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Abstract
Solid state lasers with a planar Nd,Lu,Ga:YAG/YAG waveguide structure have been created using liquid phase epitaxy. The method used to produce such epitaxial structures ensures obtaining Nd3+ ions concentration in the range from 1 at.% to 3 at.% in waveguiding thin films. Spectroscopic characterization of active films was performed. The Nd,Lu,Ga:YAG waveguide for each neodymium ions concentration exhibited lasing operation at room temperature at a wavelength of 1064 nm with impulse laser diodes pumping at 808 nm and 885 nm. The maximum average output power Pav = 2.6 mW and 72.2% slope efficiency was obtained for 1 at.% Nd3+ waveguide pumped at 885 nm.
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1. Introduction
In modern optoelectronic and telecommunications systems, control and measurement equipment or medical devices various kinds of lasers play an increasingly important role. The technological progress observed in recent years leads to miniaturization of laser devices in which, among others, solid state lasers are often used. One of the most interesting and very useful achievements in the field of optoelectronic components miniaturization is the development of diode-pumped planar waveguide and microchip solid state lasers. A planar waveguide microlaser is formed by a thin active layer usually deposited in the epitaxy process on an inactive substrate [1,2]. The waveguide that can confine and propagate the light is formed in this case by an epitaxial layer having high refractive index deposited on substrate with lower refractive index. The waveguide structures offer significant advantages over bulk lasers, such as lower generation threshold, small device dimensions, more efficient heat dissipation and high laser beam quality. A typical microchip laser consists of a thin plate of a laser material with resonator mirrors deposited directly on both plate sides [3,4].
The history of garnet planar laser structures started in 1972. This year Van der Ziel and his coworkers obtained, in epitaxial Ho3+ doped yttrium aluminum garnet (YAG) layers, generation at a wavelength of 2.1 µm with a xenon lamp as a pump source [1,2]. In the following years, a few papers were focused on technology and generation properties of Nd doped epitaxial YAG thin films [5–7]. However, the difficulty of development of efficient semiconductor laser diodes as pumping sources had stopped development of waveguide solid state lasers for more than a decade.
In the 1990s, attention had been directed to epitaxial Nd3+ [8], Yb3+ [9] and Tm3+ [10] doped YAG waveguide structure pumped AlGaAs and InGaAs laser diodes. The results presented in papers [8–10] show a significant progress in the development of diode-pumped waveguide monocrystalline YAG infrared lasers. Trivalent rear-earth ion (RE3+) doped active YAG waveguides were prepared using liquid phase epitaxy (LPE). The spectroscopic parameters of RE3+:YAG layers are comparable to parameters of best quality RE3+:YAG single-crystals grown in the standard Czochralski process. The high quality of epitaxial YAG waveguide laser structures was confirmed by high slope efficiency ∼ 70% (Tm:YAG), low generation thresholds of < 5 mW (Nd:YAG) and low-loss value of < 0.05 dB/cm (Nd:YAG) [8,9]. Apart from the LPE method, several other fabrication techniques of planar waveguide dielectric lasers are employed depending on host material. Planar garnets waveguide laser structures can also be successfully produced) using pulsed laser deposition (PLD) [11] and ion implantation into crystal matrix [12]. In the case of transparent laser ceramics, currently the most promising material for solid-state lasers, the creation of low-loss planar waveguides is provided by the type casting [13].
In recent years, particular progress has been made in the field of planar waveguides based on transparent ceramics, especially those fabricated by tape casting method. For example, in a diode pumped Nd:YAG ceramic planar waveguide produced by tape casting the maximum slope efficiency of 63% [14] and 65% [15] was achieved in continuous wave operation mode. The synthesis of transparent laser ceramics, like the liquid-phase epitaxy, ensures the fabrication of Nd:YAG material with stoichiometric composition, excellent optical quality and Nd ions concentration much higher than 1 at. % typical in Nd:YAG laser crystals. The comparison of laser performances of Nd:YAG planar waveguides produced by two such different technologies seems to be reasonable.
