Abstract:
A tunable laser source that stably outputs high-output light with reduced spontaneous emission light is to be realized. This invention is an improvement of a tunable laser source of external resonator type. This apparatus comprises a wavelength selecting unit for selecting a wavelength of incident light and emitting the light of the selected wavelength, an optical amplifier unit for making light incident on the wavelength selecting unit from one end, and a mirror for reflecting light from the other end of the optical amplifier unit directly to the wavelength selecting unit. The wavelength selecting unit feeds the light from the one end of the optical amplifier unit back to the optical amplifier unit and emits the light from the mirror as output light.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to a tunable laser source of external resonance type using a semiconductor laser, and particularly to a tunable laser source that stably outputs high-output light with less spontaneous emission light.  
         [0003]     2. Description of the Related Art  
         [0004]      FIG. 1  shows the structure of a tunable laser source of Littman layout that has been conventionally used (for example, see JP-A-2000-164980 (paragraph 0002, FIG. 4), and Karen Liu and Michael G. Littman, “Novel geometry for single-mode scanning of tunable lasers,” OPTICS LETTERS, Vol. 6, No. 3 (March 1981), pp. 117-118). In  FIG. 1 , an optical amplifier unit  10  has a semiconductor laser  11 , a first lens  12 , and a second lens  13 . The semiconductor laser  11  has an antireflection film  11   a  at its one end. The first lens  12  collimates light emitted from the one end (where the antireflection film  11   a  is provided) of the semiconductor laser  11  and emits the collimated light. The second lens  13  collimates light emitted from the other end of the semiconductor laser  11  and emits the collimated light.  
         [0005]     A wavelength selecting unit  20  has a diffraction grating  21 , a wavelength selecting mirror  22 , and a mirror rotating unit  23 . The wavelength selecting unit  20  selects a wavelength of light made incident from one end of the optical amplifier unit  10  and feeds the light of the selected wavelength back to the optical amplifier unit  10 . The diffraction grating  21  performs wavelength distribution of the light from the optical amplifier unit  10  and the light from the wavelength selecting mirror  22 . The wavelength selecting mirror  22  reflects the light that is wavelength-distributed by the diffraction grating  21 , to the diffraction grating  21 . The mirror rotating unit  23  rotates the wavelength selecting mirror  22 , thus selecting a wavelength of the light to be fed back to the optical amplifier unit  10  by the diffraction grating  21 . The rotation axis about which the rotating unit  23  rotates the wavelength selecting mirror  22  is parallel to the direction along the grooves of the diffraction grating  21 . The intersection of a line extending from the diffraction surface of the diffraction grating  21  and a line extending from the reflection surface of the wavelength selecting mirror  22  further intersects a line extending from a surface that forms an external resonator, and the center of rotation of the wavelength selecting mirror  22  is at this intersection.  
         [0006]     An optical isolator  30  transmits light made incident from the other end of the optical amplifier unit  10  and emits the transmitted light as output light. The optical isolator  30  reduces the light that is emitted from the optical amplifier unit  10 , then transmitted through the optical isolator  30  and returning to the optical amplifier unit  10  (so-called return light).  
         [0007]     The operation of this apparatus will now be described.  
         [0008]     The light emitted from the one end of the semiconductor laser  11  is collimated by the first lens  12  and becomes incident on the diffraction grating  21 . Then, the light incident on the diffraction grating  21  is diffracted by the diffraction grating  21 , then is distributed in wavelength at a different angle for each wavelength, and becomes incident on the wavelength selecting mirror  22 . Of the light incident on the wavelength selecting mirror  22 , only light having a desired wavelength is reflected on the same optical path to the diffraction grating  21 . The wavelength of light to be reflected on the same optical path is selected by the mirror rotating unit  23 .  
         [0009]     The light incident on the diffraction grating  21  is again distributed in wavelength, and only the light having the wavelength selected by the wavelength selecting unit  20  is converged on the semiconductor laser  11  by the first lens  12  and fed back. The other end of the semiconductor laser  11  and the wavelength selecting mirror  22  form an external resonator, which performs laser oscillation.  
         [0010]     On the other hand, the light emitted from the other end where the antireflection film la is not provided is collimated by the second lens  13 , then transmitted through the optical isolator  30  and emitted as output light. Moreover, as the wavelength selecting mirror  22  is rotated by the mirror rotating unit  23 , the wavelength of the light fed back to the optical amplifier unit  10  from the wavelength selecting unit  20  can be tuned, and wavelength sweep of the output light is performed when necessary.  
