Hybrid Raman-erbium random fiber laser with a half open cavity assisted by artificially controlled backscattering fiber reflectors

A hybrid Raman-erbium random fiber laser with a half-open cavity assisted by chirped artificially controlled backscattering fiber reflectors is presented. A combination of a 2.4 km-long dispersion compensating fiber with two highly erbium-doped fiber pieces of 5 m length were used as gain media. A single random laser emission line centered at 1553.8 nm with an optical signal to noise ratio of 47 dB were obtained when pumped at 37.5 dBm. A full width at half maximum of 1 nm and a 100% confidence level output power instability as low as 0.08 dB were measured. The utilization of the new laser cavity as a temperature and strain sensor is also experimentally studied.


Working principle: simulation and inscription process
The reflector based on chirped ACBFRs were manufactured by using a commercial femtosecond fiber laser chirped pulse amplifier (FLCPA) with a 1030 nm operating wavelength and 370 fs pulse duration. These pulses were tightly focused with a NA = 0.42 Mitutoyo objective lens onto the fiber to induce refractive index variations in the core of around 10 -3 . The fiber is located on a nano-resolution air-bearing XYZ stage from Aerotech for precise displacement. The pattern of the required defects along the fiber was designed using the software Fimmwave from Photon Design 23 , a vectorial-mode solver for 2D + Z waveguide structures that can model propagation in 2D and 3D structures. The reflectors had to meet two specifications: (1) they should be wide enough to allow this laser structure to have a modeless behavior, and (2) they should be centered at around 1550 nm matching the gain zone. A sequence of defects in the fiber 3 µm wide and 1 µm long separated an increasing distance from 1.08 to 1.13 µm best fitted these requirements. Due to technical limitations the ACBFR were 2.5 mm long.
A pulse energy of 4.5 µJ (before the slit) and a 100 Hz pulse repetition rate (PRR) were used for the drilling. The chirped reflector presents a period between consecutive pulses that evolves from min = 2.1423 µm to max = 2.1533 µm, which generates the 4th harmonic in the C-band. It must be noted that the slit beam shaping technique has been used, through which it is possible to modify the focal volume, and thus achieve greater control over the dimensions of each refractive index change 24 . This aspect can be seen in the inscribed optical structure shown in Fig. 1c. Figure 1a shows the schematic of the designed reflector and Fig. 1b depicts its calculated reflected output spectrum. Figure 1c presents a microscope image of the manufactured ACBFR and Fig. 1d illustrates its spectrum measured with an optical spectrum analyzer (OSA). A maximum power reflection value of − 1.37 dB centered at around 1548.4 nm and a FWHM of 7.45 nm were measured. The two chirped ACBFRs used in this work showed very similar spectra. Figure 1c shows a microscope image of one of these manufactured ACBFRs.

Experimental setup
The schematic diagram of the experimental setup of the half-open linear cavity RFL assisted by a chirped ACFBR is shown in Fig. 2. A 1445/1550 nm wavelength division multiplexer (WDM) injects the Raman pump power centered at 1445 nm into the RFL. At one of its ends, a 2.4 km length spool of DCF, acting as a distributed reflector and amplifying the randomly backscattered light was located. As aforementioned, the DCF provides additional Rayleigh scattering feedback for the RFL 2 , inducing a significant reduction of the typical gain medium length 25 . At the opposite end of the RFL, a 3 dB optical coupler divides the optical signal reflected by the DCF into two www.nature.com/scientificreports/ different branches, using a single gain medium per wavelength 26 . Each one of these branches includes 5 m of highly erbium-doped fiber (EDF) followed by a fabricated chirped artificially controlled backscattering fiber used as wavelength selected mirror. The used EDF was the M12 (980/125) from Fibercore Inc., with a theoretical peak core absorption range from 16 to 20 dB/m at 1531 nm. The reflected signals from these two branches pass through a variable optical attenuator (VOA) in order to minimize the risk of damage of the measurement devices due to high power levels. Simultaneous measurements in the optical and electrical domain were carried out by dividing this signal with another 3 dB optical coupler. One output branch was connected to an optical spectrum analyzer (OSA) with a resolution of 0.1 nm and a sensitivity of − 75 dB.
In order to experimentally verify the modeless behavior of the RFL, the other output branch was connected to a photodetector in combination with an electrical spectrum analyzer (ESA) to perform measurements in the electrical frequency domain. The RFL output, reflected from the chirped ACBFRs, were mixed with the signal of a tunable laser source (TLS, Agilent 8164B), though a 3 dB coupler to perform a heterodyne detection.
Free ends of chirped ACBFRs and DCF were 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.

