Abstract:
Provided is an athermal external cavity laser (ECL), whose output optical power and output wavelength can be kept regular irrespective of temperature changes without using additional temperature controlling components. The ECL comprises: a semiconductor amplifier; an optical fiber comprising a core in which a Bragg grating is formed and a cladding surrounding the core; and a thermosetting polymer that fixes the optical fiber to a ferrule and has a negative thermooptical coefficient, wherein the thickness of the cladding surrounding the core in which the Bragg grating is formed is smaller than the portion of the cladding surrounding the portion of the core where the Bragg grating is not formed, and the thermosetting polymer the negative thermooptical coefficient surrounds the cladding. The ECL does not need additional temperature controlling components and thus can be manufactured compact and at low cost, and thus can be used as a light source of a dense wavelength division multiplexing (DWDM) system in designing economical WDM passive optical networks (PON).

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This application claims the benefit of Korean Patent Application No. 10-2005-0119063, filed on Dec. 7, 2005 and Korean Patent Application No. 10-2006-0071649, filed on Jul. 28, 2006, in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a wavelength division multiplexing (WDM) passive optical network (PON) system, and more particularly, to an external cavity laser (ECL) used as a light source in the WDM PON system. 
         [0004]    2. Description of the Related Art 
         [0005]    In a general wavelength division multiplexing (WDM) system, the denser the channel spacing, the greater the number of channels. Accordingly, the allowable range of wavelength error of a light source that may occur with respect to the external temperature is strictly limited in the WDM system. In most WDM systems, distributed feedback-laser diode (DFB-LD) optical transmission modules are used, which are prepared in an expensive butterfly type package in which temperature controlling components such as a thermoelectric cooler (TEC) or a thermistor are mounted. 
         [0006]    For example, in an optical transmission module used for dense wavelength division multiplexing (DWDM) having a channel spacing of 50 GHz to 100 GHz (0.4 nm or 0.8 nm), the oscillation wavelength should be stabilized such that crosstalk due to interference with adjacent channels, without being affected by the variation of external temperature, is not over an allowable range, and moreover, the optical output passing through the wavelength band of multiplexing/demultiplexing used in the network should be stabilized. For this purpose, a butterfly package in which temperature controlling components such as a TEC, a thermistor, etc. are mounted is used to keep the temperature of an optical device uniform. 
         [0007]    However, since controlling components such as a TEC, a thermistor, a heat dissipating plate, a temperature controlling circuit, etc. need to be additionally attached to the DFB lasers, and an expensive butterfly type package should be selected, the amount by which the price of the optical transmission module can be reduced reaches a certain limit. Also, since it is difficult to apply the optical transmission modules including a monitoring photodetector and a wavelength fixing unit to a PON, the greatest advantage of which is its price competitiveness, a monitoring photodetector and a wavelength fixing unit should be added to stabilize the oscillation and the optical output, and this raises the price of the optical transmission modules including a monitoring photodetector and a wavelength fixing unit. Therefore, in order to replace the expensive DFB-LD, an external cavity laser is suggested as a light source. 
         [0008]      FIG. 1A  is a cross-sectional view of a conventional external cavity laser (ECL) using a Bragg grating in a TO-CAN package. 
         [0009]    Referring to  FIG. 1A , the ECL includes a semiconductor amplifier  10  as an amplifying material, a focusing lens  20 , and an optical fiber  30  of a core  32  in which a Bragg grating  34  is formed and a cladding  36  surrounding the core  32 . The Bragg grating  34  of the optical fiber  30  is stable against temperature variations, and a rear surface  11  of the semiconductor amplifier  10  and the Bragg grating  34  form an ECL. Referring to  FIG. 1A , the length of the external cavity L cavity  forming an external cavity is denoted as a double-sided arrow. 
         [0010]    The optical fiber  30  including the Bragg grating  34  is fixed in a ferrule  50  by thermosetting epoxy  40  to form an optical fiber structure  60  forming a TO-CAN package. The rear surface  11  of the semiconductor amplifier  10  is generally high reflection (HR)-coated, and a front surface  12 , that is, an exit surface, may be anti-reflection (AR)-coated. 
