JPH07162026A - Photodetector possessed of optical branching function - Google Patents

Abstract

PURPOSE:To provide a photodetector which lessens a divergent angle of diffracted light in the direction of a resonant axis and is capable of largely improving diffracted light, which is separated into its spectral components on the basis of its wavelengths, in branching limit. CONSTITUTION:A photodetector is equipped with a semiconductor optical waveguide on which light ray is incident, a diffraction grating 8 provided to the optical waveguide, and a photodetecting element row 14 which detects diffracted light 13 projected out of the waveguide. The optical waveguide where the diffraction grating is provided is possessed of an active layer 6, and electrodes 1 and 11 are provided so as to inject a current into the active layer 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photodetector having a demultiplexing function, which is used in wavelength division multiplexing optical communication.

[0002]

2. Description of the Related Art In a photodetector having a demultiplexing function,
The light incident on the waveguide in which the diffraction grating is formed is emitted from the upper surface of the waveguide by the diffraction grating. The inclination angle φ of the diffracted light from the waveguide normal direction satisfies the following relationship with the pitch Λ of the diffraction grating and the wavelength λ of the incident light. sin φ = n _eff −qλ / Λ where q is an integer and n _eff is the equivalent refractive index of the waveguide.

Therefore, if the wavelength λ of the incident light changes, the tilt angle φ also changes, so that the diffracted light is split and emitted from the upper surface of the waveguide.

The change dφ of the tilt angle with respect to the wavelength change dλ of the incident light can be obtained by differentiating the above formula. / is given by _{dλ~ (dn eff / dλ-q} / Λ) (1).

For incident light in the 0.8 .mu.m band, d.phi./d.lamda. In the AlGaAs waveguide in which a diffraction grating (q = 2, .LAMBDA..about.0.24 .mu.m) of approximately second order is formed is .about.-0.
4 deg / nm. The wavelength resolution of the diffracted light depends on the wavelength dependence dφ / dλ of the tilt angle φ and the spread angle θ _L of the diffracted light in the cavity axis direction.

When the length of the diffracted light in the cavity axis direction (light guide direction) is D, the spread of the diffracted light in the cavity axis direction is the diffraction pattern when the parallel light passes through the slit D. It can be roughly approximated, and if the direction of the point where the diffracted light intensity is 1/2 of the center is the spread angle θ _L in the axial direction, it is expressed by the following formula (see FIG. 3). sin θ _L = 0.52λ / D λ: wavelength of light (2) The wavelength resolution Δλ of diffracted light is the wavelength dependence dφ of the tilt angle φ.
It is expressed by the following equation from / dλ and the spread angle θ _{L of the} diffracted light. 1 / Δλ = dφ / dλ · 1 / θ _L (3) From equations (2) and (3), the longer the region D where the diffracted light is radiated, the smaller the spread angle θ _L of the diffracted light becomes, and the wavelength The resolution is improved.

[0007]

However, the AlGaAs waveguide has a relatively large absorption coefficient for 0.8 μm band light, and the passive output waveguide emits diffracted light. The length in the resonator axial direction is about 200 μm. Therefore, the spread angle θ _L of the diffracted light is about θ _{L to} 0.13 deg according to the equation (2), and the wavelength resolution is about Δλ to 0.3 nm according to the equation (3). Insufficient performance.

Therefore, an object of the present invention is to provide a photodetector having a demultiplexing function, in which the divergence angle of the diffracted light in the resonance axis direction is significantly reduced, and the demultiplexing limit of the wavelength-resolved diffracted light can be greatly improved. To do.

[0009]

According to the present invention, in a photodetecting device having a demultiplexing function, which is composed of a semiconductor optical waveguide having a diffraction grating and a plurality of photodetecting elements, the waveguide has an active layer. And the absorption loss of the waveguide can be greatly reduced by injecting a current into the active layer. Therefore, the region D where the diffracted light is emitted is 1200 μm or more,
The wavelength is remarkably long, and the spread angle θ _L of the diffracted light in the resonance axis direction is very narrow, about 0.02 deg. Accordingly, the wavelength resolution is greatly improved to about 0.05 nm.

