We report on a cladding-like waveguide structure in Nd:YAG crystal fabricated by the multiple carbon ion beam irradiation. After the designed multiple irradiation process, the cladding-like waveguide with triple refractive-index layers were constructed in the region near the surface of the crystal. With such a structure, the waveguiding core was compressed and refractive index profile was modified, resulting in a higher light intensity than that of the single ion-beam-irradiated monolayer waveguide. The waveguide lasing at wavelength of 1064 nm was achieved with enhanced performance in the cladding-like structures with both planar and ridge configurations by the optical pump at 810 nm.
© 2015 Optical Society of America
The cladding fiber, as a class of optical fiber, has triple layers of optical material [1,2] including an outer cladding, and inner cladding and a core. The most inner layer is called core and has the highest refractive index. The core is surrounded by the inner cladding, which is surrounded by the outer cladding. In this situation, both the signal and pumping light are confined by the inner cladding and the core, which have higher refractive index than the outer cladding. The inner cladding with a large diameter has a high coupling efficiency with the diode laser and allows the high pumping power. Meanwhile, the core doped by the rare earth ions works as the gain medium. As the core has the highest refractive index, the pumping light is easily coupled into the core from the inner cladding. Hence, optical amplifier and laser emission with high efficiency are easily to be achieved due to the high power and high intensity in the core [3–6]. The superiority of the cladding fiber is the balance of the high coupling efficiency and the high intensity of the light, which gives us a hint for the design of the novel waveguide laser with enhanced laser properties.
Waveguide laser is one of basic active devices in photonics, which works based on waveguide platforms [7–9]. In the waveguide laser, the waveguide structure is fabricated in the rare-earth doped optical material as the gain medium. The resonant cavity for the laser emission is consisted by the waveguide structure composing with laser mirrors. Owing to the better light confinement effect and better heat dissipation, the enhanced laser emission can be generated compared with the bulk laser system, such as a higher slope efficiency and a lower threshold [10,11]. The ion beam techniques, including ion implantation [11,12], swift heavy ion irradiation [13–15] and proton beam writing [16,17], have been successfully applied to produce waveguides in various gain media. The energetic ions modify the refractive indices of the material along the ions’ paths. The induced refractive index changes are related to the nuclear and electronic energy depositions of incident ions on the optical materials [18–21]. The associated lattice damage of the optical material during the irradiation process could be accumulated in case of multiple ion irradiations. This opens a way to design on-demand refractive index distributions of the waveguide through careful considerations of irradiation parameters [22–24].
In this work, we propose a cladding waveguide structure composed by an outer cladding layer (air and substrate), an inner layer cladding (width of 9 μm) and a core layer (width of 3.5 μm), which was fabricated by the multiply carbon irradiation. Compared with the monolayer waveguide, the light can be much easier coupled into the waveguide due to the confinement of the inner and the outer claddings. Along with the light propagation, the guided light was focused into the core, inducing a higher intensity density in the waveguide. Utilizing the cladding waveguide for the laser emission, the enhance laser performance with a lower threshold was observed. And due to the high output power and ensuring spatial mode control, this work proves a new way to develop cladding-pumped, double-cladding or large-mode-area planar waveguides.
The Nd:YAG crystal (2 at. % Nd3+ ion) used in this work was cut into three pieces with dimensions of 10 × 5 × 2 mm3. One biggest facet (10 × 5 mm2) of each samples were irradiated by carbon ions under different conditions. During the irradiation process, the carbon beam direction was tilted by 7° off the normal axis of the surface plane, in order to avoid the channeling effect, the sample 1 (S1) and sample 2 (S2) were irradiated at the energy (fluence) of 6 MeV (1 × 1015 ion/cm2) and 15 MeV (2 × 1014 ion/cm2), respectively. For the sample 3 (S3), the energy and fluence of the carbon ion were (6 MeV + 15 MeV) and (1 × 1015 + 2 × 1014) ion/cm2. After the irradiation process, two parallel facets (5 × 2 mm2) of samples were polished and imaged by a microscope (Olympus BX51M, Japan). Figure 1 shows the microscope images of the waveguide cross section. The region (irradiated region) with the color change corresponds to the refractive index change of the Nd:YAG crystal, in which the incident ion deposited its energy and finally stopped at the end of the ion range.
Based on S3, the Nd:YAG ridged waveguide structure was fabricated by the diamond blade dicing (DBD). During the experiment, two parallel grooves were cut by a resin-bonded blade on the irradiated surface of S3. The rotate speed of the blade and the cutting speed were set to 20 rpm and 0.1 mm/s, respectively. Between two grooves, the channel waveguide was formed. The width of the channel waveguide was modulated by the separation distance between two grooves.
The refractive index profiles of waveguides were reconstructed by the reflectivity calculation method (RCM) and the intensity profile fitting method (IPFM), respectively. For RCM, the dark-mode spectrum of the waveguide was measured by a prism coupler (Metricon 2010, USA). Based on measured dark modes, the refractive index profile was calculated by a computer code according to RCM. For IPFM, a profile consisted by two semi-Gaussian shape is manually set as the refractive index profile. According to this profile, the propagation mode is simulated by the beam propagation method (Rsoft © BeamProp 8.0). Then the calculated mode is compared with the measured one. If they are consistent with each other, we believe this profile can be characterized as the refractive index profile of the waveguide. Conversely, the manual profile should be modified and repeat the simulation and comparison steps.
Propagation modes of the Nd: YAG waveguides were detected by the end-coupling method. During the method, the continuous-wave (cw) laser at the wavelength of 1064 nm was coupled into the waveguide by a Lens (focused length 20 mm) and the output light from the end-facet was collected by a long work distance microscope objective ( × 40, N.A. = 0.5). The propagation loss of waveguides were measured by the back-reflection method .
The laser emission was also realized in the waveguide structure through an end pumping system. During the experiment, the polarized laser at wavelength of ~810 nm from a tunable cw Ti:Sapphire laser (Coherent MBR 110) was used as the pumping light and coupled into the waveguide. Mirrors with the reflectivity of >99.8% and 90% at around 1064 were adhered onto the end-facets of the waveguide as the input and output mirrors, respectively. The waveguide structure and mirrors constituted the Fabry-Perot resonant cavity.
3. Results and discussion
Characters of the cladding-like waveguide
Refractive index profiles of S1, S2 and S3 were shown in Fig. 2. For S1 (Fig. 2(a)), the refractive index of the Nd:YAG was increased near the surface constituting an enhanced well with the width of 3.5 μm. Meanwhile, there was a region of the decreased refractive index at the end of the ion range forming an optical barrier. And the waveguide structure was constructed by both the enhanced well and optical barrier. Different from S1, the light in S2 was only confined by the the enhanced well (Fig. 2(b)), which was deep inside the crystal.
Since S1 and S2 only have one layer with the increased refractive index, both of them were called the monolayer waveguide in this work. Measured propagation modes of light at the wavelength of 1064 nm in S1 and S2 were demonstrated in Fig. 2(d) and 2(e), respectively. Based on reconstructed refractive index profiles (S1 and S2), propagation mode of waveguides were also simulated by BeamProp 8.0 in Fig. 2(g) and 2(h). Compared with measured results in Fig. 2(d) and 2(e), they show a similar intensity distribution, which indicated reconstructed refractive index profiles in Fig. 2(a) and 2(b) are reasonable.
The maximum refractive index change of S1 (ΔnS1 = 0.002) is higher than S2 (ΔnS2 = 0.0012) and the width of S1 (3.5 μm) is shorter than S2 (9 μm). Hence, there is the possibility to construct the cladding shape of the refractive index distribution by overlapping the refractive index change of S1 and S2. In order to achieve this purpose, S3 was twice irradiated by same conditions of S1 and S2. The refractive index profile of S3 was reconstructed by IPFM, which was assumed to be superposed by S1 and S2 through the expression below.
Where Δn is the refractive index change; a and b are arbitrary constants. Figure 2(c) shows the reconstructed refractive index profile of S3 corresponding to a = 0.9 and b = 1. Simulated and measured intensity distributions were shown in Fig. 2(f) and 2(i) with a comparable shape with each other. In Fig. 2(c), the maximum refractive index was 1.833 at the depth of 3 μm with width of 2 μm forming the core. The width of the inner cladding was 9 μm.
Under the same coupling condition, coupling efficiencies of S2 and S3 were 40% and 41%, respectively. Meanwhile, the value of S1 was less than 5%. Intensity distributions of propagation modes were also shown in Fig. 2(g), 2(h) and 2(i). With the same power of the guided light in waveguides, the maximum energy intensity of S3 is 1.4 times and 1.9 times higher than S1 and S2, respectively. Based on discussions above, we can get the conclusion that the cladding-like waveguide structure has higher coupling efficiency and the energy intensity compared to the monolayer waveguide.
Laser performance in the cladding waveguide
We tried to excite the laser oscillation in S1, S2 and S3 under the same pumping condition. In S1, no stable laser emission was observed. As the coupling efficiency of S1 was low (less than 5%) and the power of the coupled pumping laser cannot achieve the laser threshold of S1 waveguide. In S2 and S3, the continuous laser emission at the wavelength of 1064 nm was observed, the spectrum of which was shown in the inset of Fig. 3. Figure 3 also depicted the power of the output light as a function of the power of the pumping light coupled into waveguides (input power). As one can see, the slope efficiency of laser from S2 and S3 were 9.9% and 10%, which were similar to each other. The laser threshold of S3 was 43 mW far below the value of S2 62 mW.
The decreased laser threshold (Pth) in the cladding waveguide (S3) can be explained by the difference of the energy intensity density in S2 and S3. As a four level system, the laser threshold can be expressed by the equation below:
where h is the Planck’s constant, c is the light velocity in the vacuum, λP is the wavelength of the pumping laser, δ is the round-trip cavity loss exponential factor, η is the fraction of absorbed photons, σe is the stimulated emission cross section, τ is the fluorescence lifetime, Aeff is the effective pumping area of the propagation mode. Due to the confinement of triple layers, the full width at half maximum (FWHM) of the propagation mode (Aeff) was 2.7 μm in S3, which is the almost half value of the one in S2 (width of 5 μm). As the laser threshold was proportional to Aeff. The threshold was reduced by half.
Ridged waveguide based on the cladding structure
Based on the cladding-like waveguide, the ridged waveguide was fabricated by DBD. Figure 4(a) shows the microscope image of the cross section of the ridged waveguide. The length and the width of waveguides were 5 mm and 25 μm. The propagation loss of the waveguide was around 2.5 dB/cm, which was higher than the planar waveguide (~1.5 dB/cm, in S1, S2, and S3). The extra loss was supposed to be induced by the surface rough generated during the DBD process. Coupling the pumping laser at 810 nm into the ridged waveguide, the laser emission at the wavelength of 1064 nm was obtained without any mirrors added onto end facets of waveguides. The laser model profile was shown in Fig. 4(b) at the wavelength of 1064 nm. Figure 4(c) depicted the power of the output laser as a function of the pumping power. The lasing threshold was 60 mW and the slope efficiency was 46.1%.
Here, we would like to make a comparison with the data reported in Ref . In Ref , the ridged waveguide (height d = 9 μm, width w = 22 μm, N.A. = 0.66) was fabricated by DBD based on a monolayer waveguide, which was formed by the 15 MeV carbon ion irradiation with the fluence of 3 × 1014 ions/cm2. The reported fitting curve of the output laser as a function of the absorbed power was drawn in Fig. 4(c) as the red dashed line. Although the morphology of the ridged waveguide in Ref . is similar with our work, the reported laser threshold (80 mW) is much higher than our result (60 mW). And slope efficiency is 42% slightly below ours (46.1%). The decreased laser threshold can be explained by Eq. (2) according to the previous discussion in “Laser performance in the cladding-like waveguide”. Through the comparison, we can further confirm that the laser performance is enhanced in the cladding-like waveguide structure.
We have reported on a cladding-like waveguide fabricated by the multiply ion irradiation on the surface of Nd:YAG crystal. The Nd:YAG crystal was twice irradiated by the carbon ion beam at the energy of 15 MeV and 6 MeV, respectively. A cladding waveguide with triple layers was formed after the irradiation. This structure was proved to have higher intensity density compared to the monolayer waveguide. Utilizing this waveguide for the laser oscillation, enhanced laser performances such as lower threshold were observed.
This work was supported by Young Scholars Program of Shandong University 2015WLJH20 and National Natural Science Foundation of China (Grant No. 11535008). S.Z. acknowledges the funding by the Helmholtz Association (VH-NG-713).
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