Patent Publication Number: US-2003227694-A1

Title: Laser diode module, laser diode device and optical transmitter incorporating the same

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to an uncooled type laser diode module and a laser diode device incorporating the laser diode module, and an optical transmitter incorporating the laser diode device.  
       [0002] An optical transmitter used in a cable television or the like incorporates a semiconductor laser diode as an optical signal generator. Various characteristics, such as the slope efficiency, distortion and others, of the semiconductor laser diode, greatly vary depending on a temperature of a laser diode chip. To cope with this, a cooler is contained in the laser diode device to keep the temperature of the laser diode chip at a fixed value of about 25° C., so that generation of distortion is suppressed and a stable operation of the laser diode is secured (disclosed in Japanese Patent Publication No. 5-90698A, for example).  
       [0003] In the case of the cooler contained laser diode, the chip per se is merely cooled. Accordingly, a deviation of an internal optical system, such as lenses and an isolator, which is caused by ambient temperature variation, causes a variation of an optical output level of an optical signal. Further, the laser diode is complicated in structure, large in package and hence high in cost to manufacture.  
       [0004] For this reason, in recent days, the laser diode module of the uncooled type in which the cooler is not contained has been used. The laser diode module of the uncooled type has advantages of size reduction of the package and cost reduction since it does not contain the cooler.  
       [0005] However, it is disadvantageous in that during its operation, its temperature varies, and with temperature variation, the distortion quantity and the distortion characteristic greatly vary.  
       SUMMARY OF THE INVENTION  
       [0006] It is therefore an object of the invention to provide a laser diode module, a laser diode device, and an optical transmitter in which operation temperature is kept constant to thereby prevent a distortion variation, and the cost to manufacture is made low.  
       [0007] In order to achieve the above object, according to the invention, there is provided a laser diode module, comprising:  
       [0008] a module casing;  
       [0009] a laser diode chip, disposed in the module casing; and  
       [0010] a heater, which heats the laser diode chip such that at least a temperature of the laser diode chip is kept constant.  
       [0011] Preferably, the heater is disposed outside the module casing to keep a temperature of the module casing constant.  
       [0012] Preferably, the heater is a constant-temperature heat generating element comprising a PTC (positive temperature coefficient) thermistor.  
       [0013] The heater may be disposed inside the module casing. In this case, it is preferable that the heater heats at least the laser diode chip and an optical system through which a laser beam emitted from the laser diode chip passes.  
       [0014] According to the Invention, there is also provided a laser diode device, comprising:  
       [0015] the above laser diode module;  
       [0016] a temperature detector, which detects a temperature of the laser diode module; and  
       [0017] a temperature controller, which controls a calorific value of the heater in accordance with the detected temperature of the laser diode module.  
       [0018] Preferably, the temperature controller keeps the temperature of the laser diode module at a temperature higher than an operative temperature of the laser diode device.  
       [0019] According to the invention, there is also provided a laser diode module, comprising:  
       [0020] a module casing;  
       [0021] a laser diode chip, disposed in the module casing; and  
       [0022] a Peltier element, disposed outside the module casing to keep at least a temperature of the laser diode chip constant.  
       [0023] According to the invention, there is also provided a laser diode device, comprising:  
       [0024] the above laser diode module;  
       [0025] a temperature detector, which detects a temperature of the laser diode module; and  
       [0026] a temperature controller, which controls an operation state of the Peltier element in accordance with the detected temperature of the laser diode module.  
       [0027] According to the invention, there is also provided an optical transmitter, comprising the above laser diode device in order to convert an inputted electric signal into an optical signal to be transmitted.  
       [0028] Preferably, the optical transmitter further comprises a distortion compensator, provided in an input side of the laser diode device to previously compensate a distortion to be generated at an output side of the laser diode device. Here, a compensation characteristic of the distortion compensator is optimized at the controlled temperature of the laser diode module. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0029] The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein:  
     [0030]FIG. 1 is a block diagram showing a configuration of a laser diode device according to a first embodiment of the invention;  
     [0031]FIG. 2 is a diagram showing the details of an essential portion of the laser diode device;  
     [0032]FIG. 3 is a diagram showing a configuration of a temperature controller in the laser diode device;  
     [0033]FIG. 4 is a block diagram showing a configuration of an optical transmitter incorporating the laser diode device;  
     [0034]FIG. 5 is a block diagram showing a configuration of a laser diode device according to a second embodiment of the invention;  
     [0035]FIG. 6 is a block diagram showing a configuration of a laser diode device according to a third embodiment of the invention; and  
     [0036]FIG. 7 is a diagram showing a configuration of a laser diode device according to a fourth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0037] Preferred embodiments of the invention will be described with reference to the accompanying drawings.  
