Ultra-narrow linewidth laser based on a semiconductor gain chip and extended Si3N4 Bragg grating

Narrow linewidth laser based on a sidewall grating active distributed Bragg reflector

We demonstrated a narrow linewidth semiconductor laser based on a deep-etched sidewall grating active distributed Bragg reflector (SG-ADBR). The coupling coefficients and reflectance were numerically simulated for deep-etched fifth-order SG-ADBR, and a reflectance of 0.86 with a bandwidth of 1.04 nm was obtained by the finite element method for a 500-period SG-ADBR. Then the fifth-order SG-ADBR lasers were fabricated using projection i-line lithography processes. Single-mode lasing at 1537.9 nm was obtained with a high side-mode suppression ratio (SMSR) of 65 dB, and a continuous tuning range of 10.3 nm was verified with SMSRs greater than 53 dB. Furthermore, the frequency noise power spectral density was characterized, from which a Lorentzian linewidth of 288 kHz was obtained.

Integrated self-injection-locked narrow linewidth laser based on thin-film lithium niobate

Narrow linewidth lasers have a wide range of applications in the fields of coherent optical communications, atomic clocks, and measurement. Lithium niobate material possesses excellent electro-optic and thermo-optic properties, making it an ideal photonic integration platform for a new generation. The light source is a crucial element in large-scale photonic integration. Therefore, it is essential to develop integrated narrow linewidth lasers based on low-loss LNOI. This study is based on the multimode race-track type add-drop microring resonator with multimode interferometric coupler (MMRA-MRR) of the DFB laser self-injection-locked, to achieve the narrowing of linewidth to the laser. The microring external cavity was used to narrow the linewidth of the laser to 2.5 kHz. The output power of the laser is 3.18 mW, and the side-mode suppression ratio is 60 dB. This paper presents an integrated low-noise, narrow-linewidth laser based on thin-film lithium niobate material for the communication band. This is significant for achieving all-optical device on-chip integration of lithium niobate material in the future. It has great potential for use in high-speed coherent optical communication.

Research Progress of Wide Tunable Bragg Grating External Cavity Semiconductor Lasers

In this paper, we review the progress of wide tunable Bragg grating external cavity semiconductor lasers (BG-ECSLs). We concentrate on BG-ECSLs based on the wide tunable range for multicomponent detection. Wide tunable BG-ECSLs have many important applications, such as wavelength-division multiplexing (WDM) systems, coherent optical communications, gas detection and atom cooling. Wide tunability, narrow linewidth and a high side-mode suppression ratio BG-ECSLs have attracted much attention for their merits. In this paper, three main structures for achieving widely tunable, narrow linewidth, high side-mode suppression ratio BG-ECSLs are reviewed and compared in detail, such as the volume Bragg grating (VBG) structure, fiber Bragg grating (FBG) structure and waveguide Bragg grating (WBG) structure of ECSLs. The advantages and disadvantages of different structures of BG-ECSLs are analyzed. The results show that WBG-ECSLs are a potential way to realize the integration, small size, wide tuning range, stable spectral output and high side-mode suppression ratio laser output. Therefore, the use of WBG as optical feedback elements is still the mainstream direction of BG-ECSLs, and BG-ECSLs offer a further new option for multicomponent detection and multi-atoms cooling.

Micro-transfer-printed narrow-linewidth III-V-on-Si double laser structure with a combined 110 nm tuning range

In this work, we demonstrate for the first time a narrow-linewidth III-V-on-Si double laser structure with more than a 110 nm wavelength tuning range realized using micro-transfer printing (µTP) technology. Two types of pre-fabricated III-V semiconductor optical amplifiers (SOAs) with a photoluminescence (PL) peak around 1500 nm and 1550 nm are micro-transfer printed on two silicon laser cavities. The laser cavities are fabricated in imec's silicon photonics (SiPh) pilot line on 200 mm silicon-on-insulator (SOI) wafers with a 400 nm thick silicon device layer. By combining the outputs of the two laser cavities on chip, wavelength tunability over S+C+L-bands is achieved.

