On-chip hybrid erbium-doped tellurium oxide–silicon nitride distributed Bragg reflector lasers

We demonstrate integrated on-chip erbium-doped tellurite (TeO2:Er3+) waveguide lasers fabricated on a wafer-scale silicon nitride platform. A 0.352-µm-thick TeO2:Er3+ coating was deposited as an active medium on 0.2-µm-thick, 1.2- and 1.6-µm-wide, and 22-mm-long silicon nitride waveguides with sidewall-patterned asymmetrical distributed Bragg reflector cavities. The lasers yield efficiencies between 0.06 and 0.36%, lasing threshold ranging from 13 to 26 mW, and emission within the C-band (1530–1565 nm). These results establish new opportunities for this hybrid tellurite glass–silicon nitride platform, such as the co-integration of passive components and light sources in the telecom window, and provide the foundation for the development of efficient, compact, and high-output-power on-chip erbium-doped tellurite waveguide lasers.


Introduction
Rare-earth-doped fiber lasers have achieved remarkable success over the past few decades in delivering reliable solutions for various applications, including material processing, medicine, and defense [1].They possess desirable characteristics, such as stability, high output power, narrow linewidth, low cost, and scalability [2].Among them, erbium-doped lasers stand out due to their high efficiency, compatibility with telecommunication networks, and ability to emit eyesafe radiation in the C-band [3].In the field of integrated photonics, planar glass lasers aim to bring the advantages of fibers to a chip scale, while monolithically integrating an active gain medium onto the chip.With the current mature complementary metal oxide semiconductor (CMOS) processing capabilities, it is possible to reliably fabricate small features such as Bragg gratings that can be used to define optical cavities on silicon chips.Bragg-grating-based devices are well established in both fibers and chips [2][3][4][5][6][7][8][9] and offer great control and flexibility over cavity designs, enabling precise tuning of their optical responses.These devices can be monolithically integrated into waveguides to create compact, efficient, and narrow linewidth lasers [4,10,11].They can be employed in many applications, including sensing [12], integrated LiDAR systems [13], telecommunications [14], and microwave photonics [15].
Tellurium dioxide (TeO 2 ) is a glass with a relatively high refractive index ( n TeO 2 = 2.08 at 1550 nm) and is highly trans- parent from visible to mid-infrared wavelengths.It also has strong nonlinear [16] and acousto-optic [17,18] effects, as well as high rare-earth solubility and large emission cross sections, making it an excellent candidate for a gain medium in integrated photonics [19][20][21].Additionally, it can be processed at low temperatures (< 200 °C), which is an advantage for post-processing fabrication on photonic integrated circuit platforms [22][23][24][25].Vu et al. demonstrated high gain erbium-doped tellurite waveguide amplifiers and lasing due to chip facet reflections [26].However, further development was required for etching smooth waveguides when it is doped with erbium [19] and high-resolution features suitable for integrated cavities and on-chip lasing.
Hybrid integration approaches, such as erbium-doped Al 2 O 3 on silicon nitride (SiN) waveguides, have been successfully employed to achieve distributed Bragg reflector (DBR) lasers on photonics integrated circuit (PIC) platforms [27,28].In these cases, wafer scale, low cost, and mature processing of SiN PICs are leveraged, while the 158 Page 2 of 7 gain medium is applied without the need for an etching step.SiN is a CMOS-compatible material with several advantages [29,30], including low propagation loss (< 0.1 dB/ cm) within SiN's transparency window from visible to midinfrared, high nonlinear figure of merit, low cost, small feature sizes available in large-scale production, and relatively high refractive index ( n SiN = 1.98), essential for developing compact devices [31].We have developed a hybrid silicon nitride-tellurite glass platform that combines the advantages of both materials [32].When tellurite is applied on top of thin SiN waveguides, the optical mode is expanded into the TeO 2 layer due to its slightly higher refractive index than SiN.Consequently, it is possible to achieve more than 50% of mode confinement in the gain medium, with the potential for same-chip integration with passive and nonlinear devices [16].We have demonstrated gain on this platform with erbium-doped TeO 2 (TeO 2 :Er 3+ ) [33], as well as optical amplifiers and microring lasers with thulium-doped tellurite [34,35].
Here, we demonstrate TeO 2 :Er 3+ DBR lasers monolithically integrated on a SiN chip.By varying grating period and strength, lasing is achieved at wavelengths across the C-band.Asymmetrical cavities were used to promote high emission directionality, with a maximum forward efficiency of 0.33%.These results build upon our previous work and pave the way for the development of efficient and low-cost erbium-doped tellurite glass lasers in PICs.

