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A Faraday isolator based on a new magneto-optical medium, TSAG (terbium scandium aluminum garnet) crystal, has been constructed and investigated experimentally. The device provides an isolation ratio of more than 30 dB at 500 W laser power. It is shown that this medium can be used in Faraday isolators for kilowatt-level laser powers.
© 2014 Optical Society of America
1. Introduction
Lasers with high average output power find application in different fields of activity: medicine, industry, space. They are actively used in numerous scientific projects such as ultrabright sources of radiation construction (ELI), inertial confinement fusion facilities (NIF, HiPER, Genbu), gravitational wave detection (LIGO, Virgo, Einstein Telescope) etc. The average output power of continuous wave and repetitively pulsed lasers is steadily growing, making it increasingly important to reduce thermal effects arising in various optical elements due to radiation absorption. Faraday isolators are essential parts of such laser systems as they prevent unwanted feedback and ensure safe operation of system. They are strongly affected by thermal self-action due to relatively high absorption (~10−3cm−1) in their magneto-optical elements (MOEs). Non-uniform cross-sectional temperature distribution leads to radiation wavefront distortions (thermal lens) and to linear birefringence caused by mechanical stresses in the MOE bulk (photoelastic effect). In high power lasers, the isolation ratio of FI is totally determined by the level of thermally-induced depolarization [1].
The length of a MOE and hence the value of absorbed power are given by the condition of a 45-degree rotation of the polarization plane of passing radiation. So the value of the Verdet constant of the magneto-optical medium determines, to a large extent, the main characteristic of a FI – its isolation ratio at maximum permissible radiation power. For years, there has been a search for crystals with effective Faraday rotation [2,3]. Promising materials for FIs today are various garnets as they combine the advantages of high Verdet constant, low absorption and good thermo-optical properties. Among garnets, the terbium aluminum garnet (Tb3Al5O12) TAG has the highest Verdet constant and high transmittance [4]. However, great difficulties are encountered in growing a TAG single crystal of acceptable aperture, because of its incongruent melting nature and unstable TAG phase in the Tb2O3–Al2O3 system [5]. The way out was found in the producing of TAG ceramics [6], which now begins to be used in FIs [7,8]. However, the most widely used garnet in FI today is the terbium gallium garnet TGG (Tb3Ga5O12), whose Verdet constant is significantly lower than that of TAG.
In paper [4] it has been claimed that the Verdet constant of TAG is 30% higher than that of TGG, but the graph presented in this paper leaves it unclear to which wavelength range this statement refers. The graph also gives no information about the precise value of the Verdet constant for the 1076 nm wavelength, which is of particular interest to us. In paper [6] the Verdet constant of TAG ceramics for a wavelength of 632.8 nm was measured to be 172.72 rad*T−1*m−1. Using the approximation of the dependence of the Verdet constant on the wavelength in the form V~1/(λ2- λ02) and the reported in [9] value of λ0 = 305 nm for TAG, the Verdet constant for the wavelength of 1076 nm can be calculated to be 49 rad*T−1*m−1, which is 32% higher than that for TGG.
Another interesting medium with effective Faraday rotation is a terbium scandium aluminum garnet (Tb3-xScx + yAl5-yO12) TSAG. Its advantage over the TGG crystal is a ~20% higher Verdet constant (the precise value depends on the content of scandium) and over the TAG crystal is the possibility of growing large-aperture single crystals of good optical quality [10]. All these make the TSAG a promising candidate for MOE in high power Faraday isolators.
In our paper we report the results of fabrication and experimental study of a Faraday isolator for high average power lasers based on a Tb2.88Sc1.69Al3.43O12 crystal grown at GPI RAS (Russia) by the growth procedure described in [11]. Predictions on the prospects of using this medium in kilowatt power level FIs are made.
2. Experimental results
A Faraday isolator for a wavelength of 1076 nm based on the TSAG crystal was fabricated using the traditional scheme [12]. The studied TSAG crystal is illustrated in Fig. 1 that shows its good optical quality. A permanent magnet system similar to that described in [13] was used to produce a magnetic field. This system was constructed as the set of Nd-Fe-B magnetic rings with axial and radial magnetization directions and it had the outer magnetic shield and internal pole concentrator made of magnetic conductor. The magnetic system had an aperture of 13 mm and provided a field intensity of 25 kOe in the central region. Since the Verdet constant of TGG crystal is 22% lower than that of the sample used in our study for the given wavelength, the MOE was made shorter, only 7 mm in length, to ensure the required 45 degree angle of rotation of the polarization plane.
The ability of using such short MOE is quite promising. As it clear from [13] main characteristics of FI produced using such MOEs can already compete with the characteristics of FIs with compensation of thermally induced depolarization [14].
