Dual combs with a slight difference in repetition frequency ($f_{\mathrm{rep}}$) support linear optical sampling in the time domain and multi-heterodyne spectroscopy in the frequency domain [1], which has become a precise metrology tool for rapid and precise ranging [2,3], velocity measurement [4,5], and dual-comb spectroscopy [6–9]. Dual-comb sources based on two mode-locked femtosecond lasers were demonstrated in the early work [10,11]. In recent years, multiplexing techniques involving polarization [12], wavelength [13], and propagation direction [14] in a single cavity have been developed to generate dual combs. In these schemes, the combs provide MHz-magnitude $f_{\mathrm{rep}}$ due to the limitation of cavity size, and the repetition frequency difference ($\mathrm{\Delta }f$) is determined by the intracavity group velocity dispersion or birefringence. Dual combs sharing a single cavity canceled common-mode technical noises and maintained mutual coherence, which has been used in spectroscopy [15] and ranging [16]. As for dynamic measurement, such as vibrometry or distance acquisition for moving targets, dual combs with higher $f_{\mathrm{rep}}$ exhibit characteristic advantages in high-speed acquisition [17,18]. Compared with the conventional mode-locked femtosecond lasers, the electro-optic (EO) dual combs provide higher and more flexible $f_{\mathrm{rep}}$ and $\mathrm{\Delta }f$ [19–21]. A continuous-wave (CW) laser can simultaneously feed two EO combs with intrinsic mutual coherence. Typically, the $f_{\mathrm{rep}}$ of an EO comb is less than 40 GHz due to the limitation of commercial electric devices, and several modulators in series are necessary to obtain sufficient comb lines. Thanks to the blooming of soliton combs based on microresonators, the $f_{\mathrm{rep}}$ is extended from GHz up to THz level [22,23], and various $\mathrm{\Delta }f$ with magnitude from kHz to GHz can be realized by employing a pair of microresonators with free spectral range (FSR) differences equal to the desired $\mathrm{\Delta }f$ [24]. Moreover, multiplexing techniques are also used in a single microresonator, such as space-division multiplexing of two spatial mode families in a counter-propagating way [25] and dual-soliton combs with orthogonal polarization [26].

Combs with flexible $f_{\mathrm{rep}}$ were also produced by the four-wave mixing (FWM) process of dual-pump lasers in a multistage fiber mixer consisting of highly nonlinear fibers (HNLFs) and single-mode fibers (SMFs) [27–29]. In these dual-pump schemes, the stimulated Brillouin scattering (SBS) was suppressed by applying longitudinally varying tension to the HNLF, preventing the loss of pump power. However, given the superiority of narrowband and effective gain, the SBS effect is not suppressed but is combined with Kerr nonlinearity in a resonator to boost a Brillouin comb in recent works [30–32]. Furthermore, dual Brillouin combs are demonstrated by employing multiplexing counter-propagating mode families in a microresonator under a single external pump laser [33]. The $f_{\mathrm{rep}}$ is fixed at around 110 GHz due to the limitation of the FSR, and switchable $\mathrm{\Delta }f$ ranging from 8.5 MHz to 212 MHz is realized by employing different mode-family pairs. In addition, external dual-pump lasers were used to realize a Brillouin comb with tunable $f_{\mathrm{rep}}$ based on the FWM process of the intracavity dual Brillouin lasers in a fiber loop [34–37]. The dual-pump scheme removes the limitation of cavity FSR to $f_{\mathrm{rep}}$, and the $f_{\mathrm{rep}}$ is equal to the frequency interval of the dual-pump seeds. However, in these works, a fiber loop longer than 100 m results in multi-mode lasing in the Brillouin gain spectrum. A short fiber loop (2–20 m) has been used to realize a single-mode Brillouin laser with a high optical signal-to-noise ratio (OSNR) and narrowed linewidth [38–40]. A Brillouin comb generated based on the dual-pump scheme supports flexible $f_{\mathrm{rep}}$, which provides guidance for the research on dual Brillouin combs.