         [0011]    The focusing lens  20  is used to improve the optical combination efficiency of the semiconductor amplifier  10  and the optical fiber  30 . A cross-section  37  of the optical fiber  30  is inclined at a predetermined angle to the perpendicular of an optical path P optic  to reduce residual reflection on the cross-section  37 . The optical path P optic  of the laser is denoted with a dotted line in a cavity  70 . When an external cavity is formed in the above structure, wavelengths reflected in the Bragg grating  34  among the wavelengths satisfying the phase matching conditions are oscillated and then output. 
         [0012]      FIG. 1B  is a cross-sectional view illustrating a portion I-I of  FIG. 1A , wherein the core  32 , the cladding  36 , the thermosetting epoxy  40 , and the ferrule  50  are concentrically stacked. 
         [0013]    The wavelength stability with respect to the external temperatures of the ECL in which the DFB-LD is TO-CAN packaged is as follows. In the case of the DFB laser in which the grating determining the oscillation wavelength is in a semiconductor gain region, the manufacturing cost is low, but when the temperature is not controlled, the thermooptical coefficient (∂n LD /∂T) of the semiconductor material is about 2.4×10 −4 /K, and the oscillation wavelength varies by 0.1 nm per 1° C. according to Equation 1, where n LD  refers to the refractive index of the semiconductor material of the DFB laser. 
         [0000]      ∂λ/∂ T=λ (∂ n/∂T )/ n   [Equation 1] 
         [0014]    In order to improve the wavelength stability of the light source, in the case of an ECL in which the grating is carved not in a semiconductor gain material but in an optical fiber having a low thermooptical coefficient, the wavelength stability can be improved to 0.01 nm per 1° C. 
         [0015]    However, examining the wavelength spectrums with respect to the temperature when the ECL oscillates in a single mode, mode hopping occurs. Here, mode hopping refers jumping of oscillation wavelengths at a predetermined external temperature. 
         [0016]      FIGS. 2A through 2C  are graphs illustrating mode hopping when the ECL oscillates in a single mode. 
         [0017]      FIG. 2A  is a graph showing external cavity modes  92  determined by the phase matching condition of the external cavity and the reflection spectrums  94  determined by the Bragg grating of the optical fiber. The graphs show that an m-th mode sensing the highest reflectivity in the reflection spectrums  94  among the external cavity modes  92  is the oscillation mode  90  of the ECL. 
         [0018]      FIG. 2B  is a graph showing the external cavity modes  110  and the reflection spectrums  120  varying according to the increase of temperature. Referring to  FIG. 2B , when a current is applied for a long time or the temperature of the ECL is increased due to changes in the external environment, the external cavity modes  92  and the reflection spectrums  94  change to first shifting external cavity modes  93  and first shifting reflection spectrums  95 , and accordingly, the oscillation modes  90  of the ECL change to first shifting oscillation modes  91 . 
         [0019]    Here, a shifting distance (Δm1) of the external cavity modes is represented by ∂λ ECL /∂T, and the shifting distance is determined by the thermooptical coefficient of the materials forming the ECL and the length of the optical path according to Equation 2. 
         [0000]      ∂λ ECL   /∂T =λ(Σ(∂ n   i   /∂T ) L   i )/Σ∂ n   i   L   i   [Equation 2] 
         [0020]    Also, since a shifting distance (ΔR1) of the reflection spectrum is proportional to the thermooptical coefficient of the light waveguide in which the Bragg grating is carved, thus the shifting distance (ΔR1) of the reflection spectrum can be represented by ∂λ WBG /∂T. The index WBG refers to a waveguide Bragg grating. Accordingly, in a predetermined temperature range, the first shifting oscillation mode  91  of the final output wavelength becomes the m-th mode, since the shifting distance (ΔR1) of the reflection spectrum is similar to the shifting distance (Δm1) of the external cavity modes, and the mode sensing the highest reflectivity is the m-th mode. 
