Optical Frequency Synthesis Research Articles

Wideband Balanced Photoreceivers With InP-Based Photodiodes and 65 nm CMOS TIAs for Use in Optical Frequency Synthesis Systems

Two photoreceivers, each comprised of a set of InP balanced photodiodes (PD) and one of two CMOS transimpedance amplifiers (TIA) fabricated in a 65 nm general-purpose process, are demonstrated in this paper. The balanced photodiodes achieve a responsivity of 0.65 A/W and a bandwidth of 12 GHz while driving a 50 $\bf \Omega$ load. The first photoreceiver uses a 9 GHz bandwidth regulated cascode (RGC) TIA with 59 dB $\bf \Omega$ transimpedance, achieving a maximum conversion gain of 1289 V/W and a minimum noise equivalent power (NEP) of 13 pW/ $\bf \sqrt{Hz}$ . The second photoreceiver utilizes an 11 GHz bandwidth modified current-reuse regulated cascode (CRRGC) TIA with 63 dB $\bf \Omega$ transimpedance gain, resulting in a maximum conversion gain of 2083 V/W and minimum NEP of 13 pW/ $\bf \sqrt{Hz}$ .

High-Performance Photonic Integrated Circuits on Silicon

Heterogeneous integration of III–V semiconductor photonics combined with silicon foundry technology enables low-cost, high-performance photonic integrated circuits. Highly reliable lasers using epitaxial deposition of quantum dot lasers, with >100 years at 35 C have been demonstrated at University of California, Santa Barbara (UCSB) and can be manufactured at wafer scale. Reduction in the linewidth enhancement factor allows isolator-free operation. This technology enables cost-effective photonic integrated circuits for applications such as microwave photonics and data communications. Optical frequency synthesis with ∼1.5 Hz accuracy is demonstrated using heterogeneous integration. Silicon photonics applications that will benefit from future heterogeneous integration are also demonstrated, including high dynamic range microwave photonic links and optical switching technology that scales to hyperscale datacenters with hundreds of thousands of servers.

Comb-rooted multi-channel synthesis of ultra-narrow optical frequencies of few Hz linewidth

We report a multi-channel optical frequency synthesizer developed to generate extremely stable continuous-wave lasers directly out of the optical comb of an Er-doped fiber oscillator. Being stabilized to a high-finesse cavity with a fractional frequency stability of 3.8 × 10−15 at 0.1 s, the comb-rooted synthesizer produces multiple optical frequencies of ultra-narrow linewidth of 1.0 Hz at 1 s concurrently with an output power of tens of mW per each channel. Diode-based stimulated emission by injection locking is a key mechanism that allows comb frequency modes to sprout up with sufficient power amplification but no loss of original comb frequency stability. Channel frequencies are individually selectable with a 0.1 GHz increment over the entire comb bandwidth spanning 4.25 THz around a 1550 nm center wavelength. A series of out-of-loop test results is discussed to demonstrate that the synthesizer is able to provide stable optical frequencies with the potential for advancing diverse ultra-precision applications such as optical clocks comparison, atomic line spectroscopy, photonic microwaves generation, and coherent optical telecommunications.

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.

光学频率合成器的自动化控制研究

光学频率合成器的自动化控制研究

High-Resolution Optical Frequency Synthesis Using an Integrated Electro-Optical Phase-Locked Loop

Optical frequency synthesizers have widespread applications in optical communication, sensing, and ranging systems. These synthesizers typically consist of an electro-optical phase-locked loop (EOPLL) that phase locks a widely tunable laser (TL) to the teeth of a highly stable optical frequency comb. In this paper, a partially integrated optical frequency synthesizer is reported, where a stable and low-noise frequency comb is used as the optical reference. In the proposed architecture, frequency synthesis is done in two phases. In a coarse tuning phase, a widely tunable distributed Bragg reflector laser is continuously tuned by adjusting its mirrors and phase section currents. While the laser is being tuned, an indexing system detects and counts the number of passing comb teeth. Coarse tuning is concluded when the TL reaches the target comb tooth. Following the coarse tuning phase, a heterodyne integrated EOPLL phase locks the TL to the selected comb tooth. Under the phase lock condition, fine frequency tuning is performed by adjusting the frequency of an RF local oscillator. The proposed synthesizer has the capability of synthesizing optical frequencies over a 5-THz range with a coarse tuning speed of 0.5 THz/s and coarse and fine tuning resolutions of 20 MHz and less than 1 Hz, respectively.

