Noise-like pulses with 2.15 MHz repetition rate in an all-polarization-maintaining mode-locked fiber laser at the center wavelength of 1550 nm

Fiber lasers have been used in communication, military, industrial, medical and other fields widely due to their advantages of good beam quality, high efficiency, low threshold, good heat dissipation, compact structure and so on. Since Horowitz et al first reported noise-like pulses (NLPs) in an erbium-doped fiber laser (EDFL) with nonlinear polarization rotation (NPR) [1], NLPs have attracted researchers' great interest. Different ways are developed to obtain NLPs, such as the following mode-locked techniques: nonlinear optical loop mirror (NOLM) [2–4], nonlinear amplifying loop mirror (NALM) [5–7], NPR [8–10], real saturable absorber [11, 12]. Currently, the center wavelength of NLPs is mainly distributed in 1030 nm [8] 1044 nm to 1057 nm [13], 1319.17 nm [14], 1530.5 nm [15] 1564.9 nm, 1568 nm [5], 1895 nm to 1942 nm [11] and so on.

Compared with traditional mode-locked ultrafast pulses, NLPs are wave packets composed of many ultrashort pulses that are randomly evolved in intensity and width, the interior of the pulse wave packet continues to have random spectral fluctuations with low phase coherence [10, 16]. It is called an NLP due to the pulse's overall contour is like noise. The NLP spectrum is broad and smooth without Kelly sidebands or steep edges. But the pulse profiles in the time domain from different NLPs are not exactly the same, so as the autocorrelation trace of different NLPs. In general, the pulse profiles of NLPs in the time domain can be categorized into three types: square shape, Gaussian shape and the coexistence of square and Gaussian shape [6, 7, 14, 17]. In addition, the autocorrelation traces of NLP mainly include two types: a narrow coherent peak with a wide rectangular shoulder [2, 5, 6] and a narrow spike on a wide parabolic-shape pedestal [8, 18, 19]. NLPs are easier to obtain lower coherence, higher peak energy and wider spectrum than conventional pulses [3, 20, 21]. Based on the above characteristics, NLPs have potential in many applications, such as low spectral coherence interference [22, 23], spectral domain optical coherent tomography [21, 24], optical sensing [25, 26], micromachining [27] and generating super-continuum spectrum [28–30].

In practical applications, the stability of lasers is very important. However, the structure of NPR suffers from environmental perturbations and mechanical vibrations, and thus the stability of this type of lasers will be degraded seriously. Fortunately, building a mode-locked fiber laser with an all-polarization-maintaining (all-PM) structure can effectively overcome these problems [31–35]. For instance, NLP generation in a mode-locked thulium-doped fiber laser with NALM configuration and different net anomalous dispersions was demonstrated in [34]. Besides, low threshold soliton and NLP conversion in an all-PM figure of eight cavity was demonstrated in [35] at a center wavelength of 1560 nm. In addition, reducing the pulse repetition rate is a good way to increase pulse energy and the pulse repetition rate can be reduced by increasing the length of the cavity [36 – 38]. Moshe Horowitz and Yaron Silberberg investigated noise-like operation of long-cavity lasers [39]. Recently, we have also reported a self-started mode-locked erbium-doped all-PM fiber laser that generates a Gaussian-shape NLP train with relatively low repetition rate and moderately high energy at the center wavelength of 1550 nm by using an NALM configuration [40]. This paper is an extension of our previous reports, which will help people to understand the characteristics of NLP. We observe the effect of different pump powers on the characteristics of NLPs, which confirm with the results of previous work. The single pulse energy is approximately 11.68 nJ at a repetition rate of 2.15 MHz. We generate a shorter width of the peaks on the autocorrelation trace after extra-cavity compression, which shows that the SMFs have an obvious compression effect on NLPs. Through obtaining the different radio-frequency (RF) spectra, such as the single-sideband (SSB) phase noise, 'Maxhold', 'Minhold' and 'Average' function, and other monitoring result, the stability of the laser is tested to be good.

