JPH05232536A - Wavelength conversion device - Google Patents

Abstract

PURPOSE:To constitute a small-sized, lightweight, and inexpensive light source even when variation in the wavelength of a fundamental wave is suppressed within the permissible wavelength width of a wavelength converting element at all times by precisely varying the wavelength of the light source or varying temperature of the wavelength converting element if the wavelength of the fundamental wave emitted by the light source is different from designed wavelength owing to variation in ambient temperature, etc.. CONSTITUTION:A wavelength-stabilized laser diode in structure which generates no mode pop nearby the set wavelength of the fundamental wave of the wavelength converting element when the oscillation wavelength varies with temperature is used as the light source 1 which excites a converted wave by guiding the fundamental wave to the optical waveguide G2 of the wavelength converting element 8. Consequently, the wavelength of the fundamental wave is precisely controlled or the element temperature is adjusted so that the variation in the wavelength of the fundamental wave in the wavelength converting element 8 is suppressed within the permissible wavelength width at all times.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to light from a light source having a predetermined wavelength, which is incident on a wavelength conversion element to be guided as a fundamental wave, and is converted into light having a shorter wavelength than the fundamental wave based on the fundamental wave. Wavelength converter.

[0002]

2. Description of the Related Art Conventionally, as a wavelength conversion element, for example, a second harmonic generation element having a structure as shown in FIGS. 9 and 10 has been known (see Japanese Patent Laid-Open No. 2-242236). In the harmonic generating elements shown in FIGS. 9 and 10, a light transmissive ferroelectric substrate (LiTaO ₃ , LiNbO ₃ etc.) 1 having a nonlinear optical effect is used.
On the + C plane of 01, periodic domain-inverted layers ZD1 to ZDn for inverting the phase of higher harmonics by Ti diffusion are provided, and further, the domain-inverted layers ZD1 to ZDn are crossed to form a three-dimensional optical waveguide G1. It is formed by doping such as proton exchange. In this harmonic generating element, when light from a light source having a predetermined wavelength is incident and guided as a fundamental wave from one end surface to the other end surface of the optical waveguide G1, in the guiding process, the second wave is introduced into the optical waveguide G1. Harmonics are excited. At this time, when the polarization inversion layers ZD1 to ZDn are formed in the optical waveguide G1 with a period of an even multiple of the coherent length, pseudo phase matching between the excited second harmonic wave and the fundamental wave can be achieved. Thus, the second harmonic can be efficiently emitted from the other end of the optical waveguide G1.

[0003]

By the way, in the second harmonic generation element, the wavelength allowable width Δλ of the actual light source with respect to the designed light source wavelength (fundamental wavelength) of this element is 0.2.
It is as small as ~ 0.3 nm, and the wavelength of the fundamental wave is different from the design wavelength due to changes in the light source wavelength or changes in the light source wavelength or the element temperature. If it deviates from the width Δλ, the conversion efficiency of the second harmonic wave will be significantly reduced. Therefore, as a light source used for the second harmonic generation element, a coherent light source such as a T _i : Al ₂ O ₃ laser capable of precisely changing the light source wavelength has been used.

As described above, conventionally, the light source wavelength is precisely changed with respect to the light source wavelength variation and the element temperature variation so that the variation of the fundamental wave wavelength is always within the wavelength allowable width Δλ of the element. Therefore, as a light source, a large-scale tunable solid-state laser, dye laser, etc. are required.
There is a problem that the light source becomes large in size, heavy in weight, and high in cost.

According to the present invention, when the wavelength of the fundamental wave emitted from the light source is different from the design wavelength due to changes in ambient temperature, the wavelength of the fundamental wave is changed by changing the wavelength of the light source precisely or by changing the temperature of the wavelength conversion element. It is an object of the present invention to provide a wavelength conversion device that allows a light source to have a small size, a light weight, and an inexpensive structure even when the change in the above is always within the wavelength allowable width of the wavelength conversion element.

