JP2007155487A - Space optical spectrometer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a space optical spectrometer requiring no space while ensuring flexibility in designing a spectrometer optical system. <P>SOLUTION: Light discharged from an input port 1 is collected to one point of a reflective plane grating 3, and spectrally reflected by a Littrow lens 2. The spectrally reflected light is converted into a parallel light flux through a Littrow lens 3 again, and forms a spectrum image O of the output end surface of the input port 1 on the input end surface of a relay optical system array 4. In incoming into the relay optical system array 4, the light incoming into each array element of the relay optical system array 4 is divided every constant wavelength interval, and comes into an array element corresponding to each dividing segment. The light incoming into each array element forms a complementary image I of the spectrum image formed on the input end surface of the relay optical system array 4 corresponding to central wave length of a plurality of divided wavelengths on the input end surface of an output port array 5. In this case, the distances between image points of this complementary image I are made substantially the same. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a spatial optical spectrometer.

  WDM optical communication is a technique for transmitting signals by combining different signals on different frequencies (wavelengths) separated by a predetermined frequency (wavelength) interval in order to increase signal transmission capacity. . There are various means for multiplexing and demultiplexing light of multiple wavelengths. One of them is a spectroscope using a classic spatial optical system using a wavelength dispersion element such as a grating.

  However, the wavelength dispersion characteristic of such a spatial optical spectrometer is non-linear, and the input port spectrum image imaging interval of equally spaced frequency light (approximately spaced wavelength light) is not equal. For example, when using a spatial optical spectrometer as a demultiplexer / multiplexer, an optical fiber array arranged at equal intervals is often used as an input / output port. However, with such dispersion nonlinearity, a predetermined frequency is used. It is inconvenient that the optical image row and the center of the optical fiber are shifted.

Therefore, conventionally, in a spatial optical system spectrometer using a grating, one that obtains a linear dispersion characteristic by combining a grating with a prism to cancel the nonlinearity in the wavelength dispersion characteristics of both is widely used. (Japanese Unexamined Patent Application Publication Nos. 2000-304613, 2000-304614, 2000-321135, 2001-188823)
JP 2000-304613 A JP 2000-304614 A JP 2000-321135 A JP 2001-1888023 A

  However, the incorporation of a dispersion linearization prism has the disadvantages that the degree of freedom in spectroscopic optical design is reduced, and that the optical system is easily increased in size and cost.

  The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a spatial optical spectrometer that does not take up space while ensuring a degree of freedom in designing a spectrometer optical system.

  A first means for solving the above-mentioned problems includes a single or plural input ports and a spectroscopic means, and a plurality of wavelength bands by dividing a spectrum image of the input port at a predetermined frequency interval or wavelength interval. Is set in a one-to-one correspondence with each of the plurality of wavelength bands dividing the spectrum image, and a relay optical system responsible for forming a conjugate image of the plurality of wavelength bands for each wavelength band is provided. The optical axis intervals of the individual relay optical systems are unequally spaced so that the distances between the center wavelength conjugate image points of the plurality of wavelength bands are approximately equal. This is a spatial optical spectrometer.

The second means for solving the above-mentioned problem is the first means, and the displacement amount of the optical axis installation position of each relay optical system with respect to each center point of the spectrum image multiple wavelength bands in the spectral dispersion direction. Is defined by the following equation.
L _OK = Δx _k / (1 + M)
However,

The meaning of each variable is as follows.
L _Ok : Displacement amount Δk _k of the optical axis installation position of the k th relay optical system with respect to the center point of the k th spectrum image wavelength band From the center point of the k th spectrum image wavelength band to the conjugate image of the wavelength band center point Displacement amount M along the optical axis decentering direction of relay optical system (same as spectrum wavelength dispersion direction): Relay optical system projection magnification (constant)
x _k : coordinates of the center point of the k th wavelength band
x ^′ _k : coordinates of k-th wavelength band center point conjugate image P: wavelength band center point conjugate image interval (constant)
N: Total number of wavelength bands The third means for solving the above-mentioned problem is the first means or the second means, and the relay optical system outputs substantially parallel light when the parallel light is input. It is characterized by doing.

