Abstract:
Disclosed is an external cavity tunable laser module including a substrate; a mirror surface that is formed on the substrate to reflect a laser incoming from the outside; a transmissive liquid crystal filter that is formed at a rear side of the mirror surface to select and tune a wavelength of the laser reflected through the mirror surface; and a light source chip that is formed at a rear side of the transmissive liquid crystal filter to reflect the laser that passes through the transmissive liquid crystal filter at a specific wavelength interval to form a plurality of channels and tune wavelengths of the channels.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and claims priority from Korean Patent Application No. 10-2011-0140235, filed on 2011 Dec. 22, 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 specifically, to an external cavity tunable laser module that has a high degree of integration for fast modulation and high output and includes a light source chip, a transmissive liquid crystal filter, and a mirror surface. 
       BACKGROUND 
       [0003]    As the importance of a wavelength division multiplexing-passive optical network (WDM-PON) that is capable of providing a large amount of communication service by wavelength division is increased, development of a light source which is used for the optical transmission network becomes important. The WDM-PON requires a fast modulation tunable laser module that minutely tunes and modulates a wavelength of channels at high speed while tuning the wavelength of channels having a predetermined wavelength interval. 
         [0004]    An example of a representative fast modulation tunable laser module which has been suggested so far includes a tunable laser module that uses a sampled grating distributed Bragg reflector (SG-DBR) disclosed in U.S. Pat. No. 4,896,325. The tunable laser module has a structure in which a gain section and a phase shift section are integrated between two SG-DBRs to form laser tuning and then an optical modulator is integrated at an end of one of two SG-DBRs to modulate an optical signal output from the SG-DBR. The tunable laser module that uses the SG-DBR uses a Vernier effect by two SG-DBRs in order to improve a DBR structure having a narrow tunable range of 10 nm or lower. Therefore, the tunable laser module that uses the SG-DBR requires various control circuits such as a Vernier control circuit, a control circuit for discontinuous wavelength shift, and a control circuit for a phase shift section. Thus, the control of the tunable laser module is very complex and it is hard to obtain a stable output wavelength. 
         [0005]    In the meantime, in order to substitute for the tunable laser module using the SG-DBR, a tunable laser module using two ring resonators having lightly different free spectral ranges (FSR) is announced (paper: PHOTONICS TECHNOLOGY LETTERS, Vol. 14, No. 5, p600, 2002, IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 21, No. 13, p851, 2009, IEEE Journal of Lightwave Technology, Vol. 24, No. 4, p1865, 2006). In the tunable laser module, one of ring resonators have refractive index fixed and the other one have refractive index variable so that an output wavelength of the laser is varied by an interval of the FSR. However, since the FSR difference between the two ring resonators is very small, it is very difficult to control in order to guarantee stabilization of the output wavelength of the laser. 
         [0006]    A planar lightwave circuit (PLC) based external cavity tunable laser module is an external cavity tunable laser module that combines the PLC to a reflective superluminescent diode (R-SLD) to obtain a good mass production. If a polymer based PLC is used instead of silica based PLC, since a thermo-optic coefficient of the polymer is very high, a wider wavelength region may be tuned. 
         [0007]    A polymer based external cavity tunable laser module that has a 2.5 Gbps modulation speed by direct modulation is described in detail in a recently announced paper (paper: OPTICS EXPRESS, Vol. 18, No. 6, p 5556, 2010). The polymer based external cavity tunable laser module has a semi insulating buried hetero structure which may reduce a parasitic capacitance of an R-SLD used to obtain a gain in the external cavity tunable laser module in order to have a 2.5 Gbps or higher modulation speed by the direct modulation, and a length of the R-SLD needs to be very short. Therefore, the manufacturing process of the R-SLR is very difficult and the packaging process of the external cavity tunable laser module is also complex. 
         [0008]    Even though an external cavity tunable laser module using a reflective liquid crystal filter that uses a diffraction grating is announced (paper: IEEE Photonics Technology letters, vol. 19, no. 14, pp. 1099-1101), the laser module has a structure in that the laser resonance is generated by a single side reflection of a gain chip. Therefore, no other optical elements are integrated in the gain chip. Even though there is an attempt to implement a high integration gain chip by making a gap in the gain chip, but it is difficult to control the reflectance and transmittance through the gap. 
