1. Introduction

Integration of high quality ultra-narrow-linewidth lasers on a silicon photonics platform is important for several applications, including digital coherent communications, coherent LIDAR, optical metrology, and sensing [1]. Hybrid III-V silicon lasers have been shown to generate several MHz optical linewidth with phase-shifted distributed feedback (DFB) cavities [2]. To obtain a kHz linewidth laser, an external cavity can be constructed by combining the semiconductor gain medium with a Vernier-based tunable filter [3] or a high Q resonance cavity can be used with low passive loss [4]. However, these approaches generally require complex fabrication steps, or careful temperature control. Alternatively, monolithic erbium-doped aluminum oxide (Al₂O₃:Er³⁺) lasers have been shown to achieve linewidths as low as 1.7 kHz using a DFB cavity [5]. Recently, we have demonstrated a CMOS-compatible design to integrate Al₂O₃:Er³⁺ lasers in a wafer-scale process with a single backend step [6]. The waveguide consists of a thin (~100 nm) silicon nitride (SiN_x) structure buried in a silicon dioxide (SiO₂) layer to achieve high confinement factor and mode overlap in the gain film [6–9].

In this paper, we extend the design to work for a thicker silicon nitride by using a multi-segmented waveguide structure [10–12]. This allows integration into a more general silicon photonics wafer-scale process where thicker, higher-confinement SiN_x structures might be preferred [13–15]. In addition, the design exhibits the same high confinement factor (> 85%) and a near perfect intensity overlap (> 98%) for an octave spanning range across near infrared (NIR) wavelengths (950–2000 nm).

We compare the performance of DFB lasers in discrete quarter phase shifted (QPS) cavities and distributed phase shifted (DPS) cavities [5,16]. By using a QPS-DFB configuration, we obtain single frequency lasing at 1536 nm, 1566 nm, and 1596 nm with on-chip output powers of 0.41 mW, 0.76 mW, and 0.47 mW respectively. This spans a similar emission bandwidth that has been shown previously in distributed Bragg reflector (DBR) lasers [6], covering the C and L band of the erbium gain spectrum. In a DPS cavity, we achieve an order of magnitude improvement in maximum output power (5.43 mW) for a wavelength centered at 1565 nm, corresponding to side mode suppression ratio (SMSR) of > 59.4 dB. Using a recirculating self-heterodyne delayed interferometer (R-SHDI) [17,18], we also observe a narrower linewidth for DPS-DFB at Δν_DPS = 5.3 ± 0.3 kHz, as compared to QPS-DFB at Δν_QPS = 30.4 ± 1.1 kHz. The improvement can be explained by reduction of spatial hole burning in the center of cavity and increased effective gain section [19,20].

2. Wavelength-insensitive waveguide design

Figure 1(a) shows a schematic of the wavelength insensitive waveguide design. It consists of a silicon (Si) substrate, five SiN_x segments (thickness t of 200 nm, width w of 450 nm, and gap g of 400 nm), enclosed by a SiO₂ layer (oxide gap g_ox of 200 nm), and Al₂O₃:Er³⁺ gain film (thickness t_AlO = 1100 nm). The fabrication process has been reported in references [6,21] with the only difference in the layer thickness or dopant. The distance between the Si to the bottom layer of SiN_x (>2.5μm) is sufficient to ensure the fundamental TE mode is not affected by the substrate. We use the prism coupling method to estimate the Al₂O₃:Er³⁺ gain film background loss of <0.1 dB/cm and dopant concentration of 1.0 × 10²⁰ cm⁻³ by linear fitting of measured total film loss (${\text{α}}_{tot}$) vs. the film absorption cross-section (${\sigma }_a$) around 1550 nm based on the following simplified equation:

 

Fig. 1 (a) Schematic of wavelength-insensitive laser waveguide design by multi-segmented SiN_x structure. (b) Mode-solver calculation of the intensity distribution for various near infrared wavelengths in the multi-segmented waveguide design.

