Patent ID: 12191632

DETAILED DESCRIPTION

Hereinafter, embodiments of the present disclosure will be described clearly and in detail such that those skilled in the art may easily carry out the present disclosure.

The terms used in the present specification are for describing embodiments, and are not intended to limit the present disclosure. In the present specification, the singular form also includes the plural form unless otherwise specified in the phrase. As used herein, “comprises and/or comprising” does not exclude the presence or addition of one or more other components, steps, operations and/or elements to the mentioned components, steps, operations and/or elements.

Unless otherwise defined, all terms (including technical and scientific terms) used in the present specification may be used as meanings commonly understood by those skilled in the art to which the present disclosure pertains. In addition, terms defined in the commonly used dictionary are not interpreted ideally or excessively unless explicitly defined specifically. In the present specification, the same reference numerals may refer to the same components throughout the entire text.

FIG.1is a block diagram illustrating a terahertz wave wireless communication system according to an embodiment of the present disclosure.

Referring toFIG.1, a terahertz wave wireless communication system1000according to an embodiment of the present disclosure may include a terahertz wave transmitting unit100and a terahertz wave receiving unit200.

The terahertz wave transmitting unit100may generate a terahertz wave signal S_THz for terahertz wireless communication. The terahertz wave signal S_THz may include a baseband signal including information to be transmitted. The terahertz wave transmitting unit100may include a beating light source. The beating light source may include a first light source and a second light source that generate a beating signal. Each of the first light source and the second light source may be a single mode laser. The first light source and the second light source included in the beating light source may have different oscillation frequencies to each other. A difference between the oscillation frequencies of the first light source and the second light source is referred to as a beating frequency.

The terahertz wave transmitting unit100according to an embodiment of the present disclosure may mix two beating signals having different frequencies that are generated from the first light source and the second light source. The terahertz wave transmitting unit100may mix two beating signals and may generate the terahertz wave signal S_THz, which is a continuous wave of a terahertz band, based on the mixed beating signals. The terahertz wave signal S_THz generated by the terahertz wave transmitting unit100may be radiated into free space.

The terahertz wave receiving unit200may receive the radiated terahertz wave signal S_THz. The terahertz wave receiving unit200may detect and amplify the received terahertz wave signal S_THz. The terahertz wave receiving unit200may restore a baseband signal included in the received terahertz wave signal S_THz.

Although a quantum cascade laser (QCL) capable of directly generating the terahertz wave may be used to generate the terahertz wave, the present disclosure uses the beating light source to overcome problems such as an issue of room temperature operation and limited broadband characteristics of the quantum cascade laser. In addition, to mitigate a frequency chirping phenomenon that may occur when the beating signal generated from the beating light source is directly modulated, the present disclosure uses a dual mode laser that directly applies an electric signal to a gain adjustment region. A configuration of the dual mode laser and an effect of mitigating the frequency chirping phenomenon will be described in detail later with reference toFIGS.4and9.

FIG.2is a block diagram illustrating a configuration of a terahertz wave transmitting unit according to an embodiment of the present disclosure.

Referring toFIG.2, a terahertz wave transmitting unit100aaccording to an embodiment of the present disclosure may include a terahertz wave generating apparatus110aand a transmitting antenna150. The terahertz wave generating apparatus110amay include a dual mode laser (DML)120and a photomixer140. The dual mode laser120is a kind of the beating light source.

The terahertz wave generating apparatus110amay generate the terahertz wave signal S_THz (refer toFIG.1). The dual mode laser120included in the terahertz wave generating apparatus110amay include a first light source and a second light source. The first light source and the second light source may be a single mode laser. The first light source may generate a first beating signal. The second light source may generate a second beating signal. The first beating signal and the second beating signal generated by the dual mode laser120may be incident on the photomixer140. The physical characteristics of the terahertz wave signal S_THz generated by the terahertz wave generating apparatus110amay be determined by a laser line width, noise characteristics, a tuning range, and a phase stability of the first light source and the second light source included in the dual mode laser120.

