We report on a continuous-wave alexandrite (Cr3+:BeAl2O4) microchip lasers operating at 680.4 nm and 749.5 nm laser wavelengths. Microchip resonators were realized by dielectric mirrors directly deposited on the alexandrite crystal surfaces. InGaN laser diode providing up to 3.5 W of the output power at ∼445 nm wavelength was used as a pump source. More than 210 mW and 570 mW of the laser radiation have been extracted from the microchip laser systems at 680.4 nm and 749.5 nm wavelengths, respectively. The corresponding slope efficiencies related to absorbed pump power were 15 % and 39 %.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Compact and reliable solid-state laser systems based on microchip geometry are promising radiation sources for many applications both in industry and medicine.
Alexandrite (Cr3+:BeAl2O4) is a broadly-tunable vibronic solid-state laser material  allowing laser emission mainly in the near-IR wavelength region with a peak centered around 749.5 nm. Among the family of Cr3+-doped laser crystals (e.g. Cr3+-ion doped colquiriites like Cr:LiSAF, Cr:LiCAF, Cr:LiSGaF), the alexandrite excels in thermomechanical properties. Its high thermal conductivity (23 Wm−1K−1) which is almost twice of that of Nd:YAG and five-times that of the Cr-doped colquiriites  together with a strong birefringence providing highly linearly-polarized radiation  which eliminates depolarization problems, makes the alexandrite an attractive laser material for high power laser operation . In addition, alexandrite long upper-state lifetime of 260 μs at room temperature is favorable for good energy storage in Q-switched operation  and its broad emission spectra supports generation of ultrashort pulses . The broad emission wavelength range from ∼700 nm to 858 nm [7,8] gives also possibility to realize tunable laser sources operating in the blue/UV region by employing only a single frequency conversion stage . Besides the laser generation at the vibronic transitions (around 750 nm), the alexandrite allows operation at the electronic transitions distinguished by sharp lines around 680 nm (three-level lasing mode), known as R-lines from ruby laser, which is of great interest for spectroscopic purposes . Thanks to these superior opto-mechanical and spectroscopic properties, the alexandrite was one of the most popular tunable laser material in the eighties, until the development of Ti:sapphire laser systems .
Nevertheless, in the last years, the interest in alexandrite lasers has been renewed. It is connected mainly with a great progress made in the development of efficient and reliable pump laser diodes providing laser radiation at wavelengths in the visible spectral range, corresponding with the alexandrite broad absorption bands in the blue and orange part of the electromagnetic spectrum. Direct laser diode pumping offers a potential of order-of-magnitude higher efficiency compared with well-developed but flash-lamp pumped and inefficient alexandrite laser systems in the eighties [7, 11]. Efficient alexandrite laser emission at vibronic transitions has been demonstrated under red [12–14] (record in slope efficiency of ηeff = 54 % recently reported by ), green (ηeff = 36 % under frequency-doubled Nd:YAG laser reported by , and ηeff = 26 % under laser diode pumping reported by ), and blue (ηeff = 20 %)  laser diode pumping.
In this contribution, following our previous investigation of the diode-pumped alexandrite crystal , alexandrite microchip lasers operating both at the vibronic (4T2 → 4A2) and electronic (2E → 4A2) laser transitions were designed and constructed. To the best of our knowledge, we report on the first microchip laser based on the alexandrite laser host, as well as on the first efficient diode-pumped alexandrite laser generating at the so-called R-line (2E → 4A2) at 680.4 nm wavelength. Although the first diode-pumped Alexandrite R-line emission has already been observed in ref. , it was far from the efficient laser operation (the slope efficiency related to absorbed pump power was only 0.9 %, i.e., more than 15 times lower than the slope efficiency presented in this paper).
2. Experimental arrangement and results
2.1. Laser generation at 680.4 nm
To reach an efficient continuous-wave alexandrite laser emission at 680.4 nm wavelength corresponding to lasing at the 2E → 4A2 transition, cryogenic cooling of the crystal was employed. Similarly to ruby, the metastable 2E state lies below the vibronically broadened 4T2 level, but as opposed to ruby, the energy level difference (E = 800 cm−1) between these levels is only 4 kT at room temperature, compared to 11.5 kT in ruby (E = 2300 cm−1) . In the case of alexandrite, cryogenic temperatures are required to effectively suppress the depopulation of the 2E level due to the thermal coupling to the 4T2 level. In such a case, alexandrite will behave more like ruby and it can operate at the 2E → 4A2 transition.
