OCAM4 Flashcards
What is an optical transmitter?
An optical transmitters coverts electricity into light pulses to be used for transmission.
How do optical transmitters produce light?
They produce light most commonly by Laser Diodes (LDs) which produce a concentrated beam of light and can operate beyond 100Gbps
Properties of a laser diode
1) Produce intense, powerful signal
2) Suitable for long distance
3) Used in SMF
4) Small
5) Long life
6) emit light with a consistent intensity and wavelength
Properties of a Light Emitting Diode (LED)
1) Emit cone shaped light
2) Used in some transmitters but not as common
3) Best for data rates less than 1Gbps
4) Client side support in MMF
What is a laser cavity?
A laser cavity is where LDs produce light. Laser cavities are made up of a n-type material, a p-type material, a total reflective mirror and a partially reflective mirror.
How do laser cavities produce light?
The N and P type material create a electrical potential junction between them. When a positive voltage is applied to the P type and a negative voltage applied to the N type, energy is released in the form of photons (light). These photons are trapped in the cavity and bounce between the mirrors, exciting more electrons, generating more electrical energy. Some photons escape through the partially reflective mirror to produce the last ouput
What are the types of LDs?
1) Distributed Feedback laser (DFB)
2) Fabry-Perot laser (FP)
3) Vertical Cavity surface emitting laser
Properties of DFB
1) Edge emitting
2) Spectral width of «1nm
3) High heat generation
4) High cost
5) High power consumption
6) operates at 850, 1310, 1550nm
7) Used in SM CWDM, DWDM long haul at 1-40Gbps
Properties of FP
1) Edge emitting
2) Spectral width of 5-8nm
3) Medium heat consumption
4) Medium cost
5) Moderate power consumption
6) 850-1650nm range
7) Used SM CWDM for short/medium reach service signals up to 1Gbps
Properties of Vertical Cavity Surface Emitting Lasers
1) Centre emitting
2) Wide spectral width
3) very low heat consumption
4) very cheap
5) very low power consumption
6) 850nm only
7) Used in MM short reach Gigabit ethernet (GbE) and fibre channel (FC) up to 4Gbps
Why are DFBs most commonly used in OC transport networks?
1) Produce one wavelength that is calibrated by Bragg grating
2) Wavelength can be adjusted to be any signal frequency by tuning the laser by a cooling device changing temperature of the semiconductor
3) Can also be tuned by running current through material
How to LDs deal with heat sensitivity?
LDs are usually paired with a Thermo-Electric Cooler (TEC) module and a mounting plate to dissipate heat
What factors affect LD performance?
1) Laser Chirp
2) Back -reflection
3) Temperature
What is Laser Chirp?
Laser chirp is when directly modulating the laser results in the output wavelength to shift by broadening of the wavelength as it enters the fibre. The higher the data rate the more critical this becomes. Modulating the devices externally can help prevent laser chirp
What is back-reflection?
When light that is reflected off a mirror in a laser re-enters the laser cavity resulting in unwanted light emission. Most often occurs when laser joins optical fibre or when two sections of a fibre connect
