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Here, we demonstrate a chip-scale integrated optical wavelength tracker with fast response and compact format. By exploiting the electro-optic(EO) effect on a thermally controlled silicon micro-ring resonator filter, the proposed tracker can operate over a wide wavelength range according to the thermo-optic (TO) effect; meanwhile, the tracker’s response speed is greatly improved through the EO effect (i.e. tracking within 1 ns), as compared to the traditional TO controlled methods (typical ~10 μs). With the integration of a photodiode onto the photonics chip, the compact chip is with a footprint of 0.5 mm × 1.5 mm. This tracker has potential applications for wavelength tacking in advanced DWDM network systems, tunable laser sources, and high performance optical sensors.
© 2014 Optical Society of America
1. Introduction
Optical wavelength tracking is of importance in monitoring wavelength changes and stabilization wavelength in fiber communication networks and many optical sensors applications, since the wavelength drift and/or spectral misalignments may cause significant power loss, crosstalk and signal distortion. Therefore, a wavelength tracker is essential for various optical devices and systems, such as wavelength tunable laser sources [1, 2], dense wavelength-division multiplexing (DWDM) optical network [3], silicon modulators [4], optical sensors [5, 6]. Various tracking approaches have been demonstrated, where special filters or mono-chromators are designed, such as chromatic dispersion prism [3], arrayed-waveguide grating [7], fiber Bragg grating filter [8], disk resonator filter [9]. However, these trackers are either lacking of flexibilities in integration due to their big volume or limited traceability due to the tradeoff between the tracking speed and tracking range.
In order to achieve a high performance compact wavelength tracker, a tiny filter capable of offering both fast response time and wide tuning range is required. Silicon photonics solution is a promising approach due to its conspicuous advantages, such as ultra-small footprint, flexible integration with conventional electronic integrated circuits, and high operation speed. In particular, silicon photonics based tunable filters have been extensively explored via different work mechanisms including thermo-optic effect [3, 10], optomechanical effect [11, 12], electrostatic tuning [13], free carrier effects [14] and two-photon absorption [15]. In view of broad tuning range, thermo-optic (TO) effect is preferred, but they may degrade the speed; while the electro-optic (EO) effect offers high speed operation but has an inherent limitation of small tuning range [16, 17]. In this paper, we effectively apply TO-assisted EO effect to control a silicon micro-ring resonator filter, and demonstrate a novel ultra-compact wavelength tracker.
2. Design and working principle
The proposed wavelength tracker consists of a tunable micro-ring resonator filter, a photodiode (PD) and a control circuit as shown in Fig. 1(a). To implement thermo-optic effect and electro-optic effect simultaneously, the micro-ring resonator filter is designed with rib structure and doped to form a p-i-n junction structure; meanwhile a micro-heater is placed on the top of micro-ring resonator as illustrated in Fig. 1(b). Thus, the resonance wavelength of the micro-ring resonator can be widely tuned through thermo-optic effect and rapidly adjusted via electro-optic effect. The electro-optic tuning is realized by changing free carrier concentration, which is achieved through bias current (I1) injection into the p-i-n junction. The PD is integrated onto the silicon photonics chip for monitoring the light intensity (P’(λi)). Correspondingly, the generated photocurrent (IPD) is served as a feedback signal for real-time wavelength variation tracking. When the light beam (e.g. at λi) is coupled into the tunable micro-ring resonator, it suffers a power loss induced by the round-trip scattering and the intrinsic absorption. The power loss reaches its maximum when λi meets the resonance condition of the micro-ring resonator, leading to the corresponding IPD reaching its minimum. Meanwhile the device is triggered and the corresponding injection I1 of the tunable micro-ring resonator is read out. Therefore, the input wavelength can be tracked and expressed by I1 as shown in Fig. 1(c). Response time of the tracker is dependent on the wavelengths tuning speed of the micro-ring resonator (i.e. assuming the control circuit limitation is negligible). Thus, the tracker has a chance to work at even tens of picoseconds response time because of the merit of the fast response of the electro-optic effect.
Fig. 1 Schematic and working principle of the proposed wavelength tracker with joined electro-optic and thermo-optic effects. (a) Schematic of the wavelength tracking device. (b) Cross-section view of the micro-ring waveguide (WG) in the micro-ring resonator filter. (c) Wavelength tracking principle.