         [0011]     The use of such Littman layout as shown in  FIG. 1  enables restraining mode hop at the time of tuning. In the output light, the single wavelength selected by the wavelength selecting unit  20  is dominant. However, since the output light also includes spontaneous emission light of a broad wavelength range emitted directly to the second lens  13  from the semiconductor laser  11  itself, the output light has a poor S/N ratio. Therefore, the Littman layout is not suitable for the use in measuring wavelength loss characteristics of optical components for optical communications that require a large dynamic range, such as a notch filter.  
         [0012]     Thus, to realize output light with reduced spontaneous emission light, a tunable laser source using diffracted light as output light or a tunable laser source using a tunable filter is used.  
         [0013]     First, the tunable laser source that outputs diffracted light will be described (see, for example, JP-A-11-126943(paragraphs 0021 to 0031, FIGS. 1 and 2)).  
         [0014]      FIG. 2  is a view showing the structure of the conventional tunable laser source that outputs diffracted light. In  FIG. 2 , the same elements as those shown in  FIG. 1  are denoted by the same numerals and will not be described further in detail. In  FIG. 2 , a beam splitter  40  for splitting diffracted light from the diffraction grating  21  into two light beams is provided between the first lens  12  and the diffraction grating  21 . The second lens  13  of the optical amplifier unit  10  and the optical isolator  30  are not provided.  
         [0015]     The operation of such an apparatus is substantially similar to the operation of the apparatus shown in  FIG. 1  but differs in that the beam splitter  40  splits the diffracted light from the diffraction grating  21 . One of the light beams is fed back to the semiconductor laser  11  via the first lens  12 , as in the apparatus shown in  FIG. 1 . Of the other light beam reflected at 90° by the beam splitter  40 , only light of a desired wavelength transmitted through an optical isolator, not shown, and a slit, not shown, is emitted as output light.  
         [0016]     Next, the tunable laser source with a tunable filter will be described (see, for example, JP-A-2003-69146 (paragraphs 0014 to 0017, FIG. 4)).  
         [0017]      FIG. 3  is a view showing the structure of the conventional tunable laser source with a tunable filter. In  FIG. 3 , the same elements as those shown in  FIG. 1  are denoted by the same numerals and will not be described further in detail. In  FIG. 3 , a second wavelength selecting unit  50  for selecting a wavelength of light outputted from the optical isolator  30  is provided.  
         [0018]     The second wavelength selecting unit  50  has a diffraction grating  51  and a diffraction grating rotating unit  52 . The second wavelength selecting unit  50  selects a wavelength of light outputted from the optical isolator  30  and emits the light of the selected wavelength as output light. The diffraction grating  51  performs wavelength distribution of the light from the optical isolator  30 . The diffraction grating rotating unit  52  rotates the diffraction grating  51  to adjust the direction in which the diffraction grating  51  emits light of a desired wavelength, thus selecting a wavelength. The rotation axis about which the diffraction grating rotating unit  52  rotates the diffraction grating  51  is parallel to the rotation axis of the mirror rotating unit  23 .  
         [0019]     The operation of such an apparatus is substantially similar to the operation of the apparatus shown in  FIG. 1  but differs in that the diffraction grating  51  performs wavelength distribution of the light from the optical isolator  30  including spontaneous emission light. Only light of a desired wavelength is transmitted through a slit, not shown. Thus, only the light of the desired wavelength excluding unwanted spontaneous emission light is emitted as output light. The rotations of the mirror rotating unit  23  and the diffraction grating rotating unit  52  are synchronized by a synchronizing unit, not shown, so as to select the wavelength of output light.  
         [0020]     In the apparatus shown in  FIG. 2 , since the diffracted light from the diffraction grating  21  of the wavelength selecting unit  20  is split for output light by the beam splitter  40 , unwanted spontaneous emission light can be eliminated from the output light. In the apparatus shown in  FIG. 3 , since the second wavelength selecting unit  50  selects the wavelength, unwanted spontaneous emission light can be eliminated from the output light.  
         [0021]     However, in the apparatus shown in  FIG. 2 , since a part of the light fed back to the optical amplifier unit  10  from the wavelength selecting unit  20  is split by the beam splitter  40  and used as output light, when many light beams are split and used as output light, stable laser oscillation cannot be performed. Therefore, in practice, only approximately 20% of the light from the wavelength selecting unit  20  can be acquired as output light, causing a problem of low-output light.  
