Tuneable Laser Evolution At Eindhoven University Of Technology

A team at Eindhoven University of Technology (TU/e) is spearheading advancements in the miniaturisation and scalability of tuneable lasers. Their groundbreaking and ongoing research, which is focused on developing innovative integrated photonic devices, is set to revolutionise a wide range of applications, including Optical Coherence Tomography (OCT), LiDAR, and environmental spectroscopy. With support from the National Growth Fund PhotonDelta, the team aims to unlock unprecedented performance levels in tuneable lasers, facilitating the viability and scalability of wider commercial applications.

Tuneable lasers in photonics

Tuneable lasers are versatile and powerful light sources that play a pivotal role in integrated photonics. Unlike fixed-wavelength lasers, tuneable lasers can dynamically adjust their wavelength across a specific range. This ability to selectively tune wavelengths is essential for many optical applications and enables high precision and adaptability.

The development of compact and highly efficient tuneable lasers opens up new possibilities in fields such as spectroscopy, telecommunications, sensing, and imaging. Researchers, including Stefano Tondini and Lyuben Davidov at TU/e, are particularly interested in how these lasers can be integrated into photonic systems by enhancing their capabilities and simultaneously reducing their form factor and power consumption. 

“Integrated laser diodes that are tunable over a wide range with high precision and demonstrate predictable behaviour are still extensively searched by the scientific community. ”

– Stefano Tondini, Postdoctoral Researcher, TU/e

One of the primary applications for this technology, and a key focus of the research project – is Optical Coherence Tomography (OCT).

More efficient OCT systems

Optical Coherence Tomography (OCT) is a powerful imaging technology that has been around since the 1990s. It’s very much like an ultrasound – using infrared light. Essentially, it facilitates the non-invasive examination of tissues such as the retina or skin. The technology works by shining infrared light into tissues, where it scatters and reflects off structures like cells and blood vessels. 

By collecting this scattered light, a detailed 3D scan of several millimetres deep into the tissue can be created. This allows doctors to detect abnormalities, such as skin cancers, that would be difficult or impossible to identify through standard visual inspection. Unlike X-rays, OCT uses light, making it completely safe and non-invasive.

The efficiency performance of OCT systems largely depends on the tuneable laser source of choice, which is responsible for generating the light required for imaging. 

An emerging laser source in the world of OCT is the Fourier-Domain Mode-Locked laser (FDML laser). Discovered 12 years ago, it sweeps the wavelength of the light, creating rainbow-like sweeps with repetition rates in the kHz range. 

Currently, the FDML lasers in OCT are limited by their physical components, particularly the need for km-long spools of optical fibre as well as the slow tuning speed of the Fabry-Pérot opto-mechanical filters. This makes the overall laser system bulky, costly, and difficult to assemble.

“Enhanced tunable laser sources – particularly integrated FDML lasers – will revolutionize Optical Coherence Tomography by enabling ultra-fast, high-resolution imaging. By integrating RF-tuned wavelength sweeping, we open new frontiers in real-time biomedical diagnostics and in vivo imaging.”

– Lyuben Davidov, Doctoral Candidate, TU/e

A revolution in continuous RF tuning 

The human body is in constant motion. That’s a problem for OCT technology because even the slightest movement or vibration can affect the precision resolution of OCT imaging, the 3D scans produced. The high-tuning frequencies of FDML lasers fix this problem and allow for real-time imaging. 

However, the conventional optical filters used in such systems have limitations in terms of speed and tunability and require ‘buffering stages’ to increase the frequencies of the wavelength sweeps. This makes the technology bulky, expensive, and less suitable for compact, light, and high-performance applications. For that reason, achieving superfast, high-precision on-chip integrated FDML lasers is crucial for the effective application of OCT in a greater number of fields.

Researchers at TU/e have been working to overcome these limitations by developing new laser sources with on-chip integrated tuneable filter that can be operated at much higher frequencies – while maintaining high precision. One of the main goals is to produce a laser that can achieve the same, if not better, performance at a fraction of the cost, size, and weight of existing systems. 

By co-designing the electronic driving and the Photonic Integrated Circuit (PIC), the TU/e team aims to create continuous FDML lasers with wavelength sweeps well beyond a few MHz region.

One of the most significant advancements in this area is achieving simple analogue control of an integrated optical filter that allows for performing wavelength sweeps with repetition rates in the HF, VHF, and UHF bands. The continuous tuning approach is not only simpler to implement, but also makes wavelength sweeps smoother, reducing noise, and improving accuracy. 

This innovation makes it possible to achieve high-speed tunability over a wide range of wavelengths, something previously unattainable. Current optical filters from the TU/e research group have achieved tuning frequencies up and beyond 50 MHz with wavelength range of ~20nm. There’s the very real prospect of a much wider tuning range in the near future.  And this has viable applications beyond OCT.

Broader applications and industrial relevance

While OCT remains a primary application for these advancements, the potential uses of tunable lasers extend far beyond medical imaging. For example, the same principles can be applied to LiDAR systems for 3D imaging, where the wavelength tunability helps improve angular resolution over large distances. This is particularly relevant to the automotive industry, where faster and more precise laser sources are essential for enhanced safety and performance.

Another area where continuously tunable lasers can have a profound impact is gas spectroscopy. By covering a much broader range of wavelengths, these lasers can enable pervasive pollution monitoring, and could eventually be integrated into smartphones. Such an application would be instrumental in more accurately tracking air quality and encouraging behavioural changes to improve public health.

A unique approach to laser design

The research into tuneable lasers happening at TU/e is unique for two key reasons: its focus on continuous tuning and its involvement with industry partners, including foundries such as Smart Photonics. 

By developing integrated photonic circuits at wavelengths of around 1300 and 1550 nanometers, researchers can create safer, more efficient systems optimized for various industrial and medical applications. Both wavelength regimes are good for different purposes, namely 1300 for ophthalmology, and 1550 for ocular tissue imaging. Unlike traditional systems, which rely on bulky and expensive components, this approach focuses on miniaturization, scalability, and cost reduction.

Additionally, the newly developed filters, which can be tuned at radio frequencies (RF) instead of mechanically, have already demonstrated superior performance. The ability to sweep these filters at megahertz rates, compared with the kilohertz rates of conventional filters, represents a significant leap forward. This advancement alone has the potential to revolutionize how OCT and other systems operate, making them more efficient and practical for everyday use.

The quest for faster, more efficient tunable lasers

Achieving cost-effective production methods for tunable lasers is crucial to facilitate widespread adoption. Miniaturization and integration still present technical hurdles in relation to fabrication, thermal management, and wavelength stability. 

But thanks to the research happening at TU/e the future of tunable laser technology looks promising. Currently, the research team is aiming to expand the tuning range to 50 nanometers or more while maintaining high-speed performance. Their goal is to scale this technology to meet industrial specifications, particularly for LIDAR and OCT applications, but the broader potential is huge. In fact, from microwave photonics to multi-species gas spectroscopy, the possibilities are seemingly endless.

“The impact of our research at TU/e will be further amplified when we are able to co-design and co-integrate electronics and photonics seamlessly on a single chip, avoiding the barriers of bandwidth, latency and power-consumption that currently limit the performance of PICs.”

– Stefano Tondini, Postdoctoral Researcher,TU/e

Ultimately, the pursuit of continuously tunable lasers may not only improve existing technologies but also enable entirely new applications that were previously unthinkable. As research continues, the integration of these advanced lasers into smaller, more efficient systems could open the door to a wide range of innovations across multiple industries.

Discover more about the research happening at TU/e by visiting their website: https://www.tue.nl/en/research/research-groups/photonic-integration