JPH02159784A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To enable a semiconductor laser of this design to output laser rays as small in a spectral line width as possible by a method wherein an equivalent refractive index difference in a lateral direction is set to a value as small as a refractive index waveguide structure can secure. CONSTITUTION:A equivalent refractive index difference DELTAn in a lateral direction is set to a value as small as a refractive index waveguide structure can secure, and the width of the optical waveguide region of a semiconductor laser active layer 5 is set to a value which is corresponding to the set equivalent refractive index difference DELTAn and the maximum value within a range that laser rays of modes other than a single lateral mode can be cut off. The equivalent refractive index DELTAn, which is determined by an equivalent refractive index nVeff of an active layer in a longitudinal direction and an equivalent refractive index nC of a current constriction layer 53, is required to satisfy a formula, DELTAn>=0.01, to secure a stable refractive index waveguide structure. By this setup, the junction of natural emission rays with an oscillation mode can be made small, so that a semiconductor laser which can output laser rays small in a spectral line width can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an index-guided semiconductor laser with a narrow spectral linewidth that can be used in the field of optical communication and measurement.

(Prior Art) In the field of optical communications, large-capacity communication systems of several Gbit/s have now been realized, and research is actively underway to realize even longer distance and larger-capacity communication systems.

Coherent optical communication, which is one of these research themes, is based on optical communication using the normal direct detection method (
■...It is expected that there will be a significant improvement in reception sensitivity, ■...
The frequency of the light obtained by semiconductor lasers is much higher than the electromagnetic waves traditionally used for communication and measurement, so it is attracting the most attention because it has features such as enabling even more frequency multiplexed transmission. There is.

However, in order to realize such coherent optical communication, a semiconductor laser having a very narrow spectral linewidth of I MHz or less is required. Further, such a semiconductor laser is extremely useful not only in the field of optical communication but also in fields such as coffee left optical measurement.

Therefore, research on semiconductor lasers with narrow spectral linewidth has been carried out energetically for some time, and examples of this research include the literature (Proceedings of the 1986 IEICE Semiconductor and Materials Division National Conference, p. 2-50). ) was disclosed.

This document describes the results of a study on the dependence of the spectral linewidth on the collusion length in a B-type semiconductor laser. This study was conducted by focusing on the fact that the spectral linewidth Δυ of a semiconductor laser generally decreases by increasing the cavity length, as expressed by the following formula. [(h・υO・n0sp) /
8TTPO]X (C/nEFF L) 2X
(lnRaz・L)×(10 days)×(1+α2)
・・・・・・■ However, in the formula 0, h is a blank constant, ν
. is the oscillation frequency, n0sp is the carrier density that produces spontaneous emission light coupled to the oscillation mode, P is the output power from the end face of the resonator, C is the speed of light, neff is the effective refractive index, and L is the resonator. The length and day are the end face reflectance, αL is the resonator internal loss, and α is the linewidth increase coefficient.

The actual study in the above literature is based on the oscillation wavelength or 1.551J.
This is done using a BH-DFB type semiconductor laser. FIG. 4 is a diagram for explaining a BH-DFB type semiconductor laser, and is a perspective view schematically showing the semiconductor laser with a portion thereof cut away. Note that, in order to avoid complicating the drawings, hatchings indicating cutout portions are omitted.

First, select an appropriate pitch (DFB with an oscillation wavelength of 1.55 um)
In the case of a type semiconductor laser, an n-type GaInAs is
P optical waveguide layer 13, GaInAsP active layer 15, p-type In
A laminate 21 is formed by growing the P crat layer 17 and the p-type GaInAsP layer 19 in this order.Next, this laminate 21 is formed to have a width W+ of the optical waveguide region of the active layer 15.
The laminated body 21 is processed by a photoetching technique so that the thickness is approximately 1 um and the cross section of the laminate 21 is in the shape of an inverted mesa. Next, on the n-type InP substrate 11, a p-type 1nP layer 23 and an n-type InP layer 25 are grown in this order by second crystal growth so as to embed the inverted mesa-shaped stacked body 21, and then the p-type 1nP layer 23 and the n-type InP layer 25 are grown in this order. An n-side electrode 27 is formed on the GaLn A SP cap layer 19 and the n-type InP layer 25, and an n-side electrode 29 is formed under the n-type InP substrate.

