DOI: 10.1007/s00340-007-2627-5
Appl. Phys. B 87, 437–444 (2007)
Lasers and Optics
Applied Physics B
t. schr
¨
oder
1
c. lemmerz
1
o. reitebuch
1,u
m. wirth
1
c. w
¨
uhrer
2
r. treichel
3
Frequency jitter and spectral width
of an injection-seeded Q-switched Nd:YAG laser
for a Doppler wind lidar
1
Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft- und Raumfahrt (DLR) e.V.,
82234 Weßling, Germany
2
EADS Astrium GmbH, Earth Observation & Science, Ludwig-Bölkow-Allee, 85821 Ottobrunn, Germany
3
EADS Astrium GmbH, Earth Observation & Science, 88039 Friedrichshafen, Germany
Received: 6 June 2006/Revised version: 13 December 2006
Published online: 4 May 2007 • © Springer-Verlag 2007
ABSTRACT The design of a 50 Hz single longitudinal mode,
diode-pumped and frequency-tripled Nd:YAG master oscilla-
tor power amplifier is described, and the first measurements
of output parameters are presented. The laser oscillator is
injection-seeded by a tuneable monolithic Nd:YAG ring laser
and frequency stabilized by minimising the Q-switch build-up
time. The laser system will be an integral part of an airborne in-
strument demonstrator for a first satellite based Doppler wind
lidar to measure vertical profiles of one component of the at-
mospheric wind vector. This paper focuses on the investigation
of the frequency jitter and the linewidth of the laser, which are
measured on a pulse-to-pulse basis. For this purpose a compact,
high accuracy beat frequency monitoring system has been de-
veloped at DLR. By operating the amplifier stage at half the
repetition rate (50 Hz) of the oscillator, we could reduce the
frequency stability from 10 MHz (rms) to 1.3 MHz (rms) (over
a 14 s period). We have determined a mean linewidth of 15 MHz
(FWHM) at 1064 nm. These measured laser parameters enable
wind velocity measurements in the atmosphere (0–15 km) at an
accuracy of 1 to 2 m/s.
PACS 42.55.Xi; 42.60.Lh; 42.68.Wt
1 Introduction
The measurement of atmospheric wind profiles
from ground and airborne platforms is a basic requirement in
meteorology. Particularly, reliable instantaneous global ana-
lyses of winds are needed to improve the quality of the numer-
ical weather prediction, and also to improve the understanding
of atmospheric dynamics and climate processes [1]. Initiated
by the European Space Agency ESA currently a direct detec-
tion Doppler lidar is being developed to sense global wind
fields from a satellite for the first time [2].
In order to validate the performance of this Doppler li-
dar instrument, called ALADIN (Atmospheric Laser Doppler
Instrument), and to obtain a dataset of atmospheric measure-
ments a prototype for an airborne platform was developed.
The ALADIN airborne demonstrator (A2D) is representative
for the satellite based Doppler lidar. The A2D, designed to
u Fax: +49-8153-28-1271, E-mail: oliver.reitebuch@dlr.de
fly on the Falcon 20 aircraft from DLR, operates as a di-
rect detection Doppler wind lidar at
355 nm wavelength re-
trieving aerosol and molecular backscatter signals in par-
allel. A Fabry–P
´
erot interferometer is used to measure the
Doppler shift of the spectrally broadened molecular signal
while a Fizeau interferometer is used for the narrowband
aerosol return. The main task of the A2D is the measurement
of the line-of-sight (LOS) component of the wind vector in
the atmosphere (
0–15 km) with a range resolution of 300 m to
1.2km. The projection of the line-of-sight onto the horizontal
(HLOS) should be measured with an accuracy of
1 to 2m/s
depending on range. Results of the development, the manu-
facturing of the A2D, and the campaign objectives have been
presented elsewhere [3, 4].
The laser transmitter – as key component of the Doppler
lidar system – must comply with constraints for airborne ap-
plications such as compactness, ruggedness and low power
consumption. Pulse energy, pulse length, repetition rate, spa-
tial and spectral beam properties are essential laser parameters
which determine the instrumental performance of the lidar
system. One of the most challenging goals in the develop-
ment of the laser transmitter is to achieve a frequency stability
which should be in the order of several
MHz (rms) at medium
output energies of about
70 mJ/pulse at 355 nm.Inorderto
reach the measurement goals of the A2D, the laser transmit-
ter is designed as a single mode,
70 mJ, 50 Hz pulse repetition
rate, diode-pumped and frequency tripled Nd:YAG laser in
a master oscillator power amplifier (MOPA) configuration.
