JPS61188991A - Semiconductor laser device - Google Patents

Abstract

PURPOSE:To obtain the stable and single longitudinal mode oscillation in which a wavelength chirping at direct modulation is suppressed by reducing a width of a spectrum line by integrating an active region composed of an active layer and a grating, and a light waveguide having a reflective function on the edge to compose a semiconductor laser having a composite resonator constitution. CONSTITUTION:On an N-type InP substrate 22, a grating 23 is formed and the region 28 composed of the lamination of an N-type InGaAsP light waveguide layer 24, an N-type InP isolation layer 25, an N-type InGaAsP active layer 26, and a P-type InP enclosure layer 27 is an active region and so-called a distribution feedback semiconductor laser. The region 29 where only a light waveguide layer 24 exists on a substrate 22 is a light waveguide. In this constitution, the edges 30 and 31 of a resonator are cleavage planes and an edge 32 is a plane formed by etching, all of which are the mirror planes capable of reflecting the light emission in the active layer 26 and is so-called a composite resonator constitution. The light emission in the active layer 26 is reflected by the edge 31 through the light waveguide and returns to the active region 26 again.

Description

[Detailed Description of the Invention] The field of industrial application is a semiconductor laser device that can be used as a light source for optical fiber; .

Conventional technology Semiconductor lasers are used in optical fiber communications and optical information processing.

It is expected to be used as a light source for optical measurement, and efforts are being made to improve its performance. In high-capacity, long-distance optical fiber communications, single-longitudinal mode oscillation semiconductor lasers are desired. In a multimode oscillation semiconductor laser, when light propagates through an optical fiber, the arrival time of each mode differs due to wavelength dispersion of the optical fiber.

Waveform distortion will increase. Furthermore, even a single longitudinal mode oscillation semiconductor laser often oscillates in several longitudinal modes during high-speed modulation, which is a cause of limiting the relay distance of high-speed transmission systems.

In optical communication and optical information processing, single-longitudinal mode oscillation is also required when generating coherent ultrashort optical pulses from semiconductor lasers that have no internal structure in their temporal waveforms in order to improve capacity and speed.

In addition, in optical measurement, we measure extremely small displacements and refractive index changes using interference phenomena, and measure minute vibrations and displacements of material surface shapes using holography technology.

Speed measurements using spectra take advantage of laser light's good straightness or velocity focusing ability (spatial coherence) and good monochromaticity (temporal coherence), so the spectral linewidth of each oscillation is narrow and the oscillation frequency is low. A stable semiconductor laser is required.

Semiconductor lasers that perform single-longitudinal mode oscillation include distributed feedback (DFB) semiconductor lasers and distributed reflection (DBR) semiconductor lasers in which an active layer and a grating are integrated. However, if a grating is simply created, the threshold gain will be the smallest at two symmetrical wavelengths centered on the Bragg wavelength, so excavation will occur in two modes. In addition, DFB lasers also have a 7-appli bellows mode in which both output end faces serve as resonators, so in order to obtain DFB mode oscillation even at wavelengths far from the gain peak of the semiconductor laser, it is necessary to use the 7-appli bellows mode. It is necessary to suppress it.

In order to solve the above problems and obtain stable single-longitudinal mode oscillation, 0) shift the phase by λ/4 at the center of the grating. (2) Make one end surface sloped by etching. (3) A window structure is formed between the DFB region and one cleavage plane. (4) Apply AR cocoing to one end surface. (6) There are methods such as providing a phase control mechanism.

FIG. 8 shows, as a first conventional example, a schematic cross-sectional view of a λ/4 phase shift H) FB laser as shown in Arai, Nire Research Group [Semiconductor Laser J, Winter 1985, pages 71-78]. 1 is an active layer, 2 is a grating, and in the center of the grating 2 is a phase shift region 3 having a length g. FIG. 9 shows the relationship between the excavation threshold gain gth and the deviation λ-λB of the oscillation wavelength λ from exceeding the Bragg wavelength. The period of the grating is A, the diffraction order is m, and l=o
, A/4rn.

The figure shows the case of A/2rn, :3J/4m.

