Method and system for estimating characteristics of wavelength conversion device

Abstract

The power of input light and output light in relation to wavelength conversion operation of a wavelength conversion device (10) is measured for a case where the wavelength conversion operation is performed on a secondary nonlinear medium (100) included in the wavelength conversion device (10) by means of light inputted in the forward direction with respect to the secondary nonlinear medium (100) and for a case where the wavelength conversion operation is performed on the secondary nonlinear medium (100) by light inputted in the reverse direction with respect to the secondary nonlinear medium (100), and estimation is made on the characteristics of the wavelength conversion device (10) on the basis of the measurement result.

Description

Method and system for estimating wavelength conversion device characteristics

[0001] The present disclosure relates to a method and system for estimating characteristics of a wavelength conversion device.

[0002] The wavelength conversion device is made of PPLN (periodically poled lithium niobate (LiNbO ３ For example, a waveguide device having two input and two output ports, for a total of four ports, is known.

[0003] However, to date, no technology has been proposed for estimating the characteristics of a PPLN waveguide device (which may include, for example, the coupling loss and / or wavelength conversion efficiency of each port) after assembly.

[0004] T. Umeki, O. Tadanaga, A. Takada, and M. Asobe, “Phase sensitive degenerate parametric amplification using directly-bonded PPLN ridge waveguides,” Opt. Express, Vol. 19 No. 7, pp. 6326-6332 (2011).Masaki Asobe, Osamu Tadanaga, Tsutomu Yanagawa, Hiroki Itoh, Hiroyuki Suzuki; “Reducing photorefractive effect in periodically poled ZnO-and MgO-doped LiNbO3 wavelength converters,” Appl. Phys. Lett. 21 May 2001; 78 (21): 3163-3165.MH Chou, I. Brener, KR Parameswaran and MM Fejer, “Stability and bandwidth enhancement of difference frequency conversion generation (DFGI-based wavelength conversion by pump detuning,”ELECTRONICS LETTERS 10th June 1999 Vol. 35 No. 12, pp. 978-990.

[0005] One of the objectives of the present disclosure is to enable estimation of the characteristics of a wavelength conversion device that includes a second-order nonlinear medium.

[0006] Therefore, according to one aspect of the present disclosure, a method for estimating the characteristics of a wavelength conversion device includes a measurement process for measuring the power of input light and output light related to the wavelength conversion operation of the wavelength conversion device when a second-order nonlinear medium included in the wavelength conversion device is caused to perform wavelength conversion operation by a forward light input to the second-order nonlinear medium and when a second-order nonlinear medium is caused to perform wavelength conversion operation by a reverse light input to the second-order nonlinear medium, and an estimation process for estimating the characteristics of the wavelength conversion device based on the measurement results of the measurement process.

[0007] FIG. 1 is a schematic diagram showing an example of the configuration of a PPLN device having a four-port structure according to an embodiment; FIG. 2 is a diagram for explaining a wavelength conversion operation in a PPLN waveguide according to an embodiment; FIG. 3 is a diagram for explaining a second harmonic generation (SHG) process in a PPLN waveguide according to an embodiment; FIG. 4 is a diagram for explaining a difference frequency generation (DFG) process in a PPLN waveguide according to an embodiment; FIG. 5 is a diagram for explaining a DFG process in a PPLN waveguide according to an embodiment; FIG. 6 is a diagram for explaining a DFG process in a PPLN waveguide according to an embodiment; FIG. 7 is a diagram for explaining a configuration of an exemplary measurement system for a PPLN device having a four-port structure according to an embodiment; FIG. 8 is a flowchart for explaining a measurement procedure using the measurement system illustrated in FIG. 5; FIG. 9 is a diagram showing an example of port connection relationships in a measurement system for reverse direction measurement according to an embodiment; FIG. 10 is a flowchart for explaining an example of a procedure for SHG measurement and DFG measurement according to an embodiment. 5 is a flowchart illustrating an example of a procedure for SHG measurement and DFG measurement in reverse direction measurement according to the embodiment. FIG.

[0008] Hereinafter, embodiments will be described with reference to the drawings. Note that the present disclosure is not limited to these embodiments. In addition, in the description of the drawings, the same or corresponding elements are appropriately designated by the same reference numerals. The drawings are schematic, and the dimensional relationships and ratios of each element may differ from the actual. Even between drawings, there may be parts in which the dimensional relationships and ratios differ. When numerical values ​​are described in the following description, they are merely examples, and other numerical values ​​may be used in addition or instead.

[0009] <Summary> With the recent diversification of information and communication technology services, there is a demand for further increases in transmission capacity in the optical communication systems that support the backbone of communication networks. According to Shannon's communication theory, transmission capacity C is expressed by the following equation 1 using the signal bandwidth W and the signal-to-noise (S / N) ratio.

[0010] From Equation 1, the transmission capacity is determined by the bandwidth used for transmission and the S / N ratio. However, in systems that use optical amplifiers (EDFA: Erbium-Doped Fiber Amplifier), which are currently in widespread use, it has been pointed out that there are limits to the improvement of the S / N ratio.

[0011] Examples of factors that cause such limitations include limitations on the amplification band due to the materials that make up the optical amplifier, accumulation of noise due to multiple optical amplifiers, and signal distortion due to nonlinear optical effects that occur in the transmission fiber.

[0012] Therefore, in the field of optical communication technology, research and development of technologies using nonlinear optical devices is progressing, such as wavelength band batch conversion or broadband parametric amplification technology that can overcome band limitations, or phase conjugate conversion or phase sensitive amplification technology that can improve the S / N ratio.

[0013] Various materials have been researched and developed as nonlinear optical media for use in nonlinear optical devices. For example, lithium niobate (LiNbO ３Oxide-based compound substrates such as SiO2, SiO2, and SiO2 are promising materials for nonlinear optical media because they have very high second-order nonlinear optical and electro-optic constants.

[0014] An example of an optical device utilizing the high nonlinear optical effect of lithium niobate is periodically poled lithium niobate (PPLN). An example of an optical device utilizing PPLN is a wavelength conversion element that performs wavelength conversion using second harmonic generation (SHG), difference frequency generation (DFG), and / or sum frequency generation (SFG) by PPLN.

[0015] In order to increase the efficiency of the nonlinear optical effect (e.g., wavelength conversion efficiency) of PPLN, it is effective to use an optical waveguide device. One reason for this is that wavelength conversion efficiency tends to be proportional to the power density of light propagating through a nonlinear medium, and thus forming a waveguide structure allows light to be efficiently confined within a limited range.

[0016] For this reason, various waveguides using nonlinear media have been researched and developed, and examples of waveguide structures include a diffusion-type waveguide called a Ti-diffused waveguide or a proton-exchanged waveguide, and a ridge-type optical waveguide.

[0017] In any of the above-described waveguide structures, when a PPLN waveguide is applied to optical communications, compatibility of the connection (or coupling) between the PPLN waveguide and an optical fiber is taken into consideration. Therefore, an optical device including a PPLN waveguide (hereinafter, for convenience, may be referred to as a "PPLN device") has, for example, a plurality of ports corresponding to the input and output of light, and an optical fiber is connected to each port.

