JP2002141595A - Semiconductor laser device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problem in a semiconductor laser device comprising a light receiving element for front APC disposed at the rear part of an optical path conversion reflector that the intensity of incident light from a light emitting element increases stepwise, when DVD-ROM and DVD-RAM are implemented by identical light emitting elements as the light source of an optical pickup, to cause significant variation in the output current level of the light receiving element and thereby an external control circuit for APC driving requires a very large dynamic range. SOLUTION: A variable light attenuator 4 is provided between a reflector (translucent film) 25 and a light receiving element 3 disposed in the rear thereof. Furthermore, a groove 29 is made in a light receiving element part disposed in the rear of the reflector substantially in perpendicular to the optical path in order to split the light receiving element optically in series. When the intensity of incident light is high, a light receiving element at the prestage is turned off and used as a light absorbing layer, and an output current from a post-stage light receiving element is used for control. When the intensity of incident light is low, both front and rear light receiving element are used for control.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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]

2. Description of the Related Art At present, in the fields of optical communication, optical transmission, and optical information recording, semiconductor laser devices are widely used as light sources because of the coherency of emitted light and high-speed operation, or because they are very small. ing. A semiconductor laser device emits stimulated emission light by injecting current from the outside, and the light intensity changes sensitively to heat fluctuations. However, in order to reduce the difference in the coefficient of thermal expansion between the metal and the semiconductor material constituting the semiconductor laser device, Si or A is used.
After being mounted on a substrate called a submount made of 1N or the like, it is mounted on a metal member.

A semiconductor laser device is realized by sandwiching a medium having an amplification factor of 1 or more in a resonator consisting of a plurality of reflectors. However, the reflector of the resonator can use a cleavage plane of a crystal. Development of an edge-emitting type semiconductor laser that can take a long distance to pass through an amplification medium has become mainstream. In some cases, surface-emitting laser devices that make high-reflection mirrors using semiconductors or dielectric multilayer films and emit light in the normal direction of the substrate have been put into practical use, but the technology for practical use is not sufficient. However, there are many problems such as many materials, which are still at the research stage depending on the material, that is, the wavelength of the emitted light. From this,
An edge emission type light source is used as a light source composed of most semiconductor laser elements used in products.

On the other hand, the light output of a semiconductor laser device fluctuates sensitively due to a change in environmental temperature. Therefore, both the semiconductor laser element and the mounting board 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 but heat capacity, when precise optical output control is required, a method of monitoring the actual output light and causing the drive current circuit to perform feedback control is adopted. This is called automatic light output control (Au
tomatic Power Control: AP
C). The end face emission type semiconductor laser element uses both end faces formed of a cleavage plane or the like as resonator mirrors, but output light is emitted symmetrically in the front-rear direction unless the reflectivity of the end face is specifically controlled. The above-described APC can be configured by monitoring the light output from behind with a light receiving element or the like, but the monitored light does not contribute as a signal, and thus the light use efficiency is reduced. Therefore, in a system that requires high output and high efficiency, a method of increasing the light use efficiency as much as possible by increasing the reflectance of the rear end face with a dielectric multilayer film or the like is adopted. In such a case, the light emitted from the rear that can be used for the monitor becomes small, the SN ratio deteriorates, and precise APC cannot be performed. Therefore, it is necessary to monitor a part of the light (= signal light) from the front, and this control method is called front APC (hereinafter, referred to as FAPC).

