Novel optical fibers for high power lasers

Abstract

High power fiber lasers have several applications thanks to their outstanding features such as good beam quality, all fiberized compact device size, robustness, wavelength tuning, high wall-plug efficiency, and low cost. Due to these features high power fiber lasers are replacing other solid-state lasers for several applications. Fiber lasers are being used commercially for several applications such as surgery, material processing (cutting, drilling, polishing etc.), oil and gas sensing, pumping several other lasers, and space communication etc. However, nonlinear effects restrict the output power level of fiber lasers. Although reducing power density by using large core diameter fibers can increase the threshold of non-linear effects, however large core diameter leads to multimode behavior and is prone to bend-induced effective area reduction.

Several novel large mode area fibers have been proposed to scale the output power level. However, the advantages of all-fiberized device and low cost disappear as most of these fibers involve complex fabrication and cannot be spliced to optical components such as conventional pump fibers. This thesis deals with novel large mode area fibers which are suitable for mass scale production and can offer low cost production compared to other competitive fiber designs thanks to their simple design. These novel fibers are all-solid and can be easily spliced to other fibers, thus can lead to an all-fiberized device. Moreover, some of the novel fibers proposed in this thesis offer the delocalization of powers of the higher order modes outside the core. This delocalization of the higher order modes can be useful to ensure an effective single mode operation in a double clad configuration. The proposed novel fibers offer better or competitive mode area scaling performance compared to other competitive fibers.

In this thesis, firstly conventional step-index fibers have been exploited for mode area scaling by reducing the refractive index of the actively doped core with respect to the cladding. Prior to this thesis, the lowest reported NA of a Yb-Al doped fiber was 0.048 corresponding to 0.0008 refractive index of core with respect to cladding. In this thesis, optimized solution doping process leads to a NA of 0.038 for a Yb and Al doped core corresponding to 0.0005 refractive index of core with respect to cladding. This reduction in NA of core leads to an effective area increase from ~450µm2 to ~700µm2 at 32cm bend diameter ensuring effective single mode operation. This is the lowest NA ever reported using cost-effective solution doping process to the best of author’s knowledge, which is widely used in manufacturing of rare-earth doped fiber. Further, in a 4%-4% laser configuration, a 35µm core diameter 0.038 NA fiber shows high laser efficiency (~81%) with good M2 (~1.1) value of output beam at 1040nm.

Thesis also reports a novel fiber design known as single trench fiber, where a passive Ge-doped ring has been added around the core. This ring known as resonant ring facilitates the suppression of the higher order modes thanks to resonant coupling between modes of core and ring. The combination of ultra-low NA (~0.038) and a surrounding ring can lead to an effective single mode operation of fiber having a core diameter as large as of 50µm offering an effective area of ~1,500µm2 at ~40cm bend diameter. A 40µm core diameter single trench fiber has been successfully fabricated in house and shows robust effective single mode behavior. Further, a 30µm single trench fiber has been tested in a master oscillator power amplifier configuration delivering ~23.5ps pulses at 13.5MHz repetition rate carrying up to ~3.8µJ pulse energy corresponding to >160kW peak power and ~52.3W of average power, while maintaining ~76% slope efficiency. Numerical Performance of STF has also been reported at other wavelengths such as 1550nm and 2000nm. A detailed comparative analysis has been performed with other competitive fiber designs showing the advantages of single trench fiber over other fiber designs. Further, another fiber design known as multi trench fiber has also been proposed. Multi trench fiber can scale effective area as large as of 12,000µm2 in a rod-type configuration. Multi-trench fibers offer several advantages such as easy cleaving and splicing thanks to the all solid structure; however refractive index of active core has to be same as of passive cladding. Nevertheless, this fiber has shown a strong potential for applications in ultrafast rod-type fiber lasers. A 90µm core diameter passive fiber has been fabricated in house using rod-in-tube technique in conjunction with modified chemical vapour process.

Experiments ensure an effective single mode operation. Furthermore, this fiber also shows the potential to be used for beam delivery applications with a small core diameter thanks to effective single mode operation over a wide range of bend radii. MTFs of 30µm and 20µm core diameter have been successfully fabricated and both ensure robust single mode operation over a wide range of bend radii at 1060nm and 632nm respectively. Numerical simulations show the possibility of a 10µm fiber to be effectively single moded at a wavelength of 300nm.

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Identifiers

ORCID for Jayanta Sahu: orcid.org/0000-0003-3560-6152

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