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205-µm all-ber laser source designed for CO 2 and wind
coherent lidar measurement
Julien Lahyani, Julien Le Gouët, Fabien Gibert, Nicolas Cézard
To cite this version:
Julien Lahyani, Julien Le Gouët, Fabien Gibert, Nicolas Cézard. 205-µm all-ber laser source designed
for CO 2 and wind coherent lidar measurement. Applied optics, Optical Society of America, 2021, 60
(15), pp.C12-C19. �10.1364/AO.416821�. �hal-03202735�
1
2.05-µm all-fiber laser source designed for
CO
2
and wind coherent lidar measurement
JULIEN LAHYANI
1
, JULIEN LE GOUËT
1,*
, FABIEN GIBERT
2
AND NICOLAS
CEZARD
3
1
DOTA, ONERA, Université Paris-Saclay, F-91123 Palaiseau, France
2
Laboratoire de Météorologie Dynamique (LMD), Centre National de Recherche Scientifique (CNRS),
Ecole Polytechnique, FR-91128 Palaiseau cedex, France
3
ONERA/DOTA, Université de Toulouse, F-31055 Toulouse, France
*julien.le_gouet@onera.fr
Abstract: This work reports on an all-fiber pulsed laser source for simultaneous remote
sensing of CO
2
concentration and wind velocity in the 2.05 µm region. The source is based on
a polarization maintaining Master Oscillator Power Amplifier (MOPA) architecture. Two
narrow-linewidth master oscillators for ON-line/OFF-line CO
2
DIAL operation alternately
seed a four-stage amplifier chain at a fast switching rate up to 20 kHz. The MOPA
architecture delivers laser pulses of 120 μJ energy, 200 ns duration (600 W peak power) at
20 kHz pulse repetition rate (2.4 W average power). The output linewidth is lower than 5
MHz, close to the pulse Fourier-Transform limit, and the beam quality factor is M² = 1.12.
The source also provides a pre-amplified 20 mW local oscillator with a relative intensity
noise of -160 dB/Hz that ensures optimal performance for future coherent detection.
1. Introduction
Carbon dioxide (CO
2
) is widely acknowledged as the most important anthropogenic
greenhouse gas in the atmosphere. Yet our understanding of its impact on future climate
evolution still suffers some uncertainties [1]. To improve our knowledge of the CO
2
life
cycle, and open ways to control anthropogenic emissions, it is necessary to quantify CO
2
fluxes around sources and sinks, at a local scale with ground-based instruments, and if
possible, at the global scale with space-borne instruments.
Lidar sensors are especially attractive for such tasks, and several lidar systems based on high
energy solid-state lasers already demonstrated good capabilities for CO
2
monitoring at
2 µm [2–5] and 1.5 µm [6–9], using DIfferential Absorption Lidar (DIAL) or Integrated Path
Differential Absorption (IPDA). However, solid-state architectures generally involve large
numbers of free-space optics that can raise substantial thermal and mechanical alignment
issues when designing the observation system. All-fiber laser architectures alleviate those
problems. In an assessment for a space-borne lidar (ASCENDS program), NASA Goddard
Space Flight Center [8], based on a Fibertek heritage [10,11], and the Information
Technology R&D Center of Mitsubishi Electric [12] both demonstrated for instance the
interest of an all-fiber architecture in the 1.57 µm region. As a benchmark regarding pulse
energy levels and repetition rates performed in previous works, solid-state cavities at 2.05 µm
can reach tens of mJ at moderate repetition rate (usually lower than 1 kHz) [4], while all-
fiber systems at 1.57 µm and 2.05 µm (including our system) reach hundreds of µJ at higher
repetition rate (usually tens of kHz) [10]. Both technologies thus offer different trade-offs.
For space-borne monitoring with the IPDA technique, the 2.05 µm band is attractive, for it
allows relaxing the requirement on the random error compared to1.57 µm. Indeed, the
pressure dependence of the CO
2
R30 absorption line at 2051 nm offers a more favorable
2
Weighting Function (WF) in the low troposphere [13], where sources and sinks are
localized [14]. Therefore, the development of a 2.05 µm all-fiber pulsed laser source, suitable
for CO
2
monitoring, appears highly desirable in the perspective of future space-borne lidar
system.
