We report a directly blue diode pumped Ti:Sapphire oscillator that generates 5 nJ pulses. This is five times higher pulse energy than previously reported for a directly diode pumped Ti:sapphire laser. With 460 mW of average power at 92 MHz and 82 fs pulses, its peak power reaches 61 kW, also several times higher the value than previously published. Direct diode pumping significantly reduces the complexity and therefore the footprint and the cost of the laser, while SESAM modelocking ensures reliable selfstarting and robust operation. Such a laser is ideally suited for biomedical imaging and nanostructuring applications. As a demonstration of sufficient peak power for microscopy applications, we perform different modalities of nonlinear microscopy of biological samples.
© 2017 Optical Society of America
More than any other gain media, titanium doped sapphire (Ti:Al2O3) is used as an ultrabroadband solid-state laser material for ultrashort pulse generation. The ultrashort pulses can be generated directly from a laser oscillator , which opens up a possibility to operate at higher repetition rates  and have a much simpler system compared to schemes that depend on extracavity continuum generation . Sapphire, a transparent crystal with an excellent toughness and thermal conductivity, is an indispensable host material. Besides, Ti:sapphire’s broad gain spectrum ranging between 650 and 1100 nm  allows tunability in a wide spectral range in both CW and pulsed operations. Its main disadvantage is the high saturation intensity due to its short upper state lifetime (~3.2 μs at room temperature ). Consequently, longitudinal laser pumping is necessary by a high-brightness beam mode tightly focused in the crystal.
Over the past decades, many pumping schemes were developed in the green region near the absorption maximum, but they all included complex and expensive solutions. Initially, Ar ion lasers were used [4,5]. Still, the most commonly used pump lasers today are frequency doubled neodymium lasers, but some newly developed methods include optically pumped semiconductor lasers , frequency doubled DBR-tapered lasers  and frequency doubled Yb fiber lasers . The complex pump laser adds to the cost, complexity and footprint of the whole Ti:sapphire system and limits wider use in industrial, medical or scientific environments. The most effective pump solution would be direct pumping with laser diodes. Beside much lower cost and smaller footprint, direct diode pumping provides better reliability, higher efficiency and better pointing stability.
Thanks to the developments in the laser projection technology, low-cost laser diodes emitting over 3 W in the blue region around 450 nm became available. The challenge for using these diodes as pump sources is two-fold. First, the peak absorption of Ti3+ is around 500 nm and it drops sharply on the blue edge of the spectrum . Second, the laser diodes have lower brightness compared to typically used solid-state green pump lasers. However, a breakthrough was made in 2009 when the directly blue diode pumped CW Ti:sapphire laser was demonstrated . The mode-locked operation was demonstrated in 2012. Durfee et al. used two laser diodes emitting 1 W at 445 and obtained KLM mode locked pulses of 15 fs and 34 mW average power . Roth et al. also used two diodes emitting 1 W each, but this time at 452 and 454 nm. They obtained 111 fs pulses with 101 mW of average power at 127 MHz  using SESAM mode locking . Young at al. achieved KLM mode locking at 100 MHz using two diodes emitting 2 W each at 445 nm. They reported two laser configurations with 15 fs pulses and 70 mW of average power, and 50 fs pulses and 105 mW of average power . Although having its challenges, direct blue diode pumping showed that Ti:sapphire laser performance is not compromised and ultrashort pulses can be achieved in the wide tunable spectral region. However, the pulse energy reported so far was rather low for most applications. Recently, there are two reports of a mode-locked Ti:sapphire laser directly pumped by green laser diodes [14,15]. Although 450 mW was achieved, it was at the repetition rate of 418 MHz resulting in about 1 nJ pulses . In addition, the high power green laser diodes are not yet mature technology, not commercially available, and therefore less likely to scale up in power in the near future.
In this paper, we demonstrate a high pulse energy directly blue-diode-pumped Ti:sapphire oscillator and its’ application in two-photon excited fluorescence (TPEF) and second harmonic generation (SHG) imaging. The SESAM mode locking ensured reliable self-starting and robust operation [12,16]. We present two configurations emitting 460 mW average power with 82 fs pulses and 350 mW average power with 65 fs pulses, both operating at 92 MHz pulse repetition rate. The maximum obtained pulse energy reaches 5 nJ. A laser with such a performance is ideally suitable for biomedical imaging and nanostructuring applications, where reduced cost and foot print as well as turn-key operation and industrial grade stability are required. We demonstrate the laser stability and beam quality in multicolour TPEF and SHG imaging in several biological samples.
2. Laser system characterization
The laser oscillator layout is shown in Fig. 1. The laser is pumped by two commercially available Nichia laser diodes in a counterpropgating scheme. Each diode is driven at a current of 2.3 A and emits 2.9 W CW average power around 450 nm. We did not observe any signs of potential diode damage in this pumping scheme over months of operation and therefore no particular measure was taken to block residual pump light. The pump spot sizes inside the crystal have 18 μm and 53 μm diameter in fast and slow axis, respectively, while the cavity mode size is ~30x30 μm. Therefore, pump and signal beam overlap is estimated to be ~50%. We used 4 mm long Brewster angle cut Ti:Sapphire crystal from Roditi with 0.25% dopping. The crystal has a figure of merit greater than 250. The group velocity dispersion introduced by the crystal is compensated by group delay dispersion (GDD) mirrors placed in one cavity arm. The amount of negative dispersion introduced is varied by the number of reflections on each GDD mirror. In the same arm the 7% output coupler is used as an end mirror. The modelocking is obtained by focusing the beam onto a SESAM placed at the end of the other cavity arm. The SESAMs we used are in-house designed for 800 nm central wavelength with a modulation depth of 1%.
We present two laser configurations, one with higher average power and pulse energy, which we will name further in the text Configuration 1, and one with shorter pulse duration, further named Configuration 2. Both operate around 92 MHz. They are obtained by changing the total GDD amount introduced by the chirped mirrors, exchanging OCs and SESAMs. We tested two different OC units from the same supplier (Laseroptik), one for each configuration. Both OCs have approximately 7% maximum transmition, but the peak transmission is at somewhat different central wavelengths. Similarly, we tested two SESAMs both designed in the same way, but from different growths, therefore exhibiting slightly different central wavelength, modulation depth and non-saturable losses. Consequently, the two configurations differ not only in the pulse duration, but also in the central wavelength and the output power. Configuration 1 laser exhibits the following parameters: 460 mW average power, i.e. 5 nJ pulse energy, 82 fs pulse duration, and central wavelength 784.5 nm with −900 fs2 GDD compensation, whereas with Configuration 2 we obtained 350 mW, i.e. 3.8 nJ pulse energy, 65 fs pulse duration, and central wavelength 807 nm with −700 fs2 GDD compensation. Both configurations have similar peak power, namely 61 and 58.5 kW. Table 1 summarizes all measured parameters. The CW output power for the Configuration 2 was slightly higher than in modelocked operation, namely 470 mW in CW versus 460 mW in modelocked operation, as expected for SESAM modelocked lasers, where oscillators operate in the stable regime of the cavity, and SESAMs have small non-saturable loss. To our knowledge, this is the highest pulse energy achieved, about 5 times higher than previously published, and the highest peak power reported for direct diode pumped Ti:Sapphire lasers. The central wavelength in both configurations is slightly offset from795 nm, the Ti:sapphire maximum gain wavelength, because of the offsets in lowest loss wavelength of the OCs and SESAMs.The Configuration 2 exhibits shorter pulses and lower average power due to less introduced negative GDD mirror compensation. The shortest pulse duration is limited by SESAM DBR bandwidth. However, note that both configurations generate similar peak power, which is the laser limitation due to the balance of non-linear effects. As expected, the optical to optical conversion efficiency is slightly lower for the blue diode pumped TiSa oscillator (~8%) compared to conventional green laser pump (typical 10-12%), because of the larger quantum defect when pumping at 450 nm compared to pumping at 515 or 532 nm. Despite 2-4% lower efficiency, the drastic decrease of complexity and increase of reliability is remarkable and in our opinion worth pursuing.
Pulse duration for each configuration was obtained by measuring the SHG autocorrelation (with a Femtochrome FR-103XL autocorrelator) and then fitting a sech2 function. Figure 2 displays the autocorrelation traces and optical spectra for both configurations. The optical spectra were recorded using an HP 86140A optical spectrum analyzer. The spectral bandwith is 9.4 nm for Configuration 1, and 10.2 nm for Configuration 2, demonstrating close to transform limited pulse generation.
Furthermore,RF spectra (measured with an HP 8560E RF spectrum analyzer) for both configurations are very similar and only one set of data is presented. Figure 3 shows the RF power spectra for Configuration 1 with two different frequency span and resolution settings. In the case of 500 kHz frequency span with 1 kHz resolution one observes a peak at 91.76 MHz and no side peaks with 80 dBs signal to noise ratio, which proves low noise, clean modelocking operation. In the second setting, the span is set to 200 MHz and the spectrum is recorded with 100 kHz resolution. Only peaks with 92 MHz spacing are observed with 80 dB signal to noise ratio, which demonstrates single mode operation in time and spatial mode.
The output beam quality is high with beam quality factor of M2x = 1.08 and M2y = 1.12. Figure 4 shows the output beam profile and the measurements of the beam width diameter through focus used for calculation of M2 factor.
