1. Introduction

Silicon photonics is a very promising technology for future low-cost high-bandwidth telecommunication applications down to the chip level. This is due to the high degree of integration, high optical bandwidth and large speeds coupled with the development of a wide range of integrated optical functions. These advances continue to fuel significant growth in the field. Success of this technology is due to the strong optical confinement achieved in high index contrast optical systems coupled with a well established fabrication technology that is easily integrated with CMOS systems. A wide range of devices have been added to the toolkit of silicon photonics, including all-optical switch [1], electro-optic switch [2], polarization splitter [3] and many others. Recently, generation of light on silicon using an external optical pump was demonstrated by taking advantage of the strong Raman emission in silicon [4, 5]. This approach requires tuning of the external optical pump wavelength to control emission wavelength. Other approaches use heterogeneous integration of III-V materials into a silicon platform [6]. The latter may conflict with the heterogeneous integration of photodetectors, which often require high temperature processes [7,8,9]. It is likely that material issues will arise from different processing requirements.

In this paper, an alternative approach is presented where we demonstrate a compact, scalable, planar approach that integrates for the first time on-chip silicon photonic tuning and control of lasing wavelength using an external Erbium-doped fiber amplifier as gain source. The control of the laser wavelength in this approach is accomplished by thermo-optic tuning of the refractive index [10,11], and hence, of the passband wavelength of a silicon microring-based wavelength-selective filter [12]. Repeatable and predictable tuning of the filter wavelength is achieved. The use of an Er+-doped fiber with heterogeneously broadened gain enables the generation of multiple arbitrary wavelengths by a single chip with each wavelength independently tuned. With a small area less than 20×20 µm², dozens, if not hundreds of such devices can be integrated on a single Si-photonic chip with a single external EDFA, leading to individual wavelength control of DWDM channels. Wide free-spectral-range (FSR) silicon microrings [13] with 47 nm FSR, as well as multiple wavelength tuning and multiplexing using these silicon microring filters [14] have already been demonstrated in our previous work.

2. Experiment and results

A schematic of the tunable laser experiment is shown in Fig. 1(a). The 2 main components of the tunable laser are the Erbium-doped fiber (EDF) and the silicon-based tunable filter. A piece of EDF serves as the gain media which is pumped by a 980 nm laser diode. The amplified light from the EDF is coupled to the input of the silicon-based intracavity wavelength-selective filter by a tapered fiber, which is followed by an isolator and a polarization controller. Light from the output of the filter is collected by another tapered fiber and coupled back to the input of the EDF. We use the tap port of a 1×2 coupler with a nominal splitting ratio of 1:99 as the laser output.

Figure 1(b) shows the top view microscopic image of the fabricated filter element, with light coupled from the through port to the drop port of the filter. The filter uses a 10 µm-diameter microring resonator based on single-mode silicon-on-insulator (SOI) strip waveguides operating around the telecom range of 1.55 µm. The strip waveguides has the dimension of 0.45 µm in width and 0.25 µm in height. The full-width-half-maximum (FWHM) of the microring resonator is in the range 0.1–0.2 nm and its free-spectral-range (FSR) is about 18 nm. The filter wavelength is found to be sensitive to the temperature of the microring. A linear relationship between the filter wavelength shift and microring temperature with slope of 0.095 nm/°C was demonstrated in our previous study [12]. The temperature dependence of the filter wavelength is dominated by the thermo-optic effect. The wavelength shift induced by thermal expansion of silicon microring is estimated to be 2 orders of magnitude smaller. While the thermally induced free carriers concentration does change both the refractive index and absorption coefficient of silicon [15], the changes are on the order of 10^-10 and 10^-8cm^-1, respectively, both negligible in terms of the wavelength shift and waveguide absorption. The tuning of the filter wavelength is achieved by controlling the temperature of the microring, which in turn is accomplished by applying current through a Ni-Cr micro-heater placed on top of the microring.

