1. Field of the Invention
The present invention relates to an optical wavelength conversion device used for optical information processing and optical measurement fields utilizing a coherent light source, a coherent light generator using such an optical wavelength conversion device, and an optical information processing apparatus using such a coherent light generator.
The present invention also relates to a short-wavelength light source including a semiconductor laser and an optical wavelength conversion device, and an optical disk system using such a short-wavelength light source.
2. Description of the Related Art
An optical wavelength conversion device utilizing a nonlinear optical effect has been used in a variety of fields because such a device serves to widen the range of wavelengths usable by a laser light source by converting the wavelength of light output from the laser light source.
For example, an optical wavelength conversion device utilizing second harmonic generation (SHG) converts laser light into second harmonic wave light having a half wavelength, thereby enabling to realize generation of short-wavelength light which is otherwise difficult. When parametric oscillation is utilized, it is possible to generate light beams having different wavelengths continuously from a single-wavelength light source, thereby realizing a wavelength tunable light source. When a sum frequency is utilized, it is possible to convert two light beams having different wavelengths into a light beam having a third wavelength.
In the above optical wavelength conversion utilizing a nonlinear optical effect, phase matching conditions must be satisfied between the fundamental wave light before conversion and the second harmonic wave light after conversion. Techniques for this phase match includes a birefringence method in which the propagation velocities of the fundamental wave light and the second harmonic wave light in a crystal are made identical to each other by utilizing the birefringence of the crystal, and a quasi phase match method in which a nonlinear grating is used to achieve phase match.
In reality, however, the wavelength allowance range satisfying the phase matching conditions is extremely narrow. It is therefore necessary to control the wavelength of the fundamental wave light with markedly high precision, and thus it is difficult to stabilize the output power.
Studies have been done to widen the wavelength allowance range to increase the stability of optical wavelength conversion. FIG. 33 illustrates a conventional optical wavelength conversion device for widening the wavelength allowance range (see Japanese Application No. 3-16198). Hereinbelow, generation of second harmonic wave light P2 having a wavelength of 0.42 xcexcm from fundamental wave light P1 having a wavelength of 0.84 xcexcm will be described in detail with reference to FIG. 33.
The optical wavelength conversion device shown in FIG. 33 includes an LiNbO3 substrate 1101, an optical waveguide 1102 formed on the substrate 1101, and a layer 1103 in which polarization is periodically inverted (a domain-inverted layer) formed for the optical waveguide 1102. The propagation constant of the fundamental wave light P1 is not matched with that of the second harmonic wave light P2 to be generated. This mismatch in propagation constant is compensated for by the periodic structure of the domain-inverted layer 1103, thereby enabling to generate the second harmonic wave light P2 with high efficiency.
Although the above conventional optical wavelength conversion device which performs wavelength conversion using the periodic domain-inverted layer 1103 exhibits high conversion efficiency, the allowance range of phase match wavelength in which wavelength conversion is possible is extremely narrow. In order to overcome this problem, the optical waveguide 1102 of the optical wavelength conversion device shown in FIG. 33 has portions having different propagation constants so as to widen the wavelength allowance range.
When the propagation constant of the optical waveguide 1102 is varied, the phase match wavelength in the optical waveguide 1102 varies. The phase matching conditions as used herein are conditions under which the optical wavelength conversion device can perform wavelength conversion. The phase match wavelength as used herein is a wavelength of incident light which satisfies the phase matching conditions. By dividing the optical waveguide 1102 into regions A, B, C, and D having different widths as shown in FIG. 33, the phase match wavelength varies depending on the width of the optical waveguide 1102 in the respective regions. By this construction, the phase matching conditions can be satisfied in any of the regions A, B, C, and D having different widths of the optical waveguide even when the wavelength of incident light is changed. Thus, the number of phase match wavelengths increases in the entire device. As a result, the wavelength allowance range for the optical wavelength conversion device is widened and thus a stable wavelength conversion device can be fabricated.
The phase matching conditions for the regions A, B, C, and D can also be established by varying the depth of the optical waveguide 1102 among the regions A, B, C, and D, or varying the period of the domain-inverted layer 1103 among the regions A, B, C, and D. In these cases, also, an optical wavelength conversion device having a wide wavelength allowance range is obtained.
