System for driving pulsed laser diode pump

A power supply for laser systems is configured with a DC power source having an output source voltage, an energy accumulator operatively connected to the output of the DC power source, and a pump. Coupled between the accumulator and source is a first DC to DC stage with at least one switched-mode power converter which is operative to charge the accumulator with voltage. The charged voltage may be same or different from the source voltage. The power supply further includes a second DC to DC stage with at least one switched-mode power converter coupled between the accumulator and pump and operative to discharge accumulator to the same or different output voltage. The DC to DC converters are configured so that current pulses at the input of the pump each have a peak value greater than the power source current.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Phase Application of PCT/US2011/023566 filed on Feb. 3, 2011

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The present disclosure relates to laser systems configured with a power supply which is operative to generate electrical pulses each characterized by a high peak current rating which results in long, powerful optical pulses.

2. Discussion of Known Prior Art

A laser's pumping scheme can be either continuous (cw) or pulsed. The pulsed pumping exclusively operates in brief, discrete time intervals. Traditional solid-state lasers utilize crystals configured to accumulate the energy by providing an excessive inversion for generating high power optical pulses of up to several J. Fiber lasers, in contrast to solid-state lasers, are poorly suited for storing the energy because of a relatively small core of rare-earth doped fibers.

One of the alternative methods of radiating high-energy pulses by fiber lasers includes pumping the laser material with an optical source, such as laser diodes, which are themselves pulsed. This method may impose power scaling limitations on a power source, which is designed to work in a continuous regime, since the electrical pulses are characterized by high current substantially exceeding that one in the continuous regime. If the electrical pulse is long enough, which is desirable, the overcurrent protection system limits the output current, and the power source is not able to provide the required peak power. Such an interruption leads to an inadequate performance of power source operating in the pulsed regime.

FIG. 1illustrates an attempt to somewhat minimize the above-discussed problem by coupling a capacitor “C” across the output of a power supply source. Due to a high current pulse, the voltage across the capacitor “C” slightly drops. The power source begins to compensate for the drop but is soon limited by an overcurrent circuitry operative to prevent the source overload. Subsequently, the source is either tuned off or continues to work with the limited current allowed by the design of the source.

Powerful supply sources could somewhat alleviate the disclosed problem. However, powerful sources do not provide a viable solution because the known configurations are neither compact nor cost-efficient.

A need therefore exists for a laser system with a power supply source that can handle the peak current, yet provide the normal (non-peak) operating power.

SUMMARY OF THE DISCLOSURE

The disclosed structure meets this need. In particular, the disclosed system is operative to generate high-energy electrical pulses at the input of a laser pump causing high energy light pulses at the output of a fiber laser system, and a quasi-constant load at the output of a power source.

The system includes a power source and a laser diode pump unit operative to radiate light pulses which are coupled into a laser unit. Preferably, the latter is configured as a fiber laser system, but may have other known configurations, and structured as a pulsed high power fiber laser system.

The system is further configured with an energy accumulator and a multi-stage DC-DC power converter having a first, charging stage which is located between the source and accumulator and operative to charge the latter, and a second, discharging stage which is coupled to the accumulator and pump and operative to discharge the former. Due to the stored energy on the accumulator, the converter is operative to controllably generate the desired current signal at the input of the laser pump. On the other hand, the load on the power source does not exceed the current threshold of the source. As a result, the disclosed configuration eliminates a need for oversized sources.

SPECIFIC DESCRIPTION

Reference will now be made in detail to the disclosed system and method. Wherever possible, same or similar reference numerals are used in the drawings and the description to refer to the same or like parts. The drawings are highly diagrammatic and are far from precise scale. For purposes of convenience and clarity only, the terms “connect,” “couple,” and similar terms do not necessarily denote direct and immediate connections, but also include connections through mediate elements or devices.

Referring toFIG. 2, a fiber laser system10includes a DC power source12generating a source voltage coupled to a DC-DC converter assembly14which includes an accumulator20, at least one charging stage16and at least one discharging stage18all electrically coupled to one another. The DC source12may or may not be continuously coupled to charging stage16. In either case, the charging stage16operates until accumulator20is fully charged.

The discharging stage18extracts energy from accumulator20only during a pulse duration. The pump22includes one or more laser diodes which each are a current control device. The disclosed topography allows for high peak power light pulses generated by pump unit22and therefore laser system24while shielding power source12from being overloaded.

To have high energy pulses at the input of pump unit22and therefore high power optical pulses at the output of laser system24, the energy E extracted from accumulator20during the discharge should be as high as possible. The energy E may be determine as E=C(Vc1−Vc2)Vcav, where C is the capacitance of accumulator20, Vc1and Vc2are respective voltages across the accumulator at the beginning and end of the discharge pulse, and Vcav is a mean value of voltage at the accumulator during the pulse. Accordingly, the higher the mean value of voltage and/or the higher the voltage difference, the higher the energy of accumulator20.

