Linear Optical Sampling of High-speed Optical Signals with Milliradian Phase Noise
The speed of optical communication networks continues to grow, and 40 Gb/s systems are being installed with plans for upgrade paths to 100 Gb/s. With higher data rates, the format of the data becomes more complicated as well. Phase shift-keying (PSK) allows increased spectral efficiency (data rate per spectral bandwidth) by encoding information on the optical phase of the transmitted light. As data rates grow beyond the speed of measurement electronics, there is a measurement challenge to be able to measure the phase and amplitude modulations at speeds beyond the reach of electronic detection alone. The solution is to employ optical sampling techniques to resolve the time-domain signals used in these fast PSK communication systems. One very useful technique is linear optical sampling (LOS), which uses short optical pulses to sample (in equivalent time) both the magnitude and phase of the transmitted electric field.

Paul Williams and Tasshi Dennis have assembled a LOS system using a unique phase referencing technique to remove laser phase noise, allowing a low-phase noise measurement without cumbersome curve-fitting or the bit rate dependence of existing LOS techniques. This system was demonstrated by measuring a 10 GB/s differential phase shift keyed (DPSK) signal. Results include a low-jitter bit synchronous averaging approach which allows the optical phase transmission of the modulator to be characterized to 1.2 mrad. The system bandwidth is fundamentally limited only by the pulse-width of the sampling laser itself, potentially allowing bandwidths as large as 1 THz.

Novel High-resolution, Broadband Laser Spectroscopy
A new method of spectroscopy has been demonstrated at NIST-Boulder using dual optical frequency combs. The full spectrum of a gas (both absorption and dispersion) is measured over a broad spectral region and with frequency accuracy that can reach 1 Hz. The spectroscopic measurement is equivalent to laser spectroscopy with 155,000 individual single frequency lasers, but is accomplished instead with two frequency combs. Unlike conventional Fourier Transform Spectroscopy, the system has no moving mechanical parts and minimal optical alignment. Finally, the spectrum is measured rapidly so the technique should allow for high-resolution spectroscopy on dynamical systems.

The system is based on fiber-laser frequency combs, whose output forms a comb of lines spanning a wide optical spectrum. When the frequency comb is sent through a gas cell, a given comb line will be absorbed (or phase shifted) by the gas if it lies on a resonance of one of the gas molecules. The challenge in extracting the gas spectrum lies in “reading out” the amplitude and phase change separately on each individual comb line. To solve this challenge, a second phase-locked comb was mixed with the transmitted comb, thereby translating the optical spectrum directly into the RF. The present experiment interrogates the effect of the absorption from the gas on 155,000 comb lines, spanning a wavelength range of 125 nm, with a frequency resolution 6 orders of magnitude better than other spectroscopic techniques. This work represents by far the largest number of frequency comb teeth that have been individually observed. It is described in more detail in the article by I. Coddington, W. C. Swann, N. R. Newbury, Phys. Rev. Lett., 100, 103902 (2008).

Compact Fiber Laser with GHz Fundamental Repetition Rate
Stabilized femtosecond lasers are proving to be a valuable tool in a variety of applications including optical clocks, precision measurements of range and velocity, precision spectroscopy and arbitrary waveform generation. In particular, fiber-based systems that operate around the telecommunications band are well-suited for many of these applications because they are eye-safe, compact and can exploit existing fiber-optic telecommunication components and fiber-optic networks. At NIST and elsewhere, stabilized fiber-based femtosecond lasers have been demonstrated that provide a train of pulses in the time domain with sub-femtosecond timing jitter and a frequency comb in the frequency domain with sub-hertz comb linewidths. The one current drawback of these fiber-based stabilized femtosecond lasers is their low repetition rate of about 100 MHz, which is not well-suited for many applications. Higher, gigahertz repetition-rate fiber laser systems exist but they typically employ either harmonic mode-locking or a compact proprietary doped-glass design; none of these higher repetition-rate systems have been stabilized to the same high degree as the lower repetition-rate systems.

As an important first step toward realizing a stabilized femtosecond fiber-laser system at GHz repetition rates, John McFerran in collaboration with EEEL researchers has successfully demonstrated a 2 GHz fundamentally mode-locked fiber laser. The laser is comprised of commercially available highly-doped fiber and a saturable absorber. It produces a clean pulse train that can be tightly phase-locked to either an RF or CW optical reference source. Although the optical bandwidth is limited at present to about 2.6 nm, the laser is a preliminary demonstration of an efficient, compact and cost effective frequency comb with a high repetition rate much better suited to many of the practical applications listed above.

Demonstration of Fiber Laser Frequency Comb with Hertz-level Linewidth
Optical frequency combs have been vigorously developed over the past seven years. Combs based on Ti:Sapphire lasers have been perfected at NIST in the Time and Frequency division and elsewhere and are now a tool used in high accuracy frequency measurement and in evaluating the stability of the optical clocks. In recent years, a second comb technology has emerged based on optical fiber laser frequency combs. These combs are similar in principle to the Ti:Sapphire laser-based combs but have several important distinctions. Fiber laser-based frequency combs are potentially more compact, less expensive and cover the near infrared region from 1 to 2 microns in wavelength. This region of the spectrum is particularly important as it contains the telecommunication band around 1.5 microns. Therefore, fiber laser-based frequency combs have the potential to impact telecommunication and sensing applications that use fiber optics and fiber optic devices. Unfortunately, the performance of fiber laser frequency combs has lagged considerably behind that of the original Ti:Sapphire laser-based frequency combs. Specifically, the width of the individual modes of the comb has been quite large. This broad linewidth will be a limitation on their application to the highest precision metrology needed to test optical clocks and for any time-domain experiments that exploit the carrier-envelope phase. In order to further the applications of fiber laser frequency combs in metrology, remote sensing, and time-domain experiments, it is important to improve the performance of fiber laser frequency combs, specifically to narrow the modes of the frequency comb.

Using both experimental data and our previously developed theory, we have recently identified a major cause of the broad optical linewidths as arising from white noise on the laser that pumps the fiber laser. By passively and actively removing this noise, we are able to narrow the beat note characterizing the offset frequency of the fiber laser comb to Hertz levels. The comb lines are still broadened by vibration and temperature effects, but these effects should be more easily removed by inserting the appropriate fiber stretcher or modulator in the laser cavity. Based on this work, there appears to be no fundamental reason why the fiber laser-based frequency comb cannot reach levels of performance close to that of the Ti:Sapphire laser-based frequency comb, while simultaneously providing the other distinct advantages of compactness, ease-of-use, and compatibility with fiber optic technologies.

Wavelength Calibration SRM 2519 Upgrade Measurements Completed and Prototype Developed
Bill Swann completed measurements of 25 hydrogen cyanide absorption line center wavelengths, pressure shifts, and pressure broadening. These measurements are necessary to upgrade SRM 2519 (for wavelength calibration in the 1530-1560 nm region) to higher accuracy; we expect to certify the measured line centers with 0.1 pm uncertainty, a 6-fold improvement. Bill also built and tested a prototype for the SRM upgrade, which is a simplified, single-pass design.