Spatial-spectral Holography Research Articles

Optical decoherence studies of yttrium oxyorthosilicate Y2SiO5codoped with Er3+and Eu3+for optical signal processing and quantum information applications at 1.5 microns

We report detailed studies of the optical decoherence and spectroscopic properties of the 1536-nm optical transition of the bandwidth-engineered material 0.02$%$Er:1$%$Eu:Y${}_{2}$SiO${}_{5}$. This work is motivated by the need for comprehensive understanding of new resonant optical materials for spatial-spectral holography and quantum information applications. The dependence of optical decoherence on excitation frequency, applied magnetic field strength and orientation, crystal temperature, measurement time scale, and optical excitation density was studied using stimulated photon echo spectroscopy and time-resolved spectral hole-burning techniques. The effects of weak disorder generated by the introduction of Eu${}^{3+}$ ions into the lattice was probed by comparing measurements across the static disorder-induced distribution of transition frequencies as well as by comparing with measurements on 0.02$%$Er:Y${}_{2}$SiO${}_{5}$. The results reveal that the coherence properties of Er:Y${}_{2}$SiO${}_{5}$ are preserved in the disordered system; moreover, the increased inhomogeneity produced by the disorder acts to suppress optical decoherence by inhibiting the electronic spin diffusion process, improving material properties for low magnetic field strengths and at increased temperatures desirable for practical applications.

Optical decoherence and energy level structure of 0.1%Tm3+:LiNbO3

We report the energy level structure of the ${}^{3}$H${}_{6}$ and ${}^{3}$H${}_{4}$ multiplets for Tm${}^{3+}$ doped congruent LiNbO${}_{3}$, as well as the decoherence properties and their temperature dependencies for the ${}^{3}$H${}_{6}(1){\ensuremath{\leftrightarrow}}^{3}$H${}_{4}(1a)$ transition at 794 nm. It is shown that this material provides very significant improvements in bandwidth, time-bandwidth product, and sensitivity for spatial-spectral holographic signal processing devices and quantum memories based on spectral hole burning. The available signal processing bandwidth for 0.1$%$Tm${}^{3+}$:LiNbO${}_{3}$ is 300 GHz versus 20 GHz for Tm${}^{3+}$YAG. The peak absorption coefficient for 0.1$%$Tm${}^{3+}$:LiNbO${}_{3}$ is 15 cm${}^{\ensuremath{-}1}$ at 794.5 nm compared with 1.7 cm${}^{\ensuremath{-}1}$ for 0.1$%$Tm:YAG at 793 nm, and the total absorption strength is eighty times stronger. The oscillator strength for Tm${}^{3+}$:LiNbO${}_{3}$ is about twenty-five times larger than that for Tm${}^{3+}$:YAG, making the material five times more sensitive for processing high-bandwidth analog signals. The homogeneous linewidth, which determines processing time or spectrum analyzer resolution, is 30 kHz at 1.6 K and 350 kHz at 6 K, as measured by photon echoes. Those values establish potential time-bandwidth products of 10${}^{7}$ and 7 $\ifmmode\times\else\texttimes\fi{}$10${}^{5}$, respectively. The temperature dependence of the homogeneous linewidth was explained by observation of a 7.8 cm${}^{\ensuremath{-}1}$ crystal field level in the ground multiplet and direct phonon coupling. The excited state ${}^{3}$H${}_{4}$ lifetime ${T}_{1}$ is 152 $\ensuremath{\mu}$s and the bottleneck lifetime of the lowest ${}^{3}$F${}_{4}$ level is 7 ms from photon echo measurements. These factors combine to provide a surprisingly large increase in key parameters that determine material performance for spatial-spectral holography, quantum information, and other spectral hole burning applications.

