OCT systems innovation – component specification to design for manufacture

OCT systems manufacturers need to deliver systems capable of higher resolution imaging, faster and at lower costs than ever before. You need fit for purpose components and systems that are to specification, budget and timeline. To achieve this at scale, whilst reducing costs over the product lifetime, requires efficient and effective system design.

G&H has a history of bringing instruments to life. Present day we work with OEMs across the world, advancing the state of the art in fused components and paving the future of photonic component and system development. We do this through specification, clear manufacturing pathways and a collaborative approach, enabling us to add value to OEMs from the outset.

Through investment and acquisition, we have developed our OCT manufacturing provision to become a world leader in this arena. From an initial focus on biomedical procedures such as cardiology, skin cancer investigations to ophthalmology in the 1990s, OCT is now being seriously considered for applications such as materials analysis in markets ranging from oil and gas, to food processing and automotive paint testing.

Customers’ demand for high specification yet economical components and subsystems has seen increasing development and manufacture of OCT systems by OEMs, ranging from start-ups to established multinationals.

Customers invariably require bespoke OCT system design, which is dedicated to specific applications and therefore need to carefully choose their components and supplier to maximize system performance while minimizing cost and development time.

At G&H we understand our customers’ need for high performance, as well as flexibility in design. For decades we have developed integrated subsystems, such as the critical fused fiber coupler, which was initially for the long-distance optical communications industry and is now available for OCT too. We have invested in R&D and listened to our customers’ needs, developing key components such as the optical delay line, PDR (polarization diverse receiver) and collimator for OCT interferometers, as well as subsystems and fully bespoke OCT systems.

OCT system development success is not solely dependent on an OEM choosing the optimal combination of parts but rather consideration of time to market and development and manufacture costs is paramount. The approach to design can make the difference between an OCT project reaching the market at the right time and price point or getting delayed in costly refinement cycles.

Thanks to our established buying power and design for manufacture focus, we can help customers overcome these challenges. Expertise in component selection and integration into high quality and effective OCT subsystems that can be manufactured in volume and to the highest standards is key to systems innovation.

Component selection

When an OEM is designing an OCT system, it is important to consider the performance and reliability of the following key components and subsystems in order to achieve the best overall end result following assembly and manufacture:

• Optical delay line
• PDR (polarization diverse receiver)
• Fiber coupler

(1) Optical delay line

The optical delay line is a key part of any OCT system, enabling the operator to adjust optical transmission to exactly match the reference and sample arms, such as in ophthalmology, when patients’ eyes can be different sizes.

Critical aspects of the delay line to be considered when choosing this component include:

• Environmental stability: does it perform reliably over the desired range of operating conditions and will it reduce image fading?
• Low loss (return loss): does it prevent formation of image artefacts while maximizing image contrast and maintaining power efficiency?
• Is it fast enough? A fair expectation is 50 mm/s to enable fast optimization and adjustment per measurement.

In developing its optical delay lines, G&H works to make components that can be used in any OEM’s system. So, while the OCT designs vary, the function of the delay line is consistent. Furthermore, the company’s delay lines operate at comparatively higher speeds, meaning that operators can optimize its position more rapidly, thereby improving the workflow in a clinical or industrial setting.

(2) PDR (polarization diverse receiver)

A PDR interferes the two incoming beams and splits them into their orthogonal linear polarization states for high speed, polarization-sensitive OCT measurements. Depending on the application area, three types of PDR are each specified to operate at different wavelengths:

• 1550 nm for industrial applications
• 1310 nm for cardiology and skin cancer applications
• 1060 nm for ophthalmic applications

When selecting a PDR system for integration into an OCT system, OEMs should look for the following qualities: balanced detection; a high CMRR (common mode rejection ratio) which will improve image contrast and signal to noise ratio; a relatively high bandwidth, enabling a large image depth; and dual balanced detection which will eliminate polarization fading.

Considering OCT systems based on the more recent swept laser sources, this design supports greater optical powers. The PDR is characterized and optimized at each wave band, regardless of application. The PDR design is a compact module with its photodiodes completely integrated in one package using small photodiodes on a submount. All fiber optic alignment is therefore already done for the OEM – with no further optical connections required.

(3) Fused fiber couplers

Fused fiber couplers are widely used for branching optical signals and interfacing in telecommunications, sensor and biomedical systems, including deployment in environments requiring rigorous quality and reliability standards such as OCT systems.

This component’s suitability for high-end OCT applications depends on consistency and repeatability of manufacture and assembly, given that the OEM’s objective is to market OCT systems in volume.

At G&H we manufacture a range of fused fiber couplers for visible and NIR wavelengths using single-mode and polarization maintaining fiber. With a long company history of developing couplers for undersea telecoms applications, these fused fiber components are manufactured to exacting standards, and feature low insertion loss and high extinction ratio.

Design features and specifications to be considered in a fused fiber connector are:

• Well-controlled coupling ratio for consistent unit-to-unit performance
• Awide bandwidth/flat response allowing higher image resolution and better image contrast
• Dispersion control to enable better image resolution and minimize image artefacts

Research recently conducted by G&H as part of an EU-funded project entitled Galahad, focused on ultrawideband OCT couplers. The research targeted a 300 nm bandwidth to achieve ultra-high resolution imaging and polarization-sensitive OCT for the early detection of glaucoma in patients.

The actual spectrometer is a key component in OCT detection. Typically high-resolution performance of less than 0.05 nm is required in order to image a depth range of a few millimeters in the sample. As the depth range increases, the frequency of fringes increases or the period gets much tighter. High resolution is therefore necessary to resolve this and derive the maximum information to generate the image.

Environmental stability is always an important factor considering the real-world situations where the OCT system will be deployed, whether clinical or industrial. Resistance to changing thermal conditions is paramount to maintain consistent performance.

Component integration

Besides individual specifications and component interoperability, the ability to integrate components into a compact OCT module – including PDR, delay line, coupler, polarization controllers and collimator, for example – is fundamental. Minimizing secondary assembly stages, especially making fiber optic splices, will maximize device performance.

Designing and engineering new components technology and our capability to provide integrated modules, subsystems and systems, adds value and enables our customers to focus on idea generation, safe in the knowledge that the photonics engineering and manufacture is being taken care of by the experts – providing real value through cost, time and risk reduction.

A more in depth discussion of the factors to consider in OCT system DFMA can be viewed in the webinar recording OCT systems start to finish.