High-power giant pulses can be used applied in various applications with Q-switched micro-lasers. This method can shorten the pulse duration; however, active control is currently impossible in micro-lasers. To achieve precise pulse control while maintaining compactness and simplicity, we exploit the magneto-optical effect in magnetic garnet films with micromagnetic domains that can be actively controlled by a pulsed magnetic field. Our Q-switching technique enhances the output power by a factor of 4 × 103. Moreover, the device itself is smaller than other Q-switching devices. This novel type of active Q-switch can be combined with a micro-laser to obtain megawatt-order pulses.
© 2016 Optical Society of America
High-peak-power giant pulses produced by solid-state lasers [1, 2] are useful in creating exotic phases in matter via processes such as ablation, photo ionization, shock wave generation, and Coulomb explosion. The diode-pumped solid-state laser (DPSSL) [3–5] is a more compact, long-lived, stable, and efficient device in comparison to a laser that is pumped with a flash tube. Such a device demonstrates the key advantages of the solid-state laser, such as its high power (on the order of 100 W) , Q-switching capability (pulse width on the order of picoseconds) [7,8], and mode-locked operation (stability of a few percent) [9, 10]. This DPSSL has enabled the use of lasers in various configurations, such as the fiber [11, 12] rod , array , and disk . In particular, diode-pumped microchip lasers (MCLs) have shown an active presence in the fields of laser manufacturing [14–16], vehicle engine development [17, 18], terahertz waves generation , and thruster systems for space exploration  because of their simple structure, compact size, high efficiency, and high stability in the presence of vibrations and temperature variations.
Moreover, giant pulse power has been demonstrated via passive Q-switching with a saturable absorber (e.g. Cr4+:YAG  and InGaAs ), which can lead to an output power of several megawatts . In order to obtain an active (rather than passive) Q-switched MCL whose repetition rate, pulse width, mode-lock stability, and pulse shape can be controlled, we exploited the magneto-optical (MO) effects that can be observed in ferromagnetic films that possess a garnet structure. This approach was inspired by bulk-based electro-optic (EO)  and acousto-optic (AO)  Q-switches, which are widely used in cavity laser systems. However, these components are difficult to integrate into MCLs because of their size and mechanisms. In addition, these two components usually require a large-size power supply, which hinders the entire system from downscaling.
In contrast, the MO effect exhibited by films or multiple layers that are several micrometers thick [24, 25] has been used in two-dimensional integrated arrays  and storage media  with tiny modulation powers (i.e. a current of 0.5 μA and a voltage of 20 V ). For this reason, researchers have suspected that the MO effect could be utilized in active Q-switching MCLs. However, a few recent reports on MO Q-switching have called this approach into doubt. In particular, Zhou et al. demonstrated that, for an MO Q-switch that utilizes the paramagnetic properties of rare-earth doped lasing crystals (e.g. neodymium-doped bismuth germinate), the pulse width is relatively long (~100 ns) . Furthermore, a bulk polarizer is required in the optical cavity because of the small linear MO responses of the paramagnetic material used.
Against this background, we seek to overcome the issues related to the MO effect and to demonstrate the MO Q-switching laser’s strong potential for integration into active MCLs. We provide the first demonstration of using sub-millimeter thick ferromagnetic garnet films based on fast-magnetic-domain elimination.
2. Preparation of laser cavity
Our demonstration of MO Q-switching needed to combine three essential characteristics: a DPSSL, a rapidly pulsed magnetic field generator (with two 5.30 mm diameter coils), and an MO garnet film with micrometer-scale magnetic domains (hereafter micromagnetic domains, MMDs) that can be flipped by a small magnetic field. The device structure is shown in Fig. 1(a). The laser diode (Limo, HLU32F400-808) generated light with a wavelength of 808 nm at a controlled temperature of 25°C. This light was guided to the cavity setup by a 0.4 mm aperture fiber, and then collimated and focused in the center of the lasing crystal using two lenses, whose focal lengths were 50.3 mm. The spot size (waist diameter) of the focused beam inside the crystal was 0.27 mm, and was measured by the knife-edge method. The cavity was 130 mm long, and was constructed with a dielectric multilayer-coated concave mirror (output coupler, OC) and a dielectric mirror deposited on the lasing crystal. The concave mirror (radius of curvature: 300 mm) had a reflectance of 95% and 99.8% at wavelengths of 1064 nm and 808 nm, respectively. The input-side dielectric mirror on the lasing crystal had a transmission of 0.2% and 98% at wavelengths of 1064 nm and 808 nm, respectively, which allowed for energy-efficient laser oscillation. The output-side dielectric mirror on the lasing crystal had a transmittance of 98% and 0.2% at wavelengths of 1064 nm and 808 nm, respectively.
The lasing crystal we used was a uniaxial anisotropic oxide cut along its a-axis – 0.5 at.% neodymium-doped gadolinium vanadate (Nd:GdVO4) – which emitted linearly polarized light in continuous wave (CW) operation. The temperature of the Nd:GdVO4 crystal was fixed by a proportional integral derivative (PID)-controlled Peltier cooler; additionally, it was thermally coupled with indium paper and water-cooled copper blocks. This vanadate is known to be a highly efficient laser material with a low thermal expansion coefficient, and is used in MCL experiments and many other Q-switching laser demonstrations [30, 31]. The crystal size was 4 × 3 × 3 (a × b × c) mm, and the a- and c-axes were parallel and perpendicular, respectively, to the light path. This laser’s threshold power was 2.5 W; the maximum efficiency was 20.1%; and the slope efficiency ηs was 27.6%. The oscillation peak’s quality factor (Q) was 5810. This performance was comparable to the performance measured in other reports .
3. DC output operation using MO garnet
As a next step, we inserted a 190 μm thick MO garnet film into the laser cavity. The film was epitaxially grown on a 560 μm thick gadolinium gallium garnet (GGG) substrate using the liquid phase epitaxial (LPE) method. This was cut into an 11 mm × 11 mm piece. The transmissivity was 60% at a wavelength of 1064 nm. This was not changed by applied magnetic field. Any anti-reflection coating was not formed to this sample. The absorption at the wavelength of 1064 nm was –108 dB/cm. This cavity laser’s CW performance with MO films was characterized using a germanium photo detector (Newport, 918D-IR-OD3); the results are shown in Fig. 1(b). The OC’s reflectance R at a wavelength of 1064 nm was varied 95%, 90%, and 80% in order to know the cavity’s physical parameters. The full width at half maximum (FWHM) of the output intensity of the laser using the R of 95% was 0.14 nm at 1063.57 nm (hereafter 1064 nm), as shown in Fig. 1(c). The obtained data points were not precise because of the rough resolution (~0.1 nm) of the optical spectrum analyzer we used (Advantest, Q8344A); this was measured at the temperature of the Nd:GdVO4 crystal (i.e. 20°C), and Q was 887. There were no additional oscillation peaks for other wavelengths. Furthermore, the threshold power was 1.3 W. The obtained ηs were 4.3 × 10−4%, 4.6 × 10−4%, and 5.1 × 10−4% for the R of 95%, 90%, and 80% at a wavelength of 1064 nm. Hereafter all data were obtained with the R of 95%.
The composition of the MO film used was characterized with energy-dispersive x-ray spectroscopy (JEOL, JED-2201F) measurements as follows: Tb2.0Bi1.0Fe4.8Ga0.2O12–ξ, where ξ indicates the number of oxygen vacancies. Bi and Fe were used to obtain a large MO response owing to their large spin-orbital couplings . On the other hand, Tb was used to compensate for the thermal drift of MO responses . Ga was used to obtain perpendicularly magnetized film owing to the crystalline anisotropy . This garnet film’s Faraday rotation angle loop was measured using the rotating analyzer method  (Neoark Corp., BH-M600VIR-FKR-TU), and is shown in Fig. 2(a). MO images were obtained with a polarized-light microscope, and are shown in the inset of Fig. 2(a). The MMDs (several dozen micrometers in width) were gradually flipped by the perpendicular DC magnetic field HDC. At the saturation magnetic field, the entire magnetization direction was ordered perpendicularly to the film, and thus the MMDs disappeared.
We measured the output power of the laser under various MMD conditions, which were controlled by HDC that was generated by a ring-shaped permanent magnet (SmCo, 12 mm outer diameter, 7 mm inner diameter, 3.5 mm thickness, prepared by Kogakugiken Corp.). This bias field was controlled by adjusting the distance between the MO film and the magnet, as shown in Fig. 2(b). In the case in which all the MMDs disappeared, the output power of the cavity laser was 5 mW at a pump power of 27.4 W. By way of contrast, in the case in which MMDs existed with the highest definition, the output power of the cavity laser was about 100 times smaller (~70 μW). Furthermore, the Q-factor shifted from 887 to 986 by applying HDC, which can completely remove the MMDs. These results illustrate that the population of photons inside the laser cavity can be controlled by adjusting the magnetic field, as a result of the alternation of the character of the MO film’s MMDs. Thus, to realize MO Q-switching in this context, a rapid change of the Q factor is required instead of the mechanical modulation of a permanent magnet.
4. Preparation of pulse magnetic field
For this reason, we designed a rapid magnetic field generator [37, 38] using two 5.30 mm diameter coils with 3 turns and a pulse-generating circuit. In order to flip the magnetization of the MO hysteresis loop shown in Fig. 2(a), a magnetic field of 200 Oe was required. The spot diameter of the beam at the MO film was approximately 0.46 mm, as calculated by Paraxia-Plus provided by SCIOPT. A two-coil pair configuration was chosen with a diameter and coil separation distance of 5.30 mm. The MO film was placed at the two coils’ center, whose wire diameter was 0.20 mm (the resistance was 140 mΩ) for tight turn-turn distances, and thus stronger field strength at the coaxial center. To optimize for fast magnetic switching, the inductance (and thus the number of coil turns) was minimized, and can be approximated as follows :Figure 3(a) shows the field distribution generated by the coil set connected to the electronic circuit. The field distribution was calculated using the finite-element method (Comsol version 4.3). Figure 3(b) shows field distribution in x direction at the used current of 56 A.
The electronic circuit we designed is shown in Fig. 3(c). The switching architecture chosen to meet the desired requirements was a single n-type metal-oxide-semiconductor field-effect transistor (MOSFET) switch. The coil set was connected to the drain pin of the transistor and the positive terminal of the input voltage source. A current-sense resistor was connected to the source pin and ground for measurement. A signal generator (SG) was connected to the gate pin along with a parallel impedance-matching resistor (R4). The reverse-electromagnetic field diode (D) was placed in series with a low-value resistor (R2) and in parallel with the field generation coil (L, R1) to allow for over-voltage protection and faster dissipation of magnetic energy when the driver is switched off. Bypass capacitors (C1, C2) were placed at the input source, which served to supply transient, high-amplitude current for the duration of the pulse. The specific transistor chosen for this topology was the IRL3714ZSPBF by Infineon Technologies, which has current ratings well beyond the requirements of this circuit. Simulation of the circuit described was performed using PSPICE in OrCAD Capture CIS (version 16.6). The circuit was then fabricated and inserted into the experimental system. This circuit used a current of 56 A (the applied voltage to the coil was 7.8 V) to generate a pulsed magnetic field HPulse value of 200 Oe. This HPulse value was generated with a pulse width of 3 μs and a repetition rate of 100 Hz. The repetition rate was controlled by an external signal generator (Tektronix, AFG30011). This circuit was connected to the coil set (which sandwiched the MO film) and was placed in the laser cavity.
5. MO Q-switching
We also applied HPulse to the MO film and obtained the optical signal from the laser cavity using an InGaAs-based, fast-response optical detector (Thorlabs, DET10C/M) and a digital oscilloscope (LeCroy, WaveJet 324). We observed the peak power of 30 W MO Q-switched laser oscillations with a peak width of 45 ns at a pump power of 27.4 W, as shown in Fig. 4(a). The wavelength of the light was the same as that for the CW oscillation (~1064 nm), and the peak power was enhanced by a factor of 4 × 103 as compared to that of CW operation. Figure 4(b) shows the peak power’s dependence on the pump power, and the fitting line shows that the threshold pump power was about 19.5 W. The temperatures of the MO film, the circuit, and the copper coils was not changed after an hour operation. And also, no damage was observed in the MO film. As far as we know, this represents the first evidence of MO Q-switching based on ferromagnetic garnet films. Such a demonstration of MO Q-switching allowed us to consider further decreasing the MO film’s thickness. This, in turn, would decrease the pulse width τp according to the following equation :41, 42]. The pulse width can be decreased by decreasing of L and increasing of δ. Therefore, the pulse power can also be enhanced, due to fact that the pulse power is equal to the energy divided by the pulse width.
To confirm that this MO Q-switching is indeed different from other Q-switching mechanisms, we measured the polarization state of the output light from the laser cavity without the MO garnet, with the unmagnetized MO garnet, with the MO garnet magnetized by HDC of 200 Oe, and with the MO garnet plus a HPulse of 200 Oe. As shown in Fig. 4(d), the polarization states were controlled by the magnetic field, but the polarization states were different between the cases of CW and pulsed operation. Note that the polarization state of the transmitted light through the unmagnetized MO garnet (triangles) and that through the MO garnet plus HPulse (circles) in Fig. 4(d) look similar, but they show completely different polarization states. The transmission light through the unmagnetized MO film was randomly polarized because of its maze-like magnetic domains, in contrast, that through the MO film with applying HPulse was almost circularly polarized. This difference was confirmed by inserting a quarter wave plate in front of a photo detector. This observation has not yet received a satisfactory explanation; however, the MMD’s fluctuation was observed by a polarized camera, and thus electrical pulse jitter might explain this difference. Improvements from using an impedance-matching technique on the coil and spin dynamics  might solve this issue, and should provide a much sharper picture of the change in polarization states and optical pulses.
From an engineering viewpoint, it is important to decrease the power consumption for the modulation of a pulse. For this reason, we used a bias technique. The HDC close to the saturation magnetic field (~191 Oe) generated by the permanent magnet was applied to the garnet film; simultaneously, we also applied HPulse to the garnet using the same coil set, as shown in the inset of Fig. 4(c). As expected, a similar pulse width and power due to the MO Q-switching were obtained with an applied current that was ~7 times smaller than the case without a bias technique. This result indicated that the bias field technique can decrease the power consumption. In addition, realizing such a low-power magnetic field generator can decrease the circuit footprint, and increase the repetition rate.
It is useful to compare this MO Q-switching technique against other active Q-switching techniques. For instance, Yu et al. used the same lasing material and a similar cavity length . As discussed above, the EO effect requires the polarizer to be placed in the cavity, though the MO does not. Additionally, the thickness of the MO Q-switch in the optical path was 237 times smaller than that of EO Q-switch. In comparing these approaches, it is important to note that the MMD erasure was the origin of the MO Q-switching, which can be realized with thin MO films. Moreover, other physical properties can be improved by simply decreasing the thickness and cavity size – as well as redesigning the electrical circuits – to achieve faster and sharper electronic switching. An anti-reflection coating on MO film should increase the transmissivity, increase the output power. And also, the tuning of the rare-earths in yttrium site in MO film may improve the MO responses. Such tasks are not difficult given the recent rapid development of micro- and nano-scale photonics and electronics.
In conclusion, we believe that active MCLs will soon be integrated with MO Q-switches based on the proof of concept provided in this study. The MO Q-switching approach exhibits some advantageous properties; in particular, this approach requires a lower voltage compared with the EO Q-switch, has a compact modulation circuit, and might potentially allow for the cavity length to be shortened to the lasing material’s length (at minimum). Additionally, the MO effects are ultrafast response (~100 fs) , thus the pulse width of 45 ns was not due to the speed of MO effects. And then the shortening of cavity and the increasing of the cavity loss should improve the pulse width and peak power of the MO Q-switched laser. These characteristics cannot be demonstrated with other Q-switching techniques because the majority of them use a different mechanism of Q-switching, which is based on controlling micro-domain states. Therefore, the MO effect is the most promising candidate to modulate active Q-switching MCLs, and the results of this study constitute the first evidence of MO Q-switching using ferromagnetic materials.
We acknowledge support from the Japan Society for the Promotion of Science (JSPS) KAKENHI Nos. 26706009, 26600043, 26220902, 25820124, and 15H02240, as well as the Ministry of Internal Affairs and Communications (MIC) SCOPE No. 0159-0117. We also thank Dr. Nicolaie Pave, Dr. Ryosuke Isogai, Mr. Naoki Kanazawa, Mr. Takuya Yoshimoto, Dr. Hideki Ishizuki, Mr. Kazuma Tobinaga, Mr. Yoji Haga, and Dr. Yoichi Sato for discussion and experimental supports.
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