JP2001024230A - LIGHT SOURCE DEVICE PROVIDED WITH GaN-BASED SEMICONDUCTOR LIGHT EMITTING ELEMENT AND REMOVAL METHOD FOR STRAY LIGHT - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a change in the shape of a light spot due to a change in the driving current of a GaN-based semiconductor light emitting element by forming a spatial filter by which stray light as a specific output under the maximum output of the GaN-based semiconductor light emitting element is removed from light which is emitted from the light emitting element. SOLUTION: From light which is emitted from a GaN-based semiconductor laser 20, stray light at 20% or lower of the total output under the maximum output of the semiconductor laser is removed by a spatial filter which is constituted of a slit plate 23. For example, stray light which is generated mainly in a low-current region which does not reach a laser oscillation threshold value is removed by the slit plate 23. Consequently, it is possible to prevent a change in the shape of a light spot caused by the generation of the stray light. At this time, this light source device which is provided with the GaN-based semiconductor laser can control a quantity of recording light accurately, and it is suitable for a printing field, a photographic field and a medical image field in which a high-grade gradation exposure is required.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light source device having a GaN-based semiconductor light-emitting device, and more particularly to a light source device having a function of removing stray light peculiar to a GaN-based semiconductor light-emitting device. Things.

[0002] The present invention also relates to a method for removing the above stray light.

[0003]

2. Description of the Related Art Recently, a GaN-based semiconductor laser that emits a blue laser beam by forming an active layer from a GaN-based semiconductor such as InGaN, InGaNAs, or GaNAs has been approaching practical use. Further, as shown in, for example, JP-A-11-74559, a light emitting diode having a stripe structure having an active layer made of a GaN-based semiconductor, a so-called SLD
(Super Luminescent Diode) is also known. Although this SLD does not oscillate laser, the light emitting region is limited by the stripe structure, so that it can output green light or blue light with a small light emission diameter and a narrow beam emission angle.

[0004]

However, in this GaN-based semiconductor light-emitting device (including both a semiconductor laser and a light-emitting diode), stray light peculiar to the semiconductor material is easily generated. Hereinafter, this point will be described in detail.

In a semiconductor laser or SLD using AlGaInP, AlGaAs, InGaAsP or the like as a constituent material on a GaAs substrate, GaAs serving as a substrate is an absorbing material with respect to an emission wavelength, and is formed on the opposite side to the substrate. The electrodes are also formed of a light-absorbing material such as InGaAs or GaAs. Therefore, usually several μ
Even if unnecessary stray light which is not confined in the light emitting region having the width of m is generated, the light is absorbed by the substrate or the like, and such stray light does not cause any practical problem.

On the other hand, in a GaN semiconductor light emitting device,
As the substrate material, a material transparent to the emission wavelength, such as sapphire or SiC, is used. Therefore, there may be a problem that stray light reaching the element end face on the substrate side or the counter electrode side is reflected and returns to the vicinity of the light emitting region, or stray light of various patterns is generated by multiple reflections.

When this type of semiconductor light emitting device is driven with a current equal to or higher than the laser oscillation threshold, the light intensity under laser oscillation is much higher than the intensity of spontaneous light that causes stray light. Stray light is usually not a problem. But this
In the case where a GaN-based semiconductor light emitting element is used as a recording light source for gradation image recording and direct modulation driving is performed even in a low current region where the laser oscillation threshold is not reached so that a high gradation image can be recorded, this stray light is practically used. It becomes a problem.

That is, in such a low current region, the above stray light is likely to be generated. In an extreme case, the light emission pattern is such that light is emitted not only from the stripe portion but also from the entire device. In this manner, light generated from a portion outside the stripe makes the spot shape when the recording light is narrowed, thereby lowering the coupling efficiency between the recording light and the optical system. When such a situation occurs, it is difficult to precisely control the recording light amount (exposure amount) in printing a high gradation image, and the quality of the recorded image is degraded.

In view of the above circumstances, it is an object of the present invention to provide a light source device provided with a GaN-based semiconductor light emitting element, in which a light spot shape is prevented from changing due to a change in a driving current of the light emitting element.

