JP2001257416A - DRIVING METHOD AND DEVICE OF GaN BASED SEMICONDUCTOR LASER - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce EL light generated by a bias current while an original pulse current for driving is not applied when a GaN based semiconductor laser is driven by the pulse current. SOLUTION: When the GaN based semiconductor laser 20 is driven by the pulse current Ep, a pulse bias current Eb which continues from at least a time before the rise time of the pulse current Ep until the fall time is applied to the semiconductor laser 20, synchronously with the pulse current Ep.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for driving a GaN-based semiconductor laser emitting light in a short wavelength region, and more particularly to a method for driving a GaN-based semiconductor laser having an improved bias current application method. is there.

[0002]

2. Description of the Related Art In recent years, GaN-based light emitting devices have made remarkable progress, and semiconductor lasers having an oscillation wavelength range of about 400 to 410 nm using InGaN as an active layer have come to be marketed.

[0003] On the other hand, in various applications such as image recording, optical disk, and cancer screening using a fluorescent endoscope, laser light is pulse-modulated by application of a pulse current and used. On this occasion,
Usually, a DC bias current is applied to a semiconductor laser, and an oscillation delay time from application of a pulse current to emission of laser light and a rise time of laser emission are shortened.

[0004]

However, in GaN-based semiconductor lasers, unlike GaAs-based semiconductor lasers, the GaN layers used for the sapphire and buffer layers serving as substrates are transparent to the emission wavelength. However, there is a disadvantage that stray light unnecessary for image forming applications is strong, and EL light (natural light emission) from an unnecessary portion becomes stray light particularly at a low light output, which hinders high-quality image formation.

[0005] Further, in a GaN-based semiconductor laser, the emission intensity is lower than the oscillation threshold as compared with a general GaAs-based semiconductor laser or the like. Therefore, even when the DC bias current is less than the oscillation threshold, the bias current emits light relatively strongly during the period in which the original driving pulse current is not applied.

The present inventors have further found the following problems unique to GaN-based semiconductor lasers. FIG. 1 shows the current vs. optical output characteristics of the studied semiconductor laser having an oscillation wavelength of 405 nm during DC driving. As shown here, the threshold current in DC driving is about 50 mA, but when a pulse current described later is applied, the threshold current drops by about 10% when the duty ratio is about 10%, to about 45 mA.

FIG. 2 shows that the GaN-based semiconductor laser is modulated and driven by a relatively low-frequency pulse current.
The DC current dependence of the applied current waveform and the optical output waveform is shown.
In the same figure, only the waveform of the applied current is shown,
Light output values are shown as relative values. The driving conditions at this time are: a: no DC bias, b: DC bias 10 mA, c: DC
The bias is 40 mA and the pulse peak value is 60 mA.
As shown in the figure, 80 μs (micro
There is an extremely large oscillation delay of about 2 seconds or more. This oscillation delay was reduced by applying a DC bias, and the oscillation delay time became shorter as the value of the bias current was increased, but the delay time could not be reduced to zero unless the bias current was increased up to the threshold current.

On the other hand, a normal response was exhibited during high-speed modulation. That is, the rise is 2 ns (nanosecond) or less, 8
References: S. Nakamura and G. Fasol: The Blue Laser Diode,
As described in Springer-Verlag, Berlin, 1997, p.252, the delay time shows a characteristic determined by the threshold current, the bias current, and the carrier lifetime (reference: R. N.
agarajan and JE Bowers: Semiconductor Lasers I,
Fundamentals, Academic Press, San Diego, 1999, p.
267), it was confirmed that high-speed modulation on the order of ns was possible.

Accordingly, for example, in a cancer screening application using fluorescence in an endoscope, which will be described later, when a GaN-based semiconductor laser as an excitation light source is pulse-driven in accordance with a video signal, a bias current having a considerable value is applied. Required. Then, the threshold current is about 0.5 to 1 mW.
Even when the L light is not irradiated with the pulse excitation light, it acts as background light, which hinders sample observation and the like.

On the other hand, when an image is formed by exposing a photosensitive material with pulse-modulated light, a DC bias is applied.
Since the photosensitive material is exposed by the EL light, the minimum exposure amount increases. Since the maximum exposure amount has an upper limit depending on the maximum value of the laser output, when the minimum exposure amount is large, the contrast is reduced, which causes a problem.

In various other applications, EL light generated by a GaN-based semiconductor laser by a bias current when pulse light is not generated often causes a problem as unnecessary light.

