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Optical communications > General > Flashcards

General Flashcards

1
Q

What is attenuation?

A

Attenuation limits the optical power which can reach the receiver, limiting the operating span of a system.

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2
Q

How do you calculate loss from fibre attenuation?

A

loss(dB) = fiber length (km) x attenuation(dB/km)

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3
Q

What are the three major causes of attenuation?

A
  • Material absorption loss
  • Material scattering loss
  • Bending loss
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4
Q

What causes material scattering loss?

A

It is an intrinsic loss mechanism caused by the interaction of photons with the glass itself.

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5
Q

What are the two types of absorption?

A

Intrinsic and extrinsic

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6
Q

What causes intrinsic absorption?

A

Interaction with one or more of the components of glass.

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7
Q

What causes extrinsic absorption?

A

Impurities within the glass.

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8
Q

When does absorption occur?

A

When a light particle has enough energy to interact with an electron and excite it to a higher energy level.

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9
Q

What is the main cause of intrinsic absorption in the infrared region?

A

The characteristic vibration frequency of atomic bombs.

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10
Q

What is dispersion?

A

The time domain spreading or broadening of the transmission signal light pulses as they travel through the fibre.

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11
Q

What are the negatives of dispersion?

A
  • Distorts the transmission signal.

- Dispersion limits available bandwidth.

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12
Q

What is the relationship between dispersion and bit rate?

A

The higher the dispersion, the lower the bit rate.

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13
Q

What is the principle of total internal reflection?

A

At some angle of incidence, θ1, called the critical angle θc, the angle of refraction is 90. If θ1 is greater than θc, then the ray returns or is refracted back into the high refractive index medium.

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14
Q

What is Snell’s law?

A

n1sinθ1 = n2sinθ2

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15
Q

How do you calculate available power budget?

A

Transmitter output power + receiver sensitivity

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16
Q

How do you calculate power margin?

A

Available power budget - total loss

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17
Q

What is the formula for material dispersion?

A

Dt = dispersion coefficient x fibre span x spectral width

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18
Q

How is the term power penalty defined?

A

The increase in receiver power needed to eliminate the effect of some undesirable system noise or distortion.

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19
Q

What do power penalties typically result from?

A
  • Dispersion
  • Reflection from passive components
  • Crosstalk in couples
  • Modal noise in the fibre
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20
Q

What is the condition for total internal reflection?

A

n1sinθc = n2sin(π/2)

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21
Q

In a bidirectional WDWM transmission system, the blue and red bands are separated to avoid interference, how is this done?

A

Via a guard band

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22
Q

What is the problem with a guard band?

A

Its typically about 5nm and wastes spectral space.

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23
Q

How do you calculate bit rate from time delay?

A

B = 1/δt

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24
Q

What is the expression for time delay of the slowest and fastest modes for a multimode step index fibre?

A

δt = Tmax - Tmin = (n1L/c)((n1/n2)-1)

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25
Q

How do you calculate velocity of light in a material?

A

V = c/n

where n is the refractive index

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26
Q

How can you tell if an event on a power vs distance graph is a bend loss?

A

It will be experienced to a lesser extent by materials with a smaller wavelength (unlike splice losses).

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27
Q

How can you tell the difference between a fusion splice and a mechanical splice?

A

A mechanical splice has a fresnel reflection.

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28
Q

How do you determine the transmission angle of an LED device?

A

Find the half-angle POV then double it.

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29
Q

How do you calculate optimum DC current for a signal swing of 250mA?

A

(Imax -Imin)/2 = (350 + 100)/2 = 225 mA

30
Q

What is the formula for required transmit power?

A

Pt = A x Rx power density

31
Q

What is θ0 in terms of the critical angle?

A

θ0 = π/2 - θc

32
Q

What is the numerical aperture defined as for a tapered fibre, per Snells law?

A

NA = n1sin(θi)

33
Q

How is refractive index expressed using n1 and n2?

A

n = n1/n2

34
Q

Under what reflection is refractive index greater than 1?

A

External reflection

n1 < n2 n = n2/n1 > 1

35
Q

Under what reflection is refractive index less then one?

A

Internal reflection

n1 > n2 n = n2/n1 < 1

36
Q

How do you calculate refractive index using speed of light?

