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Abstract
Dispersive mirrors are generally used as reflective elements in ultrafast laser systems. In this study, a multifunctional highly dispersive mirror (MFDM) was prepared on the fiber tip, providing high transmittance in the pump wavelength, partial reflection and dispersion compensation in the working band. To achieve this, a starting structure that combines a quarter-wavelength structure, multi-cavities, and an anti-reflection structure was proposed. A large amount of dispersion was introduced while maintaining a certain reflectivity in the range of 1,050–1,060 nm and high transmittance at 976 nm. Employing the input and output coupling mirror as a dispersive mirror for the first time, the MFDM was successfully applied to an all-fiber oscillator to generate ultrafast seed pulses. This unprecedented MFDM is expected to replace dispersion compensation fibers, opening an avenue for the development of compact fiber lasers.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultrashort fiber lasers have been developed rapidly in recent decades [1–7]. They are widely used in bio-optics, wave crest multiplexing systems, optical combs, and other fields [8–10]. For ultrashort pulse laser systems, dispersion compensation is indispensable. Dispersion compensation fibers (DCFs) and photonic crystal fibers (PCF) are commonly used all-fiber dispersion compensation components in fiber lasers. The use of fiber for dispersion compensation is suitable for long cavity oscillators. For high-repetition short-cavity systems, fibers no longer have advantages. In addition, the unit dispersion compensation capability of the fiber is weak. If the dispersion demand is large, a very long fiber is needed. High-energy laser pulses will accumulate strong nonlinear effects when propagating inside the fiber, leading to pulse distortion. Therefore, it is necessary to develop a new method for fiber laser dispersion compensation.
Dispersive mirrors (DMs) are widely used in chirped pulse amplification (CPA) systems [11]. Due to their advantages of precise dispersion compensation, compactness, and reproducibility, DMs have become a research focus in recent years. According to the different principles of dispersion compensation, DMs are divided into chirped mirrors [12–14] and Gires–Tournois (G–T) mirrors [15,16]. A chirped mirror can reflect different wavelengths of light at different depths to compensate for dispersion. Meanwhile, a G–T mirror uses the resonance effect such that the pulse reflects multiple times in the cavity of the mirror to achieve dispersion compensation. In modern DMs both effects are employed and can be present in a very complicated interplay
Generally, a DM has a film thickness of only a few microns and excellent stability. The application of DMs in fiber-based CPA systems is mature [17]; however, they are used as external dispersion compensation devices. In solid-state lasers, a DM can be used not only for extracavity compression but also for intracavity dispersion components, which is of great significance to the integration of laser systems. The DM designed by Dombi et al. was successfully applied to an optical parametric oscillator [18,19]. A DM can also be used as an input mirror in the cavity. This requires a high transmittance at the pump wavelength, and dispersion compensation is realized at the working wavelength [20,21]. Based on the integration, stability, and versatility of the DM, the application potential of the all-fiber oscillator was demonstrated. Applying DM technology to a fiber oscillator is of great significance to all-fiber oscillators and the miniaturization of fiber laser oscillators. The DM used in the all-fiber oscillator is different from the conventional highly reflective DM. It is necessary to achieve multiple functions in the same film structure, which is difficult to design. Thus, the DM must be prepared at the fiber tip; however, it is challenging to prepare a complex film structure at the fiber tip.
Herein, we present the first result of a multifunctional highly dispersive mirror (MFDM). The MFDM prepared on the fiber tip provided more than 95% transmittance at the pump wavelength (976 nm), 93 ± 2% reflection, and −5,000 fs2 negative dispersion in the working band (1,050–1,060 nm). A starting structure of the MFDM was proposed, which combines an antireflection structure (ARS), a quarter-wavelength structure, and multiple cavities, introducing high dispersion while controlling the reflectivity in the working band and antireflection in the pump wavelength. The MFDM was successfully applied to an all-fiber laser to obtain ultrafast seed pulses with a Fourier transform limit of 259 fs. This MFDM opens an avenue for the development of compact fiber lasers.
2. Design of multifunctional highly dispersive mirror (MFDM)
A diagram exhibiting the application of the MFDM is shown in Fig. 1(a). The mirror needs to be prepared on the fiber tip, which is also the incident medium; hence, the incident medium and the exit medium are both glass.
Fig. 1. (a) Diagram of multifunctional highly dispersive mirror (MFDM) application. The MFDM is in the middle of two fibers. Thus, both the incident medium and exit medium are silicon glass. (b) Model of simplified starting structure of MFDM.
The MFDM has a transmittance greater than 95% at a pump wavelength of 976 nm, reflectivity of 93 ± 2% at 1050–1060 nm, and negative dispersion of −5,000 fs2. An appropriate starting structure can help reduce design difficulty. In the proposed design, we used a modified G–T mirror structure, as shown in Fig. 1(b). The structure can be divided into four parts: the cavity introduces a large dispersion while the quarter-wavelength structure provides reflectivity. Further, an ARS close to the incident medium layer was used to increase the transmittance of the laser working and pumping bands. An ARS close to the substrate was used to increase the transmittance of the pump band. In the design process, we used three cavities to increase the dispersion of the DM. The specific structure is as follows:
where S and A denote the substrate and incident medium silica glass, respectively; H and L denote the quarter-wavelength optical thicknesses of high- and low-index materials, respectively; C = (aH bL)m is the G–T cavity structure; m is the period of alternating layer materials; and (H2LHL) and (H2L) are ARSs. In our design, a = 1, b = 1.5, and m = 4. Nb2O5 and SiO2 were chosen as the high- and low-index materials, respectively. The refractive index is described by the Cauchy formula: where λ is the wavelength (μm) and A0, A1, and A2 are the Cauchy coefficients, presented in Table 1. The Cauchy dispersion coefficient is obtained by commercial Essential Macleod.The merit function of the DM is defined as follows:
Fig. 2. Layer thickness profiles of (a) starting structure and (b) optimized MFDM. (c) Designed transmittance of the MFDM at the incident angle of 0°. (d) Designed group delay dispersion (GDD) and reflectance of the MFDM at the incident angle of 0°.
