A phase-locked frequency comb in the near infrared is demonstrated with a mode-locked, erbium-doped, fiber laser whose output is amplified and spectrally broadened in dispersion-flattened, highly nonlinear optical fiber to span from 1100 to >2200 nm. The supercontinuum output comprises a frequency comb with a spacing set by the laser repetition rate and an offset by the carrier-envelope offset frequency, which is detected with the standard f-to-2f heterodyne technique. The comb spacing and offset frequency are phase locked to a stable rf signal with a fiber stretcher in the laser cavity and by control of the pump laser power, respectively. This infrared comb permits frequency metrology experiments in the near infrared in a compact, fiber-laser-based system.

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Phase-locked, erbium-fiber-laser-based frequency comb in the near infrared.

Publisher Summary Polarization mode dispersion (PMD) is a linear effect that can be compensated in principle. In an ideal circularly symmetric fiber, the two orthogonally polarized modes have the same group delay. However, in reality, fibers exhibit a certain amount of birefringence because of imperfections in the manufacturing process or mechanical stress on the fiber after manufacture. It is noted that fluctuations in the polarization mode and fiber birefringence produced by the environment lead to dispersion that varies statistically with time and frequency. PMD causes different delays for different polarizations and when the difference in the delays approaches a significant fraction of the bit period, it leads to pulse distortion and system penalties. Environmental changes— including temperature and stress—cause the fiber PMD to vary stochastically in time. PMD, illustrating the basic concepts, the measurement techniques, the PMD measurement, the PMD statistics for first- and higher orders, the PMD simulation and emulation, the system impairments, and the mitigation methods has been summarized in the chapter. Both the optical and the electrical PMD compensations are considered.

Polarization-Mode Dispersion

We have made prototypes of three kinds of fibre Fabry-Perot (FFP) tunable filters, each of which can cover a separate free spectral range interval as determined by its resonator length. Free spectral ranges from hundreds of angstroms to tens of megahertz can be realised from the appropriate type. We have built FFPs with free spectral ranges of >1000 A, 25 GHz, 5 GHz and 2.5 GHz. Finesse up to 200 has been demonstrated. The devices are piezoelectrically tuned and are coupled to fibres in-line using standard rotary splices or ST connectors.

Pigtailed high-finesse tunable fibre Fabry-Perot interferometers with large, medium and small free spectral ranges

Recent progress in optical time-division-multiplexed (TDM) transmission technologies is reviewed including optical short pulse generation, time-division multiplexing/demultiplexing, timing extraction, and waveform measurement. The latest 400-Gbit/s transmission experiments based on optical signal processing are presented and the possibility of terabit/second TDM transmission is discussed.

Ultrahigh-speed optical time-division-multiplexed transmission technology based on optical signal processing

Metal-semiconductor-metal photodetectors (MSM PDs) with finger spacing and width as small as 25 nm were fabricated on bulk and low-temperature (LT) grown GaAs and crystalline Si using ultra-high-resolution electron-beam lithography. High-speed electrooptic characterization with a 100-fs pulsed laser showed that the fastest MSM PDs had finger spacing and width, full width at half maximum response time, and 3-dB bandwidth, respectively, of 300 nm, 0.87 ps, and 0.51 THz for LT-GaAs; 100 nm, 1.5 ps, and 0.3 THz for bulk GaAs; and 100 nm, 10.7 ps, and 41 GHz for crystalline Si. Monte Carlo simulation was used to understand the impulse response of the MSM PDs and to explore the ultimate speed limitation of transmit-time-limited MSM PDs on GaAs and Si. Factors that are important to detector capacitance were identified using a conformal mapping method. Based on the experimental data, Monte Carlo simulation, and calculation of detector capacitance, scaling rules for achieving high-speed MSM PDs are presented. >

Nanoscale tera-hertz metal-semiconductor-metal photodetectors

Optical pulses from a GaAlAs laser diode directly modulated at a frequency f0 (971 MHz) are mixed in a LiIO3 crystal with optical sampling pulses at a frequency of f0 − 10 Hz from a mode-locked GaAlAs laser diode. The optical signal obtained by sum-frequency mixing in the crystal is observed with a photomultiplier and an oscilloscope. The original pulse waveform is reproduced clearly with a temporal resolution equal to the mode-locked laser-diode pulse width and at a repetition frequency of 10 Hz. Similar results are obtained with InGaAsP laser diodes at a wavelength of 1.3 μm.

Optical waveform measurement by optical sampling with a mode-locked laser diode

We have developed a frequency-domain phase shift system for measuring the zerodispersion wavelength and the dispersion slope of single-mode optical fibers. A differential phase shift method and nonlinear four-wave mixing technique were also investigated. The frequency-domain phase shift method is used to produce Standard Reference Materials that have their zerodispersion wavelengths characterized with an expanded uncertainty (k =2 ) of 60.060 nm. sion slope; nonlinear four-wave mixing; optical fiber; optical fiber bandwidth; polarization-mode dispersion; Standard Reference Material; telecommunication systems; zero-dispersion wavelength.

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Accurate Measurements of the Zero-Dispersion Wavelength in Optical Fibers.

The National Bureau of Standards (NBS), in cooperation with the Electronic Industries Association, conducted an interlaboratory measurement comparison among six fiber manufacturers to determine the effective cutoff wavelength of single-mode fibers. Measurement techniques based on transmitted power were used to determine cutoff wavelength on four fibers designed for single-mode operation at 1300 nm. NBS also contributed results using a spectral near-field technique. One standard deviation measurement spreads for the various techniques range from 6 to 12 nm. With the appropriate data analysis, single bend attenuation and power step methods give the same results. Both techniques are easily implemented as extensions to the usual spectral attenuation measurement.

Determining the effective cutoff wavelength of single-mode fibers: An interlaboratory comparison

A calorimeter for making absolute energy measurements of high power laser pulses is described. The calorimeter, based on volume absorption in a solid, is calibrated electrically and requires no window or vacuum environment. An error analysis is included giving the systematic and random errors of the instrument for a laser measurement. Briefly, the following performance is typical of the 32-mm x 32-mm aperture calorimeter: range 0.4-15-J; random error +/-0.2% (one standard deviation); systematic error +/-2.3%; and an upper operational limit of 3 J/cm(2). Most of the volume absorber documentation is applicable for 1.06 microm; however, the calorimeter should be useful from the near ir through the visible. Absorbers for use with CO(2) lasers in the 9-11-microm range are also discussed.

Douglas L. Franzen

Papers

Optical waveform measurement by optical sampling with a mode-locked laser diode

Accurate Measurements of the Zero-Dispersion Wavelength in Optical Fibers.

Determining the effective cutoff wavelength of single-mode fibers: An interlaboratory comparison

Absolute Reference Calorimeter for Measuring High Power Laser Pulses

Absolute reference calorimeter for measuring high power laser pulses