The photomixer140may mix the first beating signal and the second beating signal generated from the dual mode laser120. The photomixer140may modulate a current supplied to the photomixer140, based on a beating frequency of the mixed beating signals. The photomixer140may generate the terahertz wave signal S_THz, based on the modulated current. The terahertz wave signal S_THz may be output to the transmitting antenna150that is connected in series to the photomixer140. The transmitting antenna150may radiate the terahertz wave signal S_THz into a free space.

FIG.3is a block diagram illustrating another configuration of a terahertz wave transmitting unit according to an embodiment of the present disclosure.

Referring toFIG.3, a terahertz wave transmitting unit100baccording to another embodiment of the present disclosure may include a terahertz wave generating apparatus110band the transmitting antenna150. The terahertz wave generating apparatus110bmay include the dual mode laser120, a semiconductor optical amplifier (SOA)130, and the photomixer140.

The terahertz wave generating apparatus110bmay generate the terahertz wave signal S_THz (refer toFIG.1). The dual mode laser120included in the terahertz wave generating apparatus110bmay include the first light source and the second light source. The first light source and the second light source may be a single mode laser. The first light source may generate the first beating signal. The second light source may generate the second beating signal. When the first light source and the second light source are semiconductor-based laser light, a light intensity of the first beating signal and the second beating signal may be weak. Accordingly, to amplify the light intensity, the first beating signal and the second beating signal generated by the dual mode laser120may be incident on the semiconductor optical amplifier130.

The semiconductor optical amplifier130may receive and amplify the first beating signal and the second beating signal that are incident from the dual mode laser120. The amplified first beating signal and the amplified the second beating signal may be incident on the photomixer140. The photomixer140may mix the amplified signals incident from the semiconductor optical amplifier130. The photomixer140may modulate a current supplied to the photomixer140, based on the beating frequency of the mixed amplified signals. The photomixer140may generate the terahertz wave signal S_THz, based on the modulated current. The terahertz wave signal S_THz may be output to the transmitting antenna150that is connected in series to the photomixer140. The transmitting antenna150may radiate the terahertz wave signal S_THz into a free space.

FIG.4is a diagram illustrating a configuration of a dual mode laser according to an embodiment of the present disclosure.

Referring toFIG.4, the dual mode laser120may include a first single mode laser122a, a gain adjustment region123, and a second single mode laser122b. The first single mode laser122aand the second single mode laser122bmay be stacked on a substrate121to be spaced apart. The gain adjustment region123may be formed between the first single mode laser122aand the second single mode laser122b. The substrate121may be a compound semiconductor substrate. For example, the substrate121may be a gallium arsenide (GaAs) substrate, an indium phosphide (InP) substrate, or a gallium nitride (GaN) substrate. The side of the dual mode laser120may be coated with anti-reflection (AR).

The first single mode laser122a, the gain adjustment region123, and the second single mode laser122bmay share an active layer124. The active layer124may include InGaAsP, InAlGaAs, or InAlAs. The active layer124may have a multi-quantum well (MQW) structure. The first single mode laser122amay include a first diffraction grating125a. A first electrode plate126amay be attached to an upper portion of the first single mode laser122a. A second electrode plate126bmay be attached to an upper portion of the gain adjustment region123. The second single mode laser122bmay include a second diffraction grating125b. A third electrode plate126cmay be attached to an upper portion of the second single mode laser122b. A diffraction period of the first diffraction grating125amay be different from a diffraction period of the second diffraction grating125b.

The first single mode laser122aand the second single mode laser122bare a distributed feedback laser diode (DFB LD), a distributed Bragg reflector laser diode (DBR LD), a sampled grating distributed Bragg reflector laser diode (SGDBR LD), or a wavelength tunable single mode laser diode. The wavelength tunable single mode laser may be a wavelength tunable single mode laser using an electro-optic (EO) effect or a thermal-optic (TO) effect in an external cavity diode laser (ECDL).

The first single mode laser122amay receive a voltage V_DFB LD1through the first electrode plate126a. The second single mode laser122bmay receive a voltage V_DFB LD2through the third electrode plate126c. The voltages V_DFB LD1and V_DFB LD2applied to the first electrode plate126aand the third electrode plate126cmay be a forward bias voltage. An active layer124aincluded in the first single mode laser122aand an active layer124cincluded in the second single mode laser122bmay operate as a gain layer. Accordingly, the first single mode laser122aand the second single mode laser122bmay respectively generate light. In this case, since the diffraction periods of the first and second diffraction gratings are different to each other, an operating wavelength of the first beating signal generated by the first single mode laser122amay be different from an operating wavelength of the second beating signal generated by the second single mode laser122b. The first single mode laser122amay output the first beating signal to the gain adjustment region123that is located at the rear end of the first single mode laser122a.

