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Abstract
We suggest and demonstrate a single-frequency fiber ring laser with an ultra-narrow linewidth based on an external weak distributed feedback. A π phase-shifted fiber Bragg grating (PSFBG) is used to improve mode selection and enable single-longitudinal mode (SLM) laser operation. The linewidth is then further strongly compressed using a signal generated by a weak distributed feedback structure (WDFS) and injected into the main laser cavity to suppress spontaneous emission. The resulting ultra-narrow linewidth fiber ring laser achieves a side-mode suppression ratio (SMSR) of ∼72 dB, and low white frequency noise of ∼10.3 Hz2/Hz, which correspond to an instantaneous linewidth of ∼32.3 Hz in the normal operating condition of the laser. Our linewidth compression mechanism not only solves the problems associated with deep linewidth compression in long-cavity fiber laser, but also fosters the development of practical and reliable all-fiber structures. Our laser source is characterized by low cost, high coherence, and low noise, which are highly desirable features in coherent optical detection, high-resolution spectrometers, microwave photonics, and optical sensing.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultra-narrow linewidth fiber lasers are usually employed as the critical light source to promote scientific and technological advancement. Also, their applications are relevant in many fields, such as gravitational wave detection, coherent communication, and precision measurement [1–6]. From a structural point of view, the most relevant feature of fiber lasers is the absence of an optical mirror in the resonator. The cavity mirror may be directly fabricated on the cross-section of the fiber, or the resonator can be formed by a fiber coupler. Compared to traditional lasers, they are adjustment-and maintenance-free and show higher stability. Currently available fiber lasers may be divided in four main categories according to the type of cavity: ring cavity [7,8], linear cavity [9,10], distributed feedback cavity [11,12] and distributed reflection cavity [13,14]. The ring cavity structure is used to impose the unidirectional operation of the optical signal in the resonator and avoid the burning of spatial holes caused by the standing-wave effect. However, fiber ring lasers with long resonators generate several closely spaced longitudinal modes and easily induce free mode-hopping, causing coherent laser degradation. Moreover, the presence of homogeneous broadening effect in rare-earth-doped gain fibers usually causes intense mode competition and thus, unstable laser output. Currently, various effective methods have been developed to achieve stable single longitudinal mode (SLM) fiber laser operations. Examples include employing a compound-ring in a resonator to increase the free spectral region (FSR) of the cavity longitudinal modes [15,16], attaching a saturable absorber in a Sagnac loop to form a narrow-band filtering effect [17,18], or inserting a π phase-shift fiber Bragg grating (PSFBG) in a laser cavity to confine the longitudinal mode region [19,20]. Nonetheless, these SLM techniques are not sufficient to guarantee an ultra-narrow linewidth fiber laser output. Therefore, to satisfy the demand for high coherence light source, it is essential to explore alternative mechanisms that can guarantee a deeper linewidth compression in fiber lasers.
Several techniques have recently been used to significantly improve the linewidth of existing SLM fiber lasers. The linewidth of a laser can be narrowed to the order of kilohertz by an external cavity self-injection with a fixed length [21,22]. Additional resonant modes might be induced by such an external cavity self-injection, which is also bound to be affected by external vibration and/or thermal disturbances. Due to these factors, the linewidth cannot be narrowed to a theoretical limit, which also restricts the improvement of the coherence. In addition, a precise tuning of the external cavity is required to match the changes of the resonant wavelength of the laser.
To be more specific, extended-cavity fiber links optical feedback can compress fiber lasers linewidth to the order of 100 Hz [14]. However, this mechanism involves a larger bulk of optical loops that inevitably introduce a larger amount of environmental thermal noise, which reduces the overall laser system robustness. Self-injection locking generated by a high-quality factor resonator provides a mean to optimize external cavity fiber lasers and compress the linewidth of single longitudinal mode fiber lasers [10–24]. However, high finesse resonators require complex fabrication processes and cannot automatically track the wavelength of the fiber master laser. Although we can obtain ultra-narrow linewidth lasers with extreme environmental control, this method is expensive and not suitable for applications [25]. Moreover, the realization of laser linewidth compression by Rayleigh scattering has been reported in our previous work [26,27]. These schemes put several kilometers of single-mode fiber (SMF-28e) into the laser cavity to provide the Rayleigh scattering signal required for linewidth narrowing. Therefore, these laser systems are complicated and susceptible to interference from the external environment. Hence, we still need to circumvent the challenges associated with the quest to compress the linewidth of fiber lasers to tens of Hz under normal conditions and at room temperature.
