Since the advent of laser-based imaging techniques in the early 2000s, image acquisition has faced a fundamental challenge: the imaging speed and signal averaging was directly tied to the firing rate of the laser. Because a minimum of one laser pulse generates a single data point, traditional flashlamp-based lasers operating at relatively low repetition rates were constrained in their ability to capture fine spatial or temporal detail quickly. For applications requiring real-time analysis or high-resolution mapping, these limitations often reduced the practicality of otherwise powerful imaging technologies.
Fortunately, the emergence of high-repetition-rate lasers based on diode-pumped solid state (DPSS) are resolving these long-standing barriers. By dramatically increasing the number of pulses per second, systems can sample and process data much faster, dramatically improving both resolution and throughput.
Similar to how pixel count defines resolution in digital photography, the number of fine laser spots analyzed increases the quality of the image for certain applications. Because each pixel requires at least one laser shot the time required to create a complete image could take hours if the repetition rate is 10 or 20 Hz.
With high-repetition-rate lasers, data collection is much faster, enabling scientists to sample and process more points in an order of magnitude less time.
This capability is essential for applications requiring rapid measurement and high spatial or temporal resolution. In medical research, mass spectrometry imaging could benefit from faster, more resolved mapping, while endoscopic photoacoustic imaging can benefit from better signal averaging of low intensity signals in vivo.
Other potential imaging applications include in-motion hyperspectral imaging and atomic force microscopy (AFM) infrared spectroscopy. Applications that require the fast and abundant removal of material such as laser-induced breakdown spectroscopy (LIBS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) could also benefit from improvements in high-repetition rate lasers.
Laser-Based Imaging
At the heart of many modern laser-based imaging systems for biological samples or subjects are nanosecond pulse width lasers capable of delivering a pulse of laser light at specific wavelengths and energies to the area of interest. Traditionally, fixed-wavelength nanosecond lasers like the Nd:YAG have been used for this purpose.
However, Nd:YAG lasers often operate outside the ideal wavelengths required to efficiently interact with the sample being imaged. For example, photoacoustic imaging is best performed at transparent wavelengths for the human body in the 650–950 nm range. Desorption and ablation techniques require high absorptivity of the sample. These wavelengths are commonly tuned to the water absorption region of 2700–3100 nm, given animals and humans are mostly made of water.
For this reason, pulsed mode Nd:YAGs that emit light at a fixed wavelength of 1064 nm or the common harmonic frequencies of 532 and 355 nm require optical parametric oscillators (OPO) to convert the Nd:YAG laser light into the optimal wavelength range for diagnostic analysis. These OPO lasers are generally referred to as tunable lasers as they offer high resolution wavelength tuning from as deep in the UV as 190 nm and as far into the IR as 3500 nm.
In addition to high resolution wavelength tuning, manufacturers such as OPOTEK have developed faster wavelength tuning techniques that enable quick scans across a wide wavelength range. Faster tuning also helps to mitigate variations in the field of view due to motion between consecutive images, which allows for more detailed imaging of moving biological processes like blood flow.
However, historically the company’s laser technology was limited to a firing rate maximum of approximately 10–20 Hz, meaning the laser could emit up to 10–20 nanosecond pulses per second.
To address this concern, OPOTEK recently introduced a family of DPSS OPO lasers that fire at 5–10 times the speed, enabling much faster operation. The Opolucis C, for example, has a repetition rate of 100 Hz and peak OPO energy of up to 45 mJ per pulse. The lasers are already being considered for potential use in applications that require both faster firing and fast wavelength tuning such as endoscopic photoacoustic imaging.
Mass Spectrometry Imaging
One clear application for this new capability is mass spectrometry imaging (MSI), a technique that benefits from higher repetition rates due to the need for faster data acquisition and increased throughput.
In traditional mass spectrometry, molecules within a sample are identified by measuring the mass-to-charge ratio of ions. MSI uses these identifications to create detailed maps of biomolecule distribution in tissue and gain insight into how molecules are organized within tissues, materials, and surfaces. Images of this type provide another level of analyzing information which can be critical in the development of medical diagnostics, drug discovery, and the production of new materials.
With MSI, the test equipment analyzes a mass of molecules in one small location, then moves to the next spot, and repeats the process until the entire area is scanned. The individual measurements are combined into a detailed image showing where each molecule appears in the sample.
