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Silicon reflectors for external cavity lasers based on ring resonators
Optics Communications 383 (2017) 453–459
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Silicon reflectors for external cavity lasers based on ring resonators Chao Wang, Xia Li, Hao Jin, Hui Yu, Jianyi Yang, Xiaoqing Jiang n College of Information science electronic engineering, Zhejiang University, Hangzhou 310013, China
art ic l e i nf o
a b s t r a c t
Article history: Received 6 July 2016 Received in revised form 16 August 2016 Accepted 18 August 2016
We propose and experimentally investigate types of silicon ring reflectors on Silicon-On-Insulator (SOI) platform. These reflectors are used for realizing the silicon hybrid external cavity lasers. A suspended edge coupler is used to connect the reflective semiconductor optical amplifier (RSOA) chip and the reflectors. The properties of the reflectors and the hybrid external cavity lasers with these reflectors are illustrated. The experimental results show that all of those reflectors have a high reflectivity and the highest reflectivity can up to be 95%. The lowest insertion loss can be as low as 0.4 dB. The output power of the hybrid external cavity lasers with these reflectors can reach mW magnitude and the highest output power is 6.1 mW. Over 30 dB side mode suppression ratio is obtained. & 2016 Published by Elsevier B.V.
Keywords: Silicon reflectors External cavity lasers Ring resonators Silicon photonic
1. Introduction In recent years, optical interconnects has developed rapidly due to the large integration of devices on silicon [1]. The urgent need of high quality light source with narrow linewidth, high power and compact size promotes the progress of optical interconnects [2–5]. Silicon photonics has emerged as a promising and commerciallyviable solution for high speed optical interconnection with low energy consumption, low latency [6,7]. Taking the good advantage of CMOS-technology, silicon material become the optimal choice for photonic products. However, the silicon is an indirect band material so the light source on a silicon photonics platform cannot be realized monolithically in a CMOS process. Many approaches have been researched for low energy consumption laser sources that can be also integrated with silicon photonics circuits [8–12]. So far, the most practical approach has been the hybrid integration of III–V with SOI. As for the hybrid integration laser, the external cavity plays a very important role because the parameters of the output in laser such as peak power, threshold current and repetition rate rely on the reflector cavity. Therefore, a good quality reflector is required. Silicon based micro-ring resonators (MRRs) possess a wide variety of functionalities as for they have very high refractive index contrast, which allows realizing ultra-compact devices. In previous works, many structures of MRRs have been applied for the wavelength filters [13,14]. However, it is also possible to create reflectors using MRRs [15–17]. Besides, the small size as well as the tunability of MRRs make it suitable to replace the grating n
Corresponding author. E-mail address: [email protected] (X. Jiang).
http://dx.doi.org/10.1016/j.optcom.2016.08.043 0030-4018/& 2016 Published by Elsevier B.V.
structures on silicon. A metallic heater is implemented on top of the ring resonator to allow thermal tuning of the resonant wavelength. Unlike the distributed bragg reflector which have a long cavity [18], ring-resonator external cavity is a good way to achieve the compact size and narrow linewidth. In this paper, we demonstrate types of silicon ring reflectors with different characteristics. Four designs are implemented: the first one shows the structure of the coupled ring reflector. The second one is made up of ring resonator and a waveguide loop mirror which is used to play a role as a comb-like reflector. The third one is the combination of micro-ring resonator and grating reflector. The last one is composed of a micro-ring resonator and a Mach–Zehnder interferometer (MZI). The experimental results show that they all have a high reflectivity and the highest reflectivity can reach 95%. The insertion loss can be as low as 0.4 dB. So these micro-ring reflectors can be used for resonance cavity. The main application of the reflectors is applied as external cavity in hybrid external cavity lasers. The laser consists of the RSOA, a spot size converter and the wavelength tunable filter. The large mismatch between the silicon ring reflector and RSOA is solved by a suspended edge coupler. We measure that all the output power of the hybrid external cavity lasers with these reflectors can reach mW magnitude and the highest laser output power is 6.1 mW. The side mode suppression ratio (SMSR) can be over 30 dB.
2. Device structure and characterization The hybrid external-cavity laser is formed by optical coupling between a silicon ring reflector and an III–V chip providing the optical gain. Wavelength selectivity is provided by the silicon ring reflector on SOI chip, while the optical gain is generated by RSOA.
