Mode-locked operation of Cr4+:YAG singlecrystal fiber laser with external cavity Shigeo Ishibashi1,* and Kazunori Naganuma2 1
NTT Photonics Laboratories, NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi-shi, Kanagawa Pref., 243-0198, Japan 2 NTT Advanced Technology Corporation, 3-1 Morinosato Wakamiya, Atsugi-shi, Kanagawa Pref., 243-0124, Japan * [email protected]
Abstract: We report what is to our knowledge the first mode-locked Cr4+:YAG single-crystal fiber laser, which generates pulses of 120-fs duration with an output power of 23 mW at a center wavelength of 1520 nm for a single pulse in a cavity-round-trip. The laser contains a single-crystal fiber multi-mode waveguide about 120 μm in diameter and 40-mm long. The fundamental transverse mode is selected with an external cavity. This design strategy turned out to be well suited for direct high-power-laserdiode pumping. ©2014 Optical Society of America OCIS codes: (140.3580) Lasers, solid-state; (140.4050) Mode-locked lasers; (140.5680) Rare earth and transition metal solid-state lasers; (140.7090) Ultrafast lasers; (060.3510) Lasers, fiber.
References and Links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Y. Ishida and K. Naganuma, “Compact diode-pumped all-solid-state femtosecond Cr4+:YAG laser,” Opt. Lett. 21(1), 51–53 (1996). S. D. Di Dio Cafiso, E. Ugolotti, A. Schmidt, V. Petrov, U. Griebner, A. Agnesi, W. B. Cho, Y. G. Zhang, S. Y. Choi, F. Rotermund, G. Reali, and F. Pirzio, “Sub-50 fs mode-locking of a Cr:YAG laser using an SWCNT-SA,” Laser Phys. Lett. 10(8), 085801 (2013). C. Chudoba, J. G. Fujimoto, E. P. Ippen, H. A. Haus, U. Morgner, F. X. Kärtner, V. Scheuer, G. Angelow, and T. Tschudi, “All-solid-state Cr:forsterite laser generating 14-fs pulses at 1.3 μm,” Opt. Lett. 26(5), 292–294 (2001). S. Ozharar, I. Baylam, M. N. Cizmeciyan, O. Balci, E. Pince, C. Kocabas, and A. Sennaroglu, “Graphene modelocked multipass-cavity femtosecond Cr4+: forsterite laser,” J. Opt. Soc. Am. B 30(5), 1270–1275 (2013). D. H. Sutter, G. Steinmeyer, L. Gallmann, N. Matuschek, F. Morier-Genoud, U. Keller, V. Scheuer, G. Angelow, and T. Tschudi, “Semiconductor saturable-absorber mirror assisted Kerr-lens mode-locked Ti:sapphire laser producing pulses in the two-cycle regime,” Opt. Lett. 24(9), 631–633 (1999). R. Ell, U. Morgner, F. X. Kãârtner, J. G. Fujimoto, E. P. Ippen, V. Scheuer, G. Angelow, T. Tschudi, M. J. Lederer, A. Boiko, and B. Luther-Davies, “Generation of 5-fs pulses and octave-spanning spectra directly from a Ti:sapphire laser,” Opt. Lett. 26(6), 373–375 (2001). A. G. Okhrimchuk and A. V. Shestakov, “Performance of YAG:Cr4+ laser crystal,” Opt. Mater. 3(1), 1–13 (1994). Y. Ishida and K. Naganuma, “Characteristics of femtosecond pulses near 1.5 µm in a self-mode-locked Cr4+:YAG laser,” Opt. Lett. 19(23), 2003–2005 (1994). A. Sennaroglu, C. R. Pollock, and H. Nathel, “Efficient continuous-wave chromium-doped YAG laser,” J. Opt. Soc. Am. B 12(5), 930–937 (1995). D. J. Ripin, C. Chudoba, J. T. Gopinath, J. G. Fujimoto, E. P. Ippen, U. Morgner, F. X. Kärtner, V. Scheuer, G. Angelow, and T. Tschudi, “Generation of 20-fs pulses by a prismless Cr4+:YAG laser,” Opt. Lett. 27(1), 61–63 (2002). E. Sorokin, S. Naumov, and I. T. Sorokina, “Ultrabroadband infrared solid-state lasers,” IEEE J. Sel. Top. Quantum Electron. 11(3), 690–712 (2005). A. J. Alcock, P. Scorah, and K. Hnatovsky, “Broadly tunable continuous-wave diode-pumped Cr4+:YAG laser,” Opt. Commun. 215(1–3), 153–157 (2003). S. Naumov, E. Sorokin, and I. T. Sorokina, “Directly diode-pumped Kerr-lens mode-locked Cr4+:YAG laser,” Opt. Lett. 29(11), 1276–1278 (2004). S. Ishibashi and K. Naganuma, “Diode-pumped Cr4+:YAG single crystal fiber laser,” in Advanced Solid State Lasers, OSA Technical Digest Series (Optical Society of America, 2000), paper MD4. S. Naumov, E. Sorokin, and I. Sorokina, “Directly diode-pumped femtosecond Cr4+:YAG laser,” in Advanced Solid-State Photonics, J. Zayhowski, ed., Vol. 83 of OSA Trends in Optics and Photonics (Optical Society of America, 2003), paper 163.
