JPH04349683A - Distribution feedback type semiconductor laser - Google Patents

Abstract

PURPOSE:To provide a distribution feedback type semiconductor laser capable of inhibiting space hole burning and having low strain properties even during high output with regards to the distribution feedback type laser. CONSTITUTION:A high reflection coat is applied to one end face whereas a low reflection coat is applied to the other end face where a first diffraction grating 7 is formed on a waveguide layer 5 on the upper part of an active layer 4 while a second diffraction grating 3 is formed on a waveguide layer 2 on the lower part of the active layer 4. The cycle of the first diffraction grating 7 is arranged to differ from that of the second diffraction grating 3 or their cycles and amplitudes are arranged to be different from each other. Assuming LAMBDA is a period corresponding to Bragg frequency and the length of a resonator is L, one of the first diffraction grating 7 or the second diffraction grating 3 has a period, LAMBDA(1+LAMBDA/4L) while the other has a period LAMBDA(1-LAMBDA/4L). It is arranged that the cycle of the first diffraction grating and the cycle of the second diffraction grating show an opposite phase on the side of the AR end faces while their cycles show the same phase on the side of the HR end faces.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a distributed feedback semiconductor laser, and more particularly to a distributed feedback semiconductor laser that reduces spatial hole burning and has low distortion characteristics.

0002

[Prior Art] In recent years, cable television (C
ATV) systems are widely used. However, since current CATV systems use coaxial cables, it is difficult to significantly expand transmission capacity and improve image quality.

[0003] Therefore, construction of optical CATV systems using optical fiber cables in place of coaxial cables is underway. It has been considered to use a semiconductor laser as a directly modulated light source for this optical CATV system. Semiconductor lasers used as directly modulated light sources are required to have low distortion characteristics, especially at high output, in order to transmit high-quality screens.

However, when a conventionally known distributed feedback semiconductor laser having one diffraction grating between the active layer and the waveguide layer is operated at high output, non-uniformity of light intensity occurs in the cavity direction. , a phenomenon called spatial hole burning occurs because the carrier density increases in areas where the light intensity is strong, resulting in non-uniform carrier distribution. Therefore, the threshold gain level at the oscillation wavelength fluctuates,
It is known that distortion characteristics deteriorate.

[0005] Therefore, in order to realize low distortion characteristics, suppression of spatial hole burning is an important issue. In order to suppress this spatial hole burning, conventionally,
A method is used in which the depth of the diffraction grating is changed to control the coupling coefficient κ so that the light intensity distribution becomes uniform.

[0006]

[Problem to be solved by the invention] However, optical CA
In distributed feedback semiconductor lasers for TVs, in order to obtain high output characteristics, an AR (anti-reflection) coating is applied to the light emitting side end face of the laser, and an HR (high reflection) coating is applied to the reflection side end face. Even if the coupling coefficient κ was controlled, the light intensity became stronger on the HR side, and it was not possible to sufficiently suppress spatial hole burning. Therefore, the present invention:
An object of the present invention is to provide a feedback semiconductor laser that can suppress spatial hole burning even at high output.

[0007]

[Means for Solving the Problems] In the distributed feedback semiconductor laser according to the present invention, one end face is coated with a high reflection coating, the other end face is coated with a low reflection coat, and the waveguide above the active layer is coated with a high reflection coating. A first diffraction grating is formed in the layer, a second diffraction grating is formed in the waveguide layer below the active layer, and the first diffraction grating and the second diffraction grating have different periods. .

In this case, the period of either the first diffraction grating or the second diffraction grating is Λ(1+Λ/4L) where Λ is the period corresponding to the Bragg wavelength and L is the resonator length.
), and the other period is Λ(1-Λ/4L),
A configuration was adopted in which the periods of the first diffraction grating and the second diffraction grating are opposite in phase on the AR end face side, and are in phase on the HR end face side.

[0009]

[Operation] FIGS. 1(A) to 1(C) are diagrams explaining the principle of the distributed feedback semiconductor laser of the present invention, with FIG. 1(A) showing the schematic structure and FIG. 1(B) showing the resonance of the equivalent refractive index neq. FIG. 1C shows the distribution of the coupling coefficient κ in the cavity direction.

