JPH0758382A - Optical pulse generator - Google Patents

Abstract

PURPOSE:To secure the safety and prevent the malfunction and further, enable low-voltage drive at occurrence of optical short pulses. CONSTITUTION:This optical pulse generator is equipped with a semiconductor laser 12, a coaxial cable 10, which has an inner conductor and an outer conductor and one end of which is open, a load resistor 13, which has the value corresponding to the property impedance of the coaxial cable 10, a MOSFET 15, and a trigger pulse generating means 16, which generates trigger pulses to be applied to the gate of the MOSFET 15. Furthermore, a drive power source is connected to the inner conductor of the coaxial cable 10 and the drain of the MOSFET 15, and the zero potential of the drive power source is connected to the anode of the semiconductor laser 12 and the source of the MOSFET 15, and the outer conductor of the coaxial cable 10 is connected to the cathode of the semiconductor laser 12 through a load resistance 13.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pulse generator which can be used in the field of performing various measurements using high-power short optical pulses.

[0002]

2. Description of the Related Art A gain switching method is known as a method of generating a short optical pulse using a semiconductor laser.
This gain switching method utilizes the characteristic of direct modulation of a semiconductor laser, and generates a short optical pulse by oscillation caused by injecting a short current pulse into the semiconductor laser.

Generally, a high-power semiconductor laser dedicated to pulse driving is used to generate a high-power optical pulse. This high-power semiconductor laser dedicated to pulse driving can obtain a high output, but has a very high threshold for laser oscillation. Therefore, in order to obtain a sufficient output, a peak current of 5 to 10 A is required. Pulses need to be injected.

Recently, it has been required to reduce the pulse width of the light pulse to several tens of nanoseconds or less, but it is necessary to turn on / off a transistor or FET that generates a current pulse that determines the light pulse width. At the switching speed, the current pulse width of about 100 nanoseconds is currently the limit.

Therefore, conventionally, the pulse width of the current pulse is adjusted by the passive device, and the switching device is used only for performing the ON or OFF operation. Figure 5
The schematic structure of the optical pulse generator which adjusts the pulse width of a current pulse by a passive device is shown. In this optical pulse generator, the anode of the semiconductor laser 1 whose cathode side is grounded is connected to a driving power source through a series circuit composed of a resistor Ra, a switch SW1 and a resistor Rb. Resistance R
The resistance value a is the same as the characteristic impedance Z of the coaxial cable 2, and the resistance value Rb is sufficiently larger than the resistance value of the resistance Ra. The coaxial cable 2 has a receiving end open and a transmitting end connected to a connection point between the drive power source and the switch SW1.

The operation of the optical pulse generator configured as described above will be described with reference to FIG. When the switch SW1 is closed while the coaxial cable 2 is charged to the voltage Vh by the driving power source, the pulse propagates from the sending end of the coaxial cable 2 to the receiving end, and the pulse reflected at the receiving end returns to the sending end. . A current pulse having a pulse width corresponding to the pulse propagation period is generated.

Since the voltage of the driving power source is applied to the coaxial cable 2 and the resistor Ra via the resistor Rb having a resistance value sufficiently larger than the resistor Ra, immediately after the switch SW1 is closed, the coaxial cable is closed. The electric charge stored in the switch 2 is connected to the switch S through the series circuit including the coaxial cable 2 and the resistor Ra without being affected by the driving power source.
It can be considered that discharge is performed via W1, the resistor Ra, and the semiconductor laser 1. At this time, Vh /, which is divided by the resistance value of the coaxial cable 2 and the resistance Ra, is provided between both ends of the resistance Ra.
2 appears as a potential difference. When viewed from the side of the coaxial cable 2, as shown in FIG. 6A, the voltage pulse of −Vh / 2 propagates toward the sending end on the coaxial cable having the potential difference Vh, and therefore, on the line after passing the voltage pulse. The potential difference is Vh / 2. Since the receiving end of the coaxial cable 2 is an open end, the reflection coefficient is 1. Therefore, theoretically, the voltage pulse reaching the receiving end of the coaxial cable 2 is reflected without a voltage drop, and the voltage pulse of −Vh / 2 propagates toward the transmitting end again. In the propagation of the reflected pulse to the sending end, the potential difference Vh
Since the voltage pulse of -Vh / 2 propagates on the line of / 2, the line potential after pulse propagation becomes zero and the voltage pulse reaches the transmission end of the coaxial cable 2 as shown in FIG. 6B. By the way, the electric potential of the coaxial cable 2 becomes completely zero.