[0018] In a PPLN waveguide, wavelength conversion such as SHG, DFG, or SFG occurs through the interaction of two types of light, for example, fundamental light and second-harmonic light, and therefore a PPLN device may have optical fiber input and output ports corresponding to the fundamental light and second-harmonic light, respectively.

[0019] For example, Non-Patent Document 1 discloses a PPLN device with a four-port structure (hereinafter, sometimes referred to as a "four-port PPLN device"). Note that "second harmonic light" corresponds to light with twice the frequency (half the wavelength) of the fundamental light, and is therefore also called "double harmonic light."

[0020] <Schematic Configuration Example of PPLN Device> Fig. 1 is a schematic diagram showing a configuration example of a PPLN device with a four-port structure. The PPLN device 10 shown in Fig. 1 includes, for example, a PPLN waveguide 100 and four ports (first to fourth) 102-1 to 102-4. Optical fibers can be coupled to each of the ports 102-1 to 102-4.

[0021] In FIG. 1, the waveguide length of the PPLN waveguide 100 is represented by L, and the wavelength conversion efficiency of the PPLN waveguide 100 is represented by η.

[0022] The first port 102-1 and the second port 102-2 are provided, for example, on one side of two short sides of the PPLN device 10. The third port 102-3 and the fourth port 102-4 are provided on the other side of the two short sides of the PPLN device 10.

[0023] Each of the ports 102-1 to 102-4 may correspond to either an optical input port or an optical output port. For example, the first port 102-1 and the second port 102-2 may correspond to an optical input port, and the third port 102-3 and the fourth port 102-4 may correspond to an optical output port.

[0024] Conversely, due to the reciprocity of the PPLN device 10, the first port 102-1 and the second port 102-2 may correspond to optical output ports, and the third port 102-3 and the fourth port 102-4 may correspond to optical input ports.

[0025] For convenience, of the four ports 102-1 to 102-4, the port through which light having the wavelength of "fundamental wave light" is input or output may be referred to as the "fundamental wave port," and the port through which light having the wavelength of "second harmonic light" is input or output may be referred to as the "second harmonic port."

[0026] For example, as will be described later, the first port 102-1 or the fourth port 102-4, which inputs or outputs the “fundamental wave light” or the “signal light,” may correspond to the “fundamental wave port.” Also, for example, the third port 102-3, which outputs the “second harmonic light,” or the second port 102-2, which inputs the “second harmonic light,” may correspond to the “second harmonic port.”

[0027] In the following description, when there is no need to distinguish between the individual ports 102-1 to 102-4, they may be simply referred to as "ports 102." Furthermore, the PPLN waveguide 100 may be simply referred to as "waveguide 100."

[0028] The performance (or characteristics) of such an optical fiber coupled optical device may vary depending on, for example, the characteristics of the PPLN waveguide 100 as well as the coupling loss (CL) between the optical fiber at each port 102 and the waveguide 100. Here, the characteristics of the PPLN waveguide 100 include, for example, wavelength conversion efficiency.

[0029] <Wavelength Conversion Efficiency of PPLN Waveguide> The wavelength conversion efficiency of the PPLN waveguide 100 can be improved, for example, by uniforming the phase matching conditions at different locations in the PPLN waveguide 100. For example, when SHG is generated in the PPLN waveguide 100, the phase matching condition between the fundamental wave light and the second harmonic light is expressed by the following equation 2.

[0030] In Equation 2, λ Ｆ is the wavelength of the fundamental light, n Ｆ is the effective refractive index for the fundamental wave light, λ ＳＨ is the wavelength of the second harmonic light, n ＳＨ represents the effective refractive index for the second harmonic wave, and Λ represents the polarization inversion period.

[0031] Here, the effective refractive indexes n Ｆ and n ＳＨmay vary depending on the width and / or thickness of the PPLN waveguide 100. For example, there may be a distribution (or variation) in the waveguide width and / or thickness across the PPLN waveguide 100 in a direction along the light propagation direction (e.g., the longitudinal direction).

[0032] In this case, a change occurs in the effective refractive index in the longitudinal direction of the PPLN waveguide 100, and therefore the wavelength that satisfies the condition of Equation 1 may differ at different points in the longitudinal direction of the PPLN waveguide 100. As a result, the characteristics of the PPLN waveguide 100, such as wavelength conversion efficiency, may be reduced.

[0033] Furthermore, even if a PPLN waveguide 100 with improved wavelength conversion efficiency can be fabricated by uniforming the phase matching conditions, the total wavelength conversion efficiency of the PPLN device 10 may be reduced due to coupling loss between the PPLN waveguide 100 and the optical fibers coupled to each port 102.

[0034] Therefore, when applying or utilizing the PPLN device 10 for communications, it is desirable to know the performance (or characteristics) of the PPLN device 10 and / or the coupling loss of each port 102 .

[0035] For example, when assembling a PPLN device 10 with a four-port structure, by evaluating in detail the optical loss before and after coupling the optical fibers, it is possible to grasp the coupling loss of each port 102 during assembly and to evaluate the characteristics of the PPLN waveguide 100 in advance.

[0036] However, even after assembling the PPLN device 10, it may be difficult in practice to maintain the coupling loss of each port 102 and / or the characteristics of the PPLN waveguide 100. The reason for this will be explained below.

[0037] <Phase-matched wavelength> One of the important characteristics of the PPLN waveguide 100 is the phase-matched wavelength, which contributes to wavelength conversion efficiency. The phase-matched wavelength may change depending on temperature. To keep the phase-matched wavelength stable, for example, the PPLN waveguide 100 may be provided with a temperature adjustment mechanism for adjusting (or controlling) the temperature of the PPLN waveguide 100 to a constant value.

[0038] When assembling the PPLN device 10, for example, the temperature of the PPLN waveguide 100 is set to an operating temperature at which a desired phase-matched wavelength is obtained, and then optical fibers are attached (e.g., coupled) to each port 102.

[0039] However, after the PPLN device 10 is assembled, for example, if the PPLN waveguide 100 is fixed to a device package using a paste material, the paste material may cause stress fluctuations in the PPLN waveguide 100. Furthermore, the photorefractive effect described in Non-Patent Document 2 may cause refractive index fluctuations in the PPLN waveguide 100, which may cause the phase-matched wavelength to deviate from the intended wavelength.

[0040] In such a case, the desired phase-matched wavelength can be obtained by changing the set temperature of the PPLN waveguide 100. However, the change in the set temperature may cause a change in thermal stress applied to the components used in the PPLN device 10, which may result in a change in the coupling loss of each port 102.

[0041] Furthermore, the PPLN device 10 may be used in a state (or under conditions) in which the phase-matched wavelength is actively or intentionally detuned by changing the set temperature, as described in, for example, Non-Patent Document 3. In such cases, the coupling loss of each port 102 may also change depending on the conditions under which the PPLN device 10 is used.

