JP2740762B2 - Integrated semiconductor laser device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an integrated semiconductor laser device, and more particularly to a mode control thereof. [Prior Art] FIG. 3 is a cross-sectional view of a conventional integrated semiconductor laser device, in which (1) is a semiconductor substrate made of P-type GaAs,
(2) is an n-type GaAs formed on the semiconductor substrate (1).
A semiconductor layer (3) is formed by etching the semiconductor layer (2) in a stripe shape at an interval (d) of 2 [μm] and forming a waveguide having a width (W) of 3.5 (μm). The difference between the refractive index of the stripe groove (3) and the equivalent refractive index between the stripe grooves (3) is 0.002. (4) is a lower cladding layer of P-type AlGaAs formed on the semiconductor layer (2) and the stripe groove (3), and (5) is a P-type AlGaAs layer formed on the lower cladding layer (4).
(6) is an active region formed in the active layer (5) on the stripe groove (3); (7)
Is an upper cladding layer made of n-type AlGaAs formed on the active layer (5). Next, the operation will be described with reference to FIG. When a voltage is applied to the laser chip, the current is blocked by the semiconductor layer (2) and flows only in the stripe groove (3). Therefore, only the active layer (5) in this portion becomes the active region (6). Light generated in the active region is vertically confined by the upper cladding layer (7) and the lower cladding layer (4), and horizontally confined by the difference in equivalent refractive index and loss between the stripe groove (3) and the outside thereof. Oscillate. The laser beams oscillated in the individual stripe grooves cause optical coupling with each other to form a unique mode as a whole, that is, a super mode. If the number of stripe grooves is N, there are N super modes, which are described by ν = 1, 2,..., N. For most integrated lasers, the highest-order supermode (8) with ν = N, which has a similar shape of the field strength distribution and the gain distribution, has the largest gain, and the fundamental mode (9) with ν = 1 next has the largest. Is known (FIG. 4). [Problems to be Solved by the Invention] Since the conventional integrated semiconductor laser device is configured as described above, it is the highest that the electrolytic strength becomes zero in the semiconductor layer (2) between the stripe grooves (3). The next super mode (8) is the easiest to stand because it has the least loss,
Since the far-field image is minimized in the normal direction to the end face from which the laser light is emitted, for example, when coupling with a fiber,
There have been problems such as extremely difficult bonding. An object of the present invention is to provide an integrated semiconductor laser device capable of maximizing a far-field image in a direction normal to an end face. [Means for Solving the Problems] The integrated semiconductor laser device according to the present invention is characterized in that the value of the difference between the equivalent refractive index in each waveguide and the equivalent refractive index difference between each waveguide adjacent to this waveguide is reduced. Correspondingly, by limiting the width of the semiconductor layer between the respective waveguides and the width of the respective waveguides, an inherent mode caused by optical coupling with each other is eliminated. [Operation] The integrated semiconductor laser device according to the present invention is configured to remove a unique mode generated by optical coupling, that is, the highest-order supermode among the supermodes. The basic mode having the largest gain next to the mode is easily set. Maxwel
The process of deriving the cutoff condition of the highest-order supermode in FIG. 2 from the equation of l will be described in detail. Maxwell's equation is as follows: ▽ × E = −∂B / ∂t ▽ × H = i + ∂D / ∂t ▽ · D = ρ ▽ · B = 0 where E: electric field, B: magnetic flux density B = μH, μ : Magnetic permeability,
H: magnetic field, i: current density i = σE, σ: conductivity, D: electric flux density D = εE. Since μ = μ ₀ (μ ₀ : magnetic permeability in vacuum) and σ = 0 in a semiconductor layer without loss, the above equation is expressed as ▽ × E−μ ₀ ∂H / ∂t ▽ × H = ε∂E / ∂t. In FIG. 5, in the case of the TE mode, the electric field in the x direction follows ∂ ² Ey / ∂x ² + (n ² k ₀ ² -β ² ) Ey = 0. Where n: refractive index of each semiconductor layer, β: ω ² μ
₀ ε, ω = 2πν, ν: frequency, k ₀ = 2π / λ ₀ λ ₀ : wavelength in vacuum. This solution is generally expressed as Ey = Aexp (jγx) + Bexp (−jγx) for each semiconductor layer, but the electric field intensity distribution (mode) that is actually allowed to exist in the entirety of the semiconductor layers I to IV is as follows. At the boundaries of, only those that have a continuous and smooth distribution. Here, A and B are constants, j is a complex number, and γ = n ² k ₀ ² + β ² . That is, since there are three boundaries in FIG. 5, at the boundary of x = 0, Ey I = Ey II, ∂Ey I / ∂x = ∂Ey II /
At the boundary of ∂xx = w, Ey II = Ey III, ∂Ey II / ∂x = ∂Ey I
II / ∂xx At the boundary of x = w + d, Ey III = Ey IV, ∂Ey III / ∂x =
Only the mode in which the six equations of ∂Ey IV / ∂x hold can actually exist. Here, Ey I, Ey II, Ey III, and Ey IV are electric fields in each semiconductor layer. The case where there are 10 stripes in FIG. 1 corresponds to the case where there are 20 boundaries between semiconductor layers. Therefore,
The electric field intensity distribution (super mode) that can be present in this array laser is only one that satisfies a total of 40 boundary conditions. Normally, when the number of stripes is 10, a super mode of 10 can be established.
