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Abstract
We present a nonlinear amplifying loop mirror-based mode-locked fiber laser. By adjusting the pump power, the proposed laser exhibits a dissipative soliton resonance (DSR)-like pulse operation with a maximum pulse width of 150 ns. Subsequently, a three-stage Tm3+-doped fiber amplifier is implemented using a single-mode double-cladding Tm3+-doped fiber to increase the DSR-like pulse output power to 52.5 W, achieving a pump slope efficiency of 47.1% in the main amplifier. A 25 m first-order Raman-gain fiber (UHNA7) is pumped by a DSR-like pulse, and 16.3 W of pure 2.135 µm first-order Raman light with a spectral purity of 73.4% is obtained. Finally, 5.4 W of 2.35 µm second-order Raman light with a spectral purity of 66% is obtained using a 10 m 98% germania-core fiber as a second-order Raman-gain fiber cascaded after UHNA7 fiber. To the best of our knowledge, this is the highest output power ever obtained from a 2.3 µm laser.
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1. Introduction
In recent years, research on 2 µm lasers has been rapidly advanced, and the results have been widely applied to many fields such as biomedicine, polymer welding, laser radar, and atmospheric remote sensing [1–4]. In addition, 2 µm fiber lasers have attracted significant interest because of their small size and compactness, good heat dissipation, and especially favorable nonlinear fiber characteristics that contribute to pulse generation. Due to the unique energy level structure of trivalent thulium ions (Tm3+), a kilowatt-order output power has been obtained from a 2 µm laser by pumping a Tm3+-doped gain medium (3F4→3H6) using a 79× nm laser diode (LD) [5–7]. The emission range of a Tm3+-doped fiber (TDF) made of silica is 1.5–2.2 µm; however, most Tm3+-doped fiber lasers (TDFLs) operate in the 1.9–2.05 µm range. This is because the 2.05–2.2 µm band deviates from the Tm3+ spectral emission peak, and the phonon energy of a silica fiber is high (1100 cm−1), leading to the weak pump efficiency of TDFs and difficulty in achieving an operating wavelength >2.05 µm [8,9]. A high output power in a laser operating in the 2.05–2.1 µm range can be achieved by either employing a TDFL-pumped holmium-doped fiber (HDF) or a 79× nm LD-pumped tunable thulium holmium co-doped fiber (THDF). The emission spectrum range of HDFs or THDFs is 1.95–2.10 µm, which can fill the emission gap of 2.05 µm. However, the emission range of HDFs or THDFs is rarely 2.1–2.2 µm [10–12].
Research on lasers operating in wavelengths 2.1–2.5 µm is currently at a relatively stagnant stage due to the lack of suitable rare-earth-doped fibers and host materials. Notably, 2.3 µm lasers are promising in various applications. For example, gas molecules, such as hydrogen fluoride, carbon monoxide, formaldehyde, and methane exhibit distinctive absorption lines in the 2.3 µm band; thus, a 2.3 µm laser light source can be applied to gas composition detection [13,14]. Furthermore, this light source is very useful in noninvasive blood glucose measurements because glucose exhibits strong absorption and relatively low water absorption at 2.3 µm [15]. Research on 2.3 µm fiber lasers is not only of high practical value but also of great significance for expanding the mid-infrared wavelength bands of 2.1–2.5 µm.
2.3 µm lasers can be implemented by employing a 79× nm LD to pump directly Tm3+-doped low-phonon energy mid-infrared glass fibers or bulk crystals (3H4→3H5) [16]. For example, Yu et al. reported a 2 µm and 2.3 µm laser employing a 794 nm LD to pump a Tm: YVO4 crystal, achieving a 1.15 W 2.3 µm continuous wave (CW) laser with a slope efficiency of 7.9% in 2022 [17]. In 2023, Li et al. reported a 2.3 µm fiber amplifier employing a 2.3 µm LD as the seed source and a 793 nm LD to pump a Tm: ZBLAN (ZrF4–BaF2–LaF3–AlF3–NaF) fiber, achieving a 1.41 W CW laser output power [18]. This is because the phonon energy of the ZBLAN fiber is only 550 cm−1, achieving a very low transmission loss in the mid-infrared band; this is important for implementing high-power lasers operating in the 2.3 µm band. A tellurite glass fiber exhibits similar properties. In 2020, Denker et al. reported a 2.3 µm Tm3+: TeO2 fiber laser employing a 794 nm LD as a pump source, achieving a 200 mW output power [19].
