Laser sources in the 2 μm wavelength region have many applications, such as light detection and ranging, spectroscopy, optical waveform generation and synthesis, material processing, communication, and trace-gas detection systems. The high water absorption at 2 μm also makes such lasers good candidates for medical applications [1]. Furthermore, 2 μm wavelength lasers in pulsed mode could be used as pumps for nonlinear processes in silicon, such as mid-IR optical parametric amplification [2]. Silicon is the material of choice for both microelectronic circuits and integrated photonics components around 1.31 and 1.55 μm. To extend the applicability of silicon photonics microsystems to the 2 μm wavelength region, complementary metal-oxide semiconductor (CMOS)-compatible integrated laser sources around 2 μm are critical. These lasers will enable highly compact devices that can perform the task of current complex bulky laboratory setups.

Among the methods to integrate lasers on a silicon platform [3–6], deposition of rare-earth-doped ${\mathrm{Al}}_2{\mathrm{O}}_3$ glass has proven to be effective due to several advantages. First, rare-earth-doped ${\mathrm{Al}}_2{\mathrm{O}}_3$ glass can be deposited as a single-step back-end-of-line process on silicon wafers without requiring any additional etch steps [7–10]. Second, common rare-earth materials such as erbium and thulium have a wide emission spectrum, which enables wavelength tunability and design flexibility [11–13]. Third, the rare-earth-doped gain media do not involve free carriers as semiconductor lasers do, enabling low losses and lasers with narrow linewidths [14–16]. Lastly, the low thermo-optic effect of the host medium enables operation over a wide temperature range [17]. The state-of-the-art for high-power lasers fabricated using this methodology was demonstrated to be 75 mW continuous-wave output power in an erbium-doped waveguide laser operating around 1.55 μm [18]. Among rare-earth elements, thulium is particularly attractive for integrated lasers on silicon since its emission spectrum is around 2 μm. Integrated thulium-doped channel waveguide lasers on tungstate were reported with high power and slope efficiencies [19,20]. However, so far work on thulium-doped high-power lasers on a CMOS-compatible platform has been absent.

Prior to this work, we have reported a thulium-doped microcavity laser with sub-milliwatt output power and multi-mode operation [21,22]. However, high output power and single-mode operation are desired for many applications. In this Letter, we demonstrate high-power thulium-doped waveguide lasers that were fabricated on silicon chips in a 300 mm CMOS foundry. An ${\mathrm{Al}}_2{\mathrm{O}}_3\text{:}{\mathrm{Tm}}^{3+}$ thin film deposited as a gain medium on top of a buried ${\mathrm{Si}}_3{\mathrm{N}}_4$ strip in ${\mathrm{SiO}}_2$ acts as a rib waveguide on a silicon substrate. The narrow reflection bandwidths of distributed feedback (DFB) and distributed Bragg reflector (DBR) structures enable single-mode output. The maximum on-chip lasing powers achieved for the DFB and DBR devices were 267 and 387 mW, with slope efficiencies of 14% and 23%, respectively.

The waveguide cross section of the DFB laser is shown in Fig. 1(a). The width and separation of the ${\mathrm{Si}}_3{\mathrm{N}}_4$ bars are optimized to be 300 and 350 nm, respectively, to provide high mode confinements for both pump and signal modes within the ${\mathrm{Al}}_2{\mathrm{O}}_3\text{:}{\mathrm{Tm}}^{3+}$ film. The confinement factors within the thulium-doped gain region for pump and signal are calculated to be 90% and 85%, respectively, using a finite-difference 2D mode solver. The oxide gap between the ${\mathrm{Si}}_3{\mathrm{N}}_4$ layer and the ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer is 200 nm. The refractive indices of the materials used in our laser at both pump and signal wavelengths are listed in Fig. 1(b). The transverse-electric (TE) field intensity of the fundamental mode is shown in Fig. 1(c). For the DBR laser, the gain waveguide cross section has seven ${\mathrm{Si}}_3{\mathrm{N}}_4$ bars instead of five. Gratings are added on both sides of the gain waveguide. We designed DFB and DBR cavities for different laser wavelengths by varying the grating periods.

 

Fig. 1. Integrated thulium DFB and DBR laser designs. (a) Cross section of the laser gain waveguide including five strips of ${\mathrm{Si}}_3{\mathrm{N}}_4$. (b) Refractive indices of the waveguide materials at both pump and signal wavelengths. (c) Fundamental TE field intensity for the pump (1600 nm) and laser output (1900 nm) wavelengths in the DFB waveguide. 3D illustrations of (d) the DFB laser and (e) the DBR laser, showing the different material layers and cavity features (not to scale).