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Fig. 1. Schematic structures of (a) reflector 1 (b) reflector 2 (c) reflector 3 (d) reflector 4.
The property of the ring reflector has a great influence on the property of the external-cavity laser. Fig. 1(a) shows the schematic diagram of the coupled ring reflector. An incident optical wave is coupled into the clockwise wave of ring 1. The clockwise wave of ring 1 is coupled into the counterclockwise wave of ring 2. Finally, the wave is coupled into the backward wave of the bus waveguide. The diameters of the two ring resonators are the same. There are two factors which effect the reflectivity of the reflector. One is the coupling ratios containing ring-ring coupling ratio and ring-bus coupling ratio.
The other is the loss coefficient. Fig. 2(a) shows the relationship between the single peak reflectivity and the coupling ratios when the loss coefficient is 2 dB/cm. From Fig. 2(a) it shows that single peak can only be obtained with an appropriate pair of ring-ring and ring-bus coupling ratios. Fig. 2(b) shows the impact of the loss coefficient to the reflectivity. The other three reflectors are the reflectors based on the single ring resonator. Fig. 1(b) shows the schematic diagram of the reflector 2. It consists of a ring resonator and a waveguide loop mirror. By tuning the voltage applied on ring resonator, the
Fig. 2. (a) the relationship between the single peak reflectivity and the coupling ratios when the loss coefficient is 2 dB/cm (b) the impact of the loss coefficient to the reflectivity (c) reflectivity response as functions of amplitude attenuation factor (d) reflectivity response as functions of coupling coefficient.
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resonant peak shift and select the center wavelength. The drop port of ring resonator is connected with a waveguide loop mirror so that the transmission of light can be reflected through this C band and then oscillate in the cavity. To fit the need of wavelength tunability, the bandwidth of the waveguide loop mirror should be broadband. Fig. 1(c) shows the structure of the reflector 3 which consists of a ring resonator and a grating. The ring resonator is used to select the wavelength. The reflectivity of grating reflector depend on the coupling efficient and the length of the grating. The bandwidth of the grating coupler is decided by the etch depth of the grating. Here, the grating is different from the DBR as mentioned before, it is broadband. The radius of the ring resonator is 10 mm and the Q-factor is about 6 104. As is shown in Fig. 1(d), the reflector 4 is composed of two Y-branches and a ring resonator. The quality (Q) factor of the micro-ring, based on symmetric coupling gaps to the ring, defines the reflection bandwidth and is designed to ensure single-mode operation while minimizing optical loss. The symmetric design also makes the device intolerant to fabrication process variations. We use the transfer-matrix method to analyze the reflection properties of all the reflectors. As for the reflectors 2, 3, 4, the influences of the coupling coefficient and the amplitude attenuation factor to them are almost the same, as shown in Fig. 2(c) and (d). To gain a high reflectivity, we design the amplitude attenuation factor of waveguide as 0.9928 and the coupling coefficient between ring and waveguide as 0.45. Except those two factors, the reflectivity of the device 2 and the device 4 is also influenced by the property of the 1 2 MMI. And the reflectivity of the device 3 is also influenced by the property of the grating. The device is fabricated on an 8-inch silicon-on-insulator (SOI) wafer with 220 nm-thick top Si layer and 2-μm-thick dioxide layer, and 248-nm deep UV photolithography is used to define the device pattern. The Si waveguides with a cross section of
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450 nm 220 nm are employed. The radii of the rings are all 10 μm . In our fabrication of the reflector 2 and reflector 4, the 1 2 MMI is used to replace the Y-branch splitter. Fig. 3 is the optical structure of the silicon ring reflectors. In Fig. 4(a), we scheme the configuration of external-cavity hybrid laser. As is known to all that a laser must contain the pump, the gain medium and the resonance cavity. Therefore, three parts are involved in our hybrid silicon external cavity lasers including a reflective semiconductor optical amplifier (INPHENIX IPRAD1501), a spot size converter (SSC), and a silicon ring reflector. The pump and the gain medium are all acted by the RSOA. The measured gain wavelength range is from 1528 nm to 1565 nm, corresponding to the C-band for optical communication. The output of RSOA is polarization-depend and the small signal gain is between 15 dB and 25 dB shown is Fig. 4(b). The resonance cavity is made up of the micro-ring reflector and the high reflectivity (HR) coating facet in RSOA. The polarization controller (PC) determine the output state of polarization. The SSC consists of suspended SiO2 waveguides and overlapped Si nano-tapers. To provide a structural support for the suspended SiO2 waveguide, the SiO2 beams are lied laterally. Based on this structure, the optical input signal is launched into the SiO2 waveguide, and then coupled into the Si nano-taper [19].
