4. PHOTODIODE CONTINUED……..
A semiconductor p-n junction device whose region of operation is
limited to reverse bias region
The application of light to the junction results in a transfer of energy
from the incidence travelling light waves to atomic structure, resulting
in an increased level of reverse current
Produces photocurrent by generating electron-hole pairs, due to the
absorption of light in the intrinsic or depletion region.
Photocurrent generated is proportional to the absorbed light intensity
7. OPTICAL CAVITY
• OPTICAL CAVITY/RESONATING CAVITY/OPTICAL
RESONATOR
• An arrangement of mirrors that forms a standing wave cavity
resonator for light waves
• Light confined in the cavity reflects multiple times producing standing
waves for certain resonance frequencies
• A beam will reflect a very large number of times with little attenuation.
• Provides positive feedback of photons by reflection at mirrors at either
end of cavity
8. CONTINUED……
• Optical cavities are designed to have a large Q factor
• Most common types of optical cavities consist of two facing plane (flat)
or spherical mirrors.
• Simplest is the plane-parallel or Fabry–Pérot cavity, consisting of two
opposing flat mirrors. While simple, this arrangement is rarely used in
large-scale lasers due the difficulty of alignment
Optical cavities are a major component of lasers, surrounding
the gain medium and providing feedback of the laser light.
9. Types of two-mirror optical cavities,
with mirrors of various curvatures,
showing the radiation pattern inside
each cavity.
10. Resonance condition
• The standing waves exist only at the frequencies for which the
distance between the mirrors is an Integral number of half of
wavelength .
𝐿 =
λq
2n
Where: L= length between mirror
n refractive index
q=number of modes
11. Values which satisfy the inequality correspond to stable resonators.
0 ≤ 1 −
𝐿
𝑅1
1 −
𝐿
𝑅2
≤ 1
L=LENGTH OF MIRROR
R1&R2 = CARVERTURE OF
MIRRORS
Only certain ranges of values
for R1, R2, and L produce stable
resonators in which periodic
refocussing of the intracavity
beam is produced.
STABILITY OF OPTICAL CAVITY
A stable output is obtained when
optical gain is exactly matched by
the losses incurred in the amplifying
medium
Major losses: absorption and
scattering in the amplifying medium
& at the mirror and Non useful
transmission through the mirror
12. WHY MICROCAVITY IN PHOTODIODE?
Progression towards higher transmission rates in optical
communication requires high speed photo diode AS A DETECTOR
For high quantum efficiency we need thick absorption region
and for larger bandwidth we need thin absorption region
Bandwidth is important phenomenon that can not be compromised
Thin absorption region with optical cavity increases both quantum
efficiency & bandwidth
In this diode a thin absorption region is placed in the middle of
resonant cavity formed by heavily doped wider band gap regions
and reflecting mirrors
13. CONTINUED….
A resonance is built up in the p-GsAs cavity at those frequency
components of the incoming light
At the resonance frequency the incoming frequency is reflected
at the two mirrors and round trip path is greatly increased
The absorption and the quantum efficiency are therefore
enhanced
This way microcavity photodiode simultaneously achieves both
large bandwidth and high possible quantum efficiency
15. DESIGN AND MATERIAL
REQUIREMENT
Mirror and the cavity materials must be non-absorbing at the detection
wavelength
The mirror should have very high reflectivity so that it gives highest
optical confinement inside the cavity
The absorption in the cavity can be limited by making the band gap of
the active region smaller than the cavity and the mirror. But a large
difference in the band gap would be a blockage in extraction of photo
generated carriers from a hetero junction
8
18. APPLICATION OF MICROCAVITY
PHOTODIODE
As a detector in optical fiber communication
Used to count items on conveyor belt
Optical communication devices
Position sensors
Bar code scanners
Automotive devices
Surveying instruments
20. • Abstract—We present a novel low–cost and low–power MEMS
gas sensor concept based on an ultrasonic resonance cavity.
The sensor consists of a capacitive micromachined ultrasonic
transducer (CMUT) embedded to an acoustic resonance cavity.
The sensor operation was demonstrated with carbon dioxide CO2
and methane CH4, the lowest resolvable concentrations are about
10 - 20 ppm – a competitive result with the existing commercially
available CO2 sensors. In addition, the sensor is able to measure
gas concentration and humidity independently, and thus can be
used as a combo sensor for gas concentrations and humidity.
