Design of Opto-Electrical Hybrid Coplanar Waveguide for Optical Module PCBs

I. Core Structure Design

1. SandwICh Stack Architecture

   | Laser Pad | <-- Opto-electronic conversion zone    | Signal Line (CPW) |    | Ground Coplanar Layer | ← Key: Gap≤λ/10 (25μm for 112Gbps)    | Ceramic Substrate (Al₂O₃/AlN) |

2. Impedance Control Golden Rule:
   $Z_0=\frac{60\pi }{\sqrt{{\epsilon }_{eff}}}\cdot \frac{K(k^{\mathrm{\prime }})}{K(k)}$
   

II. Key Hybrid Transmission Technologies

1. Opto-Electrical Isolation Design

   Parameter	Optical Channel Req.	Electrical Channel Req.
   Crosstalk Rej.	>55dB @25GHz	>40dB @25GHz
   Loss Tolerance	<0.2dB/cm	<0.5dB/cm
   *Implementation: Laser-drilled air walls (Ø80μm, pitch 200μm)*

2. Hybrid Signal Routing Optimization

   • Opto-Electrical Coaxial Transmission: Gold Ribbon Bonding

     Laser---[Gold Ribbon]---                 |---CPW Signal Line    Detector---[Gold Ribbon]---  

   • Delay Matching: ΔL ≤ 0.15mm (for 10fs jitter tolerance @112Gbps)

III. Critical Material & Process Parameters

1. Substrate Comparison

   Material	Dk	CTE	Data Rate
   Rogers 4350B	3.66	11 ppm/℃	≤56Gbps
   AlN	8.9	4.5 ppm/℃	≥112Gbps
   Al₂O₃ Ceramic	9.8	6.7 ppm/℃	25-100Gbps

2. Precision Machining Specs

   • Copper Thickness Tolerance: ±2μm (affects impedance ±3Ω)

   • Sidewall Roughness: Ra<0.3μm (reduces 10GHz IL by 15%)

IV. EMC Design Strategies

1. Grounding Optimization

   • "Star Grounding": Dedicated ground pins for laser/detector

   • CPW Ground Via Array: Ø0.1mm, pitch ≤λ/4 (1.8mm @28GHz)

2. Resonance Suppression:

   • λ/4 Open Stub: $L_{stub}=\frac{c}{4f\sqrt{{\epsilon }_r}}$
     *Example: Suppress 28GHz resonance, ε_r=3.6 → L=1.07mm*

V. SIMulation Verification Flow

Compliance Targets:

• Return Loss: >15dB @ Operating Freq.

• Insertion Loss: <0.8dB/mm @25GHz

• Eye Height: >30mV @112Gbps PRBS31

Design Case: 400G FR4 Optical Module

Performance Results:

Verification Basis:

1. Complies with IEEE 802.3bs 400GBASE-FR4

2. Validated by Ansys HFSS full-wave simulation

3. Measured data from Huawei Optical Module Lab