         [0009]    There are an external cavity tunable laser module that uses a fiber Bragg grating (FBG) having a modulation speed of 10 Gbps through direct modulation (paper: IEEE Photonics Technology letters, vol. 10, no. 12, pp. 1691-1693, IEE Electronics Letters, vol. 35, no. 20, pp. 1737-1738) and an external cavity tunable laser module (RIO corporation, USA) using silica in which a Bragg grating is formed. However, the external cavity tunable laser modules use the silica, and thus may be not used as a tunable laser module. 
         [0010]    In the polymer based external cavity tunable laser module in the related art, the laser is tuned between the polymer Bragg grating reflector and the R-SLD so that an optical modulator is not integrated in the external cavity tunable laser module but an expensive external optical modulator for fast modulation is required. 
       SUMMARY 
       [0011]    The present disclosure has been made in an effort to provide an external cavity tunable laser module that has a low power, a wide wavelength tunable wavelength range, and a high wavelength tunable speed and performs fast modulation. 
         [0012]    The present disclosure also has been made in an effort to provide an external cavity tunable laser module having a stable laser output characteristic. 
         [0013]    An exemplary embodiment of the present disclosure provides an external cavity tunable laser module including a substrate; a mirror surface that is formed on the substrate to reflect a laser incoming from the outside; a transmissive liquid crystal filter that is formed at a rear side of the mirror surface to select and tune a wavelength of the laser reflected through the mirror surface; and a light source chip that is formed at a rear side of the transmissive liquid crystal filter to reflect the laser that passes through the transmissive liquid crystal filter at a specific wavelength interval to form a plurality of channels and tune wavelengths of the channels. 
         [0014]    According to exemplary embodiments of the present disclosure, by providing an external cavity tunable laser module including a light source chip having a ring resonator and a transmissive liquid crystal filter, it is possible to minutely tune wavelengths of channels while tuning the wavelengths of channels having a predetermined wavelength interval. Further, a wide tunable range is provided at a low power consumption and a fast wavelength tunable speed is provided. Further, the ring resonator in the light source chip functions as an etalon filter so that the etalon filter does not need to be inserted at an output terminal of the laser module. 
         [0015]    By providing an external cavity tunable laser module including a light source chip in which an optical modulating unit and an optical amplifying unit are integrated in one body, an external cavity tunable laser module that allows a fast modulation and a high output is provided. 
         [0016]    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 
         [0017]      FIGS. 1 and 2  are a side view and a plan view of an external cavity tunable laser module according to an exemplary embodiment of the present disclosure. 
           [0018]      FIGS. 3 and 4  are views illustrating a front surface and an upper surface of a transmissive liquid crystal filter according to an exemplary embodiment of the present disclosure. 
           [0019]      FIG. 5  is a graph illustrating a transmission characteristic of a transmissive liquid crystal filter according to an exemplary embodiment of the present disclosure. 
           [0020]      FIG. 6  is a graph illustrating a transmission characteristic when a laser passes back and forth through a transmissive liquid crystal filter by the reflection of a mirror surface. 
           [0021]      FIG. 7  is a graph illustrating a wavelength tunable characteristic of a transmissive liquid crystal filter according to an exemplary embodiment of the present disclosure. 
           [0022]      FIGS. 8 and 9  are a plan view and a side view illustrating a structure of a light source chip according to an exemplary embodiment of the present disclosure. 
           [0023]      FIGS. 10 and 11  are plan views illustrating a configuration of an external cavity tunable laser module according to another exemplary embodiment of the present disclosure. 
           [0024]      FIGS. 12 to 14  are views illustrating an external electrode arrangement of an external cavity tunable laser module according to an exemplary embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    In the fallowing detailed description, reference is made to the accompanying drawing, which form a part hereof. The illustrative embodiments described in the detailed description, drawing, 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.|[K1] 
         [0026]      FIGS. 1 and 2  are a side view and a plan view of an external cavity tunable laser module according to an exemplary embodiment of the present disclosure. 
         [0027]    Referring to  FIGS. 1 and 2 , the external cavity tunable laser module includes a mirror surface  110 , a transmissive liquid crystal filter  120 , and a light source chip  140 . 
         [0028]    The mirror surface  110  and the transmissive liquid crystal filter  120  are actively aligned with the light source chip  140  by a first lens  130 , and the light source chip  140  is actively aligned with an optical fiber  180  by a second lens  170 . In this case, the transmissive liquid crystal filter  120  is mounted so as to be inclined at 1 to 10 degrees from an optical axis in order to reflect and remove unnecessary wavelength components other than a wavelength that penetrates through the transmissive liquid crystal filter  120 . 
