A laser-diode-pumped thulium- and holmium-doped yttrium lithium fluoride (Tm,Ho:YLF) laser that operates at a single frequency and that can be both tuned and stabilized in frequency has been demonstrated in a laboratory setup. This demonstration is a significant achievement in a continuing effort to develop frequency-stabilized, frequency-agile lasers. Such a laser could be used, for example, as a local oscillator in a coherent lidar system, wherein its capabilities would be exploited to generate a signal with a precise nominal frequency plus or minus a time-varying Doppler compensation for the relative motion of the system and its target. There may also be uses for such lasers in laser diagnostics, and in fiber-optic communication and instrumentation systems.
The laboratory setup (see figure) includes two identical laser-diode-pumped Tm,Ho:YLF lasers, denoted the injection laser (IL) and the local oscillator (LO), respectively, in correspondence to the role that each would play in a fully developed lidar, communication, or instrumentation system. Each laser comprises a Tm,Ho:YLF crystal pumped by light at a wavelength of 794 nm from a fiber-coupled laser diode. A thermoelectric cooler is used to maintain the crystal at a temperature of -10 °C, where its optical-energy-conversion efficiency is greater than it is at room temperature.
The pumped surface of the crystal is coated for high transmittance at the pump wavelength and high reflectance at a nominal output wavelength of 2,060 nm. The surface opposite the pumped surface (the output surface of the crystal) is coated to minimize reflectance at the output wavelength. The output boundary of the laser cavity is defined by a curved output-coupling mirror that is coated for 98.5-percent reflectance at the output wavelength and is mounted facing the output surface of the crystal. Two etalons are mounted in the laser cavity, between the output crystal surface and the output-coupling mirror; these etalons are angle-tuned to enforce the desired single longitudinal laser mode and broad frequency-tuning range.
In the case of the LO, the output-coupling mirror is bonded to a piezoelectric transducer (PZT). The length of the laser optical cavity, and thus the laser frequency, can be varied by varying the voltage applied to the PZT.
In this setup, the LO can be stabilized and tuned with respect to the IL, by use of the following feedback loop: The outputs of the LO and IL are mixed in a photodiode to generate a beat note denoted intermediate frequency 1 (IF1). The amplified IF1 signal is mixed with a signal with a frequency between 1 and 2 GHz generated by a voltage-controlled oscillator (VCO). The resulting beat note is denoted intermediate frequency 2 (IF2). The IF2 signal is fed to a frequency discriminator that has a center frequency of 1 GHz. Whenever IF2 differs from 1 GHz, the discriminator generates an error signal, which is fed to an analog integrator. The integrator output is amplified to a high voltage, which can be applied to the PZT to drive IF2 toward 1 GHz. If the feedback loop is thus closed, then the system strives to maintain IF1 (the difference between the LO and IL frequencies) at a frequency that differs by 1 GHz from the output frequency of the VCO.
There are three modes of operation:
- Open-Loop Operation: The feedback loop is not used. Instead, the PZT voltage is controlled at will to tune the LO frequency directly. The tuning range is ±4 GHz.
- Locked Fixed Frequency: The feedback loop is used with the VCO set at a fixed frequency. As a result, the LO is maintained at a frequency that differs from the IL frequency by a fixed amount between 0 and 1 GHz, LO jitter is reduced, and LO drift is eliminated.
- Scanning Locked Mode: The feedback loop is used and a sinusoidal or other suitable waveform with a frequency < 1 Hz is superimposed on the VCO control voltage. As in the locked-fixed-frequency mode, LO jitter is reduced and LO drift is eliminated, but in this case, IF1 varies with the VCO control voltage. The amplitude of the waveform can be chosen to scan IF1over a range of ±1 GHz.
This work was done by Hamid Hemmati, Carlos Esproles, and Robert Menzies of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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Refer to NPO-20444
This Brief includes a Technical Support Package (TSP).
Tunable, frequency-stabilized diode-pumped thulium- and holmium-doped ytrium lithium fluoride
(reference NPO20444) is currently available for download from the TSP library.
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