JPH079501B2 - Grating lens system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] The present invention is a grating lens system for focusing coherent plane wave light on one point by using two in-line type grating lenses. With a frequency distribution,
By intersecting two rays that are symmetric with respect to the optical axis in the space between the two lenses and then focusing on one point,
It is possible to obtain a good beam spot with no aberration and a stable focus position with no deviation even with respect to the wavelength variation of incident light, and thus enable a wide range of practical applications of the grating lens.

[Industrial application field]

The present invention relates to a grating lens system having a focusing function by combining grating lenses.

Recently, in an optical system that requires a function of focusing plane wave light from a coherent light source into one point, such as an optical pickup of an optical disk device, (i) downsizing of the device, (ii) shortening of access time,
(Iii) In order to realize cost reduction and the like, the use of a grating lens which is thinner, lighter and smaller than conventional optical elements and which has high mass productivity is being studied.

[Prior art]

A conventional in-line type grating lens is shown in FIG. This lens has a function of focusing only a parallel light beam having a specific wavelength λ ₀ to one point, as shown in FIG. Therefore, the wavelength λ longer than the above λ ₀
For light of (> λ ₀ ), aberration occurs as shown in FIG. 7B, good focusing performance cannot be obtained, and the focal length changes (shortened when λ> λ _0). ). Also, for wavelengths shorter than λ ₀ , aberrations occur and the focal length also changes, although the situation differs from that of FIG.

Such a phenomenon is common to lenses that bend light by diffraction. For example, a volume hologram lens (FIG. 9A), a surface relief type grating lens (FIG. 9B), and a blazed grating shown in FIG. The same applies to any of the lenses ((c) in the figure). Also, the tenth
In the case of the off-axis type grating lens as shown in the figure, as shown in (a) in the figure, even if there is no aberration when the wavelength is λ ₀ , if the wavelength changes, as shown in (b) in the figure, Not only does aberration occur, but the focus position also deviates from the optical axis.

As described above, the grating lens has advantages such as being thin, lightweight, and small, but on the other hand, the wavelength used has a predetermined value (λ
_{If it} deviates from ₀ ), aberration occurs, the focusing performance deteriorates, and the focal position also deviates.

[Problems to be Solved by the Invention] In an optical pickup of an optical disc device, a semiconductor laser is used as a coherent light source. Semiconductor lasers are generally classified into single-mode lasers and multi-mode lasers, and their oscillation wavelengths are shown in FIG. 11 (a),
It is shown in (b). In the conventional general optical pickup,
Since an ordinary optical lens is used, a beam spot change that affects reading, writing, and the like on the optical disc does not occur depending on the wavelength change of the laser light. Therefore,
Both the single mode laser and the multimode laser described above can be used. On the other hand, when a grating lens is used in the optical pickup, only a single mode semiconductor laser can be used because the wavelength variation has a great influence as described above.

However, even a single-mode semiconductor laser has a characteristic that its oscillation wavelength changes according to temperature. Figure 12 shows the relationship between the oscillation wavelength and the ambient temperature in a single-mode semiconductor laser (provided that the laser output is constant). As is clear from the figure, (a) the wavelength gradually and continuously changes with temperature change, (b) a certain temperature
There is a phenomenon that the wavelength changes discontinuously at T ₁ (so-called mode hop), and (c) there are two or more wavelengths at a certain temperature T ₂ .

Therefore, in an optical pickup using a grating lens, for example, even when a single-mode semiconductor laser is used as a light source, when the temperature changes, the wavelength also changes due to the above phenomenon. Due to this, the quality of the focused spot may be deteriorated and the focal position may change. These lead to enlargement of the beam diameter of the spot on the optical disk medium, occurrence of track deviation, occurrence of focus deviation, and the like. In particular, when the wavelength changes due to the mode hop shown in FIG. 12, the change is discontinuous, and there is a problem that the current servo mechanism cannot follow it at all.

Therefore, there is an attempt to maintain the temperature of the semiconductor laser at an appropriate value by providing an external temperature adjusting function, but since the driving current of the semiconductor laser is different between writing and reading on the optical disc, the temperature of the junction part is The temperature changes abruptly and cannot be controlled accurately with an external temperature adjustment mechanism.

In view of the above problems, the present invention can obtain a good beam spot that does not cause aberration and a stable focus position that does not deviate from a predetermined point even when the wavelength of incident light fluctuates. It is an object of the present invention to provide a grating lens system which enables application to various fields such as an optical pickup of the device.