[0042] Furthermore, the stress applied to the PPLN waveguide 100 via the paste material is not uniformly applied throughout the PPLN waveguide 100, which can result in a stress distribution within the PPLN waveguide 100. A refractive index distribution corresponding to the stress distribution can occur within the PPLN waveguide 100, which can cause the phase-matched wavelength to deviate from the desired wavelength, resulting in a decrease in wavelength conversion efficiency.

[0043] For the reasons described above, it is difficult to accurately grasp (e.g., measure or estimate) the characteristics of the assembled PPLN device (e.g., the coupling loss of each port 102 and / or the wavelength conversion efficiency of the PPLN waveguide 100).

[0044] Therefore, in the embodiments described below, a method and apparatus capable of accurately estimating the characteristics of the PPLN device 10, for example, the coupling loss of each port 102 and / or the wavelength conversion efficiency of the PPLN waveguide 100, will be described.

[0045] <Estimation of Coupling Loss> For example, the coupling loss is estimated by measuring the characteristics related to input / output or wavelength conversion of the assembled PPLN device 10. In the PPLN device 10 having a four-port structure, the unknown coupling loss to be estimated is the coupling loss for each of at least four ports 102.

[0046] Here, simply measuring or estimating the transmittance of the PPLN device 10 for the fundamental light and the second harmonic light would result in an insufficient number of measured or estimated values ​​compared to the number of unknown parameters (e.g., coupling loss), making it insufficient to estimate the coupling loss of each port 102.

[0047] Furthermore, since the coupling loss corresponds to the loss for the fundamental mode of light propagating through the optical fiber and the PPLN waveguide 100, it is difficult to accurately estimate the coupling loss for the fundamental mode by measuring the transmittance of, for example, second harmonic light. The reasons for this are as follows.

[0048] For example, the PPLN waveguide 100 is typically designed (e.g., optimized) with a core size that functions as a single-mode waveguide for the wavelength of the fundamental light. Because the second harmonic wave is half the wavelength of the fundamental light, a waveguide 100 with a core size adapted to the wavelength of the fundamental wave necessarily functions as a multi-mode waveguide for the second harmonic wave.

[0049] When external light is incident on a multimode waveguide, multiple higher-order modes are excited and the light in each mode propagates independently. Therefore, it is not easy to distinguish which mode of light is causing the amount of loss, making it difficult to estimate coupling loss, etc.

[0050] In consideration of the above circumstances, in this embodiment, the wavelength conversion in the PPLN waveguide 100 occurs for fundamental mode light, and the output light is measured when the PPLN device 10 is operating in wavelength conversion mode. Furthermore, by utilizing the bidirectionality (or reciprocity) of the PPLN device 10, the measurement is performed by switching the input / output relationship (or propagation direction) of light to / from the PPLN device 10. Based on the results of such measurements, it is possible to estimate, for example, the coupling loss for each port 102 of the PPLN device 10.

[0051] <Wavelength Conversion Operation of PPLN Waveguide> Prior to describing specific examples of this embodiment, wavelength conversion operations such as SHG or DFG in the PPLN waveguide 100 will be described with reference to Fig. 2. Fig. 2 is a diagram for explaining wavelength conversion operations in the PPLN waveguide 100. Fig. 2 shows an example of wavelength conversion efficiency due to SHG and DFG occurring in the PPLN waveguide 100, with the horizontal axis representing wavelength and the vertical axis representing normalized wavelength conversion efficiency.

[0052] The fundamental light is, for example, a laser light of a single wavelength output from a single laser light source (LD), as shown in the lower part of Fig. 2, and a phase matching line 203 is obtained for the laser light. The phase matching curves 201 and 202 shown in the upper part of Fig. 2 correspond to the phase matching bands for SHG and DFG of the PPLN waveguide 100, respectively, which satisfy Equation 1.

[0053] The bandwidth of the phase matching curve 201 for SHG is narrower than the bandwidth of the phase matching curve 202 for DFG, but is sufficiently wider than the linewidth of the phase matching line 203 for the laser light, which is the fundamental wave light.

[0054] Here, the wavelength of the fundamental light λ ０ (Frequency: ω ０ ) is 1545 nanometers (nm), the wavelength λp of the pump light is half the wavelength of the fundamental wave light (frequency ω ０ Twice as large as 2ω ０ ) is set to 772.5 nm, the DFG process of the PPLN waveguide 100 will be described.

[0055] When signal light and pump light having a wavelength corresponding to the second harmonic of the fundamental light are input to the PPLN waveguide 100, the DFG converts the wavelength λ of the fundamental light into a second harmonic, as shown in the upper part of FIG. ０ The converted light of the signal light is generated by folding back the signal light on the wavelength axis around the center. Note that, for convenience, pump light of a wavelength corresponding to the second harmonic may be abbreviated as "second harmonic pump light" below.

[0056] For example, the wavelength λs of the signal light (frequency: ω ｓ ) is assumed to be 1540 nm, 2ω ０ -ω ｓ A converted light having a wavelength of 1550 nm is generated by the DFG represented by the formula (1). As the converted light is generated, energy is transferred from the pump light to the signal light, and the signal light is amplified. This amplification is also called parametric amplification.

[0057] At this time, the quasi-phase matching condition is satisfied among the three waves of the second harmonic pump light, the signal light, and the converted light in the PPLN waveguide 100. For example, when the effective refractive indexes of the second harmonic pump light, the signal light, and the converted light in the PPLN waveguide 100 are respectively n ＳＨ , n ｓ , n ｃ , the PPLN waveguide 100 has a polarization inversion structure with an inversion period Λ that satisfies the phase matching condition expressed by the following equation 3:

[0058] When Equation 3 is satisfied, even if the signal light wavelength λs changes, the pump light and the frequency 2ω ０ -ω ｓ For example, the same conversion efficiency can be obtained between the converted light of the signal light wavelength λs (frequency: ω ｓ ) is set to 1539 nm, 2ω ０ -ω ｓ Thus, converted light with a wavelength of 1551 nm is generated.

[0059] At this time, the effective refractive index of the signal light is n ｓ and the effective refractive index of the converted light n ｃ The effective refractive index of the signal light n ｓ The effective refractive index of the converted light n ｃ Therefore, even when the signal light wavelength λs is changed, Equation 3 can be satisfied. Therefore, in parametric amplification using the DFG of the PPLN waveguide 100, it is possible to broaden the bandwidth of the phase matching curve (in other words, the wavelength conversion band) 202, as shown in the upper part of FIG.

[0060] <Estimation of Coupling Loss of Each Port> Next, an example of a method for estimating the coupling loss of each port 102 in the four-port PPLN device 10 in this embodiment will be described with reference to FIGS. 3A, 3B, 4A, and 4B.

[0061] For ease of explanation, the ports 102-1 and 102-4 of the PPLN device 10 that correspond to the "fundamental wave ports" may be referred to as "FH port #1" and "FH port #2," respectively. Also, the ports 102-2 and 102-3 that correspond to the "second harmonic ports" may be referred to as "SH port #1" and "SH port #2," respectively.

[0062] In this embodiment, for example, measurements of SHG and DFG in the PPLN device 10 are performed by changing the optical input / output relationship to the PPLN device 10 (for example, by swapping the optical input / output ports), as described below.