The schematic of the all-PM figure-of-9 EDFL for generating Gaussian-shape NLPs is shown in figure 1. The laser is composed of an NALM and a reflection cavity. The NALM consists of 1 m erbium-doped fiber (EDF) (Coractive ER35-7-PM, 35.0 dB m⁻¹ core absorption at 1530 nm) pumped by a 980 nm laser diode through a 980/1550 PM wavelength division multiplexing (WDM) coupler, a PM Phase Delay Faraday Rotator (PDFR) that ensures mode locking self-started, a 60 m PM dispersion compensation fiber (DCF) (−20 ps nm⁻¹ km⁻¹ at 1550 nm) and another 30 m PM-DCF which are used to change the dispersion of the cavity and increase the cavity length. The NALM is connected to the reflection cavity by a 45:55 PM coupler between the two DCFs. The reflection cavity contains a PM band pass filter (BPF) centered at 1550 nm with 3 dB bandwidth of 2 nm and a mirror with a reflection rate of 80%. Bidirectional pulses in the NALM interfere at the coupler, introducing disparate loss which acts as a saturable absorber. This laser works in a large positive dispersion zone. The PDFR provides an additional phase bias in bidirectional pulses and increases initial nonlinear phase shift in the cavity, which ensures self-started mode-locking. The BPF is employed to remove the enormous chirp accumulated in every pass in the cavity. Since all the components are PM, the oscillator is stable and insensitive to the environmental fluctuations and mechanical vibrations.

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Based on the above configuration, a single Gaussian-shape NLP could be easily generated with a pump power of above 200 mW when NALM is playing the role of a saturable absorber. In addition, the laser can be self-starting mode-locked by using PDFR, which introduces a certain phase difference to the cavity. Figure 2(a) shows the typical single Gaussian-shape NLP spectrum when the oscillator was pumped at 550 mW. The optical spectrum is smooth and the 3 dB spectral bandwidth is measured to be 3.1 nm. Besides, the center wavelength of the spectrum is at 1550 nm, which is in the low-loss communication window for optical communication. Figure 2(b) shows the Gaussian-shape pulse train in the time domain. A repetition rate of 2.15 MHz is obtained owing to the relatively long cavity length (shown in figure 2(c)). The signal-to-noise rate of the RF is over 50 dB, indicating that the fiber laser is stable. To further confirm that the pulse operates in NLP regime, we pass the output light through the autocorrelator and finally obtain its autocorrelation trace from the oscilloscope. The scan results show a narrow coherent peak with a wide rectangular shoulder, which is one of the typical autocorrelation trace of NLP (shown in figure 2(d)). Therefore, considering the above characteristics, the obtained pulse is a typical NLP.

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Since the whole laser is an all-PM structure, we cannot adjust polarization controllers as previous research. Here, the variation characteristics of the NLP under the all-PM structure are investigated by adjusting the pump power. When the pump power increases from 200 mW to 550 mW (mode locking starts at 200 mW and 550 mW is the maximum power of the pump source), the 3 dB spectral bandwidth of the laser slightly increases but the spectral shape is almost unchanged as shown in figure 3. It is worth noting that the center wavelength is always maintained at 1550 nm. The result of this spectral change coincides with the previous report of NLPs [4, 6, 7]. In addition, as the pump power increases from 200 mW to 550 mW, the pulse width evolves more obviously without splitting. This is because that the increase of pump energy changes the energy and quantity of pulses in the NLP wave packet, and the small pulse re-evoles into a new pulse wave packet.

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Next, we observe the change in the autocorrelation traces by adjusting the pump power and adding a spool of single-mode fiber (SMF) (17 ps nm⁻¹ km⁻¹ at 1550 nm) outside the cavity to compress the pulse, as previously carried out in [41, 42]. It can be seen from figure 4 that when the pump power increase from 200 mW to 550 mW, the height of peak and rectangular shoulder of the autocorrelation trace are constantly increasing. When using the negative-dispersion SMF to perform the extra-cavity compression on the NLP, the peak of the autocorrelation trace are obviously stretched by the 50 m SMF compression (shown in figure 4(b)). To understand the effects of extra-cavity compression ways on NLPs further, we measure the full width at half maximum (FWHM) of the peak on the autocorrelation trace under compression conditions when the pump power changes (shown in figure 5) [18]. It can be seen that the extra-cavity compression has a significant effect on the FWHM of the peak on the autocorrelation trace of NLP, which resulted from the negative dispersion provided by the SMF outside the cavity compresses the positive-dispersion pulse spread of the NLP. Figure 6 shows the NLPs under 50 m SMF compression and no compression at the pump power of 500 mW. In figure 6(a), we shift half of the peak power of the two autocorrelation traces to zero voltage. Figure 6(b) is the enlarged view of figure 6(a). It can be clearly seen that the FWHM of the peak on the autocorrelation trace with 50 m SMF is smaller significantly compared with the one without extra-cavity compression. Therefore, it can be concluded that it is effective to perform extra-cavity compression on NLP using SMFs.

