Light source directly modulated

Abstract

Title: Directly Modulated Light Source The invention relates to a directly modulated light source (DMD) comprising: - a horizontal laser diode (HD) comprising a horizontal PIN junction (HJ), the horizontal junction being formed of an intrinsic region (Zi) disposed between a first p-doped region (Zp1) and a first n-doped region (Zn1), the intrinsic region comprising quantum wells; - a vertical PIN junction (VJ) formed of said intrinsic region disposed between a second p-doped region (Zp2) and a second n-doped region (Zn2) disposed on the substrate side; the vertical junction being reverse-biased and modulating said light emitted by the horizontal laser diode by the Stark effect in the quantum wells; - a blocking layer (CB) disposed under the first p-doped region and configured to prevent current leakage between the first p-doped region and the second n-doped region. Fig. 1a

Description

Title of the invention: Directly modulated light source technical field

[0001] The invention relates to a directly modulated light source, and in particular a directly modulated light source using the Stark effect. BACKGROUND

[0002] Prior art directly modulated lasers contain laser diodes whose injection current is modulated via an analog electronic signal. In order to generate optical zeros, the injection must therefore be discontinuous, that is, alternate between 0 and a predetermined electrical current value.

[0003] This discontinuity in the injection of electric current causes changes in carrier density in the conduction band as well as in the operating temperature of the diode, due to Joule heating caused by the current injection. These phenomena, which lead to a change in the refractive index of the active material of the laser diode, result in chirps (i.e., a signal in which the frequency increases or decreases with time) and limit the bandwidth to a few tens of GHz. It is possible to improve the fiber transmission distance by using external ring resonators. However, such devices take into account two control signals (one for the laser and one for the modulator), have a higher temperature sensitivity, and are subject to ring nonlinearities at medium-high coupled optical power, which leads to signal distortion.

[0004] To mitigate these drawbacks, an externally modulated laser can be used. In an externally modulated laser, unlike a directly modulated laser, the current is continuously injected into the laser diode, and modulators (typically also diodes) are used to modulate the optical signal from the laser by means of a reverse voltage. This voltage induces a current in the modulators (typically that of a III-V PIN diode in reverse) that is lower than the current injected into the diode. Therefore, there is no major temperature change, and consequently no major change in the refractive index of the active diode material, thus reducing chirp. This is how externally modulated lasers enable optical links over distances exceeding 10 km.

[0005] However, electroabsorption modulators used for external modulation require two drivers and produce lower modulated power than direct modulation lasers, due to insertion loss in the electroabsorption modulator. Also, external modulation lasers can be difficult to This is crucial because it's essential to avoid saturating the modulator input to prevent degrading the optical modulation amplitude. Ideally, the electroabsorption modulator should operate in a linear regime and have a different material band gap to minimize insertion losses. Such requirements necessitate complex and expensive manufacturing technology with precise band gap calibration compared to directly modulated lasers. SUMMARY

[0006] In order to overcome the aforementioned drawbacks of laser diodes, the invention proposes a directly modulated light source comprising: - a so-called horizontal laser diode comprising a substrate and a first so-called horizontal PIN junction extending along an X axis of an XYZ frame defining a horizontal XY plane parallel to said substrate and a vertical axis Z, the first PIN junction being formed of an intrinsic zone arranged between a first p-type doped zone and a first n-type doped zone, the intrinsic zone comprising a stack of quantum wells arranged perpendicular to the Z axis; the horizontal laser diode is intended to be forward biased and configured to emit light along the Y axis when a direct electric current is injected into the horizontal junction; - a second PIN junction, called vertical, extending along the Z axis and formed by said intrinsic zone arranged between a second p-type doped zone, and a second n-type doped zone arranged on the substrate side; the vertical junction being intended to be reverse-biased and configured to modulate said light emitted by the horizontal laser diode when an alternating electric field along the Z-axis is applied to said intrinsic region, said modulation operating by the Stark effect in the quantum wells; and - a blocking layer disposed at least below the first p-type doped zone and configured to prevent current leakage between the first p-type doped zone and the second n-type doped zone.

[0007] In one embodiment, the blocking layer is made of insulating or semi-insulating material.

[0008] In one embodiment, the material is semi-insulating InP.

[0009] In one embodiment, the blocking layer comprises a third zone of type p and a third type n zone forming a diode configured to be reverse biased.

[0010] In one embodiment, the intrinsic area of ​​the horizontal laser diode is optically coupled to a distributed feedback network.

