1 Introduction

Commonly, single-longitudinal-mode (SLM) erbium fiber lasers also have the output features of low noise, high optical signal to noise ratio (OSNR), high beam quality and narrow linewidth (Kim et al. 2004; Wang et al. 2021). So, the erbium-doped fiber (EDF) lasers also have the SLM operation, high output stability, narrow linewidth, and simple structure and operability (Diaz et al. 2015; Wang et al. 2022). Additionally, due to its output characteristics, the EDF laser can find potential applications in many fields, such as optics sensors, optical communications, high-resolution spectroscopy, RF-photonics and bio-photonics (Wang et al. 2019; Wan et al. 2012; Tang et al. 2019). The EDF lasers based on ring cavity would be an efficient architecture to accomplish SLM operation to eschew the spatial hole burning produced by the standing wave effect (Wang et al. 2020). Nevertheless, a large number of multi-longitudinal-mode (MLM) fluctuations of EDF ring laser could be introduced through the homogeneous broadening of the erbium ion and the long fiber length of ring cavity (Liu et al. 2020). So, to suppress the MLM oscillations, arrive the stable SLM and narrow linewidth in the EDF based ring laser configurations, various related structures have been proposed in the fiber cavity, such as using the compound-ring cavity design (Wang et al. 2020; Liu et al. 2020; Lai et al. 2022; Feng et al. 2013), Rayleigh backscattering (RB) injection effect (Skvortsov et al. 2022), optical injection effect (Gao et al. 2020), unpumped EDF saturable absorber (SA) (Wang et al. 2015), and Mach–Zehnder interferometer (MZI) structure (MdAli et al. 2014). In addition, use of ultra-narrow bandwidth filter were also demonstrated, such as the chirped moiré fiber Bragg grating (FBG) filter (Reid et al. 1990), novel optical filter (Zou et al. 2013), and phase-shifted FBG filter (Chen et al. 2005). Among the above-mentioned technologies, the compound-ring structure of EDF laser is easy to construct to achieve SLM operation and narrow linewidth of kHz due to its cost-effectiveness and simplicity.

In this presentation, to achieve high OSNR, narrow linewidth and we design an EDF laser system based on a four-fiber-ring cavity to achieve stable SLM and selective wavelength output. The four-ring scheme can result in a mode-filter effect with a wider free spectrum range (FSR) based on the Vernier effect to decrease the MLM fluctuations. To complete more stable and narrower wavelength-selectable output, the EDF-SA and RB feedback injection are also added in the laser cavity. Due to the four-fiber-ring design, the available wavelength-tuning bandwidth can be spread from 1513.0 to 1581.0 nm (68 nm bandwidth) by applying the initial C-band gain-medium. Here, the optical signal to noise ratio (OSNR), output power and 3-dB linewidth of the proposed EDF four-ring laser are between 52.68 and 62.95 dB, − 9.2 to − 0.17 dBm, and 781–781 Hz over the available bandwidth of 1513.0–1581.0 nm, respectively. This EDF laser also achieve a constant power output with a fluctuation of 3 dB in the bandwidth of 1529.0–1573.0 nm. In addition, the greater instabilities of center wavelength and output power are less than 0.032 nm and 0.33 dB after 40-min observation, respectively. In past research related to DFB semiconductor laser (Shindo et al. 2021), the fluctuation of light intensity was less than 2.5 dB across whole tuning range of over 50 nm. And the variation of output power and wavelength of ref. (Shindo et al. 2021) would be better than that of the proposed EDF laser. However, the presented EDF laser can achieve 68 nm tuning range for wavelength-selection. Therefore, this EDF ring laser system not only can provide selective and stabilized SLM wavelength output, but also can achieve extend the tunable range and narrow the 3-dB linewidth. Moreover, the previous related works only achieved the single wavelength or wavelength-tunable in C-band window (Kim et al. 2004; Wang et al. 2021, 2019, 2020; Wan et al. 2012; Tang et al. 2019; Liu et al. 2020). Our proposed EDF ring laser not only achieve C + L band tuning bandwidth (1518.0–1583.0 nm), but also narrow the linewidth to sub-kHz.

