JP2007243121A - Optical apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a new optical apparatus using a hollow optical waveguide, and an optical apparatus capable of performing a continuous wavelength sweep in a wide range by a simple wavelength control system particularly when applied to a wavelength variable laser, and achieving a small-size, high optical output and high reliability. <P>SOLUTION: A gain area 42 composed of an optical waveguide structure 43 is formed on a flat substrate 41. One end face 44 of the gain area 42 has a high reflectance in an advancing direction of light propagated into the optical waveguide structure 43, and the other end face 45 is reduced in reflectance. The hollow optical waveguide 46 is constituted of a high reflection mirror 47a formed on the substrate 41 and a high reflection mirror 47b disposed in an upper part, and a DBR50 is formed in the hollow optical waveguide 46 as a phase control area 48 and a filter area 49. An upper part high reflection mirror 47b has a mechanism capable of changing a thickness of a core of the hollow optical waveguide, so that a part thereof is supported by a pillar 51. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an optical device having a hollow optical waveguide, and more particularly to an optical device having a wavelength variable function.

  A large-capacity optical communication network supports the current rapid increase in Internet traffic. This large-capacity communication network is a system called high-density wavelength division multiplexing (DWDM) that transmits multiple wavelengths at once, and it is necessary to prepare semiconductor lasers for each wavelength for each frequency grid defined by the ITU (International Telecommunication Union). There is. As described above, since a semiconductor laser is prepared for each frequency grid, there are problems in manufacturing management and inventory management in failure maintenance. In order to solve this problem, there is a tunable semiconductor laser, and at present, it is often used as a backup light source for an in-station device of a DWDM system. However, the wavelength tunable semiconductor laser is not limited to a transmission light source for DWDM, but can be developed as a key device of a photonic network as a wavelength conversion device.

  To date, various companies have been researching and developing wavelength tunable semiconductor lasers, and the types are roughly divided into three types: (1) multi-electrode DBR laser, (2) DFB laser array, and (3) external cavity laser. it can. However, each method has advantages and disadvantages.

In order to continuously change the wavelength while suppressing the mode hop, it is necessary to construct a multi-electrode laser having a normal phase control region. In recent years, multi-electrode lasers such as SSG (Super Structure Grating) having a further expanded wavelength variable range have been developed. For this reason, the number of electrodes is further increased, and furthermore, the control is not necessarily independent, so that the control is very complicated.
Moreover, it is necessary to set different control values for each laser element. Further, since the SSG oscillates at a point where the wavelengths selected by the two gratings coincide with each other, a mode hop is generated in principle. (See Patent Document 1)

  A wavelength tunable laser using the DFB laser array integrates a plurality of lasers having different wavelengths and controls the oscillation wavelength by changing the temperature. Since the amount of wavelength shift due to a temperature change of one DFB laser is limited to about 2 nm, the integrated DFB laser is switched and used. There is no method other than increasing the number of DFB lasers integrated to increase the wavelength tunable width. For example, considering wavelength tunability of 100 nm, 50 lasers are integrated. Further, since the wavelength tuning means uses temperature change, the wavelength tuning speed is slow.

  An external resonator type laser has a grating or a mirror having a movable part outside the laser, and is often a hybrid mounting type. Because of the hybrid mounting, the adjustment man-hours increase, the size of the module increases, and the reliability of movable parts and the like becomes a problem. Some lasers employ a MEMS (Micro Electro Mechanical Systems) structure for VCSELs (surface emitting lasers) that are monolithically integrated, but there are advantages in terms of low power consumption, miniaturization, and reliability. There are problems in terms of variable width and output.

The characteristics required for wavelength tunable lasers in communication applications are summarized as described above. There are wide continuous sweep width, high light output, high reliability, small size, simple wavelength control method, etc. Has not been developed. In addition, for future FTTH (Fiber to the home), it is expected to realize a small-sized photodetector with a wavelength selection function, but it is small enough to select and receive light of any wavelength from a wide wavelength range. No detector has been developed.
JP-A-6-53616

  Accordingly, the present invention has been made in view of the above, and an object of the present invention is to provide a new optical device using a hollow optical waveguide, and when this is applied to a wavelength tunable laser, a simple wavelength can be obtained. An object of the present invention is to provide an optical device that can perform continuous wavelength sweeping over a wide range by a control method, and that is small in size and has high light output and high reliability.

