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Diode-pumped Double-pulsed Ho:Tm:LuLF Laser at 2.05 µm for CO2 Differential Absorption Lidar (DIAL)

Songsheng Chen*a, Jirong Yub, Mulugeta Petrosc, Yingxin Baia, Bo C. Trieub,

Upendra N. Singhb, Michael J. Kavayab a Science Application International Corporation,

One Enterprise Parkway, Suite 370, Hampton, VA 23666 USA, b NASA Langley Research Center, MS 468, Hampton, VA 23681, USA

c Science and Technology Corporation, 10 Basil Sawyer Drive, Hampton, VA 23666 USA

ABSTRACT It has been realized that eye-safe 2-µm all-solid-state lasers are important laser sources for an accurate measurement of the CO2 concentration in the atmosphere. Served as laser transmitters, they can be integrated into ground-based, airborne-base, and spaceborne-based CO2 Differential Absorption Lidars (DIALs) to accomplish the measurement. In addition, the lasers are also ideal laser pumping sources for a ZnGeP2 (ZGP) Optical Parametric Oscillator (OPO) or an Optical Parametric Amplifier (OPA) to achieve tunable laser output in 3~5 µm. In this spectrum region, the other important greenhouse gases, water vapor (H2O), carbon monoxide (CO), and methane (CH4) in the atmosphere can be measured. In this paper, we report a diode-pumped, double-pulsed, Q-switched, eye-safe Ho:Tm:LuLF laser at 2.05 µm developed for ground-based and airborne-based CO2 Differential Absorption Lidars (DIALs). The technology can be easily transferred to a space-borne CO2 DIAL in the future. The total output pulse energy of the laser is 220 mJ and 204 mJ per pair of pulses at 2 Hz and at 10 Hz respectively. The related optical energy conversion efficiency is 6.7% and 5.9% respectively. Keywords: Diode pumped laser, 2-µm lasers, Differential Absorption Lidar (DIAL)

1. INTRODUCTION It has been recognized that the CO2 concentration in the atmosphere have been increasing over many years and high concentration level of CO2 has significant effect on the Earth’s climate system [1-3]. Accurate measurement of the CO2 concentration from the ground through troposphere in regional and/or global scale can contribute significantly to the understanding and monitoring of the carbon cycle processes in the numerical atmospheric weather and climate predictions. Generally, the CO2 measurement of 1-3 ppm (0.3-1%) over 1-2 km vertical range from the ground through troposphere is required to understand many carbon cycle processes. Two kinds optical techniques, Differential Optical Absorption Spectroscopy (DOAS) and Differential Absorption Lidar (DIAL) are suitable for the CO2 measurement in the atmosphere. Although a long-path Differential Optical Absorption Spectroscopy (DOAS) has been used in many fields and has shown a sufficient accuracy, further below 1 ppm, for the CO2 measurement, only Differential Absorption Lidar (DIAL) has the potential to provide a global three-dimensional distribution of CO2 from the ground through troposphere with high temporary resolution. Preliminary spectroscopic studies have shown the most appropriate spectral ranges are located around 1.6-µm and 2.0-µm for the CO2 measurement with a Differential Absorption Lidar (DIAL). Recently, more scientists have shown great interests in 2.0-µm CO2 Differential Absorption Lidars due to the development of laser transmitter based on all solid-state 2.0-µm lasers. As proved in the O3, H2O and other Differential Absorption Lidars, there are three key technologies, laser transmitter, detector system, and data-acquisition system, but laser transmitter is always a critical technique for the Differential Absorption Lidars. It includes all the requirements for laser pulse energy, laser pulse duration, laser wavelengths for both online and off line, laser linewidth, frequency stability, spectral purity, laser beam profile and quality. These parameters of the laser transmitter determine the maximum measurement range and accuracy of the CO2 concentration in the

*[email protected]; phone: 1 757 864-1688; fax: 1 757 864-8828

Invited Paper

Laser Radar Techniques for Atmospheric Sensing, edited by Upendra N. Singh,Proceedings of SPIE Vol. 5575 (SPIE, Bellingham, WA, 2004)

0277-786X/04/$15 · doi: 10.1117/12.568418

44

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 12/10/2013 Terms of Use: http://spiedl.org/terms

atmosphere. Laser pulse energy has to be high enough to make the measurement over whole range from the ground through troposphere. Laser duration has to be short enough to meet the required range resolution. Laser linewidth, frequency stability, and spectral purity at both online and offline wavelengths have to be sufficiently good to get the required accuracy of the measurement. Laser beam profile has to be high quality for the effective detection. Laser transmitter has to be high efficient for the airborne and spaceborne lidar system. In previous researches, we have reported 2-µm Ho:Tm:YLF and Ho:Tm:LuLF lasers and amplifiers for coherent Doppler wind lidars and coherent CO2 Differential Absorption Lidars (DIALs) [4-8]. In this paper, we present a continued development of a diode-pumped Q-switched Ho:Tm:LuLF laser, it will be integrated in a ground-based or an aircraft-based CO2 Differential Absorption Lidar (DIAL) system as a laser transmitter. The laser operated at about 2.05 µm and can be tuned over several vibrational-rotational absorption lines of CO2 in the spectral range. The overlap between CO2 absorption line and other lines of different gases, such as H2O, can be minimized by choosing suitable absorption line of CO2 gas. In the DIAL system, two close laser output pulses at two different wavelengths, one at on-line wavelength and the other at off-line wavelength, are needed. These two pulses can be obtained from two individual Q-switched lasers or from one simplified Q-switched laser, but usually this simplified laser has to be double-pulse or pair pulse pumped. The Q-switched laser pulse is required for high peak power of the laser output pulses to get efficient back scattering from the aerosols and molecules in the atmosphere. The Ho:Tm:YLF or Ho:Tm:LuLF laser has its unique property to work in double-pulse or pair-pulse output with only one pumping pulse. During one pumping cycle, the laser is Q-switched twice to get two Q-switched output pulses. The time interval between two pulses can reach several hundred microseconds and is enough to switch wavelengths between online and offline by a tunable injection-seeding laser or two injection-seeding lasers at fixed wavelengths. The total pulse energy of two pulses and the energy ratio of the two pulses depend on the Q-switch trigger starting time from the pumping diode trigger time and the delay time interval between two pulses. Practically, they can be optimized for different absorption lines according to the returned signals when the laser is integrated into a CO2 Differential Absorption Lidar (DIAL) system. The laser we report is optimized for total output pulse energy in terms of trigger starting time and the delay time interval for double-pulse operation. At repetition frequency of 2 Hz, the maximum total output pulse energy is 220 mJ per pair of pulses and optical energy conversion efficiency is 6.6 %. At repetition frequency of 10 Hz, the maximum total output pulse energy is 204 mJ per pair of pulses and optical energy conversion efficiency is 6.0 % under different parameters of the resonator. The energy ratio between first pulse and second pulse is about 1.6 at the maximum total output pulse energy.