The liquid-phase epitaxy method allows to grow Nd:YAG layers in which the concentration of neodymium ions can reach of about 10 at, % [16,17]. However, studies of active Nd:YAG waveguides have been limited to neodymium ions concentrations not exceeding of about 1.5 at. %, i.e. the most suitable for lasing operations. At higher concentrations, the interaction of Nd ions cause concentration quenching. For the spectroscopic studies and laser generation experiments, we have decided to fabricate epitaxial waveguide Nd;YAG layers with a higher concentration of neodymium ions. Such layers would provide an excellent comparison to ceramic Nd:YAG laser waveguides. We believe that, for LPE Nd:YAG epitaxial waveguides with both low and obviously high levels of Nd ions doping, the effect of direct pumping at 885 nm into the emitting level on laser generation parameters has not yet been determined.
The aim of this work was to fabricate YAG waveguide layers containing active Nd3+ ions with concentrations of 1 at. %, 2 at. % and 3 at. % by means of liquid phase epitaxy and to determine the structural, spectroscopic and generation properties of obtained waveguide structures. Generation experiments confirmed the capability of obtaining active laser material in the form of waveguide layers with different Nd3+ ions concentration. The expected generation at a wavelength of 1064 nm was observed and the efficiency of laser diode pumping at wavelengths 808 nm and 885 nm was compared. The performance of epitaxial Nd:YAG waveguide lasers was investigated in terms of threshold pump power and slope efficiency. The results were compared with those obtained in ceramic waveguide lasers.
2. LPE growth of waveguide structures
The epitaxial layer and waveguide structures Nd:YAG/YAG were prepared using liquid phase epitaxy developed previously also in the Institute of Electronic Materials Technology (now Łukasiewicz Research Network - Institute of Microelectronics and Photonics) [17–20]. The equipment for LPE growth of garnet layers was constructed at IEMT. The layers were grown from a supercooled molten garnet-flux high temperature solution. Garnet (Nd:Y3Al5O12) constituent oxides (Al2O3, Y2O3 and Nd2O3) were dissolved in flux (PbO-B2O3) to form a high temperature solution. Standard isothermal LPE dipping technique with substrate reversed axial rotation has been utilized to grow garnet layers. Using this technique Nd:YAG epitaxial layers were grown on both sides of <111 > oriented, 550 µm thick, 20 mm diameter, polished undoped YAG substrates.
The realization of Nd:YAG thin-film waveguide on YAG substrate required an increase of the refractive index of epi-film nF compared to the substrate refractive index nS by a value of about 10−2 (at λ ∼ 1000 nm). The partial substitution of Al3+ ions by Ga3+ ions in YAG lattice causes an increase in the refractive index of crystal. But, due to the larger gallium ion radius such substitution in YAG increases a lattice constant compared to the substrate lattice constant aS. Thus, it is necessary to compensate the increase of the layer lattice constant aF by introducing non optical active Lu3+ ion with ion radius smaller than the Y3+ ion radius in yttrium sites.
In the YAG waveguide layer some of the yttrium ions in the dodecahedral position are replaced by other lanthanide ions, such as lutetium or neodymium, and gallium ions replace aluminum ions in the tetrahedral and octahedral position, The value of waveguide refractive index nF is higher than the value of substrate refractive index nS. All substituting ions cause an increase of the nF value . The increase in the value of the refractive index for YAG (at λ = 633 nm) versus the concentration of selected dopant is presented in the dissertation [16]. The increase of concentration of Nd, Lu and Ga ions in the YAG layer by 1 at. % leads to the increase of refractive index value by 0.42 × 10−3, 0.095 × 10−3 and 1.1 × 10−3, respectively. The influence of gallium ions is most significant. Using the data shown above (for λ ∼ 1000 nm these values are slightly lower by about 2%) we have estimated Δn = nF - nS in obtained waveguides. The validity of such method of Δn estimating was confirmed by our earlier investigations concerning the measurement of nF and nS using Pluta’s double-refracting interference microscope [17].
