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Abstract
We demonstrate the generation of soliton and dissipative soliton in an ultrafast thulium (Tm) doped fiber laser based upon cross-phase modulation (XPM) induced mode-locking. The mode-locking is realized by periodically modulating the 2-µm signal through XPM that is activated by an injected 1.5-µm pulsed laser. Such a mechanism enables the laser to be mode-locked in various operation regimes without any real or artificial saturable absorbers. Thanks to the XPM pulling effect, the wavelength of the Tm-doped fiber laser can be tuned by adjusting the repetition frequency of the 1.5-µm pulsed laser. The maximum tuning ranges achieved in this work for the soliton and dissipative soliton regimes are respectively 11 nm and 15 nm. The outcomes of this work not only provide a continuously and controllably wavelength-tunable ultrafast laser but also offer a passively synchronized dual-color fiber laser system, which is promised for many important applications such as Raman spectroscopy, nonlinear frequency conversion systems, and multi-color pump-probe systems.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Mode-locked fiber lasers operating in the “eyesafe” spectral region around 2 µm have been intensely studied by research communities in the past decades, due to their significance in a wide range of applications such as sensing, spectroscopy, material processing, and medicine [1–5]. A number of mechanisms to realize mode-locking operation in fiber lasers have been explored. Compared with passively mode-locked fiber lasers relying on real or artificial saturable absorbers (SAs) [6–9], actively mode-locked fiber lasers exhibit superiority in the aspects which are important for practical application, such as high damage threshold and high environmental stability. Thus far, the dominant approach to realize active mode-locking is to enforce acousto-optic modulation [10] or electro-optic modulation [11] on the intracavity loss of laser. However, the large insertion loss and long response time of the modulators restrict the performance of such systems. An alternative approach is to exploit the optic-optic modulation to control the nonlinear refractive index of the propagation medium, which takes the form of the interaction between the signal laser and a pulsed laser performing as a “drive”. The typical optic-optic modulation is cross-phase modulation (XPM), and the all-optical passive synchronization based on XPM is applied to passively stabilize the signal laser cavity by drive pulses with a locked repetition frequency [12–14]. Remarkably, the tuning range of the repetition frequency in this passive synchronization is almost two orders of magnitude higher than the optical-path elongation accessed by the active synchronization [15]. The mechanism is also employed to directly induce mode-locking in a fiber laser [16]. In such a case, the mode-locking is triggered by injecting a pulsed laser (i.e., the modulation drive) that possesses a repetition frequency that is also identical to the round-trip frequency of the signal laser. The performance of XPM-induced mode-locking is superior to those of conventional active mode-locking schemes which rely on modulators, given that the deleterious effects associated with the modulators (insertion loss, risk of optical damage, slow response speed, etc.) are avoided. On the other hand, XPM-induced mode-locking still possesses the inherent advantages of active mode-locking, e.g., the high environmental stability.
It is well known that according to the net dispersion in a cavity, mode-locked lasers can operate different regimes including soliton and dissipative solitons [17], and the dissipative soliton is more preferred in certain applications due to the higher pulse energy. Hitherto the research efforts on XPM-induced mode-locking are still very limited. The only reported case is demonstrated in the soliton regime with the wavelength of drive pulses (1.54 µm) very close to signal pulses (1.56 µm) [16]. Such a strict condition for drive pulses limits further exploration on XPM-induced mode-locking. Hence, more comprehensive studies are strongly desired to demonstrate the universality of the XPM-induced mode-locking under free wavelength choice of driving pulses and for other operational regimes and give the corresponding laser performance based on this method.
In this paper, we demonstrate a Tm-doped fiber laser mode-locked via XPM. The XPM effect is activated by a 1.5-µm pulsed laser. High-quality mode-locking operation is established even though the wavelength of drive pulses is far from that of the signal. By managing the intracavity dispersion, the laser is capable of operating in both soliton and dissipative soliton regimes. A self-synchronization between the two-color pulses linked by XPM is observed: as the repetition frequency of the drive pulse is varied in a certain range, the frequency of the signal pulse will change accordingly, and the mismatch between the updated frequency and the original round-trip frequency is compensated by a spontaneous wavelength shift of the signal laser. This property associated with the self-synchronization provides a unique wavelength-tunability: the central wavelength can be tuned continuously and controllably though adjusting the repetition frequency of modulation drive without any extra wavelength-tunable elements. In the experimental works, the maximum tuning ranges operating in soliton and dissipative soliton regimes are 11 nm and 15 nm, respectively. The outcomes of this work can be informative for developing novel 2-µm ultrafast lasers and optimizing passive synchronization systems.
