Laser light emitter, laser beacon and laser image display

Description: TECHNICAL FIELD [Detailed description of the invention]

【0001】

[Technical field to which the invention belongs]

The present invention relates to a laser light generator that serves as a light source for laser light that requires an appropriately wide spectral width while exhibiting high frequency accuracy.

【0002】

[Conventional technology]

Conventionally, attempts have been made to apply high-power lasers to various industrial fields by utilizing the monochromaticity (narrow spectrum band) of laser light. In particular, the laser light oscillated by a laser light source (hereinafter simply referred to as Nd: YAG Q switch laser) using neodymium: yttrium aluminum garnet (Nd: YAG) that oscillates in vertical single mode by the Q switch method and this laser. Laser light obtained by converting light into wavelength is expected to be applied to various industrial fields because it exhibits high peak intensity.

【0003】

Such a Q-switched laser generally oscillates in vertical multimode, but when the line width (hereinafter referred to as spectral width) in the frequency component is wide, problems such as chromatic aberration are likely to occur. Therefore, the laser beam is oscillated at a single wavelength by using an injection seed technique or the like. However, on the contrary, when the laser beam is oscillated at a single wavelength, the oscillation spectrum width becomes too narrow and inconvenience tends to occur when the laser beam is used.

【0004】

Laser light with high monochromaticity, that is, narrow spectrum width, has high coherence, and when the laser light itself interferes with stray light with different propagation distances such as scattered light, it interferes with each other in an irregular phase relationship, resulting in an interference pattern. Noise generated based on the so-called speckle noise is likely to occur. On the other hand, a laser beam having low monochromaticity has low coherence, but has a wide spectrum width, so that chromatic aberration is likely to occur.

【0005】

Laser light has been applied, for example, to laser beacon devices. This laser beacon device is described in "Laser Beacon Adaptive Optics", Optics & Physics News, published in the June 1993 issue of Optics and Physics News, pages 14-19. , Pp.14-19, June, 1993), it is being studied as a means to improve the resolution of optical systems used in astronomical observation, intersatellite optical communication, etc.

【0006】

The laser beacon device emits a laser light source into the air and emits sodium atoms in the atmosphere by resonance absorption. The laser beacon device detects atmospheric disturbances in real time by detecting the light emitted by sodium atoms on the ground, and improves the resolution of the telescope by correcting the atmospheric disturbances using an adaptive optical system. There is work.

【0007】

In addition, the sodium atom resonates and emits laser light with a wavelength of around 589 nm. In order to efficiently resonantly excite sodium atoms, a high-power laser light source that accurately matches the absorption spectrum of sodium atoms in both frequency and frequency width is required.

【0008】

As a high-power light source with a wavelength of 589 nm, as shown in FIG. 9, a first laser light source 101 consisting of an Nd: YAGQ switch laser that oscillates a frequency-narrowed fundamental wave laser light with a wavelength of 1319 nm using injection seed technology. A second laser light source 102 consisting of an Nd: YAGQ switch laser that oscillates a fundamental wave laser beam with a wavelength of 1064 nm, which is similarly narrowed in frequency, and a fundamental wave laser beam with a wavelength of 1319 nm and a fundamental wave laser beam with a wavelength of 1064 nm. It has been proposed to be equipped with an LBO crystal 103 for generating those sum-wavelength laser beams.

【0009】

Further, the injection seed performed by the first laser light source 101 and the second laser light source 102 oscillates the laser light at a single wavelength, and the wavelength of the laser light after the sum frequency is generated is the wavelength of the sodium atom. This is to match the absorption wavelength exactly.

【0010】

In addition, the absorption frequency width of sodium atoms affected by the Doppler effect is about 3 GHz. Since the laser beam having a wavelength of 589 nm obtained from the high-power light source shown in FIG. 9 is frequency-narrowed by injection seeding at each light source, the laser light obtained from the high-power light source has high frequency stability. Shown. The spectral width of the fundamental wave laser light oscillated from each light source is narrowed to about 25 MHz, which is the line width of the transform limited pulse, and is only 1/120 of the absorption frequency width of the sodium atom. The resonance efficiency between light and sodium atom is low.

【0011】

Further, the laser beam is applied to, for example, a laser image display device.

【0012】

When a laser beam having a narrow spectrum width is used in the laser image display device, speckle noise is likely to occur due to the high coherence. Such speckle noise becomes granular spots in the laser image display device, and the image quality is significantly deteriorated.

【0013】

[Problems to be Solved by the Invention]

Therefore, in applying the laser beam, it is an issue to control the spectral width of the laser beam to an appropriate value so as not to cause a problem in both chromatic aberration and speckle noise.

