JPH0220087A - Distributed feedback type semiconductor laser element - Google Patents

Abstract

PURPOSE:To manufacture laser elements wherein single longitudinal oscillation is obtained at a high yield rate by optimizing the range of a normalized bonding coefficient kappaL that is proportional to the product of the depth of a diffraction grating and the length of a resonator and the range of a reflectivity at an end surface, in a distributed feedback type laser element having the diffraction grating whose phase shift part is located in the vicinity of the center of the inside and a lightguide structure. CONSTITUTION:A lightguide has a periodic structure in the direction of the axis of a resonator. One or more phase shift regions are formed in the distances from the center of the resonator having said periodic structure in both directions. The distance is less than 20% of the length of the resonator. Or a deformed lightguide region corresponding to said phase shift region in an equivalent mode is provided. The reflectivities at both end surfaces are 5-15%. The value of the product kappaL of a bounding coefficient (kappa) and the length L of the resonator is set at 0.6<=kappaL<=1.0. Within the ranges of 0.6<=kappaL<=1.0 and 5<=R(reflectivity)<=15%, a yield rate of 30-50% is obtained in the ranges characterized by relatively easy manufacture and gentle conditions can be obtained. When the reflectivities at both end surfaces are largely different, the trend becomes different. However, there is no large difference within the ranges of above described manufacturing conditions.

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial Application Field) The present invention relates to a distributed feedback semiconductor laser device that performs oscillation operation by performing optical feedback using a diffraction grating formed on an optical waveguide.

(Prior art) Distributed feedback semiconductor laser devices (D
FB-1,0:Distributed Feedba
The ck La5er Diode) has periodic concavities and convexities (diffraction grating) formed in the direction of its resonance axis, and performs optical feedback preferentially only for the longitudinal mode that matches the period of the diffraction grating, resulting in a single This enables longitudinal mode oscillation (one oscillation line). Therefore, expectations are high for this laser device, and distributed feedback semiconductor laser devices using Ga1nAsP/InP-based materials have been put into practical use, particularly as light sources for long-distance, high-capacity optical communications.

By the way, although distributed feedback semiconductor laser probes have these advantages, they are subject to significant yield constraints regarding single longitudinal mode properties due to the influence of reflective end faces at both ends that are inevitably formed during manufacturing. . That is, the positional relationship between this reflective end face and the phase of the diffraction grating has a great influence on the oscillation characteristics of the longitudinal mode. In addition, since it is technically difficult to control the positional relationship between the two, this problem is reduced to a probabilistic problem (yield).

In order to solve this problem, as shown in FIG. 6(a), we created a discontinuous part ( A structure has been proposed in which the reflectance of both end surfaces is made as close to zero as possible by providing a phase shift λ (phase shift) 2 and applying an AR coating (anti-reflection coating) to both end surfaces.

Furthermore, a phase shifter (1) structure equivalent to the phase shifter 2 has also been proposed; for example, as shown in FIG. 6(b),
The shape of the central portion 3a of the waveguide 3 is changed without providing a discontinuous portion in the diffraction grating 1. With this configuration, the phase velocity of the waveguide 3 changes at the central portion 3a, resulting in an equivalent phase shift.

(Problem to be Solved by the Invention) However, even in such a phase-shift distributed feedback laser device, the product κL of the coupling coefficient and the cavity length is
It has been pointed out that if the value of (corresponding to the amount of optical feedback by the diffraction grating) is not around 1.25, hole burning in the axial direction will occur and the single longitudinal mode characteristics will be impaired (Hida et al. Institute of Electronics, Information and Communication Engineers, Optoelectronic Electronics Study Group 0QEft6-7 PP, 49-58.1987
Year).

In addition, it is necessary to keep the reflectance of the end face extremely low.
AI with high precision? Court skills are required. Furthermore, in order to bring this reflectance reduction closer to perfection, it is necessary to adopt a window structure in which the waveguide is buried at the end face (for example, Utaka CT, AL, PP
, 23 [1-245, IEEE JOUI? NAL
OFQUANTUM IELECTRONIC8VO
L, QE-20, 1984).

