Optimized Dual-Stokes Raman Laser for 1.1 µm Emission and Temperature Sensing

Abstract

This work experimentally validates an improved Raman fiber laser, developed through cascade core variation splicing of optical fibers and the integration of fiber Bragg gratings. A continuous-wave Yb-doped fiber laser was used as the pump source, delivering up to 10 W at 1064 nm. The first Stokes emission generates laser output centered at 1119 nm, while the second Stokes emission produces a lasing mode at 1179 nm. Fiber Bragg gratings control these emissions. Furthermore, both Stokes laser emissions can be tuned by applying temperature changes, achieving a 15.07 pm/°C sensitivity. The proposed laser presents a compact and practical solution for remote temperature sensing applications using fiber lasers.

1. Introduction

Raman fiber lasers (RFLs) have attracted considerable attention due to their high spectral conversion efficiency. This property enables effective wavelength shifting through stimulated Raman scattering when a source is pumped at 1 μm sources, such as Yb-doped fiber lasers, is used [1].

One practical application that receives particular attention in fiber-optic Raman systems is long-distance distributed temperature monitoring [2,3]. In this context, temperature monitoring can be performed along an 85 km-long fiber with an accuracy of 8 °C by increasing the probe light power, thereby enhancing the intensity of the anti-Stokes Raman signal [2]. Currently, several research groups have made significant efforts to improve fiber-optic Raman systems by proposing algorithms that increase the SNR without compromising signal information [4], optimizing the frequency for Raman scattering power measurements [5], and post-processing the Raman signal to improve temperature precision [6].

Moreover, incorporating fiber Bragg gratings (FBGs) in the RFL architecture significantly enhances spectral selectivity, ensures stable operation, and minimizes noise [7]. These gratings have been widely validated under diverse operational conditions, offering reliable performance [8]. RFLs typically exhibit near-diffraction-limited beam quality, making them highly suitable for precision-demanding applications. The integration of FBGs as wavelength-selective reflectors not only improves the spectral performance but also facilitates compact, cavity-free designs. These lasers have demonstrated versatility in diverse applications, including materials processing, medical procedures (e.g., laser surgery), Raman spectroscopy, and advanced optical communication systems [9,10,11].

A FBG is a type of reflecting mirror inscribed in an optical fiber, which selectively reflects light of a certain wavelength and transmits all other wavelengths of a light beam. The wavelength of reflected light is called the Bragg wavelength and satisfies the equation λ_B = 2n_eff Λ [12], where λ_B is the Bragg wavelength, n_eff is the effective refractive index, and Λ is the period of the Bragg grating. This wavelength-selective reflection allows FBGs to detect changes in temperature, strain, and other parameters by monitoring changes in the reflected wavelength [13]. In the same vein, it has been shown that the FBG sensitivity to temperature is given by the equation sens = (λ_B) ⁄ΔT, where ∆λ_B is the Bragg wavelength difference induced by the temperature increase or decrease ΔT [14]. These selective reflection characteristics make FBGs ideal for the construction of rare-earth ion-doped fiber lasers [15], and RFLs in which the Raman gain medium is formed by special fibers [16] or optical fibers used in Telecommunications [17]. When a light beam propagates inside a silica optical fiber, it generates spontaneous Raman scattering that is radiated randomly, and this is feedback randomly distributed by Rayleigh scattering in co-propagation with the pumping power of the light beam inside the core. The photons with wavelengths corresponding to the peaks of the Raman gain spectrum start to stimulate the other photons with the same wavelengths and phases to be amplified, i.e., stimulated Raman scattering (SRS) is initiated. This SRS is also called Stokes. These first Stokes (1st Stokes) can be transformed into pump power to generate the next Stokes randomly, and so on. These lasers are known as distributed random fiber Raman lasers [18,19,20].

On the other hand, RFLs use Bragg gratings to form the resonant cavities, which are designed to operate at the wavelength corresponding to the maximum peak of the Raman gain spectrum. In this way, the energy of the reflected photon co-propagates with the pump power and stimulates the spontaneous Raman scattering photons to amplify and form the laser or Stokes signal corresponding to the Bragg wavelength of the FBG [21]. The integration of SRS and FBGs in a distributed feedback configuration which allows precise control of laser emission while enabling temperature sensing by monitoring the Stokes wavelength shifts [11]. Moreover, inserting fiber segments with differing core diameters creates localized refractive index transitions that enhance modal interference and nonlinear interactions, thus optimizing laser performance and sensor resolution [12]. These principles form the theoretical basis for the dual-Stokes laser emission and temperature-sensing behavior observed in the present study.