  In order to solve the above-described problems, according to one aspect of the present invention, a gain region having an optical gain in a desired wavelength variable width, a hollow optical waveguide that reflects light by a high reflection mirror and propagates in the air, Optical coupling means for optically coupling the gain region and the hollow optical waveguide using one or more of each of the gain region and the hollow optical waveguide, the hollow optical waveguide comprising: a core thickness variable means capable of varying a thickness of the core; and the core thickness A filter structure whose wavelength filter characteristic is changed by the variable means, and the high reflection mirror having a reflection band in which light having a wavelength determined by the filter characteristic can propagate, the optical coupling means including the gain region and the An optical device comprising a means for improving the coupling efficiency of the optical coupling portion with the hollow waveguide can be provided.

  According to the present invention, since the huge equivalent refractive index change of the hollow optical waveguide is used, when the hollow optical waveguide is used for the wavelength tunable laser, the wavelength tunable width of the conventional wavelength tunable laser can be greatly surpassed. Is possible. In addition, when integrating, the wavelength can be continuously tuned without a mode hop within a wide range with one parameter of changing the core thickness by providing a phase adjustment region based on a new principle in the hollow waveguide. Can do. Further, the use of the MEMS structure as means for changing the thickness of the core of the hollow waveguide can achieve downsizing and high reliability. Further, if an end face emission type structure is used as the gain medium, high light output is possible. With the above configuration, a wide continuous sweep is possible with a simple wavelength control method, and a high laser output, high reliability, and a small laser device can be provided. Furthermore, not only a light emitting device but also a light receiving device can provide a small photodetector with a wide wavelength selection range.

  Hereinafter, embodiments of the present invention will be described in detail. First, the hollow optical waveguide used in the present invention will be described. Conventionally, an optical waveguide represented by an optical fiber is configured by sandwiching a high refractive index core with a low refractive index clad. In such a configuration, light can propagate through the core of the high refractive index material while repeating total reflection at the interface with the cladding. The hollow optical waveguide used in the present invention propagates confined light in air (vacuum) having the lowest refractive index and no temperature dependency. As a structure in which light is guided in the low refractive index core, light can be reflected and guided by adopting a structure in which the hollow core is sandwiched between high reflectivity mirrors. Such a high reflection mirror is achieved by a multilayer film structure in which a high refractive index material and a low refractive index material are alternately laminated with a thickness of about ¼ wavelength, and the higher the number of layers, the higher the reflection mirror. It is possible. The higher the reflectivity of the mirror, the lower the loss of the waveguide can be obtained.

したがって波長１．５５μｍにおいては、コアの厚さが２μｍ程度まで小さくすることができれば１０％程度もの巨大な屈折率変化が可能である。このような特性を持つ中空導波路を集積した波長可変レーザができれば、現状の波長可変レーザの波長可変幅を大幅に凌駕できる可能性がある。しかし今まで中空導波路を波長可変レーザに使用した例はなく、その特性や設計方法は全く知られていない。A change in the equivalent refractive index of the hollow waveguide will be described. The equivalent refractive index n _eff of the fundamental mode changes approximately as follows according to the thickness D of the core of the hollow waveguide and the wavelength λ to be guided.

Therefore, at a wavelength of 1.55 μm, if the core thickness can be reduced to about 2 μm, a huge refractive index change of about 10% is possible. If a wavelength tunable laser integrated with hollow waveguides having such characteristics can be produced, there is a possibility that it can greatly exceed the wavelength tunable width of the current wavelength tunable laser. However, there has been no example of using a hollow waveguide for a wavelength tunable laser, and its characteristics and design method are not known at all.

