Introduction

Yb-doped fiber lasers are characterized by a high gain and broad emission spectrum due to the broad gain region and the small mode spacing. To achieve narrow band operation, some type of spectral filtering is therefore necessary. Fiber Bragg gratings (FBGs) have historically been used for spectral control in fiber lasers but for high powers or specialty fibers, such as large-mode area fibers and photonic crystal fibers, FBGs can be difficult to implement [1]. In recent years, volume Bragg gratings (VBGs) have attracted a great interest as robust and easily integratable spectral filters. VBGs are fabricated from holographically exposed photo-thermo-refractive (PTR) glass which has a large transparency window (350-2700 nm) [2] and can be tailored to have reflectivities up to 99.9% and bandwidths from 0.5 nm down to tens of pm [3]. Moreover, PTR-glass has a high damage threshold [4] and low thermal variations in the refractive index. VBGs have successfully been integrated as spectral filters in laser diodes [5], solid state lasers [6] and optical parametric oscillators (OPOs) [7]. Previous experiments have shown that VBGs easily can be implemented as wavelength selective reflectors in fiber lasers modules [8,9].

The high gain in Yb-doped fiber lasers make a high outcoupling possible and even preferred. A common cavity configuration is to use a highly reflective mirror on one side of the gain fiber while on the other rely on the 4% Fresnel reflections coming from the fiber end face. In a previous experiment [8], a single VBG was used instead of a mirror to achieve lasing in an Yb-doped fiber, yielding a linewidth of 5 GHz over a broad tuning range. In this work, we extend this concept by implementing two nearly identical VBGs, one on each end of the fiber. By temperature tuning one of the VBGs, we locked the laser on the overlapping peak of one VBG with the side lobes of the other. This opens up for the possibility of a high spectral selectivity with a temperature adjustable output coupling. The results are high efficiency and a reduced linewidth compared to what has previously been achieved using a single VBG. With this simple arrangement, output powers in excess of 7 W was reached with a high slope efficiency (>70%) and a linewidth below 2.5 GHz.

Experiments and results

A schematic of the experimental setup is shown in Fig. 1 . A fiber coupled laser diode emitting at 976 nm was launched through a series of lenses and signal rejection filters into the gain fiber. The pump delivery fiber had a numerical aperture of 0.22 and a diameter of 75 μm and the pump launch efficiency into the gain fiber was estimated to be 85%. The gain fiber was an Yb-doped double-clad fiber from Liekki (YB1200-20/400DC) with a core diameter of 20 µm and a cladding diameter of 400 μm. The corresponding numerical apertures were 0.06 and 0.45, respectively. Although the peak pump absorption was 3 dB/m at 976 nm, the actual pump absorption was somewhat lower due to the broad (> 6 nm) and power dependent output spectrum of the laser diode. A fiber length of 5 m was chosen to ensure that the whole fiber was adequately pumped.

 

Fig. 1 Experimental setup. In the mirror reference setup, VBG2 was exchanged for a HR-mirror and the left fiber end was perpendicularly cleaved.

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To determine where the maximum gain was located for this fiber length, a free-running reference experiment was carried out (see inset in Fig. 1). A broadband highly reflective mirror was placed on the pump side (right) and the left fiber end was perpendicularly cleaved to provide ~4% Fresnel reflection while the intra-cavity fiber end was angle-polished. In the dual-VBG experiment, both fiber-ends were manually angle-polished to 10 degrees to suppress parasitic lasing from the end faces.

The two VBGs, designated VBG1 and VBG2 in Fig. 1, were cut from the same sample to get them close to identical with respect to linewidth and peak reflectivity wavelength. This meant that the peak reflectivity of the VBGs were 99% at 1066 nm with a FWHM bandwidth of 0.22 nm. Both gratings were angle polished and AR-coated for 1 μm radiation to avoid parasitic reflections. The VBGs were placed on temperature controlled Peltier elements to enable temperature tuning and stabilization. Although a slight indirect heating from absorbed and scattered laser emission was unavoidable, care was taken to reduce the heating effects from the pump radiation. An aperture was placed between VBG1 and the collimating lens on the left side to block unabsorbed pump and VBG2 was positioned outside the pump beam path with the help of folding mirrors.

The Bragg wavelength of the VBG has a temperature dependence of roughly 8.8 pm/K [9] and could, with the available thermo-electric controllers (TECs) used, be tuned by 0.66 nm corresponding to a temperature difference of 75 °C. As can be seen in Fig. 2(a) , this is large enough to completely separate the main peaks of the VBGs. When separated, lasing occurs at the wavelength where the losses (and therefore the threshold) are lowest. This will in general appear where the main peak of the one of the gratings overlaps with a side-lobe of the other grating.

 

Fig. 2 (a) Example of spectral overlap between the VBGs. Here the temperature difference of the VBGs is 45 °C, corresponding to 0.4 nm or two-times the bandwidth. (b) Difference in output power at the two fiber ends P1 and P2 as the two VBGs are detuned from each other.

