Category: Fiber Transceivers

Fiber optic networks are networks where the transmission of data is done with the help of optical transceivers and optical cables. The optical transceivers transmit an optical light down an optical cable. As in the case with standard Ethernet copper networks, optical networks are also influenced by exterior stress and interior properties and as a consequence some power is loss. This optical power loss is called Attenuation.

Fiber optic cables consist of fiber optic glass core and cladding, buffer coating, Kevlar strength components and a protective exterior material called a jacket. Depending on the optical cable type. These components can vary in size and strength. Unlike the copper cables which use electricity to transmit data, fiber optic cables use pulses of optical light for the same function. Their core is made of an ultra-pure glass which is surrounded by a mirror like cladding. When the light hits the cable it travels down the core constantly bouncing of the cladding until it reaches the final destination. There are two types of optical cables, Multi-mode and Single-mode. From the outside they look almost the same, however their interior plays a huge role in the optical attenuation. Single-mode fibers are used for a long range, high speed connections because of their tighter core and cladding which improve the light transmission by limiting the light bouncing of the cladding. Multi-mode fibers have larger core thus the light will bounce more and more power will be lost until it reaches the destination.

However, the optical attenuation of optical fibers is not only the lost power due to the core of the cable. High optical attenuation can be caused by absorption, scattering and physical stress on the cable like bending. Signal attenuation is generally defined as the ratio of optical input power to the optical output power. As the names suggest, optical input power is the power injected in the optical cable by the optical transceiver, and optical output power is the power received by the transceiver at the other end of the cable. The unit of attenuation is described as dB/km.

Absorption is one of the biggest causes for optical attenuation. This is defined as the optical power lost due to the conversion of the optical power into another form. Absorption is typically caused by a residual water vapors. Generally absorption is defined by two factors:

Imperfection in the atomic structure of the fiber material

The extrinsic and intrinsic fiber-material properties which represent the presence of impurities in the fiber-material

The extrinsic absorption is caused by impurities like trace metals, iron and chromium, introduced into the fiber during the manufacturing process. These trace metals are causing a power loss during the process of conversion when they are transitioning from one energy level to another.

The intrinsic absorption is caused by the basic properties of the fiber material. If the optical fiber material is pure, with no impurities and imperfections, then all absorption would be intrinsic. For example in fiber optics silica glass is used due to its low intrinsic absorption at certain wavelengths ranging from 700nm to 1600nm.

Scattering losses are caused by the density fluctuations in the fiber itself. These are produced during the manufacturing process. Scattering occurs when the optical light hits various molecules in the cable and bounces around. Scattering is highly dependent on the wavelength of the optical light. There are two types of scattering loss in optical fibers:

Rayleigh scattering- this scattering occurs at commercial fibers that operate at 700-1600nm wavelengths. Rayleigh scattering occurs when the size of the density fluctuation is less than 1/10 of the operating wavelength.

Mie scattering- this scattering occurs when the size of the density fluctuation is bigger than 1/10 of the operating wavelength.

Bending the fiber cable also causes attenuation. The bending loss is classified in micro-bends and macro-bends:

Micro-bends are small microscopic bends in the fiber which most commonly occur when the fiber is cabled Macro-bends on the other hand are bends that have a large radius of curvature relative to the cable diameter.

Another type of optical power loss is the optical Dispersion. Optical Dispersion represents the spreading of the light signal over time. There are two types of optical dispersion:

Chromatic dispersion which is spreading of the light signal resulting from the different speeds of the light rays Modal dispersion which is spreading of the light signal resulting from the different propagation modes of the fiber

Modal dispersion is most commonly limiting the maximum bit rate and link length in Multi- mode fibers. The Chromatic dispersion is the main culprit for the attenuation in Single-mode fibers.

Having this in mind we should always consider, test and calculate the possible attenuation of the fibers for deploying a stable network capable for future upgrades.

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.

Since its introduction, optical fiber cables have been known to be the best transmission medium and an innovation that promised to significantly push broadband speeds. And it actually did it.