Guided-acoustic stimulated Brillouin scattering in silicon nitride photonic circuits

Coherent optomechanical interaction known as stimulated Brillouin scattering (SBS) can enable ultrahigh resolution signal processing and narrow-linewidth lasers. SBS has recently been studied extensively in integrated waveguides; however, many implementations rely on complicated fabrication schemes. The absence of SBS in standard and mature fabrication platforms prevents its large-scale circuit integration. Notably, SBS in the emerging silicon nitride (Si₃N₄) photonic integration platform is currently out of reach because of the lack of acoustic guidance. Here, we demonstrate advanced control of backward SBS in multilayer Si₃N₄ waveguides. By optimizing the separation between two Si₃N₄ layers, we unlock acoustic waveguiding in this platform, potentially leading up to 15× higher Brillouin gain coefficient than previously possible in Si₃N₄ waveguides. We use the enhanced SBS gain to demonstrate a high-rejection microwave photonic notch filter. This demonstration opens a path to achieving Brillouin-based photonic circuits in a standard, low-loss Si₃N₄ platform.

Advances in Waveguide Bragg Grating Structures, Platforms, and Applications: An Up-to-Date Appraisal

A Bragg grating (BG) is a one-dimensional optical device that may reflect a specific wavelength of light while transmitting all others. It is created by the periodic fluctuation of the refractive index in the waveguide (WG). The reflectivity of a BG is specified by the index modulation profile. A Bragg grating is a flexible optical filter that has found broad use in several scientific and industrial domains due to its straightforward construction and distinctive filtering capacity. WG BGs are also widely utilized in sensing applications due to their easy integration and high sensitivity. Sensors that utilize optical signals for sensing have several benefits over conventional sensors that use electric signals to achieve detection, including being lighter, having a strong ability to resist electromagnetic interference, consuming less power, operating over a wider frequency range, performing consistently, operating at a high speed, and experiencing less loss and crosstalk. WG BGs are simple to include in chips and are compatible with complementary metal-oxide-semiconductor (CMOS) manufacturing processes. In this review, WG BG structures based on three major optical platforms including semiconductors, polymers, and plasmonics are discussed for filtering and sensing applications. Based on the desired application and available fabrication facilities, the optical platform is selected, which mainly regulates the device performance and footprint.

Damping of relaxation oscillations, photon-photon resonance, and tolerance to external optical feedback of III-V/SiN hybrid lasers with a dispersive narrow band mirror

We address the stability of a tunable hybrid laser based on a III-V Reflective Semiconductor Optical Amplifier (RSOA) edge-coupled with a Silicon Photonic (SiPh) dispersive mirror through a model of time-delayed algebraic differential equations that accounts for the narrow band mirror. Our results allow to (i) analyze the stability of single mode lasing, (ii) quantify the impact of the mirror bandwidth on the damping of the laser relaxation oscillations and the emergence of photon-photon resonance, and (iii) study the tolerance of the laser to the external optical feedback. Thanks to this analysis, we find a mirror design that gives ultra-high stability up to an external feedback level of -10 dB. The aim of the work is providing a tool for understanding and interpreting the dynamics of these lasers and design configurations for isolator-free operation.

Optical Interconnects Finally Seeing the Light in Silicon Photonics: Past the Hype

Electrical interconnects are becoming a bottleneck in the way towards meeting future performance requirements of integrated circuits. Moore's law, which observes the doubling of the number of transistors in integrated circuits every couple of years, can no longer be maintained due to reaching a physical barrier for scaling down the transistor's size lower than 5 nm. Heading towards multi-core and many-core chips, to mitigate such a barrier and maintain Moore's law in the future, is the solution being pursued today. However, such distributed nature requires a large interconnect network that is found to consume more than 80% of the microprocessor power. Optical interconnects represent one of the viable future alternatives that can resolve many of the challenges faced by electrical interconnects. However, reaching a maturity level in optical interconnects that would allow for the transition from electrical to optical interconnects for intra-chip and inter-chip communication is still facing several challenges. A review study is required to compare the recent developments in the optical interconnects with the performance requirements needed to reach the required maturity level for the transition to happen. This review paper dissects the optical interconnect system into its components and explains the foundational concepts behind the various passive and active components along with the performance metrics. The performance of different types of on-chip lasers, grating and edge couplers, modulators, and photodetectors are compared. The potential of a slot waveguide is investigated as a new foundation since it allows for guiding and confining light into low index regions of a few tens of nanometers in cross-section. Additionally, it can be tuned to optimize transmissions over 90° bends. Hence, high-density opto-electronic integrated circuits with optical interconnects reaching the dimensions of their electrical counterparts are becoming a possibility. The latest complete optical interconnect systems realized so far are reviewed as well.

