Optical communication Chapter 2.1

Optical communicationsChapter 3.3Pham Quang ThaiPqthai.hcmut@gmail.com1ContentSemiconductor physicsLight emitting diode (LED)Laser diode

22Poor light quality of LEDSpontaneous emission: no control of emitted wavelength3

3Poor light quality of LEDSpontaneous emission: no control of phase and amplitude4

4Laser (Light Amplification by Stimulated Emission of Radiation)

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6Oscillator

7Loss -> Large output bandwidth

8Overcome loss -> Single frequency output

9Optical oscillator (optical cavity) -> photon moving between two mirrors

10Many frequencies satisfy the oscillating condition

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Video: laser longitude mode

12Absorption -> loss photon

13Stimulated emission -> create photon

14Stimulated emission -> Occur under Population inversion condition

152 energy levels -> Pump -> Transparency

163 energy levels -> pump -> population inversionVideo: light amplification

17Pump -> Population inversion -> Spontaneous emission

18Spontaneous emission -> Oscillation condition -> Stimulated emission -> LasingVideo: simple laserSemiconductor laser: pump using p-n junction

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Video: semiconductor laserP-I characteristic20

P-I characteristicRate equation

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Electron rate = injected electron - spontaneous recombination stimulated emission

Photon rate = Stimulated emission + spontaneous emission photon lossN: number of electronsI: bias currente: electron charged: active region volumesp: spontaneous recombination lifetimeC: absorption coefficient: number of photonsRsp: spontaneous emission rateph: photon lifitime

P-I characteristicAt lasing threshold: 22

: gain coefficient of cavity: loss coefficient of cavityL: cavity lengthR1, R2: mirror reflection coefficientP-I characteristic Above lasing threshold23

P-I characteristicTemperature dependent of Ith

2424P-I characteristicTemperature dependent

As temperature increases, Auger recombination increases exponentially -> N decreasesIth increases with temperature25

25Modulation characteristic

26Modulation characteristicModulation bandwidth for small signal27

Modulation characteristicModulation of large signal:28

Large change in bias current -> change in N-> change in n-> change in phase-> change in both amplitude and phase of output signal-> Chirping !Relative intensity noise (RIN)Cause by amplified spontaneous emission29

Examples for chapter 3.3Problem 4-12:A GaAs laser emitting at 800 nm has a 400-m cavity length with a refractive index n=3.6. If the gain g exceeds the total loss , throughout the range 750nm