Rare-earth-doped optical waveguide amplifiers (RDWAs) have been widely investigated over the past few years because of their low cost, compensation for optical loss, compatibility with silicon substrates, and potential applications in integrated optical systems. Among the rare-earth elements, neodymium has received significant attention because it can achieve optical amplification at 1.06 µm. 808 nm semiconductor lasers are often selected according to the intrinsic absorption of Nd³⁺ ions from ⁴I_9/2 state to ⁴F_5/2 state to achieve the population inversion of Nd³⁺ ions. However, semiconductor lasers with a high pump power (100-400 mW) cause thermal damage to the waveguides and induce the up-conversion parasitic effect. Moreover, it is difficult to reduce their commercial costs. Therefore, three low-cost light-emitting diodes (LEDs) with different central wavelengths are selected as pump sources to achieve optical gains in neodymium-complex-doped polymer waveguides.

Using thermal ion exchange technology, a group of Ag⁺-K⁺ ion-exchanged glass waveguides is fabricated in a BK-7 optical glass. A group of rectangular SU-8 polymer waveguides with a cross-section of 8 μm×5 μm is fabricated by a one-step lithography process. Next, an active polymer material, neodymium complex Nd(TTA)₃DBTDPO-doped PMMA polymer, is spin-coated as the top cladding on the surfaces of the two types of waveguides. The room-temperature absorption and photoluminescence (PL) spectra of Nd(TTA)₃DBTDPO-doped PMMA polymer films are measured. Under the excitation of three blue-violet LEDs with different central wavelengths, optical gains are achieved in waveguides based on evanescent-wave coupling.

For the evanescent-wave optical waveguide based on Ag+-K+ ion-exchanged glass with a length of 10 mm, the output optical intensity increases with the increasing pump power when the input signal power is 0.03 mW at 1.06 μm wavelength [Fig. 5(a)]. For a fixed signal power, the relative gain increases linearly with increasing pump power [Fig. 5(b)]. When the input signal power is 0.03 mW at 1.06 μm and the pump power is 225 mW, the relative gains of 3.6 dB/cm, 2.2 dB/cm, and 0.9 dB/cm are obtained under LED excitation with central wavelengths of 405, 581, and 745 nm, respectively. The relative gain under the excitation of the 581 nm LED is better than that of the 745 nm LED because of the increased absorption coefficient of Nd3+ ions. The organic ligand can realize the transition from a ground state (S0) to a high-energy singlet state (S1) by absorbing 405 nm pumped light energy and then transit the energy to a triplet state (T) by intersystem crossing. Moreover, Nd3+ ions can be excited from the ground state 4I9/2 to 4F9/2 through energy transfer from the organic ligands in the triplet state (T) (Fig. 7). After relaxation to the 4F3/2 level, the luminescence at 1060 nm (4F3/2→4I11/2) is achieved. Therefore, the relative gain under the excitation of the 405 nm LED is better than those under the excitations of the 581 nm and 745 nm LEDs. For the neodymium-doped polymer waveguide with a length of 8 mm based on evanescent-wave coupling, the relationship between the output signal intensity and pump power shows the same trend as that for the ion-exchanged optical waveguide (Fig. 6). When the pump power reaches 225 mW, a relative gain of approximately 4.1 dB/cm is obtained. Compared to the ion-exchanged waveguide, which only has a neodymium-doped polymer attached to the upper layer of the waveguide, the three sides of the polymer waveguide are surrounded by neodymium-doped polymer. Thus, the polymer waveguide exhibits an increased amplification ability.

In this study, a new LED top-pumping mode based on an evanescent-wave optical waveguide is proposed. The relative gain based on two types of optical waveguides with Nd(TTA)₃DBTDPO complex-doped PMMA polymer as the top cladding is demonstrated. An intramolecular energy transfer mechanism from the organic ligands (DBTDPO and TTA) to the central Nd³⁺ ions has been established. Using the vertical top pumping mode of the LED, the up-conversion parasitic effect and waveguide thermal damage caused by traditional laser pumping can be overcome because the incident power of the LED is almost uniformly distributed on the waveguide surface. The neodymium complex Nd(TTA)₃DBTDPO-doped PMMA polymer used in this study can be easily spin-coated on various waveguides, such as silicon on insulator (SOI), silicon nitride, polymer, and glass, to realize a loss compensation at 1.06 µm. The vertical top pumping mode of an LED based on intramolecular energy transfer can significantly reduce the commercial cost of the device and has potential market application value.