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Keywords:

  • All-Optical OFDM (AO-OFDM);
  • Optical Discrete Fourier Transform (ODFT);
  • Orthogonal Frequency Division Multiplexing (OFDM);
  • Superchannel

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Theoretical model for all optical OFDM
  5. 3. Device technologies for all-optical DFT
  6. 4. System implementations
  7. 5. Conclusions and further study
  8. Acknowledgment
  9. References
  10. Biographies

Orthogonal frequency division multiplexing (OFDM) can provide spectrally efficient communication channels because it can utilize carrier orthogonality and various impairment mitigation methods. An optical OFDM signal can be generated electronically to multiplex lower-rate carriers. In recent advancements, OFDM signals are also shown to be generated and demultiplexed by all-optical discrete Fourier transform (DFT), overcoming the speed limit of electronics for >Tbps capacity. High-performance DFT devices, such as arrayed waveguide grating (AWG) or planar lightwave circuit (PLC), are critically required to obtain strong orthogonality for scalable all-optical OFDM (AO-OFDM) system implementations. Advanced techniques such as coherent modulation and detection with digital impairment mitigation are also important for long-reach AO-OFDM transmissions. More recently, optical superchannel schemes have been introduced utilizing coherent detection for multi-Tbps AO-OFDM transmissions. This paper reviews the device and system aspects for the AO-OFDM technology, including a generalized theoretical model to provide an indepth understanding.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Theoretical model for all optical OFDM
  5. 3. Device technologies for all-optical DFT
  6. 4. System implementations
  7. 5. Conclusions and further study
  8. Acknowledgment
  9. References
  10. Biographies

The optical-fiber communications research community has achieved significant technology breakthroughs to reach fiber channel capacities beyond 10 Tbps during the past decade, and approached the physical limit of an optical-fiber channel. There are several experimental reports of 10–100 Tbps single-mode fiber transmission [1-8], while 100-Gbps single-channel coherent communication transceivers have recently become commercially available. The most important technology breakthroughs in this area include single-carrier digital coherent optical communication [3] with frequency- and time-domain equalizations [9, 10], optical OFDM [11-16], and AO-OFDM [2, 6-8, 17-33]. Single-carrier coherent communication utilizes M-ary phase and amplitude shift keying modulation, such as quadrature phase shift keying (QPSK) and quadrature amplitude modulation (QAM), combined with heterodyne detection and digital signal processing adapted from mobile communication technologies, and can successfully mitigate most fiber-optic transmission impairments. Transmission capacity can be extended even further with polarization diversity multiplexing (PDM). Recently, experimental results and field trials demonstrating Tbps data transmission over ultralong haul fiber distances have been reported by many research groups [1, 7, 8]. Meanwhile, the OFDM communication technique exploited within the framework of mobile communications has also been studied for fiber-optic transmission. With electronic OFDM multiplexing and demultiplexing, optical-fiber systems [13, 14] have shown the potential for Tbps transmission.

All the aforementioned technologies largely depend on the performance and capacity of electronic processing, which is being rapidly developed to provide higher-capacity and lower-cost implementations. However, the capacity and power-consumption limits of electronic processors present a bottleneck whenever scalability is concerned, if compared with equivalent all-optical methods. Figure 1a shows a typical schematic design of an optical OFDM system, which consists of an electronic fast Fourier transform (FFT), digital–analog and analog–digital converters (DAC and ADC, respectively), optical in-phase/quadrature-phase modulators (IQ MOD), and optical hybrid IQ detectors [15, 16]. In general, the maximum available capacity of an optical OFDM is limited by the speed of DAC and ADC, as well as the power consumption and footprints of silicon chips for FFTs and adaptive IQ detections. Even though lab experiments have demonstrated single-carrier optical OFDM capacity higher than Tbps by offline demodulation that is achieved by software processing, the true real-time optical OFDM system can achieve only up to 100 Gbps presently [17]. In order to scale the channel capacity beyond single-carrier application, we can consider multicarrier OFDM where carriers are all-optically multiplexed to have intercarrier orthogonality. Such all-optical OFDM can overcome the electronic processing limit of single-carrier systems, such as in wavelength division multiplexed (WDM) transmission that incorporates multiple time division multiplex (TDM) channels in optical fibers.

image

Figure 1. (online color at: www.lpr-journal.org) Schematic designs of optical OFDM (a) and all-optical OFDM (b). I/Q Demux(Demod) and I/Q Mux(Mod) stand for in-phase/quadrature-phase demultiplexer (demodulator) and multiplexer (modulator) in coherent communication, respectively. ADC and DAC stand for analog-to-digital and digital-to-analog converters, respectively. OH stands for optical hybrid. For IM/DD AO-OFDM communication, IQ Mod and OH can be replaced with an on–off modulator and photodetector, respectively, and DAC/ADC and M-ary Mod/Demod are not required.

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As shown in Fig. 1b, multicarrier optical data symbols can be generated individually from optical carrier sources [19]. In an AO-OFDM reference model by Lee et al. [20], FFTs and serial-to-parallel and parallel-to-serial converters are replaced with all-optical DFTs and optical delays, respectively, and hence the AO carrier multiplexing and demultiplexing are achieved at multiples of the electronics speed with nearly no power consumption. In addition, instead of using a single fast DAC/ADC, optical IQ MOD, and optical hybrid IQ demodulator, AO-OFDM is implemented with many slower devices. Carrier orthogonality can be preserved, even with arbitrary modulation formats as long as the modulation periods are equal and synchronized, providing modulation format transparency [7, 21]. Each carrier can be multiplexed by PDM to double the transmission capacity [19].

