FR2965642A1 - OPTICAL WAVELENGTH CONVERTER - Google Patents

Abstract

Wavelength optical converter comprising a quantum cascade laser, having an active region (RA) formed by an asymmetrical quantum well stack, for generating a first electromagnetic radiation, characterized in that it also comprises: injecting (L) into said active region a second electromagnetic radiation collinear with said first radiation and resonating with an interband transition of said quantum cascade laser; and means (CG, CC1, E2) for confining said first and second electromagnetic radiations within said active region. A method of converting the wavelength of electromagnetic radiation based on the use of such a converter. Use of such a converter in a wavelength division optical fiber telecommunications system.

Description

The invention relates to a wavelength optical converter, as well as to a wavelength conversion method based on the use of such a device.

Wavelength division multiplexing (WDM) is commonly used to increase the capacity of fiber optic telecommunications networks. It consists in using, in the same fiber or bundle of fibers, a plurality of optical carriers at different wavelengths, associated with respective channels. In an ideal WDM system, each network user would be characterized by its own wavelength, uniquely identifying it. However, this is not possible in large networks, and wavelength conversions must be implemented to change the routing of the signals according to the availability of the different carriers. Conventionally, these wavelength conversions are performed using an optoelectronic converter, in which the optical signal is converted into an electronic signal, and then again into an optical signal at a different wavelength. These devices are bottlenecks that severely limit the capacity of the network; for this reason, purely optical converters have been developed. Purely optical wavelength converters, which are the subject of intense research activity, exploit a non-linear interaction between an optical signal at a UNIR frequency (near infrared or visible) and a modulator beam at a frequency VTHZ "UNIR, typically in the Terahertz (THz) domain. The non-linear interaction ("three-wave mixing") produces two secondary beams at the sum (UNIR + VTHZ) and difference (UNIR-VTHZ) frequencies, of which one - or both - is amplified and injected into the replacement network. of the original signal. [Carter 2004] describes such a wavelength optical converter using, as a nonlinear medium, an asymmetric quantum well stack in which a resonant three-wave interaction occurs. In a manner well known per se, asymmetry is necessary for a second order interaction to be possible. In the [Carter 2004] device, the THz radiation is generated by a free electron laser, which is obviously incompatible with practical use. But the use of quantum cascade lasers is also envisaged. Quantum cascade lasers are sources of coherent light emitting in the middle and far infrared, up to Terahertz. They have an active region formed by a quantum well stack and exploit transitions between energetic subbands within these quantum wells. See [Faist 1994], [KOler 2002] and [Worral 2006]. In [Zervos 2006] and WO 02/13344, a quantum cascade laser is used both to generate the THz modulator beam and as a nonlinear medium. The interaction geometry used by the converters described in these documents, as well as in [Carter 2004], is non-collinear: whereas the quantum cascade laser emits parallel to the substrate, the optical signal to be converted is injected by the upper facet of the device, in oblique incidence. The secondary beam is also collected by said upper facet. This configuration is necessary to obtain a very short interaction length, so as to minimize the absorption of the signal and the secondary beam by the active region of the quantum cascade laser. However, it is hardly compatible with the use of the device in a fiber optic telecommunication system. [Dhillon 2007] describes a wavelength optical converter operating in collinear geometry, thus more easily usable in an optical fiber telecommunication system. This device in turn uses a quantum cascade laser as a THz radiation generator. But the non-linear interaction, of non-resonant type, occurs inside a GaAs layer located in the middle of the active region of the laser, where the optical signal to be converted is confined. The fact that the interaction is non-resonant eliminates the aforementioned problems of absorption of the optical signals. The main disadvantage of this device is that the non-linear optical susceptibility (x ('' non-resonant GaAs is relatively low, which requires to ensure a phase agreement to obtain a sufficiently high conversion factor.

The invention aims to remedy the aforementioned drawbacks of the prior art. According to the invention, this object is achieved by the use of a non-linear resonant type interaction within the active region of a quantum cascade laser, but in collinear geometry. This is made possible by means of guiding the optical signal (visible or near infrared), which ensure a good superposition between said signal and the modulator beam THz within said active region. The problem of the absorption of the secondary beam in the active region is eliminated by ensuring that said secondary beam is composed of photons of energy less than the width of the forbidden band of said active region.

