1 Introduction

Since 1979, fiber lasers have mainly used the optical wavelengths centered at 1.55 μm as the main band for optical communications. The main reason is that the loss of quartz fiber is very low (0.1419 dB/km) and the EDF amplifier mainly amplifies near 1550 nm. And the performance is extremely superior (Miya et al. 1979; Tamura et al. 2018). The 1550 nm band is composed of C-band, which is located in the wavelength range of 1.53–1.56 μm. Lasers are mainly divided into pulse lasers and continuous cave (CW) lasers. Compared with CW lasers, pulse lasers have attracted more and more researchers in the fields of direction measurement, material processing, optical fiber sensing, spectroscopy and other fields (Bianchi et al. 2021; Agawane et al. 2018; Fistonić and Fistonić 2018; Hu et al. 2021; Birr et al. 2017; Pan et al. 2023; Ahmad et al. 2014). In 2017, D.Z et al. used Ferro-oxide (Fe3O4) as a SA and used a single mode-multi mode-single mode fiber (SM-MM-SM) structure for tunable operation to achieve passively Q-switched with a tuning range of 7 nm. The tunable range of this experiment is narrow (Mohammed et al. 2017). In 2021, Li et al. used a single mode fiber-step-index multimode fiber-single mode fiber (SMF–SIMF–SMF) structure as SA to achieve passively Q-switched, and the single-wavelength tuning range obtained was 16 nm, but it achieves wavelength tunability through the degree of SIMF bending, which is difficult to operate (Li et al. 2021). In 2022, Peng et al. used ZrSe2 as a SA to achieve passively Q-switched and achieved a tuning range of 19.2 nm. However, the method of preparation is relatively difficult and the equipment requirements are high (Peng et al. 2022a).

Pulse lasers are mainly divided into mode-locked pulse and Q-switched pulse lasers. Because Q-switched pulse lasers have the advantages of high peak power and short pulse width, they are more widely used, such as lidar, high-precision fiber sensing, medical fields, etc. There are two main ways to achieve Q-switched pulses output, one is actively Q-switched and the other is passively Q-switched. Actively Q-switched is mainly achieved by polarization modulation (Noach et al. 2021), acousto-optic modulators (AOMs) (Wang et al. 2021), electro-optic modulators (EOMs) (Peng et al. 2022b; Chen et al. 2023), rotating mirror and pumping source modulation (Nundy 2022). These techniques require driving electronics, which significantly increase the losses in the laser resonator cavity due to their insertion, and are expensive, bulky, and complex to operate compared to passively Q-switched. Passively Q-switched is relatively simple and compact in structure. It mainly uses some materials to generate Q-switched pulses, such as semiconductor saturable absorber mirrors (SESAMs) (Fang et al. 2021; Yang et al. 2023), carbon nanotubes (CNTs) (Sun et al. 2021, 2023; Primsa et al. 2023), Fe3O4 (Mohammed et al. 2017) and black phosphorus (BP) (Zhang et al. 2020), as well as the EDF and the EYDF, etc. Among these materials, graphene (Zhang et al. 2019; Ahmad et al. 2017a), Bi2Se3 (Ahmad et al. 2015), MoS2 (Liu et al. 2022; Ahmad et al. 2017b), and other materials can achieve Q-switched pulses and mode-locked pulses with wide tuning range, but the synthesis and integration of these materials as SA are relatively difficult and expensive (Xia et al. 2015). When the EDF and the EYDF are used as SA, they are produced using mature fiber fusion splicing technology and do not change the all-fiber structure.

In this paper, the PQFL with cascaded doped fiber segments and the TBF to achieve single-wavelength tunable is proposed. When the EDF and the EYDF are non-completely pumped, they function as SA to achieve the output of passively Q-switched pulses. The laser achieves precise wavelength tuning through the TBF, which can achieve a tuning range of 1529.08–1574.12 nm. Under the combination of different pump powers, the cascaded doped fiber segments exhibit different nonlinear characteristics, resulting in different characteristics of the Q-switched pulses. The wavelength tuning range, pulse repetition rate, and pulse width are jointly controlled by the two pump powers. Stable wide tunable in C + L band single-wavelength tunable PQFL is simple and compact for use in the field of medical and bio-medical imaging (Gottschall et al. 2017).

