KR20020036015A - Fiber Bragg grating tuned fiber laser using semiconductor optical amplifier as a gain medium and the multiplexed sensor using the laser - Google Patents

Abstract

PURPOSE: An optical fiber lattice laser sensor is provided to be easy to multiplex and have higher a signal-to-noise ratio than that of manual optical fiber lattice laser sensor. CONSTITUTION: An optical fiber lattice sensing part(10) has a lattice formed along an optical fiber with which any other optical fiber lattice(11) is connected. The optical fiber lattice(11) is installed to be able to receive a strain from an exterior. An optical power divider(40) and a polarizing controller(60) are used for uniformly controlling a laser power. The laser is outputted in one direction by a unidirectional photocoupler(50). A light deflected by the optical fiber lattice sensing part(10) is optical amplified by a semiconductor optical amplifier(20) and the amplified light is provided to the optical fiber lattice(11), such that a laser suitable for a deflection wavelength of the optical fiber lattice(11) is generated.

Description

Fiber Bragg grating tuned fiber laser using semiconductor optical amplifier as a gain medium and the multiplexed sensor using the laser}

The present invention relates to a multiplexed optical fiber grating laser sensor using a semiconductor optical amplifier and a strain (or temperature) measuring device using the same. The variable multiplexed optical fiber grating laser sensor and a sensor system using the same.

Fiber gratings, which are widely used for optical communication and optical fiber sensors, are generally made by the change of refractive index in the optical fiber core generated by irradiating a strong ultraviolet laser to the optical fiber.

At this time, according to the characteristics of the refractive index modulation period of the optical fiber generated, it is classified into a short period optical fiber grating, a long period optical fiber grating, etc., and has been studied and used as a reflection and transmission filter according to the wavelength according to each characteristic.

1 shows a process for manufacturing a short period optical fiber grating using an ultraviolet laser. After placing a phase mask 2 on the optical fiber 1, ultraviolet rays are irradiated thereon, and thus an interference fringe, i.e., a core, of the optical fiber 1 is formed by a diffraction beam generated in the phase mask 2. A grid is formed.

The short period optical fiber grating manufactured as described above reflects light propagating through the core of the optical fiber. In general, the reflection band of the short period grating is 0.5 nm or less.

The optical fiber grating manufactured as described above may be applied to a sensor because the reflecting wavelength is changed by reflecting a specific wavelength and stretching or contracting by an external strain or the like. However, since the optical fiber grating is a passive element, the intensity of the reflected light is determined by the intensity of the incident light. Therefore, in order to use it as a distance measuring sensor, a strong power broadband wavelength light source must be used and to remove the influence of noise in the measured signal. An additional noise canceling device is needed.

One way to eliminate this problem is a fiber optic grating laser sensor that uses an optical fiber grating as a wavelength reflecting mirror. In this case, the wavelength of the oscillating laser is determined by the reflection wavelength of the optical fiber grating. For example, when strain is applied to an optical fiber grating, the reflection wavelength of the grating changes, which changes the wavelength of the oscillating laser.

There has been such research before. However, the conventional fiber grating laser sensor uses an erbium-doped fiber optical amplifier as a gain material for the laser, which makes it impossible to simultaneously produce multiple wavelength lasers, which makes it difficult to multiplex the sensor. This is because laser oscillation is possible only in one wavelength due to the homogeneous broadening characteristic. In order to solve this problem, a method of repeatedly scanning a laser wavelength oscillating using a tunable transmission filter has been proposed. In this case, however, there is a problem that the frequency of the measurable dynamic strain of the sensor is limited by the scanning speed of the filter. In addition, there is a method of making a multi-wavelength laser by cooling the gain material with liquid nitrogen by using the non-homogeneous property of the erbium material in the ultra low temperature (77K), but it is not practical to apply it.

Therefore, the technical problem of the present invention is to provide a large signal-to-noise ratio compared to the passive fiber grating sensor.

Another object of the present invention is to provide an optical fiber grating laser sensor that is easy to be multiplexed.

In addition, another technical problem to be achieved by the present invention is to provide a strain and temperature measuring device using such a fiber grating laser sensor.

1 is a cross-sectional view showing a fiber grating manufacturing process using an ultraviolet laser.

2 is a diagram showing the configuration of an optical fiber grating laser sensor according to an embodiment of the present invention.

3 is a measurement of the multi-wavelength laser output from the device shown in FIG.

FIG. 4 filters out some of the laser wavelength components of FIG. 3 and measures the intensity.

FIG. 5 shows the state of change of the laser output wavelength when a static strain is applied to the sensor part of the device shown in FIG.

FIG. 6 shows the rate of change of the laser output wavelength with respect to the static strain applied to the sensor portion of the device shown in FIG.

FIG. 7 shows the change in the wavelength of the laser output when dynamic strain is applied to the sensor section of the device shown in FIG.

FIG. 8 filters the wavelength components of the sensor unit to which the dynamic strain is not applied in the experiment of FIG. 7 and measures the intensity.

A multiplexed optical fiber grating laser sensor using a semiconductor optical amplifier as a gain material according to a feature of the present invention for achieving the above technical problem,

It includes a fiber laser which uses a semiconductor optical amplifier as a gain material and uses an optical fiber having a periodic grating as a reflection mirror.

The fiber laser may be any type of laser structure utilizing the wavelength selection characteristics of the fiber grating.

The optical fiber grating laser may include a directional coupler that sends broadband wavelength light at the gain material to the optical fiber grating and transmits the light reflected from the optical fiber grating back to the laser system, and transmits a portion of the laser output to the optical detector. The ratio adjustable optical power splitter and may include a polarization controller that can adjust the oscillation characteristics of the laser.

DETAILED DESCRIPTION Hereinafter, exemplary embodiments of the present invention may be easily implemented by those skilled in the art with reference to the accompanying drawings.

2 shows a structure of an optical fiber grating laser sensor using a semiconductor optical amplifier as a gain material according to an embodiment of the present invention.

As shown in FIG. 2, an optical fiber grating laser sensor according to an embodiment of the present invention includes an optical fiber grating sensor unit 10, an optical fiber grating 11, a semiconductor optical amplifier 20, and a directional coupler 30. And a ratio-adjustable optical power divider 40, a unidirectional optical coupler 50, a polarization controller 60, and a photo detector 70.

In the optical fiber grating sensor unit 10, gratings are formed in the longitudinal direction of the optical fiber, and optical fiber gratings 11 having different periods are connected in a line.

Here, the optical fiber grating 11 is formed of a grating having a constant period, but is not limited thereto. In addition, the optical fiber grating 11 may have any type of grating structure having wavelength selectivity and should have a selectivity for a wavelength different from that of the other optical fiber gratings 11 connected together.

In this embodiment of the invention, the eight optical fiber gratings 11 are connected in series and the cavity of the laser is sigma-shaped. At this time, it is possible to uniformly match the output power of the oscillating laser using the adjustable optical power distributor 40 and the polarization controller 60. In addition, the unidirectional light coupler 50 allows the output of the laser to proceed in only one direction.

The light of various wavelengths reflected from the optical fiber grating sensor unit 10 passes through the semiconductor optical amplifier and increases in intensity to be transmitted back to the optical fiber grating. As this process occurs repeatedly, a laser that matches the reflected wavelength of the optical fiber grating is oscillated. At this time, it may be a laser mode in which light of all wavelengths reflected by the heterogeneous property of the semiconductor optical amplifier is oscillated.

The optical fiber grating 11 is provided so that an external strain can be applied. Therefore, when a strain is applied from the outside, the optical fiber grating 11 is stretched or bent, and the reflection wavelength of the optical fiber grating 11 is shifted toward the longer wavelength.

As a result, the oscillation wavelength of the laser, which is determined by the reflection wavelength of the optical fiber grating, also moves together.

Therefore, the multiplexed optical fiber grating laser sensor according to the embodiment of the present invention plays a role of changing the variation of the optical fiber grating reflection wavelength due to strain or the like to the change of the laser wavelength of high power. This is because the semiconductor optical amplifier enables multi-wavelength laser oscillation, and the wavelength of the laser oscillating is determined by the reflection wavelength of the optical fiber grating.

Figure 3 shows the wavelength characteristics of the optical fiber grating laser sensor configured in an embodiment of the invention. 3, the high signal-to-noise ratio characteristics of the multi-wavelength laser can be seen. The example showed a minimum signal-to-noise ratio of 45dB. This value is a value that can be changed by the performance of the semiconductor optical amplifier 20 and the reflectance of the optical fiber grating 10 and the loss of the overall system.

FIG. 4 illustrates the characteristics on the time axis by separating one of the output wavelengths of the optical fiber grating laser sensor constructed as an embodiment of the present invention. As shown in FIG. 4, the output is stable and the continuous wave laser is operated.

