JPS62268178A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To improve oscillation spectral purity by forming a modulation region optically coupled with one of a laser region and an optical guide layer optically coupled with the other of an active layer and increasing the reflectivity of the emission end surface of the optical guide layer. CONSTITUTION:A diffraction grating 10 is shaped only on the surface of a section as a laser region 22 on an N-InP substrate l, and an N-InGaAsP optical guide layer 12, an InGaAsP active layer 13 and a P-InP clad layer 30 are grown. An SiO2 film is prepared so as to coat the whole surface on the layer 30 and patterned, the SiO2 film except the upper sections of the laser region 22 and a modulation region 23 is removed, and the P-InP layer 30 and the InGaAsP active layer 13 are etched selectively. An InGaAsP optical guide layer 50 is grown, the SiO2 film is etched, a P-InP clad layer 31 is grown on the layer 50 and flattened, and a P<+>-InGaAsP cap layer 20 is grown in an epitaxial manner in order. Accordingly, spectral purity is improved, the efficiency of frequency modulation is enhanced, and the frequency characteristics of the laser are flattened.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a semiconductor laser used in optical communication systems, optical measurement systems, etc. that use optical heterodyne detection or optical homodyne detection.

(Prior Art) In recent years, studies on coherent optical transmission systems that use information on the frequency and phase of light have been progressing in various places as one type of optical communication. In particular, the frequency shift (F
SX) In the case of the optical heterodyne optical communication system, the frequency modulation can be achieved by minute changes in the injection current of the semiconductor laser, so there is no need to use an external modulator for modulation, and a system with low loss can be constructed. It has the characteristic of being able to

However, in order to achieve high reception sensitivity in this FSK optical heterodyne communication system, the spectral purity of the light source must be sufficiently high, and current semiconductor lasers
The problem is that this demand is not necessarily met.

As a way to solve this problem of spectral purity, an optical waveguide layer is provided that is optically coupled to the semiconductor laser, and by increasing the reflectance of the output end face, a structure is added with an external mirror, which increases the spectral purity. A method is being considered. (Murata et al., 1985 Applied Physics Academic Lecture (Autumn)
Lecture Proceedings IP-M-6) (Problems to be Solved by the Present Invention) However, the semiconductor laser with this structure provided with an optical waveguide layer does not allow frequency modulation by directly modulating the laser light frequency with the press-in current. When doing so, there are disadvantages in that the frequency modulation efficiency is significantly degraded compared to a single semiconductor laser, and the frequency characteristics of the frequency modulation efficiency are frequency dependent, causing distortion in the original modulation that performs frequency modulation.

(Means for solving the problem) The present invention provides a laser region having a diffraction grating in an active layer;
a control region optically coupled to one of the laser regions, an optical waveguide layer optically coupled to the other of the active layers, and the reflectance of the output end face of the optical waveguide layer is increased. The optical waveguide layer has a structure with a reflectance, and the length of the optical waveguide layer is longer than the sum of the lengths of the laser region and the modulation region.

(Operation) The principle of the present invention will be explained below. First, the integrated laser device has a laser region with a distributed feedback configuration consisting of a diffraction grating near the active layer of the laser.
Consider a structure that does not have a modulation region but has a long optical waveguide layer optically coupled to one of the active layers and a mirror with high reflectance at the output end face of the optical waveguide layer.

In this type of integrated laser device, the mirror at the output end face of the long optical waveguide layer serves as an external mirror, so it is known that the spectral purity is significantly improved compared to a single semiconductor laser without an optical waveguide layer. ing. (Murata et al., 1985 Applied Physics Conference (Autumn) Lecture Proceedings IP-M-6) In addition, since this integrated semiconductor laser device has a distributed feedback configuration in the laser region, the wavelength selectivity of the diffraction grating is It oscillates in a single axis mode at a wavelength that matches the wavelength.