2 Experimental setup and principle analysis

2.1 Experimental setup

The experimental setup of PQFL with wide tuning range based on the EDF and the EYDF is shown in Fig. 1. The two-stage commercial EYDFA is used to provide the resonant cavity gain, and its internal structure is shown in the dashed box in the Fig. 1. It consists of two stages of amplifiers, the first stage is the EDF pumped by the 1st pump laser and second stage is the EYDF pumped by the 2nd pump laser. The optical isolator (ISO) is used to ensure the unidirectional transmission of light and avoid the damage of equipment caused by light back propagation. The wavelength tunable band-pass filter (WL Photonics #16,112,101, 120 nm tuning range, 1520–1640 nm, bandwidth of 1 nm, insertion loss is about 3.28 db.) is used to achieve wavelength tunable narrowband laser output. 20% of the coupler ports is used for the output of the laser, and the remaining light is returned to the resonator for sustained oscillation.

Fig. 1
figure 1

Experimental structure diagram of a wide-tuned in c + l band pqfl, and the internal structure diagram of the eydfa is shown in the dashed box

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The output light was measured by an optical spectrum analyzer (OSA, YOKOGAWA AQ6370D) with a maximum resolution of 0.02 nm. The time-domain characteristics of the laser were recorded by a 200 MHz digital oscilloscope (UTD4202C) through a 200 MHz photodetector (PD-200M21391). The average output power of the laser was measured by an optical power meter (PMSII-A).

2.2 Two-stage commercial EYDFA gain spectrum

The gain spectrum of the EYDFA at different pump power combinations are shown in Fig. 2 and Fig. 3. Figure 2 measures the gain spectrum at different 2nd pump powers when the 1st pump is turned off. It can be seen from Fig. 2 that the spectral amplification range is in the 1530–1570 nm band. When the 2nd pump power is increased, the gain spectrum amplification in the 1535–1550 nm band is more obvious. Figure 3 shows the gain spectrum when the 2nd pump power is fixed at 200mW and the 1st pump power is changed. It can be observed from the Fig. 3 that the gain spectrum in the 1550–1565 nm band is significantly improved.

Fig. 2
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When the 1st pump power is turned off, the gain spectrum changes with the 2nd pump power changes

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Fig. 3
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When the 2nd pump power is fixed at 200mW, the gain spectrum changes with the 1st pump power changes

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2.3 Nonlinear characterization of the EDF-EYDF

The nonlinear characteristics of a non-completely pumped the EDF and the EYDF are investigated using the average power detection by a balanced twin detector. The experimental setup is shown in Fig. 4. The input light source is a mode-locked fiber laser (MLFL) with a central wavelength of 1550 nm, a pulse duration of 240 fs and a repetition rate of 20.457 MHz. After the input light source is adjusted by the variable optical attenuator (VOA), the power is evenly distributed after passing through the 3 dB coupler. The VOA is used to adjust the power of the input light source, which can ensure that the optical power remains within a certain range and prevent equipment damage caused by excessive optical power. After the power is evenly distributed by the 3 dB coupler, the power of one optical path is directly measured by the optical power meter and recorded as power A, and the optical power of the other branch is recorded as power B after passing the EYDFA. The average output power at points A and B were measured for different pump power combinations.

Fig. 4
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Experimental device for measuring nonlinear characteristics of the EDF-EYDF

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Figure 5 shows the relationship between input power and output power with different primary pump powers. From the experimental results, it can be seen that the nonlinearity of the output power is variable with the pump power increases. The output power with different pump combinations is fitted as Eq. (1), and the parameters \(a\), \(I_{sat}\), and I0 are different for different pump combinations (Zhang et al. 2023).

$$ I_{out} = a \cdot \exp ( - {{I_{in} } \mathord{\left/ {\vphantom {{I_{in} } {I_{sat} }}} \right. \kern-0pt} {I_{sat} }}) + I_{0} $$
(1)
Fig. 5
figure 5

The nonlinear output power with 1st pump power turned off and 2nd pump power fixed at different power

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The cascaded doped fibers exhibit nonlinear gain characteristics under different pump power combinations which is shown in Eq. (2).

$$ G \propto \frac{1}{{1 + {{I_{in} } \mathord{\left/ {\vphantom {{I_{in} } {I_{sat} }}} \right. \kern-0pt} {I_{sat} }}}} $$
(2)

When the 1st pump power is turned off and the 2nd pump power is changed, the nonlinear characteristics of the EDF-EYDF is shown in Fig. 5. As can be seen from the Fig. 5, when the 2nd pump power is increased from 110 to 150 mW, the EDF-EYDF nonlinearity gradually becomes stronger, and when the 2nd pump power is increased from 150 to 190 mW, the EDF-EYDF nonlinearity gradually weakens. Figure 6 shows the nonlinear characteristics when the 2nd pump power is fixed at 150 mW and the 1st pump power is changed. The results show that when the 1st pump power is 0 mW, the nonlinear effect is the strongest. Due to the different nonlinear effects under different pump combinations, the modulation depth is also different under different pump power combinations. Table 1 records the modulation depths in the case of Fig. 5 and Fig. 6.

