JPH03110885A - Distributed feedback semiconductor laser - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a distributed feedback semiconductor laser having an optical waveguide having a periodic structure such as a diffraction grating.

(Prior art) Conventionally, distributed feedback semiconductor lasers (DF
B laser) has been developed. Two oscillation modes can exist in a normal DFB laser, and 100
It was not possible to obtain zero single longitudinal mode oscillation (SLM). Therefore, in order to obtain 100 zero single longitudinal mode oscillation,
A phase shift part of 174 (λ/4) of the wavelength of the oscillation light has been introduced into the diffraction grating.

In order to obtain high optical output in these single longitudinal mode lasers, in a normal device, as shown in FIG. A high-reflection ()IR
) The film 21 and the light extraction surface were coated with an anti-reflection (AR) film 22. Roughly speaking, in the λ/4 phase shift type laser, the position of the shift region is shifted toward the light extraction side (usually the shift position is often in the center of the resonator), and high optical output was obtained. This corresponds to the fact that the electric field is concentrated in the shift region within the resonator, so when the light extraction surface is brought closer to the shift region, the electric field at the extraction surface increases.

[Problem to be solved by the invention] However, when HR cocoing is applied to these lasers, Fabry-Perot mode oscillation may occur because the mode becomes a car mode and the reflectance at the end face increases. There were many problems. Furthermore, in the λ/4 phase shift laser, as mentioned above, the electric field concentrates in the phase shift region, so as the optical output increases, the lifetime of the injected carriers decreases in the region where the electric field concentration occurs, and the refractive index distribution becomes uneven. Due to uniformity, the problem of longitudinal mode oscillation not being maintained (spatial hole burning) has been resolved.

Therefore, an object of the present invention is to control the diffraction efficiency distribution of the diffraction grating in the resonator, increase the coupling coefficient at one end of the diffraction grating, and use this part as a reflection mirror, thereby improving the efficiency of the distribution feedback semiconductor laser. The object of the present invention is to provide a distributed feedback semiconductor laser which suppresses hole burning by making the photon density distribution and carrier density distribution as uniform as possible, and which obtains high optical output through single longitudinal mode oscillation.

[Means to solve the problem]

In order to achieve such an object, the present invention provides a distributed feedback system in which an active layer is disposed on a substrate, and a waveguide whose refractive index changes periodically is disposed along the active layer to constitute a resonator. In the type semiconductor laser, one end of the waveguide of the resonator has a refractive index that changes at a period different from that of the rest of the resonator over a length d that is less than or equal to the length of the resonator. Then, place a high-coupling region with a coupling coefficient higher than that of the remaining part, and set the product so that d is 1.0.
It is characterized by the above.

[Function] In the present invention, the diffraction grating of the DFB laser is provided with a coupling constant distribution, and in particular, by having a high region at one end of the resonator waveguide, confinement is strengthened and the oscillation threshold is reduced. Since the occurrence of hole burning can be suppressed, a narrow spectral linewidth can be obtained with a relatively short resonator.

Furthermore, according to the present invention, since the phase in the end face reflection mirror can be controlled, there is an advantage that the fractional fraction of single mode oscillation can be improved and stabilized.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

[Cause] Figures 1 (a) to (d) show an InP <001> substrate 1 on which
InGaAsP active layer 2 (0.12 urn thickness) followed by InGaAsP guide layer 3 (0.15 μm thick)
This is an example in which a diffraction grating was formed by a wet etching method on a substrate for a 1.5 μm band. By electron beam exposure method, φ-o was formed on this substrate in the <110> direction.
A diffraction grating resist pattern was formed using +ac resist 20. As seen in FIG. 1(a), this resist pattern has a region 12' with a length of 40 μm at one end.
In this case, the line is thin (the space is wide), and in the other region 11' having a length of 220 μm, the space is thin (the line is thick).

Furthermore, reference numeral 16' is a phase adjustment region between regions 11' and 12'.

Such a substrate was prepared using saturated bromine water:hydrobromic acid:water = 1
:10:40 (T, Matsuoka and H,N
agai, “InPEchant for Sub
Micron Patterns”, J, Elec
tro-chem, Soc, Vol, 133.248
When etching was performed for 20 seconds using a 0° C. ρ anisotropic etching solution (etching solution described in No. 5 (198B)), a diffraction grating as shown in FIG. 1(b) was obtained without undercuts. This is because the etching solution has almost no side etching, and once the V-shaped groove shape determined by the space width is formed, the etching rate drops significantly and the etching virtually stops. In this etching, the magnitude of the change in refractive index is proportional to the square of the space width of the resist pattern. Therefore, as shown in FIG. 1(C), in the region 12 at one end of the diffraction grating, the shape change is large, so the coupling coefficient becomes large, and in the other region 11, the shape change is small, so the refractive index change is small. Therefore, the coupling coefficient becomes smaller. Also, high area 12 and low area 11
By providing the phase adjustment region 16 near the boundary between the two lasers, these lasers are free from the spatial non-uniformity of the electric field generated by phase-shifted lasers, and a single longitudinal mode due to the HR film co-coating. Due to the absence of multiple modes, it is also possible to control the phase of the diffraction grating and obtain single longitudinal mode oscillation with a high fractional fraction.