The present invention has been made to solve the above-mentioned problems, and provides a distributed feedback semiconductor laser device that is relatively easy to manufacture and has single longitudinal mode characteristics and a relatively high crystal yield. .

[Means for Solving the Problems of the Invention] The distributed feedback semiconductor laser device of the present invention comprises an optical waveguide having a period tM structure in the direction of the cavity axis, and a shape of the circumference of the optical waveguide. 20 of the resonator length in both directions from the center of the resonator with
% or less, or a waveguide deformation region that corresponds equally to the phase shift head, and the reflectance of both end faces is 5 to 15%, and The product κL value of the coupling coefficient and the resonator length is 0,6≦κ
It is characterized in that it is configured to have an LSI of 0.

In addition, the periodic structure has a phase shift region of 1/8 of the channel wavelength at two points that approximately divide the length of the resonator having this periodic structure into three equal parts, or a waveguide deformation that equivalently corresponds to this phase shift region. In a distributed feedback semiconductor laser device having a region, the value of the product κL of the coupling coefficient and the cavity length is 0.5.
In the case of ≦κLSI, 3, the reflection q・1 ratio of both end faces is 5 to 20.
%, and when the value of κL is 1.2≦κL≦2.1, the reflectance of both end faces is set to 2% or less.

That is, in a distributed feedback laser device that has a diffraction grating and a waveguide structure that have a phase shift portion near the center of the interior,
By optimizing the range of the normalized coupling coefficient κL, which is proportional to the product of the depth of the diffraction grating and the resonance-length, and the range of the reflectance of the end facets, the single longitudinal mode yield can be increased with a simple implementation process. It is something that can be done.

(Function) The feature of the present invention is that the light intensity distribution inside the laser element is taken into consideration when determining the optimum values of the reflectance of the end face and the κL value. The instability of is taken into account. Conventionally, the gain difference Δα (-α1-α0) between the vertical mode with the lowest threshold (main mode) and the mode with the next lowest threshold (secondary mode)
It was believed that the higher the value, the better.

The present invention is characterized by paying attention to the internal light distribution to make it more realistic, and finding an optimum value different from the conventional one.

(Example) Hereinafter, examples of the present invention will be described in detail with reference to the drawings and with theory and calculation examples.

FIG. 1 is a schematic diagram showing an example of oscillation mode characteristics of a distributed feedback semiconductor laser device.

The origin of the horizontal axis shows the oscillation under the Bragg condition, and the horizontal axis shows the deviation of the propagation constant from the Bragg condition (δ−β
-βo1β0 is plotted as the product of the propagation constant under the Bragg condition) and the resonance error. Further, the vertical axis is the product of the threshold gain α corresponding to the mirror loss and the resonator length.

The white circles in the figure indicate each longitudinal mode, and the lowest threshold gain α
0 is the main mode (single longitudinal mode oscillation).

The normalized threshold gain difference ΔαL is the difference between the normalized threshold gain αOL of the main mode and the normalized threshold gain α1L of the secondary mode having the next smallest threshold gain. Conventionally, it has been believed that the larger this value is, the better the +11-longitudinal mode property is, and the larger the normalized coupling coefficient κL, the larger the ΔαL, so it has been desired to increase κL.

However, as mentioned above, it has been pointed out that the stability of the single longitudinal mode after oscillation is not only due to Δα, but is also strongly influenced by the internal light intensity distribution in the axial direction of the oscillator. This is called directional hole burning.

In other words, if there is a region with strong light intensity toward the cavity side,
Note that the incoming carrier density in that region decreases compared to other regions. This axial distribution of carrier density makes the axial refractive index distribution non-uniform through plasma effects and band gap changes.

In the case of a distributed feedback laser device, a spatial change in the equivalent refractive index means that the order of the light wave changes from place to place relative to the diffraction grating, which is equivalent to 1M.

This means that a phase shift has been formed. Such structural changes cause continuous changes in the oscillation conditions of the distributed feedback laser device. This is called axial hole burning.

