The recent dynamic development in the field of high-energy, high-average-power pulsed lasers, delivering pulse energies ranging from tens to more than 100 J at the repetition rates from units to tens of hertz, started the hunt for various optical components and devices capable of functionality in such powerful laser beams [1–4]. These lasers are aiming at the application areas spreading from advanced material processing, laser shock peening, laser induced damage threshold testing, inertial confinement fusion, or as pump lasers for high-repetition femtosecond petawatt laser systems [1,5–7]. The usage of lasers for the applications always brings the risk of unwanted back-reflections coming back from the experimental area to the power amplifier chain. These back-reflections may cause undesirable interferences, disturb the laser operation or, in the worst-case scenario, severely damage the optical components. Therefore, it is vitally important to protect the laser with a nonreciprocal device—the optical isolator [8].

The optical isolator is usually installed at the very end of the amplifier chain of the whole system and, therefore, it needs to withstand the highest pulse energies produced by the laser system. The reason is to prevent the back-reflected pulse from entering the main amplifier chain and be potentially amplified even more by the unextracted stored energy. The back-reflected pulse can be especially dangerous for the lower energy parts of the laser such as preamplifiers or even front-end. This location of the device poses highly demanding criteria on the isolator’s aperture size, laser damage threshold, and robustness from the point of view of the thermo-optical phenomena. Although there are several ways how to implement a nonreciprocal optical device [9], the current state-of-the-art of these techniques does not allow them to meet these highly demanding high-power-related criteria in any other type of medium than in the magneto-optical materials, by using the Faraday effect.

Only the Faraday isolators have been so far successfully operated at the high average powers, sometimes even exceeding $1~\mathrm {kW}$ [10]. Nevertheless, all of these previously reported devices had small apertures and have been designed and tested just for continuous-wave laser operation. Concerning the large-aperture Faraday devices, the only mention of them we were able to find in the available literature were designed for a single-shot laser [11]. In this Letter, however, we report on the first-ever demonstration of optical isolation based on the Faraday effect for a large-aperture kilowatt average power high-energy pulsed laser and, in a broader sense, the first-ever demonstration of a nonreciprocal optical device operated with a high-energy, high-average-power laser beam. The Faraday isolator has been developed to protect the Bivoj/DiPOLE100 laser system operated in HiLASE Centre and developed by the Science and Technology Facilities Council (STFC) [2] capable of delivering laser pulses with the energy exceeding $100~\mathrm {J}$ [4] at the repetition rate of $10~\mathrm {Hz}$. The final power amplifier is based on the cryogenically cooled Yb:YAG ceramics slabs amplifying the seed pulses at the central wavelength of $1030~\mathrm {nm}$. The pulse duration and shape are adjustable, around the typical duration of $10~\mathrm {ns}$. The beam profile is a high-order super-Gaussian with the $78\times 78~\mathrm {mm}^{2}$ (FWHM) dimensions. The demand on the minimal isolation ratio of the developed isolator (denoted as FI2 in Fig. 1, showing the simplified scheme of the Bivoj laser system) is given by the amplification of the main power amplifier head (MA2) and was set to $20\,\mathrm {dB}$. This value ensures that the potential back-reflection from the experimental area will not exceed the pulse energy produced by the first main amplifier (MA1) after it is amplified during the back-propagation. Such an energy level is safe for the first Faraday isolator (FI1), which separates the two main amplifier heads.

 

Fig. 1. Location of the developed Faraday isolator (FI2) in the Bivoj laser amplifier chain. Here FE+PA refers to the laser front-end and the power amplifiers, MA1 and MA2 denote the main amplifiers, FI1 and FI2 stand for the Faraday isolators. The values under the main amplifiers provide information about the output pulse energy and the size of the square-shaped beam.

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The working principle of the Faraday isolator is the nonreciprocal polarization rotation of light as a result of applying an external magnetic field on a magneto-optical material. The magnetic field for the FI2 Faraday isolator is provided by an actively shielded solenoidal superconductive magnet (Cryomagnetics, Inc.), which was optimized to provide a tunable axial magnetic field with the strength of up to $3.5~\mathrm {T}$, generated inside a large-diameter bore ($\emptyset 160$ mm). Similarly to Refs. [12–14] the purposely chosen high field strength allowed us to use much thinner magneto-optical elements, causing significantly lower thermal distortions to the passing laser beam. Furthermore, the generated magnetic field shows a high-level spatial homogeneity ($\sim 600$ ppm over a 14 cm diameter of spherical volume) over the central region of the solenoid, ensuring that exactly the same magnetic field is applied to the magneto-optical elements over the laser-incident area. As compared with a permanent magnet, which would be produced in similar dimensions with immense difficulties, the superconductive electromagnet provides a considerably stronger magnetic field and high-precision magnetic field control, including a complete change of the magnetic field direction.