The gain adjustment region123may receive a voltage V_P through the second electrode plate126b. The voltage V_P applied to the gain adjustment region may be a reverse bias voltage. An active layer124bincluded in the gain adjustment region123may operate as an absorbing layer. Accordingly, the gain adjustment region123may absorb light. The gain adjustment region123may absorb the light and may modulate the first beating signal generated by the first single mode laser122a. The gain adjustment region123may output the modulated first beating signal to the outside of the dual mode laser120. The second single mode laser122bmay output the generated second beating signal to the outside of the dual mode laser120. The dual mode laser120may output the modulated first and second beating signals to the photomixer140(refer toFIG.2) or the semiconductor optical amplifier130(refer toFIG.3) that are connected in series with the dual mode laser120.

FIG.5is a diagram illustrating a configuration of a photomixer according to an embodiment of the present disclosure.

Referring toFIG.5, the photomixer140may include a coupler141and an optical-to-electrical converter142. The photomixer140may generate the terahertz wave signal S_THz by using the beating signals generated by the dual mode laser120(refer toFIG.4). The generated terahertz wave signal S_THz may be a terahertz continuous wave. The drawing illustrated inFIG.5is intended to help understand the principle of the photomixer140and does not limit the structure of the photomixer140. As an example, the photomixer140may be a unitravelling carrier photodiode (UTC-PD), an evanescent photodiode (EC-PD), or a low temperature grown photomixer (LTG Photomixer).

The coupler141may generate a mixed beating signal S_Mix by mixing a first beating signal21generated from the first single mode laser122a(refer toFIG.4) and a second beating signal λ2generated from the second single mode laser122b(refer toFIG.4) that are included in the dual mode laser120. The frequency of the mixed beating signal S_Mix is the same as a frequency difference between the first beating signal and the second beating signal. The mixed beating signal S_Mix mixed by the coupler141may be incident on the optical-to-electrical converter142.

The optical-to-electrical converter142may generate the terahertz wave signal S_THz by modulating a photocurrent of the photodiode, based on the incident mixed beating signal S_Mix. A magnitude of the terahertz wave signal S_THz may be proportional to the dot product of a light intensity of the first beating signal λ1and the second beating signal λ2. The terahertz wave signal S_THz generated by the optical-to-electrical converter142may be output to the transmitting antenna150(refer toFIG.2) connected in series with the photomixer140and may be radiated from the transmitting antenna150to a free space.

FIG.6is a diagram illustrating a configuration of a terahertz wave receiving unit according to an embodiment of the present disclosure.

Referring toFIG.6, the terahertz wave receiving unit200may include a receiving antenna210and a terahertz wave receiving apparatus220. The terahertz wave receiving apparatus220may include a terahertz wave detector230and a radio frequency amplifier (RF Amplifier)240. The terahertz wave signal S_THz radiated into the free space may be received by the receiving antenna210. The terahertz wave signal S_THz received by the receiving antenna210may be output to the terahertz wave detector230.

The terahertz wave detector230may separate a carrier signal and a baseband signal of the input terahertz wave signal S_THz. The terahertz wave detector230may use a heterodyne reception method using a Schottky barrier diode (SBD) or a mixer and a local oscillator (LO). The Schottky barrier diode terahertz wave detector may be a terahertz wave detector having a type of III-V based Schottky barrier diode, CMOS (Complementary Metal Oxide Semiconductor) based Schottky barrier diode, or focal plane array (FPA).

The radio frequency amplifier240may be connected in series to the rear end of the terahertz wave detector230. The radio frequency amplifier240may receive the baseband signal separated by the terahertz wave detector230. The radio frequency amplifier240may amplify the separated baseband signal. The terahertz wave wireless communication system1000(refer toFIG.1) may analyze transmitted information, based on the separated baseband signal.

