1. Field of the Invention
The present invention relates to an optical switch that switches communication paths in an optical communication system, and a lens adjusting method and a lens adjusting device for the optical switch.
2. Description of the Related Art
In the third generation optical communication system (multi-ring network), because of its large communication amount, an enormous amount of work is required if replacement of fiber codes and setting of wavelength are performed manually when communication paths are switched. Therefore, an optical switch (for example, Japanese Patent Laid-Open Publication No. 2004-70053) or an optical wavelength selecting switch (for example, U.S. Pat. No. 6,549,699) that enables flexible change of paths and a remote operation is used.
FIG. 9 is a perspective view showing a configuration of a conventional optical switch. Signal light input from an input port 911 is collimated by a lens 921, and is collected on a micro electro mechanical systems (MEMS) mirror 940 by a lens 903.
The signal light collected on the MEMS mirror 940 is reflected by the MEMS mirror 940 and passes through the lens 930 and a lens 924 to be collected at an output port 914. The signal light is then output from the port 914. An optical switch controls an angle θy of the MEMS mirror 940, thereby selecting a desirable output port from which the signal light is to be output, from among output ports 912 to 914.
FIG. 10 is a perspective view showing a configuration of a conventional optical wavelength selecting switch. Wavelength division multiplexing (WDM) light that includes plural signal light beams having different wavelengths is input from an input port 1011. The WDM light is collimated by a lens 1021 and is reflected by a light dividing device 1030 such as a diffraction grating, being divided into light beams of respective wavelengths. The reflected light beams are collected on plural mirrors (not shown) in a MEMS mirror 1050, respectively.
The signal light beams collected on the respective mirrors in the MEMS mirror 1050 are reflected from the MEMS mirror 1050 to pass through the lens 1040, and reflected by the light dividing device 1030 again. The reflected light beams are collected at output ports 1012 to 1014, respectively. The optical wavelength selecting switch controls the angle of each of the mirrors in the MEMS mirror 1050 to output each of the divided light beams to a desirable output port from among the output ports 1012 to 1014. Thus, a signal light beam having arbitrary wavelength is output.
Generally, a device that converts light into collimated light with a combination of a fiber and a collimator lens is called collimator. In an optical switch and an optical wavelength selecting switch, a collimator array in which plural collimators are arranged in an array for input and output is used. Light emitted from each collimator passes a collective lens in a collimated state, and is collected on a MEMS mirror.
However, since the collimated light beams pass at different points on the collective lens in the conventional optical switch, positions at which the light beams are collected cannot be consistent due to aberration of the collective lens. Such a problem is caused because aberration of lenses varies depending on positions on lenses. Aberration usually becomes large as the position shifts away from the center.
FIG. 11 is a plan view showing a configuration of a conventional optical switch. An optical switch 1100 includes a collimator array 1110, a lens 1120, and a mirror 1130. The lens 1120 is a lens whose aberration is relatively small and curvature of field is corrected to about 0.2%, and in which f=100 millimeters (mm). An axis 1141 is a virtual axis that passes the center of the lens 1120. Collimated light beams emitted from collimators 1111 and 1112 in the collimator array 1110 pass through the lens 1120 in a state of being parallel to each other, and are collected on the mirror 1130.
The collimator 1112 emits a collimated light beam along the axis 1141. The collimator 1111 emits a collimated light beam along an axis 1142 that is positioned away from the axis 1141 for a distance L (L=10 mm in this example). The collimated light beam emitted from the collimator 1111 and the collimated light beam emitted from the collimator 1112 are collected on different points due to the aberration of the lens 1120.
In this example, suppose that the collimated light beam emitted from the collimator 1111 is collected on a position 1151, and the collimated light beam emitted from the collimator 1112 is collected on a position 1152. A distance between the positions 1151 and 1152 is approximately 200 micrometers (μm). In this case, if the mirror 1130 is positioned at the position 1151, a difference d in positions at which the respective collimated light beams enter the mirror 1130 is caused.
FIG. 12 is a graph showing relation between the distance L and the difference d. In this example, since the distance L=10 mm, there is the difference d of approximately 20 μm. Even if the mirror 1130 is positioned at a position 1153 in the middle between the positions 1151 and 1152, there is the difference d of approximately 10 μm.
FIG. 13 is a plan view showing a configuration of a conventional optical switch. If it is assumed that a collimated light beam emitted from the collimator 1111 is not influenced by the aberration of the lens 1120, this collimated light beam is refracted at a refraction angel α by the lens 1120, and passes along an optical path 1311. The light beam that has passed along the optical path 1311 travels in parallel to the core of a lens in the collimator 1113 and is collected at a port of the collimator 1113. Thus, the light beam is output from the collimator 1113.
However, the collimated light beam emitted from the collimator 1111 is influenced by the aberration of the lens 1120 in fact. Therefore, this collimated light beam is refracted by the lens 1120 at a refraction angle β (>α), and passes along an optical path 1312. The light beam that has passed along the optical path 1312 is deviated from the core of the lens of the collimator 1113 by an angle γ, and the position at which the collimated light beam is collected is not completely consistent with a position of the port of the collimator 1113. As a result, coupling loss increases.
Deviation of an optical path on an axis X that occurs in an optical switch in which plural collimators are linearly arranged has been explained. If an optical switch in which plural collimators are arranged two-dimensionally or the optical wavelength selecting switch described above is used, deviation of the optical path on a Y axis is also necessary to be considered in addition to the deviation of the optical path on the X axis.
FIG. 14 is a front view of a mirror in a conventional optical wavelength selecting switch. FIG. 14 illustrates a mirror 1050a viewed from a direction of an axis Z. The mirror 1050a is one of the mirrors in the MEMS mirror 1050 of the optical wavelength selecting switch described above. In this example, the port 1011 described above is used as an output port, and the ports 1012 to 1014 are used as input ports.
Numerals 1412 to 1414 denote positions at which light beams emitted from the ports 1012 to 1014 enter the mirror 1050a. As shown in FIG. 14, the positions at which the light beams emitted from the ports 1012 to 1014 enter the mirror 1050a vary in a direction of a Y axis due to the aberration of the lens 1040.
FIG. 15 is a graph showing relation between wavelength and coupling efficiency of signal light in the optical wavelength selecting switch. A horizontal axis indicates wavelength (frequency) of signal light, and a vertical axis indicates coupling efficiency. Numerals 1512 to 1514 indicate coupling efficiency characteristics when light beams that have been emitted from the ports 1012 to 1014, entered the MEMS mirror 1050, reflected by the MEMS mirror 1050, and enter the port 1011.
As shown in FIG. 15, an effective transmission band-width 1521 at the port 1011 when a light beam is selected from among the light beams emitted from the ports 1012 to 1014 to be output becomes narrower than an effective transmission band-width 1522 at the port 1011, for example, when only the light beam from the port 1012 is output.
As described, in the conventional optical switch using the MEMS mirror and the optical wavelength selecting switch, light beams emitted from respective ports cannot be collected at one point on the MEMS mirror due to the aberration of a lens collecting the light beams on the MEMS lens. Therefore, coupling loss at an output port increases. Accordingly, performance of module is degraded.