In this figure, 1 is the second cladding layer;
2 is a second waveguide layer, 3 is a second diffraction grating, 4 is an active layer, 5 is a first waveguide layer, 6 is a first cladding layer, 7 is a first diffraction grating, 8 is an AR (Anti-reflection) coat, 9 is HR
(highly reflective) coat.

First, to explain the structure of the distributed feedback semiconductor laser of the present invention, as shown in FIG. 1(A),
A second waveguide layer 2 and an active layer 4 are disposed on the second cladding layer 1.
, a first waveguide layer 5 and a first cladding layer 6 are laminated,
A second diffraction grating 3 is located between the second cladding layer 1 and the second waveguide layer 2, and a first diffraction grating 7 is located between the first waveguide layer 5 and the first cladding layer 6. is formed. Then, AR (anti-reflection) coat 8 on the light emission side and HR coating on the reflection side.
A (highly reflective) coat 9 is formed.

In this case, the thickness d1 of the first waveguide layer 5
and the thickness d2 of the second waveguide layer 2, the period of the first diffraction grating is Λ1, the period of the second diffraction grating is Λ2, and the distance between the active layer 4 and the center of the first diffraction grating 7 is d0, first
The amplitude of the second diffraction grating is dM1, and the amplitude of the second diffraction grating is dM1.
M2, the length of the resonator is L, and the distance in the direction of the resonator is z, then d1 = d0 - dM1sin (2πz/Λ1)
It is expressed as d2 = d0 - dM2 sin (2πz/Λ2), and if the period of the first diffraction grating and the period of the second diffraction grating are different by ±Λδ around the period Λ corresponding to the Bragg wavelength, then Λ1 = It is expressed as Λ(1+δ) Λ2 =Λ(1-δ), and in this case, if the relationship δ=Λ/4L(≪1) is established, the waveforms of d1 and d2 will be in opposite phase on the AR end face side, and the HR They are in phase on the end face side.

[0013] Looking at this with respect to the thickness of the first waveguide layer 5 and the second waveguide layer 2, they are in phase on the AR end face side,
The phase is reversed on the HR end face side. That is, the first waveguide layer 5
The change in the combined thickness of the second waveguide layer 2 and the second waveguide layer 2 is large on the AR end face side and small on the HR end face side.

FIG. 1(B) shows the distribution of the equivalent refractive index neq in the cavity direction when the configuration shown in FIG. 1(A) is adopted. Reflecting the above change in the combined thickness of A and the second waveguide layer 2, A
The change is large on the R end face side and small on the HR end face side.

The amplitude of this change in the equivalent refractive index neq is nM
is a coefficient related to the difference between dM1 and dM2,
nM ∝−[sin(2πz/Λ1)+sin (2
πz/Λ2)] −m・sin
(2πz/Λ) ∝−[sin(2πz(1−δ
)/Λ)+sin (2πz(1+δ)/Λ)]
−m・sin (2πz/Λ)
∝−2sin (2πz/Λ) [cos (2πzδ/Λ
)+m/2] ∝-2sin(2πz/Λ)[c
os (πz/2L)+m/2].

(m/2) in the equation is a term resulting from the fact that dM1 and dM2 are not equal, and becomes 0 when dM1=dM2. That is, the distribution of the equivalent refractive index neq in the laser cavity direction is [cos (πz/2L)+m/
2] changes with a period of 2πz/Λ.
/Λ), indicating that the amplitude gradually decreases with z.

FIG. 1C shows the distribution of the coupling coefficient κ in the resonator direction in this case. The coupling coefficient κ is the equivalent refractive index n
Since it is proportional to the amplitude of the alternating current component of eq, κ∝[cos(
πz/2L)+m/2], it is large at the AR end face and simply decreases toward the HR end face.

As is clear from the above explanation, depending on the optical conditions such as the reflectance of the HR coat of the distributed feedback semiconductor laser, the period of the diffraction grating or By adjusting the distribution of the coupling coefficient κ by changing the period and amplitude, concentration of light intensity near the HR end face can be avoided, spatial hole burning can be suppressed, and deterioration of strain characteristics can be reduced.

