Abstract:
The present disclosure relates to a tunable laser module including a light gain area unit for outputting an optical signal; an optical distributor for separating the optical signal output from the light gain area unit; two comb reflection units for reflecting a part of optical signals separated by the optical distributor and allow a part of the optical signals to penetrate; two phase units for changing phases of the optical signals penetrating the two comb reflection units; an optical coupler for combining the optical signals of which the phases are changed by the two phase units; and an optical amplifier for amplifying the optical signal combined by the optical coupler, wherein the light gain area unit oscillates a laser by totally reflecting the optical signals reflected by the two comb reflection units.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims priority from Korean Patent Application No. 10-2012-0026122, filed on Mar. 14, 2012, with the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to a laser module, and more particularly, to a monolithic integrated tunable laser module for high-speed modulation, high-power output, and long-term optical transmission. 
       BACKGROUND 
       [0003]    As a wavelength division multiplexing-passive optical network (WDM-PON) capable of providing a high capacity communication service through wavelength allocation becomes important, a development of light sources to be used in the WDM-PON has been significant. The WDM-PON needs a high-speed modulation tunable laser module capable of finely changing wavelengths of respective channels while changing the wavelengths of the channels having a predetermined wavelength interval, and performing high-speed modulation. 
         [0004]    A representative high-speed tunable laser module among the high-speed tunable laser modules proposed up to now is a tunable laser module using a sampled grating distributed Bragg reflector (SG-DBR) disclosed in U.S. Pat. No. 4,896,325, and has a laser structure for modulating an optical signal output from the sampled grating distributed Bragg reflector by forming resonance of laser by integrating a gain range and a phase control range between two sampled grating distributed Bragg reflectors and then integrating an optical modulator at a distal end of one sampled grating distributed Bragg reflector between the two sampled grating distributed Bragg reflectors. The sampled grating distributed Bragg reflector has a narrow tunable range of 10 nm or less, and the Vernier effect by the two sampled grating distributed Bragg reflectors is used in order to obtain a wide tunable range. 
         [0005]    Further, in order to substitute the tunable laser module using the Bragg grating reflector, a monolithic integrated tunable laser module using two ring resonators having slightly different free spectral ranges (FSR) was published (Thesis: PHOTONICS TECHNOLOGY LETTERS, Vol. 14, No. 5, p 600, 2002, IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 21, No. 13, p 851, 2009, and IEEE Journal of Lightwave Technology, Vol. 24, No. 4, p 1865, 2006). In the thesis, an output wavelength of the laser is varied by an interval of the free spectral ranges by fixing a refractive index of one ring resonator and varying a refractive index of the other ring resonator. 
         [0006]    Further, a Y-branch tunable laser module in which a laser output terminal is positioned in a gain range, which is different from the tunable laser module using the sampled grating distributed Bragg reflector, was introduced (Thesis: Proceedings Symposium IEEE/LEOS Benelux, p 55, 2003). The Y-branch tunable laser module has a structure in which an optical signal output from one output terminal in the gain range is distributed to two tunable reflectors having different free spectral ranges by using an optical distributor, and outputs a ranged signal to the remaining one output terminal in the gain range. The Y-branch tunable laser module may obtain a wide tunable range by using the Vernier effect identically to the tunable laser module using the sampled grating distributed Bragg reflector. Further, the Y-branch tunable laser module may have a structure in which an additional reflector is integrated in the output terminal of the gain range and then an optical modulator and an optical amplifier are integrated for the high-speed modulation (Thesis: IEE Electronics Letters, vol. 43, no. 9, 2007). 
         [0007]      FIG. 1  is a diagram illustrating a structure of a conventional Y-branch tunable laser module. 
         [0008]    Referring to  FIG. 1 , an optical signal output from an one-side cross section of a light gain area unit  101  is separated into two optical signals by an optical distributor  102 , the two optical signals are reflected to the light gain area unit  101  by sampled grating distributed Bragg reflectors (SG-DBRs)  104  and  105  having different free spectral ranges again, and the reflected optical signals are reflected in an opposite-side cross-section  106  of the light gain area unit  101  to oscillate a laser. Here, the two reflectors  104  and  105  reflect multi-wavelength optical signals, and the single-wavelength laser is output in one matched wavelength among the reflective wavelengths of the two reflectors  104  and  105  (Vernier effect). Further, since reflection and output need to be generated in the opposite-side cross section  106  of the light gain area unit  101 , reflectivity is required to be adjusted by coating the opposite-side cross section  106  of the light gain area unit  101  so that the optical signal is not totally reflected, but partially reflected. That is, an output characteristic is determined according to a degree of the coating of the opposite-side cross section  106  of the light gain area unit  101 . Further, one reflector between the two reflectors  104  and  105  includes a phase unit  103 , so that phases of the optical signals reflected from the two reflectors  104  and  105  are controlled. 
         [0009]      FIG. 2  is a diagram illustrating a structure of a tunable laser module in which a Mach-Zehnder optical modulators is integrated in a conventional Y-branch tunable laser module in order to use the tunable laser module for high-speed modulation and long-term transmission. 
         [0010]    Referring to  FIG. 2 , the light gain area unit  101  does not have a cross-section used for reflection and output, so that a reflection unit  206  is additionally integrated for the laser resonance. Further, in order to manufacture a Mach-Zehnder optical modulator, an optical distributor  204  for separating an optical signal into two optical signals and an optical coupler  201  for combining the two optical signals are used. In addition, the tunable laser module includes phase units  202  and  203  so that the two optical signals have different phases of 180°. Here, the characteristic of the optical modulator is determined by an accurate distribution ratio of the optical distributor  204  used for constituting the Mach-Zehnder optical modulator. Further, an optical amplifier  205  is used in order to amplify the optical signal output from the reflection unit  206 . 
         [0011]      FIG. 3  is a diagram illustrating a structure of a tunable laser module using a conventional sampled grating distributed Bragg reflector. 
         [0012]    Referring to  FIG. 3 , an optical gain unit  303  and two multi-wavelength reflection units  302  and  304  having different free spectral ranges are used, so as to vary a wide wavelength range by the Vernier effect. An optical amplifier  301  is used in order to amplify a single wavelength laser optical signal output from one of the two multi-wavelength reflection units  302  and  304 , and the optical coupler  201  and the optical distributor  204  are used in order to constitute the Mach-Zehnder optical modulator. Further, the tunable laser module includes the phase units  202  and  203  so that the two optical signals separated by the optical distributor  204  have different phases of 180°. 
         [0013]    However, the Mach-Zehnder optical modulator formed of the optical coupler, the optical distributor and two arms needs to be separately manufactured in the conventional tunable laser module, so that a structure of the tunable laser module is complex and it is difficult to achieve the monolithic integration. 
       SUMMARY 
       [0014]    The present disclosure has been made in an effort to provide a tunable laser module capable of achieving high-speed modulation, high-power output, and long-term communication. 
         [0015]    Further, the present disclosure provides a tunable laser module capable of efficiently monolithically integrating a Mach-Zehnder optical modulator. 
         [0016]    An exemplary embodiment of the present disclosure provides a tunable laser module, comprising: a light gain area unit configured to output an optical signal; an optical distributor configured to separate the optical signal output from the light gain area unit; two comb reflection units configured to reflect a part of optical signals separated by the optical distributor and allow a part of the optical signals to penetrate; two phase units configured to change phases of the optical signals penetrating the two comb reflection units; an optical coupler configured to combine the optical signals of which the phases are changed by the two phase units; and an optical amplifier configured to amplify the optical signal combined by the optical coupler, wherein the light gain area unit oscillates a laser by totally reflecting the optical signals reflected by the two comb reflection units. 
