Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2003-01-17
2004-11-02
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S203100, C359S212100, C359S900000, C359S368000, C250S234000
Reexamination Certificate
active
06813050
ABSTRACT:
BACKGROUND
1. Field of the Invention
This invention relates to delay lines in optical coherence tomographic and optical Doppler tomographic systems, and dynamic focusing mechanisms in confocal microscopy and optical coherence microscopy.
2. Prior Art Statement
Confocal microscopy, optical coherence tomography (OCT), and optical coherence microscopy (OCM) are novel optical tomography techniques which are very useful for providing subsurface high-resolution imaging of samples (including but not limited to biological and medical samples). Confocal microscopy can achieve a sub-micron resolution and a penetration depth up to a few hundred microns. Optical coherence tomography can provide a spatial resolution up to a few microns and a penetration depth up to a few millimeters. Its advantages over confocal microscopy are the higher sensitivity due to signal enhancement by optical interferences, and a faster image acquisition rate because the axial scanning is obtained by an optical delay line in the reference arm, which is generally faster than traditional mechanical scanning stages. Optical coherence microscopy is a combination of confocal microscopy and optical coherence tomography. It uses a high numerical aperture lens to reduce the spot size of the focal point in order to obtain a better spatial resolution than OCT, and low coherence interferometry to reject multiple scattering lights. However, the axial scanning range is limited by a much shorter Rayleigh range. As a consequence, an additional translation stage is always needed to achieve adequate axial scanning range.
In many potential biomedical applications, the data acquisition speed is a critical issue in suppressing motion artifacts and acquiring high resolution four dimensional images (three spatial dimensions and one temporal dimension). Rapid delay lines are necessary to achieve fast OCT, while dynamic focusing mechanism instead of mechanical scanning stages is desirable for fast confocal microscopy and OCM.
A primitive delay line is a translating mirror, which is driven by a linear motor, an actuator, or a piezoelectric transducer (PZT). As the mirror moves back and forth along the path of the received optical signal, the power consumption required to generate acceleration will increase dramatically with frequency and scanning range. This is the reason that most commercially available linear motors and actuators can only provide a repetition rate around 30 Hz when a 2-3 mm scanning range is required. Although PZT can be driven at much higher frequencies, they can provide a limited scanning range. Resonant scanners have been demonstrated to achieve a frequency of 1,200 Hz and up to a 3 mm optical length difference. The drawback is that the optical path length change is a time-dependent sinusoidal function. As a result, Doppler frequencies of interference signals are depth dependent and vary within a wide range, which may cause difficulties in signal filtering and introducing more noises.
Sophisticated delay lines require complicated arrangements of mirrors, gratings and/or lenses, as well as precise alignment. Grating based delay lines have the flexibility to adjust group delay and phase delay independently. Repetition rates of 2,000 scans/second and 4,000 scans/second have been reported for such delay lines with a galvanometer (driven with a 1-kHz triangle waveform) and a 4-kHz resonant scanner, respectively. It appears that without using resonant scanners, the vibrational motion based mechanical scanning cannot readily achieve a speed high enough to meet real time data acquisition requirements. Rotating cubes, rotating roof prisms, and a combination of a polygonal mirror and a glass cube can scan up to 28.5 kHz. However, these methods suffer from rather low duty cycles and/or considerable nonlinearity of optical path length change.
Recently, an OCT system without any moving parts for depth scanning was proposed, and a high repetition scanning rate of 500 kHz was achieved in a scanning range of 25 mm by using optical frequency comb generators. However, the depth resolution (100 microns) and signal to noise ratio of this system needs to be improved. In addition, the cost of this system is high due the use of expensive components, such as gigahertz electronics and electro-optical modulators. Some fast delay lines are linear and can achieve several kHz scanning speed. However, they suffer from wavelength dependent group velocity dispersion.
A fast scanning device is necessary for high-speed microscopic imaging methods such as optical coherence tomography (OCT) and confocal microscopy. In an OCT system, axial scanning is generally achieved with a variable optical delay line, whose repetition rate determines the image acquisition speed. In a confocal microscope, angular scanning of collimated beam is transformed into lateral scanning of focus inside a sample. Conventional scanners cannot readily achieve kilohertz repetition speed at a reasonable cost and acceptable performances.
A widely used delay line for OCT is based on a grating and a scanning mirror that has a varying tilting angle, as disclosed in U.S. Pat. No. 6,111,645A (Tearney et al.). The reported axial scanning rate was 2 kHz. The use of a grating is critical for converting angular beam scanning into optical path length change. However, dispersion of the grating may degenerate the resolution of the system and cause problems when multiple wavelengths are needed for spectroscopic information. In addition, non-linearity in scanning speed is inevitable when resonant scanners are used for kilohertz repetition rates.
A 2.58 kHz reflectometer comprised of a rotating polygon mirror was disclosed in an article entitled “Robust and rapid optical low-coherence reflectometer using a polygon mirror” by Delachenal et al. (Optics Communications, 162 (1999) pp. 195-199). The high scanning speed comes at the cost of poor linearity and a low duty factor. The same problems are related to the optical delay line with a rotating cube that was disclosed in an article entitled “Achieving variation of the optical path length by a few millimeters at millisecond rates for imaging of turbid media and optical interferometry: a new technique” by Su (Optics Letters.22, (1997), pp. 665-667).
Recently, an OCT system without any moving parts for depth scanning was disclosed in an article entitled “Ultrahigh scanning speed optical coherence tomography using optical frequency comb generators” by Lee et al. (Japanese J. of Applied Physics, Part 2, 8B, (2001), L878-880). A fairly high repetition scanning rate of 500 kHz was achieved in a scanning range of 25 mm by using optical frequency comb generators. However, the depth resolution (100 microns) and signal to noise ratio of this system cannot meet requirements for biomedical applications. In addition, the cost of this system is high due to the use of expensive components, such as gigahertz electronics and electro-optical modulators.
One example of linear scanning optical delay line was disclosed in U.S. Pat. Nos. 5,784,186A and 5,907,423A (Wang et al.). A helicoid reflecting mirror was used as a linear scanning line in an optical second-harmonic generation autocorrelator. The scanning speed was reported as 43.5 Hz. Fabrication of the spiral reflecting surface would be expensive when a high accuracy and high reflectivity are required.
Another example relates to two oppositely lying reflection means that was disclosed in U.S. Pat. No. 6,341,870B1 (Koch et al.). A movement of one mirror with respect to another of 45 microns is enough to results in a path length change of 2 mm. However, the overall path length and path length change are very sensitive to orientation of the incident beam with respect to the mirrors. Very accurate alignment and vibration control may be required.
A further example of another design relates to an optical path length scanner using moving prisms that was disclosed in U.S. Pat. No. 6,407,872B1 (Lai et al.). The design was tested with Zemax simulation but no experimental validation has been reported. It also has the dispers
Chen Nanguang
Zhu Qing
B J Associates
Phan James
Skutnik Bolegh J
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