Diode-pumped CW and passively Q-switched Er, Yb:YAB micro-laser

Er³⁺,Yb³⁺ co-doped yttrium-aluminium borate (Er, Yb:YAB) laser crystal

Er³⁺, Yb³⁺ co-doped yttrium-aluminium borate (Er, Yb:YAB) crystal is an alternative to commonly used Er, Yb:phosphate glass for application as an active medium of "eye-safe" (1,5-1,6 µm) lasers with high average output power in CW and pulsed modes. Promising Er, Yb:YAB crystal laser-based applications are high-power "eye-safe" (~1,5 μm) CW lasers for metrology, high repetition rate passively Q-switched lasers for LIDAR and LIBS systems and ultrashort mode-locked lasers for telecom systems.

Er, Yb:YAB crystals are characterized by high thermal conductivity of 7,7 and 6 W/(m⋅K) along the a-axis and c-axis, respectively. Er, Yb:YAB also features high efficiency of the Yb³⁺ to Er³⁺ energy transfer (~94%) and weak upconversion losses due to the very short lifetime of the ⁴I_11/2 excited state, (~ 80 ns), facilitated by high maximum phonon energy of the host, hv_max ~1500 cm^-1.

Figure 1. (a) π- and σ-polarized absorption cross-section spectra at room temperature in the 900-1050 nm wavelength range of Er, Yb:YAB crystal, (b) π-polarized absorption and emission cross-section at room temperature in the ~1,5-1,6 μm wavelength range of Er, Yb:YAB, (c) σ-polarized absorption and emission cross-section at room temperature in the ~1,5-1,6 μm wavelength range of Er, Yb:YAB crystal

Spectra of polarized absorption and emission spectra ar given in the figure 1. A strong (2,75x10^-20 cm²) and wide absorption band (~17 nm) in σ-polarized spectrum at 976 nm is observed that coincides with emission spectrum of commercially available InGaAs laser diodes (figure 1 (a)). The highest emission cross-section is at 1531 nm wavelength in σ-polarized spectrum with magnitude of ~2,45x10^-20 cm² (figure 1 (c)). Main lasing wavelengths of Er,Yb:YAB laser crystal are 1522 nm, 1531 nm, 1543 nm, 1550 nm and 1602 nm and it has been shown that at low inversion expected laser emission is at 1602 nm, while at higher inversion laser emission is possible from 1520 to 1630 nm [1, 2].

Setup description

Diode-pumped CW and passively Q-switched Er, Yb:YAB microlaser laser setup is based on V. E. Kisel et al. [1] article. 4Lasers provide all the necessary optical components according to previous-mentioned article to assemble Er, Yb:YAB microlaser laser for both CW and passively Q-switched regimes. Thus optomechanical and temperature control components are not in the list, 4Lasers could provide them aswell. More information can be found here Optomechanical Engineering.

Figure 2. Layout of Er, Yb:YAB microchip laser: fiber-coupled laser diode, focusing lens set, flat input mirror (for CW regime laser) or Co:Spinel (fot passively Q-swithed regime laser), Er(1,5%), Yb(12%):YAB. Heat sink of Er, Yb:YAB crystal is not previewed.

Microlaser pump source was a fiber-coupled laser diode (λ = 976 nm, NA = 0,22), which delivers non-polarized 10 W average output power laser radiation. Pump beam was delivered by focusing lens set (x2 f=35 mm Bi-convex lenses). According to the reference article [1], experimental results were achieved with 120 μm pump beam diameter inside the laser crystal. It is worth to mention that active cooling of Er, Yb:YAB is required. During experiments laser crystal was kept at 16 °C by thermoelectrically cooled heat sink.

Layout of Er, Yb:YAB microchip laser is given at figure 2. Active laser gain media is 3x3x2 mm, c-cut Er³⁺, Yb³⁺ co-doped yttrium-aluminum borate (Er(1,5 at. %), Yb(12 at.%):YAB). Laser cavity output coupler was deposited on the output face of Er, Yb:YAB laser crystal, whereas the transmission value was around 3% at 1520-1610 nm wavelength range. Other side of the crystal was coated by antireflective coatings at 976 nm (pump) + 1520-1610 nm (lasing) wavelength range. 

Input mirror for CW operation was plano/plano type and had antireflective coatings at 976 nm + 1520-1610 nm on input side and highly reflective on the other side at 1520-1610 nm wavelength region. At passive Q-switched operation plano/plano mirror was replaced by plano/plano Co:spinel saturable absorber with initial transmission of 98,5% at 1522 nm. Input side of Co:Spinel was coated by highly transparent at 976 nm and highly reflective at 1522 nm wavelengths. During the experiments Co:spinel was in mechanical contact with Er, Yb:YAB laser crystal.

Note that every optical element can be customized according to customer request.

Expected lasing parameters

4Lasers does not take any responsibility if the achieved lasing output parameters are not as described in the [1] article.

At CW regime it is expected to achieve lasing at 2 W of absorbed pump power. At 7,7 W of absorbed pump power cw lasing output power should reach 800 mW, while the slope efficiency is ~16%.

At passively Q-switched regime it is expected to observe wavelength shift from 1602 nm to 1522 nm due to wavelength dependency on the inversion density. Expected lasing parameters are up to 315 mW of average output power, 5,25 μJ pulse energy, 60 kHz pulse repetition rate, 5 ns pulse duration at 6,2 W of absorbed pump power. The spatial beam profile mode could be TEM₀₀ with M² < 1,2.

References

[1]   V. E. Kisel, K. N. Gorbachenya, A. S. Yasukevich, A. M. Ivashko, and N. V Kuleshov, “Passively Q -switched microchip Er, Yb:YAl3(BO3)4 diode-pumped laser,” vol. 37, no. 13, pp. 2745–2747, 2012.

[2]   N. A. Tolstik et al., “Efficient 1 W continuous-wave diode-pumped Er,Yb:YAl_3(BO_3)_4 laser,” Opt. Lett., vol. 32, no. 22, p. 3233, Nov. 2007.