Semiconductor laser drive circuit

1. Field of the Invention

The present invention relates to a semiconductor laser drive circuit for driving a GaN (gallium nitride)-based semiconductor laser or other high resistance semiconductor laser.

2. Description of the Related Art

Optical disks, types of information recording media, have been made increasingly high in density and large in capacity in recent years.

Along with the increase in density and larger capacity, it is required that the pulse width of emission of laser light be short and that an edge of a waveform of the laser light be sharp.

A semiconductor laser drive circuit of the related art for driving a semiconductor laser controls an output of laser light of the semiconductor laser by switching the size of the drive current.

For example, a semiconductor laser drive circuit of the related art has a plurality of constant current circuits (for plurality of channels) using transistors. It adds the output currents of the constant current circuits and supplies the result the semiconductor laser.

Summarizing the problems of the present invention, with the above semiconductor laser drive circuit of the related art, since there is a parasitic capacity at the output terminal of each constant current circuit, the total parasitic capacity becomes larger along with the increase in number of constant current circuits.

Further, since a constant current circuit is configured by transistors, a mirror effect arises by the capacity between an input and an output.

Due to these factors, the frequency characteristics and transient characteristics deteriorate in a semiconductor laser drive circuit of the related art, thus it becomes difficult to shorten the pulse width of emission of the semiconductor laser and increase the speed of the drive.

Especially, since a GaN-based semiconductor laser for emitting blue-violet laser light has a larger impedance comparing with other semiconductor lasers, it is difficult to drive it at a high speed by a current-control type semiconductor laser drive circuit of the related art.

An object of the present invention is to provide a semiconductor laser drive circuit capable of improving a driving speed of a GaN-based semiconductor laser or other high resistance semiconductor laser.

According to a first aspect of the present invention, there is provided a semiconductor laser drive circuit for driving a high resistance semiconductor laser, comprising a control circuit for generating a drive control signal and a low output impedance drive element for driving the high resistance semiconductor laser by a voltage based on the drive control signal.

Preferably, the control circuit comprises a first switch circuit supplied with a first input signal and a first switch control signal and controlling and outputting a signal level of the first input signal based on the first switch control signal; a second switch circuit supplied with a second input signal and a second switch control signal for controlling and outputting a signal level of the second input signal based on the second switch control signal; and a combining circuit for combining output signals of the first and second switch circuits to generate the drive control signal.

Preferably, the circuit further comprises a constant voltage circuit for generating a biasing use constant voltage signal. Also, the high resistance semiconductor laser is driven in accordance with a plurality of operation modes, and the combining circuit combines the constant voltage signal and an output signal of the first switch circuit when in a first operation mode and combines the constant voltage signal and outputs of the first and second switch circuits when in a second operation mode.

More preferably, the circuit further comprises an oscillation circuit for generating an oscillation signal, wherein the combining circuit combines the constant voltage signal and the oscillation signal when in a third operation mode.

Still more preferably, the circuit further comprises a resonance circuit connected between the oscillation circuit and the combining circuit. The combining circuit combines the oscillation signal passed through the resonance circuit and the constant voltage signal when in the third operation mode.

Preferably, the circuit further comprises a first level-shift circuit supplied with a third input signal for generating the first input signal by shifting the level of the third input signal and a second level-shift circuit supplied with a fourth input signal for generating the second input signal by shifting the level of the fourth input signal.

More preferably, each of the first and second level-shift circuits is a differential amplifying circuit comprised by an emitter coupled logic circuit; the first switch circuit is a current switch logic circuit with a power source connected to an emitter of differential pair transistors and with an output current of the power source set by the first switch control signal; and the second switch circuit is a current switch logic circuit with a power source connected to an emitter of differential pair transistors and with an output current of the power source set by the second switch control signal.

More preferably, the circuit further comprises a first waveform shape circuit supplied with a fifth input signal for shaping a waveform of the fifth input signal to generate the third input signal and a second waveform shape circuit supplied with a fifth input signal for shaping a waveform of the sixth input signal to generate the fourth input signal.

Preferably, the circuit further comprises a power source circuit for supplying a power source voltage to the high resistance semiconductor laser via the low output impedance drive element. The power source circuit outputs the power source voltage to the low output impedance drive element when a control signal of the power source circuit is in an enabled state, while stops outputting the power source voltage when a control signal of the power source circuit is in a disabled state.

Preferably, the circuit further comprises a detection circuit for detecting an inflowing current or an emission intensity of the high resistance semiconductor laser and a feedback circuit for feeding back an output signal of the detection circuit to the low output impedance drive element.

Preferably, the high resistance semiconductor laser comprises a GaN-based semiconductor laser; the low output impedance drive element comprises a field effect transistor; and the GaN-based semiconductor laser is driven by a source follower using the field effect transistor.

According to a second aspect of the present invention, there is provided a semiconductor laser drive circuit for driving a high resistance semiconductor laser, comprising a control circuit for generating a drive control signal; a low output impedance drive element for driving the high resistance semiconductor laser based on the drive control signal by a voltage; and a constant current circuit connected to a connection point of the low output impedance drive element and the high resistance semiconductor laser.

Preferably, the high resistance semiconductor laser and the low output impedance drive element are connected via a capacitor; the connection point of the capacitor and the low output impedance drive element is grounded via a first resistance element; and the constant current circuit is connected to a connection point of the capacitor and the high resistance semiconductor laser.

Preferably, the high resistence semiconductor laser and the constant current circuit are connected via an inductor or a second resistance element.

Preferably, the high resistance semiconductor laser comprises a GaN-based semiconductor laser; the low output impedance drive element comprises a field effect transistor; and the GaN-based semiconductor laser is driven by a source follower using the field effect transistor.

