Abstract:
A transistor-outline TO-can type optical module includes a stem, a sub-mount arranged in the stem and a laser diode (LD) is mounted in the sub-mount. A photo diode (PD), which has an inclined light incident surface, converts light emitted from the LD to current. A plurality of leads is extended through the stem while electrically being connected to the sub-mount. The inclined light incident surface of the PD permits that sufficient monitoring of photocurrent can be obtained and a p-side up bonding of a p-type electrode is allowed. Thus, the SMSR of the LOB is increased. A bias-tee is built in the TO-can to reduce heat caused by DC current and to increase opto-electric efficiency while suppressing an increase in the temperature of an LD chip.

Description:
CLAIM FOR PRIORITY  
       [0001]     This application claims priority under 35 U.S.C. § 119 to an application entitled “TO-Can Type Optical Module,” filed in the Korean Intellectual Property Office on Nov. 14, 2003 and assigned Serial No. 2003-80505, the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to an optical module. More particularly, the present invention relates to a TO-can (Transistor-Outline-can) type optical module.  
         [0004]     2. Description of the Related Art  
         [0005]     An optical module is an essential part of any system used for optical transmission. Owing to the recent rapid growth of the information industry, there are increasing proportions of information transmission traveling over an optical communication network, as well as increased demand for fast transmission and transmission of a large volume of information. The optical module must be designed to support fast and large-volumes of information transmission. Optical devices such as a laser diode (LD) or a photodiode (PD) for the optical module are usually available in TO-can packages.  
         [0006]      FIG. 1  is a perspective view of a conventional TO-can optical module package  100 . Referring to  FIG. 1 , the conventional TO-can optical module package  100  comprises a stem  101  provided at a surface thereof, with a protruding heat sink block  111 , and a plurality of leads  102 . The four leads  102  comprise two leads for driving laser diode  103  and two leads for biasing monitor photodiode  104 . The laser diode LD  103  and the monitoring photodiode MPD  104  are arranged on the surface the stem  101 . In particular, LD  103  is normally arranged on the heat sink block  111 . The LD  103  and the MPD  104  are connected to the leads  102  by, for example, wire bonding.  
         [0007]     The leads  102  are coaxially aligned via through-holes  113  penetrating both surfaces of the stem  101 , the through-holes  113  are filled with a glass sealant  105 , and the glass sealant  105  is melted, thereby fixing the leads  102  to the stem  101  and sealing the through holes  113  at the same time. Such a conventional TO-can package is the model C-13-DFB10-TJ-SLC21 manufactured and sold by Luminent Inc.  
         [0008]     However, the conventional optical module package is not practical for high-speed transmissions at 10 Gbps or over because of (1) parasitic inductance which is inherent in the leads, (2) parasitic capacitance between the leads and the stem, and (3) characteristic impedance mismatch for the RF signal passing through the leads.  
         [0009]      FIG. 2  is a perspective view of another conventional TO-can optical module package  200  featuring a ceramic feed-through. Referring to  FIG. 2 , the TO-can optical module package  200  comprises a stem  201  provided with a protruding heat sink block  211 , and a ceramic stack feed-through  203  inserted into the stem  201 . The feed-through  203 , disposed on the heat sink block  211 , has a coplanar waveguide (CPW)  202  at a surface thereof. The CPW-type package  200  receives an external RF signal through a plurality of leads  204 . TO TX PKG A2527 of Kyocera Corp. is one of such CPW-type packages.  
         [0010]     The feed-through  203  is normally fabricated in a ceramic stack structure. Since the feed-through  203  is formed by LTCC (Low Temperature Co-fired Ceramic), its processing temperature is high, for example, between 800 and 1000° C. Thus, the manufacturing costs are higher than those of conventional TO-can optical module package shown in  FIG. 1 .  
         [0011]     Moreover, when a waveguide structure is arranged with the optical module to improve the RF characteristics, the size of a sub-mount has to be increased. In this case, light emitted from the back facet of laser diode is reflected from or scattered on the surface of the sub-mount, resulting in a decrease of monitoring photocurrent. To solve this problem, Sumitomo Inc. proposed a TO-can type optical module package in which the sub-mount is shaped like “         ”. However, the conventional technology has distinctive shortcomings such as an increase in sub-mount manufacturing costs and a difficult assembly procedure. What makes it worse is that if a matching resistor is mounted on the sub-mount without any consideration of the other components, heat problems can become severe in case of un-cooled operation. When a mixture of a DC bias and an RF signal, produced from an external bias-tee, passes through the matching resistor, the heat dissipation mostly coming from DC current directly increases the operating temperature of the LD, which is located very close to the matching resistor, thereby deteriorates the performance of the TO-can type optical module in a fatal fashion.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention substantially solves many of the above-mentioned problems and/or disadvantages of conventional TO-can optical modules, and provides additional advantages described, infra. The first aspect of present invention provides an optical module that has a high frequency response characteristic while still offering the benefits of a TO-can structure.  
         [0013]     Another aspect of the present invention is to provide a TO-can type optical module that is manufactured easier than previously known structures with lower cost and higher throughput yields than those previously known.  
         [0014]     The above aspects can be achieved by providing a TO-can type optical module that includes a stem; a sub-mount arranged in the stem and a laser diode LD is arranged in the sub-mount. A photodiode PD, which has an inclined light incident surface, converts light emitted from the LD to current. A plurality of leads is extended through the stem, and is electrically connected to the sub-mount.  
         [0015]     It is preferable that a bias-tee is installed in the sub-mount so as to superpose an RF signal on the DC current for LD driving. In addition, a coplanar waveguide and a matching resistor are provided to transmit the RF signal without distortion, and an inductor serves as a choke to block the RF signal from a DC current path.  
         [0016]     The present invention also includes a method of manufacturing the TO-can module. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The above-mentioned and other aspects, features and advantages of the present invention will become more apparent from the following detailed description, when taken in conjunction with the accompanying drawings in which show that:  
         [0018]      FIG. 1  illustrates a conventional TO-can type optical module package;  
         [0019]      FIG. 2  illustrates another conventional TO-can type optical module package;  
         [0020]      FIG. 3  illustrates a TO-can type optical module showing an aspect of the present invention;  
         [0021]      FIG. 4  illustrates a side view of the structure of a PD according to the present invention;  
         [0022]      FIG. 5  is an equivalent circuit diagram of the TO-can type optical module that is cathode driven or for DML driver, as illustrated in  FIG. 3 ;  
         [0023]      FIG. 6  illustrates a TO-can type optical module that is anode driven or for EML driver according to another aspect of the present invention; and  
         [0024]      FIG. 7  illustrates a TO-can type optical module that is differentially driven according to a yet another aspect of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     Preferred aspects of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail when it is believed that they would obscure detailed description of the invention with unnecessary detail that is known by the person of ordinary skill in the art.  
         [0026]      FIG. 3  illustrates a TO-can type optical module according to an aspect of the present invention. In particular, attention is directed to the structure of a sub-mount for a DML (Direct Modulated Laser) driver or cathode driven type.  
         [0027]     Referring to  FIG. 3 , a TO-can type optical module  300  according to the present invention comprises a stem  301 , a heat sink  302 , a sub-mount  303 , an LD  304 , a PD  305 , and a plurality of leads  315  to  319 . Further included is CPW composed of  306 ,  307  and  308 , an anode line  309  for the PD  305 , a DC line  310 , a matching resistor  311 , first and second spiral inductors  312  and  313 , and a damping resistor  314 .  
         [0028]     The sub-mount  303  comprises a ceramic substrate that is die-bonded onto the heat sink  302  perpendicular to the upper surface of the stem  301 . The ceramic substrate electrically isolates the conductive stem  301  so that a variety of electrical patterns can be formed on the sub-mount  303 . Ceramic materials can include AlN, SiC, Al 2 O 3 , etc. In comparison to other ceramic materials, AlN exhibits excellent thermal conductivity and is therefore a favored material used as a sub-mount of a heat generator, such as an LD, a matching resistor and etc. The thermal conductivity of AlN is 2.1 W/cm ° C., which is about twice as high as that of silicon, 0.84W/cm ° C. Also, an AlN sub-mount can be made thinner than a silicon sub-mount. Therefore, AlN is favorable in terms of thermal conductivity to the outside.  
         [0029]     The LD  304  emits a laser beam depending on a DC bias or an RF signal. The CPW composed of  306 ,  307  and  308  together with the matching resistor, the PD line  309  and the DC line  310  are formed on the sub-mount  303  by thin-film processing. The matching resistor  311  is electrically connected to a signal line of the CPW  307 . The CPW (composed of  306 ,  307  and  308 ) and the matching resistor  311  transfer an external 10-Gbps RF signal without distortions. 50 Ω is preferred to 25 Ω as the input impedance of the sub-mount  303  in terms of power consumption. The input impedance of the sub-mount  303  is the sum of the impedance of the matching resistor  311  and the dynamic resistance of the LD  304 . The characteristic impedance of the CPW composed of  306 ,  307  and  308  is also matched to 25 Ω.  
         [0030]     The first inductor  312  is a spiral type. It is connected to the DC line  310 , functioning as a choke for blocking an RF signal from a DC path.  
         [0031]     The second inductor  313  is positioned between the LD  304  and the PD  305 . It isolates the LD  304  from the PD  305 , thereby preventing unexpected RF signal leakage. The second inductor  313  is also spiral type like the first inductor  312 . The matching resistor  311  and the first and second inductors  312  and  313  can be formed on the sub-mount  303  by thin-film processing. Therefore, a laser module package can be made compact.  
         [0032]     The damping resistor  314  is connected in parallel to the first inductor  312  and prevents LC resonance caused by parasitic capacitance.  
         [0033]     The leads  315  to  319  are inserted through the stem  301  such that one end of each of the leads  315  to  319  protrudes from one surface of the stem  301 , while the other end thereof is extended through the other surface of the stem  301 . The first lead  315  is connected to an anode of the LD  304 , the second lead  316  to an cathode RF terminal of the LD  304 , the third lead  317  to a cathode of the PD  305 , the fourth lead to an anode of the PD  305 , and the fifth lead  319  to a cathode DC terminal of the LD  304 . The first and third leads  315  and  317  are commonly connected. The leads  315  to  319  are fixed to the stem  301  by a glass sealant filled in through holes. The glass sealant is filled in the state of glass seal powder into the through holes into which the leads  315  to  319  are aligned and melted at about 500° C., thereby sealing the through holes. Alternatively, a pre-shaped glass sealant can be prepared such a way that it can hold the leads  315  to  319  and can plugged into TO-stem  301 . Then melting at about 500° C. completes the hermetic sealing of the through-hole.  
         [0034]     A solder pattern (e.g., AuSn) is formed on the sub-mount  303 , for die-bonding of the LD  304  and the PD  305 . The structure of the sub-mount  303  may vary with the type of LD driver.  
         [0035]     The PD  305  detects light emitted from the back face of the LD  304 , determines whether the LD  304  is operating normally, and correspondingly performs an automatic power control (APC) operation. The PD  305  is an RMF (Reflection Mirror Facet) type MPD having the configuration illustrated in  FIG. 4 .  
         [0036]      FIG. 4  schematically illustrates the structure of the RMF MPD  305 . The light emitted from the LD  304  is incident on the RMP MPD  305  is shown in  FIG. 4 . Referring to  FIG. 4 , because the RMF MPD  305  has an inclined light incident surface  402 , the reception efficiency of light reaching a light absorption layer  403  can be increased and so a sufficient monitoring photocurrent can be achieved.  
         [0037]     Also, the light incident surface  402  allows p-side up bonding of a p-type electrode  404  of the LD  304 , thereby increasing the SMSR (Side Mode Suppression Ratio) of the LOB (LD on Block). The SMSR of a DFB (Distributed Feed Back) laser varies with its bonding configuration. In general, it has higher SMSR in the case of p-side up bonding than p-side down bonding of a p-type electrode, showing a low bonding stress level.  
         [0038]     This PD configuration suppresses a roll-off in electro-optic response, caused by the parasitic capacitance of a pad and thus improves RF characteristics. It further allows bonding of an LD and an MPD on a plane without V grooves as in SiOB process, simplifying sub-mount fabrication and assembly and eliminating the need for flip-chip bonding during die bonding.  
         [0039]      FIG. 5  is an equivalent circuit diagram of the TO-can type optical module illustrated in  FIG. 3 . Here, the first lead  315  is shown being electrically connected to an anode of the LD  304 , the second lead  316  to an anode RF terminal of the LD  304 , the third lead  317  to a cathode of the PD  305 , the fourth lead  318  to an anode of the PD  305 , and the fifth lead  319  to a cathode DC terminal of the LD  304 . The first and third leads  315  and  317  are commonly connected.  
         [0040]      FIG. 6  illustrates a TO-can type optical module according to yet another aspect of the present invention. Specifically,  FIG. 6  is an equivalent circuit diagram of a sub-mount for an EML (Electro-absorptive Modulated Laser) driver or anode driven type.  
         [0041]      FIG. 7  illustrates a TO-can type optical module according to still another aspect of the present invention. Specifically,  FIG. 7  is an equivalent circuit diagram of a sub-mount for a differential type laser driver.  
         [0042]     As described above, the present invention has the following advantages:  
         [0043]     The use of a PD having an inclined light incident surface increases light reception efficiency and thus leads to a sufficient monitoring photocurrent. It also allows for the p-side up bonding of a p-type electrode in an LD, thereby increasing an SMSR. This PD structure suppresses a roll-off in electro-optic response caused by the parasitic capacitance of a pad, and so improves RF characteristics. Because an LD and a PD can be bonded on one plane without V grooves, assembly is simplified. Also, there is no need for flip-chip bonding for die bonding. Thus, process complexity is decreased and cost is reduced.  
         [0044]     The reduction of heat generation by incorporating a bias-tee into a TO can increases opto-electric efficiency and suppresses a temperature increase of an LD chip. Integration of an inductor and a matching resistor formed by thin-film processing on a sub-mount renders an optical module compact.  
         [0045]     While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.