JPH083445B2 - Optical spectrum detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention makes it possible to redirect and disperse light using two diffraction gratings and detect the wavelength components of the dispersed light at high speed. Optical spectrum detection device.

[Conventional technology]

To detect the wavelength component of light as a spectrum,
There are dispersion spectroscopy and interferometry.

The present invention relates to the dispersion spectroscopy among these methods, and as its conventional example, many methods of rotating the diffraction grating to detect the optical spectrum are found.

This dispersive spectroscopy can obtain good wavelength resolution by developing a diffraction grating with high-density engraved lines.
Moreover, since it can be easily realized mechanically, it is the mainstream of the present optical spectrum detection apparatus.

[Problems to be solved by the invention]

However, in the method of rotating the diffraction grating by the dispersion spectroscopy to detect the optical spectrum, there is a limit in the rotation speed of the diffraction grating due to the shape and weight of the diffraction grating and the constituent elements of the rotating mechanism, and the spectrum of one There is a drawback in that the rotation time required for detection is limited to several 10 ms to several ms.

For this reason, there is a problem that it is not suitable for detecting an optical spectrum that changes rapidly with time (several ms to several μs).

[Means for solving problems]

Therefore, the present invention has been made to solve such a problem, and a second diffraction grating is newly disposed in the middle of the measurement optical path of the optical spectrum detection device using the conventional diffraction grating. the second diffraction grating, for example, the invention according to the same applicant and the same inventors' surface acoustic wave (SAW: S urface
Diffractometer of light utilizing the A coustic W ave) (Japanese Patent Application No. Sho 60
-234812) "etc. (hereinafter referred to as acousto-optic light modulator), it is provided with a function that allows variable control of the diffraction grating interval. With this function, the second diffraction grating functions as a redirecting element that changes the direction of the light under measurement incident on the first diffraction grating, which has been conventionally used, and as a result, the first diffraction grating is relatively rotated. A similar situation can be created.

Further, since the acousto-optical light modulator can variably control the diffraction grating interval at high speed with an electric signal, it is in the same state as when the first diffraction grating is temporarily rotated at high speed. It becomes possible to detect the spectrum at high speed in a short time.

〔Example〕

Hereinafter, the present invention will be described in detail based on illustrated embodiments.

FIG. 1 is a diagram showing a configuration of an embodiment of an optical spectrum detecting device according to the present invention, and FIG. 2 is one of the diffraction gratings which is a constituent element of the present invention and whose diffraction grating interval can be variably controlled. FIG. 3 is a diagram showing the structure of an acousto-optic light modulator, FIG. 3 is a diagram showing a state in which the light to be measured is deflected by the acousto-optic light modulator of FIG. 2, and FIG. 4 is an embodiment of the present invention. FIG. 5 is a diagram showing an optical system in which the measurement optical system in the example is replaced with a transmission type, and FIG. 5 shows the tilt angles in the grating direction of the first and second diffraction gratings, which are constituent elements of the present invention, and the output optical spectrum. FIG. 6 is a diagram showing the relation of generation positions, and FIG. 6 is a diagram relating to a conventional device and showing an example of the configuration of an optical system of an optical spectrum detecting device using one diffraction grating.

 The conventional technique will be described below with reference to FIG.

In the figure, the light to be measured 1 is incident on the lens 2 and converged, and passes through the opening portion of the input side slit 3 arranged at the focal position of the lens 2.

