Tapered Fibre Optics: The Key to High-Performance Bottom-Mounted TEM Cameras

In a transmission electron microscope (TEM), the quality of the final digital image depends not only on the CMOS or CCD sensor itself, but on how efficiently the electron image is transferred from the scintillator to the sensor. For bottom-mounted TEM cameras, tapered fibre-optic coupling provides an efficient and highly stable solution for matching these two components.

Unlike a conventional 1:1 fibre-optic plate or a lens-based optical coupling system, a tapered fibre optic can deliberately change the image scale between the scintillator and the camera sensor. This makes it possible to optimise the relationship between scintillator resolution, photon collection, sensor pixel size and field of view.

How does a tapered fibre optic work?

In a TEM camera, the incoming electrons first interact with a scintillator, where they are converted into visible light. This light must then be transferred to the camera sensor.

With a conventional 1:1 fibre-optic coupling, the image is transferred at approximately the same scale from the scintillator to the sensor. The sensor therefore sees essentially the same image dimensions as the scintillator.

A tapered fibre optic, in contrast, has a larger input surface and a smaller output surface. The image is consequently demagnified as it travels through the fibre taper. This allows a relatively large scintillator area to be coupled efficiently to a smaller CMOS sensor.

The result is an increased effective pixel size at the scintillator and, importantly, a larger usable field of view without requiring an excessively large sensor.

For example, EMSIS uses carefully selected tapered fibre optics in its bottom-mounted cameras. In the QEDIRA, XAROSA and TOLARA, the tapered fibre is specifically designed to match the CMOS pixel size and scintillator, providing a large field of view while maintaining high sensitivity.

Why is matching the scintillator and sensor important?

The scintillator determines how incoming electrons are converted into photons and therefore plays a major role in the achievable resolution and sensitivity of the camera.

Modern CMOS sensors can have very small physical pixels. Simply placing such a sensor behind a scintillator does not necessarily provide the best TEM image. If the sensor pixels are significantly smaller than the useful resolution delivered by the scintillator, the system may effectively oversample the optical image without gaining corresponding spatial information.

A carefully designed fibre taper can address this mismatch. By demagnifying the scintillator image, several sensor pixels can effectively correspond to a larger area of the scintillator. This increases the amount of signal represented by each effective camera pixel and can provide an excellent balance between resolution, sensitivity and field of view.

EMSIS describes this approach as achieving a “perfect match” between pixel size, fibre optics and scintillator; its QEDIRA camera, for example, uses a tailored taper to optimise effective pixel size and field of view.

Tapered fibre versus 1:1 fibre coupling

A 1:1 fibre-optic plate offers a straightforward and efficient way of transferring the scintillator image to the sensor. However, the image scale cannot be optimised independently of the physical sensor dimensions.

A tapered fibre provides an additional degree of freedom: magnification can be designed into the fibre-optic coupling itself.

This can provide several advantages:

• Larger effective pixel size at the scintillator
• Improved matching between scintillator resolution and sensor pixel size
• Larger field of view for a given sensor format
• High photon-transfer efficiency without introducing a conventional lens system
• Compact and mechanically stable construction
• Excellent suitability for both high-resolution imaging and diffraction applications

The practical benefit is particularly relevant as CMOS sensors continue to offer higher pixel counts and smaller physical pixels. Rather than simply increasing the number of pixels, the complete detector chain can be optimised around the characteristics of the TEM scintillator.

Tapered fibre versus lens-coupled bottom-mounted cameras

An alternative approach is to transfer the scintillator image to the CMOS sensor using conventional optical lenses.

Lens coupling can provide excellent image transfer, but it introduces an additional optical system between the scintillator and detector. The optical design must control parameters such as aberrations, distortion, reflections, chromatic effects and vignetting.

A fibre-optic taper provides a direct image-transfer path. The individual optical fibres preserve the spatial information while the taper changes the image scale. This makes the camera architecture particularly compact and mechanically robust.

For a bottom-mounted TEM camera, where the available space beneath the microscope column can be limited, this compact architecture is a significant practical advantage.

The result: optimised detector performance

The key advantage of tapered fibre technology is therefore not simply higher resolution. It is the ability to optimise the complete electron-to-digital image chain.

The electron beam is converted into photons by the scintillator; the tapered fibre transfers and scales this optical image; and the CMOS sensor converts it into a digital image. When these three components are carefully matched, the camera can achieve an effective combination of sensitivity, spatial resolution, field of view and speed.

This principle is demonstrated in current EMSIS bottom-mounted cameras. The XAROSA, for example, combines a 20-megapixel CMOS sensor with tapered fibre coupling and achieves 30 fps at full resolution. The newer QEDIRA extends the same design philosophy to a 61-megapixel sensor, with the fibre optics, scintillator and pixel size carefully matched as part of the detector design.

A technology designed around the complete detector

For TEM imaging, the best detector is not necessarily the camera with the smallest pixel or the highest nominal megapixel count. What matters is how effectively the scintillator, fibre-optic coupling and sensor work together.

Tapered fibre optics provide a powerful way to optimise this relationship. By combining image transfer with controlled image reduction, they allow bottom-mounted TEM cameras to achieve a useful balance of field of view, sensitivity and resolution in a compact and stable detector architecture.

For this reason, tapered fibre technology remains an important element in the design of high-performance bottom-mounted TEM cameras.