Microresonator Frequency Combs for Integrated Microwave Photonics

Microresonator-based Kerr frequency combs (microcombs) have many outstanding features, including chip-level integration, high repetition rate, and ultrabroad bandwidth. These merits make microcombs very promising for integrated microwave photonic applications. Here, we provide a brief review on recent progress of microcomb generation and its application in microwave photonics. Both coherent and incoherent examples are discussed, including low-phase-noise microwave and optical frequency synthesis, programmable photonic microwave signal processing, true time delay beamforming, and channelized receiver. An outlook on the future research direction and key problems is also given.

Ultralow-power chip-based soliton microcombs for photonic integration

The generation of dissipative Kerr solitons in optical microresonators has provided a route to compact frequency combs of high repetition rate, which have already been employed for optical frequency synthesizers, ultrafast ranging, coherent telecommunication and dual-comb spectroscopy. Silicon nitride (Si$_3$N$_4$) microresonators are promising for photonic integrated soliton microcombs. Yet to date, soliton formation in Si$_3$N$_4$ microresonators at electronically detectable repetition rates, typically less than 100 GHz, is hindered by the requirement of external power amplifiers, due to the low quality ($Q$) factors, as well as by thermal effects which necessitate the use of frequency agile lasers to access the soliton state. These requirements complicate future photonic integration, heterogeneous or hybrid, of soliton microcomb devices based on Si$_3$N$_4$ microresonators with other active or passive components. Here, using the photonic Damascene reflow process, we demonstrate ultralow-power single soliton formation in high-Q ($Q_0>15\times10^6$) Si$_3$N$_4$ microresonators with 9.8 mW input power (6.2 mW in the waveguide) for devices of electronically detectable, 99 GHz repetition rate. We show that solitons can be accessed via simple, slow laser piezo tuning, in many resonances in the same sample. These power levels are compatible with current silicon-photonics-based lasers for full photonic integration of soliton microcombs, at repetition rates suitable for applications such as ultrafast ranging and coherent communication. Our results show the technological readiness of Si$_3$N$_4$ optical waveguides for future all-on-chip soliton microcomb devices.

Ultra-broadband dual-branch optical frequency comb with 10−18instability

Coherently bridging across wide spectral regions in the optical domain with ultra-high precision will enable progress in optical spectroscopy and time and frequency metrology. We report optical frequency synthesis at the 10−18 instability level based on a novel technique actively canceling the differential phase noise of a dual-branch Er-fiber based optical frequency comb spanning 500–2200 nm. We validate the method with an out-of-loop measurement demonstrating the stability transfer from 1560 nm to 780 nm with an instability level at 3×10−18 at 1 s averaging time and 1×10−19 at 1000 s (approximately averaging on 1×10−17/τ), limited mainly by uncompensated optical interferometers used to detect optical beatnotes. To confirm this limitation, we propose an experimental setup that eliminates the excess noise induced by uncompensated optical paths and show that the proposed method exhibits an ultimate unprecedented instability level at 6×10−19 at 1 s averaging time when two optical signals can share a common photodetector. The system realized becomes a powerful and universal tool for optical frequency comparison and spectral purity transfer across the visible and infrared domains.

Quasi-phase-matched nonlinear optical frequency conversion in on-chip whispering galleries

Chip-integrated whispering-gallery resonators (WGRs) enable compact and wavelength-agile nonlinear optical frequency synthesizers. So far, the most flexible phase-matching technique, i.e., quasi-phase matching, has not been applied in this configuration. The reason is the lack of suitable thin films with alternating crystal structure on a low-refractive-index substrate. Here, we demonstrate an innovative method of realizing thin film substrates suitable for quasi-phase matching by field-assisted domain engineering of lithium niobate, and subsequent direct bonding and polishing. We are able to fabricate high-Q on-chip WGRs with these substrates by using standard semiconductor manufacturing techniques. The Q-factors of the resonators are up to one million, which allows us to demonstrate quasi-phase-matched second-harmonic generation in on-chip WGRs for the first time, to the best of our knowledge. The normalized conversion efficiency is 9×10−4 mW−1. This method can also be transferred to other material systems.

Deterministic generation of single soliton Kerr frequency comb in microresonators by a single shot pulsed trigger.

Kerr soliton frequency comb generation in monolithic microresonators recently attracted great interests as it enables chip-scale few-cycle pulse generation at microwave rates with smooth octave-spanning spectra for self-referencing. Such versatile platform finds significant applications in dual-comb spectroscopy, low-noise optical frequency synthesis, coherent communication systems, etc. However, it still remains challenging to straightforwardly and deterministically generate and sustain the single-soliton state in microresonators. In this paper, we propose and theoretically demonstrate the excitation of single-soliton Kerr frequency comb by seeding the continuous-wave driven nonlinear microcavity with a pulsed trigger. Unlike the mostly adopted frequency tuning scheme reported so far, we show that an energetic single shot pulse can trigger the single-soliton state deterministically without experiencing any unstable or chaotic states. Neither the pump frequency nor the cavity resonance is required to be tuned. The generated mode-locked single-soliton Kerr comb is robust and insensitive to perturbations. Even when the thermal effect induced by the absorption of the intracavity light is taken into account, the proposed single pulse trigger approach remains valid without requiring any thermal compensation means.