2 Experimental setup

The diagram of the presented four-ring based EDF laser is displayed in Fig. 1. The setup of fiber laser is constructed by a commercial erbium-doped fiber amplifier (EDFA), a 3-port optical circulator (OC, Thorlabs, CIR1550PM-FC), two polarization controllers (PCs, Thorlabs, FPC561), a 2 m EDF with unpumped operation, a polarizer (POL), a tunable bandpass filter (TBF, Sentech, OTF-320-C-E), a 100 m single-mode fiber (SMF), two 1 × 2 50:50 optical couplers (OCP1), a fiber mirror (FM), and a 1 × 2 10:90 optical coupler (OCP2), respectively. The EDFA with 11 dBm saturable output power in the effective range of 1528–1562 nm, is regarded as gain-medium in the ring cavity. To lase different wavelength output, the TBF is applied in the ring cavity for tuning. The 3-dB passband and insertion loss of TBF is 0.4 nm and 6 dB over an achievable tunability range of 1510–1630 nm. The POL and two PCs are added in the ring configuration to rotate the polarization status and maintain the optimal output power.

Fig. 1
figure 1

Experimental setup of proposed four-ring based EDF laser in L-band bandwidth

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As shown in Fig. 2a–d, the designed EDF laser architecture will produce four fiber rings, representing Ring 1 to Ring 4, respectively, to meet with the Vernier effect to activate a wide and effective free spectrum range (FSR) and mode-filter effect (Wang et al. 2020; Liu et al. 2020; Lai et al. 2022; Feng et al. 2013). In this design, the fiber length of Ring 1 to Ring 4 is 234, 215, 30 and 11 m, respectively. Moreover, in this EDF ring laser, an unpumped EDF-SA of 2 m can produce an autotracking ultra-narrow bandwidth filter appearance to suppress and filter the dense longitudinal modes (Wang et al. 2015). Here, a standing wave is developed in the unpumped EDF by the propagating lightwave. The spatial-hole-burning effect of EDF will induces a slight change Δn in refractive index of the EDF. According to the standing wave theory, the spatial period of light intensity distribution in EDF is Λ = λ/2neff, where λ is the wavelength of the incident signal and 2 neff is the effective refraction index of the EDF. So, a self-induced autotracking filter is caused, when the central frequency equals to the incident signal frequency (Wang et al. 2015). As seen in Fig. 1, the FM will reflect the lasing wavelength and. And the 100 m SMF induced Rayleigh backscattering (RB) feedback injection can be used to reduce the laser linewidth (Wang et al. 2022, 2015). Therefore, by combining the above three methods in a ring cavity, a stable, tunable, narrow linewidth output SLM EDF laser will be realized. To measure the laser wavelength, an optical spectrum analyzer (OSA) is connected to the 10% output port of OCP2 for observation, as displayed in Fig. 1.

Fig. 2
figure 2

a Ring 1, b Ring 2, c Ring 3 and d Ring 4 of the proposed EDF laser architecture, respectively

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3 Result and discussion

The amplified spontaneous emission (ASE) curve of initial EDFA is measured in black dashed line of Fig. 3. The 15 dB bandwidth of ASE spectrum is between 1522.6 and 1560.3 nm. In this way, we can ensure the possible wavelength-tuning scope around the effective gain range of EDFA. To confirm the starting position of the output wavelength of this EDF laser, we adjust the passband of TBF of 1513 nm to generate a lasing wavelength around 1513.0 nm. Thus, Fig. 3 also presents the detected output spectrum of 1513.0, 1529.0, 1545.0 1565.0 and 1581.0 nm. In this measurement, a tuning bandwidth of 68 nm is achieved based on the proposed C-band EDF laser architecture. The achievable gain range of C-band EDFA is from 1528 to 1562 nm. As seen in Fig. 3, the adjustable laser wavelength starts from 1513.0 nm and ends at 1583.0 nm. The wavelength-tuning range is larger than the gain range of the original EDFA. obviously, this proposed laser configuration can suppress the effective gain and move it to both sides to increase the wavelength tunable range. Hence, this indicates that the proposed EDF laser can provide a tuning bandwidth of 68 nm from the S to L bands. Since the gain distribution on both sides of the wavelength output range is smaller, the output wavelengths of 1513.0 nm and 1581.0 nm can not suppress the ASE background noise near 1530 nm, as seen Fig. 3. And the background noise near 1530 nm at the two wavelengths of 1513.0 nm and 1581.0 nm is also higher than other output wavelengths. As the output wavelength moves toward the center of the adjustable range, its ASE background noise will become lesser due to the available gain competition, as also displayed in Fig. 3. This is because the gain is smaller on both sides of the wavelength-adjustable range.

