JPS6237899B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor laser, and particularly to a semiconductor laser that can perform multimode oscillation while maintaining high output, long life, and high reliability.

When the oscillation light of a semiconductor laser is focused and irradiated onto an information medium using an optical system, and a part of the irradiated light is reflected by the information medium and returned to the original semiconductor laser through the same optical system, the optical output of the semiconductor laser is increased. or the terminal voltage changes. This change is called the self-coupling effect of semiconductor lasers, and it is known that it can be applied to pickups for reading signals from information media such as optical disks.

On the other hand, the characteristics of semiconductor lasers have been significantly improved in recent years, and the goals of higher output and longer life have almost been achieved. Control of the oscillation spectrum has become possible through various structures and devices, and single longitudinal mode oscillation without the generation of transverse modes has become commonplace. Along with this, the coherence of semiconductor laser light has been significantly improved, and even better than H _l -N _l lasers have been obtained.

However, the improvement in coherence of semiconductor lasers has raised unexpected problems in advancing the application of semiconductor lasers. That is, in an optical memory that uses the self-coupling effect of a semiconductor laser, such as a pick-up for an optical disk, it is desirable that the distance between the semiconductor laser and the information medium be several centimeters, and in fact, such a device has been developed. This distance is within the coherence distance of the semiconductor laser, and the light reflected from the information medium and returned to the original semiconductor laser will interfere with the original oscillation light. For this reason, the optical output may change in response to minute vibrations of the optical disk, or the optical output characteristics may become non-linear with respect to the injected current.
The ratio may decrease, and in some cases, signal detection by wobbling for focus control when reading an optical disk becomes impossible. Also, in optical communication systems using optical fibers, when a semiconductor laser beam is coupled to an optical fiber, a part of the oscillated light is reflected at the end face of the optical fiber and returns to the original semiconductor laser. Regardless of whether it happens or not, there is a problem in that the self-coupling effect of the semiconductor laser occurs, causing fluctuations in the optical output.

As the single longitudinal mode oscillation of semiconductor lasers has caused various problems, it is desirable to use multi-longitudinal mode semiconductor lasers as light sources in optical communication systems in the band below 1 GHz and around 100 MHz. This is the current situation. Further, multi-mode oscillation semiconductor lasers have also become desirable when using the self-coupling effect of semiconductor lasers as pickups for optical memories such as optical disks. However, this is also a matter of satisfying various characteristics such as high output and long life, and conventional multimode lasers cannot be used as is.

This is because, as we can see from the history of improving the characteristics of semiconductor lasers, conventional multimode lasers were not designed to be multimode intentionally, but due to crystal defects and waveguide structures that occurred during semiconductor crystal growth. This was unavoidable due to the non-uniformity of the laser, and a multimode oscillation laser was the same as a laser with poor characteristics such as low output and short life.

In view of the above, it is an object of the present invention to provide a highly reliable multi-mode oscillation laser that maintains performance equal to or better than that of single longitudinal mode semiconductor lasers with improved characteristics such as high output and long life.

The present invention also actively modifies the current injection electrode as a component of recent single longitudinal mode oscillation semiconductor lasers with excellent characteristics, thereby achieving multi-mode oscillation while maintaining good characteristics. It can be said that it was a success.

Embodiments of the present invention will be described below, and FIG. 1 shows an example of a conventional single longitudinal mode semiconductor laser as a current injection type semiconductor laser to which the above modifications of the present invention are applied. .

This laser is assumed to be a GaAlAs laser, and its structure consists of an n-type GaAs substrate 1 and an n-type
GaAlAs2, GaAs active layer 3, p-type GaAlAs4, p
A positive electrode 8 is attached via an insulator 6, and a negative electrode 7 is attached to the lower side of the substrate 1. The typical thickness of the active layer 3 is about 0.2 μm, and it is important to use a double heterostructure to reduce the threshold current by confining the carrier by current injection and the light. The width W of the electrode stripe, which serves as a substantial electrode portion 10 that comes into ohmic contact with the p-type GaAs layer 5 through an opening in the insulating film 6 and injects current into the active layer 3, is set to about 10 μm. In such a conventional semiconductor laser, the width of the electrode stripe 10 is uniform, and generally the entire length from the cleavage plane on one side of the semiconductor crystal to the cleavage plane on the other side serves as an electrode. Recently, there have been cases in which it has been shown that by making the vicinity of the crystal end face transparent to laser light, it is possible to significantly raise the limit of crystal destruction by laser light, thereby increasing the optical output. Even in this case, the electrode 10 is a continuous stripe with a uniform width over the entire length except near the crystal end face.

