1. Introduction

As a key active photonic device, rare-earth-ion-doped optical waveguide amplifiers have attracted continuous interests over the last decades [1–4]. Compared with the a bulk system, the amplifiers based on waveguides are active platforms in micron scales, in which enhanced gains may be achieved due to the better light confinement and better heat dissipation [5, 6]. With the compact geometry, the waveguide amplifier can be applied in the integrated optical circuit to compensate the loss. Until now, a number of active materials have been used for optical signal amplification such as rare earth doped yttrium aluminum garnet (YAG), lithium niobate, glasses, polymers and aluminum oxides [7–11].

Rare earth doped YAG crystal has excellent lasing properties. As a gain medium, it has strong fluorescence in the near-infrared band, which makes them attractive candidates for optical amplifiers and high power solid state lasers [12–14]. Until now, the excellent performance of rare earth doped YAG crystal fiber has been reported, which is fabricated by the laser-heated pedestal growth (LHPG) method [15, 16]. For further applications in integrated optical circuits, the fabrication of the optical waveguides in YAG crystals is desirable, which have smaller size and a more compact geometry. A few techniques have been developed for fabricating low-loss waveguides based on YAG crystals, including ion irradiation/implantation [17–20], direct ultrafast laser writing [21–23]and proton beam writing [24, 25]. Among them, the proton beam writing is a relative novel technology for the waveguide structure formation in crystal, which has the intriguing advantage of direct fabrication and realizing maskless implantation due to energetic focused protons. In Nd:YAG crystal, it is proved that the fluorescence property of the crystal can be well preserved and efficient laser emission was obtained [24, 25].

In this work, we report the optical amplifier based on the proton beam written Nd:YAG waveguide. The Nd:YAG waveguide was applied as the gain medium. Under the pumping laser (wavelength tunable from 800 nm to 825 nm), the optical signal amplification was observed in the near infrared band. The maximum gain obtained was 24 dB/cm at a wavelength of 1064 nm and 6 dB/cm at 1319 nm, respectively. This work demonstrates the application of the focused proton beam writing waveguide in Nd:YAG crystal as an optical amplifier with high efficiency.

2. Experiments

The waveguide used in this work was fabricated using the proton beam writing. Nd:YAG (doped by 1 at.% Nd³⁺ ions) crystal used in this work was cut into dimensions of 4 mm × 10 mm × 2 mm (x × y × z). Two biggest facets (4 mm × 10 mm) of the crystal were optical polished and the sample is mounted onto a motorized three axis stage (Exfo inch-worm stage). The proton beam of 2 MeV energy is focused down to spot size of 1 μm. The waveguide is formed in the crystal by scanning the focused proton beam from one polished facet to the other by moving the stage in x direction with a constant speed and at the same time the beam was scanned over a width of 5 μm in y direction. Writing through x direction, the channel waveguide with the length of 4 mm was formed inside the Nd:YAG crystal with a proton fluence of 2 × 10¹⁶ ions/cm². Then, two small facets (2 mm × 4 mm) were polished vertical to the waveguide direction as the input and output facets for the waveguide. The proton beam writing is performed using facility at Centre for Ion Beam Applications, National University of Singapore, Singapore.

The fluorescence from the Nd:YAG waveguide was detected by the end-face coupling method. During the experiment, a pump laser at 810 nm with power less than 30 mW was coupled into the input of the waveguide. The output light from the waveguide was collected by a microscope objective and the spectral information was obtained using a spectrograph (measurement error of 1 nm). The experimental setup for the optical signal amplifier was shown in Fig. 1. A continuous wave (cw) tunable Ti:Sapphire laser (Coherent MBR PE) was utilized as the pump source. Meanwhile, a pulse laser at the wavelength of 1064 nm with pules width of 18 ns was used as the signal light. The power and polarization of the pump and signal light were controlled by an intensity modulator constituted by a Glan-Taylor prism and waveplates. Through a beam splitter and a lens, the pump laser and signal laser were coupled into the waveguide simultaneously. The output light from the waveguide was collected by a long working distance microscope objective. Compared with the power of the signal light with and without the pumping laser, the gain of this waveguide amplifier was measured.

 

Fig. 1 The schematic plot of the experimental setup for the signal amplification in the focused proton beam writing waveguide.

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The cross-sectional microscope image of the proton beam written channel waveguide is depicted in Fig. 2(a). The structure was constructed at the depth of ~26 μm below the sample surface, which is in good agreement with the projected range of 2MeV protons inside the YAG crystal. Figures 2(b) and 2(c) show the measured propagation modal profiles (fundamental mode) at the wavelength of 810 nm and 1064 nm, respectively, which possess good symmetry of the intensity distribution. In addition, the diameter of waist of the modes is about 4μm, which exhibits good compactness of structure dimensions. The propagation loss of the waveguide was measured to be less than 1 dB/cm at the wavelength of 1064 nm.

 

Fig. 2 (a) Optical transmission microscope image of the waveguide cross section. The measured intensity distribution of the propagation mode at the wavelength of (b) 810 nm and (c) 1064 nm, respectively.

