Optical signal generating apparatus and operating method thereof

An optical signal generating apparatus according to an embodiment of the inventive concept includes a first optical intensity modulator for modulating a first optical signal to generate a 2N-level (where N is a positive integer) second optical signal in the form of a binary signal, a first optical amplifier for amplifying the second optical signal to generate a third optical signal, and a second optical intensity modulator for modulating the third optical signal to generate a 2N+1-level fourth optical signal in the form of a binary signal. The optical signal generating apparatus according to an embodiment of the inventive concept may generate a low-cost, high-quality optical signal by using an optical device to generate a multi-level optical signal. Additionally, the optical signal generating apparatus according to an embodiment of the inventive concept may generate a multi-level optical signal by sequentially performing optical modulation and optical amplification operations.

CROSS-REFERENCE TO RELATED APPLICATION

This U.S. non-provisional patent application claims priority under 35 U.S.C. § 119 of Korean Patent Application No. 10-2017-0162246, filed on Nov. 29, 2017, the entire contents of which are hereby incorporated by reference.

BACKGROUND

The present invention relates to an optical signal generating apparatus, and more particularly, to an optical signal generating apparatus for converting an electrical signal into an optical signal in an optical transceiver module used in an optical network and an operating method thereof.

Optical communication technologies require large-capacity, high-efficiency communication means as wired and wireless convergence services expand. In order to step up the speed and capacity of such optical networks, put to use are optical techniques of a time division multiplexing (TDM) scheme for increasing the speed of an individual channel and a wavelength division multiplexing (WDM) scheme for making extensive use of optical frequency resources.

Additionally, multi-level optical signal modulation techniques based on Ethernet are being studied to connect data centers. A pulse amplitude modulation (PAM) technique is used as one of the multi-level optical signal modulation techniques. A PAM optical signal modulator involves a digital-to-analog converter (DAC) for converting a binary electrical signal, which is a digital signal, into an analog signal.

High costs can be caused by digital-to-analog conversion components of an electrical signal that are required in next-generation communication technologies based on large capacities.

SUMMARY

The present disclosure provides an optical signal generating apparatus that generates a multi-level optical signal by using an optical device instead of digital-to-analog conversion components of an electrical signal, and an operating method thereof.

An embodiment of the inventive concept provides an optical signal generating apparatus including: a first optical intensity modulator configured to modulate a first optical signal to generate a 2N-level (where N is a positive integer) second optical signal in the form of a binary signal; a first optical amplifier configured to amplify the second optical signal to generate a third optical signal; and a second optical intensity modulator configured to modulate the third optical signal to generate a 2N+1-level fourth optical signal in the form of a binary signal.

In an embodiment, the first optical intensity modulator may be configured to operate on the basis of a first bias voltage and a first RF voltage, and the second optical intensity modulator may be configured to operate on the basis of a second bias voltage equal to the first bias voltage and a second RF voltage having an amplitude equal to an amplitude of the first RF voltage.

In an embodiment, an optical intensity modulation width of the fourth optical signal may be equal to an optical intensity modulation width of the second optical signal.

In an embodiment, the first optical amplifier may be configured to reshape the second optical signal so that an optical intensity modulation width of the third optical signal becomes one-half of an optical intensity modulation width of the second optical signal.

In an embodiment, the first optical amplifier may operate on the basis of an input current, and a magnitude of the input current may be a magnitude of current that causes the first optical amplifier to operate in a nonlinear characteristic region.

In an embodiment, the first optical intensity modulator may be configured to operate on the basis of a first bias voltage and a first RF voltage, and the second optical intensity modulator may be configured to operate on the basis of a second bias voltage different from the first bias voltage and a second RF voltage having an amplitude different from an amplitude of the first RF voltage.

In an embodiment, the amplitude of the second RF voltage may be half the amplitude of the first RF voltage.

In an embodiment, the first optical amplifier may operate on the basis of an input current, and a magnitude of the input current may be a magnitude of current that causes the first optical amplifier to operate in a linear characteristic region.

In an embodiment, the highest-level optical intensity of the third optical signal may be equal to an optical intensity of the first optical signal.

In an embodiment, the optical signal generating apparatus according to an embodiment of the inventive concept may further include a second optical amplifier configured to amplify the fourth optical signal to generate a fifth optical signal, and a third optical intensity modulator configured to modulate the fifth optical signal to generate a 2N+2-level sixth optical signal in the form of a binary signal.

In an embodiment, the first optical intensity modulator may be configured to operate on the basis of a first bias voltage and a first RF voltage; the second optical intensity modulator may be configured to operate on the basis of a second bias voltage and a second RF voltage; and the third optical intensity modulator may be configured to operate on the basis of a third bias voltage and a third RF voltage, wherein the first bias voltage, the second bias voltage, and the third bias voltage are equal, and the first RF voltage, the second RF voltage, and the third RF voltage have equal amplitudes.

In an embodiment, the first optical amplifier may be configured to reshape the second optical signal so that an optical intensity modulation width of the third optical signal becomes one-half of an optical intensity modulation width of the second optical signal; and the second optical amplifier may be configured to reshape the fourth optical signal so that an optical intensity modulation width of the fifth optical signal becomes one-fourth of an optical intensity modulation width of the fourth optical signal.

In an embodiment, the first optical intensity modulator may be configured to operate on the basis of a first bias voltage and a first RF voltage; the second optical intensity modulator may be configured to operate on the basis of a second bias voltage and a second RF voltage; and the third optical intensity modulator may be configured to operate on the basis of a third bias voltage and a third RF voltage, wherein the first bias voltage, the second bias voltage, and the third bias voltage are different from each other, and the first RF voltage, the second RF voltage, and the third RF voltage have amplitudes different from each other.

In an embodiment, an amplitude of the second RF voltage may be one-half of an amplitude of the first RF voltage, and an amplitude of the third RF voltage may be one-half of an amplitude of the second RF voltage.

In an embodiment, each of the first optical intensity modulator and the second optical intensity modulator may be a Mach-Zehnder optical intensity modulator or an electro-absorption modulator.