【0014】

Several methods have been proposed so far to widen the spectral width of the laser beam. For example, it is conceivable to oscillate the laser beam in the vertical multi-mode, or to use the laser beam that oscillates in the vertical multi-mode from the beginning. In this case, not only the spectral width becomes too wide and problems such as chromatic aberration are likely to occur, but also the configuration of the laser beam generator itself needs to be changed, resulting in a decrease in luminous efficiency. That is, it was difficult to appropriately widen the spectrum width to a desired value. Furthermore, the intensity of the laser beam obtained by wavelength-converting two or more types of laser beam oscillated in vertical multimode by sum frequency mixing was unstable.

【0015】

Further, as a method of removing speckle noise, improvement of a laser beam projection system is being studied. For example, Japanese Patent Application Laid-Open No. 55-65940 proposes a laser image display device that mechanically vibrates a screen or a laser light source. However, when the screen size is large, it is difficult to realize.

【0016】

As described above, conventionally, it is difficult to control the spectral width of the laser beam to a desired value to remove speckle noise.

【0017】

The present invention has been made in view of the above-mentioned problems, and by adding a simple configuration, it is possible to widen the spectrum width of the laser beam and deteriorate the coherence to an appropriate value. The purpose is to provide the device.

【0018】

Another object of the present invention is to provide a laser beacon device and a laser image display device using the laser light generator.

【0019】

[Means for solving problems]

In order to solve the above-mentioned problems, the laser light generator according to the present invention includes a laser light source, a phase modulation means for phase-modulating the laser light emitted by the laser light source with a single or a plurality of frequency components, and the phase modulation. It is characterized by having a wavelength conversion means for converting the wavelength of the laser beam phase-modulated by the means into another wavelength.

【0020】

According to the laser light generator, the fundamental wave laser light generated by the laser light source is phase-modulated based on a predetermined modulation amplitude and modulation frequency by a phase modulation means, the spectrum width is widened, and then wavelength conversion is performed. At the same time as being converted into short wavelength laser light by means, the spectrum width is further widened. As a result, the coherent distance of the laser beam is shortened, and speckle noise is suppressed.

【0021】

Further, by performing phase modulation at a plurality of frequencies, it is possible to generate a laser beam having a continuous spectrum. The coherence of such laser light is sufficiently small, and the generation of speckle noise is suppressed. The coherence of the laser beam emitted from the wavelength conversion means is controlled by the modulation amplitude and the modulation frequency used in the phase modulation means.

【0022】

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, specific examples of embodiments of the laser beam generator according to the present invention will be described with reference to the drawings.

【0023】

FIG. 1 shows, for example, a laser beam generator that generates a laser beam of 589 nm used in a laser beacon device.

【0024】

The laser light generator includes light sources 31 and 32 that generate fundamental wave laser light, and phase modulation units 34 and 35 that phase-modulate the fundamental wave laser light from the light sources 31 and 32 with a single frequency component or a plurality of frequency components. It has a sum frequency generation unit 33 that converts the wavelength of light phase-modulated by the modulation units 34 and 35 into another wavelength.

【0025】

The phase modulation units 34 and 35 are provided with an electro-optical element, and the sum frequency generation unit 33 is provided with a nonlinear optical crystal element.

【0026】

According to FIG. 1, the fundamental wave laser beam having a wavelength of 1319 nm emitted from the first laser light source 31 is incident on the phase modulation unit 34 and undergoes phase modulation. Further, the fundamental wave laser beam having a wavelength of 1064 nm emitted from the second laser light source 32 is incident on the phase modulation unit 35 and undergoes phase modulation. At this time, in the phase modulation units 34 and 35, the fundamental wave laser light before wavelength conversion is phase-modulated at a single or a plurality of modulation frequencies, the spectral width of each fundamental wave laser light is widened, and then the sum frequency generation unit 33. The fundamental wave laser beam that has undergone this phase modulation is incident on the. In the sum frequency generator 33, sum frequency mixing, that is, wavelength conversion is performed using fundamental wave laser light having a wavelength of 1319 nm and a wavelength of 1064 nm, and the spectral width of the laser light having a wavelength of 589 nm after this wavelength conversion is larger than that of the fundamental wave laser light. Spread further. In this way, the spectral width of the laser beam having a wavelength of 589 nm is sufficiently widened through phase modulation and wavelength conversion, so that the efficiency is improved when used for resonance excitation of sodium atoms.

【0027】

First, the principle of phase modulation in an electro-optical element and the change in power spectrum due to this phase modulation will be described.

【0028】

Figure 2A shows the initial power spectrum of the fundamental wave laser beam.

【0029】

When a signal voltage having a periodic function φ (t) is applied to an electro-optical modulator equipped with an electro-optical element, the electric field E (t) of the laser beam undergoes phase modulation as shown in Eq. (1) below. In addition, f₀ Is the center frequency of the laser beam before modulation.