FIG. 4 shows a configuration when a semiconductor laser element is used in the FAPC system. Reference numeral 101 denotes a semiconductor laser element, 102 denotes an emission light splitting unit which is constituted by, for example, a half mirror, and 103 denotes a monitoring light receiving element, which constitutes a semiconductor laser device. The light emitted from the semiconductor laser element 101 is split by the splitting means 102 and a part of the light becomes output light and is emitted to, for example, an optical disk. The remaining part is input to a light receiving element for optical output monitoring and outputs a monitoring photocurrent for APC. In this configuration, when the light intensity incident from the semiconductor laser element to the light receiving element for monitoring changes stepwise greatly, for example, the DVD-ROM and the DVD-RAM can be made identical 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 greatly fluctuates, and an external control circuit for driving the APC needs a very large dynamic range. For example, a red semiconductor laser device (wavelength of about 650 nm) currently used for DVD-ROM and RAM
In the case of m), the output of the semiconductor laser in the ROM mode is about 5 mW at the maximum, while the output of the RAM is about 10 times as large as about 50 mW. Since it is inherently difficult to achieve both a large dynamic range and precise control, the control circuit system becomes complicated. Further, when the incident light intensity increases, a saturation phenomenon may occur in which the output current of the light receiving element cannot linearly follow the light intensity. When saturation occurs, it becomes necessary to control the drive current of the semiconductor laser element nonlinearly with respect to the output current from the light receiving element, and the control circuit configuration becomes complicated.
For this reason, it is necessary to separately provide a monitor light receiving element in the RAM mode and the ROM mode, or to prepare two control circuit systems. Problems arise.

As a method of controlling and using a semiconductor laser element by FAPC, a proposal as shown in FIG. 5 (for example, Japanese Patent Application Laid-Open No. 05-315700) in which a light receiving element for front APC is integrated at the rear of a reflector. Have been. A configuration in which a semiconductor laser device 1 is mounted in a concave portion 3 formed in a silicon substrate 2 as a submount, and a light beam is extracted as information by reflecting the light on a reflecting surface formed in a high-concentration n-type diffusion layer 24-2 on the wall surface of the concave portion. It is. According to this configuration, it is possible to monitor a part of the front output light. In addition, according to this configuration, the output light is reflected upward in the vicinity of the semiconductor laser element, that is, before being greatly expanded, so that the output light is kicked on the mounting surface without special consideration of the positional relationship with the mounting surface. And the output light can be extracted while the beam shape is almost maintained.

However, even with such a configuration, there is a problem essentially similar to the above-described conventional example (FIG. 4). When the intensity of light incident on the light receiving element for monitoring from the semiconductor laser element changes stepwise greatly, the output current value from the light receiving element fluctuates greatly, and an extremely large dynamic range is provided to the external control circuit for driving the APC. Is necessary.

[0008]

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 splitting means such as a reflector, a DVD-ROM and a DVD-R are used as light sources of an optical pickup.
In the case where the AM is realized by the same light emitting element, the intensity of light incident from the light emitting element greatly changes step by step, so that the output current value from the light receiving element greatly fluctuates, and the external control circuit for driving the APC is very difficult. A large dynamic range is required.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a novel semiconductor laser device which does not require a large dynamic range in an external control circuit for driving an APC.

[0010]

According to the present invention, there is provided a semiconductor laser device comprising a reflecting mirror and a light receiving element disposed at a rear portion thereof.
A variable optical attenuator is provided.

Furthermore, a groove is provided in a direction substantially perpendicular to the optical path in a light-receiving element disposed at the rear of the reflecting mirror, and the light-receiving element is optically divided in series.
The light receiving element at the preceding stage is turned off and used as a light absorbing layer, and control is performed using the output current from the light receiving element at the subsequent stage. Further, the present invention provides a semiconductor laser device which can be controlled using both front and rear light receiving elements when the intensity of incident light is small.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram for explaining the first embodiment of the present invention. 101 is a semiconductor laser element,
Reference numeral 102 denotes an outgoing light splitting unit which is constituted by, for example, a half mirror. 103 is a monitor light receiving element. 104
Is a variable optical attenuator. The variable optical attenuator may have any structure as long as the amount of attenuation can be changed by a voltage, a current, or the like, and is made of, for example, a liquid crystal. The light emitted from the semiconductor laser 101 is split by the splitting means 102 and a part of the light becomes output light and is emitted to, for example, an optical disk. The remaining part is input to the light receiving element 103 for monitoring the optical output and
The monitor photocurrent for PC is output. Here, when the light intensity incident on the monitor light receiving element is high, the attenuation of the variable optical attenuator is increased, and when the light intensity is low, the attenuation is reduced, so that the output current from the monitor light receiving element is reduced. 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 may of course be integrated and is not limited to individual components.

