Mach-Zehnder Research Articles

Broadband photonic frequency measurement based on the frequency-to-carrier phase-shifted mapping and finite extinction ratio

A novel frequency measurement is proposed based on frequency-to-carrier phase-shifted mapping and finite extinction ratio. Due to the frequency dependence of the dispersion phase shift, carrier phase-shifted double-sideband modulation and dispersive effect are combined for effective mapping. Additionally, the finite extinction ratio is introduced to mitigate the influence of unideal parameters on measurement error. To achieve this, a dual-parallel Mach-Zehnder modulator and a dispersion compensating fibre are combined, and a power minimization problem is formulated to establish a mapping table. Theoretical analysis and simulation results show the frequency measurement range over 30 GHz with errors below 180 MHz. Furthermore, the measurement error is reduced below 40 MHz after considering the finite extinction ratio.

A Tunable and Switchable Multi-Wavelength Erbium-Doped Fiber Ring Laser Enabled by Adjusting the Spectral Fringe Visibility of a Mach-Zehnder Fiber Interferometer

This paper presents a tunable, switchable multi-wavelength emission from an erbium-doped fiber ring laser, enabled by adjusting the spectral fringe visibility of a fiber interferometer filter. The filter is formed with specially designed concatenated tapered fibers to configure a Mach-Zehnder fiber interferometer (MZFI). The laser emission is highly flexible and reconfigurable, allowing for tuning between single- and dual-wavelength operation. The laser can switch sequentially from one up to six wavelengths by fixing the curvature and adjusting the polarization state. The lasing emission is generated over a stable wavelength range between 1559.59 nm and 1563.54 nm, exhibiting an optical signal-to-noise ratio (OSNR) exceeding ~35 dB. The performance of amplitude and wavelength fluctuations were evaluated, indicating an appropriate stability of ~3 dB and a shift less than 0.1 nm within a 45 min period at room temperature. A detailed comparison with the literature is given.

Highly Sensitive Gas Pressure Sensing with Temperature Monitoring Using a Slightly Tapered Fiber with an Inner Micro-Cavity and a Micro-Channel.

A highly sensitive optical fiber gas pressure sensor with temperature monitoring is proposed and demonstrated. It is based on a slightly tapered fiber with an inner micro-cavity forming an in-fiber Mach-Zehnder interferometer (MZI), and a micro-channel is drilled into the lateral wall of the in-fiber micro-cavity using a femtosecond laser to allow gas to flow in. Due to the dependence of the refractive index (RI) of air inside the micro-cavity on its gas pressure and the high RI sensitivity of the MZI, the device is extremely sensitive to gas pressure. To prevent fiber breakage, the MZI is housed in a silicate capillary tube with an air inlet. Multiple modes are excited by slightly tapering the inner micro-cavity, and the resonance dips in the sensor's transmission spectrum feature different linear gas pressure and temperature responses, so a sensitivity matrix algorithm can be used to achieve simultaneous demodulation of two parameters, thus resolving the temperature crosstalk. As expected, the experimental results demonstrated the reliability of the matrix algorithm, with pressure sensitivity reaching up to ~-12.967 nm/MPa and temperature sensitivity of ~89 pm/°C. The features of robust mechanical strength and high air pressure sensitivity with temperature monitoring imply that the proposed sensor has good practical and application prospects.

Simultaneous generation of dual 16-QAM-modulated millimeter-wave signals enabled by precoding-based optical I/Q modulation and single photodetector detection

