The Hard-Seal HeNe Laser Tube and Lasing
Figure 1
The modern high quality HeNe laser tube consists of several parts:
The glass envelope with metal end-caps provides a rigid structure to hold
the two mirrors, narrow bore or capillary, and the large aluminum
cathode, electrically connected to the metal end-cap at its end of the
tube. The anode is simply the metal end cap at the other end of the
tube. The end-caps are
attached via glass-metal seals while the mirrors are sealed with a material
called "frit", which is a sort of solder for glass. Neither of these seals
leaks significantly on any time scale that matters. So, the tube in this
laser does not need to be used periodically to maintain its health.
Like many common types of lasers, the HeNe laser tube contains mirrors
at each end.
The High Reflector (HR) reflects nearly 100 percent of the light at a
wavelength of 632.8 nm (red). The Output Coupler (OC) reflects about
99 percent. The other 1 percent exits the laser as the useful beam.
(There is also a very small amount of leakage through the back of the HR
as no mirror is perfect, but this is normally blocked by the case and is
so low as to be useless for most applications.)
And note that these mirrors are not metal coated like shaving mirrors.
They consist of a highly polished glass substrate with multiple layers
of two dielectric (transparent insulating) materials with different indices of
refraction. This results in mirrors with very low losses.
The more layers, the higher the reflectivity. The HR
with many layers has a reflectivity of greater than 99.95 percent,
while the OC with fewer layers has a reflectivity of about 99 percent.
Compare this to a normal aluminized front surface mirror which may reach
96 percent on a good day. The OC is also antireflection coated (like
a camera or binocular lens) to minimize reflections from its outer
surface. The HR substrate is slightly wedged so that the reflection from
its outer surface does not re-enter the laser.
The tube is filled with a low pressure mixture of helium (He) and neon (Ne)
with a ratio of between 7:1 and 10:1 for He:Ne. When an electrical discharge
takes place within the gases, it excites the He to an energy level
that is similar to one of those of Ne and a He atom can then transfer its
energy to a Ne atom, which is what actually does the lasing. The Ne can
remain at this upper energy level for some time (relatively speaking!).
Occasionally, a photon at 632.8 nm will be emitted at a random time in a
random direction (spontaneous emission) as the Ne drops to a lower
energy level. The difference between the two energy levels is the
energy or wavelength of the photon. (Photons at a few other
wavelengths can also be emitted,
but for a variety of reasons, one of which is that the mirrors are
optimized for 632.8 nm, they won't contribute to lasing.)
However, if by chance, the photon is emitted along the
long axis of the tube, it can pass near many other excited atoms, causing them
to emit photons of the same wavelength in the same direction (stimulated
emission). Confined to bouncing back and forth in the space between
the mirrors (the laser
cavity or laser resonator), a cascade builds up within a very short
time as more and more
photons are emitted due to stimulated emission (much less than 1
microsecond) until the losses (due to the 99 percent OC, as well as
other causes) equal the available rate at which the upper Ne energy
level is filled. Once the photons have done the lasing, they
quickly fall back down to the lowest or "ground state", where
the cycle can repeat.
For a small HeNe laser tube, this occurs when the intracavity power
(essentially, the number of photons passing a given point between
the mirrors) reaches about 100 mW, and the useful output beam has
a power of about 1 mW.
The discharge is forced to take
place in the narrow bore or capillary. By doing this, the optical gain
of the tube is maximized and the output beam is narrow and has a smooth
Gausian profile, known as TEM00 (for Transverse Electro-Magnetic Mode 0,0).
The HeNe laser power supply (described below) initiates the discharge with
a high voltage of 7 kV or more, but once the tube starts, the voltage across it
drops to under 1 kV, with a current between 3.0 and 3.5 mA.
ML-801 Power Supply Principles of Operation
The following is written with respect to the specific version of the
ML-800 described above, that found in the ML-801 "Build-A-Laser" kit,
matching the schematic below
However, most of it also applies to other versions of
the ML-800 as well as other inverter-based Metrologic HeNe laser power
supplies, except that the part numbers and some minor details differ.
