A high-gain optical waveguide amplifier has been realized in a channel waveguide platform of Nd:YAG ceramic produced by swift carbon ion irradiation with metal masking. The waveguide is single mode at wavelength of 810 and 1064 nm, and with the enhanced fluorescence intensity at around 1064 nm due to the Nd3+ ion emissions. In conjunction with the low propagation loss of the waveguide, about 26.3 dB/cm of the small signal gain at 1064 nm is achieved with an 18 ns pulse laser as the seeder under the 810-nm laser excitation. This work suggests the carbon ion irradiated Nd:YAG waveguides could serve as efficient integrated amplifiers for the signal amplification.
©2013 Optical Society of America
The rare-earth-ion-doped waveguide amplifiers are significantly interesting in the field of integrated optics over the last two decades [1–6]. Based on active integrated platforms of waveguides or fibers, the enhanced gain may be generated with respect to the bulk systems owing to the better light confinement effect and better heat dissipation. In addition, such a compact geometry has the potential to amplify optical signal at a high data rate and to compensate the connecting loss through the integrated optical circuit. As of yet, different materials, such as rare earth ion doped lithium niobate, glass, polymer and aluminum oxide [7–12], have been used to realize optical amplifications.
As a latest developed gain media for high power solid state lasers, rare earth doped yttrium aluminum garnet (YAG) ceramics have attracted continuous interests owing to the excellent lasing properties [13–15]. Compared with the single-crystalline partners, the ceramics have similar fluorescence ability and can be produced in larger homogenous volumes. Rare-earth ion doped YAG ceramics usually have broad emission bandwidth in the near-infrared band, which makes them be most attractive candidates for the optical amplifiers in telecommunications. Until now, many literatures have reported the excellent performance of the optical signal amplification based on doped YAG crystal fiber grown by the well-known laser-heated pedestal growth (LHPG) method [16,17], which can produce single crystal fibers with high-purity and low-defect-density. However, this method is not available for the YAG ceramics and has the difficulty to realize highly optical integration on a chip. To use this gain material for the waveguide amplification, we would like applying the ion beam technique, which is a more available fabrication method.
Ion beam techniques have been emerged to be an efficient way to produce waveguide structures in many gain media . During the ion irradiation process, the energetic ions penetrate the optical material surface and transfer the energy from the incident beams to the network of the target material. The energy deposition will modify the refractive index of materials and construct the waveguide structure . Depending on the energy of the incident ion, ion beam techniques can be divided into the ion implantation method (ion energy lower than 1 MeV/amu) and the heavy swift ion irradiation method (ion beam energy high than 1 MeV/amu). Until now, the ion implantation method has been applied for a variety of materials to fabricate kinds of positive and active photonic devices [20–26]. Taking the rare doped YAG ceramics for example, waveguides fabrication and waveguide laser operation have been discussed in details [27–29]. Compared with the ion implantation method, the heavy swift ion irradiation method introduces obviously reduced irradiation fluences and larger refractive index changes for the waveguide structure, which indicates the potential for highly integrated optical devices . However, the application of this technology for active devices is confined in the field of waveguide lasers [30, 31]. And there is no discussion about optical amplifiers. Besides the heavy swift ion irradiation, as a more advanced method, has not been applied to the gain medium of rare doped YAG ceramics yet.
In this work, we use the swift carbon ion irradiation method to fabricate the channel waveguide in Nd:YAG ceramic through the specially designed metal mask. Such waveguide properties as refractive index profile, propagation modes, propagation loss and fluorescence, are discussed in detail. The optical amplification in this Nd:YAG ceramic channel waveguide has been achieved for small input signals at 1.06μm.
The Nd:YAG ceramic (doped by by 2 at.% Nd3+ ions, obtained from Baikowski Ltd., Japan) sample was cut into dimensions of 2 × 8 × 7 mm3 and optically polished. Before the irradiation process, a metal mask of nickel-cobalt alloy with open slits (20 μm width, 10mm length) was prepared and put on the surface of sample to realize selective irradiation. By using a 3 MV tandem accelerator, the C5+ ion irradiation was carried out at energy of 17 MeV and at a fluence of 2 × 1014 ions/cm2. Through the open slits on the metal mask, the C5+ ion beams irradiated the sample and changed the refractive index, forming a few channel waveguides on the Nd:YAG ceramic. The length of waveguides is 7 mm. Figure 1(a) shows the sketch of the process of the selective carbon ion irradiation.
The Nd:YAG ceramic waveguide propagation loss is induced by a cooperation of the scattering from waveguide structure and the absorption from the doped neodymium ions. Both effects were analyzed in this work. At first, the waveguide scattering loss was measured at the wavelength of 1064 nm using the Fabry-Perot method . At 1064 nm, the absorption of Nd ions can be ignored, and the loss is only induced by the waveguide scattering. According to our measurement, the loss at 1064 nm was ~1.1 dB/cm. Then, we used the same method to detect the propagation loss at the wavelength of 810 nm, at which wavelength the loss is considered to be a combination of the absorption and the scattering. The measured total loss was ~11.7 dB/cm. As the waveguide scattering loss is mainly caused by the surface roughness of the waveguide walls, which is invariable with detecting wavelength. We assumed the waveguide scattering loss should have the same value at 810 nm and 1064 nm. Hence, the absorption of this waveguide was ~10.6 dB/cm at 810 nm.