Device design
Figure 1a illustrates the DBR cavities investigated in this work.The SiN thickness (t SiN ) was chosen to be 0.2 µm to allow for sufficient overlap with the active medium [32].The designed TeO 2 :Er 3+ thickness ( t TeO 2 ∶Er 3+ ) was selected as 0.35 µm to account for fabrication variability, so that the Bragg condition can be matched within the optimal tellurite thickness range of 0.25 µm < t TeO 2 ∶Er 3+ < 0.45 µm that enables lateral mode confinement and integration with tight bend radii (< 300 µm) [32].The total device length, L total = 22 mm, is the length of the chip, defined by the lithography reticle size, and is such that the cavity is long enough for the total gain to overcome the roundtrip losses.The waveguide width (w SiN ) was chosen as either 1.2 µm (close to the single mode cutoff) or 1.6 µm (further into multimode operation) [32].Single-mode operation is desirable to maximize the power coupled into the fundamental mode because the Bragg response is sensitive to the mode effective index and other modes are not reflected.However, wider waveguides tend to have lower propagation losses due to less interaction with the sidewalls [36], which can alleviate the amount of gain required to achieve lasing.
The devices were designed using a finite element method mode solver (Synopsys RSoft) to extract parameters, such as the effective index and the mode overlap fractions, with the gain medium and the SiN grating features.The grating strengths were then estimated using the transfer-matrix method [37].2D mode simulations were performed with the selected SiN waveguide geometry and TeO 2 :Er 3+ thickness to determine the effective index n ef f and the power confinement fac- tor in the grating (0 < Γ grating < 1) of the fundamental mode for several grating widths (w grating ).The grating period Λ was chosen using the first-order Bragg condition [38]: where 0 is the target laser wavelength, set to 1550 nm.The DBR cavities investigated are asymmetrical, with the length of the input grating (L in ) being longer than that of the output grating (L out ).This results in a higher reflection coefficient R at the input to promote directional laser emission through the output facet.
The grating lengths L = L in , L out were used to estimate R at 0 for each set of gratings [38]: with being the coupling coefficient, given by [38]: where n TeO 2 ∶Er 3+ is the refractive index of the TeO 2 :Er 3+ film and D = 0.5 is the duty cycle.The w grating = 50, 70, 100 nm was chosen so that R > 0.9 for a grating length of ~ 3 mm, resulting in a cavity length, L cavity , that was at least 10 mm for all devices.Within the grating region, the waveguide width varies between a minimum (w min ) and a maximum (w max ) value to match the n ef f of the straight and corrugated sections.The waveguides include an offset L offset = (0.520 ± 0.005) mm from each chip facet before the grating region starts.The 1.6-µm-wide waveguides also have a 0.25-mm-long edge coupler within this offset that is linearly tapered to a width of 1.2 µm at the chip facet, to keep the facet losses consistent among all devices.L cavity is then defined by L cavity = L total -(2L offset + L in + L out ).

Fabrication
The silicon nitride chips were fabricated at the LioniX foundry through their commercially available process, described in [39].The SiN waveguides and the grating features were defined using stepper lithography and reactive ion etching.The wafers were left unclad, then stealth diced into chips to achieve high-quality facets.As shown in Fig. 1b, due to resolution limitations, the fabricated grating features are rounded instead of the designed rectangular shape [28].Next, the chips were transferred from the foundry and deposition of the TeO 2 :Er 3+ film was carried out via radio frequency (RF) reactive co-sputtering.The deposition was performed at 150 °C, under a process pressure of 2.8 mTorr, in an Ar and O 2 atmosphere with gas flows of 12 and 10.2 sccm, respectively, using a Lesker PVD Pro 200 system.RF powers of 125 and 60 W were applied to 3″ metallic Te and Er targets, respectively.Further details about the TeO 2 :Er 3+ sputtering process can be found in [32].A 1-µm-thick Cytop fluoropolymer layer (n Cytop = 1.33) was spin-coated onto the chips as a top cladding.

Characterization
The devices were characterized on the setup illustrated in Fig. 2. A 1470 nm laser diode was used as a forward pump, followed by polarization paddles and a 1480/1550 nm wavelength division multiplexer (WDM).Next, the pump was launched into the waveguide through a tapered fiber with a spot diameter of 2.5 µm mounted on an XYZ stage.A backward pump with the same configuration was also used to perform double-sided pumping and increase the amount of power coupled to the device.The emitted laser signal at each facet was coupled to the same fibers and WDMs followed by an optical switch which was used for measuring forward and backward laser emissions separately.A free space 1500 nm edge pass filter was employed to filter the residual pump light and a 50/50 splitter was used to measure the laser power with a detector while the laser emission spectrum could be observed on an optical spectrum analyzer.The laser power was measured as a function of the pump power, which was controlled by a current source that feeds the diodes.The launched (on-chip) pump power was calculated by removing the chip, measuring the incident power at the tapered fiber output with a power meter, and subtracting the facet loss, which was determined by measuring the fiber-chip-fiber insertion loss.The amplified stimulated emission (ASE) power was evaluated by measuring the emitted signal from a straight waveguide with equal geometries and no gratings, on the same chip.The ASE power was then subtracted from the measured signal power at the detector to determine the laser output power.The on-chip laser power was determined by taking into account all the system losses from the tapered fiber to the detector as well as the facet losses.

Film and passive waveguide properties
The TeO 2 :Er 3+ deposition was carried out for 18.5 min, resulting in a 0.352-µm-thick film.A bare Si witness piece was used to measure a refractive index of 2.04 at 1550 nm wavelength through ellipsometry.The film losses were characterized on a thermal SiO 2 witness sample with a Metricon prism coupling system [40], yielding a background loss of (1.1 ± 0.3) dB/cm at 1620 nm.The same technique was applied to measure the losses at several wavelengths between 1510 and 1560 nm.The Er 3+ concentration was then estimated to be 2.4•10 20 ions/cm 3 by fitting the losses to their corresponding absorption cross sections [26].In addition, the excited-state lifetime of Er 3+ was found to be (620 ± 20) µs, by fitting the back-emitted ASE signal intensity from the waveguide as a function of time with a 50-Hz square-wave-modulated 1470-nm pump.
Point-coupled microring resonators (500-µm diameter, 1.2-µm gap) with the same tellurite and Cytop layers and waveguide geometry were used to extract the background loss of the devices at 1620 nm through Q-factor measurements [41], resulting in propagation losses of (1.0 ± 0.2) dB/cm and (0.8 ± 0.2) dB/cm for 1.2-and 1.6-µm-wide waveguides respectively.Lastly, the background loss was subtracted from the total insertion loss measured at 1620 nm to estimate a fiber-to-chip facet loss of (3.5 ± 0.5) dB/facet for each device.
Figure 3a shows normalized passive transmission measurements for the most efficient device for each grating width, measured from the same sample and with the pump off.The extinction ratios are greater than -30 dB and the flat response around -40 dB is due to the limit of detection of the power meter.The cavity transmission spectra represent the combined response of both sets of reflectors, which is dominated by the input gratings (R > 99.5%).This indicates that the overall grating strengths agree with the designed values, even though these measurements do not allow us to directly calculate the reflection coefficient of each individual set of gratings.Furthermore, the Q-factors of the resonances were estimated to be in the range of (0.5-2) × 10 5 by dividing each resonance's peak wavelength by its full width half-maximum.

Laser measurements
We observed lasing within the wavelength range of 1533-1565 nm, as shown in the normalized emission spectra in Fig. 3b.Lasing occurred in all the devices tested, except for one that was damaged during handling.Multimode lasing was observed in most of the devices, corresponding to different longitudinal modes inside the cavity.The strong grating responses caused the devices to lase at the edge of the reflection bandwidths.
In Fig. 4, the efficiency curve of the device with the lowest lasing threshold is presented.The device consisted of a 1.2-µm-wide waveguide and 50-nm-wide gratings with a period of 437 nm, and lengths L in = 6 mm and L out = 3 mm.The cavity asymmetry introduced high directionality in the output, which is evident by comparing the forward and backward laser emissions.The forward slope efficiency is η fwd = 0.26%, while the backward laser Wavelength (nm) emission is η bwd = 0.01%, for a total efficiency of 0.27%.Both forward and backward directions exhibited similar lasing thresholds (P th ) of P f wd th = 13 and P bwd th = 11 mW, respectively, as estimated through a linear fit of the experimental data.The maximum forward on-chip laser power was 0.13 and 0.28 mW for single-and double-side pumping, respectively.
Table 1 summarizes the laser results of all the devices tested.The lasing wavelengths observed are dependent on the effective index and grating period.In the high reflectivity regime, by keeping L in constant and increasing L out , the transmission coefficient of the output grating is reduced, leading to lower forward laser efficiency and, therefore, lower overall efficiency as well.
Although hybrid lasers on SiN have been demonstrated with one order of magnitude higher efficiencies [27,28], the lasing thresholds achieved here are comparable to those of similar erbium-doped aluminum oxide DBR lasers first reported in [27,28].The low efficiencies (< 1%) can be attributed mainly to the high background losses, incomplete activation of Er 3+ ions [33], and high reflection coefficient of the output gratings, which can be reduced by choosing shorter values for L out .Studying devices with just one set of uniform gratings as well as weaker reflectors will allow for further characterizing the gratings' passive response [42].The background losses reported here are dominated by the tellurite losses, which can be improved by varying the TeO 2 :Er 3+ sputtering parameters, namely the O 2 flow to adjust the film stoichiometry.Despite the fact that the TeO 2 :Er 3+ film used in this work has a relatively higher loss (> 1 dB/cm), we have demonstrated losses down to ≤ 0.1 dB/cm using the same fabrication process [32,33].We expect to achieve higher performance lasers by optimizing the Er 3+ doping concentration and fraction of active ions, waveguide cross-section geometry, and cavity parameters, such as the grating width, length, and period.Furthermore, the devices' operating wavelength can be adjusted by changing the gratings' period to achieve lasing with different rareearths, such as ytterbium, praseodymium, thulium, as well as erbium-ytterbium co-doping.

Conclusion
We demonstrate erbium DBR lasers on a TeO 2 :Er 3+ -coated SiN hybrid platform.The silicon nitride waveguides were fabricated through a standard wafer-scale foundry process and the TeO 2 :Er 3+ layer was reactively co-sputtered using a straightforward low temperature step.High-output directionality was achieved using asymmetrical cavities, with Fig. 4 Laser efficiency curve of a DBR laser with w SiN = 1.2 µm, w grating = 50 nm, Λ = 437 nm, L in = 6 mm, L out = 3 mm.A linear fit gives a total efficiency of 0.27%, of which 0.26% corresponds to forward emission, and a threshold pump power of approximately 13 mW.Inset: lasing device.Typical Er 3+ green emission can be seen due to higher order excited states because of upconversion when pumped a maximum total laser efficiency of 0.36% and minimum pump power threshold of 13 mW.Lasing at wavelengths between 1533 and 1565 nm was observed in several devices with varying waveguide widths, grating widths, periods, and lengths.These results serve as a basis for understanding the grating response in such hybrid waveguides and optimizing the cavity and the grating properties for improved performance in future designs.The simplicity and the versatility of this platform make it attractive for the integration of active and passive devices on a single chip.Overall, these results represent significant initial steps toward the realization of reliable, efficient, and high-output-power integrated erbiumdoped tellurite lasers for applications in communications and sensing.

wFig. 1 a
Fig. 1 a Diagram of a TeO 2 :Er 3+ -Si 3 N 4 DBR cavity.b Scanning electron microscope image of fabricated Si 3 N 4 waveguide gratings showing the transition between a straight section and a corrugated section for the different grating designs analyzed in this work.Inset: electric field profile for the 1550 nm fundamental TE mode in a hybrid TeO 2 :Er 3+ -Si 3 N 4 waveguide showing strong overlap with both the TeO 2 :Er 3+ gain layer and the Si 3 N 4 strip

10 Fig. 3 a
Fig. 3 a Passive transmission (unpumped) and b corresponding laser emission spectra of DBR cavities with different grating designs

Table 1
Summary of DBR laser designs and results