If Δα is large, this change will continue to occur until Δα becomes zero, resulting in undesirable non-linear phenomena such as changes in external differential quantum efficiency and adverse effects on transient waveforms. Δα
When becomes zero, the mode jumps to the next longitudinal mode, causing a kink (Hnk) in the current-light output characteristics.

Therefore, an element with a small Δα has no margin, and mode jumps easily occur after oscillation. However, even if Δα is small, if this hole burning does not occur, the single longitudinal mode property is maintained. Conversely, even in an element with a large Δα, after strong nonlinearity occurs in the current-optical output characteristics due to axial hole burning, a mode jump may occur and the single longitudinal mode property may be destroyed. Of course, it is most preferable that Δα is large, the internal light intensity distribution is flat, and axial hole burning does not occur.

In this way, the behavior of distributed feedback semiconductor laser devices after laser oscillation is being clarified both experimentally and theoretically.

Against this background, the present invention has been developed to easily control a distributed feedback semiconductor laser device that has a relatively large Δα, is less affected by axial hole burning, and actually achieves stable single longitudinal mode operation. We are proposing a structure that can be manufactured with high yield in the process.

The numerical limitations of the present invention are based on the following calculations,
A similar tendency can be confirmed experimentally as well.

That is, the normalized mode gain difference ΔαL is 0.05 or more, and the ratio F of the minimum value and maximum value of the light intensity distribution in the resonator axis direction is
R-, 1 min, / I max is 0.8 or more (the closer the FR value is to 1, the flatter the light distribution and the smaller the axial hole burning. In Perot type laser element, FR
is approximately a little over 0.8).

A distributed feedback semiconductor device that satisfies these two conditions at the same time has relatively excellent single longitudinal mode characteristics.

Incidentally, the definition of FR is illustrated in FIG.

Figures 3 and 4 show the probability that a laser element that satisfies the above two conditions can be obtained at any cleavage plane phase using the normal and single-sided reflectance values of κL as parameters. FIG. This calculation is based on the fundamental theory of distributed feedback lasers using the coupled wave equation. The value of κL is from 0.2 to 2.2 in 0.1 increments (2
1 point), and the reflectance was changed from 0 to 100% in 0.5% increments (21 points). Each point indicates the probability of obtaining an element that satisfies the conditions of FR≧0.6 and ΔαL≧0.05 when the phases of the reflective end faces are changed by 8Xs-84 (paths) on both sides (not shown). ).

Contour lines represent this probability in an easy-to-understand manner, and are a kind of yield map. This one drawing is 8 x 8 x
This corresponds to the investigation of 21X21-28224 types of distributed feedback laser elements.

The figure inserted in the figure shows the calculation of the low reflection area of 1% or less, calculated in 0.1% increments.

FIG. 3 is a yield map of a distributed feedback laser with a λ phase shifter in the center. It can be seen that a yield of 90% or more can be obtained under conventional ideal conditions, that is, zero reflection on both end faces and around κL -1.25. However, according to more detailed calculations around this condition (inset), in order to obtain a yield of 50% or more, the reflectance must be 0.2% or less (1,2≦κLS
It turns out that it is necessary to have a value of I, 4).

Obtaining a reflectance of 0.2% or less on both sides is achieved by anti-reflection (AR)
) It is difficult to control with just a coat, and the number of processes increases, such as when using it in combination with a window structure. On the other hand, even in the range of 0.6≦κLSI and 0.5≦R (reflectance) of 515%, a yield of about 30 to 50% can be obtained under relatively easy manufacturing conditions. In FIG. 3, the calculations were made assuming that the reflectances of both end faces are the same, but if the reflectances of both end faces are significantly different, the trends will be different, but within the range of the above manufacturing conditions, there is no big difference.

In this example, the structure of the distributed feedback laser is determined to satisfy this condition. In fact, when we prototyped a distributed feedback laser with a λ-shifter, even though we suppressed the reflectance to about 2% with an AR coating, we were not able to obtain as high a yield as previously expected, and the non-linear Many of the devices showed a current-optical output characteristic, and the mode jumped, resulting in bimodal oscillation. On the other hand, more elements with a relatively large reflectance of about 10% had a single longitudinal mode that was dynamically stable.