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Perspective views of the DFB and DBR lasers are illustrated in Figs. 1(d) and 1(e), respectively. For the DFB laser, the lateral gap between the grating and gain waveguide is designed to be 450 nm, and the grating width is chosen to be 260 nm in order to provide enough feedback at the designed laser wavelength. The coupling coefficient ($\kappa$) is calculated to be $1.0\times {10}^3{\mathrm{m}}^{-1}$. For the DBR laser, the grating width and gap follow the pattern of seven ${\mathrm{Si}}_3{\mathrm{N}}_4$ strips of gain waveguide between two DBR mirrors, with coupling coefficient ($\kappa$) of $1.3\times {10}^3{\mathrm{m}}^{-1}$. The cavity lengths for both lasers are 2 cm, limited by the maximum length of the chip. For a laser cavity length shorter than 2 cm, with the same pump power, the lasing power decreases. The coupling coefficient and cavity length product $(\kappa ·L)$ for DFB and DBR are 20 and 26, respectively. By characterizing the cavity response of the DBR laser, the reflectivity of the grating on each side is estimated to be 70%. Such grating reflectivity enables the laser to achieve a few hundred milliwatts of output power, while still maintaining relatively high $Q$ of the DBR cavity. The effective refractive index ($n_{\mathrm{eff}}$) of the waveguide is calculated using a finite-difference 2D mode solver, considering the grating as perturbations on both sides of the segmented ${\mathrm{Si}}_3{\mathrm{N}}_4$-rib to provide feedback. The $n_{\mathrm{eff}}$ is calculated to be 1.565. The grating period ($\mathrm{\Lambda }$) can be obtained using the following equation:

The lasers were fabricated in a state-of-the-art CMOS foundry on a 300 mm silicon wafer. The fabrication process is illustrated in Fig. 2(a). The ${\mathrm{Si}}_3{\mathrm{N}}_4$ layer is deposited on top of a ${\mathrm{SiO}}_2$ layer, both via plasma-enhanced chemical-vapor deposition (PECVD), followed by a surface polishing process to reduce optical scattering loss. Then the ${\mathrm{Si}}_3{\mathrm{N}}_4$ layer is patterned using 193 nm immersion lithography and reactive ion etching, as shown in step II. Next, in step III, we deposit a ${\mathrm{SiO}}_2$ layer and planarize the top surface via chemical-mechanical polishing. The thickness of the ${\mathrm{SiO}}_2$ layer on the ${\mathrm{Si}}_3{\mathrm{N}}_4$ layer is 200 nm. After that, the ${\mathrm{Al}}_2{\mathrm{O}}_3\text{:}{\mathrm{Tm}}^{3+}$ film is deposited at a thickness of 1100 nm via reactive co-sputtering, as shown in step IV. The substrate temperature is measured to be 350°C using a thermocouple directly in contact with chip. The background loss of the ${\mathrm{Al}}_2{\mathrm{O}}_3\text{:}{\mathrm{Tm}}^{3+}$ film is measured to be $<0.1\mathrm{dB}/\mathrm{cm}$. Fabrication runs with different doping levels revealed an optimum ${\mathrm{Tm}}^{3+}$ doping concentration of $3.0\times {10}^{20}{\mathrm{cm}}^{-3}$. At the same pump power, but lower doping concentration, the lasing power of the device decreases due to lower gain; while for higher doping concentration, the lasing power of the device decreases due to doped-ion clustering or quenching [18,23,24]. In order to visualize the ${\mathrm{Si}}_3{\mathrm{N}}_4$ pattern, after step III and before step IV, 140s hydrogen fluoride (HF) etching is used to remove the ${\mathrm{SiO}}_2$ top cladding. A scanning electron microscope (SEM) image of the gain waveguide and side gratings is shown in Fig. 2(b).

 

Fig. 2. (a) Laser fabrication steps: (I) deposition of ${\mathrm{Si}}_3{\mathrm{N}}_4$ layer via PECVD; (II) pattering of ${\mathrm{Si}}_3{\mathrm{N}}_4$ layer yielding five-segment (DFB) or seven-segment (DBR) waveguides in the center for mode confinement and additional periodic segments at each side for distributed feedback; (III) deposition of ${\mathrm{SiO}}_2$ followed by top surface planarization; (IV) deposition of the thulium-doped ${\mathrm{Al}}_2{\mathrm{O}}_3$ gain medium by reactive co-sputtering. (b) SEM image of the ${\mathrm{Si}}_3{\mathrm{N}}_4$ pattern after HF etching of the ${\mathrm{SiO}}_2$ top cladding from Step III.

