JP5981888B2 - Beam splitter and optical signal processing apparatus using the same - Google Patents

Description

  The present invention relates to a small, high-speed, large-capacity optical transmitter used in the field of optical communication and an optical component used in an optical transmission module.

  In recent years, communication techniques using optical wavelength multiplexing communication techniques such as wavelength division multiplexing (WDM) and ROADM (Reconfigurable Optical Add / Drop Multiplexer) have been widely used mainly on trunk lines. The optical wavelength division multiplexing technology (Non-patent Document 1) realizes high-speed and large-capacity information communication by simultaneously placing optical signals having a plurality of optical frequencies having different wavelengths on one optical fiber cable. In order to use this technique, a very advanced wavelength stabilization technique is required.

  Currently, a wavelength tunable semiconductor laser module is used as a transmitter for optical wavelength division multiplexing communication because it needs to be compatible with ROADM capable of changing the arrangement of wavelengths to be used. A wavelength locker is provided inside the wavelength tunable semiconductor laser module in order to improve wavelength stability. The wavelength locker is a unit having a wavelength monitoring function and a power monitoring function using a wavelength filter. These optical transmitters were high-grade transmitters used for backbone communication networks and the like several decades ago. However, in recent years, it has been widely used in lower-layer networks such as metro access systems, and a design conscious of low cost has been demanded.

  In particular, there is a significant demand for cost reduction of the wavelength tunable laser module used in the optical transmitter, and further cost reduction is desired including each optical component used in the wavelength tunable laser module. Yes.

  FIG. 1 is a diagram showing the structure of a typical tunable laser module of the prior art. FIG. 1 schematically shows an internal configuration of a semiconductor laser module 16 (see Non-Patent Document 2). The module 16 includes a tunable laser chip 1 that is a key device, a first lens 2, a broadband device, and the like. The isolator element 3, the wavelength locker 9, the second lens 10, and the optical fiber 11 are configured. The wavelength locker 9 has a function of monitoring the power and wavelength of the oscillation light. That is, the wavelength locker 9 includes a first beam splitter 4 for power monitoring and a monitoring photodiode 6 at the reflection destination. Further, the wavelength locker 9 also includes a second beam splitter 5 for wavelength monitoring, an etalon filter 7 at the reflection destination, and a photodiode 8 for wavelength monitoring. The main beam 15 that has passed through the first beam splitter 4 and the second beam splitter 5 reaches the second lens 10, is condensed, and is coupled to the optical fiber 11.

  In FIG. 1, cube-shaped beam splitters 4 and 5 which are typical as beam splitters are shown. Cube-type beam splitters have been favorably used until recently because they are mechanically stable and the light passing there is not angled. However, the beam splitter having this structure is a structure that requires an adhesion process using at least three antireflection (AR) coatings, an internal reflection film coating, and an adhesive. Due to this complicated manufacturing process, the cube-type beam splitter is an optical component that is difficult to reduce in price. Moreover, in such a structural component, as disclosed in Patent Document 1, it is known that interference fringes are generated and affect the monitor quality of oscillation light.

  FIG. 2 is a view showing the structure of a wedge-type flat beam splitter. The wedge-type flat beam splitter 20 has a structure that can be expected to reduce the price of the cube-type beam splitter. The wedge-type beam splitter 20 is provided with a reflective film on the front surface 19 (first surface) and a non-reflective coating film on the rear surface 18 (second surface), respectively, and the incident surface 19 (first surface). The back surface 18 (second surface) is not parallel and is characterized by being given an off angle of several degrees. FIG. 2 shows various effects that occur in a wedge structure in which the first surface 19 and the second surface 18 are not parallel but are inclined several degrees from parallel.

  Usually, a beam splitter has a reflective surface having a finite reflectance on a surface on which light is incident (first surface), and a non-reflective coating is applied on a surface from which light is emitted (second surface). Has a non-reflective coating surface. However, even this non-reflective coating often has a small reflectance depending on the wavelength and incident angle of light, which may cause multiple reflections and cause light interference. In order to avoid this problem, a wedge structure is often used in which the angle of the second surface (back surface) with respect to the first surface (front surface) is turned off by several degrees. When the wedge structure is used, as can be seen from FIG. 2, the reflected light 22 on the first surface 19 and the internal once-reflected light 24 that has been transmitted through the first surface 19 and then reflected by the second surface 18 are: Reflected at different ray angles due to the effect of the wedge surface. Therefore, the reflected light 22 and the internally reflected reflected light 24 are gradually separated from each other as they travel through the space on the first surface 19 without interfering with each other. When light is received by the photodiode on the extension line of the reflected light, the arrangement can be designed so that only the reflected light 22 enters the photodiode. Similarly, with respect to the transmitted light, the main beam 23 and the internal twice reflected light 25 are emitted with a certain angle difference, and thus do not interfere with each other.

  FIG. 3 is a diagram showing an arrangement configuration example of a wedge-type beam splitter that is actually used. The arrangement 30 in FIG. 3 is a typical beam splitter arrangement in the case of constituting a wavelength locker, and uses a wedge structure having an off angle of 1 degree. In order to configure the wavelength locker, first, the first beam splitter 32 has a reflecting surface (first surface) disposed at 45 degrees with respect to the traveling direction of the incident light 35, and is used for power monitoring with respect to the photodiode 31. As light, several percent of incident light is demultiplexed in a direction 90 degrees above the main beam. The light that has not been reflected and passed through the first beam splitter 32 then enters the second beam splitter 33. The second beam splitter 33 is disposed at an angle of −44.6 degrees on the side where the wavelength filter 34 such as an etalon is disposed in order to perform demultiplexing at an angle of 90 degrees downward in FIG.

  In the configuration example of FIG. 3, the angle deviation and the optical axis shift generated in the transmitted light beam are minimized by changing the directions of the wedges with the two beam splitters 32 and 33 so that the efficiency of fiber coupling does not decrease. Yes.

