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Figure 1: Example of Photonic Integrated Circuit for visible light.
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Figure 2: Examples of the basic optical functions that can be implemented in the Visible PIC technology.
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Figure 3: Interfacing option of Visible PIC: fibre coupled (left), to free space (right).
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Joost van Kerkhof, Douwe Geuzebroek – XiO Photonics, Enschede, The Netherlands
Photonic technology is increasingly used in applications in medicine, life and environmental sciences. Whereas currently many of these applications are implemented using some form of discrete (free-space) optics, much can be gained from a transition to Photonics Integrated Circuits. This follows the trends in the electronics industry where highly integrated electronic circuits have allowed the combination of many different functions in a small form factor. Just as it has done for the electronics industry, integrated optics will lead to smaller, cheaper, more reliable and more user friendly devices.
Photonic Integrated Circuits
Photonic Integrated Circuits (PICs) are devices on which several or more optical components are integrated; often together with electronic components. Equivalent to the introduction of electronic ICs many years ago, Photonic ICs offer huge advantages. Compared to a system with discrete optical components, a system with a PIC will be simpler, more robust, more reliable, more compact and lower cost. In addition, the small form factor of PICs can enable applications that are not possible to realise with existing discrete components. PICs have been used for several years already in high volumes for optical communication applications, and this use is still increasing significantly. These applications include Fttx and access networks, long-haul and transport networks, and optical datacom. PICs for optical communication typically use wavelengths between 1 and 2 μm. In applications where light is used in the visible range (400-700 nm), the use of PICs is still very rare although the potential benefits can be significant. These applications include several biophotonics applications such as confocal microscopy, flow cytometry, molecular diagnostics and spectroscopy systems. But also laser based display applications and several (food) sorting applications use visible laser light and could benefit from the use of PICs.
PIC technology for visible light
Whereas the technology of electronic ICs has evolved into a standard process (CMOS) using a single standard building block (transistor), the technology for photonic ICs is more diverse. Several materials and processes are used depending on function and application. The main technologies currently used are typically categorised as Silicon Photonics, III-V Materials and Dielectrics.
Silicon Photonics, using Silicon-on-Insulator, offers passive light manipulation at a very small footprint, allowed by the relative high contrast index of silicon. The small chip size and the CMOS fabrication compatibility result in relatively low chip prices. However, no light amplification is possible at the time, meaning that integration with other technologies will be required for active functionalities. Since silicon is not transparent for wavelengths below 1 μm, Silicon Photonics cannot be used for visible light applications.
The III-V Materials technology platform offers light amplification and detection, next to passive light manipulation as filtering, splitting or interfering. In the past 20 years, this technology has been widely used by chip manufacturers to make lasers, modulators and detectors. The main materials for this platform are Indium Phosphide (InP) and Gallium Arsenide (GaAs).
The Dielectrics technology platform offers light manipulation with very low transmission and fibre coupling losses, given the refractive index match of Silica. Also referred as Planar Lightwave Technology (PLC), it became very popular in the early 2000s, allowing for a large cost reduction because of mass production of splitters and AWGs. The main materials for this platform are Silicon Dioxide or Silica (SiO2), Silicon Nitride (Si3Ni4/SiN) and TripleX. TripleX has the advantage over the other dielectrics technologies that the contrast can be tuned depending on geometrical design. Tapers can be created that allow for efficient coupling to fibres in the low contrast regions and small bending radii allowing compact structures in the high index contrast regions.
To use PICs for visible light applications, a low loss on-chip waveguide technology is used which allows for control of wavelength, intensity, phase, mode size, polarisation and input- output geometries. The technology is optimised for wavelengths from 400 up to 2000 nm.
With this waveguide technology a number of passive and active optical functions can be made, such as wavelength combiners, power splitters, switches, filter components, variable attenuators and modulators. These functions can be combined in one PIC to realise the desired functionality in one robust and compact component. Many of these functions are known from the discrete options alternative, however new functionalities can be created by combining more on-chip functions to one. See figure 2 for some examples of the possible optical functions and their comparison with optical discrete functions.
Interfacing to PIC Devices
To interface with the optical system, the visible light PICs can be equipped with optical fibre connections or be used as a free- space optical component. Hybrid configurations with both fibre connections as well as free-space interfaces on the same device are also possible.
The devices based on the PIC for visible light have no moving parts, are robust and stable over a very wide temperature range, do not need optical adjustment and can be very compact in size.
Application Examples Integrated Laser Beam Combiner An example for the PIC device for visible light is the integrated laser beam combiner (ILBC). It can combine up to eight different (visible) input wavelengths into one single mode polarisation maintaining output fibre. Ideal for a system where light of different wavelengths has to be transported to a single target (e.g. a confocal microscope). See figure 4 above.
Variable Optical Attenuator
Another example of using PIC technology for visible light is the Variable Optical Attenuator (VOA) for visible wavelengths. This VOA enables to decrease the output power of any input laser in a measurement setup, while the laser operates at standard high output powers. This increases the stability of the total system as the lasers prefer operation at their ‘sweet-spots’, which typically are high power. However for many applications these higher powers can damage your (biological or medical) samples.
Lowering the power of the laser directly increases the noise as they no longer operate at their preferred operation point. The VOA for visible wavelengths offers the possibility to run a laser at its preferred output state and the power can be adjusted to match the level of the specific measurements.
The VOA function can also be combined with the combiner or splitter functions, which creates an adjustable combiner or splitter. With such type of function combinations, the true advantages of using PICs become apparent (figure 5).
Fringe pattern generator
An application demonstrating how PICs can be used in cooperation with freespace optics, a fringe pattern generator is developed. As the phase of the light can be controlled very accurately on the visible PIC, fringe patterns can be made and controlled that are projected on a surface. The example in figure 6 shows the principle of a simple two channel version. But the principle can be extended to more channels, allowing even more flexibility in the number of fringes and the fringe shape and pattern.
PIC technology for visible light can enable new applications which are not possible to realise with existing discrete components. We have shown several examples from our library of passive and active optical functions such as wavelength combiners, power splitters, switches, filter components, variable attenuators and modulators. Several application examples have been shown to demonstrate the creation of new functionalities by combining more on-chip function to one.