JP6024273B2 - Optical transmitter - Google Patents

Description

  The present invention relates to an optical transmission device, and more particularly, to an optical transmission device including a laser diode that outputs an optical signal and a photodiode that monitors the optical output.

  In an optical transmitter, various proposals have been made for stably controlling the intensity and extinction ratio of an optical signal output from a light emitting element such as a laser diode with an appropriate value. For example, in the laser diode (LD) drive circuit disclosed in Patent Document 1, the optical output of the laser diode (LD) is monitored by a photodiode (PD), the PD output value is compared with a reference value, and the comparison result Is negatively fed back to the LD bias current source to control the bias current. In addition, the PD output value is compared with the reference value, and the comparison result is negatively fed back to the laser drive current (pulse current) source to control the LD drive current and improve the duty ratio and ringing of the LD signal, while at high speed. An LD driving circuit capable of operation is disclosed.

JP 2006-100555 A

  In the LD drive circuit of Patent Document 1, in order to cope with high-speed operation, by supplying a compensation current for compensating the drive current of the preamplifier circuit, the slew rate is made variable, and the rise / The fall time is shortened and high-speed transmission is realized. However, if the driver output intensity decreases on the high frequency side and the bandwidth of the LD driver circuit is limited, the optical signal intensity of the high frequency component decreases. Therefore, simply improving the slew rate of the preamplifier circuit improves the eye pattern. It ’s difficult.

  Each LD has a temperature dependence of slope efficiency, and each LD has a variation in temperature dependence of slope efficiency. When this variation exists, if APC control (Automatic Power Control) is performed, there is a problem that high-frequency characteristics cannot be compensated for due to laser temperature fluctuations. For this reason, when there is variation in the characteristics of the LD, there is a problem that appropriate band compensation cannot be performed for each individual.

  The present invention has been made in view of these circumstances, and provides an optical transmission device that can maintain not only the optical output but also the frequency characteristics constant even when there are variations in the characteristics of the LD. The purpose is to provide.

An optical transmission apparatus according to the present invention includes a laser diode that outputs an optical signal and a photodiode that monitors the optical output of the laser diode, and performs optical feedback control of negative feedback of the bias current of the laser diode based on the monitor value of the photodiode. Device. The modulation current of the laser diode is generated by an individually controllable laser diode driver circuit and a pre-emphasis driver circuit, and the drive current source of the pre-emphasis driver circuit is controlled based on the bias current. The output current of the pre-emphasis driver circuit is increased .

In the present invention, the drive current source of the laser diode driver circuit may be subjected to negative feedback control based on the monitor value of the photodiode . Further, the higher the electro-optical conversion efficiency of the laser diode, the smaller the bias current.

  According to the present invention, the bias current of the laser diode is subjected to negative feedback control based on the monitor value of the photodiode, and the drive current source of the pre-emphasis driver circuit that generates the modulation current of the laser diode is controlled based on the bias current. For this reason, even when there are variations in the characteristics of the laser diode, not only the optical output but also the frequency characteristics can be maintained constant.

It is a figure which shows the example of 1 structure of the optical transmitter of this invention. It is a figure which shows the other structural example of the optical transmission apparatus of this invention. It is a figure for demonstrating the frequency dependence of the electro-optical response characteristic of a laser diode. It is a figure for demonstrating the output waveform of the electric current by pre-emphasis. It is a figure which shows the change of the eye waveform at the time of changing the amount of pre-emphasis. It is a figure for demonstrating the frequency dependence of the electrooptical conversion intensity | strength by pre-emphasis.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments according to an optical transmission device of the invention will be described with reference to the drawings. FIG. 1 is a diagram showing a configuration example of an optical transmission apparatus according to the present invention. In the figure, 1 is a laser diode (hereinafter referred to as “LD”), 2 is an LD driver circuit, 3 is a pre-emphasis driver circuit, 4 is a pre-driver circuit, 5 is a delay circuit, 6 is a bias current source, and 7 is an LD. Driver current source, 8 is a pre-emphasis driver current source, 9 is a pre-driver current source, 10 is a photodiode (hereinafter referred to as “PD”), 11 is a transimpedance amplifier (hereinafter referred to as “TIA”), 12, Reference numeral 13 denotes a comparison amplification circuit, and 14 and 15 denote reference voltage generation circuits.