FIG. 2 is a 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 edge emitting type having a resonator in the left-right direction and emitting light from an end face 11 in the figure. Reference numeral 2 denotes a submount substrate made of, for example, Si or the like, and n which is highly doped with impurities.
Layer 22 with low impurity concentration and p-type region 24 with high impurity concentration on substrate 21 of type conductivity
Is formed on a substrate formed by epitaxial growth or the like by anisotropic etching or the like. 24,2
Each of the layers 2 and 21 constitutes a pin photodiode as a light receiving element. Reference numeral 26 denotes a photodiode extraction p-electrode, which is used by applying a reverse bias voltage to a back electrode 28 (= n-electrode). The semiconductor laser device 1 is mounted in a concave portion by a conductive adhesive 5 made of solder or the like, and the emitted light 14 is a semi-transmissive film serving as a reflecting mirror made of a dielectric film or the like formed on the slope 23. At 25, a part of the light is reflected and outputted above the substrate, and a part of the transmitted light enters a 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 connected to the 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. Has been split. Transmitted light 15
Has a structure in which a part is absorbed by the photodiode 4, transmitted, and then absorbed by the photodiode 3.

To realize FAPC using this structure,
When the incident light intensity is small, the control is performed using the output currents of both the photodiodes 3 and 4, and when the incident light intensity is large, the control is performed using the photocurrent of the photodiode 3 alone. When only the photodiode 3 is used, the photodiode 4 functions as a light absorbing layer. For example, the photodiode 4 is designed to absorb 90% of the light and absorb 10% of the light in the RAM mode and the ROM mode. , Approximately the same level of photocurrent can be obtained. With such a configuration, without preparing a separate monitor light receiving element,
A large control signal (monitor output current) can be input to the drive circuit while suppressing the saturation phenomenon of the light receiving element, and it is easy to design an appropriate control circuit system as a whole. In this structure,
The light receiving element at the preceding stage has the function of both the variable optical attenuator and the light receiving element for monitoring, so that the light receiving element and the optical attenuator can be attacked with the same element structure, and the structure can be more easily integrated.

In FIG. 2, a groove 27 is formed on the side of the slope 23 serving as a reflection surface on the bottom surface of the concave portion, so that the light emitting portion (ie, the active layer) of the semiconductor laser device 1 is relatively close to the bottom surface of the concave portion. In this case, the output light beam is prevented from being kicked at the bottom of the concave portion.

This structure is realized as follows.
First, a wafer in which the low concentration layer 22 and the high concentration layer 24 are formed on the high concentration substrate 21 by epitaxial growth or the like is subjected to patterning of a thermal oxide film, a nitride film, and the like by using a photolithography method or the like. The recess is etched with a solution such as KOH until the bottom reaches the substrate 21 as a mask. Further, a groove 27 can be formed by patterning and etching a part of the bottom in the same manner. In this case, by using an anisotropic etching solution such as KOH, a specific crystal plane can be formed on the slope of the concave portion, and a flat surface 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.

The side wall of the groove 29 is passivated with a thermal oxide film or the like to suppress surface leakage. In order to reduce the reflection loss inside the groove, the passivation film is desirably formed so as to be a non-reflection film with respect to the optical axis. Then, after forming a film 25 as a semi-transmissive mirror with a dielectric film, a take-out electrode 26 and a back surface electrode 28 of the photodiode are formed and cut into chips by dicing or the like.

In this embodiment, a high-speed PIN structure is shown as the light receiving element structure, but the same effect can be obtained by using a normal PN type light receiving element. In that case, the low concentration layer 22
Is unnecessary.