Fig. 6
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The nonlinear output power with the 1st pump power fixed at different powers and the 2nd pump power fixed at 150mW

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Table 1 Modulation depth for different pump power combinations
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3 Experimental results and analysis

In the first experiment, we added PC between the isolator and the TBF in the laser structure. After experiments, it was found that by adjusting the PC, the pulse repetition frequency and pulse width did not change. In order to minimize the loss of the fiber laser, we were finally decided to remove the PC from the laser structure. We fixed the 1st pump power at 23 mW and the 2nd pump power at 200 mW. By adjusting the TBF, it is observed from the OSA that the tunable range of a single wavelength can reach 45.04 nm (1529.08–1574.12 nm), and the side mode suppression ratio (SMSR) is a minimum of 56.09 dB and a maximum of 65.37 dB, as shown in Fig. 7.

Fig. 7
figure 7

The center wavelength of the output wavelength is 1529.08–1579.52 nm

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Figure 8 shows the passively Q-switched pulses to different center wavelengths when the 1st pump power is fixed at 23 mW and the 2nd pump power is 200 mW. From the observation of the Fig. 8, it can be found that when the center wavelength is in the wavelength range of 1529.08–1550.64 nm, passively Q-switched pulse appears on the oscilloscope. The pulse repetition frequencies vary between 22.03 kHz-30.6 kHz and the pulse widths varies between 3.91 μs-3.55 μs. However, when the center wavelength lasing range is 1550.64–1579.52 nm, there is only a continuous wave state, and no passively Q-switched pulse generat.

Fig. 8
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Passively Q-switched pulses to different center wavelengths when the 1st pump power is fixed at 23 mW and the 2nd pump power is 200 mW

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As can be seen in Fig. 2, the peak power of the second stage amplification is concentrated in the wavelength range of 1535–1550 nm. It can be seen from Fig. 3 that the peak power of the first stage amplification is concentrated in the 1550–1565 nm wavelength range. That is to say, the EDF mainly provides gain to the resonant cavity in the range of 1550–1565 nm, while the EYDF mainly provides gain to the resonant cavity in the range of 1535–1550 nm.

When the 1st pump power is fixed at 13.2 mW and the 2nd pump power is 100 mW, the output center wavelength position is changed by adjusting the TBF, and the passively Q-switched pulse range can reach 31.04 nm. When the output center wavelength is within the wavelength range of 1535–1550 nm, the resonant cavity gain is mainly provided by the EYDF, and the EDF serves as SA to generate passively Q-switched pulse output. When the output center wavelength is within the wavelength range of 1550–1565 nm, the EDF provides gain for the resonant cavity, while the EYDF serves as SA and provides gain for the resonant cavity. Therefore, its saturation absorption effect is weak, and the output stability of the passively Q-switched pulse is not good, as shown in Fig. 9. When the 2nd pump power reaches 200 mW, no passively Q-switched pulse are generated in the wavelength range of 1550–1565 nm. This is because as the power increases, the EYDF gradually bleaches, its nonlinear absorption characteristics disappear, and the pulse sequence also disappears.

Fig. 9
figure 9

Passively Q-switched pulse to different center wavelengths when the 1st pump is fixed at 13.2 mW and the 2nd pump is 100 mW

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After several experiments, it is found that the changes of wavelength tuning range in pulse state, repetition frequency, pulse width and output power under the combination of different pump powers are shown in Table 2. Fixing the 1st pump power to 10 mW and changing the 2nd pump power achieve the wavelength tunable range in the pulsed state reduced from 28.40 nm to 11.55 nm, the pulse repetition frequency increased from 8.23 kHz to 36.10 kHz, and the pulse width reduced from 12.82 μs to 3.10 μs.

Table 2 Repetition frequency, pulse width and pulse range under different pump power combinations
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When the 1st pump power is 23 mW and the 2nd pump power is 200 mW, by adjusting the TBF, a single wavelength spectrum with a central wavelength of 1545 nm, a 3 dB bandwidth of approximately 0.4 nm, and an SMSR of up to 62.55 dB can be observed on the spectrometer, as shown in Fig. 10. It can be seen from Fig. 3 that the gain spectrum is the highest at 1545 nm, so the center wavelength is selected for data analysis here. When the 1st pump is turned off, the laser reaches the threshold with pulse output when the 2nd pump power exceeds 125 mW. It can be observed from Fig. 11 that the pulse repetition frequency increases with the increase of the 2nd pump power from 11.76 kHz to 44.4 kHz, and the pulse width decreases from 9.70 μs to 3.23 μs. Which is due to the fact that the EDF acts as SA to generate the pulse sequence, and the EYDF provides the gain for the resonant cavity. Fixing the 2nd pump power at 200 mW, the 1st pump power is increased from 3 mW to 89.4 mW, and the pulse repetition frequency is increased from 27.10 kHz to 46.47 kHz, while the pulse width is reduced from 3.55 μs to 2.00 μs, as shown in Fig. 12. When the 1st pump and 2nd pump powers increase to a certain power, the EDF and the EYDF gradually bleached, their nonlinear absorption characteristics disappear, and the pulse sequence also disappears. Figure 13 shows the trend of pulse repetition frequency and pulse width with the change of pump power, and its variation law is in line with the general law of PQFL.