  The optical transmission device includes an LD 1 that transmits an optical signal, and a PD 10 that monitors the optical output of the LD 1. The drive circuit of the LD 1 includes an LD driver circuit 2 that applies a modulation current to the LD 1 and a pre-emphasis driver circuit 3 that emphasizes the modulation current, and the LD drive output by the LD driver circuit 2 and the pre-emphasis driver circuit 3 is a load resistance. R2 and R3 and a coupling capacitor C are added to LD1. Modulation current pre-emphasis will be described later.

  A drive signal from a pre-driver circuit 4 (also referred to as a preamplifier circuit) is input to the LD driver circuit 2 and the pre-emphasis driver circuit 3 that generate a predetermined optical pulse output in the LD 1. The signal slightly delayed by the delay circuit 5 is input. A bias current source 6 is connected to LD 1, an LD driver current source 7 is connected to the LD driver circuit 2, a pre-emphasis driver current source 8 is connected to the pre-emphasis driver circuit 3, and the pre-driver circuit 4 is connected to the pre-driver circuit 4. Are connected to the pre-driver current source 9 in a controllable manner.

  Here, the LD driver current source 7, the pre-emphasis driver current source 8, and the pre-driver current source 9 are amplitude control current sources of the LD driver circuit 2, the pre-emphasis driver circuit 3, and the pre-driver circuit 4, respectively. Yes, the output current corresponding to each current value of the LD driver current source 7, the pre-emphasis driver current source 8, and the pre-driver current source 9 is the LD driver circuit 2, the pre-emphasis driver circuit 3, and the pre-driver circuit 4. Is output from.

  Next, the operation of the optical transmission apparatus of the present invention will be described. Most of the optical signals output from the LD 1 are coupled to an optical fiber or the like and transmitted to the optical transmission path, but the remaining part of the optical signals are received by the monitoring PD 10. The optical signal received by the PD 10 is converted into a photocurrent signal. Next, this photocurrent signal is converted into a voltage signal by a transimpedance amplifier 11 (hereinafter referred to as TIA) and output as a monitor value of the optical output of the LD 1.

  The output voltage of the TIA 11 is compared with the reference voltage V1 from the reference voltage generation circuit 14 by the comparison amplifier circuit 12, and a voltage signal corresponding to the voltage difference is generated. The voltage signal from the comparison amplifier circuit 12 is negatively fed back to the bias current source 6 of the LD1, and the DC bias current Ib is controlled so that the output voltage of the TIA 11 becomes equal to the reference voltage V1. By this negative feedback control, the optical output of LD1 is held at a constant value. As described above, the DC bias current Ib of the LD 1 is controlled by the APC circuit.

  On the other hand, LD 1 is modulated and driven by the LD driver circuit 2. Note that the modulation current that is the output of the LD driver circuit 2 may also be controlled by the APC circuit. That is, as shown in FIG. 2, the comparison amplifier circuit 16 compares the output voltage of the TIA 11 with a predetermined reference voltage from the reference voltage generation circuit 17, and a signal voltage corresponding to the voltage difference is supplied to the LD driver current source 8. By performing negative feedback, the LD driver current source 8 may be controlled so that the output voltage of the TIA 11 becomes equal to a predetermined reference voltage.

The LD 1 modulated and driven by the LD driver circuit 2 converts the electrical modulation into an optical signal at a response speed determined by a relaxation oscillation frequency (fr). Here, the relaxation oscillation frequency fr of LD1 is proportional to the product of the square root of the current and the LD-specific proportionality constant (Δfr). On the other hand, LD1 has a unique electro-optical conversion efficiency (SE). The relaxation oscillation frequency fr when the APC control is performed so that the light output Pf is obtained at a certain temperature is determined by the following theoretical formula.

