Modulation of fiber laser pumped difference frequency generator

195050/3

FIELD OF THE INVENTION

[001] The present invention relates to a system and method for modulation of an OPO by modulation of a fiber laser pump.

BACKGROUND OF THE INVENTION

[002] Fiber lasers are used for producing high average power, high beam quality, high beam stability, and highly efficient sources of laser radiation. Production of light at 1, 1.5, and 2 μm can be performed using Yb, Er/Yb, and Tm:silica fibers, respectively. Other wavelengths can be obtained using Raman lasers (generally up to 2 μm in continuous wave operation) or by wavelength conversion using bulk crystals. Such wavelength conversion processes include harmonic, sum, and difference frequency generation. Harmonic generation is used to produce shorter wavelengths at multiples of the fundamental laser frequency. An example of harmonic generation is second harmonic generation (SHG) for producing green light (532nm) from a 1064nm Yb:silica fiber laser pump. Longer wavelengths are typically produced using difference frequency generation (DFG). More commonly, DFG is performed in a cavity to enhance the efficiency, i.e. in an optical parametric oscillator (OPO). OPOs are generally used to generate longer wavelengths from a shorter wavelength and can produce a broad range of wavelengths which are longer than the fiber laser pump source.

[003] Multiple applications require high average power and high beam quality sources in the mid-infrared. In addition, such applications may also require modulation of output power or temporal characteristics. These applications include material processing (especially of glasses and plastics), spectroscopy of materials with mid-infrared signatures, remote sensing through the atmosphere, laser radar, free-space laser communications, medical applications, infrared countermeasures, and others.

[004] These applications often require the ability to quickly modulate the output radiation. For example, in a material processing cutting application, the OPO output radiation needs to be turned on and off with short transition times and stability of output power. The OPO light can be directly modulated by external modulators, such as acousto-optic or electro-optic modulators. However, modulators in the mid-IR are not commonly available with low losses and fast transition times. The modulator itself adds to the complexity and cost of the system. In addition, if the power must be modulated externally to the laser generator, then there can be a large 195050/3 amount of wasted power, both optical and electrical, as the beam is continuously generated but not used.

[005] In a paper titled “Compact, high average power, fiber-pumped terahertz source for active real-time imaging of concealed objects” by Daniel Creeden et al., published in Optics Express, Vol. 15, Issue 10, pp. 6478-6483 (2007), te authors have modeled and demonstrated a scalable, compact, fiber-pumped terahertz source based on difference frequency mixing (DFM) in zinc germanium phosphide (ZGP) capable of producing high average and peak-power pulses. Currently, our terahertz source produces 2mW of average THz power and >40W of peak power with sub-nanosecond pulses at a repetition rate of 100kHz in the range of 2-3THz without cryogenic cooling or ultra-fast optics. This high average power laser-based terahertz output enables the real-time imaging of concealed objects using an off-the-shelf uncooled microbolometer focal-plane array. With this THz system, we have imaged objects obscured inside in a common shipping envelope, demonstrating the potential of compact laser-based terahertz sources for use in security screening applications.

[006] European Patent Application No. EP 1113540 A1 teaches a cascaded optical amplifier system having at least two optical amplifying stages with two pulsed pump sources provided and two amplifying media, the first of which receives the injection and at least one pump signal wherein the injection pulsed signal is amplified to a first power and energy lever. The second amplifying medium, optically coupled to the first, amplifies the first level signal to a second level amplified, injection pulsed signal. The duty cycle of the injection pulsed signal is synchronized with the first and second pulsed pump signals with the first pulsed pump signal having a different duration in the duty cycle than the second pulsed pump signal.

[007] There is thus a need for a system and method for modulation of OPO output without the use of external modulators and that can be done in the mid-IR range.

SUMMARY OF THE INVENTION

[008] There is provided, in accordance with embodiments of the present invention, a system for modulating the output of a difference frequency generator. The system includes a pump fiber laser having at least one internal, directly modulatible component, wherein the pump fiber laser produces a pump signal, and a difference frequency generator coupled to the pump fiber laser. The difference frequency generator is configured for accepting the pump signal of the pump fiber laser and producing an output signal, wherein parameters of the output signal are 195050/3 determined based on direct modulation of the internal, directly modulatible component of the pump fiber laser.

