Optical steering of a large ring laser

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.

💡 Research Summary

In this work the authors address a long‑standing problem in large‑scale gas ring lasers: the free spectral range (FSR) of the resonator becomes much smaller than the Doppler‑broadened gain bandwidth, so many longitudinal modes can lase simultaneously or the laser can hop between modes. This multi‑mode behavior is unacceptable for precision applications such as ring‑laser gyroscopes, where a single, well‑defined mode is required for stable Sagnac detection.

The paper proposes and experimentally demonstrates a purely optical solution based on injection locking. An external diode laser (TOPTICA DL‑PRO, ≈100 kHz intrinsic linewidth) is frequency‑stabilized to a cavity resonance using a Pound‑Drever‑Hall (PDH) lock and injected into the clockwise direction of a 14 m perimeter He‑Ne ring cavity (FSR = 21.42 MHz, finesse ≈ 36 000). The injected power is modest (185 µW total, ≈11 % coupling efficiency), and the injection can be switched on and off with an acousto‑optic modulator (AOM) in a time comparable to the cavity decay time τ ≈ 270 µs.

When the injection laser is tuned to the frequency of the active laser or to one of the two adjacent longitudinal modes (i.e., f₀ ± 1 FSR), the active He‑Ne laser follows the injected frequency with 100 % probability. The transition occurs within a few milliseconds, limited by the cavity photon lifetime, and the laser remains locked to the chosen mode for many hours, only drifting when the cavity length changes enough to cause a mode hop. The success rate is independent of injection power down to the lowest usable level (≈18 µW circulating power, still about twice the active laser power), demonstrating the robustness of the technique.

If the injection frequency is set farther away (f₀ ± 2 FSR or more), the active laser does not lock to the injected frequency. Instead, it jumps to a random mode within the ±1 FSR band, indicating that the injection strength is insufficient to overcome the flat gain profile over a larger frequency offset. Consequently, the reliable steering range is limited to roughly one FSR on either side of the original mode (≈42–86 MHz for this cavity).

The authors also investigate the behavior of the counter‑clockwise (non‑injected) mode. Due to intracavity back‑scatter, the non‑injected direction can be pulled toward the injected mode, but the dynamics are more complex. After injection, the counter‑clockwise mode may initially enter a split‑mode regime where the two directions lase on different longitudinal indices, producing a beat at the FSR frequency in the PD‑FSR detector and suppressing the Sagnac beat. Over a timescale of 0.8–1.5 s, mode competition mediated by back‑scatter either collapses to a common mode (restoring the Sagnac signal) or settles into a persistent split‑mode state. Statistical analysis shows that in about two‑thirds of the cases the counter‑clockwise mode ends up on the same index as the injected direction, while the injected direction always follows the injection.

The experimental setup includes a high‑finesse square cavity (four mirrors, R = 4 m, finesse ≈ 36 000), a low‑power He‑Ne plasma cell, and a set of photodiodes for monitoring the Sagnac beat (PD SAG) and the FSR beat (PD FSR). A wavelength meter (HighFinesse WS8) provides continuous frequency readout with 1 MHz resolution at a 1.75 s sampling interval. The injection laser is frequency‑modulated at 5 MHz for PDH locking and shifted by 200 MHz with the AOM when needed.

The main conclusions are: (1) optical injection locking can deterministically select a longitudinal mode in a large gas ring laser with negligible added power; (2) the method works for any of the three adjacent modes (current, +1 FSR, –1 FSR) and is robust down to low injection powers; (3) the counter‑propagating mode can be pulled into the same index via back‑scatter, suggesting that a dual‑injection scheme could eliminate split‑mode operation entirely; (4) this technique removes the need for repetitive power cycling, thereby increasing the usable duty cycle of ring‑laser gyroscopes toward 100 %; and (5) the approach scales to even larger rings (e.g., the 100 m UG2 laser with FSR ≈ 2.5 MHz), where conventional methods would be impractical.

Overall, the paper provides a clear, experimentally validated pathway to achieve stable, single‑mode operation in large‑scale ring lasers using only brief, low‑power optical injection, opening the door to higher‑precision rotation sensing and other metrological applications that rely on long‑baseline laser cavities.