JP2006210812A - Circuit and method for light output control - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a circuit and a method for light output control that can control light output of a semiconductor light emitting device according to operation temperature, based upon a driving current of the semiconductor light emitting device. <P>SOLUTION: In the light output control circuit as an embodiment, a light emission efficiency arithmetic section calculates light emission efficiency SE through operation based upon desired light emission intensity Ptar and extinction ratio ER, a mark rate M of an input modulated signal, and a modulating current Im. A threshold generator generates a threshold through quotient operation between designated permissible light emission intensity Pmax and the light emission efficiency SE. Further, in this light output control circuit, a driving current calculator calculates the driving current Iop of the semiconductor light emitting device. When the driving current Iop exceeds the threshold, the light output control circuit stops the light output of the semiconductor light emitting device. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a light output control circuit and a light output control method for controlling light output of a semiconductor light emitting device.

  An optical data link for optical communication in which a semiconductor light emitting device such as a distributed feedback semiconductor laser (DFB-LD) is used has a safety standard that determines an upper limit of the optical output of the semiconductor light emitting device from the viewpoint of user safety protection.

As an optical output control circuit for complying with this safety standard, the following patent document 1 estimates the optical output intensity from the monitor current of a monitor photodiode (hereinafter referred to as monitor PD) that monitors the optical output intensity of a semiconductor light emitting element. A light output control circuit, a light output control circuit that estimates light output intensity from a drive current of a semiconductor light emitting element, and a light output control circuit that estimates light output intensity from a monitor current and a drive current are disclosed.
JP 2003-289172 A

  However, the light output control circuit described above cannot control the light output when the monitor PD fails. Therefore, it is not appropriate to use the output of the monitor PD in an optical output control circuit that requires high reliability to comply with safety standards.

  Further, the relationship between the driving current of the semiconductor light emitting element and the light output depends on the operating temperature of the semiconductor light emitting element. That is, the light output of the semiconductor light emitting element varies depending on the operating temperature even when the drive current is the same. Therefore, the above-described optical output control circuit cannot be employed for a non-temperature-controlled optical data link.

  Accordingly, an object of the present invention is to provide a light output control circuit and a light output control method capable of controlling the light output of a semiconductor light emitting element in accordance with the operating temperature based on the drive current of the semiconductor light emitting element.

閾値生成部は、所定の許容発光強度Ｐｍａｘと上記発光効率ＳＥとの商演算によって、閾値を生成する。駆動電流算出部は、バイアス電流Ｉｂ及び変調電流Ｉｍに基づき、半導体発光素子の駆動電流Ｉｏｐを算出する。制御部は、この駆動電流Ｉｏｐが上記閾値を超える場合に、半導体発光素子の光出力を停止させる。 An optical output control circuit according to an aspect of the present invention includes a bias current source, a driver unit, a control unit, a light emission efficiency calculation unit, a threshold generation unit, and a drive current calculation unit. The bias current source supplies a bias current Ib to the semiconductor light emitting element. The driver unit includes a modulation current source that receives an input modulation signal and supplies a modulation current Im to the semiconductor light emitting element in response to the input modulation signal. The control unit sets the bias current Ib and the modulation current Im. The light emission efficiency calculation unit calculates the light emission efficiency SE based on the following equation (1) relating to the desired light emission intensity Ptar and extinction ratio ER, the mark ratio M of the input modulation signal, and the modulation current Im.

The threshold value generation unit generates a threshold value by a quotient calculation between a predetermined allowable light emission intensity Pmax and the light emission efficiency SE. The drive current calculation unit calculates the drive current Iop of the semiconductor light emitting element based on the bias current Ib and the modulation current Im. The control unit stops the light output of the semiconductor light emitting element when the driving current Iop exceeds the threshold value.

