JP2008098395A - Wavelength scanning laser light source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength scanning laser light source which can scan wavelengths in a wide band at a high speed. <P>SOLUTION: A diffraction grating 13 is arranged on the optical axis of a laser beam emitted from a gain medium 11. In order to select a wavelength of the diffracted light diffracted by the diffraction grating 13, a condensing lens 15 and a rotary disc 17 are arranged. And, a mirror 20 is arranged to reflect the light which has passed through a slit 19 of the rotary disc 17. Due to this configuration, an external resonator is formed between the mirror 20 and a partial reflection mirror 22, and wavelengths can be scanned by rotating the rotary disc 17. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a wavelength scanning laser light source that generates monochromatic light and periodically scans its oscillation wavelength.

  In the fields of spectroscopic analysis, optical fiber sensoring, biophotonics, etc., there is an application for analyzing the response of light from an object to be measured by changing the wavelength of input light. If the wavelength can be swept at that time, dynamic responses such as the movement of the living body and the vibration of the structure can be observed, and high-speed data acquisition can be performed. High performance can be realized.

However, a conventional wavelength tunable laser used as a light source for spectroscopic analysis is generally an external resonance type using a complex variable mechanism in order to make a wide spectrum wavelength narrow with a narrow spectrum. As shown in Non-Patent Document 1, an external resonance type tunable laser is known in which a polygon mirror or the like is mechanically rotated by a motor or the like. Further, as shown in Patent Document 1, when light from a gain medium is made parallel and incident on a diffraction grating, the wavelength of the diffracted light from the diffraction grating depends on the angle. Therefore, a wavelength scanning type light source is proposed in which light of a specific wavelength selected by the slit is reflected and incident again on the diffraction grating, and the selected wavelength is changed by moving the slit.
“High-speed wavelength-swept semiconductor laser with a polygon-scanner-based wavelength filter” SHYun et al. October 15, 2003 / Vol. 28, No. 20 / OPTICS LETTERS PP1981-1983 US Patent Publication US2004 / 0264515A1

  However, such a conventional method needs to be mechanically controlled by a high-precision motor or the like, requires complicated control to stabilize the output and change the wavelength, and cannot increase the scanning speed. There were drawbacks.

  It is an object of the present invention to provide a wavelength scanning laser light source that solves the problems of the conventional wavelength scanning light source and realizes wavelength scanning at high speed and in a wide band.

  In order to solve this problem, a wavelength scanning laser light source according to the present invention includes a partial reflection mirror, a gain medium having a gain at an oscillation wavelength, a diffraction grating filter unit for selecting an oscillation wavelength, and the diffraction grating filter unit. A mirror that reflects light of a selected wavelength, and is an external resonance type wavelength scanning laser light source that forms a resonator by the partial reflection mirror and the mirror, and the diffraction grating filter unit includes: A diffraction grating that diffracts the light beam from the gain medium at different angles according to its wavelength, and reflected light that is arranged at the subsequent stage of the diffraction grating and is angle-dispersed according to the wavelength is collected at different spot positions on the mirror. A condensing lens that emits light, a rotating disk that has a slit-like opening around it, and is arranged so that the slit is positioned immediately before the focal plane of the condensing lens; and the rotating circle And a driver for driving along its central axis, and has a.

  Here, the slits are preferably arranged on the rotating disk at regular intervals, whereby a constant scanning speed can be obtained when the rotating disk is rotated at a constant rotational speed. The mirror is preferably arranged so as to reflect light incident through the slit of the rotating disk in the direction opposite to the incident direction.

  In order to solve this problem, a wavelength scanning laser light source of the present invention includes a partial reflection mirror, a gain medium having a gain at an oscillation wavelength, and a diffraction grating filter unit for selecting an oscillation wavelength, An external resonance type wavelength scanning laser light source in which a resonator is formed by a reflection mirror and the diffraction grating filter unit, and the diffraction grating filter unit is configured to change the angle of the light beam from the gain medium according to the wavelength. A diffraction grating that is diffracted into two, a condensing lens that is arranged at a subsequent stage of the diffraction grating and collects reflected light that is angularly dispersed according to the wavelength at different spot positions on the focal plane, and a plurality of slit-like reflections around it And a rotating disk disposed so that the reflecting part is positioned on the focal plane of the condenser lens, and a driving unit that drives the rotating disk along its central axis. .

  Here, it is preferable to arrange the reflecting portions of the rotating disk at regular intervals. When the rotating disk is rotated at a constant speed, a constant scanning speed can be obtained.

  According to the present invention having such characteristics, the oscillation wavelength can be periodically scanned by rotating the rotating disk at high speed. Further, the rotating disk is thinner than a polygon mirror or the like and can be rotated easily at high speed.