In this work, we designed an experimental setup in which the Raman gain medium (consisting of 4 Km Corning MetroCor fiber + 2 cm 980-HP fiber + 10 m Corning MetroCor fiber) is forced to emit at 1119 nm and 1177 nm by the insertion of FBGs through a WDM, turning the system into a random distributed feedback fiber Raman laser [22]. This experimental configuration allows the FBG-1177 nm to be subjected to temperature variations in the range of 40 to 160 °C and to record the shift in the Bragg wavelength, which, until our knowledge, had not been reported in an analysis of the FBG sensitivity around 1177 nm. With this experimental setup two FBGs operating at 1119 and 1177 nm were subjected to temperature variations in the range of 40 to 160 °C, simultaneously producing shifts at those Bragg wavelengths for which, to our knowledge, there is no reported sensitivity at those wavelengths

2. Experimental Setup and Dual Fiber Laser Emission

The experimental configuration is illustrated in Figure 1. A continuous wave Yb-doped fiber laser (model YLR-10-1064-LP from IPG Photonics) is employed as the pump source, delivering up to 10 W at 1064 nm with a linewidth of 0.5 nm in TEM00 mode. This pump is injected into Arm 1 of a wavelength division multiplexer (WDM), which has a maximum power handling capacity of 5 W at 1064 nm. From the WDM, approximately 42% of the pump power is transmitted through Arm 2, ~15% through Arm 3, and ~4.6% through Arm 4, with ~38.4% attributed to insertion losses.

Arm 2 is fusion spliced (S3) to 4 Km Corning MetroCor fiber, then to 2 cm Nufern 980-HP fiber (S4), and finally spliced to 10 m fiber Corning MetroCor (S5). The Nufern 980-HP optical fiber has an effective area of ~18 μm², while the Corning MetroCor optical fiber has an effective area of ~40 μm², introducing core discontinuities that enhance nonlinear interactions. Arm 4 is connected to two FBGs: FBG 1119 nm via splice S2 and FBG 1177 nm via splice S6, with a separation of 1 m between the FBGs. All splice losses are ~0.01 dB. When 4.3 W of pump power is coupled into Arm 2, the SRS is initiated in the 4 Km Corning MetroCor optical fiber, generating Stokes emissions near 1121 nm and 1178 nm, as previously reported [23]. Meanwhile, ~4.6% propagation of pump power through Arm 4 allowed the FBGs to reflect light at their Bragg wavelengths. A Fresnel reflection (~4%) at the end of FBG-1177 nm, due to the air–glass interface, provided additional feedback, allowing distributed feedback Raman lasing by coupling back into the gain medium, stimulating the population of glass molecules excited at the virtual level to emit photons with the same characteristics as the Bragg wavelengths, thus achieving SRS or Stokes.

The laser output is analyzed with an optical spectrum analyzer (OSA, Yokogawa model AQ6370C), and a power meter (PM, model FieldBest from 10 mW to 50 W) monitored the total output. To protect the OSA from high-power exposure, a wedge attenuator is used. Minor fluctuations in power were attributed to pointing instabilities or coating imperfections and were accounted for in the analysis.

The dual laser emission generated by the Raman fiber laser is shown in Figure 2A. Here, it can be appreciated that the emissions are centered at the Bragg wavelengths. Moreover, the laser spectra were recorded every five minutes for an hour, and the stability of the output power and wavelength of the Stokes signals can be observed in Figure 2B,C; this study is carried out at room temperature. These results show that the output power variations are ~0.02 W for the 1st Stokes and ~0.01 W for the 2nd Stokes. Concerning the wavelength at room temperature, the results for the 1st and 2nd Stokes show minimal variations; see Figure 2B,C.

To validate and discuss the improvement of the laser characteristics, comparing the fiber laser response without the FBG contribution is essential. Figure 3 shows two spectral curves for the 2nd Stokes: the 4 Km MetroCor curve (in black) corresponds to the experimental configuration of Figure 1 without FBGs and without core discontinuity, exhibiting the 2nd Stokes centered at the 1179.44 nm wavelength with a FWHM of 1.7053 nm; this is obtained randomly in free running configuration [24,25]. The maximum intensity at 1179.44 and near 1186 nm correspond, respectively, to the frequency shift peaks at 13.2 THz and 15 THz of the Raman gain spectrum for silica molecules when pumped at 1 μm [21], and the 4 Km MetroCor + 2 cm 980-HP + 10 m MetroCor curve (in red) corresponds to the experimental configuration of Figure 1; it exhibits a laser emission profile centered at 1177.1314 nm that corresponds to the FBG reflection profile, this shows a FWHM of 0.1584 nm. Both curves were acquired at room temperature and are normalized for comparative analysis. When the FBG is exposed to temperature changes, the Bragg wavelength shifts tend to take advantage of the stimulated Raman scattering produced by the 4 Km MetroCor fiber. During this shift, the Bragg wavelength occasionally coincides with lines of competition generated by stimulated Raman scattering, causing fluctuations in the second Stokes output power.

