JP2004119652A - Wide band amplified spontaneous emission light source and polarized wave variance measurement device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wide band ASE (amplified spontaneous emission) light source easily permitting the measurement of polarized wave dependence and wavelength dependence characteristics of an optical component. <P>SOLUTION: The wide band ASE light source 10 is provided with an excitation light source 11 for generating excitation light Lp, a rear earth elements loading optical fiber 12 emitting ASE lights Lb, Lf by the incidence of the excitation light Lp, an optical multiplexer/demultiplexer 14 arranged between the rear earth elements loading optical fiber 12 and the excitation light source 11 to muliplex or demultiplex the excitation light Lp and the ASE lights Lb, Lf, and a light isolator 16 equipped on the output terminal 21 of the ASE lights Lb, Lf. In such a wide band ASE light source 10, an optical device 15 having a function to separate or absorb the ASE lights Lb, Lf into orthogonally intersecting linearly polarized wave elements is provided to output linearly polarized light. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a broadband ASE light source using a spontaneous emission light generated from a rare-earth-doped optical fiber as a light source, and is suitable for use as a light source for a wavelength division multiplexing optical communication system, optical measurement, and the like.
[0002]
[Prior art]
The rare-earth-doped optical fiber has a characteristic that a large gain can be obtained for a certain wavelength by injecting pumping light in a certain wavelength range. For this reason, by transmitting signal light in a wavelength band having a gain into the rare-earth-doped optical fiber, the light intensity of the signal light can be extremely increased, and is now widely used as an optical amplifier in the field of optical communication. Have been. When the pumping light is incident on the rare-earth-doped optical fiber, the rare-earth-doped optical fiber generates a signal light gain and also generates spontaneous emission light. The light output of the generated spontaneous emission light increases under the influence of the gain. The light generated in this manner is referred to as amplified spontaneous emission light (hereinafter, referred to as ASE light).
[0003]
The rare earth-doped optical fiber can output an ASE light output with its own large gain, and can be used as a broadband light source. In recent years, with the expansion of communication capacity, wavelength-division multiplexed optical communication systems that multiplex optical signals having different wavelengths using a wide wavelength band and transmit and receive the signals have been actively studied. A broadband light source using ASE light of an added optical fiber is used as an incoherent light source for WDM and a light source for testing optical components for WDM systems.
[0004]
FIG. 10 is a diagram showing a configuration of a conventional broadband ASE light source.
[0005]
Patent Document 1 discloses a conventional broadband ASE light source shown in FIG. 10, and the configuration thereof will be described below. An excitation light source 101 for generating excitation light of a predetermined wavelength, a rare earth-doped optical fiber 102 for generating ASE light by inputting the excitation light, and a reflector 103 for reflecting ASE light emitted from the rare earth-doped optical fiber; It comprises an optical multiplexer / demultiplexer 104 and an output terminal 105, and the components are optically coupled by optical fiber fusion or splice.
[0006]
Next, the operation of the broadband ASE light source 100 will be described. Excitation light Lp emitted from the excitation light source 101 passes through the optical multiplexer / demultiplexer 104 and enters the rare-earth-doped optical fiber 102. The rare earth-doped optical fiber generates ASE light Lb and Lf by the incident pump light Lp. The ASE light Lf is reflected by the reflector 103, re-enters the rare-earth-doped optical fiber 102, and passes through the optical multiplexer / demultiplexer 104 together with the ASE light Lb. The optical multiplexer / demultiplexer 104 has a function of multiplexing or demultiplexing the light having the wavelength of the pump light Lp and the light having the wavelengths of the ASE lights Lb and Lf. The ASE light Lb + Lf transmitted through the optical multiplexer / demultiplexer 104 is an output terminal. Emitted from 105.
[0007]
By adopting a double-pass configuration in which the reflector 103 reflects the ASE light Lf in this manner, larger light can be output. Further, since the ASE light Lb is a short wavelength band light of 1530 to 1570 nm and the ASE light Lf is a long wavelength band light of 1570 nm or more, an extremely broadband light source can be obtained by reflecting the ASE light Lf and outputting it together with Lb. Realize.