An embodiment of the inventive concept provides an operating method of an optical signal generating apparatus, the operating method including: modulating a first optical signal to generate a 2N-level (where N is a positive integer) second optical signal in the form of a binary signal; amplifying the second optical signal to generate a third optical signal; and modulating the third optical signal to generate a 2N+1-level fourth optical signal in the form of a binary signal.

In an embodiment, the first optical signal may be modulated on the basis of a first bias voltage and a first RF voltage, and the third optical signal may be modulated on the basis of a second bias voltage equal to the first bias voltage and a second RF voltage having an amplitude equal to an amplitude of the first RF voltage.

In an embodiment, the first optical signal may be modulated on the basis of a first bias voltage and a first RF voltage, and the third optical signal may be modulated on the basis of a second bias voltage different from the first bias voltage and a second RF voltage having an amplitude different from an amplitude of the first RF voltage.

In an embodiment, the second optical signal may be amplified on the basis of an input current, and a magnitude of the input current may be a magnitude of current that causes an optical intensity modulation width of the third optical signal to be different from an optical intensity modulation width of the second optical signal.

In an embodiment, the second optical signal may be amplified on the basis of an input current, and a magnitude of the input current may be a magnitude of current that causes an optical intensity modulation width of the third optical signal to be equal to an optical intensity modulation width of the second optical signal.

DETAILED DESCRIPTION

Hereinafter, embodiments of the inventive concept are described in more detail with reference to the accompanying figures. In the following description, specific details such as detailed configurations and structures are described to provide more general understandings of the embodiments of the inventive concept. Therefore, various changes and modifications to the embodiments of the inventive concept can be made by those skilled in the art within the spirit and scope of the inventive concept. Additionally, descriptions of well-known functions and structures are omitted for clarity and brevity. The terms used in the following description are defined in consideration of the functions of the inventive concept, and are not limited to specific functions. Thus, the definition of the terms can be determined on the basis of the detailed description.

Modules in the following figures or detailed description can be connected to others in addition to the components illustrated in the figures or described in the detailed description. Each of the connections between modules or components can be direct or indirect. Each of the connections between the modules or components can be a communication connection or a physical connection.

Components described with reference to terms such as unit, module and layer used in the detailed description can be implemented in the form of software, hardware, or a combination thereof. By way of example, the software may be machine code, firmware, embedded code, or application software. For example, the hardware may include an electrical circuit, an electronic circuit, a processor, a computer, an integrated circuit, integrated circuit cores, a pressure sensor, an inertial sensor, a micro electro mechanical system (MEMS), a passive device, or a combination thereof.

Unless otherwise defined, all terms including technical or scientific meaning used in this specification have general meaning understood by those skilled in the art to which the inventive concept pertains. In general, terms defined in a dictionary are interpreted to have meaning equivalent to the contextual meaning in the related art and are not to be interpreted as having ideal or overly formal meaning unless explicitly defined in the specification.

FIG. 1schematically illustrates an optical signal generating apparatus according to an embodiment of the inventive concept. Referring toFIG. 1, a light source10may generate an optical signal from a current I_s. The light source10may output an optical signal of a constant intensity and transmit the same to an optical signal generating apparatus100.

The optical signal generating apparatus100may receive an optical signal from the light source10, and modulate the received optical signal to generate a 2N+1-level (N is an integer of 1 or more) (or multi-level) optical signal. That is, the optical signal generated from the optical signal generating apparatus100may represent one of 2N+1levels. Accordingly, the optical signal generating apparatus100may generate an optical signal capable of representing 2N+1values.

The optical signal generating apparatus100may include a zeroth optical intensity modulator101and at least one binary signal generating unit110. The zeroth optical intensity modulator101may receive an optical signal of a constant intensity from the light source10and modulate optical intensity. The zeroth optical intensity modulator101may generate an optical signal in the form of a binary signal (hereinafter, referred to as binary optical signal) by modulating the optical intensity. That is, the zeroth optical intensity modulator101may generate, from the optical signal of a single level, a two-level optical signal having a high level (for example, “1”) value and a low level (for example, “0”) value.

The binary signal generating unit110may receive and amplify an optical signal, and modulate the amplified optical signal to generate a binary optical signal. By way of example, the binary signal generating unit110may reshape the optical signal. When the optical signal is reshaped, an optical intensity modulation width of the binary optical signal may vary.

A first binary signal generating unit110-1may receive the two-level optical signal from the zeroth optical intensity modulator101. The first binary signal generating unit110-1may generate, from the optical signal representing two levels, a binary optical signal representing four levels.

A second binary signal generating unit110-2may receive the four-level optical signal from the first binary signal generating unit110-1. The second binary signal generating unit110-2may generate, from the optical signal representing four levels, a binary optical signal representing eight levels.

An (N)th binary signal generating unit110-N may receive a 2N-level optical signal from an (N−2)th binary signal generating unit (not illustrated). The (N)th binary signal generating unit110-N may generate, from the optical signal representing 2Nlevels, a binary optical signal representing 2N+1levels.

As illustrated inFIG. 1, the zeroth optical intensity modulator101and the at least one binary signal generating unit110may be connected in series, and sequentially modulate an optical signal to generate a multi-level optical signal. Accordingly, the optical signal generating apparatus100may generate a 2N+1-level optical signal through N binary signal generating units, and generate a multi-level optical signal through one optical intensity modulator and the at least one binary signal generating unit.

The optical signal generating apparatus according to an embodiment of the inventive concept may receive an optical signal generated from the separate light source10as illustrated inFIG. 1, but the inventive concept is not limited thereto. For example, the optical signal generating apparatus according to an embodiment of the inventive concept may include a light source and generate an optical signal in the optical signal generating apparatus.

Hereinafter, for convenience of description, the optical signal generating apparatus is assumed to receive an optical signal from a separate light source.

FIG. 2is a block diagram illustrating the optical signal generating apparatus illustrated inFIG. 1. Referring toFIGS. 1 and 2, the optical signal generating apparatus100may include the zeroth optical intensity modulator101and the at least one binary signal generating unit110. The binary signal generating unit110may include an optical amplifier111and an optical intensity modulator112.