Another way to achieve 2.3 µm wavelength expansion in fiber lasers is to employ nonlinear frequency shifting methods, such as those used in Raman fiber lasers, Raman soliton lasers, and Raman amplifiers. Raman scattering (RS) occurs when light at a fundamental frequency is scattered by molecules to produce a first- or higher-order Stokes shift, leading to wavelength expansion in the 2.1–2.5 µm range. Among the frequency shifting methods, the Raman-induced soliton self-frequency shift (SSFS) produces effective wavelength expansion; it has been reported that SSFS achieves a tunable wavelength operation in the no-gain region of rare-earth-doped fibers [20–23]. However, the Raman soliton energy is always limited by the area theorem, making it very difficult to achieve a high output power in 2.3 µm lasers. In 2011, Fortin et al. employed a ZBLAN fiber as a Raman fiber for the first time to produce 0.58 W, 2.185 µm first-order Stokes-shift light [24]. In 2012, Fortin et al. employed a fluoride glass fiber as a Raman fiber to implement a 3.66 W, 2.23 µm CW laser [25]. However, mid-infrared soft glass fibers exhibit a low damage threshold and low stability. However, laser operation usually requires a safe and stable external environment, which limits the use of such fibers in harsh environments. Highly Ge-doped fibers (HGDFs) made of silica have been widely employed in 2.1–2.4 µm fiber lasers because of their high damage threshold, low transmission loss, and high nonlinearity. HGDFs can be fusion bonded directly to standard single-mode fibers (SMFs) with low fusion loss, which favors the implementation of an all-fiberized structure [26–34]. In 2014, Liu et al. proposed a 14.3 W, 2.147 µm Raman fiber amplifier employing an HGDF as the Raman medium [28]. In 2015, Jiang et al. proposed a 0.3 W, 2.4 µm Raman pulsed laser employing an HGDF to excite a second-order Raman light with a spectral purity of 43.2% [29]. In the following year, they proposed a 3 W, 2.2 µm Raman fiber laser based on the UHNA7 fiber [30]. In 2018, Zheng et al. achieved Raman light outputs of 2.1 and 2.3 µm with a total output power of 3.4 W by using a cascade Raman structure (SMF and UHNA7 fiber), the purity of the second-order Raman spectrum is not high [31]. In 2019, Du et al. achieved a 56 mW CW output power at 2.166 µm by employing a germania-core fiber (GCF, the core doped concentration of GeO2 is 97%) with silica cladding as a Raman fiber [32]. In 2023, Liu et al. reported a high-conversion-efficiency Raman fiber laser employing a 45 m HGDF as a Raman-gain fiber pumped by a dissipative soliton resonance (DSR) pulse to produce 0.89 W of first-order Raman light at 2.15 µm with a spectral purity of 96.8% [33]. Recently, Wang et al. reported a wavelength-tunable DSR TDFL as a seed source that enables tunable second-order Raman spectroscopy from 2354 to 2395 nm [34]. The above-mentioned studies focused on the wavelength expansion of Raman fiber lasers in the 2.1–2.4 µm range. However, the HGDF transmission loss at 2.3 µm is significantly higher than that at 2.1 µm. Furthermore, first-order RS cannot expand the operational wavelength to 2.3 µm because of the limited Raman frequency shift. To achieve this expansion, it is necessary to employ second-order RS; however, the purity of the second-order Raman spectrum is low. Therefore, it is difficult to achieve a watt-order output power of 2.3 µm laser. To the best of our knowledge, there is a lack of research on high-power 2.3 µm Raman fiber lasers.
To address this issue, in this work, we set up a nonlinear amplifying loop mirror (NALM)-based mode-locked fiber laser to achieve a 10–150 ns DSR-like pulse operation. Next, the average power of the DSR-like pulse was boosted to 52.5 W by employing a three-stage Tm3+-doped fiber amplifier (TDFA), which was used to pump a cascaded Raman fiber. For this purpose, a 25 m Ultra-High NA Single-Mode (UHNA7) fiber was employed as the first-order Raman-gain fiber, and a 10 m GCF with a 98% GeO2-doped concentration was employed as the second-order Raman-gain fiber. The 25 m UHNA7 fiber was pumped using a DSR-like pulse with a 150 ns full width at half maximum (FWHM) to produce 16.3 W of 2.135 µm first-order Raman light with a spectral purity of 73.4%. Finally, 5.4 W of 2.35 µm second-order Raman light with a spectral purity of 66% was obtained cascading the UHNA7 fiber and GCF, which were pumped using 80 ns DSR-like pulses.