The AO-OFDM scheme has further advanced to AO-OFDM superchannel schemes, where optical carriers are demultiplexed orthogonally by coherent multicarrier detection using an electrical FFT, instead of demultiplexing by an all-optical DFT. In a typical embodiment of the superchannel scheme, carriers are partitioned to several groups and coherent multicarrier detection is applied locally to each group. Nonetheless, the AO-OFDM superchannel transmission utilizes the same carrier orthogonality properties as an optically demultiplexed AO-OFDM. The first AO-OFDM superchannel system was demonstrated by Masuda et al. [1], while the first terabit AO-OFDM superchannel experiment using QPSK modulation was achieved by Chandrasekhar et al. [22]. To mark a new phase of AO-OFDM superchannel technology maturity, multi-Tbit/s AO-OFDM superchannel field trials using an all-optical OFDM transmitter architecture with digital coherent carrier demultiplexing were shown in [7]. Finally, record-high AO-OFDM spectral efficiency using 16-QAM and beyond 10 000 km transmission distance was demonstrated by Huang et al. [23, 24].

This review paper is organized as follows. In Section 'Theoretical model for all optical OFDM', a simple system-level theoretical model is provided to explain how optical orthogonality can be attained by AO-OFDM schemes including superchannel concept, followed by a review of AO-OFDM multiplexer and demultiplexer device technologies in Section 'Device technologies for all-optical DFT'. In Section 'System implementations', we introduce system demonstration reviews of an AO-OFDM direct detection system and an AO-OFDM superchannel system. Section 'Conclusions and further study' concludes the review discussions with anticipated future developments and applications.

2. Theoretical model for all optical OFDM

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Theoretical model for all optical OFDM
  5. 3. Device technologies for all-optical DFT
  6. 4. System implementations
  7. 5. Conclusions and further study
  8. Acknowledgment
  9. References
  10. Biographies

2.1. All optical OFDM transmitter and receiver designs

When modulated carriers are packed too closely to each other, so that carrier spectra overlap partially, it is critical to demultiplex individual carriers while avoiding crosstalk. An OFDM receiver demultiplexes closely packed carriers, eliminating the intercarrier interference (ICI) at certain time positions. In order to investigate the principle of operation, let us consider a conceptual OFDM demultiplexer model at a receiver, as illustrated in Fig. 2a with 4 carriers (N = 4). An all-optical OFDM receiver implements two main functions: a delay array for serial-to-parallel conversion and a DFT processor [20, 25]. Denoted by inline image and inline image, respectively, the delay and DFT transfer functions of the mth circuit path can be combined to produce the frequency-domain transfer function for the nth carrier as follows:

  • display math(1)

where ω is the angular frequency, and τ is the chip period. The OFDM symbol period is T = . Accordingly, the carrier spacing in the angular frequency is defined by inline image. Neglecting the constant time delay factor of inline image, we obtain

  • display math(2)
image

Figure 2. (online color at: www.lpr-journal.org) Schematic illustrations of the principle of operation of all-optical OFDM demultiplexer consisting of delay array and DFT circuitry for the output port n = 2, (a), as an example, and its application for a receiver, (b). In (a), the sinusoidal curves indicate two carriers that are delayed and phase shifted.

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In order to gain an insight into the demultiplexing process, Fig. 2 shows two input carriers ω1 = Δ and ω2 = 2Δ, modulated by a rectangle function of duration of 2 × 4τ (two symbols in the same modulation state) arriving at the demultiplexer, as an example. From inspection of Fig. 2, it is evident that the delaying and phasing of the replicas of the carrier ω1 create destructive interference, whereas all the replicas of ω2 sum up constructively. It is also worth observing that complete orthogonality between the two carriers holds only for a certain period, where all the replicas for each carrier interfere destructively. Figure 2b is the corresponding receiver system model in which N optical detectors can be incorporated.

In principle, orthogonality is preserved with any kind of modulation format as long as i) each modulation state lasts for To = or longer, where the cyclic prefix T ′ = Kτ is defined by temporal duration in excess of , ii) the rise and fall times of modulation are short enough compared with τ, and iii) the modulation is applied to all carriers with the same modulator clock phases so that all the carriers arrive at the receiver at the same time, in order to arrange ICI from all other carriers such that it is located on the symbol boundaries in the time domain. The resulting symbol configurations are illustrated in Fig. 3, in which the ICI-free region is indicated by the white space in the time domain; Figs. 3a and b depict how ICI free periods are formed by all-optic DFT processes due to destructive interference. This property allows the use of advanced modulation formats such as M-ary phase shift key (PSK), QAM, and even another layer of optical OFDM, as well as simple nonreturn-to-zero on–off keying (NRZ-OOK) for intensity-modulation/direct-detection (IM/DD) systems and differential PSK (DPSK). In this sense, AO-OFDM is modulation-format transparent.

image

Figure 3. (online color at: www.lpr-journal.org) A schematic illustration of an OFDM symbol structure shown in the time domain; τ is the OFDM sampling space, and To = Nτ and T’ = are the minimum symbol and cyclic prefix periods, respectively, in an N = 4 carrier OFDM system. (a) and (b) represent optical carriers produced by short optical pulses; (c) and (d) by a cw source. The color codes indicate phase rotation of the carrier with respect to a reference carrer frequency, e.g. the first carrier. (b) and (d) compare how cyclic prefixes can be applied in pulsed and cw mode OFDM symbol structures. Symbol values are dectected only in the ICI-free regions.