But there is still a competition between copper and fiber. Both markets are increasing their products and potentially growing. Nevertheless fiber offers a lot of advantages over copper and is quickly replacing it, even in desktop applications.

Greater bandwidth and speed:

Optical fiber provides more bandwidth than copper, reaching speeds from 100 Mbps up to 10 Gbps and beyond. This means fiber can carry more information than copper and with better fidelity. Optical fiber speeds depends on the type of cable, which can be single-mode or multimode.

Longer distances:

When traveling over long distances, optical fiber cables experiences less signal loss than copper cables. Copper cables performance decreases after 9,328ft, while optical fiber installations can go from 984.2 ft to 24.8 miles and have an outstanding performance.

Better security:

It is possible to hack optical fiber, but it is significantly harder than hacking copper networkis. And it is really easy to monitor when a fiber cable is tapped, so you will know if someone tries to break your network security.

Immunity and reliability:

Copper, if not properly installed, produces electromagnetic currents that can cause problems on the network. While optical fiber is immune to electromagnetic interferences, thus they provide reliable data transmission. Fiber is less susceptible to temperature and can be installed underwater, too.

Lighter design:

To reach higher speeds with copper, you need to get a higher grade of cable, which usually are larger and weight more. Optical fiber cables are thin and light, which makes it easier to install because they take up little space.

Costs:

Optical fiber is more expensive than copper in the short run, but its maintaining costs are significantly lower. Fiber requires less hardware installation and lasts longer, which makes it less expensive in the long run.

With the explosion of CWDM, it is very necessary to formulate a basic testing procedure to certifying and troubleshooting CWDM networks during installation and maintenance. Today, one of the most commonly available test methods is the use of an OTDR or power source and meter, which is capable of testing the most commonly wavelengths, 1310, 1490, 1550 and 1625nm.

This article here is based on the pre-connectorized plug and play CWDM systems that allow for connecting to test equipment in the field:

In the multiplexing module of a pre-connectorized CWDM system, wavelengths are added to the network through the filters and transmitted through the common port. The transmitted wavelengths enter the COM port in the de-multiplexing module and are dropped. All other wavelengths present at the MUX/DeMux module are went through the express port.

Most of today’s OTDRs have expanded capability for testing wavelengths in addition to 1310 and 1550 nm. The OTDR allows partial testing of such system offered in test equipment source. The OTDR allows partial testing of these systems by using the flexibility of pre-connectorized solutions. This is done by switching connections within the CWDM field terminal to allow for testing portions of the non-1310/1550 nm optical paths.

To test the 1310nm, the first step is to test the downstream portion of a system at 1310 nm by connecting the OTDR to the 1310 nm input on the CWDM MUX located at the headend. Then switch the test leads over the the upstream side and repeat. Test method is the same for both the downstream and upstream paths.

1550 nm testing is performed similarly by switching the test leads to the 1550nm ports. If additional wavelengths are present, you need to follow the procedures below:

Using the 1550 nm test wavelength, switch the OTDR connection to the 1550 nm input port on the headend MUX. Have a technician stationed at the field terminal connect the drop cable leg connectors for the 1570 nm customer to the 1550 nm port on the Mux/demux device. What should be noted is that in a play and plug solution this should not require repositioning where the drop cable passes through the OSP terminal. Test the downstream 1570 nm passive link at 1550 nm, and then repeat for the 1570 nm upstream side. When testing is complete, have the technician switch the connections for the 1570 nm drop back to the 1570 nm ports on the field MUX/DeMUX device as shown in Figure 6. Repeat this process for the 1590 nm, 1610 nm drop cables and other wavelengths present. Finally, test the 1550 nm path normally with the 1550 nm drop cable connected to the 1550nm MUX/DeMUX ports.

Since the OTDRs is able to test at 1490 or 1625 nm, the drop cables under test could be connected to the EXP port of the module and tested at 1490 or 1625 nm respective wavelength, without having to connect each to the 1550 nm port. Otherwise the procedure is the same.

