We demonstrate monolithic 160-µm-diameter rare-earth-doped microring lasers using silicon-compatible methods. Pump light injection and laser output coupling are achieved via an integrated silicon nitride waveguide. We measure internal quality factors of up to 3.8 × 105 at 980 nm and 5.7 × 105 at 1550 nm in undoped microrings. In erbium- and ytterbium-doped microrings we observe single-mode 1.5-µm and 1.0-µm laser emission with slope efficiencies of 0.3 and 8.4%, respectively. Their small footprints, tens of microwatts output powers and sub-milliwatt thresholds introduce such rare-earth-doped microlasers as scalable light sources for silicon-based microphotonic devices and systems.
© 2014 Optical Society of America
Over the last several years, advances in the field of silicon photonics have lead to highly-compact, low-cost, and high-performance photonic chips for applications ranging from high-speed optical communications to lab-on-a-chip devices [1–5]. However, because of silicon’s indirect bandgap and low quantum efficiency, one of the main challenges has been the development of silicon-based lasers [6–8]. Ideally such lasers should offer high performance in addition to being compact, monolithic and fabricated using standard silicon techniques to be considered as scalable light sources for silicon microphotonic systems.
Rare-earth-doped waveguide lasers (REDWLs) have shown significant promise as silicon-based lasers because they combine efficient, continuous-wave or short pulse lasing over a wide wavelength range with inexpensive, straightforward and monolithic integration methods [9–21]. In particular, rare-earth-doped aluminum oxide (Al2O3:RE3+) has emerged as a highly versatile and robust laser medium, having recently been applied in wavelength-selective racetrack, ultra-narrow-linewidth distributed feedback (DFB) and highly-efficient distributed Bragg reflector (DBR) monolithic lasers [16–18]. Furthermore, integration of Al2O3:RE3+ DBR and DFB lasers using wafer-scale and silicon-compatible silicon nitride photonic cavity structures has been demonstrated, with up to 75 mW continuous-wave output power [19–21]. Nevertheless, integration of REDWLs into microphotonic platforms is limited by their large size and significant power consumption, with centimeter-scale cavity lengths and thresholds ranging from several to tens of milliwatts.
In an approach which drastically reduces the device size and lasing threshold, high quality factor whispering gallery mode cavities [22,23] have been doped with rare earth ions to realize visible and infrared microtoroid and microdisk lasers on silicon [24–31]. However, such lasers are isolated on the chip surface and require an external fiber to couple pump and laser light to and from the cavity, respectively. Whereas efforts have been made towards a monolithic micrometer-scale integration scheme [32–36], that is, where both microcavity and waveguide are co-integrated on the same chip, to our knowledge, no laser action has previously been demonstrated.
Here, using a design which combines Al2O3:RE3+ with silicon-compatible silicon nitride waveguides, we report on compact, low-threshold microring lasers monolithically integrated on silicon chips. The erbium- and ytterbium-doped microlasers are fully integrated with their excitation and emission bus waveguides via a flexible and high-Q cavity design and effective fabrication technique. We show sub-milliwatt threshold, single-mode lasing both at optical communications wavelengths around 1.5 µm and at 1.0 µm where biophotonic applications are of interest. Our approach allows for straightforward, dense integration of such laser sources within existing silicon-based microphotonic systems.
2. Microring laser design and fabrication
As shown in Fig. 1, our microring laser design is based on a silicon-compatible process flow which uses two silicon nitride (SiNx) layers. The microlaser structure consists of an Al2O3:RE3+-filled trench above five concentric silicon nitride rings, which forms the active microcavity, beside a SiNx bus waveguide for pump injection and laser output coupling. SiNx has a higher refractive index than Al2O3:RE3+ (n = 2 vs. n = 1.65), enabling more compact, high-refractive-index-contrast and low-loss passive photonic devices. However, to achieve net gain in the cavity and lasing, a sufficient proportion of light must propagate in the active medium (Al2O3:RE3+). Thus, we developed a composite resonator structure which supports modes with high optical intensity overlap with the Al2O3:RE3+ layer, but also permits reduced microring diameter due to the SiNx features. The trench also serves to confine the light and reduce the microring bending radius and enables silicon compatibility (the non-standard gain material can be deposited into the trenches outside of the silicon foundry). The segmented silicon nitride cavity design yields a highly wavelength-insensitive mode shape, allowing for high overlap between 980-nm pump modes and 1050-nm Yb- or 1550-nm Er-doped microring laser modes . Meanwhile, the integrated double-layer silicon nitride bus waveguide allows for a high degree of control of waveguide mode properties (i.e. for phase-matching to the microcavity pump modes and effective coupling of pump light to the ring) and low-loss guiding of pump and laser light on the chip.
We fabricated the microring laser chips using a 300-mm CMOS foundry with a 65-nm technology node. First, we deposited a 6-µm-thick SiO2 bottom cladding layer on a 300-mm silicon wafer, followed by deposition and patterning of two 240-nm-thick SiNx (n = 1.94 at 1550 nm) layers with a 100-nm-thick SiO2 layer in between. We deposited all SiO2 and SiNx layers using plasma-enhanced chemical-vapor deposition and surface-polished them after deposition to reduce optical scattering losses. We patterned both SiNx layers using 193-nm immersion lithography and reactive ion etching, yielding 0.6-µm-wide SiNx microring features with 0.4-µm separations and 160-µm outer diameter, waveguide widths, w, of 0.4 or 0.9 µm and microring-waveguide gaps, g, ranging from 0.1 to 1.0 µm (in 0.1 µm steps). Above the top SiNx level, we deposited a 4-µm-thick SiO2 layer, and patterned and etched 4-µm-deep microring trenches using the upper SiNx layer as an etch stop. After removal of the SiNx etch-stop, we deposited an additional 100-nm-thick SiO2 layer in the microring trenches. We then etched deep trenches at the edge of the chips for dicing and fiber end-coupling and transferred the wafers from the silicon foundry. Lastly, we diced the wafers into individual dies and deposited 2-µm-thick undoped, erbium-doped and ytterbium-doped aluminum oxide films over the top of the dies (shown only deposited into the microring trenches in Fig. 1 for clarity) using a reactive co-sputtering process similar to that described in . For the ytterbium-doped lasers we applied a uniform doping profile and concentration of 7 × 1020 cm−3 We selected a relatively high Yb3+ doping concentration because of the low absorption-to-emission cross-section ratio around 1050 nm and negligible concentration-quenching effects observed in Al2O3:Yb3+ . Moreover, relatively high gain was required to overcome the higher internal resonator scattering losses near 1 µm and output coupling as compared to erbium-doped devices operating near 1.5 µm. For erbium-doped lasers, the erbium concentration was varied throughout the layer in order to match the 980-nm pump mode distribution and minimize the laser threshold. We varied the sputtering power applied to the erbium target throughout the deposition, resulting in uniform lateral doping and a graded vertical concentration profile (with peak in the center of the film and approximately 1/3 the peak concentration at the top and bottom of the film). We selected peak erbium concentrations on the order of 2–3 × 1020 cm−3 – high enough to achieve higher gain than cavity losses, but low enough to maintain low threshold lasing and avoid significant concentration quenching mechanisms . We summarize the fabrication steps in Fig. 1(a) and the resulting structure is illustrated in Fig. 1(b).
We show images of the fabricated microring lasers in Fig. 2. A top view of the monolithic laser structure is displayed in Fig. 2(a), with the integrated SiNx bus waveguide visible below the microring. In Fig. 2(b), we show a top-view scanning electron micrograph (SEM) image of the microring trench structure on the silicon chip. Figure 2(c) shows a focus-ion-beam-milled cross-section of the microring-waveguide coupling region at the edge of the trench. The image displays the 2-µm-thick Al2O3:RE3+ layer on the bottom of the trench as well as a thinner Al2O3:RE3+ layer deposited on the trench sidewall due to the conformal sputtering process (the material visible above the Al2O3:RE3+ is metal deposited for imaging purposes). In Fig. 2(d), we display a close-up view of the coupling region, as indicated by the red box in Fig. 2(c). It reveals a slight offset between the top and bottom silicon nitride layers, as well as residual SiNx etch-stop material at the edge of the trench (both of which fall within the design tolerance of the lasers). Figure 2(d) clearly shows the integrated SiNx bus waveguide and indicates the waveguide width and microring-waveguide gap.
3. Microcavity and microlaser characterization
3.1 Measurement setup
We carried out transmission and laser measurements using a fiber end-coupling setup. For passive transmission measurements, light from a fiber-coupled 960–990-nm tunable laser (< 200 kHz linewidth, 1-pm minimum step size) or 1460–1630-nm tunable laser source (100-kHz linewidth, 0.1-pm minimum step size) was coupled through a polarization controller and end-coupled to the chip via a single-mode 980-nm (SM980) cleaved fiber. We coupled light from the chip using another SM980 cleaved fiber and measured the transmitted optical power using an InGaAs detector. For laser measurements, on the input side, pump light from the 960–990 nm tunable laser source or a 976-nm diode laser (1-nm linewidth) was coupled to a polarization controller, followed by a 980-nm variable-optical attenuator (VOA), a 99%/1% tap and a 980/1050 nm (Yb-doped microring lasers) or 980/1550 nm (Er-doped microring lasers) fiber-based wavelength division multiplexer (WDM). We coupled light to and from the chip using SM980 bare fibers and coupled the output fiber to another fiber WDM to separate the residual pump and laser light. We adjusted and monitored the incident pump power using the VOA and output from the 1% branch of the tap, respectively. We measured the laser output powers and optical intensity spectra by coupling the 1050 or 1550 nm branch of the WDM from each side of the chip to an optical spectrum analyzer (600–1700 nm, 20-pm resolution). Time domain measurements were carried out by coupling the laser output to an amplified 10-MHz photodetector connected to an oscilloscope.
