9.
Powering solid-state lasers
Christopher R. Hardy, Kigre, Inc., 100 Marshland Rd, Hilton Head Island, SC 29926, USA.
Keywords: Laser, Diode Pumped Solid State (DPSS), Flashlamp, Laser Power Supply,
Ionization, Pulse Forming Network (PFN), Energy Storage Unit (ESU), Laser Driver, Soft-Start,
Current Control, Snubber Circuit, Laser Diode Protection
Abstract
Methods for designing safe, reliable, and efficient power supplies for solid-state lasers are discussed. Multiple control architectures and component selection options are presented for both flashlamp and diode-pumped solid-state lasers systems. The process needed to insure good spectral overlap between a pump source’s emission and a solid-state material’s absorption spectrum is reviewed.
List of Figures
Figure 1 ........................................................................................................................................... 3
Figure 2 ........................................................................................................................................... 3
Figure 3 ........................................................................................................................................... 6
Figure 4 ........................................................................................................................................... 7
Figure 5 ........................................................................................................................................... 8
Figure 6 ......................................................................................................................................... 10
Figure 7 ......................................................................................................................................... 11
Figure 8 ......................................................................................................................................... 11
Figure 9 ......................................................................................................................................... 12
Figure 10 ....................................................................................................................................... 13
Figure 11 ....................................................................................................................................... 13
Figure 12 ....................................................................................................................................... 13
Figure 13 ....................................................................................................................................... 14
Figure 14 ....................................................................................................................................... 17
Figure 15 ....................................................................................................................................... 18
Figure 16 ....................................................................................................................................... 18
Figure 17 ....................................................................................................................................... 19
Figure 18 ....................................................................................................................................... 20
Figure 19 ....................................................................................................................................... 21
Figure 20 ....................................................................................................................................... 22
Figure 21 ....................................................................................................................................... 23
Figure 22 ....................................................................................................................................... 24
Figure 23 ....................................................................................................................................... 25
Figure 24 ....................................................................................................................................... 26
Figure 25 ....................................................................................................................................... 28
Figure 26 ....................................................................................................................................... 28
Figure 27 ....................................................................................................................................... 30
Figure 28 ....................................................................................................................................... 32
Figure 29 ....................................................................................................................................... 33
1

Figure 30 ....................................................................................................................................... 34
Figure 31 ....................................................................................................................................... 35
Figure 32 ....................................................................................................................................... 36
Figure 33 ....................................................................................................................................... 37
Figure 34 ....................................................................................................................................... 39
List of Equations
Equation 1 ....................................................................................................................................... 8
Equation 2 ..................................................................................................................................... 14
Equation 3 ..................................................................................................................................... 14
Equation 4 ..................................................................................................................................... 15
Equation 5 ..................................................................................................................................... 15
Equation 6 ..................................................................................................................................... 15
Equation 7 ..................................................................................................................................... 15
Equation 8 ..................................................................................................................................... 16
Equation 9 ..................................................................................................................................... 16
Equation 10 ................................................................................................................................... 16
List of Tables
Table 1 .......................................................................................................................................... 27
9.1.
Introduction
Lasers are often broken down into four general categories based on their lasing medium: Gas
Tube Discharge (Chemical, Excimer, Gas and Metal-vapor), Liquid (Dye), Semiconductor
(Diode), and Solid-State. Gas and Dye lasers are not considered solid-state. Although
Semiconductor lasers have their own classification, and are also not considered solid-state, their driver electronics are identical to Diode Pumped Solid State (DPSS) lasers. Therefore, in this section we elaborate on various architectures for driver electronics to power lamp-pumped and diode-pumped solid state lasers.
The introduction of flash-lamp pumped solid-state lasers in the early 1960s started a new branch in high energy power supply design (Koechner, W., 1976). Figure 1 shows a schematic of the first Gigawatt (world record) Ruby Laser developed and manufactured by Lear Siegler Laser
2

Systems Center (Myers, J. 1965). A primary power supply was used to drive the two parallel flashlamps while a secondary power supply was used to drive the flashlamp for the Q-switched element.
Figure 1
First Giga-Watt
Ruby Laser
Figure 2 shows an actual Ruby laser rod and uranium U 6+ doped glass excited state absorber Qswitch cell used in this early world record laser system.
Figure 2
Ruby Laser Rod and U 6+ doped glass excited state absorber
Q-switch
Crystalline and glass solid-state laser materials require high optical pump energies to lase and gas discharge “flash-lamps” were developed to deliver the necessary electrical-to-optical conversion. These flashlamps provide the solid-state laser designer with a broadband radiation pump source that may be electrically optimized to overlap with the laser gain material’s
3

absorption spectra. For pulsed operation, the flashlamp’s emission is optimized to match the laser’s fluorescent lifetime.
However, controlling the hundreds or even thousands of amps (often at thousands of volts) required is not trivial. Capacitors, inductors, switches, connectors, and even wire must be properly rated for safe and reliable laser power supply operation. Protective circuitry should also be added to handle current, voltage, and EMI transients. Most importantly, federal and local safety regulations need to be followed to prevent potential injury or fatality.
Diode Pumped Solid-State (DPSS) lasers, including fiber lasers, are now widely used in the industry. DPSS lasers replace the gas tube arclamp or flashlamp with a semiconductor diode laser as a pump source. Advantages of diode pumping (when compared to lamp-pumped systems) include higher efficiency, longer component lifetime, and lower maintenance requirements. DPSS systems have lower operating voltages and relatively less stored energy and are therefore much safer than flashlamp pumped systems. In some high peak power laser applications, the lamp pumped laser is still a popular option. The diode laser’s maximum peak power is the same as its average power. A 1cm pump diode laser array may provide ~ 100
Watts/pulse of pump power. A flashlamp may produce orders of magnitude more peak power than a similarly priced diode array.
9.2.
Safety
Laser power supply designers must understand and implement proper safety protocols for their product, application, and location. The designer should insure the device satisfies requirements
4