所定の許容発光強度Ｐｍａｘと該発光効率ＳＥとの商演算によって閾値を生成する工程と、バイアス電流Ｉｂ及び変調電流Ｉｍに基づき半導体発光素子の駆動電流Ｉｏｐを算出する工程と、この駆動電流Ｉｏｐが上記閾値を超える場合に半導体発光素子の光出力を停止させる工程とを含む。 An optical output control method according to another aspect of the present invention includes a step of supplying a bias current Ib to a semiconductor light emitting element and supplying a modulation current Im to the semiconductor light emitting element in response to an input modulation signal, and a desired light emission intensity. Calculating luminous efficiency SE based on the following equation (1) relating to Ptar and extinction ratio ER, mark ratio M of the input modulation signal, and modulation current Im;

A step of generating a threshold value by a quotient calculation of a predetermined allowable light emission intensity Pmax and the light emission efficiency SE, a step of calculating a drive current Iop of the semiconductor light emitting element based on the bias current Ib and the modulation current Im, and the drive current Iop And stopping the light output of the semiconductor light emitting device when the threshold value is exceeded.

  The luminous efficiency SE corresponds to the slope of the Iop-Po characteristic at the operating temperature of the semiconductor light emitting device, where Iop is the driving current of the semiconductor light emitting device and Po is the light output intensity. Therefore, the threshold value based on the quotient calculation between the predetermined allowable light emission intensity Pmax and the light emission efficiency SE corresponds to the allowable drive current Imax corresponding to the allowable light emission intensity Pmax at the operating temperature. In this light output control circuit, a threshold value is automatically generated by giving a predetermined allowable light emission intensity Pmax, a desired light emission intensity Ptar, and a desired extinction ratio ER in advance, and the threshold value is compared with the monitored drive current. As a result, the light output of the semiconductor light emitting device is stopped. Therefore, the light output control circuit can achieve light output control for complying with the safety standard only from the monitored drive current according to the operating temperature.

Note that Expression (1) is a general expression, and a value such as 50% may be set in advance as the mark rate M, for example. In this case, the luminous efficiency SE is calculated by the following equation (2).

  In addition, the light output control circuit of the present invention further includes a mark rate detection unit that obtains the mark rate M of the input modulation signal, and the light emission efficiency calculation unit uses the mark rate M to execute the calculation of equation (1). Is preferred.

  The optical output control circuit according to the present invention further includes a comparison unit that compares the drive current Iop with the threshold value. The comparison unit outputs a HIGH level when the drive current Iop exceeds the threshold value, and performs control. The unit may stop the light output of the semiconductor light emitting element according to the HIGH level.

  According to the present invention, there are provided a light output control circuit and a light output control method capable of controlling the light output of a semiconductor light emitting element according to the operating temperature based on the drive current of the semiconductor light emitting element.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals. Further, the symbols written together with the signal and current are also used as the respective levels.

  FIG. 1 is a diagram showing a configuration of a light output control circuit according to an embodiment of the present invention. 1 includes a driver unit 12, a bias current source 14, a semiconductor light emitting element 16, a semiconductor light receiving element 18, a mark rate detection unit 20, an extinction ratio control unit 22, a control unit (APC) 24, and a drive. A current calculation unit 26, a luminous efficiency calculation unit 28, a threshold generation unit 30, a storage unit 32, a comparison unit 34, and an OR logic unit 36 are provided.

  The driver unit 12 supplies a modulation current Im to the semiconductor light emitting element 16 based on the input modulation signal to modulate the intensity of the semiconductor light emitting element 16. The driver unit 12 includes an amplifier 40, a first transistor Tr1 and a second transistor Tr2 constituting a differential pair, and a modulation current source 38.

  The amplifier 40 has a differential output terminal. The amplifier 40 amplifies the differential modulation signal input from the input terminal 42 and outputs the amplified differential modulation signal from the differential output terminal.

  The differential output terminals of the amplifier 40 are respectively connected to the control terminal (gate) of the first transistor Tr1 and the control terminal (gate) of the second transistor Tr2. The differential modulation signal output from the differential output terminal of the amplifier 40 is output to the control terminals of the first transistor Tr1 and the second transistor Tr2.

  The first terminal (drain) of the first transistor Tr 1 is connected to the cathode of the semiconductor light emitting element 16. The anode of the semiconductor light emitting element 16 is connected to the first power supply line 44a. On the other hand, the first terminal (drain) of the second transistor Tr2 is connected to the first power supply line 44a.

