JPS6240789A - Laser diode driving circuit - Google Patents

Abstract

PURPOSE:To stably control the light output of a laser diode to a prescribed output value by inversely amplifying a differential current between a reference current of a current source and a photovoltaic current of a photodiode, feeding back it to the forward current circuit of the laser diode, and controlling it to a current value corresponding to the reference current. CONSTITUTION:Since a photovoltaic current Is of a photodiode PD is proportional to a light output Po, the current Is is detected to control to feed back the output Po, thereby controlling the light output Po at constant value. The output Po is controlled so that the detected current Is becomes equal to a reference current Iref to be constant. In other words, in the state of Is Iref, no a base flowing current to a control transistor Q1 is presented, and the transistor Q1 is turned OFF. This, a bias current Ib from a bias current circuit 5 flows to the base of a driving transistor Q2, which is thus turned ON, the maximum upper limit current specified by a current limiting resistor R1 flows to a laser diode LD to increase the output Po.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a laser diode drive circuit that stably controls the optical output of a laser diode to a constant output value.

[Technical Background of the Invention and Problems Therewith] Generally, a laser diode drive circuit is provided with a control circuit that controls the optical output of the laser diode to a constant output value and stabilizes the optical output of the laser diode.

Figure w412 shows such a conventional laser diode drive circuit, which is a well-known circuit described in the laser diode data book.

In the figure, LD is a laser diode, and PD is a photodiode for monitoring the optical output of the laser diode LD.
is built in one package 11, and from the package 11 there is a common connection terminal 116 that commonly connects the anode of the laser diode LD and the cathode of the photodiode PD, and a cathode terminal 1 of the laser diode D.
1b and an anode terminal 11C of the photodiode PD, a total of three terminal bins are taken out to the outside. One output surface of the laser diode LD is directed toward the photodiode PD, and an optical output PO is taken out from the other output surface.

Reference numeral 12 denotes a power supply input terminal, and the power supply voltage Vcc applied to the power supply input terminal 12 is divided into appropriate voltage values by a bias point setting circuit 13 equipped with 1 to the first operational amplifier, and is applied to the common connection terminal 11a. Supplied.

14 is a photovoltaic current detection circuit for the photodiode PD, which is equipped with a second operational amplifier A2.

15 is an arithmetic circuit for driving the transistor Q5 connected to the next stage, which inputs the output signal of the photovoltaic current detection circuit 14, performs necessary arithmetic operations on it, and drives the transistor Q5.
It outputs a control voltage of . The arithmetic circuit 15 is equipped with a third operational amplifier A3.

The transistor Q5 is connected to the forward current circuit 16 of the laser diode D, and is controlled by the arithmetic circuit 15 to define the forward current If of the laser diode LD to a predetermined current value. By regulating the forward currents I and f to predetermined current values in this manner, the optical output PO of the radar diode LD is controlled to a constant output value.

Incidentally, since the C-11 diode LD is frequently used as a light source for optical information processing, optical communications, etc., a drive circuit for driving it is required to have high-speed driving performance and to reduce costs.

However, in the above laser diode drive circuit, a total of three operational amplifiers A+, A2, and A3 are provided, which is quite large. Q: Due to the complicated circuit configuration, the cost is high, and two of the three operational amplifiers mentioned above
2. Since A3 is placed in the feedback loop of the optical output, when the optical output Po changes, the response time until it stabilizes at a constant value is relatively short, resulting in a lack of high-speed drive performance. There was a problem.

[Objective of the Invention] The present invention has been made based on the above circumstances, and therefore provides a laser that can stably control the optical output to a constant output value and has high-speed drive performance with a relatively simple circuit configuration. The purpose is to provide a diode drive circuit.

[Summary of the Invention] In order to achieve the above object, the present invention, as shown in FIG. The difference current from the basic rP lightning current ref is inverted and amplified by the inverting amplification means 2, and fed back to the forward current circuit 4 of the laser diode LD, and the forward current 1f is converted into a current value corresponding to the basic tP-current 1ref. Nii 1
til, the optical output of the laser diode LD is controlled to a constant value, and the feedback circuit 3 is provided with the inverting amplification means 2, making it possible to obtain high-speed drive performance. This is what I did.

