Electro-Optic Modulators:

Overview: Pockel Cells

Conoptics offers a complete line of Electro-Optic modulators that employ the Pockels Effect (linear electro-optics effect). The Pockels Effect is a change in phase of the induced ordinary ray, relative to the extraordinary ray when an electric field is applied. This change in phase makes the crystals a voltage variable wave plate, such that the output polarization will change from linear to elliptical to linear as a function of the applied voltage.

Installing a polarizer at the pockel cells output (crossed or parallel to the input polarization) converts the polarization change to an intensity change called “Amplitude Modulation”.

How do I allow for two rays transmit through a Pockel Cell?

For two rays to propagate through the pockel cell, the input polarization must be aligned such that it bisects the crystal axis. The input polarization should be set to 45 ° to the mechanical sides of the crystals.

What is a Transverse Field Modulator?

Conoptics modulators are designed as Transverse Field Modulators “TFM”. The electrical field is perpendicular to the light path. The TFM’s are temperature compensated by having pairs of crystals aligned such that what propagates down the fast axis in the first crystal propagates down the slow axis in the second crystal. The ½ wave voltage (minimum to maximum transmission) is a function of the crystals path length and crystal aperture size. Conoptics modulators offer apertures sizes compatible with most lasers so beam forming optics are typically not required.

Does Conoptics Offers Fiber EOM’s or Fiber Connect Option?

Conoptics is currently engineering a solution to interface input and output fibers to our pockel cells. By the end of 2011 we plan on releasing this option.

What is meant by the half-wave voltage?

This is the voltage (specified either at DC or at the maximum operating frequency) required for amplitude modulators, necessary to change from maximum transmission to maximum extinction.

What types of crystals are used?

Conoptics utilizes a number of different crystals in our electro-optic modulators. Crystal selection is dependent on wavelength, bandwidth and other requirements. Typical crystals are ADP (Ammonium Dihydorgen Phosphate), KD*P (Deuterated Potassium Phosphate), LTA (Lithium Tantalate) and Lithium Niobate.

What's the difference between a resonant modulator and a broadband modulator?

A resonant modulator operates at a specified frequency while a broadband modulator can operate over a wide frequency range.

How does a piezo resonance affect a modulator's performance?

The piezo-electric resonances are a “ringing” effect caused by applying electrical current to crystals. Because all electro-optic crystals are also piezoelectric, this effect occurs at specific frequencies most common 1-100MHz. Conoptics offers remedies to counter the most common piezo-electic resonances.

Does Conoptics offer an antireflection coating?

Conoptics does incorporate AR coatings on optics, windows and specific crystals used with our products. Our standard coating ranges are 400nm-to-80nm (Visible), 700nm-to-1200nm (NIR) and 1000nm-to-2000nm(IR). Conoptics also offers UV and DUV and V-Coat options at additional charge.

How much insertion loss can I expect?

Conoptics conservatively estimates transmission at 85%. Insertion loss is typically due to absorption, crystal cut and AR coatings. Generally speaking we expect a small loss per interface (windows and crystals). Also note that the shorter devices which use one or two crystals will have a slightly higher transmission than the four crystal devices. If you need a lower reflectivity, we offer custom coatings. Call us for more details.

How do I align the beam through your modulator?

To align the modulator, Conoptics has developed a Model 103 Alignment Tool. In order to minimize clipping through the modulator, a good rule of thumb is that the beam diameter should be less than one-third the aperture size. Conoptics has developed a step-by-step user guide for alignment provided with each sale or can be downloaded from our website.

What is the LDT for Conoptics modulators?

The Conoptics specification for LDT (Laser Damage Threshold) is as follows:

CW Laser, 500W/cm 2

Pulse Laser, 150MW/cm 2

How do I choose a modulator?

Choosing a modulator and modulation system requires and analysis of your requirements and then matching these requirements to the function and capabilities of our electro-optic modulator and modulation systems (including amplifiers). Our standard systems are available for intensity and or phase modulation with bandwidths out to 800MHz. A complete system includes matching an Electro-Optic Modulator with a Linear Amplifier. For a complete list please review the tables provided in the Modulation System Configuration section or by calling us directly.

Do you offer Resonant Tank Circuits?

Conoptics offers Resonant Tank Circuits (7:1 Step-Up) out to 50MHz for our resonant phase and amplitude modulators. The benefit of this package allows our customers to incorporate in-expensive amplifiers to drive the tank circuits meeting the voltage requirements for their application.

Can I use your phase modulator as an amplitude modulator?

Our phase modulators cannot be used as amplitude/intensity modulators, voltage variable wave-plates or polarization rotators. There are a number of reasons for this limitation. Our Phase Modulators are configured using the same crystal orientation. The input polarizations aligned along the induced ray. In addition, our amplitude modulators use at least two crystals which are aligned along their x-axis. When building a phase modulator with multiple crystals the alignment the crystals are set in-line opposed to along the x-axis.

Conoptics Amplitude Modulators

Conoptics amplitude modulators consist of an electro-optic modulator which includes an output polarizer. Internally the crystals are aligned along their x-axis. 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, simulating a voltage-tunable wave-plate. The modulation of the intensity is a sin2 function. Conoptics modulators allow for either vertical or horizontal input polarization.

Conoptics offers amplitude modulators for wavelengths from 192nm out to 2microns.

What is the extinction ratio?

The extinction ratio of an amplitude modulator is the ratio between the optical power at maximum and minimum transmission. Extinction ratios are dependent on the crystal employed. Conoptics offers three types of crystals for use in the UV, V, NIR and IR. Extinction ratios vary by model and are typically 500:1. In addition, Conoptics offers double-pass configurations allowing for 10,000:1 ER.

In order to optimize the ER ratio, fine adjustments are required in the position of the modulator aligned to the laser beam. Conoptics offers a Model 103 Alignment Tool for initial alignment, while the Model 102 Adjustable Modulator Mount is used for fine adjustment. Completed step-by-step alignment guide is available online in our support section.

High Voltage Amplifier Questions:

What type of loads can the amplifier drive?

Conoptics offers an extensive line of linear amplifiers. Drive configurations offered are Lumped Capacitance, 50ohm B.L, 50ohm S.E. and 100ohm B.L.

What is the maximum operating bandwidth of the high-voltage amplifier?

The load capacitance and output voltage together determine the maximum operating bandwidth of our linear amplifiers. Bandwidth varies by amplifier and Conoptics offers products from DC out to 500MHz.

 

Optical Isolators:

Does Conoptics offer Fiber based Optical Isolators?

At this time, Conoptics is in the engineering phase of a product release that incorporates fiber-connect for our complete line of optical isolators. We plan on releasing this product by year end.

How do I choose an Optical Isolator?

Conoptics offers both fixed wavelength and tunable Optical Isolators. Our product offers are from 350nm out to 1135nm. For best isolation, choose a model that has a tuning range max closest to your desired wavelength.

 

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