[0063] 3A and 3B, the SHG process of the PPLN waveguide 100 will be described. ＦＨｉｎ , the output optical power of the second harmonic is P ＳＨｏｕｔ When the wavelength conversion efficiency per unit length is represented as η and the length of the PPLN waveguide 100 is represented as L, it is expressed by the following equation 4.

[0064] Equation 4 represents the SHG output at the peak wavelength of the SHG phase matching curve 201 illustrated in Fig. 2. Here, as illustrated in Fig. 3A, in the four-port PPLN device 10, for example, when fundamental wave light 301 is input to FH port #1, second harmonic wave 302 is generated by SHG in the PPLN waveguide 100 and output from SH port #2.

[0065] The power of the fundamental wave light 301 input to the FH port #1 is P ＦＨ１ , the coupling loss between the FH port #1 and the PPLN waveguide 100 is α １ , the output optical power from SH port #2 is P ＳＨ２ , the coupling loss between the SH port #2 and the PPLN waveguide 100 is β ２ When these are expressed as above, the output optical power P from the SH port #2 is ＳＨ２ is exemplarily represented by the following Equation 5.

[0066] As shown in Equation 5, the output P of the SHG in FIG. ＳＨ２ is the coupling loss α between the PPLN waveguide 100 and the FH port #1, which is the input port of the fundamental wave light 301, and the SH port #2, which is the output port of the second harmonic wave 302. １ and β ２ It is expressed in a format that includes.

[0067] Thereafter, as illustrated in FIG. 3B, the port for inputting the fundamental light into the PPLN waveguide 100 is changed to FH port #2 (port 102-4), and the fundamental light is input into the PPLN waveguide 100 in the opposite direction to that in FIG. 3A.

[0068] When the fundamental wave light 303 is input to the FH port #2, a second harmonic wave 304 is generated by SHG in the PPLN waveguide 100 and output from the SH port #1 (second port 102-2).

[0069] Here, the fundamental wave optical power input to FH port #2 is P ＦＨ２ , the coupling loss between the FH port #2 and the PPLN waveguide 100 is α ２ , the output optical power from SH port #1 is P ＳＨ１ , the coupling loss between the SH port #1 and the PPLN waveguide 100 is β １ When these are expressed as above, the output optical power P from the SH port #1 is ＳＨ１ is exemplarily represented by the following Equation 6.

[0070] As shown in Equation 6, the output P of the SHG in FIG. ＳＨ１ is the coupling loss α between the PPLN waveguide 100 and the FH port #2, which is the input port of the fundamental wave light 303, and the SH port #1, which is the output port of the second harmonic wave 304. ２ and β １ It is expressed in a format that includes.

[0071] 4A and 4B, the DFG process will be described. Regarding the DFG, the input power of the second harmonic wave to the PPLN waveguide 100 is expressed as P ＳＨｉｎ When expressed as above, the gain G for the signal light ｓ , and the gain G for the wavelength-converted light ｉ are expressed by the following Equations 7 and 8, respectively.

[0072] However, in Equations 7 and 8, g is expressed by the following Equation 9. Here, the phase mismatch amount Δβ is expressed by the following equation 10.

[0073] Assuming that the signal light and the wavelength-converted light have wavelengths close to the wavelength of the fundamental light, if Δβ is set to 0 in each of Equations 6 and 7, Equations 6 and 7 are transformed into the following Equations 11 and 12, respectively.

[0074] 4A, in a four-port PPLN device 10, signal light 401 is input to port 102-1 (FH port #1), and second harmonic pump light 402 is input to port 102-2 (SH port #1).

[0075] In this case, for example, output light 404 including a signal light component 441, a light component 442 converted by DFG of the signal light 401, and an ASE light 443 is output from the fourth port 102-4 (FH port #2).

[0076] In the example shown in FIG. 4A, the coupling loss α of each of the FH port #1, the SH port #1, and the FH port #2 associated with the DFG is １ , β １ and α ２ Taking this into consideration, the above-mentioned formulas 11 and 12 are transformed into the following formulas 13 and 14, respectively.

[0077] On the other hand, as illustrated in FIG. 4B, in the four-port PPLN device 10, signal light 405 is input to the fourth port 102-4 (FH port #2) and second harmonic pump light 406 is input to the third port 102-3 (SH port #2) in the opposite direction to that in FIG. 4A.

[0078] In this case, for example, output light 407 including a signal light component 471, a light component 472 converted by DFG of the signal light 401, and an ASE light 473 is output from the first port 102-1 (FH port #1).

[0079] In the example shown in FIG. 4B, the coupling losses α of the FH port #2, the SH port #2, and the FH port #1 associated with the DFG are２ , β ２ and α １ Taking this into consideration, the above-mentioned formulas 11 and 12 are transformed into the following formulas 15 and 16, respectively.

[0080] As shown in Equations 13 to 16, the wavelength conversion gain by the DFG is calculated by multiplying the coupling loss α １ and α ２ and the coupling loss β of the input SH port #1 or SH port #2 １ or β ２ It is represented by a form that includes:

[0081] Here, in the DFG process, the wavelength-converted light generated in the PPLN waveguide 100 is single-mode light, so that the influence of unnecessary higher-order modes and the like is minimal in the measurement results for the wavelength-converted light.

[0082] Therefore, it is desirable to use the measurement results for the wavelength-converted light rather than the signal light for coupling loss estimation. This is also effective in the case where, for example, the PPLN waveguide 100 is a waveguide that excites higher-order modes even for the wavelength of the fundamental light. For these reasons, in this embodiment, the gain measurement by DFG is performed for the wavelength-converted light.

[0083] As described above, by performing SHG and DFG measurements while changing the direction of light input and output in the opposite directions, the four equations 5, 6, 14, and 16 can be obtained. However, the coupling loss α １ , α ２ , β １ , β ２ However, the number of relational expressions is insufficient to determine a total of five unknowns, including the wavelength conversion efficiency η of the PPLN waveguide 100.

[0084] Therefore, for example, attention is focused on the power (or level) of spontaneous emission (ASE) generated during the DFG process. ASE is generated within the PPLN waveguide 100 when second-harmonic pump light is input to the PPLN waveguide 100, and is output from an output port (e.g., FH port #2 in FIG. 4A or FH port #1 in FIG. 4B).

[0085] Therefore, ASE is not the coupling loss of FH ports #1 and #2 on both the input and output sides as exemplified in Equation 14 and Equation 16 of the gain by DFG, but the coupling loss of FH port #2 or FH port #1 on the output side (α ２ or α １ ) information.

[0086] Here, the ASE is output by amplifying the vacuum field through the DFG process, and the vacuum field is distributed over the entire frequency band. Therefore, for example, the level of the ASE at wavelength λs is calculated by multiplying the gain (G ｓ ) and the gain (G ｉ ) corresponds to the sum of the component that has been wavelength converted to wavelength λs.

[0087] Assuming that the wavelength of the vacuum field in question is a wavelength close to the wavelength of the fundamental wave light, and assuming that Δβ is set to 0, the gain G Ｖａｃ is expressed by the following equation 17 using equations 11 and 12.

