Wavelength conversion apparatus and two-dimensional image display apparatus

A wavelength conversion apparatus capable of stably providing high output harmonic laser light is provided. The wavelength conversion apparatus comprises an end pump fiber laser 3 containing a laser activating substance, and including a reflecting surface at one end thereof and a fiber grating in the vicinity of the reflecting surface; an excitation laser light source 1 for outputting excitation laser light; an excitation laser light introduction section 4 for introducing the excitation laser light from the excitation laser light source to the fiber laser; a wavelength conversion element 5 for converting a fundamental wave generated by the fiber laser to a harmonic; and a rear reflecting surface 6 located outside the fiber laser and forming a laser cavity together with the fiber grating.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wavelength conversion apparatus and a two-dimensional image display apparatus, and more specifically to a wavelength conversion apparatus capable of stably providing high output harmonic laser light by a combination of a fiber laser and a wavelength conversion element, and a two-dimensional image display apparatus using the same.

2. Description of the Background Art

A visible-light source providing a high, strongly monochromatic output of class W is necessary to realize large displays, high luminance displays and the like. For a red light source among RGB (red, green and blue) light sources, a red high output semiconductor laser used in DVD recorders and the like is usable as a highly productive compact light source. However, it is difficult to realize green and blue light sources with a semiconductor laser. A highly productive compact light source is demanded for these colors.

For a green or blue light source, a wavelength conversion apparatus obtained by combining a fiber laser and a wavelength conversion element is conventionally realized as a low output visible-light source. Such a wavelength conversion apparatus uses a semiconductor laser as a light source of excitation light for exciting a fiber laser and uses a nonlinear optical crystal as a wavelength conversion element.

FIG. 10schematically shows a structure of a conventional wavelength conversion apparatus. In this example, green output light is obtained using the wavelength conversion apparatus shown inFIG. 10. As shown inFIG. 10, the conventional wavelength conversion apparatus includes a fiber laser40for outputting a fundamental wave, a wavelength conversion element41for converting the fundamental wave to green laser light, and a lens42for collecting the fundamental wave on an end surface of the wavelength conversion element41.

First, a basic operation of the fiber laser40will be described. Excitation light (excitation laser light) output from an excitation laser light source43is incident on a fiber44from an end44thereof. The excitation light incident on the fiber44is absorbed by a laser activating substance contained in the fiber44and then converted to seed light of the fundamental wave inside the fiber44. The seed light of the fundamental wave is repeatedly reciprocated in a laser cavity. The laser cavity includes a fiber grating44bformed in the fiber44and a fiber grating45bformed in a fiber45as a pair of reflecting mirrors, and the seed light is reflected by, and reciprocated between, the fiber grating44band the fiber grating45b. Concurrently, the seed light of the fundamental wave is amplified by a gain provided by the laser activating substance contained in the fiber44. Thus, the light intensity is increased and also wavelength selection is performed. As a result, laser oscillation occurs. The fiber44and the fiber45are connected to each other at a connection section46. The excitation laser light source43is current-driven by an excitation laser light current source47.

The fundamental wave which is output from the fiber laser40is incident on the wavelength conversion element41via the lens42, and is converted into a harmonic by a nonlinear optical effect of the wavelength conversion element41. The obtained harmonic is partially reflected by a beam splitter48, but the rest of the harmonic is transmitted through the beam splitter48and becomes green laser light. This green laser light is the output light from the wavelength conversion apparatus.

The part of the harmonic reflected by the beam splitter48is received by a receiving element49for monitoring the output light from the wavelength conversion apparatus and converted into an electric signal to be used. An output control section50controls the excitation laser light current source47such that the electric signal obtained by the receiving element49has a desired strength, and thus adjusts the driving current of the excitation laser light source43. In this manner, the conventional wavelength conversion apparatus adjusts the intensity of the excitation light which is output from the excitation laser light source43and also adjusts the intensity of the fundamental wave which is output from the fiber laser40, and thus can provide stable output light.