To minimize resonator losses resulting in a lower threshold and a higher system efficiency, microchip geometry was proposed. The microchip resonator was formed by a dielectric pump mirror (T > 95% @ 445 nm, R > 99 % @ 680 nm) directly deposited on the alexandrite crystal surface, and by an output coupler formed by a laser quality polished opposite side of the crystal (Fresnel reflection R = 7 %).
To operate the alexandrite laser at cryogenic temperature, the active medium mounted on a liquid nitrogen cooled copper finger was placed in the vacuum chamber of the cryostat (Janis Research Co., model VPF-100) with AR-coated windows for pump and laser emission wavelength. For a fine laser system adjustment, the cryostat was fixed on a 5-axis platform.
The 0.13 at.% doped alexandrite crystal (Northrop Grumman SYNOPTICS) grown by the Czochralski method was used. The sample (3×3 mm2 cross-section, 2.5 mm long) was cut along the c-axis (according to Pbnm notation). A pump process was realized by an InGaN laser diode (LD) from NICHIA Corporation, providing a linearly polarized beam centred at ∼445 nm. The maximum output power available at this wavelength was 3.5 W. The beam quality of the LD was determined to be and with the 1.7 nm linewidth (FWHM).
The laser experiment set-up is schematically illustrated in Fig. 1. The LD output radiation was collimated by an aspherical lens (f = 4.5 mm, 0.55 NA, ARC = 400–600 nm) and after going through the beam shaping optics and achromatic half-wave plate for 400–800 nm wavelengths (Thorlabs Inc.), the radiation was focused into the active medium by a lens with the 60 mm focal length, resulting in a slightly elliptical spot with radii of about wx = 60 μm and wy = 68 μm (measured at 1/e2 peak power). The half-wave plate fixed in a rotation mount was used to precisely adjust the pump radiation polarization plane to be parallel with the alexandrite “a”-crystallographic axis to utilize the larger absorption coefficient and thus to reach a high absorption efficiency in the active medium. Laser output radiation was detected behind the cut-off filter (FEL 500, Thorlabs Inc.).
As expected for laser diodes, the wavelength generated depends on the current passing through the active layer due to thermal effects. As a result, the amount of the absorbed power in the active material differs according to the overlapping of the pump wavelength and the absorption maximum of the laser crystal. In our case, the maximal absorbed pump power in a single pass was determined to be 59 % (alexandrite absorption coefficients for different crystal temperatures as well as for E‖a and E‖b polarizations, where “a” and “b” are alexandrite crystallographic axes, can be found e.g. in ref. ).
The alexandrite microchip input-output laser characteristic at 680.4 nm is depicted in Fig. 2. The maximum output power obtained was 210 mW with the slope efficiency of 15 % (related to absorbed pump power). The laser emission was observed up to 190 K crystal temperature, as shown in Fig. 3. Spectral line shape of the radiation generated is illustrated in Fig. 4. It should be noted that the alexandrite 680.4 nm linewidth is expected to be much narrower than reported in Fig. 4, but we were limited by the resolution of our available fiber-coupled spectrometer (StellarNet BLACK-Comet C-50, resolution of about 1.5 nm). The output beam profile was multimode. However, we believe it is possible to make alexandrite microchip lase in TEM00 mode if a single transversal-mode pump beam is employed.
It should be remarked that the primary goal of this work was investigation of the alexandrite lasing capability at 680.4 nm under LD pumping rather than the effort to reach the highest slope efficiency. Therefore, based on our previous alexandrite study  when the Fresnel reflections from the non-antireflection coated crystal facets were enough to start the laser oscillations at 680.4 nm at cryogenic temperature, the alexandrite microchip output coupler was designed (in the first step) to use only the Fresnel reflection from the laser quality polished surface. It is known that in the case of a thee-level system, changes in output coupler reflectivity affect the threshold much less than in a four-level system . Nevertheless, output coupler optimization is one of our next experimental goals.
2.2. Laser generation at 749.5 nm
Concerning the alexandrite microchip laser allowing the emission at 749.5 nm wavelength, the alexandrite crystal of the same dimensions and concentration as in the previous case was used. The dielectric films on the alexandrite crystal facets forming the microchip resonator were following — the pump side (pump mirror) was highly transmitted for the incident laser diode radiation (∼445 nm) and highly reflective for the generated radiation (∼750 nm). The opposite side (output coupler) was created by a dielectric coating with approx. 98 % reflectance for the designed ∼750 nm wavelength.
It is well-known that the unique feature of the alexandrite is its increased laser performance at vibronic laser transitions at elevated temperature, which is given by the structure of the alexandrite energy levels. Based on our previous investigation of the alexandrite laser , the optimal crystal temperature for ∼750 nm generation has been found to be around 350 K. Therefore, the crystal was mount in a copper finger of the cryostat (Janis Research Co., model VPF-100) which in connection with temperature controller (Lake Shore, model 325) allows to control crystal temperature at any value within the 78–500 K range. In our case, the temperature was set to 354 K.
