Abstract:
Master oscillator power amplifier (MOPA) apparatus includes two seed-pulse sources coupled to a single fiber amplifier including one or more stages of amplification. One of the seed-pulse sources is a single-mode source generating pulses having a duration selectively variable between about 0.1 ns and 10 ns. The other seed-pulse source is a multi-mode source generating pulses having a duration selectively variable between about 1 ns and 10 μs. Selectively operating one or the other of the seed-pulse sources provides that the apparatus can deliver pulses selectively variable in a range between about 0.1 ns and 10 μs.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to master oscillator power amplifier (MOPA) laser systems including fiber-amplifiers. The invention relates in particular to pulsed MOPAs in which pulse duration is selectively variable. 
     DISCUSSION OF BACKGROUND ART 
     Extra cavity frequency-converted pulsed solid state lasers are used extensively for material processing applications such as machining, drilling, and marking. Most commercially available, pulsed, solid-state lasers are Q-switched pulsed lasers. 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 length of the resonator, the transmission of the partially-transmitting mirror, losses 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 a fiber-based MOPA in which seed pulses are generated by a master oscillator in the form of a modulated, edge-emitting semiconductor laser (diode-laser) and amplification is provided by a fiber-amplifier. A fiber-amplifier has relatively high gain, for example between about 13 decibels (dB) and 30 dB. This, combined with a low saturation power, allows a variety of low-power diode-laser seed sources to be used. Such a fiber MOPA can be operated at PRFs from less than 100 kilohertz (kHz) to 5 Megahertz (MHz) or greater, with pulse durations selected between about 0.1 nanosecond (ns) and about 1 microsecond (μs). 
     A significant problem in fiber-amplifiers is created by nonlinear effects in fibers which limit peak power and affect the spectral characteristics of the optical pulses. For harmonic generation at nanosecond pulses, spectrally-narrow light with a bandwidth between about 0.5 nanometers (nm) and 1.0 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. 
     Stimulated Raman scattering (SRS) limits the peak power in typical all-fiber systems with core diameters below 30 μm (so called LMA fibers) although larger diameters of up to 100 μm are also possible using specially designed fibers (so called photonic-crystal fibers and leaky-mode fibers). SRS is the only power-limiting factor for broadband, for example greater than 1.0 nm bandwidth, IR nanosecond pulses. Attempts to narrow the spectral bandwidth using a single-frequency seed source encountered a build-up of stimulated Brillouin scattering (SBS) resulting in optical damage of fiber components. 
     It is known that for long optical pulses, for example pulses having a duration greater than 20 ns with a bandwidth much broader than for SBS, for example between about 0.20 picometers (pm) and 0.25 pm (between about 40 and 50 MHz), the threshold power for SBS grows proportionally to the signal spectral width. This is why a common approach for SBS suppression in long pulses is to broaden the pulse bandwidth to be much larger than the SBS bandwidth of about 0.2 pm while keeping the pulse bandwidth below about 1 nm that is appropriate for frequency conversion. However, spectral broadening of nanosecond pulses due to four-wave mixing in fibers transfer the energy from the narrow spectral peak to the spectral wings at pulse peak powers above 100 W ( FIG. 1 ). This effect reduces frequency-conversion efficiency in all-fiber systems compared to solid-state lasers. 
     Another way to reduce SBS is to use pulses shorter than the SBS build-up time in fibers, which is typically close to 20 ns. For pulses having a duration less than 20 ns, SBS occurs in a transient regime with a smaller gain-factor. 
     There are two common approaches to generate pulses with variable length and pulse repetition rate. The first approach uses a directly modulated=diode-laser as a seed source. Such an approach is, in general, less expensive, and provides high peak power, for example greater than 1 Watt (W) from the seed laser. A major disadvantage of this approach is that to provide short pulses having a duration of less than 10 ns a short cavity length, for example less than about 10 millimeters (mm), is required. This, in turn, results in a single-frequency or “few-frequency mode” operation that favors SBS and limits the peak power in fiber-amplifiers. For this reason, this approach is limited to short pulses (duration less than about 10 ns), where SBS exhibits a reduced gain. Using broadband reflectors in the cavity and generating more modes helps to reduce SBS but immediately results in stronger broadening of the spectrum due to FWM, making frequency-conversion inefficient. 
     The second approach uses a continuous wave (CW) optical source or pulsed optical source with long pulses, modulated by an external modulator. In such a system, a seed source could be a diode-laser, a solid-state laser, a fiber laser, or a source generating amplified spontaneous emission (ASE source), such as a superluminescent LED, while a typical modulator is an electro-optical crystal in the waveguide Mach-Zehnder configuration or a semiconductor optical amplifier. 
     On one hand, such an approach provides less peak power (typically less than 100 mW) after modulation compared with that available from a directly modulated diode. On the other hand this approach allows generation of pulses of any length and repetition rate with a spectrum determined by an appropriately designed seed laser. For example, a diode seed laser may have a low noise operation when a fiber Bragg grating (FBG) written in a fiber placed in 1-2 m from a diode-laser chip provides an output coupler for the diode-laser cavity. With a typical FBG bandwidth of between about 0.01 nm and 1.0 nm, such a diode-laser will have a much broader spectrum than the SBS bandwidth, which helps to suppress SBS. However, due to mode-beating such a source exhibits pulse-to-pulse fluctuations, especially when operating with short pulses of less than 10 ns duration. The second approach is therefore limited to pulse durations of greater than about 10 ns. 
     For many applications it would be desirable to have a fiber MOPA with a frequency conversion stages operating at any pulse duration between about 0.1 ns and 1 microsecond. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention optical apparatus comprises an optical amplifier including one or more fiber-amplification stages. A single-mode source of optical pulses is provided and arranged to deliver optical pulses having a pulse-duration selectable in a first range of pulse durations including pulse durations between a first duration and a second duration. A multi-mode source of optical pulses is provided and arranged to deliver optical pulses having a pulse-duration selectable in a second range of pulse durations including pulse durations between about a third duration and a fourth duration, where the fourth duration is greater than the second duration. Pulses from the few mode and multi-mode pulse sources can be selectively coupled to the optical amplifier, such that the apparatus can deliver optical pulses having a duration selectable in a third range of durations between the first duration and the fourth duration. 
     In a preferred embodiment of the invention the first, second third and forth durations are respectively about 0.1 ns, 10 ns, 1 ns, and 1000 ns. Accordingly, in this embodiment of the inventive apparatus can deliver pulses having a duration selectable in a range between about 0.1 ns and 1000 ns (1 μs). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIG. 1  schematically illustrates one preferred embodiment of MOPA apparatus in accordance with the present invention having a single mode seed pulse source including a contrast-enhancing modulator and a multi-mode seed-pulse source with both sources coupled to a fiber amplifier including two stages of fiber amplification, a pulse duration and source selector cooperative with the seed pulse sources to select the source and duration of pulses delivered to the fiber amplifier, and a frequency converter for changing the wavelength of amplified pulses. 
         FIG. 2  schematically illustrates another preferred embodiment of MOPA apparatus in accordance with the present invention, similar to the apparatus of  FIG. 1 , but wherein the contrast enhancing monitor of the single-mode seed pulse source is replaced by a contrast enhancing modulator in the fiber amplifier. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, wherein like components are designated by like reference numerals,  FIG. 1  schematically illustrates one preferred embodiment  10  of a fiber-MOPA laser apparatus in accordance with the present invention. Apparatus  10  includes two optical sources  12  and  14 . The optical sources are connected via a coupler  20  to one or more fiber-amplifier stages. Two stages  22  and  24  are depicted in  FIG. 1  by way of example. The amplifier output is directed to one or more frequency conversion stages  26  for converting the fundamental wavelength of the amplified pulses to a different wavelength such as the second, third, or fourth, harmonic wavelength. Those skilled in there to which the present invention relates will recognize that in any multistage amplifier arrangement it is advisable to provide an optical isolator between stages to prevent feedback for one stage into a prior stage or from an amplifier stage back to a source. Isolators are not shown in  FIG. 1  for simplicity of illustration. Non-linear crystals can be used for the frequency conversion stage. 
     Optical source  12  is a single-mode pulsed source which provides optical pulses having a duration selectively variable in a relatively short-duration range, for example between about 0.1 ns to 10 ns. Here “single-mode” source is intended include a source that operates in one or a very few (four or less) modes. However, pure single mode operation is preferred. Optical source  12  can be a single-mode (single-frequency) directly modulated diode-laser, in particular an extended cavity diode-laser with distributed feedback (DFB) provided by distributed Bragg grating (DBG) written in an optical fiber (detail not shown). In such a directly modulated diode-laser, there is a small frequency sweep (chirp) from the beginning of the pulse to the end of the pulse. This frequency chirp helps to increase SBS threshold. Single-mode operation of source  12  also helps to reduce FWM in fiber-amplifiers and provides good pulse stability. A combination of a CW single-frequency laser with an external modulator can be used instead of a directly modulated diode-laser for source  12 . 
     Optical source  14  is a multiple-longitudinal-mode (multiple-frequency) pulsed laser providing radiation having a spectral bandwidth much broader, for example at least about ten-times broader, than the SBS bandwidth of amplifier fibers but less than about 1 nm. Preferably, the spectral bandwidth (FWHM) is between about 50 pm and 1 nm. Laser  14  provides optical pulses having a duration selectively variable in a range of durations longer than the range of laser  12 , for example between about 1 ns to 1000 ns (1 μs). For such a broadband source, the SBS threshold grows proportional to the spectral bandwidth of the pulses. Here it should be noted that there can be some overlap between the pulse duration ranges of sources  12  and  14 . 
     Optionally, the output of the source can be modulated by a modulator  16  to increase contrast between pulses and background light. This technique is described in detail in U.S. patent application Ser. No. 12/132,449, filed Jun. 3, 2008, assigned to the assignee of the present invention and the complete disclosure of which is hereby incorporated by reference. 
     Further it may be necessary to locate either an amplifier or an attenuator between either of sources  12  and  14  and the coupler, depending on the sources and other components, to approximately match average powers from both sources at the coupler. By way of example, in apparatus  10 , a fiber-amplifier  18  is included between source  14  and the coupler to compensate extra loss caused by optical modulator  16 . 
     An extended cavity diode-laser with an output mirror based on a fiber Bragg grating having a bandwidth of more than 0.01 mn and placed at least 1 meter from the diode chip is one example of a laser suitable for multimode source  14 . Another example of a laser suitable for multimode source  14  is an amplified spontaneous emission (ASE) fiber-laser including a spectral filter (for example, a fiber Bragg grating), which narrows the bandwidth of ASE source while keeping the coherence length of the source short. Such an ASE fiber-laser having short coherence length would help to reduce FWM and increase SBS threshold in fiber-amplifiers  22  and  24 . It is preferable that pulses from both sources have the same nominal wavelength (center wavelength) 
     Coupler  20  combines light from each of sources in one fiber  21  and delivers it to amplifier stages  22  and  24 . Preferably coupler  20  is a fused-fiber coupler or a fiber-pigtailed micro-optics based coupler. Only one of sources delivers pulses to fiber amplifiers  22  and  24  at any time depending on the desired pulse duration. The source and pulse duration is selected by user input to a selector  28 , which can be a component of a more universal control-electronics arrangement for operating parameters of apparatus  10 . Alternative possible coupling arrangements include such a mechanical switch between fiber output ports of each source, an electro-optical modulators such as a Mach-Zehnder (MZ) interferometer formed in a lithium niobate (LiNbO 3 ) crystal, and an acousto-optic (A-O) modulator with two input ports an one output port. 
       FIG. 2  schematically illustrates another preferred embodiment  30  of a fiber-MOPA laser apparatus in accordance with the present invention. Apparatus  30  is similar to apparatus  10  of  FIG. 1 , with an exception that modulator  16  in the single mode source arm of apparatus  10  is omitted and an optional modulator  32  is located between coupler  20  and fiber-amplifier stage  22 . Modulator  32  provides contrast enhancement for pulses from either of sources  12  and  14 , or could provide additional enhancement if either source included a contrast enhancing monitor. It should be noted, however, that the contrast-enhancement action of the monitor reduces the duration of pulses the contrast of which is being enhanced. This should be taken into account when selecting a pulse-duration from either source. Modulator  32  can also be used for temporally shaping pulses from either source. 
     The present invention is described above with reference to preferred embodiments thereof. The invention however is not limited to the embodiments described and depicted herein. Rather the invention is limited only by the claims appended hereto.