Linn F. Mollenauer

Discovery of the soliton self-frequency shift

We describe the experimental discovery of a continuous shift in the optical frequency of a soliton pulse as it travels down the fiber. The effect is caused by a Raman self-pumping of the soliton, by which energy is transferred from the higher to the lower-frequency parts of its spectrum. For 120-fsec pulses, we have observed net frequency shifts as great as 10% of the optical frequency.

Phase noise in photonic communications systems using linear amplifiers

Spontaneous emission noise limits the capacity and range of photonic communications systems that use linear optical amplifiers. We consider here the question of phase detection in such systems. Amplitude-to-phase-noise conversion occurs owing to the nonlinear Kerr effect in the transmission fiber, resulting in optimal phase noise performance when the nonlinear phase shift of the system is approximately 1 rad. Error-free state-of-the-art systems that use phase detection at multigigabit rates are thereby limited to a range of a few thousand kilometers.

Polarization multiplexing with solitons

It is shown both analytically and with numerical simulation, and confirmed experimentally in transmission over distances up to approximately 10000 km, that solitons maintain a high degree of polarization over an ultra-long distance transmission system consisting of birefringent dispersion-shifted fiber segments and erbium amplifiers. Based on that fact, the authors propose a polarization/time division multiplexing technique which should allow the single-wavelength bit-rate capacity of an ultra-long distance soliton transmission system to be doubled with little or no significant increase in bit error rate. >

The sliding-frequency guiding filter: an improved form of soliton jitter control

By gradually translating the peak frequency of guiding filters along its length, we create a fibertransmission line that is substantially opaque to noise while remaining transparent to solitons. This trick allows the use of stronger filters, and hence greater jitter reduction, without incurring the usual penalty of exponentially rising noise from the excess gain required to overcome filterloss.

The soliton laser

By incorporating a length of single-mode, polarization-preserving fiber into the feedback loop of a mode-locked color-center laser (lambda~1.4-1.6 microm), we have created a device that we call the soliton laser. Pulse width (2.0 to 0.21psec obtained to date) is determined by fiber length, in accordance with N = 2 soliton behavior. Production of <50-fsec-wide pulses is indicated for compression in an additional, external fiber.

Soliton propagation in long fibers with periodically compensated loss

With computer simulation, we study soliton propagation in an all-optical, long-distance communications system where fiber loss is periodically compensated by Raman gain. We find that distortion of the transmitted pulses from true solitons shows a peak near z_{0} = L/8 where L and z 0 are the amplification and soliton periods, respectively. We also describe optimal system design based on the exceptional pulse stability and low soliton powers obtained in the region z_{0} \gg L/8 . Typical amplification periods are in the range 30-50 km, pump powers are less than 100 mW, and for bit rates in the 10 GHz range, time average signal powers are at most a few milliwatts. The single-channel rate-length product for error rate less than 10-9is \sim29 000 GHz Km. Finally, we show that in the gain-compensated system with wavelength multiplexing, soliton-soliton collisions produce random modulation of individual pulse velocities. Nevertheless, multiplexing can yield rate-length products greater than 300 000 GHz km.

Harmonically mode-locked fiber ring laser with an internal Fabry–Perot stabilizer for soliton transmission

We describe a modulator-driven, erbium-fiber ring laser that produces chirp-free pulses with width adjustable over the range of 5 to 100 ps. A high-finesse Fabry-Perot étalon with a free spectral range equal to the 2.5-GHz laser repetition rate eliminates unwanted laser ring modes and stabilizes the pulse amplitude. Using this laser, we have extended the error-free distance of a series of soliton transmission experiments by 2000-3000 km over previous results with a mode-locked semiconductor laser.

Demonstration of soliton transmission over more than 4000 km in fiber with loss periodically compensated by Raman gain.

By recirculating 55-psec soliton pulses (λs ~ 1600 nm) many times around a closed 42-km loop with loss exactly compensated by Raman gain (λp ~ 1497 nm), we have successfully demonstrated pulse transmission, without electronic regeneration, over distances in excess of 4000 km.

Long-distance soliton propagation using lumped amplifiers and dispersion shifted fiber

It is shown both analytically and by numerical simulation, that solitons can traverse great distances through a chain of lumped amplifiers connecting dispersion shifted fiber spans. The fiber spans can also have large fractional variations in D. The resultant pulse distortions and dispersive wave radiation tend to be negligible, as long as the length scale of the variations in energy and dispersion are short relative to the soliton period. >

Effects of fiber nonlinearities and amplifier spacing on ultra-long distance transmission

It is shown that it should be possible to send error-free signals at a 2.5-Gb rate (or higher) over distances of at least 9000 km using an amplitude shift keying (ASK) soliton modulation system. To accomplish this, the amplifiers must be kept close enough that their power gain is less than 10 dB. (It is noted that timing jitter and other noise effects measured in recent soliton transmission experiments carried out at low D and with amplifier spacing of 25 km are in close accord with predictions of this work). Frequency division multiplexing of several channels over the same fiber should also be possible, as solitons of different frequencies interact very weakly, provided the distance over which they pass through one another is large compared to the amplifier spacing. >