Employing the same pump source and laser set-up as in the previous case (see section 2.1, Fig. 1), the maximum continuous-wave output power reached at 749.5 nm wavelength was more than 570 mW with the slope efficiency (related to absorbed pump power) of 39 %, as shown in Fig. 5. The corresponding spectral line shape together with the beam profile is depicted in Fig. 6. The maximal absorbed pump power in a single pass was about 64 %.
3. Discussion and conclusion
Alexandrite microchip lasers designed for operation at vibronic (4T2 → 4A2) and electronic (2E → 4A2) laser transitions at 749.5 nm and 680.4 nm, respectively, have been realized. To the best of our knowledge, this is the first demonstration of alexandrite microchip lasers, as well as the first report on the efficient diode-pumped alexandrite laser operated at the electronic transition at the so-called R-line (three-level lasing mode).
Employing blue 3.5 W InGaN laser diode (∼445 nm) as a pump source, a continuous-wave output power in the near-IR wavelength region (∼749.5 nm) exceeding 570 mW with the slope efficiency as high as 39 % (related to absorbed pump power) was extracted from the alexandrite microchip laser utilizing 98 % output coupler reflectance at the designed laser wavelength. This represents almost two times higher slope efficiency if compared with our previous results reported in , and the highest slope efficiency under the blue LD pumping so far. The laser was operated at 354 K crystal temperature, which has been found as an optimal temperature from the maximum output power point of view.
In the case of alexandrite visible laser emission at 680.4 nm, 210 mW with a slope efficiency of 15 % (related to absorbed pump power) was obtained using cryogenically cooled alexandrite microchip crystal having 7 % output coupler reflectance. Cryogenic temperature (78 K) is required to effectively suppress depopulation of the 2E level due to thermal coupling to the 4T2 level. The output beam profiles were multimode in both cases. However, there is no reason why the alexandrite microchip laser should not work in the fundamental Gaussian TEM00 mode if, e.g., a single mode pump beam profile was employed.
It should be also noted that the investigations performed in this work were not fully optimized for efficiency, especially in the case of 680.4 nm generation. Optimization of the output coupler reflectivity at 680.4 nm is expected to significantly improve the alexandrite laser performances. In addition, pumping of the alexandrite with a blue laser diode is not optimal from the point of view of optical-to-optical efficiency due to the large Stokes shift (alexandrite lasers are usually pumped by red laser diodes around 635 nm [4,13,14]). One could also expect some instabilities of the alexandrite output under a blue laser diode pumping  and/or higher inclination of the crystal to defects like e.g. color centers formation due to higher photon energies. Nevertheless, during our experiment, no degradation of crystal properties has been observed. On the contrary, if the optical-to-optical efficiency is not the main issue, the blue pumping of the alexandrite provides a potential for wavelength-multiplexed pumping  utilizing blue and red laser diodes, because the absorption maxima of the alexandrite blue and red absorption bands lie in two mutually orthogonal polarization planes.
Moreover, microchip geometry, characterized by cavity lengths of order of millimeters and less, offers a potential to produce gain-switched pulses that are much shorter than can be obtained with more conventional solid-state lasers [20–22]. The small cavity lengths lead to short cavity lifetimes and thus the possibility of generation very short pulses in nature. Our preliminarily results concerning the gain-switched alexandrite microchip laser at 680.4 nm wavelength showed increase in the peak power by three orders of magnitude, if it is compared with the classical pulse regime of operation. If the duration of the pump pulse was short enough so that there was no significant pumping after the first output spike, a single gain-switched output pulse was obtained for each pump pulse. In our case, when the alexandrite microchip laser was pumped by 170 μs laser diode pulse with 300 Hz repetition rate, a train of single output pulses (see Fig. 7) having less than 1.5 ns in duration was generated with the mean output power of 150 μW, which corresponds to the peak power in the pulse of more than 330 W. This represents three orders of magnitude higher peak power than it was possible to reach in the conventional pulse regime of operation.
To conclude, by employing cryogenic cooling together with the proper design of the resonator mirror reflectivity allowing for the laser generation both at the alexandrite R-line and vibronic transitions, it should be possible to easily realized alexandrite microchip laser operating either in the visible or near infrared spectral range by only changing the crystal temperature. This, together with the alexandrite mechanical durability, and simple and robust microchip construction, opens up the possibilities for various applications in medicine or industry.
ERDF/ESF “Center of Advanced Applied Sciences” (CZ.02.1.01/0.0/0.0/16_019/0000778).
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