JPS5941316B2 - Semiconductor laser device capable of high-speed modulation - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a semiconductor laser, and particularly to a semiconductor laser device capable of high-speed modulation when directly modulating an optical output using a signal pulse current.

Semiconductor lasers have many advantages such as small size, light weight, high efficiency, and ease of direct modulation, so they are becoming an important light source for optical communication and optical information processing. In practice, the upper limit remains at around 400 megabits per second.

The main reason for this is that the waveform of the optical output pulse is severely disturbed by the spike-like vibrations that occur at the beginning of the optical output pulse. Therefore, when attempting to perform higher-speed modulation, it is necessary to eliminate this spike-like oscillation in the optical output, and one way to do this is to inject light externally into the resonator of the semiconductor laser that is directly modulated. (See, for example, Japanese Patent Application No. 49-96737 for suppressing spike-like vibrations by injecting external light). According to this method, spike-like oscillations in the optical output are almost completely suppressed, and the delay time of the rise of laser oscillation is also reduced, making it possible to achieve good high-speed modulation, but it has the following drawbacks: It also has In other words, it is desirable to use a semiconductor laser as a light source for the light injected from the outside, and for this reason, two semiconductor laser elements are used in one semiconductor laser device. The high peak value of the spike-like vibration is effectively used because it is inferior to the case where only the spike-like vibration is used, and by suppressing the spike-like vibration, the peak value of the optical output pulse is reduced to the average value when there is no vibration. There are things that cannot be done. SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor laser device that eliminates the above drawbacks and is capable of high-speed modulation using a single semiconductor laser element.

According to this invention, a semiconductor laser element to which a high-speed signal pulse current is applied is installed on at least one output side of the semiconductor laser element, and the output light is returned and a part of it is returned to the active region of the semiconductor laser element. A semiconductor laser device comprising a reflecting mirror is obtained.

In this invention, a part of the output light of the semiconductor laser modulated by the signal pulse current is returned to the semiconductor laser after an appropriate time, thereby reducing the period of the spike-like vibration and the depression of the valley. Improve modulation characteristics.

In other words, by returning a portion of the output light in a time shorter than the period of the spike-like vibrations, the valleys in the vibrations of the output light are filled. Since this method utilizes the vibration of the output light, the high peak value of the output light pulse due to the vibration can be used effectively as is. Furthermore, since only one semiconductor laser element is required, the method is not particularly inferior to other methods in terms of element life. Since the light intensity to be returned may be a few percent or less of the output light intensity, it is easy to install a reflecting mirror, etc., and there is no problem with long-term stability. Incidentally, inventions similar to the present invention are described in JP-A-49-91386 and JP-A-49-97582.

However, as shown below, the present invention differs from the invention described in the above-mentioned publication in terms of technical idea, structure, and effect. That is, in the present invention, a reflecting mirror is added to a single semiconductor laser, and a part of the output light of the semiconductor laser is returned to the semiconductor laser. On the other hand, the invention described in the above-mentioned publication adds a reflecting mirror to a semiconductor laser to form an external cavity type semiconductor laser. In the present invention, laser oscillation is performed by a single semiconductor laser, and a portion of the output light is fed back to enable waveform shaping.

On the other hand, in the invention described in the above publication, Japanese Patent Application Laid-Open No. 49-9138
6 Publication, page 470, upper right, lines 14 to 17, ``The resonator is composed of the end face of the diode opposite to the lens, a lens, and a plane mirror...The oscillation of the eigenmode determined by this resonator ] On page 471, top right line 10, it says, ``The laser diode 1 is provided with a 'non-reflection mirror 11' by vapor deposition on one side of the cleavage.''
Publication No. 9-97582, page 485, lower left Claim 11
Lines 15 to 15: "A first reflecting member provided on one end surface of the semiconductor laser element; and a first reflecting member provided on the outer end surface of the optical transmission body so as to constitute an optical resonator together with the first reflecting member." A semiconductor laser device including a second reflective member...
...” (1) Same P. 488 Lines 8 to 9 on the upper left indicate that a laser resonator is composed of a semiconductor laser and a reflecting mirror, as shown in ``In the embodiment, the optical resonator is composed of a reflective surface 6 and a reflective surface 7.'' Therefore, a semiconductor laser alone cannot cause laser oscillation. As is clear from the above, the reflecting mirror of the present invention is a mere reflecting mirror, whereas the reflecting mirror of the invention described in the above publication is one of the reflecting mirrors constituting a laser resonator, and has a different taste.

