JPH0712648A - Wide-field Michelson Fourier spectrometer - Google Patents

Abstract

(57) [Abstract] [Purpose] A wide-field Michelson Fourier spectroscopy apparatus using a Michelson interferometer, which enables wide-field and accurate spectroscopy. A beam splitter 1 that splits incident light, a fixed mirror 2 that is fixed to the beam splitter at a constant distance, a movable mirror 3 that moves at a constant speed, and reflected light from the fixed mirror 2 and the movable mirror 3. A block is formed by the sensor 4 arranged at a position where the interference is formed to form an interference fringe and a plurality of elements 5 of the sensor 4 in a range corresponding to the instantaneous visual field Δθ ₀ .
The output signals at the times when the optical path differences are the same for the respective elements 5 in this block are time-corrected by the correction unit 7 so as to be added in the addition unit 8, and the addition output corresponding to the block by the addition unit 8 is Fourier-transformed. And a processing unit 6 for processing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wide-field Michelson Fourier spectroscopy apparatus which utilizes a Michelson interferometer and has a wide observation field. In recent years, in the field of remote sensing in the earth's atmospheric observation system such as the ozone hole in the atmosphere, development of an optical system that enables highly accurate multiband observation wavelengths and simultaneous acquisition of image data has been carried out. ing. In particular, an observation instrument for analyzing gas in the atmosphere is required to have a wide observation wavelength band and high wavelength resolution, and thus the Fourier spectroscopy method is often applied. This Fourier spectroscopy method has the advantages over the dispersive spectroscopy method in that it allows simultaneous photometry of a plurality of spectral elements and has a high utilization rate of the amount of incident light. As an interferometer for applying this Fourier spectroscopy method, a Michelson type having a beam splitter, a fixed mirror, and a movable mirror as main parts is often used. It is desired to widen the field of view in order to enable spectrum observation over a wide range by the Michelson Fourier spectroscopy device using this interferometer.

[0002]

2. Description of the Related Art FIG. 4 is an explanatory view of a conventional example, showing an optical system of a Michelson Fourier spectroscopy apparatus, in which 21 is a beam splitter, 22 is a fixed mirror, 23 is a movable mirror, 24 is a sensor, and 25 is a sensor. An aperture, 26 is a moving mechanism portion, 27 is a collimator mirror, and 28 is a condenser mirror. Also, an interferogram when the incident light has a single wavelength is shown in correspondence with the position of the movable mirror 23.

The fixed mirror 22 is fixed at a position at a distance L1 with respect to the beam splitter 21, and the movable mirror 23 is moved by a moving mechanism portion 26 in the arrow direction. The incident light is
The collimator mirror 27 is made incident through the aperture 25 to be a parallel light beam, and is made incident on the beam splitter 21,
The reflected light is split into reflected light and transmitted light, the reflected light is incident on the fixed mirror 22, and the transmitted light is incident on the movable mirror 23. Then, the reflected light from the fixed mirror 22 and the reflected light from the movable mirror 23 return to the beam splitter 21 and interfere with each other. This interference light is incident on the sensor 24 via the condenser mirror 28. The output signal I (x) of the sensor 24 is Fourier transformed by an arithmetic processing unit (not shown).

When the distance L1 between the fixed mirror 22 and the beam splitter 21 is equal to the distance L2 between the movable mirror 23 and the beam splitter 21, the optical path of the light reflected by the fixed mirror 22 and the reflection by the movable mirror 23. The difference D from the light path is D
= 2 (L2-L1) = 0. In this case, both reflected lights have the same phase in the beam splitter 21 regardless of the wavelength of the incident light, and therefore they are added. Therefore, the sensor 2
The output signal I (x) of 4 becomes maximum. The position of the movable mirror 23 in this case is shown as 0.

In the case of monochromatic light having a wavelength λ, the moving mechanism 26 moves the movable mirror 23 from the position where the optical path difference D = 0 to the beam splitter 21 side to λ / 4 or to the opposite side to the beam splitter 21. When moved by / 4, the optical path difference D becomes
D = 2 (L2-L1) = λ / 2. In this case, the two reflected lights have opposite phases in the beam splitter 21 and are thus cancelled. Therefore, the output signal I (x) of the sensor 24
Is the smallest.

That is, the interference light intensity of the incident light of the wavelength λ which is incident on the sensor 24 has an optical path difference D of n, where n is an integer.
It becomes maximum when λ, and becomes minimum when (n + 0.5) λ. Therefore, the incident light intensity P (x) on the sensor 24 can be expressed as P (x) = 2P (λ) [1 + cos (2πx / λ)] (1). Note that P (λ) is the incident light intensity of the wavelength λ.

An output signal I (x) proportional to the incident light intensity P (x) is obtained, and the interferogram in the case of a single wavelength λ is the maximum as shown corresponding to the movable mirror position. The value and the minimum value have a cosine-wave shape that periodically repeats. Further, in the case of incident light including a plurality of wavelengths, the maximum value is obtained when the movable mirror position is 0, but by moving the movable mirror 23, a curve corresponding to the light intensity of each wavelength is obtained.
If the wavelength λ in this case is λ = 1 / ν, it can be expressed as I (x) = ∫2P (ν) cos (2πνx) dν (2). In addition, ∫ indicates an integration of 0 to ∞.

In addition, it is practically difficult to make the reflectance and the transmittance of the beam splitter 21 the same over the entire wavelength region, and in consideration of this point, P (ν) is B
Assuming (ν), the equation (2) can be expressed as I (x) = ∫B (ν) cos (2πνx) dν (3). Here, ∫ indicates the integration from −∞ to + ∞. Therefore, from the relationship of the Fourier transform, B (ν) = ∫I (x) cos (2πνx) dx (4) can be expressed, and thus the output signal I of the sensor 24
The spectrum B (ν) can be obtained by using (x). That is, the Michelson Fourier spectroscopy apparatus obtains the spectrum of the incident light by Fourier transforming the interference fringes of the incident light.

[0009]

As described above, the Michelson Fourier spectroscopy apparatus of the conventional example uses the sensor 24 composed of a single element, and has a very narrow field of view. In the field of remote sensing in recent years, there is a demand to simultaneously perform imaging and spectrum analysis of an observation target, and in that case, it is necessary for the spectroscopic device to have a wide field of view corresponding to the imaging range. Become. Therefore, it is conceivable that the sensor 24 has a one-dimensional array or a two-dimensional array of a plurality of elements. However, even if the sensor 24 is simply composed of a plurality of elements, the instantaneous visual field is narrow and it is difficult to obtain a desired output signal level. If a plurality of elements corresponding to the desired viewing angle are connected in parallel in order to widen the instantaneous visual field, the interval of the interference fringes will differ depending on the spectrum of the incident light, making accurate measurement impossible. Occurs. An object of the present invention is to widen the field of view and enable accurate spectral processing.

[0010]

A wide-field Michelson Fourier spectroscopy apparatus of the present invention will be described with reference to FIG.
The beam splitter 1 splits the incident light and makes it enter the fixed mirror 2 and the movable mirror 3, respectively, and interferes the reflected light of the fixed mirror 2 and the movable mirror 3 to form an interference fringe, which forms the interference fringe. In a Michelson Fourier spectroscopy apparatus in which the sensor 4 is arranged at a position, the sensor 4 is configured by arranging a plurality of elements 5 and a block is formed by the plurality of elements 5 in a range corresponding to a desired instantaneous visual field. A calculation process of performing correction calculation on the output signal of each element 5 in the block according to the arrangement position of each element with respect to the optical axis, adding the output signal of each element in the block, and Fourier-transforming the added output signal. The part 6 is provided. In addition, 7 is a correction | amendment part, 8 is an addition part, 9 shows a condensing system.

Further, the arithmetic processing unit 6 uses the output signal of the first element in the block, which has the optical path difference x at a certain time t ₁ when the movable mirror 3 is moved at a constant speed, as the output signal. The i-th element in this block, which is placed next to the first element in sequence
The output signal of the i-th element at the time t _i when the optical path difference for the th element becomes the same as the optical path difference x for the first element is added to obtain the output signal of this block. be able to.

[0012]

The sensor 4 having the one-dimensional array or the two-dimensional array of the plurality of elements 5 is used, and the block is formed by the plurality of elements 5 in the range corresponding to the desired instantaneous visual field Δθ ₀ . The output signal of each element 5 in this block has a different arrangement position of each element 5 with respect to the optical axis, and therefore the optical path difference of each element 5 is different from the optical path difference on the optical axis. That is, even if the optical path difference on the optical axis is zero, the incident angle and the reflection angle are different from each other with respect to the optical axis at positions away from the optical axis, and the optical path difference is not zero. Therefore, each element 5 in the block
Of the output signal of the correction unit 7 according to the arrangement position with respect to the optical axis.
After being corrected by, the sum is added in the adder 8 to obtain the output signal of the block. That is, the output signal of the instantaneous visual field Δθ ₀ can be obtained. By Fourier transforming this output signal, the spectrum within the instantaneous visual field Δθ ₀ of the incident light can be obtained.

