The power of precision: how low noise light sources drive better performance

From the audio world, everyone knows what noise is – for instance, when you go to a cocktail party, and you hear all the sounds of a lot of people talking at the same time. You hear a lot of noise, but you cannot distinguish a single conversation: all the sounds disperse in a big pool of noise. Noise is also that “background hiss”, that can drown out a radio station when it is too strong, or it is the static or white snow on the TV, but probably these examples are less known in the digital world of today.

So, what is light noise? And what are noisy light sources?

3 types of noise in monochromatic laser light sources

1️⃣ Talking about noisy light sources, there are several aspects to be considered. The best known is that light can vary in intensity over time, which causes trouble in applications that require steady illumination. This is called “Intensity Noise”, because it is about variation in the intensity of the light source itself.

2️⃣ Another type of noise is “Spectral Noise”, which can manifest itself both in phase and amplitude. Light can consist of just one or of a mixture of a multitude of different wavelengths (colors) mixed together, called spectrum. Fluctuations in the spectrum, for instance caused by temperature variations of the light source, create spectral noise which can disrupt measurements and obscure fine details, it can even distort images. An important contributor of spectral noise is the amplitude noise for direct spectral power measurements such as spectroscopy, which in a way, it is a variant of intensity noise.

3️⃣ And finally, in OCT and metrology, where coherent light is being used, “Phase Noise” is relevant. Coherent light is light when all the light waves are in sync. But this synchronization may not be totally consistent, or it may drift over time. This is called “Phase Noise”, and the lower the phase noise, the higher the accuracy of the images and precision of the measurements.

Difficulties of defining noise with wideband laser light sources

Defining these different types of noise in monochromatic lasers is still relatively straightforward. However, for Super Continuum Generation (SCG) wideband lasers the situation becomes a level more complex. The consequence is that also the different types of noise sources need to be considered over a wide bandwidth and frequency range and not just at one wavelength.

Here, RIN (Relative Intensity Noise) is coming to help. For monochromatic lasers, RIN is defined as the variation of the intensity divided by (relative to) the average intensity of the laser. For a pulsed wideband laser this noise definition is more generalized to a measure of the pulse energy of the laser defined as the standard deviation of the pulse energy divided by the mean pulse energy. But the way to measure this for wideband lasers is far from trivial, and the subject of a separate paper.

👉 SuperLight Photonics whitepaper: What is the correct way to Measure RIN?

The energy fluctuations quantified by the RIN ratio are ultimately indicative of both spectral noise and phase noise in supercontinuum lasers, where the final spectrum is dependent on input pulse energy and therefore, its fluctuations. Summarizing, in lighting or illumination situations there are more relevant factors than just higher brightness. Also consistency of the spectrum itself and consistency over time are crucial. No matter what the type of noise is, less noise is always better, and light sources producing less noise generate sharper pictures and more precise measurements.

The impact of noise on applications

What does lower noise mean for the various applications for which specific light sources are required?

OCT

Optical Coherence Tomography (OCT) is widely used in medical imaging, especially in ophthalmology, and relies on capturing light reflections to create detailed, cross-sectional images of biological tissues. Lower noise light sources imply that the equipment can detect finer details within tissue structures and increased clarity is invaluable to identify subtle tissue changes, critical for early diagnosis of diseases, for instance whether tissue is cancerous or not. With a low-noise light source, OCT can capture more accurate images and identify anomalies more easily, while reducing the likelihood of artifacts and misinterpretations. It may not be surprising that lower RIN is most critical in OCT, especially for high resolution imaging and identifying fine structural details, and important for contrast and depth accuracy.

HSI

In Hyper-Spectral Imaging (HSI) the objective is to analyze a broad spectrum of light for a single pixel, which is essential for applications in the industry like process control, quality management, and beyond, in environmental monitoring, agricultural analysis, and mineral identification. Low-noise light sources suffer less from background interference, and therefore improve data quality and the precision of the collected spectral information. In this way, HSI-systems can also better distinguish between closely-related spectral signatures, which can help to identify, for instance, micro-cracks or detect even minor variations in material composition. Because HSI relies on precise wavelength discrimination, lower RIN in wideband lasers is key to be able to see even miniscule variations in contrast as relative spectral amplitude variations should be minimized.

Spectroscopy

Spectroscopy is all about understanding the interaction of light with matter to determine material composition and properties. It can be intuitively understood that noise in the light source can obscure or distort results, especially when detecting low concentrations of substances or small spectral shifts. Low-noise light sources help spectrometers to achieve a higher dynamic range and sensitivity, which enables the identification and quantification of lower concentrations, which is particularly advantageous for chemical analysis, pharmaceuticals, and environmental science. This also helps to establish accurate measurements that are critical for safety or compliance. For Spectroscopy, it comes as no surprise that lower RIN is extremely important, also indicating low spectral fluctuations that enable consistent brightness and lower the time it takes to acquire data.

Metrology

And finally in Metrology, the art of measurement, extreme accuracy is required in applications like semiconductor manufacturing, where even very small inaccuracies can lead to defective products. Lower noise light sources allow metrological instruments to perform at peak precision, because they minimize the introduction of additional noise interfering with the measurement, leading to more precise, reliable and consistent results. Probably also no surprise here, but for Metrology, lower RIN is critically important, as it is an expression of the accuracy of the measuring instrument.

Conclusion on the importance of lower noise and lower RIN

To summarize, in the above-mentioned applications lower noise light sources provide a significant advantage in helping to “see” in greater detail and with enhanced accuracy. Lower-noise clarity translates into better and more accurate “picture-quality”, which is crucial for applications that rely on detailed imaging and precise measurements. The development of lower noise light sources is a continuing challenge for setting new standards on accuracy and reliability.

At SuperLight Photonics, we are proud to advertise that the RIN of our wideband laser is five orders of magnitude lower/better than any other competing product in this space. And this fully adheres to our company slogan: Better Light, More InSight!