Spotlight on Optics: Building Lasers for the Future of Photoacoustic Imaging
The past month has been incredibly exciting 🎉. Our work on a novel laser source specifically designed for biomedical imaging was featured in Spotlight on Optics by the Optica Publishing Group ✨. Optica journals had always been a key source of knowledge and information in my research endeavors. Seeing our paper featured on the publisher’s homepage was an entirely new and thrilling experience 📢.
Spotlight selections are extremely rare—each month, from a vast number of high-quality publications, only a handful are chosen for this recognition 🌟. In fact, although numerous excellent research articles have been published in Optica journals over the past decade, this is the first time our lab has been featured in the Spotlight. That rarity made the moment even more fulfilling 🙌.
This recognition also enhanced the visibility of our work significantly. The article, originally behind a paywall as part of a subscription-based journal, was made open access as a result of the Spotlight selection 📖🔓. You can read the Spotlight article on our work here.
💡The road to Spotlight 🛣️
Coming from a background in high-power fiber lasers ⚡, transitioning to the imaging domain was a significant challenge. Our research group—the Nonlinear Photonics and High Power Lasers Laboratory at the Centre for Nano Science and Engineering, Indian Institute of Science—specializes in developing advanced high-power fiber laser systems. Over the past decade, the lab has pioneered several cutting-edge laser technologies. Our lab, supervised by Prof. V. R. Supradeepa, has gained recognition both in academia and industry, within India and across the globe🌍.
Building on the foundation we had laid over the years, I was fortunate to develop a pulsed laser source that is wavelength-switchable across a broad range—from the visible to the near-infrared window 🌈. Initially, this was more of an exploratory side project—one among many I was trying to be a part of. But over time, it became clear that the laser we had built was drawing considerable interests.
I first presented this work at the Conference on Lasers and Electro-Optics (CLEO) Pacific Rim, held in Sapporo, Japan🇯🇵, in 2022. Our paper was selected for an oral presentation in the Fiber Lasers session, chaired by Prof. Sze Yun Set from the University of Tokyo. I was overwhelmed by the positive response we received from both the conference chair and attendees. Building on the conference conversations we went on to innovate more on the laser source. This is when we extended the wavelength range from near infrared to visible. However, applying the source to an imaging system still seemed like a distant goal. We did not even dream of attempting something like that—we were far more comfortable working with high-power lasers, where we had the equipment, expertise, and confidence.
Then came January 2024, when I attended SPIE Photonics West in San Francisco, California 🇺🇸. I had the opportunity to present our latest developments in visible Raman lasers, along with another project on semiconductor metrology (more on that later). This major gathering of the global photonics community gave me valuable exposure to the fundamentals and potential of photoacoustic imaging. What truly stood out was the genuine interest shown by photoacoustic imaging experts in our laser source 🔍💬. That was the turning point—we decided it was the high time we do something about it.
We returned home with unwavering determination to demonstrate what was expected of us. But new endeavor felt daunting at first. Without the proper experience in the imaging domain, stepping in the unknown was not comfortable at all. We turned to our collaborators for necessary imaging equipments. Even then there were a thousand hurdles to cross. For a while, the photoacoustic signal seemed elusive, almost like chasing a shadow.
Our lack of experience was evident from day one. There were countless questions, gaps in understanding, and technical missteps. In hindsight, the journey may appear exciting—but at the time, it felt more like navigating in the dark. There were some major mistakes we made on the way, learnt from it. From rusting connectors to non-ideal samples, we saw it all. On multiple occasions we thought of dropping the project altogether.
But persistence paid off 🛠️. The final result we obtained would not have been possible without overcoming all those hurdles we seen. Coming through all the obstacles, we started building more understanding about photoacoustic imaging. By December, we had wrapped up the experiments and submitted a manuscript to Optics Letters. At that point, we had demonstrated photoacoustic spectroscopy of lipids—something typically achieved using far more expensive and complex optical parametric oscillator (OPO)-based systems. Our Raman laser source offered a simpler, potentially scalable alternative—bringing us one step closer to the broader deployment of photoacoustic imaging in medical diagnostics. Innovation at the laser source level plays a key role in cost reduction and system miniaturization 🏥.
We soon returned to San Francisco—this time with confirmed imaging results in hand. The excitement was palpable. The attention our work received from peers in the field was unlike anything we’d experienced before. We came back with renewed confidence in our capabilities 🚀.
