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Case Study CS-TC02 Rev. A

Frequency-Modulated, LiDAR-

Based Length and Thickness

Metrology Systems

February, 2020

Page 1

ABSTRACT

When manufacturing optics, confirming that they meet specifications is critical. Testing can cause

damage to the surfaces in accurate measurements of distance and thickness, and not all optics are

testable with non-contact measurement techniques. While thickness measurement methods tend to use

contact systems that damage the lens and length metrology techniques use methods that have problems

with surfaces that diffuse the light, Bridger Photonics provides a solution for both thickness and length

applications with a Frequency Modulated Continuous Wave (FMCW) Light Detection and Ranging

(LiDAR) technique. Without contact with the optic, Bridger Photonics can measure lengths of up to 2 m

and thicknesses of up to 60 mm with 1 µm precision. The FMCW LiDAR Metrology Systems provide

additional benefits including three dimensional (3D) mapping with the precision and accuracy needed in

laser materials processing applications and optics manufacturing.

OPTICS MANUFACTURING

Accurate and precise measurements are critical to testing

and material identification in optics manufacturing. In any

application requiring extremely accurate positions of the

optics, the thickness (optical and physical) of the lens

needs to be known to a very small degree of uncertainty.

The most demanding applications require 1 micron or better

center thickness uncertainty.1

Quick and easy, non-contact measurements can be

made with new metrology systems. High accuracy, three-

dimensional (3D) mapping can be achieved using specific

metrology systems with multiple measurement scans over

the object under test.

PROBLEMS

The most common way to measure the thickness of a lens

is by physical contact with that lens. This invariably slightly

damages the surface of the optic and requires evaluation

of the extent of the damage. Damaged optics can cause

problems in high-power laser applications. Imperfections in

the lens or on the surface of the lens can cause damage

centers.1 These can affect the beam quality of the laser and

the divergence of the laser. When the damage is too great,

testing results in scrapped optics. Human error also is a

source of inaccuracy when manually measuring the optics

using contact methods.

Non-contact

techniques

such

as

low-coherence

interferometry, optical coherence tomography, and laser

radar can achieve high levels of resolution, but they have

difficulties with diffusely scattering surfaces. This limits

measurements to only polished optics with these methods.

Additionally, they only work for very thin or flat optics. Many

manufacturers have a need for measuring thicknesses

of curved lenses. These non-contact techniques are

also limited to the size of the transparent material and

cannot measure the optical thickness and therefore

the group index. The optical thickness is related to the

phase change of light in the material and is related to the

physical thickness by a factor of the group index. This

can be used to identify the type of material that has been

processed confirming the correct blank was used. If this is

not measured alongside the physical thickness, problems

can arise from manufacturing or shipping the wrong lens

material.

In situ measurements during laser material processing

is desirable. Adding extra components to the laser setup

by using a parallel measuring system, however, is costly.

Being able to measure any optic easily and quickly,

regardless of form or surface or material, is critical for

saving manufacturers time and money.

BRIDGER PHOTONICS' SOLUTION

Bridger Photonics has created rapid, accurate, compact,

and non-contact solutions for industrial use on any type of

lens. The Thickness Metrology Station (TMS) (

Figure 1)

can measure the physical thickness, optical thickness,

and group index of the material, solving identification and

confirmation problems. The Length Metrology System

(LMS) (

Figure 2) is designed with a compact optical probe

to make integration into any system simple. It is specifically

designed for laser materials processing systems.