1. Field of the Invention
The present invention relates to a laser driver and an optical module including the same.
2. Related Background Art
Optical transceivers for transmitting and receiving optical signals and interconversion between the optical signals and electrical signals have been used in optical transmission systems constituting core networks and in communication lines between severs in data centers. Such an optical transceiver has a transmitter part (optical transmitter) and a receiver part (optical receiver) in general. The optical transmitter converts an electric signal into an optical signal and sends the optical signal to an optical transmission line including an optical fiber. Specifically, an optical transmitter of a “direct modulation” type incorporates therein a light-emitting element (laser diode) for generating an optical signal and a laser driver for driving the laser diode by a drive current.
For the optical transceivers, common specifications, called MSA (Multi Source Agreement) such as XFP (10 Gigabit Small Form-factor Pluggable), QSFP+ (Quad Small Form-factor Pluggable Plus), and GYP (C Form-factor Pluggable), have been defined, so as to set up standards for electrical and optical characteristics, communication interfaces with host devices for monitoring and controlling, terminal arrangements, outer forms (form factors), and the like. Recent steep growth in communication traffic has been demanding to increase transmission rate of the optical signals from 10 Gbps to 25 Gbps and further to 40 Gbps. For responding to such a demand, shunt drivers and push-pull drivers have been incorporated into the optical transmitters for high-speed operations.
When the conventional laser driver used for direct modulation directly drives a laser diode at a high transmission rate exceeding 20 Gbps, however, the frequency characteristic of the laser diode may have a depression in some frequency components. Such a depression often deteriorates the group delay of the optical signal emitted from the laser diode, thereby increasing jitters in the optical signal.
In view of such a problem, an object of the present invention is to provide a laser driver which restrains jitters in the optical signal and an optical module including the same.
For solving the above-mentioned problem, the laser driver in accordance with one aspect of the present invention is a laser driver for driving a laser diode (LD) by a differential signal having a pair of positive phase and negative phase components. The laser driver comprises an output terminal configured to be connected to an anode of the LD, a first circuit configured to generate a first modulation current from the positive phase component of the differential signal and provide the first modulation current to the anode of the LID through the output terminal, a second circuit configured to generate a second modulation current from the negative phase component of the differential signal and provide the second modulation current to the anode of the LD through the output terminal. The first circuit includes a frequency compensator which boosts frequency components of the positive phase component within a predetermined frequency region.
The optical module in accordance with another aspect of the present invention comprises a laser diode (LD) configured to convert a drive current to an optical signal, the laser driver having the output terminal connected to an anode of the LD. The first modulation current increasing the drive current and the second modulation current decreasing the drive current.
In the following, an optical module in accordance with a preferred embodiment of the present invention will be explained in detail with reference to the accompanying drawings. In the explanation of the drawings, the same constituents will be referred to with the same signs while omitting their overlapping descriptions.
An optical module 1 in accordance with this embodiment is a TOSA (Transmitter Optical Sub-Assembly) which outputs an optical signal in response to an electric signal input from an external device. The optical module 1 includes a driver 3 for driving a laser diode (LD) by a push-pull driving-technique.
As illustrated in this drawing, the optical module 1 mainly comprises a laser diode LD and the driver 3. An example of laser diode LD is a distributed-feedback laser diode. The driver 3 supplies a modulation current to the laser diode LD by push-pull operations described below. The laser diode LD has a cathode (negative electrode) connected to a ground and an anode (positive electrode) connected to a voltage VCC1 through a current source TB. As a consequence, the laser diode LD is supplied with a DC bias current Ibias, which is automatically maintained constant by an APC (Automatic Power Control) circuit (not depicted)An output terminal OUT of the driver 3 is connected to the anode of the laser diode LID through a bonding wire B1. In such a structure, a drive current to drive the laser diode LD is determined by the current source IB and the driver 3. The driving current is input to the anode of the laser diode LD. The laser diode LD outputs an optical signal in response to the drive current supplied.
