The present application relates to photonics. More particularly, the present application relates to pulsed laser interfacing and the control of asynchronous pulsing of amplified lasers.
Pulse to pulse energy stability is important for precision and reproducibility in certain laser-material processing applications. Pulse to pulse stability can be <1% root mean squared (RMS) in a well designed amplified laser system operating under steady state conditions. However, in many practical processing tasks, the laser must be triggered by motion control equipment that is not synchronized. Asynchronous triggering can cause transient conditions in the laser amplifier which disturb the pulse energy stability.
Pulse Energy and repetition rate are inversely related in a Master Oscillator Power Amplifier (MOPA) laser close to saturation of the amplifiers. In one method of asynchronous triggering, a MOPA laser system is pulsed at a constant repetition rate and gain level, while an optical modulator is used at the output of the laser system to gate the output pulses according to an external trigger However, in at least some applications, this approach may have disadvantages. For example, it limits the timing resolution to an integer factor of the steady state repetition rate, and it requires an optical modulator with a large enough aperture to transmit the laser output without optical damage. For high-throughput laser machining applications, both of these limitations reduce speed and increase cost.
Another method to suppress first pulses has been demonstrated in Q-switch or other pulsed lasers involving limiting the gain. Examples of US patents that relate to the field include U.S. Pat. Nos. 8,081,668; 4,337,442 and 7,876,498.
According to one example, a laser control system and method are provided. In a first aspect, the laser control system comprises a master oscillator laser configured to generate a seed laser pulse train at a first repetition rate, an optical modulator configured to receive the pulse train from the master oscillator laser and modulate the pulse train based on a received modulation signal to generate modulated seed pulses, a laser amplifier configured to amplify the modulated seed pulses to generate an amplified pulse sequence output, and a control circuit for controlling the operation of the optical modulator. The control circuit is configured to receive a clock signal synchronized with the seed laser pulse train, receive a trigger input for asynchronous modulation of the seed laser pulse train, generate the modulation signal, and communicate the modulation signal to the optical modulator. The modulation signal is configured to control the optical modulator to selectively transmit and attenuate seed pulses from the seed laser pulse train to produce modulated seed pulses corresponding to the trigger input and attenuated to maintain a predetermined amplitude envelope in the pulse sequence output.
In another aspect, the control circuit generates the modulation signal using an algorithm based on the clock signal and the trigger input.
In a further aspect, the algorithm is executed on the control circuit.
In a further aspect, the control circuit is further configured to communicate with an external processor, and the algorithm is executed on the processor.
In a further aspect, the laser control system further comprises a sensor monitoring at least one characteristic of the amplified pulse sequence output and providing feedback to the control circuit, wherein the algorithm is further based on the feedback from the sensor.
In a further aspect, the algorithm self-calibrates based on the readings from the sensor.
In a further aspect, the algorithm further comprises a learning algorithm for pulse envelope control under arbitrary triggering.
In a further aspect, the algorithm determines the amount of attenuation of the modulation signal based on a timer that resets with each pulse in the trigger input.
In a further aspect, the predetermined amplitude envelope comprises an envelope having a burst energy set point.
In a further aspect, the predetermined amplitude envelope comprises an envelope having a burst amplitude set point.
In another example, a laser control circuit is provided for controlling the output of a laser. The control circuit is configured to receive a clock signal synchronized with a seed laser pulse train, receive a trigger input for asynchronous modulation of the seed laser pulse train, generate a modulation signal for controlling an optical modulator receiving the seed laser pulse train to selectively transmit and attenuate seed pulses from the seed laser pulse train to produce modulated seed pulses corresponding to the trigger input and attenuated to maintain a predetermined amplitude envelope of a pulse sequence output (230) after being amplified by a laser amplifier, and communicate the modulation signal to the optical modulator.
In another example, a method for controlling the output of a laser is provided. The method comprises receiving at a control circuit a clock signal synchronized with a seed laser pulse train, receiving at a control circuit a trigger input for asynchronous modulation of the seed laser pulse train, generating at a control circuit a modulation signal for controlling an optical modulator receiving the seed laser pulse train to selectively transmit and attenuate seed pulses from the seed laser pulse train to produce modulated seed pulses corresponding to the trigger input and attenuated to maintain a predetermined amplitude envelope of a pulse sequence output after being amplified by a laser amplifier, and communicating the modulation signal to the optical modulator.
Further aspects and examples will be apparent to a skilled person based on the description and claims.
