1. Field of the Invention
This invention relates in general to the measurement of electrical signals of very wide frequency range. More particularly, the invention relates to the measurement of the frequential characteristics of electrical signals of very wide frequency range by a method of electrooptic measurement.
2. Description of the Prior Art
J. A. VALDMANIS and G. MOUROU recently developed a method for measuring the waveform of an electrical signal of very wide range by electrooptic sampling. This measurement method enables temporal resolution of approximately one picosecond to be achieved. In the article entitled "Subpicosecond Electrooptic Sampling:Principles and Applications" which was published in the IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-22, No. 1, JANUARY 1986, J. A. VALDMANIS and G. MOUROU describe the general configuration of an electrooptic sampling measurement system.
This electrooptic sampling measurement system utilizes the existence of birefringence in certain crystals with an electrooptic property. When a rectilinearly polarized light wave encounters an electrical wave produced by an electrical signal in a berefringent crystal, its polarization is rotated as a result of the interaction with the electrical wave. By placing itself in particular geometrical conditions, the interaction between the two waves induces a phase lag in the light wave. The light wave observed in cross-polarization then has an intensity modulated by the electrical signal and the variations in intensity of the light wave need only be measured by an optical detector to retrace the electrical signal. To achieve very good temporal resolution, a light wave formed by a pulse train of very small width is used to sample the electrical signal, it is possible to explore the temporal evolution of the electrical signal. The method is similar to stroboscopics for repetitive signals. The measurement of the intensity of the light wave is carried out at very low frequency and consequently with a conventional, very low noise and high-performance optical detector.
The temporal resolution of a system of measurement by electrooptic sampling is higher than that of conventional measurement systems of the purely electronic type such as the sampling oscilloscope. This superiority is mainly due to the fact that the polarization rotation of the light wave in the electrooptic crystal is an instantaneous phenomenon which does not have a measurable time constant as a result of which the main limitation of the temporal resolution of such a system is the width of the sampling pulses of the light wave.
However, the main drawback of electrooptic sampling measurement systems is in the fact that it is necessary to utilize a pulsing laser source which issues light pulses of very small width, i.e. subpicosecond. In fact, a laser source of this type usually has a length of several meters and is therefore very cumbersome. Moreover, it is difficult to adjust and is relatively expensive. Another major drawback of this measurement system is that it is only possible to measure signals with repetition frequencies that are multiple integers of the repetition frequency of the light pulses issued by the laser source. It can be conclued from these two drawbacks that electrooptic sampling measurement systems are as yet experimental systems that are difficult to industrialize and market.