White Paper
Short Upper State Lifetime Eliminates “Green Problem,” Setting the
Industry Standard for Lowest Overall Noise
Introduction and Overview
Demanding applications for multi-watt CW green lasers
are negatively impacted by fluctuations (noise) in the
output beam power. In many solid-state lasers based
on neodymium-doped crystals, fibers and disks, the
minimum achievable noise is often limited by so-called
“green noise.” This is an inevitable consequence of
rapid, dynamic fluctuations in the longitudinal mode
structure of these lasers. But with OPSLs, the very
short (nanosecond) upper state lifetime eliminates gain
storage, freezing the mode structure and thereby
eliminating the root cause of green noise. Since there
is no green noise component in Verdi G lasers based
on OPSL technology, they can deliver extremely low
total noise (e.g. 0.01% rms) without stabilized singlemode operation, which is the only proven method of
eliminating green noise in commercial solid-state lasers
based on Nd-doped crystals and glasses.
What is The Green Problem?
With cavity lengths measured in centimeters or even
tens of centimeters, continuous-wave, infrared lasers
can support many longitudinal cavity modes. Usually in
such lasers, the intra-cavity beam intensity is divided
between multiple longitudinal modes, each with a
slightly different frequency (see Figure 1). Although the
overall laser intensity noise can be fairly low, this
division is random and dynamic, with varying mixtures
of these modes lasing over time and competing for the
available gain. Such low-noise multi-mode operation is
typical of many legacy gas lasers, including argon ion
and helium neon lasers.
Multimode DPSS Laser
Multimode OPSL
Dynamic mode fluctuations
Static mode structure
Time
The nanosecond upper state lifetime of the
Optically Pumped Semiconductor Laser (OPSL)
gain medium eliminates dynamic mode fluctuations
which manifest as amplitude noise in the frequency
doubled (green) output of many other solid-state
lasers.
Figure 1
However, when a doubling crystal is inserted into a
fundamental intra-cavity beam with multiple longitudinal
modes, there is significant intensity noise in the
doubled output. The reason is that both secondharmonic generation (doubling the frequency of one
longitudinal mode) and sum-frequency generation
(adding the frequencies of two different longitudinal
modes) are possible. Sum-frequency generation
couples individual longitudinal modes and thereby
enables direct dynamic interactions between
longitudinal modes. The temporal dynamics from all
the pair-wise interactions of longitudinal modes,
whereby the intensity of one mode depends on the gain
of another mode, generates significant intensity noise.
This long-recognized phenomenon is called the “green
problem,” [ref 1] since the most important CW lasers
using intra-cavity doubling are green diode-pumped
solid-state (DPSS) lasers, where the laser fundamental
at 1064 nm is frequency-doubled to generate green
output at 532 nm.
Reducing/Eliminating Green Noise in DPSS Lasers
Several methods have been used in commercial multiwatt DPSS lasers at 532 nm to address the green
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problem. The first approach was to use an elongated
cavity in order to split the power over a larger number
of longitudinal modes. The idea is that the noise level
is reduced by averaging the noise effect of many more
modes. This noise- reduction approach is sufficient for
some applications, but for those that are particularly
noise-sensitive, it has proven inadequate [ref 2].
A more rigorous approach is to actually remove the
green noise at its source. The only way to do this in a
DPSS laser is to make the laser operate on a single
longitudinal mode. Doubling of this single mode is then
the only mechanism by which 532 nm light is created in
such a laser. So there are no dynamic fluctuations in
the IR → green frequency conversion process. The
green noise spectrum mirrors the IR noise spectrum,
which can be very low in a stable single-mode design.
Coherent Verdi V series lasers use this approach to
achieve specified output noise of < 0.01%.
With OPSL technology the situation is quite different.
The gain medium is a semiconductor material, albeit an
optically pumped semiconductor. Most people are
familiar with electrical pumping, where electrons and
holes are injected on opposite ends of a diode junction.
The gain is only achieved in a narrow intrinsic region
sandwiched between the p and n sections of the diode.
