Photonic chip-based low-noise microwave oscillator.

Numerous modern technologies are reliant on the low-phase noise and exquisite timing stability of microwave signals. Substantial progress has been made in the field of microwave photonics, whereby low-noise microwave signals are generated by the down-conversion of ultrastable optical references using a frequency comb1-3. Such systems, however, are constructed with bulk or fibre optics and are difficult to further reduce in size and power consumption. In this work we address this challenge by leveraging advances in integrated photonics to demonstrate low-noise microwave generation via two-point optical frequency division4,5. Narrow-linewidth self-injection-locked integrated lasers6,7 are stabilized to a miniature Fabry-Pérot cavity8, and the frequency gap between the lasers is divided with an efficient dark soliton frequency comb9. The stabilized output of the microcomb is photodetected to produce a microwave signal at 20 GHz with phase noise of -96 dBc Hz-1 at 100 Hz offset frequency that decreases to -135 dBc Hz-1 at 10 kHz offset-values that are unprecedented for an integrated photonic system. All photonic components can be heterogeneously integrated on a single chip, providing a significant advance for the application of photonics to high-precision navigation, communication and timing systems.

AlInAsSb Geiger-mode SWIR and eSWIR SPADs with high avalanche probability.

Single-photon avalanche diodes (SPADs) that are sensitive to photons in the Short-wave infrared and extended short-wave infrared (SWIR and eSWIR) spectra are important components for communication, ranging, and low-light level imaging. The high gain, low excess noise factor, and widely tunable bandgap of AlxIn1-xAsySb1-y avalanche photodiodes (APDs) make them a suitable candidate for these applications. In this work, we report single-photon-counting results for a separate absorption, charge, and multiplication (SACM) Geiger-mode SPAD within a gated-quenching circuit. The single-photon avalanche probabilities surpass 80% at 80 K, corresponding with single-photon detection efficiencies of 33% and 12% at 1.55 µm and 2 µm, respectively.

Thermal dissipation enhancement in flip-chip bonded uni-traveling carrier photodiodes.

The thermal properties of modified uni-traveling carrier (MUTC) photodiode flip-chip bonded to AlN and diamond are simulated. The thermal impedance of InGaAs is the primary source of internal heating. An n-down epitaxial structure is designed to improve thermal dissipation. Compared to the conventional p-down configuration, the n-down MUTCs bonded to diamond, or AlN submounts achieved 145% and 110% improvement in dissipated power density at thermal failure, respectively. The improved thermal characteristics presage higher RF output power before thermal failure.

Considerations for excess noise measurements of low-k-factor Sb-based avalanche photodiodes.

In applications where high sensitivity is required, the internal gain mechanism of avalanche photodiodes can provide a performance advantage relative to p-i-n photodiodes. However, this internal gain mechanism leads to an excess noise that scales with gain. This excess noise term can be minimized by using materials systems in which impact ionization is initiated primarily by one carrier type. Recently, two Sb-based materials systems, AlInAsSb and AlGaAsSb, have exhibited exceptionally low excess noise, particularly for III-V compound materials. There are four important considerations that can impact the excess noise measurements in such low-noise materials. These considerations deal with the excess noise factor calculation method, measurement RF frequency, measurement wavelength, and the gain calculation method. In this paper, each of these factors is discussed, and their implications on excess noise are considered.

Anomalous excess noise behavior in thick Al0.85Ga0.15As0.56Sb0.44 avalanche photodiodes.