  A fourth means for solving the above-mentioned problem is the third means, wherein the relay optical system is composed of a second group lens.

  ADVANTAGE OF THE INVENTION According to this invention, the space optical system spectrometer which does not take space can be provided, ensuring the freedom degree of spectrometer optical system design.

  Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram showing a configuration of a spatial optical spectrometer which is an example of an embodiment of the present invention. The light emitted from the input port 1 made of an optical fiber is collimated by the Littrow lens 2 and spectrally reflected by the reflective planar grating 3 (spectral means). The spectrally reflected light again passes through the Littrow lens 3 to form a spectrum image O of the output end face of the input port 1 on the input end face of the relay optical system array 4.

  The principal ray of the radiated light beam from the input port 1 and the spectral ray reflected by the reflective planar grating 3 are reflected back to the Littrow lens 2 and reflected so that the principal rays of the respective wavelength light beams after passing through this are parallel to each other. The distance between the expression plane grating 3 and the Littrow lens 2 is determined. In other words, the influence of the aberration of the Littrow lens 2 (spherical aberration on the principal ray group) is minimized, and the parallel condition can be satisfied as completely as possible (although it exists in the vicinity of the focal point of the Littrow lens 2). As shown, the reflection point of the reflective planar grating 3 is placed.

  When entering the relay optical system array 4, the light incident on each array element of the relay optical system array 4 is divided at regular wavelength intervals and enters the array elements corresponding to the respective divided sections. The light incident on each array element is caused by the above-described spectrum formed on the entrance end face of the relay optical system array 4 corresponding to the center wavelength of the plurality of divided wavelength bands by the action of the relay optical system array 4 as described later. The conjoint image I of the image O is formed on the input side end face of the output port array 5. At this time, the distance between the image points of the conjoint image I is substantially equal. Therefore, even when a normally used optical fiber array is used as the output port array 5, the light having the center wavelength is incident on the central point of each optical fiber with the light separated at a constant wavelength interval. Can be. Note that “substantially” means that even if a perfect equal interval is not obtained due to a manufacturing error or a design error, it is included in the scope of the present invention.

Details of the relay optical system array 4 will be described below. Relay optical system array 4, as shown in FIGS. 2, 3, the respective optical axes, individually by different amounts eccentricity in the wavelength dispersion direction from the wavelength band center point O _k in the corresponding spectrum image O is set up. As a result, each spectrum image wavelength band center point conjugate image I _k formed by each relay optical system element (positive power) 41 forms an image at a position shifted from the spectrum image wavelength band center point in the wavelength dispersion direction. . If the amount of decentration is set to an appropriate value, the wavelength band center point conjugate image interval can be corrected to an equal interval.

  FIG. 2 illustrates how the conjugate image is displaced by the relay optical system element. When the imaging magnification of each relay optical system in the relay optical system array 4 is constant in all the relay optical systems, and this is M, for the kth relay optical system in the array,

Is established.
However,
L _{Ok: k-th} x-axis direction eccentricity of the relay optical system optical axis with respect to spectrum image waveband center point O _k L _{Ik: k-th} conjugate image center point by the relay optical system of the spectral image waveband center point O _k image height Δx of I _{k k:} displacement along the x-axis from the spectrum image waveband center point O _k to the conjugate image center point I _k: k-th x-coordinate x _^'k of the spectrum image waveband center point O _k This is the x coordinate of the _kth conjugate image center point _Ik .
Conversely, the amount of eccentricity L _{Ok in} the x-axis direction of the optical axis of the relay optical system that gives the displacement amount Δx _k is:

It can be expressed.

