JP2014215518A - Performance-variable diffraction grating - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a performance-variable diffraction grating capable of varying a reflectance and a reflection wavelength band.SOLUTION: A performance-variable diffraction grating is formed on a deposit smoothly deposited on a substrate, and refraction liquid-charging grooves 12 and 13 having a width X, a length L and a depth d are formed in a part of the grating-formed substrate and a terrace part 11 having a width W is left therebetween. The refraction liquid-charging groove 13 having a periodic sawteeth shape enters parts of the terrace part 11. The refraction liquid charging grooves 12 and 13 can be charged with a liquid having an optional refractive index, and a reflectance and a reflection wavelength band being fundamental characteristics of the diffraction grating can be optionally varied by varying the refractive index of the liquid to be charged.

Description

  The present invention relates to a performance variable diffraction grating, and more particularly to a performance variable diffraction grating having a narrow band wavelength characteristic.

  Conventionally, a fiber grating (hereinafter abbreviated as FBG) in which a diffraction grating is drawn in an optical fiber has been used as one of wavelength filters. FIG. 15 shows the configuration of a conventional FBG. Since the FBG is made of the same material as the optical fiber, it has good matching with the optical fiber and is used as a wavelength filter that obtains a narrow band wavelength characteristic. For example, FBG is widely used in order to stabilize the wavelength of a 0.98 μm band semiconductor laser (hereinafter referred to as LD) used as a fiber amplifier excitation light source within a wavelength range of about 1 nm. Fabrication of the FBG is performed by irradiating an optical fiber with ultraviolet interference fringes and forming a periodic refractive index change in the optical fiber by an ultraviolet-induced refractive index change.

  On the other hand, recently, wavelength-converted light is used in fluorescent microscopes used in the bio field and the like. The wavelength-converted light can generate a wavelength that cannot be generated by a conventional LD for exciting fluorescent proteins. In particular, at wavelengths where it is difficult to realize a distributed feedback type (hereinafter referred to as DFB) structure, the LD oscillation band is narrowed using FBG.

  Moreover, since the phase matching bandwidth of the wavelength conversion element used for obtaining visible light is narrow, wavelength stabilization is an essential requirement for practical use. FIG. 16 shows an example of a phase matching condition for obtaining yellow-green light having a wavelength of 0.559 μm by sum frequency generation. The graph shows the characteristics when the temperature of lithium niobate (hereinafter, LN) is scanned, and is represented by the separation length with the wavelength that becomes the maximum output being zero. The full width at half maximum is about 0.8 ° C. When the conversion factor of 0.1 nm / ° C. when the oscillation wavelength of the 0.98 μm semiconductor laser is scanned is used, the full width at half maximum is about 80 pm, which is very narrow. For this reason, it is understood that it is necessary to suppress the oscillation spectrum of the 0.98 μm semiconductor laser to a narrow band using FBG.

  Since specifications required for wavelength-converted light are directly reduced to specifications for the pumping LD, it is necessary to optimally design the LD and FBG including the cost.

  In the pump LD for wavelength-converted light generation using FBG, problems such as generation of discontinuities called kinks appearing in the current-light output characteristics, narrowing of the oscillation spectrum width, and deterioration of noise characteristics due to pseudo-mode single oscillation There is. There are various demands in the specifications of fluorescent microscopes, such as use as continuous wave (hereinafter referred to as CW) light, mounting modulation function, low noise is required, and in addition, high-performance performance that requires high performance. Anyway, there are various needs such as low-cost use, and a combination of LD and FBG corresponding to each need is required.

  Although it is possible to obtain a suitable wavelength band for the LD in the market, differences in material systems and performance differ from manufacturer to manufacturer. Ultimately, FBG will be optimized to meet individual specifications. Need to do. The main performance of FBG is determined by (A) reflectivity, (B) reflection wavelength band, and (C) center wavelength.

Japanese Patent No. 4843506

  However, since an actual FBG is manufactured as one having a fixed characteristic by using a method such as a phase mask method or a two-beam interference method, (A) to (C), particularly (A ) And (B) cannot be made variable. Therefore, in order to determine the optimum FBG, it is necessary to prepare a plurality of FBGs, and there is an operation for connecting the FBGs each time, so that development takes a large cost and time.

  The present invention has been made in view of such problems, and an object thereof is to provide a variable performance diffraction grating capable of changing the reflectance and the reflection wavelength band.

  In order to solve the above problems, the invention according to claim 1 is a variable performance diffraction grating, and a sawtooth having periodic grooves in a terrace portion formed on a deposit deposited flat on a substrate. The saw-tooth shaped groove is a liquid reservoir that can be filled with a liquid having an arbitrary refractive index.

  According to a second aspect of the present invention, in the variable performance diffraction grating according to the first aspect, a plurality of the sawtooth-shaped grooves are formed in the terrace portion.

  The invention according to claim 3 is the variable performance diffraction grating according to claim 1 or 2, wherein spot size conversion structures are formed at both ends of the terrace portion.

  According to a fourth aspect of the present invention, in the variable performance diffraction grating according to any one of the first to third aspects, the spot size conversion structure converts the propagation light from the optical fiber connected to the diffraction grating into collimated light. It is characterized by doing.

  According to a fifth aspect of the present invention, in the variable performance diffraction grating according to any one of the first to fourth aspects, the spot size conversion structure has a tapered shape of a ridge structure.

  By using the diffraction grating according to the present invention, it is possible to change the reflectance and the reflection wavelength band without removing one diffraction grating. As a result, for example, by using an FBG having the performance determined by the diffraction grating, a semiconductor laser suitable for the specification can be manufactured with a high yield, and the specification change of the fluorescence microscope used as a measurement tool in the bio field etc. And can make a significant contribution to the development of the medical field.

It is a figure which shows the diffraction grating which concerns on Embodiment 1 of this invention. It is a figure which shows the transmission characteristic of a diffraction grating at the time of connecting to LD using the diffraction grating of Example 1 of this invention (CASE-I). It is a figure which shows the characteristic at the time of connecting to LD using the diffraction grating of Example 1 of this invention (CASE-I), (a) is the optical output characteristic with respect to an electric current, (b) is the oscillation peak wavelength characteristic with respect to an electric current. (C) is a figure which shows the LD oscillation spectrum characteristic with respect to an electric current. It is a figure which shows the transmission characteristic of a diffraction grating at the time of connecting to LD using the diffraction grating of Example 1 of this invention (CASE-II). It is a figure which shows the characteristic at the time of connecting to LD using the diffraction grating of Example 1 of this invention (CASE-II), (a) is the optical output characteristic with respect to an electric current, (b) is the oscillation peak wavelength characteristic with respect to an electric current. (C) is a figure which shows the LD oscillation spectrum characteristic with respect to an electric current. It is a figure which shows the transmission characteristic of a diffraction grating at the time of connecting to LD using the diffraction grating of Example 1 of this invention (CASE-III). It is a figure which shows the characteristic at the time of connecting to LD using the diffraction grating of Example 1 of this invention (CASE-III), (a) is the optical output characteristic with respect to an electric current, (b) is the oscillation peak wavelength characteristic with respect to an electric current. (C) is a figure which shows the LD oscillation spectrum characteristic with respect to an electric current. It is a figure which shows the diffraction grating which concerns on Embodiment 2 of this invention. It is a figure which shows the transmission characteristic of a diffraction grating at the time of connecting to LD using the diffraction grating of Example 2 of this invention (CASE-IV). It is a figure which shows the transmission characteristic of a diffraction grating at the time of connecting to LD using the diffraction grating of Example 2 of this invention (CASE-V). It is a figure which shows the transmission characteristic of a diffraction grating at the time of connecting to LD using the diffraction grating of Example 2 of this invention (CASE-VI). It is a figure which shows the diffraction grating which concerns on Embodiment 3 of this invention. It is a figure which shows the mode size of the light which propagates in the diffraction grating of this invention. It is a figure which shows the diffraction grating which concerns on Embodiment 4 of this invention. It is a figure which shows FBG which is the conventional diffraction grating. It is a figure which shows the phase matching curve of LN for obtaining the yellow-green light of wavelength 0.559micrometer by a 2nd harmonic generation.

  Hereinafter, embodiments of the present invention will be described in detail.

