Introduction

Cryogenic cooling of Yb-doped solid-state laser materials has many benefits and has previously been used to improve the performance of crystalline host materials [1, 2]. At low temperatures, the thermal expansion and the thermooptic coefficient are reduced, which in turn reduces the heat induced optical aberrations. At the same time, the thermal conductivity is increased which helps to lower the thermal stress of the material. In Yb-doped fiber lasers, these properties are at moderate powers not as critical since fibers intrinsically have good thermal handling due to the distribution of the gain medium. Moreover, the wave guiding properties of a fiber are to a large extent immune to thermal induced aberrations. The spectral properties of crystalline hosts materials are however quite different from silica hosts where the Yb-ion is subjected to strong homogeneous and inhomogeneous broadening [3].

Yb-doped fiber lasers operate between the two Stark-split manifolds ²F_5/2 and ²F_7/2. At room temperature, the thermal population in the lower laser level of ²F_7/2 results in considerable reabsorption which strongly affects the lasing properties. Pask et al. [4] used this to show that changing the fiber length (and thereby the wavelength dependent losses), lasing could be achieved for wavelengths ranging from 980 nm to 1090 nm. In a similar experiment, Brilliant et al. [5] heated a section of an Yb-doped fiber, increasing the thermal population and therefore the reabsorption losses for shorter wavelengths which shifted the preferred lasing wavelength from 1103 nm to 1121 nm.

By cooling Yb-doped fibers to cryogenic temperatures, the thermal population of the lower laser levels can be drastically reduced, eradicating the absorption tail above 1 μm [6]. This was used by Seifert et al. to amplify an external cavity diode laser at 1014.8 nm [7]. It has also been shown to efficiently suppress self pulsing [8], a phenomenon common to Yb-doped fiber lasers which occurs when the gain fiber is insufficiently pumped making part of the fiber act as a saturable absorber [9].

In this work we show that significantly more efficient lasing can be achieved at liquid nitrogen temperatures (LNT) compared to room temperature but at the cost of an increasing linewidth. By using a volume Bragg grating (VBG) [10, 11], this is mitigated without a loss in output power. With 14.5 W of launched pump, 11.4 W of temporally stable output power is extracted within a spectral window of 0.4 nm with the VBG, compared to 11.5 W in a spectral window of almost 20 nm when an ordinary broadband mirror is used, i.e. an increase in spectral power of 50 times.

Experimental setup and results

Four separate experiments were carried out using the setup shown in Fig. 1. The output characteristics were compared for configurations using either a mirror or a VBG at both LNT and room temperature. Two different fiber lengths were also used, 4 m and 11.5 m. Although the former had a suboptimal pump absorption, it was necessary to perform the room temperature experiments with a short fiber length since the longer fiber succumbed to catastrophic break-down of the fiber ends at room temperature due to self pulsing. The type of fiber used in all experiments was a double-clad Yb-doped large-mode-area (LMA) fiber from Liekki (YB1200-20/400DC). The fiber had a core diameter of 20 μm and a cladding diameter of 400 μm with corresponding NA of 0.06 and 0.22, respectively. On all fibers, one of the fiber ends was perpendicularly cleaved to provide ~4 % cavity feedback while the other was angle-polished to prevent parasitic reflections.

 

Fig. 1. Experimental setup.

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The fibers were optically pumped through the perpendicularly cleaved fiber end by a fiber coupled laser diode emitting at 976 nm. The pump light was launched at an angle of 12° with respect to the normal of the fiber end, omitting the need of a dicroic mirror in the pump beam path and thereby enabling separation of the pump and signal emission regardless of wavelength separation [10]. The launch efficiency was estimated to be 80 %.

A VBG or mirror was placed on the angle polished side of the fiber. The VBG had a bandwidth of 0.4 nm and a diffraction efficiency of 99.9 % at the center wavelength of 1029.5 nm. The surface of the VBG was AR-coated to prevent parasitic reflections. The mirror was a ordinary flat broad band highly reflective mirror with R > 99.9 % for 1020 nm to ~1100 nm.

In the first two experiments, the 4 m fiber was characterized at room temperature and at LNT, using the broad band mirror as cavity delimiter. Approximately 3.5 m of the fiber was coiled to a diameter of 13 cm and placed in a Styrofoam liquid nitrogen container. The output powers as a function of launched pump power at room temperature and LNT are shown in Fig. 2(a). Since a large part of the pump power was not absorbed with this fiber length it adversely affected the overall efficiency. It can however clearly be seen that the relative efficiency is substantially increased at LNT compared to room temperature. The output power increased from 7 W to almost 9 W or roughly 27 %. The center emission wavelength not only shifted to the shorter end of the spectrum but also broadened substantially (2 nm to 10 nm) as can be seen in Fig. 2(b). At LNT, the fiber operated with very little temporal fluctuations while at room temperature it operated in a quasi-CW regime with a repetition rate close to that of the relaxation oscillations.

 

Fig. 2. (a). Launched power vs. output power for the 4 m fiber using a HR-mirror at LNT and room temperature. (b). Emission spectrum from the 4 m fiber with the HR-mirror at room temperature and at LNT.

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To increase pump absorption, the 11.5 m fiber was mounted in the same setup and characterized with the broadband mirror at LNT. A better pump absorption was now observed together with an increase in maximum output power to 11.6 W (Fig. 3(a)). However, as can be seen in Fig. 3(b), the emission linewidth had now increased to almost 20 nm. When the mirror was exchanged for the VBG, the output power did not change noticeably, but the spectral characteristics changed considerably as the linewidth now was reduced to ~0.4 nm. With the 11.5 m fiber at LNT, the temporal characteristics showed very little fluctuations, less than a few per cent regardless of launched pump power.

 

Fig. 3. (a). Launched power vs. output power for the 11.5 m fiber at LNT using a HR-mirror and a VBG. (b) Emission spectrum from the 11.5 m fiber at LNT with a HR-mirror and a VBG.

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The slope efficiencies of the different LNT configurations can at a first glance appear to actually be higher than the theoretical limit. This is however an artifact related to the spectral properties of the pump diode. With increasing current, the pump diodes emission spectrum shifted more than 10 nm and consequently tuned over the ytterbium absorption peak at 976 nm. As the pump power increased, so did the absorption which effectively moved the lasing threshold to a higher level. An examination of the unabsorbed pump power in the mirror configuration showed that this was indeed the case. As it was not possible to measure the unabsorbed pump power in the case of the VBG due to its limited aperture, all output powers are therefore shown with respect to launched pump power.

Temperature dependence of efficiency

The Stark-split energy levels of the Yb-ion in silica are shown in Fig. 4(a). The thermal population of the energy levels in the ²F_7/2 manifold follows the Boltzmann distribution and while at room temperature, the thermal population of the lower laser level can be significant, it quickly drops of as the temperature approaches LNT. This is shown in Fig. 4(b) where the thermal population is plotted as a function of temperature for the three energy levels above ground state in the ²F_7/2 manifold. The thermal population in the lower laser level causes reabsorption of the laser radiation, increasing the necessary population inversion needed to reach threshold. The effect is also related to the absorption and emission-cross-section shown in Fig. 5(a) where we can see how the absorption tail beyond 976 nm at room temperature stretches far up above 1 μm. Even though the effect is weak, there was a slight difference in threshold pump power between the LNT and the room temperature experiments for the 4 m fiber.

 

Fig. 4. (a). Schematic energy level diagram of Yb³⁺ in silica glass [6] (b). Fractional Boltzmann population of the energy levels in the ²F_7/2 manifold for different temperatures.

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In addition to decreasing the population inversion threshold, the temperature decrease narrows and raises the emission cross-section for the laser transition. At the same time a decrease in temperature substantially lowers the absorption cross-section [12]. The effect of this can more easily be seen if we look at the gain cross-section defined as

In the expression above, σ_em and σ_abs refer to the emission and absorption cross-section whereas N₁ and N₂ are the populations in the lower and upper laser level, respectively and N is the total number of participating ions. If we plot the gain cross-section at LNT and room temperature for various inversions, we see that the gain indeed increases close to the transition lines for lower temperatures (Fig. 5(b)). One potential drawback of a narrowing of the absorption peak is that this can reduce the pump absorption if the pump radiation is too broad, but this was not noticed in our experiments.

 

Fig. 5. (a). Emission and absorption cross-section at room temperature from Ref [13]. (b). Gain cross-sections for various inversions at room temperature (black dashed) and at LNT (red) based on data from Ref [6].

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Temperature dependence of temporal and spectral behavior

The broadening of the emission linewidths at LNT with the mirror is attributed to a flattening of the inversion threshold for wavelengths above 1 μm in combination with a decreasing homogeneous broadening. Although Yb-doped glass hosts are subjected to strong homogeneous and inhomogeneous broadening, it can be shown that the material will behave in a quasi-homogeneous manner at room temperature [14]. While the inhomogeneous broadening is related to the disorder of the glass state, the homogeneous broadening is mainly related phonon transitions within the manifold and thus the temperature of the material. When the temperature is decreased to LNT, the homogeneous broadening drops up to two orders in magnitude while the inhomogeneous broadening remains constant [3]. Many more frequencies will therefore be close to reaching threshold in the experiments at LNT which is consistent with our results.

While the main cause of self pulsing in Yb-doped fibers is believed to be initiated by reabsorption at the weakly pumped part of the fiber, it has also been attributed to the interaction between the laser signal and the population inversion [15]. Nevertheless, the weakly pumped part of the fiber can act as a saturable absorber, causing pulse buildup in the cavity. As the pulses build up, they can give rise to nonlinear effects such as stimulated Brillouin scattering or may cause catastrophic break down at the fiber ends. However, there are several ways of circumventing it: One solution to this is to ensure that the gain medium in the fiber is more evenly pumped, by pumping from both directions or using a shorter than optimal fiber length, or a combination of both. Another solution is to increase the length of the fiber cavity to such a degree that the relaxation time is of the same order as the photon life time [16].

However, it turns out that cooling the Yb-doped fiber down to LNT is a simple way of getting rid of the self pulsation. When the gain fiber is cooled down there is a large reduction of the absorption saturation intensity in the part of the fiber acting as a saturable absorber. This then effectively removes the necessary initial conditions for self pulsation. This is consistent with our data as all experiments performed at LNT showed a very stable behavior with small temporal fluctuations.

Conclusions

We have shown that by cooling an Yb-doped fiber laser to cryogenic temperatures, highly efficient, temporally stable lasing can be achieved but at the expense of an increasing linewidth. However, by introducing a VBG as a cavity delimiter we also show that this emission can be harnessed in a small spectral window. With 14.5 W of launched pump power, 11.4 W is emitted with a linewidth of 0.4 nm.