JPH0269983A - Distributed feedback-type laser - Google Patents

Abstract

PURPOSE:To get a laser to operate at a stable single longitudinal mode by structuring a part of a wave guide structure whose equivalent refractive index is changing relatively with the other part so that a threshold gain difference between one mode and the other mode which are wanted to be oscillated is made large. CONSTITUTION:A secondary diffraction grating 12 is formed on an n-type InP substrate 11. On the secondary diffraction grating 12, an n-type GaInAsP optical waveguide layer 13, an undoped GaInAsP active layer 14, a p-type GaInAsP anti-meltback layer 15, a p-type InP clad layer 16 and a p<+>-type GaInAsP ohmic contact layer 17 are piled up in order. After that, a mesa.stripe section is formed by etching with the thickness of the waveguide structure uniform while the width of the central part is made narrow, forming a phase shift section 30. Then, around the phase shift section 30, a p-type InP layer 18, an n-type InP layer 19 and an undoped GaInAsP gap layer 20 are deposited in series and then are buried. By this method, a laser can be operated at a stable single longitudinal mode with the TM mode well under control.

Description

DETAILED DESCRIPTION OF THE INVENTION [Purpose of the Invention (Industrial Application Field) The present invention relates to a distributed feedback laser that performs optical feedback using a diffraction grating formed along an optical waveguide.

(Prior Art) In recent years, various semiconductor light emitting devices have been widely used as light sources for optical communications and optical disk devices.

Among these, distributed feedback semiconductor lasers (DFBs) have periodic perturbations (diffraction gratings) installed along the optical waveguide.
ibuted Feedback)
e r ) can easily realize oscillation at a single wavelength (single longitudinal mode) due to the wavelength selectivity of this diffraction grating. Currently, this device is used as a light source for long-distance, high-speed optical communications.
It has been put into practical use using nAsP/InP-based materials.

As shown in FIG. 4, the structure of this distributed feedback semiconductor laser is such that the fixed open end surface 40 is reduced in repulsion rate by AR (non-reflection), etc., and is placed in the center of the resonator. Discontinuous portion 43 of the period of the diffraction grating 42 (1/1 of the tube wavelength λ)
Structures with a phase shift corresponding to 4) are well known. This element has a plug wavelength (the
It is extremely advantageous for single longitudinal mode operation because it is possible to oscillate at a Bragg wavelength and also has a large gain difference from other longitudinal modes.

In addition, an equivalent λ/4 phase shift structure that has the same effect as this λ/4 phase shift structure is also known (for example, H, 5oda et al, Electronic
csLetters, vol, 20. pp, 1016
-10181984). This is an optical waveguide 52 partially deformed along its axial direction, as shown in FIG. 5, an example of the waveguide structure. In this case, the thickness of the waveguide is constant, and the width of the central portion 54 is set from 1 μm to 2 μm.
It is changing. The taper in the middle of the change is intended to prevent unnecessary reflections due to abrupt discontinuities. The change in width is
It works without causing a relative change in the equivalent refractive index of that part. Therefore, the phase velocity of the guided light changes, and the phase of the diffraction grating that the guided light feels changes before and after passing through that part. This produces an effect equivalent to introducing discontinuity in the period of the diffraction grating.

(Problems to be Solved by the Invention) Incidentally, a semiconductor laser has two modes with different polarizations. In other words, there is a D-F mode and a TM sub-mode.

So-called Fabrype o with both end faces formed by pleats.
In the -(Fabry-Perot) type laser, the fold surface acts so as to increase the resistance to the TE mode, so the threshold value of this mode becomes small and the laser oscillates in the TE mode.

On the other hand, in the λ/4 phase shift structure in which the reflection at both end faces is brought close to O, this mechanism does not work.

Therefore, in principle, there is no threshold difference between the two modes, E and TM, and single longitudinal mode operation is impossible. However, since the coupling coefficient is different between Ding E mode and Ding M moto,
It barely oscillates in TE mode. However, the threshold gain difference at this time is at most 3 to 4 cm. By increasing the coupling coefficient, the gain difference between TE and TM can also be increased (for example, S, Akiba
et al.

, The Transaction of the I
ECof Japan, vol, E69pp, 389-
391.1986), but the stability of the longitudinal mode deteriorates due to axial hole burning (for example, H,5od
a et al, IEEE Journal of
Quantum Electronics, QE-23,
pp. 804-814.1987). Therefore, this value is the actual limit.

