Integrated Pound-Drever-Hall laser stabilization system in silicon

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.

Modulation-free laser stabilization technique using integrated cavity-coupled Mach-Zehnder interferometer

Stable lasers play a significant role in precision optical systems where an electro-optic laser frequency stabilization system, such as the Pound-Drever-Hall technique, measures laser frequency and actively stabilizes it by comparing it to a frequency reference. Despite their excellent performance, there has been a trade-off between complexity, scalability, and noise measurement sensitivity. Here, we propose and experimentally demonstrate a modulation-free laser stabilization method using an integrated cavity-coupled Mach-Zehnder interferometer as a frequency noise discriminator. The proposed architecture maintains the sensitivity of the Pound-Drever-Hall architecture without the need for any modulation. This significantly simplifies the architecture and makes miniaturization into an integrated photonic platform easier. The implemented chip suppresses the frequency noise of a semiconductor laser by 4 orders-of-magnitude using an on-chip silicon microresonator with a quality factor of 2.5 × 106. The implemented passive photonic chip occupies an area of 0.456 mm2 and is integrated on AIM Photonics 100 nm silicon-on-insulator process.

Integrated microcavity electric field sensors using Pound-Drever-Hall detection

Discerning weak electric fields has important implications for cosmology, quantum technology, and identifying power system failures. Photonic integration of electric field sensors is highly desired for practical considerations and offers opportunities to improve performance by enhancing microwave and lightwave interactions. Here, we demonstrate a high-Q microcavity electric field sensor (MEFS) by leveraging the silicon chip-based thin film lithium niobate photonic integrated circuits. Using the Pound-Drever-Hall detection scheme, our MEFS achieves a detection sensitivity of 5.2 μV/(m[Formula: see text]), which surpasses previous lithium niobate electro-optical electric field sensors by nearly two orders of magnitude, and is comparable to atom-based quantum sensing approaches. Furthermore, our MEFS has a bandwidth that can be up to three orders of magnitude broader than quantum sensing approaches and measures fast electric field amplitude and phase variations in real-time. The ultra-sensitive MEFSs represent a significant step towards building electric field sensing networks and broaden the application spectrum of integrated microcavities.

Roadmapping the next generation of silicon photonics

Silicon photonics has developed into a mainstream technology driven by advances in optical communications. The current generation has led to a proliferation of integrated photonic devices from thousands to millions-mainly in the form of communication transceivers for data centers. Products in many exciting applications, such as sensing and computing, are around the corner. What will it take to increase the proliferation of silicon photonics from millions to billions of units shipped? What will the next generation of silicon photonics look like? What are the common threads in the integration and fabrication bottlenecks that silicon photonic applications face, and which emerging technologies can solve them? This perspective article is an attempt to answer such questions. We chart the generational trends in silicon photonics technology, drawing parallels from the generational definitions of CMOS technology. We identify the crucial challenges that must be solved to make giant strides in CMOS-foundry-compatible devices, circuits, integration, and packaging. We identify challenges critical to the next generation of systems and applications-in communication, signal processing, and sensing. By identifying and summarizing such challenges and opportunities, we aim to stimulate further research on devices, circuits, and systems for the silicon photonics ecosystem.

Resonant fiber-optic thermometry with high resolution and wide range

We report a high-resolution and wide-range thermometer using a fiber Bragg grating Fabry-Perot cavity (FBG-FP) combined with beat frequency interrogation. Two distributed feedback (DFB) lasers are locked to the FBG-FP sensing head and a hydrogen cyanide H¹³C¹⁴N (HCN) gas cell, respectively, both using the Pound-Drever-Hall (PDH) technique. The light beams from two lasers are brought together to interfere on a photodetector producing a beat frequency signal which provides a measure of the temperature change. Our sensor exhibits a dynamic range of ∼109 °C, a high resolution of 2×10^-4 °C with an averaging time of 1 s. By introducing the reference frequency, the sensor has demonstrated good long-term stability. This sensor provides a useful tool for those fields where resolving slight temperature changes is crucial, such as deep ocean temperature measurement.

Femtometer-resolved wavelength monitor based on photodiode optoelectronic chromatic dispersion with RF phase-shift amplification

The spectral sensitivity of photodiode-based optoelectronic chromatic dispersion is enhanced by phase-shift amplification using RF interferometry. With phase-shift amplification of G=4⋅10⁴, a peak phase-shift sensitivity of Δθ = 27 deg/pm is achieved, corresponding to a spectral resolution of Δλ_res = 1 fm. This all-electronic solid-state technology can serve as an on-chip inexpensive technique for femtometer-resolved wavelength monitoring.

