Custom spectrometers rise to the analytical challenge

By combining in-house manufacturing with engineering expertise, Torrent Photonics works with systems developers to create spectrometry solutions that meet the needs of demanding applications

Optical spectrometers have become a vital tool for analysing the material content within a sample, such pollutants in air and water or specific biomarkers in the blood. Most often they are integrated into a turnkey system that is tailored to a specific application, providing end users with a complete solution for preparing the sample, making the optical measurements, and analysing the spectra to produce meaningful results. However, systems engineers who are striving to deliver the best instrument for their particular application often find that an off-the-shelf spectrometer fails to deliver the performance they need.

Fortunately, help is at hand. Since 2008 Torrent Photonics has built a global business by working with equipment manufacturers to design and build optical solutions that are tailored to their specific requirements. Key to the company’s success has been an acquisition strategy that has focused on vertical integration, providing Torrent with a wealth of expertise in optical engineering and the in-house capabilities to manufacture almost all the constituent parts of high-performance spectrometers.

“People come to us when they know they have a problem, but they don’t know how to solve it,” says Hugh Garvey, the company’s Director of Spectral Devices. “We have the engineering capabilities to work closely with our customers to create a spectrometer that provides the optimal outcome for their particular application.”

Such a bespoke service often comes at a premium, either in the cost of the system or in the time needed to deliver the finished instrument. But Torrent’s internal fabrication facilities minimizes the price of critical optical components, and also provides the flexibility to modify almost any element of the spectrometer design. “Because we have access to all the parts, we can speed up the design process to create an optimal solution at an affordable price point for the customer,” says Garvey.

Particularly important is the diffraction grating, the component inside the spectrometer that splits incoming light into many different wavelengths. With a core competency in photolithography, Torrent specializes in fabricating transmissive gratings that diffract the light as it passes through a transparent structure. They are made by etching closely spaced grooves into fused silica, creating each grating as a master version rather than a replicated copy.

Such precise fabrication ensures that the gratings achieve efficient and stable light transmission across all wavelengths in the required operating range. Their transmissive nature also simplifies the optical layout, providing a more compact footprint for easy integration into a system design. Another advantage for equipment manufacturers is that transmissive gratings are easier to align with other optical components, while their output during operation is less sensitive to changes in temperature and other environmental factors such as mechanical vibrations. “A transmissive grating provides good performance in almost all situations, while a reflective design can be more susceptible to alignment errors and the polarization of the light,” says Garvey.

In-house fabrication allows Torrent to match the geometry of these transmissive gratings to the specific spectral range required by the application. In contrast, an off-the-shelf spectrometer for biomedical applications might come with a fixed operating range of 200 nm in the ultraviolet through to visible wavelengths at 700 nm. But that can limit the performance of the optical system for any application that needs to target a different part of the spectrum.

“Let’s imagine that a customer wants to go from 190 to 500 nm instead,” says Garvey. “A standard spectrometer would waste the available resolution at the top end of the operating range, but not deliver the required performance at the lower ultraviolet wavelengths. With a grating and spectrometer that are designed to operate from 190 to 500 nm, we can optimize the resolution over the desired operating range and cover the parts of the ultraviolet spectrum that would otherwise be missed.”

The key advantage, explains Garvey, is that a bespoke grating makes the most effective use of the image sensor that records the spectral output. Even the best detectors only have a certain number of pixels, and some of those will record irrelevant information if they are exposed to light beyond the wavelength range being targeted by the application. In contrast, designing the grating to disperse only the specific spectrum of interest onto the image sensor allows more useful information to be collected, enhancing the signal-to-noise ratio and boosting the effective resolution that can be achieved over the required spectral range.

In practice, the precise fabrication processes needed to create a high-performance diffraction grating makes it the most expensive element of any spectrometer. “If we needed to buy the gratings, they would cost almost as much as all the other components put together,” says Garvey. “Being able to make them ourselves cuts out the middlemen, which significantly reduces the cost of assembly and allows us to produce customized spectrometers at a competitive price.”

Even more importantly, Torrent has the internal expertise to work with customers to fine-tune the design of the diffraction grating to produce the required spectral response. “Most spectrometer companies purchase their gratings from an external supplier, which means that they need to work with a third party to get the performance they need,” says Garvey. “Because we design and fabricate all the components on site, it’s much easier for us to adapt their performance to the meet the needs of a particular application.”

Such direct interaction between Torrent’s optical engineers and their customers also speeds up the design process, while the in-house availability of almost all the components reduces the lead time needed to deliver a complete instrument and to manufacture them at the scale typically needed by system developers. Torrent also has a core competency in producing imaging devices that are commonly deployed alongside spectrometers, in particular multispectral and hyperspectral cameras, and in some cases will also collaborate with customers to create fully integrated solutions.

“Some companies are much more focused on the science of their application area, while we can offer the engineering expertise needed to build a complete optical assembly,” says Garvey. “Our vertical integration allows us to provide a full suite of capabilities for building challenging optical solutions, each one tailored to the needs of the application.”

• More information about the spectrometer solutions from Torrent Photonics can be found on the company’s website.