Wavelength converting apparatus

A wavelength converting apparatus that improves output performance of laser light subjected to wavelength conversion while improving the efficiency of laser light wavelength conversion. Wavelength converting apparatus (100) has: nonlinear optical crystal (110) converting wavelength of laser light propagating inside wavelength converting apparatus (100); right angle prism (130) deflecting laser light emanating from nonlinear optical crystal (110) and causing the laser light to be incident on nonlinear optical crystal (110) again, and to propagate in parallel and in opposite directions at a predetermined distance with respect to laser light before emanation from nonlinear optical crystal (110); and first dichroic mirror (120) separating laser light subjected to wavelength conversion inside nonlinear optical crystal (110) from the laser light before incidence on nonlinear optical crystal (110) for a second time.

CROSS REFERENCE TO RELATED APPLICATIONS

The disclosure of Japanese Patent Application No. 2008-037902 filed on Feb. 19, 2008, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The technical field relates to a wavelength converting apparatus for converting the wavelength of incident light and outputting the light using a nonlinear optical crystal.

2. Description of the Related Art

A wavelength converting apparatus that converts the wavelength of laser light using a nonlinear optical crystal is popular because the apparatus is capable of producing laser light having wavelengths that are difficult to produce using laser elements alone.

The efficiency of wavelength conversion of a nonlinear optical crystal generally increases in proportion to the square of the distance of laser light propagation. Therefore, the distance of laser light propagation inside a nonlinear optical crystal is preferably as long as possible. Meanwhile, making a longer nonlinear optical crystal results in increased cost and making large-sized apparatuses.

Then, a technique of making the distance of laser light propagation longer by deflecting laser light inside a nonlinear optical crystal is disclosed in Japanese Patent Application Laid-Open No. 2005-106954, for example.

FIG. 1is a configuration diagram of the nonlinear optical crystal disclosed in the above patent document. According to the technique disclosed in the patent document, nonlinear optical crystal10has input face11, a plurality of total reflection faces12and13, and output face14. Laser light20before wavelength conversion enters input face11at a ninety-degree incident angle. Total reflection faces12and13reverse the traveling direction of laser light20while shifting the position of the traveling direction of laser light20entering from input face11. Output face14emits laser light subjected to wavelength conversion inside nonlinear optical crystal10. This makes it possible to shuttle laser light20in the longitudinal direction of nonlinear optical crystal10and make the distance of laser light propagation longer in a state nonlinear optical crystal10is prevented from being longer.

However, according to the technique disclosed in the above patent document, there is a problem that the output of light subjected to wavelength conversion decreases.

The reason of the above problem is as follows. In nonlinear optical crystal10, laser light20becomes a mixed light of a light whose wavelength is not converted (hereinafter “unconverted light”) and of a light whose wavelength is converted (hereinafter “converted light”). However, the refractive index of a nonlinear optical crystal depends on the wavelength, and therefore phase matching is lost between converted light and unconverted light when a mixed light propagates in directions other than the direction phase matching is maintained. With the technique disclosed in the above patent document, although phase matching between converted light and unconverted light is maintained before converted light and unconverted light reflect on total reflection face12, phase matching is lost when converted light and unconverted light reflect on total reflection face12and propagate. Then, wavelength conversion is performed on the unconverted light on the return path in the state phase matching is lost. As a result, converted light subjected to wavelength conversion after reflection and converted light subjected to wavelength conversion before reflection interfere with each other, and therefore the output of converted light decreases. On the other hand, it is very difficult to make adjustment in order to prevent interference between converted light subjected to wavelength conversion after reflection and converted light subjected to wavelength conversion before reflection.

SUMMARY

An object is therefore to provide a wavelength converting apparatus that improves output performance of laser light whose wavelength is converted, while improving the efficiency of laser light wavelength conversion.

The wavelength converting apparatus adopts a configuration including: a nonlinear optical crystal that performs a wavelength conversion of laser light propagating inside the nonlinear optical crystal: a light deflection section that causes laser light that has exited from the nonlinear optical crystal to deflect and re-enter the nonlinear optical crystal, and to propagate in parallel, in an opposite direction, at a predetermined distance with respect to the laser light before exiting from the nonlinear optical crystal; and a light separation section that separates laser light whose wavelength is converted inside the nonlinear optical crystal, from the laser light before re-entering the nonlinear optical crystal.