The light that has passed through the input side slit 3 is incident on the first concave reflecting mirror 4 while spreading in a fan shape, is reflected as a parallel light flux, and is guided to the diffraction grating 5. This diffraction grating 5 has 1mm
Square-shaped optical diffraction gratings with hundreds to thousands of equally-spaced engraving lines between sides and a side length of about 2 to 4 cm are often used. The light to be measured 1 that has entered the diffraction grating by these high-density marking lines is reflected while undergoing dispersion according to its wavelength, and travels toward the second concave reflecting mirror 6. Then, the dispersed measured light 1 reflected by the second concave reflecting mirror 6 toward the output side slit 7 is converged on the surface of the output side slit 7 at a position corresponding to each light wavelength, The light intensity is measured by the photodetector 8 after passing through the opening of the slit. In this measurement, it is necessary to arrange the measured light intensities in the order of the wavelengths contained in the light to be measured, that is, according to the optical spectrum. Therefore, the position of the output side slit 7 is fixed and the diffraction grating 5 is fixed. It is rotated and the intensity distribution of dispersed light is continuously measured by the photodetector 8. When measuring dispersed light in a narrow range, the diffraction grating may be fixed and the output side slit 7 and the photodetector 8 may be moved.

In either case, while the diffraction grating 5 is rotating or the output side slit 7 and the photodetector 8 are moving,
An accurate optical spectrum cannot be detected unless the spectrum of the measured light is constant. However, in actual light, most of the spectra constantly change with time and their intervals are short. Therefore, the measurement time has been shortened by rotating the diffraction grating at high speed.

According to the present invention, instead of rotating the diffraction grating at a high speed, the optical path of the measured light incident on the diffraction grating is deflected by an acousto-optic light modulator, and the same effect as if the diffraction grating is rotated is produced. It is what

Here, the diffraction grating is the first diffraction grating in the present invention.

Since the acousto-optic light modulator is capable of electrically redirecting light at a high speed, the detection time of the optical spectrum is significantly shortened, and the acousto-optical modulator is suitable as an optical spectrum detector suitable for measuring a fluctuating spectrum. Configurable.

As shown in FIG. 1, basically, the optical system of the present invention looks similar to that of the conventional device, but a second diffractive element newly formed by an acousto-optical modulator is provided after the input side slit 3. A grating 9 is provided to diffract and change the light to be measured, and the changed light is dispersed by the first diffraction grating 5 as in the conventional example.
Then, the dispersed light is reflected by the second concave reflecting mirror 6,
It is converged and an optical spectrum is generated on the surface of the output side slit 7.

This optical spectrum is generated by diffraction when each redirected light of the measured light deflected by the second diffraction grating, that is, when the second diffraction grating 9 is a sinusoidal phase grating using SAW or the like. ± 1st order light and no diffraction 0
The light beams are redirected / separated into three light beams of the next light, but since an optical spectrum is generated for each of the redirected light beams, three optical spectra are simultaneously generated on the output side slit surface. It is known that these optical spectra differ in light quantity, but the distribution shape of the spectra is substantially the same. In the present invention, since it is necessary to spatially separate and independently measure these three optical spectra, the output side slit 7 is provided with an opening in accordance with the generation position of the three optical spectra, It is configured to include a plurality of photodetectors 8 for measuring the light passing through each opening.

Next, the deflection of the measured light and the generation of a plurality of optical spectra by the second diffraction grating will be described with reference to FIG. FIG. 2 shows an example of the structure of a SAW tunable grating that can be used as a diffraction grating whose grating spacing can be variably controlled. The grating shown in FIG. 2 has a substrate 11, an electrically insulating pedestal 12, and a piezoelectric substrate 13. It is a three-layer structure. A light transmitting window is provided in the central portion of the substrate 11 and the electrically insulating pedestal 12 so as to be aligned with each other, and light passes through the windows. The light that has passed through passes through substantially the center of the piezoelectric substrate 13 and undergoes a phase change by the SAW propagating on the substrate 11 at that time. On the surface of the piezoelectric substrate 13, a first interdigital electrode 14a having a structure in which two comb teeth for generating SAW are intertwined with each other, and generated from this electrode 14a, the piezoelectric substrate
A pair of second interdigital electrodes 14b for monitoring the SAW propagating on 13 are provided.

