As public known, there is an optical spectrum analyzer as a measurement device for measuring a wavelength component which is contained in an optical signal and a level (optical intensity) of the wavelength component. The optical spectrum analyzer disperses the optical signal into fine wavelength components by using the spectrometer, converts each optical intensity to an electric signals by receiving each wavelength component and generates a measurement display which indicates a relationship for the wavelength and the optical intensity by a horizontal axis (wavelength axis) and a vertical axis (optical intensity axis) according to these electric signals so as to show as an image. Some of these optical spectrum analyzers display an optical frequency which is indicated in the horizontal axis instead of the wavelength
When an optical signal is measured by using such an optical spectrum analyzer, an operator sets a measurement scale (measurement range) of the optical spectrum analyzer by operating and inputting the wavelength range which is supposed to be measured. In addition, an optical intensity over the above measurement scale is measured by the optical spectrum analyzer by operating and inputting the measurement starting command. When the measurement of the optical intensity over the above measurement scale is completed, the optical spectrum analyzer generates the measurement display according to the measurement result which is stored in an internal memory so as to display its image. Here, such an optical spectrum analyzer is disclosed in a commonly know document such as Japanese Patent No. 287709.
By the way, a function for changing the display range for the measurement result which is obtained in advance by changing the measurement scale after performing the measurement is provided in the above conventional optical spectrum analyzer. For example, when a display range which is larger than the measurement scale under condition that the measurement is performed is operated and input, the optical spectrum analyzer display the wavelength range for the measurement result in a contracted manner. That is, in such a case, the measurement display which is displayed on the wavelength axis is set to have a larger range than the measurement scale setting section under condition that the measurement is performed; therefore, the wavelength range for the measurement result is compressed. In contrast, a display range which is narrower than the measurement scale under condition that the measurement is performed is operated and input, the optical spectrum analyzer displays a part of the measurement result which is designated for this display range in an enlarged manner. That is, in such a case, the measurement display which is displayed on the wavelength axis is set to have a narrower range than the measurement scale setting section under condition that the measurement is performed; therefore, the wavelength range for the measurement result is enlarged.
However, in such a conventional optical spectrum analyzer, the measurement scale is changed so as to set the wavelength range for the measurement result which is displayed as the measurement display such as the display range for the measurement result; thus, it is not possible to realize a desirable operability. That is, if it is necessary to perform the measurement operation by the measurement scale which is used previously, it is necessary to set the same wavelength range as the previous measurement scale again; thus, such an operation is undesirable from an operability point of view.
Recently, in particular, a signal light (WDM: Wavelength Division Multiplex) in which a plurality of lights which have a plurality of different wavelengths are multiplexed has been used more commonly and rapidly along with an increase in the transmitted information amount for the optical information communication which is caused by a prevalence of Internet, etc., in the signal light for the optical information communication as an example for an object for measurement of the optical spectrum analyzer. In the optical communication in which the WDM is used, the lights which have different wavelengths are handled as channels respectively; thus, the transmission capacity increases by increasing the multiplexed channels. A range for the wavelength which is contained in the signal light which is used in such an optical information communication is large together with the increase of the number of the channels. In addition, if the optical spectrum for such an signal light is measured by the optical spectrum analyzer, the operator sets the measurement scale in a wide range so as to measure a plurality of channels in a single measurement operation. In addition, individual channels are displayed in an enlarged manner so as to analyze an optical level and a spectrum width of each channel after the measurement.
However, in the conventional optical spectrum analyzer, it is necessary to change the measurement scale within a displayed wavelength range so as to display each channel in an enlarged manner after the measurement. Therefore, if it is necessary to display each channel in an enlarged manner again and measure the wide range of wavelength after measuring the wide range of wavelength, it is necessary to set the measurement scale again; thus, such an operation is undesirable from an operability point of view. That is, there is a problem in the conventional technology that it is not possible to change the display range without changing the measurement scale. In other words, it is not possible to display the wavelength axis either in a contracted manner or an enlarged manner.