When the movable mirror 3 is moved at a constant speed,
At a certain time t ₁ , if the optical path difference x for the first element in the block increases at the next time, for example, the i-th position (the number of elements in the block is Assuming that m is the same as the optical path difference of the device of i = 2 to m), it occurs at time t _i before time t ₁ . Therefore, the correction unit 7 adds the output signal of the i-th element at the time t _i prior to the time t ₁ to the output signal of the first element in the addition unit 8.
The output signal of the i-th element at time t _i is set to time t ₁
Delay and correct. In contrast to the above, when the optical path difference x decreases at the next time, the optical path difference x for the first element and the optical path difference for the i-th element at time t _i after time t ₁ And are equal. Then, by adding the output signals at the times when the optical path difference is the same for each element 5 in the block, an accurate output signal can be obtained even when the interval between the dark portions of the interference fringes varies. It becomes possible to widen the field of view.

[0014]

EXAMPLE FIG. 2 is an explanatory view of an example of the present invention.
Is a beam splitter, 12 is a fixed mirror, 13 is a movable mirror, 1
4 is a sensor having a detection characteristic corresponding to the wavelength range of incident light, 15 is an element forming the sensor, 16 is an arithmetic processing unit,
Reference numeral 17 is a preamplification and sample hold unit, 18 is a movement and position detection unit, 19 is a control unit, and the control unit 19 and the arithmetic processing unit 16 can be configured by a microprocessor or the like.

The structure of the Michelson type interferometer consisting of the beam splitter 11, the fixed mirror 12 and the movable mirror 13 is the same as that of the conventional example.
Is moved at a constant speed, its moving position is detected, and the detection signal is applied to the control unit 19. The sensor 14 has a structure in which a plurality of elements 15 are arranged in a one-dimensional array or a two-dimensional array, and the plurality of elements in a range corresponding to a desired instantaneous visual field are one block.

The output signal of each element 15 of the sensor 14 is amplified and sample-held by the pre-amplification and sample-hold section 17, added to the arithmetic processing section 16, and the output signal of the element in each block is corrected. Is added,
The output signal of each block, that is, the output signal corresponding to a desired instantaneous visual field is Fourier-transformed and spectral analysis is performed. In this case, the correction process and the addition process are
It is also possible to adopt a configuration in which analog processing is performed.
It is also possible to adopt a configuration in which the values sampled and held by the preamplification and sample and hold unit 17 are AD-converted and digitally corrected and added. It is also possible to temporarily store the output signal of each element 15 in a memory or the like and perform correction processing by address control when reading the memory.

FIG. 3 is an explanatory diagram of a correction process according to the embodiment of the present invention, and schematically shows a case where a plurality of elements forming the sensor 14 are divided into three blocks, CP _{1 to} CP ₃ are correction units, AD _{1 to} AD ₃ are addition units. First, an optical path difference in the optical axis direction (θ = 0) D, the angle θ = θ ₀ + Δθ an optical path difference of an angle theta = theta ₀ with respect to the optical axis with respect to x and the optical axis (but, Δθ«θ ₀₎ When the optical path difference and x + Δx, Δx = _{[D / cos (θ 0 + Δθ} ) ] - a [D / cos [theta] _{0] ^{≒ (Dsinθ 0 / cos 2 θ}} 0) Δθ ... (5). The field of view is θ ₀ (<< 1) and the wave number resolution is Δν.
Then, it can be expressed as D≈1 / Δν.

Let Δθ be the instantaneous field of view, that is, one element
The instantaneous visual field Δθ by the child 15 is the dark area of the interference fringes.
Since it is appropriate to select less than 1/2 of the
Long λ _minFor Δx ≦ λ_min/ 2 relationship
Will be needed. Therefore, (Dsinθ₀/ Cos²θ₀) Δθ ≈ (1 / Δν) θ₀Δθ ≦ λ_min/ 2 (6) and Δθ ≦ (λ_min/ 2) Δν / θ₀And need to
Therefore, it was found that the instantaneous visual field Δθ cannot be made too large.
It

On the other hand, in the optical axis direction (θ = 0), Δx = 0
Next, in that case, [Delta] x '= D [{1 / cos (Δθ / 2 )} - 1 ] since _{^{≒ DΔθ 2/8 ≦ λ min}} / 2 ... can be expressed as (7), Δθ ≦ 2 ( λ _min Δν) ^1/2 (8), and the instantaneous visual field Δθ in the optical axis direction can be widened. That is, even if the sensor 14 is composed of the plurality of elements 15, even if the instantaneous visual field in the optical axis direction can be widened, the instantaneous visual field in the direction away from the optical axis cannot be widened.