2. Experimental setup
The diagram of the experimental setup is presented in Fig. 1(a). The laser is established in an all-fiber architecture which employs a 0.2-m-long Tm-doped fiber (Nufern SM-TSF-5/125) as the gain medium. The Tm-doped fiber is pumped by a continuous-wave (CW) erbium (Er) doped fiber laser (EDFL) operating at 1.5 µm. The 1.5-µm pump is delivered into the active fiber via a 1.5/2 µm wavelength-division multiplexer (WDM, supplied by AFR, Zhuhai, China). A polarization-independent isolator (PI-ISO, AFR) is used to guarantee the unidirectional operation of the laser. It should be noted that the pump is injected into the cavity in a opposite direction with the laser operation. A fiber polarization controller (PC, Thorlabs CPC250) is employed to manipulate the intracavity polarization state for optimizing the stability of the mode-locking. The output port of the laser is performed by the 30% arm of a 30/70 fiber optical coupler (OC, AFR) that is designed for operating at 2 µm. An all-fiber master oscillator power amplifier (MOPA) system that emits at 1.5 µm serves as the modulation drive for inducing the mode-locking through XPM. The MOPA system is composed of a seed oscillator, a preamplifier, and a final amplifier. The seed oscillator is an EDFL mode-locked using carbon nanotube (CNT). A fiberized optical delay line is inserted into the seed oscillator to adjust its repetition frequency. The output of the MOPA system is delivered into the Tm-doped fiber laser through a WDM, and is extracted through another WDM subsequently. Both WDMs are dielectric-film-based ones. As a result, XPM only occurs in the section between the two WDMs (denoted by “modulation region” in Fig. 1). Such a management excludes the interference from the gain-modulation effect, in view of that the 1.5-µm pulse is almost eliminated by the second WDM, and thus, it will not be absorbed by the Tm-doped fiber. In this work, the length of the fiber between the two WDMs (including the pigtails of the WDMs) is 2 m. All components (WDM, PC, PI-ISO, OC) in the system are fabricated using conventional single-mode fiber (SMF, Corning SMF-28). In order to investigate the performance of the laser in various dispersion regimes, there is a section in the laser cavity used for managing the dispersion. In this section we alternately use two types of fibers: convention SMF, and dispersion compensation fiber (DCF, Nufern UHNA4). The signs of the group velocity dispersion (GVD) of the SMF and the DCF are different, and thus, the total dispersion of the cavity can be changed by replacing the fiber. The fibers used for adjusting the dispersion (SMF or DCF) are of an identical length (5 m). The GVDs of the Tm-doped fiber, the SMF, and the DCF at 1.95 µm are –21ps2/km, –86 ps2/km, and 95 ps2/km [18], respectively. With a cavity length of 10.03 m, the total dispersion of the mode-locked Tm-doped fiber is estimated to be –0.85 ps2 (as SMF-28 is used in the dispersion-managing section) or 0.06 ps2 (as UHNA4 is used). As a result, the laser can operate in either anomalous-dispersion (AD) or normal-dispersion (ND) regime. It should be noted that there is no bandpass filter in the oscillator of the Tm-doped fiber laser, and the spectral filtering effect required for forming the dissipative soliton is provided by birefringence-induced filtering or the net gain bandwidth of the Tm-doped fiber [19,20]. In experimental works, the repetition frequency of the 1.5-µm modulation drive can be adjusted by managing the cavity length of the seed laser for achieving a rough matching with the 2-µm oscillator, and the optimal matching is realized by manipulating the optical delay line in the seed laser.
Fig. 1. Experimental setup. EDFL: Er-doped fiber laser; WDM: 1.5/2 µm wavelength division multiplexer; TDF: Tm-doped fiber; PC: polarization controller; PI-ISO: polarization-independent isolator; OC: 30/70 fiber coupler; SMF: single-mode fiber; DCF: dispersion compensation fiber.
The output of the mode-locked Tm-doped fiber laser is characterized with the following devices: oscilloscope (Teledyne LeCroy SDA 820Zi-B, bandwidth: 20 GHz), RF spectrum analyzer (Rohde & Schwarz FSWP), photodetector (EOT ET-5000F, bandwidth: 12.5 GHz), optical spectrum analyzer (Yokogawa AQ6376), and autocorrelator (Femtochrome FR-103 XL).
3. Results and discussion
3.1 Characteristics of modulation drive
The characteristics of the 1.5-µm drive pulse generated from the MOPA system are shown in Fig. 2. Figure 2(a) demonstrates the spectra registered under various average output power. It should be noted that the seed oscillator operates in soliton regime, whilst the Kelly sidebands are eliminated by a bandpass filter in the final-stage amplifier. Centered at ∼1562 nm, the spectrum is of a ∼5 nm full width at half maximum (FWHM) under the maximum output power (1.9 W). The autocorrelation (AC) trace of the drive pulse is presented in the inset of Fig. 2(a). The pulse duration is 58 ps assuming a sech2 pulse shape. Figure 2(b) presents the RF spectrum of the drive pulse. The repetition frequency of the modulation drive can be tuned in a span of ∼0.14 MHz (from ∼20.46 to ∼20.60 MHz) by adjusting the fiberized optical delay line. The signal-to-noise ratio (SNR) of the fundamental frequency is 65 dB, indicating moderate stability of the drive pulse. No remarkable modulation on the harmonics is observed, as shown in the inset of Fig. 2(b).
Fig. 2. (a) Optical spectrum and autocorrelation trace (inset) of the 1.5 µm pulsed laser. (b) RF spectrum of the 1.5 µm pulsed laser. Inset: RF spectrum in a 3-GHz span. Δf: tuning range of the repetition frequency.
3.2 Mode-locking in AD regime
We firstly investigate the performance of the mode-locked Tm-doped fiber laser operating in AD regime.
Fig. 3. Output characteristics of the mode-locked Tm-doped fiber laser: (a) spectrum, (b) AC trace, (c) pulse train, (d) RF spectrum around the fundamental frequency. Inset: RF spectrum in a 3-GHz span.
The laser operates in the CW mode as the pump (i.e., the EDFL) is turned on whilst the modulation drive (i.e., the MOPA) is turned off. After the output power reaches 0.64 mW (which is achieved under 207 mW pump), the drive pulse is injected into the laser cavity and gradually increased. The drive pulse periodically modifies the refractive index of the Tm-doped fiber laser, causing a periodic phase modulation, which leads to the generation of the mode-locked pulse. Through carefully adjusting the repetition frequency of the drive pulse to reach an agreement with the round-trip frequency of the Tm-doped fiber laser, mode-locking can be established as the average power of the drive pulse is raised to 280 mW. The output spectrum of the mode-locked fiber laser is presented in Fig. 3(a). It is centered at ∼1949nm with a FWHM of ∼2.4 nm. The prominent Kelly sidebands indicate that the laser operates in the soliton regime. The AC trace is presented in Fig. 3(b), based on which the pulse duration is estimated to be 1.68 ps assuming a sech2 pulse profile. The time-bandwidth product (TBP) is calculated as 0.318. The oscilloscope trace of the pulse train is shown in Fig. 3(c). Figure 3(d) presents the RF spectrum of the mode-locked Tm-doped fiber laser, from which the repetition frequency is determined as ∼20.47 MHz, with a SNR of 65 dB. No modulation on the harmonics is observed, as shown in the inset of Fig. 3(d).
The maximum achievable energy of the solitonic pulse is 103 pJ, which is acquired under 226 mW pump. Under higher pump power, the transition from soliton to other operating regimes occurs as shown in Fig. 4, given that the nonlinear effects cannot be fully balanced by dispersion.
The pulse firstly breaks into soliton molecule, of which the characteristics (recorded under a pump power of 240 mW) are presented in Fig. 5. As shown in Fig. 5(a), there is strong periodic modulation on the spectrum, which is a representative characteristic of soliton molecule. The AC trace presented in Fig. 5(b) indicates that the soliton molecule is composed of two bound solitons with an 8-ps temporal separation, which is in good agreement with the modulation period of the spectrum (∼1.5 nm) [21].
Fig. 5. Output characteristics of the generated soliton molecule: (a) spectrum, (b) AC trace, (c) RF spectrum.
As the pump power is further increased, the mode-locking operation finally evolves into the so-called noise-like pulse (NLP). NLP is a packet of individual ultrashort pulses which behaves like a single envelope in temporal domain. The spectrum of the generated NLP is demonstrated in Fig. 6(a). The AC trace is presented in Fig. 6(b). The AC trace features a double-scale structure with a narrow peak riding on a broad pedestal that is the representative characteristic of NLP [22]. The RF spectrum of the generated NLP is shown in Fig. 6(c). Large sidelobes are observed in the RF spectrum, a phenomenon that is common for NLP. The maximum achievable energy of a single NLP packet is 1.46 nJ, which is recorded under the highest available pump power (470 mW).
It should be noted that in the experimental works, as the pump power is raised, the power of the modulation drive (i.e., the 1.5 µm MOPA system) has to be increased accordingly for maintaining the effectiveness of XPM. After the XPM-induced mode-locking is established, on the other hand, further increasing the power of the modulation drive will not improve the state of the mode-locking operation. In addition, the reduction of the length of modulation region or an increase of the walk-off between the drive and the signal pulse will lead to a decrease in the nonlinear phase accumulation, which, however, can be compensated by raising the power of drive pulses. Therefore, this mode-locking method exhibits high applicability in terms of free choices of the wavelength and repetition frequency of drive pulses.
3.3 Mode-locking in ND regime
Dissipative soliton is also generated from the mode-locked Tm-doped fiber laser operating in the ND regime as the power of the pump and the modulation drive is tuned to 377 mW and 1.18 W, respectively. The blue-shifted central emission wavelength against the peak emission of the gain fiber (1950∼1970nm) and the much higher pump threshold can be partially attributed to the increased cavity loss, which is associated with the splicing between the UHNA4 fiber and the conventional SMF [23]. More nonlinear phase accumulation is required for dissipative solitons, resulting in a higher mode-locking threshold [24]. The characteristics of the generated dissipative soliton is presented in Fig. 7.
Fig. 7. Output characteristics of the generated dissipative soliton: (a) spectrum, (b) AC trace, (c) pulse train, (d) RF spectrum around the fundamental frequency. Inset: RF spectrum in a 3-GHz span.
The spectrum of the generated pulse is shown in Fig. 7(a), which exhibits a cat-ear shape that is common for dissipative solitons. The edge-to-edge width of the spectrum is ∼13.2 nm. The center of the spectrum locates at ∼1925nm. Figure 7(b) presents the AC trace of the dissipative soliton. Assuming a Gaussian profile, the duration of the uncompressed, highly chirped pulse (fitted by the blue solid line) is 49.8 ps. Figure 7(c) shows the pulse train registered using the oscilloscope. The RF spectrum is presented in Fig. 7(d), from which the repetition frequency is determined as ∼20.49 MHz, with a SNR of 60 dB. No modulation on the harmonics is observed, as shown in the inset of Fig. 7(d). We use a 150-m-long conventional SMF to compress the generated pulse. Assuming a Gaussian profile, the duration of the compressed pulse [fitted by the red solid line in Fig. 7(b)] is 1.7 ps, which is substantially larger than the Fourier limit (∼420 fs). The imperfect compression can be ascribed to the mismatched high-order dispersion and the nonlinear chirp. Unlike the case of soliton, the operation in dissipative soliton regime can be sustained in the entire tuning range of the pump power (377–470 mW). The highest average output power is 23.5 mW, corresponding to a pulse energy of 1.14 nJ, which is much higher than that of the solitonic pulse.
3.4 Synchronization and tunability of XPM-induced mode-locking
In the experimental works, it is found that the mode-locking of the signal laser is directly activated with a locked repetition frequency decided by drive pulses, and the wavelength of this mode-locked Tm-doped fiber laser is tunable in both soliton and dissipative soliton regimes. As the repetition frequency of drive pulses is changed, the repetition frequency of signal pulses varied synchronously. Such a synchronous evolution occurs in a certain range, and its consequence is that the XPM-induced mode-locking will not be weakened or terminated. Obviously, there will be a mismatch between the updated frequency and the original round-trip frequency of the laser cavity, however the mismatch is compensated by a self-driven wavelength shift of the signal laser. The variation of the repetition frequency and the wavelength of the signal laser are autogenous and instant, free of any manipulation of the laser. Such a phenomenon is attributed to the XPM pulling effect [25]. Therefore, this system can offer an inherently synchronized dual-color fiber laser system combining drive pulses and signal pulses without any extra active synchronizing components [12–15]. Meanwhile, the wavelength of the signal laser can be tuned by adjusting the repetition frequency of the modulation drive. The tunability is only available within the range where the mode-locking can be sustained by XPM pulling effect, whilst the mode-locking will be terminated outside the range. Figures 8(a) and (b) demonstrate the evolution of the output spectrum of the mode-locked Tm-doped fiber laser through the variation of the repetition frequency in AD regime (i.e., soliton) and ND regime (i.e., dissipative soliton), respectively. With an identical trend (rise or drop) in the repetition frequency, the directions of the wavelength shift of soliton and dissipative soliton are opposite, owing to their reverse GVD. Compared with previous works in which tunable spectrum filters [26–28] or polarization controllers [29,30] are essential, such a wavelength-tunability combines several advantages of continuously controllable central wavelength, free spectral width, and non-sensitive to ambient disturbances. It’s more convenient for practical applications.
Fig. 8. Evolution of the output spectrum through the rising repetition frequency of the drive pulse in (a) AD regime and (b) ND regime. Evolution of the tuning ranges for the repetition frequency and the center wavelength through the rising average power of the drive pulse in (c) AD regime and (d) ND regime. fD: repetition frequency of the drive pulse. λS: center wavelength of the signal pulse. ΔfS: tuning range of the repetition frequency of the signal pulse. ΔλS: tuning range of the center wavelength of the signal pulse. PD: average power of the drive pulse.
The dependence of the tuning range (for both repetition frequency and center wavelength) on the power of the modulation drive is discovered in the experimental works. By elevating the average power of the drive pulse, the tuning ranges of the laser in both soliton and dissipative soliton regimes are substantially increased, as demonstrated in Figs. 8(c) and (d). Such a fact stems from the enhancement of XPM pulling effect. The maximum achievable tuning ranges of the repetition frequencies for soliton and dissipative soliton are respectively ∼1.7 kHz and ∼1 kHz, which are realized under a power of 0.92 W and 1.88 W for the modulation drive, respectively. The corresponding maximum tuning ranges of the center wavelengths are respectively ∼11 nm and ∼15 nm for the soliton and dissipative soliton operation corresponding to the various repetition frequencies. Consequently, this self-synchronized dual-color fiber lasers provided in this system achieve not only the same level of tuning range for the repetition frequency but a much larger wavelength tuning range compared with the early cases of passive synchronization of dual-color mode-locked fiber lasers [12–15]. It’s worth noting that the tuning range of the XPM-induced mode-locking cannot be increased indefinitely via raising the power of the modulation drive. The upper limit is mainly relevant to the total dispersion and the gain competition effects in the laser cavity.
4. Conclusion
We experimentally demonstrate a mode-locked Tm-doped fiber laser based upon XPM-induced mode-locking, confirming the free wavelength choice of driving pulses. The laser can operate in both soliton and dissipative soliton regimes. Stable mode-locking operation is realized in the experimental works. The laser is wavelength-tunable thanks to the XPM pulling effect, and the tunability is achieved without the involvement of any filters or polarization manipulation. The maximum tuning ranges of the soliton and dissipative soliton which are realized in the experimental works are 11 nm and 15 nm, respectively. Such a system researched above can provide a flexible wavelength-tunable ultrafast fiber laser and even a tight passively synchronized dual-color fiber laser source, offering great potential for various applications such as pump-probe microscopy, Raman scattering spectroscopy, and nonlinear frequency mixing. Moreover, the demonstrated system is readily modified to other types of fiber lasers with erbium-, holmium- and ytterbium-doped gain medium.
Funding
National Natural Science Foundation of China (61775146, 61905151, 61935014, 61975136, 62105222); Basic and Applied Basic Research Foundation of Guangdong Province (2019A1515010699); Shenzhen Science and Technology Innovation Program (CJGJZD20200617103003009, JCYJ20210324094400001, GJHZ20210705141801006); Beijing Natural Science Foundation (JQ21019).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are available from the corresponding authors upon reasonable request.
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