JP 2011-123922 A Specification

Yasuharu Suematsu, Kenichi Iga “Introduction to Optical Fiber Communication”, 4th revised edition, Ohmsha, (2006/03) 11-4 Optical multiplexing system, page 249 H. Oohashi, H. Ishii, K. Kasaya, and Y. Tohmori, “Low-power-consumption Module with Tunable DFB Laser Array (TLA),” NTT Technical Review, Vol. 4, No. 5, 2006 Hiroshi Nakajima “Optical Design with EXCEL”, New Technology Communications, 2004 1.9 Reflectivity and Transmittance, page 20 Nippon Steel & Sumikin Electronics Device Co., Ltd. Catalog Optical Package http://www.nssed.nssmc.com/product/optical_tunable.html

  However, since the wedge-type beam splitter shown in FIG. 3 has a structure in which two opposing surfaces are slightly off, a slight angle deviation different from the incident angle occurs in the transmitted light. In the example of FIG. 3, in the first beam splitter 32 having an off angle of 1 degree, the outgoing light 36 is tilted slightly upward by 0.8 degrees with the optical axis shifted downward by about 200 μm. 1 beam splitter 32 is emitted. For this reason, the second beam splitter 33 is disposed with an inclination of −44.6 degrees in order to branch the outgoing light beam 36 again in the vertical direction of the traveling direction.

  As described above, in order to use the wedge-type beam splitter, it is necessary to control the mounting position and the mounting angle with high accuracy. As in the example of FIG. 3, when the light passes through the wedge surface a plurality of times, the angle deviation cannot be completely corrected. Therefore, there is a problem that some angle deviation remains in the light 37 transmitted through the beam splitter arrangement 30. is there. Furthermore, the beam splitter structure using the wedge plate has a problem that the yield is low with respect to the material to be charged at the stage of manufacture.

  FIG. 4 is a diagram for explaining the relationship between the cut portion and the thickness when manufacturing the wedge-type beam splitter. In the manufacturing process of the wedge-type beam splitter, a large-sized plate 40 which is turned off several degrees accurately is first prepared. After coating the plate with a reflective film and a non-reflective film, it is cut into a desired size and shape and used as individual beam splitters. Since the large plate 40 is formed by turning off the front surface 45 and the back surface 46 several times, the thicknesses 41, 42, and 43 change depending on the cutting location.

  Normally, a wedge-type beam splitter used in a wavelength tunable laser module or the like allows a thickness in a range from 0.4 mm to 0.7 mm, for example. Even in such a case, only a part of the manufactured large-sized material plate 40 can be used. Therefore, it is not possible to use all the material of the large plate 40, and the actual utilization rate of the input material, that is, the yield is low. In addition, the light passing through the beam splitter is shifted in position by several hundred μm before and after passing due to the difference in thickness. For this reason, in the arrangement configuration of the beam splitter of FIG. 3, the mounting position of the second beam splitter 33 arranged at −44.6 degrees is set in accordance with the thickness of the first beam splitter 32 arranged first. It must be adjusted.

  If there is a parallel shift or angle shift of the transmitted light for each beam splitter component or for each production lot of beam splitter components, the mounting position and angle are corrected at various locations in the device manufacturing process such as wavelength lockers. Must-have. This correction work may cause a new problem. As described above, the wedge-type beam splitter has an effect of suppressing optical interference due to multiple reflection, but has a poor yield when manufactured as a part. Furthermore, even when used as a component that functions as a wavelength locker, fine mounting position adjustment is required. For this reason, not only the price of a beam splitter component alone, but also an increase in the complexity and complexity of the manufacturing process of the entire device, unit, and equipment including the beam splitter, and a reduction in yield, this has a significant impact on reducing prices. ing.

  The inventors can meet the demand for cost reduction of the wavelength tunable laser module by solving the problem of the parallel plate type beam splitter rather than solving the above problem of the wedge type beam splitter. I came up with the idea of being there. Parallel plate beam splitters can be processed in large format and can be adopted from almost the same thickness, so that the product yield is high for the materials to be added and the manufacturing process is simple and the manufacturing cost is low. There is. In addition, since it is a parallel plate, the angle is preserved between the incident light and the emitted light, and no angular deviation occurs. Regarding the shift of the optical axis, a position shift is caused in parallel by the thickness of the beam splitter, but the shift amount can be predicted and controlled by the thickness. For this reason, if the thickness is manufactured with high accuracy, neither the light beam path that passes through nor the light beam path that reflects is changed, and a stable device can be manufactured.

  However, since it is a parallel plate, multiple reflection occurs depending on the accuracy of the non-reflective coating on the back surface (second surface) and the wavelength of light used. When multiple reflections occur in the beam splitter, the multiple reflected light always comes out at the same angle as the main transmitted beam and the main reflected beam. For this reason, periodic light intensity oscillations are observed in reflected light and transmitted light due to light interference. In particular, this vibration often appears in a monitoring photodiode installed at a close distance, which affects wavelength stabilization and power stabilization.

  If the problem of multiple reflection in the parallel plate beam splitter described above can be solved, there is no need to use a wedge-type beam splitter that has many problems, reducing the cost of a single beam splitter and shortening the manufacturing time of devices such as optical modules. And the yield can be improved. The present invention has been made in view of such problems, and an object thereof is to solve the problem of multiple reflected light in a parallel plate beam splitter structure. In addition, the cost of the parallel plate beam splitter alone and the unit and apparatus using the parallel plate beam splitter are reduced, and the manufacturing time is shortened and the yield is improved.

In order to achieve the above object, according to the present invention, a first surface that reflects a part of incident light is parallel to the first surface and the incident light is reflected. in There beam splitter parallel plate and a second surface which transmits said first normal and the incident angle of the incident light forms a surface theta _0, the spot size of the incident light omega _0, air Where n _{0 is} the refractive index of the beam splitter and n ₁ is the material refractive index of the beam splitter,

The thickness t between the first surface and the second surface and possess THAT determined, a perpendicular to the third plane defined by the light reflected by the incident light and the first surface An inclined end face that forms a first intersecting line perpendicular to the third face on the first face and allows the internally reflected light of the incident light to reach the second face. The inclined end face operates to diffuse or absorb the internally reflected light, and transmits the incident light through the beam splitter.