The optical intensity modulators101and112may be implemented using an electro-absorption modulator (EAM), a Mach-Zehnder optical intensity modulator or the like.

The optical intensity modulators101and112may modulate a received optical signal on the basis of a bias voltage Vbiasand an RF voltage VRF. The bias voltage Vbiasis a direct current (DC) voltage, and may be related to a reference operating point of the optical intensity modulator. The RF voltage VRFmay be related to an optical intensity modulation width of a binary optical signal generated by modulating the optical signal. By way of example, the RF voltage VRFmay be an alternating current (AC) voltage or an electrical binary signal. For example, an optical intensity reference value of the generated binary optical signal may be determined depending on the bias voltage Vbias, and the optical intensity modulation width of the binary optical signal may vary depending on the RF voltage VRF.

The optical amplifier111may amplify the intensity of a received optical signal on the basis of a current I. The optical amplifier111may reshape the optical signal by adjusting an optical intensity depth of the binary optical signal on the basis of the current I.

The zeroth optical intensity modulator101may receive a zeroth bias voltage Vbias0, a zeroth RF voltage VRF0, and an optical signal. The zeroth optical intensity modulator101may modulate the received optical signal to generate a binary optical signal, on the basis of the zeroth bias voltage Vbias0and the zeroth RF voltage VRF0. The binary optical signal generated by the zeroth optical intensity modulator101may be a two-level optical signal.

A first optical amplifier111-1may receive a first current I_1, and the optical signal generated from the zeroth optical intensity modulator101. The first optical amplifier111-1may amplify the received optical signal on the basis of the first I_1. The first optical amplifier111-1may reshape the received optical signal on the basis of the first I_1.

A first optical intensity modulator112-1may receive a first bias voltage Vbias1, a first RF voltage VRF1, and the amplified optical signal from the first optical amplifier111-1. The first optical intensity modulator112-1may modulate the received optical signal to generate a binary optical signal, on the basis of the first bias voltage Vbias1and the first RF voltage VRF1. The binary optical signal generated by the first optical intensity modulator112-1may be a four-level optical signal.

As in the first optical amplifier111-1, a second optical amplifier111-2may amplify and reshape the received optical signal on the basis of a second current I_2. Additionally, an (N)th optical amplifier111-N may amplify and reshape a received optical signal on the basis of an (N)th current I_N.

As in the first optical intensity modulator112-1, a second optical intensity modulator112-2may modulate a received optical signal to generate an eight-level optical signal, on the basis of a second bias voltage Vbias2and a second RF voltage VRF2. Additionally, an (N)th optical intensity modulator112-N may modulate a received optical signal to generate a 2N+1-level optical signal, on the basis of an (N)th bias voltage VbiasN and an (N)th RF voltage VRFN.

By way of example, magnitudes of the zeroth to (N)th bias voltages Vbias0to VbiasN may be equal, and magnitudes of amplitudes of the zeroth to (N)th RF voltages VRF0to VRFN may be equal. In this case, optical intensity modulation widths of the binary optical signals generated by the zeroth to (N)th optical intensity modulators101to112-N may be equal.

By way of example, the magnitudes of the zeroth to (N)th bias voltages Vbias0to VbiasN may be different, and the magnitudes of the amplitudes of the zeroth to (N)th RF voltages VRF0to VRFN may be different. In this case, the optical intensity modulation widths of the binary optical signals generated by the zeroth to (N)th optical intensity modulators101to112-N may be different.

As illustrated inFIGS. 1 and 2, the optical signal generating apparatus100may include N binary signal generating units. However, the optical signal generating apparatus100is not limited with respect to the number of the binary signal generating units, and thus, may generate a multi-level optical signal (e.g. a four-level optical signal) through, for example, a single binary signal generating unit.

FIGS. 3A and 3Billustrate, by way of example, a modulation operation of the optical intensity modulator according to an embodiment of the inventive concept. Specifically,FIG. 3Aillustrates an example in which the zeroth optical intensity modulator101modulates a received optical signal into a binary optical signal, andFIG. 3Billustrates an example in which the zeroth optical intensity modulator101modulates the received optical signal into the binary optical signal depending on the bias voltage Vbiasand the RF voltage VRF. For convenience of description, the optical intensity modulator according to an embodiment of the inventive concept is described with reference to the zeroth optical intensity modulator101, but other optical intensity modulators112-1to112-N may operate similarly.

Referring toFIG. 3A, a horizontal axis ofFIG. 3Arepresents time and a vertical axis thereof represents optical intensity. The optical intensity of the vertical axis represents relative magnitude of the optical intensity of an output optical signal to the optical intensity of an input optical signal, and may not mean absolute optical intensity. Similarly, optical intensity illustrated in the following figures may represent relative magnitude of optical intensity.

The zeroth optical intensity modulator101may modulate a received first optical signal OS1to generate a second optical signal OS2. By way of example, the generated second optical signal OS2may be in the form of a binary signal. For example, the second optical signal OS2may represent a value of “1” when having a higher optical intensity (i.e., p2), and may represent a value of “0” when having a lower optical intensity (i.e., p3).

As illustrated inFIG. 3A, the optical signal modulated through the zeroth optical intensity modulator101may have a reduced optical intensity due to insertion loss. For example, a second optical intensity p2of the modulated second optical signal OS2may become lower than a first optical intensity p1of the first optical signal OS1by the insertion loss.

Referring toFIG. 3B, a horizontal axis ofFIG. 3Brepresents applied voltage and a vertical axis thereof represents optical intensity.FIG. 3Billustrates an optical intensity response characteristic depending on voltages applied to the zeroth optical intensity modulator101. The zeroth optical intensity modulator101may operate on the basis of an optical intensity modulator characteristic curve. By way of example, the response characteristic (i.e., the optical intensity modulator characteristic curve) of the zeroth optical intensity modulator101may vary according to the structure and design of the zeroth optical intensity modulator101, and the response characteristic of the zeroth optical intensity modulator101may be predetermined.

