1. Introduction

Fiber lasers are one of the fastest developing fields of optics and photonics today. Their continuous wave output power has been scaled above 1kW and they are frequently used in commercial cutting and welding applications [1]. In attempts to scale power, beam combination is becoming a popular method which requires wavelength stable and powerful lasers [2, 3]. High power fiber lasers can also be potentially useful sources for nonlinear frequency conversion, but most of them suffer from an unpolarized emission and a too wide spectrum. This can to some extent be overcome by using diffraction gratings in an external cavity configuration, but it leads to substantial losses and has hence not been explored to a large extent [4]. To obtain narrow line width operation, a fiber grating can also be fused to the rare-earth doped fiber. However, it is usually difficult to splice these to large area fibers or structured gain fibers, e.g. large mode area (LMA) fibers with photonic bandgap structures and it is difficult to write gratings directly into rare-earth doped fibers. Especially microstructured optical fibers (MOF) are troublesome since only splicing in the low power regime is possible to ensure that the guiding structure does not collapse. Furthermore, when the microstructure actively guides the light, mode mismatch between the MOF and standard telecom fibers is hard to avoid [5, 6].

Volume Bragg gratings (VBGs) are made with a holographic exposure process of a photothermal glass, followed by a thermal treatment. The spectral response of the VBG, i.e. the centre wavelength, reflectivity and linewidth, can be tailored efficiently [7]. Such gratings were originally used to frequency lock diode lasers [8], but have recently also been used as out-put coupler for several types of solid-state lasers [9, 10] and optical parametric oscillators [11]. With an Er-Yb glass laser a linewidth narrower than 90 kHz was demonstrated [10] and high power single-mode lasing was obtain for a Nd:GdVO₄ micro laser [12]. The fact that the VBG can withstand high powers and at the same time have an efficient filter function with a sub-nm bandwidth opens new opportunities in many areas of laser research.

In this work we examine the possibility of using a VBG to narrow the linewidth of a neodymium doped microstructured LMA fibre. The purpose is to show that by combining fiber gain media with volume Bragg gratings, high power narrow linewidth fiber lasers can be constructed. By using this technique, problems associated with the use of fiber Bragg gratings together with LMA PCF fibers can thus be circumvented. With the open design, other optical elements can be incorporated such as electrooptic modulators and nonlinear crystals. The results are compared to an identical fibre laser with an conventional external dielectric mirror and it is found that the linewidth is reduced at least 100 times while the output power is essentially unchanged.

2. Experimental setup and results

The silica fibre in the experiments consisted of a neodymium doped (1300ppm) core surrounded by five air holes with a size of 18µm [13]. The pitch of the air holes was 19µm, which also was the size of the core. With a numerical aperture (N.A.) of 0.156 and VPCF number of 17.8, the fiber theoretically supported many modes since VPCF is well above the single mode cut-off value π. In practice however, the fiber operated in a single transverse mode. This is attributed to substantial losses for the higher order modes [13].

The VBG had an aperture of 5×2 mm and a length of 5 mm. The end facets were ARcoated for 1060 nm and also tilted 2 degrees with respect to the grating direction in order to avoid unwanted parasitic reflections. The reflectivity of the grating was 99% at 1066 nm with a FWHM bandwidth of 0.22 nm.

Several experimental setups were explored to evaluate how VBG’s compare to dielectric mirrors when used as external reflectors in fiber laser cavities. In the setup reported on here, the length of the fiber was 124 cm and it was pumped by a Ti:Sapphire laser tuned to 808nm. The fiber length was chosen to yield a pump absorption of 90%-95%. To optimize coupling efficiency and reduce losses, care was taken to match incoupling/outcoupling optics with the N.A. of the fiber. Using predominantly high N.A. lenses, this yielded an estimated pump launch efficiency of 85%. This was based on the launch efficiency of a structurally identical undoped fiber. The pump laser was linearly polarized in the same direction throughout all the experiments. The input end of the fiber was uncoated and served as one of the laser mirrors. On the other end either a VBG or a conventional plane highly reflective (R=99%) laser mirror was used as a cavity resonator. The fiber end on the opposite side was neither AR coated nor tilted, and it is hence a coupled cavity but the surface reflection was so small compared to the VBG or the mirror that its impact can be neglected. The output signal was coupled out on the input end through a dichroic mirror. The setup is depicted in Fig. 1.

 

Fig. 1. Schematic view of the experimental setup. The lenses L1and L2 constitute a telescope to mode match the Ti:Sapphire laser pump to the fiber. L3 and L4 are used to collimate the beam out of the fiber. A dichroic mirror was inserted between L2 and L3 to couple out the signal.

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The two laser set-ups performed very similarly with respect to threshold, slope efficiency and power. The threshold using the VBG was 103 mW with a slope of 51%, which yielded a maximum output power of 166 mW at a pump power of 450 mW. With the conventional dielectric mirror the threshold was 101 mW and the slope efficiency 57%, which gave a maximum output power of 188 mW, see Fig. 2. In both cases the output was unpolarized.

 

Fig. 2. The input-output characteristic of the fiber laser using mirror or VBG as the back reflector. Both the mirror and VBG had a reflectivity of R=99%.

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The spectrum obtained using the dielectric mirror was as expected quite wide (>7nm), with several longitudinal modes lasing simultaneously as can be seen in Fig. 3 (dotted line). The spectrum also fluctuated with time, most likely due to mode competition in an uncontrolled environment. The VBG-based setup on the other hand had a single line emission, also plotted in Fig. 3 (solid line). The FWHM linewidth was determined to be less than 0.07nm, limited by the resolution of the optical spectrum analyzer. Both spectrums shown in Fig. 3 were recorded at maximum pump power and it is interesting to note that despite this, the gain at 1066nm is not enough to permit lasing in the mirror based setup. The fluorescence response from the fiber was almost 50% lower at 1066nm.

The slightly different slope efficiencies is therefore attributed to that the laser with the conventional mirror operated at the maximum of the gain peak, while the VBG laser is operating far to the side of the peak where the gain was lower. In this respect, it is a bit surprising that the VBG configuration can be almost as efficient as the mirror-based laser.

 

Fig. 3. Comparison between the spectral output of the VBG and mirror setup. The dotted black curve shows the spectrum of the mirror based setup. The width of the spectrum was >7nm and fluctuating. The solid red curve shows the spectral output of the VGB based setup. The spectrum is centered around 1066nm with a linewidth of <0.07nm.

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By using the knife edge technique, an M² measurement was carried out to determine the transverse mode quality of the VBG-based laser. The beam waist was measured on several points along the axis of propagation and the data points were fitted to the beam propagation function for Gaussian beams with the beam radius and M² value as fitting parameters. It was determined that the beam had an M² value of less than 1.6. Hence, the laser operated close to a single transverse mode.

3. Discussion

Although glass is essentially an inhomogeneously broadened host medium, spectral hole burning is not a dominant effect in neodymium doped silica fiber lasers [4]. As can be seen in Fig. 2, the efficiency and corresponding maximum power of the VBG setup were only marginally lower than in the mirror setup which shows that the VBG does not add any substantial losses. This is in correspondence with what has been observed for other lasers using VBG mirrors [12]. If we introduce a figure of merit (FOM) power/linewidth [W/nm] to show how the two setups compare relative to their linewidth, we get a value of 26mW/nm and 2.37W/nm for the mirror and VBG setup, respectively. Thus, with a FOM value nearly 100 times higher, the VBG setup shows a large improvement when linewidth is an important application factor. With the narrow linewidth obtained using the VBG, the stability of the laser also improved compared with the mirror setup were the longitudinal mode stability was poor.

The VBGs, due to their distributed design, have a high damage threshold which, in turn, warrants power scalability for this type of laser solution. They were recently tested in beam combination experiments beyond 500 W of CW radiation at 1064 nm [2]. By stabilizing several fiber lasers with VBGs it should then be possible to do incoherent beam combination with them, for example using an additional VBG.

The open design with a collimated beam and an airgap between the collimating lens and the VBG would allow a Pockel cell or an acousto-optic modulator to be inserted easily in the laser to provide mode-locked or Q-switched operation.

It would further be very interesting to use the VBG as an incoupling device since it has nearly zero reflectivity for wavelengths outside of its design area. It would preserve the flexibility normally expected from fiber lasers and open up for highly efficient pumping schemes were the pump and lasing wavelengths are very close, thereby minimizing thermal effects and improving the overall efficiency. Furthermore using a VBG-retroreflector [9], it should be possible to get a narrow linewidth tunable high power fiber laser. All of this is subject to further research.

4. Conclusions

Efficient, narrow linewidth (<0.07 nm) oscillation was obtained with a Nd:fiber at 1066nm using a volume Bragg grating as one of the cavity mirrors. A high slope efficiency of 51%, substantially narrowed linewidth (>100 times lower) and increased spectral stability compared to using a conventional dielectric mirror were obtained with only marginal loss in output power. This concept opens up for several interesting applications. For example, intra-cavity second harmonic generation would be possible, providing a polarization maintaining fiber is used. Also densely separated wavelength lasers can be combined to improve beam brightness [14]. Furthermore, in applications where the splicing of fibers can be a problem, e.g. microstructured fibers, polarization maintaining fibers and rare earth doped fibers, this solution can be an attractive alternative.