Example embodiments of a laser control system and method will now be described in greater detail with reference to the accompanying drawings of example embodiments in which:
With reference to the drawings,
Direct modulation of the seed laser is an obvious alternative to modulation of the output. However, due to the excited state lifetime of the laser amplifier, prolonged periods without seed pulses lead to higher gain conditions for the leading edge of triggered pulse packets. This is known as the high energy “first pulse” effect.
Examples embodiments of the invention relate to a laser control circuit and method for enabling asynchronous, or ‘pulse on demand’ triggering of a Master Oscillator Power Amplifier (MOPA) laser system with controlled output pulse energy, by use of optical modulation and attenuation between the master oscillator (MO) seed pulses and the laser power amplifier (PA) to pre-compensate for transient gain effects in the PA in order to achieve arbitrary control of the envelope of the asynchronously modulated output pulse train.
In an example embodiment, the pump laser conditions are left constant, so as to minimize thermal relaxation effects, and the output of the laser system is modulated by controlling a fast optical attenuator between the seed laser and amplifier, with variable transmission to pre-compensate for transient gain in a laser amplifier system.
With reference to the drawings,
The modulated seed pulses 228 generated by the optical modulator 204 are in turn incident on one or more laser amplifiers. In the illustrated embodiment, there is a single laser amplifier 206 comprising a pre-amplifier 208 and a power amplifier 210 and fed by a continuous pump laser 214. The modulation signal 224 generated by the control circuit 212 is shaped to result in packets of modulated seed pulses 228 incident on the one or more laser amplifiers 206 so as to produce a desired amplified output pulse sequence 230 of amplified pulse packets with controlled amplitude, duration, frequency and phase. In the illustrated embodiment, the output pulse sequence 230 has a set burst energy point for each burst of pulses: the left burst 230(a) with the lower repetition rate has a higher pulse amplitude, while the right burst 230(b) with the higher repetition rate has a lower pulse amplitude, thus generating two burst with equivalent energy.
In other embodiments, the desired envelope of the output pulse sequence 230 could be shaped using other criteria. For example, in one embodiment the envelope of the output pulse sequence 230 would be shaped to have a set predetermined flat amplitude regardless of other burst characteristics, such as repetition rate or duration of the burst. Some embodiments could have the desired envelope characteristics preset in the control circuit 212, while others could allow a user to program their own envelope characteristics into the system using the control circuit 212 or other processors or computers attached thereto (as further set out below).
Furthermore, some embodiments may use a trigger sequence input 222 with variable amplitude. The envelope of the output 230 may take the trigger input 222 amplitude into account; for example the system may generate an output envelope with an energy set point and/or amplitude set point dependent on the amplitude of the trigger input 222.
The gain experienced by pulses in a laser amplifier with constant pumping conditions depends on the repetition rate of the modulated seed pulses 228. This is due to the lifetime of the excited state population in the laser gain material. Seeding with pulse periods shorter than the time required for re-population of the excited state results in less gain in the amplifier once the amplifier output power is saturated. Long pauses between bursts or packets of pulses can result in higher gain for the leading pulses, reducing pulse-to-pulse stability and possible optical damage to the laser amplifier. The present system and method may in some embodiments provide a method of pre-compensation of laser amplification transient characteristics by electronic controlled attenuation of the laser amplifier input pulses under steady state pumping conditions to achieve good envelope control of bursts of laser pulses.
In more detail, referring to the embodiments shown in
The following general equation describes the effect of the laser amplifier 206 on the seed pulses 228 in an example embodiment:
I
out(t)=G(t)·Iin(t)
I
in(t)=α(t)·IMO(t)
where Iout is the laser output power flux (proportional to output pulse sequence 230) in units of [W/m̂2] and Iin is the laser input power flux (seed pulses 228), a function of the IMO(t) master oscillator power flux (pulse train 226) and the modulation signal α(t) 224. G(t) is the gain of the laser amplifier 206.
In a laser control system according to an example embodiment, G may be a complicated function, and analytical description of the complex combination of nonlinear optical elements may be difficult. However, an example is described herein below to provide a basis for creating a control algorithm for an example laser control system.
With reference to
A32 606, A31 604, A21 626, are the rates of spontaneous emission, B31 610 and B21 618 are the rates of stimulated absorption and emission, ρP=U[IP(t,ω)] 612 is the energy density of the pump laser, and ρL=U[Iin(t,ω)] 614 is the energy density of the laser inside the amplifier which is a function of Iin.
In this example the gain of the 3 level laser amplifier is a function of the population inversion ΔN(t) such that
The modulation signal α(t)=A(t)·P(t) where A(t) is the time dependant attenuation produced by the control circuit algorithm and P(t) is the desired pulse sequence and pre-specified envelope. The example above illustrates one possible approach for solving (numerically or otherwise) for the time dependant attenuation required from the algorithm used by the control circuit 212.
In one example configuration, the pulse sequence P(t) is defined by the asynchronous trigger 222 and pre-specified envelope shape. In another example configuration it is entirely specified by the control input of the timing circuit 322 as seen in
In a closed loop configuration as shown in
Thus, the control circuit 212 in some embodiments includes a means of compensating for the transient changes in the laser amplifier 206 that result from changes in the timing between modulated seed pulses 228. This pre-compensation determines the amplitude of the modulation signal 224 going to the optical modulator 204, which alters the transmitted energy of the selected laser pulses.
If this uncorrected modulator signal 306 were used and transmitted to the optical modulator 204, the amplified output pulse sequence generated by the amplifier 206 would appear as an uncorrected laser amplifier output 308 having pulses 314 of variable gain producing a non-flat envelope 316, and specifically pulses wherein gain would decay over the duration of a packet and would be at its maximum at the beginning of a packet after a long interval 318 for regeneration. This is the “first pulse problem” previously discussed.
In example embodiments of the present system and method, the control circuit instead pre-compensates for these regeneration and decay effects by generating a corrected modulator signal 224 (instead of uncorrected signal 306) having attenuated gain based on the previous pulse sequence and its effects on decay and regeneration. The pulses 326 of the corrected modulator signal 224 therefore have variable gain and adjustable decay 332 depending on their position within a packet, the duration between packets, the repetition rate of the trigger input 222, and potentially other factors.
Using the corrected modulator signal 224 results in a laser amplifier output 230 having packets of pulses 328 with a flat envelope 330 (as opposed to signal 308). Pulses that would have experience higher gain than their continuously seeded counterparts would in such a pre-compensation regime be attenuated to avoid excess pulse energy after the amplification by the laser amplifier 206.
Advantages of this system and method of pre-compensation may include, in some embodiments, the ability to trigger the laser system with an external pulse sequence that is neither consistent in terms of repetition rate, nor synchronized to the master oscillator, while decoupling the output pulse energy from the external trigger timing.
Thus, some embodiments may provide a MOPA laser system with an external trigger including a control circuit 212 that can be tuned to compensate for the power amplifier 206 transient response. A specific example embodiment 500 of the control circuit 212 is shown in
In some embodiments, the gain calculations used in the pre-compensation and suppression regime are made within the control circuit 212 hardware itself, while in other embodiments the calculations are made externally, e.g. by a processor 522 or computer in communication with the control circuit 212. These calculations may take into account various factors in different embodiments, including the position of the present pulse within a packet, the duration between packets, the repetition rate of the trigger input 222, and potentially other factors. In one example embodiment, the pre-compensation gain attenuation calculation is based on the value of a timer that resets after each pulse. Some embodiments may make use of a memory to store and look up past patterns of modulation and output, and to base present pre-compensation calculations on such memory lookups.
In some embodiments, such as the variant shown in
In some embodiments, the optical modulator 204 could be implemented as two or more optical modulators operating in conjunction, either in parallel or in sequence, to produce the modulator output 228 from one or more pulse train inputs 226.
In some embodiments, the control circuit 212 could be implemented as a general purpose computer or processor, such as a general purpose computer having specialized hardware for high-speed acoustic processing.
While the described embodiments have shown the feedback signal from the photo sensor 216 as a single control signal 232, such as a sensor reading of output amplitude, some embodiments may use one or more sensors or other components to provide a plurality of control signals 232 used to train or auto-calibrate the pre-compensation algorithm used by the control circuit 212.
The present disclosure may be embodied in other specific forms without departing from the full scope of the claims as read in light of the specification as a whole, and would be understood by a person of skill in the art to encompass various sub-combinations and variants of described features. The described embodiments are to be considered in all respects as being only illustrative and not restrictive. The present disclosure intends to cover and embrace all suitable changes in technology.
The present application claims priority from U.S. provisional patent application No. 61/840,790, filed Jul. 16, 2013, the entirety of which is hereby incorporated by reference.
Number | Date | Country | |
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61846790 | Jul 2013 | US |
Number | Date | Country | |
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Parent | PCT/CA2014/050670 | Jul 2014 | US |
Child | 14996315 | US |