Pulse-generating laser

Details Pulse-generating laser Pulse-generating laser

2002-11-05

2004-08-17

Scott, Jr., Leon (Department: 2828)

Coherent light generators

Particular beam control device

Control of pulse characteristics

C372S018000, C372S011000, C372S020000, C372S006000

Reexamination Certificate

active

06778565

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to lasers, and more particularly to passively mode-locked solid-state lasers designed to operate at high repetition rates exceeding 1 GHz.
BACKGROUND OF THE INVENTION
Pulsed lasers are becoming highly important for telecom applications. As data transmission rates continue to increase, the base data transmission rate for high-end systems is moving from 10 GHz (e.g. defined by the SONET/SDH OC-192 standard among others) to approximately 40 GHz (e.g. defined by OC-768 standard among others). These higher data rates become increasing difficult due to affects of chromatic and polarization mode dispersion. State of the art systems use non-return-to-zero (NRZ) modulation format, and this format is more susceptible to degradations due to these affects than a return-to-zero (RZ) format. In addition, an RZ format allows the use of optical pulses, and ultimately the use of related soliton affects, including soliton dispersion management techniques.
Today's pulse sources for Return-to-Zero (RZ)-coding transmission are complex, require a high power radio-frequency (RF) driver and have limited power output and scalability of the approach. The widely used approach of a system of a high power continuous-wave distributed feedback (DFB) laser and a subsequent set of modulators to turn the cw output into a pulse train relies on high bandwidth high contrast ratio modulators, which are hard to get with high bandwidths (maximum working frequencies), for example with bandwidths as high as above 40 gigahertz (GHz). Alternatives are active harmonically mode-locked fiber lasers or actively mode-locked semiconductor lasers. In order to scale the repetition rate of active harmonically mode-locked fiber lasers one has to increase the harmonic, at which these lasers are operated, which has strong impacts on the jitter and on pulse-to-pulse variations. The repetition rate of mode-locked semiconductor lasers can be scaled up to several hundred GHz, but they have a fundamental power limitation due to the limited mode area in these lasers. That is why already in the 10 GHz regime, erbium-doped fiber amplifiers (EDFAs) are required for this approach to get high enough average power levels.
Also, due to the limited transmission fiber power handling capacities, as the data rate goes up, for a given average power coming from the optical source, the energy per bit goes down. This decreases the signal-to-noise ratio at the receiver end of the system, if all other parameters are assumed to be constant. Therefore, it is desirable to have increased average power at higher repetition rates to compensate for this and maintain appropriate signal-to-noise levels. The average power achievable is ultimately limited by nonlinear effects in the fiber (stimulated Brillouin scattering (SBS), self-phase modulation (SPM), related phenomena such as four-wave mixing etc.). Further, the achievable average power is also limited by maximum thermal power handling capabilities of the fiber. With a pulsed format, the amount of SPM increases due to the increased intensity at the peak of the pulse. At the same time, the threshold for SBS is increased, i.e. improved due to the increased bandwidth of the signal, which in turn are due to the shorter temporal pulses. Recently, solutions such as soliton-based and dispersion-managed soliton systems have been proposed, which require clean Gaussian or hyperbolic-secant-squared pulse shapes, to further improve transmission at high repetition rates through fiber systems.
This invention relates to the field of pulsed lasers with high repetition frequencies. Passive modelocking of solid-state lasers has been demonstrated to frequencies as high as 77 GHz (see Krainer, et. al., “77 GHz soliton modelocked NdYVO
4
laser”, Electronics Letters, vol. 36, no. 22). Passive modelocking is limited by the onset of Q-switched modelocking (QML) as e.g. described in WO 00/45480 and various scientific publications. According to the sate of the art, Nd:Vanadate is the material of choice for passively mode-locked solid-state lasers due to its excellent crystal quality, strong pump absorption, and high laser cross section which helps avoid the onset of QML.
Modelocking is a special operation regime of lasers where an intracavity modulation (amplitude or phase modulator) forces all of the laser modes to operate at a constant phase, i.e., phase-locked or “mode-locked”, so that the temporal shape of the laser output forms a continuously repeating train of short (typically in the range of picoseconds or femtoseconds) optical pulses. The repetition rate of this pulse train is set by the inverse of the laser round-trip time, or equivalently by the free spectral range of the laser, f
rep
=c/2L where c is the speed of light and L is the cavity length for a standing wave cavity. This repetition rate f
rep
is termed the fundamental repetition rate of the laser cavity, since this corresponds to only one laser pulse circulating in the cavity per round trip. The repetition rate can be scaled by integer multiples N of the fundamental repetition rate under certain conditions, and this is called harmonic modelocking. In this case, there are multiple laser pulses circulating in the cavity per round trip, which can increase the timing and amplitude jitter and which can differ from each other in the time and frequency domain (pulse-to-pulse variations. The large variety of different harmonic pulses can have different temporal and spectral shapes).
Among the available modelocking techniques, active modelockers have the disadvantages of cost and complexity. A typical device requires a precision electro-optical component, plus drive electronics which typically consists of a high-power, high-stability RF-signal (for acousto-optic modulators) or high-voltage (for electro-optic modulator) components. Additionally, feedback electronics may be required to stabilize either the drive signal for the modulator and/or the laser cavity length to achieve the necessary stability from the system (cf. U.S. Pat. No 4,025,875, Fletcher et al., “Length controlled stabilized mode-lock Nd:YAG laser” or U.S. Pat. No. 4,314,211, Mollenauer, “Servo-controlled optical length of mode-locked lasers”)
This is one reason why passive modelocking is often the technique of choice for short pulses and high repetition rates. Compared to active modelocking, passive modelocking at the fundamental repetition rate, is a much simpler, robust, and lower-cost approach to generating mode-locked pulses. Passive modelocking relies on a saturable absorber mechanism, which produces either decreasing loss with increasing optical intensity, or similarly an increase gain with increasing optical intensity. When the saturable absorber parameters are correctly adjusted for the laser system, the optical intensity in the laser cavity is enhanced such that a mode-locked pulse train builds up over a time-period corresponding to a given number of round-trips in the laser cavity.
Passive modelocking is also well-established in the state of the art (see A. J. DeMaria et al., “Self mode-locking of lasers with saturable absorbers”, Applied Physics Letters, vol. 8, pp, 174-176, 1966). The most significant developments in passive modelocking in the recent years have been Kerr-Lens Modelocking (KLM) (U.S. Pat. No. 5,163,059, Negus et al., “Mode-locked laser using non-linear self-focusing element”) for generation of femtosecond pulses from Ti:sapphire and other femtosecond laser systems, and the saturable absorber mirror device for generating picosecond and femtosecond pulses in a wide number of solid-state lasers (see U. Keller et al., “Semiconductor saturable absorber mirrors for femtosecond to nanosecond pulse generation in solid-state lasers,” Journal of Selected Topics in Quantum Electronics (JSTQE), vol. 2, no. 3, pp. 435-453, 1996).
Absorber structures suited for operation at wavelengths associated with current telecommunication applications, e.g. 1550 nm, have been demonstrated, e.g. in U.S. Pat. No. 5,701,327. Mozdy, et. al., “NaCL:OH—

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