[0011] In one embodiment, the substrate is made of InP or GaAs material.

[0012] In one embodiment, the directly modulated light source further comprises a semi-insulating layer of crystalline material disposed on the substrate and in contact with the blocking layer and the second n-type doped zone.

[0013] In one embodiment, the substrate and the semi-insulating layer are made of InP material.

[0014] The following description presents several embodiments of the device of the invention: these examples are not limiting to the scope of the invention. These embodiments present both the essential features of the invention and additional features related to the embodiments considered. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention will be better understood and other advantages will become apparent upon reading the following description, given by way of non-limiting example, and from the figures, among which:

[0016] [Fig. la] la [Fig.la] illustrates a light source directly modulated according to the invention, seen in cross-section;

[0017] [Fig.lb] [Fig.lb] illustrates a light source directly modulated according to the invention, seen from above;

[0018] [Fig.2a] [Fig.2a] illustrates an example of a directly modulated light source, cross-sectional view;

[0019] [Fig. 2b] [Fig. 2b] illustrates another example of a direct light source modulated, cross-sectional view; and

[0020] [Fig.3] [Fig.3] illustrates an equivalent electrical diagram of a light source directly modulated according to the invention. DETAILED DESCRIPTION

[0021] The invention relates to a directly modulated light source. Figures [Fig. 1a] and [Fig. 1b] illustrate a directly modulated DMD light source according to the invention, in cross-section and from above, respectively.

[0022] The directly modulated light source DMD comprises a horizontal laser diode DH including a substrate Sub and a first horizontal PIN junction JH extending along an X axis of an XYZ frame defining a horizontal plane XY parallel to the substrate and a vertical axis Z. The first PIN junction is formed of an intrinsic region Zi arranged between a first p-type doped region Zpl and a first n-type doped region Znl, the intrinsic region Zi comprising a stack of quantum wells PQ arranged perpendicular to the Z axis. The region Zi constitutes the active region of the laser diode DH.

[0023] The horizontal laser diode DH is intended to be forward biased and is configured to emit light li along the Y-axis when a direct current Cdirect is injected into the horizontal junction JH. By way of example, a current generator DCV injects a direct current Cdirect into the horizontal injunction JH. The current is injected laterally with respect to the quantum wells PQ. Typically, the direct current Cdirect is generated with a DCV generator.

[0024] The directly modulated light source DMD also includes a second PIN junction, referred to as the vertical junction JV, extending along the Z-axis and formed by the intrinsic region Zi situated between a second p-type doped region Zp2 and a second n-type doped region Zn2. The second n-type doped region Zn2 ​​is situated between the intrinsic region Zi and the substrate. Thus, the horizontal junction JH and the vertical junction JV have the same intrinsic region Zi.

[0025] The vertical junction JV is designed to be reverse-biased and is configured to modulate the light li emitted by the horizontal laser diode DH when an alternating electric field along the Z-axis is applied to the intrinsic region Zi. The modulation occurs via the quantum-confined Stark effect in the quantum wells PQ of the active region.

[0026] By way of example, an ACV square wave generator injects a square wave voltage into the vertical junction JV, thereby producing an electric field perpendicular to the quantum wells. Since the vertical junction is reverse-biased, very little current is injected through the JV junction, and carrier injection into the JH junction is not disturbed.

[0027] Furthermore, the directly modulated DMD light source includes a blocking layer CB disposed at least below the first p-type doped Zpl region and configured to prevent current leakage between the first p-type Zpl region and the second n-type Zn2 region. The width along the X-axis of the blocking layer CB is greater than that of the p-type Zpl region. In other words, the blocking layer CB is arranged so as to prevent the first p-type Zpl region and the second n-type Zn2 region from coming into contact.

[0028] The quantum wells are configured to be modulated by the quantum confinement Stark effect. In particular, in the invention, the quantum confinement Stark effect is produced when an electric field is applied perpendicularly to the quantum wells.

[0029] In general, the Stark effect occurs when an atom or molecule is placed in an external electric field. The electric field interacts with the charged particles inside the atom or molecule, causing a splitting of energy levels. The strength of the electric field determines the magnitude of the splitting. In the absence of confinement, this effect is difficult to exploit because the wave functions of the electron and the hole move apart very rapidly. In the quantum-confined Stark effect, the wave functions are confined within quantum wells, the superposition of states modified by the electric field is improved and an instantaneous reduction of the band gap of quantum wells is observed (Miller et al, Band edge electroabsorption in quantum well structures: the quantum-confined Stark effect, Phys. Rev. Lett. 53, 22, 1984).

