Thulium-Doped Fibers Lead a Charge to the 2-µm Band

NORBERTO RAMIREZ MARTINEZ CORACTIVE INC.

Optical fibers are well known and widely used as the transmission medium for optical communications. Due to the rapid increase of technology and service demand that has characterized modern optical data and telecommunications, optical fibers are necessary to use as optical amplifiers for long-haul transmission.

The most suitable example of this evolution was the development of the erbium-doped fiber amplifier. This device consists of a laser-active gain medium that enables the amplification of optical signals of 1.55 µm. Similarly, ytterbium-doped fibers (YDFs) are now an established solution supporting the growth of manufacturing and materials processing, owing to their suitability for high-power generation with a superior beam quality and thermal management.

Preform fabrication, one of two leading processes for the manufacture of optical fiber, is based on the oxidation of chemical compounds, including phosphorus oxychloride. Here, the compound is deposited via modified chemical vapor deposition. Courtesy of Coractive Inc.

As laser technology continues to mature, the development of thulium-doped fibers (TDFs) is undergoing an exponential increase in terms of both development and application. Compared with previous solutions, these fibers offer a much higher transmission capability in free space and a much larger nonlinear and fiber damage threshold (versus YDFs), and they have shown exceptional advantages in medical applications and a range of sensing and detection techniques.

Manufacture of optical fibers

Rare-earth-doped silica fiber fabrication has played an important role in optical communications and high-power fiber laser applications. The current competitive landscape of new fiber compositions and fiber designs has overtaken the standard fabrication techniques, leading companies to be active in the manufacture of specialized optical fibers. Many industry leaders have developed and are deploying innovative fabrication methods so that customers can become and remain leaders in their markets.

The main target for the incorporation of rare-earth ions into silica glass is multifaceted. The aim is to tailor the absorption and emission spectra, influence excited state properties, and improve glass-forming characteristics. Therefore, the addition of rare-earth dopants to the silica matrix is essential for the development of fiber laser technologies.

Preform fabrication is performed to achieve a modified core to cladding ratio in the finished fiber (top).
Hydroxyl group absorption and laser emission opportunities band in silica fibers (bottom). EDFA: erbium-doped fiber amplifier; HDFL: holmium-doped fiber laser; TDFL: thulium-doped fiber laser; YDFL: ytterbium-doped fiber laser. Courtesy of Coractive Inc.

Industry uses two main manufacturing processes to create an optical fiber: preform fabrication and fiber drawing. Preform fabrication is based on the oxidation of chemical compounds such as silicon tetrachloride, germanium tetrachloride, and/or phosphorus oxychloride that pass through a rotating high-purity silica substrate tube and are exposed to an external energy source to produce soot particles. These particles are deposited on the inside tube wall, then collapsed into a solid rod, called preform. The preform is then heated in a fiber drawing tower to a temperature just above the softening mark until the far end of the preform falls, thereby relying on the effects of gravity to form the optical fiber.

Adjusting the optical components of the setup configuration can broadly address most unwanted effects in a rare-earth-doped fiber laser system, such as thermal management problems and nonlinear effects. It is also possible to reduce these adverse effects by modifying the core design and selecting the proper glass host material and rare-earth concentration.

Thulium fiber facts

Thulium is the second-least abundant lanthanide, and has traditionally been widely studied for generating laser emissions in the eye-safe region between 1.7 and 2.2 µm. The first report of this type of laser was in 1967 by a team led by H.W. Gandy. The laser emitted at 1900 nm. It was not until the 1980s that significant work on thulium-doped fiber lasers emerged in R&D.

It is widely regarded that the best method to pump an active medium is to choose a pump wavelength that is close to the target wavelength due to the reduced quantum defect. To enter the 2-µm region, TDF lasers can be addressed by either high-power diodes at 79x nm, or by fiber laser pump sources at 1.55 µm. To achieve the ~1.55-µm pump, an erbium-doped fiber or ytterbium-erbium-co-doped fibers and additional high-power diodes (0.98 µm and 1.45 µm, respectively) must be added into the dynamic of the laser system.

Fortunately, since thulium exhibits a wide emission wavelength range and multiple transition lines, it is possible to pump silica-based TDFs with 79x-nm high-power diodes and benefit from a “hidden” feature that can generate a maximum laser efficiency of 80%, corresponding to a quantum efficiency of 200%. This well-known cross-relaxation process is one of the most beneficial mechanisms in operating with thulium fibers, enabling the generation of two excited ions for one pump photon when pumped by 79x-nm diodes. When the thulium concentration in a gain material exceeds ~2wt%, quan- tum efficiencies >100% in the 2-µm lasing transition can be reached.

It is worth mentioning that when the thulium concentration in a system is significantly low, the large interionic distance prevents any ion-ion interactions. These interactions are more likely to happen upon an increase to the thulium concentration and a reduction to the distance between ions.

