High-repetition rate passively mode-locked solid-state laser

Details High-repetition rate passively mode-locked solid-state laser High-repetition rate passively mode-locked solid-state laser

1999-02-01

2002-05-21

Font, Frank G. (Department: 2877)

Coherent light generators

Particular beam control device

Mode locking

C372S011000, C372S020000, C372S049010, C372S070000, C372S098000

Reexamination Certificate

active

06393035

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to lasers, and more particularly to passively mode-locked solid state lasers designed to operate at repetition rates exceeding 1 GHz.
BACKGROUND OF THE INVENTION
Solid-state lasers are known in the art. Their laser gain media are dopant ions incorporated in dilute concentrations in solid hosts. The laser gain medium can be optically excited to emit electromagnetic radiation by impinging a pumping beam on the laser gain medium. High-repetition rate lasers are desirable for a number of applications, such as for use as seed sources for driving radio-frequency photocathodes. These RF photocathodes are then used to inject high-energy electron bunches into a linear accelerator. It is often desirable to have the laser repetition rate operating at the drive frequency of the linear accelerator, which is typically at 2.8 GHz or higher. It is also possible to use high-repetition rate lasers synchronized to the drive frequency of the accelerator in diagnostic tools or in optical-electron interactions after the electrons are fully accelerated.
Other possible applications of high-repetition rate lasers are in the area of telecommunications, photonic switching, and optoelectronic testing. As networks and electronic components continue to increase in terms of bandwidth and clock frequency, optical pulsed laser sources become more important for driving, sensing, and testing of these components. One example of this application for optical clocking of integrated circuits is disclosed in U.S. Pat. No. 5,812,708 (Rao).
Mode locking is a special operation regime of lasers where an intra-cavity 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 mode locking. In this case, there are multiple laser pulses circulating in the cavity per round trip. The minimum possible pulse width of the laser is nominally set by the line width of the laser transition, following approximately the condition that t
min
≧0.44/&Dgr;f where &Dgr;f is the line width of the laser transition. For typical laser materials such as Nd:YAG or Nd:vanadate, the laser line width can support pulses to less than 10 ps. For broader-bandwidth materials such as Nd:glass or Ti:sapphire, pulse widths to below 100 fs and even below 10 fs can be generated.
DESCRIPTION OF PRIOR ART
Mode locked lasers are well known in the state-of-the-art, having been first described in the 1960's (see H. W. Mocker et al., “Mode competition and self-locking effects in a Q-switched ruby laser,” Applied Physics Letters, vol. 7, pp. 270-273, 1965). Passive mode locking using a saturable absorber was discovered almost immediately thereafter. Most mode-locked lasers have used active modulators, where the term “active” means that a source of power such as a radio-frequency signal or another electronic signal must be periodically applied to the modulator. Typical active modulators are acousto-optical modulators (AOMs, Bragg cells) or electro-optical (EOMs, Pockels cells). Active modulators can modulate the amplitude (AOMs or EOMs) or the phase (EOMs) of the optical signal to achieve mode locking.
Active mode lockers have the disadvantages of cost and complexity. A typical device requires a precision electro-optical component, plus drive electronics which typically consists of high-power, high-stability RF-signal (for AOMs) or high-voltage (for EOMs) 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”, and Lightwave Electronics, Series 131 data sheet, March 1994).
Active mode locking has been available in commercial lamp-pumped laser systems and more recently in diode-pumped laser systems at repetition rates typically of 100 MHz and extending up to 250 MHz. Research on active mode locking has been done on higher repetition rates, achieving repetition rates of approximately 2 GHz (see K. J. Weingarten et al., “Two gigahertz repetition rate, diode-pumped, mode-locked Nd:YLF laser”, Optics Letters, vol. 15, pp. 962-964, 1990), 5 GHz (P. A. Schulz et al., “5-GHz mode locking of a Nd:YLF laser”, Optics Letters, vol. 16, pp. 1502-1504, 1991), 20 GHz (A. A. Godil et al., “Harmonic mode locking of a Nd:BEL laser using a 20-GHz dielectric resonator/optical modulator”, Optics Letters, vol. 16, pp. 1765-1767, 1991), and more recently 40 GHz (A. J. C. Viera et. al., “Microchip laser for microwave and millimeter-wave generation”, IEEE MTT-S IMOC'97 Proceedings). In all cases the systems required an active modulator driven by a stable RF source and an RF amplifier. The highest repetition rates at 40 GHz were achieved with “harmonic” mode locking (see M. F. Becker et al., “Harmonic mode locking of the Nd:YAG laser”, IEEE Journal of Quantum Electronics, vol. QE-8, pp. 687-693, 1972), where the modulator is driven at some integer multiple of the fundamental laser repetition rate. This is an additional source of complexity and instability in the laser system. In general we wish to avoid harmonic mode locking if possible.
It is also possible to generate high repetition rates using other laser medium such as rare-earth-doped fiber lasers, and semiconductor lasers. Repetition rates of>10 GHz have been demonstrated in semiconductor quantum well lasers (see U.S. Pat. No. 5,040,183, Chen et al., “Apparatus comprising optical pulse-generating means”), achieving pulse repetition rates even to>100 GHz. However, their approach appears to be limited in terms of average power. Fiber lasers have also been demonstrated to high repetition rates using active or harmonic passive mode locking (see U.S. Pat. No. 5,414,725 Fermann et al., “Harmonic partitioning of a passively mode-locked laser”, and S. V. Chernikov et al., “Duration-tunable 0.2-20 ps 10-GHz source of transform-limited optical pulse based on an electro-absorption modulator”, Optics Letters, vol. 20, pp. 2399-2401, 1995) Passive mode locking at the fundamental repetition rate, on the other hand, is a much simpler, robust, and lower-cost approach to generating mode-locked pulses. Passive mode locking 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 mode locking in the recent years have been Kerr-Lens Mode locking (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 semiconductor saturable absorber mirror (SESAM) device for generating picosecond and femtosecond pulses in a wide number of solid-state lasers (see U. Keller et al., “Semiconductor saturable absorber mirrors (SESAMs) 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). Passive mode locking relies on a saturable absorber mechanism, which produces either decreasing loss with increasing optical intensity, or similarly an i

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