Stable laser emission with ultra-narrow linewidth plays an important role in making fundamental scientific breakthroughs. Here, we propose and demonstrate a new technique for the generation of an ultra-narrow linewidth and highly stable laser based on stimulated Brillouin scattering in combination with a frequency-shifted optical injection locking mechanism. The laser performance is characterized via a delayed self-heterodyne interference system, where the white frequency noise floor is ∼20 mHz2/Hz, corresponding to a fundamental linewidth of about 63 mHz. The maximum deviation in the output power is less than 1.5% over more than 10 min. The operation of the laser can be stabilized without the need for active optoelectronic feedback. The scheme presented in this work enables narrow linewidth and stable single-frequency fiber lasers in a robust and efficient way, which has shown promising potential for many applications.

Laser sources with a narrow linewidth and high stability are essential in many applications, such as coherent communications,1,2 LIDAR,3 remote sensing,4 and atomic clocks.5,6 In recent years, several techniques have been proposed to narrow the linewidth of lasers. One method to achieve narrow linewidth is to lock the laser diode to an external highly stable reference cavity via the Pound–Drever–Hall (PDH) technique.7–9 The linewidth of a laser can be narrowed to a sub-hertz level based on this method. However, a table-size vacuum chamber and a sophisticated thermal and vibration shield are generally required for these stable reference cavities, significantly increasing the system complexity and making it difficult to operate outside the laboratory.10,11 Self-injection locking (SIL) of a semiconductor distributed feedback (DFB) laser to a high-quality factor (Q factor) optical resonator has been proven to be another effective method for laser linewidth narrowing.12–17 Although it can drastically reduce the laser noise, it is difficult to narrow the linewidth to Hz level as a typical linewidth of the pump laser, i.e., the DFB semiconductor laser, is in the range of a few MHz or above. Moreover, the system based on this technique is susceptible to the system parameters or external environmental effects.

Recently, Brillouin fiber lasers (BFLs) have attracted much attention due to the unique linewidth narrowing effect of stimulated Brillouin scattering (SBS).18–22 The linewidth of BFLs can be 2–3 orders of magnitude narrower than that of pumped ones. Nonetheless, the linewidth-reduction ratio is limited as the BFL is generated by one-order SBS. A high-performance laser with linewidth at Hz level imposes a stringent requirement on the narrow linewidth of the pump laser. Moreover, any slow change in the pump frequency or the BFL cavity mode frequency caused by the environmental effect makes the generated Stokes laser prone to mode hopping, leading to an unstable operation on a long-time scale.

In this article, we demonstrate a simple and reliable approach to achieve a single-longitudinal-mode (SLM) BFL with an ultra-narrow linewidth and high stability. The operation of the laser is stabilized without the need for active optoelectronic feedback. The approach is based on a combination of stimulated Brillouin scattering and frequency-shifted optical injection locking (FSOIL). The FSOIL is employed to ensure a stable locking of the pump frequency to the Stokes wave at a frequency offset corresponding to the maximum SBS gain. Experimental results show that the FSOIL-based BFL exhibits a low white frequency noise floor of ∼20 mHz2/Hz, which is 60 dB lower than the free-running DFB pump laser. Power stability is significantly improved. Compared to the BFL without FSOIL, the maximum power fluctuation is less than 1.5% over more than 10 min. The results present a new technique to realize a single-frequency laser with an ultra-narrow linewidth and high stability, which can be promising for many practical applications.

Figure 1 illustrates the principle of our proposed FSOIL-based BFL. For the system without FSOIL, a pump laser excites SBS in the Brillouin gain fiber (BGF), and the Stokes wave, which is shown in the first stage of Fig. 1(a), is generated in the opposite direction of the pump laser. Because of the SBS effect, the generated Stokes laser beam shows an apparent spectral narrowing effect relative to the pump. However, the linewidth reduction ratio is limited and mainly determined by combining the BFL cavity loss and length. Moreover, as the pump frequency drift or the BFL cavity length slowly varied due to the environmental effect, the generated Stokes laser (first stage) is prone to mode hopping, resulting in an unstable operation on a long-time scale. When the FSOIL is applied to the BFL, the generated Stokes laser (first stage) with narrowed linewidth is frequency-shifted by an amount approximately equal to the Brillouin frequency shift and then injection-locked to the pump laser. In this case, the frequency characteristics of the generated Stokes laser are well transferred over to the pump laser due to injection locking. As a result, a narrowing of pump laser linewidth is provided, and the second stage Brillouin laser with further narrowed linewidth is generated. Similarly, the Nth stage Stokes laser linewidth characteristic will be retransferred to the pump again via the FSOIL mechanism, compressing the pump laser linewidth and then generating the Stokes laser again. Eventually, the linewidth of both the pump laser and the Stokes laser will tend to its limitation subject to the quantum noise in the BFL cavity. Figure 1(b) depicts the cyclic process of our proposed scheme, emphasizing the iterative nature of the injection-locking process in a single BFL cavity. Meanwhile, with additional FSOIL, the frequency position of the oscillating Stokes wave to the SBS gain maximum is always stably locked, as shown in Fig. 1(c). Thus, mode hopping of the Stokes laser is successfully suppressed, and a stable operation on a long-time scale can be achieved.

FIG. 1.

Principle of the Brillouin laser. (a) Cascaded stimulated Brillouin scattering. (b) Cyclic process of the proposed scheme. (c) Frequency locking between BFL longitudinal mode and pump.

FIG. 1.

Principle of the Brillouin laser. (a) Cascaded stimulated Brillouin scattering. (b) Cyclic process of the proposed scheme. (c) Frequency locking between BFL longitudinal mode and pump.

Close modal

The experimental setup of the Brillouin laser generation and Brillouin laser frequency noise measurement system is illustrated in Fig. 2. The setup comprises four main parts: a primary pump source, a BFL cavity, a FSOIL feedback loop, and a delayed self-heterodyne interference (DSHI) system. The BFL is pumped with the primary source that consists of a CW laser boosted by an erbium-doped fiber amplifier (EDFA) with the output power of 200 mw. The BFL cavity with a length of 21 m is composed of a segment of highly nonlinear fiber (HNLF) and SMF-28 fiber. The HNLF functions as an amplification medium to provide the Brillouin gain. A polarization controller (PC1) before the HNLF is used to achieve maximum Brillouin gain in the HNLF. An isolator is inserted into the cavity to ensure the running of the laser in a clockwise direction. Thus, high-order Brillouin lasing can be effectively prevented from cascading in the BFL cavity. The output of the Brillouin laser is split into two beams via a 5/95 optical coupler (OC2). The 95% is fed back into the cavity, and the 5% is separated into two portions by a 10/90 optical coupler (OC3). The 90% is injected into a 70/30 fiber coupler (OC4). The smaller portion of the OC4 is sent to the DSHI system. The remaining portion is passed through an electro-optic modulator (EOM) for shifting the frequency by 9.40GHz, which corresponds to the Brillouin frequency shift. Subsequently, the frequency shifted laser signal with the power of about −25 dBm at the pump wavelength is injected into the pump laser via the CIR1 from port1 to port2 to form a feedback loop. Subsequently, the frequency shifted laser signal is injected into the pump laser via the CIR1 from port1 to port2 to form a feedback loop. The pump beam is launched into the BFL cavity utilizing another fiber circulator (CIR2), which allows the Stokes wave to propagate in a clockwise direction while blocking the pump beam in a counterclockwise direction after one single turn. The DSHI system is used to characterize the linewidth and frequency noise of the pump laser and the generated Brillouin laser in our proposed system. The laser is divided into two beams via a 50/50 fiber coupler (OC5). One beam is delayed by a length of single-mode fiber (SMF). The other beam is frequency-shifted by 40 MHz using an acousto-optic modulator (AOM). The two signals are recombined by a 50/50 fiber coupler (OC6), and the beat signal at 40 MHz is detected by a photodiode (PD) and analyzed by an electrical spectrum analyzer (ESA).

FIG. 2.

Experimental setup of the BFL generation and DSHI system. DFB, distributed feedback laser; OC, optical coupler; CIR, optical circulator; EDFA, erbium-doped fiber amplifier; PC, polarization controller; HNLF, highly nonlinear fiber; ISO, optical isolator; RF, radio frequency; DC, direct current; EOM, electro-optical modulator; AOM, acousto-optic modulator; DF, delay fiber; PD, photodiode; and ESA, electrical spectrum analyzer. Here, the green, red, and yellow arrows denote the pump, the BFL, and the frequency shifted BFL, respectively.

FIG. 2.

Experimental setup of the BFL generation and DSHI system. DFB, distributed feedback laser; OC, optical coupler; CIR, optical circulator; EDFA, erbium-doped fiber amplifier; PC, polarization controller; HNLF, highly nonlinear fiber; ISO, optical isolator; RF, radio frequency; DC, direct current; EOM, electro-optical modulator; AOM, acousto-optic modulator; DF, delay fiber; PD, photodiode; and ESA, electrical spectrum analyzer. Here, the green, red, and yellow arrows denote the pump, the BFL, and the frequency shifted BFL, respectively.