An optical-frequency synthesizer using integrated photonics.

Optical-frequency synthesizers, which generate frequency-stable light from a single microwave-frequency reference, are revolutionizing ultrafast science and metrology, but their size, power requirement and cost need to be reduced if they are to be more widely used. Integrated-photonics microchips can be used in high-coherence applications, such as data transmission 1 , highly optimized physical sensors 2 and harnessing quantum states 3 , to lower cost and increase efficiency and portability. Here we describe a method for synthesizing the absolute frequency of a lightwave signal, using integrated photonics to create a phase-coherent microwave-to-optical link. We use a heterogeneously integrated III-V/silicon tunable laser, which is guided by nonlinear frequency combs fabricated on separate silicon chips and pumped by off-chip lasers. The laser frequency output of our optical-frequency synthesizer can be programmed by a microwave clock across 4 terahertz near 1,550 nanometres (the telecommunications C-band) with 1 hertz resolution. Our measurements verify that the output of the synthesizer is exceptionally stable across this region (synthesis error of 7.7 × 10-15 or below). Any application of an optical-frequency source could benefit from the high-precision optical synthesis presented here. Leveraging high-volume semiconductor processing built around advanced materials could allow such low-cost, low-power and compact integrated-photonics devices to be widely used.

Self-locked orthogonal polarized dual comb in a microresonator

Dual combs are an emerging tool to obtain unprecedented resolution, high sensitivity, ultrahigh accuracy, broad bandwidth, and ultrafast data updating rate in the fields of molecular spectroscopy, optical metrology, as well as optical frequency synthesis. The recent progress in chip-based microcombs has promoted the on-chip dual-comb measuring systems to a new phase attributed to the large frequency spacing and broad spectrum. In this paper, we demonstrate proof-of-concept dual-comb generation with orthogonal polarization in a single microresonator through pumping both the transverse-electric (TE) and transverse-magnetic (TM) modes simultaneously. The two orthogonal polarized pumps are self-oscillating in a fiber ring cavity. The generated dual comb exhibits excellent stability due to the intrinsic feedback mechanism of the self-locked scheme. The repetition rate of the two orthogonal combs is slightly different because of the mode spacing difference between the TE and TM modes. Such orthogonal polarized dual-combs could be a new comb source for out-of-lab applications in the fields of integrated spectroscopy, ranging measurement, optical frequency synthesis, and microwave comb generation.

Optical frequency synthesizer for precision spectroscopy of Rydberg states of Rb atoms

We have developed an optical frequency synthesizer for the precision spectroscopy of highly excited Rydberg states of Rb atoms. This synthesizer can generate a widely tunable 480 nm laser light with an optical power of 150 mW and an absolute frequency uncertainty of less than 100 kHz using a high-repetition-rate (325 MHz) Er fiber-based optical frequency comb and a tunable frequency-doubled diode laser at 960 nm. We demonstrate the precision two-photon spectroscopy of the Rydberg states of 87Rb atoms by observing the electromagnetically induced transparency in a vapor cell, and measure the absolute transition frequencies of 87Rb to nD (n = 53–92) and nS (n = 60–90) Rydberg states with an uncertainty of less than 250 kHz. It is the first direct frequency measurements of these transitions using an optical frequency comb.

Octave-spanning coherent supercontinuum generation in silicon on insulator from 1.06\u2009\u03bcm to beyond 2.4\u2009\u03bcm

Efficient complementary metal-oxide semiconductor-based nonlinear optical devices in the near-infrared are in strong demand. Due to two-photon absorption in silicon, however, much nonlinear research is shifting towards unconventional photonics platforms. In this work, we demonstrate the generation of an octave-spanning coherent supercontinuum in a silicon waveguide covering the spectral region from the near- to shortwave-infrared. With input pulses of 18 pJ in energy, the generated signal spans the wavelength range from the edge of the silicon transmission window, approximately 1.06 to beyond 2.4 μm, with a −20 dB bandwidth covering 1.124–2.4 μm. An octave-spanning supercontinuum was also observed at the energy levels as low as 4 pJ (−35 dB bandwidth). We also measured the coherence over an octave, obtaining , in good agreement with the simulations. In addition, we demonstrate optimization of the third-order dispersion of the waveguide to strengthen the dispersive wave and discuss the advantage of having a soliton at the long wavelength edge of an octave-spanning signal for nonlinear applications. This research paves the way for applications, such as chip-scale precision spectroscopy, optical coherence tomography, optical frequency metrology, frequency synthesis and wide-band wavelength division multiplexing in the telecom window.

