Milad Alemohammad

High-speed all-optical Haar wavelet transform for real-time image compression

We present a high-speed single pixel flow imager based on an all-optical Haar wavelet transform of moving objects. Spectrally-encoded wavelet measurement patterns are produced by chirp processing of broad-bandwidth mode-locked laser pulses. A complete wavelet pattern set serially illuminates the object via a spectral disperser. This high-rate structured illumination transforms the scene into a set of sparse coefficients. We show that complex scenes can be compressed to less than 30% of their Nyquist rate by thresholding and storing the most significant wavelet coefficients. Moreover by employing temporal multiplexing of the patterns we are able to achieve pixel rates in excess of 360 MPixels/s.

Conversion for the Better: A Broadband Highly Efficient Microwave Photonics Detector

Optical fibers have many advant ages over traditional electrical cables and interconnects when it comes to transmitting high-frequency signals. These advantages include lower loss, higher bandwidth, immunity to electromagnetic interference, and lower power consumption. It is due to these advantages that fiber optic links have found widespread use in applications such as radio-over-fiber and antenna remoting, where coaxial cable loss increases prohibitively versus frequency. An RF photonic link comprises a transmitter and a receiver. In the transmit stage, the optical carrier is modulated by the RF signal; in the receive stage, the optically modulated signal is demodulated using a photodetector. Despite the numerous advantages, poor efficiency of the electrical-to-optical conversion process constrains the overall link gain.

Tagging Electronic ICs using Silicon Nitride Photonic Physical Unclonable Functions

We demonstrate an on-chip photonic physical unclonable function using integrated evanescently coupled multimode spiral waveguides formed in silicon nitride with rich spectral features and validate it for use in an identification tagging application for electronic ICs.

Compressive temporal focusing microscopy (Conference Presentation)

Multiphoton microscopes are of paramount importance in capturing neural activity with cellular resolution. However, the imaging speed and field-of-view of traditional two-photon microscopes is limited by raster scanning technologies. Temporally-focused two-photon (TFTP) microscopy is a wide-field scan-free approach to increase the speed of two-photon microscopy. In conventional TFTP microscopy, wide-field depth sectioning is obtained by compressing a spatially pre-chirped pulse at the focal plane of the objective. Unfortunately, the greater imaging speed of TFTP microscopes comes at the expense of poor imaging depth in tissue due to scattering of the short-wavelength fluorescence photons en-route to the imaging camera. Here we demonstrate a compressive high-speed two-photon microscope based on wide-field temporally-focused structured illumination, which eliminates the loss of image contrast from scattering of the fluorescence signal by leveraging a single-pixel detector. Specifically, we illuminate the sample with a rapid sequence of randomly structured temporally-focused wide-field illumination pulses and integrate the net two-photon fluorescence response on a single photomultiplier tube (PMT). Notably, the longer wavelength structured illumination is significantly less susceptible to scattering and the use of integrated measurements on a single PMT provides immunity to fluorescence scattering since these measurements are solely concerned with the net fluorescence. Furthermore, our approach provides greater speed than point scanning two-photon microscopes through the use of wide-field illumination and compressive image acquisition. Experimentally we demonstrate this system operating over a 200×250-μm field-of-view and at a compression rate of 10%, which provides an order of magnitude increase in speed over a comparable point scanning architecture.

Super-achromatic microprobe for ultrahigh-resolution endoscopic OCT imaging at 800 nm(Conference Presentation)

In this paper, we report a super-achromatic microprobe made with fiber-optic ball lens to enable ultrahigh-resolution endoscopic OCT imaging. An axial resolution of ~2.4 µm (in air) can be achieved with a 7-fs Ti:Sapphire laser. The microprobe has minimal astigmatism which affords a high transverse resolution of ~5.6 µm. The miniaturized microprobe has an outer diameter of ~520 µm including the encasing metal guard and can be used to image small luminal organs. The performance of the ultrahigh-resolution OCT microprobe was demonstrated by imaging rat esophagus, guinea pig esophagus, and mouse rectum in vivo.

Broadband rotary joint for high speed ultrahigh resolution endoscopic OCT imaging (Conference Presentation)

Endoscopic OCT is a promising technology enabling noninvasive in vivo imaging of internal organs, such as the gastrointestinal tract and airways. The past few years have witnessed continued efforts to achieve ultrahigh resolution and speed. It is well-known that the axial resolution in OCT imaging has a quadratic dependence on the central wavelength. While conventional OCT endoscopes operate in 1300 nm wavelength, the second-generation endoscopes are designed for operation around 800 nm where turn-key, broadband sources are becoming readily available. Traditionally 1300 nm OCT endoscopes are scanned at the proximal end, and a broadband fiber-optic rotary joint as a key component in scanning endoscopic OCT is commercially available. Bandwidths in commercial 800 nm rotary joints are unfortunately compromised due to severe chromatic aberration, which limits the resolution afforded by the broadband light source. In the past we remedied this limitation by using a home-made capillary-tube-based rotary joint where the maximum reliable speed is ~10 revolutions/second. In this submission we report our second-generation, home-built high-speed and broadband rotary joint for 800 nm wavelength, which uses achromatic doublets in order achieve broadband achromatic operation. The measured one-way throughput of the rotary joint is >67 % while the fluctuation of the double-pass coupling efficiency during 360° rotation is less than +/-5 % at a speed of 70 revolutions/second. We demonstrate the operation of this rotary joint in conjunction with our ultrahigh-resolution (2.4 µm in air) diffractive catheter by three-dimensional full-circumferential endoscopic imaging of guinea pig esophagus at 70 frames per second in vivo.