US20080273559A1 - Multiple Output Repetitively Pulsed Laser - Google Patents

Multiple Output Repetitively Pulsed Laser Download PDF

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US20080273559A1
US20080273559A1 US11/744,625 US74462507A US2008273559A1 US 20080273559 A1 US20080273559 A1 US 20080273559A1 US 74462507 A US74462507 A US 74462507A US 2008273559 A1 US2008273559 A1 US 2008273559A1
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cavity
pulse
intracavity
laser
output
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Mikhail Grishin
Andrejus Michailovas
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Ekspla Ltd
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Ekspla Ltd
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Priority to US11/744,625 priority Critical patent/US20080273559A1/en
Assigned to EKSPLA LTD. reassignment EKSPLA LTD. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: GRISHIN, MIKHALL, MICHAILOVAS, Andrejus
Assigned to EKSPLA LTD. reassignment EKSPLA LTD. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: GRISHIN, MIKHAIL, MICHAILOVAS, Andrejus
Priority to PCT/IB2008/001460 priority patent/WO2008135859A2/fr
Publication of US20080273559A1 publication Critical patent/US20080273559A1/en
Priority to US12/649,732 priority patent/US7970026B2/en
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • H01S3/1103—Cavity dumping
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
    • H01S3/08—Construction or shape of optical resonators or components thereof
    • H01S3/08054—Passive cavity elements acting on the polarization, e.g. a polarizer for branching or walk-off compensation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/10038—Amplitude control
    • H01S3/10046—Pulse repetition rate control
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
    • H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
    • H01S3/11—Mode locking; Q-switching; Other giant-pulse techniques, e.g. cavity dumping
    • H01S3/1106—Mode locking

Definitions

  • the present invention is directed to stabilizing the output pulse energy in a continuously pumped, repetitively cavity-dumped laser.
  • a laser in its simplest configuration, operates in a “continuous wave” mode, in which the power output is relatively constant over time.
  • a pulsed laser in which the power output is extremely high over a very short period of time.
  • the output power is effectively “stored up” in the laser cavity over a period of time, then released in a short pulse.
  • the duration of the laser pulse known as the pulse width or pulse duration
  • the output power of the laser may be many orders of magnitude larger than the continuous wave output power.
  • mode locking Mode locked lasers can routinely produce pulses with durations of picoseconds.
  • cavity dumping A common technique for generating microjoule level pulses without using complex and expensive amplifier schemes is known as “cavity dumping”.
  • cavity dumping In cavity dumping, the basic idea is to keep the optical losses of the laser cavity as low as possible for some time, so that an intense pulse builds up in the cavity, then to extract this pulse within about one cavity round-trip time using an optical switch, which may be acousto-optic or electro-optic.
  • the switch may be referred to as a “cavity dumper.” Cavity dumping is explained in greater detail in the following paragraphs.
  • a laser cavity used for cavity dumping is similar to that used for Q-switching, but containing only highly reflecting mirrors (i.e., no partially transmissive output coupler mirror).
  • Output coupling is controlled with the optical switch in the cavity, typically a combination of an electro-optic modulator (EOM) and a polarizer, which is quickly turned on for pulse extraction and then directs the intracavity beam into the output. At times other than during pulse extraction, the light can circulate in the resonator with low losses.
  • EOM electro-optic modulator
  • Pulse amplification then occurs as follows. Initially, the modulator is set so that most of the light in the cavity is coupled out of the cavity. In this initial state, the power is below the laser threshold and no lasing occurs. The pump energy in the cavity is stored primarily in the gain medium. Next, the modulator is switched so that the cavity losses are reduced to small parasitic losses. In this switched state, the power in the cavity builds up quickly, typically within a few hundred cavity round-trip times. Finally, the modulator is quickly set so that most of the light is again coupled out of the cavity. In this final stage, the energy in the cavity is extracted in about one round-trip time. The cycle is then repeated.
  • An advantage of cavity dumping over Q-switching is that prior to extraction of a pulse, the energy in the cavity is stored in the electric field inside the cavity, rather than in the gain medium. As a result, the energy can be extracted much more quickly than for Q-switching, typically, in one round-trip time. This, in turn, allows for high repetition rates for cavity dumping, which is highly desirable for many applications, such as industrial machining, drilling, cutting and surface engineering.
  • Cavity dumping for ultrashort pulses is mostly used with mode-locked solid state bulk lasers, such as titanium-sapphire lasers or diode-pumped neodymium-doped or ytterbium-doped lasers.
  • the pulse energy from a cavity-dumped, mode-locked laser may typically be about an order of magnitude higher than with an ordinary mode-locked laser (i.e., typically of the order of 1 microjoule), and the pulse repetition rate can be hundreds of kilohertz or even several megahertz or higher.
  • a potential issue with cavity-dumped lasers is that the pulse energy can undesirably depend on the time between pulses, particularly for fast repetition rates.
  • the pulse energies of upcoming output pulses are generally stable over time (because energy of the output pulse is proportional to the intracavity energy at the instant of cavity dumping). In this regime, the intracavity energy at the instant of ejecting the pulse may occur in the relatively flat steady state region. If the repetition rate is changed by a user, or a pause is inserted between pulses, the intracavity energy does not change much. As a result, the energy of the next output pulse is largely unaffected. For these relatively slow repetition rates, the pulse energy is largely independent of time between pulses.
  • the next pulse ejection may occur during the oscillations.
  • the intracavity energy at the instant of ejecting the pulse may occur on a rapidly-changing oscillation, rather than on the relatively flat steady-state region. If the repetition rate is changed or a pause is inserted between pulses, the intracavity energy may vary significantly. As a result, the energy of the next output pulse may be significantly affected by the time between pulses, which may be undesirable.
  • the first pulse in each train may have a pulse energy that varies train-to-train, which is undesirable.
  • a continuously pumped, mode-locked, cavity-dumped laser 1 shown in FIG. 1 Two mirrors 4 and 9 define the laser cavity, which also typically contains a gain medium 6 , a mode locker 5 and a controllable switch 2 (or “cavity-dumper”).
  • the energy inside the laser cavity (or “intracavity energy”) takes the form of a pulse that bounces back and forth between the two mirrors. For each round-trip pass of the pulse in the cavity, the pulse passes twice through the gain medium 6 .
  • the pump for the gain medium 6 remains on throughout operation of the laser.
  • the cavity-dumper 2 can either allow the intracavity energy to remain inside the cavity, or direct the intracavity energy out of the cavity into an output beam 14 .
  • the cavity-dumper 2 typically has a polarizer 8 and an electro-optic modulator 7 , which is driven electrically by a controller 3 .
  • the electro-optic modulator 7 rotates the plane of polarization of a transmitted beam, in response to the voltage produced by the controller 3 .
  • the laser 1 output can be switched “on” and “off” by the controller 3 , where the “on” portion produces a stream of pulses, and the “off” portion produces no pulses.
  • the controller is driven by a controller driving signal 11 , a portion of which is shown in FIG. 1 .
  • the controller driving signal 11 has “on” portions, such as element 15 , and “off” portions, such as element 16 .
  • the controller 3 converts the controller driving signal to a modulator driving signal 12 , which switches the electro-optic modulator 7 for a particular duration during each pulse round-trip in the cavity.
  • the switched electro-optic modulator 7 changes the polarization state of the traveling pulse so that it is reflected by polarizer 8 and directed out of the cavity into the output beam 14 .
  • the intensity of the output beam contains streams of pulses 13 when the controller driving signal is “on”, and is effectively zero when the controller driving signal is “off”.
  • the pulses are spaced apart in time by a multiple of the round-trip time of a pulse in the cavity.
  • the energy contained in the first pulse is not necessarily constant from train-to-train. For instance, the energy in pulse 17 is less than in pulse 19 but greater than in pulse 20 .
  • the energy of the first pulse in the train depends on the length of the “off” portion that immediately precedes the train. In particular, the repetition rate of the laser 1 is fairly high, and is comparable to the relaxation rate of the cavity energy. This variation of the energy contained in the first pulse is undesirable, and the cause of this variation is explained in more detail in the paragraphs that follow.
  • FIG. 2 shows the energies in the cavity 60 a and in the output beam 60 b in greater detail, as a function of time. Prior to the time interval shown, there is a history of pulses, denoted by element 66 , which is relatively unimportant for this discussion.
  • the history of pulses is long enough so that in region 67 , the pulses have a relatively constant energy from pulse-to-pulse.
  • This region is analogous to region 18 in FIG. 1 .
  • the pulse repetition rate is f, so that the pulse-to-pulse time is 1/f.
  • the intracavity energy is E stream , and the output energy is E Const .
  • each pulse drains the cavity of a certain amount of energy, which is directed into the output beam and forms the pulse. After each pulse, the intracavity energy begins climbing again.
  • the climbing and draining amounts are roughly equal, as long as the pulse repetition rate f remains roughly constant.
  • Region 68 is analogous to the “off” region between region 18 and pulse 19 in FIG. 1 .
  • the controller driving signal is set to “off”, and the modulator driving signal is set so that the polarization of the pulse transmits through the polarizer, and the light pulse remains in the cavity.
  • the pump is always on, so that energy continuously enters the laser cavity.
  • the laser cavity includes both the gain medium and the intracavity energy.
  • the intracavity energy oscillations seen in region 68 indicate that energy “sloshes back and forth” between the gain medium, where it is stored as a population inversion, and the intracavity energy, where it is stored in the electric field.
  • the “sloshing” last for a few oscillations before the intracavity energy settles to a steady-state value, denoted by E steady-state .
  • Region 69 in FIG. 2 is analogous to the “on” region in FIG. 1 beginning with pulse 19 .
  • the controller driving signal 15 is set to “on”, the modulator driving signal switches the modulator once for each pulse, so that when switched, the beam reflects off the polarizer and is directed into the output beam.
  • the pulses in the output beam form a train, with the amplitude of the first pulse being different from the amplitudes of the other pulses in the train.
  • Pulse 61 in FIG. 2 is analogous to pulse 19 in FIG. 1 .
  • a substantial portion of the initial intracavity energy here being fairly close to E steady-state , is directed into the output beam and forms pulse 61 .
  • the energy contained in pulse 61 may vary significantly from the energy E Const of the pulses found in region 67 .
  • the intracavity energy begins climbing again, in a manner analogous to region 67 and to the leftmost portion of region 68 in FIG. 2 .
  • the pulse energy returns to E Const and the intracavity energy returns to E stream , as in region 67 .
  • An embodiment is a method of generating a laser output, comprising: amplifying a first laser pulse within a cavity of a laser to establish a second laser pulse propagating along a path within the cavity in a first direction; directing a first fraction of the second laser pulse into a first output arm, wherein a second fraction of the second laser pulse continues to propagate along the path in the first direction; amplifying the second fraction of the second laser pulse within the cavity to establish a third laser pulse propagating along the path within the cavity in a second direction opposite the first direction; directing a third fraction of the third laser pulse into a second output arm, wherein a fourth fraction of the third laser pulse continues to propagate along the path in the second direction; amplifying the fourth fraction of the third laser pulse within the cavity to establish a fourth laser pulse within the cavity; and extracting the third fraction of the third laser pulse from the second output arm to provide the laser output.
  • Another embodiment is a method of generating output pulses from a laser having a cavity bounded by a first mirror and a second mirror and containing a circulating intracavity pulse, comprising: repeating for a predetermined number of cavity round-trips the sequence of: reflecting the intracavity pulse from the first mirror; retaining the intracavity pulse in the cavity with the cavity dumper; reflecting the intracavity pulse from the second mirror; and retaining the intracavity pulse in the cavity with the cavity dumper; reflecting the intracavity pulse from the first mirror; directing a fraction of the intracavity pulse into a first output arm with a cavity dumper to form a first output pulse; retaining a fraction of the intracavity pulse in the cavity; reflecting the intracavity pulse from the second mirror; retaining the intracavity pulse in the cavity with the cavity dumper; repeating for the predetermined number of cavity round-trips the sequence of: reflecting the intracavity pulse from the first mirror
  • Another embodiment is a method of generating output pulses from a laser having a cavity bounded by a first mirror and a second mirror and containing a circulating intracavity pulse, comprising: repeating for a predetermined number of cavity round-trips the sequence of: reflecting the intracavity pulse from the first mirror; retaining the intracavity pulse in the cavity with a cavity dumper; reflecting the intracavity pulse from the second mirror; and retaining the intracavity pulse in the cavity with the cavity dumper; reflecting the intracavity pulse from the first mirror; directing the intracavity pulse to the cavity dumper, the intracavity pulse having a total power at incidence upon the cavity dumper; directing with the cavity dumper a first output percentage of the intracavity pulse into a first output arm to form a first output laser pulse; retaining a first retention percentage of the intracavity pulse in the cavity; reflecting the intracavity pulse from the second mirror; directing with the cavity dumper
  • a laser comprising: a cavity for containing intracavity light in a first direction and a second direction opposite the first direction; a cavity dumper disposed in the cavity for selectively diverting intracavity light into either or neither of a first output arm or a second output arm, comprising a first polarizer having a first pass axis, a second polarizer having a second pass axis, and a modulator disposed in the laser cavity between the first and second polarizers; and a modulator controller for selectively rotating the polarization of intracavity light traveling in the first direction away from the first pass axis so that intracavity light traveling in the first direction reflects off the first polarizer to form the first output arm, and for selectively rotating the polarization of intracavity light traveling in the second direction away from the second pass axis so that intracavity light traveling in the second direction reflects off the second polarizer to form the second output arm.
  • FIG. 1 is a schematic drawing of a repetitively pulsed laser system.
  • FIG. 2 is a graph of the intracavity energy and output energy of the laser of FIG. 1 .
  • FIG. 3 is a schematic drawing of another repetitively pulsed laser system.
  • FIG. 4 is a schematic drawing of the pulse timing sequence of the laser of FIG. 3 .
  • FIG. 5 is a schematic drawing of another pulse timing sequence of the laser of FIG. 3 .
  • FIG. 6 is a graph of one laser cycle of the intracavity energy after cavity dumping of the laser of FIG. 3 .
  • FIG. 7 is a graph of the intracavity energy and output energy of the laser of FIG. 3 .
  • FIG. 8 is a graph of the output pulse energy as a function of cavity dumping repetition rate and of cavity dumping ratio of the laser of FIG. 3 .
  • FIG. 9 is a schematic drawing of a laser pulse being directed into an output arm of the laser of FIG. 3 .
  • FIG. 10 is a schematic drawing of a laser pulse being directed into another output arm of the laser of FIG. 3 .
  • FIG. 11 is a plot of the modulator controller signals that direct the laser pulses into the output arms shown in FIGS. 9 and 10 .
  • FIG. 12 is a schematic drawing of a cavity dumper.
  • FIG. 13 is a schematic drawing of another cavity dumper.
  • FIG. 14 is a schematic drawing of another cavity dumper.
  • FIG. 15 is a schematic drawing of another cavity dumper.
  • FIG. 16 is a schematic drawing of another cavity dumper.
  • FIG. 17 is a schematic drawing of another cavity dumper.
  • a continuously pumped, mode-locked laser which includes a cavity dumper that can remove a constant fraction of the light from the cavity at every 1/f period of time, independent of the time at which the first pulse in a train is initiated.
  • the cavity dumper includes a modulator and two output arms, denoted as a primary output arm and a secondary output arm.
  • the pulses are directed to the primary output arm.
  • the pulses are directed to the secondary output arm, which terminates in an absorber or at a secondary optical system. In this manner, the energy contained in each output pulse is essentially constant, from pulse-to-pulse and from train-to-train.
  • polarization being “rotated” by an electro-optic modulator.
  • an electro-optic modulator being driven by a high voltage functions as a wave plate that introduces a phase difference between the polarization state parallel to the optic axis of the modulator crystal and polarization state perpendicular to the optic axis of the modulator crystal.
  • linearly polarized light may become elliptically polarized upon passing through the modulated crystal. It will be understood that the expression “polarization rotation of about 90 degrees” may mean the “polarization change that gives about 100% cavity dumping”.
  • polarization rotation of more or less than 90 degrees may mean the “polarization change that gives less than 100% cavity dumping”, and so forth.
  • polarization states into and out of the various modulators need not remain strictly linear, but may generally be elliptical. The phase shift between orthogonal polarization components may therefore be effectively ignored in the following discussion.
  • a laser 21 that overcomes the disadvantages described above is shown in FIG. 3 . Note that a second output arm 28 is added to the laser, using a reflection generated by a second polarizer 24 in the cavity dumper 22 .
  • the controller 23 Based on the controller driving signal, the controller 23 now generates a modulator driving signal or voltage 25 that either directs the pulses to a first output arm 14 (analogous to the “on” state described earlier), or to a second output arm 28 (analogous to the “off” state described earlier).
  • the pulses 26 in the first output arm 14 now have essentially equal energies from pulse-to-pulse.
  • the energy of the first pulse in the train is largely independent of the “off” period that immediately precedes it, and is essentially equal to the energies of subsequent pulses in the train. In this manner, the laser 21 overcomes the disadvantages described earlier.
  • FIG. 7 shows the intracavity energy 70 a , the output beam at the first output arm 70 b and output beam at the second output arm 70 c , for the laser 21 of FIG. 3 .
  • element 76 Prior to the time interval shown, there is a history of pulses, denoted by element 76 , which is relatively unimportant for this discussion.
  • Region 77 shows an “on” state, in which the pulses are directed into the first output arm.
  • Region 78 shows an “off” state, in which the pulses are directed into the second arm, which can terminate in an absorber or can propagate to a second, additional optical system.
  • Region 79 is also an “on” state.
  • the intracavity energy 70 a has a regular sawtooth-shaped pattern, rising from a minimum value to a maximum value, and dropping back down to the minimum value. Note that the peak intracavity energy is constant for all regions 77 , 78 and 79 and is equal to E stream , and that the pulses in both output arms all have a peak energy of E Const , regardless of the length of the “off” region 78 . This is in contrast to the pulse energies shown in FIG. 2 for the laser of FIG. 1 , which depend on the length of the “off” period 68 that precedes the first pulse in the train. This uniformity of the pulse energy from pulse-to-pulse and from train-to-train is quite desirable, and is a significant advantage over the laser of FIG. 1 .
  • the second output arm may or may not be used by the laser operator.
  • the pulses in the second output arm 28 may be directed to an absorber, or may be directed an additional optical system that uses the pulses, effectively doubling the potential usage of the laser 21 .
  • the polarizers 8 and 24 are similar in nature and are essentially parallel, so that the electro-optic modulator 7 can switch between transmitting through both polarizers, or reflecting off the polarizers. Because the energy in a mode-locked laser may be thought of as a pulse that oscillates between the mirrors in the laser cavity, the electro-optic modulator 7 can choose to reflect off one polarizer or the other polarizer by precisely timing the portions at which the beam polarization is rotated.
  • the electro-optic modulator 7 should rotate the polarization of the pulse as it is traveling from left-to-right in FIG. 3 as the pulse passes through the modulator 7 .
  • the electro-optic modulator 7 should rotate the polarization of the pulse as it is traveling from right-to-left in FIG. 3 as the pulse passes through the modulator 7 .
  • the intracavity energy may be directed into either output beam.
  • switching the modulator at times t 1 (or at multiples of 1/f, plus t 1 ) directs the pulses 26 into output arm 14
  • switching the modulator at times t 2 (or at multiples of 1/f, plus t 2 ) directs the 27 pulses into output arm 28 .
  • the difference (t 2 minus t 1 ) corresponds the time it takes a pulse to travel from the modulator 7 to the cavity mirror 9 and back to the modulator 7 .
  • the topmost plot 30 a in FIG. 4 is intracavity energy versus time. Note that the intracavity energy of plot 30 a is measured at a particular location in the cavity, such as the mirror 4 .
  • the actual location at which the intracavity energy is measured is relatively unimportant; one may measure that intracavity energy at any location in the cavity by horizontal shifting the x-axis in plot 30 a . In other words, while the absolute values of t 1 and t 2 do depend on the location of said measurement, the difference (t 2 minus t 1 ) does not depend on the location of said measurement.
  • the reference times 31 a - 35 a are separated by the inverse of the repetition rate, 1/f. The times 31 a - 35 a may be considered to be the time at which an intracavity pulse strikes the cavity mirror 4 , for the particular round-trip at which cavity dumping occurs.
  • the round-trip time of the pulses in the cavity is denoted by T RT .
  • the pulse repetition rate is f, with a time between pulses given by 1/f. Note that 1/f is a multiple of T RT . Pulses are released after times 32 a , 33 a and 34 a , each of which decreases the intracavity energy to a small but non-zero level.
  • the next plot down, 30 b is the modulator driving signal.
  • the modulator is switched so that the intracavity energy is directed to one of the two output arms.
  • the relative times at which the modulator is switched, relative to 32 a , 33 a and 34 a are shown in further detail in the four bottommost plots in FIG. 4 .
  • Plot 38 is the controller driving signal.
  • the pulses When the signal is at a relatively high voltage, as in regions 36 and 37 , the pulses are directed into the first output arm. This is analogous to the “on” state described above.
  • the pulses When the signal is at a relatively low voltage, the pulses are directed into the second output arm, for either absorption or for use in an additional optical system. This is analogous to the “off” state described above. It will be readily understood by one of ordinary skill in the art that the roles of “off” and “on” may be reversed, and that the first and second output arms may be reversed.
  • Plot 30 b is the modulator driving signal, produced by the controller 23 in response to the controller driving signal 38 and to a synchronization signal (not shown) generated by photodetector 10 .
  • the synchronization signal peaks once every pulse round-trip, and provides a reference for times t 1 and t 2 .
  • the delay time at which the modulator is switched, for the first pulse in each train is t 1 .
  • the delay time at which the modulator is switched, for the first pulse in each train is t 2 .
  • Plots 30 c and 30 d are the energies in the first and second output arms, respectively.
  • pulses 32 c , 33 c and 35 c are produced by switching pulses 32 b , 33 b and 35 b , respectively.
  • pulses 31 d and 34 d are produced by switching pulses 31 b and 34 b , respectively.
  • the time between pulses in the first and second output arms is roughly 1/f. Strictly speaking, the time is 1/f+/ ⁇ (t 2 minus t 1 ), although in practice the quantity (t 2 minus t 1 ) is much smaller than 1/f and may often be neglected.
  • a quantity known as the “cavity dumping ratio” is the fraction of the total intracavity energy that is extracted for each amplification cycle.
  • Typical cavity dumping ratios may be roughly 80%, although any suitable value may be used.
  • the cavity dumping ratio may be controlled in part by hardware, by setting the s-polarized reflectivity of the polarizers 8 and 24 to the cavity dumping ratio, rather than to 100%.
  • the cavity dumping ratio may be controlled in part by software, by applying a suitable modulating driving voltage that rotates the plane of polarization to an orientation slightly away from s-polarization, so that the reflectivity off the polarizers 8 and 24 equals the cavity dumping ratio. It will be understood by one of ordinary skill in the art that the cavity dumping ratio may be easily controlled in a known manner, so that the output arms receive a fraction less than 100% (equal to the cavity dumping ratio) of the intracavity energy, rather than 100%.
  • the discussion up to this point has centered around producing trains of laser pulses that all have uniform energy, from pulse-to-pulse and from train-to-train.
  • the laser 21 of FIG. 3 may also be used to produce pulses that have a variable or adjustable energy, by varying the modulator driving signal. No additional hardware is required.
  • the modulator driving signal 30 b is either at a high level ( 31 b through 35 b ), or at a low level (everything else). These levels correspond to the polarization states that reflect and transmit through the polarizers, respectively. These polarization states are 90 degrees apart, and may be referred to as s- and p-polarization states, vertical and horizontal polarization states, into the page and parallel to the page (with respect to FIG. 3 ), or any other suitable naming convention.
  • the reflectivity of the polarizer is essentially 100% and 0%, respectively, or as close as practical.
  • the high reflectivity state (corresponding to 31 b through 35 b ) may be less than 100% reflectivity (the polarization rotation of less or more than 90 degrees is produced at the electro-optic modulator), in order to retain some energy in the cavity.
  • the modulator driving signal 40 b of FIG. 5 may be at varying levels between the high level and low level of FIG. 4 . That means, reflectivities at the polarizer can vary anywhere between 0% and a predetermined value less than 100% (or as close as practical).
  • the numerical relationships between driving voltage and polarization rotation for the electro-optic modulator are easily determined, either by routine experimentation or by documentation provided by the manufacturer of the modulator.
  • the relationship between polarization orientation and reflectivity is determined by Malus's Law, which says that if the angle between the beam polarization and the reflected polarization state (typically s-polarization) is theta, then the reflected optical power is proportional to cos 2 (theta).
  • the modulator driving voltage to the reflectivity of each polarizer this relationship may be stored in a lookup table, may be calculated dynamically, or a combination of both.
  • the laser 21 may produce pulses with a desired pulse energy R (expressed as a fraction of the intracavity energy) in the following manner.
  • the modulator driving voltage is varied so that during the left-to-right pass of the pulse through the modulator 7 , the polarization is rotated to give a reflectivity of R at the polarizer 8 corresponding to the first output arm 14 .
  • the polarization is rotated to give a reflectivity of (the cavity dumping ratio minus R) at the polarizer 24 corresponding to the second arm 28 .
  • a 80% in the inset of FIG. 5 is the amplitude of the modulator driving signal (voltage) that gives a reflectivity of 80%, and so forth.
  • FIG. 5 shows the modulator driving voltage 40 b and the output energies 40 c and 40 d for the first and second output arms 14 and 28 , for various desired pulse energies.
  • the cavity dumping ratio is 80%, meaning that in the pulse round-trip during which the intracavity energy is directed into one or both output arms, 20% of the intracavity energy remains in the cavity.
  • the value of 80% is merely exemplary, and any suitable value may be used.
  • the reference times 41 a - 45 a are separated by the inverse of the repetition rate, 1/f.
  • the times 41 a - 45 a may be considered to be the time at which an intracavity pulse strikes the cavity mirror 4 , for the particular round-trip at which cavity dumping occurs.
  • the time t 1 corresponds to the time that a pulse takes to travel from the cavity mirror 4 to the modulator 7 .
  • the time t 2 corresponds to the time that a pulse takes to travel from the cavity mirror 4 to the opposing cavity mirror 9 and back to the modulator 7 .
  • the time difference (t 2 minus t 1 ) is the time it takes a pulse to travel from the modulator 7 to the mirror 9 and back to the modulator 7 .
  • the desired pulse energy is 70%.
  • the modulator driving voltage is set so that the reflectivity off polarizer 8 is 70%.
  • the modulator driving voltage is set so that the reflectivity off polarizer 24 is 10%.
  • the pulse energy 41 c in the first output arm is therefore 70%.
  • the pulse energy 41 d in the second output arm may optionally be directed to an absorber. After this round-trip pass, 80% of the intracavity energy is dumped into the output arms, and 20% remains in the cavity.
  • the desired pulse energy is 50%.
  • Pulse 43 has a desired pulse energy of 20% in the first output arm, with 60% going into the second output arm and 20% remaining in the cavity.
  • Pulse 44 has a desired pulse energy of 40% in the first output arm, with 40% going into the second output arm and 20% remaining in the cavity.
  • Pulse 45 has a desired pulse energy of 80% in the first output arm, with 0% going into the second output arm and 20% remaining in the cavity.
  • bistability is not fully understood at present. The following paragraphs provide a possible explanation of bistability, although with the caveat that this is only one possible explanation, which has not been proven.
  • FIG. 6 shows the intracavity energy versus time for a typical, continuously-pumped laser, but in the absence of any cavity dumping.
  • the gain medium is continuously pumped, so that energy enters the gain medium at a constant rate.
  • Energy “sloshes” back and forth between the gain medium, where the energy is stored as a population inversion, and the intracavity energy, where the energy is stored in the electric field.
  • the amplitude of the “sloshing” decays at a particular rate, and the intracavity energy settles to a steady-state value after a particular relaxation time, t REL .
  • the intracavity energy is decreased quickly by the cavity dumping ratio, which occurs during one round-trip pass of the pulse in the cavity.
  • the system is moved to a non-steady-state, and the “sloshing” begins again, with energy being transferred from the gain medium to the electric field in the cavity, and back.
  • the system “resetting” to a point along the curve 50 at which the energy (y-axis) is decreased. For instance, if after a cavity dump, the energy is dropped to a level near the leftmost side of the plot, then the behavior in time follows the curve shown in FIG. 6 ; the intracavity energy rises to a peak 51 , then drops to a local minimum 52 , then rises to a local maximum 53 , then a minimum 54 , then a maximum 55 , then a minimum 56 , and so forth until the oscillations become sufficiently damped and the intracavity energy settles once again back to its steady-state value.
  • the pulse repetition rate is sufficiently slow, so that there is sufficient time to settle to the steady-state value or something close to it, then the above behavior repeats in a predictable manner.
  • the intracavity energy may not even make it to its first peak 51 before the cavity is dumped. This is the case for many of the 1/f-separated pulses in FIGS. 2 and 7 ; the curved “sawtooth” behavior between pulses derives its shape from the steep, upward-curving, leftmost edge of the intracavity energy curve 50 .
  • the pulse may “reset” the curve to an intracavity energy below this leftmost edge. If this occurs, the intracavity energy may require extra time to reach the levels shown by curve 50 , which may be somewhat unpredictable. As a result, the system may become unstable, with irregularly spaced pulses, or irregular pulse energies. This region of instability occurs if the time between pulses 1/f falls near the region of the first peak 51 , and is labeled as region 57 in FIG. 5 .
  • FIG. 8 shows the effects of both pulse repetition rate and cavity dumping ratio on stability.
  • a phenomenon called “bistability” may occur near the region of the first peak (analogous to peak 51 in FIG. 6 ).
  • bistability In bistability, successive pulses in a train alternate between two different energy values, each of which may remain constant over time. Typically, the user of the laser wants only a single value for the pulse energy, not alternating values, and bistability is, in general, undesirable. Note again that the previous discussion provides only one possible explanation for bistability, a phenomenon that has been documented but at present is not well understood. This discussion of bistability should not be construed as limiting in any way.
  • the labels of 50%, 75% and 90% for the cavity dumping ratios are merely exemplary, and should not be construed as limiting in any way. Actual values may depend on the optical properties of the components in the laser cavity. Furthermore, the time scale of the x-axis is merely exemplary, and may change if the laser cavity is lengthened or compressed.
  • FIG. 9 shows a pulse to be directed into output arm 14 .
  • the pulse is initially traveling left-to-right, along with its polarization state. Initially, at stage (a), the polarization is in the plane of the page.
  • polarizers such as 24 and 8 reflect s-polarization and transmit p-polarization, although this is not a strict requirement. Either or both polarizers may alternatively reflect p- and transmit s-polarization. Furthermore, the polarizer may be “leaky”, and may transmit a fraction of p-polarized light with its s-transmission, or may reflect a fraction of s-along with its p-reflection. The initial polarization of the beam at stage (a) is generally aligned with the pass axis of the polarizer 24 .
  • the beam transmits through polarizer 24 , as shown in stage (b).
  • the polarization is still in the plane of the page.
  • the beam passes through the modulator 7 , which is driven by a modulator driving voltage (not shown).
  • the driving voltage is set so that modulator rotates the plane of polarization by about 90 degrees.
  • the polarization of the beam is out of the page.
  • the beam is s-polarized, with respect to the polarizer 8 , and reflects off polarizer 8 (generally—polarization is perpendicular to the pass axis of the polarizer 8 ).
  • the reflected beam, at stage (d) is directed into output arm 14 , as shown in FIG. 3 .
  • FIG. 10 (broken up over two pages into 10 A and 10 B) shows a pulse to be directed into the other output arm 28 .
  • the pulse is initially traveling left-to-right, along with its polarization state.
  • the polarization is in the plane of the page.
  • the beam transmits through polarizer 24 , as shown in stage (b).
  • the polarization is still in the plane of the page.
  • the modulator 7 is driven by a voltage that does not rotate the plane of polarization of the transmitted beam.
  • the beam emerges from the modulator 7 at stage (c) with its polarization still in the plane of the page.
  • the beam has transmitted through the polarizer 8 .
  • the beam has reflected off mirror 9 (not shown in FIG. 10 ) and any optional intervening optical elements, and has returned to polarizer 8 with its polarization still in the plane of the page.
  • the beam transmits through polarizer 8 , as shown in stage (f).
  • the modulator 7 is driven by a voltage that rotates the plane of polarization by about 90 degrees, so that the polarization state at stage (g) is essentially out of the page (or s-polarized with respect to polarizer 24 ).
  • the polarization rotation may be more or less than 90 degrees.
  • the beam now reflects off polarizer 24 , as in stage (h), and is directed to output arm 28 (not shown in FIG. 10 ).
  • the timing of the modulator driving signal is important. It is important to drive the modulator so that it catches the pulse traveling in one direction but not the other, so that the cavity energy may be dumped within one round-trip of the pulse in the laser cavity.
  • An exemplary set of driving voltages are shown in FIG. 11 , for the cases shown in FIGS. 9 and 10 . In each of these plots, the “low” voltage corresponds to no polarization rotation. Likewise, the “high” voltage corresponds to a sufficient rotation to direct a fraction of the light into the output arm, where the fraction is the cavity dumping ratio.
  • the polarization rotation occurs between stages “b” and “c”.
  • the polarization rotation occurs between stages “f” and “g”, with essentially no rotation between stages “b” and “c”.
  • the x-axis has no absolute “zero”, but the various stages “a” through “h” correspond to the same points in the round-trip of each pulse in the cavity. For instance, “zero” in both plots may correspond to the time at which the circulating pulse hits mirror 4 , although this is not a requirement.
  • stages “b” and “f”, or equivalently, stages “c” and “g”, is roughly equal to the round-trip time of the pulse as it travels from the modulator 7 to the mirror 9 and back. The pulse travels at the speed of light.
  • either or both may be rotated about the optical axis of the cavity, so that one or both output arms may extend out of the page.
  • the modulator 7 may be driven by a signal that produces a polarization rotation that aligns the transmitted beam with the pass axis of the respective polarizers 24 and 8 .
  • the cavity dumper 22 of FIG. 3 is shown having a single modulator 7 , located in the cavity between two inclined polarizers 24 and 8 .
  • This cavity dumper construction There are many possible variations of this cavity dumper construction, several of which are shown in FIGS. 12 through 17 .
  • the modulators are all drawn schematically as squares
  • the inclined polarizers are drawn schematically as 45-degree diagonal lines
  • the arrows show the direction of travel of a beam as it exits the cavity into an output arm.
  • FIG. 12 shows the cavity dumper 120 variation of FIG. 3 , having a single modulator 121 surrounded by two inclined polarizers 126 and 127 .
  • FIG. 13 shows a cavity dumper 130 having two modulators 131 and 132 , and two inclined polarizers 136 and 137 .
  • Each modulator 131 , 132 is paired with its own polarizer 136 , 137 , respectively.
  • the modulators 131 and 132 both direct a right-traveling pulse to the output arms.
  • FIG. 14 shows a cavity dumper 140 having two modulators 141 and 142 , and two inclined polarizers 146 and 147 .
  • Each modulator 141 , 142 is paired with its own polarizer 146 , 147 , respectively.
  • the modulator 141 directs a right-traveling pulse to an output arm, while the modulator 142 directs a left-traveling pulse to another output arm.
  • FIG. 15 shows a cavity dumper 150 having two modulators 151 and 152 , and a single inclined polarizer 156 between the modulators 151 and 152 .
  • the modulator 151 directs a right-traveling pulse to an output arm, while the modulator 152 directs a left-traveling pulse to another output arm.
  • polarizer 156 includes the functions of both polarizers 146 and 147 , while eliminating a component. In this manner, the cavity dumper may piggyback off one or more polarizers already present in the laser cavity, so that the polarizer may serve more than one function in the laser.
  • FIG. 16 shows a cavity dumper 160 having three modulators 161 , 162 and 163 , and three inclined polarizers 166 , 167 and 168 .
  • Each modulator 161 , 162 , and 163 is paired with its own polarizer 166 , 167 , and 168 , respectively. In this manner, the number of output arms may be extended to three, four, five, six or more.
  • FIG. 17 shows a cavity dumper 170 having two modulators 171 and 172 , and two inclined polarizers 176 and 177 .
  • Polarizer 176 serves two output arms, one from a left-traveling pulse and one from a right-traveling pulse.
  • the cavity dumper may piggyback off one or more modulators already present in the laser cavity, so that the modulator may serve more than one function in the laser.
  • the modulator 171 may function in the cavity dumper for a right-traveling pulse, but may be part of a switch or a cavity loss control mechanism (not shown) for a left-traveling pulse.

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US10386573B2 (en) * 2017-09-29 2019-08-20 Fujikura Ltd. Substrate-type optical waveguide and introducing method using fundamental and higher TE/TM modes
WO2019218634A1 (fr) * 2018-03-14 2019-11-21 深圳市创鑫激光股份有限公司 Procédé de commande de laser, appareil de commande électronique, laser, dispositif de perçage laser et support de stockage
CN111478173A (zh) * 2020-05-19 2020-07-31 中国科学院福建物质结构研究所 一种1.5微米被动调q激光器
US11165218B2 (en) * 2019-02-02 2021-11-02 Mks Instruments, Inc. Low repetition rate infrared tunable femtosecond laser source
WO2021260051A1 (fr) * 2020-06-26 2021-12-30 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Appareil laser et procédé de commande d'un appareil laser
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WO2021260051A1 (fr) * 2020-06-26 2021-12-30 Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V. Appareil laser et procédé de commande d'un appareil laser

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Effective date: 20070427

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