         [0017]    Another exemplary embodiment of the present disclosure provides a tunable laser module, comprising: a light gain area unit configured to output an optical signal; two comb reflection units configured to oscillate a laser by reflecting the optical signal output from the light gain area unit; two phase units configured to change phases of optical signals output from the two comb reflection units; an optical coupler configured to combine the optical signals of which the phases are changed by the two phase units; and an optical amplifier configured to amplify the optical signal combined by the optical coupler. 
         [0018]    Accordingly, the aforementioned present disclosure provides the tunable laser module using the two output terminals as the two arms of the Mach-Zehnder optical modulator, so that the tunable laser module according to the present disclosure has a simple structure and excellent output characteristics compared to the existing monolithic integrated tunable laser module. 
         [0019]    Further, the tunable laser module according to the present disclosure is capable of performing high-speed modulation and has the excellent output characteristic, thereby being capable of performing the long-term transmission. 
         [0020]    The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a diagram illustrating a structure of a conventional Y-branch tunable laser module. 
           [0022]      FIG. 2  is a diagram illustrating a structure of a tunable laser module in which Mach-Zehnder optical modulator is integrated in a conventional Y-branch tunable laser module order to use the tunable laser module for high-speed modulation and long-term transmission. 
           [0023]      FIG. 3  is a diagram illustrating a structure of a tunable laser module using a conventional sampled grating distributed Bragg reflector. 
           [0024]      FIG. 4  is a diagram illustrating a configuration of a tunable laser module according to an exemplary embodiment of the present disclosure. 
           [0025]      FIG. 5  is a diagram illustrating an example of an implementation of a tunable laser module according to an exemplary embodiment of the present disclosure. 
           [0026]      FIG. 6  is a diagram illustrating a configuration of a tunable laser module according to another exemplary embodiment of the present disclosure. 
           [0027]      FIG. 7  is a diagram illustrating an example of an implementation of a tunable laser module according to another exemplary embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. 
         [0029]    Hereinafter, exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In describing the present disclosure, a detailed description of related known configurations and functions will be omitted when it may make the essence of the present disclosure obscure. 
         [0030]      FIG. 4  is a diagram illustrating a configuration of a tunable laser module according to an exemplary embodiment of the present disclosure. 
         [0031]    Referring to  FIG. 4 , the tunable laser module according to an exemplary embodiment of the present disclosure includes a light gain area unit  401 , an optical distributor  402 , two comb reflection units  403  and  404 , two phase units  405  and  406 , an optical coupler  407  and an optical amplifier  408 . Here, the light gain area unit  401 , the optical distributor  402 , the two comb reflection units  403  and  404 , the two phase units  405  and  406 , the optical coupler  407  and the optical amplifier  408  are integrated in one substrate (not shown). 
         [0032]    The light gain area unit  401  outputs an optical signal. Further, the light gain area unit  401  oscillates a laser by totally reflecting an optical signal reflected from the two comb reflection units  403  and  404  by using a total reflection coating film  409  formed in one surface thereof. 
         [0033]    The optical distributor  402  separates the optical signal output from the light gain area unit  401 . 
         [0034]    The two comb reflection units  403  and  404  have the same reflectivity and different free spectral ranges. Accordingly, when the reflectivity of the two comb reflection units  403  and  404  is 100% or lower, the optical signal oscillated from the laser is output through the two comb reflection units  403  and  404 . Further, since the two comb reflection units  403  and  404  have the same reflectivity, output characteristics of the optical signals output from the two comb reflection units  403  and  404  are the same. Accordingly, the two optical signals having the same light strength, the same phase and the same polarized light are input in the two phase units  405  and  406  for constituting the Mach-Zehnder optical modulator. 
         [0035]    Further, each of the two comb reflection units  403  and  404  may include a grating distributed Bragg reflector or a ring resonator. 