As another light source, for example, Japanese Laid-Open Patent Publication No. 2006-19603 (hereinafter, referred to as “patent document 1”) proposes a wavelength conversion apparatus capable of stably providing output light by fixing the wavelength of the fundamental wave.FIG. 11schematically shows a structure of the conventional wavelength conversion apparatus described in patent document 1. Referring toFIG. 11, a reflecting film is provided on one end surface of a laser medium51, and a reflection preventing film is provided on an outgoing end of the laser medium51. A fundamental wave which is output from the laser medium51is collected inside a wavelength conversion element53by a lens56, and a part of the collected fundamental wave is wavelength-converted and output as a harmonic. The fundamental wave and the harmonic output from the wavelength conversion element53are collected on a surface of a wavelength selection mirror55by a lens57. The wavelength selection mirror55reflects the fundamental wave and transmits the harmonic. The fundamental wave selectively reflected by the wavelength selection mirror55is fed back to the laser medium51via the opposite path. In this manner, the oscillation wavelength of the laser medium51can be fixed to the wavelength of the fed-back light. Namely, the conventional wavelength conversion apparatus can automatically fix the oscillation wavelength of the laser medium51to the phase-matching wavelength of the wavelength conversion element53and thus can provide stable output light.

The conventional wavelength conversion apparatuses shown inFIG. 10andFIG. 11are capable of stably providing relatively low output harmonic laser light, but have a problem of not capable of easily providing high output harmonic laser light of class W.

In addition, the conventional wavelength conversion apparatuses shown inFIG. 10andFIG. 11occasionally cause the following phenomenon. When LiNbO3or LiTaO3with a polarized inversion structure is used for the wavelength conversion element41or53, a third harmonic is generated in addition to a second harmonic due to the large nonlinear optical constant of the wavelength conversion element41or53, and the third harmonic causes the second harmonic to be absorbed. Therefore, in the case where a fundamental wave of a certain power density and a second harmonic both exist, the temperature rises on and in the vicinity of an outgoing surface of the wavelength conversion element41or53. This causes a problem that the phase-matching condition is destroyed (i.e., thermal dephasing occurs) and as a result, the light emitting efficiency is lowered.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention for solving the above-described problems is to provide a wavelength conversion apparatus capable of stably providing high output harmonic laser light and a two-dimensional image display apparatus using the same.

The present invention is directed to a wavelength conversion apparatus. For achieving the above object, the wavelength conversion apparatus according to the present invention comprises an end pump fiber laser containing a laser activating substance, and including a reflecting surface at one end thereof and a fiber grating in the vicinity of the reflecting surface; an excitation laser light source for outputting excitation laser light; an excitation laser light introduction section for introducing the excitation laser light from the excitation laser light source to the fiber laser from an end surface of the fiber laser opposite to the reflecting surface; a wavelength conversion element for converting a fundamental wave generated by the fiber laser to a harmonic; and a rear reflecting surface located outside the fiber laser and forming a laser cavity together with the fiber grating. The wavelength conversion element is located between the fiber grating and the rear reflecting surface.

Owing to this, the wavelength conversion apparatus allows the fundamental wave on the forward path and the return path to contribute to the generation of a harmonic. Since the harmonic can be output both from the forward path and the return path, the light amount which causes thermal dephasing when being output from only the forward path can be output as a total light amount of an output from the forward path and an output from the return path without causing thermal dephasing.

Preferably, the rear reflecting surface has a wavelength selection function of reflecting the fundamental wave generated by the fiber laser and transmitting the harmonic generated by the wavelength conversion element; and the reflecting surface at the end of the fiber laser reflects both the excitation laser light and the harmonic. Owing to this, the wavelength conversion apparatus can output the harmonic at a higher efficiency.

The wavelength conversion apparatus further comprises a harmonic output section between an outgoing end surface of the fiber laser and the wavelength conversion element. The harmonic output section outputs the harmonic generated from the fundamental wave reflected by the rear reflecting surface. Owing to this, the wavelength conversion apparatus can output the harmonic generated on the return path even without returning the harmonic to the fiber laser.

The harmonic output section may be a coating for reflecting the harmonic, which is provided on an end surface of the wavelength conversion element closer to the fiber laser; or a coating for reflecting the harmonic, which is provided on the excitation laser light introduction section.

The rear reflecting surface is a dichroic mirror and is adjustable to rotate around an X axis and a Y axis where a Z axis is in an optical axis direction. Alternatively, the rear reflecting surface may be a coating provided on an incidence end surface of a fiber for collecting the harmonic generated by the wavelength conversion element, and the incidence end surface of the fiber may be adjustable to rotate around an X axis and a Y axis where a Z axis is in an optical axis direction. Owing to this, the wavelength conversion apparatus can easily adjust the rear reflecting surface.