In the present invention, on the other hand, the current injection electrode region 10 is structurally modified to consist of small regions in an irregular pattern. The first embodiment is shown first in FIG. 2, and the same reference numerals as in FIG. Since there is no need for any particular modification, these components 1 to 7 are omitted as appropriate in FIG. 2 as well.

In this first embodiment, in the positive electrode 8, a region which becomes a substantial current injection electrode 10 to the laser active layer is formed into small electrode regions 10 separated from each other in the length direction.
i (i= ₁ , ₂ , . . . n), and when viewed as a whole, it has an irregular electrode pattern. In general, an insulating film is interposed between each insulating portion 9j (j= ₁ , ₂ , . . . n _-1 ) between adjacent small electrode portions. Depending on the pattern, insulating parts ₉₀ (not shown) and 9n may also be provided outside the small electrode parts ₁₀₁ and 10n at both ends in the length direction.

In this embodiment, the widths of each small electrode region 10i and insulating portion 9i are all equal W, for example, 10
The length is approximately μm, but the length is varied. The length a _i of each small electrode region 10i,
To give a specific numerical example for the length b _j of the insulating portion 9j, for example, a ₁ = 8.1 μm, b ₁ = 9.8 μm.
m, a ₂ = 7.7 μm, b ₂ = 5.7 μm, a ₃ = 7.1 μm, b ₃
= 7.6 μm, a ₄ = 8.5 μm, b ₄ = 6.6 μm, etc., within a certain range of values (the upper and lower limits are fixed) and at a certain interval (even between two adjacent values). (there is always a difference), and the value is determined randomly. When current is injected into a semiconductor laser having such an irregular electrode pattern, the excited carrier density in the oscillation direction of the semiconductor laser becomes non-uniform. In this way, the oscillation spectrum can be made into multiple modes instead of a single longitudinal mode.

3A and 3B show other examples of irregular electrode patterns as the second and third embodiments, respectively, and show only the current injection electrode 10 portion in plan view. The one shown in 3A is the stripe electrode 1
The width of 0 is made different in each part in the length direction, and if you look at it from a different perspective, it is the small electrode area 10i described above.
can be said to be continuous in the length direction, and the width w _i (i= ₁ , ₂ , . . . n) of each small electrode region is varied as the length a _i changes. On the other hand, the one shown in FIG. 3B has an insulating region 9j between adjacent small electrode regions 10i and 10i+ ₁ like the embodiment shown in second drawing, but each small electrode region 10i
In addition to the length a _i and width w _i of the insulating regions, the length b _j and width s _j of each insulating region are also varied.

FIG. 4 shows still another embodiment or usage example, in which the small electrode area 10i in the second illustrated embodiment is shown.
is divided into two groups (for example, divided into a group in the area where i is an odd number and a group in the area where i is an even number), and each group is connected to a high frequency bias source 11 with different frequencies fa and fb.
It is connected to a and 11b. The current injected from one high-frequency bias source 11a is approximately one-tenth of the DC operating current, and the magnitude of the current from the other high-frequency bias source 11b is selected and injected as necessary. Let Δf be the frequency difference between the respective frequencies fa and fb, and let f be the carrier signal frequency handled by the optical memory pickup due to the self-coupling effect using this semiconductor laser, then these values include fa, fb, Δf>2f... The relationship (1) is necessary. Assuming reading of an optical disk, f = 20MHz, for example fa = 178MHz,
It is sufficient to set fb=380MHz. In this semiconductor laser, the excitation carrier density is not only spatially non-uniform but also temporally non-uniform, so the oscillation spectrum is not a single longitudinal mode but multi-mode. If the excitation carrier changes over time, the optical output will also change accordingly, but considering the application to optical disk reading, if equation (1) is satisfied, the detection signal can be processed electronically. , can be extracted as a desired signal.

FIG. 5 is a schematic diagram of an application example in which an optical pickup for a video disk is constructed by providing an antireflection film 12 on the output end facet of the semiconductor laser of the present invention and a high reflection film 13 on the opposite end facet. If an anti-reflection film is attached to the output side end face, the gain of the semiconductor laser will be reduced by about 32% of the reflectance of the cleavage plane than when no anti-reflection film is attached.
Approximately 90% is added. In this way, the gain of the semiconductor laser does not decrease, and the optical output change and terminal voltage change due to the self-coupling effect can be greatly increased, and the signal-to-noise ratio for signal detection can be greatly increased. The semiconductor laser is driven through a power source B and a resistor R. The output light from the side with the anti-reflection film added is converted into a video signal using a collimating lens L ₁ and a focusing lens L ₂ using a known method.
The optical output of the semiconductor laser changes depending on the reflected light that is focused on the surface of the disk D and reflected according to the information on the disk, so the photodiode 14 detects the light that slightly emerges from the high reflectance film at the rear. Alternatively, the voltage change at the interelectrode terminals 15, 15 of the semiconductor laser may be detected via the capacitor C.