Low-loss ultrafast and non-volatile all-optical switch enabled by all-dielectric phase change materials

All-optical switches show great potential to overcome the speed and power consumption limitations of electrical switching. Owing to its nonvolatile and superb cycle abilities, phase-change materials enabled all-optical switch (PC-AOS) is attracting much attention. However, realizing low-loss and ultrafast switching remains a challenge, because previous PC-AOS are mostly based on plasmonic metamaterials. The high thermal conductance of metallic materials disturbs the thermal accumulation for phase transition, and eventually decreases the switching speed to tens of nanoseconds. Here, we demonstrate an ultrafast switching (4.5 ps) and low-loss (2.8 dB) all-optical switch based on all-dielectric structure consisting of Ge₂Sb₂Te₅ and photonic crystals. Its switching speed is approximately ten thousand times faster than the plasmonic one. A 5.4 dB on-off ratio at 1550 nm has been experimentally achieved. We believe that the proposed all-dielectric optical switch will accelerate the progress of ultrafast and energy-efficient photonic devices and systems.

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All-dielectric phase change materials are used to achieve low loss all optical switch

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Only 15 nm phase change film is used for laser induced ultrafast switching

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Up to 7.4 dB switching contrast can be realized in the Near Infrared Spectrum

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Nano-hole array metasurface enables polarization insensitive optical filtering

Circuit-level convergence of electronics and photonics: basic concepts and recent advances

Integrated photonics is widely regarded as an important post-Moore's law research direction. However, it suffers from intrinsic limitations, such as lack of control and satisfactory photonic memory, that cannot be solved in the optical domain and must be combined with electronics for practical use. Inevitably, electronics and photonics will converge. The photonic fabrication and integration technology is gradually maturing and electronics-photonics convergence (EPC) is experiencing a transition from device integration to circuit design. We derive a conceptual framework consisting of regulator, oscillator, and memory for scalable integrated circuits based on the fundamental concepts of purposeful behavior in cybernetics, entropy in information theory, and symmetry breaking in physics. Leveraging this framework and emulating the successes experienced by electronic integrated circuits, we identify the key building blocks for the integrated circuits for EPC and review the recent advances.

High-resolution and large-dynamic-range temperature sensor using fiber Bragg grating Fabry-Pérot cavity

A high-resolution and large-dynamic-range temperature sensor adopting a pair of fiber Bragg grating as Fabry-Pérot cavity (FBG-FP) and laser frequency dither locking method is proposed and experimentally demonstrated. This sensor exhibits a temperature resolution of 7×10^-4 °C and a dynamic range of ∼46 °C. It is especially useful for applications where very small temperature changes need to be detected, such as deep ocean temperature measurement.

A Chip-Scale Optical Frequency Reference for the Telecommunication Band Based on Acetylene

Lasers precisely stabilized to known transitions between energy levels in simple, well-isolated quantum systems such as atoms and molecules are essential for a plethora of applications in metrology and optical communications. The implementation of such spectroscopic systems in a chip-scale format would allow to reduce cost dramatically and would open up new opportunities in both photonically integrated platforms and free-space applications such as lidar. Here the design, fabrication, and experimental characterization of a molecular cladded waveguide platform based on the integration of serpentine nanoscale photonic waveguides with a miniaturized acetylene chamber is presented. The goal of this platform is to enable cost-effective, miniaturized, and low power optical frequency references in the telecommunications C band. Finally, this platform is used to stabilize a 1.5 μm laser with a precision better than 400 kHz at 34 s. The molecular cladded waveguide platform introduced here could be integrated with components such as on-chip modulators, detectors, and other devices to form a complete on-chip laser stabilization system.

Self-homodyne wavelength locking of a silicon microring resonator

We propose and experimentally demonstrate a self-homodyne locking method for a silicon microring resonator (MRR). The device employs a self-homodyne detection structure and consists of a tunable MRR with two directional couplers along the ring for monitoring, two phase shifters to calibrate the phase difference between the two monitored optical signals, and a Y-branch to combine the two signals. A single photodetector is used to detect the output power of the Y-branch. If the MRR is on resonance, a destructive interference occurs in the Y-branch, therefore the monitored photocurrent is minimized. By using such a device structure and the homodyne detection scheme, the MRR with a Q factor of 1.9 × 10⁴ can be accurately locked to the signal wavelength, and the locking process is insensitive to input power variation. The wavelength locking range is larger than one free spectral range (FSR) of 6 nm, and the locking errors are ≤0.015 nm.

Frequency locking of semiconductor lasers to RF oscillators using hybrid-integrated opto-electronic oscillators with dispersive delay lines

Direct frequency locking of lasers to RF oscillators has many applications such as high resolution optical frequency synthesis, coherent optical communication, spectroscopy, sensing, and imaging. Here we present a hybrid-integrated opto-electronic loop that directly frequency locks a semiconductor laser to an RF synthesized source using an opto-electronic oscillator with a dispersive optical delay line. Cascaded ring filters, operating near the resonance frequency, provide an enhanced chromatic dispersion with a compact footprint. The electronic chip is integrated in the GlobalFoundries 180 nm CMOS SOI technology and the photonic chip is integrated in the IME 180 nm SOI technology. A tracking range of 0.5 GHz is achieved while consuming 33 mW power. The proposed scheme is used to frequency lock a commercially available DFB laser, reducing the laser frequency fluctuations by an order of magnitude compared to the free-running case.