JP2010156906A - Liquid crystal optical element and optical pickup device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a liquid crystal element including multilevel binary type diffraction element structure which is capable of electrically controlling characteristics as necessary and is more excellent in performance, and to provide an optical pickup device equipped with the element. <P>SOLUTION: The liquid crystal optical element that holds liquid crystal between two transparent substrates includes the multilevel binary type diffraction element structure on at least one of the two transparent substrates. By changing an effective refractive index of the liquid crystal according to an electrical signal, the optical characteristics of a multilevel binary type diffraction element is electrically controlled. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a liquid crystal optical element that electrically controls the optical characteristics of a multilevel binary diffraction element as required, and an optical pickup device including the same. In particular, the present invention relates to a liquid crystal optical element and an optical pickup device for electrically controlling diffraction characteristics of a one-dimensional multilevel binary diffraction element or a multilevel binary Fresnel lens having a two-dimensional structure.

  From the practical use of spherical optical elements with gradual structural changes defined only by the radius of curvature, aspherical optical elements with more complex structures, and elements with fine optical structures such as diffractive optical elements. Systems have been simplified and enhanced. In recent years, elements capable of electrically controlling optical characteristics typified by liquid crystal optical elements have been put into practical use, and have contributed to the enhancement of functions of optical pickups such as DVD devices equipped with such elements.

  Recently, a multilevel binary diffraction grating having a complicated structure obtained by stair approximation of a blazed grating as a fine optical structure has been put into practical use. In addition, a liquid crystal optical element having such a fine optical structure has been proposed (see, for example, Patent Document 1).

WO2005 / 074388 pamphlet (paragraph [0070], FIG. 3 (a) (b))

  However, multilevel binary diffraction elements have diffraction characteristics that ideally function only for a specific design wavelength. In addition, the characteristics of the multilevel binary diffraction element are sensitive to the shape. For example, if the stepped edge is broken even a little, its function is significantly deteriorated.

  The object of the present invention is to solve the above-mentioned problems, and includes a liquid crystal optical element having a multi-level binary diffraction element capable of electrically controlling diffraction characteristics and a one-dimensional multi-level binary diffraction element structure, and has little structural deterioration. Accordingly, a liquid crystal optical element with little performance deterioration and an optical pickup device including the same are provided.

  In order to solve the above problems, the liquid crystal optical element and the optical pickup device of the present invention basically adopt the following contents.

  The liquid crystal optical element in the present invention is a liquid crystal optical element in which liquid crystal is sandwiched between two transparent substrates, and at least one of the two transparent substrates has a multilevel binary diffraction element structure, The optical characteristics of the multilevel binary diffraction element are electrically controlled by changing the effective refractive index of the liquid crystal according to a signal.

  The liquid crystal optical element of the present invention is characterized in that the multilevel binary diffraction element structure is a one-dimensional multilevel binary diffraction grating. At this time, the alignment axis direction of the liquid crystal molecules and the direction of the lattice vector are preferably orthogonal.

  The liquid crystal optical element of the present invention is a two-dimensional multilevel binary Fresnel lens.

  Here, the alignment axis direction of the liquid crystal molecules is preferably parallel when no voltage is applied.

  The optical pickup device of the present invention is characterized in that the above-described liquid crystal optical element is arranged in an optical path between a laser light source and an objective lens.

  The liquid crystal optical element having the structure of the multi-level binary diffraction element according to the present invention can electrically control the diffraction characteristics of the multi-level binary diffraction element, and can cope with a change in design wavelength, wavelength variation, or design error of the diffraction structure. Correction is possible. Furthermore, in the case of a one-dimensional structure, damage to the diffraction element structure due to the alignment treatment can be reduced, and the maximum diffraction efficiency can be increased.

It is sectional drawing which shows the structural example of the liquid crystal optical element of this invention. (Example 1) It is the figure which showed the structure and effect | action of the one-dimensional multilevel binary type | mold diffraction element which is a component of this invention. (Example 1) It is the figure which showed operation | movement of the liquid crystal optical element of this invention. (Example 1) It is sectional drawing which shows the other structural example of the liquid crystal optical element of this invention. (Example 2) It is the figure which showed the design method of the two-dimensional multilevel binary type | mold Fresnel lens which is a component of this invention. (Example 2) It is drawing which shows the structure of the optical pick-up apparatus of this invention. (Example 3)

  The liquid crystal optical element shown below includes a fine optical structure of a multilevel binary diffraction element on at least one transparent substrate constituting the element, and has an internal structure of a fine optical structure having a wavelength equal to or less than incident light incident on the element. It is said. When the diffraction element has a one-dimensional structure, the direction of the one-dimensional structural change and the alignment axis direction of the liquid crystal molecules are orthogonal to each other. The specific configuration of the liquid crystal optical element in the present invention will be described below.