[0012]

SUMMARY OF THE INVENTION The present invention relates to the GaN described above.
It is made in view of the circumstances peculiar to semiconductor lasers. When driving a GaN semiconductor laser with a pulse current, the pulse response is kept good, while the original drive pulse current is applied. It is an object of the present invention to provide a method and an apparatus for driving a GaN-based semiconductor laser, which can reduce EL light generated by a bias current during a period when there is no current.

[0013]

A method of driving a GaN-based semiconductor laser according to the present invention is characterized in that, when a GaN-based semiconductor laser is driven by a pulse current, at least a time from a time before a rising time of the pulse current to a time of a falling time. A pulse-like bias current which continues until the pulse is applied to the semiconductor laser in synchronization with the pulse current.

Preferably, the pulsed bias current is a rectangular wave, a triangular wave or a sine wave.

The method of driving a GaN-based semiconductor laser according to the present invention is preferably used when the pulse current for driving the GaN-based semiconductor laser is subjected to modulation such as pulse intensity modulation, pulse width modulation, and pulse phase modulation. It is.

On the other hand, a driving apparatus for a GaN-based semiconductor laser according to the present invention is characterized in that it has a configuration for driving a GaN-based semiconductor laser by the driving method of the present invention described above.

[0017]

According to the method of driving a GaN-based semiconductor laser according to the present invention, the bias current applied to the GaN-based semiconductor laser is pulse-shaped, so that the method is different from the conventional method.
The duty ratio of the bias current is lower than when a DC bias current is applied. This makes it possible to reduce EL light generated by the bias current during a period in which the original driving pulse current is not applied.

Therefore, GaN is used in the above-mentioned fluorescent endoscope and the like.
When the method of driving a GaN-based semiconductor laser according to the present invention is applied to the case where fluorescence analysis is performed by irradiating pulsed light from a system-based semiconductor laser, background light when pulsed excitation light is not irradiated can be reduced, and sample observation can be performed Reliability can be improved.

On the other hand, when the method of driving a GaN-based semiconductor laser according to the present invention is applied to the case where an image is formed by exposing a photosensitive material with pulse-modulated light, the minimum exposure amount can be kept low.
A high dynamic range of the exposure can be secured.

In the method of driving a GaN-based semiconductor laser according to the present invention, the pulse-like bias current is supplied to the semiconductor laser such that the pulse-like bias current lasts at least from the time before the rise of the original driving pulse current to the time of the fall. Since the voltage is applied, a bias is already applied at the time when the driving pulse current rises. Therefore, the rise time and the oscillation delay time of the driving pulse current can be shortened, and the pulse response can be kept good.

[0021]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 3 schematically shows the configuration of an endoscope with a fluorescence diagnostic function provided with a GaN-based semiconductor laser driven by the method according to the first embodiment of the present invention.

As shown in the figure, the endoscope includes an inside of a living body 10,
For example, an endoscope end portion 11 inserted into an abdominal cavity or the like, a light guide 12 made of, for example, quartz glass fiber with the end portion housed in the endoscope end portion 11, and a light guide 12
The illumination lens 13 attached to the tip of the
An objective lens 14 fixed to the objective lens 11, a mirror 15 disposed inside the endoscope distal end portion 11 at a position behind (to the right in the figure) the objective lens 14, and an endoscope at an approximate position near the mirror 15 It has a CCD image pickup device 16 for image observation arranged inside the mirror tip 11.

The light guide 12 is formed by bundling a light guide 12a for excitation light and a light guide 12b for white light. The excitation light L1 emitted from the excitation light source 20 and collected by the condenser lens 21 is incident on the excitation light light guide 12a. Further, the white illumination light L2 emitted from the white light source 22 and condensed by the condenser lens 23 is incident on the white light light guide 12b.

The excitation light source 20 is composed of a GaN-based semiconductor laser, and the method of the present invention is applied to drive the excitation light source 20. The pump light source 20 in this example is, more specifically, an InGaN semiconductor laser having a multiple quantum well structure.

The excitation light source 20 is a semiconductor laser driving circuit.
Receiving the pulse current Ep generated by 24, it emits the excitation light L1 in a pulsed manner. Further, a pulsed bias current Eb generated by a bias applying circuit 25 is applied to the excitation light source 20. Semiconductor laser drive circuit 24 and bias application circuit 25
Is controlled by the controller 26. The pulse current Ep and the bias current Eb will be described later in detail. The white light source 22 is also driven by receiving a pulse-shaped drive current from a drive circuit (not shown) whose operation is controlled by the controller 26.

