Optical Multiplexing Introduction

In recent years Fiber Optic communication technologies have evolved at a rapid pace in order to support our increasingly data driven society. Many of the advancements have been in the area of finding clever ways to increase the transmission capacity of the existing fiber network infrastructure. Since optical multiplexing is a useful technique to accomplish this, researchers have explored optimizing optical multiplexing in time (TDM), space (SMX) , polarization or phase (PDM) and wavelength (WDM).

WDM (Wavelength-Division Multiplexing)

While some of the aforementioned multiplexing techniques (in particular spatial Multiplexing) hold great promise for future applications, this article focuses primarily on Wavelength-Division Multiplexing (WDM) as it is the most widely used by SanSpot.com's applications, solutions and customers. In Essence WDM creates virtual fiber pathways over a single fiber strand by combining 2 or more wavelengths (i.e. colors) of laser light into optical signals transmitted over a single fiber. This is achieved by dividing the original bandwidth available into a series of non-overlapping wavelengths or frequencies, each carrying a separate signal. Through the application of optical (WDM) filters, different wavelengths are joined together (multiplexed) into a single optical transmission signal, only to be split apart again (demultiplexed) at the receiving end. This technique reduces the need for dedicated and wavelength specific optical links and multiplies the data-carrying capacity of existing fiber by a factor of the number of wavelengths (channels) combined.

Please see the illustration below for a visualization of WDM Optical Multiplexing

Visualization of WDM Optical Multiplexing

WDM transmission systems can be grouped into the following 4 categories:

  1. Bi-Directional WDM (BWDM), 2 Channels / Wavelengths
  2. Coarse WDM (CWDM), up to 18 Channels / Wavelengths
  3. Dense WDM (DWDM), 40+ Channels / Wavelengths
  4. Next generation WDM and OTN

BWDM (Bi-Directional WDM)


Bi-Directional WDM (BWDM) systems are the most straightforward applications on the list and provide a quick, easy and cost effective way of doubling the capacity of existing duplex fiber. BWDM only provides two channels (wavelengths) of communication at standard wavelengths of ~1310nm and ~1550nm respectively. Some vendors define BWDM slightly different and refer to BWDM as Band or Band-pass WDM where instead of filtering individual channels, Band WDM will filter a group of channels which is applicable across CWDM and DWDM but mostly used in DWDM applications. To avoid any confusion Band or Bandpass WDM is not being considered in this article.

BWDM Products & Solutions

BiDirectional (BiDi) Transceivers

OptoSpan Single Fiber Bidirectional transceivers use simplex single-mode fiber to double the bandwidth without the cost. These hot-swappable single fiber transceivers provide data rates of 10G and distance up to 160 kilometers. Designed for bi-directional 10G serial optical data communications such as Ethernet and Fibre Channel. Cisco compatible and many other OEM compatible modules are available.

BiDi Media Converter Multiplexers

Optospan’s BiDi Boxes are the industry’s first bidirectional link converters with virtually no signal loss. The BiDi Box 2 to 1 Fiber Converter family instantly converts any duplex optical link into simplex suited for bi-directional communication. Available in the 3 series (operating within the 1290nm-1330nm spectrum) optical link converter featuring less than 1dB signal loss and the 5 series (operating in 1510nm-1610nm spectrum) optical link converter with less than 1.3dB loss.

CWDM (Coarse WDM)


Coarse WDM (CWDM) technology is capable to further increase the transmission capacity over a single fiber by providing up to 18 (most commercially available applications use 16) channels, across multiple wavelength bands.

For transmission equipment to accurately interpret an optical signal, it is important the signal possesses a tall, well-defined peak for each channel, allowing individual signal peaks to become easily identifiable. The space between adjacent wavelengths channels allows for this and in the case of CWDM set at 20nm as per ITU standard G.694.2.

20nm channel spacing is used to prevent cross-talk among channels due to a relatively large pass-band width in CWDM (+/- 6.5nm from center wavelength) optical filters. The pass-band or pass-band width of an individual channel is the range or band of wavelengths of light corresponding to the CWDM channel filter allowed to pass while all others are reflected.

In short: CWDM Passband = Center Wavelength +/- 6.5 nm Ie: The pass-band of Channel 1531 is 1524.5 – 1537.5 nm As per ITU-T standards, the center wavelength is defined on the “1”

Please see the below illustration of CWDM Channels, Bands and application

Each channel can operate at either 2.5, 4 or 10 gigabits per second (Gb/s). The relatively large channel spacing allows for larger filter tolerances and more cost-effective transceiver designs and makes CWDM the ideal technology for transporting large amount of data in enterprise networks. CWDM however cannot be amplified since most of the channels are located outside the operating window of the erbium-doped fiber amplifiers (EDFA) used in DWDM systems, therefore limiting the datarate and range as compared to DWDM solutions. However, due to its cost advantage over DWDM, CWDM still makes sense in many situations and what solution is most suited depends on network and user requirements.

CWDM Products & Solutions

CWDM Transceivers

OptoSpan CWDM (Coarse Wavelength Division Multiplexing) transceivers transmit multiple data channels using separate optical wavelengths onto a single fiber to increase network capacity for a fiber optical transceiver module. CWDM transceivers provide data rates of up to 4G and reach a distance of up to 200 km. OptoSpan CWDM Transceivers are Cisco compatible and many other OEM compatible modules are available.

