Indium Phosphide Membrane Nanophotonic Integrated Circuits on Silicon

3.1 Waveguide and Passive Devices

New development of passive devices is shown in Table 1, with previous results in italic font for comparison. Performance improvements of the devices are largely attributed to the use of the scanner lithography instead of the EBL. It is clear that the enhanced critical dimension control and the reduced roughness in the scanner lithography have led to a series of improved devices: lower propagation loss in straight waveguides and microrings; lowered insertion loss and reduced phase error (thus crosstalk) in arrayed waveguide gratings (AWGs).

New device types, such as photonic crystal (PhC) reflectors, reflective arrayed waveguide grating (R-AWG) and subwavelength grating couplers, have been demonstrated. It is worth mentioning that the PhC reflector has proven to be a promising building block, not only as a highly efficient and broadband on-chip mirror but also as an enabler of other complex devices, such as the R-AWG and the Fabry–Pérot lasers (see Section 3.2).

3.2 Amplifiers and Lasers

Based on the successful demonstration of an IMOS SOA at room temperature and continuous wave operation,14 the SOA building block has been combined with other passive building blocks to form various laser cavities and tuning mechanisms.15

A summary of the recently demonstrated lasers on IMOS is shown in Table 2. Distributed Bragg reflector (DBR) lasers using the aforementioned high-reflectivity PhC reflectors have all shown good threshold current density of 2 kA cm⁻², which corresponds to about 20 mA threshold current for a $500\mathrm{\mu }\mathrm{m}$ long cavity. Tunable lasers can be constructed by inserting two microring filters into the PhC DBR laser cavity. The two microrings have slightly different free spectral ranges (FSRs) such that the number of the filter passbands within the gain spectrum can be controlled by the Vernier effect. The demonstrated tunable laser has shown a SM behavior with good side-mode-suppression ratio (SMSR) of more than 30 dB over 25 nm tuning range, with only thermo-optic tuning of the microrings. The highest SMSR achieved in this tunable laser is 45 dB. The threshold current density is slightly higher than that of the conventional DBR lasers, due to the additional cavity loss induced by the two microrings, which can be reduced in the future by optimizing the ring coupling condition. The wall-plug efficiencies of the two lasers are below 1%. Possible sources of the relatively low efficiencies can be due to the high intracavity loss induced by relatively rough waveguide and underperforming mirrors due to fabrication errors in the particular run.

Index-coupled DFB lasers have been realized by controlled grating etch into the passive waveguide layer on the backside of the SOA structure.42 This approach, as shown in Figure 7, offers a flexible control over the DFB coupling strength by simply controlling the etch depth. The grating coupling coefficient κ can be tuned from 0 to 200 cm⁻¹ for an etch depth from 0 to 300 nm (complete waveguide etch). The grating etching step is independent of the SOA process flow, therefore does not compromise the SOA design and performance. The demonstrated DFB laser on IMOS contains a relatively weak grating due to a shallow etch (120 nm into the 300 nm thick waveguiding layer), with which the fiber grating couplers are also designed and realized. The resulting coupling coefficient κ is 40 cm⁻¹, which is much weaker than other InP membrane and silicon hybrid DFB lasers with κ values in the order of hundreds to thousands per centimeters.43-45 The weak coupling leads to a long laser cavity. Together with a uniform grating patterning, this contributes to an improved spectral purity.46 The demonstrated DFB laser on IMOS with 1 mm long weakly coupled cavity has shown SMSR of more than 60 dB for the current injection range between 70 and 90 mA.42 The SMSR value is the highest reported so far in DFB lasers on silicon to our best knowledge. An optical power of 1 mW in the fiber and a grating coupler loss of 11 dB (due to fabrication error) were measured, indicating a laser output power of 13 mW per facet. Considering the I–V characteristics of this laser diode,42 a wall-plug efficiency of 10% can be obtained.

It should be mentioned that although the fabricated lasers shown in Table 2 have heat sink vias (see Figure 6) connecting their p-electrodes to the silicon carrier, the performance improvement is not obvious as compared with the lasers without thermal vias. It is mainly due to the fact that the metal is too thin (only 300 nm). Future improvement includes an additional gold plating process to thicken the metal up to several micrometers.

