Question

Write notes on the following:
(a) Fax
(b) Optical detector
(c) Diode Laser
(d) Difference between amplitude modulation and frequency modulation.

Answer 1

(a) Fax: The electronic reproduction of a document at a distant place is known as facsimile telegraphy or fax. The original written document is converted into transmittable codes at the sending code. These codes are converted back into a copy of the original document at the receiving end. To send the document through fax, the following functions are performed.
(i) Optical scanning.
(ii) Conversion of data for transmission and reception.
(iii) Printing a copy of the image of the original document at the receiving end.
(b) Optical Detector: An optical detector is a device which separates the information signal and the optical carrier waves from the optical modulated signal reaching at the receiver. A photo diode is used for this purpose.
A photo diode is a $P-N$ junction diode made up of a light sensitive semiconductor. It is kept in reverse bias. The saturated current obtained in the reverse bias is called OFF current. Now light of frequency $v$ is made incident on it, such that the energy of a photon of light $hv$ must be greater than the forbidden energy $E_g$ between the valence band and conduction band of the semiconductor. As a result, the electron moves from the valence band to the conduction band and hence a hole is created in the valence band. Thus due to production of electron-hole pair, the value of saturated current flowing through the photo diode increases. It is called ON current. Since the reverse saturated current is directly proportional to the intensity of incident light, so it separates the information signal.
(c) Diode Laser: These are very convenient devices. These lasers have $p-n$ junctions made of special semiconducting material known as gallium arsenide $(Ga-As)$.
It has the following main parts:
(i) Working material: It is a $P-N$ junction semiconductor made by doping the impurities of $In-as$, $In-P$ or $In-Sb$.
(ii) Resonant cavity: The two opposite faces of semiconductor crystal behave like the resonant cavity.
(iii) Pumping arrangement: For this, $P-N$ junction semiconductor is kept in forward bias by means of a battery.
(d) Difference between Amplitude modulation and Frequency modulation:

S.No.	Amplitude modulation	Frequency modulation
(i)	In this modulation amplitude of carrier wave is changed in accordance with basic voice frequency of modulating signal.	Frequency of carrier wave is changed in accordance with basic voice frequency of modulating signal.
(ii)	The process of amplitude modulation produces two side band frequencies corresponding to original carrier frequency.	FM wave consists of a carrier and an infinite number of pair of side bands.
(iii)	Amplitude modulation is more likely to suffer from noise.	FM is free from effects of noise.
(iv)	Apparatus used in AM are simple and cheaper.	Apparatus are complex and costly.
(v)	AM broadcast operates in LW, ?MW and SW.	FM broadcast operates in very high frequency (VHF) and ultrahigh frequency (UHF).