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Abstract
Multi-gigahertz ultrafast fiber lasers are critical for many significant applications, including bioimaging, optical communications, and laser frequency combs. The gain fiber which is expected to simultaneously satisfy large mode-field area, highly gain coefficient and resistance to photodarkening, will effectively protect mode-locked materials/devices that generally possesses low damage threshold (<10 mJ/cm2) and enhance stability in the centimeter-scale fiber lasers. However, the gain fiber still remains a significant challenge. In this study, multi-element Er-Yb: silica glass fibers with large mode-field area are fabricated. Benefiting from the multi-element design, normalized frequency V-parameter of the silica glass fiber with a core diameter of around 10 µm is <2.405. Using the large mode-field area fibers, ultrafast fiber lasers with 1.6 GHz fundamental repetition rate are proposed and demonstrated. The signal-to-noise rate of the radio-frequency signal reaching up to 90 dB and the long-term stability are realized. The results indicated the fabricated large mode-field area fibers are demonstrated to be ultrafast fiber lasers with short resonant cavities, which could be extended to other rare-earth glass fiber device for exploration of high-power amplification systems.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultrafast fiber lasers with pulse repetition frequency reaching gigahertz-scale have a prominent function in optical frequency combs, lidar, non-linear biomedical imaging and material micro-processing [1–3]. However, the mode locked threshold power increases with an increase in pulse repetition rate [4]. With an increase of optical power, more thermal load is accumulated in the fiber core, which triggers a variety of non-linear effects, damages the fiber end facet and its affiliations, and induces other factors that seriously affect the stability of high-repetition-rate ultrafast fiber lasers [5,6]. For stable performance of the laser operation, the selection of a compatible gain fiber in an all-fiber laser is highly imperative. The large mode-field area fiber is a direct remedy to this challenge, as an expansion of mode field diameter results in a quadratic reduction of thermal load [7]. To accommodate the normalized frequency V-parameter requirements of single mode transmission, large mode-field area fibers with large core areas and relatively small core numerical apertures are required to culminate in output power and beam quality [8,9]. Such incredible properties underlying large mode-field area optical fibers will pave the way for innovative optical fiber devices and technology.
In addition, the realization of ultrafast fiber lasers at high repetition rate involves a problematic photodarkening phenomenon owing to the ultra-short optical resonant cavities at high pump rates. Photodarkening is a time-dependent loss caused by light irradiation and its broadband absorption wavelength ranges from visible to near-infrared [10]. It is demonstrated that the photodarkening process is related to the host material of the fiber core [11]. Quartz and high silicon-based glass materials are widely used in large mode-field area fiber amplifiers and lasers due to their high purity, stability, and low optical loss compared to low stability phosphate glasses and low mechanical strength, expensive raw fluoride materials [12–16]. Especially, Barber et al. found that photodarkening in high N. A. fluoride fiber induced by near infrared light caused significant transmission losses in the visible region [17]. In addition, photodarkening forcefully affects the optical absorption of sulphur-based materials by shifting the optical absorption edge to longer wavelengths in a metastable state [18]. In comparison, pure quartz glass fiber enhances optical performance while largely reducing the photodarkening effect [19–23]. Thus, the selection of the right fiber material is crucial to ensure the stable high repetition frequency ultrafast laser.
The performance of photodarkening has been previously reported in rare-earth doped fiber are doped with elements such as Yb3+, Tm3+, Tb3+ and Ce3+ [24–27]. In the specific theories of Yb3+-doped silica fiber, the mechanism of photodarkening has been postulated to be mainly due to the generation of colored centers, the effect of temperature, and the occurrence of clustering of Yb3+ ion in the fiber, etc [28–33]. Although the mechanism of photodarkening has not been clear revealed, numerous techniques, including design of fiber-optic materials, photobleaching, and hydrogen gas loading have been reported to suppress it [34–36]. Multiple investigations have demonstrated the benefits of phosphorus as a co-dopant in photodarkening, e.g., induced losses are more noticeable in aluminosilicate fiber than in phosphosilicate matrices [37,38]. These substantiate that photodarkening is suppressed by varying the principal component of the material. S. Jetschke et al. revealed that the co-doping of Er3+ or Tm3+ induces photodarkening parameters in Yb3+ doped silica-based fibers, causing a decrease in Yb3+ inversion and subsequently a photodarkening reduction [39].
