High-energy, nanosecond pulsed Cr:CdSe laser with a 2.25–3.08 μ m tuning range for laser biomaterial processing

: We have developed a mid-infrared (mid-IR) tunable Cr:CdSe laser with nanosecond pulse operation. A broad tuning range from 2.25 to 3.08 µm and an output energy exceeding 4 mJ at 2.64 μ m were demonstrated. The maximum energy conversion for absorbed energy reached 35% when the pump fluence was 2.1 J/cm 2 . We showed that Cr:CdSe is an attractive laser material for obtaining high-energy pulses in the mid-IR region and that the Cr:CdSe laser has high potential for laser biomaterial processing.


Introduction
Mid-infrared (mid-IR) solid-state pulsed lasers with broad tunability are of significant interest for many applications such as molecular spectroscopy, environmental remote sensing, and materials processing [1][2][3]. In particular, high-energy mid-IR lasers with the tuning range from 2 to 3 μm are expected to be applied to laser biomaterial processing. Because strong absorption peaks of the water and hydroxide ions (OH -) are included in various biomaterials appear in this wavelength range [4,5]. Mid-IR pulsed lasers oscillated at the absorption peaks of water or hydroxide ions can be used for the efficient laser ablation of biomaterials [6,7]. In addition, mid-IR pulsed lasers with wide tunability are also highly advantageous for investigating the wavelength dependence of the ablation effect of various biomaterials.
Cr 2+ -doped chalcogenide materials are effective laser materials for oscillating high-energy pulses in the mid-IR region because of their room-temperature wide tunability, broad absorption bands, and large stimulated-emission cross section [8,9]. In 1996, DeLoach et al. first reported the mid-IR lasing characteristics of Cr:ZnS and Cr:ZnSe [10]. Since then, several Cr 2+ -doped chalcogenide materials, such as Cr:Cd 1-x Mn x Te and Cr:CdSe, have been reported as tunable laser materials in the mid-IR region [11][12][13]. Cr:CdSe is an attractive laser material for direct lasing in the wavelength range from 2 to 3 μm because of its broad fluorescence spectral region and large stimulated-emission cross section [14].
MaKay et al. have demonstrated broadly tunable laser oscillation of a nanosecond pulsed Cr:CdSe laser pumped with a Q-switched Tm,Ho:YLF laser. The laser produced a tuning range of 2.32-2.88 μm and an output energy of 0.35 mJ at 2.5 µm [15]. Akimov et al. have reported a Cr:CdSe laser with a nonselective resonator and an output energy of 17 mJ was obtained around 2.65 µm by pumping with 1.94 µm, 300 µs pulses from a Tm:YAP laser. Using a dispersion prism as a wavelength-selective element, a tuning range of 2.26-3.61 μm was realized and an output energy exceeding 10 mJ was obtained at 2.65 µm [16]. However, a nanosecond pulsed Cr:CdSe laser with pulse energy exceeding several mJ and broad tuning range in the mid-IR region has not been realized. The realization of the Cr:CdSe laser greatly contributes to further progress in laser biomaterial processing utilizing material absorptions.
We previously developed a 2.01 μm high-energy Q-switched Tm:YAG laser with nanosecond pulse operation [17]. This laser has high potential as a pump source to realize wide tunability and high-energy pulses in a nanosecond pulsed Cr:CdSe laser. In our previous research, Tm:YAG lasers were used as a pump source for Cr:ZnSe, and a nanosecond pulsed Cr:ZnSe laser, which produced a tuning range of 2.12-2.71 μm and an output energy of 7.8 mJ at 2.41 µm, was demonstrated [18]. The use of a Tm:YAG laser as a pump source for Cr:CdSe is expected to accelerate the development of high-energy nanosecond pulsed Cr:CdSe lasers with broad mid-IR tunability.
In this study, we report on high-energy-pulse oscillation in the mid-IR region using Cr:CdSe pumped with a Q-switched Tm:YAG laser. We investigate the laser-induced damage threshold (LIDT) of Cr:CdSe to avoid optical damage on the Cr:CdSe surface and design a Cr:CdSe laser cavity to obtain a large fundamental-mode beam radius (~0.6 mm) on the surface for damage-free operation. The Cr:CdSe laser produced a maximum output energy of 4.4 mJ at 2.64 μm with nanosecond pulse operation. To the best of our knowledge, this is the highest output energy ever reported for a nanosecond pulsed Cr:CdSe laser. A tuning range from 2.25 to 3.08 μm was demonstrated. In addition, we applied the Cr:CdSe laser to the ablation of hard dental tissues and also demonstrated the potential use of the Cr:CdSe laser for laser biomaterial processing.