In this paper, we suggest and demonstrate a low-noise all-fiber ultra-narrow linewidth SLM ring laser based on a weak distributed feedback. In our scheme, a π PSFBG in transmission with narrow bandwidth is embedded in the laser cavity to initially realize the fiber laser SLM operation. In this way, the distributed feedback is a result of Rayleigh scattering that occurs for a wide range of wavelengths in the corresponding waveguides, and enables linewidth compression of the laser [28]. It is worth mentioning that a 50 m fiber with high scattering coefficient is used to replace several km-long SMF-28e fiber, which further reduces the probability of stimulated Brillouin scattering (SBS) and reduces the interference of feedback signals by external vibration and temperature. This signal is then injected into the SLM main cavity to further suppress the fiber laser spontaneous emission. The fiber laser linewidth can be greatly compressed, and its noise further reduced. Remarkably, the resulting fiber ring laser has an overall side-mode suppression ratio (SMSR) of more than 70 dB, and an ultra-narrow Lorentzian linewidth of ∼155 Hz. The frequency noise decreases from 3 × 107 Hz2/Hz to 1.5 × 103 Hz2/Hz at frequency offset of 1 kHz, while the high frequency white noise floor is at ∼10 Hz2/Hz level, corresponding to a fundamental linewidth of ∼32.3 Hz. Moreover, the measured relative intensity noise (RIN) is less than −130 dB/Hz at frequencies beyond 3 MHz. Our proposed scheme can be applied to fiber lasers in other wavelength bands, to further improves the performance of fiber lasers in applications.
2. Experimental setup
The schematic diagram of the ultra-narrow linewidth fiber ring laser configuration with an SLM main cavity and a weak distributed feedback structure (WDFS) is shown in Fig. 1. A commercial 980 nm laser diode (LD) with a maximum power of 400 mW is used as the optical pump. The pump signal is reversely transmitted to a 1 m highly doped-erbium fiber (EDF, Er80-8/125, LIEKKI) by a 980/1550 nm wavelength division multiplexer (WDM) to distinguish the generated laser signal from the residual pump light. An isolator (ISO) is used to determine the unidirectional working state of the laser, and to prevent standing wave hole-burning effect in the SLM main cavity, which may affect the stable operation of the laser. A variable-ratio optical coupler (VROC) is inserted into the main cavity to ensure optimal laser operation. A fiber Bragg grating (FBG) fused to the common port of the circulator (C1) serves as mode-limiting and wavelength-selecting element. In addition, a π PSFBG acting as an ultra-narrow band filter is inserted at the output port of the C1. This further reduces the bandwidth of the lasing wavelength to realize SLM operation and improve system stability. An optical coupler 1 (OC1, 80:20) is connected to the input port of C2 to direct the laser light into the WDFS leading to the laser output. Moreover, the cavity length of the main laser is approximately ∼15 m, and the corresponding free spectral region (FSR) is about ∼13.3 MHz. Here, a 50 m high scattering fiber (HSF, UHNA3, THORLABS) is used as WFDS to generate the required weak distributed feedback signal. The coupler OC2 (80:20) is used to receive the main cavity and distributed feedback signals, respectively. The output spectra of the fiber ring laser are measured by an optical spectrum analyzer (OSA, AQ6370D, YOKOGAWA). The frequency spectrum and noise characteristics of the proposed fiber laser are measured using a delayed self-heterodyne interferometry (DSHI) system.
Fig. 1. Schematic diagram of experimental setup of the proposed ultra-linewidth all-fiber ring laser. LD: laser diode; EDF: erbium-doped fiber; WDM: wavelength division multiplexer; ISO: isolator; VROC: variable-ratio optical coupler; OC: optical coupler; FBG: fiber Bragg grating; C: circulator; PSFBG: phase-shift fiber Bragg grating; WDFS: weak distributed feedback structure.
