Abstract:
A transistor, such as a FET or bipolar transistor, may be gate or base coupled to a laser driver output stage to receive a varying voltage from a driver output stage. The transistor converts the varying voltage to a varying current. The transistor, in series with the laser, may be coupled to a supply voltage on one side and ground on the other side. Thus, the current supplied to the laser diode is a function of the drive supplied to the transistor&#39;s base or gate.

Description:
BACKGROUND  
       [0001]     This invention relates generally to optical communication networks. In particular, in some embodiments, it relates to a laser modulation scheme.  
         [0002]     Typically, an optical communication network uses a light source in the form of a laser to produce optical signals that are transmitted over an optical path. The optical path may, for example, be a fiber optic cable. Typically, those signals may be wavelength division multiplexed so that a large number of different signals of distinct wavelengths may be transmitted over the same fiber.  
         [0003]     A transmitter for an optical communication network generally includes a laser diode and a driver for that diode. The laser driver modulates the laser current and, therefore, the laser light output, in accordance with the signal that is to be transmitted.  
         [0004]     A direct modulated laser may use a laser driver that includes an output termination and a damping resistor connected in series with the laser diode. This type of driver scheme may have a number of disadvantages. A relatively powerful driver, with higher voltage/current output swings, may be used since the same modulation current goes through both the laser diode and the damping resistor. As a result, power consumption is relatively high. In order to produce such high output swings, relatively expensive gallium arsenide drivers may be utilized. Moreover, the higher power driver may thermally impact the laser, such that the driver may need to be placed far away from the laser diode, resulting in transmitter radio frequency performance degradation.  
         [0005]     Thus, there is a need for better ways to provide laser modulation in optical communication systems. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]      FIG. 1  is a schematic depiction of one embodiment of the present invention;  
         [0007]      FIG. 2  is a schematic depiction of another embodiment of the present invention;  
         [0008]      FIG. 3  is a schematic depiction of another embodiment of the present invention;  
         [0009]      FIG. 4  is a schematic depiction of another embodiment of the present invention;  
         [0010]      FIG. 5  is a schematic depiction of another embodiment of the present invention;  
         [0011]      FIG. 6  is a schematic depiction of another embodiment of the present invention; and  
         [0012]      FIG. 7  is a system depiction of one embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0013]     Referring to  FIG. 1 , a laser driver  10   a,  in accordance with one embodiment of the present invention, includes a driver output stage  12 . The driver output stage allows adjustment of the modulation through a terminal  24  and receipt of the differentially driven data and data complement signals through terminals  20  and  22 .  
         [0014]     A differential circuit includes a pair of resistors  16   a  and  16   b  and a pair of transistors  18   a  and  18   b.  The differential circuit pulls the output of the stage  12  down based on the signals on the gates of transistor  18 . The transistor  18   b  receives the data complement input while the transistor  18   a  receives the data input. A transistor  26  receives the current control input which controls the current I mod as indicated.  
         [0015]     The output of the driver output stage  12  is passed through a capacitor  28  for the AC path of the transistor  34 . A laser bias adjustment voltage-may be applied through an inductor  30  for the DC path of the transistor  34 . A shunt matching resistor  32  may be used as well.  
         [0016]     The output from the output stage  12  controls the potential on the gate of a field effect transistor  34  in one embodiment of the present invention. The transistor  34  is coupled between a supply voltage and ground, in series with the laser diode  36 . The single transistor  34  acts as a simple, low cost, single stage amplifier to increase the modulation current. The gate voltage on the transistor  34  controls the amount of current applied to the laser diode  36 .  
         [0017]     A monitor photodiode  38  may be used to monitor the light output of the laser diode  36 . The signal from the diode  38  may be used to control the driver  10   a.    
         [0018]     The laser diode  36  communicates with a laser diode receiver across an optical network. In one embodiment of the present invention, the laser driver  10   a  may be implemented with field effect transistors. As one example, a pseudomorphic high electron mobility transistor (PHEMT) may be used.  
         [0019]     The laser modulation current is controlled by the voltage on the gate of the transistor  34 , which in turn is controlled by the driver output stage  12  voltage. The voltage swings at the gate do not have to be very large in order to get enough modulation current through the laser diode  36  in some embodiments. Thus, a relatively powerful output stage  12  may not be needed. As a result, smaller power supplies with lower voltage levels may be used for the entire driver  10   a  in some embodiments. The use of lower supply voltages may reduce the total power consumption. Moreover, because the transistor  34  is a lower power device, it can be placed next to the laser diode  36  without causing significant thermal impact on the laser diode  36  in some embodiments.  
         [0020]     Referring next to  FIG. 2 , the laser driver  10   b  is similar to the laser driver  10   a  shown in  FIG. 1 . However, in this case, a transistor  34   a  in the form of a bipolar transistor is utilized. The voltage on the base of the bipolar transistor  34   a  controls the amount of current applied to the laser diode  36 .  
         [0021]     Turning next to  FIG. 3 , the laser driver  10   c  is similar to the driver  10   a  shown in  FIG. 1 . However, in this example, an AC coupled matching resistor  32  includes a capacitor  40 . The AC coupled matching resistor  32  may have essentially no DC power dissipation in some embodiments. As a result, the AC coupled matching resistor  32  reduces the overall transmitter power dissipation.  
         [0022]     Referring next to  FIG. 4 , a driver  10   d,  similar to the driver  10   c  shown in  FIG. 3 , uses a bipolar transistor  34   a,  in place of a field effect transistor  34 .  
         [0023]     Referring to  FIG. 5 , the laser driver  10   e  is otherwise similar to the laser driver  10   a  except that a pair of matching resistors R 1  and R 2  are utilized. In effect, the matching resistor  32  from the previous embodiments is split in two. The ratio of the resistance of the resistor R 1  to that of the resistor R 2  is equal to the matching resistance. If the resistance of the resistor R 1  is much greater than the matching resistance and the resistance of the resistor R 2  is much greater than the matching resistance, the power dissipation of both R 1  and R 2  may be reduced.  FIG. 6  shows a similar arrangement but using a bipolar transistor  34   a  in the laser driver  10   f.    
         [0024]     Finally, referring to  FIG. 7 , a network interface, according to one embodiment of the present invention, includes a media access control  70  coupled to an encoder/decoder  60  and a serializer/deserializer  50  in one embodiment. The serializer/deserializer  50  may be coupled, on the transmitter side, to the laser driver  10 , which may be any of the embodiments illustrated herein. The driver  10  in turn is coupled to the transmitting laser diode  36 .  
         [0025]     On the receiver side, the receiving photo diode  37  is coupled to a limiting amplifier/transimpedance amplifier  40 , which in turn may be coupled to the serializer/deserializer  50 .  
         [0026]     On the transmitter side, digital data may be provided from the media access control module  70  to the encoding/decoding module  60 , where the digital data may be encoded into a format that is advantageous for conversion into optical signals. If the digital data is already in the proper form, processing by the encoder/decoder  60  may be unnecessary. Sometimes, the encoded digital data needs to be serialized or deserialized. In such case, the encoded digital data may be fed to the serializer/deserializer  50 . The output from the serializer/deserializer  50  may be fed to the laser driver  10  that may drive the laser diode  36  as described previously. Optical energy may be created and optical signals may be provided from the interface to a fiber optic line (not shown) in one embodiment of the present invention.  
         [0027]     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.