Optical decoherence and persistent spectral hole burning in Er 3+:LiNbO 3

Developing new resonant optical materials for spatial-spectral holography and quantum information applications requires detailed knowledge of the decoherence and population relaxation dynamics for the quantum states involved in the optical transitions, motivating the need for fundamental material studies. We report recent progress in studying these properties in erbium-doped lithium niobate at liquid helium temperatures. The influence of temperature, applied magnetic fields, measurement timescale, and dopant concentration were probed using photon echo spectroscopy and time-resolved spectral hole burning on the 1532 nm transition of Er 3+:LiNbO 3. Effects of spectral diffusion due to interactions between Er 3+ ions and between the Er 3+ ion and 7Li and 93Nb nuclear spins in the host lattice were observed. In addition, long-lived persistent spectral storage of seconds to minutes was observed due to non-equilibrium population redistribution among superhyperfine states.

Demonstrations of analog-to-digital conversion using a frequency domain stretched processor

The first proof-of-concept demonstrations are presented for a broadband photonic-assisted analog-to-digital converter (ADC) based on spatial spectral holography (SSH). The SSH-ADC acts as a frequency-domain stretch processor converting high bandwidth input signals to low bandwidth output signals, allowing the system to take advantage of high performance, low bandwidth electronic ADCs. Demonstrations with 50 MHz effective bandwidth are shown to highlight basic performance with approximately 5 effective bits of vertical resolution. Signal capture with 1600 MHz effective bandwidth is also shown. Because some SSH materials span over 100 GHz and have large time apertures (approximately 10 micros), this technique holds promise as a candidate for the next generation of ADCs.

Controlled compositional disorder inEr3+:Y2SiO5provides a wide-bandwidth spectral hole burning material at1.5μm

The subgigahertz spectral bandwidth of the lowest energy $1.5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ ${\mathrm{Er}}^{3+}$ $^{4}I_{15∕2}\ensuremath{\rightarrow}^{4}I_{13∕2}$ optical transition in ${\mathrm{Er}}^{3+}:{\mathrm{Y}}_{2}\mathrm{Si}{\mathrm{O}}_{5}$ has been increased to $\ensuremath{\sim}22\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$ by intentionally introducing compositional disorder through codoping with ${\mathrm{Eu}}^{3+}$ impurity ions. This illustrates a general bandwidth control technique for spectral hole burning device applications including spatial-spectral holography and quantum computing. Coherence measurements by stimulated photon echoes demonstrated that the increased disorder does not perturb the dynamical properties of the ${\mathrm{Er}}^{3+}$ transition and, thus, gives the desired bandwidth enhancement without penalty in other properties. The echo measurements and model analysis also show that phonon-driven spin flips of ${\mathrm{Er}}^{3+}$ ions in the ground state are responsible for the spectral diffusion that was observed for the optical transition. These results collectively give a better understanding of both the nature of disorder and of the ion-ion interactions in doped materials, and they also enable the high bandwidths required for signal processing and memory applications at $1.5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ based on spectral hole burning.

Storing light in optical fibers using sound waves

Increasing traffic on the Internet demands faster optical transmission networks. Future high-speed communication networks may rely largely on all-optical components that are intrinsically faster than their electronic counterparts. To achieve such networks, however, one needs to overcome a number of obstacles that exist at levels from devices to systems. For example, the lack of all-optical memory for storing optical data remains a bottleneck in modern optical information networks.1 There has been substantial progress in addressing this problem using spatial-spectral holography in systems that operate at cryogenic temperatures2 or stored light based on electromagnetically induced transparency, where optical information is impressed upon internal degrees of freedom of an ensemble of atoms or ions.3 Both of these methods, however, are limited to operating at the resonance wavelength of the ions or atoms used. These limitations have recently been partially overcome using a dynamically controlled resonator in which a single optical pulse was stored for ∼100ps.4 We recently presented a different approach: a simple storedlight approach5 in which optical pulse information is transferred onto a slowly moving hypersonic wave (∼1/40,000 of the light speed) in an optical fiber and converted back at a later time through stimulated Brillouin scattering (SBS) as illustrated in Figure 1. In the storage process, optical data pulses (of carrier frequency ν0) interact with a counterpropagating short intense ‘write’ pulse (of carrier frequency ν0 + νB, where νB is the Brillouin frequency shift of the fiber). The optical data pulses become depleted as energy is shifted to a hypersonic wave in the fiber. In the retrieval process, a short intense ‘read’ pulse at the same frequency as the ‘write’ pulse depletes the acoustic wave and releases the data back to the original optical frequency, thereby generating a replica of the incident data pulses. This stored-light approach has advantages such as room-temperature Figure 1. Stimulated Brillouin scattering allows data storage in an optical fiber. In the storage process, (a) optical data pulses interact with a ‘write’ pulse . (b) The optical data pulses become depleted as the information shifts to an acoustic wave in the fiber. To retrieve the data, (c) a ‘read’ pulse depletes the acoustic wave, (d) generating a replica of the original data pulses.5

Broadband photonic arbitrary waveform generation based on spatial-spectral holographic materials

We discuss an approach for the practical implementation of photonic arbitrary waveform generation of microwave signals. We describe and demonstrate an approach using spatial-spectral (S2) holography in rare earth ion doped crystals that has the potential to achieve extremely wide bandwidths (>40 GHz) using conventional electro-optic phase modulators and low bandwidth (<100 MHz) control electronics. We provide analysis of this approach, show simulations, and perform experimental demonstrations of the technique. We show a pulse compression factor of ~15,000 and demonstrate the largest effective bandwidth of 3.8 GHz to date for pulse compression using S2 holography. We also show control and manipulation of up to 30 independent compressed pulses for the creation of arbitrary waveforms.

Demonstration of a spatial–spectral holographic LIDAR range-Doppler processor

We present a new approach to laser interferometric Doppler and ranging (LIDAR) processing using spatial–spectral holography (SSH). In this approach, broadband optical signals from a random noise or frequency-modulated laser are transmitted and reflected off remote targets. The return signals interfere spatially and spectrally with a local copy of the original transmit signal in an SSH medium, resulting in spectral gratings that have a spectral period inversely proportional to the LIDAR target's range and a position proportional to the target's Doppler (or velocity). These gratings are subsequently read out by a slowly chirped source onto a parallel detector array, and the velocity and range of the targets are inferred. We present the theoretical framework that describes the function of the LIDAR processor, as well as proof-of-concept experimental results.

Broadband analog to digital conversion with spatial-spectral holography

A new approach to broadband photonic-assisted analog-to-digital converter (ADC) technology is proposed and analyzed. The core of the device is a spatial spectral holographic (SSH) material, which can directly record the signals of interest in the frequency domain. An SSH-ADC acts as a frequency-domain stretch processor, which leverages the high performance of conventional ADCs by converting high bandwidth input signals to low bandwidth output signals without loss of information. Analysis of a 10 GHz bandwidth SSH-ADC predicts that 10-bit performance can be achieved with currently available materials and components. SSH-ADC technology is scalable to bandwidths over 100 GHz with recently developed SSH materials. While the SSH-ADC is a transient digitizer, the spatial parallelism of SSH materials can be utilized to enable continuous digitization.

Laser frequency stabilization at 1.5 microns using ultranarrow inhomogeneous absorption profiles in Er 3+:LiYF 4

Single-frequency diode lasers have been frequency stabilized to 1.5 kHz Allan deviation over 0.05–50 s integration times, with laser frequency drift reduced to less than 1.4 kHz/min, using the frequency reference provided by an ultranarrow inhomogeneously broadened Er 3+: 4I 15/2→ 4I 13/2 optical absorption transition at a vacuum wavelength of 1530.40 nm in a low-strain LiYF 4 crystal. The 130 MHz full-width at half-maximum (FWHM) inhomogeneous line width of this reference transition is the narrowest reported for a solid at 1.5 μm. Strain-induced inhomogeneous broadening was reduced by using the single isotope 7Li and by the very similar radii of Er 3+ and the Y 3+ ions for which it substitutes. To show the practicability of cryogen-free cooling, this laser stability was obtained with the reference crystal at 5 K; moreover, this performance did not require vibrational isolation of either the laser or crystal frequency reference. Stabilization is feasible up to T=25 K where the Er 3+ absorption thermally broadens to ∼500 MHz. This stabilized laser system provides a tool for interferometry, high-resolution spectroscopy, real-time optical signal processing based on spatial spectral holography and accumulated photon echoes, secondary frequency standards, and other applications such as quantum information science requiring narrow-band light sources or coherent detection.

