Broadband 2-µm emission on silicon chips: monolithically integrated Holmium lasers

: Laser sources in the mid-infrared are of great interest due to their wide applications in detection, sensing, communication and medicine. Silicon photonics is a promising technology which enables these laser devices to be fabricated in a standard CMOS foundry, with the advantages of reliability, compactness, low cost and large-scale production. In this paper, we demonstrate a holmium-doped distributed feedback laser monolithically integrated on a silicon photonics platform. The Al 2 O 3 :Ho 3+ glass is used as gain medium, which provides broadband emission around 2 µm. By varying the distributed feedback grating period and Al 2 O 3 :Ho 3+ gain layer thickness, we show single mode laser emission at wavelengths ranging from 2.02 to 2.10 µm. Using a 1950 nm pump, we measure a maximum output power of 15 mW, a slope efficiency of 2.3% and a side-mode suppression ratio in excess of 50 dB. The introduction of a scalable monolithic light source emitting at > 2 µm is a significant step for silicon photonic microsystems operating in the grating period, we demonstrated single-mode lasers with wavelengths at 2051 nm and 2101 which are within the holmium gain bandwidth. By changing the Al 2 O 3 :Ho 3+ film thickness, the lasing wavelength can be controlled. With 1950 nm pumping, a laser output power of 15 mW was measured at a wavelength of 2050 nm with a slope efficiency of 2% and greater than 50 dB side-mode suppression ratio. This demonstration represents an important step toward high-performance on-chip silicon-based laser sources for the 2 to 2.2 µm wavelength range.


Introduction
Wavelengths in the region of around 2.0 μm have many transmission windows for atmospheric gases, strong water absorption, and highly efficient mid-infrared (IR) frequency conversions. Hence, laser sources at these wavelengths enable a wide range of applications in the fields of medicine, light detection and ranging (LIDAR) systems, remote sensing, tracegas detection, and mid-IR wavelength generation [1][2][3]. Thulium-doped waveguide lasers operate efficiently around 1.9 μm [4,5], but their efficiency at longer wavelengths is significantly reduced because of the diminishing emission cross-section of thulium-doped gain medium. In comparison, holmium-doped lasers have an emission spectrum spanning from 1.95 to 2.15 μm, allowing for signal generation in this longer wavelength range [6][7][8].
Furthermore, holmium-doped lasers have the potential to be in-band pumped using mature thulium laser technology [7]. Silicon photonics is a promising technology that has enabled production of large-scale devices [9], ultralow power modulators [10], and monolithically integrated lasers [11-14] on a single chip with a low cost and compact size. In particular, integrated lasers beyond 2.0 μm are in demand due to the diminishing two-photon absorption of silicon [15,16] while also providing a new communication band for integrated systems [1]. Among the different methods to integrate lasers on a silicon photonics platform [17][18][19][20][21][22], deposition of rare-earthdoped Al 2 O 3 glass as gain medium [11, 22-24] and utilizing complementary metal-oxidesemiconductor (CMOS)-compatible silicon nitride (Si 3 N 4 ) cavities has proven to be effective for several key reasons. First, Si 3 N 4 has high transparency and low loss from near-IR into the mid-IR wavelength regime and is a mature wafer-scale waveguide platform already applied in passive and nonlinear silicon photonic devices [25][26][27]. Second, rare-earth-doped Al 2 O 3 glass can be deposited on silicon wafers as a single-step back-end-of-line process [28]. This allows for control of the laser wavelength by changing the thickness of the gain film, which will be discussed in more detail later. Third, common rare-earth-materials such as erbium, thulium or holmium have a wide emission spectrum, enabling a wide tuning range of the laser wavelength [29-34] as well as potential for mode-locking [32 -37]. Fourth, compared to semiconductor lasers, rare-earth-ion-based lasers can provide much narrower laser linewidths because the optical pumping process involves no free carriers [30, [38][39][40]. Finally, the low thermo-optic coefficient of the host medium (Al 2 O 3 ) enables laser operation over a wide temperature range by providing good stability without active thermal control [41,42]. Rareearth-ion-based monolithic lasers integrated on a silicon platform have been demonstrated at 1.0, 1.5 and 1.9 µm wavelengths using ytterbium [24,43], erbium [14, 22, 23] and thulium [11,44] doped Al 2 O 3 glass as gain medium, respectively. However, to the best of our knowledge, monolithic integrated lasers beyond the 2 μm region have been minimally explored with no known demonstration of a holmium laser on a silicon photonics platform.
In this paper, we demonstrate a holmium-doped distributed feedback (DFB) laser fabricated on a wafer-scale silicon photonics platform. The Al 2 O 3 :Ho 3+ glass with broadband emission around 2 µm is used as gain medium. We achieve single-mode lasing at wavelength longer than 2.02 µm with a side-mode suppression ratio in excess of 50 dB. The maximum on-chip lasing power is 15 mW with a slope efficiency of 2.3%. In addition, lasing wavelength control within the gain bandwidth of Al 2 O 3 :Ho 3+ film is demonstrated by changing the gain film thickness. This is, to the best of our knowledge, the first holmiumdoped integrated laser demonstrated on a CMOS-compatible silicon photonics platform. The waveguide cross-section of the DFB laser is shown in Fig. 1(a). A wavelength-insensitive waveguide design is used, which exhibits a high confinement factor in the gain material for an octave-spanning range across near-IR wavelengths (950-2000 nm) [45]. The octave-spanning range enables the waveguide structure to be used for a broad selection of pump and lasing wavelength. It consists of five Si 3 N 4 bars buried under SiO 2 with a layer of Al 2 O 3 :Ho 3+ deposited on top. The thickness of each Si 3 N 4 bar is 200 nm, and the oxide gap between this Si 3 N 4 layer and the Al 2 O 3 layer is 200 nm. The width and separation of the Si 3 N 4 bars are optimized to be 300 nm and 350 nm, respectively, to provide high mode confinements for both pump (1.95 μm) and signal modes (~2.10 μm) within the Al 2 O 3 :Ho 3+ film. Using a vector finite-difference 2D eigenmode solver, the confinement factors within the holmiumdoped gain region for pump and signal are calculated to be 85% and 83%, respectively.

Device design and fabrication
The refractive indices of the materials used in our laser design are listed in Fig. 1(b). At both pump and signal wavelength, the refractive indices are sufficiently close that the material dispersion is negligible and excluded from the calculations. The guided modes in waveguides and the effective indices are calculated by vector finite-difference 2D eigenmode solver, with a discretization of 20 nm. The code is written in Matlab, and it solves the wave equation of the transverse electric field. After solving the eigen problem, the square of propagation constant β 2 can be obtained as eigenvalue, and hence the effective index was calculated using n eff = βλ/2π. The transverse-electric (TE) field intensity of the fundamental mode at the pump and signal wavelengths is shown in Fig. 1(c).
A perspective view of the DFB laser is illustrated in Fig. 1(d) showing the perturbations on the side of the waveguide to form the laser cavity. The lateral gap between the grating and 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 (κ) of Bragg grating is calculated to be 1.10 × 10 3 m −1 , by substituting the calculated β from mode solver into the following Eq [46]: where D is grating duty cycle, which is equal to 0.5, τ is the mode overlap within the grating region, which is calculated to be 0.21%, k is the wave number. The refractive index of n Si3N4 and n SiO2 are provided in Fig. 1(b). The total length of the cavity is 2 cm, which is limited by the length of the chip from fabrication facility. For a laser cavity length shorter than 2 cm, with the same pump power, the lasing power decreases. The mask space available for Holmium doped lasers on this fabrication run was limited to 2 cm × 0.05 cm. With a larger area available, a longer laser cavity (>2 cm) can be designed using a spiral geometry [36] for higher gain and better laser performance in terms of slope efficiency, output power and lasing threshold. The effective index (n eff ) of the waveguide is calculated to be 1.552 at 2100 nm, considering the grating as perturbations on both sides of the segmented Si 3 N 4 -rib to provide feedback. Therefore, the grating period (ᴧ) can be calculated using: where λ is the designed laser wavelength. Based on the n eff from 2D mode solver and κ obtained from Eq. (1) above, the transmission response of our DFB cavity, as shown in Fig.  1(e), is calculated by using the transfer matrix of grating [46]. A quarter-wave phase shift is introduced in the middle of the simulated section, with κ reversing sign: κ = -κ. The lasers were fabricated in a state-of-the-art CMOS foundry on a 300-mm silicon wafer. The wafer-scale fabrication process before the Al 2 O 3 :Ho 3+ film deposition is the same as that which has been reported earlier [44]. After dicing the wafer into small identical chips, a 1.14µm-thick Al 2 O 3 :Ho 3+ film was deposited on one of the chips via reactive co-sputtering as a back-end-of-line process. The doped Al 2 O 3 film was reactively co-sputtered from 2-inch diameter metallic aluminum and holmium targets in a confocal arrangement. The reaction was within an argon and oxygen atmosphere. The deposition system (AJA ATC Orion 5), as shown in Fig. 2(a) below, is equipped with two radio frequency (RF) magnetron sputtering guns. The power setting on the aluminum and holmium sputtering guns were 200 W and 18 W, respectively. The deposition was carried out at a constant pressure of 3 mTorr. The argon flow rate was kept at constant flow of 11.0 sccm, and the oxygen flow rate was adjusted from 1.1 to 1.6 sccm in order to maintain the oxygen flow -bias voltage curve at the "knee" point of the hysteresis point [47] during the entire deposition process. At this point, the film deposited is stoichiometric, while the oxygen flow is maintained below the point where the bias voltage of aluminum target and deposition rate start to drop. The substrate temperature was measured to be 415 °C. Deposition runs with different Ho 3+ doping levels revealed that a measured Ho 3+ doping concentration of 3.2 × 10 20 cm −3 , through Rutherford backscattering spectrometry (RBS) analysis, provides the best lasing performance. Given the same pump power, a lower doping concentration will suffer from lower gain while higher doping concentration will result in ion clustering or quenching [48][49][50]. In addition, the propagation loss of the passive Al 2 O 3 film (without doping) deposited on a thermally oxidized silicon substrate was measured using the prism coupling method. The propagation loss of the Al 2 O 3 film was found to be < 0.1 dB/cm.