In this paper, we demonstrate an 80-GHz-spaced dual-comb source with reconfigurable $\mathrm{\Delta }f$ by combining the intracavity dual Brillouin lasers with the cavity-enhanced cascade FWM effect in fiber loops. Two pairs of dual-pump seeds are used to drive the dual Brillouin combs based on a similar principle to that in Ref. [35]. Besides, a frequency shifter consisting of a pair of single-mode Brillouin lasers that employ different types of fibers is used to produce a frequency offset as well as an interval offset between the two dual-pump seeds, which decides the central frequency of the down-converted RF spectrum and contributes to $\mathrm{\Delta }f$ of the dual combs. By optimizing the length of fiber loops and controlling the power of each dual-pump seed, the intracavity Brillouin lasers are ensured to be single-mode lasers and then give rise to combs via the FWM effect. Furthermore, fiber Bragg gratings (FBGs) are used to improve the spectral flatness and narrow the pulse widths. Consequently, 10 comb lines in a 20-dB power deviation and a pulse width of around 650 fs are realized for every comb. More importantly, the dual-comb source supports tunable $f_{\mathrm{rep}}$ by changing the spacing of the dual-pump seeds and allows reconfigurable $\mathrm{\Delta }f$ ranging from 48 MHz to 1.486 GHz by switching the fiber pair in the frequency shifter. The proposed dual-comb source has potential applications in generating dual terahertz combs in photoconductive antennas, which can be used in terahertz frequency metrology [41,42]. In addition, the dual combs act as seeding sources that would support spectral broadening in a nonlinear fiber [35] or nonlinear optical loop mirrors system [43], increasing the possibility of application in rapid ranging.

The dual-comb source provides two Brillouin combs generated in fiber loops based on the dual-pump scheme. The $f_{\mathrm{rep}}$ of each comb is determined by the frequency interval of the corresponding dual-pump seed; thus two pairs of dual-pump seeds with a slight interval offset ($\delta$) can be used to drive dual combs with $\mathrm{\Delta }f$ almost equal to $\delta$. The experimental setup of the dual-pump seeds generator is shown in Fig. 1(a). A 12.6-kHz-linewidth continuous wave (CW) laser is modulated by a phase modulator (PM) at an RF frequency of $f_{\mathrm{RF}}$; then the $\pm 2$nd order modulation sidebands are selected by a programmable filter and serve as the dual-pump seed (DP1) with an interval of $4f_{\mathrm{RF}}$. After being amplified by an erbium-doped fiber amplifier (EDFA), DP1 passes through a 30/70 optical coupler (OC), sending 30% of its optical power to a frequency shifter and 70% to Loop3 to drive the Brillouin comb (Comb1). The frequency shifter consists of an FBG, two single-mode Brillouin fiber lasers, and a 50/50 OC. Each Brillouin fiber loop contains a section of fiber (Fiber1/Fiber2) as the Brillouin gain medium, an optical circulator (CIR), and a 10/90 OC. The frequency shifter is designed to redshift DP1 by about 10 GHz via the SBS effect and to produce another dual-pump seed (DP2) with an interval offset to DP1. Specifically, DP1 is coupled into the frequency shifter from port 1 of the four-port CIR1; then the short wavelength mode in DP1 is reflected by FBG1 and injected into Loop1, whereas the long wavelength mode passes through FBG1 and enters Loop2. Two Brillouin fiber lasers are stimulated in Loop1 and Loop2 and coupled into the 50/50 OC4 to serve as DP2. Note that the Brillouin frequency shift (BFS) depends considerably on fiber type [44]; thus Fiber1 and Fiber2 with different fiber types have different BFSs. As noted in Fig. 1, five 23-m fibers are prepared for Fiber1 and Fiber2, which provide a BFS offset of 50 MHz up to 1.49 GHz. As a result, two pairs of dual-pump seeds with an interval offset equal to the BFS offset and a relative frequency shift of about 10 GHz are obtained.

Next, the two pairs of dual-pump seeds are used to produce dual combs based on the setup in Fig. 1(b). The combs arise from the cascaded FWM effect of the dual Brillouin lasers stimulated by the dual-pump seeds. Note that sufficient powers are required for both the frequency shifter and Comb1 generation. To avoid introducing an additional EDFA, EDFA1 is not placed just before PC1 but connected with a 30/70 OC to divide the output power, which is cost-saving. The polarization controllers (PC1 and PC2) are used to regulate the polarization of the dual-pump seeds and avoid synchronous resonance of cavity modes with different polarizations. The dual combs emit from the 10% ports of OC5 and OC6, respectively. FBG2 and FBG3 are used to optimize the flatness of the dual-comb spectra. Note that multiple cavity modes in Brillouin gain bandwidth ($\sim 20\mathrm{MHz}$) can resonate simultaneously in a long fiber loop, and each comb line is endowed with the multimode feature in the FWM process [45]. Thus, to enlarge the cavity FSR and ensure single-mode comb lines, the length of each fiber loop is shortened to 24 m. Meanwhile, the fibers for Brillouin gain and all the tail fibers of the CIRs and OCs in Loop3 and Loop4 are made of SMF1 with a zero-dispersion wavelength of 1535 nm and a dispersion slope of $0.075\mathrm{ps}/({\mathrm{nm}}^2\mathrm{km})$. Consequently, the excessive insertion loss between different types of fibers caused by mode field mismatch is avoided and the roundtrip transmission losses in Loop3 and Loop4 are less than 1.5 dB.

Two pairs of dual-pump seeds with an interval offset are produced based on the setup in Fig. 1(a). First, a 20-GHz-spaced EO comb centered at 1551.1 nm is generated, and the $\pm 2$nd modulation sidebands are selected as the 80-GHz-spaced DP1. Next, DP1 is boosted by EDFA1 and injected into the frequency shifter. The two peaks of DP1 are separated by FBG1 with a center wavelength of 1550 nm, a bandwidth of 2 nm, and a reflectivity of 99%, and then stimulate the Brillouin lasers in Loop1 (Fiber1: HNLF2) and Loop2 (Fiber2: DCF) to produce DP2. When the power of DP1 coupled into the frequency shifter increases to 23.5 dBm, DP2 is generated. The optical spectra of the 20-GHz-spaced EO comb, DP1, and DP2 are plotted in Fig. 2(a). The interval offset $\delta$ between the two pairs of dual-pump seeds can be extracted from their beating RF spectrum. As plotted in Fig. 2(b), the beating signals illustrate that the BFS of the short wavelength mode in DP1 is 9.650 GHz in Loop1 and the BFS of the long wavelength mode in DP1 is 9.701 GHz in Loop2, which results in a dual-pump interval offset $\delta$ of 51 MHz.

The dual combs are generated in Brillouin fiber loops based on the setup in Fig. 1(b). Due to the nonreciprocity of CIR, DP1/DP2 does not experience any resonance condition in the clockwise direction of Loop3/Loop4, whereas the first order Brillouin lasers and their FWM effect are enhanced via the counterclockwise resonance. Note that a broadband and coherent Brillouin comb is limited when the dual Brillouin lasers operate in multiple longitudinal modes, which is related to the pump power. When the power of DP1 coupled into Loop3 is boosted to 21 dBm (70% power of EDFA1), dual Brillouin lasers are produced, simultaneously giving rise to Comb1 via the FWM effect. The optical spectra of Comb1 at the DP1 power of 27–30 dBm are plotted in Fig. 3(a) with the number of comb lines ($\mathrm{OSNR}>10\mathrm{dB}$) increasing from 14 to 56. When the DP1 power grows to 30 dBm, symmetric fluctuations on the spectrum profile and enhanced noise floor can be evidently observed, which is induced by the modulation instability [34].

The effects of pump power on the mode features of comb lines are investigated and observed in the RF domain. As the power of DP1 increases from 27 to 30 dBm, the short wavelength mode in DP1 marked by a rhombus in the inset of Fig. 3(a) is combined with the corresponding Brillouin laser in Comb1 marked by a triangle and detected by a 50-GHz photodetector (PD), and the beating signals shown in Fig. 3(b) are measured by an electrical spectrum analyzer at a resolution bandwidth (RBW) of 100 kHz. The center frequency of the RF spectra is 10.616 GHz, which corresponds to the BFS in Loop3. The spectra with only one peak at the DP1 powers of 27 and 28 dBm indicate that the Brillouin laser and all the newly generated comb lines are single-mode lasers. Since the linewidth of the pump laser ($\sim 12.6\mathrm{kHz}$) is considerably less than the typical Brillouin-gain bandwidth (10–20 MHz), the 24-m Loop3 with an FSR of 8.6 MHz allows single-mode operating of Brillouin laser. However, when the pump power is boosted to 29 dBm, the RF spectrum has three peaks with an interval of 8.6 MHz corresponding to the FSR of Loop3, which is the contribution of the multiple modes of the Brillouin laser. As the pump power increases to 30 dBm, more peaks are observed, and the signal quality deteriorates due to the increasing parametric noise induced by modulation instability. The multiple modes of the Brillouin laser might result from the degenerate FWM effect, which is similar to the generation of comb lines with a spacing equal to the cavity FSR in a microresonator under a single pump [33]. Although a broadband comb can be obtained with strong pumping, controlling the pump power is necessary to avoid multiple modes of the Brillouin laser and suppress modulation instability. Consequently, to obtain a balance between spectral width and single-mode operation of the comb lines, the output power of EDFA1 is set to 29.5 dBm, for 70% power ($\sim 28\mathrm{dBm}$) driving Comb1, and 30% power ($\sim 24\mathrm{dBm}$) driving the frequency shifter, and the output power of EDFA2 is set to 28 dBm. Note that this power setting is used in the entire work.

