Abstract

We present a high phase-shift efficienct Mach-Zehnder silicon optical modulator based on the carrier-depletion effect in a highly-doped PN diode with a small waveguide cross-sectional area. The fabricated modulator show a VπLπ of 1.8V·cm and phase shifter loss of 4.4dB/mm. A device using a 750 μm-long phase-shifter exhibits an eye opening at 12.5Gbps with an extinction ratio of 3 dB. Also, an extinction ratio of 7 dB is achieved at 4 Gbps for a device with a 2 mm-long phase shifter. Further enhancement of the extinction ratio at higher operating speed can be achieved using a travelling-wave electrode design and the optimal doping.

©2009 Optical Society of America

Research interest in silicon optical modulators has been growing. The device’s operational schemes are based on a change of refractive index [1] caused by the carrier injection effect in a p-i-n diode [2], the carrier accumulation effect in a MOS capacitor [3], and the carrier depletion effect in a PN diode [46]. The carrier-injection-based p-i-n diode modulator suffers from a slow operational speed originating from the slow carrier life time in the intrinsic region without a pre-emphasis driving scheme. This pre-emphasis driver can cause an overhead for on-chip integration of multi-channel input/output (I/O). The MOS capacitor approach shows a high speed operation because of the majority carrier dynamics. However, it requires a high driving power for high-speed operation, and shows a low modulation efficiency. On the other hand, the modulator based on the carrier-depletion effect in a PN diode shows a high-speed operation and can be driven with low power, while it has a low phase-shift efficiency originating from a small overlap between the optical field and active electrical region for a variation of carrier concentration, leading to a low extinction ratio. This drawback in a carrier-depletion PN diode modulator can be improved through an adequate design of the active electrical region. In this report, we have investigated a Mach-Zehnder modulator (MZM) with a highly-doped PN diode, based on a waveguide with a small cross-sectional area.

In order to improve the modulation efficiency of a PN diode MZM, the effects of the optical field confinement within a small area, and the PN junction with a higher doping level are considered in the device design. Figure 1(a) depicts a schematic cross-sectional view of the phase shifter of a silicon Mach-Zehnder interferometer (MZI). It is comprised of a silicon rib waveguide with a width of 500nm and height of 220nm. Here, the ridge height of the waveguide is about two or three times smaller than in previously reported cases [4,5]. An asymmetric MZI arm is used to simplify the optical characterization. The n-type doping concentration is ~1.0 x 1019 cm−3, while the p-type doping concentration are ~1.0 x 1018cm−3 for a type-(I) modulator, and ~5.0 x 1017 cm−3 for a type-(II) modulator. To insure good ohmic contact between silicon and metal, the doping concentration of the metal contact region is ~1.0 x 1020cm−3 for both dopant types. Figure 1(b) shows a micro-photography of the fabricated MZM. Here, a travelling wave electrode was not adopted. A higher doping level can introduce greater change of the carrier concentration in a PN junction interface, which contributes to a more effective index modulation. The confinement of photons and the PN junction interface within a small region lead to a strong overlap between the optical field and the active region of the carrier concentration variation, resulting in an enhancement of the effective index change, and thus leading to a higher extinction ratio.

 figure: Fig. 1

Fig. 1 (a) Cross-sectional view of a PN junction waveguide phase shifter on a Silicon-On-Insulator (SOI). (b) Micro-photograph of a fabricated MZM

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Figure 2(a) shows the measured optical transmission intensity versus applied reverse voltage characteristic of a fabricated type-(I) MZM with a 2 mm-long phase shift arm, which indicates an improvement of phase-shift efficiency using a small cross-sectional area of a rib waveguide and higher doping level. As is shown in the figure, the phase shift due to the carrier depletion effect is measured at up to a −10 V bias, at which the relative phase shift is nearly π. This indicates that the VπLπ of the type-(I) device is ~2 V·cm. For a bias of higher than −10 V, an avalanche breakdown occurs, showing that largely the increasing carrier concentration can cause a large phase shift with a small voltage variation.

 figure: Fig. 2

Fig. 2 (a). Optical output intensity of a MZM having a 2mm-long phase shifter versus voltages applied to one of the arms