3. Experimental results In our experiment, the reflectivity is a key parameter of those reflectors. A testing system is built used by a circulator shown in Fig. 5. The circulator include three ports, which only allows unidirectional transporting. When launching the light into A-port, it only goes out from B-port. However, when we launch it into B-port, only C-port detect the signal. Therefore, amplified spontaneous emission (ASE) source emit broadband light into A-port
Fig. 3. Optical micrograph of (a) reflector 1 (b) reflector 2 (c) reflector 3 (d) reflector 4.
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Fig. 4. (a) the schematic structure of hybrid silicon external cavity lasers (b) the small signal gain spectrum of RSOA.
Fig. 5. The schematic of reflectivity testing system by circulator.
and the reflectors are connected with B-port so that the light from A-port can reciprocate through B-port and final transport to C-port. The reflective spectrum of input signal is detected by the optical spectrum analyzer (OSA). The reflective spectrum includes three parts: the reflective signal, the noise caused by the circulator and the noise of the whole system. To get the accurate reflective spectrum, signal processing is needed. Because the noise is high frequency signal, so using the FFT algorithm and processed by a low-pass filter can we obtain the accurate lines. The suspended coupling loss and edge coupling loss measured in experiment are 10 dB and 3 dB, which also affect the reflectivity. The reflective spectrum of the silicon ring reflectors is plotted in Fig. 6. Fig. 6(a) shows the reflective spectrum of the coupled ring resonator. There are two resonant wavelengths at every resonant point. The reason is that the magnitude of the ring–ring coupling coefficient is about 0.08. It is not so small that the symmetric and antisymmetric modes both exist in the reflector [20], so it cannot obtain the single peak, as shown in Fig. 2(a). Even though, the reflectivity can reach 88%. The insertion loss is about 0.5 dB. The reflective spectrums of the reflector 2 and reflector 3 are plotted in Fig. 6(b) and Fig. 6(c), respectively. The FSR of the ring resonator is
8.7 nm. The insertion loss of the reflector 3 is the lowest, which is as low as 0.4 dB. The 3 dB bandwidth of the reflector 2 and reflector 3 is the same, which is 0.02 nm. Moreover, the reflectivity of them are 95% and 92% so that they can be well used for laser cavity. Fig. 6(d) shows that the reflectivity of the reflector 4 can also up to be 90%. The 3 dB bandwidth is also 0.02 nm. The Q magnitude is 6 104. However, the insertion loss of the device is about 3.5 dB. It is a little big which caused by the asymmetry of 1 2 MMI. In all of those reflective spectra, the reflectivity is different in different resonant wavelength. The reason is that the reflectivity of those devices depends on the coupling coefficient and attenuation coefficient. When the gap between the bus waveguide and the micro-ring is fixed, the coupling coefficient at different wavelength is different and the dispersion effect is wavelength-depend too, which affects the attenuation coefficient. In addition, the fabrication error will also attribute to the difference. Therefore, those factors make the reflectivity different in different resonant wavelength. Table 1 shows different performances for these silicon ring reflectors. The reflector 1 have a small device size because the mirco-ring has an advantage of compact structure. However, the reflectivity is less than 90%, so it is arguably used for a high performance laser cavity. When combining micro-ring with other structure, the reflectivity is improved but also increase the device size. As for the reflector 2–4, they have the same structure of wavelength selecting and the reflectivity are all up to 90%. It demonstrates that the highest reflectivity is obtained by the reflector 2. The 3 dB bandwidth is the same as 0.02 nm. The smaller 3 dB bandwidth represents that it has a good property to select the wavelength. But their device size is larger than double-ring structure. And the different insertion loss is a pivotal factor that affects the threshold current of the hybrid silicon external cavity lasers. Even though those parameters of four reflectors are obtained, the output of laser must be measured to judge whether those reflectors is suitable or not. In our hybrid silicon laser demonstration, the RSOA has a high reflection mirror on one side and an anti-reflection (AR) layer coated on the edge-coupling side. By
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Fig. 6. The reflective spectrum of (a) reflector 1 (b) reflector 2 (c) reflector 3 (d) reflector 4.