Integration of quadrature-amplitude-modulation (QAM) modulation and millimeter-wave (mm-wave) technology ensures a concise enhancement of transmission capacity and peak data rates in radio-over-fiber (RoF) systems. Nevertheless, with the continuous development of new-generation mobile applications, higher demands on access flexibility have also been formulated. We propose a novel, to the best of our knowledge, dual 16-QAM-modulated mm-wave signal generation scheme for the simultaneous generation and transmission of two independent 16-QAM signals with different carrier frequencies, enhancing the channel capacity, spectrum efficiency, and access flexibility of the RoF systems. In our proposed scheme, the generation of the optical dual 16-QAM-modulated mm-wave signals is implemented by a precoding-based optical in-phase/quadrature (I/Q) modulator operating at optical carrier suppression (OCS) mode, and their detection is implemented by a single photodetector (PD). At the transmitter side, two independent driving QAM vector radio-frequency (RF) signals with precoding, generated via digital signal process (DSP), are fed into two parallel Mach–Zehnder modulators (MZMs) of an I/Q modulator to implement OCS modulation. The I/Q modulator generates optical carriers carrying both information from different QAM signals simultaneously. After square-law detection in a single PD, we generate the desired dual 16-QAM-modulated mm-wave signals with photonic frequency-doubling and recover the original 16-QAM signals without information distortion. Based on our proposed scheme, we experimentally demonstrate the simultaneous generation and transmission of 2-Gbaud dual 16-QAM-modulated mm-wave signals with carrier frequencies of 26 GHz and 40 GHz, respectively. After transmission over 10-km single-mode fiber (SMF) link and 1.2-m wireless link, the dual 16-QAM signals can be successfully separated and demodulated by a common DSP. Bit-error ratios (BERs) of both recovered 16-QAM signals can reach the hard-decision forward-error-correction (HD-FEC) threshold of 3.8×10−3. Compared with the existing schemes, our scheme only requires one-half of the driving frequency to generate dual 16-QAM-modulated mm-wave signals with the same carrier frequency. The result indicates that our proposed scheme can lower bandwidth requirements for optoelectronics devices, along with implementing better spectral efficiency and receiver sensitivity, which is more applicable to the demands for increased system capacity and multi-frequency access flexibility in future systems.

Temperature-compensated optical fiber sensor for urea detection based on the femtosecond laser-inscribed process

This study demonstrates a low crosstalk temperature-compensated fiber-based urea sensor based on the hybridization of surface plasmon resonance (SPR) and Mach-Zehnder interferometer (MZI). In this sensing structure, an inscribed waveguide and a semi-open cavity are processed by a femtosecond laser in an optical fiber to excite an SPR signal sensitive to refractive index (RI) and an MZI signal highly sensitive to temperature. Based on the temperature detected by the MZI, the influence of temperature on the SPR signal’s RI detection can be calculated and compensated for using a sensitivity matrix. Based on this precise RI detection of the SPR signal, we have achieved selective urea detection by bio-functionalizing the sensor surface using the urease self-assembly method. The sensitivity and limit of detection for urea are 7.98 (nm/mmol/L) and 0.13 mmol/L, respectively, which are better than those in previous studies. Compared to previous temperature compensation studies, this SPR + MZI sensing method using femtosecond laser processing significantly improves the temperature sensitivity of the MZI signal, which reduces crosstalk and ensures a sufficient working range for the SPR signal. Therefore, this sensor is crucial for advancing high-precision temperature compensation detection, high-sensitivity biological detection, and the future application of the femtosecond laser technology.

Wavelength-Switchable Ytterbium-Doped Mode-Locked Fiber Laser Based on a Vernier Effect Filter

A wavelength-switchable ytterbium-doped mode-locked fiber laser is reported in this article. Two Mach–Zehnder interferometers (MZIs, denoted as MZI1, MZI2) with close free spectral ranges (FSRs) are connected in series to form a Vernier effect sensor. By utilizing the filtering effect of the Vernier effect sensor, the wavelength-switchable output of an ytterbium-doped mode-locked fiber laser is realized. When the 3 dB bandwidth of the Vernier effect filter is set to be 5.31 nm around 1073.42 nm, stable dissipative solitons are obtained. Stretching MZI1 horizontally, the central wavelengths of the pulses can be switched among 1073.42 nm, 1055.38 nm, and 1036.22 nm, with a total tunable central wavelength range of 37.2 nm. When the 3 dB bandwidth of the Vernier effect filter is set to be 4.07 nm, stable amplifier similaritons are obtained. Stretching MZI1 horizontally, the central wavelengths of the pulses are switchable among 1072.71 nm, 1060.15 nm, 1048.92 nm, and 1037.26 nm, with a total tunable central wavelength range of 35.15 nm. Compared with traditional fiber interference filters, the Vernier effect filter has a higher sensitivity, making wavelength switching more convenient and providing a wider tuning range for the ytterbium-doped mode-locked fiber laser.