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 figure: Fig. 2

Fig. 2 (b) Output spectra of a MZM having a 1mm-long phase shifter for various voltages applied to one of the arms

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Also, a voltage-induced wavelength shift (Δλ) measured in the transmission spectrum of the asymmetric MZI can be used to obtain a voltage-induced phase shift (Δϕ) from the relation Δϕ = 2π· Δλ /FSR [4], where FSR is the free spectral range of the asymmetric MZI. As shown in Fig. 2(b), the measured Δλ is 1.93nm with ΔV = 4V, and the measured FSR is 21.3nm for a type-(I) MZM with a 1 mm-long phase shifter. Using the obtained Δϕ = 0.18π, the figure of merit for the phase shift efficiency, VπLπ, is also obtained as 1.8 V·cm in a reverse bias range of less than 4V. Type (I) devices show phase shifter loss of 4.4dB/mm. For the type-(II) device with a lower p-type doping level, the measured VπLπ was increased to ~3 V·cm, which implies that higher doping can improve the phase-shift efficiency. For the type-(II) device, phase shifter loss was 4dB/mm.

The total on-chip loss for the type-(I) device with a 750 μm-long phase-shift arm was measured as ~7.6 dB when the MZI is on state, where maximum optical transmission occurs. This on-chip loss includes ~1.5 dB propagation loss of the passive waveguide, ~3.3 dB loss of the phase shifter, and ~2.8 dB loss of the Y-branch splitter/combiner. The phase shifter loss was obtained from a comparison with other modulators that have different phase arm lengths. The propagation loss was measured using the well-known cut back method [7]. The Y-splitter/combiner loss was determined by comparing the loss of MZI with that of a straight reference waveguide.

The high-speed performance of the modulator device has been characterized by measuring both the 3 dB frequency roll-off and the data transmission capability. For the frequency response measurement, the electrical power from a vector network analyzer was applied to one of phase shifters, sweeping the frequency from 50 MHz to more than 10GHz, and a CW laser beam was coupled into the modulator via a lensed fiber. The optical output of the MZM was converted to an electrical signal using a 40 GHz photodiode and the RF frequency spectrum of the converted electrical signal was read using a vector network analyzer. The frequency response of the modulator was obtained by de-embedding the connecting cable/connector response from the total response. Figure 3 shows the measured results for type-(I) device with a 1.5mm-long phase shifter. At a bias of −4 V and with an RF input of −2 dBm, the 3 dB roll-off frequency (f3dB) is measured as ~7 GHz.

 figure: Fig. 3

Fig. 3 A frequency spectrum of an optical response as a function of input RF frequency for a MZM [type (I)] having a 1.5mm long phase shift arm.

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The data transmission capability of the fabricated device was characterized by measuring the eye-pattern. A signal from a pseudo-random bit sequence (PRBS) generator is amplified using a commercially available RF amplifier. An amplified output signal of Vpp = 4V is combined with a −4 V DC bias using a bias-tee. The pattern length of the signal from the PRBS is 27-1. Figure 4 shows the on-chip measurement of the optical eye diagrams at a bit rate of 12.5Gbps for the device with a 750μm-long phase shifter. The measured extinction ratio is ~3dB, indicating improved values compared to previous results in a PN-diode based silicon modulator.

 figure: Fig. 4

Fig. 4 An eye diagram of a MZM [type(I)] with a 750μm-long phase shift arm at a bit rate of 12.5Gbps

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Further improvement of the extinction ratio is possible using a device with a longer phase shifter. Figure 5 shows an eye diagram of a 2 mm-long phase-shifter device at a bit rate of 4 Gbps. It exhibits a high extinction ratio of ~7 dB. However, the operating speed is limited by its capacitance, and this limit can be overcome using a travelling wave electrode design.

 figure: Fig. 5

Fig. 5 A measured eye diagram of the a type-(I) MZM with 2mm-long phase arm, showing a ~7 dB extinction ratio at a bit rate of 4Gbps.

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Using the optimum doping and travelling wave electrode design, further improvement of the extinction ratio at a higher speed operation can be achieved.