         [0021]    In  FIG. 2C , the ECL has a higher temperature than in  FIG. 2B , and the external cavity modes  92  and the reflection spectrums  94  are respectively changed into second shifting external cavity modes  93   a  and second shifting reflection spectrums  95   a.  However, since an (m−1)th mode of the oscillation modes senses greater reflectivity than an m-th mode, the oscillation mode of the final output wavelength of the ECL does not become the first shifting oscillation mode  91   a  but a second shifting oscillation mode  96 . Thus, a change in an oscillation mode according to external temperature changes is called mode hopping (H m ), and such mode hopping happens periodically according to external temperature changes. 
         [0022]      FIG. 3  is a graph showing the movement of the oscillation wavelength according to the temperature and mode hopping which occurs periodically according to the temperature changes. 
         [0023]    Referring to  FIG. 3 , the oscillation wavelength moves to the long wavelength band gradually as the temperature of the ECL increases, and the mode hopping occurs at points with a regular period  97  determined by Equation 3. 
         [0000]      Δ T=δλ   ECL /[( dλ/dT ) ECL −( dλ/dT ) WBG ]  [Equation 3] 
         [0024]    The wavelength interval of the mode hopping is an interval between the external cavity modes determined by the optical path of the external cavity. The wavelength change  99  according to the temperature of the Bragg grating is ∂λ WBG /∂T, and the wavelength change  98  according to the temperature of a substantial oscillation wavelength due to mode hopping is ∂λ ECL /∂T. 
         [0025]    In the case of the ECL operating in a single mode, the output optical power of the ECL in the mode hopping region in which the oscillation wavelength is rapidly converted is known to change by 50% or more. Such rapid change in optical power not only rapidly deteriorates the transmission quality of the WDM PON but also rapidly deteriorates long-term reliability of a device. 
         [0026]    In order to solve the mode hopping of the ECL operating in a single mode, the reflection spectrum of the Bragg grating should be kept regular regardless of temperature, and the external cavity modes should not be affected by changes in temperature. 
         [0027]    Meanwhile, in the case of the ECL operating in a multi-mode, the variation in the output optical power by mode hopping is minimal compared to a single mode. This is because the total of the output optical power of each mode is kept regular even if individual oscillation modes experience rapid light output changes during mode hopping. Accordingly, the oscillation wavelengths output from the ECL operating in a multi-mode can be designed irrespectively of external temperature changes. Also, in a multi-mode, since the width of a spectrum is broader than in a single mode, it is influenced by dispersion during transmission, and thus to reduce the influence by dispersion, the spacing and number of the oscillation modes need to be controlled. 
       SUMMARY OF THE INVENTION 
       [0028]    The present invention provides an athermal external cavity laser (ECL), whose output optical power and output wavelength can be kept regular regardless of the external temperature without using additional temperature controlling components, so as to reduce the manufacturing costs of optical transmission modules used in wavelength division multiplexing (WDM) passive optical networks (PON). 
         [0029]    According to an aspect of the present invention, there is provided an athermal external cavity laser (ECL) comprising: a semiconductor amplifier; an optical fiber comprising a core in which a Bragg grating is formed and a cladding surrounding the core; and a thermosetting polymer that fixes the optical fiber to a ferrule and has a negative thermooptical coefficient, wherein the thickness of the cladding surrounding the core in which the Bragg grating is formed is smaller than that of the cladding surrounding region of the core in which the Bragg grating is not formed, and the thermosetting polymer surrounds the cladding. 
         [0030]    Reflection spectrums of the Bragg grating may be kept athermal by the thermosetting polymer, and the output optical power of the ECL may be athermal. 
         [0031]    The thickness of the cladding around the Bragg grating may be 0.2 to 0.4 μm, the thermooptical coefficient of the thermosetting polymer may be −1×10 −4 /deg to −1.6×10 −4 /deg, and the refractive index of the thermosetting polymer may be 1.43 to 1.445. 
         [0032]    The athermal ECL may further comprise an optical path compensator between the semiconductor amplifier and the optical fiber. The athermal ECL may be a single-mode ECL, and the optical path compensator may make the length of the optical path in which the external cavity is formed athermal, so as to make the output wavelength of the ECL athermal. 