ここでグレーティング周期□は０．７８μｍである。これにより波長可変幅の広い波長可変レーザを設計する場合、波長可変の大きい５μｍ以下のコアの厚さで使用することが好ましいことがわかる。原理的にはブラッグ波長は（３）式より０近傍まで可変できる巨大可変性が示されているが、実現可能な値として２μｍ以下のコアの厚さまで可変すれば２００ｎｍ程度の波長可変は可能である。When the wavelength tunable laser is configured using a hollow optical waveguide, various configurations are conceivable. First, in the first embodiment, a fiber ring type laser using a hollow Bragg reflector incorporating a DBR in the hollow waveguide. Will be described as an example. FIG. 1 shows a configuration diagram of a wavelength tunable ring laser device. An example of characteristics of a hollow Bragg reflector in which a DBR is formed in a hollow waveguide is shown in FIG. The vertical axis indicates the wavelength, and the horizontal axis indicates the thickness of the core of the hollow waveguide. This is a plot of equation (1) and Bragg wavelength □ _g, □, where the grating period is combined with equation (2) of Bragg diffraction to obtain equation (3).

Here, the grating period □ is 0.78 μm. As a result, when designing a wavelength tunable laser having a wide wavelength tunable width, it is preferable to use a core with a thickness of 5 μm or less, which is large in wavelength tunable. Theoretically, the Bragg wavelength shows a huge variability that can be varied to near 0 from Equation (3), but if it can be varied to a core thickness of 2 μm or less as a feasible value, it is possible to tune the wavelength to about 200 nm. is there.

  A configuration diagram of the wavelength tunable ring laser device shown in FIG. 1 will be described. It is assumed that an isolator is incorporated in the gain region 11 as light propagation direction control means. After the ASE (spontaneously emitted) light emitted from the gain region 11 is cut out by the polarizer 12 so as to be in the TE mode, the light is adjusted by the polarization controller 13 and is adjusted to the Bragg reflector 14 incorporating the DBR in the hollow waveguide. input. The reason why the polarization control means is necessary for the Bragg reflector 14 is that the hollow Bragg reflector 14 has polarization dependency, and the TE mode has better wavelength tunable characteristics than the TM mode. A ring laser is configured by returning the reflected light from the hollow Bragg reflector 14 through the circulator 15 to the gain region 11 by the polarization controller 16 with the optimum polarization. The optical output is taken out by the coupler 17. In this configuration, the oscillation wavelength can be continuously changed by changing the thickness of the core of the hollow Bragg reflector 14 by the piezo element or the like as the core thickness variable means 19.

  Next, the coupling efficiency improving means 20 will be described. The coupling efficiency improving means 20 here means means for reducing the mode coupling loss due to the difference in core thickness between the optical fiber 18 and the hollow Bragg reflector 14 and the Fresnel loss generated at the interface between the two regions. is there.

  First, means for reducing the mode coupling loss will be described. The core diameter of the optical fiber 18 is about 10 μm in the case of a single mode fiber, and it is necessary to control the core of the hollow waveguide to 10 μm or less in order to expand the wavelength variable width. If the thickness of the core of the hollow Bragg reflector 14 is reduced, the mode field diameter with the optical fiber 18 will not match, and the coupling efficiency will gradually deteriorate. In addition, the light that has not entered the core of the hollow Bragg reflector 14 is reflected by its end face, giving rise to the possibility of oscillation at an unintentional wavelength. By arranging the upper mirror of the hollow Bragg reflector 14 at an angle as the coupling efficiency improving means 20 for improving this problem, the thickness of the input side of the hollow Bragg reflector 44 can be increased. In addition, by using the optical fiber 18 that enters the core of the hollow Bragg reflector 14 as a lens fiber, it is possible to effectively squeeze the light into the core of the hollow optical waveguide 14.

  Next, means for reducing the Fresnel loss will be described. In order to prevent reflection at the end face of the optical fiber 18, it is desirable to perform a treatment such as an AR (Anti-Reflectivity) coating and reduce the reflection attenuation amount to about −40 dB. As a result of applying the coupling efficiency improving means 20 as described above, when a normal single mode optical fiber is used for the fiber 48 and the upper mirror is not angled, the wavelength variable width of 35 nm is used for this structure. Thus, optical coupling with a small core thickness was improved, and a wavelength variable width of 48 nm was obtained as shown in FIG. For hollow waveguides with a variable core width and a large wavelength tunable width, improving the mode coupling efficiency at the microcore is a very effective and indispensable means for realizing a wide wavelength tunable width. .