#204315 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 17 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006764 | OPTICS EXPRESS 6764
16. J. Takayanagi, S. Kanamori, K. Suizu, M. Yamashita, T. Ouchi, S. Kasai, H. Ohtake, H. Uchida, N. Nishizawa, and K. Kawase, “Generation and detection of broadband coherent terahertz radiation using 17-fs ultrashort pulse fiber laser,” Opt. Express 16(17), 12859–12865 (2008). 17. F. D. Brunner, O.-P. Kwon, S.-J. Kwon, M. Jazbinsek, A. Schneider, and P. Günter, “A hydrogen-bonded organic nonlinear optical crystal for high-efficiency terahertz generation and detection,” Opt. Express 16(21), 16496–16508 (2008). 18. J. Stone, C. A. Burrus, A. G. Dentai, and B. I. Miller, “Nd:YAG single-crystal fiber laser: Room-temperature cw operation using a single LED as an end pump,” Appl. Phys. Lett. 29(1), 37–39 (1976). 19. J. Stone and C. A. Burrus, “Self-contained LED-pumped single-crystal Nd:YAG fiber laser,” Fiber Integrated Opt. 2(1), 19–46 (1979). 20. M. M. Fejer, J. L. Nightingale, G. A. Magel, and R. L. Byer, “Laser-heated miniature pedestal growth apparatus for single-crystal optical fibers,” Rev. Sci. Instrum. 55(11), 1791–1796 (1984). 21. S. Ishibashi, K. Naganuma, and I. Yokohama, “Cr, Ca:Y3Al5O12 laser crystal grown by the laser-heated pedestal growth method,” J. Cryst. Growth 183(4), 614–621 (1998). 22. M. E. Fermann, A. Galvanauskas, and M. Hofer, “Ultrafast pulse sources based on multi-mode optical fibers,” Appl. Phys. B 70(S1), S13–S23 (2000). 23. J. A. Au, D. Kopf, F. Morier-Genoud, M. Moser, and U. Keller, “60-fs pulses from a diode-pumped Nd:glass laser,” Opt. Lett. 22(5), 307–309 (1997). 24. F. X. Kärtner, J. Aus der Au, and U. Keller, “Modelocking with slow and fast saturable absorbers - What's the difference?” IEEE J. Sel. Top. Quantum Electron. 4(2), 159–168 (1998).
1. Introduction Solid-state lasers pumped by laser diodes (LDs), which are, in general, compact, costeffective and low-energy-consumption compared to lasers pumped by solid-state lasers, are now commonly used in various applications. On the other hand, there exist useful ultrafast solid-state lasers, such as Cr4+:YAG [1,2], Cr:forsterite [3,4], and Ti:sapphire [5,6], which are ordinarily pumped by solid-state lasers. It is highly desirable to change the pump source of these lasers to LDs in order for them to be used in medical and industrial fields. In this paper, we report a new configuration of a Cr4+:YAG laser with a single-crystal fiber, which is appropriate for a directly LD-pumped mode-locked oscillator. Cr4+:YAG has a large gain bandwidth, nearly encompassing 1.3 to 1.6 μm, and it has attracted much attention as a tunable laser and an ultrashort-pulse source. Continuous efforts have been made to develop it in a CW and a mode-locked oscillator [7–15]. This laser is optically pumped by utilizing its broad absorbing band around 1 μm. Regarding lasers using a bulk Cr4+:YAG material, there have been many attempts to pump them with solid-state lasers like Nd:YVO4 or Yb glass-fiber lasers, or with LDs directly. With a solid-state laser as a pump source, CW oscillation in the wavelength range of 1.34–1.57 μm [9], Kerr lens modelocked oscillation with a pulse width of 20 fs [10], and mode-locked oscillation started by a saturable absorber with a pulse width of 27 fs [11] have been reported. In practical use, a saturable absorber is convenient as a starter of mode-locked oscillation. An LD-pumped mode-locked Cr4+:YAG laser started with a saturable absorber has already been reported, with maximum mode-locked power of 15 mW with pulse duration of 62 fs [15]. The pump light from a standard high-power LD, whose power is more than 5 W, is diffusive. Because excessively high-concentration Cr4+:YAG sacrifice crystal quality, there is an upper limit of the dopant concentration, and hence the length of the laser crystal has a lower limit. When a bulk crystal with a reasonable length is used for direct LD-pumping, a poor overlap between the pump and oscillation beam will occur, which makes the oscillation efficiency small. The purpose of our research is to improve the power of a directly LD-pumped mode-locked Cr4+:YAG laser started with a saturable absorber. A femtosecond pulse of around 1.4 μm is suitable for THz-wave generation with organic nonlinear crystals such as DAST or OH1. Compared to 1.5-μm pumping, 1.4-μm pumping is expected to have a longer coherent length with DAST for some frequencies between 4 and 7 THz, and a longer optimum length is predicted with OH1 for frequencies between 0.3 and 2.5 THz [16,17]. As a gain medium, we have developed a Cr4+:YAG single-crystal fiber waveguide with a SiO2 clad. The fiber is about 120 μm in diameter and 40-mm long. It has enough volume for mode-locked oscillation, and compensating for its group-delay dispersion (GDD) is not difficult. This multi-mode waveguide can confine a diffusive pump beam within a small
#204315 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 17 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006764 | OPTICS EXPRESS 6765
cross-sectional area over its entire length. Addition of an external cavity enables oscillation in the fundamental transverse mode [Fig. 1]. This report confirms the capability of a singlecrystal fiber laser with an external cavity for mode-locked oscillation.
Fig. 1. Schematic of a directly LD-pumped single-crystal fiber multi-mode waveguide laser with an external cavity.
The first single-crystal fiber laser, a Nd:YAG single-crystal fiber CW laser pumped with an LED, was developed by J. Stone and associates [18,19]. The fiber for their experiment was made by using the laser-heated pedestal growth (LHPG) method. In the LHPG method, a CO2 laser is used to melt the top of a source rod. The fiber crystal is grown from the molten portion and is pulled up following a seed crystal which defines the orientation of the crystal fiber. This method was improved by M. M. Fejer et al. in the 1980s with the invention of a symmetrical focusing optics [20]. We have made Cr4+:YAG single-crystal fibers using an LHPG apparatus with almost the same focusing optics and have already reported the appropriate conditions for Cr4+:YAG fiber growth [14,21]. As one of the conditions, we select a particular growth orientation to suppress a formation of strain region. The growth atmosphere is O2, and we use a source material deposited with dopants. Annealing is done after the growth to increase the number of quadrivalent chromium ions. Using the single-crystal fiber made with the method described above, we have already reported a directly LD-pumped CW laser to oscillate in the fundamental transverse mode. Pumped with 15-W LD at 0.98 μm, an external-cavity Cr4+:YAG fiber laser oscillated in the CW mode with a power of 150 mW at 1.42 μm. The M2 value was 1.2 for both X and Y [14]. This good beam quality should be taken over to a mode-locked single-crystal fiber laser. This report is organized as follows. Section 2 reports the fiber preparation. Section 3 describes the CW oscillation, which was a test of the polarization control, and Section 4 presents the mode-locked oscillation of this fiber laser. The last section is a summary. 2. Fiber preparation For the preparation of a Cr4+:YAG single-crystal fiber using the LHPG method [14,21], we used, as an initial source, an undoped YAG single-crystal fiber whose side surface had been coated with thin layers of dopants by electron beam deposition. A 0.40-µm-thick layer of Cr2O3 and 0.12-µm-thick layer of CaO were deposited. The calcium ions provide charge compensation. The growth orientation was 15° from [100] to [110]. A Cr4+:YAG singlecrystal fiber grown in this orientation is strain-free because it exhibits no facet growth [21]. The pulling speed was 13 mm/h. The diameter was set to be about 120 μm, which is controlled by the feeding speed of the source material. Photographs of Cr4+:YAG singlecrystal fiber are shown in Figs. 2(a) and 2(b). As shown in Fig. 2(a), the cross-sectional shape of the fiber is square-like, as defined by the {100} facets; however, all sides are roundish. One of the two sets of facing sides is perpendicular to [001] orientation [Fig. 3]. After growth, the fiber was annealed at 1300°C for four hours in an O2 atmosphere in order to increase the number of quadrivalent chromium ions. The absorption coefficient at the wavelength of 1.06 μm is 1.3 cm−1.