[First Embodiment] A first embodiment of the present invention will be described below based on FIG. 2.

In FIG. 2, the same or equivalent circuit elements as those in FIG. 1 and FIG. 12 are denoted by the same reference numerals and redundant explanations will be omitted.

First, the configuration will be described. In the first embodiment, a control transistor Q1 is connected to the feedback circuit 3 as an inverting amplification means, and a driving transistor Q2 is connected to the forward current circuit 4. 5 is a bias current circuit for the drive transistor Q2, which also serves as an output circuit for the control transistor Q1. R+ is a current limiting resistor for laser diodes L and D, which is connected to prevent overcurrent.

Next, the operation will be explained with reference to FIGS. 3 and 4. Third
The figure shows forward current f and optical output p of laser diode LD.
Figure 4 is a characteristic diagram showing the relationship between o and laser diode LD.
FIG. 3 is a characteristic diagram showing the relationship between the optical output PO and the monitor current (photovoltaic current) Is generated in the photodiode PD.

From Figure 3, the laser diode LD oscillates when the forward current f reaches a ``threshold current'' of 60 mA, for example, to obtain an optical output Po, and thereafter the optical output PO is proportional to the forward current if. On the other hand, the photodiode PD
receives this optical output PO and generates a photovoltaic current 1s, but as shown in FIG. 4, the photovoltaic current IS increases in proportion to the optical output Po. To give a numerical example of the relationship between forward current 1f, optical output PO, and photovoltaic current 1s, at air temperature, forward current rf: 90 mA, optical output Po: 10 mW, photovoltaic current Is: 0.75 mA.

Therefore, according to the above numerical example, the photovoltaic current 1s and the optical output PO
The proportional relationship between .

I s/Po=0.75 <mA)/10 (mW>・
...(1) As mentioned above, the photovoltaic current Is of the photodiode PO is
Since it is proportional to the optical output po, the optical output PO can be controlled to a constant value by detecting the photovoltaic current is and controlling the optical output PO through feedback ijJ. Therefore, in the present invention, the optical output PO is controlled so that the detected photovoltaic current IS is equal to the reference ff1ilref, as shown in the next step), and is kept at a constant value.

That is, the relationship between the photovoltaic current ■S and the reference current 1ref is
In a state where s<Iref, there is no base current flowing into the control transistor Q1, and the control transistor Q1 is turned off. Therefore, the bias current tb from the bias current circuit 5 is applied to the base of the driving transistor Q2.
flows into the laser diode LD, turning it on, and the maximum upper limit current defined by the current limiting resistor R1 flows through the laser diode LD, increasing the optical output po.

When the optical output 'PO increases and the photovoltaic current Is increases accordingly, and the relationship Is>Iref' is established, the photovoltaic current I
A base current corresponding to the difference current between s and the reference current Iref flows into the control transistor Q1, and an amplified current of this difference current flows into the collector of the control transistor Q+. Therefore, the bias current rb of the bias current circuit 5 flows toward the control transistor Q1 by the amount of the amplified current, and the base bias current of the drive transistor Q2 decreases by that amount. Therefore, the driving transistor Q2 is
It will be driven by the inverted amplified current of the I control transistor Q1, and depending on the magnitude of the difference current between the photovoltaic current [S and the reference current rref, the driving transistor Q2 will be
The forward current 1'' of the laser diode LD is limited and its value is lowered.

As described above, due to the action of the feedback circuit 3, the optical output Po of the laser diode LD is changed to that of the photodiode P.
The photovoltaic current 1s of D is IR! ! The output value is controlled to be constant so as to be equal to the current Iref.

In the numerical example above, the reference current 1re of the reference current source 1
ftfi is set to 0.75 mA, and the forward current If of the laser diode LD is set to this reference current value of 0.75 m.
As a result, the optical output PO is controlled to be constant at 10 mW.

Note that as shown in the circuit configuration of FIG. 2, the forward current circuit 4
The above control current obtained by the photodiode PD
Also included is the photovoltaic current Is flowing through. Therefore, an error occurs in the control action of the forward current If by this photovoltaic current IS, but as shown in the numerical example above, the photovoltaic current Is
is a value less than 1/100 of the forward current) f, so the above error can be almost ignored, and the control current flowing through the forward current circuit 4 is the forward direction current of the laser diode LD. It can be regarded as equal to the current If.

