Dispersive mirrors for high third-order dispersion compensation in femtosecond amplification fiber laser system

Appl Phys B (2012) 108: DOI /s Dispersive mirrors for high third-order dispersion compensation in femtosecond amplification fiber laser system H.L. Niu W.D. Shen C.S. Li Y.G.
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Appl Phys B (2012) 108: DOI /s Dispersive mirrors for high third-order dispersion compensation in femtosecond amplification fiber laser system H.L. Niu W.D. Shen C.S. Li Y.G. Zhang C. Xie P. Yu W.J. Yuan B.W. Liu M.L. Hu Q.Y. Wang X. Liu Received: 29 December 2011 / Revised version: 28 February 2012 / Published online: 25 May 2012 Springer-Verlag 2012 Abstract The dispersive mirrors with a third order dispersion (TOD) of about fs 3 at the wavelength of 1040 nm and a compensation bandwidth of 20 nm are designed and fabricated with ion beam sputtering (IBS) technique. A good agreement between the measured and calculated group delay dispersion (GDD) and TOD of the mirror is achieved. These mirrors are employed in a Yb-doped femtosecond nonlinear amplification fiber laser system to compensate the residual TOD in the system. We show that after TOD compensation, the system outputs 44 fs laser pulses with little wing at 26.6 W output average power and 531 nj pulse energy, corresponding to 10.8 MW peak power. 1 Introduction Dispersive mirrors (DMs) are indispensable devices in almost all femtosecond laser systems nowadays to control dispersion. This is due to the fact that DM-coatings have lower energy loss, higher damage thresholds and better special mode preservation. In addition, DM-based dispersion compensators are compact, robust, user-friendly and easy to be inserted into laser systems. With rapid developments H.L. Niu W.D. Shen ( ) C.S. Li Y.G. Zhang P. Yu W.J. Yuan X. Liu State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou, , China Fax: C. Xie B.W. Liu M.L. Hu Q.Y. Wang Ultrafast laser Laboratory, Key Laboratory of Optoelectronics Information and Technical Science, College of Precision Instrument and Optoelectronics Engineering, Tianjin University, Tianjin, , China of design methods and the deposition technique, DM s performance has been significantly improved over the years [1 17]. In some ultrafast systems, residual TOD will cause serious deterioration to the temporal profile of the output pulses. Although low-tod compensating DMs have already been available for commercial uses at present [18], they are not suitable to be applied in some specific applications such as in photonic crystal fiber (PCF) femtosecond laser/amplification systems that require large amount of TOD compensation. In all the PCF femtosecond amplifiers, Yb-doped fiber amplifiers are widely applied to boost the average power of ultrashort pulse oscillators due to its commercial reliability and stability [19]. In order to generate higher pulse energy, bulk grating compressors are used for the compression of the stretched pulses. While the combination of the fiber stretcher and bulk grating compressor produces a significant TOD mismatch between the stretcher and compressor for large stretching ratios. Recently, some TOD compensation methods using prisms and gratings have been demonstrated to compensate the extremely large amount of TOD in the amplification system [20 23]. Here we designed and manufactured a pair of high TOD compensation mirrors with a tailored TOD value in the compensation bandwidth and demonstrated the feasibility of the mirrors in a Yb-doped femtosecond nonlinear amplification fiber laser system. After the pulse compression, the quality of the pulse is significantly enhanced and the pedestal is extremely low due to our TOD compensation. The output pulse duration is 44 fs with an average power of 26.6 W and a peak power of 10.8 MW. 610 H. Niu et al. 2 Design of the high TOD compensation mirrors The basic structure of DM can be roughly divided into two categories: chirped mirrors (CMs) [1] and Gires Tournois interferometer (GTI) mirrors [24]. Owing to the admittance mismatch, the ripples in GDD curve inevitably exist for CM mirrors [3] which results in severe oscillations of TOD, especially in the situation that a high TOD is required. It is easier to obtain linear GDD curve for GTI, which suggests the GTI is more suitable for TOD compensation. A schematic diagram of the GTI is depicted in Fig. 1. It consists of two parallel aligned reflective mirrors separated by a gap d (also called spacer layer). One of the mirrors is a perfect mirror with a reflectance R b 1 while the other is partially reflective with R a 1. According to the synthesis amplitude reflection