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Open access peer-reviewed chapter

Silicon Carbide Based Optical Nonlinear Waveguide Device

Written By

Gong-Ru Lin, Chung-Lun Wu, Chih-Hsien Cheng and Yung-Hsiang Lin

Submitted: 28 February 2015 Reviewed: 12 June 2015

DOI: 10.5772/61045

Advanced Silicon Carbide Devices and Processing IntechOpen
Advanced Silicon Carbide Devices and Processing Edited by Stephen Saddow

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Advanced Silicon Carbide Devices and Processing

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Abstract

The ultrafast nonlinear optical Kerr switch with Si quantum-dot (Si-QD) doped in amorphous a-SiC (a-SiC:Si-QD) micro-ring resonator is demonstrated. The optical nonlinearity of a-SiC can be significantly enhanced due to the enlarged oscillation strength of localized excitons in the Si-QD. The nonlinear refractive index and two-photon absorption (TPA) coefficient of a-SiC:Si-QD at 800 nm obtained from Z-scan measurements are 1.83 × 10–11 cm2/W and 4.6 × 10–6 cm/W, respectively. Although the TPA effect is severed at 800 nm, the TPA effect can be significantly suppressed by setting the operation wavelength at 1550 nm due to the small photon energy. Such a property is very important to analyze the nonlinear Kerr switch at telecommunication wavelengths without interfering with the two-photon absorption and free-carrier absorption effect. By injecting a pump pulsed laser with peak power of 3 W into the a-SiC:Si-QD micro-ring resonator at resonance condition, the transmission spectrum is dynamically red-shifted by 0.07 nm due to the nonlinear Kerr effect. By properly setting the probe wavelength at on-resonance and off-resonance of the a-SiC:Si-QD micro-ring resonator, the probe beam can be directly and inversely modulated by the injected pump source. Furthermore, the all-optical nonlinear Kerr switch delivering non-return-to-zero on-off-keying (NRZ-OOK) data format with bit-rate of 12 Gbit/s has been successfully demonstrated by using the a-SiC:Si-QD micro-ring resonator.

Keywords

  • Silicon carbide
  • Silicon quantum dot
  • Nonlinear Kerr effect
  • All-optical modulation

1. Introduction

1.1. Historical review of SiC-based optoelectronic devices

The nonstoichiometric silicon carbide (SiC) material has been investigated in recent years because of its C/Si composition ratio detuned bandgap energy [1]. In particular, the SiC has been considered as the perfect matrix for high-power electronic devices due to its unique properties of high electron velocity [2] and large breakdown electric field [3]. When combining the features of controllable n- or p-type doping concentrations in SiC films [4], the SiC material has subsequently been considered as a potential candidate for optoelectronic devices. In the past decades, most researches related to the SiC-based optoelectronic devices have been focused on light-emitting diodes (LEDs), solar cells and field-effect transistors. In view of previous works, the first report on the electroluminescence (EL) of p-i-n SiC LEDs varying from red to green color was observed by Kruangam et al. [5]. Later on, many studies on tuning the stoichiometry of SiC by changing its composition ratio were successively reported [6–8]. Tai and coworkers have even utilized the Si-rich SiC films with buried silicon quantum dots (QDs) and SiC-QDs as the active layers to improve the external quantum efficiency (EQE) of LEDs [9]. The highest EQE of Si-rich-based LEDs could be obtained as 1.58 × 10–1% [9]. Moreover, Gao et al. also reported that the a-Si1–xCx:H n-i-p-based solar cell can be used as a semitransparent solar cell in an optical-transmittance modulator [10], but such a SiC solar cell can only provide a conversion efficiency of less than 1% [10]. Moreover, Cheng et al. changed the thickness of i-SiC layer in the all Si-rich SiC-based solar cells to promote the filling factor and conversion efficiency [11]. The optimized thickness of i-SiC layer at 50 nm for all Si-rich SiC-based solar cells enhanced its filling factor and conversion efficiency to 0.25% and 1.7%, respectively [11]. On the other hand, Estrada et al. demonstrated all the SiC-based thin-film transistors with a field-effect mobility of 1.9 × 10–2 cm2 V–1 s–1 [12]. Nowadays, SiC materials are also employed for fabricating high-temperature and high-power electronic devices to keep Si-based electronics sensitive under extreme environments by the superior thermal stability and chemical inertness of SiC [13,14].

More recently, Si photonics have been developed for the application of optically interconnecting the electronic integrated chips because of bottlenecks under electrical transmission, which facilitates the development of hybrid photonic integrated chips with group IV semiconductor-based photonic and/or optoelectronic devices [15,16]. More than that, the optoelectronic photonic devices based on other dielectric materials such as Si-rich SiOx and SiNx matrices has also emerged to meet some unique demands [17–19]. Accordingly, the unique nonlinear optical properties of bulk SiC materials have been characterized for developing SiC-based waveguides and modulators [20–21]. Neidermeier et al. reported the experimental observation of second-order nonlinear coefficients of SiC with different polytypes [20]. The second-order nonlinear coefficient for 4H- and 6H-SiC materials are obtained as 18 and 24 pm/V, respectively [20]. Strait et al. stated that the strong optical rectification effect of nonstoichiometric 6H-SiC could be effectively applied for the generation of terahertz radiation [21], and the ratio of χzzz(2)/χzxx(2) for 6H-SiC was observed as –3 ± 2.6 [21]. Wu and coworker also calculated the second-order nonlinear optical susceptibility of different SiC polytypes by employing the density functional theory [22]. The χxyz(2) for 3C-SiC and χzzz(2) for 6H-SiC were simulated as 34.2 and 38.6 pm/V, respectively [22]. Except that there were few studies focused on the third-order nonlinear optical properties of SiC materials [23,24]. Only the analyses on damage threshold and optical nonlinearity of bulk SiC measured by using the femtosecond laser were studied by DesAutels et al. [23]. Both the nonlinear absorption coefficient and nonlinear refractive index of the semi-insulating SiC have been determined as 6.4 × 10–2 cm/GW and 4.75 × 10–6 cm2/GW, respectively [23]. Ding’s group also observed that the third-order nonlinear optical susceptibility of the SiC material can be significantly modified with different nitrogen (N) doping concentrations [24]. By enlarging the N doping concentration to 2 × 1017 cm–1, the real part of the third-order nonlinear susceptibility can be improved to 6.16 × 10–13 esu [24]. Technically, there are many methods to measure the nonlinear optical properties of materials, such as self-phase modulation [25], degenerate four-wave-mixing [26], nonlinear interferometry [27], nearly degenerate three-wave mixing [28], ellipse rotation [29], beam distortion [30] and Z-scan measurements [31]. Among these aforementioned techniques, the Z-scan technology is commonly used due to its high sensitivity and simple equipment. Although the bulk SiC materials with their nonlinear optical properties could be observed in previous reports, there are few works on studying the nonlinear optical properties of nanoscaled SiC film and Si-QD doped in the amorphous SiC (a-SiC) matrix.

1.2. Historical review of Si-based all-optical switching with the advantages of SiC-based nonlinear waveguide applications

To achieve an ultra-high-speed communication system, the Si-based all-optical switching devices have been widely developed. Generally, the all-optical switching devices demonstrated by Si nanowires and Si-QD-based waveguides are based on the free-carrier plasma dispersion (FCD), absorption effects (FCA) and nonlinear Kerr effect [32–39, 43]. Although the FCA cross-section in Si-QD is one-order of magnitude larger than the bulk Si [35], the free-carrier lifetime (~10 μs) is relatively longer than bulk Si (1 ns). The free-carrier lifetime of the Si-QD doped in SiOx matrix is dominated by the e–h pair recombination process but not influenced by the diffusion process (as in Si nanowire) [36]. By decreasing the Si-QD size, the carrier lifetime can be significantly enhanced up to ~100 ns due to the quantum confinement effect [37]. However, the modulation bandwidth is still lower than the typical Si nanowire. Without the isolated electrical property of SiOx host matrix, the free-carrier in the Si nanowire, can be relaxed by diffusion processes [40,41]. The modulation bandwidth of all-optical Si-based FCA modulator can be easily achieved ~1 GHz [42]. Table 1 summarizes the performance specifications of Si-based modulators in recent years. Nevertheless, the FCA effect hardly meets the demand to obtain the ultrafast all-optical modulation speed. In this case, to achieve high-speed data transmission in the near future, the all-optical switching approach based on the nonlinear optical Kerr effect must be considered for bit-rate improvement. The response time of the nonlinear Kerr effect is around subpicosecond, which is much faster than the free-carrier lifetime in Si and Si-QD.

Materials Structure Type Modulation Type/Operation Theory Performance Specifications Ref.
Si-QD doped in SiO x matrix Rib waveguide All-optical/FCA effect ⋅ 6 dB/cm FCA loss at pump photon flux of 3 × 1020 /cm2–s [34]
Si-QD doped in SiO x matrix Rib waveguide All-optical/FCA effect ⋅ FCA cross-section of 3.1 × 10–17 cm2 at 1550 nm is one order of magnitude larger than bulk Si
⋅ Modulation bandwidth increased to ~2 Mbit/s by shrinking Si-QD size to 1.7 nm		 [36, 37]
Si Channel waveguide with p-i-n diode Electro-optical/FCA effect ⋅ Reduction of the free-carrier lifetime in Si nano-waveguide from 3 ns to 12.2 ps by applying a reverse bias across an integrated p-i-n diode [38]
Si Micro-ring waveguide All-optical/FCD effect ⋅ The optical transmission of the structure is modulated by more than 97% by use of control light pulse with energy as low as 40 pJ
⋅ Response time of 450 ps		 [33]
Si3N4 Micro-ring waveguide All-optical/Kerr effect ⋅ Nonlinear refractive estimated of
2.4 × 10–15 cm2/W by using nonlinear Kerr switch
⋅ Modulation bandwidth of 1 GHz is achieved		 [39]
Si-QD doped in SiO x matrix Micro-ring and slot waveguide All-optical/Kerr effect ⋅ A modulation depth over 50% has
been achieved for on-chip optical powers of the order of 100 mW
⋅ Ultrafast modulation speed of 40 Gbit/s		 [41]

Table 1.

Historical review of Si-based modulators from recent years

Recently, the all-optical Kerr switching in the Si-QD doped in SiOx-based slot waveguide was realized [43]. By introducing the Si nanostructure into the SiO2 host matrix, the optical nonlinear property is much larger than that in stoichiometric SiO2 [44–46]. As predicted by the quantum confinement effect, the enhanced dipole polarization contributed by the localized e–h pair in the Si-QD can significantly improve the three-order susceptibility [47]. However, the low refractive index of Si-QD doped in SiOx (typical value of ~1.8) leads to poor optical confinement for the channel waveguide geometry. The peak intensity is difficult to achieve when using the channel waveguide structure. Therefore, the slot waveguide structure was utilized to enhance the optical confinement for the SiOx:Si-QD. In this case, the huge coupling loss between the slot waveguide and lensed fiber cannot be avoided due to the ultrasmall core size of the slot waveguide structure (typical core size of 50 to 100 nm). Concerning the Si nano-waveguide, the relatively low bandgap energy of Si would result in a huge two-photon absorption (TPA) by injecting the high power pulse source at 1550 nm [48]. The TPA effect would decrease the optical power inside the waveguide, and the induced free-carrier can also degrade the nonlinear Kerr effect. From this point of view, the Si-rich SiC is considered as a potential candidate. Benefiting from the relatively wide energy bandgap (for the commonly encountered polymorph of SiC is 3.05 eV) and high thermal stability [49], the SiC has the properties of low absorption coefficient at 1550 nm and the material strength at high-power operation. Furthermore, the SiC is predicted to own the high nonlinear refractive index at telecommunication wavelengths. Based on these advantages, the optical nonlinearity of SiC is expected to be improved by introducing the small Si-QD into the a-SiC host matrix.

1.3. Motivation and chapter content

In the first part of this chapter, the fabrication of Si-QD doped in a-SiC host matrix by using the hydrogen-free plasma-enhanced vapor deposition (PECVD) system is demonstrated. The atomic composition in the Si-QD doped in a-SiC is discussed. Furthermore, the nonlinear optical property of the Si-QD doped in a-SiC film is analyzed by using the femtosecond Ti:Sapphire laser-based Z-scan measurement. In the second part, the ultrafast nonlinear Kerr effect in the Si-QD doped a-SiC micro-ring resonator is investigated. The Si-QD doped a-SiC matrix all-optical switch is preliminarily demonstrated. The Si-QD doped a-SiC by low-temperature PECVD deposition process is a low-cost fabrication compared to the crystalline Si nano-waveguide fabricated by conventional CVD. Moreover, the nonlinear optical properties of a-SiC can be tuned by adjusting the atomic composition or introducing the nanostructure into the host matrix. Such atomic composition variation in the SiC cannot be obtained in the single-crystallized SiC due to the fixed atomic composition and crystal structure. Fig. 1 illustrates the deposition of Si-QD doped in a-SiC matrix by using the low-temperature PECVD system. After the E-beam lithography and reactive ion etching process, the Si-QD doped in a-SiC micro-ring resonator can be demonstrated (refer to Fig. 1). Based on the nonlinear Kerr effect, the Si-QD doped in a-SiC micro-ring resonator is utilized to demonstrate the ultrafast all-optical switch. By using a pump-probe system, the continuous-wave probe signal can be directly and inversely modulated by the injected pump pulse. With the nonlinear Kerr effect induced wavelength red-shift on the transfer function of the Si-QD doped in a-SiC micro-ring resonator, the nonlinear refractive index at near-infrared wavelengths is also preliminarily determined. According to the ultrafast response of the nonlinear Kerr effect, the refractive index inside the a-SiC ring cavity is dynamically modified by the input pump pulse. The transfer function of transmission properties then varies dynamically by the nonlinear Kerr effect, thus providing a high-speed optical switch of up to 12 Gbit/s via the cross-wavelength amplitude modulation effect.

Figure 1.

The fabrication process of a-SiC:Si-QD micro-ring resonator.

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2. Structural properties of amorphous Si-rich SiC

2.1. Composition of amorphous Si-rich SiC

The amorphous Si-rich SiC film was deposited on the Si wafer by using the PECVD with a mixed gaseous recipe of argon-diluted silane (90% Ar +10% SiH4) and methane (CH4). The fluence ratio is set at RSiC = [CH4]/([SiH4]+[CH4]) = 0.5 to facilitate the growth of amorphous Si-rich SiC film. The C/Si composition ratios of these nonstoichiometric SiC films were determined by using X-ray photoelectron spectroscopy (XPS). The atomic concentration of Si and C are obtained as 64.3% and 27.1%, respectively. In comparison with the standard SiC with a C/Si composition ratio of 50%, the amorphous Si-rich SiC grown with RSiC = 0.5 enlarges its excessive concentration up to 37.2%. The C/Si composition ratio of the amorphous Si-rich SiC film grown with the RSiC = 0.5 is 0.42. Under low temperature and weak RF plasma deposition, the SiH4 molecules is more easily decomposed compared with the CH4 molecules due to the lower dissociation energy of the SiH4 molecules (75.6 kcal/mol) [50]. Under high molecule density, each reactant molecule obtains insufficient energy from the plasma, and the decomposing rates of SiH4 and CH4 molecules cannot significantly distinguish from each other. Therefore, the Si-rich condition of SiC is easily obtained under the growth of RSiC = 0.5. In the meantime, the oxygen content in Si-rich SiC films is also maintained as 8.6% to keep the quality of the PECVD grown SiC film. The Si(2p) orbital electron related XPS spectra of the Si-rich SiC film grown with RSiC = 0.5 is shown in Fig. 2. The detected XPS spectra indicates the significant phase change of SiC films by fitting the Si(2p) orbital electron related XPS spectra with four separated Gaussian components. The binding energies of the decomposed peaks are 99.7, 100.5, 101.5 and 103.35 eV, which are attributed to the Si-Si bonds, Si-C bonds, C-Si-O bonds and Si-O bonds, respectively. The presence of Si-Si bond in Si-rich SiC films indicates that the Si-QDs existed in the Si-rich SiC film.

Figure 2.

The compositional bonds and the dependent XPS intensities within the Si(2p) spectrum.

2.2. Optical nonlinearity of amorphous Si-rich SiC

The Z-scan technology has shown its potential for the analyses of nonlinear refractive index and absorption coefficient due to its high sensitivity and simplified architecture. By fitting the transmittance variation around the focal point of the Z-scan system, the optical nonlinear properties of the a-SiC can be determined. Fig. 3 shows the configuration of the single-beam Z-scan experiment including both open- and closed-aperture experiments. The pumping laser source is outputted from the femtosecond Ti:sapphire laser at wavelength of 800 nm. The pulsewidth and repetition rate are 80 fs and 80 MHz, respectively. The minimum beam diameter of the focused pump beam is ~20 μm, and the excitation intensity is varied by moving the sample along the z-axis with a motorized translational stage. The far-field transmitted light passing through the aperture with its beam intensity is recorded by a balanced photodetector. For the open-aperture Z-scan analysis, the transmitted beam through the sample is not shut by the aperture. In that case, the nonlinear absorption (two-photon absorption or saturated absorption) can be observed in the open-aperture Z-scan analysis. In contrast, a closed-aperture Z-scan only collects the on-axis part of the divergent and diffracted beam by using an aperture placed on the z-axis in the far field.

Figure 3.

The scheme diagram of the open- (top) and closed-aperture (bottom) Z-scan measurement.

The Z-scan traces for the a-SiC films with the RSiC of 0.5 are shown in Fig. 4. For the open-aperture Z-scan analysis, the transmittance near the focal point is decreased significantly. It implies that the two-photon absorption is observed in the Si-QD doped in a-SiC matrix at a wavelength of 800 nm. The characterization of the intensity-dependent absorption for a-SiC film can be performed [51], and the nonlinear absorption coefficient of Si-rich SiC at 800 nm is ~4.6 × 10–6 cm/W determined by the open-aperture configuration. To further extract the nonlinear refractive index of the Si-rich SiC, the close/open Z-scan trace shown in Fig. 4 is obtained by dividing the closed-aperture Z-scan trace with the open-aperture Z-scan trace, and the nonlinear refractive index can be fitted with the theoretical transmittance function [46,52]:

T(z,ΔΦ)=1−4ΔΦz/z0(1+(z/z0)2)(9+(z/z0)2)=1−4n2k0IpeakLz/z0(1+(z/z0)2)(9+(z/z0)2),E1

where ∆Φ, n2 and k0 denote the phase shift, the nonlinear refractive index of the a-SiC film, and the wave number, respectively. After numerical simulation, the nonlinear refractive index of a-SiC film is obtained as 1.83 × 10–11 cm2/W for a-SiC films grown with the RSiC of 0.5. This result clearly elucidates that the excessive Si nanocluster could effectively provide large nonlinear optical properties in the Si-rich SiC than the crystalline SiC.

Figure 4.

The close/open Z-scan transmittance ratio of the Si-rich SiC films.

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3. All optical switching in Si-QD doped in a-SiC micro-ring resonator

3.1. Fabrication of Si-QD doped in a-SiC micro-ring resonator

Prior to determining the geometric structure of a-SiC-based ring resonator, the refractive index of Si-QD-doped Si-rich SiC film was obtained by fitting the reflection spectrum of a-SiC:Si-QD film deposited on Si substrate. The refractive index of a-SiC:Si-QD is calculated as ~2.63 at wavelength of 1.5 μm. For single-mode operation in a-SiC:Si-QD-based channel waveguide, the width and height of a-SiC:Si-QD core layer are set as 600 and 300 nm, respectively. For fabricating the a-SiC:Si-QD-based micro-ring channel waveguide, the a-SiC:Si-QD film is deposited on the Si substrate, which is covered with 3-μm-thick SiO2 by thermal oxidation. Subsequently, the electron beam lithography is performed to define the ring and bus waveguide. The width of the waveguide is set as 600 nm, and the gap between the ring and bus waveguide is set as 300 nm. The diameter of the ring resonator is 300 μm. To enhance the coupling efficiency between the waveguide facet and lensed fiber, the inverse taper structure is introduced into the waveguide design [53]. The width of the inverse taper is varied from 200 to 600 nm within a length of 200 μm. After the E-beam lithography, the Cr layer with 80 nm is deposited on the a-SiC:Si-QD film by using E-gun evaporation. Afterwards, the Cr hard mask is transferred on the a-SiC:Si-QD film with the lift-off process. Afterwards, the reactive-ion-etching (RIE) process with a recipe of CF4 + O2 is used to remove the unpatterned a-SiC:Si-QD and form the a-SiC:Si-QD-based micro-ring resonator. After removing the Cr mask, a 2-μm-thick SiO2 upper-cladding layer is then deposited by using the PECVD. Finally, both end-facets were cleaved and polished to minimize its coupling loss of smaller than 3 dB/facet. The polished waveguide cross-section is shown in Fig. 5(a), which reveals that the waveguide facet is very smooth, and the interface between the waveguide core and cladding can be clearly observed. The top-view images of the inverse taper and micro-ring resonator are demonstrated in Figs. 5(b) and 5(c),respectively. The diameter of the ring resonator is set as 300 μm, which is larger than ever reported. This is because the bending loss contributed by the ring waveguide is expected to be minimized.

Figure 5.

(a) The top view image of inverse taper structure of a-SiC:Si-QD-based waveguide. (b) The cross-sectional image of polished a-SiC:Si-QD-based waveguide. (c) The SEM top view image of a-SiC:Si-QD-based micro-ring resonator.

3.2. Operation of SiC ring resonator

With the presence of the micro-ring resonator, the output transmission power is modified with the dark comb-like throughput transfer function on the notched transmission spectrum, as shown in Fig. 6. The transmission spectrum of a-SiC:Si-QD micro-ring resonator shows the duel-modes at long wavelengths. This originates from the TE0 and TM0 modes in the a-SiC:Si-QD micro-ring resonator. The extension of transmission dip of the TM0 mode is lower than that of the TE0 mode due to the lower power coupling between the ring and bus waveguides of the TM0 mode. As evidence, by comparing the mode tails between TE0 and TM0 modes, the evanescent wave of the TE0 mode spreads more significantly than the TM0 mode and results in the high extension transmission dip. In order to obtain the optical property of the a-SiC:Si-QD-based micro-ring resonator, the normalized transmission spectrum of a-SiC:Si-QD micro-ring resonator can be simulated by using the equation as shown in [54].

Figure 6.

The normalized transmission spectrum of a-SiC:Si-QD micro-ring resonator.

The operation principle for demonstrating the nonlinear Kerr switch is illustrated in Fig. 7. A continuous-wave (CW) optical probe signal and a high-power optical pump data-stream at optical telecommunication wavelengths are concurrently coupled into the Si-rich SiC channel micro-ring waveguide resonator. By injecting the high-power optical pulsed data-stream at any resonance dip of the transmission spectrum of a-SiC:Si-QD micro-ring resonator, the transmission resonance dips can be dynamically red-shifted with the presence of an intensive pump due to the nonlinear Kerr effect. Such nonlinear Kerr effect causes an increased refractive index in the a-SiC:Si-QD micro-ring resonator and results in the red-shift of the notched resonant dip away from its original wavelength. As a result, the transmittance at probe wavelength is dynamically influenced by the nonlinear Kerr effect. By properly selecting the probe wavelength around the resonance dip of the a-SiC:Si-QD micro-ring resonator, the probe beam can be directly or inversely modulated by the pump pulse due to the nonlinear Kerr effect. In more detail, if the wavelength of the probe beam is set at the resonance dip of the micro-ring resonator without pump pulse injection, the transmittance of the probe beam reduces from its initial condition. When the pump pulse is injected into the ring resonator, the resonance dip is red-shifted due to the nonlinear Kerr effect. In that case, the transmittance of the probe beam is increased accordingly. Then, the probe beam can be directly modulated by the pump pulse as shown in Fig. 7(a). On the contrary, if the wavelength of the probe beam is slightly adjusted away from the resonance dip (longer than the resonant wavelength), the transmittance of the probe beam is increased without the pump pulse modulation. Once the pump pulse is injecting into the a-SiC:Si-QD micro-ring resonator, the resonance dip is dynamically red-shifted to the probe wavelength due to the nonlinear Kerr effect. That is, the transmittance of the probe beam is instantly decreased when the pump pulse is introduced into the ring cavity, resulting in the inverse modulation of the probe beam (referred to in Fig. 7(b)).

Figure 7.

The schematic diagram of nonlinear Kerr switch and operation principle of (a) direct modulation and (b) inverse modulation.

