2012. 10. 17

Kyuyoung Bae Optics and Metamaterials Lab

Paper preview

Backgrounds

Structure and Performance

Development of GeSi EA modulator

Results

Summary and Conclusion

Electro-absorption(EA) Effect

◦ Semiconductors exhibit changes in optical absorption coefficients under the influence of an electric field.

◦ High speed and relatively low power consumption

GeSi

◦ Unlike Si, bulk Ge possesses a significant electro-absorption effect

◦ Epitaxial Ge on Si

◦ Tensile strain in Ge film introduces a reduction of the direct energy gap and a decrease in the separation between the direct and indirect energy gaps.

<OM5653C-30B GaInAsP compound semiconductor>

Active device area 30𝜇𝑚2

Footprint ~200𝜇𝑚2

Peak-to-peak driving voltage (𝑉𝑝𝑝) 3.0V

Energy consumption per bit 50fJ/bit

Operation spectrum width 14nm

Extinction ratio at 1550nm 8dB

3-dB bandwidth 1.2GHz

Absorption loss 2.0±0.5dB

Coupling loss < 0.3dB

Ge-on-Si has optimum values at 1647nm wavelength

L-band 1647nm => C-band 1550nm

Add small amount of Si into Ge in order to increase its band

gap

Design GeSi EA modulator

Fabrication and measurement

Characteristic of Ge-on-Si

<S.Jongthammanurak, Appl. Phys. Lett. 89, 161115(2006)>

𝛼 = Absorption coefficient R = photodiode responsivity 𝑡𝐺𝑒= Ge depletion width under reverse bias

𝑅𝑟𝑒𝑓𝑙 = reflectance of the photodiodes

Design GeSi EA modulator

<J. Liu, Optics Express. 15, 623(2007)>

Final structure(this paper)

Energy gap change > absorption change > amplitude change

EA modulator is a vertical heterojunction p-i-n diode 50𝜇𝑚 long,

600nm wide and 400nm high, so the active device area is 30𝜇𝑚2

600nm

400nm

Small active device area

Small footprint on c-Si

Low energy consumption

Relatively large operation spectrum

bandwidth

Transmittance spectra at different reverse biases

◦ As the reverse bias

increases, the absorption edge tilts towards longer wavelength with a significant reduction in its slope.

◦ Tilting of the absorption edge is different from temperature-induced absorption because the slope changed.

1550nm

Extinction ratio at -7V bias and insertion loss at 0V bias

◦ The effective operating

spectrum range is 1,539 – 1,553nm

◦ The maximum extinction ratio is 11.2dB at 1,536nm

◦ At 1,550nm the extinction ratio is 8dB

◦ The modulator can tolerate a ~20℃ variation at room temperature

The modulation depth versus electric field

◦ A pseudo-linear regime is

observed experimentally

◦ Linear correlation factor is 0.997

◦ Threshold voltage of 𝑉𝑇 = -3.8V

◦ 𝜂 is modulation depth ,Δ𝛼 is the absolute charge in absorption coefficient, and L is length of the device

Energy consumption per bit

◦ The device only consumes 50kJ per bit for a 8-dB extinction ratio

◦ This energy consumption is even lower than a carrier-injection silicon p-i-n microring modulator.(~300fJ per bit)

C = capacitance of the device

𝑉𝑝𝑝= 𝑉𝑜𝑓𝑓 − 𝑉𝑜𝑛

𝐵 = bit rate

The transient energy consumption due to the on/off transitions

Energy consumption to maintain the on and off states

Dynamic EO response of the device with Vpp of 3V

◦ No frequency roll-off is up to 0.7 GHz, indicating that the device can still achieve ~ 8-dB extinction ratio at 0.7 GHz with Vpp=3V

◦ The 3-dB bandwidth is 1.2 GHz, and the corresponding extinction ratio is ~4dB

Silicon MZI(a) Silicon MZI(b) Microring(c) GeSi EA

Active device area (𝜇𝑚2) ~6× 103 200-400 33 30

Footprint on c-Si (𝜇𝑚2) >1× 104 ~1-2× 103 ~150 ~200

Dynamic voltage swing 𝑉𝑝𝑝 (V) 6.5 7.6 8.0 3.0

Energy consumption per bit (fJ/bit) >2.8× 104 5× 103 ~300 50

Operation spectrum width (nm) >30 - ~1 14

(a) (b) (c)

In this paper, they demonstrated a waveguide-integrated, GeSi

electro-absorption modulators for C-band applications

The GeSi EA modulator has a small active device area of 30𝜇𝑚2, a

10dB extinction ratio at 1,540nm an operating spectrum range of

1,539~1,553nm, and an ultralow energy consumption of 50fJ per bit

Low 3-dB bandwidth frequency should be improved

Liu, A. et al. High-speed optical modulation based on carrier depletion in a silicon waveguide. Opt. Express 15, 660–668 (2007)

Green, W. et al. Ultra-compact, low RF power, 10 Gb/s silicon Mach–Zehnder modulator. Opt. Express 15, 17106–17113 (2007)

Xu, Q. et al. 12.5 Gbit/s carrier-injection-based silicon microring silicon modulators. Opt. Express 15, 430–436 (2007)

Lampin, J. et al. Detection of picosecond electrical pulses using the intrinsic Franz–Keldysh effect. Appl. Phys. Lett. 78, 4103–4105 (2001)

Jongthanmmanurak, S. et al. Large electro-optic effect in tensile strained Ge-on-Si films. Appl. Phys. Lett. 89, 161115 (2006).

Liu, J. F. et al. Design of monolithically integrated GeSi electro-absorption modulators and photodetectors on an SOI platform. Opt. Express 15, 623 (2007).

http://www.photonics.com/Article.aspx?AID=11945

New prospects for high-power, high-efficiency analog electro-absorption modulators, 14 July 2008, SPIE Newsroom