A microwave permittivity sensor on a Mach–Zehnder Interferometer of: Spoof surface plasmon polariton waveguides

A microwave Mach–Zehnder Interferometer (MZI) sensor for the permittivity characterization of liquids is presented in this paper. Two spoof surface plasmon polariton (SSPP) waveguide transmission lines (TLs) composed of compact spiral units is utilized in a differential architecture. Loading a dielectric material on the sensing arm induces a phase difference between the two arms, leading to periodic interference dips on the transmission curve. The spectral positions and magnitudes of these dips change with different dielectric materials. Due to the nonlinear dispersion characteristics of the SSPP TL, the proposed SSPP-based MZI sensor achieves improved sensitivity compared with microstrip-based MZI sensor. To validate the sensor’s performance, six different oil samples and a set of adulterated oils were measured, incorporating a polydimethylsiloxane (PDMS) channel on the sensing arm. The variations in the spectral position and magnitude of the interference dip centered at 3.2 GHz are employed to determine the dielectric constants and loss tangents of the tested oils. The sensitivity in measuring the dielectric constant is 3.61 %. The maximum errors for measuring the dielectric constant and the loss tangent are 4.3 % and 6.7 %, respectively. Compared to other sensors for liquid oil detection, the proposed sensor provides a relatively high sensitivity while maintaining a low liquid volume, making it a promising candidate for permittivity measurements of liquid analytes and solid materials with similar dielectric constants.

Dual-mode 2 × 2 electro-optic switch on a SOI platform.

Mode-division multiplexing (MDM) technology is promising for enhancing the capacity of communication networks. In this Letter, we demonstrate a dual-mode 2 × 2 electro-optical switch on a silicon-on-insulator platform. The dual-mode Mach-Zehnder interferometer switch comprises of four p-i-n phase shifters and two mode-insensitive multimode interferences that can be used for TE0 and TE1, simultaneously. With π/2 phase shifters introduced, push-pull like operation enables the power consumption lower than 2.15 mW. The average insertion loss of the switch in "cross" and "bar" states are 1.31 dB ± 0.19 dB for the TE0 mode and 3.39 dB ± 0.16 dB for the TE1 mode, respectively. The cross talk is less than -16.47 dB in the C band. The compact dual-mode switch is promising to conduct a large-scale, flexible MDM system on chip.

Graphene Phase Modulators Operating in the Transparency Regime.

Next-generation data networks need to support Tb/s rates. In-phase and quadrature (IQ) modulation combine phase and intensity information to increase the density of encoded data, reduce overall power consumption by minimizing the number of channels, and increase noise tolerance. To reduce errors when decoding the received signal, intersymbol interference must be minimized. This is achieved with pure phase modulation, where the phase of the optical signal is controlled without changing its intensity. Phase modulators are characterized by the voltage required to achieve a π phase shift, Vπ, the device length, L, and their product, VπL. To reduce power consumption, IQ modulators are needed with <1 V drive voltages and compact (sub-cm) dimensions, which translate in VπL < 1Vcm. Si and LiNbO3 (LN) IQ modulators do not currently meet these requirements because VπL > 1Vcm. Here, we report a double single-layer graphene (SLG) Mach-Zehnder modulator (MZM) with pure phase modulation in the transparency regime, where optical losses are minimized and remain constant with increasing voltage. Our device has VπL ∼ 0.3Vcm, matching state-of-the-art SLG-based MZMs and plasmonic LN MZMs, but with pure phase modulation and low insertion loss (∼5 dB), essential for IQ modulation. Our VπL is ∼5 times lower than the lowest thin-film LN MZMs and ∼3 times lower than the lowest Si MZMs. This enables devices with complementary metal-oxide semiconductor compatible VπL (<1Vcm) and smaller footprint than LN or Si MZMs, improving circuit density and reducing power consumption by 1 order of magnitude.

Silicon photonics foundry fabricated, slow-light enhanced, low power thermal phase shifter

In this research, we developed a low-power silicon photonics foundry-fabricated slow-light thermal phase shifter (SLTPS) where the slow-light (SL) effect is achieved using an integrated Bragg grating (BG) waveguide. Heating the grating induces a red shift in the transmission spectrum, leading to an increased group index ng during operation, which facilitates a further reduction in the voltage needed for a π phase shift, i.e., Vπ. Additionally, we investigated a compact Mach–Zehnder Interferometer (MZI) that incorporates the SLTPS in both arms with a phase shifter length of 50 μm. A detailed theoretical analysis was conducted to address the non-idealities of the SL-MZI due to uneven optical power splitting and unbalanced loss in the two MZI arms. Vπ and power consumption for a π phase shift (Pπ) of the SL-MZI were quantified for operation in the slow-light regime, demonstrating a Vπ of 1.1 V and a Pπ of 3.63 mW at an operational wavelength near the photonic band edge. The figure of merit Pπ×τ is commonly used to assess the performance of thermal optical switches. The SL-MZI in this work has achieved a low Pπ×τ of 5.1 mW μs with an insertion loss of 4.4 dB, indicating a trade-off with the Vπ reduction.

High-Linearity Dual-Parallel Mach–Zehnder Modulators in Thin-Film Lithium Niobate

Microwave photonic (MWP) systems are inseparable from conversions of microwave electrical signals into optical signals, and their performances highly depend on the linearity of electro-optic modulators. Thin-film lithium niobate (TFLN) is expected to be an ideal platform for future microwave photonic systems due to its compact size, low optical loss, linear electro-optic effect, and high bandwidth. In this paper, we propose a TFLN modulator with a low voltage–length product (VπL) of 1.97 V·cm and an ultra-high-linearity carrier-to-distortion ratio (CDR) of 112.33 dB, using a dual-parallel Mach–Zehnder interferometer configuration. It provides an effective approach to fully suppress the third-order intermodulation distortions (IMD3), leading to 76 dB improvement over a single Mach–Zehnder modulator (MZM) in TFLN. The proposed TFLN modulator would enable a wide variety of applications in integrated MWP systems with large-scale integration, low power consumption, low optical loss, and high bandwidth.

Erbium-Doped Tunable Fiber Laser Based on a Vernier Effect Filter

A novel vernier effect filter is designed utilizing two cascaded Mach–Zehnder interferometers (MZIs). Integrating the filter into an erbium-doped fiber laser (EDFL), the tunability of laser wavelength is achieved. Each MZI comprises two sequentially interconnected 3 dB optical couplers (OCs), where the incoming light is initially split into two arms at the first OC and subsequently recombined at the second OC. Interference occurs due to the optical path difference between these two beams. Notably, the two MZIs exhibit closely matched free spectral ranges (FSRs), leading to the formation of a broadened envelope in the superimposed spectrum. By delicately adjusting the optical path difference between the two arms of one MZI, a little drift of the interference spectrum is induced. This small amount of drift, in turn, triggers a significant movement of the envelope, giving rise to the so-called vernier effect. Integrating the vernier effect filter into an EDFL, the wavelength of the fiber laser can be tuned from 1542.56 nm to 1556.62 nm, with a tuning range of 14.06 nm. Furthermore, by employing a high-precision stepper motor, a remarkable tuning accuracy of 0.01 nm is attainable. The side mode suppression ratio of all wavelengths is above 55 dB. In comparison to reported tunable fiber lasers utilizing MZI filters, the proposed fiber laser in this study offers enhanced precision and a more user-friendly tuning process.

A fixed phase tunable directional coupler based on coupling tuning

The field of photonic integrated circuits has witnessed significant progress in recent years, with a growing demand for devices that offer high-performance reconfigurability. Due to the inability of conventional tunable directional couplers (TDCs) to maintain a fixed phase while tuning the reflectivity, Mach-Zehnder interferometers (MZIs) are employed as the primary building blocks for reflectivity tuning in constructing large-scale circuits. However, MZIs are prone to fabrication errors due to the need for perfect balanced directional couplers to achieve 0-1 reflectivity, which hinders their scalability. In this study, we introduce a design of a TDC based on coupling constant tuning in the thin film Lithium Niobate platform and present an optimized design. Our optimized TDC design enables arbitrary reflectivity tuning while ensuring a consistent phase across a wide range of operating wavelengths. Furthermore, it exhibits fewer bending sections than MZIs and is inherently resilient to fabrication errors in waveguide geometry and coupling length compared to both MZIs and conventional TDCs. Our work contributes to developing high-performance photonic integrated circuits with implications for various fields, including optical communication systems and quantum information processing.

Recent Progress in Electro‐Optic Modulators: Physical Phenomenon, Structures Properties, and Integration Strategy

AbstractElectro‐optic modulators (EOMs), serving as indispensable components within photonic integrated circuits, are essential for enabling energy‐efficient, high‐speed, and high‐capacity optical communication systems. This review illustrates the principal physical phenomenon exploited in EOMs and provides a comprehensive analysis of the cutting‐edge EOMs featuring interference structures (Mach–Zehnder modulators and Michelson‐interferometer modulators) and resonance structures (microring modulators, racetrack modulators, and photonic crystal modulators). The comparative analysis of the performance merits and limitations in EOMs is presented, highlighting the combination of diverse electro‐optic material compositions with different optical structures, which reveals a promising integration strategic so as to pursue a trade‐off in modulation performance. It is contributed to the ongoing discourse on optimizing EOMs for the subsequent communication technologies and the advancement of photonic chips.

Highly Efficient Slow‐Light Mach–Zehnder Modulator Achieving 0.21 V cm Efficiency with Bandwidth Surpassing 110 GHz

AbstractHigh‐speed electro‐optic modulators are key components in modern communication networks and various applications that require chip‐scale modulation with large bandwidth, high modulation efficiency, and compact footprint. However, fundamental trade‐offs make it challenging to achieve these metrics simultaneously, and thus new methodologies must be explored. To this end, a Mach–Zehnder modulator harnessing slow‐light waveguides and capacitively loaded slow‐wave electrodes are presented on silicon‐nitride‐loaded lithium niobate on an insulator platform. The increased group index and reduced microwave loss significantly improve the modulation efficiency. With the 1‐mm‐length modulation section, a low half‐wave voltage length product Vπ·L of 0.21 V cm is obtained, which is one order of magnitude smaller than that of conventional thin film lithium niobate Mach–Zehnder modulators, and a modulation bandwidth of surpassing 110 GHz is achieved. The digital signal processor‐free non‐return‐to‐zero signal and eight‐level pulse amplitude modulation signal of up to 180 and 300 Gbps, respectively, are generated by the modulator, which provides ultra‐large bandwidth, ultra‐high efficiency, and a compact solution for next‐generation electro‐optic systems.

Real-Time Reconfigurable Radio Frequency Arbitrary-Waveform Generation via Temporal Pulse Shaping with a DPMZM and Multi-Tone Inputs

Benefitting from a large bandwidth and compact configuration, a time-domain pulse-shaping (TPS) system provides possibilities for generating broadband radio frequency (RF) arbitrary waveforms based on the Fourier transform relationship between the input–output waveform pair. However, limited by the relatively low sampling rate and bit resolution of an electronic arbitrary-waveform generator (EAWG), the diversity and fidelity of the output waveform as well as its reconfiguration rate are constrained. To remove the EAWG’s limitation and realize dynamic real-time reconfiguration of RF waveforms, we propose and demonstrate a novel approach of RF arbitrary-waveform generation based on an improved TPS system with an integrated dual parallel Mach–Zehnder modulator (DPMZM) and multi-tone inputs. By appropriately adjusting the DC bias voltages of DPMZM and the power values, as well as the center frequencies of the multi-tone inputs, any desired RF arbitrary waveform can be generated and reconfigured in real time. Proof-of-concept experiments on the generation of different user-defined waveforms with a sampling rate up to 27 GSa/s have been successfully carried out. Furthermore, the impact of modulation modes and higher-order dispersion on waveform fidelity is also discussed in detail.

