Optical Carrier-to-sideband Ratio Research Articles

Signal-to-noise ratio enhancement for digitalizing low amplitude wideband signals in photonic analog-to-digital converters.

Photonic analog-to-digital converters (PADCs) have been investigated for nearly five decades as a promising approach to overcome the bandwidth and jitter problem and bring ADC performance to new levels. However, low-amplitude signals often struggle to achieve full-scale quantization accuracy, posing a basic challenge for achieving high signal-to-noise ratio (SNR) digitization. Here, we established an optical carrier-to-sideband ratio (OCSR) based sampler model to achieve the optimal combination of the modulation, loss compensation, and photoelectric detection processes. The OCSR-based sampler features the advantages of high useful signal gain, low noise figure, and the ability to function over a very wide frequency range. The low-bias region is investigated, and the corresponding OCSR is selected as the transfer function for the Mach-Zehnder modulator (MZM). The OCSR-based sampler enables a higher gain of the radio frequency (RF) information signal sidebands. After the beating at the photodetector, the useful signal power reaches the digitizer's full scale to fully utilize the quantization accuracy, thereby enhancing the SNR of the whole system. In the experiment, a 20 GSa/s PADC with 4 interleaved sub-channels is configured out. Considerable advantages of the proposed OCSR-based sampler over conventional quadrature-biased sampler are demonstrated in comparative tests. A ∼5 dB enhancement in SNR and an increase of ∼0.8 effective number of bits (ENOB) are achieved under sinusoidal signals, and linear frequency modulation (LFM) signals with 8 GHz instantaneous bandwidth as well.

A linearized optical single-sideband modulation RoF link with tunable optical carrier-to-sideband ratio

In this paper, an optical single-sideband (OSSB) modulation radio over fiber (RoF) link with tunable optical carrier-to-side-band ratio (OCSR) and simultaneous third-order intermodulation distortion (IMD3) and power fading suppression is proposed. In the proposed link, a phase-modulated signal and an optical carrier are polarization multiplexed using a Sagnac loop. The periodic power fading caused by chromatic dispersion is avoided by filtering the lower sideband. By adjusting the polarization state, the IMD3 is suppressed and a tunable OCSR can be achieved. Compared with the traditional Mach–Zehnder modulator (MZM) based link, the spurious free dynamic range (SFDR) of the proposed link is improved by more than 18 dB with different RF frequencies. In the broadband characteristic test, the proposed link exhibits lower error vector amplitude (EVM) and higher adjacent channel power ratio (ACPR) in a larger input RF power range.

High Modulation Efficiency and Dynamic Range Optical Single Sideband Modulation Without Gain Penalty in Nonlinear Distortion Suppression

The optical single sideband (OSSB) modulation with tunable optical carrier to sideband ratio (OCSR) is a promising and popular technique in wideband analog signal transmission and microwave signal processing, which can improve modulation efficiency and combat the fiber chromatic dispersion. However, in all reported schemes for achieving OSSB modulation with tunable OCSR, the spurious free dynamic ranges (SFDR) of the systems are all limited by the third-order intermodulation distortion (IMD3). In this paper, our object is to improve the SFDR and suppress the IMD3 in a novel OSSB modulation with tunable OCSR. The proposed system consists of a primary channel and a secondary channel. In primary channel, an OSSB modulation with tunable OCSR is implemented. In secondary channel, the optical carrier and first-order sidebands are perfectly cancelled out, which can suppress the IMD3 without gain penalty at the same received optical power. In this way, a large-SFDR OSSB modulation with tunable OCSR is realized. Experimental results exhibit that the IMD3 gets effective cancellations without gain penalty under unchanged total received power, and the output microwave signal power obtains about 11-dB improvement by optimizing the OCSR. A greatly increased SFDR of over 128 dBHz4/5 is achieved in the proposed system.

An RoF system with continuously tunable optical carrier-to-sideband ratio based on polarization multiplexing

In order to improve radio over fiber (RoF) system performance and reduce system complexity, a novel RoF system scheme with continuously tunable optical carrier-to-sideband ratio (OCSR) was proposed. Using polarization multiplexing technology, the OCSR can be tuned from −36 dB to 26 dB by adjusting a polarization controller (PC). The effects of polarization angle of PC, modulation index and extinction ratio on OCSR were studied by theoretical analysis and experimental simulation, in which the results of theoretical calculation and experimental simulation are in good agreement. It is found that the receiver sensitivity can be improved 7.8 dB by tuning the OCSR from 10.41 dB to 0 dB. The power penalty is 0.07 dB for the 0 dB OCSR after 40 km transmission, which means the impact from the fiber link can be ignored.

Integrated Direct Single Sideband Modulation Utilizing Sideband Amplification Injection Locking Effect Based on Multi-Section Mutual Injection DFB Laser

An integrated single sideband (SSB) modulation method benefitting from the sideband amplification injection locking (SAIL) effect for radio-over-fiber (ROF) link is proposed and experimentally demonstrated, which is based on an integrated multi-section mutual injection (IMSMI) DFB laser. The IMSMI-DFB laser, which overcomes the drawbacks of using discrete devices, simplifies the whole structure. The SAIL effect, which is regulated by the input RF power and bias current of IMSMI-DFB laser, promises a good pathway for adjusting and controlling the modulation frequency, optical carrier to sideband ratio (OCSR), and sideband suppression ratio (SSR), meanwhile, provides high gain in the link. The characteristics of SAIL effect are investigated. The OCSR can be optimized to 0 dB, the SSR is over 26 dB, and the transmission link gain improves over 20 dB from 16 GHz to 26 GHz. The ROF transmission experiments demonstrate that the power fading introduced by fiber dispersion is dramatically eliminated, and the error vector magnitudes (EVMs) of the received 20 MSymbol/s 32 QAM signals at frequencies from17 GHz to 22.3 GHz after 50 km fiber transmission are below 4.8%. This work provides an effective way to realize integrated direct SSB modulation with outstanding features for ROF link.

Optical SSB modulator/frequency shifter with ultralow spurious sidebands.

In this paper, we report and theoretically investigate an optical single-sideband suppressed-carrier (OSSB-SC) modulator/frequency shifter with ultralow spurious sidebands. The novel, to the best of our knowledge, structure is based on three Sagnac interferometers (SIs) and two bidirectional phase modulators (PMs). The two bidirectional PMs are driven by four radio frequency (RF) signals with equal powers and appropriate phases. An OSSB-SC signal with only (4n+1)th-order sidebands, where n is an integer number, is generated. Besides, by applying appropriate RF phase shifts, an OSSB with carrier (OSSB+C) modulator can be obtained whose optical carrier-to-sideband ratio (OCSR) can be tuned by controlling the PMs modulation indices. Also, by using the proposed OSSB-SC modulator in another SI, its OCSR can be tuned by controlling the state of polarization in addition to the modulation indices of the PMs. Since the proposed structure is based on SI and no direct current (DC) bias is required for the PMs, it is robust against environmental perturbations and decreases problems associated with DC bias-drift. In addition, performance of the proposed structure under the power and phase imbalances due to the external non-ideal RF hybrid couplers and the phase imbalance of practical non-ideal nonreciprocal optical phase shifters are investigated. Simulation results using commercial software are also presented which are in good agreement with analytical results.

Bias-Free Silicon-Based Optical Single-Sideband Modulator Without 2nd-Order Sideband

In this paper, a bias-free silicon optical single-sideband (OSSB) modulator without 2nd-order sideband is proposed and theoretically investigated. The proposed modulator is based on a dual-drive Mach-Zehnder modulator (DDMZM) and silicon photonics platform. In the proposed structure, a piece of silicon waveguide is used in one of the arms of the unbalanced DDMZM which acts as a bias for the modulator. So, the operating point of the modulator can be tuned by adjusting the optical wavelength. Analytical models for all the components in the structure are derived and closed-form expressions are found for optical transmission, optical spectrum, etc. Modulator nonlinear behavior under small and large signal modulation is discussed. Conditions on the optical wavelength to have an OSSB modulated signal with/without 2nd-order sideband are found. It has also shown that optical carrier-to-sideband ratio (OCSR) can be tuned by adjusting the applied voltage. By proper adjusting the applied voltage, suppressed carrier or equal carrier to the first sideband can be obtained in OSSB modulator without 2nd-order sideband. Analytical results are verified by simulations and previously reported measurement data. The proposed analytical model is a general model that can be used for any nonlinear electro-optic modulators such as phase and absorption modulators.

High Power Efficiency and Dynamic Range Analog Photonic Link with Suppressed Dispersion-Induced Power Fading

An analog photonic link (APL) based on a dual-polarization dual-parallel Mach–Zehnder modulator (DP-DPMZM) is proposed and experimentally demonstrated. The proposed APL can suppress the nonlinearity of modulator, compensate the power fading induced by dispersion (PFID), and improve the power efficiency of link, leading to large dynamic range. The PFID and the nonlinearity are suppressed by adjusting the phase of optical carrier (OC) and polarization controller (PC), respectively. In addition, by optimizing the optical carrier to sideband ratio (OCSR) in the DP-DPMZM, the output RF power and the power efficiency of link can be significantly improved while the total optical power received by the photodetector (PD) remains constant. An experiments are performed to demonstrate the feasibility of the APL. The results show that an spurious free dynamic range (SFDR) of 103.2 dB·Hz4/5 for 13.73 GHz is achieved after compensating PFID and nonlinearity, which is 32 dB higher than that of a conventional double sideband (DSB) assisted APL. After OCSR optimizing, the output RF power achieves 13-dB improvement, and the SFDR is further raised to 114.1 dB·Hz4/5.

Optical Single-Sideband Modulation with Tunable Optical Carrier-to-Sideband Ratio Based on a Dual-Polarization Mach–Zehnder Modulator and a Polarizer

We propose and experimentally demonstrate a novel and simple approach to implementing optical single-sideband (OSSB) modulation with tunable optical carrier-to-sideband ratio (OCSR) based on a dual-polarization Mach–Zehnder modulator (DPol-MZM) and a polarizer (Pol). In this scheme, the light wave is modulated by radio frequency (RF) signal with SSB modulation in the upper arm of the DPol MZM, while the light wave in the lower arm is not modulated. Therefore, a partial polarization orthogonal SSB (PPO SSB) modulated signal is generated at the output of the DPol MZM without using an optical band-pass filter (OBPF). The PPO SSB signal is then sent to a Pol through a polarization controller. By simply adjusting the polarization state, the OSSB modulation with tunable OCSR can be realized. The experimental results show that an OSSB-modulated signal with adjustable OCSR from – 35 to 55 dB can be achieved. Besides, the proposed scheme features the merit of wideband tunability, where the radio frequency (RF) can be operated from 3 to 17 GHz.

Wideband and Dispersion Immune Microwave Photonic Phase Shifter With Tunable Optical Carrier to Sideband Ratio

A wideband microwave photonic phase shifter (MPPS) based on a dual-polarization dual-parallel Mach–Zehnder modulator (DP-DPMZM) is proposed and experimentally demonstrated. The proposed system has the features of chromatic dispersion (CD) immunity and tunable optical carrier to sideband ratio (OCSR), which can realize long fiber transmission distance with improved power efficiency. It is realized by carrier suppress double sideband (CS-DSB) plus optical carrier (OC) modulation at two orthogonal polarization states. The phases of output RF signals can be tuned continuously and independently by controlling the phases of the OCs. The experiment results show that the continuous 360° phase shifts of output RF signal over 5–25 GHz are both realized in the cases of back-to-back and fiber transmission. No CD-induced power fading and phase severe shaking of the RF signals appear after the dispersion compensating fiber (DCF) of −331 ps/nm at 1545 nm transmission. By optimizing the OCSR of the system, the output RF power efficiency of the proposed scheme realizes significantly improvement when input RF powers and output optical powers are both fixed. The possibility of generating frequency doubling signal is also verified.

Optical single sideband polarization modulator with tunable optical carrier-to-sideband ratio and its applications in microwave photonic phase shifter and optical frequency shifter.

In this paper, a novel scheme to implement an optical single sideband (OSSB) polarization modulator (PolM) is proposed and theoretically investigated. The proposed structure contains two dual-drive Mach-Zehnder modulators inside a Mach-Zehnder interferometer whose input/output optical Y-couplers are replaced by two optical polarization beam splitters/polarization beam combiners. It is shown that by applying four equal power radio-frequency signals with appropriate phases, an OSSB polarization-modulated signal is generated. In addition, $(4n+3)$(4n+3)th-order sidebands, where ${n}$n is an integer, are suppressed without the need for an optical filter. The proposed OSSB PolM can find many applications in microwave photonic (MWP) systems. For instance, by using the proposed OSSB PolM, an OSSB modulator with tunable optical carrier-to-sideband ratio (OCSR), an OSSB-suppressed carrier modulator/optical frequency shifter with ultralow spurious sidebands, and an MWP phase shifter with 360° tunable phase shifts, are proposed and theoretically investigated. Simulation results using commercial software are also presented, which are in good agreement with analytical results.

Improved OSSB modulation with FBG-based acousto-optic tunable filter and Return-to-Zero pulse transmission

Abstract An improved OSSB modulation scheme is proposed in this paper. The improvements are realized by three aspects. Firstly, the fiber Bragg grating based acousto-optic tunable filter (FBG-AOTF) is introduced to tune the Optical Carrier-to-Sideband Ratio (OCSR). Secondly, the Return-to-Zero (RZ) pulse is chosen as the data transmitting pulse. In the end, the OSSB signal with optimized OCSR is separated, and only the optical carrier is modulated by the data. When the transmission distance is within 360 km, the receiving sensitivity of the link in the improved scheme is relatively stable in the range of (−23.6 dBm, −24.1 dBm) with the increase of the distance. While for the original OSSB modulation, under the same condition, the receiving sensitivity is only about −20.0 dBm when the fiber length is 200 km. Not only the OCSR of the OSSB can be optimized, but also the problem of time shift of code edges caused by the fiber dispersion can be eliminated. As a result, both the performance and the stability of the OSSB system can be upgraded in the improved scheme. Two complete simulation systems are built by using the characteristic of the experimentally made FBG-AOTF. And the increased receiving sensitivity of 2.6 dB in the improved scheme further verifies the conclusion.

Low-drift AM–PM based optical single-sideband modulator without 2nd-order sideband with adjustable optical carrier-to-sideband ratio

We propose and theoretically investigated a novel scheme to implement optical single-sideband (OSSB) modulator without 2nd-order sideband with adjustable optical carrier-to-sideband ratio (OCSR). The proposed structure is based on a hybrid concatenated electro-optic amplitude (or intensity) modulator (AM) and phase modulator (PM). The AM consists of a bidirectional PM inside a Sagnac loop (SL). Bias-drift associated problems do not present in the proposed structure because it uses PMs which are bias-free. Furthermore, clockwise (CW) and counter clockwise (CCW) optical signals propagate in the same optical fiber inside the SL, so its robustness against environmental changes increases. The structure is fed by three microwave signals with equal amplitudes and proper phases. The OCSR of the generated OSSB modulated signal can be adjusted by tuning the optical and/or microwave phase shifters and/or the PM modulation indices. The 3rd-order sidebands can also be suppressed by setting the proper optical and microwave phase shifts and adjusting the PM modulation indices.

Chirp-adjustable optical intensity modulator with tunable OCSR based on Sagnac interferometer

Abstract In this paper a novel optical intensity modulator with tunable optical carrier-to-sideband ratio (OCSR) and adjustable chirp parameter is proposed and theoretically investigated. It is based on a bidirectional phase modulator (PM) and a tunable nonreciprocal optical phase shifter in a Sagnac interferometer (SI). Because no bias is necessary for a PM, the bias-drift problems are automatically removed. Besides, clockwise (CW) and counter clockwise (CCW) propagating lights travel in the same path so enhances robustness against environmental perturbations. In this modulator two radio frequency (RF) signals with opposite phases and tunable power ratio are applied to the bidirectional PM. Chirp parameter and OCSR of the modulator can be tuned by tuning the phase shift of the optical phase shifter and RF power ratio. Besides, this modulator can have two complementary balanced outputs. So common-mode noises such as relative intensity noise (RIN) of the laser can be suppressed in a microwave photonic link (MWPL) based on this modulator and using a balanced receiver. This novel modulator can also operate bidirectionally. An intensity modulated optical signal with tunable OCSR and chirp parameter is achieved by adjusting the optical phase shifter and the RF power ratio. The effect of the lossy SI and finite optical extinction ratio on the performance of the proposed modulator is theoretically investigated. In addition applications of the proposed chirp-adjustable modulator to overcome fiber dispersion-induced power penalty in a MWPL is investigated It is shown that by adjusting the chirp parameter, link distance for a given frequency (or bandwidth for a given link distance) doubles.

Bias-free OSSB modulator with tunable OCSR

Abstract In this paper, a novel approach to realize optical single-sideband (OSSB) modulation with tunable optical carrier-to-sideband ratio (OCSR) is proposed and theoretically demonstrated based on a bidirectional optical phase modulator (PM) in a Sagnac interferometer (SI). In the proposed OSSB modulator, two RF signals with appropriate phase shifts are applied to the bidirectional PM. An OSSB modulated signal with tunable OCSR is obtained by adjusting the optical phase difference between clockwise (CW) and counterclockwise (CCW) propagating lights inside the SI and the PM modulation index. Since no bias is necessary for a PM, the proposed scheme is bias-drift free. In addition, the two counter-propagating lights travel in the same path in the SI so enhances robustness against environmental perturbations. It is shown that the proposed OSSB modulator can operate bidirectionally with dual complementary balanced outputs. It is observed that by using a 120° RF hybrid coupler, the −2nd sideband is suppressed in addition to the + 1 st sideband, results in performance improvement of the proposed OSSB modulator. Besides, effects of the power and phase imbalances of practical non-ideal 90- or 120-degree RF hybrid couplers and lossy Sagnac loop and non-ideal lossy optical coupler with unequal power splitting/combining on the performance of the proposed scheme are investigated.

A novel method to realize optical single sideband modulation with tunable optical carrier to sideband ratio

AbstractAn optical single sideband (OSSB) modulation method with tunable optical carrier to sideband ratio (OCSR) is proposed and experimentally demonstrated based on a dual‐polarization Mach–Zehnder modulator (DPol‐MZM). An orthogonally polarized double sideband (DSB) signal output from the DPol‐MZM is filtered out the carrier and one sideband utilizing an optical bandpass filter (OBPF). Then the orthogonally polarized OSSB signal is transformed into a linear polarized OSSB signal by a polarizer (Pol). By changing the polarization direction of the Pol utilizing a polarization controller (PC), the tunable OCSR can be achieved. In the demonstrated experiments, an OSSB modulation is implemented over the wide microwave frequency range of 5–30 GHz. The OCSR is tuned from −30 dB to infinity. The proposed OSSB modulation method is advantageous in many respects as large tunable OCSR range, wide operation bandwidth, and simple structure.

