Phase noise characterization of sub-hertz linewidth lasers via digital cross correlation

Abstract

Phase noise or frequency noise is a key metrics to evaluate the short term stability of a laser. This property is of a great interest for the applications but delicate to characterize, especially for narrow line-width lasers. In this letter, we demonstrate a digital cross correlation scheme to characterize the absolute phase noise of subhertz line-width lasers. Three 1,542 nm ultra-stable lasers are used in this approach. For each measurement two lasers act as references to characterize a third one. Phase noise power spectral density from 0.5 Hz to 0.8 MHz Fourier frequencies can be derived for each laser by a mere change in the configuration of the lasers. This is the first time showing the phase noise of sub-hertz linewidth lasers with no reference limitation. We also present an analysis of the laser phase noise performance.Laser phase noise describes how the phase of a laser output electrical field deviates from an ideal sinusoidal wave. This quantity which is defined to evaluate the short term stability of a laser can also be used to estimate the line-width or coherent length of a laser. In many applications, such as coherent optical communication [1], LIDAR [2], optical fiber-based interferometeric sensors [3], high resolution spectroscopy [4], ultra-low phase noise photonic microwave generation [5], and optical atomic clock [6], the laser phase noise can profoundly impacts the limitation of a system. Thus, lasers with ultra-low phase noise are actively studied [7][8][9][10]; while the precise characterization of such ultrastable laser is becoming more important.Phase noise characterization is a comparison process. Generally, laser phase noise measurement approaches can be divided into two categories according to the comparison method. The first one is comparing the laser under test with itself through the schemes of delayed self-homodyne, delayed self-heterodyne [11,12] or Michelson interferometer [13]. Several kilometers of fiber are usually used for these optical delay line methods in order to make the delay time longer than the laser coherent time. It is difficult to characterize the phase noise of lasers with hundreds-hertz linewidth or narrower via these delay line technique as thousands of kilometers of fiber would be required, and the fiber noise itself can also become a limitation. The second category, which is called the beat-note method [14][15][16][17], implies comparing the laser under test with a reference laser whose phase noise is much lower than that of the one under test. When the phase noise of the laser under test is lower than that of any available reference, two similar lasers must be built and compared. Assuming statistical independence and equal contribution of both lasers, the phase noise is revealed after division of the beat-note phase noise by √2. However, to realize two equally good lasers is not straightforward. Cross correlation is a well know approach to characterize the phase noise of RF and microwave oscillators with ultra-low level [5,[18][19][20][21][22]. Here, we extend ...

High-precision methanol spectroscopy with a widely tunable SI-traceable frequency-comb-based mid-infrared QCL

“…The NIR frequency reference signal is given by a 1542-nm ultra-stable laser operated at LNE-SYRTE laboratory. The latter consists of a laser diode locked to an ultra-stable cavity [39,40]. It exhibits a relative frequency stability lower than 10 -15 between 0.1 s and 10 s. Its absolute frequency is measured using an OFC once per second against primary frequency standards with a total uncertainty of a few 10 -14 [41].…”

Highly-Sensitive Phase and Frequency Noise Measurement Technique Using Bayesian Filtering

“…The reason for this is that the methods reported in [3], [4], are not a solution to the MMSE problem and thereby do not employ any filtering of the measurement noise. Another widely used approach, not compared to in this paper, is based on cross-correlation [5]. The phase noise measurement based on cross-correlation does employ long-term averaging to reduce the impact of measurement noise.…”

“…It should be noted that the phase and frequency noise measurement techniques reported in [3], [4], [5] do provide accurate phase and frequency noise measurements if the optical power is sufficiently high and the frequency range of interest is not too large (up to ∼10 MHz). However, the conventional methods do not scale well for low optical signal powers and when measuring frequencies exceeding ∼10 MHz (SNR decreases for increasing measurement bandwidth).…”

“…However, the conventional methods do not scale well for low optical signal powers and when measuring frequencies exceeding ∼10 MHz (SNR decreases for increasing measurement bandwidth). Unfortunately, there is no reported work on the accuracy of phase noise measurement techniques presented in [3], [4], [5] as a function of SNR or LNR.…”

Searching for Dark Matter with an Optical Cavity and an Unequal-Delay Interferometer

“…1. A 1542 nm laser source is stabilized on an ultrastable cavity [28,29], with a locking bandwidth of a few 100 kHz. The beam power is then unevenly distributed between the three arms.…”