JP3735528B2 - Semiconductor laser device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a semiconductor laser device used as a light source in optical communication, optical transmission technology, and optical information recording technology.
[0002]
[Prior art]
Currently, semiconductor laser devices are widely used as light sources in the fields of optical communication, optical transmission, optical information recording, and the like because they can perform coherency and high-speed operation of emitted light, or are extremely small. Semiconductor laser devices are used for lead frames and metal blocks because they emit stimulated emission light by injecting current from the outside and secure a heat dissipation path because the light intensity changes sensitively to thermal fluctuations. In order to alleviate the difference in thermal expansion coefficient between the metal and the semiconductor material constituting the semiconductor laser element, it was mounted on a substrate called a submount made of Si, AlN, or the like. After that, it is mounted on a metal member.
[0003]
The semiconductor laser device is realized by sandwiching a medium having an amplification factor of 1 or more in a resonator composed of a plurality of reflecting mirrors. The development of edge-emitting semiconductor lasers that can take a long distance has become the mainstream. For some, surface-emitting laser elements that produce a high-reflecting mirror using a semiconductor or dielectric multilayer film and emit outgoing light in the normal direction of the substrate have also been put into practical use, but the technology for practical use is not sufficient However, depending on the material, that is, the wavelength of the emitted light, there are many problems such as many things still at the research stage level. For this reason, the edge emitting type light source is used as the light source composed of most semiconductor laser elements used in products.
[0004]
On the other hand, the optical output of the semiconductor laser element fluctuates sensitively due to environmental temperature changes. Therefore, both the semiconductor laser element and the mounting substrate are mounted on an element capable of controlling the temperature collectively, such as a Peltier element. However, since the mounting substrate and the submount also have a small heat capacity, when precise light output control is required, a method of monitoring the actual output light and causing the drive current circuit to perform feedback control is taken. This is called automatic light control (Automatic Power Control: APC). The end face emitting type semiconductor laser element uses both end faces formed by a scratched face or the like as a resonator mirror. However, unless the reflectance of the end face is specifically controlled, output light is emitted symmetrically in the front-rear direction. The APC can be configured by monitoring the light output from the rear with a light receiving element or the like. However, the monitored light does not contribute as a signal, so that the light use efficiency decreases. For this reason, in a system that requires high output and high efficiency, a method of increasing the light utilization efficiency as much as possible by increasing the reflectance of the rear end face with a dielectric multilayer film or the like is employed. In such a case, the emitted light from the back that can be used for the monitor becomes small and the SN ratio is deteriorated so that precise APC cannot be applied. Therefore, it becomes necessary to monitor a part of the light from the front (= signal light), and this control method is called front APC (hereinafter referred to as FAPC).
[0005]
FIG. 4 shows a configuration when a semiconductor laser element is used in the FAPC system. Reference numeral 101 denotes a semiconductor laser element, reference numeral 102 denotes an outgoing light splitting unit, which is constituted by, for example, a half mirror, and reference numeral 103 denotes a monitor light receiving element, which constitutes a semiconductor laser device. The light emitted from the semiconductor laser element 101 is divided by the dividing means 102, and part of the light becomes output light and is emitted to, for example, an optical disc. The remaining part is input to a light receiving element for optical output monitoring and outputs a monitor photocurrent for APC. In this configuration, when the intensity of light incident on the light receiving element for monitoring from the semiconductor laser element changes greatly in a stepwise manner, for example, the same DVD-ROM and DVD-RAM are used by using this apparatus as a light source of an optical pickup. In such a case, the output current value from the light receiving element fluctuates greatly, and there is a problem that a very large dynamic range is required for the external control circuit for APC driving. For example, in a red semiconductor laser element (wavelength of about 650 nm) currently used for DVD-ROM and RAM, the semiconductor laser light output in ROM mode is about 5 mW at maximum, and about 10 times in RAM is about 50 mW. . Since it is inherently difficult to achieve both a large dynamic range and precise control, the control circuit system becomes complicated. Furthermore, when the incident light intensity increases, a saturation phenomenon may occur in which the output current of the light receiving element cannot follow the light intensity linearly. When saturation occurs, it is necessary to control the drive current of the semiconductor laser element non-linearly with respect to the output current from the light receiving element, which complicates the control circuit configuration. For this reason, it is necessary to separately provide a light receiving element for monitoring in the RAM mode and the ROM mode, or to prepare two control circuit systems, which increases the number of parts, which increases the cost and hinders downsizing. Problems arise.