  FIG. 3 is a diagram for explaining the frequency dependence of the electro-optical response characteristics of the laser diode, and shows the frequency dependence of the electro-optical conversion characteristics of three types of high, medium and low LDs of electro-optical conversion efficiency. As can be seen from the figure, the band with a high electro-optical conversion efficiency SE tends to be narrow, and the band with a low electro-optical conversion efficiency SE tends to widen. An object of the present invention is to provide an optical transmitter capable of obtaining a constant high frequency response characteristic even when the electro-optical conversion efficiency SE of the LD varies as described above. The frequency response characteristic by pre-emphasis is used.

  Here, the pre-emphasis driver will be described. When a transmission signal is transmitted at a high speed of Gbps or more, a signal waveform of a high frequency component is blurred. In particular, the waveform of the rising / falling portion of the signal is rounded. Thus, if the transmission side sends the signal waveform with the rising or falling part being overshooted or undershooted in advance, the receiving side can be made to have an appropriate waveform. In this way, sending the signal waveform with emphasis before sending the transmission signal on the transmission side is called pre-emphasis.

  FIG. 4 is a diagram for explaining an output waveform of a current due to pre-emphasis. It is a figure explaining the driver current pre-emphasized and the modulation current of LD1. The modulation current Im for driving the LD 1 is generated by the output current from the LD driver circuit 2 and the output current from the pre-emphasis driver circuit 3. Each output current amplifies the pulse signal from the pre-driver circuit 4. Generated.

  FIG. 4A shows an output waveform of the driver current ILDD in the LD driver circuit 2, and a signal corresponding to this output waveform is given to the LD1. FIG. 4B shows an output waveform of the driver current IPE in the pre-emphasis driver circuit 3, and a signal corresponding to this output waveform is given to the LD1. The driver current ILDD in the LD driver circuit 2 is generated from the pulse signal from the predriver circuit 4. The driver current IPE in the pre-emphasis driver circuit 3 is generated by delaying the pulse signal from the pre-driver circuit 4 by the delay time τ and inverting the pulse signal.

  FIG. 4C shows the modulation current Im flowing through the LD1, and the modulation current Im includes the current ILDD of the LD driver circuit 2 of FIG. 4A and the current IPE of the pre-emphasis driver circuit 3 of FIG. In this modulation current Im, the rising and falling portions of the pulse-like current signal are emphasized by the output current IPE from the pre-emphasis driver circuit 3.

  The degree of rounding of the rising / falling portion of the received signal when the transmission signal is received on the receiving side by preliminarily transmitting the rising / falling portion of the transmission signal on the LD1 side as described above. Can be received in a reduced state.

  FIG. 5 is a diagram illustrating changes in the eye waveform when the pre-emphasis amount is changed. 5A shows a case where IPE / ILDD = 0, FIG. 5B shows a case where IPE / ILDD = 0.2, and FIG. 5C shows a case where IPE / ILDD = 0.4. From these figures, it can be seen that the rising of the transmission signal becomes faster as the ratio of the current IPE of the pre-emphasis driver circuit 3 to the current ILDD of the LD driver circuit 2 is increased. FIG. 6 is a diagram for explaining the frequency dependence of the electro-optical conversion intensity due to pre-emphasis. The amplitude is not amplified at the current IPE = 0 of the pre-emphasis driver circuit 3, but the response amplitude on the high frequency side increases as the IPE increases.

  From the above, the higher the output current IPE from the pre-emphasis driver circuit 3 is increased as the LD has a higher electro-optical conversion efficiency, and the lower the electro-optical conversion efficiency LD from the pre-emphasis driver circuit 3 is, the wider the band is. The output current IPE may be reduced.