  1A and 1B are diagrams showing the principle of a wavelength scanning laser light source according to Embodiment 1 of the present invention. In these figures, the gain medium 11 has a gain at the oscillation wavelength of laser oscillation, and for example, a semiconductor optical amplifier (SOA) is used. A collimating lens 12 is provided in the vicinity of one light emitting end face of the gain medium 11, and parallel light is irradiated onto the diffraction grating 13. The angle of the first-order diffracted light from the diffraction grating 13 slightly changes depending on the wavelength. Therefore, if the light reflected at any one of the angles is reflected in the reverse direction as it is, only the light of that wavelength is emitted from this laser light source. Therefore, as shown in FIG. 1A, the mirror 14 is arranged at a position for receiving the first-order diffracted light of the diffraction grating. This arrangement is a Littman arrangement. In this way, only the light of the wavelength component incident at a right angle with respect to the reflecting surface of the mirror is reflected in the incident direction as it is, and is incident on the diffraction grating 13 again. Therefore, light having this wavelength component is emitted by the external resonator type light source.

  Here, as shown in FIG. 1B, a condensing lens 15 is arranged instead of the mirror 14, and a minute mirror 16 is provided at the focal position of the condensing lens 15 and moved in the left-right direction as indicated by arrows in the figure. Then, light having a wavelength reflected by the mirror oscillates. In this embodiment, a disk having a slit is used to change the reflection wavelength at high speed.

  FIG. 2 is a diagram showing an overall configuration of the wavelength scanning light source according to Embodiment 1 of the present invention, and FIG. 3 is a side view thereof. In these drawings, the gain medium 11, the collimating lens 12, and the diffraction grating 13 are the same as those in the principle diagram described above. The first-order diffracted light reflected from the diffraction grating 13 is collected by the condenser lens 15. A rotating disk 17 is provided immediately before the focal position of the lens. The drive unit 18 shown in FIG. 3 drives the rotating disk 17 along its central axis at a constant speed. A large number of slits 19 are provided at regular intervals along the circumference of the rotating disk 17. A mirror 20 for reflecting the transmitted light is provided at a position where the light passes through the slit 19. Here, the distance from the central axis of the first-order diffracted light of the diffraction grating 13 to the condenser lens 15 and the distance from the condenser lens 15 to the mirror 18 are set equal to the focal length f of the condenser lens 15. Further, the width of the slit 19 of the rotating disk 17 is set to the same size as the condensing spot by the condensing lens 15, and the interval between the slits 19 is the same as the wavelength dispersion range on the apex surface of the lens 15. Here, the diffraction grating 13, the condensing lens 15, the rotating disk 17, and the drive unit 18 constitute a diffraction grating filter unit that selects a wavelength component of the light beam.

  A partial reflection mirror 22 is provided at the other end of the gain medium 11 via a collimator lens 21. The partial reflection mirror 22 reflects part of the light and transmits part of the light. Here, an external resonator is configured between the mirror 20 and the partial reflection mirror 22. By increasing this distance, continuous wavelength sweeping can be performed in multiple modes.

  Next, the operation of this embodiment will be described. By driving the gain medium 11, light is incident on the diffraction grating 13 via the collimating lens 12. At this time, the first-order diffracted light from the diffraction grating 13 is focused through the condenser lens 15, and only the light having the wavelength component having the angle passing through the slit 19 of the rotating disk 17 is reflected by the mirror 20 as it is. Return. Therefore, an external resonator is formed between the mirror 20 and the partial reflection mirror 22, and laser oscillation occurs. As the rotating disk 17 rotates, signals having different wavelengths reflected by the diffraction grating 13 are selected, and the laser oscillation wavelength gradually changes. When the wavelength variable range is exceeded, oscillation does not occur, and when the diffracted light can pass through the next slit by rotation, the light is reflected again by the mirror 20, and the optical path is formed again to oscillate. Therefore, the oscillation wavelength changes in a sawtooth waveform.

Now, the incident angle to the diffraction grating 13 is φ, and the outgoing angle is θ. Here, the incident angle φ is 70 °, for example. The first-order diffracted light changes from, for example, wavelengths λ1 to λ2, and the following equation is established from the equation of the diffraction grating, where the number of grating lines a is 900 / mm and the grating pitch Λ is the reciprocal thereof.
λ = Λ (sinφ + sinθ) (1)
Therefore, if 1260 nm is substituted into this equation as λ1, the emission angle θ1 at that time is 11.2 °, and if 1360 nm is substituted into this equation as λ2, the emission angle θ2 at that time is 16.5 °. This difference Δθ (= θ2−θ1) is 5.3 °.

Now, the rotating disk 17 is rotated along its axis by a driving unit 18 at a constant rotational speed. At this time, when the focal length from the condenser lens 15 to the mirror 20 is f and the slit interval s is 2.7 mm, the following equation is established.
s = 2f × tan (Δθ / 2) (2)
Here, slit interval s: 2.78 mm,
Diameter d: 60 mm
Focal length f: 30 mm.
If the rotation speed R of the rotating disk 17 is 40,000 rpm, that is, 667 Hz, the laser beam scanning speed Fs is as follows.
Fs = πD × R / s = 45 KHz (3)
Therefore, the scanning speed can be greatly increased as compared with the conventional wavelength scanning laser light source.