[0010]

According to the present invention, a light source device provided with a GaN-based semiconductor light-emitting device according to the present invention comprises a light-emitting device that emits 20% of the total output under the maximum output of the GaN-based semiconductor light-emitting device. A spatial filter for removing stray light as described below (for example, stray light generated when a drive current of a GaN-based semiconductor light-emitting element is less than a laser oscillation threshold) is provided.

When a condensing optical system for condensing light emitted from the GaN-based semiconductor light emitting device is provided, a slit plate or a slit plate disposed near the converging position by the condensing optical system is used as the spatial filter. A pinhole plate can be used, or a partial reflection mirror that partially reflects light collected near the convergence position can be used.

Further, GaN is used as the spatial filter.
A polarizing element that removes components other than the TE mode component (polarized component having an electric field vector parallel to the pn junction surface of the GaN-based semiconductor light emitting element) of light emitted from the semiconductor light emitting element can also be used.

On the other hand, the method for removing stray light according to the present invention uses GaN
In the light source device provided with a system-based semiconductor light emitting element, the GaN
From the light emitted from the system semiconductor light emitting device, a spatial filter filters out all of the power of 20
% Or less stray light.

[0014]

According to the present invention, stray light which is 20% or less of the total output under the maximum output of the semiconductor light emitting device is removed from the light emitted from the GaN semiconductor light emitting device by the spatial filter. As described above, for example, stray light mainly generated in a low current region where the laser oscillation threshold value is not reached (which has the above-described characteristics under the maximum output of the semiconductor light emitting element) is removed by this spatial filter. Therefore, a change in the light spot shape due to the generation of the stray light is prevented.

Therefore, the light source device according to the present invention can precisely control the recording light amount (exposure amount), and is suitable for the fields of printing, photography, and medical images that require high-quality gradation exposure. For example, when this light source device is applied to recording of a high-gradation image, the quality of the recorded image is improved.

[0016]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a schematic plan shape of a light source device provided with a GaN-based semiconductor laser according to a first embodiment of the present invention, and FIG. 2 schematically shows a longitudinal sectional shape of a semiconductor laser 20 used therein. It is shown in a typical way.

First, the semiconductor laser 20 will be described in detail with reference to FIG. This semiconductor laser 20 has an active layer 7
Has a double-hetero structure sandwiched between cladding layers 6 and 8, and has a stripe-shaped current injection window (portion of the cap layer 10) for confining light, and its oscillation wavelength is 400 nm. The cleavage surface of the element is a reflection surface, and a light reflection structure is formed.

Hereinafter, the layer structure of the semiconductor laser 20 will be briefly described together with the manufacturing method. After growing the n-GaN low-temperature buffer layer 2 on the sapphire c-plane substrate 1 by MOCVD, a stripe-shaped SiO ₂ mask 14 is formed. Next, an n-GaN buffer layer 3 (Si-doped, 5
μm), n-In _0.05 Ga _0.95 N buffer layer 4 (Si-doped, 0.
1 μm), n-Al _0.1 Ga _0.9 N clad layer 5 (Si-doped,
5 μm), n-GaN optical guide layer 6 (Si-doped, 0.1 μm
m), undoped active layer 7, p-GaN optical guide layer 8 (Mg doped, 0.1 μm), p-Al _0.1 Ga _0.9 N clad layer 9 (Mg doped, 0.5 μm) and p-GaN cap layer 10 (Mg doped, , 0.3 μm). After that, the p-type impurity is activated by heat treatment in a nitrogen gas atmosphere.

The active layer 7 is made of undoped ln _0.05 Ga _0.95
N (10 nm), undoped In _0.28 Ga _0.72 N quantum well layer (2.5 nm, wavelength 488 nm), undoped In _0.05 Ga _0.95 N
(5 nm), undoped In _0.28 Ga _0.72 N quantum well layer (2.5
nm), undoped ln _0.05 Ga _0.95 N (5 nm), undoped In _0.28 Ga _0.72 N quantum well layer (2.5 nm), undoped ln _0.05 Ga _0.95 N (5 nm), undoped Al _0.1 Ga _0.9 N
(10 nm).