Laser light is deflected and re-enters a nonlinear optical crystal. Then, before the wavelength of this deflected laser light is converted again, converted light is separated from the laser light, which is mixed light of unconverted light and converted light where phase difference occurs. This makes it possible to prevent mixed light that does not maintain phase matching from producing, and improve output performance of laser light whose wavelength is converted, while improving the efficiency of laser light wavelength conversion. Further, by removing converted light, it is possible to reduce the thermal lens effect of a nonlinear optical crystal and improve conversion efficiency, and, furthermore, gain an advantage of lightfastness of the crystal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 2shows a perspective view showing an outline of the wavelength converting apparatus according to Embodiment 1.

Referring toFIG. 2, wavelength converting apparatus100has nonlinear optical crystal110, first dichroic mirror120, first right angle prism130, second dichroic mirror140and second right angle prism150.

Nonlinear optical crystal110has a shape of a right prism. Hereinafter, in all of the figures, the right direction in the right and left directions on paper, that is, the longitudinal direction of nonlinear optical crystal110, is the positive direction along the Z axis. The upward direction in the up and down directions on paper is the positive direction along the Y axis. The back direction perpendicular to the right and left directions and the up and down directions on paper is the positive direction along the X axis.

Further, laser light enters nonlinear optical crystal110from the negative direction along the Z axis seen from nonlinear optical crystal110inFIG. 2, which will be described later.

First right angle prism130is placed in the positive direction along the Z axis seen from nonlinear optical crystal110. Second right angle prism150is placed in the negative direction along the Z axis seen from nonlinear optical crystal110. First dichroic mirror120is placed between nonlinear optical crystal110and first right angle prism130. Second dichroic mirror140is placed between nonlinear optical crystal110and second right angle prism150.

Nonlinear optical crystal110, which is a harmonic generation element, converts first wavelength light propagating inside nonlinear optical crystal110to second wavelength light, according to the distance of propagation. Hereinafter, the light of first wavelength light that is not converted to second wavelength light in nonlinear optical crystal110will be referred to as “unconverted light,” and second wavelength light will be referred to as “converted light.”

Nonlinear optical crystal110has first input and output face111, which is perpendicular to the Z axis and which is placed on the negative side of the Z axis, and second input and output face112, which is perpendicular to the Z axis and which is placed on the positive side of the Z axis. Referring toFIG. 3, laser light190is inputted to first input and output face111of nonlinear optical crystal110in the Z axial direction and exits from second input and output face112on the opposite side of the first input and output face.

Nonlinear optical crystal110may use the kinds harmonic generation elements according to purposes such as second harmonic generation (SHG) element, third harmonic generation (THG) element and so on.

First dichroic mirror120transmits the first wavelength light of incident light and reflects the second wavelength light of incident light. More precisely, first dichroic mirror120reflects the second wavelength light having higher reflectance than the first wavelength light.

First dichroic mirror120is placed on the optical path of laser light190exiting from second input and output face112such that its normal line direction is tilted forty five degrees in the positive direction along the Y axis with respect to the incoming direction of laser light190. That is, with regards to laser light190exiting from second input and output face112of nonlinear optical crystal110, first dichroic mirror120reflects and emits the converted light in the positive direction along the Y axis, and, transmits and emits the unconverted light in the positive direction along the Z axis.

First right angle prism130, having first and second reflecting faces131and132forming a right angle, emits the light entering first reflecting face131, from second reflecting face132, in an opposite direction from the incident direction. In first right angle prism130, first reflecting face131is placed on the optical path of the unconverted light exiting from first dichroic mirror120. Further, first right angle prism130is placed in the direction the unconverted light reflected on first reflecting face131and exiting from second reflecting face132enters perpendicularly second input and output face112of nonlinear optical crystal110. That is, first right angle prism130deflects the unconverted light exiting from first dichroic mirror120at one hundred eighty degrees at a predetermined distance in the positive direction along the Y axis, and causes the unconverted light to re-enter nonlinear optical crystal110.

As in first dichroic mirror120, second dichroic mirror140transmits first wavelength light of incident light and reflects second wavelength light.

Second dichroic mirror140is placed on the optical path of laser light190exiting from first input and output face111such that the normal line direction of second dichroic mirror140is tilted forty five degrees in the positive direction along the Y axis with respect to the incoming direction of laser light190. That is, second dichroic mirror140reflects and emits the converted light of laser light190exiting from first input and output face111of nonlinear optical crystal110, which is first wavelength light, in the positive direction along the Y axis, and, transmits and emits the unconverted light, which is second wavelength light, in the negative direction along the Z axis.

Second right angle prism150is placed on the optical path of the unconverted light exiting from second dichroic mirror140, and deflects the unconverted light in the negative direction along the Y axis, for example.

In the faces of second right angle prism150, the face on which the unconverted light exiting from second dichroic mirror140enters, is coated to totally reflect laser light. Second right angle prism150may be replaced with two total reflection mirrors corresponding to the first reflection face131and second reflection face132. Further, a right angle prism not coated as described above may be configured to be placed in a rotated or inverted manner as appropriate, to deflect laser light190using total reflection. The same configuration modification applies to other right angle prisms described later.

In wavelength converting apparatus100of this configuration, as shown inFIG. 3, laser light190enters input and output face111of nonlinear optical crystal110, propagates inside nonlinear optical crystal110and exits from second input and output face112. Although laser light190becomes a mixed light of converted light and unconverted light here, laser light190enters first dichroic mirror120to be separated into converted light and unconverted light. To be more specific, first output converted light191of laser light190, which is converted light outputted from first dichroic mirror120, is separated by reflection in the positive direction along the Y axis. Further, the remaining unconverted light of laser light190passes through first dichroic mirror120and travels toward first right angle prism130.

Laser light190, which then includes only unconverted light, is deflected and turned back by first right angle prism130, re-enters nonlinear optical crystal110from second input and output face112, and, propagates inside nonlinear optical crystal110and exits from first input and output face111. Although laser light190becomes a mixed light of converted light and unconverted light here, laser light190enters second dichroic mirror140to be separated into converted light and unconverted light. To be more specific, second output converted light192of laser light190, which is the converted light outputted from second dichroic mirror140, is separated in the positive direction along the Y axis due to reflection. Further, the remaining unconverted light of laser light190passes through second dichroic mirror140and travels toward second right angle prism150. Laser light190, which then includes only unconverted light again, is deflected by second right angle prism150, and exits as output unconverted light193in the negative direction along the Y axis, for example.

In this way, in wavelength converting apparatus100, converted light is separated from laser light190, which is a mixed light in the forward path to first right angle prism130, before wavelength conversion is performed for laser light190again.

In the deflection by reflection in first right angle prism130, phase matching between converted light and unconverted light is lost because of material dispersion, and, if wavelength conversion continues in the state phase matching is lost, the output of light whose wavelength is converted decreases. Consequently, wavelength converting apparatus100separates converted light from laser light190before wavelength conversion on the return path. By this means, even if deflection of laser light190is performed between the forward path and the return path, wavelength converting apparatus100prevents mixed light that does not maintain phase matching from being produced, and makes it possible to perform high-efficiency laser light wavelength conversion.

Material dispersion is generally caused not only in a case where the deflection means is first right angle prism130, and a case where the deflection means is a mirror coated with dielectric multilayer or other materials on the surface. Further, coating such as AR (anti-reflective) coating is applied to the input and output end faces of a nonlinear optical crystal. As shown in the following embodiments, when laser light passes through a lens system, material dispersion is caused and phase matching is lost. For that reason, it is effective to separate converted light and unconverted light before wavelength conversion is performed again.

Further, in wavelength converting apparatus100, the forward path for first right angle prism130and the return path therefrom of laser light190are placed in parallel at a predetermined distance. This predetermined distance makes it possible to prevent light on the forward path and light on the return path from having influence of optical interference. If laser light190is simply turned back, the remaining converted light not separated in first dichroic mirror120interferes with the converted light produced on the return path, and it is likely to cause output instability. Consequently, by providing a predetermined distance between the forward path and the return path of laser light190, wavelength converting apparatus100of the present embodiment makes it possible to prevent these problems and acquire stable output. However, taking into account the temperature distribution (described later) in the nonlinear optical crystal, the above-described predetermined distance is preferably provided so as to maximize a conversion efficiency of the entire wavelength converting apparatus100.