Herein, we design and manufacture multi-element Er-Yb: silica glass fibers with large mode-field area, in which the normalized frequency V-parameter for the fiber with a 10 µm core diameter is <2.405 through the multi-element design and the photodarkening reduction is realized by co-doping and using silica-based glass host. It is demonstrated that GHz high-repetition-rate all-fiber lasers have the capacity to ensure long-term operation. The signal-to-noise rate (SNR) for mode-locking operation at fundamental repetition rate of 1.57 GHz is ∼90 dB. It is also found that by decreasing average irradiance (increasing fiber field area) on the mode locker, the measurements of mode locked spectra exhibit the long-term stability, which further strengthen the reliability of the lasers. To the best of our knowledge, this is first time to use large mode-field area gain fibers in high-repetition-rate lasers.
2. Results and discussion
2.1 Fiber characterizations and analysis
The large mode-field area gain fiber could be continuously produced, and it exhibits clear structure properties (Fig. 1(a), Fig. 1(b)). For comparison, the core diameter of the gain fiber (denoted by MYE1) is approximately 12 µm, while the inner/outer cladding diameters are 130 µm and 250 µm, respectively. Figure 1(c) shows the radial refractive index profiles at a wavelength of 633 nm. The core numerical aperture (N.A.) is 0.15. The normalized frequency V-parameter is calculated to be 3.6. A slightly depressed cladding with refractive index closed to that of pure silica is produced with a small addition of phosphorus and fluorine. The cladding absorption coefficients of the fiber is 0.43 dB/m at 915 nm. The core absorption coefficient of the fiber is 30 dB/m at 1535 nm. Element mapping and linear analysis are performed, as shown in Fig. 1(d)–(f). The core region is manly composed of Si, P, Ge, Yb, and Er. The elements distribution in the fiber is determined by a commercial scanning electronic microscopy-electron probe microscopy analysis (SEM-EPMA, JXA8230). The core and cladding absorption coefficients of the fiber are measured by the cut-back method.
Fig. 1. Characteristics and element distribution analysis of the fiber MYE1: (a) Photograph of the fabricated fiber MYE1. (b) The cross-section of the fiber. (c) Refractive index difference with radial position. (d) Electron probe microanalysis (EPMA) mapping of phosphorus on the fiber. (e) EPMA mapping of ytterbium. (f) Linear scan EPMA of the constituent elements on the fiber cross section.
As shown in Fig. 2(a), the cross section is obvious, and the core diameter for the second gain fiber (denoted by MYE2) is approximately 10 µm. In order to make the normalized frequency V-parameter <2.405, a relatively small core N.A. is required, so that a pedestal with a diameter of 25 µm is incorporated in MYE2 fiber by adding germanium to the cladding. As shown in Fig. 2(b), the refractive index difference between fiber core and pedestal is measured to be 5 × 10−3 and the corresponding core N.A. value is about 0.12. The V-parameter is therefore calculated to be 2.4 for the MYE2. It is worth to note that the refractive index profiles have a central dip due to the vaporization of phosphorus and rare earth ions during fabrication of preform. Element mapping analysis of P, Ge, Yb (Fig. 2(c)–(e)) and linear scan electron probe microscopy analysis (Fig. 2(f)) show constituent elements on the MYE2 fiber cross section. The core and cladding absorption coefficients for the fiber are 2.0 dB/m at 915 nm and 36 dB/m at 1535 nm, respectively. In both fibers, Phosphorus is incorporated by gas phase doping to increase the efficiency of energy transfer from Yb3+ to Er3+ ions. In this study, the Yb-Er: silica glass preforms are produced by a modified chemical vapor deposition (MCVD) combined with solution doping technique. The fibers are drawn at 2000 °C with an ultraviolet curing adhesive as outer cladding. The refractive index profiles of the fibers are tested using an IFA-100 at 633 nm.
Fig. 2. Characteristics and element distribution analysis of the fiber MYE2: (a) The cross-section of fiber MYE2. (b) Refractive index difference with radial position. (c) EPMA mapping of phosphorus on the fiber. (d) EPMA mapping of germanium. (e) EPMA mapping of ytterbium. (f) Linear scan EPMA of the constituent elements on the fiber cross section.