When the characteristics of the device manufactured in this example were measured, the oscillation threshold current value could be made sufficiently lower than that of the device without the high region 12. Further, even when both end faces were cleaved, single-longitudinal mode oscillation with an optical output of 20 aW or more could be obtained with good reproducibility.

In this way, by changing the line and space (L/S) of the resist pattern, the etching shape (depth) of the diffraction grating can be changed, making the coupling coefficient larger at one end and smaller at the other region. can do. This makes it possible to make the electric field distribution uniform while increasing the coupling coefficient of the entire resonator, thereby making it possible to simultaneously reduce the threshold current value, narrow the spectrum, and suppress hole burning.

In order to control the photon density inside the resonator, it is sufficient that the coupling coefficient of the other low coupling coefficient region 11 is smaller than that of the high coupling coefficient region 12 at one end, but if the region is short, the high photon density The density region is short, and fewer photons are trapped, as shown in Figure 1(e). At the same time, the output power possible without hole burning is limited. For this reason, the present invention requires a low coupling coefficient region that is sufficiently long compared to the total resonator length, that is, approximately 172 or more of the total resonator length. In addition, the coupling coefficient is a value corresponding to the reflectance per unit length of the distributed reflection mirror, and the product of this value and the region length corresponds to the effective reflectance of the region. For this reason, it is sufficient that the d product in the high region is 1 or more. In addition, in this example, the coupling coefficient changes at the boundary 1 between the high region 12 and the low region 11.
However, in view of the principle of photon distribution described above, it is clear that the same effect can be obtained even if the coupling coefficient is gradually changed in the boundary region between the two regions.

In order to control the coupling coefficient as described above, a resist pattern is required according to the etching method. A resist pattern with thin lines or narrow spaces is formed in the lower part 11. The etched shape obtained in this case is shown in Figure 3(a).
, (b), the coupling coefficient of the one end portion 12 is also larger than the coupling coefficient of the other region 11.

Such control is similarly possible in the case of dry etching.

Alternatively, if the thickness of the resist 20 is large regardless of the isotropy of the anisotropy 9 and the flow of the etching solution during etching in the space is slowed down, the etched shape will be as shown in FIG. , the change in the coupling coefficient can be further increased.

Such a change in the coupling coefficient can also be achieved by changing the order of the diffraction grating. For example, as shown in FIG. 5, the resist pattern is formed such that a first-order diffraction grating is formed in the upper part 12, and a second-order or higher-order diffraction grating is formed in the other part 11. By forming this, it is possible to form a distribution in the coupling coefficient similar to that described above.

In the embodiment shown in FIG. 1(b), a diffraction grating is formed on the optical guide layer 3 disposed on the active layer 2, but the present invention is not limited to this example. As shown in FIG. 2, a diffraction grating is formed on the substrate 1 or the cladding layer 4 on the substrate 1, the optical guide layer 3 is disposed on the diffraction grating, and the active layer 2 and the cladding layer 4 are further disposed on it in this order. may be set. In this case, as shown in FIG. 1(f), a resist pattern is placed on the substrate 1 or the cladding layer 4 on the substrate 1, and then an etching process is performed to form a V-shaped groove, and then a V-shaped groove is formed on the resist pattern. A light guide layer 3 is formed to finally obtain the structure shown in FIG. 1(g).

The effect seen in Example 1 can be achieved with any DEB laser that has a diffraction grating inside the resonator, so the shape of the resonator such as the laser oscillation wavelength, material, order of lamination on the substrate, and embedding etc. It is clear that it does not depend on It is also clear that the formation of a resist pattern does not depend on the resist used.

Furthermore, as long as it is possible to form submicron patterns used in semiconductor laser diffraction gratings, there is no need to limit exposure to electron beams, and X-ray exposure using synchrotron radiation or plasma X-ray sources may also be used. The good news is clear. Furthermore, in guide layer etching (substrate etching if the stacking order is reversed), if the etching shape changes with the width of the resist pattern,
It is clear that the coupling constant can be controlled even when wet etching with an etchant other than Example 1 has a loading effect, or even when dry etching is used.