2. Experimental setup
Figure 1 shows the experimental setup of a NALM-based mode-locked fiber laser; this includes a 14 W 793 nm LD, which is used as a pump source, a (2 + 1) × 1 pump combiner that couples the pump light into the fiber resonant cavity, the signal end of the pump combiner coupled to a 1.8 m double-cladding thulium-doped fiber (Coherent, SM-TDF-10P/130-M, 10/130 µm), and an in-house made cladding power stripper that strips the residual pump light in the cladding. A 30:70 optical coupler (OC), a polarization-independent isolator (PI-ISO), a polarization controller (PC1), and a 38 m UHNA7 fiber were employed to increase the intracavity nonlinear effects and normal dispersion caused by the NALM structure. A 70:30 OC was used to connect NALM to a unidirectional ring (UR) cavity with an overall figure-of-eight cavity structure. The UR cavity consists of a polarization-independent isolator (PI-ISO), a 70:30 OC that couples 30% of the laser output power to the next stage of the laser setup, and a PC2. In the overall cavity structure, the pigtail fibers of PCs, OCs, and PI-ISO are standard single-mode fibers (Corning, SMF-28e, 8/125 µm), whereas the others are single-mode double-cladding passive fibers (Coherent, SM-GDF-10/130-15 M, 10/130 µm). In addition, a 110 m SMF-28e was used to adjust the dispersion in the cavity and the repetition frequency of the pulses. The total length of the cavity is ∼165 m, and the group velocity dispersion is ∼−9.22 ps2 in the large anomalous-dispersion regime. The NALM-based mode-locked fiber laser can achieve stable DSR-like pulse operation by adjusting the PCs, and the FWHM of the pulses can be modulated in the 10–150 ns by adjusting the pump power using a fundamental repetition frequency of 1.24 MHz and an optical spectrum central wavelength of 1.95 µm.
Figure 2 shows the experimental setup of the cascaded Raman fiber laser. To improve the output power of the 1.95 µm DSR-like pulse, which was used as the Raman pump source, and to pump the cascaded Raman fiber, the DSR-like pulse amplification system was designed as a three-stage TDFA. A 7 W, 793 nm LD, and a 1.8 m TDF was employed in the first stage of the optical amplifier. A 3 m TDF and a 22 W, 793 nm LD were used in the second stage of the optical amplifier. The first and second stages can be considered as optical preamplifiers, which were used to increase the output power level of DSR-like pulses from milliwatts to 10 W. In the third-stage optical amplifier, which is the main optical amplifier, two 50 W, 793 nm LDs were used as the pump source. This amplification stage further increased the output power of the DSR-like pulse to 52.5 W. The active fiber was placed on a water-cooled plate at 16°C to dissipate the heat. All TDFs in the optical amplification system were the same as those in the seed laser. In addition, a 25 m HGDF (Coherent, UHNA7, 2.4/125 µm) and a 10 m GCF (FORC-PHOTONICS, GDF-MM-8/125-98, 8/125 µm), which were used as first- and second-order Raman fibers, respectively, were fusion spliced after the passive fiber of the main TDFA, resulting in an all-fiber cascaded Raman laser. The Ge-concentration of the UHNA7 fiber was ∼40%, and the Raman shift was ∼430 cm−1 [33,35]. The Ge-concentration of GCF in the experimental setup was 98%; thus, the GCF transmission loss was low in the 2.3–2.4 µm range (for example, the transmission loss of a 94% GCF is ∼0.2 dB/m at 2.3 µm and 0.4 dB/m at 2.4 µm [22], and the transmission loss of the UHNA7 fiber is approximately∼0.2 dB/m at 2.3 µm and 0.7 dB/m at 2.4 µm [36]). Consequently, the total GCF transmission loss was lower than that of the UHNA7 fiber loss. Furthermore, the GCF also exhibits a high Raman-shift peak of 427 cm−1 [26]; these characteristics are suitable for a second-order Raman fiber operating in the 2.3–2.4 µm range. In addition, the output end face of the cascade Raman fiber laser was angle-cleaved 8° for preventing reflections of output laser. The output characteristics were measured by the optical spectrum analyzers (Yokogawa, AQ6375B and AQ6376), a 12.5 GHz InGaAs Photodetectors (Electro-Optics Technology Inc., ET-5000F), a 4 GHz 40 GS/s digital oscilloscope (Teledyne LeCroy, Waverunner 8404 M), an RF spectrum analyzer (Keysight, N9020B), an autocorrelator (APE, Pulsecheck 600), and an optical power meter (Ophir Centauri, FL400A-BB-50).