         [0029]    The external cavity tunable laser module according to the exemplary embodiment of the present disclosure further includes a temperature control unit  160  below the transmissive liquid crystal filter  120  and the light source chip  140  in order to stabilize an output of the laser when the output characteristic of the laser is changed depending on the temperature change of the transmissive liquid crystal filter  120  and the light source chip  140 . 
         [0030]    The external cavity tunable laser module according to the exemplary embodiment of the present disclosure may further include an RF connector  190  that applies an electric signal having a high modulation speed to an optical modulating unit  144  which will be described below. 
         [0031]    The external cavity tunable laser module according to the exemplary embodiment of the present disclosure further includes a U shaped structure  150  in order to fix the lenses  130  and  170  and the light source chip  140 . Here, the U shaped structure  150  may be formed of a metal material such as SUS having a good processability. Even though not illustrated, the external cavity tunable laser module according to the exemplary embodiment of the present disclosure may further include a structure having a high thermal conductivity between the U shaped structure  150  and the light source chip  140  in order to efficiently transmit the heat of the light source chip  140 . 
         [0032]      FIGS. 3 and 4  are views illustrating a front surface and an upper surface of a transmissive liquid crystal filter according to an exemplary embodiment of the present disclosure. 
         [0033]    Referring to  FIGS. 3 and 4 , the transmissive liquid crystal filter  120  according to the exemplary embodiment of the present disclosure is configured by a liquid crystal material  122  filled between two glass plates  124  and  126  having a high reflectance and electrodes  128  attached on the two glass plates  124  and  126 . 
         [0034]    The transmissive liquid crystal filter  120  uses a Fabry-Perot etalon effect so that a free spectral range (FSR) is determined by an interval between the two glass plates  124  and  126  and a refractive index of the liquid crystal material  122  to determine a tunable wavelength range. In other words, if a voltage is applied to the electrodes  120  which are attached on the two glass plates  124  and  126 , the refractive index of the liquid crystal material  122  is changed to change the FSR of the transmissive liquid crystal filter  120  so that the transmissive liquid crystal filter  120  tunes a wavelength of the laser. 
         [0035]      FIG. 5  is a graph illustrating a transmission characteristic of the transmissive liquid crystal filter according to the exemplary embodiment of the present disclosure and  FIG. 6  is a graph illustrating a transmission characteristic when a laser passes back and forth through the transmissive liquid crystal filter by the reflection of the mirror surface. 
         [0036]    As illustrated in  FIGS. 5 and 6 , when the laser passes back and forth through the transmissive liquid crystal filter  120  by the reflection of the mirror surface  110 , a full width half maximum (FWHM) of the transmissive liquid crystal filter  120  is reduced. For example, when a full width half maximum of the transmissive liquid crystal filter  120  is 1.4 nm, if the laser passes back and forth through the transmissive liquid crystal filter  120  by the reflection of the mirror surface  110 , the a full width half maximum of the transmissive liquid crystal filter  120  becomes 0.9 nm. Therefore, if the mirror surface  110  and the transmissive liquid crystal filter  120  are used as described in the exemplary embodiment of the present disclosure, the transmissive liquid crystal filter  120  has a narrow full width half maximum. 
         [0037]      FIG. 7  is a graph illustrating a wavelength tunable characteristic of the transmissive liquid crystal filter according to the exemplary embodiment of the present disclosure. 
         [0038]    As illustrated in  FIG. 7 , if a voltage is applied to the transmissive liquid crystal filter  120  from the outside, the refractive index of the transmissive liquid crystal filter  120  is changed by a field effect so that a transmitting wavelength is changed. Accordingly, if the transmitting wavelength of the transmissive liquid crystal filter  120  is changed by the field effect, the power consumption for tuning the wavelength is very lowered and a wavelength tunable speed is very increased. 
         [0039]      FIGS. 8 and 9  are a plan view and a side view illustrating a structure of the light source chip according to an exemplary embodiment of the present disclosure. 
         [0040]    Referring to  FIGS. 8 and 9 , the light source chip  140  according to the exemplary embodiment of the present disclosure is coupled to lenses  130  and  170  on the U shaped structure  150  and includes a phase shifter  141 , a gain unit  142 , a com reflecting unit, an optical modulating unit  145 , and an optical amplifying unit  146 . 
         [0041]    The phase shifter  141  minutely adjusts an output wavelength oscillated from the laser and stabilizes the wavelength. 
         [0042]    The gain unit  142  provides a gain for laser oscillation. 