Ultrawideband coherent noise lidar range-Doppler imaging and signal processing by use of spatial-spectral holography in inhomogeneously broadened absorbers

We introduce a new approach to coherent lidar range-Doppler sensing by utilizing random-noise illuminating waveforms and a quantum-optical, parallel sensor based on spatial-spectral holography (SSH) in a cryogenically cooled inhomogeneously broadened absorber (IBA) crystal. Interference between a reference signal and the lidar return in the spectrally selective absorption band of the IBA is used to sense the lidar returns and perform the front-end range-correlation signal processing. Modulating the reference by an array of Doppler compensating frequency shifts enables multichannel Doppler filtering. This SSH sensor performs much of the postdetection signal processing, increases the lidar system sensitivity through range-correlation gain before detection, and is capable of not only Doppler processing but also parallel multibeam reception using the high-spatial resolution of the IBA crystals. This approach permits the use of ultrawideband, high-power, random-noise, cw lasers as ranging waveforms in lidar systems instead of highly stabilized, injection-seeded, and amplified pulsed or modulated laser sources as required by most conventional coherent lidar systems. The capabilities of the IBA media for many tens of gigahertz bandwidth and resolution in the 30-300 kHz regime, while using either a pseudo-noise-coded waveform or just a high-power, noisy laser with a broad linewidth (e.g., a truly random noise lidar) may enable a new generation of improved lidar sensors and processors. Preliminary experimental demonstrations of lidar ranging and simulation on range-Doppler processing are presented.

Multidimensional spatial-spectral holographic interpretation of NMR photography

A spectral holographic interpretation arises naturally in nuclear magnetic resonance (NMR) photography from either the intrinsic chemical shift anisotropy of the spin system or the field inhomogeneity due to the applied spatial encoding gradients. We can thus think of NMR photography as arising from a "diffraction" off a spatial-spectral holographic grating. The spatial holographic component arises from a high dielectric constant (>50) of the NMR medium at high field strength (>4 T) when the excitation wavelength is commensurate with the size of the NMR sample; otherwise, it is a volume spectral holographic grating. In this paper, the NMR localized spectroscopy (imaging) equation is derived from the principles of spatial-spectral holography. Holographic properties of storage and programmable time delay and time reversal are shown to follow naturally from this viewpoint and are experimentally demonstrated in an inhomogeneously broadened NMR sample. These ideas are shown to be extendable to complex signal processing functions such as recognition, correlations, and triple products.

Optical decoherence and spectral diffusion at1.5μminEr3+:Y2SiO5versus magnetic field, temperature, andEr3+concentration

The mechanisms and effects of spectral diffusion for optical transitions of paramagnetic ions have been explored using the inhomogeneously broadened 1536 nm $^{4}\mathrm{I}_{15∕2}\ensuremath{\rightarrow}^{4}\mathrm{I}_{13∕2}$ transition in ${\mathrm{Er}}^{3+}:{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}$. Using photon echo spectroscopy, spectral diffusion was measured by observing the evolution of the effective coherence lifetimes over time scales from $1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{s}$ to 20 ms for magnetic-field strengths from 0.3 to 6.0 T, temperatures from 1.6 to 6.5 K, and nominal ${\mathrm{Er}}^{3+}$ concentrations of 0.0015%, 0.005%, and 0.02%. To understand the effect of spectral diffusion on material decoherence for different environmental conditions and material compositions, data and models were compared to identify spectral diffusion mechanisms and microscopic spin dynamics. Observations were successfully modeled by ${\mathrm{Er}}^{3+}\ensuremath{-}{\mathrm{Er}}^{3+}$ magnetic dipole interactions and ${\mathrm{Er}}^{3+}$ electron spin flips driven by the one-phonon direct process. At temperatures of 4.2 K and higher, spectral diffusion due to $^{89}\mathrm{Y}$ nuclear spin flips was also observed. The success in describing our extensive experimental results using simple models provides an important capability for exploring larger parameter spaces, accelerating the design and optimization of materials for spatial-spectral holography, and spectral hole-burning devices. The broad insight into spectral diffusion mechanisms and dynamics is applicable to other paramagnetic materials, such as those containing ${\mathrm{Yb}}^{3+}$ or ${\mathrm{Nd}}^{3+}$.