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Figure 4 shows a more precise numerical analysis to illustrate the orthogonality behavior in a 4-carrier (N = 4) transmission system with a carrier spacing inline image of 10 GHz, corresponding to a symbol period of To = = 100 ps. In order to show that ICI occurs only when a carrier is modulated, i.e. only on the boundary between symbols, unit step modulation is applied to a carrier at 195.00 THz in the system model shown in Fig. 4a. The corresponding demultiplexed optical waveforms are plotted in Fig. 4b: The target signal waveform at the 195.00-THz carrier frequency is demultiplexed to form a slowly increasing optical power waveform, and ICI waveforms for other carrier output ports of other frequencies are shown in the plots for ICIs. All ICI waveforms last only for a period of To, and hence one can infer that if carriers are modulated at every To, an interference-free time position can be found at every To, as indicated with red arrows in the plots. This behavior in two-carrier experiments was first demonstrated by Sanjoh et al. [18] and its 30 × 100 Gb/s transmission is achieved by Sano [19]. Because the interference-free interval is narrow compared to a symbol period, as shown in Fig. 3, the demultiplexed symbol should be sampled optically at each output port of an AO-OFDM demultiplexer or electrically after optical-to-electrical conversion in conjunction with digital signal processing. As depicted in Fig. 3d [18], the modulation symbol period T can be made longer than To, keeping the rise and fall times unchanged, to increase the interference-free range. This is equivalent to adding a cyclic prefix (Fig. 3b) used in conventional OFDM transmission systems [25, 26]. The cyclic prefix is also used to mitigate time-domain impairments in fading radio channels, for example. The same application can mitigate chromatic dispersion effects of the optical-fiber medium [27, 34].

image

Figure 4. (online color at: www.lpr-journal.org) A numerical example of AO-OFDM demultiplexer characteristics for a 4–10 GHz OFDM system, (a) system model where the laser is modulated to a ‘on’ state at t = 100 ps, (b) demultiplexed signal and ICIs into other carrier ports after demultiplex. Red arrows indicate the time-domain sampling positions for data recovery at the center of each symbol period that corresponds to a 10 Gbit/s data rate.

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An OFDM transmitter is required to produce frequency-locked optical carriers that exactly match the optical frequencies of the OFDM demultiplexer of the receiver. There are several transmitter designs that can generate such frequency arrangements. A comb spectrum source, produced by a modelocked laser or by a continuous wave (cw) laser modulated by an over-driven phase modulator, is the most widely adopted method. A comb spectrum is used to generate optical OFDM carriers through aggregation with a pulse-mode AO-OFDM multiplexer or a cw-mode AO carrier multiplexer.

The most comprehensive transmitter model that can present insights into AO-OFDM communication is a pulse-mode transmitter model with a pulsed optical source as shown in Fig. 5, which gives an immediate analogy with electrically generated OFDM symbols [20]. A short pulse train with a pulse width narrower or equal to τ generated by a modelocked laser diode at a repetition rate B is split into N replicas, and each of them is individually modulated and fed into each input port of the AO-OFDM multiplexer (MUX). An input pulse at an input port i is split into N replicas for all different output ports k = 0, 1, . . N–1. Pulses shown in the boxes of the figure indicate the time positions of optical signal with respect to the OFDM symbol slots. The iDFT block introduces phase shifts between input port i and output port k according to the following expression:

  • display math(3)
image

Figure 5. (online color at: www.lpr-journal.org) Conceptual schematic of all-optical ODFM multiplexer of a pulse-mode transmitter design. The spectrum illustrations on the bottom shows the modulation broadening.

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This is the same as the discrete Fourier transform (DFT) used in the electrical OFDM communication. At the outputs of the iDFT block, a set of optical delays are introduced as shown in Fig. 5. This delay array has delays of multiples of τ and N delayed signals are combined by a standard N × 1 coupler, providing parallel-to-serial conversion. The corresponding output of this OFDM symbol includes multiple carriers with designated frequencies. Applying the same analysis used to obtain Eq. (2), we find the OFDM multiplexer function for the ith carrier input as

  • display math(4)

At the receiver, the same principle of operation is applied, following the system model of Eq. (1). The symbols are delayed by a matched set of an optical delay array, followed by the DFT, output signals filtering, and photodetection to recover the input data.

In order to construct the mathematical model for pulse-mode OFDM signal multiplexing and demultiplexing, let us consider single-pulse propagation through an input port i and output port n at an AO-OFDM multiplexer and demultiplexer, respectively. The frequency-domain amplitude representation of a single pulse for the ith carrier can be defined as inline image, where inline image and inline image denote the data modulation and the modulation spectrum of the pulse, respectively. Here, the pulse width is assumed to be the same as τ. The corresponding OFDM signal is represented as

  • display math(5)

Then an optical pulse sequentially propagating through the system from the ith port of an AO-OFDM multiplexer to the nth port of a demultiplexer is obtained by

  • display math(6)

The corresponding time-domain representation of the received and demultiplexed signal can be obtained by a Fourier transform:

  • display math

Using the Fourier transform inline image, where inline image is the time-domain representation of inline image, and inline image, we obtain the optical field of the demultiplexed signal as

  • display math(7)

The evaluation of Eq. (7) for inline image is zero at inline image for integer s as long as inline image for inline image. For the case of i = n, i.e. for the target carrier being demultiplexed, we find

  • display math(8)

When N carriers are multiplexed with data modulation of each carrier inline image, where ais is the data sequence of the ith carrier, the output at the demultiplexer port is given by

  • display math(9)

It is noted that in a pulse-mode AO-OFDM transmitter, the performance of the optical data modulator can be eased because orthogonality depends on the performance of the AO-OFDM multiplexer and the shape of the pulse.