         [0033]    The thermooptical coefficient of the optical path compensator may be −1×10 −4 /deg to −2.5×10 −4 /deg, and the length of the optical path of the optical path compensator may be 500 to 2000 μm. A front surface and a rear surface of the optical path compensator may be anti-reflection (AR) coated, and the front surface and the rear surface may be inclined at a predetermined angle with respect to the perpendicular of the optical path. The inclination angle may be 1 to 3°. 
         [0034]    A cross-section of the optical fiber may be coated AR-coated; or inclined at 8° to the perpendicular of the optical path; or may be AR-coated and inclined at 8° to the perpendicular of the optical path. 
         [0035]    According to another aspect of the present invention, there is provided an athermal ECL comprising: a semiconductor amplifier; an optical fiber fixed to a ferrule by a thermosetting polymer; and a thin-film multi-layer (TFML) transmissive filter formed between the semiconductor amplifier and the optical fiber. 
         [0036]    An external cavity may be formed between the semiconductor amplifier and a cross-section of the optical fiber toward the semiconductor amplifier, and the output optical power of the ECL may be athermal. The cross-section of the optical fiber may be 3 to 5%; or 20 to 50% when a metal having a thickness of 0.1 μm or smaller is coated on the cross-section of the optical fiber; or 95% or smaller when a silicon oxide (SiO 2 ) or a metal oxide thin layer is coated on the cross-section of the optical fiber. 
         [0037]    The TFML transmissive filter may have a stack structure in which, among the silicon oxide and the metal oxide layers, at least two types of thin layers having different refractive indexes and thicknesses are alternately stacked. The stack may be formed on a glass substrate, and the degree of change of the transmission wavelength according to the temperature of the TFML transmissive filter may be smaller than 0.01 nm/deg. The degree of change of the transmission wavelength may be 0.003 nm/deg or smaller according to the temperature change. 
         [0038]    A front surface and a rear surface of the TFML transmissive filter may be AR-coated, and the front surface and the rear surface may be inclined at a predetermined angle with respect to the perpendicular of the optical path. 
         [0039]    The athermal ECL may further comprise an optical path compensator between the semiconductor amplifier and the TFML transmissive filter or between the TFML transmissive filter and the optical fiber. The ECL may be a single-mode ECL, and the optical path compensator may make a length of the optical path, in which external cavity is formed, athermal, so as to make the output wavelength of the ECL athermal. 
         [0040]    The athermal ECL may further comprise a focusing lens for improving the optical combination efficiency of the semiconductor amplifier and the optical fiber, wherein a front surface of the semiconductor amplifier, which is an exit surface through which laser light is oscillated, is AR-coated, and a rear surface of the semiconductor amplifier is high-reflection (HR) coated, and an external cavity may be formed between the rear surface of the semiconductor amplifier and the Bragg grating. 
         [0041]    The exit surface of the semiconductor amplifier may have a reflectivity of 1×10 −3  or smaller. The semiconductor amplifier may include a down-tapered optical mode converter, and a far field angle of light which is output from the exit surface is 25 degrees or less in a perpendicular direction and in a horizontal direction. 
         [0042]    In order to solve the problem of mode hopping of the ECL, the composition of the material of a waveguide, such as an optical fiber including a grating, is changed or the structure of the waveguide is modified such that reflection spectrums are not affected by the temperature to keep the output optical power regular. Also, a length of an external cavity of the ECL is fixed to fix the external cavity modes not affected by the temperature to keep the output wavelength regular. 
         [0043]    Accordingly, since the output optical power and the output wavelength of the ECL can be kept regular athermally without using additional temperature controlling components, the wavelength can be utilized to the highest extent, which is the advantage of the WDM system, and the light source module can be manufactured at low cost. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0044]    The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
           [0045]      FIG. 1A  is a cross-sectional view of a conventional external cavity laser (ECL) using a Bragg grating in a TO-CAN package; 
           [0046]      FIG. 1B  is a cross-sectional view of a portion I-I of  FIG. 1A ; 
           [0047]      FIGS. 2A through 2C  are graphs explaining mode hopping when the ECL oscillates in a single mode; 
           [0048]      FIG. 3  illustrates mode hopping according to the temperature; 
           [0049]      FIGS. 4A and 4B  are cross-sectional views of an ECL according to an embodiment of the present invention; 
           [0050]      FIGS. 5A through 5D  are graphs illustrating conditions of the Bragg grating used in the ECL in  FIGS. 4A and 4B ; 
           [0051]      FIG. 6  is a cross-sectional view of an ECL according to another embodiment of the present invention; 
           [0052]      FIG. 7  is a cross-sectional view of an ECL according to another embodiment of the present invention; and 
           [0053]      FIG. 8  is a cross-sectional view of an ECL according to another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0054]    The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. 
