10/25/2024, Fiber Optic Online News, adjustable lasers play a key role in integrated optics, especially hybrid integrated external cavity lasers (ECLs), because they can effectively combine optical amplification of direct bandgap III-V material gain elements with dimmable feedback provided by advanced passive photonic integrated circuits (PICs). This feedback circuit can be implemented through a silicon photonic (SiP) platform or a silicon nitride (Si3N4) waveguide, which has lower linear and nonlinear losses than silicon photonic circuits. This design not only improves the intrinsic Q factor based on the resonant cavity feedback circuit, but also eliminates two-photon absorption (TPA) under high power conditions, significantly reduces phase noise and ultimately achieves a small linewidth output.
However, the common integration of III-V gain elements and adjustable outer cavity circuits in chip-level packaging is still challenged. An integration method is called "heterogeneous integration", which means transferring a layer of III-V material (such as) directly to an existing silicon or silicon nitride PIC circuit, which allows high-density monolithic integration. However, the process involved is very complex and is mainly used on silicon photonic platforms, but the technical difficulty and cost are very high.
Another approach is to process the III-V chip and passive feedback circuit on different substrates separately, and then perform hybrid integration in a compact multi-chip module. This approach allows individual optimization and testing of each component and facilitates thermal decoupling between the gain element and the temperature-sensitive feedback circuit. However, the assembly of hybrid photonic multi-chip modules relies heavily on the high precision alignment of the basic optical chips. In order to ensure the efficiency of optical coupling between each chip, "active alignment" is required during the assembly process, that is, continuous adjustment and optimization. Such complex operations reduce manufacturing speed and efficiency, making it difficult to achieve large-scale, cost-effective production.
New hybrid integrated ECL based on Si3N4
This paper experimentally verifies a novel hybrid integrated ECL based on Si3N4, using photon wire bonding (PWB) technology, which is used as a coupling element between a reflective semiconductor optical amplifier (RSOA) and an external feedback circuit. PWBs are manufactured in-situ in a fully automated process, meaning they are customized directly during assembly. This custom design enables the shape of the PWB to match the pattern field size and position of the chips at both ends. Even if the cross-sectional sizes of different waveguides vary greatly, or the chip placement accuracy is limited, PWB can still achieve low coupling losses. This design reduces reliance on active alignment, thereby improving manufacturing efficiency and scalability.
In the proof-of-concept experiment, the ECL demonstrated by the team has a tuning range of 90 nm (1480 nm–1570 nm), the chip output power exceeds 12 dBm, and the edge-mode rejection ratio (SMSR) is as high as 59 dB. The research team achieved an intrinsic linewidth of 979 Hz, which is very low in a similar feedback architecture. In practical applications, the optical loss of PWB as an intra-cavity coupling element is approximately 1.6 ± 0.2 dB.
At the output, the Si3N4 feedback circuit is connected to a single-mode fiber array through a 3D printed surface microlens (FaML), which can efficiently couple the optical signal into the fiber. The advantage of 3D printing technology is that it can accurately shape the microlenses to meet the specific needs of different applications. To verify the performance and application potential of this ECL, the research team used it as a narrow linewidth tunable pump laser to generate a dissipative Kerr soliton (DKS) frequency comb, demonstrating great potential and wide application prospects in high-performance optical systems.
Figure 1. Conceptual diagram of a hybrid integrated ECL module with 3D printed intra-cavity photon wire bonding (PWB) and surface microlens (FaML)
The device consists of two chips, both mounted on a common aluminum base: one is an InP reflective semiconductor optical amplifier (RSOA) mounted on a copper radiator, and the other is an external cavity circuit based on Si3N4, with the output connected to the fiber array. The device is controlled by a PCB attached to an aluminum base, which is closely connected to the base.
Figure 1(b) Top view of InP RSOA and Si3N4 chips. The Si3N4 chip includes a ring mirror with tuned elliptical resonators R1 and R2, a cavity phase tuner (CPT), and an MZI-based adjustable output coupler. The rear end face of the InP RSOA is treated with a high reflective (HR) coating, with an anti-reflective (AR) coating on the front and cut at an angle of 9.0°. The PWB is used to fill the 306 μm gap between the front of the RSOA and the upper edge coupler (EC) of the Si3N4 chip, which consists of a tapered waveguide (WG) with an inclination angle of 19.9°. The application of PWB allows good optical coupling to be achieved even if the emission directions at both ends of the chip do not match, reflecting the flexibility of the device in chip alignment and optical connection.
In this device, the length of the CPT is 1 mm, and the circumferences of the elliptical resonators are 885.1 μm (R1) and 857.4 μm (R2), respectively. Four auxiliary waveguides (WG2 to WG5) and output waveguides (WG1) are arranged at the edge of the Si3N4 chip and are connected to the corresponding gradient EC with a spacing of 127 μm. All five waveguide ports are coupled to a single-mode fiber (SMF) array via 3D printed FaML.
Figure 1(c) shows a side view of the module. The ends of each PWB are equipped with additional connection structures for enhanced mechanical stability. The elliptical resonator, MZI and CPT of Si3N4 are adjusted by thermal phase tuners, which are used to adjust the cavity loop phase to ensure that the maximum specular reflectivity corresponds to an integer multiple of 2π. The MZI-based adjustable output coupler is used to set the ratio between the light extracted from the laser cavity and the light fed back to the RSOA.
ECL module assembly and performance characterization
In the first step of module assembly, the RSOA is pasted onto the copper radiator by conductive glue, then roughly aligned with the Si3N4 chip and fixed together on an aluminum base (as shown in Figure 1(a)). The step structure on the aluminum base is used to adjust the height of the RSOA chip and the Si3N4 outer cavity chip waveguide layer. The PWB trajectory depends on the specific position and emission direction of the end face of the RSOA and Si3N4 outer cavity chip, as shown in Figure 1(b).
The outer cavity (EC) relies on a spot size converter (SSC), which is designed by reducing the thickness of the Si3N4 top layer and the width of the two stacked Si3N4 strips while keeping the thickness of the intermediate SiO? layer unchanged. In our device, the thickness of the top Si3N4 gradually decreases from the conventional 175 nm to zero thickness close to the end face, ultimately retaining only the 75 nm thickness of the bottom Si3N4 strip.
At the chip input, the initial width of the Si3N4 strip is 2 μm and is reduced to a standard width of 1.1 μm at the end of the tapered portion. This design results in the measured elliptical mode field dimensions of 7.2 μm (horizontal) and 4.6 μm (vertical). To optimally match the pattern field dimensions of the chip end face, the rectangular cross-section of the PWB is designed with a width of 8.0 μm and a height of 5.0 μm.
In implementation, the PWB fills a 306 μm gap between the RSOA and the Si3N4 chip. The PWB on the RSOA side has an initial cross-section of 4.0 μm × 4.0 μm to match the pattern field dimensions of the RSOA end face. The cross-section of the PWB was then reduced to a conventional 2.4 μm × 2.0 μm. On the end face of the Si3N4 chip, the PWB is up-adjusted to the final cross-section above 8.0 μm x 5.0 μm to connect to the on-chip Si3N4 outer cavity. The PWB maintains a constant cross-section of 2.4 μm × 2.0 μm between these two segments, while the trajectory depicts an arc to smoothly connect edge-coupled waveguides (WGs) at both ends, although their emission directions differ significantly. The additional structure printed on the end faces of the RSOA and Si3N4 chips improves the mechanical stability of the PWB, as shown in Figure 1(c).
The output of the Si3N4 chip relies on five identical surface microlenses (FaMLs) that are printed to the end faces of the fiber array (FA). These FaMLs are designed specifically for the output of the outer cavity chip to be efficiently coupled to the fiber array (waveguide ports 1-5). The outer cavity relies again on a pair of Si3N4 strips that reduce from a standard width of 1.1 μm to the end face to a final width of 0.8 μm, the bottom Si3N4 layer maintains a constant thickness of 75 nm, while the upper layer thickness drops from 175 nm to zero.
With this design, the final mode field dimension is 7.5 μm in both directions. The length of the FaML is 70 μm, providing a working distance of 50 μm between the lens tip and the end face of the Si3N4 waveguide. Therefore, the distance between the end surface of the FA and the Si3N4 chip is 120 μm, as shown in FIG. 1(b).
Both PWB and FaML are manufactured using a negative photoresist material (, GmbH, Germany; refractive index n = 1.53) by in-situ multiphoton lithography. The manufactured PWB and FaML were developed in PGMEA and subsequently rinsed with isopropanol and blow-dried. The production of FaML on FA is a separate step after the PWB manufacturing, completed using 3D printing technology and assembled by a custom mounter.
