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We experimentally demonstrated a dual-wavelength independently mode-locked Yb:YAG ceramic laser in a single cavity. Dual-wavelength mode locking at 1033.6 and 1047.6 nm was operated simultaneously in one beam. Each pulse width was measured to be approximately 380 fs using an autocorrelator. The spectral widths were 4.50 nm centered at 1033.6 nm and 3.08 nm centered at 1047.6 nm. To the best of our knowledge, this is the first dual-wavelength mode locking achieved in Yb-doped solid-state lasers.
©2010 Optical Society of America
1. Introduction
Femtosecond mode-locked lasers are applied in various fields of physics, engineering, chemistry, biology and medicine, with applications including ultrafast spectroscopy, metrology, superfine material processing and microscopy. Specific and interesting properties of femtosecond laser pulses have been used in these applications. For example, femtosecond pulses have a very precise time resolution, and their strong electric field induces important and unique nonlinear effects. For those applications, high-power, high-efficiency and compact femtosecond lasers are required. Ceramic materials are attractive for satisfying these requirements. YAG ceramics have a 10% higher hardness than a YAG single crystal, and a fracture toughness that is more than threefold that of the YAG single crystal. Therefore, these ceramics have a higher resistance to thermal shock than the single crystal. Ytterbium (Yb3+) also has interesting properties satisfying the above requirements. Its broad absorption and emission spectra allow the realization of a directly laser-diode (LD)-pumped femtosecond laser. Moreover, its small quantum defect, absence of excited-state absorption, upconversion and cross-relaxation reduce the thermal load and enable a highly efficient operation. The emission and absorption spectra and thermal conductivity strongly depend on the host material.
Dual-wavelength mode-locked pulses are attractive for double-pulse pump-probe measurements with a single source. In these applications, an exact synchronization of these two pulses is crucial. The operation of the dual-wavelength mode can also generate ultrashort pulses in the infrared region by difference-frequency mixing in nonlinear crystals. Furthermore, the synchronization and phase locking of separate femtosecond lasers have a strong impact on the precision frequency metrology based on optical frequency combs [1,2], providing the capability of a wide-bandwidth phase-coherent frequency network covering various spectral regions. Some reports on dual-wavelength mode locking based on a single or double cavity were previously published [3–11].
Cross-mode locking has been achieved in a double-cavity dual-wavelength femtosecond Ti:sapphire laser, and synchronized 45-fs and 0.848-ps pulses have been generated [3]. Cross-phase modulation (XPM) is the dominant factor in cross-mode locking and consequently produces passive synchronization, with a timing jitter of 41 fs, between femtosecond and picosecond pulses.
The dual-wavelength operation in a single cavity of a mode-locked Ti:sapphire laser based on self-spectrum splitting was reported, without the imposed blocking of the beam at the dispersive end of the laser [5]. Increasing the bandwidth by reducing the group-delay dispersion is limited by the operation bandwidth, which results in spectrum splitting. Under this condition, the laser may operate at two distinct modes, namely, a single-wavelength mode or a dual-wavelength mode. One may obtain the transition from one mode to the other by tapping one of the cavity components. The defect of self-splitting is that the wavelength separation is small and cannot be changed.
Most dual-wavelength mode-locked operations were demonstrated using a Ti:sapphire laser. As the dual-wavelength mode-locked operation using the other laser medium, a dual-wavelength synchronously mode-locked Nd:CNGG laser based on the semiconductor saturable absorber mirror technique was reported [11]. Mode locking was achieved simultaneously on two gain bands of the crystal with a central wavelength separation of 2.4 nm. The fundamental mode-locked pulse train has a repetition rate of 88 MHz and a pulse duration of 5 ps, with an average output power of 90 mW. Autocorrelation measurements show that each of the synchronously mode-locked pulses consists of a train of quasi-periodic beat pulses with a 660 fs pulse width and a 0.63 THz repetition rate.
