High-sensitivity inter-satellite optical communications using chip-scale LED and single-photon detector hardware.

Small satellites have challenging size weight and power requirements for communications modules, which we address here by using chip-scale light-emitting diode (LED) transmitters and single-photon avalanche diode receivers. Data rates of 100 Mb/s have been demonstrated at a sensitivity of -55.2 dBm, and simulations with supporting experimental work indicate ranges in excess of 1 km are feasible with a directional gain of up to 52 dBi and comparatively modest pointing requirements. A 750 m, 20 Mb/s link using a single micro-LED has been demonstrated experimentally. The low electrical power requirements and compact, semiconductor nature of these devices offer high data rate, high sensitivity communications for small satellite platforms.

High precision integrated photonic thermometry enabled by a transfer printed diamond resonator on GaN waveguide chip.

We demonstrate a dual-material integrated photonic thermometer, fabricated by high accuracy micro-transfer printing. A freestanding diamond micro-disk resonator is printed in close proximity to a gallium nitride on a sapphire racetrack resonator, and respective loaded Q factors of 9.1 × 104 and 2.9 × 104 are measured. We show that by using two independent wide-bandgap materials, tracking the thermally induced shifts in multiple resonances, and using optimized curve fitting tools the measurement error can be reduced to 9.2 mK. Finally, for the GaN, in a continuous acquisition measurement we record an improvement in minimum Allan variance, occurring at an averaging time four times greater than a comparative silicon device, indicating better performance over longer time scales.

Synchronization-free top-down illumination photometric stereo imaging using light-emitting diodes and a mobile device.

Three dimensional reconstruction of objects using a top-down illumination photometric stereo imaging setup and a hand-held mobile phone device is demonstrated. By employing binary encoded modulation of white light-emitting diodes for scene illumination, this method is compatible with standard lighting infrastructure and can be operated without the need for temporal synchronization of the light sources and camera. The three dimensional reconstruction is robust to unmodulated background light. An error of 2.69 mm is reported for an object imaged at a distance of 42 cm and with the dimensions of 48 mm. We also demonstrate the three dimensional reconstruction of a moving object with an effective off-line reconstruction rate of 25 fps.

Combining time of flight and photometric stereo imaging for 3D reconstruction of discontinuous scenes.

Time of flight and photometric stereo are two three-dimensional (3D) imaging techniques with complementary properties, where the former can achieve depth accuracy in discontinuous scenes, and the latter can reconstruct surfaces of objects with fine depth details and high spatial resolution. In this work, we demonstrate the surface reconstruction of complex 3D fields with discontinuity between objects by combining the two imaging methods. Using commercial LEDs, a single-photon avalanche diode camera, and a mobile phone device, high resolution of surface reconstruction is achieved with a RMS error of 6% for an object auto-selected from a scene imaged at a distance of 50 cm.

Characterization, Selection, and Microassembly of Nanowire Laser Systems.

Semiconductor nanowire (NW) lasers are a promising technology for the realization of coherent optical sources with ultrasmall footprint. To fully realize their potential in on-chip photonic systems, scalable methods are required for dealing with large populations of inhomogeneous devices that are typically randomly distributed on host substrates. In this work two complementary, high-throughput techniques are combined: the characterization of nanowire laser populations using automated optical microscopy, and a high-accuracy transfer-printing process with automatic device spatial registration and transfer. Here, a population of NW lasers is characterized, binned by threshold energy density, and subsequently printed in arrays onto a secondary substrate. Statistical analysis of the transferred and control devices shows that the transfer process does not incur measurable laser damage, and the threshold binning can be maintained. Analysis on the threshold and mode spectra of the device populations proves the potential for using NW lasers for integrated systems fabrication.

Transfer printing of AlGaAs-on-SOI microdisk resonators for selective mode coupling and low-power nonlinear processes.

The transfer printing of aluminum gallium arsenide (AlGaAs) microdisk resonators onto a silicon-on-insulator (SOI) waveguide platform is demonstrated. The integrated resonators exhibit loaded ${Q}$Q-factors reaching $ 4 \times {10^4} $4×104, and the vertical assembly approach allows selective coupling to different spatial mode families. The hybrid platform's nonlinearity is characterized by four-wave mixing with a measured nonlinear coefficient of $ \gamma = 325\;{({\rm Wm})^{ - 1}} $γ=325(Wm)-1, with the devices demonstrating minimal two-photon absorption and free-carrier absorption losses that are inherent to SOI at telecommunications wavelengths.

Scalable visible light communications with a micro-LED array projector and high-speed smartphone camera.