         [0055]    It will be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. In the drawings, the thicknesses of layers and regions are exaggerated for clarity and portions not related to the description are omitted. Like reference numerals in the drawings denote like elements. The terms used are for illustrative purpose of the present application only and are not intended to limit the scope of the present invention described in the claims. 
         [0056]    For the output optical power to be athermal, reflection spectrums of a Bragg grating should be regular, and for this, the wavelength change according to the temperature of a Bragg grating  99  in  FIG. 3 , that is, ∂λ WBG /∂T, should be lowered. ∂λ WBG /∂T is reduced by varying the valid refractive index of the Bragg grating by carving a grating in a material having a low thermooptical coefficient or by modifying the structure of a light waveguide in which a grating is carved. 
         [0057]      FIGS. 4A and 4B  are cross-sectional views of an ECL  1000  according to an embodiment of the present invention, the output optical power of which is athermal. 
         [0058]    Referring to  FIG. 4A , the ECL  1000  in the current embodiment of the present invention includes a semiconductor amplifier  100 , a focusing lens  200 , and an optical fiber  300  including a core  320  in which a Bragg grating  340  is formed and a cladding  360  surrounding the core  320 . 
         [0059]    The thickness of the cladding  360  surrounding the core  320  around the Bragg grating  340  is smaller than the thickness of the other portion of the cladding  360 . That is, the thickness of an etched portion  380  of the cladding  360  is smaller than the other portion of the cladding  360 . Also, a thermosetting polymer  400  surrounding the cladding  360  has an appropriate thermooptical coefficient, for example, a negative thermooptical coefficient. The thickness t of the cladding  360  and the thermooptical coefficient of the thermosetting polymer  400  will be described in more detail with reference to  FIG. 5  as follows. 
         [0060]    The ECL  1000  in the current embodiment of the present invention is packaged in a low-priced TO-CAN package, and a length L cavity  of the optical path in which an external cavity is formed starts from a rear surface  110  of the semiconductor amplifier  100  and ends at a Bragg grating  340  of the optical fiber  300 . The semiconductor amplifier  100  may be formed of an integrated spot size converter (SSC) improving the optical coupling efficiency of the optical fiber, and an active region in which light is generated when a current is applied from the outside. Also, the active region of the semiconductor amplifier  100  may be a multi-quantum well structure. When a current is applied from the outside, the generated light is coupled through a front surface, that is, an exit surface  120 , and transmitted to a cross-section  370  of an optical fiber. 
         [0061]    Meanwhile, a rear surface  110  of the semiconductor surface may be HR-coated and the front surface, that is the exit surface  120 , may be AR-coated. The degree of AR coating of the exit surface, that is, the front surface  120 , plays an important role for the performance of an ECL, and the reflectivity of the AR-coated front surface  120  may be 10 −3  or less. Also, the cross-section  370  of the optical fiber  300  may be AR-coated or may be inclined at 8° to the perpendicular of the optical path in order to prevent a decrease in the performance of the device by residual reflection. Also, the cross-section  370  of the optical fiber  300  may be both AR-coated and be inclined at 8°. 
         [0062]    A light waveguide of the SSC of the semiconductor amplifier  100  may be down-tapered such that the size of the mode of the light generated in the active region of the semiconductor amplifier  100  increases gradually toward the exit surface  120 . The dispersion angle or the far-field angle of the light output to the exit surface  120  may be 25° or less in perpendicular/horizontal directions. 
         [0063]    Generally, when a Bragg grating is carved in an optical fiber, reflection spectrums of the Bragg grating  340  move by 0.01 nm per 1° C. of an external temperature change. To compensate this, a portion of the cladding  360  around the Bragg grating  340  of the optical fiber  300  is removed, and the cladding  360  can be inserted into a ferrule  500 . A space between the cladding  360  and ferrule  500  is filled with a thermosetting polymer  400  having an appropriate thermooptical coefficient, so as to fix the cladding  360  in the ferrule  500 . A gap between the thermosetting polymer  400  surrounding the etched cladding  360  of the optical fiber  300  and the core  320  of the optical fiber or the thickness t of the etched cladding  360  is controlled such that a variation in valid refractive indices of the optical fiber according to the external temperature changes are less than or equal to an appropriate value with respect to a thermooptical coefficient of the given polymer material. 
         [0064]    Consequently, ∂λ WBG /∂T of the ECL in the current embodiment of the present invention is lowered by reducing the thickness of the cladding  360  around a portion where the Bragg grating  340  is formed and filling the space with the thermosetting polymer  400  having a negative thermooptical coefficient. Accordingly, the reflection spectrums of the Bragg grating can be kept regular and the output optical power can be athermal. 
         [0065]      FIGS. 4B  is a cross-sectional view of a portion II-II of  FIG. 4A , wherein an optical fiber structure  600  includes a cladding  360 , a thermosetting polymer  400 , and a ferrule  500  which are stacked concentrically around a core  320 . Unlike the prior art, the thickness t of the cladding  360  around the Bragg grating  340  is reduced and the thermosetting polymer  400  at the outer layer has an appropriate coefficient as described above. 
         [0066]      FIGS. 5A through 5D  are graphs illustrating conditions for the Bragg grating used in the ECL of  FIGS. 4A and 4B . 
         [0067]      FIG. 5A  is a graph showing changes in wavelength according to temperature for various thicknesses t of the cladding  360  formed around the Bragg grating  340 , while the refractive index of the thermosetting polymer  400  is 1.43.  FIG. 5B  is a graph showing the wavelength change according to the temperature when the refractive index of the thermosetting polymer  400  is 1.435,  FIG. 5C  is a graph of when the refractive index of the thermosetting polymer  400  is 1.44, and  FIG. 5D  is a graph of when the refractive index of the thermosetting polymer  400  is 1.445. Here, the thermosetting polymer  400  is a material having a negative thermooptical coefficient, for example, −1×10 −4 /deg to −1.6×10 −4 /deg. 
         [0068]    The material of the thermosetting polymer  400  and the thickness t of the cladding  360  can be determined with reference to  FIGS. 5A through 5D . For example, in  FIG. 5C , when the refractive index of the thermosetting polymer  400  is 1.44 and the thickness t of the cladding  360  is 0.4 μm or smaller, the variation of the oscillation wavelength can be reduced to 0.1 nm or smaller at 60° C. of an external temperature change. The thickness of the cladding  360  around the Bragg grating  340  may be 0.2 to 0.4 μm. 
         [0069]    The cladding  360  surrounding the Bragg grating  340  of the optical fiber  300  may be removed by mechanical grinding or chemical etching. An example of chemical etching is dipping the optical fiber into a hydrofluoric (HF) acid aqueous solution or a buffered-oxide etched (BOE) solution for a predetermined time. Since the etching rate of the optical fiber cladding  360  according to time is regular, the degree of etching can be easily determined. 
         [0070]    The thermosetting polymer  400  is a material that can be thermoset or hardened using ultraviolet rays. When a material that can be hardened using ultraviolet rays is used, the material for the ferrule  500  should be transparent to ultraviolet rays and thus glass such as silica or a material having good transmittance in the ultraviolet ray region may be used. 
         [0071]      FIG. 6  is a cross-sectional view of an ECL, the output wavelength and the output optical power of which are athermal, according to another embodiment of the present invention. 
         [0072]    Referring to  FIG. 6 , the ECL  1000  in the current embodiment of the present invention has a similar structure in which the output optical power is athermal, as in the ECL of  FIGS. 4A and 4B . However, the ECL  1000  in the current embodiment further comprises an optical path compensator  800  to keep the output wavelength athermal. 
         [0073]    The optical path compensator  800  is formed on an optical path in which an external cavity is formed to compensate for the variation of the length L cavity  of the optical path according to external temperature changes. In other words, the length of the external cavity is fixed such that external cavity modes are athermal (are temperature independent?) in the ECL, thereby keeping the output wavelength regular. 
         [0074]    Using Equations 2 and 3, the refractive index, the thermooptical coefficient, and the length of the optical path of the optical path compensator  800  can be determined to suppress mode hopping in a required temperature range. 
         [0075]    For example, when the length of a semiconductor amplifier  100  is 600 um, the thermooptical coefficient of the semiconductor amplifier  100  is 2.2×10 −4 /deg, the length of the focusing lens  200  is 1000 um, the length of the Bragg grating  340  is 4000 um, and the thermooptical coefficient of the core  320  of the optical fiber is 0.1×10 −4 /deg, if the thermooptical coefficient of the optical path compensator  800  is −1.5×10 −4 /deg, then the length of the optical path of the optical path compensator  800  is 1200 um. The thermooptical coefficient of the material of the optical path compensator  800  may be −1×10 −4 /deg to −2.5×10 −4 /deg and the length of the optical path of the optical path compensator  800  may be 500 um to 2000 um. The length of the optical path of the optical path compensator  800  is generally the thickness of the optical path compensator; however, when the optical path compensator  800  is inclined with respect to the optical path, the length of the optical path of the optical path compensator  800  is substantially the length of the light transmission path. 
         [0076]    A front surface  820  and a rear surface  810  of the optical path compensator  800  are AR-coated and the optical path compensator  800  is inclined at a predetermined angle θ with respect to the perpendicular of the optical path, thereby reducing residual reflection on the surface. The inclination angle θ may be 1 to 3°. 
         [0077]    The ECL in the current embodiment can be useful as a single-mode ECL. In other words, since the output optical power and the output wavelength of the ECL in a single mode change rapidly by mode hopping, mode hopping can be suppressed by suppressing changes of both the reflection spectrums and the external cavity modes, and accordingly, a single-mode ECL, the output optical power and the output wavelength of which are athermal can be easily realized. 
         [0078]    In the case of a multi-mode, the output optical power is usually regular regardless of temperature variation, and thus an ECL, the output optical power and the output wavelength of which are athermal, can be realized by inserting an optical path compensator. 
         [0079]    The ECL described with reference to  FIG. 4  or  6  uses a Bragg grating as a reflection filter to form an external cavity. However, although a Bragg grating made of an optical fiber can be easily manufactured using a photosensitive optical fiber, the characteristics of a reflection filter may be substantially changed through inserting the optical fiber into a ferrule and then through a hardening process using a thermosetting polymer or epoxy. This happens because of stress or strain caused on the optical fiber grating while the thermosetting polymer is hardened. 
         [0080]    Hereinafter, an athermal ECL employing not an optical fiber Bragg grating but a novel transmissive filter will be described. 
         [0081]      FIG. 7  is a cross-sectional view of an ECL, the output wavelength and the output optical power of which are athermal, according to another embodiment of the present invention. 
         [0082]    Referring to  FIG. 7 , the ECL includes a thin-film multi-layer (TFML) transmissive filter  900  inserted into the optical path, instead of a Bragg grating as a filter selecting the oscillation wavelength. Since the transmissive TFML filter is transmissive, an additional reflector is needed. For this purpose, a cross-section of the optical fiber  370  is coated with a material having an appropriate reflectivity to form an external cavity. 
         [0083]    For example, when the cross-section  370  of the optical fiber is not coated, the reflectivity is 3 to 5%, and when the cross-section  370  of the optical fiber is coated with a single sheet of a metal film such as Cr, Au, Ag, Pt, etc. having a thickness of 0.1 um or less, the reflectivity is increased up to about 20 to 50%, and when the cross-section  370  of the optical fiber is coated with multiple sheets of metal oxides such as silicon oxide, aluminum oxide, tantalum oxide, or titanium oxide, the reflectivity is increased up to about 95%. Also, in order to reduce residual reflection on the front surface  920  and the rear surface  910  of the transmissive filter  900 , the transmissive filter  900  can be inclined at a predetermined angle φ to the perpendicular of the optical path. The inclination angle φ may be 1 to 3°. Also, in order to reduce reflection on the front surface  920  and the rear surface  910  of the transmissive filter  900 , the transmissive filter  900  can be AR-coated. 