[Means for solving problems]

The grating lens system of the present invention includes an in-line type first grating lens on which coherent plane wave light is incident, and an in-line type second grating that focuses diffracted light that has passed through the same to a predetermined point on the optical axis. The first and second grating lenses have a predetermined spatial frequency distribution rotationally symmetrical with respect to the optical axis, and the first grating lens has Diffracted light from two symmetrical points intersects each other on the optical axis, and diffracts light so that incident light farther from the optical axis is made incident on a position closer to the optical axis of the second grating lens. The second grating lens focuses light on the above-mentioned predetermined one point. Here, the “optical axis” is an “axis passing through the center of the lens and perpendicular to the incident surface of the lens”.

[Action]

In the above structure, different wavelengths λ ₀ , which are incident on one arbitrary point of the first grating lens from the same direction,
Consider two light paths of λ (λ ₀ <λ). First,
The light of the wavelength λ is diffracted by the first grating lens at a larger angle than the light of the wavelength λ ₀ , and all of these diffracted light reach the second grating lens after intersecting the optical axis. . As for the distance of the arrival point of these lights from the optical axis, the light of wavelength λ is farther than the light of wavelength λ ₀ . Next, these lights are diffracted by the second grating lens. At this time, since the light of wavelength λ is diffracted at a larger angle than the light of wavelength λ ₀ , the distance between the two lights gradually narrows. And finally meet at one point. Therefore, by giving the two grating lenses a predetermined spatial frequency distribution, the point where the two lights intersect can be placed at a predetermined point on the optical axis.

The above can be said for any light incident on any point of the first grating lens, and since the spatial frequency distribution is rotationally symmetric with respect to the optical axis, the wavelength of the incident coherent plane wave light changes. Even in this case, the light beam is focused on the above-mentioned predetermined one point on the optical axis, so that the aberration and the focal position shift do not occur.

〔Example〕

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. The present embodiment has a configuration in which the in-line first and second grating lenses 11 and 12 are arranged on the same optical axis (dashed line), and is a coherent plane wave light parallel to the optical axis (for example, from a semiconductor laser). The collimated light) is diffracted by the first grating lens 11 toward the optical axis side, crosses the optical axis once, and is then focused by the second grating lens 12 at a predetermined point P on the optical axis. is there.

The first grating lens 11 has a predetermined spatial frequency distribution that is rotationally symmetric with respect to the optical axis, and diffracted light from arbitrary two points that are symmetrical with respect to the optical axis intersect on the optical axis. . The second grating lens 12 has a predetermined spatial frequency distribution that is rotationally symmetric with respect to the optical axis, and the crossed diffracted light is focused on one point (point P) on the optical axis. is there.

Next, a specific method of determining the spatial frequency distribution of the grating lenses 11 and 12 will be described below with reference to FIG. 2 in (i) to (iv). The distance between the two grating lenses 11 and 12 is d, and the distance between the grating lens 12 and the point P is l.

(I) First, consider a ray having a wavelength λ ₀ , which is emitted from an infinite point on the optical axis (that is, parallel to the optical axis) and reaches a point R ₁ on the outermost periphery of the grating lens 11. This ray
Diffracted at the point R ₁ , the center point r ₁ of the grating lens 12
(= 0), where it is further diffracted and reaches point P (solid line a in FIG. 2). Then, by assuming the above-mentioned optical path (infinity point → R ₁ → r ₁ → P),
The spatial frequencies F ₁ , f _{1 at} R ₁ , r ₁ are determined.

(Ii) Next, consider the case where the wavelength of the light beam changes from λ ₀ to λ (> λ ₀ ). A ray of wavelength λ that has traveled from the point at infinity to point R ₁ is diffracted at point R _{1 at a} larger angle than when the wavelength is λ ₀ and reaches point r ₂ on grating lens 12 (broken line b). . Here, the condition that the wavelength is focused at a point P even when a lambda, a spatial frequency f ₂ at the point r ₂ is determined.

(Iii) Returning to the case where the wavelength is λ ₀ , it is possible to reversely determine from which point on the grating lens 11 the ray of the wavelength λ ₀ that is diffracted at the point r ₂ and reaches the point P (solid line c ). When a point on the grating lens 11 and R _2, from the condition that the diffracted light at point R ₂ reaches the point at infinity on the optical axis (i.e. parallel to the optical axis), the spatial frequency F of the point R _{2 2} is determined.