3.3 Broadband RF Lines

RF transmission lines are crucial elements for on-chip interconnection and routing of the electrodes in high-speed photonic devices to the access points of the electronics. The RF lines should be broadband and not limiting the intrinsic bandwidths of the photonic and electronic devices.47

In IMOS platform, the thick BCB/dielectric bonding layer and the PI planarization layer together form an effective decoupling of the RF transmission lines to the silicon carrier. Beneath the RF metal stack, all of the InP active–passive layerstack is also removed completely, making sure of the decoupling to photonic functions. The realization scheme of the RF transmission lines is shown in Figure 6.

Coplanar transmission lines have been fabricated on the IMOS platform, during the same metallization step for the via creation (see Section 2). About 300 nm gold metal was used. The layout of the transmission line is shown in Figure 8. The coplanar waveguides are terminated with $100\mathrm{\mu }\mathrm{m}$ pitch ground–signal–ground (GSG) probe pads to facilitate the S-parameter measurement using the vector network analyzer. Three different lengths of 0, 250, and $500\mathrm{\mu }\mathrm{m}$ were measured, and then the data were extrapolated to longer lengths of 1 and 2 mm. The S21 and S11 data are shown in Figure 8b for two different coplanar waveguide geometries with 1 mm length. As shown, the S21 only experienced ≈1 dB attenuation after 1 mm of transmission at 50 GHz frequency, indicating the broadband and low loss properties of the RF lines. The influence of different signal line and gap widths (up to 50% change) to the high-speed performance is negligible. The impedance of the transmission lines are extracted and shown in Figure 8c. Nearly frequency-independent impedance around $60\mathrm{\Omega }$ has been obtained. The impedance can be tuned by the geometrical parameters to achieve impedance matching to the connected photonic and electronic devices. The experiments have shown that broadband RF interconnections up to several millimeters in length are feasible on IMOS platform, enabling full exploitation of the intrinsic bandwidths of the membrane optoelectronics.

4.1 Optical Wireless Communication and Sensing

The grating couplers are promising candidates as integrated optical antennas for optical wireless communication (e.g., light fidelity [LiFi]) and sensing (e.g., light detection and ranging [LiDAR]) applications. High integration density and scalability in the SOI grating coupler arrays have been demonstrated and have shown optical beam forming and steering from a single chip.48-51 The unique opportunities for IMOS lie in the intrinsic integration of high-performance active devices and the performance enhancement of the grating antennas using double-sided processing.52, 53

Based on the ultrafast (>67 GHz) UTC photodiode demonstrated in IMOS platform,18 its combination with optical antennas has been exploited for the use as an integrated optical wireless receiver.52, 54, 55 The concept uses an on-chip optical antenna (e.g., a grating coupler) for efficient free-space to chip coupling, cascaded by a UTC photodiode for high-speed signal detection. The bandwidth-efficiency bottleneck in conventional free-space receivers (top illuminated photodiodes) can be eliminated,55 as shown in Figure 9. It is clear that for the waveguide-coupled case (Figure 9b), the electrical bandwidth of the photodiode can be kept at the same high value regardless of the beam size, to which the grating coupler can be relatively easily adapted. Numerical simulation has shown that the grating coupler has still a high efficiency of 50% for large beam diameters beyond $60\mathrm{\mu }\mathrm{m}$.55 For the top illuminated case (Figure 9a), the photodiode area has to increase for increased beam sizes. This leads to a reduced bandwidth. A quantitative study, using practical values of the responsivities, resistances, and capacitances and carrier dynamics for the two types of photodiodes, has shown that for beam diameters larger than $10\mathrm{\mu }\mathrm{m}$, the cascaded receiver has higher theoretical data capacity as compared with the top-illuminated counterpart.55 For beam diameters larger than $50\mathrm{\mu }\mathrm{m}$, the difference in data capacity can be more than three times. In this theoretical analysis, nonlinear phenomenon such as diode saturation was not included as the application is typically working at relatively low optical power level (milliwatts and below).

It should be noticed that the grating coupler can have its potential limitations in terms of the field of view (FoV). The interference nature of the grating puts a boundary to the range of angles within which light with a certain wavelength can be efficiently captured. The FoV also depends on the spot size of the grating. Therefore, the grating couplers may not be suited for situations with wide angle variations and very large beam diameters.

