incoming and outgoing optical power on the pigtail (Pin and Pout) and
1300 1400
OH-
peak 1390 nm
Fiber cut-off
Optical filter
Laser 1
1260 - 1360 nm Laser 2
1480 - 1580 nm
1500 nm
Application of Module for
Bidirectional Optical Transmission
Semiconductor Group 6
their corresponding photo currents in the receiver should be used for the specification of the
crosstalk attenuation:
Figure 5
Reference points for crosstalk definition
Nearly undisturbed transmission of digital signals is possible with a signal-to-noise ratio of at
least 10 dB. Depending on the attained receiver sensitivity, the necessary crosstalk
attenuation can thus be calculated. The photo current caused by crosstalk should become
ten times smaller than the minimum received photo current.
For the modules working with WDM the optical crosstalk is specified with 50 dB. Therefore a
dynamic range (output power / minimum received power) of 40 dB is possible.
On principle, modules working with a beam splitter at one wavelength have smaller crosstalk
attenuation. Typical values are in order of magnitude of 30 dB. Depending on the system
design the crosstalk attenuation can be improved by electrical means.
Difficulties can also encountered with regard to electrical crosstalk especially if a photodiode
without a preamp inside the TO-can is used. This problem can only be solved with careful
design of the external wiring of the module.
Crosstalk attenuation: 10 log Ipd(Pin) / Ipd(Pout) [dB]
P
in
Pout
Plaser
Ipd(Pin)
Ilaser
Imon
Ipd(Pout)
Application of Module for
Bidirectional Optical Transmission
Semiconductor Group 7
BIDI in different applications
The following table shows some BIDI types currently in production.
Table 2
Types of BIDIs available now
Output Input Power
Budget
Type SFH Tx wave-
length
[nm]
Power
(peak)
[dBm]
Rx wave-
length
[nm]
Respon-
sivity
[A/W]
Blocking
[nm] Cross-
talk [dB] Sensitivity
-32 dBm
[dB]
SBM51414x 1310 0 1550 0,7 1310 50 32
SBL51414x 1310 −71550 0,7 1310 50 25
SBL51214x 1310 −71310 0,4 30 20 *)
SBM81314x 1550 0 1310 0,7 1550 50 32
SBL81314x 1550 −71310 0,7 1550 50 25
*) theoretically, in practice limited by optical crosstalk
One wavelength BIDI with beam splitter
Near end optical crosstalk attenuation and the optical dynamic range are limited by internal
reflections in the module. Since the optical crosstalk attenuation in the module is 30 dB
without additional external reflections in system, working with two SBL51214x, a power
budget of at least 20 dB is possible. The crosstalk attenuation can be improved by electrical
means like TDM or FDM (time or frequency division multiplex).
WDM using the two windows
The power budget depends on the realised receiver sensitivity. This is described in the
receiver application note 3.
The modules SBL51414X and SBL81314X are designed for point to point connections. With
−32 dBm sensitivity the power budget is 25 dB.
SBM51414X and SBM81314X enables with −32 dBm sensitivity a power budget of 32 dB
suitable for a point to multipoint network with a high splitting ratio.
When the 1550 nm laser is used, fiber length and bandwidth are limited by material
dispersion of the fiber and the optical spectrum of the Fabry Perot laser.
