OPTICAL AND MICROWAVE LAB VIVA Questions :-
1. What is a fiber optic?
Fiber optics are long lenses. A cylinder or rod of transparent material forming a core surrounded by an external cladding with a slightly different material. Light, when entering the fiber, rebounds on the outer cladding towards the core. This way the light advances through the fiber in bounds or steps, until it exits at the other end.
2. Define Attenuation loss?
Attenuation loss (or path propagation loss) is the reduction in power density (attenuation) of an electromagnetic wave as it propagates through space.
3. Define Bending loss?
Attenuation occurring as a result of either a bend in an optical fibre that exceeds the minimum bend radius or an abrupt discontinuity in the core/cladding interface is called bending loss.
4. What are the sources used in optical communication?
LED’s and LASER diodes are the commonly used sources in optical communication systems, whether the system transmits digital or analog signal. It is therefore, often necessary to use linear Electrical to Optical converter to allow its use in intensity modulation & high quality analog transmission systems.
5. .SONET LAYER
- Path layer
- Line layer
- section layer,
- Physical layer
6. Define critical angle.
It is minimum angle angle of incident beyond which the total internal reflection occurs for light travelling from a medium of higher to one of lower refractive index is called critical angle.
7. Define TIR.
At angles of incident greater than the critical angle the light is reflected back into the originating dielectric medium, which is known as Total internal reflection.
8. Give a reason behind n2 > n1.
During manufacturing of fiber, certain impurities (dopants) are introduced in the core and cladding, so that the refractive index is slightly higher in core than cladding.
9. What are leaky modes in optical fiber?
A leaky mode or tunneling mode in an optical fiber or other waveguide is a mode having electric field that decays monotonically for a finite distance in the transverse direction but becomes oscillatory everywhere beyond that finite distance.
10. What are degenerated modes in optical fiber?
In weakly guiding fibers ∆ is small then HE-EH mode pair occurs which have almost identical propagation constants and electromagnetic patterns such mode are called degenerate modes.
11. Define MFD.
MFD ( mode field diameter) determines the fundamental mode by the radial extend of its electromagnetic field including losses at launching and joining, microbend losses, wave guide dispersion and width of radiation pattern.
12. What are the mechanisms of losses occurs in optical fiber?
- Absorption (fiber material)
- Scattering ( fiber material + structural imperfection)
- Radiative effects
- Pulse broadening.
13. What is macro bend and micro bend losses?
Macrobend losses occurs when radius of curvature that all large compared with fiber diameter Microbend losses are small scale fluctuation in radius of curvature of fiber axis.
14. Define Lambertian pattern.
Isotropic pattern from a surface emitter is called a lambertian pattern. In this pattern source is equally bright when viewed from any direction, but power diminishes as cosө.
15. Define modal or speckle noise.
The speckle pattern observed in multimode fiber as fluctuation which have characteristics times longer then the resolution time of the detector is know as modal or speckle noise.
16. What are the essential components of laser?
- Amplifying medium
- An optical feedback
- Pumping source.
17. What are demountable connectors?
These are the removable joints which allow easy, fast manual coupling and uncoupling of fiber analogous to electrical plugs and sockets.
18. Define polarization.
Polarization refers to the electric field orientation of a light signal, which can vary significantly along the length of a fiber polarization state is fundamental property off an optical signal.
19. What are the possible noises occur in optical receiver?
- Photo detector noise
- Quantum noise
- Dark current noise ( bulk dark current, surface dark current )
- Amplifier noise
- Thermal noise or Johnson noise
20. Define quantum limit.
It is possible to find the minimum received optical power required for a specific bit error rate performance in a digital system. This minimum received power level is known as quantum limit.