Therefore, the lasers of Publications 1 and 2 oscillate as external cavity type semiconductor lasers that include a reflecting mirror.

As a result, it is clear that none of the inventions described in the publication has the effect of suppressing relaxation vibration if it occurs in this configuration. In contrast, in the present invention, laser oscillation is performed by a single semiconductor laser, and the relaxation oscillation of the output light is suppressed by an external reflecting mirror. As described above, the present invention is clearly different from the invention described in the above-mentioned publication not only in its object but also in its structure, function, and effect.

Next, the present invention will be explained in detail with reference to the drawings.

FIG. 1 shows the basic configuration of this invention. That is, a reflecting mirror is installed on one output side of the semiconductor laser device 1 at an optical distance L apart. Optical distance refers to the product of the actual distance and the refractive index of the medium. FIG. 2 shows the effect of light feedback by the reflecting mirror 2. That is, Figure 2a
2B shows the modulated current waveform, FIG.
1 and 2 show the waveforms of the output light 3 when the light is returned by the reflecting mirror 2, respectively. By being turned back by the optical distance L, one output light 4 of the semiconductor laser element 1 becomes T=2L/C.
The light is returned to the semiconductor laser element 1 with a delay of time (C is the speed of light in vacuum). By making T smaller than the vibration period T when there is no reflector 2, the returned light works to fill the vibration valley shown in Figure 2b, and the vibration period and vibration period T are reduced as shown in Figure 2c. It is possible to reduce the drop in the valley, obtain an output optical pulse that responds well to the signal pulse current, and enable high-speed modulation. In this case, the period of vibration is approximately T. Since almost no light is fed back at the time of the rise of laser oscillation, the rise of laser oscillation is almost the same as when there is no reflecting mirror 2, and an output light pulse with a high peak value is output. Even when light returns, the peak value remains almost unchanged, increasing signal output and efficiency. Third
The figure shows a perspective view of a first embodiment of the invention. The semiconductor laser element 1 is fused with tin onto a copper heat sink 6 which also serves as an electrode, and a lead wire 7 for applying current is attached. On the heat sink 6, near one cavity end face of the semiconductor laser element 1, there is a condenser with a refractive index of length 3 TItTL1 having a refractive index of 1.6 at the center, which has a refractive index distribution that decreases approximately in proportion to the square of the distance from the center. A flexible optical transmitter 5 is installed to convert one output light 4 of the semiconductor laser element 1 into a substantially parallel light beam 8. The light beam 8 is reflected by a flat reflecting mirror 2 with a reflectance of 50 (Ft) placed 20T1Lm away from the convergent light transmitter 5 and returned to the semiconductor laser element 1. In this embodiment, the optical distance to the reflecting mirror 2 is about 25 degrees,
T = 0.17 + 1 second, which is chosen to be shorter than the period T, = 0.8 + 1 second of the spike-like vibration when there is no light feedback, so the deep valley of the spike-like vibration is filled by the effect of the returned light. As a result, a waveform that perfectly responded to the signal current waveform was obtained, making high-speed modulation possible. The manner in which the waveform of the output light of the semiconductor laser was improved by the feedback of the light emitted from itself was almost the same as shown in FIG. A monitor for detecting the intensity of the output light passing through the reflector 2 with a photodetector 9 installed behind the reflector 2 to suppress fluctuations in the output light intensity due to deterioration of the semiconductor laser element 1 or changes in ambient temperature. I got the output.

