Femtosecond lasers are essential for many applications including ultrafast spectroscopy, high speed measurement, laser micromachining and biomedical imaging. In order to make femtosecond technology more accessible outside of the laboratory, methods of reducing cost while maintaining high performance must be developed. Laser diode pumped solid state lasers are an attractive alternative to conventional pumping with expensive gas or solid state lasers. Cr:LiSAF is a widely studied laser material with a broad emission bandwidth around 850 nm that can be pumped at 670 nm with red laser diodes [1]. Previous efforts in diode pumping Cr:LiSAF used broad-stripe diodes with powers of hundreds of mW [2–6]. Pulses as short as 9.9 fs have been generated from a diode-pumped Cr:LiSAF laser [4], and pulse energies of 0.5–1 nJ, comparable to those in standard Ti:sapphire systems, can be achieved with pump powers of several hundred mW [2–6]. However, these broad-stripe pump diodes are still relatively expensive and have poor mode quality, making efficient mode matching difficult. An attractive alternative is to pump with single spatial mode diodes, which significantly improves mode matching and laser efficiency [7, 8]. Single mode diodes with powers of 50–60 mW at wavelengths ranging from 660–690 nm are available for only ~$20 each, making this pump source extremely inexpensive. Previous researchers demonstrated compact mode-locked Cr:LiSAF lasers pumped by low cost, single spatial mode diodes in several configurations [7, 8]. These lasers typically generated 20 mW output power and 120 fs pulses at 430 MHz, corresponding to a pulse energy of 0.05 nJ. To the best of our knowledge, the maximum pulse energy achieved was 0.14 nJ [8], which may be too low for some applications.

In this paper, we demonstrate an extended cavity femtosecond Cr:LiSAF laser pumped by single spatial mode diodes and using either double-chirped mirrors (DCMs) or prisms for dispersion compensation. Pulse energies of 0.75 nJ at a repetition rate of 8.6 MHz were achieved with durations of 39 fs and bandwidths of 20 nm in a prismless configuration. Using prisms for dispersion compensation, we generated 43 fs pulses with energies of 0.66 nJ and bandwidths of 18.5 nm at 8.4 MHz. A saturable Bragg reflector (SBR) was used to start and stabilize mode-locking [9]. The pulse energies and durations from this low cost femtosecond source approach those obtained with standard 100 MHz Ti:sapphire lasers.

Multi-pass cavities (MPC) providing a unity q parameter transformation of the Gaussian beam have been used to reduce repetition rates from laser oscillators and thereby increase pulse energies without requiring external amplification [10, 11]. The MPC used in this work consists of one large plane mirror and one large curved mirror, with two smaller mirrors, one plane and one curved, used to introduce and extract the laser beam from the MPC. For a desired repetition rate, the radius of the curved mirrors, number of bounces on each mirror, and distance between the two large mirrors can be optimized to give a unity q parameter transformation using ABCD matrix analysis. Low loss double chirped mirrors (DCM) [12] providing negative dispersion can also be used in the MPC to approximately cancel the large amount of air dispersion due to the many passes through the cavity. Therefore, the MPC can be introduced into the laser cavity without significantly affecting the beam parameters, cavity stability region, or cavity dispersion.

A schematic of our experimental setup is shown in Fig. 1. Initial work focused on optimizing the diode pump source, which consisted of two 50 mW diodes at 663 nm (Hitachi HL6503MG) and one 50 mW diode at 685 nm (Mitsubishi ML1013R). The diodes were microlensed (Blue Sky Research) to provide a circular output beam; however, similar results were achieved using anamorphic prisms for beam shaping. The diodes were collimated and one 663 nm diode (D1) was combined with the 685 nm diode (D2) using a dichroic mirror (DM). The other 663 nm diode (D3) was polarization rotated using a half-wave plate (WP) and the 3 beams were multiplexed with a polarizing beam splitter (PBS). This yielded a collimated beam with a total power of 137 mW incident upon the crystal when each diode was driven by a current of 117 mA. The pump spot was focused to a minimum radius of 15×18 µm using a combination of an R= -100 mm diverging lens (P1) and an R=76.3 mm antireflection coated achromatic lens (P2).

