1. Introduction

In recent years there has been increasing interest in using silicon-on-insulator (SOI) as a platform for photonic integrated circuits (PICs). Silicon processing is highly developed and the high index contrast provided by the material system allows for high optical confinement and smaller devices. Interest has also focused on hybrid III-V on SOI as a means of leveraging the passive advantages of SOI with the gain and absorption in III-V layers to create large-scale PICs [1,2]. A tunable laser is one key component for many applications, including spectroscopic measurements, variable sources for WDM systems, in Light Detection and Ranging (LIDAR), and beam-steering [3–5]. The double-ring resonator approach for widely tunable lasers was proposed by Liu et al [6]. Several groups have demonstrated tunable lasers on hybrid III-V on SOI using distributed Bragg reflectors and distributed feedback (DFB) mirrors, or a combination of ring coupling and DFB mirrors [7–9].

In this work we demonstrate a new widely tunable Vernier ring laser using hybrid III-V on silicon without the need for gratings, combining thermally tuned ring resonator wavelength filters with a ring laser design to offer wavelength selection without costly e-beam lithography. We report a tuning range of more than 40 nm with side mode suppression ratio (SMSR) greater than 35 dB, maximum continuous-wave (CW) on-chip output power of 3.3 mW, and center-wavelength linewidth of 338 kHz.

2. Design

The tunable Vernier ring laser design, as shown in Fig. 1(a), consists of two Si bus waveguides with InP gain regions tapered to transfer the mode between the Si and III-V. Two ring resonators are coupled to the bus waveguides with resistive heaters for thermal tuning and wavelength selection. A microscope image of the fabricated device is shown in Fig. 1(b). The ring resonators, Ring 1 and Ring 2, are 400 and 420 μm long with an expected free spectral range (FSR) of 1.74 and 1.66 nm respectively. The offset of the ring FSRs creates a Vernier effect as shown in Fig. 2(a). The offset in the FSR of the rings is designed to be large enough to avoid low round-trip losses in adjacent peaks but small enough so that the next Vernier overlap occurs at ± 21 FSRs, or 37 nm, which was the expected gain bandwidth – in this way single wavelength operation was assured. Figure 2(b) illustrates the single wavelength selection when the two rings are coupled and Fig. 2(c) incorporates the calculated effects of the two rings coupled with the full cavity. Transmission in Fig. 2 refers to light that remains within the cavity. Tuning one ring by the difference in the FSRs causes a selected wavelength shift of the FSR of the other ring, giving discrete tuning over the full 37nm, whereas tuning both rings simultaneously allows continuous tuning. The third ring cavity is the full round trip through the device. This has a length of 3.3 mm and a calculated FSR of 0.21 nm. Using the full cavity as a third tuning axis may give more than the designed 37nm but this will be limited by the gain bandwidth. There is no phase section for the bus waveguide region of the laser; however phase tuning is accomplished by small changes in the pump current.

 

Fig. 1 (a) Schematic view of hybrid Vernier ring laser and (b) picture of fabricated device.

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Fig. 2 (a) Calculated cavity transmission of rings superimposed to display Vernier effect assuming 3 dB/cm waveguide loss and 10% coupling to bus waveguides. (b) Calculated coupled ring transmission. (c) Calculated full cavity transmission. Insets show ring cavity designs used for calculations. Ring transmission refers to power remaining in the ring cavity.

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3. Fabrication

Rib waveguides were defined using standard photolithography in 500 nm thick Silicon-on-Insulator (SOI) with 1 μm buried oxide. The waveguides were 0.7 μm wide in the rings and bus waveguides and tapered to 2μm under the gain regions. They were dry-etched to a depth of 260 nm. InP wafers with pre-grown InGaAs quantum wells were bonded directly onto the SOI and thinned using CMP and a wet etch. The gain regions were then defined and etched using an oxide hard mask and reactive-ion etching. N-contacts were deposited in an e-beam chamber, after which 900nm of buffer PECVD oxide was deposited and vias etched over the gain region, P-contacts were deposited in the vias and the heater metal was patterned and deposited on the buffer oxide over the rings. One side of the device was diced and polished for measurement.