2. LASER DESIGN

It is difficult for a ground based CO2 Differential Absorption Lidar (DIAL) to measure CO2 concentration accurately from the ground through tropopause unless the output pulse energy reaches more than 150 mJ. For an airborne CO2 DIAL system, comparable pulse energy is expected to make the accurate measurement possible through whole troposphere. Meanwhile, the system efficiency is important, especially for a power limited airborne CO2 DIAL system. After comparing a single laser oscillator design with a master-oscillator-power-amplifier design and a diode-side-pumping configuration with a diode-end-pumping configuration, we employed the design of a diode-side-pumped single laser oscillator for the purposes of high pulse energy and high efficiency. Six conductively-cooled GaAlAs high-brightness stacked diode bars at the wavelengths around 792 nm were used as the side-pumping sources. They were mounted on particularly-designed aluminum blocks, two of the stacks side-by-side arranged along the axis of the Ho:Tm:LuLF laser rod as a pair and three pairs separately set around the circumference of the rod with a separation of 120°. Based on this configuration, the polarization of the pump beam is along the axis of the rod or an a-axis of the LuLF crystal. Although the c-axis of the LuLF crystal has much higher absorption coefficient at a pumping wavelength of around 780 nm, this alignment has much less sensitivity to pumping wavelength, which varies as the diode temperature. Each stack has six bars and can provide about 600-mJ pumping pulse energy at pumping duration of 1 ms, so total pumping pulse energy can be about 3.6 J. The active section of the Ho:Tm:LuLF laser crystal rod was 4 mm in diameter and about 20 mm in length and doped with 0.5% Holmium and 6% Thulium. Each end of the Ho:Tm:LuLF laser crystal rod was diffusion-bonded with an undoped LuLF laser crystal rod, about 15 mm in length, to support laser rod easily. There is a fused silica flowing tube, about 5 mm in internal diameter, mounted outside the laser rod for the liquid flowing cooling purpose. All laser diode bars were cooled with flowing water at about 15 °C and laser rod was cooled at 13 °C. The schematic diagram of the designed laser transmitter is shown in Fig. 1. The laser resonant cavity was designed in a ring cavity configuration to achieve a single longitudinal mode or single frequency by the injection seeding through the output coupler. The length of resonant cavity was designed at about 4 meters to keep the laser pulse width to be more

Proc. of SPIE Vol. 5575 45


than 100 ns for possible coherent detection and reduce the damage possibility of the laser crystal rod. The ring cavity consists of two curved high reflectors and two flat reflectors for a stable operation and was symmetrically configured. Two curved high reflectors made two Gaussian beam waists inside the resonator possible. When the laser crystal rod and a Brewster-angle-cut fused-silica acousto-optical Q-switch were located at the two Gaussian beam waists respectively, more active mode volume inside the resonator was utilized and the Q-switch was more efficient. The output coupler has the reflectivity of 72%. Two frequency-stabilized seeder lasers, one locked at online frequency and one locked at offline frequency, combined with a mechanical or an optical switch could be coupled into the cavity through the output coupler to force the laser in one direction operation. The active searching of resonant frequency with a detector and a PZT mounted on one of curved high reflector and the ramp-and-fire technique could be used to match the Q-switched laser pulse frequency with the continuous-wave (cw) seeder laser frequency. One laser beam expender could be used to improve the divergence of the output laser beam. These techniques have been proved in our previous researches and reported in several literatures. The schematic diagram of the laser transmitter is shown in Fig. 1. Here we are focusing our experimental results related to laser output pulse energy and optical pulse energy conversion efficiency. For these purposes, one flat high reflector was used to make the laser operate in one direction. There is no much difference (no more than 2 %) in terms of output pulse energy and optical efficiency between injection-seeded and unseeded laser operation according to our previous experiments. Thus, the experimental results we report here come from a ring-cavity laser with several longitudinal modes. It has been achieved that the pulse energy increased more than 16 % and the efficiency increased more than 22 %.

Fig. 1. Schematic diagram of the laser transmitter

3. EXPERIMENTAL RESULTS

The designed laser can operate at normal mode and double Q-switched mode. For normal mode operation, the acousto-optical Q-switch was open, no trigger pulses applied to the Q-switch, during whole cycle of the diode-pumping pulses. For double pulse operation, the Q-switch was first closed, one trigger pulse applied to the Q-switch to ensure no lasing during the whole cycle of the diode-pumping pulses. It was determined in our experiments that the trigger pulse started synchronously with the diode-pumping pulses and the duration of this pulse was 4 ms. During the duration of this blocking pulse, the Q-switch opened twice and each had a duration of 4 µs to get double pulse laser output. The starting point to open the Q-switch and the time interval between the two opening points determine the total output pulse energy and the energy ratio of these two pulses. We have determined that the first opening started at the end of the diode-pumping pulse or 1 ms after the starting point of the diode trigger pulse and the time interval between the two openings was 150 µs to get the maximum total pulse energy.

