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Improvements in the output power of a directly GaN diode-laser-pumped Ti:Al2O3 laser are achieved by using double-sided pumping. In continuous wave operation, an output power of 159 mW is reported. A tuning range of over 125 nm with output powers in excess of 100 mW is achieved. Pulses of 111 fs duration and an average power of 101 mW are demonstrated by mode locking the laser with a saturable Bragg reflector. Pumping with GaN diode lasers at wavelengths around 450 nm induces an additional parasitic crystal loss of about 1% per resonator roundtrip that is not observed at the conventional green pump wavelengths.
©2012 Optical Society of America
1. Introduction
Titanium-doped sapphire (Ti:Al2O3) has become the most widely-used broadly tunable laser material since its first demonstration in 1982 [1]. Tunable between 0.7 μm and 1.1 μm [2, 3] and capable of generating femtosecond (fs) pulses, the Ti:sapphire laser is an important tool for many applications. As a laboratory instrument, its utility is underlined most prominently by its crucial role in high-impact scientific fields such as precision spectroscopy (Nobel Prize 2005, Hänsch and Hall) and femtochemistry (Nobel Prize 1999, Zewail) [4]. This is increasingly complemented by practical applications [4, 5] in, for example, bio-medicine [5], spectroscopy [6, 7], and the generation of terahertz radiation [8]. However, today’s generation of lab-bound Ti:sapphire lasers leaves many applications unaddressed, particularly where lower costs and smaller footprints are vital. The biggest hurdle to more widespread use is the bulky, expensive and complex pump source – typically a frequency-doubled, multi-watt neodymium [2] or optically pumped semiconductor laser [9].
Replacing these intricate pump sources with direct diode-laser pumping would open up new applications. However, diode-laser pumping of Ti:sapphire [4] is difficult for two reasons: first, high-performance diode lasers have not, until recently, been available in the required blue-green spectral region; and, second, the laser properties of Ti:sapphire do not lend themselves to pumping with low-brightness sources such as diode lasers. For these reasons, considerable research effort had been devoted to alternative materials for ultrafast and broadly tunable applications that are easier to diode pump. These include Cr:LiSAF [10, 11] and a range of Ytterbium-doped media [12, 13]. Nevertheless, Ti:sapphire is still the preferred gain material for commercial systems, prompting research into alternative pump sources such as frequency-doubled tapered diodes [14], and Yb-fiber lasers [15]. However, with the extra nonlinear conversion stage, these pump sources remain relatively complex.
In 2009, we reported direct diode-laser pumping of Ti:sapphire [16], using a very low-threshold design to exploit rapid improvements in GaN diode lasers [17]. A continuous-wave (cw) output power of 19 mW was achieved using a single 1 W, 452 nm diode laser. In 2011, we improved the cw output power to 32 mW and reported mode-locked operation for the first time. Pulses as short as 114 fs and average output powers up to 13 mW were generated with a saturable Bragg reflector [18]. This work was complemented recently by the first demonstration of Kerr-lens mode-locking of a directly diode-laser-pumped Ti:sapphire laser by Durfee et al. [19]. They reported 15 fs pulses at average powers of 34 mW for pumping with two spatially combined 445 nm GaN diode lasers. The generation of ultrashort pulses suggests that with further improvements in output power, these lasers will be able to address a wide range of important applications. Indeed a number of important applications in biophotonics require pulse durations of 100 fs at average powers around 100 mW [20–22].
This paper reports the scaling of a directly diode-laser-pumped Ti:sapphire laser to output powers in excess of 100 mW for both cw and mode-locked operation. Pumping with two diode lasers is used to achieve this. In addition, the design considerations for diode-laser pumping of a Ti:sapphire laser are laid out. Pumping with GaN diode lasers at around 450 nm induces an additional parasitic loss that is not observed at the more typical green pump wavelengths. This additional loss is quantified by measurements of the resonator round-trip losses under both conventional and diode-laser pumping.