In this paper, we report a dual-wavelength independently mode-locked Yb:YAG ceramic laser in a single cavity. Dual-wavelength mode locking at 1033.6 and 1047.6 nm was operated simultaneously in one beam. Each pulse width was measured to be approximately 380 fs using an autocorrelator. The spectral widths were 4.50 nm centered at 1033.6 nm and 3.08 nm centered at 1047.6 nm. Pulse widths were obtained by inverse Fourier transformation as 298 and 341 fs for the separated wavelengths at 1033.6 and 1047.6 nm, respectively. Assuming that the two wavelengths generate one pulse, the pulse width was obtained by inverse Fourier transformation as 115 fs. These results indicate that the measured dual-wavelength mode locking was independently operated at 1033.6 and 1047.6 nm. To the best of our knowledge, this is the first dual-wavelength mode locking in Yb-doped solid-state lasers.
2. Experimental setup
The experimental setup for the dual-wavelength mode-locked Yb:YAG ceramic laser is shown in Fig. 1 . It was same configuration in Ref [12]. without the angle of M3. An x-fold cavity configuration was used. A 940 nm fiber-coupled LD was used as a pumping source. The core diameter of the fiber was 200 μm. The numerical aperture (NA) of the fiber was 0.22. The maximum pump power was 26.6 W. The pumping beam was imaged by relay to the ceramics using the lenses L1 (f = 50 mm) and L2 (f = 70 mm). The 1-mm-thick Yb:YAG (C Yb = 9.8 at.%) ceramic plate was placed at Brewster’s angle. The Yb:YAG plate was wrapped with indium foil and mounted in a water-cooled copper heat sink block. The copper block was cooled by flowing water at 20°C. The ceramic was placed between two high-reflectivity mirrors (M1, M2) that were antireflection (AR)-coated at 940 nm and had a high reflectivity at 1030 nm with a 100 mm radius of curvature (ROC). For passive mode locking, a 1 or 0.1% output coupler (OC) and a semiconductor saturable absorber mirror (SESAM, BATOP) with a 2% saturable absorption at 1030 nm, a 70 μJ/cm2 saturation fluence and a 500 fs relaxation time constant were used in the respective arms. The total cavity length was 1620 mm. The laser beam was focused onto the SESAM by a concave mirror (M3, ROC = 250 mm). The distance between the mirrors and the folded angle of the mirrors are shown in Fig. 1. The angle of M3 was only modified from 22° in Ref [12]. to 4°. Astigmatism compensation was not considered. The spot sizes of the laser mode in the laser crystal and on the SESAM were estimated to be ~61 × 53 μm and ~450 × 330 μm, respectively. An SF10 Brewster prism pair (P1, P2) with a 465 mm separation was inserted in to the other arm to compensate for the dispersion. The total negative GDD of this cavity was about –2670 fs2 per round trip.
Fig. 1 Experimental setup of mode-locked Yb:YAG ceramic laser. LD: fiber-coupled diode laser. L1, L2: focusing lenses. M1-M3: high-reflectivity mirrors. OC: output couplers. P1-P2: SF10 Brewster prisms. SESAM: semiconductor saturable absorber mirror.
4. Experimental results
Recently, we have demonstrated a diode-pumped passively mode-locked Yb:YAG ceramic laser, and passive mode locking at wavelengths of 1033.5 and 1048.3 nm was achieved [12]. This mode locking at two wavelengths was not simultaneous. In this study, we achieved the simultaneous operation of dual-wavelength mode locking at 1033.6 and 1047.6 nm.
To achieve the stable dual-wavelength mode locking, firstly, the cavity was adjusted for the stable single-wavelength mode locking at 1047.6 nm. The cw oscillation at 1033.6 nm was observed at the same time. Afterwards, the angle of SESAM was tilted gradually until mode locking at 1033.6 nm was observed while maintaining mode locking at 1047.6 nm. Unstable mode locking at 1033.6 nm was observed before the stable dual-wavelength mode locking was achieved. After mode locking at 1033.6 nm was stabilized, the angle of the SESAM was fixed.
Figure 2 shows the measured laser mode profile of the dual-wavelength mode locking. When the dual-wavelength mode locking was operated, two beams were measured. The single-wavelength mode locking at 1033.9 nm was obtained in the beam of Fig. 2(a), and the dual-wavelength mode locking at 1033.6 and 1047.6 nm was obtained in the beam of Fig. 2(b). By the observation of separate beam spatially in Fig. 2, the lasers with each of wavelength were oscillated with slightly different pass in cavity. In other word, two oscillations used different gain parts in gain medium. Therefore, dual-wavelength oscillation was independent.
