Tm3+/Ho3+ co-doped LiGd(MoO4)2 (LGM) crystals were investigated as gain media for the Ho3+ laser around 2.0 μm. Polarized spectroscopic parameters of Ho3+ ions in the crystals were calculated based on the absorption spectra by the Judd-Ofelt theory. Related fluorescence spectra and decay curves were measured and analyzed for the crystals with different Tm3+/Ho3+ co-doped concentrations, 5.4/1.4 and 4.6/0.6 at.%. Stimulated emission cross sections of the 5I7→5I8 transition of Ho3+ ions were derived according to the Füchtbauer-Ladenburg formula. End-pumped by a pulsed diode laser at 795 nm, the Ho3+ laser at 2.05 μm with a slope efficiency of 20% was realized in a c-cut crystal sample with the Tm3+/Ho3+ concentrations of 4.6/0.6 at.%.
© 2012 OSA
Ho3+-doped crystals have been widely studied for lasing around 2.0 μm via the 5I7→5I8 transition. The laser is eye-safe and matches well with the absorption lines of H2O and CO2. Therefore, a number of applications such as coherent lidar, atmospheric sensing, and surgery are possible [1–3]. Furthermore, the laser operating in ultrafast pulse regime can also be used in time-resolved spectroscopy and as pumping source for the optical parametric oscillator (OPO) in the mid-infrared region [4,5]. Research of gain media for realizing the pulse laser has recently attracted substantial attentions. Generally, Tm3+ or Yb3+ ions, which has large absorption cross section at the emitting wavelengths of the commercially available high power laser diodes (LD), is co-doped as sensitizer for improving pump absorption [6–9]. The processes of energy transfers from Tm3+ and Yb3+ to Ho3+ ions are schemed in Fig. 1 . Notably for the Tm3+/Ho3+ co-doped system, a maximal pump quantum efficiency close to 2 could be expected due to the cross relaxation (CR) of Tm3+ ions, 3H4 + 3H6→3F4 + 3F4.
Double tungstate and double molybdate crystals with a general formula of MT(XO4)2 (M = Li, Na, or K; T = Y, La, Gd, or Lu; X = W or Mo) have been proved to meet the requirement of hosts for ultrashort pulse laser [4,10]. Especially for the subgroup with M = Li or Na, which belongs to the tetragonal system with space group of . Crystals with this structure are usually characterized by a high disordered local environment due to the monovalent alkali ions and trivalent rare earth ions randomly sharing two different S4 lattice sites labeled as 2b and 2d . When active ions of rare earths substitute the trivalent cations, inhomogeneous broadening for absorption and emission bands are beneficial to being pumped by LD and generating wide tunable and ultrashort pulse lasers. Recently, a pulse laser operation as short as 191 fs has been realized in a Tm3+/Ho3+ co-doped NaY(WO4)2 crystal . Detailed spectroscopic and laser properties of another isostructural Tm3+ singly doped LiGd(MoO4)2 (LGM) crystal have been reported in our previous work . In this work, Tm3+/Ho3+ co-doped LGM crystals are studied as the gain media for Ho3+ laser around 2.0 μm. The laser has been realized in a simple plano-concave cavity with a pulsed LD as pumping source.
2. Experimental procedure
Two Tm3+/Ho3+ and one Yb3+/Ho3+ co-doped LGM single crystals were grown by the Czochralski method in air at about 1030 °C. After growth the crystals were annealed at about 850 °C for 10 hours to reduce the residual stress. The doping concentrations measured by an inductively coupled plasma atomic emission spectrometer (Ultima 2, Jobin-Yvon) are 5.4/1.4 and 4.6/0.6 at.% for the two Tm3+/Ho3+ co-doped crystals and 7.7/1.4 at.% for the Yb3+/Ho3+ co-doped crystal (hereafter 5.4/1.4 TH, 4.6/0.6 TH and 7.7/1.4 YH for short, respectively). Oriented crystal samples were cut and polished for spectral experiments with dimensions (crystallographic a × b × c) of 2.70 × 10 × 10, 1.88 × 7 × 8, and 2.20 × 5 × 7 mm3, respectively. No gas bubbles or inclusions were found in these samples. The optical quality of the crystals was examined by a ZYGO GPI optical interferometer. The interference fringes are straight and have a uniform distribution which shows the crystals have good optical homogeneity. Figure 2 shows the result for the 4.6/0.6 TH. The polarized absorption spectra were measured using a UV/VIS/NIR spectrophotometer (Lambda 900, Perkin-Elmer) with a scanning step of 0.5 nm. A deuterium lamp (UV) and a tungsten-halogen lamp (VIS/NIR) were equipped as the light sources. The polarized fluorescence spectra were measured by a TE-cooled PbS detector in the NIR region associated with a monochromator (Triax 550, Jobin-Yvon) ahead of it, a Ti:sapphire laser operating at 795 nm and an InGaAs LD operating at 970 nm were used as exciting sources for the Tm3+ and Yb3+ ions, respectively, with a measured resolution of 1.0 nm. The fluorescence decay curves were measured by a spectrometer (FLSP920, Edinburgh) with a Hamamatsu R928 PMT and an InSb detectors in the VIS and NIR regions, respectively, a tunable mid-band OPO laser (Vibrant 355II, OPOTEK) with pulse duration of about 5 ns was adopted as the exciting source. In fluorescence experiments front-surface excitation-detection was used to measure fluorescence spectra of the crystal samples in order to minimize the impact of re-absorption. Additionally, the excitation intensity was always kept as low as possible to avoid some possible amplified spontaneous emission. All of the spectral experiments were carried out at room temperature.
