JP6511919B2 - Evaluation apparatus and evaluation method of quasi phase matching (QPM: Quasi-Phase Matching) type wavelength conversion element - Google Patents

Description

  The present invention relates to an apparatus and method for evaluating a QPM wavelength conversion element.

  QPM is a method of wavelength conversion based on nonlinear optical effects. As a wavelength conversion element which realizes this QPM in an optical waveguide, there is a QPM type wavelength conversion element (see, for example, Patent Document 1). The QPM wavelength conversion element is configured by forming a periodic polarization inversion structure in an optical waveguide.

  The greater the energy density of the interacting light, and the greater the interaction length of the light (interaction length), the greater the non-linear optical effect. Therefore, in order to realize the non-linear optical effect in the optical waveguide, it is necessary to make the light propagate a sufficient distance while keeping the energy density of the light high.

Here, in the QPM type wavelength conversion element, the wavelength conversion efficiency by second harmonic generation (SHG) is proportional to the square of the interaction length and standardized [% / W · cm ² ]. It is expressed in units. However, this is a case where propagation loss of converted light and converted light is not considered. In practice, propagation loss occurs in both the converted light and the converted light. Therefore, the output intensity of the converted light is affected both by the increase by increasing the interaction length and the attenuation by the propagation loss. In wavelength conversion by SHG, since the converted light has a short wavelength of half of the converted light, the influence of the propagation loss becomes remarkable. Therefore, the interaction length is not simply set large, and it is necessary to find an optimum value for obtaining high conversion efficiency while suppressing the propagation loss. Therefore, in finding the optimum value of the interaction length in the QPM type wavelength conversion element, it is important to evaluate characteristics such as wavelength conversion efficiency and propagation loss.

  As a method of evaluating the conventional QPM type wavelength conversion element, there is a method of performing light input / output between the QPM type wavelength conversion element and an external optical element (such as an optical fiber or a lens). In this case, the output light is measured. Then, the loss and the wavelength conversion efficiency are obtained from the relationship between the input light intensity and the measured output light intensity.

  However, the loss required in this conventional method includes not only the propagation loss in the QPM wavelength conversion device but also the coupling loss between the QPM wavelength conversion device and the external optical device. This coupling loss is difficult to estimate accurately. Therefore, in the conventional method, it has been difficult to accurately evaluate the propagation loss. Also, due to the uncertainty of the coupling loss, the actual input light intensity and output light intensity to the QPM type wavelength conversion device can not be accurately known. Therefore, the evaluation of the wavelength conversion efficiency was not accurate.

  As a method of measuring propagation loss more accurately, there is a cut back method. In the cut back method, the propagation loss is obtained by comparing the input and output characteristics of a plurality of optical elements having different element lengths. However, in the cut back method, when preparing a plurality of optical elements having different element lengths, it is necessary to perform destruction such as cutting the optical elements. That is, the cutback method is a destructive inspection.

  Further, as a method of evaluating the QPM type wavelength conversion element, there is a method of forming a thin film containing a phosphor or a scatterer in an optical waveguide constituting the QPM type wavelength conversion element (see, for example, Patent Document 2). In this method, fluorescence or scattered light due to light leaked from the optical waveguide is photographed. However, this method has a problem that the number of processes for forming a thin film increases in the manufacturing process. In addition, phosphors and scatterers themselves cause propagation loss. Therefore, it is not preferable to apply to the QPM type wavelength conversion element shipped as a product. Therefore, in order to carry out this method, it is necessary to produce a QPM type wavelength conversion device for testing.

  The QPM type wavelength conversion element is difficult to manufacture. Therefore, in view of the manufacturing cost, it is not preferable to apply the cutback method, which is a destructive inspection, or the inspection that needs to prepare a test element to the QPM type wavelength conversion element.

  In addition to the methods described above, it is conceivable to apply the prism coupling method to measure the intensity of the converted light in evaluating the characteristics of the QPM type wavelength conversion element. However, in the prism coupling method, it is difficult to scan the prism in a state of being in close contact with the surface of the optical waveguide in the propagation direction. Therefore, it is difficult to obtain the intensity distribution of the converted light in the QPM type wavelength conversion element. Also, by bringing the prism into close contact, there is a risk that the optical waveguide constituting the QPM type wavelength conversion element may be damaged.