By the time I returned to India, our paper had been accepted in Optics Letters with minor revisions. Two months later, it was selected for Spotlight on Optics ✨. That’s when I decided to write this blog—not just to share the technical highlights of our innovation, but also to reflect on the journey that brought us here.
Before we dive deep into the nitty-gritty of our laser system, let’s briefly introduce photoacoustic imaging and what does in offer in the field of biomedical imaging🔬.
💡The need for photoacoustic imaging 🩻
When we talk about imaging, what first comes to mind is often imaging through visible light. Of course, we carry the best imaging apparatus with us at all times. No, it’s not our smartphones 📱. Whether it’s a budget digital camera, a $1000 iPhone, or a state-of-the-art IMAX camera—nothing quite compares to the human eye 👁️.
But no matter how impressive its feats are there are certain limitations of what eyes can do. Imaging with visible light only allows superficial imaging. In the electromagnetic spectrum, visible light occupies just a narrow range—from 400 to 700 nm. Shorter wavelengths scatter more, and as light penetrates deeper into human tissue, it gets scattered away. This leads to degradation in spatial resolution—our ability to distinguish two closely spaced features—at increasing depths.
Here, ultrasound based imaging techniques work as a savior ✨. In ultrasound imaging visualization of internal organs are done using reflected sound waves 🔊. These are high-frequency waves—beyond the range of human hearing 👂. While we can hear up to 20 kHz, ultrasound operates in the MHz range. Ultrasound based imaging techniques provide unique advantages over pure optical based imaging technique. At depth 1 mm or more, ultrasound can provide better resolution compared to pure optical imaging. Anyone who has undergone an ultrasound scan—during pregnancy or for abdominal pain—has likely seen the power of this imaging technique firsthand. It offers a window into the human body without any surgical intervention, which is an important feature of such a class of imaging technology, called non-invasive imaging modality. It’s worth noting that only a few such techniques exist, with MRI being another well-known example 🧲, whereas taking blood sample for an array of diagnosis is an invasive technique.
Well, everything is all bright and shiny with ultrasound imaging, higher penetration depth in imaging is achievable, internal organs are visible. Then what seems to be the problem, that the world needed photoacoustic imaging. The problem is the limitation of spatial resolution. While it can visualize internal organs, it falls short when it comes to resolving smaller structures like veins and arteries. This is due to the wavelength of the acoustic waves used—ranging between 0.1 to 0.75 mm for frequencies between 2 to 15 MHz, considering the speed of sound in soft tissue is roughly 1.5 mm/µs. These wavelengths act as measurement scales 📏; ultrasound can only resolve structures that are of the same order or larger. As a result, with purely ultrasonic waves only, the possible spatial resolution is ~1 cm.
On the flip side 🔄, due to shorter wavelength of light, light gets scattered from objects smaller in dimension, offering better spatial resolution at the surface. Of course light can scatter from objects smaller than 1 cm, which we can detect with our own eyes. Spatial resolution seems to be a problem only at higher imaging depth. On the other hand, light can interact with matter within the material in other ways as well, for example absorption. It’s worth noting that interactions like refraction (which occurs at interfaces) and reflection (which happens on surfaces) do not happen within the material itself.
The absorption properties of light can provide deep insights into the architecture and biochemical makeup of tissue 🧬. In the visible range, absorption corresponds to electronic transitions, while in the near-infrared region, it is related to molecular vibrations. Hence, analyzing the absorption characteristics of tissue can reveal critical physiological information. However, this is extremely challenging to achieve in vivo. The tissue must be positioned between a light source and a detector, and the absorption cannot be so high that no light reaches the detector at all. Once again, this highlights the major limitation of pure optical imaging—poor spatial resolution beyond the surface.
But what if we could combine the best of both worlds? What if we had the ability to peer inside tissue using ultrasound-based imaging while also achieving the high spatial resolution and strong absorption contrast that optical techniques offer at the surface? Imagine being able not only to look deep into tissue but also to extract molecular-level physiological information at the same time.
Here comes the magic of photoacoustic imaging. In a way, it combines the best of both worlds. It enables imaging with optical contrast with ultrasonic imaging depth, with high spatial resolution. Photoacoustic imaging is based on photoacoustic effect, first demonstrated by Alexander Graham Bell more than a century ago, in 1880. But only with the advent of lasers following Theodore Maiman’s discovery in 1960, large scale investigation of photoacoustic effect for imaging purpose was possible 🔦.
Kaminari in Japanese means light and thunder 🌩️. This is name of a company, that deals with numerous biomedical imaging equipment, including photoacoustic imaging. I recently came across this company through a connection. The name beautifully captures the essence of the photoacoustic effect: just as thunder follows lightning, sound follows light in this imaging modality ⚡🎇.