     [0038]FIG. 1 is a block diagram showing a configuration of a laser diode device according to a first embodiment of the present invention. In this embodiment, a laser diode module  11  of the uncooled type converts a high frequency analog signal, for example, into an optical signal to be outputted. A heater  12  keeps the temperature of the laser diode module  11  at a fixed value of temperature. The heater  12  may be directly mounted on the laser diode module  11 , or may be mounted on, for example, a fitting member, a board member or the like for holding the laser diode module  11 . In this case, the laser diode module  11  and the heater  12  may be provided within a specific casing.  
     [0039] A calorific value of the heater  12  is controlled by a temperature controller  13  so as to keep the temperature of the laser diode module  11  at a fixed value of temperature, which is higher than ambient temperature. In a case where given performances of an apparatus using the laser diode module  11  are guaranteed when an operative ambient temperature of the apparatus is specified within a range of −20° C. to +40° C., the temperature controller  13  operates so as to keep the temperature of the laser diode module  11  at a fixed value of temperature, for example, 65° C., which is higher than 40° C. as the upper limit value of the operative ambient temperature. By so doing, the laser diode module  11  is kept at a fixed temperature value even if the ambient temperature varies within the range of the operative ambient temperatures. A laser diode module which stably and reliably operates at a temperature higher than the operative ambient temperatures is used for the laser diode module  11 . Such a temperature is about 65° C., in this case. A laser diode module, preferably used for the laser diode module  11 , is able to operate at a temperature higher than the operative ambient temperature, about 65° C. in this case.  
     [0040] The details of the laser diode module  11  and the temperature controller  13  will be described later.  
     [0041] A high frequency signal for transmission is inputted to the laser diode module  11  through a distortion compensator  14 . The distortion compensator  14  compensates for secondary and tertiary distortions of the high frequency signal as input thereto by the utilization of the technique disclosed in Japanese Patent Publications Nos. 10-102718A and 9-126284A, for example. In particular, a compensating characteristic of the distortion compensator is selected so as to compensate for distortions generated at a temperature at which the laser diode module  11  is kept by the temperature controller  13 . The laser diode module  11  converts the high frequency input signal into an optical signal to be outputted through an optical cable.  
     [0042] The laser diode module  11  is configured as shown in FIG. 2. The laser diode module  11  is made up of an LD (laser diode) chip  111 , a monitor PD (photo diode)  112 , a first lens  113 , an isolator  114 , and a second lens  115 , and is housed in a casing  116 .  
     [0043] The LD chip  111  converts an externally-inputted high frequency signal into laser light, and outputs the same. The laser light is outputted from the laser diode module  11 , through the first lens  113 , the isolator  114 , the second lens  115  and the optical fiber  117 .  
     [0044] The monitor PD  112  receives and converts laser light that is outputted from the LD chip  111  into an electrical signal to be outputted to an automatic power controller  15 . The automatic power controller  15  controls a drive circuit of the LD chip  111  in accordance with a signal derived from the monitor PD  112 . By the control, an optical output level of an optical signal outputted from the LD chip  111  is always constant. In a case where the laser diode module  11  is constant-current driven, when temperature rises, the output level of the optical signal outputted from the laser diode module  11  decreases, and when temperature decreases, the optical output level of the optical signal increases. To cope with this, as described above, laser light outputted from the LD chip  111  is detected by the monitor PD  112 , and the drive circuit of the LD chip  111  is controlled by the automatic power controller  15  so that the output level of the optical signal output therefrom is made constant.  