In this way, the lines and lines of the resist pattern according to the present invention are
The L/S design principle can be stated as follows. Since the coupling coefficient is uniquely determined with respect to the etched shape of the diffraction grating, in an etching method with approximately controllability, the relationship between the L/S of the resist pattern and the coupling coefficient under certain conditions is as shown in Figure 2. (a)
, (b) and (c), a certain relationship (-L/S) is obtained.

Therefore, the L/S value (L/S) corresponds to h in the high coupling coefficient region and 4 in the low coupling coefficient region.
By forming a diffraction grating resist pattern in each region using h and (L/S)z, it becomes possible to obtain the diffraction grating of this example.

Furthermore, it goes without saying that the present invention is applicable not only to InP semiconductor lasers but also to other semiconductor lasers in which wavelength selection is performed using a diffraction grating in the oscillation region within a solid state.

An example of a method for producing a diffraction grating having a coupling constant distribution according to the present invention will be described below.

[Cause] Patterns as shown in FIGS. 3(a) and 3(b) were formed as the II resist pattern, and the etching solution was saturated bromine water: phosphoric acid = 2:15:1, room temperature (G, Menegh).
ini.

”Grating formation by Che
mical etching in＾lInAs fo
r MQW Devices, Electron
Lettersvol, 25,725. (1989)
Etching was performed for 5 seconds using the etching solution described in . With this etching solution, the etching depth is proportional to the etching time, so Figures 3(a) and (b)
A diffraction grating is formed, and the change in refractive index is proportional to (L-5)·d, where the line width and space width S of the resist pattern and the etching depth d. Therefore, the third
Figure (a).

For a resist pattern like that shown in FIG. 1(b), a resonator with a coupling coefficient distribution similar to that shown in FIG. 1(c) could be obtained.

Regarding the relationship between the resist pattern and the coupling coefficient as in this embodiment, if the etching shape is close to a rectangle, either anisotropic etching or isotropic etching is possible.

A pattern as shown in FIG. 6 was formed as a resist pattern using a φ-mac resist with a thickness of 50 nm.

Next, this substrate was etched for 4 seconds using the etching solution used in Example 1.

When the resist thickness is made sufficiently thin and the above etching solution is used, the etching rate is determined by the supply rate of chemically reactive active species or isotropic diffusion in the solution, and the etching rate decreases as the exposed area ratio of the substrate increases.

Due to this micro-loading effect in wet etching, the etching rate is slow for patterns with wide spaces, and the shape of the trapezoidally etched diffraction grating becomes close to a shallow rectangle. On the other hand, since the etching speed is fast for patterns with narrow spaces, the trapezoidally etched diffraction grating becomes close to a V-shape. Generally, when the depth is the same, the coupling coefficient is the largest for a rectangular shape.

Therefore, under this etching condition, the coupling coefficient reaches its maximum when L/S is about 1:l, and decreases when the shape becomes a figure 7 or when the space width becomes larger than the line width ( (See above literature). This method is applicable not only to this etching solution but also to any other etching solution that has the above-mentioned micro-loading effect, if the L/S of the diffraction grating is designed in advance by taking the etching shape into account, similar results can be obtained. The gains are clear.

A relatively thick φ-mac resist with a thickness of 300 nI11 was used to form a pattern as shown in FIG. 4 as a resist pattern. Next, this substrate was etched for 4 seconds using the etching solution used in Example 1.

At this time, since the thickness of the resist is large, the narrower the space, the lower the supply conductance of chemically active species in the groove surrounded by the resist, and the lower the etching rate. Therefore, a pattern with wide spaces will have a high etching rate, and a pattern with narrow spaces will have a slow etching rate. Therefore, an etched shape as shown in FIG. 4 is obtained. Here, the coupling coefficient becomes large in the part where the line is wide, so the coupling coefficient distribution is as shown in Figure 1 (
It will be similar to that shown in C) (see the above document).

The above-mentioned effect is not limited to the present etching solution, and similar results can be obtained even when the supply rate is controlled by the resist pattern.

Using a φ-mac resist with a thickness of 150 nm, resist patterns as shown in FIGS. 3(a) and 3(b) were formed. Then, ECR etching was performed for 2 minutes at an output of too much and an acceleration of 400 V using BCl3 as a reaction gas.

In this etching, the etching depth can be controlled by the etching time, as shown in FIG. 3(a).

