JPH09121069A - Laser light emitter, laser beacon and laser image display - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser light emitter in which the spectral width of laser light can be widened and the coherence can be lowered to an appropriate level through a convenient structure. SOLUTION: The laser light emitter comprises first and second laser light sources 31, 32, sections 34, 35 for modulating the phase of light from each light source with one or a plurality of frequency components, and a sum frequency generating section 33 for producing a light of short wavelength based on the wavelength of phase modulated light. A basic wave laser light emitted from the first laser light source 31 is subjected to phase modulation at the phase modulating section 34 based on specified modulation amplitude and frequency and the spectrum width thereof is widened before being delivered to the sum frequency generating section 33. The sum frequency generating section 33 generates a sum frequency based on the laser light received from each light source and a conversion is made into short wavelength thus widening the spectrum width furthermore. Consequently, the coherence length of outgoing laser light is shortened and speckle noise is eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser light generator that serves as a light source for laser light that exhibits a high frequency accuracy and is required to have an appropriately wide spectrum width.

[0002]

2. Description of the Related Art Conventionally, attempts have been made to apply a high-power laser to various industrial fields by utilizing the monochromaticity (narrow spectrum band) of laser light. In particular, Q
Neodymium oscillating in vertical single mode by the switch method:
Yttrium aluminum garnet (Nd: YA
The laser light oscillated by the laser light source using G) (hereinafter simply referred to as Nd: YAGQ switch laser) or the laser light obtained by wavelength-converting this laser light exhibits high peak intensity, and therefore is used in various industrial fields. Is expected to be applied.

Such a Q-switched laser generally oscillates in a longitudinal multimode, but when the line width of the frequency component (hereinafter referred to as the spectrum width) is wide, problems such as chromatic aberration easily occur. Therefore, laser light is oscillated at a single wavelength by using an injection seed technique or the like. On the contrary, when the laser light is oscillated with a single wavelength, the oscillation spectrum width becomes too narrow, which is likely to cause inconvenience in use.

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

The laser light is, for example, a laser beacon (lase
r beacon) device. This laser beacon device is described in "Laser Beacon Adaptive Optics"("L. Beacon Adaptive Optics", published on pages 14 to 19 of the June 1993 issue of Optics and Physics News).
aser Beacon Adaptive Optics ", Optics & PhysicsNew
s, pp.14-19, June, 1993),
It is being studied as a means for improving the resolution of optical systems used in astronomical observation, inter-satellite optical communication, etc.

The laser beacon device emits a laser light source into the air to cause sodium atoms in the atmosphere to emit light by resonance absorption. The laser beacon device detects the atmospheric turbulence in real time by detecting the light emitted by sodium atoms on the ground, and further improves the resolution of the telescope by correcting the atmospheric turbulence using the adaptive optical system. It has a function.

[0007] Further, the sodium atom resonantly absorbs laser light having a wavelength near 589 nm and emits light. In order to efficiently excite sodium atoms by resonance, a high-power laser light source that exactly matches the absorption spectrum of sodium atoms in both frequency and frequency width is required.

As a high-output light source with a wavelength of 589 nm, FIG.
As shown in FIG. 1, a first Nd: YAGQ switch laser that oscillates a fundamental laser beam with a wavelength of 1319 nm that has been frequency-confined by using an injection seed technique.
Laser light source 101, and Nd: YAG which oscillates fundamental-wave laser light with a wavelength of 1064 nm similarly narrowed in frequency.
A second laser light source 102 comprising a Q-switched laser,
Fundamental laser light with a wavelength of 1319 nm and a wavelength of 1064 nm
And a LBO crystal 103 for generating the sum frequency laser light by using the fundamental wave laser light.

The injection seeds used in the first laser light source 101 and the second laser light source 102 oscillate the laser light with a single wavelength to change the wavelength of the laser light after the sum frequency generation. This is for exactly matching the absorption wavelength of the sodium atom.

Further, the absorption frequency width of the sodium atom subjected to the Doppler effect is about 3 GHz. The laser light with a wavelength of 589 nm obtained by the high-power light source shown in FIG.
Since each light source performs injection seeding to narrow the frequency, the laser light obtained by the above high-power light source exhibits high frequency stability. The spectrum width of the fundamental laser light emitted 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 sodium atom. The resonance efficiency between light and sodium atoms is low.

The laser light is applied to, for example, a laser image display device.

When laser light having a narrow spectral width is used in the laser image display device, speckle noise is likely to occur because the coherence is high. Such speckle noise causes granular unevenness in the laser image display device, and significantly deteriorates the image quality.