It is comprised so that it may have the same inclination angle, It is a beam splitter characterized by the above-mentioned. Here, the spot size refers to a radius in a range from the maximum intensity to the intensity of 1 / e ² when the spatial intensity distribution of the light beam is a Gaussian type .

Invention of Claim 2 is the beam splitter of Claim 1 , Comprising: It is a 2nd inclined end surface parallel to the said inclined end surface, Comprising: The return light which passed through the same optical path as the said reflected light in the said 1st surface A beam spot of the incident light between the first intersecting line and the second intersecting line formed by the second inclined end surface and the first surface. A second inclined end face that can be arranged is further provided. Here, the return light is generated from, for example, a filter element or a light detection element of the monitor circuit. The first intersection line, the second intersection line, and the cross section that cuts the two inclined end faces are parallelograms.

A third aspect of the present invention, a beam splitter of claim 2, the distance between the second intersection line between said first intersection line is characterized in that at 3 [omega] ₀ or more.

The invention according to claim 4 is the beam splitter according to claim 2 , wherein a distance between the first intersection line and the second intersection line is 4ω ₀ or more. Said 3ω ₀ or 4ω ₀ corresponds to the effective width in the detailed description of the invention.

A fifth aspect of the present invention is a wavelength locker comprising the beam splitter according to any one of the first to fourth aspects, which is used to branch the oscillation light of a light source. The beam splitter of the present invention can also be applied to a wavelength tunable laser module.

  As described above, the present invention solves the problem of the multiple reflected light of the prior art in the parallel plate beam splitter structure, reduces the cost of a single beam splitter, and includes a unit, an optical It is possible to reduce the manufacturing time and improve the yield of devices and devices such as modules, optical transmitters, and optical signal processors.

FIG. 1 is a diagram showing the structure of a typical tunable laser module of the prior art. FIG. 2 is a view showing the structure of a wedge-type flat beam splitter. FIG. 3 is a diagram showing an arrangement configuration example of a wedge-type beam splitter that is actually used. FIG. 4 is a diagram for explaining the relationship between the cut portion and the thickness when manufacturing the wedge-type beam splitter. FIG. 5 is a diagram for explaining a problem in the parallel plate beam splitter having the configuration of the prior art. FIG. 6 is a diagram showing an outline of the configuration of an actual wavelength tunable laser module in which a beam splitter is used. FIG. 7 is a diagram for explaining the shift of the optical axis when the thickness of the parallel plate beam splitter is changed. FIG. 8 is a diagram for explaining the effect of reducing interference by changing the thickness of the parallel plate beam splitter. FIG. 9 is a diagram illustrating a case where the parallel plate beam splitter of the present invention is configured to remove a region from which internally reflected light is emitted. FIG. 10 is a diagram showing the configuration of a parallel plate beam splitter that also copes with the return light of the main reflected light according to the present invention. FIG. 11 is a diagram showing the configuration of a wavelength locker equipped with a parallel plate beam splitter manufactured using the design method of the present invention. FIG. 12 is a diagram illustrating the structure of the beam splitter mounted with the wavelength locker of the first embodiment. FIG. 13 is a diagram illustrating a configuration of an optical module in which the wavelength locker of Example 1 is accommodated in a package.

  In the present invention, the parallel plate beam splitter that can be expected to be small in size and low cost is conspicuous, and the influence of interference caused by insufficient antireflection coating on the back surface (second surface) of the splitter in the prior art is suppressed. We propose a new parallel plate beam splitter and its design method. Furthermore, a unit such as a wavelength blocker using the parallel plate beam splitter of the present invention, an optical module, an optical transmitter, and an optical signal processing device are also included.

  According to the present invention, there is provided a parallel plate beam splitter in which periodic light intensity vibrations caused by multiple reflections are suppressed, and problems that affect wavelength stabilization and power stabilization are solved. In the present invention, the geometrical optical design is used to adjust the thickness and side shape of the parallel plate beam splitter. It absorbs and diffuses the light reflected inside the beam splitter, reduces the influence on the original transmitted light and reflected light due to interference, etc., and proposes the smallest shape that can be manufactured, thereby achieving both low cost and small size. Hereinafter, first, the problems of the row plate beam splitter will be described in detail, and then the configuration and design method of the new parallel plate beam splitter of the present invention will be described.

FIG. 5 is a diagram for explaining a problem in the parallel plate beam splitter having the configuration of the prior art. FIG. 5A is a diagram showing multiple reflected light for one light beam, and FIG. 5B is a diagram showing multiple reflected light when the beam has a distribution (width). . The beam splitter shown in FIG. 5 is a parallel plate type composed of a first surface 50 that is a reflection surface on which incident light 52 and 56 is incident, and a second surface 51 that is a non-reflective coating that is a rear surface. This is a beam splitter. Referring to FIG. 5A, each arrow passing through the first surface 50 and the second surface 51 of the beam splitter perpendicularly represents a normal line of each surface. The angle between the normal line and the incident light 52, that is, the incident angle is θ ₀ , and the angle between the reflected light 53 reflected by the first surface 50 and the normal line, that is, the reflection angle is also θ ₀ .

Further, as a material of the beam splitter, a quartz glass material or the like having a small absorption effect in the wavelength band of transmitted light is usually used. Since the refractive index n ₁ of a glass material such as quartz glass material is higher than the refractive index n _{0 of} air, light can pass through the inside of the beam splitter regardless of the incident angle. At this time, the refraction angle theta ₁ of the internal beam splitter, by Snell's law, an incident angle theta _0, the refractive index n ₁ of the refractive index n ₀ and the material inside the outer air is determined from the following relationship.