WARNING: The entire power supply on the primary side (to the left) of the high
voltage transformer (T1) is directly line-connected and especially dangerous.
DO NOT touch any part of it unless it is unplugged from the AC line and a
minimum of 30 seconds has elapsed to allow the main filter capacitor, C13,
to discharge. Testing C13 with a voltmeter to make sure it is discharged is
also a really good idea! The high voltage side of the power supply to the
right of T1 has much greater voltages on it, but the available current and
energy stored in the capacitors is small. Touching the wrong points there
may result in a rather painful shock - resulting in dropping the laser or
some other involuntary reaction! - but isn't nearly and dangerous and the
AC-line side.
The ML-800 power supply consists of 5 sections:
- AC line front-end which converts 115 VAC into approximately 160 VDC.
- Power oscillator which drives the high voltage transformer.
- Voltage doubler/filter to provide the HeNe laser tube operating voltage.
- Voltage multiplier to provide the HeNe laser tube starting voltage.
- Ballast resistors to maintain stability and control current to the
HeNe laser tube.
Each of these sections will now be described in more detail.
- AC line front-end: The 115 VAC from the AC line is converted to
DC by the bridge rectifier consisting of diodes D1 through D4, and filtered
by C13. R12 limits the inrush current when power is first applied and R14
provides a high impedance path between the power supply circuitry and Earth
Ground, which is attached to the case for safety. The fuse, F1, protects
against catastrophic failure of the primary-side components.
- Power oscillator: Q1 and its surrounding components comprise a
self oscillating driver which chops the 160 VDC from the AC line front-end
and applies it to the primary winding (pins 3 and 4) of the high voltage (HV)
transformer, T1. The 2.7 V zener diode, D3, limits the peak voltage on the
base of Q1 to 2.7 V, and current flowing through Q1's emitter to ground
produces a voltage across the emitter resistance consisting of R6 in
parallel with R13 (R6||R13). Since the base of Q1 must be greater
than the emitter of Q1 by about 0.7 V for it to turn on, current flowing
in its emitter cannot become greater than that which would result in a
voltage drop of about 2 V across R6||R13. This negative feedback
implements current regulation of the peak drive to the HV transformer,
and indirectly, power transferred to the secondary and current to the
laser tube.
- Voltage doubler/filter: The high voltage transformer, T1, steps up
the chopped 160 V from the power oscillator and drives a voltage doubler
consisting of D6, D7, C6, and C10. The output from the transformer is
approximately 700 V p-p, resulting in about 1400 VDC across the safety
bleeder resistors, R3 and R4. C6, in conjunction with R5, C7, and C8
filter the operating voltage.
- Voltage multiplier: A small amount of the AC voltage from C10 is
also applied through C9 and R10 to a five stage voltage multiplier consisting
of D6 through D1 and C5 through C1. This boosts the output voltage before
the tube starts to greater than 7 kV. However, once current
starts flowing in the tube, the 1M ohm R6 and very small values of C1 through
C5 render the multiplier almost dormant so that it contributes very little
ripple to the tube current.
- Ballast resistors and HeNe laser tube: The HeNe laser tube, like
all low pressure discharge tubes including neon indicator lamps, neon signs,
and fluorescent lamps, is a negative resistance device. That is, the voltage
across it decreases with increasing current (within the range
that matters for continuous operation) rather than the other way around as
in a normal resistor. In order for the tube not to behave like a neon bulb
in a relaxation oscillator and flash or pulse, but rather be stable and on
continously, enough positive resistance must be added in series with the
tube (and its negative resistance) so the net resistance of the entire
circuit (tube plus ballast resistors) is positive. The typical negative
resistance of a HeNe laser tube is about -50k ohms. Additional ballast
beyond +50k enables the current to be controlled by varying the voltage
across the ballast/tube combination. In the ML-800, the ballast consists
of R1, R7, and R2 (66k ohms in all). This provides the stability. But R5
in the operating voltage filter gets added to that for a total of 81k for
the voltage control of current.