Biomedical Lasers

Fiber lasers in the 2 µm range, able to reach high-power (around 50 W), are drawing the attention of medical laser manufacturers as they are “eye-safe”, meaning that our eye is sensitive to such a large wavelength and instinctively closes itself when touched. That is not the case with laser of thinner wavelength that can reach deep inside our eyes and damage it irreversibly.

An interest is especially rising for thulium (Tm) doped fibers for the development of lasers for ureteroscopy, ureteric and kidney stone management. Exail has developed a fiber laser cavity (a thulium doped fiber and fiber Bragg gratings – FBG), for this innovative laser technology that may become an important milestone for kidney stone treatment.

The goal is to be minimally invasive without breaking the skin barrier. The thulium fiber laser has several advantages over the holmium YAG laser, traditionally used in urology. With a wavelength of 1940 nm, the absorption coefficient of thulium fiber laser is four times greater in water leading to outstanding tissue ablation rate (1940 nm matches the near-infrared absorption peak of liquid water at room temperature), and it enables much smaller spot sizes to be created with less powerful pulses but at a much higher rate. Thus it allows a precise and respectful surgical approach to the other bodily functions. It is also a versatile technology, minimizing blood losses during surgical operation thanks to a “mix” of cutting and coagulation.

Tm fiber laser has average and peak powers of 100 W and it does not burst tissues, providing clean and precise cutting. It allows for pulse prolongation up to 12 ms, with regular pulses. Therefore, the power produced by the laser has a constant peak, called the super pulse creation. It is perfect for surgical applications, providing even greater precision and less collateral damage. The Thulium fiber laser technology has shown excellent results during lab testing on synthetic kidney, and it has been introduced as a new revolutionary tool for other types of surgery, including thoracic & pulmonary surgery and endoscopic surgery.

The doped fibers in the Exail portfolio especially address the range of 1900 to 2090 nm (i.e 1940 nm for urology), with different absorption wavelengths available for the FBGs that can be incorporated to any kind of fiber. Exail can produce custom fibers based on core co-doped with thulium (Tm), holmium (Ho), or both (Tm/Ho), with on demand customization: any core size (4 to 25 µm), one-two or three claddings, any kind of coating (including innovative ones for high temperature or harsh environment) and any kind of doping level. The fiber can also contain a polarization-maintaining (PM).

• Read about the full range of components available for 2μm laser applications

Different new technologies of fiber lasers are being developed for application in the dermato-aesthetics field for tattoo removal:

One of them is based on a picosecond laser that delivers bursts of ultrashort pulses for tattoo removal, with enhanced efficiency and higher patient tolerance than with larger (sub-nanosecond) laser that present higher peaks of temperature. Such multi-pulse fiber lasers are available in the 1064 nm (532 when doubled) and the 1550 nm range (775 when doubled).

Another medical device for tattoo removal is a laser system based on Erbium (Er) doped fiber, which emits in the 1550 nm range. The technique consists in heating very quickly a specific target of the dermis (fibroblasts for example), while limiting the heating of the dermis or the epidermis itself, by creating coagulation micro-wells with the photothermolysis principle. It allows to act in depth and to limit the thermal diffusion.

Exail masters the fabrication of the key components necessary for a pulse fiber laser cavity in the 1,5 µm range:

• the erbium/ytterbium doped fiber as gain fiber,
• FBGs mirrors,
• and the matching passive fiber.

Bio or medical imaging relies on the ability to image, in real-time and with ultimate high-resolution, living biological materials (i.e cellular tissues at depths of about one millimeter), while being totally non-invasive. Laser sources are used in a technique called “two-photon fluorescence excitation microscopy”. It requires ultrafast pulsed lasers capable of producing high peak power with low pulse energy to avoid degradation of living cells. Fiber based lasers are ideal solutions for this application. They are much easier to install and to maintain than solid state lasers.

920 nm is one of the main wavelengths used for this application, the linewidth where scattering loss is reduced and where fluorescent proteins are available. Laser sources based on Nd-doped fiber can be used as femtosecond pulsed lasers in the 920 nm region for “two-photon microscopy”.

Following over a decade of fruitful research collaboration (with the team of Mathieu Laroche at the CIMAP lab), Exail could develop a Nd-doped fiber enabling the design of a laser source for emission in short infrared and blue/deep-UV wavelength. Exail’s portfolio now counts the sole industrial reference of Nd-doped fibers perfectly adapted to these applications:

• Visit the webpage dedicated to our Nd-doped fibers portfolio