O.C.

Flat HR Q-Switch

Curved HR

Flat HR

Curved HR

PZT Detector

Flat HR

Frequency-stabilized Seeder lasers

Switch

Controller

Isolators

Collimator

46 Proc. of SPIE Vol. 5575


The laser can operate at a repetition rate frequency of from 1 Hz through 10 Hz. The total output pulse energy of two pulses as a function of pump pulse energy at a repetition rate frequency of 2 Hz and 10 Hz for both normal-mode operation and double-pulse-mode operation was measured respectively and the results are shown in Fig. 2.

0

50

100

150

200

250

300

2.2 2.4 2.6 2.8 3 3.2 3.4 3.6

Normal mode

Double-pulse mode

Out

put p

ulse

ene

rgy

(mJ)

Pump pulse energy (J)

0

50

100

150

200

250

300

2.2 2.4 2.6 2.8 3 3.2 3.4 3.6

Normal mode

Double-pulse mode

Out

put p

ulse

ene

rgy

(mJ)


Fig. 2. Output pulse energy as a function of the pump pulse energy of the laser

Left for repetition rate frequency of 2 Hz; Right for repetition rate frequency of 10 Hz

The experimental results are summarized in Table 1 and Table 2 for normal-mode operation and double-pulse-mode operation respectively.

Table 1 Laser output parameters for normal-mode operation

Repetition Rate Frequency (Hz)

Maximum Pumping Pulse Energy (J)

Maximum Total Pulse Energy (mJ)

Thresholds (J) Slope

Efficiency (%) 2 3.3 280 2.2 26

10 3.5 290 2.4 27

Table 2 Laser output parameters for double-pulse-mode operation

Repetition Rate Frequency (Hz)

Maximum Pumping Pulse Energy (J)

Maximum Total Pulse Energy (mJ)

Thresholds (J) Slope

Efficiency (%) 2 3.3 220 2.2 20 10 3.5 204 2.4 19

The calculated optical pulse energy conversion efficiencies for different operation modes and different repetition are shown in Fig. 3. The efficiencies increase as pumping pulse energy or output pulse energy increases. The maximum efficiencies are 8.7% for normal-mode operation and 6.7% for double-pulse-mode operation at the pumping pulse energy of 3.3 J and a repetition rate frequency of 2 Hz. For a repetition rate frequency of 10 Hz, the maximum efficiencies are 8.5% for normal-mode operation and 5.9% for double-pulse-mode operation at the pumping pulse energy of 3.5 J.



1

2

3

4

5

6

7

8

9

2.2 2.4 2.6 2.8 3 3.2 3.4 3.6

Normal modeDouble-pulse mode

Opt

ical

ene

rgy

conv

ersi

on e

ffic

ienc

y (%

)


1

2

3

4

5

6

7

8

9

2.2 2.4 2.6 2.8 3 3.2 3.4 3.6

Normal modeDouble-pulse mode

Opt

ical

ene

rgy

conv

ersi

on e

ffic

ienc

y (%

)


Fig. 3. Optical pulse energy conversion efficiency as a function of the pump pulse energy

Left for repetition rate frequency of 2 Hz; Right for repetition rate frequency of 10 Hz

The pulse energy and the pulse widths of the two output laser pulses vary with the pumping pulse energy, Q-switch opening time points, and the time interval between two Q-switch opening points. Typical output laser pulses are shown in Fig. 4. At a repetition rate frequency of 2 Hz and at the pumping pulse energy of 3.35 J, the output energy of the first pulse is 138 mJ and the pulse width, Full Width Half Maximum (FWHM), is about 130 ns. The output energy of the second is 82 mJ and the width is about 180 ns.

