Laser diode has linewidth below 30 Hz
Laser diodes are used far and wide largely because they are small, rugged, and efficient sources of coherent light. Though the devices have many desirable intrinsic properties, frequency stability is not one of them; typically, their emission wavelength varies with temperature. The simplicity of these lasers, however, is driving researchers to develop precise frequency-stabilization systems for them—systems that depend on external cavity elements, external wavelength measurement, and a feedback loop to lock the wavelength in place. Researchers at the University of Hamburg (Hamburg, Germany) have now reduced the linewidth of a laser diode to below 30 Hz using a high-finesse reference cavity, reaching a regime attractive for metrology or ultrahigh-resolution spectroscopy.1
The laser itself is constructed from a commercially available chip emitting 15 mW of output power at 657 nm. An extended cavity is formed by a holographic grating placed in the Littrow configuration. Unstabilized, the laser has a spectral linewidth of a few megahertz integrated over 1 s, and of approximately 200 kHz over 1 ms. The researchers chose to stabilize the laser in reference to a 14-kHz resonance of a Fabry-Perot etalon with a finesse of 1.1 × 105. The resonator was made by optically contacting two dielectric mirrors to a hollow cylindrical spacer made of ultralow-expansion glass.
About 1 mW of the laser power is directed to the reference resonator by an electro-optic modulator that generates 40-MHz sidebands with a power of 1% of that of the carrier. Light reflected from the incoupling mirror is sensed by a photodiode; the 40-MHz beat component produced by the nonresonant sidebands and resonant carrier is demodulated in a phase detector. The resulting error signal is fed to a piezoelectric transducer that adjusts the laser's holographic grating.
Larger frequency excursions of up to 5 MHz are compensated for by a fast injection-current feedback loop. The signal is much larger when the frequency is locked than when it is unlocked, because in the unlocked case the laser frequency is driven by noise so that it dances in and out of resonance in a time shorter than the 11-µs cavity decay time.
An analysis of spectral noise density shows three maxima. One peak at 3.8 kHz arises from the resonance of the piezo-controlled grating. A second peak at 100 kHz corresponds to the imperfect spectral response of the injection-current feedback circuitry. A third very large peak at 3 MHz results from relaxation oscillations of the fast-feedback branch.
"The laser stability of 30 Hz is with respect to the etalon," explains Andreas Hemmerich, one of the researchers. "It has to be noted that this relative frequency stability applies in the resonator rest frame for that part of the beam that is coupled into the resonator. After guiding the beam through a couple of shaky mirrors on the table, for example, to some spectroscopy cell, Doppler detunings may degrade the quoted stability. A mirror shaking at 200 Hz with amplitude of 1 µm produces frequency jitter of nearly 1 kHz. This is to point out that care is needed in exploiting the quoted stability potential."
To provide absolute stability, the cavity must be isolated from all sorts of environmental influences such as acoustic noise. Optimized adjustment of the injection-current feedback loop, along with a further increase of the reference cavity finesse and servo bandwidth, could then potentially drive the laser-diode linewidth deep into the subhertz domain, say the researchers.
Battery-powered frequency standard
One possible use of the stabilized laser diode is as a portable optical frequency standard based on the 400-Hz calcium intercombination line. This transition consists of three components, one of which is nearly insensitive to magnetic and electric fields and is thus considered as a possible frequency reference in an optical clock. To exploit this line in a portable frequency standard, spectroscopy in an atomic beam has to be performed. The stabilized laser diode would replace the conventional dye-laser light source, making possible a compact, battery-driven frequency reference.
REFERENCE
- A. Schoof et al., Opt. Lett. 26 (20), 1563 (October 15, 2001).
John Wallace | Senior Technical Editor (1998-2022)
John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.