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The Faraday isolator based on a new magneto-optical medium – TAG (terbium aluminum garnet) ceramics was implemented and investigated experimentally. The magneto-optical element was temperature-stabilized using water cooling. The device provides a stable isolation ratio of 38 dB at 300 W laser power. Estimates show high performance of the device at a kilowatt laser power.
© 2014 Optical Society of America
1. Introduction
With the steady increase in the average power of both continuous and pulse-periodic lasers, the problem of reducing the thermal effects arising in different optical elements due to radiation absorption becomes increasingly more important. Faraday isolators (FIs) are optical devices that are most subject to thermal self-action associated with relatively high absorption (~10−3 cm−1) in the magneto-optical elements (MOEs). Inhomogeneous temperature distribution caused by absorption gives rise, in addition to the Faraday effect, to linear birefringence (photoelastic effect) and to radiation wavefront distortions (thermal lens). In high-power lasers, the FI isolation ratio is fully determined by the level of thermally induced depolarization [1].
Along with an increase of the maximum power of modern laser systems, there is a tendency towards an increase of the laser beam diameter to avoid damaging of optical elements when exposed to high-energy pulse-periodic radiation. But the problem of creating a wide-aperture Faraday isolator that would provide a stable isolation ratio of high average power radiation was not solved until recently. MOEs based on magneto-optical glasses that are inferior to terbium gallium garnet (TGG) single crystals (most commonly used in modern FIs) in heat conductivity, Verdet constant and other thermo-optical parameters were usually used for wide-aperture FIs. The situation changed appreciably with the development over the last decade of optical ceramics technology. Optical ceramics is a class of materials which combines the advantages of single crystals and glasses. Its thermo-optical and magneto-optical parameters are similar to those of single crystals, but the diameter has no critical limits. For now laser ceramics technology permits the fabricating of high quality optical elements of tens centimeters in diameter [2]. The first important results on manufacturing magneto-active optical ceramics were reported in [3–6] and concerned TGG ceramics. The analog of traditional magneto-active media is terbium aluminum garnet (TAG) that surpasses TGG in the value of the Verdet constant. However, it is very difficult to grow TAG single crystals with acceptable aperture because of their incongruent melting nature and unstable TAG phase in the Tb2O3-Al2O3 system [7,8]. But still, considerable efforts have been made in growing TAG single crystals from the melts: Tm3+ [9], Lu3+ [10], Ga3+ [11], Sc3+ [12], and Yb3+ [13], etc. were doped to partially substitute Tb3+ or Al3+ to obtain a congruent melting composition or a stable TAG phase. Murata Manufacturing Co., Ltd. has grown out pure TAG fiber crystals (~4 mm in diameter) by the hybrid floating zone method [14]. A possible solution to producing a larger aperture material is the manufacturing of TAG optical ceramics – the benefit of transparent ceramics is obvious: since the fabrication process of transparent ceramics is free from melting, obtaining pure TAG phase is not a big problem. The first results in manufacturing TAG ceramic samples, as well as the study of the optical quality, microstructure, magneto-optical property, and thermal conductivity were reported in [15].
The TAG ceramics used in this study were manufactured by the Shanghai Institute of Optics and Fine Mechanics (SIOM). Firstly, Tb4O7 and Al2O3 powders of high purity (99.999%) were mixed according to the Tb3Al5O12 formula with ethanol by ball milling for 24 h. A compound of 0.4 wt.% Tetraethyl orthosilicate (TEOS) and 0.09 wt.% MgO, and 1 wt.% Polyethylene glycol 400 (PEG-400) were added as the sintering aid and dispersion aid, respectively. The slurry was dried at 80°C. After meshing, the powders were uniaxially pressed into pellets under 10 MPa. The pellets were subsequently cold-isostatically pressed (into green body) under 200 MPa. The green body was pre-sintered in air at 800°C for 3 h to remove the organic ingredients. Then vacuum sintering was carried out at 1650°C under a base pressure of 1.0 × 10−3 Pa. The ceramic samples were highly transparent [15] being 7-15 mm in diameter and 3-4 mm thick, no foreign inclusions were observed. The average ceramics grain size was ~15 μm.
In this paper we present details of the design and experimental study of a Faraday isolator for a high average power laser based on TAG ceramics. Forecasts about the prospects of using this medium in kilowatt power level FIs are made.
2. Experimental results
Figure 1 shows the cross-section of the FI developed in this study. For thermal stabilization of MOE 1 water-cooled copper holder 2 was used. To provide thermal contact with the holder the lateral MOE surface was wrapped in an indium foil. Magnetic system 3 used for the FI was similar to that described in [16]; it had an aperture of 13 mm and provided a magnetic field of 2.5 T in the MOE area. Therefore, the TAG ceramic sample had the length of 7 mm only (diameter = 7 mm). A specific property of the TAG ceramic sample used as a MOE is strong radiation scattering in a relatively large spatial angle, which is a source of parasitic heating of copper holder 2, and hence an additional source of depolarization. It is exactly this effect that necessitates water cooling.
Fig. 1 Cross-section of Faraday isolator based on optical TAG ceramics. 1 – MOE, 2 – thermally stabilized copper holder, 3 – Nd-Fe-B ferromagnetic alloy magnet system, 4 – calcite wedge, 5 – half-wave plate.