Laser-induced damage test on Cr:CdSe
We investigated the LIDT of Cr:CdSe for utilization in laser cavity design. The setup for the damage test is shown in Fig. 1(a). We used a Cr:CdSe single crystal (Institute of Solid State Physics Acad. / 3photon) without an optical coating. The crystal length was 2 mm and the doping concentration of Cr 2+ was approximately 0.7 × 10 18 cm −3 , which was grown by highpressure vertical zone melting method [19]. A laboratory-built Q-switched Tm:YAG laser was prepared to induce optical damage on the sample surface. The Tm:YAG laser operated at a wavelength of 2.01 μm with a repetition rate of 10 Hz. A maximum pulse energy of 25 mJ was obtained with a pulse width of 300 ns. The output beam of the Tm:YAG laser was focused on the sample surface using a planoconvex lens (f = 1000 mm). The beam radius on the sample surface was set at 300 µm, allowing the input fluence on the sample surface to be controlled from 0 to 8.8 J/cm 2 . The beam radius was defined by the intensity at 1/e 2 from peak intensity. We performed S-on-1 damage tests. Six hundred shots were input to the sample during a radiation time of 60 s, which means the Tm:YAG laser operated at the repetition rate of 10 Hz. The input energy was gradually increased until optical damage occurred on the sample surface. The occurrence of optical damage was confirmed by monitoring the spatial profile of the transmitted beam from the sample. The beam quality deterioration of the transmitted beam indicated the optical damage generated on the sample surface. Following the above procedure, six damage tests were performed and the average damage threshold was found to be ~3.93 J/cm 2 when the Tm:YAG laser was used as a pump source. We must design a Cr:CdSe laser cavity under the condition that pump fluence on the Cr:CdSe surface is below the damage threshold.

Design of the Cr:CdSe laser cavity
A schematic diagram of the Cr:CdSe laser cavity constructed with a Z-fold configuration is shown in Fig. 2. The same Tm:YAG laser as that used in the Cr:CdSe optical damage test was used as a pump source. The laser cavity consists of an output coupler (M1), two folding mirrors (M2, M3), a total reflector (M4), and a 10-mm-long Brewster-cut Cr:CdSe crystal (3 Photon. Inc.). The Brewster angle is 67.9° for the refractive index of 2.46 and the doping concentration of Cr 2+ is approximately 0.7 × 10 18 cm −3 . The output coupler and the total reflector are flat and their reflections are 70% and 99.5% in the wavelength range from 2.2 to 3.5 μm, respectively. The folding mirrors are concave mirrors with a curvature radius of 1000 mm and a high reflection (HR) coating for the wavelength range from 2.2 to 3.5 μm. Here, the Cr:CdSe was placed at the midpoint between M2 and M3. The distance between the Cr:CdSe and M2 (M3) was set to 65 mm, and L was the length between M1 (M4) and M2 (M3). The cavity folding angle was set at about 25°. For the wavelength-tuning operation, the total reflector was replaced with a grating (Thorlabs Inc., GR25-0616). The blaze wavelength and the groove density were 1.6 μm and 600 grooves/mm, respectively. The diffraction efficiency was more than 90% in the wavelength range from 2.0 to 3.2 μm.
To realize a high-pulse-energy Cr:CdSe laser, high energy pumping of the Cr:CdSe is essential. For instance, assuming an energy conversion efficiency of 20% [15], the Cr:CdSe must be pumped with a pump energy of 25 mJ to obtain an output energy of ~5 mJ. In this case, to avoid optical damage of the Cr:CdSe, the pump beam should be incident with a spot size of more than 0.45 mm at the Cr:CdSe surface. This is because the pump fluence at the Cr:CdSe surface exceeded the damage threshold of 3.93 J/cm 2 when a pump energy of 25 mJ was focused to a spot size of less than 0.45 mm. To obtain a large fundamental-mode beam radius more than 0.45 mm on the Cr:CdSe surface, we simulated the Cr:CdSe laser cavity mode using the standard ABCD matrix method. The above values of the curvature radius and the distance between the Cr:CdSe and M2 (M3) were used in the simulation. Figure 3 shows the simulation result for the calculated fundamental-mode beam radius at the Cr:CdSe surface and on the surface of M1 (M4) as a function of the distance L. When L was longer than 160 mm, a large beam radius exceeding 0.6 mm was realized at the Cr:CdSe surface. In our experiment, we set L to 160 mm and focused the pump beam to 0.6 mm at the Cr:CdSe surface to realize mode matching between the fundamental-mode beam radius and the pump beam radius at the Cr:CdSe surface. Under this condition, the pump fluence is estimated to be approximately 2.2 J/cm 2 with a pump energy of 25 mJ. This pump fluence corresponds to less than 60% of the Cr:CdSe damage threshold. According to our cavity design, the Cr:CdSe laser cavity is a stable resonator and allows operation with no laser-induced damage up to a pump energy of 25 mJ.