for all areas where the equipment will be sold or potentially used, not just where it was developed. Regulations IEC 60825 internationally and ANSI Z136 in the United States outline requirements such as labeling, interlocks, and user safety for both the laser and laser related equipment, including the power supply. In the United States, the Occupational Safety & Health
Administration (OSHA) has standards and directives (instructions for compliance officers) related to laser hazards in the general laser industry that is specified by regulation 29CFR1910.
Class 3B and Class 4 lasers require interlocks that shut down a system whenever a safety condition is not met, such as when a room door or protective cover is opened. These interlocks are usually integrated into the laser power supply and therefore must be factored into the power supply design. An emergency stop switch must be in reach of the user and is typically located on the front panel or remote control section of the laser power supply. It is also good practice to include a key switch that can disable the laser and power supply when needed; during system maintenance, for example. A switch that allows the key to be removed in the OFF position is common as it allows the Laser Safety Officer or similar designated responsible person to insure the laser and power supply will remain off when required.
9.3.
Flash-Lamp Pumping
A properly designed power supply will insure the flashlamp efficiently converts enough electrons to photons for reliable lasing in the solid-state laser material. Xenon is typically used as the gas fill for flashlamps due to its relatively high input to output efficiency for solid-state laser material pumping. A graph of output efficiency versus wavelength for various fill gases is shown in Figure 3.
5

Figure 3
Fill Gas Comparison
Although a flashlamp’s output is relatively broadband, specific regions of the spectrum can be emphasized by controlling current through the flashlamp (Buck A. & Erickson, R. & Barnes, F.,
1963). At lower current densities, the spectral output is heavily weighted towards the visible and infrared end of the spectrum. As current densities increase, the lamp’s spectral output shifts toward the blue and ultraviolet. Example emission spectra of a xenon flash lamp operating at a high current density (6.500 kA/cm2 curve #1) and low current density (1000 kA/cm2 curve #2) are shown in Figure 4 (Penn State University, 2011).
6

Figure 4
Emission Spectra of a
Xenon Flashlamp
Higher current density pulses often produce high energy short wavelength emission extending from the ultraviolet spectral region and dissipating through the visible and near infrared. This emission results from plasma energy that is dominated by a strong “white light” continuum also described as broadband black-body or bremsstrahlung radiation emission. Bremsstrahlung is from the German bremsen
, to brake and strahlung, radiation, thus, "braking radiation" or
"deceleration radiation". This is essentially electromagnetic radiation that is produced by the acceleration and collision of charged particles (read electrons) with other charged particles such as atomic nuclei (Myers, M. & Myers, John D., Myers, A., 2008).
The flashlamp’s spectral output should match the laser’s gain material as well as possible.
Figure 5 shows a typical Nd:YAG absorption spectrum. At low power levels, the line radiation from Krypton provides a good overlap. At higher power levels, Xenon is preferred because
7

although its line structure is not perfectly matched to Nd:YAG, it is more efficient at converting electrical energy to blackbody radiation.
Figure 5
Nd:YAG Absorption
Spectrum
Flashlamp pumped power supplies may be operated either continuous (CW), modulated (quasi-
CW); or pulsed with the proper choice depending on the laser application. The design of a flashlamp pumped solid-state laser includes optimization of key components such as the laser gain element, resonator optics, pump chamber, flashlamp and power supply. The laser’s application dictates the targeted performance and operational envelope. Typical laser performance specifications include pulse repetition rate, peak power, average power, energy per pulse, power density, beam diameter, beam divergence, beam quality, electrical-optical efficiency, and wall plug efficiency.
The required input energy (E i
) is a function of the laser’s worst case estimated efficiency and is defined as laser output energy (E o
) divided by resonator efficiency (eff) as shown in Equation 1.
E i
=
E eff o
Equation 1
Input Energy
The laser application will determine the current pulsewidth requirement. Short, high energy pulses are typically best created with a Pulse Forming Network (PFN) circuit while pulses over
8

1mS typically require an Energy Storage Unit (ESU) approach. With this information, the proper energy storage, voltage, and current values can be determined.
9.3.1.
Flashlamp Ionization (Triggering)
Flashlamps are gas discharge tubes that are usually filled with Xenon and/or Krypton. Four types of triggering are commonly used: external, series, simmer, and pseudo-simmer.
9.3.1.1.
External Trigger Circuits
External triggering uses a step-up transformer to create a small arc (streamer) between the flashlamp’s electrodes. To reduce the trigger energy required for a specific flashlamp, a thin, high temperature trigger wire is typically wrapped around the outside of the flashlamp. The trigger pulse is a damped oscillation from the transformer’s secondary with peak amplitude voltage in the +/- 10KV to 30KV range. The pulse duration is usually 200nS per inch of arclength. Since there is no switch holding back the PFN energy, current will begin to flow in the flashlamp as soon as the flashlamp is ionized by the fast, high voltage trigger pulse.
Specific trigger voltages depend on arc length, bore size, fill-pressure, electrode material and capacitor potential. The trigger pulse can also be applied to a metal bar or pump-cavity element as long as the conductive metal covers the entire distance between the flashlamp’s electrodes and is close coupled to the flashlamp (typically under 6 millimeters). Because the transformer’s secondary winding is outside of the flashlamp’s high current path, the transformer can be made relatively small. An external trigger circuit is shown in Figure 6.
9

