Integrated Wavemeter with 8 Mach-Zehnder Interferometers for the Near-IR Range with Picometer Accuracy

Wavemeters are used to determine laser wavelengths and have many applications, e.g. in medical diagnostics and optical metrology. Examples of applications in the near-infrared are optical coherence tomography (OCT) and swept-source light detection and ranging (SS-LIDAR). [1]–[3] In this work, we demonstrate a chip-scale broadband wavemeter built on the Si<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf>N<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> integrated photonics platform. The fabricated wavemeter consists of eight Mach-Zehnder interferometers (MZIs) with free spectral ranges (FSRs) between 0.1 nm and 50 nm. On the wavemeter chip, the laser light is coupled in by an SU-8 edge coupler [4] and the input beam is equally distributed into all eight MZIs via three stages of 1x2 multimode interference couplers (MMIs). Each MZI output is routed to an edge coupler to illuminate a photodiode in the package. The wavemeter is characterized by first recording a reference spectrum of a tunable laser with a commercial HighFinesse wavemeter. For each wavelength the 8 output signals form an 8-tuple, which ideally is unique in the spectrum. In a following laser sweep we record a measurement spectrum in the same way. For each measurement point, i.e. wavelength, we deduce the laser emission wavelength with our wavemeter by finding the reference wavelength whose output data is the most similar to the measurement point. Technically this algorithm is implemented by calculating the root mean square (RMS) difference between all points in the reference spectrum and the measurement point. The wavelength with the minimum RMS difference is considered the determined wavelength and compared to the true result from the HighFinesse wavemeter. The difference between both wavemeter results is plotted in Fig. 1 as “deviation” and indicates the accuracy of our wavemeter. Using this method, the mean wavelength determination accuracy for the measurement range between 940 nm and 970 nm is 0.16 pm (Fig. 1b) and for the interval between 1010 nm to 1100 nm the mean accuracy is 0.78 pm (Fig. 1c).