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The output dynamics of the system was characterized for a constant pump power of 9 W by keeping VBG1 at 15 °C while the temperature of VBG2 was increased in increments of 2 °C. In Fig. 2(b), the measured output power at both fiber ends is shown. It can be seen that the output power from VBG1 (P1) increases with increasing temperature up to ~40 °C where the direction of output power became erratic. This corresponds to a detuning of ~0.2 nm which is equivalent to the bandwidth of the VBGs. The increase in power can be understood from that the cavity configuration initially is rather inefficient with a high feedback on both sides. As the VBGs are detuned, the feedback is reduced, which increases the output coupling and thus the output power.

With further temperature increase the output power still increases but the main output switches between the two fiber ends. This continues up to a temperature of 70 °C where the output again stabilizes but at a constant power level for increasing temperature. The emission wavelength was continuously monitored by coupling part of the output from VBG2 (P2) into an optical spectrum analyzer (OSA) with a resolution limit of 0.05 nm. It was found that the laser always emitted on a single wavelength, but that this wavelength changed each time the output power switched between the two fiber ends, indicating that the VBG acting as the HR-mirror changed. It was also verified that lasing indeed took place in the cavity formed by the two VBGs by misaligning each VBG by a small amount while the other remained aligned. As expected, when either VBG was misaligned, the lasing stopped and output power at both fiber ends dropped to zero.

The two VBGs were maximally detuned by heating VBG2 to 75 °C to measure the output power and slope efficiency and the results are presented in Fig. 3 . Plotted in the same image is the free-running reference slope measurement carried out using the highly reflective mirror, as mentioned earlier. It was found that the slope was ~70% with respect to launched power in both cases with a maximum output power of 7.15 W.

 

Fig. 3 Slope measurement for mirror reference configuration and dual VBG configuration with VBG2 heated to 75 °C.

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The spectral output from the fiber laser with the dual-VBG configuration and the reference mirror configuration was measured and the result is shown in Fig. 4(a) . For the free-running laser, the emission linewidth was as expected several nm broad, while it was below the resolution limit of the OSA in the dual-VBG configuration. A scanning Fabry-Perot interferometer was instead used to determine the linewidth for the dual VBG configuration and it was found to be below 2.5 GHz, as seen in the oscilloscope trace of the measurement shown in Fig. 4(b).

 

Fig. 4 (a) Emission spectrum for the two configurations. (b) Fabry Perot measurement of emission from Dual-VBG configuration.

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Discussion and outlook

In our experiments, the maximum relative detuning of the gratings was limited by the temperature controllers used. A further relative tuning would reduce the reflectivity even more, creating a higher outcoupling for the oscillating wavelength. In high power fiber lasers, a certain amount of losses is often unavoidable as bulk components such as polarizer’s, AO-modulators and even the fiber end-faces will affect the oscillating radiation. Since the slope efficiency of a laser scales linearly with the ratio of useful outcoupling to the total round-trip loss, the ability to dynamically control the outcoupling could indeed be useful, especially for fiber lasers operating far above threshold.

As the cavity is formed by two nearly identical gratings, it is not evident which of the VBGs that will act as the output coupler. Throughout these experiments, the forward direction (signal output co-propagating with the pump) was favored for large enough detuning. This was regardless of which VBG that was heated. We attribute this effect to a slight difference in peak reflectivity for the two VBGs, likely coming from an uneven exposure of the original grating they were cut from. The details of this behavior are subject to further study. For future work, by using an apodized VBG as one of the cavity delimiters, this ambiguity could be avoided.

The instabilities seen in Fig. 2(b) between the two output ports for a detuning of ∆λ = 0.2 nm to 0.5 nm is believed to be a result of an asymmetry of the side-lobes of the VBGs. That is, the side-lobes of each VBG (of the same order) will not overlap the main peak of the other VBGs, simultaneously. The VBG acting as output coupler would therefore alternate with increasing temperature. An intrinsic asymmetry exists which is related to the different wavelengths of two side-lobes on opposite side of the peak but this is rather week, usually on the order of ~10⁻³. More likely, this is a result of heat induced deformation of the Bragg structure [10], brought on by the laser irradiation.

A drawback of high output coupling is the risk of a reduced temporal stability due to self-pulsing. This has been attributed to interaction between the signal and the population inversion [11] as well as reabsorption in the more weakly pumped part of the fiber. These instabilities can be avoided by making sure the whole fiber is adequately pumped, by cooling the fiber [12] or by increasing the cavity length [13]. It has also been shown that given the right conditions, the instabilities are reduced with increasing pump power [14]. In these experiments, it was found that the temporal stability was poor for low pump powers but became quasi-continuous as the pump power increased.

Conclusions

A narrow linewidth (<2.5 GHz), spectrally locked, highly efficient Yb-doped fiber laser with an output power exceeding 7 W was built using two approximately identical highly reflective VBGs as cavity delimiters. The VBGs were detuned with respect to each other by heating and the output characteristics, spectrum and efficiency were evaluated. It was found that the output was switching erratically between the output ports when the one VBG was changed in temperature relative to the other by 20 - 50°C, corresponding to a detuning of the reflection peaks by 0.2 nm to 0.5 nm. For temperatures above this range, the output was emitted in the forward direction, i.e. co-propagating with the pump. This shows that the fiber laser could successfully be locked on the main peak of one VBG together with a side-lobe of the other. This simple arrangement also makes it possible to dynamically change the reflectivity of the outcoupler.