WN1 (Invited) 10:30 - 11:00
Nonlinear Optical Devices in Silicon B. Jalali , O. Boyraz, P. Koonath, V. Raghunathan, D. Dimitropoulos and T. Indukuri UCLA, Department of Electrical Engineering, Los Angeles, CA 90095-1594, email:
[email protected]
Silicon-on-Insulator (SOI) material system, with its high index contrast, ∆n of ~2 between Si and SiO2, provides an excellent platform for the fabrication of photonic devices, with the prospect of full integration of electronic and optical devices on the same silicon substrate. Third order nonlinearity in silicon has been investigated extensively to offer active functionalities in silicon by taking advantage of this high index contrast and hence tight beam confinement. Among the third order effects (i) Raman (ii) Kerr Nonlinearity (iii) Two Photon Absorption (TPA) are particularly strong. Compared to the optical fiber, Raman effect is 104 times stronger in silicon while the Kerr effect is 102 times stronger than that of the fiber [1,2]. Additionally, newly developed SIMOX processing has been exploited to design novel passive lightwave devices in the same platform [3]. Utilizing nonlinear properties of silicon in these novel devices will add tremendous agility to the future of silicon photonics. Among the third order nonlinear effect, the Raman effect has been proposed to bypass indirect band of silicon and to demonstrate light amplification and lasing [1]. After demonstrating spontaneous emission in 2002, stimulated emission, net light amplification and first lasing have been demonstrated in 2003 and 2004, respectively [1, 4-8]. Currently CW silicon Raman lasers [9] with electronic modulation capability [10] and silicon lossless light modulators [11] are available. Although the Raman effect is nearly 2 orders of magnitude stronger than the Kerr nonlinearity, under pulsed operation regime, with the pulse widths comparable or less than the phonon de-phasing time of a few pico second, the Raman effect is suppressed and Kerr nonlinearity dominates [12]. Additionally, the high index contrast in SOI waveguides results in tight mode confinement and improves the effective nonlinearity. In principle, it is thus possible to exploit the Kerr effect to perform chip scale supercontinuum generation [12], all optical switching [13] and four wave mixing [14]. First proof of principle on-chip continuum generation in silicon waveguides has been demonstrated in early 2004 [12]. Experimentally, we exploit Self Phase Modulation (SPM) to generate spectral broadening in silicon waveguides. Figure 3 shows the measured continuum generation in 2.5cm long silicon waveguide. The spectral broadening was achieved through SPM at moderate peak intensity levels of 2 GW/cm2. Compared to the input spectrum, a 2x spectral broadening is obtained. It is well known that SPM results in the generation of new spectral components with oscillatory power spectral density across the spectrum. This is clearly evident in the measured output spectrum. Theoretical estimations and the number of oscillations at the output spectrum reveal that >2π nonlinear phase shift is achieved in the silicon waveguide [12]. By increasing the optical intensity inside the waveguide either by increasing the input power or by reducing the waveguide dimensions, the spectral broadening factor can be enhanced up to 15x, without the free carrier contribution [12]. At high intensities, TPA, TPA-induced Free Carrier Absorption (FCA) and refraction are also prominent in semiconductor waveguides [15-16]. If the free carrier lifetime is longer than the pulse period accumulation of free carriers depletes the launched pump signal due to FCA effect. For the opposite case, the free carrier accumulation is useful for boosting spectral broadening in silicon waveguides [13]. The change in refractive index due to free carrier dynamics will result in an additional spectrum broadening which will be asymmetric about the pump wavelength [13, 17]. Together, the combination of SPM and free carrier induced refractive index change can enhance the spectral broadening factor as much as >30x [13]. Combining on-chips supercontinuum generation with SIMOX based microdisk filters will produce a chip scale Wavelength Division Multiplexing (WDM) source. Figure 2 shows the conceptual diagram of such a source. Generated broadband continuum source is spectrally carved by integrated microdisk filters to produce multiple wavelength sources. These sources can be utilized for on-chip WDM communication, or as low cost WDM sources for external applications. Add/drop multiplexer structures with different diameter microdisk filters have been demonstrated [18]. An exemplary transmission characteristic of these multiplexers is shown in Fig 3 [18]. Future work is aimed at combining these filters with efficient broadband continuum generation in silicon. Introducing pump and probe signals simultaneously generates Cross Phase Modulation (XPM) due to Kerr nonlinearity [13]. The presence of high intensity pump signal along with weak probe signal can be employed for all optical switching. All optical switching in a Mach-Zehnder interferometer configuration has been demonstrated to illustrate proof of principle operation [13]. Figure 4 shows the switching of CW signal at 1537nm by using 1 ps optical pump pulses at 1560 nm with 20 MHz repetition rate. High extinction ratio of 13 dB with 50 pJ pulse energy is measured. Free carrier induced refractive index change is an important factor in this configuration as well. As it is
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shown in Fig 4, slow recombination of generated free carriers leads to broad switching window of 7 ns [13]. Operating at lower intensity levels is necessary for a fast all optical switching without penalized by free carrier accumulation [13]. Two Photon Absorption (TPA) has been investigated earlier as a means to create a Si autocorrelator device [19]. However, for most of these nonlinear applications the TPA induced free carrier accumulation is detrimental. Reducing the device dimensions and/or adding p-i-n diode structures are proven to be very effective for reducing free carrier lifetime and hence free carrier accumulation [20]. By using micron size ring resonator structures with ~30 ps free carrier lifetime fast all optical modulators based on free carrier effect has been demonstrated very recently [21]. Measurement of other nonlinear effects in these novel devices are yet to be demonstrated. 0 -3
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Fig. 1. Measured spectrum at the input of EDFA, at the waveguide input after EDFA and broadened spectrum at the waveguide output.
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Fig. 4. Result of all optical switching at λs = 1537 nm with 13 dB extinction ratio.
References: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.
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