External mirror type wavelength tunable laser

Abstract: In an external mirror type semiconductor laser composed of a semiconductor laser having a structure wherein a reflection factor is reduced on one end plane thereof, and a lens as well as a grating type reflector disposed on the side of the end plane, respectively, the lens is shifted in the direction orthogonal to a light axis, and an incident angle of the light outputted from the end plane to the grating type reflector is changed, whereby an external mirror type wavelength tunable laser having a simple structure and tuning a radiated wavelength can be realized (Fig. 1).

Description: The present invention relates to an external mirror type wavelength tunable laser, and particularly to an external mirror type wavelength tunable laser which can tune the wavelength so as to be used suitably for optical communication. In recent years, improvements in computer performance have been remarkable so that computer-computer communications are implemented already in ordinary homes. Furthermore, the internet is developed world-wide so that circumstances in communication are changing rapidly. Under these circumstances, information is required instantly by any one who is concerned with communications. To fulfil such a request, optical-fiber telecommunication systems by which a large amount of various information can be transmitted at high-speed is utilized as the media. Recently, it has been required to transmit in real time not only character information, but also a variety of other information such as still images, dynamic images and speech data, so that high-capacity optical-fiber telecommunication systems have been developed. Moreover, in order to transmit data through an optical-fiber telecommunication system over great distances, it is required to amplify lightwave signals in the course of the optical-fiber line. In this respect, of erbium doped optical-fiber amplifiers (EDFA) are also introduced as means for light amplification. When an EDFA system is applied as a means for light amplification, a wavelength division multiplex method is performed within a wavelength band of the EDFA (about 30 nm in the current state) to make mass-storage of data possible, so that the above described transmission system is proceeding toward practical utilization. In this system, a number of light sources each having a specified wavelength are required. In the case where a plurality of light sources are used, there are disadvantages of high cost, size increase or equipment, high consumption of energy and the like, and hence, it is desired to eliminate these disadvantages. Accordingly, it is desired to realize a wavelength tunable laser in which a wavelength can be tuned arbitrarily within the wavelength band of EDFA, i.e., that of about 30 nm, so that the wavelength can be specified to a predetermined wavelength. Wavelength-tunable lasers are classified broadly into semiconductor monolithic type and external mirror type wavelength tunable lasers. As to semiconductor monolithic type wavelength-tunable lasers, a practical example which can tune the wavelength within a range of wavelengths as wide as about 30 nm corresponding to that of EDFA does not exist. On the other hand, there are also such disadvantages in that a mechanism for rotating a grating type reflector becomes too big and the like even in external mirror type wavelength tunable laser if such a gating type reflector is used as an external mirror and if an incident angle of light outputted to the grating reflector is changed for tuning the emitting wavelength, wherein for rotating the grating reflector a motor and the like is applied. An external mirror type wavelength tunable laser comprising the features of the preamble of claim 1 is disclosed in Patent Abstracts of Japan, vol. 014, no. 067 (E-0885) and JP-A-01 286 488. Accordingly, it is an object of the present invention to provide an external mirror type wavelength tunable laser having a simple structure and capable of tuning an emitting wavelength. According to the invention as defined in claim 1, an external mirror type wavelength tunable laser, comprises a semiconductor laser having one end plane of a reduced reflection factor; a lens provided to face the one end plane of the semiconductor laser; a grating type reflector for reflecting light supplied via the lens from the one end plane of the semiconductor laser; and means for shifting the lens in a direction orthogonal to a light axis of the lens to change an incident angle of the light to the grating type reflector so as to change the peak wavelength of said reflector. It is preferred that the above described shifting means is a piezoelectric element. Furthermore, it is preferred that a focal point of the lens is set on the above described end plane of the semiconductor laser. Moreover, it is preferred that the light outputted from the above described end plane is converted into parallel light. Further, it is preferred that the above described lens is an aspherical lens. The present invention will be explained in more detail in conjunction with the appended drawings, wherein: FIG. 1 is an explanatory diagram showing a constitution of the external mirror type wavelength tunable laser according to an embodiment of the present invention; FIG. 2 is a graph indicating an example of characteristics of a relationship between emitted wavelength and moving distance of a lens in the external mirror type wavelength tunable laser according to the embodiment of the present invention; FIG. 3 is a graph indicating an example of characteristics of light output versus current in the external mirror type wavelength tunable laser according to the embodiment of the present invention; and FIG. 4 is an explanatory diagram showing a constitution of the external mirror type wavelength tunable laser according to another embodiment of the present invention. FIG. 1 is an