Although the substrate 2 is formed of two epitaxial layers, an outer shape is formed using a substrate having only the low concentration layer 22, and then a p layer (24) is formed by thermal diffusion or the like. May be. In addition, although the case where the shape of the groove is vertical in the shape of a trench is illustrated, an inclined surface similar to the reflecting surface 23 may be used as long as transmitted light is incident on the light receiving element 3 at the subsequent stage. In this case, the separation groove can be formed simultaneously with the reflection surface 23, and the process is simplified.

In this embodiment, the substrate is of the n-type conductivity type. Of course, when the p-type conductivity type is used, a similar structure can be obtained by reversing the conductivity type. In particular, when forming the concave portion, when boron is used as the p-type impurity, 1 × 10 ²⁰ cm
^{If the} concentration is higher than ⁻³ , the etching rate by KOH becomes slow. Therefore, a more flat bottom surface of the concave portion can be obtained by functioning as an etching stop layer.

FIG. 3 is a diagram for explaining a third embodiment of the present invention. The difference from the structure of FIG.
And n = 4 using the n-type diffusion region 29-2. Since the separation is pn separation, the p-type diffusion region 24 is partially formed by ion implantation, thermal diffusion, or the like. This structure simplifies the process because it can be separated by a normal LSI process such as ion implantation, and there is no need to passivate the surface of the trench side wall. However,
Since all the absorption in the separation region flows to the n-type electrode and becomes a photocurrent of a slow component, it becomes noise in the case of high-speed control, and thus is suitable for low-speed control.

[0022]

The present invention is characterized in that a variable optical attenuator is provided between a reflecting mirror and a light receiving element disposed at the rear thereof. Furthermore, a groove in the direction substantially perpendicular to the optical path is provided in the light-receiving element disposed at the rear of the reflecting mirror, and the light-receiving element is optically divided in series. It is turned off and used as a light absorbing layer, and control is performed using an output current from a light receiving element at a later stage. According to the semiconductor laser device configured as described above, when the intensity of the incident light is low, it can be controlled using both the front and rear light receiving elements, and the DVD can be used as a light source for an optical pickup.
-Even when the ROM and the DVD-RAM are realized by the same light emitting element, that is, when the light intensity incident from the light emitting element changes greatly in a stepwise manner, the output current from the light receiving element is a single element. Without the value fluctuating greatly,
Since a substantially constant control signal (photocurrent) can be input to the control circuit, the entire control circuit system can be easily designed.
This has the effect of suppressing an increase in cost and size.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a second embodiment of the present invention.

FIG. 3 is a diagram illustrating a third embodiment of the present invention.

FIG. 4 is a view for explaining a conventional semiconductor laser device.

FIG. 5 is a diagram for explaining a second conventional example.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 semiconductor laser element 11 cleavage end face 14 output light beam 2 submount substrate 21 high-concentration n-type conductivity type substrate 22 low-concentration n-type epitaxial layer 23 concave slope 24 high-concentration p-type diffusion layer 24-2 high-concentration n-type diffusion layer Reference Signs List 25 semi-transmissive film 26 photodiode electrode 27 groove 28 submount back electrode 29 light-receiving element separation groove 29-2 light-receiving element separation area (n-type diffusion region) 3, 4 light-receiving element 5 adhesive

Claims (2)

[Claims] 1. A semiconductor laser device comprising: a semiconductor laser element; means for splitting light emitted from the semiconductor laser element; and a light receiving element for monitoring an optical output of the semiconductor laser element. A semiconductor laser device comprising a variable optical attenuator inserted between the semiconductor laser device and the light receiving element. 2. The semiconductor laser device according to claim 1, further comprising: a semiconductor laser element, a submount substrate on which the semiconductor laser element is mounted, and means for electrically connecting the two. One or more concave portions are formed, and at least one surface of the concave portion has a slope, and a part of light emitted from the semiconductor laser device mounted on the bottom surface of the concave portion is reflected by the slope and output. The remaining part is incident on the sub-mount substrate and is received by the photodiode formed on the substrate surface on the slope side, and the photodiode is divided into a plurality in series with respect to the optical axis. , Each of which is configured to be electrically driven separately.