In the strain measuring device having such a structure, the broadband light emitted from the light source 20 passes through the directional coupler 30 to the optical fiber grating sensor unit 10, is reflected by the optical fiber grating sensor unit 10, and then directional again. It is input through the coupler 30 to the ratio adjustable optical power splitter 40. Here, some of the light is sent to the photodetector 70, and the rest of the light is input to the semiconductor optical amplifier 20 through the unidirectional light coupler 50 and the polarization controller 60. In the photodetector, the wavelength of the detected light is measured by using a wavelength analyzer.

Here, the optical fiber grating sensor unit 10 is composed of a short period optical fiber grating 11 having eight different reflection wavelengths, each grating length is 2.5 cm, reflectance is 99%, the reflection wavelength is the center Silver S1 = 1534.44 nm, S2 = 1543.68 nm, S3 = 1546.32 nm, S4 = 1549.38 nm, S5 = 1552.56 nm, S6 = 1554.06 nm, S7 = 1556.28 nm, S8 = 1558.92 nm and the reflection band was 0.3 nm or less. The numbers given to the optical fiber gratings 10 are arbitrarily given for convenience of reference. These optical fiber gratings 11 were arranged in series so that the center wavelength toward the long wavelength toward the end of the sensor portion. This arrangement takes into account the cladding mode loss that occurs in a short-period fiber grating.

5 shows a state of change of the laser wavelength when strain is applied to the optical fiber grating sensor unit 10 according to the embodiment of the present invention. Strain was applied to the S3 fiber grating 10 and the S6 fiber grating 10. As described above, when the strain is applied, the reflection wavelength of the optical fiber grating 10 is shifted, thereby shifting the wavelength of the oscillating laser. 6 is a graphical representation of strain applied to the wavelength of the laser thus moved. As shown in FIG. 6, the oscillation wavelength of the laser moves linearly with respect to the applied strain, which is the case only for the strained optical fiber grating 10. In addition, as the strain is applied, the reflection wavelengths of the optical fiber gratings may overlap or pass each other. In this case, since the oscillation of the laser is not affected, there is no structural strain measurement limit of the present invention.

7 shows measurement results when dynamic strain is applied. As shown in FIG. 5, the oscillation wavelength of the laser is changed by the applied strain, and it can be confirmed that the change occurs only for the laser wavelength to which the strain is applied.

FIG. 8 shows the intensity of different wavelengths of light that are not strained when dynamic strain is applied. In FIG. 8, the triangular wave represents a driving voltage of a piezoelectric transducer (PZT) that applies a dynamic strain to the optical fiber grating S3. The separated light has wavelengths corresponding to the optical fiber gratings S2 and S4. It can be seen that the laser wavelength oscillating in FIG. 8 is not affected by the strain applied to other adjacent optical fiber gratings 10.

Therefore, the optical fiber grating laser sensor according to the present invention is a multiplexed sensor capable of generating the number of laser modes oscillating by using the heterogeneous characteristics of the semiconductor optical amplifier as the number of the optical fiber gratings 10, providing a high signal-to-noise ratio, Static strain can be measured.

Although the invention has been described with reference to the most practical and preferred embodiments, the invention is not limited to the embodiments disclosed above, but also includes various modifications and equivalents within the scope of the following claims.

According to the embodiment of the present invention described above, the optical fiber grating laser sensor using the semiconductor optical amplifier as a gain material and the optical fiber grating as the wavelength reflecting mirror enables multiplexing and fabrication of a sensor providing a high signal-to-noise ratio. .

Therefore, the present invention can be used as a sensor that is applied to the environment affected by loss and noise, such as remote measurement, and it is economical because there is no additional work such as noise removal process compared to the conventional method.

Claims (3)

Multiplexed sensors including fiber lasers using semiconductor optical amplifiers as gain materials and optical fibers with periodic gratings as reflection mirrors It includes a fiber laser that uses a semiconductor optical amplifier as a gain material and uses a fiber with a periodic grating as a reflection mirror, and a multiplexed sensor using the same, and is applied when a factor that changes the reflection wavelength of the fiber grating from the outside is applied. A multiplexed fiber grating laser sensor whose wavelength of the oscillating laser changes according to the set value; And Optical detector for detecting the optical signal output from the optical fiber grating laser sensor for each wavelength Measuring device comprising a. The method of claim 2, The fiber grating laser sensor Directional couplers; Adjustable rate optical power divider; Unidirectional optical coupler; A measuring device comprising a polarization regulator.