[0006]

In order to achieve the above object, a wavelength converter according to a first aspect of the present invention emits light of a predetermined wavelength and a wavelength conversion element having an optical waveguide formed of a non-linear optical medium. A light source for guiding the converted wave by guiding it as a fundamental wave in the optical waveguide of the wavelength conversion element, and setting the fundamental wave in the wavelength conversion element in the light source when the oscillation wavelength changes in temperature. It is characterized in that a wavelength-stabilized laser diode having a structure that does not cause mode pop near the wavelength is used.

Further, in the wavelength converter according to claim 2,
When the wavelength of the fundamental wave shifts from the set wavelength to the long wavelength side in the wavelength conversion element, the temperature of the light source is lowered to shift the oscillation wavelength to the short wavelength side, and the wavelength of the fundamental wave changes from the set wavelength to the short wavelength side. It is characterized in that a temperature control means for increasing the temperature of the light source to shift the oscillation wavelength to the long wavelength side is provided when the temperature shifts.

Further, in the wavelength conversion device according to claim 3,
A wavelength stabilizing laser diode having a complex resonator is used as the light source.

[0009]

In the present invention, as the light source for guiding the converted wave by guiding it as the fundamental wave in the optical waveguide of the wavelength conversion element, when the oscillation wavelength changes in temperature, the set wavelength of the fundamental wave in the wavelength conversion element is changed. A wavelength-stabilized laser diode with a structure that does not cause mode pop in the vicinity is used. As a result, the wavelength of the fundamental wave can be accurately controlled and the element temperature can be adjusted so that the change in the wavelength of the fundamental wave in the wavelength conversion element is always within the wavelength allowable width.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of an embodiment of a wavelength conversion device according to the present invention.

The wavelength conversion device of this embodiment comprises a wavelength conversion element 8, a light source 1 for emitting light of a predetermined wavelength for guiding a fundamental wave to the wavelength conversion element 8, and a temperature for changing the temperature of the light source 1. The wavelength conversion element 8 for controlling the light from the light source 1 and the control element 2
And a condenser lens 3 for condensing light. In this embodiment, the wavelength conversion element 8 is configured as a second harmonic generation element, and is embedded in a light transmissive substrate (for example, LiTaO ₃ ) 4 having a non-linear optical effect so as to be a periodically poled layer. RD1 to RDn are formed by proton exchange, annealing, etc. Further, three-dimensional optical waveguide G2 is formed by proton exchange and the like by intersecting each of the polarization inversion layers RD1 to RDn.

The pitch Λ of the polarization inversion layers RD1 to RDn
Is the polarization inversion layer Z of the harmonic generating element shown in FIGS.
Similar to the pitches of D1 to ZDn, it is preset to an even multiple of the coherent length lc according to the following equation.

[0013]

[Formula 1] Λ = 2m1c (m = 1, 2, 3 ...)

Here, the coherent length lc is given by the following equation.

[0015]

[Equation 2] lc = λ / [4 × | n _eff (SHG) −n _eff (F) |]

However, when m is an odd number, each polarization inversion layer R
The width of the layers D1 to RDn may be half the pitch Λ,
When m is an even number, it is desirable that the width is other than that. In _Equation 2, n _eff (F) and n _eff (SHG) are the equivalent refractive indices of the fundamental wave and the second harmonic in the optical waveguide G2, respectively, and λ is the wavelength of the fundamental wave in vacuum. ..

Here, for the coherent length lc, the polarization inversion layers RD1 to RDn are formed at the pitch Λ according to the equation 1 so that the pseudo phase matching condition between the fundamental wave and the second harmonic wave is satisfied. ing.

The temperature control element 2 is, for example, a heater or a Peltier element, and the temperature control element 2 changes the temperature of the light source 1 to control the oscillation wavelength of the light source 1. That is, in the present embodiment, it is intended to use a small, lightweight and inexpensive laser diode as the light source 1. In this case, in general, the laser diode has an oscillation wavelength shifted to a long wavelength side due to a rise in temperature, and Paying attention to the fact that the oscillation wavelength shifts to the short wavelength side when the temperature is lowered, the oscillation wavelength of the laser diode is changed to be the design wavelength (phase matching wavelength) of the wavelength conversion element 8. The temperature control element 2 is used for matching.