Next, the description returns to the operation of the optical transmission apparatus of the present invention. First, when the electro-optical conversion efficiency SE of the LD 1 is higher than the center value of the variation, the light output of the LD 1 becomes larger than the average value of the variation. For this reason, the output of the PD 10 also becomes larger than the average value of variation, and the bias current Ib due to the APC operation becomes smaller than the average value of variation. This bias current Ib is converted into a current / voltage by the resistor R1, and the voltage signal input to the comparison amplifier circuit 2 is reduced.

Since the comparison amplifier circuit 13 amplifies the difference between the input signal voltage and the reference voltage V2 from the reference voltage generation circuit 15, the output of the comparison amplifier circuit 2 is increased. As a result, the pre-emphasis driver current source of the pre-emphasis driver circuit 3 is increased. The current IPE of 8 is increased from the average value of variation. For this reason, the rising / falling edge of the modulation current of LD1 is more emphasized, and the band is extended even when the electro-optical conversion efficiency SE of LD1 is higher than the center value of variation and the band is narrow. It becomes possible.

As the bias current Ib is reduced, the relaxation oscillation frequency fr is reduced. By appropriately setting the reference voltage V2 and the amplification factor of the comparison amplifier circuit 13 so as to cancel this action, the optimum high frequency for transmission can be obtained. Response characteristics can be obtained. For example, the reference voltage V2 and the amplification factor of the comparison amplifier circuit 13 are set by preparing two LDs that deviate from the reference LD and the reference electro-optical conversion efficiency, and IPE and ILDD so that the optimum signal waveforms are obtained. This ratio can be obtained by adjusting the relationship between the bias current Ib and the reference voltage V2.

Next, when the electro-optical conversion efficiency of the LD 1 is lower than the center value of the variation, the bias current Ib becomes larger than the average value of the variation by the APC operation. This bias current Ib is converted into a current voltage by the resistor R1, and the voltage signal input to the comparison amplifier circuit 13 is increased. For this reason, the output of the comparison amplifier circuit 13 decreases, and as a result, the current IPE of the pre-emphasis driver current source 8 of the pre-emphasis driver circuit 3 is decreased. Further, due to the effect of pre-emphasis, since the IPE is reduced, the effect of extending the bandwidth is also reduced.

However, since the bias current Ib increases, the relaxation oscillation frequency fr increases. The band is originally wide due to the low electro-optical conversion efficiency. If the reference voltage V2 and the amplification factor of the comparison amplifier circuit 2 are appropriately set as described above, the effect of extending the band is suppressed, but transmission It is possible to obtain an optimum high frequency response characteristic. Thus, by adjusting the amplitude of the current IPE of the pre-emphasis driver circuit 3 in accordance with the bias current Ib related to the electro-optical conversion efficiency SE of the LD 1, it is possible to obtain the frequency response characteristic necessary for high-speed transmission. It becomes.

DESCRIPTION OF SYMBOLS 1 ... Laser diode (LD), 2 ... LD driver circuit, 3 ... Pre-emphasis driver circuit, 4 ... Pre-driver circuit, 5 ... Delay circuit, 6 ... Bias current source, 7 ... LD driver current source, 8 ... Pre-emphasis driver Current source, 9 ... Pre-driver current source, 10 ... Photodiode (PD), 11 ... Transimpedance amplifier (TIA), 12, 13, 16 ... Comparison amplifier circuit, 14, 15, 17 ... Reference voltage generation circuit.

Claims (3)

An optical transmission device comprising a laser diode that outputs an optical signal, and a photodiode that monitors the optical output of the laser diode, wherein the bias current of the laser diode is negatively feedback controlled by a monitor value of the photodiode,
The modulation current of the laser diode is generated by an individually controllable laser diode driver circuit and a pre-emphasis driver circuit, and a drive current source of the pre-emphasis driver circuit is controlled based on the bias current, and the bias current is An optical transmission device characterized in that the smaller the smaller, the larger the output current of the pre-emphasis driver circuit .   The optical transmission device according to claim 1, wherein negative feedback control is performed on a drive current source of the laser diode driver circuit based on the monitor value. 3. The optical transmitter according to claim 1, wherein the bias current is reduced as the electro-optical conversion efficiency of the laser diode is higher.