The isolation ratio of the manufactured device was studied experimentally using the optical measurement scheme shown in Fig. 2(a).The CW linearly polarized radiation of single-mode ytterbium fiber laser 1 at the wavelength of 1.07 microns (produced by “IPG Photonics”) was used both as heating and probe radiation. On passing through the FI consisting of MOE 4 placed in magnetic system 5, the radiation was divided by quartz wedge 6. The main part of the radiation was passed to absorber 7, and the remainder, attenuated by a factor of 103, was propagated to two measurement arrangements. In the case of depolarization measurements, Fig. 2(a), the radiation arrived at Glan prism 8 fixed in an optical table with angle scale and was recorded by CCD camera 10. The radiation recorded by camera 10 (with Pd power) is orthogonally polarized with respect to the main part of the radiation (with P0 power) reflected by the Glan prism. The radiation depolarization is given by Eq. (1)
and the isolation ratio I – the most important characteristic of the FI – is measured in decibels and is defined by Eq. (2)Fig. 2 Schemes for experimental measurements: a) thermally induced depolarization, b) thermal lens, 1 – Yb-doped fiber laser, 2 – telescope, 3 – calcite wedge, 4 – MOE, 5 – magnetic system, 6 – quartz wedges, 7 – absorber, 8 – Glan prism, 9 – measuring lens, 10 – CCD camera.
In the case of thermal lens measurements, the radiation passed through collecting lens 9 with focal length 504 mm, as shown on Fig. 2(b). Then, by moving CCD camera 10, we determined the position of the minimum cross-section of the beam (beam waist) with and without FI. Thermal lens focal length was calculated by the shift of the waist position.
The results of measuring the dependence of depolarization in the FI described here on laser power are presented in Fig. 3 (circles). Starting from 100 W, the depolarization dependence is quadratic, hence at a laser power higher than 100 W the isolation ratio is fully determined by thermally induced depolarization (Fig. 3, lines derived from the Eq. (3)), but remains higher than 38 dB up to 300 W.
Fig. 3 Depolarization versus radiation power in Faraday isolator based on TAG ceramics (circles) and TGG ceramics (squares) described in [16] and theoretical dependences (lines).
Results of the depolarization measurements in the MOE based on TGG ceramics in the FI with the same magnetic system [17] are given for comparison in Fig. 3 (squares). The depolarization level in the TGG-based FI is significantly higher (~5 times) than in the TAG-based FI. There are several reasons for that. First, because of the lower Verdet constant, MOE made of TGG is longer (9 mm); second, the thermal conductivity coefficient of TAG ceramics is higher (6.5 W/m/K [15]) than that of TGG (5 W/m/K). Also, the TAG ceramics sample is likely to have a lower absorption coefficient than the TGG ceramics sample (1.4∙10−3 1/cm [17]). To assess the absorption coefficient of the TAG ceramics sample we will make use of the equation for the thermally induced depolarization in optical ceramics [18, 19]:
where ξ is the optical anisotropy parameter, A is the beam shape parameter (A = 0.137 for the Gaussian beam [20]), p is the normalized power given by Eq. (4)where L is sample length, λ is wavelength, Pl is laser power, and Q is the thermo-optical constant responsible for thermal depolarization. The absorption coefficient α0 of the TAG ceramic sample was assessed assuming that the values of ξ and Q are get to the range between the corresponding parameters of popular garnets TGG and YAG, because of the fact that TGG, TAG and YAG have very similar chemical structure. For a TGG crystal, Q is equal to −17∙10−7 1/K [21], for YAG: 7.2∙10−7 1/K [22] and optical anisotropy parameter for a TGG medium ξ = 2.25 [23], and for a YAG medium ξ = 3.2 [22]. Estimates show that the absorption coefficient of the TAG ceramics sample is within the range of 0.9 to 1.5∙10−3 1/cm, which is not worse than commercially available TGG crystals.The measured optical strength of the thermal lens (Ft) in the TAG-based FI tested here as a function of the power of laser radiation with 2.3 mm beam diameter (by the intensity level of 1/e2) is plotted in Fig. 4.
Fig. 4 Experimental (dots) and theoretical (line) thermal lens power in the TAG-based FI versus laser power.
The focal length of thermal lens in the FI based on the TAG ceramics was 13 m at 180 W. At 300 W, it is predicted to be 8 m; such distortions can be relatively easily compensated by a system of spherical lenses.
Figure 5 shows the time dependence of the angle of rotation of the polarization plane φ at 300 W radiation power. As expected, after turning on the power, the angle firstly decreasing due to the temperature dependence of the Verde constant and after the several minutes reaches the thermal equilibrium when the heat flow through the lateral surface contact “TGG-holder” become equal to the absorbing heat power. The results indicate stable operation of the device 4 minutes after switching on of the laser radiation. This time period could be shorten with the lateral surface thermal contact improving, or making the face-end heatsink.
3. Conclusion
The FI based on the new magneto-active medium – optical TAG ceramics – providing an isolation ratio better than 38 dB at laser power up to 300 W was demonstrated for the first time. The focal length of the thermal lens arising in the device is more than 8 meters. Stable operation of the device is achieved 4 minutes after switching on of 300 W radiation.
Predictions show that the described FI will provide a stable 30 dB isolation ratio of sub-kilowatt radiation power which compares well to the best FI samples with a traditional MOE made of TGG crystals. We are now preparing for publication a paper where the possibility of elaborating TAG-based FI at the laser power of several kilowatt is studied.
Acknowledgment
This work was supported by the mega-grant of the Government of the Russian Federation No. 14.B25.31.0024.
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