In this output stabilizing effect, in this embodiment, only two transistors, a control transistor Q1 and a driving transistor Q2, and a current limiting resistor R1 are connected to the feedback circuit 3;
Compared to the conventional system shown in the figure, the number of components is extremely small and the circuit configuration is simple, so the response delay of the feedback system to changes in the optical output PO is extremely small. Appropriate stabilizing effects can also be achieved with pulsed driving. This will be further explained using the following specific example.

Next, FIG. 5 shows a specific example of the first embodiment. In this specific example, a laser diode LD is driven with a high-frequency pulsed current, and its pulsed light output is detected by an externally provided photodiode sensor DP2.

Reference number 6 in FIG. 5 is a switching circuit for switching and driving the laser diode LD, and a switching transistor Q3, resistors R5 to R8, a capacitor C2, and a diode D constitute a high-speed switching circuit. Reference numeral 7 denotes a voltage input terminal to which pulse voltage for driving the switching circuit 6 is input.

Further, a sense amplifier for amplifying the detected photovoltaic current IO and outputting a voltage output Vo from the output terminal 8 is connected to the photodiode sensor DP2. The sense amplifier consists of an operational amplifier Δ3 and a feedback resistor R3.

In this specific example, the reference current source consists of a resistor R2 with a resistance value of 8.2 Ω. If the required base-emitter voltage Vbe of the control transistor Q1 is, for example, 0.6V, the reference current fref is 0.6V/8.2, Ω=7.
It is set to 3μA.

Therefore, the optical output p of the laser diode LD.

is controlled so that the photovoltaic current is of the photodiode PD is 73 μA, which is equal to the above reference current 1 ref, so if the proportional relationship between the two in equation (1) is applied as is, the optical output is approximately 1 mW. po is laser diode L
Obtained from D.

In such a specific example, the pulse response characteristics when the laser diode LD is pulse-driven by inputting a pulse voltage with a repetition frequency of 1 MH2 as the input voltage ■i from the voltage input terminal 7 are shown in FIG. 6 (A). Shown in (B) and (C). Figure 6 (A> is a diagram showing the response characteristics between the input voltage v1 and the anode voltage (driving voltage) Vd of the laser diode LD, and Figure 6 (B) is a diagram showing the response characteristics between the input voltage vi and the forward direction of the laser diode LD. Diagram showing response characteristics between current jf, No. 6
Figure (C) is a diagram showing the response characteristic between the input voltage Vi and the external voltage output VO.

From the characteristics shown in FIG. 6(B), a forward current If of approximately 58 mA flows through the laser diode LD when emitting light.

Furthermore, the pulse output voltage VO outputted from the output terminal 8 through the external detection system consisting of the photodiode sensor #fDP2 and the sense amplifier has an amplitude value of approximately O', 2V, and a rise time tr and a fall time tf, respectively. It was confirmed that extremely high-speed pulse response of about 40 nsec and 5 onsec (q) was achieved, and the optical output PO was stably controlled to a constant value even in such pulse driving.The above rise time tr and The fall time tf is a value that includes the response delay of the sense amplifier itself.

Incidentally, when the amplitude of the output voltage is 0.2V, the photovoltaic current Io flowing to the photodiode sensor DP2 changes this output amplitude value to the feedback resistance R4 of the operational amplifier A4 = 10.
Equal to 0 divided by Ω, the value is 0.2V/100
KΩ=2μA.

[Second Embodiment] FIG. 7 shows a second embodiment of the present invention. In this embodiment, a JFET Q4 is used as a control transistor in place of the npn transistor Q1 in the first embodiment, and the control accuracy of the optical output po is further improved.

Other than the above-mentioned structure using JFETQ4, the other structure is the same as that of the first embodiment shown in FIG. 2 above.

As described in the specific example of FIG. 5, the reference current [ref
A current value of about 100μA or less is applied, and the optical output PO
The photovoltaic current IS of the photodiode PD caused by this becomes equal to this reference current Iref (Is=
Iref). The photovoltaic current 1s of the photodiode PD has a low current value as described above, so when the npn transistor Q1 is used as the control transistor as in the first embodiment, a base current of about μΔ flows through it. Since this base current component is included in the photovoltaic current ■S, a control error corresponding to the base current component occurs when controlling the photovoltaic current is t11tt' so that l5=1ref.

