Fiber Coupling Efficiency Simulator

η = exp(−2Δx²/w²) · overlap η_NA = 1 − exp(−2NA²/θ²)

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ADJUST SLIDERS TO SIMULATE COUPLING · FA & SA AXES SHOWN

Laser Diode Source

Wavelength λ 976 nm

Fast-Axis Div. θ_FA 40°

Slow-Axis Div. θ_SA 10°

Coupling Lens

Focal Length f 4.5 mm

Lens NA 0.55

M² Beam Quality 1.2

Fiber Parameters

Core Diameter 105 µm

Fiber NA 0.22

Live Readouts

Spot @ fiber (FA) —

Spot @ fiber (SA) —

NA acceptance —

Mode overlap —

Coupling Efficiency

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API Access & Educational Use

This Fiber Coupling Efficiency Simulator is available for integration into university photonics courses, laser engineering training platforms, and industrial simulation tools via API. Contact our team at contact@ephotonics.com for custom fiber presets, multi-lens system simulations, and white-label deployments for academic or industrial use.

What Is Fiber Coupling Efficiency?

Fiber coupling efficiency is the fraction of optical power from a laser source that is successfully guided into the core of an optical fiber. In practical laser diode systems, this value is rarely 100%. Power is lost due to geometric beam size mismatch, angular acceptance limits of the fiber, and Fresnel reflections at the glass surface. Understanding and maximizing coupling efficiency is critical for applications in fiber laser pumping, optical communications, medical laser delivery, and sensing systems.

This simulator lets you model the full coupling chain interactively: from the raw laser diode emission, through a collimating and focusing lens, to the fiber end face. Every parameter you adjust updates the efficiency calculation and visual in real time.

Why Laser Diodes Are Uniquely Challenging to Couple

Unlike circular Gaussian beams from solid-state lasers, laser diodes emit an asymmetric elliptical beam. The emission from the thin active layer (typically 1 µm tall) diffracts strongly, producing a Fast Axis (FA) divergence that can reach 40° full angle. The wider lateral junction produces a much narrower Slow Axis (SA) divergence, typically 8° to 15°.

This asymmetry means the beam footprint at the lens is highly elliptical, and the focused spot at the fiber face is also elliptical. For a circular fiber core, only the overlap between the beam profile and the core acceptance cone contributes to coupled power. The simulator models both axes independently and combines them geometrically, which reflects how real coupling optics behave.

The Three Loss Mechanisms Modeled

1. Mode Overlap (Geometric Capture)

For multimode fibers, the overlap is calculated as the fraction of the Gaussian beam power that physically falls within the fiber core radius. For single-mode fibers (V-number below 2.405), a stricter mode overlap integral applies:

η_overlap = [ 2 · w_beam · w_fiber / (w_beam² + w_fiber²) ]²

Here w_beam is the focused 1/e² spot radius and w_fiber is the Gaussian mode field radius of the fiber (approximately 0.85 × core radius). Maximum overlap occurs when both radii are equal, giving a theoretical limit of 100% for the overlap term alone.

2. NA Acceptance Loss

A fiber only accepts light that enters within its numerical aperture (NA) cone. Light arriving at steeper angles undergoes total internal reflection at the core-cladding interface and is lost. The fraction of a Gaussian beam's angular distribution accepted by the fiber is:

η_NA = 1 − exp( −2 · NA_fiber² / θ_beam² )

where θ_beam is the half-angle divergence of the focused beam at the fiber face, determined by M², wavelength, and spot size. A tighter focus produces higher divergence, which increases NA mismatch loss. This is the most common source of wasted power in high-NA diode coupling systems.

3. Fresnel Reflection

At each glass-air interface, approximately 4% of incident power is reflected. With two surfaces on a standard uncoated lens, the transmission factor is approximately 0.96² = 0.92, or 8% total reflection loss. Anti-reflection coatings reduce this to under 1% per surface. The simulator applies a fixed 0.92 Fresnel factor, representing an uncoated lens.

Practical note: Real-world coupling also includes losses from lateral alignment error, angular tilt, and surface contamination. A well-aligned system with AR-coated optics and a matched lens can achieve 80% to 92% coupling efficiency into a 105 µm / 0.22 NA fiber from a single-emitter diode.

How to Use This Simulator

Laser Diode Source: Set your wavelength and the full-angle fast-axis and slow-axis divergence values from your laser diode datasheet.
Coupling Lens: Enter the focal length and NA of your collimating or focusing lens. A shorter focal length focuses tighter but increases beam divergence at the fiber face.
Fiber Parameters: Use the preset buttons for common fiber types (SM 980, MM 50 µm, MM 105 µm, MM 200 µm, MM 400 µm) or enter a custom core diameter and NA.
Live Readouts: The simulator shows focused spot size for each axis, NA acceptance, mode overlap, and total coupling efficiency with a dB loss value updated in real time.

The canvas visualizes the full optical path: the diverging diode beam, the lens focusing action, the converging and re-diverging Gaussian beam, and the coupled fraction entering the fiber. The cross-section panel at the bottom left shows the elliptical beam footprint relative to the circular fiber core.

Fiber Type Selection Guide

SM 980 (3.5 µm / NA 0.12): Single-mode at 980 nm. Requires near-perfect beam quality and precise alignment. Typical coupling efficiency from a diode is below 50% without beam shaping.
MM 50 µm / 0.22 NA: Standard telecom-grade multimode. Good for low-divergence, well-collimated beams.
MM 105 µm / 0.22 NA: The most common fiber for single-emitter pump diode coupling. Balances brightness and coupling tolerance.
MM 200 µm / 0.22 NA and 400 µm / 0.39 NA: High-power delivery fibers. Easier to couple into but lower beam brightness at the output.