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An external cavity tunable laser, which is based on a silicon hybrid micro-ring resonator, is demonstrated. The heat isolate grooves around the rings effectively cuts off temperature crosstalk. Experimental results have shown that the output power of this device can reach 15.5dBm, with a linewidth less than 130 kHz. The tuning range is more than 57nm in C-band with 60 dB side mode suppression ratio (SMSR).
© 2017 Optical Society of America
1. Introduction
In recent years, coherent optical transmission is proposed as a solution to increase the spectral efficiency. As the transceiver module of coherent optical system developing in the direction toward miniaturization, the demands for minitype and low power consumption of the device are put forward, especially for most coherent optical communication system which is used for metropolitan area network (MAN), the pressure is increasing. Wavelength tunable semiconductor lasers, which act as optical power/signal supplying source devices, are the most fundamental elements in such systems. Different types of tunable lasers have been presented. Tunable DBR laser can keep stable output while the tuning range is relatively narrow. Array DFB can dynamically adjust the output wavelength, which causes crosstalk easily between each other. Shiyu Li,et al. have reported a kind of tunable laser based on a single micro-ring resonator (MRR) with tuning range of 17 nm and 14.5dBm optical power. JSTQE,Guanghua Duan,et al. have demonstrated hybrid IIIV/Si lasers with two integrated intra-cavity ring resonators can achieve a wide thermal tuning range of 35nm with a side mode suppression ratio (SMSR) higher than 50 dB and an optical power level in excess of 4 mW across the wavelength range. Tao Chu,et al. have proposed a wavelength tunable laser based on silicon photonic-wire waveguides which obtained a maximum tuning span of 38 nm at a tuning power consumption of 26mW and the SMSR observed was more than 30 dB.
The external cavity tunable laser has significant advantages of high output power, wide tunable range, side mode suppression ratio and narrow linewidth over the multiple realization forms of tunable semiconductor lasers. Therefore, the article mainly aimed at the technical research for a novel form of multi-channel external cavity tunable laser which is based on silicon waveguide, proposing a wavelength tunable laser with a semiconductor optical amplfier (SOA) and external double micro-ring resonator which is fabricated with silicon photonic waveguides. The tuning range of the device can cover the whole c-band, while the side-mode suppression ratio can reach up to 60 dB and the output power is 15.5dBm. This structure can lay a solid foundation for the future implementation of silicon-based large-scale integrated photonic devices, which has been a hot subject of this field in recent years [1–14].
In this paper, the external cavity of tunable micro-ring laser we proposed is mainly composed of an SOA, silicon-based waveguide chip and two lenses. The coupling loss between the SOA and chip can be effectively reduced for the structure of double lens, so it can get a higher optical power. There will be many resonator modes in the laser resonator, micro-ring chip based on silicon waveguide can realize single mode output which has narrow linewidth, resonator mode locking is achieved by tuning double micro-ring and straight waveguide phaser. Between them is also equipped with heat isolate grooves to prevent temperature crosstalk.
2. Design of the device
2.1 Device structure
The schematic layout of the proposed laser is shown in Fig. 1, including coupling output optical path, beam splitter, monitoring photo detector and external cavity resonator. The microscope image of the silicon micro-ring chip is shown in Fig. 2(a), mainly composed of a double micro-ring, grating coupler (GC), a multimode interference (MMI) and a spot size converter (SSC), which have been marked in the figure. The kappa of double micro-ring is about , the radius of them are 46um and 47um respectively. The 1*2 MMI is used as a beam splitter and beam combiner, the splitter ratio is 5%:95%. The return loss of MMI and GC is very large so we can neglect it. Coupling structure of MMI and double micro-ring is shown in Fig. 2(b), the ring is similar to a runway which is composed of a straight waveguide and a bend waveguide. SSC has a cantilever structure and is tilted to the edge of the silicon chip to reduce the reflection between the SOA and the silicon resonator [1,15]. Besides, the SSC can enlarge the mode size of the optical field and effectively reduce the coupling loss. The back reflection between the SOA and Si resonator is about 20dB when employed SSC. As for SOA chip, the WG angle iswith AR coating on the angled facet, the length is 1.0mm and gain BW is 40nm. The structure of double rings achieves a high-Q tunable filter which can select a single longitudinal mode from the high density of fundamental Fabry-Perot modes created by a relatively long cavity. The desired Q for this structure is no less than 8611 while the FSR of the FP modes is 90pm. The isolation magnitude provided by isolator is 45dB. Lasing wavelength is determined at the wavelength where the transmission peaks of micro-ring resonator and longitudinal mode matched. By thermally adjusting the resonant wavelength of double rings, the matched transmission peak is shifted thus the lasing wavelength is also tuned. All the external cavity laser components and temperature sensor is set above the base board made of aluminium nitride (AlN), and there is a thermo electric cooler (TEC) beneath the board, which is used to hold on the temperature around, obtaining high output power with low power consumption. TEC power consumption is about 1.21w when the temperature is kept at , it’s about 1.15w when the temperature is kept at and 1.27w when it’s .
Fig. 2 (a)The photograph of the silicon micro-ring chip. (b)Schematic diagram of chip: ①Electrodes ②Straight Waveguide ③Heat Isolate Grooves ④Sport Size Converter ⑤Double Micro-ring Structure.