Therefore, according to the present invention, the instantaneous visual field in the direction away from the optical axis can be expanded by the following processing. That is, the block is formed by, for example, the elements S _{0 to} S ₄ of the sensor 14 corresponding to the range of the desired instantaneous visual field Δθ ₀ , and the blocks are formed similarly for the other elements. Then, the output signals of the elements in each block are corrected by the correction units CP _{1 to} CP ₃ , and the addition units AD ₁ to
AD _{3 is} added to obtain an output signal corresponding to the instantaneous visual field Δθ ₀ .

For example, at time t ₀ , with respect to the optical path difference D in the optical axis direction, the sensor 1 having an angle θ = θ _{0 with} respect to the optical axis.
No. 4 element S ₀ , that is, the optical path difference x with respect to the first element S ₀ on the optical axis side in the block is D / cos as described above.
θ ₀ , and the angle θ with respect to the optical axis at that time t ₀
= Theta ₀ + element S ₁ of the [Delta] [theta], i.e., the optical path difference with respect to the second element S ₁ of the optical side of the block is x + [Delta] x.
In accordance with the movement of the movable mirror 13, when it is assumed that changes in the direction of the optical path difference D in the optical axis direction increases, the optical path difference for the element S ₁ is at a time t ₀ -.DELTA.t ₁ before time t ₀ x
Becomes That is, an optical path difference x for at element S ₀ at time t _0, and the optical path difference x is equal for at element S ₁ at time t ₀ -Δt _1.

Similarly, the element S at time t ₀ -Δt _2
The optical path difference with respect to ₂ and the element S at time t ₀ −Δt _3
The optical path difference with respect to ₃ and the element S at the time t ₀ −Δt _4
The optical path difference with respect to ₄ is the same x. That is, the respective output signals at the times when the elements S _{0 to} S ₄ exist on the circle having the radius x centered on the origin O where the optical path difference occurs are the output signals with the same optical path difference x, and they are added. However, for this reason, the correction units CP _{1 to} CP ₃
At that time will be corrected.

That is, for the correction unit CP ₁ , τ _{0 to} τ
₄ delay circuit, and τ ₀ = 0, τ ₁ = Δt ₁ , τ ₂ = Δt ₂ , τ ₃ = Δt ₃ , τ
₄ = Δt ₄ . As the delay circuit, by moving the movable mirror 13 at a constant speed, the delay times τ _{0 to} τ ₄ can be fixedly set corresponding to the arrangement positions of the elements S _{0 to} S _{4 with} respect to the optical axis. .

When the optical path difference D in the optical axis direction decreases with time, for example, the times of the elements S _{1 to} S ₄ having the same optical path difference as the optical path difference x with respect to the element S _{0 at the} time t ₀ are: each _{t 0 + Δt 1 ~t 0 +} Δt 4 , and the correction unit CP _1, when the _{τ 4 = 0, τ 3 =} Δt 4 + Δ
t ₃ , τ ₂ = Δt ₄ + Δt ₂ , τ ₁ = Δt ₄ + Δt ₁ ,
The delay circuit may have a relationship of τ ₀ = Δt ₄ .

In general terms, the elements in the block are S _{0 to} S _m, and the angles θ _i0 to θ _{im with} respect to the optical axis, respectively.
Then, the instantaneous visual field Δθ ₀ is Δθ ₀ = θ _im −θ _i0 . Also it is assumed that the movable mirror 13 as described above moves at a constant velocity v, next optical path difference x for elements of theta = theta _i0 In t = t _0, at the _{t = t 0 + Δt k θ} = θ _{Assuming that} the optical path difference for the element of _ik is also x, x = vt ₀ / cos θ _i0 = v (t ₀ + Δt _k ) / cos θ _ik (9) t ₀ cos θ _ik = (t ₀ + Δt _k ) cos θ It can be expressed as _i0 (10).

Here, θ _ik = θ _i0 + kΔθ (where 0 ≦
If k ≦ m) and _{put, cos (θ i0 + kΔθ)} / cosθ i0 ≒ 1- (k 2/2) Δθ 2 -ktanθ i0 Δθ ≒ 1 + (Δt k / t 0) ... (11) becomes, [Delta] [theta] <1 , Θ _i0 <1, the relationship Δt _k / t ₀ ≈k θ _i0 Δθ (12) holds.