2. Resonator design considerations for direct diode-laser pumping
The most powerful, commercially available GaN diode lasers operate at around 450 nm. Since the absorption cross-section of Ti:sapphire drops off sharply to the short wavelength side of its peak at 490 nm [2, 3], the pump absorption is significantly lower at 450 nm than at green pump wavelengths. Furthermore, Ti:sapphire’s absorption cross-section is relatively small and the possible doping concentrations are limited by a disproportionate increase in parasitic loss. Thus, relatively long crystals are required for effective pump absorption and acceptable parasitic loss. Also, Ti:sapphire’s short fluorescence lifetime requires high pump intensities to achieve laser oscillation. In combination with the low spatial brightness of GaN diode lasers (M2 ≈7 × 1), compared to the near diffraction-limited conventional pump sources, this imposes three conflicting challenges for direct diode-laser pumping: first, high pump intensities, necessary to overcome Ti:sapphire's intrinsically large laser threshold, have to be maintained over the length of the crystal; second, the crystal doping concentration and length must combine efficient pump absorption with acceptable parasitic loss; and third, a good overlap between the (multimode) pump and the fundamental resonator mode is required for efficient laser operation.
Laser rate-equation modeling based on the work of Alfrey [23] and reported in more detail in [16] was used to optimize the laser design. This informed the selection of a 5.2-mm-long, Brewster-cut Ti:sapphire crystal (Saint-Gobain Ltd) with a doping concentration of 0.25 wt. % and a figure of merit (ratio of the absorption coefficient at 514 nm to that at 820 nm) of 400. In a single pass, this crystal absorbs 83% of the incident, π-polarized pump light at 452 nm. The resonator design was informed by the fully astigmatic form of the Alfrey model: Fig. 1(a) shows the four-mirror resonator pumped by two GaN diode lasers through the folding mirrors (double-sided pumping). To enable mode-locking with a saturable Bragg reflector (SBR), an asymmetric, Z-folded resonator design was used to give both a tight focus on the SBR, and a near-collimated arm for the fused silica (FS) prism pair. For continuous-wave (cw) operation, the SBR was replaced by a highly reflective mirror (HR).
Fig. 1 (a) Asymmetric, Z-folded 4-mirror resonator for double-sided, diode-laser pumping. (b) Beam cross section of GaN diode laser shaped with a collimation lens (f = 4.5 mm) and a cylindrical Galilean telescope (f = −25.4 and f = 250 mm).
The two 1 W diode lasers (Nichia NDB7352E) emit at 452 nm and 454 nm respectively, with measured beam propagation factors M2x × M2y (x/y denotes the slow/fast axis) of 7.1 × 1.4 and 5.9 × 1.1. Identical pump optics were used for both diodes: an aspheric collimating lens (f = 4.5 mm), a two-element, cylindrical Galilean telescope (fx = −25.4 mm and fx = 250 mm) to shape the diode laser beam along the slow axis (see Fig. 1(b)), and a spherical focusing lens (f = 75 mm). The pump waist radii (1/e2) inside the Ti:sapphire crystal (wpx × wpy) were calculated to be 25 × 12 μm (452 nm) and 27 × 12 μm (454 nm). Each diode laser was attenuated with a half-wave plate and a polarizing beamsplitter cube to ensure a stable beam profile over the entire power range. The fundamental resonator mode waist radii (1/e2) within the Ti:sapphire crystal (wrx × wry) were calculated to be 32 x 18 μm. Deleterious feedback effects were not observed between the two diode lasers, which may, in part, have been a result of the asymmetric resonator design.
3. Laser experiments and results
With an output coupling of 3% and the emission wavelength tuned to 800 nm, the power transfer characteristic was measured, Fig. 2(a) . The diode lasers were powered up sequentially: first the 452 nm laser followed by the 454 nm laser. The slope efficiencies for each pump regime were 11.1% for the 452 nm diode laser pumping (0 - 880 mW) and 11.2% for the combined 452 and 454 nm pump regime (880 - 1770 mW). The nearly identical slope efficiencies indicate good mode matching between the two pump modes. A pump threshold of 480 mW and a maximum output power of 144 mW were measured. (N.B. All pump thresholds and slope efficiencies are with respect to pump power incident on the crystal.) The M2 parameters of the output beam were measured to be <1.2 in both principal planes. Reducing the output coupling to 0.5% resulted in a slope efficiency of 3.7% and a pump threshold of 252 mW, the lowest yet reported for a diode-laser-pumped Ti:sapphire laser.
