Low-Loss Si3N4 TriPleX Optical Waveguides: Technology and Applications Overview
Summary (6 min read)
Introduction
- This paper reports on numerous different applications of different types of silicon nitride waveguides, each with its specific and unique optical confinement and transmission properties.
- With the development of several different, standardized, building blocks that can be monolithically combined in the manufacturing process flow, complex functions can be obtained that can serve application domains such as high-speed and highcapacity optical communication, microwave photonics and medical diagnostics, to mention a few.
II. TRIPLEX WAVEGUIDES; MODELING
- TriPleX waveguides are a family of waveguide geometries that is based on an alternating layer stack consisting of two materials: Si3N4 (silicon nitride) and SiO2 (silicon dioxide) that have refractive indices of about 1.98 and 1.45, resp., at 1.55 μm wavelength.
- Most commonly used substrate is singlecrystal silicon, for some applications fused silica glass substrates have been used too (e.g., if a transparent chip in the visible light region is required).
- Table I demonstrates cross-sections and SEM photographs of four different waveguide geometries that will be described in more details in Sections II-A to II-D. An overview of the manufacturing process can be found in Section II-E.
C. Symmetric Double-Stripe or SDS
- The symmetric double-stripe (SDS) geometry consists of two stripes of Si3N4 of the same thickness at the top on top of each other, separated by an intermediate SiO2 layer.
- The optimized geometry that is also offered in the current TriPleX IR MPW runs and has been used in many optical beam-forming networks (OBFN) devices [11], that is discussed more elaborately in Section VI-C.
- A minimum bending radius of 100 μm is feasible, based on the criterion that bend losses should be below 0.01 dB/cm [14].
D. Asymmetric Double-Stripe or ADS
- The asymmetric double-stripe (ADS) has a similar waveguide geometry to the SDS: the main difference is that the thickness of the upper Si3N4 stripe is different than the lower Si3N4 stripe, = SiO2 , = Si substrate, = background material (e.g., air).
- See Fig. 2 for a schematic cross-section, while also the thickness of the intermediate SiO2 layer is significantly smaller: instead of 500 nm as for the SDS geometry, a thickness of 100 nm is used for the ADS geometry.
- Please note that the group index is not the same as for the SDS waveguide: the ADS waveguide has a slightly larger group index of 1.77.
- Propagation loss values on ADS waveguides are reproducibly measured to show similar or better values compared to SDS waveguides, so <0.1 dB/cm Table II gives an overview of the most important properties of the different TriPleX geometries discussed in this Section.
E. Manufacturing Process of the Basic TriPleX Geometries
- The general process flow is illustrated schematically in Table III for the production of the different basic TriPleX geometries.
- The left-hand column shows the production process for the box shell, the right-hand column is related to the other geometries, the double-stripe SDS and ADS.
- The current scheme contains as many batch processes as possible, using equipment that is present in most CMOS foundries: this makes production of TriPleX waveguides suitable for mass production.
- Typical deposition temperatures are between 300 °C and 400 °C.
- The stress in PECVD SiO2 layers (Table III, step 10) is much less than in LPCVD layers, allowing for much thicker layers.
A. Routing Building Blocks
- The most elementary basic building blocks are routing building blocks, that are based on regular straight waveguide building blocks (either with constant width or with varying width for lateral tapers), and on polar bend building blocks (based on bends with constant radius of curvature).
- Based on these building blocks, more advanced routing building blocks can be defined, such as S-shaped bends.
B. Spot-Size Converters
- SSCs modify the MFD of a waveguide either by using a vertical taper, as discussed in Section II-C, and/or by changing the waveguide width using a lateral taper.
- The vertical taper can be designed to locally and adiabatically change the thickness of the guiding layer(s) such that discontinuities in MFD are avoided, to minimize the excess loss of the SSC, at the cost of a length of typically several hundreds of μm.
- The SSC can be used to minimize coupling loss to e.g., a glass fiber, where the end-facet waveguide typically has a 90° angle, as the effective index of TriPleX waveguides are quite close to that of standard glass fibers for telecom).
- Another application is coupling to a waveguide from another integrated optical chip such as light sources manufactured in InP technology.
- Because of the much larger effective index of waveguides in other technologies such as InP, SOI, the SSC coupling loss can be decreased by placing it under a suitable angle different than 90°, more can be found in Section VI-A.
D. Phase Modulators
- The phase modulator that is standardly used for the examples in this paper is based on thermo-optical tuning by means of heaters on top of the top cladding above a waveguide.
- These are manufactured by defining structured metal patterns based on a chromium layer and a gold layer.
- By combining e.g., two Y-junctions and two straight waveguides, one with a phase modulator and the other without, one can define a tunable coupler based on a Mach-Zehnder interferometer (MZI).
- Thermo-optic actuators suffer from power dissipation, in the order of 300 mW per π phase-shift per modulator, and thermal cross-talk from neighboring heaters requires a distance of at least 250 μm to the nearest neighboring waveguide.