         [0022]     On the other hand, in the apparatus shown in  FIG. 3 , the wavelengths of light selected by the wavelength selecting units  20  and  50  must be synchronized, and a synchronizing unit, not shown, is required. Moreover, there is a problem that the structure is complicated by using the two moving parts.  
         [0023]     Now, a tunable laser source having a structure capable of acquiring higher output than in  FIG. 1  with fewer moving parts will be described (see, for example, JP-A-5-72499 (paragraphs 0009 to 0014, FIG. 1)). This apparatus has such a structure that light from the optical isolator  30  is made incident on an incident port of a transmission optical fiber by using a lens in the apparatus shown in  FIG. 1 . Then, the light is transmitted through the optical fiber, and the light emitted from an emission port of the optical fiber is collimated by a lens and made incident on the diffraction grating  21 . Of the diffracted light diffracted by the diffraction grating  21 , only light of a desired wavelength is transmitted through a slit, not shown. Thus, only the light of the desired wavelength excluding unwanted spontaneous emission light is emitted as output light.  
         [0024]     However, the transmission of the light from the isolator  30  through the optical fiber has the following problems.  
         [0025]     (1) Since the light is made incident on the optical fiber, even if the lens is used, insertion loss (approximately 2 to 3 [dB]) occurs and high output cannot be achieved.  
         [0026]     (2) Since the diffraction grating  21  itself is dependent on polarization, if stress, temperature change or the like is applied to the optical fiber, the polarization state of the transmitted light changes and the intensity of the output light changes. That is, even if the intensity of the light outputted from the other end of the optical amplifier unit  10  is constant, the intensity of the output light is not stable. A structure using a polarization maintaining optical fiber may be employed but the cost is high.  
         [0027]     (3) If stress, temperature change or the like is applied to the optical fiber, the polarization state changes and the wavelength distribution effect by the diffraction grating  21  changes. Therefore, unwanted spontaneous emission light is included in the output light and the S/N ratio is deteriorated.  
         [0028]     (4) For incidence on the optical fiber (particularly single-mode fiber), adjustment by several micrometers is necessary. The adjustment is difficult and susceptible to changes with the lapse of time.  
       SUMMARY OF THE INVENTION  
       [0029]     Thus, it is an object of this invention to realize a tunable laser source that stably outputs high-output light with reduced spontaneous emission light. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]      FIG. 1  is a structural view showing a conventional tunable laser source (Littman layout type).  
         [0031]      FIG. 2  is a structural view showing a conventional tunable laser source (diffracted light output type).  
         [0032]      FIG. 3  is a structural view showing a conventional tunable laser source (tunable filter type).  
         [0033]      FIG. 4  is a structural view showing a first embodiment of this invention (perspective view).  
         [0034]      FIG. 5A  is a structural view showing the first embodiment of this invention (top view).  
         [0035]      FIG. 5B  is a structural view showing the first embodiment of this invention (side view).  
         [0036]      FIG. 6  is a structural view showing a second embodiment of this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]     Embodiments of this invention will now be described with reference to the drawings.  
         [0038]      FIG. 4 ,  FIG. 5A  and  FIG. 5B  are structural views showing an embodiment of this invention.  FIG. 4  is a perspective view.  FIGS. 5A and 5B  are views showing an apparatus shown in  FIG. 4 , as viewed from different angles.  FIG. 5A  is a top view and  FIG. 5B  is a side view. In these drawings, the same elements are those shown in  FIG. 1  are denoted by the same numerals and will not be described further in detail. In  FIGS. 5A and 5B , the mirror rotating unit  23  is not shown. Of course, the center of rotation of the wavelength selecting mirror  22  is at the intersection of a line extending from the diffraction surface of the diffraction grating  21 , a line extending from the reflection surface of the wavelength selecting mirror  22 , and a line extending from a surface forming an external resonator (strictly speaking, slightly closer to the second lens  13  than to the reflection surface, which is on the other end of the semiconductor laser  11 , because of the influence of the refractive index of the semiconductor laser  11 ) (see  FIG. 5A ).  
         [0039]     In  FIG. 4 , a mirror  60  for reflecting light incident from the optical isolator  30  directly to the wavelength selecting unit  20  without causing the light to pass through the optical amplifier unit  10  is newly provided. The mirror  60  is inclined with respect to the optical axis of the optical isolator  30  so that it shifts the light from the optical isolator  30  only upward (in the direction along the grooves of the diffraction grating  21 ) and reflects the light in this manner (see  FIGS. 5A and 5B ). The reflection surface of the mirror  60  is made up of, for example, a metal coating (aluminum, silver or the like), a dielectric multilayer film or the like. The mirror  60  may reflect only light of a predetermined wavelength range (for example, around 1500 nm used in optical communications) by adjusting the thickness of the film.  
