CA2270102A1

CA2270102A1 - Wavelength monitoring system

Info

Publication number: CA2270102A1
Application number: CA 2270102
Authority: CA
Inventors: Takefumi Oguma
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1998-04-24
Filing date: 1999-04-23
Publication date: 1999-10-24
Also published as: JP3317896B2; JPH11304594A; FR2777993A1

Abstract

A laser light monitor capable of monitoring variations in laser wavelength with precision is disclosed. The monitor includes an optical analyzer and an optical splitter. The optical analyzer analyzes the laser light in a predetermined polarization direction to produce linearly polarized light.
The optical splitter splits the linearly polarized light into first linearly polarized light and second linearly polarized light in different polarization directions which are further different from the predetermined polarization direction of the analyzer. A ratio of the intensities of the first and the second linearly polarized light is used to monitor a change of the laser wavelength.

Description

WAVELENGTH MONITORING SYSTEM
BACKGROUND OF THE INVENTION
1. Field of the Invention The present invention relates to a wavelength monitoring system for monitoring the wavelength of a laser beam, and more particularly to an optical system for use in the monitoring system.

2. Description of the Related Art In recent years ( there have been widely used wavelength division multiplexing transmission by which light signals of a plurality of different wavelengths are multiplexed on a single optical fiber. The wavelength division multiplexing transmission needs a plurality of laser sources having different wavelengths. In order to secure a satisfactory transmission quality of light signals, it is necessary to control the oscillation wavelengths of these laser sources with extreme precision. A wavelength monitoring system is used to monitor variations in the wavelength of a laser beam to achieve precise wavelength control.
There has been disclosed an example of a conventional wavelength monitoring apparatusin Japanese PatentApplication Laid-Open Publication No. 10-009961. According to this wavelength monitoring apparatus, a laser beam is entered in a dielectric multilayer filter and a light component passing through this dielectric multilayer filter is received by a photodiode. Since the photo current flowing through the photodiode varies depending on the intensity of incident light , the oscillation wavelength of the laser source is monitored by monitoring the magnitude of current flowing through the photodiode.
However, accordingto the conventional laser oscillation wavelength monitoring apparatus as described above, the intensity of a laser light passing through the dielectric multilayer filter changes depending on the polarization status of incident beam, in other words , there exists a polarization dependency. Therefore, it is not possible to specify which one of a variation in the oscillation wavelength of the laser source and a variation in the polarization of laser beam causes a variation in the photocurrent. Thus, it has not been possible to achieve a precise wavelength monitoring.
Another conventional laser oscillation wavelength detector has been disclosed in Japanese Patent Application Laid-Open Publication No. 4-286925. This conventional detector is provided with a dielectric multilayer filter and a polarizing prism such as Wollaston prism which is used to separate P-polarized light and S-polarized light. Since the dielectric multilayer filter has a polarization-dependent characteristic, a wider wavelength detectable range can be .. _... ..,~~ _ . _ - .~... .. w.. ~.. _. .. .. _ F(a5-388 obtained by combining the respective linearly varying ranges of the P-polarization and S-polarization characteristics of the dielectric multilayer filter.
However, the conventional detector cannot also achieve sufficiently precise wavelength monitoring because a polarization change of incident light influences the respective intensities of the P-polarized light and the S-polarized light.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical system and a wavelength monitoring system capable of achieving the precise monitoring of variation directions and variation quantities of wavelength regardless of a polarization status of incident light.
According to the present invention, a laser light monitor includes an optical analyzer and an optical splitter. The optical analyzer analyzes the laser light in a predetermined polarization direction to produce linearly polarized light.
The optical splitter splits the linearly polarized light into first linearly polarized light and second linearly polarized light which are light components of the linearly polarized light in different polarization directions which are further different from the predetermined polarization direction of the analyzer. The first linearly polarized light and the second linearly polarized light are used to monitor the wavelength of the laser light . The different polarization directions are preferably orthogonal to each other and the predetermined polarization direction is preferably inclined 45 degrees with respect to the different polarization directions.
