Patent Description:
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> illustrates 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 turned off or continues to work with the limited current allowed by the design of the source. <CIT> discloses an electrical power supply for driving laser diodes with electrical power. A possible design of such an electrical power supply comprises a charging DC-DC converter operative to charge an accumulator while periodically supplying a diode current with a peak value which is greater than the source current to at least one laser diode emitting pump light pulses. A control circuit is used to control the operating mode of the charging DC-DC converter as an "output power limited" supply. The charging DC-DC converter is operative to step down the source voltage to the accumulator voltage while a discharging DC to DC power converter being operative to step down the accumulator voltage to the output voltage so that the accumulator voltage is lower than the source and higher than the output voltage.

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 not oversized power supply source that can handle the peak current, yet provide the normal (non-peak) operating power.

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. BRIEF DESCRIPTION OF THE DRAWINGS.

The above and other features and advantages of the disclosed system are explained in detail hereinbelow in conjunction with the following drawings, in which:.

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 to <FIG>, a fiber laser system <NUM> includes a DC power source <NUM> generating a source voltage coupled to a DC-DC converter <NUM> which includes an accumulator <NUM>, a charging stage <NUM> and discharging stage <NUM> ail electrically coupled to one another. The DC source <NUM> may or may not be continuously coupled to charging unit <NUM>. In either case, the charging unit <NUM> operates until accumulator <NUM> is fully charged.

The discharging unit <NUM> extracts energy from accumulator <NUM> only during a pulse duration. The pump <NUM> includes 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 unit <NUM> and therefore laser system <NUM> while shielding power source <NUM> from being overloaded.

To have high energy pulses at the input of pump unit <NUM> and therefore high power optical pulses at the output of laser system <NUM>, the energy E extracted from accumulator <NUM> during 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 accumulator <NUM>, Vc1 and Vc2 are 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 accumulator <NUM>.

<FIG> illustrates one of the modifications of one of the embodiments of the disclosed system in which a capacitor <NUM> functions as an energy accumulator. In addition to the capacitor, system <NUM> is configured with power source <NUM>, step-up converter <NUM>, step-down converter <NUM>, and laser diodes <NUM>.

Referring to <FIG> in addition to <FIG>, as known to those skilled in the art, step-up converter <NUM>, based on the boost topology, is operative to increase an input DC voltage Vps of power source <NUM> to a higher DC voltage Vout across capacitor <NUM> and further maintain the latter. Given only as a non-limiting example, step-up converter <NUM> includes an inductor <NUM>, a low-side switch <NUM> such as a MOSFET, and a high-side switch <NUM> configured as a diode or MOSFET. When low-side switch <NUM> is ON, the inductor current is increased; otherwise, the only path available to the inductor current is through high-side switch <NUM> and capacitor <NUM>. The amplitude of current pulses seen by As a result, the energy, accumulated during the ON period of low-side switch <NUM> is transferred into the capacitor. The high frequency operation of the switches results in the application of the limited charge current to capacitor <NUM> until voltage Vc corresponds to a reference value at the end of charging period T (t1-t1). Thereafter, low-side switch <NUM> stays OFF as long as capacitor <NUM> remains fully charged. The peak current of low-side switch <NUM> and, therefore, input current Iin at point A of <FIG> and <FIG> kept below the maximum current of the power supply by means of an appropriate duty cycle control. In summary, regardless of the concrete configuration of step-up converter <NUM>, the current Iin extracted from source <NUM> is limited.

The step-down DC to DC converter <NUM> may be based on the buck topology. An exemplary schematic of converter <NUM>, as shown, is configured as a buck converter including a high-side switch <NUM>, inductor <NUM>, diode <NUM> and an optional low-side switch <NUM>. The discharging stage or step-down converter <NUM>, for example, operates as follows. The current Iout flowing through point B and the pump diodes during a τ period (ti-t2) of <FIG> is continuously monitored and compared to the preset current Isetpoint which is nothing else but the desired form of current Iout (<FIG>) on pump diodes <NUM>. If the measured current Iout is lower than Isetpoint, then the duty cycle of PWM (Pulse- Width Modulation) pulses applied to switch <NUM> increases, until the measured current Iout matches Isetpoint. Otherwise, the duty cycle on high-side switch <NUM> decreases. The optional switch <NUM>, when present, is always out of phase with respect to high-side switch <NUM>, in other words, when switch <NUM> is ON, switch <NUM> is OFF and vice-versa. The optional switch <NUM> replaces diode <NUM> and is operative to reduce forward voltage losses which are rather significant on diode <NUM>. The Buck converter with optional switch <NUM> is known as synchronous Buck converter.

In addition, discharging stage <NUM> may be provided with a voltage control circuitry operative to monitor a capacitor voltage Vd. The control circuitry is configured to turn off step-down stage <NUM> if the measured voltage drops close to the pump diode voltage Vout.

The limitations imposed on capacitor voltage Vc are the subject to its relationship with input and output voltages. In <FIG>, the voltage Vc across capacitor <NUM> should be higher than both the source voltage Vps (Vc>Vps) and the output voltage Vd on diodes <NUM> (Vc>Vd). Accordingly, the only limitations applied to system <NUM> of <FIG> include intrinsic limitations of the components of the circuitry.

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

<FIG> illustrates another modification of the embodiment of <FIG>. In particular, system <NUM> is configured with two step-up converters <NUM> each having, for example, a boost configuration. Similarly to <FIG>, the accumulator is configured as capacitor <NUM> coupled 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 unit <NUM> (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> illustrates a further modification of the above-disclosed embodiment of system <NUM>. Here two step-down converters <NUM> are coupled to respective input and output of capacitor <NUM>. In contrast to the configuration of <FIG>, the voltage Vc across capacitor <NUM> should be lower than the voltage Vps at the output of the power source and greater than the voltage Vout at pump <NUM> (Vps>Vc>Vout).

<FIG> illustrates still a further modification of circuitry of <FIG>. The system <NUM> is configured with step-down converter <NUM> coupled to the output of the power source and step-up converter <NUM> coupled to the input of pump <NUM>. The capacitor <NUM> is coupled between respective units <NUM> and <NUM>. In this configuration, capacitor voltage should be lower than both Vps at the output of the power source and Vout at the input of pump <NUM> (Vo<Vps and Vc<Vout).

<FIG> illustrates an example the disclosed laser system which includes multiple converter units <NUM> configured in accordance with one of the schemes of <FIG>, <FIG>. The system includes a single two- or more phase charging stages <NUM> and three two-phase discharging stages <NUM>. The multi-phase charging stage <NUM> effectively reduces the input current ripple.

The discharging stages <NUM> each are loaded on a string of pump diodes <NUM>. The two high-side switches of discharging stage <NUM> are phase-shifted at a <NUM>° angle relative to one another. Such a configuration effectively reduces a ripple current on the pump diodes. In addition, three two-phase discharging stages <NUM> are shifted at a <NUM>° angle relative to one another. Although the phase-shift between discharging stages <NUM> does not result in the reduction of the ripple current through the pump diodes, since each stage <NUM> is 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 in <FIG> may include, for example, six (<NUM>) single-phase discharging stages <NUM> each loaded on a string of diodes <NUM>. All six stages <NUM> are shifted at a <NUM>° 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 stages <NUM> can be reduced to negligible values. The charging stage <NUM> may have the same configuration as that one of <FIG>.

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 to <FIG>, both converters <NUM> and <NUM> may 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 <FIG> and <FIG> includes 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> illustrates an alternative embodiment of the disclosed laser system. In particular, laser system <NUM> includes a power source <NUM>, inductor charge circuit <NUM>, inductor <NUM>, a switch <NUM> configured for example as a diode and a pump <NUM> radiating emission which is coupled into a fiber laser unit. The charger circuit <NUM> is operative to stabilize current in the primary winding of inductor <NUM> in the following manner. When the current in the primary winding of the inductor <NUM> is less than a reference value, the input voltage is applied to the primary winding of inductor <NUM> causing the flow of an increasing magnetizing current. The phasing of the single or multiple secondaries of inductor <NUM> and switch or rectifier diode <NUM> is such that the secondary does not conduct during this period. When the primary current reaches the reference value, charge circuit <NUM> stabilizes 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 unit <NUM> turns OFF the primary current, the energy stored in the magnetic field reverses the secondary winding(s) of the inductor <NUM> which causes the current flow through the forward biased diode <NUM> into 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 <FIG> and <FIG>, the circuitry of <FIG> is 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 of <FIG> over the circuitry of <FIG> and <FIG> includes 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 of <FIG> may 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.

Claim 1:
A power supply of a pulsed laser system (<NUM>) comprising:
a DC power source (<NUM>) with a source voltage and a source current;
an energy accumulator (<NUM>) in electrical communication with the DC power source (<NUM>);
a charging DC to DC converter (<NUM>) coupled between the DC power source (<NUM>) and accumulator (<NUM>) and operative to charge the accumulator (<NUM>) with a desired energy; and
a second discharging DC to DC converter (<NUM>) operative to discharge the accumulator (<NUM>) while periodically supplying a diode current with a peak value, which is greater than the source current, to at least one laser diode (<NUM>) emitting pump light pulses,
characterized in that the second DC to DC converter (<NUM>) has a feedback circuitry operative to read the diode current at an input of the laser diode (<NUM>), compare the read diode current to a reference value and regulate the diode current so as to match the diode current with the reference value.