0

0.005

0.01

0.015

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Rel

ativ

e am

plit

ude

Time (µs)

Pulse width (FWHM): 130ns

155.2 155.4 155.6 155.8 156 156.2 156.4 156.60

0.005

0.01

0.015

Time (µs)

Pulse width (FWHM): 180ns

Fig. 4. The output pulse waveforms at 2 Hz and maximum pumping pulse energy for a double pulse operation

Left for the first pulse; Right for the second pulse

4. CONCLUSIONS A diode-pumped double-pulsed Ho:Tm:LuLF laser at 2.0 µm has been developed. Integrated with previously developed injection-seeding technology, this laser can be integrated into a ground-based or an airborne based CO2 DIAL for an accurate CO2 concentration measurement. It is expected that the ground based CO2 DIAL can measure CO2

48 Proc. of SPIE Vol. 5575


concentration accurately from ground through tropopause and the airborne based CO2 DIAL can measure CO2 concentration accurately over whole troposphere. The laser is also an ideal laser pumping source of a ZnGeP2 (ZGP) Optical Parametric Oscillator (OPO) or Optical Parametric Amplifier (OPA) to achieve tunable laser output in 3~5 µm. The frequency-tunable laser pulses in this spectrum region are very important for DIALS to make the measurements of CO, H2O, and CH4 in the atmosphere.

ACKNOWLEDGEMENTS This work was supported by NASA Head Quarters Laser Risk Reduction Program. The authors gratefully acknowledge the technical support provided by E. A. Modlin and mechanical design provided by Wayne Welch in Welch Mechanical Designs, LLC.

REFERENCES

1. P. H. Flamant, C. Loth, F. M. Breon, D. Bruneau, A. Dabas, P. Desmet, T. Pain, P. Prunet, “FACTS: Future

Atmospheric Carbon Dioxide Testing from Space”, Proceedings of 22nd International Laser Radar Conference (ILRC2004), (Matera, Italy), pp. 969-972, 2004

2. G. J. Koch, M. Petros, J. Yu, J. Y. Beyon, B. W. Barnes, F. Amzajerdian, R. E. Davis, M. J. Kavaya, S. Ismail, and U. N. Singh, “2-µm Coherent DIAL Measurements of Atmospheric CO2”, Proceedings of 12th Coherent Laser Radar Conference (Bar Harbor, ME, USA), pp178-181, 2003

3. M. Uchiumi, N. J. Vasa, M. Fujiwara, S. Yokoyama, M. Maeda, and O. Uchino, “Development of DIAL for CO2 and CH4 in the atmosphere”, Proceedings of SPIE, Lidar Remote Sensing for Industry and Environment Monitoring III, (Hangzhou, China), pp. 141-149, 2002

4. J. Yu, U. N. Singh, N. P. Barnes and M. Petros, “125-mJ diode-pumped injection-seeded Ho:Tm:YLF laser” Opt. Lett. 23, 780-782, 1998

5. J. Yu, A. Braud, and M. Petros, “600mJ, double-pulse 2-µm laser”, Opt. Lett. 28, 540-542, 2003 6. S. Chen, J. Yu, U. N. Singh, M. Petros, and Y. Bai, “A diode-pumped Tm:Ho:LuLF master-oscillator-power-

amplifier (MOPA) at 2.05 µm”, Technical Digest of CLEO/QELS, (Baltimore, Maryland, USA), CThL3, 2003 7. M. Petros, J. Yu, S. Chen, U. N. Singh, B. M. Walsh, Y. Bai, and N. P. Barnes, “Diode pumped 135 mJ Tm: Ho:

LuLF oscillator”, Advanced Solid State Photonics, OSA Trends in Optics and Photonics (TOPS), Vol. 83, 315-319, 2003 8. S. Chen, J. Yu, M. Petros, U. N. Singh, and Y. Bai, “A Double-pass Diode-pumped Tm:Ho:YLF Laser Amplifier

at 2.05 ”, Advanced Solid State Photonics, OSA Trends in Optics and Photonics (TOPS),Vol. 83, pp309-314, 2003



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