electro-optic modulators (original) (raw)

Acronym: EOM

Definition: optical modulators based on the electro-optic effect

Alternative terms: electrooptic modulators, electro-optical modulators, Pockels cells

Categories: nonlinear optics, photonic devices

• optical modulators
  • acousto-optic modulators
  • electro-optic modulators
  • Pockels cells
  • electroabsorption modulators
  • liquid crystal modulators
  • intensity modulators
  • phase modulators

Related: electro-optic effectelectro-opticselectro-optic Q-switchesPockels effectPockels cellsPockels cell driverspulse pickersnonlinear crystal materialsoptical modulatorsintensity modulatorsphase modulatorscavity dumping

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Contents

What are Electro-optic Modulators?

An electro-optic modulator (EOM) (or electrooptic modulator) is a device which can be used for controlling the power (→ intensity modulators), phase (→ phase modulators) or polarization of light with an electrical control signal. Usually, such a device is based on the linear electro-optic effect (also called the Pockels effect), i.e., the modification of the refractive index of a nonlinear crystal by an electric field in proportion to the field strength. Not very common are Kerr cells, where the quadratic electro-optic effect is utilized. In some cases, modulators are based on the electro-optic effect in a wider sense, e.g. involving changes in absorption; for example, there are electroabsorption modulators based on the Franz–Keldysh effect.

Typically, an electro-optic modulator contains one or two Pockels cells, and possibly additional optical elements such as polarizers. Different types of Pockels cells are shown in Figure 1 and are described in more detail in the article on Pockels cells.

Most EOMs are operated with free-space laser beams, but there are also fiber-coupled modulators, where the Pockels cell is placed between two fiber collimators. Such devices typically have an insertion loss around 4 dB and can handle only limited power levels, e.g. 50 mW.

Frequently used nonlinear crystal materials for EOMs are potassium di-deuterium phosphate (KD*P = DKDP), potassium titanyl phosphate (KTP), beta-barium borate (BBO) (the latter for higher average powers and/or higher switching frequencies), also lithium niobate (LiNbO3), lithium tantalate (LiTaO3) and ammonium dihydrogen phosphate (NH4H2PO4, ADP). In addition to these inorganic electro-optic materials, there are also special poled polymers for modulators.

Figure 1: Pockels cells of various types.

The voltage required for inducing a phase change of ($\pi$) is called the half-wave voltage (($V_{\pi }$)). For a Pockels cell, it is usually hundreds or even thousands of volts, so that a high-voltage amplifier is required. Suitable electronic circuits can switch such large voltages within a few nanoseconds, allowing the use of EOMs as fast optical switches; such drivers need to provide substantial currents due to the electric capacitance of a Pockels cell (which should be minimized for fast switching or modulation). In other cases, a modulation with smaller voltages is sufficient, e.g. when only a small amplitude or phase modulation is required.

Apart from the above described bulk-type modulators, there are also modulators where the optical radiation is confined by a waveguide. Such devices can be realized, e.g. on lithium niobate (LiNbO3), which has substantial electro-optic coefficients. Due to the small electrode distances, such devices can work with relatively low electrical voltages, and they can also allow for quite high modulation frequencies. There are also modulators based on semiconductor materials such as aluminum gallium arsenide (AlGaAs) or indium phosphide (InP).

Types of Electro-optic Modulators

Phase Modulators

The simplest type of electro-optic modulator is a phase modulator containing only a Pockels cell, where an electric field (applied to the crystal via electrodes) changes the phase delay of a laser beam sent through the crystal. The polarization of the input beam often has to be aligned with one of the optical axes of the crystal, so that the polarization state is not changed.

Many applications require only a small (periodic or nonperiodic) phase modulation. For example, this is often the case when one uses an EOM for monitoring and stabilizing a resonance frequency of an optical resonator. Resonant modulators (see below) are often used when a sinusoidal modulation of fixed frequency is required, and make possible a large modulation depth with a moderate drive voltage. The modulation depth can in some cases be so high that dozens of sidebands are generated in the optical spectrum (comb generators, → frequency combs).

Note that an electro-optic modulator is not suitable for frequency modulation, or precisely speaking only for limited short-term frequency changes. For example, it could not be used to generate a constant change in optical frequency of an optical signal, since that would imply a linearly increasing phase delay (without any limit to the phase excursion).

Polarization Modulators

Depending on the type and orientation of the nonlinear crystal, and on the direction of the applied electric field, the phase delay can depend on the polarization direction. A Pockels cell can thus be seen as a voltage-controlled waveplate, and it can be used for modulating the polarization state. For a linear input polarization (often oriented at 45° to the crystal axes), the output polarization will in general be elliptical, rather than simply a linear polarization state with a rotated direction, but a 90° change in linear polarization direction is possible with a relative phase change of ($π$) between the two axes of the modulator. With a random drive signal, one may realize a polarization scrambler.

Amplitude or Intensity Modulators

Combined with other optical elements, in particular with polarizers, Pockels cells can be used for other kinds of modulation. In particular, an amplitude modulator (Figure 2) is based on a Pockels cell for modifying the polarization state and a polarizer for subsequently converting this into a change in transmitted optical amplitude and power.

Figure 2: Electro-optic intensity modulator, containing a Pockels cell between two polarizers.

An alternative technical approach is to use an electro-optic phase modulator in one arm of a Mach–Zehnder interferometer to obtain amplitude modulation. This principle is often used in integrated optics (for photonic integrated circuits), where the required phase stability is much more easily achieved than with bulk optical elements.

optical switches are modulators where the transmission is either switched on or off, rather than varied gradually. Such a switch can be used, e.g., as a pulse picker, selecting certain pulses from a train of ultrashort pulses, or in cavity-dumped lasers (with an EOM as cavity dumper) and regenerative amplifiers.

Technical Details

Temperature Drifts; Thermally Compensated Devices

In configurations where the induced relative phase change between two polarization directions is used, thermal influences can be disturbing. They result in a drift of the operation point, which may have to be compensated with an automatically adjusted bias voltage. Additional electronics may be used for such purposes, deriving the required bias voltage from some optical signals.

Some electro-optic modulators contain two matched Pockels cells in an athermal configuration where the temperature dependence of the relative phase shift is largely canceled. There are also configurations with four crystals of exactly the same length, canceling both birefringence effects and spatial walk-off. Various types of multi-crystal designs are used, depending on the material and the exact requirements.

Resonant Versus Broadband Devices

For some applications, a purely sinusoidal modulation with a fixed frequency is required. In that case, it is often beneficial to use an electrically (not mechanically) resonant electro-optic modulator, containing a resonant LC circuit. The input voltage of the device can then be substantially lower than the voltage across the electrodes of the Pockels cell. A high ratio of these voltages requires a high Q-factor of the LC circuit and reduces the bandwidth in which strong resonant enhancement can be achieved. The disadvantage of using a resonant device is that one loses flexibility: changing the resonance frequency requires the exchange of at least one electric component.

Broadband modulators are optimized for operation in a wide frequency range, which typically starts at zero frequency. A high modulation bandwidth typically requires a Pockels cell with a small electric capacitance, and excludes the exploitation of a resonance.

Traveling-Wave Modulators

For particularly high modulation bandwidths e.g. in the gigahertz region, integrated optical traveling-wave modulators are often used. Here, the electric drive signal generates an electromagnetic wave (microwave) propagating along the electrodes in the direction of the optical beam. Ideally, the phase velocities of both waves are matched (through an appropriate electrode design) so that efficient modulation is possible even for frequencies which are so high that the electrode length corresponds to several wavelengths of the microwave.