A diffraction grating as shown in (b) is formed, and the change in refractive index is proportional to (L-5)·d, where the line width of the resist pattern, the space width S, and the etching depth d. Therefore, for resist patterns such as those shown in FIGS. 3(a) and 3(b), a resonator with a coupling coefficient distribution similar to that shown in FIG. 1(C) could be obtained.

The relationship between the resist pattern and the coupling coefficient as shown in this example is not limited to ECR etching, and other dry etching methods such as ion etching (Ill) and reactive ion etching (RIE) can also be used. One thing is clear.

Furthermore, the etching gas may be any other gas as long as it is capable of etching the substrate, and is not limited to acts.

As a resist pattern, a first-order diffraction grating pattern was formed in a region 12 at one end of the resonator, and a second-order diffraction grating pattern was formed in the other region 11, as shown in FIG.

Even if the second-order diffraction grating is etched to the same depth as the first-order diffraction grating, the coupling coefficient will be significantly smaller (
(See above literature).

Therefore, when etching was performed using such a resist pattern as a mask, a coupling coefficient distribution similar to that shown in FIG. 1(C) could be realized.

When the etching shape is close to a rectangle, the low-order diffraction grating has a large coupling coefficient, so there is no particular need for a combination of primary and secondary, and the region 12 at one end is smaller than the other region 11. What is necessary is to make it a higher-order diffraction grating.

Furthermore, even if the etched shape is significantly different from a rectangle, as seen in the above literature, the value of the coupling coefficient differs depending on the order even at the same etching depth. It is clear that it can be designed to

As mentioned in the prior art section of the convoluted body, the photon density is concentrated in the mirror part where the reflection coefficient is large and in the phase shift part. Since a diffraction grating with a large coupling coefficient can be regarded as a type of distributed mirror due to a change in refractive index, if the coupling coefficient of one end 12 is significantly increased in the resonator structure mentioned in Examples 1 to 6, the optical output operation state will be reduced. Light may be concentrated in these areas, causing the oscillation mode to be strongly influenced by the phase of the end facets and causing hole burning.

Figures 7(a) and (b) show the DFB configured in consideration of this.
- shows the structure of the LD, where 5 is an electrode pad placed on the substrate 1 side, 6 is a main injection electrode, 7 is a main injection region placed on the cladding layer 4 and connected to the electrode 6, and 8 is a suppressed injection Electrode 9 is an injection suppression region disposed on cladding layer 4 and connected to electrode 8 . The coupling coefficient of the one-end region 12 is larger than that of the other region 11. Therefore, since both ends of the diffraction grating act as passive mirrors, electric field concentration on both end faces is suppressed, and the influence of the phase of the cleavage plane is suppressed. Furthermore, in order to solve the above-mentioned drawback, in the outer part 9 of the region 12 at one end having a high coupling coefficient, a suppressing injection electrode 8 different from the main injection electrode 6 of the main injection region 7 is used to connect the main injection region 7 with a suppressing injection electrode 8. Also, current injection is suppressed or the active layer 2 is removed as in the embodiment shown in FIG. 7(b). This allows concentration of light to be controlled by controlling the amount of current injection from the outside.

Furthermore, in order to avoid the influence of the phase of the diffraction grating on the cleavage end facet, it is also effective to make the region of the diffraction grating shorter than the resonator length. This is because when a diffraction grating is exposed on the end face, element characteristics such as oscillation threshold and optical output vary depending on the phase of the diffraction grating, but this structure has the advantage of sufficiently reducing variations in element characteristics. be.

When a device with this structure was actually fabricated, single longitudinal mode oscillation was maintained even in the high optical output range compared to conventional lasers, and the variation was kept sufficiently small.

0 Tsujidan In Examples 1 to 7, if the coupling coefficient is increased in the region 12 at one end of the resonator, the confinement becomes stronger, but the reflectance may become smaller at the optical output end where the fiber etc. is connected. , laser oscillation may become unstable due to the incidence of light from the outside. Therefore, as shown in FIG. 8(a), in order to suppress the influence of reflected light at the light output end, it is conceivable to provide a high area at the light output end as well. Here, it is necessary to achieve high confinement of light at the non-light output end, whereas at the light output end, it is necessary to derive the light inside the resonator, so the coupling efficiency is affected by the product of the region length d. d needs to be small at the optical output end. FIG. 8(a) shows the structure of an example in which, based on the above-mentioned guidelines, 13 regions are provided at the light output end of the element of Example 7.

1. 2. , are the coupling coefficients of the light output end 13, the resonator central part 11, and the non-light output end 12, respectively, and dl,
dz and ds are the diffraction grating region lengths of the respective portions.

In the example described so far, ni8-ni2kuni.