Table 1 Specifications for silicon ring reflectors. Item Typography Reflectivity 3 dB Bandwidth (nm)
Device size (mm2)
Insertion Loss (dB)
Reflector Reflector Reflector Reflector
0.06 0.022 0.45 0.09 1.47 0.025 1.05 0.025
0.5 0.5 0.4 3.5
1 2 3 4
88 95 92 90
0.35 0.02 0.02 0.02
Fig. 7. L–I curve of hybrid silicon external cavity lasers.
tuning the current of RSOA, the pump power change at the same time and initiate stimulated emission in external laser cavity. When the power increase the threshold power, the laser beam is
generating. The relationships between the RSOA injection current and the output power of the hybrid external cavity lasers with these reflectors are shown in Fig. 7. The wall-plug efficiency and output power of reflector 4 is lowest among four devices due to its relatively high insertion loss. The coupling loss between the SOI chip and the RSOA (see Fig. 4) is 5.4 dB. The output power of the hybrid laser is measured through a lensed fiber connected to the power meters. The measured L–I curves shows different lasing threshold current, different maximum optical power and different SMSR. Fig. 8(a)–(d) give the spectra of the laser output for different silicon ring reflectors at the same current of 150 mA. The lasing wavelength is not at the wavelength of which the reflectivity is the highest. The reason is that the lasing wavelength is determined both by the reflectivity of the reflector and the RSOA gain property. According to the Fig. 4(b) and Fig. 6, the gain peak is not at the wavelength corresponding to the highest reflectivity. The output is actually low polarization sensitivity in this system so the mode competition and internal motivation cause different polarization of signals and random mode hopping in cavity. When tuning the polarization controller at a certain angle, the specific polarization state is selected to obtain a single mode output. As shown in the embed figure, the single wavelength lasing is obtained except the first one. The reason is that the resonant wavelength of the two rings in reflector 1 is not the same. Table 2 are some data comparison for the four different reflectors used in the hybrid external cavity lasers. It shows that the output power of the lasers can all reach mW magnitude and the lowest threshold current and the highest SMSR are for the reflector 1 because of its small device size and low device loss. The highest output power is for the reflector 2 and the lowest output power is for reflector 4. We make two conclusions from Table 1
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Fig. 8. The spectrum of the lasers' output power. (a) for the reflector 1 (b) for the reflector 2 (c) for the reflector 3 (d) for the reflector 4.
Table 2 Specifications for hybrid external cavity lasers. Item Typography Threshold current (mA)
Output power (mW)
SMSR (dB) Tunable range (nm)
Reflector Reflector Reflector Reflector
3.2 6.1 5.8 1.4
35 35 32 30
1 2 3 4
75 82 90 100
10.1 nm (a FSR) 8.7 nm (a FSR) 8.7 nm (a FSR) 8.7 nm (a FSR)
and Table 2. One is that the threshold current is determined by both the device size and insertion loss. Small size and low loss device can have a low threshold current, which is suitable for the on-chip sources. Another conclusion is the insertion loss and reflectivity of reflector both affect the output power so that reflector 2 have a highest wall-plug efficiency and output power among those four structures, because of its high reflectivity and low device loss. In addition, the tunable range of reflector1 is larger than other three reflectors because there is a phase change through the coupler and the resonance wavelength shift broad in this coupledring structure [20].
4. Conclusions In conclusions, we have demonstrated four types of silicon ring reflectors and discussed the factors that influence their reflectivity. The characteristics, such as the reflectivity, 3 dB bandwidth, sizes and insertion loss are measured. We also have measured the property of the hybrid silicon external cavity lasers with the
reflectors. The output power of the lasers can all up to be mW magnitude. Among those four reflectors, we conclude several choice strategies. The micro-ring structure has a compact device size and low loss such as reflector 1, however, the reflectivity is a little low. Therefore, this type of reflector is convenient to design and usually applied for the on-chip system which requiring low threshold and not much high power. Another three devices which combining ring resonator with other waveguide structures can obtain reflectivity over 90% at the expense of device size. The reflector 2 can get the reflectivity up to 95% and the output power is 6.1 mW. This type of devices is better suitable for the chip-chip system which need less density, high output power and wide tunable range. In summary, our results can be as a reference for the design of the external cavity lasers or other applications.
Acknowledgments This work is supported by the Nature Basic Research Program of China (2013CB632105) and the National Natural Science Foundation of China (61307074).
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Silicon reflectors for external cavity lasers based on ring resonators Chao Wang, Xia Li, Hao Jin, Hui Yu, Jianyi Yang, Xiaoqing Jiang n College of Information science electronic engineering, Zhejiang University, Hangzhou 310013, China
art ic l e i nf o
a b s t r a c t
Article history: Received 6 July 2016 Received in revised form 16 August 2016 Accepted 18 August 2016
We propose and experimentally investigate types of silicon ring reflectors on Silicon-On-Insulator (SOI) platform. These reflectors are used for realizing the silicon hybrid external cavity lasers. A suspended edge coupler is used to connect the reflective semiconductor optical amplifier (RSOA) chip and the reflectors. The properties of the reflectors and the hybrid external cavity lasers with these reflectors are illustrated. The experimental results show that all of those reflectors have a high reflectivity and the highest reflectivity can up to be 95%. The lowest insertion loss can be as low as 0.4 dB. The output power of the hybrid external cavity lasers with these reflectors can reach mW magnitude and the highest output power is 6.1 mW. Over 30 dB side mode suppression ratio is obtained. & 2016 Published by Elsevier B.V.
Keywords: Silicon reflectors External cavity lasers Ring resonators Silicon photonic
1. Introduction In recent years, optical interconnects has developed rapidly due to the large integration of devices on silicon [1]. The urgent need of high quality light source with narrow linewidth, high power and compact size promotes the progress of optical interconnects [2–5]. Silicon photonics has emerged as a promising and commerciallyviable solution for high speed optical interconnection with low energy consumption, low latency [6,7]. Taking the good advantage of CMOS-technology, silicon material become the optimal choice for photonic products. However, the silicon is an indirect band material so the light source on a silicon photonics platform cannot be realized monolithically in a CMOS process. Many approaches have been researched for low energy consumption laser sources that can be also integrated with silicon photonics circuits [8–12]. So far, the most practical approach has been the hybrid integration of III–V with SOI. As for the hybrid integration laser, the external cavity plays a very important role because the parameters of the output in laser such as peak power, threshold current and repetition rate rely on the reflector cavity. Therefore, a good quality reflector is required. Silicon based micro-ring resonators (MRRs) possess a wide variety of functionalities as for they have very high refractive index contrast, which allows realizing ultra-compact devices. In previous works, many structures of MRRs have been applied for the wavelength filters [13,14]. However, it is also possible to create reflectors using MRRs [15–17]. Besides, the small size as well as the tunability of MRRs make it suitable to replace the grating n
Corresponding author. E-mail address: [email protected] (X. Jiang).
http://dx.doi.org/10.1016/j.optcom.2016.08.043 0030-4018/& 2016 Published by Elsevier B.V.
structures on silicon. A metallic heater is implemented on top of the ring resonator to allow thermal tuning of the resonant wavelength. Unlike the distributed bragg reflector which have a long cavity [18], ring-resonator external cavity is a good way to achieve the compact size and narrow linewidth. In this paper, we demonstrate types of silicon ring reflectors with different characteristics. Four designs are implemented: the first one shows the structure of the coupled ring reflector. The second one is made up of ring resonator and a waveguide loop mirror which is used to play a role as a comb-like reflector. The third one is the combination of micro-ring resonator and grating reflector. The last one is composed of a micro-ring resonator and a Mach–Zehnder interferometer (MZI). The experimental results show that they all have a high reflectivity and the highest reflectivity can reach 95%. The insertion loss can be as low as 0.4 dB. So these micro-ring reflectors can be used for resonance cavity. The main application of the reflectors is applied as external cavity in hybrid external cavity lasers. The laser consists of the RSOA, a spot size converter and the wavelength tunable filter. The large mismatch between the silicon ring reflector and RSOA is solved by a suspended edge coupler. We measure that all the output power of the hybrid external cavity lasers with these reflectors can reach mW magnitude and the highest laser output power is 6.1 mW. The side mode suppression ratio (SMSR) can be over 30 dB.
2. Device structure and characterization The hybrid external-cavity laser is formed by optical coupling between a silicon ring reflector and an III–V chip providing the optical gain. Wavelength selectivity is provided by the silicon ring reflector on SOI chip, while the optical gain is generated by RSOA.
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Fig. 1. Schematic structures of (a) reflector 1 (b) reflector 2 (c) reflector 3 (d) reflector 4.