     [0045] The heater  12  is mounted on the outside of the laser diode module  11 , i.e., the outside of the casing  116 . Further, a heat sensitive element, e.g., an NTC (negative temperature coefficient) thermistor (referred to simply as a thermistor)  26 , is mounted on the laser diode module  11 . Similarly, the thermistor  26  may be directly mounted on the laser diode module  11 , or may be mounted on, for example, the fitting member or the board member for holding the laser diode module  11 .  
     [0046] The thermistor  26  is an element for detecting temperature of the laser diode module  11 , and a detection signal is inputted to the temperature controller  13 . The temperature controller  13  controls the heater  12  in accordance with the detection signal from the thermistor  26 , and keeps the temperature of the laser diode module  11  at a fixed value of temperature, for example, 65° C.  
     [0047] The temperature controller  13  is configured as shown in FIG. 3 Specifically, a voltage Vcc supplied from a DC power source circuit (not shown) is inputted to a collector of a power transistor  21 , through the heater  12 , and further to a power line  23  through a resistor  22 . A parallel circuit including a constant-voltage diode  24  and a capacitor  25  is connected between the power line  23  and ground, whereby a voltage on the power line  23  is kept constant.  
     [0048] A series circuit including the thermistor  26  and bias resistors  27  and  28 , which are connected in series, is connected between the power line  23  and ground. A divided voltage appearing at a node between the thermistor  26  and the bias resistor  27  is inputted to the base of the amplifying transistor  29 .  
     [0049] The amplifying transistor  29  receives at the collector a voltage on the power line  23  via a load resistor  30 , and is grounded at the emitter. The collector voltage of the amplifying transistor  29  is inputted to the base of the power transistor  21  through the bias resistor  31 . The power transistor  21  is grounded at the emitter.  
     [0050] In the temperature controller thus constructed, when the temperature of the laser diode module  11  is low, temperature of the thermistor  26  is also low and its resistance is large. Accordingly, a divided voltage at the node between the thermistor  26  and the bias resistor  27  is low, and a base current of the amplifying transistor  29  reduces. As a result, a collector voltage of the amplifying transistor  29  becomes high. A base current of the power transistor  21  reduces. A current flowing into the heater  12  reduces, and a heating temperature for the laser diode module  11  increases.  
     [0051] When the temperature of the laser diode module  11  becomes high, the temperature of the thermistor  26  also becomes high, and its resistance decreases. The divided voltage at the node between the thermistor  26  and the bias resistor  27  increases, and a base current to the amplifying transistor  29  increases. As a result, the collector voltage of the amplifying transistor  29  becomes low, the base current of the power transistor  21  decreases, the current flowing into the laser diode module  11  decreases, and the heating temperature for the laser diode module  11  decreases.  
     [0052] As described above, the current to the heater  12  is controlled so that the temperature of the laser diode module  11  is kept constant. A set temperature for the laser diode module  11  may be adjusted by appropriately selecting the values of the bias resistors  27  and  28 . Accordingly, the values of the bias resistors  27  and  28  are selected so that the laser diode module  11  is kept at an appropriate temperature, e.g., 65° C.  
     [0053] As described above, the heater  12  and the temperature controller  13  cooperate to keep the temperature of the laser diode module  11  at a fixed value of temperature. As a result, a distortion generated in the laser diode module  11  may be kept constant. In this state, the laser diode module  11  and the distortion compensator  14  are combined, and a distortion compensation by the distortion compensator  14  is optimized at a temperature (e.g., 65° C.) at which the laser diode module  11  is kept, whereby great distortion improvement is secured.  
     [0054] By keeping the entire temperature of the laser diode module  11  constant, deviation of not only the LD chip  111  but also the optical system, such as the first lens  113 , the isolator  114  and the casing  116 , may be suppressed, whereby a tracking error is surely prevented. Accordingly, an output level of an optical signal at the output terminal of the laser diode module  11  may be kept constant.  
     [0055] When comparing with the a laser diode module containing the cooler, the laser diode module  11  is smaller in size, and simple in structure, and the package per se is very low in cost. Accordingly, even if the externally mounted heater  12  is provided, the laser diode module  11  may be manufactured at low cost.  
