1. Introduction

Passively mode-locked fiber lasers with a multi-gigahertz repetition rate are key components of many applications, including high precision frequency metrology, arbitrary waveform generation, nonlinear optical microscopy techniques, and polarized electron beams for electron accelerators [1–4]. For example, the high repetition rate laser pulses at a wavelength range of 1.2-1.35 µm, where most live biological specimens have a low light attenuation [5], are expected to increase two-photon excitation imaging speed for the higher contrast ratio image in deep specimens [6]. The previous work has demonstrated that the energy at the wavelength of 1 µm can be readily transferred to low frequency components at these wavelengths by intrapulse Raman scattering [7]. Ultrafast fiber lasers with the fundamental repetition rate exceeding 1 GHz have been experimentally realized over the past ten years [8–21]. At 1.5 µm telecom wavelengths, Martinez et al. [12] reported the highest repetition rate of 19.45 GHz in a passively mode-locked Er-fiber laser in 2011. In the 2 µm spectral region, Cheng et al. [18] have recently demonstrated the highest fundamental pulse repetition rate of 1.6 GHz using a 5.9 cm heavily Tm³⁺-doped barium gallo-germanate glass fiber. In the 1 µm wavelength region, where the gain fibers generally experience positive group velocity dispersion (GDD), the highest fundamental repetition rate is reported to be 3 GHz [21]. However, bulk optics were employed in the cavity, eliminating the advantage offered by the all-fiber configuration.

In contrast to typical mode-locked fiber lasers, high-repetition-rate lasers have generally suffered from instability in ultrashort laser cavities. Recently, the instability of Q-switched mode-locking and pulsation in ultrashort oscillators has been theoretically and experimentally investigated [22]. It has revealed that the regime of Q-switched mode-locking dramatically expands with increasing the repetition rate.

In this letter, to the best of our knowledge, we experimentally demonstrate the highest fundamental pulse repetition rate in a passively mode-locked Yb-doped fiber laser, reaching 5 GHz. The threshold pump power and radio-frequency signal-to-noise ratio for the oscillator were optimized to be 42 mW and 90 dB, respectively. The laser operated with a peak wavelength of 1058.7 nm and pulse duration of 2.60 ps. Moreover, the wavelength and repetition rate of the mode-locked oscillator can be continuously tuned by increasing the laser cavity temperature. By implementing temperature control on the gain fiber and the absorber, long-term stability of the high-repetition-rate oscillator is realized.

2. Experimental setup

The heavily Yb³⁺-doped fiber (YDF) used in the experiment was fabricated using a fiber-drawing tower by the rod-in-tube technique. The Yb³⁺ doping concentration in the core region reaches 15.2 wt%, and the net gain coefficient of the fiber was measured to be 5.7 dB/cm. The YDF has 5/125 µm core/cladding diameters with a numerical aperture of 0.14. More details about the fiber are described in [23].

A schematic of the experimental setup is shown in Fig. 1(a). A 2-cm YDF was pumped by a 974-nm laser diode (LD) through a wavelength division multiplexer (WDM). The YDF was inserted and glued in a ceramic ferrule with an inner diameter of 125 µm, both end facets of which were then perpendicularly polished. One end of the YDF was butt-coupled to a fiber-type dielectric mirror, which was spliced to the common port of the WDM. The mirror was fabricated by directly coating multiple-layer SiO₂/Ta₂O₅ dielectric films onto a fiber ferrule using a plasma sputter deposition system. The dielectric films have a high transmittance at a pump wavelength of 974 nm as well as a reflectivity of 99.75% at a wavelength of 1059 nm. The other end of the YDF was connected to a semiconductor saturable absorber mirror (SESAM), which is capable of being sandwiched between the YDF and fiber ferrule because of its compact size. The SESAM, with a chip area of 1.0 × 1.0 mm and thickness of 450 µm, has a modulation depth of 5%, a non-saturable loss of 3%, a recovery time of 1 ps, and a saturation fluence of 40 µJ/cm² at 1040 nm (Batop GmbH). The reflectivity of the non-saturated SESAM at a laser wavelength of 1059 nm is 93.64%. A polarization controller was utilized for optimization of the signal-to-noise ratio of the fundamental repetition rate signal. The net GVD of the laser cavity is estimated to be ~350 fs².