Figure 6
External Trigger
Circuit
9.3.1.2.
Series Trigger Circuits
Series triggering also uses a step-up transformer to create a small arc between the flashlamp’s electrodes. However, unlike the external trigger circuit, the series design places the transformer
“in series with” the flashlamp. The series trigger transformer is larger and heavier than the external trigger design because the secondary winding must carry the full flashlamp current. The secondary winding also adds impedance to the PFN discharge circuit, which must be factored into the design.
However, by choosing the proper series transformer, it can replace the inductor element in the
PFN circuit. Because the trigger pulse is applied directly to the flashlamp’s electrode, the trigger voltage and energy can often be less than required by the external trigger circuit. This results in less radiated Electro-Magnetic Interference (EMI) and more reliable flashlamp ionization. As with the external trigger circuit, current will begin to flow in the flashlamp as soon as the flashlamp is ionized by the trigger pulse from the transformer. A series trigger circuit is shown in Figure 7.
10

Figure 7
Series Trigger
Circuit
9.3.1.3.
Simmer Trigger Circuits
The simmer trigger circuit uses a separate DC current source (simmer power supply) to maintain a continuous DC current through the flashlamp. This small DC current (typically 100-500mA) keeps the flashlamp ionized as soon as the external trigger pulse is applied. Because of the switch element, a Silicon-Controlled Rectifier (SCR) for example, the PFN energy will not discharge into the flashlamp even though the lamp is ionized. Current from the PFN will flow through the flashlamp only when the separate flash control pulse is applied to the SCR switch.
Because the high voltage trigger pulse has to be applied only once, a simmer circuit is very advantageous for repetition rates above 1 Hertz. However, simmering the lamp adds heat (often in the tens of watts) that must be removed from the flashlamp and laser pump cavity. A simmer trigger circuit is shown in Figure 8.
Figure 8
Simmer Trigger
Circuit
9.3.1.4.
Pseudo-Simmer Trigger Circuits
11

Pseudo-Simmer trigger circuits typically use a high power resistor and transistor switch instead of a separate simmer power supply. Although simple in its approach, the series simmer (pass) element can dissipate tens of watts. So this circuit often includes a secondary switch (not shown) in series with the resistor to enable the pseudo-simmer current just before the transistor, such as a
Metal Oxide Semiconductor Field-Effect Transistor (MOSFET) or Insulated Gate Bipolar
Transistor (IGBT), is turned on.
The time it takes for the flashlamp to start conducting current will depend on the lamp, PFN, and resistor value but is generally in the 1-100uS range. A pseudo-simmer trigger circuit is shown in
Figure 9.
Figure 9
Pseudo-Simmer
Trigger Circuit
9.3.2.
Pulsed Forming Network (PFN) Design
A PFN design typically includes the AC-DC or DC-DC capacitor charging power supply, high voltage capacitor, air-core, wire-wound inductor, Silicon Controlled Rectifier (SCR) switch, flashlamp, and related control circuitry. It is usually best to design the system by selecting the flashlamp, then calculating the proper PFN capacitor and inductor values to get the proper current pulse profile. A typical PFN circuit is shown in Figure 10.
12

Figure 10
PFN Circuit
In most practical flashlamp circuits, the inductance, capacitance, and capacitor voltage are carefully chosen so that the energy is transferred to the flashlamp in a critically damped pulse.
Critical damping is important to insure the most efficient transfer of energy from the capacitor to the flashlamp. For a flashlamp, a damping factor of 0.8 is considered to be optimal. A graph of a critically damped pulse is shown in Figure 11.
Figure 11
Critically-damped Pulse
Damping factors over 0.8 are considered “over-damped” and result in low peak current and power. A graph of over-damped ratios for Normalized Current versus Normalized Time is shown in Figure 12 (ILC Technology, 1983).
Figure 12
Over-damped Pulse
13

Damping factors under 0.8 are “under-damped” and result in high peak current, lower peak power, and lower efficiency energy transfer. Under-damped circuits also produce current reversal (ringing) which is detrimental to flashlamp lifetime. A graph of under-damped ratios for
Normalized Current versus Normalized Time is shown in Figure 13.
Figure 13
Under-damped Pulse
Designing a critically damped flashlamp circuit would be straightforward if it could be treated as a traditional RLC circuit. However, unlike a linear resistor, the flashlamp has dynamic impedance designated by K o
. The lamp impedance calculation is shown in Equation 2.
K o
=
1 .
28 *
%
Xe
450
+
%
Kr
805
⎟
1 / 5
* l d
Ω
A
Equation 2
Lamp Impedance
Fill pressure is in torr; ℓ = arc length in cm; d
= bore diameter in cm. Units are Ohms*Squareroot of Amps. The capacitance calculation is shown in Equation 3.
C =
⎡
⎢
⎢
⎢
⎣
2 *
E o
*
4
K o
4
* t
3
2
⎥
⎥
⎤
1
⎥
⎦
/ 3
Equation 3
Capacitance
C = capacitance in Farads; E o
= energy stored in capacitor in Joules; α = unit-less damping parameter = 0.8 for critical damping; and t = desired current pulse-width at 10% points. The inductance calculation is shown in Equation 4.
14