  The second terminal (source) of the first transistor Tr1 and the second terminal (source) of the second transistor Tr2 are connected to the common node 41.

  The modulation current source 38 includes a third transistor Tr3. The first terminal (drain) of the third transistor Tr3 is connected to the common node 41. The second terminal (source) of the third transistor Tr3 is connected to the second power supply line 44b. The control terminal (gate) of the third transistor Tr3 is connected to the output terminal 24oa of the control unit 24. The third transistor Tr3 sets the peak value of the modulation current Im by the modulation current control signal Vm received at the control terminal.

  The bias current source 14 includes a fourth transistor Tr4. The first terminal (drain) of the fourth transistor Tr4 is connected to the cathode of the semiconductor light emitting element 16 via an inductor 46 for blocking high frequency. The second terminal (source) of the fourth transistor Tr4 is connected to the second power supply line 44b. The control terminal (gate) of the fourth transistor Tr4 is connected to the output terminal 24ob of the control unit 24. The fourth transistor Tr4 sets the bias current Ib by the bias current control signal Vb received at the control terminal. The bias current source 14 can supply a stable bias current to the semiconductor light emitting element 16 without being affected by the modulation current by the inductor 46.

  The cathode of the semiconductor light receiving element 18 is connected to the first power supply line 44 a, and the anode of the semiconductor light receiving element 18 is connected to the input terminal 24 ib of the control unit 24. The semiconductor light receiving element 18 receives a part of the light emitted from the semiconductor light emitting element 16 and outputs a current Impd corresponding to the intensity of the light. For example, a photodiode (PD) is used for the semiconductor light receiving element 18. Note that another part of the light emitted from the semiconductor light emitting element 16 is output to the optical fiber 48.

  The mark rate detection unit 20 receives an input modulation signal at the input terminal 20i. The mark rate detection unit 20 obtains the mark rate M of the modulation signal by monitoring the input modulation signal. The mark rate detection unit 20 outputs the mark rate M to the control unit 24 and the light emission efficiency calculation unit 28 from the output terminal 20o.

  The extinction ratio control unit 22 controls the extinction ratio of the light output of the semiconductor light emitting element 16 according to the temperature of the semiconductor light emitting element 16. Here, the temperature dependence of the optical output of the semiconductor light emitting element 16 will be described.

  FIG. 2 is a diagram showing the drive current Iop-light output intensity Po characteristic of the semiconductor light emitting element 16. In FIG. 2, Tld is the operating temperature of the light emitting element, Ph is the maximum value of light output, Ptar is the required light output, Pl is the minimum value of light output, Ith is the threshold current, and Im4 and Im6 are Tld = 40 ° C. And modulation current when Tld = 60 ° C., and Ib4 and Ib6 indicate bias currents when Tld = 40 ° C. and Tld = 60 ° C., respectively. The extinction ratio ER is obtained by ER = Ph / Pl.

  The optical data link is required to have a constant optical output Ptar and extinction ratio ER. However, when the temperature Tld of the light emitting element changes, the Iop-Po characteristic of the semiconductor light emitting element 16 changes. Even if the temperature Tld of the light emitting element changes, in order to obtain a desired light output Ptar and extinction ratio ER, it is necessary to adjust the modulation current Im and the bias current Ib according to the temperature.

  Specifically, as shown in FIG. 2, in order to obtain a light output Po1 with constant Ptar and ER at both Tld = 40 ° C. and Tld = 60 ° C., the drive current is set to Iop4, Tld at Tld = 40 ° C. At = 60 ° C., it is necessary to change the drive current as Iop6. That is, it is necessary to change the ratio between the modulation current and the bias current to Im4 / Ib4 at Tld = 40 ° C. and Im6 / Ib6 at Tld = 60 ° C.

  Therefore, the extinction ratio control unit 22 controls the extinction ratio by outputting an Im / Ib ratio corresponding to the temperature of the semiconductor light emitting element 16 to the control unit 24. In the present embodiment, the extinction ratio control unit 22 stores data of the Im / Ib ratio for each environmental temperature instead of the temperature of the light emitting element. The Im / Ib ratio is a ratio between the modulation current Im and the bias current Ib set in advance so that the semiconductor light emitting element 16 has a desired light emission intensity Ptar and a desired extinction ratio ER at the ambient temperature. The extinction ratio control unit 22 receives a temperature signal Ta monitored by a temperature sensing element such as a thermistor at the input terminal 22i, and outputs an Im / Ib ratio corresponding to the temperature signal Ta from the output terminal 22o to the control unit 24. .

  The control unit 24 receives the mark rate M at the input terminal 24ia and the current Impd at the input terminal 24ib. Further, the control unit 24 receives the Im / Ib ratio at the input terminal 24ic, and receives the PD current control target value Itpd at the input terminal 24id. The control unit 24 generates the modulation current control signal Vm and the bias current control signal Vb so that the current Impd becomes equal to the PD current control target value Itpd at the mark ratio M and Im / Ib ratio. The control unit 24 outputs the modulation current control signal Vm from the output terminal 24oa, and outputs the bias current control signal Vb from the output terminal 24ob. The control unit 24 sets the modulation current Im and the bias current Ib based on the modulation current control signal Vm and the bias current control signal Vb, and performs negative feedback control on the semiconductor light emitting element 16.

  The PD current control target value Itpd is a target value of the current Impd that is set in advance so that the light output of the semiconductor light emitting element 16 becomes the desired light emission intensity Ptar and extinction ratio ER. The PD current control target value Itpd is set, for example, as a current Impd when the light output of the semiconductor light emitting element 16 becomes a desired light emission intensity Ptar and extinction ratio ER before shipment, and is stored in the storage element.

  Hereinafter, a configuration related to over-emission control in the light output control circuit 10 will be described. The bias current source 14 further includes a bias current monitor unit 50. The bias current monitor unit 50 outputs a first signal Vibm corresponding to the bias current Ib to the drive current calculation unit 26. The bias current monitor unit 50 includes, for example, a transistor having the same characteristics as the fourth transistor Tr4 and a resistor. In this case, the bias current monitor unit 50 generates the first signal Vibm by performing current-voltage conversion on the mirror current of the bias current Ib.

  The modulation current source 38 further includes a modulation current monitor unit 52. The modulation current monitor unit 52 outputs a second signal Vimm corresponding to the modulation current Im to the drive current calculation unit 26 and the light emission efficiency calculation unit 28. The modulation current monitor unit 52 includes, for example, a transistor having the same characteristics as the third transistor Tr3 and a resistor.

  The drive current calculator 26 receives the first signal Vibm at the input terminal 26ib, and receives the second signal Vimm at the input terminal 26ia. The drive current calculation unit 26 calculates the drive current Iop of the semiconductor light emitting element 16 and the third signal Vop corresponding to the current based on the first signal Vibm and the second signal Vimm. The drive current calculation unit 26 outputs the drive current Iop from the output terminal 26oa, and outputs the third signal Vop from the output terminal 26ob.

  For example, the drive current calculation unit 26 calculates the modulation current Im by performing an inverse operation of the operation corresponding to the current-voltage conversion of the modulation current monitor unit 52. If the modulation current monitor unit 52 performs current-voltage conversion equivalent to the calculation of Vimm = Im × a (a is a constant. In the circuit of FIG. 1, a is a conversion gain based on the resistance value of the modulation current monitor unit 52) The drive current calculation unit 26 obtains the modulation current Im by calculation of Im = Vimm / a. Similarly, the drive current calculation unit 26 obtains the bias current Ib. The drive current Iop is obtained by adding the modulation current Im and the bias current Ib. The drive current calculation unit 26 calculates the third signal Vop from the drive current Iop by current-voltage conversion.

  The luminous efficiency calculation unit 28 receives the second signal Vimm at the input terminal 28ia and the extinction ratio ER at the input terminal 28ib. Further, the light emission efficiency calculation unit 28 receives a desired light emission intensity Ptar at the input terminal 28ic, and receives the mark rate M of the modulation signal at the input terminal 28id. Similar to the drive current calculation unit 26, the light emission efficiency calculation unit 28 obtains a modulation current Im obtained by converting the second signal Vimm into a current value. The light emission efficiency calculation unit 28 calculates the light emission efficiency SE by performing the calculation of the above formula (1) based on the desired light emission intensity Ptar and extinction ratio ER, the modulation signal mark ratio M, and the modulation current Im. The luminous efficiency SE is output from the output terminal 28oa to the threshold value generator 30. When the mark ratio M of the modulation signal is 50%, the light emission efficiency calculation unit 28 can calculate the light emission efficiency SE by the above formula (2) instead of the above formula (1).

