Technical Note
New Focus™ offers a broad line of optical modulators and drivers that are versatile, reliable and easy to use. All our optical modulators are based on the electro-optic or Pockels’ effect—the linear dependence of the index of refraction on an applied electric field. Applying a voltage across
the electrodes of an electro-optic crystal changes the effective refractive index
and thus the phase of light as it passes through the crystal.
Our amplitude and phase modulators span the frequency range from DC to 9.2 GHz, and feature low drive voltages, low insertion losses, and high maximum optical powers. They use lithium niobate (LiNbO3), magnesium-oxide-doped lithium niobate (MgO:LiNbO3), and KTP crystals which have large electro-optic coefficients minimizing required drive voltages. In addition, the small loss tangents at RF frequencies of LiNbO3 and KTP permit operation of these devices over a broad range of frequencies from DC to 9.2 GHz. LiNbO3
and KTP are also non-hygroscopic, and have high maximum optical-power limits and low optical insertion loss.
When choosing a modulator, keep in mind whether you need phase or amplitude modulation, broadband versus resonant (or single-frequency) operation, and over what wavelength range you are operating. The last will determine the AR coatings on the crystal.
Transmitted intensity spectrum from a scanning Fabry-Perot optical spectrum analyzer. The 1.06-µm laser was phase modulated with a Model 4003 resonant phase modulator. The driving frequency was 7.94 MHz and the peak voltage was 3 V.Phase Modulators
Phase modulators are used to vary the phase of an optical beam. When driven sinusoidally, phase modulators can generate frequency sidebands on a cw optical beam. Sinusoidal phase modulation at a frequency Ω generates frequency sidebands at multiples of Ω about the central optical frequency, w.
Given a sinusoidal phase modulation at frequency Ω and a peak phase modulation m, the phase variation is ø(t)=msin(Ωt). The electric field of the optical beam after passing through the modulator can be written:
Top: When a phase modulator is used, the laser beam should be well collimated and its polarization should be oriented vertically to within 1°. For an unpolarized laser, the polarizer should have an extinction ratio greater than 100:1. We recommend our
Glan-Thompson polarizers (page 308) or our low-cost sheet polarizers (page 309).
Bottom: In our amplitude modulators, we mount the crystals at 45˚. The input beam should be either vertically or horizontally polarized.
NOTE: Polarizers (pages 308–309) are available separately.
he amplitude of the kth sideband is proportional to Jk(m), where Jk is the Bessel function of order k. The fraction of optical power transferred into each of the first-order sidebands is [J1(m)]2, and the fraction of optical power that remains in the carrier is [J0(m)]2.
For example, imposing a phase modulation with peak phase shift of 1 radian will transfer 19% of the initial carrier power to each of the first-order sidebands and leave 59% of the power in the carrier. The maximum power that can be transferred to the first-order sidebands is about 34%, which requires a peak phase shift of 1.8 radians.
Top: This spectrum of a phase-modulated electric field is given by Bessel functions. The optical intensity of each sideband is proportional to the square of the electric field amplitude.
Bottom: The transfer function of an amplitude modulator between crossed polarizers is a sin2 function. You can achieve linear amplitude modulation with small modulation voltages by biasing the modulator at the 50% transmission point, either with a quarter-wave plate or by applying a DC voltage to the modulator.
Amplitude Modulators
A bulk electro-optic amplitude modulator consists of a voltage-tunable wave plate followed by a polarizer.† Thus, the modulation of the intensity is a sin2 function. (See figures at bottom left.) If the input polarization is oriented at 45° to
the crystal axes, the applied voltage will produce a variable phase delay between the ordinary and extraordinary field components.
New Focus™ simplifies your optical setup by mounting the crystal at 45°. Thus, the input polarization can be either vertical or horizontal.
In order to suppress birefringence variations due to temperature changes, we use two matched crystals arranged in series with their applied electric fields oriented at 90° relative to each other. Our amplitude modulators exhibit less than 1 mrad/°C of temperaturedependent polarization rotation. We cancel thermal birefringence while doubling the electro-optically induced polarization rotations by reversing the crystal axes such that both polarization components travel equal optical paths in the ordinary and extraordinary orientations.
†We do not recommend using a general-purpose phase modulator as an amplitude modulator. This will result in a slowly varying amplitude modulation, due to the temperature-dependent birefringence of the phase-modulator crystal.
Broadband Versus Resonant Modulators
Our modulators are available in both broadband and resonant configurations. Broadband modulators can be driven over a range of frequencies, while resonant modulators operate at a single customer-specified frequency.
The advantage of the broadband devices is that they can be operated from DC to 100 MHz (200 MHz for the Model 4104 amplitude modulator), making them appropriate for applications where modulation over a broad frequency range is required. However, since the input drive voltage is applied directly across the crystal electrodes, these devices require a relatively high drive voltage, making it difficult to achieve large modulation depths.
For applications requiring modulation at a single frequency, resonant modulators are preferred because much higher modulation can be achieved with a given drive voltage.
To compare the advantages of resonant enhancement, we use the half-wave voltage, Vπ, which is the voltage required to produce a π phase shift. The Vπ of a broadband phase modulator at 1.06 µm is 210 V, corresponding to a modulation depth of 0.015 rad/V. These values scale with wavelength, so at 532 nm, Vπ is 105 V and the modulation depth is 0.03 rad/V. In contrast, the Model 4001 and 4003 resonant phase modulators have much higher modulation depths: Vπ at 1.06 µm is typically 10–31 V, corresponding to a modulation depth of 0.1–0.3 rad/V.
NOTE: For these modulators, the modulation depth is typically 0.3 rad/V in the low-frequency range (0.01–20 MHz) and drops to 0.1 rad/V at high frequencies (120–200 MHz).