         [0040]     The operation of such an apparatus will now be described.  
         [0041]     Light emitted from one end of the semiconductor laser  11  is collimated by the first lens  12  and becomes incident on the diffraction grating  21 . The light incident on the diffraction grating  21  is diffracted by the diffraction grating  21 , then distributed in wavelength at a different angle for each wavelength, and becomes incident on the wavelength selecting mirror  22 . Of the light incident on the wavelength selecting mirror  22 , only light of a predetermined wavelength is reflected on the same optical path to the diffraction grating  21 . The wavelength of the light to be reflected on the same optical path is selected by the mirror rotating unit  23 .  
         [0042]     Then, the light incident on the diffraction grating  21  is again distributed in wavelength, and only light of a wavelength selected by the wavelength selecting unit  20  is converged on the semiconductor laser  11  by the first lens  12  and thus fed back. The other end of the semiconductor laser  11  and the wavelength selecting mirror  22  form an external resonator, which performs laser oscillation.  
         [0043]     On the other hand, light emitted from the other end (end surface where the antireflection film  11   a  is not provided) of the semiconductor laser is collimated by the second lens  13 , then transmitted through the optical isolator  30  and becomes incident on the mirror  60 . The light is substantially totally reflected by the mirror  60  and becomes incident directly on the diffraction grating  21  of the wavelength selecting unit  20  without passing through the optical isolator  30  and the optical amplifier unit  10 . The reflected light from the mirror  60  becomes incident at a position on the diffraction grating  21  that is shifted only upward from the position on the diffraction grating  21  at which the transmitted light from the first lens  12  becomes incident.  
         [0044]     The light incident on the diffraction grating  21  is diffracted by the diffraction grating  21 , then distributed in wavelength at a different angle for each wavelength, and becomes incident on the wavelength selecting mirror  22 . Then, of the light incident on wavelength selecting mirror  22 , only light of a predetermined wavelength is reflected on an optical path on the diffraction grating  21  shifted only upward. Moreover, the light incident on the diffraction grating  21  is again distributed in wavelength and emitted, and only light of a desired wavelength is transmitted through a slit, not shown. Thus, only the light of the desired wavelength excluding unwanted spontaneous emission light is emitted as output light.  
         [0045]     Since the wavelength selecting mirror  22  is rotated by the mirror rotating unit  23 , selection of the wavelength of the light fed back to the optical amplifier unit  10  from the wavelength selecting unit  20  and selection of the wavelength of the output light emitted from the wavelength selecting unit  20  can be varied, and wavelength sweep of the output light is performed when necessary.  
         [0046]     In this manner, the mirror  60  reflects the light incident from the other end of the optical amplifier unit  10  via the optical isolator  30 , directly to the wavelength selecting unit  20 . Since the wavelength selecting unit  20  selects the wavelength of the light from the mirror  60  and emits the light of the selected wavelength as output light, unwanted spontaneous emission light can be eliminated from the output light. This enables stable output of high-output light with reduced spontaneous emission light by using few moving parts.  
         [0047]     Additionally, since the single wavelength selecting unit  20  can perform both the selection of an oscillation wavelength and the filtering of spontaneous emission light synchronized with the oscillation wavelength, it is not necessary to provide two wavelength selecting units having moving parts. Therefore, the structure is simplified and the cost can be reduced.  
         [0048]     Moreover, compared with the case where the optical fiber transmits the light from the optical isolator  30  to the diffraction grating  21 , the following features can be achieved.  
         [0049]     (1) Since the mirror  60  reflects the light from the optical isolator  30  to the diffraction grating  21 , substantially total reflection can be performed and a high output with reduced loss can be acquired.  
         [0050]     (2) Since the mirror  60  reflects the light from the optical isolator  30  to the diffraction grating  21 , the polarization state does not change and the intensity of the output light remains constant and stable. Moreover, the use of the mirror enables reduction in the cost, compared with the use of the optical fiber.  
         [0051]     (3) Since the mirror  60  reflects the light from the optical isolator  30  to the diffraction grating  21 , the polarization state does not change and the wavelength distribution effect by the diffraction grating  21  is constant. Therefore, unwanted spontaneous emission light can be eliminated and the S/N ratio can be improved.  
         [0052]     (4) Since the mirror  60  reflects the light from the optical isolator  30  to the diffraction grating  21 , adjustment of incident light becomes easier and is more resistant to changes with the lapse of time.  
