We report the design and characterization of external-cavity DBR lasers built with a III-V-semiconductor reflective-SOA with spot-size converter edge-coupled to SOI waveguides containing Bragg grating mirrors. The un-cooled lasers have wall-plug-efficiencies of up to 9.5% at powers of 6 mW. The lasers are suitable for making power efficient, hybrid WDM transmitters in a CMOS-compatible SOI optical platform.
©2012 Optical Society of America
Silicon photonics is rapidly emerging as a key technology in the realization of low-cost, high performance components that can be used to meet the ever-increasing demands for bandwidth in optical communications systems, data center networks, and microprocessor optical interconnects [1–3]. Wavelength-division-multiplexing (WDM) offers high-bandwidth, high-density, and inherent parallelism in these systems and therefore is of interest for scaling optical solutions to meet advanced needs. We have been engaged in the development of a complete suite of low-power-consumption and high-density WDM transceiver and transport technologies suitable for meeting these needs on a 3-micron SOI platform [4, 5].
In the absence of practically efficient lasers achievable directly in Silicon (Si) or other group IV materials, Si-photonic transmitter sources must be made by hybrid integration with III/V gain materials. External-cavity (EC) lasers are well suited for making stable, bandwidth- and wavelength-controlled WDM lasers. Single-mode EC lasers with sufficiently narrow linewidth for a data link can be made either in distributed-feedback (DFB) or distributed-Bragg-reflector (DBR) configurations. However, the power efficiency of the laser source is now one of the key requirements driving technology selection, as it is becoming a significant portion of the energy cost-per-bit in data transmission. DBR lasers are inherently more power efficient as the optical losses within the laser cavity and the electrical resistance are inherently lower. The EC architecture using separate gain and passive chips also allows for the flexibility to optimize the grating mirror reflectance, bandwidth, and laser cavity length in the passive chip, and optimize the loss and efficiency in gain material, independently from each other.
To date, Si-photonic or PLC-based hybrid-integrated DBR lasers reported recently have had wall-plug-efficiencies (WPEs) ranging from ~1% to 6.2% [6–14]. Here, we report design and experiment details on proof-of-concept hybrid EC continuous-wave (CW) DBR lasers , with up to 9.5% waveguide-coupled WPE, made by butt-coupling a III-V reflective-SOA (RSOA) chip with spot-size converter to passive 3-µm-platform silicon-on-insulator (SOI) chips containing Bragg grating mirrors of varying lengths. Flip-chip-integrated versions of these lasers are suitable for integration with our other passive and active high-speed devices built on the 3 µm SOI platform [16,17] to create a power-efficient, compact Si-photonic WDM transceiver chip.
2. Sample structures and fabrication
The RSOA gain chip was a 600-µm-long buried-heterostructure (BH) bulk InGaAsP/InP amplifier with horizontal spot-size converting tapers on both ends. The tapers produce an expanded mode with an electric-field 1/e diameter of 3 µm, symmetric in both vertical and horizontal directions, which is well matched to the SOI waveguide. The back facet of the RSOA was HR coated to be 90-95% reflective. At the front facet, the BH waveguide was bent at an angle of 10 degrees to the facet normal, and the facet was AR coated, to minimize the intensity of back-reflected light. The RSOA chip was bonded to a carrier with front end protruding off of the carrier.
The Si passive chip was fabricated on a SOI platform with a 3-μm-thick Si layer and 0.4-μm-thick buried oxide layer. The waveguide height, width, and slab thickness are µm, µm, and µm, respectively (see Fig. 1a ). The SOI ridge waveguide was tilted 9.3 degrees with respect to the polished facet at the input (on the RSOA side), to match the emission angle of the RSOA. The SOI facet was also anti-reflection (AR) coated for the off-normal incidence angle to minimize the back reflection. The waveguide facet at the laser output side was normal to the polished chip surface and normal-incidence AR-coated. The SOI and RSOA waveguides were single-moded in all regions.
3. Bragg grating design
The Bragg gratings are formed by a periodic corrugation on top of the ridge waveguide (see Fig. 1b). The grating period Λ of 224.9 nm has been chosen based on the Bragg condition for a passband center wavelength at nm . Here, where is the mark width, is the effective index of the unperturbed ridge waveguide of height and is the effective index of the etched waveguide of height The duty cycle of the gratings is 84%.
Coupled mode theory has been used to design the grating in terms of the length and etch depth for the desired bandwidth and reflectivity . The reflectance spectrum of the grating is given as:Eq. (1) can be obtained as:
For the laser demonstrations we have fabricated Bragg gratings of µm etch depth having three different lengths L = 0.7, 1, and 1.5 mm. The grating bandwidth decreases for larger lengths, and the maximum reflectivity increases. The 3-dB bandwidths are 0.46, 0.34, and 0.25 nm, and the maximum peak reflectances are −8.6, −5.5, and −3 dB, for gratings A, B, and C, respectively (Fig. 2 ). The Bragg gratings were located 0.24 mm from the input facet of the chip, such that when the RSOA is edge-coupled to the SOI waveguide, the effective laser cavity length is 1.4 mm, creating longitudinal modes with a free-spectral-range (FSR) of 0.25 nm.
The grating reflectance spectra were characterized with an ASE source connected to a fiber circulator collecting back-reflected light. Figure 2 compares the measured spectra (solid lines) with the simulated spectra (dashed lines). There is a good match between measured and simulated spectra for reflectances above the −23 dB sensitivity floor of the measurement.
4. Experimental results and discussion
Lasing experiments were performed by mounting the RSOA chip-on-carrier and Si passive chips on precision 6-axis alignment stages, and edge coupling the two waveguides, as shown in Fig. 3(a) . Alignment for optimum coupling was found by maximizing the lasing power above threshold. Emission from the laser at the output of each passive chip was collected with a lensed fiber mounted on a third alignment stage. A fiber-optic isolator with isolation of >50 dB was placed between lensed fiber and detector to ensure back-reflected light from sources beyond the output of the Si passive chip are suppressed. The RSOA chip was un-cooled during the experiment. A picture of the RSOA and Si chips after alignment is shown in Fig. 3(b).
Figure 4(a) shows the light versus current (LI) and voltage versus current (VI) curves measured from the three lasers. The threshold currents were 15.3, 14.0 and 13.0 mA for the lasers with gratings A, B, and C respectively. The kinks in the LI curves are caused by mode-hops, and occur when the preferred longitudinal mode of the cavity jumps to an adjacent mode. This is caused by a cavity optical path length change due to the thermo-optic effect as the laser core temperature rises .
Figure 4(b) plots the WPE defined as the output optical power divided by the electrical power injected into the RSOA, WPE = Pout/(IbiasVbias), versus bias current, for the three lasers We find that the laser with grating A, the grating with the shortest length L = 0.7 mm, having reflectance of R = 15%, and 3-dB bandwidth of 0.46 nm, produces the highest output power and achieves 9.5% efficiency, while maintaining an SMSR of > 45 dB. 9.5% WPE is achieved at 50 mA with an output power of 6 mW. To our knowledge, this is the highest WPE reported for hybrid Si/III-V guided-wave lasers, and the power level at maximum efficiency is practical for silicon photonic link budgets. The grating A laser also has the fewest and lowest-magnitude kink discontinuities.
We performed modeling of the laser LI curves to quantify the laser physical parameters and LI roll-off characteristics. The output power Pout for our EC laser is related to the RSOA-SOI-waveguide coupling efficiency η and external quantum efficiency ηext according to [19,20]:
Here, Rg is the peak grating reflectance, α, L, αg, and Lg,out are the effective waveguide scattering loss in the cavity, effective laser cavity length, grating loss, and length of the grating outside the cavity, respectively. Rr is the RSOA rear-facet reflectance, ηi is the RSOA internal quantum efficiency, h is plank's constant, qe is the elementary charge, and f is the laser frequency. Basic thermal saturation of the laser power is modeled by the terms involving the thermal impedance ZT and the power absorbed by the RSOA, PD – Pout, where PD = IbiasVbias. The coupling efficiency η was determined to be 2 dB for all three lasers, by fitting Eq. (3) to the measured LI data for all three lasers, keeping the other physical parameters fixed near their estimated values. The saturation of the LI curves at high powers is accounted for by thermal roll-over characterized by the factor ZT/T1. The fits approximate well the threshold currents, slope efficiency, and thermal roll-off for the three lasers up to the first mode-hop. More detailed temperature dependence and thermal modeling will be performed on future integrated embodiments.
Figure 5(a) shows the lasing spectra measured at two bias currents for the laser A, overlaid with the grating reflectance spectrum. The spectra were collected with an Optical Spectrum Analyzer (OSA) with a 0.01 nm resolution bandwidth. Figure. 5(b) shows a contour plot of the spectra measured at currents from 20 mA to 150 mA in steps of 5 mA, with the wavelength axis aligned to Fig. 5(a). Lasing occurs between 1556.65 to 1556.9 nm, within the 1-dB passband of the grating. Jumps in the lasing longitudinal mode due to mode hops are clearly observed. The two lasing spectra shown in Fig. 5(a) are at 95 mA, before the first mode hop, and at 100 mA, after the first mode hop. The 3-dB bandwidth of the laser lines seen in Fig. 5(a) are 0.01 nm and are limited by the OSA resolution bandwidth, thus the actual line-width of the laser is expected to be much narrower than 0.01 nm. The inset in Fig. 5(a) shows a zoom-out of the 95-mA lasing spectrum over the entire wavelength range collected by the OSA (8 nm), and no additional lasing modes were observed above the −60 dBm noise floor in wider range scans. This confirms single-mode lasing with an SMSR of >45 dB.
Figure 5(c) also shows the LI curve for the laser with bias current axis aligned to the contour plot. It can be seen that the LI curve kinks happen near 95 mA and 135 mA, approximately the same currents as the mode hops in Fig. 5(b). Small mismatches between the mode hop currents in the spectra and the kinks in the LI curves are due to the fact that the RSOA chip heats differently during the spectra measurements because they are collected over a slower timescale compared to the LI measurement. We also believe parasitic cavities from non-zero back-reflections from the SOI chip facets play a role in causing the minor LI discontinuities.
A high WPE of 9.5% is achieved in our laser by designing a low output mirror grating reflectance of R = 15% to provide a high slope efficiency, as well as by achieving a low RSOA-Si-chip coupling loss of 2 dB, low waveguide losses of < 0.3 dB/cm and grating losses of < 0.5 dB/mm which result in a low 15 mA threshold. The next-highest WPE of 6.2% (up to 7.5% at 40 mA) recently reported in  used a similar III/V-SOI flip-chip edge-copuled scheme, and also had a low 1.5 dB chip coupling loss, but used a 0.25-μm SOI platform, and required expanding the waveguide modes to 3 µm using spot-size converting tapers in the SOI chip. The slope efficiency in this report is lower compared to our slope efficiency of 0.18 W/A, and therefore their output power and WPE is reduced compared to our laser. In this scheme we believe the spot-size-converter tapers required for expanding the mode adds some inherent cavity loss. The evanescent- and vertical-taper-coupled wafer-bonded hybrid devices which have recently demonstrated WPEs of 1% to 3.5% [11–14] also have lower efficiencies because of inherent losses in the evanescent- and taper-coupling regions, as well as lower differential gain because i) the gain region has a lower confinement factor and ii) the full length of the gain material is not used. We thus believe our 3-µm SOI platform is inherently suited for higher efficiency hybrid lasers as no SOI spot-size converters or coupling between multiple layers is required, and the gain region efficiency can be optimized independently from the SOI chip.
We have designed and characterized actively-aligned un-cooled III/V-Si external-cavity DBR lasers. We showed single-mode lasing with an SMSR > 45 dB, a 9.5% wall plug efficiency for an output power of 6 mW, and an LI curve with small-magnitude mode-hop discontinuities. This was achieved with a low grating output mirror reflectance of 15%, low 2-dB coupling loss between the RSOA and SOI waveguide, a high RSOA gain, and low waveguide and grating losses. The lasers are suitable for flip-chip hybrid integration into a 3 µm SOI platform to create power-efficient, compact, Si-photonic WDM transceivers.
This work is supported in part by DARPA under Agreements HR0011-08-09-0001. The authors thank Dr. Jagdeep Shah of DARPA MTO for his inspiration and support of this program. The views expressed are those of the author and do not reflect the official policy or position of the Department of Defense or the U.S. Government. Approved for public release. Distribution Unlimited.
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