CWDM Multiplexers

OptoSpan CWDM multiplexers provide the most robust and low-cost bandwidth upgrade for your current fiber optic communication networks. All CWDM Mux Demux systems are based on thin film filter technology and metal bonding micro optics packaging. OptoSpan provides high quality CWDM with flat channel bandwidth, flexible channel configuration, low insertion loss and high isolation. All OptoSpan products are epoxy-free in the optical path. Each CWDM module supports ESCON, ATM, Fibre Channel, and Gigabit Ethernet over each port. Available in 4+2, 8+1, and 16 channel.

DWDM (Dense WDM)


Dense WDM (DWDM) uses much narrower pass-band widths (+/- 0.25nm), allowing for much denser channel spacing (+/- 0.8nm) and as a result 16 DWDM channels will fit within the passband of one single CWDM channel! This allows for a lot more channels of optical communication and a lot higher transmission capacity. Due to the lowest attenuation, most commercial DWDM applications operate solely within the C-Band (1530 nm-1565 nm) transmission spectrum and even though channel designs vary, a typical DWDM system would use up to 40 channels at 100 GHz spacing. Latest technologies and amplification options are capable of even denser spacing (ultra or super dense WDM) or the extension of usable wavelengths into the L-band spectrum promising a future of 100+ channels

DWDM Products & Solutions

DWDM Transceivers

OptoSpan DWDM (Dense Wavelength Division Multiplexing) transceivers and pluggables use C-band wavelengths to multiplex optical signals onto a single fiber. These transceivers are intended for high capacity, long distance transmissions. 4G Fibre Channel DWDM transceivers reach distances of up to 200km. OptoSpan offers 30 different modules including, DWDM SFP Cisco compatible and many other OEM compatible modules.

DWDM Multiplexers

OptoSpan DWDM Mux Demux multiplexers provide the most robust and low-cost bandwidth upgrade for your current fiber optic communication networks. All our DWDM systems are based on thin film filter technology and metal bonding micro optics packaging. OptoSpan provides high quality DWDM with flat channel bandwidth, flexible channel configuration, low insertion loss and high isolation. All OptoSpan products are epoxy-free in the optical path. Each DWDM Mux Demux module supports ESCON, ATM, Fibre Channel, and Gigabit Ethernet over each port. Available in 8+1, 16+1, 32 and 40 channel.

Next generation WDM and OTN


Advanced or Next Generation WDM technology is not so much based on channel expansion or condensing of spacing but essentially integrates all the aforementioned technologies and related protocols into a single platform acting as a converged transport layer. Next generation WDM communicates based on standardized digital wrapper technology and follows the Optical Transport Network (OTN) definition as set forth by the ITU-T.

OTN is defined in ITU G.709 as a set of Optical Network Elements (ONE) connected by optical fiber links, able to provide functionality of transport, multiplexing, switching, management and supervision of optical channels carrying client signals. This allows network operators to converge networks through seamless transport of the numerous types of legacy protocols, yet providing the flexibility required to support future client protocols.

Optical Transport Network (OTN) is the ideal technology to bridge the gap between next generation IP and legacy Time Division Multiplexing (TDM) networks (i.e. SONET/SDH) by acting as a converged transport layer for newer packet-based and existing TDM protocols.

Key features & Benefits of OTN:

  • Improves network efficiency by performing optical multiplexing for optimum transmission capacity.
  • Enables network convergence and scalability as well as support for dedicated Ethernet services.
  • Protects the network against an uncertain or undesirable service mix by providing a native transport of signals.
  • Delivers multi-layer performance monitoring and enhanced maintenance capabilities.
  • Improves system performance through Forward Error Correction (FEC).
  • Provides enhanced alarm handling capability.

Next Generation DWDM and OTN Products & Solutions

OTS-10 Advanced WDM 160km 10G/40G Long-haul Optical System

At the frontier of multi-service Optical Transport Systems (OTS), the OTS-10 is designed for enterprises, carriers and service providers alike, who desire a cost effective long distance, high bandwidth and reliable network.

Its unique platform integrates WDM and SDH multiplexing with onboard transponders, eliminating the need or requirement of expensive OEM transceivers for long distance links. This design provides seamless transport of 10G/40G ethernet or OC48 and OC192 SONET services.

OTS-100 Versatile 100G+ Long-haul Optical Transport System

The next generation of all-in-one Optical Transport Systems, the OTS-100 represents the future of cost-effective 100G WDM networking. With similar core functionality and capabilities as the OTS-10 system it extends its functionality by additionally supporting various 100G client services including 100GBASE-LR4, 100GBASE-ER4, and 100GBASE-SR10.

The system is a turnkey 40G/100G solution for 80km box-to-box transmission but can be stacked for single or dual fiber connections to achieve bandwidth expansion up to 400G transmission across 160km (with EDFA).

Optical Multiplexing Summary

As outlined in this article, great progress has been made in increasing the data-carrying capacity of existing fiber by means of Optical Multiplexing Technologies. The next exciting phase however, is one of convergence of different technologies and protocols that will lay the foundation for true Optical Transport Networks and usher in a future of affordable long distance, high bandwidth networking!

Please contact us for more information on Advanced WDM or OTN solutions and see what these can do for your organization: Request More Info...