Fig. 2 (a) Ti:sapphire laser output power at 800 nm and 3% output coupling as a function of pump power from two GaN diode lasers (452 and 454 nm). (b) Tuning curve of the Ti:sapphire laser pumped with 880 mW at 452 nm and 2% output coupling (one-sided pumping) or with a combined 1770 mW at 452 and 454 nm and 3% output coupling (double-sided pumping).
The tuning of the Ti:sapphire laser was recorded at maximum pump power for single- and double-sided pumping, Fig. 2(b). The output couplings of 2% (one-sided pumping) and 3% (double-sided pumping) were chosen to maximize the output power. By pumping with both diode lasers, output powers greater than 100 mW were maintained over 125 nm (715 – 840 nm). The maximum output power of 159 mW occurred at 765 nm rather than at Ti:sapphire’s peak gain wavelength of 795 nm due to the wavelength characteristic of the output coupler. The tuning range is limited by reduced output coupler reflectivity for wavelengths below 720 nm and above 850 nm rather than Ti:sapphire’s gain spectrum.
The flat mirror in the short arm of the resonator was replaced with a saturable Bragg reflector (SBR) to obtain robust mode locking. The SBR was designated for use in a low-threshold Cr:LiSAF laser at around 825 nm. As a result, the laser had to be tuned to this wavelength region, away from the peak gain wavelength of Ti:sapphire at 795 nm, and 765 nm where the maximum output power in cw operation was achieved. The SBR consists of 30 pairs of Al0.15Ga0.85As and AlAs quarter-wave layers on a 0.5-mm-thick GaAs substrate. Saturable absorption is provided by a 5-nm-thick In0.08Ga0.92As quantum well in the middle of the topmost AlGaAs layer after the broadband SBR technique of Zhang et al. [24]. Conventional high-temperature growth was used to produce an SBR with low nonsaturable losses (typically less than 0.5% in such devices, dominated by the transmission of the rear mirror [25]). The resonator mode radii (1/e2) on the SBR were calculated to be 20 × 21 μm. The tip-to-tip separation of the fused-silica prism pair was set to 60 cm. With a 2% output coupling, an average output power of 101 mW in pulses of 111 fs duration (FWHM) was achieved under double-sided pumping (1770 mW of pump power), see Fig. 3 . The spectral bandwidth (FWHM) was 8.0 nm (3.6 THz) with a centre wavelength of 818 nm as shown in Fig. 3, giving a duration-bandwidth product of 0.40. The repetition rate was 127 MHz.
Fig. 3 Interferometric autocorrelation (a) and spectrum (b) of the pulses at maximum double-sided diode-laser pump power (1770 mW) and 101 mW of average output power (2% output coupling). The divergence from the ideal 8:1 ratio resulted from an asymmetry in the interferometer used for the autocorrelation. The centre wavelength was 818 nm and the repetition rate was 127 MHz.
These results are a considerable improvement over the highest powers reported to date from directly diode-laser-pumped Ti:sapphire lasers. Doubled-sided pumping and improvements in the resonator design have enabled a five-fold increase in the cw output power [18]. Similarly the average output power under mode-locked operation is eight times higher than that reported by Roth et al. [18] for single-diode pumping and SBR mode locking, and 3-fold higher than that reported by Durfee et al. [19] for Kerr-lens mode locking and pumping with two spatially combined diode lasers. This increase in average power opens the way for using directly diode-laser-pumped Ti:sapphire lasers in important biophotonics applications [4, 5] such as two-photon microscopy [20], optical transfection [21] and cellular microsurgery [22]. The conventional Ti:sapphire lasers that are currently used typically provide ~100 fs pulses at ~100MHz repetition rates with an average power at the sample of a few tens of milliwatts [20–22]. Thus, for practicality and to allow for losses in the optical system, average output powers from the laser of >100 mW are preferred. Similarly, Stone et al. have reported the generation of cw THz radiation using ~100 mW of output power from a cw Ti:sapphire laser [8].