  Further, the light emission efficiency calculation unit 28 calculates a ratio Ptar / SE between the desired light emission intensity Ptar and the light emission efficiency SE, and outputs this Ptar / SE to the threshold value generation unit 30 from the output terminal 28ob.

  The light emission efficiency calculation unit 28 is configured by an arithmetic circuit using a device such as a microcontroller, for example. The desired light emission intensity Ptar and extinction ratio ER are stored in advance in the storage unit 32.

The threshold value generator 30 receives the luminous efficiency SE at the input terminal 30ia and Ptar / SE at the input terminal 30ib. Further, the threshold generation unit 30 receives the allowable light emission intensity Pmax at the input terminal 30ic, and receives the drive current Iop at the input terminal 30id. The threshold generation unit 30 generates a threshold signal (threshold) Vmax based on the ratio between the allowable light emission intensity Pmax and the light emission efficiency SE. Specifically, the threshold generation unit 30 is calculated by the drive current Iop calculated by the drive current calculation unit 26, the allowable light emission intensity Pmax and the desired light emission intensity Ptar stored in the storage unit 32, and the light emission efficiency calculation unit 28. Using the luminous efficiency SE and Ptar / SE, the allowable drive current Imax is obtained by the following equation (3), and the allowable drive current Imax is subjected to current-voltage conversion to calculate the threshold signal Vmax. The threshold generation unit 30 outputs the threshold signal Vmax from the output terminal 30 o to the comparison unit 34. The threshold generation unit 30 is configured by an arithmetic circuit using a microcontroller.

  Here, since the luminous efficiency SE corresponds to the slope of the Iop-Po characteristic of the semiconductor light emitting element 16 shown in FIG. 2, Ptar / SE corresponds to the current contributing to the light emission of the semiconductor light emitting element 16. Therefore, the threshold current Ith of the semiconductor light emitting element 16 is obtained by subtracting Ptar / SE from the drive current Iop.

When the threshold current Ith is sufficiently smaller than the allowable drive current Imax, the threshold generation unit 30 may calculate the allowable drive current Imax by the following equation (4) instead of the above equation (3).

  The comparison unit 34 receives the threshold signal Vmax at the minus input terminal and the third signal Vop at the plus input terminal. The comparison unit 34 outputs the fourth signal Vcomp based on the comparison between the third signal Vop and the threshold signal Vmax. In the present embodiment, when the third signal Vop exceeds the threshold signal Vmax, the HIGH level is output to the OR logic unit 36.

  The fourth signal Vcomp is input to one input terminal of the OR logic unit 36, and the external control terminal 54 is connected to the other input terminal of the OR logic unit 36. The OR logic unit 36 performs an OR operation between an external shutdown signal for stopping the light output of the semiconductor light emitting element 16 via the external control terminal 54 and Vcomp.

  The control unit 24 receives the output signal of the OR logic unit 36 at the input terminal 24ie. When the signal from the OR logic unit 36 is at the HIGH level, the control unit 24 lowers the modulation current control signal Vm and the bias current control signal Vb to the ground potential. Thereby, the modulation current source 38 and the bias current source 14 are stopped, and the light output of the semiconductor light emitting element 16 is stopped.

  The operation of the light output control circuit 10 of the present invention will be described below. In the light output control circuit 10, the driver unit 12 intensity-modulates the semiconductor light emitting element 16 based on the input modulation signal, so that light is output from the semiconductor light emitting element 16. Part of the light output from the semiconductor light emitting element 16 is received by the semiconductor light receiving element 18.