3.2 Passive transmission measurements
Figures 3–5 summarize 980-nm and 1550-nm transmission measurements in undoped (passive) microrings. In Fig. 3, we show the resonantly-coupled 980-nm pump power vs. microring-waveguide gap. We measured the coupling for the transverse-electric- (TE-) and transverse-magnetic- (TM-) like pump modes near 980 nm (labeled TE1 and TM1, see Fig. 4
) and w = 0.4 µm (the same waveguide width as for Er- and Yb-doped microring lasers). For both polarizations we observed optimum coupling near g = 0.5 µm, where the internal quality factor of the resonator, Qi, matched the external quality factor, Qe .
Figure 4 (top) shows 980-nm transmission measurements for a device with w = 0.4 µm and g = 0.7 µm and both TE and TM polarizations (left and right, respectively). The insets show the intensity profiles of the optimum pump modes (TE1 and TM1) calculated using a finite element mode solver, with their resonances indicated on the plots. Their resonances were differentiated from those of the lowest order TE and TM modes (TE0 and TM0), which calculations show are strongly confined in the SiNx layer, and higher-order, lossier modes by their free-spectral ranges, 1.10 and 1.12 nm, respectively. By fitting the high-resolution transmission responses of the under-coupled resonator using a Lorentzian function (Fig. 4, bottom) , we obtain internal quality factors, Qi, of 3.8 × 105 and 2.7 × 105 (TE1 and TM1, respectively). These Qi correspond to propagation losses per unit length of 1.3 and 1.8 dB/cm at the pump wavelength in the cavity.
Figure 5 (top) shows 1550-nm transmission measurements in similar ring structures with w = 0.9 µm (phase matched to the 1550-nm resonator modes) and g = 1.0 µm. The inset images show the calculated intensity profiles of the TE1 and TM1 modes. By fitting the resonances (Fig. 5, bottom), we obtain Qi on the order of 5.7 × 105 and 4.2 × 105 for TE and TM polarization, respectively. These quality factors correspond to propagation losses of 0.5 and 0.7 dB/cm. Given the peak gain coefficients of 2.0 dB/cm at 1532 nm and 2.5 dB/cm at 1020 nm measured previously in Er- and Yb-doped Al2O3 waveguides [39,40], we determined such passive resonator losses to be sufficiently low so as to enable round-trip net gain and lasing in the rare-earth-doped microrings.
3.3 Microring laser measurements
We observed laser emission in both Er- and Yb-doped microrings. In Fig. 6 we show an optical microscope image of a resonantly-pumped Er-doped microring laser. The image displays the typical green spontaneous emission from excited Er3+ ions due to excited state absorption and ion-ion interactions. At different gaps we observed laser modes spanning a wavelength range of 1530–1565 nm and both multi-mode and single-mode lasing. The laser spectrum was typically multi-mode at low pump powers (i.e. the maximum power obtainable with the tunable laser), while one or two dominant modes emerged under higher powers (typically requiring 976-nm diode laser pumping). As shown in Fig. 7, for a device with peak Er concentration, NEr,peak, of 3 × 1020 cm−3 and g = 0.3 µm we observed single-mode lasing at 1559.82 nm (with side-mode suppression > 30 dB) and up to 27-µW total laser power in the silicon nitride waveguide (limited by the maximum diode laser pump power). By comparing to the resonances observed in 1550-nm transmission measurements in the same chips, we determined the laser modes to be TM-like. The free-space −3dB linewidth was < 20 pm (below the limit of the optical spectrum analyzer used to measure the laser spectrum). We obtained the lowest laser threshold in devices with g ≈0.5 µm (at optimum pump coupling) and at reduced erbium concentration. In Fig. 8, we show the laser power curve of a device with NEr,peak = 2 × 1020 cm−3 and g = 0.5 µm measured under resonant pumping at 978.84 nm, near the Er absorption peak. The laser exhibits a threshold of 0.5 mW, and double-sided slope efficiency of 0.3%, with up to 2.4 µW output power (total power measured from both outputs) coupled into the SiNx waveguide. Time-domain measurements in several devices revealed that the lasers operated in a pulsed mode at frequencies on the order of 1 MHz, as observed in previous erbium-doped lasers [42–44].