As CWDM network become more and more common the data they carrying has also become critical. The procedure introduced here allows for testing modular pre-connectorized CWDM systems with standard optical test equipments. Relative channel power can be measured with a wide-band fiber optic power meter at the filter outputs or at other points in the network with the aid of a wavelength selective test device or with an optical spectrum analyzer.

Fiber optic networks are networks where the transmission of data is done with the help of optical transceivers and optical cables. The optical transceivers transmit an optical light down an optical cable. As in the case with standard Ethernet copper networks, optical networks are also influenced by exterior stress and interior properties and as a consequence some power is loss. This optical power loss is called Attenuation.

Fiber optic cables consist of fiber optic glass core and cladding, buffer coating, Kevlar strength components and a protective exterior material called a jacket. Depending on the optical cable type. These components can vary in size and strength. Unlike the copper cables which use electricity to transmit data, fiber optic cables use pulses of optical light for the same function. Their core is made of an ultra-pure glass which is surrounded by a mirror like cladding. When the light hits the cable it travels down the core constantly bouncing of the cladding until it reaches the final destination. There are two types of optical cables, Multi-mode and Single-mode. From the outside they look almost the same, however their interior plays a huge role in the optical attenuation. Single-mode fibers are used for a long range, high speed connections because of their tighter core and cladding which improve the light transmission by limiting the light bouncing of the cladding. Multi-mode fibers have larger core thus the light will bounce more and more power will be lost until it reaches the destination.

However, the optical attenuation of optical fibers is not only the lost power due to the core of the cable. High optical attenuation can be caused by absorption, scattering and physical stress on the cable like bending. Signal attenuation is generally defined as the ratio of optical input power to the optical output power. As the names suggest, optical input power is the power injected in the optical cable by the optical transceiver, and optical output power is the power received by the transceiver at the other end of the cable. The unit of attenuation is described as dB/km.

Absorption is one of the biggest causes for optical attenuation. This is defined as the optical power lost due to the conversion of the optical power into another form. Absorption is typically caused by a residual water vapors. Generally absorption is defined by two factors:

Imperfection in the atomic structure of the fiber material

The extrinsic and intrinsic fiber-material properties which represent the presence of impurities in the fiber-material

The extrinsic absorption is caused by impurities like trace metals, iron and chromium, introduced into the fiber during the manufacturing process. These trace metals are causing a power loss during the process of conversion when they are transitioning from one energy level to another.

The intrinsic absorption is caused by the basic properties of the fiber material. If the optical fiber material is pure, with no impurities and imperfections, then all absorption would be intrinsic. For example in fiber optics silica glass is used due to its low intrinsic absorption at certain wavelengths ranging from 700nm to 1600nm.

Scattering losses are caused by the density fluctuations in the fiber itself. These are produced during the manufacturing process. Scattering occurs when the optical light hits various molecules in the cable and bounces around. Scattering is highly dependent on the wavelength of the optical light. There are two types of scattering loss in optical fibers:

Rayleigh scattering- this scattering occurs at commercial fibers that operate at 700-1600nm wavelengths. Rayleigh scattering occurs when the size of the density fluctuation is less than 1/10 of the operating wavelength.

Mie scattering- this scattering occurs when the size of the density fluctuation is bigger than 1/10 of the operating wavelength.

Bending the fiber cable also causes attenuation. The bending loss is classified in micro-bends and macro-bends:

Micro-bends are small microscopic bends in the fiber which most commonly occur when the fiber is cabled

Macro-bends on the other hand are bends that have a large radius of curvature relative to the cable diameter.

Another type of optical power loss is the optical Dispersion. Optical Dispersion represents the spreading of the light signal over time. There are two types of optical dispersion:

Chromatic dispersion which is spreading of the light signal resulting from the different speeds of the light rays

Modal dispersion which is spreading of the light signal resulting from the different propagation modes of the fiber

Modal dispersion is most commonly limiting the maximum bit rate and link length in Multi- mode fibers. The Chromatic dispersion is the main culprit for the attenuation in Single-mode fibers.

Having this in mind we should always consider, test and calculate the possible attenuation of the fibers for deploying a stable network capable for future upgrades.

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.