It is desirable that an outgoing end surface of the fiber laser is cut such that a propagation direction of the fundamental wave going out from the fiber laser forms a Brewster's angle with respect to a direction vertical to the outgoing end surface of the fiber laser. Owing to this, the wavelength conversion apparatus can provide single polarization as the fundamental wave going out from the fiber laser. In this case, a single-mode fiber with no polarization-maintaining function is usable as the fiber laser, and thus the wavelength conversion apparatus can be structured at low cost.

The present invention is also directed to a two-dimensional image display apparatus. The two-dimensional image display apparatus comprises a screen; and a plurality of laser light sources. The laser light sources respectively emit at least red light, green light and blue light. Either one of the above-described wavelength conversion apparatuses is used as at least the light source for emitting the green light among the laser light sources.

Owing to this, the two-dimensional image display apparatus can generate an output from the green light source at a high efficiency and thus provide a high luminance image. Such an increase in the output can provide, for example, the following effects. The fiber laser may be shortened to reduce the cost, or the light amount of the excitation light source may be decreased to reduce the power consumption.

As described above, the wavelength conversion apparatus according to the present invention includes a wavelength conversion element located in the laser cavity including the fiber grating and the rear reflecting surface, so as to cause the fundamental wave both on the forward path and the return path to contribute to the generation of a harmonic. In addition, since the harmonic can be output both from the forward path and the return path, the light amount which causes thermal dephasing when being output from only the forward path can be output as a total light amount of an output from the forward path and an output from the return path without causing thermal dephasing. As a result, a high output harmonic can be provided stably.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

FIG. 1is a block diagram showing an exemplary structure of a wavelength conversion apparatus according to a first embodiment of the present invention. As shown inFIG. 1, the wavelength conversion apparatus includes an excitation light source1, an end pump fiber laser3(hereinafter, referred to simply as the “fiber laser3”), an excitation laser light introduction section4, a wavelength conversion element5, a rear reflecting surface6, a collection lens11, a collection lens12, and a collimator lens13. The fiber laser3contains a laser activating substance, and includes a fiber grating2for reflecting a fundamental wave9and a reflecting surface7for reflecting excitation laser light8at an end of the fiber.

The excitation laser light8which is emitted from an excitation light source1is reflected by the excitation laser light introduction section4and introduced to the fiber laser3via the collimator lens13. The excitation laser light introduction section4is located between the excitation light source1and the fiber laser3and acts to introduce the excitation laser light8to the fiber laser3. The excitation laser light8introduced to the fiber laser3is propagated in the fiber laser3while being absorbed by the laser activating substance. After passing through the fiber grating2, the excitation laser light8is reflected by the reflecting surface7at the end of the fiber and is propagated in the opposite direction while being absorbed by the laser activating substance. The excitation laser light8is absorbed by the laser activating substance and extinguished almost entirely before reciprocating once in the fiber laser3and going out from the fiber laser3.

The seed light of the fundamental wave generated during this period is amplified by the excitation laser light8while reciprocating in a laser cavity including the fiber grating2and the rear reflecting surface6, and thus causes laser oscillation as a high output fundamental wave. In this laser cavity, the fiber grating2is provided at one end of the cavity. Owing to this structure, it becomes possible to select any desired central reflection wavelength and to select any desired central oscillation wavelength. It also becomes possible to generate a fundamental wave of a narrow band of 0.05 to 0.2 nm. If the fiber grating2is formed of a reflecting mirror such as a dielectric multi-layer film or the like, it is difficult to generate a fundamental wave of this level of narrow band. In this case, it is also difficult to cause laser oscillation at a desired wavelength because the oscillation occurs at a high gain wavelength, at which the oscillation occurs easily. The wavelength conversion element5is located in the cavity including the fiber grating2and the rear reflecting surface6.

Next, a function of the wavelength conversion element5will be described. The fundamental wave9going out from the fiber laser3as described above (i.e., the fundamental wave on the forward path) is collimated by the collimator lens13, then collected by the collection lens11, and incident on the wavelength conversion element5. The fundamental wave9is partially converted by the nonlinear optical effect of the wavelength conversion element5and is output as a harmonic10having a wavelength which is ½ of that of the fundamental wave9. The harmonic10output from the wavelength conversion element5is generally collimated by the collection lens12and transmitted through the rear reflecting surface6to be taken outside. This harmonic10is output from the wavelength conversion apparatus as harmonic laser light. The fundamental wave9transmitted through the wavelength conversion element5without being used for the wavelength conversion is also generally collimated by the collection lens12and is reflected by the rear reflecting surface6to be returned on the same path.

