1. Introduction

Silicon based photonic devices built on silicon-on-insulator (SOI) substrate are attractive for dense photonic device integration based on high-refractive-index-contrast silicon waveguides. Active laser devices fabricated on SOI using a CMOS compatible process will enable the realization of Electronic-Photonic Integrated Circuits (EPICs) in which the photonic devices and waveguides on an SOI based electronic chip enable broadband chip-to-chip, chip-to peripheral, or computer-to-computer optical interconnects. To enable active photonic devices with optical gain, recently, heterogeneous integration scheme using evanescent field from the propagating optical mode in SOI waveguide to interact with the optical gain medium in a III-V bonded layer has been demonstrated [1–3]. This approach has led to the demonstration of electrically pumped Fabry-Perot, microdisk, racetrack, and DFB lasers on Si platform [4–6].

The employment of WDM for the photonic integrated circuits on SOI will further enable ultra-broadband optical interconnects. WDM devices on SOI platform reported in the literature are mainly passive WDM devices such as arrayed waveguide gratings (AWG) [7,8]. A laser source that can be integrated on the SOI platform capable of emitting at wavelengths corresponding to the DWDM ITU grid would be of great interest. One way to achieve that is with use of AWG in the optical feedback path [9]. AWG, however, suffers from the problem of high optical loss when integrated on InP or Si platform. As a result, laser fabricated with AWG on InP platform has low output power and poor laser performances [9]. The same can be expected for laser fabricated with AWG on Silicon platform. An alternative scheme is to use integrated Echelle-Rowland diffraction grating. Implementation of integrated Echelle-Rowland grating based lasers, however, also suffers from the problem of high optical loss due to the large centimeter-size of the Echelle-Rowland grating used [10]. In addition, the large size of the Echelle-Rowland grating used made it difficult to achieve single wavelength operation due to its dense FP modes.

In this paper, we reported an electrically pumped Silicon/III-V heterogeneously integrated laser based on a novel super-compact ultra-large-angle curved diffraction grating on silicon [11]. This “super-compact grating” (SCG) is small in size (~1mm) while capable of DWDM resolution and has the potential for realizing ultra-compact multi-wavelength integrated laser sources on SOI chip for WDM application in EPICs. These lasers will be referred to as SCG Si/III-V lasers. To our knowledge, this is the first time the curved diffraction grating is integrated with active devices on the heterogeneous integrated silicon/III-V platform.

2. Laser design principle

Before we go into the details, let us first describe the general design for a laser chip based on the super-compact diffraction grating (SCG). A typical chip layout is shown in Fig. 1 . The laser cavity is formed by mirror M and the SCG. The mirror M is at the end of the gain waveguide arm GW. Light from M first passes through a gain waveguide (GW) section, and then propagates to a waveguide mouth with width W_SL, which serves as an entrance slit to SCG. SCG then diffracts the beam from slit W_SL back to W_SL and re-enter waveguide arm GW. The mirror M can be formed by a cleaved facet and serves as the output coupler.

 

Fig. 1 schematic diagram of the grating laser employing super compact grating (SCG).

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In this paper, the diffraction grating laser is realized on a Silicon/InP heterogeneously integrated platform. The diffraction grating is fabricated on the silicon layer and the waveguide section that provides the optical gain is fabricated on an evanescently coupled Si/InP region

The key to achieving single-wavelength lasing in such diffraction grating based lasers is to have very narrow passband for the diffraction grating. Let the center peak frequency of the grating pass band be ν_gr and the 3dB passband bandwidth of the grating be dν_gr(3dB) as shown in Fig. 2(a) . Let the center mode of the Fabry-Perot (FP) resonance frequency be ν_FP and intermode spacing (i.e. free spectral range FSR) be dν_FPMS. To achieve single-mode operation, it is desirable to have dν_gr(3dB) to be at most a few times of dν_FPMS as shown in Fig. 2(a) and 2(b) so that there is only a few dominant mode inside the grating passband and the two adjacent FP side modes ν_FP+ and ν_FP- experience low transmission through the grating compare to the on-resonance mode at ν_FP=ν_gr. The total FP cavity length is the length of the gain waveguide and the grating propagation length. For example, with gain waveguide length ~400 µm and grating propagation length ~600 µm (with grating waveguide mouth opening size of 1.2 µm, grating angle of 55° and grating radius R = 760 µm), the total cavity length will be 1mm. The FP cavity’s intermode spacing (FSR) will be dν_FPMS~50GHz. The grating 3dB passband bandwidth will be dν_gr(3dB) ~100GHz so that the Fabry Perot side modes will experience 3dB extinction compared to the central mode, thus providing enough spectral filtering to facilitate single mode lasing.

