Different Technologies Used in DWDM Multiplexer

Dense wavelength division multiplexing (DWDM) is an advancing fiber-optic transmission technology which combines multiple optical channels at different light wavelengths and then transmits them over the same optical fiber. Theoretically, the DWDM technology can multiplex hundreds and even thousands of separate wavelengths into a single optical fiber. To achieve this goal, the DWDM multiplexer is one of the most important parts needing to be improved. This article will discuss several commonly used technologies in the wavelength-selective mechanism of the DWDM multiplexer.

Key Optical Parameters of the DWDM Multiplexer

To better understand those technologies, we need to have a basic knowledge of the DWDM multiplexer’s main optical parameters which are center frequency, insertion loss, pass-band and cross talk.

Center frequency is usually defined as either the arithmetic mean or the geometric mean of the lower cutoff frequency and the upper cutoff frequency of a band-pass system or a band-stop system. Insertion loss is the input-to-output power loss caused by the insertion of a device in a transmission line or optical fiber. A pass-band is the portion of the frequency spectrum that is transmitted (with minimum relative loss or maximum relative gain) by some filtering device. Wider and flatter pass-band makes the system more tolerable to wavelength drift of the laser transmitter. The energy leakage from adjacent and nonadjacent channels in the DWDM system is defined as cross talk.

Technologies Used in the Wavelength-selective Mechanism of the DWDM Multiplexer

Interference filter, bulk grating, array waveguide grating (AWG), and Mach-Zehnder (MZ) interferometer are the four common-used technologies in the DWDM multiplexer.

Interference filter

Consisting of multiple thin layers of dielectric material having different refractive indices, the interference filter reflects one or more spectral bands or lines and transmits others. The dielectric thin-film filter is one of the most popular technologies use to make the DWDM multiplexer.

bulk grating

Being a classical optics technology that has been in existence for over 100 years, bulk grating is now one of the newest technological forerunners in the field of DWDM.

AWG

As the AWG is capable of multiplexing a large number of wavelengths into a single optical fiber, it is commonly used as optical (de)multiplexers in wavelength division multiplexed (WDM) systems, which considerably increases the transmission capacity of optical networks.

MZ interferometer

As a basic interference device, the MZ interferometer is consist of two couplers connected by two waveguides of different length. It is widely used in various fibre-optic communications applications.

A high-performance DWDM multiplexer should have small polarization-dependent loss (PDL), low polarization mode dispersion (PMD), and the same performance under hostile environment temperature. A series of DWDM MUX/DEMUX modules are provided by Fiberstore, which is with as more as 96 channels (50GHz, 100GHz, or 200GHz spaced) in simplex or duplex configurations. All the DWDM modules are available with three types of packaging: ABS Pigtailed Box, Rack Chassis and LGX Cassette.

What Is CWDM SFP Transceiver

Being a kind of compact optical transceiver, the CWDM SFP Transceiver is widely used in optical communications for both telecommunication and data communication. Basing on the SFP form factor which is a MSA standard build, it is designed for operations in Metro Access Rings and Point-to-Point networks using Synchronous Optical Network (SONET), SDH (Synchronous Digital Hierarchy), Gigabit Ethernet and Fibre Channel networking equipment.

The CWDM SFP Transceiver is a cost effective module with high performance, supporting data-rate of 1.25 Gbps and 80km transmission distance. It has a specific laser which emits a “color” defined in the CWDM International Telecommunication Union (ITU) grid that is defined from 1270 to 1610nm and has steps of 20nm. Thus its available wavelength includes 1270nm, 1290nm, 1310nm, 1330nm, 1350nm, 1370nm, 1390nm, 1410nm, 1430nm, 1450nm, 1470nm, 1490nm, 1510nm, 1530nm, 1550nm, 1570nm, 1590nm and 1610nm.

The CWDM SFP transceiver is consist of an uncooled CWDM Distributed Feed Back (DFB) laser transmitter, a Pin photodiode integrated with a Trans-impedance Preamplifier (TIA) and a Microprogrammed Control Unit (MCU). The DFB laser used in the CWDM SFP transceiver transmitter, which is a 18 CWDM DFB wavelengths laser, is well suited for high capacity reverse traffic. Obeying the standard diode equation for low frequency signals, The PIN photodiode has a 80km transmission distance. And the MCU is a high-speed, executive, input-output (I/O) processor and interrupt handler for the Network Restructuring Language (NRL) Signal Processing Element (SPE).