  FIG. 4 is a conceptual diagram of a laser device according to the second embodiment of the present invention. 4A is a cross-sectional view thereof, and FIG. 4B is a view seen from an oblique direction. Currently, a DFB (Distributed Feedback) laser or a DBR (Distributed Bragg Reflector) laser, which is excellent in single mode property, is the mainstream as a semiconductor laser for communication applications. Therefore, in the second embodiment, a DBR type laser integrated with hollow waveguides will be described as an example. As shown in FIG. 4A, a gain region 42 made of a gain medium is formed on a substrate 41 having flatness. An optical waveguide structure 43 is formed in the gain region 42 so that light can propagate. A structure having reflectivity with respect to the traveling direction of light propagating in the optical waveguide structure 43 is applied to one end face 44 of the gain region 42, and usually has a cleavage plane from the viewpoint of lowering the threshold value and efficiency. It is desirable that the reflectivity is higher than the reflectivity, and HR (High Reflectivity) processing in which multilayer films are alternately laminated is performed. This reflecting surface forms one mirror of a Fabry-Perot resonator as a laser. One end face 45 is provided with a reflectance reduction means such as an AR process. The hollow optical waveguide 46 is composed of a high reflection mirror 47a formed on the substrate 41 and a high reflection mirror 47b disposed on the upper portion. A DBR 50 is formed in the hollow optical waveguide 46 as a phase control region 48 and a filter region 49. The upper high reflection mirror 47b has a mechanism capable of changing the thickness of the core of the hollow optical waveguide, and a part of the upper high reflection mirror 47b is supported by the pillar 51 as shown in FIG. 4B. .

The high reflection mirrors 47a and 47b forming the hollow optical waveguide 13 can be constituted by a dielectric multilayer film in which, for example, Si and SiO ₂ are alternately laminated. When a current is to be passed, a semiconductor multilayer film or a gold film is used. Can do. In the case of a dielectric multilayer film in which films having a large difference in refractive index such as Si and SiO ₂ are alternately laminated, almost 100% reflection is obtained in a wide wavelength range from 1 μm to 2 μm in about 6 layer pairs. A three-dimensional reflecting mirror such as a photonic crystal can also be used.

The DBR 50 formed in the filter region 49 in the hollow optical waveguide 46 forms another reflection mirror that forms a Fabry-Perot resonator. The DBR 50 is a form of filter and can be considered as a band elimination filter. In the second embodiment according to the present invention, since the hollow optical waveguide 46 is assumed to be a two-dimensional slab waveguide, the DBR 50 is constituted by a circular diffraction grating. By using such a circular diffraction grating, light spread from the incident point is reflected and returns to the incident point again. This DBR 20 can be formed of, for example, SiO ₂ . SiO ₂ can be formed by a sputtering apparatus or the like. After applying a resist, EB exposure is performed and development is performed, and SiO ₂ is etched. The height of SiO ₂ determines the degree of coupling of the grating together with the thickness of the core, and the reflectivity and reflection band are determined. Therefore, the SMSR (side mode suppression ratio) of the wavelength tunable laser is determined to have desired characteristics. There is a need to. The characteristic example of the hollow Bragg reflector in which the DBR 51 is formed in the hollow waveguide region 46 is the same as that of the first embodiment and is shown in FIG.

  The core thickness varying means 52 capable of varying the thickness of the core of the hollow optical waveguide 16 is basically the same as that used in a normal MEMS device suitable for integration. For example, one mirror 47b can be moved by a bimorph effect in which two different types of materials are superposed on each other and deformed by the temperature, or an electrostatic force generated between two opposing electrodes. More specifically, the phenomenon is used in which the substrate is removed, gold is vapor-deposited on the highly reflective mirror 47b which has been thinned, and bending occurs when heat or voltage is applied. It should be noted here that when using electrostatic force according to the principle of MEMS, it is known that only 1/3 of the distance between two electrodes can be controlled, so that it meets the single mode condition described later. Thus, it is necessary to design the distance between the two electrodes. In addition, since the thing using a temperature change has a bad influence on a laser area | region, it is more preferable to change the thickness of a core by a method like an electrostatic force if possible.

