Lasers Flashcards
How do you calculate the frequency range, Δν, from a corresponding range of wavelengths Δλ?
Differentiate ν = c/λ with appropriate approximations, to get Δν = c * Δλ / λ^2
What are the two types of emission in lasers?
Spontaneous and stimulated.
What does LASER stand for?
Light Amplification by the Stimulated Emission of Radiation.
Which type of emission produces coherent light?
Stimulated.
How does stimulated emission occur?
A passing photon triggers the emission of an exact copy when an excited atom matches the energy of the photon - same phases & direction
Name the parts of a laser
- power supply
- gain material
- resonator mirrors
What is the simplest model of laser system? What is its drawback?
Two-level system, cannot achieve population inversion.
Equation for the rate of absorption?
dN1/dt = - B * ρ(ν)* N1 B = probability a photon will cause a 1 -> 2 transition ρ(ν) = photon density at frequency ν
Equations for the rates of emission?
Spontaneous: dN2/dt = - A(21)N2
Stimulated: dN1/dt = - B * ρ(ν) * N2
Total emission = sum of spontaneous + stimulated
What are the Einstein equations?
Equations for the emission and absorption rates in lasers
What are the Einstein coefficients?
The coefficients B12, B21, and A21 in the Einstein equations.
Equation for the ratio R of stimulated and spontaneous emissions?
R = e^(hν/k{B}T) - 1
What is population inversion?
When N2 > N1`
Which transition in a three level system is the fast one?
N2 -> N3
Equation for the maximum efficiency of a three-level laser?
η{max} = λ[absorbed] / λ[emitted]
What is the equation for the intensity of a signal going through a gain medium?
I = I{o} * e^(α*x)
What is the laser threshold and its equation?
The laser threshold is the condition where I2 = I1 through the gain medium - there is no amplification. Occurs when R1 * R2 * e^(2αL) = 1
What are the limitations of a three-level system?
- Laser transition ends in ground state
- Ground state is well populated
- Lasing transition can undergo absorption
- Emitted photons lost to re-absorption
Name the transition sequence of an atom in a four level system
N1 -> absorption -> N2 -> non-radiative transmission -> N3 -> emission -> N4 -> fast non-radiative transmission -> N1
What is a doped gain material?
A crystalline/glass matrix with a dopant that provides it with excitable electrons, denoted dopant:matrix
What is the resonance condition?
When the wave in the laser is a standing wave, at m * λ{m} = 2 * L{opt}
What is the optical length of a standing wave cavity?
L{opt} = n * L
n = refractive length
L = physical length of resonatorThe spacing between two adjacent resonator modes is…
… the free spectral range, delta nu. Given by: Δν= c / 2*L{opt}
What types of gain materials exist?
- solid state gain media
- liquid gain media (dye lasers)
- gaseous gain media
- semiconductor gain media