- For these reasons, alternative types of phase modulators have been developed, as is mentioned in the first paragraph of Section III, namely stress-optic modulators (see the next section).
E. Stress-Optic Actuator
- To drastically decrease the power dissipation, ultra-lowpower stress-optic phase actuators are implemented in the TriPleX platform for visible light [18] as well as for the telecommunication C-band [21].
- When the PZT on top of the waveguide is exposed to an electric field, the PZT expands in the direction along the electric field while simultaneously contracting in the two other directions resulting in stress in the waveguide structure.
- The stress-optic actuators were fabricated using a symmetric double stripe TriPleX geometry (see Section II-C).
- The PZT layer with a thickness 2 μm was grown using pulsed laser deposition allowing for the growth of high-quality layers on large wafer sizes and at commercial throughput [22].
- Finally, platinum top electrodes with a thickness of 100 nm were deposited.
A. Ring Resonator Filter
- An important functionality that can be realized with low loss TriPleX waveguide is optical filtering via micro-ring resonators ROELOFFZEN et al.: LOW-LOSS SI3 N4 TRIPLEX OPTICAL WAVEGUIDES: TECHNOLOGY AND APPLICATIONS OVERVIEW 4400321 (MRRs).
- The ring waveguide behaves thus as a wavelength selective optical cavity that stores, enhances and drops out only resonant light, while all other light is transmitted to the output.
- In order to increase the cavity life-time and Qfactor, the internal losses of the cavity, of which propagation loss is a main contribution, need to be substantially reduced.
- The authors calculated the threshold power [28] for which they used the Q-factor given in Fig. 5 and the resonator length of 3400 μm.
- Because the confinement is only moderate with a mode field area of about 10μm2 , the gain is mainly due to the lower nonlinear index of the SiO2 cladding [29], and the mode volume is relatively large.
C. Ring Resonator Based Delay Lines
- Tunable delay lines are essential building blocks for implementing optical signal processing functions on chips.
- In particular, microwave photonic applications like optical beamforming.
D. AWG Based Wavelength Independent Switch
- Output shows three outputs instead of one, because of higher order spatial modes.
- Fig. 9. Switching between 10 different output ports of the AWG, while measuring the output power of all 10 ports simultaneously.
- As example, in Fig. 8(a) zero-order AWG design is shown, containing 3 inputs, 103 arms and 80 output waveguides without tuning elements with red light coupled into the waveguide.
- This building block can be designed for many required output numbers.
A. Switched Delay Line
- Switched delay lines are commonly used in OBFNs, signal processers, signal routers and buffering [35].
- The schematic layout is shown in Fig. 10.
- The optical cross talk, being the light travelling through an unwanted delay path interfering with the wanted delay path, is twice the amount of optical suppression of a tunable coupler.
- The absolute difference per delay, which the authors call the delay error, is smaller than ±0.5 ps, as is shown in Fig. 12, where each dot indicates a separate measurement.
- As is clear from these measurements, lowloss TriPleX is very suitable for fabricating switched delay lines.
C. High-Granularity WDM Filter
- TriPleX circuits also promise a significant impact on the next generation elastic optical communication networks, where technologies enabling high spectral efficiency and fine granularity transmissions are of significant use to increase the network capacity and flexibility.
- One reason for this is that the low-loss feature of the waveguide supports designs of filter structures with long delay paths, e.g., multiple cm, and therefore enables spectral processing with resolution finer than 1 GHz, which in contrast is challenging to implement using free-space optics.
- One interesting function is a Nyquist-filtering (de)interleaver [38] implemented using a ring resonator-assisted Mach-Zehnder interferometer circuit that comprises an asymmetric MZI with each arm coupled with a ring resonator as shown in Fig. 14.
- The design presented here employs a total of seven tuning elements, i.e., two tunable couplers at the input and output of the MZI, two tunable couplers of the two ring resonators, and three tunable phase shifters in the ring loops and MZI delay line.
- Using this combination, a WDM superchannel ROADM was enabled, which supports a sub-channel spacing of 12.5 GHz, a factor of four smaller than the current 50 GHz DWDM grid.
D. High-Order Ring Resonators
- Another interesting application of high-order ring resonators is input and output multiplexers (IMUXs and OMUXs) of satellite transponders in the field of satellite communications.
- Currently, the implementation of these functions relies on bulky electronics.
- As an important milestone of the waveguide technology, this filter shows an excellent combination of frequency selectivity, power rejection, and insertion loss when using the longest waveguides that were ever applied in such filters, i.e., each ring loop having an optical length of 21 cm.
- A single pass through all resonators thus corresponds to about 1.7 m, and taking into account that multiple roundtrips are performed in the.
E. Programmable Photonic Integrated Circuits for General Purpose Optical Processor Chip
- Integrated optical signal processors promise a wide range of applications in several different fields including optical communication networks, microwave photonics, optical sensing, biophotonics, and quantum optics.
- In practice, this paradigm may have issues of cost and flexibility, which are determining factors from a commercial perspective.
