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Integrated optics

Integrated optics

Smooth photonic waveguides keep propagation losses low

08 Oct 2020 Isabelle Dumé
chip-photo
Lead author Junxia Zhou with the reported OTDL device. Courtesy: Junxia Zhou

Circuits that carry signals via visible and infrared light rather than electric currents are desirable for many applications because they transmit data faster and use less energy. The problem is that current programmable photonic integrated circuits (PICs) are volatile and suffer from high optical signal losses – both of which prevent them from maintaining their programmed state. A team of researchers in China has now succeeded in fabricating metre-scale single-mode waveguides that boast optical losses of only 0.03 dB/cm. The researchers also used their waveguides to construct optical true delay lines (OTDLs), which are important components of many photonic devices – including future quantum information processors and sensors.

Current fabrication techniques for PICs produce devices with highly variable final properties. This variability limits the techniques’ yield and reduces the devices’ configurability. Existing techniques also produce devices with a high surface roughness, leading to high optical losses.

The excellent optical and electro-optical properties of lithium niobate on insulator (LNOI) offer a possible way around this problem. The material has recently emerged as an attractive substrate material for PICs, and it shows promise for making circuits with lower losses, higher density and greater tunability than previous devices. However, while researchers had previously fabricated various photonic structures – including waveguides, microresonators and photonic crystal cavities – on LNOI, no group had succeeded in using it to fabricate high-quality, low-loss OTDLs.

Low-loss OTDLs on lithium niobate

A team of researchers led by Ya Cheng of East China Normal University has now done just that using a new technique called photolithography-associated chemo-mechanical etching (PLACE). The technique can produce smooth, metre-long waveguides on the LNOI that can be integrated with micro-electrodes, which allow the devices to be tuned electro-optically at a later stage.

Cheng explains that PLACE requires five major steps. The first step is to apply a thin coating of chromium of the top surface of lithium niobate thin film using a technique called magnetron sputtering. Next, the researchers pattern the chromium film into a waveguide mask using space-selective femtosecond laser ablation. They then use a chemo-mechanical polishing (CMP) process to selectively remove the lithium niobate. The CMP process produces extremely smooth walls, leading to ultralow light propagation losses in the finished waveguides. The fourth and fifth steps involve removing the chromium mask layer using chemical wet etching and depositing a titanium oxide (Ti2O5) film on the fabrication lithium niobate waveguide as the cladding layer.

Ultra-low propagation losses

To measure the propagation losses in the OTDLs, the researchers used a high-precision loss measurement method that relies on comparing propagation losses in two waveguide arms of different lengths in a beamsplitter. They used a wavelength-tuneable laser as the light source, tuning it to a wavelength of 1550 nm and coupling it into the waveguide through a fibre taper. They first collected the light transmitted from the waveguide using a lens and then recorded its intensity with either a power meter or an infrared CCD.

The devices have propagation losses of just 0.03 dB/cm, meaning that after propagating in a one-metre long waveguide, half of the light power of the laser beam can be preserved, Cheng explains. They are also reconfigurable thanks to the good electro-optical properties of lithium niobate.

The researchers, who report their work in Chinese Physics Letters, say they now hope to reduce propagation losses even further by refining their fabrication technique. “We will also be looking into applications for the OTDLs, probably beginning with quantum PICs,” Cheng tells Physics World.

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