Optics: measuring and testing – By light interference – Having polarization
Reexamination Certificate
2002-04-30
2002-10-08
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Having polarization
C356S453000
Reexamination Certificate
active
06462827
ABSTRACT:
FIELD OF THE INVENTION
The invention is directed to systems and methods for measuring and controlling wavelength, and more particularly to such systems and methods that accurately measure and control the output wavelength of CW and pulsed lasers.
BACKGROUND OF THE INVENTION
Measuring and controlling a laser wavelength accurately over a large continuous wavelength range at low continuous or pulsed input power and/or at high pulse repetition rate is a difficult and important problem in many areas of technology, including excimer laser microlithography and wavelength division multiplexed (WDM) fiber optic communication systems. Previously disclosed approaches for measuring laser wavelengths suffer from one or more limitations, including poor sensitivity, difficult calibration, long measurement times, or the physical size of the equipment.
At present there exist at least four methods for measuring laser wavelength: grating spectrometers, etalon-based wavelength meters, and scanning Michelson interferometers, as well as a recently introduced phase sensitive wavelength meter. While all four methods can be used to measure continuous lasers, only grating spectrometers and etalon-based wavelength meters have been found acceptable for pulsed laser applications.
The resolution of a grating spectrometer is equal to the product of the diffraction order number m and the number of grooves N:
&lgr;/&Dgr;&lgr;=
mN
(1)
Equation (1) implies that the resolving power of the grating is equal to the number of wavelengths in the path difference between rays that are diffracted in the direction of the center wavelength of the laser line from the extreme ends of the gratings. In laser wavelength metrology it is often required to only locate the center of the laser spectrum with extreme accuracy, not to resolve closely spaced individual spectral lines. Still, in order to achieve the required linewidth line center localization of 0.1 pm, or better, very large grating spectrometers are required. In addition, the key to accurate wavelength measurement is the incorporation of very accurate and stable calibration sources into the spectrometer. Stable laser sources of known wavelength, such as frequency doubled Argon Ion laser light at 244 nm, or stabilized Helium-Neon lasers at 632.99 nm may be used. Another technique uses known absorption lines that are backlit by a broad band light source and the resulting absorption spectrum may impinge on the recording plane of the spectrometer onto a charge coupled device (CCD) or photodiode array detector.
Etalon-based wavelength meters are significantly smaller than high-resolution grating based spectrometers, and normally have higher resolution as well. A fixed Fabry-Perot etalon is composed of parallel, high reflectivity, mirrors separated by a defined spacing. Other types of etalons such as Fizeau or scanning Fabry-Perot etalons maybe used, however, since many of their performance characteristics are the same as fixed Fabry-Perot etalons they will not be reviewed here. We review the performance characteristics of the Fabry-Perot etalon based wavelength meter as an example of this category of wavelength meters. If the Fabry-Perot etalon is illuminated by collimated light, only a very narrow wavelength will be in resonance with the cavity and be transmitted. This geometry is not generally used in wavelength meter applications. If a diverging beam illuminates the etalon, transmission occurs only at angles of incidence such that the path length between mirrors is an integral number of incident wavelengths. This geometry is routinely used in laser wavelength meters.
An Etalon is characterized by a figure of merit called its finesse F which equals:
𝔍
=
π
R
1
-
R
(
2
)
Where R is the reflectivity of each mirror, assuming both cavity mirrors have the same reflectivity. For Fabry-Perot etalons illuminated with a diverging beam the resolving power &Dgr;&lgr;/&lgr; equals:
λ
λ
=
0.97
*
m
*
𝔍
(
3
)
where m is the order number representing an integer number of wavelengths that separate the mirrors. At near normal incidence m≈2n&lgr;/&lgr;, where n is the refractive index of the medium between the mirrors at the wavelength X, and h is the physical separation of the mirrors. In order to use an etalon properly one must know the spacing between the mirrors to within an order number, or else there is ambiguity in determination of wavelength. The wavelength range between consecutive orders is called “Free Spectral Range” (FSR) of the etalon, where:
FSR=&lgr;/m=&lgr;
2
/2
n&lgr;
(4)
Thus as can be seen from equation (4) the FSR decreases as the separation between the mirrors h increases. The resolution of the etalon also increases as the FSR increases. It is known that etalons themselves and the optics associated with an etalon-based wavelength meter are not absolutely stable. Therefore, the etalon spacing and angles of incidence of the laser beam must be continuously calibrated either with a stable reference laser, such as a HeNe laser, or with a backlit absorption cell having well-calibrated absorption lines. The calibration light should be well reflected by the etalon mirrors, and with adequate, such that the resolution of the reference lines is equivalent to that of the unknown laser lines. The spectra of the reference laser and of the excimer laser are both recorded by an array detector, usually a charge-coupled device (CCD), or a photodiode array. Ideally, a single array is used that is sensitive to both the reference and laser wavelengths to be measured.
A single etalon may be used to improve the measurement accuracy of the laser wavelength to 1 part in 50-100 of the Etalon's FSR. It the laser wavelength is not known to the accuracy of about ½ the FSR, and then one or more additional etalons of reduced resolution can be used to more accurately estimate the wavelength before finally determining the laser wavelength with the etalon having the longest spacing.
Both spectrometer and etalon based wavelength measurement techniques suffer from serious drawbacks in wavelength measurement. Both techniques can use array detectors to measure an excimer laser spectrum. Array detectors can be read out at the 1-4 kHz sample rates required, but are inherently noisy during high speed scanning operation. This results in reduced signal detectivity and thus poor accuracy at higher data rates. Slower data rates result in a better signal-to-noise ratio, but may not properly record all the laser pulses. Spectrometers of adequate resolution for high-resolution laser wavelength measurement are large instruments, have very large gratings, are difficult to absolutely calibrate. Etalon spectrometers are inefficient. In transmission, light at only the correct angle produces interference orders. Therefore only about 1-5% of the incident light is used in measuring wavelength. If more than one etalon is required, due to inadequate initial knowledge of the laser wavelength, the efficiency drops rapidly as the incident power must be divided into several beams.
High-resolution scanning Michelson wavelength meters are often used to measure the wavelength of continuous or very high (greater than megahertz) repetition rate lasers. They cannot be used to measure pulsed lasers of low repetition rate (kHz). In a scanning Michelson interferometer the laser to be measured is split into 2 paths by a beamsplitter. At the end of each path, called an arm of the interferometer, is a reflecting mirror that directs the beams back to the beamsplitter. The 2 beams are recombined and the signal is recorded on a photodiode. The path length of one arm of the interferometer is continuously varied and a time sequence of the intensity of the two interfering beams is recorded by the digitized output of the photodiode. If the path length of the light in each arm differs by an integral number of wavelengths of the reference beam than the two beams will combine constructively. If the path length differs by ½ of a wavelength, th
Chromaplex, Inc.
Font Frank G.
Natividad Phil
Ropes & Gray
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