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ACCOMPLISHMENTS |
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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 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 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
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
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| Standard Reference Materials (SRMs) |
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SRM
2513 SRM
2514 SRM
2515 SRM
2517a SRM
2519 SRM
2520 Additional SRMs for optical fiber communications produced by NIST: SRMs 2522 & 2523 for optical fiber ferrule geometry and SRMs 2553-2555 for optical fiber coating diameter. Please visit our SRM page for more information. |
| Page updated: 02/10/2009 |
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