Method and apparatus for driving laser diode

[0002] The invention relates to a method of driving a laser diode which may be utilized in high rate data transmission or the like, for example, and a driving apparatus using the driving method.

[0003] FIG. 11 shows an example of a conventional laser diode driver arrangement in its simplest form. A pair of transistors Q₁and Q₂in differential connection have their emitters connected to a current source Isd in common. A fixed bias voltage Vb is connected to the base of one of the transistors, Q₂. Accordingly, the transistors Q₁and Q₂operate as switching elements in a manner such that when one of them is turned on, the other is turned off. In the example shown, a laser diode LD is connected to the collector of the transistor Q₁, the base of which is connected with an input terminal T_INwhich is fed with a pulse drive signal V_INof a positive polarity. Each time a pulse in the signal V_INrises to assume a high level, the transistor Q₁. is turned on to cause a light emission from the laser diode LD, thus radiating radiation P_OUT. When the pulse applied to the input terminal T_INfalls, the transistors Q₁is turned off, while the transistor Q2 is turned on. The pair of transistors Q₁and Q₂are turned on and off in a differential manner to achieve a high rate switching operation.

[0004] In FIG. 12, there is illustrated a relationship between the current If passing through the laser diode LD and an output radiation P_OUT, which are depicted at A and B, respectively. A flow of bias current Ib which is close to an emission initiating current Ith is maintained through the laser diode LD by a current source Isb. A sophistication is made such that as soon as the transistors Q₁is turned on, the drive current If immediately exceeds the emission initiating current Ith, thus minimizing a lag in the light emission. The drive current If is shown at A in FIG. 12 while a response of radiation P_OUTis shown at B in FIG. 12. A lag of radiation P_OUTwith respect to the rising and the falling edge of the drive current If is shown exaggerated at T_DLY(ON)and T_DLY(OFF)as will be noted in the graph B of FIG. 12.

[0005] A lag in the termination of radiation P_OUTis attributable to the fact that a carrier concentration which remains in the active layer of the laser diode LD fails to return to zero rapidly when the drive voltage is interrupted. A difficulty that is caused by the lag in reducing the carrier concentration is illustrated in FIG. 13

[0006] For example, as illustrated in FIG. 13A, as the OFF interval T₁of the drive current I^for the extinction interval becomes shortened, the drive current I_fwould begin to flow at a timing when the carrier concentration is not reduced sufficiently, as shown in FIG. 13B, the time interval T_DLY(ON)until the next emission is reached would be shorter or T_DLY(ON1)>, T_DLY(ON2), as illustrated in FIG. 13C. In the example shown in FIG. 2, it is longer again in the OFF interval T₂, whereby the delay of the emission with respect to the immediately following rising edge of the drive current, or T_DLY(ON3), is longer than T_DLY(ON2). This means that a jitter is produced depending on the length of the extinction period T_OFF, and the occurrence of a jitter presents a fault to the high rate transmission in a disadvantageous manner. This problem is discussed in“Effect of Bit-Rate, Bias and Threshold Currents on Turn-on Timing Jitter in Lasers Modulated with Uncoded and Coded Waveforms,” IEEE Photonics Technology Letters, Vol. 8, No. 3, March 1996, pp 461-463. This literature indicates that the turn-on time T_ONof the laser diode LD after extinction period T_OFFis given as follows: $\begin{array}{cc}{T}_{\mathrm{ON}}=\tau \ln \left\{1+\frac{\mathrm{Ith}-\mathrm{Ib}}{\mathrm{Im}-\mathrm{Ith}}\left[1-\exp \left(-T_{\mathrm{OFF}}/\tau \right)\right]\right\}& \text{(1)}\end{array}$

[0007] where τ represents a residual carrier concentration (carrier lifetime) in the active layer of the laser diode LD. FIG. 14 shows the relationship as represented by the equation (1) where turn-on time T_ONand the extinction period T_OFFare normalized with τ, respectively. Curves 12A, 12B and 12C depicted in FIG. 12 correspond to values of Im which are equal to 2Ith, 3Ith and 4Ith, respectively, for Ib=0.5 Ith. It will be seen from FIG. 14 that the turn-on time T_ONvaries with the extinction period T_OFFfor a range of T_OFFfrom 0 to nearly three of four times τ, but remains constant for any subsequent change in the extinction period T_OFF. The lifetime τ of residual carriers of a laser diode is usually on the order of 2 to 3 ns, and thus it is seen that if a time interval from a preceding pulse is less than 10 ns, a jitter occurs unavoidably. However, the jitter itself (or T_ON) can be reduced if the drive current If is increased.

