External Intensity Modulator Research Articles

10 Hz Stable Mode-locked Er-Fiber Laser

Abstract: In this paper, a stable output at the reduction repetition rate from 80 MHz mode-locked Er-doped fiber laser (EDFL) was demonstrated by adjusting the operating point for external LiNbO3 Mach-Zehnder‎ electro-optic intensity modulator to suppress harmonic mode-locked pulses. More than 70% of the laser beam was launched into the external pigtail Mach-Zehnder intensity modulator using single mode fiber with collimating lens and fiber (ferrule connector) FC adapter. The optimum values of Mach-Zehnder modulator bias voltages V0 and Vπ (half wave voltage) which resulted in maximum and minimum laser output power to leave the modulator were found at 6.5 and 0.29 V, respectively. The best modulation was achieved for the RF modulating signal with an opening time of about 8 ns. For driving voltage equal to Vπ, a clean mode-locked pulse train was generated with a pulse shape and energy similar to the input pulse, albeit with a low repetition rate. The minimum repetition rate reached as low as 10 Hz, resulting in pulse energy and peak power of 0.32 nJ and 1.5 kW, respectively. Second-harmonic generation of the low repetition rate pulse train was also produced by using a PPLN crystal. This opens up the potential for using this laser in single photon detection experiments and as a seed laser for many applications. Keywords: Er-fiber laser, Passively mode-locked fiber laser, Nonlinear polarization rotation, Mach-Zehnder modulator, Fast electro-optic modulator. PACS: Fiber lasers, 42.55.Wd, Mode locking, 42.60.Fc.

Practical Analysis of Sending or Not-Sending Twin-Field Quantum Key Distribution with Frequency Side Channels

The twin-field quantum key distribution (TF-QKD) and its variants can overcome the fundamental rate-distance limit of QKD. However, their physical implementations with the side channels remain the subject of further research. We test the side channel of a type of external intensity modulation that applies a Mach–Zehnder-type electro-optical intensity modulator, which shows the distinguishability of the signal and decoy states in the frequency domain. Based on this security loophole, we propose a side-channel attack, named the passive frequency-shift attack, on the imperfect implementation of the sending or not-sending (SNS) TF-QKD protocol. We analyze the performance of the SNS protocol with the actively odd-parity pairing (AOPP) method under the side-channel attack by giving the formula of the upper bound of the real secret key rate and comparing it with the lower bound of the secret key rate under Alice and Bob’s estimation. The simulation results quantitatively show the effectiveness of the attack on the imperfect devices at a long distance. Our results emphasize the importance of practical security at the light source and might provide a valuable reference for device selection in the practical implementation of the SNS protocol.

Performance optimization of RSOA based mm-wave radio-over-fibre access network

In this paper, a Reflective Semiconductor Optical Amplifier based, Radio-over-Fibre access network configuration has been proposed to feed future millimeter-wave radio systems. The system architecture combines several approaches to overcome the challenges of millimeter-wave signal transmission. Reflective semiconductor optical amplifier modulator realizes a colorless and relatively cost-effective Remote Antenna Unit. The same optical carrier is used for both downlink and uplink. Optical single-sideband modulation is used at the downlink, which is robust against chromatic dispersion, but the complex realization of this modulation format is not possible at the Remote Antenna Unit. Optical intermediate frequency transmission is applied at the uplink direction, and the required local oscillator signal originates from the central station. The critical element is the reflective optical amplifier, as it compensates for the optical loss and works as an external intensity modulator. The operation of the reflective optical amplifier is modeled by multisection rate and wave equation-based description. The amplification and modulation behaviors of an available reflective optical amplifier are also measured. The experimental work validated the colorless operation and the quality of the modulation versus bias current and input optical power. Finally, system simulation was realized. The uplink and downlink power budgets were balanced, and optimal values for the optical coupling rate and RSOA bias current have been selected.