When a DFB laser is modulated with high-speed PIT 1~RAE 1~ and used for long distance use, a gain difference of 6 cm<-1> or more is desired, but a sufficient gain difference cannot be obtained at present. In the λ/4 phase shift structure in which the reflection on both end faces is close to ○, there is a gain difference of about 20 cm-1 from the submode of the same polarization (particularly, the suppression of TM mode is insufficient). can be said to be fatal.

The present invention overcomes the above-mentioned drawbacks of the prior art, and provides a distributed feedback laser that can suppress the TM sub-mode, realize stable single longitudinal mode operation, and can perform high-speed, high-efficiency modulation. That is, the present invention provides a distributed feedback laser exhibiting the following characteristics.

(2) The threshold gain difference (Δα) with the vertical mode (secondary mode) of the same polarization (TE) is large.

■Threshold gain difference (△
αTM> is also large.

■Less effect of axial hole burning.

Structure of the Invention (Means for Solving the Problems) The present invention provides a distributed feedback laser in which one or more partial structures in which the equivalent refractive index of the waveguide structure changes relative to other portions is used. There are two or more of them, and the phase shift amount of the equivalent phase non-shifting portion is optimized in the D-E mode, and the threshold value is increased in the TM sub-mode.

(Function) In the phase shift by the diffraction grating, the phase discontinuity is almost 1
However, since the equivalent phase shift section has a certain length, it is possible to differentiate the amount of phase shift between the T mode and the TM sub-mode. By increasing the threshold value of the TM submode, it is possible to realize a distributed feedback laser that suppresses the TM submode and operates in a stable single longitudinal mode.

More generally speaking, in the present invention, for example, in the TE mode, the shift amount is λ/4, and in the TM sub-mode, the shift amount is λ/4.
It is provided with an equivalent phase shift section having a phase shift of 10 or less.

Thereby, the gain difference due to the coupling coefficient is added up, and the minimum value of 5 cm −1 can be achieved.

Now, Figure 2 shows the normalized sub-mode gain difference Δ in TE mode.
Calculate the normalized gain difference ΔαLTM between αL and TM submode (a negative value indicates that the TM submode has a lower threshold value), and use the phase shift region length Lo as the horizontal axis.
~It is a figure. However, the difference due to the difference in L between the mode E mode and the mode T M "E-mode" is not taken into consideration.

In the same figure (a), the resonator length is 300 μm and the waveguide width is 1
The width of the μm1 phase shift portion is 0.5 μm, and both end faces of the resonator are non-reflective. From this figure, L −40μ
It can be seen that both ΔαL and ΔαLT)l can be increased near m. That is, at this time ΔαL=0.5,
ΔαL, , = 0.15.

When converted to α, they are 17 cm and 5 cm', respectively.

In particular, when considering the difference in L, △αTM is 8~9c
m, exceeding the safe range of 6 cm.

In addition, as shown in FIG. 2(b), when the convergence width of the central phase shift part is 2 μm, the active layer thickness is 0.05 μm.
Even if m, Δαt−rq can only be obtained at most about 0.05, and in that case, the glue.

The value of is also far from the maximum value of ΔαL. Therefore, the desired purpose cannot be achieved. Active layer thickness is 0.10
There is almost no effect when it is μm.

In short, when the amount of phase shift felt by the TE mode has a large gain difference from that of the sub-mode, the difference Δφ between the amount of phase shift of the TE mode and the amount of phase shift felt by the TM sub-mode is ±π/
A value close to 2(λ/4) can increase the gain difference with the 7-mode. In other words, ideally, in TE mode, λ/
4. It is desirable that the shift is 1 or more in TM submode ○.

In FIG. 1, the value of the difference Δφ between the amount of phase shift in the D-F mode and the amount of phase shift felt by the TM-D is plotted using the active layer width and the active layer thickness as parameters. The same figure (a
) shows the case where the width of the center part is 0.5 μm smaller than the other parts, and the figure (b) shows the case where the width of the center part is larger by 0.5 μm. When the width of the central part is 0.5 μm smaller, △φ of 0.3π or more is obtained over a fairly wide range, and 3 cm
A gain difference of -1 or more can be ensured. On the other hand, if the width at the center is 0.5 μm larger, the active layer width is 0.6 μm.
m or less, and the active layer thickness must be 0.15 μm or more.