Close modal
Figure 3(a) shows the dependence of the BFL laser power (at port a in Fig. 2) as a function of Brillouin pump power (at port B in Fig. 2). It can be clearly observed that the output power of the generated Brillouin laser is proportional to the pump power as expected and there existed a power threshold at ∼115 mw, below which no Brillouin power can be detected. Figure 3(b) shows the recorded optical spectra of the FSOIL-based BFL (at port A in Fig. 2) and the Brillouin pump (at port B in Fig. 2). The BFL is redshifted by a wavelength spacing of ∼0.076 nm relative to the pump, corresponding to a Brillouin frequency shift of ∼9.40 GHz in the HNLF. We use the DSHI system to characterize the linewidth performance of the free-running DFB and the generated Brillouin laser without FSOIL, as shown in Fig. 3(c). For the free-running DFB laser, the delay fiber length is 5000 m. The beat spectrum is shown as a blue curve, and its Lorentz fitting overlaid as a red curve characterizes a 3 dB width of 800 kHz, indicating that the spectral linewidth of the DFB is 400 kHz.23,24 For the generated Brillouin laser (without FSOIL) with linewidth at the kHz level, the coherence length is quite long, and the traditional demodulation method based on DSHI does not apply here. This is mainly because of the Gaussian noise introduced by the excessively long delay, leading to an incorrect estimation of the actual laser linewidth. To address this limitation, a short fiber delay (1006 m) and strong coherent envelope demodulation are used.25,26 The laser linewidth (Δf) is related to the difference in power between the second peak and the second trough of the strongly coherent envelope (ΔS) as follows:
ΔS=12.82Δf+38470Δf+1685.
(1)
FIG. 3.

(a) Brillouin laser-output power as a function of pump power. (b) Optical spectrum of the pump laser and the Brillouin laser. (c) Optical self-heterodyne beat frequency spectrum of DFB without FSOIL. (d) Partial coherent self-heterodyne spectrum of the BFL without FSOIL.

FIG. 3.

(a) Brillouin laser-output power as a function of pump power. (b) Optical spectrum of the pump laser and the Brillouin laser. (c) Optical self-heterodyne beat frequency spectrum of DFB without FSOIL. (d) Partial coherent self-heterodyne spectrum of the BFL without FSOIL.

Close modal

As shown in Fig. 3(d), S = 17.1 dB. By bringing S into Eq. (1), the laser linewidth Δf can be calculated to be ∼2300 Hz.

The frequency noise of the lasers was measured27 by using the DSHI system part of Fig. 2. Specifically, the laser frequency noise Lvf can be expressed as
Lvf=LRFf+20logf2sinπfτ0,
(2)
where LRFf is the measured RF signal phase noise and τ0 is the delay between the two arms. From Eq. (2), we can obtain the frequency noise of the laser from the measured RF signal phase noise acquired by the ESA. The use of different delays allows us to trade off measurement sensitivity and measurement range. Here, we use the delay fiber length of 50 m for characterizing the ECDL, DFB, and BFL without FSOIL. In the case of the BFL with FSOIL and the BFL pumped by the ECDL, a longer delay fiber length of 1006 m is employed. The frequency noise of the Brillouin laser without FSOIL is decreased by around 22 dB relative to that of the free-running pump (DFB), as shown in Fig. 4(a). When the BFL has an additional FSOIL feedback loop, there is a significant improvement in the frequency noise performance because of the cascading narrowing mechanism involved in the system. The white noise floor is suppressed by 60 dB compared with that of the DFB pump, and it is approaching ∼20 mHz2/Hz corresponding to a fundamental linewidth of ∼63 mHz.28 The cascade mechanism in the FSOIL-based BFL system may play an essential role in the narrowing process. The mechanism is verified by comparing the frequency noise of Brillouin lasers created by a commercial external cavity diode laser (ECDL, TOPTICA CTL 1550) and the FSOIL-based BFL. The frequency noise of the ECDL and its first stage Brillouin laser are shown for reference in Fig. 4(b). For a better comparison of the frequency noise, we use a 70/30 OC instead of 5/95 OC in the BFL cavity, thus the higher frequency noise for the first stage Brillouin laser. It can be observed that although the ECDL exhibits a lower white noise floor compared to the Brillouin laser at the first stage without FSOIL, the white frequency noise floor corresponding to the Brillouin laser created by the ECDL is higher than that of the FSOIL-based BFL. The findings suggest that the FSOIL-based BFL experiences more than twice the Brillouin narrowing effect, thus demonstrating the cascading narrowing mechanism in the FSOIL-based BFL system. For the frequency noise at a low offset frequency (<2 kHz), the Brillouin laser created by ECDL almost overlaps with the FSOIL-based BFL. This is mainly because the external impacts may exceed the limit of this frequency noise-measuring technology.
FIG. 4.

(a) Frequency noise of the pump and Brillouin lasers with or without FSOIL (OC2: 5/95 or 70/30). (b) Frequency noise of the ECDL laser and the Brillouin laser with or without FSOIL.

FIG. 4.