Al0.85Ga0.15As0.56Sb0.44 has recently attracted significant research interest as a material for 1550 nm low-noise short-wave infrared (SWIR) avalanche photodiodes (APDs) due to the very wide ratio between its electron and hole ionization coefficients. This work reports new experimental excess noise data for thick Al0.85Ga0.15As0.56Sb0.44 PIN and NIP structures, measuring low noise at significantly higher multiplication values than previously reported (F = 2.2 at M = 38). These results disagree with the classical McIntyre excess noise theory, which overestimates the expected noise based on the ionization coefficients reported for this alloy. Even the addition of 'dead space' effects cannot account for these discrepancies. The only way to explain the low excess noise observed is to conclude that the spatial probability distributions for impact ionization of electrons and holes in this material follows a Weibull-Fréchet distribution function even at relatively low electric-fields. Knowledge of the ionization coefficients alone is no longer sufficient to predict the excess noise properties of this material system and consequently the electric-field dependent electron and hole ionization probability distributions are extracted for this alloy.

Narrow bandgap Al0.15In0.85As0.77Sb0.23 for mid-infrared photodetectors.

Mid-IR is a useful wavelength range for both science and military applications due to its low atmospheric attenuation and ability to be used for passive detection. However, many solutions for detecting light in this spectral region need to be operated at cryogenic temperatures as their required narrow bandgaps suffer from carrier recombination and band-to-band tunneling at room temperature leading to high dark currents. These problems can be alleviated by using a separate absorption, charge, and multiplication avalanche photodiode. We have recently demonstrated such a device with a 3-µm cutoff using Al0.15In0.85As0.77Sb0.23, as the absorber, grown on GaSb. Here we investigate Al0.15In0.85As0.77Sb0.23 as a simple PIN homojunction and provide metrics to aid in future designs using this material. PL spectrum measurements indicate a bandgap of 2.94 µm at 300 K. External quantum efficiencies of 39% and 33% are achieved at 1.55 µm and 2 µm respectively. Between 180 K and 280 K the activation energy is ∼0.22 eV, roughly half the bandgap of Al0.15In0.85As0.77Sb0.23, indicating thermal generation is dominant.

Frequency behavior of AlInAsSb nBn photodetectors and the development of an equivalent circuit model.

We report the frequency response of Al0.3InAsSb/Al0.7InAsSb nBn photodetectors. The 3-dB bandwidth of the devices varies from ∼ 150 MHz to ∼ 700 MHz with different device diameters and saturates with bias voltage immediately after the device turn on. A new equivalent circuit model is developed to explain the frequency behavior of nBn photodetectors. The simulated bandwidth based on the new equivalent circuit model agrees well with the bandwidth and the microwave scattering parameter measurements. The analysis reveals that the limiting factor of the bandwidth of the nBn photodetector is the large diffusion capacitance caused by the minority carrier lifetime and the device area. Additionally, the bandwidth of the nBn photodetector is barely affected by the photocurrent, which is found to be caused by the barrier structure in the nBn photodetector.

Dynamic-quenching of a single-photon avalanche photodetector using an adaptive resistive switch.

One of the most common approaches for quenching single-photon avalanche diodes is to use a passive resistor in series with it. A drawback of this approach has been the limited recovery speed of the single-photon avalanche diodes. High resistance is needed to quench the avalanche, leading to slower recharging of the single-photon avalanche diodes depletion capacitor. We address this issue by replacing a fixed quenching resistor with a bias-dependent adaptive resistive switch. Reversible generation of metallic conduction enables switching between low and high resistance states under unipolar bias. As an example, using a Pt/Al2O3/Ag resistor with a commercial silicon single-photon avalanche diodes, we demonstrate avalanche pulse widths as small as ~30 ns, 10× smaller than a passively quenched approach, thus significantly improving the single-photon avalanche diodes frequency response. The experimental results are consistent with a model where the adaptive resistor dynamically changes its resistance during discharging and recharging the single-photon avalanche diodes.

Near 100% external quantum efficiency 1550-nm broad spectrum photodetector.

We report InGaAs/InP based p-i-n photodiodes with an external quantum efficiency (EQE) above 98% from 1510 nm to 1575 nm. For surface normal photodiodes with a diameter of 80 µm, the measured 3-dB bandwidth is 3 GHz. The saturation current is 30.5 mA, with an RF output power of 9.3 dBm at a bias of -17 V at 3 GHz.