         [0036]    The two phase units  405  and  406  change the phases of the optical signals penetrating the two comb reflection units  403  and  404 , so that an output characteristic of the optical coupler  407  is determined when the optical signals penetrating the two comb reflection units  403  and  404  are combined by the optical coupler  407 . Specifically, when the two optical signals having the same light strength, the same phase and the same polarized light are combined by the optical coupler  407 , the optical signal is input from the optical coupler  407  by constructive interference, and when a phase difference between the two optical signals is made to be 180° by using at least one phase unit between the two phase units  405  and  406 , the optical signal is not output in the optical coupler  407  by destructive interference. That is, the two phase units  405  and  406  serve as the Mach-Zehnder optical modulator. 
         [0037]    The optical coupler  407  combines the optical signals of which the phases are changed by the two phase units  405  and  406 . Here, the optical coupler  407  may be a splitter, a gap coupler or a multimode interference coupler in which a waveguide is separated into two waveguides. 
         [0038]    The optical amplifier  408  amplifies the optical signal combined by the optical coupler  407  in order to increase an output signal of the laser. Here, a spot size converter may be integrated in the output terminal of the optical amplifier  408  in order to maximize the output signal of the laser by increasing a ratio of the combination with the outside. 
         [0039]    In addition, the tunable laser module according to the present disclosure may include a temperature controller (not shown) in a substrate (not shown) in order to stabilize the operation characteristic of the tunable laser module. 
         [0040]    Accordingly, while the tunable laser module of  FIG. 3  light-modulates the optical signal output from one reflection unit between the two reflection units  302  and  304  through the Mach-Zehnder optical modulator  201 ,  202 ,  203  and  204  and does not use the optical signal output from the remaining reflection unit, the tunable laser module according to the present disclosure uses all of the optical signals output from the two comb reflection units  403  and  404 . 
         [0041]    Further, while the Y-branch tunable laser module of  FIG. 2  needs to use the additional reflection unit  206  in order to integrate the Mach-Zehnder optical modulator and also requires the optical distributor  102  for the Y-branch and the additional optical distributor  204  for constituting the Mach-Zehnder optical modulator, the tunable laser module according to the present disclosure uses the output terminals of the comb reflection units  403  and  404  as the arms of the Mach-Zehnder optical modulator, thereby achieving the very simple structure. 
         [0042]      FIG. 5  is a diagram illustrating an example of an implementation of a tunable laser module according to an exemplary embodiment of the present disclosure. 
         [0043]    Referring to  FIG. 5 , two sampled grating distributed Bragg reflectors  501  and  502  having different free spectral ranges are used as the two comb reflection units  403  and  404  of the tunable laser module according to the exemplary embodiment of the present disclosure. Here, when one wavelength component of multi wavelength components output from the two sampled grating distributed Bragg reflectors  501  and  502  is reflected in the sampled grating distributed Bragg reflectors  501  and  502 , the one wavelength component is oscillated in the laser. 
         [0044]      FIG. 6  is a diagram illustrating a configuration of a tunable laser module according to another exemplary embodiment of the present disclosure. 
         [0045]    Referring to  FIG. 6 , the tunable laser module according to another exemplary embodiment of the present disclosure includes a light gain area unit  601 , two comb reflection units  602  and  603 , two phase units  604  and  605 , an optical coupler  606  and an optical amplifier  607 . Here, the light gain area unit  601 , the two comb reflection units  602  and  603 , the two phase units  604  and  605 , the optical coupler  606  and the optical amplifier  607  are integrated in one substrate (not shown). 
         [0046]    The light gain area unit  601  outputs an optical signal. 
         [0047]    The two comb reflection units  602  and  603  have the same reflectivity and different free spectral ranges. Here, the two comb reflection units  602  and  603  oscillate a laser by reflecting the optical signal output from the light gain area unit  601  and outputs the optical signal oscillated from the laser. In this case, since the two comb reflection units  602  and  603  have the same reflectivity, output characteristics of the optical signals output from the two comb reflection units  602  and  603  are the same. Accordingly, the two optical signals having the same light strength, the same phase and the same polarized light are input in the two phase units  604  and  605  for constituting the Mach-Zehnder optical modulator. 
