Investigating the Impact of SMF on Brillouin Fiber Laser Performance

Using erbium-doped fiber, a steady Q-switched erbium-doped laser (EDFL) was successfully demonstrated. With a 150 m of Single Mode Fiber to generate the Brillouin Fiber Laser and increase the non-linearity inside the cavity. In the absence of a Brillouin pump, a compact Brillouin fiber laser (BFL) is realized by combining an Erbium Doped Fiber (EDF) and a Single Mode Fiber (SMF), which serve as both the Brillouin and linear gain media. It is accomplished by self-Brillouin production in a laser cavity from 4.5 m of EDF and 150 m of SMF with strong nonlinearity. The SMF inside the ring cavity was used to generate self-generating BFL. The BFL in the linear cavity produced more Brillouin stokes at a lower pump threshold. In the experiments, the average of spacing is 0.088 nm was generated at the EDF pump power of 93.468 to 259.493 mW for the 150 m of SMF. This compact BFL has total cavity length (160.34 m). Eventually, this is the most recent gain medium length recorded for Brillouin erbium fiber laser production without the use of a Brillouin pump.


Introduction
A Brillouin fiber laser is a type of laser that utilizes the phenomenon of Brillouin scattering to generate coherent light.It operates by exploiting the interaction between a high-power pump laser and an optical fiber that contains a gain medium capable of exhibiting stimulated Brillouin scattering [1].Brillouin scattering is a nonlinear optical effect that occurs when light propagates through a material and interacts with its acoustic vibrations [2].It leads to the scattering of photons by acoustic phonons, resulting in the generation of a new optical wave with a slightly shifted frequency called the Brillouin scattered wave [3].In a Brillouin fiber laser, a pump laser with a high-power level is coupled into an optical fiber [4].The pump laser creates a strong optical intensity within the fiber, which induces acoustic vibrations due to the electrostriction effect [5].These acoustic vibrations interact with the pump light, leading to Brillouin scattering [6].This scattering process generates a backward-propagating Brillouin scattered wave with a frequency shift equal to the Brillouin frequency shift, which is dependent on the fiber's properties [7].The backward-propagating Brillouin scattered wave can be reflected back into the fiber by a fiber Bragg grating or other means [8].As it propagates in the opposite direction of the pump laser, it experiences further amplification through the process of stimulated Brillouin scattering [9].This feedback mechanism sustains the laser oscillation and allows the generation of coherent light at the Brillouin frequency [10].Brillouin fiber lasers have unique properties, including narrow linewidth and low intensity noise, making them useful in various applications such as fiber optic sensing, optical communications, and microwave photonics [11].
To the best of our knowledge, add a single-mode fiber (SMF) to a Brillouin fiber laser, it can have several effects on the laser's performance: Mode Conditioning: The SMF can act as a mode conditioning element for the laser.It helps to shape the output beam profile by modifying the transverse mode structure of the laser beam.This can be beneficial in applications where a specific beam profile or mode quality is required [11].Increased Fiber Length: By adding an SMF, the overall length of the laser cavity is increased [12].This can have an impact on the laser's performance, such as altering the threshold pump power required for lasing or affecting the laser's output power and efficiency.The longer fiber length may also introduce additional losses and dispersion, which can influence the laser's spectral characteristics [13].Enhanced Brillouin Gain: The addition of an SMF can increase the Brillouin gain in the laser cavity [14].The Brillouin gain is dependent on the fiber's properties, such as its length, composition, and effective area [15].By introducing an SMF with specific properties, such as a longer length or higher Brillouin gain coefficient, the Brillouin scattering process can be enhanced, leading to higher output power and improved laser performance [16].Increased Stability: The addition of an SMF can help stabilize the laser's operation [17].It can provide a feedback mechanism to suppress unwanted cavity fluctuations and improve the laser's output stability [18].The SMF can act as a distributed feedback element, reducing mode hopping and enhancing the laser's coherence properties [19].Dispersion Effects: The SMF may introduce dispersion effects into the laser cavity.Dispersion refers to the dependence of the refractive index on the light's wavelength, and it can influence the laser's pulse duration, chirp, or spectral width.Depending on the specific characteristics of the SMF, such as its dispersion profile and length, these effects can be tailored to meet specific application requirements.It is important to note that the specific impact of adding an SMF to a Brillouin fiber laser will depend on various factors, including the properties of the SMF, the design of the laser cavity, and the intended application.Careful consideration and optimization are required to achieve the desired performance enhancements or modifications [20].In conclusion, the Brillouin fiber laser is a type of laser that utilizes the phenomenon of Brillouin scattering to generate coherent light.It operates by exploiting the interaction between a high-power pump laser and an optical fiber containing a gain medium capable of exhibiting stimulated Brillouin scattering.Brillouin fiber lasers have unique properties such as narrow linewidth and low intensity noise, making them useful in various applications [20].When adding a single-mode fiber (SMF) to a Brillouin fiber laser, several effects can occur.The SMF can act as a mode conditioning element, shaping the output beam profile.It can increase the overall fiber length, which may impact the laser's performance, threshold pump power, output power, and efficiency.The SMF can enhance the Brillouin gain, leading to higher output power and improved laser performance.It can also contribute to stability by providing feedback and suppressing unwanted cavity fluctuations.Additionally, the SMF may introduce dispersion effects, which can influence the laser's pulse duration, chirp, or spectral width [21].However, the specific impact of adding an SMF to a Brillouin fiber laser depends on factors like the SMF's properties, laser cavity design, and intended application [22].Careful consideration and optimization are necessary to achieve the desired performance enhancements or modifications [23].Experimental setup (Methodology) Figure 1.illustrates the SBS laser oscillator The nonlinear gain is achieved by combining 4.5 meters of erbium-doped fiber (EDF) with 150 meters of single-mode fiber (SMF).The Erbium-doped fiber (EDF) has a linear gain that amplifies the Brillouin signal and lowers the threshold for SBS generation.While increase the non-linearity for the cavity by using the SMF.When the gain exceeds the loss in the ring resonator, the Stokes wave is formed and oscillated to produce a Brillouin fiber laser (BFL) in the reverse direction.
The gain medium (EDF) I-25 (980-125) cut off wavelength (nm) is (900-970) and numerical aperture is (0.23-0.26).Using a WDM Coupler, it was backward pumped by a 980 nm Laser Diode (LD:LC96A74P-20R).It measures 4.5 meters in length and boasts a high concentration of erbium ions.In order to combine the pump from a 980 nm laser diode with the Brillouin pump (BP) signal at 1550 nm, a wavelength division multiplexer, also known as WDM, is utilized.Within the ring resonator, the EDF is utilized to provide assistance for the creation of SBS.In order to tap into the output of the laser, a 20 dB output coupler is utilized.An optical spectrum analyzer (OSA) with a resolution of 0.015 nm is used to characterize the output laser's spectral characteristics, and a digital oscilloscope is used to describe the output laser's temporal characteristics.
Inside the ring cavity, an optical isolator is utilized to prevent the BP from oscillating in the resonator.Because the cavity layout used allows for the counter-propagation of the pump and Stokes fields, Kerr four-wave mixing and cascaded SBS may be avoided.When a highly concentrated EDF is utilized to generate BEFL, a total cavity loss can be reduced, which in turn results in an increase in output power.Additionally, the EDF possesses a significant nonlinearity, which enables the generation of steady pulses at around the Brillouin threshold pump power.

Results and Discussion
At a pump power of 94 mW, the threshold of Brillouin Fiber Laser (BFL) operating at a wavelength of 1565.916nm is seen.With the power of the pump increasing from 10 mW to 236 mW, a passive BFL is detected.
The spectral and temporal performances of the Brillouin Fiber laser are depicted in After incorporating the SMF into the cavity, the peak laser wavelength was blue-shifted from 1565.904 nm to 1556.64 nm by 0.08 nm and red-shifted from 1565.9.409 nm by 0.08 nm.A spectral bandwidth of 0.08 nm at -3.15 dB is available from the Qswitched laser.Since the loss increment in the cavity causes the blue-and red-shifted wavelengths, the lasing will emit shorter wavelengths in order to increase amplification gain and offset the additional losses.

Conclusion
In this work, we were able to show that a Brillouin fiber laser can be made in SMF by using EDF that has been mechanically exfoliated as the gain medium.It was made in cavity that was set up with a 150 m of SMF.The BFL can be made with the cavity setup.With the addition of EDF, cavity setup of 150 m of SMF can make BFL work.In this setup, the BFL pulsed better because it had a higher pulse repetition rate and a narrower pulse width.The SMF contained within the ring cavity was utilized to produce self-generating BFL.At a lower pump threshold, the BFL in the linear cavity produced more Brillouin stokes.the trials, the average nm for the 150 m of SMF at an EDF pump power of 93.468 to 259.493 mW.The entire cavity length of this tiny BFL is 160.34 m.Finally, this is the most recent gain medium length documented for the manufacture of Brillouin erbium fiber lasers without the usage of a Brillouin pump.

Figure 1 .
Figure 1.Schematic of the proposed BEFL

Figure 2 .
(a) Depicts the laser output wavelength of the cavity operation produced with 150 lengths of (SMF) at a maximum pump power of 236 mW as shown in Figure 2. (b).

Figure 2 .
(c) depicts the oscilloscope trace of Q-switched pulses at 236 mW pump power.The oscilloscope trace demonstrates that the Q-switching process is extremely consistent across the time span.The temporal profile of two neighbouring pulses is depicted in Figure 2. (d), where the pulse width and pulse interval between the Q-switched pulses are observed to be 28.6 s and 45.72 s, respectively.The pulse interval of 19.7 microseconds corresponds to a repetition rate of 19.7 kilohertz.In addition, the RF spectrum was analyzed to evaluate the pulsing's stability, as shown in Figure2.(e).At a fundamental frequency of 19.7 kHz, the RF spectrum of the Q-switched pulse demonstrates exceptional stability, with a signal-to-noise ratio (SNR) of around 64.82 dB.