And by adjusting the value of the refractive index n of each region,
The number of super modes satisfying these 40 boundary conditions can be reduced to nine, where the highest-order super mode is no longer allowed. Embodiment An embodiment of the present invention will be described below with reference to FIG. In the figure, (1) to (7) are the same as those of the prior art, and (11) is a 10-waveguide consisting of 10 stripe grooves formed by etching the semiconductor layer (2). The waveguide (11) has a value of a difference between an equivalent refractive index in the waveguide (11) and an equivalent refractive index between the waveguides (11) adjacent to the waveguide (11), that is, an equivalent refractive index difference (Δn ) To 0.
002, and each of the above waveguides (1
The width (W) of (1) is 2 [μm], and the width (d) of each semiconductor layer (2) between the waveguides (11) is 2 [μm]. The super mode is removed. That is, the unique mode is uniquely determined if the refractive index distribution and the wavelength of light are determined. That is, when Maxwell's equation, which is a basic equation of electromagnetics, is solved under the boundary condition of the refractive index distribution, a propagation constant that determines each mode is obtained. However, when the width (W) of the waveguide (11) and the interval (d) thereof are reduced with respect to a certain refractive index difference Δn, there is no solution for a higher-order mode. That is, the mode is cut off and the propagation constant no longer exists. This is why unique modes can be eliminated. The integrated semiconductor laser device configured as described above is configured to remove the highest-order supermode, so that the highest-order supermode is never excited no matter how high the mode gain is. Therefore, oscillation occurs in the fundamental mode with the second highest mode gain, ν = 1, whereby the far-field image is maximized in the direction normal to the end face. The equivalent refractive index difference (Δn) is selected in consideration of a decrease in the refractive index due to injection of carriers into the active region in order to maintain stable oscillation, and the width of the stripe groove (11) serving as a waveguide ( Since W) and the interval (d) are of a size that can be easily formed by existing technology, they can be manufactured with good yield and reliability. In the above embodiment, the equivalent refractive index difference (Δn) is
Although 0.002, the width (W) of the waveguide (11) and the interval (d) thereof were set to 2 [μm], the present invention is not limited to these values, and the highest-order supermode derived from Maxwell's equation What is necessary is to satisfy the condition for removing the refractive index. For example, as shown in FIG. 2, the equivalent refractive index difference (Δ
The width (W) and interval (d) of the waveguide (11) are selected on the left side of each curve drawn when n) is set to 0.0015, 0.0034, 0.002, and the stripe groove (11) is formed with the values. It is good to do. In the above embodiment, the waveguide (11) is formed by a stripe-shaped groove, but the waveguide may be formed by a protrusion called a ridge. [Effects of the Invention] As described above, the present invention relates to the above-described waveguides corresponding to the difference between the refractive index of each waveguide and the equivalent refractive index difference between the waveguides adjacent to the waveguide. By limiting the width of the semiconductor layer and the width of each of the above-mentioned waveguides, a unique mode caused by optical coupling with each other, that is, the highest-order supermode is removed. Next, a fundamental mode having the next largest gain is easily formed, which has an effect that the far-field image can be maximized in the normal direction to the end face.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 (a) is a sectional view of an integrated semiconductor laser device showing one embodiment of the present invention, and FIG. 1 (b) is the refraction of the device shown in FIG. 1 (a). FIG. 2 is a diagram showing a refractive index distribution, FIG. 2 is a diagram showing the relationship between the width of a waveguide capable of removing an inherent mode, the width of a semiconductor layer between waveguides, and the equivalent refractive index difference, and FIG. FIG. 4 is a diagram showing a fundamental mode and a highest-order super mode, and FIG. 5 is a schematic diagram showing coordinate axes of a semiconductor layer. In the figure, (2) is a semiconductor layer, (5) is an active layer, (8) is an inherent mode, and (11) is a waveguide. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

(57) [Claims] In an integrated semiconductor laser device in which a laser beam generated in an active layer propagates through a plurality of waveguides formed by a plurality of stripes, a refractive index of each waveguide and a semiconductor layer between each waveguide adjacent to the waveguide. The width of the semiconductor layer between the waveguides and the width of the waveguides corresponding to the value of the difference between the refractive index and the width of the waveguide are defined as conditions for removing the highest-order supermode derived from Maxwell's equation. An integrated semiconductor laser device wherein the highest order mode among super modes generated by optical coupling with each other is removed by determining to satisfy the condition.