今、フィルタ領域４９のコアの厚さが変化して、その変化したブラッグ波長□_ｇで発振し続けると仮定する。その仮定のもとではフィルタ領域４９の伝搬定数は（５）式となり、コアの厚さＤ_ＤＢＲを変えても一定となり変化しない。

そのとき、利得領域４２の伝搬定数は、（６）式のようになる。

一方、位相調整領域４８の伝搬定数β_ｐはＤ_ｐ＝Ｄ_ＤＢＲ−Ｄ_ｓを満足しながら変化するので（７）式が成り立つ。

フィルタ領域４９のコアの厚みＤ_ＤＢＲを小さくすることを考えると、（６）式より利得領域４２の伝搬定数は大きくなるが、位相調整領域４８の伝搬定数は（７）式より小さくなるので、位相補償効果があることがわかる。実際に利得領域長ｌ_ａ＝１００μｍ、位相調整領域長ｌ_ｐ＝３１０μｍ、ＤＢＲの有効長Ｌ_ｅｆｆ＝１００μｍ、グレーティングの周期Λ＝０．８μｍ、δをブラッグ波長λ_ｇからの伝搬定数の差としたとき（８）式の発振位相条件からブラッグ波長λ_ｇに一番近い波長を求め、図５に図示する。
２（β_ａ・ｌ_ａ＋β_ｐ・ｌ_ｐ＋Ｌ_ｅｆｆ・δ）＝２ｍπ ｍは正の整数 …（８）
また、図５には位相調整領域４８がある場合と位相調整領域４８がない場合も比較のため示してある。位相調整領域４８がない場合はモードホップを生ずるが、位相調整領域４８がある場合は中空コアの厚さが２．５μｍから３．５μｍまで、波長変化でいうと、１．５２５μｍから１．５６５μｍの３５ｎｍ（Ｃバンド）の範囲内を連続可変できる可能性があることがわかる。このような単純な構成によって３５ｎｍのモードホップなしの連続可変が可能であるが、これ以上の連続可変幅が必要な場合はさらに０から２πまで任意の値が設定できる新たな位相調整領域を付加してもよいが、この場合の位相調整領域はかならずしも中空導波路内部に形成する必要はなく従来のような利得領域に形成しても構わない。A structure and method capable of continuously changing the wavelength according to a new principle will be described. In the phase adjustment region 48 of the hollow optical waveguide 46, in this embodiment, the phase adjustment can be performed by using the characteristic characteristic of the hollow waveguide. The control parameter of one of which forms a phase adjusting region 48 stepped D _s of the thickness of the core of the filter region 19 yelling form a DBR50 into the hollow waveguide 46, moving the same upper mirror 47b The Bragg wavelength is varied and the phase is adjusted at the same time, so that the wavelength can be continuously varied without mode hopping. This will be explained using mathematical expressions. As described above, the upper refractive mirror 47b is moved up and down to change the core thickness, whereby the equivalent refractive index n _eff changes in the filter region 49, and the Bragg wavelength λg is equal to the core thickness D _DBR of the filter region 49. And the Bragg period Λ, as shown in equation (4).

Now, it is assumed that the core thickness of the filter region 49 changes, and oscillation continues at the changed Bragg wavelength □ _g . Under the assumption, the propagation constant of the filter region 49 is expressed by equation (5), and is constant and does not change even if the core thickness _DDBR is changed.

At that time, the propagation constant of the gain region 42 is as shown in Equation (6).

On the other hand, since the propagation constant β _p of the phase adjustment region 48 changes while satisfying D _p = D _DBR −D _s , the equation (7) is established.