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In order to verify the stability of the laser, the frequency responses of the NLP at the pump power of 550 mW are observed by an electronic spectrum analyzer through a 15 GHz photodetector (PD). Figure 7(a) shows the measured RF spectrum of the NLP in a span of 50 MHz. It can be seen that the modes in the entire spectrum are independent. In addition, the longitudinal mode spacing is constant and the height is basically the same, indicating that the laser is well mode-locked. Figure 7(b) is a measured and fitted curve of the measured SSB phase noise. The phase noise at the carrier frequency of 2.15 MHz is −32 dBc Hz⁻¹ and the calculation of the jitter is approximately 0.33 ms (in the 3 Hz–1 MHz frequency range). The sudden jump at ∼100 kHz is primarily due to relaxation oscillations. The relatively large timing jitter is due to that the integration is performed starting from a very low starting frequency (3 Hz), so that slow fluctuations (<kHz) are typically very large and dominate in the jitter calculation. Figure 7(c) demonstrate the output electrical power stabilities of the NLP signal by using the 'Maxhold', 'Minhold' and 'Average' function of the RF spectrum analyzer [43]. Within a measuring time of 20 min. The peak power of the three tracks almost coincides and the jitter is extremely small, indicating that the stability of the signal power is very excellent. Besides, we have monitored the output power of the designed laser for 60 h (shown in figure 8), and the fluctuation of 1.27% (RMS) represents that the output power of the laser is stable overall.

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Finally, we measured the change of output optical power of the laser without any SMF when the pump power increase from 200 mW to 550 mW (shown in figure 9(a)). Obviously, the output power of the NLP varies linearly with the pump power. Interestingly, as reported in previous paper [44], a small mutation occurred somewhere in the process of linear change. Since the repetition rate of the laser does not vary with the pump power, the corresponding output pulse energy of the laser can be calculated. When the pump power is increased from 200 mW to 550 mW, the output power changes from 5.19 mW to 25.11 mW, and the pulse energy changes from 2.41 nJ to 11.68 nJ. It can be concluded that increasing the pump power is an effective way to obtain high NLP energy (shown in figure 9(b)).

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In conclusion, we have reported a self-started mode-locked erbium-doped all-PM fiber laser that generates a Gaussian-shape NLP train with a relatively low repetition rate and a moderately high energy at the center wavelength of 1550 nm by using an NALM configuration. We investigated the influence of pump power and extra-cavity dispersion on the evolution characteristics of NLP, the pump power can not only change the spectrum and pulse wave packet, but also has a positive correlation effect on NLP energy. Extra-cavity SMF is beneficial to the compression of NLP due to the negative dispersion provided by the SMF outside the cavity compresses the positive-dispersion pulse spread of the NLP. In addition, we verified that the NLP has excellent stability, which can be attributed to the all-PM figure-of-9 structure. More importantly, the center wavelength of the NLP laser is 1550 nm, which is an important band in the field of optical communication. Furthermore, we effectively reduced the repetition rate and obtained a moderately high pulse energy. This scheme provides a new promising idea for building stable NLP fiber laser at the center wave of 1550 nm.

This work was supported by National Natural Science Foundation of China (NSFC) (61801037, 61625104, 61527820, 61671071, 61675031, 61501051, 61431003); The Open Research Fund of State Key Laboratory of Space-Ground Integrated Information Technology (2015_SGIIT_KFJJ_DH_02); Fund of the Key Laboratory of Astronomical Optics & Technology, CAS (CAS-KLAOT-KF201503), and Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), P. R. China.