lasers 01

SETAM (OFC / 7th Semester) 1

Lasers (7th October 2014)

07-Oct-14

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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.


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• 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)


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• 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)


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• 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.


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Threshold condition for laser oscillation (Lasing conditions)

• Please refer notes


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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)


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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)


lasers 01

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