JPH0290688A - Distributed feedback type semiconductor laser - Google Patents

Abstract

PURPOSE:To obtain a semiconductor laser which is excellent in a low threshold operation and a high temperature operation, high in yield, and capable of oscillating in a single wavelength by a method wherein the wavelength composition of a guide layer and the shape (height and shape of crest) of the periodic irregularities of a diffraction grating are so determined as to enable a coupling coefficient which indicates the coupling strength between a periodic structure and light to be a specified value. CONSTITUTION:A primary diffraction grating 3 provided with a phase shift region 2 is formed on an n-InP substrate 1, and then an n-InGaAsP guide layer 4, a non doped InGaAs active layer 5, a p-InP clad layer 6, and a P<+>-InGaAsP cap layer 7 are grown in crystal respectively. Electrodes 8 and 9 are provided onto the cap layer 7 and under the substrate 1 respectively, anti-reflective coatings 10 and 11 are provided to both the end faces perpendicular to the active layer 5. The height of the crest of the diffraction grating 3 is 400Angstrom and a resonator length is 300mum. Here, a coupling coefficient (KL) can be regulated through the wavelength composition of the guide layer 4 and the height of the crest of the diffraction grating 3. KL needs to be within the range of 1.5-2.5 so as to obtain a semiconductor laser which is operable at a low threshold, high in yield, and capable of oscillating in a single wavelength.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a distributed feedback semiconductor laser with excellent stability in single wavelength oscillation.

[Conventional technology]

With the development of long-distance, large-capacity optical fiber communications, demand for distributed feedback semiconductor lasers (hereinafter referred to as DFB-LD), which operate stably at a single wavelength even during high-speed modulation, is rapidly increasing. A DFB-LD is characterized in that it oscillates at a single wavelength by utilizing the periodicity of a diffraction grating formed inside the device. Among them, the λ/4 shift type DFB-LD, which has a structure in which the periodicity of the diffraction grating is shifted by 1/4 of the propagation wavelength at the center of the element, has a larger gain difference between the main axis mode and the sub-axis mode than other DFB structures. Therefore, it is being most actively developed as a light source that can provide single wavelength oscillation with high yield. In the λ/4 shift type DFB-LD, the light emitted from the active layer is effectively confined inside the resonator due to the distributed feedback of light by the diffraction grating, especially in the center of the element where the periodicity of the diffraction grating is shifted. The light field is concentrated near the phase shift area (near the phase shift area). Therefore, as pointed out in the literature (K9ta et al., Institute of Electronics, Information and Communication Engineers, Quantum Electronics Research Group Proceedings, 0QE86-7), carriers near the center of the element are caused by the spatial hole burning phenomenon along the resonator direction. There are many cases where the density decreases and the resulting change in refractive index leads to bimodal oscillation. Such spatial hole burning phenomenon can be prevented by uniformizing the optical field distribution along the resonance direction;
It is necessary to set a small coupling coefficient between the diffraction grating and the light (KL, where KL is the coupling efficiency, and L is the resonator length). According to the above literature, as a result of theoretical analysis taking into account the hole burning phenomenon, in order to obtain single wavelength oscillation with the highest yield in a λ/4 shift type DFB-LD, the coupling coefficient (KL) must be set to 1.
．． It is said that it is desirable to set it to about 2 to 1.3.

Based on this design guideline, the conventional λ/4 shift type DFB
-LD, the height of the peaks of the diffraction grating was adjusted so that KL=1.2 to about 13.

[Problem to be solved by the invention]

However, in the conventional λ/4 shift type DFB/LD, although almost no bimodal oscillation due to the hole burning phenomenon has been observed, the oscillation threshold current is high and the optical output decreases during high temperature operation. Saturation tendency is other DFB-L
There was a problem that it was larger than the D tank structure.

The object of the present invention is to have excellent low threshold operation and high temperature operation;
Another object of the present invention is to provide a ^/4 shift type DFB-LD which can obtain single wavelength oscillation with high yield.

[Means to solve the problem]

The distributed feedback semiconductor laser according to the present invention is characterized by having a semiconductor double heterostructure consisting of an active layer that emits light and a cladding layer sandwiching the active layer, which has a larger energy gap than the active layer. In between, there is a guide layer whose layer thickness changes at a period of an integral multiple of half the wavelength of the propagating light, and whose energy gap is larger than the active layer and smaller than the cladding layer, and the periodic structure is located at the center of the element. In a phase-shifted distributed feedback semiconductor laser shifted by an integral multiple of 1/4 of the wavelength of light propagating in , the coupling coefficient KL, which indicates the strength of coupling between the periodic structure and light, is set from 1.5 to 2.5. It was set between

[Effect]

In the λ/4 shift type DFB-LD, the reflectance of the two end faces is suppressed, so the feedback (
(or reflection) will rely solely on distributed feedback by the diffraction grating. Therefore, if the coupling coefficient KL is small, the feedback rate of this light will also decrease, leading to an oscillation threshold current. Therefore λ
In order to keep the oscillation threshold current of the /4 shift type DFB-LD low, it is necessary to set KL to a certain degree or more. This is also in the opposite direction to the direction of suppressing the hole burning phenomenon described above, and therefore it is necessary to perform an optimal design that fully considers the effects on both.

First, a standard for KL is set from the viewpoint of oscillation threshold current. The oscillation threshold gain of the DFB-LD can be calculated and is generally expressed as αthL. DFB
- When the LD is replaced with a general Fabry-Perot type LD, the reflectance of the isolateral end face is expressed by the following equation using αthL.

Rerr=exP(2αthe-) Here, αtbL can be determined as the relationship between the coupling coefficient vlKL and the actual end face reflectance R. Fabry-Perot type L
Using the relationship between the end face reflectance and the oscillation threshold current I th obtained experimentally in D and the R err obtained from calculation, the end face reflectance R in the case of λ/4 shift type DFB/LD is calculated. The relationship with the oscillation threshold current Ith was determined using KL as a parameter.

The results are shown in FIG. In the figure, the oscillation threshold current ■τt of the DFB-LD is standardized by the oscillation threshold BP of a Fabry-Perot type LD with an end face reflectance of 30%. From the figure, 1℃ increases as R becomes smaller, but the degree of increase is K
It can be seen that the larger L is, the smaller it is. In an actual λ/4 shift I type DFB-LD, R is about 0.5 to 1%, and within this range I? Reduce the increase in h (+”: /■,
'h < 1.5) To suppress K L is 1.5
That's all there is to it.

Next, KL required to suppress the hole burning phenomenon.
This section describes the standards. We actually fabricated prototype λ/4 shift type DFB-LDs with various coupling coefficients KL, and investigated the frequency of occurrence of two-mode oscillation due to the hole burning phenomenon.

The coupling coefficient KL was adjusted by changing the wavelength composition of the guide layer close to the active layer. The results are shown in FIG. From this figure, the hole-burning two-mode oscillation is KL
It can be seen that the probability of occurrence increases rapidly when the value exceeds 2.5. It has been found that in order to suppress the probability of occurrence of such two-mode oscillation to 10% or less, it is necessary to satisfy KL<2.5.

As mentioned above, KL = 1.2 to 1.3 was theoretically considered to be appropriate for hole burning suppression, but based on this experimental result, KL = around 2.5 is actually acceptable. It turned out that it would be done.

As described above, from the viewpoint of reducing the oscillation threshold current and suppressing the hole burning phenomenon, it can be said that the optimum range for the coupling coefficient KL is 1.5 to 2.5. In the examples shown below, the results of device fabrication with the median value of KL=2.0 as the target will be described.

〔Example〕

Embodiments according to the present invention will be described in detail below with reference to the drawings. FIG. 1 is a sectional view taken along a resonator of a semiconductor laser according to an embodiment of the present invention. λ of oscillation wavelength 1.3ttm
/4 shift type DFB-LD. n-InP substrate 1
A first-order diffraction grating 3 having a period of 2015A and a phase shift region 2 at the center of the element is formed on the surface of the grating by interference exposure method and chemical etching method, and then the wavelength composition is 1.10 μm.
n-I nGaAsP guide layer 4, wavelength composition 1.31
μm non-doped rnGaAsP active layer 5, p-InP
The cladding layer 6 and the P''-1nGaAsP cap layer 7 have thicknesses of 0.1 μm and 0.1 μm, respectively.

It forms a semiconductor double heterostructure in which crystals are grown in the order of 2 μm and 0.5 μm. On top of the cap layer 7 and on the substrate 1
An electrode 8.9 is provided below, and anti-reflection (AR> coating 10.11) is applied to the two end faces perpendicular to the active N5.The height of the peaks of the diffraction grating 3 is 400A, and the resonator The length is 300 μm.Here, the coupling coefficient KL is the guide N4.
The wavelength composition can be adjusted by the wavelength composition and the height of the peaks of the diffraction grating 3. In the example, the wavelength composition was set to 1.10 μm and the height of the diffraction grating was set to 400 m, with KL=2 as the target.

As a result of actually estimating the KL of this device from the spectrum characteristics when biased below the oscillation threshold current, it is approximately 2.1
The value was very close to the target value. 100
When the oscillation spectrum characteristics of each element were evaluated, hole-burning two-mode oscillation was hardly observed, and the occurrence rate was 2%. Further, the oscillation threshold current was 17 mA on average, which was slightly lower than the average value of 20 mA of the conventional Fabry-Perot type LD. It also has excellent high-temperature operating characteristics, and was able to obtain CW oscillation up to 110°C.

In this example, we have described an element structure aiming at KL ~ 2.0, but KI is not limited to this, and in order to obtain single wavelength oscillation with a low threshold and high yield. It is sufficient that KL falls within the range of 1.5 to 2.5.