Table 1. Cauchy parameters of the materials
3. Fabrication of MFDM
The MFDM was fabricated via a dual-ion-beam sputtering technique with two ion sources (16-cm main ion source and 12-cm assistant ion source). The high-energy argon ions produced by the 16-cm main source bombard the target. The main function of the 12-cm ion assistant source is to increase the coating density and achieve a firmer bond with the substrate. The deposition rates of SiO2 and Nb2O5 are 0.18 and 0.20 nm/s, respectively. A low deposition rate improved the density of the coating. The fiber was fixed on the planetary turntable, and the fiber tip was placed vertically upward. A schematic of this is shown in Fig. 4(a), and the fiber tip coated with the MFDM is shown in Fig. 4(b).
The spectra were measured using a PerkinElmer spectrophotometer (Lambda-1050) with an accuracy of ±0.05% [24]. The GDD value was measured using a white-light interferometer (KMLabs CHromatis) [25]. The incident medium of the MFDM is silicon glass, but in the measurement process, air must be used. To judge whether our preparation is successful, we changed the incident medium in the design to air. The design results of the air and SiO2 incident media are shown in Figs. 5(a) and (b). The measured results of the air incident medium are shown in Figs. 5(c) and (d). A significant correspondence between the theoretical and measured transmittance spectra of the air incident medium can be seen in Fig. 5(c). Meanwhile, the GDD and reflectance measurements at 1,010–1,100 nm are shown in Fig. 5(d). The corresponding theoretical results are also presented. Considering the high sensitivity of the GDD to deposition errors, the agreement between the theory and experiment appears to be satisfactory.
Fig. 5. (a) Designed spectra of air and SiO2 incidence media. (b) Designed GDD results of air and SiO2 incidence media. (c) Theoretical spectrum (black line) and measured spectrum (red dashed line) of the MFDM in the range of 800–1,600 nm. (d) Theoretical reflectance, measured reflectance, theoretical GDD, and measured GDD of the MFDM in the range of 1,010–1,100 nm. The incident medium of the designed MFDM is silicon glass, but in the measurement process, we had to use air. Therefore, to judge whether our preparation was successful, we changed the incident medium in the design result to air.
4. MFDM in fiber laser system
Furthermore, an application in ultrafast fiber laser of the fabricated MFDM was demonstrated, the schematic of the fiber oscillator is shown in Fig. 6. In the oscillator, the gain was provided by a piece of highly Yb3+-doped fiber (YDF) with a length of 6.8 cm, and both ends of the YDF were perpendicularly polished. The YDF was pumped by a single-mode laser through a wavelength division multiplexer (WDM). One end of the YDF was coated with the MFDM, and was connected to a pigtail, which was spliced to the common port of the WDM. One end of the YDF was butt-coupled to a commercially available semiconductor saturable absorber mirror (SESAM), for self-starting mode-locking operation. A continuous wavelength (CW) operation of the oscillator started at a pump power of 450 mW, and once the pumped power increased to 750 mW, the self-starting mode-locking operation of the oscillator was achieved.
Fig. 6. Schematic of the laser system. SESAM, semiconductor saturable absorber mirror; YDF, Yb3+-doped fiber; WDM, wavelength division multiplexer.
The Fig. 7 illustrates the output optical spectrum of the oscillator measured by an optical spectrum analyzer (YOKOGAWA AQ6370B). The optical spectrum of the oscillator using the MFDM has a central wavelength of 1054 nm, and a spectral bandwidth at the full-width half maximum (FWHM) of 4.45 nm, corresponding to a transform limited pulse duration of 259 fs (Sech2-fitting assuming), as shown in Fig. 7(b). The output optical spectrum without the MFDM is 2.64 nm, as shown in Fig. 7(a).
5. Conclusion
In summary, we demonstrated a new application mode of DMs as well as highly dispersive mirrors with a GDD of −5,000 fs2 in an all-fiber oscillator. In addition to providing dispersion compensation within the working bandwidth, the MFDM can provide extremely high transmittance in the pump wavelength and maintain partial reflection in the working band to achieve laser amplification output. Therefore, this type of DM has three simultaneous functions: input coupling mirror, output coupling mirror, and DM. Based on the function of this DM, we proposed a starting structure to achieve the design goal quickly. The results obtained using the prepared DMs are highly consistent with the design results, and the DM was successfully applied in laser systems. Finally, seed pulses with a spectral bandwidth at the FWHM of 4.45 nm were obtained, and the Fourier transform limit was 259 fs.
Funding
National Key Research and Development Program of China (2018YFE0118000); National Natural Science Foundation of China (11904376, 11975052, 61805263); Shanghai Sailing Program (18YF1426400); NSAF Fund Jointly set up by the National Natural Science Foundation of China and the Chinese Academy of Engineering Physics (U1630140); the Strategic Priority Research Program of CAS (XDB1603); Youth Innovation Promotion Association of the Chinese Academy of Sciences (2017289).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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