Siddharth Tallur

A monolithic radiation-pressure driven, low phase noise silicon nitride opto-mechanical oscillator

Cavity opto-mechanics enabled radiation pressure (RP) driven oscillators shown in the past offer an all optical Radio Frequency (RF) source without the need for external electrical feedback. However these oscillators require external tapered fiber or prism coupling and non-standard fabrication processes. In this work, we present a CMOS compatible fabrication process to design high optical quality factor opto-mechanical resonators in silicon nitride. The ring resonators designed in this process demonstrate low phase noise RP driven oscillations. Using integrated grating couplers and waveguide to couple light to the micro-resonator eliminates 1/f(3) and other higher order phase noise slopes at close-to-carrier frequencies present in previous demonstrations. We present an RP driven opto-mechanical oscillator (OMO) operating at 41.97 MHz with a signal power of -11 dBm and phase noise of -85 dBc/Hz at 1 kHz offset with only 1/f(2) noise down to 10 Hz offset from carrier.

A Low Power Impulse Radio Design for Body-Area-Networks

This paper presents a low power radio design tailored to the short distance, low data rate application of body area networks. In our analysis we consider a comparison between traditional continuous wave radios and ultra wide band impulse radios for this application space. We analyze the energy/bit requirement for each of the architectures and discuss how a duty-cycled radio is better suited to low data rate applications due to practical design considerations. As a proof-of-concept we present the design and measured results of a duty-cycled, noncoherent impulse radio transceiver. The designed transceiver was measured to consume only 19 μW at a data-rate of 100 kbps. The design gives a BER of 10-5 and works for a range of 2.5 m at an average Rx-sensitivity of -81 dBm. The designed transceiver enables both OOK and BPSK schemes and can be configured to use a pseudocoherent self-correlated signature detection and generation mechanism. This added functionality helps distinguish different types of pulses such as timing and data-pulses in real time. The transceiver was designed in a 90 nm CMOS process and occupies 2.3 mm2 area.

An Ultralow-Power Dual-Band UWB Impulse Radio

In this brief, we present a dual-band ultralow-power ultrawideband (UWB) impulse radio (UWB-IR) transceiver in a 90-nm CMOS process for low-data-rate UWB on-off keying (OOK) communications. The dual-band topology enables separation of timing and data impulses for simplified synchronization and duty cycling. The receiver can dynamically switch between two 500-MHz bands centered at 3.5 and 4.5 GHz with an interband isolation of 30 dB. With a fast turn-on time of ~1-2 ns, the total power requirement of the duty-cycled receiver (Rx) block is measured to be 12 μW at 100 kb/s for a Rx sensitivity of -87 dBm and a bit error rate (BER) of 10-3. The dual-band impulse-based transmitter (Tx) is designed using a duty-cycled LC oscillator topology enabled only when a transmission is requested and consumes only 8 μW at 100 kpulse/s. For a BER of 10-3 , the instantaneous signal-to-interference ratio was measured to be better than -50 dB for a 2.4-GHz narrowband interferer. The total power for the transceiver at 20 μW is an order of magnitude better than state-of-the-art designs for comparable performance.

A silicon electromechanical photodetector.

Optomechanical systems have enabled wide-band optical frequency conversion and multichannel all-optical radio frequency amplification. Realization of an on-chip silicon communication platform is limited by photodetectors needed to convert optical information to electrical signals for further signal processing. In this paper we present a coupled silicon microresonator, which converts near-IR optical intensity modulation at 174.2 MHz and 1.198 GHz into motional electrical current. This device emulates a photodetector which detects intensity modulation of continuous wave laser light in the full-width-at-half-maximum bandwidth of the mechanical resonance. The resonant principle of operation eliminates dark current challenges associated with convetional photodetectors. While the results presented here constitute a purely classical demonstration, the device can also potentially be extended to the quantum regime to realize a photon-phonon translator.

