Nicolas Volet

An optical-frequency synthesizer using integrated photonics.

Integrated-photonics microchips now enable a range of advanced functionalities for high-coherence applications such as data transmission, highly optimized physical sensors, and harnessing quantum states, but with cost, efficiency, and portability much beyond tabletop experiments. Through high-volume semiconductor processing built around advanced materials there exists an opportunity for integrated devices to impact applications cutting across disciplines of basic science and technology. Here we show how to synthesize the absolute frequency of a lightwave signal, using integrated photonics to implement lasers, system interconnects, and nonlinear frequency comb generation. The laser frequency output of our synthesizer is programmed by a microwave clock across 4 THz near 1550 nm with 1 Hz resolution and traceability to the SI second. This is accomplished with a heterogeneously integrated III/V-Si tunable laser, which is guided by dual dissipative-Kerr-soliton frequency combs fabricated on silicon chips. Through out-of-loop measurements of the phase-coherent, microwave-to-optical link, we verify that the fractional-frequency instability of the integrated photonics synthesizer matches the 7.0x10^(−13) reference-clock instability for a 1 second acquisition, and constrain any synthesis error to 7.7x10^(−15) while stepping the synthesizer across the telecommunication C band. Any application of an optical frequency source would be enabled by the precision optical synthesis presented here. Building on the ubiquitous capability in the microwave domain, our results demonstrate a first path to synthesis with integrated photonics, leveraging low-cost, low-power, and compact features that will be critical for its widespread use.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 transmission1, highly optimized physical sensors2 and harnessing quantum states3, 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.An optical-frequency synthesizer based on stabilized frequency combs has been developed utilizing chip-scale devices as key components, in a move towards using integrated photonics technology for ultrafast science and metrology.

Wafer-Fused Optically Pumped VECSELs Emitting in the 1310-nm and 1550-nm Wavebands

1300-nm, 1550-nm, and 1480-nm wavelength, optically pumped VECSELs based on wafer-fused InAlGaAs/InP-AlGaAs/GaAs gain mirrors with intracavity diamond heat spreaders are described. These devices demonstrate very low thermal impedance of 4 K/W. Maximum CW output of devices with 5 groups of quantum wells shows CW output power of 2.7 W from 180  μm apertures in both the 1300-nm and the 1550-nm bands. Devices with 3 groups of quantum wells emitting at 1480 nm and with the same aperture size show CW output of 4.8 W. These VECSELs emit a high-quality beam with 𝑀 2 beam parameter below 1.6 allowing reaching a coupling efficiency as high as 70% into a single-mode fiber. Maximum value of output power of 6.6 W was reached for 1300 nm wavelength devices with 290  μm aperture size. Based on these VECSELs, second harmonic emission at 650 nm wavelength with a record output of 3 W and Raman fiber lasers with 0.5 W emission at 1600 nm have been demonstrated.

Heterogeneous Silicon/III–V Semiconductor Optical Amplifiers

We report high output power and high-gain semiconductor optical amplifiers integrated on a heterogeneous silicon/III-V photonics platform. The devices produce 25 dB of unsaturated gain for the highest gain design, and 14 dBm of saturated output power for the highest output power design. The amplifier structure is also suitable for lasers, and can be readily integrated with a multitude of silicon photonic circuit components. These devices are useful for a wide range of photonic integrated circuits. We show a design method for optimizing the amplifier for the desired characteristics. The amplifier incorporates a low loss and low reflection transition between the heterogeneous active region and a silicon waveguide, and we report transition loss below 1 dB across the entire measurement range and parasitic reflection coefficient from the transition below 1 · 10-3.

Heterogeneous integration of lithium niobate and silicon nitride waveguides for wafer-scale photonic integrated circuits on silicon

An ideal photonic integrated circuit for nonlinear photonic applications requires high optical nonlinearities and low loss. This work demonstrates a heterogeneous platform by bonding lithium niobate (LN) thin films onto a silicon nitride (Si3N4) waveguide layer on silicon. It not only provides large second- and third-order nonlinear coefficients, but also shows low propagation loss in both the Si3N4 and the LN-Si3N4 waveguides. The tapers enable low-loss-mode transitions between these two waveguides. This platform is essential for various on-chip applications, e.g., modulators, frequency conversions, and quantum communications.