The composition of waveguide layers was specified based on the results of high-resolution X-ray diffraction measurements (HRXRD). The fractional mismatch Δa/aS (where Δa = aS – aF) between layer and substrate was calculated from X-ray rocking curves, From fractional misfit Δa/as the concentration of Nd3+, Lu3+ and Ga3+ ions in Nd,Lu,Ga:YAG waveguides was estimated using the empirical formulas that allow to calculate the YAG lattice constants depending on the concentration of ions occupying the sites of Y and Al ions [21].
According to XRD measurements, the value of Δa in obtained waveguide structures varies from - 0.001 Å (1 at. % Nd ions) to – 0.008 Å (3 at. % Nd ions), which means that the Nd,Lu,Ga:YAG waveguide films were in slight compression (aF > aS) and had exhibited an elastic accommodation to the YAG substrate [17]. The neodymium, lutetium and gallium ions in fabricated waveguides causes the lattice constants mismatch Δa, which falls in the range from – 0.02 Å to + 0.01 Å. This mismatch range provides growth of the monocrystalline layer [22,23].
The Nd,Lu,Ga:YAG/YAG structures for planar waveguide lasers were obtained from the melts of the specified proper composition. The concentration of gallium ions in all waveguide layers was estimated at about 12 at %. With an increase in the concentration of Nd ions, the concentration of Lu ions decreased. The concentration of Lu ions was about 30 at. % for Nd 1 at. %, ∼ 28 at. % for Nd 2 at. % and ∼ 26 at. % for Nd 3 at. %, respectively. For example, the composition of the Nd,Lu,Ga:YAG layer with Nd ions concentration of 3 at. % can be presented as follows: Y2.13Nd0.09Lu0.78Al4.4Ga0.6O12.
The calculated increase of Δn values of about 1.6 × 10−2 for all Nd,Lu,Ga:YAG layers proves that we have grown waveguide layers.
3. Spectroscopic characterization
The thin epitaxial Nd:YAG layers and Nd,Lu,Ga:YAG waveguide layers deposited on YAG substrates were investigated. LPE technology has granted us access to the set of samples with active Nd3+ ions concentration in the range from 0.5 at. % to 3 at. %. The thickness, activator concentration and the type of layers used in the spectroscopic measurements are summarized in Table 1.
Table 1. Tested layers – basic data
Emission spectra were measured in room temperature using Acton SpectraPro 2300i grating monochromator. In all cases the samples were excited by laser diodes generating at wavelengths of 808 nm and 885 nm. The normalized emission spectra of neodymium doped active layers are presented on Fig. 1.
Fig. 1. Emission spectra of Nd:YAG and Nd,Lu,Ga:YAG layers: a,c - excitation at 808 nm and b,d – excitation at 885 nm.
The analysis of recorded emission characteristics allows us to notice only subtle differences in the spectra shown in Figs. 1(a),(b). By limiting the spectrum registration range to the region close to the wavelength of expected laser action (λ = 1064 nm) a shift of the position of the main emission peaks for waveguide layers by about 1 nm can be observed (Fig. 1(c),(d)). The wavelength shift is evidently caused by gallium and lutetium ions introduced into the waveguide layer. The lutetium and gallium ions in Nd:YAG crystal contribute to the shift of the 4F3/2 emission wavelength in opposite directions. For Nd:Lu3Al5O12. crystal, compared to Nd:YAG, the 4F3/2 emission wavelength increases by 0.1 nm while for Nd:Y3Ga5O12 it decreases by 1.5 nm [24]. The concentration of neodymium ions in the compared crystals was about 1 at. %. The above mentioned dependencies explain the position shift of the emission peak related to the 4F3/2 → 4I11/2 transition toward shorter wavelengths. However, for the concentration of Ga ions around 12 at. % a smaller peak shift would be expected.