The CWDM SFP is a hot-swappable, transceiver component that can be plugged into a SFP-compatible uplink port. And each SFP port must match the wavelength specifications on the other end of the cable which must not exceed the stipulated cable length for reliable communications.

Applied to the access layer of Metropolitan Area Network (MAN), The CWDM SFP is a low-cost Wavelength Division Multiplex (WDM) transmission technology. Adopting the CWDM technology, it can combine optical signals with different wavelength together and transmit them with one fiber strand. It can largely save the fiber resources. The CWDM SFP allows enterprise companies and service providers to provide scalable and easy-to-deploy Gigabit Ethernet and Fibre Channel services in their networks, enabling the flexible design of highly available, multiservice networks.

Fiberstore’s CWDM SFP transceiver provides a convenient and cost-effective solution for the adoption of Gigabit Ethernet and Fiber Channel (FC) in campus, data-center, and metropolitan-area access networks. With its maximum optical budget of 41dB, Fiberstore CWDM SFP modules support data rates from 100 Mbps up to 4.25 Gbps.

A Brief Introduction to CWDM Mux/Demux

CWDM Mux/Demux is the short name for Coarse Wavelength Divison Multiplexer/Demultiplexer Module, which is a flexible, low-cost solution enabling the expansion of existing fiber capacity. It is based on dielectric thin-film technology designed for integration in low cost Metro and Access networks. Combining with highly reliable passive optics certified for environmentally hardened applications, the CWDM Mux/Demux enables the operator to make full use of available fiber bandwidth in local loop and enterprise architectures. This module can either combine (added) or separated (dropped) 4, 8 16 or 18 channels .

What is Mux/Demux
The Mux is a module that makes it possible to share several signals in one device or resource such as an analog-to-digital converter (A/D converter) or one communication line, instead of having one device per input signal. A Mux is also called a data selector, which is mainly used to increase the amount of data that can be sent over the network within a certain amount of time and bandwidth.
On the contrary, the Demux, which is connected to the single input, takes a single input signal and selecting one of many data-output-lines. A complementary Demux is often used with a Mux on the receiving end.

How does CWDM Mux/Demux work
Through the use of Mux, several optical channels of different wavelengths can be combined, allowing multiple services to be transmitted together via fiber without interference. This can be possible for the reason that different light colours (wavelengths) do not affect each other. For transmission, light colours are multiplexed onto an fiber using a wavelength-specific filter (multiplexing). At the other (receiving) end of the line, the wavelengths are divided or demultiplexed again by the Demux. Hence, any transmission line consists of a multiplexer and a demultiplexer.
Being a universal device capable of combining nine optical signals into a fiber pair, the CWDM Mux/Demux is designed to support a broad range of architectures, ranging from scalable point-to-point links to two fiber-protected rings.

What are the features of CWDM Mux/Demux
First of all, as a passive CWDM optical mux/demux design, the CWDM Mux/Demux has a long transmission distance coverage of multiple signals on a single fiber strand. Secondly, it can support various types of signals such as 3Gbps/HD/SD, AES, DVB-ASI, Ethernet, etc. Furthermore, its ambient operating temperature is from -40°C to 85°C. In another word, CWDM Mux/Demux is suitable for outside plant applications. Last but not least, it requires no powering because of thermally stable passive optics.

The CWDM MUX/DEMUX modules are used to combine and split the multi-wavelength optical signals of CWDM networks. Fiberstore provides a series of CWDM MUX/DEMUX modules with as more as 18 channels (20nm spaced) in simplex or duplex configurations. All the CWDM modules are available with three types of packaging: ABS Pigtailed Box, Rack Chassis and LGX Cassette.

