The present invention relates in general to seed pulse generators in master oscillator power amplifier (MOPA) laser systems. The invention relates in particular to MOPAs in which seed pulses are generated by modulating the output of a diode-laser.
Pulsed frequency converted solid-state lasers are used extensively for material processing applications such as machining, drilling and marking. Most commercially available, pulsed, solid-state lasers are operated by the well known technique of Q-switching. Q-switched pulsed lasers include a laser-resonator having a solid-state gain-element and selectively variable-loss device located therein. The laser resonator is terminated at one end thereof by a mirror that is maximally reflecting at a fundamental wavelength of the gain-element, and terminated at an opposite end thereof by a mirror that is partially reflecting and partially transmitting at the fundamental wavelength. Such a laser is usually operated by continuously optically pumping the gain element while periodically varying (switching) the loss caused by the variable loss device (Q-switch) between a value that will prevent lasing in the resonator and a value that will allow lasing in the resonator. While lasing is allowed in the resonator, laser radiation is delivered from the partially transmitting mirror as a laser pulse.
The pulse repetition frequency (PRF) of a Q-switched solid-state laser is determined by the frequency at which the Q-switch is switched. The pulse duration is determined for any particular gain-medium by factors including the transmission of the partially-transmitting mirror, any loss in the Q-switch in a lasing-allowed condition, the optical pump power, and the PRF. A pulse repetition rate and pulse duration that are optimum for an operation on any one material will usually not be optimum for another operation or another material. Accordingly, an “ideal” pulsed laser would have independently variable PRF and pulse-duration to allow an optimum combination to be selected for most operations on most materials.
One type of laser system in which the PRF can be varied without a variation in pulse duration is an optical-fiber based MOPA in which seed pulses are generated by a modulated single-mode, edge-emitting semiconductor laser diode-laser. High gain per a fiber amplification stage, for example between about 13 and 30 decibels (dB), together with a low saturation power allows using a variety of low power diode seed sources. Such a fiber MOPA can be operated at pulse-repetition frequencies (PRFs) from less than 100 kilohertz (kHz) to 5 megahertz (MHz) or greater with pulse duration selected between about 0.1 nanosecond (ns) and about 1 microsecond (its).
A major problem with fiber MOPAs is due to nonlinear effects that limit peak power and adversely affect spectral characteristics of the optical pulses. For harmonic generation from nanosecond pulses spectrally narrow light having a bandwidth of between about 0.5 nanometers (nm) and 1 nm is required. Stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), and spectral-broadening of nanosecond pulses due to four-wave mixing (FWM) in fibers significantly narrow the available space of optical parameters acceptable for frequency conversion.
There are two approaches to generation of pulses with variable length and pulse repetition rate. The first approach uses directly modulated diode-lasers as a seed source. Such an approach is in general less expensive, and provides high peak power (above 1 W) from the seed laser. A major disadvantage of this approach is that in order to provide short pulses of less than 10 ns, a short cavity length, for example less than about 10 mm is required. This, in turn, results in a single-frequency or a few frequency mode operation that favors to SBS and limits a peak power in fiber amplifiers. Another problem of few-frequency mode operation is a strong mode-beating effect resulting in significant pulse-to-pulse fluctuations. For longer cavity lengths, for example between about 10 centimeters (cm) and 30 cm, the pulse spectrum changes across an optical pulse at it comes to a steady state spectral width after many round-trips, for example between about 3 and 8 round trips. That is why at direct diode modulation with long cavities, an optical pulse has a spectral width narrowing toward the end of the pulse.
A second approach uses a continuously operating (CW) optical source modulated by an external modulator. In such an approach, a seed-source could be a diode laser, a solid-state laser, or a fiber laser. Typical modulators include an electro-optical crystal in a waveguide Mach-Zehnder configuration or a diode-laser amplifier. On one hand, such an approach provides less peak power, typically less than 100 milliwatts (mW) after modulation compared to a directly modulated diode-laser. On the other hand this approach allows pulses of any length and repetition rate to be generated with a spectrum determined by an appropriately designed seed-laser such as a low-noise seed laser. By way of example, a diode seed-laser having low-noise operation can be realized by incorporating a fiber Bragg grating (FBG) in a long fiber coupled to a diode-laser chip, with the FBG between about 1 meter (m) and 2 m from the diode-laser chip to form a cavity including the chip. The FBG provides an output coupler for the cavity.