We also measured power stability over 30 days of continuous operation. We found a 14% drop of output power that we could recover by realigning the cavity. We did not observe any output degradation due to blue diode induced absorption.
3. Multiphoton imaging capabilities
The laser in Configuration 1, was used as an excitation source for multimodal nonlinear microscopy experiments . The size of the prototype laser, including the proper pump collimation and focusing, is approximately 20x40 cm, where only half of that size is the TiSa oscillator. The output beam of the laser was injected into a multiphoton microscope having a commercial inverted Nikon C1-Si confocal microscope as a base. For this work we have employed only the multiphoton branch of the microscope. The laser beam was expanded to fill the back aperture of a high numerical aperture oil immersion objective (Nikon, 60X, 1.4 NA), and a variable attenuator was employed to adjust the average power of the beam to be 5 mW at the microscope’s sample plane. Two biological imaging modalities were demonstrated by using the laser in the Configuration 1: two-photon excited fluorescence (TPEF) microscopy and second harmonic generation (SHG) microscopy. Figure 5 shows the three-color TPEF imaging capabilities of the microscope by using only our laser as excitation source.
The first sample, Fig. 5(a), is a section of mouse intestine were the goblet cells (connective tissue) were stained with Alexa Fluor 350, shown in blue, the nuclei with SYTOX Green, shown in green, and, the actin filaments with Alexa Fluor 568, shown in red. The second sample, Fig. 5(b), is made of fixed bovine pulmonary artery endothelial (BPAE) cells where the nuclei were stained with DAPI, shown in blue, the actin filaments with BODIPY-FL, shown in green, and the mitochondria with Red CMXRos, shown in red. These results show that our laser can be effectively used for three-color TPEF imaging with different combinations of commercial fluorescent markers, without the need of having a different laser for each fluorophore.
Besides, our preliminary results also show the benefit of imaging with sub-100 fs pulses: as expected, we found a 1.5 to 2-fold increase of the TPEF signal when our laser source is used for imaging compared to a standard facility-use femtosecond laser source tuned at the same wavelength, with the same average power, but with longer pulses. Longer pulses degrade the Figure of Merit for TPF imaging .
Finally, Fig. 6 shows the endogenous SHG microscopy data of collagen type-I from a commercial tendon sample taken using our laser with same average power as in the previous TPEF experiment. The integration time for all images in Figs. 5 and 6 was set to 1 s. All images were taken with a pixel resolution of 512x512, with a dwell time per pixel of 3.81 μs. This results in approximately 351 pulses-per-pixel if the repetition rate 92 MHz of the laser is taken into account. Note that Fig. 5(a) is the maximum intensity projection over 28 z-stack images, Fig. 5(b) is the maximum intensity projection over 13 z-stack images and Fig. 6 is a 3D view of a data set composed of 45 z-stack images. The stack step employed for all data sets was 350 nm. All images composing the 3D stacks are the average of two captured frames in order to increase SNR of the final images.
We have observed an increase of the SHG signal, in an amount similar to the TPEF case, by using our laser as compared to the reference long-pulsed laser. The different 3D fiber structure arrangements can be clearly distinguished in the 3D views of the sample, thanks to the good SNR of the obtained images.
The characterization of the arrangement of Collagen and Myosin fibrils of biological tissues is gaining relevance in many biomedical fields as it may provide important information about tissue structure and health [19–21]. The use of the laser here presented can help to reduce the time and/or the average power delivered to the sample reducing the photodamage induced to the precious biological sample.
We presented a directly blue diode pumped Ti:sapphire laser with a compact footprint and reliable and robust SESAM modelocking. We obtained two sets of data, both at about 92 MHz operation. In the first configuration, we obtain 460 mW average power with 82 fs pulses, which corresponds to 5 nJ pulses and 61 kW peak power. This is five times higher pulse energy then previously reported and the highest peak power reported for directly diode pumped Ti:sapphire lasers. In the second configuration, we obtain shorter pulses and lower average power, i.e. 65 fs and 350 mW. The pulse energy and peak power are still high, namely, 3.8 nJ and 58.5 kW, respectively. We expect power scaling of blue laser diodes to further drive power scaling of directly pumped Ti:Sapphire lasers.
This is the first directly diode pumped Ti:sapphire laser delivering multi-nJ pulse energies with sub-100 fs pulses, which is sufficient for multiple real-world applications. The laser with such a performance is highly desirable in multimodal nonlinear microscopy and nanosurgery [22–24], nanostructuring , ultrafast spectroscopy , seeding amplifiers , and other applications. Even shorter pulses could be achieved with quantum-dot-based SESAMs . We showed its application for three-color TPF and SHG imaging and benefit of sub-100 fs pulses in terms of nonlinear signal generation.
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