3. Results and discussion
3.1 Absorption and Judd-Ofelt analysis
Absorption spectra of the 7.7/1.4 YH in a range of 350−2200 nm were measured for two different polarizations of σ (E⊥c, E represents the electric field direction of incident light) and π (E//c) and are shown in Fig. 3 . It can be found that the absorption of Ho3+ ions exhibits a strong anisotropic behavior. The Yb3+ ions have only an intense absorption band around 980 nm and do not influence the absorption of Ho3+ ions in other measured spectral region. The Judd-Ofelt (J-O) theory [13,14] was applied to determine the intermultiplet spontaneous transition rates of Ho3+ ions in LGM crystal. The J-O intensity parameters Ωt (t = 2, 4, 6) were fitted by adopting the six easily distinguishable absorption bands corresponding to the transitions from the ground 5I8 multiplet to the excited 5I7, 5I6, 5F5, 5F4 + 5S2, 5F1 + 5G6, and 3G5 multiplets. The calculation procedure can be found in the literature [15,16] and the refractive index has been estimated in . The contribution of the magnetic dipole (MD) transition of 5I8→5I7 was excluded before the calculation. The values of the reduced matrix elements of unit tensor operators and the coefficients of the intermediate coupling wavefunctions were taken from  and , respectively. The mean wavelength (), the experimental oscillator strength (), and the oscillator strength calculated from the J-O intensity parameters () for each band are listed in Table 1 . The root mean square (rms) deviations between the experimental and calculated oscillator strengths are 0.95 × 10−6 and 2.42 × 10−6 for σ and π polarizations, respectively. Both are in the typical error range of the J-O fitting . The intensity parameters for Ho3+ ions in LGM crystal are listed in Table 2 and the effective J-O intensity parameters were calculated according to . These parameters are comparable to those of Ho3+ in KGd(WO4)2 crystal .
The intermultiplet electric dipole (ED) spontaneous transition rates () of Ho3+ were calculated from the intensity parameters for both polarizations (q = σ and π) and are listed in Table 3 . Taking the MD transition rates () into account, the average spontaneous transition rates could be calculated by with . Then the fluorescence branching ratios (β) and the radiative lifetimes () could be further calculated and the results are also listed in Table 3.
The polarized absorption spectra were also measured for the Tm3+/Ho3+ co-doped LGM crystals in a range of 750−850 nm. Since the absorption cross sections corresponding to the 3H6→3H4 transition of Tm3+ ions are nearly identical for both 5.4/1.4 TH and 4.6/0.6 TH, only the spectra of the latter are shown in Fig. 4 for brevity. The peak absorption cross sections were 4.66 × 10−20 cm2 for σ polarization at 795 nm and 1.93 × 10−20 cm2 for π polarization at 781 nm, with band widths of 8 and 37 nm, respectively. Both the peak values are slightly larger than those of the Tm3+ singly doped LGM, respectively . Taking into account the typical wavelength drift with temperature is about 0.3 nm/K and the spectral bandwidth is 3 to 6 nm for commercial LD pumping source, the stronger absorption of Tm3+/Ho3+ co-doped LGM in σ polarization should be ideal for LD pumping.