2.2 Fluorescent performance and laser resonator
The fluorescent performance of the MYE1 and MYE2 fibers is measured at first. The amplified spontaneous emission (ASE) spectra are shown in panel (a) and (b) of Fig. 3, respectively. Pumped by a laser diode (LD) operating at 974 nm, the ASE spectrum of MYE1 covers two ranges including 900–1170 nm and 1470–1640 nm, while the spectrum of MYE2 covers almost same range but the intensity shows different to some extent, indicating a successful fabrication. The design of the GHz laser resonator with large mode-field area gain fibers is illustrated in Fig. 3(c). The overall cavity lengths are less than 10 cm defined by the lengths of gain fibers MYE1 and MYE2. Development of the laser resonator is completed by depositing a multilayer dielectric coating directly onto the exposed face of the passive fiber section and affixing a semiconductor saturable absorber mirror (SESAM) to the opposite end of the fibers. A paragraph of the resonator is further shown in the inset of Fig. 3(c). The resonator section is pumped through a 980/1550 nm wavelength division multiplexer (WDM) by a LD at a wavelength of 974 nm with a maximum output power of 950 mW. The terminals of the Yb-Er: glass fibers are inserted and glued into a ceramic ferrule with an inner diameter of 130 µm and the two end facets are polished vertically. Then the common port of the WDM is connected to a fiber-type dielectric mirror, which is attached to one end of the gain fiber. The mirror surface is made by plating a multilayer SiO2/Ta2O5 dielectric film directly onto the fiber sleeve using a plasma sputter deposition system. The dielectric films exhibit high light transmission at the pumping wavelength of 974 nm and essentially flat over the 100 nm region of interest, enabling a reflectivity of 93% at 1550 nm. The other end of the gain fiber is coupled to a SESAM, which possesses a modulation depth of 18%, a non-saturable loss of 12%, a recovery time of 5 ps and a saturation fluence of 50 µJ/cm2 at 1550 nm (Batop GmbH). The GHz pulse train is coupled out via the signal port of the WDM, which is sequentially spliced to a fiber-type isolator operated at C- and L-band. Measurement of the pulse train temporal behavior is by an oscilloscope and a photodiode having bandwidths of 6 GHz and 25 GHz, respectively.
Fig. 3. Emission characteristics of the MYE1 and MYE2 fibers and the laser resonator construction: (a) Measured amplified spontaneous emission (ASE) spectrum of the fiber MYE1 with length of 8.0 cm pumped by laser diode (LD) at 974 nm; (b) Measured ASE spectrum of the fiber MYE2 with length of 6.3 cm pumped by LD at 974 nm; (c) Schematic setup of the ultrafast laser using the developed large mode-field area gain fibers, and the inset shows the photograph of the miniature laser resonant cavity. LD, laser diode; WDM, wavelength division multiplexer; DF, dielectric film; ISO, isolator, and SESAM, semiconductor saturable absorber mirror.
2.3 Generation of gigahertz ultrashort pulse
Figure 4 summarizes the measurements of ultrafast laser performances when the laser using 8.0-cm fiber MYE1. The dependence of the average output power on launched pump power is illustrated in Fig. 4(a) and, as indicated, the laser of Fig. 3(c) operates in a Q-switched, mode-locking regime when the pump power is decreased below 205 mW, while in a pulsation regime when the pump power increases above 359 mW. Importantly, the laser threshold for CW mode-locking regime lies at 205 mW and the variation of output power with input power remains linearly in CW-mode locked regime but the slope efficiency reduces. At pump power of 249 mW, the mode-locked spectrum (Fig. 4(b)) shows almost symmetrical extending from below 1530 nm to beyond 1540 nm. Panel (c) in Fig. 4 shows the measurement of autocorrelation trace of the pulse and its temporal width (FWHM) is found to be 2.2 ps, assuming the intensity profile to be Gaussian. The pulse train temporal behavior, as illustrated in Fig. 4(d), shows output pulses separated by ∼ 803 ps, corresponding to the fundamental repetition rate of 1.25 GHz. Figure 4(e) shows radio-frequency (RF) spectrum recorded between 1.2445 GHz and 1.2465 GHz. A single peak at 1.245 GHz is observed and the background noise is suppressed by 82 dB. In order to demonstrate the long-term stability, the spectra are recorded and are shown in false color in Fig. 4(f) for 220 minutes (a datum recorded per two-minutes). Throughout the measurement, the launched pump power is held constant at 240 mW. With few exceptions; however, the laser spectrum is very stable, indicating that the developed MYE1 fiber as a gain fiber largely reduces the undesirable perturbation of the laser and ensures the stability of the laser in long-term mode-locked operation.