explanatory diagram showing a constitution of an external mirror type wavelength tunable laser according to an embodiment of the invention wherein a non-reflection coating is applied to one end plane 11 of a semiconductor laser 1, whereby each reflection factor of input light and output light is reduced in the end plane 11. The light outputted from the semiconductor laser 1 is converted into parallel light by means of a lens 2 a focal point of which is placed so as to face the end plane 11 of the semiconductor laser 1. The lens 2 is movable in a plane orthogonal to the direction of the laser beam (light axis) outputted from the end plane 11 of the semiconductor laser 1. When the position of the lens 2 is changed, the direction of laser beam outputted from the end plane 11 can be varied. For example, when the lens 2 is shifted up to a position represented by the broken line in FIG. 1 as a result of transferring the lens 2 along the direction represented by arrow D in the figure, an outputting direction of light changes with an angle  shown in FIG. 1. A grating type reflector 3 separates spatially the laser beam outputted from the end plane 11 of the semiconductor laser 1 and converted into parallel light by means of the lens 2 to respective wavelengths. Moreover, another lens 5 is disposed on the side of the opposite end plane of the semiconductor laser 1. The lens 5 functions to collect the laser beam outputted from the above described end plane of the semiconductor laser 1. The laser beam outputted from the semiconductor laser 1 is propagated through an optical fiber 4. A non-reflection coating is not applied on the opposite end plane of the semiconductor laser 1, as in the case of the end plane 11, so that a resonator is composed of the present end plane and the grating type reflector 3. As mentioned above, the grating type reflector 3 functions to separate spatially the laser beam outputted from the end plane 11 of the semiconductor laser 1 and has been converted into parallel light by means of the lens 2 into the respective wavelengths. Thus, the wavelengths satisfying the resonating condition required by the resonator are restricted, so that laser beams of prescribed wavelengths can be obtained. Furthermore, since an angle at which the parallel light is inputted to the grating type reflector 3 can be varied by changing the position of the lens 2 as mentioned above, a wavelength of the laser beam outputted becomes variable. The above-mentioned grating type reflector exhibits remarkable wavelength dependency, so that it has a significant reflection factor at a specified wavelength. In this respect, a wavelength at which the reflection factor reaches its peak varies by changing the incident angle  of the light to be inputted to the grating type reflector 3. When a wavelength at which the reflection factor reaches its peak is represented by λ, a relationship between the peak wavelength λ and the angle  of incident light is given by the following equation (1) : dλ/d = cos /Nm where N is the number of grooves per 1 mm of grating, and m is a degree of diffraction. The present embodiment is characterized by changing the angle  of the incident light applied to the grating type reflector 3 by shifting the position of the lens 2 in the direction orthogonal to the light axis. In the present embodiment, the lens 2 is disposed at a position away from the end plane 11 of the semiconductor laser 1 exhibiting a reduced reflection factor by a focal length f of the lens 2, and in addition, the laser beam outputted from the end plane 11 is converted into parallel light by means of the lens 2 as mentioned above. In this arrangement, a relationship between a lens moving distance dx with respect to the light axis of the lens 2 and the angle  of incident light to the grating type reflector 3 is given by the following equation (2): d/dx = 1/f From the above described equations (1) and (2), the following equation (3) is obtained: dλ/dx = cos /(Nmf) According to equation (3), the peak wavelength of the grating type reflector 3 may be changed by shifting the position of the lens 2 in the direction orthogonal to the light axis. In the following, the invention will be described by employing specific parameters. The grating type reflector 3 is used in Littrow mounting wherein primary diffraction is applied in a wavelength band of 1550 nm. The parameters in this situation are as follows. cos  = 0.893 N = 500 [1/mm] m = 1 Further, an aspherical lens having a focal distance f = 600 mm is used as the lens 2. When these numerical values are assigned to the equation (3), it yields: dλ/dx = 3 x 10 -3 An example of characteristics of the external mirror type wavelength tunable laser according to the embodiment of the present invention will be described hereinafter. A semiconductor laser used in the embodiment had a gain peak wavelength of around 1560 nm. A reflection factor of the end plane 11 to which a non-reflection coating has been applied to reduce the reflection factor has about 0.5%. Measurement of the characteristics was made by utilizing the light outputted from the optical fiber in FIG. 1. FIG. 2 is a graph indicating an example of characteristics of a relationship between the emitted wavelength and the moving distance of the lens 2 in the external mirror type wavelength tunable laser according to the embodiment of the present invention wherein marks "o" indicate measured values. As is apparent from FIG. 2, radiation of laser was confirmed within an extremely wide range of about 100 nm extending from 1510 nm to 1620 nm. A rate of change dλ/dx in emitted wavelength with respect to the moving distance of the lens 2 is about 3.9 [nm/mm] = 3.9 x 10 -3 . The result is somewhat larger than the above-mentioned calculated value, but the emitted wavelength varies in proportion to the moving distance of the lens, whereby the appropriateness of the external mirror type wavelength