[009] There is provided, in accordance with embodiments of the present invention, a method for modulating the output of a difference frequency generator. The method includes providing a pump fiber laser having at least one internal, directly modulatible component, modulating the internal, directly modulatible component, obtaining a pump signal from the modulated component of the pump fiber laser, providing the obtained pump signal to a difference frequency generator, and obtaining an output signal from the difference frequency generator, wherein parameters of said output signal are determined based on the modulation of the internal, directly modulatible component of the pump fiber laser.

[0010] In further embodiments of the present invention, the pump laser may be an “all-fiber” fiber laser or a hybrid fiber laser. The internal, directly modulatible component is at least one of: a power amplifier pump diode, an amplifier pump diode, or the oscillator. When the internal, directly modulatible component is an oscillator, the difference frequency generator output signal may have an “on” mode or an “off” mode, and the oscillator is modulatible by keeping average power of the pump fiber laser constant while providing an oscillator signal having an amplified peak power above a difference frequency generator threshold for the “on” mode and below the difference frequency generator threshold for the “off” mode. This may be accomplished by modulating at least one of: pulse duration, peak power, repetition rate, or pulse shape of the oscillator signal.

[0011] In additional embodiments, when the directly modulatible component is an oscillator, the difference frequency generator output signal may have a modified energy or repetition rate, and the oscillator is modulatible by keeping average power of the pump fiber laser constant while providing an oscillator signal having a modulated pulse energy or a modulated repetition rate.

[0012] In yet additional embodiments of the present invention, the pump fiber laser is a MOPA fiber laser, and the internal, directly modulatible component is either a power amplifier pump diode or an amplifier pump diode. The difference frequency generator produced output signal may be obtained by modulating the power amplifier pump diode while keeping the pump signal constant prior to its entry into the power amplifier pump diode. Alternatively, the difference frequency generator produced output signal may be obtained by modulating the amplifier pump diode. 195050/3 [0013] In some embodiments, the internal, directly modulatible component may include two or more of: a power amplifier pump diode, an amplifier pump diode, or an oscillator. The difference frequency generator is at least one of: an OPO, an OPA or an OPG.

[0014] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. In case of conflict, the patent specification, including definitions, will control. In addition, the materials, methods, and examples are illustrative only and not intended to be limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The above and further advantages of the present invention may be better understood by referring to the following description in conjunction with the accompanying drawings in which: FIG. 1 is a schematic illustration of a system 10 for modulation of OPO output, in accordance with embodiments of the present invention; FIG. 2A is a graphical illustration of pump power with modulation of pulse sequences; FIG. 2B is a graphical illustration of OPO power with the modulation of FIG. 2A; FIG. 3A is a graphical illustration of pump power with modulation of pulse duration; FIG. 3B is a graphical illustration of OPO power with the modulation of FIG. 3A; FIG. 4 is a schematic illustration of a MOPA fiber laser with pump diodes which can be used in the present invention; FIGS. 5A and 5B are graphical illustrations of output power and gain during a simulation; and FIG. 6 is a graphical illustration of OPO output during the simulation of FIGS. 5A and 5B.

[0016] It will be appreciated that for simplicity and clarity of illustration, elements shown in the drawings have not necessarily been drawn accurately or to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity or several physical components may be included 195050/3 in one functional block or element. Further, where considered appropriate, reference numerals may be repeated among the drawings to indicate corresponding or analogous elements. Moreover, some of the blocks depicted in the drawings may be combined into a single function.

DETAILED DESCRIPTION

[0017] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be understood by those of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and structures may not have been described in detail so as not to obscure the present invention.

[0018] The present invention is directed to a system and method for modulating output of a difference frequency generator, such as an OPO, OPA or OPG. In addition, it should be understood that the same or similar method exists for harmonic generation, sum frequency generation, and any other nonlinear process driven by a fiber laser. The principles and operation of a system and method according to the present invention may be better understood with reference to the drawings and accompanying descriptions.

