EXAM

Question 1

Poisson equation

Answer 1

P(r)=N^r/r! e^-N

Question 2

N in the poisson equation

Answer 2

Describes the expectation value, or the average number of events one can expect to occur in a given timeframe

Question 3

P(r)

Answer 3

is the probability that an event occurs r times

Question 4

what events does poisson distribution describe

Answer 4

Poisson distribution describes discrete and uncorrelated events

Question 5

standard distribution of poisson

Answer 5

The standard deviation of poisson distribution is given by sqrt(N)

Question 6

what is Poisson mainly used for

Answer 6

used to describe rare events

Question 7

poisson and Gauss

Answer 7

for a large number of events the poisson and gauss distribution look very similar

Question 8

shot noise and Poisson

Answer 8

Shot noise is a type of noise which originates from discrete nature of events such as electric charge, particles occurring in radioactive decay or photon counting in optical devices, due to particle nature of light. Shot noise is dominate for rare events hence is well described by the poisson distribution.

Question 9

How silicon pn junction acts as a photodetector by converting photons into electron-hole pairs

Answer 9

Incident light is absorbed by the semiconductor as long as the incoming photon energy is higher than the bandgap energy of the semiconductor. The absorbed photon energy promotes electrons from the valence band to the conduction band. THe in-build-field of the diode separates the electrons from the holes.

Question 10

NEP (noise equivalent power)

Answer 10

NEP describes the noise power spectral density of a photodetector and relates to the noise power
p_noise = NEP x sqrt(B)

With B the bandwidth of the detecro

Question 11

Detectivity D*

Answer 11

Detectivity is the inverse of the NEP normalised to the area of the detector,
D* = sqrt(A_det)/NEP

Question 12

White noise

Answer 12

White noise refers to the fact that noise is independent of frequency. This means that each frequency component contributes equally to the noise power generated. by the detector.

Question 13

why noise current of a photodetector depends on the sqrt of the bandwidth

Answer 13

As noise power scales linearly with bandwidth and since current is proportional to the sqrt of the power, the photocurrent noise scales as the sqrt of the bandwidth

Question 14

How the cut off frequency of a photodector depends on the reverse voltage

Answer 14

A photodetector can be understood as a capacitor. The doped n and p regions represent the two capacitors plates and the depletino region represents the spacer. THe frequency is then given by the RC time constant of the device. As the fromula shows and increase in Vext will increase . As d is increased te capacitance drops according to C=e0erA/d as C decreases the RC time constant becomes shorter so the bandwidth increases.

Question 15

sampling theroem

Answer 15

if a continuous function contains no frequencies higher than fmax it is completely determined by its value at a series of points taken less than 1/2fmax apart

Question 16

nature of alpha particle and charge state

Answer 16

Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus. positively charged

Question 17

nature of beta particles and charge state

Answer 17

Beta particles are high-energy speed electrons or positrons. Beta particles are also cahrged eiether positively (positron) or negatively (electrons)

Question 18

gamma particles

Answer 18

Gamma particles are high energy photons so they are not charged.

Question 19

Neutrons

Answer 19

Are one of the fundamental building blocks of the atomic nucleus, together with protons. Neutrons are not charged.

Question 20

Quantum efficiency

Answer 20

Describes the efficiency of the detector of converting photons into electron hole pairs.

Question 21

Responsivity

Answer 21

The responsivity measures the ability of the detector to convert an optical power into a photocurrent

Question 22

difference between responsivity and quantum efficiency

Answer 22

The difference is that the responsivity takes the erngy of the photon into account while the quantum efficiency does not. Therefore the maximum value of the quantum efficiency is unity or 100% (1 photon in =1e-h pair out) while the responsivity can assume any positve value up to infinity

Question 23

Transducer

Answer 23

converts energy from one form into another

Question 24

examples of transducers

Answer 24

Scales measure weight via the compression of a spring. Spring is a transducer

Question 25

signal

Answer 25

combined multiple measurements in time to get a sequence of measurments

Question 26

noise

Answer 26

additional unwanted signal

Question 27

SNR

Answer 27

signal to noise ratio used to describe the quality of a signal

Question 28

Noise in digital siganl

Answer 28

Noise is less of a problem in digital signal. Noise usually disturbs the amplitude. In a digital signal the information is not contained in the amplitude

Question 29

sigma in gauss equation

Answer 29

describes the width of the curve

Question 30

mu in gauss equation

Answer 30

expectation value

Question 31

Gaussion function

Answer 31

describes the likelyhood of a particular argument

Question 32

shot noise

Answer 32

type of electronic noise which can be modeled by a poisson process.

Question 33

shot noise caused by

Answer 33

enegetic particles not emitting in regular intervals.

Question 34

key property of poisson

Answer 34

sigma =sqrt(N)

Question 35

noise and signal strength

Answer 35

As a signal get’s stronger the noise gets weaker

Question 36

Signal power proportional to

Answer 36

to signal amplitude squared

Question 37

Bandwidth

Answer 37

describes the maximum data transfer rate of a network or internet connection.

Question 38

Bandwidth equation

Answer 38

B= I / delT

Question 39

colour of shot noise

Answer 39

Shot noise is white

Question 40

shot noise can be mitigated how>

Answer 40

by increasing the signal strength, as the SNR is proportional to sqrt(signal) or by decreasing the dector bandwidth as shot noise power like any white noisesscales as the bandwidth

Question 41

Thermal noise

Answer 41

Arises from the presence of thermal energy in the detecor e.g. brownian motion of the electrons. some electrons thereby assume sufficient energy to mimic a detection event. THe spectral density of thermal noise is flat. i.e. thermal nnoise is white TO mitigate decrease bandwidth or cool down

Question 42

1/f noise

Answer 42

also called, flicker or pink noise. occurs in many physical, biological and economic systems. It does not have a single origin. A key characteristic is that there is equal energy in all octaves. Since 1/f noise drops as the bandwidth, it is best mitigated by decreaseing the bandwidth of the detector.

Question 43

Lock in detection

Answer 43

improves the SNR by modulating the signal at a particular frequency and using a bandpass filter that coincides with the lock in frequency. The spectral width of the bandpass filter is given by the bandwidth of the signal to. be measured.

Question 44

three ways lock in detection reduces noise

Answer 44

1) the bandpass filter reduces Johnson noise, which is proportional to the detected bandwidth.
2) The frequency offset provided by the modulation does reduce the 1/f noise, as it shifts the detection to higher frequencies, where the 1/f noise is lower
3) Background noise sources are not modulated and are therefore eliminated from the measurement. 50/100Hz noise arising from the mains and background light is a particular problem in this respect, and lock in detection, for example allow for the use of daylight in laboratories.

Question 45

frequency filter that uses RC

Answer 45

Highpass and lowpass filters are voltage dividers that use the fact that impedance of a capacitor depends on frequency RC=1/omegaC.

For low frequencies Rc>R so most or all of the voltage drops across the capacitor while for high frequncies the impedance of the capacitor is low so the voltage drops across the resistor.