Solid-state camera systems are now widely available in portable consumer electronics, providing potential receivers for visible light communications in every device. Typically, data rates with camera receivers are limited by the 60 fps frame rate of both the image sensor and projector systems. Recent developments in high-frame rate microdisplays and slow-motion cameras for smartphones now permit high-speed, spatially structured signals to be transmitted and captured. Here, we present a method for transmitting data to a smartphone using a CMOS-controlled micro-LED projector system. Spatial patterns are projected onto a wall at a refresh rate of 480 Hz, which can be captured by the smartphone's 960 fps camera. Data transfer is performed over meter scale distances, and the use of an alignment frame gives the system a level of tolerance to motion and misalignment. The current system allows data transmission at a peak rate of 122.88 kb/s using a 16 × 16 micro-LED array, which can be readily scaled to Mb/s rates with a higher resolution transmitter.

Technique for the measurement of picosecond optical pulses using a non-linear fiber loop mirror and an optical power meter.

A method for measuring picosecond pulse width by using only fiber components and optical power meters is presented. We have shown that the output power splitting ratio of a non-linear fiber loop mirror can be used to extract the full-width half maximum of the optical pulse, assuming a known slowly varying envelope shape and internal phase structure. Theoretical evaluation was carried out using both self-phase and cross-phase modulation approaches, with the latter showing a twofold sensitivity increase, as expected. In the experimental validation, pulses from an actively fiber mode-locked laser at the repetition rate of 10 GHz were incrementally temporally dispersed by using SMF-28 fiber, and then successfully measured over a pulse width range of 2-10 ps, with a resolution of 0.25 ps. This range can be easily extended from 0.25 to 40 ps by selecting different physical setup parameters.

Multispectral time-of-flight imaging using light-emitting diodes.

Multispectral and 3-D imaging are useful for a wide variety of applications, adding valuable spectral and depth information for image analysis. Single-photon avalanche diode (SPAD) based imaging systems provide photon time-of-arrival information, and can be used for imaging with time-correlated single photon counting techniques. Here we demonstrate an LED based synchronised illumination system, where temporally structured light can be used to relate time-of-arrival to specific wavelengths, thus recovering reflectance information. Cross-correlation of the received multi-peak histogram with a reference measurement yields a time delay, allowing depth information to be determined with cm-scale resolution despite the long sequence of relatively wide (∼10 ns) pulses. Using commercial LEDs and a SPAD imaging array, multispectral 3-D imaging is demonstrated across 9 visible wavelength bands.

High accuracy transfer printing of single-mode membrane silicon photonic devices.

A transfer printing (TP) method is presented for the micro-assembly of integrated photonic devices from suspended membrane components. Ultra thin membranes with thickness of 150nm are directly printed without the use of mechanical support and adhesion layers. By using a correlation alignment scheme vertical integration of single-mode silicon waveguides is achieved with an average placement accuracy of 100±70nm. Silicon (Si) µ-ring resonators are also fabricated and show controllable optical coupling by varying the lateral absolute position to an underlying Si bus waveguide.

Integration of Semiconductor Nanowire Lasers with Polymeric Waveguide Devices on a Mechanically Flexible Substrate.

Nanowire lasers are integrated with planar waveguide devices using a high positional accuracy microtransfer printing technique. Direct nanowire to waveguide coupling is demonstrated, with coupling losses as low as -17 dB, dominated by mode mismatch between the structures. Coupling is achieved using both end-fire coupling into a waveguide facet, and from nanowire lasers printed directly onto the top surface of the waveguide. In-waveguide peak powers up to 11.8 µW are demonstrated. Basic photonic integrated circuit functions such as power splitting and wavelength multiplexing are presented. Finally, devices are fabricated on a mechanically flexible substrate to demonstrate robust coupling between the on-chip laser source and waveguides under significant deformation of the system.

Silicon photonic filters with high rejection of both TE and TM modes for on-chip four wave mixing applications.

Wavelength selective filters represent one of the key elements for photonic integrated circuits (PIC) and many of their applications in linear and non-linear optics. In devices optimised for single polarisation operation, cross-polarisation scattering can significantly limit the achievable filter rejection. An on-chip filter consisting of elements to filter both TE and TM polarisations is demonstrated, based on a cascaded ring resonator geometry, which exhibits a high total optical rejection of over 60 dB. Monolithic integration of a cascaded ring filter with a four-wave mixing micro-ring device is also experimentally demonstrated with a FWM efficiency of -22dB and pump filter extinction of 62dB.

High-extinction-ratio TE/TM selective Bragg grating filters on silicon-on-insulator.

We report on the design and fabrication of TE and TM polarization selective Bragg grating filters in the form of sinusoidal perturbations on the waveguide sidewall and etched holes on the top of the waveguide, respectively. Combining the two geometries on a silicon-on-insulator waveguide resulted in Bragg grating filters with high extinction ratios of approximately 60 dB.

Tunable Q-factor silicon microring resonators for ultra-low power parametric processes.