1 = o, that is, phase. If there is no shift region, the threshold gain will be equal in two modes symmetric with respect to the Bragg wavelength, resulting in bimodal oscillation, but if a phase shift region is provided, A difference in threshold gain occurs between these two modes. When l=A/2m=λB/4, the oscillation wavelength matches the Bragg wavelength, the threshold gain becomes the minimum, and the threshold gain difference between the main mode and the secondary mode becomes the maximum, so
Stable single-longitudinal mode excavation can be obtained.

FIG. 10 shows a schematic sectional view of a second conventional example shown in SSS IEICE University 944 by Itaya et al.

One end surface 4 of the DFB laser is a cleavage surface, and the other end surface 5 is manufactured by etching, and the slope of the surface 6 is about 560 degrees from the horizontal plane, and the reflectance of this surface for the waveguide mode is compared to that of the cleavage surface. It is approximately 1/100.

FIG. 11 shows the relationship between the oscillation threshold gain gth and the deviation λ-λB of the oscillation wavelength from the Bragg wavelength λB. As the coupling constant K becomes larger, the threshold gain becomes smaller and the separation of oscillation modes becomes more noticeable. Furthermore, since the reflectances of both end faces are greatly different, the gain curve becomes asymmetrical with respect to the Bragg wavelength, and the Fabry-Bello mode is also suppressed, realizing single-longitudinal mode oscillation.

Figure 12 shows a schematic sectional view of the third conventional example shown in S57 Telecommunications Optical and Radio Division 278. 6 is DF
Area B, 7, is a window area.

Since one end face of the DFB region 6 is embedded with a material that is transparent to the oscillation light, reflection at the boundary is negligibly small, and since the window region does not have a waveguide structure, the window region The proportion of light that is reflected at the end faces of the region and returns to the active layer is extremely small. Therefore, like the second conventional example, single mode oscillation occurs.

FIG. 13 shows a schematic sectional view of a fourth conventional example disclosed in Yamaro et al., S59 IEICE University 1o26.

One end face of the DFB laser is coated with a λ/4 thick SiN film 8 using the plasma CVD method, with a reflectance of 2%.
The following low reflection end face is used, and the second. Similar to the third conventional example, single-longitudinal mode oscillation is obtained.

FIG. 14 shows a schematic cross-sectional view of a sixth conventional example disclosed in Kitamura et al., S69 IEICE University 1o24.

This connects the semiconductor laser to the DFB region 9 and the phase control region 10.
By changing the control current 11, the refractive index is changed and the phase of the end face on the side of the control area is controlled, thereby obtaining single-longitudinal mode oscillation.

On the other hand, as shown in S59 IEICE All Dog 1022°, that is, in FIG. 16, Yamaro et al. -Longitudinal mode oscillation can be obtained.

In the first to sixth conventional examples described above, stable single-longitudinal mode oscillation is performed even during high-speed modulation, and the oscillation wavelength does not jump even with temperature changes.

FIG. 16 shows a schematic sectional view of a seventh conventional example disclosed in Japanese Patent Laid-Open No. 52-68390. 1e is an active layer, 17 is a grating, and the electrode 18 is formed only in the active region 19 where the grating is formed. Laser oscillation occurs in region 19, passes through layer 16 of optical waveguide region 2o, is reflected at cleavage plane 21, and returns to the laser section again. In this conventional example, the length of the optical waveguide is L-S-C/2 L X O, 2T where C is the speed of light, n is the refractive index, and T is the period of relaxation oscillation.

This is different from the present invention because the purpose is to suppress relaxation oscillation when the drive current is modulated in a pulsed manner.

Problems to be Solved by the Invention As mentioned above in the prior art, a semiconductor laser with an integrated active region and grating performs stable single-longitudinal mode oscillation, but when direct modulation is performed in optical communication,
A broadening of the oscillation spectrum called wavelength chirping occurs, which causes waveform distortion due to wavelength dispersion of optical fibers during long-distance transmission.

Further, when reflected light from the near end face of an optical fiber, the connector portion of an optical transmission line, or the disk surface of a multicolored optical disk system is fed back to the semiconductor laser, noise increases. In particular, when used as a light source for a video disc system that performs analog optical communication or analog signal processing, there are extremely strict requirements regarding the noise level, and noise reduction has become an important issue. When the reflectance of one end face of the semiconductor laser is low as in the third conventional example, noise is likely to occur due to this reflected return light.