Further, in the passive optical output waveguide, since the intensity of the guided light is exponentially attenuated from the incident end, there is also an intensity distribution in the radiation area of the diffracted light.
Since the intensity of the guided light is almost constant, the radiation area of the diffracted light becomes uniform, and it becomes possible to obtain diffracted light with excellent uniformity and reproducibility.

That is, in the photodetector having the demultiplexing function of the present invention, the semiconductor optical waveguide into which light is incident, the diffraction grating formed in the waveguide, and the diffracted light emitted to the outside of the waveguide. A plurality of portions for detecting light, the optical waveguide in which the diffraction grating is formed has an active layer, and the intensity of the guided light in the waveguide is substantially constant. A current is injected into the active layer so that

More specifically, a part of the waveguide has at least one optical amplifier for amplifying incident light by injecting a current, or a waveguide light is provided on the opposite side of the light incident end face of the waveguide. Or a reverse bias is applied to the active layer as a means for absorbing the guided light.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a drawing which best shows the characteristics of the first embodiment, in which the photodetector array 1 is arranged at a position where the diffracted light 13 emitted from the semiconductor optical waveguide having the diffraction grating 8 can be received.
4 are arranged. In the device having the semiconductor optical waveguide having the diffraction grating 8, the GaAs buffer layer (not shown), A
lGaAs lower optical confinement layer 4, GaAs / AlGaAs
After stacking the multi-quantum well active layer 6 and the AlGaAs optical waveguide layer 7, after forming the secondary diffraction grating 8 by the interference exposure method or the like, the AlGaAs upper optical confinement layer 9 and AlGa are further formed.
The As contact layer 10 is laminated. After that, the AlGaAsBH buried layer 5 for confining light in the lateral direction is formed, and further the metal electrodes 1 and 11 for current injection are formed.
A window 12 through which diffracted light is emitted is formed in the upper electrode 11.

Incident light passes through a dielectric antireflection film 2 (not shown, but an antireflection film also exists on the opposite end face) formed on the end face of the waveguide and is coupled to the optical waveguide layer 7. However, in the state where no current is injected into the active layer 6, the absorption loss of the waveguide and the radiation of the diffracted light cause a sharp attenuation, so that the region where the diffracted light is emitted is -100 μm from the incident end.
The spread angle θ _L of the diffracted light in the resonance axis direction is
It was 0.2 deg. On the other hand, the change in the tilt angle φ of the diffracted light with respect to the change in the wavelength λ of the incident light is dφ / dλ to −0.4.
Since it is deg / nm, the demultiplexing limit by the wavelength-resolved diffracted light was about 0.5 nm.

However, by applying a forward bias between the electrodes 1 and 11 and injecting a current into the active layer 6, the guided light is amplified. By adjusting the bias, the intensity of the guided light is kept substantially constant in the waveguide, and the diffracted light 13
The region where is emitted is greatly expanded to about 1200 μm or more. Moreover, since the radiation intensity is almost constant over the entire area,
The spread angle θ _L of the diffracted light 13 in the resonance axis direction is about 0.02.
It became very narrow with deg.

On the other hand, the current injection into the active layer 6 causes a plasma effect, so that the equivalent refractive index of the waveguide region changes,
Therefore, the inclination angle φ of the diffracted light 13 also changes, but the wavelength dependence does not change, so the demultiplexing limit of the wavelength-resolved diffracted light 13 is greatly improved to about 0.05 nm.

The diffracted light 13 is detected by the photodetector array 14 (the length of each light receiving surface in the cavity axis direction corresponds to the spread angle θ _L of the diffracted light 13 in the cavity axis direction). By doing so, it becomes possible to demultiplex and detect a high-density wavelength-multiplexed optical signal.