On the other hand, the CCD image pickup device 16 for image observation is connected to a reading circuit 27, and the image signal S
p is input to a display means 28 such as a CRT display device.

Hereinafter, the image observation using the endoscope having the above-described configuration will be described first, from the observation of a normal image. When observing a normal image, the white light source 22 is driven, and white illumination light L2 is emitted therefrom in, for example, a (1/60) s cycle in a pulsed manner. This white illumination light L2 is a light guide 12b for white light.
To irradiate the observation site 30 inside the living body 10.
The white illumination light L2 reflected at the observation site 30 is collected by the objective lens 14, reflected by the mirror 15, and guided to the CCD image pickup device 16. Thereby, the observation site by the white illumination light L2
Thirty images (normal images) are captured by the CCD image sensor 16.

The output signal S of the CCD 16 is read by a reading circuit.
27, where it undergoes processing such as amplification and image processing. The image signal Sp output from the reading circuit 27 is input to the display unit 28, and the display unit 28 displays a normal image of the observation region 30.

Next, observation of a fluorescent image will be described.
When observing the fluorescent image, the excitation light source 20 is driven, and the excitation light L1 is emitted in a pulse form therefrom. This excitation light L
Numeral 1 has a wavelength of about 400 to 410 nm, and propagates through the excitation light guide 12a to irradiate an observation site 30 inside the living body 10. For example, a photosensitizer having tumor affinity is previously administered to the observation site 30. The photosensitizer is excited by the excitation light L1 in the above-mentioned wavelength region and emits fluorescence L3.

This fluorescent light L3 is collected by the objective lens 14,
The light is reflected by the mirror 15 and guided to the CCD image pickup device 16. As a result, an image (fluorescence image) of the observation site 30 by the excitation light L1 is captured by the CCD imaging device 16. Then, as in the case of normal image observation, based on the output signal S of the CCD image pickup device 16, an observation region 30 is displayed on the display means 28.
Are displayed. In this embodiment, the display means
At 28, the fluorescent image and the above-described normal image are simultaneously displayed on each half of the screen.

Next, referring to FIG. 4, the drive current waveforms of the excitation light source 20 and the white light source 22 and the signal transfer timing of the CCD 16 will be described in detail. (1) in the figure,
(2), (3) and (4) respectively show the drive current waveform of the white light source 22, the drive current waveform of the excitation light source 20, the signal transfer timing for the normal image of the CCD image pickup device 16, and the same.
5 shows the signal transfer timing for the fluorescent image.

FIG. 5 shows a case where a conventional DC bias current is applied, in the same manner as in (1), (2) and (3).
And (4) respectively show the drive current waveform of the white light source 22, the drive current waveform of the excitation light source 20, the signal transfer timing of the CCD image sensor 16 for the normal image, and the signal transfer timing of the CCD image sensor 16 for the fluorescent image.

As shown in FIG. 4, in the present embodiment, each of the normal image and the fluorescent image is imaged at an imaging frequency of 60 Hz of the video rate, that is, at a period of (1/60) s. Then, during this (1/60) s, the imaging of the normal image and the imaging of the fluorescence image are performed once. In addition, CCD
Since the output signal of the image sensor 16 is transferred with a delay of one field, the irradiation of the white illumination light L2 and the signal transfer for the normal image are shifted by one field. This is the excitation light L1
The same applies to the illumination and the signal transfer for the fluorescent image.

In this embodiment, the drive current waveform of the excitation light source 20 is as shown in FIG. That is, the pulse current Ep for originally driving the excitation light source 20 and the pulsed bias current Eb indicated by hatching in the figure are superimposed and applied to the excitation light source 20 made of an InGaN semiconductor laser. Note that the bias current Eb in this example is a point in time before the rising point of the pulse current Ep (for example, 0.01 to 1).
The pulse is applied to the excitation light source 20 in synchronism with the pulse current Ep from the pulse shape (approximately s before) to the falling point.

When the application of the DC bias current, which has been conventionally performed, is adopted, the driving current waveform of the excitation light source 20 is shown in FIG.
It is as shown in. In such a case, the bias current is continuously applied to the excitation light source 20 even during the period in which the white illumination light L2 is irradiated on the observation site 30, and the wavelength of 400 to 410 n
The observation light 30 is irradiated with the excitation light L1 of about m.
Due to the above-described circumstances peculiar to the GaN-based semiconductor laser, even if the bias current is less than the oscillation threshold and the pump light L
Even if 1 is EL light (natural light emission), it becomes background light of considerably high intensity, which causes problems such as deterioration of color reproducibility of a normal image.

