JP2723966B2 - Michelson interferometer - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a Michelson interferometer for measuring a spectrum of light to be measured.

(Prior Art) FIG. 2 shows a conventional Michelson interferometer. In the figure, 1 is a light source, 2 is a beam splitter, 3 is a quarter-wave plate, and 4, 5 are total reflection prisms (hereinafter, referred to as “hereafter”). 6 is a polarization beam splitter, and 7, 8, and 9 are photodetectors.

Light (hereinafter, referred to as reference light) 10 emitted from a light source 1 such as a He-Ne laser is linearly polarized by a polarizing plate (not shown), and is bisected by a beam splitter 2. One of the two divided reference lights 10 is circularly polarized by the quarter-wave plate 3 and is incident on the prism 4, and the other is directly incident on the prism 5, is totally reflected, and is multiplexed again by the beam splitter 2. Is done. The multiplexed reference light 10 enters the polarization beam splitter 6, where it is bisected and received by photodetectors 7 and 8, respectively.

At this time, the outputs of the photodetectors 7 and 8 are V ₀ Sinφ and V ₀ Cos
becomes φ. Here, φ = 2πx / λ, λ is the wavelength of the reference light 10 (here, 0.0328 μm), x is the optical path of the light returning from the beam splitter 2 through the prism 4 to the beam splitter 2 again, and the beam splitter 2 This is the difference in length from the optical path of the light returning to the beam splitter 2 through the prism 5 again. Therefore, the optical path difference x can be obtained from the outputs of the photodetectors 7 and 8.

On the other hand, the measured light 11 is incident on the beam splitter 2 in parallel with the reference light 10, and is split into two in the same manner as described above.
The light is totally reflected again, is multiplexed again, and is received by the photodetector 9. At this time, when the prism 4 or 5 moves in the light incident direction from the beam splitter 2 or the light emitting direction to the beam splitter 2 (hereinafter, referred to as a beam direction),
An interferogram is generated in the measured light 11 received by the photodetector 9.

Note that the interferogram is an interference fringe generated within the interference intensity when an optical delay is given to one of the two split lights when the light is split by a beam splitter and recombined.

The interferogram of the measured light 11 with respect to the movement of the prism 4 or 5 is as follows.

R (τ) = ∫G (ν) Cos2πντdν (1) where G (ν) is the spectrum of the light to be measured, ν is the frequency, and τ = x / c (c is the speed of light).

As described above, since the optical path difference x can be obtained from the outputs of the photodetectors 7 and 8, the interferogram R (τ) of the measured light 11 measured by the photodetector 9 in the above equation (1) is calculated. The spectrum G (ν) can be obtained by performing the inverse Fourier transform.

Incidentally, the output of the photodetector 9 contains various noise components in addition to the interferogram. In order to remove this noise component, that is, to improve the S / N ratio, the output of the photodetector 9 may be passed through a band-pass filter having a narrow bandwidth. However, if the bandwidth is too narrow, the value of the interferogram itself may be reduced. Have an effect.

The prism 4 or 5 has a constant velocity v, for example, v = 1000 μ
When moving at m / sec, the center wavelength of the measured light 11 is λ = 0.8
With μm, an interferogram with a center frequency of 2v / λ = 2.5 kHz results. To limit the bandwidth without affecting the value of the interferogram itself, a low-pass filter with a bandwidth of at least 2.5 kHz had to be used.

Therefore, in order to improve the S / N ratio by reducing the band width of the band-pass filter, it is necessary to reduce the moving speed of the prism 4 or 5. For example, speed v
If = 5 μm / sec, the center frequency of the generated interferogram is 2v / λ = 12.5 Hz, so that a band-pass filter having a bandwidth narrower than two digits can be used.

(Problems to be Solved by the Invention) However, at low frequencies around 10 Hz as described above, noise increases due to 1 / f noise and various low-frequency noises, and an improvement in the S / N ratio cannot be expected. . For this reason, there is a problem that it is difficult to measure the spectrum of the weak light to be measured with the above-mentioned Michelson interferometer.

The present invention has been made in view of the above problems, and has as its object to provide a Michelson interferometer capable of measuring a spectrum of a weak light to be measured.

(Means for Solving the Problems) In the present invention, in order to achieve the above object, a light to be measured is referred to a demultiplexing / combining optical system including a beam splitter and a pair of reflecting mirrors or a total reflection prism with good coherence. Light is incident together with the light, and either one of the pair of reflecting mirrors or the total reflection prism is moved in the beam direction. At this time, interference fringes of the light to be measured emitted from the demultiplexing / multiplexing optical system are simultaneously emitted. In the Michelson interferometer for measuring together with the reference light, one of the pair of reflecting mirrors or the total reflection prism is slightly displaced in the beam direction at a predetermined frequency f, and the light is emitted from the demultiplexing / multiplexing optical system. The frequency f and 2f components are extracted from the received light of the reference light to be detected, and the moving amount detecting means for obtaining the moving amount of the reflecting mirror or the total reflection prism from the phase relationship thereof, and the light emitted from the demultiplexing / multiplexing optical system. A Michelson interferometer provided with interference output detection means for mixing an AC signal having a frequency f with a received light signal of the light to be measured and extracting a component below the center frequency of the interference fringe from the mixed signal. Proposes a Michelson interferometer provided with a processing means for sampling an interference output for each fixed amount of movement, storing the interference output, and performing signal processing.

