Tip of the Week – 14 Sep 2013

Tipe of the Week – Back to Basics

Here are a few useful units you may come across when measuring optical signals in your fiber network.

Fiber loss or component insertion loss, unit: dB
Typical measurement instrument: power meter and light source
Common use: fiber span and component optical loss measurement (such as DWDM or CWDM mux)
Note: loss in dB is always indicated as a positive value. Example, fiber loss of 5 dB, not -5 dB. Also optical fiber loss is dependent on transmission wavelength. Fiber loss is higher at 1310nm than 1550nm for the same fiber length. Be sure to use the proper wavelength laser source and power meter. When recording optical loss, always record wavelength, such as 15 dB @1310nm.

Fiber attenuation, aka attenuation coefficient, unit: dB/km (loss per 1 km unit length of fiber)
Typical measurement instrument: OTDR
Common use: fiber cable specifications
Note: always positive value. Typical attenuation for a good fiber is 0.35 dB/km at 1310nm and 0.23 dB/km at 1550nm

Optical power, unit: dBm (referenced to 1 milli-Watt of optical power, 0 dBm = 1 mW)
Typical measurement instrument: power meter and light source (test source or transceiver)
Common use: absolute optical power measurement
Note: can be positive or negative value. Negative value means the power is less than 1 mW.

Chromatic dispersion “CD”, unit: ps/nm
Typical measurement instrument: CD test set
Common use: fiber span or component measurement
Note: can be positive or negative value.

Chromatic dispersion coefficient unit: ps/(nm.km) (chromatic dispersion per 1 km unit length of fiber)
Typical measurement instrument: CD test set
Common use: fiber cable specification
Note: can be positive or negative value

Polarization mode dispersion “PMD”, unit: ps
Typical measurement instrument: PMD test set
Common use: fiber span and component measurement
Note: always positive value.

Polarization mode dispersion coefficient, unit: ps/√km (PMD per square root 1 km unit length of fiber)
Typical measurement instrument: PMD test set
Common use: fiber cable specification
Note: always positive value.

Optical signal to noise ratio “OSNR”, unit: dB/nm (dB with resolution bandwidth “RBW” in nm)
Typical measurement instrument: optical spectrum analyzer (OSA)
Common use: OSNR measurement in fiber span for DWDM systems
Note: always positive value.

OSA peak power, unit: dBm/nm (dBm power with resolution bandwidth “RBW” in nm)
Typical measurement instrument: optical spectrum analyzer (OSA)
Common use: optical power measurements in DWDM system. Not necessarily the same value as total power measured with power meter.
Note, can be positive or negative value

Transmission rate: Gbps or Gb/s, Mbps or Mb/s, bps or b/s

Typical measurement instrument: BER test set
Common use: equipment specifications, link design (x bits per second)
Note, this is the physical later transmission rate and is not the same as data rate

Data rate: GBps or GB/s, MBps or MB/s, Bps or B/s
Typical measurement instrument: Data test set
Common use: information throughput (x bytes per second)
Note, this is the rate information is being transferred and is not the same as the transmission rate

Notes:
dB = decibel, relative measurement between input and output, dB = -10log(PowerOut/PowerIn)
dBm = absolute measurement referenced to 1 mW, 0dBm = 1mW, equation dBm = 10log(Power/1mW)
ps = picosecond
nm = nanometer
km = kilometer
mW = milliwatt
bps or b/s = bits per second
Bps or B/s = bytes per second

Pertinent Products/Services:

If you have a CWDM system installed, you may be interested in measuring CWDM channel loss. Our CWDM Lite Laser Source is the proper light source to allow you to measure power loss over all 18 CWDM Channels.

Benefits:

  • Accurately measure optical power loss over all 18 CWDM channels 1270nm to 1610nm (CWDM Power Meter also required).
  • Determine if high loss channels at fiber water peak wavelengths can be used for communication.
  • Measuring optical loss at 1610nm may reveal tight bend loss information, since optical loss increases at tight bends at this wavelength.
  • Identify loss of each CWDM channel. Useful in debugging CWDM problems, specially where multiple OADM are deployed.

Applications:

  • CWDM networks
  • Fiber characterization measurements

More details are avaible by visiting our CWDM Lite (TM) CWDM Laser Source page.