JPS6412115B2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a semiconductor laser that exhibits stable laser oscillation characteristics in a single longitudinal mode, and in particular to a wavelength-controlled semiconductor laser that suppresses fluctuations in oscillation state due to externally reflected light. Related to laser oscillation method.

[Technical background of the invention and its problems]

Wavelength-controlled semiconductor lasers such as DBR (distributed blur reflection) lasers and DFB (distributed feedback) lasers can achieve stable laser oscillation in a single longitudinal mode, so they are attracting attention as light sources for wavelength multiplexed optical transmission and optical applied measurement. has been done.

In principle, a wavelength-controlled semiconductor laser uses the single wavelength selectivity of a Bragg reflector due to periodic changes in refractive index to use only one wavelength as feedback light for laser oscillation. A very sharp oscillation spectrum can be obtained. Furthermore, since this oscillation wavelength is fixed by the reflection wavelength of the Bragg reflector, it has the characteristic that wavelength fluctuations due to temperature, modulation, etc. are extremely small.

However, this type of laser has the following problems. In other words, because the wavelength selectivity is too sharp, the fluctuation of the laser oscillation state due to reflected light is large. This occurs because reflected light that occurs at a coupling part such as an optical fiber causes a feedback phenomenon as an external fluctuation, and the narrower the oscillation spectrum, the greater the effect.

Conventionally, methods such as suppressing reflected light using a beam splitter or non-reflective coating, or expanding the spectral width using high frequency superposition have been used as countermeasures against this problem.
However, these methods have problems such as difficulty in integration and miniaturization and the complexity of the modulation device. Further, the method of suppressing reflected light alone does not provide a sufficient countermeasure effect, and also has drawbacks such as lowering the output light intensity.

[Purpose of the invention]

An object of the present invention is to provide a method for oscillating a wavelength-controlled semiconductor laser that can suppress fluctuations in the laser oscillation state due to externally reflected light without hindering integration, miniaturization, etc.

[Summary of the invention]

The reason why wavelength-controlled semiconductor lasers are easily affected by reflected light is that their spectral widths are narrow, as described above.However, the present invention reduces the influence of reflected light by widening the spectral width of wavelength-controlled semiconductor lasers. It is something to improve. That is, the gist of the present invention is to obtain a wide spectrum width by partially changing the coupling constant of propagating light.

Hereinafter, an overview of the present invention will be explained with reference to the drawings. Note that the explanation will be given here using a DFB laser as an example.

Figure 1a is a perspective view showing the schematic structure of a conventional DFB laser, Figure 1b is a perspective view showing the diffraction grating portion of this laser, and Figure 1c is a side view showing the light extraction surface. 2 is a substrate, 2 is a waveguide layer, 3 is an active layer, 4 is a cladding layer, and 5 is a diffraction grating.
The DFB laser has a diffraction grating 5 provided on a substrate 1.
distributed optical feedback is performed. This enables optical amplification and laser oscillation. Here, since the light to be fed back is limited to a wavelength that satisfies the Bragg reflection condition of the diffraction grating 5 as described above,
As shown in FIG. 2a, oscillation is possible only at discrete values. Moreover, it is the Bragg reflection wavelength λb
Oscillation is possible across a forbidden band width Wb centered around λb, and the farther a mode is from λb, the more difficult it is to oscillate.
n in FIG. 2a is a value indicating discrete mode numbers. Normally, near the oscillation threshold, two modes, n = 1 and n = -1, try to oscillate, but the mode with even a slightly larger gain oscillates first, and remains as a single longitudinal mode. Laser oscillation occurs. This is the oscillation principle of the DFB laser, and the wavelength at which the laser oscillates is determined by the Bragg reflection wavelength λb and the forbidden band width Wb, as described above. On the other hand, the forbidden band width Wb is determined by the equivalent refractive index of the optical waveguide (cladding layer 2 and active layer 3) of the DFB laser, the coupling coefficient K between the traveling wave and the reflected wave, etc. (hereinafter referred to as Wb) is as shown in Figure 2b. In other words, the larger K is, the wider Wb becomes.

The present inventors focused on this point, and by making one laser have two values (or more) for the forbidden band width Wb and keeping each value sufficiently close, the wavelength can be slightly reduced. We found that oscillation occurs in two different modes. Furthermore, since these wavelengths are close enough, the oscillation spectrum becomes such that they are optically coupled and oscillate in one mode.
It was found that the width of the oscillation spectrum was widened. Further, according to further intensive research by the present inventors, as a means for making the forbidden band width Wb have two or more values, the coupling coefficient K We found that it is effective to control the value of .

That is, the present invention provides a wavelength-controlled semiconductor laser in which an optical waveguide is provided with a diffraction grating of a predetermined period.
2, which is at least approximate to the coupling coefficient K between the traveling wave and the reflected wave of the laser beam in a direction perpendicular to the laser traveling direction in the plane in which the above-mentioned diffraction grating is provided.
It is designed to have two values.