 

Fig. 1. (a). Experimental setup of the tunable laser (b) top view microscopic picture of fabricated switch element

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The fabrication of the filter began with an SOI wafer with 3 µm buried oxide layer and 0.25 µm silicon layer. The waveguide pattern was defined by a VB6-HR 100kV ebeam lithography system with 5 nm beamstep into hydrogen silsesquioxane (HSQ) resist with dose of 1100 µC/cm² and transferred by Cl₂ reactive ion etching (RIE) process. Etching was followed by deposition of ~1.4 µm top cladding using plasma enhanced chemical vapor deposition (PECVD) process, and incorporated the HSQ resist. The micro-heater was generated through a process sequence of photolithography, thermal deposition of ~0.1 µm Ni-Cr and liftoff. The micro-heater size is 50 µm×50 µm. The Ti/Au contact pads and feed lines were generated by the same sequence except deposited by sputtering. Finally, the input and output waveguides were polished to the edge of the chip where waveguides are finished with nanotapers, which can theoretically reduce insertion loss to below 1 dB [16].

The overall fiber-chip-fiber insertion loss was measured to be ~15dB, higher than previously reported 10.4 dB of a very similar structure in ref. [17], mainly due to observed imperfections of the fabricated waveguides and nanotapers. Round-trip losses include additional contributions from the polarization controller and the 1×2 coupler which are measured to be 0.6 dB and 1.0 dB, respectively. The gain obtained from EDF at λ=1.55 µm, which exceeds 20 dB as shown in Fig. 3 (inset) overcomes the losses stated above; therefore lasing was expected in this band with the proposed silicon microring filter.

 

Fig. 2. (a). Transmission spectrum of the filter using unpolarized ASE source. Lasing spectra of: (b) TE mode; (c) TM mode; and (d) simultaneous lasing of both TE and TM modes. Insets show corresponding top view infrared images of the ring area during lasing.

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The birefringent nature of the microring resonator leads to separated quasi-TE resonances and quasi-TM resonances observed within a FSR of the filter [18]. Thus, the filter transmission spectrum is sensitive to input polarization. Our device is optimized at quasi-TE resonances with a much narrower linewidths (FWHM=0.1-0.2 nm) compared to quasi- TM ones (FWHM=1-2 nm). Figure 2(a) shows the transmission spectrum around 1.55 µm of the filter was taken using broadband unpolarized ASE source when the chip was at room temperature (~24°C). The quasi-TE and quasi-TM resonance wavelengths were measured to be 1555.56 nm and 1550.93 nm respectively. Similar peak transmissions and extinction ratios were achieved for both quasi-TE and quasi-TM resonances. The extinction ratios were measured to be over 10 dB. The double resonance characteristic of the TE peak has been attributed to sidewall back scattering on the microring [19]. By tuning the input polarization state using the polarization controller, the polarization-dependent loss can be controlled to switch on/off the TE or TM lasing modes. Figure 2(b,c) shows lasing spectra at 1555.57±0.01 nm or 1550.90±0.1 nm corresponding to the quasi-TE or quasi-TM filter resonances, respectively. Simultaneous operation at two-wavelengths is demonstrated by adjusting the input polarization at 45° to the chip normal with lasing observed at both TE and TM wavelengths (Fig. 2(d)). Significantly narrower FWHMs of 0.02 nm were observed in both lasing modes, approaching the resolution limit of the optical spectrum analyzer (0.01nm). The lasing wavelength of TM mode was observed to shift ±0.1 nm around 1550.90 nm due to the broad peak linewidth of the quasi-TM resonance of the filter. However, for TE mode, the lasing wavelength shift was ±0.01 nm which is significantly reduced compared to TM lasing mode and approaches the wavelength resolution of the OSA. An interesting phenomenon was observed when comparing the topview infrared images captured of the microring filter at corresponding lasing modes as shown in the insets of Fig. 2(b–d). The top view imaging setup was configured by mounting a 20× long working distance objective above the chip. An infrared camera was used to monitor and capture the top view image with exactly the same camera settings. From the images, greater scattering around the ring area was observed when lasing at the TE mode compared to the TM mode although slightly higher lasing output was measured at the TM mode. This is due to the increased off-plane scattering of TE mode with electric field parallel to the surface of the sample. The speckle pattern observed around the filter area is due to reflections from the unpolished bottom surface of the chip. The TM mode preferentially scatters along the plane of the sample, hence the reduced intensity collected by the top viewing camera. Other filter resonances within EDF gain bandwidth are found at 1537.45 nm (quasi-TE) and 1533.81nm (quasi-TM), however, lasing was never observed on those wavelengths even with highest EDF pumping current provided. This is because the corresponding gain at those wavelengths are at least 6 dB lower than at 1550 nm band for our EDF and thus below the lasing threshold.