[0088] In the four-port PPLN device 10, for example, as shown in FIG. 4A, when second harmonic pump light 402 is input from SH port #1 (port 102-2), Equation 17 expresses the coupling loss β １ and α ２ Taking this into consideration, it is expressed by the following Equation 18.

[0089] The expectation value of the power of the vacuum field is expressed as hν / 2, where h is the Planck constant and ν is the frequency of the photon energy. Therefore, the power P per unit frequency width (for example, 1 Hz width) of the ASE is ＡＳＥ is expressed by the following formula 19, for example. ｓ and ν c represent the frequencies of the photon energy at wavelengths λs and λc, respectively.

[0090] Assuming that the ASE level is to be measured in the vicinity of the wavelength of the excitation light having a wavelength corresponding to the wavelength of the fundamental wave, v s ≒ν c Then, Equation 19 is expressed by the following Equation 20. Note that the vicinity of the wavelength of the excitation light exemplarily means a wavelength in a range where Equation 10 can be regarded as zero.

[0091] On the other hand, as shown in FIG. 4B, when the second harmonic pump light 406 is input from the SH port #2 (port 102-3), the power P ＡＳＥ is the coupling loss β of the input side SH port #2 and the output side FH port #1 ２ and α １ Taking this into consideration, it is expressed by the following Equation 22.

[0092] In this way, a total of six relational expressions are obtained for the five unknowns, which are the four expressions (Equation 5, Equation 6, Equation 14, and Equation 16) plus Equation 21 and Equation 22 relating to the ASE level. Here, the coupling loss α of each port 102 is １ , α ２ , β １ , β ２ and the wavelength conversion efficiency η of the PPLN waveguide 100 can be obtained by measurement.

[0093] Therefore, by simultaneously solving the above six relational expressions based on the measured values ​​of the SHG and the DFG, the coupling loss α of each port 102 can be calculated. １ , α ２ , β １ , β ２and the wavelength conversion efficiency η of the PPLN waveguide 100 can be estimated.

[0094] Next, an example will be described with reference to Fig. 5 to Fig. 10. Fig. 5 is a diagram showing the configuration of an exemplary measurement system 500 for the four-port PPLN device 10 described above, and Fig. 6 is a diagram showing an example configuration of the four-port PPLN device 10 according to the example.

[0095] <Configuration Example of Measurement System> As illustrated in FIG. 5 , a measurement system 500 includes, for example, a wavelength-tunable light source 502, a power meter 504, a second harmonic excitation light source 506, power meters 508 and 510, an optical spectrum analyzer (OSA) 512, and an arithmetic circuit 514.

[0096] The wavelength-tunable light source 502 is, for example, an example of a light source that generates fundamental light or signal light, and is optically coupled to port 102-1 (FH port #1) of the PPLN device 10 via, for example, a fundamental light source input port 502a.

[0097] The power meter 504 exemplarily has a light receiving band corresponding to the fundamental light, and measures the power of the fundamental light input from the wavelength tunable light source 502 to the FH port # 1 of the PPLN device 10 .

[0098] The second harmonic pump light source 506 is an example of a light source that generates pump light corresponding to twice the frequency (half wavelength) of the fundamental light, and is optically coupled to port 102-2 (SH port #1) of the PPLN device 10, for example, via second harmonic light source port 506a.

[0099] The power meter 508 illustratively has a receiving band corresponding to the second harmonic and measures the power of the second harmonic pump light input from the second harmonic pump light source 506 to port 102-2 (SH port #1) of the PPLN device 10.

[0100] The power meter 510 illustratively has a receiving band corresponding to the second harmonic and is optically coupled to port 102-3 (SH port #2) of the PPLN device 10 via the second harmonic receiving port 510a to measure the output optical power of SH port #2.

[0101] One or both of the light sources 502 and 506 may be configured to include, for example, a laser diode (LD) and a current controller for driving the LD. Also, each of the power meters 504, 508, and 510 may be configured to include, for example, a photoreceiver or photodetector (PD) that outputs an electrical signal corresponding to the received light power to the arithmetic circuit 514.

[0102] The OSA 512 is illustratively optically coupled to the port 102-4 (FH port #2) of the PPLN device 10 via the fundamental wave receiving port 512a, and measures the spectrum of the light output from the FH port #2.

[0103] The power meters 504, 508, 510 and the OSA 512 are each an example of a measurement device that constitutes a measurement section. The set of power meters 504 and 510 is illustratively used in SHG measurements. The set of power meters 504, 508 and the OSA 512 is illustratively used in DFG measurements. In other words, the power meter 504 may be common to both SHG and DFG measurements.

[0104] The calculation circuit 514 is an example of an estimation unit, and estimates the characteristics of the PPLN device 10 based on, for example, the measurement results from the power meters 504, 508, 510, and the OSA 512 and the six relational equations (Equation 5, Equation 6, Equation 14, Equation 16, Equation 21, and Equation 22) listed above.

[0105] The arithmetic circuit 514 may be configured to include hardware such as a CPU (Central Processing Unit), a DSP (Digital Signal Processor), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), and / or an FPGA (Field Programmable Gate Array), for example.

[0106] The arithmetic circuit 514 may also include a memory (not shown). The memory may store, for example, data corresponding to the six relational expressions (Equation 5, Equation 6, Equation 14, Equation 16, Equation 21, and Equation 22) described above.

[0107] <Configuration Example of PPLN Device> As shown in FIG. 6, for example, the PPLN device 10 includes a first free-space optical system 104, a PPLN waveguide 100, a second free-space optical system 106, and a Peltier device 108.

[0108] The first spatial optical system 104 spatially couples, for example, light of different wavelengths (e.g., fundamental light or signal light and pump light corresponding to the second harmonic) input from one or both of the FH port #1 and the SH port #1 to the PPLN waveguide 100. Furthermore, the first spatial optical system 104 spatially demultiplexes light of different wavelengths output from the PPLN waveguide 100 by, for example, optical reciprocity, and spatially couples the demultiplexed light to the FH port #1 and the SH port #1.

[0109] To this end, the first spatial optical system 104 includes lenses 141 , 142 , and 143 , and dichroic mirrors 144 and 145 .

[0110] Lens 141, for example, focuses light (e.g., fundamental wave light or signal light) incident from FH port #1 and outputs it to dichroic mirror 144, while focusing light reflected by dichroic mirror 144 and incident on it and outputs it to port 102-1.

[0111] The dichroic mirror 144 reflects the light incident from the lens 141 and guides it to the dichroic mirror 145 , and also reflects the light incident from the dichroic mirror 145 and guides it to the lens 141 .

[0112] Lens 142, for example, focuses light incident from SH port #1 and outputs it to dichroic mirror 145, while also focusing light that has passed through dichroic mirror 145 and entered it (for example, light with a wavelength corresponding to the second harmonic) and outputs it to SH port #1.

[0113] Dichroic mirror 145, for example, transmits light incident from lens 142 and guides it to lens 143. Furthermore, dichroic mirror 145 transmits, for example, light of a wavelength corresponding to the second harmonic among the light incident from lens 143 to lens 142, and reflects light of other wavelengths including the fundamental wave and guides it to dichroic mirror 144.

