Kai Wang

High-power amplifiers are of great importance in many optical systems deployed in optical sensing, ranging, medical surgery and more. Likewise, high-gain, low-noise amplifiers with low polarization dependence are critical components of long-range optical communication systems. Integrated photonic solutions show great potential in challenging application fields thanks to their drastic reduction in size, weight and cost, but coming at the expense of low optical power due to reduced energy storage capacity in small devices. Recently, the large mode area (LMA) technology, which is known for dramatically increasing the energy storage capacity and saturation power of fiber amplifiers, has been brought to the chip-level, allowing for watt-level amplification directly from the chip. Here we demonstrate a silicon photonic LMA amplifier capable of both high-power amplification with output saturation powers > 115 mW and high small-signal gain up to 30 dB with a 3.6 dB noise figure on-chip, which we achieved by suppressing parasitic lasing with index-matching glue on the chip facets. Furthermore, we present a method to tune and completely nullify the polarization dependent gain (PDG). We believe that the PDG-tunability combined with the low-noise, high-gain and high-power amplification positions it as a potentially disruptive technology for next-generation integrated amplifiers across a range of applications.

Fiber-to-fiber gain is demonstrated in waveguide amplifiers fabricated from Al ₂ O ₃ :Nd ³⁺ films. The Nd-doped aluminium oxide film is deposited using reactive sputtering, resulting in a polycrystalline film with an approximate ion concentration of 2.2 × 10 ²⁰ ions/cm ³ . An amplifier with a length of 5.5 cm is optically pumped at 805 nm, following a bi-directional pumping scheme. For an incident signal input power of -14.9 dBm, the amplifier shows an external gain at a signal wavelength of 1064 nm as large as 14.5 dB, limited by the available pump power. Under those conditions, a noise figure of 8.5 dB was measured. These results pave the way towards on-chip amplification for applications including biomedical imaging, spectroscopy, atmospheric LiDAR, and quantum technologies.

High power sources are desired in integrated photonics to utilize their full potential for various applications such as in telecom, medical, and remote sensing. However, the signal power from the integrated photonic sources has been quite limited. One of the main reasons for this is the usage of small optical mode area waveguides, which, although they help in keeping the footprint small and increasing the gain efficiency, limit the energy storage and extraction capability of the signal from an integrated device, thus limiting the output power. By increasing the optical mode area, one allows the photons to interact with a significantly larger number of gain ions, allowing large energy storage and power extraction. In light of this, we show a high-power continuous wave laser based on a recently demonstrated large-mode-area gain waveguide, which requires no external amplifier. The maximum power generated from the distributed Bragg reflector laser is close to 1.35 W with a slope efficiency of more than 50%, and it operates around the long wavelength window (∼1.85 μm) relevant for medical, space, and defense applications. The power level demonstrated here enables silicon photonics based continuous wave light sources comparable to their benchtop counterparts and brings them significantly closer to mass production and deployment for desired applications.

High-power amplifiers are critical components in optical systems spanning from long-range optical sensing and optical communication systems to micromachining and medical surgery. Today, integrated photonics with its promise of large reductions in size, weight and cost cannot be used in these applications, owing to the lack of on-chip high-power amplifiers. Integrated devices severely lack in output power owing to their small size, which limits their energy storage capacity. For the past two decades, large mode area (LMA) technology has played a disruptive role in fibre amplifiers, enabling a dramatic increase of output power and energy by orders of magnitude. Owing to the ability of LMA fibres to support significantly larger optical modes, the energy storage and power handling capabilities of LMA fibres have significantly increased. Therefore, an LMA device on an integrated platform can play a similar role in power and energy scaling of integrated devices. In this work, we demonstrate LMA waveguide-based watt-class high-power amplifiers in silicon photonics with an on-chip output power exceeding ~1 W within a footprint of only ~4.4 mm². The power achieved is comparable and even surpasses that of many fibre-based amplifiers. We believe that this work has the potential to radically change the integrated photonics application landscape, allowing power levels previously unimaginable from an integrated device to replace much of today’s benchtop systems. Moreover, mass producibility, reduced size, weight and cost will enable yet unforeseen applications of laser technology.