The introduction of Lu and Ga ions with different ionic radii compared to the basic Y and Al cations in the YAG lattice not only changes the lattice constant but can also cause local randomly distributed distortions and stresses. These local lattice perturbations affect the crystal field around optically active Nd ions. The consequence of such micro-scale disturbances is the emission of a series of lines with slightly different frequencies. As a result, a broadening of the peak emitted by the Nd,Lu,Ga:YAG waveguide layers can be observed.
A fundamental parameter characterizing the laser crystal is the fluorescence lifetime. The lifetime value depends on the interaction of the active ion with the crystal lattice and the interactions between the active centers. The fluorescence lifetime of 4F3/2 emitting level of Nd3+ ion was measured at room temperature using laser diodes oscillating at 808 nm and 885 nm as excitation sources. The recorded fluorescence dynamics profiles depending on Nd3+ ions concentration are shown on Fig. 2. The measured lifetimes are presented in Table 2.
Table 2. The 4F3/2 level fluorescence lifetimes t under 808 nm and 885 nm laser diodes excitation.
The estimated 4F3/2 level fluorescence lifetime for the lowest concentration of Nd3+ ions in YAG epitaxial layers is about 275 µs. With increasing neodymium ions concentration fluorescence decay times decrease as a result of concentration quenching. The measured values of the lifetimes in Nd:YAG layers and Nd, Lu, Ga:YAG waveguides reported in Table 2 for both applied IR excitation wavelengths are close within the measurement error. The excitation wavelength does not affect the fluorescence lifetime of emitting level. The recorded lifetimes are higher compared to literature data for low and highly doped Nd:YAG crystals [25,26].
4. Generation properties and discussion
Generation experiments were performed for samples of planar Nd,Lu,Ga:YAG/YAG waveguide structures typically of 7.0 × 10.0 mm cut from wafers with deposited epitaxial layers. A YAG over-cladding layer was not grown on the top surface of the waveguide layer so the measurements were taken on an asymmetrical waveguide formed by: air – Nd,Lu,Ga:YAG layer – YAG substrate. The surface of the end edges of the rectangular samples were polished to laser quality.
As stated above, the estimated refractive index difference between the Nd,Lu,Ga:YAG layer and YAG substrate of about 1.6 × 10−2 (λ = 1000 nm) indicates the creation of a planar waveguide structure in which, for layer thickness in the range of about 60 µm to 90 µm, multi-mode generation can be expected. The laser resonator was formed by depositing input mirror (HR = 100% at λ = 1064 nm, HT > 90% in the range of λ = (800 - 900) nm and output mirror (HR > 99% in the range of λ = (800–900) nm, HT > 3% at λ = 1064 nm) on the polished end faces of the waveguide sample as shown in Fig. 3. The scheme of the experimental setup used to obtain laser oscillation is sketched also in Fig. 3.
Fig. 3. Laser setup and edge photography of the Nd,Lu,Ga:YAG/YAG/Nd,Lu,Ga:YAG (2 at. %) structure with 60 µm thick waveguide layers with deposited mirror coatings: a) Laser setup: 1.output mirror, 2. active materials, 3. cooling’s system, 4. input mirror, 5. coupling optics, 6. laser diode pump, b) input mirror.
It should be noted that for the first time, as the authors of this paper believe, an internal resonator arrangement in the form of specially designed mirrors deposited on polished edges of the waveguide structure was used in the study of the generation of Nd:YAG epitaxial waveguide.
