1970 W 1030 nm Single-Mode All-Fiber Master Oscillator Power Amplifier with ~3.2 GHz Linewidth Based on Ultra-Low NA Active Fiber

Abstract

A high-power narrow-linewidth fiber laser with single-mode beam quality is experimentally demonstrated. By employing a cascaded phase modulation strategy, the stimulated Brillouin scattering (SBS) threshold of the laser is effectively increased from 973 W to 1970 W. High-order modes are well suppressed during power scaling, benefiting from the significant bending loss of a low numerical aperture (NA) ytterbium-doped fiber. A maximum output power of 1970 W is achieved, with a linewidth of 3.2 GHz and a beam quality factor M² of 1.14. To the best of our knowledge, this represents the highest reported output power for narrow-linewidth fiber lasers (linewidth < 10 GHz) operating at wavelengths below 1040 nm.

1. Introduction

High-power narrow-linewidth fiber laser systems demonstrate significant potential in a wide range of applications, including beam-combining systems [1,2], coherent lidar [3], remote communication [4], and nonlinear frequency conversion [5,6]. Previous studies have demonstrated that enhancing the output power and expanding operational wavelengths of such lasers are critical for improving the performance of their application systems [7,8]. Consequently, substantial efforts have been devoted to these areas, yielding remarkable achievements. For example, the record output power of narrow-linewidth systems with 100 GHz level linewidths has exceeded 5 kW in the conventional wavelength (1050–1080 nm) [9,10,11,12]. However, expanding the wavelength to the short wavelength (<1040 nm) induces amplified spontaneous emission (ASE) accumulation in the conventional wavelength, severely limiting power scaling [13]. Furthermore, spectral gain overlap between ASE and stimulated Raman scattering (SRS) lowers the SRS threshold during power amplification [14]. This makes it significantly more challenging to simultaneously mitigate ASE, nonlinear effects (SRS and stimulated Brillouin scattering (SBS)), and transverse mode instability (TMI) in short wavelength systems [15]. Linewidth compression is another essential objective for high-power narrow-linewidth lasers. Nevertheless, narrower linewidths exacerbate the trade-off between nonlinear effects and TMI [16,17]. When the linewidth is reduced to sub-10 GHz, the maximum output power of conventional wavelength systems is constrained to ~3 kW [18,19,20,21,22,23,24]. For short-wavelength systems, ASE and SBS effects limit the maximum power to approximately 1 kW [25,26]. Specifically, Schmidt et al. (2011) utilized a 40/200 μm photonic crystal fiber to amplify an ASE seed source (3.5 GHz linewidth), achieving 697 W at 1030 nm [25]. Naderi et al. (2016) demonstrated a 1034 nm laser at kilowatt-level power using a 6.4-m 25/400 μm ytterbium-doped fiber (YDF) with a 3.5 GHz linewidth [26]. To date, power scaling of short-wavelength narrow-linewidth lasers (<10 GHz linewidth) has progressed slowly. Recently, our group reported a 1.4 kW all-fiber laser at 1030 nm (3.5 GHz linewidth) by combining cascaded sinusoidal phase modulation with a confined Yb-doped fiber (core/cladding: 30/250 μm) [27].

In this study, experiments are conducted focusing on the power scaling of a 1030 nm narrow-linewidth fiber laser. Cascaded phase modulation, together with a low numerical aperture (NA) YDF, is introduced into the amplifier to enhance the SBS threshold and maintain near-diffraction-limited beam quality. Consequently, a 1030 nm single-mode fiber laser with a record high power of ~2 kW and a linewidth of ~3.2 GHz is demonstrated.