Further, in wavelength converting apparatus100, the traveling directions of laser light190are opposite between the forward path and there turn path. Another advantage because the forward path and the return path are provided in parallel and opposite directions at a short predetermined distance, will be explained using a drawing.

FIG. 4illustrates for explanation showing a distribution of heat of nonlinear optical crystal110shown inFIG. 3. Here, nonlinear optical crystal110serves as a second harmonic generation (SHG) element.

A nonlinear optical crystal generally has light absorption properties and produces heat caused by propagation of light. Further, a nonlinear optical crystal absorbs more energy from light having a shorter wavelength, and the amount of heat increases more. Furthermore, as light propagates inside a nonlinear optical crystal, the proportion of converted light increases, so that a distribution of heat (i.e. differences in the amount of heat) is produced between the input side and the output side of the nonlinear optical crystal.

Here, the amounts of heat per unit distance at a position immediately after laser light190aon the forward path enters, a position immediately before laser light190aon the forward path exits, a position immediately after laser light190bon the return path enters and a position immediately before laser light190bon the return path exits, are Q1to Q4[J: joule] in order. Due to the above reason, the amounts of heat Q1to Q4fulfill the relationships represented by the following equations 1 and 2.

In general, to perform high-efficiency wavelength conversion, it is necessary to keep nonlinear optical crystal110at a constant temperature. That is, it is not preferable that a distribution of heat (i.e. differences in the amount of heat) is produced on an optical path and causes the distribution of temperatures (i.e. differences in temperature).

Then, nonlinear optical crystal110of the present embodiment is formed such that the forward path of laser light190aand the return path of laser light190bare in parallel and in opposite directions at a short predetermined distance. This makes it possible to facilitate conduction of heat, cancel out the distribution of temperature and reduce the differences in temperature from the propagation region of laser light190bon the return path to the propagation region of laser light190aon the forward path. That is, it is possible to improve the efficiency of wavelength conversion.

As described above, according to the present embodiment, the reflection in first right angle prism130allows laser light190exiting from nonlinear optical crystal110to deflect and re-enters nonlinear optical crystal110, and to propagate in parallel and in opposite directions at a short predetermined distance with respect to laser light190before exiting from nonlinear optical crystal110. Further, first dichroic mirror120separates first output converted light191whose wavelength is converted in nonlinear optical crystal110from laser light190before re-entering. This makes it possible to separate converted light from laser light190before wavelength conversion is performed again for deflected laser light190, prevent mixed light not maintaining phase matching from entering nonlinear optical crystal110and prevent output instability due to interference and forming resonator. That is, it is possible to improve output performance of laser light whose wavelength is converted, while improving the efficiency of laser light wavelength conversion.

Further, the forward path and return path of laser light190are in parallel and in opposite directions at a short predetermined distance. This makes it possible to average the distribution of temperature and improve the efficiency of wavelength conversion.

FIG. 5is a perspective view showing an outline of the wavelength converting apparatus according to Embodiment 2, and corresponds toFIG. 2of Embodiment 1. The same parts as inFIG. 2are assigned the same numerals, and therefore the explanation thereof will be omitted.

Referring toFIG. 6, first dichroic mirror220and second dichroic mirror240are placed on both the forward path and the return path of laser light190for and from first right angle prism130.

As shown inFIG. 6, in wavelength converting apparatus200of this configuration, laser light190having passed through first dichroic mirror220on the forward path re-enters first dichroic mirror220. Then, the remaining converted light, not separated on the forward path, is separated as surplus converted light194, in the negative direction along the Y axis by reflection. This makes it possible to separate the remaining converted light on the return path, not separated on the forward path, before wavelength conversion is performed again, and it is possible to reduce the possibility of mixing converted light produced on the forward path into laser light190on the return path and reduce the output.

Further, in Embodiment 1 as explained earlier, the size of first dichroic mirror120and second dichroic mirror140needs to be smaller so as to cross only one of the forward path and the return path of laser light190. However, if the size of first dichroic mirror120and second dichroic mirror140is small, cases occur where the cost of manufacturing increases and where accurate positioning is difficult. By contrast with this, according to the present embodiment, it is possible to make first dichroic mirror220and second dichroic mirror240larger than or equal to the size like dichroic mirrors220and240cross both the forward path and the return path of laser light190. Consequently, it is possible to reduce the cost of manufacturing and improve the accuracy of the wavelength converting apparatus.

