Optical Frequency Control Research Articles

Broadband serrodyne phase modulation for optical frequency standards and spectral purity transfer.

We perform low phase noise, efficient serrodyne modulation for optical frequency control and spectral purity transfer between two ultrastable lasers. After characterizing serrodyne modulation efficiency and its bandwidth, we estimate the phase noise induced by the modulation setup by developing a novel, to the best of our knowledge, composite self-heterodyne interferometer. Exploiting serrodyne modulation, we phase locked a 698 nm ultrastable laser to a superior ultrastable laser source at 1156 nm by means of a frequency comb as a transfer oscillator. We show that this technique is a reliable tool for ultrastable optical frequency standards.

Precise Optical Modulation and Its Application to Optoelectronic Device Measurement

Optoelectronic devices which play important roles in high-speed optical fiber networks can offer effective measurement methods for optoelectronic devices including optical modulators and photodetectors. Precise optical signal modulation is required for measurement applications. This paper focuses on high-speed and precise optical modulation devices and their application to device measurement. Optical modulators using electro-optic effect offers precise control of lightwaves for wideband signals. As examples, this paper describes frequency response measurement of photodetectors using high-precision amplitude modulation and wavelength domain measurement of optical filters using fast optical frequency sweep. Precise and high-speed modulation can be achieved by active trimming which compensates device structure imbalance due to fabrication error, where preciseness can be described by on-off extinction ratio. A Mach-Zehnder modulator with sub Mach-Zehnder interferometors can offer high extinction-ratio optical intensity modulation, which can be used for precise optoelectronic frequency response measurement. Precise modulation would be also useful for multi-level modulation schemes. To investigate impact of finite extinction ratio on optical modulation, duobinary modulation with small signal operation was demonstrated. For optical frequency domain analysis, single sideband modulation, which shifts optical frequency, can be used for generation of stimulus signals. Rapid measurement of optical filters was performed by using an optical sweeper consisting of an integrated Mach-Zehnder modulator for optical frequency control and an arbitrary waveform generator for generation of a source frequency chirp signal.

Performance of digital servos in an optical frequency transfer network.

We demonstrate the use of three kinds of flexible digital servos for the stabilization of the optical fiber link, the interferometer temperature, and the polarization of the transmitted light at the remote site, respectively. The main fiber noise cancellation digital servo provides a large phase detection range (∼210π radians), automatic relock function, and low cycle-slip rate over a 62 km field-deployed fiber link achieved by utilizing a feedback optical actuator of an acousto-optic modulator fed by a voltage-controlled oscillator. The temperature control and polarization control digital servos enable that the temperature of the interferometer can be stabilized at a stability of 0.01K and the data uptime is enhanced from 85.5% to 99.9% by implementing the polarization controller. The results demonstrate that the performance of the three digital servos is sufficient for high-precision optical frequency transfer applications and indicates comparable performance to existing analog optical frequency control systems. The full digital controlled optical frequency transfer method demonstrated here provides guidance for the development of a low-cost, low-complexity, and high-reliability optical frequency transfer system.

Self-starting bi-chromatic LiNbO3 soliton microcomb

For its many useful properties, including second and third-order optical nonlinearity as well as electro-optic control, lithium niobate is considered an important potential microcomb material. Here, a soliton microcomb is demonstrated in a monolithic high-Q lithium niobate resonator. Besides the demonstration of soliton mode locking, the photorefractive effect enables mode locking to self-start and soliton switching to occur bi-directionally. Second-harmonic generation of the soliton spectrum is also observed, an essential step for comb self-referencing. The Raman shock time constant of lithium niobate is also determined by measurement of soliton self-frequency-shift. Besides the considerable technical simplification provided by a self-starting soliton system, these demonstrations, together with the electro-optic and piezoelectric properties of lithium niobate, open the door to a multi-functional microcomb providing f-2f generation and fast electrical control of optical frequency and repetition rate, all of which are critical in applications including time keeping, frequency synthesis/division, spectroscopy and signal generation.

Distributed Aggregation of Spectrally Efficient Single- and Dual-Polarization Super-Channels by Optical Frequency Conversion in Fiber

Recently developed upgrade options for distributed ultra-dense frequency-division multiplexing toward large aggregation capacity are discussed. The concept, originally demonstrated for distributed coherent optical orthogonal frequency-division multiplexing, is based on optical frequency conversion in fiber. It enables precise frequency allocation of the successively multiplexed subcarriers within the super-channel without the need of absolute optical frequency control, i.e., allowing for use of free-running lasers at the individual multiplexing nodes. Follow-up experiments proved the applicability of this concept also to Nyquist wavelength-division multiplexing (WDM) with extremely small guard bands by successfully demonstrating the distributed aggregation of a spectrally efficient 400-Gb/s single-polarization zero-guard-band Nyquist-WDM super-channel using 4× 28-GBd 16-ary quadrature-amplitude modulation (16QAM) subcarriers. Recently, we further extended the concept to distributed generation of dual-polarization super-channels. This extension allowed for the distributed aggregation of a 400-Gb/s Nyquist-WDM super-channel using 4× 28-GBd polarization-division multiplexed quaternary phase-shift keying (PDM-QPSK) subcarriers. In this invited contribution, we review and compare in detail both experiments enabling 400-Gb/s aggregation capacity. Additionally, we discuss a viable option for operation under random fiber birefringence regardless of the polarization-dependent nature of the underlying optical frequency conversion process.