The above first and second aspects of the present invention drive the high resistance semiconductor laser by a low output impedance drive element, so can effectively supply power to the high resistance semiconductor laser.

Also, by supplying power to the high resistance semiconductor laser via a low output impedance drive element, they can suppress the mirror effect and can improve the driving speed of the high resistance semiconductor laser.

By being configured to be powered by a source follower, the high resistance semiconductor laser can suppress the mirror effect in a field effect transistor and can improve the driving speed a GaN-based semiconductor laser or other high resistance laser.

Also since the semiconductor laser drive circuit according to the second aspect of the present invention is provided with a constant current circuit connected to a connection point of a low output impedance drive element and a high resistance semiconductor laser, it can supply a constant current to the high resistance semiconductor laser and can output laser light corresponding to the constant current under normal conditions.

These and other objects and features of the present invention will become clearer from the following description of the preferred embodiments given with reference to the accompanying drawings, in which:

FIG. 1 is a schematic block diagram of the configuration of a semiconductor laser drive circuit according to a first embodiment of the present invention;

FIG. 2 is a circuit diagram of a semiconductor laser drive circuit of FIG. 1;

FIG. 3 is a schematic block diagram of the configuration of a semiconductor laser drive circuit according to a second embodiment of the present invention;

FIG. 4 is a schematic block diagram of the configuration of a semiconductor laser drive circuit according to a third embodiment of the present invention;

FIG. 5 is a schematic circuit diagram for explaining a principle of a semiconductor laser drive circuit according to a fourth embodiment of the present invention;

FIG. 6 is a schematic circuit diagram of an example of a semiconductor laser drive circuit according to the fourth embodiment of the present invention;

FIG. 7 is a schematic circuit diagram of another example of a semiconductor laser drive circuit according to the fourth embodiment of the present invention; and

FIG. 8 is a schematic circuit diagram of another example of a semiconductor laser drive circuit according to the fourth embodiment of the present invention.

Below, preferred embodiments will be described with reference to the accompanying drawings.

First Embodiment

FIG. 1 is a schematic block diagram of the configuration of a semiconductor laser drive circuit according to a first embodiment of the present invention.

In this embodiment, a semiconductor laser drive circuit 100 is installed in a recording/reproducing apparatus for recording information on an optical disk and reproducing information stored on an optical disk.

The semiconductor laser drive circuit 100 comprises a power source 1 , a source follower (drain ground circuit) 4 , and a control circuit 5 for generating a drive control signal Sc.

The source follower 4 comprises a low output impedance drive element 2 and a high resistance semiconductor laser 3 .

The low impedance drive element 2 is comprised of, for example, an enhancement type n-channel field effect transistor (FET).

The high resistance semiconductor laser 3 is comprised of, for example, a GaN-based semiconductor laser (GaN-based semiconductor laser diode) for outputting blue-violet laser light.

Next, the case will be explained where the low output impedance drive element 2 is an FET and the high resistance semiconductor laser 3 is a GaN-based semiconductor laser.

A drain of the FET 2 is connected to a power source circuit 1 and a source is connected to the GaN-based semiconductor laser 3 .

The control circuit 5 comprises an oscillation circuit 10 , a resonance circuit 20 , a combining circuit 30 , a buffer circuit 40 , a level-shift circuit 50 , a switch circuit 60 , and a constant voltage circuit 70 .

The buffer circuit 40 comprises a first waveform shaping circuit 41 and a second waveform shaping circuit 42 .

The level-shift circuit 50 comprises a first level-shift circuit 51 and a second level-shift circuit 52 .

The switch circuit 60 comprises a first switch circuit 61 and a second switch circuit 62 .

The first waveform shaping circuit 41 receives as an input fifth input signals S 4 and S 5 . The fifth input signals S 4 and S 5 are in a relationship of an inverted signal and a non-inverted signal and form a pair.

The second waveform shaping circuit 42 receives as an input sixth input signals S 6 and S 7 . The sixth input signals S 6 and S 7 are in a relationship of an inverted signal and a non-inverted signal and form a pair.

Note that in the case of a first operation mode, that is, in the case of erasing information stored on an optical disk (at the time of erasure), the input signals S 4 and S 5 are supplied only to the first waveform shaping circuit 41 among the first and second waveform shaping circuits 41 and 42 .

Also, in the case of a second operation mode, that is, when writing for storing information on the optical disk (at the time of recording), the input signals S 4 and S 5 are supplied to the first waveform shaping circuit 41 , while the input signals S 6 and S 7 are supplied to the second waveform shaping circuit 42 . The emission output (emission intensity) of the GaN-based semiconductor laser 3 becomes larger at the time of recording than at the time of erasure.

Further, in the case of a third operation mode, that is, when reading information stored on the optical disk (at the time of reproducing), input signals are not supplied to the first and second waveform shaping circuits 41 and 42 .

The first waveform shaping circuit 41 performs waveform shaping on the fifth input signals S 4 and S 5 to generate signals S 4 A and S 5 A and supplies the signals S 4 A and S 5 A as third input signals to the first level-shift circuit 51 .

The second waveform shaping circuit 42 performs waveform shaping on the fifth input signals S 6 and S 7 to generate signals S 6 A and S 7 A and supplies the signals S 6 A and S 7 A as fourth input signals to the first level-shift circuit 52 .

The first and second waveform shaping circuits 41 and 42 receive as input a signal of a p-ECL level, output a signal of a p-ECL level, and shape a signal with a blunt edge to a signal with a sharp edge.

By configuring the circuit as such, the buffer circuit 40 can prevent the waveform of an input signal from becoming blunt when the transfer path of the input signal is extended.

The first level-shift circuit 51 shifts the levels (signal levels) of the third input signals S 4 A and S 5 A to generate signals S 4 B and S 5 B and supplies the signals S 4 B and S 5 B as first input signals to the first switch circuit 61 .