The overall concept of the cascaded receiver, however, is still very promising. The first-stage light capturing is not limited to grating-type antennas. The grating couplers used in this first demonstration can be replaced by other integrated optical antenna concepts with wider FoV. Potential candidates include metasurfaces56 and on-chip microlenses.57

In the system experiment of the first-generation cascaded receiver, 40 Gbit s⁻¹ on–off keying (OOK) signal transmission has been achieved under a simulated wireless scenario: 2.4 km of SM fiber and 2 m of free-space path. Furthermore, the sensitivity to the wavelength and the angle of the incident beam were investigated, revealing a −3 dB FoV of 7.5° and an optical bandwidth of 30 nm.55

The successful demonstration of the single receiver can be scaled up to an optical wireless wavefront receiver (OWWR) with an grating antenna array to sample the incident wavefront of the free-space optical beam. The sampled optical signals in each channels will contain both intensity and phase information of the sampled wavefront. The signals can be further processed to reconstruct both the intensity (the signal) and phase profile (the incident angle) of the incident beam. The additional phase information is extremely valuable for an active beam tracing.52 A proposed OWWR circuit layout, consisting of $4\times 4$ compact grating antenna array, thermo-optic phase shifters, optical power combiners, and a UTC photodiode, is shown in Figure 10. The proposed antenna array is designed to sample fixed points of a large free-space incoming beam. The sampled optical signals from the grating array are combined by the power combiners and eventually fed to the UTC photodiode. The phase front of the incoming beam can be reconstructed by reading out the applied voltages (i.e., the tuned phases) on each of the phase shifters, at the moment when the latter are tuned to achieve maximum photocurrent. This extra angle information can be valuable to create awareness of relative positions between the transmitter and the receiver. A fabricated grating element used in the $4\times 4$ array is also shown in the figure, featuring a compact footprint of only $3\times 4\mathrm{\mu }{\mathrm{m}}^2$.

Reconfigurable optical wireless transceivers have also been proposed,58 taking advantage of the reconfigurable gating behavior in a p-i-n SOA structure. The operation speed of such a p-i-n diode with QWs as the absorption medium will be limited to about 20–30 GHz in theory. It cannot replace the UTC photodiodes for high-performance optical wireless receivers, but can offer a cost-effective (no additional regrowth and processing of the UTC layerstack) approach to realizing both transmitter and receiver functions on a single chip.

The double-sided processing technology can enhance the efficiency of an individual grating antenna by incorporating a backside metal layer.20, 21 Such flexibility is not easily accessible in SOI platforms. The efficiency of the grating can be boosted with the assistance of the metal layer, which reflects all the downward light to upward direction. A grating-to-fiber coupling efficiency of 75% has been achieved,20 which is a significant boost comparing with the 30 % efficiency in conventional grating designs without metal reflector.12

The intrinsic on-chip amplification and light sources that IMOS technology offers can provide high scalability (e.g., WDM transceivers) and reconfigurability (e.g., reconfigurable transceivers) solutions on a single chip. Moreover, the combination of nanophotonic circuits with ultrafast optoelectronic devices can enable a seamless cointegration of optical wireless and microwave systems.59 Several key devices, such as the UTC photodiode and tunable laser, are essential for both optical wireless and microwave photonics.60, 61 The sharing of the key devices across different frequency bands can lead to powerful wireless systems-on-chip.

4.2 Integrated Quantum Photonics

The nanophotonic waveguides in the IMOS platform has opened up new opportunities for studying and utilizing strong light-matter interactions in InP. The index contrast and dimensions of the SM InP membrane waveguide are very similar to those of the SOI waveguides, as shown in Table 3. Such submicrometer-dimensioned waveguides provide tight optical confinement, resulting in a highly concentrated optical field profile within the InP core as well as on the InP/air (or dielectrics) interfaces (see Figure 2).