FIG. 7is a configuration diagram of the wavelength converting apparatus according to Embodiment 3, and corresponds toFIG. 6of Embodiment 2. The same parts as inFIG. 6are assigned the same numerals, and therefore the explanation thereof will be omitted.

Referring toFIG. 7, wavelength converting apparatus300has second right angle prism350of larger size, which is replaced with second right angle prism150shown inFIG. 6, and further has half-wave plate360and polarizing beam splitter370.

Second right angle prism350has the expanded size so as to cross not only laser light190exiting from first input and output face111of nonlinear optical crystal110, but second output converted light192exiting from second dichroic mirror240. Second right angle prism350has third reflecting face351such that its normal line direction is tilted forty five degrees in the positive direction along the Z axis with respect to the incoming direction of second output converted light192exiting from second dichroic mirror240. That is, second right angle prism350deflects second output converted light192exiting from second dichroic mirror240in the positive direction along the Z axis.

half-wave plate360rotates ninety degrees its polarization plane of incident light. That is, half-wave plate360rotates ninety degrees the polarization plane of second output converted light192, and causes the polarization direction of second output converted light192to be perpendicular to the polarization direction of first output converted light191exiting from first dichroic mirror220.

Polarizing beam splitter370combines the incident lights from two predetermined directions and emits combined light. Here, polarizing beam splitter370is placed at the position corresponding to the vertex of first output converted light191exiting from first dichroic mirror220and second output converted light192exiting from half-wave plate360. Further, polarizing beam splitter370is placed in the direction so as to combine the first output converted light191and second output converted light192that have entered, and output the combined light in the positive direction along the Y axis. That is, polarizing beam splitter370emits combined converted light195, which combines first output converted light191and second output converted light192, in the positive direction along the Y axis.

First output converted light191and second output converted light192do not interfere with each other when propagating with an optical path difference longer than the coherence length. In this case, speckle noise generally decreases. Consequently, by configuring wavelength converting apparatus300such that a longer optical path difference than the coherence length is generated, it is possible to acquire a beam light whose wavelength is converted with reduced speckle noise.

Further, second right angle prism350deflects the unconverted light and converted light exiting from second dichroic mirror240, so that it is possible to reduce the number of the components.

Further, light deflection elements may be provided individually to deflect the unconverted light and converted light exiting from second dichroic mirror240. Further, it is possible to rotate the polarization plane of first output converted light191, instead of the polarization plane of second output converted light192. Further, surplus converted light194outputted from first dichroic mirror220may be combined with first output converted light191, second output converted light192or combined converted light195, using half-wave plate360and polarizing beam splitter370. Furthermore, it is possible to receive surplus converted light194by, for example, a photodiode, and use this reception result in power control of laser light190and temperature control of nonlinear optical crystal110.

FIG. 8is a perspective view showing an outline of the wavelength converting apparatus according to Embodiment 4, and corresponds toFIG. 2of Embodiment 1. The same parts as inFIG. 2are assigned the same numerals, and therefore the explanation thereof will be omitted.

Referring toFIG. 8, wavelength converting apparatus400has first cylindrical lens480aand second cylindrical lens480bon the both sides of nonlinear optical crystal110in the Z axial direction, in addition to the configuration of wavelength converting apparatus100shown inFIG. 2.

First cylindrical lens480acollects parallel light entering from the cylinder surface into the focal point on the plane surface, and, meanwhile, changes light incident from this focal point to parallel light, to emit the parallel light from the cylinder surface. Here, first cylindrical lens480ais placed such that its plane surface opposes first input and output face111of nonlinear optical crystal110and its optical axis matches with the Z-axial direction. Further, the focal point of first cylindrical lens480ais positioned in plane113that passes the center of nonlinear optical crystal110in the Z-axial direction and that is perpendicular to the Z axis (hereinafter “crystal center plane”).

Second cylindrical lens480band first cylindrical lens480aare placed so as to have a plane-symmetric shape with respect to crystal center plane113of nonlinear optical crystal110.

Referring toFIG. 9, first cylindrical lens480aand second cylindrical lens480bare large such that their central axes cross both the forward path and the return path of laser light190.

