Variable output coupling laser

Details Variable output coupling laser Variable output coupling laser

1999-09-22

2003-03-04

Ip, Paul (Department: 2828)

Coherent light generators

Particular resonant cavity

C372S025000, C372S027000, C372S029010, C372S030000, C372S073000, C372S099000

Reexamination Certificate

active

06529540

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to the coupling of light out of a laser cavity in a controlled manner.
BACKGROUND OF THE INVENTION
A laser includes two reflecting surfaces, typically one surface being more reflective than the other, an active medium between the reflecting surfaces, and a power source capable of exciting the active medium.
The power source is capable of pumping the active medium to promote a portion of its constituents to an excited state. Pumping can be electrical or optical. The active medium can be atoms or molecules in a gas, a liquid, a glass, or a crystalline solid. What is important is that the excited species be capable of emitting radiation by the process of stimulated emission; for example, that a photon passing in proximity to an excited species can stimulate that species to emit a second photon identical in energy, phase, and direction to the first.
A laser begins to oscillate when a first randomly emitted photon leaves the medium, strikes one of the reflecting surfaces and is reflected back through the medium; it then stimulates a stream of identical photons to travel along with it in an amplified beam. If these photons are reflected back into the medium, they will each be amplified. As long as the gain per round trip exceeds the losses per round trip, the result is a build-up of circulating photons. However, the population of excited state species in the medium will become depleted, due to the transfer their energy to the photon beam oscillating between the reflecting surfaces. If the pumping is continuous, the laser may settle down to a continuous operation, but if the pump is pulsed or occurs at too low a rate, the photon beam will deplete the medium and the laser will turn off again after emitting a pulse of light.
In general, a laser will begin to oscillate as soon as the gain supplied by the pump exceeds the losses imposed by scattering, transmission through the output coupling mirror, and so on, This is referred to as “reaching threshold”.
A useful laser output signal is usually obtained by having one of the mirrors coated so as to transmit some fraction of the light falling on it. The fraction depends upon the nature of the medium and the rate of pumping and is a critical factor in optimizing performance. If there is 0% output coupling, no output will be obtained from the laser; in this instance the light will oscillate and be very intense inside the laser cavity, but none will exit the cavity to be useful. In contrast, if there is 100% coupling, the effect is as if there are no reflectors at all; and, the laser will not operate, as it has no feedback. For any laser system, the optimum lies somewhere between these extremes of 0% and 100% output coupling.
Some media are capable of storing energy for a relatively long time; hence, their excited state species are long-lived. In this instance, it can be beneficial to block one of the reflectors to prevent photons from circulating within the cavity, while pumping the medium for an extended predetermined period of time. In this manner, the pump energy can be integrated, and when the reflector is unblocked, the intensity of the resulting laser pulse increases more than it would were the laser simple allowed to begin oscillating on its own. This process is known as Q-switching.
“Q” refers to the cavity quality factor; a value of Q=1 implies no losses at all, and a value of Q=0 implies 100% loss. The idea is that a very low Q-cavity will allow the build-up of very large gain, and if the Q is suddenly raised, the gain will greatly exceed the loss and the laser will produce a hugely amplified output until the gain is depleted.
A common way of Q-switching is to construct a laser cavity as is shown in prior art FIG.
1
. An active medium in the form of a rod
12
with Brewster angle ends is disposed between a high reflecting surface in the form of a mirror
14
a
, preferably being 100% reflective and an output coupling mirror having a reflectivity less than 100%. A polarizer
16
is shown between the active medium
12
and the output-coupling mirror
14
b
. A crystalline material in the form of a Pockels cell
18
is disposed between the polarizer
16
, and a quarter-wave plate
19
is located between the Pockels cell
18
and the polarizer
16
. In operation, these elements ensure that light polarized in the plane of the page of
FIG. 1
can oscillate in the cavity. The oscillation of the light can be effectively blocked by having the quarter-wave plate
19
in the cavity and adjusting it so that two passes through it result in a 90 degree rotation of the plane of polarization. Thus, horizontally polarized light passing from the rod towards the output coupling mirror strikes the mirror
14
b
and returns towards the rod; since the light is vertically polarized it will be reflected out of the cavity by the polarizer and cannot get back to the rod to be amplified. Hence, the cavity is effectively blocked.
The Pockels cell functions as a variable wave plate activated by an applied voltage. Using a suitable voltage the net rotation on a round trip through both the wave plate
19
and the Pockels cell
18
can be made to vanish, and the laser cavity thus switches from low to high Q.
The polarizer
16
, quarter-wave plate
19
, and Pockels cell
18
form a Q-switch in combination with required control circuitry. The laser is operated by ensuring that the Q-Switch blocks or prevents oscillations in a controlled manner while a pulsed light source is used to pump the laser rod
12
. Typically the Q-switch is maintained in a closed state where Q=0 until the pump pulse is terminated, and the rod has accumulated a significant portion of the energy from the pump. By opening the Q-switch such that Q >0, the light can circulate and due to the very high laser gain, the buildup in the circulating intensity is very fast and very large. The partially transmissive output coupling mirror
14
b
allows a portion of this circulating power to escape on each round trip; this constitutes a useful laser output signal. The pulse intensity and duration are only indirectly controllable, and result from a combination of how quickly the Q-switch can open, and how quickly the gain stored in the laser rod
12
is depleted by the circulating laser beam.
One drawback to the prior art system described heretofore, is that is suffers from having an output pulse profile that is determined by the opening time of the Q-switch and the depletion of the gain from the rod; this is typically a single large spike of a few tens of nanoseconds in duration. For many applications this is too much energy too quickly and has been known to cause problems in various material processing situations.
U.S. Pat. No. 4,630,275 in the name of Rapoport discloses a controlled slow Q-switch. By applying a staircase-shaped control signal to a laser Q-switch, a plurality of laser pulses are emitted with controlled energy and time separation. Compared with conventional Q-switching, the invention enables the laser to emit pulses with shorter time intervals, narrower line widths, higher output energy, and more uniform power density across the laser beam cross section. Rappaport provides a tri-level voltage input signal in order to obtain three short uniform Q-switched pulses from the output of the laser cavity.
U.S. Pat. No. 4,660,205 which discloses a technique for achieving extremely short laser pulses uses pulse-transmission mode (PTM) Q-switches. This device also uses a Pockels cell and polarizer to rapidly drain the energy from a laser cavity. In essence, the apparatus amounts to a high-speed, voltage-variable mirror, whose reflectivity can be changed rapidly between 0 percent and 100 percent (see Solid-State Laser Engineering, W. Koechner, Springer-Verlag, N.Y. (1976), pp. 441 ff). A similar technique is used for cavity dumping of CW-pumped solid-state lasers (op. cit., pp. 444ff). Notwithstanding, the variable voltage mirror disclosed in the '205 patent does not provide the useful functionality of a voltage vari

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