Kriti Charan

A single-pixel terahertz imaging system based on compressed sensing

We describe a terahertz imaging system that uses a single pixel detector in combination with a series of random masks to enable high-speed image acquisition. The image formation is based on the theory of compressed sensing, which permits the reconstruction of a N-by-N pixel image using much fewer than N2 measurements. This approach eliminates the need for raster scanning of the object or the terahertz beam, while maintaining the high sensitivity of a single-element detector. We demonstrate the concept using a pulsed terahertz time-domain system and show the reconstruction of both amplitude and phase-contrast images. The idea of compressed sensing is quite general and could also be implemented with a continuous-wave terahertz source.

Advanced Fiber Soliton Sources for Nonlinear Deep Tissue Imaging in Biophotonics

Optical imaging plays a major role in both basic biological research and clinical diagnostics, providing noninvasive or minimally invasive microscopic imaging capability to investigate biological tissues. Optical image acquisition through significant depths of biological tissues, however, presents a major challenge since tissue is extremely heterogeneous and the strong scattering of the various tissue components has restricted high-resolution optical imaging to superficial layers. Multiphoton microscopy (MPM) has significantly extended the penetration depth of high-resolution optical imaging, particularly for in vivo applications. Multiphoton imaging critically depends on ultrafast technologies, particularly pulsed excitation sources. In this paper, the basics of deep tissue MPM and its improvements utilizing soliton self-frequency shift (SSFS) are reviewed. Wavelength tunable, high-energy soliton generation through SSFS in large-mode-area (LMA) fibers and photonic crystal rods is presented. The application of these solitons to MPM enables noninvasive imaging in biological tissues with unprecedented depth. The main characteristics of the excitation source for deep tissue MPM, such as wavelength, pulse energy, and repetition rate, are discussed.

Time‐lens based hyperspectral stimulated Raman scattering imaging and quantitative spectral analysis

We demonstrate a hyperspectral stimulated Raman scattering (SRS) microscope through spectral-transformed excitation. The 1064 nm Stokes pulse was from a synchronized time-lens source, generated through time-domain phase modulation of a continuous wave (CW) laser. The tunable pump pulse was from linear spectral filtering of a femtosecond laser output with an intra-pulse spectral scanning pulse shaper. By electronically modulating the time-lens source at 2.29 MHz, hyperspectral stimulated Raman loss (SRL) images were obtained on a laser-scanning microscope. Using this microscope, DMSO in aqueous solution with a concentration down to 28 mM could be detected at 2 μs time constant. Hyperspectral SRL images of prostate cancer cells were obtained. Multivariate curve resolution analysis was further applied to decompose the SRL images into concentration maps of CH₂ and CH₃ bonds. This method offers exciting potential in label-free imaging of live cells using fingerprint Raman bands.

Generation of intense 100 fs solitons tunable from 2 to 4.3 μm in fluoride fiber

There is great interest in sources of coherent radiation in the mid-wave infrared (3–5 μm), and instruments based on fiber can offer major practical advantages. This range, and much broader, can be covered easily by supercontinuum generation in soft glass fibers, but with low power spectral density. For applications that require intense ultrashort pulses, fiber sources are quite limited. In this Letter, we report a fiber-based system that generates 100 fs pulses with 5 nJ energy, continuously wavelength-tunable over 2–4.3 μm through the soliton self-frequency shift (SSFS) in fluoride fibers. The pulse energies are 2 orders of magnitude higher than those previously achieved by SSFS, around 3 μm, and the range of wavelengths is extended by 1000 nm. Peak power ranges from 20 to 75 kW are achieved across the tuning range. Numerical simulations are in good agreement with the experimental results, and indicate the potential for few-cycle soliton generation out to 5.6 μm. Fiber-integrated sources of femtosecond pulses tunable across this region should be valuable for mid-infrared applications.

Three-photon excited fluorescence imaging of unstained tissue using a GRIN lens endoscope

We present a compact and portable three-photon gradient index (GRIN) lens endoscope system suitable for imaging of unstained tissues, potentially deep within the body, using a GRIN lens system of 1 mm diameter and 8 cm length. The lateral and axial resolution in water is 1.0 μm and 9.5 μm, respectively. The ~200 μm diameter field of view is imaged at 2 frames/s using a fiber-based excitation source at 1040 nm. Ex vivo imaging is demonstrated with unstained mouse lung at 5.9 mW average power. These results demonstrate the feasibility of three-photon GRIN lens endoscopy for optical biopsy.

Higher-order-mode fiber optimized for energetic soliton propagation.

We describe the design optimization of a higher-order-mode (HOM) fiber for energetic soliton propagation at wavelengths below 1300 nm. A new HOM fiber is fabricated according to our design criteria. The HOM fiber is pumped at 1045 nm by an energetic femtosecond fiber laser. The soliton self-frequency shift process shifts the center wavelength of the soliton to 1085 nm. The soliton has a temporal duration of 216 fs and a pulse energy of 6.3 nJ. The demonstrated pulse energy is approximately six times higher than the previous record in a solid core fiber at wavelengths below 1300 nm.

Intermodal four-wave mixing in a higher-order-mode fiber.

We demonstrate intermodal four-wave mixing in an all-fiber system between the LP01 and LP02 mode of a higher-order-mode fiber. Anti-Stokes and Stokes light with 2 nJ pulse energy is generated with 20% conversion efficiency.

Intermodal Čerenkov radiation in a higher-order-mode fiber.

We demonstrate an intermodal Čerenkov radiation effect in a higher-order-mode (HOM) fiber with a mode crossing (i.e., two guided modes having the same propagation constant at the same wavelength). A frequency-shifted soliton in the vicinity of the mode-crossing wavelength emits a phase-matched dispersive wave in a different propagation mode. We develop a theoretical explanation for this nonlinear optical effect and demonstrate that the mode crossing in HOM fibers can be utilized to achieve simultaneous wavelength and mode conversion; the strength of this intermodal nonlinear interaction can be tuned by controlled fiber bending.

Nonresonant background suppression for coherent anti-Stokes Raman scattering microscopy using a multi-wavelength time-lens source.

We demonstrate a robust, all-fiber, two-wavelength time-lens source for background-free coherent anti-Stokes Raman scattering imaging. The time-lens source generates two picosecond pulse trains simultaneously: one at 1064 nm and the other tunable between 1040 nm and 1075 nm (~400 mW for each wavelength). When synchronized to a mode-locked Ti:Sapphire laser, the two wavelengths are used to obtain on- and off-resonance coherent anti-Stokes Raman scattering images. Real-time subtraction of the nonresonant background in the coherent anti-Stokes Raman scattering image is achieved by the synchronization of the pixel clock and the time-lens source. Background-free coherent anti-Stokes Raman scattering imaging of sebaceous glands in ex vivo mouse tissue is demonstrated.

A single-pixel terahertz camera

We describe a single-pixel, pulsed terahertz camera which uses random patterns to enable high-speed image acquisition. Our method requires no raster scanning of objects, nor detection using a focal-plane array.