In conclusion, the effect of the high doping concentration in a PN diode with a smaller cross-sectional area rib waveguide has been investigated in a carrier-depletion PN-diode Mach-Zehnder silicon modulator. The measured phase-shift efficiency of the fabricated modulator, VπLπ, is 1.8 V·cm, indicating a high phase-shift efficiency. The device with a 1.5 mm-long phase shifter shows a 3-dB bandwidth of 7 GHz. The extinction ratio of 3 dB at a bit rate of 12.5Gbps is obtained in a device with a 750 μm-long phase-shifter, showing an enhanced extinction ratio. Also the device with a 2 mm-long phase shifter exhibits a high extinction ratio of ~7 dB at a bit rate of 4 Gbps. With the addition of a travelling wave electrode and optimum doping design, further improvement of the extinction ratio and the operating speed of the modulator can be possible.

References and links

1. R. A. Soref and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. of Quant. Elec. 23(1), 123–129 (1987). [CrossRef]  

2. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005). [CrossRef]   [PubMed]  

3. A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004). [CrossRef]   [PubMed]  

4. A. Liu, L. Liao, D. Rubin, H. Nguyen, B. Ciftcioglu, Y. Chetrit, N. Izhaky, and M. Paniccia, “High-speed optical modulation based on carrier depletion in a silicon waveguide,” Opt. Express 15(2), 660–668 (2007). [CrossRef]   [PubMed]  

5. D. Marris-Morini, L. Vivien, J. M. Fédéli, E. Cassan, P. Lyan, and S. Laval, “Low loss and high speed silicon optical modulator based on a lateral carrier depletion structure,” Opt. Express 16(1), 334–339 (2008). [CrossRef]   [PubMed]  

6. A. Huang, C. Gunn, G. Liang, Y. Liang, S. Mirsaidi, A. Narasimha, and T. Pinguet, “A 10Gb/s photonic modulator and WDM MUX/DEMUX integrated with electronics in 0.13mm SOI CMOS”, ISSCC, 13.7 (2006).

7. G. T. Reed, and A. P. Knights, Silicon Photonics: an introduction (John Wiley, Chichester, 2004)

References

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  1. R. A. Soref and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. of Quant. Elec. 23(1), 123–129 (1987).
    [Crossref]
  2. Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005).
    [Crossref] [PubMed]
  3. A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004).
    [Crossref] [PubMed]
  4. A. Liu, L. Liao, D. Rubin, H. Nguyen, B. Ciftcioglu, Y. Chetrit, N. Izhaky, and M. Paniccia, “High-speed optical modulation based on carrier depletion in a silicon waveguide,” Opt. Express 15(2), 660–668 (2007).
    [Crossref] [PubMed]
  5. D. Marris-Morini, L. Vivien, J. M. Fédéli, E. Cassan, P. Lyan, and S. Laval, “Low loss and high speed silicon optical modulator based on a lateral carrier depletion structure,” Opt. Express 16(1), 334–339 (2008).
    [Crossref] [PubMed]
  6. A. Huang, C. Gunn, G. Liang, Y. Liang, S. Mirsaidi, A. Narasimha, and T. Pinguet, “A 10Gb/s photonic modulator and WDM MUX/DEMUX integrated with electronics in 0.13mm SOI CMOS”, ISSCC, 13.7 (2006).
  7. G. T. Reed, and A. P. Knights, Silicon Photonics: an introduction (John Wiley, Chichester, 2004)

2008 (1)

2007 (1)

2005 (1)

Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005).
[Crossref] [PubMed]

2004 (1)

A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004).
[Crossref] [PubMed]

1987 (1)

R. A. Soref and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. of Quant. Elec. 23(1), 123–129 (1987).
[Crossref]

Bennett, B. R.

R. A. Soref and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. of Quant. Elec. 23(1), 123–129 (1987).
[Crossref]

Cassan, E.

Chetrit, Y.

Ciftcioglu, B.

Cohen, O.

A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004).
[Crossref] [PubMed]

Fédéli, J. M.

Izhaky, N.

Jones, R.

A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004).
[Crossref] [PubMed]

Laval, S.

Liao, L.

