JP2019201162A - Optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To broaden the band of an optical device composed of a semiconductor used as a light source. SOLUTION: The active part 101 has a quantum well structure, and the active part 101 is composed of a well layer 111 made of InGaAlAs and a barrier layer made of InGaAlAs made of carbon-doped p-type and sandwiching the well layer 111. And the layer 112. Further, the first optical confinement layer 103 made of n type InGaAlAs is formed on the first cladding layer 102 made of n type InP, and the active portion 101 is formed on the first optical confinement layer 103. . A second optical confinement layer 104 made of p-type InGaAlAs is formed on the active portion 101, and a second cladding layer 105 made of p-type InP is formed on the second optical confinement layer 104. [Selection diagram]

Description

  The present invention relates to an optical device including an active portion having a quantum well structure.

  2. Description of the Related Art In recent years, the amount of information handled by optical communication has been rapidly increasing along with an increase in demand for browsing movies on the Internet. The signal speed of the Ethernet (registered trademark) standard that has been developed based on computers and networks has also been increased to 1 Gbps, 10 Gbps, and 100 Gbps one after another.

  The Ethernet standard “100 GbE-LR4” with a speed of 100 Gbps and a transmission distance of 10 km, which is mainly used in and between data centers, is an optical communication system of an intensity modulation / direct detection method using wavelength multiplexing technology. In this standard, an optical signal having a signal speed of 25 Gbps is multiplexed in four wavelengths, and the signal speed is set to 100 Gbps. As a light source for transmission, a direct modulation type distributed feedback (DFB) laser or a DFB laser integrated with an electroabsorption (EA) modulator capable of modulation at a speed of 25 Gbps is used.

  Studies on signal speeds of 400 Gbps and 1 Mbps aiming at higher speeds have begun, but the issue here is the operating speed of the transmission light source. In order to realize a system with a signal speed of 400 Gbps or 1 Mbps, it is necessary to increase the operation speed of the modulation light source to 50 Gbps or more and to perform multi-value intensity modulation, and it is an issue to widen the modulation light source.

W. Kobayashi et al., "50-Gb / s Direct Modulation of a 1.3-μm InGaAlAs-Based DFB Laser With a Ridge Waveguide Structure", IEEE Journal of Selected Topics in Quantum Electronics, vol. 19, no. 4, 1500908 , 2013.

  There are several solutions for increasing the bandwidth of semiconductor light sources. For example, one method is to optimize a quantum well structure used as an active layer of a semiconductor laser by band engineering. There is also a method of optimizing the structural parameters of the laser resonator such as the laser resonator length and the reflectivity of the diffraction grating. Non-Patent Document 1 is an example of a direct modulation laser manufactured by optimizing a quantum well structure or a laser resonator structure, and realizes a modulation speed of 50 Gbps. However, even with this laser, the frequency band is insufficient to perform multi-level intensity modulation, and a wider band is required.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to broaden the bandwidth of an optical device composed of a semiconductor used as a light source.

  An optical device according to the present invention includes an active portion having a quantum well structure, and the active portion is composed of a well layer composed of InGaAlAs and InGaAlAs doped with carbon to be p-type, and is disposed with the well layer interposed therebetween. A barrier layer.

  In the above optical device, the active part includes a resonance part composed of a diffraction grating to constitute a distributed feedback laser.

  The optical device includes an optical amplifier including a second active part integrated in series with a distributed feedback laser, and the second active part has the same quantum well structure as the active part.

  The optical device may include an electroabsorption optical modulator integrated in series with the distributed feedback laser.

In the optical device, the concentration of carbon doped in the barrier layer may be in the range of 6 to 8 × 10 ²⁴ cm ⁻³ .

  As described above, according to the present invention, since the barrier layer constituting the active part of the quantum well structure is composed of p-type InGaAlAs doped with carbon, the optical device composed of a semiconductor used as a light source Thus, an excellent effect that a wider band can be obtained is obtained.

FIG. 1 is a cross-sectional view showing a configuration of an optical device according to an embodiment of the present invention. FIG. 2 is a band diagram showing a band structure of the optical device according to the embodiment of the present invention. FIG. 3A is a cross-sectional view showing a configuration of a semiconductor laser serving as an application example of the optical device according to the embodiment of the present invention. FIG. 3B is a cross-sectional view showing a configuration of a semiconductor laser serving as an application example of the optical device according to the embodiment of the present invention. Figure 4 is a characteristic diagram showing a case where the optical device and a semiconductor laser, when varying the doping concentration of carbon in the barrier layer, the change in the threshold carrier density N _th in the embodiment. 5, in a case where the optical device in an embodiment with a semiconductor laser, when varying the doping concentration of carbon in the barrier layer is a characteristic diagram showing the change of the threshold differential gain ∂g _th / ∂ _n. FIG. 6 is a cross-sectional view showing the configuration of another optical device according to the embodiment of the present invention.

