Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2001-04-27
2003-08-19
Porta, David (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S339010
Reexamination Certificate
active
06608309
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for measuring the concentration of trace amounts of OH groups that are contained in quartz glass, especially in quartz glass to be used in preforms of optical fibers, cells for use in spectroscopy and lamps.
In particular the invention relates to a method for measuring a low concentration of OH groups (less than 1 ppm) at high precision.
BACKGROUND OF THE INVENTION
Much attention is paid on the content of OH groups that are incorporated in quartz glass, for instance, in optical fibers, because OH groups increase the transmission loss of light. Conventionally, an infrared spectrophotometer has been used to measure the concentration of the OH groups contained in quartz glass, and by measuring the height of the absorbance peak in the near infrared region attributed to the OH groups, the concentration is calculated in accordance with the following equation:
C
sample
=&Dgr;A/L&egr;×
(
M
H
+M
O
)/
×10
3
(1),
wherein C
sample
represents the concentration of OH groups that are present in the sample test piece (in wt·ppm); &Dgr;A represents the peak height (ABS); L represents the length (cm) of optical path; &egr; represents the absorption coefficient (1·mol
−1
·cm
−1
),
M
H
represents the atomic weight of hydrogen; M
O
represents the atomic weight of oxygen; and
represents the density (g·cm
−3
) of quartz glass.
However, in the measuring method above, the intensity of the absorbance peak decreases with decreasing concentration of OH groups making the accurate measurement of the concentration difficult. Thus, even if the absorbance peak having the highest sensitivity at a wavelength of 2720 nm is used, the detection limit of the concentration for an optical path length of 1 cm is said to be about 1 ppm. Accordingly, since the height of the absorbance peak increases in proportion to the length of optical path, it has been proposed to make the optical path longer, but a longer optical path length results in an increase in absorbance for the longer wavelength side due to the increase in base absorption of the quartz glass. This led to a loss in measurement precision due to the baseline inclining upward with increasing wavelength. Although it is possible to circumvent the interference of this base absorption by selecting the absorbance peak at a wavelength of 1380 nm attributed to OH groups, it was still unfeasible to perform measurements with high precision if a conventional test specimen was used, because the sensitivity was lowered to about {fraction (1/160)}. In the light of such circumstances, the concentration of OH groups that are present in trace quantity in quartz glass has been determined by preparing optical fibers of the quartz glass, and by then selecting the less sensitive absorbance peak at a wavelength of 1380 nm by taking advantage of the superior light transmission characteristics and the increase in optical path length.
However, in the conventional method described above, the measurement was made possible only after the quartz glass is drawn into an optical fiber, and it was impossible to determine the concentration of the OH groups that are incorporated in trace quantities in the preforms. Furthermore, it was not feasible to measure the concentration of the OH groups contained in the quartz glass that are used in applications other than optical fibers such as the cells for use in spectroscopy, quartz glass tubes for lamps, etc.
SUMMARY OF THE INVENTION
In the light of the circumstances above, an object of the present invention is to provide a simple method for measuring the concentration of OH groups that are contained in trace quantity in quartz glasses of any type, yet at high precision.
Furthermore, another object of the present invention is to provide a method for measuring the concentration of OH groups that are present in the quartz glass at a quantity of less than 1 ppm, yet at a high precision.
To achieve the objects above in accordance with the present invention, there is provided, in measuring the concentration of OH groups contained in quartz glass by means of infrared spectroscopy, a method for measuring the concentration of OH groups that are contained in trace quantity in quartz glass, which comprises preparing a set of test pieces comprising a reference test piece from a blank of a quartz glass body whose OH group content is known and a sample test piece from a quartz glass body whose OH group content is to be measured, whereby the test pieces each have two planar planes faced to each other; setting the sample test piece and the reference test piece in an infrared spectrophotometer; successively irradiating perpendicular to one of the two planar planes an incident infrared radiation in a wavelength region of approximately 2500 nm to approximately 2950 nm, while simultaneously detecting the outgoing radiation spectrum from the other plane; obtaining the difference of the outgoing radiation spectrum of each of the test pieces; selecting the absorbance peak assigned to OH groups at a wavelength of 2720 nm to obtain the peak height thereof; and calculating the concentration of OH groups from the peak height in the sample test piece.
For calculating of the concentration of OH groups that are present in a trace quantity in the sample test piece the following equation is suitable:
C
sample
=C
blank
+&Dgr;A/L&egr;×
(
M
H
+M
O
)/
×10
3
(2),
wherein, C
sample
represents the concentration (wt·ppm) of OH groups that are present in the sample test piece; C
blank
represents the concentration (wt·ppm) of OH groups that are present in the reference test piece; &Dgr;A represents the peak height (ABS.); L represents the length (cm) of optical path; &egr; represents the absorption coefficient (1·mol
−1
·cm
−1
), M
H
represents the atomic weight of hydrogen; M
O
represents the atomic weight of oxygen; and
represents the density (g·cm
−3
) of quartz glass.
In the measurement method according to the present invention, the baseline inclined by the presence of base absorption can be set horizontally by calculating the concentration of OH groups that are present in a trace quantity in the quartz glass by obtaining the difference spectrum. Furthermore, by selecting the highly sensitive OH groups absorbance peak at the wavelength of 2720 nm, the concentration of OH groups that are present in a trace quantity of lower than 1 ppm can be obtained. Moreover, since the method does not require preparing an optical fiber therefrom, the concentration of OH groups that are present in trace quantity can be measured on any type of quartz glass bodies.
The method according to the present invention requires a quartz glass body to use as a blank whose OH group content is known. However, to measure the concentration of OH groups contained in the sample test piece at high precision, it is preferred to use a blank test peace made from a quartz glass body free from OH groups, i.e., a quartz glass body having an OH content of “0” percent. “A quartz glass body free from OH groups” signifies a quartz glass body whose part is made into a fiber and whose spectrum is measured to confirm that there is no absorption at 1380 nm ascribed to the presence of OH groups. By using this quartz body, the concentration of OH groups that are present in trace quantity can be obtained with high precision.
The precision of measurement can be increased by providing two planes disposed faced to each other and through which the light is passed in such a manner that are placed in parallel with each other and providing them as mirror finished planes, or polished planes or cut planes coated with an oil for matching the refractive index or a reflection preventive agent, such that they yield a center line average roughness R
a
of 10 &mgr;m or less. A reflection preventive agent is an organic substance free from OH groups and having an effect of reducing the reflection index. Furthermore, an increase in planar precision enables measuring OH gr
Fukui Yasushi
Humbach Oliver
Ikeda Toru
Ochiai Hidenori
Sattmann Ralph
Heraeus Tenevo AG
Milde & Hoffberg LLP
Porta David
Sung Christine
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