[0114] The lens 143 collects the transmitted light and reflected light from the dichroic mirror 145 and couples them into the PPLN waveguide 100 , while collecting the output light from the PPLN waveguide 100 and making it incident on the dichroic mirror 145 .

[0115] The PPLN waveguide 100 has, for example, a waveguide length L and a wavelength conversion efficiency η as described above, and performs wavelength conversion by SHG or DFG in response to incident light from the first spatial optical system 104 (e.g., lens 143) or the second spatial optical system 106 (e.g., lens 161 described later).

[0116] The second spatial optical system 106, for example, spatially separates light having a wavelength corresponding to the second harmonic from light having other wavelengths including the fundamental wave among the output light from the PPLN waveguide 100, and couples the light having the wavelength corresponding to the second harmonic to the SH port #2 and couples the light having other wavelengths to the FH port #2. Furthermore, the second spatial optical system 106 spatially couples light of different wavelengths incident from the SH port #2 and the FH port #2, for example, by using optical reciprocity, and inputs the combined light into the PPLN waveguide 100.

[0117] Therefore, the second spatial optical system 106 includes, for example, lenses 161 , 162 and 163 and dichroic mirrors 164 and 165 .

[0118] Lens 161, for example, focuses light incident from SH port #2 and outputs it to dichroic mirror 164, while focusing light that has passed through dichroic mirror 164 (for example, light with a wavelength corresponding to the second harmonic) and outputs it to SH port #2.

[0119] Dichroic mirror 164 transmits, for example, light incident from lens 161 (for example, light having a wavelength corresponding to the second harmonic) and guides it to lens 163. Furthermore, dichroic mirror 164 transmits, for example, light having a wavelength corresponding to the second harmonic out of the light incident from lens 163 to lens 161, and reflects light of other wavelengths including the fundamental wave and guides it to dichroic mirror 165.

[0120] The lens 162, for example, collects light incident from FH port #2 and outputs it to the dichroic mirror 165, while collecting light reflected by the dichroic mirror 165 and incident thereon and outputs it to the FH port #2.

[0121] The dichroic mirror 165 , for example, reflects the light incident from the lens 162 and guides it to the dichroic mirror 164 , and also guides the light reflected by the dichroic mirror 164 and incident thereon to the lens 162 .

[0122] The lens 163 collects the transmitted light and reflected light from the dichroic mirror 164 and makes them incident on the PPLN waveguide 100 , while collecting the output light from the PPLN waveguide 100 and making them incident on the dichroic mirror 164 .

[0123] The Peltier device 108 is, for example, thermally coupled to the PPLN waveguide 100 and controls the operating temperature of the PPLN waveguide 100 to a constant value. Note that, as a non-limiting example, the PPLN waveguide 100 has a waveguide length L of 45 millimeters (mm) and a phase-matched wavelength of 1545 nm at a temperature of 50° C. However, these numerical values ​​are merely examples, and other numerical values ​​may also be applied.

[0124] <Example of Measurement Procedure> Next, an example of a measurement procedure using the above-described measurement system 500 will be described with reference to FIGS.

[0125] 7, the measurement system 500 first performs measurements when the PPLN device 10 is operated by SHG and when it is operated by DFG, as illustrated in FIGS. 3A and 3B (S702). In S702, the measurement during SHG operation may correspond to an example of a first measurement, and the measurement during DFG operation may correspond to a second measurement.

[0126] 4A and 4B, the measurement system 500 performs measurements when the PPLN device 10 is operated by SHG and when it is operated by DFG using optical input in the direction opposite to that of S702 (S704). For convenience, this measurement may be referred to as a "reverse direction measurement." In S704, the measurement during SHG operation may correspond to an example of a third measurement, and the measurement during DFG operation may correspond to a fourth measurement.

[0127] Here, Fig. 8 shows an example of port connection relationships in the measurement system 500 for reverse direction measurement. As shown in Fig. 8, in reverse direction measurement, the input / output relationships of the individual ports 102 are swapped from the configuration in Fig. 5, and the devices connected to the individual ports 102 are changed accordingly.

[0128] For example, the wavelength-tunable light source 502 is optically coupled to the FH port #2 of the PPLN device 10 via the fundamental light source input port 502a, and the power of the fundamental light input to the FH port #2 of the PPLN device 10 is measured by the power meter 504.

[0129] In addition, the second harmonic pump light source 506 is optically coupled to the SH port #2 of the PPLN device 10, for example, via the second harmonic light source port 506a, and the power of the second harmonic pump light input to the SH port #2 of the PPLN device 10 is measured by the power meter 508.

[0130] The power meter 510 is optically coupled to the SH port #1 of the PPLN device 10 via, for example, the second harmonic receiving port 510a, and measures the output optical power of the SH port #1.

[0131] The OSA 512 is optically coupled to the FH port #1 of the PPLN device 10 via, for example, the fundamental wave receiving port 512a, and measures the spectrum of the output light output from the FH port #1.

[0132] In each of S702 and S704 in FIG. 7, the order in which the SHG operation and the DFG operation are performed does not matter, and the measurement during the SHG operation may be performed after the measurement during the DFG operation, or vice versa.

[0133] Then, the measurement system 500, for example in the calculation circuit 514, derives the characteristics of the PPLN device 10, such as the coupling loss and wavelength conversion efficiency of each port 102, based on the measurement results in S702 and S704 and the six relational equations described above (S706).

[0134] <Example of Measurement During SHG Operation and DFG Operation> Next, with reference to FIGS. 9 and 10, an example of measurement during SHG operation and DFG operation will be described.

[0135] 9 , in SHG measurement, fundamental light is input from the wavelength-tunable light source 502 to the FH port #1 (S902). Here, for example, the wavelength of the light output from the wavelength-tunable light source 502 (e.g., 1545 nm) is swept in a wavelength range of about ±2 nm around that wavelength in increments of about 0.01 nm (S904). This wavelength sweeping operation can be performed, for example, by the arithmetic circuit 514 controlling the wavelength-tunable light source 502.

[0136] During the wavelength sweep operation of the fundamental light, the power of the second harmonic pump light output from the SH port #2 by the SHG process is measured by the power meter 510 (S906). By such wavelength sweep measurement, a phase matching curve of the SHG (see, for example, FIG. 2) is obtained in, for example, the arithmetic circuit 514.

[0137] The arithmetic circuit 514 obtains a relational expression equivalent to Equation 5 based on the peak value of the obtained phase matching curve and the input optical power of the fundamental light corresponding to the peak value (measured by the power meter 504).

[0138] In this example, the wavelength sweep range is set to ±2 nm and the increment is set to 0.01 nm, but this is not limited to these and may be set to any value that allows the peak value of the SHG phase matching curve to be obtained.

[0139] <DFG Measurement> After the above SHG measurement, the measurement system 500 inputs the signal light from the wavelength-tunable light source 502 to the FH port #1, and inputs second harmonic pump light (e.g., wavelength 772.5 nm) from the second harmonic pump light source 506 to the SH port #1 at a desired power (e.g., 1 watt) (S908).

[0140] The wavelength of the signal light is set by the arithmetic circuit 514 to a wavelength close to the phase matching wavelength (for example, a wavelength of 1544 nm) so that Δβ expressed by the above-mentioned formula 10 becomes Δβ ≈ 0. Furthermore, the input power of the second harmonic pump light to the SH port #1 can be adjusted to a desired power by, for example, monitoring the measurement result by the power meter 508 in the arithmetic circuit 514 and having the arithmetic circuit 514 control the second harmonic pump light source 506.

[0141] When the signal light and second-harmonic pump light are input to the FH port #1 and the SH port #1, respectively, the signal light, wavelength-converted light, and light including ASE are generated by the DFG process in the PPLN waveguide 100. This light is output from the FH port #2, and the spectrum of the output light is measured by the OSA 512 (S910).

[0142] The arithmetic circuit 514 obtains a relational expression equivalent to Equation 14 by, for example, obtaining the difference between the power of the wavelength-converted light generated at 1546 nm and the input signal light power at 1544 nm from the output light spectrum obtained in the OSA 512.

[0143] The arithmetic circuit 514 obtains a relational expression equivalent to Equation 21 by monitoring the ASE level at a wavelength of 1544.5 nm in the output light spectrum obtained by the OSA 512, for example.

[0144] In this example, the resolution of the OSA 512 is set to 0.1 nm, for example, but is not limited to this value, and other numerical resolutions may be set in the OSA 512. Also, in this example, the wavelength of the signal light input to the FH port #1 is set to 1544 nm, but is not limited to this value, and may be any wavelength near the phase matching wavelength.

[0145] The above measurements were made with FH port #1 and SH port #1 as input ports and FH port #2 and SH port #2 as output ports in the SHG and DFG, respectively, and correspond to the measurements illustrated in Figures 3A and 3B and S702 in Figure 7.

[0146] <Reverse Direction Measurement> Next, in the reverse direction measurement illustrated in S704 of Fig. 7, the relationship between the optical input and output is reversed as illustrated in Fig. 4A, Fig. 4B, and Fig. 8. For example, FH port #2 and SH port #2 are set as input ports, and FH port #1 and SH port #1 are set as output ports, and SHG and DFG measurements are performed in the same manner.

[0147] For example, the measurement system 500 performs the process (S1002, S1004, and S1006) shown in Fig. 10 as SHG measurement. The SHG measurement in Fig. 10 corresponds to the process in the SHG measurement in Fig. 9, where FH port #1 as the input port is replaced with FH port #2, and SH port #2 as the output port is replaced with SH port #1.

[0148] The measurement system 500 also performs the process (S1008 and S1010) shown in Fig. 10 as DFG measurement. The DFG measurement in Fig. 10 corresponds to the process in the DFG measurement in Fig. 9 where the FH port #1 and the SH port #1 as input ports are replaced with the FH port #2 and the SH port #2, respectively, and the FH port #2 as output port is replaced with the FH port #1.

[0149] By performing measurements in which the relationship between the input port and the output port is swapped, for example, in the arithmetic circuit 514, a relational expression equivalent to Equation 6 is obtained by SHG measurement, and relational expressions equivalent to Equations 16 and 22 are obtained by DFG measurement.

[0150] Through the above-described measurements, six relational expressions corresponding to Equations 5, 6, 14, 16, 21, and 22 are obtained in the arithmetic circuit 514. Therefore, by simultaneously solving these relational expressions, the arithmetic circuit 514 can derive the coupling loss of each port 102 of the PPLN device 10 and the wavelength conversion efficiency of the PPLN waveguide 100.

[0151] As described above, in the embodiments including the examples described above, measurements of optical inputs and outputs related to wavelength conversion operations (for example, SHG and DFG) of the PPLN device 10 are performed by switching the relationship between the optical inputs and outputs.

[0152] Therefore, based on the measurement results, it is possible to estimate the characteristics of the PPLN device 10, such as the coupling loss of each port 102 and / or the wavelength conversion efficiency of the PPLN waveguide 100, from the relational equation for wavelength conversion by SHG, the relational equation for wavelength conversion gain by DFG, and the relational equation for the ASE level. Therefore, it is possible to accurately estimate the characteristics of the PPLN device 10, for example, even after the PPLN device 10 has been assembled.

[0153] <Modification of Measurement System> Figure 11 is a diagram showing an example of the configuration of a measurement system 500A that is a modification of the measurement system 500 shown in Figure 5. The measurement system 500A shown in Figure 11 differs from the configuration shown in Figure 5 in that it additionally includes two 2x2 optical switches 522 and 524 that can switch between straight coupling and cross coupling. Note that the arithmetic circuit 514 is not shown in Figure 11.

[0154] In this modification, by switching between straight coupling and cross coupling of the two 2×2 optical switches 522 and 524, it is possible to switch between the port connection relationship during forward measurement (see, for example, FIG. 5) and the port connection relationship during reverse measurement (see, for example, FIG. 8).

[0155] For example, in the straight coupling of one of the 2x2 optical switches 522, the FH#1 port corresponding to the fundamental wave port of the PPLN device 10 is optically coupled to the wavelength-tunable light source 502, and the SH#2 port corresponding to the second harmonic port of the PPLN device 10 is optically coupled to the power meter 510.

[0156] Similarly, in the straight coupling of the other 2x2 optical switch 524, the SH#1 port corresponding to the second harmonic port of the PPLN device 10 is optically coupled to the second harmonic pump light source 506, and the FH#2 port corresponding to the fundamental wave port of the PPLN device 10 is optically coupled to the OSA 512.

[0157] In contrast to the above, in the cross-coupling of one of the 2x2 optical switches 522, the SH#2 port, which corresponds to the second harmonic port of the PPLN device 10, is optically coupled to the wavelength-tunable light source 502, and the FH#1 port, which corresponds to the fundamental wave port of the PPLN device 10, is optically coupled to the power meter 510.

[0158] Similarly, in the cross-coupling of the other 2x2 optical switch 524, the FH#2 port, which corresponds to the fundamental wave port of the PPLN device 10, is optically coupled to the second harmonic pump light source 506, and the SH#1 port, which corresponds to the second harmonic port of the PPLN device 10, is optically coupled to the OSA 512.

[0159] 7, for example, in the forward direction measurement of S702, the 2×2 optical switches 522 and 524 are both set to straight coupling, and in the reverse direction measurement of S704, the 2×2 optical switches 522 and 524 are both set to cross coupling. This makes it possible to perform SHG measurements and DFG measurements in both the forward and reverse directions. Therefore, with the measurement system 500A according to the modified example, port switching for forward and reverse measurements can be easily achieved compared to the measurement system 500 described above.

[0160] The setting or switching of the straight coupling and cross coupling of each of the 2x2 optical switches 522 and 524 may be controlled, for example, by the arithmetic circuit 514, or may be controlled by a control circuit (not shown) separate from the arithmetic circuit 514.