Next, in order to form a ridge stripe having a width of 6 μm, the epitaxial layer other than the ridge stripe portion is removed from the cap layer 10 to the middle of the cladding layer 9 by reactive ion beam etching (RIBE) using chlorine ions. Next, an SiN film 11 is formed on the exposed surface including the upper portion of the ridge stripe portion by plasma CVD. After that, in order to form an n-side electrode, the epitaxial layer other than the light emitting region including the ridge stripe is removed by etching until the n-GaN buffer layer 3 is exposed by photolithography and RIBE using chlorine. At this time, a resonator end face is formed.

Thereafter, a stripe-shaped window (10 μm width) for current injection is formed in the SiN film 11 on the upper surface of the ridge portion, and Ni / Au is used as the p-side electrode 12 so as to cover the stripe window. Ti / A as an n-side electrode 13 on the exposed portion of the buffer layer 3
After vacuum deposition of l, annealing is performed in nitrogen to form an ohmic electrode.

The dimensions of the semiconductor laser 20 shown in FIG. 2 are, for example, W1 = 2 μm and W2 = 300 μm.
m, H1 = 0.5-1 μm, H2 = 3-5 μm, H3 = 100
μm.

Next, the light source device of FIG. 1 provided with the semiconductor laser 20 will be described in detail. As shown in the drawing, the light source device includes a semiconductor laser 20, a condensing lens 22 for condensing a laser beam 21 having a wavelength of 400 nm emitted in a divergent light state from the semiconductor laser 20, and a laser beam 21 by the condensing lens 22. And a slit plate 23 disposed at the convergence position. Reference numeral 30 in the drawing is a photodetector for detecting the light amount of the laser beam 21.

In FIG. 1, the semiconductor laser 20 has its pn
It is arranged so that the joining surface is parallel to the paper surface. On the other hand, the slit plate 23 is disposed such that its elongated slit 23a extends perpendicularly to the paper surface. Also condensing lens
A numerical aperture NA = 0.75 is used as 22 and the optical loss due to lens insertion is suppressed to about 10%.

In order to confirm the effect of the slit plate 23, as shown in FIG. 3, a system for directly receiving a laser beam 21 emitted from a semiconductor laser 20 in a diverging light state by a photodetector 30 was prepared. In the system shown in FIG. 3 and the light source device shown in FIG. 1, the drive current of the semiconductor laser 20 was changed, and the resulting change in the optical output was measured by the photodetector 30.
FIG. 4 shows the measurement results. In FIG. 4, the curve a shows the case without the slit (the configuration of FIG. 3), and the curves b and c each show the width of the slit 23a of FIG.
m, 0.7 mm.

In the example of FIG. 4, the threshold current of laser oscillation is about 38 mA. The light output in the region equal to or larger than the threshold current, that is, in the laser oscillation region, has a value that does not substantially change whether or not the slit plate 23 is provided. That is, it can be said that the amount of oscillated light emitted from the stripe portion of the active layer 7 of the semiconductor laser 20 is hardly impaired by the slit plate 23.

On the other hand, in the region below the threshold current, that is, in the natural light emission region, the light output when the slit plate 23 is provided is reduced to about の of that when the slit plate 23 is not provided.
That is, in the natural light emitting region, it is considered that stray light emitted from portions other than the stripe portion of the active layer 7 of the semiconductor laser 20 is cut by the slit plate 23.

As is apparent from FIG. 4, in the natural light emission region,
About half of the amount of light emitted from the semiconductor laser 20 is stray light. Such a large amount of stray light results in a poor spot shape when the laser beam 21 is narrowed. When the light source device is used as a recording light source of a high gradation image recording device, the recording light amount (exposure amount) is precisely adjusted. Control becomes difficult, and the quality of the recorded image is degraded. However, if such stray light can be cut by the slit plate 23, such a problem can be avoided.

If the width of the slit 23a is set to be almost the width of the light emission, it becomes difficult to adjust the optical system, and the tolerance for mechanical vibration and the like is reduced. However, as described above, the width of the slit 23a is set to 1 mm or 0.7 mm. Even when the distance is relatively large, a remarkable stray light removing effect is obtained. Generally, if the slit width is about twice or less the light spot diameter at the converging portion, a clear stray light removing effect can be obtained. In the case of the configuration shown in FIG. 1, when the width of the slit 23a is set to 0.5 mm or less, the amount of light passing therethrough decreases sharply.