Dynamics of Semiconductor Lasers under External Optical Feedback from Both Sides of the Laser Cavity

To increase the spectral efficiency of coherent communication systems, lasers with ever-narrower linewidths are required as they enable higher-order modulation formats with lower bit-error rates. In particular, semiconductor lasers are a key component due to their compactness, low power consumption, and potential for mass production. In field-testing scenarios their output is coupled to a fiber, making them susceptible to external optical feedback (EOF). This has a detrimental effect on its stability, thus it is traditionally countered by employing, for example, optical isolators and angled output waveguides. In this work, EOF is explored in a novel way with the aim to reduce and stabilize the laser linewidth. EOF has been traditionally studied in the case where it is applied to only one side of the laser cavity. In contrast, this work gives a generalization to the case of feedback on both sides. It is implemented using photonic components available via generic foundry platforms, thus creating a path towards devices with high technology-readiness level. Numerical results shows an improvement in performance of the double-feedback case with respect to the single-feedback case. In particularly, by appropriately selecting the phase of the feedback from both sides, a broad stability regime is discovered. This work paves the way towards low-cost, integrated and stable narrow-linewidth integrated lasers.

High-performance lasers for fully integrated silicon nitride photonics

Silicon nitride (SiN) waveguides with ultra-low optical loss enable integrated photonic applications including low noise, narrow linewidth lasers, chip-scale nonlinear photonics, and microwave photonics. Lasers are key components to SiN photonic integrated circuits (PICs), but are difficult to fully integrate with low-index SiN waveguides due to their large mismatch with the high-index III-V gain materials. The recent demonstration of multilayer heterogeneous integration provides a practical solution and enabled the first-generation of lasers fully integrated with SiN waveguides. However, a laser with high device yield and high output power at telecommunication wavelengths, where photonics applications are clustered, is still missing, hindered by large mode transition loss, non-optimized cavity design, and a complicated fabrication process. Here, we report high-performance lasers on SiN with tens of milliwatts output power through the SiN waveguide and sub-kHz fundamental linewidth, addressing all the aforementioned issues. We also show Hertz-level fundamental linewidth lasers are achievable with the developed integration techniques. These lasers, together with high-Q SiN resonators, mark a milestone towards a fully integrated low-noise silicon nitride photonics platform. This laser should find potential applications in LIDAR, microwave photonics and coherent optical communications.

Hybrid external-cavity lasers (ECL) using photonic wire bonds as coupling elements

Combining semiconductor optical amplifiers (SOA) on direct-bandgap III–V substrates with low-loss silicon or silicon-nitride photonic integrated circuits (PIC) has been key to chip-scale external-cavity lasers (ECL) that offer wideband tunability along with small optical linewidths. However, fabrication of such devices still relies on technologically demanding monolithic integration of heterogeneous material systems or requires costly high-precision package-level assembly, often based on active alignment, to achieve low-loss coupling between the SOA and the external feedback circuits. In this paper, we demonstrate a novel class of hybrid ECL that overcome these limitations by exploiting 3D-printed photonic wire bonds as intra-cavity coupling elements. Photonic wire bonds can be written in-situ in a fully automated process with shapes adapted to the mode-field sizes and the positions of the chips at both ends, thereby providing low-loss coupling even in presence of limited placement accuracy. In a proof-of-concept experiment, we use an InP-based reflective SOA (RSOA) along with a silicon photonic external feedback circuit and demonstrate a single-mode tuning range from 1515 to 1565 nm along with side mode suppression ratios above 40 dB and intrinsic linewidths down to 105 kHz. Our approach combines the scalability advantages of monolithic integration with the performance and flexibility of hybrid multi-chip assemblies and may thus open a path towards integrated ECL on a wide variety of integration platforms.

Widely tunable, low linewidth, and high power laser source using an electro-optic comb and injection-locked slave laser array

We propose and implement a tunable, high power and narrow linewidth laser source based on a series of highly coherent tones from an electro-optic frequency comb and a set of 3 DFB slave lasers. We experimentally demonstrate approximately 1.25 THz (10 nm) of tuning within the C-Band centered at 192.9 THz (1555 nm). The output power is approximately 100 mW (20 dBm), with a side band suppression ratio greater than 55 dB and a linewidth below 400 Hz across the full range of tunability. This approach is scalable and may be extended to cover a significantly broader optical spectral range.

