A digital-to-analog converter (DAC) includes multiple electro-optical converters to generate multiple first optical signals in response to multiple input signals, multiple optical attenuators to attenuate intensities of the first optical signals and to generate multiple second optical signals, an optical coupler to combine the second optical signals and to generate a third optical signal, and a photodetector to convert the third optical signal into an electrical analog signal.

TECHNICAL FIELD

The described technology generally relates to digital-to-analog converters (DACs) and, more particularly, to photonic DACs.

BACKGROUND

In general, data in digital format has a low noise level when being transmitted, stored and processed, and thus is used in various fields of electronics which require stable signal processing. However, the data in digital format needs to be converted into data in analog format when the digital data is applied to analog devices such as radars and displays. Here, a digital-to-analog converter (DAC) may be employed for converting a digital electrical signal into an analog electrical signal. The DAC should have a data conversion rate that is high enough to enable high-speed data transmission, storage and processing. Recently, research on high-speed DACs, including attempts to apply photonics technology to DACs, is being actively conducted.

A photonic DAC (PDAC) has high-speed sampling, a wide bandwidth and reduced interference, and thus is an attractive candidate for next-generation DACs. An example of a PDAC technology is described in the Institute of Electrical and Electronics Engineers (IEEE) Photonics Technology Letters, v. 15, n. 1, p. 117, January 2003, by Araz Yacoubian, et al., which describes a PDAC technique employing a weighted 1×N coupler and a multiple number of electro-optic polymer modulators. Mach-Zehnder modulators (MZMs) are used as the electro-optic polymer modulators. The weighted 1×N coupler divides a continuous wave (CW) laser beam into N beams having various intensities. Another example of a PDAC technology is described in Electronics Letters, v. 43, n. 19, p. 1044, September 2007, by X. Yu, et al., which describes a PDAC technique employing a weighted 1×N coupler and a multiple number of MZMs.

DETAILED DESCRIPTION

In one embodiment, a DAC includes multiple electro-optical converters to generate multiple first optical signals in response to multiple input signals, multiple optical attenuators to attenuate intensities of the first optical signals and to generate multiple second optical signals, an optical coupler to combine the second optical signals and to generate a third optical signal, and a photodetector to convert the third optical signal into an electrical analog signal.

In another embodiment, a method for converting a digital signal into an analog signal includes generating multiple first optical signals in response to multiple input signals, attenuating intensities of the first optical signals and generating multiple second optical signals, combining the second optical signals and generating a third optical signal, and converting the third optical signal into an electrical analog signal.

FIG. 1shows a schematic of an illustrative embodiment of a digital-to-analog converter (DAC). As illustrated, a DAC100includes a multiple number of electro-optical converters110-1,110-2,110-3, . . . , and110-N that generate a multiple number of first optical signals (N is an integer greater than or equal to 2), a multiple number of optical attenuators120-1,120-2,120-3, . . . , and120-N that attenuate intensities of the first optical signals to generate a multiple number of second optical signals, an optical coupler130that combines the second optical signals to generate a third optical signal, and a photodetector140that convert the third optical signal into an electrical analog signal. In some embodiments, the DAC100may optionally further include an amplifier150which couples the optical coupler130with the photodetector140to amplify the intensity of the third optical signal.

The electro-optical converters110-1,110-2,110-3, . . . , and110-N may generate a multiple number of first optical signals PSA1-1, PSA1-2, PSA1-3, . . . , and PSA1-N, respectively, in response to a multiple number of input signals DB-1, DB-2, DB-3, . . . , and DB-N generated, for example, by an external source. The input signals DB-1, DB-2, DB-3, . . . , or DB-N control the working modes of the electro-optical converter110-1,110-2,110-3, . . . , or110-N, respectively, so that the electro-optical converters110-1,110-2,110-3, . . . , and110-N may be in an on-state or an off-state. By way of example, when an input signal DB-1is “1”, the corresponding electro-optical converter110-1may be in the on-state. When the input signal DB-1is “0”, the corresponding electro-optical converter110-1may be in the off-state. Likewise, when some of the input signals DB-2, . . . , and DB-N are “1”, the corresponding electro-optical converters110-2,110-3, . . . , and110-N may be in the on-state, and when some of the input signals DB-2, . . . , and DB-N are “0”, the corresponding electro-optical converters110-2,110-3, . . . , and110-N may be in the off-state.

In one embodiment, the first optical signals PSA1-1, PSA1-2, PSA1-3, . . . , and PSA1-N generated in the on-state may have substantially the same intensity as each other. Here, substantially the same intensities of the first optical signals PSA1-1, PSA1-2, PSA1-3, . . . , and PSA1-N indicate that intensities of the first optical signals PSA1-1, PSA1-2, PSA1-3, . . . , and PSA1-N are exactly the same or within a predetermined permissible error range. Substantially zero intensities of the first optical signals PSA1-1, PSA1-2, PSA1-3, . . . , and PSA1-N generated in the off-state indicate that the intensities of the first optical signals PSA1-1, PSA1-2, PSA1-3, . . . , and PSA1-N are exactly zero or within a predetermined permissible error range from zero. The predetermined permissible error may change according to requirements for an application field employing the first optical signals PSA1-1, PSA1-2, PSA1-3, . . . , and PSA1-N. By way of example, a permissible error of an electro-optical converter receiving a most significant bit (MSB) may be less than a permissible error of an electro-optical converter receiving a least significant bit (LSB).

In another embodiment, the first optical signals PSA1-1, PSA1-2, PSA1-3, and PSA1-N generated in the on-state may have substantially the same wavelengths as each other. Here, substantially the same wavelengths of the first optical signals PSA1-1, PSA1-2, PSA1-3, . . . , and PSA1-N indicate that wavelengths of the first optical signals PSA1-1, PSA1-2, PSA1-3, . . . , and PSA1-N are exactly the same or within a predetermined permissible error range. The permissible error may change, for example, according to requirements for an application field employing the first optical signals PSA1-1, PSA1-2, PSA1-3, . . . , and PSA1-N, or according to the type of the optical coupler130.

The electro-optical converter110-1,110-2,110-3, . . . , or110-N may have a driver160-1,160-2,160-3, . . . , or160-N and a laser diode170-1,170-2,170-3, . . . , or170-N, respectively. The driver160-1,160-2,160-3, . . . , or160-N may apply a drive signal to its corresponding laser diode170-1,170-2,170-3, . . . , or170-N. The drive signal may correspond to the input signal DB-1, DB-2, DB-3, . . . , or DB-N. By way of example, the drive signal may have a type of voltage or current.

In one embodiment, the driver160-1may apply a predetermined voltage (e.g., 5 V) that is higher than a threshold voltage to the laser diode170-1when the input signal DB-1is “1”. The driver160-1may apply a predetermined voltage (e.g., 0 V) that is lower than the threshold voltage to the laser diode170-1when the input signal DB-1is “0”. The threshold voltage denotes a minimum voltage applied into the laser diode170-1,170-2,170-3, . . . , or170-N from, for example, an external source in order to drive the laser diode170-1,170-2,170-3, . . . , or170-N. Likewise, when some of the input signals DB-2, DB-3, . . . , and DB-N are “1”, the corresponding drivers160-2,160-3, . . . , and160-N may apply the predetermined voltage (e.g., 5 V) that is higher than the threshold voltage to the corresponding laser diodes170-2,170-3, . . . , and170-N. Also, when some of the input signals DB-2, DB-3, . . . , and DB-N are “0”, the corresponding drivers160-2,160-3, . . . , and160-N may apply the predetermined voltage (e.g., 0 V) that is lower than the threshold voltage to the corresponding laser diodes170-2,170-3, . . . , and170-N.

