The spectral response of a distributed-feedback resonator with a thermal chirp is investigated. An Al<sub>2</sub>O<sub>3</sub> channel waveguide with a surface Bragg grating inscribed into its SiO<sub>2</sub> top cladding is studied. A linear temperature gradient along the resonator leads to a corresponding variation of the grating period. We characterize its spectral response with respect to wavelength and linewidth changes of the resonance peak. Simulated results show good agreement with the experimental data, indicating that the resonance wavelength is determined by the total accumulated phase shift. The calculated grating reflectivities at the resonance wavelength largely explain the observed changes of the resonance linewidth. This agreement demonstrates that the linewidth increase is caused by the increase of resonator outcoupling losses.
Distributed-feedback (DFB) laser resonators are widely recognized for their advantage of generating laser emission with extremely narrow linewidth. Our investigation concerns ytterbium-doped amorphous Al<sub>2</sub>O<sub>3</sub> channel waveguides with a corrugated homogeneous Bragg grating inscribed into its SiO<sub>2</sub> top cladding, in which a λ/4 phase-shift provides a resonance and allows for laser emission with a linewidth as narrow as a few kHz. Pump absorption imposes a thermal chirp of the grating period, which has implications for the spectral characteristics of the resonator. Thermal effects on the spectral response of a DFB passive resonator were investigated via simulations using Coupled Mode Theory by considering (i) a constant deviation of the grating period or (ii) a chirp with a linear profile. We report an increase of the resonance linewidth up to 15%. This result is due to two factors, namely changes of the grating reflectivity at the resonance frequency up to 2.4% and of the shift of resonance frequency up to 61 pm due to an accumulated phase shift imposed on the grating by the chirp profile. The linewidth decrease due to gain is on the order of 10<sup>6</sup>, which is a much larger value. Nevertheless, according to the Schawlow-Townes equation the linewidth increase of the passive resonator due to a thermal chirp quadratically increases the laser linewidth.
Spiral-waveguide amplifiers in erbium-doped amorphous aluminum oxide are fabricated by RF reactive co-sputtering of
1-μm-thick layers onto a thermally-oxidized silicon wafer and chlorine-based reactive ion etching. The samples are
overgrown by a SiO<sub>2</sub> cladding. Spirals with several lengths ranging from 13 cm to 42 cm and four different erbium
concentrations between 0.5−3.0×10<sup>20</sup> cm<sup>-3</sup> are experimentally characterized. A maximum internal net gain of 20 dB in
the small-signal-gain regime is measured at the peak emission wavelength of 1532 nm for two sample configurations
with waveguide lengths of 13 cm and 24 cm and erbium concentrations of 2×10<sup>20</sup> cm<sup>-3</sup> and 1×10<sup>20</sup> cm<sup>-3</sup>, respectively.
The obtained gain improves previous results by van den Hoven et al. in this host material by a factor of 9. Gain
saturation as a result of increasing signal power is investigated. Positive net gain is measured in the saturated-gain
regime up to ~100 μW of signal power, but extension to the mW regime seems feasible. The experimental results are
compared to a rate-equation model that takes into account migration-accelerated energy-transfer upconversion (ETU)
and a fast quenching process affecting a fraction of the erbium ions. Without these two detrimental processes, several
tens of dB/cm of internal net gain per unit length would be achievable. Whereas ETU limits the gain per unit length to 8
dB/cm, the fast quenching process further reduces it to 2 dB/cm. The fast quenching process strongly deteriorates the
amplifier performance of the Al<sub>2</sub>O<sub>3</sub>:Er<sup>3+</sup> waveguide amplifiers. This effect is accentuated for concentrations higher than
We report the fabrication and optical characterization of long, spiral-shaped erbium-doped aluminum oxide (Al<sub>2</sub>O<sub>3</sub>:Er<sup>3+</sup>) channel waveguides for achieving high overall signal amplification on a small footprint. Al<sub>2</sub>O<sub>3</sub>:Er<sup>3+</sup> films with Er<sup>3+</sup> concentrations in the range between 0.44−3.1×10<sup>20</sup> cm<sup>-3</sup> were deposited by reactive co-sputtering onto standard, thermally oxidized silicon substrates. Spiral-shaped waveguides were designed and structured into the films by chlorinebased reactive ion etching. In the current design, each spiral waveguide occupies an area of 1 cm<sup>2</sup>. Typical background propagation losses near 1500 nm are (0.2±0.1) dB/cm. A commercially available, pigtailed diode laser at 976 nm was employed as the pump source. The erbium-doped waveguide amplifiers were characterized in the small-signal-gain regime at the peak-gain wavelength (λ = 1532 nm) of Al<sub>2</sub>O<sub>3</sub>:Er<sup>3+</sup>. A maximum of 20 dB of internal net gain was measured for a 24.5-cm-long spiral waveguide with an Er<sup>3+</sup> concentration of 0.95×10<sup>20</sup> cm<sup>-3</sup>. Similar results were obtained for a shorter spiral with an Er<sup>3+</sup> concentration about twice as high. Samples with lower concentration exhibited lower gain because of insufficient pump absorption, while samples with higher concentration showed less gain because of migration-accelerated energy transfer up-conversion and, more importantly, a fast quenching process.
We report on diode-pumped distributed-feedback (DFB) and distributed-Bragg-reflector (DBR) channel waveguide lasers in Er-doped and Yb-doped Al<sub>2</sub>O<sub>3</sub> on standard thermally oxidized silicon substrates. Uniform surface-relief Bragg gratings were patterned by laser-interference lithography and etched into the SiO<sub>2</sub> top cladding. The maximum grating reflectivity exceeded 99%. Monolithic DFB and DBR cavities with Q-factors of up to 1.35×10<sup>6</sup> were realized. The Erdoped DFB laser delivered 3 mW of output power with a slope efficiency of 41% versus absorbed pump power. Singlelongitudinal- mode operation at a wavelength of 1545.2 nm was achieved with an emission line width of 1.70 0.58 kHz, corresponding to a laser Q-factor of 1.14×10<sup>11</sup>. Yb-doped DFB and DBR lasers were demonstrated at wavelengths near 1020 nm with output powers of 55 mW and a slope efficiency of 67% versus launched pump power. An Yb-doped dualwavelength laser was achieved based on the optical resonances induced by two local phase shifts in the DFB structure. A stable microwave signal at ~15 GHz with a –3-dB width of 9 kHz and a long-term frequency stability of ± 2.5 MHz was created via the heterodyne photo-detection of the two laser wavelengths. By measuring changes in the microwave beat signal as the intra-cavity evanescent laser field interacts with micro-particles on the waveguide surface, we achieved real-time detection and accurate size measurement of single micro-particles with diameters ranging between 1 μm and 20 μm, which represents the typical size of many fungal and bacterial pathogens. A limit of detection of ~500 nm was deduced.
Erbium-doped aluminum oxide channel waveguides were fabricated on silicon substrates and their characteristics were
investigated for Er concentrations ranging from 0.27 to 4.2 × 10<sup>20</sup> cm<sup>-3</sup>. Background losses below 0.3 dB/cm at 1320 nm
were measured. For optimum Er concentrations in the range of 1 to 2 × 10<sup>20</sup> cm<sup>-3</sup>, internal net gain was obtained over a
wavelength range of 80 nm (1500-1580 nm) and a peak gain of 2.0 dB/cm was measured at 1533 nm. Integrated
Al<sub>2</sub>O<sub>3</sub>:Er<sup>3+</sup> channel waveguide ring lasers were realized based on such waveguides. Output powers of up to 9.5 μW and
slope efficiencies of up to 0.11 % were measured. Lasing was observed for a threshold diode-pump power as low as 6.4
mW. Wavelength selection in the range 1530 to 1557 nm was demonstrated by varying the length of the output coupler
from the ring.