M-QAM signal transmission at the photonically generated K-band over thermal-induced turbulent FSO links with different turbulence distributions

We present a theoretical and experimental study on the impact of different thermal-induced free-space turbulence distributions on the M-quadrature amplitude modulation (M-QAM) signal transmission in radio frequency K-band over hybrid optical links of standard single mode fiber (SSMF) and free-space optics (FSO). Frequency multiplication using an external intensity modulator biased at the null transmission point has been employed to photonically generate radio signals at a frequency of 25 GHz , included for the frequency bands for fifth-generation (5G) mobile networks. Moreover, extensive simulations have been performed for 10Gb/s with 4-, 16-, and 64-QAM over 5 km of SSMF and 500 m long FSO channels under scenarios with different turbulence levels and distributions. Proof-of-concept experiments have been conducted for 20 MHz with 4- and 64-QAM over 5 km of SSMF and 2 m long FSO channels under turbulence conditions. Both theoretical and experimental systems have been analyzed in terms of error vector magnitude (EVM) performance showing feasible transmission over the hybrid links in the received optical power range. Non-uniform turbulence distributions are shown to have a different impact on M-QAM modulation formats, i.e., turbulence distributions with higher strength in the middle of the FSO link reveal a 1.9 dB penalty when using 64-QAM signals compared to a 1.3 dB penalty using 4-QAM signals, whereas higher penalties have been measured when 4-QAM format is transmitted over turbulence distributions with larger magnitude in the second half of the FSO link. The results have been validated by theoretical predictions and lead to practical consequences on future networks' deployment.

InAsSb-based heterostructures for infrared light modulation

We demonstrate the strong modulation of the long wave infrared transmission of GaInSb/InAsSb/AlInAsSb heterostructures under carrier injection. This results in the population of states in the conduction band of the narrow-gap layer and changes the absorption and refractive index over a broad wavelength range. At λ = 8.6-μm, a single-pass intensity modulation depth up to 9% was demonstrated at T = 77 K for a 1-μm–thick InAs0.58Sb0.42 absorber. By modeling the structure, we show that this corresponds to the electron quasi-Fermi level rising up to 30 meV above the conduction band edge. Due to the strong band-to-band absorption, the change in the quasi-Fermi level is accompanied by a modulation of the refractive index by up to 0.06 in the spectral range below the energy gap of the alloy. This change is orders of magnitude greater than what is achievable in conventional electro-optic materials and allows, for example, the external intensity modulation of long-wave infrared laser sources with a high extinction ratio and a nanosecond-scale time response. Low power requirements make it possible to develop arrays of integrated devices for optical beam steering and shaping.

Off-set Filtering Effect in SOA Based Optical Access Network

In-line and reflective semiconductor optical amplifiers (SOA and RSOA) can be effectively utilized as external optical intensity modulators. However the limited modulation bandwidth affects the application possibilities, because next generation access networks desire high trans- mission speed. Bandwidth enhancement can be achieved by means of optical equalization thanks to the chirping char- acteristics of semiconductor optical amplifier. The limited modulation transfer function can be combined with the converted phase modulation transfer function. Investiga- tion of optical filter with optimal slope is a crucial chal- lenge in this approach. The presented simulations describe the slope dependence of the modulation band. The model can take into account the different type of realizable opti- cal filters. However there is a trade-off between modula- tion efficiency and modulation bandwidth due to the extra insertion loss of the set-off optical filter. In the paper theo- retical and simulation investigations are presented with the result of improved modulation bandwidth.

Wideband Analog Transmission System Based on the External Intensity Electro-Optic Modulator

The work deals with design and integration of an analog electro-optic transmission system suitable for some specialized tasks of diagnostics and measurements. The system is based on principle of external intensity modulation of fiber guided laser radiation. Besides wideband and almost lossless transmission the system tolerates extreme length of the transmitting medium and ensures ultimate galvanic barrier between the input and output.

Direct and External Intensity Modulation in OFDM RoF Links

Radio-over-fiber (RoF) systems comprise light modulation and transmission of millimeter-wave signals over fiber links. The aim of this study is to investigate the performance of external and direct intensity modulation in RoF links and to analyze the drawbacks induced by different components of the optical system. In external modulation, the Mach-Zehnder modulator (MZM) is used, whereas the vertical-cavity surface-emitting laser (VCSEL) is utilized in direct modulation. Both modulation schemes are tested for a vector modulation format, i.e., the quadrature amplitude modulation (QAM), where an orthogonal frequency-division multiplexing (OFDM) scheme is used to generate signal subcarriers. The simulations are carried out with the same values of common global parameters for both schemes of intensity modulation. Although VCSEL is a promising device for future RoF systems, the external modulation shows a more robust performance compared with that of VCSEL when implemented with the OFDM modulation technique.

W-Band PDM-QPSK Vector Signal Generation by MZM-Based Photonic Frequency Octupling and Precoding

We experimentally demonstrate polarization-division-multiplexing quadrature-phase-shift-keying (PDM-QPSK) modulated vector signal generation at the W-band, adopting Mach–Zehnder-modulator-based (MZM-based) photonic frequency octupling and phase precoding techniques. The MZM biased at its maximum transmission point is driven by a 12-GHz precoded vector signal, which is generated by MATLAB programming and carries up to 4-GBd QPSK transmitter data. The phase of the 12-GHz precoded vector signal is one eighth that of the regular QPSK symbol to overcome the phase octupling effect accompanying the frequency octupling during square-law photodiode (PD) detection. Only one polarization beam splitter is needed to implement optical polarization diversity. The generated 96-GHz vector signal can carry up to 4-GBd PDM-QPSK data. To the best of our knowledge, this is the first time that the generation and reception of the polarization multiplexing vector signal has been recognized by one external intensity modulator at the W-band.

Symmetric 40-Gb/s, 100-km Passive Reach TWDM-PON with 53-dB Loss Budget

A truly passive long-reach, symmetric 40-Gb/s time and wavelength division multiplexed passive optical network (TWDM-PON) with a high loss budget is demonstrated, using direct modulation and direct detection in both upstream and downstream directions. Thermally tuned directly modulated lasers (DMLs) are employed to serve as both upstream and downstream transmitters, not only owing to their low cost, but also, as a carrier-less modulation method, where the signal generated by direct modulation is demonstrated to be more robust to high launch power induced fiber nonlinearities compared with external intensity modulation formats with strong carrier power. Therefore, DML is suitable for applying in TWDM-PON to achieve a high loss budget. Moreover, the frequency chirp induced dispersion of directly-modulated signal is managed thanks to the combination of optical spectral reshaping and dispersion supported transmission effects, which makes it possible for the directly-modulated signal to reach a distance of 100 km and still with a good quality. As a result, a system loss budget of 53 dB is achieved, supporting more than 1000 users with 100-km purely passive reach, which is the first demonstration of high loss budget, long reach TWDM-PONs to our best knowledge.

Optical microwave generation using modified sideband injection synchronization with wide line-width lasers for broadband communications

In future generation space based phased arrays and in pico-cellular broadband mobile communication systems operating at microwave frequencies optical techniques for transport and generation of electrical signals are widely used. In general, there are two main classes of optical transmitting and generating microwave frequencies: (1) methods using direct or external intensity modulation of lasers and (2) heterodyning method using two coherent optical waves. The present paper dwells on the second method. The present paper overcomes the major practical limitations of optical sideband injection locking. Simulation results have been given in support of the analytical result.

Multiwavelength diode-laser absorption spectroscopy using external intensity modulation by semiconductor optical amplifiers

A novel opto-electronic scheme for line-of-sight Near-IR gas absorption measurement based on direct absorption spectroscopy (DAS) is reported. A diode-laser-based, multiwavelength system is designed for future application in nonintrusive, high temporal resolution tomographic imaging of H2O in internal combustion engines. DAS is implemented with semiconductor optical amplifiers (SOAs) to enable wavelength multiplexing and to induce external intensity modulation for phase-sensitive detection. Two overtone water transitions in the Near-IR have been selected for ratiometric temperature compensation to enable concentration measurements, and an additional wavelength is used to account for nonabsorbing attenuation. A wavelength scanning approach was used to evaluate the new modulation technique, and showed excellent absorption line recovery. Fixed-wavelength, time-division-multiplexing operation with SOAs has also been demonstrated. To the best of our knowledge this is the first time SOAs have been used for modulation and switching in a spectroscopic application. With appropriate diode laser selection this scheme can be also used for other chemical species absorption measurements.

Ultrashort pulse generation using fiber FM laser

Ultrashort optical pulse generation using a fiber FM laser is presented and analyzed in detail. Fiber FM laser operation is realized using a fiber ring with an internal phase modulator and an erbium-doped fiber amplifier. To compress FM laser pulses, an external dispersive single-mode fiber is employed. Furthermore, by external intensity modulation, the pulse background is removed. The background ratio of the generated ultrashort pulse is calculated and compared with the experimental results. The experimental results show an output optical pulse width of 1.77 ps and a spectral bandwidth of 0.5 THz.

Generation of Flat Optical Frequency Comb Based on FM Laser Operation of Fiber Ring Laser

We obtained flat optical frequency combs by using the FM laser operation of a fiber ring laser and external intensity modulation. Extremely wide FM spectra can be easily obtained by the moderate internal phase modulation of an FM laser. We used an external intensity modulator to extract a linearly chirped part from the FM light in order to obtain flat spectra. In our experiments, we obtained a flat optical frequency comb with a spectral bandwidth of about 0.5THz and a power deviation of less than ±1.5dB.

