.webp "Fiber Mux & OADM")
The Passive Optical Networks - PON are having a great expansion these days with the increased demand of the business and consumer of the Ethernet bandwidth. The expansion of the network it is possible if the total cost of ownership is lower than the revenues generated by customers.
In such PON systems, burst-mode optical transceivers are essential components. They are constructed by integrating transceiver-circuit, optical device, and module technologies. The configuration of a typical optical transceiver is based on an optical sub-assembly (OSA). The transmitter and receiver modules are called TOSA and ROSA, respectively. A TOSA contains a semiconductor laser diode (LD), while a ROSA contains a photodiode (PD), optical lens, preamplifier, and passive electrical parts.
They are designed in small form-factor with some integrated optical sub-assemblies which can be suitable for high-density network. The major cost components of a transceiver module are the transmitter optical sub-assembly (TOSA), which converts an electrical and the receiver optical sub-assembly (ROSA). However, inside a BiDi (Bi-Directional) transceiver, there is a component with called “BOSA” (Bi Directional Optical Sub-Assemblies) which acts the role of TOSA and ROSA but with different principle.
In a PON system, one optical fiber is used for bidirectional transmission to reduce the network cost by using optical wavelength division multiplexing (WDM). For bidirectional transmission, a WDM filter is used: it lets transmitted optical signals pass through and reflects received optical signals. The optical bidirectional (BIDI) module is composed of a TOSA, a ROSA, and a WDM filter it lets transmitted optical signals pass through and reflects received optical signals.
TOSA Structure
The TOSA consists of a laser diode, optical interface, monitor photodiode, metal and/or plastic housing, and electrical interface. Depending upon the required functionality and application, other components may be present as well including filter elements and isolators. It is used to convert signal into an optical signal coupled into an optical fiber.
The transmitter mainly consists of an LD and its driver circuit with an automatic optical
power control (APC) circuit. Fabry-Pérot LDs (FP-LDs) and distributed feedback LDs (DFB-LDs) are widely used in optical transmission systems. FP-LDs are inexpensive and commonly used in ONUs. On the other hand, the LD of the OLT should provide a narrower wavelength for the optical signal than that of the ONU. The standardized allocation of optical wavelengths in a PON system is upstream around central frequency 1310nm and downstream data around central frequency 1490nm and video data downstream around 1540nm. To achieve this accuracy for the OLT transmitter, we used a DFB-LD, which can provide a narrow optical wavelength spectrum.
ROSA Structure
The ROSA consists of a photodiode, optical interface, metal and/or plastic housing, and electrical interface. Depending upon the required functionality and application, other components may be present as well including amplifiers. It is used to receive an optical signal from a fiber and convert it back into an electrical signal.
The receiver consists of a PD, which converts a received optical signal to an electrical current signal, and amplifiers. The amplifiers reshape input signals degraded by long-distance transmission. The amplifier circuit consists of a preamplifier and a post-amplifier. The preamplifier converts a current signal to a voltage signal and amplifies the converted signal. The post-amplifier equalizes the output signal of the preamplifier to an amplitude level suitable for input to the following digital circuit. The PD and preamplifier are assembled in a ROSA module because the preamplifier is very sensitive to mounting conditions. The ROSA module makes it easy to handle the optical module and better performance is obtained.
BOSA Structure
The BOSA consists of a TOSA, a ROSA and a WDM filter so that it can use bidirectional technology to support two wavelengths on each fiber. The most valuable advantage of BiDi transceivers is saving much cost on fibers.
Transceiver assembly
A small-form-factor pluggable (SFP) chassis, which can be attached to and detached from an electrical substrate. It contains a BIDI optical module with metal-can-type TOSA and ROSA, an LD driver IC, and a post-amplifier IC.
A burst-mode optical transceiver for gigabit-per-second-class PON systems. The use of various optical and electrical module techniques along with our developed receiver ICs enabled us to obtain high performance with an inexpensive PIN Photodiode. The transceiver is built on a small SFP chassis. It achieved a sensitivity of –29.7 dBm and an output optical power of more than +5 dBm. This optical transceiver will enable us to decrease the cost of gigabit-per-second-class PON systems.
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