     [0056] An arrangement of an optical transmitter using the laser diode module  11  described above will be described with reference to FIG. 4. A high frequency signal inputted to an input terminal  41  is amplified by an amplifier  42 , wave-shaped by a waveform shaper  43 , and gain-adjusted by a gain controller  44 . After then, the high frequency signal is inputted to the laser diode module  11 , through an amplifier  45 , the distortion compensator  14  and an amplifier  46 . The laser diode module  11  converts the inputted high frequency signal into an optical signal, and outputs the same through an optical cable (not shown) via an optical connector  48  provided on a front panel  47 .  
     [0057] The laser diode module  11  is kept at a fixed value of temperature by the combination of the heater  12  and the temperature controller  13 , and is controlled, by the automatic power controller  15 , to produce an optical signal having always a fixed output level.  
     [0058] An alarm generator  50  is connected to the temperature controller  13  and the automatic power controller  15 . The alarm generator  50  monitors operation statuses of the temperature controller  13  and the automatic power controller  15 , and when it detects an abnormality, it transmits an alarm signal to an LED (light emitting device) driver  51 .  
     [0059] The LED driver  51  controls the turning-on/off of a plurality of status LEDs  52  provided on the front panel  47  in accordance with a signal outputted from the alarm generator  50 . The status LEDs  52  indicate a power status (POWER), temperature (TEMP) of the laser diode module  11 , an output status of the optical signal (OPT PWR), an LD bias status (BIAS) by the automatic power controller  15 , or the like.  
     [0060] Therefore, a high performance optical transmitter which is remarkably reduced in distortion occurrence and low in cost may be constructed not using the cooler-contained laser diode module. Further, great distortion improvement is secured by combining the laser diode module  11  and the distortion compensator  14  such that a distortion compensation by the distortion compensator  14  is optimized at a temperature (e.g., 65° C.) at which the laser diode module  11  is kept.  
     [0061] A second embodiment of the present invention will be described with reference to FIG. 5. As shown, a constant-temperature heat generating body  60  incorporating a PTC thermistor therein is directly mounted on a casing of the laser diode module  11 . Electric power is supplied from a power source, e.g., an AC power source  61 , to the PTC thermistor. By utilizing heat generated by the PTC thermistor, the laser diode module  11  is kept at a temperature higher than operative ambient temperature, for example, a fixed temperature of 65° C. Distortion of the laser diode module  11  is compensated by the distortion compensator  14 .  
     [0062] The PTC thermistor is a semiconductor resistor element, and has a nature that its resistance steeply increases with increase of temperature, and its switching temperature may be set by appropriately selecting its material compositions. The PTC thermistor has such a characteristic that at low temperatures, a resistance value of the thermistor is low and large current flows therethrough, but at temperatures higher than a given temperature, its resistance value steeply increases and little current flows therethrough. Accordingly, the PTC thermistor may be used as a heat generator of the constant-temperature heat generating body  60  by the utilization of the temperature characteristic mentioned above.  
     [0063] In such a configuration, the temperature controller may be simplified in construction when the temperature of the laser diode module  11  is kept at a fixed value of temperature by use of the constant-temperature heat generating body  60 . The constant-temperature heat generating body  60  may be directly mounted on the laser diode module  11 , or may be mounted on, for example, a fitting member, or a board member for holding the laser diode module  11 .  
     [0064] When the laser diode module  11  is kept at a fixed value of temperature and the distortion compensation by the distortion compensator  14  is optimized at that temperature value, great distortion improvement is secured.  
     [0065] In the second embodiment, the constant-temperature heat generating body  60  is externally mounted on the casing of the laser diode module  11 . In an alternative, the constant-temperature heat generating body  60  is provided within the casing of the laser diode module  11 , viz, near the LD chip  111 . And the temperature of the LD chip  111  is kept at a fixed value of temperature, for example, 65° C., as described above. In this case, a lead wire of the constant-temperature heat generating body  60  is led out of the casing of the laser diode module  11 , and connected to a power source, for example, the AC power source  61 , as in the second embodiment.  
     [0066] A third embodiment of the invention will be described with reference to FIG. 6. In this embodiment, a cooler element, for example, a Peltier element  71 , is used in place of the heater  12  in the first embodiment. A configuration of the laser diode module  11  is similar to that of the first embodiment shown in FIG. 2. Hence, like or equivalent portions are designated by like reference numerals in FIG. 2.  
