In this new white paper, Dr Stuart Coomber, Global Head of OEM Sales, discusses the use of periodically poled lithium niobate (PPLN) in medical lasers.
The Use of Lasers in Medicine
The US Food and Drug Administration (FDA) website states that: ‘Medical lasers are medical devices that use precisely focused light sources to treat or remove tissues.’ 1 In recent years, lasers have been used for the diagnosis and treatment of an increasing number of medical conditions. Ongoing research into laser therapeutics by medical institutions and commercial companies continues to produce novel, effective and patient-friendly alternatives to more traditional care pathways. In addition, pathology testing is set to be revolutionized by photonics based, miniature chips within the next 10 to 15 years.
According to a 2022 report published by Research and Markets, the global medical lasers market reached a value of US$ 3.70 Billion in 2021 and is expected to increase to US$ 7.01 Billion by 2027.2 The uplift in value of this sector highlights the new treatments and medical conditions being successfully and cost-effectively treated by laser technologies.
In the UK, the NHS currently employs 25,000 people in pathology laboratories all of which use photonics-based instruments to test patient samples. This costs the NHS US$ 2 billion annually, or the equivalent of 4% of its budget.3
Medical Conditions Diagnosed and Treated with Lasers
Lasers can be used for a range of medical procedures as the beam itself is so small and accurate it allows surgeons to safely and effectively treat tissue without injuring the surrounding area.
Cosmetic dermatologists use lasers for the effective removal or treatment of tattoos, scars, stretch marks, sunspots, wrinkles, birthmarks, spider veins and unwanted hair. Other specialists in the field of cosmetics have also more recently adopted the use of lasers e.g., for dental tooth whitening.
Outside of the cosmetic market, lasers are used for an increasingly broad range of medical treatments. Ophthalmologists have been using lasers for eye surgery since the late 1980’s, using the technology to treat a range of conditions including refractive errors, posterior capsular opacity, glaucoma, diabetic eye disease and retinal tears. Other branches of surgery are also successfully using lasers to aid in the treatment of certain illnesses, tumours, kidney stones and prostate glands are all routinely removed using lasers in surgery.
There are a number of benefits to using laser treatments as supposed to more traditional surgical techniques. Laser treatments carry the same risks as open surgery including pain, bleeding and scarring but the recovery time for the patient, and hence the post recovery cost for the hospital, has been shown to be much reduced. In addition, the laser light beam does not pose health risks to the patient or medical team in the way that other treatments e.g., radiation therapy might do.
The Future of Lasers in Medicine
With an increasing, aging population in many countries globally, the traditional central testing pathology laboratories are both unaffordable and unsustainable. It is envisaged that by 2035, with advances in integrated photonics, point of care tests will be taken at the GP surgery or bedside using miniature chips. It is already estimated that point of care testing will reach US$ 31 billion by 2025, reducing pathology laboratory testing requirements by up to a quarter.4
The use of lasers in medicine offers a host of other potential applications for the future e.g., the use of spectroscopy to monitor blood glucose, highly localized irradiation for light-activated cancer treatments via key-hole surgery are amongst a vast array of other diagnostic and treatments currently already being used or in active trials.5, 6
Ensuring that these pioneers in healthcare have the correct wavelengths and other laser light properties to meet the continuing development of medical applications is an important focus area for laser and medical equipment manufacturers.
Wavelength Engineering Using Non-Linear, Optical Crystals
Non-linear optical (NLO) crystals provide an enormously flexible solution for generating new wavelengths from existing, off-the-shelf laser sources. Although there are a wide variety of commercially available laser sources covering the extended optical spectrum it is still not always possible to find a direct or cost effective light source for all applications. It is in these cases
where a practical, direct source is not available that wavelength conversion using highly efficient, non-linear optical crystals provides a powerful solution.
When considering non-linear optical, crystal materials, lithium niobate (LiNbO3) is a particularly attractive option since it has a very high non-linear coefficient . With its high efficiency, ability to be periodically poled and broad optical transmission, MgO-doped, periodically poled lithium niobate (MgO:PPLN) becomes a highly flexible solution for the generation wavelengths from 400nm to 5μm.