3.2 Fluorescence and stimulated emission cross sections
When the Yb3+ ions were excited to the 2F5/2 multiplet at 970 nm, the polarized fluorescence spectra of 7.7/1.4 YH around 2.0 μm were measured and are shown in Fig. 5(a) . The emission bands in the range of 1900−2100 nm with peaks around 2050 nm are attributed to the 5I7→5I8 transition of Ho3+ ions. Under excitation at 795 nm, the Tm3+ ions were excited to the 3H4 multiplet and the polarized fluorescence spectra of both 5.4/1.4 TH and 4.6/0.6 TH were measured and are shown in Figs. 5(b) and 5(c), respectively. The emission bands of Tm3+ ions corresponding to the transition of 3F4→3H6 can still be found in the range of 1650−1950 nm, which suggests that the energy transfer from Tm3+ to Ho3+ is not complete. Furthermore, the ratio of the integrated fluorescence intensity of Ho3+ ions to that of both Tm3+ and Ho3+ ions, i.e., , in the entire emission band of 1650−2100 nm in one co-doped crystal, can be used as a measure for the net energy transfer efficiency from Tm3+ to Ho3+. The ratio in 5.4/1.4 TH is larger compared with that for 4.6/0.6 TH just indicates a preferred energy distribution on Ho3+ ions in the higher co-doped case.
The fluorescence decay curves for the 3H4 multiplet of Tm3+ ions were measured for both 5.4/1.4 TH and 4.6/0.6 TH and are shown in Fig. 6 in semi-log scale. The exciting and monitoring wavelengths were 783 and 810 nm, respectively. Both decay curves display a departure from the single exponential behavior. The mean fluorescence lifetimes are estimated 18 and 28 μs for 5.4/1.4 TH and 4.6/0.6 TH, respectively, according to where is the fluorescence intensity at time t . Since an intrinsic lifetime τ0 = 134 μs for the 3H4 multiplet has been reported in a 0.79 at.% Tm3+ singly doped LGM crystal , the energy transfer efficiency (η) for Tm3+ ions on the 3H4 multiplet were estimated 87% and 79% for 5.4/1.4 TH and 4.6/0.6 TH, respectively, by and the energy transfer should be firstly attributed to the cross relaxation (3H4 + 3H6→3F4 + 3F4) between Tm3+ ions.
The fluorescence decay curves for the 3F4 multiplet of Tm3+ ions and the 5I7 multiplet of Ho3+ ions were also measured for the Tm3+/Ho3+ co-doped LGM crystals and are shown in Fig. 7 . The exciting and monitoring wavelengths were 1700 (2000) nm and 1750 (2050) nm for the 3F4 (5I7) multiplet of Tm3+ (Ho3+) ions, respectively. Similar kinetic behaviors can be found for both the 5.4/1.4 TH and 4.6/0.6 TH. Notably for the curves of Tm3+ ions, a sharp decay in the early time was recorded responsible for the fast energy transfer of 3F4(Tm3+) + 5I8(Ho3+)→3H6(Tm3+) + 5I7(Ho3+). Since the energy transfer is nearly resonant and always accompanied by its inverse process of 5I7(Ho3+) + 3H6(Tm3+)→5I8(Ho3+) + 3F4(Tm3+), the two multiplets, 3F4 and 5I7, will share their excitations and be rapidly coupled into an equilibrium system after excitation . Therefore, the decay rates of both the Tm3+ and Ho3+ ions become nearly identical in the late decay period. The mean fluorescence lifetimes were estimated 3.9 ms for the 3F4 multiplet and 4.2 ms for the 5I7 multiplet in 5.4/1.4 TH and 2.1 and 2.3 ms in 4.6/0.6 TH, respectively. By the way, the fluorescence decay curve for the 5I7 multiplet of Ho3+ ions was also measured from the 7.7/1.4 YH and is shown in Fig. 8 . The lifetime fitted from the single exponential decay curve is 4.9 ms which is longer than the above Tm3+/Ho3+ co-doped systems. Meanwhile, the fluorescence lifetime for the 3F4 multiplet of Tm3+ ions was previously reported about 1 ms in Tm3+ singly doped LGM crystals , the distinct coupled decay behaviors for the Tm3+/Ho3+ co-doped systems just embodies the energy transfers between Tm3+ and Ho3+ ions. The coupled lifetimes are closely related to the doping concentrations, especially, related to the ratio of Tm3+ to Ho3+ concentrations .The stimulated emission cross section () for the 5I7→5I8 transition of Ho3+ ions is an important factor related to the 2.0 μm laser operation. According to the Füchtbauer-Ladenburg (F-L) formula Fig. 9 . The maximum emission cross sections are 0.75 × 10−20 cm2 at 2050 nm for σ polarization and 1.67 × 10−20 cm2 at 2045 nm for π polarization.