Stable single-mode operation and tuning is achieved by injec-
tion seeding the oscillator in combination with the Q-switch
build-up time (QBUT) minimisation technique [5, 6] to match
the axial mode frequency of the slave laser to the seeder fre-
quency. Medium power single-mode Nd:YAG laser systems
nearest to the optical specifications for the A2D are the sys-
tems reported by Ehret et al. [7] using the QBUT technique
and Ostermeyer et al. [8], applying amodifiedPound–Drever–
Hall scheme. The latter was claimed to achieve a frequency
stability of better than a few MHz at
1064 nm, but no measure-
ments were presented.
The knowledge of the frequency stability and spectral
width of the laser and the compliance during the wavelength
calibration of the receiver as well as during the wind meas-
urements requires the deployment of an appropriate moni-
toring system. The optical heterodyning detection method,
438 Applied Physics B – Lasers and Optics
which is used in coherent Doppler lidar systems [9] for meas-
uring the Doppler-shift allows the recording of the shot-to-
shot frequency jitter of a pulsed laser with high accuracy.
By some authors the optical heterodyning method was ap-
plied for studying the instantaneous frequency fluctuations of
nanosecond single mode laser pulses for precision laser spec-
troscopy [10–13]. Compared to methods based on a spectrum
analyzer or a Fabry–P
´
erot etalon [14] the spectrum of indi-
vidual pulses can be determined, as opposed to the average
spectrum of an ensemble of pulses.
This paper is organized as follows. Section 2 outlines es-
sential optical requirements such as pulse energy, frequency
stability and linewidth and describes in detail the optical lay-
out of the A2D laser transmitter. In Sect. 3 we describe the
design of the optical heterodyne measuring system. Section 4
presents output parameters and measurements of the fre-
quency fluctuations and the linewidth of the laser transmitter
on a pulse-to-pulse basis. The results are discussed related to
the required characteristics of the airborne demonstrator. Fi-
nally, Sect. 5 summarizes the experimental results.
2 Laser transmitter design
2.1 Optical design
The optical design shown in Fig. 1 consists of the
following main parts: a tuneable seed laser and a quasi fixed-
FIGURE 1 Schematic of the laser setup including the fibre coupling to the heterodyne unit
frequency “reference laser”, a low power oscillator (LPO),
two amplifier stages, and a frequency conversion stage. The
reference laser head was developed at Innolight GmbH, Ger-
many; the frequency tripled Nd:YAG MOPA was designed at
Thales Laser S.A., France. The LPO is injection seeded in
order to achieve single longitudinal mode operation and fre-
quency tuning. The amplification is achieved in two stages
in order to reach the required pulse energy. Second and
third harmonic generators (SHG, THG) convert the IR radi-
ation to
532 nm and 355 nm. The laser base plate is made
of an iron-nickel alloy (Invar) which guaranties high thermo-
mechanical stability. In order to cope with the spatial re-
strictions within the aircraft the laser head was constructed
in a compact setup with dimensions of
344 mm (width) ×
780 mm (length) ×352 mm (height). The optical heterodyne
unit used for the laser frequency diagnostic gets the optical
signals of the seed laser and of the residual pulsed IR laser
beam via polarization-maintaining (PM) single mode fibres.
The seed laser power is split up in a ratio of
50 : 50 in order to
have a local oscillator signal for the heterodyne detection unit
(see Sect. 3).
2.2 Reference laser head
The reference laser head (RLH) consists of two
identical laser systems (reference laser and seed laser) and
SCHRÖDER et al. Frequency jitter and spectral width of an injection-seeded Q-switched Nd:YAG laser for a Doppler wind lidar 439
a frequency stabilisation scheme. The reference laser – being
the frequency reference – operates as a quasi fixed-frequency
laser while the seed laser is continuously frequency tunable.