The above-mentioned literature describes the spectral linewidths of BH-DFB lasers with a cavity length L% of 300 um when operated to obtain various optical outputs, and the cavity length L a of 1200 um. The horizontal axis represents the reciprocal of the optical output P-'%, and the vertical axis represents the spectral linewidth Δν. A plotted characteristic curve diagram as shown in Fig. 5 is published. According to this, by increasing the resonator length from 300 um to 1200 um, △ν becomes about 1/4, and L = 1200
A minimum spectral linewidth of 2 to 3 MHz has been obtained in the case of a semiconductor laser with a diameter of m.

(Problem to be solved by the invention) However, in the case of a DFB type semiconductor laser, when the cavity length is lengthened, the distribution of light intensity in the cavity length direction becomes non-uniform, resulting in spatial ball-burning. Multi-mode oscillation becomes easier. Therefore, the resonator length cannot be made very long.

In fact, even if the cavity length L is set to 1200 um, the yield of a semiconductor laser that oscillates in a single longitudinal mode will be extremely small, and it is not practical to lengthen the cavity length. Not on target.

For this reason, the method of increasing the resonator length requires that the minimum spectral linewidth obtained is 2 to 3 MHz, which is required for a light source for coherent optical communication.
It is not possible to obtain a semiconductor laser that can output laser light with a spectral line width of MHz or less.

This invention was made in view of these points,
Therefore, an object of the present invention is to provide an index-guided semiconductor laser that can output laser light with a narrower spectral linewidth.

(Means for Solving the Problem) In order to achieve this objective, the inventor of this application has made extensive studies. First, we reexamined the causes of widening the spectral linewidth of semiconductor lasers, and first examined whether there were any factors other than the resonator length that could narrow the spectral linewidth. This will be explained first.

The most fundamental cause of widening the spectral line width of semiconductor lasers is spontaneous emission. In other words, even during laser oscillation, spontaneous emission light is randomly generated, and a portion of it is coupled to the laser oscillation moat and becomes noise that widens the spectral line width. CR is the proportion of spontaneously emitted light generated during laser oscillation that is coupled to the laser oscillation moat. In other words, C
Defining R (amount of spontaneous emission light converted to one resonator moat)/(total amount of spontaneous emission light generated), this CR can be expressed as the following formula in the vicinity of the laser oscillation wavelength. .

where " is the optical confinement coefficient, the input is the oscillation wavelength, Va is the volume of the optical waveguide region of the active layer, and △λ is the half-width of the spontaneous emission light intensity spectrum. This Va is expressed in detail by the following formula 0. It is defined by

Va=Wa-d, -L-...■ Where, W8 is the width of the optical waveguide region of the active layer, and d8 is the thickness of the optical waveguide region of the active layer.

Here, from the meaning of the definition, the above OR has a relationship with n0sp (carrier density that generates spontaneous emission light coupled to the oscillation mode) in the equation 0 already explained as shown in the following equation 0. .

where h is a blank constant and ν. is the oscillation frequency, ν is the frequency of one resonant mode, n5p(ν) is the carrier density that produces spontaneous emission light coupled to the moat of frequency υ, and D(ν) is the radiation electromagnetic field mode density.

Therefore, it can be seen that from the above equations ■, ■, ■ and 0, the following relationship can be derived (however, ``.

is the optical confinement coefficient in the horizontal direction, and ヮ is the optical confinement coefficient in the vertical direction.

Therefore, as can be understood from Equation 0, it is possible to change Δv by changing the width W8 of the optical waveguide region of the active layer.

However, it is also true that in order to stably oscillate a semiconductor laser in a single fundamental transverse mode, the width W of the optical waveguide region of the active layer must be a condition that can cut off the transverse higher-order modes. be. If Wa is wider than this cutoff condition, transverse higher-order modes will also be guided.

Therefore, the inventor of this application focused on and studied the horizontal equipolarized refractive index difference that can constitute a refractive index waveguide. Figure 2 shows an example of the results of this study.
BH-D explains the structure of a semiconductor laser using Fig. 4.
The structure is an FB type semiconductor laser, the energy key cap wavelength input of the active layer is y @ 1.55 LI 171, the energy key gap wavelength λ of the optical waveguide layer is %1.310 m, and the thickness of each of these layers is 0. lum and using the refractive index nc of the current confinement layer that embeds the sides of the active layer as a parameter, calculate r s / W when Wa is changed using the well-known equipolarized refractive index method (determined from this). , the result - is plotted with W8 on the horizontal axis and rH/W, ! on the vertical axis. Incidentally, nc is 3, +7.3. +
The values were set to 9°3.2+, 3.23, and 3.25. Further, each point indicated by a mark in FIG. 2 is a cutoff condition for the transverse higher-order mode when the refractive index nc of the current confinement layer has the above-mentioned values. As can be understood from Fig. 2, by increasing the refractive index nc of the current confinement layer, the equal polarized refractive index difference Δn in the horizontal direction (' in the case of a BH-DFB laser, ′ in the vertical direction of the active layer It has been found that by decreasing the difference Δn) between the homopolarized refractive index of the current confinement layer and the homopolarized refractive index of the current confinement layer, the width that satisfies the cutoff condition of the transverse high-order moat can be increased. It goes without saying that if the equipolarized refractive index difference Δn becomes too small by 1, it will not be possible to secure a waveguide structure that utilizes the refractive index difference.