Here’s how it works: a pulsed laser deposits energy onto a tissue sample. This causes a rapid thermal expansion of the absorbing region. Due to periodic expansion and subsequent contraction due to cooling give rise to a pressure wave. This wave is an ultrasonic sound wave, typically in the megahertz range. If one can hear that sound using a microphone, a ultrasonic transducer in this case, one can deduce, whether energy is deposited or not. If the sample absorbs in the wavelength range the laser is emitting, it will give rise to sound wave, otherwise not. Due to this, photoacoustic effect can give important informations on the absorption feature of samples. And since ultrasound is used for detection, imaging at greater depths is possible—something not feasible with light alone .
Photoacoustic imaging, based on photoacoustic effect, opens a new paradigm in biomedical engineering 🏥. It enables volumetric functional imaging, offering an invaluable tool for diagnosis. One of the widespread use of this technique is in oxygen metabolism, where by measuring the absorption contrast between oxygenated and de-oxygenated blood, the oxygen contents in a blood vessel can be estimated. Photoacoustic imaging also enabled monitoring of angiogenesis, a fancy name for formation of new blood vessels. Armed with these capabilities, photoacoustic imaging has revolutionized early breast cancer detection. With continuous advancements in transducer design, image reconstruction algorithms, and laser sources, the technology is scaling new heights. (More on photoacoustic imaging is available on my website)
It should be now clear that the laser source is a core component of any photoacoustic imaging module. The requirements are demanding as well. The pulse energy must be sufficiently high to excite detectable acoustic waves. The wavelengths should atch the absorption characteristics of the target tissue. Different endogenous chromophores (fancy name for absorbers inside tissues, such as fats, hemoglobin etc) absorb at different wavelengths. Hemoglobin absorbs visible light (as evident from the color of blood🩸) whereas fats absorb near infrared light (wavelengths beyond red, and hence invisible to the eye 👀). It is a significant challenge to build lasers that operate in such a wide range of wavelengths. Additionally pulsing nature of the laser has to fast enough for real-time imaging.
All of this presents a golden opportunity for innovation in laser design. Being laser engineers, that was our moment to shine🌟. We harnessed our skills and experience to build a new, compact and versatile laser tailored specifically for photoacoustic imaging💡.
💡A shiny new laser source
It turns out that for most instruments requiring a laser source, the laser itself is often the most expensive component. Strong innovation in laser design—be it in reducing cost or miniaturizing the system—can lead to large-scale deployment of the associated equipment. Building on our conference discussions and independent literature reviews, we realized that there is enormous scope for innovation in laser sources tailored for photoacoustic imaging. It turns out, the Raman fiber laser we have been developing provides a significant leap forward in terms of the required characteristics.
As we have already discussed, the photoacoustic imaging targets absorption contrast of biological substances inside tissue. Even many in the scientific community fails to recognize some times that absorption is highly wavelength dependent. Different tissue types—due to variations in composition—exhibit different absorption characteristics. For instance, hemoglobin absorbs at specific visible wavelengths 🩸, while lipids (the primary components of fat cells) absorb strongly in the near-infrared. Naturally, a problem arises here: to image different tissues, multiple laser sources at different wavelengths are required. If one is to develop a universal photoacoustic imaging module, it must be powered by a wavelength-agile and versatile laser source. This is where our expertise in building high-power fiber lasers opens a unique opportunity💡.
Over the past decades, we have refined our skills in developing an array of fiber lasers, ranging from narrow linewidth kilowatt class laser system ⚠️ to wavelength-agile high power supercontinuum and Raman fiber laser sources🌈. At this junction, when the lab is completing a decade in CeNSE, we are presented with a unique opportunity to put our laser building skills to use in biomedical imaging applications, a domain we hadn’t explored before 🧬.
Our laser sources provide distinctive advantages over existing options in photoacoustic imaging.On one hand, we have optical parametric oscillator (OPO) based laser sources. Such sources are a corner stone of scientific research, generating high pulse energy across multiple wavelength bands. But such laser sources are quite bulky, unsuitable for commercial deployment. Then we have low pulse energy supercontinuum sources, often dubbed as white light source. Unlike monochromatic, that is single colored, laser sources, they provide sufficient power across a wide wavelength range ⚪️. Just like a pure white light, it constitutes all the colors. Of course, a near infrared supercontinuum would contain near infrared wavelength. Let alone showing white color, they are invisible to human eyes. Due to the broad spectral range, supercontinuum sources work impressively for multi-spectral photoacoustic imaging, but due to the low energy, the image quality is often compromised. (You can read more about them on my supercontinuum page).
