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Rapid Wavelength Tracking with Silicon Photonics

Writer's picture: Latitude Design SystemsLatitude Design Systems
Introduction

For emerging applications like light detection and ranging (LiDAR), biomedical sensing, imaging, and optical communications, there is a need to monitor and track the wavelength of rapidly tuning lasers. Traditional approaches using etalons or wavemeters have limitations in speed, often operating in the millisecond range or slower. This tutorial discusses a new silicon photonic wavemeter that can measure wavelength changes with response times under 200 nanoseconds, enabling real-time tracking of fast wavelength excursions.

Device Design

The wavemeter is based on an array of silicon Mach-Zehnder interferometers (MZIs) with varying free-spectral ranges from 156 GHz to 11.7 THz, as shown in Figure 1(a). This design provides both broad wavelength coverage of over 80 nm and accurate measurements with errors below 1 GHz. The use of both silicon and silicon nitride MZIs makes the device essentially insensitive to temperature changes.

silicon photonic wavemeter and electronic adapter board
Figure 1: (a) Diagram of silicon photonic wavemeter and electronic adapter board. (b) Photograph of assembled wavemeter module. (c) Photograph of adapter board.

The wavemeter chip is assembled into a module (Fig. 1(b)) and connected to a custom adapter board (Fig. 1(c)) via coaxial cables. The board includes transimpedance amplifiers (TIAs) to measure the photocurrents from the integrated germanium photodiodes on the wavemeter chip.

Measuring Fast Wavelength Bursts

To demonstrate the high-speed capability, the wavemeter measured the wavelength excursions of a C-band distributed feedback (DFB) laser operating in burst mode. The laser was driven with a 100 kHz square wave with 50% duty cycle, resulting in 5 μs optical bursts.

Figure 2(a) shows the measured optical frequency relative to 1549.25 nm during one of these bursts. The wavemeter exhibits a 10-90% response time of 180 ns for the falling edge (laser turn-off), while the rise time is around 320 ns. The insets show the TIA output signals used to calculate the optical frequency.

optical frequency excursion of a DFB laser
Figure 2: (a) Measured optical frequency excursion of a DFB laser during a short burst, relative to 1549.25 nm. Insets show TIA output signals before frequency calculation. (b) Maximum frequency excursion for varying duty cycles at 100 kHz repetition rate.

The slower rise time is attributed to carrier effects in the laser causing an initial wavelength shift of opposite sign to the subsequent thermal shift. Frequency excursions up to 10 GHz were measured from 100 ns into the burst until the end.

Figure 2(b) shows that longer burst durations resulted in larger frequency excursions until a thermal equilibrium was reached at higher duty cycles. Direct connection to analog-to-digital converters (ADCs) could further improve measurement speeds currently limited by the oscilloscope connection.

Tracking Linear Frequency Ramps

The wavemeter was also used to track linear frequency chirps generated by applying a triangular waveform to the laser bias. Figure 3(a) shows the measured optical frequency modulation during one chirp period.

Measured optical frequency of the laser relative to 1549.25 nm for a triangular function at 100 kHz repetition rate
Figure 3: (a) Measured optical frequency of the laser relative to 1549.25 nm for a triangular function at 100 kHz repetition rate. (b) Maximum frequency excursion for varying repetition rates.

The peak-to-peak frequency excursion was measured for varying chirp rates by changing the repetition rate of the triangular waveform, as plotted in Figure 3(b). This measurement capability could be useful for characterizing and improving the linearity of laser chirps for FMCW LiDAR systems.

Potential Applications

With its high measurement speed, wide optical bandwidth, and accurate frequency tracking, this silicon photonic wavemeter could find numerous applications:

  • Monitor wavelength excursions and improve laser performance for LiDAR, biomedical sensing/imaging, and optical communications

  • Characterize laser behavior such as thermal transients, mode-hopping, and chirp linearity

  • Enable new techniques and algorithms for ranging, sensing, and communications by providing real-time wavelength feedback

  • Integrated low-cost solution leveraging silicon photonics manufacturing

Conclusion

We have demonstrated a silicon photonic wavemeter capable of measuring wavelength changes with sub-200 ns response times across over 80 nm bandwidth. Its ability to rapidly track laser wavelengths opens up new possibilities for applications requiring fast monitoring and feedback of optical frequency dynamics. As an integrated photonic solution, it could enable advanced ranging, sensing, imaging and communication techniques at low cost.

Reference

[1] B. Stern, B. Farah, K. Kim, R. Borkowski, K. Vijayan, F. Ashtiani, and D. Bitauld, "Rapid wavelength measurements with a silicon photonic wavemeter," Nokia Bell Labs, Murray Hill, NJ, USA, and III-V Lab (a joint lab of Nokia Bell Labs, Thales & CEA-LETI), Palaiseau, France, 2024.

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