The second level-shift circuit 52 shifts the levels (signal levels) of the fourth input signals S 6 A and S 7 A to generate signals S 6 B and S 7 B and supplies the signals S 6 B and S 7 B as second input signals to the second switch circuit 62 .

The first and second level-shift circuits 51 and 52 are comprised by a differential amplifying circuits comprised of emitter coupled logic (ECL) circuits.

The first switch circuit 61 amplifies the first input signals S 4 B and S 5 B based on the first switch control signal S 3 to generate a first amplification signal S 61 and supplies the first amplification signal S 61 to the combining circuit 30 .

The amplification degree of the first switch circuit 61 can be adjusted by a first amplification control signal (first switch control signal) S 3 .

The second switch circuit 62 amplifies the second input signals S 6 B and S 7 B based on a second switch control signal S 8 to generate a second amplification signal S 62 and supplies the second amplification signal S 62 to the combining circuit 30 .

The amplification degree of the second switch circuit 62 can be adjusted by a second amplification control signal (second switch control signal) S 8 .

The first and second switch circuits 61 and 62 are comprised by a current switch logic (CSL) circuit comprised by an ECL circuit.

The constant voltage circuit 70 generates a constant voltage signal S 70 for biasing based on a setting signal S 9 and supplies the constant voltage signal S 70 to the combining circuit 30 .

A bias voltage of a gate input signal of the FET 2 can be adjusted by the setting signal S 9 and an output power of the GaN-based semiconductor laser 3 can be set.

The oscillation circuit 10 generates an oscillation signal S 10 of a predetermined frequency based on an oscillation control signal S 2 and supplies the oscillation signal S 10 to the resonance circuit 20 .

The oscillation circuit 10 generates an oscillation signal S 10 when the oscillation control signal S 2 is in a first state, while stops generating the oscillation signal S 10 when the oscillation control signal S 2 is in a second state.

The oscillation control signal S 2 of the first state is supplied to the oscillation circuit at the time of reproduction.

The resonance circuit 20 is comprised by an LC resonance circuit comprised of a capacitor C and an inductor L connected in series, passes a signal S 20 of a specific frequency from the oscillation signal 10 , and supplies the passing signal S 20 to the combining circuit 30 .

The combining circuit 30 is supplied with the passing signal S 20 , first and second amplification signals S 61 and S 62 , and constant current signal S 70 , selectively combines the signals to generate a combined signal Sc, and supplies the combined signal Sc to the source follower 4 .

At the time of erasure, the combining circuit 30 adds the first amplification signal S 61 and the constant voltage signal S 70 to generate a combined signal Sc and supplies the combined signal Sc to the gate of the FET 2 .

At the time of recording, the combining circuit 30 adds the first and second amplification signals S 61 and S 62 and the constant voltage signal S 70 to generate a combined signal Sc and supplies the combined signal Sc to the gate of the FET 2 .

At the time of reproduction, the combining circuit 30 adds the passing signal S 20 and the constant voltage signal S 70 to generate a combined signal Sc and supplies the combined signal Sc to the gate of the FET 2 .

The power source circuit 1 generates a power source voltage Vp for driving the GaN-based semiconductor laser 3 based on the power source control signal S 1 and supplies the power source voltage Vp to a drain of the FET 2 of the source follower 4 .

When the power source control signal S 1 is in an enabled state (one state), the power source circuit 1 generates a power source voltage Vp and the GaN-based semiconductor laser 3 outputs laser light.

When the power source control signal S 1 is in a disabled state (other state), the power source circuit 1 stops generating the power source voltage Vp to make the output voltage to the FET 2 0 V and the GaN-based semiconductor laser 3 stops outputting laser light.

The FET 2 is supplied with the combined signal Sc as a gate input signal and supplies power to the GaN-based semiconductor laser 3 based on the combined signal Sc.

The signal level of the combined signal Sc at the time of recording is larger than that at the time of erasure. Accordingly, a larger voltage is applied to the GaN-based semiconductor laser 3 at the time of recording than at the time of erasure. As a result, the emission output of the GaN-based semiconductor laser 3 at the time of recording becomes larger than that at the time of erasure.

Since the combined signal Sc at the time of reproducing is comprised of a signal obtained by adding the passing signal S 20 of a specific frequency and the constant voltage signal S 70 , a specific frequency can be superimposed onto the laser light from the GaN-based semiconductor laser 3 .

Also, the laser light output from the GaN-based semiconductor laser 3 can be made larger compared with the case where the combined signal Sc at the time of reproducing is comprised of a constant voltage signal S 70 , thus it is possible to reduce the laser light noise at the time of low output of laser light.

FIG. 2 is a circuit diagram of the semiconductor laser drive circuit 100 of FIG. 1, wherein the buffer circuit 40 is omitted. Note that the reference GND in the figure indicates a ground potential (ground level).

The input signal S 4 A is input as an input voltage Va 1 to the first level-shift circuit 51 .

The input signal S 5 A is input as an input voltage Va 2 to the first level-shift circuit 51 .

The input signal S 6 A is input as an input voltage Vb 1 to the second level-shift circuit 52 .

The input signal S 7 A is input as an input voltage Vb 2 to the second level-shift circuit 52 .

The first switch control signal S 3 is input as an input voltage V 61 to the first switch circuit 61 .

The second switch control signal S 8 is input as an input voltage V 62 to the second switch circuit 62 .

The power source control signal S 1 is input as an input voltage Vd to the power source circuit 1 .

The oscillation control signal S 2 is input as an input voltage Vsw to the oscillation circuit 10 .

The setting signal S 9 is input as an input voltage Vin to the constant voltage circuit 70 .

