Background

This document covers work carried out under a Phase II Small Business Innovative Research (SBIR) Program entitled "Tandem OPO for Mid-IR DIAL," Air Force Contract F29601-97-C0120. The effort was in response to Topic AF95-100, "Optical Parametric Oscillator Based Lidars for Remote Sensing of Chemicals," whose stated objective was to "develop OPO based lidar technology applicable to environmental compliance monitoring of chemical clouds/plumes and chemical agent defense." In response to this topic we developed a concept for a "tandem" optical parametric oscillator (OPO) based on the use of a Nd-doped pumped laser, specifically Nd:YLF, followed by two OPOs in series, where the idler beam of the first OPO acts as a pump for the second OPO. Figure 1 is a schematic diagram of the system concept. The seed sources, based on narrow-linewidth lasers, are used when necessary to obtain the narrow linewidths required to sense some gases.

Figure 1. Block diagram of tandem-OPO IR source.

Papers Presented

There have been several papers presented on aspects of this work, and the reader is invited to go to the following links to read summaries of the papers.

Tandem OPO Source Generating 1-10-um Wavelengths
Multi-wavelength, 1.5-10-um tunable, tandem OPO
Injection-Seeded, Pump-Enhanced, Tunable KTA OPO

Phase I

In the Phase I program we showed feasibility of our tandem OPO concept by successfully operating the Q-switched laser and OPO building blocks, specifically:

  1. A Q-switched, Nd:YLF, 1053-nm laser suitable for use as the first-stage OPO pump source. The system produced 200-mJ, 20-ns pulses at a 20-Hz rate and was based on a lamp-pumped, low-divergence oscillator followed by a single-stage amplifier.
  2. An efficient KTA OPO based on an x-cut crystal, operated at a fixed angle with non-critical phase-matching (NCPM) to generate signal and idler wavelengths of 1520 and 3450 nm. The conversion efficiency from pump to combined signal and idler energy was 42% at the full pump energy.
  3. A CdSe NCPM OPO pumped by the idler output of the KTA OPO. At the maximum pump energy of 22.5 mJ the CdSe OPO generated 4.5 mJ of signal energy at a measured signal wavelength of 5.12 mm; the resultant idler was at 10.58 mm. Our calculated signal wavelength was 5.25 mm, in good agreement with the data.

We proposed in the Phase II effort to further develop the tandem OPO system. The intent of the Phase II program was to establish the capabilities and limitations of the technology through laboratory experiments and demonstrations. The results of the Phase II effort are summarized below. Personnel involved in the effort included Principal Investigator Dr. Yelena Isyanova, Dr. Peter Moulton, Mr. John Flint, and Drs. Alex Dergachev and Dicky Lee.

Summary of Phase II results

In the performance of the effort we designed, assembled and characterized a seed source, a single-frequency cw 1053-nm, diode-pumped Nd:YLF laser, and an injection-seeded, Q-switched, lamp-pumped Nd:YLF master-oscillator integrated with an amplifier (MOPA system) that generated 200-mJ, 25-ns pulses at a 20-50 Hz rate, with a diffraction-limited, single-frequency beam. After several design iterations, for the master oscillator we used a ring-laser with image rotation to provide 75-mJ-energy pulses at pulse rates in the 5-50-Hz range, for a lamp pump energy of 28 J. We measured the oscillator-beam M2 to be in the range 1.2-1.3.

Figure 2. Q-switched Nd:YLF ring laser with image rotation and injection locking.

In order to lock the master oscillator to the frequency of the seed laser, we used the pulse buildup time reduction technique. Appropriate electronics measured the delay time of the oscillator pulse with respect to the Q-switch opening and controlled a PZT-mounted mirror in the oscillator to minimize the delay. Figure 3 shows the output pulse of the oscillator for three different cases, unseeded, with seeding but without locking and seeding with locking.

Figure 3. Unseeded, seeded with and without locking operation of the ring oscillator.

The amplifier utilized a laser head and power supply similar to that of the oscillator. We used a 5-mm diameter, 100-mm long a-cut Nd:YLF rod with faces wedged at 30. The maximum input energy from the power supply was equal to 38 J, and at this level we obtained 200 mJ of energy from the amplifier.

We used the MOPA system to pump a variety of angle-tuned KTA OPOs, which generated signal wavelengths from 1480 nm to 2060 nm, and idler wavelengths from 2150 nm to 3620 nm. The maximum slope efficiency and combined signal and idler energies were 50% and 60 mJ, respectively. Figure 4 shows the angle-tuning data obtained with the following crystal orientations: 1)q =600, j =0, (x-cut) 2) q =900, j =0 (x-cut, NCPM) and 3) q =900, j =900 (y-cut, NCPM)

Figure 4. Angle-tuning curve of KTA OPO pumped by Nd:YLF laser.

With the x-cut (q =600) KTA crystal, we were able to tune the OPO to cover the spectral ranges from 2066 nm to 1573 nm for the signal, and 2167 nm to 3184 nm for the idler. While angle tuning the KTA crystal from 500 to 700 , we took data on output pulse energies at 200 mJ of pump pulse energy. In Fig. 5 we show experimental data on signal and idler output pulse energies versus tuning angles when the pump laser, as well as the OPO, were not seeded. Also shown are corresponding signal and idler wavelengths.

Figure. 5. Unseeded KTA OPO output pulse energies and corresponding wavelengths versus tuning angle.

We demonstrated single-frequency operation of an angle-tuned KTA OPO using the seeded MOPA source, combined with a tunable diode-laser seed source. We found that seeding combined with pump resonance increases the efficiency by a factor of 2. Figure 6 shows the system layout for the seeded KTA OPO system. Once seeding occurred, the spectral width of the signal decreased from a few lines to a single line, as observed by a Fabry-Perot interferometer. Seeding was stable for several minutes with no adjustments, down to 0.4 mW of injected power from the tunable diode laser. We were able to tune the system such that the idler wave was in resonance with a methane absorption line at 3250.037 nm

Figure. 6. Single-frequency KTA OPO system layout.

A further increase of the KTA OPO idler efficiency, by ~ 25%, was achieved through difference frequency generation of the signal and idler waves mixed in a second KTA crystal placed into the KTA OPO resonator, as shown in Fig. 7. We also demonstrated that the KTA OPO/DFG system extends the tuning range of the KTA OPO beyond 4 mm.

Figure 7. Layout of the OPO - DFG experiment.

We then used the idler-beam output of the KTA OPO to pump a NCPM CdSe OPO, which produced signal wavelengths from 3.66 m m to 5.12 m m and idler wavelengths 7.99 m m to 10.57 m m. The CdSe OPO operated with a slope efficiency of 27% and a threshold of 1 mJ. Figure 8 shows the tuning data we obtained with the CdSe OPO, in comparson with theory

Figure 8. Composite angle-tuning for x-cut KTA-CdSe tandem OPO. Solid lines are calculated and points are experimental.

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