Figure 1(b) shows the experiment setup for the investigation of the Nd:YAG ceramic waveguide amplifier. The pump source was a continuous wave (cw) tunable Ti:Sapphire laser (Coherent MBR 110) with 21 mW (absorption power 17.7 mW) at wavelength of 810 nm. Two pairs of waveplates and a Glan-Taylor prism allowed the control of the light intensity and polarization. Through a convex lens (with the focal length of 25 mm), the pump light was coupled into the waveguide. The coupling efficiency was assumed to be ~10% yielding a good agreement of waveguide propagation loss measurement. At the same time, a train of 18 ns pulse laser at a central wavelength of 1064 nm was used as the signal source. The signal light was spliced by the beam splitter and coupled into the waveguide by the convex lens. The output light from the waveguide was collected by a long work distance microscope objective ( × 20, f = 25 mm). To avoid the disturbance from the reflected light, the incident light was not perpendicular to the input facet of waveguide on purpose.
3. Results and discussion
The reconstructed refractive index distribution is depicted in Fig. 2(a) following the method described in reference  at the wavelength of 810 nm. The waveguide is nearly a buried one. The maximum index change is estimated to be ~0.004 below the surface of sample. Different from pure surface waveguides, the buried waveguide holds the propagation mode with a relative symmetry shape along the direction vertical to the surface. Besides, this structure is less sensitive to the crystal imperfection on the surface. According to the reconstructed refractive index distribution, the propagation mode of the waveguide is simulated by the beam propagation method (Rsoft© BeamProp 8.0) as shown in Fig. 2(b). Compared with the measured one in Fig. 2(c), it shows a good agreement between the experimental and simulated modal profiles, which suggests that the refractive index distribution is reasonable.
Figure 2(d) shows the measured propagation mode profile at the wavelength of 1064 nm. The FWHM is ~5 μm parallel to the surface and ~2.5 μm in vertical direction. As can be observed between the pump and signal light, the shape of the propagation mode is almost the same at 810 nm and 1064 nm. Moreover, the FWHM of propagation modes has a fluctuation ~0.2 μm within the wavelength range of 810 nm - 1064 nm. It means that this structure is a stable single mode waveguide with the operating range including the wavelength of pump (810 nm) and signal (1064 nm) light. Good overlap of pump and signal modes suggest a significant reduction of the signal reabsorption.
The fluorescence performance and the potential of the 1064 nm signal amplifier are observed by pumping with the 810 nm laser from a Ti-sapphire laser. The power of the absorbed pump light is modulated to ~17.7 mW. In this way, the Nd ions are excited from the ground state (4I9/2) to 4F5/2, and rapidly transit to the metastable state (4F3/2). The subsequent transition to the ground state gives rise to the 4F3/2→4I11/2 luminescence band. Figure 3 compares the room-temperature luminescence spectra obtained from the Nd:YAG ceramic waveguide (red dash line) and the bulk (blue solid line). An emission spectrum is observed with several peaks at 1052 nm, 1061 nm, 1064 nm, 1068 nm, 1073 nm and 1077 nm, which indicates the potential of Nd:YAG ceramic works as optical amplifier for a broadband spectrum. Comparing the emission from the waveguide and the bulk, it can be observed that the fluorescence in the waveguide is enhanced at the 1064 nm range. We believe the fluorescence improvement is coming from the contribution of the waveguide resonant, as the waveguide structure with two optical polished end facets can works as a resonant cavity.
The waveguide amplifier is characterized by measuring the gain (the ratio of the amplified and the input signal power) as a function of the input power depicted in Fig. 4(a). The measured maximum gain is around 50 with 0.7 μW of the input signal. Considering the continuous wave steady-state pump and signal conditions, the performance of the amplifier can be established by the theoretical model .
where Ii is the power of input signal; G is the measured gain; Is is the saturation power; G0 is the signal gain when the power of signal is far less than Is; g is the small signal gain; L is the length of sample. Is and G0 are constants decided by the optical amplifier. Based on Eq. (1), the measured experiment results are fitted in Fig. 4(a) and the G0 (and Is) is obtained to be 70 (and 35 μW). G0 is considered as the theoretical maximum effective gain induced by this Nd:YAG ceramic waveguide amplifier. According to Eq. (2), the small signal gain of this optical amplifier is 26.3 dB/cm for the signal amplification at 1064 nm, which is 18.4 dB considering the 0.7 cm length of the waveguide. Besides the amplification of the pulse signal, the experiment result reveals an interesting narrowing of the signal pulse (Fig. 4(b)). Without the pump laser, the duration of the signal laser is around 18 ns. After amplification, it drops down to 9 ns. The phenomenon was observed in literatures .
In conclusion, we have fabricated the optical waveguide amplifier in the Nd:YAG ceramic through the swift carbon ion irradiation. The 1064-nm pulse laser amplification (g = 26.3 dB/cm) has been realized by the enhanced florescence in conjunction with the excellent guiding properties in the waveguide. The net small signal gain value of this 7-mm long waveguide was determined to be ~18.4 dB. It suggests that the carbon ion irradiated Nd:YAG ceramic waveguides may serve as new integrated optical amplifiers.
This work is carried out under the support by the National Natural Science Foundation of China (No. 10925524) and the 973 Project (no. 2010CB832906) of China. Tan acknowledges the support by the Independent Innovation Foundation of Shandong University (IIFSDU, No. 104222012GN056 / 11160072614098) and China Postdoctoral Science Foundation (Grant No. 2013M530316). The work at Helmholtz-Zentrum Dresden-Rossendorf is supported by the Helmholtz-Gemeinschaft Deutscher Forschungszentren (HGF-VH-NG-713).
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