Such nanophotonic waveguides have attracted much attention in the application of quantum optics, taking advantage of the high third-order (χ⁽³⁾) nonlinearity. Recent open access to the silicon photonics foundry services has led to a range of SOI devices and circuits for entangled photon pair generation72 and quantum distribution.73

IMOS can potentially offer several advantages over SOI, for instance, the intrinsic on-chip quantum light sources, as well as the access to the InP material system which has higher third-order nonlinearities. The optical Kerr coefficient in InP is about 5–10 times higher than that of the silicon, as shown in Table 3. The higher Kerr coefficient can enable stronger processes at lowered optical power level. First experiments on nonlinearities in IMOS devices have been conducted in the study by Thourhout et al.74 In that work, an IMOS microring resonator (Q > 40 000) was used as an entangled photon pair source using the spontaneous four-wave mixing (SFWM) process. The photon pair generation rate (PGR) obtained from this InP microring resonator is 145 MHz mW⁻², which is comparable with the state-of-the-art SOI microring resonator (PGR 149 MHz mW⁻²75) with a doubled Q factor and halved radius. This revealed promising aspects of IMOS nanophotonic circuitry for the quantum photonics applications.

Similar to the silicon, other nonlinear processes such as the two photon absorption (TPA) and free carrier absorption (FCA) coexist with the χ⁽³⁾ process in InP waveguides, first reported in the study by Johnson and Painter.76 The TPA and FCA coefficients in InP are higher than those in silicon (see Table 3). Therefore, the benefit of higher χ⁽³⁾ in InP will be restricted to relatively low optical power level. At higher power level, TPA starts to dominate and the free carriers generated by TPA will cause FCA. The consequences include reduced Q factor and blue shift of the resonant peaks,74, 77 which reduce the single photon generation rate.

Free carrier lifetime plays a critical role in the FCA process. The free carrier lifetime in InP waveguides can be shorter than that in SOI waveguides, due to the direct bandgap, as shown in Table 3. However, both are in the (sub)nanosecond range (0.5 ns for InP and 3–4 ns for SOI). Note that the 0.5 ns value comes from bulk InP crystal.69 It is known that the actual free carrier lifetime in highly confined waveguides will be even shorter due to enhanced surface recombination.68, 78 Therefore, the actual free carrier lifetime in the InP membrane waveguides will be shorter than the bulk value of 0.5 ns. Reducing the carrier lifetime is a viable route to further suppressing the impact of TPA and FCA. In SOI waveguides, several approaches have been proposed and experimented, such as intentional recombination centers,68 biased p-i-n diode70 and enhanced surface recombinations.78 Reduced carrier lifetime down to as short as 12.2 ps has become feasible.

Similar approaches can be applied to IMOS waveguides as well to reduce the carrier lifetime. Moreover, it is easier to implement doping profile and p-i-n diode structure in InP than in silicon. Doping can be done straightforwardly during epitaxial growth of InP material systems, without the need of ion implantation. Moreover, it is discovered that at the same n-dopant concentration, InP membrane waveguides experience three times lower absorption loss than SOI waveguides.71 This is attributed to the higher electron mobility in InP than in silicon at the same n-dopant concentration.71, 79 A quantitative comparison is shown in Table 3. These evidences have shown high potential of creating low-optical-loss and highly compact p-i-n diode structure to reduce the free carrier lifetime in IMOS nanophotonic waveguides.

4.3 Ultralow Energy Optical Crossconnects

Optical micro-electro-mechanical system (MEMS) is regarded as a promising technology for large-scale optical crossconnects for the rapidly growing optical networks.80 The optical MEMS (especially the NOEMS) has the advantage of high scalability and low energy consumption. The advantages of realizing MEMS and NOEMS devices on IMOS platform have been discussed in Section 2.

Recently, a NOEMS phase modulator has been demonstrated on IMOS platform,32 based on a twin-guide layer structure similar to the one previously used for SOAs.12 The p-InP layer and the passive InP (as well as n-InP layer) forms the top and bottom waveguides of the NOEMS phase modulator, respectively, whereas the original InGaAsP active layer in between is removed wet chemically. Based on it, a Mach–Zehnder interferometer (MZI) optical switch has been fabricated, as shown in Figure 11. Since the top and bottom waveguides are doped, an electrostatic force will be applied as the structure is biased. As a result, the top waveguide can be actuated and cause significant change in the mode effective index and phase shift (see Figure 11c). The first demonstration already showed promising results: maximum $4.8\pi$ phase shift with a phase modulator of only $140\mathrm{\mu }\mathrm{m}$ long and a 15 dB extinction ratio of the MZI switch. Megahertz-level actuation frequency is expected. Such giant phase shift can find applications in switch networks and sensors which require low energy consumption.