By the way, the relationship between the oscillation wavelength and the temperature of a laser diode having a normal Fabry-Perot resonator is shown in FIG.
It becomes like (a). That is, when the resonator length of the laser diode is L, the oscillation center wavelength is λ ₀ , and the effective refractive index is n _eff ′, the longitudinal mode interval Δλ ′ of the laser diode is given by the following equation.

[0020]

## EQU3 ## Δλ '=-λ ₀ ² / (2n _eff ' _.L ) n _eff '= n _eq {1- (λ ₀ / n _eq ). (Dn _eq / dλ ₀ )}

Here, n _eq is the equivalent refractive index of the optical waveguide layer including the active layer of the laser diode. In the case of a laser diode having a Fabry-Perot resonator, since the resonator length L is usually about 300 μm, the longitudinal mode interval Δλ ′ becomes about 0.3 nm when the oscillation wavelength is near 0.8 μm, and the element temperature of the laser diode changes. As a result, the oscillation wavelength causes mode jumps (mode pops) in the longitudinal mode at intervals Δλ ′ of 0.3 nm, and the oscillation wavelength changes stepwise as shown in FIG. Further, as can be seen from FIG. 2A, the oscillation wavelength of this laser diode differs between the time of temperature increase and the time of cooling even at the same temperature, so that hysteresis occurs.

On the other hand, the wavelength conversion element 8 (in the present case, the second
In order to efficiently generate the converted wave (SH light), the harmonic generation element) is said to have a permissible width Δλ of the wavelength shift of the fundamental wave of about 0.2 nm as described above. Therefore, when using a normal laser diode having a Fabry-Perot resonator as the light source 1, the wavelength conversion element 8
When the temperature of the light source 1 is changed and the oscillation wavelength is changed so that the wavelength shift of the fundamental wave guided through the optical waveguide G2 falls within the allowable width Δλ (≈0.2 nm), Mode pops with a width of Δλ or more (Δλ '= 0.3
However, the oscillation wavelength cannot be set and controlled so that the deviation of the wavelength of the fundamental wave falls within the allowable width Δλ. Even if the temperature of the wavelength conversion element 8 is changed so that the wavelength shift of the fundamental wave is within the allowable width Δλ, if the light source 1 causes a mode pop due to a change in ambient temperature, the converted wave is stably generated. Cannot be generated.

In order to overcome the problems as described above, the inventor of the present application has selected a laser diode having a predetermined structure and characteristics from among various laser diodes, which is wavelength-stabilized laser diode. And select this as the light source 1
Was applied to the wavelength converter. The application to this type of wavelength-stabilized laser diode and wavelength conversion device will be described below.

FIG. 3 shows DFB (Distributed)
(Feedback) It is a figure which shows the structural example of a laser diode. This DFB laser diode has a semiconductor substrate 10
An optical waveguide layer 11 including an active layer, a gap layer, and an electrode 12 are formed on the upper surface by crystal growth or the like, and a waveform for modulating the equivalent refractive index of the optical waveguide layer is formed near the active layer or in the active layer itself. The grating structure 13 is formed and oscillates in a single mode or two modes. In this DFB laser diode, the relationship between the oscillation wavelength and the temperature increases or decreases almost monotonously as shown in FIG. 2B, and the mode jumps like a normal laser diode using a Fabry-Perot resonator. And no hysteresis occurs. Therefore, when this DFB laser diode is used as the light source 1, even if the temperature of the light source 1 is raised or lowered by the temperature control element 2, the oscillation wavelength changes substantially linearly with temperature without causing mode jump or hysteresis. In the wavelength conversion element 8, if the design wavelength of the fundamental wave is set within the oscillation wavelength range of this DFB laser diode (corresponding to the temperature range in which oscillation is possible),
By changing the temperature of the DFB laser diode, the fundamental wave and the converted wave (in this case, the second harmonic) can be accurately phase-matched in the wavelength conversion element 8, more specifically, in the optical waveguide G2. .. As a result, the converted wave can always be stably and efficiently converted and generated.