(Operation) According to the present invention, when one of the pair of reflecting mirrors or the total reflection prism is slightly displaced in the beam direction at the frequency f by the driving means, the optical path length in the demultiplexing / combining optical system is changed to the frequency f.
, And the reference light and the measured light are subjected to phase modulation of the frequency f. The frequency f and 2f components of the received signal of the reference light to which the phase modulation has been applied are extracted by the movement amount detecting means, and the movement amount of the reflecting mirror or the total reflection prism is obtained from the phase relationship. Further, the received signal of the measured light to which the phase modulation has been applied is mixed with the AC signal of the frequency f by the interference output detecting means, and the components below the center frequency of the interference fringes are extracted among them, and only the interference output is output. Is done.

Further, according to the present invention, the interference output is sampled and stored for each fixed amount of movement of the reflecting mirror or the total reflection prism by the processing means, and further subjected to signal processing to obtain the spectrum of the measured light.

(Embodiment) FIG. 1 shows an embodiment of the Michelson interferometer of the present invention, in which the same components as those of the conventional example are denoted by the same reference numerals. That is, 1 is a light source, 2 is a beam splitter,
4, 5 are total reflection prisms (hereinafter simply referred to as prisms), 8, 9 are photodetectors, 10 is reference light, 11 is measured light, 1
2 is a vibration device, 13 is an AC signal source, 14 and 15 are lock-in amplifiers, 16 is a trigger circuit, 17 is a mixer, 18 is a waveform storage circuit (memory), and 19 is a computer.

The prism 4 is oscillated in the beam direction at a predetermined frequency f, here 1 kHz, by an oscillating device 12. The prism 5 can be moved in the beam direction at a speed v = 5 μm / sec by a driving device (not shown). The output of the photodetector 8 is sent to lock-in amplifiers 14 and 15, and the output of the photodetector 9 is sent to a mixer 17.

The vibrating device 12 includes, for example, a coil such as a speaker and a magnet, and vibrates the prism 4 in one direction, here a beam direction, according to an AC signal supplied from an AC signal source 13. The AC signal source 13 sends the AC signal of the frequency f to the vibrating device 12, the lock-in amplifiers 14, 15 and the mixer 17 (however, the signals sent to the lock-in amplifiers 14, 15 have a phase difference of 90 ° from each other. Shall be.). The outputs of the lock-in amplifiers 14 and 15 are sent to a trigger circuit 16, and the output trigger pulse of the trigger circuit 16 is stored in a memory 18
To be sent to The mixer 17 has a built-in low-pass filter to be described later. The output of the mixer 17 is sent to a memory 18, and the contents of the memory 18 are read and processed by the computer 9.

 Next, the operation of the device will be described.

The reference light 10 emitted from the light source 1 is applied to the beam splitter 2
And incident on the prisms 4 and 5, respectively,
Is reflected. Here, the reference light 10 reflected by the prism 4
Is (4πν ₀ / c) ΔxCos2πft due to the vibration of the prism 4.
(Where ν ₀ is the frequency of the reference beam 10 and Δx is the maximum value of the displacement of vibration in the prism 4). The reference light 10 is combined with the light from the prism 5 by the beam splitter 2 and received by the photodetector 8.
The output is as follows:

I = I ₀ {1 + Cos (φ ₀ Cos2πft + ψ)} (2) where φ ₀ = 4πν ₀ Δx / c, ψ = 2πν ₀ x / c
It is. When the equation (2) is expanded by a Bessel function, the f component is as follows.

V ₁ = −2I ₀ J ₁ (φ ₀ ) SinψCos 2πft (3) V ₂ = 2I ₀ J ₂ (φ ₀ ) CosψCos 4πft (4) Note that J ₁ and J ₂ are first-order Bessel functions.

Therefore, the output of the photodetector 8 is connected to the lock-in amplifiers 14 and 15.
, An output proportional to Sin ψ, Cos ψ is obtained. These signals are input to a trigger circuit 16. The trigger circuit 16 detects, for example, its zero cross point, and based on this, a trigger pulse is generated each time the optical path difference x changes (increases or decreases) by λ / 4. And sends it to the memory 18.

On the other hand, the light to be measured 11 is also split into two by the beam splitter 2 as in the case of the conventional example, and enters the prisms 4 and 5, respectively, and is reflected. Here, the measured light 11 reflected by the prism 4 undergoes the same phase modulation as the reference light 10. Therefore, the interferogram generated when the beam splitter 2 multiplexes the light from the prism 5 and detected by the photodetector 9 is as follows.