The fundamental wave9reflected by the rear reflecting surface6(i.e., the fundamental wave on the return path) proceeds on exactly the same path as taken from the fiber laser3, in the opposite direction; i.e., is transmitted through the collection lens12and is incident on the wavelength conversion element5to contribute to the generation of a harmonic. The fundamental wave9which is transmitted through the wavelength conversion element5still proceeds on the same path as taken from the fiber laser3, in the opposite direction; i.e., is transmitted through the collection lens11, the excitation laser light introduction section4and the collimator lens13to return to the fiber laser3and thus contribute to the generation of the laser oscillation of the fundamental wave. In a solid-state laser such as YAG laser or the like, the reflecting surface provided at an end of the laser medium included in the laser cavity is usually required to have a reflectance close to 100%. In the laser fiber, by contrast, stable laser oscillation is obtained even with a reflectance of as low as about 20% at an outgoing end of the fiber. Therefore, it is made possible to provide the wavelength conversion element5inside the laser cavity as in the present invention.

As the wavelength conversion element5, an SHG element formed of a nonlinear optical crystal having a periodical polarized inversion structure is preferable. Available as the SHG element having a periodical polarized inversion structure are, for example, KTiOPO4, LiNbO3, LiTaO3, Mg-doped LiNbO3or LiTaO3, or stoichio LiNbO3or LiTaO3. The crystals of these materials have a high nonlinear constant and therefore can be wavelength-converted at a high efficiency. These materials are also advantageous in allowing the phase-matching condition to be freely set by changing the periodical structure.

The harmonic10generated inside the wavelength conversion element5from the fundamental wave9, which is reflected by the rear reflecting surface6and proceeds on the return path, is again collimated by the collection lens11, and collected by the collimator lens13via the excitation laser light introduction section4, and incident on the fiber laser3. The fiber laser3, in the case of being Yb-doped as described above, absorbs light at an excitation laser wavelength of 915 nm, but is almost transparent to a harmonic of about 530 nm. Therefore, the harmonic10is propagated in the fiber laser3without being absorbed. In this structure, the reflecting surface7for reflecting the excitation light is provided at the end of the fiber. Therefore, the harmonic10passes through the fiber grating2and then is reflected by the reflecting surface7to return. The harmonic10reciprocated once in the fiber laser3goes out from the fiber laser3, is collimated by the collimator lens13, passes through the excitation laser light introduction section4, and is transmitted through the collection lens11, the wavelength conversion element5, the collection lens12and the rear reflecting surface6. Thus, the harmonic10can be output from the wavelength conversion apparatus, like the above-described harmonic10generated on the forward path.

In this embodiment, the excitation laser light introduction section4and the rear reflecting surface6may be each formed of a dichroic mirror. In this case, it is preferable that the excitation laser light introduction section4is provided with a characteristic of reflecting the excitation laser light8and transmitting the fundamental wave9, and that the rear reflecting surface6is provided with a characteristic of reflecting the fundamental wave9and transmitting the harmonic10. Usually, these characteristics can be provided by coating the excitation laser light introduction section4and the rear reflecting surface6with a dielectric multi-layer film. The reflecting surface7at the end of the fiber is preferably provided with a characteristic of reflecting both the excitation laser light8and the harmonic10.

As described above, the harmonic10generated in the wavelength conversion element5from the fundamental wave9, which is reflected by the rear reflecting surface6to be returned on the same path, can be output from the wavelength conversion apparatus by being returned to the fiber laser3. Alternatively, as shown inFIG. 2, the harmonic10may be output from a harmonic output section14which is inserted between the fiber laser3and the wavelength conversion element5. In this case, the harmonic output section14is provided with a characteristic of transmitting the fundamental wave9and reflecting the harmonic10, and thus the reflected harmonic10can be output. Namely, the harmonic10reflected by the harmonic output section14is output from the wavelength conversion apparatus after passing through the collection lens11, the wavelength conversion element5, the collection lens12, and the rear reflecting surface6.