The first level-shift circuit 51 comprises collector resistors R 5 and R 6 supplied with a power source voltage Vcc, base resistors R 1 and R 2 supplied with input voltages Va 1 and Va 2 , emitter resistors R 3 and R 4 , differential pair transistors Q 2 and Q 3 , a transistor Q 1 , and a grounded load resistor R 9 .

The transistors Q 1 , Q 2 , and Q 3 are npn transistors.

The resistance values of the collector resistors R 5 and R 6 are equal, the resistance values of the base resistors R 1 and R 2 are equal, the resistance values of the emitter resistors R 3 and R 4 are equal, and the characteristics of the transistors Q 2 and Q 3 are equal.

The collector of the transistor Q 1 is connected to a connection point of the emitter resistors R 3 and R 4 , the emitter is connected to the load resistor R 9 , and the base is supplied with an input voltage V 51 . The collector current I 1 of the transistor Q 1 is set by the input voltage V 51 .

A constant current source for making a collector current of the transistor Q 1 constant is formed by the transistor Q 1 and the load resistor R 9 . The constant current source, depending on the input voltage V 51 , outputs a constant current at the time of erasure (first operation mode) and recording (second operation mode) and stop outputting a current at the time of reproduction (third operation mode).

The base of the transistor Q 2 is connected to the base resistor R 1 , the collector is connected to the collector resistor R 5 , and the emitter is connected to the emitter resistor R 3 .

The base of the transistor Q 3 is connected to the base resistor R 2 , the collector is connected to the collector resistor R 6 , and the emitter is connected to the emitter resistor R 4 .

The first level-shift signal S 4 B is supplied as a collector voltage Va 3 of the transistor Q 2 to the base resistor R 8 in the first switch circuit 61 .

The first level-shift signal S 5 B is supplied as a collector voltage Va 4 of the transistor Q 3 to the base resistor R 7 in the first switch circuit 61 .

The second level-shift circuit 52 comprises collector resistors R 55 and R 56 supplied with a power source voltage Vcc, base resistors R 51 and R 52 supplied with input voltages Vb 1 and Vb 2 , emitter resistors R 53 and R 54 , differential pair transistors Q 52 and Q 53 , a transistor Q 51 , and a grounded load resistor R 59 .

The transistors Q 51 , Q 52 , and Q 53 are npn transistors.

The resistance values of the collector resistors R 55 and R 56 are equal, the resistance values of the emitter resistors R 51 and R 52 are equal, the resistance values of the emitter resistors R 53 and R 54 are equal, and the characteristics of the transistors Q 52 and Q 53 are equal.

A collector of the transistor Q 51 is connected to a connection point of the emitter resistors R 53 and R 54 , the emitter is connected to the load resistor R 59 , and the base is supplied with an input voltage V 52 . The collector current of the transistor Q 51 is set by the input voltage V 52 .

A constant current source for making a collector current of the transistor Q 51 constant is formed by the transistor Q 51 and the load resistor R 59 . The constant current source, depending on the input voltage V 52 , outputs a constant current at the time of recording (second operation mode) and stops outputting a current at the time of erasure (first operation mode) and at the time of reproduction (third operation mode).

The base of the transistor Q 52 is connected to the base resistor R 51 , the collector is connected to the collector resistor R 55 , and the emitter is connected to the emitter resistor R 53 .

The base of the transistor Q 53 is connected to the base resistor R 52 , the collector is connected to the collector resistor R 56 , and the emitter is connected to the emitter resistor R 54 .

The second level-shift signal S 6 B is supplied as a collector voltage Vb 3 of the transistor Q 52 to the base resistor R 26 in the second switch circuit 62 .

The second level-shift signal S 7 B is supplied as a collector voltage Vb 4 of the transistor Q 53 to the base resistor R 25 in the second switch circuit 62 .

The first switch circuit 61 comprises a load resistor RIO supplied with a power source voltage Vcc, a transistor Q 4 , differential pair transistors Q 6 and Q 7 , and base resistors R 7 and R 8 .

The transistors Q 4 , Q 6 , and Q 7 are pnp transistors.

The resistance values of the base resistors R 7 and R 8 are equal and the characteristics of the transistors Q 6 and Q 7 are equal.

The collector of the transistor Q 4 is connected to a connection point of the emitters of the differential pair transistors Q 6 and Q 7 , the emitter is connected to the load resistor R 10 , the base is supplied with an input voltage V 61 , and a collector voltage of the transistor Q 4 is set by the input voltage V 61 .

A constant current source for making a collector current of the transistor Q 4 constant is formed by the transistor Q 4 and the load resistor Q 10 . The constant current source, depending on the input voltage V 61 , outputs a constant current at the time of erasure and recording and stops current output at the time of reproduction.

The emitter of the transistor Q 7 is connected to the collector of the transistor Q 4 , the base is connected to the base resistor R 7 , and the collector is connected to the resistor R 13 .

The emitter of the transistor Q 6 is connected to the collector of the transistor Q 4 , the base is connected to the base resistor R 8 , and the collector is connected to the signal combining resistor R 12 .

The amplification signal S 61 is supplied as a collector voltage or a collector current i 2 of the transistor Q 6 to the resistor R 12 .

The second switch circuit 62 comprises a load resistor R 11 supplied with a power source voltage Vcc, a transistor Q 5 , differential pair transistors Q 8 and Q 9 , base resistors R 26 and R 25 , and a grounded collector R 13 .

The transistors Q 5 , Q 8 , and Q 9 are pnp transistors.

The resistance values of the base resistors R 25 and R 26 are equal and the characteristics of the transistors Q 8 and Q 9 are equal.

The collector of the transistor Q 5 is connected to a connection point of both emitters of the differential pair transistors Q 8 and Q 9 , the emitter is connected to the load resistor R 11 , an input voltage V 62 is supplied to the base, and a collector voltage of the transistor Q 5 is set by the input voltage V 62 .

