Laser Stabilization at 1536 nm Using Regenerative Spectral Hole Burning

Laser frequency stabilization giving a 500-Hz Allan deviation for a 2-ms integration time with drift reduced to 7 kHz/min over several minutes was achieved at 1536 nm in the optical communication band. A continuously regenerated spectral hole in the inhomogeneously broadened 4I15/2(1)!4I13/2(1) optical absorption of an Er31:Y2SiO5 crystal was used as the short-term frequency reference, while a variation on the locking technique allowed simultaneous use of the inhomogeneously broadened absorption line as a long-term reference. The reported frequency stability was achieved without vibration isolation. Spectral hole burning frequency stabilization provides ideal laser sources for high-resolution spectroscopy, real-time optical signal processing, and a range of applications requiring ultra-narrow-band light sources or coherent detection; the time scale for stability and the compatibility with spectral hole burning devices make this technique complementary to other frequency references for laser stabilization

Rare-Earth-Doped Materials with Application to Optical Signal Processing, Quantum Information Science, and Medical Imaging Technology

Unique spectroscopic properties of isolated rare earth ions in solids offer optical linewidths rivaling those of trapped single atoms and enable a variety of recent applications. We design rare-earth-doped crystals, ceramics, and fibers with persistent or transient “spectral hole” recording properties for applications including high-bandwidth optical signal processing where light and our solids replace the high-bandwidth portion of the electronics; quantum cryptography and information science including the goal of storage and recall of single photons; and medical imaging technology for the 700-900 nm therapeutic window. Ease of optically manipulating rare-earth ions in solids enables capturing complex spectral information in 105 to 108 frequency bins. Combining spatial holography and spectral hole burning provides a capability for processing high-bandwidth RF and optical signals with sub-MHz spectral resolution and bandwidths of tens to hundreds of GHz for applications including range-Doppler radar and high bandwidth RF spectral analysis. Simply stated, one can think of these crystals as holographic recording media capable of distinguishing up to 108 different colors. Ultra-narrow spectral holes also serve as a vibration-insensitive sub-kHz frequency reference for laser frequency stabilization to a part in 1013 over tens of milliseconds. The unusual properties and applications of spectral hole burning of rare earth ions in optical materials are reviewed. Experimental results on the promising Tm3+:LiNbO3 material system are presented and discussed for medical imaging applications. Finally, a new application of these materials as dynamic optical filters for laser noise suppression is discussed along with experimental demonstrations and theoretical modeling of the process

Highly multimode memory in a crystal

We experimentally demonstrate the storage of 1060 temporal modes onto a thulium-doped crystal using an atomic frequency comb (AFC). The comb covers 0.93 GHz defining the storage bandwidth. As compared to previous AFC preparation methods (pulse sequences i.e. amplitude modulation), we only use frequency modulation to produce the desired optical pumping spectrum. To ensure an accurate spectrally selective optical pumping, the frequency modulated laser is self-locked on the atomic comb. Our approach is general and should be applicable to a wide range of rare-earth doped material in the context of multimode quantum memory

Material Optimization of Er3+Y2SiO5 at 1.5 μm for Optical Processing, Memory, and Laser Frequency Stabilization Applications

Spatial-spectral holography using spectral hole burning materials is a powerful technique for performing real-time, wide-bandwidth information storage and signal processing. For operation in the important 1.5 μm communication band, the material Er3+:Y2SiO5 enables applications such as laser frequency stabilization, all-optical correlators, analog signal processing, and data storage. Site-selective absorption and emission spectroscopy identified spectral hole burning transitions and excited state T1 lifetimes in the 1.5 μm spectral region. The effects of crystal temperature, Er3+-dopant concentration, magnetic field strength, and crystal orientation on spectral diffusion were explored using stimulated photon echo spectroscopy, which is the “prototype” interaction mechanism for device applications. The performance of Er3+:Y2SiO5 and related Er3+ materials has been dramatically enhanced by reducing the effect of spectral diffusion on the coherence lifetime T2 through fundamental material design coupled with the application of an external magnetic field oriented along specific directions. A preferred magnetic field orientation that maximized T2 by minimizing the effects of spectral diffusion was determined using the results of angle-dependent Zeeman spectroscopy. The observed linewidth broadening due to spectral diffusion was successfully modeled by considering the effect of one-phonon (direct) processes on Er3+ - Er3+ interactions. The reported studies improved our understanding of Er3+ materials, explored the range of conditions and material parameters required to optimize performance for specific applications, and enabled measurement of the narrowest optical resonance ever observed in a solid—with a homogeneous linewidth of 73 Hz. With the optimized materials and operating conditions, photon echoes were observed up to temperatures of 5 K, enabling 0.5 GHz bandwidth optical signal processing at 4.2 K and providing the possibility for operation with a closed-cycle cryocooler

Quadratic Zeeman Spectral Diffusion of Thulium Ion Population in a Yttrium Gallium Garnet Crystal

The creation of well understood structures using spectral hole burning is an important task in the use of technologies based on rare earth ion doped crystals. We apply a series of different techniques to model and improve the frequency dependent population change in the atomic level structure of Thulium Yttrium Gallium Garnet (Tm:YGG). In particular we demonstrate that at zero applied magnetic field, numerical solutions to frequency dependent three-level rate equations show good agreement with spectral hole burning results. This allows predicting spectral structures given a specific hole burning sequence, the underpinning spectroscopic material properties, and the relevant laser parameters. This enables us to largely eliminate power dependent hole broadening through the use of adiabatic hole-burning pulses. Though this system of rate equations shows good agreement at zero field, the addition of a magnetic field results in unexpected spectral diffusion proportional to the induced Tm ion magnetic dipole moment and average magnetic field strength, which, through the quadratic Zeeman effect, dominates the optical spectrum over long time scales. Our results allow optimization of the preparation process for spectral structures in a large variety of rare earth ion doped materials for quantum memories and other applications