Key Takeaways
- This case study examines the high numerical aperture (NA) infinity conjugate long working distance microscope objective, suitable for precision applications such as optical tweezers.
- The microscope objective's 16 mm focal length, 14 mm working distance, and 0.7 NA provide a good blend of resolution and depth.
- It supports laser sources with wavelengths of 420 to 900 nm. The lens corrects for a three millimeter quartz window and needs precision engineering due to its sensitivity to tolerance levels.
- Performance testing adjusts eccentricity and air gaps, ensuring optimal imaging quality.
Overview of Optical Tweezers Technology
Optical tweezers use highly focused laser beams to catch and manipulate ultra-small matter like cells and nanoparticles. The tweezers generate force by transferring light, allowing for non-contact control of small objects.
The focused laser beam generates a strong gradient field near its focal point, drawing microscopic particles to the area of high light intensity and enabling collection.
Optical tweezers can control these tiny particles without damaging the materials and have shown promising results in a range of fields. In atom capture experiments, lasers typically must converge to the micron level to effectively catch these particles.
To do so, optical tweezers require microscope objectives with excellent resolution. Increasing the numerical aperture (NA) is one way to effectively produce high-resolution objective lenses.
Because samples for atom capture experiments are frequently placed in a vacuum chamber, the microscope objective must have a long working distance.
In recent years, microscope objective manufacturers have emphasized the importance of obtaining both high NA and long working distances. High NA microscope objectives are widely used in biology, materials research, and semiconductor detection because of their excellent resolution and light-gathering capabilities.
Microscope objectives with high NA typically have shorter working distances. Achieving high NA and long working distances requires correcting multiple higher-order aberrations during the design stage. This, in turn, leads to more complex structures, larger diameters, tighter tolerances, and significant challenges in both design and manufacturing.
Description of the High NA Objective
This high NA microscope objective has a numerical aperture of 0.7 and a working distance of 14 mm, which results in a bigger diameter than standard objectives. Its broad operating band, spanning wavelengths of 420 nm to 900 nm, makes it suitable for a variety of laser sources.
It is important to note that this objective is intended to correct a three millimeter thick quartz window. Any deviation from this thickness during operation may reduce the objective's performance. Working distance is the distance between the observed or processed object and the front end of the lens.
When choosing a microscope objective for any practical application, it is critical to consider working distance. Typically, a larger working distance allows for better application versatility.
At a fixed NA, however, increasing the working distance requires a larger lens size, which increases advanced aberrations in the optical path and complicates the manufacturing process.
In addition, reducing the focal length to working distance ratio might result in higher spherical error from the rear lens as the optical path aperture expands, complicating design efforts. Taking these effects into consideration, this lens is built with a focal length that roughly matches the working distance (16 mm) and has a low magnification.
Specification of the NA0.7 infinitely conjugated long working distance microscope objective. Source: Avantier Inc.
| |
|
| Focal length |
16 mm |
| NA |
0.7 |
| Wavelength |
420-900 nm |
| FOV |
Φ0.5 mm |
| Working distance |
14 mm (including 3 mm fused silica) |
Image Credit: Avantier Inc.
NA0.7 Infinite conjugate long working distance microscope objective design structure. Image Credit: Avantier Inc.
Image Credit: Avantier Inc.
Spot and WFE performance. Image Credit: Avantier Inc.
Performance Criteria of the Microscope Objective
The microscope objective's performance is assessed using the size of the dispersion spot and the trans-wavefront error. The lens' optical axis has a spot radius of less than 0.4 µm, indicating effective micron-level spot convergence.
The transmit-wavefront diagram shows that this goal achieves diffraction-limited performance at many wavelengths, except for 421 nm, where the off-axis performance slightly exceeds the limit.
Focal shift curve. Image Credit: Avantier Inc.
Chromatic Aberration and Wavelength Range
This lens is remarkable as it operates across a huge range of wavelengths, from violet to near infrared. While chromatic aberration correction does not satisfy the diffraction limit, this objective lens is specifically designed to work effectively with laser sources.
Minor post-focusing adjustments are needed when using diverse light sources, which help to reduce performance loss caused by chromatic aberration. The transmission wavefront design performance for the 421 nm band after focusing is shown below.
transmission wavefront error@421 nm. Image Credit: Avantier Inc.
Design Summary of High NA and Long Working Distance Microscope Objective Lenses
This micro-objective lens has a modest magnification of about 12X when combined with a 200 mm tube lens. The numerical aperture is considerable, reaching 0.7, and the working distance is 14 mm, accounting for a three millimeter thick quartz window.
The microscope objective operates across a wide wavelength range (421 to 900 nm), is compatible with a variety of laser sources, making it a unique form of high-end objective.
Optical Component Machining
Given the specificity of this objective lens, the effect of tolerance is extremely sensitive, requiring accurate machining tolerance for all components. The high NA and long working distance increase the difficulties associated with producing such optical components.
A frame for adjusting this micro objective lens. Image Credit: Avantier Inc.
Objective Focusing and Performance Testing
When modifying the micro-objective lens, it is essential to take an image of the object and then change the eccentricity and air gap based on the imaging results. This method helps to reduce coma and spherical aberrations caused by manufacturing errors.
The objective in question is a low-magnification microscope lens with a big entrance pupil and a high numerical aperture (NA). When used with a normal microscope frame, imaging results may be insufficient, making proper adjustment difficult. The microscope must be aligned with the object under observation to guarantee optimal performance.
Measurement Graph: MTF vs. Frequency. Image Credit: Avantier Inc.
Customize Long Working Distance Microscope Objective Lenses
The NA0.7 infinity conjugate long working distance microscope objective is a specially designed lens intended for sophisticated optical applications, requiring high resolution and long working distance.
Its wide wavelength adaptability and diffraction-limited performance make it particularly useful in materials science and quantum research.
Its complex design needs accurate optical machining and cautious alignment to reduce aberrations. To ensure optimal performance, fine-tuned modifications are required, particularly in eccentricity and air gaps.
This case study highlights the necessity of striking the right balance between numerical aperture, working distance, and manufacturing precision when designing high-end microscope objectives for cutting-edge applications.
This information has been sourced, reviewed, and adapted from materials provided by Avantier Inc.
For more information on this source, please visit Avantier Inc.