FIG.7Ais a diagram illustrating a principle of generating a light signal when an electric signal is applied to a single mode laser.FIG.7Bis a diagram describing a frequency chirping phenomenon occurring inFIG.7A.

FIG.7Ais a diagram illustrating that an injection current is directly applied to a single mode laser122. When the injection current is directly applied to the single mode laser122, the beating light signal generated from the single mode laser122may be directly modulated by the injection current. The single mode laser122has a unique effective refractive index depending on a gain medium, but the operating wavelength of the single mode laser122may be determined based on the period of the diffraction grating included in the single mode laser122and the injection current. When the diffraction grating included in the single mode laser122is a Bragg grating, an oscillation wavelength may be determined based on Equation 1 below. In Equation 1, λ is the oscillation wavelength, neffis the effective refractive index of the gain medium, and A is the period of the diffraction grating.
λ=2·neff·Λ  [Equation 1]

In this case, when a high-speed injection current is directly applied to the single mode laser122, the effective refractive index of the gain medium of the single mode laser122changes. When the effective refractive index of the gain medium is changed by the injection current, frequency chirping characteristics of the oscillation wavelength may appear. The chirping refers to a phenomenon in which the oscillation frequency is changed depending on the change in the effective refractive index. A dotted line ofFIG.7Bindicates an amount of injection current applied to the single mode laser122over time. A solid line ofFIG.7Bindicates a frequency chirp of the oscillation wavelength over time. When a current is directly injected into the single mode laser122, the frequency chirping phenomenon occurs in which the frequency of the light signal continuously changes over time.

FIGS.8A and8Bare diagrams describing how a frequency chirping phenomenon affects a light signal.

InFIGS.8A and8B, a dotted line represents an optical intensity of the light signal, and a solid line represents a waveform of the frequency chirped light signal. The light signal ofFIG.8Ais a signal of 40 Gb/s, and the light signal ofFIG.8Bis a signal of 10 Gb/s. When the laser diode directly modulates the light signal, a carrier density has a dynamic change due to the dynamic characteristics of the laser diode. When the gain changes due to the dynamic change in the carrier density, it may cause a change in the refractive index inside a laser diode resonator. When the refractive index is dynamically changed, the resonant frequency of the laser diode resonator is changed, resulting in the chirping phenomenon of the carrier frequency.

The frequency chirping phenomenon may be associated with a color dispersion effect of an optical fiber during high-speed modulation, resulting in a deterioration in the performance of the optical communication system. In detail, when the current is directly injected into the single mode laser, the refractive index seen by the light wave decreases as the gain momentarily increases due to the sudden injection of the carrier. In this case, the light wave generated under a resonance condition may be higher than a normal frequency. This frequency shift may contribute to a spread of a band. Therefore, to improve the quality of the wireless communication system, it is necessary to mitigate the frequency chirping phenomenon.

FIG.9is a diagram describing an effect of mitigating a frequency chirping phenomenon of a light signal generated by a dual mode laser according to an embodiment of the present disclosure.

FIG.9is an embodiment of the present disclosure, unlike the single mode laser illustrated inFIG.7A, and illustrates a characteristic in which an applied voltage flows into the gain adjustment region123included in the dual mode laser120(refer toFIG.4). Since there is no electrical signal directly applied to the first single mode laser122a, the first beating signal generated by the first single mode laser122ais not directly modulated by the laser diode. However, it is possible to modulate the first beating signal output from the first single mode laser122aby directly applying the electric signal to the gain adjustment region123. The second single mode laser122bmay output the second beating signal λ2as it is.

The dual mode laser120may output the modulated first beating signal λ1and the second beating signal λ2. In the embodiment of the present disclosure, the electric signal is directly applied to the dual mode laser120, but since the light signal is not modulated by directly applying the electric signal to the single mode laser diode, the frequency chirping phenomenon may be mitigated. As the frequency chirping phenomenon is reduced, the degradation of the transmission quality of the terahertz wave wireless communication system to which the present disclosure is applied may be alleviated.

FIG.10is a flowchart describing a terahertz wave generating method according to an embodiment of the present disclosure.