A construction combining a periodic domain-inverted structure and a phase control section is also disclosed (see Japanese Application No. 4-070726). FIG. 34 illustrates a conventional optical wavelength conversion device having such construction for widening the wavelength allowance range.
The optical wavelength conversion device shown in FIG. 34 includes a plurality of domain-inverted regions 1105 and phase control sections 1106 between the adjacent domain-inverted regions 1105, which are both formed on a nonlinear optical crystal 1101. In this device, it is attempted to widen the allowance range of phase match wavelengths by utilizing the difference in the phase matching conditions of the respective domain-inverted regions 1105. It is also attempted to reduce a variation of the output power of second harmonic wave light P2 (i.e., the SHG output power) caused by a wavelength variation of fundamental wave light P1 by adjusting phase mismatch generated between the domain-inverted regions 1105 by the phase control sections 1106.
It is possible to further widen the allowance range of phase match wavelength by increasing the number of the domain-inverted regions 1105. For example, FIGS. 35A and 35B show tuning curves representing the relationship between the wavelength of fundamental wave light and the SHG output power when the domain-inverted layer is segmented into three regions and four regions, respectively. It is observed from these tuning curves that the wavelength allowance range can be greatly widened by increasing the number of segmented regions.
A method for widening the allowance range of phase match wavelength by modulating a periodic structure of domain-inverted is also disclosed.
For example, Suhara et al., IEEE Journal of Quantum Electronics, vol. 26 (1990) pp. 1265-1276 discloses a method for widening the allowance range of phase match wavelength by changing a periodic structure of domain-inverted in a chirping fashion. Specifically, this method uses a linear chirping structure where the period of domain-inverted is increased in proportion to the distance. In this method, since phase displacement linearly changes in the domain-inverted structure, the allowance range of phase match wavelength can be widely increased.
On the other hand, in recent years, optical disk systems using a near-infrared semiconductor laser having a 780 nm wavelength band and a red semiconductor laser having a wavelength of 670 nm have been vigorously developed. On the other hand, in order to realize high-density optical disks, it is desirable to use a small spot to reproduce information from optical disks. To achieve this, it is necessary to increase the numeral aperture (NA) of a light focusing lens and to short the wavelength of a light source.
As one of techniques of shortening the wavelength of a light source, a second harmonic generation (SHG) technique using a near-infrared semiconductor laser and a quasi phase match (OPM) domain-inverted type optical waveguide device (Yamamoto et al., Optics Letters, Vol.16, No.15 (1991) p.1156) is known.
FIG. 15 shows a schematic construction of a conventional short-wavelength light source (an SHG blue laser) using a domain-inverted type optical wavelength conversion device.
Referring to FIG. 15, the short-wavelength light source includes: a 0.85 xcexcm band, 100 mW class, AlGaAs wavelength tunable distributed Bragg reflector (DBR) type semiconductor laser 38; a collimator lens 39 having NA of 0.5; a focusing lens 40 having NA of 0.5; and an optical wavelength conversion device 41. The wavelength tunable DBR semiconductor laser 38 includes a DBR region for fixing an oscillation wavelength and an active region for actually generating laser oscillation. The DBR region includes an internal heater for tuning the oscillation wavelength. The oscillation wavelength can be tuned by applying a current to the DBR region. Typically, a 2 nm wavelength tunable region is obtained by applying a current of 100 mA.
The construction of the optical wavelength conversion device 41 will be described.
The optical wavelength conversion device 41 includes a proton exchange optical waveguide 43 formed on an x-cut, Mg-doped LiNbO3 substrate 42 and periodic domain-inverted regions 44 formed by a two-dimensional electric field application method. The periodic domain-inverted regions 44 are formed in the following manner. First, a comb-shaped electrode having a period of 3.2 xcexcm and a parallel electrode are formed on the top surface of the +X substrate 42. A Ta film is deposited on the bottom surface of the +X substrate 42 as a bottom electrode. While a 4 V voltage is being applied between the top surface and the bottom surface of the substrate 42, a 0.4 V pulse voltage is applied to the top surface of the +X substrate 42 at a pulse width of 100 ms. In this way, the periodic domain-inverted regions 44 are formed.