The laser transmitter must be tuneable in frequency over the
spectral range
±5 GHz of the two interferometers of the A2D
receiver for wavelength calibration of the A2D. Reference and
seed laser are both based on a laser diode pumped monolithic
non-planar Nd:YAG ring oscillator with single longitudinal
mode operation [15]. A phase-locked-loop controller is used
in the RLH to hold and tune the seed laser at a defined differ-
ence frequency from the frequency stable reference laser. Fre-
quency tuning of the seed laser is realized by a combination of
a slow laser crystal temperature variationand a fast pump laser
diode current modulation. The seed laser tunes mode-hop free
over
15 GHz. The frequency stability of the reference laser
was measured by heterodyning its reference radiation against
an iodine stabilised Nd:YAG laser. The measured frequency
stability was
234 kHz (rms) over 25 min [16]. The maximum
seed laser power was
90 mW measured at the fibre output.
2.3 Low power oscillator (LPO)
The pump chamber of the linear resonator is com-
posed of an Nd:YAG rod transversally pumped by three stan-
dard diode stacks in a threefold radial symmetric configura-
tion. Each stack provides a peak power of
350 W for 175 µs
pulse duration at 100 Hz. The highly reflective mirror (HR) is
concave with a reflectivity of
99%. A convex Gaussian vari-
able reflectivity mirror (VRM) is used as output coupler to
ensure the required output beam parameters. With that config-
uration and a cavity length of about
30 cm pulse durations of
more than
30 ns were obtained. To prevent the effect of spatial
hole burning in the laser crystal, two quarter-wave-plates are
placed in front of and after the pump chamber to have a circu-
lar polarisation within the rod (twisted mode technique). For
the generation of actively controlled laser pulses a polarizer,
a quarter-wave-plate and a Pockels cell as Q-switch are in-
serted. The output coupler mirror of the LPO is mounted on
a piezoelectric translator (PZT) to allow control of the cavity
length. The seed laser beam is mode matched to the oscil-
lator cavity by means of a focusable fibre collimator (FC).
The coupling occurs through a polarizer. A beam expand-
ing telescope (BE) positioned behind the LPO fits the beam
divergence and size for optimum power amplification. Two
Faraday isolators (FI) protect the LPO and the seed laser from
destabilizing feedback or actual damage from back-reflected
light.
2.4 Active frequency control
The active frequency control of the LPO is achieved
by injection seeding in combination with the QBUT minimi-
sation technique. This is currently the most common method
to ensure resonance of the slave laser cavity with the seed laser
frequency. The principal relies on the fact, that the build-up
time (time between actual pulse emission and Q-switch trig-
ger) of the Q-switched pulses is shortest when a longitudinal
mode of the oscillator corresponds to the seed laser frequency.
By translating the output coupler by means of the PZT, the
laser cavity is dithered by a small fraction of one free spec-
tral range (FSR) of the resonator about the locked position
FIGURE 2 Measured Q-switch build-up time (including an offset of about
550 ns) during scanning the PZT. The LPO is seeded with a power of about
40 mW. The PZT dithers between two subsequent laser shots (odd and even)
typically by about 5% of one FSR (532 nm). The measurement was per-
formed using the data acquisition unit for the heterodyne detection unit
(Sect. 3)
at 50 Hz rate. The difference in the build-up times of the two
translator positions is fed back to correct the average transla-
tor position.
The measured Q-switch build-up time of the laser pulse
vs. the cavity length change is shown in Fig. 2. If the os-
cillator cavity is in resonance with the seed laser frequency
the Q-switch build-up time is minimised. On the first (odd)
pulse the resonator is changed to a slightly shorter length
from its average length, and then changed to a slightly longer
length by the same amount on the subsequent (even) pulse.