2. Experimental Setup

The high-power narrow-linewidth all-fiber amplifier system operating at 1030 nm adopts a master oscillator power amplifier (MOPA) configuration, as schematically depicted in Figure 1. The seed source comprises a single-frequency fiber laser with an emission wavelength of 1030 nm, output power of 40 mW, and 3 dB linewidth of ~21 kHz. The modulation stage is driven by two-stage sinusoidal-phase RF signals applied to the LiNbO₃ electro-optic modulator (EOM). The first EOM is driven by a sinusoidal RF signal with a modulation depth of 0.58π and a driving frequency of 1 GHz. The linewidth is measured by a Fabry–Perot interferometer (FPI) with a free spectral range (FSR) of 10 GHz and calculated to be approximately 3.28 GHz. Then, the cascaded second EOM is also driven by a sinusoidal RF signal with a modulation depth of 3π and a driving frequency of 60 MHz. After two-stage sinusoidal phase modulations, the seed linewidth is broadened to 3.66 GHz [27]. Subsequently, the modulated seed signal is amplified using two-stage pre-amplifiers. The first-stage pre-amplifier employs a commercial pre-amplifier to scale the seed signal power above 2 W. A 2 nm bandpass filter (BPF-1) centered at 1030 nm is then employed to suppress spectral sidelobes and ASE induced during the scaling process. After passing through BPF-1, the optical signal-to-noise ratio (OSNR) reaches 62 dB and narrow band component OSNR suppression of ~44 dB at maximum output power. Meanwhile, a polarization-insensitive isolator (ISO) with >30 dB isolation is employed to attenuate back-propagating light, thereby protecting upstream components. The second-stage pre-amplifier utilizes a 2.5 m long YDF with 10/125 µm core/cladding diameter dimensions, pumped through a (2 + 1) × 1 combiner via two 976 nm laser diodes (LDs). Subsequently, a circulator with >20 dB isolation and a 2 nm bandpass filter (BPF-2) are connected in series to monitor back-propagating light and filter out the ASE, respectively. Through a two-stage pre-amplification architecture, the system achieves a final output power exceeding 20 W while maintaining ASE suppression > 64 dB over 1050–1090 nm and narrow band component OSNR suppression of ~44 dB. This high-OSNR seed source enables ASE suppression in the subsequent main amplifier.

The main amplifier employs a forward-pumped configuration, utilizing six 976 nm LDs as the pump source coupled through a (6 + 1) × 1 fiber combiner. The combiner features a 15/130 µm signal input port and a 25/250 µm output port, with pump delivery fibers of 135/155 µm diameter (NA = 0.22). The active fiber consists of an ultra-low NA YDF developed by our group, exhibiting core/cladding diameters of approximately 35/250 µm and a cladding absorption coefficient of ~6 dB/m at 976 nm. The core NA is ~0.04, resulting in a normalized cutoff frequency (V) that is calculated to be 4.27. So, only LP₀₁ mode, LP₁₁ mode, LP₂₁ mode, and LP₀₂ mode are supported, with an effective mode field area exceeding 760 μm². The amplifier incorporates a 3 m length in this ultra-low NA YDF, optimized for efficient pump absorption to maximize optical-to-optical conversion efficiency. Meanwhile, the ultra-low NA YDF is properly coiled around a water-cooled plate with a diameter of 40 cm and a cooling temperature of 20 °C. Additionally, the bending loss coefficient for the LP₀₁ is ~3.4 × 10⁻³ dB/m, while the LP₁₁ mode exhibits ~6 dB/m. This indicates that such bending effectively suppresses higher-order modes. Following the YDF, a cladding power stripper (CPS) removes residual pump power and cladding modes, succeeded by a quartz block holder (QBH). The total distance from the active fiber output to QBH is ~0.9 m. The collimated output beam is directed to a high-reflectivity (HR) mirror, where ~99.9% of the optical power is reflected to a power meter. The transmitted portion (~0.1%) is utilized for comprehensive output characterization, including spectral analysis via optical spectrum analyzer (OSA), temporal trace measurement using a photodetector (PD) and oscilloscope (OSC), linewidth characterization with a Fabry–Perot interferometer (FPI), and beam quality assessment via laser quality monitor (LQM).

3. Experimental Results

3.1. Single-Stage Sinusoidal Phase Modulation

The output power characteristics of the unmodulated single-frequency seed source are first characterized. As shown in Figure 2a, the output power exhibits linear scaling with pump power, achieving a slope efficiency of 82.3%. Furthermore, the backward-propagating power detected via the circulator demonstrates nonlinear growth with increasing pump power, suggesting the onset of SBS. Established experimental criteria indicate that a backward power ratio (R) between 0.01% and 0.1% signifies amplifier operation near the continuous-wave SBS threshold, where R is defined as the ratio of backward power to output power [28,29]. For an operational definition, we designate the SBS threshold as the output power corresponding to R = 0.1%. Consequently, the SBS threshold for this amplifier is estimated at approximately 206 W. Figure 2b confirms effective suppression of ASE noise in the main amplifier, yielding an optical OSNR of ~49 dB, while the narrowband component exhibits an OSNR of ~41 dB. Longitudinal mode characteristics at maximum output power are analyzed using an FPI with a free spectral range (FSR) of 1.5 GHz (Figure 2c). The absence of secondary peaks between the FPI’s principal resonances confirms single-frequency laser operation.

To elevate the amplifier’s SBS threshold, sinusoidal phase modulation is applied to broaden the seed laser linewidth. Initial broadening employed a 60 MHz sinusoidal RF signal with 3π modulation depth. The resultant amplifier output power characteristics are represented by the cyan triangular markers in Figure 3a. Under identical SBS threshold evaluation criteria, this phase-modulated configuration demonstrates a substantially increased threshold of 764 W relative to the unmodulated case. To achieve further enhancement, the single-frequency seed laser is modulated using a 1 GHz sinusoidal RF signal with 0.58π modulation depth. The corresponding output power characteristics (magenta circular markers, Figure 3a) yielded an elevated SBS threshold of 973 W. Temporal traces and corresponding Fourier transform spectra at maximum output power under 60 MHz and 1 GHz modulation are presented in Figure 3b. Notably, no TMI is observed below 1000 W output power, indicating SBS as the fundamental limitation for further power scaling. Figure 4 displays FPI-measured scanning spectra at maximum amplifier output power under sinusoidal RF modulation. Following linewidth calibration and frequency-domain conversion, the processed spectra yielded calculated linewidths of 1.29 GHz and 2.94 GHz at maximum output power, respectively.

The experimental results demonstrate that implementing single-stage sinusoidal phase modulation with elevated drive frequency effectively broadens the spectral linewidth of the single-frequency seed laser while simultaneously increasing the SBS threshold in the amplifier stage. Nevertheless, this approach exhibits two fundamental limitations regarding SBS suppression efficacy. First, the maximum attainable linewidth broadening through this technique is inherently constrained by the modulation parameters. Second, spectral analysis reveals a progressive transition in laser operation regime with increasing drive frequency, from the stable single-longitudinal-mode dominance illustrated in Figure 4a to the unstable multi-longitudinal-mode competition regime shown in Figure 4b. Quantitative measurements indicate that despite achieving a linewidth enhancement from 1.29 GHz to 2.94 GHz through frequency modulation, the corresponding SBS threshold improvement remains modest (ΔP_SBS = 209 W). Previous investigations [30,31] have established that spectral profiles approaching rectangular distributions with uniform longitudinal mode intensities demonstrate superior SBS suppression characteristics. To realize such optimized spectral properties, we implement a cascaded phase modulation architecture comprising two distinct stages. The initial spectral linewidth is broadened through high-frequency, low-modulation-depth EOM. Subsequent spectral homogenization is accomplished through secondary modulation using a low-frequency, high-modulation-depth EOM [27].

3.2. Two-Stage Sinusoidal Phase Modulation

Figure 5a presents the relationship between the output power, backward-propagating power, and pump power of the amplifier. The output power demonstrates a linear dependence on pump power, with a consistently maintained slope efficiency of 83.3%. The temporal traces and corresponding Fourier transform spectra recorded at different powers are shown in Figure 5b. At 1817 W, the temporal trace remains stable, and no distinct spectral peaks are observed in the Fourier transform spectrum. However, at 1970 W, significant intensity fluctuations emerge in the temporal domain, accompanied by pronounced characteristic peaks in the Fourier spectrum. These observations confirm the occurrence of the TMI effect.

The SBS effect is monitored by analyzing backward-propagating light (Figure 5a), where the ratio of backward power to signal power remains as low as 0.07‰. Figure 5c shows the backward-propagating laser power spectrum, revealing the emergence of Stokes peaks above 1900 W of output power. At 1970 W, the Stokes peak intensity exceeds the Rayleigh peak by 3.8 dB, confirming SBS-induced spectral broadening. Figure 5d displays the optical spectrum at the maximum output power of 1970 W, revealing an OSNR of ~59 dB with no detectable ASE. Additionally, the OSNR of the narrowband component remains at ~36 dB.

Beam quality (M² factor) as a function of output power is presented in Figure 6. The M² factor exhibits only minor fluctuations in the power-scaling process, indicating excellent beam stability. At the maximum output power (1970 W), the M² factor is measured to be 1.14. Notably, although time–frequency characteristics degrade and output power fluctuations occur at higher pump powers, beam quality does not deteriorate. This suggests that high-order modes generated in the system are effectively suppressed by fiber losses.

Spectral linewidth measurement is presented in Figure 7. At maximum output power, the linewidth is measured at 3.20 GHz after calibration. However, waveform distortion occurs due to inherent signal instability from two-stage sinusoidal phase modulation and longitudinal mode competition. Despite this, the two-stage modulation method demonstrates superior SBS suppression compared to single-stage modulation, as evidenced by the spectral stability and power scalability achieved.

4. Discussion

Different from the broadband fiber source, for the power scaling of GHz-level all-fiber amplifiers, the balance of SBS and TMI thresholds has still been challenging. Further power enhancement still requires comprehensive optimization from both the seed laser and amplifier aspects. Regarding the seed laser, in this work, for SBS suppression, a cascaded phase modulation approach is employed to generate a quasi-rectangular spectrum, which demonstrated a higher SBS threshold compared to Lorentzian, sinc², and Gaussian spectra [32]. However, due to the influence of spectral distortion in the generation and amplification process, the SBS enhancement factor is still lower than the ideal rectangular result. In order to achieve a higher SBS threshold, two aspects are necessary: (i) precise calibration of the electro-optic transfer function within the phase modulation system to generate a more regular rectangular modulation spectrum; (ii) mitigation of spectral distortion induced by self-phase modulation during amplification, for which adaptive optimization techniques demonstrate significant potential as a viable solution [33,34].

For the main amplifier, a low-NA fiber must achieve two key benefits: on the one hand, it must increase the fundamental mode field area, thereby suppressing the SBS effect; on the other hand, it must leverage the high bending loss of higher-order modes in low-NA fiber to mitigate the TMI effect, then a breakthrough of nearly 2 kW has been achieved. However, the system remains constrained by the dual limitations of SBS and TMI effects. To address the SBS effect, combining a low-NA fiber with tapered fiber drawing presents a promising technical pathway [35]. To address the TMI effect, more precise control of the refractive index profile, such as through confined doped [27], single-trench [36], or bat-type designs [37], holds promise for further increasing the loss of higher-order modes and achieving a higher threshold. Additionally, the exploration of fibers with even lower NA remains a research aspect worth investigating. This involves challenges related to the stability and reproducibility of the fiber-drawing process that require further investigation.

5. Conclusions

In summary, we demonstrate a high-power, single-mode, narrow-linewidth laser operating at 1030 nm. The variation in the SBS threshold with sinusoidal phase modulation parameters applied to a single-frequency seed laser is experimentally investigated. Cascaded sinusoidal phase modulation featuring a broadband quasi-rectangular spectrum confers significant advantages for SBS suppression. Consequently, a maximum output power of 1970 W is achieved, with the laser exhibiting high slope efficiency (83.3%), a narrow linewidth (~3.2 GHz), and excellent beam quality (M² = 1.14). Further power scaling is jointly limited by the TMI and SBS effects. These results establish a novel, efficient laser architecture for high-power spectral beam combining and frequency conversion systems. Future work will prioritize optimization of phase modulation schemes and fiber specifications to enable higher-power operation.