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Professor Emir Salih Magden updates on plans to commercialise an on-chip broadband optical filter announced last year

Optical filters are encountered in nearly every aspect of the optics, photonics and imaging industries, featuring in applications such as spectroscopy, microscopy, lidar, astronomy and optical communications, in addition to a multitude of others.

These devices are used to separate one light source into two separate outputs: one reflecting unwanted wavelengths – or colours – and the other transmitting desired wavelengths. As an example, instruments that require infrared radiation will use optical filters to remove any visible light in order to achieve cleaner infrared signals.

Existing optical filters have trade-offs and disadvantages. Discrete (off-chip) broadband filters – called dichroic filters – process wide portions of the light spectrum, but are large, can be expensive, and require many layers of optical coatings that reflect certain wavelengths. Alternatively, integrated (on-chip) filters can be produced in large quantities inexpensively, but they typically cover a very narrow band of the spectrum, so many must be combined to efficiently and selectively filter larger portions of the spectrum.

Last year, researchers from MIT’s Research Laboratory of Electronics announced the first on-chip filter that, essentially, matches the broadband coverage and precision performance of bulky dichroic filters, while being manufacturable using traditional silicon-chip fabrication methods.

Assistant Professor Emir Salih Magden, a former PhD student at MIT and faculty member at Koç University’s College of Engineering, said: ‘This new filter takes an extremely broad range of wavelengths in its bandwidth as input, and efficiently separates it into two output signals, regardless of exactly how wide or at what wavelength the input is. That capability didn’t exist before in integrated optics.’

Magden was the first author on the paper describing the filters published in Nature Communications last year. His work presents a novel approach to creating integrated photonic spectral filters, with high-pass and low-pass characteristics. Additionally, many of these broadband filters can be implemented in one system to flexibly process signals across the entire optical spectrum, including splitting and combing signals from multiple inputs to outputs.

With additional developments we plan to make to this technology over the next couple of years, it will take a form much more readily accessible

This could pave the way for sharper ‘optical combs’, a relatively new invention of uniformly spaced femtosecond pulses of light from across the visible light spectrum resulting in thousands of individual lines of radio-frequency signals that resemble teeth of a comb. Broadband optical filters are key to combine different parts of the comb, which reduces unwanted signal noise and produces very fine teeth at exact wavelengths.

Because the speed of light is known and constant, the teeth of the comb are used like a ruler to measure light emitted or reflected by objects for various purposes.

A promising use for the combs is powering ‘optical clocks’ for GPS satellites that could pinpoint the location of a mobile phone user down to a centimetre, or even better detect gravitational waves. Other uses include high-precision spectroscopy, enabled by stable optical combs combining different portions of the optical spectrum in one beam, which, Magden says, could be used to access unmeasurable phenomena.

Making it ready for market

While the device is the first of its kind to enable such a high level of performance in an integrated platform, reducing its footprint to that of conventional integrated devices is key for making it a commercial application.

Magden is now further developing the technology with his research group Photonic Architecture Laboratories at Koç University in Istanbul. The group has been working on reducing the footprint of the technology, and has already succeeded in doing so by approximately an order of magnitude, bringing the filter much closer to more conventional footprints.

‘With additional developments we plan to make to this technology over the next couple of years, it will take a form much more readily accessible,’ said Magden. ‘We are considering the option of making the device available as a packaged component that can be dropped in an existing communications network, or be part of a development kit that can be picked and placed in a communication or sensor network – before it is fabricated as a full on-chip integrated circuit. These applications will vastly benefit from the flexibility allowed by such filters, where a full range of spectral transmission properties will be fully controlled by the designer.’

The group can also see the improved optical filter technology used in optical sensing applications, such as medical imaging, spectroscopy, gas detection or chemical concentration metrology. As well as flexibility allowed by the filter characteristics, the broadband coverage of these devices can enable simultaneous detection at a wide range of wavelengths, addressing a typical challenge in many sensor systems. Magden said: ‘We are focussing efforts on making sure our new generation devices are compatible with the most common demands in sensor and imaging, as well as the communication space.’