Demonstration of a continuous scanner and time-integrating correlator using spatial–spectral holography

We present a new approach to time-integrating correlation using spatial–spectral holography in which an optically modulated temporal input is spatially scanned by diffracting it from an appropriately programed spatial–spectral grating. The grating required for this scanning operation is programed into the inhomogeneously broadened medium by interfering the image of the traveling-wave diffracted light from an acousto-optic deflector (AOD) with the temporally modulated light from an acousto-optic modulator, thereby recording a spatial–spectral holographic copy of the traveling-wave scanning characteristics of the AOD. This scrolling diffracted field is interfered with a temporally modulated correlation reference and accumulated on a 1-D CCD photodetector array, and is thus quite analogous to an acousto-optic time-integrating correlator (TIC), but without the inherent bandwidth limits of acoustic-wave propagation. This spatial–spectral holographic TIC is experimentally demonstrated at bandwidths of about 30 MHz with 150 resolvable spots and 10 ms of coherent integration time, and the prospect for much wider bandwidth, longer integration time, larger processing gain, and higher resolution are explored.

Semiconductor lasers stabilized to spectral holes in rare earth crystals to a part in 10 13 and their application to devices and spectroscopy

Single-frequency diode lasers have been stabilized to 200 Hz at 1.5 μm and independently to 20 Hz at 793 nm over 10-ms integration times using narrow spectral holes in the absorption lines of Er 3+- and Tm 3+-doped cryogenic crystals as frequency references. Kilohertz stability over 100-s integration times is provided by these techniques, and that performance should be extendable to long integration times with further development. The achieved frequency stabilization provides ideal lasers for high-resolution spectroscopy in the time and frequency domains, real time analog optical signal processing based on spatial-spectral holography, interferometry, and other applications requiring ultra-narrow-band light sources or coherent detection. The stabilized lasers have enabled demonstrations of analog optical signal processing in Er 3+ materials at 0.5 GHz bandwidths at temperatures of 4.2 K, and they will be important for electromagnetically induced transparency and quantum information demonstrations.

The distribution of impurities in silicon epitaxial films : O. Planer, Tesla Electron., April (1969), p. 117

We present a new approach to time-integrating correlation using spatial–spectral holography in which an optically modulated temporal input is spatially scanned by diffracting it from an appropriately programed spatial–spectral grating. The grating required for this scanning operation is programed into the inhomogeneously broadened medium by interfering the image of the traveling-wave diffracted light from an acousto-optic deflector (AOD) with the temporally modulated light from an acousto-optic modulator, thereby recording a spatial–spectral holographic copy of the traveling-wave scanning characteristics of the AOD. This scrolling diffracted field is interfered with a temporally modulated correlation reference and accumulated on a 1-D CCD photodetector array, and is thus quite analogous to an acousto-optic time-integrating correlator (TIC), but without the inherent bandwidth limits of acoustic-wave propagation. This spatial–spectral holographic TIC is experimentally demonstrated at bandwidths of about 30MHz with 150 resolvable spots and 10ms of coherent integration time, and the prospect for much wider bandwidth, longer integration time, larger processing gain, and higher resolution are explored.