High Power Fiber Lasers : An Enabling Tool for Futuristic Applications

Fiber Lasers are an elegant tool for several applications in defence and industrial sectors. Tactical counter-measure defence platforms, advanced medical systems, precision material processing tools for automotive and industrial use, to mention a few, are the emerging areas where fiber lasers play a pivotal role. Because of their 'speed of light' support and high precision, laser-based Directed Energy Weapons (DEWs) are the preferred choice of next-generation military systems. Globally, laser-based weapons are becoming integral part of the defence forces, with newest example being the 300kW class laser recently introduced by the US Defence.

Fiber lasers are a type of laser in which the gain medium is doped (active) fibers. Fiber lasers are preferred over other laser sources due to their compactness, energy efficiency, and reliability. A high-power, multi-kW system consists of the following components: Laser Engine Optical Assembly, Pump Diode Assembly, Diode Driver, Thermal Management Module, Control Electronics and Embedded Control Software, Rugged Enclosure, and the graphical user interface. Fiber Laser system manufacturing requires a vertically integrated manufacturing facility with production capabilities in Fiber Optics, Electronics, Mechanical, and laser testing facilities.

In high power fiber lasers, the active fiber is typically double clad, with the rare-earth-doped core surrounded by a considerably larger and higher NA inner cladding. High-power multimode pump diodes can efficiently pump light into the inner cladding. Pump propagates through the cladding and is absorbed by the core. The inner cladding of fiber is hexagonal in shape, which aids in enhancing the pump absorption efficiency. The signal with a low numerical aperture propagates through the core and is amplified, preserving the benefits of single mode operation.

Selection of the pump wavelength depends on the absorption and emission characteristic of active fiber. For Yb doped fibers, the two distinct absorption peaks are at 915nm and 976nm. Absorption at 915nm is broad, and is less efficient, while the 976nm absorption region is very narrow, however its efficiency is multi-fold compared to 915nm. Even though, pumping at 976nm requires more control on the pump wavelength stability, it helps to enhance the efficiency.

Fiber Laser cavity is often realized as a single oscillator with wavelength matched fiber bragg gratings on both the ends of the active fiber. Combiners are used at either ends for coupling the pump light into the cavity. Distributed spatial filtering with a special routing pattern of the gain fiber is used to suppress the higher order modes except the fundamental mode to achieve single mode, diffraction-limited operation to ensure beam quality. Stimulated Raman Scattering (SRS) is one of the major non-linear effects in fiber laser sources that can be catastrophic to the laser cavity, if not managed properly. This can be controlled by optimizing the cavity length and the delivery cable.

Optimizing the splice is important for effective signal coupling to the output, low signal cross-coupling to the pump ports and also to minimize the initiation of non-linear effects. Poor splicing can degrade the beam quality and can cause hot points and create burn-outs. During splicing, the active fiber is rotated so that the flat surface of hexagonal cladding comes perpendicular to the electrode. Also, it is to be ensured that cleave angle and fiber alignment angle are well within limits to get the best splice and to ensure signal propagation in the core with minimal loss. Beyond the loss estimated by the splicing machine during splicing, the splice performance is often checked through IR viewer or thermal camera. With process optimization, an optical-to-optical efficiency greater than 85% and electrical to optical efficiency >40% is achieved, which is the best as per industry standards. Efficiency is better, because of the optimized splices, and accurate wavelength control and efficient diode drivers.

Heat from the pump diodes and also from the gain fiber is managed through liquid cooling. For example, in 2kW laser, a heat load of 3kW is managed. In the optical cavity, more heat is at the splice points, combiners and at the mode strippers. Thermal management of the active fiber needs symmetric heat dissipation and thus fiber is mounted on special grooves on a cold plate with appropriate thermal interface materials. Fiber Laser Systems are realized with several safety interlocks based on multiple inbuilt sensors such as flow sensor, pressure sensor, temperature sensors, humidity sensor and optical sensors.

Critical parameters of a laser are the power, beam quality, wavelength, and its efficiency. The beam quality (M2 Value) less than 1.2 is preferred for most of the high-end applications. Moreover, the system shall meet highest level of ruggedness for defence and industrial use. Each unit undergoes Environmental Stress Screening (ESS) tests with pre-thermal vibration, thermal cycling and post-thermal vibration.

The demand for power scaling of lasers is increasing depending on new applications. Methods employed for power scaling includes spectral beam combining (SBC) and coherent beam combining (CBC) and a combination of these methods to offer several 100’s of kW Fiber Laser. Such power combining schemes need narrow linewidth fiber lasers. With maturity in the technology, component availability, and also the level of ruggedization, fiber lasers are rapidly getting accepted as the preferred choice for many applications.

About SFO:

SFO Technologies Pvt. Ltd is an ODM company with its headquarters in Cochin, India. SFO’s diversified design and manufacturing capabilities include Photonics, Electronics, RF, Mechanical, Power Electronics, Firmware, Software etc. SFO started working in optical amplifiers from 2004 onwards and completed development of air-cooled low power Fiber Lasers in 2010. SFO jointly developed high power lasers with DRDO. This journey continues and now SFO owns the only Indian facility that has realized multi-kW single cavity lasers at 1μm. SFO’s contribution to development of such niche technologies in the country is recognized by SIDM, ELCINA etc. This facility is used by various global customers for their manufacturing requirements.

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