Fig. 2 Transverse intensity beam profile at 20 cm from output coupler (OC) when dual-wavelength mode locking was observed. (a) Single-wavelength mode locking at 1033.9 nm and (b) dual-wavelength mode locking at 1033.6 and 1047.6 nm.
We categorized the mode locking into three cases and measured them. The first case is the single-wavelength mode locking at 1033.9 nm, as shown in Fig. 2(a). The second case is the dual-wavelength mode locking at 1033.6 and 1047.6 nm, as shown in Fig. 2(b). The third case is the state just before the stable dual wavelength mode locking was achieved. Only mode locking at 1047.6 nm was stable.
4.1 Single-wavelength mode locking at 1033.9 nm
The single-wavelength mode locking at 1033.9 nm was obtained in Fig. 2(a). Figure 3 shows the intensity autocorrelation trace and spectrum of mode-locked pulses in Fig. 2(a). The autocorrelation trace and spectrum were measured by cutting the beam of the dual-wavelength mode locking in Fig. 2(b) using an aperture. The sech2-fitted pulse width was 345 fs and the spectral width was 4.04 nm at the center wavelength of 1033.9 nm, which results in a time-bandwidth product of 0.391, slightly above the Fourier limit for a sech2 pulse (0.315). The average output power was 3 mW at a pump power of 26.6 W. The repetition rate was 91 MHz. The pulse energy and peak power were 32.8 pJ and 95.3 W, respectively. When the dual-wavelength mode locking in Fig. 2(b) was operated, no mode-locked or cw component in the 1050 nm region was observed in Fig. 2(a).
Fig. 3 (a) Measured autocorrelation trace and sech2 fitting, and (b) spectrum of mode-locked pulses in beam in Fig. 2 (a).
4.2 Dual-wavelength mode locking at 1033.6 and 1047.6 nm
With appropriate alignment of the cavity, mode locking was simultaneously achieved on the two strongest gain lines of the laser, centered at 1033.6 and 1047.6 nm wavelengths. The two separate gain lines were initially independently mode locked, and two individually separated mode-locked pulses were formed in the single cavity, as shown in Fig. 1. Experimentally, by a careful optimization of the cavity alignment (e.g., changing the distance and angle of the SESAM), the two independent dual-color pulses were then obtained.
Figure 4 shows the intensity autocorrelation trace and spectrum of dual-wavelength mode-locked pulses in Fig. 2(b). The autocorrelation trace and spectrum were measured by cutting the beam of the single-wavelength mode locking in Fig. 2(a) using an aperture. The sech2-fitted pulse width was 380 fs. The spectral widths were 4.50 nm centered at 1033.6 nm and 3.08 nm centered at 1047.6 nm. These result in time-bandwidth products of 0.480 at the wavelength of 1033.6 nm and 0.321 at the wavelength of 1047.6 nm. The time-bandwidth product of 0.480 at the wavelength of 1033.6 nm increased by 52% from the Fourier limit for a sech2 pulse (0.315). This indicates that two dual-color autocorrelation traces combined and the large time-bandwidth product at 1033.6 nm was calculated using longer pulses at 1047.6 nm. On the other hand, the small time-bandwidth product at the wavelength of 1047.6 nm was calculated using shorter pulses at 1033.6 nm. The average output power was 8 mW at a pump power of 26.6 W. The repetition rate was 91 MHz.
Fig. 4 (a) Measured autocorrelation trace and sech2 fitting, and (b) spectrum of mode-locked pulses when dual-wavelength mode locking was operated.
Figure 5 shows the SH spectrum of dual-wavelength mode-locked pulses. There are two independently spectra of 517 and 524 nm in Fig. 5. There is no signal from SFG of 1033.6 nm and 1047.6 nm located in between the two peaks due to poor overlap of two beams at 1033.6 nm and 1047.6 nm. This phenomenon is confirmed by separation of beam angle in Fig. 2. Therefore, the dual-wavelength mode locking was independently operated at 1033.6 and 1047.6 nm.