L = ⎝
⎜
⎛ t
3 ⎠
⎟
⎞
2
C
L = inductance in Henries. The capacitor voltage calculation is shown in Equation 5.
Equation 4
Inductance
V = 2 *
E
C
Equation 5
Voltage
V = initial capacitor voltage in volts. The circuit impedance calculation is shown in Equation 6.
Z = L
C
Equation 6
Impedance
Z = circuit impedance in ohms. In reality, the exact capacitor and inductor values determined by calculation are rarely available off-the-shelf. So it is important to re-calculate the actual damping factor using your actual values for the capacitor and inductor. The actual damping factor calculation is shown in Equation 7, where
= the actual damping factor.
=
V o
K o
* Z o
Equation 7
Damping Factor
If necessary, lamp impedance can be adjusted to insure a critically damped current pulse is achieved. As seen in formula 1, the lamp’s K o
can be increased by increasing fill pressure.
However, this will also affect the current pulse waveform. Therefore, actual circuit components should be selected to insure lamp impedance between 0.7 and 0.8. An anti-parallel shunt diode must be added directly across the PFN capacitor to eliminate the negative current swing of a potentially under-damped circuit.
The peak current delivered by the PFN to the flashlamp should be calculated to insure all of the circuit components are properly rated. The peak current calculation is shown in Equation 8.
15

I pk
= 0.94*
⎡
⎢
( e
− 0 .
77 * α
)
* V o
Z o
⎤
⎥ Equation 8
Peak Current
I pk
= peak current in amps. Flashlamps do not operate under standardized conditions and therefore cannot be given a specific lifetime rating. Instead, flashlamp lifetime is a function of bore size, arc length, input energy, and pulse-width and is calculated in terms of total number of shots (flashes). The maximum input energy is referred to as explosion energy. The explosion energy calculation is shown in Equation 9.
Ex = 90 * d
* ℓ * t
Formula 9
Explosion Energy d = bore diameter in mm; ℓ = arc length in inches; and t = current pulse-width in milliseconds.
The term explosion energy is used because this is the energy at which the envelope is likely to fracture. Flashlamp lifetime is a function of the ratio of the input energy to explosion energy.
The flashlamp lifetime calculation is shown in Equation 10.
Lifetime =
⎛
⎜⎜
E in
E x
⎞
⎟⎟
− 8 .
5
Equation 10
Flashlamp Lifetime
E in
= input energy from capacitor; E x
= explosion energy calculation. In practice, actual lamp lifetimes may also be limited by electrode life due to sputtering of electrode material onto the flashlamp wall surface. In this case, light output drops gradually throughout the lamp’s lifetime.
These PFN equations can be modeled using various computer programs to calculate ideal values for the application. Computer modeling also allows the user to try “what if” scenarios to potentially better optimize the laser system using off-the-shelf components.
Many types of PFN capacitor styles exist, including oil filled, metalized and discrete foil, film and paper dielectric. Companies such as CSI Technologies , Dearborn Electronics , and General
16

Atomics Electronic Systems offer a wide capacitor selection and usually include helpful design guides and data-sheets on their web-sites. Metallized capacitors, shown in Figure 14, are widely used in the pulsed laser industry.
Figure 14
Metallized HV
Capacitors
Purchasing the proper PFN inductor can often prove tricky so most companies manufacture their own. Thankfully the construction process is not too complex and simple loops of magnet wire around a plastic form will suffice. Magnet wire is available from companies such as
MWS Wire
Industries and is usually available through standard electronic product distributors. Polyimide coated magnet wire is preferred for PFN applications. Transformer encapsulation material is generally available from companies such as Lord Corporation, Master-bond, and Solar
Compounds Corporation
.
A small, custom PFN is shown in Figure 15. 20AWG magnet wire is wrapped around a 20uF,
1KV metal foil capacitor to provide 20uH of inductance.
17

Figure 15
20uF, 20uH, 1KV
PFN Assembly
This PFN assembly is part of a laser system used by
StellarNet Inc.
in their PORTA-LIBS 2000
Laser Induced Breakdown Spectroscopy (LIBS) instrument. LIBS systems focus a high peakpower laser onto a small area at the surface of the specimen to create plasma. This permits realtime qualitative identification of trace elements in solids, gases, and liquids via optical detection of elemental emission spectra. The PORTA-LIBS 2000 instrument is shown in Figure 16.
Figure 16
StellarNet PORTA-LIBS
2000 LIBS Instrument
A similar PFN assembly was used by
RCA Corporation
(Burlington, MA) in their model
AN/GVS-5 Near-Infrared (NIR) laser rangefinder. This unit uses a pulse Time-of-Flight (ToF) approach to determine accurate distance measurements to a remote target. A ToF laser rangefinder consists of a high peak power, pulsed laser transmitter, optical receiver, and range
18

computer. Eye safe and non-eye safe laser wavelengths may be used depending on the application. The AN/GVS-5 PFN assembly can be seen in Figure 17.
Figure 17
RCA Corporation
AN/GVS-5 NIR Laser
Rangefinder
The high current SCR thyristor switch is available from manufactures such as
International
Rectifier , Powerex, and Semikron . SCR thyristors conduct current only after they have been switched on by the gate terminal. Once conduction has started in the SCR, the device remains latched in the “on” state, even without additional gate drive, as long as sufficient current continues to flow through the device’s anode-cathode junction. In a PFN application, the SCR will stay in conduction as long as the current flowing to the lamp from the capacitor exceeds the
SCR’s latching current specification. As soon as current drops below the latching current rating, the SCR will stop conducting and will stay off until the next trigger pulse is received at its gate.
SCR latching currents are often under one amp, so it is important to turn off (quench) the high voltage power supply just before the trigger command is sent to the SCR. The quenching time is typically a few milliseconds to insure both the SCR and flashlamp come fully out of conduction.
The power supply voltage and power rating is determined by the energy per pulse, quench time, and the repetition rate.
19