The media used for the nonreciprocal polarization rotation are terbium gallium garnet (TGG) single crystals (Anhui Crystro Crystal Materials Co., Ltd.), shaped in the form of $69\times 69~\mathrm {mm}^{2}$ slabs, each of them 3.6 mm thick. The slabs with these dimensions represent the current technological limit, the largest obtainable crystalline magneto-optical media with the optical quality that is sufficient for this application. Due to this clear-aperture limitation of the available TGG crystals, the original output beam needed to be demagnified to $59\times 59~\mathrm {mm}^{2}$ before it enters the isolator. This increases the incident fluence to $\sim 3~\mathrm {J cm}^{-2}$, which is, according to our laser-induced damage threshold testing, a safe value for all of the dielectric multilayer AR coatings (Manx Precision Optics, Ltd.) applied on the slabs’ surfaces. For the designed thickness of the slabs, the optimal working point of the magnet would be around $3~\mathrm {T}$ in order to provide the desired 45$^{\circ }$ Faraday rotation. As it follows from our calculations, a further increase in the magnetic field strength (leading to a possible further reduction of the slab thicknesses) does not significantly improve the performance any more. The slabs are becoming very thin, making them very unsuitable for handling, and the slabs thinner than the used ones do not reduce the thermal issues significantly. The whole optical layout of the isolator is depicted in Fig. 2. Apart from the nonreciprocal magneto-optical media (TGGs), there is a quartz reciprocal $67.5^{\circ }$ polarization rotator (QR, Union Optic, Inc.) installed in between the two slabs to compensate for the thermal stress-induced birefringence [15]. The isolator further consists of a half-wave plate (HWP, Manx Precision Optics, Ltd.) placed in front of the entrance to the magnet’s bore, maintaining the input–output polarization plane orientation in the same direction, and two Brewster-type polarizers (P1 and P2, Optoman).

 

Fig. 2. Optical layout of the Faraday isolator: TGG, magneto-optical media made of TGG crystal, each delivering $-22.5^{\circ }$ Faraday rotation; QR, $+67.5^{\circ }$ quartz reciprocal rotator; HWP, half-wave plate; P1 and P2, polarizers. The cyan arrows are indicating the directions of the cooling airflow through thin ducts; the rotation angles are defined for the open direction of the isolator (in source view notation).

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The TGG slabs are actively cooled by dry air flowing through thin ducts between the slabs and the reciprocal rotator. The cooling was designed on the basis of the finite element method simulations (COMSOL Multiphysics) of the heat exchange between the slab and the cooling air. The simulations led to the overall heat deposition of $\sim 0.5~\mathrm {W}$ which resulted to the maximal temperature difference within the TGG slab $< 1~\mathrm {K}$. The polarization and wavefront distortions caused by the heating of the slabs were evaluated using our in-house developed codes for thermo-optical calculations [16–18]. This thermo-optical and magneto-optical design is based on the detailed knowledge of the material properties of the TGG crystal, especially its temperature-dependent Verdet constant [19,20], thermo-optical coefficient [21,22], and thermo-mechanical properties [22–25]. Based on the calculations, the optimal air-flow velocity which ensures sufficient heat removal was found to be $36~\mathrm {m s}^{-1}$. In that case, the pressure of the cooling air should be maintained just at the level which provides sufficient airflow velocity, somewhere between 1 and 2 bars.

Placement of the Faraday isolator in the laboratory is shown in Fig. 3 the left-hand photograph shows the input side, whereas the right-hand photograph shows the output side of the isolator. The laser beam propagates from the left, first going through the polarizer P1 and the HWP, then it enters the magnet’s bore containing the magneto-optical media and the reciprocal rotator fixed in the air-cooled optomechanical mount. The mount is attached to the magnet’s body on the output side, just before the output polarizer P2. The beam reflections from the polarizers are captured into beam dumps (BD) placed in the proximity of the P1 and P2 polarizers. The photographs depicted in Fig. 3 were taken during the transmission/isolation ratio measurements and hence there are two beam dumps missing in the photographs, but they will be present for the regular operation of the isolator. These additional BDs will be placed to the opposite sides of both polarizers to be capable of damping of the reflections from the polarizers produced by the back-reflected beam.

 

Fig. 3. Faraday isolator in the laboratory: (left-hand) the input side and (right-hand) output side.