According to this wavelength converting apparatus400, laser light190is narrowed down in the X axial direction in the area between first cylindrical lens480aand second cylindrical lens480bso as to form a beam waist in crystal center plane113. By this means, the density of incident light in the region that laser light190inside nonlinear optical crystal110passes increases.

Generally, the efficiency of wavelength conversion by a nonlinear optical crystal improves when its incident light density increases, if the incident light intensity is the same. Consequently, by causing laser light190to converge inside nonlinear optical crystal110, the efficiency of wavelength conversion further improves.

Further, cylindrical lenses480are used as the light converging elements, so that it is possible to use one component on the forward path and return path, and reduce the number of lens system components, compared to providing components separately for the forward path and the return path.

Further, cylindrical lenses480are placed in the Y-axial direction with high flexibility and easily positioned with high accuracy using a plurality of plane parts, so that it is easy to adjust cylindrical lenses480optically.

Laser light190propagates as parallel light outside the area between first cylindrical lens480aand second cylindrical lens480bon the forward path and the return path. Consequently, the presence of first cylindrical lens480aand second cylindrical lens480bdoes not have influence on the separation of converted light and the deflection of laser light190in this area.

Although a configuration has been described with the present embodiment where laser light190converges on both the forward path and the return path, a configuration may be adopted where laser light190converges in only one of the forward path and the return path.

Further, it is possible to place first cylindrical lens480aonly and not place second cylindrical lens480b. In this case, the focal point of first cylindrical lens480ais preferably adjusted as appropriate such that the output sum value of first output converted light191and second output converted light192is maximum, taking into account the length of nonlinear optical crystal110and the position of first right angle prism130.

FIG. 10is a perspective view showing an outline of the wavelength converting apparatus according to Embodiment 5, and corresponds toFIG. 8of Embodiment 4. The same parts as inFIG. 8are assigned the same numerals, and therefore the explanation thereof will be omitted.

Referring toFIG. 10, wavelength converting apparatus500has first microlens element580ato fourth microlens element580dinstead of first cylindrical lens480aand second cylindrical lens480bshown inFIG. 8. To be more specific, first microlens element580aand second microlens element580bforming a lens array (not shown) are placed on the side of second right angle prism150of nonlinear optical crystal110, and third microlens element580cand fourth microlens element580dforming another lens array (not shown) are placed on the side of first right angle prism130of nonlinear optical crystal110. First microlens element580ato fourth microlens element580deach has the same configuration.

Microlens elements580each condense parallel light entering from one side to the focal point on the other side, and, each emit light entering from the focal point on one side, as parallel light from the other side.

InFIG. 11, first microlens element580aand second microlens element580bare placed on the forward path and the return path respectively such that one face opposes first input and output face111of nonlinear optical crystal110and its optical axis matches with the incoming direction of laser light. Further, the focal points of first microlens element580aand second microlens element580bare positioned on crystal center plane113of nonlinear optical crystal110.

Third microlens element580cand fourth microlens element580d, and first microlens element580aand second microlens element580bare placed so as to have a plane-symmetric shape with respect to crystal center plane113of nonlinear optical crystal110, respectively.

According to this wavelength converted apparatus500, the diameter of laser light190is narrowed down in the area between first microlens element580aand third microlens element580con the forward path so as to form a beam waist in crystal center plane113. Further, the diameter of laser light190is narrowed down in the area between second microlens element580band fourth microlens element580don the return path so as to form a beam waist in crystal center plane113. By this means, the density of incident light of laser light190inside nonlinear optical crystal110increases.

Further, although in Embodiment 4 as explained earlier, laser light converges in the X-axial direction only, with the present embodiment, laser light converges in two directions, that is, in the X-axial direction and in the Y-axial direction, so that the efficiency of wavelength conversion improves further.

As in Embodiment 2, first dichroic mirror120and second dichroic mirror140may be large so as to cross both the forward path and the return path of laser light190. Further, as in Embodiment 3, it is possible to acquire combined converted light195by providing half-wave plate360and polarizing beam splitter370.

FIG. 12is a perspective view showing an outline of the wavelength converting apparatus according to Embodiment 6, and corresponds toFIG. 8of Embodiment 4. The same parts as inFIG. 8are assigned the same numerals, and therefore the explanation thereof will be omitted.