In the above embodiment, the present invention is applied to the positive electrode 8 of the pair of positive and negative electrodes 8 and 7. but,
Depending on the case, an irregular electrode pattern configuration may be adopted for the negative electrode 7 or for both electrodes.

Here, the reasons why the configuration of the present invention can cause multimode oscillation will be briefly summarized again.

As mentioned at the beginning, in the early days of semiconductor lasers, the crystal growth technology was immature, so small defects were scattered within the crystal, and the current injection was inevitably non-uniform, resulting in non-uniform excitation carriers.
Single longitudinal mode oscillation was difficult. As it became possible to make intracrystalline defects extremely small, it became possible to create a single longitudinal mode. As the injection current is further increased, the strongest Fabry-Perot cavity mode determined by both cleavage planes of the semiconductor laser grows due to frequency pooling, a principle taught by quantum electronics associated with stimulated emission, and a single longitudinal mode is generated. becomes stronger. This frequency pooling effect is extremely strong in semiconductor lasers compared to other gas lasers, and for this reason, the temperature dependence of the oscillation frequency of semiconductor lasers is not proportional to the temperature as initially thought, but rather has a stepped shape with mode hops. Experiments have shown that there is a relationship of change. This step-like change is the reason why the single longitudinal mode property of a semiconductor laser, and thus its coherence, are far beyond common sense. As a semiconductor laser with excellent coherence, buried heterostructure (BH type), channel, stripe,
Renormalized waveguide structures such as the planar structure (CSP type) are famous. In these cases, the transverse mode is easily controlled, and the single longitudinal mode is easily achieved. However, even these semiconductor lasers become multi-mode when an optical arrangement that uses the self-coupling effect is used. In this case, the returned light makes the excitation carrier distribution of the semiconductor laser non-uniform, which causes a change in the refractive index inside the crystal, causing several Fabry-Perot cavity modes to oscillate.
This can be explained as multi-mode oscillation.

Therefore, when the excited carrier density inside the semiconductor crystal is modulated at least spatially and, depending on the embodiment, also temporally, as in the configuration of the present invention, a large number of Fabry-Perot resonances occur simultaneously or momentarily. When the oscillation mode oscillates and the oscillation spectrum is observed within a certain period of time, it can be seen as multimode oscillation. In the manner described above, multimode oscillation can be made possible while controlling the transverse mode.

Finally, regarding the manufacturing method, the main difference between the manufacturing method of the semiconductor laser of the present invention and the conventional semiconductor laser is that an irregular electrode pattern is manufactured as opposed to the conventional uniform electrode. ,this is,
It is sufficient to simply prepare a special mask when producing the electrode. This mask production has been fully developed using photoresist, electron beam exposure equipment, and other technologies for producing semiconductor devices such as LSIs, and recently it has become fully possible to produce masks with a resolution of 0.1 to 0.2 μm. It is possible to produce an electrode shape having a width and length determined by random numbers as described above. Furthermore, it is easy to add an antireflection film or a high reflectance film to the end face of a semiconductor laser using well-known techniques.

As described above in detail, according to the present invention, a stable and highly reliable multi-mode oscillation semiconductor laser can be provided, and the self-coupling effect of the semiconductor laser can be used.
This can greatly contribute to the practical application of optical disk reading technology.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a conventional single longitudinal mode oscillation semiconductor laser, Fig. 2 is a schematic block diagram of main parts of the first embodiment of the present invention, and Figs. FIG. 4 is an explanatory diagram of still another embodiment, and FIG. 5 is an explanatory diagram of an example of actually assembling an application device. In the figure, 3 is a laser active layer, 10 is a current injection electrode region to the active layer, 10i is each small electrode region, and 11
a and 11b are high frequency bias sources.

Claims (1)

[Scope of Claims] 1. In a current injection semiconductor laser, at least one of the positive and negative current injection electrode regions for injecting current into the laser active layer is composed of a plurality of small electrode regions having an irregular pattern as a whole, A semiconductor laser characterized in that the small electrode regions are provided continuously or with an insulating part interposed between the small electrode regions, and the width and length of the small electrode regions are variously different. 2. The semiconductor laser according to claim 1, wherein the small electrode region is divided into at least two groups, and bias current sources with different frequencies are connected to each group.