Results and discussion
Spectral characterization. To evaluate the output spectra of the laser, the half-open cavity RFL was pumped with powers ranging from 32.5 up to 37.5 dBm. Figure 3a, b show the measured optical and electrical spectra, respectively, when pumped at 32.5 dBm. Both cases illustrate that modeless behavior was not obtained for such a low pump power level. On the other hand, when Raman pump power is increased up to 37.5 dBm, random laser behavior is experimentally ratified as Fig. 3c, d illustrate.
In all cases, the backscattered light forms a multitude of resonant modes with random frequencies 17 . Nevertheless, the reflection from the chirped ABCFs was dominant in the half-open cavity and only the resonant modes of these reflectors reached their threshold (when the combination of the EDFA and Raman gain overcame the cavity loss).
The spectrum shape presented in Fig. 3a shows a great number of wavelength emission lines from 1552 up to 1570 nm with output power levels of around -30 dBm. However, when pump power was increased to 37.5 dBm, a random laser emission line centered at 1553.8 nm was measured, as it can be in Fig. 3c. The output power level obtained from this RFL was − 6.5 dBm with a full width at half maximum (FHWM) of 1 nm, and an optical signal to noise ratio (OSNR) of 47 dB. Figure 3b, d illustrate the frequency spectra corresponding to the frequency domain conversion, when a photodetector in combination with an ESA was used to perform the measurements. There is no indication of longitudinal mode beating in Fig. 3d, when pumped at 37.5 dBm. Nevertheless, Fig. 3b clearly shows the appearance of longitudinal mode beating, which varies as a function of the pump power level. These results support the modeless theory of random fiber lasers reported in 27 .
In order to characterize the dependency of the output power level, FWHM and modeless laser behavior with the pump power a sweep from 32.5 up to 37.5 dBm in 0.5 dB steps was carried out. Figure 4a displays the experimental results of the measured output power level as a function of the inserted pump power. The central wavelength emission of the RFL was maintained around 1553.8 nm along the measurement range. However, this is not the case for the FHWM since it widens for higher pump power levels. Figure 4b shows the output power and the FHWM as a function of pump power level. It can be clearly seen that the obtained RFL does not reach a modeless laser behavior until the pump power does not reach 35 dBm, which corresponds to a FWHM value of 0.76 nm.
Output power stability. The output power level and wavelength stability of a fiber laser are key parameters to be assessed, since these output power variations have high influence on the precision of sensor systems. High  www.nature.com/scientificreports/ intensity stability guarantees high resolution operation of the intensity sensor 28 . Figure 5 shows the output power instability of the half-open RFL as a function of the inserted pump power. The measured data was stored each 10 s for one hour considering a confidence level (CL) of 100%. It was observed that the peak power averaged at room temperature presented a variation as low as 0.08 dB when a pump power level of 37.5 dBm was inserted. These results present a remarkable improvement compared to previous and recent works 29 . As Fig. 5 shows, an exponential relationship between pump power and output power instability was experimentally evaluated, with an error of R 2 = 0.9967. Therefore, as the output spectrum becomes wider, that is, the FWHM increases, the output power stability increases considerably.
To quantify the improvement when using chirped ACFBRs, the measurements were repeated using two fiber Bragg gratings (FBGs) centered at 1551 nm as reflectors. Figure 6a shows the measured output power stability when pumped at 37.5 dBm by using FBGs as reflectors to be compared to the results provided by the chirped ACBFRs, presented in Fig. 6b. In both cases, measured data was stored each 10 s over one hour and a confidence level (CL) of 100% was considered. Even though the optical spectrum shape obtained with the FBGs does not present lateral lobes, its stability is notably worse. Moreover, both measured emission lines, centered at 1551 nm (Fig. 6a) and 1553.8 nm (Fig. 6b) deal with the same gain shape. The output power level presented a variation of 0.53 dB when using FBGs at room temperature, but only 0.08 dB when using ACFBRs. Therefore, the use of ACFBRs implies a remarkable increase of the output power level stability under identical circumstances: around six time higher.
Sensing response. The utilization of this half-open RFL in sensing applications was also analyzed. We tested the chirped ACBFRs as temperature and strain sensor heads. The response of the reflectors to the tem-  www.nature.com/scientificreports/ perature was checked by heating them in a climatic chamber from 25 to 150 °C. As it can be seen in Fig. 7a, the center wavelength shift for the laser presents a clear linear behavior (the mean square error is 0.9966, very close to 1) and a temperature sensitivity of 9.6 pm/°C. Next, these sensor heads based on chirped ACBFs, were placed in a high precision single-axis motorized stage (MS) in order to evaluate the wavelength shift produced by their mechanical stain. The sensor head characterization consisted of 28 steps of about 790 µɛ. As Fig. 7b illustrates, the strain response of these reflectors, when axial strain from 0 to 2100 µε was applied, also presented a linear behavior with a strain sensitivity of 0.8 pm/ µε and an error of R 2 = 0.9896.
Typically, FBG-based sensing systems offer a temperature sensitivity of 11 pm/°C and a strain sensitivity of 1.2 pm/µε 30,31 . These measurement sensitivities have been enough to establish this kind of sensors as a flagship in sensing along two decades or more. Here, both temperature and strain sensitivity measured values are not as good as the FBG-based ones. However, the measured values together with the achieved stability both in terms of output power and wavelength allows this RFL system assisted by chirped ACBFRs to be used as a suitable option for a number of sensing applications.

Conclusions
In this work, a new hybrid Raman-erbium random fiber laser with a half-open cavity, assisted by chirped artificially controlled backscattering fiber reflectors has been presented. A combination of a 2.4 km-long dispersion compensating fiber with two highly erbium-doped fiber segments of 5 m were used as a gain media. By using these chirped ACBFRs, a single random laser emission line centered at 1553.8 nm and an optical signal to noise ratio of 47 dB have been measured when pumped at 37.5 dBm. A full width at half maximum of 1 nm and an output power instability as low as 0.08 dB, with a confidence level of 100% have been measured. Its sensor response as a function of temperature and strain variations has been also experimentally demonstrated.