SCRs for laser power supply applications typically come in three distinct package styles: stud mount, disc (hockey puck), and module. Modules are generally the easiest to use since their mounting surface tends to be electrically isolated from the SCR’s anode and cathode terminals.
Therefore, the module’s heatsink does not electrical isolation from the laser power supply enclosure. Stud and disc styles handle tremendous amounts of current, but their anode and cathode connections are the mounting surfaces, and are therefore not electrically isolated.
Heatsinks for these types of devices must provide the necessary electrical hold-off for the application. Important specifications to consider when using an SCR include anode-to-cathode heat dissipation, maximum current, voltage, and dv/dt rating.
SCR gate drive circuits typically consist of a simple transformer to provide necessary high voltage isolation between the logic trigger pulse and the SCR’s gate and cathode terminal. A typical SCR gate drive circuit is shown in Figure 18.
Figure 18
SCR Gate Circuit
Gate protection elements include an anti-parallel diode and resistor-capacitor (R-C) filter for transient absorption. The diode is often a high speed zener to clamp voltage spikes at the SCR’s gate. The snubber circuit consists of a high current, anti-parallel diode and a series connected resistor and capacitor with values selected for the necessary frequency response for the application.
20

9.3.3.
Multiple-Section PFN Design
Some applications require a square current profile instead of the standard Gaussian shape provided by a critically damped PFN. In this case, multiple PFN sections can be added together to create a “Mesh” circuit network. Three (or more) sections, with equal capacitor and inductor combinations, combined in a transmission line arrangement will result in a reasonably square current pulse profile. A three-mesh PFN developed for Fermi National Accelerator Laboratory in
Batavia, IL is shown in Figure 19.
Figure 19
3-Mesh PFN
This PFN network was able to deliver 1700V, 3200A, 1.5mS current pulses for a Nd:Glass Slab laser system. The custom multi-tapped inductor is located above the energy storage capacitors.
The SCRs, current monitor circuits, discharge circuits, and fire control circuitry are located on the panel above the inductor. Each of these mesh PFN assemblies included optical transceivers to communicate with the user’s fire control system. The laser system included four of these
21

mesh PFN units, providing 32 Kilowatts of average power and a large liquid cooling system for the Nd:Glass Slab and four large flash lamps. An oscilloscope trace of the 3200A, 1.5mS current pulse is shown in Figure 20. The current scale is 500A/division. The time scale is 500uS/div.
The current was measured using a Pearson Electronics model 5623, 0.001V/A, Hall-effect current probe. When using Hall-effect current monitors, it is important to use a probe with sufficiently rated current-time product for your application or waveform distortion, due to probe saturation, will result. DC probes are ideal for measuring long current pulses but are often limited by their relatively low peak current ratings.
Figure 20
3200A Current Pulse using a 3-Mesh PFN
Network
9.3.4.
Energy Storage Unit (ESU) Design
An ESU design typically includes the AC-DC or DC-DC capacitor charging power supply, medium voltage energy storage capacitor bank, high current switch, flashlamp, and related control circuitry. It is usually best to design the system by selecting the flashlamp, then calculating the proper ESU capacitor bank values to get the proper current pulse profile. And
22

unlike a fixed PFN design, the ESU circuit also allows for variable current pulse-widths. A typical ESU circuit is shown in Figure 21.
Figure 21
ESU Circuit
ESU capacitor banks are typically a parallel or series-parallel combination of high energy density low Equivalent Series Resistance (ESR) aluminum electrolytic capacitors. Companies such as
Cornell Dubilier Electronics
,
Illinois Capacitor
,
TDK-Epcos
, and
Panasonic
offer a wide capacitor selection and usually include helpful design guides and data-sheets on their web-sites.
Laminated buss-bar assemblies can provide a reliable connection between energy storage capacitors. Due to their low impedance power path, buss-bars can reduce circuit losses and will help insure uniform current distribution. The high current IGBT thyristor switch is available from manufactures such as
International Rectifier
,
Microsemi
, and
Powerex.
Unlike SCRs, IGBTs have the ability to turn off via the gate control signal. This allows the power supply designer to create a specific current pulse shape without the added loss of an inductor element. The on-time of the IGBT’s gate drive pulse will determine the flashlamp’s current pulse-width. A single 600A, 1400V IGBT from
Powerex
, model CM600HA-28H, is shown in Figure 22.
23

Figure 22
Powerex Single
IGBT module
There are two important things to notice in this figure. First, the module’s gate terminals are shorted to prevent damage during storage. Secondly, there is a large letter “H” written on the top of the device. This letter is added by the manufacturer and corresponds to the measured onresistance of the device. Specific on-resistance can vary from device to device, so it is extremely important to use modules that have the same letter when using them in parallel circuits for high current applications. This insures relatively even current sharing through each IGBT in the parallel circuit. Modules with multiple IGBTs inside are also available.
High voltage, high current switches are widely used in ESU designs. Due to their relatively low gate charge, MOSFETs are often used in high frequency designs while IGBTs are typically preferred for low frequency (<20 KHz) applications. For a given package size, IGBTs generally have lower on resistance and therefore lower drain-to-source voltage and are the choice for very high pulsed current applications.
IGBT gate drive circuits are required to properly get the IGBT into and out of conduction.
Depending on the location of the IGBT in the ESU circuit, the gate drive circuit may need to be isolated. Small DC-DC convertors are used to provide this isolation, but not all convertors
24

