Light-Linked AI, Chip-Powered Future: High-Performance CW Lasers for the AI Era!

Driven by the rapidly rising demand for AI computing power, the high-speed optical module market is undergoing continuous iteration, with mainstream products shifting from 400G to 800G/1.6T. Lightcounting’s latest forecast indicates that 2026 will see explosive growth in 800G module demand. In AI optical interconnects, single-mode solutions are typically adopted for link distances above 100 meters, and the underlying laser-chip technologies mainly fall into two categories: traditional EML solutions and emerging silicon photonics (SiPh) solutions. EML solutions, known for their excellent transmission performance, are primarily used in FR and other wavelength-multiplexed scenarios, offering strong cost advantages for transmission distances up to 2 km. Silicon photonics, with its inherent strengths in integration, is mainly applied to DR and other parallel multi-lane scenarios, where 500-meter transmission is most common.

 

Figure 1: 400G Silicon Photonics Optical Module Architecture (Source: Intel)

 

Due to the inherent limitations of silicon as a material, silicon photonics solutions still rely on external light sources to deliver continuous and stable optical input into the silicon-based chip. This creates significantly higher requirements for the external laser source: stable single-mode output, excellent high-temperature performance, and high reliability. Today’s silicon photonics external light sources are typically O-band continuous-wave (CW) distributed-feedback (DFB) semiconductor laser chips. As the core emitting component, the chosen CW DFB technology route— including material system (InGaAsP vs. InGaAlAs), waveguide structure (BH vs. RWG), and integration strategy— directly shapes the optical module’s performance ceiling, reliability profile, and cost structure. The following section provides a systematic analysis of these key influencing factors and the technological evolution paths behind them.

 

Waveguide Structure Design of CW DFB Light Sources

 

In CW DFB laser chips, achieving stable single-transverse-mode output requires the use of effective lateral optical field confinement and carrier confinement structures. Today, the two mainstream waveguide architectures are the Buried Heterostructure (BH) and the Ridge Waveguide (RWG). These structural choices fundamentally influence the laser’s performance, manufacturing complexity, and application suitability. The following comparison summarizes the advantages and trade-offs of both approaches:

 

Comparison Dimensions	Buried Heterostructure (BH)	Ridge Waveguide (RWG)
Core Structure	This is realized through two or more epitaxial growth steps. After the first epitaxial growth of the active region, photolithography and etching are used to form a ridge mesa structure. A second epitaxial growth is then performed, during which a current-blocking layer (e.g., reverse-biased PN junction) laterally “buries” the mesa of the active region, providing strong optical and carrier confinement.	A ridge structure is etched above the active region, and the effective refractive index contrast between the ridge and the surrounding air (or low-index material) provides lateral optical confinement.
Fabrication Complexity	The fabrication process is extremely complex, particularly the regrowth interface in the second epitaxial step, which is difficult to control and prone to defects, posing potential risks to the long-term reliability of the chip.	The fabrication process is simple, requiring only a single epitaxial growth and standard semiconductor etching techniques, avoiding the complex regrowth steps and the associated interface quality issues.
Advantages	It provides strong carrier confinement and low threshold current. The far-field divergence is approximately circular, offering high compatibility with the optical spot of silicon photonics interfaces.	It offers higher reliability, improved manufacturing yield, and cost advantages.
Disadvantages	The complex fabrication process results in lower manufacturing yield and higher production costs.	Its carrier confinement is weaker than that of BH structures; the far-field divergence is relatively elliptical, resulting in lower coupling efficiency with silicon photonics waveguides.

Figure 2: Schematic Comparison of BH and RWG Structures

 

Active Region Material Selection for CW DFB Light Sources

 

In addition to waveguide design, the choice of active region material system plays a decisive role in laser performance, particularly in high-temperature operation. Currently, there are two mainstream material systems for the active region: InGaAlAs (Indium Gallium Aluminum Arsenide) and InGaAsP (Indium Gallium Arsenide Phosphide). The following table compares the advantages and disadvantages of these two material systems:

 