Fig. 5 SH spectrum of mode-locked pulses when dual-wavelength mode locking was operated in beam in Fig. 2 (b).
Figure 6 shows the expected transform-limited pulses obtained by an inverse Fourier transformation of (a) the dual wavelength that is assumed to operate one mode locking with the spectrum between 1033.6 and 1047.6 nm, and the separated wavelengths at (b) 1033.6 and (c) 1047.6 nm. The full widths at half maximum (FWHMs) of the numerically obtained pulse duration are t p = 115, 298 and 341 fs for Figs. 6(a) – 6 (c), respectively. The pulse width in Fig. 6(a) is 70% below the measured pulse width (380 fs) of the dual-wavelength mode locking in Fig. 4(a), and the pulse duration of Fig. 6(a) has two large pedestals. These pedestals were not quenched in the case including a large group-delay dispersion. On the other hand, the pulse widths in Figs. 6(b) and 6(c) are respectively 22% and 10% below the measured pulse width (380 fs) of the dual-wavelength mode locking in Fig. 4(a); these values are close to the measured dual-wavelength pulse width of 380 fs. These results indicate that the measured dual-wavelength mode locking was independently operated between 1033.6 and 1047.6 nm.
Fig. 6 Expected transform-limited pulses numerically obtained by inverse Fourier transformation of (a) dual wavelength and separated wavelengths at (b) 1033.6 and (c) 1047.6 nm.
The synchronization of dual-wavelength ultrashort pulses induced the generation of ultrahigh-repetition-rate pulses by optical beating [11,13]. Furthermore, the XPM that induced cross-mode locking in a dual-wavelength femtosecond laser caused self-synchronization with a timing jitter of a few femtoseconds [6] and exhibited a passive synchronization that differs in mechanism from an active synchronization [14]. In our dual-wavelength mode locking, no generation of ultrahigh-repetition-rate pulses by optical beating or XPM was observed. This indicates that our dual-wavelength mode locking was independently operated at 1033.6 and 1047.6 nm.
4.3 State just before generation of dual-wavelength mode locking, with mode locking at 1047.6 nm and cw component at 1032.8 nm
Figure 7 shows the intensity autocorrelation trace and spectrum of mode-locked pulses when the mode locking at 1033.6 nm was stopped. The sech2-fitted pulse width was 384 fs and the spectral width was 3.02 nm centered at 1047.6 nm, which results in a time-bandwidth product of 0.317, almost the Fourier limit for a sech2 pulse (0.315). The average output power was 7 mW at a pump power of 26.6 W. The repetition rate was 91 MHz. The pulse energy and peak power were 76.6 pJ and 19.9 W, respectively. Figure 8 shows the SH spectrum of mode-locked pulses. There is no spectrum at 517 nm in Fig. 8. Therefore, the spectral component at 1032.8 nm is not related to mode locking.
Fig. 7 (a) Measured autocorrelation trace and sech2 fitting, and (b) spectrum of mode-locked pulses when mode locking at 1033.6 nm was stopped in beam in Fig. 2 (a).
Fig. 8 SH spectrum of mode-locked pulses when mode locking at 1033.6 nm was stopped in beam in Fig. 2 (b).
5. Conclusion
A dual-wavelength femtosecond Yb:YAG ceramic laser was demonstrated. We successfully achieved out simultaneous independent generation of mode locking on the two strongest gain lines of the laser, centered at 1033.6 and 1047.6 nm wavelengths. When the dual-wavelength mode locking was operated, two beams were measured and the dual-wavelength mode locking was observed in one of the beams. Each pulse width was measured to be approximately 380 fs using an autocorrelator. The spectral widths were 4.50 nm centered at 1033.6 nm and 3.08 nm centered at 1047.6 nm. The pulse widths were obtained by inverse Fourier transformation as 298 and 341 fs for the separated wavelength at 1033.6 and 1047.6 nm, respectively. By assuming that the two wavelengths generate one pulse, the pulse width was obtained by inverse Fourier transformation as 115 fs. These results indicate that the measured dual-wavelength mode locking was independently operated between 1033.6 and 1047.6 nm. To the best of our knowledge, this is the first dual-wavelength mode locking achieved in Yb-doped solid-state lasers.
Acknowledgments
This work was partially supported by KAKENHI (21604002) of Grant-in-Aid for Scientific Research (C).
References and links
1. Th. Udem, J. Reichert, R. Holzwarth, and T. W. Hänsch, “Absolute Optical Frequency Measurement of the Cesium D1 Line with a Mode-Locked Laser,” Phys. Rev. Lett. 82(18), 3568–3571 (1999). [CrossRef]
2. S. A. Diddams, D. J. Jones, J. Ye, S. T. Cundiff, J. L. Hall, J. K. Ranka, R. S. Windeler, R. Holzwarth, T. Udem, and T. W. Hänsch, “Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb,” Phys. Rev. Lett. 84(22), 5102–5105 (2000). [CrossRef] [PubMed]
3. C. J. Zhu, J. F. He, and S. C. Wang, “Generation of synchronized femtosecond and picosecond pulses in a dual-wavelength femtosecond Ti:sapphire laser,” Opt. Lett. 30(5), 561–563 (2005). [CrossRef] [PubMed]
4. M. Betz, F. Sotier, F. Tauser, S. Trumm, A. Laubereau, and A. Leitenstorfer, “All-optical phase locking of two femtosecond Ti:sapphire lasers: a passive coupling mechanism beyond the slowly varying amplitude approximation,” Opt. Lett. 15(29), 629–631 (2004). [CrossRef]
5. Z. Zhang and T. Yagi, “Dual-wavelength synchronous operation of a mode-locked Ti:sapphire laser based on self-spectrum splitting,” Opt. Lett. 18(24), 2126–2128 (1993). [CrossRef] [PubMed]
6. A. Leitenstorfer, C. Fürst, and A. Laubereau, “Widely tunable two-color mode-locked Ti:sapphire laser with pulse jitter of less than 2 fs,” Opt. Lett. 20(8), 916–918 (1995). [CrossRef] [PubMed]
7. J. M. Evans, D. E. Spence, D. Burns, and W. Sibbett, “Dual-wavelength self-mode-locked Ti:sapphire laser,” Opt. Lett. 18(13), 1074–1076 (1993). [CrossRef] [PubMed]
8. M. R. X. de Barros and P. C. Becker, “Two-color synchronously mode-locked femtosecond Ti:sapphire laser,” Opt. Lett. 18(8), 631–633 (1993). [CrossRef] [PubMed]
9. D. R. Dykaar, S. B. Darack, and W. H. Knox, “Cross-locking dynamics in a two-color mode-locked Ti:sapphire laser,” Opt. Lett. 19(14), 1058–1060 (1994). [CrossRef] [PubMed]
10. D. R. Dykaar and S. B. Darack, “Sticky pulses: two-color cross-mode-locked femtosecond operation of a single Ti:sapphire laser,” Opt. Lett. 18(8), 634–636 (1993). [CrossRef] [PubMed]
11. G. Q. Xie, D. Y. Tang, H. Luo, H. J. Zhang, H. H. Yu, J. Y. Wang, X. T. Tao, M. H. Jiang, and L. J. Qian, “Dual-wavelength synchronously mode-locked Nd:CNGG laser,” Opt. Lett. 33(16), 1872–1874 (2008). [CrossRef] [PubMed]
12. H. Yoshioka, S. Nakamura, T. Ogawa, and S. Wada, “Diode-pumped mode-locked Yb:YAG ceramic laser,” Opt. Express 17(11), 8919–8925 (2009). [CrossRef] [PubMed]
13. M. D. Pelusi, H. F. Liu, D. Novak, and Y. Ogawa, “THz optical beat frequency generation from a single mode locked semiconductor laser,” Appl. Phys. Lett. 71(4), 449–451 (1997). [CrossRef]
14. L.-S. Ma, R. K. Shelton, H. C. Kapteyn, M. M. Murnane, and J. Ye, “Sub-10-femtosecond active synchronization of two passively mode-locked Ti:sapphire oscillators,” Phys. Rev. A 64(2), 1–4 (2001). [CrossRef]