In operation S110, the dual mode laser120(refer toFIG.4) of the terahertz wave generating apparatus110a(refer toFIG.2) may receive a voltage from the outside. The forward bias voltage may be applied to the first single mode laser122a(refer toFIG.4) and the second single mode laser122b(refer toFIG.4) included in the dual mode laser120. The reverse bias voltage may be applied to the gain adjustment region123(refer toFIG.4) included in the dual mode laser120. The first single mode laser122aand the second single mode laser122bmay generate the first beating signal and the second beating signal by the applied forward bias voltage. The first beating signal generated by the first single mode laser122amay be output to the gain adjustment region123.

In operation S120, the gain adjustment region123may modulate the incident first beating signal by using the applied reverse bias voltage. The active layer included in the gain adjustment region123may function as an absorbing layer. Accordingly, the first beating signal may be absorbed in the active layer of the gain adjustment region123. The amount of light of the absorbed first beating signal may be proportional to the intensity of the reverse bias voltage applied to the gain adjustment region123. The first beating signal may be modulated by absorption of light in the gain adjustment region123.

In operation S130, the first beating signal modulated in the gain adjustment region123and the second beating signal generated by the second single mode laser122bmay be output from the dual mode laser120. The modulated first beating signal and second beating signal may be incident on the photomixer140(refer toFIG.2) that is connected in series with the dual mode laser120. When the light intensity of the modulated first beating signal and the second beating signal is weak, the modulated first beating signal and the second beating signal may be incident on the semiconductor optical amplifier130(refer toFIG.3) and may be amplified, and then may be incident on the photomixer140.

In operation S140, the photomixer140may mix the modulated first beating signal and the second beating signal. Alternatively, the coupler141(refer toFIG.5) included in the photomixer140may mix the modulated first beating signal and the second beating signal that are amplified by the semiconductor optical amplifier130. The mixed beating signal S_Mix may be incident on the optical-to-electrical converter142(refer toFIG.5). The optical-to-electrical converter142may generate the terahertz wave signal S_THz by modulating the current flowing through the photodiode in proportion to the light intensity of the mixed beating signal S_Mix.

FIGS.11A and11Bare diagrams describing a light output depending on an applied voltage in a gain adjustment region of a dual mode laser according to an embodiment of the present disclosure.

FIG.11Aillustrates a light output spectrum of the dual mode laser120depending on the applied voltage V_P (refer toFIG.4) of the gain adjustment region123(refer toFIG.4) included in the dual mode laser120(refer toFIG.4). When the applied voltage V_P of the gain adjustment region123is the forward bias voltage (V_P=+0.2 V), it may be seen that the first beating signal λ1and the second beating signal λ2are not output with a single wavelength. When the applied voltage V_P of the gain adjustment region123is the reverse bias voltage (V_P=−0.7, −1, −2, −3 V), it may be seen that the first beating signal λ1and the second beating signal λ2are output with a single wavelength. In addition, as long as the applied voltage V_P of the gain adjustment region123is the reverse bias voltage (V_P=−0.7, −1, −2, −3 V), the output wavelength of the first beating signal21and the second beating signal22depending on the magnitude of the applied voltage V_P does not change.

In contrast,FIG.11Billustrates the output light intensity of the first beating signal21depending on the applied voltage V_P of the gain adjustment region123. Referring toFIG.11B, it may be seen that as the magnitude of the applied voltage V_P of the gain adjustment region123increases, the output light intensity of the first beating signal21decreases. Therefore, to derive a desired light output in the terahertz wave generating apparatus1000according to an embodiment of the present disclosure, the reverse bias voltage is applied to the gain adjustment region123, but the magnitude of the reverse bias voltage should be appropriately adjusted.

According to an embodiment of the present disclosure, a terahertz wave generating apparatus may generate a modulated terahertz wave by directly applying an electric signal to a gain adjustment region included in a dual mode laser.

According to an embodiment of the present disclosure, a terahertz wave generating apparatus may prevent deterioration of call quality in wireless communication by mitigating a frequency chirping phenomenon.

While the present disclosure has been described with reference to embodiments thereof, it will be apparent to those of ordinary skill in the art that various changes and modifications may be made thereto without departing from the spirit and scope of the present disclosure as set forth in the following claims.