3. Results and discussion
3.1 52.5 W DSR-like pulse generation at 1.95 µm
In the experiment, by increasing the pump power of the 793 nm LD to 2.7 W and adjusting the PCs, the seed laser achieved a DSR-like pulse operation in the anomalous-dispersion resonant cavity [37,38]. The characteristics of the DSR-like pulses are shown in Fig. 3. The output spectra of the DSR-like pulses in the seed laser are shown in Fig. 3(a). We observe that by continuously increasing the pump power, the center wavelength of the spectra is maintained at 1950nm, and the FWHM is slightly increased from 5.8 nm to 6.4 nm. The intensity of the spectra gradually increases as the output power increases. Furthermore, an obvious water absorption line appears in the shortwave region of the spectra. Figure 3(b) shows the variation of the DSR-like single-pulse envelope as the pump power increases. We observe that when the pump power increases from 2.7 W to 13.7 W, the FWHM of the DSR-like pulses increases from 10 ns to 150 ns due to the peak power clamping effect to maintain the stability of the pulse peak power; this is a typical feature of the DSR-like pulse operation [39–41]. Furthermore, the DSR-like single-pulse envelope exhibits an asymmetric distribution. The intensity of the pulse leading edge is slightly higher than that of the pulse trailing edge; this is more obvious in power optical amplifiers. We also measured the autocorrelation trace of 150 ns DSR pulse in the 150 ps scanning range and did not observe any pulse spikes as shown in Fig. 3(c), which shows that the DSR-like pulse is a nanosecond pulse with rectangular shape. Figure 3(d) shows the radio-frequency (RF) spectrum of the mode-locked pulse. The fundamental repetition frequency of the pulse is 1.246 MHz, and the signal-to-noise ratio (SNR) is 60 dB, demonstrating the good stability of the DSR-like pulse. The RF spectrum in the 0–100 MHz range is shown in Fig. 3(e); we observe that the modulation period of the broadband spectrum is 6.6 MHz, which is consistent with the reciprocal of the 150 ns DSR-like pulse width. Figure 3(f) shows that the output power of the seed laser is linearly related to the pump power, and the maximum output power is 52.6 mW. The mode-locked fiber oscillator has a low slope efficiency about 0.5%. The main reasons are considered as follows: The length of used-TDF (DC-TDF, 10/130) is ∼1.8 m, so the short length leads to low pump efficiency; the ∼165 m of total fiber length is used in the oscillator, which leads to a relatively high transmission loss; the large insertion loss caused by the fiber components, such as pump combiner (∼0.13 dB for signal port), isolator (∼1.2 dB), optical coupler (∼0.16 dB); the fusion loss between SMF-28e and UHNA7, passive fiber and TDF also needs to be considered [42,43].
Fig. 3. DSR-like pulse characteristics of the NALM-based mode-locked fiber laser. (a) Variation of the optical spectrum as the pump power increases from 2.7 W to 13.7 W. (b) Variation of the DSR-like single-pulse envelope as the pump power increases from 2.7 to 13.7 W. (c) Autocorrelation trace in the 0–150 ps scanning range. (d) RF spectrum at the fundamental frequency of 1.246 MHz. (e) RF spectrum in the 0–100 MHz bandwidth. (f) Output power versus pump power.