Therefore, the output signals of the respective elements at the times corresponding to k = 0 to k = m are expressed by Δ
Add after delaying t _k . That is, θ _{i0 at} time t ₀
The output signal of the corresponding element in _{V (θ i0 (t 0)} ), the time t ₀ -
The output signal V of the element corresponding to θ _i0 + Δθ at Δt _{1.
(Θ i0 + Δθ (t 0} -Δt 1)), the time t ₀ -.DELTA.t output signal V of the corresponding element ₂ in at θ _i0 + 2Δθ (θ _i0 +2
Δθ (t ₀ −Δt ₂ )), and the likewise at time t ₀ −Δt _m
The output signal V (θ of the element corresponding to θ _i0 + mΔθ in
_{i0 + mΔθ (t 0 -Δt m} )) is added for.

Therefore, the output signal V corresponding to the block is V = V (θ _i0 (t ₀ )) + V (θ _i0 + Δθ (t ₀ −Δt ₁ )) + ... + V (θ _i0 + mΔθ (t ₀ −Δt _m )) (13) The output signal V corresponding to this block is output from, for example, the addition units AD _{1 to} AD ₃ in FIG. 3, the Fourier transform processing is performed on each output signal V, and the spectrum analysis is performed.

The above generalized contents are, for example,
If one block is constituted by the m-th element from the first, the time of the i-th element having the same optical path difference and the optical path difference x with respect to the first-th element in a time t ₁ t
It shows that the output signal of the i-th element at _i (i = 2 to m) is added to the output signal of the first element.

When the output signals of the respective elements 15 of the sensor 14 are sampled and held by the preamplification and sample and hold section 17, the values are sequentially stored in the arithmetic processing section 16 (not shown) or an external memory, Since the output signal of each element 15 for each sample time is accumulated,
By controlling the read address, the correction unit CP of FIG.
_{As in 1 to} CP ₃ , it becomes possible to perform time correction of the output signal of each element 15 corresponding to each block.

[0031]

As described above, according to the present invention, a block is constituted by a plurality of elements 5 of the sensor 4 in the range corresponding to the desired instantaneous visual field Δθ _0, and the output signal of each element 5 in the block is calculated. This is corrected by the correction unit 7 of the processing unit 6 and added by the addition unit 8 to obtain an output signal for each block. An accurate output signal can be obtained even when the instantaneous visual field Δθ ₀ is widened, and a plurality of elements can be obtained. Since the output signals of 5 are added,
There is an advantage that the signal level becomes large and the S / N can be improved. Therefore, simultaneously with the imaging of the observation target, it is possible to observe the spectrum of the observation target by widening the field of view. Further, since the arithmetic processing in the arithmetic processing unit 6 does not become complicated, even if the number of elements 5 of the sensor 4 is increased and the number of blocks is increased, the arithmetic processing unit 6 is operated by a microprocessor or the like.
It is possible to easily realize the function of.

[Brief description of drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is an explanatory diagram of an example of the present invention.

FIG. 3 is an explanatory diagram of a correction process according to the embodiment of this invention.

FIG. 4 is an explanatory diagram of a conventional example.

[Explanation of symbols]

 1 Beam Splitter 2 Fixed Mirror 3 Movable Mirror 4 Sensor 5 Element 6 Arithmetic Processing Unit

Claims (2)

[Claims] 1. A beam splitter (1) splits incident light into a fixed mirror (2) and a movable mirror (3), respectively, and reflects the fixed mirror (2) and the movable mirror (3). In a Michelson Fourier spectroscopy apparatus in which light is interfered to form an interference fringe and a sensor (4) is arranged at a position where the interference fringe is formed, the sensor (4) is arranged with a plurality of elements (5). Multiple elements in a range corresponding to the desired instantaneous field of view (5)
To form a block, and each element (5) in the block
Correction signal according to the arrangement position of each element (5) with respect to the optical axis is added to the output signal of each element (5), the output signal of each element (5) in the block is added, and the added output signal is Fourier-transformed. A wide-field Michelson Fourier spectroscopy apparatus characterized by comprising an arithmetic processing section (6). 2. The arithmetic processing unit (6) is the first one in the block in which the optical path difference x becomes an optical path difference x at a certain time t ₁ when the movable mirror (3) is moved at a constant speed. The time t _{i at which the} optical path difference for the i-th element in the block sequentially arranged adjacent to the first element is the same as the optical path difference x for the first element in the output signal of the element. 2. The wide-field Michelson Fourier spectroscopy apparatus according to claim 1, further comprising a configuration in which an output signal of the i-th element in the above is added to obtain an output signal of the block.