The experiments were carried out with classic end pumping setup with a pump beam formed by two plane-convex lenses with focal length of 20 mm. Two types o laser diodes were used as pump sources a Spectra Physics SDL-3490 laser diode operating at 805 nm and a diode manufactured at the former Institute of Electronic Materials Technology generating at 885 nm [27], respectively. In order to reduce the thermal effects on the active material in tested structures pulsed operating was used for both pumping wavelengths. The samples covered on both sides by indium tape were mounted on a thermoelectrically cooled XYZ stage by means of a properly profiled cooper clamp to ensure uniform heat dissipation during generation experiments.
To deliver the pump power the beam generated by the laser diodes was focused in the resonator about 1 mm downstream of the input mirror to a spot about 100 µm in diameter. Figure 4 shows for the tested waveguide structures the relationships of average output power versus the average input power.
Fig. 4. Laser average output power versus average input pump power for Nd,Lu,Ga:YAG/YAG waveguide structures under pulsed laser diode pumping at: a) 808 nm, b) 885 nm.
The comparison of the observed laser parameters for waveguide with different concentrations of Nd3+ ions is presented in Table 3. The lasing operation was observed at 1064 nm for all tested waveguide samples when pumped at 808 nm and 885 nm.
Table 3. Slope efficiency η, incident threshold power Pth and maximum average output power Pav under pulsed laser diode pumping at 808 nm and 885 nm wavelengths
The maximum value of average laser output power Pav = 2.6 mW with the slope efficiency η = 72.2% was recorded for the 1 at. %-doped sample at excitation at λ = 885 nm. For the same sample when pumped at λ = 808 nm the maximum value of Pav and η were recorded as 1.5 mW and 33.8%, respectively.
An increase of the concentration of active neodymium ions causes, in each case, a decrease of conversion efficiency and reduction of output power. The influence of Nd3+ ions concentration increasing on the degradation of laser performance is significantly greater for pumping at 808 nm to the strong absorption level of 4F5/2. The parasitic quantum defect between the pump and laser emission wavelength contributes to laser performance degradation. According to the data presented in Table 3 pumping at λ = 885 nm directly to laser upper level 4F3/2 for all samples reveals the favorable influence of this means of pumping on laser emission threshold, output power and slope efficiency. The excitation to the 4F3/2 level of Nd3+ ion reduces the value of pumping beam absorbed power and, as a result of better energy matching and lower thermal losses in the laser resonator, leads to a significant increase of the slope efficiency. The highest values of average generated power of 2.6 mW and 2.0 mW were obtained at 1 at. % and 2 at. % Nd3+ concentration with comparable incident threshold power Pth values of 2.4 mW and 2.45 mW, respectively.
The comparison of the main features of laser emission obtained for various waveguides structures is difficult due to the use of different setups in their investigation. In Nd:YAG waveguide generation studies have been applied different resonator configurations, pumping diodes of different powers, out-coupling mirrors of various transmittance values and have employed several ways of introducing the pumping beam into the waveguide. Therefore, we accepted the slope efficiency as a parameter that allows to quite objective comparison of Nd:YAG waveguide lasers fabricated by different technologies.
The conventional pumping into the strongly absorbing 4F5/2 level led to weaker laser parameters compared to waveguide ceramic lasers. The slope efficiency of 1 at. % Nd,Lu,Ga:YAG waveguide laser under 808 nm pumping was 33.8%. For higher concentrations, lower slope values were recorded. Probably the use an output mirror with a higher transmittance could increase the slope efficiency.
In a diode pumped at 808 nm YAG/Nd:YAG/YAG ceramic planar waveguide with Nd doping concentration of 2. at.% produced by tape casting, the maximum slope efficiency of 63% [14] and 65% [15] was achieved in continuous wave operation mode. For comparison, the slope efficiency in a buried channel laser waveguide fabricated in a Nd:YAG (2 at. %) ceramic by femtosecond laser writing is about 60% [28]. The comparable slope efficiency value of 61% was obtained for a Nd:YAG (1 at. %) ceramic novel bonded planar waveguide laser [29].