However, in this example, as shown in Fig. 8(b), dl is set to 3°d3, 1 is set to 3, and 2 is set to 2 at the optical output end. Since the confinement is stronger than the confinement at the non-light output end, the photon density distribution inside the resonator becomes as shown in Figure 8(c), and the energy injected into the resonator is divided into guided emission and from the light output end. It can be seen that it works efficiently for emitting light. Furthermore, by slightly weakening the confinement at the non-light output end, it is possible to extract light output for output monitoring using a pin diode or the like.

〔Effect of the invention〕

As explained above, according to the present invention, the coupling coefficient of the diffraction grating provided in the resonator of the DFB-LD is distributed, and in particular, by providing a region with a high coupling coefficient at one end of the resonator waveguide, Since it is possible to strengthen confinement, reduce the oscillation threshold, and suppress the occurrence of hole burning, it is also possible to obtain a narrow spectral linewidth with a relatively short resonator.

Furthermore, according to the present invention, since the phase in the end face reflection mirror can be controlled, there is an advantage that the fractional fraction of single mode oscillation can be improved and stabilized.

[Brief explanation of drawings]

FIG. 1(a) shows a DFB-L according to an embodiment of the present invention having a diffraction grating with a distribution of coupling coefficients by anisotropic wet etching.
FIG. 1(b) is a cross-sectional view showing a resist pattern for producing D. FIG. 1(b) is a cross-sectional view showing a diffraction grating wet-etched using the resist pattern of FIG. 1(a). FIG. The coupling coefficient distribution diagram of the diffraction grating in (b), and Figures 1 (d) and (e) are for explaining how to determine the length of the low coupling coefficient region in the present invention with respect to the total resonator length. Photon density distribution diagrams of Fig. 1(f) and (
g) is a sectional view showing another embodiment of the present invention, and Figures 2(a) to (C) show the relationship between the coupling coefficient and L/S in order to explain the principle of resist pattern design in the present invention. Figure 3(a) shows the resist pattern and etched diffraction grating when the line width of the resist pattern in the lower part is narrowed when forming a distributed diffraction grating with a coupling coefficient by isotropic wet etching. Figure 3(b) is a cross-sectional view of the case where the space width of the resist pattern in the low region is narrowed, and Figure 4 is a cross-sectional view of the case where there is a loading effect (reduction in conductance due to resist film thickness). A cross-sectional view showing a resist pattern and an etched diffraction grating according to an embodiment of the present invention in which a distributed diffraction grating is formed on a coupling coefficient by wet etching. FIG. A cross-sectional view showing a resist pattern and an etched diffraction grating according to an embodiment of the invention. FIG. 6 shows an embodiment of the invention in which a distributed diffraction grating is formed with a coupling coefficient by wet etching when there is a loading effect (rate-limiting supply of reactive species). FIG. 7(a) is a cross-sectional view showing the structure of a distributed DFB-LD according to an embodiment of the present invention in which current injection at the non-light output end is suppressed. Figure 8(b) is a cross-sectional view showing the structure of a distribution DFB-LD according to an embodiment of the present invention in which the active layer at the non-light output end is excluded and the gain is suppressed, and Figure 8(a) is a high coupling coefficient region also at the light output end. FIG. 8(b) is a coupling coefficient distribution diagram of the embodiment of FIG. 8(a), FIG. FIG. 8(a) is a photon density distribution diagram of the embodiment, and FIG. 9 is a cross-sectional view showing the structure of a conventional single-end high-reflection DFB-LD. DESCRIPTION OF SYMBOLS 1... InP substrate, 2... Active layer, 3... Guide layer, 4... Clad layer, 5... Electrode pad, 6... Main injection electrode, 7... Main injection region , 8... Suppressing injection electrode, 9... Injection suppressing region, 10... Diffraction grating, 11... Low area, 11'... Resist pattern area, 12... High area, 12'...Resist pattern area, 13...Inside area, 15...Phase shift, l6...Phase adjustment area, 16'...Resist pattern area, 20...Resist, 21 ...High reflective coating, 22...Non-reflective coating.

Claims (1)

[Claims] 1) A distributed feedback semiconductor laser in which a resonator is configured by disposing an active layer on a substrate and disposing a waveguide whose refractive index periodically changes along the active layer, comprising: At one end of the waveguide of the resonator, the refractive index changes at a period different from that of the remaining part of the resonator over a length d that is 1/2 or less of the length of the resonator, and the refractive index changes at a period different from that of the remaining part of the resonator. 1. A distributed feedback semiconductor laser characterized in that a high coupling region having a coupling coefficient k higher than that of a portion is arranged, and the product k·d is 1.0 or more.