Fig. 10
figure 10

Single-wavelength spectrum with fixed 1st pump power of 23 mW and 2nd pump power of 200 mW, with the center wavelength at 1545.00 nm

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Fig. 11
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Passively Q-switched pulse with 1st pump turned off and 2.nd pump power varied from 125 to 250 mW. (output center wavelength at 1545.00 nm)

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Fig. 12
figure 12

Passively Q-switched pulse when the 2nd pump power is fixed at 200 mW and the 1.st pump power is changed from 3.0 mW to 89.4 mW. (output center wavelength is 1545.00 nm)

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Fig. 13
figure 13

Variation of pulse repetition frequency and pulse width (a) Turn off the 1st pump power and varying the 2nd pump power, and (b) fix the1st pump power at 200 mW and varying the 2nd pump power

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Under fixed combinations of different pump powers, the average output power within the allowable error range is shown in Fig. 14. Through multiple experimental data records, it can be concluded that when the 2nd pump power is fixed, as the 1st pump power increases, the average output power increases, and the average output power fluctuates less.

Fig. 14
figure 14

Variation of average output power at the center wavelength of 1545 nm for different pump power combinations

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In order to test the stability of the laser, under the conditions of fixing the 1st pump power to 23 mW, the 2nd pump power to 200 mW and the fixed central wavelength to 1545 nm, the output spectrum was recorded every 10 min within 1 h. The output spectrum of the laser is shown in Fig. 15, which is a recorded plot of wavelength drift and power fluctuations. The maximum wavelength drift in the output spectrum is 0.04 nm and the maximum power fluctuation is 0.038 dBm. The results show that the output spectrum fluctuation of the laser is very stable.

Fig. 15
figure 15

When the output center wavelength is 1545 nm, the stability of the output spectrum is tested every 10 min for one hour; the inset shows the center wavelength drift and peak power fluctuation

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Table 3 compares the performance indicators of PQFL produced by different SA. As can be seen from the table, the PQFL obtained in this experiment has the advantage of a wide tuning range and other indicators are relatively superior.

Table 3 Q-switched erbium-doped fiber lasers by different SA
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By adjusting the TBF, the central wavelength was fixed at 1545 nm. CW is observed when the EYDFA reaches a certain power, while stable passively Q-switched pulse is observed when the two cascaded doped fiber segments were non-completely pumped. Figure 16 shows the area diagram of the passively Q-switched pulse state and the CW state under different pump power combinations. The green area is the passively Q-switched pulse state, and the white area is the CW state of the laser. It can be clearly seen from Fig. 16 that in the area where the 2nd pump power is greater than 200 mW, as the 2nd pump power increases, the variation range of the 1st pump power in the passively Q-switched pulse area decreases. The opposite is true in the area where the 2nd pump power is less than 200 mW. As the 1st and 2nd pump power increases to a certain power, the passively Q-switched pulse disappears. Table 4 demonstrate and analyzes the causes of pulse generation under different pump power combinations. When the center wavelength changes, its Q-switched pulse and CW area maps also change. Because under the same pump power, different centers wavelengths have corresponding pulse sequences.

Fig. 16
figure 16

Range of pulse output at a fixed center wavelength of 1545 nm for two pump powers combined

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Table 4 Analyzes the causes of pulse generation under different pump power combinations
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4 Conclusion

In this paper, the PQFL with cascaded doped fiber segments and the TBF to achieve all-fiber, low-threshold, single-wavelength tunable fiber is proposed. The two-stage commercial EYDFA not only provides cavity gain, but also acts as SA to achieve passively Q-switched pulse output when the doped fiber is non-completely pumped. Under different pump power combinations, stable pulses with repetition frequencies from 8.23 kHz to 46.47 kHz and pulse widths from 12.82 μs to 2.00 μs are achieved. The output single-wavelength SMSR can reach up to 65.37 dB. The wavelength tunable range in pulse state is up to 31.04 nm. The laser has the characteristics of simple structure, wide tuning range, wavelength and pulse stability, and has broad application prospects in fields such as biomedicine and laser imaging.