On the other hand, in the present embodiment, since the bias current Eb is pulsed as described above, it is possible to reduce the EL light generated by the bias current Eb during the period when the original driving pulse current Ep is not applied. it can.
Therefore, it is possible to reduce the background light when the excitation light L1 is not irradiated, prevent problems such as deterioration of color reproducibility of a normal image, and improve reliability of sample observation.

The pulsed bias current Eb is:
Since the waveform has a waveform that continues from the time before the rising time of the driving pulse current Ep to the falling time, a bias is already applied at the time of the rising of the driving pulse current Ep. As a result, the rising time or the oscillation delay time of the driving pulse current Ep can be shortened, and the pulse responsiveness can be kept good.

The bias current Eb in this embodiment is
Is a waveform that lasts until the falling point of the driving pulse current Ep, but it may be a waveform that continues to a point slightly later than this falling point.

Next, a second embodiment of the present invention will be described with reference to FIGS. In FIG. 6, the same elements as those in FIG. 3 are denoted by the same reference numerals, and the description thereof will be omitted unless otherwise necessary.

FIG. 6 schematically shows the configuration of an endoscope with a fluorescence diagnostic function provided with a GaN-based semiconductor laser driven by the method according to the second embodiment of the present invention. This endoscope differs from the endoscope shown in FIG. 3 in that a configuration for capturing a fluorescent image is provided independently of a configuration for capturing a normal image.

That is, the distal end portion 11 of the endoscope accommodates the distal end portion of the fluorescent image fiber 40, and the image (fluorescent image) by the fluorescent light L3 collected by the fluorescent image objective lens 41.
Is transmitted through the image fiber 40. The fluorescent image is formed on the imaging lens 42 via the excitation light cut filter 45, and is captured by the CCD image sensor 43. The output signal S of the CCD image sensor 43
2 is input to the reading circuit 44, and the image signal Sq output from the reading circuit 44 is input to the display means 28 to be used for displaying a fluorescent image.

In the second embodiment, a fluorescent image by the weak fluorescent light L3 can be captured with high sensitivity by the action of the excitation light cut filter 45 and the high-sensitivity CCD image sensor 43. In this case, another CCD imaging device 16 is used exclusively for capturing a normal image.

FIGS. 7A and 7B respectively show the excitation light source driving current and the high sensitivity CC for the fluorescence image in this embodiment.
6 is a time chart of image signal transfer of a D imaging element 43.
In this example, in order to shorten the time from the generation of the fluorescent light L3 to the transfer of the image signal, a pulse-like excitation light L1 is irradiated during the blank time of the video signal, and the image signal is transferred immediately in the next field. are doing. Such a reduction in time can reduce dark noise as a background, so that the S / N of the fluorescent image can be increased.

A driving pulse current Ep having a pulse width of about several milliseconds is applied to the excitation light source 20 to generate the excitation light L1, and from the rising point of the driving pulse current Ep.
A pulse-like bias current Eb from about 20 μs to 0.1 ms before
Is applied. Thus, even when the bias current Eb having a relatively short pulse width is used, it is possible to perform steep pulse excitation without oscillation delay.

Here, an example in which a short-pulse bias current Eb is used in a mode in which only a fluorescent image is obtained is shown. And an operation mode for acquiring a normal image.

Next, a third embodiment of the present invention will be described with reference to FIG. FIG. 8 is a time chart showing the waveform of the excitation light drive current applied to the third embodiment. In this example, the GaN-based semiconductor laser is driven by receiving a drive current modulated by pulse intensity. This is a method applied to the case of controlling the exposure amount by modulating the intensity of pulse light emitted from a GaN-based semiconductor laser in applications such as image exposure.

For example, in pulse current modulation with a frequency of 10 MHz and a duty of 10%, the pulse width is 10 ns. On the other hand, from the rising point of the driving pulse current Ep, 15
By applying the pulsed bias current Eb about ns before, a good pulse response can be realized, and at the same time, the minimum exposure amount due to light emission by the bias current can be prevented from increasing and the dynamic range of the exposure amount can be increased. Will be possible.

Next, a fourth embodiment of the present invention will be described with reference to FIG. FIG. 9 is a time chart showing the waveform of the excitation light drive current applied to the fourth embodiment. In this example, the GaN-based semiconductor laser is driven by receiving a drive current whose pulse width is modulated. This is a method applied to a case where the exposure amount is controlled by modulating the width of a pulse light emitted from a GaN-based semiconductor laser in applications such as image exposure.