FIG. 3illustrates one of the modifications of one of the embodiments of the disclosed system in which a capacitor38functions as an energy accumulator. In addition to the capacitor, system10is configured with power source12, step-up converter26constituting charging stage16, step-down converter30constituting discharging stage18, and laser diodes45.

Referring toFIG. 4in addition toFIG. 3, as known to those skilled in the art, step-up converter26, based on the boost topology, is operative to increase an input DC voltage Vps of power source12to a higher DC voltage Vout across capacitor38and further maintain the latter. Given only as a non-limiting example, step-up converter26includes an inductor32, a low-side switch36such as a MOSFET, and a high-side switch34configured as a diode or MOSFET. When low-side switch36is ON, the inductor current is increased; otherwise, the only path available to the inductor current is through high-side switch34and capacitor38. As a result, the energy, accumulated during the ON period of low-side switch36is transferred into the capacitor. The high frequency operation of the switches results in the application of the limited charge current to capacitor38until voltage Vc corresponds to a reference value at the end of charging period. Thereafter, low-side switch36stays OFF as long as capacitor38remains fully charged. The peak current of low-side switch36and, therefore, input current Iin at point A ofFIGS. 3 and 4kept below the maximum current of the power supply by means of an appropriate duty cycle control. All of the above disclosed functions of switch36are monitored and controlled at the input side by a controller35. In summary, regardless of the concrete configuration of step-up converter26, the current Iin extracted from source12is limited.

The step-down DC to DC converter30may be based on the buck topology. An exemplary schematic of converter30, as shown, is configured as a buck converter including a high-side switch43, inductor42, diode44and an optional low-side switch40. The discharging stage16includes step-down converter30, for example, and operates as follows. The current Iout flowing through point B and the pump diodes during a τ period (t1−t2) ofFIG. 4is continuously monitored by controller37and compared to the preset current Isetpoint which is nothing else but the desired form of current Iout (FIG. 4) on pump diodes45. If the measured current Iout lower than Isetpoint, then the duty cycle of PWM (Pulse-Width Modulation) pulses applied to switch43increases, until the measured current lout matches Isetpoint. Otherwise, the duty cylcle on high-side switch43decreases. The optional switch40, when present, is always out of phase with respect to high-side switch43, in other words, when switch43is ON, switch40is OFF and vice-versa. The optional switch40replaces diode44and is operative to reduce forward voltage losses which are rather significant on diode44. The Buck converter with optional switch40is known as synchronous Buck converter.

In addition, discharging stage18including converter30may be provided with a voltage control circuitry, such as controller37, operative to monitor a capacitor voltage Vc. The control circuitry is configured to turn off converter30if the measured voltage drops close to the pump diode output voltage Vout.

The limitations imposed on capacitor voltage Vc are the subject to its relationship with input and output voltages. InFIG. 3, the voltage Vc across capacitor38should be higher than both the source voltage Vps (Vc>Vps) and the output voltage Vout on diodes45(Vc>Vout). Accordingly, the only limitations applied to system10ofFIG. 3include intrinsic limitations of the components of the circuitry.

A start-up is a stressful time for power supply12, the output current may jump up to the overcurrent limit, because the diode34is forward biased during a start-up. To ramp the power supply at a slower rate, system10may have a soft start circuit. For example, it may be a MOSFET. Alternatively, it may be a resistor31and a switch33shunting the resistor. A controller35is configured to measure the current through the current limiting element, such as resistor31. As this current drops below a reference value, controller35is operative to close switch33so as to have the current flow through the closed switch, and to enable the converter's charging stage.

FIG. 5illustrates another modification of the embodiment ofFIG. 3. In particular, system10is configured with two step-up converters26each having, for example, a boost configuration. Similarly toFIG. 3, the accumulator is configured as capacitor38coupled between the converters. The voltage Vc at the capacitor should be higher than the source voltage Vps and lower than the voltage Vout at pump unit22(Vps<Vc<Vout). In accordance with the concept of the disclosed system, the energy stored on the capacitor is a function of the difference between voltages across the capacitor at the beginning and end of the pulse lout, respectively.

FIG. 6illustrates a further modification of the above-disclosed embodiment of system10. Here two step-down converters30are coupled to respective input and output of capacitor38. In contrast to the configuration ofFIG. 5, the voltage Vc across capacitor38should be lower than the voltage Vps at the output of the power source and greater than the voltage Vout at pump22(Vps>Vc>Vout).

FIG. 7illustrates still a further modification of circuitry ofFIG. 3. The system10is configured with step-down converter30coupled to the output of the power source and step-up converter26coupled to the input of pump22. The capacitor38is coupled between respective converters30and26. In this configuration, capacitor voltage should be lower than both Vpc at the output of the power source and Vout at the input of pump22(Vc<Vps and Vc<Vout).

FIG. 8illustrates an example the disclosed laser system which includes multiple converter units14configured in accordance with one of the schemes ofFIGS. 3,5-7. The system includes a single two- or more phase charging stages16and three two-phase discharging stages18. The multi-phase charging stage16effectively reduces the input current ripple.

The discharging stages18each are loaded on a string of pump diodes22. The two high-side switches of discharging stage18are phase-shifted at a 180° angle relative to one another. Such a configuration effectively reduces a ripple current on the pump diodes. In addition, three two-phase discharging stages18are shifted at a 120° angle relative to one another. Although the phase-shift between discharging stages18does not result in the reduction of the ripple current through the pump diodes, since each stage18is coupled to its string of diodes, it certainly affects the optical ripple reduction, because all the pump diodes are optically coupled to the same laser system.

The modification of the system shown inFIG. 8may include, for example, six (6) single-phase discharging stages18each loaded on a string of diodes22. All six stages18are shifted at a 60° angle relative to one another to effectively reduce the optical ripple described above. This configuration functions as if a switching frequency were six times greater. Taking into account the reaction time of the active fiber itself, the final optical noise which is measured at the output of the laser system and associated with the fundamental switching frequency of the discharging stages18can be reduced to negligible values. The charging stage16may have the same configuration as that one ofFIG. 8. As well understood, stages16and18each may operate with one or multiple converters/channels.

The scope of the disclosure is not limited to the above-disclosed converter topologies. One of possible DC-DC converters may include the use of a single-ended primary-inductor converter (SEPIC) well known to one of ordinary skill in the switched mode power supplies art. The SEPIC can have configurations allowing the voltage at its output, which is controlled by the duty cycle of the control transistor, to be greater than, less than, or equal to that at its input. Accordingly, returning toFIG. 2, both converters16and18may have the topology of the SEPIC. Still another DC-DC converter may be based on a Cuk topology which, like that of the SEPIC may have an output voltage either greater than or less than the input voltage. Finally, a buck-boost topology also can be utilized in the DC-DC converter of this disclosure. One of the advantages of the above-referred topologies over standard DC-DC converters of FIGS.3and5-7includes practically the maximum possible capacitor voltage change within the pulse cycle which leads to high energy light pulses. Still another advantage, because of a wide operating range of the capacitor voltage, a soft-start circuitry may be necessary only in some of the disclosed embodiments.

FIG. 9illustrates an alternative embodiment of the disclosed laser system. In particular, laser system55includes a power source48, inductor charge circuit50, inductor51, a switch53configured for example as a diode and a pump52radiating emission which is coupled into a fiber laser unit. The charger circuit50is operative to stabilize current in the primary winding of inductor51in the following manner. When the current in the primary winding of the inductor51is less than a reference value, the input voltage is applied to the primary winding of inductor51causing the flow of an increasing magnetizing current. The phasing of the single or multiple secondaries of inductors51and switch or rectifier diode53is such that the secondary does not conduct during this period. When the primary current reaches the reference value, charge circuit50stabilizes the primary current at the reference value. This can be done for example by introducing any limiting element such as a controlled Mosfet, or with the aim of DC/DC technique, or by another solution. Thus, in contrast to a regular transformer, the energy is not immediately transferred, but is accumulating in a core or air gap.

When the system is requested to generate an output pulse, the internal high-speed switch of charge unit50turns OFF the primary current, the energy stored in the magnetic field reverses the secondary winding(s) of the inductor51which causes the current flow through the forward biased diode53into pump diodes. The above-described algorithm is very similar in operation to Flyback-topology known in the DC/DC switching circuits. Similarly to the embodiment disclosed in FIGS.3and5-7, the circuitry ofFIG. 9is operative to first charge the accumulator and then discharge the latter.

The particularity of the disclosed system is that the required current in the secondary winding or windings may or may not be greater than stabilized primary current. Constructively this can be attained by determining the ratio between the number of turns in the secondary and primary windings.

One of the advantages of the system ofFIG. 9over the circuitry of FIGS.3and5-7includes the fast (microseconds and even nanoseconds depending on the switch design and the configuration of the inductor) current rise in the pulse. Accordingly the situations requiring relative low energy and short duration, the configuration ofFIG. 9may be a preferable solution. A relatively low energy stored in the inductor and linear decay of current in the secondary winding may be considered disadvantageous under certain circumstances if compared to the capacitor-based scheme.

Although the preferred embodiments of the present invention have been described herein, the above description is merely illustrative. The power supply systems disclosed herein are particularly beneficial to fiber laser systems, but may be used for powering a solid state laser systems. Further modification of the invention herein disclosed will occur to those skilled in the respective arts and all such modifications are deemed to be within the scope of the invention as defined by the appended claims.