As the demands placed on optical systems have grown, traditional spherical optical systems have become larger and heavier as more lenses have been combined to increase the precision and functionality of the system. To avoid this awkward and bulky way of increasing performance, modern systems now exploit diffractive optics and aspheric designs instead.
A Diffractive or aspheric lens can replace multiple spherical lenses meaning the weight of the optical system is lower, the system is more compact and the cost is reduced.
Aspheric optics are used to eliminate or minimize ‘spherical aberration’, to improve the focus quality, and diffractive lenses are usually used for the correction of ‘chromatic aberration’. Diffractive optics facilitate the design of advanced lens which can provide high quality data for optical systems.
Asphero-diffractive lenses reduce the number of lenses needed to build an optical system and they also greatly reduce spherical aberration and chromatic errors via compensation techniques – diffractive zones combined with the refractive properties of the lens can be used to compensate for chromatic aberrations.
Key Parameters – Roughness and Lens Form
The surface roughness of a lens impacts its performance meaning the roughness of a lens should be measured and monitored. The form of the lens is also an incredibly important design parameter, with the form of lens used to manipulate the performance of aspheric and asphero-diffractive optics.
In this article we take a look at different examples of using contact (PGI) and non-contact (CCI) metrology to measure asphero-diffractive lenses, beginning with an introduction to each technique.
Advanced Metrology for Optics
As optics continues to evolve there is a requirement for appropriate advanced metrology tools for the characterization of increasingly complex lenses which can vary in size, shape and material constitution.
Several different metrology tools can be used for the measurement of aspheric and asphero-diffractive lenses, such as non-contact interferometry based Coherence Correlation Interferometry (CCI) and contact stylus profilometry based Phase Grating Interferometry (PGI).
PGI is a form of profilometry, which uses a stylus, and it can offer a greater gauge range to resolution when compared to conventional profilometers such as laser interferometers and inductive gauges.
CCI systems are used to collect a 3D image of a surface without making contact. The technique is rapid and reliable and delivers information such as 3D form and roughness, and 2D profile measurements.
CCI - Coherence Correlation Interferometry
Figure 1 shows a schematic of a typical scanning interferometer. The instrument works by directing a beam of light by an upper beam splitter towards an objective lens, before it reaches the lens the beam is split into two by the lower beam splitter.
Figure 1. Schematic of a scanning interferometer system
One beam of light is directed to an internal reference and the other towards the sample. The two beams recombine and are sent to the detector.
The interferometric objective is scanned on the z-axis so interference between the combined beams occurs when they have the same path length. The detector records the beam intensity, taking periodic recordings as the beam is scanned over the sample to create an intensity map of the sample, which can then be used to create a 3D surface image.
Several techniques are available to control the interferometers motion and to determine surface parameters of the sample. The reliability and accuracy of the measurement is dependant on the correct control of the scanner and the calculation of surface properties from the interference data.
Coherence Correlation Interferometry is establishing itself as an important measurement technique in applications as it delivers:
- Quantitative surface characterization at high accuracy
- A resolution of sub-Angstroms in any scanning range
- Non-destructive measurements with automated control
- Rapid set up and sample loading
- Ability to analyze wide variety of materials
- The analysis of step-height and roughness from one measurement
- Ability to measure film thickness and interfacial surfaces
- Measurements which are very repeatable
Phase Grating Interferometry
The PGI gauge is Taylor Hobson’s advanced interferometry gauge, which uses a special grating to function. A stylus is ran over the sample surface and laser photodiodes are used to detect any interference signals relating to the movement of the stylus.
When compared to conventional inductive gauges and laser interferometer gauges the PGI can deliver an extremely high gauge range to resolution whilst also being a smaller instrument. The PGI represents a new ear in gauging technology, being able to achieve a typical range and resolution of 12.5 mm and resolution down to 0.2 nm.
- High resolution and large range
- The resolution of 0.2 nm is independent of gauge range
- Very high linearity and accuracy
- Signals returned from the sensors are processed radiometrically
- Results unaffected by changes in laser wavelength of signal level
The PGI Dimension
The PGI Dimension uses different technologies to deliver – High accuracy roundness and aspheric profilometry:
- Slope angles up to 85 deg
- Sags of up to 50 mm
- Up to 300 mm diameter
- Gauge noise
- High accurate alignment of the rotate part axis
- Patented proven fusion method
- Patented calibrations
Case Study 1 – Using PGI to Measure A Large and Shallow Optical Lens
PGI Dimension was used to test a large (75 mm diameter) and shallow (0.5 mm sag) asphero-diffractive lens. As the sag : diameter ratio is below 0.01 leveling and centering the measurement would be extremely difficult using conventional equipment.
The PGI uses a unique method to align samples, which uses symmetry to align the sample with the rotational axis of the instrument, which means that any profiles that are taken run over the samples true aspheric axis. Measurements must be taken over the aspheric axis for steep-sided or small lenses as off-axis measurement results in erroneous results. This alignment feature makes the measurement of shallow and large lenses possible.
Case Study 2 – Lens Form Compensation with PGI Dimension
A large (60 mm diameter) asphero-diffractive convex lens with a 3 µm step height was analyzed using PGI Dimension before and after lens compensation. The results can be seen in Figure 3.
The results demonstrate how manufacturers have precise control over lens parameters throughout the manufacturing process when using PGI Dimensions. This helps manufacturers manufacture the best quality and performing lenses, whilst keep production costs low.
Case 3 – Using CCI to Measure Optical Lens Form
One of the most important lens design features is the lens form as this determines the aspheric and asphero-diffractive optics quality.
CCI allows operators to take fast and reliable 3D morphological images of a sample, whilst also providing information on surface roughness and form error, all in one measurement, which takes only 15 seconds.
The availability of both contact and non-contact profiling tools provides modern optics and metrology researchers with the tools required to develop a comprehensive understanding of optical components.
PGI, with its extremely low resolutions (sub-nanometer vertical and sub-micron lateral) facilitates the characterization of optics with challenging structures such as those with steep sides or shallow concaves.
PGI Dimension is a market-leading metrology system for the form analysis of lenses, their molds, and other spherical and aspherical systems. Using PGI Dimensions manufacturers can exhibit control over parameters such as step height, form error and zoning throughout the manufacturing process and allowing the instrument to be used in a many optics related applications.
Non-contact CCI provides fast and reliable 3D morphology data at a sub-angstrom vertical resolution. This data can be used to complement PGI measurements providing information on structured surfaces, fragile surfaces and miniaturized optical lenses.
This information has been sourced, reviewed and adapted from materials provided by Taylor Hobson.
For more information on this source, please visit Taylor Hobson.