JP 2005-70529 A Unexamined-Japanese-Patent No. 5-248991

  Therefore, an object of the present invention is to provide an evaluation apparatus and an evaluation method capable of evaluating the characteristics of a QPM type wavelength conversion element nondestructively and without preparing a test element.

  In order to achieve the above-mentioned object, the evaluation apparatus of the QPM type wavelength conversion device of the present invention has the following features.

That is, the evaluation apparatus of the QPM type wavelength conversion element is converted by the QPM type wavelength conversion element leaked from the optical waveguide and a light source that sends the converted light of QPM wavelength to the optical waveguide constituting the QPM type wavelength conversion element to be evaluated. intensity conversion light converted from light, an objective lens for focusing a magnification which is set according to the width of the optical waveguide, and a detector for detecting the intensity of light objective lens is condensed, the detection unit detects And a processing unit for acquiring the intensity distribution of converted light leaking from the optical waveguide along the light propagation direction in the optical waveguide .

  Further, the evaluation method of the QPM type wavelength conversion element of the present invention includes the following steps.

In other words, the evaluation method of the QPM type wavelength conversion element includes a first step of sending to be converted light QPM wavelength to the optical waveguide constituting the QPM wavelength converting element to be evaluated, leaks from the optical waveguide, the QPM wavelength converting element The second process of condensing the converted light converted from the light to be converted with a magnification set according to the width of the optical waveguide , the third process of detecting the intensity of the condensed light, and the detection in the third process And a fourth process of obtaining an intensity distribution of converted light leaking from the optical waveguide along the light propagation direction in the optical waveguide based on the intensity .

  In the evaluation apparatus and the evaluation method of the QPM type wavelength conversion element of the present invention, the intensity of light (leakage light) leaking from the optical waveguide is detected. Since the intensity of leaked light corresponds to the intensity of light propagating through the optical waveguide, the characteristics of the QPM type wavelength conversion element can be evaluated based on the intensity of leaked light.

  Further, in the evaluation apparatus and the evaluation method of the QPM type wavelength conversion element of the present invention, the intensity of leaked light can be detected by condensing the leaked light by the objective lens. Therefore, it is not necessary to destroy the QPM type wavelength conversion element or to form a thin film or adhere closely to the QPM type wavelength conversion element. Therefore, in the evaluation apparatus and evaluation method of the QPM type wavelength conversion element of the present invention, the characteristics of the QPM type wavelength conversion element can be evaluated nondestructively and without the need to prepare a test element. .

It is the schematic which shows the evaluation apparatus of QPM type | mold wavelength conversion element by this invention. It is a figure which shows intensity distribution of the leaked light along the light propagation direction of the optical waveguide acquired using the evaluation apparatus and evaluation method of QPM type | system | group wavelength conversion element by this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shapes, sizes, and arrangement relationships of respective components are merely schematically shown to the extent that the present invention can be understood. In addition, although preferred embodiments of the present invention will be described below, materials and numerical conditions of each component are merely preferred embodiments. Therefore, the present invention is not limited to the following embodiments, and many changes or modifications that can achieve the effects of the present invention can be made without departing from the scope of the configuration of the present invention.

(Evaluation equipment of QPM type wavelength converter)
Referring to FIG. 1, an evaluation apparatus (also referred to simply as an evaluation apparatus) of a QPM type wavelength conversion element according to an embodiment of the present invention will be described. FIG. 1 is a schematic view showing an evaluation device.

  In the evaluation apparatus 100, light is input to the QPM type wavelength conversion element 200 to be evaluated. Then, the intensity of the light leaking from the optical waveguide constituting the QPM type wavelength conversion element 200 is detected. The evaluation apparatus 100 obtains the wavelength conversion characteristic of the QPM type wavelength conversion element 200 based on the detected intensity of leaked light.

The QPM type wavelength conversion element 200 to be evaluated is configured by forming a periodic polarization inversion structure in the optical waveguide 250. The QPM type wavelength conversion element 200 is, for example, periodically poled lithium niobate (PPLN: Periodically-Poled LiNbO ₃ ). The QPM type wavelength conversion element 200 wavelength-converts the converted light of the QPM wavelength according to the periodical polarization inversion structure by SHG.

  Here, in the QPM type wavelength conversion element 200, light input from one end 250a of the optical waveguide 250 is output from the other end 250b. Therefore, the light input to the QPM type wavelength conversion element 200 propagates the optical waveguide 250 in the direction from the one end 250 a to the other end 250 b. When the light to be converted of the QPM wavelength is input from the one end 250a, the converted light obtained by wavelength conversion from the light to be converted is output from the other end 250b. In FIG. 1, the light propagation direction in the optical waveguide 250 is indicated by an arrow R. The light guide 250 extends, for example, linearly along the light propagation direction.

  The evaluation apparatus 100 includes a light source 10, an objective lens 20, a beam splitter 30, a detection unit 40, a processing unit 50, an imaging unit 60, and an image display unit 70. Further, the QPM type wavelength conversion element 200 to be evaluated is mounted on a mounting table (not shown) provided in the evaluation apparatus 100.

  The light source 10 generates light and sends it to the optical waveguide 250 that constitutes the QPM wavelength conversion element 200. Here, the light source 10 generates converted light of QPM wavelength and sends it to the optical waveguide 250. As the light source 10, for example, a laser such as an LD (Laser Diode) can be used.

  The light source 10 and the optical waveguide 250 are connected by an optical fiber 91, for example. The light generated by the light source 10 passes through the optical fiber 91 and is input to the optical waveguide 250 from one end 250 a.

  The objective lens 20 is disposed to face the side surface 250 c of the optical waveguide 250 along the light propagation direction R. Then, the objective lens 20 condenses the light leaked from the optical waveguide 250 (leakage light). In this embodiment, among the converted light generated by wavelength conversion from the converted light in the optical waveguide 250, the objective lens 20 condenses the converted light leaking from the side surface 250c of the optical waveguide 250. The magnification of the objective lens 20 is set in accordance with the width of the optical waveguide 250 (the dimension in the direction orthogonal to the light propagation direction R).

  Further, one or both of the mounting table on which the QPM wavelength conversion element 200 is mounted and the objective lens 20 can move horizontally along the light propagation direction R. Therefore, the relative position of the optical waveguide 250 and the objective lens 20 along the light propagation direction R can be changed. Therefore, the objective lens 20 can sweep along the light propagation direction R to condense the leaked light from the optical waveguide 250. In this embodiment, the objective lens 20 condenses the leaked light at each position while changing the position opposed to the optical waveguide 250 along the light propagation direction R. Therefore, the objective lens 20 condenses the leaked light from a plurality of positions between the one end 250 a and the other end 250 b of the optical waveguide 250.

  The objective lens 20 sends the collected leaked light to the beam splitter 30. The beam splitter 30 splits the leaked light sent from the objective lens 20 into two. The beam splitter 30 sends one of the branched branched light to the detection unit 40 sequentially through the filter 81, the first connection lens 83, and the optical fiber 93.

  The filter 81 removes light in wavelength bands other than the converted light to be detected as leaked light. Therefore, even if, for example, the objective lens 20 undesirably condenses the light other than the converted light, it is possible to remove the light that is undesirably condensed by the filter 81.

  The first connection lens 83 condenses the leaked light sent from the filter 81 and sends it to the optical fiber 93.

  The optical fiber 93 is connected to the detection unit 40, and transmits leaked light sent from the first connection lens 83 to the detection unit 40.

  The detection unit 40 detects the intensity of the leaked light. As the detection unit 40, for example, a photodetector such as PD (Photo Diode) can be used. Here, the detection unit 40 detects the intensity of the converted light as leaked light. Therefore, it is preferable to use, as the detection unit 40, a photodetector optimized according to the wavelength of the converted light. The detection unit 40 sends information on the detected intensity of leaked light to the processing unit 50.

  The processing unit 50 records the intensity information sent from the detection unit 40. For example, a computer or the like can be used as the processing unit 50. Here, as described above, in this embodiment, the objective lens 20 condenses the leaked light from a plurality of positions along the light propagation direction R of the optical waveguide 250. Therefore, information on the intensity of leaked light from each position of the optical waveguide 250 is sent to the processing unit 50. The processing unit 50 records the received information on the intensity of leaked light for each position of the optical waveguide 250. Then, the processing unit 50 acquires the intensity distribution of leaked light along the light propagation direction R of the optical waveguide 250 based on the information of the intensity of leaked light at each position.

  Further, the above-described beam splitter 30 sends the other of the two branched leaked light to the imaging unit 60 via the second connection lens 85.

  The second connection lens 85 condenses the leaked light sent from the beam splitter 30 and sends it to the imaging unit 60.

  The imaging unit 60 images the leaked light sent from the second connection lens 85. In addition, the imaging unit 60 captures an image of the side surface 250 c of the optical waveguide 250 which is coupled by a system of the objective lens 20, the beam splitter 30, and the second connection lens 85. The imaging unit 60 generates image information of the optical waveguide 250 from the captured leaked light and the image of the optical waveguide 250. As the imaging unit 60, for example, a CCD (Charge Coupled Device) camera can be used. The imaging unit 60 sends the generated image information to the image display unit 70.

  The image display unit 70 visibly displays the image information sent from the imaging unit 60. For example, a known monitor or the like can be used as the image display unit 70. The image information of the optical waveguide 250 displayed on the image display unit 70 can be used, for example, for alignment between the objective lens 20 and the optical waveguide 250, setting of the magnification of the objective lens 20, and the like.

(Evaluation method of QPM type wavelength converter)
A method of evaluating a QPM wavelength conversion element (also referred to simply as an evaluation method) according to an embodiment of the present invention will be described. The evaluation method according to this embodiment is performed using the above-described evaluation apparatus 100.

  The light source 10 is used to send light to the optical waveguide 250 that constitutes the QPM type wavelength conversion element 200 to be evaluated. Here, the converted light of the QPM wavelength is sent to the optical waveguide 250. The converted light is converted into converted light by the SHG while propagating through the optical waveguide 250. The converted light is output from the other end 250 b of the optical waveguide 250. The converted light is not only output from the other end 250 b of the optical waveguide 250, but also leaks as leakage light from the side surface 250 c of the optical waveguide 250. This leaked light is collected using an objective lens 20. Then, the intensity of the converted light as leaked light collected by the objective lens 20 is detected using the detection unit 40.

  The objective lens 20 condenses leaked light from a plurality of positions between one end 250 a and the other end 250 b of the optical waveguide 250. In the evaluation method according to this embodiment, the processing unit 50 is used to acquire the intensity distribution of leaked light along the light propagation direction R based on the information on the intensity of leaked light at each position.

  The intensity of the converted light as leaked light is proportional to the intensity of the converted light generated by wavelength conversion from the light to be converted in the optical waveguide 250. Therefore, by acquiring the intensity distribution of leaked light along the light propagation direction R, the relationship between the position along the light propagation direction R in the optical waveguide 250 and the intensity of converted light generated at that position can be known. Therefore, based on the intensity distribution of the leaked light, the relationship between the length of the optical waveguide 250 along the light propagation direction R and the wavelength conversion efficiency can be known.

  Therefore, in the evaluation apparatus and the evaluation method according to this embodiment, it is possible to evaluate the wavelength conversion characteristics in the QPM type wavelength conversion element nondestructively and without preparing a test element. In the evaluation apparatus and evaluation method according to this embodiment, the appropriate length of the optical waveguide 250 (ie, the QPM type wavelength) is determined based on the relationship between the length of the optical waveguide 250 along the light propagation direction R and the wavelength conversion characteristic. The element length of the conversion element can be determined.

(Characteristics evaluation)
The inventor conducted an experiment to evaluate the QPM type wavelength conversion device using the evaluation apparatus and the evaluation method according to this embodiment.

  In this experiment, PPLN having a length of 56 mm and a width of 9 μm for the optical waveguide 250 was used as the QPM type wavelength conversion element 200 to be evaluated. Further, the wavelength of the converted light to be input to the optical waveguide 250 is set to 1549.75 nm corresponding to the QPM wavelength of PPLN. In this case, in the QPM wavelength conversion element 200, the wavelength of converted light generated by SHG is about 775 nm. Then, the objective lens 20, the filter 81, the first connection lens 83, and the detection unit 40 are designed by optimizing the dimensions of the optical waveguide 250 and the wavelength of the converted light. Here, the magnification of the objective lens 20 is set to 10 times, and the width of the area where light can be collected is about 10 μm. Then, leaked light (converted light) was collected at a plurality of positions along the light propagation direction R of the optical waveguide 250. The intensity of the leaked light at each of these positions was detected, and the intensity distribution of the leaked light along the light propagation direction R was acquired.

  The intensity distribution obtained in this experiment is shown in FIG. In FIG. 2, the vertical axis indicates the intensity as leaked light (converted light) in arbitrary units. Further, on the horizontal axis, the length of the optical waveguide 250 corresponding to the focusing position is shown in mm units as the focusing position by the objective lens 20. The origin (0 mm) on the horizontal axis corresponds to the position of one end 250 a of the optical waveguide 250.

  Each point shown in FIG. 2 is an actual measurement value of the intensity of leaked light at each position along the light propagation direction R of the optical waveguide 250. As described above, these intensities correspond to the intensities of converted light generated at each position of the optical waveguide 250.

  Moreover, the curve shown in FIG. 2 is an approximation line calculated | required from the intensity | strength of each point which is an actual value. Here, in determining the curve shown in FIG. 2, the curve shown in FIG. 2 was determined by fitting the change rate based on the intensity of each point and the propagation loss of converted light as parameters.

  As shown in FIG. 2, the intensity of converted light as leaked light increases as the optical waveguide 250 becomes longer. Then, the light is saturated at a position where the length of the optical waveguide 250 is around 47 mm. Therefore, in the QPM type wavelength conversion element 200 used in this experiment, the wavelength conversion efficiency can be maximized by designing the length of the optical waveguide 250 to 47 mm or more.

  Here, as described above, in this experiment, the QPM type wavelength conversion element 200 in which the length of the optical waveguide 250 is 56 mm was used. From the results shown in FIG. 2, it is found that the wavelength conversion efficiency is maximized when the length of the optical waveguide 250 is designed to be at least 47 mm. Therefore, the maximum wavelength conversion efficiency can be obtained even if the length of the optical waveguide 250 is shortened by about 16% as compared with the QPM type wavelength conversion device 200 used in the experiment. Therefore, it is possible to reduce the size of the QPM wavelength conversion element 200 while securing the maximum wavelength conversion efficiency.

  Thus, by using the evaluation apparatus and evaluation method according to this embodiment, it is possible to evaluate the wavelength conversion efficiency and obtain the optimum value of the length of the optical waveguide 250 (that is, the element length of the QPM type wavelength conversion element). it can.

10: light source 20: objective lens 30: beam splitter 40: detection unit 50: processing unit 60: imaging unit 70: image display unit 81: filter 83: first connection lens 85: second connection lens 91: 93: light Fiber 100: Evaluation device 200: QPM wavelength conversion element 250: Optical waveguide

Claims (2)

A light source for sending converted light of QPM wavelength to an optical waveguide constituting a QPM type wavelength conversion element to be evaluated;
An objective lens that condenses converted light that has been converted from the converted light by the QPM type wavelength conversion element and that leaks from the optical waveguide at a magnification that is set according to the width of the optical waveguide ;
A detection unit that detects the intensity of light collected by the objective lens ;
A processing unit for acquiring an intensity distribution of the converted light leaking from the optical waveguide along the light propagation direction in the optical waveguide based on the intensity detected by the detection unit; Evaluation device for optical wavelength converter. A first process of sending converted light of QPM wavelength to an optical waveguide constituting a QPM type wavelength conversion device to be evaluated;
A second process of condensing the converted light converted from the converted light by the QPM type wavelength conversion device, which leaks from the optical waveguide, at a magnification set according to the width of the optical waveguide ;
A third step of detecting the intensity of the collected light ;
Acquiring the intensity distribution of the converted light leaking from the optical waveguide along the light propagation direction in the optical waveguide based on the intensity detected in the third step; and QPM type wavelength conversion Element evaluation method.

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