Wavelength-switchable L-band ﬁ ber laser assisted by random re ﬂ ectors

. A wavelength-switchable L-band erbium-doped ﬁ ber laser (EDFL) assisted by an arti ﬁ cially controlled backscattering (ACB) ﬁ ber re ﬂ ector is here presented. This random re ﬂ ector was inscribed by femtosecond (fs) laser direct writing on the axial axis of a multimode ﬁ ber with 50 l m core and 125 l m clad-ding with a length of 17 mm. This microstructure was placed inside a surgical syringe to be positioned in the center of a high-precision rotation mount to accurately control its angle of rotation. Only by rotating this mount, three different output spectra were obtained: a single wavelength lasing centered at 1574.75 nm, a dual wavelength lasing centered at 1574.75 nm and 1575.75 nm, and a single wavelength lasing centered at 1575.5 nm. All of them showed an optical signal-to-noise ratio (OSNR) of around 60 dB when pumped at 300 mW.


Introduction
Femtosecond (fs) laser writing has become an effective way to process any type of transparent optical material, such as silica optical fibers [1]. This technology enables the microfabrication of numerous fiber structures with excellent properties for a wide range of practical applications [2]. In particular, the development of fiber-optic microstructures based on refractive index (RI) modification under fs-laser irradiation has resulted in different implementations in the field of optical fiber sensors: surrounding refractive index sensors [3], strain sensors [4], curvature sensors [5], or multiparameter sensors [6], among others. These artificially controlled backscattering (ACB) fiber reflectors have been shown to have similar temperature sensitivity to traditional FBGs. However, the strain sensitivity can be improved by more than an order of magnitude when comparing these quasi-randomly distributed reflective microstructures with FBG-based sensors [7].
In addition to the above, these fiber-optic microstructures also have interesting applications in areas such as fiber-optic lasers [8], fiber Bragg gratings (FBGs) [9], long period fiber gratings (LPGs) [10], tilted fiber Bragg gratings [11], interferometers [12][13][14], couplers [15] or birefringence adjustable elements [1,16]. In further, changes in the birefringence of a fiber can be caused by its twist [17]. Investigation of twist induced birefringence has been a topic of investigation back to the late 1970s, early 1980s [18][19][20] and continued up to recent years [21,22]. One of the most important parameters in these studies is the evolution of the polarization dependent loss (PDL) response of the fiberbased reflector with respect to the applied twist [23]. As presented in [23], the PDL response of a fiber grating structure has higher twist sensitivity than that of the reflected or transmitted amplitude spectrum. Moreover, PDL shows two distinct lobes whose changes with the increase/decrease of twist angle are significant (both in counter clock and clockwise twist), providing meaningful information for the twist effects.
In this work, an artificially controlled backscattering (ACB) fiber reflector inscribed by fs laser direct-write technique is used into an L-band erbium doped fiber laser (EDFL). This random reflector was inscribed on the axial axis of a 50/125 multimode fiber (MMF), with a length of 17 mm, and located into a high-precision rotation mount to control its angle of twist rotation. Only by rotating in a range of 8°the MMF sample where the RFG was inscribed, this wavelength-switchable EDFL can be switched among three different lasing spectra: a single wavelength lasing centered at 1574.75 nm, a dual wavelength lasing centered at 1574.75 nm and 1575.75 nm, or a single wavelength lasing centered at 1575.5 nm. An OSNR of 60 dB and 58 dB were measured for single and dual-wavelength operation respectively, when pumped at 300 mW.