In Yb-doped microrings we observed lasing on multiple TE- and TM-like modes and at wavelengths in the range 1020–1045 nm. In Figs. 9 and 10, we show typical Yb-doped microring laser measurements, obtained for g = 0.4 µm. We observed lowest-threshold lasing at a pump wavelength of 970.96 nm when tuned onto the TM-like resonance. This wavelength is blue-shifted from the Yb3+ absorption peak around 975 nm because the absorption at 975 nm is too strong, thus significantly reducing both Qi and the coupled pump power. The laser spectrum under maximum pump power is displayed in Fig. 9. A single laser line is evident at 1042.74 nm with a side-mode suppression of > 40 dB (inset). The laser power curve for the same microring laser is shown in Fig. 10. We observed lasing at a threshold of 0.7 mW, a total output power of > 100 µW coupled into the SiNx waveguide and double-sided slope efficiency of 8.4%. As with the erbium-doped microring lasers, we observed pulsing behavior at pump powers close to threshold, with pulse frequencies up to 2 MHz. With increasing pump power, however, the pulsing behavior was eventually suppressed and we observed continuous-wave, single-mode lasing in Yb-doped microrings.
With these results we have realized highly-compact monolithic rare-earth-doped devices on silicon. Previously, the smallest bend radius obtained in an Al2O3:Er3+ device embedded in SiO2 was 250 µm, in that case a compact amplifier . Here, using SiNx features we reduce the bend radius to 80 µm, thus suggesting a path towards more compact integrated amplifiers as well as lasers. Compared to previous Al2O3:Er3+ racetrack lasers , we have decreased the device footprint by a factor of ~500. In addition, we show single-mode operation, which is much more easily obtained in a smaller resonator structure. Meanwhile, the total cavity length of the microrings is 20 times shorter than that of DFB and DBR devices [17–21], and the thresholds reported here are more than 10 times smaller.
While in this work we demonstrate a proof of principle, a full investigation of the parameter space can lead to enhanced microring laser performance. Continuous-wave operation of erbium-doped microlasers can be achieved by injecting higher pump powers (as demonstrated in the ytterbium-doped microring lasers and ) or by adjusting the Al2O3:Er3+ layer thickness and doping concentration . In addition, Er-Yb co-doping or in-band pumping at 1480 nm can be explored to increase the erbium laser efficiency. Furthermore, the high efficiencies of DFB and DBR REDWLs [17,18] and ultra-low thresholds demonstrated in fiber-coupled microcavities [24–31] indicate that higher efficiencies and lower thresholds can be obtained by optimizing the resonator mode and optical coupling properties and increasing the cavity Q.
By using standard silicon wafer-scale processing, we demonstrate that these microring lasers can be implemented in silicon-based photonic circuits (i.e. enabling integration with high-performance active and passive silicon, germanium and silicon nitride devices). A single off-chip fiber-coupled pump source or heterogeneously-bonded on-chip laser diode pump could efficiently power multiple microring lasers. Due to their low threshold and single-mode operation around 1.5 µm, arrays of Er-doped lasers are promising as chip-scale multi-wavelengths communications sources. Meanwhile, Yb-doped lasers emit in the low water absorption window, can operate in water and can act as highly effective nanoparticle sensors [46,47]. Thus, their implementation in lab-on-a-chip or integrated biophotonic applications is of interest. Furthermore, the reported cavity structure can easily be adapted for additional rare earth dopants (Nd3+, Tm3+, etc.) with different pump and laser wavelengths.
In summary, we have shown lasing in fully-integrated, monolithic microring structures on silicon chips. The rare-earth-doped microring lasers are enabled by a multilayer design which is silicon-compatible, straightforward and can be easily adapted to realize laser sources emitting from the visible to mid-infrared wavelengths on a full silicon photonics process flow. The low lasing threshold observed in both erbium- and ytterbium-doped lasers suggest that a single, low-cost diode laser pump source could excite densely-integrated arrays of lasers on a chip for wavelength division multiplexing applications. Optimization of optical coupling parameters and microring mode properties can lead to even lower thresholds, higher efficiencies and smaller device footprints. These new, compact, inexpensive and efficient lasers are a viable alternative to existing silicon-based laser technologies and have implications for wide-ranging applications of integrated photonic systems, including optical computing, classical and quantum communications, sensing, biophotonics and fundamental micro- and nanoscale research.
This work was supported by the Defense Advanced Research Projects Agency (DARPA) of the United States under the E-PHI project, grant no. HR0011-12-2-0007. Fabrication was carried out in part at the College of Nanoscale Science and Engineering at the State University of New York and the Microsystems Technology Laboratories at the Massachusetts Institute of Technology. We thank K. Broderick and T. Yu for their assistance with fabrication and imaging.
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