In addition, the spectral linewidth of semiconductor lasers is much wider than that of gas lasers, and the actually measured value is several M) Iz
That's all. Similar values were reported for DFB and DBR lasers by Mito et al. in IEICE Technical Report 0QE84-55 (July 1984), but they are also used for coherent optical measurement such as high-resolution spectroscopy and precision measurement, and for coherent optical communication. Now, it is required to further narrow the spectral linewidth.

The present invention solves the above-mentioned conventional problems by integrating an active region consisting of an active layer and a grating, and an optical waveguide having a reflective function on the end facets on the same substrate to form a semiconductor laser into a composite resonator configuration. As a result, the spectral linewidth is narrow and wavelength chirping is suppressed even during direct modulation.
The purpose is to provide a semiconductor laser device that performs longitudinal mode oscillation, does not generate noise due to reflected return light, and can be used in a wide range of applications as a light source for analog transmission, high-speed digital transmission, coherent optical communication, optical disks, coherent optical measurement, etc. It is said that

Means for Solving the Problems The semiconductor laser device of the present invention includes a compound semiconductor substrate, at least an active layer partially formed on the substrate, a confinement layer, and a grating provided close to the active layer. an active region of a semiconductor laser including: an optical waveguide formed on the substrate adjacent to the active region and capable of guiding light emitted from the active region; and an end face of the optical waveguide remote from the active region; The semiconductor laser has a reflector that reflects the light emitted from the active region and is formed as a resonator surface of the semiconductor laser. The active layer may have a multiple quantum well structure.

Operation The present invention provides a semiconductor laser device having a composite resonator configuration in which the active region is a distributed feedback semiconductor laser and a reflector is provided at the end face of the optical waveguide, due to the above-described configuration. The distributed feedback semiconductor laser part has an asymmetric structure by coupling an optical waveguide on one side, and a difference occurs in the threshold gain of two symmetrical modes centered on the Bragg wavelength, resulting in stable single mode oscillation. .

The oscillation phase condition of a semiconductor laser having a composite resonator configuration is given by the following equation.

tanψ1=ro(r1'-1)1nψoAr1(1+
ro2)+ro(1+r12)CO5ψ0〕・・・・・・
・・・・・・(1) ・・・・・・・・・(2) eo is the optical waveguide length, 11 is the active region length, “Ol”1
are the refractive index of the optical waveguide and the active layer, respectively, and rolr
l is the amplitude reflectance of the optical waveguide end face and the coupling portion between the optical waveguide and the active layer, respectively. When a semiconductor laser is directly modulated with current, the refractive index of the active layer changes by Δn and the single oscillation wavelength changes by Δλ, which results in wavelength chirping. The wavelength change when there is no optical waveguide is Δλ−(Δn1/
n1) λ0, and the degree of suppression of wavelength chirping with respect to modulation can be determined by Δλ/Δλ'.

FIG. 2 shows the relationship of Δλ/dλ' for 'nO"O/"1 false, with ro as a parameter. As can be seen from the figure, in order to suppress wavelength chirping, it is necessary to shorten the active region length, increase the optical waveguide length, increase the reflectance of the optical waveguide end face, and improve the coupling between the active layer and the optical waveguide. It can be seen that it is sufficient to reduce the reflectance of the area.

It has been reported by Saito et al., IEICE Technical Report, that the oscillation spectrum linewidth can also be reduced by using a double-valve oscillator configuration.
○It is shown in QEa1-62 etc. FIG. 3 shows the relationship between the feedback parameter X and the amount of decrease Δν/Δν' in the spectral linewidth. As can be seen from the figure, the larger the X parameter is, the shorter the active region length is, and the longer the optical waveguide length is, as in the case of suppressing wavelength chirping. The smaller the reflectance of the coupling portion with the optical waveguide, the smaller the spectral linewidth.

R, Wyatt: 8th Conf, on
OpL Fiber Comm.

Tup 2, Feb., 1985. San D
In California, it has been reported that the spectral linewidth was reduced to 1 KHz using a composite cavity configuration with a mirror placed outside the semiconductor laser. However, when considering miniaturization of the device, mechanical stability, stability against temperature changes, and reliability, it goes without saying that it is more desirable to integrate the composite resonator on one substrate.