Therefore, the present invention provides, in an index-guided semiconductor laser, a lateral equipolarized refractive index difference Δnu, which is set to a smaller value within the range that can be secured by the refractive index waveguide structure. The width of the optical waveguide region is set to the maximum dimension within a range that corresponds to the set equipolarized refractive index difference and can cut off modes other than the single fundamental transverse mode.

(Function) According to such a configuration, the horizontal width of the optical waveguide region of the active layer, which is the cutoff condition for the higher-order moat, can be increased by the reduction of the equipolarized refractive index difference in the lateral direction. Therefore, Wa in the above equation 0 can be made thicker, and as a result, the subeuttle line width Δ
υ becomes a small value.

(Example) Examples will be described below using an example of a refractive index waveguide type semiconductor laser uBH-DFB type semiconductor laser. FIG. 1 is a diagram for explaining a DFB-type semiconductor laser according to an embodiment, and is a perspective view schematically showing a partially cut away semiconductor laser. Note that, in order to avoid complicating the drawings, hatchings indicating notches are omitted.

The semiconductor laser of this example has gratings 41 with a predetermined pitch (in this example, a DFB laser with an oscillation wavelength of 1.55 um is used, so a pitch of approximately 2400 people).
On the n-type InP substrate 41 on which a is formed, an n-type GaIn
A stacked body 51 consisting of an AsP optical waveguide layer 43, a GaInAsP active layer 45, a p-type LnP crat layer 47, and a p-type GaInAsP cap layer 49 and having an inverted mesa shape in cross section taken in a direction perpendicular to the laser stripe is inspected. A current confinement layer is provided in which a p-type GaInAsP layer 53 and an n-type GaInAsP layer 55 are grown in this order so as to bury this stacked body 51. Roughly, p-type Ga1nAs
A p-side electrode 57 is provided on the P camp layer 49 and the n-type GaInAsP layer 55, and an n-type electrode is provided below the n-type LnP substrate 41.
A side electrode 59 is provided. And in this example,
The active layer 45 has an energy cap wavelength λ9 of 1.55
Using a GaI and nAsP layer with a thickness of O and Ium,
The optical waveguide layer 43 has an energy key wavelength λ9 or λ1.
31un and thickness 0. A GaInAsP layer of Ium is used. Therefore, the equipolarized refractive index n of the active layer 45 in the vertical direction
veff is approximately 3.26.

In addition, in order to realize (ensure) a stable refractive index waveguide structure in a semiconductor laser having this structure, it is necessary to have a lateral equipolarized refractive index difference Δn defined as shown in FIG. The vertically equipolarized refractive index nv8ff and the current confinement layer 5
Equipolar refractive index n of 3. Which difference determines the equal polarized refractive index difference △n, △n≧0. It seems necessary to satisfy the old requirements. Therefore, the current confinement layer has a uniform refractive index nC or n. ≦3
．． Although it is necessary to satisfy 25, it is preferable to use a material that has a larger value. Therefore, in this embodiment, the energy cap wavelength λ9 is used as the current confinement layer 53:55.
By using a GaInAsP layer with a thickness of 1.05 um, the equipolarized refractive index n. constitutes a current confinement layer of 3.25.

Note that 61 in FIG. 3 indicates guided light.

In addition, the equipolar refractive index n. Width Wa of the optical waveguide region of the active layer of a BH-DFB semiconductor laser when using a current confinement layer with a current confinement layer of 3.25 (in the case of this example, the active layer 45
This corresponds to the width W2 of the portion in contact with the p-type InP cladding layer 47. ) for H/W a changes as shown in the characteristic curve ■ shown in Figure 2. Therefore, this characteristic curve ■
As is not clear, the cutoff condition for the lateral high-order moat of the NC-3,25 semiconductor laser is Wa≦2.8um, and in this case, it can be seen that r,/Wa is 40.31. . Based on this, in this embodiment, the width W2% of the active layer 45 is approximately 3 um. On the other hand, if we consider the conventional semiconductor laser explained using FIG. 4, it uses InP for the current confinement layer, so its refractive index n,
is 3.17, so Wa and r, /W.