[0006]
As a method of controlling and using a semiconductor laser element by FAPC, a proposal as shown in FIG. 5 in which a light receiving element for front APC is integrated in the rear part of a reflecting mirror (for example, Japanese Patent Laid-Open No. 05-315700) has been proposed. . A configuration in which the semiconductor laser element 1 is mounted in a recess 3 formed in a silicon substrate 2 as a submount, reflected by a reflecting surface formed in a high concentration n-type diffusion layer 24-2 on the wall surface of the recess, and a light beam is extracted upward. It is. According to this configuration, it is possible to monitor a part of the front output light. Also, according to this configuration, the output light is reflected in the vicinity of the semiconductor laser element, that is, before it spreads greatly, so that it is kicked on the mounting surface without special consideration of the positional relationship with the mounting surface. The output light can be taken out with almost no beam shape.
[0007]
However, even such a configuration has essentially the same problem as the conventional example (FIG. 4). When the light intensity incident on the light receiving element for monitoring from the semiconductor laser element changes greatly in a stepwise manner, the output current value from the light receiving element fluctuates greatly, and the external control circuit for driving APC has a very large dynamic range. There is a problem that is necessary.
[0008]
[Problems to be solved by the invention]
As described in detail above, in a semiconductor laser device in which a light receiving element for front APC is arranged at the rear of an optical path dividing means such as a reflecting mirror, a DVD-ROM and a DVD-RAM are used as a light source for an optical pickup with the same light emitting element. In such a case, since the intensity of light incident from the light emitting element changes greatly in steps, the output current value from the light receiving element greatly fluctuates, and a very large dynamic range is required for the external control circuit for APC driving. There was a problem of becoming.
[0009]
The present invention has been made in response to the above-described problems, and provides a novel semiconductor laser device that does not require a large dynamic range in an external control circuit for driving APC.
[0010]
[Means for Solving the Problems]
The semiconductor laser device of the present invention is characterized in that a variable optical attenuator is provided between the reflecting mirror and the light receiving element disposed at the rear part thereof.
[0011]
Furthermore, a groove in a direction almost perpendicular to the optical path is provided in the light receiving element portion arranged at the rear of the reflecting mirror, and the light receiving elements are optically divided in series. It is used as a light absorption layer in the off state, and control is performed using the output current from the light receiving element at the subsequent stage. Also provided is a semiconductor laser device that can be controlled by using both front and rear light receiving elements when the incident light intensity is low.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram for explaining a first embodiment of the present invention. Reference numeral 101 denotes a semiconductor laser element, and reference numeral 102 denotes outgoing light splitting means, which is composed of, for example, a half mirror. Reference numeral 103 denotes a monitor light receiving element. Reference numeral 104 denotes a variable optical attenuator. The variable optical attenuator may be of any structure that can change the attenuation amount by voltage, current, etc., and is composed of, for example, liquid crystal. The light emitted from the semiconductor laser 101 is divided by the dividing means 102, and part of the light becomes output light, for example, emitted to an optical disk. The remaining part is input to the light receiving element 103 for monitoring optical output and outputs a monitor photocurrent for APC. Here, the output current from the monitor light receiving element is increased by increasing the attenuation amount of the variable optical attenuator when the light intensity incident on the monitor light receiving element is high, and decreasing the attenuation amount when the light intensity is low. The value can be controlled appropriately. Thus, it is not necessary to prepare another light receiving element, and the control circuit can be easily designed. In this figure, each element is depicted as a separate element, but of course may be integrated and is not limited to individual components.