The pumping at 885 nm directly to the laser emitting 4F3/2 level caused a significant increase in slope efficiencies of Nd,Lu,Ga:YAG waveguide lasers for all neodymium ions concentration. The highest slope efficiency at 885 nm pumping were obtained in 1 at. % waveguide. This laser slope efficiency of 72.8% is one of the highest reported not only for a Nd:YAG epitaxial waveguide laser. For the 1 at. %, 2 at. % and 3 at. % Nd,Lu,Ga:YAG waveguides the slope efficiencies are over 2 - 3 times larger than achieved under 808 nm pumping. The slope efficiency for a 2 at. % waveguide has a value of 56% and is comparable to the best laser slope efficiencies reported from 2 at. % ceramic waveguides under 808 nm pumping.
The favorable effects of resonant pumping at 885 nm into 4F3/2 level for a 1 at.% Nd:YAG laser bulk crystal were presented by the authors of the paper [30]. In this case the slope efficiency of 72,4% was achieved with pumping by a fiber-coupled laser diode.
A comparison of our results with the laser parameters obtained for Nd:YAG laser crystals with almost the same concentration of neodymium ions CNd proved interesting and useful information in relation to the influence of CNd on laser performance. For the 1 at. % Nd:YAG crystal sample the slope efficiency was 54%, the threshold of absorbed power was 31 mW. The increase of CNd causes the deterioration of laser performances. For the 2,4 at. % and 3,5 at. % samples slope efficiency was 50% and 48%, respectively and the threshold absorbed power increased to 45 mW and 65 mW [31].
In low and highly doped (1 at. %, 2.4 at. % and 3.5 at. %) Nd:YAG crystals, the character of changes caused by an increase of the neodymium ions concentration is similar to that observed in Nd,Lu,Ga:YAG waveguides we investigated. As the CNd increases, the slope efficiency decreases. The authors of paper [31] explained the influence of concentration on the laser parameters in relation to the emission quantum efficiency. The quantum emission efficiency calculated from fluorescence decay also depends on the concentration of Nd ions. The increase of CNd causes the reduction of emission quantum efficiency, which is not compensated by the enhanced absorption. As a result, there will be a decrease in laser performance. We believe that such a far-simplified description can explain the influence of increased concentration of neodymium ions on laser performance observed in Nd,Lu,Ga:YAG waveguide layers.
5. Conclusions
In this paper we report on the laser emission in low and highly doped Nd,Lu,Ga:YAG epitaxial laser waveguides diode pumped at 808 nm and 885 nm. In order to obtain a proper material for investigation we have successfully grown, by means of LPE technique Nd:YAG epitaxial and Nd,Lu,Ga:YAG waveguide layers with Nd3+ ions concentrations of 1 at. %, 2 at. % and 3 at. %. respectively.
The laser generation at wavelength of 1064 nm was achieved in Nd,Lu,Ga:YAG waveguides for all Nd3+ ions concentrations and both pumping wavelengths. The performed studies of laser generation in a Nd,Lu,Ga:YAG waveguide showed the beneficial effect of direct pumping into the emitting level on laser threshold and slope efficiency. Laser diode pumping at 885 nm relative to 808 nm is much more efficient and results in more than doubling the slop efficiency, which reaches 72.2% at about three times reduced threshold of absorbed average pump power. Such slop efficiency value of 72.2% obtained for the 1 at. % Nd,Lu,Ga:YAG waveguide is the highest ever reported for any Nd:YAG based waveguide laser.
We have demonstrated a high-efficiency low-power Nd,Lu,Ga:YAG epitaxial waveguide laser operating under 885 nm laser diode pumping. The presented results prove the potential of the epitaxial Nd,Lu,Ga:YAG waveguides to realize efficient micro-laser sources pumped by diode lasers.
Funding
The National Centre for Research and Development (N N515 342036).
Acknowledgements
The authors would like to thank Professor R. Buczyński for helpful discussions
Disclosures
The authors declare no conflicts of interest.
Data availability
The data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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