High-power tunable lasers are intensely pursued due to their vast application potential such as in telecom, ranging, and molecular sensing. Integrated photonics, however, is usually considered not suitable for high-power applications mainly due to its small size which limits the energy storage capacity and, therefore, the output power. In the late 90s, to improve the beam quality and increase the stored energy, large-mode-area (LMA) fibers were introduced in which the optical mode area is substantially large. Such LMA fibers have transformed the high-power capability of fiber systems ever since. Introducing such an LMA technology at the chip-scale can play an equally disruptive role with high power signal generation from an integrated photonics system. To this end, in this work we demonstrate such a technology, and show a very high-power tunable laser with the help of a silicon photonics based LMA power amplifier. We show output power reaching 1.8 W over a tunability range of 60 nm, spanning from 1.83 µm to 1.89 µm, limited only by the seed laser. Such an integrated LMA device can be used to substantially increase the power of the existing integrated tunable lasers currently limited to a few tens of milliwatts. The power levels demonstrated here reach and surpass that of many benchtop systems which truly makes the silicon photonics based integrated LMA device poised towards mass deployment for high power applications without relying on benchtop systems.

Low-stress stoichiometric silicon nitride (Si₃N₄) waveguides with an unprecedented thickness of up to 1350 nm and a width in the range of 2.2 - 2.7 µm are fabricated using a single LPCVD step on sapphire substrates (SiNOS). Optical characterization of proof-of-concept ∼1.35 µm thick waveguides show propagation losses in the order of 0.30 ± 0.01 dB/cm at 1600 nm. The proposed process offers a simple route to high confinement Si₃N₄ waveguides, enabling applications in nonlinear and mid-IR integrated photonics.

Femtosecond laser pulses enable the synthesis of light across the electromagnetic spectrum and provide access to ultrafast phenomena in physics, biology, and chemistry. Chip-integration of femtosecond technology could revolutionize applications such as point-of-care diagnostics, bio-medical imaging, portable chemical sensing, or autonomous navigation. However, current chip-integrated pulse sources lack the required peak power, and on-chip amplification of femtosecond pulses has been an unresolved challenge. Here, addressing this challenge, we report >50-fold amplification of 1 GHz-repetition-rate chirped femtosecond pulses in a CMOS-compatible photonic chip to 800 W peak power with 116 fs pulse duration. This power level is 2–3 orders of magnitude higher compared to those in previously demonstrated on-chip pulse sources and can provide the power needed to address key applications. To achieve this, detrimental nonlinear effects are mitigated through all-normal dispersion, large mode-area and rare-earth-doped gain waveguides. These results offer a pathway to chip-integrated femtosecond technology with peak power levels characteristic of table-top sources.

High power amplifiers are critical components in optical systems spanning from long range optical sensing and optical communication systems to micromachining and medical surgery. Today, integrated photonics with its promise of large reductions in size, weight and cost cannot be used in these applications, due to the lack of on-chip high power amplifiers. Integrated devices severely lack in output power due to their small size which limits energy storage capacity. For the last two decades, large mode area (LMA) technology has played a disruptive role in fiber amplifiers enabling a dramatic increase of output power and energy by orders of magnitude. Thanks to the capability of LMA fiber to support significantly larger optical modes the energy storage and power handling capability has significantly increased. Therefore, an LMA device on an integrated platform can play a similar role in power and energy scaling of integrated devices. In this work, we demonstrate LMA waveguide-based CMOS compatible watt-class high power amplifiers with an on-chip output power reaching beyond ~ 1 W within a footprint of only ~ 4 mm². The power achieved is comparable and even surpasses many fiber-based amplifiers. We believe this work has the potential to radically change the integrated photonics application landscape, allowing power levels previously unimaginable from an integrated device replacing much of today’s benchtop systems. Moreover, mass producibility, reduced size, weight and cost will enable yet unforeseen applications for laser technology.

Chip-scale, high-energy optical pulse generation is becoming increasingly important as integrated optics expands into space and medical applications where miniaturization is needed. Q-switching of the laser cavity was historically the first technique to generate high-energy pulses, and typically such systems are in the realm of large bench-top solid-state lasers and fibre lasers, especially in the long wavelength range >1.8 µm, thanks to their large energy storage capacity. However, in integrated photonics, the very property of tight mode confinement that enables a small form factor becomes an impediment to high-energy applications owing to small optical mode cross-sections. Here we demonstrate a high-energy silicon photonics-based passively Q-switched laser with a compact footprint using a rare-earth gain-based large-mode-area waveguide. We demonstrate high on-chip output pulse energies of >150 nJ and 250 ns pulse duration in a single transverse fundamental mode in the retina-safe spectral region (1.9 µm), with a slope efficiency of ~40% in a footprint of ~9 mm². The high-energy pulse generation demonstrated in this work is comparable to or in many cases exceeds that of Q-switched fibre lasers. This bodes well for field applications in medicine and space.