1. Introduction

Holmium lasers emitting at wavelengths around 2 μm are promising source of laser radiation for coherent Doppler lidar system and nonlinear optical generation of tunable mid-infrared radiation [1,2]. To meet the stringent requirement for laser lidar transmitter and driving ZnGeP₂ optical parametric oscillator (OPO), significant progress in Ho lasers based on LuLF, YAG and YLF host materials has been made recently [3–5]. Lu₂SiO₅ host has several properties that make it well suited for development of a solid-state laser source around 2 μm. Crystal growth of undoped or doped LSO by the Czochralski technique is quite easy to perform due to its congruent melting behavior. As a monoclinic biaxial crystal (class 2/m, space group c2c), the strong natural birefringence overwhelms the thermally induced stress birefringence that is the source of thermal depolarization observed in isotropic media such as YAG. The thermal conductivity of pure LSO is 3.67 Wm⁻¹K⁻¹ at 300K [6], which is smaller than 7.2 Wm⁻¹K⁻¹ for YLF and 11.2 Wm⁻¹K⁻¹ for YAG at 299 K [7]. However, the weak molar mass difference of 3.5% between Tm, Ho and Lu makes a weak decrease in thermal conductivity of LSO with thulium and holmium doping [8].

In the Tm:Ho system the Tm³⁺ ions are pumped by a diode laser into the ³F₄ manifold, followed by energy transfer to populate the Ho ⁵I₇ upper laser manifold. Emission near 2.1 μm is then generated from Ho ${}^5{I}_7{\to }^5{I}_8$ transition. Another alternative approach is to pump Ho ions directly into the upper laser manifold of ${}^5{I}_7$ by a Tm-doped laser. In this paper we report what is to our knowledge the first Tm, Ho-codoped and Ho-doped LSO lasers operating around 2 μm, pumped by laser diodes near 790 nm and a Tm laser at 1.9 μm, respectively.

2. Absorption and emission spectrum

Singly-doped-Ho (1 at. %) and Tm (6 at. %):Ho(0.4 at. %)-codoped LSO boules were grown by the standard Czochralski technique. All the Ho:LSO samples utilized to measure spectroscopic parameters have the same size of 15 × 20 mm² in aperture and 1 mm in length. The room-temperature polarized absorption cross-section spectra corresponding to Tm ${}^3{H}_6{\to }^3{H}_4$ and Ho ${}^5{I}_8{\to }^5{I}_7$ bands, recorded on a Shimadzu UV-3100PC spectrophotometer at 0.2 nm resolution, are shown in Fig. 1 . The absorption spectra of thulium in LSO host is broad, which extends from 760 to 810 nm. The absorption features, coinciding with emission wavelengths of commercially available high-power laser diodes, are located at 786 (σ_abs = 4.2 × 10⁻²¹cm², linewidth 4 nm) for $E//D_1$ and 791 nm (σ_abs = 5.8 × 10⁻²¹cm², linewidth 6 nm) for$E//<010>$. As also shown in Fig. 1, for Ho:LSO along the crystallographic D₁ axis the absorption peaks occur at 1.91, 1.94, and 1.99 μm, which are consistent with the emission wavelengths of Tm lasers such as Tm:YLF (1.91 μm), Tm:YAP (1.94 μm) and Tm fiber (1.8~2.1 μm). Meanwhile, there is serious reabsorption at the lasing wavelengths around 2 μm. To reduce the reabsorption losses, it is necessary to maintain Tm,Ho:LSO medium at lower temperature such as 77 K for minimizing the fractional Bolzman population of the holmium ⁵I₈ lower laser Stark level in LSO. And, the low Ho ions concentrations of less than 0.5 at. % and short crystal length to several millimeters could also be preferable for decreasing reabsorption when operating in room temperature [9].

 

Fig. 1 Room-temperature polarized absorption cross sections of the Tm ³H₄ manifold in LSO.

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The room-temperature polarized fluorescence spectra, which correspond to Ho ${}^5{I}_7{\to }^5{I}_8$ transition, were taken by a 0.3-m single-grating (300 lines/mm, blazing at 2.0 μm) WDM1-3 monochrometer with a resolution of 0.8 nm. The singly-doped Ho:LSO sample was excited at 1.91 μm by a cw Tm:YLF laser. The luminescence signal was detected by an InGaAs detector and processed by a SRS-830 lock-in amplifier. The lifetime of Ho ⁵I₇ manifold for LSO was 3.3 ms, measured by using a 2.05-μm Tm,Ho:GdVO₄ laser (10 Hz PRF, 20-ns pulse duration) as an exciting source, comparing with 8 ms for Ho:YAG and 12 ms for Ho:YLF. Figure 2 shows the polarized effective emission cross-section spectrum of Ho:LSO determined by Fuchtbauer-Ladenburg equation. The maximum effective emission cross sections are $2.02\times {10}^{-20}$cm⁻² at 2.03 μm ($E{\text{//D}}_{\text{1}}$) and $1.59\times {10}^{-20}$cm⁻² at 2.09 μm ($E{\text{//D}}_{\text{1}}$) in Ho:LSO, compared with that of $1.13\times {10}^{-20}$cm² at 2.09 μm in Ho:YAG [10] and $1.55\times {10}^{-20}$cm² (π polarization) at 2.05 μm in Ho:YLF [11]. In the orthosilicate LSO, holmium ions can lie in the two rare earth sites, respectively, six- and seven-coordinated, both of which are of low symmetry and very distorted [12]. Consequently, a broad emission band arises, which extends from 1.93 to 2.11 μm, as shown in Fig. 2.

