JP2005347668A - Wavelength scanning type fiber laser optical source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength variable type fiber laser optical source which can vary a wavelength at a fast speed. <P>SOLUTION: A loop is formed by providing a gain medium with a gain in an oscillation wavelength to an optical fiber loop by an optical fiber. An optical circulator 13 is provided to the optical fiber loop. Light picked up by the optical circulator 13 is enlarged by a collimate lens 21, and a band path filter 24 is provided between it and a mirror 25. Selectivity is raised by selecting a wavelength which penetrates through the band path filter 24 twice. The incidence angle of the band path filter 24 is varied at a fast speed and a selection wavelength is varied. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a wavelength scanning fiber laser light source that generates single light and periodically scans its emission wavelength.

  Conventionally, a broadband light source is used as a light source of an analysis apparatus that irradiates a measurement object with light and analyzes the measurement object. In the spectroscopic analysis, a method is widely used in which broadband light is projected onto a measurement target, and the reflected light or transmitted light is spatially decomposed into wavelength components by a grating or the like, or Fourier-transformed into frequency components by an interferometer. It has been. As such a light source, for example, a white light source or an ASE light source using erbium-doped fiber (EDF) has been used. However, in such spectroscopic analysis, since the light output intensity density with respect to the wavelength is low, the level of light that can be used in spectroscopy is small. For this reason, even if Fourier transform analysis is performed, the detection light signal is buried in noise, which makes it difficult to analyze.

  There is also a method of using a wavelength-variable light source that changes a single spectrum of a strong level in a desired band as a light source of the analyzer. In this method, the wavelength of light having strong single light is changed to irradiate the measurement object, and light that passes through or reflects the measurement object is received by the light receiving element as it is. In this method, since the light output intensity density with respect to the wavelength of the light source is high, the level of detection light and the signal-to-noise ratio are sufficiently high, and sufficient measurement accuracy can be realized.

  Conventional wavelength tunable light sources include an external resonator laser and a fiber ring laser. The external resonator type laser uses a gain medium, for example, a semiconductor laser, forms an external resonator between one end face of the semiconductor laser and an external mirror, and a wavelength tunable filter such as a grating in the external resonator. By changing the oscillation wavelength, a wavelength-tunable light source is obtained. In the external resonator type laser light source, the external resonator type length is relatively small, for example, 50 mm, and the longitudinal mode interval is wide, for example, 30 GHz. Therefore, simply changing the wavelength of the tunable filter makes it unstable between the longitudinal modes. For example, discontinuous mode hops may occur between modes, or oscillation may occur in multiple modes. Therefore, in order to continuously change the wavelength in a single mode and stabilize the output, the external resonator length must be delicately controlled using a piezo element or the like, and complicated control is required. . In addition, there is a drawback that it is difficult to change the wavelength at high speed because the wavelength and the external resonator length are controlled in synchronization with the mechanical operation.

Non-Patent Document 1 also proposes a wavelength tunable light source using a ring laser using an erbium-doped fiber. In this method, an erbium-doped fiber (EDF) and a fiber amplifier that excites the erbium-doped fiber (EDF) are used as a gain medium, and a wavelength-tunable bandpass filter is provided between the optical fiber loops to change the wavelength of the bandpass filter. Thus, a tunable light source is obtained. In this case, since the resonator length of the optical fiber loop can be increased to, for example, 30 m, the longitudinal mode interval can be reduced. Therefore, the influence of the mode hop can be eliminated without changing the resonator length. Therefore, although it is not strictly single mode oscillation, it is possible to tune the wavelength continuously in a pseudo manner simply by changing the selected wavelength of the bandpass filter.
YAMASHITA ET AL., IEEE JOURAL ON SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL.7, NO.1 JANUARY / FEBRUARY 2001, PP41 ~ 43

  When a wavelength tunable light source using this ring laser is used as a light source of an analyzer, it is required as characteristics of the bandpass filter to change the wavelength at high speed and to narrow the width of the oscillation spectrum.