[0013]

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

The method of expanding the spectral width of laser light is as follows:
Several proposals have been made so far. For example, it is conceivable to oscillate the laser light in the longitudinal multimode or to use laser light that oscillates in the longitudinal multimode from the beginning. In this case, not only is the spectrum width too wide and problems such as chromatic aberration are likely to occur, but it is also necessary to change the configuration of the laser light generator itself, resulting in a decrease in luminous efficiency. That is, it was difficult to appropriately widen the spectrum width to a desired value. Further, the intensity of the laser light obtained by wavelength-converting two or more types of laser light oscillated in the longitudinal multimode by the sum frequency mixing is unstable.

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

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

The present invention has been made in view of the above problems, and by adding a simple structure, it is possible to broaden the spectral width of laser light and degrade coherence to an appropriate value. An object is to provide a laser light generator.

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

[0019]

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

According to the above laser light generator, the fundamental wave laser light generated by the laser light source is phase-modulated by the phase modulator on the basis of the predetermined modulation amplitude and modulation frequency to widen the spectrum width. At the same time, the wavelength conversion means converts the laser light into a short wavelength laser light, and at the same time, the spectrum width is further expanded. As a result, the coherence length of the laser light is shortened and speckle noise is suppressed.

Further, by performing phase modulation at a plurality of frequencies, laser light having a continuous spectrum can be generated. The coherence of such laser light is sufficiently small, and the generation of speckle noise is suppressed. The coherence of the laser light emitted from the wavelength converting means is controlled by the modulation amplitude and the modulation frequency used in the phase modulating means.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific examples of embodiments of a laser light generator according to the present invention will be described below with reference to the drawings.

FIG. 1 shows a laser light generator for generating a laser light of 589 nm used in a laser beacon device, for example.

The laser light generating apparatus has light sources 31 and 32 for generating fundamental wave laser light and phase modulators 34 and 35 for phase-modulating the fundamental wave laser light from the light sources 31 and 32 with a single or a plurality of frequency components. And a sum frequency generator 33 that converts the wavelength of the light phase-modulated by the phase modulators 34 and 35 into another wavelength.

The phase modulators 34 and 35 are provided with electro-optical elements, and the sum frequency generator 33 is provided with a nonlinear optical crystal element.

According to FIG. 1, the fundamental laser light having a wavelength of 1319 nm emitted from the first laser light source 31 is incident on the phase modulator 34 and undergoes phase modulation. Further, the fundamental wave laser light having a wavelength of 1064 nm emitted from the second laser light source 32 is incident on the phase modulator 35 and undergoes phase modulation. At this time, in the phase modulators 34 and 35, the fundamental wave laser light before wavelength conversion is subjected to phase modulation at a single or a plurality of modulation frequencies to widen the spectrum width of each fundamental wave laser light, and then the sum frequency generator 33. The fundamental wave laser light that has undergone this phase modulation is incident on. In the sum frequency generation unit 33, sum frequency mixing, that is, wavelength conversion is performed using the fundamental wave laser light having a wavelength of 1319 nm and a wavelength of 1064 nm, and the spectrum width of the laser light having a wavelength of 589 nm after the wavelength conversion is greater than Further spread. In this way,
Since the spectrum width of the laser light having the wavelength of 589 nm is sufficiently widened through the phase modulation and the wavelength conversion, the efficiency is improved when it is used for the resonance excitation of sodium atom.

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

FIG. 2A shows the initial power spectrum of the fundamental laser light.

When a signal voltage of the periodic function φ (t) is applied to the electro-optical modulator having the electro-optical element, the electric field E (t) of the laser light undergoes phase modulation as shown in the following equation (1). receive. Note that f ₀ is the center frequency of the laser light before modulation.

[0030]

(Equation 1)

Where Φ (t) is the phase modulation function,
It is proportional to the signal φ (t) applied to the electro-optical modulator. That is, it becomes as shown in the equation (2). In particular, Φ
When (t) is a sine wave having an amplitude m and a frequency f _m as shown in the following formula (3), the electric field E (t) is expressed by the Bessel function sequence J as shown in the following formula (4). It can be developed using _k (m).

[0032]

(Equation 2)

At this time, the power spectrum is as shown in FIG.
As shown in B of FIG. 7, the sum is the sum of the spectra of the intensity J _k ² (m) and the frequency (f ₀ + kf _m ). here,
k is an integer. Due to the phase modulation, the spectrum intensity of the center frequency f ₀ is reduced, and {J ₀ (m)} ² is smaller than that before the modulation.
Double.

When the 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 related to the frequency. Acts additively and synergistically in strength.

[0035]

(Equation 3)

For example, the modulation amplitude m ₁ and the modulation frequency f _m1
Intensity J _k ² (m ₁ ) generated by the phase modulation at
The spectrum consisting of the frequency (f ₀ + kf _m1 ) and the intensity J _k ² (m ₁ ) J _k ² (m ₂ ) is further increased by the phase modulation at another modulation frequency f _m2 as shown in FIG. 2C. Slitting into a spectrum composed of frequencies (f ₀ + kf _m1 + lf _m2 ) results in an aggregate of a large number of spectra. here,
k and l are integers.

