Changho Chong

Three-dimensional and high-speed swept-source optical coherence tomography for in vivo investigation of human anterior eye segments

A two- and three-dimensional swept source optical coherence tomography (SS-OCT) system, which uses a ready-to-ship scanning light source, is demonstrated. The light source has a center wavelength of 1.31 mum, -3 dB wavelength range of 110 nm, scanning rate of 20 KHz, and high linearity in frequency scanning. This paper presents a simple calibration method using a fringe analysis technique for spectral rescaling. This SS-OCT system is capable of realtime display of two-dimensional OCT and can obtain three-dimensional OCT with a measurement time of 2 s. In vivo human anterior eye segments are investigated two- and three-dimensionally. The system sensitivity is experimentally determined to be 114 dB. The three-dimensional OCT volumes reveal the structures of the anterior eye segments, which are difficult to observe in two-dimensional OCT images.

A 5-V operated MEMS variable optical attenuator by SOI bulk micromachining

We report the design, fabrication, and successful demonstration of microelectromechanical variable optical attenuator (VOA) using an electrostatic microtorsion mirror (0.6 mm in diameter) combined with a fiber-optic collimator. The VOA operates at low voltages (dc 5 V or less) for large optical attenuation (40 dB, corresponding to mirror angle of 0.3/spl deg/) and a fast response time (5 ms or faster). The mirror made of a bulk-micromachined silicon-on-insulator wafer has been designed to be shock resistant up to 500 G without any mechanical failure. We also have suppressed temperature dependence of optical performance to be less than /spl plusmn/0.5 dB at 10-dB attenuation in the range of -5/spl deg/C-70/spl deg/C by mechanically decoupling the parasitic bimorph effect from the electrostatic operation.

Optically Modulated MEMS scanning endoscope

This letter presents a novel configuration of a microelectrical-mechanical system (MEMS) scanning endoscope that is actuated by external optical modulation. Light at a wavelength in the 1550-nm range is used to modulate a scanning MEMS mirror. The instrument also provides for the delivery of a second beam at a wavelength of 1310 nm, commonly used for medical diagnostic applications such as optical coherence tomography. The two wavelengths are combined with using a wavelength-division multiplexing filter and launched into a single-mode fiber. This novel approach provides for the operation of a scanning endoscope without the need for directly powering up the scanning element. In this way a less hazardous apparatus for in vivo diagnosis is possible. The design of the module and MEMS scanning mirror is discussed. A fabricated module is demonstrated in which a MEMS mirror scans at the resonant frequency of 350 Hz, generating an optical scanning angle of 8/spl deg/.

Large coherence length swept source for axial length measurement of the eye

We present and demonstrate a swept source with a large coherence length using a quasi-phase continuous tuning (QPCT) technique. QPCT is a method of minimizing the phase shift per round trip with respect to the tunable filter so that the resonance of lasing becomes high, resulting in high finesse of lasing during a rapid sweep. The demonstrated swept source consists of a fiber ring extended cavity laser with a diffraction grating and a polygon scanner-based tunable filter configuration. The projected beam on the diffraction grating is expanded with a multiple of beam expanders to achieve high finesse of the filter. The source demonstrated an 18 nm swept range at 1060 nm wavelength, 28 mm coherence length, and 6.2 mW peak power at a 2.5 kHz swept rate. OCT imaging results showed that a coherence length of 28 mm enables the measurement of the axial length of a pigs eye with 20 mm length in physical size.

Spectral narrowing effect by quasi-phase continuous tuning in high-speed wavelength-swept light source

This paper reports on a technique to improve the coherence length of a high-speed wavelength swept laser. The wavelength swept laser comprises a pigtailed semiconductor optical amplifier and a wavelength-scanning filter in a fiber extended cavity configuration. The laser operates in the 1310 nm wavelength region. The tunable filter consists of a diffraction grating and polygon mirror scanner. Littrow arrangement of external cavity in a specific geometry realizes the quasi-phase continuous tuning over wavelength range emphasizing coherent amplification of cavity modes resulting in spectral narrowing of the instantaneous linewidth to about 0.06 nm. Improvement by a factor of two is confirmed in comparison with coherence length without using this technique. Peak power is 12 mW and wavelength swept range is 55 nm, from 1271 nm to 1326 nm. Measured coherence lengths of over 30 mm and 17 mm were achieved at scanning rates of 2.5 kHz and 20 kHz, respectively. Correlation of laser cavity parameters with spectral linewidth is also discussed by introducing the rate equations for multi-mode laser operation. Shorter cavity length is considered effective to further improve the coherence length in terms of shorter roundtrip time as well as higher mode suppression ratio because of higher carrier concentration on cavity modes around the filter center.

