Here, we describe the operation of a SiN integrated photonic circuit containing optical phased arrays. The circuits are used to emit low divergence laser beams in the near infrared and steer them in two dimensions.
To develop photonic integrated circuits built on silicon, we need a characterization method that's fast, automated, and at the wafer scale. Our protocol allows us to evaluate our beam-steering circuits on the wafer using a lightly modified prober station, which is a standard piece of equipment in the micro-electronics industry. Demonstrating the procedure, we have Sylvain Guerber, a post-doc researcher from our laboratory.
To begin, load the wafer on your probe station. To align the fibers, use a light microscope to carefully lower the fiber until it touches the wafer surface, away from the input grating coupler, before moving the fiber up about 20 micrometers. To maximize the light intensity at the output gratings, begin sweeping the fiber position over the optical phased array input grating coupler.
The light exiting at the optical phased array output gratings should be visible on the image. When light is observed from the optical phased array antennas, adjust the polarization to maximize the light intensity at the output gratings, taking care to avoid any movement or vibration of the input fiber. For OPA output imaging, switch to the far field imaging sensor and carefully adjust both the exposure time of the sensor and the laser power in such a way that the OPA output is clearly visible on the camera, but the beam does not saturate the sensor.
If necessary, cover the setup, so that the background light does not interfere with the image from the optical phased array beam. To block the reflections, place a highly reflecting sheet between the reflection and the camera. An OPA is by definition extremely sensitive to phase variation.
Therefore, all sources of noises must be suppressed, including input fiber vibrations, polarization instabilities, and parasitic light. To perform beam steering in two directions, first connect the electric circuit for the phase control to a multichannel electric probe and use the microscope to connect the pins of electric probe to the metal contact pads of the optical circuit. Then, switch to far field sensor to image the output.
To select the parallel emission angle theta using the switching network, observe the far field image of the output while varying the voltages applied to phase shifters at the ring resonators. With the correct voltage applied to each resonator, a different area on the sensor will be illuminated, corresponding to a certain theta value. To select the orthogonal emission angle phi by optimizing the optical phased array phases, select a small pixel area corresponding to the desired phi angle that should be illuminated with a focused output beam and shift the phase of one of the optical phased array channels in small increments.
After each shift, record the integral of the brightness in the pixel area inside and outside of the selected area, and calculate the ratio of the inside light divided by the outside light. After a full phase shift cycle between zero and two pi, apply the phase shift with the highest recorded brightness ratio. Then, switch to the next channel and repeat the previous steps until the optimization process in saturated and a focused output beam is visible.
To steer the output beam to a different phi angle, select a new pixel area and repeat the optimization process. Once the optimization has been performed for several output phi angles, the beam can be steered. To image the beam divergence, optimize the position of the input fiber and record the image of the OPA output in the far field.
Make sure that at least two clear interference maxima are visible and use the alignment system to move the wafer to align the next device to the input fiber. Using precision positioners, light from a fiber is able to be efficiently coupled to the integrated optical circuit to obtain a high-intensity output beam. The use of a multichannel probe allows all of the electrical connections to be made simultaneously.
Using an optimization algorithm, a nicely shaped beam can be obtained on the phi access. The use of a ring-based switch allows the proper selection of an emission angle and the theta direction. Once the OPA has been calibrated, the beam can be arbitrarily steered in both dimensions and the steering range and beam divergence, the main figures of merit in an OPA, can be accurately characterized.
It's essential, as much as possible, to eliminate any electronic, mechanical, or optical instabilities during the calibration procedure. Once satisfactory circuits have been identified and calibrated, we can integrate them with the other parts of the LIDAR system, to perform some rudimentary special imaging.
