Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-08-10
2004-03-02
Davie, James (Department: 2828)
Optical waveguides
With optical coupler
Input/output coupler
Reexamination Certificate
active
06701044
ABSTRACT:
RELATED APPLICATIONS
This application is related to application entitled “Compound Light Source Employing Passive Q-switching and Nonlinear Frequency Conversion” and application entitled “Pulse Sequencing for Generating a Color Image in Laser-Based Display Systems”, both of which are being filed on the same day as this application.
FIELD OF THE INVENTION
The present invention relates generally to solid state light sources employing lasers with passive Q-switches and nonlinear frequency converters to generate light in the UV wavelength range for writing Bragg gratings and for other photolithographic applications.
BACKGROUND OF THE INVENTION
Fiber Bragg gratings are quietly revolutionizing modern telecommunication systems and are enabling new types of optical fiber sensors which have the potential to displace equivalent electrical sensor devices. Therefore, it is important to develop suitable apparatus and methods for producing Bragg gratings efficiently and reliably.
Typically, Bragg gratings are written in a photosensitive core of a fiber by illuminating it with an exposure beam at a UV wavelength within a photosensitive range of the core. For example, a Bragg grating is written in a core containing an oxygen deficient matrix in glass (e.g., the core has germanium oxygen deficient centers). Such matrix is highly photosensitive in a range between 240 to 250 nm, where it has an absorption band peaking at about 242 nm. Hence, most commonly employed source of radiation in the UV wavelength range have exposure wavelengths between 240 and 250 nm.
At present, methods for writing Bragg gratings include interferometric techniques, phase mask techniques and point-by-point techniques. There are many variants for each of these three methods, and each requires a suitable light source for generating an exposure beam in the UV wavelength range. Among the most common light source employed for writing Bragg gratings are UV laser sources such as frequency-doubled optical parametric oscillators, narrowed-linewidth 248 nm KrF excimer lasers, intracavity frequency-doubled Argon ion lasers, frequency doubled Ca vapor lasers, frequency quadrupled Nd:YAG lasers. Meanwhile, frequency-doubled optical parametric oscillators pumped by a frequency tripled Q-switched Nd:YAG laser have been used to make fiber Bragg gratings, but such systems tend to be complex and expensive. We note that such systems can be all-solid-state and diode-pumped.
Besides writing Bragg gratings, many materials processing applications include a photolithographic step during which a processed material is exposed to UV radiation. The light sources used for the exposure should be stable, efficient and spectrally pure high-power light sources. For efficient exposure the power level of such light sources should be in the range of several hundred milliWatts and more, e.g., 1 Watt or more. Furthermore, such light sources should be inexpensive to produce and they should generate light in the appropriate portion of the UV wavelength range between 200 nm and 330 nm.
Currently, the most commonly used sources of UV radiation for photolithographic applications such as processing of semiconductor wafers employ excimer lasers of various wavelengths. Excimer lasers at 248 are also the most commonly used UV sources for producing fiber Bragg gratings. Meanwhile, frequency doubled Argon laser emitting at 488 nm (yielding an exposure wavelength of 244 nm) provides the best performance for producing fiber Bragg gratings. Unfortunately, this source is very bulky, cumbersome and expensive to use. For more general information on photolithography using UV radiation the reader is referred to U.S. Pat. No. 5,367,588 to Hill et al. and to U.S. Pat. No. 5,940,568 to Losch et al. addressing the application of photolithographic methods as applied to writing Bragg gratings in fibers.
The prior art teaches various types of light sources for generating light in the visible and UV ranges. A number of these sources rely on a nonlinear frequency conversion operation such as second harmonic generation (SHG) to transform a frequency outside the visible range, e.g., in the IR range, to the desired deep blue or UV frequency. For example, U.S. Pat. No. 5,751,751 to Hargis et al. teaches the use of SHG to produce deep blue light. Specifically, Hargis et al. use a micro-laser which has a rare earth doped microlaser crystal and emits light at about 914 nm to drive SHG in a crystal of BBO producing output at about 457 nm.
U.S. Pat. No. 5,483,546 to Johnson et al. teaches a sensing system for high sensitivity spectroscopic measurements. This system uses a passively Q-switched laser emitting light at a first frequency. The light from the laser is transmitted through a fiber and converted to output light at a second frequency in the UV range. The conversion is performed by two frequency doubling crystals disposed far away from the Q-switched laser.
U.S. Pat. No. 6,185,236 to Eichenholz et al. teaches a self frequency doubled Nd:doped YCOB laser. The laser generates light of about 400 mW power at about 1060 nm and frequency doubles it with the aid of a frequency doubling oxyborate crystal to output light in the green range at about 530 nm. Eichenholz et al. combine the active gain medium and the frequency doubler in one single element to produce a compact and efficient light source.
In U.S. Pat. Nos. 5,745,284 and 5,909,306 Goldberg et al. teach a solid-state spectrally pure pulsed fiber amplifier laser system for generating UV light. This system has a fiber amplifier in a resonant cavity and an acousto-optic or electro-optic modulator incorporated into the cavity for extracting high-peak-power, short-duration pulses from the cavity. These short pulses are then frequency converted in several non-linear frequency conversion crystals (frequency doubling crystals). The addition of the modulator into the cavity for extracting the pulses and placement of the fiber amplifier within the resonant cavity renders this system very stable and capable of delivering a spectrally-pure pulse. Unfortunately, this also makes the system cumbersome and expensive.
U.S. Pat. No. 5,740,190 to Moulton teaches a three-color coherent light system adapted for image display purposes. This system employs a laser source and a frequency doubling crystal to generate green light at 523.5 nm. Moulton's system also generates blue light at 455 nm and red light at 618 nm by relying on frequency doubling and the nonlinear process of optical parametric oscillation.
Q-switched lasers operating on the 3-level ~980 nm transition of Yb have been demonstrated. For example, in ‘Three-level Q-switched laser operation of ytterbium-doped Sr
5
(PO
4
)
3
F at 985 nm’ (A. Bayramian, et. al., Opt. Lett. Vol 25, No. 9, Pg. 622-625, May 1, 2000) the authors showed that Yb:SFAP can be Q-switched on this transition, however they had to resort to a complex and inefficient pumping scheme. The authors point out the usefulness of the 2
nd
and 3
rd
harmonic of this laser wavelength, but fail to identify the 4
th
harmonic at 246 nm as attractive. Additionally, they do not indentify writing of fiber Bragg gratings or other photolithographic applications.
Unfortunately, the light sources described above and various other types of light sources taught by the prior art can not be employed to make stable, low-cost, efficient sources of light delivering UV radiation of sufficient power for writing Bragg gratings and other photolithographic applications. This is in part due to the fact that frequency conversion, e.g., frequency doubling in crystals, is not a very efficient operation. If the frequency doubling crystal had extremely high non-linearity, then low power continuous wave (cw) lasers could be efficiently doubled to generate output power levels near 1 Watt. However, in the absence of such frequency doubling crystals high-peak-power, short pulse lasers are necessary to obtain frequency doubled light at appreciable power levels. It should also be noted that providing such high-peak-power short pulses adds complexity to the design of
Arbore Mark A.
Kane Thomas J.
Kmetec Jeffrey D.
Davie James
Lightwave Electronics
Lumen Intellectual Property Services Inc.
LandOfFree
Solid state laser generating UV radiation for writing fiber... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Solid state laser generating UV radiation for writing fiber..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Solid state laser generating UV radiation for writing fiber... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3204295