A constant current source for making a collector current of the transistor Q 5 constant is formed by the transistor Q 5 and the load resistor Q 11 . The constant current source, depending on the input voltage V 62 , outputs a constant current at the time of recording and stops current output at the time of erasure and reproduction.

The emitter of the transistor Q 9 is connected to the collector of the transistor Q 5 , the base is connected to the base resistor R 25 , and the collector is connected to the resistor R 13 .

The emitter of the transistor Q 8 is connected to the collector of the transistor Q 5 , the base is connected to the base resistor R 26 , and the collector is connected to the signal combining resistor R 12 .

The amplification signal S 62 is supplied as a collector voltage or a collector current i 3 of the transistor Q 8 to the resistor R 12 .

The constant voltage circuit 70 comprises a resistor R 16 supplied with a power source voltage Vcc, a resistor R 17 , grounded resistors R 18 and R 19 , and transistors Q 12 and Q 13 .

The transistors Q 12 and Q 13 are pnp transistors.

The power source voltage Vcc is divided by the resistors R 16 , R 17 , and R 18 connected in series.

An emitter of the transistor Q 12 is supplied with an input voltage Vin, the base is connected to a connection point of the resistors R 17 and R 18 and supplied with a constant voltage, and the collector is connected to the resistor R 19 .

The emitter of the transistor Q 13 is connected to a connection point of the resistors R 16 , R 17 , and R 12 , the base is connected to the resistor R 19 , and the collector is grounded.

The collector of the transistor Q 12 is connected to the base of the transistor Q 13 .

The constant voltage circuit 70 makes an emitter voltage (emitter potential) of the transistor Q 13 a constant voltage V 1 and supplies the constant voltage V 1 as a constant voltage signal S 70 to the resistor R 12 .

The oscillation circuit 10 comprises a signal source Vf for generating a sine signal of a predetermined frequency and a grounded switching element SW. The switching element SW is controlled by an input voltage Vsw.

The signal source Vf is connected to the switching element SW.

The switching element SW becomes on at the time of reproduction. As a result, the signal source Vf oscillates to generate an output voltage V 3 . The oscillation signal S 10 is supplied as an output voltage V 3 to the resonance circuit 20 .

The switching element SW becomes off at the time of recording and erasure. As a result, the signal source Vf stop oscillating.

The resonance circuit 20 comprises an inductor L 1 and a capacitor C 1 . The inductor L 1 and the capacitor C 1 are connected in series.

The resonance circuit 20 passes the signal voltage of a specific frequency pass from the output signal from the oscillation circuit 10 and supplies it as the passing signal S 20 to the resistor R 12 .

The combining circuit 30 comprises a resistor R 12 .

One end of the resistor R 12 is connected to an emitter of the transistor Q 13 and is supplied with a constant voltage V 1 .

The other end of the resistor R 12 is connected to collectors of transistors Q 6 and Q 8 , the inductor L 1 , and a connection point of the resistors R 14 and R 15 .

The resistance values of the resistors R 12 and R 13 are equal.

The power source circuit 1 comprises resistors R 20 and R 21 supplied with a power source voltage Vcc, transistors Q 14 , Q 15 , and Q 16 , a base resistor R 24 , and grounded resistors R 22 and R 23 .

The transistors Q 14 and Q 16 are npn transistors and the transistor Q 15 is a pnp transistor.

The power source voltage Vcc is divided by the resistors R 21 and R 22 .

The collector of the transistor Q 14 is supplied with a power source voltage Vcc, the emitter is connected to drains of FETs 2 A and 2 B, and the base is connected to the resistor R 23 .

The collector of the transistor Q 15 is connected to the resistor R 23 , the emitter is connected to the resistor R 20 , and the base is connected to a connection point of the resistors R 21 and R 22 and supplied with a constant voltage.

The collector of the transistor Q 16 is connected to the resistor R 23 , the emitter is grounded, and the base is connected to the base resistor R 24 .

The base of the transistor Q 14 and the collectors of the transistors Q 15 and Q 16 are connected to each other.

The power source circuit 1 makes an emitter voltage of the transistor Q 14 a constant power source voltage Vp and supplies the power source voltage Vp to the source follower 4 .

The source follower 4 comprises gate resistors R 14 and R 15 , FETs 2 A and 2 B, and a GaN-based semiconductor laser 3 . If examining correspondence with the FET 2 in FIG. 1, the FETs 2 A and 2 B are used as the FET 2 .

The resistance values of the gate resistors R 14 and R 15 are equal and the characteristics of the FET 2 A and FET 2 B are equal.

The FETs 2 A and 2 B are GaAs metal semiconductor field effect transistors (MESFET) and are capable of operating at a high speed.

The gate of the FET 2 A is connected to the gate resistor R 14 , the drain is connected to the emitter of the transistor Q 14 , and the source is connected to an anode of the GaN-based semiconductor laser 3 .

The gate of the FET 2 B is connected to a gate resistor R 15 , the drain is connected to the emitter of the transistor Q 14 , and the source is connected to an anode of the GaN-based semiconductor laser 3 . The cathode of the GaN-based semiconductor laser 3 is grounded.

A connection point of the resistors R 14 and R 15 is connected to the resistor R 12 .

By providing the resistors R 14 and R 15 , there is an advantage that it is possible to prevent a current from flowing in a forward direction between the gate and source of the FETs 2 A and 2 B when the voltage V 2 rises and to absorb individual differences between the FETs 2 A and 2 B.

Next, the operation of the circuit will be explained.

The first and second level-shift circuits 51 and 52 are differential amplifying circuits.

An operation point is decided by a collector current I 1 of the transistor Q 1 and a collector resistance (load resistor) R 5 . The operation point voltage Va can be expressed by the formula (1) below. Note that a resistance value of the load resistor R 5 is expressed as R 5 .

