JP3489720B2 - Tunable quasi-phase matching element - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for realizing a nonlinear optical medium plate that efficiently converts wavelength. In particular, the present invention provides a quasi-phase matching element that can be replaced as it is with an angle phase matching crystal that has been widely used in conventional measuring instruments, and that has a higher efficiency.

[0002]

2. Description of the Related Art A wavelength conversion technique using a non-linear optical medium has now been applied to a wide range of fields.
Of these, what the present invention is particularly concerned with is application to a measuring instrument, and as an example thereof, a correlator used for observing an ultrahigh-speed optical signal in the picosecond to femtosecond region has been taken up. The principle of the correlator and the usage condition of the nonlinear optical medium therein will be described below.

For ultrafast optical signals in the picosecond to femtosecond range, there is no photodetector with sufficient time resolution, and therefore, observation by a correlator is currently being performed. In such a correlator, a signal light to be measured and an optical pulse to be used as a reference are made incident and focused in a non-linear medium, and the integrated intensity of the generated light is measured as a time relationship (delay time) between the signal light to be measured and the reference light pulse. It is measured as a function of. The time resolution in this case is determined by the response time of the nonlinear medium and the time width of the reference light pulse, not by the response time of the photoelectric converter or the like.

Of these, when the reference light pulse is called optical sampling using an optical pulse that is synchronized with the signal light to be measured and has a sufficiently short time width compared to the signal to be measured, the response time of the nonlinear medium is sufficient. In that case, the intensity waveform of the signal light to be measured can be directly obtained. In many cases, the signal light to be measured itself is used as the reference light pulse, the obtained signal is called autocorrelation, and the device that measures it is called an autocorrelator (autocorrelator). With, the waveform of the signal under measurement can be estimated.

There are two factors that determine the response time of another nonlinear medium. The first one is the followability when the nonlinear polarization changes in response to the change of the incident / focused input electric field. In the non-resonant non-linear effect, which is usually used without actual transition, the response of non-linear polarization is the electron orbit time around the nucleus, that is, about 1 to 2 fs, and the part related to this factor shows a practical instantaneous response. I can see it. The second one relates to phase matching. In order for the non-linear polarization generated in various places in the medium of finite length to contribute to the output photoelectric field in-phase, it is necessary that the phase of the non-linear polarization is exactly aligned with the propagation phase of the output light.

Since the phase of the non-linear polarization in each place is defined by the propagation phase of the input light, this condition can be finally reapplied to the relationship between the propagation phases of the input light and the output light. That is, when the output light of the wavelength ω ₃ (ω ₃ = ω ₁ ± ω ₂ ) is obtained from the two input lights of the angular frequencies ω ₁ and ω ₂ , the phase mismatch Δk Δk = k ₁ ± k ₂ −k ₃ ... If (1) is zero, the contribution of nonlinear polarization from various parts of the medium is strengthened and the output light becomes maximum. Here, in the composite, the + case corresponds to the sum frequency generation, and the − case corresponds to the difference frequency generation. Furthermore, using the fact that each wave number k _i is expressed as k _i = n _i ω _i / c using the refractive index n _i and the speed of light c in vacuum, the above phase mismatch becomes zero. That is, in order to achieve the phase matching, it is necessary that all three refractive indices are equal or that the refractive index for the maximum angular frequency is sandwiched between the other two. For example, the relationship is ω ₁ <ω ₃ <ω ₂ with respect to the angular frequency ω ₃ .

In this case, in the latter case, since the refractive index monotonically increases with respect to the frequency in the wavelength region where the medium is transparent, phase matching can never be achieved as long as a single refractive index is considered. However, in an anisotropic optical crystal, there is a birefringence phenomenon in which the refractive index changes depending on the direction of the optical field. Utilizing this opens the possibility of achieving phase matching, and is called angular phase matching. In the birefringent optical crystal having the principal refractive indices n _x , n _Y , and _nz , the angle (direction) vector s (V
_The refractive index n felt by the light propagating to _ec s) is Fresnel's velocity equation (2) [Equation 1]

[Equation 1] And two refractive indices are generally obtained for one propagation direction. These different refractive indices correspond to linearly polarized waves that are orthogonal to each other. In order to achieve phase matching, at least the light for the maximum angular frequency must be propagated in the polarized wave giving the low refractive index, and one of the remaining two lights must be propagated in the polarized wave giving the high refractive index. Is. For the remaining light, there is a choice between using a high refractive index and a low refractive index, which are called type I and type II phase matching, respectively. In this way, after selecting the combination of the polarizations of the respective lights, the common propagation direction V _ec s where the phase mismatch of the previous expression (2) becomes zero is searched.