3. Experimental results and discussion
A variable-ratio optical coupler is used in the main cavity to explore and optimize the laser system operation. The behavior of the output power as a function of the coupling ratio with or without weak distributed feedback is shown in Fig. 2(a). As it is apparent from the plot, we see that when the splitting ratio of the coupler reaches the 60%, the laser output power achieves its maximum. In addition, when the laser injected into the WDFS reaches a certain intensity, an effective distributed feedback signal is excited and interact with the main cavity laser signal. The laser output power as a function of the pump power (for the optimal coupling ratio) is shown in Fig. 2(b). Apparently, the laser system can only operate in a stimulated resonance mode only when the pump power is larger than a threshold power of 225 mW. Notice that given the limitations on the pump LD maximum output power, our laser system is still far from saturation. The measured reflection spectrum (blue line) of an ordinary FBG and a transmission spectrum (red line) of an π PSFBG are shown in Fig. 2(c). The ordinary FBG center wavelength is 1549.996 nm, while the maximum reflectivity is approximately 99%. Although the ordinary FBG 3 dB reflection bandwidth is approximately 0.18 nm, it is still relatively larger than the FSR of the main resonator. In addition, the π PSFBG has a bandwidth of approximately 17 pm, with a central wavelength of 1550.08 nm, and a transmissivity larger than 90%. We also notice that the π PSFBG transmission peak is situated within the ordinary FBG reflection peak. It is also close to its right sideband falling edge, thus reducing the filtering bandwidth. Hence, the master laser achieves SLM output if the filter bandwidth is smaller than the longitudinal mode spacing. Figure 2(d) shows the spectral evolution from unstable to stable lasing oscillations when the pump power is larger than the threshold. Here, the measured laser central wavelength with a pump power of 400 mW is 1550.098 nm, which is close to the π PSFBG central wavelength.
Fig. 2. (a) Evolution of laser output power with coupling ratio. (b) Dependence of laser output power on LD pump power. (c) Measured reflection spectrum of the ordinary FBG (black line) and transmission spectrum of the π PSFBG (red line). (d) Optical spectra at the output port of the fiber laser for different pump powers.
In principle, the main ring cavity with the π PSFBG can achieve independent SLM operation in the proposed laser configuration. Hence, it is essential to further explore the frequency spectral performance of the master laser. The radio frequency (RF) spectrum measured by an electrical spectrum analyzer (ESA, FSV30, ROHDE&SCHWARZ) without weak distributed feedback is shown in Fig. 3(a). Here, the SMSR of the main ring cavity is 43 dB, suggesting that the master laser maintains SLM operation. Notwithstanding, there are still some irresolvable oscillating side modes in the spectrum. Hence, to further enhance mode selection effect on the laser system, a weak distributed feedback is introduced into the main cavity to achieve an SLM output with an SMSR of 72 dB and no side mode excitation, as shown in Fig. 3(b). The SMSR of the laser is improved by approximately 29 dB by means of the weak distributed feedback. The experimental results also show that the central mode has core competitiveness in the process of gain competition due to weak distributed feedback. In addition, Pan et al. [29] found that the power of the excitation source also affects the longitudinal mode operation in a fiber laser resonator. Hence, we explored the stable laser spectral evolution output at different pump powers (300-400 mW). Figure 4(a) shows the variation of the RF spectra with the pump power for a narrowband filtering with only PSFBG. In this case, our fiber ring laser achieves SLM operation at different pump powers, but there are still some accompanying side modes. On this basis, a mode selection mechanism (weak distributed feedback) with stronger performance is introduced into the laser system to realize SLM operation without any side mode at different pump powers [30], as shown in Fig. 4(b). The SMSRs of the laser activation cavity with or without weak distributed feedback at different pump powers are shown in Fig. 4(c). This further confirms that the weak distributed feedback significantly improves the signal-to-noise-ratio (SNR) of the laser system.
Fig. 3. The RF spectrum of the proposed fiber laser when the pump power is 400 mW. (a) without weak distributed feedback. (b) with weak distributed feedback.