 

Fig. 3. Laser output power versus the EDF pumping bias current. Inset shows the EDF gain at 1550 nm for different input power at pumping current from 100 mA to 400 mA

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Figure 3 plots the laser output power versus the EDF pump bias current with the tuning filter chip maintained at room temperature. In this experiment, the input polarization state was adjusted by a manual polarization controller that was set to TE mode at the highest pumping current. The pump current was decreased to below threshold and an ANDO AQ6317B optical spectrum analyzer (OSA) was used to record the emission spectra. The peak position and power were noted at each pump current. From the plot we can determine a lasing threshold of ~260 mA. We estimate the power in the loop after the EDF to be ~0.3 mW with pumping current of 400 mA. The small value is due to the high losses in our current silicon tunable filter chip. It is important to note that losses in silicon photonics have been shown to be as low as 3.6±0.1 dB/cm [20]. The output power was observed to fluctuate significantly for pumping currents above 300 mA, however, lasers built with same EDF and FBG grating filters showed a stable output. This fluctuation is currently under investigation and is believed to be caused by the interplay of several factors including the modulation of losses in the fiber-to-chip coupling; self-modulation of the ring resonator filters and gain saturation effects in the EDF.

 

Fig. 4. (a) Filter spectra and (b) lasing spectra at applied heating current of 0, 1, 3 and 5mA (c) filter and lasing peak wavelength shift versus applied heating power.

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Tuning of the lasing wavelength was achieved by adjusting the current into the microheater to tune the silicon microring resonance. To drive the micro-heater on top of the ring, two electrical probes were used to provide ohmic contact to the pads. Previously we demonstrated pulsed operation of the silicon microring tunable filter up to 6.4 nm, corresponding to an increase in temperature of 60 K [10]. However, continuous operation is restricted to 5mA at which point the probe contacts are not reliable. The probes are connected to a current source which provides heating current. The heater resistance was determined to be ~800 Ω. Figure 4(a,b) show the filter transmission spectra and the laser emission spectra, for a fixed polarization controller position, taken at the same microheater currents of 0, 1, 3 and 5 mA after thermal equilibrium had been reached. Peak intensity changes are attributed to uncorrected drifts in polarization and fiber-to-chip coupling, in addition to the fluctuations mentioned earlier. Both spectra were found to red-shift with constant current heating as previously demonstrated [12–13]. The peak shift was found to be proportional to the electrical heating power as seen in Fig. 4(c) which show wavelength shifts versus applied heating power with a measured slope of ~0.067 nm/mW. A thermo-optic tunable range of over 1.3 nm is achieved with low heating power of 20 mW, which corresponds to a change in temperature of approximately 14 K, although much wider tunable range should be achievable as shown in our previous work on silicon microring filters [12].

3. Conclusion

In summary, we have demonstrated a new application of silicon photonics for the control of on-chip wavelengths through the use of an external gain source. Tuning of 1.3 nm was demonstrated with 20 mW electrical power with wider tuning ranges of at least up to 6.2 nm possible with larger currents. Simultaneous lasing at two different wavelengths was achieved using the natural birefringence of the devices. The use of parallel ring configurations will enable simultaneous lasing wavelengths to be selected and tuned across the whole Er+ range. This integration of the wavelength control of optical sources into silicon photonics is an important step towards wavelength division multiplexing in intra-chip and inter-chip communications.