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The performance of the isolator was tested using the laser beam of the Bivoj laser system up to the full pulse energy of $100~\mathrm {J}$, at the $10~\mathrm {Hz}$ repetition rate, $10~\mathrm {ns}$ pulse duration. At first, the isolator was tested without the TGG slabs and the quartz rotator installed inside the magnet, in order to evaluate the extinction ratio of the polarizers and the half-wave plate. The measurement at the full energy has confirmed that the limiting value given by this non-magneto-active part of the isolator is $> 33~\mathrm {dB}$, proving that there is more than an order of magnitude safe margin to the 20 dB target for the isolation ratio.

As a next step, the transmission measurement of the whole isolator assembly was taken in the open direction. The transmission was evaluated as a ratio of the pulse energy measured at the output from the isolator behind the polarizer P2 to the pulse energy transmitted by the input polarizer P1. The transmission through the input polarizer P1 was lowered due to the polarization distortions of the incident laser beam for approximately $9.5\%$. Combined with the losses on prior optical components guiding the beam from the amplifier MA2 to polarizer P1 the throughput of P1 was equal to $87\%$ of the energy leaving MA2. The polarization distortions of the input beam are planned to be further suppressed using the in-house developed Muller-matrix polarimetric method [26]. The transmission measurement run was taken for 35 min (more than 20 000 pulses) and resulted in the transmission of $97\%$. The transmission losses consist mainly from the reflections from coatings, losses due to polarization changes in the magneto-optical part of the isolator and the slight clip of the of the beams’ marginal areas given by the limited clear aperture of the TGG slabs. The last mentioned is planned to be investigated in more detail by using the smaller beam, which will still be allowed according to the LIDT tests performed.

The possible deterioration of the beam quality caused by the presence of the isolator has been monitored by the diagnostic branch located at the output from the whole system. Both the near field and the far field beam profiles are continuously monitored there to verify the beam quality needed for the experiments or applications. Since the applications of the Bivoj laser (e.g., laser shock peening, laser-induced damage threshold measurements, etc.) are using exclusively the near field beam profile, the most attention was paid to the near field beam profile. Based on the observation using the diagnostics inclusion of the Faraday isolator changed neither the near field nor the less important far field beam profile. The measurement of the wavefront distortions is planned but has not been performed so far. Based on the performed numerical calculations the thermal part of the wavefront distortions should not exceed 0.1 waves peak-to-valley. The beam pointing stability (BPS) without the isolator was measured to be $45~\mathrm{\mu} \mathrm {rad}$ for the laser running at full power. With the presence of the isolator, this value changed to approximately $70~\mathrm{\mu} \mathrm {rad}$ in both horizontal and vertical directions, however, these fluctuations are most probably caused mainly by the vibrations of the superconductive magnet transferred via the optomechanical mount on the TGG slabs and the reciprocal rotator. Since there was no statistically significant difference between the BPS values measured with the cooling air switched on and off the contribution of the airflow to BPS decrease was found to be insignificant. Even though the level of the BPS is still acceptable for a good laser performance it is planned to improve the stability with the next-generation optomechanical mount.

The major characteristic of the developed isolator is its isolation ratio, defined as a ratio between the leakage in the closed direction to the energy transmitted through the input polarizer P1. The closed direction of the isolator was realized by the reversal of the magnetic field direction. The isolation ratio has been measured in an approximately one-hour-long run (36 301 pulses); the results of the isolation ratio measurement are shown in Fig. 4. The upper plot shows the energy of the laser pulses incident to the polarizer P1 (black) and the energy of the pulses leaking through the closed direction of the isolator. The lower plot depicts the isolation ratio of the Faraday isolator calculated as a ratio of the leaked pulse energy to the pulse energy transmitted through the polarizer P1. The isolator provided an isolation ratio of $30.46~\mathrm {dB}$ in the course of an hour-long testing run at full power without any noticeable decrease due to the thermal effects.

 

Fig. 4. Isolation ratio measurement in the closed direction of the isolator. Input pulse energy (black) is the energy delivered to the polarizer P1. Pulse energy leak (red) is the leakage through the isolator in the closed direction.

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This clearly demonstrates reliable and stable operation of the designed Faraday isolator, proving that this technology may be used for the protection of large-aperture high-energy pulsed high-average-power laser amplifier chains against any back-reflections coming from the target site. To the best of our knowledge, this is the first-ever demonstration of the optical isolation for such a powerful high-energy high-repetition-rate laser beam. It opens up the possibilities for this type of laser to be used for a number of industrial and scientific applications, which would be otherwise too risky to perform.