4. Pump-induced loss
In order to achieve laser oscillation in a Ti:sapphire laser with a GaN diode-laser pump, the resonator must be optimized for low pump thresholds. A frequency-doubled Nd:YVO4 pump laser emitting at 532 nm was used to optimize the resonator before switching to diode laser pumping. However, on switching from frequency-doubled Nd:YVO4 to diode laser pumping, the Ti:sapphire laser’s output power deteriorated over the first few minutes before reaching a steady state. This unexpected behavior was described in an earlier publication [16]. In the absence of laser oscillation no degradation of the fluorescence intensity was observed under diode-laser pumping. This suggests an increase in parasitic loss rather than a reduction in gain [16]. For a pump wavelength above some threshold between 458 and 477 nm, this deterioration does not occur [16]. After a few minutes, once the deterioration effect has fully set in, the laser performance remains stable, and returns immediately to this level even after being switched off for several hours [16]. After the first time the diode lasers are used to pump a particular area of the crystal, no further performance deterioration occurs and the laser can be used from day-to-day with repeatable, steady-state performance. Thus, although this effect reduces the efficiency of directly diode-laser-pumped Ti:sapphire lasers, it is not a bar to their use in applications. It is the lower output power, steady-state results that are of interest from a laser engineering perspective and which were reported in the previous section. The deterioration can be reversed by pumping the laser at 532 nm for tens of minutes [16].
Previously, the losses due to this effect have only been quantified under argon-ion laser pumping [16]. In order to quantify the pump-induced loss under direct diode-laser pumping, Findlay-Clay [26] and Caird [27] analyses of the overall resonator loss were undertaken for 452 nm (single 1 W, GaN diode laser) and 532 nm (Nd:YVO4 laser, Elforlight HPG5000, 5 W, M2 = 1.8 × 1.2) pumping using the resonator shown in Fig. 1(a). The Nd:YVO4 pump laser was focused to a waist size (1/e2 half-width) of 20 × 9 μm while the calculated half-widths of the fundamental resonator mode were 24 × 14 μm. The measured resonator round-trip losses were 1.2% from the Findlay-Clay analysis and 1.1% from the Caird method for 532 nm pumping. GaN diode-laser pumping resulted in resonator round-trip losses of 2.6% for both methods. This suggests a pump-induced increase in parasitic crystal loss of 1.4% to 1.5% per cavity round-trip under 452 nm diode laser pumping – an increase in the overall resonator round-trip loss by a factor of 2.2 to 2.4 compared to conventional pumping. These additional losses compromise both the threshold and the slope efficiency of the laser. Hoffstädt observed pump-induced loss in flashlamp-pumped Ti:sapphire lasers [28] which might be the result of the same effect. He tentatively attributed this loss to charge transfer interactions in Ti3+-Ti4+ pairs. However, Durfee et al. did not observe such an effect [19].
5. Conclusions
The design and characterization of a Ti:sapphire laser suitable for direct diode-laser pumping has been described. In order to achieve higher output powers, double-sided pumping with two GaN diode lasers was employed. Cw output powers of more than 100 mW were obtained over a tuning range of 715 - 840 nm. For mode-locked operation, the maximum output power was 101 mW at a pulse duration of 111 fs. This may open the way for Ti:sapphire lasers that are more compact, less expensive and hence better suited to a wider range of applications.
The laser performance is markedly lower under GaN diode laser pumping when compared to pumping with conventional 532 nm lasers. This is partly due to the larger quantum defect, lower pump absorption at short pump wavelengths and the GaN diode lasers inferior beam quality. Additionally, a pump-induced increase in parasitic loss occurs at pump wavelengths below some threshold between 458 and 477 nm. Resonator loss measurements with a highly-doped, low-parasitic loss crystal put the difference in parasitic crystal loss between 532 nm and 452 nm pumping at around 1% per cavity roundtrip.
Nonetheless, the output powers already achieved have the potential to open up new applications by reducing the cost and complexity of Ti:sapphire lasers. Furthermore, a marked improvement in the performance of diode-laser pumped Ti:sapphire lasers can be expected as longer wavelength GaN diode-lasers become available.
Acknowledgments
This work was supported by the UK Engineering and Physical Science Research Council under grant number EP/0367421/X.
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