Typically, the powers of comb lines decrease with the line order because of the inherent properties of the Brillouin comb generated from the FWM process [34–37]. As shown in the upper blue curve in Fig. 4(a) with the optical spectrum of Comb1 before FBG2, the central two Brillouin lasers are nearly 15 dB stronger than other comb lines. To optimize the flatness, the central lines of Comb1 and Comb2 are suppressed by FBG2 and FBG3 with the central wavelength, bandwidth, and reflectivity of 1551.2 nm, 2 nm, and 99%, respectively. The dual-comb spectra after FBGs are plotted in the lower region of Fig. 4(a). Each comb provides 22 lines with OSNRs over 10 dB and 10 lines in a 20-dB power deviation with high OSNRs exceeding 50 dB. The spectral profiles are sunken in the center due to the excessive reflectivity of FBGs. The spectral flatness can be further optimized by utilizing FBGs with specially designed reflectance spectra matching the spectral profile of the combs.

The frequency shifter not only gives rise to DP2 with an interval of 51 MHz greater than DP1 but also introduces a relative frequency offset between the dual combs based on the Brillouin effect, which ensures an unambiguous down-conversion in the RF domain after dual-comb beating. The beating signals of the dual combs are detected by a 50-GHz PD and shown in Fig. 4(b) with 10 beat notes separated at a spacing of 48 MHz corresponding to $\mathrm{\Delta }f$ of the dual combs. Note that the repetition frequency difference $\mathrm{\Delta }f$ is not strictly equal to the interval offset $\delta$ between the two pairs of dual-pump seeds. The frequencies of the central dual Brillouin lasers of each comb are redshifted relative to the corresponding dual-pump lasers via the SBS effect in the same fiber, but BFS is a function of pump wavelength [44]. In addition, the intracavity Brillouin laser tends to lie between the Brillouin gain center and the neighboring cold cavity mode due to the mode-pulling effect [46,47], which also contributes to the slight deviation between $\mathrm{\Delta }f$ and $\delta$. In the RF domain, only 10 pairs of lines can be detected synchronously due to the limited sensitivity and saturation power of the PD. The unambiguous beating signals at an RBW of 200 kHz prove that every comb line of the dual combs is a single-mode laser. Moreover, the left third beat note (arrow marked) is measured in a span of 4 kHz, and it is still resolution-limited at an RBW of 300 Hz as shown in Fig. 4(c).

Other beating signals have almost the same narrow linewidth of less than 300 Hz. As a result, a dual-comb source with $f_{\mathrm{rep}}$ of 80 GHz and $\mathrm{\Delta }f$ of 48 MHz is realized successfully based on intracavity Brillouin lasers. As added benefits, the Brillouin lasers avoid the amplified spontaneous emission noise from EDFA, which ensures the combs with ultrahigh OSNRs. For a dual-comb source, the stability of the relative frequency between the dual combs is important but a complex locking system is required [8,11], which has not been implemented in this work. The frequency of the free-running CW laser in the dual-comb system drifts over time inevitably, which induces frequency jitters to the dual-pump seeds and the Brillouin combs. In future efforts, the CW laser will be locked to a high-finesse, ultra-stable Fabry–Perot cavity to decrease the frequency jitters [48], and servo controlling for the frequency offset of the two pairs of dual-pump seeds [49] can be used to enhance the coherence of the dual combs.

