ISO 13142:2021 Optics and photonics — Lasers and laser-related equipment — Cavity ring-down method for high-reflectance and high-transmittance measurements

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Optics and photonics — Lasers and laser-related equipment — Cavity ring-down method for high-reflectance and high-transmittance measurements是于2021-06-10发布的ISO标准，适用于全球范围。

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INTERNATIONAL STANDARD ISO 13142:2021

Optics and photonics — Lasers and laser-related equipment — Cavity ring-down method for high-

reflectance and high-transmittance measurements

1 Scope

This document specifies measurement procedures for the precise determination of the high reflectance or high transmittance (>99 %) of optical laser components.

The methods given in this document are intended to be used for the testing and characterization of high reflectance of both concave and plane mirrors or high transmittance of plane windows used in laser systems and laser-based instruments. The reflectance of convex mirrors or transmittance of positive or negative lenses can also be tested by taking into consideration the radius of curvature of the mirror surface or the focal length of the lens. This document is complementary to ISO 15368 which specifies the measurement procedures for the determination of reflectance and transmittance of optical components with spectrophotometry. ISO 15368 is applicable to the measurements of reflectance and transmittance in the range from 0 % to 100 % with a typical accuracy of ±0,3 %, and is therefore not applicable to the precise measurements of reflectance and transmittance higher than 99,9 %.

2 Normative references

The following documents are referred to in the text in such a way that some or all of their content constitutes requirements of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 11145, Optics and photonics — Lasers and laser-related equipment — Vocabulary and symbols

ISO 14644-1, Cleanrooms and associated controlled environments — Part 1: Classification of air cleanliness by particle concentration

ISO 80000-7, Quantities and units — Part 7: Light and radiation

3 Terms and definitions

For the purposes of this document, the terms and definitions given in ISO 11145 and ISO 80000-7 and the following apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https://www.iso.org/obp

— IEC Electropedia: available at http://www.electropedia.org/

3.1

reflectance

ratio of the reflected radiant or luminous flux to the incident flux in the given conditions

3.2

transmittance

ratio of the transmitted radiant or luminous flux to the incident flux in the given conditions

ISO 13142:2021

4 Symbols used and units of measure

Table 1 — Symbols used and units of measure

Symbol Term Unit

c speed of light in measurement environment m/s

c speed of light in vacuum m/s

0 h(t) impulse response of the ring-down cavity

h (t) Instrumental response function

0

L length of the initial cavity m

0 L length of the test cavity m

ΔL measurement error of the initial cavity length m

0

ΔL measurement error of the test cavity length m

n refractive index of air in measurement environment

R average reflectance of the concave cavity mirrors, equals square root of R ×R

1 2

R reflectance of the test sample

s

T transmittance of the test sample

s

R , R reflectance of two concave cavity mirrors

1 2

R reflectance of the planar cavity mirror

3

τ instrumental response time s

inst

t time s

u(t) negative-step function

the overall optical loss coefficient (absorption plus scattering) of the gases inside the cavity −1

α m

at the laser wavelength

n refractive index of the transmissive test sample substrate

s

d thickness of the transmissive test sample or central thickness of a lens to be tested m

δ(t) delta function

θ angle of incidence of the test sample rad

ρ radius of curvature of concave surface of the cavity mirror m

τ decay time of the initial cavity s

0 τ decay time of the test cavity s

Δτ measurement error of the decay time of the initial cavity s

0 Δτ measurement error of the decay time of the test cavity s

5 Test principles

5.1 General

The conventional reflectance/transmittance measurement techniques (spectrophotometry and laser ratiometry) are based on measuring the relative changes of light power reflected/transmitted by the test sample. The measurement accuracy is limited by the power fluctuations of the light sources. The cavity ring-down (CRD) technique, on the other hand, is based on the measurement of the decay rate of laser power trapped in a ring-down cavity consisting of at least two highly reflective mirrors. It is therefore totally immune to the power fluctuations of the light sources. The CRD technique can achieve a measurement accuracy that far exceeds the limit set by the power fluctuations of the light sources.

5.2 Decay time of initial cavity and reflectance of cavity mirrors

When a laser beam is coupled into the ring-down cavity, it will gradually leak out of the cavity as a small fraction of the light is transmitted through the cavity mirrors at each reflection. The temporal

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