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Diode-pumped, high-average-power femtosecond Cr3+ : LiCAF laser Umit Demirbas,1 Alphan Sennaroglu,2 Andrew Benedick,1 Aleem Siddiqui,1 Franz X. Kärtner,1 and James G. Fujimoto1 1

Department of Electrical Engineering and Computer Science and Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA 2 Laser Research Laboratory, Department of Physics, Koç University, Rumelifeneri, 34450 Istanbul, Turkey Received August 20, 2007; revised October 9, 2007; accepted October 10, 2007; posted October 15, 2007 (Doc. ID 86706); published November 6, 2007 We demonstrate a high-average-power continuous wave (cw) and cw mode-locked Cr3+ : LiCAF laser pumped by broad-area laser diodes. In cw lasing experiments, up to 580 mW of output was obtained with 4.35 W of incident pump. A semiconductor saturable absorber mirror was used to initiate stable, self-starting, mode locking. In the cw mode-locked regime, the Cr3+ : LiCAF laser produced nearly transform-limited, 67 fs long pulses near 800 nm with an average output power of 300 mW. The pulse repetition rate was 120 MHz, with a pulse energy of 2.5 nJ. © 2007 Optical Society of America OCIS codes: 140.3460, 140.5680, 140.3580, 140.3600, 140.3480, 140.4050.

Cr3+-doped colquiriite gain media (Cr3+ : LiSAF, Cr3+ : LiSGaF, and Cr3+ : LiCAF) [1,2] are a potential low-cost alternative to femtosecond Ti:sapphire lasers in the 800 nm region because they can be directly pumped by red diodes [3–9]. Initially, most of the fs-laser development with Cr3+-doped colquiriites focused on Cr3+ : LiSAF due to its lower scattering loss [10] and higher peak emission cross section [3]. However, Cr3+ : LiSAF suffers from thermal problems, and for diode-pumped systems, this either limits the average power levels to the sub-100 mW level or requires carefully designed laser cavities with an asymmetric laser mode to prevent heating due to upconversion processes [3]. Using a specialized cavity consisting of cylindrical cavity mirrors and a very thin crystal with optimized cooling, the highest power performance from a Cr3+:colquiriite laser was reported by Kopf et al., who obtained 110 fs pulses with 500 mW of average power (also 50 fs, 340 mW pulses) from a Cr3+ : LiSAF laser pumped by a 15 W laser diode array [3]. Cr3+ : LiCAF has higher intrinsic slope efficiency [3], higher thermal conductivity [3], and lower thermal lensing [11] than Cr3+ : LiSAF. Most importantly, thermal quenching of fluorescence for Cr3+ : LiCAF occurs at a much higher temperature 共255° C兲, compared with Cr3+ : LiSAF 共69° C兲 [12]. However, Cr: LiCAF is more susceptible to cavity losses due to its lower gain [3]. Despite the low gain, however, Cr3+ : LiCAF has recently attracted attention because of its superior thermal properties [4,6–9]. To date, diode-pumped, Kerr-lens mode-locked (KLM) Cr3+ : LiCAF systems generating 10 fs pulses with 40 mW average power [6] and 50 fs pulses with 75 mW of average power [4] have been demonstrated. The main disadvantage of pumping with broad-area laser diodes is that it is difficult to achieve stable KLM. In the development of high-power Cr3+ : LiCAF lasers, it is very important to understand the role of thermal effects in power scaling and whether output powers comparable to Cr3+ : LiSAF can be achieved. 0146-9592/07/223309-3/$15.00