coefficient of the multi-reflected pulses that is given by [25], the phase shift of the pulses reflected by the GTI can be obtained as [26]: [ ] (1 R a ) sin ωt 0 ϕ(ω) = arctan 2. (1) R a (1 + R a ) cos ωt 0 Here t 0 is the roundtrip time of the pulse in the spacer layer. The TOD are derived by calculation of the third derivative of ϕ with respect to ω: TOD = d3 ϕ dω 3 = { t0 3 Ra (1 R a )[2 R a cos 2 ωt 0 + (1 + R a ) cos ωt 0 4 R a ] } [ (1 + R a 2 R a cos ωt 0 ) 3] 1. (2) From Eq. (2), we can see that the TOD of the GTI depends strongly on the reflectance R a of the partial-reflection mirror and the optical thickness nd of the resonant cavity. We numerically analyzed the TOD as a function of nd and R a in Fig. 2. As depicted in Fig. 2(a), the curves are calculated for three different optical thicknesses nd of the spacer layer (λ 0 = 1040 nm). Here the R a is fixed at 27 %. It is indicated that this simple and compact optical element can provide considerably higher TOD along with the increase of the cavity thickness at the price of significant decrease of the compensation bandwidth. While Fig. 2(b) shows the dispersion features of GTI mirrors for three different R a of the partial-reflection film stack. In these three cases, the optical thickness of the cavity is fixed at nd = λ 0 /2. We can see the TOD at the central wavelength improves by several orders of magnitude when R a increases from 27 % to 77 %, which shows that the influence Fig. 1 (a) The meaning of denotation and (b) a schematic diagram of Gires Tournois interferometer Fig. 2 TOD curves of different (a) cavity thickness: nd = λ 0 /2(dotted curves), λ 0 (dashed curves), 3λ 0 /2(solid curves), (b) reflectivity of the partial-reflection mirror: R a = 27 % (dotted curves), 56 % (dashed curves), 77 % (solid curves) Dispersive mirrors for high third-order dispersion compensation in femtosecond amplification fiber laser 611 Table 1 Sellmeier formula coefficients for the layer materials (wavelength in the Sellmeier formula should be expressed in microns), and refractive index values at 1.04 µm A 0 A 1 A 2 A 3 A 4 nm Nb 2 O e e e SiO e e e Fig. 3 The features of the optimized mirrors: (a) layer structure, (b) reflectivity, (c) GDD curve, (d) TOD curve of R a on TOD is more remarkable than that of nd. AGTI can be considered a special case of a Fabry Perot cavity, it should be noticed that there would be a considerable decline of the reflectivity of the GTI mirror at the resonant wavelength when R a is comparable with R b. Hence, for different requirements of TOD compensation there is a necessity to choose a suitable partial-reflection film stack and an appropriate nd of spacer layer with concerns of bandwidth and amount of TOD compensation. In this paper, the mirrors were designed and fabricated for a Yb-doped photonic crystal fiber amplification system [27]. The TOD target is fs 3 at 1040 nm and a compensation bandwidth of 20 nm (centered at 1040 nm), and the reflectivity should be higher than 99.9 % in this bandwidth to reduce the insertion loss. Taking these requirements into account, the initial GTI design consists of a high-reflecting 25-layer quarter-wave stack, a half-wave low-index spacing layer and a partially-reflecting 3-layer quarter-wave stack: Sub/(HL) 12 H2LHLH/Air We consider BK7 glass as a substrate, Nb 2 O 5 and SiO 2 as high-index and low-index coating materials, respectively. Refractive indices of the layer materials are specified by Sellmeier formula n(λ) = A 0 + A 1λ 2 λ 2 A 2 + A 3λ 2 λ 2 A 4 with coefficients presented in Table 1. Based on this initial structure, we used both the needleoptimization [2] and gradual-evolution algorithms [9] with a commercial software, Optilayer (Optilayer Ltd.). These algorithms offer the best performance in terms of approaching the global optimum pursued. The optimized structure was achieved after just a few tens of iterations. The thin film structure together with its designed dispersion properties are showninfig.3 and the numerical values of the layer thicknesses obtained after computer optimization is presented in Table 2. The final design consists of 37 layers with a total thickness of 5.2 µm and a thinnest layer of 52 nm. 612 H. Niu et al. Table 2 Numerical values of the layer thicknesses obtained after computer optimization Layer number Material Physical thickness (nm) 1 Nb 2 O SiO Nb 2 O SiO Nb 2 O SiO Nb 2 O SiO Nb 2 O SiO Nb 2 O SiO Nb 2 O SiO Nb 2 O SiO Nb 2 O SiO Nb 2 O SiO Nb 2 O SiO Nb 2 O SiO Nb 2 O SiO Nb 2 O SiO Nb 2 O SiO Nb 2 O SiO Nb 2 O SiO Nb 2 O SiO Nb 2 O Experimental results Among all the deposition techniques, ion beam sputtering (IBS) is extremely stable and is currently considered to be one of advanced technologies available for the production of dispersive optics. We fabricated the designed mirrors by our home-made Dual-IBS machine equipped with 16 and 12 cm ion sources (VEECO). To guarantee the deposition accuracy of the coatings, quartz crystal was used to control the thickness of the thin film during the depositing process. It can be much more accurate than the method of time-control, especially in depositing complicated thin film stacks. Table 3 presents the detail fabrication parameters in our experiment. These parameters are obtained and optimized in the longterm production practice of our group. Two 5 mm thick BK7 glass samples with a diameter of 50 mm and a radius of curvature of 2 m were used as the substrates. And we adjusted the tooling-factor of the quartz crystal just before the experiment. The transmission and dispersion spectra of the fabricated mirrors were measured with a spectrophotometer and a home-made white light interferometer (WLI) [28]. WLI is a Michelson interferometer in the frequency domain that records the interference fringes of the light and then obtains the phase information through inverse Fourier Transform. Figure 4(a) shows a good match between the measured and the designed transmission spectra, indicating our good control precision of the thickness. Figures 4(b) and 4(c) present the measured GDD and TOD curves of the mirrors. We can notice that the measured GDD curve is linear enough in the wavelength range nm. Therefore, a good agreement between the designed and measured TOD is achieved in this bandwidth. The manufactured mirrors were employed at the final stage of a femtosecond nonlinear amplification fiber laser system (Fig. 5) based on double-cladding ytterbium-doped large mode area (LMA) photonic crystal fiber as described in [27]. There are three main origins of TOD in our experimental setup. First of all, for typical LMA fibers, the dispersion is dominated by the material, resulting in a third-order propagation constant of β 3 = fs 3 m 1. The length of PCF in our experiment is 3 m, then the total TOD introduced by the PCF is ϕ 3 = fs 3 m 1 3m = Table 3 Fabrication parameters of the Dual-IBS Main ion source Assistant ion source Vacuum Beam current /ma Voltage /V Gas flow /sccm Beam current /ma Voltage /V Gas flow /sccm Background vacuum /Pa Deposition vacuum /Pa (Ar) (Ar) + 12(O 2 ) (Nb 2 O 5 ) (SiO 2 ) Dispersive mirrors for high third-order dispersion compensation in femtosecond amplification fiber laser 613 Fig. 4 Measured results of the mirrors: (a) transmission curves with designed curve and measured curve, (b) GDD curve, (c) TOD curve Fig. 5 Schematic of the amplification system The autocorrelation trace is depicted in Fig. 6. Dramatic variation in the pulse quality is observable: the pulse pedestal is effectively suppressed after TOD compensation. The pulse duration is 44 fs with little pedestal of 26.6 W average power (71 % compression efficiency (includes the grating pair) of 37.4 W output power from the amplifier) and 50 MHz repetition rate, 531 nj pulse energy, corresponding to 10.8 MW peak power. This simple and effective DM-based TOD compensator shows a promising potential in fiber optical amplification system. Fig. 6 Measured autocorrelation trace of the output pulses from the fiber amplifier before (dashed curve)andafter(solid curve)todcompensation fs 3. Conventional gratings compressor is also a main source of TOD. According to our calculation, it induces a positive TOD of ϕ 3 = fs 3. The contribution from the fiber and the gratings compressor is therefore approximately fs 3. The third source of TOD derives from SPM of spectrally asymmetric pulses. However, this contribution is hard to evaluate due to the fact that SPM effect is related to the spectral intensity as well as the precise shape in the amplifier, which is dependent on many factors. Based on the calculated TOD in the system and the measured TOD of the mirrors, we finally make 50 (25 2) bounces of laser pulse off the mirrors to compensate the TOD. 4 Conclusion In conclusion, a pair of mirrors capable of compensating high third-order dispersion by applying the coating-based GTI mirror over a bandwidth from 1030 to 1050 nm have been designed, realized, and successfully tested in a nonlinear amplification femtosecond fiber laser system. Analysis shows that the temporal profile of pulse will be seriously distorted because of the residual TOD in the system. Our implementation of mirrors in the system significantly enhanced the quality of the recompressed pulses. 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