In order to suppress wavelength chirping and spectral linewidth, if the reflectance of the optical waveguide end face is increased, the amount of light that is returned from the optical waveguide side to the active region increases, and a small amount of light is returned from the output end face side of the active region. Even if the laser beam returns, it is less affected by it, and the noise caused by the reflected return light, which has been a big problem with conventional semiconductor lasers, is no longer generated.

The above-mentioned suppression of wavelength chirping, narrowing of the spectral linewidth, and suppression of noise due to reflected return light are effects obtained by using a composite resonator configuration that returns light to the active region of the semiconductor laser. . Conventional distributed reflection type (
DBR) Semiconductor laser or the semiconductor laser shown in the seventh conventional example has a composite resonator configuration, but DB
With the R laser, the amount of light returned to the active region was not very large.
+A? The seventh conventional example aims to suppress relaxation oscillations, so the optical waveguide length is limited to a specific value, and the amount of light returned to the active region is limited. This is not particularly defined and is different from the present invention.

Embodiment FIG. 1 is a sectional view of an embodiment of the present invention. In FIG. 1, a grating 23 is formed on an n-type InP substrate 22, an n-type InGaAsP optical waveguide layer 24 (band gap Eg=1.18 eV), an n-type InP isolation layer 25, an n-type 1nGaAsP active layer 28 (Eq. =0.95eV)
, a region 28 having a structure in which p-type InP confinement layers 27 are laminated is an active region, and is a so-called distributed feedback semiconductor laser. On the other hand, a region 29 where only the optical waveguide layer 24 exists on the substrate 22 is an optical waveguide. In this structure, the end surfaces 30 and 31 are cleaved surfaces, and the end surface 32 is a surface formed by etching, and both surfaces are mirror surfaces capable of reflecting light emitted from the active layer 26, forming a so-called composite resonator configuration. ing.

The light emitted from the active region 26 passes through the optical waveguide to the end surface 31.
is reflected and returned to the active region 26 again.

The amount of light returned is determined by the waveguide efficiency of the optical waveguide 24 and the coupling efficiency between the active region 28 and the optical waveguide region 29, but these values can be changed arbitrarily to some extent by changing the structural parameters of the element. It is. In order to increase the amount of feedback light, it is also possible to easily coat the optical waveguide end face 31 with a reflection increasing film to increase the reflectance.

It goes without saying that the cross-sectional structure in the direction perpendicular to the resonator direction is desirably formed into a striped structure by a conventional method. Further, in this example, the material was InGaAsP crystal on an InP substrate, but other compound semiconductor materials such as A I Ga on a GδAs substrate were used.
The structure of the present invention can be easily realized even with As-based crystals or InGaAsP-based crystals on a substrate.

FIG. 4 shows the oscillation spectrum when the semiconductor laser of the example is directly modulated and the modulation frequency is varied. Bias current b=13Ith, modulation depth 100ch. As shown in the figure, there is no spectrum broadening due to multimode oscillation, and adjacent submodes are completely suppressed.

Figure 6 shows the temperature characteristics of the oscillation wavelength. The oscillation wavelength changed smoothly with no mode jump, and stable single-longitudinal mode oscillation was obtained.

FIG. 6 shows the relationship between modulation current and chirping width (full width at half maximum) measured for elements with different optical waveguide lengths. The modulation frequency is 10 Q MH2. As is clear from the figure, a device with a long optical waveguide length was effective in suppressing wavelength chirping. Furthermore, even if the optical waveguide length is the same,
When the optical waveguide end face was coated with Au to increase the amount of light returned to the active region, wavelength chirping could be further suppressed. It is thought that these wavelength chirpings can be further suppressed by changing the structural parameters of the device.

In addition, the spectral linewidth was measured by the delay self-heterodia method using a delay fiber of 5 km, and the results showed that the longer the optical waveguide length, the narrower the linewidth.It became even narrower by coating with Au, and it was easy to reach I MHz.
We were able to obtain the following spectral linewidths.

Next, a 2.2 m long optical fiber was coupled to the output end face, and the results of observing noise due to reflected return light from the fiber will be described. Figures 7a and 7b show the results of measuring relative noise intensities for an element without an optical waveguide and an element with an optical waveguide, in which the near end of the optical fiber is processed into a spherical lens and the far end is made into a vertical end face. As shown in Figure 7a, in the device without an optical waveguide, noise due to reflected return light from the fiber far end is observed at frequencies corresponding to the round trip frequency of 45 MH2 and its harmonics up to the optical fiber far end. It was done. On the other hand, in the device of this example with an optical waveguide, no noise due to reflected return light was observed, as shown in FIG.

