1. Field of the Invention
The present invention relates to optical attenuator and optical power meter calibration systems with an optical-pulse conversion and averaging
In addition, the present invention relates to an optical attenuation calibration method and apparatus, and an optical power meter calibration apparatus for calibrating the optical attenuation or linearity of an object to be calibrated and, more particularly, to an optical calibration apparatus mainly used to calibrate the optical attenuation of an optical component such as an optical attenuator and to calibrate the linearity of an optical power meter
Furthermore, the present invention relates to an apparatus capable of setting an accurate optical attenuation in calibration requiring high-precision measurement and to an optical attenuation calibration method and apparatus, and an optical power meter calibration apparatus for performing calibration by using the set optical attenuation as a standard optical attenuation.
2. Description of the Related Art
There is a great demand for accurate measurement of a low-level optical power. Such measurement has been performed in the following manner.
a) An optical absolute power for calibration is calibrated at a high level point The optical absolute power is then attenuated by an accurate optical attenuator having no errors. A low-level optical power is calibrated and measured by comparison with this optical absolute power. However, an accurate optical attenuator having no errors which can be used in this method is not available. Therefore, satisfactory accuracy cannot be obtained.
b) If a low-level optical power is to be directly measured, a large error is inevitably caused by the influences of the linearity of a photodetector.
Currently, no calibration service for optical attenuation is available from any public institution in any country.
More specifically, in a conventional method, measurement of the optical attenuation of an optical attenuator is generally performed by an arrangement shown in FIG. 10A. In the arrangement shown in FIG. 10A, if an optical power value indicated by an indicator 44 when light from a light source 41 is converted into an electrical signal by a photodetector 43 without causing it to pass through using an optical attenuator 42 is represented by P.sub.0 (W) as indicated by a dotted line, and an optical power value indicated by the indicator 44 when light passes through the optical attenuator 42 is represented by P.sub.1 (W) as indicated by a solid line, an optical attenuation A can be given by the following equation: EQU A=10.times.log(P.sub.0 /P.sub.1) (unit: dB)
In this case, since the photodetector 43 detects different optical powers, the linearity of the photodetector 43 causes measurement errors. If a photodiode sensor is used as the photodetector 43, the linearity is influenced by the diode characteristics. If a thermoelectric conversion type sensor (a Peltier effect element, thermopile, or the like) is used, the linearity is influenced by the thermoelectric conversion characteristics, and heat radiation and convection.
If such linearity of the photodetector 43 is calibrated in advance, correction can be performed. For this purpose, however, a calibration standard having an accurate optical attenuation is required.
Assume that a standard optical attenuator 45 having an accurate optical attenuation is available. In this case, if the attenuator 45 is inserted between the optical attenuator 42 and the photodetector 43, as indicated by FIG. 10B, so as to perform measurement (or calibration) by a serial substitution method, since the photodetector 43 receives substantially the same optical power, measurement is substantially free from the influences of the linearity.
The serial substitution method will be briefly described below with standard to FIG. 10B. If, for example, a nominal value of 10 dB of an optical attenuator 42 is to be calibrated, the standard optical attenuator 45 having an attenuation which can be accurately changed to 0 dB and 10 dB is inserted in series. The attenuation of the standard optical attenuator 45 is set to be 10 dB, and an optical signal from the light source is measured through the attenuator 45 without the optical attenuator 42, as indicated by the dotted line. At this time, a value P.sub.2 (W) indicated by the indicator 44 is read. Then the attenuation of the standard optical attenuator 45 is set to be 0 dB, and the optical attenuator 42 is inserted in series, as indicated by the solid line. If the value indicated by the indicator 44 at this time is P.sub.3 (W), an accurate attenuation of the optical attenuator 42 is given by [10+10.times.log(P.sub.2 /P.sub.3)]dB.
According to this calibration technique, the standard optical attenuator is indispensable. Currently, however, such a standard optical attenuator is very difficult to obtain.
A method of setting a standard optical attenuation in calibration is disclosed in Japanese Patent Application No. 62-193068 (Published Unexamined Japanese Patent Application No. 64-35323) assigned to the same assignee as that of the present application. An outline of this method will be described below with standard to FIG. 11. Referring to FIG. 11, first and second light amount adjusting units 53 and 54 are adjusted such that the optical powers of first and second light from light sources 51 and 52 for generating light beams having wavelengths close to each other become equal to each other. Assume that, after this operation, a value of power received by an optical power meter 58 under calibration when one of first and second switches 55 and 56 is turned on is represented by P.sub.A (W), and a value of power received by the optical power meter 58, which received synthesized light from a light synthesizing section 57 when both the switches 55 and 56 are turned on, is represented by P.sub.AB (W). In this case, the ratio of the optical powers (P.sub.AB /P.sub.A) is 3.01013 dB (P.sub.AB /P.sub.A =2). The linearity of the power meter 58 is calibrated by a 2.sup.N method disclosed in Published Unexamined Japanese Patent Application No. 64-35323 by using the ratio of the optical powers as a standard optical attenuation. In this case, the 2.sup.N method is a method of measuring linearity in a level range of 2.sup.N (or 2.sup. -N) by sequentially accumulating 2 times (or 1/2 times) a light amount (by means of addition of equal light amount) in measurement steps.
In the case shown in FIG. 11, as the first and second light sources 51 and 52, coherent light sources must be selected, which do not interfere with each other and have a wavelength gap enough to be free from the influences of the wavelength characteristics of the optical power meter 58. In the technique disclosed in Published Unexamined Japanese Patent Application No. 64-35323, since formation of optical attenuations other than 3.0103 dB is not proposed, in order to perform calibration in a wide range, complicated control such as the 2.sup.N method is required.
Note that prior arts associated with the present invention include an experimental apparatus wherein an average level of transmitted light is changed in accordance with a time in which a window formed in a rotating sector crosses the transmitted light (Journal of Research of the National Bureau of Standards, Vol. 76A, No. 5 September - October 1972, pp 437-453). However, no prior arts associated with a calibration apparatus and method of the present invention (to be described below) have been disclosed.
As described above, in the conventional methods, in order to realize a calibration apparatus for an optical attenuation, an optical attenuator having an accurate optical attenuation as a standard is required. However, such an attenuator is not currently available. Therefore, a complicated arrangement is required to calibrate the optical attenuation or insertion loss of an optical component such as an optical attenuator or to calibrate an optical power meter. In addition, the conventional methods require sophisticated techniques.