Because of this, the output frequency of the oscillator will al-
ternate between two succeeding shots. The dither amplitude
for achieving long term single frequency operation is typic-
ally about
5% of the FSR [5]. According to the LPO’s FSR
of
400 MHz, this corresponds to an expected dither of the
output frequency of
20 MHz. An alternation of the frequency
between two subsequent shots by this amount is not toler-
able compared with the requirement of
< 1.3MHz(rms) at
1064 nm. In order to considerably increase the shot-to-shot
frequency stability the amplifier stages were operated at half
of the oscillator repetition rate, i.e., at
50 Hz, while the LPO
operates at
100 Hz.
2.5 Amplifier stages
A slab geometry has been selected as baseline for
the two amplifier stages. This geometry allows effective re-
moval of the strong thermal load deposited in the slab by the
pump laser light. The Nd:YAG slab dimensions have been de-
termined through optical and thermo-optical simulations to
ensure an effective extraction. When double passed, eleven
internal reflections have been determined as an optimum fill
factor at given beam diameter and slab dimensions. Each slab
is side-pumped by eight standard stacks (
1000 W/stack for
about
185 µs pulse duration at 50 Hz, synchronized on the os-
cillator). Between the two amplifier stages, three beam turning
mirrors (BTM) rotate the beam spatially by
90
◦
after each pass
440 Applied Physics B – Lasers and Optics
for compensation of thermally induced astigmatism. After
double pass amplification the beam is quasi-circular so that
spherical optics can be used. A Galilean telescope in front of
the harmonic generators reduces the spot size and adapts the
divergence of the pump beam in order to optimize the UV con-
version efficiency.
2.6 Harmonic generators
The nonlinear optical crystals for second and third
harmonic generation are LBO (
LiB
3
O
5
) crystals used in crit-
ical phase-matching orientation and temperature stabilized at
35
◦
C.TheSHGcrystalis18 mm long, while the THG crys-
tal is
16 mm long. The section is 7 ×7mm
2
for both crys-
tals. The high quality anti-reflection coatings at the entrance
and exit faces of the SHG crystal (double band coating at
1064 nm and 532 nm) and THG crystal (triple band coating
at
1064 nm, 532 nm and 355 nm) minimise the optical losses.
Four heaters allow heating the mount symmetrically. A ther-
mistor is attached to the mount as part of the temperature
control circuit.
3 Optical heterodyne method
The spectral linewidth and the frequency jitter of
the laser were measured by optical heterodyning the pulsed
laser and the seed laser. Figure 3 illustrates the basic prin-
ciple of the heterodyne detection. We consider two linearly
polarized plane waves from the seed and from the pulsed laser
which are mixed on a fast photodiode. The electric field from
the seed laser is frequency shifted by the carrier frequency
Ω
AOM
from an acousto-optic modulator (AOM). The electric
fields are expressed as
E
ref
(t) = A
ref
exp
[
iω
ref
t
]
+c.c., (1)
E
p
(t) = A
p
(t) exp
iω
p
t +iΦ(t)
+c.c., (2)
A
ref
: electric field amplitude of reference signal;
A
p
(t): electric field amplitude of pulsed signal;
FIGURE 3 Schematic of the optical heterodyne method for measuring the
frequency jitter and the linewidth on pulse-to-pulse basis
ω
p
: centre frequency of pulsed laser;
ω
ref
: frequency of seed laser shifted by Ω
AOM
;
Φ(t): phase perturbations of pulsed light relative to the refer-
ence field;
c.c.: complex conjugated.
The intensity of the detected signal (
I) is proportional to the
product of the sum of the signals and their complex conjugate.
The mixed signal contains the sum and the difference frequen-
cies of the two components. The sum cannot be detected by
the photodiode due to its limited bandwidth. But the differ-
ence is a low-frequency signal that can be determined with
high accuracy. The time averaged detector signal results in
I∼A
2
ref
+ A
2
p
(t) + A
p
(t)A
ref
cos
(ω
ref
−ω
p
)t +Φ(t)
. (3)
The first term in (3) is the direct current (DC) component
of the cw reference signal. The second term represents the
intensity envelope of the optical pulse, and the third term
is the optical heterodyne beat signal. The beat frequency
ω
ref
−ω
p
=Ω
AOM
−δω yields the deviation δω between the
frequencies of the seed laser and the pulsed laser. An analysis
of the beat waveform by a fast-Fourier transform (FFT) tech-
nique provides the spectral intensity profile. From the deriva-
tion of the phase perturbations
Φ(t) the frequency chirp of the
optical pulse can be derived [12, 13]. It should be emphasised
that the determination of the absolute frequency drift and jit-
ter of the pulsed laser requires the heterodyning against a more
frequency stable source such as a iodine stabilized Nd:YAG
laser.