Room-temperature bandwidth of 2-µm AlInAsSb avalanche photodiodes.

We investigate the room-temperature bandwidth performance of AlInAsSb avalanche photodiodes under 2-µm illumination. Parameter characterization denotes RC-limited performance. While measurements indicate a maximum gain-bandwidth product of 44 GHz for a 60-µm-diameter device, we scale this performance to smaller device sizes based on the RC response. For a 15-µm-diameter device, we predict a maximum gain-bandwidth product of approximately 144 GHz based on the reported measurements.

Efficient absorption enhancement approaches for AlInAsSb avalanche photodiodes for 2-µm applications.

Recently, advances in imaging and LIDAR applications have stimulated the development of high-sensitivity receivers that operate at wavelengths of ≥ 2 µm, which has driven research on avalanche photodiodes (APDs) that operate in that spectral region. High quantum efficiency is a key performance parameter for these photodetectors. Increasing the thickness of the absorption region is a straightforward approach to increase the quantum efficiency. However, the primary source of dark current is the narrow-bandgap material used for 2-µm detection. Increasing its thickness results in higher noise. In this paper, we describe two approaches to enhance the quantum efficiency, both of which are superior to a conventional anti-reflection (AR) coating. For normal incidence at 2 µm, finite-difference time-domain (FDTD) simulations show the absorption can be enhanced by more than 100% with a triangular-lattice photonic crystal, and nearly 400% by applying a metal grating. This is achieved by coupling normal incidence light into the laterally propagating modes in the device. Moreover, the significantly higher absorption of the metal grating compared to the photonic crystal is due to the high coupling efficiency provided by the metal grating. This work provides promising methods and physical understanding for enhancing the quantum efficiency for 2-µm detection without increasing absorber thickness, which also enables low dark current and high bandwidth.

Photonic generation of pulsed microwave signals in the X-, Ku- and K-band.

Photonic microwave generation of high-power pulsed signals in the X-, Ku- and K-band using charge-compensated MUTC photodiodes is demonstrated. The impulse photoresponse without modulation showed a maximum peak voltage of 38.3 V and full-width at half-maximum of 30 ps. High power pulsed microwave signals at 10, 17 and 22 GHz with peak power up to 44.2 dBm (26.3 W), 41.6 dBm (14.5 W) and 40.6 dBm (11.5 W) were achieved, respectively.

Coherent optical clock down-conversion for microwave frequencies with 10-18 instability.

Optical atomic clocks are poised to redefine the Système International (SI) second, thanks to stability and accuracy more than 100 times better than the current microwave atomic clock standard. However, the best optical clocks have not seen their performance transferred to the electronic domain, where radar, navigation, communications, and fundamental research rely on less stable microwave sources. By comparing two independent optical-to-electronic signal generators, we demonstrate a 10-gigahertz microwave signal with phase that exactly tracks that of the optical clock phase from which it is derived, yielding an absolute fractional frequency instability of 1 × 10-18 in the electronic domain. Such faithful reproduction of the optical clock phase expands the opportunities for optical clocks both technologically and scientifically for time dissemination, navigation, and long-baseline interferometric imaging.

Triple-mesa avalanche photodiodes with very low surface dark current.

The dark current of a photodetector is a key parameter for high-sensitivity optical receivers. We report low-dark-current, triple-mesa avalanche photodiodes that have ~50 times lower dark current than conventional single-mesa devices, and suppress surface leakage. The tolerances of triple-mesa avalanche photodiode parameters are presented.

III-V on silicon avalanche photodiodes by heteroepitaxy.

We demonstrate a III-V avalanche photodiode (APD) grown by heteroepitaxy on silicon. This InGaAs/InAlAs APD exhibits low dark current, gain >20, external quantum efficiency >40%, and similar low excess noise, k∼0.2, as InAlAs APDs on InP.

