Monolithically integrated erbium-doped tunable laser on a CMOS-compatible silicon photonics platform

: A tunable laser source is a crucial photonic component for many applications, such as spectroscopic measurements, wavelength division multiplexing (WDM), frequency-modulated light detection and ranging (LIDAR), and optical coherence tomography (OCT). In this article, we demonstrate the first monolithically integrated erbium-doped tunable laser on a complementary-metal-oxide-semiconductor (CMOS)-compatible silicon photonics platform. Erbium-doped Al 2 O 3 sputtered on top is used as a gain medium to achieve lasing. The laser achieves a tunability from 1527 nm to 1573 nm, with a >40 dB side mode suppression ratio (SMSR). The wide tuning range (46 nm) is realized with a Vernier cavity, formed by two Si 3 N 4 microring resonators. With 107 mW on-chip 980 nm pump power, up to 1.6 mW output lasing power is obtained with a 2.2% slope efficiency. The maximum output power is limited by pump power.

Compared to lasers using III-V semiconductor gain media, lasers based on erbium-doped gain media have a wide gain bandwidth across the C and L bands. Such wide emission spectrum enables a wide tuning range of the laser wavelength [29][30][31] as well as potential for mode-locking [31][32][33][34]. Additionally, erbium-doped lasers can achieve a narrow linewidth with large side mode suppression ratios (SMSRs) due to homogeneously-broadened gain [35,36]. Since erbium can be co-sputtered with a host (e.g. silica, alumina or phosphate glass) [37][38][39], integration into a CMOS-compatible silicon photonics platform is straightforward as a back-end step. Monolithically integrated erbium-doped waveguide lasers have been demonstrated with both continuous-wave and pulsed lasing using erbium co-sputtered with Al 2 O 3 as a host [33,[40][41][42][43][44][45]. The low thermo-optic coefficient of Al 2 O 3 enables robust operation of the laser over a wide temperature range [46,47], important for control-free WDM systems [48]. However, previously demonstrated lasers could not be actively tuned. Lasers using erbium-doped fiber as gain media with integrated silicon microdisk cavities have been demonstrated with passive [49] and active [50,51] wavelength tuning. However, these demonstrations were mostly fiber based and not fully integrated on-chip.
In this article, we present the first tunable monolithically integrated erbium-doped laser on a CMOS-compatible silicon photonics platform. Wavelength tunability is achieved by utilizing a Vernier structure, which is formed by two Si 3 N 4 microring resonators. Erbiumdoped Al 2 O 3 is deposited as a back-end step and used as the gain medium, and metal layers for thermal tuning are deposited as heaters and contacts. Wavelength tuning over 46 nm (from 1527 to 1573 nm) with more than 40 dB SMSR is achieved. With 107 mW on-chip 980 nm pump power, up to 1.6 mW output lasing power is obtained with a 2.2% slope efficiency. The fine-tuning capability of the lasing wavelength is demonstrated by tuning a separate heater in the gain cavity that shifts the longitudinal cavity modes. The signal/heater response time is measured to be around 200 µs and 35 µs for coarse and fine tuning, respectively. In addition, the laser linewidth is measured to be 340 kHz by using a self-delay heterodyne method. Furthermore, the laser signal is stabilized by continuous locking to a mode-locked laser (MLL) over 4900 seconds. The peak-to-peak frequency deviation of the signal remains below 10 Hz during the measurement period. The system with stabilized tunable optical frequency can be further developed as an optical frequency synthesizer.

Integrated tunable laser design
A schematic perspective view of the tunable laser design is shown in Fig. 1(a), which includes all the layers used in the silicon photonics platform. It has a compact footprint of 0.23 cm 2 (1 mm × 2.3 cm). Figure 1(a) includes the zoom-in view of the wavelength tuning components, which consists of two microring filters in a Vernier configuration and a gaincavity longitudinal-mode phase shifter with metal heater layer on top. The rings are made of 200 nm thick and 1.6 µm wide Si 3 N 4 with a radius of 100 µm and 104.6 µm, thereby giving a free spectral range (FSR) of 2.23 nm and 2.13 nm respectively. In a Vernier configuration, this gives a combined FSR of 50 nm for wide tuning within the erbium gain bandwidth. The coupling gap is designed to be 563 nm, resulting in 8.75% calculated power coupling for the signal wavelength while no 980 nm pump light couples. This is due to the large confinement of the pump in the Si 3 N 4 bus waveguide, thereby obtaining a low-loss pump/signal combiner inside the laser cavity. Power coupling more than 8.75% leads to a wider bandwidth for each resonance of the microrings, and hence side mode extinction will be lower for the laser. In contrast, power coupling lower than the optimized value leads to more energy trapped in microring, and, hence, the intrinsic loss of the ring will dominate. Therefore, the 8.75% power coupling is chosen to balance between the side-mode extinction and intrinsic loss of the ring during the laser design.
The length of each gain-cavity longitudinal-mode phase shifter is 500 µm and a 2π phase shift can be readily achieved with about 200 mW electrical power applied on it. Both the gain cavity phase shifters and rings can be thermally tuned separately using a TiN/Al alloy metal layer above the waveguide. The heater layer has a width of 5 µm, and is located 2 µm above the Si 3 N 4 layer to ensure optical isolation. The resistivity of the heater metal alloy is designed to be 15 Ω/sq. The heater layer is connected to the contact pads through vias. Both contact pads and vias form the M1 layer, which is made of a low resistance TiN/Al alloy for electrical routing.
The optical mode in the Si 3 N 4 layer is coupled to the laser gain waveguide through an adiabatic transition. The transition loss is measured to be 0.3 dB/transition. The gain waveguide is formed by a 1.1 µm thick Al 2 O 3 :Er 3+ film deposited within a 4 µm deep and 5 µm wide tren pump is show thermal tunin waveguide cr layer as an confinement f fundamental solver. The re waveguide is shows the mo mode mismat is simulated t measurements The simu provides an F filter respons longitudinal-m lasing mode h  Fig. 1 color fluoresce ). The electrica shows a scann he depth of the illustrated in he Al 2 O 3 :Er 3+ e calculated to es used in the for a >4 cm lon the signal wav ese modes is re with a minim nd loss is estim e of two com m. Figure 2 with the lase is 2.5 GHz, fo ctivity over the ion of an integrat nd gain cavity pha bium green color f waveguide cross and signal wavele de at the signal wav ence due to the al probe is in c ning electron m e trench is accu n Fig. 1 Fig. 1(a) Figure 5(b wave signal wi onential functi re 5(c) shows by a square w mes obtained ar for the gain ca mal conductivi f-pulsing are no hold of the dev tup: the tunable l r. The beat signal wer applied on g response tim ected by a PD nce of time d power is slight havior with a fr lained by the i d to beyond la equency is inc in Fig. 5(a) Fig. 7(b). Du relative to th g 10 −16 after 10 rogrammable g system with a s al frequency sy plication [5].
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Conclusio
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Funding
Defense Adv Synthesizer (D