JP2001324653A - Thin type optical communication module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical communication monitoring module which confirms the presence or absence of communication for every channel of wave length multiplexed communication and which is made thin. SOLUTION: This module is provided at least with a diffraction grating for separating multiplexed lights and the module is made thin as a whole by making the spot of an incident light to be formed on the surface of the diffraction grating of the diffraction grating flat while making the angle (x) of incidence of a light with respect to the diffraction grating to be within 50 deg.-75 deg., (50 deg.<=x<=75 deg.) and making the difference between the angle (x) of incidence of the light and the output angle (y) from the diffraction grating to be equal to or larger than 10 deg. and smaller than 37 deg., (10 deg.<=x-y<37 deg.).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to optical communication technology, and
The present invention relates to an optical module for WDM.

[0002]

2. Description of the Related Art With the development of the information-oriented society in recent years, it is required to increase the capacity of communication media. Until now, communication cables were mainly coaxial cables, but with the expansion of bandwidth, optical cables have been used as backbone cables. As a result, the transfer capacity has been greatly expanded. However, with the recent increase in the amount of data transferred between computers such as the Internet, a communication method using wavelength multiplexing technology has been used instead of passing information on one wavelength. We are starting. This multiplexing method is referred to as DWDM (Dense Wave Division Mu).
ltiplexing). The advantage of this method is that
The point is that the communication capacity can be expanded without adding optical cables already laid.

In this communication system, information is superimposed on light by using a wavelength variable laser as a light source and passing light from the laser light source through a module for modulating light. The optical multiplexing is completed through an optical multiplexer for bundling these into several tens of channels. The multiplexed light travels through one optical cable, and is transmitted to the destination while being amplified by an optical fiber amplifier on the way. At the destination, the multiplexed light is passed through an optical demultiplexer, separated into original light of each wave, and then subjected to optical / electrical (O / E) conversion to demodulate the signal.

Here, a wavelength monitor is required to grasp the state of each channel in the multiplexed optical signal. By introducing this device into an optical fiber amplifier, for example, it is possible to control the amplification factor of the optical fiber amplifier or to control the optical fiber amplifier from running away when the input signal is interrupted due to a line failure. Become.

When a module is incorporated in such an optical communication device, the optical communication device is often required to be thin because of the space of the housing. However, in the case of a DWDM-related optical communication module, the multiplexed optical signal needs to be separated once for each channel, so that an optical element or an optical component having some kind of spectral function must be introduced. For such optical components, elements utilizing the wavelength dependence of light transmission and refraction, such as a wavelength filter, an AWG (Arrayed Waveguide Grating), a prism, and a diffraction grating, are used.

Among them, a system using a plurality of wavelength filters (F ₀ , F ₁ , F ₂ ,..., F _n ) shown in the simplified configuration diagram of FIG. A wavelength filter is prepared corresponding to the wavelength of each channel, and the state of each channel can be monitored by passing the multiplexed light demultiplexed by the optical demultiplexer through each wavelength filter. However, the problem here is that if the number of channels increases, the light amount loss in the optical demultiplexer increases, which is not suitable for monitoring a large-capacity optical communication system.

For this purpose, a system generally used at present is an AWG-based system having a configuration as shown in FIG. This is because the optical waveguide is formed in an arch shape, and the slab waveguide through which light of different wavelengths is transmitted is different by gradually changing the optical path length of the arch portion for each waveguide. It is possible to obtain a separated light output for each. According to this method, a compact optical module can be formed because the waveguide itself is thin.

However, in this method, a high technique is required for producing a refractive index distribution of the waveguide, and since such a refractive index distribution changes with temperature, temperature control becomes important. For this reason, the system using the AWG is generally expensive, so that the applicable range is actually limited.

Under these circumstances, in order to realize an inexpensive and thin channel monitor for DWDM, it is necessary to use a spectral device which is thin but easy to manufacture.
We focused on a diffraction grating (reflection grating) as a device that meets this requirement. Diffraction gratings form fine grooves on a substrate made of quartz or silicon, and diffracted light generated in such grooves interferes with each other, so that light of a specific wavelength is emitted in a specific direction. is there. As the groove forming technique, a photolithography technique used in semiconductors can be used, so that a very accurate groove can be formed. In addition, it is easy to form a replica using a transfer technique using the diffraction grating once formed as a master. Therefore, it can be said that the diffraction configuration device is suitable for mass production.