In the second embodiment, a JFET with a high gate input impedance is used as a control transistor, thereby making the gate current almost zero and eliminating the above control error.

[Third practice - flow example] FIG. 8 shows a third embodiment of the invention.

In this embodiment, a voltage comparator vC is connected between the connection point between the photodiode PD and the reference current source 1 and the base of the control transistor Q1 to further improve control accuracy.

The reference voltage yret' of the voltage comparator vC is set to the ground level in the illustrated example.

In this embodiment as well, the signal valve corresponding to the difference current between the reference current IS of the reference current source 1 and the photovoltaic current IS of the photodiode PD is inverted and amplified by the control transistor Q1 via the voltage comparator VC. The amplified output is fed back to the forward current circuit 4, and is controlled to a current value corresponding to the forward current 1fh'X reference current Tref of the laser diode LD. This kind of basic V work is
This is the same as in the first embodiment.

In the above control operation, in this embodiment, the voltage at the anode of the photodiode PD is set to be equal to the base Q-lightning voltage ref of the voltage comparator VC, that is, the ground level.

When the anode of the photodiode PD is directly connected to the base of the control transistor Q1 as in the case of the first embodiment, the potential of the anode of the photodiode PD is affected by the temperature characteristics of the control transistor Q1. The laser diode driving circuit has a temperature dependence, and the stabilization effect of the leading horn of the laser diode drive circuit also has a temperature dependence.

However, in this embodiment, the photodiode PD
The voltage is defined by the reference voltage Vref of the voltage comparator C, and the influence of the temperature characteristics of the control transistor Q1 is removed.

Further, for example, if the potential of the anode of the laser diode LD is 1.5V and the base-emitter voltage vbe of the control transistor Q1 is 0.6V, in the case of the first embodiment, the photodiode 1' PD is 1.5V-0,
6V=0.9V (7) Operates with reverse bias voltage.

However, in this third embodiment, the photodiode P
The anode of D is connected to the reference voltage Vr of the voltage comparator VC.
Since it is at the ground level which is ef, the above-mentioned reverse bias voltage is 1.5V-OV=1.5V, which is also set to a large value in the case of the first embodiment. For this reason, the detection sensitivity of the photodiode PD reaches an upper limit, and combined with the effect of eliminating the influence of temperature characteristics mentioned above, the laser diode L
The optical output Po stabilizing effect of D is performed with higher precision.

[Fourth Embodiment] In the fourth embodiment, and the fifth and sixth embodiments described below, the laser diode LD and the photodiode PD are built into one baggee 11 as described above, Examples of configurations in which the laser diode LD and photodiode PD can be arbitrarily connected in the circuit configuration by removing the restriction on the connection mode that the anode of the laser diode LD and the cathode of the photodiode PD are commonly connected are shown below. It shows.

First, FIG. 9 (Δ) shows a fourth embodiment of the present invention.

This embodiment uses a pnp transistor as the driving transistor Q2, and a reference current source 1 and a photodiode P.
The connection point of D is directly connected to the base of the driving transistor Q2, thereby omitting the provision of a transistor corresponding to the control transistor Q1 in the first embodiment, and further simplifying the circuit configuration. This is the main focus. Laser diode LD is driving transistor Q2
The driving transistor Q2 is connected to the emitter circuit of the transistor Q2 in an emitter follower connection.

To explain the effect, the relationship between the photovoltaic current IS and the reference current Iref is [in the state of s<Iref, this photovoltaic current i
A base current corresponding to the difference current between s and the reference current 1ref flows through the base of the driving transistor Q2, and a forward current If corresponding to the amplified current of this difference current flows through its emitter circuit. The optical output Po of the laser diode LD is increased by this amplified forward current If. In this manner, in this embodiment as well, the forward current If is reversed and increased in response to the difference current between the photovoltaic current ``S'' and the reference current 1ref, and the optical output po is increased.

When the optical output PO increases and the photovoltaic current Is increases accordingly, and the relationship is > Iref, the base current of the driving transistor Q2 disappears, and the driving transistor Q2 turns off and turns off. Cut off the directional current If,
Optical output PO decreases.

Due to the feedback action described above, the optical output Po of the laser diode LD is controlled to be constant at an output value such that the photovoltaic current Is of the photodiode PD is equal to the reference current ref. In this embodiment, since the number of circuit elements is small, an even better effect can be obtained in terms of high-speed drive performance.

In FIG. 9(B), the driving transistor Q2' is np
This is a complementary circuit shown in Fig. 9 (A>) using n transistors.The operation is shown in Fig. 9 (A).
This is the same as in case A).

[Fifth Embodiment] FIG. 10(A) shows a fifth embodiment of the present invention. In this embodiment, pnp transistors are used as the driving transistor Q2, and the reference current section 1 and photodiode PD
This is similar to the case of the fourth embodiment shown in FIG. 9<A) above, in that the connection point with the drive transistor Q2 is connected to the base of the driving transistor Q2. The difference is that the laser diode L
This is the point where D is connected to the collector circuit of the driving transistor Q2.

The photovoltaic current IS corresponding to the optical output po is detected by the photodiode PD, and this is fed back to the previously published JP.
The operation of controlling O to a constant output value such that the photovoltaic current IS is equal to the reference current 1ref is almost the same as in the fourth embodiment.

In FIG. 10(B), the driving transistor Q2' is npn.
This circuit uses transistors to form the complementary circuit shown in FIG. 10 <A). The action is the same as in the case of FIG. 10 (A>).

[Sixth Embodiment] FIG. 11(A) shows a sixth embodiment of the present invention. This embodiment shows a circuit configuration where there is a restriction that the cathode of the laser diode LD and the cathode of the photodiode PD are commonly connected and a common connection terminal is drawn out from the connection point.

As mentioned above, except for the fact that the cathode of the photodiode PD is connected to the cathode of the laser diode LD, the other circuit configuration is the same as that of the fourth embodiment shown in FIG. 9(A). It is similar to

In this example, a control current of 1 m flows through the driving transistor Q2, and a photovoltaic current IS flows through the photodiode PD.
The sum of the currents is the forward current I in the laser diode LD.
flows as f. Therefore, an error of 1.8 photovoltaic current occurs between the forward current 1f and the control current. However, as mentioned above, the photovoltaic current IS is a forward current [f of 10
Since the value is less than 1/0, the above error can be almost ignored, and the control current flowing through the driving transistor Q2 and the forward current 1f of the laser diode LD can be considered to be equal.

Therefore, the stabilization control effect of the optical output PO according to this embodiment is almost the same as that of the fourth embodiment.

FIG. 11(B) shows p as the driving transistor Q2'.
This circuit uses np transistors and is configured as the complementary circuit shown in FIG. 11(A). The action is almost the same as that shown in FIG. 11 (A>) above.

[Effects of the Invention] As explained above, according to the present invention, a base current source is connected to the output circuit of a photodiode that monitors the optical output of a laser diode, and the reference current of this reference current source and the photodiode are The difference current from the photovoltaic current of Therefore, the optical output of the laser diode can be stably controlled to a constant output value. Further, since the feedback circuit has a relatively compact circuit configuration and has extremely low response delay, it has the advantage of achieving high-speed drive performance and further reducing costs.

[Brief Description of the Drawings] Fig. 1 is a circuit diagram showing the basic configuration of a radar diode drive circuit according to the present invention, Fig. 2 is a circuit diagram showing a first practical example of the invention, and Fig. A characteristic diagram showing the relationship between forward current and optical output of the laser diode used in the first embodiment of the same as above, FIG.
FIG. 5 is a characteristic diagram showing the relationship between the light output of the diode and the monitor current of the photodiode. FIG. 5 is a circuit diagram showing a specific example of the present invention. FIG. FIG. 8 is a circuit diagram showing a second embodiment of the invention, FIG. 8 is a circuit diagram showing a third embodiment of the invention, and FIG. 9 is a circuit diagram showing a fourth embodiment of the invention. 10 is a circuit diagram showing a fifth embodiment of this invention, FIG. 11 is a circuit diagram showing a sixth embodiment of this invention, and FIG. 12 is a circuit diagram showing a conventional laser diode drive circuit. . DESCRIPTION OF SYMBOLS 1...Reference current source, 2...Inverting amplification means, 3...Feedback circuit, 4...Forward current circuit, LD...Laser diode, PD...Photodiode, Q+, Q4・...Control transistor, Q2...Drive transistor, VC...Voltage comparator. Figure 1 Figure 2 Tools 110,000 Fl, I↑(mA-U-Figure 8 = L h Pd, mW) → Figure 4 Figure 6 (A) Figure 6 (B) 6 Figure (C) Figure 9fA) Figure 10 (A) Old Figure 9) Figure 10 (B) Procedure Nerti (properly written spontaneously) April 2, 1986 Director General of the Patent Office Uga Tsuri Department 1 , Indication of the case 1985 Patent Application No. 179255 2 Name of the invention Laser diode drive circuit 3 Relationship with the person making the amendment 1 Patent applicant address (residence) Horiyo-cho, Saiwai-ku, Yozaki City, Kanagawa Prefecture Address 72 Name Toshiba Corporation Representative Watari Sugi - Department 4, Agent Postal code 105 Address
5F, Toranomon Building, 1-2-3 Toranomon, Minato-ku, Tokyo (Date of shipment: Month, Day, 1927) 6, Subject of amendment (1) “Claims” column of the specification (2)
"Detailed Description of the Invention" column of the specification (3) Drawing 7, Contents of amendment (1) The "Claims" of Book 111I will be amended as shown in the attached sheet. (2) Mei 18, page 5, line 6 to line 18 of the same page, ``This invention is...''.
[In order to achieve the above object, the present invention, as shown in FIG. Electromotive current IS and reference current T flowing through base #-current source 1
The current controller stage 2 controls the forward current of the forward current circuit 4 of the laser diode LD so that the forward current If becomes equal to the reference current 1 ref. A relatively simple circuit in which the current control means 2 is provided in the feedback circuit 3 for controlling the optical output of the laser diode LD to a constant value and controlling the forward current If. (3) From page 22, line 19 to page 23, line 12 of the specification.
As explained in the second line, according to this invention...
There are advantages. ” [As explained above, according to the present invention, the reference current t$i is connected to the output circuit of the photodetection means for detecting the optical output of the laser diode, and the base r¥ of this reference current source is By controlling the forward current of the forward current circuit of the laser diode so that the photovoltaic current of the lightning current photodiode is equal, the forward current is controlled to a current value corresponding to the base 1M current. , the optical output of the laser diode can be stably controlled to a constant output level. In addition, the feedback circuit for controlling the forward current has a relatively simple circuit configuration to minimize response delays, resulting in high-speed drive performance and, moreover, the advantage of being able to reduce costs. be. ” he corrected. (4) Amend Figure 1 of the drawings as shown in the attached sheet. 8. List of attached documents (1) Claims (2) Drawings (Figure 1) Claims (1) A laser diode and a photovoltaic current outputting a value corresponding to the optical output of the laser diode. a reference current source connected to the output circuit of the photodetection means and whose current value is set to a predetermined reference current value; a photovoltaic current of the photodetection means and a reference current of the reference current source; A laser diode drive circuit characterized in that it has a current control means for controlling a forward current supplied to the laser diode so that the forward current supplied to the laser diode is equal to an inverting amplifying means for inverting and amplifying the difference current between the electromotive current and the reference current of the reference current source; and an inverting amplifying output from the inverting amplifying means is fed back to the forward current circuit, and the forward current is converted into the Ql current. 1. The laser diode drive circuit according to claim 1, further comprising a feedback circuit that controls the current value to a value corresponding to the current value.

Claims (1)

[Scope of Claims] A forward current circuit that supplies a forward current to a laser diode; a photodiode that monitors the optical output of the laser diode and outputs a photovoltaic current having a value corresponding to the level of the optical output; , a reference current source connected to the output circuit of the photodiode and having a current value set to a predetermined reference current value, and inverting and amplifying the difference current between the photovoltaic current of the photodiode and the reference current of the reference current source. and a feedback circuit that feeds back an inverted amplified output from the inverting amplification means to the forward current circuit and controls the forward current to a current value corresponding to the reference current. Laser diode drive circuit.