2.2 Coupling efficiency analysis
It is important to improve efficient coupling between SOA and silicon micro-ring chip. Structural parameters were optimized as the Fig. 3(a) shows, the beam divergence angle of the SSC is xwhile the beam divergence of SOA is 16°(lateral) and 30°(transverse). A couple of collimating lenses are used for coupling between the micro-ring chip and SOA to increase the coupling efficiency and the alignment tolerance. Coupling loss of external cavity is abou1.5dB. The loss will be less at central wavelength than at extreme wavelengths.
The schematic diagram of cantilever is shown in Fig. 3(b). Such coupling structure of external cavity can effectively improve coupling efficiency, reduce the cavity loss which is conducive to increase the output power [16,17].
2.3 Tuning range analysis
The silicon micro-ring chip acts as a wavelength tunable filter. Since the free Spectral Ranges (FSR) of the double-ring resonators differ slightly, the synthetic spectra of the double-ring resonators forms matched and unmatched transmission peaks of the external resonator spectrum. By changing drive voltage of electrodes around Ring-1 and Ring-2, the resonating wavelength of the ring resonators can be adjusted. In this paper, we pick out larger FSR1 and the difference between FSR2 and FSR1 is small. The FSR1 of Ring-1 is 2.296nm (287GHz), FSR2 of Ring-2 is 2.400nm (300GHz). The synthetic FSR is shown in Fig. 4(a). Assume the two FSRs coincide at the position a,b and c, the distance between a and b,a and c is FSR3. It can be calculated as follow [18]:
Then we can get the value of FSR3 is about 53nm which presents a very satisfactory agreement with 57nm obtained by the experiments as Fig. 4(b) shows. It’s easy to know from the formula that the smaller difference between FSR1 and FSR2 is, the larger FSR3 we will get. If the difference is too small to make the main peak differ by 3dB from sub-peak, then we need to choose proper difference though we want to get as large tuning range as possible.The maximum tuning range is the wavelength spacing between the two neighboring matched peaks, which is significantly enlarged via the Vernier effect, compared with a single-ring resonator [14].
2.4 SMSR improvement analysis
Data based on the test of the chip were analyzed so that we can optimize the design to improve the performance of the laser. As is shown in Fig. 5(a), it’s easy to see the main peak differs by 3.4dB from sub-peak with narrow transmission bandwidth, 3dB bandwidth is about 0.084nm (more than one cavity mode and less than two). As a result, it has a Q as high as 19324 as Fig. 5(b) shows. When used in the external cavity laser, it can achieve a high SMSR output especially at central wavelength as Fig. 5(c) shows.
Fig. 5 Output characteristic of the chip. (a)Spectrum of chip. (b)The high Q value of chip. (c)SMSR of each wavelength.
3. Experiment
The characteristics of tunable laser this paper proposed are shown in Fig. 6. To further evaluate the performance of proposed laser, we implement experiments to test some key indicators. Before starting experiment we need to adjust optical coupling between SOA and micro-ring chip and set a proper temperature of TEC. By changing drive current of electrodes around Ring-1 and Ring-2, the output wavelength of laser can be tuned. During the experiment, we try to keep only one of the FSRs fixed and the other one is tuned. By this approach, the wavelength can be tuned with a certain step size.As for the output properties of proposed laser, it is shown in Fig. 6(c), when current of SOA is 280mA, the output power of different peak wavelength can reach 15.5dBm and side-mode suppression ratio is close to 60dB at same time. Relationship between output power and current of the proposed laser is shown in Fig. 6(a). The threshold current of this tunable laser is about 65 mA with slope efficiency of 0.155mW/mA when the output wavelength is 1550nm. The linewidth of this tunable laser is measured by delayed self-heterodyne method with a delay fiber of 10 km length and a LiNbO3 intensity modulator to shift the carrier frequency by 200MHz [19]. The signal is captured by RF spectrum analyzer and the 20-dB spectra width is less than 130 kHz as shown in Fig. 6(b).
Fig. 6 Experimental results of proposed laser. (a)L-I curve of the proposed laser. (b)Self heterodyne linewidth spectrum. (c)Tunning spectrum of the proposed laser.
We adopt a new project to achieve wavelength locking. MCU can get optimum wavelength output and maximum sampling value of optical power at the same time, it will calculate the difference between current optical power and former when sampling value changes, if the difference exceed the threshold level, MCU will change the current loaded into phasing straight waveguide to achieve phase compensation.
4. Conclusion
In summary, we have demonstrated a compact silicon narrow linewidth tunable laser based on a double micro-ring resonator. The external cavity of tunable laser we proposed is mainly composed of a semiconductor optical amplifier (SOA), silicon-based waveguide chip and two lenses. Due to the high Q value and narrow linewidth of synthetic spectrum, the laser can get single mode output whose power is 15.5dBm and linewidth is about 130KHZ. The tuning range is 57 nm which covers C-band with 60 dB side-mode-suppression-ratio (SMSR). The heat isolate grooves around rings effectively cut off temperature crosstalk. The use of straight waveguide phase modulation and MPD implements a more flexible and lower power consumption method to lock wavelength. Our future work will focus on laser wavelength calibration, improving the accuracy of output wavelength.
Funding
2016 General program of Natural Science Foundation of Hubei Province (2016CFB597).
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