Examples of Wavelengths Required for Medical Application
Lasers are used in ophthalmology more than in any other medical specialty. The transparent nature of the human eye makes it possible to target intraocular structures without the need for endoscopy or separate surgery. Wavelengths of interest include 689nm used in photodynamic therapy (PDT), PDT is a treatment that involves light-sensitive medicine and a light source to destroy abnormal cells. PDT is used to treat age-related macular degeneration (AMD)7. Laser light at 810 nm is used in transpupillary thermotherapy (TTT) which is the most common type of laser treatment for eye melanoma. TTT uses the infrared light to heat and kill the tumor.8
Flow cytometry is a widely used method in biomedical research and increasingly in clinical diagnostics.9 It is a powerful and rapid technique to analyze physical and chemical properties of single cells or particles as they are suspended in liquid and pass in a narrow line across laser beams. Fluorescence together with scattered laser light is then filtered, detected, and analyzed. In addition to analysis, many flow cytometers can also sort and purify cell populations of interest for downstream analysis based on the identified properties of cells or particles. Lasers are exclusively used for flow cytometry due to their power, uniform, and focused illumination properties. Multiple monochromatic laser wavelengths provide multiparametric detection possibilities with the use of many different fluorescent labels. The most commonly used antibody labels in biosciences fluoresce at the following wavelengths 405, 445, 488, 532, 561, 633, 640, 660, and 810nm.10
In both of these examples the wavelengths of interest cover the visible and NIR regions of the optical spectrum. Non-linear wavelength conversion provides a powerful method for the generation of visible wavelengths from IR laser sources and can therefore be used to ‘fill’ the wavelength gaps that exist between direct laser sources.
Visible Wavelength Generation
Wavelengths covering the visible region of the optical spectrum can be generated via Second Harmonic Generation (SHG), or Sum Frequency Generation (SFG). With appropriate choice of the pump lasers either fixed or tunable wavelength output can be produced.
Second harmonic generation (SHG), or frequency doubling, is the most commonly used second order non-linear process. In SHG, two input pump photons with the same wavelength λP are combined through a nonlinear process to generate a third photon at λSHG, where, λSHG = λP/2 (or in terms of frequency fSHG = 2fP).
MgO:PPLN SHG crystals can be fabricated to work with a wide range of commercially available pump laser wavelengths from 976nm to 2100nm, allowing generation of frequency doubled light between 488nm and 1050nm.
Example:High efficiency SHG of 1064nm light using PPLN can generate 532nm light at Watt levels of power suitable for skin treatment including removal of port wine stains, birthmarks, melanomas, tattoo & hair removal.
Sum frequency generation (SFG) combines two input photons at λP and λS to generate an output photon at λSFG, where λSFG = (1/ λP + 1/ λS) -1 (or in terms of frequency fSHG = fP + fS). By combining readily available fixed (e.g. 1550nm) and tunable (e.g. 780/810nm) pump laser sources MgO:PPLN SFG crystals can provide tunable output light between 500-700nm.
Example: High efficiency SFG using PPLN can combine tunable 1560nm and fixed 1064nm sources to generate light around 633nm for use in flow cytometry.
Ease of Use
MgO:PPLN can be readily manufactured into a variety of forms from bulk crystal to waveguide providing both a wide application range as well as enhanced conversion efficiency. Wavelength conversion chips, either using bulk crystal or waveguide forms, can then easily be packaged with fiber-coupled input and output – for enhanced ease of use. The combination of a fiber-coupled package together with a high precision temperature controller provides a plug and play wavelength conversion solution ready for benchtop use or OEM integration.
In conclusion, the use of PPLN provides a practical solution for the generation of a wide range of wavelengths that are of importance in medical applications. It offers an alternative solution to existing, costly laser sources and a solution for wavelengths that are not readily accessible via direct laser sources. This highly efficient material can be packaged into components ready for integration into OEM lasers and medical equipment.
As a leading supplier of PPLN-based wavelength conversion products Covesion is able to offer advice on customer specific solutions, as well as technical support in their set-up, use and optimization. With an extensive portfolio of COTS products, as well as custom design capabilities, Covesion is well placed to support the widest range of wavelength conversion applications.
For further information please contact;
Dr Stuart Coomber Global Head of OEM Sales, Covesion Ltd
- M. Houe et al, J. Phys. D Appl. Phys., 28:1747–1763, 1995
- Photodynamic Therapy for Age-Related Macular Degeneration | Johns Hopkins Medicine
- B. Faisting et al, Medical Laser Application, Volume 25, Issue 4, November 2010, Pages 214-222
- How a Flow Cytometer Works | Thermo Fisher Scientific – UK
- Trends in flow cytometry lasers call for more new wavelengths | Laser Focus World