High-Speed Wavelength-Swept Laser Source With High-Linearity Sweep for Optical Coherence Tomography

This paper reports on a high-speed, wide-range, wavelength-swept laser operating at 1310 nm with highly linear sweep performance for optical coherence tomography (OCT) applications. The simple and robust laser comprises a pigtailed semiconductor optical amplifier and a wavelength-scanning filter in an extended fiber cavity configuration. The tunable filter consists of a diffraction grating and polygon mirror scanner. A photodiode is used to generate a start trigger signal synchronized to start of each frequency sweep. Intracavity prism beam expanders are aligned to provide constant laser linewidth and linear frequency sweep. The laser exhibits a peak power of 15 mW. The tuning range of the laser is 117 nm maximum, and 100 nm full-width at half-maximum at a scanning frequency of 20 kHz. OCT signal in time-constant fast Fourier transform mode is compared with OCT signal using conventional wavelength rescaled processing. Coherence length was measured to be 5 mm. This approach realizes a practical and robust laser system suitable for deployment in swept-source-based OCT real-time imaging systems.

A high speed MEMS scanner for 140-kHz SS-OCT

We present an electrostatic vertical comb-drive MEMS optical scanner with the angle magnifier mechanism developed for a 140-kHz wavelength tunable-laser in the swept-source OCT system of a 2 ms/frame visualization capability.

MEMS scanner based swept source laser for optical coherence tomography

We developed a swept source laser using a micro electro mechanical systems(MEMS) scanner mirror, and demonstrated optical coherence tomography. To enable both the wide tuning wavelength range and high scanning frequency, we introduced 2-degree-of-freedom(2-DOF) MEMS scanner mirror. A tunable optical filter is composed of a MEMS scanner mirror and a diffraction grating which is arranged in Littrow configuration. We built a swept source laser which has a wavelength range of 143 nm, center wavelength of 1304 nm, and a peak power of 16 mW. OCT measurements are performed at a rate of 17.9 kHz and doubled 35.9 kHz at unidirectional and bidirectional sweeps, respectively. The system sensitivity is 101.5 dB.

Design and fabrication of a MEMS 1-D optical scanner using self-assembled vertical combs and scan-angle magnifying mechanism

We report a novel one-axis MEMS optical scanner with a scan-angle magnifying mechanism. Small actuator-motion was amplified through the two-mass-two-spring mechanism, and a large mirror angle was obtained

Non-Harmonic Analysis Applied to Optical Coherence Tomography Imaging

A new processing technique called non-harmonic analysis (NHA) is proposed for optical coherence tomography (OCT) imaging. Conventional Fourier-domain OCT employs the discrete Fourier transform (DFT), which depends on the window function and length. The axial resolution of the OCT image, calculated by using DFT, is inversely proportional to the full width at half maximum (FWHM) of the wavelength range. The FWHM of wavelength range is limited by the sweeping range of the source in swept-source OCT and it is limited by the number of CCD pixels in spectral-domain OCT. However, the NHA process does not have such constraints; NHA can resolve high frequencies irrespective of the window function and the frame length of the sampled data. In this study, the NHA process is described and it is applied to OCT imaging. It is compared with OCT images based on the DFT. To demonstrate the benefits of using NHA for OCT, we perform OCT imaging with NHA of an onion skin. The results reveal that NHA can achieve an image resolution equivalent that of a 100-nm sweep range using a significantly reduced wavelength range. They also reveal the potential of using this technique to achieve high-resolution imaging without using a broadband source. However, the long calculation times required for NHA must be addressed if it is to be used in clinical applications.