The behavior of the relaxation oscillation when the drive current is modulated in a pulsed manner is thought to change depending on the structure of the element, etc.
In this example, no significant relaxation oscillation was observed in any of the elements with different optical waveguide lengths.

In this example, if the active layer has a multiple quantum well structure, wavelength chirping during direct modulation can be further suppressed.
The threshold current is lower, the temperature dependence of the threshold current value is also reduced, and more stable laser oscillation can be obtained with lower power consumption.

Effects of the Invention As described above, the present invention has an active region consisting of an active layer and a grating, and an optical waveguide having a reflective function on the end face is integrated on the same substrate to form a semiconductor laser into a composite resonator configuration. , it is possible to realize stable single-longitudinal mode oscillation by reducing the spectral linewidth and suppressing wavelength chirping during direct modulation, and furthermore, it is possible to suppress noise due to reflected return light. Therefore, a wide range of applications are possible, including analog transmission systems that require noise reduction, high-speed digital communications, coherent optical communications, and coherent optical measurements.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a semiconductor laser device according to an embodiment of the present invention;
Figure 2 is a diagram showing the theoretical value of the wavelength chirping suppression degree of this example, Figure 3 is a diagram showing the theoretical value of the amount of spectral linewidth reduction in this example, and Figure 4 is a diagram showing the oscillation spectrum of this example. FIG. 6 is a diagram showing the temperature characteristics of the excavation wavelength of this example. FIG. 6 is a diagram showing the measurement results of the chirping width of this example. FIG. Figure 8 shows the measurement results of the relative noise intensity of the laser. Figure 8 is a schematic cross-sectional view of the first conventional example. Figure 9 shows the oscillation threshold gain and deviation of the oscillation wavelength from the Bragg wavelength of the first conventional example. A diagram showing the relationship, FIG. 10, is a schematic cross-sectional view of the second conventional example. 22... Compound semiconductor substrate, 23... Grating, 24... Optical waveguide layer, 25...
... Separation layer, 26 ... Active layer, 27 ...
...Confinement layer, 28...Active region, 29...
...Optical waveguide region, 30, 31°32...
Resonator end face. Fig. 1 Fig. 2 nρ no σ//I, Nol 3rd illustration included () resistance Fig. 5 Fig. 6/R key/f, A (7F+, 4P-P) 3SION
EAIIV73M Section 8 Fig. 3 Phase shift 4 Page 9 Fig. 9 σ λ-person e Fig. 10 2 R1-TEL 1 Fig. U λ-λB Fig. 12 Fig. 13 Fig. 14 @ Figure 15 Procedural amendment form) l Indication of the case 1985 Patent application No. 29608 2 Name of the invention Semiconductor laser device 3 Person making the amendment Relationship to the case Patent application Address 1006 Oaza Kadoma, Kadoma City, Osaka Prefecture Address name
(582) Matsushita Electric Industrial Co., Ltd. Representative Yama
Toshihiko Shimo 4 Agent 571 Address 1006 Oaza Kadoma, Kadoma City, Osaka Matsushita Electric Industrial Co., Ltd. June 28, 1985 Wyatt: 8th Conf, on Opt, Fib
er C0mm,,Tup2, Feb+, 1985
, San DLeqo, Ca1ifornia
J. R. Wyatt 8th Conference on Fiber Optic Communications, Tup 2. February 1985, San Diego, California (R. Wyatt: 8 th
Conf, onOpt, Fiber Comm
,,Tup2. Feb, 1986, SanD
iego, Cat 1fornia) Corrected to J.

Claims (2)

[Claims] (1) A compound semiconductor substrate, an active region of a semiconductor laser including at least an active layer partially formed on the substrate, a confinement layer, and a grating provided close to the active layer, and close to the active region. an optical waveguide formed on the substrate and capable of guiding light emitted from the active region; 1. A semiconductor laser device comprising: a reflector formed as a resonator surface; (2) The semiconductor laser device according to claim 1, wherein the active layer of the semiconductor laser has a multiple quantum well structure.