Phase noise modeling of opto-mechanical oscillators

We build upon and derive a precise far from carrier phase noise model for radiation pressure driven opto-mechanical oscillators and show that calculations based on our model accurately match published phase noise data for such oscillators. Furthermore, we derive insights based on the equations presented and calculate phase noise for an array of coupled disk resonators, showing that it is possible to achieve phase noise as low as −80 dBc/Hz at 1 kHz offset for a 54 MHz opto-mechanical oscillator.

Motional impedance analysis: Bridging the ‘Gap’ in dielectric transduction

This paper presents an analytical model to estimate the motional resistance for partial air gap capacitively-transduced MEMS resonators. This model serves as a link between the well formulated analytical models for conventional air gap and internal dielectric transduction schemes, thereby helping decide which scheme is optimal for a given design frequency. Using this model, we simulate and experimentally verify the motional resistance for a 303MHz polysilicon disk resonator within a 5% range of accuracy.

A silicon nitride optomechanical oscillator with zero flicker noise

We present an integrated chip-scale Radiation-Pressure driven Opto-Mechanical Oscillator (RP-OMO) in silicon nitride with excellent close-to-carrier phase noise. We illustrate a process to micro-fabricate optomechanical resonators, waveguides and grating couplers in silicon nitride and demonstrate an RP-OMO operating at 41.95MHz, with phase noise of -85dBc/Hz at 1kHz offset. The phase noise does not show 1/f3 or other higher order slopes all the way down to 10Hz offset from carrier. Using a lower optical quality factor resonance, we demonstrate improvement of 6dB in phase noise.

Partial Gap Transduced MEMS Optoacoustic Oscillator Beyond Gigahertz

Electrostatically actuated microelectromechanical system (MEMS) oscillators are limited to few megahertz-gigahertz range on account of transduction inefficiency at higher frequencies. Piezoelectric transduction affords lower motional impedances at high frequencies, however mass-loading on account of metal electrodes imposes practical limits on the mechanical quality factor of piezoelectric resonators in the gigahertz frequency regime. In this paper, we present a silicon optoacoustic oscillator operating at 2.05 GHz with signal power 18 dBm and phase noise -80 dBc/Hz at 10-kHz offset from carrier. We employ displacement amplification and partial air gap capacitive transduction to enhance the transduction efficiency. An unconventional photolithography step is performed on a released MEMS structure, which greatly simplifies the fabrication process and allows electrical contact with the electrodes. Built-in nonlinear optomechanical modulation provides noiseless up-conversion of the oscillation signal all the way up to 16.4 GHz with -45-dBm signal power. We develop a phase noise model for the oscillator and identify the photodetector shot noise to be the dominant noise source. Using a high gain and low noise avalanche photodetector enables reduction of the far-from-carrier phase noise floor by 15dB. The phase noise model provides insights into understanding the influence of laser detuning on the oscillator noise performance, which has not been studied to date.

Comparison of f-Q scaling in wineglass and radial modes in ring resonators

Low phase noise MEMS oscillators necessitate resonators with high f-Q. Resonators achieving high f-Q (mechanical frequency-quality factor product) close to the thermo-elastic damping (TED) limit have been demonstrated at expense of feed-through. Here we present a study comparing frequency scaling of quality factors of wineglass and radial modes in a ring resonator using an opto-mechanical two port transmission measurement. Higher harmonics of the wineglass mode show an increasing trend in the f-Q product, as compared to a saturation of f-Q for radial modes. The measured f-Q of 5.11×1013 Hz at 9.82GHz in air at room temperature for a wineglass mode is close to the highest measured values in silicon resonators.

Electromechanically Induced GHz Rate Optical Frequency Modulation in Silicon

We present a monolithic silicon acousto-optic frequency modulator (AOFM) operating at 1.09 GHz. Direct spectroscopy of the modulated laser power shows asymmetric sidebands, which indicate coincident amplitude modulation and frequency modulation (FM). Employing mechanical levers to enhance the displacement of the optical resonator resulted in greater than 67X improvement in the optomechanical FM factor over earlier reported numbers for silicon nanobeams.