As described above, by performing the phase modulation a plurality of times, for example, twice at different modulation frequencies, it can be seen that the initial power spectrum is separated into a large number of spectrums and the spectrum intervals become fine. At this time, the spectrum interval is equal to or smaller than the smallest modulation frequency among the plurality of set modulation frequencies. When the least common multiple between the modulation frequencies is sufficiently large, the number of spectra is approximately (2m ₁ +1).
X (2m ₂ +1).

For example, a high-frequency voltage signal (phase modulation function Φ (t)) having a frequency f _m1 = 350 MHz and a voltage amplitude such that the phase modulation amplitude m ₁ is 2 radians is subjected to the first phase modulation, and the frequency f _{When the} second phase modulation is performed with a high frequency voltage signal having a voltage amplitude such that m ₂ = 350 MHz and the phase modulation amplitude m ₂ becomes 2 radians, the spectrum interval becomes 1
00MHz, the number of spectra is (2 × 2 + 1)
× (2 × 2 + 1) = 25.

In the phase modulation performed here, the spectral line width of the fundamental laser light itself does not spread, but
It only splits into a number of spectra and widens the overall spectrum. Hereinafter, the width of the envelope of the aggregate of the separated spectra is defined as the entire spectrum width. Therefore, the coherent length is shortened and the speckle noise is reduced by widening the width of the entire spectrum.

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

The high-frequency signal generators 36 and 37 generate a signal voltage of a periodic function φ (t) for obtaining the above-mentioned phase modulation function Φ (t) in the phase modulators 34 and 35, for example, a single or a plurality of frequency components. The sine wave voltage signal composed of is generated as a high frequency signal and is output to the amplifiers 38 and 39. The amplifiers 38 and 39 amplify the input high frequency voltage and output it to the phase modulators 34 and 35.

The phase modulator 34 is composed of a plurality of potassium titanium oxide phosphates (KTiOPO ₄ :
(KTP) crystal is used, as shown in FIG. Two KTPs 41a, 41 on the mount 42
b is installed, one electrode of each KTP 41a, 41b is connected to the amplifier 38 via the connector 43, and the other electrode is grounded. Thereby, the high frequency voltage from the high frequency signal generator 36 is applied to the electrodes of the KTPs 41a and 41b.

Further, the phase modulator 34 converts the incident fundamental laser light having a wavelength of 1319 nm into the high frequency signal generator 3
6 is phase-modulated by a phase modulation function obtained based on the sinusoidal voltage signal from 6 to widen the entire spectrum width of the fundamental laser light having a wavelength of 1319 nm to Δf ₁ (Δf ₁ ≈2 ×
f _m1 × m ₁ ). That is, the electric field E ₁ (t) of the fundamental wave laser light at this time has a value represented by the following expression (6).

[0044]

(Equation 4)

Similarly, the phase modulator 35 phase-modulates the incident fundamental wave laser light having a wavelength of 1064 nm by a phase modulation function obtained based on the sinusoidal voltage signal from the high frequency signal generator 37. The entire spectral width of the fundamental wave laser light having a wavelength of 1064 nm is expanded to Δf ₂ (Δf ₂
≈2 × f _m2 × m ₂ ). That is, the electric field E ₂ (t) of the fundamental laser light at this time has a value represented by the following expression (7).

[0046]

(Equation 5)

The sum frequency generator 33 is a nonlinear optical crystal element such as lithium boron oxide (LiB ₃ O ₅ : L).
(BO) crystal is used to generate and emit a laser beam having a wavelength of 589 nm in accordance with the principle of sum frequency generation described later, using the fundamental wave laser beam having a wavelength of 1319 nm and a wavelength of 1064 nm that has been subjected to the above-mentioned phase modulation. . At this time, the overall spectrum width Δf ₃ of the laser light wavelength-converted to the wavelength of 589 nm is approximately the same as the sum of the overall spectrum widths Δf ₁ and Δf ₂ of each fundamental wave laser light before wavelength conversion. That is, Δf ₃ = Δf ₁ + Δf ₂ . Moreover, since the allowable 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 spectrum width. The electric field E ₃ (t) of the laser light obtained by the wavelength conversion has the value shown in the following equation (8).

[0048]

(Equation 6)

Here, the principle of sum frequency generation will be described. In the nonlinear optical crystal, nonlinear polarization that is not proportional to the magnitude of an externally applied electric field occurs. In this nonlinear polarization, if the second-order nonlinear susceptibility is not 0, the frequency is ν ₃ when two lights of frequencies ν ₁ and ν ₂ enter the crystal (however, ν ₃ = ν ₁ + ν ₂ ). Non-linear polarization is induced in the crystal. This polarization emits light with a frequency of ν ₃ . That is, when the laser light having the wavelengths λ ₁ and λ ₂ is incident, the wavelength λ _{3 of} the light having the sum frequency emitted from the nonlinear optical crystal satisfies the relation of the following expression (9).