An object of the invention is therefore a wavelength optical converter comprising a quantum cascade laser, having an active region formed by an asymmetrical quantum well stack, for generating a first electromagnetic radiation, characterized in that it comprises also: - injection means, in said active region, a second electromagnetic radiation collinear with said first radiation and resonant with an inter-band transition of said quantum cascade laser; and means for confining said first and second electromagnetic radiation within said active region. In particular, said asymmetric quantum well stack can be deposited on a substrate and in which said first and second electromagnetic radiations propagate parallel to said substrate.

Said means for confining said first and second electromagnetic radiations may comprise at least one layer, interposed between said active region and said substrate, having a refractive index lower than those of said active region and said substrate. This layer may in particular be semiconductive. In particular, the means for confining said first and second electromagnetic radiations may comprise a first layer for confining said second electromagnetic radiation, and a second layer for confining said first electromagnetic radiation. Alternatively, a metal layer may be provided to confine both the first and the second electromagnetic radiation. Advantageously, the converter may also comprise a metal layer deposited on or above said active region, on the opposite side to that of said substrate. This layer then has a triple function: it serves as an electrical contact and it ensures the confinement of both the first and the second radiation. Another object of the invention is a method of frequency conversion of electromagnetic radiation, comprising the following operations: electrically or optically pump the quantum cascade laser of a converter as described above, so as to inducing the generation of said first electromagnetic radiation; Injecting into the active region of said quantum cascade laser said second electromagnetic radiation, with a propagation direction parallel to that of said first radiation; and extracting from said converter at least one radiation obtained by mixing three waves of said first and said second radiation. Advantageously, the band structure of said quantum cascade laser and the wavelength of said second radiation may be chosen such that the radiation obtained by difference in frequency of said first and second radiations consists of photons of energy less than forbidden band of the active region of said quantum cascade laser.

Said first electromagnetic radiation may be medium or far infrared radiation, or Terahertz radiation, while said second electromagnetic radiation may be near-infrared or visible radiation.

Yet another object of the invention is the use of such a wavelength converter in a wavelength division optical fiber telecommunications system. The following is meant: - For "visible radiation", electromagnetic radiation of wavelength between 380 nm and 780 nm; - For "near-infrared radiation" means electromagnetic radiation with a wavelength between 780 nm and 3 pm; the near infrared includes the spectral band used in optical telecommunications (1250 - 1700 nm, and more particularly 1530 - 1565 nm); - For "mid-infrared radiation", electromagnetic radiation of wavelength between 3 pm and 50 pm; - For "far-infrared radiation", electromagnetic radiation of wavelength between 50 pm and 1 mm - for "Terahertz radiation", far-infrared electromagnetic radiation having more precisely one wavelength between 100 pm and 1 mm (frequency between 25 300 GHz and 3 THz approximately). Other characteristics, details and advantages of the invention will emerge on reading the description given with reference to the appended drawings given by way of example and which represent, respectively: FIG. 1, the structure of a length converter Wave according to one embodiment of the invention; FIG. 2, the optical intensity distribution and THz inside the structure of FIG. 1; and FIG. 3, a simplified band diagram of the converter of FIG. 1, illustrating the energy transitions involved in the wavelength conversion process. Figure 1 shows the structure of a wavelength converter according to one embodiment of the invention. The device is obtained by molecular beam epitaxial growth on a SS semi-insulating GaAs substrate having a thickness of 500 μm. A CG layer of Al0, 5Gao, 5As of 300 nm thick is deposited on a so-called upper surface of the SS substrate; as will be explained in detail later, it serves to guide the input optical signal, or second electromagnetic radiation, as well as the signal converted to wavelength, or secondary beam. Above this guiding layer is deposited an n-doped GaAs layer CC1 of a thickness of 600 nm intended to provide electrical contact with the lower electrode E1. At the same time, this layer CC1 forms a plasmonic waveguide providing a partial confinement of the modulation THz radiation (see the aforementioned article [Kbhler 2002]). Above is deposited a stack of AIGaAs / GaAs quantum wells forming the active region of a quantum cascade laser for emission at 2.1 THz. This stack is asymmetrical, in the sense that it has no plane of symmetry parallel to the surface of the substrate. A stack of this type is described by the aforementioned article [Worral 2006]. Above the active region is deposited a second contact layer CC2 n-doped GaAs (70 nm) on which is deposited a metal layer (Au) forming the upper electrode E2. The latter, in addition to its electrical function, contributes to the confinement of both modulation radiation and input optical signals and converted to wavelength. The structure has a total thickness of 15 μm above the upper surface of the substrate. The assembly is subjected to wet etching to isolate the active regions of the different devices (width: 220 μm) and to expose the layer CC1, on which a metal deposit is formed forming the lower electrodes E1. Subsequently, the SS substrate is thinned to a thickness of 200 μm and a metal layer SM is deposited on its lower surface. The various devices, a length of 3 mm (it will be noted that Figure 1 is not to scale) are then separated and welded on a copper support with gold coating. The optical input signal SOE, which propagates parallel to the substrate, is injected into the device by a lateral facet F1 through the lens L; the signal converted into wavelength SOC, which is also propagated parallel to the substrate, is extracted from the opposite facet F2. These two facets also define the cavity of the quantum cascade laser, the reflection being provided by the optical index jump between the active region and the vacuum. The lens L is preferably aspherical to optimize the coupling with the guided mode inside the converter.