With the above, the phase mismatch can be reduced to zero
Once achieved, one can further consider the band of phase matching.
it can. Under the plane wave approximation, the angular frequency ω₃Output light intensity
Degree P ₃The phase mismatch Δk and the crystal length L depends on         P₃∝L²sinc²(ΔkL / 2) (3) Follow In this formula (3), the function sinc x is (sin x) / x
Represent This output light intensity is half the value when the phase mismatch is zero.
The value of ΔkL / 2 that becomes the minute is 1.392, enter this
If you re-adjust to the band (full width at half maximum) about light,         ΔΩ_i= 4 × 1.392 | (τ_i−τ₃) L |^-1      … (4) Is obtained. Where τ₁, Τ₂, Τ₃For each light
Is the group delay time per unit length of the medium. Absolutely in the formula
The values in the figure are the values that are received when the input light and output light respectively pass through the crystal.
It is the difference in the group delay time, and may be called group delay mismatch.
it can. By convention, we have the absolute value of this group delay mismatch.
The response time of the nonlinear medium to the incident light
is doing.

In one non-linear medium, if the medium length L in the above formula is shortened, the response time becomes short, and at the same time, if the viewpoint is changed, the band of convertible input light is expanded. Go. Therefore, if a very short medium is used, it is theoretically unnecessary to re-adjust the phase even if the wavelength of the input light changes. However, according to the equation (3), as the medium length is shortened, the output light intensity is rapidly reduced in proportion to the square of the medium length, and the measurement sensitivity of the correlator is lowered. More specifically, this square dependence depends on the propagation of the beam diameter in the medium without change:
That is, it is a property that holds in a medium having a channel-type waveguide structure, and in the case of propagation in a bulk medium, at least theoretically, the beam diameter is changed to an optimum beam diameter proportional to 1/2 of the medium length. By doing so, the dependence of the output light intensity on the medium length can be relaxed to the first power, but in any case, the shortening of the medium length does not reduce the measurement sensitivity, and excessive shortening is not desirable. .

Therefore, the longest medium length that gives the necessary time resolution is selected according to the measured optical signal to be measured,
When the wavelength of the input light changes, the method of adjusting the propagation direction according to the change and re-matching the phase is generally taken. For the adjustment of the propagation direction, the incident optical axis is usually fixed, and the non-linear optical medium is rotated to find an appropriate propagation direction. From the above description, the use condition of the non-linear optical medium in the current correlator is summarized as follows: the medium is held on a pedestal with a rotation mechanism to adjust the propagation direction to achieve angular phase matching, The medium and the pedestal are detachably joined so that crystals having different thicknesses (thicknesses) can be exchanged.

Recently, however, a quasi-phase matching technique has been noticed in which a structure in which a nonlinear constant is periodically changed is artificially formed in a nonlinear medium, and the periodicity is utilized to achieve phase matching. Has been done. According to this quasi phase matching,
Even in a non-linear crystal such as lithium niobate that has been conventionally used, a large component such as d _zzz, which cannot be used in the angular phase matching using birefringence, can be utilized among the non-linear constant tensors, and higher efficiency can be achieved. It is possible to perform wavelength conversion by using, or it is possible to perform phase matching in a wavelength range where angular phase matching cannot be achieved. In addition, as the polarization of the input and output light can be taken in the direction of the principal axis of the crystal, problems such as the limitation of the interaction length due to the birefringence phenomenon generally associated with the angular phase matching or the deterioration of the output light beam shape. Can be avoided. In addition, the possibility of phase matching is newly opened for material systems such as semiconductors and glass where angular phase matching was originally impossible.

For this quasi-phase matching, it is ideal that the non-linear constant changes sinusoidally, but in reality it is difficult to make such a change. Therefore, a step-like non-linear constant change is used. Has been. The method of creating this nonlinear constant change varies depending on the material. For example, in the case of a ferroelectric crystal such as lithium niobate, the nonlinear constant is changed by reversing the polarization by applying an external electric field. Create a structure in which the sign of changes periodically. For semiconductor materials, a structure in which the sign of the nonlinear constant also changes periodically is formed by a method such as bonding or by preparing a substrate whose surface state changes periodically and then performing epitaxial growth. For glass and organic molecules, using means such as periodic thermal polarization and periodic etching,
The part of the non-linear constant that is non-zero and zero is created.

The effective non-linear constant d _eff that accompanies such a step-like non-linear constant change is generally given as d _eff = (d _s / π) sin (πD) (5). Here, d _s is the total swing width of the nonlinear constant. If the nonlinear constant swings between d and −d due to the reversal of the sign, 2 d, and the nonlinear constant swings between d and zero. If it is, the value of d is taken. Further, D is a ratio occupied by a portion having a high nonlinear constant in one cycle. It can be seen from the equation (5) that D becomes 1/2 when the level of the nonlinear constant repeats exactly 1: 1 and the largest effective nonlinear constant is obtained. The change period Λ of the nonlinear constant required for the quasi-phase matching is obtained from the condition that the phase mismatch of the above equation (1) is just canceled by the wave number corresponding to the period, and is given as follows. Λ = 2π / | k ₁ ± k ₂ −k ₃ | (6) Again, in the composite, + is for sum frequency generation and − is for difference frequency generation in the composite.

FIG. 5 shows a lithium niobate crystal.
The inversion period of the nonlinear constants required in the case of second harmonic generation _{(ω 1 = ω 2 = ω} 3/2), shows the crystal temperature as a parameter. From this, it is possible to change the wavelength at which the quasi-phase mismatch can be obtained in a wide range by changing the inversion period.
It can be seen that even if the temperature is changed by 60 ° from 70 ° C to 70 ° C, the phase matching wavelength moves only 5.5 nm. As described above, the quasi-phase matching crystal has high wavelength conversion efficiency, but has poor wavelength tunability.As a result, its application is limited to the aspect of wavelength conversion for a laser light source with a fixed wavelength, and it is tuned to the wavelength of the measured light. At present, it is not applied to measuring instruments such as a correlator that needs to be performed.

Therefore, in order to change the wavelength of the quasi-phase matching element, a non-linear optical medium plate provided with fan-out polarization so that the period changes in the direction perpendicular to the optical path in the element. The method used is Optics Letters, Volume 16 (1991).
・ Published on pages 375-377. FIG. 6 is a diagram showing this conventional example. However, in the above-mentioned document, a large number of channel waveguides parallel to each other are formed on the plate, and the incoming and outgoing light propagates through one of them, but in that case, the wavelength can be changed only discretely. Therefore, this figure shows a configuration in which the channel waveguide is removed and the incident / emitted light is modified so as to propagate within the bulk thickness.

In this example, the nonlinear optical medium plate 601
Is a flat-polished entrance end face 604 and exit end face 605.
With. Incident light enters the incident end face 604 along the incident optical axis 602 and propagates inside the plate along the in-plate optical axis 606. The converted light generated at various points on the in-plate optical axis 606 propagates in the plate along the in-plate optical axis 606, is emitted from the emission end face 605, and after emission, travels along the emission optical axis 603. In this example, the incident end face 604 and the emitting end face 605 are both in the plate optical axis 6
As a result of being perpendicular to 06, the incident optical axis 602 and the outgoing optical axis 60
3 and the in-plate optical axis 606 are coaxial with each other. The non-linear optical medium plate 601 has an in-plate optical axis 606.
A fan-like polarization reversal whose period changes in the direction perpendicular to is built in. Further, the nonlinear optical medium plate has an in-plate optical axis 60.
It is held on a pedestal with a moving mechanism so that it can move in parallel in a moving direction 611 perpendicular to the direction 6.

Now, the non-linear optical medium plate 601 is made of lithium niobate crystal, the central period is 18.85 μm, and the period changes by 1.07 μm per 1 mm in the direction perpendicular to the in-plate optical axis 606. It is assumed that the polarization inversion is applied. In this case, when the crystal temperature is 20 ° C., the wavelength at the center is 1.545 μm, and the phase matching wavelength can be changed by 0.04 μm every 1 mm movement along the movement direction 611. In order to see this characteristic, in FIG. 5, the incident light wavelength corresponding to the inversion period on the vertical axis at the temperature may be read. Thus, for example, the moving direction 61
If the movement amount of ± 1 mm can be taken for 1, 1.505
Wavelengths from μm to 1.585 μm can be covered by a single lithium niobate crystal plate.
Sufficient wavelength tunability can be obtained for measurement of optical signals in m-band optical communication.

[0018]

However, the conventional wavelength tunable quasi phase matching element described above has the following problems. The wavelength tunable quasi-phase matching element described above requires a mechanism for translating the nonlinear optical medium plate in a direction perpendicular to the in-plate optical axis in order to change and adjust the phase matching wavelength. This is not a problem when a correlator incorporating a wavelength tunable quasi phase matching element is newly designed and manufactured.

However, this is a major obstacle to applying the wavelength tunable quasi-phase matching element to the correlometers that are already in circulation / deployment for the purpose of improving the sensitivity. This is because, as described above, in the current correlator, only the mechanism for rotating the nonlinear optical medium plate is installed in order to achieve the angle phase matching. This is because it does not have a parallel movement mechanism in a direction perpendicular to the optical axis. In the current correlator, the non-linear medium and the holding / rotating mechanism of the non-linear medium occupy a very small amount in terms of space and price, and are actually quite small, being less than 10%. is there.
In order to apply the conventional wavelength tunable quasi-phase matching element, it is necessary to change such a small part, and as a result, when the whole correlator device is newly designed and manufactured, it is a waste of time and resources. I don't get. On the contrary, if a wavelength tunable quasi-phase matching element that can change and adjust the phase matching wavelength by using the rotation mechanism provided in the conventional correlator is realized, replace it with the non-linear medium of the current correlator as it is. Therefore, the measurement sensitivity can be improved at an extremely low cost.

Therefore, the present invention provides a wavelength tunable quasi phase matching element that can change and adjust the phase matching wavelength by a rotating mechanism instead of a parallel moving mechanism, and as a result, can directly replace the conventional angle phase matching element. The purpose is to do.