[0010]

When a diffraction grating or a reflection type diffraction grating is used for a spectral device, it is necessary to apply a somewhat large beam diameter to the diffraction grating surface. In order to irradiate the light on the diffraction grating, a space corresponding to the space is required for the housing for accommodating them. However, as described above, since the optical communication module is required to be thin, it is not possible to select a sufficiently large luminous flux diameter for a required wavelength resolution, and as a result, a desired demultiplexing performance is obtained. There is a technical problem that it cannot be obtained. On the other hand, when the light demultiplexed by the diffraction grating is condensed for each channel and coupled to a light receiving device such as a light receiving element array, the light is condensed to an arbitrary channel and to an adjacent channel due to a decrease in demultiplexing performance. Light having the required wavelength leaks, causing a problem of deteriorating crosstalk between channels.

[0011] In DWDM optical communication, there is no mechanism for adjusting the polarization of propagating light. Therefore, usually, there is no way to know in advance the polarization state of the light emitted from the fiber. Therefore, in many cases, polarization independence is required for the optical communication module.

Optical polarization dependence is based on whether the vibration plane of the electric field component of the electromagnetic field is perpendicular (p-wave) or parallel (s-wave) to the plane of incidence.
Changes the state of reflection and refraction. In the optical module, reflected light is often handled from the point of efficiency.In the diffraction grating, it is difficult to fabricate a polarization-independent element because the interference state of the diffracted light varies greatly depending on the incident angle. is there.

[0013]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has as its object to provide an optical communication monitor module employing a low-cost and easily manufactured diffraction grating as a spectral element. According to the present invention, it is possible to reduce the thickness of the entire module housing while maintaining the demultiplexing performance of the diffraction grating, and an optical communication monitor module having a thin and high-performance demultiplexing function suitable for mass production. Is provided.

Therefore, the optical communication monitor module according to the present invention is an optical communication monitor module for demultiplexing / demultiplexing a desired wavelength for each channel of wavelength multiplexed communication. At least a diffraction grating for dispersing light is provided. The diameter of a light beam propagating in parallel light is made smaller than the spatial height in the thickness direction of the module, and the incident angle x of light on the diffraction grating is in the range of 50 ° to 75 °. (50 ° ≦ x ≦ 75 °), and the difference from the output angle y from the diffraction grating is set to 10 ° or more and less than 37 ° (10 ° ≦ xy <
37 °), the overall thickness of the module is reduced by making the incident light spot formed on the diffraction grating surface of the diffraction grating flat.

The optical communication monitor module according to the present invention uses a diffraction grating to separate incident light, but such a diffraction grating has polarization dependence. In the present invention, the problem of polarization dependence is solved by installing a mirror that compensates for the characteristics of the diffraction grating while keeping the module thin.

Therefore, the optical communication monitor module according to the present invention is an optical communication monitor module for confirming the presence or absence of communication for each channel of the wavelength multiplex communication, and for separating the multiplexed light. By controlling the polarization dependence in advance so that the diffraction grating and the light before or after diffraction by the diffraction grating are incident, the polarization grating has the opposite property to the polarization dependence of the diffraction grating. And a mirror having a dielectric multilayer film formed as described above, and the PDL of the diffraction grating is minimized, thereby realizing polarization independence.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. Note that the same reference numerals indicate the same elements throughout the drawings.

FIG. 1 is a plan view showing a schematic configuration of a thin optical communication module 100 according to a first embodiment of the present invention with an upper plate removed. The thin optical communication module 100 is configured by, for example, arranging various optical elements in a substantially rectangular casing 10 in a plan view. In the embodiment shown in FIG. 1, the housing 10 includes a bottom plate, two sets of side walls that are opposed to each other in parallel,
And an upper plate (FIG. 1 is omitted).