Differences between CWDM and DWDM

Coarse Wavelength Division Multiplexing (CWDM) and Dense WaveLength Division Multiplexing (DWDM) are two kinds of wavelength-division multiplexing (WDM) which is a technology multiplexing a number of optical carrier signals onto a single optical fiber by using different wave lengths of laser light. Seeing from their development, DWDM came well before CWDM. DWDM appeared only after a booming telecommunications market, driving prices to affordable lows. Whereas CWDM breaks the spectrum into big chunks. Here is some information related to the differences between CWDM and DWDM.

 

Definitions

CWDM is a method of combining multiple signals on laser beams at various wavelengths for transmission along fiber optic cables. Its WDM system has less than 8 active wavelengths per optical fiber.

DWDM is a fiber-optic transmission technique that employs light wavelengths to transmit data parallel-by-bit or serial-by character. Its WDM system has more than 8 active wavelengths per optical fiber.

Capacities

Each CWDM wavelength, which can be expanded to 10Gbps support, typically supports up to 2.5Gbps. This transfer rate is sufficient to support GbE, Fast Ethernet or 1/2/4/8/10G FC, STM-1/STM-4/STM-16 / OC3/OC12/OC48 and other protocols. However, since the optical amplifiers cannot be used due to the large spacing between channels, the CWDM is typically deployed at networks up to 80Km.

Providing up to 96 wavelengths (at 50GHz) of mixed service types, the DWDM systems can transport to distances up to 3000 km through the deployment of amplifiers and dispersion compensators, which increases the fiber capacity by a factor of x100. Today’s DWDM solution is often embedded with Reconfigurable Optical Add Drop Multiplexer (ROADM). The ROADM enables the building of flexible remotely managed infrastructure in which any wavelength can be added or dropped at any site.

prices

The function of CWDM is to help carriers make the best of their network capacity in the regional, metro and access network sectors. Comparing with DWDM, CWDM supports fewer wavelengths; but it is much cheaper than DWDM. Thus CWDM is the perfect choice for those areas having average traffic growth projections.

Since the laser of DWDM is more precise and stable, it tends to be more expensive at the sub-10G rates. However, for 10G service rates, it is a more appropriate solution,which provides large capacity data transport and connectivity over long distances at affordable costs.

Filters

Due to fewer number of layers in the filter design, the filter of CWDM is inherently less expensive to make than that DWDM. Typically, there are over 100 layers required for a 200 GHz filter design as used in metro DWDM products, where there are only 50 layers in a 20 nm filter used in Metro CWDM products. The result is shorter manufacturing time, less materials and higher manufacturing yields for CWDM filters. As a result, CWDM filter costs are generally less than 50 percent of the cost of comparable DWDM filters.

CWDM networks use CWDM modules such as CWDM MUX/DEMUX and CWDM OADM. While DWDM MUX/DEMUX and DWDM OADM are used in DWDM networks. A series of WDM modules are provided in Fiberstore, which enable the expansion of existing fiber capacity.

What Is Optical Transceiver

What Is Optical Transceiver

As a central component of optical communication, optical transceiver is a kind of optical-electric/electric-optical converter. It is also called fiber optic transmitter and receiver. An optical transceiver consists of a transmitter on one end of a fiber and a receiver on the other end. The transmitter end takes in and converts the electrical signal into light, after the optical fiber transmission in the fiber cable plant, the receiver end again converts the light signal into electrical signal. Both the receiver and the transmitter ends have their own circuitry and can handle transmissions in both directions.

The structure of optical transceiver

Optical transmitter: since the sources used for fiber optic transmitters need to be at the correct wavelength, be able to be modulated fast enough to transmit data and be efficiently coupled into fiber, LEDs, fabry-perot (FP) lasers, distributed feedback (DFB) lasers and vertical cavity surface-emitting lasers (VCSELs) are the four commonly used types of sources. Thought these decvices all convert electrical signals into optical signals, they are quite different from each other.
Optical receiver: the optical receiver uses semiconductor detectors (photodiodes or phodetectors) to convert optical signals to electrical signals. Silicon photodiodes are used for short wavelength links (650 for plastic optical fiber and 850 for glass MM fiber). Long wavelength systems usually use indium gallium arsenide (InGaAs) detectors as they have lower noise than germanium which allows for more sensitive receivers. Very high speed systems sometimes use avalanche photodiodes (APDs) that are biased at high voltage to create gain in the photodiode.