An additional quality of the new optical filters, which according to Magden wasn’t previously possible in such a small footprint, is their ability to be cascaded to create arbitrary filter shapes. ‘Unlike many other integrated filters, our filters do not rely on the principle of interference, they work purely from a geometric shaping principle,’ he said. ‘This enables arbitrary wavelengths to be transmitted through these filters that hadn’t been possible before.’

The parameters of these filters will be adjustable to levels that have not previously been possible, Magden said, which will be useful for those looking to build adaptive communication networks or adaptive sensor systems.

Where to first?

It is possible that the first adopters of the group’s new filter technology will be in the communications market, due to the flexibility and the convenience it allows for a wide range of wavelengths in comparison to existing technology. However, according to Magden, the maturity of this market means well-established technologies are often favoured. He believes the challenges faced by the communications industry could be solved more effectively with the group’s new filters: ‘Our filters could be more favourable to those building new communication networks from scratch, and would grant them more flexibility than is available from current technologies used in that market.’

In contrast, Magden continued, new technologies are trialled for medical imaging and sensing every day, with not one individual optical technology yet succeeding in dominating that market. ‘The medical sensing field, where you have to separate different wavelengths or build certain arbitrary types of filters, will be easier to apply this technology for future development,’ he said. ‘Firms creating medical sensing or imaging devices that benefit from multi-wavelength capability would therefore be ideal partners for us.’

He concluded by explaining that, depending on the type of interest his group receives in its developing filter technology, it could either licence the technology to a commercial partner, or alternatively establish a start-up that could either work alongside commercial partners, or operate on its own.

Featured product: CeNing Optics

CeNing Optics provides a variety of optical filters, including coloured glass, narrowband interference, and bandpass.

Its coloured glass filters cover the UV, visible and NIR wavelength regions. These filters serve as bandpass, broadband or long-wave pass filters. Specifications are as follows: materials including Schott, Hoya coloured glass or equivalent; dimension tolerances ±0.15mm; parallelism 3 arcmin; surface flatness of  λ/2 per 25mm; surface quality of 60-40 S/D; and clear apertures >85 per cent.

CeNing’s band pass filter products are divided into short-pass and long-pass filters according to whether they transmit below or above the transition wavelength. The transition wavelength or cut-on/off wavelength is the wavelength at which the transmission is 50 per cent of its peak value.
The specifications for bandpass filters include: BK7, fused silica materials; dimension tolerances of ±0.1mm; 3 arcmin; parallelism 3 arcmin; flatness λ/4 @633nm; and surface quality of 60-40 S/D.

Latest commercial products

New optical filter products include Delta Optical Thin Film’s custom continuously variable bandpass filters (CVBPFs) for mid-size and full-frame CCD/CMOS sensors in hyperspectral imaging (HSI). They offer very high transmission and are fully blocked in the light-sensitive wavelength range of silicon-based detectors. The combination of CVBPFs with silicon detectors allows the design of compact, robust and affordable HSI detectors that offer benefits over conventional approaches, such as large aperture and high transmission compared to gratings and prisms; short measurement time and high suppression of stray light.

For fluorescence microscopy applications, Laser Components provides both stock and bespoke excitation and emission filters, as well as precision dichroic mirrors, which facilitate imaging onto the detector. Thanks to the range of coating technologies, these filters enable exceptionally broad fluorescence detection. Laser Components currently offers a stock range of these filters, compatible with all major microscope manufacturers.

Broadband UV-NIR neutral density filters are available from Teledyne Acton Optics, which deliver constant transmission from 190 to 1,700nm, consistent attenuation from the UV to the NIR, and superior optical performance and wavefront quality.

These neutral density filters are often used in precision optical systems, spectrometers, and medical systems. These filter coatings can be deposited on custom-sized substrates for OEM applications and can also be designed for other optical density values, if required.

Iridian has recently expanded its filter capacity for lidar sensing applications, which make use of common laser wavelengths including 532nm, 905nm, 1,064nm and 1,550nm. The filters feature narrow bandwidth, high transmission and low angular wavelength shift for a wider angle of incidence range.

High-performance characteristics of the  filters include high transmission at the laser wavelength (more than 95 per cent); narrow bandwidths (less than 1nm to over 20nm dependent on system requirements); deep blocking (OD 4-6 over detector range); high stability, reliability and environmental robustness; and a wide angle of incidence range of 0 to 15 degrees. They can also be manufactured at high volume for low cost.