[0013]
FIG. 2 is a cross-sectional view for explaining a second embodiment of the present invention. In FIG. 2, reference numeral 1 denotes a semiconductor laser element, which is an end-emitting type in which a resonator is provided in the left-right direction in the drawing and light is emitted from the end surface 11. Reference numeral 2 denotes a submount substrate made of, for example, Si, and a low impurity concentration layer 22 and a high impurity concentration p-type conductivity region 24 are formed on an n-type conductivity type substrate 21 doped with a high concentration of impurities. A concave portion is formed by anisotropic etching or the like on a substrate formed by epitaxial growth or the like. Each layer of 24, 22, and 21 constitutes a pin type photodiode that is a light receiving element. Reference numeral 26 denotes a photodiode lead-out p electrode, which is used by applying a reverse bias voltage to the back electrode 28 (= n electrode). The semiconductor laser element 1 is mounted in a recess by a conductive adhesive 5 made of solder or the like, and the emitted light 14 is a transflective film that becomes a reflecting mirror made of a dielectric film or the like formed on the slope 23. In 25, a part of the light is reflected and output above the substrate, and a part of the transmitted light enters the depletion layer spread in the low concentration layer 22 and is absorbed to become a photocurrent. At this time, the light receiving element formed behind the semi-transmissive film 25 is separated into two photodiodes 3 and 4 by the separation groove 29 so as to be in series with the optical axis of the transmitted light 15 of the emitted light 14 of the semiconductor laser. It is divided. A part of the transmitted light 15 is absorbed by the photodiode 4, is transmitted, and then is absorbed by the photodiode 3.
[0014]
In order to realize FAPC using this structure, when the incident light intensity is low, control is performed using the output currents of both the photodiodes 3 and 4, and conversely, when the incident light intensity is large, the photocurrent of only the photodiode 3 is controlled. Control using. When only the photodiode 3 is used, the photodiode 4 functions as a light absorption layer. Therefore, for example, by designing to absorb 90% of light and absorb 10% by the photodiode 3, the RAM mode and the ROM mode. Almost the same photocurrent can be obtained. With this configuration, a large control signal (monitor output current) can be input to the drive circuit while suppressing the saturation phenomenon of the light receiving element without preparing a separate light receiving element for monitoring. The entire circuit system can be easily designed. In this structure, the light receiving element in the previous stage has the functions of both a variable light attenuator and a light receiving element for monitoring, so that the light receiving element and the light attenuator can be configured with the same element structure, and a structure that is easier to integrate It is.
[0015]
In FIG. 2, a groove 27 is formed on the side of the inclined surface 23 serving as a reflection surface of the bottom surface of the recess, and the light emitting portion (that is, the active layer) of the semiconductor laser element 1 is relatively close to the bottom surface of the recess. In addition, the output light beam is prevented from being kicked at the bottom of the recess.
[0016]
This structure is realized as follows. First, a wafer having the low concentration layer 22 and the high concentration layer 24 formed on the high concentration substrate 21 by epitaxial growth or the like is patterned using a photolithography technique or the like using a thermal oxide film or a nitride film, and these films are formed. The recess is etched with a solution such as KOH as a mask until the bottom reaches the substrate 21. Further, the groove 27 can be formed by patterning and etching a part of the bottom by the same method. In this case, by using an anisotropic etching solution such as KOH, the slope of the recess can give a specific crystal face, and a flat face can be obtained. Further, a separation groove 29 reaching the substrate 21 is formed by RIE or the like using a resist or an oxide film as a mask.
[0017]
The sidewall of the groove 29 is passivated with a thermal oxide film or the like in order to suppress surface leakage. In order to reduce the reflection loss inside the groove, the passivation film is preferably formed so as to be a non-reflective film with respect to the optical axis. Thereafter, after the film 25 is formed of a dielectric film as a semi-transmissive mirror, the extraction electrode 26 and the back electrode 28 of the photodiode are formed and cut into chips by dicing or the like.
[0018]
In the present embodiment, a high-speed PIN structure is shown as the light receiving element structure, but the same effect can be obtained even if a normal PN light receiving element is used. In that case, the low concentration layer 22 is unnecessary.
[0019]
Further, although the substrate 2 is formed of two epitaxial layers, the p layer 24 may be formed after heat diffusion or the like after forming the outline using the substrate having only the low concentration layer 22. Moreover, although the case where the shape of the groove is a vertical shape in the form of a trench is illustrated, the groove may be an inclined surface similar to the reflecting surface 23 as long as the transmitted light is incident on the light receiving element 3 in the subsequent stage. In this case, the separation groove can be formed simultaneously with the reflecting surface 23, and the process is simplified.