#204315 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 17 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006764 | OPTICS EXPRESS 6766
Fig. 2. Photographs of Cr4+:YAG single-crystal fiber. (a) End view. White light enters the fiber from the back end. The fiber is surrounded by {100} facets. (b) Side view.
Fig. 3. Growth orientation and a cross-sectional shape of the Cr4+:YAG single-crystal fiber.
Fig. 4. Photograph of the Cr4+:YAG single-crystal fiber, with both ends polished at the angle of 85.5°.
In this experiment, we use a 40-mm-long single-crystal fiber with SiO2-clad thickness of 1.3 μm. The refractive indices of the SiO2 glass and YAG are 1.45 and 1.82 at around 1.0 μm, respectively. This large difference produces a high numerical aperture and thus confines highpower LD light. The clad thickness of 1.3 μm is large enough to prevent attenuation of the oscillation light but still thin enough to allow the transfer of heat from the gain medium to a heatsink. Both ends of the fiber were polished at an angle of 85.5° to the fiber axis to prevent the formation of a sub-cavity in the laser oscillator [Fig. 4]. All ends were coated for antireflection (AR) for wavelengths from 1.48 to 1.62 μm. In the mode-locking experiment, we intended to oscillate at the wavelengths above 1.5 μm, where the absorbance of water vapor is relatively low, and the saturable absorber used in our experiment functions well. 3. CW Laser oscillation with a linear cavity The Cr4+:YAG single-crystal fiber was set between two copper fiber-holding blocks with indium foil inserted to fill the space between the fiber and the blocks. In order to control the oscillation polarization, the cross section of the grooves on the holding blocks was designed to be the shape of half of the rounded rectangle [Fig. 5]. It imposed a stress perpendicular to
#204315 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 17 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006764 | OPTICS EXPRESS 6767
the [001] orientation, along which gain is maximum. When the fiber was held in the blocks in another manner, such as with V-grooves, the oscillation polarization rotated by some degrees at random. This rotation was suspected to come from the nonsymmetrical birefringence to the plane containing the [001] orientation and the fiber axis which is generated by the stress imposed by the holding blocks.
Fig. 5. Schematic of the single-crystal-fiber holding blocks used for polarization control.
We examined CW laser oscillations using a simple cavity composed of two spherical mirrors with a 100-mm radius of curvature [Fig. 6]. The output coupling was 1%. The centers of the spherical mirrors were positioned almost at the fiber ends. At this stage of our research project, we used a Nd:YVO4 laser as a pump sources for convenience. Its power was 10 W in CW. A lens with 120-mm focal length was used to focus the pump beam. The holding blocks of the fiber were maintained at 21°C by water-cooling. In the configuration without a tuning element, the oscillation wavelength is altered depending on the modification of the cavity alignment; however, the tunability is not controllable. With the alignment of the oscillation at 1526 nm, this laser generates output power of 200 mW with the absorbed pump power of 4.6 W. Its oscillation polarization was parallel to the [001] orientation as intended. A fairly good value of 18 dB was obtained for the extinction ratio of the polarization. The same characteristics of the oscillation polarization and extinction ratio were maintained across the modification of alignments.
Fig. 6. Schematic of two-mirror cavity with a birefringent filter for the evaluation of the tunability of the Cr4+:YAG single-crystal fiber laser.
The wavelength dependence of the output power was systematically measured with a birefringent filter (BF). Incident pump power was fixed at 5.6 W. The alignment of the cavity was adjusted at each rotation angle of the BF. The measured spectrum is shown in Fig. 7(a). The observed oscillation bandwidth is 180 nm (1386–1566 nm). As far as we know, this is the largest tuning range ever reported for a single-crystal fiber laser. The maximum output is 180 mW at 1501 nm when the absorbed pump power is 4.6 W. The dependence of output power on the absorbed pump power at this rotation angle of the BF is shown in Fig. 7(b). The absorbed pump power at a threshold is 2.1 W. The measured M2 values are 1.1 for X and 1.2 for Y. The X axis and the [001] orientation are on the same plane. A good beam quality was achieved as expected. These results confirm the validity of our method for polarization control.
#204315 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 17 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006764 | OPTICS EXPRESS 6768
Fig. 7. (a) Oscillation-wavelength dependence of output power of Cr4+:YAG single-crystal fiber laser. (b) Output power from Cr4+:YAG single-crystal fiber laser with a birefringent filter (solid line). The broken line indicates the oscillation wavelength.
4. Z-fold cavity for mode-locked oscillation For glass material, mode-locked oscillation using a multi-mode glass fiber has already been reported [22]. The fundamental transverse mode was selected by means of two single-mode fiber pig-tails spliced onto both ends of the multi-mode fiber. As mentioned above, we investigated mode-locked operation using a multi-mode single-crystal fiber waveguide, and we tried to select the fundamental transverse mode using an external cavity.