[Embodiments of the invention]

FIG. 3 is a side view showing a schematic structure of a DFB laser according to an embodiment of the present invention. 11 in the figure is N
- An InP substrate, on which a striped groove 12 is formed as shown in a perspective view in FIG. 4, and furthermore, a diffraction grating 13 is formed on the substrate 11 including this groove. On the substrate 11 on which such grooves 12 and diffraction gratings 13 are formed, N
InGaAsP waveguide layer 14 (λ=1, 2 μm), InGaAsP
An active layer 15 (λ=1, 3 μm) and a P—InP cladding layer 16 are laminated in sequence. Then, a P-side electrode is deposited on the cladding layer 16 via a contact layer, and an N-side electrode is deposited on the lower surface of the substrate 11.
A DFB laser is configured.

Next, the operation of the embodiment laser configured as described above will be explained.

Since the groove portion 12 is formed on the substrate 11, the distance from the active layer 15 to the diffraction grating 13 is
In the direction perpendicular to the resonator direction, the central portion and both ends are different. If we consider this separately, the distance between the active layer 15 and the diffraction grating 13, that is, the thickness of the optical waveguide 14, is thin at both ends, as shown in Figure 5a, and the coupling coefficient at this time is
Let's call it K1. On the other hand, since the thickness of the optical waveguide layer 14 is thick in the central part, as shown in FIG. 5b,
The coupling coefficient at this time is K2. Although K1 and K2 are similar, K1>K2, and K1 has a slightly wider forbidden band width than K2. In other words, K1
is slightly further away from the oscillation frequency λb than K2.

Therefore, the example laser using the diffraction grating 13 that combines FIG. 5 a and b is a mixture of K1 and K2.
It becomes a DFB laser. As shown in FIG. 6a, the mode characteristics at this time are such that a mode is generated that is shifted by the wavelength determined by the difference between K1 and K2. Here, assuming that the DFB laser shown in Figure 3 oscillates in the n=1 mode in Figure 6, the two wavelengths λ1 and λ2 shown in Figure 6a will cause laser oscillation. become. As shown in Figure 6b, if the wavelength difference between λ1 and λ2 is only a few [Å] and the intensity of the oscillation spectra is almost the same, then the two oscillation frequencies λ1 and λ2
are optically coupled by their slight wavelength spread, resulting in an oscillation spectrum as if they were oscillating at one wavelength, as shown by the solid line. This makes it possible to obtain a DFB laser with a broadened oscillation spectrum width while having an oscillation wavelength fixed by the Bragg reflection conditions of the diffraction grating.

Thus, according to this embodiment, although it is a DFB laser, its oscillation spectrum width can be widened. Therefore, fluctuations in the laser oscillation state due to externally reflected light can be suppressed, and various applications such as DAD are possible. Furthermore, since there is no need to provide a beam splitter, antireflection film, etc., it is extremely effective for integration and miniaturization.

Note that the present invention is not limited to the embodiments described above. For example, a similar effect can be obtained by using a DFB laser in which a striped convex portion is provided in the portion where the diffraction grating is to be formed, as shown in FIG. 7, instead of the groove portion. Furthermore, the coupling coefficient does not necessarily have to have two values, and may have more than two values.
In other words, in addition to the striped grooves and protrusions, the eighth
As shown in the figure, the flatness of the portion where the diffraction grating is to be formed may be varied continuously to vary the coupling coefficient K in an arcuate manner. Furthermore, the same phenomenon effect can be obtained not only with DFB lasers but also with DBR lasers, and it is possible to apply the present invention to various other wavelength-controlled lasers. In addition, various modifications can be made without departing from the gist of the present invention.

[Brief explanation of drawings]

Figures 1 a to c are perspective views and side views showing the structure of a conventional DFB laser, and Figures 2 a and b are the above.
A schematic diagram to explain the operating characteristics of a DFB laser,
FIG. 3 is a side view showing a schematic structure of a DFB laser according to an embodiment of the present invention, FIG. 4 is a perspective view showing the shape of a diffraction grating used in the laser of the above embodiment, and FIG.
Figures a and b and Figures 6a and 6b are schematic diagrams for explaining the laser action of the above embodiment, respectively, and Figures 7 and 8 are side views respectively showing the structure of a modified example. DESCRIPTION OF SYMBOLS 11... N-InP substrate, 12... Striped groove part, 13... Diffraction grating, 14... N-InGaAsP optical waveguide layer, 15... InGaAsP active layer, 16... P-InP
Cladding layer, 17...Striped convex portions.

Claims (1)

[Claims] 1. A diffraction grating with a predetermined period is provided in the optical waveguide,
Using a wavelength-controlled semiconductor laser in which the coupling coefficient between the traveling wave of the laser beam and the reflected wave has at least two similar values in a direction perpendicular to the traveling direction of the laser beam in the plane in which the diffraction grating is provided. A method for oscillating a wavelength-controlled semiconductor laser, characterized by causing two laser oscillations that are optically coupled and oscillate in one mode, with substantially the same intensity and slightly different wavelengths. . 2 In the plane in which the diffraction grating is provided,
2. The method according to claim 1, wherein stripe-shaped grooves are formed along the traveling direction of the laser beam, and the diffraction grating is formed including the grooves. 3 In the plane in which the diffraction grating is provided,
The method according to claim 1, wherein striped convex portions are provided along the traveling direction of the laser beam, and the diffraction grating is formed including the striped convex portions. .