 

Fig. 1 (a) Schematic setup of the 5 GHz Yb³⁺-doped all-fiber oscillator. (b) Picture of the laser cavity, along with the temperature control modules. LD, laser diode; PC, polarization controller; WDM, wavelength division multiplexer; DF, dielectric films; YDF, Yb³⁺-doped fiber; SESAM, semiconductor saturable absorber mirror; and TC, temperature control.

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In addition, a thermo electric cooler was used as the temperature control (TC1) for stabilizing and tuning the laser wavelength and repetition rate by maintaining constant temperature of the YDF. TC2, connected to the copper ferrule on which the SESAM was placed, will remove the dissipated heat on the absorber to guarantee long-term stability in operation. A picture of the laser cavity as well as the temperature control modules is shown in Fig. 1(b).The laser light is coupled out via the signal port of the WDM. The optical spectrum of the output laser was measured by using an optical spectrum analyzer (YOKOGAWA AQ6370B). The temporal waveform was detected by using a 25 GHz photodetector (PD) and a 20 GHz bandwidth digital oscilloscope (Keysight DSOV204A). The pulse duration was measured by an autocorrelator (APE Pulsecheck USB 50). The radio-frequency (RF) spectrum was detected by using a signal analyzer (Rohde&Schwarz FSWP26).

3. Results and discussion

Measurements of the dependence of the output power of the oscillator on the launched pump power are summarized in Fig. 2(a). The continuous wave (CW) laser oscillation starts at a pump power of P = 16 mW. In the range of 16 ≤ P ≤ 42 mW, the output power increases linearly with increasing pump power. Once the pump power threshold of P = 42 mW is reached, self-started mode locking of the oscillator is achieved and the output power increases linearly but with a slightly higher slope. The threshold pump power of 42 mW is remarkably low and can be explained by the high reflectivity of the dielectric film at the laser wavelength.

 

Fig. 2 (a) Variation of the output power with the pump power, and the mode-locked pump threshold observed was as low as 42 mW. (b) Optical spectrum of the fundamental mode-locking operation at a pump power of 60 mW. (c) Measured autocorrelation trace of mode-locked pulse (blue square) and Sech² fit trace (red line). (d) Typical pulse train of fundamental mode-locked fiber laser with 4.95 GHz repetition rate. (e) Measured RF spectrum of the fundamental mode-locking operation. Inset: the broad-span RF output spectrum.

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Figure 2(b) illustrates the optical spectrum of the CW mode-locking as the pump power increased to 60 mW. The optical spectrum extends from 1057 to 1063 nm, having a full width at half maximum (FWHM) of ~1.0 nm and a peak wavelength of 1058.7 nm. The spectrum presents asymmetric shapes on the top, which is different from the trapezoid-spectrum shape of a typical normal-dispersion mode-locked fiber laser [24]. It could be attributed to the nonlinear absorption of the SESAM with a recovery time of 1 ps, resulting in the asymmetric absorption on the leading and trailing edges of the temporal optical pulse [25,26]. Particularly for the case of positive chirping, the leading part of the optical pulse will experience more loss because of unsaturated absorption of the SESAM, leading to lower intensity on the red side of the optical spectrum. In order to measure the pulse autocorrelation trace, the laser was amplified to ~4 mW. As shown in Fig. 2(c), if a hyperbolic secant pulse profile is assumed with a deconvolution factor of 1.54, the pulse width is 2.60 ps. The pedestals in the autocorrelation trace originate from the amplification process. Furthermore, an additional positive chirp is to be expected from the amplification fibers. Correspondingly, the mode-locked waveform of the oscilloscope trace reveals that the pulses have a temporal period of 201.95 ps as shown in Fig. 2(d), which matches the cavity round-trip time for the 2 cm cavity length and indicates the fundamental cavity frequency of 4.95 GHz. The RF spectrum of the mode-locked fiber laser recorded between 4.94368 GHz to 4.94768 GHz at a 10 Hz resolution bandwidth is shown in Fig. 2(e). The fundamental frequency is located at a repetition rate of 4.94568 GHz with a signal-to-noise ratio (SNR) of up to 90 dB. The SNR is considerably high, illustrating that the noise in the oscillator was well suppressed. The inset to Fig. 2(e) shows the fundamental repetition frequency and its harmonics out to a frequency range of 25 GHz (limited by the bandwidth of the PD), recorded at a 10 kHz resolution bandwidth. The clear intensity demonstrates that the laser is operating well at CW mode-locking, without spurious Q-switched instabilities.