Since the light transmitted from the first surface 50 into the beam splitter is a parallel plate, it reaches the second surface 51 on the back surface at an incident angle θ ₁ , and the main beam enters the air from the second surface 51. It is going to jump out at the refraction angle θ _0. It can be seen that the incident angle and the outgoing angle are the same, and the parallel plate beam splitter has a characteristic that the angle of the light beam is preserved. This is one advantage of adopting a parallel plate beam splitter.

The second surface 51 on the back surface is usually provided with a non-reflective coating, and the reflectance is sufficiently lowered. However, the reflectance cannot be completely reduced to zero. If the wavelength band to be used is wide, a reflectance of the order of 0.1% remains depending on the wavelength used. In such a case, a part of the main beam that has reached the second surface 51 at the incident angle θ ₁ is reflected at the same angle θ ₁ with respect to the normal of the second surface 51 and transmitted through the inside of the beam splitter. As a result, the first surface 50 is reached. At this time, the incident angle incident on the reflecting surface of the first surface 50 is θ ₁ because it is a parallel plate. A part of the beam that has reached the first surface 50 is reflected again at θ ₁ equivalent to the incident angle because the reflectance is set on the first surface, and the others are reflected as reflected light 55 as a refraction angle. Jump into the air at θ ₀ .

  The light reflected again inside the beam splitter then repeats the same reflection. However, if the light is reflected a plurality of times and proceeds inside the beam splitter, the light intensity immediately decreases to −60 dB or less, and becomes a negligible level. The problem is the first internally reflected light 55. If the first surface 50 is a reflective surface having a reflectivity of about 3%, and the reflection from the non-reflective coating surface of the second surface 51 is about 0.1%, the first interior in the second surface 51 Most of the reflected light (referred to as the first internally reflected light) comes out as it is. That is, the internal reflection light 55 occupies about 3% of the entire reflection light (the total of the main reflection light 53 and the internal reflection light 55).

  FIG. 5B shows the multiple reflected light when the incident light has a distribution with a predetermined width. The incident light 56 is distributed in a range from the auxiliary light beam 56b to the auxiliary light beam 56c with the position of the main light 56a as the center. Compared with the case of FIG. 5A, the number of arrows is different, but the mutual relationship between the main reflected light 57, the main beam 58 transmitted through the beam splitter, and the multiple reflected light 59 is different. Is the same as (a) of FIG. In FIG. 5B, the right side portion of the main reflected light 57 in the drawing and the left side portion of the multiple reflected light 59 are completely overlapped, and the monitor light detected by the photodiode is completely reflected by the multiple reflected light. I am receiving interference.

  Thus, in the conventional parallel plate beam splitter, the main reflected light is mixed with light having different reflection paths at a ratio of several percent. Further, problems in a module such as an actual wavelength tunable laser module in which a parallel plate beam splitter is used will be described below.

  FIG. 6 is a diagram showing an outline of the configuration of an actual wavelength tunable laser module in which a beam splitter is used. The module 65 of FIG. 6 is a small package for a wavelength tunable laser called an optical package (for tunable) that has been widely used in recent years (Non-Patent Document 4), and incorporates a semiconductor laser and a wavelength locker. This small package is characterized in that the width is reduced to about half compared to a conventional butterfly-shaped standard package and the electrode pins are arranged on one side of the rear surface. Inside the package 65, a laser element 67, a lens 68 and a wavelength locker 69 are provided. In the wavelength locker 69, one of the beam splitters splits about 2% to 5% of light from the main beam, monitors the optical power using the branched light, and simultaneously monitors the wavelength of the light. Yes.

  When a parallel plate beam splitter is used in such a small wavelength tunable laser package 65 or wavelength locker 69, the main reflected light used for monitoring each time the oscillation light wavelength from the wavelength tunable laser changes. Then, reflected light that has passed through different paths included at a ratio of several percent interferes, and interference fringes, that is, light intensity fluctuations of several percent are observed. This is one problem when using a parallel plate beam splitter.

  As described above, the cause of this interference problem is the internally reflected light generated by the first (first) internal reflection on the second surface. As a means for reducing the first internally reflected light, there is a method of increasing the thickness of the beam splitter. However, as the beam splitter becomes thicker, another problem of the shift amount 71 of the optical axis occurs.

FIG. 7 is a diagram for explaining the shift of the optical axis when the thickness of the parallel plate beam splitter is changed. (A) and (b) of FIG. 7 show the case where the light 72 incident on the first surface of the parallel plate beam splitter is transmitted through the inside of the splitter, and the emitted lights 73 and 74 as the main beams are obtained, respectively. Show. And the shift amount 71a of the optical axis between the outgoing light and the incident light when the thickness t ₁ of the beam splitter (a), outgoing light and the incident light when the thickness t ₂ of the beam splitter (b) Is clearly different from the shift amount 71b of the optical axis. That is, in the case of FIG. 7B where the beam splitter is thicker, the shift amount 71b of the optical axis becomes larger.

  FIG. 8 is a diagram for explaining the effect of reducing interference by changing the thickness of the parallel plate beam splitter. The beam splitter 80 of FIG. 8 has a configuration in which a part of the incident light 83 is reflected as reflected light 84 and the remaining incident light is transmitted through the beam splitter and emitted from the splitter to the space as outgoing light 85. The case where the incident beam 83 is distributed with the width | variety of the range of the auxiliary ray 83b to the auxiliary ray 83c is shown. As shown in FIG. 8, when the thickness t of the beam splitter 80 is increased, the position of the internally reflected light 86 is shifted to the right in FIG. 8, and the main reflected light 84 and the internally reflected light 86 are spatially changed. Can be separated. However, when the thickness t is increased without limitation, the optical axis shift described with reference to FIG. 7 occurs before and after passing through the beam splitter.