Fig. 8. (a) Schematic of Cr4+:YAG single-crystal fiber laser with an external cavity. (b) Near field beam profiles of the output from the single-crystal fiber laser.
As illustrated in Fig. 8(a), the cavity has a Z shape and consists of the Cr4+:YAG singlecrystal fiber, a block of fused silica (53.4 mm) with Brewster angles, and a semiconductor saturable absorber mirror (SESAM). The fused silica block compensates for the GDD of the single-crystal fiber [8]. At 1520 nm, the GDDs of the fused silica block and the YAG fiber for a round trip are −2608 and 706 fs2, respectively. The reflection angle of the two inner spherical mirrors with a 100-mm radius of curvature is designed to compensate for the astigmatism of the fused silica block. Figure 8(b) shows the beam profile of CW oscillation at the focal point with a 50-mm focusing lens. The measured M2 value is 1.1 for both X and Y orientations. The fundamental transverse mode was clearly selected. The beam diameter inside the fiber is estimated to be about 60 μm. We conducted a wavelength-tuning experiment with a BF and a 1-% output coupler. The tuning bandwidth was 158 nm (1385–1543 nm) and the maximum output was 110 mW without re-aligning the cavity during BF rotation.
#204315 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 17 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006764 | OPTICS EXPRESS 6769
Mode-locked oscillation was investigated with a SESAM placed at the cavities’ end opposite to the entrance of the pump beam. A 1-% output coupler was replaced with the spherical mirror at the entrance. A modulation depth of this SESAM is 0.6%. The reflectance for low-intensity light from 1500 to 1600 nm is more than 98% which is sufficiently large for laser oscillation. From 1500 to 1580 nm, the absolute values of GDD are small (< 300 fs2); however, the GDD is larger than 1500 fs2 at 1480 nm according to the catalogue of the SESAM (model SAM-1550-1from BATOP GmbH). Stable mode-locked operation was achieved with a repetition frequency of 240 MHz. Figure 9(a) shows an intensity autocorrelation. The full width at half maximum is 186 fs. Assuming a hyperbolic secant squared intensity profile (solid line), the pulse duration is calculated to be 120 fs. The spectrum bandwidth is 20.6 nm for wavelength, and it is 2.7 THz for frequency as shown in Fig. 9(b). The center wavelength is 1520 nm. The time-bandwidth product is 0.32, indicating that this pulse is almost transform-limited.
Fig. 9. (a) Intensity autocorrelation of the mode-locked Cr4+:YAG single-crystal fiber laser. Dots indicate measured values. The solid line is a calculation for a hyperbolic secant squared intensity pulse shape. (b) Oscillation spectrum of the mode-locked Cr4+:YAG single-crystal fiber laser. The solid line indicates measured values. The broken line is a calculation of a hyperbolic secant squared intensity profile on frequencies.
Compared to the simulation assuming a hyperbolic secant squared intensity profile on oscillation frequencies (broken line), the measured intensity is slightly excessive at around 1480 nm. A positive GDD value caused by the SESAM is considered to raise the intensities at these wavelengths. The recorded repetition frequency matches the optical length of the cavity of 0.63 m. M2 values were measured to be 1.2 for X and 1.3 for Y. The fundamental transverse mode was also selected in the mode-locked operation. When increasing pump power, the mode-locked oscillation changes from a single pulse to multiple pulses within a round-trip. With 1-% output coupling, the maximum output power of the mode-locked laser with a single pulse is 23 mW at the absorbed pump power of 3.9 W. We achieved the maximum mode-locked power of 48 mW with four pulses in a round trip at the absorbed pump power of 4.7 W. Such multi-pulse oscillations occur because of the limitation of the gain bandwidth. The reflectivity of the SESAM decreases when single pulse oscillation changes to multi-pulsing. This is a consequence of saturation recovery caused by lower energy of each pulses. On the other hand, the gain is increased with multi-pulsing, which comes from shrinking of the oscillation bandwidth into the limited gain bandwidth. At the moment the increase of gain surpass the decrease of the reflectivity of the SESAM, multipulsing occurs [23,24]. If the short-wavelength limits of both the AR on the fiber ends and the reflection bandwidth of the SESAM were moved to shorter wavelength, the gain bandwidth would become broader, hence more power and shorter pulse duration should be achievable with a single pulse. The pulse fluence of the single pulse at 23 mW is 340 μJ/cm2, which is #204315 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 17 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006764 | OPTICS EXPRESS 6770
4.9 times the saturation fluence of this SESAM (70 μJ/cm2). An increase of pulse energy is also expected by adjusting parameters of a SESAM used. 5. Summary In conclusion, we have demonstrated a mode-locked Cr4+:YAG single-crystal fiber laser for the first time as far as we know, — not only for Cr4+:YAG fiber but also for a fiber of any other kind of laser crystal to the best of our knowledge. The combination with a single-crystal fiber multi-mode waveguide and an external cavity enables oscillation in the fundamental transverse mode. The specially designed holding blocks generate a proper birefringence for the control of the oscillation polarization. Based on these features, a Z-fold cavity with a SESAM generated a stable 120-fs pulse train at 23 mW for a single pulse in a round trip. The pulse profile was almost transform-limited. For multi-pulse operation, the maximum modelocked power was 48 mW with four pulses in a round trip. These experiments were conducted using a Nd:YVO4 laser as a pump source. As mentioned above, the purpose of our project is to develop a directly LD-pumped mode-locked Cr4+:YAG laser using a single-crystal fiber. The feasibility of such laser is firmly supported by the achievements reported here. A mode-locked oscillation with the center wavelength of around 1.4 μm would also be possible with an appropriate dispersion compensator, a saturable absorber, and a purged cavity to avoid absorption of water vapor. The combination with a single-crystal fiber and an external cavity is not restricted to the Cr4+:YAG laser. We believe that this design strategy is also applicable to a direct-LD-pumped ultrafast laser using another laser crystal, such as Cr:forsterite and Ti:sapphire.