Among the above, x _k is a value determined by the wavelength dispersion characteristic of the reflective planar grating 3 and the positional relationship between the reflective planar grating 3 and the Littrow lens 2. Therefore, set to be equal intervals x _'k, determine the [Delta] x _k, (3) only _{L Ok} from the position of the x-axis direction eccentricity _{L Ok} to seek spectral image waveband center point _{O k} by equation the displaced position, by setting the optical axes of the relay optical system elements, the conjugate image center point I _k can be in equal intervals.

  Next, conditions for minimizing the total amount of eccentricity in the relay optical system array 4 will be described.

First, in the spectrum image O _k formed by the demultiplexer, from the center point one-dimensional coordinate values the end of a plurality of wavelengths bands as determined by the predetermined wavelength band separated,
x ₁ , x ₂ , x ₃ ,..., x _N
Suppose that Assume that these spectrum image wavelength band conjugate image center points formed by the relay optical system array are formed at the following coordinate positions. Note that the coordinate origin is the same as the one-dimensional coordinate of the center point of a plurality of wavelength bands.
x ′ ₁ , x ′ ₂ , x ′ ₃ ,..., x ′ _N
The difference between them, that is, the amount of deviation along the x-axis of the image is
Δx _k = x ′ _k −x _k (k = 1 to N) (4)
It is.

  When each conjugate image center point interval takes a constant value P, the following equation is established.

Using P and x ′ ₁ as variables, a condition that minimizes the sum S of Δx _k ² for k is obtained.

Therefore, when the extreme value is obtained by partial differentiation of S with P and x ₁ ′, the result is as follows.

If P and x ₁ ′ are employed, the image lateral shift amount Δx _k (k = 1 to N) can be minimized as a whole.

  Next, the conditions of the relay optical system that are more convenient for the apparatus will be described. In a spectrum image of a spatial optical spectrometer used in WDM optical communication, an optical design is often made so that chief rays of each wavelength are parallel. For example, when an optical fiber array is used as the output port, in order to achieve the same maximum coupling efficiency with any optical fiber, the principal rays of any wavelength light beams need to be parallel to each other.

  For this purpose, the principal ray incident on the relay optical system and the emitted principal ray should be kept in a parallel relationship, for example, a paraxial power structure as shown in FIG.

In the paraxial optical system shown in FIG. 3, positive powers 42 and 43 having a function as a so-called field lens are arranged at the entrance end and the exit end of the relay optical system, and the positive power 41 having a relay function is provided at the center of the optical system. It is arranged. The rear focus of the positive power 42 coincides with the front focus of the positive power 43, and the positive power 41 is arranged at these focal positions. The positive power 42 is on a plane including the incident point O _k, positive power 43 is on a plane including the image point I _k.

  An example of this is shown in FIG. The relay optical system 1 element is composed of two biconvex lenses 401 and 402. The first surface 42 of the biconvex lens 401 corresponds to the positive power 42 in FIG. The biconvex lens second surface 43 corresponds to the positive power 43 in FIG. The positive power 41 shown in FIGS. 2 and 3 is constituted by the two surfaces of the biconvex lens second surface 411 and the biconvex lens first surface 412. The biconvex lenses 401 and 402 are disposed so as to be in close contact with each other with the optical axis as a coaxial state. In FIG. 4, a two-point difference line (imaginary line) indicates a complete lens, but this portion is a portion that does not actually exist in order to avoid interference between the biconvex lenses 401 and 402.

  The relay optical system elements having the above-described configuration are arranged with their optical axes in parallel to form a relay optical system array. Each optical axis interval has a value specific to the individual interval as described above. As described above, if the relay optical system is constituted by two groups of elements (lenses), it is sufficient to use two sets of lens arrays in combination, which facilitates manufacture. In the above description, the duplexer is taken as an example, but it goes without saying that the same functions as a multiplexer if the light travels in the opposite direction.