【0030】

[Number 1]

<img file="JPH09121069A_D0001.tif" />【0031】

Here, Φ (t) is a phase modulation function and is proportional to the signal φ (t) applied to the electro-optical modulation device. That is, it becomes as shown in Eq. (2). In particular, Φ (t) has the amplitude m and frequency f as shown in Eq. (3) below._m In the case of a sine wave of, the electric field E (t) is the Bessel function sequence J as shown in Eq. (4) below._kIt can be expanded using (m).

【0032】

[Number 2]

<img file="JPH09121069A_D0002.tif" />【0033】

At this time, each of the power spectra has an intensity J as shown in B of FIG._k²(m), frequency (f)₀ + kf_m ) Is the sum of the spectra. Where k is an integer. Center frequency f by phase modulation₀ The spectral intensity of {J₀(m)}²Double.

【0034】

Further, when phase modulation is performed at a plurality of frequencies, the phase modulation function Φ (t) is as shown in the following equation (5), and the phase modulation at each frequency is additive with respect to the frequency. It also acts synergistically in strength.

【0035】

[Number 3]

<img file="JPH09121069A_D0003.tif" />【0036】

For example, modulation amplitude m₁And modulation frequency f_m1Intensity J generated by phase modulation in_k²(m₁ ) And frequency (f₀ + kf_m1) And the other modulation frequency f_m2Due to the phase modulation in, the intensity J is further increased, as shown in C in FIG._k²(m₁ ) J_k²(m₂ ) And frequency (f₀ + kf_m1+ lf_m2) Slits into a spectrum consisting of), and becomes an aggregate of a large number of spectra. Where k and l are integers.

【0037】

In this way, it can be seen that by performing a plurality of, for example, two phase modulations at different modulation frequencies, the initial power spectrum is separated into a large number of spectra and the spectral intervals become dense. The spectral interval at this time is equal to or less than the smallest modulation frequency among the plurality of set modulation frequencies. Also, when the least common multiple of the modulation frequencies is sufficiently large, the number of spectra is approximately (2 m).₁ +1) × (2m₂ +1).

【0038】

For example, frequency f_m1= 350MHz, phase modulation amplitude m₁ The first phase modulation is performed with a high-frequency voltage signal (phase modulation function Φ (t)) with a voltage amplitude such that is 2 radians, and the frequency f_m2= 350MHz, phase modulation amplitude m₂When the second phase modulation is performed on a high-frequency voltage signal with a voltage amplitude such that is 2 radians, the spectrum interval becomes 100 MHz, and the number of spectra is (2 × 2 + 1) × (2 × 2 + 1). = 25 pieces.

【0039】

Here, the phase modulation performed does not widen the spectral line width of the fundamental wave laser beam itself, but only widens the entire spectrum by separating into a large number of spectra. Hereinafter, the width of the envelope of the aggregate of spectra after this separation is defined as the total spectrum width. Therefore, by widening the width of the entire spectrum, the coherent length is shortened and speckle noise is reduced.

【0040】

Returning to FIG. 1, the first laser light source 31 is an Nd: YAGQ switch laser in which a laser beam having a wavelength of 1319 nm is frequency-narrowed by an injection seed. The second laser light source 32 is an Nd: YAGQ switch laser in which a laser beam having a wavelength of 1064 nm is frequency-narrowed by an injection seed.

【0041】

The high frequency signal generation units 36 and 37 are signal voltages of the periodic function φ (t) for obtaining the phase modulation function Φ (t) described above in the phase modulation units 34 and 35, for example, a sine wave composed of one or more frequency components. The wave voltage signal is generated as a high frequency signal and output to the amplifiers 38 and 39. The amplifiers 38 and 39 amplify the input high frequency voltage and output it to the phase modulation units 34 and 35.

【0042】

The phase modulation unit 34 may be used as a plurality of potassium phosphate potassium (KTiOPO) as an electro-optical crystal, for example.₄ : KTP) It is configured as shown in Fig. 3 using crystals. Two KTP41a, 41b are installed on the mount 42, one of the electrodes of each KTP41a, 41b is connected to the amplifier 38 via the connector 43, and the other electrode is grounded. As a result, the high frequency voltage from the high frequency signal generator 36 is applied to the electrodes of KTP41a and 41b.

【0043】

Further, the phase modulation unit 34 phase-modulates the incident fundamental wave laser beam having a wavelength of 1319 nm with a phase modulation function obtained based on the sinusoidal voltage signal from the high-frequency signal generation unit 36, and the fundamental wave having a wavelength of 1319 nm. Δf for the overall spectral width of the laser beam₁ Spread to (Δf₁ 2 × f_m1× m₁ ). That is, the electric field E of the fundamental wave laser beam at this time.₁ (t) is the value shown in the following equation (6).

【0044】

[Number 4]

<img file="JPH09121069A_D0004.tif" />【0045】

Similarly, the phase modulation unit 35 phase-modulates the incident fundamental wave laser beam having a wavelength of 1064 nm with a phase modulation function obtained based on the sinusoidal voltage signal from the high-frequency signal generator 37, and has a wavelength of 1064 nm. Δf for the overall spectral width of the sine wave laser beam₂ Spread to (Δf₂ 2 × f_m2× m₂ ). That is, the electric field E of the fundamental wave laser beam at this time.₂ (t) is the value shown in the following equation (7).

【0046】

[Number 5]

<img file="JPH09121069A_D0005.tif" />【0047】

Further, the sum frequency generation unit 33 is a nonlinear optical crystal element, for example, lithium boron oxide (LiB).₃O₅: LBO) It has a crystal, and uses the above-mentioned phase-modulated fundamental wave laser light with a wavelength of 1319 nm and a wavelength of 1064 nm to generate and emit a laser light with a wavelength of 589 nm according to the principle of sum frequency generation described later. To do. At this time, the entire spectral width Δf of the laser beam whose wavelength has been converted to 589 nm.₃ Is the total spectral width Δf of each fundamental wave laser beam before wavelength conversion.₁ , Δf₂ It is about the same as the sum of. That is, Δf₃ = Δf₁ + Δf₂ Will be. Further, since the permissible wavelength width of each laser beam used for this wavelength conversion is sufficiently wide, the wavelength conversion efficiency does not decrease due to the widening of the entire spectral width. Electric field E of laser light obtained by this wavelength conversion₃ (t) is the value shown in the following equation (8).

【0048】

[Number 6]

<img file="JPH09121069A_D0006.tif" />【0049】

Here, the principle of sum frequency generation will be described. In a nonlinear optical crystal, nonlinear polarization that is not proportional to the magnitude of the electric field applied from the outside occurs. If the second-order nonlinear susceptibility of this nonlinear polarization is not 0, the frequency ν₁ , Ν₂ When the two lights of are incident on the crystal, the frequency is ν.₃ (However, ν₃ = ν₁ + ν₂ ) Non-linear polarization is induced in the crystal. Due to this polarization, the frequency is ν₃ The light that is is emitted. That is, the wavelength is λ₁ , Λ₂ Wavelength λ of the sum frequency generated light emitted from the nonlinear optical crystal when the laser beam of₃ Will satisfy the relationship of Eq. (9) below.

【0050】

[Number 7]

<img file="JPH09121069A_D0007.tif" />【0051】

That is, the wavelength of the laser beam obtained by incident a laser beam having a wavelength of 1319 nm and a laser beam having a wavelength of 1064 nm into a nonlinear optical crystal and generating a sum frequency is 1 / (1/1319 + 1/1064) = 589 nm. ..

【0052】

Further, consider a case where phase modulation is performed using a sine wave having a single frequency component as a phase modulation function. A fundamental wave laser beam with a wavelength of 1319 nm, for example, f_m1To 350MHz, m₁ When phase-modulating as 2 radians, Δf₁ Is 2 x 350 MHz x 2 = 1.4 GHz. That is, the entire spectral width of the fundamental wave laser beam having a wavelength of 1319 nm has been widened to about 1.4 GHz.

【0053】

Also, a fundamental wave laser beam with a wavelength of 1064, for example, f_m2To 350MHz, m₂ When phase-modulating as 2 radians, Δf₂ Is 2 x 350 x 2 = 1.4 GHz. That is, the entire spectral width of the fundamental wave laser beam having a wavelength of 1064 nm has also been widened to about 1.4 GHz.

【0054】

Furthermore, the overall spectral width Δf of the laser beam obtained by sum frequency generation₃ Is Δf₁ And Δf₂ Since it is the sum of and, the spectrum width is about 1.4GHz + 1.4GHz = 2.8GHz, which is almost the same as the absorption line width of the sodium atom subjected to the Doppler effect, and the sodium atom can be excited efficiently.

【0055】

In the above laser beam generator, each phase modulation effect is made to act additively by generating a sum frequency laser beam after performing isomodulation on the fundamental wave laser beam before wavelength conversion. The spectral width of the laser light before wavelength conversion may be half the spectral width desired for the laser light after wavelength conversion.

【0056】

By the way, the singular modulation frequency f_m When performing phase modulation with, the power spectrum has a frequency interval of f._m Is a collection of spectra. Therefore, the frequency interval of the spectrum of the laser beam obtained by performing phase modulation using modulation frequencies of the same magnitude for both of the two types of wavelength laser beams to generate a sum frequency is f._m Degree.

【0057】

Here, in the case of the above-mentioned specific example, there are only about eight spectra at an interval of 350 MHz within a spectrum width of 2.8 GHz. On the other hand, in order to efficiently excite sodium atoms, it is desirable to generate a continuous spectrum with a spectrum width of about 3 GHz.

【0058】

Therefore, in order to obtain such a spectrum by performing phase modulation, it is necessary to reduce the frequency interval to generate a large number of spectra. Further, since the spectrum width of the laser light emitted from the laser light source is about 25 MHz, a continuous spectrum can be obtained when the frequency interval is set to about 25 MHz. To achieve with a single modulation frequency, the modulation frequency needs to be reduced to 25MHz. To set the spectral width to 2.8 GHz, set the modulation amplitude to about 28, set the spectral width of the laser beam of each wavelength to 2 x 28 x 25 MHz 1.4 GHz, and further double the spectral width through the sum frequency generation process. It should be. However, it is practically impossible to obtain such a large value of modulation amplitude.

【0059】

Therefore, by performing a plurality of phase modulations using a lower modulation frequency component, it is possible to generate a spectrum having a frequency interval that is finer than the spectrum obtained by phase modulation using a single frequency component.

【0060】

Specifically, phase modulation is performed on each of the fundamental wave laser beams having two types of wavelengths so that the modulation frequency is 350 MHz and the modulation amplitude is 2 radians. Further, one or both of the fundamental wave laser beams before wavelength conversion are subjected to phase modulation such that the modulation frequency is 100 MHz and the modulation amplitude is 2 radians, and the spectrum is separated into a precise spectrum with a frequency interval of 100 MHz. Further, phase modulation is performed so that the modulation frequency is 25 MHz and the modulation amplitude is 2 radians.

【0061】

The spectrum of the laser beam obtained by performing the above phase modulation becomes an aggregate of a large number of spectra having a frequency interval of 25 MHz, and a substantially continuous spectrum can be realized. Therefore, by performing phase modulation using a plurality of frequencies, it is possible to reduce the frequency interval without increasing the modulation amplitude. As a result, it is possible to generate a continuous spectrum, and for example, the excitation efficiency of sodium atoms can be improved.

【0062】

Further, as a method of performing phase modulation with the plurality of frequency components, there are a method of arranging a plurality of electro-optical crystals in series and applying voltages of different frequencies to each, and a method of having a plurality of frequency components in one electro-optical crystal. There is a method of applying a signal. In particular, in the former case, as will be described later, it is possible to reduce the drive voltage of the phase modulation unit by arranging a circuit that electrically resonates at a frequency applied to each electro-optical crystal. Further, in the latter case, the number of electro-optical crystals can be reduced, which can be realized at low cost.

【0063】

As described above, performing phase modulation at a plurality of frequencies also exhibits an effective effect in removing speckle noise, as will be described later.

【0064】

Further, as the electro-optical crystal, in addition to KTP, all electro-optical crystals that transmit near-infrared light can be used. In particular, MTiOXO, which is a derivative of KTP.₄ (M = K, Rb, Tl, NH₄ , Cs, X = P, As) are effective for high-power lasers because they show a large electro-optical effect and are less likely to be damaged by relatively high-power laser light.

【0065】

Further, in the above specific example, the fundamental wave laser light having a wavelength of 1319 nm and the fundamental wave laser light having a wavelength of 1064 nm are combined, and phase modulation is performed on two fundamental wave laser lights at the same time by one phase modulation unit. It is also possible. The modulation frequency and modulation amplitude of the phase modulation applied to each fundamental wave laser beam cannot be set independently, but the spectrum width can be effectively widened by selecting an appropriate modulation frequency and modulation amplitude. Can be done.

【0066】

Further, the effect of the present invention can be obtained even if values other than the present specific examples are used for the modulation frequency and the modulation amplitude. The absorption line width of the sodium atom affected by the Doppler effect is quite wide at 3 GHz, but the frequency of a general frequency-stabilized solid-state Q-switched laser is only a few tens of MHz.

【0067】

Even if the frequency width of the laser beam is not expanded to 3 GHz, the resonance excitation efficiency can be sufficiently improved by expanding it to about 500 MHz. It is desirable that the shape of the spectrum is continuous, but even if a laser beam consisting of four or more spectra whose respective spectral intensities are 30% or less of the sum of the total spectral intensities is used, sufficient improvement in excitation efficiency is achieved. Is possible.

【0068】

FIG. 4 shows a specific configuration in which the laser light generator according to the present invention is applied to a laser image display device.

【0069】

Generally, an image display device requires three color light sources of red, green, and blue. Therefore, according to the laser image display device, for example, it has a first laser light source 51 having an Nd: YAGQ switch laser that generates a laser beam having a wavelength of 1319 nm, and an Nd: YAGQ switch laser that has a laser beam having a wavelength of 1064 nm. A second laser light source 52 and a third laser light source 53 having an Nd: YAGQ switch laser that generates a laser beam having a wavelength of 946 nm are used as light sources. The second harmonic of the fundamental wave laser beam emitted from the first laser light source 51 is a red laser beam, and the second harmonic of the fundamental wave laser beam emitted from the second laser light source 52 is a green laser beam. The second harmonic of the fundamental wave laser beam emitted from the third laser light source 53 is a blue laser beam.

【0070】

Further, the fundamental wave laser light emitted from each laser light source is phase-modulated at a single or a plurality of frequencies by the phase modulation unit 54, and is wavelength-converted by the second harmonic generator 55 to be the second harmonic. It becomes a wave and the spectrum width is widened. That is, the temporal coherence is reduced. Further, after being modulated by the image signal by the intensity modulator 56, the decrease in temporal coherence is converted into a decrease in spatial coherence by the decoherer 57, and the speckle noise of the laser beam is reduced. .. Generally, the screen 511 side is moved to reduce speckle noise, but this is not necessary here.

【0071】

Here, the phase modulation unit 54 is a portion that is independently provided corresponding to each laser light source and has an electro-optical crystal, for example, KTP, and depends on a predetermined modulation frequency and modulation amplitude applied from the outside. Phase modulation is applied to the incident fundamental wave laser beam to widen the spectrum width as described above. Arbitrary coherence can be achieved by appropriately selecting the modulation frequency and modulation amplitude.

【0072】

Like the phase modulation section 54, the second harmonic generation section 55 is provided independently for each laser light source, and is a section having nonlinear optical crystals such as LBO and BBO, and is phase-modulated. The resulting fundamental wave laser beam is converted into a second harmonic, that is, the wavelength of the fundamental wave laser beam is converted into 1/2 wavelength. For example, if the spectral width of the fundamental wave laser beam is expanded to 500 MHz by phase modulation, the spectral width of the laser beam after wavelength conversion is doubled to 1 GHz, which is twice that of the fundamental wave. The coherent length is about 0.2m.

【0073】

The decoherer 57 is independently provided for each laser light source, divides the laser beam, gives the divided laser beam a difference in optical path length equal to or greater than the coherent length, and then synthesizes the laser beam again. ..

【0074】

The polygon mirror 58 is a rotating member having a series of planar reflecting surfaces around it, and together with the galvano mirror 59 described later, constitutes a deflection optical system for scanning the light from each laser light source on the screen 511. The combined laser light emitted from the three decoherers 57 corresponding to each laser light source is reflected and incident on the galvano mirror 59 via the projection lens 510.

【0075】

The galvano mirror 59 is rotatably supported by the galvano motor 512, and rotates at high speed in response to the laser light sequentially sent according to the rotational operation of the polygon mirror 58, and emits the incident laser light. It is reflected and projected on the screen 511.

【0076】

In order to reduce speckle noise, it is necessary to sufficiently reduce the coherence of the laser beam where there is a difference in the optical path length equal to or greater than the coherent length. For this purpose, it is desirable that the spectrum of the laser beam after wavelength conversion is composed of at least four spectra having a non-negligible intensity, and the respective spectral intensities are 30% or less of the total intensity. This will be described below.

【0077】

Even if the spectrum of the laser beam after wavelength conversion is separated by phase modulation, it is specified when the number of spectra is, for example, 3 or less, or the spectral intensity of one of them is as large as 30% or more of the total intensity. Since the spectral intensity is concentrated at the frequency of, the laser beam cannot be said to be completely multimode, and the coherence is not sufficiently reduced. In this case, coherence appears again at a distance of about twice the coherent length. This is a disadvantage in the design of the decohera.

【0078】

On the other hand, when the intensity of each spectrum is 30% or less of the total intensity and is composed of four or more spectra, the spectrum of the laser beam is separated into many spectra in a well-balanced manner, so that the coherence is effective. Decrease. The distance at which the coherence increases again is advantageous in the design of the decoherer because it is farther than the coherent length.

【0079】

This will be described with reference to the example of FIG. FIG. 5 shows the laser beam spectrum and the coherent curve when phase-modulated to a vertical single-mode laser with a high-frequency voltage signal of a certain frequency. The phase modulation amplitude m is changed from 1.2 m 2.4. When phase modulation is performed at a single frequency with a relatively small modulation amplitude as in this example, the coherence becomes zero once with a certain optical path length difference (coherent length), but the optical path length difference becomes long. It gets bigger again as it gets bigger. This is because the intervals between the individual spectra are equal, so that the coherence between adjacent spectra appears strongly.

【0080】

In particular, when the modulation amplitude is small and m = 1.2 to 1.4, the coherence becomes maximum again with an optical path length difference of about 2.5 times the coherent length. That is, in the example of m = 1.4, the coherent length is 80 cm, and the optical path length difference is 200 cm, and the coherence is maximized again. It is difficult to remove speckle noise using a decoherer with a light source that maximizes coherence again with an optical path length difference of only about 2.5 times the coherent length.

【0081】

On the other hand, when the modulation amplitude is m = 1.6 or more, the spectrum is composed of four or more spectra having a non-negligible intensity, and the respective spectral intensities are 30% or less of the total intensity. In the case of this example, the optical path length difference at which coherence is maximized again is more than three times the coherent length. In the example of m = 1.6 in Fig. 5, the coherent length is 60 cm, and the optical path length difference is 200 cm, and the coherence is maximized again. If the optical path length difference, which maximizes coherence again, is long enough compared to the coherent length, speckle noise can be effectively removed by optimally designing the decoherer. If the maximum optical path length difference is about 3 times the coherent length, it is not always sufficient, but it is still easy to realize the effect of speckle noise removal.

【0082】

Further, it is desirable that the entire spectral width of the laser beam after phase modulation and wavelength conversion is 500 MHz or more. This is because the size of the decoherer becomes too large if the coherent length is too long due to the design of the decoherer. If the spectrum width is 500MHz or more, the coherent length will be about 0.4m, and a decoherer can be designed with a practical size.

【0083】

Further, when the phase is modulated at a plurality of frequencies, a larger effect can be obtained. When phase modulation is performed at a single frequency, there is always an optical path length difference in which the coherent length or more increases the coherence again. This situation is shown in A of FIG. This is because the distance between each spectrum of the laser is different, so that the coherence does not increase again due to the optical path length difference equal to or larger than the coherent length. This is shown in Fig. 6B. This is advantageous in removing speckle noise.

【0084】

Here, a specific example in which the present invention is applied to a laser image display device has been described, but the laser beam is not limited to the above-mentioned specific example and can be changed in various ways.

【0085】

For example, a Q-switched laser is used as a light source, and a solid-state laser element Nd: YAG is used as a specific example of the Q-switched laser. For example, a gas laser such as a krypton gas laser or a laser diode is used as a red light source. A semiconductor laser such as the above may be used.

【0086】

Further, in the above example, the spectrum width is further widened by using the wavelength conversion process, but it is also possible to directly phase-modulate the light source after the wavelength conversion. A laser light source that does not have a wavelength conversion process can also realize the effects of the present invention, such as low coherence and reduction of speckle noise.

【0087】

When a krypton gas laser or laser diode is used as the red light source and wavelength conversion is not performed, the laser light from these light sources has a shorter wavelength than the near-infrared laser light, so that it is large at a low voltage during phase modulation. A modulation effect can be obtained.

【0088】

In addition, an example was given in which the laser light from the three color light sources of red, green, and blue is phase-modulated independently, but after the three laser lights are combined, one phase modulation unit has three colors. It is possible to perform phase modulation at the same time. In this case, the phase-modulated laser beam is separated by a color separation filter and then incident on the second harmonic generator, which is a wavelength conversion unit.

【0089】

In addition, since it is possible to control the spectral width by controlling the modulation amplitude and modulation frequency when performing phase modulation processing, it is possible to realize a laser beam in which both speckle noise and chromatic aberration have reached a practical level. Will be possible.

【0090】

Although the specific example in which the present invention is used as the light source of the laser beacon device and the laser image display device has been shown above, it may be used as the light source of another optical device.

【0091】

Further, as the light source of the fundamental wave laser light for phase modulation, a light source that oscillates in the vertical single mode is used, but a laser light source in the vertical multimode may also be used. For example, when the Nd: YAGQ switch laser oscillates in multiple modes with a frequency interval of about 400 MHz, the laser light is phase-modulated at a modulation frequency of 400 MHz or less (for example, 100 MHz) to make the power spectrum of the laser light as continuous as possible. By bringing it closer to the spectrum, the coherence can be made sufficiently small.

【0092】

Further, although a laser light source that oscillates with a Q switch is used as a light source of the fundamental wave laser light that performs phase modulation, a laser light source that oscillates continuously may be used. In this case, phase modulation is applied to the laser beam after wavelength conversion. Generally, when the frequency of the fundamental wave laser beam is matched with the resonance frequency of the resonator containing the nonlinear optical crystal, wavelength conversion is performed with high conversion efficiency, but when the spectrum is widened before wavelength conversion, the wavelength conversion efficiency is improved. This is because it drops significantly.

【0093】

As a configuration example of the phase modulation unit, as shown in FIG. 3, an example in which two KTPs are arranged in series in the longitudinal direction is given, but as shown in FIG. 7, for example, the longitudinal direction is 30 mm and the other side is Using KTP62, which is all 2 mm, an electrode 63 is formed on one side surface of this KTP62, and the distance s from the electrode 63 on one surface adjacent to the electrode 63 of the KTP62, for example, 0.5 mm inside, and the distance between the electrodes is s. As described above, the groove electrode 64 may be provided, the electrode 63 may be grounded, and the groove electrode 64 may be connected to the amplifier 38. In this case, a phase-modulated laser beam is incident between the two electrodes. With this configuration, the voltage generated by the high-frequency signal generator 36 and input to the KTP 62 can be reduced. Further, when the laser damage does not occur in the crystal even if the beam diameter of the incident laser light is sufficiently reduced, the phase modulation unit as shown in FIG. 7 is particularly effective.

