Historically, multimode fibre (MMF) optical channel links utilised light emitting diode (LED) transceivers for data communications up to 640 Mb/s and distances up to 2 km. Early MMF types included OM1 and OM2 as specified in IEC/ISO 11801 generic cabling standards. Dr. Rick Pimpinella, Panduit Fellow, researcher in multimode and single-mode optical fibre technology looks at future options.
The development of laser optimised MMF enabled the transmission of optical signals at data rates of 1 Gb/s and beyond. Today, VCSELs can modulate optical signals up to 25 GBaud, and with modifications to the VCSEL’s resonant cavity and material system, VCSELs can be tuned to emit longer wavelengths.
Laser optimised MMF
The maximum distance or reach of multimode fibre is limited by three optical power penalties: modal dispersion; chromatic dispersion and optical attenuation. When light from the VCSEL is coupled into the core of a MMF, due to the wave properties of light the signal propagates through the fibre core along multiple discrete optical paths, called modes. An ideal MMF has a graded index of refraction so that light traveling close to the core axis encounters a higher refractive index thereby slowing down the speed of the light taking the shortest optical path (low order modes). Conversely, light traveling along paths that come close to the outer regions of the fibre core and is reflected back into the core spiralling around the core central axis, traverses a longer overall optical path. Consequently, because of the variation in arrival times of the light traveling along these different optical paths (modes), the width of the output signal at the output of the fibre is broadened. This is called modal dispersion and for traditional MMF channels, modal dispersion has been the primary optical penalty limiting the maximum reach.
Chromatic dispersion is caused by the wavelength dependence of the refractive index. Since a VCSEL emits a narrow spectrum of light (a spectral width of the order of 0.5 nm) the different wavelengths (colours) of light comprising the optical pulse travel at different speeds thereby broadening the output signal, in this case due to chromatic dispersion.
In addition to dispersion, impurities in the fibre’s silica glass core cause a small percentage of the optical signal to scatter and radiate out of the fibre core. This causes a reduction in optical power, or optical attenuation on the order of 2.5 dB/km for 850 nm transmission. The reduction in optical power at the receiver’s photo-detector reduces the signal to noise ratio (SNR), thereby degrading the channel’s performance.
As a result of continued process improvements in the fabrication of MMF, laser optimised OM3 and OM4 were introduced which have improved controlled core refractive index profile, providing significantly lower modal dispersion compared to OM1 and OM2. To better characterise the modal dispersion of OM3 and OM4, a new metric calculated from the modal dispersion measurement, Effective Modal Bandwidth (EMB) specified in units of MHz·km was introduced. In production, laser optimised fibres with an EMB between 2000 MHz·km and 4699 MHz·km are sorted as OM3, and fibres with EMB greater than 4700 MHz·km are sorted as OM4.
Higher performance multimode fibres
In the development and specification of OM3 and OM4 fibre, it was assumed that VCSELs launched the same optical spectrum into each of the MMF modes. However, in 2008 researchers at Panduit Labs discovered that the spectral emission pattern of VCSEL caused different wavelengths to couple into different fibre modes. Consequently, in addition to modes undergoing modal dispersion, due to the spectral differences between modes, the temporal separation between modes also undergo a chromatic dispersion. Therefore, the modal and chromatic dispersion of MMF cannot be treated separately, but instead the channel bandwidth must be determined by the interaction of modal and chromatic dispersions.
This discovery gave way to a new generation of MMF with significantly higher total bandwidth. By selecting a subset of OM4 compliant fibres that have a uniquely specified refractive index profile, the modal and chromatic dispersions can be compensated thereby reducing the total dispersion in a VCSEL-MMF channel. In 2008 Panduit introduced its proprietary MMF, which provides dispersion compensation enabling longer reaches and larger channel insertion loss.
Another important feature of the proprietary fibre is its ability to support wavelengths longer than 850 nm, as currently specified in application standards. In collaboration with Cisco’s transceiver group during the development of the 40 GbE Bi-Direction (BiDi) transceiver, this latest fibre technology became the first multi-wavelength (dual-wavelength) MMF, supporting 850 nm and 910 nm transmission based on short wavelength division multiplexing (SWDM).
Future proofing your network
For single-mode applications, CWDM (course WDM) and DWDM (dense WDM) technologies have been deployed for more than two decades. However, to achieve higher data rates over MMF, parallel optics (using 8, 16, and 32 fibres) have been the technology of choice due to the simplicity of scaling the data rate. To achieve data rates beyond 200 Gb/s using 8 fibres or less, will require the use of SWDM technology.
Table 1 (above) shows a roadmap for ethernet data rates beyond single-wavelength transmission. It is important to note that parallel optics is required to breakout high density switch ports to four server I/O ports. The solutions all use the same structured cabling and support standards based network architectures.