A scanning electron microscopy (SEM) image of Si 3 N 4 layer is illustrated in Fig. 2(b). In order to remove the top SiO 2 cladding layer for SEM imaging, prior to Al 2 O 3 deposition, the device was placed into buffered hydrogen fluoride (HF) for 140 s and coated with 5 nm thin gold layer at top to avoid charging.

Device characterization
The common pumping scheme of holmium is shown in Fig. 3(a). For direct pumping of singly-doped holmium, the absorption bands of interest lie at wavelengths of 1.15 μm ( 4 I 7 level) and 1.95 μm ( 5 I 7 level). Pumping of the 4 I 7 level can be addressed by 1.12 μm laser diodes [8] or long wavelength operation of ytterbium fiber lasers [51]. However, the low quantum efficiency of this pumping scheme limits its efficiency and power scaling potential. Alternatively, the 5 I 7 level can be accessed by thulium fiber or on-chip lasers, which can provide a high-power pump source at 1.95 μm.
The measurement setup for characterization of the lasers is illustrated in Fig. 3(b). A highpower fiber laser source at 1950 nm was used for optical pumping. A polarization controller was used to ensure that the pump light is coupled into the fundamental TE mode of the gain waveguide. A cleaved single-mode SM-2000 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 setup and fiber-to-chip coupling losses were first determined using a 3-mm-long passive waveguide, which had an undoped Al 2 O 3 film at top but was otherwise identical to the laser gain waveguide, and SM-2000 fibers on each side of the chip for both pump and lasing wavelengths. The fiber-to-chip coupling losses for SM-2000 were measured to be 7.5 dB at the pump wavelength, and 6.9 dB at the laser output wavelength. The output signal was coupled into an optical spectrum analyzer (Yokogawa AQ6375) to capture the spectrum.  First, we compare the spontaneous emission spectra of thulium-and holmium-doped Al 2 O 3 films, which both emit near 2 µm. We deposited Al 2 O 3 :Tm 3+ and Al 2 O 3 :Ho 3+ films on top of the Si 3 N 4 waveguide design illustrated in Fig. 1(a). A low power pump at 1.614 μm (at the Tm 3+ absorption peak) was used to generate spontaneous emission in the Al 2 O 3 :Tm 3+ waveguide on the 3 F 4 → 3 H 6 transition. The spectrum recorded by an optical spectrum analyzer is shown in Fig. 4(a). It shows that the emission range is from 1680 nm up to 2020 nm, with 3-dB bandwidth of 340 nm. Lasing within this emission range has been reported by using the same Al 2 O 3 :Tm 3+ thin film and varying the grating period [44]. In comparison, the spontaneous emission spectrum of the Al 2 O 3 :Ho 3+ film is shown in Fig. 4(b). A low power laser source at 1.12 μm is used as the pump. The emission range has a 3-dB bandwidth of 200 nm, spanning from 1930 nm up to 2130 nm, which is beyond the upper limit of the Al 2 O 3 :Tm 3+ emission. Lasing within this emission range is achieved by varying the grating period as well as Al 2 O 3 thin film thickness, as shown in the following parts of this article.