The driver 3, which includes voltage-controlled current sources(first and second circuit) VCCS1, VCCS2, increases and decreases the drive current for the direct modulation responding to a differential input signal having a pair of positive phase and negative phase signals (positive phase and negative phase components) from the outside. The voltage-controlled current source VCCS1 is connected between an input terminal INP and the output terminal OUT. The voltage-controlled current source VCCS1 generates a modulation current Ip in response to a positive phase signal Vinp (the positive phase component of the differential input signal) input through the input terminal INP. The modulation current Ip is pushed out toward the laser diode LD through the bonding wire B1. The voltage-controlled current source VCCS2 is connected between an input terminal INN and the output terminal OUT. The voltage-controlled current source VCCS2 generates a current In in response to a negative phase signal Vinn (the negative phase component of the differential input signal) input through the input terminal INN. The current In is pulled in from the laser diode LD through the bonding wire B1.
The driver 3 generates a drive current ILD to drive the laser diode LD by superimposing the modulation currents Ip and In with the bias current Ibias. Therefore, the drive current ILD equals the bias current Ibias plus the modulation current Ip minus the modulation current In (where a positive current corresponds to a current flowing from the output terminal OUT to the laser diode and a negative current corresponds to a current flowing from the laser diode to the output terminal OUT). In other words, the voltage-controlled current source VCCS1 increases the drive current ILD as the positive phase signal Vinp increases, and the voltage-controlled current source VCCS2 decreases the drive current IUD as the negative phase signal Vinn increases. These modulation currents Ip, In directly modulate the laser diode LD to which the bias current Ibias is constantly applied. Thus, the driver 3 pushes the modulation current Ip into a load circuit (laser diode LD and pull the current In from the load circuit (laser diode LD) complementarily depending on the differential input signal. Such complementary driving operations are referred to as push-pull operations, and a driver for driving the load circuit (laser diode LD) by the push-pull operations according to an input signal is called a push-pull driver.
The structure of the driver 3 will now be explained in more detail.
The input terminals INP is connected to a termination node through a terminator R1 and the input terminal INN is also connected to the termination node through a terminator R2. Each of the terminators R1, R2 has a resistance value of 50 for example. The termination node is grounded through a capacitor C1 in order to lower common-mode impedance and biased to a reference potential Vref0 by a voltage source 5.
The voltage-controlled current source VCCS1 is constituted by an NPN bipolar transistor Q0, a current source I0, a bandpass filter (frequency compensator) 7, an nMOS transistor (n-type Metal-Oxide-Semiconductor Field-Effect Transistor) M0 which is an n-type field-effect transistor, and a resistor Rb. The NPN bipolar transistor Q0 has a base connected to the input terminal INP, an emitter grounded through the current source I0, and a collector connected to a supply voltage VCC0. The emitter of the NPN bipolar transistor Q0 is also connected to a gate of the nMOS transistor M0 through the bandpass filter 7. The nMOS transistor M0 has a drain connected to the supply voltage VCC0 and a source connected to the output terminal OUT through the resistor Rb.
In the voltage-controlled current source VCCS1, the emitter follower constituted by the NPN bipolar transistor Q0 receives the positive phase signal Vinp, while the output of the emitter follower VCCS1 is input to the gate of the nMOS transistor M0 through the bandpass filter 7. The gate of the nMOS transistor M0 is further connected to the supply voltage VCC0 through a resistor Ra (which will be explained later) within the bandpass filter 7. The nMOS transistor M0 and resistor Rb output the modulation current Ip toward the output terminal OUT according to the positive phase signal Vinp. That is, the modulation current Ip increases with the positive phase signal Vinp. Here, the bandpass filter 7 makes the frequency components of the positive phase signal VinP in a predetermined frequency region pass through and suppress the other frequency components outside of the predetermined frequency region. As a result, the frequency response of the modulation current Ip with respect to the differential input signal is boosted in the predetermined frequency region.
The voltage-controlled current source VCCS2 is constituted by an NPN-bipolar transistor Q1, a current source I1, an NPN bipolar transistor Q2, and a resistor Re. The NPN bipolar transistor Q1 has a base connected to the input terminal INN, an emitter grounded through the current source I1, and a collector connected to the supply voltage VCC0. The emitter of the NPN bipolar transistor Q1 is also connected to a base of the NPN bipolar transistor Q2. The NPN bipolar transistor Q2 has a collector connected to the output terminal OUT and an emitter grounded through the resistor Re.