As described above, the coupling coefficient KL can be adjusted by adjusting the height of the diffraction grating 3 and the wavelength composition of the guide layer 4. Figure 2 (a) shows a 1.3 μm body λ/4 shift DF using a first-order diffraction grating.
An example will be shown in which the relationship between the peak height h of the diffraction grating 3 and KL in B-LD (resonance wavelength 3006 m) is calculated using the wavelength composition λg of the guide layer 4 as a parameter. The height of the diffraction grating 3 that can be realized in terms of device fabrication is h<6008, and the composition of the guide layer 4 allowed to effectively confine carriers in the active layer 5 is λg<1.2 μm. This allows KL=
The range in which 1.5 to 2.0 can be achieved is the area surrounded by the thick solid line in the figure. Therefore, according to the present invention, by setting h and λg within this range, a 1.3 μm band λ/4 shift DFB-LD that exhibits a low threshold and stable single wavelength oscillation can be obtained. Here, between the parameters h and λg, h has poorer controllability. Therefore, it is preferable to set λg so that the tolerance range for h is the widest, and λ
In that sense, g=1.10 μm is optimal. The aim at this time is h=4008. Based on these design guidelines, in this example, the wavelength composition of guide N4 is set to λg=
The height of the diffraction grating 3 was 400A.

According to the present invention, it is possible to similarly design a device aiming at KL=1.5 to 2.0 in a λ/4 shift DFB-LD with an oscillation wavelength of 1855 μm. FIG. 2(b) shows the relationship between KL and h with λg as a parameter for a 1.55 μm band element having a first-order diffraction grating 3.

Although 1.3 μm InGaAsPJl is used,
In order to effectively confine carriers in this active layer 5, it is necessary to set the wavelength composition of the guide layer 4 to λg - 1.30 μm. Considering these constraints, KL=1.5~2.5
The range of h and λg that can achieve this is shown by the thick line in the figure. h
1, since it has the widest tolerance range for
．． In the 55 μm band, λg=1.20 μm is optimal, and the target value of h at this time is h-4008.

In this way, the wavelength 1.55 with a first-order diffraction grating. czm
In the band λ/4 shift DFB, LD, the wavelength composition of the guide layer 4 is λg = 1.20 μm, and the height of the diffraction grating 3 is h = 4.
If it is set to 00A, a device with a low threshold value and an excellent single wavelength oscillation yield can be obtained with good reproducibility. 1.55 μm λ/4 shift DFB/LD manufactured based on this design guideline
, the average value of the oscillation threshold current is 19 mA,
Good results were obtained with a probability of occurrence of hole-burning two-mode oscillation of 4%.

In the above description of this example and device design, 1.
Although only an element having the following diffraction grating 3 has been described, the present invention is also effective for an element having a second-order or third-order diffraction grating. In that case, the larger the number of characters in the diffraction grating 3, the lower the coupling coefficient KL becomes.
As the number of characters increases, it is necessary to change the wavelength composition λg of the guide N4 to the longer wavelength side.

Furthermore, in this example, the diffraction grating 3 was formed under the guide layer 4, but the guide layer 4 was formed on the active layer 5, and the diffraction grating 3 was formed on the guide layer 4. The present invention is also effective for devices of this type.

The graph shown in Figure 2 is obtained by making a large number of guide layers with diffraction gratings with various heights of the peaks of the diffraction gratings and varying the composition of the guide layers, measuring the coupling coefficient of each, and plotting the measured values. By doing this, it can be determined for any composition.

〔Effect of the invention〕

In the λ/4 shift DFB-LD according to the present invention,
Since the oscillation threshold current is low and the yield of single wavelength oscillation is excellent, the yield of device coupling is significantly improved compared to the conventional method. Furthermore, the above two improvements bring about the advantages of reducing the drive current to obtain a constant output and improving the stability of the single wavelength oscillation characteristics, thereby reducing the λ/4 shift D.
The long-term reliability of FB/LD will also be further improved.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a distributed feedback semiconductor laser which is an embodiment of the present invention, and in the figure, 1...n-InP substrate, 2...phase shift region, 3.
...Diffraction grating, 4-・n-I n Ga As
P guide layer, 5--InGaAsP active layer, 0--
-p-InP cladding layer, 7...P” -InGa
As P cap layer, 8.9... Electrode, 10.11.
...AR coating film. FIG. 2 is a diagram showing the relationship between the coupling coefficient KL and the height h of the diffraction grating using the wavelength composition λg of the guide layer as a parameter. FIG. 3 is a diagram showing the relationship between the probability of occurrence of hole-burning two-mode deepening and the coupling coefficient KL. FIG. 4 is a diagram showing the relationship between the normalized oscillation threshold current and the end face reflectance using the coupling coefficient L as a parameter.

Claims (1)

[Claims] It has a semiconductor double heterostructure consisting of an active layer that emits light and a cladding layer with a larger energy gap than the active layer sandwiching it, and the layer thickness between the active layer and the cladding layer is the wavelength of the light that propagates. The periodic structure changes at an integer multiple of half of the period, has a guide layer whose energy gap is larger than the active layer and smaller than the cladding layer, and the periodic structure has a period of 1/4 of the wavelength of light propagating at the center of the element. In the phase-shifted distributed feedback semiconductor laser shifted by an integer multiple, the composition of the guide layer is adjusted so that the coupling coefficient KL, which indicates the strength of the coupling between the periodic structure and light, is between 1.5 and 2.5. A distributed feedback semiconductor laser characterized by a defined periodic uneven shape (height and shape of the peaks).