The relationship between the two corresponds to the characteristic curve II shown in FIG. Therefore, as is clear from this characteristic curve II, the cutoff condition for the transverse high-order mode of the conventional semiconductor laser is W8≦0.9um, and at this time, r, /Wa
# It can be seen that it is 0.95. In reality, the width W1 of the active layer of the conventional semiconductor laser shown in FIG. 4 is lu
It is less than m.

Therefore, as is clear from comparing the two, if the current confinement layer is constructed using a material with nC or 3.25, then n
It can be seen that me/w is about 1/3 compared to the case where C is 317.

Roughly speaking, since there is a relationship expressed by the equation 0, if the current confinement layer is constructed using a material with an nC of 3.25, the Δ difference will be reduced compared to the case where a material with an nC of 3.17 is used. It turns out that it is possible to make it /3.

Incidentally, although the above embodiment is explained using an n-type 1nP substrate, the same effect as in the embodiment can be obtained even if the substrate is a p-type and the conductivity type of each semiconductor layer is opposite to that of the embodiment. It will be done.

In general, the present invention can also be applied to semiconductor lasers using materials other than InP.

Further, in the above embodiment, the refractive index waveguide type semiconductor laser 78
Although the description has been made using an example of a BH-DFB type semiconductor laser, the present invention can also be applied to index guided type semiconductor lasers having other structures.

For example, PCW (Piano Convex Wave
When applying the present invention to a guide type semiconductor laser, the difference between the equipolarized refractive index of the optical waveguide portion of the active layer and the equipolarized refractive index of the portions on both sides of the active layer must be within a range where the refractive index waveguide structure can secure the difference. It is sufficient to set the width of the optical waveguide portion of the active layer to the maximum dimension within the range that satisfies the cutoff condition for transverse higher-order modes.

Also, for example, VIPS (V-qrooved Inne
When applying the present invention to a semiconductor laser of the r5tripe) type, the difference between the average equipolarized refractive index of the crescent-shaped active layer portion and the equipolarized refractive index of the current confinement layer is determined by the refractive index waveguide structure. The width of the crescent-shaped active layer portion may be set to the maximum dimension within the range that satisfies the cut-off condition for the transverse higher-order mode.

(Effects of the Invention) As is clear from the above description, according to the present invention, in an index-guided semiconductor laser, the range in which the refractive index-guide structure can ensure the equipolarized refractive index difference Δn in the lateral direction Since it is set to a smaller value within the range, it is possible to increase the width of the optical waveguide region of the active layer, which is the cutoff condition for higher-order modes.

Therefore, it is possible to widen the width of the optical waveguide region of the active layer while maintaining one transverse fundamental modulus, thereby reducing the coupling of spontaneously emitted light to the oscillation mode, and as a result, the spectral linewidth of the semiconductor is smaller than that of conventional semiconductors. Laser is obtained.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a cutaway perspective view showing a semiconductor laser according to an embodiment, FIG. 2 is a diagram for explaining the present invention, and FIG. A diagram explaining the definition of the refractive index difference Δn, FIG. 4 is a cutaway perspective view showing a conventional semiconductor laser, and FIG. FIG. 41...n-type 1nP substrate, 41a... grating 43-n-type GaInAsP optical waveguide layer 45/GaInAsP active layer, 47-o-type InP crat layer 49-o-type Ga1nAsP cap layer 51... laminate 53- o-type GaInAsP current confinement layer 55, n-type GaInAsP current confinement layer 57... n-side electrode, 59... n-side electrode 61... guided light. Patent applicant: Oki Electric Industry Co., Ltd.

Claims (2)

[Claims] (1) In a refractive index waveguide type semiconductor laser, the lateral equivalent refractive index difference Δn is set to a smaller value within the range that can ensure the refractive index waveguide structure, and the optical waveguide region of the active layer of the semiconductor laser is A semiconductor laser characterized in that the width of the semiconductor laser is set to a maximum size within a range that corresponds to the set equivalent refractive index difference and can cut off modes other than the single fundamental transverse mode. (2) The index-guided semiconductor laser is a buried semiconductor laser, and the equivalent refractive index in the thickness direction of the active layer of the semiconductor laser,
Difference Δn from the equivalent refractive index of the current confinement layer that embeds the active layer
2. The semiconductor laser according to claim 1, wherein is set to a smaller value within a range that can ensure a refractive index waveguide structure.