Since the analyzer produces the linearly polarized light of the predetermined polarization direction different from the two different polarization directions of the splitter, a change of polarization direction of the laser light causes the respective intensities of the first linearly polarized light and the second linearly polarized light to concurrently change, canceling out the polarization change of the laser light.
Therefore, a change in wavelength of the laser light can be accurately monitored regardless of variations in polarization of the laser light.
The monitor may further include an optical band-pass filter provided on an optical path, wherein the optical band-pass filter has a polarization-dependent filter characteristic. The optical band-pass filter may be placed between the analyzer and the splitter. Alternatively, the optical band-pass filter may be placed at an output side of the splitter, corresponding to each of the first and second linearly-polarized light beams.
As described above, according to the present invention, it is possible to achieve a high-precision monitoring of variations in laser wavelength regardless of a polarization status of the laser light.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing an arrangement of a laser wavelength monitoring apparatus according to a first embodiment of the present invention;
FIG. 2 is a schematic diagram showing a main optical system of the laser wavelength monitoring apparatus of FIG.
1;
FIG. 3 is a diagram showing transmission loss - wavelength characteristics of an optical band-pass filter employed in the first embodiment;
FIG. 4 is a diagram showing variations in photo current flowing through two photo detectors receiving two different polarized outputs of a polarizing prism, respectively; and FIG. 5 is a diagram showing an arrangement of a laser wavelength monitoring apparatus according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
As shown in FIG. 1, a single-mode optical fiber 11 guides a laser beam emitted from a laser source (not shown) and outputs the laser beam 100 to a lens 12 which is designed to covert the laser beam 100 into a parallel beam 101. The parallel beam 101 is output to an analyzer (hereinafter, called polarizer) 13 having a predetermined polarization direction. The polarizer 13 polarizes the parallel beam 101 to produce a beam of linearly polarized light 102. The linearly polarized light 102 is output to an optical band-pass filter 14 composed of a dielectric multilayer film, for example. The optical band-pass filter 14 passes only a light component of a predetermined wavelength band through and the output light 103 is entered into a polarization beam splitter (here, a polarizing prism) 15.
The polarizing prism 15 separates a beam of the linearly polarized light 103 into two linearly-polarized light beams 104 and 105 of orthogonal polarization directions, which are output to two photo detectors 21 and 22, respectively. As will be described later, the polarization direction of the polarizer 13 is set at an inclination between the two polarization directions provided in the polarizing prism 15.
The respective linearly polarized light beams 104 and 105 are converted to electric signals IpDl and IPD2 by the photo detectors 21 and 22. A monitor 23 calculates a ratio between IPpl and IPpz, which is used to monitor the wavelength of the incident laser beam regardless of the polarization status thereof as will be described later.
The lens 12 is designed to convert the laser beam 100 emitted from the optical fiber 11 into the parallel beam 101 having a beam diameter of 300 um, for example. In the present embodiment, a non-spherical lens is used as the lens 12 for restricting a reduction in the coupling efficiency.
As the polarizes 13, there may be used a dichroic glass polarizes, PolarcorTM, of Corning Inc.. Such a polarizes is preferably employed to absorb polarization components other than transmission polarization component to obtain a high extinction ratio. The transmission polarization direction of this polarizes 13 is preferably set to 45 degrees with respect to the X axis and Y axis.
The optical band-pass filter 14 is a narrow-band optical band-pass filter having polarization-dependent transmittance characteristics , which is composed of a dielectric multilayer film on a glass substrate having a thickness of 0.3 mm, for example. A half-band width in the non-polarization status is set at 0.4 nm and a transmission center wavelength is adjusted to a desired laser oscillation wavelength ( 1, 550 . 00 nm in this case ) . Here , the optical band-pass filter 14 is disposed with a slope of B - 16 degrees (desirably within around 10 to 30 degrees) with respect to the X axis in FIG. 1.

The polarizing prism 15 is a polarization beam splitter (PBS) which is disposed to be able to separate polarization components parallel to the X axis and the Y axis , respectively (see FIG. 2). For the polarizes 13 and the polarizing prism 15, there may be used a double-refractive material such as rutile. The photo detectors 21 and 22 are p-i-n photodiodes with effective aperture diameter of 500 um, for example.
Referring to FIG. 2, the transmission polarization direction of the polarizes 13 is sloped by 45 degrees with respect to the X axis and the Y axis, respectively. Therefore, a linearly-polarized light 102 which is polarized by 45 degrees with respect to the X axis and the Y axis is produced from the unpolarized light 101.
The linearly-polarized light 102 is incident to the optical band-pass filter 14 having the polarization-dependent transmittance characteristic, which produces different effective refractive indexes depending on a polarization direction of light component. Therefore, the optical band-pass filter 14 has different filtering characteristics for different linearly-polarized light components of the linearly-polarized light 103, which will be described in detail.
A beam of linearly-polarized light 103 transmitted through the optical band-pass filter 14 is separated by the polarizing prism 15 into a first linearly-polarized light 104 which is polarized in the X direction and a second linearly-polarized light 105 which is polarized in the Y
direction. As described before, the linearly-polarized light 102 is polarized in the direction inclined 45 degrees with respect to the X and Y directions. Therefore, the first and second linearly-polarized light beams 104 and 105 have the substantially same intensity of light regardless of the polarization status of the laser beam 101.
The first and second linearly-polarized light beams 104 and 105 are incident to the photo detectors 21 and 22, respectively. Thus, photo currents corresponding to the intensities of the first and second linearly-polarized light beams 104 and 105 are output from the photo detectors 21 and 22, respectively.
As described before, the optical band-pass filter 14 has the polarization-dependent transmittance characteristic, which produces different effective refractive indexes depending on a polarization direction of light component.
Assuming that the linearly-polarized light 102 is white light , transmission spectrums of the first and second linearly-polarized light beams 104 and 105 are different.
As shown in FIG. 3, more specifically, a first transmission spectrum 201 for the first linearly-polarized light 104 is shifted more to a relatively short wavelength side than a second transmission spectrum 202 for the second linearly-polarized light 105.
In this graph, taking as an example the case where the wavelength of the laser light 100 or a desired oscillation wavelength of the laser source is 1,550.00 nm, the filter transmission center wavelength of the first linearly-polarized light 104 is 1549.50 nm and that of the second linearly-polarized light 105 is 1550.50 nm. Therefore, the optical band-pass filter 14 has different filtering characteristics 201 and 202 for the first and second linearly-polarized light components of the linearly-polarized light 103.
As shown in FIG. 4, the respective intensities of the beams 104 and 105 incident to the photo detectors 21 and 22 change, or the photo currents IPpl and IPDZ change, depending on the wavelength of the laser light 100 in different way.
The intensities of these beams 104 and 105 also change depending on the intensity or the polarization of the laser light 100. Since the polarizer 13 produces the linearly-polarized light 102 polarized in the direction inclined 45 degrees with respect to the X and Y directions, however, the ratio between the intensities of these beams 104 and 105 is kept constant even when the intensity or the polarization of the laser light 100 changes.
Hereinafter, an operation of the optical system as described above will be described taking as an example the case where the wavelength of the laser light 100 or a desired oscillation wavelength of the laser source is 1,550.00 nm.
As shown in FIG. 3, when the wavelength of the laser beam 100 is 1,550.00 nm, the transmission losses of the first and second linearly-polarized light beams 104 and 105 in the optical band-pass filter 14 are the same of about 8 dB. In this case, the photo currents IPDl and IPDZ obtained by the photo detectors 21 and 22 are about 130 ~.tA, respectively, as shown in FIG. 4.
Next , when the wavelength of the laser light 100 shifts to 1,549.90 nm, the transmission loss of the first linearly-polarized light 104 in the optical band-pass filter 14 becomes smaller as shown by the curve 201 of FIG. 3 while the transmission loss of the second linearly-polarized light beam 105 becomes larger as shown by the curve 202 of FIG. 3.
In this case, the transmission loss of the first linearly-polarized light beam 104 is about 7 dB while that of the second linearly-polarized light beam 105 is about 10 dB. Therefore, the intensities of the beams 104 and 105 incident to the photo detectors 21 and 22 are different, and the photocurrents Ippl and IPpz obtained also change accordingly. As shown in FIG. 4, the photocurrent IPDl obtained by the photo detector 21 becomes 152 ,u A while the photocurrent IPD2 obtained by the photo detector 22 becomes 110 ,uA.
As explained above, the respective photocurrents IPD1 and IPDZ obtained by photo detectors 21 and 22 become different as the wavelength of the light 100 is shifted from the target wavelength of 1550.00 nm. When the light 100 shifts to the short wavelength side, the photocurrent IPD1 of the photo detector 21 increases and the photocurrent IPDZ of the photo detector 22 decreases . Contrarily, when the light 100 shifts to the long wavelength side, the opposite trends are observed.
Because of these behaviors, the ratio between the photo currents IPD1 and Ipp2 changes depending on the direction of a wavelength variation of the laser light 100 and the variation amount thereof. Therefore, it becomes possible to electrically monitor in high precision wavelength variations of the laser light 100 including the variation direction and the variation quantity thereof, by monitoring the photocurrents IPD1 and IPD2 and calculating the ratio thereof .
On the other hand, when the polarization status of the laser light 100 changes, the light intensity of the linearly-polarized light beam 102 produced by the polarizer 13 changes because the polarizer 13 is set at the predetermined polarization direction (here, inclined 45 degrees).
Therefore, the light intensities of the linearly-polarized light beams 104 and 105 after having been transmitted through the polarizing prism 15 also change in the same manner, and thereby the photocurrents IPD1 and IPD2 also change accordingly.
However, there is no change in the transmission loss due to the optical band-pass filter 14. Therefore, the intensity ratio of the linearly-polarized light beams 104 and 105 does not change, and the ratio of the photocurrents IPD1 and IPD2 does not change either. From the above fact, it becomes possible to compensate for a change of a polarization status by monitoring the ratio of the photo currents IPD1 and IPD2. Further, for the same reason, it also becomes possible to compensate for changes in the intensity of the laser light 100 by using this monitoring method.
Further, unpolarized light can be used to easily adjust the center wavelength of the optical band-pass filter 14 to an operating point (laser oscillation wavelength). This also has an effect that it is easy to manufacture a laser oscillation wavelength monitoring system.
Referring to FIG. 5, there is shown a second embodiment which is provided with optical band-pass filters 16 and 17 between the polarizing prism 15 and the photo detectors 21 and 22, respectively, instead of the optical band-pass filter 14 of FIG. 1.
In the second embodiment, the optical band-pass filters 16 and 17 have the same characteristic as the characteristic of the optical band-pass filter 14 as shown in FIG. 3. The transmission center wavelengths of the optical band-pass filters 16 and 17 are adjusted to l, 549. 50 nm and 1, 550. 50 nm, respectively. Further, in this case, the respective beam incident angles of the optical band-pass filters 16 and 17 are adjusted to B 1 = 10 degrees and 8 2 = 8 degrees, respectively.
The wavelength of the laser source is 1,550.00 nm.
In the above-described structure, the respective intensities of the beams incident to the photo detectors 21 and 22 change depending on a change of the wavelength, the intensity, or the polarization status of the laser light 100.

The ratio of the intensities of these beams, however, does not change in the case where only the polarization status changes .
Therefore, it becomes possible to obtain the same effect as obtained by the first embodiment.
It is needless to mention that the present invention can also be applied to other modes than those described in the above embodiments. For example, in the structure shown in FIG. 1, the optical band-pass filter 14 may be disposed between the lens 12 and the polarizes 13, instead of the layout as shown in FIG. 1. The polarized light components produced by the polarizes 13 and the separated light components produced by the polarizing prism 15 may also be changed freely within the range of limit caused by relations among them.

Claims

1. A laser light monitor comprising:
an optical analyzer for analyzing laser light in a predetermined polarization direction to produce linearly-polarized light; and an optical splitter for splitting the linearly-polarized light into first linearly-polarized light and second linearly-polarized light which are light components of the linearly-polarized light in different polarization directions which are further different from the predetermined polarization direction of the optical analyzer, wherein the first linearly-polarized light and the second linearly-polarized light are used to monitor the wavelength of the laser light.

2. The laser light monitor according to claim 1, further comprising:
an optical band-pass filter provided between the optical analyzer and the optical splitter, the optical band-pass filter having a polarization-dependent filter characteristic.

3. The laser light monitor according to claim 2, wherein the optical band-pass filter is inclined toward the optical path by a predetermined amount to provide a desired polarization-dependent filter characteristic.

4. The laser light monitor according to claim 1, further comprising:
an optical band-pass filter provided at an output side of the splitter, corresponding to each of the first linearly-polarized light and the second linearly-polarized light, the optical band-pass filter having a polarization-dependent filter characteristic.

5. The laser light monitor according to claim 4, wherein the optical band-pass filter is inclined toward the optical path by a predetermined amount to provide a desired polarization-dependent filter characteristic for each of the first linearly-polarized light and the second linearly-polarized light.

6. The laser light monitor according to claim 1, wherein the optical analyzer is a polarizes for absorbing polarized light components other than the predetermined polarization direction to produce the linearly-polarized light.

7. A method for monitoring laser light, comprising the steps of:

analyzing the laser light in a predetermined polarization direction to produce linearly-polarized light;
passing a light component of the linearly-polarized light according to a polarization-dependent band-pass filter characteristic;
splitting passed linearly-polarized light into first linearly-polarized light and second linearly-polarized light which are light components of the passed linearly-polarized light in different polarization directions which are further different from the predetermined polarization direction;
detecting the first linearly-polarized light and the second linearly-polarized light to produce a first detection signal and a second detection signal, respectively;
and monitoring the laser light based on the first and second detection signals.

8. The method according to claim 7, wherein the laser light is monitored by calculating a ratio of amplitudes of the first and second detection signals.

9. A method for monitoring laser light, comprising the steps of:
analyzing the laser light in a predetermined polarization direction to produce linearly-polarized light;

splitting the linearly-polarized light into first linearly-polarized light and second linearly-polarized light which are light components of the passed linearly-polarized light in different polarization directions which are further different from the predetermined polarization direction;
passing a light component of each of the first linearly-polarized light and the second linearly-polarized light according to a preset band-pass filter characteristic set to produce first passed linearly-polarized light and second passed linearly-polarized light;
detecting the first passed linearly-polarized light and the second passed linearly-polarized light to produce a first detection signal and a second detection signal, respectively; and monitoring the laser light based on the first and second detection signals.

10. The method according to claim 9, wherein the laser light is monitored by calculating a ratio of amplitudes of the first and second detection signals.

11. A monitor for monitoring laser light, comprising:
an optical system comprising an optical band-pass filter provided on an optical path, wherein the optical band-pass filter has a polarization-dependent filter characteristic, the optical system further comprising::

an analyzer for analyzing the laser light in a predetermined polarization direction to produce linearly-polarized light; and a splitter for splitting the linearly-polarized light into first linearly-polarized light and second linearly-polarized light which are light components of the linearly-polarized light in different polarization directions which are further different from the predetermined polarization direction of the analyzer;
a pair of detectors for detecting the first linearly-polarized light and the second linearly-polarized light to produce a first detection signal and a second detection signal, respectively; and a monitor for monitoring the laser light based on a ratio of the first and second detection signals.

12. The monitor according to claim 11, wherein the optical band-pass filter is provided between the analyzer and the splitter.

13. The monitor according to claim 12, wherein the optical band-pass filter is inclined toward the optical path by a predetermined amount to provide a desired polarization-dependent filter characteristic.

14. The monitor according to claim 11, wherein the optical band-pass filter is provided at an output side of the splitter, corresponding to each of the first linearly-polarized light and the second linearly-polarized light.

15. The monitor according to claim 14, wherein the optical band-pass filter is inclined toward the optical path by a predetermined amount to provide a desired polarization-dependent filter characteristic for each of the first linearly-polarized light and the second linearly-polarized light.

16. The monitor according to claim 11, wherein the analyzer is a polarizer for absorbing polarized light components other than the predetermined polarization direction to produce the linearly-polarized light.

17. The laser light monitor according to claim 1, wherein the different polarization directions of the optical splitter are orthogonal to each other and the predetermined polarization direction of the optical analyzer is inclined 45 degrees with respect to the different polarization directions of the optical splitter.

18. The method according to claim 7, wherein the different polarization directions are orthogonal to each other and the predetermined polarization direction is inclined 45 degrees with respect to the different polarization directions.

19. The method according to claim 9, wherein the different polarization directions are orthogonal to each other and the predetermined polarization direction is inclined 45 degrees with respect to the different polarization directions.

20. The monitor according to claim 11, wherein the different polarization directions of the splitter are orthogonal to each other and the predetermined polarization direction of the analyzer is inclined 45 degrees with respect to the different polarization directions of the splitter.

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