A. Liu, L. Liao, D. Rubin, H. Nguyen, B. Ciftcioglu, Y. Chetrit, N. Izhaky, and M. Paniccia, “High-speed optical modulation based on carrier depletion in a silicon waveguide,” Opt. Express 15(2), 660–668 (2007).
[Crossref] [PubMed]

A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004).
[Crossref] [PubMed]

Lipson, M.

Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005).
[Crossref] [PubMed]

Liu, A.

A. Liu, L. Liao, D. Rubin, H. Nguyen, B. Ciftcioglu, Y. Chetrit, N. Izhaky, and M. Paniccia, “High-speed optical modulation based on carrier depletion in a silicon waveguide,” Opt. Express 15(2), 660–668 (2007).
[Crossref] [PubMed]

A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004).
[Crossref] [PubMed]

Lyan, P.

Marris-Morini, D.

Nguyen, H.

Nicolaescu, R.

A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004).
[Crossref] [PubMed]

Paniccia, M.

A. Liu, L. Liao, D. Rubin, H. Nguyen, B. Ciftcioglu, Y. Chetrit, N. Izhaky, and M. Paniccia, “High-speed optical modulation based on carrier depletion in a silicon waveguide,” Opt. Express 15(2), 660–668 (2007).
[Crossref] [PubMed]

A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004).
[Crossref] [PubMed]

Pradhan, S.

Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005).
[Crossref] [PubMed]

Rubin, D.

A. Liu, L. Liao, D. Rubin, H. Nguyen, B. Ciftcioglu, Y. Chetrit, N. Izhaky, and M. Paniccia, “High-speed optical modulation based on carrier depletion in a silicon waveguide,” Opt. Express 15(2), 660–668 (2007).
[Crossref] [PubMed]

A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004).
[Crossref] [PubMed]

Samara-Rubio, D.

A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004).
[Crossref] [PubMed]

Schmidt, B.

Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005).
[Crossref] [PubMed]

Soref, R. A.

R. A. Soref and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. of Quant. Elec. 23(1), 123–129 (1987).
[Crossref]

Vivien, L.

Xu, Q.

Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005).
[Crossref] [PubMed]

IEEE J. of Quant. Elec. (1)

R. A. Soref and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. of Quant. Elec. 23(1), 123–129 (1987).
[Crossref]

Nature (2)

Q. Xu, B. Schmidt, S. Pradhan, and M. Lipson, “Micrometre-scale silicon electro-optic modulator,” Nature 435(7040), 325–327 (2005).
[Crossref] [PubMed]

A. Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O. Cohen, R. Nicolaescu, and M. Paniccia, “A high-speed silicon optical modulator based on a metal-oxide-semiconductor capacitor,” Nature 427(6975), 615–618 (2004).
[Crossref] [PubMed]

Opt. Express (2)

Other (2)

A. Huang, C. Gunn, G. Liang, Y. Liang, S. Mirsaidi, A. Narasimha, and T. Pinguet, “A 10Gb/s photonic modulator and WDM MUX/DEMUX integrated with electronics in 0.13mm SOI CMOS”, ISSCC, 13.7 (2006).

G. T. Reed, and A. P. Knights, Silicon Photonics: an introduction (John Wiley, Chichester, 2004)

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Figures (6)

Fig. 1
Fig. 1 (a) Cross-sectional view of a PN junction waveguide phase shifter on a Silicon-On-Insulator (SOI). (b) Micro-photograph of a fabricated MZM
Fig. 2
Fig. 2 (a). Optical output intensity of a MZM having a 2mm-long phase shifter versus voltages applied to one of the arms
Fig. 2
Fig. 2 (b) Output spectra of a MZM having a 1mm-long phase shifter for various voltages applied to one of the arms
Fig. 3
Fig. 3 A frequency spectrum of an optical response as a function of input RF frequency for a MZM [type (I)] having a 1.5mm long phase shift arm.
Fig. 4
Fig. 4 An eye diagram of a MZM [type(I)] with a 750μm-long phase shift arm at a bit rate of 12.5Gbps
Fig. 5
Fig. 5 A measured eye diagram of the a type-(I) MZM with 2mm-long phase arm, showing a ~7 dB extinction ratio at a bit rate of 4Gbps.

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