The wavelength conversion apparatus does not need to include the harmonic output section14as an additional element. As shown inFIG. 3, the excitation laser light introduction section4may be provided with a coating15for outputting the harmonic. Thus, the harmonic10can be output from the excitation laser light introduction section4. In the example ofFIG. 3, the wavelength conversion apparatus can allow the harmonic10to be output from both the rear reflecting surface6and the excitation laser light introduction section4.

The wavelength conversion apparatus may also allow the harmonic10to be output by, as shown inFIG. 4, providing a coating16, at an end surface of the wavelength conversion element5closer to the fiber laser3, for transmitting the fundamental wave9and reflecting the harmonic10.

In order to return the fundamental wave9reflected by the rear reflecting surface6to the fiber laser3, the wavelength conversion apparatus needs to collimate the fundamental wave9on the forward path by the collection lens12, or to form a covalent point with respect to the outgoing end of the fiber laser3on the rear reflecting surface6. In the case where the rear reflecting surface6is formed of a dichroic mirror, it is simple to use and preferable to adjust the position of the collection lens12such that the fundamental wave9is collimated. The reason is that the harmonic10is also collected by the collection lens12.

The rear reflecting surface6may be formed of a dichroic mirror as described above, but alternatively, as shown inFIG. 5A, the rear reflecting surface6may be omitted and the harmonic10may be collected by a fiber17. In this case, an incidence end surface of the fiber17may be provided with a coating18for reflecting the fundamental wave9and transmitting the harmonic10. In this structure, it is preferable to locate the collection lens12such that the harmonic10is collected on the incidence end surface of the fiber17. In any way, the wavelength conversion apparatus is structured such that the fundamental wave9reflected by the incidence end surface of the fiber17is returned on the same optical path as the forward path, in the opposite direction. As shown inFIG. 5B, the wavelength conversion apparatus may include another collection lens12abetween the rear reflecting surface6and the fiber17, so that the harmonic10is collected on the incidence end surface of the fiber17. In this case, it is not necessary to provide the coating18at the incidence end surface of the fiber17as shown inFIG. 5A.

In order to efficiently return the fundamental wave9reflected by the rear reflecting surface6to the fiber laser3, the wavelength conversion apparatus needs to propagate the fundamental wave9from the fiber laser3on an optical path as similar as possible to the forward path, in the opposite direction. For realizing this, it is preferable to attach the rear reflecting surface6to be rotatable around X and Y axes (where Z axis is in the optical axis direction). Also in the example ofFIG. 5A, it is preferable to attach the fiber17to be rotatable around the X and Y axes.

In a wavelength conversion apparatus shown inFIG. 6A, including the same elements as those inFIG. 1, the excitation laser light8from the excitation light source1is incident on the fiber laser3after being transmitted through, not reflected by, an excitation laser light introduction section19. By contrast, the fundamental wave9is reflected by the excitation laser light introduction section19and introduced to the wavelength conversion element5. In order to realize this structure, the excitation laser light introduction section19is provided with a coating for transmitting the excitation laser light8and reflecting the fundamental wave9. Owing to this structure, the optical path for returning the fundamental wave9and the harmonic10to the fiber laser3can be adjusted more freely. It is preferable that the excitation laser light introduction section19and the rear reflecting surface6are adjustable around the X and Y axes where the Z axis is in the optical axis direction. By adjusting the position of the excitation light source1, the excitation laser light8can be incident on the fiber laser3efficiently.

In the example ofFIG. 6A, the rear reflecting surface6is formed of a dichroic mirror. Alternatively, the structure shown inFIG. 6Bmay be adopted. InFIG. 6B, the harmonic10is collected by the fiber17, and the incidence end surface of the fiber17is provided with the coating18for reflecting the fundamental wave9and transmitting the harmonic10. By providing the coating of the excitation laser light introduction section19with a characteristic of reflecting the wavelength of the harmonic10, the efficiency of the harmonic output can be increased.

As described above, the conventional wavelength conversion apparatuses occasionally cause the phenomenon that when LiNbO3or LiTaO3with a polarized inversion structure is used for the wavelength conversion element, a third harmonic is generated in addition to a second harmonic due to the large nonlinear optical constant of the wavelength conversion element, and the third harmonic causes the second harmonic to be absorbed. Therefore, in the case where a fundamental wave of a certain power density and a second harmonic both exist, the temperature rises on and in the vicinity of an outgoing surface of the wavelength conversion element. This causes the problem that the phase-matching condition is destroyed (i.e., thermal dephasing occurs) and as a result, the light emitting efficiency is lowered. The wavelength conversion apparatus according to the present invention can output the second harmonic from both the forward path and the return path as described above. Therefore, the light amount which causes thermal dephasing when being output from only the forward path can be output as a total light amount of an output from the forward path and an output from the return path without causing thermal dephasing. As a result, the total light amount of the harmonic laser obtained from the forward path and the return path can be increased.

Specifically, for example, a wavelength conversion apparatus, conventionally capable of outputting light of about 2.8 W from the forward path, can output light of about 2.2 W from the forward path and light of about 1.2 W from the return path without causing thermal dephasing when adopting the structure of the present invention. Light of about 3.4 W is obtained in total. Such an increase in the output can provide, for example, the following effects. The fiber laser3may be shortened by the increased amount of light emission to reduce the cost, or the light amount of the excitation light source1may be decreased by the increased amount of light emission to reduce the power consumption.

Now, means for obtaining single polarization as the fundamental wave9going out from the fiber laser3will be described.FIG. 7shows the relationship between an end surface20of the fiber laser3and the angle of the wavelength conversion element5. In the wavelength conversion element5formed of LiNbO3or the like, the relationship between the crystalline axis at which the wavelength conversion is performed efficiently and the direction of electric field of the incident fundamental wave is generally determined. For example, when the wavelength conversion element5is formed of LiNbO3or the like, in order to raise the output efficiency of the harmonic10, it is preferable that the fundamental wave9going out from the fiber laser3is as close as possible to the single polarization and that the polarization direction thereof (the amplitude direction of the electric field) and the Z axis are the same.

In order to make the fundamental wave9going out from the fiber laser3closer to the single polarization, it is conceivable to define the angle θout of the fundamental wave9going out from the fiber laser3. Specifically, the end surface20of the fiber laser3is cut such that the outgoing angle θout of the fundamental wave9with respect to the direction vertical to the end surface20of the fiber laser3is the Brewster's angle or the vicinity thereof.FIG. 8shows the transmission angle dependence of S polarization and P polarization of the fundamental wave9at the end surface20of the fiber laser3. In this structure, in the case where the outgoing angle θout of the fundamental wave9from the fiber laser3is set to 55°, almost 100% of a P polarization component of the fundamental wave9goes out from the fiber laser3, where as only about 87% of an S polarization component of the fundamental wave9goes out from the fiber laser3. When the fundamental wave9is, for example, reflected by the rear reflecting surface6to return toward the fiber laser3and is transmitted through the end surface20of the fiber laser3again, the ratio of the S polarization component going out from the fiber laser3is further lowered to about 75%. In this manner, merely by cutting the outgoing surface of the fiber laser3to a predetermined angle, the P polarization component can be output at a larger ratio with respect to the S polarization component.

In this embodiment, the polarization is controlled by cutting the end surface20of the fiber laser3to form the Brewster's angle or the vicinity thereof. The same effect is provided by cutting an end surface of the wavelength conversion element5, on and from which the fundamental wave9is incident and goes out, to form the Brewster's angle or the vicinity thereof. By cutting a plurality of surfaces to form the Brewster's angle or the vicinity thereof, the ratio of the P polarization component with respect to the S polarization component can be increased.

In this case, it is conceived to use, as the fiber laser3, a double-clad polarization-maintaining fiber capable of propagating the excitation laser light8of high output. However, as compared to a single-mode fiber with no polarization-maintaining function (hereinafter, referred to simply as the single-mode fiber), a polarization-maintaining fiber is disadvantageous in being more expensive and lower in propagation efficiency. Now, the reason why the propagation efficiency of the polarization-maintaining fiber is lower will be described. The propagation efficiency of a fiber depends on the structure thereof. For example, a polarization-maintaining fiber such as a PANDA fiber or the like includes a stress application section on both sides of a core with respect to the propagation direction. Owing to such a structure, birefringence is induced in the core by the optical elasticity effect and thus the polarization of the fundamental wave propagating in the core is maintained. However, this stress application section slightly acts as a scattering source of the oscillated fundamental wave on the are a where the fundamental wave is propagated and thus a propagation loss of the fundamental wave is generated. Therefore, the propagation efficiency of the polarization-maintaining fiber is lower than that of the single-mode fiber.

