Optics: measuring and testing – By light interference
Reexamination Certificate
2000-05-15
2002-09-24
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
Reexamination Certificate
active
06456380
ABSTRACT:
This application is based on Japanese Patent Application Nos. 11-139087 (1999) filed May 19, 1999 and 11-260816 (1999) filed Sep. 14, 1999, the contents of which are incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for measuring a temporal waveform of the electric field of an optical signal, and particularly to a technique for accurately measuring temporal variations in intensity and phase of ultra-fast optical signals whose temporal waveforms cannot be measured with an ordinary means because of their much faster changes than the response time of existing high-speed optical detectors or electronic circuits. Generally, a simultaneous measuring of the intensity and phase enables full determination of the characteristics of an optical signal as a classical electromagnetic wave. Therefore, such types of measuring methods are referred to as a method for measuring a temporal waveform of the electric field of an optical signal.
2. Description of the Related Art
Ultra-fast optical signals in a picosecond to femtosecond order, which have no suitable optical detector with a sufficient time resolution, have been observed by correlators in the recent twenty years. Such correlators make an optical signal to be measured and a reference optical pulse incident and focused onto a nonlinear medium, and measure the integrated power of the generated light in terms of a function of a time relationship (delay time) between the optical signal to be measured and the reference optical pulse. In this case, the time resolution is determined only by the response time of the nonlinear medium and the width of the reference optical pulse, completely independently of the response time of an optical detector or the like for carrying out optic-electrical conversion of the generated light. Using the optical signal to be measured itself as the reference optical signal provides a simplest and versatile measuring method. The signal obtained in this measurement is called an autocorrelation signal, and the apparatus for measuring it is called an autocorrelator.
It was once considered difficult to obtain the waveform of the optical signal to be measured from the autocorrelation signal without imposing any conditions.
At the present, however, it is known that by complementing the data by the spectrum of the optical signal to be measured, an iterative calculation enables the simultaneous calculation of the intensity and phase of the optical signal to be measured. Such a method is disclosed in Japanese Patent Application Publication No. 5-2252 (1993) (Japanese Patent Application No. 61-211100 (1986) “Method for measuring and estimating ultrashort optical pulses” by Kazunori Naganuma and Juichi Noda, or U.S. Pat. No. 4,792,230 “Method and apparatus for measuring ultrashort optical pulse”, by Kazunori Naganuma and Juichi Noda.
Another analogous methods are carried out which observe a spectrum of light generated in an nonlinear medium, and calculate the intensity and phase of an optical signal to be measured by performing iterative calculation using two-dimensional data of two variables, the spectrum component of the generated light and the time delay. For example, such a method is described in detail in R. Trebino, K. W. Delong, D. N. Fittinghoff, J. N. Sweetser. M. A. Krumbuegel and B. A. Richman “Measuring ultrashort laser pulses in the time-frequency domain using frequency-resolved optical gating”, Review of Scientific Instrument, Vol. 68 (1997), pp. 3277-3295.
However, such methods for measuring waveforms of optical signals have an ineradicable drawback in that they cannot deal with optical signals with complicated waveforms, and hence cannot fulfill the needs of the measurement in the recent optical information processing field. For example, as a simplest example of an optical signal waveform that those methods cannot deal with, is known a pair of pulses with different phases.
Furthermore, since the iterative calculation is nonlinear, it is very difficult to estimate the effect. of noise or error in the data used for the calculation on the calculation results. In a worst case, the calculation falls into a chaotic behavior so that a slightest variation in the input can result in an entirely different output. None of the foregoing iterative calculations are guaranteed that such behavior does not take place. Thus, the quality and accuracy of measuring apparatuses based on such methods cannot be warranted because of the intrinsic ambiguity of the iterative calculation.
Let us consider here the reason why the measurement of the temporal waveform of the electric field of an optical signal requires the iterative calculation. It is well known that Fourier transform can bring any temporal waveform into one-to-one correspondence with a spectrum in a frequency domain. In other words, once a spectrum of a signal is known, its temporal waveform can be obtained by inverse Fourier transform. As for the electric field of optical signals, their power spectra are usually measured using a common optical spectrum observing method, and hence it is not difficult to obtain the magnitudes (amplitudes) of the spectra of the optical signals. However, without acquiring the phases of the spectra, the corresponding temporal waveforms cannot be obtained from the spectra using the inverse Fourier transform. Here, with the optical signals, there are circumstances that the measurement of the spectral phases is not obvious at all.
Such circumstances that although the magnitude of some physical quantity is known, its phase is difficult to obtain are often encountered in various scientific fields. Thus, to mathematically handle the circumstances, there has been established a distinct academic branch called phase retrieval problem. The foregoing iterative calculation is considered as an application of a method for a phase retrieval problem to the measurement of optical signals.
Direct observations of the spectral phase will enable temporal waveforms of electric field of optical signals to be obtained without using the iterative calculation involving the complicated phase retrieval problem. Recently, such a direct measuring method of the spectral phase has attracted attention, and some proposals have been made that can be called frequency shearing interferometer.
A classical interferometer detects in a direct current (DC) manner an interference signal that is obtained by splitting an optical signal into two waves, and by recombining and interfering them again. In this case, the spectral components of the same frequency are superimposed so that those frequencies are mutually nullified, and thus the DC component is generated. However, since the phases of the spectral components are completely canceled out at the same time, the spectral phases can never be observed. Even if a fast optical detector of today is used to observe the interference signal, because it cannot follow the ultra-fast optical signal in a picosecond to femtosecond order to be observed here, it cannot exceed the realm of the DC observation, providing only the same result.
In view of this, a frequency shearing interferometer is proposed that splits an optical signal into two portions, shifts the frequency of one of them, and recombines them to interfere with each. other. Assuming the shift amount to be &Dgr;&ngr;, the spectral components with the frequency shift &Dgr;&ngr; superimpose on each other to cancel out the frequency components, thereby generating a DC component. The magnitude of the DC component depends on the phase difference between the spectral components with the frequency shift &Dgr;&ngr;. This enables the spectral phase to be directly observed in terms of the difference using &Dgr;&ngr; as a step. The principle of such a frequency shearing interferometer is disclosed in V. Wong and I. A. Walmsley “Analysis of ultrashort pulse-shape measurement using linear interferometers”, Optics Letters, Vol. 19 (1994), pp. 287-289.
If the shift amount &Dgr;&ngr; is too small, the phase diffe
Nippon Telegraph and Telephone Corporation
Ostrolenk Faber Gerb & Soffen, LLP
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