PDMS-assisted Mach–Zehnder interferometer for simultaneous sensing of strain and temperature based on hollow-core fibers

A PDMS-assisted Mach-Zehnder interferometric (MZI) fiber-optical sensor based on hollow-core fibers (HCF) is proposed and experimentally demonstrated for the simultaneous detection of strain and temperature. The transmission spectral evolution in response to the applied strain and temperature sensor are investigated. Experimental results suggest that our proposed sensor exhibits strain and temperature sensitivities up to −40.95 dB/N and 2.5 nm/°C for the sensing ranges of 0.01 N to 0.08 N and 35 °C to 42 °C, respectively. Moreover, crosstalk analyses were performed to discriminate the strain and temperature measurement. The proposed sensor possesses desirable advantages such as high sensitivity, easy-fabrication, and low-crosstalk, which paves new way toward developing photonic devices for strain/temperature dual-parameter sensing and related applications.

Performance Comparison of Seed Generation Techniques of Stimulated Brillouin Scattering-Based Microwave Photonics Amplifier and Filter

This work presents a Brillouin amplification performance comparison of seed generation techniques using double-sideband suppressed carrier (DSB-SC) and single-sideband suppressed carrier (SSB-SC) modulations. The SSB-SC is obtained using an optical bandpass filter (OBPF) and in-phase and quadrature Mach-Zehnder modulator (IQ-MZM). All three techniques provide high amplification performance with optical signal-to-noise ratio (OSNR) enhancement of 37.47 dB, 33.14 dB, and 32.67 dB using DSB-SC, SSB-SC/OBPF, and SSB-SC/IQ-MZM, respectively. The best seed generation technique is using the DSB with a signal amplification of 62.47 dB. The technique presents ~4 dB higher OSNR enhancement due to the dual-energy transfer obtained from the beating process of the DSB than SSB. A ~3 dB OSNR reduction is found when pump linewidth (LW) was changed from 1kHz to 50 MHz, which suggests using a low-cost pump source whenever the OSNR reduction is not critical. The work also shows that the three techniques required 10 dBm stimulated Brillouin scattering threshold (SBST) to stimulate the process. An additional analysis of DSB-SC shows that a high-carrier suppression during the seed generation technique using MZMs is insignificant to the amplification performance. The high-carrier suppression produces a high seed signal power that distorts the Brillouin gain spectrum (BGS) and the pump depletion region, hence reducing the Brillouin gain (BG). Since carrier suppression is not a primary consideration, a cost-effective MZM with a modest extinction ratio requirement is allowed. The relaxed requirement of the pump’s linewidth and MZM’s extinction ratio suggest a cost-effective development of the SBS-based optical amplifier with narrow filter bandwidth.

Si-waveguide-based optical power monitoring of a 2 × 2 Mach-Zehnder interferometer based on a InGaAsP/Si hybrid MOS optical phase shifter.

Transparent in-line optical power monitoring in Si programmable photonic integrated circuits (PICs) is indispensable for calibrating integrated optical devices such as optical switches and resonators. A Si waveguide (WG) photodetector (PD) based on defect-mediated photodetection is a promising candidate for a transparent in-line optical power monitor, owing to its simplicity and ease of integration with a fully complementary metal-oxide-semiconductor (CMOS)-compatible process. Here, we propose a simple optical power monitoring scheme for a 2 × 2 Mach-Zehnder interferometer (MZI) optical switch based on InGaAsP/Si hybrid MOS optical phase shifters. In the proposed scheme, a low-doped p-type Si WG PD with a response time of microseconds is utilized as a transparent in-line optical power monitor, and the ground terminal of the MOS optical phase shifter is shared with that of the Si WG PD to enable the simple monitoring of the output optical power of the MZI. Based on this scheme, we experimentally demonstrate that the output optical power of a 2 × 2 MZI can be simply monitored by applying a bias voltage to the Si slabs formed at the output WGs of the MZI without excess optical insertion loss.

Detecting optical frequency in weak signal light using injection-induced frequency modulation

In fiber-optic sensing, long-distance transmission and demodulation of weak signal light (SL) are critical. This study improves the system by relocating an external cavity tunable laser (ECTL) to the transmission end. Directly injecting the SL into the ECTL modulates the output light and uses the high-power ECTL light for transmission, reducing loss. At the demodulation end, an orthogonal Mach-Zehnder interferometer (MZI) and an H13C14N gas absorption cell (GAC) extract SL wavelength information. Results demonstrate effective modulation with SL intensity as low as 1 nW and a demodulation resolution of up to 10 MHz, offering significant improvements in long-distance fiber-optic sensing.