Fabrication and characterization process
An ACB multimode fiber reflector was inscribed by using ultrafast laser writing, performed by a Cazadero fiber laser (Calmar Laser) that delivers 370 fs laser pulses at a central wavelength of 1030 nm. It is worth noting that, as presented in [24], the light absorbed non-linearly by the fiber has a wavelength of 515 nm, due to the second harmonic generation (SHG) introduced in the setup. The laser pulses were tightly focused into the Ø50 lm core of the MMF using a 0.42 NA, 50Â objective lens from Mitutoyo. A pulse energy of 0.75 lJ and a pulse repetition rate (PRR) of 150 Hz were used, and the MMF sample was translated through the laser focus using a motorized nanoresolution XYZ stage from Aerotech. This ACB MMF reflector had a random period between K min = 1.61085 lm and K max = 1.64230 lm, with a length of 17 mm. It was also written on the axial axis of the MMF. Although this is a random fiber grating (MMF-RFG) [25], the almost total periodicity of the optical structure gives rise to Bragg resonances which, in the third order (m = 3) and with an effective refractive index (n eff ) of around 1.4528, present reflections in the following spectral band: Before placing this MMF-RFG inside the surgical syringe to be positioned in the center of a high-precision rotation mount, its backscattered optical power was characterized with an optical frequency domain reflectometer (OFDR). This ultra-high spatial resolution optical backscattered reflectometer (OBR 4600, from LUNA) is commonly used not only for optical sensing of strain and temperature variations but to test fiber optic components, optical fibers and optical fiber sensors [26]. Free termination of this fiber-based reflector was immersed into indexmatching oil to avoid undesired reflections. Figure 1a presents the backscattered optical power as a function of fiber length for the inscribed MMF-RFG. As this figure shows, the MMF sample was located about 1.88 m from the connector of the OBR and it showed an amplitude between 40 and 50 dB above the noise level. Figure 1b illustrates the reflection spectrum of the MMF-RFG reflector. This spectrum shows a non-flat response, with a maximum value of around À10 dB at around 1575.5 nm, so it is expected that the emission wavelengths of the generated lasers will be mainly determined by the shape of this spectrum. Figure 2 illustrates a schematic diagram of the L-band linear-cavity fiber laser experimental setup. As this figure shows, the 976-nm pump power (Fig. 2a) was injected into the linear-cavity EDFL by means of a 980/1550 nm wavelength division multiplexer (WDM) (Fig. 2b). The gain medium was 5 m of highly erbium-doped fiber (EDF) I25 (980/125, Fibercore Inc.) (Fig. 2c), suitable for C-band amplifiers with a core composition optimized for EDF amplifiers (EDFAs) in dense-WDM (DWDM) networks and a peak core absorption ranges from 7.7 to 9.4 dB/m at 1531 nm [27]. This EDF was connected to the common port of the WDM and followed by a 3-ports optical circulator (Fig. 2d) in which ports 3 and 1 were connected to conform a fiber loop mirror (FLM), as in [27]. After passing through the highly EDF section again, the reflected signal from the FLM arrived to the 1550 nm-port of the WDM up to the optical coupler (Fig. 2e). Then, the signal is divided into two branches where 90% of the signal reached the MMF-RFG-based reflector and the other 10% was visualized with an optical spectrum analyzer (OSA) (Fig. 2f) with a resolution of 30 pm and a sensitivity of À75 dBm. The section of fiber where the MMF-RFG was inscribed was placed inside a surgical syringe so that it could be positioned in the center of the high-precision rotation mount (Fig. 2g) to accurately control its angle of rotation. A photograph of the RFG reflector inscribed by femtosecond laser writing is shown in Figure 2h. As in previous studies, free end of the ACB fiber-based reflector was immersed in refractive index-matching gel to avoid undesired reflections. All the experimental measurements were carried out at room temperature, and no vibration isolation or temperature compensation techniques were employed.  Figure 3b presents the same relation between pump power levels versus output power but when simultaneous dual-wavelength emission centered at 1574.75 nm (blue line) and centered at 1575.75 nm (red line) was reached. As Figure 3b illustrates, the power level of both lasers increases evenly as pumping power increases. Moreover, a pump power threshold of around 50 mW with and an optical efficiency of 0.26% were measured when the EDFL was tuned to obtain a single-longitudinal laser emission. This pump power threshold value increased 5 mW when dual-wavelength emission is obtained, while maintaining the same total optical efficiency than the previous case, as shown in (Fig. 3b). Figure 4 shows the output spectra of the linear cavity EDF pumped at 300 mW when (a) single-wavelength laser emission centered at 1574.75 nm, (b) dual-wavelength laser emission centered at 1574.75 nm and 1575.53 nm, or (c) single-wavelength laser emission line at 1575.53 nm were obtained. By rotating the high-precision rotation mount where the multimode fiber-based reflector was located into a syringe, these three configurations can be easily reached. Output power levels of À1.02 dBm (Fig. 4a) and À1.85 dBm (Fig. 4c) were measured when single-wavelength laser emission was attained, both presenting an OSNR of 60 dB. On the other hand, when dual-wavelength laser emission was achieved, an output power level around À5 dBm and an OSNR of 58 dB was measured (Fig. 4b). As expected, by increasing the number of lasing wavelengths the output power values of the lasing emission lines decrease [28]. Figure 4d illustrates the frequency spectrum corresponding to the frequency domain conversion, when a photodetector in combination with an ESA was used to evaluate the longitudinal laser mode behavior. As in [29], heterodyne detection was carried out by using a 3-dB optical coupler for mixing the signal from a tunable laser source   (TLS) whose full width at half-maximum (FWHM) linewidth was 100 kHz, with the reflected signal from the MMF-RFG-based reflector. This measured frequency spectrum clearly shows the appearance of multiple longitudinal mode beating, demonstrating its multimode operation.

Results and discussion
Such multimode lasers are usually more unstable that single mode ones in their output power levels. However, in this case, an output power level variation of 0.28 dB, with a confidence level (CL) of 95% was measured at room temperature. Measured data was stored each 10 s for 1 h, when single wavelength lasing pumped at 300 mW with a FWHM of 28.05 pm was obtained. The central emission wavelength of this laser showed a variation of 23 pm during the same period but with a CL of 100%.

Conclusions
In this work, a new wavelength-switchable L-band erbiumdoped fiber laser is proposed and experimentally characterized. The laser is assisted by an artificially controlled backscattering fiber reflector, inscribed by femtosecond laser writing. This reflector is a random fiber grating that was written on the axial axis of a 50/125 multimode fiber with a length of 17 mm. This quasi-distributed fiber reflector was placed inside a surgical syringe to be positioned in the center of a high-precision rotation mount to precisely control its angle of rotation. Only by rotating this mount, three different output spectra were obtained: a single wavelength lasing centered at 1574.75 nm, a dual wavelength lasing centered at 1574.75 nm and 1575.75 nm, and a single wavelength lasing centered at 1575.5 nm. All of them showed an optical signal-to-noise ratio of 60 dB when pumped at 300 mW.