         [0053]     Furthermore, since the optical isolator  30  reduces the return light to the optical amplifier unit  10 , laser oscillation can be stabilized.  
         [0054]     It is to be noted that this invention is limited to this embodiment but the following structures can also be employed.  
         [0055]     While the apparatus shown in  FIG. 4  has the structure in which the optical isolator  30  reduces the return light to the optical amplifier unit  10 , the light emitted from the other end of the optical amplifier unit  10  may be made incident directly to the mirror  60  without providing the optical isolator  30 .  
         [0056]     Also, while the apparatus shown in  FIG. 4  has the structure in which the reflected light distributed in wavelength by the diffraction grating  21  is made incident on the slit, not shown, the light may be made incident on an optical fiber and the incident light may be used as output light.  
         [0057]     Moreover, while the apparatus shown in  FIG. 4  has the structure in which the diffraction grating  21  diffracts the reflected light from the mirror  60  twice and the diffracted light is emitted as output light, the light from the mirror  60  may be diffracted once by the diffraction grating  21  and then emitted as output light, as shown in  FIG. 6 . Specifically, the diffraction grating  21  performs wavelength distribution of the reflected light from the mirror  60  and only light of a desired wavelength is transmitted through a slit, not shown. That is, the wavelength selecting unit  20  twice diffracts only the light from the one end of the optical amplifier unit  10  by using the diffraction grating  21  and feeds the diffracted light back to the optical amplifier unit  10 , while the wavelength selecting unit  20  diffracts the light from the mirror  60  only once by using the diffraction grating  21  and then emits the diffracted light as output light. The slit, not shown, moves on the optical path of the selected wavelength synchronously with the mirror rotating unit  23 .  
         [0058]     In this manner, the mirror  60  reflects the light incident from the other end of the optical amplifier unit  10  via the optical isolator  30 , directly to the wavelength selecting unit  20 . Then, the wavelength selecting unit  20  selects a wavelength of the light from the mirror  60  and emits the light of the selected wavelength as output light. Therefore, unwanted spontaneous emission light can be eliminated. This enables stable output of high-output light with reduced spontaneous emission light.  
         [0059]     Moreover, while the apparatus shown in  FIG. 4  has the wavelength selecting unit  20  in which the wavelength selecting mirror  22  reflects the diffracted light from the diffraction grating  21  again to the diffraction grating  21 , the diffracted light from the diffraction grating  21  may be directly fed back to the optical amplifier unit  10  without providing the wavelength selecting mirror  22 . Of course, a diffraction grating rotating unit for rotating the diffraction grating  21  is provided instead of the mirror rotating unit  23 . This diffraction grating rotating unit performs wavelength selection of the light to be fed back to the optical amplifier unit  10  by the diffraction grating  21  and wavelength selection of output light emitted from the diffraction grating  21 .  
         [0060]     In this manner, the mirror  60  reflects the light incident from the other end of the optical amplifier unit  10  via the optical isolator  30 , directly to the wavelength selecting unit  20 . The wavelength selecting unit  20  selects a wavelength of the light from the mirror  60  and emits the light of the selected wavelength as output light. Therefore, unwanted spontaneous emission light can be eliminated. This enables stable output of high-output light with reduced spontaneous emission light by using fewer moving parts.  
         [0061]     Moreover, since the single wavelength selecting unit  20  can perform both the selection of an oscillation wavelength and the filtering of spontaneous emission light synchronized with the oscillation wavelength, it is not necessary to provide two wave length selecting unit shaving moving parts. Therefore, the structure is simplified and the cost can be reduced.  
         [0062]     This invention has the following effects.  
         [0063]     The mirror reflects the light from the other end of the optical amplifier unit, directly to the wavelength selecting unit. The wavelength selecting unit selects a wavelength of the light from the mirror and emits the light of the selected wavelength as output light. Therefore, unwanted spontaneous emission light can be eliminated. This enables stable output of high-output light with reduced spontaneous emission light.  
         [0064]     Since the optical isolator reduces the return light to the optical amplifier unit, laser oscillation can be stabilized.  
         [0065]     Since single wavelength selecting unit can perform both the selection of an oscillation wavelength and the filtering of spontaneous emission light synchronized with the oscillation wavelength, it is not necessary to provide two wavelength selecting units having moving parts. Therefore, the structure is simplified and the cost can be reduced. This enables stable output of high-output light with reduced spontaneous emission light by using fewer moving parts.