In another embodiment, the driver160-1may apply a predetermined current corresponding to the on-state to the laser diode170-1when the input signal DB-1is “1”. The driver160-1may apply a current of substantially zero to the laser diode170-1when the input signal DB-1is “0”. Likewise, when some of the input signals DB-2, DB-3, . . . , and DB-N are “1”, the corresponding drivers160-2,160-3, . . . , and160-N may apply the predetermined current corresponding to the on-state to the corresponding laser diodes170-2,170-3, . . . , and170-N. Also, when some of the input signals DB-2, DB-3, . . . , and DB-N are “0”, the corresponding drivers160-2,160-3, . . . , and160-N may apply the current of substantially zero to the corresponding laser diodes170-2,170-3, . . . , and170-N.

The laser diodes170-1,170-2,170-3, . . . , and170-N may be, for example, commercially available laser diodes such as semiconductor laser diodes. The semiconductor laser diodes may be monolithic semiconductor laser diodes integrated on a single semiconductor substrate. Since the monolithic semiconductor laser diodes can be fabricated using a batch process, the monolithic semiconductor laser diodes may generate optical signals having substantially the same wavelengths and intensities as each other. The monolithic semiconductor laser diodes can be mass-produced at low cost using a semiconductor process. Instead of the laser diode170-1,170-2,170-3, . . . , or170-N, a light-emitting device such as a light-emitting diode (LED) or organic light-emitting diode (OLED) may be used.

The optical attenuators120-1,120-2,120-3, . . . , and120-N may attenuate intensities of the first optical signals PSA1-1, PSA1-2, PSA1-3, . . . , and PSA1-N, and generate a multiple number of second optical signals PSA2-1, PSA2-2, PSA2-3, . . . , and PSA2-N. The optical attenuator120-1,120-2,120-3, . . . , or120-N may differentially attenuate the first optical signal PSA1-1, PSA1-2, PSA1-3, . . . , or PSA1-N from each other. The attenuation of the optical attenuator120-1,120-2,120-3, . . . , or120-N may be expressed by a difference between the intensity of the second optical signal PSA2-1, PSA2-2, PSA2-3, . . . , or PSA2-N and the intensity of the corresponding first optical signal PSA1-1, PSA1-2, PSA1-3, . . . , or PSA1-N. As used herein, the intensity difference refers to the optical attenuation of an optical attenuator.

The optical attenuation of the optical attenuator120-1,120-2,120-3, . . . , or120-N may be affected by the input signal DB-1, DB-2, DB-3, . . . , or DB-N. The optical attenuation of the optical attenuator120-1,120-2,120-3, . . . , or120-N may be determined based on, for example, a weight of the corresponding input signal DB-1, DB-2, DB-3, . . . , or DB-N. In some embodiments, the optical attenuators120-1,120-2,120-3, . . . , and120-N may correspond to the input signals DB-1, DB-2, DB-3, . . . , and DB-N, respectively. By way of example, the input signal DB-1may correspond to a least significant bit (LSB) of a digital signal, and the input signal DB-N may correspond to a most significant bit (MSB) of the digital signal. In this case, the optical attenuator120-N,120-(N−1),120-(N−2), . . . , or120-1may have the optical attenuation M−3(N−1) dB, M−3(N−2) dB, . . . , or M dB, respectively. Here, M is a predetermined value greater than 3(N−1), and dB (decibel) is a logarithmic unit used to denote the intensity of an optical signal. By way of example, when an optical signal has an intensity that is 3 dB greater than another optical signal, the intensity of the optical signal is approximately double the intensity of the other optical signal. The optical attenuation may sequentially decrease by 3 dB from M dB of the optical attenuator120-1to M−3(N−1) of the optical attenuator120-N. Accordingly, the optical attenuators120-1,120-2,120-3, . . . , and120-N may generate attenuation patterns where the intensities of the optical attenuation sequentially decrease by about 2 times (a factor of 2) from the optical attenuator120-1to the optical attenuator120-N.