[0050]

(Equation 7)

That is, the wavelength of the laser light obtained by injecting the laser light having the wavelength of 1319 nm and the laser light having the wavelength of 1064 nm into the nonlinear optical crystal to generate the sum frequency is 1
/ (1/1319 + 1/1064) = 589 nm.

Consider a case where phase modulation is performed by using a sine wave having a single frequency component as a phase modulation function. Wavelength 131
The fundamental wave laser light of 9 nm, for example, f _m1 350 MHz
And when phase modulation is performed with m _{1 set} to 2 radians, Δf
₁ becomes 2 × 350 MHz × 2 = 1.4 GHz. That is, the entire spectrum width of the fundamental wave laser light having a wavelength of 1319 nm is expanded to about 1.4 GHz.

Further, the fundamental wave laser light of wavelength 1064 is
For example, when f _m2 is 350 MHz and phase modulation is performed with m ₂ being 2 radians, Δf ₂ is 2 × 350 × 2 = 1.4.
It becomes GHz. That is, the entire spectrum width of the fundamental wave laser light having the wavelength of 1064 nm is also expanded to about 1.4 GHz.

Further, the total spectral width Δf ₃ of the laser light obtained by the sum frequency generation is the sum of Δf ₁ and Δf _2, and therefore 1.4 GHz + 1.4 GHz = 2.8 GHz
The spectrum width is about z, which is approximately the same as the absorption line width of the sodium atom subjected to the Doppler effect, and the sodium atom can be efficiently excited.

In the above laser light generator, each phase modulation effect is additively exerted by generating the sum frequency laser light after the modulation of the fundamental wave laser light before wavelength conversion. . The spectrum width of the laser light before wavelength conversion may be half the spectrum width desired for the laser light after wavelength conversion.

By the way, when phase modulation is performed at a single modulation frequency f _m , the power spectrum is a set of spectra having frequency intervals f _m . Therefore, the frequency interval of the spectrum of the laser light obtained by performing the phase modulation using the modulation frequencies of the same magnitude for both the laser lights of two types of wavelengths to generate the sum frequency is about f _m .

Here, in the case of the above-mentioned specific example, there are only about 8 spectra at intervals of 350 MHz within the spectrum width of 2.8 GHz. On the other hand, in order to efficiently excite sodium atoms, the spectral width is 3 GHz.
It is desirable to generate a continuous spectrum of the order.

Therefore, in order to obtain such a spectrum by performing phase modulation, it is necessary to reduce the frequency interval and generate a large number of spectrums. In addition, the spectral width of the laser light emitted from the laser light source is 25 MHz.
Since it is about z, a continuous spectrum can be obtained by setting the frequency interval to about 25 MHz. To realize with a single modulation frequency, it is necessary to reduce the modulation frequency to 25 MHz. To set the spectral width to 2.8 GHz,
The modulation amplitude may be set to about 28, the spectrum width of the laser light of each wavelength may be set to 2 × 28 × 25 MHz≈1.4 GHz, and the spectrum width may be further doubled through the sum frequency generation process. However, it is practically impossible to obtain such a large modulation amplitude.

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

Specifically, phase modulation is performed on each of the fundamental laser light beams having two types of wavelengths so that the modulation frequency is 350 MHz and the modulation amplitude is 2 radians. Furthermore, for one or both of the fundamental laser light before wavelength conversion, the modulation frequency is 100 MHz and the modulation amplitude is 2
Phase modulation is performed in radians and frequency interval is 1
Separated into a dense spectrum of 00 MHz. Furthermore, phase modulation is performed so that the modulation frequency is 25 MHz and the modulation amplitude is 2 radians.

The spectrum of the laser light obtained by performing the above phase modulation becomes an aggregate of a large number of spectrums 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 becomes possible to reduce the frequency interval without increasing the modulation amplitude. As a result, it is possible to generate a continuous spectrum and improve the excitation efficiency of sodium atoms, for example.

As a method of performing phase modulation with a plurality of frequency components, a method of arranging a plurality of electro-optic crystals in series and applying voltages of different frequencies to each, and a method of applying a plurality of frequencies to one electro-optic crystal are used. There is a method of applying a signal having a component. Especially in the former case, as described below,
By disposing a circuit that electrically resonates at the frequency applied to each electro-optic crystal, it is possible to reduce the drive voltage of the phase modulator. In the latter case, the number of electro-optic crystals can be reduced and the cost can be realized.

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

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 ₄ which is a derivative of KTP
(M = K, Rb, Tl, NH ₄ , Cs, X = P, As)
Shows a large electro-optic effect and is less likely to be damaged by a relatively high power laser beam, and is therefore effective for a high power laser.

