JPS60111488A - Wavelength control type semiconductor laser - Google Patents

Abstract

PURPOSE:To inhibit the variation of the state of laser oscillation by external reflected beams by giving a coupling coefficient between the progressive waves and reflected waves of laser beams at least two values and widening the spectral width of a wavelength control type semiconductor laser. CONSTITUTION:Groove sections 12 are formed on a substrate 11, and a distance up to a diffraction grating 13 from an active layer 15 differs at a central section and both end sections in the direction orthogonal to the direction of a resonator. The thickness of an optical waveguide 14 is thinned at both ensections and thickened at the central section, and the coupling coefficients of both end sections and the central section are made K1 and K2. A laser using the diffractin grating 13 is brought to a DFB laser having a coupling coefficient in which K1 and K2 are mixed. When the DFB laser is laser-oscillated at a mode of n=1, two wavelengths of lambda1 and lambda2 are oscillated. When the difference of the wavelengths of lambda1 and lambda2 is slight, two oscillating wavelengths of lambda1 and lambda2 is slight, two oscillating wavelengths couple by several slight wavelength extent, and oscillating spectrum, the width thereof is widened, which looks as through being oscillated by one wavelength is obtained.

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 the oscillation state due to externally reflected light. Regarding lasers.

[Technical background of the invention and its problems]

Wavelength-controlled semiconductor lasers such as DBR (distributed Bragg reflection) lasers and DFB (distributed feedback) lasers can achieve stable laser oscillation in a single longitudinal mode. It is possible to obtain a very sharp oscillation spectrum. Moreover, since this oscillation wavelength is fixed by the reflection wavelength of the Bragg reflector, wavelength fluctuations due to temperature, modulation, etc.
It has the characteristic that p is always 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 variable W4 device. In addition, the present invention provides a wavelength-controlled semiconductor laser that can suppress fluctuations in the laser oscillation state due to externally reflected light without reducing the output light intensity (5'), which would be insufficient to obtain a sufficient countermeasure effect by suppressing reflected light. Do: rhP.

(Summary of the invention) This invention improves the effects of light. 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 by taking a DFB laser as an example.

FIG. 1(a) is a perspective view showing the schematic structure of a conventional DFB laser, FIG. 1(b) is a perspective view showing the diffraction grating portion of this laser, and FIG. 1(C) is a side view showing the light extraction surface. In the figure, 1 is a substrate, 2 is a waveguide layer, 3 is an active layer, 4 is a cladding layer, and 5 is a diffraction grating. In the DFB laser, distributed optical feedback is performed by a diffraction grating 5 provided on the substrate 1. This enables optical amplification and laser oscillation. Here, as mentioned above, the reflected light is limited to a wavelength that satisfies the Bragg reflection condition of the diffraction grating 5, and can oscillate with a forbidden band width wb centered at λb), and from λb to The distant mode with a larger mode weight causes oscillation first, and single-longitudinal mode laser oscillation occurs.This is the oscillation principle of the DFB laser, but as mentioned above, the wavelength of laser oscillation is the Bragg reflection wavelength (λb ) and forbidden band width (Wb).

On the other hand, the forbidden band width (Wb>) is the equivalent refractive index of the optical waveguide (cladding layer 2 and active layer 3) of the DFB laser.

It is a quantity determined by the coupling coefficient between the traveling wave and the reflected wave, etc., and the relationship between K and forbidden band width (hereinafter referred to as wb) is shown in Figure 2 (
b). In other words, the larger 5- is, the wider wb becomes.

The inventors of the present invention focused on such points, and the above-mentioned prohibited band width w
It has been found that by providing two (or more) values for b in one laser and keeping each value sufficiently close, oscillation occurs in two modes with slightly different wavelengths. Furthermore, we have found that these are effective.

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, in which a traveling wave of a laser beam and a reflection are generated in a direction perpendicular to the laser traveling direction in a plane in which the diffraction grating is provided. The wave coupling coefficient K has at least two values.

6- [Embodiment of the Invention] FIG. 3 is a side view showing a schematic structure of a DFB laser according to an embodiment of the invention. In the figure, reference numeral 11 denotes an N-InP substrate. On this substrate 11, a striped groove 12 is formed as shown in a perspective view in FIG. ing. On the substrate 11 on which the groove portion 12 and the diffraction grating 13 are formed, an N-TnGaASP waveguide layer 14 (λ=i, 2 μm>,
InGaASP active layer 15 (λ-1, 3 μm) and %P-
1nP cladding layers 16 are sequentially laminated. Then, a contact layer is placed on the cladding layer 16.
jB′F, and then deposit the P-side electrode, and apply N on the lower surface of the substrate 11.
A DFB laser is constructed by attaching side electrodes.

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 different between the central portion and a portion of both ends 7 in the direction perpendicular to the resonator direction. 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 n14, is thin at both ends, as shown in FIG. 5(a), and the coupling coefficient at this time can be calculated as follows. Set to 1. On the other hand, in the central portion, the optical waveguide layer 14 is thick, as shown in FIG. 5(b), and the coupling coefficient at this time is set to 2. The relationship between K1 and 1<2 is 1>K2, and K1
has a wider forbidden band width than that of 2. In other words, K1
is further away from the oscillation wavelength λb by 2.

Therefore, in FIG. 5, the diffraction grating 1 combining (a>(b))
In the example laser using 3, a mode group is generated with a wavelength difference determined by the difference between 1 and 2 of K2. Here, we will move on to the age shown in Figure 3. Moreover, λ1. If the wavelength difference between λ2 is only a few [X], the two oscillation wavelengths λ1゜λ
2 are combined by a slight wavelength offset, resulting in an oscillation spectrum as if they were oscillating at one wavelength, as shown by the solid line in FIG. 6(b). This makes it possible to obtain a DFB laser with a wide oscillation spectrum width while having an oscillation wavelength fixed by the Bragg reflection condition 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.

Although the present invention is limited to the above-mentioned embodiments, the portion where the diffraction grating is to be formed may have a stripe l −9− −< or more as shown in FIG.

In other words, in addition to the striped grooves and protrusions, the eighth groove may be used. Further, the same phenomenon effect can be obtained not only with a DFB laser but also with a DBR laser, 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 schematic diagrams explaining the operating characteristics of the DFB laser. , 3rd
The figure is a side view showing the schematic structure of a DFB laser according to an embodiment of the present invention, FIG. 4 is a perspective view showing the shape of the diffraction grating used in the laser of the above embodiment, and FIGS. 6th
Figures (a) and 10-(b) are schematic diagrams for explaining the laser action of the above embodiment, respectively, and Figures 7 and 8 are respectively modified. Figure 4 Figure 3 Figure 5 (a) (b) 1 to 2 Figure 6 λB (7 but 5 skin length) (b) λ2λ1

Claims (1)

[Scope of Claims] (1) In a wavelength-controlled semiconductor laser in which an optical waveguide is provided with a diffraction grating of a predetermined period, the traveling direction of the laser beam in the plane in which the diffraction grating is provided is the same as the traveling direction of the laser beam. 2. The wavelength-controlled semiconductor laser according to claim 1, wherein a stripe-shaped groove is formed along the groove, and the diffraction grating is formed including the groove. (3) Striped convex portions are provided in the plane on which the diffraction grating is provided along the traveling direction of the laser beam, and the diffraction grating is formed including the striped convex portions. A wavelength-controlled semiconductor laser according to claim 1, characterized in that: