1. Introduction

Widely tunable lasers are key devices in, e.g., optical communication, gas sensing, lidar and nonlinear optical applications [1–8]. Rare-earth doped fiber lasers are well known for their high beam quality, efficiencies and compactness. Common rare-earth laser ions used for active optical fibers, such as Erbium (Er), Ytterbium (Yb), and Thulium (Tm), provide wideband gain and emission in silica glass, enabling continuous tuning over a large range of wavelengths. A common tuning technique utilizes a fiber-integrated Fabry Perot (FP) etalon in which the tuning can be controlled by an applied voltage. The technique is fast and reliable, and tuning in excess of 100 nm near 1.55 µm, and 200 nm near 2 µm, have been reported [9,10]. Introducing a wavelength dependent loss using a birefringent crystal, i.e. using a Lyot filter, is another widely employed technique providing tuning in the range of 40 nm [11,12]. Using fiber Bragg gratings (FBGs) as narrow-band feedback mirrors, monolithic lasers can easily be realized. The center wavelength of the FBGs can easily be tailored to any wavelength region overlapping the emission band of the rare-earth ion, providing a compact all-fiber based solution. Such lasers can be mechanically tuned by, e.g., stretching or compressing the FBG [13]. With this method more than 110 nm of wavelength tuning has been reported [14]. In a recent study a method to embed FBGs into thermoplastic bars was demonstrated by fused deposition modeling (FDM) 3D printing [15]. Using mechanical tuning the center Bragg wavelength could be tuned over 60 nm with maintained bandwidth and reflectivity.

In this paper we demonstrate a simple and inexpensive method to achieve stable and wide wavelength tunability of a narrow linewidth Er:Yb fiber ring laser (EYFL). Output wavelength was controlled through mechanical tuning of an FBG, which was embedded in a polyethylene terephthalate glycol (PETG) beam using a commercially available FDM 3D printer. The laser had a linewidth of less than 0.07 nm, limited by the resolution of the optical spectrum analyzer (OSA), and could be continuously tuned from 1543 nm to 1574 nm with a power variation below 1 dB. The tuning range can be further tailored by modifying the tunable FBG design.

2. Fabrication of the tunable FBG filter

A key component of the laser is the tunable FBG filter, which should provide both a narrowband reflection and a wide tuning range. Gratings with a center wavelength near 1.55 µm were inscribed in photosensitive single-mode fibers (GF3, Thorlabs) using a two-beam interferometer and a UV laser operating at a wavelength of 213 nm (Xiton Photonics, IMPRESS 213). Prior to inscription, the coating was removed at the location of the grating. We then embedded these fibers in cuboid shaped beams that were 3D printed using an FDM printer (FlashForge Dreamer). The beam dimensions (l×h×w) were 100 mm × 10 mm × 8 mm, with the FBG placed centrally along the beam at a distance d = 3 mm from the middle plane, as shown in Fig. 1(a). The FBG was embedded into the 3D printed beam by pausing the layer-by-layer printing at a predetermined height of 8 mm (0.5h + d). Once paused the FBG was inserted and clamped into place, after which printing was resumed to complete the print.

 

Fig. 1. (a) Schematic of the tunable FBG beam, and (b) the 3-point bending layout.

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In order to find a suitable material for the printed beam, different filament materials were evaluated, including acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), PETG and nylon. PETG was chosen due to a combination of higher stiffness, structural strength and low hysteresis when being bent. Once printing was complete the as-printed beam was thermally annealed at 110°C for 5 min to reduce non-uniform residual stresses induced by the 3D printing process. Mechanical tuning was achieved using a 3-point bending setup, schematically shown in Fig. 1(b), where the dashed line indicated the middle plane of the beam. More details of the fabrication and tuning of these FBG filters can be found in Ref. [15]. The cost of the materials and the processes involved in making this filter is low compared to many commercially available tunable filters.

To evaluate the impact of FBG length on the spectral properties during tuning, gratings with lengths of l = 3 mm and l = 10 mm were initially tested. Figure 2(a) shows the spectral reflectivity and the bandwidth of the two gratings prior to embedding into the 3D printed beam, while Fig. 2(b) and (c) show the spectral change during mechanical tuning of the 3 mm and 10 mm grating, respectively. It is clear that the 3 mm long FBG can be tuned over almost 34 nm without any significant change in reflectivity or bandwidth, while the 10 mm long FBG gets a strongly distorted spectral response already at a tuning 3 nm away from the central peak. As the fiber is not re-coated before being embedded into the 3D print, small voids were formed at the transition from coated to bare fiber due to the thickness of the coating and relatively low viscosity of the thermoplastic. Combined with a weaker bonding of the thermoplastic to the bare fiber, compared to a coated fiber, these voids can cause inhomogeneous stress and strain distribution across the uncoated fiber section. The longer FBG, with the grating structure being located closer to these voids, will therefore be more distorted [15]. Due to the better performance, the 3 mm long embedded FBG was used as the end mirror in the construction of the tunable fiber laser. This FBG had a central wavelength of 1554.3 nm designed to match the emission spectrum of Er, a reflectivity of R = 85.3%, and a full width half maximum (FWHM) linewidth of Δλ_FWHM ∼ 0.45 nm. Within the tuning range from 1543 nm to 1582 nm, the linewidth remained below 0.5 nm, while the reflectivity was reduced approximately 5% at the extreme points, i.e. at maximum compression and maximum tension. When further stretched beyond 1584 nm, the reflectivity dropped about 10% and multiple side lobes appeared in the spectrum, as seen in Fig. 2(b).