Figure 4 shows the characteristics of the 150 ns DSR-like pulse obtained at the output of the three-stage TDFA system. Figure 4(a) shows the variation of the DSR-like pulse optical spectrum as the output power increases from 25.56 W to 52.5 W. We observe that the center wavelength is continuously shifted from 1952 to 1954nm, and the FWHM of the spectrum is slightly increased from 4.9 to 5.1 nm due to the self-phase modulation effect. Compared with the seed laser characteristics, the central wavelength at the output of the three-stage amplifier is shifted from 1950nm to 1954nm, and the FWHM of the spectra is decreased from 6 nm to 5 nm for the maximum pump power at 793 nm. Figure 4(b) shows that the DSR-like pulse intensity increases as the pump power increases from 11.42 to 25.56 W. Furthermore, the pulse width does not vary, which satisfies the nanosecond-pulse optical amplification characteristics. As the pump power continues to increase, the intensity of the pulse leading edge is significantly higher than that of the pulse trailing edge; this is because more inversion particles are consumed at the pulse leading edge than those at the pulse trailing edge, resulting in insufficient gain at the trailing edge, which further enhances the asymmetry of the DSR-like pulse. The SNR of the amplified DSR-like pulse at 1.24 MHz is 60 dB, as shown in Fig. 4(c), indicating that compared with the DSR-like pulse in the seed laser, the amplified DSR-like pulse exhibits good stability. Figure 4(d) shows the RF spectrum in the 0–100 MHz bandwidth; we observe that the modulation period is 6.6 MHz, which is consistent with the reciprocal of the 150 ns pulse width. The output power of the DSR-like pulse at the output of the main amplifier versus the pump power of the 793 nm LD is shown in Fig. 4(e), where the 793 nm LD increases the average power of the DSR-like pulse to 52.5 W with a pump slope efficiency of 47.7%. At the output of the three-stage amplifier, the DSR-like single-pulse energy and peak power are increased to 42.3 µJ and 282.2 W, respectively.
Fig. 4. DSR-like pulse characteristics at the output of the third-stage TDFA. (a) Spectral variation of the DSR-like pulse as the output power increases from 25.56 W to 52.5 W. (b) Variation of the 150 ns DSR-like single-pulse envelope as the output power increases from 11.42 W to 25.56 W. (c) RF spectrum of fundamental frequency located at 1.24 MHz. (d) RF spectrum in the 0–100 MHz bandwidth. (e) DSR-like pulse output power versus 793 nm LD pump power.
3.2 16.3 W, 2.135 µm first-order Raman laser
We initially measured the output characteristics of the first-order Raman laser operating at 2.135 µm. The 25 m UHNA7 fiber, which was used as a first-order Raman-gain fiber, was fused after the master oscillator power amplifier (MOPA). The DSR-like MOPA was used as the Raman pump source to obtain the first-order Raman fiber laser output characteristics shown in Fig. 5. The 52.5 W, 150 ns DSR-like pulse was used to pump the UHNA7 fiber; eventually, a total output power of 22.2 W was obtained. The variation in the first-order Raman spectra with the Raman pump power is shown in Fig. 5(a); we observe that the spectra of the fundamental-frequency light at 1954nm are dominant at a low Raman pump power. As the DSR-like pulse power continuously increases, the fundamental-frequency light is transformed into a 2.135 µm first-order Stokes-shift light, and the weak component of the second-order Stokes-shift light is maintained at 2.35 µm. The calculated Raman shift of UHNA7 fiber was 430 cm−1, which is consistent with the reported data. Figure 5(b) shows the relation between the total output power, the net output power at 2.135 µm, the spectrum purity of the first-order Raman laser, and the pump power of the DSR-like pulses. We observe that as the pump power increases, the first-order Raman pulse output power and its spectral purity gradually increase. When the pump power is 52.5 W, the FWHM of the first-order Raman spectrum is 24.8 nm, and the center wavelength is 2.135 µm, achieving the highest conversion rate of 73.4%. The output power of the pure 2.135 µm laser is 16.3 W, and the conversion efficiency of the first-order Raman pump is 31%. The peak power of the DSR-like pulse can be increased by reducing the pulse width to improve the conversion efficiency of the second-order Raman light; however, the transmission loss of the second-order UHNA7 fiber in the Raman laser is large and can produce a serious thermal effect. It is difficult to obtain purer spectra and higher power for the second-order Raman light by simply employing the UHNA7 fiber.
Fig. 5. (a) Variation of output spectra for different Raman pump powers. (b) Output power of the first-order Raman laser versus Raman pump power.