[0021]

The present invention which achieves the above object has the following matters for specifying the invention. 1) A wavelength tunable quasi-phase matching element made of a plate-shaped non-linear optical medium, which is converted into an incident end face that is a side face on which converted light is incident and a side opposite to the incident end face. with the outgoing end face a side where light is emitted, a vertically with respect to the optical path for propagating light incident on the said element, and the upper surface of the element
In the wavelength tunable quasi-phase matching element in which alternating polarization is applied so that the period changes linearly, the converted light is provided with a rotation center on a line extending the propagation optical path before reaching the incident end surface, rotatable der in a two-dimensional plane wherein the device comprises a top surface of the device with respect to the rotation center is, the shortest distance from the center of rotation to the incident end face r _0, the object converted light of the nonlinear optical medium Where n is the refractive index with respect to, the shape of the incident end face is a polar coordinate display in which the radius vector with respect to the rotation center is r and the azimuth angle is θ, and r = r ₀ (n−1) / (n cos θ−1 ) Is a hyperbola. 2) In the above 1), the radius (n = n is about the point (r = nr ₀ , θ = 0) in the polar coordinate display so that the shape of the incident end face approximates the hyperbola in the shape of an arc near its apex. -1) It is characterized by having an arc shape of r ₀ . 3) In the above 1) or 2), when there are two converted lights, a rotation center is provided on the bisector of each propagation optical path before each converted light reaches the incident end face,
3. The wavelength tunable quasi phase matching element according to claim 1, wherein an average value of refractive indexes of the respective non-linear optical media with respect to converted light is n. 4) In the above 1), 2), or 3), the shape of the exit end face corresponds to the shape of the entrance end face that is translated in parallel along the optical path along which the incident light propagates. . 5) In any one of 1) to 4) above, an antireflection film for the converted light is provided on the incident end face. 6) In any one of 1) to 5) above, an antireflection film for the converted light is provided on the emission end face. 7) One of the above 1) to 5) is characterized in that it is a slab waveguide so that the light propagating through the variable wavelength quasi phase matching element is confined within a two-dimensional plane.

[0022]

[0023]

[0024]

[0025]

[0026]

[0027]

The nonlinear optical medium plate generally has a refractive index greater than 1. For example, in the above-mentioned lithium niobate crystal plate, the refractive index n for light having an electric field oriented in the optical axis direction, which is usually used in quasi-phase matching, is about 2.14 in the 1.5 μm wavelength band. When light is obliquely incident on an optical medium having such a large refractive index, the optical axis is bent at the incident interface according to Snell's law. The basic idea of the present invention is to utilize this phenomenon to convert the rotation of the quasi-phase matching element with respect to the incident optical axis into translation of the optical axis within the element.

[0029]

BEST MODE FOR CARRYING OUT THE INVENTION Here, an embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a diagram showing the configuration of the variable wavelength quasi-phase matching element of the present invention. A non-linear optical medium plate 101, which is a flat plate, is provided with an incident end face 104 and an outgoing end face 105 that are shaped and polished into a shape described later. . Incident light is incident on the incident end surface 104 along the incident optical axis 102,
It propagates in the plate along the in-plate optical axis 106. The converted light generated at various points along the in-plate optical axis 106 travels inside the plate.
Propagates along the output optical axis 103, exits from the output end surface 105, and travels along the output optical axis 103 after the output. The non-linear optical medium plate 1
A fan-shaped polarization whose period changes in the direction perpendicular to the in-plate optical axis 106 is given to 01. The nonlinear optical medium plate is held on a pedestal equipped with a rotation mechanism so that it can rotate within the same flat plate plane about a rotation center 107 on the extension line of the incident optical axis. In the present invention, it is necessary that this rotation center 107 is not on the incident end face 104.

FIG. 1 (a) shows a condition when tuning to the phase matching wavelength in the center of the variable range. In this case, the incident end face 104 is perpendicular to the incident optical axis 102 at the incident point with respect to the nonlinear optical medium plate 101. As a result, the in-plate optical axis 106 is also perpendicular to the incident end face. Further, at the emission end face 103, at the emission point in this case, as a result of the emission end face being perpendicular to the in-plate optical axis, the emission optical axis 103 is also perpendicular to the emission end face. As a result of these, in the present state, the incident optical axis 10
2, the output optical axis 103, and the in-plate optical axis 106 are coaxial with each other. Here, the straight lines on which these three ride are named the vertical symmetry axes 109 of the nonlinear optical medium plate of the present invention.

FIG. 1B shows the nonlinear optical medium plate 10 described above.
1 shows a state in which 1 rotates counterclockwise around the rotation center 107 by a rotation angle 108. Since the center of rotation is not on the incident end face 104, as a result of the rotation, the incident point moves on the incident end face. At the incident point after the movement, the incident optical axis 102 is obliquely incident on the incident end face 104, and as a result, the incident optical axis is bent and becomes the in-plate optical axis 106. In the present invention, the shape of the incident end face 104 is selected so that the refracted in-plate optical axis 106 is always parallel to the symmetry axis 109. Since the non-linear optical medium plate 101 is provided with a fan-shaped polarization so that the period changes in a direction perpendicular to the in-plate optical axis 106, that is, the symmetry axis 109, the in-plate optical axis 106 described in the previous stage. As a result of the parallel movement of, the period of polarization along the in-plate optical axis changes, and eventually the phase matching wavelength changes. Thus, the object of the present invention to change / adjust the phase matching wavelength by the rotating mechanism is realized.