A

n = c/v

v = velocity of light in medium

37
Q

What is the relationship between refractive index and wavelength of light in the medium?

A

Refractive index, n, decreases as wavelength increases.

38
Q

What is the law of reflection?

A

Angle of incidence, θi = angle of reflection, θr

39
Q

What is the core refractive index represented by?

A

n1

40
Q

What is the cladding refractive index represented by?

A

n2

41
Q

Why is the refractive index of the core greater than that of the cladding in a fibre?

A

So that the light gets trapped in the core

42
Q

What is a meridional ray?

A

Light rays that pass through the fibre centre axis

43
Q

What is the formula for Rayleigh scattering loss?

A

αr = Ar / λ(μm)^4

where Ar is the Rayleigh scattering coefficient

44
Q

What is the formula for rayleigh scattering coefficient for a graded index near parabolic profile fibre?

A

0.63 + 1.75(NA) = Ar

45
Q

If noise is the same, what can you say about the SNR ratio between the corner of the room and the back of the room?

A

SNR(centre) / SNR(corner) = Prx(centre) / Prx(corner)

46
Q

What is the formula for transmit power, in terms of minimum received power?

A

Pt = (Pr . Ar)/Ac

Where Ar = coverage area
Ac = collection area

47
Q

How do you convert dBm to watts?

A

(W) = (1W x 10^(P(dBm)/10))/1000

48
Q

When does the transmit power = the total received power?

A

When there is no loss

49
Q

A typical SNR > 15.6 will lead to a bit error rate of what?

A

<10^(-6)

50
Q

What is the formula to calculate the wavelength of the fourth wave in four wave mixing?

A

w4 = w1 + w2 - w3

51
Q

In four wave mixing, when does a channel cause interference?

A

When their wavelength is equal to one of the individual components.

52
Q

What is the acronym to remember a functional system diagram?

A

DMLLO C OPTD C

53
Q

How do you calculate signal to noise ratio (SNR)?

A

SNR = Pr/Pn

Pr = received power
Pn = noise power
54
Q

What are the three main functional blocks of a system block diagram?

A
  • Transmitter
  • Channel
  • Reciever
55
Q

In a system block diagram, what blocks are contained within the transmitter block?

A
  • Data source
  • Modulation
  • LED/LD driver DMLLO
  • LED/LD emitter
  • Optics
56
Q

In a system block diagram, what blocks are contained within the channel block?

A

Channel

57
Q

In a system block diagram, what blocks are contained within the receiver block?

A
  • Optics
  • photodetector
  • Transimpedance amplifier OPTDC
  • Demodulation
  • Clock and data recovery
58
Q

In a system block diagram, what is the purpose of the data source block?

A

Send the data signal

59
Q

In a system block diagram, what is the purpose of the modulation block?

A

To modulate the data to a specific format.

60
Q

In a system block diagram, what is the purpose of the LED/LD driver block?

A

To provide sufficient current to the LED/LD

61
Q

In a system block diagram, what is the purpose of the LED/LD emitter block?

A

The light source

62
Q

In a system block diagram, what is the purpose of the optical lens block?

A

Lens to shape the beam profile.

63
Q

In a system block diagram, what is the purpose of the channel block?

A

Medium the light propagates through

64
Q

In a system block diagram, what is the purpose of the 2nd optical filter block?

A

To limit the ambient light and increase collection area.

65
Q

In a system block diagram, what is the purpose of the photodetector block?

A

To convert the light power to electrical photocurrent.

66
Q

In a system block diagram, what is the purpose of the transimpedance amplifier block?

A

To convert the electrical photocurrent to voltage level.

67
Q

In a system block diagram, what is the purpose of the demodulation block?

A

To demodulate the signal.

68
Q

What is a Lambertian source?

A

An optical source that obeys Lambert’s cosine law: it has an intensity directly proportional to the cosine of the angle from which it is viewed.

69
Q

What is cutoff wavelength?

A

Singlemode operation only takes place above a theoretical cutoff wavelength.

70
Q

How do you calculate geometry path loss due to beam divergence?

A

receiver collection area / beam coverage area

Optical communications (1 decks)

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