3.3 5.4 W, 2.35 µm cascaded Raman laser
The transmission loss of GCF and its equivalent Raman shift are relatively lower than those of the UHNA7 fiber in the 2.3–2.4 µm range. Therefore, the 10 m GCF was selected as a second-order Raman-gain fiber and cascaded after the 25 m UNHA7 fiber. In addition, to implement a second-order Raman laser generation, the DSR-like pulse usually needs a high peak power to excite the second-order Raman effect; thus, we set the fitting pulse width of the DSR-like operation to 80 ns. Figure 6(a) shows the spectra of the GCF, which was pumped using an 80 ns DSR-like pulse with a 12.8–52.5 W Raman pump power. We observe that as the power of the pump pulse increases, the first-order Raman light at 2.135 µm initially appears; then, the first-order Raman laser appears to be self-pumped as the Raman pump power continues to increase. as evidenced by the intensity of the 2.135 µm spectrum, which gradually decreases and shifts and the spectral intensity at 2.35 µm, which significantly increases in the 2.3–2.4 µm optical spectrum range. The spectrum FWHM is ∼30 nm, and the calculated Raman shift of the cascaded Raman fiber is ∼430 cm−1. The total output power characteristics, the net output power at 2.35 µm, and the purity of the second-order Raman spectra for the cascaded Raman laser are shown in Fig. 6(b) as a function of the pump power. At the highest pump power, the second-order Raman spectral purity is 68%, and the net output power of the 2.35 µm laser is 5.08 W. It is important to note that when the pump power is the highest, the 2.35 µm laser cannot produce its highest output power. Due to the increase in the peak power of the DSR-like pulse, the second-order Stokes-shift light reaches the Raman threshold, thus producing a 2.6 µm third-order Stokes-shift light. However, the transmission loss of the GCF in the 2.6 µm laser is 1 dB/m, resulting in more pump light converted into thermal losses. Although the spectral purity of the second-order Raman laser is improved, the total output power of the Raman laser is significantly decreased. In the subsequent experiment, we used another optical spectrum analyzer with a 1500–3400 nm measurement range to detect the third-order Raman light at 2.6 µm when pump with a 4.5 W DSR-like pulse. The results are shown in Fig. 6(c). We observe that the third-order Raman light does not occur at 2.6 µm by adjusting the DSR-like pulse width in the 20–10 ns range to improve the peak power and to stimulate the third-order RS effect. This result demonstrates that the low-order Raman light is converted into thermal losses and cannot form a third-order Raman light due to the GCF loss at 2.6 µm.
Fig. 6. (a) Spectral variation of the cascaded Raman fiber laser for different Raman pump power. (b) Output power of the cascaded Raman fiber laser versus Raman pump power. (c) Cascaded Raman spectrum from 1900nm to 2700 nm pumped by a 4.5 W, 10–20 ns DSR-like pulse width.
Therefore, when the Raman pump power is 46.3 W, the cascaded Raman fiber laser produces a total output power of 8.23 W. A 5.4 W, 2.35 µm Raman light with a spectral purity of 66% and a Raman pump conversion efficiency of 11.6% was ultimately obtained. In Table 1, we compare our results with those obtained from other recent studies on high-power 2.3 µm lasers. It is evident that the cascaded Raman fiber lasers exhibit a high potential in the design of high-power 2.3 µm laser sources and wavelength expansion in the 2.1–2.4 µm range. It's worth noting that if a wavelength-tunable Raman pump source operating around 2.0 µm is used, the wavelength tunable first-order and second-order Raman fiber laser can be realized, which will be beneficial to achieving wavelength expansion in the wavelength range of 2.1–2.4 µm.
Table 1. Comparison of our results with those obtained from other recent studies on ∼2.3 µm laser
4. Conclusion
In this work, an all-fiberized 2.35 µm cascaded Raman laser achieving a 5.4 W net output power was proposed. Initially, a seed laser based on the NALM structure was built to obtain tunable DSR-like pulses in the 10–150 ns. Next, a three-stage TDFA system was used to increase the average power of the DSR-like pulses to 52.5 W. Subsequently, a 16.3 W, 2.135 µm laser output pulse was obtained using 150 ns DSR-like pulses to pump a 25 m UHNA7 fiber. Finally, a 10 m long GCF was employed as a second-order Raman-gain fiber cascaded after the UHNA7 fiber to form a cascaded Raman laser, and a 5.4 W, 2.35 µm second-order Raman optical output was obtained by pump with 80 ns DSR-like pulses. This work demonstrates that a cascaded Raman fiber laser can be very useful in expanding the wavelength expansion in the no-gain and low-gain region of rare-earth-doped optical fibers.
Funding
Shenzhen Pingshan District Science and Technology Innovation Fund (KY2022QJKCZ001, PSKG202003, PSKG202007); Basic and Applied Basic Research Foundation of Guangdong Province (2023A1515111114); Fundamental research project of Department of Education of Guangdong Province (2021ZDJS106); National Key Research and Development Program of China (2022YFB3605800); National Natural Science Foundation of China (62105225, 62275174).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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