[0008]
Although not shown in Japanese Patent Publication No. 7-117669, a polarization-independent optical isolator is generally arranged in front of the output terminal 105. The role of the optical isolator is to remove the reflected light returning to the rare-earth-doped optical fiber 102, thereby suppressing the parasitic oscillation due to the reflected light and the decrease in the gain due to the multiple reflection of the ASE light.
[0009]
Such a broadband ASE light source is widely used in combination with an optical spectrum analyzer as a light source for measuring the wavelength characteristics of an optical component such as an optical fiber or an optical isolator, utilizing its broadband property and high output power.
[0010]
[Patent Document 1]
Tokuhei 7-117669 No.
[Problems to be solved by the invention]
The broadband ASE light source 100 shown in the above-described prior art emits non-polarized light due to the nature of spontaneous emission light, and thus has a problem that it is difficult to measure the polarization dependency of an optical component.
[0012]
[Means for Solving the Problems]
The present invention has been made to solve these problems, and includes an excitation light source that generates excitation light, a rare-earth-doped optical fiber that generates ASE light by inputting excitation light, the rare-earth-doped optical fiber, and the pump. A broadband ASE light source provided between the light source and having an optical multiplexer / demultiplexer for multiplexing and demultiplexing the excitation light and the ASE light, and an optical isolator at an output end of the ASE light; An optical device having a function of separating or absorbing linearly polarized light components is provided, and linearly polarized light is output.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, a broadband ASE light source according to the present invention will be described as an example.
[0014]
FIG. 1 is a configuration diagram showing an embodiment of a broadband ASE light source 10 of the present invention. The pumping light Lp emitted from the pumping light source 11 passes through the optical multiplexer / demultiplexer 14 and enters the rare-earth-doped optical fiber 12. The rare earth-doped optical fiber 12 generates ASE light Lb and Lf by the incident pump light Lp. The ASE light Lf is reflected by the reflector 13, re-enters the rare-earth-doped optical fiber 12, and passes through the optical multiplexer / demultiplexer 14 together with the ASE light Lb. The optical multiplexer / demultiplexer 14 has a function of multiplexing or demultiplexing the light having the wavelength of the pump light Lp and the light having the wavelength of the ASE light Lb + Lf, and the ASE light Lb + Lf transmitted through the optical multiplexer / demultiplexer 14 passes through the optical isolator 16. Demultiplexed in the direction. The optical isolator 16 is disposed in the forward direction with respect to the incident Lb + Lf light, has a function of removing unnecessary reflection to the rare-earth-doped optical fiber 12, suppresses parasitic oscillation accompanying optical gain, and increases the output of ASE light. Having. The ASE light Lb + lf incident on the optical isolator 16 passes through the optical isolator 16 and is incident on the polarizer 15 in a random polarization state. Here, the polarizer 15 has a function of transmitting only polarized light in a certain direction and absorbing or separating other polarized waves. Therefore, the light transmitted through the polarizer 15 becomes linearly polarized light and exits from the output terminal 21.
[0015]
The pumping light source 11 generates pumping light Lp having a wavelength in, for example, a 1.48 μm band or a 0.98 μm band. The rare-earth-doped optical fiber 12 is a general doped optical fiber in which erbium or the like, which is a rare-earth element, is doped into the core of the optical fiber. Rare-earth atoms in the rare-earth-doped optical fiber 12 are excited by the excitation light Lp. Generates ASE light. ASE light is a combination of light traveling in random directions, and its polarization direction is also random. Only the modes that can propagate in the core propagate through the rare-earth-doped optical fiber 12. Here, since the waveguide portion of the optical fiber 12 is circular, it is isotropic with respect to the polarization, and the polarization direction propagates at random.
[0016]
The propagating ASE light is light in the 1.55 μm band, and exists in both the backward and forward directions. The backward propagating light is indicated by Lb, and the forward propagating light is indicated by Lf. The ASE light Lf of the forward-propagating light is re-absorbed in the latter half of the rare-earth-doped optical fiber 12, thereby producing stimulated emission in the 1.58 μm band. The ASE light Lf is further reflected by the reflector 13 and propagates through the rare-earth-doped optical fiber 12 in the opposite direction. Due to this reflection, the probability that the ASE light is re-absorbed is increased, and light in the 1.58 μm band is amplified with high excitation efficiency. Further, the ASE light Lb of the backward propagation light has a small amount to be re-absorbed. Therefore, most of the ASE light Lb propagates toward the optical multiplexer / demultiplexer 14 as 1.55 μm band light.
[0017]
Here, the reflector 13 has a function of reflecting at least a part or all of the ASE light Lf, such as a reflection mirror made of a dielectric multilayer film, an optical fiber grating, a fiber loop mirror, or a Fresnel reflection from an end face of an optical fiber. It is possible to use
[0018]
As the optical isolator 16, a polarization-independent type in-line optical isolator is used, and a two-stage L-band optical isolator having a 1.58 μm band isolation of 50 dB or more is preferable. Generally, the optical isolator 16 is a composite optical module in which optical components such as a lens, a birefringent crystal, and a magneto-optical crystal are arranged in a space.
[0019]
FIG. 2 is a diagram illustrating a polarization state of the ASE light that enters and exits the polarizer 15.
[0020]
The polarizer 15 is an absorption polarizer, 17 represents incident light, 18 represents outgoing light, and arrows represent the polarization state of the light. An arrow 19 indicates the transmission polarization direction of the polarizer 15. The ASE incident light 17 in a randomly polarized state is incident on the polarizer 15, and only the component in the transmission polarization direction 19 is transmitted through the polarizer 15, and the other components are absorbed and cut off. Therefore, the ASE emission light 18 emits a single linearly polarized light indicated by an arrow.
[0021]
Here, as the polarizer 15, an absorption polarizer in which metal particles such as silver or copper are dispersed in a dielectric material such as glass and stretched in one direction is used. In this case, the polarization component in the extending direction is absorbed and cut off.
[0022]
FIG. 3 is a configuration diagram showing a second embodiment of the present invention.
[0023]
This is a two-port broadband ASE light source that uses a separation type polarizer 25 as a polarizer and outputs separated orthogonal polarized light components from two output terminals 21 respectively. The mechanism and configuration of the ASE light generation up to the optical isolator is the same as in the first embodiment. The randomly polarized ASE light transmitted through the optical isolator 16 is incident on the split polarizer 25. The ASE light of random polarization is separated into orthogonal polarization components by the separation polarizer 25, and each is output from the output terminal 21.
[0024]
In this way, by adopting the two-port configuration, about one-half of the other orthogonal polarization components that were wasted in the one-port output can be effectively output, and the function of two units can be provided.
[0025]
Here, the separation type polarizer 25 is a birefringent crystal such as rutile or calcite, a polarizing beam splitter obtained by depositing a dielectric multilayer film on glass, or a coupler type polarizing device obtained by melting and stretching two polarization maintaining fibers. The child is used.
[0026]
Here, the state of polarization separation when a birefringent crystal is used as the separation polarizer 25 will be described. The birefringent crystal is a parallel plate, which is cut out so that the normal to the plane of incidence and the optical axis of the birefringent crystal have a certain angle. A linear polarization perpendicular to the plane including the optical propagation direction of the light and the optical axis of the birefringent crystal (referred to as a main section) is called an ordinary ray, and a linear polarization parallel to the direction is an extraordinary ray. The optic axis of a birefringent crystal is directional in the orientation of the atoms that make up the crystal, so the direction of light propagation differs between ordinary light and extraordinary light propagating in the birefringent crystal, and polarization is separated. . By coupling the separated polarized lights to two different fibers, the polarization separation shown in the present invention is realized. Further, when only one polarized light is coupled to one fiber, the same effect as the absorption polarizer shown in the first embodiment of the present invention can be obtained.
[0027]
In addition, in the case of a polarizing beam splitter in which a dielectric multilayer film is deposited on glass, polarized light is separated using the difference in Brewster conditions depending on the polarization direction, and in the case of a coupler type polarizer, two beams are used. Polarized light is separated by utilizing the difference in the propagation constant depending on the polarization direction at the fusion part of the fiber.
[0028]
FIG. 4 is a configuration diagram showing a third embodiment of the present invention.
[0029]
The polarizer 15 is rotatable about an optical axis indicated by a dotted line in the figure, and can extract a desired linearly polarized light depending on its rotation state. The desired linearly polarized light that has passed through the polarizer 15 passes through the light branching unit 22 that branches and emits the light at a predetermined ratio, is disposed at one end of the light branching unit 22, and is emitted from the light branching unit 22. Light measuring means 23 for measuring linearly polarized ASE light.
[0030]
For example, the optical branching means 22 is composed of a coupler-type branching unit in which two optical fibers are melted and drawn at a branching ratio of 5% or a module-type branching unit made of a dielectric multilayer film. As the light measuring means 23 for measuring the ASE light, a photodiode for converting the amount of received light into a current value is used. Further, the output of the excitation light source 11 is automatically controlled so that the light receiving power of the measuring means 23 becomes constant.
[0031]
As described above, by adding the rotation movable mechanism of the polarizer 15 and the auto power control function of the excitation light source by the light measuring means, the broadband ASE light source of the present invention can always output a desired linearly polarized light with a constant output. Obtainable.
[0032]
FIG. 5 shows a first measurement example using the broadband ASE light source of the present invention.
[0033]
By connecting the linearly polarized light emitted from the broadband ASE light source 10 to the polarization scrambler 31, various polarization states are generated at random with respect to time, and are incident on the DUT 33. The light emitted from the device under test is received by the power meter 32, and the polarization-dependent loss of the device under test 33 can be measured by determining the difference between the maximum light reception amount and the minimum light reception amount within a certain specific time. .
[0034]
Here, the polarization scrambler 31 rotates the half-wave plate and the quarter-wave plate, which are independently held, or twists and rotates the optical fiber as a plurality of loop bundles, thereby making the polarization scrambler 31 movable. This is a device that continuously generates a large number of polarized lights in seconds.
[0035]
When the degree of linear polarization of the light incident on the polarization scrambler 31 is low, the polarization scrambler 31 cannot generate light in various polarization states, and the polarization dependence of the DUT 33 with sufficient accuracy. Loss cannot be measured. Therefore, by using the linearly polarized light broadband ASE light source 10 of the present invention as a light source, extremely accurate measurement becomes possible, and the measurement time is completed in a few seconds.
[0036]
FIG. 6 shows a second measurement example using the broadband ASE light source of the present invention.
[0037]
Generally, a broadband ASE light source is used as a light source for measuring the wavelength characteristic of loss of an object to be measured, in combination with an optical spectrum analyzer that measures the wavelength and power of light. Here, a system that simply measures the polarization dependence in addition to the wavelength characteristics of the loss of the device under test using the broadband ASE light source 30 and the optical spectrum analyzer 34 described in the third embodiment will be described.
[0038]
The broadband ASE light source 30 can output a desired linearly polarized light, and its output power is constant. The broadband linearly polarized light output from the broadband ASE light source passes through the DUT 33, is received by the optical spectrum analyzer 34, and measures the loss wavelength characteristic of the DUT. Further, the output polarization state of the broadband ASE light source 30 is changed, and the optical wavelength analyzer similarly measures the loss wavelength characteristic. This can be measured at several points within a range of 180 degrees of the transmission polarization direction of the polarizer, and the minimum value can be subtracted from the maximum value of the measurement to measure the wavelength characteristic of the loss polarization of the DUT 33. .
[0039]
According to this measurement method, the wavelength characteristic and the polarization dependence of the device under test can be measured simultaneously, and the measurement time can be greatly reduced.
[0040]
FIG. 7 shows a third measurement example using the broadband ASE light source of the present invention.
[0041]
A polarization mode dispersion (PMD) measurement by a combination of the broadband ASE light source 30, the optical spectrum analyzer 34, and the polarization analyzer 35 disposed therebetween, which is called a fixed analyzer method. is there. The linearly polarized light output from the broadband ASE light source 30 is delayed due to the polarization component of the light due to the PMD of the device under test 33, and the output light from the device under test 33 is an elliptical polarization having a wavelength cycle. Become. This wavelength period is converted into optical power by a polarization analyzer, and measured by an optical spectrum analyzer, so that the PMD of the device under test can be calculated.
[0042]
In the fixed analyzer method, the PMD value can be calculated by equation (1) using this wavelength cycle as the phase difference 2π and the peak wavelength λ2 adjacent to the first peak wavelength λ1. C is 2.998 × 10 ⁸ m / s in light speed.
[0043]
PMD = (λ1 × λ2) / (C × (λ2-λ1)) (1)
Since the broadband ASE light source 30 of the present invention has a wide band, a large and constant optical power, and can easily output various linearly polarized waves, the PMD measurement system can be further simplified. In addition, by using the broadband ASE light source of the present invention as a measurement light source, there is an advantage that the wavelength range of measurement is widened and the measurement accuracy of PMD is improved.
[0044]
Here, the device under test for measuring the polarization dependent loss and PMD is, for example, a passive device such as an optical fiber coupler or an optical isolator, or an optical fiber such as a polarization maintaining fiber.
[0045]
In this embodiment, the configuration of the high-efficiency and wide-band broadband ASE light source using the reflection mirror has been described. However, the present invention is not limited to this. The same effect can be obtained for all the ASE light sources.
[0046]
The polarizer 15 may be an in-line type configuration in which a lens and an input / output optical fiber port are connected to an absorption type polarizer or a birefringent crystal, a configuration in which the polarizer 15 is directly built in the output terminal 21, or a polarization type in the optical isolator 16. There are various mounting methods such as a configuration incorporating the child 15.
[0047]
【Example】
As an example of the broadband ASE light source of the present invention, a prototype of the broadband ASE light source shown in FIG. 4 was manufactured. Each component and configuration will be described below.
[0048]
The excitation light source 11 had an optical output of about 120 mW at a wavelength of 1.48 μm. The optical multiplexer / demultiplexer 14 is a fiber fusion drawing type, and has a function of multiplexing and demultiplexing light in the 1.48 μm band and 1.58 μm band. The rare-earth-doped optical fiber 12 is of a sufficiently long length for re-absorption of light in the 1.55 μm band, and has a length of about 80 m in the prototype. As the reflector 13, a product composed of a dielectric multilayer film and an optical fiber and having a reflectance of 90% or more was used. Between the output terminal 21 and the optical multiplexer / demultiplexer 14, a two-stage polarization-independent optical isolator, a polarizer 15 with a rotation adjusting mechanism, a monitor coupler 22 for branching 5% of linearly polarized ASE light, A photodiode 23 that receives 5% of branched light is arranged. The output of the excitation light source 11 is adjusted so that the amount of light received by the photodiode 23 is constant. The output terminal 21 is made of an FC / SPC connector and has a return loss of 45 dB or more.
[0049]
FIG. 8 is a spectrum waveform of a prototype broadband ASE light source of the present invention.
[0050]
The total output of the ASE light was +12 dBm, and the output was reduced by about 3 dB from the conventional ASE light source, but the power was sufficient for the measurement. Further, the polarizer 15 was rotated every 5 degrees within a range of 180 degrees, and the total output of 36 points and the spectrum waveform were measured. As a result, the fluctuation value of the total output is 0.01 dB or less, the maximum fluctuation value of the spectrum waveform is 0.05 dB or less in a range of 1530 to 1610 nm, the output is very stable in a wide band, and the polarization rotation is possible in a wide band. An ASE light source has been realized.
[0051]
Next, the PMD shown in FIG. 7 was measured using the prototype broadband ASE light source 30.
[0052]
The device under test 33 was a single-mode fiber having a length of 20 km. First, reference data is measured without the polarization analyzer 35. Next, the polarization analyzer 35 is arranged immediately before the optical spectrum analyzer 34 to record measurement data. The value obtained by subtracting the reference data from the measurement data is taken in as PMD data. The PMD data is data having a certain wavelength period, and the polarizer 15 and the polarization analyzer 35 incorporated in the broadband ASE light source 30 are rotationally adjusted so that the amplitude becomes maximum.
[0053]
FIG. 9 shows the measured PMD data.
[0054]
From the data, the peak wavelength λ1 = 1540 nm, the peak wavelength λ2 = 1580 nm, and the period λ2−λ1 = 40 nm were read and substituted into the equation (1) to calculate the PMD. As a result, the PMD of the 20-km single mode fiber as the measured object was It was measured as 0.203 ps.
[0055]
As described above, the PMD can be easily measured by using the broadband ASE light source of the present invention.
[0056]
【The invention's effect】
As described above, according to the invention, an excitation light source that generates excitation light, a rare-earth-doped optical fiber that generates ASE light by incidence of excitation light, and the rare-earth-doped optical fiber and the excitation light source are arranged between the rare-earth-doped optical fiber and the excitation light source. In a broadband ASE light source including an optical multiplexer / demultiplexer for multiplexing and demultiplexing pump light and ASE light and an optical isolator at an output end of the ASE light, the ASE light is separated or absorbed into orthogonal linearly polarized components. By providing the optical device having the function of performing the above, desired linearly polarized light can be easily obtained in a wide band. Further, there is an effect that characteristics such as insertion loss polarization dependence, wavelength dependence, and PMD of an object to be measured can be easily measured using the broadband ASE light source of the present invention.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing an embodiment of a broadband ASE light source of the present invention.
FIG. 2 is a diagram illustrating a polarization state of ASE light entering and exiting a polarizer 15. FIG. 3 is a configuration diagram illustrating a second embodiment of the present invention.
FIG. 4 is a configuration diagram showing a third embodiment of the present invention.
FIG. 5 is a first measurement example using the broadband ASE light source of the present invention.
FIG. 6 is a second measurement example using the broadband ASE light source of the present invention.
FIG. 7 is a third measurement example using the broadband ASE light source of the present invention.
FIG. 8 is a spectrum waveform of a prototype broadband ASE light source of the present invention.
FIG. 9 shows PMD data measured using the polarization dispersion measuring apparatus of the present invention.
FIG. 10 is a diagram showing a configuration of a conventional broadband ASE light source.
[Explanation of symbols]
10, 20, 30, 100: Broadband ASE light source 11, 101: Pump light source 12, 102: Rare-earth-doped optical fiber 13, 103: Reflector 14, 104: Optical multiplexer / demultiplexer 15, Polarizer 16, Optical isolator 17 : Incident light 18: output light 19: transmission polarization direction 21: output terminal 22: optical branching means 23: optical measuring means 25: separation type polarizer 31: polarization scrambler 32: power meter 33: DUT 34: Optical spectrum analyzer 35: Polarization analyzer

Claims (4)

An excitation light source that generates excitation light, a rare-earth-doped optical fiber that generates ASE light when the excitation light is incident, and a light-distribution element that is disposed between the rare-earth-doped optical fiber and the excitation light source to combine the excitation light and the ASE light. In a broadband ASE light source including a wave, an optical multiplexer / demultiplexer for demultiplexing, and an optical isolator at an output end of the ASE light, an optical device having a function of separating or absorbing the ASE light into a linearly polarized component is provided. A broadband ASE light source that outputs linearly polarized light. 9. The optical device according to claim 8, wherein the optical device includes a separation polarizer having a function of separating light into two polarization components orthogonal to each other, and outputs the separated polarization lights from two different output terminals. 2. The broadband ASE light source according to 1. A light splitting means for splitting the ASE light at a predetermined ratio and emitting the light; and a measuring means arranged at one end of the light splitting means for measuring the ASE light emitted from the light splitting means. The broadband ASE light source according to claim 1. A broadband ASE light source, a polarization analyzer having a function of outputting only a specific polarization component, and a polarization dispersion measuring apparatus including an optical spectrum analyzer for measuring the wavelength and power of light, wherein the broadband ASE light source is An optical device having a function of separating or absorbing linearly polarized light components, and the optical device has a rotation mechanism about an optical axis, and the direction of linearly polarized light to be output is variable. Polarization dispersion measurement device.

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