Figure 4 is a yield map for a structure with a λ/8 phase shifter in the center, and the ideal condition is that the reflectance on both end faces is zero and is set around κL -1,35. . In this structure, according to detailed calculations near the ideal conditions, a yield of 50% can be obtained in the range of 1.2≦κLSI, 6, and R≦0.4%. Although this condition is less strict than that for the λ ha shift structure, it is still a fairly difficult control condition. Even in this structure, 0, 6≦κLSI, which is far from the ideal condition,
, 30~ which is relatively achievable in the range of 0.5≦R≦15%.
A 50% yield region exists.

Now, FIG. 3 is a yield map of a structure in which λ/8 phase shifters are provided at two points that divide the resonator length into three approximately equal parts. In this case, the conditions for obtaining a stable single longitudinal mode are considerably relaxed. In this case, according to detailed calculations,
A yield of 40% can be obtained over a wide range of 1.2≦κL≦2.1 and R52%. Within this range, the feasibility is quite high. Further, as in the previous two examples, when 0.5≦κLSI, 3, the yield can be made approximately 30 to 40% by setting the reflectance R of both end faces to 5%≦R≦20%. By providing the λ/8 shifters at two trisecting points in this manner, a high single longitudinal mode yield can be achieved with a wide range of structural parameters. In fact, a device with this structure is InGaAsP/
As a result of trial manufacturing using InP-based materials, stable single longitudinal mode operation was confirmed with relatively high yield.

[Effects of the Invention] As explained above, according to the distributed feedback semiconductor laser device of the present invention, it is possible to manufacture a laser device capable of single longitudinal mode oscillation at a high yield. The present invention is based on the essential characteristics of distributed feedback semiconductor laser devices, and is intended to facilitate mass production of laser devices.

Therefore, it can greatly contribute to cost reduction and widespread use.

[Brief explanation of the drawing]

FIG. 1 is an αL-δL diagram for explaining ΔαL, FIG. 2 is a diagram showing an example of the internal light intensity distribution of a distributed feedback semiconductor laser device for explaining the definition of FR, and FIG.
The figure shows that when the resonator has a λ phase shifter in the center, when both end face reflectances (vertical axis) and the value of κL (horizontal axis) are changed, when the value of ΔαL is 0.05 or more, the FR (internal light intensity Figure 4 is a contour line representation of the yield obtained for devices with a parameter (parameter indicating flatness of distribution) of 0.6 or more, with λ/
A similar diagram showing contour lines of the yield in the case of having 8 phase shifters. Figure 5 shows the contour lines of the yield in the case of having λ/8 phase shifters at two points dividing the resonator into three equal parts. FIG. 2 is a plan view and a cross-sectional view showing a waveguide structure of a distributed feedback semiconductor laser device having a phase shift structure. Applicant: Toshiba Corporation

Claims (1)

[Claims] (1) An optical waveguide having a periodic structure in the direction of the resonator axis and an optical waveguide formed between the center of the resonator having the periodic structure and a distance of 20% or less of the resonator length in both directions. It has one or more phase shift regions or a waveguide deformation region that equivalently corresponds to the phase shift regions, and the reflectance of both end faces is 5 to 5.
15% and the product κL value of the coupling coefficient κ and the resonator length L satisfies the following: 0.6≦κL≦1.0. (2) A waveguide whose periodic structure corresponds to a phase shift region of 1/8 of the channel wavelength or equivalently corresponds to this phase shift region at two points that approximately divide the length of the resonator having this periodic structure into three equal parts. In a distributed feedback semiconductor laser device having a deformation region, when the value of the product κL of the coupling coefficient κ and the cavity length L is 0.5≦κL≦1.3, the reflectance of both end faces is set to 5 to 2.
0%, and when the value of κL is 1.2≦κL≦2.1, the reflectance of both end faces is set to 2% or less.