Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
1999-01-19
2001-06-19
Spyrou, Cassandra (Department: 2872)
Optical waveguides
Optical fiber waveguide with cladding
C359S199200
Reexamination Certificate
active
06249630
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an optical pulse delivery system for various types of optical devices, such as an optical measurement system, requiring ultrashort pulses with high peak power. More particularly, the present invention relates to an optical pulse delivery system which employs an optical fiber and which is capable of compensating for various dispersion effects within the system (including those within the optical device, if desired) in order to deliver high peak power pulses.
2. Description of the Related Art
Ultrashort optical pulse sources are presently known to be capable of creating pulses having pulse widths of picosecond to sub-picosecond duration at a variety of wavelengths, pulse energies, and repetition rates up to the GHz regime. Such optical pulse sources are commonly used in measurement and imaging applications that require time gating or excitation by a high peak power or high intensity. Ultrashort optical pulses provide both high spatial and high temporal resolutions, as well as high peak powers in a focusable beam necessary for the excitation of certain non-linear events (such as the excitation of a multi-photon fluorescent medium). These capabilities find use in applications including biological and medical imaging, metrology, terahertz generation, photoconductive and electro-optical sampling, and optical time domain reflectometers.
Current techniques for the delivery of ultrashort optical pulses to a device under test or a measurement point include the use of optical components such as mirrors, lenses, optical fiber, beamsplitters, and dichroic elements. Ultrashort optical pulses passing through a delivery system made of such elements will experience a change in peak power as well as distortions in their temporal profile. These distortions may result in a reduction in resolution, or a degradation in signal-to-noise ratio. The changes in peak power and temporal shape of an ultrashort optical pulse signal propagating through an optical system are caused by losses and dispersion. In addition, at high peak powers, non-linear effects can distort the optical pulse.
An ultrashort optical pulse is made up of a certain range of optical frequencies (or wavelengths), which constitutes its bandwidth. The shortest pulse for a given bandwidth (the bandwidth-limited pulse) has all of its frequency components perfectly overlapped in time. In propagation through a system, the different wavelength components of a pulse experience different delays. These different delays will cause the above-mentioned distortion in temporal shape and change in peak power of ultrashort optical pulses. The result is a frequency chirped pulse, where instantaneous frequency is a function of time along the pulse.
Propagation through a common optically transparent material used to deliver optical signals, is such as glass, will generally result in very small loss. However, due to the frequency dependent refractive index n(&ngr;) of the medium, which gives the velocity, v, of propagation of the optical signal by the relationship v=c/ n(&ngr;), where c is the speed of light in a vacuum, different wavelengths, &lgr;, experience different velocities in the material, where wavelength is related to frequency by &lgr;=/c&ngr;. This effect is referred to as chromatic dispersion. Through the interaction of a pulsed optical signal and such a material, pulse broadening can occur due to group velocity dispersion (GVD). This effect causes the lower frequency components and the higher frequency components of the bandwidth to arrive at different times after passing through the dispersing medium. The effect may be that the lower frequency components arrive earlier or later, depending upon the sign of the dispersion. In glass, for wavelengths shorter than the zero-dispersion wavelength (~1300 nm), the sign of the dispersion is positive, and higher frequencies of the optical pulse travel more slowly than lower frequencies. Above the zero-dispersion wavelength, the sign of the dispersion is negative, and lower frequencies of the optical pulse travel more slowly than higher frequencies. Therefore, any optical element through which the ultrashort optical pulse is transmitted may potentially have a distorting effect.
Dispersion manipulation may be performed with several well known optical elements and systems. These include glass prisms, diffraction gratings, fiber gratings, and optical fiber. These elements allow for both signs of dispersion to be reached at any wavelength, as well as allowing for compensation of frequency chirp. Glass prism pairs can be used to create a dispersive delay line, where, by varying the distance between the two prisms, different amounts of dispersion can be achieved. Similarly, using either reflective or transmissive diffraction gratings, variable amounts of both positive and negative dispersion can be provided. Fiber gratings are chirped Bragg gratings written in the core of an optical fiber. In a chirped fiber grating, dispersion is achieved by reflecting different wavelengths at different locations in space, thereby adding different time shifts to different wavelength components. Specialty fibers can be made for wavelengths longer than ~1300 nm. These fibers use waveguide dispersion in conjunction with material dispersion to create tailored dispersion which may be positive, negative, or close to zero.
Of the commonly used optics for beam steering in an optical system, optical fibers are a convenient method of delivery in practical systems, particularly those where the laser source is bulky. Optical fibers offer increased reliability and robustness, by allowing for stable pre-alignment of components. By providing confinement of the laser light, optical fiber delivery allows for placement of the laser source in more diverse environments than the typical laser laboratory, as well as allowing for convenient placement of the source of light with respect to the rest of the system, providing more flexibility in system design. Additionally, the optical fiber can be disconnected without disturbing the alignment of the laser source and the optical device; thus, the two systems can be pre-aligned and shipped separately in different boxes. However, optical fibers can distort the temporal profile of ultrashort optical pulses, as described below.
Optical fibers can be characterized as being single-mode (capable of propagating a single spatial mode) or multi-mode (capable of supporting the propagation of many spatial modes) for wavelength &lgr;. Considering the single-mode case, the properties of optical fiber pulse propagation include: a frequency dependent loss, material dispersion giving rise to pulse broadening, and waveguide dispersion. At the “zero dispersion” point where the material dispersion changes sign (for instance, in standard telecommunications fiber, at ~1300 nm) pulses may propagate without significant broadening. However, as the material dispersion effect decreases, waveguide dispersion becomes significant, arising from the confinement of the mode at the core-cladding interface. In multi-mode fiber, the situation is further complicated by the addition of many spatial modes which may produce further temporal broadening. However, multi-mode fiber is of interest in a number of applications due to its higher tolerance to misalignment.
In long-haul fiber-optic telecommunications systems, there exists the problem of high bit-error-rates due to broadening of optical signal pulses along the long optical fiber delivery lengths. This problem has been addressed using various schemes, including dispersion compensation by using specially designed optical fibers, pre-chirping of the pulses, possibly using optical fiber gratings for either of these techniques. However, the peak powers of the signals used in these systems are below the onset of non-linear effects; these systems do not address the delivery of high peak power (high peak power is herein defined as >1 kW) pulses through the optical fiber.
One sys
Fermann Martin E.
Galvanauskas Almantas
Harter Donald J.
Stock Michelle L.
Sucha Gregg D.
Imra America Inc.
Spyrou Cassandra
Sughrue Mion Zinn Macpeak & Seas, PLLC
Treas Jared
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