Application Note:
Phase and Amplitude Modulator Applications

Laser Frequency Stabilization

A Model 4001/3 or 4061/3 (U.S. Patent 5,189,547) resonant phase modulator is the ideal component to use in a Pound-Drever-Hall laser frequency stabilization system. This optical FM frequency discriminator technique* is used to lock the optical frequency of a laser to a stable Fabry-Perot reference cavity. The system consists of a single-frequency laser beam that is sinusoidally phase modulated and coupled into an axial mode of the Fabry-Perot resonator cavity. The stabilization signal is fed back to a high-voltage amplifier that drives a piezoelectric transducer (PZT). The frequency-stabilized light transmitted by the cavity is clean spatially as well as spectrally. *R.W.P. Drever, et al. “Laser Phase and Frequency Stabilization Using an Optical Resonator,” Appl. Phys. B31, pp. 97–105 (1983).

2006 PDH laser stabilization
Diagram for a Pound-Drever-Hall laser-frequency-stabilization system.

High-Frequency Optical Chopping

One application of amplitude modulators is high-frequency optical chopping. Although mechanical choppers are frequently used, they modulate the optical intensity at rates of only a few kilohertz. This frequency is often not high enough to get away from the 1/f noise of the detection system. An optical amplitude modulator, such as the Model 4103, can be used to chop the beam at 1 MHz, thus giving you shot-noise-limited detection.

Terahertz Optical Comb Generator


Researchers at JILA, University of Colorado, National Institute of Standards and Technology, and New Focus™ have used a prototype Model 4851 that resonated at 10.5 GHz to generate a spectrum of sidebands over 3-THz wide around a stabilized 633-nm HeNe laser source.** (That's over 250 sidebands at 10.5-GHz spacings!) Typically, phase modulation produces only a few sidebands. (See page 56 for a discussion of sideband generation.) However, by combining the efficiency of the Model 4851 with an optical resonator, the researchers were able to produce the spectrum shown in the graph below. Called optical comb generation, this technique produces a series of equally spaced spectral lines that extends over a wide frequency range around a cw optical carrier. One important application of optical comb generators is for high-resolution spectroscopy of various molecular and atomic transitions. These studies are usually performed with a laser locked to a well-known frequency reference, which limits one to studying transitions close to the laser frequency. In contrast, because optical comb generation enables wide frequency intervals to be coherently linked, a frequency-stabilized laser with an optical comb generator can be used to study transitions that are far from the laser's center frequency. For instance, a stabilized HeNe laser and an optical comb generator can be used to study several molecular iodine absorption lines around 633 nm as well as a neon transition that occurs at 633.6 nm. **J. Ye, L.-S. Ma, T. Day, and J. L. Hall, “Highly selective terahertz optical frequency comb generator,” Optics Letters, 22, No. 5 (1997).

99-485X Comb Generator Graph
The output spectrum of the comb generator measured using a Fabry-Perot cavity with a 2-THz free-spectral range.

Searching for the Fine-Structure Constant

One of our first Model 4851 9.2-GHz modulators was used by researchers at Stanford University to perform an atom-interferometry experiment. Under the direction of Nobel laureate Steven Chu, their goal was to precisely determine the fine-structure constant, a. Their method involved measuring the ratio of Planck's constant to the mass of the cesium atom and then combining their results with precision measurements of other physical quantities. (The fine-structure constant is the dimensionless coupling constant specifying the strength of the interaction between charged particles and photons.)

Although the fine-structure constant can be determined from quantum electrodynamic calculations involving the electron's Landé g factor, this new method provided a simpler determination. The atom interferometers in the experiment required two laser frequencies separated by the 9.2-GHz ground-state hyperfine splitting in cesium. By using the carrier frequency of a Ti:sapphire laser and one of the first-order sidebands generated by the phase modulator, a single laser was used as the light source for both frequencies, thus eliminating the need for an additional independent laser.
Inside view of a Model 4001 resonant phase modulator. U.S. Patent 5,189,547