The variable performance diffraction grating according to the present invention will be described with reference to FIG. This performance variable diffraction grating is formed on a quartz-based compound SiO ₂ that is flatly deposited on a silicon substrate. A refractive liquid having a width X, a length L, and a depth d is formed on a part of the formed substrate. The filling grooves 12 and 13 are dug, and the terrace portion 11 having a width W is left between them. The refractive liquid filling groove 13 has a periodic saw-tooth shape and enters a part of the terrace portion 11. Here, a case where X = 50 μm, L = 20 mm, W = 5 μm, and d = 1 μm will be described. In addition, the diffraction grating described here is for a wavelength of 0.98 μm, and a groove of 0.34 μm, which is a primary diffraction grating, should be used originally. It was produced at 0.68 μm.

(Embodiment 1)
FIG. 1 shows a diffraction grating according to Embodiment 1 of the present invention. FIG.1 (b) is sectional drawing in PQ of Fig.1 (a). By suspending the refractive liquid A having a refractive index of 1.454 in the groove portion 13 as CASE-I, the refractive index 1.45 of the ridge portion is higher by about 1.3 × 10 ⁻⁵ at the groove portions 12 and 13 with respect to the propagating light. The condition for refractive index was realized. As a result, a grating characteristic with a transmittance of 55% (reflectance of 45%) and a half width of 18 pm as shown in FIG. 2 was obtained. The dotted line in the figure is an auxiliary line for reading the half width. An optical fiber is connected to both ends of this diffraction grating as collimated light, and a 0.98 μm band LD is connected to the tip of one optical fiber at a distance of 1 m, and a connector is connected to the tip of the other optical fiber to perform LD evaluation. As a result, the basic characteristics shown in FIGS. 3A to 3C were obtained. 3A shows the optical output characteristics with respect to the current, FIG. 3B shows the oscillation peak wavelength characteristics with respect to the current, and FIG. 3C shows the LD oscillation spectrum characteristics with respect to the current.

Next, as CASE-II, the refractive liquid A is removed from the groove portion by using a cleaning agent, and then the refractive liquid B having a refractive index of 1.452 is filled in the groove portion, and the refractive index of the ridge portion at the groove portions 12 and 13 with respect to the propagating light. The condition that the refractive index is higher by 1.0 × 10 ⁻⁵ than 1.45 is realized. As a result, a grating characteristic with a transmittance of 67% (reflectance of 33%) and a half width of 18 pm as shown in FIG. 4 was obtained. The dotted line in the figure is an auxiliary line for reading the half width. As basic characteristics of the LD at this time, the characteristics as shown in FIGS. 5A to 5C were obtained. 5A shows the optical output characteristics with respect to the current, FIG. 5B shows the oscillation peak wavelength characteristics with respect to the current, and FIG. 5C shows the LD oscillation spectrum characteristics with respect to the current.

Similarly, the groove portion is filled with the refraction liquid C having a refractive index of 1.451 as CASE-III, and only about 7.5 × 10 ⁻⁶ from the refractive index 1.45 of the ridge portion in the groove portions 12 and 13 with respect to propagating light. Realized the condition that high refractive index. As a result, a grating characteristic with a transmittance of 95% (reflectance of 5%) and a half width of 18 pm as shown in FIG. 6 was obtained. The dotted line in FIG. 6 is an auxiliary line for reading the half width. Characteristics as shown in FIGS. 7A to 7C were obtained as basic characteristics of the LD at this time. FIG. 7A shows the optical output characteristics with respect to the current, FIG. 7B shows the oscillation peak wavelength characteristics with respect to the current, and FIG. 7C shows the LD oscillation spectrum characteristics with respect to the current. You can see how the spectrum width increases with current.

  Of these, the kink is not so large, the change in wavelength due to the current is small, and the spectral peak wavelength characteristics with respect to the current with a small spectrum width are close to the specifications that the characteristics of the refractive liquid B are most necessary, and are equivalent to this configuration. Decided to adopt FBG. In this way, multiple types of commercially available refracting liquids are prepared, and the reflectance can be arbitrarily changed with one FBG only by changing the refracting liquid to be filled. Therefore, the diffraction grating specifications at the research and development stage can be easily determined. Is possible.

  In this case, the refracting liquid A may or may not hang down in the groove portion 12, or the groove portion 12 does not have to be made if the propagation light is set so as not to be affected by the groove 12 as shown in FIG. 13.