In the configuration of FIG. 1 described above, stray light spreading in the direction perpendicular to the pn junction surface of the semiconductor laser 20 (the direction perpendicular to the paper) cannot be removed by the slit plate 23. In order to remove such stray light, a pinhole plate may be used instead of the slit plate 23.

The same effect can be obtained by using a thin partially reflecting mirror for partially reflecting the laser beam 21 near the convergence position of the laser beam 21 instead of the slit plate 23.

Next, another embodiment of the present invention will be described. FIG. 5 shows a schematic plan view of a light source device provided with a GaN-based semiconductor laser according to a second embodiment of the present invention. In FIG. 5, elements that are the same as the elements in FIG. 1 are given the same reference numerals, and overlapping descriptions thereof will be omitted (hereinafter the same).

In the second embodiment, a laser beam 21 having a wavelength of 400 nm emitted from a semiconductor laser 20 is collimated by a collimator lens 40 and then passed through a Glan-Thompson prism 41. It has become so. The laser beam 21 that has passed through the Glan-Thompson prism 41 is condensed by the condenser lens 42 and received by the photodetector 30.

The semiconductor laser 20 is arranged in FIG. 5 such that its pn junction surface is parallel to the plane of the drawing. On the other hand, the Glan-Thompson prism 41 as a polarizing element is arranged so as to transmit only the TE mode component (polarized component having an electric field vector parallel to the pn junction plane) of the laser beam 21 and remove other polarized components. I have.

In order to confirm the effect of the Glan-Thompson prism 41, the light source device shown in FIG. 5 and the light source device shown in FIG.
In each of the systems, the drive current of the semiconductor laser 20 was changed, and the resulting change in optical output was measured by the photodetector 30. FIG. 6 shows the measurement results. In FIG. 6, the curve a shows the case without the Glan-Thompson prism 41 and the slit plate 23 (the configuration of FIG. 3), and the curve d shows the case where the Glan-Thompson prism 41 is provided (the configuration of FIG. 5). . Further, here, for reference, the characteristic when the width of the slit 23a is 0.7 mm in the configuration of FIG. 1 is shown by a curve c.

In the example shown in FIG. 6, the threshold current of laser oscillation is about 38 mA. The light output in the region above this threshold current, that is, in the laser oscillation region, is the Glan-Thompson prism 41
The value is almost the same whether or not there is. That is, the amount of TE mode oscillation light emitted from the stripe portion of the active layer 7 of the semiconductor laser 20 is hardly impaired by the Glan-Thompson prism 41.

On the other hand, in a region below the threshold current, that is, in a natural light emitting region, the Glan-Thompson prism 41
The light output in the case where there is is significantly reduced as compared with the case where it does not exist. In other words, in this natural light emission region, it is considered that the stray light emitted from other than the stripe portion of the active layer 7 of the semiconductor laser 20 and randomly polarized is largely cut by the Glan-Thompson prism 41.

In the example shown in FIG. 6, the stray light removing effect is higher when the slit plate 23 is inserted than when the Glan-Thompson prism 41 is inserted. It depends on. Therefore, the effects of the present invention can be optimized by selecting and combining elements for removing stray light. When a slit plate is used, a condensing optical system for converging the laser beam is necessary, and precise optical adjustment is also required.However, when a polarizing element is used, the optical adjustment is rough. Also, the degree of freedom of the element insertion position is high.

The two embodiments described above have only the basic structure that is the core of the light source device.
The light source device according to the present invention can, of course, constitute an optical scanning system in combination with a polygon mirror (rotating polygon mirror) for optical scanning, a galvanometer mirror, or the like. In such a case, necessary optical elements such as lenses may be appropriately combined to form an optical system as shown in FIGS. 7 and 8, for example.

In the third embodiment shown in FIG. 7, in addition to the configuration shown in FIG. 1, a condenser lens 50 for condensing the laser beam 21 after passing through the slit plate 23, and the laser beam 21 An optical system including a correction cylindrical lens 51 for condensing light only in a vertical direction is employed.