tunable laser according to the present embodiment was confirmed. FIG. 3 is a graph indicating an example of characteristics of light output versus electric current in the external mirror type wavelength tunable laser according to the embodiment of the present invention. In FIG. 3, since the gain peak wavelength of the semiconductor laser 1 is in the vicinity of 1560 nm, a threshold current value in the vicinity of that wavelength is the lowest and the light output is also significant, while it is remarkable that the threshold current value increases and the light output decreases at a wavelength away from that wavelength of, for example, 1515 nm and 1616 nm. As described above, about 100 nm is attained as a wavelength tunable range of the external mirror type wavelength tunable laser according to the present embodiment. However, it is considered to be reasonable to use the external mirror type wavelength tunable laser within a range of wavelength of around 50 nm in view of practical aspects taking light output and threshold current value characteristics into consideration. An external mirror type wavelength tunable laser according to another embodiment of the present invention will be described hereinafter. FIG. 4 is an explanatory diagram showing a constitution of an external mirror type wavelength tunable laser according to another embodiment of the invention wherein the same or like parts in the present embodiment are designated by the same reference numbers as those of the former embodiment shown in FIG. 1, and the explanation therefor is omitted. The external mirror type wavelength tunable laser according to the present embodiment of the invention shown in FIG. 4 differs from that of the former embodiment shown in FIG. 1 in that a piezoelectric element 6 is utilized as the means for shifting the position of the lens 2. An element from which a shifting amount of around 10 µm is obtained by changing an applied voltage from 0 volt to 100 volts was employed as the piezoelectric element 6. As an actual tunable range of wavelengths, about 40 nm ranging from 1540 nm to 1580 nm was confirmed. In order to achieve a wider tunable range of wavelengths with a smaller moving distance, reduction of the number N of grooves in the grating type reflector 3 based on the above-mentioned equation (3) as well as reduction of a focal distance f of the lens 2 are effective. However, it is not practical to reduce the number N, because it results in a decrease in wavelength resolving power as well as in diffraction efficiency of the grating type reflector. On the other hand, reduction of the focal distance f can be realized by employing a microlens having a shorter focal distance f. For instance, when a plate microlens is used, a wavelength tunable range of around 40 nm can be realized by a lens moving distance of 2 to 3 µm, since such a plate microlens has a short focal distance of around a little less than 100 µm. While the present invention has been explained in conjunction with the above described embodiments, it is not limited only thereto. For instance, although an aspherical lens has been employed as the lens 2 in the embodiment described in relation to FIG. 1, the same advantageous effects can also be obtained by employing another type of lens. Furthermore, while the lens 2 has been disposed at a position away from the end plane 11 of the semiconductor laser 1 having a reduced reflection factor by a distance corresponding to the focal distance of the lens 2 in the same embodiment, the position is not limited thereto. Moreover, while the case where the piezoelectric element 6 is used as means for shifting electrically a position of the lens 2 has been described in the latter embodiment shown in FIG. 4, the invention is not limited thereto. As described above, according to the present invention, an external mirror type semiconductor laser composed of a semiconductor laser having a structure wherein a reflection factor is reduced on one end plane thereof, a lens, and a grating type reflector, both the members being disposed on the side of the end plane, comprises further means for shifting the aforesaid lens in a direction orthogonal to a light axis to change an incident angle of the light outputted from the aforesaid end plane to the grating type reflector. As a result, the invention exhibits such an advantage of being capable of realizing a wavelength tunable laser tunable over a wide range of wavelengths of several tens nm or more.

Claims: An external mirror type wavelength tunable laser, comprising: a semiconductor laser (1) having one end plane of a reduced reflection factor (11); a lens (2) provided to face said one end plane (11) of said semiconductor laser (1); a grating type reflector (3) for reflecting a light supplied via said lens (2) from said one end plane (11) of said semiconductor laser (1); characterised in that said wavelength tunable laser comprises also means (6) for shifting said lens (2) in a direction orthogonal to a light axis of said lens (2) to change an incident angle of said light to said grating type reflectors (3) so as to change the peak wavelength of said reflector (3). The external mirror type wavelength tunable laser as defined in claim 1, wherein said means (6) for shifting the lens (2) is a piezoelectric element. The external mirror type wavelength tunable laser as defined in claim 1 or 2, wherein a position of either of the focal points of said lens (2) is set onto said end plane (11). The external mirror type wavelength tunable laser as defined in claim 1, 2 or 3, wherein said lens (2) converts the light outputted from said end plane (11) into parallel light. The external mirror type wavelength tunable laser as defined in claim 1, 2, 3 or 4, wherein said lens (2) is an aspherical lens.

Inventors: ABE YUJI, ABE, YUJI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1998
Main classification: H01S5
IPC codes: H01S5