21. LED
In optical fiber communication system electrical signal is first converted into optical signal with the help of electrical to optical conversion device as LED
22. .PHOTO DETECTOR
Optical signal is transmitted through optical fiber it is retrieved in its original electrical form with the help of optical to electrical conversion device such as photo detector
23. .NECESSITY OF CLADDING FOR AN OPTICAL FIBER
- Provide proper light guidance inside the core
- Avoid leakage of light from the fiber
24. .USES OF OPTICAL FIBER
- To transmit the information which are in the form of coded signals of the telephone communication, computer data
- To transmit the optical images(ex: endoscopy)
25. ACCEPTANCE ANGLE
The maximum angle ɸmax with which a ray of light can enter through the entrance end of the fiber and still be totally internally reflected
26. .MERIDIONAL RAYS
Rays following the zigzag path when they travel through fiber and for every reflection it will cross the fiber axis
27. SKEW RAYS
Rays following the helical path around the fiber axis when they travel through the fiber and they would not cross the fiber axis at any time
28. MIE SCATTERIN
The scattering created by such inhomogeneities is mainly in the forward direction
29. DISPERSION
Dispersion of the transmitted optical signal causes distortion for both digital and analog
transmission along optical fiber
30. DISPERSION FLATTENING
The reduction of fiber dispersion by spreading the dispersion minimum out over a widen rage this approach
31. CUTOFF WAVELENGTH OF FIBER
The cutoff wavelength is defined as the minimum value of wavelength that can be
transmitted through the fiber. The wavelength greater than the cutoff wavelength can be
transmitted
32. MENTION THE LOSSES RESPONSIBLE FOT ATTENUATION IN OPTICAL FIBERS
- Absorption
- scattering
- Bending
33. FIBER SPLICING
A Permanent joint formed between two individual optical fibers in the field or factory is known a fiber splicing
34. FIBER COUPLER
An optical fiber coupler is a device that distribute light from a main fiber into one or more branch fibers. To combine and split optical signals in an optical network a directional coupler is used
35. ADVANTAGES OF LED
Simpler fabrication
-Cost-The simpler construction of the LED leads to much reduced cost
-Reliability
36. .WHY DO WE PREFER LASER DIODE OVER LED’S FOR COMMUNICATION APPLICATIONS
- High intensity radiation
- Narrow spectral width of the laser source is the preferable feature of laser compared to LED
37. DARK CURRENT NOISE
The dark current noise arises from electrons/holes which are thermally generated in the PN junction of the photodiode. This current continues to flow through the bias circuit of the device when no light is incident on the photodiode
38. .REQUIREMENTS OF OPTICAL FIBER
- Light detector
- Equalizer
- Signal discriminator circuits
39. DEFINE THRESHOLD LEVEL
A Decision circuit compares the signal in each time slot with a certain references voltage
40. DEFINE BER
An approach is to divide the number .No .of. errors occurring over a certain time interval t by the number N of pulses transmitted during this interval.
41. ADVANTAGES OF PREAMPLIFIER
- Low noise level
- High bandwidth
- High gain
42. .DEFINE SONET/SDH
SONET is a set of standards defining the rates and formats for optical networks. A similar
standard, synchronous digital hierarchy (SDH) has also been established in Europe
43. DEFINE ANOMALIES
An anomaly is the smallest discrepancy that can be observed between the actual and desired characteristics of an item
44. ADVANTAGES OF SDH
- High transmission rate
- Reliability
- High availability and capacity
45. .BROADCAST AND SELECT NETWORK
A broadcast and select network is a second generation network in which each transmitter sends messages at a fixed frequency to the central star coupler through fiber path
46. .SINGLE HOP NETWORK
-All network nodes are connected to the central hub node
-Single hop WDM networks are based on central passive star coupler(PSC) or array
waveguide grating(AWG)
47. DEFINE LIGHT PATH
A connection between two end nodes whose traffic is switched only in the optical domain at intermediate nodes is referred to as a lightpath
48. DEFINE WDM
WDM is wavelength division multiplexing .The optical beam consists of different wavelength and several channel information is transmitted over a single channel
49. IMPORTANT PHOTO DETECTOR MATERIAL
- InGaAs
- InAlGaAs
50. VARIOUS ERROR SOURCES
- Quantum noise
- Bulk dark current noise
- surface leakage current noise
51. State Snell’s law.
It gives relationship between arrange of incident and refraction for a wave impinging on a interface between two different media with different refractive index.