 

Fig. 2 (a) Single frequency lasing condition with dν_gr(−3dB) and ν_FPMS. (b) Grating filtering bandwidth and FP cavity intermode spacing.

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In other words, in order to achieve a single wavelength lasing, the grating passband must be narrow enough to filter out the undesirable FP modes. This requires the waveguide mouth opening size W_SL to be around one micron and the diffraction angle of the light beam coming towards the grating to be very large. Such a high spatial resolution cannot be achieved using the usual Echelle-Rowland grating design [11].

The Echelle-Rowland grating design is based on what is referred to as a constant parallel cord design in which a cord to the circle is displaced with constant distance repeatedly and the intersections of the parallel cords generated with the Rowland circle then give the grating teeth locations. The Echelle-Rowland grating can have good focusing only for rays at small angle of divergence (i.e. when the waveguide mouth is large so the beam from the mouth has small diffraction angle). When the divergence angle is large (larger than 10 degrees), the large-angle rays do not converge well onto the same spot, even for the designed wavelength, leading to spatial aberration, which will enlarge the focused spot. This spatial aberration gives loss and limits the spectral resolution. When one reduces the waveguide mouth size to achieve higher spectral resolution, the diffraction angle becomes large. Thus one will see aberration from Echelle-Rowland grating when the waveguide mouth size is approaching the optical wavelength. The SCG grating teeth are computationally generated to correct for spatial aberrations even for rays at large angle of divergence, and do not follow the Rowland’s parallel cord design for curved grating [11–14]. For multi-channel operations, the SCG grating design is capable of minimizing the spatial aberration across a wide wavelength range (>100nm). Other variants of such aberration corrected design have also been reported [15]. The large angle capability for this grating enables very small focused spot size at the waveguide mouth, resulting in high spectral resolution for a small grating size. Comparing to diffraction grating fabricated on InP, the laser grating is fabricated on SOI wafer with much shallower etching requirement. Thus, it is much easier to achieve high quality etching at the grating reflection surface. The high refractive index contrast of the silicon core and the SiO₂ cladding also reduced the requirement on the etching verticality [16], allowing higher performance grating to be realized on chip.

The wafer structure started from the top surface is made up of a 10nm thick n-type InP bonding layer with a dopant density of 1 × 10¹⁸cm⁻³. Then two sets of super lattice layers of N-In_0.573Ga_0.1As_0.327P and N-InP of 7.5nm thickness for each layer were grown to buffer the roughness on the bonding surface. On top of that, N-InP layer of 110nm with a dopant density of 1 × 10¹⁸cm⁻³ was used for N-type contact. The quantum well consists of 9 barrier layers of 10nm thick Al_0.080Ga_0.467In_0.453As with photoluminescent peak at 1.3µm, and 8 layers of 7nm thick Al_0.063Ga_0.280In_0.657As quantum well layers with photoluminescent peak at 1.545µm. On top of the quantum well, 250nm thick SCH layer of Al_0.141Ga_0.356In_0.533As with a p-type dopant density of 1 × 10¹⁷ cm⁻³ was used. Then a 1.5µm thick P-InP layer with a dopant density of 1 × 10¹⁸ cm⁻³ was used to form the top cladding. Lastly, a p-type In_0.53Ga_0.47As layer with a dopant density of 1 × 10¹⁹ cm⁻³ for p-type contact was grown right above the substrate.

3. Laser fabrication

The schematic of the heterogeneously integrated SCG Si/III-V laser we fabricated is illustrated in Fig. 3 . A vertical cross-sectional views at A-A’ (Si WG region), B-B’ (Gain region), and C-C’ (Planar waveguide and grating region) are shown in Fig. 3. The integrated device is fabricated on the silicon evanescent device platform with an AlGaInAs multiple quantum well (MQW) structure based active layer structure wafer bonded to silicon waveguide. The thickness of the buried oxide and top Si device layer are t_BOX=1µm and t_Si=0.7µm, respectively. The silicon waveguide width W_Gain-Si at the gain region is 2.5μm and its length L_Gain is 1600 μm. The mesa width of the III-V gain structure is W_Gain-III/V= 75μm. About 83% of the optical mode overlaps with the SOI waveguide while the evanescent tail of the mode is overlapping with the MQWs in the bonded III-V epi-layers that provide optical gain. The silicon waveguide is tapered down from 2.5μm at the gain section to W_SL-Si=1μm at the waveguide mouth. The taper length L_Tap-Si is 30μm. A 1-micometer trench T_tran-Si on both sides of the silicon waveguide confines the optical modes laterally (see cross-section A-A’). The grating is operated in the 8th order and with incident angle of 55 degree. The size of the grating is 260μm by 310μm.