  Further, in the wavelength tunable laser module described with reference to FIG. 6, the shift amount by which the position of the optical axis of the main beam traveling through the wavelength locker 69 including the beam splitter is shifted downward in the drawing of FIG. 71 will increase. When the wavelength locker is mounted in the optical package 65 that has been further miniaturized as shown in FIG. 6, there arises a problem that it is difficult to secure the beam path of the main beam. The small package 65 shown in FIG. 6 has a width of about 6 mm to 8 mm, and the minimum width inside the package is about 5 mm. If the beam splitter installed at 45 degrees with respect to the incident light has a thickness of about 1 mm, and the material refractive index of the beam splitter is 1.5, the optical axis of the transmitted light beam is about 0. 0. Shift by 5 mm. Assuming that the beam splitter is mounted at the center of the package 65, the shift amount of 0.5 mm is a large value with respect to the size of the entire package.

  When the light (main beam) that has passed through the beam splitter is shifted by 0.5 mm from the center, the distance to the inner wall of the package is 2 mm, and the distance to the outer wall is 2.5 mm. For this reason, it is necessary to shift the mounting position of the second lens 70 mounted on the small package 65 by 0.5 mm. In this case, if the external width of the small package 65 is 6 mm, the external dimensions of the lens that can be used must be a radius of 2.5 mm and a diameter of 5 mm or less. In an actual module manufacturing process, a margin for adjusting the lens position is also required. Therefore, if the adjustment amount is +/− 0.2 mm, it is forced to use a lens having a diameter of 4.6 mm or less. Thus, the setting of the beam splitter thickness affects other component designs, and in some cases may limit the miniaturization of the package size itself. Therefore, in the parallel plate type beam splitter, it is important to determine the minimum value of the beam splitter thickness required to separate the internally reflected light from the main reflected light.

  According to the present invention, there is provided a new beam splitter structure design method in consideration of interference of internally reflected light and shift of the optical axis of the parallel plate beam splitter. The thickness t of the parallel plate beam splitter of the present invention described later defines the minimum thickness for spatially separating the internally reflected light and the main reflected light. Hereinafter, a method of designing the parallel plate beam splitter of the present invention will be described using each parameter shown in FIG.

Referring again to FIG. 8, the spot size of the main beam 83 incident on the first surface 81 of the parallel plate beam splitter 80 is ω ₀ . Here, the spot size means a radius in a range from the maximum intensity to 1 / e ² when the spatial intensity distribution of the light beam is a Gaussian type. In calculating the amount of transmitted light and reflected light, if a range of about 1.5 times the spot size ω ₀ is taken into account, 98% or more of the beam power is included. Regarded as the range of light transmitted through this width effectively, (in terms of the diameter) the effective width of the main beam incident on the beam splitter twice 1.5Omega _0, i.e., the 3 [omega] _0. The effective width of 3ω ₀ takes into account the distribution of most of the light that passes through the beam splitter. In FIG. 8, the range between the auxiliary ray 83b and the auxiliary ray 83c with respect to the center 83a of the main beam corresponds to the effective width 3ω ₀ . As described above, the effective width means the width of the beam including the majority (eg, 98%) of the beam power, and corresponds to the beam diameter, not the spot size ω ₀ corresponding to the radius. Please note that.

If light having an effective width 3ω ₀ is incident on the parallel plate beam splitter at an incident angle θ ₀ , the light incident region 2ω ₁ on the first surface 81 (reflection surface) can be obtained by the following equation.

Also, taking more widely effective width, if the light of the width of 4Omega ₀ Considering approximately twice the range of the spot size omega ₀ is incident at an incident angle theta _0, the light entrance region 2 [omega _1, the following equation Is required.

However, in the above equation, θ ₀ is the angle of incidence on the first surface 81. The refraction angle θ ₁ entering the beam splitter 80 from the first surface is obtained by the following equation from Snell's law.

Here, n ₀ is the refractive index of air, and n ₁ is the material refractive index of the beam splitter.

8, the point at which the auxiliary light beam 83b reaches the first surface 81 R0, auxiliary light 83b is penetrated with refraction angle theta ₁ to the internal beam splitter, a point that has reached the rear surface of the second surface 82 R1 'And. Further, a perpendicular line (normal line) is drawn perpendicularly to the second surface 82 from R1 ′, the point intersecting with the first surface 81 is R1, and the auxiliary light beam 83b reaching the second surface 82 is the second A point that is reflected by the surface 82 and reaches the first surface 81 again is defined as R2. From the principle of reflection, the reflection angle between the normal of the two surfaces passing through R1 ′ and R1 and the reflected light is θ ₁ . Accordingly, the corresponding two sides of the right triangle R0R1′R1 and the right triangle R1′R1R2 which are geometrically congruent, the lengths of R0R1 and R1R2 are equal, and this length L is expressed by the following equation.

L represents the beam width converted on the first surface corresponding to the effective beam width. Here, if L is larger than ω ₁ (2L> 2ω ₁ ), the main reflected light 84 and the internal reflected light 86 have no intersecting portions and are spatially separated. This relationship can be expressed by the inequality concerning L as follows.

It is only necessary to find the thickness t of the beam splitter where the inequality of equation (6) holds. When Expression (2) (when the effective width is in the range of about 1.5 times the spot size) and Expression (5) are used in Expression (6), the following expression is transformed as an inequality for thickness t. can get.

  If the thickness t of the beam splitter is determined by the equation (7), the main reflection light 84 and the internal reflection light 86 can be spatially separated, and a parallel plate beam splitter having a minimum thickness can be manufactured.

Accordingly, the present invention provides a parallel plate beam splitter having a first surface that reflects a part of incident light and a second surface that is parallel to the first surface and transmits the incident light. The incident angle between the normal of the first surface and the incident light is θ ₀ , the spot size of the incident light is ω ₀ , the refractive index of air is n ₀ , and the material refractive index of the beam splitter is n ₁ . and when,

で決定される前記第１の面と前記第２の面と間の厚さｔを有することを特徴とする平行平板ビームスプリッタである。

A parallel plate beam splitter having a thickness t between the first surface and the second surface determined in (1).