Plasmonic Modulators

Plasmonic modulators are a special type of electro-optic modulators which exploit the formation of plasmons (a special type of electromagnetic excitation) at metal surfaces, which lead to surface plasmon polaritons (SPPs). They can be extremely fast while having a low energy consumption.

Important Properties

A number of properties should be considered before purchasing an electro-optic modulator:

• The device must have a sufficiently large open aperture, particularly in cases with high peak powers. A high crystal quality and appropriate electrode geometry are required for uniform switching or modulation across the full open aperture. The price can substantially rise for increasing aperture sizes.
• For switching ultrashort pulses, effects of Kerr nonlinearity and chromatic dispersion may be relevant, which depend on the crystal material and length and also on the beam radius. (Significant effects of this kind often cannot be avoided and thus have to be taken into account in the design of, e.g., a regenerative amplifier.)
• Depending on the device design, the polarization of the incoming beam may or may not be maintained in the output.
• A phase modulator may generate unwanted amplitude modulation, and vice versa. This depends strongly on the design.
• As electro-optic materials are also piezoelectric, the applied voltage can introduce mechanical vibrations, which themselves can affect the refractive index via the elasto-optic effect. Around certain mechanical resonance frequencies, the modulator response may be strongly modified. This can be a problem particularly for broadband modulators. In switching applications, unwanted ringing effects can occur. Such effects depend strongly on the crystal material, dimensions, orientation and mechanical design.
• Both high optical average powers and high switching frequencies can induce thermal problems. The thermal handling and thus the power and frequency capabilities depend on various construction details.
• The crystal(s) should have high-quality anti-reflection coatings, designed for the required range of operation wavelengths, and of course a good material transparency, to minimize the insertion loss.
• Rejected optical beams may be absorbed within the modulator device, or (particularly for high-power devices) leave the device at some location and direction.
• The switching speed (rise time, fall time) depends on properties of both the modulator (e.g. via its capacitance) and the electronic driver.
• Electro-optic modulators can be purchased in fiber-coupled form, with different types of connectors and fibers (e.g. single-mode or multimode).

Note that a proper mechanical mount is also required, often with means to align the modulator precisely in various directions.

Electronic Drivers

It is important to use an electronic driver which is both well matched to the EOM and suitable for the particular application. For example, different kinds of EOMs require different drive voltages, and the driver should also be designed for the given electrical capacitance of the EOM. Some drivers are suitable for a purely sinusoidal modulation, whereas broadband devices work in a large range of modulation frequencies. Many problems can be avoided by purchasing an electro-optic modulator together with the electronic driver from the same supplier because the responsibility for the overall performance is then in one place.

See also the article on electronics for photonics.

Applications

Some typical applications of electro-optic modulators are:

• modulating the power of a laser beam, e.g. for laser printing, high-speed digital data recording, or high-speed optical communications
• in laser frequency stabilization schemes, e.g. with the Pound–Drever–Hall method
• active mode locking (where the EOM modulates the resonator losses or the optical phase with the round-trip frequency or a multiple thereof)
• switching pulses in pulse pickers, regenerative amplifiers and cavity-dumped lasers

For Q-switching of solid-state lasers (where the modulator serves to block the laser resonator before the pulse is to be emitted), specialized electro-optic Q-switches are used.

Frequently Asked Questions

What is an electro-optic modulator?

An electro-optic modulator (EOM) is a device that uses an electrical signal to control the power, phase, or polarization of a light beam. It is typically based on the electro-optic effect in a nonlinear crystal.

How does an electro-optic modulator work?

Most EOMs utilize the Pockels effect, where an applied electric field linearly changes the refractive index of a crystal. This alters the phase of light passing through it, which can be used to modulate the light's properties.

What is the half-wave voltage of an EOM?

The half-wave voltage, or ($V_{\pi}$), is the voltage needed to induce a phase shift of ($\pi$). For typical Pockels cells, this is hundreds or thousands of volts, requiring a high-voltage amplifier.

What are the main types of electro-optic modulators?