The graph shows the absorption-emission cross section of thulium-doped fibers. Thulium exhibits a wide emission wavelength range and multiple transition lines. It is possible to pump silica-based thulium-doped fibers (TDFs) with 79x-nm high-power diodes to achieve a “hidden” feature that can generate a maximum laser efficiency of 80% and correspond to a quantum efficiency of 200%. Courtesy of Coractive Inc.

The innovation and adaptation of companies to meet demands is enabling TDFs to approach the well-established YDFs operating in the 1-µm region. In many cases, they are reaching laser efficiencies of ~70% by possessing better eye safety, relaxed nonlinear limits, and the flexibility to work in continuous wave, nanosecond, picosecond, and femtosecond laser regimes. At the same time, this significant growth of the thulium fiber lasers has opened opportunities for R&D to continue its search of more efficient laser solutions across cutting-edge applications and emerging markets.

Medical applications

The widely recognized origin of the use of lasers in health care applications traces back 50 years, where an argon laser beam was pointed at the bladder wall for an experiment. Advancements in scientific research have enabled modern laser technologies to find use in minimally invasive surgeries as well as in urology procedures. These and many other applications achieve the required high levels of precision and efficiency. As a result, they are leading to opportunities for laser-based innovation for established and emerging medical markets.

The simplified thulium (Tm) energy level diagram shows the two-for-one cross-relaxation process (top). Charting the thulium-to-holmium (Ho) energy transfer process (bottom). The 3F4 manifold describes an upper laser level in ions of thulium ions involved in the cross-relaxation process. Courtesy of Coractive Inc.

Two significant aspects of 2-µm laser sources render it a suitable candidate for clear-cut surgical applications: the high absorption in water in combination with the penetration depth that leads to a minimal damage around the exposed area, and the coagulation effect caused by the 2-µm radiation that helps mitigate unnecessary bleeding during the intervention.

The Holmium:YAG (Ho:YAG) laser is the most commonly used laser in urology due to its multipurpose ability to achieve desired cutting and tissue coagulation. Despite this ubiquity, however, common diodes for its main absorption band at 1.9 µm do not exist. Therefore, these lasers are typically flashlamp-pumped — and generate considerable heat waste because the pumping of the laser crystal is not effective. On the other hand, TDF lasers can be pumped using commercial diodes, which offers several advantages compared with flashlamps because they can operate at a lower power and less robust laser configurations (see table).

Courtesy of Coractive Inc.

Since the water absorption coefficient determines the level of absorption of an infrared radiation source, users can localize the most efficient wavelength for stone ablation and retropulsion, for example, as well as for other clinical applications. The energy of TDF lasers has a water absorption coefficient ~4× higher than the Ho:YAG laser energy, which is clinically advantageous because it leads to a more efficient intervention while minimizing photothermal damage to surrounding tissue.

TDF lasers and their holmium-doped fiber laser counterparts have experienced significant improvements in the longer side of the NIR region. This progress for both sources has arrived at a point at which holmium-doped fiber lasers are now the favored option for operation at longer wavelengths (>2.1 µm). As mentioned, these holmium laser systems do not have absorption bands where high-power diodes are available.

As an alternative, TDF lasers operating at ~1.95 µm are traditionally used for pumping holmium-doped fiber lasers. Therefore, the overall optical-to-optical conversion efficiency of the holmium laser system depends on the efficiency of TDF lasers. This dynamic brings additional challenges to fabrication processes due to the need for an all-glass fiber structure with a fluorine-doped cladding for low-loss pump guidance. Low-index polymer(s) used in standard double clad fibers will incur a strong absorption in the 2-µm wavelength region.

Co-doping silica fibers with thulium and holmium can be considered as an alternative to the in-band pumping scheme. The 79x-nm pump can be used to excite thulium ions and promote the two-for-one cross-relaxation process, followed by a dominant donor-acceptor energy transfer mechanism from thulium to holmium. Laser sources based on thulium:holmium co-doped silica fibers have been reported with slope efficiencies of up to 56%.

Room for future innovation

In addition to 2-μm medical applications, 2-μm laser systems are also promising options for the permissible power transmission in free space that can achieve several orders of magnitude greater than at 1 μm. This potential is allowing thulium technology to rapidly mature and achieve improvements in different fiber laser regimes. For example, pulsed laser systems may be used either for lidar, or for conversion into the mid- and far- infrared, remote sensing, and spectroscopy. Despite the application potential, it is still the power scaling of continuous-wave thulium fiber lasers that offers the most evidence for the improvements to the fiber technology.

Not long ago, it was expected that 100-W class, 2-μm mid-infrared fiber lasers would overthrow the use of YDFs for certain applications. Now, with the constant improvement to TDF laser technology, offering eye safety from 1.7 to 2.2 μm — and the ability to convert available diode sources into emission features for numerous applications — this idea has become a reality.

Meet the author

Norberto Ramirez Martinez is a researcher in product development at Coractive, focused on the fabrication of optical fibers for 1-μm, 1.5-μm, and 2-μm applications. He holds a Ph.D. (optoelectronics) from the University of Southampton; email: norberto.ramirez@coractive.com.