OCAM2 Flashcards
What is the speed of light?
2.9979X10^8m/s
What is equation for frequency?
f = c/lambda
F: Frequency
C: Speed of light
Lambda: wavelength
What is Electro Magnetic Radiation?
EMR consists of waves and particles that both have an electrical and magnetic component. Light is a form of EMR
Define light in terms of optical communications
Light is EMR that occurs in a specific region of the electromagnetic spectrum (optical spectrum). This includes ultraviolet, visible and infrared light. The spectral bands used in optical transmission are located between 850-1675nm
What are the wave properties of light?
1) Has an electrical and magnetic field which have direction and amplitude. These fields are orbited 90-degrees to each other and so light energy is in the form or a lightwave
2) The fields oscillate in amplitude and direction, giving a sine wave shape. The length of one complete cycle gives the wavelength of the light
3) These fields can also be known as polarisation modes
What is a photon?
1) Basic unit of light is a elementary packet called a photon
2) Photons have no mass and travel close to the speed of light
How are a photons created?
When electrons transition from a high energy state to a low energy, the energy released is emitted as photons
What are the particle properties of light?
Photon bundles, although not physical particles exhibit similar characteristics. The classic demonstration of this is the photoelectric effect
What is the photoelectric effect?
When matter emits electrons in response to the absorption of energy from light
What is a decibel?
A decibel is a measurement that expresses the ratio between two power values. We use dB to measure optical loss
What is dBm?
A measure of power referenced to 1mW. We use dBm to measure optical power. 1mW=0dBm
What are the three components used in optical fibre?
1) Core - pure silicon dioxide, providing transmission path for optical signals
2) Cladding - nearly pure silicon dioxide, confining light to the core
3) Buffer - durable resin, protecting core and cladding from moisture and physical damage
What is light propagation?
Propagation is the ability for light to travel over the transmission medium (fibre) by reflecting and maintaining light into the core
What is refraction?
Refraction is the bending of light at the intersection between two dissimilar materials
What is refractive index?
RI is the ratio of the velocity of the speed of light in an optical fibre versus the speed of light in a vacuum
What is reflection?
Reflection is the immediate change in direction of a light ray at the intersection of two different media. In terms of optical fibre, the core must have a higher RI than the cladding for the light to remain in the core
What is Snell’s law?
Snell’s Law states the ratio of the sines of the angles of incidence and refraction is equivalent to the ratio of phase velocities in the two media
What is the Cone of Acceptance?
Light entering the core must enter within the cone of acceptance to enable propagation down the fibre. If light enters outside of the cone, light is refracted into the cladding. This is specified by Numerical Aperture.
What is NA?
The NA is based on the refractive indices of the core and and cladding material of the fibre
What is Total Internal Reflection?
When the refractive index of the core is greater than the refractive index of the cladding medium i.e the angle of incidence is past the point called the critical angle. This means all light energy is reflected and no light enters the cladding
How does the cladding have lower RI than the core?
Fibre manufactures dope the cladding with germanium
What are optical impairments?
Optical impairments contribute to and/or cause a reduction in optical signal i.e a loss
What are the types of optical impairments?
1) Linear
2) Non-linear
What is dispersion?
Signal distortion that results in the spreading or broadening of a light pulse as it travels through the fibre. It can limit the distance a signal can travel through a fibre
What are the types of dispersion?
1) Intermodal or multimode dispersion (only in MMF)
2) Intramodal dispersion
What are the types of Intramodal Dispersion?
1) Chromatic dispersion
2) Polarisation mode dispersion
What is CD?
CD is the most common type of dispersion and occurs when different wavelengths travel at different speeds causing the light to spread and creating inter-symbol interference. The different speeds mean the received signal is distorted. It is made up of material and waveguide dispersion.
What factors affect CD?
1) Spectral width of the laser used
2) Bit rate
3) Temporal pulse separation
4) Type of fibre used
What is material dispersion?
