Letter
Vol. 41, No. 15 / August 1 2016 / Optics Letters
3599
Broadband tunable microwave photonic phase shifter with low RF power variation in a high-Q AlN microring XIANWEN LIU,1 CHANGZHENG SUN,1,* BING XIONG,1 JIAN WANG,1 LAI WANG,1 YANJUN HAN,1 ZHIBIAO HAO,1 HONGTAO LI,1 YI LUO,1 JIANCHANG YAN,2 TONG BO WEI,2 YUN ZHANG,2 AND JUNXI WANG2 1
Tsinghua National Laboratory for Information Science and Technology, State Key Lab on Integrated Optoelectronics, Department of Electronic Engineering, Tsinghua University, Beijing 100084, China 2 R&D Center for Semiconductor Lighting, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China *Corresponding author:
[email protected] Received 21 June 2016; revised 6 July 2016; accepted 6 July 2016; posted 7 July 2016 (Doc. ID 267533); published 28 July 2016
An all-optically tunable microwave photonic phase shifter is demonstrated based on an epitaxial aluminum nitride (AlN) microring with an intrinsic quality factor of 3.2 × 106 . The microring adopts a pedestal structure, which allows overcoupling with 700 nm gap size and facilitates the fabrication process. A phase shift for broadband signals from 4 to 25 GHz is demonstrated by employing the thermo-optic effect and the separate carrier tuning technique. A phase tuning range of 0°–332° is recorded with a 3 dB radio frequency (RF) power variation and 48 mW optical power consumption. In addition, AlN exhibits intrinsic second-order optical nonlinearity. Thus, our work presents a novel platform with a low propagation loss and the capability of electro-optic modulation for applications in integrated microwave photonics. © 2016 Optical Society of America OCIS codes: (130.3120) Integrated optics devices; (230.1150) All-optical devices; (140.4780) Optical resonators; (050.5080) Phase shift. http://dx.doi.org/10.1364/OL.41.003599
Microwave photonics (MWP) that allows generation, transmission, and processing of radio frequency (RF) signals in the optical domain has attracted significant attention due to its inherent low loss, wide bandwidth, and immunity to electromagnetic interference [1]. In MWP applications such as reconfigurable microwave filters [2] and phased-array antennas [3], high-performance tunable MWP phase shifters with large phase tuning range, broad bandwidth, low RF power variation, and high tuning speed are of particular importance. Conventional microwave phase shifters based on ferrites and ferroelectrics materials suffer from narrow bandwidth, bulky size, and high power consumption [4]. To overcome these drawbacks, various phase-shifting schemes based on photonic components have been proposed, such as semiconductor optical amplifiers [5], electro-optic modulators [6], and nonlinear effects in optical fibers [7,8]. However, as these configurations 0146-9592/16/153599-04 Journal © 2016 Optical Society of America
are based on discrete optoelectronic or fiber components, they suffer from large size, power consumption, and inflexibility. To meet the ever-increasing demand for compact size, low power consumption, and high flexibility, implementing MWP functionalities in a photonic integrated circuit has emerged as a promising field, which is called integrated MWP (IMWP) [9]. Up to now, chip-scale MWP phase shifters have been demonstrated in silicon-on-insulator (SOI) [10–12] or silicon nitride (Si3 N4 ) microrings [13], hybrid III-V/SOI microdisks [14], integrated SOI waveguide Bragg gratings [15], and stimulated Brillouin scattering (SBS) in As2 S3 waveguides [16]. Among them, microring-based approaches have shown great potential for both phase shift and true time delay applications. By employing the separate carrier tuning (SCT) technique, microrings are capable of processing wideband microwave signals [17]. Meanwhile, phase shift with a low RF power variation has been demonstrated with cascaded microring structures [12]. However, the phase tuning for SOI or Si3 N4 microrings is implemented by the thermo-optic effect due to their centrosymmetric crystal structures, which leads to a relatively slow tuning speed (∼1 μs) [10]. To overcome this limitation, phase tuning as fast as hundreds of picoseconds is realized through carrier injection in a hybrid III-V/SOI microdisk but with a large RF power variation [14]. Recently, sputtered aluminum nitride (AlN) was reported to exhibit a low propagation loss [18], and electro-optic modulation has been realized by taking advantage of its second-order optical nonlinearity [19]. Here, we demonstrate a novel tunable MWP phase shifter in a highquality-factor (high-Q) epitaxial AlN microring by utilizing the thermo-optic effect. A large phase-shifting range with low RF power variation and small optical power consumption is obtained. By employing the SCT technique, broadband RF signal processing has been realized. To fabricate the high-Q ring resonators, approximately 1 μm thick AlN film is grown on a c-plane sapphire substrate by metal organic chemical vapor deposition with a measured refractive index of ∼2.09 at 1.55 μm. An x-ray diffraction (XRD) measurement exhibits a full width at half-maximum
Vol. 41, No. 15 / August 1 2016 / Optics Letters
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Fig. 1. Scanning electron microscopy images of (a) the coupling section and (b) the cleaved waveguide facet of the fabricated device.
