Optical Communication Set 4

Answer: nitrogen

Explaination:
an indirect band-gap semiconductor may be made into an electro- luminescent material by the addition of impurity centers which will convert it into a direct band-gap material. the introduction of nitrogen as an impurity into gallium phosphide makes it an effective emitter of light. such conversion is only achieved in materials where the direct and indirect band- gaps have a small energy difference.

Answer: heavy doping of both p-type and n-type material

Explaination:
population inversion at p-n junction is obtained by heavy doping of both p-type and n-type material. heavy p-type

Answer: 660 ma

Explaination:
the threshold current is denoted by ith. it is given by-

Answer: 0.32

Explaination:
the reflectivity for normal incidence of the plane wave on the gaas-air interface is given by-

Answer: false

Explaination:
the photo-emissive properties

Answer: two

Explaination:
hetero-junctions are classified into an isotype and an-isotype. the isotype hetero-junctions are also called as n-n or p-p junction. the an-isotype hetero-junctions are called as p-n junction with large band-gap energies.

Answer: gaas/alga as dh

Explaination:
for dh device fabrication, materials such as gaas, alga as are used. the band-gap in this material may be tailored to span the entire wavelength band by changing the alga composition. thus, gaas/ alga as dh system is used for fabrication of lasers and leds for shorter wavelength region (0.8μm-0.9μm).

Answer: low

Explaination:
planer leds are fabricated using liquid or vapor phase epitaxial processes. here p-type is diffused into n-type substrate which creates junction. forward current flow through junction provides lambertian spontaneous emission. thus, device emits light from all surfaces. however a limited amount of light escapes the structure due to total internal reflection thus providing low radiance.

Answer: higher, greater, reduced

Explaination:
in dome leds, the diameter of dome is selected so as to maximum the internal emission reaching surface within critical angle of gaas. thus, dome leds have high external power efficiency. the geometry of dome leds is such that dome is much larger than active recombination area, so it has greater emission era and reduced of radiance.

Answer: true

Explaination:
burros and dawson provided a technique to restrict emission to small active region within device thus providing high radiance. etched well in a gaas substrate is used to prevent heavy absorption of emitted region and physically accommodating the fiber. these structures provide low thermal impedance allowing high current densities of high radiance.

Answer: dh structures

Explaination:
dh structures provide high efficiency from electrical and optical confinement. along with efficiency, they provide less absorption of emitted radiation.

Answer: very low

Explaination:
the larger band gap confining layers and the reflection coefficient at the back crystal space is high in dh surface emitter burros type leds. this provides good forward radiance. thus these structure leds have very less internal absorption.

Answer: more coupled optical power

Explaination:
the optical power coupled into a fiber depends on distance, alignment between emission area and fiber, sled emission pattern and medium between emitting area and fiber. all these parameters if considered, reduces refractive index mismatch and increases external power efficiency thus providing more coupled optical power.

Answer: lost

Explaination:
optical power from an incoherent source is initially coupled into large angle rays falling within acceptance angle of fiber but have more energy than meridional rays. energy from these rays goes into the cladding and thus may be lost.

Answer: 0.03

Explaination:
optical power launched can be computed by

Answer: to reduce current spreading

Explaination:
the planar structures of burros-type led allow lateral current spreading specially for contact diameters less than 25 μm.this results in reduced current density and effective emission area greater than contact area. this technique to reduce current spreading in very small devices is mesa structured sleds.

Answer: length strip geometry

Explaination:
the short striped structure of these leds around 100 μmimproves the external efficiency of leds by reducing internal absorption of carriers. these are also called truncated strip e-leds.

Answer: zn

Explaination:
zn doping reduces the minority carrier lifetime. thus this improves the device modulation bandwidth hence active layer is doped in zn in e-leds.

Answer: optical source

Explaination:
an optical source is an active component in an optical fiber communication system. it converts electrical energy into optical energy and allows the light output to be efficiently coupled into the optical fiber.

Answer: three

Explaination:
three main types of optical light sources are available. these are wideband sources, monochromatic incoherent

Answer: stimulated emission

Explaination:
in stimulated emission, the photon produced is of the same energy to the one which cause it. hence, the light associated with stimulated photon is in phase and has same polarization. therefore, in contrast to spontaneous emission, coherent radiation is obtained. the coherent radiation phenomenon in laser provides amplification thereby making laser a better optical source than led.

Answer: 1.47×10-11

Explaination:
the ratio of the stimulated emission rate to the spontaneous emission rate is given by-

Answer: thermal equilibrium

Explaination:
under the conditions of thermal equilibrium, the lower energy level contains more atoms than the upper level. to achieve optical amplification, it is required to create a non-equilibrium distribution such that the population of upper energy level is more than

Answer: light amplification

Explaination:
laser emits coherent radiation of one or more discrete wavelength. lasers produce coherent light through a process called stimulated emission. light amplification is obtained through stimulated emission. continuation of this process creates avalanche multiplication.

Answer: 2.2×105

Explaination:
the number of longitudinal modes is given by-