Mid-altitude wind measurements with mobile Rayleigh Doppler lidar incorporating system-level optical frequency control method

A mobile Rayleigh Doppler lidar based on double-edge technique is developed for mid-altitude wind observation. To reduce the systematic error, a system-level optical frequency control method is proposed and demonstrated. The emission of the seed laser at 1064 nm is used to synchronize the FPI in the optical frequency domain. A servo loop stabilizing the frequency of the seed laser is formed by measuring the absolute frequency of the second harmonic against an iodine absorption line. And, the third harmonic is used for Rayleigh lidar detection. The frequency stability is 1.6 MHz at 1064 nm over 2 minutes. A locking accuracy of 0.3 MHz at 1064 nm is realized. In comparison experiments, wind profiles from the lidar, radiosonde and European Center for Medium range Weather Forecast (ECMWF) analysis show good agreement from 8 km to 25 km. Wind observation over two months is carried out in Urumqi (42.1°N, 87.1°E), northwest of China, demonstrating the stability and robustness of the system. For the first time, quasi-zero wind layer and dynamic evolution of high-altitude tropospheric jet are observed based on Rayleigh Doppler lidar in Asia.

Rapid, Precise, and Stable Control of Optical Frequency for SG-DBR LD Using Quartz Etalon

This paper presents rapid, precise, and stable control of optical frequency for sampled-grating distributed Bragg reflector (SG-DBR) laser diode (LD) using a Z-cut quartz etalon supported at the middle point. The optical frequency of SG-DBR LD was controlled by the mode-hop-free three-electrode control method. As a result, stable mode operation and wide continuous tuning range without mode hopping have been realized. A 0.2-ms optical frequency tuning speed with a 4.35-THz tuning range and a 130-MHz control error of the optical frequency was demonstrated. A 1.1-MHz Allan deviation was reached at an integration time of 12 times 103 s.

Comparision of a Kr-stabilized Laser and a Holographic Filter for use as an Optical Frequency Control Element for High Dense WDM Systems

Article Comparision of a Kr-stabilized Laser and a Holographic Filter for use as an Optical Frequency Control Element for High Dense WDM Systems was published on February 1, 2000 in the journal Journal of Optical Communications (volume 21, issue 1).

Increasing acceptable number of signals in subcarrier multiple access optical networks in the presence of high optical beat interference

This paper theoretically and experimentally demonstrates that our proposed method improves the carrier-to-noise ratio (CNR) of subcarriers and significantly increases the number of acceptable optical signals in subcarrier multiple access (SCMA) systems using distributed feedback laser diodes (DFB-LD's) with the same center optical frequency. This method reduces optical beat interference by using the multiplied product of the subcarriers and a low-frequency signal. From the experimental results using 1.3 /spl mu/m DFB-LD's, the CNR sufficiently satisfies the required level even if the number of optical signals is three. Theoretically, the number of acceptable optical signals increases as LD's with wider optical line-width or with greater chirp coefficient are used. For example, assuming a laser chirp coefficient of 540 MHz/mA, the maximum number of LD's that can be used while still meeting the minimal CNR requirement is 12. This number corresponds to 66 optical beat interferences. Thus, this proposed beat-interference reduction technique is very effective in SCMA systems using DFB-LD's without optical frequency control.

A new automatic frequency-control method for fast frequency switching of a coherent optical receiver

The authors propose a new automatic optical frequency-control method for fast frequency switching. Simulation and experimental results for the new method are presented. In the experiment, the frequency of an optical coherent receiver was changed freely and accurately and was not affected by the thermal effect of the laser diode. By this method, switching time was reduced to less than one tenth that of an uncontrolled laser diode.

622 Mbit/s‐sixteen‐channel FDM coherent optical transmission system using two‐section MQW DFB‐LDs

AbstractA light source and optical frequency control technique needed for realizing the optical frequency division multiplexing transmission (Optical FDM) is developed and experiments of a 622 Mbit/s‐sixteen‐channel FDM coherent optical transmission based on the heterodyne detection are conducted. the transmission is configured such that sixteen 1.55 μm wavelength two‐section MQW DFB‐laser diode (LD) are multiplexed by a star coupler. the wavelength of each LD was spaced at 10 GHz intervals with the frequency fluctuation kept under + 100 MHz by the optical frequency control circuit that uses a scanning Fabry‐Perot interferometer. the receiver configuration has a delay‐line demodulator, polarization diversity, and a frequency discriminator with no dead bands. For the station light source, the two‐section MQW DFB‐LD was tuned, which is the same as the transmitter, using current and temperature control. Random channels could selectively be received with a receiver sensitivity of ‐39.3 dBm. It was verified that 16 channels of 622 Mbit/s high‐vision (Japanese standard HDTV) signal could be transmitted through 110 km of 1.3 μm zero dispersion fiber without signal degradation.

Optical frequency control for DWDM networks using sum-frequency generation in multilayer waveguides

Future high bit-rate DWDM networks will require an ensemble of laser sources whose frequencies can be set accurately. Considering the excellent short-term wavelength stability of today's DFB semiconductor lasers, network frequency management could be accomplished through periodic measurement and correction of essentially free-running sources. In this paper, we present experimental results on a novel technique for measuring and controlling the wavelength of a semiconductor laser using the non-linear properties of a multilayer waveguide. Using the described calibration procedure, absolute optical frequency control of a 1.3 /spl mu/m DFB laser within better than 2 GHz is demonstrated. Our technique allows full flexibility in channel frequency assignment, is already compatible with projected DWDM network wavelength control requirements and lends itself to monolithic optical integration. >