Since a potential difference of Vh / 2 is generated between both ends of the resistor Ra while the above voltage pulse is propagating, the semiconductor laser 1 has a pulse width of 2 L / as shown in FIG. 6C. A current pulse of vg will be injected.
Here, L is the length of the coaxial cable, and vg is the group velocity of the pulse in the line.

Therefore, the pulse width of the current pulse injected into the semiconductor laser 1 can be adjusted by the length of the coaxial cable 2. In the conventional optical pulse generator, as a switching device of the switch SW1, a thyratron,
Thyristors and mercury relays were used. High-speed power MOSFE instead of these switching devices
If T is used, a high response speed can be secured.

When a high-speed power MOSFET is applied to the switching device of the above-mentioned optical pulse generator, the structure shown in FIG. 7 is expected. This optical pulse generator uses a power MOS for the cathode of the semiconductor laser 1.
The FET 3 is grounded via its drain and source, the trigger pulse generator 4 is connected to the gate of the power MOSFET 3, and a switching operation is caused by applying the trigger pulse to the gate.

[0011]

However, the optical pulse generator shown in FIG. 7 has the following problems. (1) As shown in FIG. 8, a general semiconductor laser is housed in a case 4, and its anode is at ground potential. Therefore, when the semiconductor laser is driven with a positive high voltage, the positive high voltage applied from the driving power source may appear in the case potential when the power MOSFET 3 is off, which is not preferable for safety. Also, devices with low withstand voltage can be destroyed by high positive voltage.

(2) If a high voltage is applied to the drain side when the power MOSFET 3 is turned off, the power MO
When the SFET 3 is turned off at the falling edge of the trigger pulse, a pulse current is generated to oscillate the semiconductor laser, resulting in a double pulse.

(3) Since the peak value of the current pulse injected into the semiconductor laser is calculated by Vo / 2Zo, an extremely high power supply voltage is required to obtain a sufficient output.
Note that Vo is the voltage of the driving power supply, and Zo is the characteristic impedance of the coaxial cable. For example, when the characteristic impedance Zo of the coaxial cable is 50 ohms, a high voltage of 1000 V is required to obtain a peak current value of 10 A.

The present invention has been made in view of the above circumstances, and a positive high voltage does not appear in the case potential of a semiconductor laser, which is excellent in safety and eliminates the risk of device destruction. An object is to provide a pulse generator.

It is another object of the present invention to provide an optical pulse generator which prevents double pulses from occurring, prevents malfunctions and improves reliability. Another object of the present invention is to provide an optical pulse generator capable of generating a sufficiently large current pulse without increasing the voltage of the driving power source.

[0016]

According to a first aspect of the present invention, there is provided an optical pulse generator having a semiconductor laser which generates an optical pulse, an inner conductor and an outer conductor, and one end of which is open. Two coaxial cables, a load resistance having a resistance value corresponding to the characteristic impedance of the coaxial cable, a MOSFET for switching to inject a current pulse into the semiconductor laser, and the MOSF
Trigger pulse generating means for generating a trigger pulse to be applied to the gate of ET, the drive power source is connected to the inner conductor of the coaxial cable and the drain of the MOSFET, and the zero potential of the drive power source is connected to the semiconductor laser. The anode and the source of the MOSFET are connected, and the outer conductor of the coaxial cable is connected to the cathode of the semiconductor laser via the load resistor.

According to a second aspect of the present invention, in the optical pulse generator of the present invention, n coaxial cables of the same length are connected in parallel, and the resistance value of the load resistance is set for each coaxial cable. It was set to 1 / n of the characteristic impedance.

In the optical pulse generator of the present invention corresponding to claim 3, a current limiting resistor is connected between the drive power source and the drain of the MOSFET, and the resistance value R of the current limiting resistor is used to generate the optical pulse. The period is t _p , the width of the trigger pulse is t _w , and the equivalent capacitance of the current limiting resistor viewed from the drain side of the MOSFET is C, and t _w <2.2RC <t _p is selected.

[0019]

In the optical pulse generator according to the first aspect, since the series circuit including the MOSFET, the semiconductor laser, and the load resistor is connected from the inner conductor side to the outer conductor side, the coaxial cable is connected to the inner conductor side to the positive side. It can be regarded as a capacitance portion in which the outer conductor side is the negative electrode side. Therefore, in the state where the coaxial cable is charged by the driving power source, the trigger pulse is given from the trigger pulse generating means to the MOSFE.
When T is turned on, a positive voltage pulse divided into two by the characteristic impedance and the load resistance propagates toward the receiving end at the transmitting end of the coaxial cable. The positive voltage pulse is reflected by the receiving end which is an open end and reaches the sending end again, and then the propagation is terminated. While the voltage pulse is propagating through the coaxial cable, a current flows through the semiconductor laser, and pulsed light having a pulse width corresponding to the current pulse is generated.

