Automatic Counter of Interference Fringes

In many problems of interferometry, such as, for example, the comparison of standards of length with one another or with a wavelength, it becomes necessary to count an enormous number of interference fringes passing through the field of view. It is obvious that automation of this extremely laborious work is highly necessary for the purposes of metrology, especially in the invisible regions of the spectrum. At the same time, it would make it possible to substantially broaden the possibilities of measurements of this kind, in any case with respect to their speed or wider use. However, this task is greatly complicated by the following circumstance. Owing to unavoidable vibrations and imperfections of the mechanical construction of the interferometer, it is impossible to eliminate completely the backward motion of the interference fringes, especially in the case of comparatively rapid counting, for which the automatic device is intended. Thus, a counter of interference fringes must distinguish fringes moving in opposite directions and, accordingly, perform addition or subtraction operations. The paper under review is devoted to the description of such a counter*.

Fig. 1.

First of all, it is necessary to choose a path for distinguishing the direction of displacement of the fringes. This can be done if the intensity is measured

* E. R. Peck and S. W. Obetz, JOSA 43, No. 6, 505 (1953).

interference pattern at two points displaced relative to one another by approximately one quarter of the fringe width. In this case the maximum brightness of a fringe recorded at one point will correspond either to a decrease or to an increase in brightness at the other point, depending on the direction of motion of the fringes (Fig. 1). However, such a method is not directly applicable in view of the change in the scale of the fringes as the order of interference increases. The authors therefore resorted to a certain complication. They used an interferometer (Fig. 2) that simultaneously gives two identical interference patterns, differing only by a constant phase shift between the interfering beams. In fact, the phase difference introduced by the separating plates for the pattern perceived by photocell \(A\) is equal to \(\varphi_{\mathrm{st}}-\varphi_{\mathrm{air}}\), where \(\varphi_{\mathrm{st}}\) and \(\varphi_{\mathrm{air}}\) are the phase shifts upon reflection from the metallized surface on the glass side and on the air side. For the pattern perceived by photocell \(B\), this quantity is equal to \(2\varphi_{\mathrm{st}}-2\varphi_{\mathrm{met}}\), where \(\varphi_{\mathrm{met}}\) is the phase shift in transmission of light through the metallic film. Consequently, the phase shift between the two interference patterns is \(\varphi=2\varphi_{\mathrm{met}}-\varphi_{\mathrm{st}}-\varphi_{\mathrm{air}}\). Thus, between corresponding portions of the two interference patterns there is a phase shift \(\varphi\), independent of the scale of the interference pattern and, consequently, of the order of interference.

Fig. 2. Interferometer giving a doubled interference pattern.

Let us now suppose that the signal from photocell \(A\), after amplification, is fed to a counting circuit that gives one positive and one negative count if the photocurrent decreases from the value \(I_1\) to the value \(I_2\), and one negative count if the photocurrent increases from \(I_2\) to \(I_1\). At the same time, the photocurrent from photocell \(B\) is fed to a blocking circuit, which suppresses the count of the first circuit if the photocurrent from photocell \(B\) is greater than \(I_4\), and permits the first circuit to produce a count if this photocurrent is less than \(I_3\). If the photocurrent from photocell \(B\) has a value between \(I_3\) and \(I_4\), then the count of the first circuit is prohibited or permitted depending on the preceding value of the photocurrent. In Fig. 3 the amplitude of the photocurrent from photocell \(A\)

Fig. 3. Interval of an interference fringe that produces a count.

Fig. 4. Block diagram of the gating device.

Fig. 5. Circuit of a typical counter stage giving positive and negative counts.

as a function of the position of the interference pattern. The solid line corresponds to the region in which the counting circuit operates with a plus or minus sign, depending on the direction of displacement of the fringe.

The block diagram for connecting photoelements \(A\) and \(B\) is shown in Fig. 4. Rectangular pulses are formed by a flip-flop circuit that selects the photocurrent interval from \(I_1\) to \(I_2\) for photoelement \(A\), and from \(I_3\) to \(I_4\) for photoelement \(B\). Flip-flop \(A'\) operates in the interval from \(I'_1\) to \(I'_2\) \((I'_1 > I_1,\ I'_2 < I_2)\); moreover, if the counting pulse is generated at \(I_1\), circuit \(A'\) opens the forward-counting circuit and locks the backward-counting circuit, whereas if the counting pulse is generated at \(I_2\), the reverse is true.

Figure 5 shows the circuit of a typical counter cascade giving both positive and negative counts. The authors constructed a counter consisting of fourteen binary cascades and making it possible to perform 16,384 counts in one cycle. Testing the system with a rotating disk with slots showed that the counter operates in both directions at a rate of up to 1,000 counts per second; this limit is determined not by the design of the counter, but by the locking device. Tests showed that the counter makes it possible to count interference fringes even in the presence of strong interference. The authors propose using it to measure wavelength, as well as the refractive indices of gases in the infrared region of the spectrum.

P. G.