  In the first embodiment, the wavelength scanning speed can be changed by the rotation speed of the rotating disk 17, and the wavelength scanning can be performed at a high speed by rotating at a high speed. As the rotating disk, one used for a linear encoder or the like can be used. Since the rotating disk is smaller in weight than a polygon mirror, it can be easily rotated at a high speed.

  Next, a second embodiment of the present invention will be described with reference to FIG. In the first embodiment, a slit is formed in the rotating disk, and the light passing through the slit is reflected by the mirror. However, in the second embodiment, the rotating disk 31 corresponds to a slit instead of the slit. The reflection part 32 is formed at the position. In this case, the interval is set so that the focal length f is between the condenser lens 14 and the rotating disk 31. Also in this case, the rotating disk 31 is rotated by the drive unit 18 (not shown). In this way, an external resonator is formed by the partial reflection mirror 22 and the reflection section 32, and the wavelength can be scanned at a high speed according to the rotation speed of the rotating disk without using a mirror.

  Next, Embodiment 3 of the present invention will be described with reference to FIG. In the wavelength scanning laser light source of the third embodiment, a loop is formed by the optical fiber 41. A semiconductor optical amplifier, an optical circulator 43, an optical coupler 44, and a polarization controller 45 are provided as a gain medium 42 in a part of this loop. The optical circulator 43 regulates the direction of light passing through the optical fiber 41 in the direction of the arrow as shown. That is, the input terminals 43a and 43b of the optical circulator 43 are connected to the optical fiber loop, and light incident from the input terminal 43a is emitted from the terminal 43c of the optical circulator. The light incident from the input terminal 43c of the optical circulator is emitted from the terminal 43b. Light incident from the terminal 43b is emitted from the terminal 43a. The optical coupler 44 extracts a part of the light in the optical fiber loop, and the polarization controller 45 defines the polarization direction of the light transmitted through the optical fiber loop in a certain direction.

  The collimating lens 12 is connected to the terminal 43 c of the optical circulator 43 through the optical fiber 46 as shown in the figure. Other configurations are the same as those of the first or second embodiment. In FIG. 5, it is the same as in the first embodiment. In this case, since the optical path length can be expanded by the optical fiber loop, the wavelength can be continuously scanned in a wide band without mode hops.

  In each of the embodiments described above, slits 19 are provided at equal intervals in the rotating disk 17. Accordingly, if the rotation speed is constant, the wavelength can be scanned with a constant period. When changing the period, the interval between the slits does not need to be constant. Also in the second embodiment, the interval between the mirrors corresponding to the position of the slit is not necessarily equal.

  The present invention can provide a scanning laser light source whose wavelength changes at high speed. Therefore, it can be applied to various analytical instruments, for example, a high-resolution medical image diagnostic apparatus for the epidermal layer in medical applications, and medical images in real time can be obtained.

It is a principle diagram of the wavelength scanning laser light source of the present invention. It is a principle diagram of the wavelength scanning laser light source of the present invention. It is a block diagram which shows the wavelength scanning type laser light source by Embodiment 1 of this invention. It is a side view of this Embodiment. It is a figure which shows the whole structure of the wavelength scanning laser light source by Embodiment 2 of this invention. It is a figure which shows the whole structure of the wavelength scanning laser light source by Embodiment 3 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Gain medium 12,21 Collimating lens 13 Diffraction grating 15 Condensing lens 17,31 Rotating disk 18 Drive part 19 Slit 20 Mirror 22 Partial reflection mirror 32 Reflection part 41 Optical fiber 42 Gain medium 43 Optical circulator 44 Optical coupler 45 Polarization controller

Claims (2)

A partially reflective mirror,
A gain medium having a gain at the oscillation wavelength;
A diffraction grating filter section for selecting an oscillation wavelength;
A mirror that reflects light of a wavelength selected by the diffraction grating filter unit, and an external resonance type wavelength scanning laser light source that forms a resonator by the partial reflection mirror and the mirror,
The diffraction grating filter section is
A diffraction grating that diffracts the light beam from the gain medium at different angles depending on its wavelength;
A condensing lens that is arranged at a subsequent stage of the diffraction grating and collects reflected light that is angularly dispersed according to wavelength at different spot positions on the mirror;
A rotating disk having a slit-like opening around it, and arranged so that the slit is located immediately before the focal plane of the condenser lens;
A wavelength scanning laser light source having a drive unit for driving the rotating disk along its central axis. A partially reflective mirror,
A gain medium having a gain at the oscillation wavelength;
An external resonance type wavelength scanning laser light source having a diffraction grating filter section for selecting an oscillation wavelength, and forming a resonator by the partial reflection mirror and the diffraction grating filter section,
The diffraction grating filter section is
A diffraction grating that diffracts the light beam from the gain medium at different angles depending on its wavelength;
A condensing lens that is arranged at the subsequent stage of the diffraction grating and collects the reflected light that is angularly dispersed according to the wavelength at different spot positions on the focal plane;
A rotating disk having a plurality of slit-shaped reflecting portions around and disposed so that the reflecting portions are located on the focal plane of the condenser lens;
A wavelength scanning laser light source having a drive unit for driving the rotating disk along its central axis.

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