 

Fig. 2 Room-temperature polarized emission cross-section spectra of Ho:LSO ⁵I₇ manifold.

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3. Diode-pumped Tm,Ho:LSO laser

The schematic diagram of cryogenic Tm, Ho:LSO laser is shown in Fig. 3 . The laser crystal, cut along the <010> crystallographic axis, doped with 6 at. % Tm³⁺ and 0.4 at. % Ho³⁺, had a dimension of $4\text{ mm}\times 4\text{ mm}\left(\text{in cross section}\right)\times \text{10 mm}$(in length), of which both end surfaces were anti-reflection (AR) coated at the pump wavelength of 786 nm (R<0.5%) and the lasing wavelengths of 2.0-2.1μm (R<0.2%). This active medium was wrapped in indium foil and held in a copper heat-sink connected with a small dewar filled with liquid nitrogen, resulting in a significant decrease of ground-state absorption and reduction of thermal-optic effects at low temperature [13]. The pump source is a fiber-coupled (0.4 mm core diameter, 0.22 NA) laser diode with a maximum output power of 15 W at 786 nm. The collimated output from the diode was split in half for dual end-pumping, allowing for the heat generated in the Tm, Ho:LSO crystal dissipating along the entire length uniformly, and then relay-imaged into the gain medium with a magnification of 1.5. The absorption spectral profile of Tm,Ho:LSO corresponding to the Tm^{3+ 3}H₆→³H₄ transition becomes narrower at 77 K compared to that in room temperature, hence the pump absorption efficiency is sensitive to the diode emission wavelength, which must be held at constant value by controlling its operating temperature. About 90% diode power was absorbed by the 10-mm long Tm,Ho:LSO crystal. The L-shaped resonator with physical length of about 120 mm, is constituted by a flat mirror with 99.5% reflectivity in the wavelength range 2.0-2.1 μm and 95% transmission at the pump wavelength of 786 nm, a flat 45° dichroic mirror with reflectivity of 99.8% at 2.0-2.1 μm and a transmissivity of 95% at 786 nm, and is a plano-concave output coupler (OC) with a radius of curvature of 300 mm.

 

Fig. 3 Experiment setup of dual-end-pumped cryogenic Tm,Ho: LSO laser. DM, dichroic mirror (HR@2.0-2.1 μm and HT@791 nm), CL, collimating lens (focal length of 25 mm), FL, focusing lens (focal length of 38 mm).

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Power performance of diode pumped cryogenic Tm, Ho:LSO laser is shown in Fig. 4 . The maximum cw output power of 3 W is achieved by use of the 9% output coupling at ~11-W incident pump power, corresponding to an optical-to-optical efficiency of 27%. The maximum slope efficiency of 35%, measured with respect to the incident diode power, is yielded by linear fit of the experimental data obtained with 16% transmission mirror. The Loss L and quantum efficiency ${\eta }_q$ can be derived from the measured slope efficiency ${\eta }_s$ and the formula ${\eta }_s={\eta }_q{\nu }_L/{\nu }_P\cdot T/\left(L+T\right)$, where ${\nu }_L$ and ${\nu }_P$ are laser and pump frequency [14]. A least-squares fit to this formula yields ${\eta }_q=\text{1 .20}\pm \text{0 .15}$ and$L=\text{4 .2}\pm 1.\text{6}\%$. The cross relaxation process between Tm³⁺ ions accounts for the greater than unit quantum efficiency. Figure 4 inset shows the lasing spectrums of Tm, Ho:LSO laser. The three emission spectral profiles corresponding to three different transmission OCs, which are centered at 2089, 2071 and 2062 nm, have FWHM linewidths of 11.1, 5.3, and 4.9 nm, respectively. The output wavelengths dependence on the OC losses are related to the gain cross section of Tm,Ho:LSO [15]. The nearly linearly polarized light with contrast ratio of 9 dB was achieved owing to the difference of the effective emission cross-sections for E// D₁ and E//D₂ polarizations. At the maximum 3-W output power level the beam quality of M² factor was measured to be ~1.30 for cryogenically cooled Tm,Ho:LSO laser.

 

Fig. 4 Output power of cryogenically cooled Tm,Ho:LSO laser versus pump power with different output coupling. Inset, output laser spectrum with different OC transmissivity.