         [0084]    The TFML transmissive filter  900  may have a stack structure in which two different types of SiO 2  or metal oxide layers such as Al 2 O 3 , Ta 2 O 5 , or TiO 2  having different thicknesses and refractive indices are alternately stacked on a glass substrate. Desired transmission broadband, transmission center wavelength, and transmittance can be obtained by controlling the thickness of the TFML transmissive filter  900  and the number of layers. 
         [0085]    The degree of change of the transmission wavelength of the TFML transmissive filter  900  according to external temperature may be smaller than 0.01 nm/deg, preferably 0.003 nm/deg or smaller. When the degree of change of the transmission wavelength is 0.003 nm/deg or smaller, the temperature stability of the oscillated wavelength is 0.3 nm or smaller, when the external temperature is changed by 100 degrees. 
         [0086]    The ECL in the current embodiment of the present invention can also keep reflection spectrums regular as in the case of the ECL in  FIGS. 4A and 4B  by using a TFML and thus the output optical power is athermal. 
         [0087]      FIG. 8  is a cross-sectional view of an ECL  1000  according to another embodiment of the present invention, the output wavelength and the output optical power of which are athermal to the external temperature. 
         [0088]    Referring to  FIG. 8 , the ECL  1000  in the current embodiment of the present invention has a similar structure to the ECL in  FIG. 7  in which the wavelength is athermal to the external temperature. However, the ECL  1000  in the current embodiment further includes an optical path compensator  800  as the ECL in  FIG. 6 . 
         [0089]    The optical path compensator  800  is formed on at least one of the left and right optical paths of the TFML transmissive filter  900  to compensate for changes in the length of the optical path according to the external temperature. A front surface  820  and a rear surface  810  of the optical path compensator  800  are AR-coated, and the optical path compensator  800  is inclined at a predetermined angle θ to the perpendicular of the optical path, thereby reducing residual reflection. 
         [0090]    Also, using Equations 2 and 3, the refractive index, thermooptical coefficient, and the length of the optical path can be determined to suppress mode hopping within the desired temperature range. For example, when the length of the semiconductor amplifier  100  is 600 um, the thermooptical coefficient of the semiconductor amplifier  100  is 2.2×10 −4 /deg, the length of the focusing lens  200  is 1000 um, the thickness of the TFML transmissive filter  900  is 1000 um, the thermooptical coefficient of the TFML transmissive filter  900  is 0.04×10 −4 /deg, and the thermooptical coefficient of the optical path compensator  800  is −1.5×10 −4 /deg, the length of the optical path of the optical path compensator  800  is about 1000 um. 
         [0091]    The thermooptical coefficient of the material of the optical path compensator  800  may be −1.0×10 −4 /deg to −2.5×10 −4 /deg and the length of the optical path of the optical path compensator  800  may be 500 um to 2000 um. 
         [0092]    The ECL in the current embodiment can also be applied to a single-mode is ECL such as the ECL in  FIG. 6 . That is, the ECL in the current embodiment uses TFML and an optical path compensator to suppress mode hopping in a single mode, the output optical power and the output wavelength of which are athermal. 
         [0093]    As described above, the ECL in the present invention changes the composition of the material of a waveguide in which a grating is carved, changes the structure of the waveguide, or uses a novel filter to make reflection spectrums athermal to keep the output optical power regular, and fixes the length of the external cavity of the ECL to keep the output wavelength regular. 
         [0094]    Also, the ECL in the present invention does not use expensive temperature control modules as in a butterfly type package, to reduce the cost of the temperature control modules and the packaging cost. 
         [0095]    Furthermore, the ECL in the present invention has regular output wavelength and output optical power that is athermal, and thus can be used as a light source of a dense wavelength division multiplexing (DWDM) system in which the width between the channels can be reduced by more than 10 times compared to coarse wavelength division multiplexing (CWDM). 
         [0096]    While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.