We demonstrate the lasing performance in the Nd:YAG ceramic channel waveguide produced by the carbon ion irradiation, including the continuous-wave (cw) and graphene Q-switched configurations. The highest slope efficiency of 56% and the lowest threshold of 40 mW have been obtained for the cw waveguide laser. With graphene as a saturable absorber, the Q-switched laser produces stable pulses with 57 ns pulse duration and 77 nJ pulse energy, respectively. Under the variation of the pumping power, the repetition of the pulse laser could be modified from 1.5 MHz to 4.1 MHz.
© 2014 Optical Society of America
The Q-switched pulse lasers with high peak power density and good flexibilities have been applied in various fields, such as remote sensing, range finding, medicine, laser processing, and telecommunications. According to the switching techniques, the pulse laser could be achieved by the passive Q-switching and the active Q-switching. The performance of actively Q-switched laser is controlled by an external controller. And the passive Q-switching depends on a saturable absorber. Compared with the active Q-switching, the passive Q-switching has the attractive advantages of compactness, simplicity, and flexibility in designs. The saturable absorbers used for the passive Q-switching could be made by a number of materials [1–3], such as transition metal-doped crystals, semiconductor saturable absorber mirrors (SESAMs), and single-wall carbon nanotubes (SWNTs). The SESAMs and SWNTs are considered as the ideal media for the passive Q-switched lasers. Nevertheless, the cost of SESAMs is relative high due to the complex fabrication technologies and expensive packaging. In addition, the SWNT has problems of a low damage threshold, poor stability, and poor long-term reliability. To overcome drawbacks of both of them, recently graphene has been suggested to be an outstanding candidate as the saturable absorber [4–6].
Graphene is a two-dimensional material composed of a single layer of carbon atoms. Since 2004 , there is an increasing attribution due to its great prospect in electronics and excellent nonlinear absorption properties. Owing to the intriguing nonlinear properties, it could be used as a saturable absorber for photonic applications. Different from conventional saturable absorbers, the absorption band of graphene has a wide range from visible to terahertz (THz) with weak wavelength dependence. Additionally, it possesses some advantages, such as high damage threshold and low cost, over other saturable absorbers [8, 9]. As a result, graphene-based Q switching lasing is with great potential for constructing high-performance and cost-effective laser systems.
Channel waveguides confine light fields in small volumes, in which high optical intensities could be reached. Based on the waveguide platform, the lasing could be with low pump thresholds and enhanced efficiency, owing to the high intra-cavity light intensities [10–12]. In addition, the performance of the waveguide lasers also relies on the features of the gain material. Rare-earth-ion doped yttrium aluminum garnet (Re:YAG) ceramics possess excellent lasing properties and attract wide interests in the scientific community in the past two decades, which have been considered to be the most promising gain media for next-generation solid state lasers . Compared with YAG single crystals, the ceramics could be manufactured to be with larger volumes as well as with similar fluorescence features. Taking the neodymium doped yttrium aluminum garnet (Nd:YAG) as an example, it has been proved to be an excellent medium for high-power cw , Q-switching , mode-locking  bulk lasers . Recently, waveguide lasers have been realized in Nd:YAG ceramics [18, 19].
The ion beam technique is an efficient way to produce the waveguide structure, which is applicable for many materials. Depending on the energy of the incident ion beam, this technique could be divided into the ion implantation method (ion energy lower than 1 MeV/amu) and the swift heavy ion irradiation (ion beam energy higher than 1 MeV/amu), respectively. Different from the normal ion implantation, the swift heavy ion irradiation may induce larger refractive index change [20, 21]. For Nd:YAG ceramics, the ion implantation decreases the refractive index of Nd:YAG ceramics at the end of the ion range and slightly increases the value along the trajectory [18, 22]. The waveguides fabricated by the swift heavy ion irradiation have negligible refractive index change near the surface but significant increase along the ion path . Compared with the implanted waveguides, the enhancement of the refractive index of Nd:YAG ceramics induced by the swift heavy ion irradiation is almost three times of magnitude.
In this work, we investigate both the cw and pulse laser performances of the swift carbon ion irradiated Nd:YAG ceramic channel waveguide. With neither the graphene nor mirrors in the cavity, the cw lasing of the waveguide was achieved with ~56% slope efficiency and 65 mW laser threshold, under 810 nm optical pumping. With both of the graphene and a mirror for the waveguide resonating cavity, the Q-switched pulse laser was obtained and the repetition rate could be tuned from 1.5 MHz to 4.1 MHz. The pulse duration and pulse energy were 57 ns and 77 nJ, which have a variation less than 5% with the power of the pumping laser.
The Nd:YAG ceramic used in this work was doped by 2 at.% Nd3+ ions, obtained from Baikowski Ltd. (Japan). The sample was cut into pieces with dimensions of 2 × 10 × 7 mm3. And the biggest facets (10 × 7 mm2) of the sample were optically polished. By using a 3 MV tandem accelerator, the biggest facet of the sample was irradiated by the C5+ ions at the energy of 15 MeV and at a fluence of 2 × 1014 ions/cm2. During the irradiation process, a metal mask of nickel-cobalt alloy with open slits (with 20-μm width and 10-mm length) was put on the surface of the sample in order to limit the irradiation to certain regions. In the irradiated region, the refractive index of the sample was modified and the waveguide structure was generated. And facets perpendicular to the waveguide channels were optically polished and paralleled to each other. To construct the resonant cavity of the pulse laser, the graphene and output mirror (reflectivity of >99% at 1064 nm) were adhered to the output and input facets, respectively. In this work, the graphene (~6 layers) was used as the saturable absorber. At first, the graphene was grown by chemical vapor deposition (CVD) on copper and nickel, which has layers of ~6 measured by the Raman spectroscopy. The graphene was transferred to a transparent fused silica glass with a thickness of ~2 mm. Figure 1 shows the experimental setup for the pulse laser generation. The pump laser beam at the wavelength of 810 nm was coupled into the waveguide by a convex lens. The polarization of the pumping laser was perpendicular to the biggest facet of the sample. The output light from the waveguide structure was collected by a long-work-distance microscope objective (MO).
3. Results and discussion
Color centers would be generated during the irradiation process, which may have the feature of the additional saturated absorption. To avoid this effect from the color center, the Nd:YAG ceramic waveguide used for lasing in this work was an annealed one. As shown in Fig. 2(a), the refractive index distribution of the waveguide was reconstructed after the annealing at 180 °C for 30 min. Compared with the sample without the annealing, the maximum refractive index of the annealed waveguide was slightly decreased (Δn ≈-0.0005). The decreased refractive index change of the sample was supposed to be induced by the removal of defects induced by the irradiation. In addition, the propagation loss was decreased to ~0.7 dB/cm at the wavelength of 1064 nm. The reduced loss also proved that color centers were reduced by the annealing process, as the absorption of color centers was one of main factors for the loss generation of ion irradiated waveguides. As depicted in Fig. 2(b), the propagation mode of the waveguide is still a single mode.
Firstly, the waveguide was pumped with neither the graphene nor mirror. Figure 3 (a) shows the spectrum of the output light collected by the long work distance MO. As one can see, there is a peak at the wavelength of 1064 nm with a full-width at the half maximum (FWHM) around 2 nm. Considering the measurement error of the spectrograph (2 nm), it means there is the laser oscillation at 1064 nm in the waveguide. As no mirror was added to the end facets, the output coupler (OC) of this waveguide was assumed to be ~99%. As depicted in Fig. 3 (b), the laser threshold of ~56% was demonstrated and the waveguide laser delivered a maximum output power of 72 mW with the pumping power of 192 mW. Compared to other ion irradiated / implanted waveguide laser cases in Ref [19, 22], the slope efficiency obtained in this work is even higher, showing better performance of the lasing features.
Secondly, both of the graphene absorber and the mirror were inserted onto the output facets of the waveguide for generation of the passive Q-switching output pulse laser. Considering about the reflection of the end facet, the OC of this waveguide laser was assumed to be ~80%. Figure 3(b) shows the comparison of laser characteristics of the waveguide laser with different OC. With OC ≈80%, the laser threshold was around 45 mW and the laser slope efficiency was assumed to be ~30.7%. Although the laser operation was observed with pumping power above 45 mW, the stable Q-switched pulse trains were not obtained until pumping power more than 100 mW.
Figure 4(a) illuminates the typical Q-switched pulse trains under the pumping power of ~130.5 mW. The modulation depth of the Q-switched pulses was ~50% and Q-switched pulses have the pulse duration of 57 ns. In this case the average output power of the laser was 24 mW, the corresponding Q-switched pulse energy was ~11 nJ with the repetition of 2.2 MHz. When the pumping power was increased, the modulation of the pulse repetition was observed in Fig. 4(b). At ~108 mW, the repetition rate was around 1.5 MHz, whilst the pump power was set at ~192 mW, the repetition rate was increased to 4.1 MHz. Obviously, there is a linear variation with the repetition and the power of the launched pumping laser.
In Fig. 5, we show the pulse duration and energy of each pulse as a function of the pumping power. It is clear that the pulse energy and pulse duration were slightly changed with increasing pump power. At the highest and lowest pumping power, the pulse energy was ~10.5 nJ. And the variation of the pulse energy was less than 0.5 nJ with pumping power changed from ~110 mW to 190 mW. Meanwhile, the duration of the pulse was around 60 ns. And the variation was less than 5 ns within the pumping region. The pulse parameters were not shown in Fig. 5 with pumping power less than 100 mW, as the pulse laser was not stable.
We reported on the laser performance based on the Nd:YAG ceramics waveguide produced by swift carbon irradiation. The laser slope efficiency as high as ~56% was observed for cw lasing regime. With the graphene as the saturable absorber, the pulse laser was generated by the passive Q switching technique. The pulse energy and duration were around 10.5 nJ and 60 ns. The repetition rate can be tuned from 1.5 MHz to 4.1 MHz through the variation of the pumping power.
This work is carried out under the support by the National Natural Science Foundation of China (No. U1332121) and the 973 Project (No. 2010CB832906) of China. Y. Tan acknowledges the support by the National Natural Science Foundation (No. 11305094) and China Postdoctoral Science Foundation (Grant No. 2013M530316). The work at Helmholtz-Zentrum Dresden-Rossendorf is supported by the Helmholz-Gemeinschaft Deutscher Forschungszentren (HGF-VH-NG-713).
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