Considering that the core thickness _DDBR of the filter region 49 is reduced, the propagation constant of the gain region 42 is larger than the equation (6), but the propagation constant of the phase adjustment region 48 is smaller than the equation (7). It can be seen that there is a phase compensation effect. Actually, gain region length l _a = 100 μm, phase adjustment region length l _p = 310 μm, DBR effective length L _eff = 100 μm, grating period Λ = 0.8 μm, δ is the difference in propagation constant from Bragg wavelength λ _g seeking a wavelength closest to the Bragg wavelength lambda _g from the time (8) of the oscillation phase condition, illustrated in Figure 5.
2 (β _a · l _a + β _p · l _p + L _eff · δ) = 2mπ m is a positive integer (8)
FIG. 5 also shows a case where the phase adjustment region 48 is present and a case where the phase adjustment region 48 is not present for comparison. When there is no phase adjustment region 48, a mode hop occurs, but when the phase adjustment region 48 is present, the thickness of the hollow core is 2.5 μm to 3.5 μm, and in terms of wavelength change, 1.525 μm to 1.565 μm. It can be seen that there is a possibility of being continuously variable within the range of 35 nm (C band). With such a simple configuration, continuous variable without mode hops of 35 nm is possible, but if a continuous variable width of more than this is required, a new phase adjustment region that can set any value from 0 to 2π is added. However, the phase adjustment region in this case is not necessarily formed inside the hollow waveguide, and may be formed in a conventional gain region.

  Next, the coupling efficiency improving means 53 will be described. The coupling efficiency improving means 53 is means for reducing mode coupling loss due to the difference in core thickness between the gain region 42 and the hollow waveguide region 46 and Fresnel loss generated at the interface between the two regions.

  First, means for reducing the mode coupling loss will be described. Since the upper high reflection mirror 47b of the hollow waveguide 46 is movable, it tends to have a structure in which a gap is formed at the coupling portion between the semiconductor gain region 42 and the hollow waveguide 46. Of course, it is preferable in terms of coupling efficiency that there is no gap between the gain region 42 and the hollow waveguide 46, and the light emission point of the gain region 42 is positioned so as to sink under the high reflection mirror 47 b in the hollow waveguide 16 if possible. Better to do. Even if there is a gap, the coupling loss can be suppressed to about 1 dB if the calculation result is 3 μm or less. Also, the structure in which the core thickness remains constant at the junction between the gain region 42 and the hollow waveguide 46, and the core thickness is changed by increasing the angle with a taper structure as the distance from the junction increases. This can be inferred from the above embodiments, but is a very effective means.

  Next, the influence of Fresnel loss and a method for reducing it will be described. The semiconductor core formed of the optical waveguide structure 43 in the gain region 42 and the air core of the hollow optical waveguide 46 have a large refractive index difference, and when directly coupled, Fresnel reflection occurs. This Fresnel reflection not only deteriorates the coupling efficiency, but also increases the number of reflection points in the laser resonator, so that it becomes a composite resonator. It is not preferable that there is. This will be explained below based on experimental data.

  FIG. 6 shows a laser structure in which the gain region 61 is directly coupled to the hollow Bragg reflector 62. First, in order to confirm the influence of Fresnel reflection, a Fabry-Perot semiconductor 30 μm stripe laser (FP-LD) was manufactured as the gain region 61 and a coupling experiment was performed. As a result, in addition to the oscillation of the FP-LD itself, the oscillation of the DBR mode reflected by the hollow Bragg reflector 62 is observed, and the wavelength of the DBR mode changes with the change of the core, and the oscillation of the FP-LD itself does not change. Results were obtained. For this reason, in order to reduce the reflectivity of one side of the gain region 61, a similar experiment was attempted by fabricating a device in which a 30 μm stripe was tilted 7 ° from the (011) crystal direction and an end surface was coated with an AR coating. The oscillation of only the DBR mode as shown in FIG. 7 was confirmed, and it was confirmed that prevention of Fresnel reflection occurring at the interface between the gain region 61 and the hollow Bragg reflector 62 was effective. When we looked at how the wavelength was tuned in detail, a large mode hop was observed without being continuously variable. This is shown in FIG. This is considered that the slab hollow waveguide is a multimode waveguide, and a mode in which the gain of the gain region 61 is high is selected from the fundamental mode and the higher order mode and oscillates. In order to prevent this, it is necessary to consider the single mode condition of the hollow Bragg reflector 62. In addition, in the micro core region where the core thickness is 5 μm or less, the light is reflected on the wall of the hollow Bragg reflector 62 and oscillates at the peak of the ASE, that is, the maximum point of gain. I found out that it was necessary.