Next, time-domain pulses of the dual combs are measured by an optical pulse analyzer that employs the frequency resolved optical gating technology. As shown in Fig. 5, the autocorrelation trace of Comb1 before FBG2 (blue dashed curve) features approximately a sine wave with a period of 12.5 ps, corresponding to the comb line spacing of 80 GHz. Because the intensity ratios between the central dual Brillouin lasers and the newly generated frequency lines exceed 15 dB, the central dual Brillouin lasers dominate almost all the optical power of the comb. The time-domain pulse of the Brillouin comb has a relatively large duration, which is similar to that of dual-frequency lasers [50]. To realize spectral reshaping and pulse narrowing via a simple approach, FBGs are induced to suppress the central dual Brillouin lasers. The flatness of the spectrum of each comb is improved after spectral reshaping, and the autocorrelation traces of Comb1 (top solid curve) and Comb2 (bottom solid curve) after the FBGs are depicted in Fig. 5 with significantly narrowed pulses grown with low-power pedestals. When the pedestals are unconsidered, the calculated pulse widths are 645 fs for Comb1 and 648 fs for Comb2 at a Gaussian fitting. The pedestals are attributed to the uncompensated quadratic and higher-order frequency chirps. Although the Brillouin comb generated from the FWM process features a sinusoidal pulse, the self-phase modulation (SPM) cannot be ignored. Due to the SPM effect, the peak part of the sinusoid produces a positive linear frequency chirp whereas the part near the half-maximum induces quadratic and higher-order frequency chirps. Simultaneously, a negative linear frequency chirp is added to the pulse when it propagates in the fiber loop comprising SMF1 with anomalous dispersion, which only compensates for the linear chirp induced by the SPM. After passing the FBG, the spectral flatness of the comb is optimized, and the pulse width is compressed. However, the quadratic and higher-order frequency chirps are still in the pulse, which induces the parasitic pedestals. In future attempts, pulse shaping components outside the Brillouin loops will be used to carve the nonlinear frequency chirp and eliminate the parasitic pedestals, such as introducing nonlinear optical/amplifying loop mirrors to carve the pulse [51].

Furthermore, the absolute linewidths of the comb lines are measured by the delayed self-heterodyne technique using a 40-km delay fiber and a 200-MHz acousto-optic modulator. Given the broadening effect of the $1/f$ noise with a Gaussian curve near the center of the delay self-heterodyne beating RF spectrum, the linewidth will be overestimated by fitting the 3-dB width with a Lorentzian function. However, more precise 3-dB linewidth can be extracted from the 20-dB width that is dominated by the Lorentzian contribution, which is equal to $1/(2\sqrt{99})$ of the 20-dB width [52]. The orders of the central dual Brillouin lasers at short and long wavelengths are defined as $N=-1$ and $N=1$, respectively, and the absolute value of $N$ for the newly generated comb lines increases sequentially. The linewidths of the central 10 lines in each comb are extracted from the 20-dB width of the delay self-heterodyne beating spectra. As depicted in Fig. 6(a), the linewidths of comb lines from $N=-5$ to $N=5$ are all around 1.5 kHz, and the fluctuation of the linewidth is caused by errors in measurement and fitting. The delay self-heterodyne beating RF spectra of the comb lines with $N=-5\mathrm{in}$ Comb1 and $N=-1\mathrm{in}$ Comb2 are shown in Figs. 6(b) and 6(c) with extracted linewidths of 1.54 kHz and 1.38 kHz, respectively. Typically, for an EO comb, the linewidth of comb lines increases linearly with the line order number due to the phase noise accumulation of the RF source [53]. In our experiment, the 20-GHz RF signal features a relatively low phase noise of $-100\mathrm{dBc}/\mathrm{Hz}$ at 10-kHz frequency offset. The RF phase noise accumulation on the $\pm 2$nd modulation sidebands (DP1) is unremarkable, and the linewidths of DP1 are similar to that of the CW laser. The delay self-heterodyne beating RF spectra of the CW laser and the short wavelength mode of DP1 are plotted in Fig. 6(d), indicating almost equal linewidths to the extracted 3-dB linewidth of 12.6 kHz. Note that the phase noise of DP1 is not directly added to the Brillouin comb lines, because the combs arise from the FWM process of secondary dual pumps (the dual Brillouin lasers), which further weakens the effect of the RF phase noise on the noise performance of Brillouin combs. The experimental results illustrate that the linewidths of the dual Brillouin lasers ($N=\pm 1$) are compressed by more than eight times compared to that of DP1. Besides, for an FWM-generated comb, the linewidth broadening versus the line order number can be almost eliminated by using dual-pump lasers with correlated phases [28]. Each Brillouin comb has dual Brillouin lasers generated in the same fiber loop, which ensures correlated phases, and their linewidth features are transmitted to the comb teeth generated from the FWM process.

