Department of Physics (SET-I) is a pictorial representation of _________ ? 3rd Semester (2 Years M. Sc. in Physics) Examination-2022 (Regular Paper) Paper Name: QUANTUM ELECTRONICS (Code: PHY/711040) Full Marks: 60 Time: 2 Hrs. (a) Spontaneous emission (b) Spontaneous Absorption (c) Stimulated emission (d) INSTRUCTIONS: Stimulated Absorption 1. Question paper is divided into two Sections A and B. Attempt the 4. Which factors lead to short pulses from a 2 required number of questions as per the instruction mentioned in each passively Q-switched laser? Section. 2. Numbers showing in the right side indicating marks. (a) a high pump intensity (b) a large laser resonator (c) a small amount of saturable Section: A (Answer all questions) (Total 20 marks) absorption (d) a large amount of saturable absorption S. No Question Marks 5. Consider sum frequency generation with 2 wavelengths of 2 μm and 1 μm. The 1. Calculate the Q factor of a LASER with a 2 wavelength of the generated wave will be central wavelength of 450 nm and spectral (a) 0.666 μm (b) 3 μm (c) 1 μm (d) 0.333 FWHM of 1.5 GHz. μm −4 4 (a) 2. 249 × 10 (b) 2. 249 × 10 5 3 6. If three wave components co-propagate at 2 (c) 4. 446 × 10 (d) 1. 443 × 10 angular frequencies of ω1, ω2 & ω3, as a 2. Calculate the coherence length (𝐿𝑐) for a 2 result a new wave is being generated at an beam traveling in water (refractive index, angular frequency of ω4, which is given 𝑛 = 1. 33) for which the emission by? linewidth is 1500 MHz. (a) ω4 = ω1 − ω2 − ω3 (a) 0.07 cm (b) 7.0 μm (c) 15 cm (d) 15 μm
1 (b) ω4 = ω1 + ω2 + ω3 10. The induced index change has its 2 (c) ω4 = ω1 + ω2 − ω3 proportionality with respect to _________ in Kerr type effect? (d) ω4 = ω1 − ω2 + ω3 (a) square of electric field (b) cube of electric field (c) cube root of electric field 7. What is the higher order harmonics of 2 (d) one-fourth power of electric field order 5 of a waveform with fundamental frequency of 50Hz? (a) 10μHz (b) 10Hz (c) 200Hz (d) 250Hz
8. State which the following processes are 2
automatically phase matched? (a) Self phase modulation (b) Four wave Section: B (Answer any eight questions) (Total 40 marks) mixing (c) Second harmonic generation (d) Third harmonic generation S. No Question Marks
9. Is the following statement true: What 2
11. Calculate the ratio of the rates of 5 happens when a degenerate optical spontaneous and stimulated emission (R) parametric oscillator (OPO) is exactly the for the light emitted by an electric time-reversed version of resonant discharge in a gas such as Neon in the frequency doubled. Helium-Neon (He-Ne) LASER. If the (a) Yes, why not? discharge temperature is 370 K for the (b) No, these processes rely on different red-line produced by this laser, which has nonlinearities. 14 (c) No, these processes differ in terms of a frequency of ν = 4. 74 × 10 𝐻𝑧. phase matching. 12. Starting from a beam with the following 5 (d) No, as a time-reversed frequency frequency (ω) components doubler would have two input waves, −𝑖ω1𝑡 −𝑖ω2𝑡 whereas the OPO has only one. 𝐸(𝑡) = 𝐸1𝑒 + 𝐸2𝑒 + 𝑐. 𝑐, compute explicitly the second order
2 components of the nonlinear polarization 18. Derive the expression for non-degenerate 5 (2) 𝑃 (𝑡) = 𝜖0𝜒 𝐸(𝑡) (2) 2 and write down spontaneous parametric (frequency) down conversion and discuss the conditions for what each frequency component parametric amplification and attenuation. represents. 19. Write short notes on Sum Frequency 5 13. (a) In a dispersive medium the refractive 5 Generator (SFG) and Difference index varies with wavelength. We can Frequency Generator (DFG). define a group refractive index by the relation, 𝑛𝑔 = 𝑛 − λ 𝑑𝑛 , prove that 20. What is Chirped Pulse Amplification? 5 𝑑λ 𝑑𝑛 What is the core of the method that really 𝑛𝑔 = 𝑛 + ν 𝑑ν . makes it tick? What pre-existing (b) At what temperature would the problems did its introduction solve? stimulated and spontaneous emission rates be equal for particles in a cavity and a transition at a wavelength of 1 μm.
14. With the help of energy level diagrams, 5
explain the working of a vibronic LASER by Franck Condon principle.
15. Differentiate between spatial coherence 5
and temporal coherence.
16. What is the cause of natural broadening? 5
Find an expression for it using usual notations.
17. Find an expression for the ratio of output 5
and input electric field of a Febry-Perot resonator cavity.