The effective waveguide parameters for M-mode suppression are:
Depends on the individual DFB structure - cannot be specifically described. That is, the waveguide structure must be designed comprehensively because it is relatively related to other parts than the phase shift part. Regarding the waveguide and its phase shift structure, the width,
Many combinations of layer structure including thickness and refractive index are possible. However, the equivalent λ of InGaASP/InP system
/4 Schiff l-D F B Sea 1A, the total thickness of the waveguide groove layer including the active layer is 0.3 μm or less,
and when the thickness of the active layer is 0.1 μm or less and the width of the main part of the waveguide structure layer (peripheral normal part) is 1 μm or less,
If the width of the central phase shift region length is made smaller than the main part by 0.5 μ... or more, the gain difference from the D-M mode can be increased with easy accuracy. Note that the first half of the dimension definition is a necessary condition for obtaining a stable fundamental transverse mode. In connection with this, FIG. 1 also shows cut-off conditions for higher-order transverse modes.

(Example) Below, an example in which the present invention is applied to a λ/4 phase shift type distributed feedback laser using GaInASP/InP-based materials will be described with reference to the drawings.

FIG. 3 shows horizontal and vertical sectional views and a plan view of the distributed feedback laser according to the embodiment.

First, a second-order diffraction grating 12 is formed on an n-type InP substrate 11, and an n-type Ga1nASP optical waveguide layer 13 (λ-
1,27 μm band composition, 0.1 μm thickness), undoped Ga
InASP active layer 14 (λ-1, 55 μm band composition, 0.
1 μm thick), p-type GaIr1ASP angel meltback layer 15 (λ-1.27 μm band composition, 0.05 μm thickness)
, p-type InP cladding layer 16, p+-type GaInASP ohmic contact layer 17 (λ-1, 15 μm band composition)
are sequentially stacked.

Thereafter, mesa stripes are formed by etching. At this time, the waveguide structure has a uniform thickness and a length in the center. The width was narrowed over . Outer uniform part 22
The width of the narrow phase shift portion 30 is 1 μm, and the width of the narrow phase shift portion 30 is 0.0 μm.
It was set to 5 μm. Also, the length of the phase shift section 30. is 4
It was set to 0 μm.

Next, the p-type InP layer 18, the n-type In2 layer 1
9. Undoped GaInASP cap layer 20 (λ-1
, 15 μm band composition) is continuously grown and embedded. At this time, since the current is blocked by the pn reverse bias junction 21 in the buried region, the current is efficiently injected only into the active layer stripe 14.

The length of the resonator was 300 μm, and the end faces of the resonator were coated with anti-reflection coats 1 to 1. Also, the value of L for the normalized coupling coefficient is
It was adjusted to around 1.25, where the influence of hole burning in the axial direction is least.

According to the distributed feedback laser of this example, stable single longitudinal mode operation was obtained by sufficiently suppressing the TM submode.

Although the above-mentioned embodiment describes the case where there is one phase shift section, the present invention is not limited to this, and can be similarly applied to a case where there are two or more phase shift regions. It is also applicable to a method in which the phase shift region does not change in shape and the refractive index of that portion is changed by independently controlling injection excitation from the outside.

[Effects of the Invention] According to the present invention, stable single longitudinal mode operation can be realized while suppressing the TM submode. Furthermore, this provides a distributed feedback laser capable of high-speed, high-efficiency modulation.

[Brief explanation of the drawing]

Figure 1 is a diagram in which the value of the difference Δφ between the amount of phase shift in the TE mode and the amount of phase shift felt in the TM submode is plotted using the active layer width and active layer thickness as parameters. Figure 2 shows the case where the width of the central part of the waveguide structure is 0.5 μm smaller than the other parts, and the same figure (b) shows the case where the width of the central part of the waveguide structure is 0.5 μm wider than the other parts.
The normalized sub-mode gain difference ΔαL of the mode and the normalized gain difference ΔαLTM of the TM sub-mode are the length of the phase shift region. is plotted on the horizontal axis. Figure (a) shows the case where the width at the center of the waveguide structure is 0.5 μm smaller than the width, and figure (b) shows the case where the width at the center of the waveguide structure is 0.5 μm smaller than the center width. 0. than others.
Figure 3 is a diagram showing horizontal and vertical cross-sectional views and a plan view of the distributed feedback laser of the example, Figure 4 is a schematic cross-sectional structure of a waveguide structure with a λ/4 phase shift structure. FIG. 5 is a plan view showing an example of a waveguide structure having an equivalent λ/4 phase shift structure. Agent Patent Attorney Nori Chika Yudo Kikuo Takehana

Claims (1)

[Claims] In a distributed feedback laser that performs optical feedback using a diffraction grating provided along an optical waveguide, one or more partial structures in which the equivalent refractive index of the waveguide structure changes relative to other parts are used. The partial structure is set so that the threshold gain difference between one mode to be oscillated and the other mode is large among two oscillation modes having different polarizations and propagating through this partial structure. A distributed feedback laser characterized by: