Spectrometers point to the next mixed-signal trend
Article By : Bill Schweber
Just as analog merged with digital circuitry for mixed-signal ICs, electronics and optics are being fabricated as single-package/chip device.
The phrase “mixed signal” traditionally referred to a blend or hybrid of classical analog circuity with digital functions. Now there’s a new definition of the term gaining traction: integration of advanced optical functions and devices with analog/digital electronics.
First, we’re seeing progress in integrating optical building blocks with electronic ones. This can be done with co-packaged optics (CPOs), where optical and electronic components are housed in the same tiny package, or on-board optics (OBO), where the optical waveguides and other structures are fabricated using layers within the PCB, such as being developed by Vario-Optics AG in Switzerland (Figure 1).
Figure 1 By using special inner layers of the circuit board as optical waveguides and other functional blocks, a type of integrated electro-optics called on-board optics (OBO) is being developed. Source: Vario-Optics AG
Of course, the goal is to eventually have a monolithic device which functions as a photonic integrated circuit (PIC) with an optical source and receiver supporting multi-Gbps data rates. That’s what companies such as Ayar Labs are now doing while using CMOS processing to develop high-speed, high-density, low-power optical interconnect “chiplets” and lasers to replace traditional electrical I/O CMOS processing (Figure 2).
Figure 2 There’s been significant progress in using the same substrate for electronic and optical devices, primarily for monolithic high-speed interconnects. Source: Ayar Labs
However, there’s more to integrated electronics and optics than a light source, optical link, waveguides, and optical receiver: there’s spectral analysis, as well. For this, the primary instrument is the spectrometer. By assessing the optical spectral content, a system provides analytical data and performs other needed functions.
For these reasons, we’re seeing R&D efforts toward making miniatured spectrometers which are largely monolithic, or at least highly integrated devices. Two recent examples demonstrate the efforts.
Infrared spectrometer
A multi-institute team lead by researchers at the prestigious Swiss Federal Laboratories for Materials Science and Technology (EMPA) has devised a miniaturization process for IR spectrometers based on a quantum dot photodetector, which can be integrated on a single chip. Potential applications include use of smartphones for assessing food quality, detection of hazardous chemicals, air pollution monitoring and wearable electronics, and detection of counterfeit medical drugs as well as of greenhouse gases such as methane and CO2.
Their proof-of-concept miniaturized Fourier-transform waveguide spectrometer incorporates a subwavelength photodetector as a light sensor, consisting of colloidal mercury telluride quantum dot (Hg Te). It’s compatible with standard CMOS technology (Figure 3).
Figure 3 The EMPA spectrometer design uses multiple optical layers and structures on a common substrate along with an embedded controllable, solid-state mirror. Source: EMPA
In this design, the photodetector is fabricated on top of a LiNbO3 substrate to create an optical waveguide. This comprises a gold electrode at the bottom, functioning as a scattering center; a photoactive layer, consisting of colloidal HgTe quantum dots as a mirror; and a top gold electrode. By moving the mirror, the measured photocurrent maps the light intensity of the standing wave of the IR light. A Fourier transformation of the measured signal gives the optical spectra.
The total active spectrometer volume is under 100 μm × 100 μm × 100 μm. This ultracompact spectrometer design allows the integration of optical/analytical measurement instruments in consumer electronics and space devices. Full details are in their paper “Integrated photodetectors for compact Fourier-transform waveguide spectrometers” published in Nature Photonics.
Computational spectrometer
Another approach has been devised by researchers at Oregon State University working with Aalto University in Finland and others; it’s based on a high-performance computational spectrometer. They use a single van der Waals (vdW) junction with an electrically tunable spectral response, and combine this tunable junction, measuring just 22 × 8 μm, with a computational-reconstruction algorithm (Figure 4).
Figure 4 In a very different approach, this ultra-miniaturized spectrometer concept uses a gate-tunable van der Waals junction to distinguish peak wavelengths of monochromatic light and computational processing to analyze the spectral information of the images. Source: Aalto University
Three steps are required to implement their spectrometer concept. First, measure the gate-tunable spectral responses with multiple known incident spectra. It’s the learning process. Second, measure the gate-tunable photocurrent of the unknown incident light to be analyzed. It’s the testing process. Third, compute the spectral information of the unknown incident light based on the results obtained in learning and testing processes with the reconstruction algorithm and it constitutes the reconstructing process.
If all this sounds complicated and hard to explain…well, it is. Their full paper “Miniaturized spectrometers with a tunable van der Waals junction” is published in Science but it’s behind a paywall. Fortunately, Aalto University has posted a preprint at its site.
Making spectrometers feasible
What’s interesting about both very different miniature spectrometers is that they rely on a significant amount of computational effort to extract useful data, as they are not just spectrum-oriented filters. Perhaps that’s the extra boost needed to make these sorts of highly integrated electronic-optical mixed-signal devices feasible, at least at this time. If so, that’s not a bad price to pay for what it yields.
Do you see an increasing role for such PICs and instrumentation, going beyond point-to-point data links? Is this the leading edge of what may be a major transformative technology in a decade or two?
This article was originally published on Planet Analog.
Bill Schweber is an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical website manager for multiple EE Times sites and as both Executive Editor and Analog Editor at EDN. At Analog Devices, he was in marketing communications; as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these. Prior to the marcom role at Analog, Bill was Associate Editor of its respected technical journal, and also worked in its product marketing and applications engineering groups. Before those roles, he was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls. He has a BSEE from Columbia University and an MSEE from the University of Massachusetts, is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. He has also planned, written, and presented online courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.