Transverse mode discrimination in long-wavelength wafer-fused vertical-cavity surface-emitting lasers by intra-cavity patterning

Transverse mode discrimination is demonstrated in long-wavelength wafer-fused vertical-cavity surface-emitting lasers using ring-shaped air gap patterns at the fused interface between the cavity and the top distributed Bragg reflector. A significant number of devices with varying pattern dimensions was investigated by on-wafer mapping, allowing in particular the identification of a design that reproducibly increases the maximal single-mode emitted power by about 30 %. Numerical simulations support these observations and allow specifying optimized ring dimensions for which higher-order transverse modes are localized out of the optical aperture. These simulations predict further enhancement of the single-mode properties of the devices with negligible penalty on threshold current and emitted power.

Numerical Analysis of Mode Discrimination by Intracavity Patterning in Long-Wavelength Wafer-Fused Vertical-Cavity Surface-Emitting Lasers

This paper presents an extensive numerical analysis of 1.3-μm wavelength wafer-fused vertical-cavity surface-emitting lasers (VCSELs) incorporating intracavity patterning. Using a 3-D, self-consistent model of the physical phenomena in VCSELs, supported by experimental results used for parameter calibration, we investigate the influence of arch-and ring-shaped intracavity features with a broad range of geometrical parameters on the modal behavior of the VCSEL. To design and optimize the devices, we used intracavity patterning that provides very strong discrimination of higher order modes, pushing them out from the active region. This mechanism makes possible single mode operation under a broad range of currents and could potentially enhance the single-mode output power of these devices.

High performance wafer-fused semiconductor disk lasers emitting in the 1300 nm waveband

We report for the first time on the performance of 1300 nm waveband semiconductor disc lasers (SDLs) with wafer fused gain mirrors that implement intracavity diamond and flip-chip heat dissipation schemes based on the same gain material. With a new type of gain mirror structure, maximum output power values reach 7.1 W with intracavity diamond gain mirrors and 5.6 W with flip-chip gain mirrors, using a pump spot diameter of 300 µm, exhibiting a beam quality factor M(2)< 1.25 in the full operation range. These results confirm previously published theoretical modeling of these types of SDLs.

High-power optically-pumped VECSELs emitting in the 1310-nm and 1550-nm wavebands

1300-nm, 1550-nm and 1480-nm wavelength, optically-pumped VECSELs based on wafer-fused InAlGaAs/InPAlGaAs/ GaAs gain mirrors with intra-cavity diamond heat-spreaders demonstrate very low thermal impedance of 4 K/W. Maximum CW output of devices with5 groups of quantum wells show CW output power of 2.7 W from 180μm apertures in both 1300-nm and 1550-nm bands. Devices with 3 groups of quantum wells emitting at 1480 nm and with the same aperture size show CW output of 4.8 W. These devices emit a high quality beam with M² beam parameter below 1.6 allowing reaching a coupling efficiency into a single mode fiber as high as 70 %. Maximum value of output power of 6.6 W was reached for 1300nm wavelength devices with 290μm aperture size.

Heterogeneous Integration for Mid-infrared Silicon Photonics

Heterogeneous integration enables the construction of silicon (Si) photonic systems, which are fully integrated with a range of passive and active elements including lasers and detectors. Numerous advancements in recent years have shown that heterogeneous Si platforms can be extended beyond near-infrared telecommunication wavelengths to the mid-infrared (MIR) (2–20 μm) regime. These wavelengths hold potential for an extensive range of sensing applications and the necessary components for fully integrated heterogeneous MIR Si photonic technologies have now been demonstrated. However, due to the broad wavelength range and the diverse assortment of MIR technologies, the optimal platform for each specific application is unclear. Here, we overview Si photonic waveguide platforms and lasers at the MIR, including quantum cascade lasers on Si. We also discuss progress toward building an integrated multispectral source, which can be constructed by wavelength beam combining the outputs from multiple lasers with arrayed waveguide gratings and duplexing adiabatic couplers.

Wafer-fused VECSELs emitting in the 1310nm waveband

Optically pumped wafer fused 1310 nm VECSELs have the advantage of high output power and wavelength agility. Gain mirrors in these lasers are formed by direct bonding of InAlGaAs/InP active cavities to Al(Ga)As/GaAs DBRs. We present for the first time Watt-level 1310 nm wafer-fused VCSELs based on gain mirrors with heat dissipation in the “flip-chip” configuration. Even though output power levels in this approach is lower than with intra-cavity diamond heat-spreaders, the “flip-chip configuration demonstrates higher quality optical emission and is preferable for industrial applications in optical amplifiers, intra-cavity doubled lasers, etc.