R (τ) = ∫G (ν ) Cos {2πντ + φ 0 (ν / ν 0) Cos2πft} ...... (5) Here, phase modulation (4πνΔx / c) Cos2πft =
φ ₀ (ν / ν ₀ ) Cos2πft is used.

The above interferogram is multiplied by the mixer 17 with the AC signal Cos2πft. On the other hand, in _{Cos {2πντ + φ 0 (ν} / ν 0) Cos2πft} = Cos (2πντ) Cos {φ 0 (ν / ν 0) Cos2πft} -Sin2πντSin {φ 0 (ν / ν 0) Cos2πft} ...... (6) Yes, Becomes _{Therefore, Cos {2πντ + φ 0 (} ν / ν 0) Cos
When the low-pass filter for removing the frequency f, 2f,... Nf component of the product of 2πft｝ and Cos2πft is incorporated in the mixer 17, the output becomes −2J ₁ (φ ₀ ) according to the equations (7) and (8).
ν / ν ₀ ) Sin2πντ. That is, the interferogram after detection output from the mixer 17 is proportional to R (τ) = ∫G (ν ) J 1 (φ 0 ν / ν 0) Sin2πντ ...... (9).

The beat frequency in the interferogram is
Since 2v / λ = 12.5 Hz, the bandwidth of the low-pass filter built in the mixer 17 can be set to about 20 Hz.
Therefore, narrowing of the band by about two digits can be realized in a high detection frequency region (here, 1 kHz) as compared with the conventional Michelson interferometer.

The interferogram obtained by the mixer 17 is sampled, that is, stored in the memory 18 in accordance with the trigger pulse output from the trigger circuit 16, and further processed by the computer 19 to obtain the spectrum of the measured light.

FIG. 3 shows an example of the results of measurement of the interferograms by the conventional device and the device of the present invention. FIG. 3 (a) shows the results measured by the conventional device, and FIG. 3 (b) shows the same results. The result of having measured the measured light with the apparatus of this invention is shown. As shown in the drawing, the S / N of the apparatus of the present invention is significantly higher than that of the conventional apparatus.
It can be seen that the ratio is improved (the moving speed of the prism is 1000 μm / sec and 5 μm / sec, respectively).

In the embodiment, the light is reflected using the total reflection prism, but an ordinary reflecting mirror may be used.

(Effects of the Invention) As described above, according to the present invention, one of a pair of reflecting mirrors or a total reflection prism of a demultiplexing / combining optical system is displaced in the beam direction at a frequency f to generate a reference light and The measured light is subjected to phase modulation, the amount of movement is detected from the received light signal of the modulated reference light, and an AC signal of frequency f is mixed with the modulated received light signal of the measured light, thereby obtaining interference fringes. Since components below the center frequency are extracted,
An interference output can be obtained in a narrow band in a high frequency region, so that an interference fringe having a good S / N ratio can be obtained, and a weak spectrum of the light to be measured can be detected.

In addition, the interference output is sampled for each fixed movement amount,
According to the one provided with a means for storing and processing the signal,
An interference output value corresponding to the movement amount, that is, the change in the optical path difference in the demultiplexing / combining optical system is immediately obtained, whereby the spectrum of the light to be measured can be easily measured.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of the Michelson interferometer of the present invention, FIG. 2 is a block diagram showing a conventional Michelson interferometer, and FIGS. 3 (a) and 3 (b) show the conventional and the present invention. It is a figure showing an example of a measurement result of an interferogram by a device. DESCRIPTION OF SYMBOLS 1 ... Light source, 2 ... Beam splitter, 4,5 ... Prism, 8,9 ... Photodetector, 10 ... Reference light, 11 ... Light to be measured, 12 ... Vibration device, 13 ... AC Signal source, 14, 15 ... lock-in amplifier, 16 ... trigger circuit, 17 ... mixer, 18
…… Memory, 19 …… Computer.

Claims (2)

(57) [Claims] 1. A light to be measured is incident on a demultiplexing / combining optical system comprising a beam splitter and a pair of reflecting mirrors or a total reflection prism together with a reference light having good coherence, and said pair of reflection mirrors or a total reflection prism is provided. Is moved in the beam direction, and at this time, in a Michelson interferometer that measures interference fringes of light to be measured emitted from the demultiplexing / combining optical system together with reference light emitted simultaneously, a pair of A driving means for slightly displacing one of the reflecting mirror and the total reflection prism in the beam direction at a predetermined frequency f; and a frequency f and 2f component from the received light signal of the reference light emitted from the demultiplexing / combining optical system. Moving amount detecting means for calculating the moving amount of the reflecting mirror or the total reflection prism from the phase relationship, and an AC signal having a frequency f as a light receiving signal of the light to be measured emitted from the demultiplexing / multiplexing optical system. mixture Michelson interferometer, characterized in that a interference output detecting means for extracting a center frequency around the following components of the interference fringes than this. 2. The Michelson interferometer according to claim 1, further comprising processing means for sampling the interference output for each fixed movement amount, storing the interference output, and processing the signal.