     [0067] The Peltier element  71  is mounted on the outside of the laser diode module  11 , for example, on the outside of the casing  116 . And a heat sensing element, e.g., the thermistor  26 , is also mounted on the casing  116 . The Peltier element  71  and the thermistor  26  may be mounted on, for example, a fitting member or a board member for holding the laser diode module  11 .  
     [0068] The thermistor  26  detects temperature of the laser diode module  11 , and transmits a detection signal to the temperature controller  13 . In turn, the temperature controller  13  controls the Peltier element  71  in accordance with the detection signal from the thermistor  26  so that the temperature of the laser diode module  11  is kept at a fixed value of temperature, for example, 25° C.  
     [0069] By controlling the Peltier element  71  by the temperature controller  13 , the temperature of the whole of the laser diode module  11  may be kept at a fixed value of temperature. As in the first embodiment, deviation of not only the LD chip  111  but also the optical system may be suppressed to thereby prevent the tracking error and to keep an output level of an optical signal constant.  
     [0070] When comparing with the a laser diode module containing the cooler, the laser diode module  11  is simple in structure, and the package per se is very low in cost. Accordingly, even if the externally mounted Peltier element  71  is provided, the laser diode module may be manufactured at low cost.  
     [0071] Further, by keeping the temperature of the laser diode module  11  at a fixed value of temperature, a distortion generated in the laser diode module  11  may be kept constant. Further, great distortion improvement is secured by combining the laser diode module  11  and the distortion compensator  14  as in the first and second embodiments, so that a distortion compensation by the distortion compensator  14  is optimized at the controlled temperature. The resultant optical transmitter is low in cost and high in performance.  
     [0072] The laser diode module  11  is normally set and kept at about 25° C. by the temperature controller  13 . If necessary, the temperature at which the laser diode module is kept may be higher than 25° C., for example, 65° C.  
     [0073] A fourth embodiment of the invention will be described with reference to FIG. 7. In this embodiment, a heater  81  is contained in a laser diode module  80  of the uncooled type. A laser diode module which stably and reliably operates at a temperature higher than the operative ambient temperature is used for the laser diode module  80 . Such a temperature is about 65° C., for example.  
     [0074] The laser diode module  80  contains an LD chip  111 , a monitor PD  112 , a first lens  113 , an isolator  114 , a second lens  115  and a heater  81  for heating the LD chip  111 , and is contained in a casing  116 . A heat sensitive element, for example, a thermistor  26 , is disposed near the LD chip  111  within the casing  116 , and detects a temperature of or near the LD chip  111 .  
     [0075] Laser light outputted from the LD chip  111  is led out through the first lens  113 , the isolator  114 , the second lens  115  and the optical fiber  117 .  
     [0076] The monitor PD  112  receives and converts the laser light outputted from the LD chip  111  into an electrical signal to be outputted to the externally-provided automatic power controller  15 . The automatic power controller  15  controls an output level of an optical signal outputted from the LD chip  111  in accordance with a signal outputted from the monitor PD  112  so that the optical output level is kept constant.  
     [0077] The thermistor  26  is connected to the temperature controller  13  provided outside the laser diode module  80 . The thermistor  26  detects a temperature within the casing  116 , for example, a temperature of or near the LD chip  111 , and the temperature controller  13  controls a heat value of the heater  81  by use of its drive current based on the detected temperature, and keeps the temperature of the heater  81  at a fixed value of temperature, for example, 65° C.  
     [0078] A laser diode module which stably and reliably operates at a temperature higher than normal temperature is used for the laser diode module  80 . By so doing, the temperature of the laser diode module  80  containing the heater  81  may be kept at a fixed value of temperature. As a result, the temperature control is considerably readily carried out with a simple construction, leading to cost reduction.  
     [0079] Further, great distortion improvement is secured by combining the laser diode module  80  and the distortion compensator  14  as in the first and second embodiments, and a distortion compensation by the distortion compensator  14  is optimized at the controlled temperature. The resultant optical transmitter is low in cost and high in performance.  
     [0080] Although the present invention has been shown and described with reference to specific preferred embodiments, various changes and modifications will be apparent to those skilled in the art from the teachings herein. Such changes and modifications as are obvious are deemed to come within the spirit, scope and contemplation of the invention as defined in the appended claims