Next, the single mode condition will be described. The Bragg diffraction conditions of a waveguide in which a DBR is formed in a hollow waveguide are as follows.
β _m + β _n = 2qπ / Λ m and n are integers (9)
Here, βm and βn are propagation constants of the m-order traveling wave mode and the n-order backward wave mode, respectively. q is the order of the grating and is 1 in this embodiment. FIG. 9 illustrates an example of how to obtain the single mode condition. Although the modes given by equation (9) are infinite, the basic mode of m = n = 1 and the case of m = 1 and n = 2 can be considered. FIG. 9 is a diagram showing a method for obtaining a single mode condition. First, the intersection of the fundamental mode and the secondary mode is obtained from the shortest wavelength side of the wavelength variable range. In this example, the process proceeds from the 1500 nm position to the right. The value of the X axis of these two intersections becomes the variable range of the core thickness. The wavelength at this time gives a wavelength variable range. In the example of FIG. 9, it is 2.8 μm to 4.3 μm. When the core thickness is 4 μm, there are two points of 1490 nm and 1530 nm, but it is necessary to select a gain medium having a gain difference that does not oscillate at 1490 nm. If it is the range of the core calculated | required in this way, only a fundamental mode will exist in a hollow waveguide.

  The light polarization control means will be described. If the polarization has the same structure as the semiconductor laser structure, the TE mode is normally guided, so that the polarization control is automatically performed.

Finally, the design of the wavelength tunable laser using the hollow waveguide will be summarized. FIG. 10 shows a design flow chart of a wavelength tunable laser using a hollow waveguide. First, a desired variable wavelength range, that is, a variable band is determined. (ST101). Next, since it is necessary for the gain medium to have a sufficient gain in this wavelength range, the gain medium is selected so that the gain peak comes near the center of the wavelength variable range. (ST102) Next, it is verified whether or not a band in which light is guided as a hollow waveguide is sufficient. The band to be guided depends on the difference in refractive index of the material to be formed. Therefore, when forming a hollow waveguide with a difference in refractive index that is not so large as in a semiconductor multilayer film, it may be narrower than the desired wavelength band. Because there is, attention is necessary (ST103). Next, the variable width of the core that satisfies the single mode condition of the hollow Bragg reflector is obtained. Since the Bragg wavelength value can be shifted with the same core thickness by changing the grating period Λ, an optimum core variable width is set using Λ as a parameter (ST104). When the core variable width is determined, the MEMS structure is designed accordingly. Here, a structure that improves the coupling efficiency in accordance with the device structure is examined (ST105). On the other hand, since the grating period Λ is determined, the depth of the grating is determined. The degree of coupling as a DBR is determined by the depth of the grating and the variable width of the core. If the degree of coupling is too strong, the Bragg band becomes wide, and multimode oscillation occurs or the side mode suppression ratio (SMSR) decreases (ST106). As described above, since the filter characteristics as the DBR are determined, the effective length l _{eff of the} DBR and the mirror loss can be known. (ST107) Next, the amount of step D _s formed in the hollow waveguide and the phase adjustment region length l _p are determined as conditions for enabling continuous phase variation (ST108). The gain of the gain region, the gain region length l _a and the mirror loss, determine the laser oscillation condition from loss of each area (ST 109), a determination of whether the oscillation threshold current, SMSR, laser characteristics, such as wavelength tunable width is satisfied (ST110). If the desired laser characteristics cannot be obtained, review the trade-off items and repeat the same.

  As described above, according to one embodiment of the present invention, since the huge equivalent refractive index change of the hollow optical waveguide is utilized, when the hollow optical waveguide is used for the wavelength tunable laser, It is possible to greatly exceed the wavelength tunable width. In addition, when integrating, the wavelength can be continuously tuned without a mode hop within a wide range with one parameter of changing the core thickness by providing a phase adjustment region based on a new principle in the hollow waveguide. Can do. Further, the use of the MEMS structure as means for changing the thickness of the core of the hollow waveguide can achieve downsizing and high reliability. Further, if an end face emission type structure is used as the gain medium, high light output is possible. With the above configuration, a wide continuous sweep is possible with a simple wavelength control method, and a high laser output, high reliability, and a small laser device can be provided.