The developed heterodyne detection unit completely uses
fibre optics which greatly simplifies measurements and solves
many optical alignment problems. For maintaining the polar-
isation state of the optical fields we used PM single mode
fibres for beam guiding. A fraction of the seed laser beam
(about
10 mW) is used as reference input for the heterodyne
measurement. The optical pulse, whose frequency spectrum
has to be analysed, is attenuated by the leakage of the two
45
◦
dichroic mirrors within the power laser head and an ad-
justable threaded radial screw located at the input port of
the beam combiner within the heterodyne detection unit. The
peak power of the pulsed optical signal was set to a level
comparable to the reference signal. The seed laser frequency
is shifted by
200 MHz using a fibre pigtailed AOM. A 1 :1
PM beam combiner combines both optical signals which are
mixedbya
3.5 GHz-bandwidth InGaAs PIN photodiode fol-
lowed by an amplifier. The output of the photodetector ampli-
fier is digitized by a high speed digitizer card at a sampling
rate of
2GSamples/s. The resulting digitized signal is trans-
ferred to a computer for processing. A FFT algorithm using
the Hanning window type provides the spectral intensity of
the seeded laser pulse. The centre beat frequency is given by
the centre of gravity of the beat spectrum considering the four-
fold spectral bandwidth around the maximum. The AOM used
as frequency shifter is driven by a
200 MHz signal generated
by a quartz oscillator having
5 ×10
−5
frequency stability. As
a result, the systematic error of a beat frequency measurement
is
< 10 kHz.
A typical heterodyne beat signal – recorded and digitized
by a
1 GHz analogue bandwidth oscilloscope at 16 GSam-
ples/s – is shown in Fig. 4. A FFT of this signal leads to the
SCHRÖDER et al. Frequency jitter and spectral width of an injection-seeded Q-switched Nd:YAG laser for a Doppler wind lidar 441
FIGURE 4 Heterodyne beat signal of a typical laser pulse with injection
seeding at 1064 nm recorded by a 1 GHz analogue bandwidth oscilloscope at
16 GSamples/s sampling rate
FIGURE 5 Fast-Fourier transform of a single heterodyne beat signal, show-
ing the positive portion of the power spectrum. The spectrum represents the
intensity term at 0 Hz and the interference term at 210 MHz
corresponding power spectrum shown in Fig. 5. The peak at
0Hzis associated with the pulse intensity term A
2
p
(t) of (3),
and the peak at
210 MHz is associated with the interference
term. The measured signal frequencies are below the Nyquist
frequency of
8 GHz ensuring alias-free sampling. The offset
of the centre beat frequency compared to the
200 MHz fre-
quency shift of the seed frequency depends on the PZT dither
amplitude.
4 Measured laser parameters
The master oscillator provides a pulse with nearly
diffraction limited beam profile and an output energy of
10 mJ
at 100 Hz pulse repetition rate. The measured IR pulse en-
ergy after double pass amplification exceeds
200 mJ (Fig. 6)
at
50 Hz repetition rate. Without injection seeding the pulse
shape shows strong modulations due to mode beating (Fig. 7).
With injection seeding the mode beating is suppressed and the
pulses show a smooth shape as expected for single mode op-
eration. The measured FWHM pulse duration at
1064 nm was
35 ns. After frequency tripling the pulse length was reduced
FIGURE 6 IR output energy of the injection seeded Nd:YAG laser vs. peak
current of the amplifier pump diodes
FIGURE 7 Temporal pulse shapes of the laser transmitter at 1064 nm in
seeded and unseeded operation. When the laser is seeded its Q-switch build-
up time was reduced by 60 ns. The pulse shape was measured with a 2 GHz
InGaAs photodiode and a 1 GHz analogue bandwidth oscilloscope. The IR
pulse duration (FWHM) is 35 ns at 200 mJ/pulse output energy
to 25 ns. The UV conversion efficiency was measured to be
30% which corresponds to E = 60 mJ/pulse output energy at
355 nm.
As has been shown in simulations of the A2D per-
formance [17] the maximum expected wind speed random
error is
< 0.9m/s (700 accumulated laser pulses) for E =
60 mJ/pulse for an airborne system at 12 km flight altitude.
This value is still below the envisaged wind speed measure-
ment accuracy of
1–2m/s (HLOS).
Table 1 lists the output performance values which have
been measured including frequency stability and linewidth
which are described in the following subsections.
4.1 Frequency stability
The analysis of the frequency jitter refers to
a recording interval of
14 s (700 shots) which is the averag-
ing time of the ALADIN lidar receiver for one line-of-sight
wind profile measurement. The Q-switch build-up time min-