Fiber lidar systems also have the advantage to facilitate the use of coherent detection to
perform range-resolved measurements of the wind speed. In the perspective of ground-based
lidar systems, a fiber pulsed laser source at 2.05 µm could therefore offer a robust solution to
measure simultaneously range-resolved profiles of the CO
2
Volume Mixing Ratio (VMR) and
the wind speed, which is ideal for autonomous CO
2
flux rate measurements. Such dual
function DIAL-Doppler lidars have already been reported for CO
2
[2,3] using solid-state
lasers.
These systems could also find industrial interests. For example, the Physics Department of
Montana state University [15] reported surface monitoring of CO
2
sequestration sites with a
1.57 µm fiber-based DIAL lidar, but they used direct detection and could not perform wind
measurement simultaneously. Recently, our research group reported a fiber-based DIAL-
Doppler lidar at 1.64 µm for industrial methane leaks monitoring, and demonstrated
simultaneous CH
4
/wind range-resolved profiles [16].
This study follows a bottom-up approach, with multiple potential applications that will not be
discussed in details here. Of course, space requirements are very demanding and could not be
fulfilled using the presented system. Ground-based industrial applications would generally
require relaxed requirements, but with large variations depending of the scenario. Whatever
the final application, building a powerful all-fiber laser source is a step forward, simpler and
easier-to-deploy, for future lidar systems.
In this paper we report on the design and performance of a high peak-power, narrow-
linewidth, all-fiber pulsed laser source at 2.05 µm, designed to be suitable for standalone km-
range ground-based CO
2
/wind measurement using coherent detection. To the best of our
knowledge, this is the first all-fiber laser system at 2.05 µm that allows for such possibilities.
In the first part, we describe and justify the objectives we had for the 2 µm fiber laser source,
and remind relevant results of the literature in that field. In the second part, we describe the
laser design. The last part is dedicated to experimental characteristics obtained with the laser
source, in the perspective of upcoming DIAL-Doppler measurements.
2. Laser objectives
The CO
2
absorption line centered on 2050.97 nm (R30) has been identified in previous
studies as one of the most promising for a space-borne lidar instrument [17]. According to the
HITRAN database, the CO
2
R30 transition is about 4.3 GHz wide (at Full Width Half
Maximum - FWHM) at 1 bar/293 °K in standard atmosphere, and is separated by 40 GHz
from the nearest CO
2
absorption line. Therefore, the laser must offer narrow-linewidth in
comparison to the absorption line width, and a tuning range of 20 GHz is desirable to allow
full coverage of the R30 absorption line sideband. The ON and OFF wavelengths must also
be close enough (typically < 1nm) to guarantee similar backscattering amplitude by the
atmospheric aerosols or by hard-target surfaces.
To maximize the measurement accuracy and the lidar range, high laser pulse energy is
necessary. Typically hundreds of µJ are required for range-resolved DIAL measurement with
kilometer range in the boundary layer [2]. The pulse length should be between 100 ns and
1 µs, for the range-resolution to be between 15 and 150 m. The Pulse Repetition Rate (PRF)
should be below 50 kHz to raise the ambiguity range up to 3 km. Fast wavelength switching
is also required to ensure high atmospheric correlation between ON and OFF signals. For the
3
atmosphere to be considered as ‘frozen’, a switching rate of 1 kHz or more is typically
required for ground-based systems [18]. In Table 1, we summarize the main laser features.
Table 1: Main laser features
Feature
Results in this study
Laser wavelength
2.05 µm
Spectral tunability
70 GHz (1 nm)
Pulse energy
120 µJ
Peak – Average power
600 W – 2.4 W
Pulse duration
200 ns
PRF
20 kHz
ON-OFF switch rate
Up to 20 kHz
Heterodyne measurement requires a Local Oscillator (LO) with low RIN (Relative Intensity
Noise) in the analysis bandwidth. The fiber laser should exhibit a nearly Fourier-transform
limited linewidth, a beam quality close to the diffraction limit (M
2
=1) and a linear
polarization. All these features play an important role on the Carrier-to-Noise Ratio (CNR)
(see section 4). To minimize the bias made on the CO
2
VMR measurement, the spectral drift
within the measurement time should be limited (or at least be monitored) to keep the bias as
low as possible. High cross-talk isolation between ON-line and OFF-line beams and high
Side-Mode Suppression Ratio (SMSR) are also required. In Table 2, we show the measured
characteristics for all these parameters, with the associated error budgets in terms of CNR loss
(random error) and CO
2
VMR bias budget (systematic error). These numbers are discussed in
section 4.