Figure 3 shows how light is diverted when SAW is used as a phase grating.
The SAW generated on the surface of the piezoelectric substrate 13 by the electric signal of the sine wave of f ₀ has a spatial period d corresponding to the lattice constant and progresses at the velocity v in the direction of the arrow. The piezoelectric substrate 13 has a light-transmitting property, and when light incident from the left side of the figure passes through this piezoelectric substrate 13, the incident light is a sinusoidal wave on the surface of the piezoelectric substrate 13 due to SAW. The unevenness and the change in the density just below the surface of the piezoelectric substrate 13, that is, the change in the refractive index cause phase modulation.

Since this phase modulation is periodic due to the repetition of the spatial period d, when these lights are imaged on the focal plane 16 of the lens by the lens 15, similar to those transmitted through a normal phase grating, a diffracted image is formed. Occurs. Here, if the incident light is a monochromatic light of wavelength λ, the diffraction image has ± 1 determined by the lattice constant d.
The next diffraction bright spot occurs. The position where the diffracted bright spot occurs is a position separated from the optical axis of the focal plane 16 by a distance α, and the direction thereof is equal to the SAW propagation direction.

If the focal length of the lens 15 is F, the value of the distance α is It is represented by. Here, assuming that the frequency of the sine wave electric signal changes ± Δf / 2 around f ₀ , the displacement amount Δα of the ± 1st-order diffracted bright spot on the focal plane 16 is Becomes As is clear from the equation (2), the displacement amount Δα becomes larger as the SAW propagation speed is slower, the lens 15 has a longer focal length, and the light wavelength λ is longer.

If such a SAW tunable grating is placed in the measurement optical path of the optical spectrum detector,
Three optical spectra centering on the first-order diffracted bright spot and the zero-order light that is not diffracted will be generated simultaneously. FIG. 4 shows the optical system according to the present invention shown in FIG. Is a transmissive type, that is, the convex lenses 17 and 18 are considered instead of the first and second concave reflecting mirrors 4 and 6 in FIG.
The diffracted light deflected by the SAW tunable grating is changed in angle by the optical system in which the reflection type first diffraction grating 5 is replaced with the transmission type diffraction grating 19 and is incident on the first diffraction grating 5 again, and again 3 It shows that two bright spots are connected. The dotted line shown in FIG. 4 indicates the optical axis of the light beam deflected by the SAW. In FIG. 1, when the light passes through the first diffraction grating, the 0th time shown by the solid line in FIG. In fact, in practice, the first diffraction grating is of the reflective type, so that if the incident angle of the incident light beam changes, this has the same effect as if the diffraction grating itself were rotated. As for the diffracted light deflected by the SAW tunable grating described above, the direction of deflection, that is, the angle of incidence on the diffraction grating changes by changing the SAW frequency. Although the directions of rotation of the diffraction grating are opposite to each other, it is the same as the rotation of the diffraction grating itself when viewed from both diffracted lights.

Now, the three simultaneously generated spectra are
If the grating directions of the first and second diffraction gratings 5 and 9 are the same, they are arranged in a substantially straight line, and it is difficult to separate each spectrum. Also, when the grating directions are perpendicular to each other, the spectra are arranged in a vertical relationship and are easy to separate, but the SAW frequency is swept and the grating spacing of the second diffraction grating is changed to ± 1.
Even if the light spectrum centering on the second-order diffracted light is spatially scanned, it does not move in the arrangement direction of the light spectrum, so that there is no effect. Therefore, the grating directions of the first and second diffraction gratings need to be inclined.