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To achieve operation of Tm,Ho:LSO laser in room temperature, a shorter crystal of 1 mm long was pumped by a high brightness fiber coupled LD (DILAS M1F1S22), which was collimated and refocused to form a beam waist radius of 250 μm inside the gain medium. The input side of the crystal was coated with unity reflectivity near 2 μm and approximately 90% transmissivity at 0.79 μm, while the output surface was 99% reflecting for the lasing wavelength and highly transmitting for the pump wavelength. The diode output was temperature tuned to 791 nm, at which approximately 30% light power was absorbed by the 1-mm long crystal. The laser output power as a function of the absorbed pump power for the microchip Tm,Ho:LSO at 12 °C is shown in Fig. 5 . An output power of 80 mW at 2.08 μm was obtained for 1.1 W of absorbed power, which corresponds to 4 W of incident power. The threshold was at 0.61 W of absorbed power. The slope efficiency for the output relative to the absorbed pump powers was 26%, compared to 25.4% slope efficiency obtained by the similar Tm, Ho:YLF microchip laser operating at −5 °C [16]. Due to the serious thermal effects the output power would not increase with greater than 1.1 W of absorbed pump power by the active medium.

 

Fig. 5 Output power of the room-temperature Tm,Ho:LSO laser versus incident pump power.

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4. Room-temperature resonantly pumped Ho:LSO laser

Tm, Ho-codoped crystal is difficult to obtain cw output in multi-Watt power level due to its low energy transferring efficiency and strong cooperative upconversion. To achieve cw operation of Ho:LSO laser at room temperature, an in-band-pumping configuration was adopted in our experiment. The laser schematic diagram is shown in Fig. 6 . A diode-pumped Tm:YLF laser with an emission wavelength of 1.91 μm and a beam quality parameter of M²≈1.1 was utilized as a pump source. By use of a 200-mm focal length mode-matching lens, pump spot size of ~380 μm in diameter was formed inside the gain medium. The Ho:LSO crystal is 20 mm in length and 4 × 4 mm in cross section, doped with 1 at. % Ho and antireflection coated at the pump and laser wavelengths. At the wavelength of 1.91 μm the single-pass pump absorption efficiency is about 50% by the 20 mm long crystal. The In foil wrapped Ho:LSO laser crystal was clamped in a copper block, which was mounted onto a thermoelectric cooler (TEC) to allow for pump generated heat removal and precise temperature control. The crystal temperature was held at 18°C. The L-shaped Ho:LSO laser cavity consists of a flat mirror with R>99.8% at 2.1 μm, a 45° dichroic mirror with R>99.8% at 2.1 μm and T>98% at 1.91 μm, and a concave output coupler with a radius of curvature of 100 mm and 5% transmission at 2.1μm. The total physical length of the resonator is ~50 mm, resulting in a calculated TEM₀₀ beam waist diameter of ~350 μm.

 

Fig. 6 Schematic diagram of a resonantly pumped Ho:LSO laser operating in room temperature.

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The cw laser data obtained using resonantly pumping configuration are shown in Fig. 7 . The maximum output power is 2.8 W for 12 W of absorbed pump power, corresponding to an optical-to-optical conversion efficiency of 23%, comparing with that of 32% for Ho:YLF laser pumped by a 1.94 μm Tm fiber laser [5] and 50% for Ho:YAG laser pumped by a Tm:YLF laser [2]. The scattering losses inside the crystal and the thermally induced mode mismatch between the pump and resonator beam (less than 350 μm in diameter) could account for the lower conversion efficiency. A linear regression fit to the data yields a slope efficiency of 35% and a threshold pump power of 4 W. The beam quality of the in-band pumped Ho:LSO laser was measured to be M²≈1.1 at the maximum output power level. The output of Ho:LSO laser with 5% transmission OC becomes linearly polarized along the D₁ crystallographic axis. The spectral output of room-temperature cw Ho:LSO laser, measured by means of a WDM1-3 monochrometer with a resolution of 0.8 nm, is shown in Fig. 6 inset, in which the emission profile is centered at 2106 nm with FWHM linewidth of 3.3 nm. The longer wavelength of Ho:LSO operation in room temperature is due to the higher gain cross section at 2106 nm for the longer (20 mm compared to 10 mm) and higher Ho³⁺ concentration (1% compared to 0.4%) medium. For higher optical conversion efficiency the Ho:LSO could be pumped at 1.94 μm where the strongest absorption exists.

 

Fig. 7 Laser output from resonantly pumped Ho:LSO. Inset, laser spectrum of Ho:LSO with OC transmissivity of 5%.

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5. Summary

In summary, we report the first laser performance of Ho:LSO crystals. The maximum cw output power of 3 W and 80 mW have been achieved by using diode pumped Tm,Ho:LSO lasers, operating in cryogenic temperature regime and in room temperature, respectively. In addition, a maximum cw output power of 2.8 W at 2106 nm, and a slope efficiency of 35% have been demonstrated in resonantly pumped Ho:LSO laser operating in room temperature.