A simple coherent laser sensor

September 10, 2015

The Red Pitaya is a small network-attached instrument that can be thought of as a programmable oscilloscope. It features two analog inputs and two analog outputs, both with 14 bits resolution and 125 MHz sampling frequency. Although this is not the most powerful card on the market, its versatility and its price tag make it an ideal candidate to prototype custom instruments.

The Red Pitaya is based on the Xilinx Zynq SoC, which combines a Dual ARM Cortex-A9 and a FPGA on the same chip. Custom real-time algorithms can be implemented on the Programmable Logic (FPGA), while the Processing System (ARM) can host a fully featured Linux OS. Unlike boards such as the Raspberry Pi or the BeagleBone, the Red Pitaya does not have HDMI output. However, it can easily host a modern network stack to communicate with the client through TCP, HTTP or the WebSocket protocol.

We designed a small laser board which fits on top of the Red Pitaya. The board contains a 1550 nm DFB laser diode and a transimpedance amplified photodetection. Laser current can be modulated through the analog outputs of the Red Pitaya. We used the laser board to setup a simple fibered Mach-Zehnder interferometer as shown in the picture below.

Mach-Zehnder interferometer

The laser linewidth is about 10 MHz, which corresponds to a coherence length of about 30 m. A 1 m fiber patch cable has been inserted on one interferometer arm. This corresponds to a delay small enough to observe interference fringes without being disturbed by the laser phase noise. The fringes can be observed temporally by modulating the laser diode current. When the current increases, the laser chip heats up and expands, which has the effect of increasing the laser wavelength. Interference fringes are scanned as the laser wavelength varies, as shown in the animation below:

Fringes animation