A compact silicon ring resonator is demonstrated that allows simple electrical tuning of the ring coupling coefficient and Q-factor and therefore the resonant enhancement of on-chip nonlinear optical processes. Fabrication-induced variation in designed coupling fraction, crucial in the resonator performance, can be overcome using this post-fabrication trimming technique. Tuning of the microring resonator across the critical coupling point is demonstrated, exhibiting a Q-factor tunable between 9000 and 96,000. Consequently, resonantly enhanced four-wave mixing shows tunable efficiency between -40 and -16.3 dB at an ultra-low on-chip pump power of 0.7 mW.

Bandpass integrated Bragg gratings in silicon-on-insulator with well-controlled amplitude and phase responses.

Bandpass filters with square shape amplitude responses and well-controlled dispersion characteristics are achieved by accurate apodization of Bragg grating structures in silicon-on-insulator waveguides. For these devices, precise tailoring of their frequency response typically requires low coupling coefficients and relatively long on-chip propagation lengths. These challenges are addressed by implementing apodization by phase-modulation and using wider strip waveguides to reduce phase noise. This design approach is demonstrated with a dispersion-less narrowband filter and a chirped bandpass filter.

Multiwavelength super-structured Bragg grating laser for tunable repetition rate mode-locked operation.

A multiwavelength laser based on a super-structured Bragg grating is designed and fabricated on multiquantum well AlGaInAs-InP. This laser exhibits phase locking via mutual injection of the neighboring cavities assisted by four wave mixing. We present optical and electrical characterization of its emission regimes showing a complex dynamic behavior. More specifically, this paper focuses on a pulsed regime with a quasi-continuous tunable repetition rate from 32 GHz to 49 GHz.

Multi-wavelength filters in silicon using superposition sidewall Bragg grating devices.

Multiple-filter stopbands, with the potential to be nonuniformly spaced in frequency, are realized in a single integrated Bragg grating device on silicon. By utilizing a superposition of sidewall relief grating functions, N individual filter responses can be fabricated with a device length N× shorter than the equivalent serial set of gratings. Arbitrary combinations of eight-basis filter responses were demonstrated as selected by an eight-bit pseudorandom number generator, showing the flexibility of the complex Bragg grating device design.

Additive GaN Solid Immersion Lenses for Enhanced Photon Extraction Efficiency from Diamond Color Centers.

Effective light extraction from optically active solid-state spin centers inside high-index semiconductor host crystals is an important factor in integrating these pseudo-atomic centers in wider quantum systems. Here, we report increased fluorescent light collection efficiency from laser-written nitrogen-vacancy (NV) centers in bulk diamond facilitated by micro-transfer printed GaN solid immersion lenses. Both laser-writing of NV centers and transfer printing of micro-lens structures are compatible with high spatial resolution, enabling deterministic fabrication routes toward future scalable systems development. The micro-lenses are integrated in a noninvasive manner, as they are added on top of the unstructured diamond surface and bonded by van der Waals forces. For emitters at 5 µm depth, we find approximately 2× improvement of fluorescent light collection using an air objective with a numerical aperture of NA = 0.95 in good agreement with simulations. Similarly, the solid immersion lenses strongly enhance light collection when using an objective with NA = 0.5, significantly improving the signal-to-noise ratio of the NV center emission while maintaining the NV's quantum properties after integration.

Waveguide-Integrated Colloidal Nanocrystal Supraparticle Lasers.

Supraparticle (SP) microlasers fabricated by the self-assembly of colloidal nanocrystals have great potential as coherent optical sources for integrated photonics. However, their deterministic placement for integration with other photonic elements remains an unsolved challenge. In this work, we demonstrate the manipulation and printing of individual SP microlasers, laying the foundation for their use in more complex photonic integrated circuits. We fabricate CdSxSe1-x/ZnS colloidal quantum dot (CQD) SPs with diameters from 4 to 20 µm and Q-factors of approximately 300 via an oil-in-water self-assembly process. Under a subnanosecond-pulse optical excitation at 532 nm, the laser threshold is reached at an average number of excitons per CQD of 2.6, with modes oscillating between 625 and 655 nm. Microtransfer printing is used to pick up individual CQD SPs from an initial substrate and move them to a different one without affecting their capability for lasing. As a proof of concept, a CQD SP is printed on the side of an SU-8 waveguide, and its modes are successfully coupled to the waveguide.

Ultra-low power generation of twin photons in a compact silicon ring resonator.

We demonstrate efficient generation of correlated photon pairs by spontaneous four wave mixing in a 5 µm radius silicon ring resonator in the telecom band around 1550 nm. By optically pumping our device with a 200 µW continuous wave laser, we obtain a pair generation rate of 0.2 MHz and demonstrate photon time correlations with a coincidence-to-accidental ratio as high as 250. The results are in good agreement with theoretical predictions and show the potential of silicon micro-ring resonators as room temperature sources for integrated quantum optics applications.