Lasers are a key part of the process. In one of the most common MSI methods, called MALDI (Matrix-Assisted Laser Desorption/Ionization), a UV laser beam strikes a sample coated with a matrix that absorbs laser energy and assists in desorbing and ionizing analytes.
Other MSI methodologies employ mid-infrared lasers in various ways to vaporize, ionize, or desorb molecules before transferring them into the mass spectrometer for analysis. These approaches enable direct, in situ analysis within the sample’s native environment, requiring minimal preparation and eliminating the need to remove samples for laboratory processing.
This is where the higher repetition rate becomes a critical advantage. With faster pulsing in the range of 100 Hz or higher, the system can sample and process more pixels per second. This translates to much faster overall scan times especially when increasing pixel resolution by focusing the light to very small spot sizes or overlapping spots spatially in a process called oversampling.
With the high-repetition rate, fast tuning OPO style lasers, scientists can adjust the power, wavelength, and pulse frequency to optimize performance for different types of samples.
Endoscopic Photoacoustic Imaging
Another application driving the need for faster laser systems is endoscopic photoacoustic imaging, which combines optical and acoustic modalities to enable some of the most promising medical research today. This approach can render extremely detailed diagnostic imaging for cardiovascular diseases, cancer, chronic respiratory diseases, and diabetes.
With photoacoustic imaging, short bursts of laser light are used to heat tiny areas of tissue. This causes the tissue to expand slightly and create sound waves, which are then picked up by ultrasound detectors to form an image.
The laser’s firing speed has been one of the main limitations of photoacoustic imaging, explaining that each laser pulse can generate one “frame” of acoustic data but due to low intensity photoacoustic response it is desirable to average many laser pulses per frame. If the repetition rate is low, photoacoustic images are formed slowly, which is impractical inside the body due to tissues constantly moving. A higher repetition rate means you can fire many pulses per second, gather enough acoustic signals quickly, and reconstruct an image closer to real time.
Over the past decade, other innovations have had an impact on photoacoustic imaging, specifically enabling mobile operation on a cart, fiber delivery, fast tuning, and real time energy monitoring. These are now standard features on all the company’s photoacoustic imaging lasers including the Phocus II.
In the past, the goal with photoacoustic imaging was to deliver as much energy as possible in a single pulse to maximize penetration depth. However, this can damage tissue, which limits its use in many clinical and research settings. This has caused researchers to shift toward endoscopic techniques that deliver light through a fiber inserted into the body through veins or other access points.
He adds that bringing the light source closer to the target tissue endoscopically can make it easier to detect cancers, identify abnormal structures, and guide treatments within the gastrointestinal tract, lungs, and blood vessels.
Fast Firing with High Energy Pulse
Another key distinction of the Opolucis and Phocus II series is that it can provide high pulse energy at high repetition rates. In combination, this means the laser produces a much greater average power, a key measurement of performance.
Most tunable high-rep-rate systems deliver relatively low pulse energy, which limits its use to low-impact applications. When material needs to be significantly ablated, the new lasers can deliver significant energy output while firing rapidly. This makes the system more viable for applications that involve direct interaction with the material.
Techniques such as LIBS and LA-ICP-MS that rely on laser ablation to vaporize material so it can be analyzed.
LIBS works by analyzing the plasma formed by focusing a high intensity laser beam onto a surface and using a spectrometer to detect the light emitted from the plasma. LA-ICP follows a related principle, where the laser vaporizes material that is then carried into a plasma source for elemental analysis using a mass spectrometer.
Both techniques require rapid, high-energy laser pulses and could be well supported by the capabilities of a fast-firing laser. The development of tunable high-repetition-rate lasers marks a significant step forward in imaging capabilities, providing faster acquisition speeds, improved resolution, and greater adaptability across multiple scientific domains.
By overcoming some of the traditional constraints in pulse frequency, energy output, and tuning speed, tunable high-repetition rate OPO lasers will enable researchers to develop next-generation imaging modalities that capture much higher resolution images in a fraction of the time.
This article was written by Mark Little, PhD, Director of Sales and Support Services at OPOTEK, Carlsbad, CA. For more information, e-mail
