The supercontinuum (SC) has both the wide spectral characteristics of traditional broadband light and the high spatial coherence of lasers. Because of this advantage, the SC has a wide range of applications, such as spectroscopy, biomedicine, and fiber optic communications^[1–4], and high-power operation is required to increase the system performance and capability^[5–7].

Currently, the mainly used ways to generate high-power SC can be categorized two types. The first way is to use high-power continuous wave or pulsed fiber laser to pump photonic crystal fiber (PCF)^[8–14]. With the help of a high nonlinear coefficient of PCF, the pump laser spectrum will be greatly broadened in it and become an SC. Recently in 2021, researchers from the China Academy of Engineering Physics achieved >300W SC laser based on the “pulsed fiber laser + PCF” method^[8]. The output spectrum extended from 390 to 2400 nm with good flatness. The other way is to directly generate SC in the nonlinear fiber amplifier^[15–18]. It usually employs a high peak-power pulsed laser as the seed laser for injection into the fiber amplifier, in which the laser peak power can be boosted and induce the strong nonlinear effects to realize SC output. In 2013, Song et al.^[18] realized a 177 W SC laser based on “nanosecond fiber laser + three-stage nonlinear fiber amplifiers.” The spectrum covered 1064–2000 nm with a −10dB spectral width of 740 nm. In addition, a new method of generating SC has emerged, which is about using random fiber lasers (RFLs) to directly generate near-infrared SC^[19–22]. Researchers from the University of Electronic Science and Technology of China firstly, to the best of our knowledge, reported this method in 2018^[19], and they used a 1365 nm Raman fiber laser to pump an RFL with a working wavelength of 1461 nm. The collective effects of RFL, stimulated Raman scattering (SRS) and cross-phase modulation (XPM) finally lead to spectral broadening, and the SC was generated with −20dB bandwidth of 145 nm.

Here, we demonstrated a novel scheme to generate high-power SC. Different from previous methods, this scheme is based on a simple fiber Bragg gratings (FBGs)-based laser cavity. When the bandwidth of the FBGs pair is narrow, random self-pulses will be generated with the peak power much higher than the average power^[23–27], which will cause nonlinear effects and broaden the laser spectrum. In the experiment, we built a narrow-linewidth laser cavity with a pair of polarization-maintaining (PM) FBGs. Applying temperature control to FBGs, the SC with an average power of ∼80W was achieved, and the optical-to-optical conversion efficiency was 43.2%. The output spectrum ranged from<600nm to 1600 nm, and the −10dB bandwidth was up to 420.3 nm with 1250 nm as the central wavelength.

The structure of the fiber laser cavity is shown in Fig. 1. This cavity was constructed through cross splicing a pair of PM FBGs, in which the reflectivity of the highly-reflected FBG (HR-FBG) and output-coupling FBG (OC-FBG) was 99.5% and 10%, respectively. Cross-splicing techniques made the laser with a linear polarization state parallel to the fast axis of HR-FBG^[28]. The −3dB bandwidths were 0.28 nm and 0.08 nm, corresponding to the HR-FBG and OC-FBG. The laser gain was provided by a piece of 4.5 m PM ytterbium-doped fiber (YDF) with a core-clad diameter of 10/125 µm and a core numerical aperture of 0.07. The cladding absorption coefficient of YDF was 4.95 dB/m at 976 nm. The cavity was directly pumped by a 976 nm laser diode (LD) through a combiner, and the pump power could reach ∼185W. The unabsorbed pump power was removed by a cladding power stripper (CPS). While the temperature of HR-FBG was maintained at the room temperature of 21°C, the OC-FBG was set to 12°C by a thermal electrical cooler (TEC). It was found in the experiment that such a temperature difference can ensure that the laser can resonate normally^[29] and play a key role in the generation of SC. Both ends of the fiber laser cavity were cleaved with 8° to prevent parasitic oscillation.

