A wavelength tunable laser with an SOA and external double micro-ring resonator, which is fabricated with silicon photonic-wire waveguides, is demonstrated. To date, it is the first wavelength tunable laser fabricated with silicon photonic technology. The device is ultra compact, and its external resonator footprint is 700 × 450 μm, which is about 1/25 that of conventional tunable lasers fabricated with SiON waveguides. The silicon resonator shows a wide tuning range covering the C or L bands for DWDM optical communication. We obtained a maximum tuning span of 38 nm at a tuning power consumption of 26 mW, which is about 1/8 that of SiON-type resonators.
©2009 Optical Society of America
Wavelength tunable semiconductor lasers, which act as optical power/signal supplying source devices, are the most fundamental elements in optical wavelength division multiplexing (WDM) communication systems. They are now commonly used to replace conventional distributed feed back (DFB) lasers. They make network systems more efficient and flexible. For example, the extra set of DFB-laser-installed interface boards is not required as a backup for device fault, and they significantly reduce the cost of selection of wavelength-suitable laser devices and inventory of laser devices for industrial production [1,2].
Usually, wavelength tunable semiconductor lasers are constructed in two different ways; with/without monolithically integrated resonators. Monolithically integrated wavelength tunable lasers, including DFB arrays and sampled-grating distributed Bragg reflector (SGDBR) lasers, are compact since all functional elements are fabricated on one chip [3,4]. Nevertheless, lasers constructed with semiconductor optical amplifiers (SOAs) and external wavelength tunable resonators can be more easily fabricated since the SOAs and the external resonators can be optimally designed and fabricated individually, and they have relatively larger wavelength tunable ranges and output power [1,2]. Therefore, these SOA and external resonator constructed lasers are highly expected for use in actual optical network systems. However, the external resonators of these lasers are currently designed with thermo-optically tuning ring resonators, which are based on SiON waveguides [1,2]. This leads to larger external resonator footprints since SiON waveguides cannot bend with a smaller bending radius. Moreover, the electric power needed for thermo-optical wavelength tuning is high due to the low thermo-optic efficiency of SiON materials and the large resonator footprint. The large device footprint and high tuning power limit applications, such as being installed in transmitter optical subassemblies (TOSA) of a 10-Gigabit small form-factor pluggable (XFP), due to the space limit of the resonator and its cooling unit. Thus, reducing the device footprint and tuning power consumption are highly recommended to enlarge the application area of wavelength tunable lasers and to make more flexible optical communication systems.
On the other hand, silicon photonics is widely regarded as the most promising technology in fabricating ultra-compact, low-power consumption, and low-cost optical devices [5–7]. To date, many novel optical devices, including the first hybrid integrated silicon evanescent laser and thermally tunable ring resonator, have been demonstrated on silicon-on-insulator substrates [8–16], which are compatible with available CMOS technology. Compared with conventional optical devices based on silica materials, silicon photonic devices based on silicon waveguides with silica/air claddings can be ultra compact and exhibit low-power consumption due to their small waveguide bends, benefiting from the waveguide high light confinement and 20-times higher thermo-optic efficiency of silicon material [13,17,18].
Therefore, we propose a silicon-based wavelength tunable laser, whose external resonator is fabricated with silicon photonic-wire waveguides on a SOI substrate. The device is ultra compact and the resonator footprint is less than 700 x 450 μm, which is about 1/25 that of those made of SiON material [1,2]. The maximum wavelength tuning span is currently measured to be 38 nm, which covers optical communication C (1530 - 1565 nm) or L (1565 – 1610 nm) bands. The tuning power consumption for a 38-nm wavelength tuning of this laser is also significantly reduced to 26 mW, which is about 8 times less than that of the SiON types reported previously [1,2]. To our knowledge, this is the first wavelength tunable laser fabricated with silicon photonic technology.
2. Device Structure, Fabrication and Operating Principle
2.1 Device Structure
The wavelength tunable laser we propose is constructed with an SOA and an external resonator, which is fabricated with silicon photonic-wire waveguides on a SOI substrate. A schematic of this tunable laser is illustrated in Fig. 1, in which the SOA was designed to be mounted on the same SOI substrate for future devices and was temporally fixed on an automatically moving stage separated from the external resonator during the experiment.
The SOA with gain and phase control sections has InP-based buried hetero structure waveguides with different compositional wavelengths. The phase section is used to control the optical phase to satisfy lasing oscillation condition.
The external resonator is a double micro-ring resonator. Ring resonators are used because they have a wide free spectra range (FSR) due to the fact that they have short cavity lengths. Furthermore, compared with ring resonators made of SiON material [1,2], ring resonators made with silicon photonic-wire waveguides have a much wider FSR due to their smaller bend radius of several micrometers. Therefore, it is much easier to obtain a larger gain difference needed for single mode lasing oscillation and larger wavelength tuning range with silicon ring resonators. This increases design flexibility and operation reliability.
The external resonator is based on silicon-photonic wire waveguides. The waveguides have a core section size of 450 × 220 (height) nm, which are clad with a 1.8-μm-thick top cladding layer and a 3-μm-thick bottom cladding layer. The propagation loss of the waveguide is about 0.8 dB/mm for TE-like light used in our experiment. All the bending radius of the waveguides are 10 μm, whose bending losses are negligible. An optical spot-size converter (SSC) is introduced into the device to reduce the transmitting and reflecting loss at the coupling between the SOA and the external resonator. The SSC includes waveguides of a silica cladding SiON waveguide, which converts light field to that of the SOA, and a SiON-cladding tapered silicon waveguide, which converts the light field of the silicon waveguide to that of the SiON waveguide. The section size of the SiON waveguide core is 1.5 × 1.4 (height) μm, while the core-end section size and the length of the tapered silicon waveguide is 100 × 220 nm and 200 μm, respectively. Following the SSC at the input/output port of the external resonator, a 3-dB 1 × 2 multi-mode interferometer (MMI) is used for power splitting and combining. Together with the MMI, two racetrack-ring resonators compose a loop-back reflecting resonator. For all the waveguide opening ends, tapered waveguide terminals, whose end-sections are about 80 × 220 nm, are equipped for eliminating optical reflections occurring at the waveguide ends. Directional couplers (DCs), whose power splitting ratio can be flexibly designed, are used for each ring resonator for power splitting at certain power splitting ratios. The DC coupling length and the gap-width between the DC waveguides are 12 and 0.3 μm, respectively. Since the external resonator is thermally adjusting, thin-film micro-heaters are formed over each ring with thermal insulating grooves formed at both sides of the waveguide. The double-ring resonators differ slightly on the ring perimeters, resulting in the difference in FSRs. The ring perimeters are 114.8 and 126.8 μm, respectively.
The external resonator is fabricated on a SOI substrate. First, the silicon photonic-wire waveguide cores were formed by etching the silicon top layer down to the buried oxide (BOX) layer with an inductively coupled plasma (ICP) dry etcher. Then, a SiON layer was buried. Next, both the SiON waveguide cores and the SiON claddings of the SSC were formed by etching the SiON layer down to the BOX layer with a reactive ion etch (RIE) dry etcher. Next, another silica layer was buried over to make the top cladding layer for both SiON and silicon waveguides. Finally, over the top silica cladding layer, thin film heaters and their electrode pads were sputtered onto ring resonators, while thermal insolating grooves beside the waveguides of the ring resonators were etched through all cladding layers down to the bottom supporting silicon substrate. The device sample bar was sliced from the SOI wafer and the SiON waveguide end was polished for optical coupling with the SOA. An optical microscopy photograph of the fabricated external wavelength tuning resonator is shown in Fig. 2. The footprint of the external resonator is 700 × 450 μm, which is about 1/25 that of ones made of SiON material [1,2].
2.3 Laser Wavelength Tuning Using Ring Resonators
The operation principle of the wavelength tunable laser with an external double-ring resonator, which acts as a wavelength filter, is shown in Fig. 3. Since the FSRs of the double-ring resonators differ slightly, the superposition of the spectra of the double-ring resonators forms matched and unmatched transmission peaks of the external resonator spectrum. Using the gain difference between the matched and unmatched transmission peaks, lasing wavelength is determined at the wavelength where the transmission peaks matched. By thermally adjusting the resonating wavelength of the ring resonators, the matched transmission peak is shifted through the Vernier effect, and therefore, the lasing wavelength is tuned [1,2,19]. The lasing wavelength tuning span is the wavelength range between the two neighboring matched peaks, which is significantly enlarged via the Vernier effect, compared with a single-ring resonator . The FSRs of the double-ring resonators are 610 and 680 GHz, respectively.
3. Optical Measurements
In our experiment, the SOA and external resonator were bonded on different ceramic substrates, which were fixed on separate metal stages for optical coupling adjustment. Between the SOA and the external resonator, polymer gel was inserted to reduce the reflection resulting from the index difference. Laser emitting lights from the back end of the SOA were collected with a tapered lens fiber and measured with a spectrum analyzer.
Figure 4 shows superimposed spectra of the wavelength tunable laser. The lasing spectra were measured with a C-band SOA and an L-band SOA, respectively. Since the SOAs we had were optimally designed for certain C or L bands, their inherent optical gain characteristics limited us in using one of them at both C and L bands. Nevertheless, the external resonator, which is designed for either C or L optical communication bands, exhibited good lasing and wavelength tuning characteristics at both bands with different SOAs. The SOA current was set to 50 mA. The fact that the lasing output powers differ at different wavelengths might be arising from the heating induced optical misalignments. The lasing spectra were measured while only one ring resonator was heated, which resulted in the discreteness in wavelength tuning with a certain wavelength interval. If both of the ring resonators were heated and their heating powers were finely adjusted, the lasing wavelength could be continuously tuned through the total wavelength tuning span. The side-mode suppression ratio (SMSR) observed was more than 30 dB.
The wavelength tuning characteristics dependence on thermal tuning power is shown in Fig. 5(a), in which laser oscillation was measured at only one FSR period. A linear tuning characteristic of a single-mode oscillation without mode hoping was obtained. A maximum tuning span of 38 nm was obtained at a tuning power of 26 mW. The tuning power is about 1/8 of that needed for ones fabricated with SiON waveguides due to small resonator footprint and high silicon thermal-optic efficiency. Due to fabrication errors on the micro-ring resonators, extra heating power were needed for wavelength fine calibration of the double-ring resonators, which resulted in the tuning power not starting from 0 mW.
Figure 5(b) shows the laser output dependence on SOA current, while the lasing wavelength was fixed at 1590-nm. The laser oscillation threshold current was found to be 30 mA. A 1.6-mW lasing optical output was obtained at a SOA current of 100 mA.
A wavelength tunable laser with an SOA and an external double micro-ring resonator, which is fabricated with silicon photonic-wire waveguides, was demonstrated. To date, it is the first wavelength tunable laser fabricated with silicon photonic technology. The device is ultra compact and its external resonator footprint is 700 × 450 μm, which is about 1/25 that of conventional tunable lasers fabricated with SiON waveguides. In experiment, the silicon resonator shows a wide tuning range covering the C or L band for dense wavelength division multiplex (DWDM) optical communication. Due to the bandwidth limits of gain of the SOAs we used, we obtained a maximum tuning span of 38 nm at a tuning power consumption of 26 mW, which is about 1/8 that of the cost for SiON-type resonators previously reported. The lasing threshold SOA current is 30 mA. Compared with conventional SiON types, the wavelength tunable laser based on silicon photonic-wire waveguides is compact and exhibits low-power consumption. It meets with the size and power demands for being included in pluggable optical transmitters. It is considered as a promising device in building future photonic network systems.
The authors would like to thank Yutaka Urino, Shigeru Nakamura, Dr. Masatoshi Tokushima, Dr. Takashi Matsumoto, Akira Suzuki, and Hiroyuki Yamazaki for fruitful discussions, and Emiko Saito, Sekizen Takaesu, Akio Kamei, Masataka Noguchi, and Junetsu Sone for help in device manufacture processing. We also thank Dr. Takahiro Nakamura, Dr. Keiishi Ohashi, and Dr. Shuichi Tahara for their kind support. This work belongs to “Next-generation High-efficiency Network Device Project” that OITDA contracted with the New Energy and Industrial Technology Development Organization (NEDO).
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