And here comes our shiny new pulsed cascaded Raman fiber lasers 🌈. Significantly more compact and less resource heavy (in terms of electricity and water cooling) compared to OPO sources. Of course, the pulse energy is lower, in the order of micro-Joule compared to mili-Joule in OPO sources. But it turns out, for most of the imaging applications, such pulse energy is good enough. But the pulse energy is better than supercontinuum sources, enabling better image quality. It turns out we have built a source that has the potential to work perfectly filling all the gaps in the existing technology.
So, what exactly is a Raman laser? It’s a special class of multi-color laser system. Traditional lasers emit at specific wavelengths based on the gain medium—a material where electronic transitions produce laser light. However, not all desired wavelengths have a suitable gain medium. This is where Raman scattering comes into play. In a Raman laser, the wavelength of a high-power pump laser is shifted to longer wavelengths via interactions with the medium—this energy loss is the Raman shift. This process can repeat multiple times, creating a cascade of new wavelengths 🔁.
As a student working on Raman fiber lasers at the Indian Institute of Science, it’s a proud moment to connect our work with the legacy of 1930 Physics Nobel Prize winner, Sir C. V. Raman, the discoverer of Raman scattering and the first Indian director of IISc 🇮🇳. (More details on Raman lasers can be found in my website.)
Unlike some Raman fiber laser technologies already exist in the market or in the research domain, we made our laser wavelength agile in true sense. The existing Raman lasers would definitely generate multiple colors, but they would be discrete, one color here and one color there. We figured to truly exploit the power of photoacoustic imaging, a source with continuous wavelength tuning is indispensable. Then only it will be able to capture the absorption feature of biological tissue, a prerequisite for functional and diagnostic imaging 🩻. Only with this capability can we truly exploit the power of photoacoustic.
With our laser packaged and optimized, we moved toward a demonstration of photoacoustic imaging. We looked for a biological tissue that is challenging to image with the existing laser technology. It turned out to be lipids, the constituents of fats. One of its absorption band is near 1200 nm. There exist only a few promising high pulse energy multi-color laser technology in this band. Seizing this opportunity, we demonstrated photoacoustic spectroscopy of lipids using commercial-grade cholesterol 🧈. (Photoacoustic spectroscopy, by the way, involves extracting absorption features via photoacoustic signals.)
Though the current experiments were done ex-vivo (outside of living body), it paves way for extracting absorption feature of live tissue in-vivo. It will enable functional live imaging of fats contains inside tissue like arteries. Why is spectroscopy of lipids so important? It turns out that different types of lipids exhibit different absorption features, and this opens the door to differentiating between cholesterol, adipose tissue, and other lipid types 💡.
While others have demonstrated lipid spectroscopy using bulky and expensive OPO sources, our experiment marks the first successful demonstration using a Raman laser source ✨. By achieving comparable results with a compact and accessible viable system, we are laying the foundation for large-scale deployment of photoacoustic imaging modules . We are quite happy with the development and actively looking for scopes to improve our systems. More technical details on the work can be found here in our published paper.
💡The scope and the future of the project
As I prepare to present our work at the prestigious Conference on Lasers and Electro-Optics (CLEO) 2025, to be held in Munich, Germany 🇩🇪, I have been seriously considering the development of a full-fledged imaging module. Several ambitious ideas are taking shape. One of our primary goals is to build a system capable of producing volumetric, three-dimensional images that offer functional information across different tissue depths🔬. By keeping the motto of “one laser solution for every application” in mind, , we plan to extend the current system to target a broader range of biological chromophores—including hemoglobin, melanin, and DNA. Our current works are limited to only lipids imaging. That is going to be the highlight of my talk in CLEO, titled “Photoacoustic Spectroscopy Using a Widely Tunable Pulsed Cascaded Raman Fiber Laser”.
CLEO 2025 promises to be a vibrant gathering of experts and peers from across the different domains. I am particularly looking forward to the discussions and feedback that often shape the trajectory of ongoing research. In the past, such interactions at conferences have played a pivotal role in refining our scientific direction. Adding to the excitement is the opportunity to reconnect with former lab mates and friends during my time in Germany. Here’s to a productive and memorable experience ahead. Cheers to a great time ahead 🎉.
Image: Demonstrating our packaged laser system in the cleanroom corridor at CENSE, IISc.