- A radically different approach in contrast to ASPICs is so-called programmable optical chips [40], i.e., universal signal processors integrated on optical chips.
- In principle, when provided with a sufficiently large network scale, one can program those Mach-Zehnder couplers to implement an arbitrary interferometric circuit topology for signal processing purposes, e.g., FIR and IIR filters, with full control of circuit parameters (amplitude and phase of each optical path in the circuit).
- As the network size scales up, waveguide loss plays an increasingly significant role for the device performance.
F. All-Optimized Integrated MWP Notch Filter
- Microwave photonics (MWP) is an emerging technology of which photonic technologies are used to generate, distribute, process, and measure RF and microwave signals [42].
- Recently, there is a strong paradigm shift towards the incorporation of photonic circuits in MWP leading to integrated MWP devices and systems [43].
- The waveguide geometry used in this work was the symmetric double-stripe waveguide (Section II-C) with loss of 0.1 dB/cm and ring bend radius of 125 microns.
- This leads to a ring filter with FSR of 25 GHz and quality factor of >900,000 allowing high resolution filtering of 150 MHz.
- The measured RF gain, noise figure and dynamic range of the filter in the range of 1–12 GHz is shown in Fig. 17(c).
G. Planar Waveguide Based Sensor
- The monitoring of the wavelength of light is crucial for many applications such as in optical communications [46], wavelength-division-multiplexing [47], specifically for realtime wavelength control and tuning of on-chip hybrid lasers [48].
- An advantage of using MRR is the possibility to realize wavelength meters via high-Q resonators that can offer a smaller footprint and a much higher resolution as compared to other integrated optic approaches such as, e.g., Mach-Zehnder interferometers.
- The named estimation range, however, would not be sufficient for many applications, e.g., monitoring on-chip hybrid lasers across the entire C-band that comprises tens of nanometers.
- It can be clearly seen that the FSR has become much wider compared to previous work, reaching a value of 43 nm.
- The wavelength meter was first calibrated with 1001 different known input wavelengths, by.
B. Tunable Hybrid Lasers With High Coherence
- Enabled by the low-loss properties and the mature tapering options that offer high chip-to-chip coupling efficiency, TriPleX is a highly suitable waveguide platform to realize external feedback circuits with long optical cavity lengths for bandwidth narrowing of diode lasers.
- As a result, such hybrid lasers are able to provide laser light with a much higher spectral purity as compared to typical DFB and DBR lasers, i.e., single-mode oscillation with narrow spectral linewidth.
- In the experimental realization, the laser comprises an InP reflective semiconductor optical amplifier (RSOA) coupled to the external TriPleX waveguide circuit.
- The RSOA usually possess a HR coated back-facet to reduce cavity losses, and a low reflectivity front facet to impose lasing via the feedback from the external waveguide resonator circuit.
- Here the circuits are used as a highly frequency selective feedback mirror for imposing single-frequency operation at a narrow spectral linewidth.
C. Binary Tree Optical Beamforming Network
- Phased array antenna systems can be found in multiple applications as radar systems, radio astronomy and communication systems like the upcoming 5G mobile networks systems.
- Optical beamforming networks (OBFN)s can be used to control the beam shape and direction.
- The use of cascaded ring ROELOFFZEN et al.: LOW-LOSS SI3 N4 TRIPLEX OPTICAL WAVEGUIDES: TECHNOLOGY AND APPLICATIONS OVERVIEW 4400321 resonators has two benefits [55].
- First, for a certain target delay value the maximum delay bandwidth increases approximately linear with the number of ring resonators.
- The manufactured OBFN has ring resonators with an FSR of 14 GHz.
VII. OUTLOOK
- The activities in the past decade have provided great insights in the possibilities and the capabilities of the TriPleX platform.
- Challenges and solutions evolve out of combined expertise of multidisciplinary teams.
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Cites background or methods from "Low-Loss Si3N4 TriPleX Optical Wave..."
...The single- and double-stripe geometries are part of the TriPleX foundry platform [13], [52] and used in the ultralow-loss waveguides developed at the University of California Santa Barbara (UCSB) [53]....
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...A wide variety of building blocks have been realized in the Si3N4 platform including bends, crossings, gain blocks, and directional couplers [52], [59]....
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...Further details of the fabrication processes for the TriPleX and the Damascene platforms can be found in [52] and [54]....
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References
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"Low-Loss Si3N4 TriPleX Optical Wave..." refers background in this paper
...which photonic technologies are used to generate, distribute, process, and measure RF and microwave signals [42]....
[...]
1,052 citations
592 citations
422 citations
"Low-Loss Si3N4 TriPleX Optical Wave..." refers background in this paper
...Switched delay lines are commonly used in OBFNs, signal processers, signal routers and buffering [35]....
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389 citations
"Low-Loss Si3N4 TriPleX Optical Wave..." refers background in this paper
...bination of straight-propagation loss and curvature loss via selecting an appropriate cross-section (Section II) [8]....
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...thickness in the range of several tens of nm, and by optimizing the stripe width [8]....
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