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The measurement setup for characterizing the lasers is illustrated in Fig. 3(a). A laser source at 1612 nm together with an L-band erbium-doped fiber amplifier (EDFA) were used for optical pumping. A polarization controller was used to ensure that the pump light was coupled into the fundamental TE mode of the gain waveguide. A cleaved single-mode SMF-28 fiber was used to butt-couple the pump light onto the chip, and another cleaved single-mode SM-2000 fiber was used to butt-couple the output signal of the laser from the chip. The fiber-to-chip coupling losses were measured to be 7.1 dB for SMF-28 at the pump wavelength, and 7.9 dB for SM-2000 at the laser output wavelength. The output signal was coupled into an optical spectrum analyzer (Yokogawa AQ6375) to capture the spectrum. The grating period design variations are 581, 587, 594, 601, 608, and 625 nm. They were calculated using Eq. (1), with corresponding wavelengths at 1820, 1840, 1860, 1880, 1900, and 1950 nm, respectively. The measured optical spectra of the corresponding DFB laser designs are shown in Fig. 3(b). Within the broad gain bandwidth of the ${\mathrm{Al}}_2{\mathrm{O}}_3\text{:}{\mathrm{Tm}}^{3+}$, we are able to precisely control the laser wavelength by choosing the proper grating period.

 

Fig. 3. (a) Measurement setup: a 1612 nm laser source together with a high-power EDFA, followed by a polarization controller to ensure that the fundamental TE mode is coupled into the gain waveguide of the laser. An optical spectrum analyzer is used to capture the spectrum; (b) output spectrum of DFB lasers with different grating periods, showing single-mode lasing at 1820, 1840, 1860, 1880, 1900, and 1950 nm.

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From the DFB and DBR laser grating period design variations, we selected the devices that operate near the peak of the thulium emission spectrum for the lasing slope efficiency measurements. The slope efficiency curves for DFB and DBR lasers are shown in Figs. 4(a) and 4(b), respectively. The maximum on-chip laser output power of the DFB and DBR lasers were measured to be 267 and 387 mW, respectively. The powers were measured from a single output end of the lasers, and the fiber coupling losses are calibrated out. An equivalent level of power could be obtained when pump is launched from the other side of the laser, considering the symmetry of the cavity. Using linear curve fitting, the single-sided slope efficiencies for the DFB and DBR lasers were found to be 14% and 23%, with laser thresholds of 96 and 65 mW at 1612 nm pumping. The peak laser wavelengths with up to 2 W on-chip pump power for the DFB and DBR lasers were recorded by the spectrum analyzer, as shown in Figs. 4(c) and 4(d), respectively. Single-mode operation was enabled by the narrow bandwidth grating. The side-mode suppression ratio reached more than 70 dB.

 

Fig. 4. (a) DFB and (b) DBR laser output curves, showing 14% and 23% slope efficiency, 267 and 387 mW maximum output power, and 96 and 65 mW lasing threshold, respectively. The output spectra of (c) DFB and (d) DBR lasers at 1861 nm and 1881 nm, respectively, obtained with up to 2 W on-chip pump power and showing side-mode suppression ratios $>70\mathrm{dB}$.

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In summary, we have designed, fabricated, and characterized high-power thulium DFB and DBR lasers integrated on a silicon chip. A CMOS-compatible segmented ${\mathrm{Si}}_3{\mathrm{N}}_4$ rib waveguide was used to form the laser cavity. Gratings were added on both sides of the segmented ${\mathrm{Si}}_3{\mathrm{N}}_4$ rib waveguide to provide feedback and thulium-doped ${\mathrm{Al}}_2{\mathrm{O}}_3$ glass was used as the gain medium. By varying the grating period, single-mode lasers with wavelengths from 1800 to 1950 nm have been demonstrated within the thulium gain bandwidth. The highest DFB and DBR output powers of 267 and 387 mW under 1612 nm pumping were measured at 1861 nm and 1881 nm wavelengths, with slope efficiencies of 14% and 23%, respectively. More than 70 dB side-mode suppression, which enables high signal-to-noise ratio for chip-scale communication and spectroscopic applications, was observed.