The performance of optical transceiver

Just as with copper wire or radio transmission, the performance of the optical data link can be determined by how well the reconverted electrical signal out of the receiver matches the input to the transmitter. The discussion of performance on datalinks applies directly to transceivers which supply the optical to electrical conversion.

Every manufacturer of optical transceivers specifies their product for receiver sensitivity (perhaps a minimum power required) and minimum power coupled into the fiber from the source. Those specifications will end up being the datalink specifications on the final product used in the field.

All datalinks are limited by the power budget of the link. The power budget is the difference between the output power of the transmitter and the input power requirements of the receiver. The receiver has an operating range determined by the signal-to-noise ratio (S/N) in the receiver. The S/N ratio is generally quoted for analog links while the bit-error-rate (BER) is used for digital links. BER is practically an inverse function of S/N.

Three primary types of optical transceiver

There are many different kinds of optical transceivers that can be used in telecommunications applications. The different specs and designs are widely used to meet the changing needs of designers. The small form-factor (SFP), the small form-factor pluggable (SFP+) and the XFP transceiver are the three primary kinds. SFP is available for designers to use in various applications. SFP+ , which is an enhanced version of an SFP, is designed to support data rates up to 10 Gbit/s. While the XFP transceiver can operating at wavelengths of 850nm, 1310nm, and 1550nm, which are capable of operating at a single wavelength.

How to Select A Right SFP Transceiver

The small form-factor pluggable (SFP) transceiver is also called mini Gigabit Interface Converter (MINI-GBIC) module due to its smaller size and is the second generation product after the standardized pluggable optical module GBIC. It is a compact, “hot-pluggable” optical transceiver used in optical communications for both telecommunication and data communications application. Nowadays SFP transceivers are being more and more widely used for bi-directional transmission of data between an electrical interface and an optical data link. And SFP transceivers are available with a variety of transmitter and receiver types. Here are some tips for users to select their appropriate SFP tansceiver.

According to different types of optical fiber, SFP transceiver can be divided into the following categories:

1) For multi-mode fiber, with black or beige extraction lever

Only one type of SFP transceiver is available, whose specs is SX-850 nm, for a maximum of 550 m at 1.25 Gbit/s (Gigabit Ethernet) or 150m at 4.25 Gbit/s (fiber Channel).

2) for single-mode fiber, with blue extraction lever

There are many kinds of SFP transceiver available for this type of fiber. For example:

① LX-1310 nm, for distances up to 10 km.

②BX-1490 nm/1310 nm, Single Fiber Bi-Directional Gigabit SFP Transceivers, paired as BS-U and BS-D for Uplink and Downlink respectively, also for distances up to 10 km 1550 nm 40 km (XD), 80 km (ZX), 120 km (EX or EZX).

③Coarse Wavelength Division Multiplexing (CWDM) and Dense WaveLength Division Multiplexing (DWDM) transceivers at various wavelengths achieving various maximum distances.

3) for copper twisted pair cabling

1000BASE-T SFP Transceiver Modules are available, which incorporate significant interface circuitry and can only be used for Gigabit Ethernet, as that is the interface they implement. They are not compatible with (or rather: do not have equivalents for) fiber channel or SONET.

According to the wavelength of SFP transceiver, users can select an appropriate one among 850 nm/1310 nm/1550 nm/1490 nm/1530nm/1610 nm. To be specific, the 850nm wavelength is SFP multimode, and the transmission distance is 2 km below. While the 1310/1550 nm is SFP single-mode, and the transmission distance is longer than 2 km. In the market, the prices of 850 nm/1310 nm/1550 nm SFP transceiver are relatively cheaper than that of the other three.

The SFP transceiver modules are designed to support Synchronous Optical Network (SONET)/Synchronous Digital Hierarchy (SDH), Fast Ethernet, Gigabit Ethernet, fiber Channel and other communications standards. By choosing the appropriate SFP transceiver module, the same electrical port on the switch can connect to fibers of different types (multimode or singlemode) and different wavelengths. Fiberstore manufactures and supplies a complete range of SFP transceiver modules which can be Customized. In addition, all of its SFP transceiver modules come with a lifetime advance replacement warranty and are 100% functionally tested.