Numerous applications demanding precise control over laser output necessitate frequency modulation of emitted light. Frequency modulation techniques, notably integral to the Pound-Drever-Hall (PDH) stabilization scheme, find widespread use in stabilizing lasers to cavities or atomic transitions. With Quantum Technology's emergence, the use of stabilized lasers is growing, particularly in systems where photon interaction with atomic transitions underpins desired functionalities. This spans applications such as quantum computing based on ions and atoms, and quantum sensors like atomic clocks, atom interferometers, and Rydberg sensors.
In PDH locking, the achievable stability through locking the laser to a reference (cavity or atomic cell), hinges on several parameters that determine the signal-to-noise ratio (SNR) of the error signal, crucial for maintaining a stable lock. While there's a ceiling to the intensity of the signal that can be used in such a system, minimizing noise is the most beneficial route to enhance SNR. Among various noise sources—electronic circuitry and optical implementations—fluctuations in relative amplitude modulation (RAM) pose a significant challenge to minimize and are a major source of stress to the system designers. RAM in PDH schemes arises from unequal sidebands imprinted by electrooptic modulators on laser light, manifesting as unequal amplitude or phase, lor both. This results in additional noise around the modulation frequency (frequency offset noise) which limits the achievable SNR in the error signal, and thus the quality of the lock. Additionally, RAM limits the accuracy of cavity or atomic line center determination.
Since sideband fluctuations relate to light polarization in the modulator—which is sensitive to temperature variations—and linearity with applied frequency, passive approaches to reducing RAM involve meticulous modulator selection and temperature stabilization. Active techniques, like specialized circuitry employed in a feedback configuration, can reduce RAM by 60 dB (W. Zhang et al., OPTICS LETTERS / Vol. 39, No. 7, April 1, 2014). This, however, is obtained at the cost of increased complexity, size, power consumption and cost of the system.
OEwaves lasers overcome the limitations of RAM while providing ultra-low noise (sub-Hz linewidth) output in a compact and cost-effective package. One of the attractive features of OEwaves ultra-narrow linewidth lasers is that they can be modulated with RAM at the level of 80 dB below the signal level or smaller. This is achieved by imparting the modulation of light through changing the index of refraction of the crystalline whispering gallery mode resonator (WGM) that serves to produce the ultra-low noise output. In this configuration, when the frequency of the cavity mode changes, the frequency of the laser also changes.
Attaching a PZT element to the resonator and actuating it with voltage induces stress and changes the resonator’s index of refraction and the resonator mode frequency. This process is highly linear, resulting in equal sidebands in amplitude and frequency (phase) on the laser carrier and ensuring the extremely low RAM levels mentioned above.
For further information on how OEwaves can reduce your stress associated with solving laser stabilization challenges, please contact sales@oewaves.com.