Study of microcomb threshold power with coupling scaling

We model the generation threshold and conversion efficiency of microcombs by scaling the cavity coupling. With the Lugiato–Lefever equation (LLE), quantitative analysis of threshold is established in the parameter space of pump power and coupling. Considering the large detuning and Kerr-induced phase shift, the threshold power is numerically solved with the minimum at over-coupling, in agreement with that from the traveling wave theory. Furthermore, the coupling dependence on microcomb generation is discussed, providing the accessibility of high-efficient, stable combs (≥ 40%) around the threshold. This work offers universal guidelines for the design of microcombs with low-power and high-efficient operation.

Back-side-on-BOX heterogeneously integrated III-V-on-silicon O-band discrete-mode lasers

We demonstrate foundry-fabricated O-band III-V-on-silicon discrete-mode lasers. The laser fabrication follows the back-side-on-buried-oxide laser integration process and is compatible with complex, multilayer, silicon-on-insulator based platforms. A series of devices were characterized, with the best devices producing on-chip powers of nearly 20 mW with Lorentzian linewidths below 20 kHz and a side mode suppression ratio of at least 60 dB.

Effects of nonlinear loss in high-Q Si ring resonators for narrow-linewidth III-V/Si heterogeneously integrated tunable lasers

High-Q Si ring resonators play an important role in the development of widely tunable heterogeneously integrated lasers. However, while a high Q-factor (Q > 1 million) is important for ring resonators in a laser cavity, the parasitic high-power density in a Si resonator can deteriorate the laser performance at high power levels due to nonlinear loss. Here, we experimentally show that this detrimental effect can happen at moderate power levels (a few milliwatts) where typical heterogeneously integrated lasers work. We further compare different ring resonators, including extended Si ring resonators and Si₃N₄ ring resonators and provide practical approaches to minimize this effect. Our results provide explanations and guidelines for high-Q ring resonator designs in heterogeneously integrated tunable lasers, and they are also applicable for hybrid integrated butt-coupled lasers.

Deuterated silicon dioxide for heterogeneous integration of ultra-low-loss waveguides

Ultra-low-loss waveguide fabrication typically requires high-temperature annealing beyond 1000°C to reduce the hydrogen content in deposited dielectric films. However, realizing the full potential of an ultra-low loss will require the integration of active materials that cannot tolerate high temperature. Uniting ultra-low-loss waveguides with on-chip sources, modulators, and detectors will require a low-temperature, low-loss dielectric to serve as a passivation and spacer layers for complex fabrication processes. We report a 250°C deuterated silicon dioxide film for top cladding in ultra-low-loss waveguides. Using multiple techniques, we measure propagation loss below 12 dB/m for the entire 1200-1650 nm range and top-cladding material absorption below 1 dB/m in the S, C, and L bands.

On-chip optical narrowband reflector based on anti-symmetric Bragg grating

We propose an on-chip optical narrowband reflector (NBR) based on two cascaded Bragg gratings (BGs). A π phase shifted anti-symmetric Bragg grating (π-PS-ASBG) and a rear uniform Bragg grating (UBG), are in-line connected. The π-PS-ASBG provides a hybrid mode resonance between the even- and odd TE (TE₀ and TE₁) modes, while the UBG is used as a rear reflector to reflect the TE₀ mode that transmitted from the π-PS-ASBG. Different from traditional UBG, the reflection bandwidth decreases when the coupling coefficient increases. The calculated 3-dB bandwidth is 0.16 nm when the whole grating length is 400 µm. The proposed NBR can be applied in the cases requiring narrow reflection such as narrow linewidth semiconductor lasers.

Hybrid Integrated Semiconductor Lasers with Silicon Nitride Feedback Circuits

Hybrid integrated semiconductor laser sources offering extremely narrow spectral linewidth, as well as compatibility for embedding into integrated photonic circuits, are of high importance for a wide range of applications. We present an overview on our recently developed hybrid-integrated diode lasers with feedback from low-loss silicon nitride (Si 3 N 4 in SiO 2 ) circuits, to provide sub-100-Hz-level intrinsic linewidths, up to 120 nm spectral coverage around a 1.55 μ m wavelength, and an output power above 100 mW. We show dual-wavelength operation, dual-gain operation, laser frequency comb generation, and present work towards realizing a visible-light hybrid integrated diode laser.