  However, it has been difficult to obtain a high Q value and a high Q value simultaneously with the conventional filter technology. For example, a wavelength tunable filter using the photoacoustic optical effect (AO) has a drawback that the suppression ratio other than the transmission wavelength is not sufficient, and stable oscillation cannot be performed. Further, when a Fabry-Perot etalon is formed using a piezo element as a bandpass filter, there is a problem that the wavelength variable speed is as low as several Hz or less and there is hysteresis. When a grating is used for the bandpass filter, there are disadvantages that the adjustment of the optical axis is difficult and expensive. Further, although an optical interference filter may be used as a bandpass filter, there is a drawback that the spectrum cannot be narrowed much because the Q of the filter is low only by passing the filter once.

  The present invention has been made in order to eliminate such drawbacks, and an object of the present invention is to provide a wavelength scanning type fiber laser light source capable of scanning a narrow band light source at high speed.

  In order to solve this problem, a wavelength scanning fiber laser light source of the present invention includes an optical fiber loop, a gain medium that is provided in the optical fiber loop and has a gain with respect to an oscillation wavelength, and first to third terminals. The first and second terminals are connected to an optical fiber loop, and an optical circulator for controlling the direction of light incident on each terminal is parallel to the light obtained from the third terminal of the optical circulator. A collimating lens for light, a mirror having a reflecting surface perpendicular to the optical axis formed by the collimating lens, and a band that is disposed between the collimating lens and the mirror and changes a transmission wavelength depending on an incident angle. A pass filter, an angle control unit that periodically changes transmitted light by changing an incident angle of light with respect to the band pass filter, and the optical filter. Formed in Ibarupu, it is characterized in that it comprises an optical coupler for taking out a part of the light passing through the optical fiber loop, a. The angle control unit may be configured by a galvanometer that changes the angle between the bandpass filter and the optical axis.

  Here, the gain medium may be an optical fiber amplifier constituting at least a part of the optical fiber loop. The optical fiber amplifier includes, for example, an erbium-doped fiber doped with erbium, a pumping laser that drives the erbium-doped fiber, and a coupler that inputs an output from the pumping laser to the erbium-doped fiber. You may do it. It is also possible to adjust the gain wavelength band by selecting a material to be doped.

  Here, the gain medium may be a semiconductor optical amplifier that amplifies light.

  Here, a polarization controller that is provided in the optical fiber loop and controls the polarization direction of light in the optical fiber loop, and a polarizer that is provided between the collimator lens and the mirror and has a polarization direction constant. You may make it have further.

  Here, the optical fiber loop may include a polarization-preserving optical fiber.

  According to the present invention having such a feature, a part of the light of the optical fiber loop is taken out by the optical circulator and emitted as parallel light to the space by the collimating lens, and the bandpass filter is provided between the collimating lens and the mirror. It is arranged. In this way, the light that has been transmitted twice through the bandpass filter is oscillated by the optical fiber loop, and laser light with high wavelength selectivity can be oscillated. The angle of the band pass filter is changed by the angle control unit. By making the angle control speed sufficiently high, it is possible to obtain an effect that the wavelength can be varied at a high speed. The light collimated by the collimating lens passes through the bandpass filter, and the light beam reflected by the mirror follows the same optical path regardless of the angle of the bandpass filter and enters the optical fiber loop from the collimating lens. There is no change in the output level with the change of. In addition, by configuring the optical fiber loop to include a polarization-maintaining optical fiber, an effect that the configuration can be simplified can be obtained.