         [0048]    Further, each of the two comb reflection units  602  and  603  may include a grating distributed Bragg reflector or a ring resonator. 
         [0049]    The two phase units  604  and  605  change the phases of the optical signals penetrating the two comb reflection units  602  and  603 , so that an output characteristic of the optical coupler  606  is determined when the optical signals penetrating the two comb reflection units  602  and  603  are combined by the optical coupler  606 . Specifically, when the two optical signals having the same light strength, the same phase and the same polarized light are combined by the optical coupler  606 , the optical signal is output from the optical coupler  606  by constructive interference, and when a phase difference between the two optical signals is made to be 180° by using at least one phase unit between the two phase units  604  and  605 , the optical signal is not output in the optical coupler  606  by destructive interference. That is, the two phase units  604  and  605  serve as the Mach-Zehnder optical modulator. 
         [0050]    The optical coupler  606  combines the optical signals of which the phases are changed by the two phase units  604  and  605 . Here, the optical coupler  606  may be a splitter, a gap coupler or a multimode interference coupler in which a waveguide is separated into two waveguides. 
         [0051]    The optical amplifier  607  amplifies the optical signal combined by the optical coupler  606  in order to increase an output signal of the laser. Here, a spot size converter may be integrated in the output terminal of the optical amplifier  607  in order to maximize the output signal of the laser by increasing a ratio of the combination with the outside. 
         [0052]    In addition, the tunable laser module according to the present disclosure may include a temperature controller (not shown) in a substrate (not shown) in order to stabilize the operation characteristic of the tunable laser module. 
         [0053]      FIG. 7  is a diagram illustrating an example of an implementation of a tunable laser module according to another exemplary embodiment of the present disclosure. 
         [0054]    Referring to  FIG. 7 , two ring resonators  701  and  705  having different free spectral ranges are used as the two comb reflection units  602  and  603  of the tunable laser module according to another exemplary embodiment of the present disclosure. 
         [0055]    One ring resonator  701  includes two optical distributors  702  and  703  provided with two input terminals and one output terminal or two input terminals and two output terminals and a phase unit  704  for changing a phase within a curve waveguide and the ring resonator. The other ring resonator  705  includes two optical distributors  706  and  707 , a curve waveguide and a phase unit  708 . Accordingly, the optical signal output from the light gain area unit  601  is reflected by the two ring resonators  701  and  705 , the laser is resonated in a wavelength matching to the reflective wavelengths of the two ring resonators  701  and  705 , and the phase of the optical signal is varied by using the phase units  704  and  708  included in the two ring resonators  701  and  705 , so that an output wavelength of the laser may be changed by changing the wavelength matching with the reflective wavelength. 
         [0056]    Further, one output terminal between the two output terminals of the optical distributors  702  and  707  constituting the ring resonators  701  and  705  may be connected to the phase units  604  and  605  constituting the Mach-Zehnder optical modulator, and the other output terminal may be connected to a sensor for monitoring the output of the laser or an optical absorber in order to prevent the output characteristic of the laser from being deteriorated by internal reflection. 
         [0057]    The exemplary embodiments disclosed in the present disclosure are intended to illustrate the scope of the technical idea of the present disclosure, and the scope of the present disclosure is not limited by the embodiment. The scope of the present disclosure shall be construed on the basis of the accompanying claims so that all of the technical ideas included within the scope equivalent to the claims belong to the present disclosure. 
         [0058]    From the foregoing, it will be appreciated that various embodiments of the present disclosure have been described herein for purposes of illustration, and that various modifications may be made without departing from the scope and spirit of the present disclosure. Accordingly, the various embodiments disclosed herein are not intended to be limiting, with the true scope and spirit being indicated by the following claims.