At this time, since the cathode side of the semiconductor laser becomes a negative voltage due to the switching of the MOSFET, a positive high voltage does not appear in the case potential of the semiconductor laser, the risk can be eliminated, and the device destruction can be prevented.

An optical pulse generator according to claim 2 is
Since n coaxial cables are connected in parallel, the characteristic impedance is equivalently 1 / n. Assuming that a potential ½ of the power supply voltage V _E is generated across the load resistance, the current value I injected into the semiconductor laser is I = V _E · n / Z. Therefore, it is possible to generate a current pulse having a large current value according to the number n connected in parallel.

In the optical pulse generator according to the third aspect, by setting the resistance value R of the current limiting resistor in the above range, when the gate of the MOSFET is turned off,
The voltage applied between the drain and source of the MOSFET is still low. Then, the power supply voltage is gradually raised and is charged up to nearly 100% by the time of the next switching. Therefore, when the MOSFET is off, the voltage between the drain and the source is low, so that no current pulse is generated when the MOSFET is off, and the double pulse is prevented.

[0023]

EXAMPLES Examples of the present invention will be described below. FIG. 1 is a configuration diagram of an optical pulse generator according to an embodiment of the present invention. In the optical pulse generator of this embodiment, n coaxial cables 10 of the same length are connected in parallel, and a drive power source (not shown) is connected to the inner conductor of the coaxial cable 10 by a current limiting resistor 1.
1 is connected. On the other hand, a semiconductor laser 12 including a laser diode is provided on the outer conductor of the coaxial cable 10.
The cathode of is connected via a load resistor 13. In the semiconductor laser 12, a diode 14 is connected in antiparallel between its anode and cathode, and the anode is connected to the zero potential of the driving power supply. The load resistance 13 is a combined characteristic impedance Z of all the coaxial cables 10 connected in parallel.
The value is equal to o / n.

A power MO is connected between the anode of the semiconductor laser 12 connected to the zero potential of the driving power source and the driving power source.
The SFET 15 is connected via its source and drain. A trigger pulse generator 16 is connected to the gate of the power MOSFET 15, and a trigger pulse divided by two resistors Zs equal to the signal source impedance is applied to the gate.

In the present embodiment configured as described above, after the coaxial cable 10 is charged to the voltage V _E by the driving power source, the trigger pulse generated by the trigger pulse generating means 16 is applied to the gate of the power MOSFET 15. M
When the gate of the OSFET is turned on, the inner conductor side of the coaxial cable 10 becomes the ground potential, and -V _E appears on the outer conductor side of the coaxial cable 10.

At this time, at the sending end of the coaxial cable 10,
Immediately after the MOSFET 15 is turned on, as shown in FIG. 2A, the characteristic impedance Zo / n and the load resistance 13
The positive voltage pulse of V _E / 2 divided by and is -V _E
Propagate on the railroad track toward the receiving end. The positive voltage pulse is reflected at the receiving end, which is an open end, and propagates toward the sending end again (FIG. 2 (b)). Since it terminates in the characteristic impedance at the transmitting end, the voltage pulse reaching the transmitting end stops propagating there without being reflected further.

While the voltage pulse is propagating through the coaxial cable, a current (pulse current) flows through the semiconductor laser 12, and the semiconductor laser 12 generates pulsed light. The operation waveforms during this period are shown in FIG. As shown in the figure, a current pulse having a predetermined pulse width is generated at the rising edge of the trigger pulse, and an optical pulse having a pulse width corresponding to the current pulse is generated.

In this embodiment, the current injected into the semiconductor laser 12 is a pulse current whose ON state lasts for the time period during which the V _E / 2 voltage pulse travels back and forth through the coaxial cable 10. The propagation speed of the voltage pulse in the coaxial cable 10 is v
If g and the length per coaxial cable are L, the pulse width τ of the current pulse is τ = 2L / vg.

For example, in a typical coaxial cable with a characteristic impedance of 250 ohms, the propagation speed vg of the voltage pulse is vg.
Is 20 cm / nsec, the length of the coaxial cable 10 can be determined from the required pulse width τ based on the above equation.

Further, in this embodiment, since n coaxial cables 10 are connected in parallel and the equivalent impedance is Zo / n, the load resistance 13 having a resistance value matched with the characteristic impedance Zo / n. Since a potential of −V _E / 2 is generated between both ends of the semiconductor laser 1,
The injection current I into 2 is I = V _E · n / Zo. Therefore, it is possible to generate pulses having a large current value by the number of coaxial cables connected in parallel.