 

Fig. 1. Experimental setup of the extended cavity femtosecond Cr:LiSAF laser. A multi-pass cavity (MPC) reduces the repetition rate while leaving the Gaussian beam invariant.

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The laser was first optimized in a standard four-mirror X cavity with no prisms, MPC, or SBR, in order to maximize the cw output power when using the diode pump source. The end mirror for this cavity is labeled “short cavity HR” in figure 1. Two curved mirrors (M1 and M2, R=10 cm) were used to tightly focus the laser mode to a spot size of 20 µm radius within the 5 mm long Brewster cut, 1.5 % doped Cr:LiSAF crystal (CR). Mirrors M3 and M4 were plane mirrors used to increase the arm length. The reflectivity bandwidth of mirrors M1–M4 extended from 765 to 915 nm. The 3 mm thick output coupler (OC) had a measured transmission of 1% at 860 nm. A cw output power of 28.5 mW was obtained from this cavity.

The next step was to remove the end mirror and replace it with the multi-pass cavity, consisting of one plane DCM (M6) and one R=4 m DCM (M7) separated by 2 meters, with a flat DCM (M5) and a curved R=4 m DCM (M8) to introduce and extract the beams. The reference planes for the unity q transformation on one pass through the cavity are immediately before the first bounce on M5 and at M9, 2 meters after striking M8. The beam makes 16 passes through the MPC in one round trip, resulting in a total added cavity length of 32 meters. The DCMs provided -42 fs² group delay dispersion (GDD) per bounce around 860 nm and had a high reflectivity bandwidth of 695–895 nm, with reflectivity estimated to be greater than 99.9%. The total dispersion of the MPC is approximately -32 fs² for 32 m of air and 16 bounces on the MPC mirrors. The cw output power was maximized and 15 mW was obtained when using M9 as the end mirror.

The cavity was then set for mode-locked operation using prisms for dispersion compensation. Two fused silica prisms (PR1 and PR2) separated by 50 cm were placed in the output coupler arm. They were set to compensate the dispersion of the crystal (~225 fs²) and tune the dispersion operating point. The dispersion of the SBR and non-DCM plane mirrors was near zero. The output coupler arm was 70 cm and the MPC arm was 90 cm. The SBR was similar to that described in ref. 9, with a reflectivity of 99.5% from 825–900 nm. It was placed in an extra fold after the MPC, with an R=20 cm mirror (M10) used to focus the laser mode to a ~75 µm radius spot on the SBR. The distances from M9 (the MPC reference plane) to M10 and from M10 to the SBR were made equal to the focal length of M10. This set the extra fold for a unity q parameter transformation as calculated by ABCD matrix analysis.

In this configuration, the laser generated 43 fs pulses (assuming sech² pulse shape) with 18.5 nm bandwidth, centered at 856 nm at an 8.4 MHz repetition rate (Fig. 2). The pulses were nearly transform-limited with a time-bandwidth product of 0.327. The pulse duration was measured by intensity autocorrelation in a 500 µm KDP crystal. The average output power was 5.5 mW, corresponding to a pulse energy of 0.66 nJ. Mode-locking was initiated by shaking the SBR stage and was occasionally self-starting. The SBR and M2 positions could be varied over nearly the full cavity stability range without significantly affecting the output spectrum and pulse duration, although the output power decreased when the mirrors were not at their optimum positions. The dispersion was varied by inserting one of the prisms into the beam, and mode-locking was obtained in both negative and positive dispersion over the full range of prism insertion.

 

Fig. 2. Autocorrelation (a) and spectrum (b) of the extended cavity Cr:LiSAF laser using prisms for dispersion compensation. The output power in this configuration is 5.5 mW at an 8.4 MHz repetition rate, resulting in pulse energies of 0.66 nJ.