A problem with such a MOPA is that between seed pulses there is a very low but finite CW background emission from the diode laser, for example between about 20 dB and 30 dB less than pulse peak power. A typical electro-optical waveguide modulator based on Mach-Zehnder waveguide in a lithium niobate (LiNbO3) crystal has a contrast ratio of between about 18 dB and 25 dB. While on first consideration this may seem insignificant it must be recognized that the background exists considerably longer than the pulses. By way of example, for pulses having a duration of 1 ns at a PRF of 100 KHz the background duration is ten-thousand times longer than the pulse duration.
The background level between pulses is amplified in the power amplifier in addition to the pulses being amplified. Amplifying the background takes energy from whatever gain medium is used in the amplifier. This reduces the efficiency of amplification of the pulses and results in a relatively low contrast ratio (ratio of pulse-intensity and background-intensity) in the amplified output. There is need for improving the efficiency of amplification and increasing the output contrast-ratio in diode-laser seeded MOPAs.
In one aspect, apparatus in accordance with the present invention comprises a diode-laser arrangement arranged to provide a first sequence of optical pulses, the pulses in the first sequence thereof having a first duration. A modulator arrangement is provided for modulating the first sequence of optical pulses to provide a second sequence of optical pulses. The pulses in the second sequence thereof have a second duration, the second duration being shorter than the first duration.
In one embodiment of the inventive apparatus, the diode-laser arrangement includes a diode-laser driven by a first sequence of current pulses such that the output of the diode-laser is the first sequence of optical pulses. The modulator arrangement may include a modulated semiconductor optical amplifier or an electro-optic modulator. In another embodiment of the inventive apparatus, the diode-laser arrangement includes a first diode-laser arranged to provide continuous wave (CW) radiation and a semiconductor optical amplifier arranged to modulate the CW radiation to provide the first sequence of optical pulses.
The accompanying drawings, which are incorporated in and constitute a part of the specification, schematically illustrate a preferred embodiment of the present invention, and together with the general description given above and the detailed description of the preferred embodiment given below, serve to explain principles of the present invention.
Referring now to the drawings, wherein like components are designated by like reference numerals,
The term “directly modulated” as used here with reference diode-laser 12 means that the diode-laser is driven by a pulsed electric current. The output (from FBG 16) of the extended cavity diode-laser is a sequence of radiation pulses having about the form of the driving-current pulses. The fastest rise-time is determined by diode-laser package design and typically varies from about 100 picoseconds (ps) to about 5 ps. For longer cavity lengths, for example between about 1 m and 2 m, between about 3 and 8 round trips (between about 30 ns and 160 ns) are required to establish a pulse bandwidth determined by the FBG. The pulses preferably have a duration between about 300 ns and 500 ns, so at the end of the pulse, the pulse spectral-width corresponds to a steady-state spectrum.
The pulses enter a circulator 18 via port-1 thereof and exit the circulator via port-2 thereof to be transported by an optical fiber 20 to a diode-laser 22, here arranged as a semiconductor optical amplifier. Diode-laser 22 is also driven by a pulsed electric current, wherein the current pulses are of a shorter duration than the current pulses driving diode-laser 12 and determine the duration of output pulses of the MOPA. Preferably the pulses have a duration between about 1 ns and 100 ns. The PRF of current pulses driving diode-laser 22 is exactly the same as the PRF of the current pulses driving diode-laser 12. The current pulses driving diode-laser 22 are synchronized to occur within the period of an optical pulse entering diode-laser 22. Diode-lasers 12 and 22 preferably have the same peak-gain wavelength.
This situation is illustrated schematically in
Continuing now with reference again to
Typically an E-O modulator for operation at about 1000 nm wavelength is at least two times more expensive than a diode-laser. However, an E-O modulator provides sharper edges of an optical pulse since its rise time is typically between about 100 ps and 300 ps. A diode-laser provides simultaneous gain and modulation functions while an E-O modulator introduces high insertion loss, for example between about 4 and 6 dB. To compensate for this loss, fibers 20 or 34 can be or include gain fibers. Gain fibers will, however, require corresponding pump-diode lasers (not shown).
In each of the embodiments of the present invention described above there are two stages of modulation with at least one stage of modulation being in a double-pass configuration. The first stage of modulation provides pulses of a relatively long duration. These pulses are modulated in the second stage of modulation to provide pulses of a shorter duration. Those skilled in the art will recognize that it possible to include, without departing from the spirit and scope of the present invention, more than two modulation stages, in single or double-pass configuration, with pulses being of shorter duration after each stage.
In summary, present invention is described above in terms of a preferred and other embodiments. The invention is not limited, however, to the embodiments described and depicted. Rather, the invention is defined by the claims appended hereto.