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Why is etalon effect (fringing) reduction important in spectroscopic devices

Abstract image with diagonal streaks in blue, yellow, pink, and orange on a dark blue background, creating a vibrant, dynamic pattern with gradient color transitions.

Etalon effect - also known as optical fringing - reduction is critical because it causes severe signal fluctuations and heavily degrades performance, remaining a key limiting factor in laser absorption spectroscopy [1,2]. 

Minimizing this effect is crucial to unlocking the full potential of optical spectroscopic techniques, which offer rapid and highly precise gas detection capabilities. Solving this problem is particularly urgent today, as the widespread availability of QCL and ICL sources accelerates the deployment of spectroscopic instruments in key areas like security, industrial control, environmental monitoring, and healthcare. 

 

What optical fringing effect is?

Optical fringing is a result of the interference of radiation reflected on the optical elements. In photonic infrared detectors, the window and the chip are the components most critical to the formation of interference fringes. As radiation passes through them, the transition between media causes its energy to split, resulting in partial transmission and reflection (see Fig. 1). The co-propagating rays interfere with one another, and their optical path difference determines whether the interference is constructive or destructive, thereby amplifying or attenuating the signal. An example of signal fluctuations caused by fringing is illustrated in Fig. 2. 

 

Diagram showing radiation from a source passing through a window, becoming focused, and then hitting a chip; arrows indicate the direction of radiation through each stage.
FIGURE 1. Schematic of radiation splitting within the detector components.
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FIGURE 2. Illustration of optical fringing/etalon effect

A characteristic feature of etaloning is a periodic variation in the signal. The period of this sinusoidal modulation depends on the optical path length within the medium, while its amplitude is determined by the intensities of the transmitted and reflected rays [3]. Reflectivity at the interface of two media is defined by the equation:

 

The image shows the equation: R = ((n₁ - n₂) / (n₁ + n₂))², representing the reflection coefficient formula.
where n1, n2- refraction index of the media.

Material for the window should not absorb radiation. In the case of infrared detectors the most common materials are presented in the Tab. 1.

Common materials used in infrared detectors.

Material Refraction index Reflectivity* [%] Transmittance spectra [um]
Silicon 3.4 29.75 0.8-6.5
Germanium 4.2 37.87 1.8-16
ZnSe 2.3 15.52 0.5-22
GaAs 3.3 28.61 0.8-18
Sapphire 1.7 6.72 0.2-5.5

*reflectivity at the material/air interface.

As can be seen, materials used in the infrared region exhibit quite high refractive indices, which result in high reflectance. This effect can be mitigated by antireflective coatings.

 

The amplitude of the interference fringes in infrared detectors increases significantly for radiation wavelength above peak wavelength. This is due to the weaker absorption of long wavelengths in the absorber, so that more radiation is reflected from the metallization of the detector structure, contributing to greater etalon effect (see FIGURE 3).

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FIGURE 3. Theoretical calculations of fringing vs. wavelength in MWIR standard IR detector illuminated by coherent radiation

How anti-fringing technology work?

VIGO anti-fringing technology means internal modification of the detector structure. After the growth the wafer is characterized and specially processed to manufacture detection structures immune to the generating of interference fringes. This results in the fringing 10 – 40 times smaller compared to the standard IR detector (see FIGURE 4, FIGURE 5, FIGURE 6  and FIGURE 7).

Line graph showing Fringing, 1 versus Wavelength, µm from 2.5 to 5.0. The dark blue line rises sharply after 4.5 µm, while the yellow line stays low and fairly flat.

FIGURE 4. Fringing in an exemplary MWIR standard detector (dark blue line) and detector with anti-fringing technology applied (orange line)

Line graph showing fringings (Y-axis) versus wavelength in micrometers (X-axis, 2.0–11.0 µm) for two data series: one black line rising sharply after 8 µm, and a yellow line with a smaller increase.

FIGURE 5. Optical fringing in an exemplary LWIR standard detector (dark blue line) and detector with anti-fringing technology (orange line)

Plot of intensity versus wavelength (µm) with two overlaid lines, one yellow and one dark blue, showing a sharp dip near 4.3 µm. An inset zooms into the 4.2–5.1 µm region, highlighting detailed oscillations.

FIGURE 6. Normalized spectral characteristics of an exemplary MWIR standard detector (dark blue line) and detector with anti-fringing technology applied (orange line)

Plot of intensity versus wavelength (2.5–15 μm) with two data series: yellow and blue. An inset magnifies the 9–14 μm range, showing detailed differences between the two curves.

FIGURE 7. Normalized spectral characteristics of an exemplary LWIR standard detector (dark blue line) and detector with anti-fringing technology (orange line)

Additionally, this modified detection structure can be anti-reflection coated. It slightly improves fringes reduction, but it may be of importance in very high accuracy applications, i.e. trace gas detection.

We can apply anti-fringing technology to all our detectors without immersion microlenses.

Anti-fringing solutions

Radiation decoherence Reflectance reduction
Wedged window AR coating on the window
Wedged cap AR coating on the active structure
Modification of the internal structure

Examples of infrared detectors with anti-fringing solution:

 

 

Author: Justyna Chrzanowska-Giżyńska PhD Senior Optical & Process Engineer, VIGO Photonics

 

 

Q&A Section

Question 1: What is the etalon effect (fringing) in infrared detectors?

Answer: Etalon effect (fringing) is a phenomenon that appears as signal fluctuations. It results from the interference of radiation that undergoes multiple reflections with the parallel optical components, which act as a Fabry-Perot ethalon.

Question 2: Which detector components are most susceptible to etaloning?

Answer: The most critical components susceptible to etaloning are the detector window and the chip (active structure) itself. This is because materials used in the infrared region exhibit high refractive indices, which generate strong reflections at the air interface.

Question 3: Why is optical fringing reduction so important in laser-based gas spectroscopy 

Answer: Absorption spectroscopy (e.g., using QCL and ICL sources) relies on the precise measurement of light intensity passing through a gas. Sinusoidal fluctuations caused by optical fringing overlap with the gas absorption lines, drastically reducing detection sensitivity and accuracy, which ultimately prevents the detection of low concentrations (ppm/ppb). 

Question 4: What determines the amplitude and period of the interference fringes (fringing)? 

Answer: The period of the signal variations is closely linked to the optical path length (i.e., the component's thickness and its refractive index). On the other hand, the amplitude of the interference fringes (the depth of the modulation) depends on the intensities of the transmitted and reflected rays at the media interfaces. 

Question 5: What are the main strategies for mitigating fringing in the detector architecture? 

Answer: Strategies for fringing reduction can be divided into two main groups: radiation coherence reduction (e.g., by using wedged windows or wedged caps) and surface reflectance reduction (by depositing anti-reflection coatings). 

 

References:

  1. K. Bogumil et al., J. Photoch. Photobiol. A: Chem. 157 (2003) 167–184 
  2. P. Kluczynski et al., Appl. Phys. B 103 (2), 451-459 (2010)
  3. K. K. Sharma, Optics: Principles and Applications, Burlington, MA, USA: Academic Press, 2006