Further, in this embodiment, the current limiting resistor 11 connected between the driving power source and the drain of the power MOSFET 15 prevents the double pulse from being generated. The resistance value of the current limiting resistor 11 is set in the range described later.

The light pulse generation period is t _p (sec), and the pulse width of the trigger pulse of the power MOSFET 15 is t _w.
Let (sec) be the capacitance between the grounds (the sum of the capacitance of the coaxial cable 10 and the feedback capacitance of the power MOSFET 15) seen from the current limiting resistor 11 when the power MOSFET 15 is turned on, and let C be the resistance of the current limiting resistor 11. Value R
Is selected to satisfy the following equation.

T _w <2.2 RC <t _p By selecting the resistance value R of the current limiting resistor 11 so as to satisfy the above, as shown in FIG.
Since the drain voltage of the FET 15 is almost 0 when the power MOSFET 15 is off (at the falling edge of the trigger pulse), a current pulse is not generated when the power MOSFET 15 is off, and a double pulse is not generated. Moreover, after that, the power MOSFET 1
The drain voltage of No. 5 gradually rises, and rises to almost 100% by the time when the next optical pulse is generated.

As described above, according to the present embodiment, since the semiconductor laser 12 is driven with a negative high voltage, a positive high voltage is induced in the case of the semiconductor laser 12 to cause a dangerous state. Can be prevented, and safety can be improved. Further, since a positive high voltage is not induced in the case of the semiconductor laser 12, it is possible to prevent device breakdown due to the positive high voltage.

Further, according to this embodiment, the coaxial cable 10
N parallel connection, it is possible to generate a high-current current pulse at a low voltage, and a high-speed power MOSFE with a relatively low breakdown voltage.
Since T can be used, a short light pulse having a short pulse width can be stably generated. In addition, the low voltage drive makes the device smaller,
It has the effect of reducing power consumption.

Further, according to this embodiment, the current limiting resistor 1
Since the resistance value R of 1 is set in the above range, the occurrence of double pulse can be prevented, the malfunction of the device can be prevented, and the reliability can be improved. The present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

[0037]

As described above in detail, according to the present invention, a positive high voltage does not appear in the case potential of a semiconductor laser, which is excellent in safety and eliminates the risk of device destruction. Can be provided.

Further, according to the invention, it is possible to provide an optical pulse generator which prevents double pulses from occurring, eliminates malfunction and improves reliability. Further, according to the present invention, it is possible to provide an optical pulse generator capable of driving a semiconductor laser at a low voltage, downsizing the device, and reducing power consumption.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical pulse generator according to an embodiment of the present invention.

FIG. 2 is a diagram showing a coaxial cable potential when a voltage pulse propagates through a coaxial cable.

FIG. 3 is a diagram for explaining a rising delay action by a current limiting resistance.

FIG. 4 is an operation waveform diagram of the above embodiment.

FIG. 5 is a configuration diagram of a conventional optical pulse generator.

6 is an explanatory view of a current pulse generation principle of the optical pulse generator shown in FIG.

FIG. 7 is a configuration diagram of an optical pulse generator including a MOSFET as a switching device.

FIG. 8 is a diagram showing a grounded state of a semiconductor laser.

[Explanation of symbols]

10 ... Coaxial cable, 11 ... Current limiting resistance, 12 ... Semiconductor laser, 13 ... Load resistance, 14 ... Diode, 15 ...
Power MOSFET, 16 ... Trigger pulse generator.

Claims (3)

[Claims] 1. A semiconductor laser for generating an optical pulse, at least one coaxial cable having an inner conductor and an outer conductor, one end of which is open, and a load having a resistance value corresponding to the characteristic impedance of the coaxial cable. Resistance,
The semiconductor laser includes a MOSFET that performs switching for injecting a current pulse, and trigger pulse generation means that generates a trigger pulse to be applied to the gate of the MOSFET, and the drive power source is an inner conductor of the coaxial cable and the Connected to the drain of the MOSFET, the zero potential of the driving power source is connected to the anode of the semiconductor laser and the MOSFE.
An optical pulse generator characterized in that it is connected to the source of T and the outer conductor of the coaxial cable is connected to the cathode of the semiconductor laser via the load resistor. 2. The coaxial cables of the same length are connected in parallel, and the resistance value of the load resistor is set to 1 / n of the characteristic impedance of each coaxial cable. Optical pulse generator. 3. A current limiting resistor is connected between the drive power source and the drain of the MOSFET, and the resistance value R of the current limiting resistor has a light pulse generation period t _p and a trigger pulse width t _w. the equivalent capacitance of the current limiting resistor viewed from the drain side of the MOSFET as C, t _w <2.2RC <according to claim 1 or claim 2, wherein the selected possible to satisfy the t _p Optical pulse generator.