Referring toFIG. 12, wavelength converting apparatus600has first lens element680aand second lens element680b, instead of first cylindrical lens480aand second cylindrical lens480bshown inFIG. 9. Further, wavelength converting apparatus600has first dichroic mirror620, instead of first dichroic mirror120and first right angle prism130, and has second dichroic mirror640, instead of second dichroic mirror140and second right angle prism150. First lens element680aand second lens element680bhave the same configuration.

Here, first lens element680ais placed such that one face opposes first input and output face111of nonlinear optical crystal110and its optical axis matches with the Z-axial direction.

Second lens element680band first lens element680aare placed so as to have a plane-symmetric shape with respect to crystal center plane113of nonlinear optical crystal110.

InFIG. 13, contrary to first dichroic mirror120shown inFIG. 9, first dichroic mirror620reflects first wavelength light (i.e. unconverted light) and transmits second wavelength light (i.e. converted light). Further, first dichroic mirror620is placed in the direction perpendicular to the Z axis, at the position of focal point681bof second lens element680bon the opposite side of nonlinear optical crystal110with respect to second lens element680b.

Contrary to second dichroic mirror140shown inFIG. 9, second dichroic mirror640reflects first wavelength light (i.e. unconverted light) and transmits second wavelength light (i.e. converted light). Further, second dichroic mirror640is placed in the direction perpendicular to the Z axis, on the opposite side of nonlinear optical crystal110with respect to first lens element680a. However, second dichroic mirror640is placed at the position farther from first lens element680athan focal point681aof first lens element680apositioned on the opposite side of nonlinear optical crystal110with respect to first lens element680a. Further, second dichroic mirror640is placed on a virtual optical path, and, the virtual optical path and the optical path of unconverted laser light190are placed so as to have a line symmetric shape with respect to the optical axis of first lens element680a. Unconverted laser light190is inputted on first lens element680athrough focal point681a.

If the focal length of first lens element680aand second lens element680bis f and the difference between the refractive indices of nonlinear optical crystal110and air is ignored, the distance between first lens element680aand second lens element680bis 2f. That is, first lens element680aand second lens element680bare placed such that their focal points on the side of nonlinear optical crystal110are positioned at the same position on crystal center plane113in nonlinear optical crystal110.

In this wavelength converting apparatus600, laser light190exiting from first lens element680atravels in the positive direction along the Z axis while laser light190is narrowed down as a beam waist on crystal center plane113in nonlinear optical crystal110, and enters second lens element680b. Then, laser light190exiting from second lens element680bbecomes parallel light passing focal point681b, and is separated into first output converted light191and unconverted light in first dichroic mirror620.

The unconverted light turned back by first dichroic mirror620passes focal point681b. Consequently, as in the forward path to first dichroic mirror620, this unconverted light is narrowed down inside nonlinear optical crystal110by second lens element680b, and becomes parallel light passing focal point681a, by first lens element680a. Then, laser light190exiting from first lens element680ais separated into second output converted light192and output unconverted light193.

Here, a case will be explained where difference between the refractive indices of nonlinear optical crystal110and air is taken into account.

If there is a parallel medium of length L and refractive index n along the optical axis, the focal point of lens element680is shifted α=(1−1/n)×L farther. Consequently, taking into account the difference between the refractive indices of nonlinear optical crystal110and air, if the length of nonlinear optical crystal is Lp, distance D between first lens element680aand second lens element680bis 2α longer than 2f, that is, distance D should be set to fulfill the following equation 3.

FIG. 14is a perspective view showing an outline of the wavelength converting apparatus according to Embodiment 7, and corresponds toFIG. 2of Embodiment 1. The same parts as inFIG. 2are assigned the same numerals, and therefore the explanation thereof will be omitted.

Referring toFIG. 14, wavelength converting apparatus700has first dichroic mirror720and total reflection mirror730, instead of first dichroic mirror120and first right angle prism130.

First dichroic mirror720is placed in the same position and the same direction as first dichroic mirror120shown inFIG. 2.

Total reflection mirror730is placed on the positive side of the Y axis from first dichroic mirror720.

InFIG. 15, contrary to first dichroic mirror120shown inFIG. 2, first dichroic mirror720reflects first wavelength light (i.e. unconverted light) and transmits second wavelength light (i.e. converted light). That is, first dichroic mirror720reflects unconverted light in the positive direction along the Y axis and converted light in the positive direction along the Z axis.