Fig. 3
figure 3

Observed output spectrum of 1513.0, 1529.0, 1545.0 1565.0 and 1581.0 nm of the presented EDF laser and ASE curve of C-band EDFA, respectively

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At that time, we also perform the output power with corresponding optical signal to noise ratio (OSNR) of the exhibited EDF ring laser over in the range of 1513.0–1581.0 nm. In the observation, the OSNR of each wavelength is measured directly through the OSA with a resolution of 0.07 nm. And the output power of each wavelength is detected over the full bandwidth. The measurable output power and OSNR is in the range of − 9.2 to − 0.17 dBm and 52.68–62.95 dB over the whole bandwidth, respectively, as seen in Fig. 4. The maximum output power of − 0.17 dBm with 61.71 dB OSNR is observed at the wavelength of 1541.0 nm. The change of OSNR at each wavelength is related to the output power. So, the measured OSNR and output power will become smaller on both sides of the effective wavelength output bandwidth due to smaller gain operation, as seen in Fig. 4. Furthermore, the proposed EDF laser architecture also can deliver a flatter power output bandwidth from 1529.0 to 1573.0 nm with 0.3 dB power fluctuation [ΔP = − 0.17 − (− 0.47) = 0.3 dB]. As a result, the presented EDF laser can not only extend the tuning bandwidth from 1513.0 to 1581.0 nm by applying C-band EDFA gain-medium, but also reach a flattened power range of 1529.0–1573.0 nm under 0.3 dB power variation.

Fig. 4
figure 4

Obtained output power and OSNR of the EDF laser in the wavelengths from 1513.0 to 1583.0 nm

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The instabilities of output power and center wavelength of the designed EDF laser are also significant issue in this study. First, the output wavelength of 1581.0 nm is applied to measure the fluctuations of power (ΔP) and central wavelength (Δλ) through 40-min observation, as shown in Fig. 5a. The results show that the largest changes of output power and central wavelength are 0.22 dB and 0.024 nm. Then, we select eighteen output wavelengths of 1513.0–1581.0 nm with 4 nm channel interval for the observation of output stability over the entire tuning range. As exhibited in Fig. 5b, the measured output power and wavelength fluctuations of the eighteen chosen wavelengths are between 0.11 and 0.33 dB and 0.016 and 0.032 nm after 40-min short-term observation, respectively. Due to the long ring fiber cavity length, 3-dB bandwidth of TBF with 0.4 nm, and environmental vibration and temperature changes, slightly wavelength fluctuations will still be caused. Thus, based on the results obtained from the above measurements, the demonstrated EDF ring laser also achieves better output stability (ΔP ≤ 0.33 dB and Δλ ≤ 0.032 nm) over the total wavelength-selective bandwidth.