By way of example, a 4-bit digital signal including the input signals DB-1, DB-2, DB-3and DB-4having a bit pattern “1001” may be provided to the optical attenuators120-1,120-2,120-3and120-4. In this case, the input signals DB-1, DB-2, DB-3and DB-4may be “1”, “0”, “0” and “1”, respectively. When M is 10, the optical attenuators120-1,120-2,120-3and120-4may have optical attenuations of 10 dB, 7 dB, 4 dB and 1 dB, respectively. Accordingly, when receiving the input signals DB-1and DB-4having a bit “1”, the optical attenuators120-1and120-4having the optical attenuations of 10 dB and 1 dB, respectively, may attenuate the first optical signals PSA1-1and PSA1-4into the second optical signals PSA2-1and PSA2-4, respectively. That is, when intensities of the first optical signals PSA1-1and PSA1-4are the same as I0, the optical attenuators120-1and120-4may generate the second optical signal PSA2-1having an intensity of 0.1×I0that is attenuated by 10 dB from I0, and the second optical signal PSA2-4having an intensity of about 0.794×I0that is attenuated by 1 dB from I0.

By way of another example, a 4-bit digital signal including the input signals DB-1, DB-2, DB-3and DB-4having a bit pattern “1100” may be provided to the optical attenuators120-1,120-2,120-3and120-4. In this case, the input signals DB-1, DB-2, DB-3and DB-4may be “0”, “0”, “1” and “1”, respectively. When M is 10, the optical attenuators120-1,120-2,120-3and120-4may have optical attenuations of 10 dB, 7 dB, 4 dB and 1 dB, respectively. Accordingly, when receiving the input signals DB-3and DB-4having a bit “1”, the optical attenuators120-3and120-4having the optical attenuations of 4 db and 1 db, respectively, may attenuate the first optical signals PAS1-3and PSA1-4into the second optical signals PSA2-3and PSA2-4, respectively. That is when intensities of the first optical signals PSA1-3and PSA1-4are the same as I0, the optical attenuators120-3and120-4may generate the second optical signal PSA2-3having an intensity of about 0.4×I0attenuated by 4 dB from I0, and the second optical signal PSA2-4having an intensity of about 0.794×I0attenuated by 1 dB from I0.

The optical attenuators120-1,120-2,120-3, . . . , and120-N may be known, commercially available optical attenuators. At least one of the optical attenuators120-1,120-2,120-3, . . . , and120-N may be other types of optical attenuators such as variable optical attenuators.

The optical coupler130may combine the second optical signals PSA2-1, PSA2-2, PSA2-3, . . . , and PSA2-N together, thus generating a third optical signal PSA3. An intensity of the third optical signal PSA3may be determined based on intensities of the second optical signals PSA2-1, PSA2-2, PSA2-3, . . . , and PSA2-N and an insertion loss of the optical coupler130. In some embodiments, the optical coupler130algebraically combines the intensities of the second optical signals PSA2-1, PSA2-2, PSA2-3, . . . , and PSA2-N. As described above, the second optical signal PSA2-1, PSA2-2, PSA2-3, . . . , or PSA2-N may have various intensities according to the corresponding input signals signal DB-1, DB-2, DB-3, . . . , or DB-N and according to the corresponding attenuation of the optical attenuator120-1,120-2,120-3, . . . , or120-N. Therefore, the third optical signal PSA3may have various discrete intensities by algebraically combining the various discrete intensities of the second optical signals PSA2-1, PSA2-2, PSA2-3, . . . , and PSA2-N. By way of example, when a 4-bit digital signal including the input signals DB-1, DB-2, DB-3and DB-4is provided to the DAC100, the third optical signal PSA3may have 24discrete intensities that are different from each other. As discussed above, in the case where the 4-bit digital signal has a bit pattern “1001”, the second optical signal PSA2-1having an intensity of 0.1×I0, and the second optical signal PSA2-4having an intensity of about 0.794×I0may be produced through the optical attenuators120-1and120-4having the optical attenuations of 10 dB and 1 dB, respectively. The optical coupler130algebraically combines the intensities of the second optical signals PSA2-1and PSA2-4, and generates the third optical signal PSA3having the intensity of about 0.894×I0. Likewise, the optical coupler130generates the third optical signal PSA3having intensities of 24discrete intensities in response to the 24bit patterns of the input signals DB-1, DB-2, DB-3and DB-4and in response to the corresponding optical attenuations 10 dB, 7 dB, 4 dB and 1 dB.