The experimental investigation of the isolation ratio of the device was made with the help of the measurement optical scheme shown in Fig. 2. A cw linearly polarized beam from a ytterbium fiber laser 1 (produced by IPG Photonics) operating at a wavelength of 1076 nm was used as the source of heating and probing radiation simultaneously. The beam passing through a FI consisting of a MOE 4 placed inside a magnetic system 5, was split by a quartz wedge 6. The main part of the beam then went to a light absorber 7, whereas the other part attenuated by a factor of ~103 was directed to a Glan prism 8 fixed in an optical table with angle scale and was then recorded by a CCD camera 10. The beam recorded by the CCD camera 10 (power Pd) was polarized orthogonally to the main beam part (power P0) reflected by the Glan prism.
Fig. 2 Schematic of experimental measurements of thermal depolarization: 1 – ytterbium 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.
The depolarization of the orthogonally polarized beam is given by
and the isolation ratio, the most important characteristic of the FI, is measured in decibels, and is defined by
It should be noted that γ may be fully determined by both the thermal effects and the ‘cold’ depolarization that depends on the MOE quality and inhomogeneity of the magnetic field. As it shown in [1], the value of the thermally induced depolarization can be found by
where L is the length of the sample, λ is the wavelength, P is the laser power, is the thermal conductivity, and Q is the thermo-optical constant. In the general case, the proportionality coefficient in the Eq. (3) depends on the laser beam profile, orientation of the crystallographic axes in the MOE and the optical anisotropy parameter.
The characteristic structure of the depolarized component of radiation and the transmitted laser beam profile at a power of ~200 W are shown in Fig. 3. It is evident that the depolarized component distribution has a cross-like structure due to the photoelastic effect [15]. Good quality of the transmitted beam and cross-like structure of depolarized radiation component prove good optical quality of the sample (the absence of impurities, uniformity of radiation absorption and heat sink).
Fig. 3 Intensity of the depolarized component of radiation (a) and laser beam (b) used in experiment.
The results of measurements of the depolarization versus laser power in the FI are shown in Fig. 4 (diamonds). At low powers (below 100 W) γ is determined by the “cold” depolarization and is comparable with γ of the TGG crystal. Starting from 200 W, the depolarization dependence becomes quadratic and hence the isolation ratio at such powers is totally determined by the thermally induced depolarization. It can be seen that up to ~500 W an isolation ratio of more than 30 dB is provided.
Fig. 4 Thermally induced depolarization versus radiation power in Faraday isolators based on TSAG crystal (diamonds) and TGG crystal (squares).
For comparison, Fig. 4 (squares) shows the results of measurements for depolarization in a 9 mm length TGG-based MOE [13] incorporated in a traditional scheme FI with a similar magnetic system. Dashed lines indicate corresponding quadratic dependences obtained by approximating of experimental data. The experimental data fit well the quadratic dependence that satisfies the Eq. (3). The results for TSAG, despite the shorter crystal length, are worse, though slightly, than the results for a commercial TGG. To a certain degree, this can be due to a higher absorption coefficient in TSAG, which accounted to αTSAG = 2.5*10−3 cm−1 according to our estimates (αTGG = 1.3*10−3 cm−1 for TGG used in [13]). Advances in the crystal growth technology generally lead to improvement of optical quality of crystals and reduction of the absorption coefficient. As it can be seen from [16] and [13], in case of TGG the absorption coefficient of commercially available crystals was reduced by more than two times in the past years. A double decrease in the absorption coefficient of TSAG, as follows from Eq. (3) and shown in Fig. 4 (dash-dotted line), will allow the creation of a kilowatt-power-level FI based on the traditional scheme.
In addition, the orientation of the crystallographic axes of the studied TSAG sample was close to the [111] orientation, in contrast to the [001] orientation used in TGG, which is known [15] to be optimal with regard to the reduction of thermally induced depolarization. By using the [001] orientation it may be possible to reduce the thermally induced depolarization by a factor of ((2ξ + 1)/3)2 [15], where ξ is the optical anisotropy parameter. In the nearest future we intent to investigate the ξ parameter as well as other thermo-optical constants of the TSAG crystal, which, to our mind, is a very promising material for FI for high average power lasers.
3. Conclusion
In this study a Faraday isolator based on a new magneto-active medium - TSAG crystal - providing an isolation ratio of more than 30 dB at laser power up to 500 W has been constructed for the first time. Measurements and estimations demonstrate that with the expected reduction in the absorption coefficient of grown crystals and the use of optimal orientation of crystallographic axes it would be possible to construct a traditional FI for a laser power of more than 1 kilowatt.
Acknowledgments
This work was supported by the mega-grant of the Government of the Russian Federation No. 14.B25.31.0024 executing in the Institute of Applied Physics RAS.
References and links
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