  The semiconductor light receiving element 18 generates a current Impd corresponding to the intensity of the received light. Upon receiving this current Impd, the control unit 24 corrects the signal with the signal indicating the mark rate M output from the mark rate detection unit 20 and the Im / Ib ratio output from the extinction ratio control unit 22, while the monitor current Impd is The modulation current control signal Vm and the bias current control signal Vb are output so as to be equal to the current control target value Itpd.

  The modulation current source 38 sets the modulation current Im according to the modulation current control signal Vm, and the bias current source 14 sets the bias current Ib according to the bias current control signal Vb. Negative feedback control is performed on the semiconductor light emitting element 16 by the modulation current and the bias current set in this way, and the average value of the light output and the extinction ratio ER are kept constant.

  In the light output control circuit 10, the drive current calculation unit 26 generates the third signal Vop corresponding to the drive current Iop. In addition, the light emission efficiency calculation unit 28 calculates the light emission efficiency SE by performing the calculation of the above formula (1) or (2), and the threshold value generation unit 30 calculates the above formula (3) or (4). The threshold signal Vmax is generated by performing the calculation. That is, the light output control circuit 10 automatically generates the threshold signal Vmax corresponding to the temperature of the light emitting element only by giving the allowable light emission intensity Pmax, the desired light emission intensity Ptar, and the desired extinction ratio ER in advance. Is done.

  When the semiconductor light emitting element 16 is over-lighted, that is, when the light emission output exceeds the allowable light emission intensity Pmax, the third signal Vop is larger than the threshold signal Vmax, so the fourth output Vcomp of the comparison unit 34 is It becomes HIGH level, and the output of the OR logic unit 36 becomes HIGH level.

  Upon receiving this HIGH level signal, the control unit 24 stops the light output of the semiconductor light emitting element 16 by lowering the modulation current control signal Vm and the bias current control signal Vb to the ground potential.

  The light output control circuit 10 can similarly stop the light output of the semiconductor light emitting element 16 by setting the external shutdown signal to HIGH level.

  As described above, the light output control circuit 10 compares the threshold signal Vmax that is automatically generated according to the temperature of the light emitting element and the third signal Vop corresponding to the monitor drive current. The light output can be stopped. That is, the light output control circuit 10 emits light output control for complying with safety standards only from the monitored drive current value by setting the allowable light emission intensity Pmax, light emission intensity Ptar, and extinction ratio ER in advance. This can be achieved depending on the temperature of the device. Note that the storage unit 32 can be configured by a small-capacity storage element in order to store the allowable emission intensity Pmax, the emission intensity Ptar, and the extinction ratio ER.

  The light output control method according to the embodiment of the present invention will be described below. In this light output control method, first, a bias current Ib is supplied to the semiconductor light emitting element 16. The bias current Ib is supplied by, for example, the bias current source 14.

  At the same time, the modulation current Im is supplied to the semiconductor light emitting element 16 in accordance with the input modulation signal. The modulation current Im is supplied by the driver unit 12 and the modulation current source 38, for example.

  The bias current Ib and the modulation current Im are set so that the light output and extinction ratio ER of the semiconductor light emitting element 16 are constant. The bias current Ib and the modulation current Im are set by negative feedback control using the control unit 24, for example.

  Next, the drive current Iop of the semiconductor light emitting element 16 is calculated based on the bias current Ib and the modulation current Im. The drive current Iop is calculated by the drive current calculation unit 26, for example.

  Next, the light emission efficiency SE of the semiconductor light emitting element 16 is calculated based on the desired light emission intensity Ptar and extinction ratio ER, the mark ratio M of the modulation signal, and the above equation (1) or (2) relating to the modulation current Im. The luminous efficiency SE is calculated by, for example, the luminous efficiency calculation unit 28.

  Next, the threshold value Vmax is generated based on the ratio between the allowable light emission intensity Pmax and the light emission efficiency SE. The threshold value Vmax is generated by the threshold value generator 30 based on the above formula (3) or (4), for example.

  When the drive current Iop exceeds the threshold value Vmax, the light output of the semiconductor light emitting element 16 is stopped. The light output stop of the semiconductor light emitting element 16 is achieved by stopping the supply of the bias current Ib and the modulation current Im using, for example, the control unit 24, the comparison unit 34, and the OR logic unit 36.