Table 2: Laser-induced CNR loss/VMR bias budget
Features
Results in this study
CNR loss budget
RIN (around AOM
frequency shift)
-160 dB/Hz
< 1 dB
Spectral Linewidth
<5 MHz
< 1 dB
Signal -LO beat frequency
stability @ 10 ms
100 kHz
Negligible
Beam quality M
2
1,12
0.5 dB
Polarization, Polarization
Extinction Ratio (PER)
Linear, PER > 16 dB
< 0.11 dB
VMR bias budget
Frequency drift over 10 min
< 50 MHz (peak-to-peak)
<0.2%
Cross-Talk
-23 dB
<0.1% up to 3 km
SMSR
> 45dB
Negligible
Usually, power scaling of fiber-laser is limited by the extractible power or Stimulated
Brillouin Scattering (SBS). As illustrated in [19], Tm-doped fiber amplifiers can deliver very
high powers (up to 1 kW), and SBS can be partly circumvented by numerous methods [20].
In [21], a monolithic all-fiber amplifier delivering a high peak power of 10 kW for 100 ns
pulses at 2.05 µm has also been demonstrated, but using a non-single-frequency seeder. We
previously developed a 2.05 µm, single-frequency, 110 µJ, 110 ns, 20 kHz fiber laser source
(2.2 W average power) [22]. However, this source was developed for optical parametric
oscillator pumping, was mono-wavelength, and did not provide any LO output. Moreover,
4
this previous source unfortunately exhibited high RIN and also suffered power instabilities.
The following sections explain how the source has been re-designed, upgraded, and fully
characterized to comply with our objectives for combined lidar measurement of CO
2
and
wind.
3. Experimental setup
The amplification chain is shown on Fig 1 and is detailed in following subsections. The
source architecture is based on a Polarization Maintaining (PM) Master Oscillator Power
Fiber Amplifier (MOPFA) made of four Thulium Doped Fiber Amplifiers (TDFA) pumped at
793 nm. The MOPFA is seeded alternatively by two narrow-linewidth Distributed Feed-Back
Laser Diodes (DFB-LD) using an Optical Switch (OS). The pre-amplifier (TDFA1, detailed
in section 3.1) delivers a continuous signal. The optical power at its output is split in two
parts, one delivering the LO power and one seeding the second amplifier. An AOM (Acousto-
Optic Modulator) shapes the signal into pulses and adds an optical frequency offset for
heterodyne detection. The TDFA2 and the pulse shaping system are described in section 3.2.
At its output, a Mach-Zehnder Electro Optic Modulator (EOM) is used as a time-gated
attenuator to filter amplified AOM parasitic spikes. Since the last two amplifiers, (including
ASE filtering, Stimulated Brillouin Scattering (SBS) monitoring, and fiber strain gradient for
SBS gain reduction) are identical to those reported in [22], only general features of TDFA3
and TDFA4 are reminded in section 3.3. Finally, the output of TDFA4 is spliced to a simple-
clad LMA fiber that is crimped into a FC-APC connector, allowing for an easy coupling to
the lidar emission optics.
Fig 1: Schematic of the MOPFA. Energy values are given for 200 ns pulses at 20 kHz
repetition rate. DFB-LD: Distributed Feedback Laser Diode, OS: Optical Switch, LO: Local
Oscillator, TDFA: Thulium Doped Fiber Amplifier, AOM: Acousto-Optic Modulator, EOM:
Electro-Optic Modulator, ASE: Amplified Spontaneous Emission, HR-FBG: High Reflectivity
Fiber Bragg Grating, SBS: Stimulated Brillouin Scattering, LMA: Large Mode Area.
3.1 Continuous wave pre-amplification for LO derivation
In many coherent fiber lidar designs, a fraction of the CW (Continuous Wave) laser seed is
directly derived to be used as LO, and the rest is shaped before amplification [23]. In our
case, a LO power of 20 mW (able to provide 1 mW power at the detector after a 95:5 coupler)
is necessary for the heterodyne measurement. However, the power delivered by the DFB-LD
is limited to 10 mW, and the optical switch introduces losses. Moreover, DIAL measurement
may also require a wavelength calibration or monitoring channel that would require another
fraction of the DFB-LD CW power. Therefore, it was necessary to pre-amplify the signal,
without degrading the intensity noise. The seeders, OS and CW pre-amplifier are illustrated in
Fig 2.