(a) Frequency noise of the pump and Brillouin lasers with or without FSOIL (OC2: 5/95 or 70/30). (b) Frequency noise of the ECDL laser and the Brillouin laser with or without FSOIL.

Close modal

To evaluate how FSOIL improves power stability in BFL, the output power of the BFL with or without FSOIL is recorded with a power meter. The results are shown in Fig. 5. For the BFL without FSOIL, the BFL cavity mode associated with the oscillating Stokes wave is not locked to the SBS gain maximum. Driven by an environmental effect, the BFL cavity length, the cavity mode frequency and the pump frequency slowly (and almost independently) drift in time, forcing the output power of the BFL to walk between its minimal and maximal values. As shown in Fig. 5(a), power variation is up to 15% due to the varied detuning of the oscillating Stokes wave with respect to the SBS gain maximum. From Figs. 5(b) and 5(c), the beat frequency difference corresponding to the maximum and minimum laser power is approximately equal to the frequency spacing between adjacent longitudinal modes, which indicates that the power jump is caused by mode hopping. Note that the power variation is related to the cavity length/mode spacing and it can be reduced by using a longer cavity length (or a smaller mode spacing), but the increased cavity length will lead to stronger mode-hopping to the system. When the FSOIL is engaged, it can be observed from the yellow curve in Fig. 5(a) that the optical power trace is almost flat and exhibits nearly no fluctuations within more than 10 min. The maximum deviation in the output power is less than 1.5%, indicating a greatly improved power stability. Such a high performance can be attributed to the stable frequency-position locking between the generated Brillouin laser and the Brillouin pump, which is verified by the almost overlapped three beat frequency signals at different times, as shown in Fig. 5(e).

FIG. 5.

(a) Characterization of the power stability. (b) Zoom-in views of the gray-dashed box region in (a). (c) Beat signals corresponding to the maximum and minimum BFL power, respectively. (d) Zoom-in views of the red-dashed box region in (a). (e) Recorded beat signals between the generated Brillouin laser and the Brillouin pump at three different times.

FIG. 5.

(a) Characterization of the power stability. (b) Zoom-in views of the gray-dashed box region in (a). (c) Beat signals corresponding to the maximum and minimum BFL power, respectively. (d) Zoom-in views of the red-dashed box region in (a). (e) Recorded beat signals between the generated Brillouin laser and the Brillouin pump at three different times.

Close modal

The BFL, in combination with the FSOIL mechanism, works against external perturbations, enabling robust laser operation. Two optical switches (OS) are placed in the regions of a and b in Fig. 2, respectively. To verify the laser robustness, we deliberately introduce disturbances to the system by periodically switching on and off every 0.5 s the BFL pump and the FSOIL feedback loop, respectively. The results in Figs. 6(a)6(d) show the system response to external disturbances. The laser output power dropped to zero and then reliably recovers to the original level. Such a behavior indicates that the stable locked state consistently reappears even after substantial system disruptions induced by external perturbations.

FIG. 6.

BFL system response to external perturbations by switching on and off (a) the pump power and (b) the FSOIL feedback loop, respectively. (c) Zoom-in views of the gray-dashed box region in (a). (d) Zoom-in views of the gray-dashed box region in (b).

FIG. 6.

BFL system response to external perturbations by switching on and off (a) the pump power and (b) the FSOIL feedback loop, respectively. (c) Zoom-in views of the gray-dashed box region in (a). (d) Zoom-in views of the gray-dashed box region in (b).

Close modal

In summary, an ultra-narrow linewidth Brillouin laser with highly stable SLM operation has been demonstrated. The white frequency noise floor was ∼20 mHz2/Hz, corresponding to a fundamental linewidth of ∼63 mHz. The proposed FSOIL-based BFL possesses high stability, and the output power fluctuation variation is less than 1.5% in long-term observations. With a superior performance in terms of linewidth, stability, and robustness of the laser output, the proposed FSOIL-based BFL will have a wide range of applications in highly coherent fields.

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (62075150), the Natural Science Research Project of Jiangsu Higher Education Institutions of China (21KJA510002), Gusu Youth Leading Talent (ZXL2022491), and Startup Funding of Soochow University (NH15900123).

The authors have no conflicts to disclose.

Mingzhao Chen and Yin Xu contributed equally to this paper.

Mingzhao Chen: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Writing – original draft (equal). Yin Xu: Formal analysis (equal); Writing – review & editing (equal). Zhexin Zhang: Data curation (equal); Formal analysis (equal); Writing – original draft (equal); Writing – review & editing (equal). Xiaojie Luo: Data curation (equal); Writing – review & editing (equal). Hualong Bao: Conceptualization (equal); Formal analysis (equal); Funding acquisition (lead); Project administration (lead); Supervision (lead); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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