Comparison Dimensions	InGaAlAs	InGaAsP
Features	Quaternary Material System Containing Aluminum (Al)	Traditional InP-based quaternary material systems have a high level of process maturity.
High-Temperature Performance	The InGaAlAs material, lattice-matched to InP, has a larger conduction band offset (ΔEc/ΔEg ≈ 0.7). This provides more effective electron confinement, significantly suppressing carrier leakage at elevated temperatures, thereby achieving excellent high-temperature performance, lower threshold current, and higher characteristic temperature (T₀).	This material system has a relatively small conduction band offset (ΔEc). Under high-temperature operating conditions, electrons can more easily overcome the heterojunction barrier, leading to significant carrier leakage, which in turn results in higher threshold current, lower characteristic temperature (T₀), and reduced slope efficiency.
Fabrication Challenges	Aluminum is chemically active and easily oxidizes in air, posing significant challenges for epitaxial growth and subsequent processes, particularly buried regrowth.	/

 

Figure 3: Schematic Energy Band Diagram of InGaAlAs and InGaAsP Material Systems (Source: FUJITSU)

 

Comparison of CW Laser Technology Routes

 

Considering the above waveguide designs and material systems, the mainstream chip design schemes currently mainly include the following three types:

 

Material-Structure Combination	Synergistic Effect
BH  +  InGaAsP	The performance is stable, but due to the intrinsic physical properties of InGaAsP, its high-temperature performance is somewhat limited.
BH  +  InGaAlAs	The performance is excellent; however, during the second epitaxial growth in BH structures, the aluminum-containing active region is highly prone to forming a dense oxide layer when exposed to the chamber environment. This leads to numerous defects at the regrowth interface, making large-scale production extremely challenging. In particular, for high-current, high-power applications matched to silicon photonics, reliability issues are difficult to overcome in the short term.
RWG  +  InGaAlAs	The performance is well-balanced. The single-step epitaxial process of the RWG structure avoids the regrowth issues associated with aluminum-containing materials, allowing the excellent high-temperature performance of the InGaAlAs material system to be fully realized. This results in outstanding high-temperature optical output and compensates for the coupling losses caused by the elliptical far-field beam.

 

Therefore, the two viable combinations currently on the market are BH + InGaAsP and RWG + InGaAlAs.

 

Differences in CW DFB Laser Technology Routes from a Reliability Perspective

 

The typical failure modes of DFB lasers can be broadly categorized into cavity failures and facet failures, with reliability risk points varying across different technology schemes. The figure below illustrates the typical failure modes of semiconductor laser chips.

 

Figure 4: Typical Failure Modes of Laser Chips. Source: M. Fukuda / Microelectronics Reliability 47 (2007) 1619–1624

 

Cavity Reliability Challenges of the BH + InGaAsP Scheme

• Reliability Risk: Defects at the regrowth interface during the second epitaxial step

• Failure Mechanism: The complex buried regrowth and etching processes introduce interface states, lattice defects, and other imperfections within the cavity, which can act as non-radiative recombination centers, leading to gradual device performance degradation

• Quality Control: Highly dependent on extremely precise epitaxial growth control and interface treatment techniques

 

 

Facet Reliability Solutions of the RWG + InGaAlAs Scheme

• Risk Focus: Facet oxidation issues of aluminum-containing materials

• Solution: Using specialized facet treatment equipment and processes, such as in-situ passivation techniques, the facet is cleaned and a dielectric film is deposited in an ultra-high vacuum environment, significantly enhancing facet robustness

 

Practical experience has shown that, by optimizing facet processing, the RWG + InGaAlAs scheme can achieve an extremely high reliability level.

 

Everbright CW Laser Source Product Series

 

The RWG + InGaAlAs scheme offers excellent overall performance, low process complexity, and good manufacturability, making its advantages highly significant. However, one technical challenge remains: aluminum at the laser facet may oxidize after dissociation, potentially causing gradual power degradation or catastrophic optical damage (COD), meaning long-term reliability is still a critical issue to address.

With over a decade of R&D and product experience in high-power diode lasers, Everbright’s high-power product series all adopt RWG (Ridge Waveguide) structures. To solve the facet oxidation problem in aluminum-containing high-power diode chips, Everbright has successfully developed an ultra-high vacuum dissociation plus in-situ facet passivation technology, ensuring that the facet is perfectly “sealed” in its cleanest state, fundamentally eliminating oxidation issues. Using this technology, Everbright’s high-power diode chips have achieved cumulative shipments in the hundreds of millions, and long-term market validation has widely recognized the mature process and product stability.

Building on over a decade of expertise in high-power diode lasers, Everbright has leveraged the mature RWG design and facet passivation process to develop silicon photonics laser chips, effectively addressing long-term reliability challenges. The company has successfully launched wide-temperature 70 mW / 100 mW / 200 mW CW DFB diode products, continuously providing high-performance light source solutions for high-speed silicon photonics modules.