[0020]
In this embodiment, the substrate is an n-type conductivity type. Of course, when the p-type conductivity type is used, the same configuration can be obtained by reversing the conductivity type. In particular, when boron is used as a p-type impurity at the time of forming a recess, the etching rate by KOH becomes slow at a high concentration exceeding 1 × 10 ²⁰ cm ^−3. A flat concave bottom surface can be obtained.
[0021]
FIG. 3 is a diagram for explaining a third embodiment of the present invention. The difference from the structure of FIG. 2 is that the n-type diffusion region 29-2 is used to separate the light receiving elements 3 and 4. Since the separation is pn separation, the p-type diffusion region 24 is partially formed by ion implantation or thermal diffusion. In this structure, separation by a normal LSI process such as ion implantation is possible, and it is not necessary to passivate the surface of the groove sidewall, thereby simplifying the process. However, since all absorption in the separation region flows to the n-type electrode and becomes a slow component photocurrent, it becomes noise in the case of high-speed control, and is suitable for low-speed control.
[0022]
【The invention's effect】
The present invention is characterized in that a variable optical attenuator is provided between the reflecting mirror and the light receiving element disposed at the rear part thereof. Furthermore, a groove in a direction almost perpendicular to the optical path is provided in the light receiving element portion arranged at the rear of the reflecting mirror, and the light receiving elements are optically divided in series. It is used as a light absorption layer in the off state, and control is performed using the output current from the light receiving element at the subsequent stage. According to the semiconductor laser device configured as described above, when the incident light intensity is low, it can be controlled using both the front and rear light receiving elements, and the DVD-ROM and DVD-RAM can be used as the light source of the optical pickup. Even when realized with the same light emitting element, that is, when the intensity of light incident from the light emitting element changes greatly in steps, the output current value from the light receiving element does not vary greatly with a single element. Since a substantially constant control signal (photocurrent) can be input to the control circuit, the entire control circuit system can be easily designed, and there is an effect of suppressing an increase in cost and an increase in size.
[Brief description of the drawings]
[1] Figure of order to explain the first embodiment of the present invention.
FIG. 2 is a diagram for explaining a second embodiment of the present invention.
FIG. 3 is a diagram for explaining a third embodiment of the present invention.
FIG. 4 is a diagram for explaining a conventional semiconductor laser device.
FIG. 5 is a diagram for explaining a second conventional example.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Semiconductor laser element 11 Big end face 14 Output light beam 2 Submount substrate 21 High concentration n type conduction type substrate 22 Low concentration n type epitaxial layer 23 Recessed slope 24 High concentration p type diffusion layer 24-2 High concentration n type diffusion layer 25 Semi-transmissive film 26 Photodiode electrode 27 Groove 28 Submount back surface electrode 29 Light receiving element separation groove 29-2 Light receiving element separation region (n-type diffusion region)
3, 4 Light receiving element 5 Adhesive

Claims (2)

What is claimed is: 1. A semiconductor laser device comprising: a semiconductor laser element; means for dividing light emitted from the semiconductor laser element; and a photodiode for monitoring the light output of the semiconductor laser element. A submount substrate to be mounted, and a means for electrically connecting both, wherein at least one recess is formed in the submount substrate, and at least one surface of the recess has a slope, A part of the emitted light of the semiconductor laser element mounted on the bottom surface of the recess is reflected by the inclined surface and output, and the remaining part is incident on the submount substrate and formed on the substrate surface on the inclined surface side. Light is received by a photodiode, and is divided into a plurality of portions in series with respect to the optical axis of light incident on the submount substrate. The semiconductor laser device, characterized in that the drivable configuration s in. The semiconductor laser element emits light in an emitted light mode with different light output by the same element, and the preceding photodiode on the semiconductor laser element side among the divided photodiodes is a latter stage when the incident light intensity is high. The light-absorbing layer of the present invention operates as a light absorption layer. 1 The semiconductor laser device described.

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