Quantifying the Noise Floor Limitations of Laser Frequency Noise Measurements

The impact and limitations of the electrical noise floor on a laser frequency noise spectrum measurement technique are investigated in detail. Understanding the measurement noise floor is crucial to measure the linewidth of sub-kilohertz lasers. All of the sources of electrical noise are analysed to evaluate the measurement noise floor: quantisation noise in the oscilloscope, thermal noise in the photoreceiver, shot noise and laser relative intensity noise. We show that quantisation noise in the oscilloscope can be made sufficiently small such that the thermal noise in the photoreceiver or the laser relative intensity noise dominates. In our analysis, we show that the frequency noise floor of the measurement is very sensitive to the relative intensity noise due to the optical heterodyning. This work allows us to predict the noise floor level and thus to specify relevant technical requirements in the measurement system, such as the responsivity of the photoreceiver and the effective number of bits of the oscilloscope. This work is crucial for constructing a highly sensitive laser frequency noise measurement system with the most basic components. The accurate characterization of laser phase noise is also a vital parameter when employing these lasers in applications such as LIDAR, quantum sensors, gas monitoring, etc.