(FWHM) of ∼100 arcsec along the [002] crystal orientation, indicating high crystalline quality. After depositing a thin SiNx mask on the AlN film by plasma-enhanced chemical vapor deposition (PECVD), the microring and associated bus waveguide are defined by electron beam lithography (EBL) with ZEP520A resist. The pattern is first transferred to the SiNx film by reactive ion etching, and inductively coupled plasma (ICP) dry etching of AlN is then carried out in optimized Cl2 ∕BCl3 ∕Ar mixtures [20]. Finally, the integrated microrings are embedded in 3 μm thick PECVD silicon dioxide (SiO2 ) for protection, and no additional annealing process is performed. Figure 1(a) shows the coupling section of the fabricated device. The microring has an outer radius of 80 μm and a width of 3.2 μm, whereas the width of the bus waveguide is 1.3 μm. Meanwhile, the coupling gap is as wide as 700 nm. To improve the fiber-to-chip coupling efficiency, linearly lateral tapers are adopted at both ends of the bus waveguide to shrink the waveguide width to 0.3 μm. The waveguide facets are cleaved with a scriber machine after thinning the sapphire substrate down to 150 μm [20]. As shown in Fig. 1(b), smooth waveguide facets have been obtained and the waveguide sidewall angle is ∼80°. Typically, a microring-based MWP phase shifter works at the overcoupled condition to achieve a continuous and large phase tuning range [11]. A high-Q microring with a small resonance extinction ratio (ER) is preferred to achieve an abrupt phase-shifting slope and a low RF power variation. However, a small resonance ER in the overcoupled regime normally implies a narrow coupling gap between the microring and the bus waveguide, which will result in a reduced Q factor due to the increased excess coupling loss [21]. To expand the coupling gap while maintaining the overcoupling condition, a pedestal microring is adopted in this work to increase the coupling coefficient [22]. As shown in Fig. 1(b), through accurate control of the etch depth, a pedestal structure is successfully fabricated with a 430 nm thick unetched AlN layer at the ridge bottom. Since the AlN film is only partially etched, it also helps improve the Q factor due to the reduced sidewall scattering loss. To investigate the coupling strength of the fabricated pedestal microring, the power coupling coefficient κ is calculated with FIMMPROP based on the eigenmode expansion (EME) method [23]. The coupling quality factor Q C can then be deduced as [21]
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2π · neff · L ; (1) λ·κ where λ, neff , and L are the wavelength, effective refractive index, and microring circumference, respectively. As illustrated in Fig. 2, the fundamental transverse-electric (TE0 ) and transverse-magnetic (TM0 ) modes exhibit distinct Q C as the coupling gap increases. The reason is that the TE0 mode extends further into the unetched AlN layer, which enhances the modal field overlap between the bus waveguide and the microring. Assuming that the fabricated microring has an intrinsic quality factor Q int of 2 × 106 (Q int > Q C for overcoupling), the resonance ER is obtained based on the following expression [22]: 2 Q Q ERdB 10 · log10 1 − int ∕= 1 int : (2) Q Q QC
C
C
As shown in Fig. 2, TE0 mode exhibits a smaller resonance ER as the coupling gap increases. Thus, by choosing a gap size of 700 nm, we can obtain a high-resonance ER for the TM0 mode and a much smaller one for the TE0 mode. As a result, a thermo-optically tunable MWP phase shifter with low RF power variation as well as small optical power consumption can be realized by employing TE0 and TM0 modes as signal and control lights, respectively. Furthermore, if phase tuning is to be implemented by the electro-optic effect, a RF power variation of ∼1 dB can be realized with a gap size of 400 nm, which can be readily fabricated with the current EBL technique. To characterize the performance of the fabricated microring, a transmission measurement is carried out by employing a tunable laser (Santec TSL-510) as the light source and lensed fibers to couple light into and out of the chip. As illustrated in Fig. 3(a), the microring supports three resonance modes, corresponding to TE0 , TM0 , and first-order TM1 modes. The insertion losses are ∼10 and ∼8 dB for TE- and TM-polarized light, respectively. The resonance ER for the TE0 mode at ∼1549 nm is less than 3 dB, whereas it is around 13 dB for the TM0 mode at ∼1553 nm. According to the phase measurement [24], both TE0 and TM0 modes are overcoupled, whereas the TM1 mode is undercoupled. Accordingly, the proposed all-optically tunable MWP phase shifter can be realized by employing a TE0 mode at ∼1549 nm and a TM0 mode at ∼1553 nm as the signal and control lights, respectively.
Vol. 41, No. 15 / August 1 2016 / Optics Letters
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Fig. 3. (a) Transmission spectra of the microring for TE- and TMpolarized light. (b), (c) High-resolution resonant linewidth measurement with the extracted Q factors for TE0 and TM0 modes at ∼1549 and 1553 nm, respectively.
