Yunsu Sung Yunsu Sung Special Topics in Optical Engineering - PowerPoint PPT Presentation

Mar 12 2015 Yunsu Sung Yunsu Sung Special Topics in Optical Engineering II(15/1)

Contents • Two-port model • Rate equation and damping • Small signal response • Conclusion Yunsu Sung Special Topics in Optical Engineering II(15/1)

Two Port Model  I:Current  V:Voltage  P: Optical Power  ∆ν : Optical frequency shift • Model summarize parasitic effects and overall response • This model are valid in single frequency DFB lasers Yunsu Sung Special Topics in Optical Engineering II(15/1)

Two Port Model P: Optical Power Δν: Optical frequency shift • 3 sections of laser model – 1. package or mount parasitic • Bonding wire inductance, capacitance between input terminal – 2. semiconductor chip parasitic • parasitic capacitance, resistance with semiconductor material – 3. Intrinsic laser(active layer & cavity) Yunsu Sung Special Topics in Optical Engineering II(15/1)

Two Port Model • Signal response of semiconductor laser  p j ( )  IM  I ( j ) A    ( j )  FM  I ( j ) A • Parasitic: Lower high frequency of signal response • Intrinsic Laser: Resonance peak Yunsu Sung Special Topics in Optical Engineering II(15/1)

Parasitic • Chip cross section Yunsu Sung Special Topics in Optical Engineering II(15/1)

Parasitic • Circuit model of parasitic • L p : bondwire inductuce • R p : Small loss resistance • C p : Pad capacitance • C s : Shunt capacitance • R s : Series resistance • I L : Leakage current Yunsu Sung Special Topics in Optical Engineering II(15/1)

Rate Equations and Damping • Single mode rate equation • N: electron density • S: photon density • Γ : optical confinement factor dN I N • τ p : photon lifetime       A g ( N N )(1 S S )  g 0 0 • τ n : electron lifetime dt qV act n • V act : Volume of active layer     • β : Fraction of spontaneous emission dS 1 N            g ( N N )(1 S ) S coupled into the laser mode   0 0 g   dt   • ε : gain compression characteristic p n absorption Spontaneous emission Stimulated emission • N,S are assumed constant across active layer Yunsu Sung Special Topics in Optical Engineering II(15/1)

Rate Equations and Damping • Cause of damping in the modulation response – Spontaneous emission coupled into the lasing mode – Spatial hole burning combined with carrier diffusion – Nonlinear due to spectral hole burning – Nonlinear absorption Yunsu Sung Special Topics in Optical Engineering II(15/1)

Small Signal Response • Intensity Modulation    p j ( ) I     th ' M j ( )  qV i ( j ) act A g S     2 2 B 0 0 M j ( )   0 0   p         M (0) ' 1 ' S             2   2 h 0 ( j ) j S ( g ) B       M (0) 0 0 0   S S   2 q 0 n p n 0 n p Damping term  M j ( ) 1  With some approximation 2     M (0)   j j       1       0 m Yunsu Sung Special Topics in Optical Engineering II(15/1)

Small Signal Response • Damping of resonance   ' 1    Damping term: S ( g )   0 0 S 0 n p Damping term ↓  peak ↑ , ω p ≈ ω 0 Damping term ↑  peak ↓ , ω p ≠ ω 0 Low S 0 (Low output power)  Spontaneous emission term dominate Large S 0 (Large output power)  gain compression damping term( ε ) Yunsu Sung Special Topics in Optical Engineering II(15/1)

High Frequency limitations  M j ( ) 1 • Recall  2       M (0) j j       1       0 m  2 2 4       • Then   1   p    0   0           2 m m m 2  2  4 4              p p  3 dB       0              m m m m 1  2 M p 2 4       1   0   0        4 m m Yunsu Sung Special Topics in Optical Engineering II(15/1)

High Frequency limitations • ω 0 proportional to output power • ω p ≈ ω 0 at low output power( ω 0 / ω m <<1) • ω p / ω m max at ω 0 / ω m =1, zero at ω 0 / ω m = √ 2 • ω 3dB / ω m max at ω 0 / ω m = √ 2 • M p =0(no peak) at ω 0 / ω m = √ 2 – Second order Butterworth Yunsu Sung Special Topics in Optical Engineering II(15/1)

Design for Wide-Band Laser • ω 3dB / ω m max at ω 0 / ω m = √ 2 g S   2 0 0  • Make large ω 0 (up to √ 2) for large bandwidth 0 p • 1.Increse S 0 – Decrease the width of the optical field distribution – Design low threshhold current • 2. Increase g 0 – Decrease temperature • 3. Reduce photon lifetime – Reduce cavity length Yunsu Sung Special Topics in Optical Engineering II(15/1)

Small Signal Response • Frequency Modulation    g N    0  4   ( j )   F j ( )  i ( j ) A  j  m 1   2 F j ( )  0     F (0) j j   2   ( ) 1     0 m Yunsu Sung Special Topics in Optical Engineering II(15/1)

Small Signal Response FM IM • Difference between IM,FM – FM has much larger peak – IM slope decade -40dB – FM slope decade-20dB Yunsu Sung Special Topics in Optical Engineering II(15/1)

Conclusion • Semiconductor Laser response modeling was described • Bandwidth of direct modulator can control by small signal model Yunsu Sung Special Topics in Optical Engineering II(15/1)