provide high isolation voltage, so it is important to refer to the manufacture’s datasheet to insure a sufficient isolation rating for your application. Gate drive circuits can also provide short circuit protection by monitoring the saturation voltage of the switching device. A typical IGBT gate drive circuit is shown in Figure 23.
Figure 23
IGBT Gate
Drive Circuit
9.4.
Laser Diode Pumping
Using semiconductor diode lasers instead of flashlamps as an optical pump source for solid state lasers offers significant advantages such as higher optical efficiency and longer pump source lifetime. In the past decade, and particularly in the last 3 years, semiconductor laser diodes have made huge advances in power handling and reliability. Laser efficiency improvements have led system designers to expect ever higher efficiencies from their laser diode power supplies.
Depending on the application, a properly designed DPSS laser can typically exceed 1 billion pulses. Therefore, the weak link in system design is now typically the driver – especially the energy storage and filter capacitors – not the flashlamp.
As is the case with a flashlamp-based power supply, a properly designed DPSS laser driver is extremely important. The exact voltage and current control methods will greatly depend on the
25

application. High average power systems typically require a switch-mode DC-DC convertor with current feedback to the switching control circuitry. Low average power applications can often use a simpler linear current control circuit as long as the circuit’s heat dissipation is acceptable.
The decision to use an isolated or non-isolated driver will also depend on the laser design and potential for external wiring faults, such as a short circuit to chassis ground.
A typical DPSS driver will include input, soft-start, energy storage, current control, and laser diode protection circuits. An example of a 0 to 500 amp, 100uS to 3mS pulsed laser driver from
Kigre, Inc.
is shown in Figure 24. Low profile energy storage capacitors (to handle the peak current discharge) are underneath the board.
Figure 24
0-500 Amp Pulsed
DPSS Laser
Driver
9.4.1.
Pump Diode Selection
Efficient optical pumping requires good spectral matching of the pump diode wavelength with the absorption spectrum of the laser material. Unfortunately, many laser materials have narrow absorption peaks so careful attention is needed to insure the pump wavelength remains in the
26

absorption band for all operating conditions. Table 1 lists the peak pump absorption wavelength and typical output wavelength for some of the most common laser materials.
LASER MATERIAL Er:YAG Nd:YAG Yb:YAG Ho:Cr:Tm
:YAG
PEAK ABSORPTION
WAVELENGTH (25
OUTPUT
WAVELENGTH o C)
Nd:Glass
(Phosphate)
Er:Glass
(Phosphate)
940nm 808nm 940nm 781nm 803nm 975nm
2940nm 1064nm 1030nm 2097nm 1054nm 1535nm
Table 1
Pump Diode Absorption Wavelengths
It is important to consider the laser material’s absorption spectra for the entire temperature range of your application. The absorption spectrum should be compared with the pump diode’s center wavelength and wavelength shift specifications. The laser pump diode’s wavelength will shorten at lower temperatures and lengthen at higher temperatures. Therefore, the best pump diode choice may not have its center wavelength at the laser material’s peak absorption wavelength.
Compromises must also be made with respect to standard pump diode wavelengths as most diode bars operate in the 780-860nm or 940-980nm wavelength regions with 808nm, 940nm, and
975nm being the most common.
Wavelength shift due to temperature fluctuations can be especially troublesome in Nd:YAG and other crystalline host DPSS lasers as the width of their pump band tends to be very narrow.
However, laser glass host gain materials provide relatively broad pump band widths allowing them to function over wide temperature ranges without the need for diode thermal conditioning.
A typical specification for diode wavelength drift with temperature is 0.25nm/ o C. Figure 25 illustrates the difference in pump band width and how it affects the thermal stability of
27

Neodymium doped Yttrium Aluminum Garnet (YAG) crystal and glass host lasers (Michael J.
Myers, 2007).
Figure 25
Effective diode drift range for Nd:YAG and Nd:Glass pump bands
A large number of individual diode emitters can be attached to a sub-mount and packaged into a diode bar array. Multiple diode bars can also stacked vertically on the same sub-mount package as shown in Figure 26. This approach can yield Quasi-CW (pulsed) output powers in the hundreds of watts.
Diodes
Figure 26
Diode bars stacked in vertical array
Low power diode bars often use Indium/Tin (52% In, 48% Sn) solder due to its low melting point of 118 o C. The low melting point allows the manufacture to solder the laser diode emitters to the substrate without risking damage to the semiconductor material. However, high power
28

diode bars are typically assembled using high temperature (hard) Gold/Tin (80% Au, 20% Sn) solder rather than lower temperature Indium/Tin solder. The higher melting point of Gold/Tin
(281 o C) allows the diode bar to operate at much higher temperatures, and therefore higher peak powers, than modules using Indium/Tin.
AuSn is a gold-based eutectic solder that does not need flux. In production, dry nitrogen is often used to displace oxygen during the Gold/Tin soldering process to insure oxidation does not occur. The diodes are mounted to a low coefficient of expansion substrate such as BeO ceramic.
Because the beam quality of high power diode bars can be poor, their low brightness is an issue.
Therefore, collimation optics can be added to efficiently couple the diode bar’s output to the laser material. Collimation of the diode bar’s output to a fiber is commonly used in applications where the pump source needs to be remotely located from the laser host material. A Volume
Bragg Grating can also be used to narrow the pump diode’s emission spectrum.
9.4.2.
Input Protection
Along with EMI transient protection, the input circuit should provide reverse voltage, overvoltage, and under-voltage protection.
Depending on the application, EMI protection can be handled by a combination of low, medium, and high frequency protection. Gas Discharge Tubes (GDTs) can handle low frequency, high energy transients while Transient Voltage Suppression (TVS) devices, such as Metal Oxide
Varistors (MOVs), can handle medium frequency, medium energy transients. Low-pass circuits of inductor elements, RF cores, and capacitors offer high frequency protection.
29

