Dr Yaakov Glick | Brightness Enhancement with Raman Fibre Lasers

Both Raman fibre lasers (lasers based on stimulating molecules to emit photons at a given frequency shift from the pump laser) and Rare Earth fibre lasers (which use rare earth elements to emit light) work as fibre-based laser sources. Scientists have become interested in Raman fibre lasers because Rare Earth lasers have power limitations, due to the excess heat generated by the lasing process. Dr Yaakov Glick and his colleagues in the Applied Physics Division, at Soreq Nuclear Research Centre in Yavne, Israel, collaborating internationally with other groups, have worked to increase the power of Raman fibre lasers, while simultaneously enhancing their brightness.

Brightness Enhancement

For much of his career, Dr Yaakov Glick has worked to increase power and brightness in lasers. Laser brightness of a light source beam depends on its power, cross sectional area, and angular divergence — so, it is possible to increase brightness in a laser either by increasing its power or by decreasing its geometrical attributes (diameter and angle). Brightness enhancement can be achieved by beam cleanup, in other words, by removing the higher order mode components of the laser beam, whilst enhancing the power in the lower (brighter) order modes of the laser beam (see figures 1 and 5).

Although Rare Earth fibre lasers have historically been more powerful than Raman fibre lasers, they cannot be as efficient, since they have higher inherent efficiency loss in relation to the pump laser. Therefore, Raman fibre lasers are promising in terms of reaching high brightness levels.

High-Power Raman Fibre Lasers

Raman fibre lasers are used in studies of molecular structures, materials science, and microscopy. Unlike Rare Earth fibre lasers, they are not limited to emitting photons in a specific region of the optical spectrum. These lasers are based on Stimulated-Raman Scattering, working at the quantum level to stimulate a molecule to emit a photon at a given frequency-shift from the pump laser wavelength, so that they are applicable at a wider spectral range than rare earth fibre lasers. However, this is a non-linear process, whereby multiple photons are required, so it is mostly applicable at high power levels, and may potentially be even more efficient at higher power levels. In addition, the process can create simultaneous brightness enhancement of the output, compared to the input pump laser source. Furthermore, they have lower levels of quantum defect than Rare Earth fibre lasers, so they may be more efficient.

Quantum defect refers to the energy difference between the input pump photon and the lower energy output photon, generated in the laser process. Because energy cannot be created or destroyed, the energy loss in this laser process has to go somewhere; in this case, it is lost as generated heat. This excess heat must be dealt with and removed efficiently from the system, to prevent damage due to over-heating. The energy loss, or quantum defect, is typically over 10% in conventional Rare Earth fibre lasers, but it is only around 5% in silica Raman fibre lasers.

Additionally, Rare Earth fibre lasers can suffer from photodarkening. Photodarkening occurs when intense light irradiates a medium and causes optical absorption. Optical absorption is another source of loss and heating in optical setups, since the laser loses some of its original energy, which is converted into heat. Photodarkening does not occur in Raman fibre lasers, making this another of their advantages over Rare Earth fibre lasers.

Another barrier to increased power in fibre lasers is a phenomenon known as thermal modal instability (TMI), which causes beating between the competing modes in the fibre (i.e., a continuous transfer of power between the lower mode and the higher modes, see figure 5) and ultimately a decrease in the power of the fibre’s lowest order mode (the mode with the highest brightness). TMI can be caused by thermal heat scattered along the fibre’s length. Because they typically use longer fibre lengths than conventional fibre lasers, Raman fibres are less prone to this problem, since the heat is distributed along a longer fibre length, thus creating less excess thermal density.

Figure 2: Various fibre types. A) Single–mode. B) Multi-mode. C) Double-clad

Graded Index Fibre in Bulk Optics Cavity

In 2016, Dr Glick’s group measured the power output of a Raman fibre laser with a standard graded index (GIF) MM fibre in a bulk optics cavity. GIF are standard, readily available fibres used in some optical communication systems, containing a core whose refractive index decreases continuously with distance from the centre. In this optical setup, composed of separate optical elements, the team started by collimating the laser pump output. Collimation is when the light output remains parallel instead of dispersing outwards from the source, as light often tends to do. The light was then imaged through lenses, and focused into the fibre. The light traversed the graded index fibre, which was part of a cavity configuration, which also contained reflecting mirrors. The power output in this laser setup was 154 Watts, with 65% optical efficiency. At the time, it represented both record power and record efficiency in a Raman fibre laser, allowing brightness enhancement. In this case, brightness enhancement was obtained due to the unique fibre structure, as the output signal source was confined to a smaller cross-sectional diameter of the fibre than the initial pump source had originally populated.

Figure 3: Schematic showing clad pumping principle of operation.

Standard Graded Index Fibre in All-Fibre Cavity

The Raman fibre laser with a graded index fibre in a bulk optics cavity was just the beginning. The following year, Dr Glick and his collaborators reported the first Raman GIF output from an all-fibre cavity. All-fibre systems are inherently more rugged than bulk optics systems. The reason is that the light remains within the fibre throughout the system and is therefore unaffected by mechanical vibrations. In bulk optics systems, where light travels between separate components such as mirrors and lenses, any vibration or movement causes some light to scatter away from the system, thus lowering the ultimate power output. Here, the fibre coupled pump light is directed into the cavity through fibre couplers without the need of auxiliary bulk optical elements. The cavity was based on a GIF and the mirrors are fibre mirrors, (fibre Bragg gratings, or FBGs), so the light never leaves the fibres until the system output. The Raman shifted optical output power was measured in this all-fibre laser setup and reached 135 Watts. This power level was lower than the 154 Watts achieved in the previous setup, however the efficiency measured in this case was 68% relative to the 65% measured previously. More significantly, this was the first ever demonstration of brightness enhancing Raman fibre pumping in an all-fibre configuration using a GIF. At the same time, it was also the highest brightness and highest efficiency ever measured in any Raman optical setup based on a graded index fibre. Thus, Dr Glick’s team found the results of this experiment encouraging, noting that what had limited the total efficiency was the available laser pump power. They hypothesized that increasing the power at the pump source in this setup, could result in higher efficiency and less loss, allowing it to compete with conventional optical setups.

Figure 4: Typical fibre laser configuration, includes gain fibre, pump modules, coupling components and reflection mirrors (FBGs).

Triple Clad Fibre in All Fibre Cavity

Another method to enhance the brightness with Raman fibre lasers is to use multi-clad fibres. Cladding in optical fibres means that the glass fibre consists of a centrosymmetric glass clad with a low refractive index material, surrounding a central core region with a higher refractive index (see figure 2). Similar to standard Rare Earth fibre laser setups, the pump source is injected into the low brightness (large diameter) fibre clad, while the signal output light is generated in the centrally located (small diameter) core of the fibre. Thus, brightness enhancement is achieved due to the inherent geometrical structure of the optical fibre. Despite the seeming advantages of the Raman fibre lasers, they cannot yield high efficiency unless the employed fibre has a relatively small ratio between the inner cladding and the core diameters. This small ratio limits the maximum possible amount of brightness enhancement, and requires fibres that are especially designed for Raman fibre lasers.

In 2018, Dr Glick and his colleagues used a custom designed triple clad fibre to increase the power output in a Raman fibre laser in an all-fibre cavity. This optical setup (see figure 4) was conceptually similar to that of the standard GIF in the all-fibre cavity (except of course for the fibre type). The pump source, which was itself fibre coupled, was inserted with fibre couplers directly into the clad of the triple clad fibre, which was part of an all-fibre cavity with FBG mirrors. The laser output of this setup, which was generated in the fibre core, was 1.2 kilowatts (1200 Watts), at 85% efficiency. This was an exciting result, as there was no previously recorded output greater than 1 kilowatt in any brightness enhancement fibre laser that was not a Rare Earth fibre laser. This was also the highest output power and highest efficiency ever observed in a Raman fibre laser, in a brightness enhancing optical setup.

Dr Glick and the teams have proven that Raman fibre lasers can work as high-power, high-brightness laser sources. They anticipate that Raman fibre lasers could scale to even higher power and brightness levels. However, there are still some limiting factors, which need to be dealt with properly. These include thermal management (the way the system gets rid of excess heat), and the prevention of backreflection (the light that reflects back to the pump source from the Raman laser), which can potentially cause instabilities to the pump laser source and hence to the entire system.

Figure 5: Many optical fibre systems employ single mode (SM) fibres with low power. In a SM fibre (see top left panel, LP01), the light is able to propagate in only one mode (in the central region of the fibre core). However, due to the relatively small diameter of this core, the amount of power these fibres can sustain before approaching their damage threshold is limited.
To increase the power capability of the fibre, it is necessary to increase the diameter of the fibre core, rendering it a multi-mode fibre (MM; see all other panels for beam shape examples). However along with the increased ability to sustain power, MM fibre also allows multiple paths in the fibre (higher modes), causing the cross section of the beam to be structured, as opposed to the clean SM beam.

Continuing and Future Work

Dr Glick and his collaborators continue to experiment with Raman fibre lasers as well as with Rare Earth all-fibre cavities and amplifiers. They have used different types of clad fibres to continue extending output powers beyond the kilowatt range. Specifically, they have used fibres with increased higher order mode loss which help to inhibit the fibre’s higher modes, while enhancing the generation of light in the fundamental (central) mode of the fibre. Their work has succeeded in creating power outputs beyond the kilowatt range, with their highest pulse peak power recorded at 420 kilowatts (this is 420,000 Watts) in a single-mode beam. Their findings prove that fibre lasers in all-fibre configurations have the potential to operate at high output average and peak powers. These findings can apply to new advances in micromachining, free space communications, remote sensing, and directed energy.