The generated second optical signal OS2may be determined on the basis of the optical intensity modulator characteristic curve. As illustrated inFIG. 3B, a reference point of a modulated optical intensity may vary depending on the bias voltage Vbias. For example, when the bias voltage Vbiasbecomes smaller, the modulated optical intensity may become higher, and when the bias voltage Vbiasbecomes larger, the modulated optical intensity may become lower.

Additionally, an optical intensity modulation width of the generated optical signal may vary depending on the amplitude of the RF voltage VRF. For example, when the amplitude of the RF voltage VRFbecomes larger, the optical intensity modulation width may become larger, and when the amplitude of the RF voltage VRFbecomes smaller, the optical intensity modulation width may become smaller.

By way of example, the bias voltage Vbiasand the RF voltage VRFmay be applied so that the zeroth optical intensity modulator101may operate in a linear characteristic region of the optical intensity modulator characteristic curve. When the zeroth optical intensity modulator101operates in the linear characteristic region, the optical intensity modulation width may be easily controlled by adjusting the applied bias voltage Vbiasand RF voltage VRF.

FIGS. 4A and 4Billustrate, by way of example, an operation of an optical amplifier according to an embodiment of the inventive concept. Specifically,FIG. 4Aillustrates an example in which the first optical amplifier111-1amplifies a received optical signal, andFIG. 4Billustrates an example for describing a method of determining the extent of optical amplification outputted from the first optical amplifier111-1. For convenience of description, the optical amplifier according to an embodiment of the inventive concept is described with reference to the first optical amplifier111-1, but other optical amplifiers111-2to111-N may operate similarly.

Referring toFIG. 4A, a horizontal axis represents time and a vertical axis represents optical intensity. The first optical signal OS1may be modulated into the second optical signal OS2by the zeroth optical intensity modulator101, and the optical intensity of the second optical signal OS2may be amplified by the first optical amplifier111-1to generate a third optical signal OS3.

By the first optical amplifier111-1, the second optical intensity p2of the second optical signal OS2may be amplified to a fourth optical intensity p4of the third optical signal OS3, and the third optical intensity p3of the second optical signal OS2may be amplified to a fifth optical intensity p5of the third optical signal OS3.

The first optical amplifier111-1may not only perform amplification of optical intensity, but also change the optical intensity modulation width of the generated optical signal (i.e., reshape the optical signal). Accordingly, optical intensity modulation widths before and after amplification may be different. For example, the first optical amplifier111-1may amplify an optical signal so that the optical intensity modulation widths before and after the amplification are equal. Alternatively, the first optical amplifier111-1may amplify an optical signal so that the optical intensity modulation width after the amplification becomes smaller than that before the amplification.

Referring toFIG. 4B, a horizontal axis represents input optical intensity and a vertical axis represents output optical intensity. The first optical amplifier111-1may amplify the optical intensity of an inputted optical signal on the basis of an optical amplifier characteristic curve. According to the optical amplifier characteristic curve, the second optical intensity p2may be amplified to the fourth optical intensity p4, and the third optical intensity p3may be amplified to the fifth optical intensity p5.

The optical amplifier characteristic curve may have a linear characteristic region and a nonlinear characteristic region (or saturation region). When optical intensity is amplified in the linear characteristic region, output optical intensity may be determined in proportion to input optical intensity. Optical intensity modulation widths before and after the amplification may be equal. Accordingly, when an optical signal is only amplified, the optical amplifier may operate in the linear characteristic region.

When optical intensity is amplified in the nonlinear characteristic region, output optical intensity may not be proportionate to input optical intensity. An optical intensity modulation width after the amplification may become smaller than that before the amplification. Accordingly, when an optical signal is reshaped, the optical amplifier may operate in the nonlinear characteristic region.

The first optical amplifier111-1may perform amplification operation on the basis of an input current. When the input current changes, an operation characteristic of the first optical amplifier111-1may change. In other words, the optical amplifier characteristic curve illustrated inFIG. 4Bmay be changed. For example, the input current may determine whether the first optical amplifier111-1operates in the linear or nonlinear characteristic region. Accordingly, depending on the input current, the first optical amplifier111-1may amplify an optical signal in the linear or nonlinear characteristic region.

FIG. 5is a block diagram illustrating an optical signal generating apparatus according to an embodiment of the inventive concept. Referring toFIG. 5, an optical signal generating apparatus200may receive an optical signal to output a four-level optical signal. The optical signal generating apparatus200may include a zeroth optical intensity modulator201and a binary signal generating unit210. The binary signal generating unit210may include a first optical amplifier211and a first optical intensity modulator212. Because the zeroth optical intensity modulator201, the first optical amplifier211and the first optical intensity modulator212perform operations similar to those of the optical intensity modulators and the optical amplifiers described inFIGS. 1 to 4B, detailed description thereof will not be given.

The zeroth optical intensity modulator201may operate on the basis of a zeroth bias voltage Vbias0and a zeroth RF voltage VRF0, and the first optical intensity modulator212may operate on the basis of a first bias voltage Vbias1and a first RF voltage VRF1. The zeroth bias voltage Vbias0and the first bias voltage Vbias1may be equal, and amplitudes of the zeroth RF voltage VRF0and the first RF voltage VRF1may be equal. That is, the zeroth optical intensity modulator201and the first optical intensity modulator212may be operated by the equal bias voltages Vbiasand the RF voltages VRFhaving the equal amplitudes.

As illustrated inFIG. 3B, when the bias voltages Vbiasapplied to the zeroth and first optical intensity modulators201and212are equal, and amplitudes of the RF voltages VRFapplied thereto are equal, optical signals having equal optical intensity modulation widths may be generated. Accordingly, an optical intensity modulation width of an optical signal generated by the zeroth optical intensity modulator201may be equal to that of an optical signal generated by the first optical intensity modulator212.