In this Letter, we describe a diode-pumped, modelocked, high-average-power Cr3+ : LiCAF laser producing 67 fs pulses at a 120 MHz repetition rate. Mode-locking was initiated with a semiconductor saturable absorber mirror (SESAM) [13], also known as saturable Bragg reflectors (SBRs) [14]. The SESAM mode-locked laser was self-starting, immune to environmental fluctuations, did not require careful cavity alignment, and enabled turn-key operation. Our results also show that power scaling with pumping greater than 4 W is possible. In particular, cw operation with up to 580 mW of output power at ⬃790 nm wavelengths was obtained with ⬃4.35 W of incident pump power. In mode-locking experiments, 67 fs pulses with ⬃300 mW of average power, and 50 fs pulses with 150 mW of average power, could be obtained. To the best of our knowledge, these are the highest cw and mode-locked powers demonstrated from a diode-pumped Cr3+ : LiCAF laser system, and peak powers 共⬃35 kW兲 are comparable to KLM mode-locked Cr3+ : LiCAF lasers [6]. Also, SESAMinitiated mode locking was used for the first time in Cr3+ : LiCAF, thus making the system highly stable and robust. Figure 1 shows a schematic of the diode-pumped Cr3+ : LiCAF laser cavity. Five linearly polarized 1 W single-emitter diodes (D1–D5) (n-Light Photonics) were used for pumping. The diodes had an emitter size of 1 ␮m ⫻ 150 ␮m and used cylindrical microlenses to collimate the light along the fast axis (perpendicular to the plane of the junction). Diode output was diffraction limited along the fast axis and multimode along the slow axis (parallel to plane of the junction) with an M2 of ⬃10. Two of the 680 nm diodes (D1–D2) were combined with polarization multiplexing, using a half-wave plate 共␭ / 2兲 and a polarizing beam splitter (PBS) cube, to pump the crystal from one side. For pumping from the other side, a dichroic mirror was used to combine the diodes D3 共680 nm兲 and D4 共665 nm兲, and an additional diode D5 共680 nm兲 was combined by polarization multiplex© 2007 Optical Society of America

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Fig. 1. (Color online) Schematic of the diode-pumped Cr3+ : LiCAF laser system. Dashed lines indicate the cw cavity.

ing. The pump beams were then collimated with 10 cm lenses and focused to a spot size of ⬃25 ␮m ⫻ 70 ␮m using achromatic doublets with focal lengths of 45 mm (left side) and 63.5 mm (right side). Shown with dashed lines in Fig. 1, the cw laser resonator was a standard x-folded cavity, consisting of two curved pump mirrors (M1 and M2, R = 75 mm), a highly reflecting end mirror (M3), and a flat output coupler (OC). Arm lengths of 35 cm (OC arm) and 40 cm were used to obtain a laser mode size of 25– 30 ␮m inside the crystal. Cw and mode-locked laser performance were measured with seven different output couplers (1%, 1.4%, 1.9%, 2.4%, 3.4%, 4.3%, and 6.2 % at ⬃800 nm). The gain medium was a 2 mm long, 3 mm thick, Brewster-cut, 10% Cr doped Cr3+ : LiCAF crystal, which was mounted with indium foil in a copper holder and water-cooled at 15° C. Although cooling the crystal from 15° C to 3 ° C resulted in a ⬃10% increase in the output powers, the temperature was kept at 15° C to reduce risk of damaging the crystal due to condensation. The absorption of the crystal was ⬎90% for both TE and TM polarized light at 665 nm and 680 nm. In mode-locking experiments, two double-chirped mirrors (DCM-1 and DCM-2) with a group velocity dispersion (GVD) of ⬃−50 fs2 per bounce were used in the cavity. The Cr3+ : LiCAF crystal 共GVD ⬃ + 25 fs2 / mm兲, and the intracavity air path produced a dispersion of ⬃ + 140 fs2. The estimated total round-trip cavity dispersion was ⬃−50 to −100 fs2. To initiate mode-locking, a SESAM was used. The SESAM consists of a 20 pair AlAs/ Al0.17Ga0.83As Bragg stack centered at 800 nm with a ⬃60 nm bandwidth and five 6 nm thick GaAs quantum wells that provide saturable absorption. The measured modulation depth of the SESAM was 4.5%. Nonsaturable loss was below 0.5%, which is very important for efficient operation, since Cr3+ : LiCAF has low gain. The measured saturation energy fluence of the SESAM was ⬃35 ␮J / cm2 and two-photon absorption (TPA) effects were observed for incident energy fluences above ⬃1.5 mJ/ cm2. Mirrors (M4) with R = 75 and 100 mm were used to vary the spot size on the SESAM between a ⬃25 and 40 ␮m radius. Figure 2 shows the measured cw performance of the laser using 1.4%, 3.4%, and 4.3% output couplers. The diodes were turned on in sequence from D1 to D5. The slope efficiencies for each diode were similar and did not vary by more than 20%. The best power

performance was obtained with a 1.4% OC, with up to 580 mW of output power generated at 4.35 W of incident pump power. The corresponding threshold pump power and the slope efficiency were 200 mW and ⬃17%, respectively. The relatively low slope efficiency is mainly due to the mode mismatch between the laser and pump modes. The performance with the 1%, 1.9%, and 2.4% output couplers were similar to those with 1.4% transmission, where the cw power level was ⬎450 mW (not shown in Fig. 2). For the 3.4% and 4.3% output couplers, cw performance decreased dramatically due to the low gain of Cr3+ : LiCAF. The free-running cw output was ⬃790 nm for all output couplers. By using a fused silica prism in the high-reflector arm, the cavity could be tuned continuously from 765 to 850 nm using the 1% output coupler. The cavity loss was estimated to be ⬃1% using the threshold data from the different output couplers. As shown in Fig. 2, thermal effects were observed for pump powers above ⬍3.5 W, where the decrease in laser performance is due to thermal quenching of fluorescence lifetime (starting above 200° C [12]). Parasitic upconversion produces a high thermal load on the crystal [11], thus scaling as the square of the upper-state population density. The 10% Cr3+ : LiCAF crystal had ␣ = ⬃ 11.5 cm−1, which caused high population inversion densities and thus increased thermal loading due to upconversion.

Fig. 2. (Color online) Continuous-wave efficiency curves for the diode pumped Cr3+ : LiCAF laser, taken with the 1.4%, 3.4%, and 4.3% output couplers.

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In mode-locking experiments, focusing mirrors with different radii of curvature (M4 in Fig. 1), and different output couplers, were used to vary the spot size and incident energy fluence on the SESAM to optimize performance. For the cavity with an R = 75 mm M4 and a 2.4% output coupler, the cw lasing threshold increased from 430 to 850 mW using the SESAM. The cavity operated cw for incident pump powers up to 1.5 W, and generated Q-switched mode-locked pulses for pump powers between 1.5 and 2 W. Stable cw mode-locking was obtained for pump powers ⬎2 W. Figure 3 shows examples of spectra and background-free autocorrelation traces at ⬃4 W pump. Mode-locking was self-starting and stable against environmental disturbances. Figure 3(a) shows the spectrum and autocorrelation trace with an R = 75 mm M4 and a 2.4% output coupler. With this configuration, 67 fs pulses with 300 mW of average power and 11 nm of bandwidth ⬃800 nm were obtained at 120 MHz repetition rate (a pulse energy of 2.5 nJ and a peak power of ⬃35 kW). The time bandwidth product is ⬃0.35; close to the transform limit (0.315) for sech2 pulses. The intracavity pulse energies were ⬎100 nJ, thus producing an estimated nonlinear phase shift of ⬃0.1 rad/ round-trip. For this focusing configuration (R = 75 mm, M4), it was not possible to further decrease the pulsewidths by using lower output coupling, since, with the lower OCs, the incident energy fluence levels on the SESAM caused TPA. Multiple pulsing instabilities were also observed for low output couplings. Changing the focusing mirror from R = 75 mm to 100 mm increased the spot size on the SBR, thus enabling lower output coupling to be used. In that case, using a 1% OC, pulses as short as 50 fs could be obtained with

Fig. 3. (Color online) Spectra and background-free intensity autocorrelation taken using a (a) 75 mm radius M4 [see Fig. 1] and a 2.4% output coupler and (b) 100 mm radius M4 and a 1% output coupler. Output powers were (a) 300 mW and (b) 150 mW, corresponding to (a) 2.5 nJ and (b) 1.25 nJ pulse energies for the 120 MHz repetition rate cavity.

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average powers of ⬃150 mW (⬃1.25 nJ pulse energy). Figure 3(b) shows the spectra and autocorrelation trace (time–bandwidth product ⬃0.4). We also inserted a fused silica prism pair into the HR arm to continuously tune the GVD. This did not produce shorter pulse widths, possibly due to the limited bandwidth of the SESAM. In conclusion, our results clearly demonstrate that, due to the superior thermal properties of Cr3+ : LiCAF, similar power levels as in Cr3+ : LiSAF [3] can be obtained without specially designed laser cavities. By using a diode-pumped Cr3+ : LiCAF laser, we demonstrated up to 580 mW of cw output. Using a SESAM for mode-locking, sub-70 fs pulses with 300 mW of average power and 50 fs pulses with 150 mW of average power, could be obtained. Advantages of this SESAM-initiated configuration include self-starting mode-locking, robust operation against environmental fluctuations, and reduced requirements for precise cavity alignment. We expect that with the advent of higher-power diodes near 650 nm, comparable or higher powers can be achieved with a simpler pumping configuration. This project was supported by the National Science Foundation (ECS-0456928 and ECS-0501478), Air Force Office of Scientific Research (FA9550-040-10046 and FA9550-040-1-0011), and the Scientific and Technical Research Council of Turkey (Tubitak, project 104T247). References 1. S. A. Payne, L. L. Chase, H. W. Newkirk, L. K. Smith, and W. F. Krupke, IEEE J. Quantum Electron. 24, 2243 (1988). 2. S. A. Payne, L. L. Chase, L. K. Smith, W. L. Kway, and H. W. Newkirk, J. Appl. Phys. 66, 1051 (1989). 3. D. Kopf, K. J. Weingarten, G. Zhang, M. Moser, M. A. Emanuel, R. J. Beach, J. A. Skidmore, and U. Keller, Appl. Phys. B 65, 235 (1997). 4. K. M. Gabel, P. Russbuldt, R. Lebert, and A. Valster, Opt. Commun. 157, 327 (1998). 5. J. M. Hopkins, G. J. Valentine, B. Agate, A. J. Kemp, U. Keller, and W. Sibbett, IEEE J. Quantum Electron. 38, 360 (2002). 6. P. Wagenblast, R. Ell, U. Morgner, F. Grawert, and F. X. Kärtner, Opt. Lett. 28, 1713 (2003). 7. A. Isemann and C. Fallnich, Opt. Express 11, 259 (2003). 8. P. Wagenblast, U. Morgner, F. Grawert, V. Scheuer, G. Angelow, M. J. Lederer, and F. X. Kärtner, Opt. Lett. 27, 1726 (2002). 9. P. LiKamWa, B. H. T. Chai, and A. Miller, Opt. Lett. 17, 1438 (1992). 10. D. Klimm, G. Lacayo, and P. Reiche, J. Cryst. Growth 210, 683 (2000). 11. J. M. Eichenholz and M. Richardson, IEEE J. Quantum Electron. 34, 910 (1998). 12. M. Stalder, M. Bass, and B. H. T. Chai, J. Opt. Soc. Am. B 9, 2271 (1992). 13. U. Keller, K. J. Weingarten, F. X. Kärtner, D. Kopf, B. Braun, I. D. Jung, R. Fluck, C. Honninger, N. Matuschek, and J. A. derAu, IEEE J. Sel. Top. Quantum Electron. 2, 435 (1996). 14. S. Tsuda, W. H. Knox, S. T. Cundiff, W. Y. Jan, and J. E. Cunningham, IEEE J. Sel. Top. Quantum Electron. 2, 454 (1996).