FIG. 5 shows this state. The arrangement of the second diffraction grating (SAW tunable grating) is such that the grating direction of the first diffraction grating is the y-axis direction and the direction perpendicular to the grating direction is the x direction. The state is shown in (a) of the figure, the state of the first diffraction grating and its incident light beam is shown in (b) of the figure, and the output distribution state of the optical spectrum by these arrangements is shown in (c). . The angle θ is the tilt angle of the second diffraction grating, but in the present invention, the frequency of the SAW forming the second diffraction grating is swept to obtain the optical spectrum of the ± 1st-order diffracted light on the output side slit 7 surface. Move it in the direction of the spectrum,
Since the high-speed scanning photometry is performed by the fixed aperture of the slit, it is necessary to move the optical spectrum more effectively with respect to the frequency change of SAW. Therefore, the angle θ is set to three values on the slit. It is necessary to minimize the light spectrum within a spatially separable range. The portion surrounded by the dotted line in FIG. 5 (c) shows an optical spectrum 20 capable of high-speed scanning photometry by ± first-order diffracted light, and an optical spectrum 21 for normal photometry which is measured by rotating the first diffraction grating. There is. In the present invention, the output side slit 7 having an opening at an appropriate portion within the dotted line is used, and the photodetector 8 is used as necessary.
The optical spectrum is detected using a plurality of. Regarding the optical spectrum display, in addition to the conventional method of displaying the wavelength axis by the rotation angle of the first diffraction grating, the wavelength axis is displayed by the SAW generation electric signal frequency added to the second diffraction grating. Method. Various combined methods can be considered depending on the type or property of the light to be measured, but the most general and simplest method is to perform wideband scanning by rotating the first diffraction grating at a low speed and perform narrowband high speed scanning at a specific portion. It seems that the method of timely using the diffraction grating of No. 2 will be used.

Also, of course, even if the order of the first diffraction grating and the second diffraction grating is interchanged, the same operation can be performed in principle.

〔effect〕

As described above, according to the present invention, the diffraction grating whose distance between the gratings can be variably controlled is used to redirect light at a high speed, and the diffraction grating used for dispersion similar to that used in the conventional device is applied to the diffraction grating. The incident angle of the measurement light is changed, and as a result, the same effect as when the diffraction grating for dispersion is rapidly rotated is obtained.
It has become possible to greatly reduce the measurement time required for optical spectrum detection. As a result, it is possible to detect the spectrum of light with a short fluctuation period, and it is possible to observe the changing state of the light spectrum.

[Brief description of drawings]

FIG. 1 shows the configuration of a measurement optical system in an embodiment of an optical spectrum detecting apparatus using the second diffraction grating of the present invention. FIG. 2 shows an acousto-optic light modulator (SA) which is one of the diffraction gratings whose grating spacing is variably controllable, which is a constituent element of the present invention.
W tunable rating). FIG. 3 shows a light deflection state by the SAW tunable grating shown in FIG. FIG. 4 shows an optical system diagram in which the measurement optical system in one embodiment of the present invention is rewritten into a transmission type. FIG. 5 shows the relationship between the inclination θ of the first and second diffraction gratings, which are the constituent elements of the present invention, in the grating direction and the generation position of the output optical spectrum. FIG. 6 shows an embodiment of the measuring optical system of the optical spectrum detecting device in the conventional device. In the figure, 1 is the light to be measured, 2 is the lens, 3 is the input side slit, 4 is the first concave reflecting mirror, 5 is the first diffraction grating, 6 is the second concave reflecting mirror, and 7 is the output side slit. , 8 is a photodetector, 9 is a second
Diffraction grating, 11 is a substrate, 12 is an electrically insulating pedestal, 13 is a piezoelectric substrate, 14a is a first interdigital electrode, 14b is a second interdigital electrode, 15 is a lens, 16 is a focal plane, 17 and 18 are convex lenses, 19 is a transmission type diffraction grating, 20 is an optical spectrum capable of high-speed scanning photometry, and 21 is an optical spectrum for normal photometry.

Claims (1)

[Claims] 1. An optical spectrum detecting device for dispersing incident light via a first diffraction grating and a second diffraction grating and detecting the optical spectrum thereof, wherein the first diffraction grating and the second diffraction grating are provided. One of the diffraction gratings is a diffraction grating with a fixed diffraction grating interval for dispersing the incident light to obtain an optical spectrum, and the other diffraction grating is used for the incident light. Is a diffraction grating with a controllable variable diffraction grating spacing for spatially diverting the optical path of, and the diffraction grating spacing is deflected by a controllable variable diffraction grating and the diffraction grating spacing is fixed. An optical spectrum detecting device comprising a spatial filter for passing a part of an optical spectrum dispersed by a diffraction grating.