Optical single sideband modulation with tunable optical carrier-to-sideband ratio using a modulator in a Sagnac loop

We propose and experimentally demonstrate a cost-effective approach to achieve optical single sideband (OSSB) modulation with tunable optical carrier-to-sideband ratio (OCSR) based on a bi-directional use of an OSSB modulator. The key concept of the proposed scheme is to use an OSSB modulator which is incorporated in a Sagnac loop where a light wave is modulated in the form of SSB modulation along the clock-wise direction while the orthogonal light is not modulated along the counter clock-wise direction due to the velocity mismatch. The signals along two directions are combined together at the output of the Sagnac loop via a polarization beam splitter (PBS). Thus a partial orthogonal SSB (PO-SSB) modulated signal is obtained. The generated signal is then sent into a polarization controller (PC) followed by a polarizer. An OSSB-modulated signal with tunable OCSR from −10 to 27dB is achieved by adjusting the PC in the experiment. The proposed scheme is wavelength independent, which can support low microwave bandwidth and achieve flexible wavelength tunability.

All optical multi-wavelength single-sideband modulated WDM radio-over-fiber systems by introducing a Sagnac loop filter

Abstract All optical multi-wavelength single-sideband modulated WDM radio-over-fiber system by introducing a homemade Sagnac loop filter is proposed and analyzed. The process of the Sagnac loop filter improves the link performance by balancing optical carrier-to-sideband ratio (OCSR). Numerical analysis and simulations are carried out to verify the theory. It is found that the receiver sensitivity is improved by 2.16 dB with OCSR reducing from 4.40 dB to 1.13 dB. In theory, the optimal value of the OCSR can also be achieved. Besides, periodic fluctuation of the RF power is mitigated.

Simultaneous Realization of Frequency Multiplication and Single Sideband Modulation by Exploiting Nonlinear Birefringent Effect

We theoretically analyze and experimentally demonstrate a photonic scheme to realize light-controlled microwave frequency multiplication and single sideband (SSB) modulation simultaneously. By properly choosing operational parameters of the orthogonal polarized pump light to stimulate the nonlinear birefringent effect in highly nonlinear fiber, the proposed scheme can realize different functions. One is to generate a high-frequency microwave signal with a tunable multiplication factor among 1, 2, 4, and 8. The other is to obtain an SSB signal with tunable optical carrier to sideband ratio (OCSR) or simultaneous suppression of the − first- and + second-order sidebands. We demonstrate the frequency multiplication function theoretically. A frequency-octupled signal at 144 GHz is obtained. The SSB modulation from 10 to 40 GHz with OCSR ranging from −8.7 to 26.7 dB is demonstrated by experiment. The SSB modulation with simultaneous suppression of the − first- and + second-order sidebands is also observed. The proposed approach operates easily and no wavelength-depended device is required. Results show that it has potential applications in improving the transmission of photonic microwave signal processing systems.

Optical single sideband modulation based on a high-order birefringent filter using cascaded Solc-Sagnac and Lyot-Sagnac loops.

We propose and experimentally demonstrate a simple and flexible photonic approach to implementing single sideband (SSB) modulation based on optical spectral filtering. The high-order birefringent filter is realized through the cascaded Solc-Sagnac and Lyot-Sagnac loops. By adjusting the rotation angle of the polarization controller (PC), the notch position to remove undesired sidebands changes. The frequency for SSB modulation varies accordingly. The periodical response of the filter spectrum allows both the carrier wavelength and the optical carrier to sideband ratio (OCSR) to be tunable. SSB modulation over a frequency range from 5 to 40 GHz and tunable OCSR ranging from -9.174 to 34.408 dB are obtained. The significant merits of the proposed approach are the simple structure, easy operation, large frequency range, tunable OCSR, and wavelength independence. The approach has potential applications in optimizing the transmission performance of photonic microwave signal processing systems.