Universal loss and gain characterization inside photonic integrated circuits

Abstract

Integrated photonics has undergone tremendous development in the past few decades, transforming many fields of study in science and technology. Loss and gain are two fundamental elements in photonic circuits and have direct impacts on nearly all key performance metrics. Surprisingly, the tools to characterize the optical loss and gain inside photonic integrated circuits (PICs) are very limited. This is because, unlike free-space or fiber optics, integrated circuits cannot be nondestructively disassembled. Here, we report a universal method to see inside the photonic integrated circuits and measure loss and gain on the component level nondestructively. The method leverages nonlinear optical devices as optical power discriminators to retrieve the loss and gain information inside the PICs. Our method has a precision better than 0.1 dB, and can characterize the loss of individual fiber-chip coupling facet and general unknown devices under test. As a demonstration of applications, we measured the true on-chip quantum efficiency of a quantum PIC consisting of heterogeneously integrated balanced photodiodes, a critical building block for integrated quantum technology. Our method can be implemented on different photonic platforms, and can be used to understand gain and loss in complex photonic circuits, which is essential to optimize circuit design and to create large-scale systems with predictable, reproducible performance.

Figures (3)

• Figure 1: Illustration of universal loss and gain measurement method for photonic integrated circuits. The method is based on the general concept that if light experiences different losses when it arrives at a nonlinear device from the left path and the right path, then this difference should be reflected by the optical nonlinearity phenomena, e.g., OPO threshold. If $\Delta$ dB more laser power is needed to reach the same nonlinear phenomenon when the laser is injected from the left than from the right, then it means there is $\Delta$ dB more loss on the left-side optical path than on the right-side optical path. The method is illustrated for two general scenarios: (a) Fiber chip coupling loss characterization. Measurement of the total optical loss ($\alpha$), and the in-fiber OPO threshold pumping from the left facet ($P_L$) and right facet ($P_R$) can unveil the individual facet loss of the left ($\alpha_L$) and the right ($\alpha_R$). (b) The general method of measuring an unknown loss/gain ($\alpha_X$) of an on-chip device under test (DUT). (c) An example of characterizing individual facet loss and an unknown component loss (waveguide crossing) in the PIC. All variables are in log scale (dBm or dB).
• Figure 2: Validation of the loss and gain measurement method.(a) Simplified setup for facet loss measurement. The photonic chip is pumped by an amplified single-wavelength laser. An optical switch is used to route the laser to pump the left facet or the right facet. To measure the OPO comb threshold conveniently, a wavelength division multiplexer (WDM) is used to separate the pump wavelength from the comb spectrum. The right coupling facet can be intentionally misaligned to incur additional loss. (b) Optical spectrum of the microresonator output when the pump laser power is below (blue) and above (red) the OPO threshold. Inset: microscopic image of the OPO microresonator. (c) OPO threshold measurements when additional loss is deliberately incurred at the right coupling facet. The OPO threshold when pumping the left facet (blue) stays unchanged, while the OPO threshold when pumping the right facet (red) increases with the incurred loss. (d) The measured left facet loss (blue) and right facet loss (red) versus the total coupling loss when additional loss is incurred at the right facet. The left facet loss stays unchanged, while the right facet loss increases linearly with the incurred loss, with a slope of 1. This validates that our method can clearly identify that the incurred loss is coming from the right facet.
• Figure 3: Measurement of on-chip efficiency of a quantum photonic circuit using thermal-optics nonlinearity.(a) Circuit illustration of the quantum photonic chip. A Kerr microcavity serves as a broadband entanglement and squeezing source, followed by a ring filter to pick up quantum modes at selective wavelengths. The drop-port of the filter is then combined with a local oscillator arm on a 50/50 MMI coupler, and the MMI output goes into a pair of balanced photodiodes heterogeneously integrated on the photonic chip. The efficiency after the Kerr microcavity (the dashed box) directly impacts the entanglement and squeezing quality. This efficiency can be conveniently extracted after the loss measurement of the left coupling facet. (b) Microscopic pictures of the heterogeneously integrated photonic chip, and a zoom-in image of a typical pair of integrated balanced photodiodes. (c) Transmission of the Kerr microcavity when pumping from the left facet (blue, 16.6 dBm in fiber power) and right facet (red, 20.1 dBm in fiber power). The triangle-shaped resonance transmission is caused by the thermal-optics effect of the pump laser. (d) In-fiber pump power versus thermal broadened full-width at half maximum (FWHM) of the transmission resonance. On average, the pump power at the right facet needs to be 3.36 dB higher than the pump power at the left facet to reach the same thermal broadening of the resonance transmission. (e) Photocurrent on the balanced photodiodes versus the on-chip optical power. On-chip optical power is obtained by correcting the in-fiber power with the 3.3 dB left facet loss.