Compact, Energy Efficient Simultaneous Transmission of Analog and Different Digital Signals With a Single Ring Modulator

We present a compact, agnostic transmitter capable of simultaneously transmitting all types of signals (analog and digital with different modulation formats) with a single-ring modulator with a radius of 10 micrometers and a bandwidth of around 18 GHz. To the best of our knowledge, we show for the first time, experimental results for transmitting three different Nyquist BPSK signals with an aggregated baud rate of 24 GBd in such a compact device. Additionally, we show the simultaneous transmission of a 3 GHz analog signal together with normal and Nyquist-shaped 4 GHz BPSK signals. All results were achieved without any power-consuming pre- or postprocessing to adapt the signals to the transfer function of the modulators. The simultaneous transmission of analog and digital signals in a device with a small footprint and low power consumption may be especially advantageous for future access networks.

High-bandwidth photonic and electronic components are the backbone of high data rate transmission systems.However, due to the utilization of massive digital signal processing (DSP) components, their energy consumption rises with their bandwidth, which results in challenges for integration into a low-cost platform [2].To overcome the limitations of an all electrical-based access network, a hybrid access network combining both electrical and optical structures has been proposed [2], [3].Several optical methods have been developed to enhance transmission data rates by establishing optical super-channels.These methods include orthogonal frequency division multiplexing (OFDM) [4], [5], [6], [7] and Nyquist wavelength division multiplexing (Nyquist-WDM) [8].However, despite their effectiveness, these methods still require high-bandwidth digital-to-analog converters (DACs), analog-to-digital converters (ADCs), and complex DSP techniques, especially for OFDM.
In [9], the concept of an agnostic sampling transceiver was introduced.This idea involves the simultaneous transmission of any kind of signals at high data rates, whether they are analog or digital (normal or Nyquist-shaped).An optical super-channel, even faster than Nyquist symbol rates, can be achieved by combining n WDM channels with k time-division multiplexing (TDM) channels, resulting in an aggregated data rate in the range of Tbit/s [10].It does not require any optical filtering or additional DSP.This approach may give a solution for hybrid access networks.Although the method is straightforward and does not require any complex components, it primarily relies on integrated Mach-Zehnder modulators (MZM) with dimensions in the millimeter scale and power consumption in the range of pj/bit [11], [12].
To reduce power consumption and footprint by orders of magnitude, an optical ring modulator may replace the MZM.The ring modulator has dimensions in the micrometer range and consumes a power of only a few fJ/bit [12], [13], [14].We have validated this concept through simulations [15], [16], [17] and here, we present experimental demonstrations in a fabricated device.To the best of our knowledge, this is the first experimental study of a compact agnostic transmitter based on an optical ring modulator.We present the simultaneous generation of three different binary phase shift keying (BPSK) signals, each limited to its Nyquist bandwidth of 4 GHz, achieving an aggregated data rate of 24 GBd in a rectangular optical bandwidth of 24 GHz.We also show the simultaneous generation and multiplexing of a 3 GHz analog signal, a 4 GBd normal-shaped BPSK signal, and an 8 GBd Nyquist BPSK signal.Combining the integrated ring modulator with the method presented in [17], [18], [19] may result in an ultra-compact agnostic transceiver with extremely low power consumption for access networks.

II. RING MODULATOR-BASED AGNOSTIC TRANSMISSION CONCEPT
The agnostic transceiver system is depicted in Fig. 1.The transceiver can simultaneously generate and receive any kind of band-limited signal.Therefore, the signal in the sub-channels c 1 to c k can be an analog or any kind of digital one with different modulation formats [9].According to the sampling theorem, all bandwidth-limited signals can be described as a superposition of orthogonal sinc pulses as shown in Fig. 2(a).Utilizing the orthogonality between the different pulses, in the receiver each single data point can theoretically be extracted by a multiplication of the signal with a sinc pulse with the correct bandwidth and time shift.However, each sinc pulse is unlimited in time and as such not realizable in practice.Alternatively, the same signal can be represented error-free as a superposition of sinc-pulse sequences (SPS), as shown in Fig. 2(b) [20].As can be seen in the spectrum depicted in Fig. 2(b), each single SPS in the frequency domain corresponds to the spectrum of the sub-signal (rectangular with B b /k) spread over the whole baseband bandwidth of the signal B b .Since all three SPS are phase shifted by 120°in the same spectral bandwidth, the whole spectrum is the superposition of all these spectra, leading to the signal spectrum of Fig. 2(a).Such an SPS is represented by a flat frequency comb in the frequency domain and can be realized quite simply [19].The difference to the single sinc pulse, where every single pulse represents a single sampling point, is that each SPS represents periodic sampling points, as shown with the green, orange, and purple points in Fig. 2(b).
Each single channel c k of the transmitter can be seen as sampled and represented by an SPS with the right bandwidth and time shift [9].As for the single sinc pulse, the bandwidth of the SPS is the inverse of the duration between the maxima and the first zero crossings.To fulfill the orthogonality between the SPS, this bandwidth corresponds to the total sampling rate Δf s of all channels together.So, the next channel (SPS) has to be in the zero crossing of the previous.Therefore, the bandwidth and time shift of the SPS are defined by the required Nyquist sampling rate for the total transmission signal [9], [21].
In the presented concept, the SPS generation and sampling for all k channels are simultaneously carried out by a singlering modulator, driven by an electrical network (S I and S Q in Fig. 1).A radio frequency source (RF) generates a flat radio frequency comb with n = (k-1)/2 frequencies.For k = 3 a single sinusoidal wave is sufficient.In each branch, the corresponding SPS, represented by this frequency comb, is multiplied by the data of the single channel to be transmitted.This can be an analog signal or any kind of data signal.However, for I-Q signals, one ring modulator and network for the I and another one for the Q component are required as seen in Fig. 1.Before multiplication, in each branch the single SPS is time-shifted so that it is in the zero crossings of all others.For k = 3 instead of the time shift a phase shift would be sufficient.All these branches are added up in the electrical domain and used to feed the ring modulator.To simultaneously generate the SPS and sample the data with the ring modulator, the key point is to adjust one of the ring modulator resonances to the wavelength of the laser diode (LD) [14], [15], [16], [17].
To achieve an optical wavelength-domain multiplexing plus time-domain multiplexing (WDM-TDM) superchannel, a comb source and cascaded optical ring modulators can be employed, as demonstrated in [17].The comb generates n wavelength lines and each ring modulator works as a filter, selecting one of the wavelengths, and as a modulator, modulating k TDM channels.The TDM channels can be Nyquist-shaped with a rectangular bandwidth.This way, a rectangular, guard-band-free n x k WDM-TDM optical superchannel can be accomplished [10].The yellow box on the transmitter side of Fig. 1  At the receiver side, depicted in the green dashed box of Fig. 1, each branch is equipped with an MZM.These MZMs are driven by (k-1)/2 RF frequencies, each adjusted to have a phase shift equivalent to that of the transmitted signal, facilitating its demultiplexing.For higher-order modulation formats, a single-intensity modulator is sufficient [22], [23].It is important to highlight that another single-ring modulator can be employed instead of the MZM.In this case, the ring modulator should be connected directly with the local oscillator (LO) as presented in [18] to prevent carrier suppression.To detect the whole information of the transmitted signal, a coherent detector (CD) with a bandwidth of B b can be used.

III. EXPERIMENTAL SETUP
For the proof of concept, we have used the experimental setup shown in Fig. 3(a).It follows the concept in Fig. 1, except that we have generated the k = 3 signals -as described in the yellow dashed box of Fig. 1 -by Mathematica software.The data was encoded with a pseudo-random bit sequence (PRBS-7).The result was directly fed into the arbitrary waveform generator (AWG) (Tektronix AWG70001A).Additionally, we transmit only one component (either I or Q) since we have just one integrated optical ring modulator.At the receiver side, only one branch is presented due to laboratory equipment limitations.We have demultiplexed the three different channels subsequently by changing the phase of the single RF frequency driving the modulator to 0°, 120°, and 240°.
The optical ring modulator was manufactured by a multiproject wafer run in AMF Singapore.It features a PN junction operating under a reverse bias configuration with a radius of 10 µm and a bandwidth of around 18 GHz [24], [25].The ring modulator has a modulation efficiency of around 0.85 V•cm.To prevent the resonance wavelength from shifting due to heat dissipation, we maintained the chip's temperature at 25 °C by a Peltier element within the temperature compensation system illustrated in Fig. 3(b).
Before performing the transmission experiments, we swept the wavelength of the 1544 nm tunable laser to measure the intensity transfer function of the ring modulator for a 0 V DC bias voltage.The optical power injected into the chip was adjusted to 0 dBm by a 7 dB optical attenuator to mitigate nonlinear effects [26].The response of the chip was measured by an optical power meter (Agilent 81638).Fig. 3(c) shows a photo of the front side of the chip.As demonstrated in Fig. 3(d), the resonance wavelength of the ring modulator was at 1543.29 nm with an optical power of approximately −23.5 dBm at a DC bias of 0 V.
For the first experiment, we generated three different BPSK signals with Nyquist shaping.The 8 GBd signals with 4 GHz bandwidth were multiplied in each branch by a sinusoidal frequency of 8 GHz.Orthogonality was ensured by adjusting the phase of the 8 GHz signal to 0°in the first 120°in the second and 240°in the third branch.After adding the branches up with an aggregated baud rate of 24 GBd, the signal was fed to the optical ring modulator with a peak-to-peak driving voltage of 500 mV.The laser wavelength was set near the resonance point (around 1543.33 nm).This point shifts with the frequency and power of the RF signal [27].
To demultiplex the data signals, the bias of the MZM at the receiver side is adjusted properly and driven by an 8 GHz sinusoidal frequency with the correct phase [28].That way, the incoming 24 GBd signal is multiplied by a sinc-pulse sequence with k = 3.A CD together with a LO was used to receive the data signals.For this CD a bandwidth of only 4 GHz is required.Since this was not available for our experiments, we used a CD with a bandwidth of 33 GHz and restricted the bandwidth of the down-converted baseband signal to 4 GHz by a software filter.It is important to note that clock recovery is necessary between the transmitter and receiver, however, the phase synchronization is not that strict.
To mitigate the high coupling losses of the proof-of-concept chip, it was necessary to amplify the signal and subsequently filter out the amplified spontaneous noise with erbium-doped fiber amplifiers (EDFAs) and optical bandpass filters (BPFs).All experiments were carried out without a DC bias voltage for the optical ring modulator.

IV. EXPERIMENTAL RESULTS
As depicted in Fig. 4(a), the optical spectrum of the multiplexed 24 GBd signal exhibits deviations from a rectangular shape.These deviations can be attributed to the limited resolution of our optical spectrum analyzer and the imperfections in the ring setup due to temperature fluctuations from the temperature control system.A temperature control system is necessary for the ring modulators to keep the frequency resonances from drifting and thus stabilize the modulation point.This temperature control can be avoided by athermal optical ring modulators [29], [30].
Fig. 4(b) illustrates the optical spectrum of the received signal for one of the 8 GBd channels after the MZM.The multiplication of the B = 24 GHz rectangular signal spectrum with the 24 GHz rectangular spectrum of the SPS, results in a triangular-shaped convolution between the spectra.For the optical spectra in Fig. 4(a) and (b), the data for all three channels are Nyquist bandwidth limited BPSK signals.In addition, the electrical spectrum of a mixed signal (8 Gbd Nyquist BPSK, 4 Gbd BPSK and 3 GHz sinusoidal) after the demultiplexing of the analog signal is displayed in Fig. 4(c).The bandwidth of the detector and the electronics is only B b , which results in the 4 GHz depicted in the blue box in Fig. 4(b).
In Fig. 5(a), (b), and (c), the received eye diagrams for each Nyquist BPSK data signal are presented.Each signal has an optical power of 7 dBm.The received signals have similar Qfactors of 14 dB, 13.7 dB, and 13.5 dB, respectively.The BER was calculated according to [31].The corresponding BERs are 7.79E-45, 5.07E-43, and 7.81E-42, respectively.
For the second experiment, we generated three different signals simultaneously, namely a 3 GHz analog signal, a Nyquist 8 GBd BPSK, and a normal 4 GBd BPSK signal.As shown in Fig. 6(a), the analog signal was received accurately.We attribute the differences to a clear sinusoidal signal to problems of the coherent receiver software, which occur if no data signal is received.Additionally, clear eye diagrams were obtained for Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.All received Q-factors and BER from the proposed system meet the standard values for access networks, which are a Qfactor of 6 and a BER of 1E-9 [32], [33].The Agnostic effect and bandwidth advantages hold true for any number of k as we have shown previously [9], [21], [34].However, scaling the system to a higher number of branches k comes with more complexity.Additionally, since at the receiver side the power is split by k, it is dependent on the available signal power and especially signal-to-noise ratio.
The noise in Figs. 5 and 6 is mainly attributed to inaccuracies in the temperature compensation system, as the temperature of the optical ring modulator is not accurately stabilized.Additionally, there are optical coupling losses.Integrating the whole system on a chip with an embedded heater to accurately stabilize the temperature would lead to much lower noise levels.It is important to note, that we did not use any electronic pre-or post-processing to adapt the data signals to the transfer function of the modulators.