MD is when different wavelengths travel at different velocities, with the slower wavelengths beginning to lag, causing light to spread. Over great distances the lagged wavelengths overlap the faster wavelengths in the following signal. This degrades and destroys the signal.
What is waveguide dispersion?
WD occurs when the light passes through the cladding as well as the core. Over long distances this can become a factor for dispersion
How to compensate for CD?
1) Using fibre with different dispersion characteristics on optical span
2) Dispersion Compensation modules (DCMs)
3) Dispersion Sloped Compensation Modules (DSCMs)
4) Electronic dispersion compensation
What is polarisation mode dispersion?
1) When extremely minor changes in the fibre content results in the two polarisation modes (waveforms) travel at different speeds. This delay is known as Differential Group Delay (DGD). When the receiver adds the modes together, the light is expanded.
PMD is measured in picoseconds
What factors contribute to PMD?
1) Bit rate of the signal
2) Fibre core symmetry
3) Environmental factors
4) Bends or stress in the fibre
When does PMD become an issue?
PMD only needs to compensated for at bit rates higher than 40gbps and in spans over 1600km
How do you compensate for PMD?
1) Next generation optical transmitters and receivers
2) Use of improved fibre
3) Use of manufacturer’s recommended installation techniques of fibre
When can Non-linear effects cause optical impairments?
1) High bit rates (over 2.5gbps)
2) High power levels
What are the nonlinear effects?
1) Stimulated Brillouin Scattering (SBS)
2) Stimulated Rama Scattering (SRS)
3) Self-phase Modulation (SPM)
4) Cross- Phase Modulation (CPM)
5) Four-Wave Mixing (FWM)
What is SBS?
SBS is backscattering caused by electrical field acoustic vibrations resulting fluctuations of the fibres refractive index. This causes scattering of the transmitted wave (pump wave) and the reflected wavelength propagates in the opposite direction (stoke wave)
What effects does SBS have optical transmission?
1) Attenuation of the transmitted signal
2) Power saturation - SBS increases as power increases, eventually SBS cancels out any increase of power on the transmitted signal
3) Noise in amplifies and transmitters
How can we limit SBS?
1) Using phase modulation instead of amplitude modulation
2) Limit optical power per channel to a level lower than when SBS becomes a problem
3) Increase line width of the transmitted signal
What is SRS?
SRS is caused by acoustic molecular vibrations in the fibre, resulting in more than one channel being effected. The scattering happens in both directions.
How does SRS impair optical transmission?
SRS transfers energy from shorter wavelengths to higher wavelengths when separated by 100nm. This suppress shorter wavelengths in multi channel transmission, with the more closely spaced optical channels being more affected
What is SPM?
SPM is a result of the refractive index being slightly higher at the intensity peak than it is at the leading and trialing edges of the light pulse. This results in the broadening of the pulse spectrum. This interacts with CD to change the rate at which the wavelengths broaden as the pulse travels through the fibre
How to overcome SPM?
1) Use a lower optical power
2) Use optical fibre characterised by dispersion greater than zero in the operating wavelength range. This means SPM compresses an optical pulse which compensates for the effect of CD
What is CPM?
CPM is similar SPM except for it involves multiple wavelengths. A power fluctuation in one signal causes phase fluctuations in others
How to overcome CPM?
Increasing the effective area of the fibre reduces the effects by decreasing in optical intensity
What is FWM?
FWM is the result of nonlinearity in the refractive index giving rise to signals other than the transmitted. If there are two transmitted wave, a new additional wave is generated, this interferes with the other wavelengths via cross talking. The more wavelengths, the more interfering products, these are called ghost channels.
What influences FWM?
1) Close channel spacing
2) CD - less CD means more FWM
How to overcome FWM?
1) Forward Error Correction - allows for lower power levels but same distance
2) Increased channel spacing
3) Lower signal power
4) Use asymmetrical channel spacing
5) Reduce number of channels transmitted
6) Introduce CD