The insertion loss denotes the power lost while the second optical signals PSA2-1, PSA2-2, PSA2-3, . . . , and PSA2-N pass through the optical coupler130. The insertion loss may have different values according to the type of the optical coupler130. The optical coupler130may be, for example, a known, commercially available optical coupler such as an N×1 coupler. The N×1 coupler receives as input N optical signals and outputs one optical signal.

The photodetector140converts the third optical signal PSA3into an electrical analog signal ASA. An intensity of the electrical analog signal ASA may vary in proportion to the intensity of the third optical signal PSA3. By way of example, the photodetector140may convert the third optical signal PSA3having 2Ndifferent intensities into the electrical analog signal ASA having 2Ndifferent intensities. The electrical analog signal ASA may be a type of, for example, current or voltage. The photodetector140may be, for example, a known, commercially available photodetector such as a photodiode. The photodiode may be, for example, a positive-intrinsic-negative (PIN) photodiode.

In some embodiments, the amplifier150may couple the optical coupler130with the photodetector140, and amplify the intensity of the third optical signal PSA3. The third optical signal PSA3having the amplified intensity may be provided to the photodetector140. A gain of the amplifier150may denote the ratio of output intensity to input intensity. The gain of the amplifier150may be determined based on the intensity of the third optical signal PSA3and a threshold value of the photodetector140. The threshold value of the photodetector140indicates a minimum intensity of the amplified third optical signal PSA3in order to drive the photodetector140to generate the electrical analog signal ASA. The amplifier150may amplify the intensity of the third optical signal PSA3before the third optical signal PSA3is received at the photodetector140in order to maintain the amplified intensity of the third optical signal PSA3over the threshold value of the photodetector140. In this case, the minimum gain of the amplifier150may be expressed by the ratio of the threshold value of the photodetector140to the corresponding intensity of the third optical signal PSA3. In other embodiments, when the intensity of the third optical signal PSA3outputted from the optical coupler130is maintained to be greater than or equal to the threshold value of the photodetector140, the DAC100may not include the amplifier150before the photodetector140.

In some embodiments, the amplifier150may be disposed behind (that is, after) the photodetector140to amplify the electrical analog signal ASA output by the photodetector140over a predetermined value. The predetermined value may vary according to the type of electronic device employing the electrical analog signal ASA. In other embodiments, when the intensity of the electrical analog signal ASA is greater than or equal to the predetermined value of the electronic device receiving the electrical analog signal ASA, the DAC100may not include the amplifier150behind the photodetector140.

As described above, the DAC100according to some embodiments may include the electro-optical converters110-1,110-2,110-3, . . . , and110-N and the optical attenuators120-1,120-2,120-3, . . . , and120-N. The electro-optical converters110-1,110-2,110-3, . . . , and110-N may generate the first optical signals PSA1-1, PSA1-2, PSA1-3, . . . , and PSA1-N in response to the input signal DB-1, DB-2, DB-3, . . . , and DB-N. The first optical signal PSA1-1, PSA1-2, PSA1-3, . . . , or PSA1-N may be generated by the laser diode170-1,170-2,170-3, . . . , or170-N driven by the driver160-1,160-2,160-3, . . . , or160-N. The laser diodes170-1,170-2,170-3, . . . , and170-N may be separate from each other, and thus the first optical signals PSA1-1, PSA1-2, PSA1-3, . . . , and PSA1-N can have a random phase difference from each other. The first optical signals PSA1-1, PSA1-2, PSA1-3, . . . , and PSA1-N having the random phase difference from each other can have incoherent characteristics. The DAC100using the first optical signals PSA1-1, PSA1-2, PSA1-3, . . . , and PSA1-N having the incoherent characteristics can have low interference characteristics.

FIG. 2shows a schematic of another illustrative embodiment of a DAC. As illustrated, a DAC200may include a multiple number of electro-optical converters210-1,210-2,210-3, . . . , and210-N that generate a multiple number of first optical signals, a multiple number of optical attenuators220-1,220-2,220-3, . . . , and220-N that attenuate intensities of the first optical signals to a multiple number of second optical signals, an optical coupler230that combines the second optical signals to generate a third optical signal, and a photodetector240that converts the third optical signal into an electrical analog signal. In some embodiments, the DAC200may optionally further include an amplifier250which couples the optical coupler230with the photodetector240to amplify the intensity of the third optical signal.

The electro-optical converter210-1,210-2,210-3, . . . , or210-N may include a driver260-1,260-2,260-3, . . . , or260-N, a laser diode270-1,270-2,270-3, . . . , or270-N, and a light coupler280-1,280-2,280-3, . . . , or280-N. The light coupler280-1,280-2,280-3, . . . , or280-N may divide a laser signal LSA-1, LSA-2, LSA-3, . . . , or LSA-N of the laser diode270-1,270-2,270-3, . . . , or270-N into a first optical signal PSB1-1, PSB1-2, PSB1-3, . . . , or PSB1-N and a control signal CSA-1, CSA-2, CSA-3, . . . , or CSA-N. A ratio of an intensity of the first optical signal PSB1-1, PSB1-2, PSB1-3, . . . , or PSB1-N to an intensity of the control signal CSA-1, CSA-2, CSA-3, or CSA-N may be predetermined and adjusted by the light coupler280-1,280-2,280-3, . . . , or280-N. The control signal CSA-1, CSA-2, CSA-3, . . . , or CSA-N may be applied to the driver260-1,260-2,260-3, . . . , or260-N. The driver260-1,260-2,260-3, . . . , or260-N may adjust power applied to the laser diode270-1,270-2,270-3, . . . , or270-N, based on the control signal CSA-1, CSA-2, CSA-3, . . . , or CSA-N. In some embodiments, the driver260-1,260-2,260-3, . . . , or260-N may have a photodetector (not shown) and a comparator (not shown). The photodetector may receive the control signal CSA-1, CSA-2, CSA-3, . . . , or CSA-N and convert the control signal CSA-1, CSA-2, CSA-3, . . . , or CSA-N into an electrical signal. The comparator may compare the electrical signal with a predetermined reference value. When an intensity of the electrical signal is less than the predetermined reference value, the comparator may increase the power applied to the laser diode270-1,270-2,270-3, . . . , or270-N. As the power increases, an intensity of the laser signal LSA-1, LSA-2, LSA-3, . . . , or LSA-N and an intensity of the first optical signal PSB1-1, PSB1-2, PSB1-3, . . . , or PSB1-N may also increase. When the intensity of the electrical signal is greater than the predetermined reference value, the comparator may reduce the power applied to the laser diode270-1,270-2,270-3, . . . , or270-N. When the power decreases, an intensity of the laser signal LSA-1, LSA-2, LSA-3, . . . , or LSA-N and an intensity of the first optical signal PSB1-1, PSB1-2, PSB1-3, . . . , or PSB1-N may also decrease. In some embodiments, the comparator adjusts the power applied to the laser diode270-1,270-2,270-3, . . . , or270-N such that the first optical signal PSB1-1, PSB1-2, PSB1-3, . . . , or PSB1-N may have substantially the same intensity as the predetermined intensity. Here, the first optical signal PSB1-1, PSB1-2, PSB1-3, . . . , or PSB1-N being substantially the same as the predetermined intensity indicates that the intensity of the first optical signal PSB1-1, PSB1-2, PSB1-3, . . . , or PSB1-N is exactly the same as the predetermined intensity or within a predetermined permissible error range. By way of example, a permissible error of an intensity of a first optical signal corresponding to an MSB may be less than a permissible error of an intensity of a first optical signal corresponding to an LSB. The light coupler280-1,280-2,280-3, . . . , or280-N may be, for example, a 1×2 optical splitter. Using the 1×2 optical splitter, one optical signal as input can be divided into two optical signals as output.

The structure and operation of the optical attenuators220-1,220-2,220-3, . . . , and220-N outputting second optical signals PSB2-1, PSB2-2, PSB2-3, . . . , and PSB2-N, the optical coupler230outputting a third optical signal PSB3, the photodetector240outputting an electrical analog signal ASB, the amplifier250amplifying the third optical signal PSB3, the drivers260-1,260-2,260-3, . . . , and260-N, and the laser diodes270-1,270-2,270-3, . . . , and270-N are substantially the same as those of the optical attenuators120-1,120-2,120-3, . . . , and120-N, the optical coupler130, the photodetector140, the amplifier150, the drivers160-1,160-2,160-3, . . . , and160-N, and the laser diodes170-1,170-2,170-3, . . . , and170-N described above with reference toFIG. 1, and thus the descriptions of the structure and operation will not be reiterated.

FIG. 3shows a schematic of still another illustrative embodiment of a DAC. As illustrated, a DAC300may include a multiple number of electro-optical converters310-1,310-2,310-3, . . . , and310-N that generate a multiple number of first optical signals, a multiple number of optical attenuators320-1,320-2,320-3, . . . , and320-N that attenuate intensities of the first optical signals to generate a multiple number of second optical signals, an optical coupler330that combines the second optical signals to generate a third optical signal, and a photodetector340that converts the third optical signal into an electrical analog signal ASC. The structure of the DAC300is substantially the same as the structure of the DAC100except that an N×1 arrayed waveguide grating (AWG) is used as the optical coupler330.

The electro-optical converters310-1,310-2,310-3, . . . , and310-N may generate a multiple number of first optical signals PSC1-1, PSC1-2, PSC1-3, . . . , and PSC1-N having various wavelengths with each other. The electro-optical converter310-1,310-2,310-3, . . . , or310-N may include a driver360-1,360-2,360-3, . . . , or360-N and a laser diode370-1,370-2,370-3, . . . , or370-N. The structure and operation of the electro-optical converters310-1,310-2,310-3, . . . , and310-N are substantially the same as the structure and operation of the electro-optical converter110-1,110-2,110-3, . . . , and110-N described above with reference toFIG. 1, and thus the descriptions of the structure and operation will not be reiterated.

The optical coupler330may combine a multiple number of second optical signals PSC2-1, PSC2-2, PSC2-3, . . . , and PSC2-N having various wavelengths and generate a third optical signal PSC3. As shown in the drawing, the optical coupler330is an N×1 AWG. The N×1 AWG receives as input N optical signals having various wavelengths and outputs one optical signal. An insertion loss of the N×1 AWG may be affected not by N, which is the number of the received optical signals, but by an insertion loss of the N×1 AWG. The operation of the optical coupler330is substantially the same as the operation of the optical coupler130described above with reference toFIG. 1, and thus the description of operation will not be reiterated.

As described above, the DAC300according to some embodiments includes an N×1 AWG as the optical coupler330. The N×1 AWG may receive as input second optical signals PSC2-1, PSC2-2, PSC2-3, . . . , and PSC2-N having various wavelengths and output the third optical signal PSC3. An insertion loss of the N×1 AWG is not affected by N, which is the number of the received second optical signals PSC2-1, PSC2-2, PSC2-3, . . . , and PSC2-N originated from the input signals DB-1, DB-2, DB-3, . . . , and DB-N. Consequently, the N×1 AWG of the DAC300can have a low insertion loss so that the DAC300can obtain the third optical signal PSC3having the increased intensity.

FIG. 4shows a schematic of a further illustrative embodiment of a DAC. As illustrated, a DAC400may include a multiple number of electro-optical converters410-1,410-2,410-3, . . . , and410-N that generate a multiple number of first optical signals, a multiple number of optical attenuators420-1,420-2,420-3, . . . , and420-N that attenuate intensities of the first optical signals to generate a multiple number of second optical signals, an optical coupler430that combines the second optical signals to generate a third optical signal, and a photodetector440that converts the third optical signal into an electrical analog signal ASD. The electro-optical converter410-1,410-2,410-3, . . . , or410-N may include a driver460-1,460-2,460-3, . . . , or460-N, a laser diode470-1,470-2,470-3, . . . , or470-N, and a light coupler480-1,480-2,480-3, . . . , or480-N.

The structure and operation of the DAC400are substantially the same as the structure and operation of the DAC300described above with reference toFIG. 3, except that the electro-optical converter410-1,410-2,410-3, . . . , or410-N further includes the light coupler480-1,480-2,480-3, . . . , or480-N. The light coupler480-1,480-2,480-3, . . . , or480-N may divide a laser signal LSB-1, LSB-2, LSB-3, . . . , or LSB-N of the laser diode470-1,470-2,470-3, . . . , or470-N into a first optical signal PSD1-1, PSD1-2, PSD1-3, . . . , or PSD1-N and a control signal CSB-1, CSB-2, CSB-3, . . . , or CSB-N. The operation of the light coupler480-1,480-2,480-3, . . . , or480-N is substantially the same as the operation of the light coupler280-1,280-2,280-3, . . . , or280-N described above with reference toFIG. 2.

As described above, the DAC400according to some embodiments may adjust an intensity of the control signal CSB-1, CSB-2, CSB-3, . . . , or CSB-N and an intensity of the first optical signal PSD1-1, PSD1-2, PSD1-3, . . . , or PSD1-N using the light coupler480-1,480-2,480-3, . . . , or480-N. Consequently, even if the laser signals LSB-1, LSB-2, LSB-3, . . . , and LSB-N have intensities that are different from each other, the electro-optical converters410-1,410-2,410-3, . . . , and410-N of the DAC400can provide the first optical signals PSD1-1, PSD1-2, PSD1-3, . . . , and PSD1-N having substantially the same intensity as each other. Also, an N×1 AWG is used as the optical coupler430to combine a multiple number of second optical signals PSD2-1, PSD2-2, PSD2-3, . . . , and PSD2-N having various wavelengths with each other and generate a third optical signal PSD3. An insertion loss of the N×1 AWG is not affected by N, which is the number of the received second optical signals PSD2-1, PSD2-2, PSD2-3, . . . , and PSD2-N originated from the input signals DB-1, DB-2, DB-3, . . . , and DB-N. Consequently, the N×1 AWG of the DAC400can have a low insertion loss so that the DAC400can obtain the third optical signal PSD3having the increased intensity.

FIG. 5is a flowchart of an illustrative embodiment of a method for converting a digital signal into an analog signal. Referring toFIG. 5, in block510, multiple electro-optical converters generate a multiple number of first optical signals in response to a multiple number of input signals. The first optical signals may have substantially the same intensity as each other. The electro-optical converters may generate the first optical signals in response to the corresponding input signals. In block520, multiple optical attenuators attenuate the intensities of the first optical signals and generate a multiple number of second optical signals. The optical attenuators may differentially attenuate the intensities of the first optical signals according to weights of the corresponding input signals and generate the second optical signals. The optical attenuators may respectively attenuate the intensities of the first optical signals by M−3(N−1) dB, M−3(N−2) dB, . . . , and M dB according to the weights and generate the second optical signals. In block530, an optical coupler combines the second optical signals and generates a third optical signal. For example, the optical coupler may be an N×1 coupler or an N×1 AWG. In block540, a photodetector converts the third optical signal into an electrical analog signal. For example, the photodetector may be a photodiode.