When the laser region of this integrated semiconductor laser device is biased above the threshold value and the magnitude of the sinking current is slightly changed, the refractive index within the laser medium changes due to fluctuations in the carrier density within the laser medium, causing the laser to emit light. Frequency modulation is applied to the output light. This is the same as the conventional direct frequency modulation method used in ordinary semiconductor lasers. Since the carrier density fluctuation in this case is influenced by the relaxation oscillation of the semiconductor laser, the amount of carrier fluctuation varies depending on the modulation frequency. Therefore, the modulation characteristics during direct frequency modulation also become non-uniform, and in the range of 10 MHz to several GHz, a phenomenon is observed in which the higher the modulation frequency, the wider the frequency shift. An example of the frequency characteristics of the frequency shift in this case is shown in FIG. For this reason, waveform distortion occurs when the current applied to the laser region is AM-modulated with pulses to obtain FM-modulated laser light and this laser light is demodulated.

On the other hand, in a laser with a modulation region as described in Niss, Yamazaki et al., Electronics Raters, Vol. 21, No. 7, 1985, pages 283-285, a bias current of a constant value below the oscillation threshold is applied to the laser region. Then, by slightly changing the magnitude of the current applied to the modulation region using a modulation signal, it is possible to frequency modulate the output light. Furthermore, it is known that at this time, the modulation efficiency is greater than that of a semiconductor laser without a modulation region, and the dependence on the modulation frequency is flat. Frequency modulation in this case is realized by the following principle.

Generally, one of the factors that determines the oscillation frequency of a distributed feedback semiconductor laser is the phase relationship between the resonator end face of the semiconductor laser and the diffraction grating. When a current is applied to the modulation region of the integrated semiconductor laser probe used in the present invention, the carrier density in the modulation region changes. This gives us, isometrically,
As the phase condition of the modulation region side end face as seen from the distributed feedback region (!111) changes, the oscillation frequency changes, and as a result, frequency modulation is applied to the output light.At this time, the modulation region is not an active region such as a laser region. Uniform frequency modulation characteristics can be obtained because the carrier density is not affected by carrier density fluctuations.Also, in the laser region, the carrier density is clamped to the laser oscillation threshold state, so the carrier density fluctuations for humans are On the other hand, in the modulation region, most of the applied current contributes to carrier density fluctuations, making it possible to realize efficient frequency modulation.

Since the integrated semiconductor laser device of the present invention also has a modulation region optically coupled to the laser region, the modulation efficiency is higher than that of an integrated semiconductor laser device that does not have a modulation region. Its frequency dependence can be made flat. Furthermore, since the semiconductor laser device of the present invention has a long optical waveguide layer, the purity of the oscillation spectrum is also increased.

Here, the purity of the oscillation spectrum increases in proportion to the length of the integrated semiconductor laser, so if the length of the optical waveguide layer is made longer than the sum of the lengths of the modulation region and the laser region, the purity of the oscillation spectrum increases. is improved by more than twice compared to the case without the optical waveguide layer.

(Example) Examples of the present invention will be described below with reference to the drawings. 1st
The figure is a block diagram of a laser beam direct frequency modulator used in an embodiment of the present invention, and Figure 2 shows the modulation characteristics of a conventional laser with a structure in which the modulation region is removed from the integrated semiconductor laser device of Figure 1. FIG. 3 is a diagram showing modulation characteristics when frequency modulation is performed in the modulation region according to an embodiment of the present invention.

The integrated semiconductor laser device 21 used in this example was constructed as follows. First, a diffraction grating 10 with a period of about 2400 was formed only on the surface of the portion that would become the laser region 22 on the n-InP substrate 1 . Then n-InGaAsP
The thickness of the optical waveguide layer 12 is 0.2 pm, the thickness of the InGaAsP active layer 13 is 0.1 pm, and the thickness of the p-InP cladding layer 30 is 0.2 pm.
Grow 2pm. SiO2 is placed on top of this to cover the entire surface.
A film is created and patterned using a photoresist, and the Si is removed in areas other than the laser area 22 and modulation area
The O2 film is removed and the p-InP layer 30 and InGaAs
The P active layer 13 is selectively etched. After this InG
aAsP optical waveguide layer 50 (1.15p in terms of emission wavelength)
5i02 film was etched, and p-InP ground film 31 was grown to 0.8 pm on top of the 5i02 film.
After growing and flattening the p+-InGaAsP cap layer 2
0 is sequentially epitaxially grown. Furthermore, a drive electrode 61 and a modulation electrode 62 are formed on the cap layer 20 of the laser region 22 and the modulation region 23, respectively, and the InP substrate 1
An n-side electrode 8 was formed below. Further, a groove 90 deeper than the cap layer 20 was formed between the drive electrode 61 and the modulation electrode 62 in order to improve electrical isolation between the two electrodes. Further, an insulating film S is provided at the output end of the optical waveguide layer 50.
By forming the i○2 film 91 and the Au film 92 thereon, the light reflectance was made 95% or more. Through the above steps, a semiconductor laser 21 as shown in FIG. 1 can be manufactured.