AlxIn1-xAsySb1-y photodiodes with low avalanche breakdown temperature dependence.

We report AlxIn1-xAsySb1-y PIN and Separate Absorption, Charge and Multiplication (SACM) avalanche photodiodes (APDs) with high temperature stability. This work is based on measurements of avalanche breakdown voltage of these devices for temperatures between 223 K and 363 K. Breakdown voltage temperature coefficients are shown to be lower than those of APDs fabricated with other materials with comparable multiplication layer thicknesses.

Hybrid integration of modified uni-traveling carrier photodiodes on a multi-layer silicon nitride platform using total reflection mirrors.

We demonstrate hybrid integration of modified uni-traveling carrier photodiodes on a multi-layer silicon nitride platform using total reflection mirrors etched by focused ion beam. The hybrid photodetectors show external responsivity of 0.15 A/W and bandwidth of 3.5 GHz for devices with a diameter of 80 µm. The insertion loss of the waveguide is 3 dB and the coupling efficiency of the total reflection mirror is -3 dB. The highest RF output power is -0.5 dBm measured at 3 GHz with 9 mA photocurrent and -9 V bias.

Breaking the buildup-time limit of sensitivity in avalanche photodiodes by dynamic biasing.

Avalanche photodiodes (APDs) are the preferred photodetectors for direct-detection, high data-rate long-haul optical telecommunications. APDs can detect low-level optical signals due to their internal amplification of the photon-generated electrical current, which is attributable to the avalanche of electron and hole impact ionizations. Despite recent advances in APDs aimed at reducing the average avalanche-buildup time, which causes intersymbol interference and compromises receiver sensitivity at high data rates, operable speeds of commercially available APDs have been limited to 10Gbps. We report the first demonstration of a dynamically biased APD that breaks the traditional sensitivity-versus-speed limit by employing a data-synchronous sinusoidal reverse-bias that drastically suppresses the average avalanche-buildup time. Compared with traditional DC biasing, the sensitivity of germanium APDs at 3Gbps is improved by 4.3 dB, which is equivalent to a 3,500-fold reduction in the bit-error rate. The method is APD-type agnostic and it promises to enable operation at rates of 25Gbps and beyond.

Broadband noise limit in the photodetection of ultralow jitter optical pulses.

Applications with optical atomic clocks and precision timing often require the transfer of optical frequency references to the electrical domain with extremely high fidelity. Here we examine the impact of photocarrier scattering and distributed absorption on the photocurrent noise of high-speed photodiodes when detecting ultralow jitter optical pulses. Despite its small contribution to the total photocurrent, this excess noise can determine the phase noise and timing jitter of microwave signals generated by detecting ultrashort optical pulses. A Monte Carlo simulation of the photodetection process is used to quantitatively estimate the excess noise. Simulated phase noise on the 10 GHz harmonic of a photodetected pulse train shows good agreement with previous experimental data, leading to the conclusion that the lowest phase noise photonically generated microwave signals are limited by photocarrier scattering well above the quantum limit of the optical pulse train.

Optical amplification and pulse interleaving for low-noise photonic microwave generation.

We investigate the impact of pulse interleaving and optical amplification on the spectral purity of microwave signals generated by photodetecting the pulsed output of an Er:fiber-based optical frequency comb. It is shown that the microwave phase noise floor can be extremely sensitive to delay length errors in the interleaver, and the contribution of the quantum noise from optical amplification to the phase noise can be reduced ∼10 dB for short pulse detection. We exploit optical amplification, in conjunction with high power handling modified unitraveling carrier photodetectors, to generate a phase noise floor on a 10 GHz carrier of -175 dBc/Hz, the lowest ever demonstrated in the photodetection of a mode-locked fiber laser. At all offset frequencies, the photodetected 10 GHz phase noise performance is comparable to or better than the lowest phase noise results yet demonstrated with stabilized Ti:sapphire frequency combs.