lasers 01
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SETAM (OFC / 7th Semester) 1
Lasers (7th October 2014)
07-Oct-14
2
Contents
• Resonant Frequencies• Lasing Conditions• Rate equations• Efficiency
07-Oct-14 SETAM (OFC / 7th Semester)
3
Resonant Frequencies (1/4)
• Forms a resonant cavity.• PI exists in the amplifying medium.
Radiation builds up and becomes established as standing waves between the mirrors.
• These standing waves exists only at frequencies for which the distance between the mirror is an integral number of half wavelengths.
07-Oct-14 SETAM (OFC / 7th Semester)
4
Resonant Frequencies (2/4)
• The resonant condition along the axis of the cavity is: L=λq/2n -------- (1)
(λ:emission wavelength, n: RI of the amplifying medium, q:integer, L: spacing between the mirrors)
• Discrete emission frequencies ‘f’ defined by f = qc/2nL
-------- (2)
(The frequencies represents longitudinal modes or axial modes as L is along the longitudinal axis of the structure (Fig: 1))
c: velocity of light
q: determines different oscillation frequencies and each constitutes a resonance or mode.
• The modes are separated by δf=(c/2nL) -------- (3)
07-Oct-14 SETAM (OFC / 7th Semester)
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Resonant Frequencies (3/4)
• The mode separation in terms of free space wavelength assuming δf << f is: δλ = λδf/f = λ2/c δf -------- (4)
• δλ = λ2/2nL (after subs. 3 into 4) -------- (5)
07-Oct-14 SETAM (OFC / 7th Semester)
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Resonant Frequencies (4/4)
• Large number of modes generated (Fig. a).
• The spectral output defined by the gain curve (Fig. b).
• Laser emission includes only the longitudinal modes contained within the spectral width of the gain curve.
07-Oct-14 SETAM (OFC / 7th Semester)
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Threshold condition for laser oscillation (Lasing conditions)
• Please refer notes
07-Oct-14 SETAM (OFC / 7th Semester)
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Rate equations
• Describes the behavior of the semiconductor laser for electrons and photon density in the active layer of the device.
• Gives an understanding of laser electrical and optical performance characteristics.
• Represents an average behavior for the active medium within the laser cavity.
• Used to study both the steady and transient state behavior of the semiconductor laser (more concerned with the steady state)
• Please refer notes (for the remaining)
07-Oct-14 SETAM (OFC / 7th Semester)
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Efficiency
• Defines the operational efficiency of the semiconductor laser.
• Can be defined: Differential external quantum efficiency.• Ratio of the increase in photon output rate for a given
increase in the no. of injected electrons. • Gives a measure of the rate of change of the optical
output power with current and hence, the slope of the output characteristics (Slope quantum efficiency)
• Please refer notes (for the remaining)
07-Oct-14 SETAM (OFC / 7th Semester)
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