#204315 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 17 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006764 | OPTICS EXPRESS 6771
NTT Photonics Laboratories, NTT Corporation, 3-1 Morinosato Wakamiya, Atsugi-shi, Kanagawa Pref., 243-0198, Japan 2 NTT Advanced Technology Corporation, 3-1 Morinosato Wakamiya, Atsugi-shi, Kanagawa Pref., 243-0124, Japan * [email protected]
Abstract: We report what is to our knowledge the first mode-locked Cr4+:YAG single-crystal fiber laser, which generates pulses of 120-fs duration with an output power of 23 mW at a center wavelength of 1520 nm for a single pulse in a cavity-round-trip. The laser contains a single-crystal fiber multi-mode waveguide about 120 μm in diameter and 40-mm long. The fundamental transverse mode is selected with an external cavity. This design strategy turned out to be well suited for direct high-power-laserdiode pumping. ©2014 Optical Society of America OCIS codes: (140.3580) Lasers, solid-state; (140.4050) Mode-locked lasers; (140.5680) Rare earth and transition metal solid-state lasers; (140.7090) Ultrafast lasers; (060.3510) Lasers, fiber.
References and Links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Y. Ishida and K. Naganuma, “Compact diode-pumped all-solid-state femtosecond Cr4+:YAG laser,” Opt. Lett. 21(1), 51–53 (1996). S. D. Di Dio Cafiso, E. Ugolotti, A. Schmidt, V. Petrov, U. Griebner, A. Agnesi, W. B. Cho, Y. G. Zhang, S. Y. Choi, F. Rotermund, G. Reali, and F. Pirzio, “Sub-50 fs mode-locking of a Cr:YAG laser using an SWCNT-SA,” Laser Phys. Lett. 10(8), 085801 (2013). C. Chudoba, J. G. Fujimoto, E. P. Ippen, H. A. Haus, U. Morgner, F. X. Kärtner, V. Scheuer, G. Angelow, and T. Tschudi, “All-solid-state Cr:forsterite laser generating 14-fs pulses at 1.3 μm,” Opt. Lett. 26(5), 292–294 (2001). S. Ozharar, I. Baylam, M. N. Cizmeciyan, O. Balci, E. Pince, C. Kocabas, and A. Sennaroglu, “Graphene modelocked multipass-cavity femtosecond Cr4+: forsterite laser,” J. Opt. Soc. Am. B 30(5), 1270–1275 (2013). D. H. Sutter, G. Steinmeyer, L. Gallmann, N. Matuschek, F. Morier-Genoud, U. Keller, V. Scheuer, G. Angelow, and T. Tschudi, “Semiconductor saturable-absorber mirror assisted Kerr-lens mode-locked Ti:sapphire laser producing pulses in the two-cycle regime,” Opt. Lett. 24(9), 631–633 (1999). R. Ell, U. Morgner, F. X. Kãârtner, J. G. Fujimoto, E. P. Ippen, V. Scheuer, G. Angelow, T. Tschudi, M. J. Lederer, A. Boiko, and B. Luther-Davies, “Generation of 5-fs pulses and octave-spanning spectra directly from a Ti:sapphire laser,” Opt. Lett. 26(6), 373–375 (2001). A. G. Okhrimchuk and A. V. Shestakov, “Performance of YAG:Cr4+ laser crystal,” Opt. Mater. 3(1), 1–13 (1994). Y. Ishida and K. Naganuma, “Characteristics of femtosecond pulses near 1.5 µm in a self-mode-locked Cr4+:YAG laser,” Opt. Lett. 19(23), 2003–2005 (1994). A. Sennaroglu, C. R. Pollock, and H. Nathel, “Efficient continuous-wave chromium-doped YAG laser,” J. Opt. Soc. Am. B 12(5), 930–937 (1995). D. J. Ripin, C. Chudoba, J. T. Gopinath, J. G. Fujimoto, E. P. Ippen, U. Morgner, F. X. Kärtner, V. Scheuer, G. Angelow, and T. Tschudi, “Generation of 20-fs pulses by a prismless Cr4+:YAG laser,” Opt. Lett. 27(1), 61–63 (2002). E. Sorokin, S. Naumov, and I. T. Sorokina, “Ultrabroadband infrared solid-state lasers,” IEEE J. Sel. Top. Quantum Electron. 11(3), 690–712 (2005). A. J. Alcock, P. Scorah, and K. Hnatovsky, “Broadly tunable continuous-wave diode-pumped Cr4+:YAG laser,” Opt. Commun. 215(1–3), 153–157 (2003). S. Naumov, E. Sorokin, and I. T. Sorokina, “Directly diode-pumped Kerr-lens mode-locked Cr4+:YAG laser,” Opt. Lett. 29(11), 1276–1278 (2004). S. Ishibashi and K. Naganuma, “Diode-pumped Cr4+:YAG single crystal fiber laser,” in Advanced Solid State Lasers, OSA Technical Digest Series (Optical Society of America, 2000), paper MD4. S. Naumov, E. Sorokin, and I. Sorokina, “Directly diode-pumped femtosecond Cr4+:YAG laser,” in Advanced Solid-State Photonics, J. Zayhowski, ed., Vol. 83 of OSA Trends in Optics and Photonics (Optical Society of America, 2003), paper 163.
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Received 7 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 17 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006764 | OPTICS EXPRESS 6764
16. J. Takayanagi, S. Kanamori, K. Suizu, M. Yamashita, T. Ouchi, S. Kasai, H. Ohtake, H. Uchida, N. Nishizawa, and K. Kawase, “Generation and detection of broadband coherent terahertz radiation using 17-fs ultrashort pulse fiber laser,” Opt. Express 16(17), 12859–12865 (2008). 17. F. D. Brunner, O.-P. Kwon, S.-J. Kwon, M. Jazbinsek, A. Schneider, and P. Günter, “A hydrogen-bonded organic nonlinear optical crystal for high-efficiency terahertz generation and detection,” Opt. Express 16(21), 16496–16508 (2008). 18. J. Stone, C. A. Burrus, A. G. Dentai, and B. I. Miller, “Nd:YAG single-crystal fiber laser: Room-temperature cw operation using a single LED as an end pump,” Appl. Phys. Lett. 29(1), 37–39 (1976). 19. J. Stone and C. A. Burrus, “Self-contained LED-pumped single-crystal Nd:YAG fiber laser,” Fiber Integrated Opt. 2(1), 19–46 (1979). 20. M. M. Fejer, J. L. Nightingale, G. A. Magel, and R. L. Byer, “Laser-heated miniature pedestal growth apparatus for single-crystal optical fibers,” Rev. Sci. Instrum. 55(11), 1791–1796 (1984). 21. S. Ishibashi, K. Naganuma, and I. Yokohama, “Cr, Ca:Y3Al5O12 laser crystal grown by the laser-heated pedestal growth method,” J. Cryst. Growth 183(4), 614–621 (1998). 22. M. E. Fermann, A. Galvanauskas, and M. Hofer, “Ultrafast pulse sources based on multi-mode optical fibers,” Appl. Phys. B 70(S1), S13–S23 (2000). 23. J. A. Au, D. Kopf, F. Morier-Genoud, M. Moser, and U. Keller, “60-fs pulses from a diode-pumped Nd:glass laser,” Opt. Lett. 22(5), 307–309 (1997). 24. F. X. Kärtner, J. Aus der Au, and U. Keller, “Modelocking with slow and fast saturable absorbers - What's the difference?” IEEE J. Sel. Top. Quantum Electron. 4(2), 159–168 (1998).
1. Introduction Solid-state lasers pumped by laser diodes (LDs), which are, in general, compact, costeffective and low-energy-consumption compared to lasers pumped by solid-state lasers, are now commonly used in various applications. On the other hand, there exist useful ultrafast solid-state lasers, such as Cr4+:YAG [1,2], Cr:forsterite [3,4], and Ti:sapphire [5,6], which are ordinarily pumped by solid-state lasers. It is highly desirable to change the pump source of these lasers to LDs in order for them to be used in medical and industrial fields. In this paper, we report a new configuration of a Cr4+:YAG laser with a single-crystal fiber, which is appropriate for a directly LD-pumped mode-locked oscillator. Cr4+:YAG has a large gain bandwidth, nearly encompassing 1.3 to 1.6 μm, and it has attracted much attention as a tunable laser and an ultrashort-pulse source. Continuous efforts have been made to develop it in a CW and a mode-locked oscillator [7–15]. This laser is optically pumped by utilizing its broad absorbing band around 1 μm. Regarding lasers using a bulk Cr4+:YAG material, there have been many attempts to pump them with solid-state lasers like Nd:YVO4 or Yb glass-fiber lasers, or with LDs directly. With a solid-state laser as a pump source, CW oscillation in the wavelength range of 1.34–1.57 μm [9], Kerr lens modelocked oscillation with a pulse width of 20 fs [10], and mode-locked oscillation started by a saturable absorber with a pulse width of 27 fs [11] have been reported. In practical use, a saturable absorber is convenient as a starter of mode-locked oscillation. An LD-pumped mode-locked Cr4+:YAG laser started with a saturable absorber has already been reported, with maximum mode-locked power of 15 mW with pulse duration of 62 fs [15]. The pump light from a standard high-power LD, whose power is more than 5 W, is diffusive. Because excessively high-concentration Cr4+:YAG sacrifice crystal quality, there is an upper limit of the dopant concentration, and hence the length of the laser crystal has a lower limit. When a bulk crystal with a reasonable length is used for direct LD-pumping, a poor overlap between the pump and oscillation beam will occur, which makes the oscillation efficiency small. The purpose of our research is to improve the power of a directly LD-pumped mode-locked Cr4+:YAG laser started with a saturable absorber. A femtosecond pulse of around 1.4 μm is suitable for THz-wave generation with organic nonlinear crystals such as DAST or OH1. Compared to 1.5-μm pumping, 1.4-μm pumping is expected to have a longer coherent length with DAST for some frequencies between 4 and 7 THz, and a longer optimum length is predicted with OH1 for frequencies between 0.3 and 2.5 THz [16,17]. As a gain medium, we have developed a Cr4+:YAG single-crystal fiber waveguide with a SiO2 clad. The fiber is about 120 μm in diameter and 40-mm long. It has enough volume for mode-locked oscillation, and compensating for its group-delay dispersion (GDD) is not difficult. This multi-mode waveguide can confine a diffusive pump beam within a small
#204315 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 17 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006764 | OPTICS EXPRESS 6765
cross-sectional area over its entire length. Addition of an external cavity enables oscillation in the fundamental transverse mode [Fig. 1]. This report confirms the capability of a singlecrystal fiber laser with an external cavity for mode-locked oscillation.
Fig. 1. Schematic of a directly LD-pumped single-crystal fiber multi-mode waveguide laser with an external cavity.
The first single-crystal fiber laser, a Nd:YAG single-crystal fiber CW laser pumped with an LED, was developed by J. Stone and associates [18,19]. The fiber for their experiment was made by using the laser-heated pedestal growth (LHPG) method. In the LHPG method, a CO2 laser is used to melt the top of a source rod. The fiber crystal is grown from the molten portion and is pulled up following a seed crystal which defines the orientation of the crystal fiber. This method was improved by M. M. Fejer et al. in the 1980s with the invention of a symmetrical focusing optics [20]. We have made Cr4+:YAG single-crystal fibers using an LHPG apparatus with almost the same focusing optics and have already reported the appropriate conditions for Cr4+:YAG fiber growth [14,21]. As one of the conditions, we select a particular growth orientation to suppress a formation of strain region. The growth atmosphere is O2, and we use a source material deposited with dopants. Annealing is done after the growth to increase the number of quadrivalent chromium ions. Using the single-crystal fiber made with the method described above, we have already reported a directly LD-pumped CW laser to oscillate in the fundamental transverse mode. Pumped with 15-W LD at 0.98 μm, an external-cavity Cr4+:YAG fiber laser oscillated in the CW mode with a power of 150 mW at 1.42 μm. The M2 value was 1.2 for both X and Y [14]. This good beam quality should be taken over to a mode-locked single-crystal fiber laser. This report is organized as follows. Section 2 reports the fiber preparation. Section 3 describes the CW oscillation, which was a test of the polarization control, and Section 4 presents the mode-locked oscillation of this fiber laser. The last section is a summary. 2. Fiber preparation For the preparation of a Cr4+:YAG single-crystal fiber using the LHPG method [14,21], we used, as an initial source, an undoped YAG single-crystal fiber whose side surface had been coated with thin layers of dopants by electron beam deposition. A 0.40-µm-thick layer of Cr2O3 and 0.12-µm-thick layer of CaO were deposited. The calcium ions provide charge compensation. The growth orientation was 15° from [100] to [110]. A Cr4+:YAG singlecrystal fiber grown in this orientation is strain-free because it exhibits no facet growth [21]. The pulling speed was 13 mm/h. The diameter was set to be about 120 μm, which is controlled by the feeding speed of the source material. Photographs of Cr4+:YAG singlecrystal fiber are shown in Figs. 2(a) and 2(b). As shown in Fig. 2(a), the cross-sectional shape of the fiber is square-like, as defined by the {100} facets; however, all sides are roundish. One of the two sets of facing sides is perpendicular to [001] orientation [Fig. 3]. After growth, the fiber was annealed at 1300°C for four hours in an O2 atmosphere in order to increase the number of quadrivalent chromium ions. The absorption coefficient at the wavelength of 1.06 μm is 1.3 cm−1.
#204315 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 17 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006764 | OPTICS EXPRESS 6766
Fig. 2. Photographs of Cr4+:YAG single-crystal fiber. (a) End view. White light enters the fiber from the back end. The fiber is surrounded by {100} facets. (b) Side view.
Fig. 3. Growth orientation and a cross-sectional shape of the Cr4+:YAG single-crystal fiber.
Fig. 4. Photograph of the Cr4+:YAG single-crystal fiber, with both ends polished at the angle of 85.5°.
In this experiment, we use a 40-mm-long single-crystal fiber with SiO2-clad thickness of 1.3 μm. The refractive indices of the SiO2 glass and YAG are 1.45 and 1.82 at around 1.0 μm, respectively. This large difference produces a high numerical aperture and thus confines highpower LD light. The clad thickness of 1.3 μm is large enough to prevent attenuation of the oscillation light but still thin enough to allow the transfer of heat from the gain medium to a heatsink. Both ends of the fiber were polished at an angle of 85.5° to the fiber axis to prevent the formation of a sub-cavity in the laser oscillator [Fig. 4]. All ends were coated for antireflection (AR) for wavelengths from 1.48 to 1.62 μm. In the mode-locking experiment, we intended to oscillate at the wavelengths above 1.5 μm, where the absorbance of water vapor is relatively low, and the saturable absorber used in our experiment functions well. 3. CW Laser oscillation with a linear cavity The Cr4+:YAG single-crystal fiber was set between two copper fiber-holding blocks with indium foil inserted to fill the space between the fiber and the blocks. In order to control the oscillation polarization, the cross section of the grooves on the holding blocks was designed to be the shape of half of the rounded rectangle [Fig. 5]. It imposed a stress perpendicular to
#204315 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 17 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006764 | OPTICS EXPRESS 6767
the [001] orientation, along which gain is maximum. When the fiber was held in the blocks in another manner, such as with V-grooves, the oscillation polarization rotated by some degrees at random. This rotation was suspected to come from the nonsymmetrical birefringence to the plane containing the [001] orientation and the fiber axis which is generated by the stress imposed by the holding blocks.
Fig. 5. Schematic of the single-crystal-fiber holding blocks used for polarization control.
We examined CW laser oscillations using a simple cavity composed of two spherical mirrors with a 100-mm radius of curvature [Fig. 6]. The output coupling was 1%. The centers of the spherical mirrors were positioned almost at the fiber ends. At this stage of our research project, we used a Nd:YVO4 laser as a pump sources for convenience. Its power was 10 W in CW. A lens with 120-mm focal length was used to focus the pump beam. The holding blocks of the fiber were maintained at 21°C by water-cooling. In the configuration without a tuning element, the oscillation wavelength is altered depending on the modification of the cavity alignment; however, the tunability is not controllable. With the alignment of the oscillation at 1526 nm, this laser generates output power of 200 mW with the absorbed pump power of 4.6 W. Its oscillation polarization was parallel to the [001] orientation as intended. A fairly good value of 18 dB was obtained for the extinction ratio of the polarization. The same characteristics of the oscillation polarization and extinction ratio were maintained across the modification of alignments.
Fig. 6. Schematic of two-mirror cavity with a birefringent filter for the evaluation of the tunability of the Cr4+:YAG single-crystal fiber laser.
The wavelength dependence of the output power was systematically measured with a birefringent filter (BF). Incident pump power was fixed at 5.6 W. The alignment of the cavity was adjusted at each rotation angle of the BF. The measured spectrum is shown in Fig. 7(a). The observed oscillation bandwidth is 180 nm (1386–1566 nm). As far as we know, this is the largest tuning range ever reported for a single-crystal fiber laser. The maximum output is 180 mW at 1501 nm when the absorbed pump power is 4.6 W. The dependence of output power on the absorbed pump power at this rotation angle of the BF is shown in Fig. 7(b). The absorbed pump power at a threshold is 2.1 W. The measured M2 values are 1.1 for X and 1.2 for Y. The X axis and the [001] orientation are on the same plane. A good beam quality was achieved as expected. These results confirm the validity of our method for polarization control.
#204315 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 17 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006764 | OPTICS EXPRESS 6768
Fig. 7. (a) Oscillation-wavelength dependence of output power of Cr4+:YAG single-crystal fiber laser. (b) Output power from Cr4+:YAG single-crystal fiber laser with a birefringent filter (solid line). The broken line indicates the oscillation wavelength.
4. Z-fold cavity for mode-locked oscillation For glass material, mode-locked oscillation using a multi-mode glass fiber has already been reported [22]. The fundamental transverse mode was selected by means of two single-mode fiber pig-tails spliced onto both ends of the multi-mode fiber. As mentioned above, we investigated mode-locked operation using a multi-mode single-crystal fiber waveguide, and we tried to select the fundamental transverse mode using an external cavity.
Fig. 8. (a) Schematic of Cr4+:YAG single-crystal fiber laser with an external cavity. (b) Near field beam profiles of the output from the single-crystal fiber laser.
As illustrated in Fig. 8(a), the cavity has a Z shape and consists of the Cr4+:YAG singlecrystal fiber, a block of fused silica (53.4 mm) with Brewster angles, and a semiconductor saturable absorber mirror (SESAM). The fused silica block compensates for the GDD of the single-crystal fiber [8]. At 1520 nm, the GDDs of the fused silica block and the YAG fiber for a round trip are −2608 and 706 fs2, respectively. The reflection angle of the two inner spherical mirrors with a 100-mm radius of curvature is designed to compensate for the astigmatism of the fused silica block. Figure 8(b) shows the beam profile of CW oscillation at the focal point with a 50-mm focusing lens. The measured M2 value is 1.1 for both X and Y orientations. The fundamental transverse mode was clearly selected. The beam diameter inside the fiber is estimated to be about 60 μm. We conducted a wavelength-tuning experiment with a BF and a 1-% output coupler. The tuning bandwidth was 158 nm (1385–1543 nm) and the maximum output was 110 mW without re-aligning the cavity during BF rotation.
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Received 7 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 17 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006764 | OPTICS EXPRESS 6769
Mode-locked oscillation was investigated with a SESAM placed at the cavities’ end opposite to the entrance of the pump beam. A 1-% output coupler was replaced with the spherical mirror at the entrance. A modulation depth of this SESAM is 0.6%. The reflectance for low-intensity light from 1500 to 1600 nm is more than 98% which is sufficiently large for laser oscillation. From 1500 to 1580 nm, the absolute values of GDD are small (< 300 fs2); however, the GDD is larger than 1500 fs2 at 1480 nm according to the catalogue of the SESAM (model SAM-1550-1from BATOP GmbH). Stable mode-locked operation was achieved with a repetition frequency of 240 MHz. Figure 9(a) shows an intensity autocorrelation. The full width at half maximum is 186 fs. Assuming a hyperbolic secant squared intensity profile (solid line), the pulse duration is calculated to be 120 fs. The spectrum bandwidth is 20.6 nm for wavelength, and it is 2.7 THz for frequency as shown in Fig. 9(b). The center wavelength is 1520 nm. The time-bandwidth product is 0.32, indicating that this pulse is almost transform-limited.
Fig. 9. (a) Intensity autocorrelation of the mode-locked Cr4+:YAG single-crystal fiber laser. Dots indicate measured values. The solid line is a calculation for a hyperbolic secant squared intensity pulse shape. (b) Oscillation spectrum of the mode-locked Cr4+:YAG single-crystal fiber laser. The solid line indicates measured values. The broken line is a calculation of a hyperbolic secant squared intensity profile on frequencies.
Compared to the simulation assuming a hyperbolic secant squared intensity profile on oscillation frequencies (broken line), the measured intensity is slightly excessive at around 1480 nm. A positive GDD value caused by the SESAM is considered to raise the intensities at these wavelengths. The recorded repetition frequency matches the optical length of the cavity of 0.63 m. M2 values were measured to be 1.2 for X and 1.3 for Y. The fundamental transverse mode was also selected in the mode-locked operation. When increasing pump power, the mode-locked oscillation changes from a single pulse to multiple pulses within a round-trip. With 1-% output coupling, the maximum output power of the mode-locked laser with a single pulse is 23 mW at the absorbed pump power of 3.9 W. We achieved the maximum mode-locked power of 48 mW with four pulses in a round trip at the absorbed pump power of 4.7 W. Such multi-pulse oscillations occur because of the limitation of the gain bandwidth. The reflectivity of the SESAM decreases when single pulse oscillation changes to multi-pulsing. This is a consequence of saturation recovery caused by lower energy of each pulses. On the other hand, the gain is increased with multi-pulsing, which comes from shrinking of the oscillation bandwidth into the limited gain bandwidth. At the moment the increase of gain surpass the decrease of the reflectivity of the SESAM, multipulsing occurs [23,24]. If the short-wavelength limits of both the AR on the fiber ends and the reflection bandwidth of the SESAM were moved to shorter wavelength, the gain bandwidth would become broader, hence more power and shorter pulse duration should be achievable with a single pulse. The pulse fluence of the single pulse at 23 mW is 340 μJ/cm2, which is #204315 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 17 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006764 | OPTICS EXPRESS 6770
4.9 times the saturation fluence of this SESAM (70 μJ/cm2). An increase of pulse energy is also expected by adjusting parameters of a SESAM used. 5. Summary In conclusion, we have demonstrated a mode-locked Cr4+:YAG single-crystal fiber laser for the first time as far as we know, — not only for Cr4+:YAG fiber but also for a fiber of any other kind of laser crystal to the best of our knowledge. The combination with a single-crystal fiber multi-mode waveguide and an external cavity enables oscillation in the fundamental transverse mode. The specially designed holding blocks generate a proper birefringence for the control of the oscillation polarization. Based on these features, a Z-fold cavity with a SESAM generated a stable 120-fs pulse train at 23 mW for a single pulse in a round trip. The pulse profile was almost transform-limited. For multi-pulse operation, the maximum modelocked power was 48 mW with four pulses in a round trip. These experiments were conducted using a Nd:YVO4 laser as a pump source. As mentioned above, the purpose of our project is to develop a directly LD-pumped mode-locked Cr4+:YAG laser using a single-crystal fiber. The feasibility of such laser is firmly supported by the achievements reported here. A mode-locked oscillation with the center wavelength of around 1.4 μm would also be possible with an appropriate dispersion compensator, a saturable absorber, and a purged cavity to avoid absorption of water vapor. The combination with a single-crystal fiber and an external cavity is not restricted to the Cr4+:YAG laser. We believe that this design strategy is also applicable to a direct-LD-pumped ultrafast laser using another laser crystal, such as Cr:forsterite and Ti:sapphire.
#204315 - $15.00 USD (C) 2014 OSA
Received 7 Jan 2014; revised 20 Feb 2014; accepted 25 Feb 2014; published 17 Mar 2014 24 March 2014 | Vol. 22, No. 6 | DOI:10.1364/OE.22.006764 | OPTICS EXPRESS 6771