The property of the ring reflector has a great influence on the property of the external-cavity laser. Fig. 1(a) shows the schematic diagram of the coupled ring reflector. An incident optical wave is coupled into the clockwise wave of ring 1. The clockwise wave of ring 1 is coupled into the counterclockwise wave of ring 2. Finally, the wave is coupled into the backward wave of the bus waveguide. The diameters of the two ring resonators are the same. There are two factors which effect the reflectivity of the reflector. One is the coupling ratios containing ring-ring coupling ratio and ring-bus coupling ratio.
The other is the loss coefficient. Fig. 2(a) shows the relationship between the single peak reflectivity and the coupling ratios when the loss coefficient is 2 dB/cm. From Fig. 2(a) it shows that single peak can only be obtained with an appropriate pair of ring-ring and ring-bus coupling ratios. Fig. 2(b) shows the impact of the loss coefficient to the reflectivity. The other three reflectors are the reflectors based on the single ring resonator. Fig. 1(b) shows the schematic diagram of the reflector 2. It consists of a ring resonator and a waveguide loop mirror. By tuning the voltage applied on ring resonator, the
Fig. 2. (a) the relationship between the single peak reflectivity and the coupling ratios when the loss coefficient is 2 dB/cm (b) the impact of the loss coefficient to the reflectivity (c) reflectivity response as functions of amplitude attenuation factor (d) reflectivity response as functions of coupling coefficient.
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resonant peak shift and select the center wavelength. The drop port of ring resonator is connected with a waveguide loop mirror so that the transmission of light can be reflected through this C band and then oscillate in the cavity. To fit the need of wavelength tunability, the bandwidth of the waveguide loop mirror should be broadband. Fig. 1(c) shows the structure of the reflector 3 which consists of a ring resonator and a grating. The ring resonator is used to select the wavelength. The reflectivity of grating reflector depend on the coupling efficient and the length of the grating. The bandwidth of the grating coupler is decided by the etch depth of the grating. Here, the grating is different from the DBR as mentioned before, it is broadband. The radius of the ring resonator is 10 mm and the Q-factor is about 6 104. As is shown in Fig. 1(d), the reflector 4 is composed of two Y-branches and a ring resonator. The quality (Q) factor of the micro-ring, based on symmetric coupling gaps to the ring, defines the reflection bandwidth and is designed to ensure single-mode operation while minimizing optical loss. The symmetric design also makes the device intolerant to fabrication process variations. We use the transfer-matrix method to analyze the reflection properties of all the reflectors. As for the reflectors 2, 3, 4, the influences of the coupling coefficient and the amplitude attenuation factor to them are almost the same, as shown in Fig. 2(c) and (d). To gain a high reflectivity, we design the amplitude attenuation factor of waveguide as 0.9928 and the coupling coefficient between ring and waveguide as 0.45. Except those two factors, the reflectivity of the device 2 and the device 4 is also influenced by the property of the 1 2 MMI. And the reflectivity of the device 3 is also influenced by the property of the grating. The device is fabricated on an 8-inch silicon-on-insulator (SOI) wafer with 220 nm-thick top Si layer and 2-μm-thick dioxide layer, and 248-nm deep UV photolithography is used to define the device pattern. The Si waveguides with a cross section of
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450 nm 220 nm are employed. The radii of the rings are all 10 μm . In our fabrication of the reflector 2 and reflector 4, the 1 2 MMI is used to replace the Y-branch splitter. Fig. 3 is the optical structure of the silicon ring reflectors. In Fig. 4(a), we scheme the configuration of external-cavity hybrid laser. As is known to all that a laser must contain the pump, the gain medium and the resonance cavity. Therefore, three parts are involved in our hybrid silicon external cavity lasers including a reflective semiconductor optical amplifier (INPHENIX IPRAD1501), a spot size converter (SSC), and a silicon ring reflector. The pump and the gain medium are all acted by the RSOA. The measured gain wavelength range is from 1528 nm to 1565 nm, corresponding to the C-band for optical communication. The output of RSOA is polarization-depend and the small signal gain is between 15 dB and 25 dB shown is Fig. 4(b). The resonance cavity is made up of the micro-ring reflector and the high reflectivity (HR) coating facet in RSOA. The polarization controller (PC) determine the output state of polarization. The SSC consists of suspended SiO2 waveguides and overlapped Si nano-tapers. To provide a structural support for the suspended SiO2 waveguide, the SiO2 beams are lied laterally. Based on this structure, the optical input signal is launched into the SiO2 waveguide, and then coupled into the Si nano-taper [19].
3. Experimental results In our experiment, the reflectivity is a key parameter of those reflectors. A testing system is built used by a circulator shown in Fig. 5. The circulator include three ports, which only allows unidirectional transporting. When launching the light into A-port, it only goes out from B-port. However, when we launch it into B-port, only C-port detect the signal. Therefore, amplified spontaneous emission (ASE) source emit broadband light into A-port
Fig. 3. Optical micrograph of (a) reflector 1 (b) reflector 2 (c) reflector 3 (d) reflector 4.
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Fig. 4. (a) the schematic structure of hybrid silicon external cavity lasers (b) the small signal gain spectrum of RSOA.
Fig. 5. The schematic of reflectivity testing system by circulator.
and the reflectors are connected with B-port so that the light from A-port can reciprocate through B-port and final transport to C-port. The reflective spectrum of input signal is detected by the optical spectrum analyzer (OSA). The reflective spectrum includes three parts: the reflective signal, the noise caused by the circulator and the noise of the whole system. To get the accurate reflective spectrum, signal processing is needed. Because the noise is high frequency signal, so using the FFT algorithm and processed by a low-pass filter can we obtain the accurate lines. The suspended coupling loss and edge coupling loss measured in experiment are 10 dB and 3 dB, which also affect the reflectivity. The reflective spectrum of the silicon ring reflectors is plotted in Fig. 6. Fig. 6(a) shows the reflective spectrum of the coupled ring resonator. There are two resonant wavelengths at every resonant point. The reason is that the magnitude of the ring–ring coupling coefficient is about 0.08. It is not so small that the symmetric and antisymmetric modes both exist in the reflector [20], so it cannot obtain the single peak, as shown in Fig. 2(a). Even though, the reflectivity can reach 88%. The insertion loss is about 0.5 dB. The reflective spectrums of the reflector 2 and reflector 3 are plotted in Fig. 6(b) and Fig. 6(c), respectively. The FSR of the ring resonator is
8.7 nm. The insertion loss of the reflector 3 is the lowest, which is as low as 0.4 dB. The 3 dB bandwidth of the reflector 2 and reflector 3 is the same, which is 0.02 nm. Moreover, the reflectivity of them are 95% and 92% so that they can be well used for laser cavity. Fig. 6(d) shows that the reflectivity of the reflector 4 can also up to be 90%. The 3 dB bandwidth is also 0.02 nm. The Q magnitude is 6 104. However, the insertion loss of the device is about 3.5 dB. It is a little big which caused by the asymmetry of 1 2 MMI. In all of those reflective spectra, the reflectivity is different in different resonant wavelength. The reason is that the reflectivity of those devices depends on the coupling coefficient and attenuation coefficient. When the gap between the bus waveguide and the micro-ring is fixed, the coupling coefficient at different wavelength is different and the dispersion effect is wavelength-depend too, which affects the attenuation coefficient. In addition, the fabrication error will also attribute to the difference. Therefore, those factors make the reflectivity different in different resonant wavelength. Table 1 shows different performances for these silicon ring reflectors. The reflector 1 have a small device size because the mirco-ring has an advantage of compact structure. However, the reflectivity is less than 90%, so it is arguably used for a high performance laser cavity. When combining micro-ring with other structure, the reflectivity is improved but also increase the device size. As for the reflector 2–4, they have the same structure of wavelength selecting and the reflectivity are all up to 90%. It demonstrates that the highest reflectivity is obtained by the reflector 2. The 3 dB bandwidth is the same as 0.02 nm. The smaller 3 dB bandwidth represents that it has a good property to select the wavelength. But their device size is larger than double-ring structure. And the different insertion loss is a pivotal factor that affects the threshold current of the hybrid silicon external cavity lasers. Even though those parameters of four reflectors are obtained, the output of laser must be measured to judge whether those reflectors is suitable or not. In our hybrid silicon laser demonstration, the RSOA has a high reflection mirror on one side and an anti-reflection (AR) layer coated on the edge-coupling side. By
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Fig. 6. The reflective spectrum of (a) reflector 1 (b) reflector 2 (c) reflector 3 (d) reflector 4.
Table 1 Specifications for silicon ring reflectors. Item Typography Reflectivity 3 dB Bandwidth (nm)
Device size (mm2)
Insertion Loss (dB)
Reflector Reflector Reflector Reflector
0.06 0.022 0.45 0.09 1.47 0.025 1.05 0.025
0.5 0.5 0.4 3.5
1 2 3 4
88 95 92 90
0.35 0.02 0.02 0.02
Fig. 7. L–I curve of hybrid silicon external cavity lasers.
tuning the current of RSOA, the pump power change at the same time and initiate stimulated emission in external laser cavity. When the power increase the threshold power, the laser beam is
generating. The relationships between the RSOA injection current and the output power of the hybrid external cavity lasers with these reflectors are shown in Fig. 7. The wall-plug efficiency and output power of reflector 4 is lowest among four devices due to its relatively high insertion loss. The coupling loss between the SOI chip and the RSOA (see Fig. 4) is 5.4 dB. The output power of the hybrid laser is measured through a lensed fiber connected to the power meters. The measured L–I curves shows different lasing threshold current, different maximum optical power and different SMSR. Fig. 8(a)–(d) give the spectra of the laser output for different silicon ring reflectors at the same current of 150 mA. The lasing wavelength is not at the wavelength of which the reflectivity is the highest. The reason is that the lasing wavelength is determined both by the reflectivity of the reflector and the RSOA gain property. According to the Fig. 4(b) and Fig. 6, the gain peak is not at the wavelength corresponding to the highest reflectivity. The output is actually low polarization sensitivity in this system so the mode competition and internal motivation cause different polarization of signals and random mode hopping in cavity. When tuning the polarization controller at a certain angle, the specific polarization state is selected to obtain a single mode output. As shown in the embed figure, the single wavelength lasing is obtained except the first one. The reason is that the resonant wavelength of the two rings in reflector 1 is not the same. Table 2 are some data comparison for the four different reflectors used in the hybrid external cavity lasers. It shows that the output power of the lasers can all reach mW magnitude and the lowest threshold current and the highest SMSR are for the reflector 1 because of its small device size and low device loss. The highest output power is for the reflector 2 and the lowest output power is for reflector 4. We make two conclusions from Table 1
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Fig. 8. The spectrum of the lasers' output power. (a) for the reflector 1 (b) for the reflector 2 (c) for the reflector 3 (d) for the reflector 4.
Table 2 Specifications for hybrid external cavity lasers. Item Typography Threshold current (mA)
Output power (mW)
SMSR (dB) Tunable range (nm)
Reflector Reflector Reflector Reflector
3.2 6.1 5.8 1.4
35 35 32 30
1 2 3 4
75 82 90 100
10.1 nm (a FSR) 8.7 nm (a FSR) 8.7 nm (a FSR) 8.7 nm (a FSR)
and Table 2. One is that the threshold current is determined by both the device size and insertion loss. Small size and low loss device can have a low threshold current, which is suitable for the on-chip sources. Another conclusion is the insertion loss and reflectivity of reflector both affect the output power so that reflector 2 have a highest wall-plug efficiency and output power among those four structures, because of its high reflectivity and low device loss. In addition, the tunable range of reflector1 is larger than other three reflectors because there is a phase change through the coupler and the resonance wavelength shift broad in this coupledring structure [20].
4. Conclusions In conclusions, we have demonstrated four types of silicon ring reflectors and discussed the factors that influence their reflectivity. The characteristics, such as the reflectivity, 3 dB bandwidth, sizes and insertion loss are measured. We also have measured the property of the hybrid silicon external cavity lasers with the
reflectors. The output power of the lasers can all up to be mW magnitude. Among those four reflectors, we conclude several choice strategies. The micro-ring structure has a compact device size and low loss such as reflector 1, however, the reflectivity is a little low. Therefore, this type of reflector is convenient to design and usually applied for the on-chip system which requiring low threshold and not much high power. Another three devices which combining ring resonator with other waveguide structures can obtain reflectivity over 90% at the expense of device size. The reflector 2 can get the reflectivity up to 95% and the output power is 6.1 mW. This type of devices is better suitable for the chip-chip system which need less density, high output power and wide tunable range. In summary, our results can be as a reference for the design of the external cavity lasers or other applications.
Acknowledgments This work is supported by the Nature Basic Research Program of China (2013CB632105) and the National Natural Science Foundation of China (61307074).
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