  FIG. 1 shows a cross-sectional structure of a liquid crystal optical element having a multilevel binary diffraction element structure corresponding to a one-dimensional fine optical structure in this embodiment.

  As shown in FIG. 1, in the liquid crystal optical element 100a of this embodiment, a transparent substrate 101a is added with the structure of a multilevel binary diffraction element 102 having a fine optical structure, and a transparent electrode is formed on the surface of the diffraction element. 103a and an alignment film 104a made of polyimide. Here, the multi-level binary diffraction element 102 has a one-dimensional structure, which will be described later, and has a four-step structure in which the direction of the grating vector is the Y-axis direction and the height per step is d. . The alignment axis 105 of the alignment film 104a provided on the diffraction element surface is the X-axis direction, and is set to be orthogonal to the direction of the grating vector in the multilevel binary diffraction grating 102. The direction of this lattice vector corresponds to the direction in which the one-dimensional optical structure changes (the Y-axis direction in the figure).

A transparent electrode 103b and an alignment film 104b are formed on the surface of the transparent substrate 101b opposite to the transparent substrate 101a on which the multilevel binary diffraction element 102 is formed. The transparent electrode 103b is a solid electrode on the entire surface, and the alignment film 104b is an organic alignment film formed of polyimide that has been rubbed and aligned so as to be parallel to the alignment film 104a. In this way, the alignment films 104a and 104b are arranged in parallel alignment, so that before and after applying voltage, the incident linearly polarized light of the incident light 106 is emitted from the element without changing its direction. Phase modulation can be realized.

  In addition, this drawing shows an example in which a transparent electrode 103a is divided into a plurality of parts for each multilevel binary diffraction grating 102 having a four-step staircase structure, and each of these transparent electrodes 103a is individually provided. Power can be supplied.

  With such a configuration, it is possible to arbitrarily form a region that supplies power to the transparent electrode 103a and a region that does not supply power, and the liquid crystal molecules contained in the liquid crystal layer 112 sandwiched between the transparent substrates 101a and 101b. The orientation of the liquid crystal molecules 107a oriented in the direction perpendicular to the drawing and the liquid crystal molecules 107b oriented in the horizontal direction can be adopted. Of course, in normal use, the transparent electrode 103a is not divided into a plurality of parts, but the whole surface is solid, and the liquid crystal is driven uniformly to electrically control the optical characteristics of the diffraction grating.

  Next, the structure and characteristics of a one-dimensional multilevel binary diffraction element 102, which is one of the components of the liquid crystal optical element 100a, will be described. FIG. 2 is a diagram showing a Y-axis cross-sectional structure of the multilevel binary diffraction element 102, and shows a one-dimensional diffraction grating having no structural change in the X-axis direction.

  As shown in FIG. 2, the multilevel binary diffraction element 102 has a stepped structure in the Z-axis direction, which is the height direction, as described above. Here, the case of four stages is shown. The four-step staircase structure is a diffraction grating that repeats at a pitch P in the Y-axis direction. This is the direction in which the optical structure changes. The Y-axis direction at this time is defined as the direction of the grating vector.

  In FIG. 2, the staircase height 108 is constant at d. At this time, if the refractive index of the medium constituting the multilevel binary diffraction element 102 is n, the optical path length of the medium portion per step is n × d. On the other hand, if the refractive index of air is 1, the optical path length in the air portion per step of the staircase is d, so the optical path length difference between them is (n−1) × d, and the staircase-shaped optical path length distribution. You can see that

  Moreover, since the staircase structure is composed of four steps, the maximum optical path length difference is represented by 4 × (n−1) × d. If the number of steps is sufficiently large at this time, it can be seen that the staircase structure has a triangular prism shape that smoothly and linearly changes. This is a blazed grating that can theoretically realize 100% diffraction efficiency for a specific wavelength.