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Subsequently, the cavity was reconfigured to operate without prisms, using only DCMs for dispersion compensation. This was done by replacing mirror M1 by a R=10 cm DCM and M3, M4, and M9 by flat DCMs. The prisms were removed and M10 was replaced by an R=30 cm non-DCM curved mirror. The output coupler arm was 40 cm and the MPC arm was 80 cm. These modifications resulted in a total of eight bounces on DCMs outside the MPC, providing -336 fs² GDD. This compensated the dispersion of the crystal and excess dispersion from air in the cavity, setting the dispersion operating point for the whole cavity at approximately -83 fs². In this prismless cavity, the laser generated 39 fs pulses (assuming sech² pulse shape) with 20 nm bandwidth centered at 867 nm at an 8.6 MHz repetition rate (Fig. 3). The pulses were transform-limited, with a time-bandwidth product of 0.315. The average output power was 6.5 mW, corresponding to a pulse energy of 0.75 nJ. We believe that the small improvement in output power using DCMs compared to the prism dispersion compensation may be due to variations in the SBR reflectivity for different alignments or due to reduced aberration in the prismless cavity.

The laser threshold and slope efficiency were measured in the prismless cavity with different combinations of pump diodes. The lowest threshold of 69 mW was measured for a combination of 21.8 mW from D2 and 47 mW from D1, with D3 off. The slope efficiency was also measured while varying the power of each diode and keeping the other two at full power. The largest slope efficiency was 12.8%, measured for D1 while keeping D2 and D3 at a total power of 90 mW. The slope efficiency was 11.6% for D2 and 8.3% for D3. D1 is expected to have the largest slope efficiency since the absorption of the Cr:LiSAF crystal is maximum for this pump wavelength and polarization.

We believe the pulse duration is primarily limited by the SBR bandwidth in both configurations. This hypothesis was tested by removing the MPC and putting an extra fold set for a unity q transformation in the previously described short cavity (replacing the “short cavity HR”) to focus onto the SBR. The laser produced bandwidths of ~20 nm in a cavity with no DCMs that used only prisms for dispersion compensation. The broad reflectivity bandwidth of the other cavity mirrors implied that the SBR bandwidth was the main limitation on the generation of shorter pulses. The use of broader bandwidth and lower loss SBRs should result in improvements in both pulse duration and output power. It should ultimately be possible to improve output powers and pulse durations if short pulses can be generated using pure Kerr lens modelocking without SBRs. We could initiate KLM in our laser, but were not able to obtain sustained modelocked operation without an SBR. This may be due to the fact that the Kerr lens effect is weak because the nonlinear index of Cr:LiSAF is small and the laser intensity is still relatively small compared to that obtained using broad area diode pump lasers.

 

Fig. 3. Autocorrelation (a) and spectrum (b) of the extended cavity Cr:LiSAF laser using only DCMs for dispersion compensation. The output power in this configuration is 6.5 mW at an 8.6 MHz repetition rate, resulting in pulse energies of 0.75 nJ.

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In conclusion, low cost single spatial mode diodes were used to pump an extended cavity femtosecond Cr:LiSAF laser mode-locked with a saturable Bragg reflector. These inexpensive diodes provided efficient matching of the pump and laser modes as compared to broad stripe pump diodes. A multi-pass cavity was demonstrated to lower the repetition rate and improve the pulse energy. Using prisms for dispersion compensation, 43 fs pulses with 18.5 nm bandwidth were produced at an 8.4 MHz repetition rate. The pulse energy was 0.66 nJ in this configuration. In a prismless cavity using DCMs, the laser generated transform-limited 39 fs pulses with 20 nm bandwidth. The repetition rate was 8.6 MHz and the pulse energy was 0.75 nJ. Slope efficiencies up to 12.8% were measured, indicating that laser output powers can be scaled to higher output powers and pulse energies as higher power pump diodes become available. We have demonstrated a low cost source of femtosecond pulses with pulse energies and durations that make this a cost-effective alternative to standard Ti:sapphire lasers in certain applications.

We thank U. Morgner, A. Tucay Zare, E. P. Ippen and V. Sharma for helpful assistance. This research is supported in part by NSF contract ECS-0119452, AFOSR contract F49620-01-1-0084, AFOSR Medical Free Electron Laser Program contract F49620-01-1-0186. A. M. Kowalevicz, Jr. is also with the Division of Applied Sciences, Harvard University. Y. Hirakawa is currently with Hiroshima University, Japan.