Optical Transmitters and Receivers

Question 1

IPAD Q: Draw coherent and incoherent light

Answer 1

x

Question 2

Explain the Fabry-Perot LASER model iPAD q

Answer 2

X

Question 3

What are tunable lasers and why are they so useful?

Answer 3

1. Flexibility; one size fits all, reduces number of spare lasers needed in WDM
2. Reconfigurable; choose WL most suitable to network

Question 5

The idea is to change the resonant cavity. How can we do this?

Answer 5

1. Mechanically; physically increase the size of the cavity
2. Electronically; electrical current in the laser changes the value of the refractive index, and thus the electrical size.

Question 6

Explain signal modulation

Answer 6

Modulation is used to add information to a carrier signal

Question 7

Explain the difference between direct laser modulation and external modulation

Answer 7

1. Direct laser modulation; switching the power current on and off.
2. External modulation; obscuring intermittently the light generated by a laser

Question 8

Explain what happens at the receiver

Answer 8

At the end of the link, the optical signal is converted to electrical to be processed by network elements (switchers, routers) or by end-used equipment.

Question 9

Which element is used as a receiver

Answer 9

A photodetector

Question 10

Explain how the electron goes from valence to conduction band -> IPAD Q

Question 11

What happens in optical transmission based on direct detection

Answer 11

Only process the amplitude information of the signal

Question 12

A laser is:

A. A device that emits
coherent light

B. A resonant cavity
where a current can
be pumped to induce
population inversion

C. A device used for
cutting steel in heavy
industry

D. A telecommunication
device used to
transmit information

Answer 12

A, C, D

Question 13

How can receivers be wavelength dependent?

Answer 13

Different materials have different bandgap

Question 14

What are avalanche photodiodes?

Answer 14

Use large electric field to accelerate and generate a chain reaction, so that each photon can generate more than one electron/hole.

Question 15

What is the drawback of avalanche photodiodes?

Answer 15

The variance of the generated current also increases, creating additional noise.

Question 16

What does the receiver sensitivity tell us?

Answer 16

The receiver sensitivity indicated the minimum i/p optical power at the receiver necessary to achieve a certain Bit Error Rate

Question 17

Where are coherent systems common?

Answer 17

In core and metro network (more expensive than DD) with channels of 100Gb/s and above

Question 18

What are the advantages of coherent systems

Answer 18

1. Higher sensitivity compared to DD (LO provides gain to the received signal)
2. Phase of signal is known, DSP can be applied to counteract the dispersion of the channel.
3. LO is tuned to the receiver frequency, amplifying it -> operates as a filter
4. User of higher order modulation enables higher data rate for the same baud rate.
5. Spectral shaping: pre-processing to match the channel response in frequency

Question 19

An optical source needs to be modulation:

A. In order to transmit
information
B. In order to reduce
the transmission
laser linewidth
C. To increase its
propagation speed
D. In order to be
detected

Answer 19

A

Question 20

Where do you think was coherent
transmission technology used first
A. In access network
transmission
B. In metro network
transmission
C. In core network
transmission
D. It hasn’t been used
yet

Answer 20

C,D