Shubhashish Datta

Photonic microwave signals with zeptosecond-level absolute timing noise

Ultralow-noise microwave signals are generated at 12 GHz by a low-noise fibre-based frequency comb and cutting-edge photodetection techniques. The microwave signals have a fractional frequency stability below 6.5 × 10–16 at 1 s and a timing noise floor below 41 zs Hz–1/2. Photonic synthesis of radiofrequency (RF) waveforms revived the quest for unrivalled microwave purity because of its ability to convey the benefits of optics to the microwave world1,2,3,4,5,6,7,8,9,10,11. In this work, we perform a high-fidelity transfer of frequency stability between an optical reference and a microwave signal via a low-noise fibre-based frequency comb and cutting-edge photodetection techniques. We demonstrate the generation of the purest microwave signal with a fractional frequency stability below 6.5 × 10−16 at 1 s and a timing noise floor below 41 zs Hz−1/2 (phase noise below −173 dBc Hz−1 for a 12 GHz carrier). This outperforms existing sources and promises a new era for state-of-the-art microwave generation. The characterization is achieved through a heterodyne cross-correlation scheme with the lowermost detection noise. This unprecedented level of purity can impact domains such as radar systems12, telecommunications13 and time–frequency metrology2,14. The measurement methods developed here can benefit the characterization of a broad range of signals.

GRIN Lens Coupled Top-Illuminated Highly Linear InGaAs Photodiodes

We report a graded index (GRIN) lens-coupled highly linear InGaAs photodiode with a third-order harmonic output intercept point of 54 dBm and an estimated third output intercept point of 49.2 dBm. Optical beam shaping through GRIN lens coupling enhances the third-order intercept by 8 dB as compared to single-mode fiber coupled photodiodes.

A monolithically integrated long-wavelength balanced photodiode using asymmetric twin-waveguide technology

We demonstrate a long-wavelength monolithically integrated photodiode pair balanced to within 0.4 dB. The (15/spl plusmn/1) GHz bandwidth balanced photodiode pair has minimum dark current of 5 nA, a responsivity of (0.32/spl plusmn/0.02) A/W at 1.55-/spl mu/m wavelength, linearity up to +3-dBm input optical power, and a common mode rejection ratio of (34/spl plusmn/2) dB. The asymmetric twin-waveguide architecture used makes it possible for integration of the detector pair with other optical components such as semiconductor optical amplifiers, lasers, modulators, and in-plane waveguide-filters.

Dual InGaAs Photodiodes Having High Phase Linearity for Precise Timing Applications

We report highly linear InGaAs p-i-n dual photodiodes having a power-to-phase conversion factor of < 3.2 rad/W up to 1.5-V peak radio-frequency amplitude. These matched photodiodes, each having a 3-dB bandwidth of 22 GHz, demonstrate nearly identical performance leading to a differential power-to-phase conversion factor of < 1.5 rad/W.

Modeling of nonideal volume Bragg reflection gratings in photosensitive glass using a perturbed transmission matrix approach

Reflection spectra of volume Bragg gratings written in bulk photosensitive silicate glass for wavelength division multiplexing applications are modeled using a transmission matrix approach. This allows for the examination of the effects of spatial perturbations along the grating due to chirp, apodization, compositional inhomogeneities, and index contrast saturation leading to reflection spectra that are asymmetric about the Bragg peak. Effects of the nonlinear relationship between index contrast in the glass and exposure flux on the reflection spectrum is also studied. Volume Bragg gratings are fabricated in UV exposure-sensitive silicate glasses containing alkali-halide nano-crystalline domains, and their reflection spectra are compared with calculation. The magnitude and uniformity of the index changes observed in our glass gratings make them useful in a wide range of wavelength multiplexing applications.

A monolithically integrated optical heterodyne receiver

We demonstrate a monolithically integrated optical heterodyne receiver consisting of an input fiber waveguide, a single frequency distributed Bragg reflector (DBR) laser, a multimode interferometer-based 3-dB coupler, and a pair of semiconductor optical amplifier (SOA)/p-i-n modules using a single epitaxial growth based on asymmetric twin-waveguide (ATG) technology. The input signal was coupled into a fiber waveguide and mixed with the DBR local oscillator laser through the 3-dB coupler, and then coupled into the SOA/p-i-n modules for detection. Using this monolithically integrated optical receiver with a wide-band rectifier narrow-band amplifier receiver, a 3-GHz frequency modulated coherent analog optical link is demonstrated.

A high bandwidth analog heterodyne RF optical link with high dynamic range and low noise figure

We demonstrate a 1.55-/spl mu/m wavelength 5-GHz analog heterodyne optical link employing an integrated balanced photodiode (BPD) with a noise figure of (20/spl plusmn/1) dB and spurious-free dynamic range of (102/spl plusmn/2) dB/spl middot/Hz/sup 2/3/. A wide-band rectifier narrow-band receiver architecture is employed to allow for phase noise cancellation. The advantage of BPD over a single photodiode is also explored.

High-speed, large-area, p-i-n InGaAs photodiode linear array at 2-micron wavelength

We present 16-element and 32-element lattice-mismatched InGaAs photodiode arrays having a cut-off wavelength of ~2.2 um. Each 100 um × 200 um large pixel of the 32-element array has a capacitance of 2.5 pF at 5 V reverse bias, thereby allowing a RC-limited bandwidth of ~1.3 GHz. At room temperature, each pixel demonstrates a dark current of 25 uA at 5 V reverse bias. Corresponding results for the 16-element array having 200 um × 200 um pixels are also reported. Cooling the photodiode array to 150K is expected to reduce its dark current to < 50 nA per pixel at 5 V reverse bias. Additionally, measurement results of 2-micron single photodiodes having 16 GHz bandwidth and corresponding PIN-TIA photoreceiver having 6 GHz bandwidth are also reported.

Low-Noise Large-Area Quad Photoreceivers Based on Low-Capacitance Quad InGaAs Photodiodes

We report a large-area quad photoreceiver (2 times 2 array) having an equivalent input current noise density of <3.2 pA/radicHz per quadrant up to a 3-dB bandwidth of ~20 MHz. The low-noise photoreceiver performance is enabled by a 1-mm diameter quad p-i-n InGaAs photodiode having 2.5-pF capacitance per quadrant at 5-V reverse bias.

Improved Impulse Response of Top-Illuminated InGaAs Photodiodes using GRIN Lens Coupling

We report a top-illuminated, GRIN lens-coupled InGaAs photodiode with an impulse response linearity exceeding 2.5 V peak-to-peak at 7.5% duty cycle and 100% modulation depth. A -100% improvement over single mode fiber-coupled photodiodes is experimentally demonstrated.