In the voltage controlled current source VCCS2, the base of the NPN bipolar transistor Q2 is biased to a bias potential determined by the voltage source 5 through the terminator R2 and an emitter follower constituted by the NPN bipolar transistor Q1. Letting Ib1 be the base current of the NPN bipolar transistor Q1, and Vbe1 be the base-emitter voltage, for example, the bias voltage is Vref0-R2*Ib1-Vbe1. The collector of the NPN bipolar transistor Q2 is biased to the on-state voltage of the laser diode LD. The negative phase signal Vin is received by the emitter follower constituted by the NPN bipolar transistor Q1, and the output of the emitter follower is input to the base of the NPN bipolar transistor Q2. The NPN bipolar transistor Q2 and the resistor Re pull in the current In from the output terminal OUT according to the negative phase signal Vinn. That is, the current In increases with the negative phase signal Vinn.
The gain for the negative phase signal Vinn in the voltage-controlled current source VCCS2 is set greater than the gain for the positive phase signal Vinp in the voltage-controlled current source VCCS1. The following is a reason therefor. That is, while it is necessary to decrease the resistance of the resistor Rb in order to increase the gain on the voltage-controlled current source VCCS1, when the resistance is set too low, the output resistance of the voltage-controlled current source VCCS1 becomes comparable to the impedance of the laser diode LD. As the resistance of the resistor Rb can be seen in parallel with the impedance of the laser diode LD from the voltage-controlled current source VCCS2, the current In is harder to flow to the laser diode LD (some component of the current In is pulled in from the voltage-controlled current source VCCS1). At the same time, a plurality of parasitic capacitances Cgd, Cds, and Cdb (drain-body capacitance) of the nMOS transistor M0 become more influential, so that they deteriorates the electrical-to-optical response in a high frequency region and so the high-speed performance of the optical module 1. Setting a greater gain for the voltage-controlled current source VCCS2 prevents such a disadvantageous state.
In this bandpass filter 7, the output impedance of the emitter follower and the capacitor Ca form a low-pass filter, the capacitor C0 and the resistor Ra form a high-pass filter, and these filters are combined together so as to constitute a bandpass filter. That is, it is constructed such that the positive phase signal Vinp input from the input terminal INP passes the low-pass filter unit 9 and then the high-pass filter unit 11. This can make a gain greater in a frequency region between the frequency (second frequency) set by the high-pass filter unit 11 and the frequency (first frequency) set by the low-pass filter unit 9 than in the other frequency regions.
The driver 3 explained in the foregoing increases and decreases the drive current for the laser diode LD as the positive phase signal Vinp and negative phase signal Vinn increase, respectively. Here, the voltage-controlled current source VCCS1 for controlling the drive current according to the positive phase signal Vinp is equipped with the bandpass filter 7, which makes the gain for the positive phase signal Vinp in the voltage-controlled current source VCCS1 greater in a predetermined frequency region than in a frequency region other than the predetermined frequency region. As a result, the frequency characteristic of the electrical-to-optical response of the laser diode LD can be compensated and made flatter by the bandpass filter 7. This can improve the group delay of optical output signals generated by the laser diode LD and reduce jitters in the optical signals.
The bandpass filter 7, which includes the low-pass filter 9 and high-pass filter 11, is constructed such that the positive phase component Vinp passes the low-pass filter 9 and then the high-pass filter unit 11. Such a structure can make the gain for the positive phase signal Vinp in the voltage-controlled current source VCCS1 greater in the predetermined frequency region by a simple circuit configuration.
In the following, the electrical-to-optical response in this embodiment will be explained, in comparison with a comparative example.
According to these responses characteristics, the optical module 901 has a substantially flat response on the positive phase from 1 GHz to 15 GHz. Here, the gradient occurring at 15 GHz and above results from the circuit such as elements and parasitic components. On the other hand, the optical module 1 has a response characteristic on the positive phase component forming a peak from near 2 GHz to near 10 GHz. In the total characteristic combining the positive phase and negative phase components, the optical module 901 has a depression from 0 GHz to 10 GHz, whereas the optical module 1 has an improved flatness by compensating the depression.
Though a preferred embodiment in accordance with the present invention is illustrated and explained in the foregoing, the present invention is not limited to the above-mentioned specific embodiment. That is, it is easy for one skilled in the art to understand various modifications and changes are possible within the scope of the gist of the present invention set forth in the claims.
A characteristic curve CC6 in
Various circuit structures can be used to constitute the voltage source 5 in
Number | Date | Country | Kind |
---|---|---|---|
2014-143916 | Jul 2014 | JP | national |