 

Fig. 3 Top view schematics of the grating laser showing the layout of the electrodes.

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The waveguide and grating are defined by electron beam lithography on a PMMA resist. The pattern is transferred to PECVD oxide hard mask using CHF₃ based reactive ion etching process. The Si layer is then etched using Cl₂-based inductively couple plasma etching using the oxide as etch mask. Figure 4(a) shows the optical microscope photo of the waveguide and grating on the SOI wafer and (b) shows the detailed scanning electron microscope (SEM) image of the grating teeth.

 

Fig. 4 (a) Optical microscope image of the waveguide and grating structures on SOI; (b) SEM image of the super compact grating; (c) Optical image of the AlGaInAs quantum-well based active layer structure bonded to SOI wafer with pre-fabricated silicon waveguides and SCG grating.

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Prior to bonding, the III-V layer and SOI wafer pre-fabricated with the silicon waveguide and grating, were cleaned with chemicals. Oxygen plasma activation was performed on both surfaces. The wafers were then bonded together under 0.3MPa pressure at 220°C temperature for three hours. These pressure and temperature are comparatively lower than previous work [1]. The InP substrate was then selectively removed by selective chemical etching that self stop at InGaAs layer. Figure 4(c) shows the optical image of a 1cm-by-1cm III-V thin film (about 3μm in thickness after InP substrate removal) tightly bonded to 2cm-by-2cm processed SOI wafer. A 75μm-wide mesa was etched down to n-InP by using combination of ICP dry etching and selective wet etching. The facet of III-V near the grating is etched with a 7° slanted angle in order to deflect the light and eliminate the unwanted optical cavity formation. Metals have been deposited on top of p-InGaAs and exposed n-InP surface for the p and n contacts. Lastly, the thickness of the SOI substrate was thinned down to 150 μm by lapping and the end facets was cleaved for device experimental measurements. Figure 5(a) shows the microscope image of a SCG/III-V laser device fabricated showing the grating and the gain waveguide region. A high resolution TEM image of the interface shown in Fig. 5(b) indicates bubble-free atomically bonded Si/III-V interfaces. The image reveals the presence of a 5-nm-thick amorphous layer at the interface introduced during the bonding process by oxygen plasma activation, which has no significant effect on the optical mode.

 

Fig. 5 (a) Optical microscope image of the SCG Si/III-V laser; (b) High resolution TEM image of the bonded interface

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4. Results and discussion

To measure the lasing properties, the laser is driven by 1 μs wide current pulse with a repetition rate of 10 kHz. The laser output is collected by a single-mode fiber to an optical spectrum analyzer. As shown in Fig. 6(a) , single-wavelength lasing is observed when the laser is driven at 248 mA with a lasing peak at 1551nm. From the lasing spectrum shown in Fig. 6(a), the side mode suppression ratio of the lasing mode is better than 30dB. Figure 6(b) shows the laser output power as a function of the injection current. As can be seen from the figure, laser threshold current is ~150mA and the maximum power output from the facet is >2.3mW. The I-V curve shown in Fig. 6(c) indicates a turn-on voltage of about 1V and the structural resistance is about 20 Ω after diode turns on.

 

Fig. 6 (a) laser spectrum at pumping current of 248mA; (b) P-I curve; c) I-V curve; of the grating laser.

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In summary, we reported the design, fabrication, and measurement of a heterogeneously integrated III-V-on-Silicon laser based an aberration-free ultra-large-angle super-compact grating (SCG). The SCG has high spectral resolution and small physical size, enabling a small number of Fabry-Perot modes under its filtering curve, which is needed for achieving single wavelength lasing. The passive low loss SCG is fabricated on the silicon layer of an SOI substrate. The optical gain is provided by a silicon waveguide bonded with III-V materials. The evanescence field from the silicon waveguide in the III-V gain material results in optical gain. Experimental measurements of the SCG Si-III/V laser fabricated show single-wavelength lasing at 1550nm with a lasing threshold of 150mA and an output power of up to 2.3mW. The SCG laser can be designed to provide multiple lasing wavelengths for wavelength-division-multiplexed applications enabling transmission of high data rate between or within electronic-photonic integrated circuits, which will open up various applications for ultra-high-bit-rate chip-to-chip, computer-to-computer, or intra-chip optical interconnects.