Q.No:1 CSIR June-2015
He-Ne laser operates by using two energy levels of Ne separated by 2.26 eV. Under steady state conditions of optical pumping, the equivalent temperature of the system at which the ratio of the number of atoms in the upper state to that in the lower state will be 1/20, is approximately (the Boltzmann constant \(k_{B} = 8.6 \times 10^{-5}eV/K)\)
(1)
\(10^{10}\) K
(2)
\(10^{8}\) K
(3)
\(10^{6}\) K
(4)
\(10^{4}\) K
Check Answer
Option 4
Q.No:2 CSIR Dec-2015
For a two level system, the population of atoms in the upper and lower levels are \(3\times 10^{18}\) and \(0.7\times 10^{18}\), respectively. If the coefficient of stimulated emission is \(3.0\times 10^5 \text{ m}^3/\text{W-s}^3\) and the energy density is \(9.0 \text{ J}/\text{m}^3\text{-Hz}\), the rate of stimulated emission will be
(1)
\(6.3\times 10^{16} \text{ s}^{-1}\)
(2)
\(4.1\times 10^{16} \text{ s}^{-1}\)
(3)
\(2.7\times 10^{16} \text{ s}^{-1}\)
(4)
\(1.8\times 10^{16} \text{ s}^{-1}\)
Check Answer
Option 3
Q.No:3 CSIR June-2016
The separation between the energy levels of a two-level atom is \(2 eV\). Suppose that \(4\times 10^{20}\) atoms are in the ground state and \(7\times 10^{20}\) atoms are pumped into the excited state just before lasing starts. How much energy will be released in a single laser pulse?
(1)
\(24.6 J\)
(2)
\(98 J\)
(3)
\(22.4 J\)
(4)
\(48 J\)
Check Answer
Option 4
Q.No:4 CSIR Dec-2016
A two level system in a thermal (black body) environment can decay from the excited state by both spontaneous and thermally stimulated emission. At room temperature (\(300 K\)), the frequency below which thermal emission dominates over spontaneous emission is nearest to
(1)
\(10^{13} Hz\)
(2)
\(10^{8} Hz\)
(3)
\(10^{5} Hz\)
(4)
\(10^{11} Hz\)
Check Answer
Option 1
Q.No:5 CSIR June-2017
If the coefficient of stimulated emission for a particular transition is \(2.1\times 10^{19} m^3 W^{-1} s^{-3}\) and the emitted photon is at wavelength \(3000\) Angstrom, then the lifetime of the excited state is approximately
(1)
\(20 ns\)
(2)
\(40 ns\)
(3)
\(80 ns\)
(4)
\(100 ns\)
Check Answer
Option 3
Q.No:6 CSIR June-2018
The electronic energy level diagram of a molecule is shown in the following figure.
Let \(\Gamma_{ij}\) denote the decay rate for a transition from the level \(i\) to \(j\). The molecules are optically pumped from level 1 to 2. For the transition from level 3 to level 4 to be a lasing transition, the decay rates have to satisfy
(1)
\(\Gamma_{21}> \Gamma_{23}> \Gamma_{41}> \Gamma_{34}\)
(2)
\(\Gamma_{21}> \Gamma_{41}> \Gamma_{23}> \Gamma_{34}\)
(3)
\(\Gamma_{41}> \Gamma_{23}> \Gamma_{21}> \Gamma_{34}\)
(4)
\(\Gamma_{41}> \Gamma_{21}> \Gamma_{34}> \Gamma_{23}\)
Check Answer
Option
Q.No:8 CSIR Dec-2018
The volume of an optical cavity is \(1 cm^3\). The number of modes it can support within a bandwidth of \(0.1 nm\), centred at \(\lambda=500 nm\), is of the order of
(1)
\(10^3\)
(2)
\(10^5\)
(3)
\(10^{10}\)
(4)
\(10^7\)
Check Answer
Option 3
Q.No:9 CSIR June-2019
The cavity of a \({He}-{Ne}\) laser emitting at \(632.8 nm\), consists of two mirrors separate by a distance of \(35 cm\). If the oscillations in the laser cavity occur at frequencies within the gain bandwidth of \(1.3 GHz\), the number of longitudinal modes allowed in the cavity is
(1)
\(1\)
(2)
\(2\)
(3)
\(3\)
(4)
\(4\)
Check Answer
Option 4
Q.No:10 Assam CSIR Dec-2019
An atom of mass \(m kg\) is in a laser field of wavelength \(800 nm\). The Einstein \(A\) coefficient of the atom is \(2\times 10^7 s^{-1}\). The natural line width at low pressure is equal to the Doppler line width when the temperature is \(\alpha m/k_B\), where \(\alpha\) is