Using the absorption and emission cross sections derived above, the wavelength dependence of the gain cross sections can be calculated by Figure 10 shows the calculated gain cross sections for several values of P (P = 0.3, 0.4, …, 0.7) for both polarizations. To take the P = 0.5 for an example, it can be seen that the gain for π polarization is larger than that for σ polarization and the theoretical tunable range of Ho3+ laser covers from 1975 to 2100 nm.
3.3 Laser demonstration
In order to assess the laser performance of the Tm3+/Ho3+ co-doped LGM, an end-pumped plano-concave resonator was adopted with a fiber-coupled pulsed LD emitting at 795 nm as the pumping source. The pump beam was modulated with a duty cycle of 2% and a modulation frequency of 10 Hz. It passed through an input coupler which had a 90% transmission at 795 nm and a reflectivity of R > 99% in the region between 1900 and 2100 nm. The crystal sample, without any antireflection coating, was held in an aluminum mount and positioned to the input coupler as close as possible. The diameter of pump beam inside the gain medium was about 115 μm. Three output couplers with transmissions TOC = 0.7%, 2.4%, and 3.9% around 2.0 μm and the same curvature radius of 100 mm were adopted in the experiment, respectively. The length of the plano-concave cavity was set close to the curvature radius of the output couplers.
Taking into account the polarization-dependent absorption of the Tm3+ ions at the pump wavelength, the c-cut Tm3+/Ho3+ co-doped LGM crystal samples with dimensions (a × b × c) of 7 × 10 × 0.87 and 7 × 7 × 1.10 mm for 5.4/1.4 TH and 4.6/0.6 TH were used as laser gain media, respectively. However, the Ho3+ laser was only realized in the 4.6/0.6 TH with lasing wavelength at 2.05 μm. Within a low-power pump, the single-path pump absorption efficiency was measured about 80% which is in agreement with the calculated 79% from absorption cross section. The absorption efficiency was almost kept the same in our experiment. Figure 11 shows the average output power versus the average absorbed pump power at different output coupler transmissions, the laser operated at a relative low-power level. The maximum output power of 25 mW was achieved when the absorbed pump power was 138 mW for the TOC = 3.9%. The related slope efficiency (ηS) was about 20%. Moreover, it can be found that, since the slope efficiency has monotonically increased from 10% to 20% with the increment of TOC from 0.7% to 3.9%, a higher efficient laser can be expected by optimizing the output coupler transmission.
The Ho3+ laser around 2.0 μm operating at room temperature is a quasi-three-level system. The reason for the lack of laser demonstration in the higher co-doped 5.4/1.4 TH sample may be mainly attributed to the stronger re-absorption loss.
A detailed description of the spectroscopic properties for the Ho3+ ions in LGM crystals was performed. Comparative study on the spectroscopic properties related to the 2.0 μm laser of Ho3+ has been carried out for the Tm3+/Ho3+ co-doped LGM crystals with concentrations of 5.4/1.4 and 4.6/0.6 at.%. Although the fluorescence spectra suggest that a more efficient energy transfer from Tm3+ to Ho3+ ions has occurred in the 5.4/1.4 TH, laser operation has been realized only in the 4.6/0.6 TH. The main reason for the lack of laser operation in the higher co-doped sample should be attributed to the stronger re-absorption loss as the Ho3+ concentration is more than twice as high in the lower co-doped case.
A 2.05 μm Ho3+ laser has been realized in a 1.10-mm-thick c-cut 4.6/0.6 TH sample with a fiber-coupled pulsed LD end-pumped at 795 nm. The slope efficiency of 20% was achieved at the output coupler transmission of 3.9% and the maximum average output power was 25 mW when the absorbed pump power was 138 mW. Since a continuous wave Ho3+ laser with slope efficiency as high as 48% has been realized in another isostructural 5/0.25 at.% Tm3+/Ho3+ co-doped NaY(WO4)2 crystal when a V-type astigmatically-compensated resonator has been adopted , a better result can be expected for the Tm3+/Ho3+ co-doped LGM crystal by further optimizing the concentrations of Tm3+/Ho3+ ions and configuration of laser cavity.
This work has been supported by the National Natural Science Foundation of China (grant 50972142), the Knowledge Innovation Program of the Chinese Academy of Sciences (grant KJCX2-EW-H03-01), and the Chinese National Engineering Research Center for Optoelectronic Crystalline Materials.
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