(Iv) Consider the case where the wavelength becomes λ again, and in the same way as (ii) above, point r ₃ on the grating lens 12 (not shown)
And its spatial frequency f ₃ . Then, the wavelength is returned to λ ₀ , and the grating lens 11 is operated in the same manner as (iii) above.
Find the upper point R ₃ (not shown) and its spatial frequency F ₃ . In this way, by repeating the above steps (ii) and (iii) until the point Rn (n = 1,2,3, ...) Reaches the center of the grating lens 11,
The radial spatial frequency distribution at 2 is determined. The diameter of the second grating lens 12 is determined by the position of the point rn and is usually smaller than the diameter of the first grating lens 11.

By determining the spatial frequency distributions of the grating lenses 11 and 12 as described above, even if the plane wave light incident parallel to the optical axes thereof has a wavelength λ different from the reference wavelength λ ₀ , Can be focused on the point P with practically no aberration.

Next, in order to clarify the characteristics of the spatial frequency distribution determined as described above, as shown in FIG. 3, the diffraction function of the grating lenses 11 and 12 is divided into an intermediate side and an outer side thereof. Think. Then, the second grating lens 12 can be equivalently divided into two grating lenses 12a and 12b by a plane wave traveling in the optical axis direction. At the same time, if the spatial frequency of the grating lens 12 is f, then f is divided into fa and fb, and the relationship of f = fa + fb is established. Where the grating lens 12a
Is an in-line type grating lens that focuses a plane wave on one point, and is equivalent to the conventional one shown in FIG. On the other hand, the grating lens 12b and the first grating lens 11 on the other side have a special spatial frequency distribution as shown below.

FIG. 4 shows an example of specific spatial frequency distributions of the first and second grating lenses 11 and 12. In this example, the radius of the first grating lens 11 is R = 1.5 mm, the radius of the second grating lens 12 is r = 0.95 mm, and the focal length is 1 =
The design wavelength is λ ₀ = 830 nm when the distance between both lenses is 1.8 mm and d = 2.5 mm.

In the figure, the first and second grating lenses 11 and 1
The spatial frequency (F, f) distribution of 2 is a distribution that increases smoothly from the lens center toward the outer periphery. Also, the second
Although the grating lens 12 can be considered to be divided as described above, the spatial frequency (fa) distribution of one of the grating lenses 12a is zero at the lens center and increases linearly toward the outer periphery. is there. The spatial frequency (fb) distribution of the other grating lens 12b is
The distribution has a band (430 to 620 mm ⁻¹ ) equal to the spatial frequency band of the first grating lens 11, and has a distribution that decreases smoothly from the lens center toward the outer periphery. In the second grating lens 12 (12a, 12b), the relationship of f = fa + fb is established as described above.

Therefore, in order to confirm the compensation effect for the wavelength variation in the grating lens system of the present embodiment, the RMS value and the maximum value of the wavefront aberration generated when the wavelength of the incident light is shifted from the central wavelength 830 nm by Δλ, are calculated, The results are shown in FIG. In addition, the grating lens 11,12
Has determined the spatial frequency distribution by the method shown in FIG. 2 so that there will be no aberration for light of wavelengths 830 nm and 830.3 nm.
In FIG. 5, it can be seen that practically no aberration can be maintained for the wavelength of 830 ± 14 nm under the condition that can be regarded as practically no aberration (RMS value <0.07λ, MAX value <0.25λ). This wavelength range (830 ± 14 nm) is a range that can cover the oscillation wavelength (including wavelength variation due to temperature) of almost all semiconductor lasers currently on the market. In addition, in the grating lens system of the present embodiment, there is no focal position shift.

From these results, according to the present embodiment, it is possible to obtain a good beam spot and a stable focus position even with respect to the wavelength change due to the temperature change of the semiconductor laser, the mode hop, and even for the multimode laser. Therefore, the practical application of the grating lens to an optical pickup or the like can be realized.

The grating lens system of the present embodiment can be easily integrated by utilizing the fact that the grating lenses 11 and 12 are thin, small and light, and have an axisymmetric structure. An example of the integration is shown in FIG. This is one in which two grating lenses 11 and 12 are fitted and fixed on both bottom surfaces of a cylindrical outer frame portion 20 so that their central axes coincide with the optical axis.