Here, it is assumed that the incident light 106 having the wavelength λ ₁ is vertically incident on the multilevel binary diffraction element 102 shown in FIG. At this time, if m is an arbitrary natural number and the following equation (1) is satisfied, 100% primary diffracted light 109 can be obtained theoretically. In the equation (1), the symbol || represents an absolute value. The diffraction angle θ of the first-order diffracted light 109 at this time is expressed by equation (2) using the previous period P.

_{m × λ 1 = 4 × |} (n-1) | × d ··· (1)
P × sin (θ) = λ ₁ (2)

Here, it is assumed that the incident light 106 having the wavelength λ ₂ is vertically incident on the multilevel binary diffraction element 102 shown in FIG. At this time, if L is an arbitrary natural number and the optical path length difference with the air per step is the same as the wavelength λ ₂ or an integral multiple thereof, as shown in the equation (3), the staircase structure for the light It can be considered the same as not having. This is because light is a wave that repeats with a wavelength as a period. Accordingly, the incident light 106 at this time passes through the liquid crystal optical element 100 a as it is, and becomes zero-order light 110.

_{L × λ 2 = | (n} -1) | × d ··· (3)

As described above, the binary diffraction element 102 can realize an optical characteristic that functions 100% with respect to a specific wavelength λ ₁ and does not function at all with respect to another specific wavelength λ ₂ .

  Next, the structure and operation of the liquid crystal optical element 100a will be briefly described with reference to FIG. FIG. 3A shows the behavior of the liquid crystal molecules in the liquid crystal optical element 100a viewed from the direction rotated by 90 ° from the direction shown in FIG. 1, and FIG. 3B shows the behavior from the same direction as FIG. The state of liquid crystal molecules was shown.

  As shown in FIGS. 3A and 3B, transparent electrodes 103a and 103b are formed on a pair of transparent substrates 101a and 101b, and the transparent electrodes 103a formed on one transparent substrate 101a are vertically arranged in this figure. It is electrically divided into two. Further, it is assumed that an alignment film 104a is further applied on the transparent electrode 103a and the alignment axis 105 is set to be in the X-axis direction.

3A and 3B, it is assumed that no potential difference is given to the liquid crystal layer 112 sandwiched between the transparent electrodes 103a and 103b in the upper region in the drawing. At this time, the rugby ball-shaped liquid crystal molecules 107 a align their molecular major axes 111 in the direction of the alignment axis 105 in the vicinity of the alignment film 104 a. Therefore, the group of liquid crystal molecules 107a that behave as a continuum forms a liquid crystal layer 112 in which the molecular long axes 111 are aligned substantially parallel to the X-axis direction. Here, the liquid crystal molecules 107a shown in FIG. 3A are slightly tilted with respect to the X axis (alignment axis 105) because the pretilt angle is set, and the angle is about several degrees. is there. In this region, the incident light 106 having a linear polarization component in the X-axis direction propagates through the liquid crystal layer 112 while looking at the refractive index n _e of approximately molecular long axis 111.

On the other hand, when a sufficiently high voltage is applied to the liquid crystal layer 112 sandwiched between the transparent electrodes 103a and 103b in the lower region of both drawings, the liquid crystal molecules 107b have a molecular major axis 111 extending in the direction of the electric field Z. Stand still in the axial direction. In this case the incident light 106 propagates while watching the approximate molecular refraction index _{n o} of the minor axis 113. By adjusting the voltage applied transparent electrodes 103a, in 103b, it can be adjusted the tilt of the liquid crystal molecules 107 b, _{n o} the refractive index for incident light 106 of the liquid crystal layer 112 by this voltage, approximately _{n e} Can be changed continuously.

Therefore, it is possible to provide a refractive index distribution according to the shape of the multilevel binary diffraction element 102 formed on the surface of the transparent substrate 101a, and the depth of the refractive index distribution can be controlled by voltage. Here, the polarization direction of the incident light 106 must be the same as the direction of the orientation axis 105, more precisely, the direction of the molecular major axis 111 before applying a voltage. This is because with respect to the X-axis direction of the linearly polarized light regardless of the inclination of the liquid crystal molecules 107 b, propagates while watching the refractive index n _o of the molecular minor axis 113.

This phenomenon is the same as replacing the air layer with the liquid crystal layer 112. Therefore, if the refractive index of the medium constituting the multi-level binary type diffraction element 102 is n, any in the aforementioned (1) (3), instead of the refractive index 1 of air, from n _e to n _o Can be electrically selected, and is expressed by the following equations (4), (5), and (6). This shows that the design wavelength can be changed electrically.
m × λ ₁ = 4 × | (n−n _θ ) | × d (4)
L × λ ₂ = | (n−n _θ ) | × d (5)
n _o <n _θ <n _e (6)

When n is a typical value as an example 1.5, _{n e} is 1.7, _{n o} is 1.5, and d is a 2 [mu] m, when no given voltage to the liquid crystal optical element 100a is (3) When L is 1 from the equation, the wavelength of 400 nm passes, and when m is 2 from equation (1), almost 100% of the first-order diffracted light with respect to the wavelength of 800 nm can be obtained.

Further, if a voltage is applied to the liquid crystal optical element 100a and the effective refractive index of the liquid crystal molecules 107a and 107b with respect to the incident light 106 is set to 1.6, light having a wavelength of 200 nm passes similarly and light having a wavelength of 400 nm. In contrast, almost 100% first-order diffracted light can be obtained.
To be exact, the refractive indexes and thicknesses of the transparent electrodes 103a and 103b and the alignment films 104a and 104b must be taken into account, but these thicknesses are neglected here because they are about 200 nm or less.

  The liquid crystal optical element 100a described above can electrically change the design wavelength, but also plays an important role in terms of correction of the design wavelength. That is, the multi-level binary diffraction element 102 is sensitive to a deviation from the design wavelength, has a subtle error in the staircase height, variation in the oscillation wavelength of the laser beam, laser oscillation wavelength shift due to temperature, and expansion / contraction of the staircase structure. Affected by. As a result, the efficiency of a light beam having a wavelength to be diffracted is reduced, and diffraction noise of a light beam having a wavelength that should not be diffracted occurs. In such a case, by controlling the voltage applied to the liquid crystal layer 112 and correcting the design wavelength described above, these effects can be almost completely removed. In addition, according to this embodiment, the step structure of the multi-level binary diffraction element 102 having a fine structure that is easily broken by contact or the like is inherently protected in the liquid crystal optical element 100a, which is convenient.

  Further, as described above, in the multi-level binary diffraction element 102, a blazed shape, that is, a triangular prism-shaped slope (indicated by a broken line in FIG. 2) is approximated stepwise. It is known that the theoretical maximum diffraction efficiency obtained at this time is about 82% in the case of four stages shown in FIG. 2 and about 95% in the case of eight stages.

  Here, as an example of the effect of the present invention, the maximum diffraction efficiency of the liquid crystal optical element 100a including the multi-level binary diffraction element 102 having a pitch of 10 μm, a step d of 0.285 μm, and the number of steps of 8 was measured. In this measurement, a laser beam having a wavelength of 650 nm was perpendicularly incident on the liquid crystal optical element 100a, and the first-order diffracted light intensity with respect to the incident light intensity was measured to obtain the ratio. In measuring the light intensity, a commercially available laser power meter was used. As a result, in the liquid crystal optical element 100a shown in this example, the value of the first-order diffracted light with respect to the design wavelength was 82%, which was a value relatively close to 95% of the theoretical maximum efficiency. In the conventional liquid crystal optical element subjected to the alignment treatment in the orthogonal direction, only about 60% of the first-order diffracted light was obtained.

  In this way, if the liquid crystal optical element 100a is set so that the alignment axis 105 in the alignment film 104a is orthogonal to the direction of the lattice vector of the multilevel binary diffraction element 102, not only the rubbing alignment process but also the optical alignment In the alignment process such as the process or the vapor deposition alignment process, it was confirmed that the step structure of the multi-level binary diffraction grating 102 is hardly damaged and the maximum diffraction efficiency is dramatically improved as compared with the conventional one.

  In the above description, the configuration of the liquid crystal optical element 100a in which the alignment films 104a and 104b are aligned in parallel is shown. However, in the initial state, a vertical alignment type in which the liquid crystal molecular major axis stands substantially perpendicular to the transparent substrate 101. The same effect as that of the present invention can be obtained even in the optical element form.

  In the above description, an example in which the multi-level binary diffraction element 102 is disposed only on the transparent substrate 101a is shown, but this may be provided on both surfaces of the transparent substrates 101a and 101b as necessary. Absent. In this way, by providing the multilevel binary diffraction element 102 with the same grating and the same pitch on the surfaces of both transparent substrates 101a and 101b, the depth carved into each grating can be halved, and the manufacture of the element is facilitated. . Further, when different diffraction gratings are provided on both substrates, one element can have two types of diffraction functions.

  In the above description, the transparent electrode 103a is arranged on the surface of the multilevel binary diffraction element 102. However, the multilevel binary diffraction element 102 is provided on the transparent substrate 103a provided on the surface of the transparent substrate 101a. It does not matter as a configuration.

  Next, another configuration example of the liquid crystal optical element will be described with reference to FIG. FIG. 4 shows a cross-sectional structure of a liquid crystal optical element having a two-dimensional multilevel binary Fresnel lens 114, and has a rotationally symmetric structure with respect to the optical axis 115.

  As shown in FIG. 4, the liquid crystal optical element 100b shown in the present embodiment has a multi-level binary Fresnel lens 114 having a four-stage multistage structure on the transparent substrate 101a as in the embodiment shown in FIG. Similarly to the embodiment shown in FIG. 1, the transparent electrode 103a and the alignment film 104a are coated on the multilevel binary Fresnel lens 114, and the transparent electrode 103b and the alignment film 104b are formed on the surface of the opposing transparent substrate 101b. Are formed, and the liquid crystal molecules 107a are aligned in parallel.

  Here, a design method of the multilevel binary Fresnel lens shown in FIG. 4 will be described. FIG. 5 is a diagram showing a structure design method of the multi-level binary Fresnel lens 114.

  FIG. 5A shows a cross-sectional structure of a normal spherical lens 116 (including an aspheric lens). First, for this spherical lens 116, as shown in FIG. 5B, the structure of the spherical lens 116 in FIG. 5A is sliced at an equal pitch in the Z-axis direction, and a uniform thickness is omitted. 117. Next, as shown in FIG. 5C, the structure of the Fresnel lens 117 shown in FIG. 5B is subjected to multi-stage quantization at the height pitch d to obtain the structure of the multilevel binary Fresnel lens 114. Although FIG. 5 shows an example of slicing at equal pitch in the Z-axis direction, it may be sliced at equal pitch in the X-axis direction and subjected to multistage quantization to form a multi-level binary Fresnel lens.

The multi-level binary liquid crystal element in this embodiment configured as described above functions as a 100% lens with respect to a specific wavelength λ ₁ by the same action as the embodiment in FIG. For λ ₂ , optical characteristics that do not function at all can be realized, and as a result, the design wavelengths λ ₁ and λ ₂ can be electrically controlled.

  Also in the form shown in this example, the same effect as in the present invention can be obtained not only in the parallel alignment type but also in the vertical alignment type optical element form as in Example 1.

  Next, an example in which the liquid crystal optical element shown in Embodiment 2 is applied to an optical pickup device will be described. FIG. 6 is a diagram illustrating a configuration example of an optical pickup device using the liquid crystal optical element in the present embodiment.

The optical pickup device 200 shown in this embodiment includes a laser light source 118 that oscillates laser light of a predetermined wavelength, a collimator lens 120 that makes the laser light oscillated from the laser light source parallel light, and an optical disk 123 that has the parallel light. And a liquid crystal optical element 100b arranged in the middle of the optical path between the collimating lens 120 and the objective lens 122.

  In this optical pickup device 200, laser light 119 oscillated from a laser light source 118 having oscillation wavelengths of 405 nm and 650 nm is converted into parallel light by a collimator lens 120, then transmitted through a liquid crystal optical element 100 b, and optical discs by an objective lens 122. 123 is condensed. Here, the liquid crystal optical element 100b is a Fresnel lens having the basic structure shown in FIG. The numerical aperture of the objective lens 122 is 0.85 for BD (Blu-ray Disc). In reproducing a BD disc, the liquid crystal optical element 100b does not function as a lens. That is, the liquid crystal is controlled by the drive circuit 124 so that it does not function for a wavelength of 405 nm.

  On the other hand, in reproducing a DVD, the liquid crystal optical element 100b is controlled by a drive circuit 124 so as to function as a multi-level binary Fresnel lens, and functions as a DVD objective lens system by interacting with the objective lens 122. Here, when the oscillation wavelength of the laser beam fluctuates, the drive circuit 124 can correct the design wavelength of the liquid crystal optical element 100b.

  In this way, by arranging the liquid crystal optical element 100b shown in the second embodiment in the optical path between the laser light source 118 and the objective lens 122 in the optical pickup device 200, the lens state and the raw glass state can be switched reliably. Thus, the deviation of the focal position of the optical disk 123 is corrected.

  In the above description, the example in which the liquid crystal optical element 100b having the multilevel binary Fresnel lens shown in the second embodiment is mounted on the optical pickup device 200 has been described. However, the liquid crystal optical element 100b is replaced with the first embodiment. It is also possible to mount the liquid crystal optical element 100a having the multilevel binary diffraction grating shown. The case where the liquid crystal optical element 100a is mounted on the optical pickup device 200 will be described below.

  The optical pickup device equipped with the liquid crystal optical element 100a generates three beams of zero-order light and ± first-order light from the laser light 119 emitted from the laser light source 118 by a multilevel binary diffraction grating. For example, assume that the laser light source 118 is a light source that oscillates laser light having two wavelengths of 785 nm and 650 nm, and at the time of CD reproduction, the 785 nm laser light 119 emitted from the laser light source 118 is 3 The beam is converted into laser beam 119. Then, the optical disc 123 (CD) is irradiated with the three-beam laser light, the disc information is read using the 0th order light of the reflected light from the optical disc 123, and the tracking signal is obtained from the ± 1st order light. Read accurate information on CD.

  When reproducing a DVD, the 650-nm laser light incident on the liquid crystal optical element 100a is passed as it is, and the optical disk 123 (DVD) is irradiated with one beam as it is to read accurate information on the DVD.

  As described above, in the optical pickup device, the multi-level binary diffraction grating of the liquid crystal optical element 100a is used so as to function as a diffractive element at a CD wavelength and not to function as a diffractive element at a DVD laser wavelength. be able to.

Further, according to the above configuration, even if the oscillation wavelength of the laser beam 119 oscillated from the laser light source 118 changes due to temperature fluctuation, for example, the diffraction efficiency varies according to the fluctuation amount by the liquid crystal optical element 100a. Thus, there is also an advantage that stable three-beam laser light can be generated at all times.

100a, 100b Liquid crystal optical element 101a, 101b Transparent substrate 102 Multi-level binary diffraction element 103a, 103b Transparent electrode 104a, 104b Alignment film 105 Alignment axis 106 Incident light 107a, 107b Liquid crystal molecule 108 Stair height 109 First order diffracted light 110 0th order Light 111 Molecular long axis 112 Liquid crystal layer 113 Molecular short axis 114 Multi-level binary Fresnel lens 115 Optical axis 116 Lens 117 Fresnel lens 118 Laser light source 119 Laser light 120 Collimating lens 122 Objective lens 123 Optical disk 124 Drive circuit 200 Optical pickup device

Claims (6)

In a liquid crystal optical element that sandwiches liquid crystal between two transparent substrates,
At least one transparent substrate has a multi-level binary diffraction element structure,
A liquid crystal optical element, wherein an optical characteristic of the multilevel binary diffraction element is electrically controlled by changing an effective refractive index of the liquid crystal by an electric signal. The liquid crystal optical element according to claim 1, wherein the multilevel binary diffraction element is a one-dimensional multilevel binary diffraction grating. The liquid crystal optical element according to claim 2, wherein an alignment axis direction of liquid crystal molecules of the liquid crystal and a direction of a lattice vector of the one-dimensional multilevel binary diffraction grating are orthogonal to each other. The liquid crystal optical element according to claim 1, wherein the multilevel binary diffraction element is a two-dimensional multilevel binary Fresnel lens. The liquid crystal optical element according to any one of claims 2 to 4, wherein the alignment axis directions of the liquid crystal molecules are parallel when no voltage is applied. An optical pickup device comprising the liquid crystal optical element according to any one of claims 2 to 5 disposed in an optical path between a laser light source and an objective lens.

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