The multiplexed light is guided into the housing 10 by the optical fiber 2, and is shaped into a light beam having a predetermined diameter by the collimator lens 3. The beam diameter here is the case 1
0 is the largest diameter corresponding to the thickness direction of the housing 10 allowed. Next, the light beam is incident on the diffraction grating (reflection grating) 4 as shown in FIG. 2, and the incident angle x at this time is adjusted so as to be within a range of 50 ° to 75 °. As a result, the angle difference between the incident angle x and the exit angle y can be increased, and the light beam reflected (emitted) from the diffraction grating 4 is orthogonal to the groove of the diffraction grating in the spot on the diffraction grating surface. In this case, the width in the direction of movement can be increased. Therefore, the light beam shape becomes an elliptical shape or an elliptical shape extended along the surface direction of the housing 10.

The light of each channel spectrally reflected by the diffraction grating 4 in accordance with the wavelength dispersion characteristic is reflected by the mirror 5 and then condensed by the condenser lens 6 to narrow the light beam along the housing surface. Focused spots separated and focused for each channel are obtained on the light receiving element array 7. Each of these condensed spots is an oval or elliptical spot that is long in the thickness direction of the housing 10. Thereby, by using the light receiving element array formed by arranging a plurality of light receiving elements each long in the housing thickness direction along the housing surface direction, the separation degree is good while using the thin housing. It is possible to form a channel monitor.

FIGS. 3 and 4 clarify the difference in the shape of the light beam spot formed on the diffraction grating surface due to the difference in the incident angle with respect to the diffraction grating of the conventional example and the present invention. That is, FIG. 3A shows a state in which a light beam enters at an incident angle x ₁ and is reflected at an exit angle y _{1 according} to the conventional example, and FIG. 3B shows a diffraction grating surface in that case. A substantially circular spot shape formed thereon is shown. On the other hand, FIG. 4 shows a state in which a light beam enters at an incident angle x ₂ and is reflected at an exit angle y _{2 according} to the present invention.
(B) shows a spot shape of a substantially elliptical (or elliptical) shape formed on the diffraction grating surface in that case. In FIG. 3B, there is no sufficient number of grooves in the light beam, whereas in FIG. 4B, a sufficient number of grooves can be included in the light beam for a diffraction grating of the same thickness. Become.

Next, a specific example of designing an optical communication monitor module in consideration of the above points will be described. The wavelength used is in the 1.55 μm band. The center wavelength is 1552.52
It is determined as 4 nm. The channel pitch is 0.8 nm. The interval between the light receiving elements in the light receiving element array is 50μ.
m. The diffraction order of the diffraction grating is set to 25 order, and the grating constant is determined to be 24.7 μm so that the incident angle is 71.5 ° and the emission angle is 38.5 °. The outer dimensions of the housing are 85 ×
80 x 8.5 mm, and the total thickness of the upper and lower side plates of the housing is 4 mm. Therefore, the space available for the optical path has a thickness of 4.5 mm. Therefore, the light from the fiber is designed to be 4.05 mm by the collimator lens. The light beam emitted from the collimator lens impinges on the diffraction grating.
0 mm. This is emitted (reflected) at an emission angle of 38.5 °
Therefore, the short-axis length of the condensed spot condensed by the light-receiving element array 7 is about 36% of the long axis. This makes it possible to realize a thin module having a sufficient wavelength resolution to realize a channel monitor of about 20 channels.

In the above-described first embodiment, in order to form an elliptical or elliptical light beam spot on the diffraction grating surface, the diffraction grating is left as it is as a method of reducing the incident angle on the diffraction grating. In the state where the fiber and the collimator / lens were held in this state, the incident angle was adjusted by moving the direction of emission of the light beam from the fiber and the collimator / lens relative to the diffraction grating. In the present invention, on the contrary, while holding the fiber and the collimator lens as they are, the diffraction grating having the inclination angle (α) of the diffraction grating surface in which the grooves are cut as shown in FIG. By replacing with a diffraction grating having an inclination angle (β) as shown in FIG. 5B (α <β), as a result, it is possible to increase the incident angle as in the first embodiment. .

Further, in order to form an elliptical or elliptical light beam spot on the diffraction grating surface, as shown in FIG. 6, an optical fiber as a multiplexed light emitting source and a diffraction grating are used. An oblong spot can be formed on the surface of the diffraction grating by interposing a cylindrical lens therebetween.

Next, a thin optical communication monitor module according to a second embodiment of the present invention will be described. In general, the diffraction efficiency of a diffraction grating becomes maximum in a region called a blaze wavelength. However, in a diffraction grating having a shallow grating groove where the Franhofer approximation does not hold, the diffraction phenomenon by wave optics affects the diffraction efficiency, and a polarization dependence called anomaly occurs (FIG. 8). Generally, near the blaze wavelength and on the long wavelength side, the p-wave shows higher diffraction efficiency. The decibel notation of the ratio of the diffraction efficiency between the p-wave and the s-wave is PDL (Pol
arization Dependence Loss). When this value is close to 0 dB, the polarization dependence is reduced.

On the other hand, in a mirror coated with a dielectric or the like having a structure shown in the partially enlarged cross-sectional view of FIG. 9B, the wavelength is deviated from the normalized wavelength as shown in FIG. 9A. In the region, the reflectance of the p-wave decreases. By utilizing this phenomenon, the PDL characteristic of the diffraction grating can be canceled.

Referring to FIG. 7, a thin optical communication monitor module 200 according to the second embodiment will be described. As in the first embodiment described with reference to FIG. 1, the multiplexed light is guided into the module by an optical fiber 22, and is shaped into a beam having a predetermined diameter by a collimator lens 23. The beam diameter here is the largest diameter allowed for the housing 20. Next, a dielectric multilayer mirror 25 having a cross section as shown in FIG. 9B is set at a predetermined angle. The reflection film coated on the mirror 25 has a PDL characteristic that can sufficiently cancel the PDL characteristic of the diffraction grating 24 at a desired incident reflection angle. The multiplexed light reflected by the dielectric mirror 25 is
Spectroscopy. The split light is condensed on the light receiving element array 27 by the condensing lens 26, where it is subjected to O / E conversion to be processed. Incidentally, like the mirror 5 and the diffraction grating 5 shown in FIG. 1 in the first embodiment, the order of the dielectric mirror 25 and the diffraction grating 24 may be changed in the second embodiment.

Since the PDL of the diffraction grating is about 1 to 2 dB under the same numerical limitation conditions as in the specific example of the design of the optical communication monitor module described earlier in the first embodiment, for example, a dielectric multilayer In the case of film coating, the ratio of the design wavelength of the reflective film to the center wavelength is 1.10 to 1.15, or
The reflection film is designed to be in the range of 0.86 to 0.90. Further, the PDL characteristics of the mirror can be finely adjusted also by the incident reflection angle.

The position accuracy between the optical components constituting the monitor module of the present invention is required to be several μm or less. Therefore, when the housing thermally expands due to a change in the ambient temperature, the characteristics tend to be unstable. Therefore, it is necessary to use a material having a small linear expansion coefficient for the housing. However, it is desirable that the material is metal from the viewpoint of processability and product strength. An invar material was used for the housing of the monitor module of the present invention. The linear expansion coefficient of this material at room temperature is 0.13 × 10 ⁻⁶ , which is smaller than that of quartz glass or the like. The thickness of the housing made of Invar was 2.5 to 3 mm. Length of one side of the housing, 85m
Since the positional deviation of the light spot allowed on the light receiving element array with respect to m is about 5 μm, use of the invar material having the above-mentioned coefficient of linear expansion allows use of the module in a temperature range of, for example, −40 to 85 ° C. Is sufficiently stable even if is required. Each optical component was bonded on the bottom surface of the housing formed of the Invar material. Use low expansion resin adhesive for bonding,
Laser fusion can be used.

[0030]

According to the present invention, a diffraction grating or
By using the combination of the diffraction grating and the dielectric multilayer mirror, a DWDM thin optical communication monitor module with improved PDL characteristics can be provided at low cost without using an advanced spectroscopic element such as AWG.

[Brief description of the drawings]

FIG. 1 is a schematic plan view showing an internal configuration of a thin optical communication monitor module according to a first embodiment of the present invention by removing an upper plate.

FIG. 2 is a schematic perspective view of the diffraction grating showing a relationship between incident light and output light with respect to the diffraction grating.

FIG. 3A is a side view showing a state of incident light and outgoing light at a conventional angle with respect to the diffraction grating surface, and FIG. 3B is a substantially circular incident light spot formed on the diffraction grating surface; FIG.

FIG. 4 (a) is a side view showing the state of incoming and outgoing light at an angle according to the present invention with respect to the diffraction grating surface, and FIG.
(B) is a plan view showing an oblong incident light spot formed on the diffraction grating surface.

FIGS. 5A and 5B are side views showing a state in which the grating surface inclination angles α and β of the diffraction grating are changed with respect to incident light having a constant angle, and the inclination angles α <β It is.

FIG. 6 is a schematic configuration diagram showing a partially modified embodiment in which a cylindrical lens is disposed instead of the collimator lens in the first embodiment of the present invention.

FIG. 7 is a schematic plan view showing the internal configuration of the thin optical communication monitor module when the arrangement order of the mirror and the diffraction grating shown in FIG. 1 is reversed, with the upper plate removed.

FIG. 8 is a graph showing diffraction efficiency for various wavelengths of s-polarized light and p-polarized light.

FIG. 9A is a graph showing the reflection efficiency of a dielectric multilayer mirror that varies according to various wavelengths,
FIG. 9B is a partially enlarged cross-sectional view of a mirror provided with a dielectric multilayer coating.

FIG. 10 is a schematic configuration diagram of an optical communication monitor module using a number of wavelength filters.

FIG. 11 is a schematic configuration diagram of an optical communication monitor module using an AWG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Multiplexed light 2,22 Optical fiber 3,23 Collimator lens 4,24 Diffraction grating 5,25 Mirror 6,26 Condensing lens 7,27 Light receiving element array 10,20 Housing 100,200 Optical communication monitor module

Claims (9)

[Claims] 1. An optical communication monitor module, comprising: an optical fiber for transmitting multiplexed light; lens means for parallelizing multiplexed light from an open end of the optical fiber to form a light beam; Having a diffraction grating surface for receiving light emitted from the same at a predetermined incident angle, diffracting the light in accordance with the wavelength dispersion characteristic, and emitting the light; and a condenser lens for condensing the light emitted from the diffraction grating. A light-receiving element array for receiving a light-converged spot for each channel separated and condensed from the light-condensing lens; and an optical fiber opening end, a lens means, a diffraction grating, a light-condensing lens, and a light-receiving element array. An optical communication monitor module, comprising: a housing that includes: 2. An incident angle of the incident beam with respect to the diffraction grating surface is tilted such that a spot formed on the diffraction grating surface of the incident beam has an elliptical shape. An optical communication monitor module according to item 1. 3. An incident angle of the incident beam with respect to the diffraction grating surface is in a range of 50 ° to 75 °, and a difference from an emission angle from the diffraction grating is 10 ° or more and less than 37 °. The optical communication monitor module according to claim 1, wherein 4. The optical communication monitor module according to claim 1, wherein said lens means is a collimator lens. 5. The optical communication monitor module according to claim 1, wherein said lens means is a cylindrical lens. 6. The apparatus according to claim 1, wherein an inclination angle of the diffraction grating surface with respect to the incident beam is selected such that a spot formed on the diffraction grating surface of the incident beam has an oblong shape. An optical communication monitor module according to item 1. 7. A mirror having a dielectric multilayer reflective film made to have a property opposite to the polarization dependence of the diffraction grating by controlling the polarization dependence in advance, and the mirror is further provided. The optical communication monitor module according to claim 1, wherein the PDL of the diffraction grating is minimized by making reflected light from the light incident on the diffraction grating. 8. A mirror having a dielectric multilayer reflective film made to have a property opposite to the polarization dependence of the diffraction grating by controlling the polarization dependence in advance, and The optical communication monitor module according to claim 1, wherein the light emitted from the grating is incident on the mirror to minimize the PDL of the diffraction grating. 9. The optical communication monitor module according to claim 1, wherein the material of the housing is Invar.