Optical steering of a large ring laser

TL;DR

Large gas ring lasers suffer multi-mode operation because the cavity spacing $f_{FSR}=c/P$ becomes small relative to the Doppler-broadened gain width, leading to mode hops that undermine metrology. The authors introduce an all-optical injection-locking method using an external diode laser to steer lasing to a chosen longitudinal mode index, with the injected mode following on a timescale set by the cavity decay. In a 14 m ring ( $f_{FSR}=21.42\,\mathrm{MHz}$ ), the injected mode tracks the external frequency within the $f_{FSR}$ window, robustly down to roughly $18\,\mu\mathrm{W}$ and on timescales of about $270\,\mu\mathrm{s}$, while injections at $2f_{FSR}$ do not lock and can produce backscatter-driven split-mode dynamics. The results imply that extended injection schemes could stabilize both directions and substantially increase uptime for large-ring gyroscopes, a virtue that grows as ring size—and thus the number of potential modes—increases (e.g., $f_{FSR}$ down to a few MHz for very long perimeters).

Abstract

A common approach to reduce the linewidth of a laser is an increase of its resonator length. In large gas lasers, however, the frequency spacing between longitudinal modes of the resonator easily becomes significantly smaller than the Doppler-broadened width of the gain profile. As a consequence, the laser might operate on a multitude of modes simultaneously, or jump between modes. Such unstable operation cannot be tolerated in metrological or sensing applications, such as ring laser gyroscopes. Here, we propose and demonstrate a method to establish stable operation on a chosen mode index by optically steering the ring laser to a desired mode index through injection locking with an external laser. The injected mode reliably follows the external steering. Intra-cavity backscattering can even cause the counter-propagating, non-injected mode to follow the external steering as well.

Figures (3)

• Figure 1: Schematic view of the 14m perimeter ring laser cavity. The Sagnac and $f_{\text{FSR}}$ beats are measured with different photodiodes at the north corner. The external laser is injected in clockwise direction at the east corner. At the west corner, the clockwise transmission is coupled into a fiber to be evaluated by a wavelength meter and the counter-clockwise transmission is used to stabilize the lasing intensity of the active ring laser.
• Figure 2: Wavelength meter measurement of the active ring laser frequency (red) and the frequency of the diode laser beam, when it is injected into the ring cavity (blue). a) The active laser modes follow the frequency of the injected beam within $f_{0}\pm 1f_{\text{FSR}}$. b) The active laser does not follow injections at frequencies of $f_{0}\pm2f_{\text{FSR}}$, jumping to a frequency within $f_{0}\pm1f_{\text{FSR}}$.
• Figure 3: a) Amplitude swelling in the PD$_{\text{SAG}}$ signal after injections. The first and second datasets show an intact Sagnac signal after about 1s. The third dataset shows an injection that yields no Sagnac signal after mode competition dynamics over many seconds. b) Insert showing the Sagnac beat of about 312Hz, observed during common mode operation. c) Example of the PD$_{\text{FSR}}$ signal showing the $f_{\text{FSR}} =$ 21.42M Hz beat if split mode operation is present, as in the indicated position in a).