However, in this structure which defines only the thickness of the bi chromatography beam splitter, a main reflected light 84 and the internally reflected light 86 are distributed adjacent, is still depending on the size and position of the photodiode for receiving light, at the same time two One light can be received. Even if the light is not directly received by the photodiode, the internally reflected light 86 is emitted from the beam splitter to the space. The internally reflected light 86 is reflected by a wall surface in the package as shown in FIG. 6 and may reach another monitor photodiode. Therefore, in order to more effectively suppress interference due to multiple reflection in the parallel plate beam splitter, it is necessary to remove or diffuse the reflected light itself emitted from the inside of the beam splitter.

  Inside the beam splitter having the minimum beam splitter thickness t given by equation (7), the path through which the main beam passes through the first surface 81 and the second surface 82 and the reflection path of the internally reflected light are: , Has been separated for the time being. Here, if the region itself until the internally reflected light generated on the second surface 82 reaches the first surface 81 again is eliminated, the internally reflected light is not emitted outside the beam splitter, and the main reflected light is lost. The same route as 84 is no longer taken.

FIG. 9 is a diagram for explaining a configuration in which a region where internally reflected light is emitted is removed in the parallel plate beam splitter of the present invention. The beam splitter 90 of FIG. 9 has a configuration in which a part of the incident light 93 is reflected as reflected light 94 and the remaining incident light is transmitted through the beam splitter and emitted from the splitter to the space as outgoing light 95. The incident angle θ ₀ of the incident light 93 to the parallel plate beam splitter 90, the reflection angle θ ₁ of the internally reflected light on the second surface 92, the signs R 0, R 1, R 2, etc. representing each point are the same as in FIG. That is, in FIG. 9, an auxiliary ray 93 b corresponding to the effective width of the main beam 93 incident on the beam splitter 90 is transmitted through the beam splitter, reflected by the second surface 92, and again reflected on the first surface 91. The return position is R2. R2 is a limit position closest to the main reflected light 94 when the internally reflected light 96 is about to be emitted from the first surface 91.

As shown in FIG. 9, when machining the right part than R2 points so as to form an internal refraction angle theta ₁ between parallel inclined end surface 97, the main beam is passed through without loss beam splitter 90, the internal The reflected light 96 is diffused and absorbed by the processed inclined end surface 97 and blocked. When the reflected light does not return from the point branched by the beam splitter, the configuration of FIG. 9 can sufficiently remove the multiple internally reflected light 96 generated when the incident light passes through the parallel plate beam splitter. .

Therefore, the configuration of the beam splitter of the present invention shown in FIG. 9 is perpendicular to the third surface defined by the incident light 93 and the reflected light 94 on the first surface 91, and is on the first surface 91. to form a first intersection line perpendicular to the third surface, internally reflected light 96 of the incident light in the second surface 92 is an inclined end surface 97 that can be reached, the inclined end face 97 operates to diffuse or absorb the internally reflected light 96 and transmits the incident light 93 through the beam splitter.

It is comprised so that it may have the same inclination angle. It can be said that the third surface is a surface including the incident light 93 and the reflected light 94. Further, the first intersection line corresponds to the ridgeline between the first surface and the inclined end surface 97 corresponding to the position of R2 in FIG.

The position of the main beam 93 incident on the beam splitter 90 configured as shown in FIG. 9 needs to be controlled. When the effective width is in the range of about 1.5 times the spot size, the incident position of the main beam 93 on the beam splitter is the effective width 1 of the beam from the point R0 that is returned by 2L to the left from the point R2 in FIG. It is necessary to be within a range from 0.5ω ₀ to a point returned in the R2 direction. In addition, when the effective width is set to be about twice the spot size, the beam is returned to the R2 direction by the effective width 2ω ₀ minutes or more from the point R0 which is returned by 2L to the left from the point R2 in FIG. There must be. At this time, Expression (3) (when the effective width is in the range of about twice the spot size) and Expression (5) are used in Expression (6), and the following expression corresponding to Expression (7) is used. Note that the beam splitter thickness t is determined.

  With the configuration of FIG. 9, the multiple internally reflected light in the parallel plate beam splitter can be sufficiently removed. By using a parallel plate type beam splitter, the need for a wedge type beam splitter is eliminated, and the yield in manufacturing the beam splitter itself is greatly improved. In addition to reducing the cost of a single beam splitter, position and angle adjustments corresponding to individual splitters are greatly simplified, making it possible to shorten manufacturing time and improve yield in devices or apparatuses such as optical modules.

  As described above, the configuration shown in FIG. 9 can prevent the internally reflected light generated on the second surface of the parallel plate beam splitter from being emitted to the outside of the beam splitter. As a result, when monitoring the main reflected light, periodic light intensity oscillations are not observed due to light interference. However, when the beam splitter having the configuration shown in FIG. 9 is used for a wavelength locker or the like, reflected light from an etalon filter or the like mounted in the wavelength locker returns as return light on the same ray path as the main reflected light 94. In order to cope with this return light, it is preferable to further modify the configuration of FIG.

FIG. 10 is a diagram showing the configuration of a parallel plate beam splitter that also copes with the return light of the main reflected light according to the present invention. 10 has a configuration in which a part of incident light 103 is reflected as reflected light 104 and the remaining incident light is transmitted through the beam splitter and emitted from the splitter to the space as outgoing light 105. The parallel plate beam splitter 100 is further configured to prevent multiple reflections of the return light from the optical component ahead of the main reflected light 104. That is, not only the inclined end surface 107 in the direction in which the internally reflected light reaches, but also the opposite side surface is cut as the second inclined end surface 108 at the same refraction angle θ ₁ as shown in FIG. preferable. The cutting position of the inclined end surface 108 is 2L, which is twice the length L derived by the equation (5) from the point R0 ′ where the auxiliary light beam 103c of the main beam 103 reaches the reflecting surface of the first surface 101. R3 on the left side. By cutting from the position of R3, it is possible to obtain a return light removal effect that is substantially the same as the removal effect of internally reflected light on the main beam. The second inclined end surface 108 operates to diffuse and absorb light in the same manner as the processed inclined end surface 107.