A stripe-shaped mask is then formed after etching away the electrodes, and proton exchange is performed under pyrophosphoric acid to form the optical waveguide 43. The optical waveguide 43 typically has a width of 4 xcexcm, a depth of 2 xcexcm, and a length of 10 mm. An end face of the optical waveguide 43 is covered with a non-reflection coat.
The wavelength conversion characteristic of the optical wavelength conversion device 41 having the above construction with respect to the wavelength of fundamental wave light was evaluated. As a result, it was found that the full width at half maximum where the power level of blue light (second harmonic wave light) is halved is 0.08 nm.
Laser light output from the wavelength tunable DBR semiconductor laser 38 is coupled to the optical waveguide 43 of the optical wavelength conversion device 41 via the collimator lens 39 and the focusing lens 40. Typically, out of the laser output power of 100 mW, 70 mW laser light is coupled to the optical waveguide 43. Then, about 15 mW blue light (second harmonic wave light) is obtained by controlling the current amount injected to the DBR region of the wavelength tunable DBR semiconductor laser 38 and fixing the oscillation wavelength within an allowable wavelength range of phase match wavelength of the optical wavelength conversion device 41.
A control circuit is added to the above construction, in some cases, in order to stabilize the blue light (second harmonic wave light) power. In such a case, first, a current is injected from the control circuit to the active region of the semiconductor laser so that the intensity of output laser light is a preset value (e.g., 100 mW). Thereafter, SHG light obtained by wavelength conversion is detected by an optical detector to stabilize the SHG output power. An Si-PIN photodiode may be used as the optical detector.
More specifically, when a current injected to the DBR region is changed to vary the oscillation wavelength of the wavelength tunable DBR semiconductor laser, the oscillation wavelength shifts toward a longer wavelength with an increase in the injected current while repeating mode hopping. Accordingly, while the injected current is changed within the range of 0 mA to 100 mA, the oscillation wavelength is scanned and a signal output from the optical detector is detected, to store a current injected to the DBR region, Idbr, at which the blue light (second harmonic wave light) power is maximum in the control circuit. For example, assume that the maximum power 5 mW of the second harmonic wave light was obtained when the injected current Idbr is 50 mA. In such a case, the injected current Idbr is first reduced to 40 mA which is lower than 50 mA, and then increased to the stored current value (50 mA), so that the oscillation wavelength of laser light is fixed to the phase match wavelength of the optical wavelength conversion device.
By the operation described above, 5 mW maximum blue light (second harmonic wave light) power is stably obtained.
In such a short-wavelength light source where laser light output from a semiconductor laser is input into an optical wavelength conversion device as fundamental wave light to convert the fundamental wave light into second harmonic wave light, phase matching conditions need to be satisfied in order to realize high-efficiency wavelength conversion. The allowable wavelength width of the phase matching conditions is generally small. Accordingly, in a short-wavelength light source including a semiconductor laser and an optical wavelength conversion device, a wavelength tunable DER semiconductor laser is Used as the semiconductor laser which supplies fundamental wave light. The wavelength tunable DBR semiconductor laser includes a DBR region for fixing and tuning the wavelength. By applying a current to the DBR region, a wavelength tunable region of about 2 nm is obtained.
The aforementioned conventional optical wavelength conversion devices using a domain-inverted structure have the following problem in attempting to widen the allowance range of phase match wavelength. That is, the phase matching characteristic. i.e., the SHG output power characteristic obtained when the phase match wavelength has been tuned, greatly varies near the peak thereof.
In the conventional method described above, where the optical wavelength conversion device having the domain-inverted layer is segmented into two or more regions to have different phase matching conditions between the respective segmented regions thereby to increase the wavelength allowance range for the optical wavelength conversion device, second harmonic wave light (SHG light) is generated over a wide wavelength range since the phase match wavelength is different between the respective segmented regions. In this case, however, the second harmonic wave light beams generated in the respective regions interfere with each other, resulting in increasing the variation of the SHG output power near the peak thereof in response to the wavelength variation of fundamental wave light, as shown in FIGS. 35A and 35B.
