Takashi Buma

NDT of fiber-reinforced composites with a new fiber-optic pump-probe laser-ultrasound system.

Laser-ultrasonics is an attractive and powerful tool for the non-destructive testing and evaluation (NDT&E) of composite materials. Current systems for non-contact detection of ultrasound have relatively low sensitivity compared to contact peizotransducers. They are also expensive, difficult to adjust, and strongly influenced by environmental noise. Moreover, laser-ultrasound (LU) systems typically launch only about 50 firings per second, much slower than the kHz level pulse repetition rate of conventional systems. As demonstrated here, most of these drawbacks can be eliminated by combining a new generation of compact, inexpensive, high repetition rate nanosecond fiber lasers with new developments in fiber telecommunication optics and an optimally designed balanced probe beam detector. In particular, a modified fiber-optic balanced Sagnac interferometer is presented as part of a LU pump–probe system for NDT&E of aircraft composites. The performance of the all-optical system is demonstrated for a number of composite samples with different types and locations of inclusions.

A new fiber-optic non-contact compact laser-ultrasound scanner for fast non-destructive testing and evaluation of aircraft composites

Laser ultrasonic (LU) inspection represents an attractive, non-contact method to evaluate composite materials. Current non-contact systems, however, have relatively low sensitivity compared to contact piezoelectric detection. They are also difficult to adjust, very expensive, and strongly influenced by environmental noise. Here, we demonstrate that most of these drawbacks can be eliminated by combining a new generation of compact, inexpensive fiber lasers with new developments in fiber telecommunication optics and an optimally designed balanced probe scheme. In particular, a new type of a balanced fiber-optic Sagnac interferometer is presented as part of an all-optical LU pump-probe system for non-destructive testing and evaluation of aircraft composites. The performance of the LU system is demonstrated on a composite sample with known defects. Wide-band ultrasound probe signals are generated directly at the sample surface with a pulsed fiber laser delivering nanosecond laser pulses at a repetition rate up to 76 kHz rate with a pulse energy of 0.6 mJ. A balanced fiber-optic Sagnac interferometer is employed to detect pressure signals at the same point on the composite surface. A- and B-scans obtained with the Sagnac interferometer are compared to those made with a contact wide-band polyvinylidene fluoride transducer.

On the Role of Intrinsic and Extrinsic Forces in Early Cardiac S-looping

Background: Looping is a crucial phase during heart development when the initially straight heart tube is transformed into a shape that more closely resembles the mature heart. Although the genetic and biochemical pathways of cardiac looping have been well studied, the biophysical mechanisms that actually effect the looping process remain poorly understood. Using a combined experimental (chick embryo) and computational (finite element modeling) approach, we study the forces driving early s‐looping when the primitive ventricle moves to its definitive position inferior to the common atrium. Results: New results from our study indicate that the primitive heart has no intrinsic ability to form an s‐loop and that extrinsic forces are necessary to effect early s‐looping. They support previous studies that established an important role for cervical flexure in causing early cardiac s‐looping. Our results also show that forces applied by the splanchnopleure cannot be ignored during early s‐looping and shed light on the role of cardiac jelly. Using available experimental data and computer modeling, we successfully developed and tested a hypothesis for the force mechanisms driving s‐loop formation. Conclusions: Forces external to the primitive heart tube are necessary in the later stages of cardiac looping. Experimental and model results support our proposed hypothesis for forces driving early s‐looping. Developmental Dynamics 242:801–816, 2013.

Near-infrared spectroscopic photoacoustic microscopy using a multi-color fiber laser source

We demonstrate a simple multi-wavelength optical source suitable for spectroscopic optical resolution photoacoustic microscopy (OR-PAM) of lipid-rich tissue. 1064 nm laser pulses are converted to multiple wavelengths beyond 1300 nm via nonlinear optical propagation in a birefringent optical fiber. OR-PAM experiments with lipid phantoms clearly show the expected absorption peak near 1210 nm. We believe this simple multi-color technique is a promising cost-effective approach to spectroscopic OR-PAM of lipid-rich tissue.

Photoacoustic microscopy with a tunable source based on cascaded stimulated Raman scattering in a large-mode area photonic crystal fiber

Conventional tunable pulsed lasers for photoacoustic microscopy (PAM) are bulky and expensive. We have previously demonstrated a compact tunable source using cascaded stimulated Raman scattering (SRS) in an ordinary single-mode fiber. In this paper we report an improved tunable source with both higher pulse energy (over 200 nJ), repetition rate (30 kHz), and extended tuning range (532 - 610 nm). The key feature is a large mode area photonic crystal fiber (LMA-PCF). The large mode area, pure silica composition, and honeycomb cladding of the LMA-PCF produce a much higher optical damage threshold than ordinary fiber. We have found that cascaded SRS with very high Raman gain occurs in the LMA-PCF to produce both discrete and a continuum of wavelengths between 560 and 610 nm, offering the possibility of rapid tunability within this physiologically important wavelength range. Our Q-switched Nd:YAG laser produces 2 ns duration pulses at 532 nm with 10 uJ of energy at a 30 kHz repetition rate. The laser pulses are coupled into a 30 meter long LMA-PCF. The multi-color fiber output goes through a band pass filter (10 nm width), where the selected wavelength is sent to a 50 MHz photoacoustic microscopy system employing optical focusing. The individual pulse energy is 270, 360, 520, 530, and 400 nJ at wavelengths of 532, 546, 568, 589, and 600 nm, respectively. Test imaging experiments are performed on a silicone tube phantom containing red and blue ink. To our knowledge, this is the first demonstration of a tunable pulsed optical source using a LMA-PCF. The pulse energy and wavelength range of our system are suitable for oxygenation measurements. We believe these results can significantly benefit the development of functional PAM systems.