Finally, the reconfigurability of the dual-comb source is investigated by changing the interval offset between DP1 and DP2. The interval of DP1 is fixed at 80 GHz, and switchable DP2 is realized by changing the fiber pairs in the frequency shifter. Note that every fiber provides a single-peak Brillouin gain spectrum to enable robust operation of the single-mode Brillouin laser. When Fiber1/Fiber2 is set to SMF1/SMF2, HNLF1/HNLF2, DCF/SMF1, and HNLF1/SMF2, the beating RF spectra of the two pairs of dual-pump seeds are shown in Fig. 7(a), corresponding to the interval offset $\delta$ of 202, 323, 916, and 1486 MHz, respectively. When the mode pulling effect and the dependence of BFS on wavelength are not considered, different fiber pairs in the frequency shifter can be used to realize dual combs with reconfigurable $\mathrm{\Delta }f$ approximately from 50 to 1490 MHz. The reconfigurability is confirmed by investigating a dual-comb source with $\mathrm{\Delta }f$ of 322 MHz with Fiber1/Fiber2 set to HNLF1/HNLF2. The optical spectra of the dual combs and the corresponding beating RF spectrum are shown in Figs. 7(b) and 7(c), respectively. As expected, an 80-GHz-spaced dual-comb source with 10 pairs of lines in a 20-dB power deviation and a 322-MHz-spaced RF comb are obtained successfully. Based on the same principle and process, various $\mathrm{\Delta }f$ can be realized by assembling the frequency shifter with different Fiber1 and Fiber2. Practically, as $\mathrm{\Delta }f$ increases, the $f_{\mathrm{rep}}$ of the dual combs should be further enlarged to maintain a relatively high spectral compression factor of $m=f_{\mathrm{rep}}/\mathrm{\Delta }f$, which is desirable in high-speed acquisition applications [25]. In the proposed scheme, $f_{\mathrm{rep}}$ can be easily enlarged to much higher than the bandwidth of the RF source and the PM modulator by selecting two higher-order modulation sidebands. In addition, the dual combs with a high $\mathrm{\Delta }f$ over 1 GHz are not suitable for high-speed acquisition applications but are potential in broadband photonic RF channelization [54,55].

In summary, we have demonstrated an 80-GHz-spaced dual-comb source with nonlinear Brillouin fiber loops, featuring a reconfigurable repetition frequency difference ranging from 48 MHz to 1.486 GHz. A dual-pump seed with a frequency interval of 80 GHz is obtained from a 20-GHz EO comb, and it gives rise to another dual-pump seed with an interval offset via the SBS effect in two different fiber loops. Each dual-pump seed with the power of 28 dBm is injected into a 24-m fiber loop with a round-trip loss of 1.5 dB to boost the corresponding dual Brillouin lasers and then produce a comb via the FWM effect. Consequently, each comb achieves 22 single-mode comb lines with OSNR exceeding 10 dB and a pulse width of approximately 650 fs. The RF signals derived from the dual-comb beating feature narrow linewidths of less than 300 Hz, and the comb lines feature absolute linewidths of around 1.5 kHz. The dual Brillouin combs feature repetition frequencies much higher than those of dual electro-optic combs and inherit the tunability of repetition frequency from the electro-optic scheme. Moreover, specific repetition frequency differences ranging from 48 MHz to 1.486 GHz are switchable by changing the pluggable fibers in the frequency shifter. Although the repetition frequency difference is less flexible than that of the EO modulation scheme, it avoids the need for excessive EO modulators and microwave sources. The feasibility of the dual-comb source based on the dual pump scheme has been experimentally validated. In prospect, more efforts would be used to improve the performance of the dual combs and enhance the potentiality in various applications, such as terahertz frequency metrology, ranging, and broadband photonic RF processing. Particularly, a nonlinear spectral broadening system can be connected to the output port of each Brillouin loop to broaden the comb bandwidth [35]. And the repetition frequency difference can be fine-tuned within several MHz by controlling the Brillouin frequency shift based on its temperature [56] or stress features [57].