In the above specific example, the wavelength 1319
It is also possible to combine a fundamental wave laser beam having a wavelength of 10 nm with a fundamental wave laser beam having a wavelength of 1064 nm and perform phase modulation on two fundamental wave laser beams at the same time by one phase modulation unit.
The modulation frequency and the modulation amplitude of the phase modulation applied to each fundamental wave laser light cannot be set independently, but the spectrum width can be effectively expanded by selecting an appropriate modulation frequency and modulation amplitude. You can

The effects of the present invention can be obtained by using values other than those in this example for the modulation frequency and the modulation amplitude. The absorption line width of sodium atoms subjected to the Doppler effect is quite wide as 3 GHz, but the frequency of a general solid-state Q-switched laser whose frequency is stabilized is only several tens MHz.

Even if the frequency width of the laser beam is not expanded to 3 GHz, if it is expanded to about 500 MHz, the resonance excitation efficiency can be sufficiently improved. It is desirable that the shape of the spectrum is continuous, but even if a laser beam composed of four or more spectra, each of which has a spectral intensity of 30% or less of the sum of the total spectral intensities, sufficient excitation efficiency is improved. Is possible.

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.

Generally, the image display device requires light sources of three colors of red, green and blue. Therefore, according to the laser image display device described above, for example, the first laser light source 51 having an Nd: YAGQ switch laser that generates a laser beam with a wavelength of 1319 nm and the N laser beam with a wavelength of 1064 nm are generated.
A second laser light source 52 having a d: YAGQ switch laser, and Nd: Y for generating laser light having a wavelength of 946 nm.
A third laser light source 53 having an AGQ switch laser is used as the light source. The second harmonic of the fundamental laser light emitted from the first laser light source 51 is a red laser light, the second harmonic of the fundamental laser light emitted from the second laser light source 52 is a green laser light, The second harmonic of the fundamental laser light emitted from the third laser light source 53 is blue laser light.

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

Here, the phase modulating section 54 is a section provided independently corresponding to each laser light source and having an electro-optic crystal such as KTP, and is modulated with a predetermined modulation frequency applied from the outside. The fundamental wave laser light incident on the basis of the amplitude is subjected to phase modulation to widen the spectrum width as described above. Arbitrary coherence can be realized by appropriately selecting the modulation frequency and the modulation amplitude.

The second harmonic generation section 55 includes a phase modulation section 54.
In the same manner as above, it is a part provided independently for each laser light source and having a nonlinear optical crystal such as LBO, BBO, etc., and converts the phase-modulated fundamental laser light into the second harmonic. That is, the wavelength of the fundamental laser light is converted into a half wavelength. For example, if the spectrum width of the fundamental laser light is expanded to 500 MHz by phase modulation, the spectrum width of the laser light after wavelength conversion is 2
Double up to 1GHz. The coherent length is about 0.2 m.

The decoherer 57 is provided independently corresponding to each laser light source, divides the laser light, gives the divided laser light an optical path length difference equal to or longer than the coherent length, and then synthesizes again. It is a thing.

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

The galvano mirror 59 is used for the galvano motor 5
It is rotatably supported by 12 and rotates at a high speed corresponding to the laser light sequentially sent according to the rotation operation of the polygon mirror 58, and the incident laser light is reflected on the screen 511 and projected. To do.

In order to reduce the speckle noise, it is necessary that the coherence of the laser light is sufficiently reduced where there is a difference in optical path length equal to or longer than the coherent length. To this end, the spectrum of the laser light after wavelength conversion is composed of at least four or more spectra having a non-negligible intensity,
Further, it is desirable that each spectrum intensity is 30% or less of the total intensity. This will be described below.

Even if the spectrum of the laser light after wavelength conversion is separated by phase modulation, the number of spectrums is, for example, 3 or less, or the spectrum intensity of one of them is as large as 30% or more of the total intensity. In this case, since the spectrum intensity is concentrated on a specific frequency, it cannot be said that the laser light is completely multimode, and the coherence is not sufficiently reduced. In this case, the coherence again appears strongly at a distance of about twice the coherent length. This is because of the design of the decoherer,
Disadvantageous.

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

Description will be made with reference to the example of FIG. FIG.
3 shows a laser light spectrum and a coherence curve when a longitudinal single mode laser is phase-modulated with a high frequency voltage signal of a certain frequency. The phase modulation amplitude m is 1.2 ≦ m ≦ 2.
It is changed up to 4. When phase modulation is performed at a single frequency and a relatively small modulation amplitude as in this example, the coherence temporarily becomes zero at a certain optical path length difference (coherent length), but the optical path length difference becomes long. Grows bigger again. This is because the individual spectra are equally spaced,
This is because the coherence of the spectrum between adjacent ones appears strongly.