Fig. 4. Distribution of RF spectral at different pump powers (a) without and (b) with weak distributed feedback. (c) The overall SMSRs as a function of the pump power with and without feedback.
In order to assess the purifying ability of the weak distributed feedback on the fiber ring laser signal we detect the output linewidth by the DSHI system. Figures 5(a) and 5(b) show the RF spectra of DSHI signals with and without weak distributed feedback when the LD pump power is fixed at 350 mW. At this point, it is easy to observe the spectral narrowing since the frequency span of 50 kHz remains the same in both cases. All RF spectra are fitted by Lorentzian curves to calculate the laser output linewidths. The 1/f noise of the entire laser system causes Gaussian broadening at the center frequency of the spectrum. Hence, the laser linewidth is obtained by calculating the bandwidth below the 20 dB peak in the Lorentzian curve to eliminate the noise effect on the measurement accuracy. As a result, the 20 dB Lorentz bandwidth of the free-running laser without weak distributed feedback is estimated to be 28.9 kHz, and the corresponding 3-dB laser linewidth is 1.45 kHz, which is illustrated in Fig. 5(a). Similarly, we evaluate the Lorentz linewidth of the laser with weak distributed feedback which is 155 Hz, as shown in Fig. 5(b). Figure 5(c) shows the laser linewidth as a function of the pump power, and the linewidth compression clearly emerges. It is observable that the linewidth values measured with weak distributed feedback (red dots) are in a smaller order of magnitude compared to the blue dots representing the linewidth values of the free-running laser at equal pump power. Notice that the linewidth compression effect does not depend on the operating pump power. It can be seen from Fig. 4(c) that the SMSR of the laser spectrum at low pump power (300-350 mW) shows irregular fluctuation in the case without feedback, which is caused by the unstable oscillation of the laser at low power operation. Therefore, this instability caused by low pump power leads to broadening of the laser spectral lines. Eventually, there is a certain dependence of the laser linewidth on the low operating pump power (300-350 mW). In addition, the longitudinal mode of the laser reaches stable operation with the increase of pump power. The dependence of the linewidth on the pump power will be weakened, and it will remain basically unchanged within the allowable range of the measurement error, as shown in Fig. 5(c). Therefore, the operation of the laser mode can be stabilized using this distributed feedback.
Fig. 5. RF spectra measured by the DSHI system with the span of 50 kHz and Lorentz fitting curves (a) without and (b) with feedback. (c) Corresponded linewidths with different pump powers.
In principle, for an accurate linewidth measurement (of the order of 100 Hz), thousands of kilometer delay lines should be added to the DSHI system to break the coherence of the beat frequency signal [31]. However, a long fiber length would introduce severe 1/f noise and transmission loss, which cannot be realized in practical applications. Hence, we decided to use a 50 km delay fiber to measure the laser Lorentz linewidth to minimize the impact of adverse factors. A longer delay fiber would be advantageous for the laser’s intrinsic white noise, i.e., to generate the ideal Lorentz line-shape, and the error caused by a shorter delay fiber can cause oscillation broadening of the self-heterodyne spectra, and in turn increases the estimated Lorentz linewidth [32]. On the other hand, in an actual measurement, the frequency spectrum obtained by the delayed self-heterodyne method is the Lorentzian spectrum convoluted with a Gaussian due to the unavoidable 1/f noise induced by a 50 km delay fiber. In general, it is arduous to distinguish these two kinds of line shapes and extract an accurate Lorentz linewidth. Therefore, we report the overall result, including the Gaussian contributing, and accept a slightly increased Lorentzian linewidth. Although the measured results exceed the actual linewidth, the beat spectrum has a satisfactory Lorentz profile, and these results can be regarded as a conservative representation of the natural laser linewidth. In order words, our results confirm the linewidth narrowing due the distributed feedback mechanism. In addition, the laser frequency noise measurement can be realized using a coherent DSHI system [33,34] to further confirm the above linewidth narrowing phenomenon. We also measure the frequency noise of our fiber ring laser using differential phase interferometry induced by a 10 m delay fiber with a 100 MHz carrier frequency. Figure 6(a) shows the single-sided frequency noise spectra obtained by the DSHI system. It can be clearly seen that the frequency noise curve decreases when the weak distributed feedback is introduced into the main cavity. The frequency noise in the low frequency region is suppressed by ∼43 dB from 3 × 107 Hz2/Hz to 1.5 × 103 Hz2/Hz when the frequency offset is approximately 1 kHz. The higher noise peaks around 1 kHz originate from the external environment, and this means that this effect may be avoided by properly shielding the laser system. Moreover, the flat region of high frequency white noise in the laser frequency noise spectrum can be used to estimate the Schawlow–Townes–Henry linewidth determined by the well-known Lorentzian line shape [35,36]. However, the linewidth calculated by this method can only represent the influence of the spontaneous emission in the laser gain and the linear loss of the laser resonator on the quantum noise of the optical field in the cavity on the laser linewidth. In general, lasers are affected by flicker noise caused by random fluctuations of electrons and thermodynamic noise caused by temperature fluctuations and vibrations in low frequency regions. Therefore, the laser linewidth is determined by a combination of low frequency noise and high frequency noise. Herein, the β-separation line (S(f) 8 × ln2 × f/π2) divides the frequency noise spectrum into two regions, corresponding to high and low modulation index regimes, respectively [37]. Here, only those spectral components whose frequency noise is above the β-separation line (region of high modulation index) contribute to the linewidth. Hence, the laser linewidth calculated by the β-separation line is obtained by integrating the part affecting the linewidth in the frequency noise spectrum, which is also called the integrated linewidth. The calculation result includes the influence of the combined effect of the low frequency part and the high frequency part on the line width. Based on this, the linewidth calculated in this way can better describe the linewidth level of ultra-narrow linewidth lasers.
Fig. 6. (a) Frequency noise spectra. Blue curve: frequency noise spectra of the free-running fiber laser for reference; red curve: frequency noise spectra of the fiber laser with feedback; olive curve: β-separation line for evaluation of linewidth. (b) RIN noise spectra of the laser with and without feedback. The inset is a local magnification in the low frequency range from 0 to 800 kHz.
The long cavity length in a ring fiber laser will increase the photon lifetime, which results in a laser with a narrow intrinsic linewidth. Here, this intrinsic linewidth is calculated from high frequency white noise flat region in the frequency noise spectrum. Here, it can be seen from Fig. 6(a) that the flat regions of the frequency noise spectrum are 10.3 Hz2/Hz and 51.2 Hz2/Hz with or without feedback, and the corresponding fundamental linewidth are 32.3 Hz and 160.8 Hz, respectively. The estimated intrinsic linewidth is lower than the beat frequency results measured with the DSHI method. Nonetheless, this is still a significant compression of the laser linewidth. However, the high frequency white noise flat region frequency ranges with and without feedback correspond to f >3 × 104 Hz and f > 6.5 × 105 Hz, respectively. In addition, the integrated linewidth including low frequency noise with or without feedback calculated by β-separation line are 2.3 kHz and 56.2 kHz, respectively. Therefore, it can be obtained that the proposed distributed feedback also suppresses the frequency noise in the low frequency region of the laser. It is worth mentioning that the high frequency white noise flat region is used to calculate the linewidth of the laser in order to explore the limit of laser linewidth compression, which provides guidance for further compression of the linewidth.
In addition, the jitter of laser power in each frequency band is characterized by relative intensity noise (RIN), which is an important index to evaluate the laser output power stability. The RIN is suppressed by 10 dB when the laser is assisted by weak distributed feedback, as shown in Fig. 6(b). As shown in the red curve line, the RIN floor of the laser output can be as low as -132 dB/Hz for frequencies beyond 3 MHz. Moreover, the inset in Fig. 6(b) shows the RIN spectrum of the laser with and without feedback in the low frequency region from 0 to 800 kHz. Herein, the frequency corresponding to the first peak is 50 kHz. At the same time, the frequency of the noise caused by the driving of the pump laser diode is in the frequency range of tens of kHz. Hence, it is further proved that the oscillation peaks appearing in the RIN spectrum are not caused by laser relaxation oscillations. It is suggested that these peaks appearing in the low frequency region are caused by the instability of the pump laser. As indicated in Fig. 6(b), when the weak distributed feedback is further incorporated into the laser system, the oscillation peaks in the noise spectrum are all effectively suppressed. The above results show that the proposed distributed feedback mechanism exhibits a suppressive effect on both high and low frequency regions of laser intensity noise.