FIG. 7 shows the integrated grating lens system A described above.
Is an example in which an optical pickup is configured by mounting the above as an objective lens on an actuator B. The actuator B is fixed to the rotary plate C as in the conventional case, and can perform focusing and tracking in the arrow direction. In the above configuration, for example, the collimated light L from the semiconductor laser is reflected upward by the mirror M and is focused by the grating lens system A at a point on the optical disc medium (not shown). At this time, even if the wavelength of the semiconductor laser light fluctuates, the beam spot on the optical disk medium is not affected by the compensation effect of the two grating lenses 11 and 12 described above. In addition, there are advantages that the objective lens is light in weight, small in size, and low in price, as compared with the conventional objective lens including the optical lens group.

As the method for producing the grating lens according to the present invention, the electron beam drawing method is the most reliable,
A desired grating lens can be created by inputting the calculated value of the spatial frequency as data. In this case, the efficiency of the grating lens can be further improved by performing the blazing of the grating. It is also possible to make a holographic image by creating a desired wavefront with an optical element. In this case, the distance d between the two lenses is made small and the spatial frequency band is made high, whereby the efficiency of the lenses can be further enhanced.

In addition, the grating lens system of the present invention is not applied only to the optical pickup of the optical disc device,
It can be applied to various optical systems that require a function of focusing coherent plane wave light on one point.

〔The invention's effect〕

According to the grating lens system of the present invention, it is possible to obtain a good beam spot with no aberration and a stable focus position that does not deviate from a predetermined one point. It has become possible to put it into practical use in various fields.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing a method of determining spatial frequencies of the first and second grating lenses 11 and 12 according to the embodiment, and FIG. Explanatory drawing of the spatial frequency distribution characteristic in an Example, FIG. 4 is a figure which shows an example of a spatial frequency distribution of the 1st, 2nd grating lenses 11 and 12, FIG. 5 is a wavefront aberration and wavelength shift amount in the said Example And FIG. 6 is a sectional view showing an integrated example of the grating lens system of the above embodiment, and FIG. 7 shows an example of an optical pickup configured by using the integrated grating lens system. 8A and 8B are perspective views showing the function and drawbacks of the conventional in-line type grating lens, and FIGS. 9A to 9C are schematic configurations showing various conventional grating lenses. Figure, Figure 10 (a) and (b) Is a diagram showing the function of the conventional off-axis type grating lens and its drawbacks. FIGS. 11 (a) and 11 (b) are diagrams showing the oscillation wavelength states of a single-mode semiconductor laser and a multi-mode semiconductor laser, respectively. The figure shows the temperature dependence of the oscillation wavelength in a single-mode semiconductor laser. 11 …… First grating lens, 12 …… Second grating lens.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hiroyuki Ikeda 1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture, Fujitsu Limited (72) Inventor, Yushi Inagaki 1015, Kamiodanaka, Nakahara-ku, Kawasaki, Kanagawa Prefecture, Fujitsu Limited ( 56) References JP-A-59-192207 (JP, A) JP-A-59-160166 (JP, A)

Claims (4)

[Claims] 1. An in-line type first grating lens (11) on which coherent plane wave light is incident, and an in-line type that focuses diffracted light transmitted through the first grating lens at a predetermined point on the optical axis. A second grating lens (12) on the same optical axis, wherein the first grating lens has a predetermined spatial frequency distribution rotationally symmetric with respect to the optical axis, Diffracted light from two arbitrary points that are symmetrical with respect to the optical axis are intersected on the optical axis, and incident light that is farther from the optical axis is incident on a position closer to the optical axis of the second grating lens. The second grating lens has a predetermined spatial frequency distribution that is rotationally symmetric with respect to the optical axis, and the second grating lens has a predetermined spatial frequency distribution. A grating lens system, wherein diffracted light from a lens is focused on the predetermined one point. 2. The grating lens according to claim 1, wherein the spatial frequency distributions of the first and second grating lenses are distributions that smoothly increase from the lens center toward the outer periphery. system. 3. The spatial frequency distribution of the second grating lens is a combination of the spatial frequency distribution of an in-line type grating lens that focuses a plane wave and a spatial frequency distribution that smoothly decreases from the lens center toward the outer periphery. The second aspect of the present invention is that
Grating lens system according to item. 4. The band of the spatial frequency distribution that smoothly decreases is equal to the band of the spatial frequency distribution of the first grating lens.
Grating lens system according to item.