Download Full Size | PDF

We perform analysis of the multi-segmented structure for a broad selection of wavelengths relevant to rare-earth emission in the NIR range; 800 nm, 900 nm, 950 nm and 980 nm (diode pump lasers for Er and Yb), 1050 nm (Yb- and Nd-doped lasers), 1300 nm (Nd-doped lasers), 1550 nm (Er-doped lasers), and 2000 nm (Tm- and Ho-doped lasers). The intensity distributions of the fundamental TE modes at the various wavelengths are calculated by a vector finite-difference 2D Eigen mode solver (in-house Matlab code), as shown in Fig. 1(b). By inspection of the modes in Fig. 1(b), we observe that the mode distribution at shorter wavelengths tends to concentrate in the silicon nitride layer. Starting from 950 nm, the mode is distributed almost exclusively in the gain layer.

We define the confinement factor (γ) and overlap factor (Γ) in the active medium by the following equations:

 

Fig. 2 (a) Calculated confinement factor in the multi-segmented waveguide for typical NIR diode pump and rare earth laser wavelengths. (b) Calculated intensity overlap between 980-nm mode and NIR diode and rare earth laser modes.

Download Full Size | PDF

3. Al₂O₃:Er³⁺ DFB lasers

We apply the five-segment wavelength-insensitive design to realize single-frequency and ultra-narrow-linewidth Al₂O₃:Er³⁺ lasers. We investigate two different phase shift configurations for 2 cm long DFB cavity. In QPS-DFB, a discrete quarter phase shift is formed at the center of the cavity with a sharp frequency resonance at the Bragg wavelength. The intense electric field concentrated around the phase shifted region may limit the performance of the laser due to spatial hole burning. Alternatively, the phase shift in DPS-DFB cavity is continuously distributed in a wider region, thus improving the uniformity of the field distribution and increasing the length of the effective gain section. We compare the performance of the lasers in the following sections.

3.1 QPS-DFB

The grating unit in the QPS-DFB cavities is formed by placing additional periodic pieces on both sides of the five-segment SiN_x structure, as shown in Fig. 3(a). These periodic side pieces have width wg of 300 nm with the grating strength κ adjusted by varying the gap distance d_wg. We fabricated a total of 9 devices with 3 grating period variations (Λ = 482 nm, 492 nm, and 502 nm) and 3 grating strength variations (d_wg / κ = 600 nm / 0.6 mm⁻¹, 350 nm / 0.9 mm⁻¹, and 200 nm / 1.2 mm⁻¹).

 

Fig. 3 (a) Design of Al₂O₃:Er³⁺ QPS-DFB (not to scale). The cavity structure consists of five continuous SiN_x segments with grating perturbation provided by two additional periodic side pieces. (b) Optical spectrum of Al₂O₃:Er³⁺ QPS-DFB lasers at various grating periods. (c) On-chip laser power of Al₂O₃:Er³⁺ QPS-DFB lasers vs. pump power.

Download Full Size | PDF

To characterize their performance, we pumped the DFB lasers from both sides using fiber pigtail laser diodes at 978 nm and 976 nm. We obtain the best laser performance from devices with d_wg = 350 nm or κ = 0.9 mm⁻¹ for all three wavelengths. Figure 3(b) shows the spectra of DFB lasers operating at 1536 nm, 1566 nm, and 1596 nm, which covers a similar emission bandwidth that has been shown previously in distributed Bragg reflector (DBR) lasers [6]. The highest on-chip output power of 0.76 mW is achieved at 1566 nm. The laser operating at 1536 nm has the lowest output power, corresponding to > 46.1 dB side mode suppression ratio (SMSR) for all devices. We characterize the slope efficiency and threshold power of the lasers, as shown in Fig. 3(c). We obtain slope efficiencies η = 0.3%, 0.6%, and 0.6% and threshold powers P_th = 55 mW, 65 mW, and 105 mW for lasers at wavelengths 1536 nm, 1566 nm, and 1596 nm, respectively.

The thermal stability of our DFB is investigated in [22,23] showing a temperature-dependent shift of 0.02 nm/°C, which is more than two times lower than the thermal wavelength shift reported for hybrid lasers [24–27]. Such small wavelength shift is contributed by the low thermo-optic coefficients of the Al₂O₃ film and the SiN_x waveguide. The laser stability over time is mainly affected by mechanical misalignment of the fiber coupling for pump and lasing signal. A short term solution is to isolate the laser chip by covering up the test setup, so that the coupling fiber will be minimally affected by environmental fluctuations. Meanwhile, a long term solution is to integrate the diode pump onto the system so that fiber coupling is not necessary.

3.2 DPS-DFB

The DPS-DFB cavities are formed using an asymmetric design which includes a continuous segment with varying width w_n(x) on one side of the waveguide and periodic pieces with spacing d_κ(x) on the other side, where x is the axis along the cavity, as shown in Fig. 4(a). This allows accumulation of phase shift by a gradual sinusoidal change of the effective refractive index Δn_eff(x) [16] while maintaining a constant grating strength κ in the phase shifted region with length L_ps. We use the coupled mode theory [28] to determine the right combination of w_n(x) and d_κ(x) for fixed gap distance d_n = 250 nm and periodic pieces of width w_κ = 300 nm. We fabricated two different L_ps (0.2 cm and 0.4 cm) for grating period at 492 nm and κ = 0.7 mm⁻¹. For the 0.2-cm DPS-DFB, w_n(x) varies from 168 to 351 nm and d_κ(x) varies from 119 to 162 nm. For the 0.4-cm DPS-DFB, w_n(x) varies from 168 to 271 nm and d_κ(x) varies from 140 to 162 nm.

 

Fig. 4 (a) Design of Al₂O₃:Er³⁺ DPS-DFB laser with five-segment SiN_x waveguide (not to scale). The cavity structure consists of five continuous SiN_x segments with grating perturbation provided by two additional side pieces, one with a phase shift region (top) and the other with periodic segments (bottom). (b) Optical spectrum of Al₂O₃:Er³⁺ DPS-DFB lasers at various grating periods. (c) On-chip laser power of Al₂O₃:Er³⁺ DPS-DFB lasers vs. pump power.

Download Full Size | PDF

Figure 4(b) shows the spectrum of the best DPS-DFB with L_ps = 0.4 cm and emission centered at λ = 1565 nm. A maximum on-chip output power of 5.43 mW is obtained, corresponding to a SMSR > 59.4 dB. Figure 4(c) shows the comparison of power performance of the lasers. The longer L_ps DPS-DFB laser has almost double the output power at maximum pump. The threshold power is > 4 times lower than the QPS design (P_th = 14 mW), with close to 5 times improvement in the slope efficiency (η = 2.9%). These improvements can be attributed to a more uniform and longer active gain section in DPS-DFB. Meanwhile, compare with QPS structure, DPS structure may have slightly higher loss from (a) the asymmetry of the grating (due to transition from symmetric waveguide to asymmetric design), and (b) the tapered structure of the waveguide on a longer region that is not observed in traditional QPS. This may deviate the performance of DPS structure from theoretical expectation.

4. Ultra-narrow-linewidth measurement

For an accurate linewidth measurement in a self-heterodyne interferometer, a fiber delay length larger than the laser coherence length is required (L_delay > L_coherence) [29,30]. From the Schawlow-Townes formula, the fundamental (quantum) linewidth limit of an Al₂O₃:Er³⁺ DFB laser can reach the sub-kHz level [31]. If we assume Δν = 1 kHz and the speed of light c/n = 2 × 10⁸ m/s, then the minimum L_delay needs to be at least 200 km. Such a long fiber requirement can be alleviated in a recirculating SHDI (R-SHDI) configuration [17,18], as shown in Fig. 5. The setup is similar to a standard SHDI, but one of the branches couples to a multipass cavity that consists of a fiber delay and an acousto optic modulator (AOM) for frequency shifting (f_AOM = 44 MHz). Thus, the spectrum at frequency n × f_AOM corresponds to an auto-correlation of the input light after passing through the equivalent delay of n × L_delay. Lastly, an erbium doped fiber amplifier (EDFA), an optical isolator, and a tunable filter are included to compensate for roundtrip loss.

 

Fig. 5 Recirculating self-heterodyne delayed interferometer for ultra-narrow-linewidth measurement.