The DFB laser diode is GaAs
/ AlGaAs system, the oscillation wavelength λ is 0.8
In the vicinity of μm, the equivalent refractive index n _eq is about 3.5 to 3.6, and the effective refractive index n _eff ′ is about 4.3 according to Formula 3. At this time, in the single mode oscillation, the temperature change dλ / dT of the oscillation wavelength λ becomes a ratio of about 0.07 nm / ° C. On the other hand, during multimode oscillation of a laser diode having a normal Fabry-Perot resonator, the temperature change dλ / dT of the oscillation wavelength λ is about 2.3 to 3Å / ° C.

FIG. 4 shows another wavelength stable laser diode,
That is, it is a diagram showing a configuration example of a DBR (Distributed Bragg reflection type) laser diode. This DBR laser diode includes an optical waveguide layer 21 including an active layer 25, a gap layer and an electrode 22 on a semiconductor substrate 20.
Are formed by crystal growth or the like, and the optical waveguide layer 2 before and after the active region of the active layer 25 (region where current flows) is removed.
A grating structure 23 is formed on or near 1. This grating structure 23 functions as a mirror of the laser diode, and by selecting the grating period, it oscillates in a single mode and suppresses mode pop of the oscillation wavelength due to temperature change, as in the above-mentioned DFB laser diode. It is possible to obtain the same temperature change characteristic as that of 2 (b), and it is possible to obtain the same effect as when the DFB laser diode is used.

FIG. 5 is a diagram showing a structural example of still another wavelength stable laser diode. In this laser diode, a laser diode 31 having a normal Fabry-Perot resonator and a block 32 are provided on a heat sink 30 with a predetermined distance d (several micrometers to several tens of micrometers) (for example, the heat sink 30). It is provided by being bonded on top). In the laser diode 31, an optical waveguide layer 34 including an active layer, a gap layer and an electrode 35 are formed on a substrate 33, and a light emitting end face 36 is formed.
Is provided with a low-reflection coating layer (coating layer that reduces reflectance). In addition, an end surface 38 of the block 32 facing the laser diode 31 is provided with an optically highly reflective layer having a mirror surface by metal coating or the like.

In the wavelength stable laser diode shown in FIG.
The laser diode 31 itself functions as a normal Fabry-Perot resonator, and also functions as another resonator between the end surface 36 of the laser diode 31 and the end surface 38 of the block 35, which is defined by the distance d, and thus the total of 2 It is constructed as a compound-cavity laser with two resonators. As a result, the longitudinal mode interval Δλ ′ of the oscillation mode is
It is almost determined by the longitudinal mode interval of the short resonator having the resonator length L (that is, the resonator determined by the interval d), and the longitudinal mode interval Δλ ′ is calculated according to the equation 3 to be about 10 in the case of only the ordinary Fabry-Perot resonator. It can be more than doubled. As a result, the oscillation wavelength of this wavelength stable laser diode can be changed in temperature as shown in FIG. 2C, and a sufficiently wide wavelength range (wavelength tolerance of the wavelength conversion element 8 is allowed in the vicinity of the set wavelength of the wavelength conversion element 8. It is not necessary to cause a mode pop in the width Δλ of at least several times. Therefore, by changing the temperature of the wavelength stable laser diode and changing the oscillation wavelength, the optical waveguide G2 of the wavelength conversion element 8 can be changed in the same manner as the laser diode of FIGS.
The fundamental wave and the second harmonic can be accurately phase-matched, and the converted wave can always be stably and efficiently converted and generated. In the above example, two resonators are provided, but instead of this, a combination of one resonator and an etalon or the like can also constitute a compound resonator laser of this kind.

FIG. 6 is a diagram showing an example of the structure of another wavelength stable laser diode, that is, a GC (groove coupling type) laser diode. In this GC laser diode, a groove 43 is formed by dry etching or the like at one place of a normal laser diode having a substrate 40, an optical waveguide layer 41 including an active layer, a gap layer and an electrode 42, and thus, substantially 2 is formed. It is a kind of compound resonator laser having one resonator, that is, two laser diodes 44 and 45. At this time, since the width w of the groove 43 is as small as about the order of wavelength, the two resonators 44 and 45 are coupled to each other,
The laser diode 45 having the shorter cavity length oscillates at the longitudinal mode interval Δλ ′. As a result, the temperature change of the oscillation wavelength can be made to have the characteristic shown in FIG. 2B, and the same effect as that of the laser diode of FIGS. 3 and 4 can be obtained. In the above example, the groove 43 is formed at one location of the ordinary laser diode, but instead of this, two Fabry-Perot resonator type laser diodes each having two cavity lengths, short and long, are placed close to each other. You may arrange | position and be comprised.

