With the explosive growth of computing power driven by large‑scale AI models, short‑reach interconnections between data‑center servers have once again become an industry hotspot. Two primary interconnection schemes exist: electrical interconnection (servers linked via copper cables) and optical interconnection (servers linked via optical fibers).

Copper‑cable loss rises with transmission distance, and high‑frequency attenuation worsens at higher data rates. This gives optical interconnection distinct advantages: optical‑fiber transmission features far lower loss, larger capacity and lower latency. For distances ranging from 100 m to 500 m, optical links represent the only viable solution. Optical modules perform the critical electro‑optical signal conversion. Common optical‑module package styles include COB, TO and BOX, differing mainly in optoelectronic‑device packaging architecture.

Gold‑finger pin formats cover SFP / DSFP / SFP‑DD / QSFP / QSFP‑DD / OSFP. Package form factors define the number of electrical lanes and consequently determine physical‑layer rates. SFP/SFP+ provide one electrical lane; DSFP/SFP‑DD provide two electrical lanes; QSFP provides four electrical lanes; QSFP‑DD / OSFP provide eight electrical lanes (supporting schemes such as LPO / LRO / DPO). Reach variants evolve as: SR (100 m) → DR (500 m) → FR (2 km) → LR (10 km) → ER (40 km) → ZR (80 km).

Mind‑map for optical‑module SI topics

Taking conventional optical‑module hardware as an example:

Full TX signal chain: ODSP(DAC) → DSP package substrate → solder ball → PCB via & trace → wire bond (WB) → Driver pad (stack‑up included) → Driver S‑parameter / IBIS‑AMI model → WB → PIC pad (stack‑up included) → PIC S‑parameter / PIC small‑signal equivalent‑circuit.

Full RX signal chain: ODSP(ADC) → DSP package substrate → solder ball → PCB via & trace → AC‑coupling capacitor → WB → TIA pad (stack‑up included) → TIA S‑parameter / IBIS‑AMI model → WB → PD(APD) pad (stack‑up included) → PD(APD) S‑parameter / small‑signal equivalent‑circuit.

Core responsibilities of optical‑module SI work

SI engineers design, simulate and optimize end‑to‑end signal‑integrity performance across the complete optical‑module link, ensuring high‑speed signals arrive at receivers with minimal distortion and satisfy overall system specifications.

Major work items:

1. Optimize board‑level interconnection structures; optimize passive OE / EO structures inside optoelectronic devices (wire bonds, pads, flip‑chip FC, traces); design ceramic packages, COC substrates, organic substrates and connector interfaces; evaluate overall end‑to‑link performance.

2. Perform physical‑layer measurements. Operate instruments such as VNA and lightwave component analyzers (LCA) to characterize passive S‑parameters and EO/OE responses. Master calibration, de‑embedding and calibration‑fixture design techniques.

General simulation workflow

1. Split the passive channel into multiple segments at uniform transmission‑line locations. Optimize passive performance for each segment individually. Focus heavily on impedance‑discontinuity structures: gold‑finger connectors, vias, coupling capacitors and EE / EO / OE passive interfaces. This phase consumes the largest portion of engineering effort.

2. After passive‑network optimization, cascade passive‑channel models with active chip models (DSP / PIC / EIC). Perform full‑link TX / RX co‑simulation within circuit‑simulation tools such as ADS or domestic alternatives (e.g., Xinhe). Evaluate eye diagrams and EO / OE frequency‑domain responses.

Key SI performance metrics for optical modules

1. S21 (Insertion Loss, EO/OE response)

Dominated by conductor loss and dielectric loss. Evaluation focuses on in‑band flatness, resonance, ripple and roll‑off position (roll‑off should fall outside target frequency bands).

Transmission‑line loss exhibits low‑pass filtering behavior. High‑frequency loss arises from conductor loss (governed by conductivity, copper roughness, cross‑sectional area and skin effect) and dielectric loss (determined by Dk and Df; higher Df introduces larger loss). Common countermeasures: adopt low‑Dk / low‑Df dielectrics; shorten trace lengths; employ high‑conductivity, low‑roughness metals; increase trace width subject to impedance constraints and layout feasibility.

2. S11 (Return Loss, EO/OE reflection)

Caused by impedance discontinuities along the signal path. Passive‑channel design targets impedance matching to suppress in‑band reflection and comply with protocol requirements.

Important note: Perfectly flat impedance does not guarantee excellent wide‑band return loss because impedance is frequency‑dependent. TDR measurements represent impedance at a specific frequency. Well‑optimized impedance usually yields good low‑frequency return loss at the cost of degraded high‑frequency performance. According to energy‑conservation principles, engineers may intentionally relax low‑frequency return‑loss performance to lift high‑frequency return loss, aiming for roughly uniform return‑loss maxima across the whole frequency band.

Single‑ended and differential characteristic impedances must be selected according to source and load impedance conditions to guarantee end‑to‑end matching. Typical values: single‑ended: 25 Ω / 50 Ω; differential: 100 Ω / 90 Ω / 85 Ω / 75 Ω.

3. Crosstalk (S31/S32/S41/S42)

Includes near‑end crosstalk (NEXT) and far‑end crosstalk (FEXT). Higher data rates and shrinking channel pitch strengthen electromagnetic coupling between traces and worsen crosstalk. Crosstalk mitigation ranks among the most challenging and critical SI tasks for optoelectronic devices.

Common mitigation approaches: enlarge channel spacing; shorten return‑current paths; deploy GSSG differential routing; reduce parallel coupling length. (A dedicated follow‑up article will elaborate on this topic.)

4. Eye diagram (large‑signal time‑domain response)

Key observables: eye height, eye width, TDECQ, extinction ratio (ER), margin and SNR.

High‑speed evolution trends for optical‑module SI

1. Dielectric‑material upgrade: Deploy low‑Dk / low‑Df substrates. Lower Dk reduces parasitic capacitance between signal and return paths and shifts bandwidth roll‑off toward higher frequencies; lower Df cuts dielectric loss.

2. Minimize interconnection parasitics: Adopt FC and die‑to‑die (D2D) interconnections.

3. Shrink electrical reach between ASIC and optics: Form‑factor roadmap: pluggable modules → NPO → CPO → OIO.

   

4. Adopt advanced‑packaging technologies: TMV, TSV, TGV, 3D‑IC, 2.5D‑IC and interposer‑based architectures.

   Closing remarks

   Signal‑integrity engineering has a relatively low entry barrier, yet deep expertise requires extensive hands‑on project experience. SI engineers need solid theoretical knowledge, practical experience and proficiency in simulation tools.

   Beyond transmission‑line theory, reflection and crosstalk fundamentals, practitioners must understand packaging principles, equalization algorithms (FFE / CTLE / DFE), system architectures (LPO / CPO / NPO / OIO), and industry specifications (OIF, IEEE 802.3). Tools serve only as analysis aids; over‑reliance on simulation software should be avoided. Practical SI design adopts field‑circuit co‑simulation workflows.