The primary types are phase modulators, which directly alter the light's phase; polarization modulators, which act as voltage-controlled waveplates; and amplitude (or intensity) modulators, which use a polarizer to turn polarization changes into power changes.

What are common applications of electro-optic modulators?

EOMs are used for high-speed power modulation in communications, active mode locking and Q-switching in lasers, and for switching optical pulses in pulse pickers and regenerative amplifiers.

What is the difference between a bulk and a waveguide EOM?

Bulk modulators are for free-space laser beams passing through a crystal. Waveguide modulators confine light in a small channel on a chip, allowing for lower operating voltages and higher modulation frequencies.

What is a resonant EOM?

A resonant EOM is designed for modulation at a specific, fixed frequency. It uses an electrical resonant circuit to achieve a large modulation depth with a moderate drive voltage, but it is not suitable for broadband operation.

Why do EOMs require a special electronic driver?

A special driver is needed to supply the high voltages (often hundreds of volts) required by the EOM and to rapidly charge and discharge the device's electrical capacitance for fast switching or high-frequency modulation.

Suppliers

Sponsored content: The RP Photonics Buyer's Guide contains 48 suppliers for electro-optic modulators. Among them:

⚙ hardware

GWU's portfolio of electro-optical devices can meet even demanding requirements. The BBO or KD*P Pockels cells offer a high extinction ratio, fast switching capability and a high damage threshold. They can be configured in flexible design, either free-space or fiber-coupled. We also offer driver electronics.

⚙ hardware🧩 accessories and parts🧴 consumables🔧 maintenance, repair📏 metrology, calibration, testing💡 consulting🧰 development

Hangzhou Shalom EO offers Pockels cells and based on DKDP, BBO, LiNbO3, and MgO:LiNbO3 crystals. Shalom EO’s Pockels cells, available in stock and custom versions, deliver the competitive edges of excellent optical transmission at your wavelengths of interest, low half-wave voltage, high damage threshold, and low insertion loss for ideal applications like pulse picking and laser cavity dumping. Various standard and custom extinction ratios are available. Recently, we have succeeded in developing a 3-pin connector DKDP Pockels cells for 755 nm alexandrite lasers manufactured using 1 pce of 99% deuteration DKDP crystal with 3 gold plating.

In addition, Shalom EO also provides off-the-shelf and custom electro-optic crystals for EO modulation. The crystals feature rich dimension/orientation options, and large apertures, while also boasting high-precision polishing and different kinds of coatings. Cr–Au electrodes can be deposited. Various substrate materials are available: DKDP, BBO, LiNbO3, MgO:LiNbO3, TeO2, and HGTR-KTP crystals. EO modulation crystals could be offered in the forms of crystal boules, crystal blanks, and crystals with AR coatings and electrodes.

⚙ hardware

Electro-optic crystals like LiNbO3, LiTaO3, DKDP, BBO, KTP and RTP crystals in conjunction with HF drivers are available for phase or amplitude modulation. ALPHALAS has a large stock of Pockels cells and phase modulators for most of the standard laser wavelengths.

⚙ hardware

Raicol Crystals offers electro-optic modulators based on RTP (with a temperature compensating design) or BBO.

Besides, we can supply super-polished LBO crystals which can be used as the center piece of an electro-optic modulator. With its high resistance to laser-induced damage, LBO is very suitable for high-power applications, and its transparency range is large (160 nm — 2600 nm). We offer highly homogeneous crystals with up to 100 × 100 mm2 and very low bulk absorption.

⚙ hardware

With the introduction of the OM6N series of high-speed optical modulators, Thorlabs has designed and manufactured a new all-reflective technology based on deformable mirrors for modulating laser power in the 700 — 1350 nm range. This Pockels cell or AOM alternative introduces nearly zero dispersion to ultrafast laser pulses. This solution extends Thorlabs’ existing collection of adaptive optics and EO modulators.

Bibliography

(Suggest additional literature!)

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