Fig. 5
figure 5

a Observed fluctuations of output power and central wavelength at the lasing wavelength of 1581.0 nm through 40-min observation. b Detected oscillations of output power and center wavelength of the EDF laser in the total tunability range

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Next, we can apply use delayed self-homodyne and self-heterodyne setups to verify the SLM performance and measure the laser linewidth, respectively. The two setups are constructed by a dual-arm MZI configuration, which consists of two 1 × 2 50:50 OCPs, a 73 km fiber delay line, an acousto-optic modulator (AOM) and a polarization controller (PC), respectively. And the experimental setup can be found in ref. (Wang et al. 2022). And the experimental setup can be found in ref. (Wang et al. 2022). To demonstrate the SLM implementation of the EDF laser, all eighteen output wavelengths are measured. So, Fig. 6 displays the detected electrical spectrum of each lasing wavelength from 1513.0 to 1581.0 nm in a frequency bandwidth of 0–1 GHz via self-homodyne observation, respectively. The whole observed electrical spectra are free of any longitudinal mode oscillations within the 1 GHz bandwidth, as exhibited in Fig. 6. This observed phenomenon proves the SLM output of the EDF laser system. Additionally, during 40-min measurement, the entire selective wavelengths of the EDF laser also can be run on the SLM operation.

Fig. 6
figure 6

Detected electrical spectrum of the wavelengths from 1513.0 to 1583.0 nm with 4 nm channel intervals in the frequency span of 0–1000 MHz, respectively

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Figure 7 indicates the observed electrical linewidth profile at 1513.0 nm wavelength via the self-heterodyne detection, when a 55 MHz shift frequency is applied on AOM to obtain a beat signal. To attain the actual linewidth of the EDF laser, a Lorentzian curve is used to fit the measurement result as plotted in the blue line of Fig. 7. The 3- and 10-dB linewidth at the 1513.0 nm wavelength is 781 and 1875 Hz through the Lorentzian fitting, respectively. We can then apply the same experiment to other resulting wavelengths for linewidth measurement. Over the obtainable wavelength-selecting bandwidth of 1513.0–1581.0 nm, the completed 3-dB and 10-dB linewidth of the EDF laser are between 781 and 781 Hz and 1562 and 2343 Hz, respectively, as exhibited in Fig. 8. When we only use the RB feedback injection with 100 m SMF, the output linewidth of 6.184 kHz was achieved in the previous study (Wang et al. 2022). Hence, the demonstrated EDF laser with EDF-SA, RB feedback injection and quad-ring cavity can realize the output characteristics of SLM operation, narrow linewidth, wide wavelength-tuning bandwidth and high stability. Compared with the previous studies (Wang et al. 2019, 2021, 2022; Tang et al. 2019; Feng et al. 2013, 2016; Song et al. 2001; Wan et al. 2020), as seen in Table 1, although not every output characteristic (containing wavelength-tuning range, OSNR, linewidth, power stability and flattened power output of this proposed EDF laser architecture is the best. But the overall performance is beneficial, including a wavelength adjustable range of 68 nm, an lasing linewidth of 781 Hz, and a flat power output range of 44 nm.

Fig. 7
figure 7

Electrical spectrum measured and fitted at 1513.0 nm wavelength through self-heterodyne setup

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Fig. 8
figure 8

Measured 3- and 10-dB Lorentzian linewidth of the designed EDF laser over the entire tunability range

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Table 1 Comparison between the proposed EDF ring laser and other previous works
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4 Conclusion

To realize a tunable and stable erbium-doped fiber ring laser with single-longitudinal-mode wavelength output, four-ring configuration also together with RB feedback injection and unpumped erbium-doped fiber saturable absorber was presented. In the experiment, a C-band erbium-doped fiber gain-medium was applied in laser cavity to produce a wide bandwidth wavelength output of 1513.0–1581.0 nm (68 nm bandwidth) in part of the S-band, the complete C-band and part of the L-band. This is because the four-ring apparatus can cause the mode-filter to suppress and increase the valuable gain bandwidth by using C-band gain-medium. In the measurement, the obtained output power and optical signal to noise ratio were in the range of − 9.2 to − 0.17 dBm, 52.68 and 62.95 dB over the available bandwidth of 1513.0–1583.0 nm, respectively. And the 3-dB linewidth for all wavelengths was 781 Hz. This fiber laser also reached a constant power output with a variation of 0.3 dB in a tunable range of 1529.0–1573.0 nm. Furthermore, the observed stabilities of output power and center wavelength over the output bandwidth were between 0.11 and 0.33 dB and 0.016 and 0.032 nm through 40-min measurement, respectively.