In the tracking process, thermal tuning works as the coarse tracking over a broadband wavelength range, while electrical tuning acts as the mainly tracking approach for precisely matching the resonance wavelength of the micro-ring resonator to the input wavelength. When the input wavelength is λi with an optical power of P(λi), its transmission through the tunable micro-ring resonator is expressed as [18]
where is the minimum transmission power, which is determined by the coupling loss (η) with , and loss coefficient of the micro-ring (α); F is a constant related to the finesse of the micro-ring resonator and independent with the input wavelength. ϕ is the total phase accumulated over one round trip of the micro-ring resonator, as can be expressed with [18]where Neff is the effective refractive index of the micro-ring, L is the circumference of the micro-ring resonator. Based on the Kramers-Kronig theory and the free-carrier plasma dispersion effect, the effective refractive index change (ΔNeff) due to electro-optic effect can be expressed by [19, 20],where ΔN and ΔP are the free electron and hole concentrations, respectively. f(I1) is a function determined by the doping concentration and the profile of the doping region [13], and it directly links ΔNeff to I1. In the wavelength tracking process, by adjusting the effective refractive index with thermo-optic effect and electro-optic effect, the micro-ring resonance is tuned, and the input wavelength λi can always satisfy the corresponding resonance condition (i.e. ϕ = 2kπ, k is an integer). When the temperature on the micro-ring is fixed, the input wavelength as a function of I1 can be expressed aswhere λ0 is the initial resonance wavelength of the micro-ring resonator and Δλe is the wavelength change due to electro-optic effect. By sweeping the bias current of the micro-ring and monitoring the readout current IPD, the minimum IPD is recorded and the corresponding I1 represents the input wavelength information. Therefore, the input wavelength can be tracked and expressed in terms of I1, with IPD at its minimum.3. Device fabrication and integration
The wavelength tracker device was fabricated by using CMOS compatible fabrication technology. Figure 2(a) shows a top-view of the device, integrated with a PD bonded by using flip-chip technology. Fabrication process started from a silicon-on-insulator (SOI) wafer with a structure layer of 220 nm and a buried oxide layer of 2 μm. Optical waveguides were patterned with 248 nm deep-UV lithography and partially etched by reactive ion etching process. Then, the internal and external areas of the micro-ring resonator were doped with Boron (concentration ~5 × 1017 cm−3) and Phosphorus (concentration ~5 × 1017 cm−3), respectively, with multiple lithography steps defining the two doping areas. Those high-energy ion implantation and rapid thermal annealing defined the p-i-n junction for electro-optic effect tuning. Following the implantation, a 1.2 μm thick oxide layer was deposited as the top cladding layer. Later, a 120 nm thick titanium nitride (TiN) film was deposited and patterned as a micro-heater for thermo-optic effect tuning. The width and the length of the heater are 500 nm and 450 μm, respectively. The zoom-view of the micro-ring resonator which can be tuned by double effects is shown in Fig. 2(b), with its schematic cross-section view illustrated in Fig. 2(c). The chip has a footprint of 0.5 mm × 1.5 mm. This compact size allows people to make a tracker array in a single chip with multi-channel tracking ability. The bus waveguide and the micro-ring waveguide have a dimension of 450 nm × 220 nm (width × height) with a slab height of 80 nm. The radius of the micro-ring resonator is 25 μm and the coupling gap is 200 nm. Waveguide grating couplers at the two ends of the bus waveguide have a period of 630 nm with a uniform filling fact and depth of 70 nm. One grating couples the light into the chip, and the other grating couples the light from the waveguide into the PD [21].
Fig. 2 (a) SEM image of the fabricated optical wavelength tracker. (b) Zoom view of the tunable micro-ring resonator filter, with P/N-doping and the TiN-heater structures. (c) Schematic cross-section of the tunable micro-ring resonator.
4. Experimental results and discussion
The transmission spectrum of the tunable micro-ring resonator was characterized by using an optical spectrum analyzer. In the experiment, a broad-band light beam was coupled into the chip. The free spectral range (FSR) of this micro-ring is measured as 3.9 nm, and the power to tune one whole FSR is about 40 mW. Figure 3(a) shows the transmission spectra with responding to different electrical power applied on the TiN-heater (e.g. power increases from 0 to 30 mW with a step of 3.3 mW). The corresponding resonance wavelength is linearly red-shifted from 1560.48 nm to 1563.71 nm as the heating power increases. Figure 3(b) shows the relation of the resonance wavelength as a function of the heating power. The dark square blocks denote the experimental results, and the red line is the linear fitting curve with the slope of 0.108. In other words, 1 mW electrical power can induce a 0.108 nm wavelength red-shift.
Fig. 3 Transmission spectra under thermo-optic effect. (a) Resonance wavelengths shift with increasing electrical heating power. Wavelength is changed from 1560.48 nm to 1563.71 nm, and electrical heating power is from 0 to 30 mW with an increasing step of 3.3 mW. (b) The resonance wavelength as a function of electrical heating power.
The transmission spectra of the tunable micro-ring resonator at different injection bias currents of the p-i-n junction were also measured. When the bias current varied from 0 to 3.2 mA, a tuning range of 1.04 nm was obtained with a resolution of 0.4 pm/μA, and the corresponding power consumption increased from 0 to 2.8 mW, as shown in Fig. 4. The corresponding resonance wavelength shift can be expressed with, Δλe = −0.561I1 + 0.144I12 −0.023 I13, as shown Fig. 4(b). The red square boxes denote the experimental result and the black line is the polynomial fitting curve. In most of wavelength tracking applications, the wavelength variation is smaller than 1 nm [1–6], thus 3 mA injected bias current is sufficient to meet the requirement.
Fig. 4 (a) Measured transmission spectra of the tunable micro-ring resonator at different bias currents applied on the p-i-n junction. (b) The resonance wavelength as a function of bias current, together with the numerical fitting line.
In another filter characterization, a light beam at 1560.35 nm was coupled into the chip. It is 0.13 nm shorter than the nearest resonance dip (1560.48 nm). Transmission spectra of the filter at different bias current are plotted in Fig. 5(a). The black line shows its original transmission spectrum which possesses the strongest transmission power due to the low loss. When I1 = 0.25 mA, an obvious power drop is recorded (i.e. blue line), corresponding to the status of wavelength overlap between input wavelength and the resonance wavelength. When I1 is further increased, the transmission intensity increases again (e.g. I1 = 0.51 mA), as plotted with the red line in Fig. 5(a).
Fig. 5 Wavelength detection and tracking process. (a) Wavelength detection process for a target wavelength of 1560.35 nm. (b) The input wavelength versus the bias current when the input wavelengths are variation around 1560.35 nm.
The wavelength tracking process was demonstrated, and the results were recorded with the PD bias current, as shown in Fig. 5(b). The input light hovered around 1560.35 nm at a constant power of 1 mW. The bias current of p-i-n junction was chosen as the control parameter while the photocurrent of the PD was adapted as the output parameter to monitor the tracking process. When the bias current swept from 0 to 2 mA, the photocurrent generated an obvious dip at certain specific bias current value for each input wavelength, for example 0.27 mA for 1560.34 nm, 0.78 mA for 1560.12 nm, 1.29 mA for 1559.95 nm, 1.81 mA for 1559.78 nm, as indicated in Fig. 5(b). Therefore, by recording the photocurrent, the corresponding wavelength information can be derived through its relation to the bias current. It should be noted that the current tracker has demonstrated a limited tracking range of 3.2 nm. To improve the performance in terms of tracking range, one possible solution is to reduce the diameter of the micro-ring, resulting in a larger FSR.
The transient measurement was conducted to qualify the response time. The resonance condition was switched between on-tracking state and off-tracking state through the bias current applied on the micro-ring resonator. The switching speed was estimated through the detected PD current, as shown in Fig. 6. Response times of 0.4 ns and 0.3 ns are for the rising edge and falling edge, respectively. It should be noted here that such fast response is mainly based on EO effect, although the response of traditional TO effect is around ~10 μs [10].
Fig. 6 Dynamic change of the transmission in response to injection current change. The rise time (t1) and the relaxation time (t2) of the wavelength are 0.4 ns and 0.3 ns, respectively.
5. Conclusions
In summary, a compact wavelength tracker is demonstrated through a tunable micro-ring resonator filter and a PD configuration. Combination of the electro-optic and thermo-optic effects tuning mechanisms on the same chip would enable the well tracking performance at high speed over a wide tracking range of 3.2 nm. Especially, the electro-optic effect offers a fast response time of less than 1 ns (rise time ~0.4 ns and fall time ~0.3 ns). By integration of the PD onto the silicon photonics chip, the compact chip has a footprint of only 0.5 mm × 1.5 mm, making it much suitable for integration with other photonics devices (e.g. switches, modulators, AWGs, filters). The proposed tracker has potential applications in advanced DWDM communication system and high performance optical sensors.
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