Besides, the stability of lasing center wavelength and output power is an important index to evaluate the performance of laser system. Hence, the optical lasing wavelength and output power of the proposed fiber ring laser without any significant variation can be obtained from the laser output spectra of 13 times repeated scans at 10-min intervals. These results are monitored and measured by the OSA with a resolution of 0.02 nm at a pump power of 400 mW, as shown in Fig. 7(a). It can be seen from a series of spectrograms obtained by monitoring that the laser output spectrum does not degrade over time. Moreover, to fully illustrate the long-term stability of the proposed fiber laser system based on external weak distributed feedback, we further monitor the evolution of lasing center wavelength and output power over time. Figure 7(b) presents the high stability of the center wavelength variation, and the output power fluctuation is less than 0.017 nm and 0.026 mW, respectively, in a 10-min interval of 120 minutes observation. The above results demonstrate that the fiber laser assisted by weak distributed feedback exhibits remarkable long-term stability.
Fig. 7. (a) Optical spectra of 13 times repeated scans at 10-min intervals. (b) Temporal stability of the output power and wavelength at the pump power of 400 mW in a 10-min interval over 120 min.
The proposed laser linewidth compression mechanism based on weak distributed feedback greatly improves the coherence of single-frequency fiber ring lasers. It also provides a mechanism that can be exploited in other gain-wavelength light sources to achieve extreme control of laser parameters. Moreover, we can also achieve wavelength tuning based on the output of ultra-narrow linewidth lasers by simultaneously adjusting the center wavelengths of both the PSFBG and the FBG. Herein, the HSF employed in this experiment is merely an approach to verify that the weak distributed feedback signal can compress the laser linewidth. Therefore, using a small-sized artificial waveguide structure with high scattering coefficient and low optical transmission attenuation can further improve the laser performance. In addition, this laser system has a long and exposed fiber link which is prone to interference by external vibration and thermal noise that alters the stability of the main cavity laser and degrades the compression effect. Therefore, the laser and the detection system should be packaged with shock-proof and temperature control technologies to further optimize the performance of the overall devices.
5. Conclusion
We have suggested and demonstrated an ultra-narrow-linewidth single-frequency fiber ring laser using a π PSFBG and assisted by weak distributed feedback. The π PSFBG is used as a narrowband filter element for the preliminary SLM operation of the fiber ring laser. In order to overcome the existing limitations associated with the linewidth compression of traditional fiber ring lasers, we have proposed a linewidth high-compression based on an excited weak distributed feedback signal from an external WDFS. The feedback signal is generated by the WDFS, and is injected into the single longitudinal mode main cavity. It can operate independently to suppress the spontaneous emission of the laser, and consequently decrease the phase fluctuations, in order to obtain a narrow linewidth laser. Eventually, the laser SMSR has been improved from 43 dB to 72 dB, and the output linewidth has been reduced from 1.45 kHz to 155 Hz when the feedback is present. A very low white frequency noise at 10.3 Hz2/Hz has been measured, corresponding to a fundamental linewidth of ∼32.3 Hz. The measured RIN is low as –132 dB/Hz for frequencies beyond 3 MHz. Overall, the properties of our all-fiber system structure make it very attractive for applications in telecommunications and optical sensing, thereby providing a new solution for all-fiber ultra-narrow linewidth low-noise laser sources.
Funding
National Natural Science Foundation of China (61927818, 62075020); National Science Fund for Distinguished Young Scholars (61825501); Chongqing Natural Science Foundation of Innovative Research Groups under Grant (cstc2020jcyj-cxttX0005).
Acknowledgements
The authors would like to express their gratitude to EditSprings (https://www.editsprings.cn) for the expert linguistic services provided.
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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