Particularly, the modulation amplitude is small and m = 1.2 to 1.
In the case of 4, the coherence is maximized 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 optical path length difference is 200 when the coherent length is 80 cm.
The coherence is again maximized in cm. It is difficult to remove speckle noise using a decoherer in a light source in which coherence is maximized again with an optical path length difference of only about 2.5 times the coherent length.

On the other hand, when the modulation amplitude is m = 1.6 or more,
The spectrum is composed of four or more spectra with intensities that cannot be ignored, and the intensity of each spectrum is 30% or less of the total intensity. In the case of this example, the optical path length difference at which the coherence becomes maximum again is three times or more the coherent length. In the example of m = 1.6 in FIG. 5, the coherent length 60c
For m, the coherence is maximized again when the optical path length difference is 200 cm. When the optical path length difference that maximizes the coherence again is sufficiently long as compared with the coherent length, speckle noise can be effectively removed by optimally designing the decoherer. It is not always sufficient when the optical path length difference is about 3 times the coherent length to maximize the difference, but it is still easy to realize the effect of removing speckle noise.

Further, it is desirable that the entire spectrum width of the laser light which has been wavelength-converted after being phase-modulated is 500 MHz or more. This is because the size of the decoherer becomes too large when the coherent length is too long due to the design of the decoherer. Spectral width is 500MHz
With the above, the coherent length is about 0.4 m, and the decoherer can be designed with a practical size.

Furthermore, when phase modulation is performed at a plurality of frequencies, a greater effect can be obtained. When phase modulation is performed with a single frequency, there is always an optical path length difference at which the coherence becomes greater than the coherent length. This state is shown in FIG.
Shown in This is because the intervals between the spectra of the lasers are different, and therefore the coherence does not increase again due to the optical path length difference of the coherence length or more. This state is shown in B of FIG. This is advantageous in removing speckle noise.

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 specific example described above, and various modifications can be made.

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

In the above example, the spectrum width is further widened by utilizing the wavelength conversion process, but it is also possible to directly phase-modulate the light source after wavelength conversion. A laser light source having no wavelength conversion process can also achieve low coherence and reduction of speckle noise, which are the effects of the present invention.

When a krypton gas laser or a 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 laser light in the near infrared, so that it is low during phase modulation. A large modulation effect can be obtained with voltage.

In addition, an example has been given in which the laser light from the light sources of three colors of red, green and blue is independently phase-modulated.
It is possible to perform phase modulation of three colors at the same time with one phase modulation unit after multiplexing the laser lights of the book. In this case, the phase-modulated laser light is separated by the color separation filter and then made incident on the second harmonic generation device which is the wavelength conversion unit.

Further, the spectrum width can be controlled by controlling the modulation amplitude and the modulation frequency at the time of performing the phase modulation processing, so that both the speckle noise and the chromatic aberration reach the practical level. Can be realized.

The specific examples in which the present invention is used as the light source of the laser beacon device and the laser image display device have been described above, but the light source of other optical devices may be used.

Further, although the light source which oscillates in the longitudinal single mode is used as the light source of the fundamental wave laser light for performing the phase modulation, the laser light source of the longitudinal multimode may be used. For example, Nd:
When the YAGQ switch laser oscillates in multiple modes at a frequency interval of about 400 MHz, this laser light is 400 M
Coherence can be sufficiently reduced by performing phase modulation at a modulation frequency of 100 Hz or less (for example, 100 MHz) and making the power spectrum of the laser light as close to a continuous spectrum as possible.

Although a laser light source that oscillates Q-switch is used as the light source of the fundamental wave laser light that is phase-modulated, it may be a laser light source that continuously oscillates. In this case, phase modulation is applied to the laser light after wavelength conversion. Generally, when the frequency of the fundamental laser light is matched with the resonance frequency of the resonator including the nonlinear optical crystal, the wavelength conversion is performed with high conversion efficiency.However, if the spectrum is expanded before the wavelength conversion, the wavelength conversion efficiency is increased. This is because it will be greatly reduced.

As an example of the configuration of the phase modulator, two KTPs are arranged in series in the longitudinal direction as shown in FIG. 3, but as shown in FIG.
An electrode 63 is formed on one side surface of the KTP 62 by using the KTP 62 in which all other sides are 2 mm in m.
The groove electrode 64 is provided on one surface adjacent to the electrode 63 of the electrode 63 at a distance s, for example, 0.5 mm, from the electrode 63 so that the inter-electrode distance is s, the electrode 63 is grounded, and the groove electrode 64 is connected to the amplifier 38. You may let me. In this case, phase-modulated laser light is incident between the two electrodes. With this configuration, the KTP6 is generated by the high frequency signal generator 36.
The voltage input to 2 can be reduced. Further, in the case where laser damage does not occur in the crystal even if the beam diameter of the incident laser light is made sufficiently small, the phase modulation section as shown in FIG. 7 is particularly effective.

