Download PULSED LASERS

PULSED LASERS
A pulsed laser is pumped at high intensity for a short period of time. As a result, the active
medium gain, and the loop gain are much higher than for continuous wave laser, so the output power is higher.
Pulse Shape Out of a Pulsed Solid-State Laser
Figure 5.10 describes the shape of a single pulse out of a Ruby laser, compared to the pumping pulse from the
flash lamp.
Figure 5.10: single pulse out of a Ruby laser, compared to the pumping pulse from the flash lamp
The output laser pulse is about 1 millisecond, and it is composed of hundreds or thousands of small pulses.
Each of the small pulses is called a spike, and last about a microsecond.
The spikes appear randomly in time, and differ from each other in its length and peak power.
Usually only the entire pulse is measured, without consideration of each spike.
The average power per pulse is calculated by timing the entire pulse, and measuring its energy.
In figure 5.10 it can be seen that the laser pulse starts after a short time from the pumping pulse. This is
the time it takes the active medium to arrive at the threshold value for lasing.
Analysis of a single pulse from a solid state laser
The linewidth of a laser beam from a solid state laser is more than 30 [GHz] (3*1010 [Hz]). Each line has
hundreds of longitudinal modes in it. For each of these modes, the process described in figure 5.11 applies.
Figure 5.11: Gain and output power from a pulsed solid state laser.
Figure 5.11 describes a simple case of constant pumping of the active medium that starts at time t1.
1. Starting from t1, the active medium gain and the loop gain increase rapidly as a result of continuous
strong pumping.
2. At time t2, the active medium gain arrive to the threshold value, and the loop gain arrive to “1” lasing starts. The active medium gain and loop gain continue to rise since the output power has not
reach the saturation value that cause “hole burning” in the gain curve.
3. Until time t3, the high value of the loop gain causes intense pulse of laser radiation. Thus, the active
medium gain drops below the threshold value. When the loop gain is below “1”, lasing stops, and the
whole process starts again as long as the pumping continue.
Each longitudinal laser mode starts at a different time, with a different photon. There is a competition between
the longitudinal modes on the energy inside the active medium. Thus, the random nature of the spikes:
Each spike has its own peak power and duration.
Q switched Lasers
Q switch is a method of using optical switch inside the optical cavity.
An optical switch has two states:
1. Open - When the radiation pass through the switch undisturbed.
2. Closed - When the radiation can not pass through the switch.
Switching means transferring the system from “open” to “close” or the other way.
Q (Quality) Factor
Quality (Q) factor of an optical cavity is a quantity that measure the capability of the optical cavity to
store electromagnetic energy inside.
The way this energy is stored is by forming standing waves between the laser mirrors.
Q Factor is proportional to the ratio of the energy stored inside the standing wave and the wasted energy
from the wave during round trip between the laser mirrors.
High Q Factor value means that the energy is stored well inside the cavity.
Low Q Factor value means that the energy is emitted from the cavity rapidly.
Q Switch
Short pulses from gas lasers, and solid state lasers, are usually created by using switching inside the
laser cavity to change the quality (Q) factor of the laser cavity.
Such switching is called “Q Switch”.
Q switch determines the capability of the optical cavity to store electromagnetic energy.
In a laser without Q switch, the atoms are excited to the lasing level at certain rate. Lasing starts as soon
as “population inversion” is created.
In a laser with Q switch, the feedback which help establishing the population inversion is blocked, thus as
long as the switch is “on” there is no lasing.
The continuous pumping, transfer more and more atoms into the excited state.
At the moment the Q switch opens, all the excited state atoms creates a short laser pulse with high energy.
In a Q switched laser the high value of the Q factor is maintained throughout the excitation of the active
medium, until high energy is stored inside the laser cavity.
Then the Q factor is lowered quickly, and all the energy stored in the cavity is emitted as a short pulse.
Figure 7.18 describe schematically the structure of a Q switched laser.
Figure 7.18: Schematic description of the structure of a Q switched laser.
Q switch acts as a shutter that can be open suddenly inside the laser cavity.
When the switch is closed, the laser radiation can not move between the mirrors. Lasing can not occur, but the
excitation continue to pump energy into the cavity.
The Q switch is timed to open when the gain of the active medium reach its maximum value.
All the energy stored in the active medium is emitted in a single pulse of electromagnetic radiation with high
power
Explanation of Q Switch as a function of time
Figure 7.19 describe the Q switch action as a function of time.
Figure 7.19: Q switch action as a function of time.