[0030] In the present invention, when a square wave alternating voltage (as shown in [Fig. 1a]) oscillating between two predetermined values ​​is applied to the quantum wells, an electric field perpendicular to the quantum wells is generated. Under the influence of this electric field, the electronic state of the quantum wells is modified. Thus, the band gap of the quantum wells is rapidly reduced, thanks to the quantum confinement Stark effect. This makes it possible to modulate the light li emitted by the horizontal diode DH, by absorbing or allowing the light li to pass through the quantum wells. The light li is therefore modulated directly using the quantum confinement Stark effect within the light source DMD. Modulating the electric field across the vertical junction in the intrinsic region Zi thus makes it possible to modulate the light li emitted by the horizontal diode DH.Typically, alternating voltage is produced by an ACV square wave generator.

[0031] The invention thus makes it possible to obtain a directly modulated DMD light source using the quantum confinement Stark effect, which can be integrated into a laser source. Advantageously, the use of the quantum confinement Stark effect directly in the light source allows for the use of a continuous and constant current injection into the horizontal laser diode DH while achieving effective intensity modulation thanks to the vertical electric field, all in a single component. The continuous current avoids the changes in refractive index normally generated by a discontinuously injected current (as is the case for directly modulated lasers in the prior art). The continuous current thus makes it possible to reduce the chirp normally generated in directly modulated lasers in the prior art.

[0032] Furthermore, the use of the quantum confinement Stark effect directly in the light source allows for simplified fabrication and design, without external modulators, unlike externally modulated lasers. Thus, the present invention makes it possible to obtain a simple, low-cost laser source with reduced chirp.

[0033] The directly modulated laser according to the invention therefore does not have the disadvantages of directly modulated lasers according to the prior art, while retaining the advantages, such as simplicity of manufacture and design, low cost and high power.

[0034] In one embodiment, the substrate Sub is made of InP or GaAs material. Advantageously, the different layers of the DMD light source are formed by epitaxy from the substrate. Thus, an InP or GaAs substrate allows the epitaxial growth of quantum wells PQ directly on the substrate Sub.

[0035] The choice of substrate and associated layers depends on the desired emission wavelength. An InP substrate allows the implementation of the DMD light source at a telecommunications wavelength, i.e., at wavelengths greater than 1 pm.

[0036] In another embodiment, the Sub substrate is arranged on a second silicon substrate, thus allowing the use of the DMD light source in a chip for example.

[0037] The horizontal laser diode DH includes an optical cavity that allows the emission of light li along the Y-axis when a direct current Cdirect is injected into the horizontal junction JH. In one embodiment, the optical cavity can be defined by means of two mirrors of the distributed Bragg reflector (DBR) type, by facet (i.e., by cleaving the component to expose a facet which is treated with an anti-reflective coating whose deposited material thicknesses are adjusted to obtain a defined reflection coefficient), or by using a resonator extending along the Y-axis. The optical cavity can also be defined by a distributed-feedback (DFB) array in a III-V material or in an optical waveguide made of another material (silicon, silicon nitride, lithium niobate, or other), extending along the Y-axis.The intrinsic Zi region of the horizontal laser diode DH is thus optically coupled to the distributed feedback network DFB.

[0038] Advantageously, the distributed feedback Bragg network DFB makes it possible to obtain a single-mode laser emission useful for optical communications.

[0039] In an embodiment illustrated in [Fig. 2a], the blocking layer CB comprises a third p-type Zp3 region and a third n-type Zn3 region forming a DI diode configured to be reverse-biased. Advantageously, the third p-type Zp3 region is in contact with the second n-type Zn2 region, such that the second n-type Zn2 region is not in contact with the first n-type Zn1 region, thus preventing current from flowing from the first p-type Zp1 doped region to the second n-type Zn2 doped region. Advantageously, the reverse-biased diode allows for a thin blocking layer.

[0040] In another embodiment illustrated in [Fig. 2b], the blocking layer CB is made of insulating or semi-insulating SI2 material. For example, the material is semi-insulating InP. Advantageously, the semi-insulating InP layer has a width greater than the width of the p-type Zpl doped zone.

[0041] In one embodiment, the crystal structure of the blocking layer CB allows the growth by epitaxy of layers arranged above the blocking layer, and in particular the first doped zone of type p Zpl.