 

Fig. 2. (a) Reflection spectra of the 10-mm and 3-mm FBG; (b) Tuning of the 3-mm FBG; (c) and the 10-mm FBG.

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3. Experimental design and setup

A schematic of the laser is shown in Fig. 3. A ring laser design was chosen to avoid spatial hole burning [16,17], but more importantly to be able to use the grating as a single cavity mirror, enabling smooth tuning while bending the FBG beam. The gain medium was a 10-meter-long Er:Yb double-clad fiber (SM-EYDF-6/125-HE, Nufern), which was pumped by a fiber coupled 976 nm laser diode (BMU10A-975-02-R, II-VI Laser Enterprise). The pump beam was coupled to the laser cavity using a fiber coupler and an integrated isolator (ISO) prevented feedback to the pump diode and potential damage. Emission from the Er:Yb fiber was coupled into the FBG through an optical circulator and the reflected light was then returned to the cavity though the circulator to ensure unidirectional light propagation within the cavity. A fiber coupler (FC) was used as the output port for the laser.

 

Fig. 3. Configuration of the widely tunable Er: Yb fiber ring laser.

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Wavelength tuning was achieved by bending the 3D printed beam as schematically shown in Fig. 3. To optimize the laser output several FCs with different coupling efficiencies were evaluated. A 3-dB FC, i.e., with 50% of the power coupled out, was found to give best performance and stable output power. An OSA (HP86140A, resolution bandwidth 0.07 nm) and a power meter (PM, Gentec-EO XLPF12-3S-H2-D0) were used to monitor the output power and the wavelength spectrum during operation, as schematically shown in Fig. 3. In most cases, for ease of operation, the wavelength spectrum was recorded by monitoring the transmitted light through the back end of the FBG, as shown in Fig. 3.

4. Experimental results

A tuning range of 30 nm, from 1543 nm to 1573 nm, with maintained narrow linewidth was achieved both at low and high pump power, as is shown in Fig. 4. In order to protect the FC and the circulator from optical damage, the maximum pump power used was limited to 2.28 W, which resulted in a maximum output power of 245 mW with the lasing wavelength set to 1565 nm. An output power variation of less than 1 dB was measured over the full tuning range, as shown in Fig. 4. To verify the spectral properties of the laser output, the OSA was connected to the FC ports of the laser. Five overlapping spectra measured for different wavelength settings are shown in Fig. 5(a), having a signal-to-background value exceeding 50 dB.

 

Fig. 4. Power variation at different pump powers.

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Fig. 5. (a) Output spectrum from the FC end and (b) from the FBG end.

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A clear advantage of the configuration used is that amplified spontaneous emission (ASE), which appeared in the gain fiber was coupled out through the FBG, as shown in Fig. 5(b). The FBG filter has hence two important functions. Besides acting as a wavelength tunable feedback signal for the laser, the FBG also acts as a drop filter removing unwanted ASE from being further amplified within in the ring cavity. The ASE filtering results in clean spectral output with high signal-to-background performance.

Within the wavelength range from 1543 nm to 1573 nm the laser could be continuously tuned, emitting at a single wavelength displaying narrow spectrum and maintained spectral shape. The tuning range was limited mainly by two factors: the spectral gain characteristics and the tuning range of the FBG filter. When the FBG was tuned towards the edges of the Er-emission region, where the gain was lower, more energy was lost to ASE. With a tuning range of the current FBG filter limited from 1543 nm to 1582 nm, distortion of the spectral properties of the grating at the extremes further reduced the gain and gave an increased ASE limiting the tuning range of the laser.

The 3-dB linewidth of the laser was below the resolution of the OSA (0.07 nm). The optical circulator ensured the unidirectional propagation, thereby avoiding standing wave induced spatial hole burning and cross-gain effects [16,17] in the gain fiber, which is normally observed in linear cavities. As a result, the peak wavelength of the laser was very stable. When measuring the laser spectrum (at 1558 nm) in 10-minute intervals over a 50-minute period, wavelength variations were within the repeatability of the OSA (± 0.003 nm). The recorded spectra are shown in Fig. 6.

 

Fig. 6. Spectral stability measured at 10-minute intervals during 50 mins from the FBG end.

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5. Conclusion

In this work we demonstrate a simple and cost-effective technique to construct a tunable, low noise, narrow linewidth (Δλ < 0.07 nm) Er:Yb fiber laser using a ring configuration with a FBG based tunable filter embedded in a 3D printed beam. By bending the beam the laser wavelength could be continuously tuned from 1543 nm to 1573 nm, with a variation of output power below 1 dB. The ring cavity configuration gave a stable laser operation, in both output power and laser wavelength, with excellent signal-to-background values (> 50 dB). The laser design, in combination with the tunable FBG filter, provides a simple and versatile platform for tunable narrow-linewidth lasers that can be extended to other types of active fibers, such as Yb-doped fiber laser operating in the 1 µm wavelength region, or Tm-doped fiber lasers operating near 2 µm.