  Hereinafter, an optical device according to an embodiment of the present invention will be described with reference to FIG. This optical device includes an active portion 101 having a quantum well structure. The active part 101 includes a well layer 111 made of InGaAlAs and a barrier layer 112 made of p-type InGaAlAs doped with carbon and sandwiching the well layer 111. In this example, two well layers 111 are provided.

  In the embodiment, the first light confinement layer 103 made of n-type InGaAlAs is formed on the first clad layer 102 made of n-type InP, and the active portion 101 is formed on the first light confinement layer 103. Is forming. A second optical confinement layer 104 made of p-type InGaAlAs is formed on the active portion 101, and a second cladding layer 105 made of p-type InP is formed thereon.

  In the embodiment, the active portion 101 is disposed between the first optical confinement layer 103 and the second optical confinement layer 104. The first optical confinement layer 103, the active part 101, and the second optical confinement layer 104 are disposed so as to be sandwiched between the first cladding layer 102 and the second cladding layer 105.

  Next, an energy band in the stacking direction of the optical device in the embodiment will be described with reference to FIG.

  The carbon doped in the barrier layer 112 made of p-type InGaAlAs acts as an acceptor and emits holes into the valence band, but these holes 203 accumulate in the well layer 111 with low energy. Further, even if no voltage is applied to the p-i-n diode structure, the holes 203 are distributed in the well layer 111 due to the doping effect. The well layer 111 also retains electrons 201 and holes 202 injected by applying a voltage to the diode structure. As a result, a change occurs in the optical gain characteristics when a voltage is applied to the diode composition and electrons and holes are injected into the well layer 111.

  As described above, since the holes are originally distributed in the quantum well structure in the embodiment, carrier inversion distribution is likely to occur, and the optical gain and the gain coefficient can be increased. These have long been known as modulation doping, but the feature of the present invention is that this technique is applied to an InGaAlAs quantum well structure.

  Conventionally, InGaAsP has been widely used in a quantum well structure that constitutes a laser having a long wavelength band (1.3 to 1.6 μm) using an InP substrate. In the case of InGaAsP, zinc (Zn) is generally used as a p-type dopant. However, since Zn has a large diffusion coefficient, it is easy to diffuse during the production of a laser structure by crystal growth. Even if modulation doping is performed, Zn moves due to diffusion, and a desired doping distribution can be obtained. There was no problem.

  In contrast, in the present invention, modulation doping is realized in the active portion 101 of the quantum well structure by doping carbon (C) into the barrier layer 112 made of InGaAlAs. Since C has a diffusion coefficient that is two orders of magnitude smaller than that of Zn, a doping distribution capable of obtaining a band state as shown in FIG. 2 can be realized. For InGaAsP, since C is an n-type dopant, this configuration cannot be applied to the quantum well structure serving as the active portion 101 in the above-described embodiment.

The optical device in the embodiment can be a semiconductor laser, for example. An important parameter that determines the direct modulation band of this semiconductor laser is the relaxation oscillation frequency _fr, which is expressed by the following equation.

In the above equation, τ _ph is a photon lifetime, ∂g _th / ∂ _n is a threshold differential gain, I is an injection current, and I _th is a threshold current.

The use of modulation doping structure by doping with carbon in the barrier layer, to reduce the threshold current I _th, it is possible to increase the threshold differential gain ∂g _th / ∂ _n, increasing the relaxation oscillation frequency f _r is Be expected. As a result, the modulation band also increases.

  Next, a case where the optical device in the above-described embodiment is a semiconductor laser will be described with reference to FIGS. 3A and 3B. FIG. 3A shows a cross section parallel to the optical waveguide direction, and FIG. 3B shows a cross section perpendicular to the optical waveguide direction.

  This semiconductor laser has a quantum well structure comprising a well layer made of InGaAlAs and a barrier layer made of p-type InGaAlAs doped with carbon on a first cladding layer 300 made of n-type InP. Active portion 302 is formed. For example, the active portion 302 is scattered with the number of well layers being eight. Further, the well layer has a composition such that a compressive strain of about 1.5% is applied.

  A guide layer 303 made of InGaAsP is formed on the active portion 302, and a second cladding layer 301 made of p-type InP is formed thereon. In the guide layer 303, a diffraction grating 304 is formed to serve as a resonance part of a distributed feedback laser that oscillates in a single mode. The laser cavity length is 150 μm.

  Further, an n-side electrode 306 made of Au is formed on the back surface side of the first cladding layer 300, and p made of Au is formed on the second cladding layer 301 via a contact layer 305 made of p-type InGaAsP. A side electrode 307 is formed. Further, the side portion of the guide layer 303 on which the active portion 302 and the diffraction grating 304 are formed is buried by a buried layer 310 made of semi-insulating InP, and has a current confinement structure for efficiently injecting current. Yes. A high reflection film 308 is formed at one end of the active portion 302 in the waveguide direction, and an antireflection film 309 is formed at the other end.

  Each structural parameter described above is a value optimized to increase the modulation frequency band. Laser oscillation is obtained by applying a voltage between the p-side electrode 307 and the n-side electrode 306 and passing a current through the active portion 302.

FIG. 4 shows changes in the threshold carrier density N _th when the carbon doping concentration in the barrier layer of the semiconductor laser is changed, and FIG. 5 shows changes in the threshold differential gain ∂g _th / ∂ _n . The threshold gain is 300 ^-1, 400 ^-1, which shows the case of 500 cm ^-1. The oscillation wavelength is 1300 nm. Threshold carrier density N _th with increasing doping concentration decreases. The threshold differential gain ∂g _th / ∂ _n increases monotonically to a doping concentration p ₀ = 5 × 10 ²⁴ m ⁻³ and then shows a tendency to saturate. Further, when the threshold gain is in the range of about 300 to 500 cm ⁻¹ , the characteristics do not change so much. If the doping concentration is excessively increased, the crystallinity may be lowered. Therefore, it can be said that the doping concentration is preferably about 6 to 8 × 10 ²⁴ m ⁻³ . The threshold differential gain at this time increases to about 1.5 times.

  The semiconductor laser described above has a DFB structure in which a diffraction grating is formed on the active portion 302, but the present invention is not limited to this. For example, a so-called distributed Bragg reflector (DBR) laser in which a diffraction grating is formed in an optical waveguide other than the active portion may be used. Further, the structure of the optical waveguide is a buried structure, but it can also be applied to a ridge structure.

  Next, a configuration in which the optical device according to the above-described embodiment is configured by combining a semiconductor laser with an optical amplifier and an optical modulator will be described with reference to FIG. FIG. 6 shows a cross section parallel to the direction in which light is guided.

  In this example, a semiconductor laser 351, an electroabsorption optical modulator 352, and an optical amplifier 353 are integrated in this order in series. The configuration of the semiconductor laser 351 is the same as that of the semiconductor laser described with reference to FIGS. 3A and 3B. In all regions, the first cladding layer 300 and the second cladding layer 301 are common.

  The optical modulator 352 includes an active core portion 314 having a multiple quantum well structure made of InGaAsP on the first cladding layer 300, and the p-side via a contact layer 305 made of p-type InGaAsP on the second cladding layer 301. An electrode 311 is formed.

  In addition, the optical amplifier 353 includes an active portion 302 having a quantum well structure with a barrier layer made of InGaAlAs doped with carbon to be p-type by doping carbon on the second cladding layer 301, and the p-type cladding is formed on the second cladding layer 301. A p-side electrode 312 is formed via a contact layer 305 made of type InGaAsP. The optical amplifier 353 includes the same active portion 302 as that of the semiconductor laser 351, but does not include a resonance structure.

  Further, the semiconductor laser 351 and the optical modulator 352 and the optical modulator 352 and the optical amplifier 353 are optically connected by an optical waveguide with a passive core 313 made of InGaAsP.

  In this optical device, the modulation frequency band is determined by the optical modulator 352 including the active core portion 314 not having the modulation dope structure, so that the modulation frequency band is not affected by the modulation doping. However, since the threshold current of the semiconductor laser 351 is lowered by the active portion 302 having the modulation doping structure and the gain of the optical amplifier 353 is increased, it is possible to operate with low power consumption.

  As described above, according to the present invention, since the barrier layer constituting the active part of the quantum well structure is composed of p-type InGaAlAs doped with carbon, light composed of a semiconductor used as a light source The apparatus can be made to have a wider bandwidth.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious.

  DESCRIPTION OF SYMBOLS 101 ... Active part, 102 ... 1st clad layer, 103 ... 1st optical confinement layer, 104 ... 2nd optical confinement layer, 105 ... 2nd clad layer, 111 ... Well layer, 112 ... Barrier layer.

Claims (5)

An optical device comprising an active part of a quantum well structure,
The active part is
A well layer composed of InGaAlAs;
An optical device comprising: a barrier layer made of p-type InGaAlAs doped with carbon and sandwiching the well layer. The optical device according to claim 1.
The optical device according to claim 1, wherein the active portion includes a resonating portion including a diffraction grating to constitute a distributed feedback laser. The optical device according to claim 2.
An optical amplifier comprising a second active part integrated in series with the distributed feedback laser;
The optical device, wherein the second active part has the same quantum well structure as the active part. The optical device according to claim 2 or 3,
An optical apparatus comprising an electroabsorption optical modulator integrated in series with the distributed feedback laser. The optical device according to any one of claims 1 to 4,
The concentration of carbon doped in the barrier layer is in the range of 6-8 × 10 ²⁴ cm ⁻³ .

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