  According to the present invention, spectral wavelength dispersion linearization by a spatial optical system can be realized in a small space independently of the spectroscopic body. The relay optical system array used for chromatic dispersion linearization can be manufactured by applying microlens array manufacturing technology, and can be procured at a very low cost if mass-produced. Further, if the base of the relay optical system array is made of a low linear expansion coefficient material such as quartz glass, the temperature characteristics can be stabilized. The present invention has a wide range of applications other than the demultiplexer / multiplexer shown in this embodiment. For example, if a MEMS mirror array is installed instead of the output port array, it can be applied to WSS (Wavelength Selective Switch).

  An optical system configured to split light emitted from the input port 1 made of an optical fiber and output the light to an output port 5 made of an optical fiber array by an optical system as shown in FIG.

  Table 1 shows numerical examples of this optical system. The optical system was calculated as having no aberration. Eleven wavelength bands were set by dividing the wavelength range from 495 nm to 605 nm at 10 nm intervals, and the pectogram image coordinate values for each wavelength band center wavelength were calculated. The coordinate origin was on the Littrow lens optical axis, and the x-axis was set along the wavelength dispersion direction. This is a direction parallel to the plane of the paper in FIG.

The relay optical system element for calculating the conjugate image point interval P and the first conjugate image point coordinate x ₁ ′ from equations (9) and (10), and then calculating all the desired conjugate image point coordinates and realizing them. Individual eccentricity L _Ok was calculated from equation (3). The relay magnification was 0.8 times. Referring to Table 1, despite the spacing of the X _k is changed, the distance X k _'is substantially constant, the substantially equally spaced center wavelengths conjugate between image point distance of several wavelength bands It can be seen that the function of this spatial optical spectrometer is performed.

1 is a schematic diagram illustrating a configuration of a spatial optical spectrometer that is an example of an embodiment of the present invention. FIG. It is a figure which shows how to shift the conjugate image by the relay optical system element. It is paraxial structure explanatory drawing of a relay optical system element. It is a figure which shows the example which actualized the relay optical system element shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Input port, 2 ... Littrow lens, 3 ... Reflection type planar grating, 4 ... Relay optical system array, 5 ... Output port array

Claims (4)

When a plurality of input ports and a spectroscopic means are provided, and a plurality of wavelength bands are set by dividing a spectrum image of the input port at a predetermined frequency interval or wavelength interval, a plurality of wavelengths delimiting the spectrum image A relay optical system that exists in a one-to-one correspondence with each of the bands and has an action of forming a conjugate image of the plurality of wavelength bands for each of the wavelength bands; and the center of the plurality of wavelength bands A spatial optical system spectrometer characterized in that the optical axis intervals of the individual relay optical systems are unequal so that the distances between the wavelength conjugate image points are substantially equal. 2. The spatial optical system according to claim 1, wherein a displacement amount of the optical axis installation position of each relay optical system with respect to each center point of a spectrum image in a plurality of wavelength bands in the spectral dispersion direction is determined by the following equation. Spectroscope.
L _OK = Δx _k / (1 + M)
However,

The meaning of each variable is as follows.
L _Ok : Displacement amount Δk _k of the optical axis installation position of the k th relay optical system with respect to the center point of the k th spectrum image wavelength band From the center point of the k th spectrum image wavelength band to the conjugate image of the wavelength band center point Displacement amount M along the optical axis decentering direction of relay optical system (same as spectrum wavelength dispersion direction): Relay optical system projection magnification (constant)
x _k : coordinates of the center point of the k th wavelength band
x ^′ _k : coordinates of k-th wavelength band center point conjugate image P: wavelength band center point conjugate image interval (constant)
N: Total number of wavelength bands The spatial optical spectrometer according to claim 1, wherein the relay optical system outputs substantially parallel light when parallel light is input. 4. The spatial optical spectrometer according to claim 3, wherein the relay optical system includes a second group lens.

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