In actuality, when the fiber laser3is formed of a polarization-maintaining fiber having the same length as that of the single-mode fiber, the output from the fiber laser3with the same structure is lowered due to the lower propagation efficiency, and occasionally laser oscillation does not occur. For example, in the case of a laser oscillating at 1064 nm, the optimum length of the laser is 30 meters when the usual single-mode fiber is used but is shortened to 18 to 20 meters when the propagation-maintaining fiber is used, and the conversion efficiency from the excitation light to the oscillation light is lower by 10 to 15% with the propagation-maintaining fiber than with the single-mode fiber. The data is confirmed by experiments. As described above, however, in the case where the end surface20of the fiber laser30is cut to form the Brewster's angle or the vicinity thereof to control the direction of the polarization, the single-mode fiber which costs low and provides a high propagation efficiency can be used. Therefore, the fundamental wave9is propagated at a high efficiency and as a result, the harmonic is output at a high efficiency.

Thus, in the case where, for example, the wavelength conversion element5is formed of LiNbO3crystal or LiTaO3, the fundamental wave can be converted to the harmonic at a high efficiency by locating the electric field P polarization component Ep of the fundamental wave mainly including the P polarization component in the same direction as the Z axis direction. In the case where wavelength conversion element5is formed of KTiO4, the fundamental wave can be converted to the harmonic at a high efficiency by allowing the beam to be incident on the wavelength conversion element5at an angle which is 90° with respect to the Z axis of the crystalline axis and 23.5° with respect to the X axis, such that the electric field Ep is 45° with respect to the XY plane. The range of the incident angle θout is designed to be preferably within ±10°, and more preferably within ±5° from the Brewster's angle.

As described above, the wavelength conversion apparatus according to the first embodiment of the present invention includes the wavelength conversion element5located in the laser cavity including the fiber grating2and the rear reflecting surface6, so as to cause the fundamental wave both on the forward path and the return path to contribute to the generation of a harmonic and thus increase the efficiency of wavelength conversion. In addition, since the harmonic can be output both from the forward path and the return path, the light amount which causes thermal dephasing when being output from only the forward path can be output as a total light amount of an output from the forward path and an output from the return path without causing thermal dephasing. As a result, a high output harmonic can be provided stably.

Second Embodiment

FIG. 9is a block diagram showing an exemplary structure of a two-dimensional image display apparatus (laser display) according to a second embodiment of the present invention. As shown inFIG. 9, red (R), green (G) and blue (B) laser light sources21a,21band21care used as light sources. As the R light source21a, an AlGaInP/GaAs-system semiconductor laser of a wavelength of 638 nm is used; and as the B light source21c, a GaN-system semiconductor laser of a wavelength of 465 nm is used. As the G light source21b, the wavelength conversion apparatus according to the first embodiment is used. Laser beams emitted from the R, G, and B light sources21a,21band21care respectively collected by collection lenses22a,22band22cand then are caused to scan diffusion plates25a,25band25cby two-dimensional beam scanning means23a,23band23c.

Image data is divided into R data, G data and B data. Signals respectively regarding the R data, the G data and the B data are focused by field lenses26a,26band26cand input to spatial optical modulation elements27a,27band27c. Then, the signals are combined by a dichroic prism28to form a color image. A beam for the color image is projected on a screen31after passing through projection lenses20and30. On an optical path for allowing the beam from the G light source21bto be incident on the spatial optical modulation element27b, a concave lens24is inserted in order to make the spot size of the G light at the spatial optical modulation element27bequal to those of the R light and the B light.

Thus, the two-dimensional image display apparatus according to the second embodiment of the present invention uses laser light sources as the R, G and B light sources, so as to be high in luminance and thin in structure. By using the wavelength conversion apparatus according to the first embodiment as the G light source, the efficiency of outputting the G light is increased to provide a high luminance image. The increased efficiency provides the following effects. For example, the fiber laser may be shortened to reduce the cost, or the light amount of the excitation light source may be decreased to reduce the power consumption. The two-dimensional image display apparatus according to the second embodiment of the present invention may be provided in the form of a rear projection display for projecting an image from the rear side of the screen, as well as the laser display described above.

A wavelength conversion apparatus according to the present invention is usable as a high output visible-light source or the like, and is applicable to a display or the like.