When the injected pump power was 27.3 W, the laser power reached the power of 13.8 W. At this power, the mean normalized laser timing sequences and spectrum were measured and are shown in Figs. 2(a) and 2(b). All of the laser timing sequences in this Letter were gathered by a high-speed photodetector with 5 GHz bandwidth and an oscilloscope with 5 GSa/s. The sampling time interval was 0.2 ns, and the measurement time window was 40 µs. It should be noted that the y axes’ units of laser timing sequences in Fig. 2 are the relative voltage value, and the relationship between power and voltage can be descripted as P∝U2. The random self-pulses were observed at the power of 13.8 W, in which the peak-power values were 9–25 times the average power. At this time, the output spectrum had narrow linewidth, and the −3dB bandwidth was 58 pm.

With the further increase of pump power, the high peak-value pulse occurred at the laser power of 44.1 W, as shown in Fig. 2(c). The peak power can reach as high as the kilowatt (kW) level. Within 40 µs, there were statistically ∼35 pulses with peak power of >100 times the average power. These high-power pulses induced the nonlinear effects and led to spectral broadening. The Raman Stokes light was observed in the spectral diagram, as shown in the Fig. 2(d), and there had been some signs in the spectrum about the generation of SC.

When the pump power was increased to 183.9 W, the laser power reached 79.5 W with the optical-to-optical conversion efficiency of 43.2%. The peak power of the random pulse had astonishingly reached>104 times the average power, which means that the instantaneous laser power had reached as high as the 0.8 MW level, as shown in Fig. 2(e). High peak-value pulses appeared more frequently, and ∼1779 pulses with peak power of >2.5×103 times the average power occurred within 40 µs. These pulses’ duration presented random characteristics and fluctuated within the range of 1–10 ns. Such intense and frequent pulses caused more severe nonlinear effects, and the SC was generated, as shown in Fig. 2(f). The spectral range was from<600nm to 1600 nm. The detection of shorter wavelengths was limited by the wavelength range of the optical spectrum analyzer (600–1700 nm). The −10dB bandwidth was 420.3 nm with 1250 nm as the central wavelength. The SRS was the most obvious nonlinear phenomenon in our experiment, and its classic formula of the power threshold is depicted as the following^[30]: Pth=16Aeff/gRLeff,(1)where Pth is the SRS threshold, Aeff is the effective area of the fiber laser, gR=10−13m/W is the peak value of the SRS gain spectrum, and Leff is the effective nonlinear interaction length; the SRS threshold calculated in our fiber cavity should be at the 2 kW level. In other words, it is impossible to observe Raman Stokes light on the spectrum with laser power of 80 W. However, the SRS response time is in the order of hundreds of femtoseconds^[30], and the nanosecond (ns)-scale self-pulses peak power had reached the 0.8 MW level. So, the Raman Stokes light observed in the spectrum was attributed to these strong self-pulses. The physical mechanism behind these random strong self-pulses is intriguing, and we speculated that they are especially related to the stimulated Brillouin scattering (SBS)^[31–33].

Since the wavelength of SC had been extended to the visible light band, the visible red laser output was observed in the experiment and photographed, as shown in Fig. 3. In addition, it can be seen from Fig. 4 that the laser power increased linearly with pump power, indicating that this kind of SC laser has potential for further power boosting.

For testing the stability of SC generated through this simple narrow-bandwidth FBGs-based laser cavity, the measured spectra of ∼70W SC laser at the different dates (May 31, 2021 and Sep 15, 2021) are shown in Fig. 5. It turns out that the two output spectra agree very well before the wavelength of 1420 nm, and the difference is only reflected in the longer wave band, which was attributed to the random characteristic of self-pulses or optical rogue waves^{[23,26,27,31–33]}.

In conclusion, we demonstrated a novel way to generate high-power SC by employing the self-pulsing effect in a narrow-linewidth fiber laser cavity. With the help of temperature control of FBGs, the ∼80W SC was finally achieved. The output spectrum ranged from<600nm to 1600 nm, and the −10dB bandwidth was 420.3 nm with 1250 nm as the central wavelength. In the future, we will work on a theoretical model about the production mechanism of those high peak-power random pulses and further boost the SC output power through bidirectional pumping.