In more compact embodiments, the input optical signal can be fed through an optical fiber, coupled to the converter by means of ball lenses or simply by abutting. The converted beam is then filtered to suppress unwanted spectral components (at the initial frequency, at the sum frequency if it is desired to recover the difference frequency, or vice versa), amplified, shaped and / or injected into an optical fiber. . Figure 2 shows the distribution of near infrared optical intensity (input and converted optical signals) - NIR - e curve Terahertz (modulation radiation, emitted by the quantum cascade laser) - THZ curve - at FIG. 1 shows that the near-infrared radiation is confined within the active region by means of the guiding layer CG and the upper metal layer E2, which at these wavelengths a refractive index lower than that of the active region. The THz radiation is also confined, although less efficiently, thanks to the conductive layer CC1 whose dielectric constant is negative in this spectral region. This effect of guiding the THz radiation is explained by [Kohler 2965642 s 2002]. The converter structure thus forms a "dual waveguide", confining both near-infrared (or visible) radiation and far-infrared / THz radiation. Figure 3 illustrates the three-wave resonant mixing process that occurs within the active region of the converter of the invention. In this figure, the references BC and BV respectively denote the conduction band and the valence band, "e" denotes the electrons and "h" the holes. Each layer of GaAs, of thickness "a" of a few nanometers and sandwiched between layers of AIGaAs of greater forbidden band, forms a quantum well (only one is represented in the figure) which confines the carriers; the latter thus occupy discrete energy levels, or "sub-bands". 11 h) indicates the state of the lowest energy holes, by 11 e) the state of electrons of higher energy, by 12e) a state of electrons of higher energy, by Eh the energy of confining the holes, 15 by Ee the confinement energy of the electrons and by EG the width of the forbidden band in the quantum well. The input optical signal has a photonic energy EmR, resonant with the interband transition 11n) - 11e); the laser transition, EQCL energy, occurs between states 12e) and 11e). Under these conditions, a resonant interaction occurs which generates two secondary beams at the sum and difference frequencies. Given the large length of the interaction (3mm), the beam at the sum frequency - whose photon energy is greater than the width G of the forbidden band - is absorbed almost entirely. On the other hand, the difference frequency beam - whose photon energy is less than EG - is only weakly attenuated. As shown in FIG. 3, the generation of the difference frequency can be interpreted as a resonant process between two real states, 11e) and 11h), and a virtual state Iv) located inside the forbidden band. In contrast, in this configuration, the generation of the sum frequency involves three real states, 11 h), 11 e) and 12 e). It would also be possible to generate a sum frequency from an energy input optical signal EN, R resonant with the transition I1h) lv) and a resonant energy EQcL with the transition lv) -> Il e); but even in this case, the radiation at the sum frequency would be likely to be absorbed by the active material, with a loss of conversion efficiency.