3. Temperature Sensing Application

To investigate the thermal response of the Raman fiber laser system (4.3 W), the FBG-1177 nm is gradually heated from 40 °C to 160 °C in 10 °C steps using a home-made Temperature Control System (TCS). The system comprised a TR44 Class C refractory furnace, equipped with a REX-C700 controller and resistive heaters capable of reaching 700 °C. For each temperature increment, the laser spectrum and output power were recorded. Figure 4A displays the output spectra across different temperatures. The residual pump at 1064 nm is negligible, indicating efficient conversion into the 1st Stokes at 1119 nm, which subsequently acted as a pump for the 2nd Stokes emission centered at 1177.13 nm. As the temperature increased, two main effects were observed: (1) the central wavelength of the 2nd Stokes shifted to longer wavelengths (redshift), and (2) moderate variations occurred in its output power (Figure 4B). The 1st Stokes wavelength remained stable.

Figure 5 shows the Bragg wavelength shift for the FBG-1177 nm as temperature increased from 26 °C to 160 °C and back. A shift of approximately 2.02 nm is measured, corresponding to a thermal sensitivity of 15.07 pm/°C. A hysteresis of ~0.24 nm is observed when the temperature decreases. It is important to recall that the wavelength shift generated by the FBG is related to the thermal expansion coefficient (0.55 × 10⁻⁶/°C) and the thermo-optic coefficient (8.6 × 10⁻⁶/°C) of silica optical fiber [8]. In addition, these fibers can withstand significant thermal variations without damage [13].

The power distribution among the residual pump, 1st Stokes, and 2nd Stokes is analyzed (Figure 6). A power exchange behavior is observed: as the 2nd Stokes power dropped at 120 °C, the 1st Stokes power increased, suggesting energy redistribution between orders. For example, at 120 °C, the 2nd Stokes transfers power to the 1st Stokes. Across all of the spectral measurements taken during this study, the output power of the laser system consistently maintained a value of 0.53 W. The detailed breakdown of this output power, illustrating its distribution among the unabsorbed pump light (residual pump), the first-order Stokes emission, and the second-order Stokes emission, is clearly depicted in Figure 6. This figure provides a visual representation of how the total output power is partitioned across these different spectral components.

From 40 °C to 110 °C, the 1st Stokes remained relatively constant, followed by a peak at 120 °C, a decrease at 130 °C, and another rise at 160 °C. The 2nd Stokes exhibited a sharp drop at 120 °C, recovery at 130 °C, stability until 150 °C, and a final drop at 160 °C.

Following our analysis of the impact of heating solely the FBG-1178 nm, we now move on to the next phase of our investigation, which involves simultaneously heating both FBGs. Here, the two FBGs were located over the refractory furnace; then, a fiber was arranged and bent to set both FBGs. This includes the FBG-1119 nm and, once more, the FBG-1178 nm, across a temperature range spanning from 35 °C to 160 °C. Figure 7A presents the resulting output spectra obtained when both FBGs were subjected to this controlled heating. As the temperature increased, the shifts in the respective Bragg wavelengths became clearly distinguishable. Specifically, for the FBG-1119 nm, we observed a shift in its central wavelength from an initial value of 1119.17 nm to a final value of 1120.93 nm. This resulted in a total wavelength difference of 1.76 nm which corresponds to a sensitivity of 14.08 pm/°C. This sensitivity value is further illustrated in Figure 7B. Notably, in Figure 7C, the FBG-1178 nm demonstrates a temperature sensitivity of 15.07 pm/°C, which is consistent with the sensitivity previously observed and detailed in Figure 4.

In this sense, our experimental results show that changes in the displacement of the 2nd Stokes profile can be detected when the FBG is exposed to temperature or simultaneously in the 1st and 2nd Stokes when both FBGs are heated. The shape of the first Stokes profile at 1119 nm is affected by the bending indicated when both fibers are located over the refractory furnace. This bending adds to the Bragg wavelength phase, resulting in a reflection band with small ripples.

The discontinuity of the core of the Raman gain medium optimizes the output power as shown in Figure 8, this exhibits two output power curves for the second Stokes with and without Nufern 980-HP fiber segment, these curves were obtained at 4.3 W, a safety pump power limit to protect the WDM itself, because our WDM is limited to 5 W of pump power. Note that the output power curve without the Nufern 980-HP fiber segment fluctuates and even disappears in the 100–120 °C range. However, if we apply power above 4.3 W, the second Stokes curve is fully visible. By inserting the short segment of Nufern 980-HP fiber, the difference in effective area allows for superposition of the fundamental modes of pump power at 1064 nm and the first and second Stokes power at 1119 and 1177 nm [23], respectively.

4. Discussion

Raman fiber lasers (RFLs) are exciting because they enable flexible wavelength switching by appropriately choosing the Raman gain medium, pump source, and reflective cavity elements. Then, several configurations have been proposed and demonstrated. For instance, specific wavelength bands that are challenging to access with traditional rare-earth-doped fibers can be achieved by seeding a tunable random Raman fiber laser and high-power pigtailed pump diodes [26]. Other prior alternatives employ a high-power Ytterbium fiber laser as a seed element to excite SRS [27]. The above-mentioned works show that the reflective devices involved play an important role and can be adjusted according to the RFL demands. However, choosing a suitable high-power pump source makes it possible to propose RFL with minimal elements involved, which can be more suitable for practical applications [28]. Recently, some groups have explored different all-fiber optic filters to control RFL output. Here, Yuxi Ma et al., introduce a few-mode all-fiber filter to achieve multiwavelength generation in different NIR bands from 1.1 µm to 1.5 µm [29]. Meanwhile, it was demonstrated that by a suitable design of long-period fiber grating, it is possible to control the emission in a 5 kW Raman fiber laser [30].

In this work, we propose to include the minimal elements required in Raman fiber lasers and employ a high-power pump source at 1064 nm. Using 4 km of metro-core fiber as a Raman gain medium and incorporating a short segment of 980-HP, it is possible to obtain a Raman laser emission centered at 1177.13 nm. Moreover, incorporating two FBGs makes it possible to obtain dual-laser wavelength emissions that are challenging to achieve using Ytterbium-doped fiber laser systems. Furthermore, by using these reflective devices (FBGs), it is possible to explore the sensing capabilities of the RFL, which makes it possible to be a competitive alternative and explore further sensing applications for strain [31], volatile organic compounds detection [32], and high-sensitive temperature monitoring [33]. In these applications, and considering the laser line, it is possible to achieve high-resolution fiber laser sensors. In addition, the proposed technique in this work is a competitive temperature sensor alternative in terms of sensitivity. The capabilities of the proposed RFL in this work are compared in

Table 1. Comparative review of the latest developments in temperature sensors based on Raman scattering mechanisms.

Sensing Head	Sensitivity (pm/°C)/Temperature Range (°C)	Wavelength Operation (µm)	Ref/Year
FBG	13.38/22–500	1.5	[34]/2005
FBG	9.7/25–70	1.5	[35]/2016
All-Fiber Interferometer	10/30–80	1.5	[36]/2024
LPFG	8.61/35–60	1.5	[37]/2024
FBG	11.9/20–150	1.5	[38]/2024
FBG	15.07/35–160	1.1	This Work

.

5. Conclusions

This work experimentally demonstrated a compact Raman fiber laser configuration designed for high-sensitivity temperature sensing. By integrating cascaded fiber segments with different core diameters and employing precisely aligned fiber Bragg gratings (FBGs), a dual-Stokes lasing system was achieved, enabling narrowband emission at 1119 nm and 1177 nm.

The most significant contribution of this study is the demonstration of a reliable method to thermally tune the second-order Stokes emission via controlled heating of the FBG-1177, achieving a thermal sensitivity of 15.07 pm/°C. This result highlights the capability of this setup to function as a distributed temperature sensor with high spectral accuracy.

Additionally, the incorporation of a 2 cm Nufern 980-HP segment proved critical in enhancing spectral selectivity and emission stability. The deliberate core mismatch enabled mode-field interactions that narrowed the second Stokes linewidth by an order of magnitude.

The work also revealed power exchange dynamics between Stokes orders under varying thermal conditions, providing valuable insight into intermodal energy redistribution, which is often overlooked in simplified Raman laser models.