Va=Vcc −( I 1 × R 5 )/2  (1)

Further, if the differential input voltage Via is made |Va 2 −Va 1 | and the differential output voltage Voa is made |Va 4 −Va 3 |, the relationship between the differential input voltage Via and the differential output voltage Voa is shown by the formula (2). Note that the resistance value of the load resistor R 3 is expressed as R 3 .

Voa=Via× ( R 5 / R 3 )  (2)

The first and second level-shift circuits 51 and 52 prevent the transistors Q 6 to Q 9 of the first and second switch circuits 61 and 62 from being saturated even if a high voltage is required for the control voltage V 2 of the source follower 4 by shifting the levels of the input voltages Va 1 , Va 2 , Vb 1 , and Vb 2 .

Also, the first and second level-shift circuits 51 and 52 amplify the input voltages Va 1 , Va 2 , Vb 1 , and Vb 2 to voltages for turning on and off the pn junctions of the transistors Q 6 to Q 9 so that the first and second switch circuits 61 and 62 can perform accurate switching.

Assuming that an input voltage of the buffer circuit 50 of the semiconductor laser drive circuit 100 is directly input to the switch circuit 60 , collector voltages (collector potentials) of the transistors Q 6 and Q 8 of the first and second switch circuits 61 and 62 become higher along with a source terminal voltage of the FET.

As a result, there is a possibility that a forward voltage will be applied between the collector and base of the transistors Q 6 and Q 8 , the transistors Q 6 and Q 8 will become saturated, and the output signal waveform will become deviated.

Furthermore, when the amplitude of the input voltage is small, the transistors Q 6 and Q 8 may no longer turn on or off. Even if the transistors Q 6 and Q 8 turn on or off, the output signal waveform may become blunted due to level-shifting in the switch circuit 60 .

Therefore, it is preferable that the level-shift circuit 50 be connected before the switch circuit 60 .

The first and second switch circuits 61 and 62 are current switch logic (CSL) circuit comprised by an ECL circuit. When one of the differential pair transistors Q 6 and Q 7 is in on, the other is off. Also, when one of the differential pair transistors Q 8 and Q 9 is on, the other is off.

The collector current of the transistor Q 4 is equal to the collector current of the on transistor Q 6 or Q 7 , while the collector current of the transistor Q 5 is equal to the collector current of the on transistor Q 8 or Q 9 .

The control voltage V 2 of the FETs 2 A and 2 B is determined by the collector current i 2 of the transistor Q 6 and the collector current i 3 of the transistor Q 8 . The control voltage V 2 is expressed by the formula (3) below. Note that the resistance value of the resistor R 12 is expressed as R 12 .

V 2 =( i 2 + i 3 )× R 12 + V 1   (3)

The constant voltage circuit 70 is a sink type voltage source and configured by one transistor amplifying circuit and buffer amplifier.

In the constant voltage circuit 70 , a feedback resistor R 17 is provided so that the output voltage V 1 does not change by a load current I 13 . The output voltage V 1 is kept constant by the resistor R 17 unless the input voltage Vin changes.

Then the constant voltage circuit 70 supplies a gate bias voltage of the FETs 2 A and 2 B in order to fulfill an operation voltage of the GaN-based semiconductor laser 3 .

The output voltage V 1 of the constant voltage circuit 70 can be expressed by a formula (4). Note that a base-emitter voltage of the transistor Q 12 is indicated as Vbe 1 and resistance values of the resistors R 16 , R 17 , and R 18 are indicated as R 16 , R 17 , and R 18 . Also, the input voltage V 1 satisfies formula (5) below.

V 1 =( Vin−Vbe 1 )×(1+ R 17 / R 18 )  (4)

V 1 ≦( R 17 + R 18 )× Vcc /( R 16 + R 17 + R 18 )  (5)

The signal combining resistor R 12 converts a sum of the output currents (i 2 +i 3 ) generated in the current switch (CSL) circuits 61 and 62 to a voltage and generates a control voltage V 2 to which a constant voltage V 1 is superimposed. The control voltage V 2 is expressed by a formula (6) next by using the above formulas (3) and (4).

V 2 =( i 2 + i 3 )× R 12 +( Vin−Vbe 1 )×(1+ R 17 / R 18 )  (6)

The source follower 4 drives the GaN-based semiconductor laser 3 by a voltage.

In the DC characteristic, a load current Io flowing into the GaN-based semiconductor laser 3 is determined by voltage-current characteristics of the GaN-based semiconductor laser 3 and gate-source voltage-drain current characteristics of the FETs 2 A and 2 B.

The load current Io is expressed by a formula (7) below. Note that in formula (7), Vo is a terminal voltage of the GaN-based semiconductor laser 3 , Vt is a threshold voltage of the FETs 2 A and 2 B, and Idds is a drain saturation current.

Io=Idds×{ 1+( V 2 − Vo )/ Vt} 2   (7)

In the power source circuit 1 , a constant current source having a transistor Q 15 and a saturation switch circuit having a transistor Q 16 are connected in a push-pull structure. Also, the saturation switch circuit controls a destination of flow of a constant current I 5 from the constant current source so as to allow or prevent the flow of the constant current I 5 to the load resistor R 23 .

The output voltage Vp of the power source circuit 1 is determined by the current I 23 flowing into the load resistor R 23 and expressed by a formula (8) below. Note that the base-emitter voltage of the transistor Q 14 is expressed as Vbe 2 and a resistance value of the load resistor R 23 is expressed as R 23 .

Vp=I 23 × R 23 − Vbe 2   (8)

As explained above, when the transistor Q 16 is on, the current I 23 becomes 0 or substantially 0 and the transistor Q 14 becomes off. As a result, the GaN-based semiconductor laser 3 becomes completely off.

