Objective The amplification function that describes an optical signal can be realized in rare-earth-doped polymer optical waveguide amplifiers based on the stimulated radiation of rare-earth ions when they experience excitation at the pump source. As an active device, polymer optical waveguide amplifier can be integrated with multiplexer/demultiplexer, beam splitter, optical switch, and other devices to compensate for various losses in the optical field that may occur during device transmission. To fabricate optical waveguide amplifiers, we typically use an SU-8 photoresist polymer and polymethyl methacrylate (PMMA) as the doping matrices for rare-earth ions. Further, to ensure population inversion of the produced rare-earth ions, pump sources are usually required. A majority of the research spanning the past three decades has focused on selecting semiconductor lasers as the pumping sources. Compared with the end-coupled pumping method using semiconductor lasers as its pumping source, the use of a low-power and low-cost light emitting diode (LED) is a new development trend that can effectively solve the problems of up-conversion problems and polymer waveguide damage caused by high-power semiconductor laser pumping (200--400 mW) sources. Additionally, this development greatly reduces the commercialization costs involved in fabricating these devices and is expected to replace the traditional pumping method of semiconductor lasers. The absorption of the pump source by the polymer matrix material directly affects the gain performance of the rare-earth-doped polymer optical waveguide amplifier. However, SU-8 and PMMA materials are seldom reported to negatively impact absorption performance during the excitation of ultraviolet (UV)-visible LEDs. Based on this point, we utilized SU-8 and PMMA materials in a core layer and fabricated these materials via lithography and reactive-ion etching processes to form passive polymer optical waveguides. We demonstrated the absorption characteristics of polymer optical waveguides with pump sources derived from four different wavelengths of LEDs.
Methods Using a one-step lithography process, a rectangular SU-8 polymer waveguide and a Mach-Zehnder waveguide with a cross-section of 5 μm×5 μm were fabricated. A rectangular PMMA waveguide as core material was prepared via lithography and reactive-ion etching. Next, the morphology of these waveguides was characterized using scanning electron microscopy. Using a vertical top pumping mode, the absorbabilities of the SU-8 and PMMA polymer waveguides were measured at 1064, 980, and 635 nm wavelengths under the excitation wavelength of 310, 365, 405, and 525 nm for the LED-based approach as well as an excitation wavelength of 808 nm using the vertical top pumping mode.
Results and Discussions For the polymer SU-8 waveguide with a cross-section of 5 μm×5 μm and a length of 20 mm, the optical intensity attenuation reached ~91.7%, 48.3%, and 26.7% at 1064-nm wavelength laser under the LED with excitation wavelength of 310, 365, and 405 nm and 50-mW pump power. The optical intensity could remain stable under the excitation wavelength of 525 and 808 nm using LED and laser, respectively [Fig. 5 (a)]. The optical intensity attenuation reached ~70.8%, 41.1%, and 24.2% [Fig. 6(a)] at 980-nm wavelength laser under the LED with excitation wavelength of 310, 365, and 405 nm, respectively. There was no obvious attenuation of the optical intensity under laser pumping at 635-nm wavelength [Fig. 6(b)]. For an SU-8 polymer waveguide with a length of 20 mm, a thickness of 5 μm, and the widths of 4, 6, and 8 μm, the optical intensity attenuations of approximately 53.1%, 65.1%, and 70.6%, respectively, were obtained at laser with wavelength of 1064 nm for LED with an excitation of 310 nm and 80-mW pump power (Fig. 8). Turning to the SU-8 polymer Mach-Zehnder waveguide, using a 50-mW LED-based pump source, the optical intensity attenuations of approximately 98.9%, 38.1%, and 24.1% were obtained at a 1064-nm wavelength laser for LED with excitation wavelength of 310, 365, and 405 nm, respectively. The optical intensity remained stable for an excitation wavelength of 525 nm using the LED-based approach [Fig. 10(a)]. There was no obvious optical intensity attenuation for the LED-based approach at 635-nm wavelength [Fig. 10(b)], which aligns with results obtained for the rectangular straight waveguide. Finally, for the PMMA polymer waveguide, the optical intensity remained stable for the excitation wavelength of 310, 365, and 405 nm using the LED-based approach (Fig. 11).
Conclusions In this study, we measure the optical absorption performance of SU-8 and PMMA polymer optical waveguides with excitation wavelength of 310, 365, 405, and 525 nm for an LED-based pump source, as well as an excitation wavelength of 808 nm for the traditional laser-based approach. Our experimental results show that when pumped by a blue-violet LED, the resulting optical intensity of the SU-8 polymer waveguide sharply decays at the wavelengths of 980 and 1064 nm. Additionally, we observe that the optical intensity attenuation weakens with a red shift of the center wavelength of the LED pump source and a decrease in the size of the polymer waveguide. For the SU-8 polymer waveguide with a cross-section of 5 μm × 5 μm and a length of 20 mm, using a 50-mW LED-based pump source, we attain an optical intensity attenuation of approximately 91.7%, 48.3%, and 26.7% at laser source with wavelength of 1064 nm for the LED with excitation wavelength of 310, 365, and 405 nm, respectively. Conversely, both the 525-nm LED-based approach and the 808-nm traditional laser-based approach, the resulting optical intensity of the SU-8 polymer waveguide remains stable. Further, for the PMMA polymer waveguide, no obvious optical intensity attenuation was observed under the excitation of LEDs. Therefore, we conclude that in rare-earth-doped SU-8 polymer optical waveguide amplifiers pumped by blue-violet LEDs, single-mode and small-size waveguides with a low-power LED pump source should be used to effectively avoid optical intensity attenuation. Here, we note that it is easier to achieve the optical gain using a PMMA polymer as the host for the rare-earth solution when pumped by LEDs.