Needless to say, the other output light 3 of the semiconductor laser element 1 can be coupled to another optical system, such as an optical fiber, without any restrictions. If the light return time T is shorter than the period T1 of the spike-like vibration, the output light waveform will be improved, but in order to obtain the most desirable waveform,
Since it is known that TTlTl must be in the range of 1≦T≦-, it is desirable to set the optical distance to the reflecting mirror so that T falls within this range when implementing the present invention.

FIG. 4 shows a sectional view of a second embodiment of the invention.

This consists of a focusing optical transmitter 5 with a length of 3Tnm and a length of 16Tnm.
A cylindrical glass 10 having a reflective film 11 formed by vapor deposition of a multilayer film of SiO 2 and TiO 2 on one end face is placed on a heat sink 6 in close proximity to one output side of the semiconductor laser device 1. The refractive index of the convergent light transmitter 5 and the cylindrical glass 10 are both about 1.6, the optical distance is about 30 mm, T=0.2+1 seconds, and the period of spike-like vibration TlO. It was smaller than 8+1 seconds, and the spike-like vibration of optical output 3 was suppressed to a level that poses no practical problem, making high-speed modulation possible. FIG. 5 shows a perspective view of a third embodiment of the invention.

In the third example, the cylindrical glass 10 used in the second example
Instead, an optical fiber 12 with a larger refractive index at the center than at the periphery is used, and the length of the convergent light transmitter 5 is set to 4.5 mm.
1, the output light from one side of the semiconductor laser device 1 is focused and coupled to the optical fiber 12. Similar to the second embodiment, a reflective film 11 is attached to one end face of the optical fiber 12. In this example as well, by making T smaller than the period T1 of the spike vibration, the spike-like vibration of the optical output was suppressed and high-speed modulation could be realized.
Since the optical fiber 12 is highly flexible, it can be rolled up and inserted into the package of the semiconductor laser device 1, for example. Although typical embodiments have been described above, the present invention requires only one semiconductor laser element, so the reliability of the apparatus is increased compared to, for example, a case where a plurality of semiconductor laser elements are used in the same apparatus.

Further, since the intensity of the light to be returned is small, it is easy to install a reflecting mirror or the like. In addition to the above-described typical embodiments, several modifications of the present invention are possible.

The focusing light transmitting body 5 used to focus the output light 4 toward the reflecting mirror 2 may be replaced with another light focusing element such as a lens. Further, the reflecting mirror 2 does not necessarily have to be a plane mirror, and may be a spherical mirror, for example, as long as it can return light to the semiconductor laser element 1. Further, since a spherical mirror itself has a focusing effect, there is no need to use any other light focusing element. Further, in the embodiment, the optical distance between the reflecting mirror 2 (or the reflecting film 11) and the resonator end face of the semiconductor laser element 1 is fixed, but it does not need to be constant. It is also possible to install a variable optical distance so that the output light waveform can be adjusted for the best.

[Brief explanation of the drawing]

Fig. 1 is a basic configuration diagram of this invention, Fig. 2 is a diagram for explaining the effects of this invention, Fig. 3 is a perspective view of a first embodiment of this invention, and Fig. 4 is a diagram of this invention. FIG. 5 is a cross-sectional view of the second embodiment of the present invention, and FIG. 5 is a perspective view of the third embodiment of the present invention. In the figure, 1 is a semiconductor laser element, 2 is a reflecting mirror,
3 and 4 are the output lights of the semiconductor laser element 1, 5 is a focusing light transmitter, 6 is a heat sink, 7 is a current application leader line,
8 is a light beam, 9 is a photodetector, 10 is a cylindrical glass, 11
1 is a reflective film, and 12 is an optical fiber.

Claims (1)

[Claims] 1. A semiconductor laser device that emits output light with spike-like vibrations when a signal current is applied, and a semiconductor laser device installed on at least one output side of the device to emit at least a part of the output light with a period shorter than the period of the spike-like vibrations. A semiconductor laser device capable of high-speed modulation, comprising means for feeding back to the semiconductor laser element within a short period of time.