Optical Circuits
“The number of transistors incorporated in a chip will approximately double every 24 months”- Gordon Moore
The above law was predicted to fail until the development of the photonic integrated circuits.
Communications through copper wire had many disadvantages including attenuation, distortion, electromagnetic interference(EMI), crosstalk, etc. The invention of fibre cables that transported information through glass wires was a breakthrough in telecommunications. Since information passing through the glass fibres were in the form of light signals, it could overcome all the problems caused by transporting through copper wires.
Optical circuits, just like the name suggests, utilize light signals to function, and can be used for communication. Integrated optical circuits also known as photonic integrated circuits(P.I.C) are devices on which several optical components that use photons to function, including photonic sensors are integrated similar to the conventional integrated circuit(I.C),where several electronic components that use electric signals to function are integrated. Photonics refers to the physical science of photon(light) generation, detection, and manipulation through emission, transmission, modulation, signal processing, switching, amplification, and sensing.
Technology used to manufacture the chip
Fabrication of optical circuits is done by using a wafer-scale technology or wafer-scale integration in which large integrated circuits are built using an entire substrate wafer to produce a single chip. The devices integrated on the chip include low loss interconnect waveguides, power splitters, optical amplifiers, optical modulators, filters, lasers and detectors.
Optical circuits can be of different types, depending on the function required. For example, silica (SiO2) based PICs have properties suitable for making AWGs (Arrayed Waveguide Gratings).
AWG is used as optical (de)multiplexers in wavelength division multiplexed (WDM) systems. Devices with WDM systems are capable of multiplexing many wavelengths into a single optical fiber, hence increasing the transmission capacity of optical networks considerably. Since light waves of different wavelengths do not interfere linearly with each other, light can be multiplexed easily. AWGs can be used as both multiplexer(at the transmission end) and demultiplexer(at the receiving end) in an optical communication network.
The incoming light (1) traverses a free space (2) enters a bundle of optical fibers or channel waveguides (3). The fibers have different lengths and thus apply a different phase shift at the exit of the fibers. The light then traverses another free space (4) interferes at the entries of the output waveguides (5) in such a way that each output channel receives only light of a certain wavelength. The orange lines only illustrate the light path. The light path from (1) to (5) is a demultiplexer, from (5) to (1) a multiplexer.
Silicon On Insulator(SOI) and Lithium Niobate On Insulator(LNOI) are commonly used.
Silicon On Insulator(SOI)
The study and application of photonic systems which use silicon as an optical medium is called silicon photonics. Silicon is transparent at wavelengths typically used for optical communication transmission, due to its intrinsic band gap of 1.1 eV. Silicon photonic devices in a CMOS (complementary metal oxide semiconductor environment) makes it possible to create low-cost photonic solutions.
The combination of both photonic and electronic devices on a silicon wafer is known as a hybrid photonic integrated circuit.
Silicon-on-Insulator chip (26 × 11 mm2) with about 800 test structures: waveguides, Fabry-Pérot modulators, raman-lasers and -amplifiers, frequency-to-time-conversion, Mach-Zehnder-modulators etc.
The chip is made by placing a layer of insulating material (SiO2) between two layers of silicon substrate that have thickness 220 nm and 500 μm. The index contrast of the silicon device layer compared to the insulating oxide layer can be used to fabricate low loss optical waveguides with nanometer size. These small waveguides may be bended with radii down to only 5 micrometer which can be used for many processes including optogenetics(a biological technique that involves the use of light to control neurons).
Cross-section of a waveguide(metal tube confining microwaves) in SOI, SEM(Scanning Electron Microscope) image of an SOI waveguide with hole engineering (mirror-spacer-mirror).
Resonators and optical cavities play an important role in a photonic integrated circuit. To control light for trapping and emission of photons, lasing, switching, and optical filtering, ultra-small cavities are required in PICs. Optical resonators are used for optical modulation. Optical modulation is the process of encoding information on a carrier optical wave.Most popular components are ring resonators and photonic crystal microcavities.
One-dimensional (1D) micro-cavities have a very small footprint(amount of space on an optical component occupied by a light beam) and are, therefore, well suited for highly dense packaging. High performance electro-optical modulation in silicon has mostly been demonstrated using Mach-Zehnder modulators. Those modulator devices have lengths in the mm range.
A Mach-Zehnder modulator is used for controlling the amplitude of an optical wave. The input waveguide is split up into two waveguide interferometer arms. If a voltage is applied across one of the arms, a phase shift is induced for the wave passing through that arm. When the two arms are recombined, the phase difference between the two waves is converted to an amplitude modulation.
Electro-optic modulators are based on ring resonators. Ring resonator is a set of waveguides in which at least one is a closed loop coupled to light input and output, and functions as a filter.
Size comparison between modulators for the integrated optics
Fabry-Perot modulators are the modulators with two highly reflecting mirrors or parallel surfaces.
Ring resonators possess certain setbacks.
The small modulators have ring diameters down to 12 μm and modulation frequencies greater than 10 GB/s. If the size of the rings is further reduced to less than 10 μm, the guided mode is leaking out of the ring waveguide. Therefore, reduction of the footprint of the modulator is limited. (Leaky mode, also known as, tunnelling mode in an optical fibre or other waveguide is a mode having an electric field that decays monotonically for a finite distance in the transverse direction but becomes oscillatory everywhere beyond that finite distance.)
The high temperature sensitivity of the ring resonators can be solved by reducing the footprint of electro-optic modulators. This can be done by using 1D micro-resonators.
Lithium niobate on insulator(LNOI)
Compared to other materials like silicon, LiNbO3 (Lithium Niobate)has several natural advantages, including strong electro-optic effect, large refractive index, wide transparency wavelength (from 400 nm to 5 μm), and stable physical and chemical characteristics, thus making it the one of the best material for integrated photonics.
Schematic view of an integrated LNOI photonics chip
Similar to silicon on insulator (SOI), LN on insulator (LNOI) consists of a sub-micrometer LN film on a silica buried layer, which is on top of a substrate made from silicon or LN. A typical LNOI wafer has an LN film thickness of hundreds of nanometers (typically 300–900 nm) and a diameter of 3 or 4 inches.
To fabricate an LNOI, a single-crystal sub-micrometer LN film is first obtained by ion slicing a bulk crystal and then is bonded to a low index substrate by using benzocyclobutene bonding or crystal bonding. Several techniques including wet etching, dry etching, chemical mechanical polishing (CMP), diamond dicing and femtosecond laser direct writing can be used to nanostructure LNOI. These techniques enable fabricating LN ridge waveguides or structures on LNOI platform.
Wet Etching
Wet etching is used for the direct etching of LN ridge waveguides. A chromium film is deposited on top of an LN film as a mask for the waveguide and is patterned by lithography. Then, the LN is wet etched by corrosive liquid, for example, a solution of 40% HF and 100% HNO3.
Wet etching is used not only for etching LN thin films, but also for nanostructuring titanium-diffused or proton-exchanged LN thin films. Wet etching is also used to improve the sidewall roughness of a waveguide caused by the residual of photoresist mask. Since wet etching processes are chemical and isotropic, a tilted sidewall is created. Due to this drawback, dry etching is considered to be a better alternative.
Dry Etching
Dry etching is the most popular method for fabricating a high-quality waveguide in thin film LN. Dry etching technique acquires sub-micron scale waveguide with low loss on LNOI.
First, an etching mask is deposited on the top of the LNOI wafer. According to different processes, different etching masks like amorphous silicon or other resists(a resistant substance applied as a coating to protect a surface during a process), can be used.
Second, photonic patterns are defined in the etching mask using different methods including electron-beam lithography (EBL) or photolithography.
Electron-beam lithography (EBL) is the process of scanning a focused beam of electrons to draw custom shapes on a surface covered with an electron-sensitive film called a resist.
Photolithography, also called optical lithography, is used to pattern parts on a thin film or the bulk of a substrate (also called a wafer). It uses light to transfer a geometric pattern from a photomask (also called an optical mask) to a photosensitive (light-sensitive) chemical photoresist on the substrate.
In order to improve the selectivity of etching or to improve the sidewall roughness, annealing is performed. Annealing refers to the process of heating a substance (usually metal or glass) and allowing it to cool slowly, in order to remove internal stresses and toughen it.
The third and the most important step is etching LNOI thin film for obtaining the waveguide structure. Reactive ion etching (RIE) and/or inductively coupled plasma RIE (ICP-RIE) is commonly used for dry etching LNOI.
A standard dry etching process of LNOI
An LNOI wafer (A) first deposited with an etching mask, (B)The etching mask patterned by using electron-beam lithography, ©Defining the geometry of structure. Using reactive ion etching (RIE) or inductively coupled plasma RIE (ICP-RIE), the lithium niobate outside the waveguide area is etched away, (D)Etching mask is removed (E) Waveguide is formed.
Because of the chemical stability of LN, argon(Ar) plasma is used for physical etching. Recently, Ar+ is used for physical etching because it induces a more vertical sidewall, despite the fact that Ar+ extends the etching time. Other etching parameters, such as ICP power, RIE power, gas flow rate, pressure, and temperature, influence the processing speed, etching depth, surface roughness, and sidewall angle. After dry etching, the etching mask is removed and the fabrication process is complete.
Chemical mechanical polishing (CMP)
Chemical mechanical polishing is an alternative to dry etching that can provide a surface as smooth. Chromium layer is used as a mask in CMP method and is patterned by femtosecond laser ablation to define the shape of the waveguide.
Femtosecond laser ablation is a high-precision technique allowing a fine material texturation (ripples, cones, cavities) with multiple geometries (period, orientation, structure density) using lasers.
Next, by using a wafer polishing machine, the exposed LN film can be removed to form the waveguide. Finally, the chromium layer on top of the LN waveguide can be removed by using wet etching with HF solution.
If the upper surface of the waveguide needs to be very smooth, a second CMP process is performed.
A standard fabrication process of chemical mechanical polishing (CMP).
(A) Cr thin film is first deposited on a LNOI wafer and is ablated by femtosecond laser (B) to define the pattern. Then, CMP is implemented © to polish lithium niobate thin film. Finally, Cr mask is removed by wet etching and secondary CMP is implemented for a smoother surface (D). (E) shows a schematic diagram for the CMP process.
Diamond Dicing
Diamond dicing, A direct machining method, can also be used to fabricate ridge waveguides on LNOI.
The sidewall angle of waveguides made by diamond dicing is defined by the shape of the diamond cutting blade. A waveguide with a high aspect ratio can be made by diamond dicing, but the width of the waveguide and the minimum distance between two neighbouring waveguides are largely limited by the shape of the cutting blade .
A standard fabrication process of diamond dicing.
(A–F) Show the standard fabrication process of diamond dicing and (G) shows a scanning electron microscope photograph of the waveguide made by using diamond dicing.
Femtosecond laser direct writing has also been used in nanostructuring LNOI, which is a laser-based precise three-dimensional (3D)micro/nanofabrication method. This technique allows large-scale surface patterning.
Schematic diagram of laser surface texturing with femtosecond laser
To achieve high performance integrated optoelectronic devices like electro-optic modulators, low-loss waveguide is required. The two major sources of losses for integrated LN waveguides are radiation loss (caused by waveguide bending) and scattering loss (induced by surface roughness).
The loss of integrated LN waveguide can be measured using several methods:
Q-factor method
It is the most popular method used to measure the quality (Q) factor of a microring whose cross section is the same as the waveguide. A higher Q factor means that the resonator can “lock” more energy for a given input-output coupling efficiency. Factors like material and structure of the resonator, the fabrication precision, and coupling efficiency determines the Q factor.
When a continuous wave (CW) light is critically coupled into the microring, waveguide loss can be calculated via α=2πneff/(Qλ), where neff is the mode index, and λ is the resonant frequency. This method is particularly suitable for low-loss waveguides, corresponding to high-Q microrings whose Q factor can be measured accurately.
Cut-back method
Cut-back method is an alternative and simple way for measuring waveguide loss. By measuring transmission power of waveguides with different lengths, the propagation loss is obtained by calculating the slope of transmission versus waveguide-length curve.
Interferometric method
Interferometric method is another way for measuring waveguide loss. A waveguide with two polished end faces can be treated as a Fabry–Pérot resonator (resonator with two highly reflecting mirrors or parallel surfaces). So, the transmitted power changes periodically as a function of the round-trip phase, which can be tuned via thermo-optic or electro-optic effect.
An electro-optic effect is a change in the optical properties of a material in response to an electric field that varies slowly compared with the frequency of light. The change in optical properties of a material because of heat radiation is known as thermo-optic effect.
The attenuation constant α of the waveguide is obtained by measuring the contrast K (also called visibility or modulation depth) of the Fabry–Perot interference.
The three methods, (A)Q-factor method, (B)Cut-back method, ©Interferometric method
There are two major designs of resonators; microdisk and microring.
Microdisk
Microdisk resonators are also known as whispering gallery mode (WGM) resonators because this type of resonators based on LNOI has the same physical principle as WGM resonators based on other platforms, such as optical fibers. The structure of the resonator is a solid microdisk made from LN thin film with a diameter of tens of micrometers. In order to improve the input and output coupling efficiency of waveguide and to confine the WGM mode within the cavity, the surface of the silica underneath the LN microdisk is removed by wet etching after processing the microdisk.
(A) Optical microscope image of the microdisk from the top, (B) scanning electron microscope(SEM) image showing the overview, © The edge of the microdisk
Microring
Compared to microdisk resonators, microring resonators have a significant advantage. A microring can be made by closing the loop of a waveguide. Such an advantage brings a great degree of flexibility to the design of microring. Due to this, microring’s shape can be made according to the particular requirement of the device. For example, to achieve an electro-optic frequency comb with long electrodes, a racetrack-shaped microring is designed.
(A) Scanning electron microscope(SEM) of a typical LNOI microring. (B), © Scanning electron microscope (SEM) images of waveguides constituting microrings. (D) Optical microscope images of racetrack resonators with different lengths
LN modulator’s response time to the modulation signal (on femtosecond timescales) is much shorter than that of silicon modulator (on nanosecond timescales), enabling an ultrahigh-speed modulation. High-speed LNOI electro-optic modulator is a traveling wave modulator, avoiding the long resistor-capacitance response time originated from the P-N junction of integrated silicon modulators.
General Applications
Photonic integrated circuits or integrated optical circuits have a wide range of applications in different sectors including healthcare diagnostics, processing industry, mobility, safety and security, and agro-food. They are used in the manufacture of metrology and optical sensors. They are also used to manufacture Externally Modulated Lasers(EML) and biomedical computing. Photonic integrated circuits are used to make optical transceivers. A transceiver is the interface between the electronic data processing and the optical data transmission line.
Future of photonic integrated circuits
Since PICs offer high speeds and high bandwidth, they can be used to handle the quickly growing data traffic in the near future where pure copper based electronics will fail to satisfy the requirements of bandwidth and distance. In fact, the global Photonic Integrated Circuit (IC) market is expected to reach approximately US$ 2.25 Billion by 2025 growing at a CAGR (Compound And Annual Growth) of 27.6% from 2019–2025.
Current status of integrated optical circuits
Photonics has high potential for development, but it still requires a thorough scientific research to perform all operations.
References
- LaserFocusWorld
- Wiley
- Circuits Today
- Edmund Optics
- Mangalmay
- Wikipedia
- Degruyter
- Comsol
- The Free Dictionary
- ScienceDirect
- Lehigh University
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