V. CONCLUSION
In conclusion, this work introduces an agnostic transmitter utilizing an optical ring modulator.We experimentally validated this concept for the first time, to the best of our knowledge with a ring modulator with a radius of 10 µm and a bandwidth of 18 GHz.We successfully generated and received simultaneously three different 8 GBd Nyquist BPSK signals with an aggregated symbol rate of 24 GBd.Additionally, we have shown the simultaneous multiplexing and demultiplexing of a 3 GHz analog signal, a Nyquist 8 GBd BPSK signal, and a 4 GBd BPSK normal-shaped signal.All experiments were obtained at 0 V DC bias for the optical ring modulator and very low RF power.The concept can be used to generate high data rate superchannels with an extremely compact and low-power system.We believe that such a transceiver may be of great interest, especially for applications in access networks.

Fig. 1 .
Fig. 1.Concept of the agnostic transceiver system.The proposed transmitter (pink dashed box) can be utilized to transmit higher-order modulation format signals through the I and Q branches.It is based on optical ring modulators to modulate k signals.One of the ring modulator resonances should be adjusted to the wavelength of the LD.The multiplexing of k signals, each of which with a sampling rate of Δf s /k and a baseband bandwidth of B b = Δf s /(2k), is shown in the yellow box.The agnostic receiver (green dashed box) utilizes coherent detectors with a bandwidth of B b to detect the transmitted k signals in k branches.Here the orange and black lines represent the optical and electrical connections, respectively.TX: transmitter, RX: receiver, LD: laser diode, RF: radio frequency source, mix: Mixer, Sum: Summation, CD: Coherent detector, and DSP: Digital signal processing both with a bandwidth of B b .

Fig. 2 .
Fig. 2. Bandwidth-limited random signal fully represented by single sincpulses (a), and (b) by three sinc-pulse sequences in the time (left) and frequency domain (right).The brighter part are the mirror frequencies that will occur below the carrier after modulation.The superposition of the three spectra in (b) leads to the spectrum in (a).

Fig. 3 .
Fig. 3. (a) Experimental setup for the proposed agnostic transceiver system.An optical ring modulator modulates three different TDM channels at the transmitter side.At the receiver side, an MZM is used to demultiplex the proposed data signal in the optical domain for one branch by adjusting the phase of the RF signal.A coherent detector (CD) with a local oscillator (LO) is utilized to obtain the required data signal.The orange lines represent the optical connections, and the black lines are the electrical ones.(b) Illustration of the temperature compensation system used to stabilize the temperature of the ring modulator.(c) Demonstration of the front-side view of the experimental setup.(d) Measured intensity transfer function of the ring modulator at 0 V DC bias voltage.AWG: Arbitrary waveform generator, GS probe: Ground-signal probe, DC: Direct current, LD: Laser diode, Atten: Optical attenuator, RM: ring modulator, EDFA: Erbium-doped fiber amplifier, BPF: Optical bandpass filter, PC: Polarization controller, MZM: Mach-Zehnder modulator, OSA: Optical spectrum analyzer, RFG: Radio frequency generator, LO: Local oscillator, CD: Coherent detector, and OSC: Electrical oscilloscope.
illustrates the multiplexing of k signals.In each of the k branches the bandwidth B b is divided by k, leading to B b /k.Since the optical bandwidth B is twice the baseband bandwidth, the total optical bandwidth of the generated signal becomes B = 2B b .

Fig. 4 .
Fig. 4. Optical spectrum of the 24 GBd 3-channel BPSK TDM signal (a), and (b) for one 8 GBd demultiplexed data signal after the MZM.(c) Electrical spectrum at the receiver for the demultiplexed analog signal from the mixed analog and digital signal.In (a), the deviation from the rectangular form is due to the non-idealties of the ring setup and the limited resolution of our optical spectrum analyzer.