  The present invention is not limited to the above-described embodiment as it is, and can be embodied without departing from the spirit of the invention at the stage of implementation. For example, in the present embodiment, only the wavelength tunable laser has been described in detail, but considering the enormous equivalent refractive index change and filter characteristics of the hollow waveguide, various new configurations with the same configuration as shown in the present embodiment are provided. An optical functional device can be considered. For example, it is possible to manufacture a new laser that controls the propagation delay of transmission light or transmits light with dispersion applied in advance. Further, as can be easily understood from the fact that the laser becomes a photo detector when the laser is reverse-biased, a photo detector capable of selecting the wavelength can be configured by replacing the photo detector with an absorption gain instead of the gain medium.

1 is a configuration diagram of a wavelength tunable ring laser device according to a first embodiment of the present invention. It is an example of the characteristic of the hollow Bragg reflector which formed DBR in the hollow waveguide. It is the figure which showed one characteristic example of the wavelength variable characteristic in 1st Embodiment. FIG. 3 is a conceptual diagram of a laser device according to a second embodiment of the present invention. (A) is the sectional view, and (b) is a view as seen from an oblique direction. It is a block diagram of the wavelength tunable ring laser apparatus in 2nd Embodiment which concerns on this invention. It is a figure which shows the mode of the wavelength variable when not incorporating a phase adjustment area | region. It is a block diagram of a DBR laser in which a gain region and a hollow Bragg reflector are directly coupled. It is the figure which showed one characteristic example of the wavelength variable characteristic of the DBR laser which directly couple | bonded the gain area | region and the hollow Bragg reflector. It is the figure which showed the example of the characteristic of the oscillation mode of the DBR laser which couple | bonded the gain area | region and the hollow Bragg reflector directly. It is the figure which showed 1 method which calculates | requires the single mode conditions based on this invention. It is the figure which showed the design flowchart of the wavelength tunable laser using the hollow waveguide based on this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 12 ... Polarizer 13, 16 ... Polarization controller 14, 62 ... Hollow Bragg reflector 15 ... Circulator 17 ... Coupler 18 ... Optical fiber 19, 52 ... Core thickness variable means 20, 53 ... Coupling efficiency improvement means 41 ... Substrate 42, DESCRIPTION OF SYMBOLS 11, 61 ... Gain region 43 ... Optical waveguide structure 44, 45 ... End surface 46 ... Hollow waveguide 47a, 47b ... High reflection mirror 48 ... Phase adjustment region 49 ... Filter region 50 ... Diffraction grating (DBR)
51 ... pillar

Claims (8)

Using a gain region having an optical gain in a desired wavelength tunable width, a hollow optical waveguide that reflects light by a highly reflective mirror and propagates in the air, and one or more of the gain region and the hollow optical waveguide, respectively An optical coupling means for optical coupling,
The hollow optical waveguide is capable of propagating light having a wavelength determined by the filter characteristics, a core thickness variable means capable of changing the thickness of the core, a filter structure whose wavelength filter characteristics are changed by the core thickness variable means A high reflection mirror having a reflection band;
An optical apparatus, wherein the optical coupling means includes means for improving the coupling efficiency of an optical coupling portion between the gain region and the hollow waveguide. The optical apparatus according to claim 1, wherein the optical gain is positive or negative. 2. When a ring resonator is constituted by the optical coupling means, polarization control means for light propagating through the ring resonator and propagation direction control means for selecting a light propagation direction are provided. Optical device. The optical device according to claim 1, wherein the hollow optical waveguide further includes a region in which the thickness of the core is changed in a part of the hollow waveguide. The core thickness variable range that is changed by the core thickness variable means is a core thickness range in which only the fundamental mode of the hollow waveguide is selected with respect to the desired wavelength variable width. Item 4. The optical device according to Item 1. 2. The optical apparatus according to claim 1, wherein the core thickness varying means varies the thickness of the core by a strain stress or electrostatic force due to piezoelectricity and heat. 2. The optical device according to claim 1, wherein the filter structure includes one or more circular or parallel diffraction gratings or one or more diffraction gratings obtained by shifting a part of the diffraction grating by a quarter wavelength. 2. The optical apparatus according to claim 1, wherein the high reflection mirror is a multilayer film in which two materials having a high refractive index difference are alternately laminated, gold, or a photonic crystal.

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