We find that, interestingly, the wavelength of the mode-locked optical spectrum can be continuously tuned at different cavity temperatures, which is depicted in Fig. 3(a). The peak wavelength increases from 1056.7 to 1060.9 nm as the temperature of TC1 increases from 7 to 35°C. In the process, the intracavity energy becomes higher because of the SESAM presents a higher reflectivity at a longer wavelength, which will induce that intracavity pulses experience the higher phase change and raise a positively chirped component in the normal GVD system [26]. The component will further be shaped by the self-absorption modulation in the SESAM and can lead to the spectral broadening. Meanwhile, the repetition rate correspondingly decreases from 4.945874 to 4.945496 GHz, as shown in Fig. 3(b). The tuning process in the experiment is reproducible as well as reversible. In particular, for high-repetition-rate ultrafast lasers, the gain fiber length, l, is generally shortened to a few centimeters (e.g., ~2 cm in the experiment); thus, an absolute length change ∆l has a stronger impact on a short laser cavity than on a long cavity in a traditional ultrafast fiber laser.

 

Fig. 3 Spectra of wavelength (a) and repetition rate (b) tunable mode-locked operation at different YDF temperatures controlled by TC1. (c) False color map of optical spectra of high-repetition-rate mode-locked oscillator recorded for 48 h, one datum per half-hour. While recording the data, TC1 and TC2 were maintained at 25°C and 17°C, respectively.

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With a long-term mode-locking operation, the increase of the laser cavity temperature was observed in the experiment because of the heat accumulation on the YDF. Therefore, to guarantee the stability of the wavelength during long-term mode-locked operation, TC1 is implemented to remain at a constant temperature to make the laser cavity free from temperature perturbation and ensure that the wavelength is stable. In addition, TC2 is further exploited to remove the dissipated heat on the absorber from the relevant intensity modulation process as well as the residual pump. As a result, the spectra recorded for the present oscillator are shown in false color in Fig. 3(c) for 48 h (a datum recorded per half-hour), with few exceptions; however, the laser spectrum in Fig. 3(c) is very stable. Throughout these experiments, TC1 and TC2 were fixed at 25°C and 17°C, respectively.

4. Conclusion

In conclusion, we have demonstrated a 5 GHz fundamental repetition rate, wavelength tunable, all-fiber passively mode-locked Yb-doped fiber laser. To the best of our knowledge, the repetition rate of 5 GHz is the highest among the previously reported Yb-doped fiber lasers. With regard to the temperature-sensitive, high-repetition-rate lasers, the CW mode-locked pump threshold power is optimized to as low as 42 mW and the SNR of the RF spectrum is 90 dB. The mode locked oscillator has a peak wavelength of 1058.7 nm and pulse duration of 2.60 ps. In particular, the wavelength and repetition rate of the high-repetition-rate oscillator can be continuously tuned by controlling the laser cavity temperature. Furthermore, the mode-locked operation has long-term stability from exploiting two temperature controls, both on the laser cavity and the absorber. In further endeavors, the output power of the oscillator is expected to be scaled up with an appropriate reflectivity of the fiber-type dielectric mirror.