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The fluorescence from the Nd:YAG waveguide was shown in Fig. 3, in which the Nd ions were excited from the ground state ⁴F_3/2 to ⁴I_11/2 (~1064 nm Fig. 3(a)) and ⁴I_13/2 (~1.3 μm Fig. 3(b)), respectively. The measured spectra were compared with the room-temperature luminescence from the Nd:YAG crystal under the same measurement conditions. In Fig. 3(a), several peaks at 1053 nm, 1064 nm, 1073 nm and 1113 nm were observed indicating the potential for the optical signal amplifier at multiple wavelengths. Compared with the bulk material, the intensity of the fluorescence was decreased within the 1064 nm range. Meanwhile, peaks at 1319 nm and 1335 nm were also obtained in ~1.3 μm band. However, the peak intensity of the fluorescence was increasing compared with Nd:YAG crystal.

 

Fig. 3 Comparison of the room temperature luminescence emission spectra correlated to Nd³⁺ ions at ⁴F_3/2 to ⁴I_11/2 (a) and ⁴F_3/2 to ⁴I_13/2 (b) transitions obtained from the channel waveguide (red line) and the bulk (blue line).

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The variation of the luminescence intensity was also reported in Ref [26, 27]. On the one hand, the ion irradiation process would generate defects within the waveguide region. And defects will induce the quenching of the emission lines through radiative energy transfer and scattering processes [26]. On the other hand, the waveguide structure has higher refractive index than the substrate. The emission light could be confined to the waveguide region. So the detected intensity will be higher [27]. In this situation, we believe the fluorescence variation in the waveguide was a combination of these two effects, which dominate in different emission bands.

Figure 4(a) shows the shape of the pulse train of the signal light at the wavelength of 1064 nm (λ_s). As one can see, there were weak and strong pulses alternately. The intensity ratio of the weak and strong pulse was ~0.56 and the average power of the signal light was controlled to be less than 10 μW. Without the pumping laser, the pulse train of the output signal light was similar to the input light and the power was slightly decreased (Fig. 4(b)). With the pump laser, the intensity of the pulse train was increased and the strength ratio remain stable ~0.56. It indicates the signal light at 1064 nm was amplified in the waveguide structure with the same waveform.

 

Fig. 4 (a) The pulse train of the input signal at the wavelength of 1064 nm. The pulse train of the output light without (b) and with (c) pumping light.

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Figure 5 shows the gain variation along with the incident pump laser. At first, the wavelength of the pump wavelength was fixed at 815 nm and the pump power was changed from 20 mW to 100 mW. A linear variation for the gain (λ_s = 1064 nm) was observed with pump power as shown in Fig. 5(a). The maximum value of gain was 24 dB/cm (a net gain of 9.6dB for a 4-mm long sample), corresponding to the 100 mW pump laser. Then, the pump power remained the same at 100 mW and the wavelength of the pump laser was changed from 800 nm to 825 nm. The gain spectrum was shown in Fig. 5(b). The value of gain was around 23 dB/cm within the wavelength range of 803 nm ~817 nm. This result indicates that the Nd:YAG waveguide has a broad absorption band which enabled the stable optical signal amplification.

 

Fig. 5 (a) Measured gain (λ_s = 1064 nm) as a function of the pumping power (λ_p = 815 nm). (b) The variation of gain along with the wavelength of the pumping light at the pumping power of 100 mW.

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To measure the optical absorption and gain spectra, an unpolarized chopped light (λ_s 1.26~1.38 μm) from a diode laser was utilized as the signal light and coupled into the waveguide using a fiber. The output light was monitored by a monochromator. By setting the pumping power to zero, an absorption spectrum was measured in the wavelength range of 1260 ~1380 nm. As depicted in Fig. 6, the absorption was increased from 1 dB/cm to 4 dB/cm along with the wavelength variation from 1260 nm to 1380 nm. Optical amplification was observed when the pump power was increased to 80 mW (λ_p = 808 nm). At the signal wavelength of 1319 nm, 1336 nm and 1356 nm, the gain was 6 dB/cm, 4 dB/cm and 1.4 dB/cm, respectively. The gain is much lower than the one at 1064 nm, as the fluorescence intensity of Nd ion was higher at 1064 nm.

 

Fig. 6 Measured absorption and gain spectrum in the wavelength range of 1260 nm – 1380 nm (λ_p = 808 nm).

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4. Conclusions

We reported on the optical amplification based on the Nd:YAG waveguide. The waveguide structure was produced by focused proton beam writing. Pumping by a cw laser within the wavelength of 800 nm ~825 nm, the signal amplification at the wavelength of 1064 nm and the range of 1330 nm ~1360 nm was observed. The maximum gain at 1064 nm and 1319 nm was as high as ~24 dB/cm and 6 dB/cm, respectively. This work indicates the focused proton beam writing as an excellent candidate to fabricate the waveguide amplifier with high efficiency.