Under voltage (UV), over voltage (OV), and reverse supply protection are best handled with an
IC controller such as the
Linear Technology
# LTC4365. This device controls the gate voltages of a pair of back-to-back external N-channel MOSFETs to ensure the output stays within a safe operating range. Two precision comparators are used to monitor for over-voltage and undervoltage conditions. If the input supply rises above the OV threshold or falls below the UV threshold, the gate of the MOSFET is quickly turned off – disconnecting the output (load). An example input protection circuit is shown in Figure 27.
Figure 27
Input Protection
Circuit
9.4.3.
Soft-Start
Soft-start circuits are required to control the in-rush of current from the power source to the driver’s energy storage circuitry. The circuitry must be designed to protect both the driver and the power source. Understanding the power source’s specifications and limitations is critical to a properly designed laser driver. Important specifications include acceptable voltage range, peak
30

and average current and power ratings, and Equivalent Series Resistance (ESR) values over the entire operating temperature range.
Thankfully, there are a variety of control ICs from manufactures such as Linear Technology ,
Maxim Integrated Products
, and
Micrel, Incorporated
. Control ICs are available for isolated, non-isolated, step-up (boost), step-down (buck), inverter (fly-back) and Single-Ended Primary
Inductor Converter (SEPIC) topologies.
Many DPSS laser applications are now powered from Lithium batteries, such as the Energizer
EL123AP (Energizer Holdings, Inc., 2011). This cylindrical lithium battery is nominally 3.0 volts and has a typical capacity of 1500mAh down to 2.0 volts. It is important to note that below
2.0 volts, the usable energy and current quickly diminishes. Manufacturers such as
Energizer now often include simulated application test data on their battery datasheets. Pulsed DPSS laser applications are similar to photoflash applications, so battery requirement calculations for a particular application are straight-forward.
9.4.4.
Energy Storage
Pulsed DPSS drivers often are required to deliver tens or even hundreds of amps to the pump diodes inside the laser. However, most power sources, including batteries, cannot directly handle this type of current surge. Therefore, DPSS drivers should include energy storage to handle the laser’s peak current pulse requirements. The capacitors used must be low ESR types and are generally aluminum electrolytic, hybrid, or super capacitor types.
31

The latest super capacitors have extremely high energy density, but often have too much internal resistance for DPSS driver applications. Relative to aluminum electrolytic capacitors, super capacitors also have low voltage ratings. Series or series-parallel combinations of super capacitors can be used for laser drive applications, but they must be properly balanced to avoid over-voltage conditions. A BEST-CAP super-capacitor from
AVX
is shown in Figure 28.
Figure 28
Super Capacitor
A variety of control ICs are available to efficiently manage series connected super capacitors including the
Linear Technology
LTC3225, a 2-cell, 150mA super capacitor charger. The
Texas
Instruments
(TI) BQ33100 is a super capacitor “fuel gauge” that includes individual capacitor monitoring and voltage balancing for up to 9 series-connector super capacitors. A system partitioning diagram for the BQ33100 is shown in Figure 29.
32

Figure 29
BQ33100 System
Partitioning Diagram
Voltage requirements for the energy storage section are a combination of the forward voltage of the laser diode at its operating current plus any voltage drops due to wiring, high current switch, and capacitor ESR resistances. This overall voltage drop, due to system losses, is typically referred to as “compliance voltage” or “over-head voltage”. Sufficient energy storage is required to insure the bank voltage remains above the compliance voltage.
9.4.5.
Current Control
The laser power supply designer has to determine the best current control method for the application. Contributing factors include space claim, compliance voltage, peak current, and heat dissipation requirements. Overall efficiency is also important especially when operating from batteries.
9.4.6.
Linear Current Control
33

Perhaps surprisingly, linear current control is often the best choice for a laser driver application.
Linear control is non-switching and produces very low radiated EMI. It is also very immune to conducted EMI due to its inherent circuit impedance. Protecting the output against short circuit faults is also straight forward with a linear control approach and many devices such the Texas
Instruments
(
TI)
OPA549 provide bullet-proof over-current protection. In addition, the OPA549 provides an accurate, user-selected current limit and the device senses the load indirectly so no power resistor in series with the output current path is required. This allows the current limit to be adjusted from 0A to 10A with a simple potentiometer or controlled digitally with a Digital-to-
Analog Converter (DAC). Figure 30 demonstrates a basic 8A (10A peak) driver circuit using the
OPA549.
Figure 30
0-8 Amp Linear
Controller using an
OPA549
Linear devices are also easily parallel. An example laser drive circuit using two parallel
TI
OPA548 surface mount devices is shown in Figure 31. This circuit can provide 0-10 amps output with simple 0-5 volt user input.
34

Figure 31
0-10 Amp Linear
Controller using parallel OPA548s
Of course, the designer must be aware of the disadvantages of using linear current control, especially with respect to overall efficiency. One way to improve efficiency is to minimize the voltage drop from the input to the output. Adding an adjustable DC-DC converter to the front end of a linear current controller can maximize efficiency. A feedback circuit incorporating a programmable micro-controller or I 2 C DAQ, such as the
Maxim
DS4432, can be implemented.
The feedback circuit should keep the linear driver’s input voltage just above the minimum required compliance voltage value.
9.4.7.
Switch-mode Current Control
A more common approach in DPSS laser power supply design is to use switch-mode control with feedback to regulate the laser driver’s output current. Switch mode designs offer three main advantages when compared to linear regulator circuits. Their switching efficiency can be better; they can be smaller; and they can provide electrical isolation. Since less energy is lost in the transfer, smaller components and less thermal management are required. Current feedback can be used with boost, buck, fly-back, and SEPIC control topologies. Current sensing can be achieved with low value, ultra-stable power sense resistors or via non-contact devices such as
Hall-effect current sensors.
35

Low average and/or peak current designs typically use a sense-resistor with a low resistive value in series with the output path. Current flowing through the precision resistor to the laser diode creates a voltage that can be measured by the feedback circuit. Series sense-resistor circuits are relatively small and low cost but can dissipate significant power at higher average power levels.
High current designs generally use a non-contact approach in feedback circuits to minimize power loss and to provide electrical isolation between the current carrying conductor and the output signal. Hall-effect types can be either closed or open loop style and should be sized for the peak current capability and saturation value. Higher repetition rate applications should consider a DC rated Hall-effect sensor to prevent degradation of the output signal due to sensor field saturation. However, Hall-effect sensors are relatively large when compared to series sense-resistor circuits. A clamp-on 70A DC current probe from
Tektronix
, model A622, is shown in Figure 32. This model is easy to use and connects directly to an oscilloscope. Its fast response time and 70 amps DC current rating makes it ideal for most DPSS laser power supply current measurements.
Figure 32
Tektronix A622
DC Current Probe
9.4.8.
Laser Diode Protection
As with all semiconductor devices, laser diodes are very sensitive to over voltage and over current transients and proper protection is required. Laser driver designs should include a shorting relay, snubber circuit, EMI and transient protection. A sample output protection circuit is shown in Figure 33.
36

Figure 33
Laser Diode Protection
9.5.
Control Features
Flashlamp and Diode pumped laser controllers have many common features including various internal and external controls. Control signals are generally a mix of inputs and outputs and can be in either analog or digital form.
9.5.1.
Inputs
The laser power supply designer should consider user inputs such as power on and off, laser standby and ready, and fire control settings (including repetition rate and burst). System inputs include feedback from the laser such as output pulse detection, temperature, laser energy and/or laser power. Other system signals recognized by the controller as inputs may include coolant flow, pressure and temperature indication, current, voltage, or charge levels or system diagnostics such as board level DC-DC power supply status.
37

9.5.2.
Outputs
In addition to controlling the flashlamp or pump diode current and energy, the laser power supply may also need to control hardware such as a Q-switch driver, cooling unit, or secondary laser amplifier system. Control of internal or external power supplies and/or energy storage devices is also common. Using a standard communication protocol, such as gigabit Ethernet or controller area network (CAN) allows microcontrollers, PLCs, and similar devices to communicate with each other without a host computer.
9.5.3.
Software and Hardware Platform
Real-time software and hardware platforms are often necessary for laser systems that need reliability, timing precision and/or fast input/output (I/O) response times. Real-time operating systems from companies such as
QNX Software Systems
,
LynuxWorks
or
Wind River Systems
are readily available to the laser power supply designer. Development tools, such as
National
Instruments LabVIEW, that automatically integrate real-time operating system software are also available.
Systems that don’t require real-time control can often use simple programmable logic controllers
(PLC) or off-the-shelf Data Acquisition (DAQ) modules from companies such as
Measurement
Computing
or
National Instruments
. These manufacturers also provide graphical programming languages that communicate directly with their DAQ modules for stand-alone or personal computer (PC) control via Ethernet, USB, IEEE-488 (GPIB) or PCI interface. A sample graphical user interface (GUI) from
Kigre, Inc.
is shown in Figure 34.
38

Figure 34
Graphical User
Interface (GUI)
9.6.
Summary and Outlook
As output energy and power density of DPSS lasers increase, they will continue to replace flashlamp-based laser systems. And because DPSS laser systems typically require lower operating voltages and energies, their power supplies are inherently safer than flashlamp-based supplies of the past.
Although laser system power supplies have been around for over 50 years, the latest designs and component materials are hardly mature. Powerful, low cost, off-the-shelf microcontrollers have pushed control and communication levels higher and higher. Semiconductor switch products are significantly improved and datasheets are readily available to the designer. Capacitor and battery chemistries are evolving rapidly and the power supply designer must stay abreast of current technology to insure a competitive, efficient, and reliable product.
39

Due to the incredible advancement of laser applications, laser power supplies can be found in the medical, industrial, communication, environmental, research, and military fields. Wherever there are lasers you will find laser power supplies and the driver designs often vary as greatly as the laser application itself. From extremely small laser endoscopes to very large directed energy weapons, lasers continue to find a home in a variety of unique applications.
Reference List
Buck A. & Erickson, R. & Barnes, F. (1963). Design and Operation of Xenon Flashtubes.
Journal of Applied Physics
, 34, 2115-2116.
Energizer Holdings, Inc. (2011). Energizer 123 Datasheet . [Online] Available at: http://data.energizer.com/PDFs/123.pdf
[Accessed 11 July 2011]
ILC Technology. (1983).
A guide to Flashlamps for Pulsed Solid State Lasers.
Sunnyvale: ILC
Technology.
Koechner, W. (1976). Solid-State Laser Engineering.
New York: Springer-Verlag.
Myers, J. D. (1965).
First GigaWatt (World Record) Ruby Laser.
Ann Arbor: Lear Siegler Laser
Systems Center.
Myers, M. & Myers, John D., Guo, B. (2007). Practical internal combustion engine laser spark plug.
SPIE Optics & Photonics 2007
(pp. 3-4). San Diego: SPIE.
Myers, M. & Myers, John D., Myers, A. (2008).
Laser-induced Breakdown Spectroscopy (LIBS).
Weinheim: Wiley-VCH.
Penn State University (2011).
Pulsed Light & PEF.
University Park: Penn State University,
Department of Architectural Engineering. Available at: http:// www.engr.psu.edu/iec/abe/control/pulsed_light.asp
[Accessed 15 July 2011]
40