[0161] <Supplementary Notes> In the embodiments including the examples described above, after the SHG measurement and DFG measurement are performed, the optical input / output relationship is changed and the SHG measurement and DFG measurement are performed in the reverse direction, but the order of these four measurements (first to fourth measurements) does not matter and may be changed as desired. However, by performing the SHG measurement and DFG measurement as a set in the forward direction and the reverse direction, it is possible to minimize the number of times the optical input / output relationship is changed.

[0162] Furthermore, in the embodiments including the above-described examples, an example has been described in which the arithmetic circuit 514 derives the characteristics of the PPLN device 10 through calculation using six relational expressions (Equation 5, Equation 6, Equation 14, Equation 16, Equation 21, and Equation 22). Alternatively, the characteristics of the PPLN device 10 may be derived by storing table-format data generated in advance based on the six relational expressions in a memory or the like, and the arithmetic circuit 514 searching for or looking up corresponding characteristic values ​​in the data based on measurement results.

[0163] In the above-described embodiment, the second harmonic pump light source 506 is an example of a light source that directly outputs pump light of a wavelength in the second harmonic band. Alternatively, pump light in the second harmonic band may be generated indirectly by using, for example, a light source that outputs fundamental light, an optical amplifier (e.g., EDFA) that amplifies the fundamental light, and a (second) PPLN waveguide that converts the amplified fundamental light into pump light in the second harmonic band by SHG.

[0164] Furthermore, in the above-described embodiment, the phase matching wavelength of the PPLN waveguide 100 is 1545 nm, but this is not limitative, and a wavelength other than 1545 nm may be used as the phase matching wavelength of the PPLN waveguide 100 .

[0165] In the above-described embodiment, the PPLN device 10 uses spatial optical systems 104 and 106 (see FIG. 6 ) for multiplexing or demultiplexing the fundamental wave light and the second harmonic pump light, and for optical coupling between the PPLN waveguide 100 and the optical fiber.

[0166] However, the present disclosure is not limited to this, and for example, the optical fiber and the PPLN waveguide 100 may be coupled using an optical waveguide made of a material different from that of the PPLN waveguide 100, which has the function of multiplexing or demultiplexing the fundamental light and the second harmonic pump light. Furthermore, the function of multiplexing or demultiplexing the fundamental light and the second harmonic pump light may be implemented using, for example, an optical waveguide monolithically integrated on the PPLN waveguide 100.

[0167] Furthermore, in the above-described measurement system 500, a configuration using a plurality of wavelength filters may be used instead of the OSA 512. For example, the power of the signal light, the converted light, and the ASE may be measured using a plurality of wavelength filters that transmit light of wavelengths corresponding to the signal light, the converted light, and the ASE to be measured, respectively, and a plurality of power meters that detect the transmission power of each wavelength filter.

[0168] Furthermore, the PPLN waveguide 100 may include, by way of example and not limitation, LiNbO ３ , LiTaO3, or LiNb (x) Ta (1-x) O ３ (0≦x≦1), or a nonlinear optical medium containing at least one selected from the group consisting of Mg, Zn, Sc, and In as an additive may be used.

[0169] <Terminology> In the embodiments including the above-described examples, the term "measurement" may be appropriately interpreted as other terms suitable for those skilled in the art, such as "detection" or "monitoring," depending on the context. Furthermore, the term "estimation" may be appropriately interpreted as other terms suitable for those skilled in the art, such as "determination," "judgment," "determination," "confirmation," "calculation," "derive," or "evaluation," depending on the context.

[0170] Any reference to an element followed by a designation such as "first...," "second...," etc. does not limit the quantity or order of those elements. These designations are merely used as a convenient way to distinguish between two or more elements. For example, a reference to a first and a second element does not imply that only two elements may be employed, nor does it imply that the first element must precede the second element in any physical quantity.

[0171] Although the present disclosure has been described in detail above, it is clear to those skilled in the art that the spirit and scope of the present disclosure are not limited to the contents described throughout the present disclosure. The present disclosure can be implemented in modified and altered forms without departing from the spirit and scope of the present disclosure as defined by the claims. Therefore, the description of the present disclosure is intended for illustrative purposes only and does not have any limiting meaning on the spirit and scope of the present disclosure.

[0172] The present disclosure is useful, for example, in a technique for measuring the characteristics of a wavelength conversion device that utilizes a nonlinear optical effect.

Claims

1. A method for estimating the characteristics of a wavelength conversion device, comprising: a measurement process for measuring the power of input light and output light related to the wavelength conversion operation of a wavelength conversion device when a second-order nonlinear medium included in the wavelength conversion device is caused to perform wavelength conversion operation by a forward light input to the second-order nonlinear medium, and when a second-order nonlinear medium is caused to perform wavelength conversion operation by a reverse light input to the second-order nonlinear medium; and an estimation process for estimating the characteristics of the wavelength conversion device based on the measurement results of the measurement process.

2. The method of claim 1, wherein the wavelength conversion operation includes a second harmonic generation operation and a difference frequency generation operation, and the measurement process includes: a first measurement for measuring input optical power and output optical power when the second-order nonlinear medium is subjected to the second harmonic generation operation by the forward optical input; a second measurement for measuring input optical power and output optical power when the second-order nonlinear medium is subjected to the difference frequency generation operation by the forward optical input; a third measurement for measuring input optical power and output optical power when the second-order nonlinear medium is subjected to the second harmonic generation operation by the backward optical input; and a fourth measurement for measuring input optical power and output optical power when the second-order nonlinear medium is subjected to the difference frequency generation operation by the backward optical input.

3. The wavelength conversion device has first and second input ports and first and second output ports optically coupled to the second-order nonlinear medium, the first measurement includes measuring the power of fundamental light input to the first input port and the power of second-harmonic light output from the first output port, the second measurement includes measuring the power of signal light input to the first input port, the power of pump light having the wavelength of the second-harmonic light input to the second input port, and the power of converted light of the signal light by the difference frequency generation operation and spontaneous emission light output from the second output port, and the third measurement includes measuring the power of fundamental light input to the second output port and the power of second-harmonic light output from the second input port, and 3. The method of claim 2, wherein the fourth measurement includes measuring the power of signal light input to the second output port, the power of pump light having the wavelength of the second harmonic light input to the first output port, and the power of converted light and spontaneous emission light of the signal light by the difference frequency generation operation output from the first input port.

4. The method of claim 2 or 3, wherein in the measurement process, the third measurement and the fourth set of measurements are performed after or before the first measurement and the second set of measurements are performed.

5. The method of claim 3, wherein the characteristics of the wavelength conversion device include at least one of the individual coupling losses of the first input port, the second input port, the first output port, and the second output port, and the wavelength conversion efficiency of the second-order nonlinear medium.

6. A system for estimating the characteristics of a wavelength conversion device, comprising: a measurement unit that measures the power of input light and output light related to the wavelength conversion operation of a wavelength conversion device when a second-order nonlinear medium included in the wavelength conversion device is caused to perform wavelength conversion operation by a forward light input to the second-order nonlinear medium, and when a second-order nonlinear medium is caused to perform wavelength conversion operation by a reverse light input to the second-order nonlinear medium; and an estimation unit that estimates the characteristics of the wavelength conversion device based on the measurement results by the measurement unit.

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