(1)
\(\frac{32}{\ln{2}} m^2 s^{-2}\)
(2)
\(\frac{8}{\pi^2 \ln{2}} m^2 s^{-2}\)
(3)
\(\frac{16}{\ln{2}} m^2 s^{-2}\)
(4)
\(\frac{4}{\pi^2 \ln{2}} m^2 s^{-2}\)
Check Answer
Option 1
Q.No:11 CSIR June-2020
The energies of the \(3\) lowest states of an atom are \(E_0=-14 eV, E_1=-9 eV\) and \(E_2=-7 eV\). The Einstein coefficients are \(A_{10}=3\times 10^{8} s^{-1}, A_{20}=1.2\times 10^{8} s^{-1}\) and \(A_{21}=8\times 10^{7} s^{-1}\). If a large number of atoms are in the energy level \(E_2\), the mean radiative lifetime of this excited state is
(a)
\(8.3\times 10^{-9} s\)
(b)
\(1\times 10^{-8} s\)
(c)
\(0.5\times 10^{-8} s\)
(d)
\(1.2\times 10^{-8} s\)
Check Answer
Option c
Q.No:1 GATE-2021
To sustain lasing action in a three-level laser as shown in the figure, necessary condition(s) is(are)
(A)
lifetime of the energy level 1 should be greater than that of energy level 2
(B)
population of the particles in level 1 should be greater than that of level 0
(C)
lifetime of the energy level 2 should be greater than that of energy level 0
(D)
population of the particles in level 2 should be greater than that of level 1
Check Answer
Option A_B
Q.No:2 GATE-2021
Consider the atomic system as shown in the figure, where the Einstein \(A\) coefficients for spontaneous emission for the levels are \(A_{2\to 1}=2\times 10^7 s^{-1}\) and \(A_{1\to 0}=10^8 s^{-1}\). If \(10^{14} atoms/cm^3\) are excited from level 0 to level 2 and a steady state population in level 2 is achieved, then the steady state population at level 1 will be \(x\times 10^{13} cm^{-3}\). The value of \(x\) (in integer) is __________.
Check Answer
Ans 2
Q.No:3 GATE-2022
Frequency bandwidth \(\Delta \nu\) of a gas laser of frequency \(\nu \hspace{1mm}\text{Hz}\) is
\[
\Delta \nu=\frac{2\nu}{c}\sqrt{\frac{\alpha}{A}}
\]
where \(\alpha=3.44\times 10^6\hspace{1mm}\text{m}^2\text{s}^{-2}\) at room temperature and \(A\) is the atomic mass of the lasing atom. For \(ce{^{4} He}\)-\(ce{^{20} Ne}\) laser (wavelength \(=633 \hspace{1mm}\text{nm}\)), \(\Delta \nu=n\times 10^9 \hspace{1mm}\text{Hz}\). The value of \(n\) is ------------ (Round off to one decimal place)
Check Answer
Ans 1.2-1.4
Q.No:1 JEST-2015
Which of the following excited states of a hydrogen atom has the highest lifetime?
(a)
\(2p\)
(b)
\(2s\)
(c)
\(3s\)
(d)
\(3p\)
Check Answer
Option c
Q.No:2 JEST-2020
A continuous \({He}-{Ne}\) laser beam (\(\lambda=632.8 nm\)) is 'chopped', using a spinning aperture, into \(1 \mu s\) square pulses. The order-of-magnitude estimate of the spectral width \(\Delta \lambda\) of the emerging 'pulsed' light is
(A)
\(10^{-9} m\)
(B)
\(10^{-12} m\)
(C)
\(10^{-15} m\)
(D)
\(10^{-18} m\)
Check Answer
Option C
Q.No:1 TIFR-2014
A beam of atoms moving in a certain direction can be slowed down if they absorb photons from a laser beam moving in the opposite direction and subsequently spontaneously emit photons isotropically. For a beam of Sodium atoms (mass number \(A=23\)) with speed \(600 ms^{-1}\), if a laser beam of wavelength \(589 nm\) is used, the number of such absorption and emission cycles needed to bring a Sodium atom to rest would be approximately
(a)
\(1.3\times 10^5\)
(b)
\(1.3\times 10^4\)
(c)
\(2.1\times 10^3\)
(d)
\(2.1\times 10^4\)
Check Answer
Option d
Q.No:1 TIFR-2014
A continuous monochromatic (\(\lambda=600 nm\)) laser beam is chopped into \(0.1 ns\) pulses using some sort of shutter. Find the resultant linewidth \(\Delta \lambda\) of the beam in units of \(10^{-3} nm\).