Quasi-phase-matched multispectral Kerr frequency comb.

We study a new type of Kerr frequency comb where the momentum conservation law is fulfilled by azimuthal modulation of the waveguide dispersion. The concept can expand the parametric range in which a Kerr frequency comb is obtained. In a good agreement with the theoretical analysis, we demonstrate a multispectral Kerr frequency comb covering important fiber-optic communication bands. Comb coherence and absence of a sub-comb offset are confirmed by continuous-wave heterodyne beat note and amplitude noise spectra measurements. The device can be used for achieving broadband optical frequency synthesizers and high-capacity coherent communication.

Stably accessing octave-spanning microresonator frequency combs in the soliton regime.

Microresonator frequency combs can be an enabling technology for optical frequency synthesis and timekeeping in low size, weight, and power architectures. Such systems require comb operation in low-noise, phase-coherent states such as solitons, with broad spectral bandwidths (e.g., octave-spanning) for self-referencing to detect the carrier-envelope offset frequency. However, accessing such states is complicated by thermo-optic dispersion. For example, in the Si3N4 platform, precisely dispersion-engineered structures can support broadband operation, but microsecond thermal time constants often require fast pump power or frequency control to stabilize the solitons. In contrast, here we consider how broadband soliton states can be accessed with simple pump laser frequency tuning, at a rate much slower than the thermal dynamics. We demonstrate octave-spanning soliton frequency combs in Si3N4 microresonators, including the generation of a multi-soliton state with a pump power near 40 mW and a single-soliton state with a pump power near 120 mW. We also develop a simplified two-step analysis to explain how these states are accessed without fast control of the pump laser, and outline the required thermal properties for such operation. Our model agrees with experimental results as well as numerical simulations based on a Lugiato-Lefever equation that incorporates thermo-optic dispersion. Moreover, it also explains an experimental observation that a member of an adjacent mode family on the red-detuned side of the pump mode can mitigate the thermal requirements for accessing soliton states.

Towards chip-scale optical frequency synthesis based on optical heterodyne phase-locked loop.

An integrated heterodyne optical phase-locked loop was designed and demonstrated with an indium phosphide based photonic integrated circuit and commercial off-the-shelf electronic components. As an input reference, a stable microresonator-based optical frequency comb with a 50-dB span of 25 nm (~3 THz) around 1550 nm, having a spacing of ~26 GHz, was used. A widely-tunable on-chip sampled-grating distributed-Bragg-reflector laser is offset locked across multiple comb lines. An arbitrary frequency synthesis between the comb lines is demonstrated by tuning the RF offset source, and better than 100Hz tuning resolution with ± 5 Hz accuracy is obtained. Frequency switching of the on-chip laser to a point more than two dozen comb lines away (~5.6 nm) and simultaneous locking to the corresponding nearest comb line is also achieved in a time ~200 ns. A low residual phase noise of the optical phase-locking system is successfully achieved, as experimentally verified by the value of -80 dBc/Hz at an offset of as low as 200 Hz.

Microresonator Kerr frequency combs with high conversion efficiency

AbstractMicroresonator‐based Kerr frequency comb (microcomb) generation can potentially revolutionize a variety of applications ranging from telecommunications to optical frequency synthesis. However, phase‐locked microcombs have generally had low conversion efficiency limited to a few percent. Here we report experimental results that achieve conversion efficiency ( on‐chip comb power excluding the pump) in the fiber telecommunication band with broadband mode‐locked dark‐pulse combs. We present a general analysis on the efficiency which is applicable to any phase‐locked microcomb state. The effective coupling condition for the pump as well as the duty cycle of localized time‐domain structures play a key role in determining the conversion efficiency. Our observation of high efficiency comb states is relevant for applications such as optical communications which require high power per comb line. image

Long-term Absolute Wavelength Stability of Acetylene-stabilized Reference Laser at 1533 nm

The second harmonic generation process in Periodically Poled Lithium Niobate (PPLN) has been applied in order to measure frequency of reference laser locked to acetylene absorption peak 12C2H2 (P13) (1533 nm) against optical frequency synthesizer. The measurement results have been compared to the results obtained using different techniques for the same reference laser during the past 10 years in other laboratories.