Total reflection mirror730reflects first wavelength light (i.e. unconverted light) and second wavelength light (i.e. converted light). Total reflection mirror730is placed on the optical path of the unconverted light exiting from first dichroic mirror720such that its normal line direction is tilted forty five degrees in the negative direction along the Z axis with respect to the incoming direction of laser light190. That is, by combining first dichroic mirror720, total reflection mirror730deflects the unconverted light exiting from nonlinear optical crystal110at one hundred eighty degrees at a predetermined distance in the positive direction along the Y axis, and causes the unconverted light to re-enter nonlinear optical crystal110.

According to this wavelength converting apparatus700, first right angle prism130is no longer necessary, and it is possible to shorten the light path length of laser light190in the Z-axial direction. Further, first right angle prism130, which at least has to be larger than the distance between the forward path and the return path of laser light190, can be replaced with smaller total reflection mirror730. Consequently, it is possible to make miniaturization, simplification and cost reduction of the wavelength converting apparatus.

A dichroic mirror that reflects unconverted light and transmits converted light, or a micro right angle prism may be placed instead of total reflection mirror730. When a dichroic mirror to reflect unconverted light and transmit converted light is placed, it is possible to separate in a higher rate converted light from laser light190to re-enter nonlinear optical crystal110.

Further, a total reflection mirror to reflect unconverted light and converted light may be placed instead of first dichroic mirror720, and meanwhile, a dichroic mirror to reflect unconverted light and transmit converted light may be placed instead of total reflection mirror730. In this case, the direction to emit converted light in can be the positive direction along the Y axis. This makes it possible to employ an optical system for combining converted light by half-wave plate360and polarizing beam splitter370of Embodiment 3 shown inFIG. 7. Further, aright angle prism applied total reflection coating may be used instead of the total reflection mirror by replacing with first dichroic mirror720.

Further, a dichroic mirror to reflect unconverted light and transmit the converted light may only be replaced with total reflection mirror730. By this means, one more process of removing converted light is added, so that it is possible to reduce the proportion of converted light included in light to re-enter.

FIG. 16is a perspective view showing an outline of the wavelength converting apparatus according to Embodiment 8, and corresponds toFIG. 14of Embodiment 7. The same parts as inFIG. 14are assigned the same numerals, and therefore the explanation thereof will be omitted.

Referring toFIG. 16, wavelength converting apparatus800has second total reflection mirror830, in addition to the configuration diagram of wavelength converting apparatus700shown inFIG. 14.

Second total reflection mirror830is placed in the positive direction along the Z axis with respect to first dichroic mirror720.

InFIG. 17, second total reflection mirror830totally reflects first wavelength light (i.e. unconverted light) and second wavelength light (i.e. converted light). Second total reflection mirror830is placed in parallel with first dichroic mirror720, on the optical path of converted light exiting from first dichroic mirror720. That is, second total reflection mirror830deflects converted light exiting from first dichroic mirror720in the positive direction along the Y axis.

According to this wavelength converting apparatus800, as in Embodiment 1, it is possible that the direction of emitting first output converted light191outputted from first dichroic mirror120is the positive direction along the Y axis. This makes it easy to employ an optical system for combining converted light by half-wave plate360and polarizing beam splitter370of Embodiment 3 shown inFIG. 7.

Further, this wavelength converting apparatus emits converted light in the Y-axial direction at the position placing a distance in the positive direction along the Z axis from the part deflecting light, so that it is possible to improve flexibility of configuration of the wavelength converting apparatus. For example, other units may be placed in positions close to the Y axial direction of the part deflecting light. Further, it is possible to configure to set up the optical path difference between output converted light191and output converted light192longer and adjust the optical path difference easily between output converted light191and output converted light192.

As described above, the wavelength converting apparatus according to the present invention is suitable for use as a wavelength converting apparatus that improves output performance of laser light whose wavelength is converted, while improving the efficiency of laser light wavelength conversion. To be more specific, laser light having a short wavelength and oscillating with difficulty can be easily outputted, so that, by combining other laser light sources adequately, it is possible to achieve a high-quality and high-output white laser light source. This white laser light source is applicable to, for example, various display apparatuses including light source apparatuses of projectors and backlight apparatuses of liquid crystal display, and, various function elements used in optical communication systems including wavelength multiplexing communication systems.