Optical Generation of Modulated Millimeter Waves Based on a Gain-Switched Laser

We propose and demonstrate two configurations for optical millimeter-wave (mm-wave) generation and transmission of 3-Gb/s downstream data based on a gain-switched laser (GSL). The first configuration generates an optical comb spectrum from a GSL that can be appropriately filtered to generate two optical sidebands with 60-GHz separation. These sidebands are modulated with baseband data by using an external intensity modulator and then transmitted via optical fiber to the remote antenna unit (RAU). The second configuration produces a modulated optical frequency comb by driving the laser with both RF local oscillator and data streams coupled together and then followed by the same optical filters to generate two modulated optical sidebands. At the RAU, these two sidebands are heterodyned using a photodetector to generate the electrical modulated mm-wave signal. We investigate the distribution of these two methods over 3-km fiber with 2-m wireless link and demonstrate the system simplicity and cost efficiency for mm-wave over fiber systems. Both configurations are simulated to verify our obtained results and show system performance at higher bit rates.

L-band erbium-doped fiber laser with polarization-maintaining ring cavity

A new, simple and efficient L-band erbium-doped fiber (EDF) laser with polarization-maintaining ring cavity is proposed and experimentally demonstrated. The laser offers the unique advantage of being a polarization-maintaining lasing, which meets the requirement of external electronic-optical intensity modulation for DWDM communication and interferometric fiber sensors. Theory has been established. Experimental results are given to show that the laser has long-term stability over a couple of hours with 35 dB signal-to-noise ratio at 1598.644 nm and tunable from 1572.167 to 1608.480 nm.

Polarization-Insensitive All-Optical Upconversion for Seamless Integration Optical Core/Metro/Access Networks With ROF Systems Based on a Dual-Pump FWM Scheme

We have proposed and experimentally demonstrated a novel scheme to realize polarization-insensitive all-optical upconversion for single-channel or wavelength-division multiplexing signals. This scheme is based on the copolarized-pump four-wave mixing effect in nonlinear media. The copolarized pump is generated by using optical carrier suppression in an external intensity modulator; therefore, the two pumps are always parallel and phase locked. This scheme has some unique advantages for the integration between optical transport networks and radio-over-fiber systems, such as a polarization-insensitive, low-frequency bandwidth requirement for optical and electrical components, and high wavelength stability.

Polarization insensitive all-optical up-conversion for ROF systems based on parallel pump FWM in a SOA

We have proposed and experimentally investigated polarization insensitive all-optical up-conversion for ROF system based on FWM in a semiconductor optical amplifier (SOA). The parallel pump is generated based on odd-order optical sidebands and carrier suppression using an external intensity modulator and a cascaded optical filter. Therefore, the two pumps are always parallel and phase locked, which makes system polarization insensitive. After FWM in a SOA and optical filtering, similar to single sideband (SSB) 40 GHz optical millimeter-wave is generated only using 10 GHz RF as local oscillator (LO). The receiver sensitivity at a BER of 10(-9) for the up-converted signals is -28.4 dBm. The power penalty for the up-converted downstream signals is smaller than 1 dBm after 20 km SSMF-28 transmission.

High Dynamic Range Microwave Photonic Links for RF Signal Transport and RF-IF Conversion

Two different techniques were used to extend the dynamic range of analog optical links operating at microwave frequencies. The link noise figure was reduced by adjusting the bias point of an external Mach-Zehnder intensity modulator. The link distortion was reduced by linearizing the transfer function of the intensity modulator. Frequency conversion was implemented using an additional optical modulator and electrical local oscillator rather than a conventional electronic mixer. A microwave frequency signal transport link was demonstrated with a spur-free dynamic range (SFDR) of 71 dB in a 500-MHz noise bandwidth. A link with down-conversion from a microwave frequency to an intermediate frequency was demonstrated with an SFDR of 64 dB in a 500-MHz noise bandwidth.

Low-Noise and High-Frequency Resolution Electrooptic Sensing of RF Near-Fields Using an External Optical Modulator

We developed an electrooptic (EO) sensing system using an external optical intensity modulator for radio frequency (RF) electric near-field measurements. This low-noise, high-frequency resolution system is based on the photonic heterodyning technique in which a RF near-field signal is mixed with modulated light by an EO crystal and downconverted to the difference frequency. Because the optical modulator has a bandwidth extending from DC to several gigahertz, the frequency resolution of this system is determined by that of the RF synthesizer it uses. The intensity noise of the modulated light is quite low, and the noise level of the EO signal can be reduced to almost the shot-noise level by using a balanced detection scheme.