The range of phase match wavelength can be greatly widened by adopting the linear chirping structure for the domain-inverted period. However, this method also generates large rippling near the peak of the tuning curve.
Thus, the conventional methods fail to provide a flat power characteristic near the peak of the tuning curve. This causes a change in the SHG output power in response to a small variation of the wavelength of fundamental wave light within the allowance range of phase match wavelength, resulting in failing to obtain stable power.
Another problem of the conventional devices is as follows. In the conventional optical wavelength conversion device described above, interference between second harmonic wave light beams generated in the respective domain-inverted regions is reduced by providing the phase control section between the domain-inverted regions, and thus a variation of the SHG output power in response to the wavelength variation of fundamental wave light is reduced. Even in such a conventional optical wavelength conversion device, the SHG output power still varies by 10% or more. It is therefore difficult to obtain a flat portion near the peak of the tuning curve, which is required to stabilize the output power.
Still another problem of the conventional devices is as follows. In the conventional optical wavelength conversion device, although the wavelength allowance range can be easily widened, the conversion efficiency markedly decreases. For example, the conversion efficiency decreases to {fraction (1/9)} or less for the crystal of the same length, and the conversion efficiency decreases to {fraction (1/10)} with the 10 times wide wavelength allowance range. In other words, in the conventional optical wavelength conversion device, it is difficult to widen the wavelength allowance range while high-efficiency wavelength conversion characteristic is maintained to obtain the stable SHG output power.
In a conventional optical wavelength conversion device where a voltage is applied to an entire waveguide, in order to obtain a high-efficiency conversion characteristic, the applied voltage distribution must be highly uniform. It is therefore difficult to change the phase match wavelength while maintaining high-efficiency conversion efficiency. Moreover, in order to maintain a constant temperature over the entire optical wavelength conversion device, temperature control over the wide area is required, resulting in increasing power consumption.
Furthermore, in order to obtain stable power by varying the phase match wavelength of the optical wavelength conversion device, it is required to control the wavelength using the applied voltage and fix the applied voltage after selection of a predetermined wavelength. However, a material having an electro-optic effect, such as LiNbO3 and LiTaO3, has a problem of DC drifting, giving rise to the problem that electric charge offsetting the applied voltage is generated between electrodes when a constant voltage is applied and thus the applied voltage gradually changes. This makes it difficult, not only to apply a constant DC voltage to the entire waveguide, but also to form a uniform electric field distribution over the entire optical waveguide.
Since the refractive index changeable depending on the electro-optic effect is of the order of 10xe2x88x924, the resultant range of the phase match wavelength capable of being modulated is limited to about 0.1 nm or less.
The above-described conventional short-wavelength light source has also the following problems.
In the short-wavelength light source including a wavelength tunable DBR semiconductor laser and a domain-inverted type optical wavelength conversion device, the oscillation wavelength of the wavelength tunable DBR semiconductor laser shifts toward a longer wavelength while repeating mode hopping (mode interval; 0.11 nm). In the domain-inverted type optical wavelength conversion device, the allowable wavelength width of phase match wavelength in the operation characteristic (tuning curve characteristic) for a 10 nm device length is 0.05 nm in the full width at half maximum, and the wavelength width in which the power level is 95% of the peak power is as small as 0.02 nm.
FIGS. 16A and 16B schematically show the relationship between the oscillation wavelength A at each mode of the wavelength tunable DBR semiconductor laser as the X-axis and the tuning curve characteristic B of the domain-inverted type optical wavelength conversion device as the Y-axis.
In FIG. 16A, the wavelength in a certain oscillation mode of the wavelength tunable DBR semiconductor laser is located near the peak of the tuning curve characteristic of the domain-inverted type optical wavelength conversion device. It is therefore possible to fix the oscillation wavelength of the wavelength tunable DBR semiconductor laser to the phase match wavelength with which the maximum conversion efficiency is obtained (corresponding to the peak of the characteristic B in FIG. 16A).
However, when the oscillation wavelength is largely displaced from the peak of the tuning curve characteristic as shown in FIG. 16B, the conversion efficiency of only about 50% of the maximum value is obtained since the longitudinal mode interval of the wavelength tunable DBR semiconductor laser is 0.11 nm, for example. In the latter case, it is necessary to adjust the oscillation wavelength of the wavelength tunable DBR semiconductor laser to match with the phase match wavelength with which the maximum conversion efficiency is obtained.
One method for adjusting the oscillation wavelength is as follows. When a driving current for the wavelength tunable DBR semiconductor laser (a current injected to an active region) is changed, the oscillation wavelength changes in the order of 0.01 nm. For example, for an AlGaAs wavelength tunable DBR semiconductor laser, the oscillation wavelength changes at a rate of 0.02 nm/10 mA.
However, as described above with reference to FIGS. 16A and 16B, the wavelength width in which the power level is 95% of the peak power is xc2x10.01 nm. Accordingly, when the driving current is controlled under auto power control (APC) to make constant the fundamental wave light power and the SHG output power, the oscillation wavelength changes. In other words, the control loop is in a diverging direction, not in a converging direction. For example, when the fundamental wave light power is changed by 5% to change the SHG output power by about 10%, the oscillation wavelength changes by about 0.02 nm, decreasing the conversion efficiency by about 20%. Thus, it is actually very difficult to employ APC driving for the conventional short-wavelength light source including the wavelength tunable DBR semiconductor laser and the domain-inverted type optical wavelength conversion device.
Alternatively, it is possible to control the temperature of the entire module on which the wavelength tunable DBR semiconductor laser and the domain-inverted type optical wavelength conversion device are mounted, by means of an electric cooling element and the like, to adjust the oscillation wavelength of the wavelength tunable DBR semiconductor laser to the phase match wavelength with which the maximum conversion efficiency is obtained.
In relation with the above, FIG. 17 shows the temperature dependency of each of the operation characteristics of the wavelength tunable DBR semiconductor laser and the domain-inverted type optical wavelength conversion device. Typically, the oscillation wavelength of the wavelength tunable DBR semiconductor laser shifts toward a longer wavelength with the temperature increase at a rate of 0.068 nm/xc2x0 C., and the phase match wavelength of the domain-inverted type optical wavelength conversion device on a Mg:LiNbO3 substrate shifts toward a longer wavelength with the temperature increase at a rate of 0.055 nm/xc2x0 C. Accordingly, as the temperature of the module changes, the relationship between the oscillation wavelength and the phase match wavelength changes at a rate of 0.013 nm/xc2x0 C.
Utilizing this relationship, it in possible to fix the oscillation wavelength of the wavelength tunable DBR semiconductor laser at the peak of the phase match wavelength of the domain-inverted type optical wavelength conversion device by raising the module temperature, for example, by 3xc2x0 C.
However, while the wavelength tuning by the current application to the DBR region is performed at a speed of the order of milliseconds, the temperature control by a Peltier element as described above is performed at a speed of the order of seconds. In consideration that the start-up time of a light source is desirably as short as possible when the light source is applied to an optical disk device, a display, and the like, the above response speed characteristic is not satisfactory. Moreover, a power on the order of several volts and several amperes is required to be applied to operate the Peltier element, increasing power consumption of the light source. This is not desirable for personal-use application, either.
In general, the relationship of SHG output power with respect to the wavelength of fundamental wave light of a domain-inverted type optical wavelength conversion device (the phase match wavelength curve, i.e., the tuning curve) has a profile of a sinc function. The tuning curve characteristic has no flat portion even in the vicinity of the peak of the phase match wavelength. Accordingly, the SHG output power varies with even a slight temperature change of the module. It is therefore necessary to continuously control the temperature of the module.
Thus, in the short-wavelength light source including the optical wavelength conversion device and the wavelength tunable DBR semiconductor laser, the wavelength tunable characteristic is discontinuous when a general wavelength tunable DBR semiconductor laser is used. The wavelength is actually varied while repeating mode hopping at intervals of about 0.1 nm. It is therefore extremely difficult to keep stabilizing the resultant SHG output power. In particular, it is very difficult to stabilize the SHG output power by AFC driving.
A wavelength tunable DBR semiconductor laser of a three-electrode type which includes a phase region in addition to the active region and the DBR region has been developed. However, it is still difficult to control the power and the wavelength stably.
It has been studied to widen the allowable wavelength width of the phase match wavelength by shortening the length of the optical wavelength conversion device. However, in this method, the device length must be shortened to the order of several millimeters in order to widen the allowable wavelength width to the mode hopping interval of the semiconductor laser. This greatly reduces the conversion efficiency and thus is not practical.
Another problem of the power stabilization relates to stabilization of the wavelength of fundamental wave light and the phase match wavelength.
That is, when the wavelength of fundamental wave light and the phase match wavelength of the optical wavelength conversion device vary due to a factor such as a temperature change, it is necessary to monitor the variation amount to stabilize the output power by the feedback of the wavelength of the fundamental wave light. In the conventional optical wavelength conversion device, however, the idea of providing a monitor function to optimize the phase match state is not taken into consideration. The conventional device also has a structural problem that makes it difficult to provide a monitor function.
A short-wavelength light source of the present invention includes at least an optical wavelength conversion device and a wavelength tunable semiconductor laser, wherein the optical wavelength conversion device receives light output from the wavelength tunable semiconductor laser as a fundamental wave light, and outputs a second harmonic wave light obtained by converting a wavelength of the fundamental wave light, and an output power characteristic of the second harmonic wave light of the optical wavelength conversion device has a flat portion near the maximum power, and an oscillation wavelength of the wavelength tunable semiconductor laser is fixed to the flat portion of the output power characteristic of the optical wavelength conversion device.
An optical wavelength conversion device of the present invention includes: two or more non-linear optical crystals each having approximately identical phase matching conditions for a fundamental wave light and a second harmonic wave light; and a phase adjusting section inserted between the adjacent non-linear optical crystals, wherein the phase adjusting section has a dispersion characteristic which is different from that of the non-linear optical crystals, and the phase adjusting section is formed so as to allow at least one of a refractive index or a length thereof to be modulated.
Another optical wavelength conversion device of the present invention includes a non-linear optical crystal and a refractive index modulating section formed in a portion of the non-linear optical crystal, wherein the refractive index modulating section is formed in a region having a length of xc2xd or less of a whole length of the nonlinear optical crystal.
Yet another optical wavelength conversion device of the present invention includes two or more non-linear optical crystals and a phase adjusting section inserted between the adjacent non-linear optical crystals, wherein the non-linear optical crystals have phase matching conditions substantially identical to each other.
Yet another optical wavelength conversion device of the present invention includes: a non-linear optical crystal; a periodic domain-inverted structure formed in the non-linear optical crystal and segmented into two or more is regions; and a phase adjusting section disposed between the adjacent segmented regions of the periodic domain-inverted structure, wherein the respective segmented regions of the periodic domain-inverted structure have periods substantially equal to each other.
Yet another optical wavelength conversion device of the present invention includes a non-linear optical crystal in which a fundamental wave light having a wavelength of xcex is converted into a second harmonic wave light having a wavelength of xcex/2, wherein a propagation loss for the fundamental wave light is substantially xc2xd of that for the second harmonic wave light in the non-linear optical crystal.
Yet another optical wavelength conversion device of the present invention includes a non-linear optical crystal in which a wavelength conversion is performed between first and second light respectively having wavelengths of xcex1 and xcex2 and a third light having a wavelength of xcex3, wherein the wavelengths satisfy the relationship of 1/xcex3=1/xcex1+1/xcex2, and propagation losses for the first, second, and third lights are substantially equal to each other in the non-linear optical crystal.
A coherent light generator of the present invention includes: a semiconductor laser having a function to tune an oscillation wavelength; and an optical wavelength conversion device which receives light output from the wavelength tunable semiconductor laser as a fundamental wave light and performs a wavelength conversion for the fundamental wave light to output a second harmonic wave light, wherein a characteristic curve indicating a relationship between a wavelength of the fundamental wave light and an output of the second harmonic wave light has a flat portion in the vicinity of the maximum output of the second harmonic wave light, and a width of the flat portion is designed to be larger than an interval of longitudinal oscillation modes of the semiconductor laser.