In the fourth embodiment shown in FIG. 8, a collimator lens similar to that used in the configuration of FIG.
In addition to 40 and the condenser lens 42, the condenser lens 60 for converging the laser beam 21 collimated by the collimator lens 40 at the position of the slit plate 23, and the laser beam 21 after passing through the slit plate 23 are parallelized. An optical system provided with a collimator lens 61 that converts to light is employed.

[Brief description of the drawings]

FIG. 1 is a schematic plan view showing a light source device according to a first embodiment of the present invention.

FIG. 2 is a schematic view showing a vertical cross-sectional shape of a semiconductor light emitting element used in the light source device.

FIG. 3 is a schematic plan view showing a light source device as a comparative example of the present invention.

FIG. 4 is a graph showing a semiconductor laser driving current-optical output characteristic in the light source device of FIG. 1 together with a characteristic of a comparative example.

FIG. 5 is a schematic plan view showing a light source device according to a second embodiment of the present invention.

6 is a graph showing a semiconductor laser driving current-light output characteristic in the light source device of FIG. 5 together with a characteristic of a comparative example.

FIG. 7 is a schematic plan view showing a light source device according to a third embodiment of the present invention.

FIG. 8 is a schematic plan view showing a light source device according to a fourth embodiment of the present invention.

[Explanation of symbols]

Reference Signs List 1 sapphire c-plane substrate 2 n-GaN low-temperature buffer layer 3 n-GaN buffer layer 4 n-In _0.05 Ga _0.95 N buffer layer 5 n-Al _0.1 Ga _0.9 N clad layer 6 n-GaN optical guide layer 7 undoped active layer 8 p-GaN optical guide layer 9 p-Al _0.1 Ga _0.9 N cladding layer 10 p-GaN cap layer 11 SiN film 12 p-side electrode 13 n-side electrode 14 SiO ₂ mask 20 semiconductor laser 21 laser beam 22 condenser lens 23 slit plate 30 Photodetector 40, 61 Collimator lens 41 Glan-Thompson prism 42, 50, 60 Condensing lens 51 Cylindrical lens

Continued on front page F-term (reference) 5F041 AA14 CA04 CA05 CA33 CA34 CA35 CA36 CA39 CA40 CA46 CA65 CA74 CA82 CA92 CB05 EE11 EE22 EE25 FF16 5F073 AA11 AA74 AB21 AB27 AB29 BA09 CA07 CB04 CB05 CB07 CB22 DA05 EA07

Claims (7)

[Claims] 1. A light source device provided with a GaN-based semiconductor light-emitting device, wherein stray light which is 20% or less of the total output under the maximum output of the semiconductor light-emitting device is removed from light emitted from the GaN-based semiconductor light-emitting device. A light source device comprising a GaN-based semiconductor light-emitting element, wherein a spatial filter is provided. 2. A light condensing optical system for condensing light emitted from the GaN-based semiconductor light emitting element, wherein the spatial filter is disposed near a converging position of the light by the light condensing optical system. 2. The Ga according to claim 1, wherein the Ga is made of a plate or a pinhole plate.
Light source device equipped with N-based semiconductor light emitting element. 3. A condensing optical system for condensing light emitted from the GaN-based semiconductor light emitting device, wherein the spatial filter partially divides the light near a converging position of the light by the condensing optical system. 2. The method according to claim 1, wherein the partial reflection mirror is configured to partially reflect light.
A light source device provided with a GaN-based semiconductor light emitting element. 4. The GaN-based semiconductor light-emitting device according to claim 1, wherein the spatial filter is a polarizing device for removing a component other than a TE mode component of light emitted from the GaN-based semiconductor light-emitting device. Light source device provided with. 5. The GaN-based semiconductor according to claim 1, wherein the stray light is generated when a drive current of the GaN-based semiconductor light-emitting element is less than a laser oscillation threshold. A light source device including a light emitting element. 6. A light source device comprising a GaN-based semiconductor light-emitting element, wherein a light emitted from the GaN-based semiconductor light-emitting element is reduced by a spatial filter to 20% or less of the total output under the maximum output of the semiconductor light-emitting element. A method for removing stray light, comprising: removing stray light. 7. The method according to claim 6, wherein the stray light is stray light generated when a drive current of the GaN-based semiconductor light emitting device is less than a laser oscillation threshold.