As an example of reducing the voltage input to KTP, as shown in FIG. 8, two KTPs 62a,
62b are arranged on separate mounts 42a and 42b, respectively, and are arranged in series in the longitudinal direction.
An example is that an electrode 65 is provided on the surface of the mount 2a opposite to the mount 42a, and the coil 63 is inserted and connected between the electrode 65 and the amplifier 38a.

Generally, the capacitance of an electro-optic crystal is C _xtal.
And the inductance of the coil whose one end is connected to the electrode arranged on the electro-optic crystal is L, the following (1
0) as shown in the expression, when the high frequency signal of the frequency f _r to the other end of the coil is input to the electro-optic crystal and the coil to form an electrical resonance system.

[0096]

(Equation 8)

Therefore, a signal of a single frequency is applied from the first high-frequency signal generator 36a or the second high-frequency signal generator 36b to each phase modulator, and each phase modulator is calculated based on the equation (10). By setting the inductance of the coil so as to electrically resonate according to the frequency and applying a voltage of about 1/10 to the KTPs 62a and 62b, it is possible to obtain a modulation amplitude of an equivalent magnitude. is there.
As an electro-optic crystal, potassium dihydrogen phosphate (KH ₂ P), which has a low dielectric loss, that is, a high electric resistance, is used.
O ₄ ), ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ),
β-barium borate (β-BaB ₂ O ₄ , β-BBO), rubidium arsenate titanate (RbTiOAsO ₄ ), cesium titanate arsenate (CsTiOOAsO ₄ ), lithium niobate (LiNbO ₃ ), lithium tantalate (LiTa).
It is particularly effective when O ₃ ) or the like is used.

When performing phase modulation with a so-called high-frequency signal of 400 MHz or more in order to widen the spectrum width at high frequency and small modulation amplitude, the phase modulation should be performed so that the electro-optic crystal is arranged inside the microwave waveguide. You may comprise a part. In this case, if the microwave waveguide is resonated when a voltage is applied to the electro-optic crystal, the applied voltage can be small.

Further, a pair of mirrors may be provided with the electro-optic crystal sandwiched therebetween. In this case, the working distance for phase modulation is lengthened by repeating the laser light incident on the crystal in the crystal. The required voltage can be reduced. It should be noted that this mirror may be arranged outside, or a high reflection film may be formed by depositing on the crystal surface and a low reflection film may be formed on a part of this surface. Alternatively, a total reflection phenomenon that occurs by adjusting the incident angle may be used.

[0100]

As described above, according to the laser light generator of the present invention, the optical device has a simple structure for controlling the modulation frequency and the amplitude when the fundamental laser light is phase-modulated. It is possible to realize the coherence required for this optical device and reduce the speckle noise by adding it to the light source.

When performing phase modulation at a plurality of modulation frequencies, it becomes possible to generate a continuous spectrum and further reduce speckle noise.

If phase modulation is performed before wavelength conversion,
Since the laser light incident on the electro-optic crystal is light in the near infrared region, it is possible to suppress laser damage to the electro-optic crystal.

[Brief description of the drawings]

FIG. 1 is a diagram showing a specific configuration when a laser light generator according to the present invention is used as a light source of a laser beacon device.

FIG. 2 is a diagram illustrating phase modulation performed by the laser light generator.

FIG. 3 is a diagram showing a specific configuration of a phase modulator in the laser light generator.

FIG. 4 is a diagram showing a specific configuration when the laser light generator is used as a light source of a laser image display device.

FIG. 5 is a graph showing a laser light spectrum and a coherence curve.

FIG. 6 is a graph showing coherence over a coherent length of laser light phase-modulated by the phase modulator.

FIG. 7 is a diagram showing a configuration example of a second specific example of the phase modulation unit.

FIG. 8 is a diagram showing a configuration example of a third specific example of the phase modulation unit.

FIG. 9 is a diagram showing a configuration of a conventional laser light generator.

[Explanation of symbols]

 31 First Laser Light Source 32 Second Laser Light Source 33 Sum Frequency Generation Section 34, 35 Phase Modulation Section 36, 37 High Frequency Signal Generation Section 36a First High Frequency Signal Generation Section 36b Second High Frequency Signal Generation Section 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 Coil

Claims (18)