[0042] In one embodiment, the directly modulated light source DMD further comprises a crystalline semi-insulating layer CSI disposed on the substrate and in contact with the blocking layer CB and the second n-type doped region Zn2. The CSI semi-insulating layer prevents current from flowing from the vertical junction JV to the substrate Sub. The crystalline structure of the CSI semi-insulating layer allows the epitaxial growth of layers disposed above the CSI semi-insulating layer, and in particular the blocking layer as well as the second p-type doped region Zp2. In one example, the CSI semi-insulating layer is made of InP, thus allowing its use at telecommunication wavelengths.

[0043] For example, the substrate and the semi-insulating layer CSI are made of crystalline InP, thus enabling emission at telecommunication wavelengths. Figure 3 illustrates an example of the equivalent circuit of a directly modulated DMD light source according to the embodiment of Figure 2a. The equivalent circuit includes a first forward-biased diode DI corresponding to the junction between the first p-doped region Zp1 and the third n-doped region Zn3, a reverse-biased diode D2 corresponding to the junction between the third n-doped region Zn3 and the third p-doped region Zp3, and a third forward-biased diode D3 corresponding to the junction between the third p-doped region Zp3 and the second n-doped region Zn2. The equivalent circuit also includes two DCV and ACV generators. The DCV generator allows the direct current Cdirect to be injected into the horizontal junction.The ACV square wave generator allows the oscillating voltage between two predetermined values ​​to be injected into the vertical junction JV. The resistor R represents the resistance of the vertical junction JV and the capacitor C represents the capacitance induced by the vertical junction. The resistors RI and R2 represent the intrinsic resistance of the quantum wells, with resistance RI representing the left side of the quantum wells (before contact with the second p-doped region Zp2) and resistance R2 representing the right side of the quantum wells.

[0044] Although the invention has been illustrated and described in detail using a preferred embodiment, the invention is not limited to the disclosed examples. Other variations can be deduced by a person skilled in the art without departing from the scope of protection of the claimed invention.

Claims

Demands

1. A directly modulated light source (DMD) comprising: - a horizontal laser diode (DH) comprising a substrate (Sub) and a first horizontal PIN junction (JH) extending along an X axis of an XYZ frame defining a horizontal XY plane parallel to said substrate and a vertical axis Z, the first PIN junction being formed of an intrinsic region (Zi) disposed between a first p-type doped region (Zpl) and a first n-type doped region (Znl), the intrinsic region (Zi) comprising a stack of quantum wells (PQ) disposed perpendicular to the Z axis; the horizontal laser diode being intended to be forward biased and configured to emit light (li) along the Y axis when a direct electric current (Cdirect) is injected into the horizontal junction;- a second PIN junction, called the vertical junction (VJ), extending along the Z-axis and formed by said intrinsic zone (Zi) disposed between a second p-type doped zone (Zp2) and a second n-type doped zone (Zn2) disposed on the substrate side; the vertical junction (VJ) being intended to be reverse-biased and configured to modulate said light emitted by the horizontal laser diode when an alternating electric field along the Z-axis is applied to said intrinsic zone (Zi), said modulation being carried out by the Stark effect in the quantum wells; and - a blocking layer (CB) disposed at least below the first p-type doped zone (Zpl) and configured to prevent current leakage between the first p-type doped zone (Zpl) and the second n-type doped zone (Zn2).

2. Directly modulated light source (DMD) according to claim 1, wherein the blocking layer is made of insulating or semi-insulating material.

3. Direct modulated light source (DMD) according to the preceding claim, wherein the material is semi-insulating InP.

4. Direct modulated light source (DMD) according to claim 1, wherein the blocking layer comprises a third p-type zone (Zp3) and a third n-type zone (Zn3) forming a diode (DI) configured to be reverse biased.

5. Directly modulated light source (DMD) according to any one of the preceding claims, wherein the intrinsic area (Zi) of the horizontal laser diode (DH) is optically coupled to a distributed feedback network (DFB).

6. Light source according to any one of the preceding claims wherein the substrate is made of InP or GaAs material.

7. Direct modulated light source (DMD) according to any one of the preceding claims, the direct modulated light source (DMD) further comprising a semi-insulating layer (CSI) of crystalline material disposed on the substrate and in contact with the blocking layer (CB) and the second n-type doped zone (Zn2).

8. Direct modulated light source (DMD) according to the preceding claim, wherein the substrate and the semi-insulating layer are made of InP material.

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