The optical wavelength conversion device can be the one provided in accordance with the present invention.
Another coherent light generator of the present invention includes: an optical wavelength conversion device according to the present invention; and a laser light source, wherein a wavelength of an output light from the laser light source is converted by the optical wavelength conversion device.
Yet another coherent light generator of the present invention includes: a non-linear optical crystal; a wavelength tunable laser light source; and first and second optical detectors, wherein a first light emitted from the laser light source is converted into a second light in the optical wavelength conversion device. With respect to a selected light among the first and second lights, the first optical detector detects an intensity of scattered light from the non-linear optical crystal, and the second optical detector detects an intensity of the selected light in the vicinity of an output portion of the non-linear optical crystal. An oscillation wavelength of the wavelength tunable laser light source is controlled based on the detected result of the first and second optical detectors.
An optical wavelength conversion device of the present invention includes: a nonlinear optical crystal; and a periodic domain-inverted structure formed on the nonlinear optical crystal, wherein the domain-inverted structure includes a single period portion having a single period xcex90 and chirping period portions having gradually changing periods.
Another optical wavelength conversion device of the present invention includes: a nonlinear optical crystal; and a periodic domain-inverted structure formed on the nonlinear optical crystal. A period of the domain-inverted structure is a domain-inverted period represented by
xcex9xe2x88x92m, xcex9xe2x88x92(mxe2x88x921), . . . , xcex9xe2x88x922, xcex9xe2x88x921, xcex90, xcex91, . . . , xcex9mxe2x88x921, xcex9m,
the domain-inverted period has a distribution f(z) of phase mismatch amount, the distribution f(z) satisfying relationships:
f(i★*xcex90)=(xcex91+xcex92+ . . . +xcex9i)xe2x88x92i★xcex90 and
f(xe2x88x92i★xcex90)=(xcex9xe2x88x921+xcex9xe2x88x922+ . . . +xcex9xe2x88x921)xe2x88x92i★xcex90
wherein i=1, 2, 3, . . . , and f(z)=0 being established when z is in the vicinity of zero. The distribution f(z) further satisfies a relationship of f(i★xcex90)xe2x88x92f(xe2x88x92i★xcex90), and a quadratic differential coefficient becomes larger at a position closer to ends of the nonlinear optical crystal.
Yet another optical wavelength conversion device of the present invention includes: a plurality of nonlinear optical crystals having the same domain-inverted structure; and a phase adjusting section disposed between the nonlinear optical crystals, wherein the phase adjusting section includes a domain-inverted structure having a period different from that of the nonlinear optical crystals.
According to another aspect of the present invention, a coherent light generator is provided, which includes: an optical wavelength conversion device according to the present invention; and a laser light source, wherein a wavelength of light output from the laser light source is converted by the optical wavelength conversion device.
According to still another aspect of the present invention, an optical information processing apparatus is provided, which includes: a coherent light generator according to the present invention; and a focusing optical system, wherein coherent light output from the coherent light generator is focused by the focusing optical system.
Thus, the invention described herein makes possible the advantages of (1) providing an optical wavelength conversion device having a phase matching characteristic having a wide flat portion near the maximum of a tuning curve (a flat peak phase matching characteristic), (2) providing an optical wavelength conversion device capable of stably changing the wavelength allowance range of phase match wavelength over a wide range, (3) providing an optical wavelength conversion device having a stable wavelength conversion characteristic by widening the wavelength allowance range while maintaining a phase matching characteristic having a wide flat portion near the peak (a flat peak characteristic), (4) providing a coherent light generator including such an optical wavelength conversion device and a semiconductor laser, which stabilizes a variation of the oscillation wavelength of the semiconductor laser to have a stable power characteristic, (5) providing a short-wavelength light source including a wavelength tunable DBR semiconductor laser and a domain-inverted type optical wavelength conversion device, which can be applied to optical disk systems, display systems, and the like and realizes a stable SHG output power characteristic under any ambient temperature or operation state, and (6) providing an optical information processing apparatus using such a coherent light generator or short-wavelength light source.