When the transistor Q 16 is off, the current I 23 is equal to the constant collector current I 5 , and the transistor Q 14 becomes on and supplies the output voltage Vp to the source follower 4 .

The source follower 4 is configured to drive the GaN-based semiconductor laser 3 by voltage. The output resistance (output impedance) is small, and the output capacity is small enough to be negligible.

Further, by using the FETs 2 A and 2 B in parallel, the output resistance can be made further smaller which also enables high speed driving.

Also, since the source follower 4 can also suppress the mirror effect, it is possible to suppress a delay of a pulse response even when the resistance value of the load resistor is large.

Further, for example, even when a capacitor is connected in parallel to the load resistor, a current can be supplied to the load resistor almost regardless of the capacitance.

Since the GaN-based semiconductor laser 3 is configured so that the capacitor is connected in parallel to the load resistor if ignoring the inductance of the lead, a current can be supplied to the load resistor without being affected much at all by the capacitance of the capacitor, an output waveform of the laser light can be prevented from becoming blunt, and thus high speed driving is realized from this point as well.

Second Embodiment

FIG. 3 is a schematic block diagram of the configuration of a semiconductor laser drive circuit according to a second embodiment of the present invention.

The semiconductor laser drive circuit 200 comprises the semiconductor laser drive circuit 100 in FIG. 1 provided with a detection circuit 92 and a feedback circuit 8 .

Note that the same reference numerals are assigned to identical components as those in the semiconductor laser drive circuit 100 in FIG. 1 and the explanation of the identical components will be omitted.

A schematic equivalent circuit is illustrated inside the control circuit 5 in FIG. 3. A signal generating source 6 of a drive control signal Sc and a variable voltage source 7 are connected in series.

The control circuit 5 supplies the drive control signal Sc to the feedback circuit 8 .

The detection circuit 92 is connected between a power source circuit 1 and a source follower 4 , detects a current flowing into the source follower 4 from the power source circuit to generate a detection signal Sk, and supplies the detection signal Sk to the feedback circuit 8 .

The feedback circuit 8 comprises an adder circuit 81 and an amplifying circuit 82 .

The adder circuit 81 is supplied with the drive control signal Sc and the detection signal Sk, subtracts the detection signal from the drive control signal Sc to generate an adder signal S 81 , and supplies the adder signal S 81 to the amplification circuit 82 .

The amplification circuit 82 is configured for example by an operational amplifier, a transistor, an FET, etc., amplifies the adder signal S 81 to generate an amplification signal Sg, and supplies the amplification signal Sg as a gate input signal to the source follower 4 .

In the semiconductor laser drive circuit 200 , the detection circuit 92 , the feedback circuit 8 , and the source follower 4 form a feedback group.

The operation of the GaN-based semiconductor laser 3 is stabilized by detecting the current flowing to the GaN-based semiconductor laser 3 inside the source follower 4 , converting the level and voltage of the detected current value to generate a detection signal Sk, and feeding back the detection signal Sk.

The semiconductor laser drive circuit 200 is configured to detect a value of the current flowing to the GaN-based semiconductor laser 3 and capable of controlling the inflowing current without depending on voltage-current characteristics of the GaN-based semiconductor laser 3 .

Also, even in a case where a pulse current is flowing into the source follower 4 from the power source circuit 1 , the inflowing current can be controlled by setting a broad frequency bandwidth for the detection circuit 91 and the feedback circuit 8 to operate.

Third Embodiment

FIG. 4 is a schematic block diagram of the configuration of a semiconductor laser drive circuit according to a third embodiment of the present invention.

The semiconductor laser drive circuit 300 is comprised of the semiconductor laser drive circuit 200 in FIG. 3 without the detection circuit 92 , with a power source circuit 1 which supplies power directly to the source follower 4 , and with a detection circuit 93 .

Note that the same reference numerals are assigned for identical components as those in the semiconductor laser drive circuit 200 in FIG. 3 . The explanation of the identical components will be omitted.

The detection circuit 93 detects an emission output of a GaN-based semiconductor laser 3 to generate a detection signal Sk and supplies the detection signal Sk to a feedback circuit 8 .

For example, the detection circuit 93 splits laser light output from the GaN-based semiconductor laser 3 by using a beam splitter, receives the split laser light by a light receiving element, converts it to an electric signal, and amplifies the electric signal to obtain the detection signal Sk.

Fourth Embodiment

Next, a semiconductor laser drive circuit according to a fourth embodiment of the present invention will be explained.

FIGS. 5 to 8 are schematic circuit diagrams of the semiconductor laser drive circuit according to the fourth embodiment of the present invention.

FIG. 5 is a schematic circuit diagram for explaining a principle of the semiconductor laser drive circuit according to the fourth embodiment of the present invention.

The semiconductor laser drive circuit 400 comprises a switching element 401 , a diode 402 , a constant voltage source 403 , a GaN-based semiconductor laser 3 , and a constant current circuit 410 .

The constant current circuit 410 supplies a constant current I 410 to the GaN-based semiconductor laser 3 .

In the semiconductor laser drive circuit 400 , the switching element 401 , the diode 402 , and the constant voltage source 403 are connected in series. The switching element 401 becomes on or off by a drive control signal Sc.

The constant voltage source 403 supplies a constant voltage to the GaN-based semiconductor laser 3 when the switching element 401 is on.

In the semiconductor laser drive circuit 400 , a biasing use direct current is generated in the constant current circuit 410 , while at the time of high speed driving, a pulse voltage is generated by using the switching element 401 and the pulse voltage is supplied to the GaN-based semiconductor laser 3 to enable high speed response.

In the semiconductor laser drive circuit 400 , at the time of high speed driving, it is possible to drive the GaN-based semiconductor laser 3 without depending on voltage-current characteristics of the GaN-based semiconductor laser 3 .