(Embodiment 2)
8A and 8B show a diffraction grating according to Embodiment 2 of the present invention. FIG. 8B is a cross-sectional view taken along the PQ in FIG. The difference between the second embodiment and the first embodiment is that the groove 12 is divided into a groove 12A, a groove 12B, and a groove 12C, and the groove 13 is divided into a groove 13A, a groove 13B, and a groove 13C. The periodic grooves of the groove portion 13A, the groove portion 12B, and the groove portion 13C are formed on the same optical path.

The length _L A of each _{groove, L B,} L _C, respectively 6.7 mm, 6.7 mm, is 6.6 mm, L = 20 mm is the same as Embodiment 1. An optical fiber is connected to both ends of the diffraction grating as collimated light, a 0.98 μm band LD is connected 1 m away from one optical fiber, and a connector is connected to the tip of the other optical fiber to perform LD evaluation. Is the same as in the first embodiment. However, in the second embodiment, an LD different from the LD used in the evaluation of the first embodiment is used.

  Under these conditions, CASE-IV is filled with the refractive liquid C having a refractive index of 1.451 in the grooves 13A, 12B, and 13C, so that the grating has a reflectivity of 5% that is almost the same as CASE-III and a half width of about 10 pm. Characteristics (FIG. 9) were obtained. In addition, the dotted line in FIG. 9 is an auxiliary line for reading a half value width.

Next, after removing the refracting liquid B from all the grooves by using a cleaning agent as CASE-V, the refracting liquid C having a refractive index of 1.451 is filled in the grooves 13A and 12B, and the refraction having a refractive index of 1.45 is filled in the groove 13C. Liquid X was filled (as a relative refractive index, ˜0.0%). In this case, the portion covered with the refractive liquid filling groove 13C does not have a difference in refractive index depending on the presence or absence of the groove, and therefore has the characteristics of the terrace portion 11 itself. In other words, the diffraction grating length is not 30 mm, but L _A + L _B is 20 mm. As a result, a grating characteristic (FIG. 10) having a reflectance of 5% but a half width of about 20 pm was obtained. The dotted line in the figure is an auxiliary line for reading the half width. The refractive liquid C may be filled into the groove portions 12A and 13B, and the refractive liquid X may be filled into the groove portion 12C.

Similarly to CASE-VI, the refractive liquid B is removed from all the grooves by using a cleaning agent, and then the refractive liquid C having a refractive index of 1.451 is filled only into the groove 13A, and the grooves 12B and 13C have a refractive index of 1.45. Refractive liquid X was filled (as a relative refractive index, -0.0%). In this case, the portions where the refractive liquid filling grooves 12B and 13C are applied have the characteristic of the terrace portion 11 itself because there is no difference in refractive index depending on the presence or absence of the grooves. That not 30mm as a diffraction grating length, operates as 10mm for _{L A} only. As a result, a grating characteristic (FIG. 11) having a reflectance of 5% but a half width of 48 pm was obtained. The dotted line in the figure is an auxiliary line for reading the half width.

  Thus, by preparing a plurality of commercially available refractive liquids and using the same refractive liquid as the diffraction grating of the material constituting the diffraction grating, the half width of the diffraction grating can be changed. The longer the diffraction grating portion, the narrower the reflection characteristics can be obtained, but the manufacturing variation tends to occur from the viewpoint of uniformity. Therefore, it is effective to find the optimum length of the diffraction grating because it is not desired to make it longer than necessary.

  Since the full width at half maximum can be arbitrarily changed with one FBG only by changing the refractive liquid to be filled, it is possible to easily determine the diffraction grating specifications at the research and development stage. In addition, although the groove part was divided into three here, if it is divided into two or more, the same effect is obtained. Further, in the second embodiment, considering the ease of changing the refractive liquid, the groove notches into the terraces 11 in A, B, and C are alternately changed. The effect is the same.

(Embodiment 3)
FIG. 12 shows a diffraction grating according to Embodiment 3 of the present invention. In the third embodiment, optical fibers 15 and 17 are mounted on both ends of the diffraction gratings of the first and second embodiments. In order for the effect of the diffraction grating of the present invention to work effectively, the light propagating through the diffraction grating needs to be effectively transmitted between the groove portions 12 and 13, so that the propagating light is desirably collimated. Therefore, in the third embodiment, a configuration is adopted in which the propagation light is collimated using the lenses 14 and 16 between the optical fibers 15 and 17 and the diffraction grating.