         [0043]    The com reflecting unit according to the exemplary embodiment of the present disclosure includes an optical coupling unit  143  and a ring resonator  144 . Two input terminals of the ring resonator  144  are coupled to two output terminals of the optical coupling unit  143  to reflect the laser at a specific wavelength interval. One of the two output terminals outputs the laser and the other one outputs a reflection signal. In this case, since the reflection signal generated from the other output terminal of the ring resonator  144  lowers the output characteristics of the laser, the reflection signal is removed by an absorbing unit  147 . 
         [0044]    Accordingly, the external cavity tunable laser module according to the exemplary embodiment of the present disclosure forms laser resonance for oscillating the laser by the reflection by the mirror surface  110  and the transmissive liquid crystal filter  120  and the reflection by the com reflecting unit. Accordingly, the external cavity tunable laser module according to the exemplary embodiment of the present disclosure uses the transmissive liquid crystal filter  120  and the com reflecting unit together so as to output the more stable single wavelength component at a specific wavelength interval as compared with the external cavity tunable laser module that uses only the transmissive liquid crystal filter  120 . 
         [0045]    The com reflecting unit according to the exemplary embodiment of the present disclosure further includes a minute phase shift  148  to minutely change the wavelength output from the external cavity tunable laser module while changing the phase of the com reflecting unit. 
         [0046]    In the meantime, if the reflectance of the input terminal and the output terminal of the light source chip  140  is high, an internal reflection mode is generated by the internal reflection, which affects the stability of the output wavelength of the external cavity tunable laser module and deteriorates the performance of the external cavity tunable laser module due to the internal damage. Therefore, in the exemplary embodiment, in order to reduce the reflectance of the input terminal and the output terminal of the light source chip  140 , the input terminal and the output terminal are non-reflectively coated and waveguides of the input terminal and the output terminal are inclined so that the reflectance becomes 0.1% or lower. As described above, if the waveguides of the input terminal and the output terminal of the light source chip  140  are inclined, the optical axes of the input terminal and the output terminal of the light source chip  140  are varied. Therefore, the position of the first lens  130  that aligns the transmissive liquid crystal filter  120  and the light source chip  140  and the position of the second lens  170  that aligns the light source chip  140  and the optical fiber  180  are varied. 
         [0047]    In the case of a general external cavity tunable laser module, mode hopping is generated by the external resonance mode to change the output wavelength of the laser. In contrast, the external cavity tunable laser module according to the exemplary embodiment of the present disclosure includes the phase shift  141  in the light source chip  140  in order to compensate the change in the output wavelength of the laser to stabilize the output characteristic of the laser. 
         [0048]      FIGS. 10 and 11  are plan views illustrating a configuration of an external cavity tunable laser module according to another exemplary embodiment of the present disclosure. 
         [0049]    Referring to  FIG. 10 , the external cavity tunable laser module according to the exemplary embodiment of the present disclosure uses a spectrometer  1010  to diverge a part of laser output from the output terminal of the light source chip  140  and uses a monitor detector (photo detector: PD)  1020  to measure an intensity of the diverged laser to control the output characteristic. 
         [0050]    Referring to  FIG. 11 , the external cavity tunable laser module according to the exemplary embodiment of the present disclosure includes a monitor detector  1110  between the transmissive liquid crystal filter  120  and the first lens  130  and uses the monitor detector  1110  to detect a laser reflected from the transmissive liquid crystal filter  120  disposed at an angle to control the output characteristic. In other words, when the laser is oscillated by the light source chip  140 , the transmissive liquid crystal filter  120 , and the mirror surface  110 , the external cavity tunable laser module detects the oscillated laser by the monitor detector  1110  to control the output characteristic. Therefore, the external cavity tunable laser module of  FIG. 11  does not diverge the output, which is different from the external cavity tunable laser module of  FIG. 10 , so that the output characteristic is controlled without losing the output of the laser. 
         [0051]      FIGS. 12 to 14  are views illustrating an external electrode arrangement of an external cavity tunable laser module according to an exemplary embodiment of the present disclosure. 
         [0052]    Referring to  FIG. 12 , in the external cavity tunable laser module of  FIG. 2 , as external electrodes,  15  electrode pins  192  and one RF connector  190  are disposed around the external cavity tunable laser module with an regular interval. In contrast, in the external cavity tunable laser module of  FIG. 12 , an external electrode  1210  is disposed on one side of the external cavity tunable laser module. 
         [0053]    As illustrated in  FIGS. 13 and 14 , the external electrode  1310  may be disposed at a rear side of the external cavity tunable laser module. Such arrangement of the external electrode  1310  may reduce the width of the external cavity tunable laser module, which is suitable for a small-sized laser module. 
         [0054]    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.