A cw-mode AO-OFDM transmitter generates a multiplexed OFDM signal either from a comb spectrum source or by a set of frequency-controlled cw sources. Figure 6a shows the key principles of AO-OFDM superchannel generation that can be demultiplexed and demodulated by digital coherent carrier multiplexing [1, 8, 22, 27], which has been widely exploited in high-speed AO-OFDM transmission experiments. At the transmitter, a multitone generator can be used as an optical multicarrier source with phase-locked optical tones at integer multiples of the driving source clock frequency B. Such a multicarrier source can be realized using a mode-locked laser synchronized at its fundamental frequency [28], concatenated overdriven optical phase modulators [6, 35], or a recirculating loop where a new tone is generated at each pass through the loop [23]. In the case of multicarrier tone generation via wideband frequency modulation, the optical powers of the generated carriers will follow an nth-order Bessel function of the first kind, such that intercarrier power variation can be large. A flexible band WSS [36] can then be employed with an all-optical carrier demultiplexer to equalize the intercarrier powers, while an optical carrier demultiplexer, such as an AWG, can be used to separate the orthogonal frequency components.

image

Figure 6. (online color at: www.lpr-journal.org) Conceptual schematic of cw-mode all-optical ODFM transmitter designs by the use of (a) phase-locked tone generation for superchannel generation, and (b) individual free-running cw carrier sources with precise wavelength control. The AO carrier MUXs in both designs can be achieved by various design options. Tones and carriers are generated with exact carrier spacing B. The modulators can use advanced coherent modulation formats at the B baud rate.

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A comprehensive understanding of orthogonality can be extended from the fact that orthogonality is independent of carrier–carrier phase correlation, as inferred from Fig. 4. As shown in Fig. 6b, a set of free-running cw lasers can produce AO-OFDM carriers as long as the laser frequencies are precisely tuned to the receiver AO-OFDM demultiplexer frequencies and the modulation clock phases are aligned to produce orthogonality. In this configuration, however, the instantaneous power waveform of an OFDM symbol is not deterministic because the carriers are not phase-locked. Subsequently, when coherent detection with electrical adaptive equalization by digital signal processing is used at a receiver, the impact of ICI may not be eliminated as it manifests fast beat noise fluctuation due to fast phase rotations of free-running sources. In addition, fiber nonlinearity impairments may not be deterministic, making nonlinear penalty equalization by receiver digital signal processing difficult. Nonetheless, this scheme can also be modulation-format transparent.

A generalized mathematical model for cw-mode AO-OFDM can be obtained by modification of Eqs. (5) and (6). A modulated carrier i with a modulation spectrum Si(ω) = aiΦ(ω−iΔ), where Φ(ω) is the modulation spectrum due to optical data modulation, propagating through the OFDM system to an AO-OFDM demultiplexer at the receiver through the nth port is denoted by

  • display math(10)

Using the similar derivation for Eq. (7), we obtain the corresponding time-domain representation of the optical field of the demultiplexed signal as

  • display math(11)

The evaluation of Eq. (11) with step-function modulation in the N = 4 example is shown in Fig. 4. For i = n, Eq. (11) is simplified as

  • display math(12)

The carrier modulation symbol ai can be detected by direct or coherent detection. For inline image, Eq. (11) is zero at t = sT for integer s as long as inline image for inline image. This is an important modulator requirement for a cw-mode AO-OFDM transmitter.

2.2. Subrate OFDM Transmission

In an optical OFDM system, transmission impairments can cause two-fold penalties of intersymbol interference (ISI) and intercarrier interferences (ICI). The ISI penalty in an OFDM system is the same as that of a single-carrier system. On the other hand, the ICI degrades the orthogonal property of an OFDM system, and hence can be more critical than the ISI penalty. As an example, orthogonality degradation due to fiber chromatic dispersion (CD) penalty is more severe if the OFDM symbol bandwidth exceeds 100 GHz. In order to model an example of CD penalty in an AO-OFDM demultiplexing, Eq. (10) can be modified as

  • display math(13)

Here, the fiber CD transfer function inline image is used, where inline image; D, L, λ, and c, are the fiber CD coefficient, fiber length, wavelength, and speed of light, respectively. Using the system model of Fig. 4a with a fiber inserted in front of the demultiplexer, the ISI and ICI behaviors are evaluated by a numerical Fourier transform of Eq. (13). This data shows a 4-carrier example where the carrier at 195.00-THz turns on at t = 100 ps, and the corresponding ISI and ICIs are observed at the receiver AO-OFDM demultiplexer ports as shown in Fig. 7. From the data plots, one can see the ISI and ICI are spread in the time domain, making the orthogonal property poor. In order to mitigate such interference penalties, the OFDM carrier baud rate BC can be lowered below B so that detection points can be sampled at a lower rate as can be seen in comparison of Fig. 4b and Fig. 7. Such subrate OFDM transmission can be achieved by cyclic prefix insertion and subrate (BC <B) modulation in the pulse-mode [27, 34] and cw-mode [18, 25, 26] AO-OFDM transmission systems, respectively. The use of subrate modulation is also equivalent to the insertion of a temporal guard band between symbols.

image

Figure 7. (online color at: www.lpr-journal.org) AO-OFDM symbol propagation in dispersive fiber observed at different output ports of the AO-OFDM demultiplexer with 20 km of SMF28 fiber inserted in the setup shown in Fig. 4a, for comparison with the waveforms of Fig. 4b. Red arrows indicate a reasonable choice of the subrate sampling positions for data recovery.

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2.3. Superchannel digital coherent demultiplexing

A superchannel can be regarded as a bank of AO-OFDM carriers that can be generated by the scheme discussed in Fig. 6a of Section 'All optical OFDM transmitter and receiver designs'. Figure 8 illustrates the key principle of digital coherent carrier demultiplexing of AO-OFDM superchannel, which has been widely exploited in high-speed AO-OFDM transmission experiments. In this scheme, optical carriers are modulated in parallel using a bank of optical modulators (Fig. 6a). As aforementioned, the typical modulation format used for each optical carrier can be arbitrary; specifically, either OFDM or single-carrier (SC) modulation in the form of M-ary PSK (M-PSK) or M-ary quadrature amplitude modulation (M-QAM) can be used on each optical carrier, without loss of generality. In the case of SC transmission per optical carrier, the baud rate per carrier inline image should be less than or equal to the frequency spacing of the all-optical carriers B to ensure digital carrier demultiplexing can be performed with low distortion, as aforementioned. For the special case of zero-guard-interval OFDM, inline image. It is also noted that, apart from the all-optical tone generator, the AO-OFDM approach with SC per-carrier modulation can use identical transmitters and receivers as those developed for 100 Gb/s coherent systems, which is attractive from the system-upgrade point of view. The aggregate data rate of the AO-OFDM superchannel can then be scaled by simply increasing the number of optically generated carriers and SC transceivers.

image

Figure 8. (online color at: www.lpr-journal.org) Digital coherent AO-OFDM carrier demultiplexing. LO: local oscillator; PD: photodetector; ADC: analog-to-digital converter; FDE: frequency domain equalizer; TDE: time-domain equalizer; CR: carrier recovery; SD: symbol detection.

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At the receiver side, to detect an all-optical carrier of interest, a cw local oscillator (LO) laser can be tuned near the carrier center frequency, essentially downconverting the target optical signal to the electrical baseband by an optical hybrid. Following such LO-based downconversion, a standard coherent receiver front-end can be used for digital coherent carrier demultiplexing [8]. If the bandwidth of the coherent receiver front-end is sufficiently wide, it is also possible to detect multiple carriers per transceiver by placing the LO midway between the target carriers and using DSP to accomplish any residual baseband shifting prior to digital impairment compensation.

To process the target AO-OFDM carriers with low distortion, the analog-digital converter (ADC) bandwidth and oversampling rate must be high enough to capture sufficient energy of main and side lobes (Fig. 8). It is also noted that the ADC bandwidth and oversampling rate requirements also increase with constellation size. Following digitization, residual CD must be first compensated using a frequency-domain equalizer (FDE) [9] to restore orthogonality between the carriers. For a DSP sampling rate that is M times the carrier symbol baud rate, the carrier of interest can be demultiplexed digitally by initializing an adaptive time-domain equalizer (TDE) [10] with coefficients that are equal to the first column of an M-point FFT matrix. In other words, the TDE coefficients are initialized so as to have a frequency response of an M-point FFT, where M < N is equivalent to the number of AO-OFDM carriers being jointly processed in DSP. This is done in order to have a sufficiently high oversampling rate so as to perform the partial digital coherent carrier demultiplexing without incurring too much penalty. It is noted that this differs from conventional OFDM, wherein the entire OFDM band is captured and a single-joint FFT is used for carrier demultiplexing. It is also noted that the TDE both demultiplexes the carrier(s) of interest and equalizes the residual impulse response of the channel. The TDE coefficients can subsequently be adapted using any commonly used algorithm for SC systems, such as common modulus or decision-directed algorithms [10]. Finally, carrier recovery (CR) and symbol detection (SD) are performed. Consequently, notwithstanding potentially higher oversampling requirements and the need for proper initialization of the adaptive TDE, digital carrier demultiplexing for AO-OFDM can be performed largely using the same DSP algorithms as those exploited in current-generation 100 Gb/s digital coherent transmission systems.

3. Device technologies for all-optical DFT

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Theoretical model for all optical OFDM
  5. 3. Device technologies for all-optical DFT
  6. 4. System implementations
  7. 5. Conclusions and further study
  8. Acknowledgment
  9. References
  10. Biographies

There are several device technologies that provide the all-optical DFT function for OFDM multiplexing and demultiplexing, including the multimode coupler design used in arrayed waveguide gratings (AWG), the waveguide-and-coupler mesh (WCM) design, and the fiber Bragg gratings (FBG) design. In this section, we review the design principles of AWG and WCM devices, as these are widely adopted device technologies.

3.1. AWG-based DFT device design

A decade ago, when OFDM was not yet widely studied as a research topic for optical communications, an AO-OFDM-capable AWG device was first designed to be used not for DFT, but for generation and processing of optical codes in packet switching or optical code-division multiple access (OCDMA) systems [37, 38]. A specially designed AWG with a two-slab coupler connected by an array of waveguides (Fig. 9) can produce and detect OCDMA codes with Fourier coding, which is equivalent to OFDM symbols, as discussed in [20].

image

Figure 9. (online color at: www.lpr-journal.org) AWG configuration consisting of delay array and DFT.

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The first slab waveguide distributes the input from the single port to the waveguide array consisting of N waveguides with incremental delay differences of τ to perform serial–parallel conversion as illustrated in Fig. 2a. The waveguide ports on the second slab coupler are carefully placed to produce the Nth-order DFT phase relations between N input and N output ports, i.e. the DFT of a parallel optical field input signal am (m = 0,1, . . N–1) produces a parallel optical field output bn (n = 0,1, . . N–1) as depicted in Fig. 10a, according to the definition of DFT:

  • display math(14)

It is interesting to observe that the slab coupler of Fig. 10a performs the spatial DFT equivalently to a two-lens system separated by a focal length f, as illustrated in Fig. 10b, where the curvature radius R of the slabs equals the focal length f. In this design, the light distribution in the output plane is a Fourier transform of the input object, and this concept forms the basis of the entire optical information processor. As an example, we can obtain the DFT function Eq. (14), by equally spacing the input and output waveguides. In [37], a relevant configuration of the AWG design for the multiport encoder/decoder (or DFT device) has been realized: In this design, the array pitches d and do of Fig. 10a have to satisfy the following condition:

  • display math(15)

where l is the slab length. In addition, the delay difference τ introduced by two adjacent waveguides should satisfy the condition τ = 1/BN, where B is the OFDM symbol rate. The same AWG architecture has been described in [26, 31, 33]. In some previous publications, the confocal slab couplers that perform the DFT have been replaced by a set of phase shifters [18, 20].

image

Figure 10. (online color at: www.lpr-journal.org) Slab coupler configuration (a), and its equivalent two-lens imaging system model (b).

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The same AWG device can also be used to generate OFDM symbols. If a single short laser pulse is fed backward to one of the output ports of an AWG of Fig. 9, the corresponding Fourier code is simultaneously generated at the device input port. Each optical Fourier code is equivalent to an AO-OFDM symbol that corresponds to a carrier and the orthogonality between the code detection is used to demultiplex OFDM carrier at the receiver.

3.2. Coupler-based DFT device design

A device that performs DFT in the optical domain can be attained by different physical implementations. The first architecture that performs the DFT was proposed by Marhic [39] and revisited in a work by Siegman [40]. The WCM composed of passive couplers and phase shifters shown in Fig. 10a performs the DFT of a parallel optical field input signal am (m = 0,1, . . N–1) to produce a parallel optical field output bn (n = 0,1, . . N–1), according to Eq. (4). The Nth-order DFT of the parallel input signal is evaluated by a chain of log2N stages, wherein each stage is composed of 2 × 2 3-dB couplers that are equivalent to the second-order DFT as shown in Fig. 11b.

image

Figure 11. Waveguide and coupler mesh schematic diagram (a) composed of couplers and phase shifters that perform the DFT of N = 8 parallel inputs. The phase shifters are represented by the boxes with phase-shift values –2πq/N, where q is indicated inside the boxes; and (b) the mathematical model for the 2 × 2 coupler.

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These schemes make the DFT have a parallel input. To be used in optical communications, where data symbols are serially transmitted on a fiber, a serial-to-parallel converter, composed of a splitter and a set of delay lines, is required, as shown in Fig. 12. To realize a complete AO-OFDM demultiplexer design, this schematic should be incorporated with optical delay arrays as shown in Fig. 12, which was fabricated for the first time by Takiguchi et al. [41, 42]. The same principle has also been used for 2 × 2 DFT signal processing in an all-optical OFDM [21]. In addition, a generalization of the architectures of Figs. 11 and 12, replacing the 2 × 2 couplers with M × M hybrids has been proposed [43].

image

Figure 12. (online color at: www.lpr-journal.org) From [42]: Planar waveguide circuit for the 8 × 8 DFT. The time delay introduced by ΔL corresponds to τ, where Δ and L are the effective refractive index and the unit length difference of the delay array waveguide, respectively.

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The DFT schemes of Figs. 11 and 12 are highly sensitive to small phase errors on all waveguides since a WCM is a complex interferometer, wherein multiple Mach–Zehnder interferometers (MZI) are nested with one another, as studied in [44]. According to this report, a WCM device is more sensitive to device-fabrication errors of waveguide phase-shift control than an equivalent AWG device. In order to overcome this drawback, a new configuration of the waveguide DFT approach was formulated for a discrete serial input by Cincotti [45], and reinvestigated in [46]. In this novel design as shown in Fig. 13, the time delay array is repositioned into MZIs, and MZIs are separated from one another through a tree architecture, as shown in Fig. 13: The Nth-order DFT is then obtained by a chain of log2N stages, each of which consists of an MZI with an output coupler.

image

Figure 13. (online color at: www.lpr-journal.org) Cascaded optical FFT composed of MZIs with couplers and phase shifters, which provides AO-OFDM demultiplexing equivalent to combined functions of serial-to-parallel conversion, delay arrays, and DFT. The time delays and phase shifters are represented by the boxes with time-delay values (multiples of τ) and phase shift values (fractions of π), respectively.

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This architecture has been fabricated by Hillerkeuss et al. [43] (Fig. 14) and used in the record-breaking 26 Tbit/s transmission experiment [6]. In this design, N carriers can be demultiplexed in M (< N) carrier AO-OFDM, and hence each output of the AO-OFDM output port has N/M carriers, showing a cyclic pattern in the spectrum. The multiple carriers of a port are separated by a coarse WDM demultiplexer. Overall, this hybrid design of a cascaded optical FFT and WDM demultiplexer has demonstrated device efficiency as well as strong performance.

image

Figure 14. From [6]: Cascaded optical-FFT and coarse-WDM AO-OFDM receiver model for a 325-carrier 26-Tbps transmission demonstration. Note that a conventional WDM demultiplexer is used before electroabsorption modulators (EAMs). EAMs are used to time sample the optical symbol at the orthogonal points.

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4. System implementations

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Theoretical model for all optical OFDM
  5. 3. Device technologies for all-optical DFT
  6. 4. System implementations
  7. 5. Conclusions and further study
  8. Acknowledgment
  9. References
  10. Biographies

As discussed throughout this review paper, several advanced all-optical techniques exist for all-optical DFT/IDFT processing, which can benefit from the advantages of modulation format transparency, compact all-optical implementation, and high-speed all-optical signal generation and detection beyond the limit of the bandwidth of electronic components.

4.1. System demonstrations of IM-DD AO-OFDM transmission

In the early development of AO-OFDM, there have been a few reports of two-carrier IM-DD AO-OFDM demonstrations [18], followed by several reports of system performance studies [20]. Nonetheless, experimental demonstrations for practical applications have been achieved only recently [25, 26]. More recently, a reference AO-OFDM design with IM/DD that is depicted in Fig. 5 [20] has been demonstrated experimentally by Shimizu et al. [33].

Figure 15a shows the experimental schematic of 12.5-Gbps 8-carrier AWG-based AO-OFDM transmitter and receiver [33]. An optical comb for eight carriers with 12.5 GHz frequency spacing is generated from a cw light source with two cascaded EAMs modulated by 25 GHz and 12.5 GHz clocks, respectively. The frequency of the cw light source is 193.4875 THz. The optical comb is then split into two replicas and modulated by 12.5 Gbit/s pseudorandom bit sequence (PRBS) in an NRZ-OOK format with pattern lengths of 215–1 and 231–1 for odd and even channels, respectively. Each modulated data signal is split into four branches and sent to the AWG-based OFDM multiplexer. Each branch has a different delay for data pattern decorrelation. The polarizations and powers of all the carriers are aligned by polarization controllers and variable optical attenuators, respectively. The AWG used in this experiment has 16 ports with 12.5 GHz carrier spacing. At the receiver side, the signal is sent to the AWG-based OFDM demultiplexer. The demultiplexed signal is applied to the EAM to extract the orthogonal point in the symbol time slot. The corresponding demultiplexed output eye diagram of the target carrier is shown in Fig. 14b. In this design, a waveform reshaping modulator is employed at the receiver side, which provides time-domain equalization (TDE) to compensate for the AWG demultiplexing error. As a result, TDE can achieve efficient ICI cancellation at the sampling point as shown in Fig. 15, restoring orthogonality. This experiment confirms that the BER is reduced to 10−6 from 10−4 by the use of TDE.

image

Figure 15. (online color at: www.lpr-journal.org) Pulse-mode AWG AO-OFDM system; (a) system schematic for IM-DD AO-OFDM transmission of 8 × 12.5 Gbps, (b) demultiplexed receiver output optical eye diagram of carrier 5, (c) ICI waveform in carrier 5 output from carrier 6, and (d) BER performance. LD: laser diode, EAM: electroabsorption modulator, LN-IM: LiNbO3 intensity modulator, AWG: arrayed waveguide grating, VOA: variable optical attenuator, PC: polarization controller, Rx: receiver, PPG: pulse pattern generator, BERT: bit error rate tester.

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4.2. System demonstrations of high-speed AO-OFDM superchannel transmission

In recent years, significant advances in high-speed AO-OFDM superchannel technology and transmission capabilities have occurred. Such optical superchannels using AO-OFDM are thus quite promising for realizing aggregate transmission data rates beyond 100 Gb/s, by enabling flexible capacity scaling via parallel addition of transmitters and receivers. At the transmitter side, the AO-OFDM superchannel approach is implemented based on optical methods to generate phase-locked optical carriers, followed by individual modulation of each carrier as a parallel channel. At the receiver side, the optical carriers can either be demodulated optically via an optical DFT circuit, or electrically by first downconverting a spectral slice of the AO-OFDM to the electrical domain with a digital coherent receiver, then applying a digital fast Fourier transform (FFT). However, since CD destroys orthogonality between carriers, precise all-optical CD compensation must be performed prior to optical detection if an optical Fourier transform circuit is used, which can be a challenge to fully accomplish in the optical domain. On the other hand, the digital FFT can be implemented in receiver-side DSP as a complement to functions already implemented in typical 100 Gbit/s systems, including such as electronic CD compensation. Consequently, existing optical and electrical hardware developed for 100 Gb/s systems can to a large extent be reused, such that software-defined upgrades to beyond 100 Gb/s transmission become possible. In this section, we consider experimental system-level considerations for high-speed AO-OFDM superchannel transmission.

The system-level setup of the 21.7-Tbit/s AO-OFDM field trial over 1503 km of installed standard single-mode fiber (SSMF) achieved in [7] is shown in Fig. 16. In total, 22 external cavity lasers (ECLs) were used as seed lasers to produce 330 AO-OFDM carriers, with either polarization multiplexed QPSK or 8-QAM modulation used on each carrier, depending on the underlying channel quality. The optical tone generator consisted of separate phase modulators overdriven with sine-wave inputs, such that phase-locked optical carriers at 25 GHz spacing were created for each seed laser. A flexible band WSS was used to equalize the optical carrier powers, as well as it combined the carriers from the “odd” and “even” groups into a single output. On each carrier, the symbol rate Rs = 12.5 Gbaud was exploited, with the per-carrier modulation format decided based on spectral efficiency requirements and optical signal-to-noise ratio (OSNR) conditions.

image

Figure 16. (online color at: www.lpr-journal.org) System-level experimental setup of 21.7Tbit/s AO-OFDM field trial with transmitter and receiver side spectra and experimental bit error rate (BER) results [7].

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Following field-installed SSMF transmission, as in Fig. 16, a WSS was used to filter out the target superchannel, while an ECL LO was tuned to downconvert the target AO-OFDM carriers for digital carrier demultiplexing. Specifically, the LO was tuned midway between two optical carriers, which were jointly processed in offline DSP as described in Fig. 8 with 30 GHz input bandwidth and 6 times (M = 6) oversampling. Additional details of the system demonstration may be found in [7]. As shown by the BER results in the inset of Fig. 16, 18 PDM 8-QAM superchannels were successfully transmitted over 1503 km of field-deployed SSMF within the BER limit of 4.5 × 10−3 for 7% overhead hard-decision forward error correction, while 4 PM QPSK superchannels achieved the same transmission distance with more margin. By exhibiting both the highest field-trial capacity (21.7 Tbps) and the highest capacity–distance product (32.6 Pbps km) to date, the experimental demonstration confirms the promising spectral efficiency and aggregate transmission rate capabilities of AO-OFDM for future >Tbit/s optical transmission systems.

5. Conclusions and further study

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Theoretical model for all optical OFDM
  5. 3. Device technologies for all-optical DFT
  6. 4. System implementations
  7. 5. Conclusions and further study
  8. Acknowledgment
  9. References
  10. Biographies

All-optical OFDM techniques have rapidly gained interest in the optical data transmission community in recent years, even though research efforts on this topic were initially started on academic grounds. Nowadays, the principle understandings of orthogonality in multicarrier optical data transmission form the basis for Tbps superchannel and other highly spectrally efficient transmission systems for next-generation applications. This paper has discussed recent developments in AO-OFDM techniques, including a theoretical investigation of optical system requirements, a review of AO-OFDM multiplexer/demultiplexer device technologies, and recent state-of-the-art system demonstrations of AO-OFDM IM/DD and superchannel systems.

An AO-OFDM transmission system is modulation-format transparent, meaning that different carriers can be configured in different ways, enabling a mixture of low-cost direct detection and more advanced coherent detection for short-haul and long-haul transmission scenarios, respectively. Such flexibility with high spectral efficiency can be scaled to enable flexible hybrid optical networks that can be utilized for optical flow switching and optical waveband switching. In addition, AO-OFDM processing can significantly reduce overall energy consumption, and overcome current energy consumption limits to enable beyond-Tbps optical transmission systems.

Although AO-OFDM technology has rapidly advanced with proof-of-concept demonstrations and field trials, there are many unsolved problems to be addressed in practical applications. First, various optical DFT device and system technologies need to be investigated in order to maximize AO-OFDM carrier orthogonality in nonideal conditions. A good example in this avenue was reported in [6], where a high-order optical DFT was achieved by a combination of a low-order optical DFT and a conventional WDM demultiplexer. In addition, orthogonality can also be improved with optical signal processing as discussed in [33], where optical time-domain equalization is used to compensate for the AO-OFDM demultiplexer design errors. Moreover, there exist various device-design solutions that can improve the accuracy of filter functions of optical DFT devices. Secondly, fiber transmission impairment mitigation technologies have to be exploited to optimize system performance against fiber chromatic dispersion and nonlinearity. For example, fiber chromatic dispersion penalty can be mitigated by optical or electrical compensation methods. Self-phase modulation and intercarrier four-wave mixing penalty can likewise be managed by phase control among carriers, or mitigated by coherent detection and digital signal processing (e.g. nonlinear backpropagation [47]). All of these anticipated developments will add to critical enabling technologies for practical realizations of AO-OFDM for beyond-Tbps optical transmission systems.

Acknowledgment

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Theoretical model for all optical OFDM
  5. 3. Device technologies for all-optical DFT
  6. 4. System implementations
  7. 5. Conclusions and further study
  8. Acknowledgment
  9. References
  10. Biographies

The authors are grateful to Professor Gabriella Cincotti of University Roma TRE, Italy, for indepth guidance and technical discussions.

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  2. Abstract
  3. 1. Introduction
  4. 2. Theoretical model for all optical OFDM
  5. 3. Device technologies for all-optical DFT
  6. 4. System implementations
  7. 5. Conclusions and further study
  8. Acknowledgment
  9. References
  10. Biographies
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Biographies

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Theoretical model for all optical OFDM
  5. 3. Device technologies for all-optical DFT
  6. 4. System implementations
  7. 5. Conclusions and further study
  8. Acknowledgment
  9. References
  10. Biographies
  • Image of creator

    June-Koo K. Rhee is an associate professor at the Korea Advanced Institute of Science and Technology and a graduate of Seoul National Univ. with a B.E. (1988) and M.Sc. (1990), and the Univ. of Michigan, Ann Arbor, with a Ph.D. (1995), all in electrical engineering. Previously, he was with Princeton Univ. (1995), NEC Res. Inst. (1996), Corning Inc. (1998), and Samsung Adv. Inst. of Techno. (2003). His research has made contributions in the areas of ROADM, DPSK DWDM transmission, and WDM optical protection, green optical networking, and optical and flow switching.

  • Image of creator

    Neda Cvijetic received the Ph.D. degree in electrical engineering from the University of Virginia, Charlottesville, VA, in 2008. Since 2008, she has worked as a Research Staff Member in the Broadband and Mobile Networking Department at NEC Laboratories America, Princeton, NJ. She is also currently a member of the adjunct faculty at the Department of Electrical Engineering at Columbia University. Her research interests include advanced modulation, detection and digital signal processing (DSP) for high-speed optical transmission, optical-wireless convergence, and next-generation optical access networks.

  • Image of creator

    Naoya Wada received the B.E., M.E., and Dr. Eng. degrees in electronics from Hokkaido University, Sapporo, Japan, in 1991, 1993, and 1996, respectively. In 1996, he joined the Communications Research Lab, Japan. He is currently the Research Manager of the Photonic Network Group, National Inst. of Info. and Comm. Technology (NICT), Japan. His research interests include photonic networks and optical communications, such as OPS network, optical processing, and OCDMA system.

  • Image of creator

    Ting Wang received the M.S. degree in electrical engineering from the City University of New York, New York, and the Ph.D. degree in electrical engineering from Nanjing University of Science and Technology, Nanjing, China. Since 1991, he has been with NEC Laboratories America, Princeton, NJ, where he is currently the Department Head of optical networking research.