STh3I.7.pdf
CLEO:2015 © OSA 2015
Frequency instability and phase noise characterization of an integrated chip-scale optomechanical oscillator Yongjun Huang1,2,3,*, Jiagui Wu1,2,4, Xingsheng Luan2, Shu-Wei Huang1,2, Mingbin Yu5, Guoqiang Lo5, DimLee Kwong5, Guangjun Wen3, and Chee Wei Wong1,2,* 1
Mesoscopic Optics and Quantum Electronics Laboratory, University of California at Los Angeles, Los Angeles, CA 90095, USA 2 Optical Nanostructures Laboratory, Columbia University, New York, NY 10029, USA 3 Centre for RFIC and System Technology, School of Communication and Information Engineering University of Electronic Science and Technology of China, Chengdu, 611731, China 4 School of Physics, Southwest University, Chongqing 400715, China 5 The Institute of Microelectronics, 11 Science Park Road, Singapore 117685, Singapore Author e-mail address:
[email protected];
[email protected]
Abstract: We characterize the frequency instability and single-sideband phase noise of chip-scale optomechanically-driven oscillators, with integrated Ge photoreceivers. At 400-μW, an open-loop frequency instability at 10-8 is observed, with -125 dBc/Hz phase noise at 10-kHz offset. OCIS codes: (130.3120) Integrated optics devices; (220.4880) Optomechanics; (230.4910) Oscillators
1. Introduction Recently the emergence of optomechanical oscillators (OMO) [1] provides an alternative approach towards stable chip-scale frequency references [2], comparing with classical quartz crystal oscillators. With carefully-designed optical cavities, large radiation pressure forces [3] can be possible in the optomechanical oscillator, modifying the motion of micro/nano-mechanical resonators [4], and such mechanical resonance can become a self-sustained oscillator when drive the optical power exceeding the intrinsic mechanical damping losses. The periodic motion of the mechanical resonator perturbs the optical cavity resonance, resulting in a stable modulation of the intracavity optical field, thus making an on-chip photonic-based radio-frequency (RF) clock [2,3]. Moreover, a fully monolithically-integrated OMO with on-board photodetector and electronics [5] can provide a portable frequency clock, and allows as much RF power into the detector as possible for signal processing. Here we characterize the frequency instability and single-sideband phase noise of the integrated optomechanically-driven oscillators. At 400μW, an open-loop frequency instability at 10-8 is observed, with -125 dBc/Hz phase noise at 10-kHz offset. 2. Chip characterizations and results 70 um a
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Fig. 1 (Color online) (a) Designed OMO with Ge detector chipset layout. (b) Zoom-in scanning electron micrograph (SEM) of input silicon WG with buried silicon inverse taper. (c)-(d) Zoom-in SEMs of air-bridged PhC slot cavity, along with optimized design input/output slot waveguides, and formed by differential perturbative shifting of the nearest neighbor holes from a periodic lattice and denoted by the arrows (red: 5 nm; green: 10 nm; blue: 15 nm). (e)-(g) Zoom-in SEMs of designed Ge detector with tapered silica waveguide and tapered electrode contact pads. (h) Setup for characterizing the DC I-V and RF bandwidth properties of the integrated Ge detector. A RF probe contacts slightly the electrodes as shown in inset. (i) Setup for the optical transmission and mechanical resonance/oscillation measurements.
Fig. 1a shows the designed layout of fully integrated optomechanical cavity oscillator with on-chip Ge detector, with the tapered optical Si waveguide (WG) path denoted in light blue. Laser is first coupled from free-space lenses into a low-loss inverse oxide coupler at the chip facet (Fig. 1b), propagating then from the oxide coupler into a silicon waveguide. The transmitted light is split equally into two paths: one into the integrated Ge detector and the other coupled out from the inverse oxide coupler to off-chip lenses for external test diagnostics. The proposed OMO is a slot-type PhC cavity consisting of two air-bridged PhC slabs separated by a narrow 120 nm air slot as shown in Fig. 1c and 1d. Figs. 1e to g show the integrated Ge detector which analyzed systematically in our previous works [6].