The first optical amplifier211may receive a first current I_1, and amplify and reshape the optical signal received from the zeroth optical intensity modulator201. The first optical amplifier211may amplify and reshape the received optical signal so that intervals between adjacent signal levels of the four-level optical signal outputted from the optical signal generating apparatus200are equal. As illustrated inFIG. 4B, the first optical amplifier211may operate in a nonlinear characteristic region so as to amplify and reshape a received optical signal. Accordingly, the first current I_1may be a current that causes the first optical amplifier211to operate in the nonlinear characteristic region.

By way of example, when the first optical amplifier211reshapes the inputted optical signal, an output optical intensity modulation width may be one-half of an input optical intensity modulation width.

The first optical intensity modulator212may modulate an amplified and reshaped two-level optical signal to generate the four-level optical signal. Accordingly, the optical signal generating apparatus200may output the four-level optical signal (i.e., a multi-level optical signal).

FIG. 6illustrates, by way of example, an optical signal output of the optical signal generating apparatus illustrated inFIG. 5. A horizontal axis ofFIG. 6represents time and a vertical axis thereof represents optical intensity. Referring toFIGS. 5 and 6, the zeroth optical intensity modulator201may modulate a first optical signal OS1to generate a second optical signal OS2. A two-level second optical signal OS2having a second optical intensity p2and a third optical intensity p3may be generated from a first optical signal OS1having a first optical intensity p1. The zeroth optical intensity modulator201may generate the second optical signal OS2having a first optical intensity modulation width W1, on the basis of the zeroth bias voltage Vbias0and the zeroth RF voltage VRF0.

The first optical amplifier211may amplify the second optical signal OS2to generate a third optical signal OS3. The first optical amplifier211may generate the third optical signal OS3on the basis of the first I_1. The third optical signal OS3may be a two-level optical signal having a fourth optical intensity p4and a fifth optical intensity p5.

The third optical signal OS3may have a higher optical intensity and a smaller optical intensity modulation width than the second optical signal OS2. By way of example, even though illustrated differently inFIG. 6, the first optical amplifier211may amplify the second optical signal OS2so that the fourth optical intensity p4of the third optical signal OS3becomes equal to the first optical intensity p1of the first optical signal OS1. The first optical amplifier211may reshape the second optical signal OS2so that a second optical intensity modulation width W2of the third optical signal OS3becomes half the first optical intensity modulation width W1.

The first optical intensity modulator212may modulate the third optical signal OS3to generate a fourth optical signal OS4, on the basis of the first bias voltage Vbias1and the first RF voltage VRF1. The first bias voltage Vbias1may be equal to the zeroth bias voltage Vbias0, and the amplitude of the first RF voltage VRF1may be equal to that of the zeroth RF voltage VRF0.

The fourth optical signal OS4may be a four-level optical signal having a sixth optical intensity p6, a seventh optical intensity p7, an eighth optical intensity p8and a ninth optical intensity p9. The first optical intensity modulator212may generate the fourth optical signal OS4having the sixth optical intensity p6and the eighth optical intensity p8from the third optical signal OS3having the fourth optical intensity p4. The first optical intensity modulator212may generate the fourth optical signal OS4having the seventh optical intensity p7and the ninth optical intensity p9from the third optical signal OS3having the fifth optical intensity p5. That is, the first optical intensity modulator212may generate the four-level fourth optical signal OS4from the two-level third optical signal OS3.

The first optical intensity modulator212may generate the fourth optical signal OS4having a third optical intensity modulation width W3from the third optical signal OS3having the fourth optical intensity p4. The first optical intensity modulator212may generate the fourth optical signal OS4having a fourth optical intensity modulation width W4from the third optical signal OS3having the fifth optical intensity p5. The third optical intensity modulation width W3and the fourth optical intensity modulation width W4may be equal, and the third optical intensity modulation width W3and the fourth optical intensity modulation width W4may be equal to the first optical intensity modulation width W1. Additionally, intervals of the optical intensities p6to p9of the fourth optical signal OS4may be equal.

In this specification, the optical intensity modulation width may mean the optical intensity depth of a binary optical signal generated through the optical intensity modulator from a constant-level optical signal. Accordingly, the third optical intensity modulation width W3or the fourth optical intensity modulation width W4of the fourth optical signal OS4may have meaning different from the intervals of the optical intensities p6to p9of the fourth optical signal OS4.

FIG. 7is a flowchart illustrating an operating method of the optical signal generating apparatus illustrated inFIG. 5. In step S101, referring toFIGS. 5 and 7, an optical signal generating apparatus200may modulate, depending on a zeroth bias voltage Vbias0and a zeroth RF voltage VRF0, an optical signal to generate a binary optical signal. In step S102, the optical signal generating apparatus200may amplify and reshape the generated binary optical signal. In step S103, the optical signal generating apparatus200may modulate, depending on a first bias voltage Vbias1and a first RF voltage VRF1, the reshaped binary optical signal to generate a multi-level optical signal. The first bias voltage Vbias1may be equal to the zeroth bias voltage Vbias0, and the amplitude of the first RF voltage VRF1may be equal to that of the zeroth RF voltage VRF0.

The optical signal generating apparatus according to an embodiment of the inventive concept is not limited to what is illustrated inFIG. 7, and may generate an optical signal having more levels by repeating steps S102and S103. For example, when steps S102and S103are performed N times, the optical signal generating apparatus may generate a 2N+1-level optical signal.

FIG. 8is a block diagram illustrating an optical signal generating apparatus according to an embodiment of the inventive concept. An optical signal generating apparatus300may include a zeroth optical intensity modulator301, a first binary signal generating unit310and a second binary signal generating unit320. The first binary signal generating unit310may include a first optical amplifier311and a first optical intensity modulator312, and the second binary signal generating unit320may include a second optical amplifier321and a second optical intensity modulator322. Because operations of the zeroth optical intensity modulator301and the first binary signal generating unit310are similar to those of the zeroth optical intensity modulator201and the binary signal generating unit210ofFIG. 5, detailed description will not be given.

The optical signal generating apparatus300may generate an eight-level optical signal through the zeroth optical intensity modulator301and the first and second binary signal generating units310and320. Equal bias voltages (i.e., Vbias0=Vbias1=Vbias2) and RF voltages of equal amplitudes (i.e., VRF0=VRF1=VRF2) may be applied to the zeroth optical intensity modulator301, and the first and second optical intensity modulators312and322. A first current I_1and a second current I_2may be respectively inputted to the first optical amplifier311and the second optical amplifier321.

FIG. 9illustrates, by way of example, an optical signal output of the optical signal generating apparatus illustrated inFIG. 8. A horizontal axis ofFIG. 9represents time and a vertical axis thereof represents optical intensity. Referring toFIGS. 8 and 9, the zeroth optical intensity modulator301may modulate, on the basis of the zeroth bias voltage Vbias0and the zeroth RF voltage VRF0, a first optical signal OS1to generate a second optical signal OS2having a first optical intensity modulation width W1.

The first optical amplifier311may amplify and reshape the second optical signal OS2to generate a third optical signal OS3, on the basis of the first I_1. A second optical intensity modulation width W2of the reshaped third optical signal OS3may be different from the first optical intensity modulation width W1. By way of example, the second optical intensity modulation width W2may be half the first optical intensity modulation width W1. The highest-level optical intensity of the third optical signal OS3may be equal to the optical intensity of the first optical signal OS1.

The first optical intensity modulator312may modulate, on the basis of the first bias voltage Vbias1and the first RF voltage VRF1, the third optical signal OS3to generate a fourth optical signal OS4having a third optical intensity modulation width W3. The first bias voltage Vbias1may be equal to the zeroth bias voltage Vbias0, and the amplitude of the first RF voltage VRF1may be equal to that of the zeroth RF voltage VRF0. By way of example, the third optical intensity modulation width W3may be equal to the first optical intensity modulation width W1, and intervals between adjacent signal levels of the fourth optical signal OS4may be equal.

The second optical amplifier321may amplify and reshape the fourth optical signal OS4to generate a fifth optical signal OS5, on the basis of the second current I_2. The fifth optical signal OS5may have a fourth optical intensity modulation width W4different from the third optical intensity modulation width W3. By way of example, the fourth optical intensity modulation width W4may be one-fourth of the third optical intensity modulation width W3. Accordingly, the optical signal generating apparatus300according to an embodiment of the inventive concept may reshape an optical signal so that the optical intensity modulation width becomes further reduced as the optical signal amplification steps progress. For example, the optical signal generating apparatus300may reshape the optical signal modulation width to one-half in the first optical amplifier, to one-fourth in the second optical amplifier, and to one-eighth in the third optical amplifier. The highest-level optical intensity of the fifth optical signal OS5may be equal to the highest-level optical intensity of the third optical signal OS3.

The second optical intensity modulator322may modulate, on the basis of the second bias voltage Vbias2and the second RF voltage VRF2, the fifth optical signal OS5to generate a sixth optical signal OS6having a fifth optical intensity modulation width W5. The second bias voltage Vbias2may be equal to the zeroth bias voltage Vbias0, and the amplitude of the second RF voltage VRF2may be equal to that of the zeroth RF voltage VRF0. By way of example, the fifth optical intensity modulation width W5may be equal to the first and third optical intensity modulation widths W1and W3, and intervals between adjacent signal levels of the sixth optical signal OS6may be equal.

The optical signal generating apparatus300may output the sixth optical signal OS6generated from the second optical intensity modulator322. The optical signal generating apparatus300may output an eight-level optical signal therefrom. The eight-level optical signal outputted from the optical signal generating apparatus300may indicate values corresponding to bits assigned as inFIG. 9and the following Table 1.

TABLE 1Eight-Value of the zerothValue of the first RFValue of the secondlevelRF voltage of thevoltage of the firstRF voltage of theopticalzeroth opticaloptical intensitysecond opticalsignalintensity modulatormodulatorintensity modulator000000001100010010011110100001101101110011111111

As in Table 1, when the eight-level optical signal is received from the optical signal generating apparatus300, a receiving terminal may recognize a value indicated by the optical signal depending on the optical signal level as one of “000” to “111”. For example, when a zeroth RF voltage VRF0corresponding to “1” is inputted to the zeroth optical intensity modulator301, a first RF voltage VRF1corresponding to “0” is inputted to the first optical intensity modulator312, and a second RF voltage VRF2corresponding to “0” is inputted to the second optical intensity modulator322, the receiving terminal may receive an optical signal indicating “001”.

When a zeroth RF voltage VRF0corresponding to “0” is inputted to the zeroth optical intensity modulator301, a first RF voltage VRF1corresponding to “1” is inputted to the first optical intensity modulator312, and a second RF voltage VRF2corresponding to “1” is inputted to the second optical intensity modulator322, the receiving terminal may receive an optical signal indicating “110”.

Bit values may be assigned as in Table 1 for the eight-level optical signal, but the inventive concept is not limited thereto, and bit values corresponding to respective levels may vary.

As described in detail, the optical signal generating apparatus according to embodiments of the inventive concept may generate a multi-level optical signal by sequentially modulating optical signals through a plurality of optical intensity modulators operating on the basis of equal bias voltages Vbiasand RF voltages VRFof equal amplitudes.

FIG. 10is a block diagram illustrating an optical signal generating apparatus according to an embodiment of the inventive concept. Referring toFIG. 10, an optical signal generating apparatus400may receive an optical signal to output a four-level optical signal. The optical signal generating apparatus400may include a zeroth optical intensity modulator401and a binary signal generating unit410. The binary signal generating unit410may include a first optical amplifier411and a first optical intensity modulator412. Because the zeroth optical intensity modulator401, the first optical amplifier411and the first optical intensity modulator412perform operations similar to those of the optical intensity modulators and the optical amplifiers described inFIGS. 1 to 4B, detailed description thereof will not be given.

The zeroth optical intensity modulator401may modulate an optical signal by using a zeroth bias voltage Vbias0and a zeroth RF voltage VRF0as inputs, and the first optical intensity modulator412may modulate an optical signal by using a first bias voltage Vbias1and a first RF voltage VRF1as inputs. As illustrated inFIG. 3B, when the bias voltages Vbiasapplied to the zeroth and first optical intensity modulators401and412are different, and amplitudes of the RF voltages VRFapplied thereto are different, optical signals having different optical intensity modulation widths may be generated.

The first optical amplifier411may receive a first current I_1, and amplify an optical signal received from the zeroth optical intensity modulator401. The first optical amplifier411may amplify the optical signal while keeping an optical intensity modulation width equal. As illustrated inFIG. 4B, the first optical amplifier411may operate in a linear characteristic region to amplify the optical signal. Accordingly, the first current I_1may be a current that causes the first optical amplifier411to operate in the linear characteristic region.

The first optical intensity modulator412may modulate an amplified two-level optical signal to generate a four-level optical signal. Accordingly, the optical signal generating apparatus400may output the four-level optical signal (i.e., a multi-level optical signal).

FIG. 11illustrates, by way of example, an optical signal output of the optical signal generating apparatus illustrated inFIG. 10. A horizontal axis ofFIG. 11represents time and a vertical axis thereof represents optical intensity. Referring toFIGS. 10 and 11, the zeroth optical intensity modulator401may modulate a first optical signal OS1to generate a second optical signal OS2. The zeroth optical intensity modulator401may generate a two-level second optical signal OS2having a second optical intensity p2and a third optical intensity p3from a first optical signal OS1having a first optical intensity p1. The zeroth optical intensity modulator401may generate the second optical signal OS2having a first optical intensity modulation width W1, on the basis of the zeroth bias voltage Vbias0and the zeroth RF voltage VRF0.

The first optical amplifier411may amplify the second optical signal OS2to generate a third optical signal OS3, on the basis of the first I_1. The third optical signal OS3may be a two-level optical signal having a fourth optical intensity p4and a fifth optical intensity p5. The first optical amplifier411may amplify the second optical signal OS2so that the fourth optical intensity p4of the third optical signal OS3becomes equal to the first optical intensity p1of the first optical signal OS1. A second optical intensity modulation width W2of the third optical signal OS3may be equal to the first optical intensity modulation width W1.

The first optical intensity modulator412may modulate the third optical signal OS3to generate a fourth optical signal OS4, on the basis of the first bias voltage Vbias1and the first RF voltage VRF1. The fourth optical signal OS4may be a four-level optical signal having a sixth optical intensity p6, a seventh optical intensity p7, an eighth optical intensity p8and a ninth optical intensity p9. The first optical intensity modulator412may generate the fourth optical signal OS4having the sixth optical intensity p6and the seventh optical intensity p7from the third optical signal OS3having the fourth optical intensity p4. The first optical intensity modulator412may generate the fourth optical signal OS4having the eighth optical intensity p8and the ninth optical intensity p9from the third optical signal OS3having the fifth optical intensity p5. Accordingly, the first optical intensity modulator412may generate the four-level fourth optical signal OS4from the two-level third optical signal OS3.

The first optical intensity modulator412may generate the fourth optical signal OS4having a third optical intensity modulation width W3and a fourth optical intensity modulation width W4from the third optical signal OS3. The first optical intensity modulator412may modulate the third optical signal OS3so that intervals between adjacent signal levels of the fourth optical signal OS4are equal (i.e., the third optical intensity modulation width W3, the fourth optical intensity modulation width W4and a fifth optical intensity modulation width W5are equal.). For example, the intervals between the adjacent signal levels of the fourth optical signal OS4may be equal by applying a first RF voltage VRF1having an amplitude half that of the zeroth RF voltage VRF0.

As illustrated inFIG. 11, the optical signal generating apparatus400may generate a multi-level optical signal similarly to the optical signal generating apparatus200ofFIG. 5, but may generate other form of multi-level optical signal.

FIG. 12is a flowchart illustrating an operating method of the optical signal generating apparatus illustrated inFIG. 10. In step S201, referring toFIGS. 10 and 12, an optical signal generating apparatus400may modulate, depending on a zeroth bias voltage Vbias0and a zeroth RF voltage VRF0, an optical signal to generate a binary optical signal. In step S202, the optical signal generating apparatus400may amplify the generated binary optical signal. In step S203, the optical signal generating apparatus400may modulate, depending on a first bias voltage Vbias1and a first RF voltage VRF1, the amplified binary optical signal to generate a multi-level optical signal. The first bias voltage Vbias1may be different from the zeroth bias voltage Vbias0, and the amplitude of the first RF voltage VRF1may be different from that of the zeroth RF voltage VRF0.

An operating method of the optical signal generating apparatus according to an embodiment of the inventive concept is not limited to that illustrated inFIG. 12, and the optical signal generating apparatus may generate an optical signal having more levels by repeating steps S202and S203. For example, when steps S202and S203are performed N times, the optical signal generating apparatus may generate a 2N+1-level optical signal.

FIG. 13is a block diagram illustrating an optical signal generating apparatus according to an embodiment of the inventive concept. An optical signal generating apparatus500may include a zeroth optical intensity modulator501, a first binary signal generating unit510and a second binary signal generating unit520. The first binary signal generating unit510may include a first optical amplifier511and a first optical intensity modulator512, and the second binary signal generating unit520may include a second optical amplifier521and a second optical intensity modulator522. Because operations of the zeroth optical intensity modulator501and the first binary signal generating unit510are similar to those of the zeroth optical intensity modulator401and the binary signal generating unit410ofFIG. 10, detailed description will not be given.

The optical signal generating apparatus500may generate an eight-level optical signal through the zeroth optical intensity modulator501, and the first and second binary signal generating units510and520. Different bias voltages Vbias0to Vbias2and different RF voltages VRF0to VRF2may be applied respectively to the zeroth optical intensity modulator501, and the first and second optical intensity modulators512and522. For example, an amplitude of the first RF voltage VRF1inputted to the first optical intensity modulator512may be half that of the zeroth RF voltage VRF0, and an amplitude of the second RF voltage VRF2inputted to the second optical intensity modulator512may be half that of the first RF voltage VRF1. In other words, the magnitude of the amplitude of the RF voltage VRFinputted to the optical intensity modulator may be reduced at a constant rate compared with that of the RF voltage VRFinputted to the optical intensity modulator of a previous step.

A first current I_1may be inputted to the first optical amplifier511, and a second current I_2may be inputted to the second optical amplifier521. The first and second optical amplifiers511and521may respectively receive the first and second currents I_1and I_2to amplify optical signals.

FIG. 14illustrates, by way of example, an optical signal output of the optical signal generating apparatus illustrated inFIG. 13. A horizontal axis ofFIG. 14represents time and a vertical axis thereof represents optical intensity. Referring toFIGS. 13 and 14, the zeroth optical intensity modulator501may modulate, on the basis of the zeroth bias voltage Vbias0and the zeroth RF voltage VRF0, a first optical signal OS1to generate a second optical signal OS2having a first optical intensity modulation width W1.

The first optical amplifier511may amplify the second optical signal OS2to generate a third optical signal OS3, on the basis of the first I_1. The third optical signal OS3may have a second optical intensity modulation width W2equal to a first optical intensity modulation width W1. By way of example, the highest-level optical intensity of the third optical signal OS3may be equal to the optical intensity of the first optical signal OS1.

The first optical intensity modulator512may modulate, on the basis of the first bias voltage Vbias1and the first RF voltage VRF1, the third optical signal OS3to generate a fourth optical signal OS4having a third optical intensity modulation width W3. Intervals between adjacent signal levels of the fourth optical signal OS4may be equal to the third optical intensity modulation width W3. For example, a first RF voltage VRF1having an amplitude half that of the zeroth RF voltage VRF0may be applied to the first optical intensity modulator512so that the intervals between the adjacent signal levels of the fourth optical signal OS4may be equal.

The second optical amplifier521may amplify the fourth optical signal OS4to generate a fifth optical signal OS5, on the basis of the second current I_2. The amplified fourth optical signal OS4may have a fourth optical intensity modulation width W4equal to the third optical intensity modulation width W3. By way of example, the highest-level optical intensity of the fifth optical signal OS5may be equal to the highest-level optical intensity of the third optical signal OS3.

The second optical intensity modulator522may modulate, on the basis of the second bias voltage Vbias2and the second RF voltage VRF2, the fifth optical signal OS5to generate a sixth optical signal OS6having a fifth optical intensity modulation width W5. Intervals between adjacent signal levels of the sixth optical signal OS6may be equal to the fifth optical intensity modulation width W5. For example, a second RF voltage VRF2having an amplitude half that of the first RF voltage VRF1may be applied to the second optical intensity modulator522so that the intervals between the adjacent signal levels of the sixth optical signal OS6are equal.

The optical signal generating apparatus500may output the sixth optical signal OS6generated from the second optical intensity modulator522. The optical signal generating apparatus500may output an eight-level optical signal therefrom. The eight-level optical signal outputted from the optical signal generating apparatus500may indicate values corresponding to bits assigned as inFIG. 14and the following Table 2.

TABLE 2Eight-Value of the zerothValue of the first RFValue of the secondlevelRF voltage of thevoltage of the firstRF voltage of theopticalzeroth opticaloptical intensitysecond opticalsignalintensity modulatormodulatorintensity modulator000000001001010010011011100100101101110110111111

As in Table 2, when the eight-level optical signal is received from the optical signal generating apparatus500, a receiving terminal may recognize a value indicated by the optical signal depending on the optical signal level as one of “000” to “111”. For example, when a zeroth RF voltage VRF0corresponding to “0” is inputted to the zeroth optical intensity modulator501, a first RF voltage VRF1corresponding to “0” is inputted to the first optical intensity modulator512, and a second RF voltage VRF2corresponding to “1” is inputted to the second optical intensity modulator522, the receiving terminal may receive an optical signal indicating “001”.

When a zeroth RF voltage VRF0corresponding to “1” is inputted to the zeroth optical intensity modulator501, a first RF voltage VRF1corresponding to “1” is inputted to the first optical intensity modulator512, and a second RF voltage VRF2corresponding to “0” is inputted to the second optical intensity modulator522, the receiving terminal may receive an optical signal indicating “110”.

Bit values may be assigned as in Table 2 for the eight-level optical signal, but the inventive concept is not limited thereto, and bit values corresponding to respective levels may vary.

As described in detail, the optical signal generating apparatus according to embodiments of the inventive concept may generate a multi-level optical signal by sequentially modulating optical signals through a plurality of optical intensity modulators operating on the basis of bias voltages Vbiasdifferent from each other and RF voltages VRFof amplitudes different from each other.

The optical signal generating apparatus according to embodiments of the inventive concept may generate a multi-level optical signal by sequentially performing optical modulation and optical amplification operations. That is, the optical signal generating apparatus according to embodiments of the inventive concept may generate a multi-level optical signal by arranging a plurality of optical intensity modulators in series. In the case of generating a multi-level optical signal by arranging a plurality of optical intensity modulators in parallel, a separate device may be required to combine optical signals outputted from respective optical intensity modulators. Accordingly, the optical signal generating apparatus according to embodiments of the inventive concept may generate a multi-level optical signal using only the optical intensity modulator and optical amplifier without a separate device to combine optical signals.

In addition, the optical signal generating apparatus according to embodiments of the inventive concept may generate a multi-level optical signal using an optical device without using a multi-level electrical signal. Accordingly, the optical signal generating apparatus according to embodiments of the inventive concept may generate a low-cost, high-quality optical signal.

The detailed description above is about specific embodiments for implementing the inventive concept. The inventive concept will include embodiments that are not only described above in detail, but also may be simply redesigned or easily changed. In addition, the inventive concept will also include techniques that may be readily modified and implemented using the embodiments. Therefore, the scope of the inventive concept is defined by the following claims or the equivalents other than the foregoing detailed description of the exemplary embodiments.