4. Characterization

Transmission measurements of the ring resonators were performed on polished and anti-reflection coated passive devices using an external tunable laser, two single-mode lensed fibers, and a power meter. Light was fiber coupled to a bus waveguide which was coupled to a ring resonator and a wavelength sweep was performed. The output of the bus waveguide with two ring spectra superimposed is shown in Fig. 3(a) where the resonant wavelengths of the rings show a drop in throughput power. The data show individual ring FSRs of 1.60 and 1.53 nm and a coupled FSR greater than 32 nm. The FSRs of both rings are lower than designed, resulting in a lower expected tuning range, but 40 nm tuning may still be achieved by aligning the FSR peaks of the full laser cavity. Fine tuning data of a single peak is shown in Figs. 3(b)-3(c) with a least squares fit modeling transmission. Ring 1 and 2 have unloaded round trip losses of 0.20 dB and 0.15 dB respectively (5.1 and 3.5 dB/cm) and power coupling coefficients to the bus waveguides of 2.6% and 1.7%. The FSR of the full passive cavity is measured to be 0.16 nm.

 

Fig. 3 (a) Transmission data for full FSR of coupled rings. (b) Fine tuning data from ring filter 1 with round trip loss of 0.20 dB, coupling loss of 2.6% and FWHM of 0.022 radians. (c) Fine tuning data from ring filter 2 with round trip loss of 0.15 dB, coupling loss of 1.7% and FWHM of 0.020 radians.

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Loss measurements of the silicon waveguides were performed using the cutback method. Light from an external laser source was passed through waveguides of increasing length to distinguish between fiber coupling and waveguide losses. Waveguide and fiber coupling losses were measured to be 0.58 cm⁻¹ and 11.5 dB respectively. The tapered transitions from silicon waveguides to III-V gain sections as well as the gain section itself were characterized by Davenport et al [10]. Transition losses were measured to be 10 dB per transition at threshold (there were four such transitions in this device). Gain section losses were measured to be 11.7 cm⁻¹. The total intracavity loss is calculated to be 45 dB which is largely dominated by tapered transition losses. As a result of such high losses, room temperature operation of this laser resulted in insufficient gain for widely tunable operation. Active measurements were taken with the chip mounted on a thermally controlled chuck maintained at 10⁰ C.

A maximum on-chip output power of 3.3 mW from a single output was achieved with a threshold current of 160 mA at 10⁰ C. This value was measured using a lensed fiber and adjusted to subtract out the measured coupling loss. Threshold current density was 2350 A/cm² for the combined gain element length of 1360 μm. Such high current density is attributed to high losses in the tapered transition sections. Linewidth measurements were made by the delayed self-heterodyne method using a delay length of 5.5 km of fiber and a modulation frequency of 100 MHz. The beat signal captured using an RF spectrum analyzer is shown in Fig. 4(a). A Lorentzian function was fitted to the data with a 3 dB bandwidth of 676 kHz corresponding to a laser linewidth of 338 kHz at 10⁰ C and 260 mA pump current. Figure 4(b) shows single wavelength emission at 1575 nm with SMSR greater than 45 dB under the same conditions. Emission is nearly identical for clockwise and counterclockwise propagating modes for both power and wavelength.

 

Fig. 4 (a) Beat signal from a delayed self-heterodyne measurement of laser measuring 338 kHz linewidth at 100 C and 260 mA pump current. (b) Single wavelength lasing showing SMSR greater than 45 dB at 100 C and 260 mA pump current.

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Wavelength tuning is accomplished by pumping current through a resistive heater above one or both of the ring resonators. Wavelength tuning was measured using a single-mode tapered fiber connected to an optical spectrum analyzer with resolution of 0.06nm. Fine wavelength tuning was demonstrated with an arbitrarily chosen resolution of 0.1 nm and is shown in Fig. 5(a) for a range of 2.5 nm by tuning both rings simultaneously. The heater tuning powers used are shown in Fig. 5(b); the trend is generally linear with power for Heater 1. No clear trend was shown for Heater 2 which is attributed to thermal crosstalk. Thermal tuning over 40 nm was achieved with a SMSR greater than 35 dB and on-chip output powers greater than 0.45 mW at an operating temperature of 10⁰ C as shown in Fig. 5(c).

 

Fig. 5 (a) Superimposed spectra showing 0.1 nm resolution tuning over 2.5 nm. (b) Wavelength tuning as a function of heater power. (c) Superimposed spectra of coarse wavelength tuning over more than 40 nm with SMSR greater than 35 dB.

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5. Conclusions

We have demonstrated a tunable Vernier ring laser with more than 40 nm of tuning, SMSR greater than 35 dB, linewidth of 338 kHz, and a maximum of 3.3 mW on-chip output power using a CMOS compatible hybrid silicon platform.