[0018]

[Other Embodiments] FIG. 2 shows a second embodiment of the present invention.
The same functional units as those in FIG. 1 are denoted by the same numbers. Further, the diffracted light and the photodetector array for receiving the diffracted light are omitted.

An optical amplifying portion is formed on the light incident side of the diffracted light emitting portion described in the first embodiment, and a light absorbing portion is formed on the opposite side, so that each can be biased independently. Between the electrodes 11, 111, 112, a separation groove 113,
It is electrically separated by 114.

The incident light enters a forward-biased optical amplification section having no diffraction grating near the waveguide, is amplified there, and then is guided to the diffracted light emission section. The diffracted light emitting section is forward-biased to compensate for the absorption loss of the waveguide and the amount of light emitted outside the waveguide as diffracted light, and diffracted light of substantially uniform intensity is emitted from the diffracted light emitting window 12 to the photodetector element. Emitted to the row (not shown). On the other hand, although the guided light is guided to the light absorbing portion, the light absorbing portion is reverse-biased and has a very large absorption loss, so all the guided light is absorbed. The light amplification section improves the minimum reception level of the photodetector, and the light absorption section removes the influence of the return light due to the end face reflection.

In this embodiment, the one having both the light amplifying portion and the light absorbing portion is described, but either one may be provided. Further, since the function of the light absorbing section does not depend on the presence or absence of the diffraction grating, the light absorbing section may or may not have the diffraction grating.

[0022]

As described above, in the photodetector according to the present invention, the optical waveguide in which the diffraction grating is formed has the active layer, and current is injected into the active layer to absorb the waveguide. The loss and the amount of the diffracted light emitted to the outside of the waveguide are compensated. Therefore, the length of the diffracted light in the cavity axis direction is significantly increased, and the spread angle θ _L of the diffracted light in the resonance axis direction is also significantly reduced. The wave limit can be greatly improved.

Therefore, it becomes possible to accurately select and receive an optical signal of arbitrary one or more waves from the optical signal that is wavelength-divided in a significantly high density.

[Brief description of drawings]

FIG. 1 is a partially cutaway perspective view of a first embodiment of a photodetector having a demultiplexing function according to the present invention.

FIG. 2 is a partially cutaway perspective view of a second embodiment of a photodetector having a light amplification function and a light absorption function.

FIG. 3 is a diagram showing the spread of diffracted light in the cavity axis direction.

[Explanation of symbols]

 1, 11, 111, 112 Metal electrode 2 Antireflection film 3 Substrate 4, 8 Optical confinement layer 5 Embedded layer 6 Active layer 7 Optical waveguide layer 8 Diffraction grating 10 Contact layer 12 Diffracted light emission window 13 Diffracted light 14 Photodetection Element row 113, 114 Electrical isolation groove

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location G02F 1/035 H01S 3/10 Z 8106-2K G02B 6/28 D

Claims (4)

[Claims] 1. A semiconductor optical waveguide into which light is incident, a diffraction grating formed in the waveguide, and a photodetection unit including a plurality of portions for detecting diffracted light emitted to the outside of the waveguide. In the photodetector having, the optical waveguide in which the diffraction grating is formed has an active layer, and a current is injected into the active layer so that the intensity of guided light in the waveguide is substantially constant. A photodetector having a demultiplexing function, characterized in that it has a means for 2. A part of the waveguide is provided with at least one optical amplification section that is electrically separated from the current injection means and that amplifies incident light by current injection. A photodetector having the described demultiplexing function. 3. A means for absorbing guided light is provided on the opposite side of the light incident end face of the waveguide.
Alternatively, a photodetector having the demultiplexing function described in 2. 4. The demultiplexing function according to claim 3, wherein, as a means for absorbing the guided light, a reverse bias is applied to the active layer by an electrode electrically separated from the current injection means. A photodetector having a.