  As mentioned above, although embodiment of this invention was described, this invention is not limited to this embodiment, A various deformation | transformation aspect can be comprised. For example, in the light output control circuit 10, the driver unit 12 and the semiconductor light emitting element 16 may be connected via a resistor.

  Further, in the above-described embodiment, the configuration in which the driver unit 12 DC drives the semiconductor light emitting element 16 is exemplified. However, the driver unit 12 may be configured to AC drive the semiconductor light emitting element 16. FIG. 3 is a diagram showing a light output control circuit 10a when the semiconductor light emitting element 16 is AC driven. In the light output control circuit 10 a shown in FIG. 3, the driver unit 12 and the semiconductor light emitting element 16 are connected via a capacitor 56. Therefore, since the DC current of the modulation current source 38 is not supplied to the semiconductor light emitting element 16, the drive current calculation unit 26 receives only the first signal Vibm from the bias current source 14, and based on the first signal Vibm, A current value (Iop = Ib) of the drive current Iop and a third signal Vop having a level corresponding to the current value are calculated. Other configurations and operations of the light output control circuit 10a are the same as those of the light output control circuit 10.

It is a figure which shows the structure of the light output control circuit concerning embodiment of this invention. It is a figure which shows the drive current Iop-light output intensity Po characteristic of a semiconductor light-emitting device. It is a figure which shows the light output control circuit in the case of carrying out AC drive of the semiconductor light-emitting device.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10 ... Light output control circuit, 12 ... Driver part, 14 ... Bias current source, 16 ... Semiconductor light emitting element, 18 ... Semiconductor light receiving element, 20 ... Mark rate detection part, 22 ... Extinction ratio control part, 24 ... Control part, 26 DESCRIPTION OF SYMBOLS ... Drive current calculation part, 28 ... Luminous efficiency calculating part, 30 ... Threshold generation part, 32 ... Memory | storage part, 34 ... Comparison part, 36 ... OR logic part, 38 ... Modulation current source, 40 ... Amplifier, 42 ... Input terminal, 46: Inductor, 48: Optical fiber, 50: Bias current monitor, 52: Modulation current monitor, 54: External control terminal, 56: Capacitor

Claims (4)

A bias current source for supplying a bias current Ib to the semiconductor light emitting device;
A driver unit having a modulation current source for receiving a modulation signal and supplying a modulation current Im to the semiconductor light emitting element in response to the modulation signal;
A control unit for setting the bias current Ib and the modulation current Im;
A light emission efficiency calculation unit that calculates the light emission efficiency SE based on the desired light emission intensity Ptar and extinction ratio ER, the mark ratio M of the modulation signal, and the following equation (1) relating to the modulation current Im;

A threshold generation unit that generates a threshold based on a predetermined allowable light emission intensity Pmax and the light emission efficiency SE;
A drive current calculation unit that calculates a drive current Iop of the semiconductor light emitting element based on the bias current Ib and the modulation current Im;
With
The control unit stops the light output of the semiconductor light emitting element when the drive current Iop exceeds the threshold.
Light output control circuit. A mark rate detection unit for obtaining a mark rate M of the modulation signal;
The light output control circuit according to claim 1, wherein the light emission efficiency calculation unit executes the calculation of the formula (1) using the mark rate M. A comparator for comparing the driving current Iop with the threshold;
The comparison unit outputs a HIGH level when the drive current Iop exceeds the threshold,
The control unit stops the light output of the semiconductor light emitting element according to the HIGH level.
The light output control circuit according to claim 1. Supplying a bias current Ib and the modulation current Im to the semiconductor light emitting device;
Calculating the luminous efficiency SE based on the following equation (1) relating to the desired emission intensity Ptar and extinction ratio ER, the mark ratio M of the modulation signal, and the modulation current Im;

Generating a threshold based on a predetermined allowable light emission intensity Pmax and the light emission efficiency SE;
Calculating a drive current Iop of the semiconductor light emitting element based on the bias current Ib and the modulation current Im;
Stopping the light output of the semiconductor light emitting device when the drive current Iop exceeds the threshold;
A light output control method including:

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