FIG. 7 shows another wavelength-stabilized laser diode, that is, ITG (Integrated Twin).
(Guide) It is a figure which shows the structural example of a laser diode. This ITG laser diode has an optical waveguide layer 5 including an active layer.
1 and the optical waveguide layer 52 for emitting light are arranged in parallel, and are coupled and coupled to each other. In such a configuration,
In the longitudinal and transverse modes of the optical waveguide layer with a short cavity length and the optical waveguide layer with a long cavity length, only the common mode or a mode having an effective refractive index close to it is excited,
Actually, it becomes the vertical mode interval when the resonator length is short,
The oscillation wavelength is shown in FIG. 2 (c) or FIG.
It becomes like (b). As a result, the same effect as in FIG. 5 or FIGS. 3, 4, and 6 can be obtained.

FIG. 8 is a diagram showing a structural example of still another wavelength stable laser diode. The laser diode shown in FIG. 8 is called a surface emitting laser diode and emits light in a direction perpendicular to the substrate 60. This type of laser diode is used for DBR in addition to the usual Fabry-Perot type.
It is also possible to manufacture the active layer 61 formed on the substrate 60 in the thickness direction, although
A short resonator having a film thickness order (several μm) is formed.
This allows the longitudinal mode interval to be wide enough,
The temperature change characteristic of the oscillation wavelength as shown in FIG. 2B or FIG. 2C can be obtained, and the same effect as that of each wavelength stable laser diode described above can be obtained.

In the above-mentioned embodiment, for example, LiTaO ₃ is used for the substrate 4 of the wavelength conversion element 8, but the present invention is not limited to this, and a nonlinear optical material such as LiNbO ₃ or KTP can be used. When a material such as LiNbO ₃ or KTP is used for the substrate 4, it is not always necessary to form the periodic domain-inverted layers RD1 to RDn on the substrate 4, and it is only necessary to form the optical waveguide G2 on the substrate 4 to guide the fundamental wave. It is also possible to achieve phase matching between the wave mode and the guided mode of the converted wave. Further, a substrate is formed of a material other than LiTaO ₃ , LiNbO _3, etc., and LiTaO ₃ , Li
It is also possible to form a material such as NbO ₃ by sputtering, liquid layer growth, etc., and make it function as an optical waveguide.

In any of the above cases, the wavelength stable laser diode described above can be used as the light source 1, and the same effect as described above can be obtained.

As described above, in this embodiment, when the oscillation wavelength changes with temperature, a predetermined wavelength range near the set wavelength of the wavelength conversion element 8 (at least several times the wavelength allowable width Δλ).
Since a wavelength stable laser diode having a structure that does not cause mode popping is used as the light source 1, the wavelength of the fundamental wave can be accurately controlled in the wavelength conversion element 8 by changing the oscillation wavelength with temperature. Even if the temperature changes, the oscillation wavelength changes smoothly without mode pop, so that the temperature of the wavelength conversion element 8 can be changed to change the set wavelength to the vicinity of the oscillation wavelength. This allows the difference between the oscillation wavelength and the set wavelength to be within the allowable width Δλ.
It is possible to always keep good pseudo-phase matching between the fundamental wave and the converted wave by suppressing it within (≈0.2 nm), thereby always stabilizing and increasing the conversion efficiency to the converted wave. You can

That is, when the wavelength of the fundamental wave shifts from the set wavelength to the long wavelength side in the wavelength conversion element 8, the temperature control element 2 lowers the temperature of the light source 1 to shift the oscillation wavelength to the short wavelength side. When the wavelength of the fundamental wave deviates from the set wavelength to the short wavelength side, the temperature control element 2 causes
Although the temperature of the light source is raised to shift the oscillation wavelength to the long wavelength side, mode pop does not occur in the oscillation wavelength when the temperature rises and falls, and therefore the change in the fundamental wavelength is within the wavelength tolerance range. The wavelength of the fundamental wave can be accurately controlled so that Further, although only the temperature control element 2 is shown as the temperature control means in FIG. 1, it is possible to automatically detect the change of the fundamental wave in the wavelength conversion element and control the temperature control element 2 according to the change. The temperature control means may be configured as a simple temperature control device.

When the oscillation temperature of the light source 1 is deviated from the set wavelength of the wavelength conversion element 8 due to changes in the ambient temperature and the like,
Instead of changing the temperature of the light source 1, the temperature of the wavelength conversion element 8 can be changed to match the set wavelength (phase matching wavelength) at which the harmonic is generated with the oscillation wavelength of the light source 1.
In this case, conversely to the temperature change of the laser diode, when the temperature is raised, the wavelength for phase matching tends to be short, and when the temperature is lowered, the wavelength for phase matching tends to be long.

[0036]

As described above, according to the first aspect of the invention, the oscillation wavelength of the light source for guiding the converted wave by guiding it as the fundamental wave in the optical waveguide of the wavelength conversion element changes with temperature. When using a wavelength-stabilized laser diode with a structure that does not cause mode pop near the set wavelength of the fundamental wave in the wavelength conversion element, a compact, lightweight, and inexpensive light source can be used, while the ambient temperature changes. Even if the oscillation wavelength changes smoothly without mode pop, the temperature of the wavelength conversion element can be changed and the set wavelength can be accurately tracked to the vicinity of the oscillation wavelength and converted into a converted wave. Efficiency can always be stabilized and increased.

Further, according to the second aspect of the present invention, since the temperature control means is provided in the light source, the fundamental wave is kept so that the change of the wavelength of the fundamental wave in the wavelength conversion element is always within the allowable wavelength range. The wavelength can be controlled more accurately, and the conversion efficiency of the converted wave can be constantly stabilized and increased.

According to the third aspect of the invention, since the wavelength-stabilized laser diode having a complex resonator is used as the light source, the light source has a relatively simple structure and can be manufactured at low cost. Therefore, an inexpensive light source can be used.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of a wavelength conversion device according to the present invention.

2 (a), (b) and (c) are diagrams showing a temperature change of an oscillation wavelength of a light source.

FIG. 3 is a diagram showing a configuration example of a DFB laser diode.

FIG. 4 is a diagram showing a configuration example of a DBR laser diode.

FIG. 5 is a diagram showing a configuration example of a composite resonator laser.

FIG. 6 is a diagram showing a configuration example of a GC laser diode.

FIG. 7 is a diagram showing a configuration example of an ITG laser diode.

FIG. 8 is a diagram showing a configuration example of a surface emitting laser diode.

FIG. 9 is a perspective view of a conventional harmonic wave generating element.

FIG. 10 is a plan view of the harmonic generating element of FIG.

[Explanation of symbols]

 1 light source 2 temperature control element 4 substrate 8 wavelength conversion element RD1 to RDn polarization inversion layer G2 optical waveguide Λ polarization inversion layer pitch

Claims (3)

[Claims] 1. A wavelength conversion element having an optical waveguide formed of a non-linear optical medium, and for emitting light of a predetermined wavelength and guiding the light as a fundamental wave in the optical waveguide of the wavelength conversion element to excite the converted wave. And a light source of,
A wavelength conversion device characterized in that a wavelength-stabilized laser diode having a structure that does not cause mode pop near the set wavelength of the fundamental wave in the wavelength conversion element when the oscillation wavelength changes with temperature. 2. The wavelength conversion device according to claim 1, wherein
When the wavelength of the fundamental wave shifts from the set wavelength to the long wavelength side in the wavelength conversion element, the temperature of the light source is lowered to shift the oscillation wavelength to the short wavelength side, and the wavelength of the fundamental wave changes from the set wavelength to the short wavelength side. The wavelength conversion device is provided with temperature control means for increasing the temperature of the light source and shifting the oscillation wavelength to the long wavelength side when the temperature shifts. 3. The wavelength conversion device according to claim 1, wherein the light source is a wavelength stabilization laser diode having a complex resonator, in particular.