Fig. 4. Laser characteristics of the mode-locked fiber laser based on the MYE1 fiber: (a) Measured variation of the laser average output power with the launched pump power (974 nm). (b) Optical spectrum of the laser at a pump power of 249 mW. (c) Autocorrelation trace of mode-locked pulse (red line) and Gauss fitting trace (blue short dash dot). (d) Laser waveform measured with an oscilloscope and a photodiode having bandwidths of 6 GHz and 25 GHz, respective. The interval between intensity peaks is 803 ps. (e) Spectrum of the photodiode signal in the 2 MHz region acquired with an RF spectrum analyzer. (f) False color map of the mode-locked spectra recorded for 200 minutes. In recording the data, the launched pump power was fixed at 240 mW.
Using the fiber MYE2, the CW mode-locking is further demonstrated in a 6.3 cm-long laser cavity. The variation of the laser output power with the launched pump power is shown in Fig. 5(a). In this case, the pump threshold for CW mode-locking increases to be 488 mW, and the instability regime for the Q-switched mode-locking is extended to the range of 80 ≤ P ≤ 488 mW. The average power suffer severely disturbed in Q-switched mode-locking regime but it could recover to a linear slope when the laser operated in CW mode-locking regime. Representative mode-locked spectrum for the MYE2 case is measured and shown in Fig. 5(b). A peak wavelength maintains at 1535 nm and a “dip” segment at 1538 nm arises. As shown in Fig. 5(c), if a Gaussian pulse profile is assumed with a deconvolution factor of 1.54, the pulse width is 2.3 ps. Correspondingly, the mode-locked waveform of the oscilloscope trace reveals that the pulses have a temporal period of 636 ps, as shown in Fig. 5(d), which matches the cavity round-trip time for the 6.3 cm cavity length and indicates the fundamental cavity frequency of 1.57 GHz. The RF spectrum of the laser recorded between 1.5697 and 1.5717 GHz at a 10 Hz resolution bandwidth is shown in Fig. 5(e). The SNR of 89 dB is obtained. The SNR is considerably high, illustrating that the noise in the oscillator is well suppressed. Furthermore, the false color map of the optical spectrum of the mode-locked operation is recorded in Fig. 5(f), implying improved long-term stability of the laser that employed the large mode-field area gain fiber MYE2 as the gain medium.
Fig. 5. Mode locked laser characteristics based on the MYE2 fiber: (a) Measured variation of the laser output power with the launched pump power. (b) Optical spectrum of the mode-locked laser at a pump power of 533 mW. (c) Measured autocorrelation trace of the ultrafast pulse (red) and Gaussian fitting (blue). (d) Measured laser waveform with a fundamental repetition rate of 1.575 GHz. (e) Measured RF spectrum in the 2 MHz region acquired with an RF spectrum analyzer with RBW of 10 Hz. (f) False color map of optical spectrum for 200 minutes. While recording the data, the launched pump power was maintained at 535 mW.
3. Conclusion
We have demonstrated gigahertz all-fiber ultrafast lasers that can be operated in long-term stability by using the developed large mode-field area Er/Yb: silica glass fibers. The multi-element (Si, P, Ge, Yb, and Er in the core region) show relatively small numerical apertures to make V-parameter <2.405. Moreover, the photodarkening effect is further suppressed in the silica glass host. In resonant cavity, the mode-locker could be effectively protected by the large mode-field area fibers. The results highlight that the multi-element design may stimulate new concepts for creation of large-more field single-mode gain fibers, providing new directions to improve the stability for gigahertz ultrafast fiber lasers. In further endeavors, the laser operation in higher repetition rate could be realized based on the large more-field area strategy. Moreover, the large-mode field gain fiber with double-cladding structure is expected to realize higher average power in high power laser systems.
Funding
National Natural Science Foundation of China (61905205); Key Programs of the Chinese Academy of Sciences (KGFZD-145-22-13).
Disclosures
The authors declare no conflicts of interest.
Data availability
The Data that support the findings of this study are available from the corresponding author upon reasonable request.
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