【0094】

As an example of reducing the voltage input to the KTP, as shown in FIG. 8, the two KTP62a and 62b are arranged on the separate mounts 42a and 42b and in series in the longitudinal direction, for example, the mount of the KTP62a. An electrode 65 may be provided on the surface opposite to the 42a, and a coil 63 may be inserted and connected between the electrode 65 and the amplifier 38a.

【0095】

Generally, the capacitance of an electro-optical crystal is C._xtalAssuming that the inductance of the coil whose one end is connected to the electrode arranged on the electro-optical crystal is L, the frequency f is attached to the other end of the coil as shown in the following equation (10)._r When the high frequency signal of is input, the electro-optical crystal and the coil form an electrical resonance system.

【0096】

[Number 8]

<img file="JPH09121069A_D0008.tif" />【0097】

Therefore, a signal of a single frequency is applied to each phase modulation unit from the first high-frequency signal generation unit 36a or the second high-frequency signal generation unit 36b, and according to each frequency based on Eq. (10). By setting the inductance of the coil so that it resonates electrically, it is possible to obtain a modulation amplitude of the same magnitude by applying a voltage of about 1/10 to the KTP62a and 62b. As an electro-optical crystal, potassium dihydrogen phosphate (KH), which is a crystal having a small dielectric loss, that is, a large electrical resistance.₂PO₄), Ammonium dihydrogen phosphate (NH₄H₂PO₄ ), Β-barium borate (β-BaB)₂O₄, β-BBO), Rubidium Titanate (RbTiOAsO)₄), Cesium Titanate (CsTiOAsO)₄ ), Lithium niobate (LiNbO)₃ ), Lithium tantalate (LiTaO)₃ ) Etc. are especially effective.

【0098】

Further, when phase modulation is performed with a so-called high frequency signal of 400 MHz or more in order to widen the spectrum width with a high frequency and a small modulation amplitude, the phase modulation unit is configured so as to arrange an electro-optical crystal inside the microwave waveguide. You may. In this case, if the microwave waveguide is resonated when the voltage is applied to the electro-optical crystal, the applied voltage can be small.

【0099】

Further, a pair of mirrors may be provided so as to sandwich the electro-optical crystal. In this case, the laser beam incident on the crystal is repeatedly advanced in the crystal to prolong the working distance at which phase modulation is performed and the required voltage. Can be reduced. The mirror may be arranged outside, or may have a high-reflection film adhered to the crystal surface and a low-reflection film formed on a part of the surface. Further, the total reflection phenomenon generated by adjusting the incident angle may be used.

【0100】

[Effect of the invention]

As described above, according to the laser light generator according to the present invention, a simple configuration for controlling the modulation frequency and the modulation amplitude when phase-modulating the fundamental wave laser light is added to the light source of the optical device. This makes it possible to realize the coherence required for this optical device and reduce speckle noise.

【0101】

Further, when phase modulation is performed at a plurality of modulation frequencies, it is possible to generate a continuous spectrum and further reduce speckle noise.

【0102】

Further, if the phase is modulated before the wavelength conversion, the laser light incident on the electro-optical crystal is the light in the near-infrared region, so that the laser damage of the electro-optical crystal can be suppressed.

[Simple explanation of drawings]

[Figure 1]

It is a figure which shows the specific structure when the laser light generator which concerns on this invention is used as a light source of a laser beacon apparatus.

[Figure 2]

It is a figure explaining the phase modulation performed by the said laser beam generator.

[Fig. 3]

It is a figure which shows one specific structure of the phase modulation part in the said laser light generator.

[Fig. 4]

It is a figure which shows the specific structure when the said laser light generator is used as a light source of a laser image display device.

[Fig. 5]

It is a graph which shows the laser beam spectrum and the coherence curve.

[Fig. 6]

It is a graph which shows the coherence at the coherent length or more of the phase-modulated laser beam by the phase modulation part.

[Fig. 7]

It is a figure which shows the structural example of the 2nd specific example of the said phase modulation part.

[Fig. 8]

It is a figure which shows the structural example of the 3rd specific example of the said phase modulation part.

[Fig. 9]

It is a figure which shows the structure of the conventional laser beam generator.

[Explanation of symbols]

31 First laser light source 32 Second laser light source 33 Sum frequency generator 34, 35 Phase modulator 36, 37 High frequency signal generator 36a 1st high frequency signal generator 36b 2nd high frequency signal generator 41a, 41b KTP 51 First laser light source 52 Second laser light source 53 Third laser light source 54 Phase modulator 55 Second harmonic generator 62 KTP 62a, 62b KTP 64 groove electrode 63, 64 coils