Due to the resonant nature of the nonlinear interaction, a satisfactory conversion efficiency (of the order of 10-2 or more for an input power of 1 mW) can be obtained without phase agreement. By way of example, the quantum cascade laser of the converter of FIG. 1 generates a modulation radiation at 2.1 THz, which corresponds to a photonic energy EQcL = 8.7 meV and at a wavelength of 143. pm. The input optical signal may have a wavelength of 817 nm (ENIR = 1.5177 eV); in this case, the two secondary beams generated by mixing with three waves have a wavelength of 812.3 nm (summed frequency, energy EN, R + EQcL = 1.5264 eV) and 821.7 nm (difference frequency, ENIR-EQcL energy = 1.5090 eV). The emission energy of the quantum cascade laser, and therefore the wavelength shift induced by the converter, can be tuned within certain limits by modifying the supply current injected through the electrodes E1 and E2.

The converter of the invention was tested at 60K, in a cryostat cooled with liquid helium. However, THz quantum cascade lasers have been demonstrated to be up to 186 K, and future developments are expected to achieve 240K operation, in which case cooling could be done very simply by Peltier effect. Smaller wavelength quantum cascade lasers can operate at room temperature; lasers of this type are likely to introduce larger wavelength shifts, allowing for example to transfer a signal from one optical telecommunications band to another.

The invention has been described with reference to a specific example, but several variants are possible. First, different types of quantum cascade lasers operating in various spectral regions of the far and middle infrared can be used. Their energy bands can be adjusted to accommodate different wavelength signals (in particular around 1.55 μm) and to introduce more or less significant offsets. The dimensions of the device can be modified without substantially modifying the operation thereof. Other means of containment than those described herein may be used. For example, the guiding layer CG may be made of another semiconductor material, such as GaInP. Alternatively, it may be replaced by a metal layer, which has the advantage of improving the THz radiation guidance.

Claims (10)

REVENDICATIONS1. Optical wavelength converter comprising a CLAIMS1. Wavelength optical converter comprising a quantum cascade laser, having an active region (RA) formed by an asymmetrical quantum well stack, for generating a first electromagnetic radiation, characterized in that it also comprises: injecting (L) into said active region a second electromagnetic radiation collinear with said first radiation and resonating with an interband transition of said quantum cascade laser; and means (CG, CC1, E2) for confining said first and second electromagnetic radiations within said active region. An optical wavelength converter according to claim 1, wherein said asymmetric quantum well stack is deposited on a substrate (SS) and wherein said first and second electromagnetic radiation propagates parallel to said substrate. An optical wavelength converter according to claim 2, wherein said means for confining said first and second electromagnetic radiations comprises at least one layer (CG) interposed between said active region and said substrate having a lower refractive index. those of said active region and said substrate. An optical wavelength converter according to claim 3, wherein said means for confining said first and second electromagnetic radiations comprises a first layer (CG) for confining said second electromagnetic radiation, and a second layer (CC1) for confining said second electromagnetic radiation first electromagnetic radiation. An optical wavelength converter according to claim 3, wherein said layer is a metal layer. 6. optical wavelength converter according to one of claims 2 to 5, further comprising a metal layer (E2) 12 deposited on or above said active region, the opposite side to that of said substrate. 7. A method of converting the wavelength of an electromagnetic radiation, comprising the following operations: electrically or optically pump the quantum cascade laser of a converter according to one of the preceding claims, so as to induce the generation said first electromagnetic radiation; injecting into the active region of said quantum cascade laser said second electromagnetic radiation, with a propagation direction parallel to that of said first radiation; and extracting from said converter at least one radiation obtained by mixing three waves of said first and said second radiation. 15 A method of wavelength conversion of electromagnetic radiation according to claim 7, wherein the band structure of said quantum cascade laser and the wavelength of said second radiation are chosen such that the radiation obtained by The difference in frequency of said first and second radiations consists of photons of energy less than the band gap of the active region of said quantum cascade laser. 9. A method of wavelength conversion of electromagnetic radiation according to one of claims 7 or 8, wherein: - said first electromagnetic radiation is a radiation in the middle or far infrared, or Terahertz radiation; and said second electromagnetic radiation is near-infrared or visible radiation. 10. Use of a wavelength converter according to one of claims 1 to 6 in a wavelength division optical fiber telecommunications system. 25 30