For example, when the modulation range of the pulse width is 1% to 90% at the time of 1 MHz modulation, the pulse width is 10 ns to 90%.
0 ns. On the other hand, by applying the pulsed bias current Eb about 10 ns before the rising point of the driving pulse current Ep, a good pulse response is realized, and at the same time, the minimum exposure amount due to light emission by the bias current is increased. By doing so, the dynamic range of the exposure amount can be increased.

Next, a fifth embodiment of the present invention will be described with reference to FIG. FIG. 10 is a time chart showing the waveform of the excitation light drive current applied to the fifth embodiment. In this example, a triangular pulsed bias current Eb is employed.

Next, a sixth embodiment of the present invention will be described with reference to FIG. FIG. 11 is a time chart showing the waveform of the excitation light drive current applied to the sixth embodiment. In this example, a sinusoidal pulsed bias current Eb is employed.

As shown in the above two embodiments, in the present invention, a pulsed bias current having a waveform other than a rectangular wave can be applied. In such a case, a circuit simpler than the rectangular wave generating circuit for applying the bias current can be employed to reduce the cost.

The present invention is not limited to the case where the pulse intensity modulation and the pulse width modulation are performed, but is similarly applied to the case where other pulse modulation methods such as modulation by changing the pulse position (phase) are employed. It is possible. Further, the present invention can be similarly applied to, for example, a case where a higher gradation image exposure is performed by combining pulse intensity modulation and pulse width modulation.

The method of driving a GaN-based semiconductor laser according to the present invention is not limited to a GaN-based semiconductor laser used as an excitation light source in a fluorescent endoscope, but may be used in various systems such as optical recording and optical information transmission. The same can be applied to a system semiconductor laser.

[Brief description of the drawings]

FIG. 1 is a graph showing an example of a driving current versus optical output characteristic of a GaN-based semiconductor laser.

FIG. 2 is an explanatory diagram showing a DC bias dependence of an optical output response to a pulse drive current of a GaN-based semiconductor laser.

FIG. 3 is a schematic view of a distal end portion of an endoscope with a fluorescence diagnostic function to which the method according to the first embodiment of the present invention is applied.

FIG. 4 is a time chart showing a relationship between a semiconductor laser drive current waveform and a drive signal related thereto according to the first embodiment.

FIG. 5 is a time chart showing a relationship between a semiconductor laser drive current waveform and a related drive signal in a conventional device.

FIG. 6 is a schematic diagram of a distal end portion of an endoscope with a fluorescence diagnostic function to which the method according to the second embodiment of the present invention is applied.

FIG. 7 is a time chart showing a relationship between a drive current waveform of an excitation light source and transfer of a fluorescent image signal from an image sensor in the second embodiment.

FIG. 8 is a time chart showing a semiconductor laser driving current waveform according to a third embodiment of the present invention.

FIG. 9 is a time chart showing a semiconductor laser driving current waveform according to a fourth embodiment of the present invention.

FIG. 10 is a time chart showing a semiconductor laser driving current waveform according to a fifth embodiment of the present invention.

FIG. 11 is a time chart showing a semiconductor laser driving current waveform according to a sixth embodiment of the present invention.

[Description of Signs] 11 Endoscope tip 12 Light guide 13 Illumination lens 14 Objective lens 15 Mirror 16 CCD image sensor for image observation 20 Excitation light source 21, 23 Condensing lens 22 White light source 24 Semiconductor laser drive circuit 25 Bias application circuit 26 Controller 27 Readout circuit 28 Display means 30 Observation site 40 Fluorescent image image fiber 41 Fluorescent image objective lens 42 Imaging lens 43 High-sensitivity CCD image sensor for fluorescent image 44 Reading circuit 45 Excitation light cut filter L1 Excitation light L2 White illumination light L3 fluorescence

Claims (4)

[Claims] When a GaN-based semiconductor laser is driven by a pulse current, a pulse-like bias current that lasts at least from a point in time prior to a rising point of the pulse current to a point in time of falling is synchronized with the pulse current. A method for driving a GaN-based semiconductor laser, wherein the method is applied to the semiconductor laser. 2. The method of driving a GaN-based semiconductor laser according to claim 1, wherein the pulsed bias current is a rectangular wave, a triangular wave, or a sine wave. 3. The method according to claim 1, wherein the pulse current for driving the GaN-based semiconductor laser is modulated by at least one of pulse intensity modulation, pulse width modulation and pulse phase modulation. A method for driving a GaN-based semiconductor laser. 4. A driving device for a GaN-based semiconductor laser having a configuration for driving a GaN-based semiconductor laser by the method according to claim 1.