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Broad-Band Continuous-Wave Parametric Wavelength Conversion in Silicon Nanowaveguides

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Broad-band continuous-wave parametric wavelength conversion in silicon nanowaveguides
Mark A. Foster1 , Amy C. Turner2 , Reza Salem1 , Michal Lipson2 , and
Alexander L. Gaeta1
1 School
2 School

of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853 of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853 alg3@cornell.edu Abstract:
We demonstrate highly broad-band frequency conversion via four-wave mixing in silicon nanowaveguides. Through appropriate engineering of the waveguide dimensions, conversion bandwidths greater than 150 nm are achieved and peak conversion efficiencies of -9.6 dB are demonstrated. Furthermore, utilizing fourth-order dispersion, wavelength conversion across four telecommunication bands from 1477 nm
(S-band) to 1672 nm (U-band) is demonstrated with an efficiency of -12 dB.
© 2007 Optical Society of America
OCIS codes: (190.4380) Four-wave mixing; (130.5990) Semiconductors; (130.4310) Nonlinear; (130.3060) Infrared

References and links
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37. M. A. Foster, K. D. Moll, A. L. Gaeta, “Optimal waveguide dimensions for nonlinear interactions,” Opt. Express
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1.

Introduction

Reducing the power requirements and size of nonlinear optical devices is critical to their implementation in optical systems. Benefiting from a large refractive index, large nonlinear response, and a mature and low-cost fabrication process, silicon has recently emerged as a highly attractive material for nonlinear photonic integration. Compact all-optical devices based on twophoton absorption [1–3], free-carrier dispersion [4, 5], the Raman effect [6–12], and the Kerr effect [1,2,13–16] have all been demonstrated in silicon. For parametric processes such as selfphase modulation [1, 13, 14, 17, 18], cross-phase modulation [15, 19, 20], and FWM [21–29] the group-velocity dispersion (GVD) [1, 30–32] is a critical parameter, which defines conditions, for example, for solitonic or non-solitonic behavior and phase-matching [33]. In fact, a pulsed experiment using anomalous-GVD in appropriately scaled silicon waveguides recently demonstrated broad-bandwidth parametric gain via FWM [26]. In experiments, optimization of continuous-wave (CW) FWM in silicon has focused on the waveguide length and free carrierlifetime [23–25,27,29], but little attention has been paid to optimizing phase-matching through
GVD [22, 26, 28].
In this paper, we experimentally demonstrate highly broad-band frequency conversion using
FWM in silicon nanowavegudies. We demonstrate 3-dB conversion bandwidths as large as 150 nm with peak conversion efficiencies of -9.6 dB. Utilizing a waveguide with low third-order dispersion (TOD) and low GVD, we are able to tune the pump throughout the C-band while maintaining conversion bandwidths > 100 nm. Pumping close to the zero-GVD point of one waveguide, we find the phase-matching bandwidth is determined not only by the GVD but also by the fourth-order dispersion (FOD) [34, 35]. Using higher-order dispersion phase-matching, we convert signals from 1477 nm to 1672 nm with an efficiency of -12 dB. To demonstrate the utility of silicon wavelength converters, we convert a 10-Gb/s NRZ data train across the C-band from 1535 nm to 1566 nm with minimal degradation of the signal quality.
2.

Theory

Efficient FWM requires minimal phase-mismatch of the four interacting waves [33, 36]. Considering a degenerate pump and including the effects of cross- and self-phase modulation, this mismatch ∆k is given by,
(1)
∆k = 2γ Ppump − ∆klinear , where γ = 2π n2 /λ Ae f f is the effective nonlinearity, n2 is the nonlinear refractive index, λ is the wavelength of light, Ae f f is the mode area, Ppump is the pump power, ∆klinear = 2k pump − ksignal − kidler is the linear phase-mismatch, and k pump , ksignal , and kidler are the pump, signal, and idler propagation constants. Including the effects of dispersion up to fourth-order, the linear

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phase-mismatch is approximately given by,
∆klinear = −β2 (∆ω )2 −

1 β4 (∆ω )4 ,
12

(2)

where β2 = d 2 k/d ω 2 is the GVD parameter at the pump wavelength, β4 = d 4 k/d ω 4 is the fourth-order dispersion (FOD) parameter at the pump wavelength, and ∆ω is the frequency difference between the pump and signal waves. Only the even-order dispersion terms play a role in the phase-mismatch due to the symmetry of the FWM process. A commonly used GVD parameter D is related to β2 by D = −2π cβ2 /λ 2 . The conversion efficiency Gidler is given by [36],
2
out
Pidler
γ Ppump
Gidler = in
(3)
sinh(gL) ,
=
g
Psignal
where g = γ Ppump ∆klinear − (∆klinear /2)2

1
2

(4)

out is the parametric gain parameter, Ppump is the pump power, Pidler is the output power in the in idler wave, Psignal is the input power of the signal wave, and L is the interaction length. The maximum efficiency Gmax occurs when ∆k = 0 and is given by, idler Gmax = sinh2 (γ Ppump L). idler (5)

The conversion bandwidth can be estimated as the bandwidth for which |∆kL| < π [33]. This definition provides a bandwidth slightly larger than the 3-dB bandwidth. In the small-gain limit,
2γ Ppump L 100 nm [40, 41]. Achieving these maximal bandwidths in silicon will likewise require careful design of the waveguide geometry for minimal GVD.
3.

Dispersion and phase-matching

The material GVD of silicon is large and normal near 1550 nm, with a value of approximately
D = −1, 000 ps/(nm·km). The large index contrast of silicon-on-insulator waveguides (3.5 to
1.5) allows for a large waveguide contribution to the dispersion for highly confining waveguides. It was recently shown that by finely adjusting the waveguide dimensions, the GVD near
1500 nm can be varied to any value from large and anomalous [i.e. D = 5, 000 ps/(nm·km)] to large and normal [i.e D = −15, 000 ps/(nm·km)] [30–32]. In this paper, we consider silicon waveguides of uniform 300-nm height with widths ranging from 500 nm to 750 nm. The GVD
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(a)
2000

D [ps/(nm·km)]

1500
1000
500
0
-500
-1000

(b)

TE
TE
TE

TM 300 x 750 nm
TM 300 x 650 nm
TM 300 x 500 nm

2

∆k linear ·L/π

1
0
-1
-2

Conversion Efficiency [dB]

(c)

TE
TE

TM 300 x 650 nm (1550 nm pump)
TM 300 x 500 nm (1550 nm pump)
TM 300 x 500 nm (1585 nm pump)

-10
-15
-20
-25
-30
-35
1480 1500 1520 1540 1560 1580 1600 1620 1640 1660 1680 1700
Wavelength [nm]

Fig. 1. (a) Simulated group-velocity dispersion D of the TE and TM modes for three of the waveguide cross-sections used in this investigation, (b) the acquired phase mismatch after 1-cm of propagation, and (c) the predicted conversion efficiency for 100-mW pump power and 1-cm interaction length. All curves assume a pump wavelength of 1550 nm except the black curve which has a pump wavelength of 1585 nm. In the small-gain limit, the conversion bandwidth corresponds to the range of wavelengths for which the magnitude of this linear mismatch is less than π , as indicated by the grey region in (b).
The waveguides with the lowest GVD at the pump wavelength have the largest conversion bandwidth.
When the pump is tuned near the zero-GVD point to 1585-nm (dashed black curve), fourth-order dispersion adds two additional phase matching points away from the pump wavelength.

of three of these waveguides for the TE and TM polarization modes are shown in Fig. 1(a). The range of sizes yields GVD values from D = 50 ps/(nm·km) to D = 1, 000 ps/(nm·km) at 1550 nm. For a pump at 1550 nm and for a 1-cm interaction length, the linear phase-mismatch for the
500-nm wide and 650-nm wide waveguides are shown in Fig. 1(b). As explained in the previous section, in the small-gain limit the conversion bandwidth is approximately the wavelength range over which the magnitude of this linear mismatch is less than π . For TE (TM) polarization the larger (smaller) waveguide has the lowest magnitude of GVD. Correspondingly, the conversion bandwidth for the TE (TM) polarization is largest in the larger (smaller) waveguide. Assuming a 100-mW CW pump and a 1-cm interaction length, the predicted conversion bandwidths are shown in Fig. 1(c). Under these conditions, conversion efficiencies of -10 dB are expected with bandwidths approaching 60 nm.
If the pump wavelength is tuned near the zero-GVD point of a waveguide, the FOD plays an important role in the phase-mismatch. For TM polarization in the 300-nm by 500-nm waveguide, a pump wavelength of 1585 nm demonstrates this behavior. The phase mismatch under
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these conditions is shown in Fig. 1(b), in which two phase-matching regions appear; one extremely broad region near the pump is due to the GVD, and a second set further from the pump is due to FOD. The position of the FOD phase-matched region can be calculated from Eq. (2) and is given by,
∆ω =

12 |β2 |
,
|β4 |

(7)

assuming β2 and β4 are of opposite sign. The predicted conversion efficiency for a 100-mW pump and 1-cm interaction length is shown in Fig. 1(c). Such a scheme should allow wavelength conversion over a 200-nm range with efficiencies of approximately -10 dB.
The analysis of this section neglects the nonlinear losses of two-photon absorption (TPA) and free-carrier absorption (FCA) present in silicon waveguides. For a pump power of 100-mW and a propagation length of 1-cm the losses due to TPA and FCA is calculated to be less than 0.7 dB and therefore will have a small effect on the conversion efficiency. Furthermore, in this small-gain limit the conversion bandwidth depends solely on the GVD and will not depend on nonlinear absorption, pump power, or variations in the effective nonlinearity of the waveguides.
However, for higher pump powers or effective nonlinearities, the nonlinear losses will lead to saturation of the peak conversion efficiency.

(a)

Conversion Efficiency [dB]

-10

-15

-20

-25

-30

(b)

TE polarization
300 x 500 nm (130 mW)
300 x 550 nm (100 mW)
300 x 600 nm (100 mW)
300 x 650 nm (350 mW)

Conversion Efficiency [dB]

-10

-15

-20

-25

-30

TM polarization
300 x 500 nm (140 mW)
300 x 550 nm (150 mW)
300 x 600 nm (210 mW)
300 x 650 nm (350 mW)
1550

1560

1570
1580
Conjugate Wavelength [nm]

1590

1600

Fig. 2. Experimentally measured conversion efficiency in the (a) TE and (b) TM polarization modes of the four waveguides with the pump wave at 1550 nm. The TM mode of the smallest waveguide and the TE mode of the largest waveguide have the lowest GVD magnitude and consequently have the largest conversion bandwidths.

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4.

Experiment

The embedded silicon waveguides in this investigation are fabricated as previously described
[26, 32]. Five cross-sectional sizes are utilized which are all 300-nm tall with widths from 500 nm to 750 nm. The four waveguides with widths from 500 nm to 650 nm are 1.8-cm long and have linear propagation losses ranging from 1 to 1.5 dB/cm. The 750-nm wide waveguide is
2-cm long and has a 3-dB/cm propagation loss. We use two tunable lasers to form the pump and signal waves. The pump wave is amplified in an EDFA and subsequently filtered and combined with the signal in a wavelength-division multiplexer. The two waves are coupled into the silicon waveguide using a tapered-lens fiber and an inverse-taper mode converter [42]. A fiber polarization controller before the tapered-lens fiber allows for selection of TE or TM polarization. The waves exiting the waveguide are collimated, filtered by a polarizer, and collected with a single-mode fiber or free-space power meter for analysis. The coupling loss was measured using a low power input (less than 5 mW) to avoid nonlinear loss mechanisms and comparing to the output power corrected for the propagation loss. We measured coupling losses ranging from -7 dB to -13 dB and obtained better coupling in waveguides with larger dimensions.
The experimentally measured conversion efficiencies for the 500 to 650-nm-wide waveguides pumped at 1550 nm are shown in Fig. 2. For comparison with previous research
[23–27, 29], we define conversion efficiency as the converted power exiting the waveguide with the pump on divided by the signal power exiting the waveguide with the pump off. For these measurements, the power inside the waveguide ranges from 100 mW to 350 mW due to the respective coupling efficiencies for each of the waveguides. For TE polarization (see Fig.
2(a)), we see an increase in conversion bandwidth with increased cross-sectional waveguide size due to the corresponding decrease in GVD magnitude and we observe a maximum conversion efficiency of -10.4 dB and a maximum 3-dB conversion bandwidth of 58 nm, including the symmetric lobe to the short wavelength side. In comparison, for TM polarization (see Fig.
2(b)) we see an increase in the 3-dB conversion bandwidth with decreased cross-sectional waveguide size which is also due to a corresponding decrease in GVD magnitude and we observe a maximum conversion efficiency of -9.6 dB and a maximum 3-dB conversion bandwidth of
48 nm, including the symmetric lobe. For both polarizations the conversion bandwidth is critically dependent on the magnitude of GVD in this small-gain limit. Using the waveguides and

Conversion Efficiency [dB]

-10

TM Polarization
300 x 500 nm
140 mW
100 mW
60 mW
20 mW

-15

-20

-25

-30
1550

1560

1570
1580
Conjugate Wavelength [nm]

1590

1600

Fig. 3. Experimentally measured conversion efficiency for various pump powers in the TM polarization of the 300-nm by 500-nm waveguide. While the maximum efficiency is highly dependent on pump power, the conversion bandwidth is not.

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Conversion Efficiency [dB]

-14
-16
-18
-20
-22
-24
-26

Pump Wavelength (100 mW)
1538 nm
1550 nm
1565 nm

-28
-30
0

10

20

30
40
50
60
70
Pump-Signal Detuning [nm]

80

90

100

Fig. 4. Experimentally measured conversion efficiency for the TE mode of the 300-nm by 750-nm waveguide for three pump wavelengths spanning the C-band. The 3-dB conversion bandwidth remains
> 100 nm for signal wavelengths tuned relative to these pump wavelengths demonstrating that any
C-band signal can be converted to any other C-band wavelength by tuning the pump wavelength.

polarizations with lowest magnitude of GVD allows us to convert across the entire C-band with conversion efficiencies higher than -11 dB.
The dependence of conversion efficiency on pump power is shown in Fig. 3 for the TM polarization in the 300-nm by 500-nm waveguide. Although the conversion bandwidth is not highly dependent on pump power, the conversion efficiency is determined primarily by the coupled pump power. Interestingly, the conversion efficiency does not saturate for the powers investigated here which indicates that with improved coupling efficiency higher conversion efficiencies can be expected. This result is consistent with our observation of minimal nonlinear losses for the power levels used in this experiment.
While third-order dispersion (TOD) does not influence the FWM bandwidth, it does limit the ability to tune the pump wavelength and maintain a large bandwidth. The tunability of the pump wavelength is an important factor for wavelength conversion of a fixed signal to an arbitrary wavelength. By choosing the TE mode of the 300-nm by 750-nm waveguide, which exhibits low TOD and low GVD, we are able to choose three pump wavelengths throughout the C-band
(a)

(b)
-20

-12
Spectrum [dBm]

Conversion Efficiency [dB]

-10
-14
-16
-18
-20
-22

-30
-40
-12 dB
-50
-60

-24
-26

-70
1580

1600
1620
1640
1660
Conjugate Wavelength [nm]

1680

1480

1520

1560 1600 1640
Wavelength [nm]

1680

Fig. 5. (a) Experimentally measured conversion efficiency pumping at 1568 nm in the TM mode of the
300-nm by 500-nm waveguide. This pump wavelength is near the zero-GVD point of the waveguide allowing phase-matching through fourth-order dispersion further from the pump. (b) This fourth-order phase matching yields conversion across four telecommunication bands from 1477 nm to 1672 nm with -12 dB efficiency

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Signal [arb. units]

Input at 1535 nm

Converted Output at 1566 nm

Fig. 6. Eye diagrams associated with conversion of a 10-Gb/s signal from 1535 nm (blue) to 1566 nm
(red). The converted output shows minimal degradation of the data quality.

while maintaining a 3-dB conversion bandwidth > 100 nm as the signal is tuned relative to these pump wavelengths (see Fig. 4). Since the zero-GVD point occurs to the short wavelength side of our pump-tuning range, the largest 3-dB conversion bandwidth of 150 nm occurs for the shortest pump wavelength. We were unable to tune the signal to within 12-nm of the 1538nm pump due to the WDM used to combine the two waves. However, similar conversion is clearly expected since the phase-mismatch will only decrease for these small detunings. The ability to tune the pump throughout the C-band while maintaining large conversion bandwidths demonstrates that we can convert any C-band signal to any other C-band wavelength by solely tuning the pump wavelength.
To observe larger signal-idler detunings, we tune the pump wavelength to 1568 nm, close to the zero-GVD wavelength of the TM mode in the 300-nm by 500-nm waveguide. Figure
5(a) shows the conversion efficiency for this pump wavelength, and its spectral dependence is determined by both GVD and FOD, due to the proximity of the zero-GVD point. The low GVD magnitude yields a 3-dB conversion bandwidth of 100 nm near the pump wavelength, including the symmetric lobe to shorter wavelengths. Fourth-order dispersion leads to a second 40-nm wide region of efficient conversion further from the pump wavelength, including the symmetric lobe. As illustrated in Fig. 5(b), this region enables conversion across four telecommunications bands from 1477 nm (S-band) to 1672 nm (U-band) with an efficiency of -12 dB.
To demonstrate that the FWM process in silicon nanowaveguides does not appreciably degrade an optical signal, we convert 10-Gb/s NRZ data from 1535 nm to 1566 nm using the
TM-polarization mode of the 300-nm by 500-nm waveguide. The choice of a 1535-nm input is limited by the EDFA bandwidth and not the conversion bandwidth of the process, as shown in Fig. 2. For this measurement, the converted signal is detected with no post amplification.
Figure 6(a) shows the eye diagrams of the input signal (1535 nm) and the converted output
(1566 nm), which is measured using a 231 − 1 pseudo-random bit sequence. Both eye diagrams are measured with an input signal of -20 dBm. Although time-dependence to the loss mechanisms such as free-carrier absorption or thermal effects may be a concern, minimal degradation of the signal quality occurs on the converted output as has been demonstrated over narrower bandwidths [25, 27, 29]. Since the pump is CW, any nonlinear absorption mechanisms will not be time dependent and thus will not degrade the signal.
5.

Conclusion

We demonstrate continuous-wave four-wave mixing in silicon nanowaveguides over an extremely broad bandwidth, allowing for conversion across four telecommunications bands from
1477 nm to 1672 nm. These demonstrations are enabled by combining the large effective nonlinearity of these waveguides (five orders of magnitude larger than single-mode fiber) with
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(C) 2007 OSA

Received 15 Jun 2007; revised 13 Aug 2007; accepted 13 Aug 2007; published 24 Sep 2007

1 October 2007 / Vol. 15, No. 20 / OPTICS EXPRESS 12957

the ability to engineer the GVD through the dominating contribution of waveguide dispersion.
These broad bandwidths illustrate the ability to tune the GVD in designing silicon parametric wavelength converters. The combination of large conversion bandwidths and low pump powers allow the porting of existing parametric optical processing technology from silica fibers to photonic integrated circuits.
Acknowledgments
We gratefully acknowledge discussions with David F. Geraghty. This work is funded by the
Center for Nanoscale Systems, supported by the NSF and the New York State Office of Science, Technology and Academic Research, and the DARPA DSO Slow-Light Program. M.A.F. acknowledges support through the IBM Graduate Fellowship Program.

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(C) 2007 OSA

Received 15 Jun 2007; revised 13 Aug 2007; accepted 13 Aug 2007; published 24 Sep 2007

1 October 2007 / Vol. 15, No. 20 / OPTICS EXPRESS 12958…...

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