In this case, an equivalent resistance when an alternating small signal is input to the GaN-based semiconductor laser 3 becomes a current feedback resistance.

FIG. 6 is a circuit diagram of a preferable semiconductor laser drive circuit showing the semiconductor laser drive circuit 400 in FIG. 5 more specifically.

The semiconductor laser drive circuit 460 comprises an FET 404 , a signal generating source 405 for generating a drive control signal Sc, a GaN-based semiconductor laser 3 , and a constant current circuit 410 .

The switching element 401 and the diode 402 in FIG. 5 are configured to be included in the FET 404 as an example of a low output impedance drive element.

The GaN-based semiconductor laser 3 is made to be driven by the source follower.

The constant current circuit 410 comprises an emitter resistor 411 supplied with a power source voltage Vcc, a transistor 412 , and a variable voltage source 413 . The transistor 412 is a pnp transistor.

The emitter of the transistor 412 is connected to the emitter resistor 411 , the collector is connected to an anode of the GaN-based semiconductor laser 3 , and the base is connected to the variable voltage source 413 so as to generate a constant collector current I 410 .

The FET 404 is an enhancement type n-channel field effect transistor. For example, a GaAs FET is used.

The drain of the FET 404 is supplied with a power source voltage Vcc, the gate is connected to the signal generating source 405 , and the source is connected to an anode of the GaN-based semiconductor laser 3 . A cathode of the GaN-based semiconductor laser 3 is grounded.

When the FET 404 is on, a drive voltage from the FET 404 is supplied to the GaN-based semiconductor laser 3 and the GaN-based semiconductor laser 3 outputs a blue-violet laser light.

When the FET 404 is off, the drive voltage from the FET 404 is not supplied to the GaN-based semiconductor laser 3 . The GaN-based semiconductor laser 3 stops outputting light or emits light of a low output and/or a constant output based on the constant current I 410 .

FIG. 7 is a circuit diagram of a modification of the semiconductor laser drive circuit 460 in FIG. 6 .

Note that the same reference numerals are assigned for identical components as those in the semiconductor laser drive circuit 460 in FIG. 6 and the explanation of the identical components will be omitted.

The semiconductor laser drive circuit 470 is comprised of the semiconductor laser drive circuit 460 provided with a resistor (first resistor element) 406 and a capacitor 407 . A source of the FET 404 is grounded via the resistor 406 .

Also, a source of the FET 404 is connected to an anode of the GaN-based semiconductor laser 3 via the capacitor 407 .

Since the resistor 406 and the capacitor 407 are provided, when a pulse voltage is output from the source of the FET 404 , a high frequency component of the pulse voltage can be supplied to the GaN-based semiconductor laser 3 by a capacitor 407 , a low frequency component of the pulse voltage can be blocked, and a high frequency voltage can be supplied to the GaN-based semiconductor laser 3 at the time of high speed driving.

FIG. 8 is a circuit diagram of a modification of the semiconductor laser drive circuit 470 in FIG. 7 .

Note that the same reference numerals are assigned for identical components as those in the semiconductor laser drive circuit 470 in FIG. 7 and the explanation of the identical components will be omitted.

The semiconductor laser drive circuit 480 is comprised of the semiconductor laser drive circuit 470 provided with an inductor 408 such as a choke coil between the constant current circuit 410 and the GaN-based semiconductor laser 3 .

Since the inductor 408 is provided, when the output capacitor of the constant current circuit 410 (collector capacitor in FIG. 8) is at least the junction capacity of the GaN-based semiconductor laser 3 , it is possible to prevent the pulse current from the capacitor 407 from flowing into an output terminal of the constant current circuit 410 .

Note that a resistor (second resistor element) may be provided instead of the inductance between the constant current circuit 410 and the GaN-based semiconductor laser 3 and that the resistor and the inductance may be provided connected in series.

Also, the control circuit 5 may be connected to the gate of the FET 404 so as to supply a drive control signal Sc to the gate.

A bipolar transistor may be used as a low output impedance drive element and an emitter follower may be used instead of the source follower 4 .

Further, the above embodiments are illustrations of examples of the present invention. The present invention is not limited to the above embodiments.

Summarizing the effects of the invention, in the semiconductor laser drive circuit according to the first aspect of the present invention, since a drive voltage is supplied to a high resistance semiconductor laser via the low output impedance drive element, power can be efficiently supplied to the high resistance semiconductor laser.

Also, the mirror effect can be suppressed by the low output impedance drive element, and the drive speed of the high resistance semiconductor laser can be improved by making the frequency of the drive control signal high.

Furthermore, since the control circuit comprises first and second switch circuits and a combining circuit for combining an output signal of the first and second switch circuits to generate the drive control signal, the control circuit is capable of changing a signal level of the drive control signal in many stages and controlling an emission intensity of the GaN-based semiconductor laser in many stages.

In a semiconductor laser drive circuit according to the second aspect of the present invention, since the drive voltage is supplied to the high resistance semiconductor laser via the low output impedance drive element, power can be effectively supplied to the high resistance semiconductor laser.

Also, the mirror effect can be suppressed by the low output impedance drive element, and the driving speed of the high resistance semiconductor laser can be improved by raising the frequency of the drive control signal.

Furthermore, an operation point of the high resistance semiconductor laser can be set by a constant current circuit connected to the high resistance semiconductor laser and a laser light intensity of the high resistance semiconductor laser during normal output can be set.

Also, by connecting the low output impedance drive element and the high resistance semiconductor laser via a capacitor, it is possible to supply a high frequency component in an output voltage of a low output impedance drive element to the high resistance semiconductor laser, to block a low frequency component, and to change the emission intensity of the GaN-based semiconductor laser to a pulse.

While the invention has been described with reference to specific embodiment chosen for purpose of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention.