Microchips that operate faster and with less energy than their electronic equivalents have long been the promise of the photonics branch. However, building such a circuit has proven difficult over time. Having enough output power to provide a strong enough signal is one of the main challenges. However, the performance of the new chip-scale lightweight power amplifier is nearly identical to that of similar devices used in commercial telecommunications.
There are now ultra-wideband fiber optic networks connecting the world. In this communication, erbium doped fiber amplifiers are used, enabling ultra-fast data rates worldwide.
Signals from optical fibers and other network components have to be amplified many times over as optical signals are sent over long distances.
Erbium-doped fiber amplifiers were first developed in the 1980s. These are used to amplify optical pulses without first having to convert them to electrical signals. These devices are able to specifically improve light in the wavelength range of 1.55 micrometers or 1,550 nanometers, which is the range with the lowest transmission losses for optical fibers.
What is Photonics and What Are the Applications of Photonics?
Photonics is the physical science and application of light (photon) production, detection and manipulation through emission, transmission, modulation, signal processing, switching, amplification and sensing. Although it covers all technical applications of light across the entire spectrum, most photonic applications are in the visible and near infrared light range. The term photonics was developed as a result of the first practical semiconductor light emitters invented in the early 1960s and optical fibers developed in the 1970s.
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Swiss Federal Institute of Technology Tobias Kippenberg explains the purpose of the study as follows.
“The most exciting part of this study is how well the amplifiers work and are on par with commercial amplifiers despite being only a few hundred microns in any size.”
Similar amplifiers capable of operating inside photonic microchips have been a research target for many years.
However, attempts to create such chip-scale amplifiers were mainly used to carry light inside the chips.
But due to losses from waveguides, it has produced devices with output power often lower than one milliwatt – too low for many real-world applications.
The manufacture of these amplifiers was not compatible with contemporary photonic integrated circuit fabrication techniques.
A chip-scale erbium-doped fiber amplifier has now been created by researchers. With only 2,61 mW of input power and a small signal gain of over 30 dB, the new device has a record high output power of over 145 milliwatts.
This resulted in a telecommunications band amplification that worked more than a thousand times continuously.
This is already equivalent to that of high-end commercial erbium-doped fiber amplifiers.
The most exciting part of this work, according to optical engineer Kippenberg, is how well the amplifiers work. Although they are only a few hundred microns in any dimension, they are on par with commercial amplifiers.
Additionally, this device's erbium-doped waveguide, which can be up to half a meter long, is compressed into a spiral with a tiny footprint of just 1,2 millimeters by 3,6 millimeters. In addition, the device has a high power conversion efficiency of about 60%.
The ultra-low loss chip-scale photonic waveguide based on silicon nitride, which is already widely used in the semiconductor industry, forms the basis of this innovative technology. Recently, Kippenberg and colleagues created meter-long, ultra-low loss silicon nitride waveguides. Next, they looked at the possibility of creating optical amplifiers by embedding erbium in such waveguides.
Kippenberg claims that light can only be strengthened very softly by erbium ions. Gain is possible only when incorporated into extremely low loss optical fibers and interacting with light over very long distances, usually measured in metres.
In tests, the researchers demonstrated that they can increase the output power of soliton microcombs by 100 times. Soliton micro combs can be used in spectroscopy, metrology and other fields, but due to their output power of only tens to hundreds of microwatts, nearly all of these fields need to amplify the signal.
The scientists also revealed that their device can directly amplify more than 1 wavelength division multiplexing channels for data transmission over a 20-kilometer-long fiber optic link. This indicates that it can be used in chip-scale amplification for telecommunications networks.
The researchers stressed that an external pump laser is still required for their optical amplifiers. As a result, the entire unit is still not fully integrated. According to Kippenberg, future hybrid integration will be necessary to address this important shortcoming.
Ultimately, the researchers expect their optical amplifiers to contribute to the development of chip-scale, mode-locked lasers that can produce bursts as short as femtoseconds. Such devices can have a wide variety of functions, such as lidar, according to lead author Yang Liu, an optical engineer at the Swiss Federal Institute of Technology Lausanne.
The femtosecond mode-locked laser is arguably the holy grail, and that's what Kippenberg and his team are currently focusing on.
“The femtosecond mode-locked laser is clearly the holy grail, and it's what we're eyeing right now,” Kippenberg says.