FIG. 1 is a diagram showing a configuration of a wavelength scanning fiber laser light source according to an embodiment of the present invention. The wavelength scanning fiber laser light source 10 of the present embodiment includes an optical fiber 11 to form a loop. A gain medium 12, an optical circulator 13, an optical coupler 14, and a polarization controller 15 are provided in a part of this loop. The gain medium 12 includes an erbium-doped fiber 16 doped with erbium ions (Er ³⁺ ) provided in a part of an optical fiber loop, and a fiber-pumped semiconductor laser 17 that makes pump light incident on the erbium-doped fiber 16. And a WDM coupler 18. This optical fiber loop is assumed to have a length of 30 to 50 m, for example. The pumping semiconductor laser 17 has a wavelength of 1480 nm or 980 nm, for example, and amplifies the light transmitted through the erbium-doped fiber 16. The optical circulator 13 regulates the direction of light passing through the optical fiber 11 in the direction of the arrow as shown. That is, the input terminals 13a and 13b of the optical circulator 13 are connected to the optical fiber loop, and the light incident from the input terminal 13a is emitted from the terminal 13c of the optical circulator. The light incident from the optical circulator 13c is emitted from the terminal 13b. Light incident from the terminal 13b is emitted from the terminal 13a. The optical coupler 14 extracts a part of the light in the optical fiber loop, and the polarization controller 15 defines the polarization direction of the light transmitted through the optical fiber loop in a certain direction.

  The terminal 13c of the optical circulator 13 is connected to the collimating lens 22 through the optical fiber 21 as illustrated. The collimating lens 22 converts the light from the optical fiber 21 into parallel light, and a polarizer 23, a band pass filter 24, and a mirror 25 are provided on the optical axis. The mirror 25 is a total reflection mirror in which the reflection surface is arranged perpendicular to the optical axis formed by the collimating lens 22. The band-pass filter 24 is, for example, an optical interference type dielectric multilayer filter in which a large number of dielectric multilayer films are stacked, and is disposed inclined with respect to the optical axis. The transmission wavelength of the dielectric multilayer filter changes as the incident angle changes. This band pass filter 24 is connected to a galvanometer 26. The galvanometer 26 is an angle control unit that changes the angle of the bandpass filter 24 with respect to the optical axis at high speed. For example, the incident angle of light with respect to the filter 24 is rotated at a speed of several KHz within a range of 40 °. The polarizer 23 defines the polarization direction of light transmitted through the optical axis between the collimator lens and the mirror 25 in a predetermined direction.

  Next, the operation of this embodiment will be described. In this embodiment, the above-described pumping semiconductor laser 17 is driven to pump the optical fiber loop through the WDM coupler 18. FIG. 2A shows the gain of the gain medium 12. In this way, light added from the terminal 13 a by the action of the optical circulator 13 enters the optical fiber 21 from the terminal 13 c and becomes parallel light by the collimating lens 22. Then, light having a wavelength that passes through the bandpass filter 24 enters the mirror 25, and only the light that is reflected by the mirror 25 and passes through the filter 24 again is added to the optical fiber loop from the circulator 13 via the collimator lens 22. The polarization controller 15 adjusts the polarization of light transmitted through the optical fiber loop in a certain direction. FIG. 2B shows the external resonance longitudinal mode determined according to the optical length determined by the length of the optical fiber loop and the refractive index of the optical fiber. For example, when the optical wavelength is 30 m, there are longitudinal modes with an interval of about 10 MHz. Here, the curve A in FIG. 3 is a diagram showing the transmission characteristics of the band-pass filter 24 once, and the curve B is a diagram showing the characteristics of the band-pass filter that has been narrowed by two transmissions. FIG. 2 (c) shows the filter characteristic B1 due to the two transmissions, and a multimode oscillation including a longitudinal mode as shown in FIG. 2 (d) at a wavelength selected by passing through the filter. To do. The oscillation wavelength is 1550 nm, for example. A part of the laser light oscillated in the optical fiber loop in this way, for example, light having a level of 10% of the laser light is extracted through the optical coupler 14. Optical signals in multimode oscillation are a problem when transmitted by optical wavelength division multiplexing. However, in spectral analysis, optical fiber sensing, optical component evaluation, etc., the oscillation line width (strictly, in multimode oscillation) If the half-value width of the envelope of the spectrum is sufficiently narrower than the resolution of the object to be measured, this is not a problem.