With optical pumping, the whole thickness of the
structure can be made of intrinsic material and the
electron and holes are directly generated only where
they are needed - more precisely in narrow layers
known as quantum wells. The emission wavelength can
be engineered by properly choosing composition and
thickness.
Typical Verdi G Noise Data
(10 Hz‐ 100 MHz)
0.04
Verdi G – No Green Problem to Eliminate
In a DPSS the upper state lifetime is in the
microsecond regime. Because the lifetime is many
orders of magnitude longer than the cavity round trip
time, the laser medium has the ability of storing energy;
this is an advantage for achieving high pulse energy
and power in industrial lasers by means of Q-switching,
but translates into a problem for intracavity doubled CW
lasers. In a multimode IR DPSS laser, the modes
flicker up and down randomly even though the overall
power can be quite constant. By inserting a second
harmonic crystal in the cavity we introduce a
mechanism for the different modes to affect each other.
Because of the long storage time provided by the gain
medium, these effects build up over several cavity
roundtrips, and the final result is a chaotic situation,
where different modes alternate between being
dominant and lurking at low power while stealing gain
from the dominant mode (see Figure 2).
Multimode OPSL
(intracavity doubler)
IR Output Power
IR Output Power
IR Output Power
Output
Time
Green Output Power
Output
Multimode DPSS
(intracavity doubler)
Output
Multimode DPSS
(fundamental only)
Time
Green Output Power
Time
RMS Noise (%)
0.03
0.02
0.01
0
1
2
3
4
5
6
7
8
Output Power (Watts)
Figure 3
In a semiconductor, radiative and non-radiative
recombination of charge carriers are both very fast
processes. So in an OPSL, the effective upper state
lifetime is a few nanoseconds or less, i.e. on the
timescale of the cavity trip time. On the laser mode
timescale there is thus no stored gain, only
instantaneous gain. The behavior of the individual
cavity modes therefore is determined solely by the
cavity, the gain just follows along. If the cavity is
properly aligned and stable, as in the case of Verdi G
series lasers, then even if an OPSL is operating on
multiple longitudinal modes, there is no green noise in
the frequency doubling process. As a result, the typical
output noise of a Verdi G laser is < 0.01% rms over the
range 10 Hz – 100 MHz (see Figure 3). This makes
these lasers an optimum choice for even the most
noise-sensitive applications.
Figure 2
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In practice, Verdi G lasers incorporate a birefringent
filter (BRF) wavelength tuning element in the cavity.
This serves two purposes. The gain bandwidth of the
OPSL gain chip is fairly broad and the factory
adjustable BRF enables the laser output wavelength to
be set precisely to 532 nm, which can be important in
applications such as Raman spectroscopy of low
frequency vibrations. In addition, the output bandwidth
is narrowed to a handful of longitudinal modes making
the laser very monochromatic. With this multimode
format, Verdi G lasers deliver noise characteristics as
good or better than a single-mode DPSS laser.
Conclusion
In the past, it has often seemed the case that new laser
technologies have been developed and promoted
simply because the technology is new. But today’s
economic realities and the diverse demands of laser
applications mean that new technology must now
deliver tangible and valuable benefits if it is to
successfully compete with, and ultimately displace,
existing entrenched laser types. OPSLs certainly meet
this litmus test. Indeed, because of their unique
advantages, OPSLs have rapidly become the dominant
technology of choice in applications as diverse as
bioinstrumentation and crime-scene forensics. Now the
Verdi G family of multi-watt green lasers is bringing key
OPSL advantages to demanding scientific applications.
Together with other Verdi G advantages such as low
cost of ownership, high reliability and smoothly
adjustable output power, the complete absence of
green noise makes these lasers the optimum choice for
the majority of applications.
References
1. T. Baer, Large amplitude fluctuations due to
longitudinal mode coupling in diode-pumped
intracavity-doubled Nd:YAG lasers, J. Opt. Soc. Am. B,
vol 3, 9, pp 1175-1180 (1986).
2 S. Witte, R.T. Zinkstok, W. Hogervorst, and K.S.E.
Eikema, Control and Precise Measurement of Carrier
Envelope Phase Dynamics, Appl. Phys B 78, 5-12
(2004).
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