Technology

Coherent modulated lidar/LADAR systems, allow much higher detection sensitivity and accuracy compared to the more common pulsed-beam based lidar systems. The FMCW illumination beam, for instance, is constant in intensity, but swept in optical frequency. The crucial operating principle of FMCW lidar is the coherent ‘mixing’ of a portion of the modulated laser light that is split off from the transmit beam, called the local oscillator, with the light received from the target to produce a beat signal whose frequency is proportional to the target range. This unique “matched filter” correlation blocks any background radiation or signal interference. By using appropriate modulated waveforms and signal processing, the Doppler signature (i.e. velocity) of the target can be measured simultaneously with the range. For FMCW lidar, the signal-to-noise ratio is proportional to the total number of received photons and not to the peak power. Thus, FMCW lidar has several orders of magnitude greater sensitivity than pulsed time-of-flight lidar, with much lower peak powers.

The advantages of FMCW lidar are:

• High Selectivity
• High Sensitivity
• High Dynamic Range
• High Bandwidth and Spatial resolution

Holography is an interferometric-diffractive imaging technique, which allows recording and reconstruction of three-dimensional (3D) images of objects. The two-step process involves recording the interference pattern formed by a reference beam and an object beam (reflected from or transmitted by the object), followed by image reconstruction. The interference pattern of the reference coherent wavefront with the wavefront scattered from the object's surface forms an intensity pattern on the surface of a high-resolution recording film. This recorded interference pattern (called a hologram) contains information about both the amplitude and phase of the object's wave field. The stored information can be reconstructed by illuminating the hologram with a reference wave, resulting in an image with three-dimensional features exhibiting all the effects of perspective and depth of focus that the object would exhibit. This remarkable technology has found many applications in diverse fields such as displays, vibrometry, and microscopy. Producing holograms requires strict phase coherence of the object and reference beam. Laser noise, turbulence, or object movement or vibration can significantly degrade the hologram.

The development of fast and high-resolution optoelectronic devices, such as CCD and SWIR cameras, has enabled the replacement of holographic film by digital cameras. In Digital Holography (DH), the hologram is recorded digitally and image reconstruction is done numerically. As in the case of classical holography, the light scattered from an object illuminates the image sensor along with a reference wave, and the interference pattern is digitally recorded. A 2D Fourier transform of the recorded data localizes the object-wave information. After masking of the Fourier transform to extract the necessary amplitude and phase information, the result is inverse 2D Fourier transformed to reveal the complex light field of the object.  Access to the phase and amplitude of the object’s return light allows for additional capabilities not possible with standard holography, such as post-recording image focusing and phase front correction (e.g. turbulence mitigation).

At the core of this technology are Spatial Spectral Holographic (SSH) materials that have unique intrinsic frequency selective properties, as well as high spatial resolution. The materials typically contain rare earth ions like Tm (793 nm) or Er (1.5 microns) that are doped into a crystal or glass. Typical hosts include oxide crystals like YAG, a common laser material, or, more recently, LiNbO₃, as well as other crystals, glasses, and organic polymers.

The frequency selectivity of SSH materials comes from the sharp optical resonance of individual ions doped into the host material. The line-widths of individual dopants (the homogeneous line-width) can be less than a kilohertz at cryogenic temperatures (2-6 K). Due to microscopic defects in the crystal, the resonant frequency of each dopant is shifted from its nominal value. These shifts are randomly distributed and lead to a smooth inhomogeneous absorption profile that ranges from 20 GHz to over 200 GHz in crystals to several THz in glasses and organics. These shifts do not broaden the individual dopant resonances.

As a result, an inhomogeneously broadened SSH material acts like a broadband multi-channel spectral filter with an incredible number of channels. The ratio of the inhomogeneous bandwidth to the homogeneous bandwidth yields the time-bandwidth product of the processor. For example, Tm:YAG has a measured time-bandwidth product (TBP) exceeding 10⁶. Other promising processing materials that can provide over 300 GHz of bandwidth with sub-MHz resolution at 4K have also been developed.

At temperatures of 4K and lower, these materials are one of the leading candidate systems for a number of quantum information applications, including multiplexed quantum memories and reversible microwave-optical transduction.