  FIG. 13 shows the cross section of the diffraction grating as in FIGS. 1 (a) and 8 (a), but the transmission region 20 of the propagating light has the terrace portion 11 so that the effect of the diffraction grating can be efficiently exhibited. The position is adjusted in a region where a periodic groove is formed. It should be noted that the reflectance and the half-value width can be adjusted by changing the coupling efficiency of the diffraction grating that the light feels by adjusting the position.

  In the configuration shown in FIG. 12, the number of lenses is one on one side, but a plurality of configurations may be used.

(Embodiment 4)
FIG. 14 shows a diffraction grating according to Embodiment 4 of the present invention. By providing spot size changing structures 18 and 19 having tapered ridge structures on both sides of the diffraction grating portion on the substrate on which the diffraction grating is formed, the spot size of the light beam being propagated is converted as in the third embodiment. Thus, the position of the light beam 20 can be adjusted in the region where the periodic grooves of the terrace portion 11 are formed, and the propagation light in the diffraction grating can be made collimated light without using a lens.

  In the first to fourth embodiments, the method of changing only the reflectance of the diffraction grating and the method of changing only the half width are described. However, the reflectance and the half width can be optimized simultaneously by combining the two. .

  Further, in the evaluation in the present specification, a secondary value is used as a period for the diffraction grating for the sake of convenience, but the same result can be obtained even if fabrication is performed using a primary value that requires processing accuracy. .

  Further, the present invention can be arbitrarily used with respect to the wavelength, and the same applies to the case where the optical components are spatially connected via a lens or the like.

  In the present specification, the optical component based on quartz is described, but the same effect can be obtained by forming the terrace portion 11 and the grooves 12 and 13 for refraction liquid filling in a waveguide using LN or a semiconductor. Play.

  Furthermore, by using the refractive liquid as a solid refractive index material by curing the refractive liquid, it can be used as it is as a diffraction grating instead of FBG, and the refractive liquid can be cured or sealed so as not to leak out. It can also be used as a general-purpose component instead of.

  The center wavelength can be varied by about sub-nm to several nm by controlling the temperature of the diffraction grating itself.

  Further, in FIG. 14, a tapered waveguide structure is shown as the spot size conversion structure. However, the spot size is converted to adjust the position of the light beam 20 in the region where the groove notches of the terrace portion 11 are formed, and the lens is mounted. If the propagating light in the diffraction grating is set to collimated light without using it, the structure is arbitrary.

  As described above, according to the present invention, the reflectance and the reflection wavelength band, which are basic characteristics of the diffraction grating, can be arbitrarily changed. Although the cost of the FBG itself has been considerably reduced, it is difficult to prepare a reflectance and a reflection wavelength band for a test for optimization in view of development costs. The method of optimizing the combination with, for example, an LD using such a diffraction grating, the LD characteristic obtained by the wavelength is gusseted, and it is difficult to prepare several types of FBGs each time. There is a big effect on the acceleration of.

DESCRIPTION OF SYMBOLS 11 Terrace part 12, 13 Refractive liquid filling groove | channel 14, 16 Lens 15, 17 Optical fiber 18, 19 Spot size change structure 20 Transmission area of propagation light

Claims (5)

  A liquid in which a sawtooth-shaped groove having periodic grooves is formed in a terrace portion formed on a deposit deposited flat on a substrate, and the sawtooth-shaped groove can be filled with a liquid having an arbitrary refractive index. A variable performance diffraction grating characterized by being a reservoir.   The variable performance diffraction grating according to claim 1, wherein a plurality of the serrated grooves are formed in the terrace portion.   The variable performance diffraction grating according to claim 1, wherein spot size conversion structures are formed at both ends of the terrace portion.   4. The variable performance diffraction grating according to claim 1, wherein the spot size conversion structure uses collimated light as propagation light from an optical fiber connected to the diffraction grating.   5. The variable performance diffraction grating according to claim 1, wherein the spot size conversion structure has a tapered shape of a ridge structure.

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