[0008] It is an object of the invention to provide a method of driving a laser diode which is capable of suppressing the occurrence of a jitter and a laser diode driver arrangement which utilizes the driving method.

[0009] An apparatus for driving a laser diode according to a first aspect of the invention comprises:

[0010] a laser diode;

[0011] power supply means for supplying a driving power to the laser diode;

[0012] switching means connected in series with the laser diode for controlling a drive current which passes through the laser diode in accordance with a drive signal which repeats an ON and an OFF interval to switch the laser diode between a light emission and an extinction;

[0013] time measuring means for measuring the OFF interval of the drive signal and delivering a time signal having a level which corresponds to the interval; and

[0014] current control means for controlling the drive current in accordance with a control signal which corresponds to the time signal such that the shorter the OFF interval, the longer the rising of the drive current is retarded.

[0015] An apparatus for driving a laser diode according to a second aspect of the invention comprises:

[0016] a laser diode;

[0017] power supply means for supplying a driving power to the laser diode;

[0018] switching means connected in series with the laser diode for controlling a drive current which passes through the laser diode in accordance with a drive signal to switch the laser diode between a light emission and an extinction; and

[0019] differential current generating means for generating a differential current from the drive signal and for superimposing the differential current upon the drive current through the laser diode.

[0020] A method of driving a laser diode according to the first aspect of the invention comprises the steps of:

[0021] (a) measuring an OFF interval in the drive signal and producing a time signal of a level which corresponds to the OFF interval; and

[0022] (b) controlling the drive current on the basis of the time signal such that the shorter the measured interval, the longer the rising time which the drive current is retarded.

[0023] A method of driving a laser diode according to the second aspect of the invention comprises the steps of:

[0024] (a) generating a differential current from the drive signal; and

[0025] (b) superimposing the differential current upon the drive current so that the differential current from the falling edge of the drive current is effective in reducing residual carriers.

[0026] With the method and apparatus for driving a laser diode according to the invention, the shorter the time interval since a preceding drive current has been interrupted, the greater the magnitude of a reverse current that is superimposed upon the drive current at the next time the drive current rises. In this manner, the next emission is controlled to be occurring in a retarded direction. Consequently, a rising of the emission is restricted, affording an advantage that the occurrence of a jitter in the data that is optically transmitted is suppressed.

[0027] FIG. 1 is a circuit diagram illustrating a first embodiment of the invention;

[0028] FIG. 2 is a series of timing charts illustrating the operation of the embodiment of FIG. 1;

[0029] FIG. 3 is a circuit diagram illustrating a second embodiment of the invention;

[0030] FIG. 4 is a series of timing charts illustrating the operation of the embodiment of FIG. 3;

[0031] FIG. 5 is a circuit diagram illustrating a third embodiment of the invention;

[0032] FIG. 6 is series of timing charts illustrating the operation of the embodiment shown in FIG. 5;

[0033] FIG. 7 is a circuit diagram illustrating a fourth embodiment of the invention;

[0034] FIG. 8 is a series of waveform diagrams illustrating the operation of the embodiment shown in FIG. 7;

[0035] FIG. 9 is a circuit diagram showing a fifth embodiment of the invention;

[0036] FIG. 10 is a series of waveform diagrams illustrating the operation of the embodiment shown in FIG. 9;

[0037] FIG. 11 is a circuit diagram illustrating a prior art;

[0038] FIG. 12 are waveform diagrams illustrating the operation of FIG. 11;

[0039] FIG. 13 is a series of waveform diagrams illustrating a disadvantage of the prior art; and

[0040] FIG. 14 graphically shows a relationship between the extinction period and turn-on time.

[0041] Referring to FIG. 1, a method and an apparatus for driving a laser according to one embodiment of the invention will be described. It should be noted that similar parts as those shown in FIG. 11 are designated by same reference characters in FIG. 1 as used in the description of FIG. 11. The present invention is characterized by the provision of a time measuring unit 11 which measures the length of time which elapsed since the interruption of a drive current, and a current controller 12 which controls a rising of a next drive current in a manner such that the shorter the length of time measured by the time measuring unit 11, the longer it takes for the next drive current to rise. The embodiment of FIG. 1 controls the turn-on time T_ONof the laser diode by controlling the current Im appearing in the equation (1).

[0042] In the example shown in FIG. 1, the time measuring unit 11 determines the lapse of time since the falling edge of a drive signal V_INthrough a time constant circuit TMC, and an arrangement is illustrated here in which the shorter the OFF interval to a next rising, the less the magnitude of an amount by which the next rising driving current exceeds the emission initiating current Ith is restricted for a given interval by the current controller 12.

[0043] A drive signal V_INas shown in FIG. 2A is input to the time constant circuit TMC from an input terminal T_INthrough a delay element DLY and a buffer amplifier BUF. The time constant circuit TMC includes a resistor R, and a voltage VH of H-logic representing a drive signal V_INis applied to one end of the resistor R. The time constant circuit TMC also includes a capacitor C, and a junction A between the capacitor C and the resistor R of the time constant circuit TMC is connected to the anode of a diode D, the cathode of which is supplied with VH−Vf where Vf represents a forward voltage across the diode D. Accordingly, when the drive signal V_INrises to H-logic, a voltage at the junction A rises in retarded relationship by a delay time T_DYproduced by the delay element DLY, as shown in FIG. 2B, thus clipping the voltage V_Aat the junction A at voltage level VH as it tends to exceed the voltage level VH. Accordingly, a differentiated waveform of positive polarity, shown in dotted lines in FIG. 2B, which is developed across the resistor R is removed.

[0044] The current controller 12 may comprise a transistor Q₃having its collector connected to the common emitter junction of the pair of transistors Q₁and Q₂which are turned on and off in a differential manner. The transistor Q₃has a base, to which a given forward bias voltage Vcont from a bias voltage source 9 is applied through an adder ADD. In addition, a time signal of a negative polarity −Vdt (reverse bias voltage) which is generated by the time measuring unit 11 at the falling edge of the drive signal V_INis also applied to the adder ADD so that the shorter the length of time elapsed since the interruption of the previous current, the greater the magnitude of a reverse bias voltage (time signal −Vdt) applied to the current controller 12 at the rising edge of the drive signal V_IN, as shown in FIG. 2B.

[0045] In the embodiment of FIG. 1, as the drive signal V_INfalls, the voltage at the junction A is biased in the negative direction as referenced to VH (which will be hereafter referred to as“a reverse bias”) with a delay time T_DYproduced by the delay element DLY, beginning to charge the capacitor C through the resistor R in the opposite sense from the previous operation. Accordingly, the reverse bias voltage −Vdt applied to the junction A decreases with a time constant determined by RC, allowing the voltage V_Ato approach VH. It will be seen from FIG. 2 that the shorter the OFF interval of the drive signal V_IN, the greater the magnitude of the reverse bias voltage (a negative voltage as referenced to VH) which remains to be applied to the junction A at the time when the drive signal V_INrises for the next time. As a consequence, it will be noted from FIG. 2C that the magnitude of an amount by which the drive current If exceeds the emission initiating current Ith at the time when the drive current V_INrises is reduced in a corresponding manner, and because the voltage V_Aat the junction A rises with the delay time T_DYfrom this point in time, it follows that an amount of carriers injected into the active layer of the laser diode by the time the drive current If rises to the maximum current is reduced in a corresponding manner. A product of the magnitude of the reverse bias voltage −Vdt applied to the junction A at the rising time of the drive signal V_INand the interval T_DYduring which the bias is applied, which is shown in FIG. 2C as an area of a hatched area, represents an amount of carrier injection suppressed by the arrangement of the invention, or an amount of carrier injection which would take place during the interval T_DYif the drive current If is allowed to rise at the same time as the rising edge of the drive signal V_IN. It will be seen that the greater the amount of carrier injection thus suppressed, the longer the time until the laser diode LD is actually driven into oscillation. Consequently, in the example illustrated in FIG. 2C, the amount of carrier injection suppressed at the rising edge of the drive signal which immediately follows the OFF interval, and hence the delay time until the emission, is the greatest for the OFF interval T₁, and then sequentially decreases for the OFF intervals T₂and T₃.