The active medium gain increase continuously, while the loop gain and the output power remains zero, until
the Q switch is open at time t2.
The dash line in figure 7.19 describes the process in the same active medium if the Q switch were not
present.
Q switch is used when a short pulse with high peak power is needed.
The payment for the high peak power is the decrease in the total energy emitted by the laser.
Q switch method is applicable only for lasers with long lifetime of the upper laser energy
level.
Example: Peak Power in a Q Switch Laser
Ruby laser operating without Q switch creates pulse that last 0.5 [msec], and its energy is 5 [J].
The same laser operating with the Q switch creates a pulse that lasts 10 [nsec], and have energy of 1 [J].
Assuming a triangular pulse shape, calculate the peak power of the pulse in both cases.
The peak power of the pulse without the Q switch:
P = E/∆
∆ t1/2 = 5 [J]/(5*10-4 [sec]) = 104 [W]=10 [KW]
The peak power of the pulse with the Q switch:
P = E/∆ t1/2 = 1 [J]/10-8[sec] = 108 [W] = 100 [MW]
Thus, the peak power increase 10,000 times (!!!).
Different Methods for Q Switch
Figure 7.20 describes some of the methods for Q switching.
Figure 7.20: Methods for Q switching.
•
Turning mirror at the end of the optical cavity - Only when the mirror is facing the other
mirror, there is lasing. To increase the number of pulses per second, a special drum with many mirrors
is turning. Thus for every turn of the drum, many pulses are emitted, since each mirror on the drum is
facing the cavity separately.
•
•
•
Electro-Optic transducer - Change the transmission through the device by electric voltage.
Acousto-Optic transducer - Change the transmission through the device by acoustic signal.
Saturable absorber - An absorber that becomes transparent when it reach saturation. It is usually a
dye solution which prevents lasing by absorption. When the radiation arrives at a certain level, this
absorber comes to saturation, and since it can no longer absorb radiation, it becomes transparent. At
that moment lasing can occur, and all the stored energy inside the cavity is emitted as a single pulse.
Mode-Locked Lasers.
There are applications that require very short pulses, with maximum peak power in each pulse:
1. In scientific research of high speed processes - in physics, chemistry, biology etc.
2. In optical communication - As the pulse width get shorter, higher number of pulses per second can be
transmitted.
3. In optical computer - As the pulse width get shorter, higher number of operations per second that can
be done.
To achieve shorter laser pulses for these applications, mode locking of the laser modes is done.
Most lasers operate on many longitudinal modes at the same time. Thus, the laser output contains several
frequencies, as can be seen in figure 7.22a.
Figure 7.22a: Output laser power without Mode Locking as a function of frequency.
All laser modes operates without specific constant phase relation among them.
Since the electric field of each mode oscillate independently, there is no point inside the cavity where all the
modes are in phase.
The instantaneous output power is proportional to the square of the electric field at each moment, thus it
fluctuate randomly in time as seen in figure 7.22b.
Figure 7.22b: Output laser power without Mode Locking as a function of time.
Locking the Longitudinal Modes of the Laser
Locking the longitudinal optical modes inside the cavity is achieved by locking the relative phase of all the
optical modes, such that at a certain point they all have the same phase.
At this point a constructive interference occur between all the laser modes, and the result is a single pulse,
with very short width and very high peak power, which move between the mirrors of the cavity.
This moving pulse cause the laser output to be orderly chain of pulses.
The length of each pulse is from 1 [psec] (10-12 [sec]), up to 1 [nsec] (10-9 [sec]).
In figure 7.23 the output laser radiation of a mode locked laser can be seen.
Figure 7.23: output laser radiation of a mode locked laser.
Mode Lock Optical Switch
The element locking the laser modes is an optical switch inside the cavity.
This switch is opened for a very short time equal to the length of the pulse, and than close for a period of
time equal to the time of round trip of the pulse inside the cavity.
The location of the switch is near one of the end mirrors.
The switch allows the pulse to pass to the mirror and back, and than close to disable other pulses from
building up.
The switch opens again when this particular pulse arrive again from the other mirror.
This is a synchronous switching which accumulates all the energy in a single pulse moving back and forth
between the cavity mirrors. Each time it reaches the output coupler, a single pulse is emitted.
The mode locking is done with an Acousto-optical modulator, and the frequency of its operation is determined
by the travel time of the pulse between the mirrors.
From: "Laser Adventure " by Rami ARIELI
http://www.phys.ksu.edu/perg/vqm/laserweb/index.htm