Download Full Size | PDF

We measured the linewidth of the Al₂O₃:Er³⁺ QPS-DFB laser centered at λ = 1566 nm and DPS-DFB laser centered at λ = 1565 nm, as shown in Fig. 6. For QPS-DFB laser, the spectrum was collected at a center frequency of f_c = 132 MHz (n = 3), which corresponds to a total delay length of 105 km (L_delay = 35 km). For the DPS-DFB laser, n = 15, f_c = 660 MHz, and the effective L_delay = 525 km. To differentiate the 1/f frequency noise contribution, the measured spectra are fitted with Voigt functions [32,33]. The self-heterodyne spectra are plotted around f_c with the QPS-DFB laser presented in red color and the DPS-DFB laser in blue color.

 

Fig. 6 Self-heterodyne spectra of Al₂O₃:Er³⁺ QPS- (red) and DPS- (blue) DFB lasers in (a) linear and (b) dB scale. The solid lines of the same color are the fits of the corresponding measurements (dots).

Download Full Size | PDF

By fitting the QPS-DFB spectrum, we obtain a full width half maximum (FWHM) of the Voigt function of FWHM_Voigt = 66.1 ± 2.5 kHz. The Voigt linewidth is further decomposed into the Gaussian component FWHM_Gauss = 18.4 ± 7.9 kHz and Lorentzian component FWHM_Lorentz = 60.7 ± 2.2 kHz. As the self-heterodyne measurement is an autocorrelation process whose FWHM is 2 times the laser linewidth, the optical linewidth Δν can be estimated from half the Lorentzian width of the spectrum, thus Δν_QPS = ½ × FWHM_Lorentz = 30.4 ± 1.1 kHz. For the DPS-DFB laser, with the same analysis above we obtain FWHM_Voigt = 23.8 ± 0.7 kHz, FWHM_Gauss = 17.5 ± 1 kHz, FWHM_Lorentz = 10.5 ± 0.5 kHz, and thus Δν_DPS = 5.3 ± 0.3 kHz. The linewidth improvement in the DPS-DFB laser (Δν_QPS = 5.73 × Δν_DPS) can be attributed to a higher output power and reduction in the spatial hole burning effect.

To our knowledge, this is one of the first demonstrations of a sub-10-kHz-linewidth monolithically integrated laser in a CMOS-compatible silicon photonics platform [2,3,5]. The lowest linewidth for an erbium-doped Al₂O₃:Er³⁺ DFB waveguide laser was obtained in [5]. The linewidth difference between our laser and the ultra-narrow-linewidth laser reported in ref [5], might be explained by differences in the waveguide and cavity dimensions, the wavelength and linewidth quality of the pump laser and/or the mechanical and environmental stability of the experiment. By careful optimization each of these properties we expect that further reduction of the linewidth can be obtained.

5. Conclusion

We demonstrate narrow linewidth Al₂O₃:Er³⁺ DFB lasers using a multi-segmented SiN_x silicon-compatible waveguide design. The waveguide design has > 85% confinement factor and > 98% intensity overlap at wavelengths from 950 to 2000 nm, showing octave-spanning wavelength-insensitivity in the NIR range and enabling efficient pumping at broadly-spaced pump and signal/laser wavelengths. We apply the design to QPS-DFB and DPS-DFB cavities. In the QPS-DFB configuration, we obtain maximum output powers of 0.41 mW, 0.76 mW, and 0.47 mW at widely spaced wavelengths within both the C and L bands of the erbium gain spectrum (1536 nm, 1566 nm, and 1596 nm). In a DPS cavity, we achieve an order of magnitude improvement in maximum output power (5.43 mW) for a wavelength centered at 1565 nm, corresponding to a side mode suppression ratio (SMSR) of > 59.4 dB. Finally, we measure the optical linewidths with an R-SHDI setup to obtain Δν_QPS = 30.4 ± 1.1 kHz and Δν_DPS = 5.3 ± 0.3 kHz. The overall 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. Even narrower linewidth can be achieved by mechanical stabilization of the setup, increasing the pump absorption efficiency, increasing the output power, or enhancing the cavity Q. The Q can be enhanced by accounting for Al₂O₃:Er³⁺ film thickness variation across the cavity, reducing the SiN_x loss and optimizing the DFB grating strength.