Figure 2(a) shows microscopic images of a fully assembled ECL module, including PWB in a false color and scanning electron microscope (SEM) images of corresponding cross-sections, as well as microscope images of five FaMLs on FA.
Figure 2. Implementation and performance characterization of ECL modules
(a) Microscopic image of the ECL module assembled according to FIG. 1(a). Inset (i) shows scanning electron microscopy (SEM) images that are treated with false shading, showing the connection of photon wire bonding (PWB) between RSOA and Si3N4 chips. Through cross-sectional markings at different locations, the shape changes of PWB during light propagation and its effect on optical coupling can be observed. Inset (ii) shows microscopic images of five surface microlenses (FaML) on a fiber array (FA). They are 127 μm apart and are directly opposite to the corresponding waveguide ports on the Si3N4 chip (Ports 1 to 5). This design ensures efficient coupling between the optical fiber and the waveguide, reducing optical loss.
(b) The relationship between the output power (Pout) of the ECL module and the injection current (I) at 1550 nm wavelength. It can be observed that the threshold current is 19 mA and the average slope efficiency is 132 mW/A (fitted with a dashed line).
(c) ?The laser spectrum overlay of ECL is shown when the injection current is 100 mA. The tuning range of the laser is 1480 nm to 1570 nm, and is tuned with a stride of 5 nm. Around 1525 nm, the maximum chip output power reaches 12 dBm.
(d) The FM noise spectrum is presented, containing the fitted blue dotted lines. The inherent (Lorentz) line width is δf = πS0 = 979 Hz, and this value is obtained by fitting the solid line.
The key role of PWB and 3D printing optical microlens technology in hybrid integrated ECL
This study also verifies the versatility of hybrid integrated ECL, which was successfully used as a narrow linewidth tunable pump laser and has achieved the generation of a single soliton Kerr frequency comb in a high-quality Si3N4 microresonator for the first time (see OF, Vol. 41, Issue 11, June 2023 for details). The hybrid integrated ECL with adjustable frequency has potential application prospects in the fields of coherent communication, optical coherent tomography (OCT).
This paper explores the key role of PWB and FaML technologies in hybrid integrated ECL, and analyzes their contributions to improving laser performance, reducing manufacturing costs, and simplifying the assembly process. PWB technology achieves seamless coupling between III-V gain elements and passive photonic integrated circuits through efficient optical connections, eliminating the dependence on high-precision alignment in traditional assembly. This technology adopts a multi-photon lithography process, which enables the shape to be customized in situ during the manufacturing process, adapting to the pattern field size and position differences between different chips, thus ensuring low coupling losses.
FaML technology uses advanced 3D printing technology to manufacture micro-optical structures, achieving ultra-high positioning accuracy and excellent beam shaping effect, significantly reducing light loss and achieving higher transmission efficiency. This technology can accurately design the geometry and structure of microlens according to specific application requirements, thereby optimizing the optical coupling effect and ensuring consistency in product quality.
The combination of PWB and FaML technologies provides strong support to meet the growing market demand, promote the development of photonic integrated circuits, and enhance the overall competitiveness of optoelectronic devices.
For more information about these two technologies and related products, please contact Lingyunguang Company.
References: Maier, , et al. “Sub-kHz- - Laser (ECL) With Si3N4 Used as a Pump for a Kerr Comb.” * of *, vol. 41, no. 11, June 2023, pp. 3479–3490.
About the Company
Founded in 2017, the company is located in Karlsruhe, Germany. It is an incubator of the Karlsruhe Institute of Technology (KIT) in Germany. The company has created 3D-printed photonic wire bonding (Wire, PWB) and micro-optical component technology, focusing on photonic integrated chip coupling and packaging applications. The equipment and technologies are widely used in the field of optoelectronic integrated chip packaging manufacturing, including telecommunications/data communication high-speed optical modules, 3D sensing, optical computing and other directions.
about
Lingyun Light (stock code:) is a high-tech enterprise based on optical technology innovation and conducts business around machine vision and fiber optics. The company is committed to becoming a global leader in the fields of visual artificial intelligence and optoelectronic information. In the field of fiber optics, Lingyun Optics has been deeply engaged in five major application fields, including scientific communications, telecommunications communications, data communications, fiber lasers and fiber sensing. Combining international advanced technologies and independent research and development products, it provides customers with high-quality fiber optic devices and instrument solutions, helping China's optical technology field scientific and technological progress and industrial development. For more details, please visit Lingyunguang's official website or call Lingyunguang.