  Then, the band pass filter 24 is rotated by the galvanometer 26. By doing so, the angle between the light beam and the bandpass filter 24 changes, so that the transmission wavelength of the bandpass filter 22 changes in the range of B2 to B3 in FIG. By appropriately adjusting this angle by driving the galvanometer 26, the transmission wavelength of the filter can be changed at a scanning speed of several KHz within a wavelength range of, for example, 100 nm. Therefore, the oscillation wavelength of the fiber laser light source can be changed within a range of 100 nm at a scanning speed of several KHz. FIG. 4 is a graph showing temporal changes in the oscillation wavelength.

  In the case of the oscillation according to this embodiment, the oscillation is in a multimode state as shown in FIG. However, as shown in FIG. 2B, since the longitudinal mode interval is extremely narrow and the mode hop is fast and fast, there is no adverse effect associated with the mode hop as in the case of an external cavity semiconductor laser, and the oscillation wavelength can be changed. Can do. Also, by using a galvanometer, an effect that a fiber laser light source can be put into practical use at a relatively low cost can be obtained.

  FIG. 5 is a diagram showing the relationship between the change in the optical axis between the collimating lens 22 and the mirror 25 and the tilt angle of the bandpass filter. As shown in FIGS. 5A and 5B, the optical axis between the bandpass filter 24 and the mirror 25 is shifted up and down by the inclination of the bandpass filter 24. However, the light reflected by the mirror 25 enters the collimating lens through the same optical path via the band pass filter 24. Therefore, in this embodiment, the light emitted from the optical fiber 21 and the light incident on the optical fiber 21 have the same optical axis. That is, even if the angle of the bandpass filter 24 changes, there is no change in coupling loss on the optical axis of the light transmitted through the bandpass filter 24, and the loss can be flattened over the entire wavelength range. Therefore, the gain band of the gain medium can be used more effectively and the output wavelength range can be widened.

  On the other hand, FIG. 6 shows a wavelength scanning fiber laser light source shown as a comparative example of the present embodiment. In this comparative example, a band pass filter 29 is directly inserted into the optical fiber loop via the collimating lenses 27 and 28 instead of the circulator. Other configurations are the same as those of the present embodiment. FIG. 7 is a diagram showing changes in the optical axes of the collimating lenses 27 and 28 and the bandpass filter 29 in this case. When the bandpass filter 29 is rotated by the galvanometer 30 to change the selected wavelength, the optical axis fluctuates left and right depending on the angle of the filter as shown in FIG. 7, so that the level of light incident on the optical fiber changes. Therefore, there is a problem that the loss in the resonator fluctuates over the wavelength change region and the output level changes according to the wavelength. In contrast, this embodiment can eliminate such a problem.

  FIG. 8 is a diagram showing a wavelength scanning fiber laser light source according to the second embodiment of the present invention. In this embodiment, a semiconductor optical amplifier (SOA) 31 is used as a gain medium in a part of the optical fiber loop. In the fiber loop, a loop is formed only by the ordinary optical fiber 11. Further, 15a and 15b are inserted as polarization controllers. Other configurations are the same as those of the first embodiment. The semiconductor optical amplifier 31 has output surfaces at both ends, and amplifies light by injecting current. However, there is no specific resonance wavelength, and an optical fiber is connected to both ends as in the present embodiment to loop. As a result, the external resonance mode shown in FIG. 2B is obtained. Similarly to the first embodiment, by connecting the bandpass filter 24 via the optical circulator 13, oscillation can be obtained at the wavelength of the bandpass filter 24 as in the above-described embodiment. By driving with the galvanometer 26, the oscillation wavelength can be changed at high speed.

  FIG. 9 is a diagram showing a wavelength scanning fiber laser light source according to the third embodiment of the present invention. In this embodiment, a fiber laser light source loop is formed by using a polarization-maintaining optical fiber 32 in an optical fiber loop. In this embodiment as well, the semiconductor optical amplifier 31 is used as a gain medium as in the second embodiment. Also, the use of the optical circulator 13 and the optical coupler 14 is the same as in the above-described embodiment. In this embodiment, since the polarization plane preserving optical fiber 32 is used, the polarization plane of light oscillating around the loop is constant in a predetermined direction. Therefore, the polarization controller 15 and the polarizer 23 between the collimating lens 22 and the mirror 25 as in the first embodiment are not necessary. Other configurations are the same as those of the above-described embodiment, and similar effects can be obtained with a relatively simple configuration.

  In each of the above-described embodiments, a galvanometer is used as the angle control unit. However, any device that can change the angle of the bandpass filter 24 at a high speed is sufficient.

  In the present invention, the wavelength can be varied at high speed, and the wavelength can be varied at high speed while improving the wavelength selection characteristics of the bandpass filter. Further, since no grating is used, it can be realized at a relatively low cost. Accordingly, the present invention can be applied to medical analysis equipment, for example, a skin cross-section monitor device. When measuring strain using a grating fiber, the wavelength scanning fiber laser light source of the present invention can be used as a light source.

It is the schematic which shows the wavelength scanning fiber laser light source by the 1st Embodiment of this invention. It is a graph which shows the gain of the gain medium of the optical fiber laser light source of this Embodiment, an oscillation mode, a band pass filter, and an oscillation output. It is a graph which shows the characteristic of the band pass filter of this Embodiment. It is a graph which shows the time change of the oscillation wavelength of this Embodiment. It is a figure which shows the change of the rotation angle and optical axis of the mirror by this Embodiment. It is the schematic of the wavelength scanning fiber laser light source by a comparative example. It is a figure which shows the angle of the band pass filter by a comparative example, and the change of an optical axis. It is the schematic which shows the wavelength scanning fiber laser light source by the 2nd Embodiment of this invention. It is the schematic which shows the wavelength scanning fiber laser light source by the 3rd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Wavelength scanning fiber laser light source 11 Optical fiber 12 Gain medium 13 Optical circulator 14 Coupler 15 Polarization controller 16 Erbium doped fiber 17 Semiconductor laser 18 WDM coupler 21 Optical fiber 22 Collimating lens 23 Polarizer 24 Band pass filter 25 Mirror 26 Galvanometer 31 Semiconductor optical amplifier 32 Polarization plane preserving optical fiber

Claims (5)

An optical fiber loop;
A gain medium provided in the optical fiber loop and having a gain with respect to an oscillation wavelength;
An optical circulator having first to third terminals, the first and second terminals being connected to an optical fiber loop, and controlling the direction of light incident on each terminal;
A collimating lens that collimates the light obtained from the third terminal of the optical circulator;
A mirror having a reflecting surface perpendicular to the optical axis formed by the collimating lens;
A bandpass filter that is disposed between the collimating lens and the mirror and changes a transmission wavelength according to an incident angle;
An angle control unit that periodically changes transmitted light by changing an incident angle of light with respect to the bandpass filter;
An optical coupler that is formed in the optical fiber loop and extracts a part of the light that passes through the optical fiber loop.   2. The wavelength scanning fiber laser light source according to claim 1, wherein the gain medium is an optical fiber amplifier constituting a part of the optical fiber loop.   2. The wavelength scanning fiber laser light source according to claim 1, wherein the gain medium is a semiconductor optical amplifier that amplifies light. A polarization controller that is provided in the optical fiber loop and controls a polarization direction of light in the optical fiber loop;
2. The wavelength scanning fiber laser light source according to claim 1, further comprising a polarizer provided between the collimator lens and the mirror and having a constant polarization direction.   The wavelength scanning fiber laser light source according to claim 1, wherein the optical fiber loop includes a polarization-preserving optical fiber.

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