Fiber-device-fiber gain from a sol-gel erbium-doped waveguide amplifier

Abstract

Abstract-An erbium-doped silica-on-silicon planar waveguide optical amplifier is described. The active core is a topographic guide formed from aluminophosphosilicate glass doped with erbium and ytterbium. The buffer is formed from silica deposited by thermal oxidation and the cladding from borophosphosilicate glass obtained by plasma-enhanced chemical vapor deposition. The use of low process temperatures allows relatively heavy doping and careful control of the core etching allows low background insertion losses to be obtained. Spontaneous emission and gain measurements are given and 5.4-dB fiber-device-fiber gain is demonstrated using a 5-cm-long chip pumped using a 980-nm laser diode at 175-mW pump power.Index Terms-Erbium, optical amplification, sol-gel, waveguides.T HE ERBIUM-DOPED fiber amplifier (EDFA) has been highly successful as a low-noise gain block at 1535-nm wavelength. Er-doped waveguide amplifiers (EDWAs) have also attracted attention, because of their potential for integration with pump lasers and filters [1]. The main difficulty in constructing an EDWA lies in providing suitably high gain in a short length, which requires high Er doping without concentration-dependent effects.Erbium-doped amplifiers and lossless splitters have been formed by ion exchange in silicate [2] at 980 nm, high pump powers are required. However, pump powers are reduced by codoping with Yb , which has a large cross section and which may transfer energy nonradiatively to the I level of Er . Although erbium may be incorporated in silica by PECVD, difficulties are caused by the low vapor pressure of erbium compounds. There is, therefore, a need to investigate alternative processes for depositing Er-doped silicate glasses, which can also incorporate the range of codopants needed to tailor the Er environment. One possibility is the sol-gel process. Thick layers of silicate glasses can be formed as multilayers, using repetitive cycles of spin-coating and rapid thermal annealing (SC-RTA), with careful control of the annealing temperature [9]. These layers may then be formed into buried channel guides by the conventional route of reactive ion etching and cladding.Early sol-gel guides were mainly based on TiO -SiO . However, TiO is unsuitable for sol-gel optics, because of its high process temperatures and tendency to form crystalline phases, including erbium titanate. Since then, improved guides have been developed using a range of dopants such as P O , B O , Al O , and Ga O . Some dopants have been used to control process temperatures in addition to altering the refractive index. Rare earth dopants such as Nb O , Er O , and Yb O have also been incorporated. Some progress has been made toward fabricating EDWAs by SC-RTA. For example, relative gains of 1.1 dB/cm have been obtained using Yb-Er-codoped silica-titania, but only in a striploaded geometry with fiber coupling and propagation losses so high that no external gain was observed [10]. In this letter, we demonstrate the first practical sol-gel EDWA. The device is based on a ...

Molecular homogeneity in erbium-doped sol-gel waveguide amplifiers

“…This was done by measuring the absorption spectrum using single-mode fiber (SMF) at device input and output and again using a multimode fiber (MMF) from the output to the spectrometer, assuming show that the coupling losses of the amplifier to SMF were of the order of 0.1-0.2 dB per facet. Similar results were obtained by using the cut back method in [17]. Waveguide propagation losses were measured at 1620 and 1300 nm, well outside the erbium absorption bands, by subtracting coupling loss (determined as above) from overall insertion loss.…”

“…These buried channel sol-gel guides showed low background loss levels of about 0.1 dB/cm, with high Er O doping levels of 0.25 mol%. This system was the first to achieve net fiber-fiber gain in a sol-gel based Er-doped waveguide amplifier, this net gain being 5.75 dB in a 5-cm-long waveguide [17]. Per unit length, this we believe is the highest reported in the silica-on-silicon format.…”

“…Devices were fabricated as in [17], with only the active guiding layer made by sol-gel. The buffer layer was formed by steam oxidation of the silicon substrate, over which the Er-doped layer was deposited.…”

Sol-gel silica-on-silicon buried-channel EDWAs

“…The peak value of the ASE power spectral density is 41.5 dBm/nm. This figure is similar to a value given in [87], but a correction of 3.5 dB to cover waveguide-fiber joint loss, WDM coupler loss, fiber connector loss, and OSA power accuracy has been made here. Subtracting the ASE from the signal, a corrected variation of gain with pump power may be obtained, decreasing the peak gain from 5.75 to 5.5 dB.…”

“…However, high fiber coupling and propagation losses prevented any net gain being achieved, the quoted gain (1.1 dB/cm) being relative. Recently, the first sol-gel silica-on-silicon EDWA with net fiber-device-fiber gain was demonstrated (again, by us), based on a buried channel guide in Er/Yb : AlPSG [87]. This achievement is a significant advance for sol-gel processing.…”