  Accordingly, the beam splitter of FIG. 10 is a second inclined end face 108 parallel to the inclined end face 107, and diffuses or absorbs the return light 106 that has passed through the same optical path as the reflected light 104 on the first face 101. The beam spot of the incident light 103 can be disposed between the first intersecting line and the second intersecting line formed by the second inclined end surface 108 and the first surface 101. A second inclined end face 108 is further provided. Here, the second intersection line is a ridge line formed by the second inclined end surface 108 and the first surface 101, and corresponds to the position R3 in FIG.

  With the configuration shown in FIG. 10, since the secondary cutting direction in the case of manufacturing the beam splitter is two parallel surfaces 107 and 108, only one kind of inclined end surface is sufficient. For this reason, the manufacturing cost of the beam splitter can be reduced as compared with the configuration of FIG. 9 having two different cutting directions for cutting only one side at the inclined end face. Since the manufacturing process can be performed by optical polishing of parallel plates, there is no variation in the thickness and the lot dependency is small. The position of the optical axis after transmission does not vary depending on the individual beam splitter, and there is little dependency between lots. Furthermore, the manufacturing process of the wavelength locker provided with the parallel plate beam splitter of the present invention is simplified, the cost of manufacturing the unit can be reduced, and the yield of the entire wavelength locker can be greatly improved.

When the package size to be mounted is large and the size of components to be mounted can be increased, or when a margin for mounting positioning is ensured, the effective width of the main beam can be made wider. That is, 1.5 times the 1.5Omega ₀ spot size omega ₀ a effective width of the main beam _(effective width, 3 [omega] ₀ and equivalent _diameter) based on the minimum design of the formula (2) with consideration of the first rather than computing the conversion beam width L and the beam splitter thickness t in terms twice the 2 [omega ₀ spot size omega ₀ a effective width of the main beam _(effective width, 4Omega ₀ to equivalent _diameter) to It can be calculated based on the design of equation (3) expanded to When the demand for downsizing is not strong, it is appropriate to adopt a wider effective width in consideration of up to about twice the spot size ω ₀ with a margin. This case has already been shown as equation (8).

Furthermore, when a wider range of beam widths can be tolerated, for example, when considering a range up to N times the spot size ω ₀ as the width of the main beam, the thickness t of the beam splitter is given by Can be generalized to:

  L, which determines the cutting position of the side surface of the beam splitter, can be obtained as a minimum L value by using the same equation (5) for the minimum thickness t obtained by the above equation. A parallel plate type beam splitter that does not cause so-called vignetting can be manufactured by efficiently removing the internally reflected light of the beam splitter and reflecting and transmitting the main beam light without loss. Hereinafter, more specific examples of the parallel plate beam splitter obtained by the designing method of the present invention will be described.

  FIG. 11 is a diagram showing the configuration of a wavelength locker equipped with a parallel plate beam splitter manufactured using the design method of the present invention. The wavelength locker 110 includes an isolator, a first parallelogram beam splitter 112, a second parallelogram beam splitter 113, an etalon filter 114, and two photodiodes 115 and 116 on an aluminum nitride substrate of about 4 mm square. It has a configuration equipped with.

  The main beam 117a is incident on the wavelength locker 110 from the left side of the drawing, passes through a 1.5-stage isolator 111 in which an analyzer, a Faraday rotator, an analyzer, a Faraday rotator, and an analyzer are stacked in this order. The first parallelogram beam splitter 112 having a plane arranged at 45 degrees is reached. On the surface of the first surface of the first parallelogram beam splitter 112, a reflection film of 5% is disposed, and 95% of the incident light passes through the first parallelogram beam splitter 112 as it is, Coupled to a lens and optical fiber external to the wavelength locker 110. 5% of the light reflected in the vertical direction (upward in the drawing) by the first parallelogram beam splitter 112 subsequently reaches the second parallelogram beam splitter 113 arranged at −45 degrees.

  The second parallelogram beam splitter 113 splits the light beam at a ratio of 2: 1, that is, a ratio of 67% reflection and 33% transmission. The reflected light of 67% is guided to the etalon filter 114 and further to the wavelength monitoring photodiode 115. On the other hand, 33% of the branched light 119 passes through the second parallelogram beam splitter 113 and is guided to the photodiode 116 for power monitoring.

  By using the parallelogram beam splitters 112 and 113 of the present invention, the internally reflected light 121 is absorbed and diffused at the inclined end face, and the return light 122 from the etalon filter 114 is also absorbed and diffused at the inclined end face. The semiconductor laser (not shown in FIG. 11) which is the source of the incident light 117a has an emission angle (far field pattern) of 35 degrees in the vertical direction (perpendicular to the drawing in FIG. 11) and the horizontal direction (in FIG. 11). It has a full width at half maximum of 19 degrees in the plane of the drawing (perpendicular to the incident light 117a). The spot size of the collimator beam diameter produced by the combination with a lens having a numerical aperture NA of 0.6 is about 0.4 mm in the vertical direction and about 0.2 mm in the horizontal direction.

The wavelength locker of FIG. 11 requires beam splitters 112 and 113 that branch in the horizontal direction, and the spot size of the incident light 117a in the horizontal direction is about 0.2 mm. Therefore, when the effective beam diameter of 1.5 times the 0.3mm spot size omega _0, and were designed for two as in the case of double 0.4mm spot size omega _0. As a material (glass material) for the beam splitter, lead-free glass having a refractive index of 1550 nm and a wavelength of about 1.5 was used. Assuming a parallel plate splitter with an incident angle of 45 degrees, ω ₁ is obtained as 0.4243 mm and 0.5657 mm from the expressions (2) and (3), respectively, on the first surface inclined 45 degrees.

Since the splitter width (length of the line of intersection of the main beam and the surface of the first surface including a main reflection light) is required 2 [omega ₁ be less effective beam diameter of the spot size omega ₀ 1. For two types of 5 times and 2 times, 0.8486 mm and 1.1314 mm or more are required, respectively. From Equation (7) and Equation (8), the minimum value of the beam splitter thickness t is about 0.8 mm and about 1.05 mm. At this time, the shift amounts of the optical axes of the light beams passing through the beam splitters 112 and 113 are 0.43 mm and 0.56 mm, respectively. When the shift amount of the optical axis exceeds 0.5 mm, the position of the output light beam described above is limited. For this reason, in this embodiment, although the margin for mounting components is somewhat small, the beam splitter was manufactured with a thickness of 0.8 mm in both designs.

  FIG. 12 is a diagram illustrating the structure of the beam splitter actually mounted in the first embodiment. 12A shows the first beam splitter 112, and FIG. 12B shows the second beam splitter 113. In FIG. 12A, the surface 124 of the first beam splitter is an incident surface, which is coated with a reflective film having a reflectance of 5%. In FIG. 5B, the surface 125 of the second beam splitter is an incident surface and is coated with 66% reflective film. The opposing surfaces 123 of the surfaces 124 and 125 are coated with antireflection. The widths of the incident surfaces 124 and 125 are designed to be as wide as 1 mm in consideration of workability during mounting. The processing of the side surface was a combination of an internal angle of 62 degrees and 118 degrees in consideration of a refraction angle of 28 degrees calculated from the refractive index.

  FIG. 13 is a diagram illustrating a configuration of an optical module in which the wavelength locker of Example 1 is accommodated in a package. Inside the optical package, a laser 131, a first lens 132, and a wavelength locker 133 are included. Output light 135 is emitted from the second lens 136. The wavelength locker 133 includes the parallel plate beam splitter 134 of the present invention. In the configuration of FIG. 13, since the parts of the parallel plate beam splitter 134 of the present invention are manufactured in a constant shape, the variation in the shift amount of the optical axis is very small and almost constant.

  The insertion loss of the wavelength locker shown in FIG. 11 is equivalent to 5% (0.2 dB) corresponding to the reflectance of the first parallelogram beam splitter 112. The measured insertion loss of the manufactured wavelength locker is about 5.8% (0.26 dB), and there is an excess loss of about 0.8% (0.06 dB). It was. Moreover, 1 mW incident light 117a was given by the design in which about 1/4 of the reflected 5% of the monitor light is incident on the photodiode as a whole. The peak intensity of the photodiode was 5 μA for the first beam splitter 112 and 12 μA for the second beam splitter 113, respectively, and a light reception level almost as expected was obtained. Further, although the wavelength was swept in the range of 1530 nm to 1560 nm, the wavelength dependent amplitude fluctuation observed with the wavelength monitoring photodiode 115 was hardly observed below the measurement limit.

  As described above, it was confirmed that the parallelogram-shaped parallel plate beam splitter manufactured using the parallel plate beam splitter design method of the present invention functions as a wavelength locker with sufficient performance. By making the beam splitter into a parallelogram, the yield with respect to the material to be introduced is greatly improved and the manufacturing process itself is simplified, so that the manufacturing cost of the beam splitter can be suppressed. Since the beam splitter of the present invention can be manufactured by optical polishing processing of parallel plates, there is no variation in thickness and the dependence on the production lot is small. The position of the optical axis of the transmitted light does not vary depending on the individual beam splitter and the production lot. Furthermore, the manufacturing process of a unit / device such as a wavelength locker provided with the beam splitter of the present invention is simplified. With the beam splitter of the present invention, the cost of manufacturing the wavelength locker can be reduced, and the overall yield of the wavelength locker can be greatly improved.

  As described above in detail, the present invention solves the problem of multiple reflected light in the parallel plate beam splitter structure of the prior art, reduces the cost of the beam splitter alone, and includes the unit having the beam splitter inside. It is possible to reduce the manufacturing time and improve the yield of devices and devices such as optical modules, optical transmitters, and optical signal processors.

  The present invention is generally applicable to an optical communication system. In particular, it is used for an optical transmitter and an optical transmission module.

1, 67, 131 Laser light source 2, 10, 132, 136 Lens 20, 32, 33, 40 Wedge beam splitter 21, 83, 93, 103 Incident light 22, 84, 94, 104 Reflected light 23, 85, 95, 105 Emission light 50, 81, 91, 101 First surface 51, 82, 92, 102 Second surface 80, 90, 100 Parallel plate beam splitter 97, 107, 108 Inclined end surface 110, 133 Wavelength locker 114 Etalon 115 116 photodiode

Claims (5)

A first surface for reflecting a portion of the incident light, the beam splitter of said parallel plate and a second surface on which the incident light is transmitted which is parallel to the first surface,
The incident angle formed by the normal of the first surface and the incident light is θ ₀ , the spot size of the incident light is ω ₀ , the refractive index of air is n ₀ , and the material refractive index of the beam splitter is n ₁ . and when,

The thickness t between the first surface and the second surface possess THAT determined,
A first intersection line that is perpendicular to the third surface defined by the incident light and the reflected light at the first surface and is perpendicular to the third surface on the first surface Comprising an inclined end surface that can be internally reflected by the incident light on the second surface,
The inclined end surface operates to diffuse or absorb the internally reflected light, and transmits a transmission angle of the incident light that passes through the beam splitter.

The beam splitter is configured to have the same tilt angle . A second inclined end surface parallel to the inclined end surface, wherein the return light that has passed through the same optical path as the reflected light on the first surface operates to diffuse or absorb, and the first intersection line; It further comprises a second inclined end face capable of arranging a beam spot of the incident light between the second inclined end face and a second intersecting line formed by the first face. The beam splitter according to claim 1 . Wherein a distance between said second intersection line first intersection line, the beam splitter according to claim 2, characterized in that at 3 [omega] ₀ or more. Wherein a distance between said second intersection line first intersection line, the beam splitter according to claim 2, characterized in that at 4Omega ₀ or more. Is used to branch the oscillation light of the light source, the wavelength locker, characterized in that it comprises a beam splitter according to any of claims 1 to 4.

Images