To determine the Q factors of the signal and control lights, a high-resolution resonant linewidth measurement is implemented with the fine tuning function of the Santec TSL510 tunable laser, which is frequency calibrated by beating with another tunable laser and detected with a high-speed photodetector (PD) in connection with an electrical spectrum analyzer. As shown in Figs. 3(b) and 3(c), the extracted intrinsic Q factors are as high as ∼3.2 × 106 and 1.2 × 106 for the signal and control lights, respectively. Such high Q factors not only ensure an abrupt phase-shifting slope and allow wideband RF signal processing but also reduce the optical power consumption. In addition, the extracted Q C for signal light is ∼5.6 × 105 , which is in good agreement with the simulation result in Fig. 2 for a gap size of 700 nm. The experimental setup for the all-optically tunable MWP phase shifter is illustrated in Fig. 4. The RF signal to be phase
Fig. 4. Experimental setup for the proposed all-optically tunable MWP phase shifter. TLS, tunable laser source; FPC, fiber polarization controller; MZM, Mach–Zehnder modulator; EDFA, erbium-doped fiber amplifier; BPF, bandpass filter; PD, photodetector; VNA, vector network analyzer.
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shifted is generated from a vector network analyzer (VNA) and modulated onto a continuous-wave (CW) light via a Mach– Zehnder modulator (MZM). A single-sideband (SSB) modulated signal is obtained by suppressing one sideband from the MZM’s output with a bandpass filter (BPF2). The modulated signal light is adjusted to TE polarization with a fiber polarization controller (FPC2) and amplified with an erbium-doped fiber amplifier (EDFA2). Meanwhile, the control light from another tunable laser is aligned to TM polarization using FPC1 and amplified by EDFA1. Then, both the modulated signal light and the control light are sent into the AlN microring through a 3 dB coupler. After removing the control light from the output of the microring with BPF1, the RF signal is finally recovered with a photodetector and sent back to the VNA for the extraction of phase and power variation. Figure 5(a) shows the measured phase and power variation when the control light is turned off. An abrupt phase-shifting slope is realized due to the high Q factor of the signal light resonance. For frequency detuned by 5 GHz from the resonance, a maximum phase variation of ∼354° and a low RF power fluctuation of close to 3 dB are recorded. To implement phase tuning, the power of the control light is increased while its wavelength is tuned deep into resonance, which heats up the AlN microring and induces a redshift of the signal light resonance due to thermo-optic effect. As illustrated in Fig. 5(b), the signal light resonance exhibits a linear frequency shift with a slope of 67 1.16 MHz∕mW as the control light power increases. By employing the SCT technique [17] to introduce a phase shift to the optical carrier, broadband RF signal processing can be realized. As shown in Fig. 5(c), a frequency-independent phase shift for RF signals from 4 to 25 GHz is demonstrated as the control light power in bus waveguide (P in ) increases from 2 to 48 mW. Theoretically, the lower and upper frequency limits for the phase shifter are determined by the Q factor of the microring and the frequency spacing between adjacent resonances, respectively. Thanks to the high Q factor of the AlN microring, a lower frequency of 4 GHz is demonstrated, which is mainly limited by the transition band of BPF2 for SSB modulation. On the other hand, the upper frequency of 25 GHz is limited by the bandwidth of the MZM and PD adopted in our experiment. Figure 5(d) depicts the phase shift and RF power variation versus the control light power for a RF signal at 19 GHz. The maximum phase shift is ∼332° with the control light power of 48 mW. A quasi-linear phase shift range of ∼260° is achieved for the control light power ranging from ∼14 to 23 mW. Furthermore, a less than 3 dB RF power fluctuation is maintained over the entire tuning process, which is of great significance in practical applications. By adopting cascaded high-Q microrings, an enhanced phase shift range of a full 360° can be realized [12]. In conclusion, an all-optically tunable MWP phase shifter in a high-Q epitaxial AlN microring has been demonstrated, which provides a novel platform for IMWP applications. A pedestal microring has been adopted to achieve a high coupling coefficient at a gap size of 700 nm. Processing broadband RF signals from 4 to 25 GHz has been realized. A phase tuning range of 0°–332° and less than 3 dB RF power variation have been recorded. Moreover, since AlN has an intrinsic secondorder optical nonlinearity due to its noncentrosymmetric
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crystal structure, there is the potential to realize fast and continuous phase tuning with the electro-optic effect [19]. Funding. National Basic Research Program of China (2012CB315605, 2014CB340002); National Natural Science Foundation of China (NSFC) (61210014, 61321004, 61307024, 61574082, 51561165012); High Technology Research and Development Program of China (2015AA017101); Tsinghua University Initiative Scientific Research Program (20131089364, 20161080068, 20161080062); Open Fund of State Key Laboratory on Integrated Optoelectronics (IOSKL2014KF09).
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Fig. 5. (a) Phase and power variation for the resonance of the signal light. (b) Resonance shift of the signal light with increased control light power. (c) Phase shift for broadband RF signals by employing the SCT technique. (d) Measured phase shift and RF power variation for a 19 GHz microwave signal.