[Claims] 1. A laser light source, phase modulation means for phase-modulating the laser light emitted from the laser light source with a single or a plurality of frequency components, and the wavelength of the laser light phase-modulated by the phase modulation means Laser generating device having a wavelength converting means for converting to another wavelength. 2. An electro-optical crystal having a laser light source, a high-frequency signal generating means for generating and outputting a high-frequency electrical signal having a single or a plurality of frequency components, and an electro-optical crystal, to which the high-frequency electrical signal is applied. Has a phase modulating means for modulating the phase of the laser light by transmitting the laser light emitted from the laser light source, and a nonlinear optical crystal, and the phase is modulated by the phase modulating means in the nonlinear optical crystal. And a wavelength conversion means for converting the wavelength of the laser light by transmitting the modulated laser light. 3. The laser light source is a Q-switched laser having a solid-state laser element.
The laser light generator described. 4. The laser light generator according to claim 2, wherein the laser light source is a laser light source that continuously oscillates. 5. The laser light generator according to claim 2, wherein the laser light source is a laser light source that oscillates laser light in a longitudinal single mode. 6. The laser light generator according to claim 2, wherein the laser light source is a laser light source that oscillates laser light in a longitudinal multimode having two or three spectra. 7. The electro-optic crystal is at least one of potassium titanate, potassium titanate derivative, β-barium borate, lithium niobate, lithium tantalate, potassium dihydrogen phosphate and ammonium dihydrogen phosphate. The laser light generator according to claim 2, wherein the laser light generator is composed of: 8. The laser light generator according to claim 2, wherein the phase modulator comprises a plurality of phase modulators operating at different frequencies, which are installed in series. 9. The phase modulation means includes a coil and an electro-optic crystal, and a resonance system formed by the coil and the electro-optic crystal has a function of amplifying a drive voltage applied to the electro-optic crystal. The laser light generator according to claim 2, wherein 10. The phase modulation means includes a microwave waveguide and an electro-optic crystal, and a microwave resonance system formed by arranging the electro-optic crystal in the microwave waveguide is applied to the electro-optic crystal. 3. The laser light generator according to claim 2, which has the function of amplifying the drive voltage. 11. The laser light generator according to claim 2, wherein the phase modulation means is configured such that the input fundamental wave laser light is repeatedly transmitted through the electro-optic crystal a plurality of times. 12. The phase modulating means includes a groove formed on the surface of the electro-optic crystal along an axis through which the modulated laser light is transmitted, and one electrode is formed on an inner side surface of the groove. The laser light generator according to claim 2, which is characterized in that. 13. The non-linear optical crystal is selected from potassium titanate, potassium titanate derivative, β-barium borate, lithium niobate, lithium tantalate, potassium dihydrogen phosphate, ammonium dihydrogen phosphate, and lithium cesium borate. 2. The laser light generator according to claim 1, further comprising at least one of the above. 14. A laser beacon device which operates by resonantly exciting atoms in the atmosphere, comprising: a laser light source; and 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. A laser beacon device comprising: a laser light generator including: a wavelength conversion means for converting the wavelength of the laser light phase-modulated by the phase modulation means into another wavelength. 15. A laser image display device for scanning visible laser light on a screen, comprising: a laser light source; a phase modulation means for phase-modulating the laser light emitted by the laser light source with a single or multiple frequency components; A laser image display device, comprising: a laser light generator having a wavelength conversion means for converting the wavelength of the laser light phase-modulated by the modulation means into another wavelength. 16. A laser beacon device that operates by resonantly exciting atoms in the atmosphere, a laser light source, a high-frequency signal generating means for generating and outputting a high-frequency electric signal having a single or plural frequency components, and an electro-optic crystal. And a phase modulating means for modulating the phase of the laser light by transmitting the laser light emitted from the laser light source into the electro-optic crystal to which the high-frequency electric signal is applied, and a nonlinear optical crystal. And a wavelength conversion means for converting the wavelength of the laser light by transmitting the laser light phase-modulated by the phase modulation means in the nonlinear optical crystal. A laser beacon device having. 17. A laser image display device for scanning visible laser light on a screen, comprising: a laser light source; a high frequency signal generating means for generating and outputting a high frequency electric signal having a single or plural frequency components; and an electro-optic crystal. Then, the electro-optical crystal to which the high-frequency electric signal is applied has a phase modulation means for modulating the phase of the laser light by transmitting the laser light emitted from the laser light source, and a nonlinear optical crystal. Then, a laser light generator having wavelength conversion means for converting the wavelength of the laser light by transmitting the laser light phase-modulated by the phase modulation means is provided in the nonlinear optical crystal. A laser image display device characterized by. 18. A laser light source, a high-frequency signal generating means for generating and outputting a high-frequency electric signal having a single or a plurality of frequency components, and an electro-optical crystal, and the electro-optical crystal to which the high-frequency electric signal is applied. By transmitting the laser light emitted from the laser light source and modulating the phase of the laser light, the number of spectral lines is 4 or less, and each spectral intensity is 30 of the total intensity of all spectral intensities. % Or less,
And a phase modulation means for generating laser light having an overall spectrum width of 500 MHz or more, and a wavelength conversion for converting the wavelength of the laser light phase-modulated by the phase modulation means into another wavelength by using a nonlinear optical crystal element. And a laser beam generator having means.