STh3I.7.pdf
CLEO:2015 © OSA 2015
The DC I-V diode characterization and frequency response of the Ge detector are characterized firstly with the setup shown in Fig. 1h. As shown in Fig. 2a, the measured dark current is 500 nA at -1 V bias while a dark current of 1 μA is the typical upper bound for high-bandwidth detectors [6]. The measured 3-dB bandwidth of the detector is 9 GHz at 0 V bias and 18.5 GHz at -1 V bias as shown in Fig. 2b. The oscillator-integrated detector responsivity is measured as 0.58 A/W at 0 V and 0.62 A/W near -0.5 V under 1550 nm illumination of 200 μW. Fig. 2c illustrate the measured optical transmission spectra of the slot cavity resonances with the setup shown in Fig. 1i. The modeled |E|2 field distributions of the resonance is shown in Fig. 2c inset, with intrinsic quality factor of ~ 800,000 for the fundamental mode. Figs. 2d and 2e show the measured RF spectra of the integrated optomechanical oscillator at blue detuning and below/above the threshold power, respectively. For a dropped-in power about -15 dBm, the measured RF spectra for both detectors in room temperature and atmosphere show the fundamental mechanical resonance at 110.3 MHz and a cold cavity mechanical quality factor of about 480. The modeled modal resonance displacement field is shown in the inset of Fig. 2d. By comparing results from external detector and integrated detector, our integrated Ge detector has low background noise floor that can go down to approximately -98 dBm (Fig. 2d). When driven (~ 400 μW) above threshold (~ 127 μW), the intrinsic mechanical energy dissipation is overcame and the optomechanical resonator becomes a self-sustained OMO with narrow linewidths (~11 Hz), as illustrated in Fig. 2e. The vacuum optomechanical coupling rate is determined as ~ 0.8 MHz which is much larger than other non-PhC optomechanical cavities [5], important to reduce the OMO threshold and improve the SNR.
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Fig. 2 (Color online) (a) Measured DC I-V curve for integrated Ge detector under dark and illumination conditions with different laser powers. (b) Integrated Ge detector bandwidth under different reverse biases. (c) Transmission spectra of the fundamental optical resonance mode with loaded Q at 75,200. Inset: |E|2-field distribution of the fundamental resonances. (d) RF spectra with integrated Ge detector and external photodetector, of cold cavity regime before oscillation. Inset: Finite-element model of the fundamental eigenmode. (e) RF spectra with integrated Ge detector and external photodetector of another device which shows mechanical mode centered near 112.7 MHz, under larger input laser power above threshold for oscillation mode. (f) Phase noise results for both integrated Ge detector and external photodetector at 400 μW dropped-in power. The phase noise of signal generator and the phase noise floor of the Agilent instrument are shown in this panel for comparisons. (g) The corresponding Allen deviation results converted from the measured phase noise results for both detectors. (h) Timing jitter for the OMO measured by integrated Ge detector and external photodetector respectively.
Fig. 2f shows the single-sideband phase noise spectra of our free-running OMO chipset, for a 112.7 MHz carrier. In room temperature and atmospheric non-vacuum, our integrated OMO chipset exhibits a phase noise of approximately -103 dBc/Hz at 1 kHz offset and -125 dBc/Hz at 10 kHz offset, one of the lowest noise to date in reported OMOs. We note that at higher frequency offsets (such as 1 MHz or more), the noise floor is limited only by our detector currently as the phase noise measured simultaneously by external detector can get as low as -165 dBc/Hz at 10 MHz offset. As we can see, for offsets close-to-carrier frequency, our free running OMO has significant amount of 1/f 3 whereas for offsets far-from-carrier frequency (f >100 kHz). Fig. 2g shows the Allan deviations calculated from raw phase noise and power-law fitted phase noise for the free-running OMO, and an open-loop frequency instability at 10-8 is observed. The consistency between different methods can be seen in Fig. 2g where there is small phase noise discrepancy at the close-to-carrier and far-from-carrier offsets. As shown in Fig. 2h, the timing jitter of the oscillator is calculated from the measured phase noise integrating from 100 Hz to the carrier frequency, and should be 3.42 ps for the integrated detector and 10.01 ps for the external photodetector, with performance close to commercial electronic frequency standards. 3. References [1] T. J. Kippenberg, and K. J. Vahala, Science 321, 1172–1176 (2008). [2] M. Hossein-Zadeh, and K.Vahala, IEEE J. Sel. Top. Quantum Electron. 16, 276–287 (2010). [3] D. Van Thourhout, and J. Roels, Nature Photonics 4, 211–217 (2010). [4] J. D. Thompson, et al., Nature 452, 72–75 (2008). [5] X. Sun, K. Xu, and H. X. Tang, Opt. Lett. 39, 2514–2517 (2014). [6] K. Ang, T. Liow, and M.Yu, IEEE J. Sel. Top. Quantum Electron. 16, 106–113 (2010).
