Research Progress in Chalcogenide Glass Fibers for Infrared Laser Delivery

Abstract

Significance: As the performance of mid-infrared lasers continues to improve, there occurs an increasing demand for their applications in laser surgery, military, materials processing, and other fields. Compared with spatial optical systems, the use of infrared fibers for laser transmission can greatly reduce the size of an optical system and improve the compactness and reliability of the whole system. For example, in the medical field, a considerable amount of laser surgery is performed using 2.94 μm Er: YAG lasers. The Er: YAG laser radiation absorption is very strong because this laser wavelength is practically in the center of the maximum absorption band of cellular water. Since biological tissues contain up to 70%~90% water, the Er: YAG laser is extremely efficient for their high precision cutting and vaporization. In addition, in the military field, another important application for 2 μm to 5 μm short-wave mid-infrared lasers is infrared countermeasures (IRCM) or laser tactical systems. Transmitting high power infrared lasers by infrared fibers can deflect or dazzle the infrared target seeking system. This application puts a high demand on the power handling capability of the fiber (typically tens of watts). For 512 μm long-wave mid-infrared lasers, high power CO (5.4 μm) and CO2 (10.6 μm) lasers can be used for laser surgery, industrial cutting, and welding applications. In addition, the transmission of laser power through optical fibers enables remote operation. Due to the characteristics of wide infrared transmission, good physical and chemical stabilities, and easily fiberized performances, chalcogenide glass is one of the best materials for infrared laser power delivery fiber. Therefore, as an important infrared fiber, the fabrication and application of chalcogenide glass fiber have been paid much attention at home and abroad. This review introduces the research progresses of domestic and foreign research groups in the preparation and application of chalcogenide fibers (including the step-index fibers and micro-structured fibers) for infrared laser power delivery. Progress: Step-index chalcogenide glass fibers are the earliest and most mature chalcogenide fibers. For 25 μm short-wave mid-infrared lasers, researchers first studied chalcogenide multi-mode fibers. In 1998, the US Naval Laboratory reported the successful transmission of a 2.94 μm wavelength medical free electron laser (MFEL) using an As40S60 multi-mode fiber. This result showed that laser surgery should be possible using a chalcogenide multi-mode fiber. Both CW and pulsed laser transmissions through chalcogenide multi-mode fibers were then subsequently reported. With the continuous development of mid-infrared lasers, while large-core multi-mode fibers can transmit higher power lasers, laser transmission quality and transmission modes still had to be considered, which require the development of small-core single-mode fibers. In 2018, the University of Central Florida examined the potential of chalcogenide fibers to handle high power mid-infrared lasers. The AR-coated As40S60 single-mode fiber enables the delivery of 10.3 W laser at 2.053 μm (Fig. 1). For 512 μm long-wave mid-infrared lasers, a multi-hundred-watt CO laser has been successfully delivered through chalcogenide glass fibers under gas cooling conditions (Fig. 2). And Te-based chalcogenide fibers can deliver a CO2 laser with tens of watts. The laser transmission of different chalcogenide step-index fibers has been summarized (Tabled 1). It can be seen that chalcogenide step-index fibers have made a great progress in mid-infrared laser transmission. At present, a multi-mode step-index fiber can achieve multi-hundred-watt laser transmission, while a single-mode step-index fiber can basically meet the demand for laser transmission within 10 W, and has been experimentally demonstrated in the fields of laser surgery and laser processing. Although a great progress has been made in chalcogenide step-index fibers, traditional step-index fibers are incapable due to material limitations with the increase in transmission power. In order to achieve a high power laser delivery, increasing the mode field area is the most direct and effective solution, and at the same time to ensure the beam quality, the transmission in fibers is required to be single-mode. A hollow-core micro-structured optical fiber (HC-MOF) and a large-mode-area photonic crystal fiber (LMA PCF) have become effective approaches to enhance the capability of power handling of chalcogenide fibers. Hollow-core micro-structured fibers can be divided into hollow-core Bragg fibers, hollow-core photonic crystal fibers (HC-PCFs) and hollow-core anti-resonant fibers (HC-ARFs). As researchers continue to study the numerical simulation and fabrication of micro-structured optical fibers, HC-ARFs have become the most promising infrared fibers for a high power laser delivery. The development of HC-ARFs is an encouraging advance in the fiber technology which combines the low theoretical loss over a wide bandwidth with a high tolerance to fabrication imperfections. A tolerance to fabrication imperfection is particularly important for chalcogenide fibers. At present, the minimum loss of HC-ARFs is 2.1 dB/m at 10 μm. This result indicates that HC-ARFs have already some practical value. Conclusions and Prospects: Chalcogenide fibers have been used in laser processing, laser surgery, and homeland security. The development of chalcogenide fibers with low loss and high laser damage threshold has great scientific value and application prospects. In particular, hollow-core micro-structured fibers are a technological race for the next generation of infrared fibers and related applications.