Near-infrared multispectral photoacoustic microscopy using a graded-index fiber amplifier

We demonstrate optical resolution photoacoustic microscopy (OR-PAM) of lipid-rich tissue using a multi-wavelength pulsed laser based on nonlinear fiber optics. 1047 nm laser pulses are converted to 1098, 1153, 1215, and 1270 nm pulses via stimulated Raman scattering in a graded-index multimode fiber. Multispectral PAM of a lipid phantom is demonstrated with our low-cost and simple technique.

Multispectral photoacoustic microscopy of lipids using a pulsed supercontinuum laser

We demonstrate optical resolution photoacoustic microscopy (OR-PAM) of lipid-rich tissue between 1050-1714 nm using a pulsed supercontinuum laser based on a large-mode-area photonic crystal fiber. OR-PAM experiments of lipid-rich samples show the expected optical absorption peaks near 1210 and 1720 nm. These results show that pulsed supercontinuum lasers are promising for OR-PAM applications such as label-free histology of lipid-rich tissue and imaging small animal models of disease.

Optoacoustics for biomedical applications

We are developing optical techniques to generate and receive ultrasound for various biomedical applications including high frequency 2D arrays, molecular imaging, and microfluidic devices. A 2D synthetic receive array uses a HeNe laser to probe the surface displacements of a thin reflective membrane. Images with near optimal resolution and wide fields of view have been produced at 10 to 50 MHz. A 75 MHz transmitting 2D array element relies on the thermoelastic effect. A 10 ns laser pulse is focused onto a 25 μm thick black polydimethylsiloxane (PDMS) film spin coated on a pure PDMS substrate. Our work in optoacoustic molecular imaging combines ultrafast lasers with high frequency ultrasound. When ultrafast laser pulses are focused into transparent media, laser induced optical breakdown (LIOB) produces acoustic emission and cavitation bubbles. A real‐time acoustic technique has been developed to characterize LIOB in dendrimer nanocomposite (DNC) solutions. Lamb waves propagating in thin membranes have foun...

Photoacoustic microscopy using four-wave mixing in a multimode fiber

Many applications in photoacoustic microscopy (PAM) use contrast agents that preferentially absorb light at wavelengths other than 532 nm. This usually requires the use of expensive dye or optical parametric oscillator (OPO) lasers. We have previously demonstrated a cost-effective alternative using an inexpensive 532 nm laser and a photonic crystal fiber, where multiple wavelengths are produced by stimulated Raman scattering (SRS). In this paper, we demonstrate a new source using four-wave mixing (FWM) in SMF-28e, an industry standard telecommunications fiber. Our system is based on a Q-switched Nd:YAG laser producing 2 ns duration pulses at 532 nm with 15 uJ of energy at a 20 kHz repetition rate. The laser pulses are coupled into 100 meters of SMF-28e, a step-index fiber that is designed to be single-mode at 1310 and 1550 nm but supports a few optical modes at 532 nm. The fiber output spectrum contains both discrete lines below 625 nm and a broad continuum from 625 to 900 nm. This continuum is most likely due to phase matched FWM between multiple fiber modes, particularly the LP01, LP11, and LP02 modes. The fiber output goes through a dielectric filter, where the selected spectral window is sent to a PAM system employing optical focusing and laser scanning. PAM imaging of a USAF target produced an image resolution of 7 μm using 800 nm light. Imaging experiments are performed on a dye phantom, where one tube is filled with diluted red ink and the other with 200 μM of IR-820 dye. Dielectric filters were used to select either a 750-900 nm (1.3 uJ pulse energy) or 532-700 nm (1.0 uJ pulse energy) spectral window. The resulting images easily differentiate between the two absorbers. The advantages of this multi-wavelength optical source are the very simple apparatus, the flexibility in choosing the desired spectral range, and the use of a very inexpensive optical fiber.

Spectroscopic photoacoustic microscopy in the 1064–1300 nm range using a pulsed multi-color source based on stimulated Raman scattering

Photoacoustic microscopy (PAM) provides excellent image contrast based on optical absorption. Lipidrich tissue, such as atherosclerotic plaques and myelinated nerve fibers, exhibits an optical absorption peak near 1210 nm. Unfortunately, pulsed lasers operating in this wavelength range use expensive optical parametric oscillator (OPO) systems. We demonstrate a simple approach to convert an inexpensive 1064 nm pulsed laser into a multi-wavelength source suitable for spectroscopic PAM of lipid-rich tissue. Wavelength conversion is achieved by using stimulated Raman scattering (SRS) in an optical fiber. A sufficiently intense laser pulse nonlinearly interacts with the internal vibrations of the glass molecular structure to produce a series of down-shifted frequency components (Stokes lines). We use a Q-switched Nd:YAG microchip laser producing 0.6 ns duration pulses at 1064 nm with 8 uJ of energy at a 7.4 kHz repetition rate. The laser pulses are coupled into a 20 meter long single-mode fiber. The multi-color fiber output goes through a band pass filter, where the selected wavelength is sent to a photoacoustic microscopy system employing optical focusing. Pulse energies above 200 nJ are produced within spectral bands at 1064, 1100, 1175, 1225, 1275, and 1325 nm. Imaging experiments with phantoms containing butter clearly show the expected absorption peak near 1210 nm. A major advantage of our technique is the simple arrangement to convert a single-wavelength laser into a multi-color source for spectroscopic PAM. We believe this multi-color technique is a promising method to achieve high-speed spectroscopic photoacoustic microscopy of lipid-rich tissue.