[0019]

[Examples] Examples of the present invention will be described below. Figure 2 shows
FIG. 1 is an explanatory diagram of a configuration of a distributed feedback semiconductor laser according to an embodiment of the present invention. In this figure, 11 is an n-InP substrate that also serves as a second cladding layer, 12 is a second waveguide layer made of p-InP, 13 is an active layer made of InGaAsP, and 14 is a first waveguide layer made of InGaAsP. , 15 is p-I
First cladding layer made of nP, 16 AR coat, 1
7 is an HR coat.

In the distributed feedback semiconductor laser of this embodiment, the surface of the n-InP substrate 11, which also serves as the n-type second cladding layer and has a length L of 300 μm, has a depth of 30 mm with a period Λ2.
Corrugations of 0 Å (2 dM2) are provided by interference exposure and etching to form a first diffraction grating, and on top of that, p-In with a thickness of 0.15 μm (dguide2) is formed.
A second waveguide layer 12 made of P is formed, on which an active layer 13 made of InGaAsP with a thickness of 0.13 μm (dact) is formed, and on top of this, an active layer 13 made of InGaAsP with a thickness of 0.15 μm is formed.
A first waveguide layer 14 made of InGaAsP (dguide1) is formed on its surface with a period Λ1 and a depth of 30
0 Å (2 dM1) corrugation is provided to form a second diffraction grating, and on top of that, a 2.0 μm thick p-InP
AR coat 16 is formed on the end face of the light emission side (right side in the figure), and on the reflection side (left side of the figure).
It is constructed by forming an HR coat 17 on the end face.

[0021] Then, a distributed feedback semiconductor laser is completed in which a stripe structure is formed in the direction of propagation of light, and electrodes are formed above and below to modulate the degree of coupling κ. In this example, since the wavelength λ is aimed at 1.31 μm, the period Λ corresponding to the Bragg wavelength is 2022.0 Å, and the length L of the resonator is 300 μm, so δ is 1.685×1
The score is 0-4. Therefore, the period Λ1 of the diffraction grating
is 2022.34 Å, and the period Λ2 is 2021.
It becomes 66 Å. In this way, spatial hole burning is reduced by modulating the coupling coefficient κ with a desired distribution.

[0022]

[Effects of the Invention] According to the present invention, spatial hole burning can be reduced and deterioration of distortion characteristics at high output can be prevented, contributing to the field of transmission and processing technology for signals, information, etc. using optical fibers. There's a lot to do.

[Brief explanation of drawings]

FIGS. 1A to 1C are diagrams illustrating the principle of a distributed feedback semiconductor laser according to the present invention.

FIG. 2 is a diagram illustrating the configuration of a distributed feedback semiconductor laser according to an embodiment of the present invention.

[Explanation of symbols]

1 Second cladding layer 2 Second waveguide layer 3 Second diffraction grating 4 Active layer 5 First waveguide layer 6 First cladding layer 7 First diffraction grating 8 AR (anti-reflection) coat 9 HR (high reflection) coat

Claims (3)

[Claims] Claim 1: One end face is coated with a high reflection coating, the other end face is coated with a low reflection coat, a first diffraction grating is provided on the waveguide layer above the active layer, and a first diffraction grating is provided on the waveguide layer below the active layer. A distributed feedback semiconductor laser characterized in that a second diffraction grating is formed in a waveguide layer, and the periods of the first diffraction grating and the second diffraction grating are different from each other. Claim 2: The period of either the first diffraction grating or the second diffraction grating has a period corresponding to the Bragg wavelength, Λ
, where L is the resonator length, Λ(1+Λ/4L),
The other period is Λ (1-Λ/4L), and the period of the first diffraction grating and the second diffraction grating are opposite in phase on the AR end face side, and are in phase on the HR end face side. 2. The distributed feedback semiconductor laser according to claim 1. 3. A first diffraction grating is formed in a waveguide layer above the active layer, a second diffraction grating is formed in a waveguide layer below the active layer, and the first diffraction grating and the second diffraction grating are formed in a waveguide layer below the active layer. A distributed feedback semiconductor laser characterized in that a second diffraction grating has a different period and amplitude.