The nonlinear Kerr switch is characterized by a pump-probe analysis. An external modulation method is used to generate a high-power optical pump pulse. An electrical pulse with a duration of 83 ps and a repetition rate of 12 MHz was employed to modulate a tunable laser through a Mach-Zehnder modulator. The pump pulse was amplified by using an erbium-doped fiber amplifier (EDFA) to obtain a peak power of 3 W. To inject the pump/probe beam into the a-SiC:Si-QD micro-ring resonator, two beams are combined by using a 50/50 coupler injected into the waveguide via a lensed fiber. Moreover, the modulated probe beam and pump pulse are collected from another waveguide facet by using the lensed fiber. In order to analyze the modulated probe signal without the contribution of the pump pulse, the optical bandpass filter is utilized to eliminate the pump pulse. Subsequently, the modulated probe signal is detected by using a high-speed photodetector, and the modulated probe trace is displayed by using the digital sampling oscilloscope. Fig. 8 shows the time-domain traces of a single bit shape for the modulated probe signals with different operating wavelengths. Firstly, when selecting the probe wavelength at the resonance dips of the a-SiC:Si-QD micro-ring resonator, the probe beam can be directly modulated by the original optical pump data-stream and with the maximum positive modulated amplitude. Moreover, the modulated amplitude of the probe beam is gradually decreased with the red shift of the probe wavelength. The probe beam becomes inversely modulated when the wavelength of probe beam is shifted. In comparison with the time-domain trace of the high-power pump pulse, the converted probe and the inverted probe perfectly match the bit shape to the optical pump pulse without distortion.

Figure 8.

The preserved and inverted single bit shapes of the modulated probe signals at different operating wavelengths.

To realize the practical NRZ-OOK modulation by using the a-SiC:Si-QD micro-ring resonator, the optical pump source is encoded by an arbitrary waveform generator (AWG) with the NRZ-OOK data format. The bit-rate of the pump signal is 12 Gbit/s and the time-domain trace is shown in Fig. 9(a). Similar to the previous experimental condition, the wavelength of the pump signal is selected at resonance dip of the micro-ring resonator of 1551.08 nm to induce the nonlinear Kerr effect. As expected, when setting the probe wavelength at on-resonance (λprobe = 1559.59 nm) and off-resonance (λprobe = 1559.65 nm) in the adjacent transmission dip of the micro-ring resonator, the probe beam can be directly and inversely modulated accordingly. The corresponding modulated probe signal traces with preserved and inverted NRZ-OOK data-stream are shown in Figs. 9(b) and 9(c),respectively. Moreover, there is no signal distortion by comparing the pump data-stream with the modulated probe signal traces, indicating that the 12 Gbit/s NRZ-OOK data-stream can be modulated by using the a-SiC:Si-QD micro-ring resonator. Furthermore, modulation on the probe signal is dominated by the red-shift on the resonance dip of the a-SiC:Si-QD micro-ring resonator, which is solely contributed by the nonlinear Kerr effect but not the TPA or FCA effect.

Figure 9.

The time-domain traces of the 12 Gbit/s NRZ-OOK data-streams measured from the ports of (a) the optical pump input at λ = 1551.08 nm, (b) the probe output at on-resonant dip wavelength of 1559.59 nm and (c) the probe output at off-resonant dip wavelength of 1559.65 nm.

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4. Conclusion

The optical nonlinearity of a-SiC is enhanced by doping the Si-QD into the host matrix by using the PECVD. Based on the Z-scan measurement, the nonlinear refractive index and two-photon absorption coefficient at 800 nm of a-SiC:Si-QD are 1.83 × 10–11 cm2/W and 4.6 × 10–6 cm/W, respectively. Furthermore, the a-SiC:Si-QD is utilized to fabricate the micro-ring resonator to demonstrate the optical nonlinear Kerr switch at ~1550 nm. The fabricated micro-ring waveguide resonator is obtained with Q = 22,117 and the transmittance drop of nearly 60% at a wavelength of 1551.08 nm. By injecting the high-power pump pulse of 3 W at 1551.08 nm into the micro-ring resonator, the refractive index change induced by the nonlinear Kerr effect is utilized to modify the transmittance of the micro-ring resonator. Therefore, the CW probe beam can be modulated by the pump pulse due to the nonlinear Kerr effect. Finally, the all-optical modulation in the a-SiC:Si-QD micro-ring resonator with 12 Gbit/s NRZ-OOK data format has been successively realized in the present work. The modulated probe signal traces show no distortion in comparison with the pumping data-stream, indicating that the modulation process is dominated by the nonlinear Kerr effect without the contribution of TPA and FCA effects. Such a-SiC:Si-QD-based ultrafast all-optical Kerr switch shows great potential for the nonlinear optical waveguide-based data processing applications.

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Written By

Gong-Ru Lin, Chung-Lun Wu, Chih-Hsien Cheng and Yung-Hsiang Lin

Submitted: 28 February 2015 Reviewed: 12 June 2015

© The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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