[0019] Before explaining at least one embodiment of the present invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.

[0020] It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

[0021] Reference is now made to FIG. 1, which is a schematic illustration of a system 10 for modulation of OPO output, in accordance with embodiments of the present invention. Generally, an OPO is pumped by a pulse from a pulse source. The pulse source may be, for example, a fiber laser or a Q-switched bulk laser. In the present invention, pulse source 12 is a fiber laser, and more specifically, a MOPA 195050/3 (master oscillator power amplifier) fiber laser 13. MOPA fiber laser 13 is used to pump an OPO 14. The use of a MOPA fiber laser allows for direct control over the output pump laser properties.

[0022] As shown in FIG. 1, MOPA fiber laser 13 includes a master oscillator 18 for providing a seed pulse and an amplifier 20. The seed pulse is sent to amplifier 20, which in some embodiments may comprise a string of amplifiers, as shown in FIG. 1. Master oscillator 18 may be a laser diode with or without an externally modulated switch, for example. Modulation of OPO output is done internally within MOPA fiber laser 13, as will be described in greater detail hereinbelow. Modulated output from MOPA fiber laser 13 is sent to OPO 14, resulting in IR radiation, as shown by arrow 16.

[0023] For the purposes of the present invention, the following terms are defined as follows: [0024] “Pulse signal” is a pulse generated within a pulse source such as a MOPA fiber laser.

[0025] “Pulse parameter” refers to any parameter of the pulse signal itself, including, for example, pulse shape, pulse duration, pulse frequency, peak power, intensity, etc.; [0026] “Pulse source component” refers to any physical component within the pulse source which is used to generate or to modify the pulse signal, for example, oscillators, diodes, amplifiers, modulators (e.g. electo-optic, acousto-optic, electro-absorption); [0027] In embodiments of the present invention, modulation is done by modulating an internal, directly modulatible component of a pump fiber laser, and may include modulation of a pulse source component or a pulse parameter, as defined above.

[0028] Reference is now made to FIGS. 2A-2B and 3A-3B, which are graphical illustrations of examples of modulation of pulse parameters, in accordance with a first embodiment of the present invention. In this embodiment, modulating OPO output is done while keeping average output power of pulse source 12 constant. The main reason for keeping the laser output average power constant is to avoid transient behavior in the laser. If, for example, the pulse train to the amplifier chain is stopped and then restarted, the additional accumulated energy in the amplifier from the pump diodes will lead to a high energy pulse followed by a transient until steady state behavior is obtained. This high energy pulse may 195050/3 damage the laser or the OPO, which often works close to damage threshold to obtain high conversion efficiencies. Keeping the average power of the laser the same (over fractions of the gain media relaxation time) eliminates the transients allowing for fast switching times on the order of the pulse repletion rate. It is also advantageous to keep the peak power of the OPO constant in order to maximize conversion efficiency by the OPO design as in FIG 3. Higher peak powers may lead to damage, whereas lower peak powers may lead to less efficient conversion. As shown in FIG. 2A, modulation of pulse parameters may be done by changing of pulse sequences. Thus, for example, one part of a sequence might include high peak power pulses 30 while another part of a sequence might include low peak power pulses 32. However, during the low peak power portion of the sequence, a high repetition rate will be used so as to keep average output power constant. An OPO will only convert pulses with intensities above the OPO threshold. Thus, by modulating the pulse sequences as in FIG. 2A, resulting OPO power will have an “on” state 34 for high peak power pulses and an “off” state 36 for low peak power pulses, as shown in FIG 2B. This configuration provides short on/off transition times.

[0029] Modulation of the pulse duration is often times required by the application. For example, in a materials processing application high energy pulses may be used for ablating whereas low energy high repetition rate pulses maybe used for welding. As shown in FIG. 3A, modulation of pulse parameters may be done by changing of pulse duration and the pulse spacing, or period. This method provides modulation of energy per pulse while keeping average power constant. Thus, for example, a pulse or series of pulses might have a long duration 38 while another pulse or series of pulses has a short duration 40. The long duration pulses may have longer periods 39 - spaced further apart -- while the short duration pulses have shorter periods 41, to keep average power of the pulse source constant. As shown in FIG. 3B, modulation of pulse duration and periods provides modulation of energy per pulse, and results in increased pulse energy 42 and decreased pulse energy 44 for OPO output.

[0030] Reference is now made to FIG. 4, which is a schematic illustration of a MOPA fiber laser 13 with pump diodes which can be used in the present invention. An example of modulating OPO output by direct modulation of components within pulse source 12 (MOPA fiber laser 13) is now described.

[0031] MOPA fiber laser 13 includes an oscillator 18 and an amplifier 20, wherein amplifier 20 may include a power amplifier 21 and one or several additional amplifiers 23. MOPA fiber laser 13 further typically includes at least one power 195050/3 amplifier pump diode 50 and at least one amplifier pump diode 52 for each of amplifiers 23. Modulation of components of MOPA fiber laser 13 can include modulation of oscillator 18, one or multiple power amplifier pump diodes 50 and/or one or multiple amplifier pump diodes 52.

[0032] Often times it is desired to conserve overall laser power, meaning that if there is not OPO output, the laser should be turned off. Most of the power consumption occurs in the pump diodes 50 of the power amplifier. Thus, modulating this diode directly may result in the desire OPO output characteristics while at the same time decreasing the laser overall power consumption, or increasing the electrical efficiency. However, modulating the pump diodes will result in slower transition times than modulating the oscillator and keeping the output power of the fiber laser constant.

[0033] Modulating the pump diode 50 results in modulation of the MOPA fiber laser output. The OPO thus may be turned off by bringing the output power of the MOPA fiber laser below the OPO threshold. By turning off the power amplifier pump diode 50, MOPA fiber laser 13 will be in an “off” state when no OPO output is required, thereby reducing overall power consumption and thermal load. However, as stated above, any of the pump diodes 52 or oscillator 18 may be modulated as well to achieve desired system performance parameters, such as minimum power consumption or rise time. Generally, fast rise and fall times may be achieved by modulating the pulse characteristics of the oscillator, as shown in FIG 2. Modulating both the pump diodes 50 and the oscillator 18 may result in both a highly efficient laser source and fast transition times.

[0034] In summary, tailoring of the pump source output can be used to 1) modulate the OPO output with fast transition times, and 2) operate the OPO different pulse energies. More specifically, fiber laser modulation of the OPO allows for OPO modulation with no additional optical components. In addition, in low duty cycle applications, power savings can be obtained by only operating the pump source when OPO output is required.

EXAMPLE

[0035] A simulation of a method of directly modulating power amplifier pump diode 50 was performed. A 50W amplifier based on LMA-YDF-25/250 fiber (available from, for example, Nufern or Liekki) was pumped with 100W of diode power at 915nm. The input signal was a train of 100ns pulses at 200kHz repetition rate. The output power and gain are shown in FIGS. 5A and 5B, respectively. The 9 195050/3 pump was turned on at time zero and off at 250 ps. Full amplification was reached within 50με. After the pump was stopped, the pulses slowly trailed off.

[0036] If this laser output is then sent to an OPO which operates at three times threshold with at 12.5% slope efficiency, then the output is shown in FIG. 6. 5 The rise time and fall time of the OPO is much less than the rise time of the laser due to the thresholding effect of the OPO. If faster rise times are required, 1) the laser can be pumped harder thereby increasing the output power, 2) the input pulses can be turned off in order to obtain a faster inversion, or 3) the pulses from the oscillator can be modulated in such a way to allow for fast transition times, as in FIG. 2.

10 [0037] To conclude, these preliminary simulations indicate that fast OPO switching, rise and fall times <50us, can be performed by modulating the pump diodes of the power amplifier of a MOPA chain. Such modulation will result in a power consumption reduction proportional to the duty cycle. 15