[0046] When a choice is made in the equation (1) given above so that the following equality:

[0047] (Im−Ith)=Id=k[1−exp(−T_OFF/τ)]  (2)

[0048] applies where k represents a constant of proportionality, the turn-on time T_ONcan be made to be constant. Accordingly, the time constant R·C=τ of the time constant circuit TMC is chosen to be substantially equal to the lifetime τ of carriers in the active layer of the laser diode LD to be driven. Thus, when the time signal −Vdt returns to VH volt, it follows that the carrier concentration substantially reverts to 0. If the drive signal V_INthen rises to its H-logic at this point in time, the current If which passes through the laser diode LD rises with a given rising time T_ON.

[0049] By contrast, if the drive signal V_INrises to its H-logic at the time when the length of time elapsed since the interruption of the previous drive current is short, a time signal −Vdt of a higher reverse bias is applied to the current controller 12, and the H-logic signal will be applied under this condition. This means that it takes a longer time for the current passing through the laser diode LD to rise to the emission initiating current Ith, as shown in FIG. 2C, thus retarding the timing when the light emission occurs from the laser diode LD by a length of time which corresponds to the contribution of any residual carrier concentration to accelerating the initiation of the light emission.

[0050] Accordingly, when the drive signal V_INhave successive off times T₁, T₂and T₃, which are related such that T₁<T₂<T₃, the length of time T₁₁, T₂₂and T₃₃, which is required for the current If passing through the laser diode LD to reach the emission initiating current value Ith since the rising time of the drive signal V_INwill be related such that T₁₁>T₂₂>T₃₃, as shown in FIG. 2C. In other words, the length of time from the inversion of the drive signal V_INto its H-logic until the drive current If actually exceeds the emission initiating current value Ith will be increasingly shorter when the off terms T₁, T₂and T₃are increasingly longer.

[0051] As a consequence, the timing when the light emission from the laser diode LD occurs can be made uniform if the off times of the laser diode LD are related such that T₁<T₂<T₃, allowing the laser diode to be driven into emission at substantially an equal timing from the rising edge of the drive signal V_IN. Where data is transmitted in accordance with a flashing operation of the laser diode LD, there is obtained an advantage that the occurrence of a jitter can be suppressed.

[0052] FIG. 3 shows a second embodiment of the invention which is intended to provide a constant turn-on time T_ON. by controlling the bias current Ib which appears in the equation (1). In this embodiment, a current controller 12 is provided as a current source which supplies a bias current Ib to the laser diode LD, and comprises a transistor Q₃, the current flow Ib through which is controlled in accordance with the time signal −Vdt which is determined by the time measuring unit 11. However, it is to be noted that there is no element which corresponds to the delay element DLY in the time measuring unit 11 shown in the embodiment of FIG. 1. In addition, the emitters of the transistors Q₁and Q₂in the differential connection are connected in common to a constant current source Isd which provides a current Id. In other respects the arrangement is similar to the embodiment shown in FIG. 1.

[0053] It will be noted from FIG. 4B that the waveform of the voltage V_Aat the junction A does not contain a delay T_DY, as illustrated in FIG. 2B, at the time the drive signal V_INis applied as shown in FIG. 4A. Accordingly, the drive current If rises at the same timing as the drive signal V_INrises, as indicated in FIG. 4C. For each of the OFF intervals T₁, T₂and T₃, the time signal −Vdt is applied to the base of the transistor Q₃in the current controller 12 through an adder ADD as a reverse bias voltage which decreases with a time constant CR, as indicated in FIG. 4B.

[0054] The shorter the length of time since the interruption of the drive current If passing through the laser diode LD, the greater the bias voltage which is applied to the base of the transistor Q₃as shown in FIG. 2B, and accordingly, if the drive signal V_INreverts to its H-logic in a shorter interval since the falling edge thereof to its L-logic, it will be seen that the bias current Ib which is applied as the drive current If during the OFF interval rises from a small value, as shown in FIG. 4C. It will be apparent from the equation (1) given above that if the H-logic of the drive signal V_INprovides the drive current Im=Ib+Id of a given magnitude, it takes a longer turn-on time T_ONfor the laser diode when the bias current Ib has a small magnitude, thus retarding the time when the oscillation is entered.

[0055] When the drive signal V_INreverts to its H-logic at a timing when a length of time on the order of the lifetime τ of carriers in the active layer of the laser diode LD or longer has passed since the interruption of the drive current If through the laser diode LD, it will be seen that the time signal −Vdt delivered from the time measuring unit 11 is approaching VH volt or has already reached VH volt, and this means that the high bias current Ib is already passing through the laser diode, and accordingly, the laser diode is driven into oscillation in a reduced length of time in response to the reversion of the drive signal V_INto its H-logic.

[0056] FIG. 5 shows a third embodiment of the invention which is intended to control the turn-on time T_ONby controlling the OFF interval of the drive signal which is applied to the transistor Q₁actually driving the laser diode in accordance with the length of the OFF interval of the input drive signal V_INas if appears in the equation (1). In this embodiment, the current controller 12 shown in FIG. 1 is replaced by a fixed current source Isd as shown in FIG. 3, and a variable delay circuit V_DLYis connected between the base of the transistor Q₁and the input terminal T_INso that the combination of the variable delay circuit V_DLYand the transistor Q₁constitutes together a current controller 12. By controlling a delay time provided by the variable delay circuit V_DLYby a time signal −Vdt delivered from the time measuring unit 11, a delay time for the drive signal V_INto be transmitted from the input terminal T_INto the base of the transistor Q₁can be controlled.

[0057] The time measuring unit 11 is constructed in the similar manner as in FIG. 1, and accordingly, the voltage V_Aat the junction A represents a negative differentiated waveform with a time delay Td provided by the delay element DLY with respect to the input drive signal VIN shown in FIG. 6A, in the similar manner as shown in FIG. 2B. The delay time Td of the variable delay circuit V_DLYis controlled in a manner such that the greater the absolute magnitude of the time signal −Vdt in the negative direction, the longer the delay time. Accordingly, an output voltage from the variable delay circuit V_DLYor an input base voltage of the transistor Q₁is such that its rising time, which corresponds to the rising edge of the drive signal, is delayed by an interval Td which corresponds to the value of the time signal −Vdt which prevails at the rising edge of the drive signal, even though its falling edge coincides with the falling edge of the drive signal V_IN, as illustrated in FIG. 6C. As a consequence, when the drive signal V_INhas OFF intervals related such that T₁<T₂<T₃as indicated in FIG. 6A, if follows that the negative voltage −Vdt at the end of the respective OFF interval (or at the rising edge of the drive signal) is sequentially smaller, and accordingly, the delay time provided by the variable delay circuit V_DLYfor the rising edge of the drive signal is also sequentially smaller, as indicated in FIG. 6C. Accordingly, if the drive signal V_INreverts to its H-logic in a short length of time from the interruption of drive current If passing through the laser diode LD, the time measuring unit 11 delivers the time signal −Vdt of a negative polarity and having a greater magnitude, whereby the variable delay circuit V_DLYis controlled to increase the delay time, and thus the reversion of the drive signal V_INapplied to the input terminal T_INto its H-logic undergoes a longer delay before it is transmitted to the base of the transistor Q₁. As a consequence, a rising of the drive current If through the transistor Q₁is retarded, as is the light emission from the laser diode LD.

[0058] When the drive signal reverts to its H-logic at a timing when a time comparable to the lifetime τ of carriers in the active layer of the laser diode LD has passed since the interruption of the drive current If through the laser diode LD, the time signal −Vdt delivered from the time measuring unit 11 is nearly VH volt or has already reached the VH volt. Accordingly, in this instance, the delay time provided by the variable delay circuit V_DLYis at its minimum, and the input applied to the input terminal T_INas a reversion to H-logic is transmitted with a small lag to the transistor Q₁, allowing the drive current If through the laser diode LD to rise with a small lag, and thus allowing the laser diode LD to be driven into light emission in a reduced length of time.

[0059] FIG. 7 shows a fourth embodiment of the apparatus for driving a laser diode according to the invention which controls the turn-on time T_ONof the laser diode by controlling the drive current If=Im which appears in the equation (1) in the similar manner as the embodiment of FIG. 1, but in which unlike the embodiment of FIG. 1, a control current Idt is superimposed on the drive current If=Ib+Id. In this embodiment, there is provided a differential current generator 13 which differentiates the drive signal V_INto generate a differential current, which is then superimposed upon the drive current If through the laser diode LD. In the example shown in FIG. 7, the differential current derived from the drive current promotes the falling edge of the drive current If or is a reverse current to the laser diode LD at the time of extinction of the laser diode LD, thus contributing to rapidly reducing carriers which may be injected into the active layer of the laser diode LD. In the example shown in FIG. 7, the laser diode LD is driven in phase with the drive signal V_IN, and the differential current generator 13 comprises a buffer BUF and a capacitor C. Specifically, the buffer BUF generates a duplicate drive signal of the same polarity as the drive signal, which is applied to the capacitor C, thus charging and discharging the capacitor C to generate a differential current. In response to the drive current If shown in FIG. 8A, the generator 13 generates a differential current Idt shown in FIG. 8B, which is then superimposed upon the drive current If through the laser diode in a manner shown in FIG. 8C. When the positive differential current is superimposed on the rising drive current If, there is achieved the effect of reducing the turn-on time T_ONby increasing the current Im as mentioned above in connection with FIG. 4. The differential current in a negative direction which is indicated in a region shown hatched is a reverse current with respect to the forward direction of the laser diode and is effective to reduce any residual carriers in the laser diode LD rapidly, thus allowing an influence of such residual carriers to be reduced from the next rising of the drive current.

[0060] FIG. 9 shows a modification of the embodiment shown in FIG. 7 where the laser diode LD is driven in opposite phase to the drive signal V_IN. In this instance, a differential current generator 13 comprises a cascade connection of an inverter INV and a capacitor C. The inverter INV generates a duplicate drive signal of the opposite polarity from the drive signal, which is applied to the capacitor C to cause a charging and a discharging of the capacitor to generate a differential current. The drive current V_IN, as shown in FIG. 10A, is supplied to the cascade connection of the inverter INV and the capacitor C, and a differential current Idt as shown in FIG. 10C is derived at its output and is superimposed upon the drive current If, shown in FIG. 10B of the laser diode LD, as indicated in FIG. 10D.

[0061] Again, the differential current which represent a reverse flow with respect to the laser diode LD reduces residual carriers in the laser diode LD, which then operates to reach a light emission with a given time interval from the rising edge of the drive current If independently from the off time of the laser diode LD.

[0062] In the embodiments shown in FIGS. 7 and 9, a positive differential current is also superimposed, providing an advantage that the length of time until the light emission from the laser diode can be initiated can be reduced.

[0063] As discussed above, in accordance with the invention, a jitter which may occur on an optical pulse can be eliminated by utilizing the characteristic of a laser diode that the timing of next light emission varies depending on the length of time elapsed since the extinction. As a consequence, the invention allows a high quality data transmission when it is applied to a high rate data transmission which utilizes optical pulses of a reduced pulse width. For example, it may be utilized in a data delivery and reception within an IC testing equipment with a great advantage.