The shape of the emission end face 105 is not involved in the above basic operation of the present invention. However, assuming the use of the element of the present invention in which the conventional angle phase matching element is directly substituted, it is desirable that the incident optical axis 102 and the outgoing optical axis 103 are always kept parallel. In order to impart this property, the shape of the emitting end face 105 may be selected so as to match the incident end face 104 with the above-mentioned symmetry axis, or the same as the parallel translation along the in-plate optical axis 106. . At this time, both ends of the in-plate optical axis, that is, the entrance point and the exit point become corresponding points on the respective end faces, and as a result, the angle formed by the in-plate optical axis and the exit end face is the It is always equal to the angle formed. Here, ignoring the difference in the refractive index of the nonlinear optical medium plate due to the difference in the wavelengths of the converted light and the converted light, refraction just opposite to the incident point occurs at the emission point, and eventually the emission optical axis 103 is incident. It is always kept parallel to the optical axis 102.

As described above, the present inventor analytically derived the incident end face shape such that the in-plate optical axis 106 refracted at the incident point is always parallel to the symmetry axis 109. The derivation will be outlined below with reference to FIG. As described above, in actual use, the incident optical axis is fixed, and the nonlinear optical medium plate is rotated around the center of rotation, but for the purpose of deriving the incident end face shape, the nonlinear optical medium is used. It does not matter if the medium plate is fixed and the incident optical axis is rotated around the center of rotation. Figure 2
In this way, the incident optical axis 202 is rotated about the rotation center 207.
Around the symmetry axis 209 is shown rotated by an angle θ.

The shape of the incident end face 204 is changed to the rotation center 207.
It is expressed as r (θ) using the two-dimensional polar coordinates where r is the radius vector and θ is the azimuth angle. As orthonormal system representing any vector at each point on the plane, radial unit vector V _ec e _r and the azimuth angle direction of the unit vector V _ec E.theta
To adopt. At the point of incidence, the angles φ and φ ′ formed by the incident optical axis and the in-plate optical axis with the normal vector V _ec n are
It is connected by the well-known Snell's law, sin φ = sin φ ′. Here, n is the refractive index of the above-mentioned nonlinear optical medium plate. This equation uses the unit vectors V _ec s and V _ec s ′ in the directions of the incident optical axis and the in-plate optical axis, and is a vector equation V _ec s × V _ec n = r · V _ec s ′ × V _ec n ... ( It can be expressed in the form of 7). In general, the normal vector V _ec n of the curve r (theta), using the preceding V _ec e _r and V _ec E.theta, and the derivative r '= dr / dθ, the following equation (8) Number 2]

[Equation 2] Can be expressed as Subsequently, the requirements of the present invention will be taken into consideration. First, the incident optical axis V _ec s is, in this case, since the vector directed to the rotation center, the condition _{V ec s = -V ec e r} ... (9) is apparent. Further, in the case of the present invention, the in-plate optical axis _Vec
s'should be parallel to the axis of symmetry (baseline). This condition is expressed by equation (10) [Equation 3].

[Equation 3] give.

The above three equations (8), (9), (10)
By substituting [Equation 3] into the equation (7) and performing an elementary operation, a differential equation r ′ = rn sin θ / (n cos θ−1) to be satisfied by the end surface shape r (θ) is obtained. ) Is derived. It is easy to solve this differential equation, and as a result, a hyperbola represented by r = r ₀ (n−1) / (n cos θ−1) (12) is obtained as the end face shape. Here, the integration constant r ₀
If a geometrical meaning is given to, it can be said that it is the shortest distance from the rotation center 207 to the incident end surface 204.
This is equal to the displacement of the intersection of the symmetry axis 209 and the incident end surface 204 (that is, the apex of the hyperbola) from the rotation center 207.

Up to this point, in FIGS. 1 and 2, the center of rotation is
The case has been shown where it is deeper than the incident end face when viewed from the incident optical axis. In this case, the expression (12) is a concave incident end face which is recessed toward the incident optical axis 102 as seen in both figures. In a conventional correlator using an angular phase matching element, the center of rotation is usually located at the center in the thickness direction of the nonlinear optical crystal. Will be done. However, the situation covered by equation (12) is wider, and it is also effective when the center of rotation is in front of the incident end face as viewed from the incident optical axis, that is, outside the element. In this case, r and r
Both ₀ may be negative numbers. Thus, the incident end face obtained in this case has a convex shape protruding toward the incident optical axis, contrary to the above.

The change of the azimuth angle is converted into a parallel movement of the in-plate optical axis 206, that is, an optical axis displacement 210 by the end face shape thus guided. The functional relationship between the azimuth angle θ and the optical axis displacement y is clearly y = r sin θ, and if this is written down explicitly, y / r ₀ = (n−1) sin θ / (n cos θ−1). (13) is obtained.

Since there is no demand for producing a hyperbolic type end face in the current optical component manufacturing industry, mass production technology is incomplete, and therefore the end face shape given by the above equation (12) representing the hyperbola is
Strictly forming and polishing is expensive. A too expensive non-linear optical medium plate, even if its effect is clear, cannot be expected to become popular in general and ends up being industrially worthless. Therefore, it is necessary to study a method of approximating the above-mentioned hyperbolic end face with a shape that can be industrially manufactured at a lower cost. Therefore, in the non-linear optical medium plate of the present invention, paying attention to the fact that the incident point is limited to the vicinity of the apex of the hyperbolic incident end face, and in the vicinity of the apex, the hyperbola can be approximated by an arc. it can. The approximate shape at this time is a part of the inscribed circle at the apex of the hyperbola given by the above equation (12), and its radius ρ is ρ = (n−1) r ₀ (14 ) And it is sufficient. Such an arc end surface can be created and polished using a cylindrical polishing tool, and can be realized at a sufficiently low cost, as can be seen from the fact that a large number of cylindrical lenses are currently manufactured by a similar processing method.

<Example> Nonlinear optical medium plate 101 of FIG.
Similarly to the conventional example, a central period Λ ₀ is 18.85 μm, and ΔΛ = 1.07 μ per 1 mm in the direction perpendicular to the in-plate optical axis 106.
A fan-like polarization reversal was performed so that the period was changed by m.
In this embodiment, the shape of the emission end face 105 is the incident end face 10
4 was formed so as to correspond to the shape in which it was translated by 2 mm along the vertical axis of symmetry 109. As a result, the length of the in-plate optical axis, that is, the propagation distance L in the crystal is 2 mm. At this time, the group delay mismatch amount, which is a measure of the response time of the nonlinear optical medium plate, is (τ
₃₋ τ ₁ ) L = 0.6 ps. In FIG. 1, the center of rotation 107 is placed at the center of the nonlinear optical medium plate 101 (the bisector of the in-plate optical axis on the axis of symmetry 109) for convenience of understanding. 107 can be placed at any point on the extension of the incident optical axis 102. It has already been described above that it may be placed outside the nonlinear optical medium plate. In this embodiment, the rotation center 107 is placed on the emitting end face 105. As a result, the shortest distance r ₀ from the rotation center 107 to the incident end surface 104 is 2 mm, which is equal to the propagation distance L in the crystal.

As is apparent from the radius of curvature at the apex given by equation (14), the radius of curvature at each point of the hyperbola given by equation (12) is proportional to the shortest distance r ₀ of the rotation center.
When the radius of curvature of the end face is large, the processing is easy in the case of the mm scale involved in the present invention, and the undesirable lens effect on the end face can be reduced. Therefore, it is recommended to make r ₀ large. The method of placing the rotation center 107 on the emission end face 105 as in this embodiment is r ₀ compared to the case of placing the rotation center at the center of the nonlinear optical medium plate.
Can be doubled, and further, when the non-linear optical medium plate is attached, it is easy to visually align with the rotation center (rotation axis).

FIG. 3 is a diagram showing wavelength tunable characteristics obtained by this embodiment. The left vertical axis of FIG. 3 is a scale showing the right side of the equation (13) that gives the relationship between the rotation angle and the optical axis displacement. The reading on this scale is normalized by r ₀ , and therefore gives a value that is generally applicable to a quasi-phase-matched lithium niobate crystal having an incident end surface shape according to the present invention. From this, in the hyperbolic end face, ± 0.65r ₀ is obtained as the amount of movement of the in-plate optical axis along with the rotation angle change of ± 30 °, and even if the end face shape is replaced by a cheaper arc, ± It can be seen that a movement amount of 0.45r ₀ can be obtained.

In FIG. 3, a tangent line that touches both the hyperbolic end face and the arc end face in the vicinity of the rotation angle of zero is added.
This tangent line is represented by y / r ₀ = θ. Since this does not depend on the refractive index of the nonlinear optical medium plate, it is the most common one that is common to all wavelength tunable quasi phase matching elements according to the present invention. When the reading y / r _{0 on the} left vertical axis is obtained, the shortest distance r _{0 of the} center of rotation and the median Λ of the polarization inversion period
_{Using 0} and the rate of change ΔΛ, the polarization inversion period Λ on the in-plate optical axis of the device of this example is obtained by Λ = Λ ₀ + (y / r ₀ ) r ₀ ΔΛ (15). Crystal temperature 20 with respect to this polarization inversion period
The phase-matched wavelength at ° C is obtained, and the scale on the right vertical axis in Fig. 3 is created. From the above, it can be seen from the hyperbolic end face that the rotation angle changes ± 30 ° from 1.492 μm to 1.598 μm.
It can cover up to wavelengths. Even in the case of a substitute end face formed by a circular arc, wavelength variability between 1.510 μm and 1.580 μm can be obtained. In this way, with any of the end face shapes, wavelength tunability sufficient for measuring an optical signal in the current 1.5 μm band optical communication is realized. Further, it goes without saying that if the change rate ΔΛ of the polarization inversion period is made larger, wavelength tunability can be obtained in a wider range.

The tunable quasi-phase matching element of the present invention is characterized in that it can be directly replaced with the angle phase matching element in the existing correlator, and it is an object of the present invention. Then, next, FIG. 4 shows a comparison of the performance of the correlator before and after replacing the produced element of the present invention with the conventional angle phase matching element.

The correlator used in this example is conventionally 2 m.
The second harmonic generation effect by angle phase matching in m-thick lithium niobate crystal is used, and the generated second harmonic light is photoelectrically converted by the photomultiplier tube, and the amplification sensitivity is 5 × 1.
It is converted into a voltage value and output by a 0 ⁴ V / A current amplifier. An antireflection film in the 1.5 μm wavelength band is deposited on the incident end face of this lithium niobate crystal, and an antireflection film in the 0.75 μm band, which is the second harmonic wavelength, is deposited on the exit end face. ing. In this correlator, an angle of 5.7 ° is formed between the incident direction of the signal light to be measured and the incident direction of the reference light to the crystal, and the second harmonic light individually generated from only one light is It has a configuration that does not fit in the detector. The configuration of such a non-coaxial correlator is called a background-free correlator. In the following, the correlator is operated in the autocorrelation mode in which the incident signal light is divided into two and the reference light is supplied by itself.

The measurement result by the conventional angular phase matching of the autocorrelation waveform of the pulse having a wavelength of 1.564 μm and a time width of 1.5 ps emitted from the optical filter having a transmission wavelength width of 2 nm is shown as the lower curve in FIG. It was The average power of this incident pulse is 0.15 mW and the peak power is 550 mW. At this time, 450 mV is obtained as the maximum value of the autocorrelation signal. It is known that the output of a correlator using such a second harmonic generation effect is proportional to the product of the average power of the incident pulse and the peak power. The sensitivity of the correlator can be expressed as 5.5 mV / mW ² .

Here, the angle phase matching element was replaced with the element of the present invention described above. As already mentioned, the propagation distance in the inventive example element is 2 mm, which is equal to the replaced angular phase matching element. In order to make the conditions even more uniform, the device of the present invention was prepared by depositing antireflection films in the wavelength band of 1.5 μm and the wavelength band of 0.75 μm on the output facet. The element of the embodiment of the present invention has a thickness of 0.3 mm and a lateral width of 3 mm, which is much smaller than the entrance surface dimension of the conventional angle phase matching element of 5 mm × 5 mm. Therefore, a metal frame having the same size as the incident surface of the conventional element was formed, and the element of the example of the present invention was attached to it, and then the metal frame was attached to the correlator. If you do this,
The elements can be easily replaced on-site without changing the correlator itself. When a wavelength conversion element is used in an ordinary measuring instrument, since the incident light is focused on the element by the lens, the beam diameter inside the element is small. In this correlator, the incident light is narrowed down to about 10 μm by a 20 mm lens. Therefore, the fact that the thickness of the element of the embodiment of the present invention is only 0.3 mm hinders practical use at all, except during mounting work. Does not mean

The center wavelength matching wavelength of the embodiment element of the present invention is
Since it is designed to have a wavelength of 1.545 μm, it is not possible to observe a correlation signal for an incident wavelength of 1.564 μm here under the condition of straight incidence. However, the phase matching could be realized by adjusting the rotation angle. The rotation angle at that time is about 6 °, which is 12 ° expected from FIG.
It was out of alignment. It is considered that this is because in FIG. 3, the incident light is calculated under the condition of the coaxial, whereas in the present correlator, an angle is formed between the two incident lights. When there is an angle between the incident lights, the equivalent in-plate optical axis becomes its bisector, and the refractive index on the converted light side is reduced to the projection amount to the in-plate optical axis. As a result, for a certain incident wavelength, a small polarization change period is required for quasi phase matching, and the scale on the right vertical axis in FIG. 3 shifts downward. If such a correction is performed according to the conditions of each installed correlator, the phase matching wavelength with respect to the rotation angle of the device of the example of the present invention can be predicted with a practically sufficient accuracy.

The measurement result of the autocorrelation waveform in the case of using the element of the embodiment of the present invention is shown as the upper curve in FIG.
The autocorrelation waveform resembles with the case of the conventional angle phase matching element with very high accuracy, and it can be seen that the wavelength tunable quasi phase matching crystal of the present invention can be completely applied to the waveform measurement in the picosecond region. What should be noted here is that the maximum value of the autocorrelation signal is 10 V, which is much larger than that obtained by the conventional angle phase matching element. If the sensitivity in this case is calculated in the same manner as before, it is 121 mV / m
It can be represented as W ² . That is, the sensitivity is 22 times higher than that of the conventional angle phase matching element. The reason for this is that the nonlinear constant tensor used in quasi-phase-matched lithium niobate crystals is d _zzz ,
This is because it is possible to use a tensor component which is about 6 times larger than d _zxx used in the conventional angle matching lithium niobate element.

As a result of the above, the shape of the incident end face is represented by polar coordinates with r being the radius vector with respect to the rotation center and θ being the azimuth angle, and r = r ₀ (n-1) / (n cos θ-1). By forming the hyperbola represented, the phase matching wavelength change
Adjustment can be performed by a rotating mechanism, and when there are two converted lights, the center of rotation is provided on the bisector of the propagation optical path before the converted light reaches the incident end face, and the average value of the refractive index of the medium is set. Is set to n, and the vicinity of the apex of the hyperbola may be a circular arc that approximates the hyperbola, and the shape of the emitting end face is the same as the shape of the incident end face, or the reflectance is reduced to one of the incident end face and the emitting end face. A film can be added, and a slab waveguide can be formed as a nonlinear optical medium plate.

[0050]

The variable wavelength quasi phase matching element of the present invention is
The phase matching wavelength can be changed / adjusted by the rotating mechanism instead of the parallel moving mechanism. Therefore, the wavelength tunable quasi-phase matching element of the present invention can be directly replaced by the non-linear medium of the current correlator, and the measurement sensitivity can be improved very easily. It can provide a path for improved performance. Further, the wavelength tunable quasi-phase matching element of the present invention can be manufactured at low cost, so that a great industrial effect can be obtained.

[Brief description of drawings]

FIG. 1 is an example of an embodiment of the present invention, in which (a) is a state diagram tuned to a central wavelength, and (b) is a state diagram tuned to a shorter wavelength.

FIG. 2 is an explanatory diagram for deriving an end face shape.

FIG. 3 is a diagram showing wavelength tunable characteristics.

FIG. 4 is a diagram showing an output signal of a correlator.

FIG. 5 is a diagram showing an inversion period necessary for generating a second harmonic.

FIG. 6 is an example of a configuration of a conventional variable wavelength quasi-phase matching element, (a) is a state diagram tuned to a central wavelength, (b)
Is a phase diagram tuned to a shorter wavelength.

[Explanation of symbols]

101 Non-linear optical medium plate 102 incident optical axis 103 Output optical axis 104 incident end face 105 exit end face 106 In-plate optical axis 107 center of rotation 108 rotation angle 109 axis of symmetry 202 Incident optical axis 204 Incident end face 206 In-plate optical axis 207 center of rotation 209 symmetry axis 210 Optical axis displacement 601 Non-linear optical medium plate 602 incident optical axis 603 Output optical axis 604 Incident end face 605 exit end face 606 In-plate optical axis 611 Movement direction

─────────────────────────────────────────────────── ───Continuation of front page (56) Reference JP-A-6-132595 (JP, A) JP-A 1-502229 (JP, A) JP-A 6-505100 (JP, A) US Patent 5355247 (US , A) P.P. E. POWERS, et. a. , OPTICS LETTERS, February 1, 1998, Vol. 23, No. 3, pp. 159-161 (58) Fields investigated (Int.Cl. ⁷ , DB name) G02F 1/37 JISST file (JOIS)

Claims (7)

(57) [Claims] 1. A wavelength tunable quasi-phase matching element comprising a plate-shaped non-linear optical medium, wherein an incident end face that is a side face on which converted light is incident and a side opposite to the incident end face are converted. with the emission end face is a side through which light is emitted, a vertically with respect to the optical path for propagating incident light, and periodically in the upper surface of the element an alternating polarization to vary linearly granted in the device In the wavelength tunable quasi phase matching element, a two-dimensional surface including a rotation center on a line extending the propagation optical path before the converted light reaches the incident end surface, the element including the upper surface of the element with respect to the rotation center Ri rotatable der an inner, r ₀ the shortest distance from the center of rotation to the incident end face, wherein when the refractive index said with respect to the converted light of non-linear optical medium is n, the shape of the incident end face, the Radial to the center of rotation is r A wavelength tunable quasi-phase matching element having a hyperbola represented by r = r ₀ (n-1) / (n cos θ-1) in polar coordinates with a position angle of θ. 2. A radius (n−) having a point (r = nr ₀ , θ = 0) in the polar coordinate display as a center so that the shape of the incident end face approximates the hyperbola in an arc shape near its apex.
1) The wavelength tunable quasi-phase matching element according to claim 1, which has an arc shape of r ₀ . 3. When there are two converted lights, each of the converted lights has a rotation center on the bisector of each propagation optical path before reaching the incident end face, and each of the nonlinear optical media has a center of rotation. 3. The wavelength tunable quasi-phase matching element according to claim 1, wherein the average value of the refractive index with respect to the converted light is n. 4. The shape of the emission end face corresponds to the shape of the incident end face that is translated in parallel along the optical path along which the incident light propagates. The tunable quasi-phase matching element described. 5. The tunable quasi-phase matching element according to claim 1, wherein an antireflection film for the converted light is provided on the incident end face. 6. The wavelength tunable quasi phase matching element according to claim 1, wherein an antireflection film for the converted light is provided on the emission end face. 7. The tunable quasi-wavelength according to claim 1, wherein the slab waveguide is configured so that light propagating through the tunable quasi-phase matching element is confined in a two-dimensional plane. Phase matching element.