To demonstrate lasing using Al 2 O 3 :Ho 3+ as a gain medium, two DFB lasers with different grating periods were fabricated. The two grating periods were 659 nm and 677 nm, which based on Eq. (2), correspond to wavelengths around 2051 nm and 2101 nm, respectively. The measured optical spectra of the corresponding DFB laser designs are shown in Figs. 5(a) and 5(b). Due to the narrow reflection bandwidth of the DFB structure, single-mode operation is demonstrated and more than 50 dB side-mode suppression ratio is achieved for both designs. A 50 pm (~3.4 GHz) optical spectrum analyzer resolution used in the experiment confirms the single mode operation of the laser with ~5 GHz free spectral range. Within the broad gain bandwidth of Al 2 O 3 :Ho 3+ , we are able to control the laser wavelength by choosing a proper grating period based on Eq. (2).  The device that operates at the higher emission cross-section of the holmium gain spectrum, which is 2050 nm, was selected for the lasing slope efficiency measurement shown in Fig. 6. The maximum on-chip output power of the DFB laser was measured to be 15 mW, limited by the on-chip pump power of 800 mW. The output power was measured from a single side of the laser and the fiber-to-chip coupling loss was calibrated out. An equivalent level of power can be obtained if the pump is launched from the other side of the laser due to the symmetry of the cavity design. Using linear curve fitting to the experimental data, we demonstrate a single-sided slope efficiency of 2.0% and a laser threshold of 130 mW with 1950 nm pumping. In order to find the effective lasing threshold with respect to absorbed pump power, the residual pump power was measured near the lasing threshold of the same device. The absorbed pump power is obtained by subtracting the measured residual pump from the on-chip pump power. The laser power near the threshold with respect to absorbed pump power is shown in Fig. 6(b). The lasing threshold with respect to absorbed pump power is found to be 50 mW, and the slope efficiency is measured to be 2.3%, after linear curve fitting of experimental data. The slope efficiency reported here is limited by the loss in the laser cavity, which is mainly attributed to the waveguide surface roughness and loss from the materials (Si 3 N 4 , Al 2 O 3 , SiO 2 ). The loss of the laser cavity can be reduced by improving fabrication process (e.g. using LPCVD instead of PECVD Si 3 N 4 ). In addition, the laser performance in terms of slope efficiency, output power and lasing threshold can be improved by using a distributed-phase-shifted (DPS)-DFB with optimized grating coupling coefficient (κ) instead of a quarter-wave phase shift (QPS)-DFB [45]. The improvement of the DPS-DFB cavity can be attributed to the reduction of spatial hole burning in QPS-DFB cavity and a longer effective gain section.

Lasing wavelength shift by changing Al 2 O 3 :Ho 3+ film thickness
Since the deposition of the Al 2 O 3 :Ho 3+ thin film is a back-end-of-line process, the lasing wavelength of our DFB laser can be controlled by changing the thickness of the film. Using the same Si 3 N 4 grating periods (659 nm and 677 nm), the thickness of the film is reduced from 1.14 μm to 0.91 μm by reducing the sputtering time in a new Al 2 O 3 :Ho 3+ deposition run. The lasing wavelength is shifted from 2050.2 nm to 2022.7 nm and 2101.4 nm to 2072.6 nm, respectively, as shown in Fig. 7(a) below. In addition, the effective index of the gain waveguide with reduced Al 2 O 3 :Ho 3+ film thickness is calculated using the 2D eigenmode solver, and hence the expected lasing wavelengths are calculated by substituting the n eff and grating period into Eq. (2). A comparison of the calculated lasing wavelength from simulation and the measured lasing wavelength from the experiment is shown in Fig. 7(b). They agree to an accuracy of 1 nm. The difference may be caused by fabrication variation of the Si 3 N 4 layer, as well as by the Al 2 O 3 :Ho 3+ film thickness non-uniformity along the 2-cm-long DFB cavity [52]. In addition, the thermal shift of the device, which was reported to be 0.02 nm/°C [42], may also have an effect.
Lasing was observed with film thicknesses ranging from 0.85 μm up to 1.25 μm. If the film thickness is less than 0.85 μm, the laser device experiences low gain. This is due to the fact that as the film gets thinner, the pump and signal mode overlap decreases, and therefore the gain decreases. Meanwhile, if the film thickness is more than 1.25 μm, the laser device experiences high round-trip loss. This is mainly due to the fact that as the film gets thicker, the horizontal mode confinement (provided by Si 3 N 4 bars) is weaker and hence the expanded mode will experience more scattering loss. Additionally, with a gain film thickness in the range of 0.85 μm to 1.25 μm, there is an optimum thickness value to provide the highest net gain giving the best laser performance in terms of output power, slope efficiency and lasing threshold.

Conclusion
In summary, we have designed, fabricated and characterized holmium doped DFB lasers monolithically integrated on a silicon chip. The holmium-doped Al 2 O 3 glass, which provides broadband emission from 1930 nm to 2130 nm, was used as gain medium. A CMOScompatible segmented Si 3 N 4 rib-waveguide was used to form the laser cavity. Gratings were added on both sides of the segmented Si 3 N 4 rib waveguide to provide feedback. By varying the grating period, we demonstrated single-mode lasers with wavelengths at 2051 nm and 2101 nm, which are within the holmium gain bandwidth. By changing the Al 2 O 3 :Ho 3+ film thickness, the lasing wavelength can be controlled. With 1950 nm pumping, a laser output power of 15 mW was measured at a wavelength of 2050 nm with a slope efficiency of 2% and greater than 50 dB side-mode suppression ratio. This demonstration represents an important step toward high-performance on-chip silicon-based laser sources for the 2 to 2.2 µm wavelength range.