The lengths of the laser region 22, modulation region 23, and optical waveguide layer 50 are 3009 m, 300 Pm, and 2.0 m, respectively.
mm.

Further, as peripheral circuits, a drive circuit 11 for injecting current into the laser region 22, a modulation circuit 42 for injecting current into the modulation region j, and a signal generator 43 for generating a modulation signal for frequency modulation are provided. Prepared.

The laser region 22 is biased to a constant value of the oscillation threshold value, and a signal from a signal generator 43 is applied to the modulation region 23 via the modulation circuit 42 to perform frequency modulation. The modulation characteristics are shown in FIG. Frequency modulation in the modulation region 23 had good modulation efficiency, and the modulation characteristics had no modulation frequency dependence.

Next, a 100 Mb/s NRZ random signal was generated by the signal generator 43 and added to the modulation area 23 via the modulation circuit 42 to perform binary FM modulation. Here, the modulated output light 24 from the integrated semiconductor laser device 21 was directly subjected to optical frequency discrimination detection by passing through a Fabry-Perot etalon, and the output waveform was observed. The obtained waveform almost faithfully reproduced the modulated waveform, and there was no waveform distortion due to frequency modulation in the modulation region 23. Further, the spectral width at this time was measured by heterodyne detection and was found to be approximately IMHz.

In addition to the above-described embodiments, various modifications of the present invention can be considered. For example, in this embodiment, an integrated semiconductor laser device 21 may be used in which the diffraction grating 10 for distributed j feedback is located above the active layer 2 in the laser region 22. In addition, in the present invention, the diffraction grating 10 is formed into grooves only in a part of the laser region 22, and the region where the diffraction grating]0 is set as a non-injected region, that is, the laser region j422 is constituted by a distributed reflection type laser. An integrated semiconductor laser device can also be used. The swipe groove structure of the integrated semiconductor laser device 21 is not limited to the buried heterostructure of this embodiment, but can take various structures such as a planar stripe structure and a transverse junction stripe structure. Further, the oscillation wavelength of the integrated semiconductor laser device 21 is not limited to 1.55 pm, and can be set to various wavelengths such as 1.3 pm, for example. Alternatively, the integrated semiconductor laser device 21 may be one in which the end face on the modulation region 23 side is coated to reduce reflectance. In this case, the optical power of the modulated output light 24 can be increased.

(Effects of the Invention) As described above, when the semiconductor laser having the structure of the present invention is used,
It is possible to provide characteristics such as high spectral purity, high frequency modulation efficiency, and flat frequency characteristics.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a laser beam direct frequency modulation device used in an embodiment of the present invention, Fig. 2 is a diagram showing the modulation characteristics of a conventional laser, and Fig. 3 is a diagram showing the frequency in the modulation region according to an embodiment of the present invention. FIG. 7 is a diagram showing modulation characteristics when modulation is performed. 13: Active layer 21: Integrated semiconductor laser element 22: Laser region 23: Modulation region 50: Optical waveguide traversing 92: Au Moon Figure 2 [GHz/mA l 1M 10M 100M iG iOG modulation frequency (Hzl) Figure 3 1M iOM ioOM iG IOG modulation frequency f Hz l

Claims (1)

[Claims] a laser region having a diffraction grating along an active layer, a modulation region optically coupled to one of the laser regions, and an optical waveguide layer optically coupled to the other of the active layer; A semiconductor laser characterized by having a structure in which the output end face of the optical waveguide layer has a high reflectance.